bluuebunny/crossref_metadata_2025 huggingface dataset:
Zur Reaktion von Dicyclopentadiendiepoxid mit Acrylsäure
Authors: Heublein Günther , Burckhardt Astrid
Published: December 2004

On the Reaction of Dicyclopentadiene Diepoxide with Acrylic Acid

The esterification of dicyclopentadiene diepoxide as a bifunctional epoxide with acrylic acid was investigated. The influence of solvent polarity, reaction temperature and the catalytic system used are discussed. It was found, that the reaction rate increased with increasing solvent polarity. By using tertiary amines as catalysts, only a small catalytic effect was observed. On the contrary, a good catalytic effect was achieved with Lewis‐acids. By means of FeCl3, a vinylester containing epoxide groups and CC‐double‐bonds is obtained, which polymerizes via various mechanisms.
Kinetischer H/D‐Isotopeneffekt der Photooxidation von 3,5‐Dimethyl‐1,1‐dioxo‐2‐(p‐tolyl)‐1,2‐thiazin durch Singulettsauerstoff
Authors: Fanghänel E. , Bode B. , Bedemann K.‐H. , Radeglia R.
Published: December 2004

Kinetic H/D Isotope Effect of the Photooxidation of 3,5‐Dimethyl‐1,1‐dioxo‐2‐(p‐tolyl)‐1,2‐thiazine by Singlet Oxygen

The hydrogen atoms of the thiazine ring and of the methyl groups of 3,5‐dimethyl‐1,1‐dioxo‐2‐(p‐tolyl)‐1,2‐thiazine 1a can be exchanged by deuterium in basic or acidic deutero methanol (CH3OD). In CD3OD/D2SO4 the rate of the H/D‐exchange of the different protons of 1a was determined by 1H‐n.m.r. spectroscopy.

A kinetic isotope effect of 1.5–1.8 was determined from the rate of the oxygen consumption and also by using a competitive method. The results are in accordance with an “ene” reaction of 1a with singlet oxygen.
Chinolinanaloge Chalkone: Wittig‐Synthese, 13C‐NMR‐Spektren und Bestimmung von σ‐Konstanten für die 2‐, 3 und 4‐Position des Chinolins
Authors: Bödeker J. , Fieblinger D. , Radeglia R.
Published: December 2004

Quinolineanalogous Chalcones: Wittig Synthesis, 13C‐N.M.R. Spectra and Estimation of σ‐Constants for the 2‐, 3‐ and 4‐Position of Quinoline

The Wittig type reaction of the quinolinealdehydes 2a—c with benzoylmethylentriphenylphosphorane (1) gives stereoselectively the E‐configurated quinolineanalogous chalcones 3a—c. These compounds react with methyl iodide to the N‐methyl‐quinolinium salts 4a—c. The complete 13C‐n.m.r. analysis of 3a—c is reported. From the chemical shift of the Cα‐signals and on the basis of a Hammett relationship σ‐constants were calculated for the 2‐, 3‐ and 4‐position of quinoline. These values are compared with those from kinetic measurements; the observed deviations are chiefly attributed to the extent of the resonance interaction in 3a—c.
Vibronic Spectral Behaviour of Molecules: Theoretical study on the vibrational structure of 9‐phenyl anthracene S1S0 absorption and fluorescence
Authors: Gustav K.
Published: December 2004

Underlying the completely‐optimized S0 and S1 molecular geometries of 9‐phenylanthracene, the vibronic ‘line’ spectra of the S0S1 absorption and fluorescence have been calculated within the Condon approximation. On this basis the corresponding theoretical spectrum contours as obtained by superposition of individual Gaussian line width functions are presented. For the spectroscopic peaks an analysis of the vibronic levels is given. The theory/experiment comparison of the respective vibrationally structured spectra of 9‐phenyl‐anthracene turns out to be satisfactory. It confirms likewise the geometric structure data calculated for both electronic states.
Spaltung von 1,2,4‐Thiadiazol‐3‐onen mit Triphenylphosphan: Triphenylphosphonio‐thioimidazolate und ihre Folgereaktionen
Authors: Tittelbach Franz , Martin Dieter
Published: December 2004

Fission of 1,2,4‐Thiadiazol‐3‐ones by Triphenylphosphane: gewidinet Triphenylphosphonio Thioimidazolates and Their Consecutive Reactions

Treatment of benzimidazo[1,2‐d][1,2,4]thiadiazol‐3(2H)‐ones (1) and imidazo[1,2‐d]‐[1,2,4]thiadiazol‐3(2H)‐ones (17) with triphenylphosphan leads to the liberation of isocyanate and the formation of triphenylphosphonio‐thiobenzimidazolat (4) and triphenylphosphonio‐thioimidazolat (18), respectively. 2,4‐Diphenyl‐5‐phenylimino‐1,2,4‐thiadiazol‐3‐one (23) is desulfurized with Ph3P and decomposed into phenyl isocyanate and diphenylcarbodiimide whereas the N‐unsubstituted imino‐thiadiazol‐3‐one (25) is attacked at the exocyclic imino group by Ph3P to give the N‐phosphoranylidene thiourea (26). The structure of 4 can be rationalized as a dynamic system between polar and apolar valence isomeres. Reactions of 1 and 17 with cyanates, isocyanates, carbon disulfide, acetic anhydride, amines, benzyl chloride, grignard compounds, and CH acidic compounds are described.
Action of Grignard Reagents on 6‐Arylpyridazin‐3(2H)‐thiones
Authors: Fekry Ismail M. , Kaddah A. M. , Elkafrawy A. F. , Sayed Fekria S. , El‐Sawy O. M.
Published: December 2004

6‐Arylpyridazin‐3(2H)‐thiones (1a—c) react with Grignard reagents by 1,4‐addition to give either 4‐substituted 6‐aryl‐4,5‐dihydropyridazin‐3(2H)‐thiones (2a—c) or their dehydrogenated products, namely 4‐substituted 6‐arylpyridazin‐3(2H)‐thiones (3d—k). The reaction of Grignard reagents with 6‐aryl‐4‐methylpyridazin‐3(2H)‐thiones (3d—f) proceeds also by 1,4‐addition yielding 4,4‐disubstituted 6‐aryl‐4,5‐dihydropyridazin‐3(2H)‐thiones (5). Structural assignments are based on analytical and spectral data and also on the synthesis of authentic samples in some cases.
The “normalizing factor” for the tomato mutant chloronerva. XXX. Synthesis of 3,4‐Seconicotianamine
Authors: Ripperger H.
Published: December 2004

3,4‐Seconicotianamine (7) was synthesized by stepwise reductive alkylation of (S)‐N‐methylalanine ethyl ester (3) with ethyl (S)‐4‐oxo‐2‐(trifluoroacetylamino)butanoate (4). 3,4‐Seconicotianamine showed biological activity with regard to the phenotypical normalization of the tomato mutant chloronerva, but to a lesser degree than nicotianamine (1).
Nucleophile Substitutionen an Bischlornitrosoverbindungen. IV. Sulfonylaminooxime als Extraktionsmittel für Kupfer(II)
Authors: Beger J. , Neumann R. , Gloe K. , Mühl P.
Published: December 2004

Nucleophilic Substitutions with Bischloronitroso Compounds. IV. Sulfonylaminooximes as Extracting Agents for Copper(II)

New vicinal sulfonylaminooximes 5a–5v are synthesized from bischloronitroso compounds 1 or 2‐chlorooximes 2 and sulfonamides 4 under alkaline conditions. In several cases oligomeric products 6 result instead of 5. 2‐Sulfonylaminobenzophenoneoximes 8a–g are also prepared from 2‐aminobenzophenones and sulfonyl chlorides and subsequent reaction with hydroxylamine. The pH0,5 values for the extraction of copper‐(II)‐ions into a chloroformic solution are between 2,8 and 4. Compounds 8 are comparably weaker extragents then oximes 5.
Alkylierung und Umalkylierung von Aromaten an Alumosilicaten
Authors: Vogt F. , Wendlandt K.‐P. , Isakov Ja. I. , Isakova T. A. , Minačev Ch. M.
Published: December 2004

Alkylation and Transalkylation of Aromatics on Alumosilicates

The alkylation of toluene (benzene) with methanol and the isomerization of metaxylene on alumina‐silica and modified zeolites (HNaY, HM, HZSM‐5) are acid‐catalyzed proceed via a same intermediate. Except for the samples HZSM‐5, the alkylation of toluene (benzene) can be explained by activity — selectivity — correlations.

The referential diffusion of para‐xylene in the channels of zeolithe ZSM‐5 leads to a high product selectivity in alkylation and transalkylation reactions. Suggesting the supposition of a triangular reaction scheme.
The improved Synthesis of 8‐Methylene‐des‐AB‐cholestan‐9‐one
Authors: Morzycki Jacek W. , Rodewald Wladyslaw J. , Jurek Jaroslaw
Published: December 2004

An improved synthesis of the title compound 6 is described. The starting cholesterol was transformed into 3β‐methoxy‐5β‐methyl‐19‐nor‐9,10‐secocholestane‐6,9,10‐trione (6E)‐monooxime derivative (2b). Its fragmentation under acidic conditions afforded 2‐cyano‐4‐methoxy‐2‐methylcyclohexanone (8) and 8‐methylene‐des‐AB‐cholestan‐9‐one (6). The mechanisms of this reaction and the isomerization of the latter compound to 8‐methyl‐des‐AB‐cholest‐8(14)‐en‐9‐one (7) are discussed.
Tetrazolverbindungen. 2. (1‐Aryl‐1H‐tetrazol‐5‐yl)‐acetaldehyd‐Derivate aus 1‐Aryl‐5‐(2‐dimethylamino‐vinyl)‐1H‐tetrazolen
Authors: Fischer G. W.
Published: December 2004

Tetrazole Compounds. 2. 1‐Aryl‐1H‐tetrazole‐5‐acetaldehyde Derivatives from 1‐Aryl‐5‐(2‐dimethylamino‐vinyl)‐1H‐tetrazoles

1‐Aryl‐5‐(2‐dialkylamino‐vinyl)‐1H‐tetrazoles 2, easily accessible by the reaction of 3‐chloro‐propeniminium salts 1 with excess sodium azide in refluxing alcohols, represent enamines of the hitherto unknown 1‐aryl‐1H‐tetrazole‐5‐acetaldehydes 3. Attempts to isolate the latter after acid‐catalyzed hydrolysis of 2 were unsuccessful. However, generated in situ, they can be trapped by suitable aldehyde reagents. Thus, starting from 1‐aryl‐5‐(2‐dimethylamino‐vinyl)‐1H‐tetrazoles 4, the 4‐nitrophenylhydrazones 5a‐‐hyphen;i, 2,4‐dinitrophenylhydrazones 5j‐‐hyphen;r, and thiosemicarbazones 6 of the corresponding 1‐aryl‐1H‐tetrazole‐5‐acetaldehydes 3 were obtained. Analogously, acid‐catalyzed hydrolysis of 4 in the presence of 1,2‐dianilinoethane resulted in 1‐aryl‐5‐(1,3‐diphenyl‐2‐imidazolidinylmethyl)‐1H‐tetrazoles 7. ‐‐hyphen; The structure of the aldehyde derivatives 5, 6, and 7 was proved by 1H n.m.r. spectroscopy.
Synthese einiger neuer 3,4‐Diarylsydnone
Authors: Csongar Ch. , Müller I. , Slezak H. , Klebsch H.‐J. , Tomaschewski G.
Published: December 2004

Synthesis of Some New 3,4‐Diarylsydnones

The synthesis of some new sydnones 4 is described. Their characteristic absorption maxima, mass spectroscopic fragmentation and CO‐valence vibrations are given. The sydnones 4 are synthezid by nitrosation of C, N‐diarylglycines 3 (from strecker synthesis) and subsequent cyclization with Ac2O. The primarily formed C,N‐diaryl‐α‐aminoacetic acid nitrils (1) are decomposed with conc. HCl yielding glycines (3) in good yields.
Iminoformylierung substituierter Crotonnitrile. III. Zur Stereochemie und innermolekularen Beweglichkeit von 4‐Amino‐1,1‐dicyanbuta‐1,3‐dienen
Authors: Michalik Manfred , Zahn Kerstin , Köckritz Peter , Liebscher Jürgen
Published: December 2004

Iminoformylation Reaction of Substituted Crotonnitriles. III. Stereochemistry and Intramolecular Mobility of 4‐Amino‐1.1‐dicyano‐buta‐1.3‐dienes

1H‐ and 13C‐n.m.r. spectroscopic investigations show the preference of s‐trans configuration and the push‐pull character of the investigated 4‐amino‐1,1‐dicyano‐buta‐1,3‐dienes. Dynamic 1H‐ and 13C‐n.m.r. data for the rotation process about the CN bond are determined. The activation energies are discussed with respect to electronic and steric effects of substituents and to solvents polarity.
Preparation and cyclisation reactions of 3‐Carboxy‐4‐naphthylbut‐3‐enoic Acids
Authors: Guirguis Nadia R. , Awad Boshra M. , El‐Mariah Fatma A. A.
Published: December 2004

The condensation of naphthalene‐1‐ or 2‐carbaldehyde with dimethyl 2,2‐dimethyl‐succinate (Stobbe conditions) gives predominantly the (Z)‐hemiesters 3 and 11a. The hemiester 3 contains a small amount of the (E)‐isomer. Similar condensation of naphthalene‐2‐carbaldehyde with dimethyl‐2‐methylsuccinate gave the hemiester (E)‐12a. Their configuration and the ratios of the (Z:E)‐hemiesters, their derived acids, anhydrides, indenones and phenanthrene derivatives are inferred by high resolution 1H‐n.m.r. spectroscopy.
Verbesserte Einsatzmöglichkeiten des 4‐Methoxybenzylrestes als 2′‐OH‐Schutz bei Ribonucleotidsynthesen durch TFA‐Acidolyse
Authors: Losse G. , Pechstein Birgit
Published: December 2004

An Improved Method for the Application of the 4‐Methoxybenzyl Group to Protect the 2′‐Hydroxyl Group in the Ribonucleotide Synthesis by TFA‐acidolysis

The cleavage of the 4‐methoxybenzyl group from the 2′‐OH‐position of ribonucleosides by the hydrogenation with different Pd‐catalysts as well as trifluoracetic acid has been studied in detail. During hydrogenation, side reactions at the base residue of cytidine‐occurred which, however, could be extensively suppressed by PdCl2 catalysis. More practicable results were obtained with trifluoracetic acid in the presence of cation scavengers, allowing smoothly to convert a series of 2′‐methoxybenzyl ribonucleotides to the homogeneous deprotection products.
Substitution Reactions of 2‐(Aroyl, Methylidene)‐thiazolidine‐4‐on Derivatives
Authors: Zaleska Barbara , Winnik Witold
Published: December 2004

The reaction of 2‐(aroyl, methylidene)‐3‐arylthiazolidine‐4‐ones 1 with trifluoromethylsulfenyl chloride yields a mixture of geometric (Z, E) isomers of 2(aroyl, trifluorosulfenylmethylen)‐3‐aryl‐thiazolidine‐4‐ones 2. The bromination of 1 leads to 2‐(aroyl, methylidene)‐3‐aryl‐5‐bromothiazolidine‐4‐ones 3 which on reaction with nitrosobenzene formes nitrones 4. The structures of the compounds 2–4 were determined by means of n.m.r. and i.r. spectroscopy.
N‐Arylsulfonyl‐N′‐(pyrimidin‐4‐yl)ureas‐synthesis and reaction with dimethyl sulfoxide
Authors: Zeuner F. , Niclas H.‐J.
Published: December 2004

The N‐arylsulfonyl‐N′‐(pyrimidin‐4‐yl)ureas (3a–1) were synthesized by reaction of arylsulfonyl isocyanates (1) with 4‐amino‐2,6‐dialkylpyrimidines (2). Treatment of 3 with alkali or of 1 with an excess of 4‐amino‐2,6‐dimethylpyrimidine (2a) gave the salts 4a–e. Reaction of the ureas 3a–c with dimethyl sulfoxide at 90 to 120°C afforded N,N′‐bis(2,6‐dimethylpyrimidin‐4‐yl) ureas 8a–c. The reaction is assumed to involve a four‐centre mechanism with 5 as intermediate.
1H‐ und 13C‐NMR‐Spektren von N‐Methyl‐chinoliniumchalkonen: Bestimmung von σ‐Konstanten für die 2‐, 3‐ und 4‐Position des N‐Methyl‐chinoliniumrings
Authors: Radeglia R. , Bödeker J. , Fieblinger Dagmar
Published: December 2004

1H‐ and 13C‐N.M.R. Spectra of N‐Methyl‐quinolinium Chalcones: Determination of σ‐Constants for the 2‐, 3‐ and 4‐Position of the N‐Methyl‐quinolinium Ring

For the N‐methyl‐quinolinium chalcones 1a–c the 1H‐ and 13C‐n.m.r. data are reported. The unambiguous assignment of signals is based upon the 1H/1H homoshift and the 13C/1H heteroshift correlated 2D‐n.m.r. spectra. Using a Hammett relationship we have determined σ‐constants for the 2‐, 3‐ and 4‐position of the N‐methyl‐quinolinium ring from the 13C‐α chemical shifts of 1a–c. Their comparison with the σ‐values for the equivalent positions in the pyridine, N‐methyl‐pyridinium and quinoline ring indicates the additivity of anellation and quaternization effect.
Substituenteneffekte in den13C‐NMR‐Spektren von diastereomeren Chalkondihalogeniden. VIII. Untersuchungen an α‐ und β‐Halogendihydrochalkonen und Berechnung von13C‐chemischen Verschiebungen bei diastereomeren Chalkondihalogeniden und Chalkonhalogenhydrinen
Authors: Weber F. G. , Radeglia R.
Published: December 2004

Substituent Effects in13C‐N.M.R. Spectra of Diastereomeric Chalcone Dihalogenides. VIII. Investigations of α‐ and β‐Halogene Dihydrochalcones and Calculation of13C‐chemical Shifts of Diastereomeric Chalcone Dihalogenides and Chalcone Halogenohydrins

The chemical shifts of C‐α and C‐β in the13C‐n.m.r. spectra are determined and compared with calculated values from an incremental system and Taft relationship. The correlations are extended for erythro and threo isomers of halohydrins and diastereometric dihalogenides6–18.

The chemical shifts of C‐α and C‐β atoms of6–18are described in the dependence of the shifts calculated by an incremental system. Excellent correlations allowed the computation of chemical shifts of C‐α and C‐β for diastereomeric α,β‐disubstituted dihydrochalcone derivatives likely6–18in advance.
New Red Diazonium Compounds based on 2‐amino benzene diazonium salts
Authors: Mustroph Heinz , Haessner Rainer
Published: December 2004

Using PMO theory the influence of substituents on the visible spectra of benzene diazonium salts is discussed. The theory explains the experimental structure‐colour relationships and allows to predict new basic structures of coloured benzene diazonium salts. The synthesis and spectral behaviour of the new class of superfast diazonium salts are described.
Fotochemische Primärprozesse von Xanthenfarbstoffen. X. Zum Einfluß ionischer Mizellen auf die Löschung der Fluoreszenz von Xanthenfarbstoffen durch Diazoniumionen
Authors: Ortmann W. , Wassiljuk I. , Fanghänel E.
Published: December 2004

Photochemical Primary Processes of Xanthene Dyes. X. The Influence of Ionic Micelles on the Fluorescence Quenching of Xanthene Dyes with Diazonium Ions

The steady‐state and the time‐resolved fluorescence quenching of xanthene dyes by diazonium salts is investigated. The fluorescence of anionic dyes is more efficiently quenched than that of cationic dyes, due to the opposite charge. In the presence of anionic micelles the fluorescence quenching of cationic dyes by diazonium ions is more effective than in water, because the dye and diazonium ions are solubilized in the Stern region of the micelles. In the presence of cationic micelles the fluorescence of anionic dyes is effectively quenched by p‐dodecyloxybenzene‐diazonium salt solubilized in the micelles like a tensid ion. The equilibrium constants for the distribution of the diazonium ions between the micelles and the water phase are determined. The importance of the singlet state for the sensitized photolysis of diazonium salts is discussed.
The knoevenagel reaction of cyanoacetamide with β‐ketoanilides: Synthesis of polyfunctionally substituted pyridine and coumarine derivatives
Authors: Mohareb R. M. , Ibrahim N. S.
Published: December 2004

Cyanoacetamide undergoes a Knoevenagel condensation reaction with β‐ketoanilides 2a–c to give the acyclic products 3a–c. The reactivity of 3a towards the active methylene reagents, aldehydes, aryldiazonium salts and cinnamonitrile derivatives were studied.
Studies on 4‐thiazolidinones. VIII. The role of substituents at position‐3 on the mode of cleavage of 5‐Arylmethylene‐2,4‐dioxothiazolidines
Authors: Omar M. T. , Habashy M. M. , Youssef A. M. , Sherif F. A.
Published: December 2004

Hydrazine hydrate, morpholine and benzylamine react with 3‐aryl‐5‐arylmethylene‐2,4‐dioxothiazolidines 1a–d to give mixtures of thiolopropenamides 2, ethylideneamino‐biuret 3 and/or 4‐arylaminocarboxamides 4 in varying proportions. However, 3‐benzyl‐5‐arylmethylene 2,4‐dioxothiazolidines 1e–f afford mixtures of pyrazolinones 5 and 4‐benzyl‐hydrazinocarboxamide 6 upon treatment with hydrazine hydrate. The formation of these products is rationalized and discussed on the basis of the role of the substituent at the 3‐position. Structures 2–6 have been established.
Studies on the Autoxidation of Phenyl Cycloalkanes
Authors: Batke Birgit , Lauterbach Gerlinde , Pritzkow Wilhelm , Voerckel Volkmar , Belyakov Vladimir A.
Published: December 2004

The products of the autoxidation of phenyl cyclopropane (I), phenyl cyclobutane (II), phenyl cyclopentane (III), phenyl cyclohexane (IV), phenyl cycloheptane (V) and phenyl cyclooctane (VI) were analyzed after reduction of the reaction mixtures with LiAlH4. As products of the attack on the α‐CH bonds the corresponding 1‐phenyl cycloalkanols and 1‐phenyl alkan‐1‐ols were found. In the case of phenyl cyclopropane some SR2 ring opening probably takes place.

The oxidabilities \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm k}_{\rm p} /\sqrt {{\rm k}_{\rm t}} $\end{document}, the chain termination constants kt, the absolute chain propagation constants kp and the relative chain propagation constant (kp)rel were determined for the phenyl cycloalkanes I—VI. As it is to be expected on the basis of the I‐strain concept the autoxidation rate of phenyl cyclopentane (III) is considerably higher than that of phenyl cyclobutane (II) and phenyl cyclohexane (IV).
Studies on aminoazoles: Synthesis of Pyrazolo[1,5‐a]‐pyrimidines and their aza derivatives
Authors: Elgemeie Galal H. , Elghandour Ahmed H. , Elshimy Hamdy M.
Published: December 2004

The reaction of 4‐arylazo‐3‐aminopyrazol‐5‐ones (1a–i) with α,β‐unsaturated nitriles, active methylene reagents and nitrile imines are reported. They lead to new polyfunctional derivatives of pyrazolo[1,5‐a]pyrimidine (5a–c, 6, 10a–i, 13a–c, 14a–c, 17a–d, 15), pyrazolo‐[5,1‐c]‐1,2,4‐triazine (22a–i) and pyrazolo[5,1‐c]‐1,2,4‐triazole (25a–c).

The structures of these products and the mechanisms of their formation are reported.
Zur elektrochemischen Darstellung des 1,1‐Dichlor‐4‐methylpenta‐1,3‐diens
Authors: Fechtel U. , Matschiner H.
Published: December 2004

Electrochemical Synthesis of 1,1‐Dichloro‐4‐methylpenta‐1,3‐diene

Reaction of 1,1,1‐trichloro‐4‐methylpent‐3‐en‐2‐ol (2) with inorganic acid halides yielded several polychloroalkenes (3–5). These compounds were utilized for the electrochemical synthesis of 1,1‐dichloro‐4‐methylpenta‐1,3‐diene (1).
Azole. 22.. 1H‐ und 13C‐NMR Untersuchungen an isomeren 3‐ und 5‐Amino‐pyrazol‐Derivaten
Authors: Bernard M. K. , Wrzeciono U.
Published: December 2004

Azoles. 22. 1H‐ and 13C‐N.M.R. Investigation of Isomeric‐3 and 5‐Amino‐Pyrazole Derivatives

The 1H‐ and 13C‐n.m.r. spectra of nine pairs of isomeric 3‐ and 5‐amino‐pyrazole derivatives 1a—1i and 2a—2i and some of their acetylderivatives have been assigned and discussed. It has been found that 1H‐ and 13C‐n.m.r. spectroscopy is a useful tool for differentation of the above mentioned 3‐ and 5‐amino‐isomers.
Inter‐ und intramolekulare Acylwanderungen an 1(9)H‐2,3‐Dihydro‐imidazo[1,2‐a]benzimidazolen
Authors: Graubaum H. , Ozegowski R.
Published: December 2004

Inter‐ and Intramolecular Acyl Transfer on 1(9)H‐Imidazo[1,2‐a]benzimidazoles

The new imidazo[1,2‐a]benzimidazoles (3) react with electrophiles like aroyl chlorides, isocyanates, cyanates, chloroformates and chlorothioformates to give the 9‐acyl derivatives (4a–1).

At higher temperatures intermolecular acyl transfer takes place to give the l‐acyl compounds (5a–m). In the case of 1‐carbamoyl imidazo[1,2‐a]benzimidazoles (5) was observed intramolecular acyl transfer in the presence of strong bases and 1‐benzimidazol‐2‐yl‐hydantoins (6) were isolated.

The structures of the prepared compounds could be inferred from their 1H and 13C n.m.r., mass spectra and were corroborated by the comparison with the data of similar derivatives as well as by chemical means.
Synthese und Reaktionen von N‐Phthalimido‐keten‐S,S‐acetalen
Authors: Dölling W. , Herrmann Cornelia , Augustin M. , Giering Susanne
Published: December 2004

Synthesis and Reactions of N‐Phthalimido‐ketene‐S,S‐acetals

N‐substituted phthalimido derivatives 1a–f react in the presence of bases (NaH, t‐BuONa) with carbon disulfide to the N‐phthalimido‐ketene‐S,S‐acetals 3, 4, 5 and 6, respectively. 2‐Amino‐ketene‐S,S‐acetals 7a–c are formed by reaction of 3a, b or 5a with hydrazine hydrate. 3c produces with phenylhydrazine the pyrazole 8, and with piperidine and pyrrolidine the E/Z mixture of the corresponding ketene‐S, N‐acetals 9a,b. 1c is transformed into 10a–c (E/Z‐mixture)by treatment with phenylisothiocyanate under basic condition followed by alkylation.
Diskussionsbeitrag zur Normung der Ermittlung der Sprengenergie unter Wasser
Authors: Christmann W. , Lingens P.
Published: December 2004

Der Versuchsaufbau zur Ermittlung der Sprengstoffenergie unter Wasser wird zur Vereinheitlichung vorgeschlagen. Die Form und das Gewicht der Sprengladung werden unter Berücksichtigung der verfügbaren Größe des Wasserbehälters diskutiert. Die Messung der Zeit zwischen der Detonation der Sprengladung und dem ersten Zusammenschlagen der Gasblase und die Auswertung der Meßergebnisse mit Hilfe der Formel von Cole werden ebenfalls zur Normung empfohlen. Die Messung der Stoßwellenenergie und die Auswertung der Meßergebnisse können noch nicht vereinheitlicht werden. Hierzu müssen noch weitere Arbeiten durchgeführt werden.
Application of Safety Testing Methods of Explosives to Firedamp and coal dust and comparison to blasting safety methods in collieries
Authors: Gorol C. , Sobala J.
Published: December 2004

Requirements are discussed which must be fulfilled by laboratory and field tests in methane und coal dust before an explosive is permitted for use in mine. Based on results of explosive testing done in recent years in Poland and on results of our continual research, basic differences were found in the ignition ability of methane and coal dust. The differences were also found in laboratory tests using steel mortars which simulate boreholes, and in field tests in underground roadways. With respect to these findings the safety of mining explosives in methane and coal dust and their applicability in the mining practice are discussed.
Characterization of BAMO/NMMO Copolymers
Authors: Kimura Eishu , Oyumi Yoshio , Kawasaki Hidetoshi , Maeda Yuko , Anan Toshiyuki
Published: December 2004

The compositions and microstructures, thermolyses, and sensitivities of 3,3‐bis(azidomethyl)oxetane (BAMO) copolymers with 3‐nitratomethyl‐3‐methyloxetane (NMMO) were characterized. The molar ratios of monomers fed in the copolymerizations were: BAMO/NMMO = 8/2, 7/3, and 6/4. The observed compositions were 81/19, 68/32, and 60/40, respectively. The relative compositions of trade microstructures were also exactly the same as theoretical values. The copolymerization of BAMO with NMMO behaved like an ideal system in the sense that the copolymer has approximately the same composition as the monomer fed. The microstructures of copolymers were also randomly arranged. Poly BAMO showed side chain decomposition in the initial stage of the thermolysis, on the other hand, backbone decomposition controlled the thermolysis of poly NMMO. The BAMO units and the NMMO units appeared to decompose more or less independently of one another and retained their individual decomposition characteristics despite being copolymerized. However, the heat generated by the NMMO decomposition, which occurred at 40°C lower than that of BAMO. activated the reaction of BAMO. Poly B/N(7/3) showed very interesting characteristics in sensitivities, mechanical properties, and the heat of decomposition.
Synthesis of N, N′ ‐Bis(2, 4‐Dinitrobenzofuroxanyl)‐3, 5‐Dinitro‐2, 6‐Diaminopyridine
Authors: Naixing Wang , Boren Chen , Yuxiang Ou
Published: December 2004

N,N′‐Bis(2,4‐dinitrobenzofuroxanyl) ‐3,5‐dinitro‐2,6‐diaminopyridine has been synthesized from 2,6‐diaminopyridine and trinitrodichlorobenzene. For this compound, the structure has been determined by elemental analysis, IR,1HNMR and MS spectroscopies.
Thermal decomposition features of ammonium nitrate and its boron mixture under high pressures
Authors: Koroban V. A. , Burtsev Yu. N. , Alimov F. R. , Haustov A. D. , Dubovik V. A. , Teselkin V. A.
Published: December 2004

The kinetics of ammonium nitrate thermal decomposition was studied by the differential thermal analysis in the pressure range from atmospheric to 1.0 GPa. The decomposition under pressure was shown to obey the zero‐order equation with a constant proportional to the initial HNO3 concentration in the sample. The structural transitions in ammonium nitrate were found to affect its exothermic interaction with boron. The mechanism of this phenomenon was interpreted. The experimental data allowed to establish volume changes on melting ammonium nitrate, equal to 0.072 cm3/g.
Evaluation of pyrotechnic smoke for anti‐infrared and anti‐laser roles
Authors: Singh Amarjit , Avachat S. G. , Joshi S. A. , Singh Haridwar
Published: December 2004

This paper describes the experimental setup for evaluation of pyrotechnic smoke compositions for anti‐laser and anti‐infrared roles using the continuous wave 10.6 μm CO2 laser (4 watt) and presents the results for a few pyrotechnic smoke compositions. The particle sizes of the aerosol from pyrotechnic smoke compositions were also determined using Quartz crystal Microbalance ‐ Cascade Impactor. Two smoke compositions containing hexachloroethane, anthracene/naphthalenee, potassium perchlorate and magnesium powder showed more than 97% attenuation of CO2 laser, whereas a composition containing zinc oxide, aluminium, iron oxide along with hexachlorocthane was least effective.
Wellenmechanische Absolutrechnungen an Molekülen und Atomsystemen mit der SCFMOLC(LCGO) Methode. I. Das Cyclopentadienylanion (C5H)
Authors: Preuss H. , Diercksen G.
Published: December 2004

Das C5H wird im Gleichgewichtsabstand der Zentren unter Berücksichtigung aller 36 Elektronen ab initio mit Hilfe des Allgemeinen Programmsystems/SCFMOLC(LCGO) Methode berechnet. Es ergibt sich eine Gesamtenergie von −189,42 a.E. Die Ionisierungsenergie (Elektronenaffinität des C5H5) beträgt 0,9 eV. Bei den Einteilchenenergien sind in energetischer Folge den π‐Zuständen σ‐Zustände eingelagert.
Extended Hartree–Fock calculations for the helium ground state
Authors: Lefebvre Roland , Smeyers Yves G.
Published: December 2004

The extended Hartree–Fock (EHF) wave function of an n‐electron system is defined (Löwdin, Phys. Rev. 97, 1509 (1955)) as the best Slater determinant built on one‐electron spin orbitals having a complete flexibility and projected onto an appropriate symmetry subspace. The configuration interaction equivalent to such a wavefunction for the 1S state of a two‐electron atom is discussed. It is shown that there is in this case an infinite number of solutions to the variational problem with energies lower than that of the usual Hartree–Fock function, and with spin orbitals satisfying all the extremum conditions. Two procedures for obtaining EHF spin orbitals are presented. An application to the ground state of Helium within a basic set made up of 4(s), 3(p0), 2(d0) and 1 (f0) Slater orbitals has produced 90% of the correlation energy.
Wellenmechanische Absolutrechnungen an Molekülen und Atomsystemen mit der SCFMOLC(LCGO) Methode. VII. Das System (LiH2)−
Authors: Preuss H. , Diercksen G.
Published: December 2004

Das System (LiH2)− wurde für eine Reihe von Kernlagen unter Berücksichtigung aller 6 Elektronen mit dem Allgemeinen Programmsystem SCFMOLC(LCGO) Mathode berechnet. Es ergab sich, dass für die lineare Anordnung (RLiH = 3,5 ± 0,2 a.E.) ein metastabiles Molekül vorliegt, dessen Gesamtenergie um 0,12 a.E. tiefer liegt als das System der getrennten Atome. Dagegen kann (LiH2)− über einen Potentialberg in Li− + H2 zerfallen, welches um 0,01 a.E. tiefer liegt. Die Höhe des Potentialberges beträgt 0,15 a.E. Die Energiehyperfläche wird graphisch dargestellt und im Einzelnen die Bildung (und der Zerfall) von (LiH2)− diskutiert.
Wellenmechanische Absolutrechnungen an Molekülen und Atomsystemen mit der SCFMOLC(LCGO) Methode. X. Das Methylkation (CH3)+
Authors: Von Bünau G. , Diercksen G. , Preuss H.
Published: December 2004

Das (CH3)+ wurde unter Berücksichtigung aller 8 Elektronen ab initio mit dem Allgemeinen Programmsystem/SCFMOLC(LCGO) Verfahren berechnet. Es ergab sich nach Variation des CH Abstandes und der Lage des C Atoms, dass (CH3)+ eben gebaut ist, mit einem Bindungsabstand RCH = 2,05 a.E. Die beiden Kraftkonstanten (Atmungsschwingung, Winkelschwingung) ergeben sich dabei zu k1 = 18,9 mdyn/Å und k2 = 3,3 mdyn/Å mit den zugehörigen Wellenzahlen ω1 = 3256 cm−1 und ω2 = 1526 cm−1. Die Ionisierungsenergie wurde zu I = 25,75 eV bestimmt. Die Elektronenaffinität wurde zu A = 5,4 eV abgeschätzt.
Molecular‐orbital studies of the mechanism of xanthine oxidase‐catalyzed oxidation of purines, especially 2‐chloropurine
Authors: Song Pill‐Soon , Moore Thomas A.
Published: December 2004

A detailed study to the xanthine oxidase‐catalyzed reactions of 2‐chloropurine and other substrate bases with various molecular‐orbital techniques such as HMO, ω‐SCFHMO, and ppp semiempirical SCFLCAOMO has shown that the enzyme reactions can be understood in terms of electronic reactivity indices. Furthermore, it appeared possible to suggest the enzyme specificity from a systematic analysis of discrepancy between mo theoretically predicted and observed reaction sites in substrates with 2‐ and 8‐oxy substituents. In other words, the discrepancy does not necessarily indicate the failure of the MO melthodes for such substrates, but it is possible to utilize the result in correlating with binding specificity of the ES complex. This has been done specifically for 2‐chloropurine.

Among several electronic reacxtivity indices, frontier orbital density, superdelocalizability, and localization energy have been proved to be very useful. Diferent MO methods gave, in general, consistent results.
Note on the usual perturbation theory
Authors: George C.
Published: December 2004

The Kato perturbative scheme is recovered and generalized. The procedure followed is inspired by the technique used in statistical mechanics to study the asymptotic evolution of the density matrix.
Nature of the chemisorption bond on semiconductor surfaces
Authors: Tomášek M. , Koutecký J.
Published: December 2004

Results obtained by various quantum‐mechanical approaches in studying localized states on crystal surfaces are summarized and discussed. The one‐ and many‐electron aspects of the problem are compared and shown to lead to similar results. Emphasis is laid upon localized chemisorption states on intrinsic semiconductors. The problem of the calculation of chemisorption heat on solid surfaces is also mentioned.
Study of the convergence radius of the Rayleigh–Schrödinger perturbation series for the delta‐function model of H
Authors: Claverie Pierre
Published: December 2004

The convergence radius of the series expansion for the energy of H2+ in the δ‐function model (in terms of the perturbation parameter μ/λ, where μ is the charge of the perturbing nucleus and λ the fixed charge of the other nucleus) is investigated. A lower bound of this convergence radius (possibly equal to it) previously defined by Robinson [5] is studied analytically as a function of the internuclear distance R and computed numerically. The results differ strikingly from those previously obtained by Robinson who used a simplified but poorer lower bound: in contrast with this poorer bound, the one studied in the present paper is larger than for I every R, from which fact it may be concluded that, contrary to Robinson's previous result, the series expansion of the energy, in the δ‐function model under consideration, still converges when μ = λ for every R.
Spin–other–orbit and spin–orbit interactions in ƒ2 and ƒ3 electron configurations
Authors: Saxena K. M. S. , Malli Gulzari
Published: December 2004

Expressions of the matrix elements of the spin–other–orbit and spin–orbit interactions for the various multiplets of all the states of ƒ2‐ and ƒ3‐electron configurations are reported and used to evaluate the Hartree–Fock values of these interactions in the neutral atoms Ce(4ƒ2), Pr(4ƒ3), Ho(4ƒ11) and Er(4ƒ12). The required values of the spin–spin parameters M, and the spin‐orbit parameter ζ for these atoms were obtained using numerical Hartree–Fock wave functions.
Electronic absorption spectra for free hydrazyl radicals, their ionic forms and initial hydrazines
Authors: Gubanov V. A. , Pereliaeva L. A. , Chirkov A. K. , Yastchenko G. N. , Matevosian R. O.
Published: December 2004

The results of the calculations of π–π* transition energies of some free hydrazyl radicals, their ionic forms and initial hydrazines by the SCF–MO–LCAO method with restricted configuration interaction are presented. The comparison of the data obtained with the electronic spectra experimentally found, revealed that the absorption of radical solutions in the visible area was due to the electronic structure of radical molecules, the contribution of ionic forms to the absorption being negligible.
Frequency distribution of radiation emitted from excited atomic systems
Authors: Hearn Dwight D.
Published: December 2004

General expressions for the time‐dependent probability amplitudes of the quantum states of two arbitrary, interacting atoms are calculated when one atom is initially in an excited p state and the other atom is in an s ground state. The lifetimes of the excited states and the line shape of the emitted radiation are obtained as functions of both the atomic separation and the energy difference between the excited states of the two atoms. The emission line shape is shown to be doubly peaked and to agree with the line shape of the radiation scattered by a system of two interacting atoms. The expressions for the lifetimes of the excited states are found to be identical to those obtained for the radiation scattering situation.
A theoretical investigation of the singly excited two‐electron atomic states 1s2s3S and 1s3s3S
Authors: Lyslo Gudmund , Aashamar Kjell , Midtdal John
Published: December 2004

The singly‐excited two‐electron states 1s2s 3S and ls3s 3S have been investigated by means of the variational perturbation theory procedure. The wave functions have been constructed as linear combinations of Hylleraas terms with hyperbolic factors in t, and the results obtained by carrying the computations through to 10th perturbation order and with 36(37)‐ 57(58)‐ and 85(86)‐term basic sets, respectively, are reported. These results compare favourably with the corresponding best values from previous conventional variational calculations.
Théorie quantique des phénomènes de biréfringence electrique et magnétique des molécules
Authors: Locqueneux R.
Published: December 2004

A general formalism is set up to apply the variation‐perturbation method to the quantum theory of non‐linear optical effects, in particular to the electric and magnetic birefringence (Kerr and Cotton‐Mouton effects).

The quantities describing these effects are obtained from the wave function Ψ of the ground state of the unperturbed molecule and the different vectors and tensors which are determined by a variation method.

With this method an approximate computation of the Kerr and Cotton‐Mouton constants of some simple molecules has been made.
The green's functions of the Schrödinger equation for the simplest systems
Authors: Vetchinkin S. I. , Bachrach V. L.
Published: December 2004

The Green's functions for the simplest quantum mechanical systems the linear harmonic oscillator, the three‐dimensional isotropic oscillator, the Morse oscillator, the Kratcer potential, and the double‐minimum potential V(x) = (mw2/2)(/x/−R)2 are presented in closed analytical forms.
The general scheme of using non‐orthogonal radial orbitals in a complex electronic configuration of the atom
Authors: Jucys A. P. , Grudzinskas J. J.
Published: December 2004

A theory for using non‐orthogonal radial orbitals between shells with identical orbital quantum numbers, in the case of complex configurations, is presented. Construction of the antisymmetric wave function of the whole configuration, with the help of antisymmetrical wave functions of individual shells, is described. General methods of calculating matrix elements of one‐ and two‐electron operators are given.
The cusp condition: Constraint on the electron density matrix
Authors: Clinton W. L. , Massa L. J.
Published: December 2004

Attention is called to the attractive cusp conditions as useful constraints in fixing the elements of the electron density matrix. An equation for determination of pure state densities satisfying arbitrary constraints is reviewed, and a detailed formalism for using cusp conditions with this equation is displayed. A calculation is done using the nitrogen molecule as an example.
Study of the electronic structure of molecules. XV. Comments on the molecular orbital valency state and on the molecular orbital energies
Authors: Clementi Enrico , Routh Anne
Published: December 2004

The valency state (vs) concept is analyzed in the Hartree–Fock approximation. A valency state “standard” is defined for atoms at infinite separation. A molecular orbital valency state (Movs) is defined from a partitioning technique (bond energy analysis) previously introduced for the Hartree–Fock molecular wave functions. The Movs for a given atom in a molecule is much higher in energy than the vs and its energy varies from molecule to molecule depending on the exact field of the surrounding atoms. The examples selected in the discussion are the CH4 CH3F, CH2F2, CHF3 and CF4 molecules.

An analysis of the orbital energies is then given in terms of the bond energy. The importance of the rearrangement effects following ionization of inner shell electrons (simulation of ESCA type experiments) is illustrated with computations of the positive ion for methane and its fluoroderivatives. It is concluded that rearrangement following ionization from inner shells is as important as rearrangements following ionization from valency electrons. A direct consequence is that the orbital energies should not be equated to the inner shell ionization potentials. The computation of such ionization potentials agrees to about 99.5% with ESCA data, when the energy of both the neutral and ionic species are computed; the use of the orbital energies limits this agreement to about 95%.
Some developments in the spin‐extended Hartree‐Fock method
Authors: Mestechkin M. M.
Published: December 2004

All the second‐order density matrix spin components for the spin‐extended Hartree‐Fock method are obtained. The coefficients in the final formulae are only ωsM, ωsM±1, ωsM±2, where ωsM are the weights of pure states of spin s in the initial unprojected determinant with spin projection M. The eigenvalue problem for the best electron density natural orbitals in the spin‐extended method is formulated. All the second‐order transition density matrix spin components between pure spin basis functions built of orthogonal orbitals and distinguished by different core choice are also found. This basis may be used on CI calculations.
A study of the AMO method as applied to the lithium metal. III. A method for calculating the total electronic energy of crystals in the LCAO approximation
Authors: Sperber Gunnar
Published: December 2004

A method is given for accurate calculation of the expectation value of the Hamiltonian of a crystal with respect to a single‐determinant wave function of LCAO type. The method is partly based on numerical integration over one Wigner‐Seitz cell. In principle, the calculations involve infinite summations over lattice sites; suitable truncation methods are devised.
Optimization of molecular orbitals: Easy parametrizations of little rotations suitable for functional iteration
Authors: Gianinetti Ermanno
Published: December 2004

The method of functional iteration applied to orbital optimization is proposed. The main problem, i.e., the most economical parametrization of the rotations of the molecular orbitals, is resolved. First and second derivatives of rotations with respect to the parameters are given. The fact that the orbital space can be decomposed in subspaces such that energy is invariant against transformations of these subspaces into themselves is explicitly taken into consideration.
Gaussian orbitals optimized for lower bounds of hydrogenic atoms
Authors: Johnson J. W. , Poshusta R. D.
Published: December 2004

Various optimization criteria are compared for the hydrogen atom to find orbitals which improve lower bounds computed from the Weinstein, Temple, and Stevenson‐Crawford formulas. Minimization of squared energy deviation, “variance,” is recommended because the resulting lower bound orbitals give excellent lower bounds, converge to the exact wave function, are relatively easy to optimize, and are insensitive to the estimated energy eigenvalue. New linear combinations of Gaussian orbitals which minimize the variance are presented for the 1s, 2s, 2p, 3s, 3p, and 3d orbitals. These orbitals are compared with previous linear combinations with regard to their expectation values and local properties.
Interactions between molecules in the neighborhood of a long linear molecule
Authors: Krizan John E. , Lloyd Brian F.
Published: December 2004

The third‐order interaction between two nonpolar molecules due to the instantaneous induced dipole effect of an infinitely long linear molecule is assessed following arguments similar to earlier research on three‐body effects. It is found that an attractive force favors small polar angles in approach to the linear molecule; such behavior could be of interest to processes such as those involved in replication of DNA.
Alternative equivalent icosahedral irreducible tensors
Authors: Boyle L. L. , Schäuffer C. E.
Published: December 2004

Some sets of icosahedral irreducible tensors can only be defined with respect to one of two seemingly equivalent choices of axes. The problem is explained and the cases when such an alternative choice may cause mixing of sets belonging to different irreducible representations are determined. It is found that the amount of mixing is a property of the spherical basis sets and is independent of the rank and of the permutation symmetry of the tensorial sets. It is further found that certain operators which change the handedness of the coordinate axes can similarly affect these sets, notwithstanding the centrosymmetry of the icosahedron.
Comments on kets not strongly symmetry adapted
Authors: Klein D. J.
Published: December 2004

Various instances in which weakly or nonsymmetry‐adapted kets may arise in the description of electronic systems are discussed. These cases include some types of selfconsistent field kets, exchange perturbation kets, and multistate kets.
Molecules‐in‐molecules calculations with fixed resonance integrals
Authors: Kiss A. I. , Martin A.
Published: December 2004

A simple equation for the evaluation of resonance integrals from overlap integrals and ionization potentials of the molecular fragments is suggested for molecules‐in‐molecules π electron calculations. The singlet π → π* transition energies of some benzene derivatives containing donor substituents were calculated. The best results were obtained if in the expression of the resonance integral the first experimental ionization potential of the methyl derivative of the donor groups is used.
Approximate valence‐shell electron unrestricted Hartree‐Fock calculations with orthogonalized atomic orbitals: Proton hyperfine splittings in benzyl and related radicals
Authors: Nanda D. N. , Narasimhan P. T.
Published: December 2004

A modified INDO procedure has been used to calculate the proton hyperfine splittings in benzyl and the isoelectronic anilino, phenoxy and 2‐azabenzyl as well as 2‐ and 3‐thenyl radicals. The present procedure differentiates between s‐, p‐ and d‐orbitals on an atom in estimating various integrals involving them, satisfies the rotational invariance requirements and employs an orthogonalized basis set of atomic orbitals for obtaining core‐Hamiltonian matrix elements. The calculations based on using the exponents which depend only on the type of orbital and the nature of atom fail to provide correct relative order of ortho and para proton splittings in benzyl as well as anilino, phenoxy and 2‐azabenzyl radicals. On the other hand, use of the exponents which are modified according to the charge densities in various orbitals leads to a high absolute value for para proton splitting compared to that for ortho proton splitting which in case of all these radicals is in agreement with experiment. A spin density calculation on benzyl, anilino and phenoxy radicals considering the variation of one‐center one‐electron and one‐center two‐electron integrals for different protons with their charges is found to yield further improvement in the relative order of ortho and para proton splittings in all these radicals. In 2‐ and 3‐thenyl radicals the role of 3d‐orbitals on sulfur has also been examined. To our knowledge, no unrestricted INDO calculations including 3d‐orbitals on sulfur have been reported in the literature so far.
Interaction of two H2 molecules: Comparative study of various perturbation procedures
Authors: Basilevsky M. V. , Berenfeld M. M.
Published: December 2004

A variational calculation of the interaction between two H2 molecules using minimal basis set of 1s functions is performed to check the efficiency of various versions of the perturbation theory for intermolecular interactions. The matrix procedure starting with the zero‐order Hamiltonian which is symmetric with respect to intermolecular electron permutations shows better convergence than the procedures using nonsymmetric zero approximations. This conclusion follows from the calculations of ground state and four lowest excited states for three geometric configurations of the H4 system. The behaviour of the potential curves is interpreted in terms of symmetric perturbation theory. The various contributions to the interaction energy are considered in detail. The importance of charge transfer states for the description of the intermediate range of intermolecular separations is specially emphasized.