bluuebunny/crossref_metadata_2025 huggingface dataset:
X‐Ray Molecular Structure of a Red Bilin Derivative from Chlorella protothecoides. 4th communication on chlorophyll catabolism
Authors: Engel Norbert , Gossauer Albert , Gruber Karl , Kratky Christoph
Published: December 2004

The molecular structure of a chlorophyll a catabolite isolated from the culture medium of bleached Chlorella protothecoides cells has been determined by X‐ray diffraction; the crystal structure shows a distorted helical arrangement of the pyrrole rings and consists of symmetrical dimers located at a crystallographic diad, which are stabilized by six intermolecular H‐bonds.
Synthesis, Structure, and Reactivity of Secosteroids Containing a Medium‐Size Ring. Part 35. Photooxidations of some (Z)‐ and (E)‐1(10)‐unsaturated 5,10‐secosteroids in acetone solution
Authors: Lorenc Ljubinka B. , Pavlović Vladimir D. , Mihailović Mihailo Lj. , Krstić Natalija M.
Published: December 2004

UV Irradiation of (Z)‐ and (E)‐1(10)‐unsaturated 5,10‐secosteroids 1–4 in acetone solution effected, besides (Z/E)‐isomerization, (i) a stereospecific epoxidation (only in the presence of O2), which, depending on the configuration ((Z) or (E)) in the starting steroid, gave cis‐epoxides 5 and 8 (from the (Z)‐compounds 1 and 3) or trans‐epoxides 6,9, and 10 (from the (E)‐compounds 2 and 4), and (ii) oxidative acetone addition to the olefinic double bond producing 1‐acetonyl derivatives 7 and 11a, b.
Rhodium(I)‐Catalyzed ‘Metallo‐Ene’ Cyclizations/β‐Eliminations
Authors: Oppolzer Wolfgang , Fürstner Alois
Published: December 2004

Octadienyl carbonates 5 provide cyclic 1,4‐dienes 6 when treated with Rh1 complexes (1–10 mol‐%) at 80°. Similar cyclization of cyclohexenyl acetate 8 affords cis‐ fused hexahydroindene 9. Analogous ring closures of nonadienyl carbonate 10 yield preferably the cis‐divinypyrrolidine 11 with Rh1 catalysis but the trans‐isomer 12 when catalyzed by Pd0. Azaoctadienyl carbonate 5a undergoes elimination with [RhH(PPh3)4] (5 mol‐%, 80°) in MeCN giving acyclic triene. 7.
Phenylenbis(silandiyl‐triflate) als neuartige Synthesebausteine für vielfältig strukturierte Organosilizium‐Polymere
Authors: Uhlig Wolfram
Published: December 2004

Phenylenebis(silandiyl triflates) – New Synthetic Building Blocks for Variously Structured Organosilicon Polymers

Ortho‐, meta‐, and para‐substituted phenylenebis(silandiyl triflates) are prepared as new useful building blocks for the synthesis of polymers with a regular alternating arrangement of an organosilicon unit and a π‐electron system (phenylene or ethynylene) in the backbone. Such polymers can be used as photoresists, semiconducting materials or precursors of silicon carbide. The phenylenebis(silandiyl‐triflates) 5a–c are obtained by protodesilylation of the corresponding (allylsilyl)‐ or [(diethylamino)silyl]benzenes 3a–c and 4a–c, respectively, with trifluoromethanesulfonic acid. Reactions with dinucleophiles like Li2C2 and Ph2Si(OH)2 lead to variously structured organosilicon polymers (see Eqns. 11 and 12), which are characterized by spectroscopic methods.
The Protonation of [Pt3(m̈‐CO)3(PCy3)3]
Authors: Dahmen Klaus‐Hermann , Imhof Daniel , Venanzi Luigi M.
Published: December 2004

The cluster [Pt3(m̈‐CO)3(PCy3)3] can be protonated with HBF4 · OEt2 to form the cluster [Pt3(m̈‐CO)3(PCy3)3](m̈3‐H) +BF (2). This unstable compound was isolated and characterised by NMR and IR spectroscopy.
β ‐Cyclodextrin‐Mediated Regioselective Hydrolysis of 5,10,15,20‐tetrakis[2,4‐bis(pivaloyloxy)phenyl]‐21H,23H ‐porphine
Authors: Matile Stefan , Hansen Thorsten , Stocker Achim , Woggon Wolf‐Dietrich
Published: December 2004

The four peripheral ester moieties of the title compound 5 are regioselectively hydrolyzed under mild conditions in the presence of β ‐cyclodextrin (10), providing a new entry to tetraphenylporphyrin derivatives with differently substituted Ph groups in the meso‐positions. UV, 1H‐NMR and mass spectroscopy of the inclusion complex 9 of 2,6‐O‐dimethyl‐β‐cyclodextrin (12) and 5 indicate a stoichiometry of 2:1 for 10/5. Moreover, calculations confirm NOE experiments consistent with the fact that the cyclodextrins 10 and 12 approach the Ph groups of the porphyrin with the small opening of the cavity (primary face).
Enantioselective Reduction of Electrophilic CC Bonds with sodium tetrahydroborate and ‘semicorrin’ cobalt catalysts
Authors: Misun Marian , Pfaltz Andreas
Published: December 2004

‘Semicorrin’ cobalt complexes, prepared in situ from cobalt(II) chloride and the corresponding ligands, are efficient catalysts for the enantioselective reduction of electrophilic CC bonds with NaBH4. The best selectivities (> 90% ee) are achieved with α,β‐unsaturated carboxamides and carboxylates. Analogous α,β‐unsaturated nitriles, sulfones, and phosphonates afford enantiomeric excesses of 50–70%. Interestingly, in the reduction of α,β‐unsaturated sulfones, the highest enantioselectivities were obtained with unsymmetrical ‘semicorrins’, whereas in all other cases C2‐symmetric ‘semicorrins’ proved to be superior.
Acid‐Catalysed Backbone Rearrangement of cholesta‐6,8(14)‐dienes
Authors: Liu Rong‐Ming , Chillier Xavier F. D. , Kamalaprija Philippe , Burger Ulrich , Gülaçar Fazil O.
Published: December 2004

Rearrangement of 5α‐ and 5β‐cholesta‐6,8(14)‐dienes (13a and 13b, resp.) in the presence of anhydrous toluene‐4‐sulfonic acid in acetic acid leads to 5α‐ and 5β‐12(13 → 14)‐abeo‐cholesta‐8,13(17)‐dienes (15a and 15b, resp.) via 5α‐ and 5β‐cholesta‐8,14‐dienes (14a and 14b, resp.), respectively. Epimerization at C(20) of the spirosteradienes 15a and 15b occurs with increasing reaction time. Molecular‐mechanics calculation of the relative stabilities of these compounds and of congeners thereof is in agreement with the observed reaction pathway.
Metal Complexes with Macrocyclic Ligands. Part XLII. Tetraazamacrocyclic nickel(II) complexes with a methylthio or a methoxy pendant chain as model for cofactor F430
Authors: Schmid Claudio L. , Kempf Christian , Taubert Andreas , Neuburger Markus , Zehnder Margareta , Kaden Thomas A. , Bujno Krzysztof , Bilewicz Renata
Published: December 2004

The 14‐membered macrocyclic Ni2+ complexes of 1 and 2, with a methylthio pendant chain, and those of 3 and 4, with a methoxy pendant chain, have been synthesized and their chemistry has been studied. Solution spectra in H2O, MeCN, and DMF indicate no participation of the side‐chain donor group in metal coordination. This is also the case in the solid state as shown by the X‐ray structures of the Ni2+ complexes with 1 and 2, in which a tetrahedrally distorted square‐planar geometry around the Ni2+ results by the coordination of the four N‐atoms of the macrocycle. Cyclic voltammetry of these complexes in MeCN reveals that Ni2+ is reversibly reduced to Ni+ between −0.7 and −0.8 V vs. SCE. For the complexes with 1 and 2, the thioether bond is cleaved at more negative potentials, whereby a thiol group is formed. This thiol group is then oxidized at ca. +0.7 V vs. SCE, when a glassy C electrode is used, or at ca. 0 V vs. SCE at a dropping Hg electrode. No cleavage of the ether bond in the complexes with 3 and 4 is observed under similar conditions. Reduction of the Ni2+ complexes of 1 and 2 with Na‐amalgam in DMF produces small amounts of methane only in the case of 1, indicating the importance of the proximity between the NiI centre and the MeS group.
The X‐Ray Crystal Structure and Packing of a Hexakis‐adduct of C60: Temperature dependence of weak CH…︁O interactions
Authors: Seiler Paul , Isaacs Lyle , Diederich François
Published: December 2004

The crystal structure and packing of the C60 hexakis‐adduct 1 has been determined from accurate X‐ray diffraction measurements. The largest deformations on the C60 surface occur within the bridged pyracylene subunits; the bridgehead 6–6 bond lengths of the four cyclopropane and the two cyclohexene rings, attached in pseudo‐octahedral positions, are close to 1.6 Å. Significant deformations also occur in the remaining part of the C60 skeleton, not bridged to any of the functional groups. The crystal packing, analyzed at 270, 230, and 180 K is characterized by a three‐dimensional network of 14 weak CH…︁O interactions. For most of these, H…︁O and C…︁O distances decrease on cooling the crystal specimen, indicating that they are attractive in nature. On the other hand, the shortest, nearly linear C(sp3)H…︁O contact seems to be repulsive, since cooling of the crystal specimen leads to significantly longer H…︁O and C…︁O distances and to a reduction of the CH…︁O linearity. Probably as a result of this intermolecular expansion, strain is produced in the crystal. Depending on the crystal‐mounting procedure, this can lead either to a disordered structure (below ca. 200 K), or to destruction of the crystal specimen.
Preparation of α,α‐Dibromocyclobutanones from Olefins: A simple procedure for the regioselective functionalization of olefins
Authors: Chaumeil Hélène , Le Drian Claude
Published: December 2004

Dibromoketene is prepared by in‐situ Zn dehalogenation of tribromoacetyl bromide in the presence of POCl3. Under these conditions, the cycloaddition of dibromoketene with olefins 5 yields α,α‐dibromocyclobutanones 2, which are cleaved with alkoxides to yield β‐dibromomethyl esters 4. Several examples are given. For monosubstituted olefins, the functionalization is regioselective, the methoxycarbonyl group being introduced at the terminal C‐atom.
Asymmetric Synthesis of the Alkaloids mayfoline and N(1)‐acetyl‐N(1)‐deoxymayfoline
Authors: Kuehne Paul , Linden Anthony , Hesse Manfred
Published: December 2004

The total syntheses of the spermidine alkaloids (−)‐mayfoline (11) and (+)‐N(1)‐acetyl‐N(1)‐deoxymayfoline (12) are described. These macrocyclic lactams belong to the most interesting conjugates of the polyamine derivatives very commonly found in nature. The enantioselective syntheses were achieved through resolution of the methyl 3‐amino‐3‐phenylpropanoate (2) by recrystallization of its (+)‐L‐tartrate salt. Construction of the 13‐membered ring ensued through condensation, reductive ring expansion (internal bond cleavage), and finally a transamidation reaction involving a second ring expansion.
Nucleic‐Acid Analogs with Restricted Conformational Flexibility in the Sugar‐Phosphate Backbone (‘Bicyclo‐DNA’). Part 6. Probing the influence of torsion angle γ on DNA‐duplex stability: Syntheis and properties of oligodeoxynucleotides containing [(3′S,5′S)‐2′‐deoxy‐3′,5′‐ethano‐β‐D‐ribofuranosyl]adenine and ‐thymine (‘5′‐epi‐bicyclodeoxynucleosides’)
Authors: Litten J. Christopher , Leumann Christian
Published: December 2004

The synthesis of the thymine‐ and adenine‐containing 5′‐epi‐bicyclodeoxynucleosides 7 and 8 as well as of the corresponding building blocks 13 and 14 for oligonucleotide syntheis according to the phosphoramidite methodology is described. A conformational analysis of 7 and 8 by 1H‐NMR spectroscopy, refined by molecular modeling, shows the preferred conformation of the furanose unit in these nucleosides to be of the 1′‐exo/2′‐endo type. The 5′‐OH group on the carbocyclic ring prefers to be axially oriented, thus placing torsion angle γ in the unusual –syn‐clinal (–sc) range. These epi‐bicyclodeoxynucleosides were successfully incorporated into DNA decamers. From UV/melting curves of such decamers with DNA and RNA complements, a duplex destabilization of −2 to −9° per residue was observed. An oligonucleotide built completely from 5′‐epi‐bicyclothymidine shows no detectable affinity to its DNA or RNA complement anymore. CD Spectra of duplexes containing 5′‐epi‐bicyclodeoxynucleotide units are very similar to the natural reference systems, indicating no major structural changes. A molecular‐dynamics simulation of a heptamer duplex containing one 5′‐epibicyclothymidine residue in the center reveals a conformational change of its carbocyclic unit placing torsion angle γ in the (for the free mononucleoside unfavorable) –anti‐clinal (–ac) conformation in the duplex. The role of torsion angle γ on DNA duplex stability is discussed.
The Synthesis and Characterization of New Optically Active ‘dimeric’ ‘pineno’‐[4,5]‐fused 2,2′‐bipyridines linked without spacer or by small spacer groups
Authors: Fletcher Nicholas C. , Keene F. Richard , Ziegler Marco , Stoeckli‐Evans Helen , Viebrock Heiko , von Zelewsky Alex
Published: December 2004

Linked chiral bipyridines 2–4 are prepared by combining two optically active ‘pineno’‐[4,5]‐fused 2,2′‐bipyridines in a stereoselective reaction (Scheme 1). These potential ligands are new members of the ‘chiragen’ family, and are characterized by NMR spectroscopy and, in the case of 2 and 3 by single‐crystal X‐ray analysis. A new synthesis of ‘dipineno’‐[4,5;4′,5′]‐fused 2,2′ ‐bipyridine 8 is described, which, when coupled, gives additional four chiral centres to the analogous ‘chiragen’ series ( → 9). Analysis of the CD spectra allowed conformational information about the solution species to be determined.
The Enantioselective Synthesis of β‐Amino Acids, their α‐hydroxy derivatives, and the N‐terminal components of bestatin and microginin
Authors: Jefford Charles W. , McNulty James , Lu Zhi‐Hui , Wang Jian Bo
Published: December 2004

L‐Aspartic acid by tosylation, anhydride formation, and reduction with NaBH4 was converted into (3S)‐3‐(tosylamino)butan‐4‐olide (8; Scheme 1). Tretment of 8 with ethanolic trimethylsilyl iodide gave the N‐protected deoxy‐iodo‐β‐homoserine ethyl ester 9. The latter, on successive nucleophilic displacement with lithium dialkyl‐cuprates ( → 10a–e), alkaline hydrolysis ( → 11a–e), and reductive removal of the tosyl group, produced the corresponding 4‐substituted (3R)‐3‐aminobutanoic acids 12a–e (ee > 99%). Electrophilic hydroxylation of 8 ( → 19; Scheme 3), subsequent iodo‐esterification ( → 21; Scheme 4), and nucleophilic alkylation and phenylation afforded, after saponification and deprotection, a series of 4‐substituted (2S, 3R)‐3‐amino‐2‐hydroxybutanoic acids 24 including the N‐terminal acids 24e ( =3) and 24f ( =4) of bestatin and microginin (de > 95%), respectively.
Singlet‐Oxygen (1Δg) Production by Ruthenium(II) complexes containing polyazaheterocyclic ligands in methanol and in water
Authors: Garcìa‐Fresnadillo David , Georgiadou Yiola , Orellana Guillermo , Braun André M. , Oliveros Esther
Published: December 2004

In the context of our studies on ruthenium(II) complexes containing polyazaheterocyclic ligands, we have determined the rate constants of quenching by molecular oxygen (kq) of the metal‐to‐ligand charge‐transfer‐excited state of a series of homoleptic [RuL3] complexes (where L stands for 2,2′‐bipyridine (bpy), 1,10‐phenanthroline (phen), 2,2′‐bipyrazine (bpz), 4,7‐diphenyl‐1,10‐phenanthroline (dip), diphenyl‐1,10‐phenanthroline‐4,7‐disulfonate (dpds), and 1, 10‐phenanthroline‐5‐octadecanamide (poda)) in H2O and in MeOH. These compounds are singlet‐oxygen (O2(1Δg)) sensitizers, and quantum yields of singlet‐oxygen production (ΨΔ) in both solvents are also reported. Values of kq and ΨΔ depend on the nature of the ligand L and on the solvent, ΨΔ values showing a large range of variation (0.2 to 1.0). In MeOH, the only pathway for quenching of the excited [RuL3] complexes by molecular oxygen is energy transfer: the fraction of quenched excited states yielding singlet oxygen (ƒ) is unity for all compounds in the series investigated. Changing from MeOH to H2O has several remarkable effects: higher kq and lower ΨΔ values are observed; ƒ drops to ca. 0.5 except for [Ru(bpz)3]2+. In fact, [Ru(bpz)3]2+ is by far the weakest reductant in the series and behaves differently from the other complexes, with lowest kq and ΨΔ values and a ƒ equal to 1 in both solvents. Results are interpreted on the basis of the role played by charge‐transfer interactions between the sensitizer excited state and molecular oxygen in the quenching mechanism. RuII Complexes based on the 4,7‐diphenyl‐1, 10‐phenanthroline (dip) ligand are very efficient and stable singlet‐oxygen sensitizers with ΨΔ values close to unity in air‐saturated MeOH.
Photoaddition of Ethyl 2,2‐Dimethyl‐5‐oxo‐5,6‐dihydro‐2H‐pyridine‐1‐carboxylate to 2,3‐dimethylbut‐2‐ene
Authors: Jeandon Christophe , Constien Ralf , Sinnwell Volker , Margaretha Paul
Published: December 2004

On irradiation (350 nm) in the presence of excess 2,3‐dimethylbut‐2‐ene, the newly synthesized title compound 5 affords as main products the unexpected cyclopropylpyrrolidine 10 (50%) and the spiro‐oxetane 9 (25%).
Influence of Lewis Acids on the [4 + 2] Cycloaddition of N,N′‐Fumaroylbis[(2R)‐bornane‐10,2‐sultam] to Cyclopentadiene and application to various dienes
Authors: Bauer Tomasz , Chapuis Christian , Kucharska Anna , Rzepecki Piotr , Jurczak Janusz
Published: December 2004

Among seventeen different Lewis acids, TiCl4 was found to be the best catalyst for the [4 + 2] cycloaddition of cyclopentadiene to N,N′‐fumaroylbis[(2R)‐bornane‐10,2‐sultam] ((−)‐1). Independently of the TiCl4 molar concentration, almost constant and complete (98–89% d.e.) diastereofacial π‐selection was achieved in the Diels‐Alder addition of (−)‐1 to cyclopentadiene, cyclohexadiene, isoprene, and 2,3‐dimethylbuta‐1,3‐diene.
Observations sur la photolyse de l'oxyde biazotique (protoxyde d'azote) à l'état comprimé
Authors: Briner E. , Karbassi H.
Published: December 2004

En nous servant de la lampe à vapeur de mercure comme source de rayons ultraviolets, nous avons étudié la photolyse de l'oxyde biazotique (protoxyde d'azote) comprimé à des pressions allant de 5 à 40 atm.
Viskosität und Ladung des Albuminions
Authors: Pauli Wolfgang
Published: December 2004

1. Das mit steigendem HCl‐Zusatz zu hochgereinigtem Seralbumin unter starkem Anstieg der Viskosität gebildete positive Proteinion besitzt im Optimum von η eine Ladungszahl Lz ∼ 80 je Teilchen und diente den Versuchen zum Studium seiner Feldwirkung.
Zur Dehydrierung der Hexahydro‐benzoesäure im Tierkörper
Authors: Bernhard Karl , Caflisch‐Weill Henriette
Published: December 2004

Frühere Befunde über die Umwandlung der Hexahydro‐benzoesäure zu Benzoesäure im Tierkörper erfuhren eine Erweiterung. Da der Mensch und z. B. das Kaninchen in wechselnden Mengen Benzoe‐ bzw. Hippursäure als normales Stoffwechselprodukt ausscheiden, ist ein vermehrtes Auftreten derselben nach Gaben von Cyclohexan‐carbonsäure kein sicherer Beweis für stattfindende Dehydrierung. Mit Hilfe von Deuterium als Indikator, d. h. durch Herstellung und Verabreichung d‐haltiger Hexahydro‐benzoesäure konnte gezeigt werden, daß Mensch, Hund und Kaninchen zu dieser Dehydrierung befähigt sind. Die Aromatisierung des Cyclohexanringes ist im Tierkörper in vielen Fällen leicht möglich.
Untersuchungen über die Sulfierung des Naphthalins
Authors: Fierz‐David Hans Eduard , Richter Carl
Published: December 2004

1. Das Isomerenverhältnis ist bei direkter Sulfierung ein kompliziertes. Bei 130° konnten, von Naphthalin ausgehend, alle nach der Armstrong'schen Regel möglichen Disulfosäuren nebeneinander nachgewiesen werden, so auch die 1,5‐ neben der 2,6‐Säure.
Etudes sur les matières végétales volatiles XXXIII. Sur l'évaluation des alcools tertiaires dans les huiles essentielles
Authors: Naves Y. R.
Published: December 2004

La méthode imaginée par Schryver en vue du titrage des phénols, fondée sur le dégagement d'ammoniac au contact de l'amidure de sodium n'est pas applicable à l'évaluation des alcools. En dehors de l'hydroxyle, d'autres éléments de la molécule régissent et les valeurs obtenues sont, de ce fait, excessives. Par contre, la méthode de Tschugaeff‐Zerewitinow, selon la technique de Soltys, permet le titrage satisfaisant des alcools, dans l'espace de trente minutes.
Hydrogen bonding in simple π‐electron systems II. Pyridine–pyrrol
Authors: Sabin John R.
Published: December 2004

Previous work in this laboratory concerning the properties of hydrogen bonds in the base pairs of DNA [1–6] has led to considerable interest in the properties of hydrogen bonds in π‐electron systems. The first paper in this series [7] has investigated the usefulness of the LCAO–MO–SCF method and the Pariser–Parr–Pople approximation as applied to this problem, by calculation on the ideal pyridine–pyridinium complex. In this paper, a relation with experiment will be established by comparison of the results obtained from this method of calculation with the properties of the experimentally observable pyridine–pyrrol system.
Might the difference between Schrödinger and Ruedenberg's local contributions to the kinetic energy represent elastic deformations of the electron?
Authors: Jørgensen Chr. Klixbüll
Published: December 2004

In Schrödinger's equation, the local contribution to the kinetic energy is sometimes negative, but it allows a conservative potential in contrast to an alternative, never negative, energy proposed by Ruedenberg. The total kinetic energy evaluated in certain characteristic volumes, and in all of space, is identical in the two cases. Dirac's suggestion in 1962 of a bubble‐surface representing the electron is applied in a far‐reaching, but incomplete, attempt to identify external kinetic energy with a first‐order relativistic effect and the internal energy constituting the difference between Schrödinger and Ruedenberg's proposals is related to oscillations of the surface.
On the calculation of London–Van der Waals interactions in a monopole‐bond polarizability approximation with application to interaction between purine and pyrimidine bases
Authors: Rein Robert , Claverie Pierre , Pollak Michael
Published: December 2004

Methods for calculating intermolecular interaction energies are evaluated in a specific application to double‐stranded DNA‐like configurations between bases. The total interaction is decomposed in such a way, that existing methods can be hybridized so that each method can be utilized to its best advantage. In addition, a new method is suggested for the computation of the dispersion interaction between the σ‐electron system of one molecule and the π‐electron system of the other. Results of various approaches applied to base pairs and double‐stranded dimers are summarized and compared in a number of tables.
Wellenmechanische Untersuchungen an der Wasserstoffbrücke zwischen zwei Berylliumatomen
Authors: Janoschek R. , Diercksen G. , Preuss H.
Published: December 2004

The systems Be2H+ and Be2H− have been investigated for different nuclear positions, the H atom being situated between the Be atoms, taking all electrons into account, using the Allgemeines Programmsystem/SCF–MO–LC(LCGO) Verfahren. For Be2H+ there results a minimum total energy of −29.3824 a.u. in the linear symmetric configuration with a bond distance of 1.609 Å. The ionization energy was estimated to be 12.37 eV. The formation of Be2H+ can be interpreted as an addition of Be to BeH+ with an exotherm heat of reaction of 7.0 kcal/mole. The electron affinity of BeH+ (ionization energy of BeH) was estimated to be approximately 7.24 eV. All force constants of Be2H+ and BeH+ have been computed. Using SCF results, the Be2H− was found to be unstable.
Molecular‐orbital calculations on electrophilic substitution and relative basicities in pyrrole, indole, furan, and benzofuran
Authors: Hermann Robert B.
Published: December 2004

Two molecular‐orbital methods—extended Hückel theory and CNDO/2—have been used to estimate the relative electrophilic reactivities of the alpha and beta positions in the nonalternant heterocyclic compounds pyrrole, indole, furan, and benzofuran by comparing total transition‐state energies. The order of basicities of three positions of indole and pyrrole are predicted. Charge densities are also calculated and evaluated in the light of n.m.r. chemical shift data.
Convergence studies in configuration interaction calculations
Authors: Pipano A. , Shavitt I.
Published: December 2004

A series of CI calculations for the ground state of BH3 in which various levels of excitation from the ground state are included are compared with each other and with the full‐CI(196 configurations) result. The comparisons cover calculations in terms of two different molecular orbital bases—the canonical SCF basis and a simple, arbitrarily chosen, symmetry orbital basis. As expected, single and, to a lesser extent, triple excitations are of little importance in the SCF case but cannot be ignored for the arbitrary basis. However, as soon as all excitations ≦4 are included, there is practically no difference in the results for the two bases, both giving energies quite close to the full‐CI value. In fact, the energies for the two bases are in close agreement also in the (0 + 1 + 2 + 3)‐excitation calculation and in qualitative agreement in the (0 + 1 + 2) case.

Two methods are tested for the selection of the important higher‐excitation configurations, and it is found that results very close to full CI can be obtained with substantially fewer functions. Particularly promising is the application of the “unlinked cluster” approach, based on the ideas developed by Sinanoğlu and others, to the prediction of the coefficients of evenly excited configurations from those of the doubly‐excited functions in a limited‐CI calculation.
An application of perturbation theory ideas in configuration interaction calculations
Authors: Gershgorn Z. , Shavitt I.
Published: December 2004

Configuration interaction calculations of electronic wave functions for atoms and molecules have generally been limited to relatively small basis sets because of the exponential increase in the number of configurations as basis functions are added. While higher than quadruply excited configurations are of negligible importance in CI wave functions, it is shown that the effect of triple and quadruple excitation configurations can be substantially included even when the matrix elements between such configurations are neglected, leaving only their diagonal elements and the elements connecting them with the single and double excitations. This approximation is seen to be formally practically equivalent to a first‐order perturbation expression for the wave function (second‐order for the energy) based on an optimum linear combination of the zero, single, and double excitation configurations as the zero‐order function. If suitable procedures are used, the amount of computational effort involved in such a calculation is roughly proportional to the fourth power of the number of basis functions employed, thus preventing the CI stage of the calculation from increasing in magnitude much faster than the stages involving the calculation and manipulation of the elementary integrals.
Upper and lower bounds to polarizabilities and van der Waals forces I. General theory
Authors: Goscinski Osvaldo
Published: December 2004

Upper and lower bounds to polarizabilities, and long‐range interactions of atoms and molecules are derived using operator inequalities.

The formulae for the dispersion and the three‐body non‐additive forces are given in terms of the S(k), related by sum rules to properties of the separated systems.

Some known approximations are obtained but with additional information about their nature, as well as several new inequalities. Their interest is that they give explicit and rigorous bounds involving quantities available from experimental data.
Numerical studies for a theoretical analysis of semiempirical LCAO–CI methods
Authors: Carpentieri M. , Porro L. , Del Re G.
Published: December 2004

A numerical investigation of Del Re and Parr's formulas [1] for the treatment of π systems has been preformed in the case of five‐membered rings, using two different expressions for the core Hamiltonian and different values for the effective charges. The results obtained are discussed by analysing the three stages of the calculation: (a) a non‐iterative LCAO–MO calculation; (b) the same calculation with corrections for exchange and repulsion terms arising from fluctuations of the orbital populations; (c) configuration interaction. The calculations are interesting also because they do not involve the zero differential overlap approximation; a calculation without inclusion of overlap hse been carried out for pyrrole and the results have been compared with those including S.

The main conclusions hold also for σ electrons, and can serve to assess better the validity of simple σ calculations.
The electronic structure of tetrahedral III–V compounds: The ground state of boron nitride
Authors: Coulson C. A. , Doggett G.
Published: December 2004

The bond‐orbital theory of III–V compounds, previously described by Coulson, Redei and Stocker, is used to calculate the effective atomic charges and the binding energy per bond in boron nitride. The theory is reformulated in a manner which is convenient for performing both ab initio and semiempirical calculations. Two different choices for the atomic‐orbital exponents are considered and, in both cases, the results obtained from the ab initio method are at variance with the earlier calculations in predicting an electronic charge displacement from nitrogen to boron. The magnitude of the effective charges is found to vary according to the method of partitioning the overlap charge between the nitrogen and boron atoms. The use of orthogonalized Slater 2s functions is also examined.

The semiempirical calculations are performed with an explicit inclusion of the Madelung energy from the outset. The ionicity in the bond is shown to be determined by the competition between the difference in orbital electronegativities and the difference in Madelung potential across the ends of the bond. Unfortunately, the semiempirical theory breaks down because the energy per bond passes through a maximum at the optimum value of the polarity parameter. The reasons for this behaviour are examined and discussed.
Self‐consistent‐field method including correlation effect for atomic systems. Three‐ and four‐electron atomic systems
Authors: Modrak P.
Published: December 2004

The correlation effect for three‐ and foru‐electron atomic systems has been taken into account by modifying the potentials of the electron interactions appearing in the Hartree–Fock equations. The correlation energies obtained for Li, Be+, B2+, Li−, Be and B+ differ by less than 25 percent from the exact values.
Theory of intermolecular interactions: The long range terms in the dipole–dipole, monopoles–dipole, and monopoles–bond polarizabilities approximations
Authors: Claverie Pierre , Rein Robert
Published: December 2004

The problem of evaluating the long range terms (electrostatic, polarization, dispersion) of the interaction energy between molecules at intermediate distances (i.e. distances of the order of magnitude of the molecular dimensions) is considered. Instead of being approximated by its dipole part, the exact interaction Hamiltonian is treated as proposed by Longuet‐Higgins [11], i.e. the matrix elements are interpreted as electrostatic interactions between state and transition charge distributions. These charge distributions are approximated in a systematic way by sets of point charges (localized on the atoms) or sets of dipoles (localized on the bonds). The various contributions to the energy may then be expressed in terms of atomic net charges and bond polarizabilities. More refined approximations of the charge distributions could be used and correspondingly improved formulae could be derived: as an example, a formula for the σ‐π dispersion energy is derived, where the σ charge distributions are approximated by bond transition dipoles (leading to σ bond polarizabilities in the final formula) while the π charge distributions are approximated by atomic charges.
On the choice and definition of atomic orbital‐bases. II. Promoted Slater orbitals and best‐atom hydrogen‐like orbitals
Authors: Rastelli Augusto , Del Re Giuseppe
Published: December 2004

Single‐exponential functions of the forms 1s̄ = 1s, 2s̄ = N1(2s + b1s + a3s), 2p̄ = N2(2p + c3p) are introduced as an alternative to the Slater orbitals 1s, 2s, 2p for the atoms of the first period. The orbitals are intended for molecular calculations and the parameters a and c should be chosen in order to make the basis consistent with a given preliminary description of the molecule: see Part I, G. Del Re, Intern. J. Quantum Chem. 1, 293 (1967), but, first of all, a study of their behavior in atomic systems is necessary. Therefore orbital exponents and total atomic energies are determined by optimization: the promotion coefficients a and c are zero for best‐atoms and the functions become strictly hydrogen‐like orbitals. Comparisons are allowed with Slater's, Roothaan's, Ransil's and SCF orbitals.

Given promoted 2s̄ and 2p̄ orbitals on carbon atoms in a carbon‐carbon sigma bond, we fix the coefficient a and c in order to obtain one center sp orbitals orthogonal to one another. These are discussed, mapped and compared with the two center hybrids obtained by the Löwdin orthogonalization.
Quadratically convergent iteration procedure for geminal determinations
Authors: Loter Joe , Christoffersen Ralph E.
Published: December 2004

A general formalism is presented for the determination of the optimum natural orbitals within the various strongly orthogonal geminals of a wavefunction describing a 2N‐electron closed‐shell molecule or atom. The relationship to Hartree–Fock–Roothaan theory is established; the algorithm that is developed is quadratically convergent to the desired result, and does not ignore off diagonal Lagrangian multipliers, or require an infinite series of 2 × 2 orthogonal transformations of the original basis.
Many‐Electron problems. II. Energy expansions and internal fields
Authors: Gáspár R.
Published: December 2004

Energy relations are derived for neutral atoms and isoelectronic sequences. It is shown that the field of the electrons at the position of the nucleus follows a very simple law and the diamagnetic shielding constant may be given in simple analytic form as a function of the atomic number.
On the theoretical foundation of Walsh's rules of molecular geometry in terms of the Hellmann–Feynman theorem
Authors: Coulson C. A. , Deb B. M.
Published: December 2004

A new interpretation of the ordinate in a Walsh diagram for a polyatomic molecule is suggested in terms of the Hellmann–Feynman theorem. This makes use of the fact that in a single‐configurational MO wave function the total one‐electron density is the sum of individual densities in the occupied orbitals. Walsh‐type diagrams have been constructed for three different molecules, water, ammonia and hydrogen peroxide.

In H2O and NH3 calculation of the force, and thus of the energy, in terms of the valence angle, is made on the assumption that the central (heavy) atom is kept fixed while each of the lighter atoms moves in a plane containing the principal symmetry axis and the relevant bond, in a totally symmetric fashion; for H2O2 the two oxygen atoms are kept fixed.

The angular correlation diagrams obtained reproduce the general features of those obtained by plotting Hartree–Fock MO energies as functions of the valence angles. The conclusion emerges that the force formulation provides a satisfactory pictorial basis for understanding molecular geometry in terms of the balance between the electron–nucleus attractive forces resulting from the charge densities in the occupied MO'S, and the nuclear repulsive forces. However, in the absence of highly accurate charge distributions such an approach is unsuitable for the quantitative prediction of molecular quantities such as valence angles, force constants or energy barriers.
Valence‐Bond calculations and matrix elements between two tableau functions of non‐orthogonal orbitals
Authors: Gallup G. A.
Published: December 2004

An integral formula is derived involving a new type of determinantal function which reproduces the effect of the Young operator θNPN. This result is used to calculate matrix elements between tableau functions of non‐orthogonal orbitals. Matrices which transform this representation into the traditional valence‐bond scheme are also given.
Use of molecular symmetry in SCF calculations
Authors: Elder M.
Published: December 2004

A method is described whereby molecular symmetry properties may be used to reduce the numbers of one‐ and two‐electron integrals that need to be calculated and stored in the course of a molecular SCF calculation. The method is a generalization of a previously reported procedure, extending the earlier work to cover those molecules belonging to point groups which have complex representations. The practical application of the method is discussed and an illustrative example given. The quite extensive tables of molecular symmetry properties which the method uses may be computer generated in a straightforward manner. A procedure for doing this using a minimum amount of input data is presented.
Projection between basis sets: A means of defining the electronic structure of functional subunits
Authors: Fink William H.
Published: December 2004

The general problem of the transfer of the representation of a set of orthonormal functions from one basis to a different, nonequivalent one is considered and specialized to a matrix formulation convenient for use in molecular electronic structure calculations. A procedure is suggested for treating problems where the transfer of representation breaks into a subset of most interest and one of less interest as for example the occupied and virtual orbitals of a Hartree‐Fock SCF calculation. These techniques are then applied to obtain a representation of a methyl group from an SCF wave function for methane.
Spin‐free quantum chemistry. XIII. Spin waves
Authors: Matsen F. A. , Suger J. E. , Picone J. M.
Published: December 2004

We have obtained the Bloch spin wave dispersion formula using the methods of spin‐free quantum chemistry. The spin‐free eigenvectors are waves in spin‐free space. This development makes the point that Bloch spin waves are dynamically spin‐free. The neutron diffraction transition moment for spin waves is calculated employing the antisymmetrized projections of vector products of spin‐free eigenkets and spin kets and is found to be agreement with results of Moorhouse.
Spin‐free quantum chemistry. XIV. The infinite interaction range model for ferromagnetism
Authors: Matsen F. A. , Cosgrove J. G. , Picone J. M.
Published: December 2004

The infinite interaction range model (IIRM) for ferromagnetic systems is presented in its spin‐free formulation. In this formulation the states are labelled by partitions which provide a natural variable for thermodynamic computation. We have extended the calculations of Kittel and Shore by computing to a practical thermodynamic limit (N ∼ 100,000). The heat capacity, magnetic susceptibility and the magnetization of the first two functions exhibit a critical temperature while the magnetization is zero at zero field for all temperatures. Spontaneous magnetization is obtained by linear extrapolation from high field or equivalently by a polarized partition function. Relationships are explored among IIRM, the Heisenberg model and the mean field model. Application to IIRM of the Yang‐Lee condition for a phase transition yields a critical temperature identical to that obtained by the direct calculation.
Brillouin‐Wigner perturbation theory and the generalized eigenvalue equation
Authors: Laughlin C. , Woodward M. R. , Amos A. T.
Published: December 2004

A Brillouin‐Wigner perturbation expansion is derived for the generalized eigenvalue equation (F0 + F1)Ψ = μAΨ. The theory is applied through second order to calculate the ground‐state energies of the helium atom and the hydrogen molecular ion. The results are compared with the corresponding Rayleigh‐Schrödinger expansion. For the examples we consider, the Brillouin‐Wigner results through second order are generally superior to the Rayleigh‐Schrödinger ones.
SCF equations for pure spin states with many‐particle interactions
Authors: Lim T. K.
Published: December 2004

SCF equations for any pure spin state are given for a spin‐free system with many‐particle interactions. The equations are very simple and explicit. Due to the use of different antisymmetric requirement our equations are different from some of the other methods. In our method, the abstract group theory formalism is converted into some explicit and straightforward equations which makes the many‐particle interaction problem easier to handle.
Conditional probability amplitudes in wave mechanics
Authors: Hunter Geoffrey
Published: December 2004

The total wave function of a system ψ(x, y) is expressed as a product of a marginal amplitude function f(y) and a conditional amplitude function ϕ(x | y). The original Schrödinger equation Hψ = Eψ is reduced to a Schrödinger equation for the marginal amplitude H′f = Ef. The factorization of ψ is in principle exact. It achieves a partial separation of the variables x and y.
Computation of Fourier transform quantities in Hartree–Fock calculations for simple crystals
Authors: Graovac Ante , Monkhorst Hendrik J. , Glasser M. L.
Published: December 2004

An efficient method is presented to compute the Fourier transforms of lattice sums over Slater‐type orbital products that arise in crystal Hartree–Fock calculations. Introduction of one‐dimensional integral representations for the Fourier transforms of the STO'S enables a separation of the three infinite sums under the (two‐dimensional) integral sign. This, in turn, makes it possible to calculate and store quantities associated with the three Fourier transform vector components separately. The orbital transform integrations are then performed numerically. The method is very advantageous when lattice sum transforms are needed for a large number of transform vectors. Formulas and results are presented for simple‐cubic crystals when 1s, 2s, and 2p Slater orbitals are involved.
The use of partitioning technique in the solution of operator equations for ionizations and excitations
Authors: Nerbrant Per‐Olof
Published: December 2004

An inhomogeneous operator equation is associated with the equations of motion for ionization‐ and excitation operators. The partitioning technique is applied to solve the equation. Formal expressions for the ionization‐ and excitation operators are obtained and examples of approximations to these are given.
Errors in the calculations of first‐order and second‐order energy perturbations
Authors: Hameka Hendrik F. , Svendsen E. Nørby
Published: December 2004

We consider the calculation of first‐order and second‐order atomic and molecular properties from approximate ground‐state wave‐functions by using variational procedures for solving the first‐ and second‐order perturbation equations. We evaluate the errors in the final results due to the error in the unperturbed wave‐function and to the errors caused by the approximations in solving the perturbation equations. By combining slightly different results we can eliminate all first‐order errors. Our analysis covers situations where perturbation expansions of the error in the ground‐state wave‐function are not feasible and it includes the effects due to approximations in solving the perturbation equations.
The spin‐density‐functional formalism for quantum mechanical calculations: Test on diatomic molecules with an efficient numerical method
Authors: Gunnarsson O. , Johansson P.
Published: December 2004

We have applied the spin‐density‐functional (SDF) formalism with the local‐spin‐density (LSD) approximation to a number of small molecules with the primary aim of testing the approximation for molecular applications. A new numerical method to solve the one‐electron wave equation is developed, utilizing the special features of the SDF formalism. We have calculated energy curves, dissociation energies, and equilibrium distances for some diatomic molecules [H (2Σ, 2Σ), H2(1Σ, 3Σ), He (1Σ), and He2(1Σ)] and the vibrational frequencies of H2. The deviations from the experimental results are typically 1/2 eV for the energies and ≤ 0.1 Å for the distances. We discuss the LSD approximation using the concept of an exchange‐correlation hole and make predictions about the applicability to other molecules. The LSD approximation is compared with the Hartree‐Fock and multiple‐scattering‐Xα methods and some difficulties in the latter methods are pointed out. It is argued that the SDF formalism within the LSD approximation has physical advantages compared to the Hartree‐Fock and Xα methods and that it should provide a simple and useful method for a broad range of applications.
Spin‐Extended Hartree–Fock calculation for hydrocarbon π‐radicals
Authors: Klimo Viliam , TiňO Jozef
Published: December 2004

An iterative method of solving the Mestechkin spin‐extended Hartree–Fock equations is proposed. On this basis projected and spin‐extended values of energies and spin densities for some hydrocarbon π‐radicals are calculated. Relations among UHF, ASA, AA, projected, and spin‐extended energies are discussed.
Calculations of electric dipole polarizabilities of polyatomic molecules
Authors: Lazzeretti Paolo , Cadioli Beniamino , Pincelli Ugo
Published: December 2004

Coupled and uncoupled Hartree–Fock theories are used to compute the electric dipole polarizability of water, ammonia, and methane with three different GTO basis sets. Bounds for the geometric approximation to uncoupled polarizabilities are also computed to examine the accuracy of calculated values. The results are compared with those obtained by a variational‐perturbation method proposed by Rebane. The numerical tests provide some information on the correlation terms affecting total computed polarizabilities. The computed values are shown to be in fair or good agreement with experimental data for the largest basis sets. The reliability of Rebane's method with respect to coupled HF procedure is also discussed.
I. Lineare Fehlerminimisierung Ein Verfahren zur Eigenwertberechnung bei Schrödinger‐Operatoren
Authors: Kleindienst H. , Altmann W.
Published: December 2004

A variational principle is derived from the spectral theorem of self‐adjoint operators. The principle allows to calculate simultaneously upper and lower bounds for each point of the spectrum. For Schrödinger operators this method of error minimization is reduced to a series of matrix diagonalizations using an iterative process and is linearized in this way. The convergence of this method and the equivalence with the nonlinear problem of variation are proven.
II. Eigenwertberechnung beim Dreikörperproblem
Authors: Kleindienst H. , Baumgarten E. , Altmann W.
Published: December 2004

The convergence and the quality of approximation of the variational method developed in part I are tested using the He atom as an example. For the three‐body problem the inner products and the norms were explicity expressed as functions of the optimization parameters. The method permits calculation of upper and lower bounds for the ground state of helium.
The instabilities of the Hartree–Fock solutions and the lattice instabilities for conjugated hydrocarbons: The bond‐order and bond‐length alternations
Authors: Toyota A. , Tanaka T. , Nakajima T.
Published: December 2004

The instabilities, especially the singlet instabilities, of the conventional Hartree–Fock (HF) solutions for a variety of alternant and nonalternant hydrocarbons, some of which have been known to show lattice instabilities (bond‐length alterations), are examined. The HF solutions for nonalternant hydrocarbons in the pentalene series larger than heptalene and [4n + 2]‐annulenes larger than C22H22 are found to be singlet unstable and there appear new solutions lower in energy than the conventional HF solutions and characterized by charge‐density waves exhibiting bond‐order alterations. It is found that such symmetry‐breaking solutions are energetically further stabilized by distorting the nuclear framework so that it may match up with the distribution of bond‐order matrix elements of the charge‐density wave, which means that in conjugated systems the singlet instability of the HF solution is always accompanied with the lattice instability. Further, it is shown that in conjugated systems, even when the HF solution is singlet stable, if it is not sufficiently stable as, for example, in pentalene and heptalene, there is every possibility for the occurrence of lattice instability. It is also shown that the singlet instability as well as the lattice instability arises from the existence of a sufficiently low‐lying singlet excited state.