bluuebunny/crossref_metadata_2025 huggingface dataset:
Evaluation of the conformational, hydrogen bonding and crystal packing preferences of acyclic imides
Authors: Reutzel Susan M. , Etter Margaret C.
Published: January 2005

The molecular recognition properties of 18 acyclic imides were studied to evaluate the relative contributions of conformational, hydrogen bonding and crystal packing forces to the stabilization of specific aggregate patterns in the solid state. The crystal structure of diisobutyramide and the aggregate patterns of the 18 imides are presented. The stabilization by hydrogen bonding was found to override the conformational preferences of imides, while packing forces often precluded the formation of the most stable hydrogen bonded aggregate. The aggregate patterns of imides were also found to be a function of the type of substituents present as R groups. Imides with R groups of similar shape and size prefer to pack as bifurcated hydrogen bonded chains, whereas dimers or singly hydrogen‐bonded chains form when the R groups have significantly different spatial requirements. Analysis of imide aggregate patterns revealed the similar spatial requirements of isopropyl and phenyl groups. The molecular recognition properties of acyclic imides are summarized as a set of hydrogen bond rules, which can be used to design new imide aggregates.
Hydrolyses of terpenoid diphosphates. Effects of azide ion on products of hydrolysis
Authors: Alarcon Maritza , Cori Osvaldo , Rojas M. Cecilia , Pavez Hernan , Bacaloglu Radu , Bunton Clifford A.
Published: January 2005

Hydrolysis of geranyl diphosphate (GPP) at pH 7 in water gives largely linalool (LOH) + geraniol (GOH) in the ratio of 3:1. Added N−3 generates mixed acylic allylic azides and increases the LOH GOH ratio to 15:1 in 2 M NaN3, but does not speed up the overall reaction. Hydrolysis of neryl diphosphate (NPP) gives largely α‐terpineol (TOH) +p LOH, but their ratio is not very sensitive to NaN3 concentration although acyclic azide and small amounts of α‐terpinyl azide (TN3) are formed. Hydrolysis of α‐terpinyl diphosphate (TPP) gives large amounts of the cyclic alkenes, limonene and terpinolene. Added N−3 does not change the amount of elimination, but increases the ratio of limonene to terpinolene, and diverts some substitution product to TN3. Trapping of carbocationic species from GPP by N−3 is sharply increased by addition of Mn2+, which also catalyzes the overall reaction. Products of reaction of GPP are derived from acyclic intermediates and of NPP from acyclic and cyclic intermediates, and ionizations of the three substrates do not generate common carbocationic species.
Molecular modelling of organophosphorus podands and their complexes with alkali metal cations
Authors: Varnek A. A. , Morosi G. , Gamba A.
Published: January 2005

A molecular mechanics study on some organophosphorus podands, with both rigid and flexible terminal groups, and on their complexes with Li+ and Na+ was carried out. Na+ causes smaller deformations of the ligands than Li+, but its complexes are less stable, as the interaction energy with the podands is weaker. The ligands with rigid terminal groups are, in general, more pre‐organized for complex formation. The cation‐ligand interaction energies, including also the changes in steric energies, are always higher in complexes of flexible molecules than in those containing rigid ligands. The lower entropy losses which accompany the formation of complexes of rigid molecules with alkali metal cations probably account for the observed higher stabilities of the complexes.
Cycloaddition of heterofunctionalized allenes with tert‐butylthioacrylonitrile. Determination of activation parameters for diradical intermediate formation and the detection of reversible ring closure
Authors: Pasto Daniel J. , Kong Wei
Published: January 2005

The cycloaddition reactions of 4‐methylphenylthio‐, methoxy‐, 4‐methoxyphenyl‐, phenyl‐, chloro‐ and cyanoallene with tert‐butylthioacrylonitrile (BTA) in toluene‐d8 solution were investigated. The relative reactivity sequence of these substituted allenes is 4‐methylphenylthio ∼ methoxy‐ > 4‐methoxyphenyl‐ > phenyl‐ > chloro‐ > cyanoallene. Activation parameters were measured for diradical intermediate formation of BTA with 4‐methylphenylthio‐, methoxy‐ and 4‐methoxyphenylallene, giving average values of Ea and ΔS± of 14.2–16.8 kcal mol−1 and ca. ‐33 eu, respectively. The relative reactivity sequence is consistent with a FMO allene‐HOMO, BTA‐LUMO dominant interaction for diradical intermediate formation. However, the regioselectivity of attack on the substituted allene appears to be thermodynamically controlled. The formation of the diradical intermediates in these cycloaddition processes appears to be irreversible. However, the ring closure of the diradical intermediates formed from 4‐methylphenylthio‐, methoxy‐ and 4‐methoxyphenylallene is reversible under the conditions of the kinetic experiments. The kinetically controlled ring closure of the diradical intermediates is allyl radical SOMO controlled, while the final cycloadduct distribution is thermodynamically controlled.
Enhancement or suppression of CIDNP intensities by a second magnetic nucleus: Interplay between g factor difference, external magnetic field, and hyperfine coupling constants
Authors: Hwang Kuo Chu , Roth Heinz D. , Turro Nicholas J. , Welsh Kevin M.
Published: January 2005

The enhancement factors of the 1H polarization induced in radical pairs generated by α‐cleavage of 12CO‐ and CO‐labelled ketones deviate from the ratios expected on the basis of the relative abundances of the respective carbon isotopomers. For dibenzyl ketone, the ratio of absolute CIDNP intensities of the 1H(13C) doublet (90% 13CO) to the 1H(12C) singlet (10% 12CO) was found to be >9:1 at 58·3 kG and <9:1 at 21 kG. Similar deviations were observed for other 13C labelled ketones, viz. deoxybenzoin (13CO) and acetophenone (13CO or 13CH3). This novel enhancement or suppression effect on the CIDNP enhancement can be ascribed to the presence of a second hyperfine coupled nucleus in the intermediate radical pair. The individual contributions of the g factor difference (Δg), the external magnetic field strength (Ho), and the hfc constants of the observed (aA) and the interacting nucleus (ax) in the enhancement‐suppression are exemplified. The experimental results are consistent with theoretical calculations of the mutual effect of Δg, H0, aA and ax on the CIDNP enhancement factor of the observed nucleus.
O‐versus S‐protonation in diaryl sulfoxides: A semi‐empirical mo investigation: Comparison with solution studies in superacids
Authors: Laali Kenneth K. , Houser John J.
Published: January 2005

The protonation of diphenyl sulfoxide and several substituted diphenyl sulfoxides was probed by the MNDO‐PM3 method to gain a theoretical insight into the experimentally observed preference for stable diphenyl sulfoxonium ion formation (O‐protonation) in 1:1 FSO3H—SbF5 (Magic Acid)—SO2. In agreement with solution studies, O‐ protonation is uniformly favored (by ca 17 kcal mol −1) over S‐protonation. The differences in the heats of formation of protonated and unprotonated diphenyl sulfoxides are increased by electron‐withdrawing substituents (F, CF3) and decreased by electron‐donating groups (Me, OMe). Variations in the SO bond length and bond order in the onium ions are compatible with simple resonance arguments and the oxonium/sulfonium character of both the O‐ and S‐protonated onium ions. Surprisingly, the O‐ and S‐protonated dibenzothiophene S‐oxides are predicted to have identical energetics as compared with the corresponding protonated diphenyl sulfoxides. On structure optimization dibenzothiophene S‐oxide itself, if the initial geometry is somewhat twisted, rearranges by ring expansion to give a new heterocycle with lower energy. A rotational barrier study on the parent O‐protonated diphenyl sulfoxide showed two minima, separated by 1.3 kcal mol−1, at HOSC dihedral angles of 60° and 240°. The two conformations correspond to syn and anti orientations of the OH proton relative to the aromatic rings, and support the lowtemperature solution observations of the presence of two distinct sulfoxonium ions in solution. The rotational barriers for diphenyl sulfoxonium cation are compared with those of O‐protonated dimethyl sulfoxide.
Stabilization of molecular association: An AM1/PM3 study of tetrahalomethane–arene molecular complexes
Authors: Mager Thomas , Bertagnolli Helmut
Published: January 2005

An AM1/PM3 study of several tetrahalomethane–arene molecular complexes is presented. The results indicate the existence of weak attractive non‐dispersion interactions in some of these complexes, which can be rationalized in terms of multipole—multipole and multipole—induced dipole interactions. No evidence was found for the occurrence of charge transfer in the ground states of these complexes.
Quantitative aspects of chirality. I. Method of dissymmetry function
Authors: Kuz'min V. E. , Stel'makh I. B. , Bekker M. B. , Pozigun D. V.
Published: January 2005

To study molecular features connected with chirality, a procedure for the quantitative estimation of the chirality level of compounds of different classes is needed. A procedure for estimating the molecular asymmetry level relative to mirror‐reflection axes of symmetry, S1, S2, S4 and S6, has been developed. The geometrical mean of these parameters is the disymmetry function (DF). To calculate the DF, the molecule must be fixed in the coordinate system, transferred to the main axes of inertia.
Photochemical reactions of 2,2′‐furil. Solvent dependent photoreduction via the lowest excited singlet and triplet states
Authors: Inoue Hiroyasu , Sakurai Tadamitsu , Hoshi Toshihiko , Ono Isao , Okubo Jun
Published: January 2005

2,2′‐Furil (di‐2‐furyl diketone) undergoes photoreduction to give furoin on irradiation at 366 nm in triethylamine‐containing benzene and ethers. On the other hand, in ethanol the photoirradiation leads to the formation of an enediol type of compound, 1,2‐di(2‐furan)ethene‐1,2‐diol. The results of quenching and sensitization both for the photoreactions and the phosphorescence demonstrate that furoin and the enediol are formed via the lowest excited singlet and the triplet states of 2,2′‐furil, respectively.
Using theoretical descriptors in quantitative structure activity relationships: Some physicochemical properties
Authors: Famini George R. , Penski Carl A. , Wilson Leland. Y.
Published: January 2005

The application of computational techniques to biology, chemistry and physics is growing rapidly. Quantitative structure–activity relationships (QSAR) have been used widely to relate biological activities and physicochemical properties to molecular structural features. A difficulty in this approach has been non‐uniformity of parameter sets resulting in the inability to examine contributions across properties and data sets. Linear solvation energy relationships (LSER) developed by Kamlet and Taft successfully utilize a single set of parameters to correlate a wide range of biological, chemical and physical properties. The empirical LSER solvatochromic parameters have been replaced with theoretically determined parameters to permit greater ease in a priori property prediction. These TLSER descriptors have given good correlations and interpretations for some biological activities. This paper discusses the application of these descriptors to six physicochemical properties involving equilibria, kinetics and spectra. The results show good correlation and physical interpretation.
O‐17 NMR studies on α‐diazoketones
Authors: Cerioni Giovanni , Culeddu Nicola , Saba Antonio
Published: January 2005

The O‐17 NMR spectra of 13 α′‐alkyl‐ and aryl‐substituted α‐diazoketones were measured. The results obtained show a clear discrimination between them. A good correlation in a series of para‐substituted α‐diazoacetophenones between O‐17 chemical shifts and both α+p and dual substituent parameters indicated a greater contribution of resonance than inductive effects.
Survey of the DNA binding properties of natural and synthetic polyamino compounds
Authors: Stewart Kent D. , Gray Thomas A.
Published: January 2005

Using a flourescence‐detected ethidium displacement assay, the calf thymus DNA complexation properties of 27 mono‐, di‐, tri‐, tetra‐ and hexacationic polyamines were determined. The DNA‐binding affinity of these polyamine compounds increased with increasing cationic charge on the polyamine. Although most of the compounds exhibited no base pair binding selectivity, two of the tricationic polyamines possessing additional neutral amine groups exhibited approximately tenfold GC binding selectivities.
Potassium fluoride for the promotion of β‐cyclodextrin‐induced regioselective PO(3′) cleavage of adenosine 2′, 3′‐cyclic phosphate
Authors: Sawata Shinya , Komiyama Makoto
Published: January 2005

Both the regioselectivity and the reaction rate for the β‐cyclodextrin (β‐CyD)‐induced P–O(3′) cleavage of adenosine 2′, 3′‐cyclic phosphate (A > p) to adenosine 2′‐phosphate are greatly promoted by KF. The promoting activity of KF is much larger than that of KCl. KF amplifies the difference in the chemical circumstance between the P–O(2′) and the P–O(3′) bonds of A > p, induced by β‐CyD on complex formation with A > p. The higher activity of KF is ascribed to absence of complex formation of F− ion with β‐CyD, which results in deeper penetration of A > p into the cavity of β‐CyD.
Re‐evaluation of 1,4‐Non‐bonded interactions in molecular mechanics
Authors: Hirano Tsuneo , Miyajima Takeshi , Imai Keisuke , Fujiyoshi‐yoneda Teruyo , Ōsawa Eiji
Published: January 2005

Analysis of the sum of two‐centre terms obtained from semi‐empirical molecular orbital calculations into atom‐pair interactions revealed a unique behaviour of 1,4‐non‐bonded atom‐pair interactions compared with 1,5‐ and longer range interactions. Explicit inclusion of a special 1,4‐non‐bonded interaction term in an MM2‐like molecular mechanics scheme gave a test force field with good performance.
Solvent effect on the hammett reaction constant for the electroreduction of substituted benzophenones
Authors: Jaworski Jan S. , Malik Marcin , Kalinowski Marek K.
Published: January 2005

Solvent effects on the Hammett ρ value for the cathodic reduction of substituted benzophenones were determined. The electrochemistry of a series of 11 compounds was studied in acetonitrile, acetone, dimethyl sulphoxide, propylene carbonate, N,N‐dimethylformamide, N,N‐dimethylacetamide, N,N‐diethylformamide and hexamethylphosphoric triamide. The ρ values for the reversible one‐electron transfer are described by the Lewis acid‐base model ρ = − 0.006AN + 0.003DN + 0.391, where AN = solvent acceptor number and DN = solvent donor number.
Dynamic behaviour of [2] metacyclo [2] (3,4) thiophenophane
Authors: Takeshita Michinori , Tsuge Akihiko , Tashiro Masashi
Published: January 2005

The conformational analysis of dimethyl [2] metacyclo [2] (3,4)thiophenophane (1), which is the thiophene analogue of [2.2] orthometacyclophane, is described. The comparison between the dynamic NMR spectra of 1 and the computer‐simulated spectra led to the energy barrier between syn and anti conformers of 1. This is the first conformational analysis of a [2,2] orthometa‐type cyclophane.
Binding of lithium ion isotopes with redox‐active macrocyclic and macrobicyclic crown ethers
Authors: Chen Zhihong , Echegoyen Luis
Published: January 2005

Three anthraquinone‐containing crown ether macrocycles and one macrobicycle were reduced coulometrically to their corresponding anion radicals under vacuum. The resulting anion radical solutions in CH2CL2 were stirred with an excess of a solid 6LiClO4−7LiClO4 mixture of known isotropic composition in order to form the appropriate Li+ complexes. After equilibration, the solution was separated from the excess solid and the [6Li+]/[7Li+] ratio was determined by atomic absorption spectrometry. The values were converted into the corresponding separation factors (α) after division by the original composition ratio of the solid mixture. The values were in the range 1.04–1.18. Except for one compound, 2, all of the reduced systems studied exhibited a much larger α value than the 1.057 reported for 12. crown‐4 at 0° C in a liquid–liquid extraction system.
Solution conformations and hydrolytic stability of 2′ ‐ and 3′ ‐substituted 2′,3′‐dideoxyribonucleosides, including some potential inhibitors of human immunodeficiency virus
Authors: Oksman Pentti , Hakala Harri , Zavgorodny Sergei , Polianski Michael , Azhayev Alex , van Aerschot Arthur , Herdewijn Piet , Lönnberg Harri
Published: January 2005

Sugar ring conformations of a number of 2′ ‐ and 3′ ‐substituted 2′,3′‐dideoxyribonucleosides were determined in 2H2O by 1H NMR spectroscopy. First‐order rate constants for the cleavage of their N‐glycosidic bond in aqueous acid were measured. The dependence of sugar ring conformation and hydrolytic stability on the polar nature of the 2′/3′‐substitutent is discussed.
Effects controlling the conformational selectivity and association parameters of H‐bonded assemblies between di‐ and triaminotriazines and bemegride
Authors: Willner Itamar , Rosengaus Jacqueline , Eichen Yoav
Published: January 2005

2,4,6‐Tris(aminocyclohexyl)triazine (1) and 2,4‐bis(aminocyclohexyl)‐6‐methoxytriazine (2) are present in solution in two and three equilibrating conformations, respectively. The activation barrier for interconversion of the conformers 1a ⇌ 1b is 14·55 ± 0·2 kcal mol−1 (1 kcal = 4·184 kJ) and the activation barrier for interconversion of the conformers 2a ⇌ 2b ⇌ 2c is 14·2 ± 0·2 kcal mol−1. The conformational analyses of 1 and 2 were also followed by molecular mechanics calculations. Compounds 1 and 2 form H‐bonded intermolecular complexes with bemegride (3). Association of 3 proceeds by selection of a specific conformation of 1 and 2, i.e. 1b and 2a, respectively. The association constants of 3–1b and 3–2a are to Ka = 915 and 450 l mol−1, respectively. Molecular mechanics calculations for the H‐bonded intermolecular assemblies support the experimental observations of the selective conformational association.
Ambident reactivity of aryloxide ions towards 1,3,5‐trinitrobenzene, low‐temperature characterization of the elusive Oxygen‐bonded σ‐complexes by 1H and 13C NMR spectroscopy
Authors: Buncel Erwin , Manderville Richard A.
Published: January 2005

The ambident reactivity of phenoxide ion towards 1,3,5‐trinitrobenzene (TNB) was re‐examined by means of a novel reaction system(CD3CN–glyme‐d10) which allows the investigation of species formed at low temperatures (−40°C), contrasting with previous studies in dimethyl sulfoxide (DMSO) at ambient temperature. The new method coupled with 400 MHz NMR spectroscopy has allowed the definitive observation of both O‐ and C‐bonded phenoxide σ‐complex adducts for the first time, confirming the formation of the former through kinetic control and of the latter through thermodynamic control. The corresponding O‐bonded σ‐adduct in the TNB–mesitoxide system (whereC‐bonded σ‐adduct formation is not possible but where there is competing nitro group displacement) has also been characterized by 1H and 13C NBR. Another O‐bonded aryloxide adduct characterized is that from the reaction of TNB with3,5‐di‐tert‐butylphenoxide; in this system there is also competing NO2 displacement. Stereoelectronic factors in the O‐bonded σ‐adducts and aryl ethers are
discussed.
Molecular rearrangements in homogeneous gas‐phase elimination reactions. Pyrolysis kinetics of alkyl methanesulphonates
Authors: Chuchani Gabriel , Martin Ignacio , Dominguez Rosa M. , Rotinov Alexandra , Pekerarm Sara , Gomez Matilde
Published: January 2005

The rates of elimination of several alkyl methanesulphonates were determined in a seasoned, static reaction vessel over the temperature range 300–420°C and the pressure range 28–163 Torr. The reactions are homogeneous, unimolecular and follow a first‐order rate law. The overall rate coefficients are given by the following equations: for isobutyl methanesulphonate, log k1 (s−1) = (12·51 ± 0·38) − (177·0 ± 2·1) kJ mol−1 (2·303RT)−1; for 2‐phenyl‐1‐propyl methanesulphonate, log k1(s−1) = (12·62 ± 0·04) − (176·2 ± 0·5) kJ mol−1 (2·303RT)−1; for neopentyl methane‐sulphonate, log k1(s−1) = (13·35 ± 0·42) − (198·2 ± 5·2) kJ mol−1 (2·303RT)−1; and for 3‐chloro‐2,2‐dimethyl‐1‐propyl methanesulphonate, log k1(s−1) = (13·87 ± 0·42) − (218·2 ± 5·4) kJ mol−1 (2·303RT)−1. Rearrangements in these methanesulphonate pyrolyses may proceed via an intimate ion‐pair type of mechanism. Consequently, the results appear to confirm that intramolecular migration through autosolvation is possible in gas‐phase elimination reactions of certain types of organic molecules.
Conformational interconversion of [1.1] ferrocenophane: A molecular mechanics study
Authors: RudzińAski Jerzy M. , ŌSawa Eiji
Published: January 2005

A molecular mechanics scheme developed for bridged ferrocene derivatives was applied to study the conformational energy surface of [1.1] ferrocenophane (1). The mechanism of the intramolecular interconversion of 1 is shown to be different from the mechanism of the syn–syn exchange proposed earlier for this ‘molecular acrobat’.
Multinuclear NMR and ab initio MO studies of 7‐methyl‐7H‐pyrrolo [2,3‐b]pyridine and related compounds
Authors: Minakata Satoshi , Itoh Shinobu , Komatsu Mitsuo , Ohshiro Yoshiki , Yokomichi Yasunori
Published: January 2005

The contribution of the polarized resonance structures to 7‐methyl‐7H‐pyrrolo[2,3,‐b]pyridine and 4‐methyl‐4H‐pyrrolo [3,2‐b] pyridine, which have recently attracted much attention in physico‐chemical studies, was considered based on multinuclear (1H, 13C and 15N) NMR spectroscopy and MO calculation. Comparison of the chemical shifts of the compounds with those of other relevant compounds and the effects of concentration and solvents observed by multinuclear NMR suggested that the contribution of the non‐polarized structures to both compounds predominates over that of the polarized structures. This result was also supported by ab initio MO calculations using the 6–31G basis set.
Mechanism of SYN addition of molecular fluorine to ethylene. An ab initio MO study
Authors: Iwaoka Tomoyasu , Kaneko Chikara , Shigihara Atsushi , Ichikawa Hiroshi
Published: January 2005

The whole process for the syn addition of molecular fluorine to ethylene was analysed at the MP2/6–31 + G level with IRC calculation. The analysis indicates that fluorine approaches the CC double bond vertically to form a perpendicular complex as the intermediate, which then reorientates to a rhombic‐type complex as the transition state to give the final syn addition product. This shows that the square‐type complex previously proposed is not involved in any stage of the reaction.
Substituent effects on the kinetic solvent isotope effect in solvolyses of arenesulphonyl chlorides
Authors: Koo In Sun , Lee Ikchoon , Oh Jieun , Yang Kiyull , Bentley T. W.
Published: January 2005

The Hammett type plot of kinetic solvent isotope effect (KSIE = kSOH/kSOD), log(KSIE) vs σ, can be a useful mechanistic tool for solvoltic reactions. The slopes of such straight line plots for para‐substituted benzenesulphonyl chlorides in methanol (0·15), water (0·05) and methanol–water (0·05) are interpreted in terms of different reaction channels (general base‐catalysed and SN2), in contrast to simple Hammett plots, log kSOH (or log kSOD) vs σ, invariably exhibiting non‐linear concave curves.
Crystal and moecular structure of N‐nitro‐N‐methyl‐p‐nitroaniline: Analysis of substituent effects on the ring geometry and estimation of the Hammett substituent constant for the N‐methylnitramino group
Authors: Anulewicz Romana , Krygowski Tadeusz Marek , Gawinecki Ryszard , Rasała Danuta
Published: January 2005

The crystal and molecular structure of N‐nitro‐N‐methyl‐p‐nitroaniline was solved with good precision: R = 0·058 and average estimated standard deviation for bond lengths = 0·003 Å. Analysis of the geometry reveals that the NO2 attached to the N atom is strongly conjugated. Application of the HOSE model to its geometry and to geometries of 12 other para‐substituted nitrobenzenes yields good linear regressions between canonical structure weights and Hammett σ values by interpolation, σ for the N(Me)NO2 group is estimated to 0·36.
Hydrogen bonding of aromatic amines in hydroxylic solvents. Part 1. Absorption spectroscopy of substituted anilines and hydrogen bond‐induced rehybridization of trivalent nitrogen atoms.
Authors: Nakagaki Ryoichi , Aoyama Iwao , Shimizu Kazuhisa , Akagi Masao
Published: January 2005

The longest wavelength absorption band of substituted anilines containing electron‐accepting groups is characterized as an intramolecular charge‐transfer (ICT) transition. Dual absorption peaks observed for the ICT transition of some para‐substituted anilines in a strongly hydrogen‐bonding solvent, hexafluoropropan‐2‐ol (HFP) can be attributed to two inequivalently hydrogen‐bonded species with differently hybridized aniline nitrogens in HFP. The hydrogen bond‐induced rehybridization causes appreciable changes in the ICT absorption spectra of highly polar aromatic amines. Rehybridization of aromatic amino nitrogens depends on the OH acidity in the solvent molecule and the basicity of the substituted anilines. The geometry of trivalent nitrogens in aryl amines is critically dependent upon the ionization potential of the amino group, the electron affinity of the electron‐accepting substituent and the substitution site of the electron‐withdrawing group with respect to the amino moiety.
The Marcus cross relation for hydrogen‐transfer reactions
Authors: SüHnel JüRgen
Published: January 2005

The Marcus cross relation provides a tool to calculate cross rates from the corresponding identity rates and the equilibrium constant. Assumptions made in deriving the Marcus cross relation and conclusions which can be drawn from agreement or disagreement between experimental and calculated cross rates are discussed. For a small variation in identity rate constants, agreement between experimental and calculated cross rates are discussed. For a small variation in identity rate constants, agreement between experimental and calculated cross rates simply means that the rate constant of a hypothetical thermoneutral cross reaction has a value within the range covered by the identity rates. Whether or not this value corresponds to the mean value of logarithmic identity rate constants can only be checked for a large variation in identity rates. Therefore, a newly defined index M is used to relate the difference in experimental and calculated logarithmic rate constants to the variation in the logarithmic identity rates. Provided that the Marcus cross relation is not obeyed, this indicates non‐additivity of kinetic parameters. Applying this analysis to protontransfer reactions between different transition metal acids and between 9‐alkylflourenes and (9‐alkylfluorenyl)lithium shows that claims about agreement or disagreement of experimental rate constants with the rate constants calculated by means of the Marcus cross relation have to be revised in part.
Hydrogen‐bond basicity of nitriles
Authors: Berthelot Michel , Helbert Maryvonne , Laurence Christian , Questel Jean‐Yves Le
Published: January 2005

A thermodynamic hydrogen‐bond basicity scale, pKHB, and a spectroscopic hydrogen‐bond basicity scale, Δν(OH), were measured which permitted the construction of the solute hydrogen‐bond basicity scale, β, for 71 nitriles embracing a wide range of structures from trichloroacetonitrile to cyanamides. Field, resonance, and polarizability contributions of the X substituents to the hydrogen‐bond basicity of XCN compounds were established. Steric effects do not contribute to the hydrogen‐bond basicity of nitriles.
Interaction of 3,4‐diphenyl‐1,2,5‐thiadiazole 1,1‐dioxide with proton donor solvents
Authors: Mirífico M. V. , Caram J. A. , Vasini E. J. , Sicre J. E.
Published: January 2005

The UV spectra of 3,4‐diphenyl‐1,2,5‐thiadiazole 1,1‐dioxide (I) and that of its thiadiazoline and thiadiazolidine derivatives were measured in several aprotic and protic solvents. Strong specific interactions of I with protic solvents are observed and the formation of stable carbinolamine type derivatives of I with methanol or ethanol is proposed. Spectroscopic data (UV, 1H and 13C NMR) and electrochemical evidence for their formation are given and a new thiadiazoline derivative of I (3‐ethoxy‐2‐methyl‐3,4‐diphenyl‐1,2,5‐thiadiazoline 1,1‐dioxide) was synthesized. The equilibrium constant for the reaction of I with ethanol is reported.
Metal ions that promote the hydrolysis of nucleoside phosphoesters do not enhance intramolecular phosphate migration
Authors: Kuusela Satu , Lönnberg Harri
Published: January 2005

The effects of several metal ions and metal ion complexes on the hydrolysis and interconversion of uridylyl(2′,5′)uridine and its 3′,5′‐isomer were studied as a function of pH and metal ion concentration. The hydrolysis was shown to be markedly accelerated by Zn2+, Cd2+ and trivalent lanthanide ions and by tri‐ and tetraaza complexes of Zn2+. In contrast, none of these species appreciably promotes the interconversion of the 2′,5′‐ and 3′,5′‐isomers, in spite of the fact that this reaction proceeds through the same pentacoordinated intermediate as the hydrolysis. Lanthanide ions also promote the hydrolytic dephosphorylation of uridine 2′‐ and 3′‐monophosphates, but have a barely noticeable effect on their interconversion. The mechanisms of the metal ion‐promoted reactions are discussed.
Kinetic solvent isotope effect studies on the methanolysis of 1‐phenylethyl chlorides
Authors: Lee Ikchoon , Lee Won Heui , Lee Hai Whang
Published: January 2005

The negative slope (ΔρY+ < 0) of the Hammett‐type plot using kinetic solvent isotope effect, log kSOH/kSOD versus σ+, for methanolysis of 1‐(Y‐phenyl)ethyl chlorides is rationalized by an ion‐pair mechanism in which a solvent molecule attacks the relatively stable carbocation formed in the pre‐equilibrium.
Tuning intramolecular carbenic insertions
Authors: Moss Robert A. , Liu Weiguo , Ge Chuan‐Sheng
Published: January 2005

The 1,2‐carbenic hydride shift of neopentylmethoxycarbene was suppressed by the methoxy substituent. Thermally activated hydride shifts were observed with phenoxymethylmethoxycarbene and phenoxymethyltrifluoro‐ethoxycarbene, where appropriate potentiating substituents were introduced at both the migration origin and the migration terminus. Similarly, the 1,3‐CH insertion reaction of tert‐butylfluorocarbene could be induced by thermal activation.
Kinetic studies of solvent and pressure effects on thermal isomerizations ofN‐phenyl‐ andN‐methylrhodaminelactams
Authors: Sueishi Yoshimi , Sugiyama Yukie , Yamamoto Shunzo , Nishimura Norio
Published: January 2005

Solvent and pressure effects were examined for the isomerization rates ofN‐phenyl‐ andN‐methylrhodaminelactams (RL‐Ph and RL‐CH3) from their zwitterionic to the spiro forms. From the pressure dependence of the reaction rate, the activation volumes were estimated to be about 5 cm3mol−1for RL‐Ph and 3–10 cm3mol−1for RL‐CH3. It is proposed that there are two rotameric isomers for the coloured zwitterionic forms, and the role of these rotamers in the kinetic effects of solvent and pressure is discussed. The results are consistent with a reaction mechanism involving heterolytic ring closure on activation.
Dipole moments and electronic structure of 2,6‐bis(dialkylamino) derivatives of 4H‐pyran‐4‐one(thione) and 4H‐thiopyran‐4‐one(thione)
Authors: Schroth Werner , Spitzner Roland , Minkin Vladimir
Published: January 2005

Dipole moments of a series of 2,6‐bis(dimethylamino) and 2,6‐bis(pyrrolidino) derivatives of 4H‐pyran‐4‐one (2), 4H‐thiopyran‐4‐thione (3), 4H‐pyran‐4‐thione (4) and 4H‐thiopyran‐4‐thione (5) were determined in benzene and dioxane solutions at 25°C. High values of the dipole moments (6·44–8·44 D) and mesomeric moments originated from delocalization of the nitrogen lone pairs into the π‐system of the heterocyclic moieties (1·11–3·08 D) indicate a strong polarization of the molecules 2–5 due to a tendency to stabilize pentamethinium and pyrylium (thiopyrylium) ionic structures. The MNDO‐calculated electronic distribution in 2–5 compared with that for the parent 4H‐pyran(thiopyran)‐4‐one(thione)s is in line with this conclusion.
Theoretical studies of the effects of ortho‐methyl substitution on SN1 and SN2 processes at primary and secondary benzylic carbons
Authors: Lee Ikchoon , Kim Nam Doo , Kim Chang Kon
Published: January 2005

The effects of o‐methyl substitution on the transition‐state (TS) structure and reactivity of SN1 and SN2 processes were investigated for primary and secondary benzyl (α‐tert‐butyl) derivatives MO theoretically using the AM1 method. The o‐methyl groups have both stabilizing (by electron donation) and destabilizing (steric) effects on the SN2 TS; the former prevails for the primary whereas the latter effect becomes dominant for the secondary compounds. In contrast, complex interplay of the two effects are operative in the SN1 TS of both primary and secondary derivatives leading to the reactivity order which is consistent with that observed by solvolytic experiments, o,o′‐dimethyl < H < o‐methyl. The bond length–reactivity rule applies only for the primary benzyl SN2 series for which steric effects are insignificant. For reactions of benzyl derivatives is general the coplanarity between the carbenium sp2 centre and the benzene ring, π–σ, is of the utmost importance in stabilizing TS structure for which other structural requirements may be relaxed.
Efficient formation of some amidines by the reactions of aryliminodimagnesium with weakly electron‐accepting benzonitrile, N,N‐dimethylformamide and related compounds. Features of reactions governed by σ‐complexation
Authors: Okubo Masao , Matsuo Koji , Tsurusaki Nobuyuki , Niwaki Keiji , Tanaka Mikio
Published: January 2005

In many reactions of magnesium reagents, including Grignard and aryliminodimagnesium [ArN(MgBr)2, IDMg], the formation of abnormal products via reactions such as radical dimerization, hydrogen abstraction and conjugate 1,4‐addition orginating from efficient single electron transfer (SET) is not excluded. In IDMg reactions with benzonitrile and N,N‐dimethylformamide in tetrahydrofuran, however, clean reactions took place affording N‐arylbenzamidine and N,N′‐diarylformamidine via exclusive 1,2‐addition and condensation plus replacement, respectively. Some alkyl and arylvinyl cyanides gave the corresponding amidines without 1,4‐addition or hydrogen abstraction. These results are the first observation of such reactions of magnesium reagents, and are attributed to a combination of the weak electron‐donating ability (EDA) of IDMg with the weak electron‐accepting ability (EAA) of nitriles. Although ESR is therefore inapplicable, factors governing the product yields were studied on the basis of effects caused by the addition of strongly coordinating hexamethylphosphoramide and pyridine to the reagent solution, and by addition of nitrobenzenes having a strong EAA. From the results, a large participation of σ‐complexation followed by implicit SET in the inner sphere of the complex is proposed.
Calculation of non‐linear optical properties of pyridinium cyclopentadienylides
Authors: Pranata Julianto , Murray Christopher J.
Published: January 2005

Finite field/PM3 calculations of the dipole moments, polarizabilities, hyperpolarizabilities, and second‐order hyperpolarizabilities of pyridinium cyclopentadienylide and some nitro and amino derivatives suggest that these compounds are good candidates for non‐linear optical materials.
A comparison of the relative magnitudes of the interaction energies between functional groups with zero, one and two intervening 2p AOs on Carbon, and substituent effects on the relative ground‐state energies of E‐1,2‐disubstituted ethenes
Authors: Pasto Daniel J.
Published: January 2005

Ab initio theoretical calculations have been carried out on directly bonded donor–acceptor (Ẍ–Y) species and on variously substituted E‐1,2‐disubstituted ethanes, and π interaction energies have been estimated. In conjunction with the previously calculated radical stabilization energies of mono‐ and disubstituted methyl radicals, a comparison is made of the relative magnitudes of the π interaction energies between groups with zero, one and two intervening 2p AOs on carbon atoms. It is concluded that the stabilizing effect of the π interaction between directly bonded donor Ẍ and acceptor Y groups (zero intervening 2p AOs) is approximately 2·1 times that in the disubstituted methyl radicals (one intervening 2p AO), while in the E‐1,2‐disubstituted ethenes (two intervening 2p AOs) the stabilizing effect is ˜ 0·7 of that calculated for the disubstituted methyl radicals for both acceptor–donor and acceptor–acceptor disubstituted systems. No correlation is observed with the π stabilization energies of the ‘cross‐conjugated’ 1,1‐disubstituted ethenes.
Theoretical solute parameters and correlations of log P and pKa. A comment on a recent development and statistical protocol
Authors: Herndon William C.
Published: January 2005

Physical–chemical properties can be correlated and predicted using linear free energy or linear solvation energy relationships (LSER). Procedures to obtain theoretical parameters from calculated molecular structures that can be used in LSER have recently been evaluated [G R. Famini, C. A. Penski and L. Y. Wilson, J. Phys. Org. Chem. 5, 395–408 (1992)]. Among other applications, these procedures were applied to correlate octanol–water partition coefficients and pKa values for two groups of structurally diverse solutes with very good reported results. In this paper, the statistical results of those studies are re‐examined. The corrected statistical parameters do not provide tenability for the appropriateness of the methodology in these applications. However, valid multilinear relationships of the theoretical LSER parameters with the experimental properties are found which do substantiate the original conclusions of Famini et al.
σ–π Energy diagrams for substituted rings
Authors: Jug Karl , Matuschewski Marta , Köster Andreas M.
Published: January 2005

The recently developed σ–π energy separation technique was applied to substituted monocyclic ring systems. Modified correlation diagrams are presented which allow the influence of the substituent on the σ and π energies to be assessed. The slopes of the σ and π electron energy curves can be used to classify aromatic, non‐aromatic and anti‐aromatic compounds. A two‐dimensional resonance coordinate representation shows a clustering of compounds of the three classes in distinctly different regions of this plane.
AM1 and polarized‐π frontier molecular orbital (PPFMO) studies of facial selectivity in hydrogen transfer to substituted adamantyl radicals
Authors: Huang Xiao Ling , Dannenberg J. J.
Published: January 2005

Fully optimized transition states for hydrogen transfer from methane and propene to substituted 2‐adamantyl radicals were calculated using the AM1 molecular orbital method. Methane and propene were chosen to provide an early (propene) and a late (methane) transition state. For 5‐substituted radicals (F, Cl, Ph, CH3, CF3) the enthalpic differences between syn and anti reactions was found to be small [<0·1 kcal mol−1 (1 kcal = 4·184 kJ)]. Other radicals with nitrogen or boron included directly in the adamantyl fragment showed greater selectivities, with aza substitution favoring syn and bora substitution favouring anti attack, as did the 4,9‐difluoro derivative. The calculated selectivities are all in qualitative agreement with experimental results (where available) on bromine atom transfer. PPFMO calculations showed the polarization of the SOMO to be generally in accord with the AM1 results, whereas the polarization of the LUMO's was less indicative. The bond orders for the bonds syn and anti to the incipient CH bond in the adamantyl fragments indicated that the anti bonds were always weaker than those syn, in agreement with the suggestion by Cieplak that has been used to explain the experimental selectivities. However, the bond lengths of the incipient CH bonds are always shorter for the side of preferred attack, in apparent agreement with the suggestion by Anh.
Semi‐empirical calculations on heterocumulene–heterodiene cycloaddition reactions: Ketene + 1‐azabutadiene
Authors: Fabian Walter M. F. , Kollenz Gert
Published: January 2005

Semi‐empirical (AM1) calculations on the 12 isomeric products and the corresponding transition states of ketene–1‐azabutadiene [4 + 2] and [2 + 2] cycloaddition reactions as a model system for the reaction of ketenes with 4‐iminobenzylfuran‐2,3‐diones are presented. A [4 + 2] type of reaction of the ketene CC double bond leading to a six‐membered lactam compound is found to be highly favoured both thermodynamically and kinetically. [2 + 2] Cycloadducts generally have significantly higher activation energies. Reactions involving the formation of a carbon–nitrogen bond proceed in most cases via attack of the nitrogen lone pair leading to a zwitterionic intermediate. Depending on the respective cycloadduct, both two‐step and concerted, albeit asynchronous, processes were obtained. Based on the structures of the various transition states, some predictions with respect to substituent effects are made. Similarities to and differences from the analogous reaction of ketenimines with oxa‐1,3‐dienes are discussed.
N‐alkylation and N‐arylation of anilines starting from a mild NMg reagent: Its activation causing the ‘NC’ coupling to extend the unified structure–reactivity relationship
Authors: Matsuo Koji , Shichida Yoshiaki , Nishida Hiroshi , Nakata Satsuki , Okubo Masao
Published: January 2005

New N‐alkylation and N‐arylation procedures starting from anilinomagnesium (ArNHMgBr) are reported. For N‐alkylation with alkyl bromides, addition of hexamethylphosphoramide to an ArNHMgBr solution in tetrahydrofuran (THF) is effective. After heating at 55°C, N‐monoalkylation product was obtained in 60–90% yield, slight dialkylation taking place. The combined use of aryliminodimagnesium [ArN(MgBr)2] with α,ω‐dibromoalkanes led to N‐arylazacycloalkanes. For N‐arylation with iodobenzene, replacement of THF with pyridine and additional use of copper(I) iodide are effective. After heating at 115°C with iodobenzene, mono‐ and diarylation products were obtained, the former being predominant. The combined use of ArNHMgBr and N,N,N′,N′‐tetramethylethylenediamine as the ligand of copper species is effective for elimination of diarylation and other undesired products, and leads to diarylamines in excellent yield. The method is of advantage over the conventional Ullmann and Chapman methods. The polar solvents and copper salt are effective additives for inducing ‘inert combinations’ of ArNHMgBr or ArN(MgBr)2 with alkyl and aryl halides into NC coupling, to extend the unified view proposed for the reactivity of magnesium reagents. The difference in the roles of NMg and NCu species is discussed.
Variation of ρx(ρnuc) with the extent of bond making in SN2 transition states
Authors: Lee Ikchoon , Koh Han Joong , Lee Byung Choon
Published: January 2005

The magnitude ρx(ρnuc) is shown to decrease with increase in the extent of bond making estimated by the kinetic isotope effect for the reactions of Y‐benzoyl chlorides with anilines, XC6H4NH2, in acetonitrile at 25·0°C.
The influence of solvents on the basicity of dipolar amines
Authors: Headley Allan D. , McMurry Mike E.
Published: January 2005

The relative basicities of six monosubstituted dipolar trimethylamines (XCH2NMe2) were determined in nine different solvents from potentiometric titration. Solvation effects on basicity variations were analyzed by the solvatochromic parameters (π*, α and β). Non‐specific solvation of the substituted dimethylammonium ions is not reflected adequately by the solvent's dipolarity–polarizability constant, π*. A better representation of non‐specific interaction between solvents and the monosubstituted dipolar trimethylammonium ions is gained from the product of π* and the solvent dipole moment (μ). For these ions where the dipolar substituents are separated from the reaction center by one methylene unit, there is an interaction between the charge of the ammonium ion and the dipolar substituent. As a result, solvent molecules must not only solvate the dipolar substituents, but must also gain access between the lines of force of the dipolar substituent and the charge. Bulky solvents, such as nitrobenzene, cannot access adequately the region between the dipolar substituents and the reaction center to accomplish similar solvation as less bulky solvents.
Mechanisms of aromatic nucleophilic substitution reactions by amines in solvents of low relative permittivity
Authors: Hirst Jack
Published: January 2005

The evidence for the mechanisms proposed for aromatic nucleophilic substitution reactions by primary and secondary amines in aprotic solvents of low relative permittivity is reviewed.
Kinetics and mechanism of electron‐transfer reaction of aquothallium(III) and coordinated thallium(III). Oxidation of metol with thallium(III) in acid perchlorate medium
Authors: Nahar Sharmila , Sharma P. D.
Published: January 2005

The kinetics of oxidation of metol (N‐methyl‐p‐aminophenol) by thallium(III) have been studied in a perchloric acid medium. This organic compound is employed extensively in photographic work. The stoichiometry of the reaction is represented by: C6H4(OH)NHCH3 + TIIII → C6H4(O)NCH3 + TII + 2H+The kinetics indicate the rate law:

where k′ is the pseudo‐first‐order rate constant. The rate constants and thermodynamic parameters for the rate‐limiting step have been evaluated.
Calibration of a semi‐empirical procedure for predicting the ground‐state spin multiplicities of open‐shell molecules. Applications to new systems
Authors: Ichimura Andrew S. , Koga Noboru , Iwamura Hiizu
Published: January 2005

A semi‐empirical configuration interaction (CI) algorithm is presented for predicting the ground‐state spin multiplicity of open‐shell systems. The computed energy gaps ΔE(LSHS) of model systems, Y‐phenyl‐X‐phenyl‐Y, where Y is a nitrene or nitroxide radical and X is an exchange coupling unit, are critically compared with the available experimental data. The current method has attenuated the deficiencies of the earlier algorithm proposed by Lahti and Ichimura and the predictions are qualitatively clearer, facilitating the classification of high‐ and low‐spin topologies for isomeric species. The relative effectiveness of different exchange couplers can be discerned semi‐quantitatively, and several new exchange couplers are described. The reasons for the qualitative failure of the earlier algorithm are discussed.
Linear free energy relationships in radical reactions. II. hydrogen abstraction from substituted toluenes by Tert‐Butyl, Tert‐Butoxyl and Tert‐Butylperoxyl radicals
Authors: HÉBerger KÁRoly
Published: January 2005

Calculations were carried out to study the validity of σ‐scales for hydrogen abstraction from substituted toluenes by tert‐butyl, tert‐butoxyl and tert‐butylperoxyl radicals. Rate constants were compiled and evaluated from the literature for meta‐ and/or para‐substituted toluenes. The substituents were characterized by factored ionic sigmas (σI, σR, σ), ionic scales (σ, σ+) and various radical sigmas (σσ). The dependence of log k values on these substituent descriptors was investigated using ‘stepwise linear regression’ and ‘all possible regression’ methods. The following predictive equations can be recommended:
for tert‐butyl radicals, at 321 K:

for tert‐butoxyl radicals, at 313 K:

and for tert‐butylperoxyl radicals, at 303 K:

The results suggest that there is no unversal radical scale for hydrogen abstraction reactions, that the rate is primarily influenced by polar factors (inductive, resonance) and that only two radical scales (σĊ and σ) are appropriate, showing a small, yet significant, role of radical stabilization.