Photochemistry of azidostyrylquinolines: 1. Quantum-chemical study of the structure in the ground and the lower excited singlet states

The structures of isomeric 2-and 4-azidostyrylquinolines and their protonated forms in the ground (S ₀) and the lowest excited singlet (S ₁) states were calculated by the PM3 semiempirical method and the density functional theory (DFT) using the B3LYP/6-31G^* basis set. It was shown that the σ _NN ^* molecular orbital, which is localized on the azide group and is antibonding for the N-N₂ bond, is populated in the S₁ state of these azides in both neutral and protonated forms. Based on this result, it was assumed that the test azides would be photoactive in both forms, i.e., would have a photodissociation quantum yield of φ > 0.1. The calculation of absorption spectra by the TD B3LYP/6-31G^* method showed that the long-wavelength absorption bands of the protonated forms are shifted to visible spectral region, thus suggesting that azidostyrylquinolines in the protonated form will be sensitive to visible light.     

Quantum-chemical study of the protonation of pyrrolo[2,1-b]thiazole and its selenium and tellurium analogs

The possibility of proton attack on various centers in pyrrolo[2,1-b]thiazole (1) has been evaluated. The results of semiempirical (MNDO, AM1, and PM3) andab initio (6-31G^*) calculations were compared. The MNDO and 6-31G^* methods give “chemically proper” and qualitatively coincident results. Analysis of the intramolecular (geometric and electronic) reorganization of molecule1, depending on the protonation center, has been carried out. The most probable attack centers, depending on the mechanism of electrophilic reaction, have been recognized. The energy parameters of intramolecular prototropic rearrangements in cation1 and the “blocking” factor value of methyl groups reducting the corresponding complex stability have been evaluated. It has been established that the relative stability of the protonated forms does not change on going to pyrrolo[2,1-b]selenium- and telluriumazoles, but the range of variations is considerably narrowed in the series S>Se>Te. Published inIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1707–1711, October, 2000.     

Multiple bond migration with participation of a protophilic agent

The pathways of migration of the double bond in the 1-methoxy-2-propene molecule with participation of a hydroxide ion were investigated by theab initio RHF/6-31+G^* and MP2/6-31+G^* methods. Stationary points corresponding to complexes between the molecule under study and the hydroxide ion and between the corresponding carbanion and a water molecule were found on the potential energy surfaces of the proton transfer reactions. As in the propene molecule, migration of the double bond in the 1-methoxy-2-propene molecule can occur in the gas phase by the mechanism of intramolecular proton transfer involving the proton of the hydroxide ion. Conformational isomerism of the initial molecule and reaction products was considered. The proposed mechanism of 1,3-hydrogen shift involving the proton-containing base suggests the formation of bothE- andZ-products with predominance of the latter irrespective of the nature of the substituent. In this case the direction of multiple bond migration will be completely determined by the energy difference between the initial reagents and final products.     

Multiple bond migration with participation of a protophilic agent

The pathways of migration of the multiple bond in propene and propyne molecules involving the hydroxide ion were investigated by theab initio (RHF/6-31+G^* and MP2/6-31+G^*) methods. Stationary points corresponding to stable complexes between the molecules under study and the hydroxide ion and between corresponding carbanions and water molecule were found on the potential energy surfaces of the proton transfer reactions. In the presence of hydroxide ion, migration of the multiple bond can occur by an “intramolecular” mechanism of the proton transfer involving the proton of hydroxide ion bound in the complex with propene or propyne molecule. For the propene system, such a mechanism seems to be quite realistic and more preferable energetically than a traditional two-stage mechanism with a passage of the proton into the medium. For the system with the triple bond, an equal expenditure of energy is required to follow any mechanism (without taking into account the effects of solvation and the interaction with a cation), whereas the formation of the stable [H₂C=C=CH·H₂O]⁻ complex can prevent further transformations. For Part 1, see Ref. 1. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 35–41, January, 1999.     

Structural relaxations and energy effects of intramolecular motions inN,N-dimethylnitramine: A theoretical study

The equilibrium geometry of theN,N-dimethylnitramine molecule and changes in the energy and structural parameters due to the internal rotation of the nitro group and the inversion of the N atom in the amino fragment were calculated by the restricted Hartree-Fock (RHF) method and at the second-order Møller-Plesset (MP2) level of perturbation theory with inclusion of electron correlation using the 6–31 G^* and 6–31 G^** basis sets. The one-dimensional potential functions of these motions calculated at the RHF/6–31 G^* level were approximated by a truncated Fourier and power series, respectively. The frequencies of torsional and inversion transitions were determined by solving direct vibrational problems for a non-rigid model,i.e., taking into account the molecular geometry relaxation. The equilibrium conformation of the molecular skeleton ofN,N-dimethylnitramine is nonplanar. Transition states of the internal rotation of the nitro group and inversion of the amine N atom are characterized by pronounced concerted changes in its bond angles and the length of the N?N bond. In the MP2/6–31 G^* approximation, the height of the barrier to internal rotation calculated taking into account the difference in the zero-point vibrational energies is equal to 9.7 kcal mol^?1. Inversion in the amino fragment is accompanied by a relatively small energy change at the barrier height of ?1.0 kcal mol^?1 calculated in the same approximation.     

A potential function for computer simulation studies of proton transfer in acetylacetone

A potential energy model is developed to study the intramolecular proton transfer in the enol form of acetylacetone. It makes use of the empirical valence bond approach developed by Warshel to combine standard molecular mechanics potentials for the reactant and product states to reproduce the interconversion between these two states. Most parameters have been fitted to reproduce the key features of an ab initio potential surface obtained from 4-31G^* Hartree-Fock calculations. The partial charges have been fitted to reproduce the electrostatic potential surface of 6-31G^* Hartree-Fock wave functions, subject to total charge and symmetry constraints, using a fitting procedure based on generalized inverses. The resulting potential energy function reproduces the features most important for proton transfer simulations, while being several orders of magnitude faster in evaluation time than ab initio energy calculations. © 1997 by John Wiley & Sons, Inc.     

Electronic structure of testosterone: A semiempirical and ab initio assessment

Testosterone (17β-hydroxy-4-androsten-3-one) was studied by the semiempirical AM1 and PM3 and ab initio STO-3G^*, 3–21G^*, and 6–31G^* methods. The goals were to compare those methods and to know the electronic structure of the hormone. Full geometry optimization was performed, and two crystal conformers (T1 and T2), and experimental dipole moment in solution were used for comparison. One conformer with a dipole moment similar to the solvated conditions was generated. Total energy, entalphies, dipole moments, charges, electrostatic potentials, and highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbital (LUMO) were calculated. Root-mean-square (RMS) index of the theoretical molecules against T1 and T2 showed best results with the 3–21G^* and 6–31G^* methods, while AM1 gave better energies than PM3. Dipole moments were directed toward the OH group and the botton face of the A ring. The frontier orbitals were located along the C4–C5 π bond, particularly the LUMO was split between C4 and C5, predicting the action of enzymes at C5 yielding to 5α and 5β-reduced androgens. Electrostatic potentials might be also of biological importance since they are coincidental with the dipole-moment orientation. Finally, it is interesting that the solvatedlike conformer, its properties, and the OH group laid between the same group of T1 and T2 and with a total energy between the crystals and the gas phase or in vacuo conditions. This results might also explain the biology of testosterone and use them to model the hormone-receptor interaction. © 1997 John Wiley & Sons, Inc.     

Empirical force field and ab initio calculations on allyl cations

Allyl cation geometries optimized using an extended version of MMP2, newly parameterized for localized and delocalized classical cations, compare favorably with those obtained at the MP2(full) /6–31G^* level. Hence, the force field should provide good starting structures for ab initio calculations. The π-electron densities obtained by these two very different methods are quite similar. The relative energies of various isomers at MP4/6–31G^*//MP2(full)/6–31G^* are reproduced well by the force-field calculations. The heats of formation calculated by MMP2, as well as those predicted from the ab initio data, agree with experimentally determined values. The force-field method provides interpretive capabilities. Energy differences between isomers can be separated into electronic and steric contributions, reasonable estimates of resonance energies are given, and nonbonded resonance energies in delocalized cations can be evaluated. The stabilizing 1–3 π-interactions in allyl cations are quite significant, but are reduced by alkyl groups hyperconjugatively and sterically. © 1997 by John Wiley & Sons, Inc.     

Multiple bond migration with participation of a protophilic agent

Ab initio study of the pathways of migration of the double bond in the 3-methylthioprop-1-ene (1) and 3-methoxyprop-1-ene (2) molecules with participation of hydroxide ion was carried out by the RHF/6-31+G^* and MP2/6-31+G//RHF/6-31+G^* methods. Conformational isomerism of the initial molecules and reaction products was considered. The distinctions are discussed in the spatial and electronic structure of intermediate carbanions stabilized (for1) due to the negative hyperconjugation. Stationary points corresponding to complexes between the molecules under study and the hydroxide ion and between the corresponding carbanion and water molecule were localized on the potential energy surfaces of the proton transfer reactions. For2, the single-stage mechanism of prototropic rearrangement involving the H atom of the hydroxide ions was found to be more energetically preferable than the two-stage mechanism, whereas both mechanisms are expected to be equiprobable for1. For Parts 1–3, see Refs. 1–3. Translated fromIzvestiya Akademii Nauk. Seriya Kimicheskaya, No. 3, pp. 407–413, March, 2000.     

AB Initio and MNDO calculations of atomic interactions in the CH3OCOCl molecule

The CH₃OCOCl molecule is calculated by ab initio methods using the split-valence basis sets at RHF/3-21G//RHF/3-21G, RHF/6-31G^*//RHF/6-31G^*, and RHF/6-311G^*//RHF/6-31G^* levels of theory and in the MNDO approximation. The optimized geometry of the molecule is consistent with the experimental data. The populations of the p-AOs of this molecule and the MO compositions show that the electron distribution in this molecule should be interpreted without considering the conjugation between the lone electron pairs of the Cl or O atoms and the π-electron system of the carbonyl group. The asymmetry parameters of the electric field gradient on the³⁵Cl nucleus were calculated using the Cl p-AO populations and compared with the corresponding experimental value. Instite of Technical Chemistry, Ural Branch, Russian Academy of Sciences. Translated fromZhurnal Struktumoi Khimii, Vol. 37, No. 4, pp. 646–651, July–August, 1996. Translated by I. Izvekova     

Conformational stability of peroxynitric acid molecule and the barriers to internal rotation about the O−O and N−O bonds

The structure of conformers and potential curves of the internal rotation (PCR) about the O?O and N?O bonds in peroxynitric acid (PNA) were calculated by the unrestricted Hartree-Fock-Roothaan method. The standard valence-split 6–31G and 6–31G^* basis sets were used. The presence of two maxima on every curve has been shown. To refine the values of barriers to the internal rotation in the regions of minima and maxima of PCR, calculations taking into account the electron correlation energy have been carried out at the second- and fourth-order Møller-Plesset level of perturbation theory (MP2 and MP4, respectively). At the MP4/6-61G^* level of approximation, the barriers to the rotation about the O?O bond are equal to 8.6 kJ mol^?1 and 14.7 kJ mol^?1, and both barriers to the rotation about the N?O bond are equal to 33.5 kJ mol^?1. The results are compared with those published for PCR in hydrogen peroxide and peroxynitric acid.     

A new scaling procedure to correct semiempirical MEP and MEP-derived properties

Summary The MNDO, AM1, PM3, and ab initio 6–31G^* and 6–31+G^* MEPs for 21 neutral and 12 charged molecules were computed in layers ranging from 1.2 to 2.0 times the van der Waals radii of atoms. Semiempirical and ab initio MEPs for each layer and two groups of layers were compared to gain insight into the relationships between semiempirical and ab initio MEPs. A detailed statistical study allowed us to obtain a new set of scaling coefficients able to correct the semiempirical MEPs to provide better representations of the ab initio values. The corrected semiempirical MEPs were used to obtain electrostatic charges, whose quality was tested by the comparison between semiempirical Coulombic MEPs and ab initio quantum mechanical MEPs.     

Molecular Structures and Thermodynamic Stabilities of Cycloalkadienes and Their Methoxy Derivatives: An Ab Initio and DFT Study

The geometry-optimized molecular structures and total energies of 4- to 6-membered cycloalkadienes, and of a number of their monoand dimethoxy derivatives, have been calculated by ab initio (HF/6-31G^*, MP2/6-31G^*//HF/6-31G^*) and DFT (B3LYP/6-31G^*) methods. By comparison with available experimental data, the reliability of these computational methods for an estimation of the relative stabilities (enthalpies) of the isomeric forms of the title compounds was tested. The experimental enthalpies of isomerization proved to agree best with the respective theoretical data based on the mean of the HF/6-31G^* and B3LYP/6-31G^* energies. The theoretical calculations were then extended to several isomeric methoxy-substituted cycloalkadienes, for which no previous thermodynamic data exist. Some structural features of the title dienes were also discussed.     

芳香性氨基酸的二次谐波非线性光学性质

采用密度泛函理论(DFT)-B3LYP方法在6-31G^*基组水平上,对芳香性氨基酸分子体系(Phe, [Phe―H]^-,PheH^*, Tyr, [Tyr―H]^-, TyrH⁺, Trp, [Trp―H]^-和TrpH⁺)进行结构优化. 在优化所得构型的基础上, 利用含时密度泛函理论(TDDFT)-B3LYP在6-31G^*基组上计算了它们的激发态性质,并结合态求和方法研究了它们在二次谐波过程中的二阶极化率值. 同时讨论了二次谐波非线性光学响应的起源及其产生变化的原因. 计算结果表明,相对于中性的氨基酸分子, 去质子化和质子化后的氨基酸分子的二阶极化率值都有明显的增加, 且符合规律Phe < PheH⁺ < [Phe―H]^-和Tyr < TyrH⁺ < [Tyr―H]^-. 通过对它们电极化起源的分析, 我们得到对于中性氨基酸分子, 侧链芳香环上的π→π^*跃迁对二阶极化率起主要贡献; 对去质子化和质子化后的氨基酸分子, 吲哚环上的π→π^*电荷跃迁和α碳原子相连的氨基和羧基基团内电荷跃迁对二阶极化率起到同样重要的贡献.     

Proton affinities of polybenzenoid aromatic hydrocarbons and those with five-membered rings

Proton affinities of PAHs including one five-membered ring are calculated by using the AM1 Hamiltonian for the determination of ΔH_f^o of the neutral and protonated molecules. The calculated PAs are compared to experimental PAs measured by chemical ionization mass spectrometry, using a new method based on competition between charge transfer and proton transfer occurring during the ionization process. A procedure is proposed to validate AM1-calculated PAs from experimental PAs after rescaling the calculated and measured PA values. The site of protonation is first determined on the criterion of the lowest loss of aromaticity, then on the criterion of the largest HOMO coefficient. For indene, the corrected result is compared to an ab initio calculation at the MP2/6-31G^*//HF/6-31G^* level and to a DFT calculation at the B3LYP/6-31G^* and the B3LYP/6-311 + G^** levels. Five new PAs are thus established and one published experimental PA is revised. © 1997 by John Wiley & Sons, Inc.     

A theoretical study of the Si-O bond in disiloxane and related molecules

Summary A comparison of semi-empirical (MNDO) and ab initio (GAUSSIAN) calculations for disiloxane and related molecules is given. The STO-3G^* basis set well reproduced the observed geometries of disiloxane (*), DZP, TZVP) gave much poorer agreement with the observed geometries.Comparison of the STO-3G^* and the STO-3G basis sets demonstrates the necessity of including d-orbitals on the silicon. However, the semi-empirical MNDO program gave, despite the absence of d-orbitals, a better approximation to the molecular geometry than the complex ab initio basis sets.Force field parameters have been calculated for k_SiOSi, k_OSiO, 0.089 and 0.73 mdyneÅ/rad², and the SiOSiO torsion which has a V₁ potential of –0.68 kcal/mol. In addition, the HSiOH torsion is shown to have a three-fold potential of 0.78 kcal/mol. These are profoundly different from the analogous carbon-oxygen force constants, demonstrating that C-O parameters cannot be transferred to the corresponding Si-O systems.     

Ab initio investigation of the diels-alder reaction between 2H-phosphole and phosphaethene: A model for phosphole dimerization

All four possible Diels-Alder reactions between 2H-phosphole and phosphaethene were examined at various theoretical levels, including HF, MP4SDQ, CCSD(T), and CASSCF. MP2/6-31G^* geometry optimizations could not be employed since the potential energy surface is qualitatively incorrect at this level of theory, due to the inherent underestimation of the activation energies (ameliorated at higher-order MP or coupled-cluster levels). Solvent effects were examined employing the Onsager, polarized continuum, and isodensity and surface polarized continuum models. At MP4SDQ/6-31G^*//HF/6-31G^* these reactions are exothermic by 34–38 kcal mol⁻¹ and have very low activation energies, 5–7 kcal mol⁻¹. The P P/C C regioisomer products are lower in energy than the C P isomers and, within each pair, the exo isomer is lower in energy. At low computational levels the smallest activation energy is for the reaction leading to the C P endo product. Larger basis sets, electron correlation, and solvent favor the transition state leading to the experimentally observed P P/C P endo isomer. The dimerization of phosphole is, therefore, kinetically controlled. Based on geometric and electron density analysis, the reactions are concerted and synchronous. © 1997 by John Wiley & Sons, Inc.     

Ab initio calculations of methionines and their protonated forms

Ab initio calculations of molecular and electronic structures of neutral molecules and protonated forms of methionine and its derivatives in the gaseous phase were carried out by the Hartree-Fock method using the 6–31G^* basis set with full geometry optimization. Proton affinities of methionine (1), methionine sulfoxide (2), and methionine sulfone (3) were calculated for different modes of coordination of the proton. The results of calculations demonstrated that in protonated forms of 1 and 3, bonding between the proton and the N atom is most favorable, while in protonated form of 2, bonding between the proton and the O atom of the SO group is most favorable. The proton affinities of the amino acids are as follows: 223.2 (1), 241.2 (2), and 221.5 (3) kcal mol⁻¹,i.e., methionine sulfoxide 2 exhibits the highest proton affinity in the series of the amino acids under consideration. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1487–1490, August, 1998.     

Theoretical study on the pyrolysis mechanism and kinetics of β-hydroxyketones

The mechanism and kinetics for the decomposition of β-hydroxypropaldehyde, primary and secondary β-hydroxyketones, were studied by using ab initio RHF/6–31G and RHF/6–31G^* methods. The activation barriers of these reactions were refined to be 39.57, 40.10, and 36.80 kcal mol⁻¹ at the MP2/ /RHF/6–31G^* level, respectively. The calculated results show that each decomposition is a concerted process with hydrogen transferring and bond breaking via a six-membered cyclic transition state. The thermal rate constants of the decomposition of primary and secondary β-hydroxyketones were obtained by calculating microcanonical probability fluxes through each transition state. It is theoretically confirmed that methyl substitution at the hydroxyl carbon of β-hydroxyketones causes a small enhancement in rates. The theoretical investigations of the mechanism and the rate constants are in agreement with the experimental results. © 1997 John Wiley & Sons, Inc.     

Bridged and linear dilithioacetylenes - Two minima on the potential energy surface?

Vibrational frequency calculations at the 6–31G^*, 6–31G and STO-3G levels in addition to an STO-3G basis set with only a 1s orbital on lithium reveal that both bridged and linear dilithioacetylenes are minima on the potential energy surface.