Alkylhydrazides and their protonated forms: Physical methods andab initio calculations

The basicity of hydrazides of the highest aliphatic carboxylic acids RC(O)NHNH₂ (R = C_nH_2n+1,n = 5-12) has been studied by potentiometric titration, and IR and¹H NMR spectroscopy.Ab initio Hartree-Fork calculations using the 6–31G^* basis set with full optimization of geometry were carried out on the simplest acy1hydrazines and their possible protonated forms. Based on these calculations, and the 1R and¹H NMR spectra, the tautomerism of alkylhydrazides and the structures of their protonated forms are discussed.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2645–2649, November, 1996.     

Ab initio calculations of of pyridine and its 2- and 3-chloroderivatives

Ab initio calculations of the C₅H₅N, 2-. and 3-ClC₅H₄N molecules by the RHF method in the valence split 6-31G^* basis set with full optimization of the geometry have been carried out. The alternation of the charges on the atoms of the pyridine ring and of the populations of their p _x -orbitals is in agreement with the noninductive through -the-field interaction of the geminal atoms. The³⁵Cl NQR frequencies and the electric field gradient asymmetry parameters at the³⁵Cl nuclei in 2- and 3-CIC₅H₄N were estimated using the populations of the valentp-orbitals of the Cl atoms and their components. The³⁵Cl NQR frequency for the first compound is lower than that for the second one, mainly due to the higher p-electron population of its Cl atom.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2641–2644, November, 1996.     

Ab initio calculations of atomic interaction in the ClCH2OCH3 molecule

Ab initio calculations of both ClCH₂OCH₃ and ClCH₂CH₃ molecules and various ClCH₂OCH₃ structures with fixed angles of rotation of the methoxy group about the C−O bond were performed by the restricted Hartree-Fock method in the valence-split 6–31 G^* basis set with full optimization of the geometry. The populations of the valent p-orbitals of the chlorine atoms in these molecules have been analyzed. The³⁵Cl NQR frequencies and the asymmetry parameters of the electric field gradient (EFG) at the³⁵Cl nuclei have been calculated. Good agreement with experimental NQR frequencies was obtained for the calculations where only the populations of the less diffuse 3p-components of these orbitals were used. The³⁵Cl NQR frequency in ClCH₂OCH₃ is lower than that in ClCH₂CH₃ due to the higher population of the less diffuse component of the p_σ-orbital of the Cl atom in the former molecule. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 431–434, March, 1997.     

Scopolamine and atropineN-methyl diastereomer stability.Ab initio calculations onO-formylscopine andO-formyltropine model compounds

Ab initio geometry optimizations at the RHF-21G basis set level were calculated forequatorial andaxial N-methyl diastereomers ofO-formyltropine andO-formylscopine esters and other model compounds. These optimized geometries were then utilized as input for single-point energy calculations using the higher level RHF/6-31G^* basis set to afford a more precise estimation of the total energies and atomic charges. Ethano bridge pinching of theN-protonated tropanyl piperidine moiety pushes the smalleraxial N-proton closer toward the neighboring twoaxial C-H bonds compared with the analogous case for a bulkyaxial N-methyl. Increasedcis 1,3-diaxial interactions in theaxial N-methyl diastereomer destabilize this epimer in favor of theequatorial N-methyl counterpart [e.g., 2.121 kcal/mol lower energy for theequatorial N-methylO-formyltropineN-protonated diastereomer (12) than for theaxial epimer (13)]. Lower pyramidality at nitrogen in the free base maintains the relative stability of theequatorial N-methyl free base diastereomer (14) (1.120 kcal/mol more stable than theaxial free base15). A nonprotonated carbon atom at the apex of a three-membered ring fused to the 6,7-positions of theO-formyltropine skeleton results in severe transannular nonbonding steric interactions involving the neighboringequatorial N-methyl group inN-protonated16 (3.335 kcal/mol less stable than theaxial N-methyl epimer17, where these transannular interactions are reduced due to the smallerequatorial N-H proton). Oxygen atom occupation of the apex of a similar fused three-membered ring retains the same severe transannular nonbonding steric interactions involving the neighboringequatorial N-methyl group inN-protonated18. These transannular interactions now become electrostatically attractive in theN-protonatedaxial N-methyl epimer19 (2.031 kcal/mol more stable than theequatorial epimer). Reduced pyramidality at nitrogen in theO-formylscopine free base reduces the repulsive transannular interaction with the neighboringequatorial N-methyl group compared to that in theN-protonated form. Lowered pyramidality also reduces thecis-1,3-diaxial interactions in theaxial N-methyl epimer, but the nitrogen lone pair is pushed close to the transannular oxygen lone pair as a result (theequatorial N-methyl free base20 is 3.870 kcal/mol more stable than theaxial epimer21). Theseab initio-calculated models ofO-formyltropines andO-formylscopineN-methyl diastereomeric protonated cations and free bases provide stereochemical insight into the relative stabilities of solution-state atropine and scopolamineN-methyl species previously observed by NMR spectroscopic methods.     

Vibrational and electronic spectra of benzophenone in different phase states: Ab initio calculations and experiment

An ab initio method has been used to perform quantum mechanical calculations of the formation energy of different conformers of benzophenone: planar molecule, twisted molecule, planar molecule dimer, twisted molecule dimer; electronic and vibrational spectra of these conformers were also obtained. An assessment of the medium (solvent) influence on the optimal geometry, dipole moment and stability of different forms of benzophenone was performed in the self-consistent reaction field approximation. It is shown that the twisted conformer is more stable than the planar one (the difference of free energies is 32 kJ/mol for free molecules) and it becomes even more stable with the increase in solvent polarity. The calculated electronic and vibrational spectra agree well with the experimental data and properly reflect the complication of the vibrational spectrum when passing from the gaseous phase to the condensed state of benzophenone. The difference between spectral properties of the two dimer forms allows their identification from the spectra and qualitative explanation of the observed peculiarities of phosphorescence of the amorphous phase of benzophenone by the stabilization of different conformers.     

Ab initio calculations of activation energy of the reaction of hydrogen exchange on strongly acidic centers

Ab initio calculations of fragments of the potential energy surfaces of hydrogen exchange reactions between H₂, CH₄, and alanine molecules and the H₃O⁺ ion were performed by the restricted Hartree-Fock method, at the second-order Møller-Plesset level of perturbation theory, and by the method of coupled clusters using the 6–31G^* and aug-cc-pVDZ basis sets. The one-center synchronous mechanism of hydrogen exchange reaction was studied and the activation energies and structures of transition states were determined. It was found that the geometric parameters of the H₂ and CH₄ molecules in the transition states are close to those of the H₃ ⁺ and CH₅ ⁺ ions. The higher the proton affinity of the reacting molecule in the reaction studied the lower the activiation energy of hydrogen exchange. The one-center mechanism studied can be used to describe the high-temperature solid-state catalytic isotope exchange (HSCIE) reaction. The results ofab initio calculations of synchronous hydrogen exchange between the H₃O⁺ ion and hydrogen atoms in different positions of the alanine molecule are in good agreement with experimental data on the regioselectivity and stereoselectivity of the HSCIE reaction with spillover-tritium.     

Ab initio calculations ofo- andm-Cl2C6H4 molecules and their35CI NQR parameters

The MNDO andab initio (Hartree—Fock method, split 6-31G^* basis set, full optimization of geometry) calculations for theo- andm-dichlorobenzene molecules have been performed. The populations of the valent p-orbitals of the carbon and chlorine atoms have been analyzed. Estimations of the³⁵Cl NQR frequencies and the asymmetry parameters of the electric field gradient at the³⁵CI nuclei have been made. It was confirmed that these values are not determined by the contribution of the whole valent p-shell, but only by its less diffuse component.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 2180–2183, September, 1996.     

Ab initio studies of tetrafluoroethylene and tetratrifluoromethylethylene molecules in the ground and “twisted” states

Ab initio calculations with full optimization of geometry have been carried out with the 6–31 G^* basis set on tetrafluoroethylene (with the unrestricted Hartree-Fock method—UHF and the second-order Moller-Plesset perturbation theory—MP2) and tetratrifluoromethylethylene (with UHF) molecules in the singlet ground and triplet biradical states. The symmetry of the tetrafluoroethylene molecule in the triplet biradical state was demonstrated to differ from that of ethylene (D _2d ) due to the deviation of fluorine atoms from CCFF plane. The MP2 optimized geometries of ethylene and tetrafluoroethylene were used for higher level calculations (MP3, MP4, CCSD). The energy of the ground state singlet-biradical triplet splitting decreases in the series: ethylene>tetrafluoroethylene> tetratrifluoromethylethylene. These data on energy splitting explain the increase in reactivity toward the [2+2]-cycloaddition on going from ethylene to tetrafluoroethylene. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 605–607, April, 1998.     

Quantum-chemical gas-phase calculations on the protonation forms oftrans- andcis-urocanic acid

The neutral, cationic, and anionic structures of both prototropic tautomers oftrans- andcis-urocanic acid [(E)- and (Z)-3-(1H-imidazol-4(5)-yl)propenoic acid, respectively] were studied by using semiempirical andab initio gas-phase calculations. Potential energy surfaces of the structures were calculated by using the semiempirical AM1 method, and the geometries corresponding to global minima on these surfaces were optimized up to the MP2/6-31G^* level of theory. The calculated protonation forms of each urocanic acid isomer have a planar molecular structure due to a delocalized -electron system, and all of them prefer thes-trans conformation with respect to the bond between the imidazole and the propenoic acid moieties. Thecis-urocanic acid structures are stabilized by an intramolecular hydrogen bond. The chargedcis-urocanic acid isomers have a lower molecular energy than the correspondingtrans-isomers, whereas the neutral molecules have, after inclusion of thermodynamic corrections, approximately the same energy. The cationic urocanic acid structures have about 2500 kJ mol^–1 lower energy than the anionic ones and about 1000 kJ mol^–1 lower energy than the neutral ones. The nonzwitterionic forms of the neutral urocanic acid isomers have about 200 kJ mol^–1 lower energy than the zwitterionic ones. These energy differences are explained by the proton affinities of the imidazole and the propenoic acid moieties of the urocanic acid structures.     

Ab initio calculation of the band structure of some boron polymers

Ab initio calculations using a STO-3G basis set have been performed on the polymer systems (HBX) _n where X = Be, BH, CH₂, NH, and O. Energy band diagrams and accompanying density-of-states plots have been obtained. The highest filled orbital of (HBNH) _n and (HBO) _n occurs at the X-point and possesses character while the - framework orbital at the X- point is the highest filled level for (HBBe) _n , (HBBH) _n , and (HBCH₂) _n . The conduction band for all five species has symmetry and the band gap of the (HBX) _n species increases in the order X = Be < BH < NH < O < CH₂. An estimate of the energy of polymerisation of the (HBX) _n systems suggests that HBNH is particularly stabilised by polymerisation. The electron distribution in (HBBe) _n shows a - electron drift towards boron, while in the other four systems the net electron transfer is directed away from boron. There is significant electron back-donation to boron in (HBO) _n and (HBNH) _n .     

Ab initio study of oxaphosphaalkyne P=C-OH and its isomers

Anab initio study on HP=C=O PC-OH isomerization has been performed at the 3-21G(d(P)), 4–31G(d(P)) and 6–31G** levels. In contrast to analogous transformation of 1-aza-3-phosphaallene, this process is endothermic in the gas phase. It proceedsvia a [1,3]H shift without an intermediate. The route, including two subsequent [1,2]H shifts, leads to phosphinidene.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 172–173, January, 1994.     

DFT(B3LYP) calculations of silatranes and their radical cations. First ionization potentials

DFT(B3LYP) and 2 quantum chemical calculations have been performed for 1-substituted silatranes XSi(OCH₂CH₂)N (X = H, CH₃, CH₂Cl, F), their radical cations, and first ionization potentials (IP₁) of these silatranes. The calculated values of IP₁ agree well with the experiment and make it possible to assign the first band to IP₁ in the photoelectron spectra. Analysis of spin density distribution and electronic charges in the radical cations suggests that ionization occurs mainly due to the lone electron pair of nitrogen, participating in intramolecular coordination. The N → Si interaction is broken, and the N...Si distance increases to 335–340 pm.     

Ab initio calculations ofp-Cl2C6H4 molecule and its35Cl NQR parameters

Ab initio calculations ofp-dichlorobenzene molecule were carried out using the Hartree-Fock method in the 6–31 G* valence-split basis set. The molecule was also calculated by the MNDO method in the valence sp-basis set for comparison. The populations of the valent p-orbitals of the C and CI atoms were analyzed. The optimized geometry of the molecule as well as its³⁵Cl NQR frequency and the asymmetry parameter of the electric field gradient at the³⁵CI nuclei calculated using the populations of the less diffuse components of the valent p-orbitals of the Cl atoms are in agreement with the corresponding experimental values for the -modification of 1,4-Cl₂C₆H₄.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 2177–2179, September, 1996.     

Structural features of muscimol, a potent GABAA receptor agonist, crystal structure and quantum chemicalab initio calculations

Muscimol, a constituent of the mushroomAmanita muscaria, is a semirigid analogue of the inhibitory neurotransmitter 4-aminobutyric acid (GABA). X-ray structure determinations and quantum chemicalab initio calculations (HF/6-31G^*) have been carried out on the muscimol zwitterion. The solid-state conformations of the muscimol zwitterion are calculated to be 1.6–2.2 kcal/mol higher in energy than that of the calculated minimum energy structurein vacuo. A comparison of the calculated and experimental structures indicates that the hydrogen bonding network in the solid state significantly affects the geometry of the molecular structure. This conclusion is supported by results ofab initio calculations on binary complexes between muscimol and an ammonium ion and between muscimol and a methoxide anion, simulating observed hydrogen bonding in the crystal structures.     

Ab initio study of molecular structure and internal rotation in methyldicyanophosphine and methyldiisocyanophosphine. The Raman spectrum of methyldicyanophosphine and its interpretation with the use of scaling ofab initio force fields

The equilibrium geometric parameters and structures of transition states of internal rotation for the molecules of methyldicyanophospine MeP(CN)₂ and its isocyano analog MeP(NC)₂ were calculated by the RHF and MP2 methods with the 6–31G^* and 6–31G^** basis sets. At the MP2 level, the total energy of cyanide is −35 kcal mol⁻¹ lower than that of isocyanide and the barriers to internal rotation of methyl group for MeP(CN)₂ and MeP(NC)₂ are 2.2 and 2.7 kcal mol⁻¹, respectively. For both molecules, the one-dimensionalab initio potential functions of internal rotation approximated by a truncated Fourier series were used to determine the frequencies of torsional transitions by solving direct vibrational problems for a non-rigid model. The Raman spectrum of crystalline MeP(CN)₂ was recorded in the range 3500–50 cm⁻¹. The vibrational spectra of this compound were interpreted by scalingab initio force fields calculated by the RHF and MP2 method. The vibrational spectrum of methyldiisocyanophosphine was predicted with the use of the obtained scale factors. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1703–1714, September, 1998.     

Ab initio molecular orbital calculations of the infrared spectra of hydrogen-bonded complexes of water,ammonia and hydroxylamine

Summary The geometries of three hydrogen-bonded dimers of hydroxylamine have been optimized, at the MP2 level of theory, using the 6-31G** basis set. These calculations yielded three separate local minima on the dimer potential energy surface. The interaction energies of these three species have been calculated, and corrected for basis set superposition error. The infrared band wavenumbers and intensities have been computed, and the monomer-dimer wavenumber shifts and intensity enhancements rationalized in terms of the types and strengths of hydrogen bonds present. The predicted wavenumbers have been correlated with those measured in a recent matrix isolation spectroscopic study, and an argument for the structure of the preferred dimer has been presented.     

FTIR and FT-Raman spectra, assignments, ab initio HF and DFT analysis of xanthine

The molecular vibrations of xanthine were investigated in polycrystalline sample, at room temperature by Fourier transform infrared (FTIR) and FT-Raman spectroscopies. The spectra of the molecule have been recorded in the regions 4000-50 cm(-1) and 3500-100 cm(-1), respectively. Theoretical information on the optimized geometry, harmonic vibrational frequencies, infrared and Raman intensities were obtained by means of ab initio Hartree-Fock (HF) and density functional theory (DFT) gradient calculations with complete relaxation in the potential energy surface using 6-311++G(d,p) basis set. The vibrational frequencies which were determined experimentally from the spectral data are compared with those obtained theoretically from ab initio and DFT calculations. A close agreement was achieved between the observed and calculated frequencies by refinement of the scale factors. The infrared and Raman spectra were also predicted from the calculated intensities. Thermodynamic properties like entropy, heat capacity, zero point energy have been calculated for the molecule. Unambiguous vibrational assignment of all the fundamentals was made using the potential energy distribution (PED).     

Ab initio study of the ground state and conformational stability of peroxyacetyl nitrate in internal rotation about the peroxide bond

The molecular and electronic structure of the ground state of peroxyacetyl nitrate (PAN) was calculated by the unrestricted Hartree-Fock-Roothaan method with the use of the standard 3–21G and 6–31G basis set. The potential curve of the internal rotation about the peroxide bond of PAN was calculated with the 6–31G basis set. The curve contains two maxima. The ground state of PAN is characterized by a structure in which groups of atoms adjacent to the peroxide bond lie in planes that are perpendicular to each other (the dihedral angle ϱ(COON) is 89.9°). The calculated barriers to rotation are 19.6 and 66.8 kJ mol⁻¹. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 600–604, April, 1998.     

Ab initio investigation of tautomeric stability,molecular structure,and internal rotation of methylphosphonic dicyanide,methoxydicyanophosphine, and their isocyano analogs

The equilibrium geometric parameters and structures of the transition states of internal rotation for MeP(O)(CN)₂, McOP(CN)₂, and their isocyano analogs, MeP(O)(NC)₂ and MeOP(NC)₂, have been calculated by theab initio SCF method and with inclusion of electron correlation effects according to the second-order Muuller-Plesset perturbation theory (MP2). At both levels the 6-31G* basis set has been used. The estimation of relative stability of these tautomeric forms depends largely on the calculation level. The total energies of the cyanides calculated by the MP2 method are 25–30 kcal mol^–1 lower than those of the corresponding isocyanides. The oxo-tautomeric forms containing four-coordinate phosphorus are 15–25 kcal mol^–1 more stable than the three-coordinate phosphorus aci-derivatives. The internal rotation potential curves of the aci-forms are characterized by a deep minimum for thetrans-arrangement of the methoxy group and phosphorus lone electron pair. Two additional less clearly pronounced minima are located symmetrically on both sides of the weak maximum, which corresponds to thecis-arrangement. The equilibrium oxo-form structures have a staggered configuration of the methyl group with respect to the phosphorus atom bonds.Translated from izvestiyaAkademii Nauk. Seriya Khimicheskaya, No. 5, pp. 1104–1115, May, 1996.