A priori constraints in the force field calculations of polyatomic molecules

An inverse problem on calculating the force field parameters of a molecule from experimental data and its solution using regularizing algorithms are considered The choice of a priori constraints that may be imposed on solutions of the inverse vibrational problem is analyzed in terms of quantum mechanical calculations of some polyatomic molecules at different levels of theory, including electron correlation. Translated from Zhumal Struktumoi Khimii, Vol. 38, No. 2, pp. 221–239, March–April, 1997. Original article submitted June 14, 1996.     

Conformational analysis of 5-substituted 1,3-dioxanes

Quantum-chemical study on the potential energy surface of 5-alkyl- and 5-phenyl-1,3-dioxanes at the RHF/6-31G(d) level of theory revealed two pathways for conformational isomerizations of the equatorial and axial chair conformers. Potential barriers to this process were estimated. The Gibbs conformational energies ΔG° of substituents at C⁵ in the 1,3-dioxane ring were determined on the basis of experimental (¹H NMR) and theoretical vicinal coupling constants, which turned out to be consistent with published data.     

Vibrational spectra and stable conformations of N-methylacetamide and (1S,5S,6R)-6-acetylamino-(5′-methoxyindolo[2,3-b])bicyclo[3.2.1]oct-2-ene

IR absorption and Raman spectra of N-methylacetamide and (1S,5S,6R)-6-acetylamino-(5′-methoxyindolo[2,3-b])bicyclo[3.2.1]oct-2-ene are investigated. Optimized structures and harmonic force fields of stable conformers of both compounds are obtained by means of MP2 and DFT (B3LYP) methods using the 6-31+G** basis set. Based on quantum mechanical calculations, a detailed interpretation of spectra is proposed and vibrational frequencies of the most stable conformations are assigned. Correlations between structures and the spectra of N-methylacetamide and (1S,5S,6R)-6-acetylamino-(5′-methoxyindolo[2,3-b])bicyclo[3.2.1]oct-2-ene were considered and analytical spectral regions were proposed.     

Molecular dynamics. Vibrational and35Cl NQR spectra of a 1,1,1,3,3-pentafluoro-3-chloropropane crystal

Journal of Applied Spectroscopy -     

Conformational analysis of 5-bromo-5-nitro-1,3-dioxane

Conformational analysis of 5-bromo-5-nitro-1,3-dioxane has been performed by computer simulation in terms of HF/pVDZ, hybrid DFT PBE/3ξ, and RI-MP2/λ2 quantum chemical methods. The global minimum on the potential energy surface corresponds to the chair conformer with axial nitro group, and the other two minima have been identified as chair conformer with equatorial nitro group and 2,5-twist. All transition states on the route of conformational transformations of the title molecule have been revealed.     

Pathways of conformational isomerization of 4-phenyl-1,3-dioxane

A study of the potential energy surface of 4-phenyl-1,3-dioxane by non-empirical quantum chemical RHF/STO-3G and 6-31G(d) approximations reveals six energetically inequivalent pathways of conformational isomerization of equatorial and axial chair forms.     

Conformational analysis of <Emphasis Type="Italic">cis-</Emphasis> and <Emphasis Type="Italic">trans</Emphasis>-isomers of 2,5-dimethyl-1,3-dioxane

The conformational isomerization of cis- and trans-isomers of 2,5-dimethyl-1,3-dioxane was investigated by means of ab initio RHF//6-31G(d) and PBE//ccpVDZ quantum-chemical methods. It has been shown that in comparison with cis-form the potential energy surface of trans-isomer has a less number of stationary points and includes the main minimum belonging to the diequatorial chair form. In the case of cisisomer the main minimum corresponds to the C 2e5a chair form. The values of potentional barriers of conformational isomerization were also determined. On the basis of experimental (NMR ¹H) and theoretical vicinal coupling constants we determine the value ΔG ⁰ of chair-chair inversion for the trans-isomer molecule.     

Ab initio study of the structure of 2,2-difluoroethanal in the ground and lowest excited triplet electronic states

The molecular structure of 2,2-difluoroethanal (DFE) in the ground (S₀) and lowest excited triplet (T_i) electronic states was investigated byab initio quantum-chemical methods. In the S₀ state, the DFE molecule exists as the only stablecis conformer. The T_i↓S₀ electronic excitation is accompanied by the rotation of the top and the deviation of the carbonyl fragment from planarity. For the DFE molecule in the T_i state, six minima corresponding to three pairs of enantiomers were found on the potential energy surface. Based on this potential energy surface, the problems on torsion and inversion nuclear motions were solved in the one- and two-dimensional approximations, and the interaction between these motions was revealed. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 989–995, June, 2000.     

Structure and Conformational Analysis of 5-Methyl-2,2-diphenyl-1,3-dioxane

Russian Journal of Organic Chemistry - The structure of 5-methyl-2,2-diphenyl-1,3-dioxane has been studied by1H and 13C NMR spectroscopy and X-ray analysis. Its molecules in crystal, as well as in...