Theoretical study of stereochemistry of methoxy(methylthio)methane as a model of thioacetal segment in thiosaccharides

The stereochemistry of methoxy(methylthio)methane - a model of the thioacetal segment in thiosaccharides - has been studied by semiempirical quantum -chemical methods. The conformational map of the rotation around the central C-O and C-S bonds has been calculated by the PCILO method. Decomposition of the energy for the rotation around the C-O (C-S)bond (the methyl group being fixed in the antiperiplanar position) in terms of the Fourier series gives the values 7.12 (2.53) kJ mol-1 for the respective rotational barrier. The geometry of five stable conformers, whose stability decreases in the order (sc,sc) > (ap,sc) > (-ac,sc) > (ap,ap), has been optimized by the MNDO method. Ratio of the individual conformers in an isolated molecule calculated on the basis of the MNDO energies is 92 : 5 : 2 : 1 : 0, the mean dipole moment being 3.6 . 10-30 Cm. The solvent effect has been calculated by a method involving electrostatic and dispersion interactions and cavity contribution. Population of the conformers in diluted solutions depends on nature of the solvent, proportion of the antiperiplanar positions being increased with increasing polarity. The calculated ratios of the conformers are 85 : 8 : 3 : 4 : 0 and 63 : 14 : 7 : 16 : 0 in acetonitrile and water, respectively. The calculated values of the Gibbs energies have also been used for assessment of magnitude of anomeric and exo-anomeric effects in α- and β-1-thiopyranosides and α- and β-5-thiopyranosides.

A quantum-chemical study of phenol self-association of open and cyclic dimers and trimers

Itermolecular hydrogen bond in the open/linear and cyclic/closed dimers and trimers of phenol has been studied by means of the quantum-chemical PCILO method. Our calculations have shown that the open /linear associate is more stable in the case of the dimers. On the contrary, the cyclic/closed associate is more stable in the case of the trimers. Positive cooperativity has been observed by formation of both the open/linear and cyclic/closed trimers

PCILO study of non-bonding interactions

The PCILO method has been used for study of non-bonding interactions in the systems water-water and water-ethylene. The PCILO method is unable to account correctly for the intermolecular non-bonding interactions of the types lone electron pair - lone electron pair and lone electron pair-π bond. The potential energy curves, which should be strongly repulsive, are predicted as being attractive. The results obtained are discussed from the point of view of energy partitioning in terms of PCILO theory.

MO study of molecular association of local anaesthetics of carbamate type

The quantum-chemical PCILO method has been used to study of molecular association of piperidinoethyl alkoxyphenylcarbamates. The self-association of this type of compounds has been studied with methyl phenylcarbamate as a simple model. The PCILO calculations indicate the open dimer with N-H...O=C hydrogen bond to be the most stable complex. Moreover, the PCILO method has been used fro study of intermolecular hydrogen bonds formed between oxygen and nitrogen atoms of -NH-COO- group of the title compounds and N-methylacetamide, N,N-dimethylacetamide, and phenol. The calculations have shown that the most stable hydrogen bonds are formed by carbonyl oxygen atom. Somewhat weaker hydrogen bonds are formed by N-H group. The weakest hydrogen bonds are formed by methoxyl oxygen atom of the -NH-COO- part of the drug.

Theoretical study of the conformational arrangement of 3-benzylidene-2,4-pentanedione and its substructures

The quantum chemical PCILO method was employed for calculation of conformational maps of 3-benzylidene-2,4-pentanedione. The analogous conformational maps of simpler substructures were also investigated to enable investigating the respective interactions. Steric interactions of the acetyl group with the phenyl group and of both acetyl groups were found to play a dominant role. One acetyl group is turned by 60° from the s-cis position, the other is in an s-trans conformation when being in the energetically more favourable sterical arrangement. The obtained data were compared with the experimental results.

Quantum-chemical study of N-methylacetamide and N,N-dimethylacetamide as models for peptidic bond in hydrogen-bond interaction with water, methanol and phenol

The PCILO quantum-chemical method with geometry optimization has been used to study rotation barriers of methyl groups in N-methylacetamide and N,N-dimethylacetamide. In all the cases studied, the eclipsed conformation have been found to be the most stable. Cis form of N-methylacetamide is less stable than the corresponding trans form by 2.0 kJ mol-1. Moreover, the PCILO method has been used to study linear n-mers (n = 4) of N-methylacetamide. On going from the dimer to tri- and tetramers, the hydrogen-bond energies have been found non-additive, and positive cooperativity has been observed. Finally, hydrogen-bond complexes have been studied which were formed by C=O groups of N-methylacetamide and N,N-dimethylacetamide with water, methanol or phenol as proton-donors. The said proton-donors have been found to act as breakers of inter-peptide hydrogen bonds N-H...O=C. The hydrogen bonds formed by methanol are somewhat stronger than those formed by water. In accordance with experiment, the strongest hydrogen bonds with the studied proton-acceptors are formed by phenol.

Calculation of stabilization energy of parallel hexane molecules

Molecular-mechanics method has been used for calculation od stable configurations of n-hexane pairs and triads in extended all-trans conformations with full translational and rotational freedom of the molecules during optimization. The calculated stabilization energies and equilibrium distances have been compared with the experimental data obtained for molecular crystals of paraffins. The comparison enables to distinguish the effects characteristical of the collective packing forces in the crystal. The optimum configurations of some hexane pairs have also been calculated by the quantum-chemical PCILO method. The results indicate superiority of MMC to the quantum-chemical methods and other empirical calculation procedures for the purposes of the stabilization energy determination.