Conformational Analysis and Electronic Interactions of Some 4'-Substituted-2-ethylthio-phenylacetates.

The conformational analysis of various 4'-substituted-2-ethylthio-phenylacetate compounds bearing the substituents NO2 (1), Cl (2), H (3), Me (4), and OMe (5) was performed using infrared (IR) spectroscopic analysis of the carbonyl stretching band (νCO) supported by B3LYP/6-31G(d,p), NBO, QTAIM, and SM5.42R calculations for compounds 1, 3, and 5. The IR spectra in n-hexane indicate the presence of three components, whose intensities decrease upon increasing frequency. In solvents with high permittivity, while the low intensity component at higher frequency disappears, the relative intensity of the component at the intermediate frequency changes with respect to the lower frequency component with differing trends for the various derivatives. It can be observed that the intensity does not vary for compounds 1 and 2, which bear an electron-withdrawing substituent at 4', while it increases in intensity for compounds 3-5. The computational results predict the presence of three gauche conformers, defined by the orientation of the C-S bond with respect to the carbonyl group, whose intensities and νCO frequencies are in agreement with the experimental results. In solvents with increasing permittivity, the SM5.42R solvation model results reproduce the experimental trend observed for the two components in the low frequency region, while it overestimates the amount of the higher frequency conformer. NBO analysis suggests that all the conformers are stabilized to the same extent in the gauche conformation via σC-S → π*CO and πCO → σ*C-S orbital interactions. The different stability can be attributed to the geometrical arrangement of the C(O)-CH2-S-CH2-CH3 moiety, which assumes a six-membered chair-like geometry in the g1 conformer, a six-membered twisted-chair-like geometry in the g2 conformer, and a seven-membered chair-like ring in the g3 conformer. Quantum theory of atoms in molecules (QTAIM) indicates that the ring geometries were formed and stabilized from short contacts between the oppositely charged carbonyl oxygen and the methylene/methyl hydrogen atoms, which interact through unusual intramolecular electrostatic hydrogen bonding that satisfies the Popelier criteria.

Spectroscopic and theoretical studies of some 3-(4'-substituted phenylsulfanyl)-1-methyl-2-piperidones.

The analysis of the IR carbonyl bands of some 3-(4'-substituted phenylsulfanyl)-1-methyl-2-piperidones 1-6 bearing substituents: NO2 (compound 1), Br (compound 2), Cl (compound 3), H (compound 4) Me (compound 5) and OMe (compound 6) supported by B3LYP/6-31+G(d,p) and PCM calculations along with NBO analysis (for compound 4) and X-ray diffraction (for 2) indicated the existence of two stable conformations, i.e., axial (ax) and equatorial (eq), the former corresponding to the most stable and the least polar one in the gas phase calculations. The sum of the energy contributions of the orbital interactions (NBO analysis) and the electrostatic interactions correlate well with the populations and the νCO frequencies of the ax and eq conformers found in the gas phase. Unusually, in solution of the non-polar solvents n-C6H14 and CCl4, the more intense higher IR carbonyl frequency can be ascribed to the ax conformer, while the less intense lower IR doublet component to the eq one. The same νCO frequency trend also holds in polar solvents, that is ν(CO)(eq)< ν(CO)(ax). However, a reversal of the ax/eq intensity ratio occurs going from non-polar to polar solvents, with the ax conformer component that progressively decreases with respect to the eq one in CHCl3 and CH2Cl2, and is no longer detectable in the most polar solvent CH3CN. The PCM method applied to compound 4 supports these findings. In fact, it predicts the progressive increase of the eq/ax population ratio as the relative permittivity of the solvent increases. Moreover, it indicates that the computed ν(CO) frequencies of the ax and eq conformers do not change in the non-polar solvents n-C6H14 and CCl4, while the ν(CO) frequencies of the eq conformer become progressively lower than that of the ax one going from CHCl3 to CH2Cl2 and to CH3CN, in agreement with the experimental IR values. The analysis of the geometries of the ax and eq conformers shows that the carbonyl oxygen atom of the eq conformer is free for solvation, while the O[CO]…H[o-Ph] hydrogen bond that takes place in the ax conformer partially hinders the approach of the solvent molecules to the carbonyl oxygen atom. Therefore, the larger solvation that occurs in the carbonyl oxygen atom of the eq conformer is responsible for the observed and calculated decrease of the corresponding frequency. The X-ray single crystal analysis of 2 indicates that this compound adopts the most polar eq geometry in the solid. In fact, in order to obtain the largest energy gain, the molecules are arranged in the crystal in a helical fashion due to dipole moment coupling along with C-H…O and C-H…π(Ph) hydrogen bonds.

Spectroscopic and theoretical studies of some 2­(methoxy)­2­[(4­substituted)­phenylsulfanyl]­(4'­substituted) acetophenones.

The conformational analysis of some 2­(methoxy)­2­[(4­substituted)­phenylsulfanyl]­(4'­substituted) acetophenones was performed through infrared (IR) spectroscopic analysis of the carbonyl stretching band (νCO), supported by B3LYP/6-31+G(d,p) calculations and X-ray diffraction. Five (1-5) of the seven studied compounds (1-7) presented Fermi resonance (FR) on the νCO fundamental transition band. Deuteration of these compounds (1a-5a) precluded the occurrence of FR, revealing a νCO doublet for all compounds in all solvents used. The computational results indicated the existence of three conformers (c1, c2 and c3) for the whole series whose relative abundances varied with solvent permittivity. The higher νCO frequency c1 conformer was assigned to the higher frequency component of the carbonyl doublet, while both c2 and c3 were assigned to the lower frequency one. Anharmonic vibrational frequencies and Potential Energy Distribution (PED) calculations of compound 3 indicated that the combination band (cb) between the methyne δCH and one skeletal mode couples with the νCO mode giving rise to the FR on the c2 conformer in vacuum and on the c1 one in non-polar solvents. The experimental data indicated a progressive increase in c1 conformer stability with the increase of the solvent dielectric constant, which is in good agreement with the polarizable continuum model (PCM) calculations. The higher νCO frequency and the stronger solvation of the c1 conformer is a consequence of the repulsive field effect (RFE) originated by the alignment and closeness of the Cδ+Oδ- and Cδ+Oδ- dipoles. Finally, the balance between orbital and electrostatic interactions dictates the conformational preferences. X-ray single crystal analysis for compound 6 revealed the c1 geometry in the solid state and its stabilization by CH…O hydrogen bonds.

Crystal structure of 2-meth-oxy-2-[(4-meth-oxy-phen-yl)sulfan-yl]-1-phenyl-ethanone.

In the title ß-thio-carbonyl compound, C16H16O3S, the adjacent meth-oxy and carbonyl O atoms are synperiplanar [the O-C-C-O torsion angle is 19.8 (4)°] and are separated by 2.582 (3) Å. The dihedral angle between the rings is 40.11 (16)°, and the meth-oxy group is coplanar with the benzene ring to which it is connected [the C-C-O-C torsion angle is 179.1 (3)°]. The most notable feature of the crystal packing is the formation of methine and methyl C-H⋯O(carbon-yl) inter-actions that lead to a supra-molecular chain with a zigzag topology along the c axis. Chains pack with no specific inter-molecular inter-actions between them.

Crystal structure of 2-meth-oxy-2-[(4-methyl-phen-yl)sulfan-yl]-1-phenyl-ethan-1-one.

In the title ß-thio-carbonyl compound, C16H16O2S, the carbonyl and meth-oxy O atoms are approximately coplanar [O-C-C-O torsion angle = -18.2 (5)°] and syn to each other, and the tolyl ring is orientated to lie over them. The dihedral angle between the planes of the two rings is 44.03 (16)°. In the crystal, supra-molecular chains are formed along the c axis mediated by C-H⋯O inter-actions involving methine and methyl H atoms as donors, with the carbonyl O atom accepting both bonds; these pack with no specific inter-molecular inter-actions between them.