The analysis of the IR carbonyl bands of some 3-(4′-substituted phenylsulfanyl)-1-methyl-2-piperidones
1–
6 bearing substituents: NO
2 (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 ν
CO frequencies of the
ax and
eq conformers found in the gas phase. Unusually, in solution of the non-polar solvents
n-C
6H
14 and CCl
4, 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 ν
CO 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 CHCl
3 and CH
2Cl
2, and is no longer detectable in the most polar solvent CH
3CN. 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-C
6H
14 and CCl
4, while the ν
CO frequencies of the
eq conformer become progressively lower than that of the
ax one going from CHCl
3 to CH
2Cl
2 and to CH
3CN, 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.
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