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.

3,3-Bis[(4-meth-oxy-phen-yl)sulfan-yl]-1-methyl-piperidin-2-one.

The piperidone ring in the title compound, C(20)H(23)NO(3)S(2), has a distorted half-chair conformation with the central methyl-ene atom of the propyl fragment lying 0.696 (1) Šout of the plane defined by the other five atoms (r.m.s. deviation = 0.071 Å). One of the S-bound phenyl rings is almost perpendicular to the mean plane through the piperidone ring, whereas the other is splayed [dihedral angles = 71.95 (6) and 38.42 (6)°]. In the crystal, C-H⋯O and C-H⋯π inter-actions lead to the formation of supra-molecular layers in the ab plane.

1-Methyl-3,3-bis-(phenyl-sulfan-yl)piperidin-2-one.

The piperidone ring in the title compound, C(18)H(19)NOS(2), is in a distorted half-chair conformation, distorted towards a twisted boat, with the central methyl-ene C atom of the propyl backbone lying 0.606 (2) Šout of the plane defined by the other five atoms (r.m.s. deviation = 0.1197 Å). One of the S-bound phenyl rings is almost perpendicular to the least-squares plane through the piperidone ring, whereas the other is splayed [dihedral angles = 75.97 (6) and 44.21 (7)°, respectively]. The most prominent feature of the crystal packing is the formation of helical supra-molecular chains along the b axis sustained by C-H⋯O inter-actions. The chains are consolidated into a three-dimensional architecture via C-H⋯π inter-actions whereby one S-bound phenyl ring accepts two C-H⋯π contacts.

1-Methyl-3,3-bis-[(4-methyl-phen-yl)sulfan-yl]piperidin-2-one.

The piperidone ring in the title compound, C(20)H(23)NOS(2), has a half-chair distorted to a twisted-boat conformation [Q(T) = 0.5200 (17) Å]. One of the S-bound benzene rings is almost perpendicular to the least-squares plane through the piperidone ring, whereas the other is not [dihedral angles = 75.28 (5) and 46.41 (5) Å, respectively]. In the crystal, the presence of C-H⋯O and C-H⋯π inter-actions leads to the formation of supra-molecular layers in the ab plane.

3,3-Bis[(4-chloro-phen-yl)sulfan-yl]-1-methyl-piperidin-2-one.

The piperidone ring in the title compound, C(18)H(17)Cl(2)NOS(2), has a distorted half-chair conformation. The S-bound benzene rings are approximately perpendicular to and splayed out of the mean plane through the piperidone ring [dihedral angles = 71.86 (13) and 46.94 (11)°]. In the crystal, C-H⋯O inter-actions link the mol-ecules into [010] supra-molecular chains with a helical topology. C-H⋯Cl and C-H⋯π inter-actions are also present.