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Molecules 2013, 18(7), 7492-7509; doi:10.3390/molecules18077492

Spectroscopic and Theoretical Studies of Some 3-(4′-Substituted phenylsulfanyl)-1-methyl-2-piperidones

Conformational Analysis and Electronic Interactions Laboratory, Institute of Chemistry, University of São Paulo, USP, CP 26077, 05513-970 São Paulo, SP, Brazil
Institute for Environmental, Chemical and Pharmaceutical Sciences, Federal University of São Paulo, UNIFESP, Diadema 09972-270, SP, Brazil
Chemistry Department, Federal University of São Carlos, CP 676, 13565-905, São Carlos, SP, Brazil
Department of Chemistry, University of Ferrara, Ferrara 44100, Italy
Author to whom correspondence should be addressed.
Received: 31 May 2013 / Revised: 20 June 2013 / Accepted: 21 June 2013 / Published: 27 June 2013
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The analysis of the IR carbonyl bands of some 3-(4′-substituted phenylsulfanyl)-1-methyl-2-piperidones 16 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 νCO (eq)< νCO (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 νCO frequencies of the ax and eq conformers do not change in the non–polar solvents n-C6H14 and CCl4, while the νCO 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-HO and C-HπPh hydrogen bonds. View Full-Text
Keywords: conformational analysis; infrared spectroscopy; theoretical calculations; 3-(4′-substituted phenylsulfanyl)-1-methyl-2-piperidones; X-ray crystal structure conformational analysis; infrared spectroscopy; theoretical calculations; 3-(4′-substituted phenylsulfanyl)-1-methyl-2-piperidones; X-ray crystal structure

Figure 1

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MDPI and ACS Style

Olivato, P.R.; Santos, J.M.M.; Contieri, B.; Cerqueira, C.R., Jr.; Rodrigues, D.N.S.; Vinhato, E.; Zukerman-Schpector, J.; Colle, M.D. Spectroscopic and Theoretical Studies of Some 3-(4′-Substituted phenylsulfanyl)-1-methyl-2-piperidones. Molecules 2013, 18, 7492-7509.

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