Ab initio molecular orbital study of the conformational energies (??G� orA values) of 4-alkyltetrahydro-2H-thiopyrans (tetrahydrothiopyrans, thiacyclohexanes, thianes)

Ab initio 6–31G* and MP2/6–31G*//6–31G* methods were used to calculate the relative energies of the rotamers in the chair conformations of 4-alkyltetrahydro-2H-thiopyrans (tetrahydrothiopyrans, thiacyclohexanes, thianes; CH3, C2H5, i-C3H7, t-C4H9, neo-C5H11, SiMe3). The MP2/6–31G*//6–31G* conformational energies (−ΔG° or A values, kcal mol-1) of the 4-alkylthiacyclohexanes (Me = 1.94; Et = 1.72; i-Pr = 1.62; t-Bu = 5.49; neo-Pent = 1.39; SiMe3 = 2.76) are similar to those calculated for the corresponding alkylcyclohexanes. Plots of the calculated conformational energies for the 4-alkylthiacyclohexanes versus the calculated conformational energies for the corresponding alkylcyclohexanes are linear (slope = 1.01 and r = 0.999 for 6–31G* and slope = 1.09 and r = 0.990 for MP2/6–31G*//6–31G*). The conformational energies are strongly influenced by the magnitude and number of repulsive nonbonded hydrogen–hydrogen interactions and the gauche (synclinal) effects in the conformers. The carbon–sulfur bond lengths are mostly in the narrow range of 1.815–1.818 Å. The C—S—C angles vary from 97.1° to 99.4° and the C(3)—C(4)—C(7) angle in the most stable axial conformer is larger that the corresponding angle in its most stable equatorial conformer.