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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; CH 3 , C 2 H 5 , i-C 3 H 7 , t-C 4 H 9 , neo-C 5 H 11 , SiMe 3 ). The MP2/6-31G * //6-31G * conformational energies (ÀDG°o r 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; SiMe 3 =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.

Ab initio 6-31G* and MP2/6-31G*// 6-31G* methods and density functional (pBPDN**) theory were used to calculate the geometries and relative energies of the chair, boat, and twist-boat conformations of tetrahydro-2H-thiopyrans (tetrahydrothiopyrans, thiacyclohexanes, thianes). The chair conformation of thiacyclohexane is 5.3 and 8.0 kcal mol À1, respectively, (1 kcal = 4.184 kJ) more stable than the twist-boat and boat structures at the MP2/6-31G*// 6-31G* level. Ab initio methods were used to calculate the relative energies of the rotamers in the chair conformations of 2-alkylthiacyclohexanes (CH 3 , C 2 H 5 , i-C 3 H 7 , t-C 4 H 9 , neo-C 5 H 11 , SiMe 3 ). The MP2/6-31G*// 6-31G* conformational energies (ÀDG°or A values, kcal mol À1 ) of the 2-alkylthiacyclohexanes (Me = 1.00; Et = 1.16; i-Pr = 1.02; t-Bu = 3.56; neo-Pent = 1.04; SiMe 3 = 2.05) are smaller than those calculated for the corresponding alkylcyclohexanes and 4-alkylthiacyclohexanes. Plots of the calculated conformational energies (ÀDG°) for the 2-alkylthiacyclohexanes versus the calculated conformational energies for the corresponding alkylcyclohexanes are linear (slope = 0.680 and r = 0.983 for 6-31G* and slope = 0.667 and r = 0.989 for MP2/6-31G*// 6-31G*). The C(2)-S(1) bond lengths are in the range 1.827-1.840 Å and the C(6)-S(1) bond lengths are in the range 1.815-1.819 Å . With the exception of the axial 2-t-Bu substituent (103.0°), the C-S-C angles vary from 98.6°to 100.4°. The S(1)-C(2)-C(7) angle in the most stable axial conformer is larger that the corresponding angle in its most stable equatorial conformer.

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