An ab initio molecular orbital study of the conformational energies of 3-alkyltetrahydro-2H-thioprans (tetrahydrothiopyrans, thiacyclohexanes, thianes)

摘要

Ab initio 6-31G~* basis sets and density functional theory (pBP/DN~**) were used to calculate the geometry of the chair conformer of tetrahydro-2H-thiopran (tetrahydrothiopyran, thiacycloheane, thiane). 6-31G~* geometry optimization and MP2/6-31~*//6-31G~* single point energy methods were used to calculate the relative energies and conformational energies (-#DELTA#G~#omicron# or A ualues, Kcal/mol) of 3-alkyltetrahydro-2H-thiopyrans (Me=1.56; Et=1.22; i-Pr=0.91; t-Bu=4.54; neo-Pent=1.06; SiMe_3=2.01). The MP2/6-31G~*//6-31G~* calculated conformational energies of the 3-alkylthiacyclohexanes are gener-ally smaller than those for the corresponding alkylcyclohexanes and 4-alkythiacyclohexanes, but are similar to those calculated for 2-alkylthiacyclohexanes. Plots of the calculated conformational enerigies of the 3-alkythiacyclohexanes versus the calculated -#DELTA#G~#omicron# of the corresponding alkylcyclohexanes (slope=0.901 and r=0.997 for 6-31G~* and slope=0.845 and r=0.994 for MP2/6-31G~*//6-31G~*). 2-alkylthiacyclohexanes (slope=1.273 and r=0.981 for 6-31G~* and slope=1.273 and r=0.986 for MP2/6-31G~*//6-31G~*). and 4-alkylthiacyclohexanes (slope=0.888 and r=0.997 for 6-31G~* and slope=0.839 and r=0.994 for MP2/6-31G~*//6-31G~*) are linear. Both carbon-surlfur bond lengths in the 3-alkylthiacyclohexanes are generally in the narrow range of 1.815 and 1.818 A and the C(2)-H_(ax) and C(6)-H(ax) bond lengths are slightly longer than the respective C(2)-H_(cq) and C(6)-H_(eq) bond lengths. The C-S-C bond angles vary form 97.1 to 99.0 and the C(2)-C(3)-C(7) and C(4)-C(3)-C(7) bond angles in the most stable axial conformer are larger that the corresponding angles in its most stable equatorial conformer. 1999 Elsevier Science B.V. All rights reserved.