Ab initio molecular orbital calculations for 3,6‐dihydro‐1,2‐dithiin and 3,6‐dihydro‐1,2‐dioxin

Abstract

Optimized geometries and total energies for the conformers of 3,6-dihydro-l,2-dithiin (2) and 3,6-dihydro-l,2-dioxin (3) were calculated at several ab initio MO levels: RHF/3-21G(*), RHF/631G*, MP2/6-31Gt, and MP2/ 6-31G*//RHF/3-21G(*). For the dioxin, in addition to the above levels the corresponding nonextended basis sets ab initio methods were also carried out. The dithiin results are compared with those of simple disulfanes, HSSH and (CH3),Sz, whose optimized geometries agree closely with the observed structures, which is the gauche (C, symmetry). For the disulfanes, the gauche geometries from RHF/3-21G(*) are in good agreement with the observed structure while the RHF/3-21G results best fit the dioxin. Pertinent structural data at the RHF/3-21G(*) for the half-chair (C,) dithiin are: bond lengths, -SS--,-CS--, -CC=, and -C=C-, 2.050, 1.817,1.515, and 1.317A, respectively; bond angles, CSS, =CCS, and C=CS, 98.0,114.2, and 127.8", respectively; CSSC dihedral angle of 63.2"; and twist angle of 36.5". The total energy for half-chair dithiin at MP2/6-31G*/ /RHF/3-21G(*) is less than the planar (C,,) and the half-boat (Cs) structures by 69.67 and 29.05 kJ/mol, respectively. The calculated structural data (vs. observed) at RHF/3-21G for the half-chair dioxin are: bond lengths, -00-, -CO-, -CC=,

Molecular orbital study of the conformational energies (−ΔG° orA values) of 2-alkyltetrahydro-2H-thiopyrans (tetrahydrothiopyrans, thiacyclohexanes, thianes)

“…5 The larger the conformational energy, the greater is the preference for the equatorial position. The replacement of carbon by another element in cycloalkanes or cycloalkenes [7][8][9][10][11] (F. Freeman, Z. M. Tsegai and W. J. Hehre, submitted; N. L. Allinger and K. H. Chen, personal communication) produces changes in several structural parameters and consequently affects the conformational characteristics of the molecules. For example, introduction of a nitrogen, an oxygen or a sulfur atom for a methylene group in cyclohexane will lead to changes in bond lengths (C-C 1.54 Å ; C-N 1.47 Å ; C-O 1.43 Å ; C-S 1.82 Å ) and in bond angles.…”

“…[7][8][9][10][11] This study was undertaken in order to help elucidate the influences of replacing a methylene group in cyclohexane with a sulfur atom by comparing the calculated geometries (6-31G*) and single point energies (MP2/6-31G*// 6-31G*) of the conformers [chair, C s point group (2); boat, C s point group (4); boat (5); twist-boat (skew-boat, C 2 point group (6); planar C 2v point group (7)] of tetrahydro-2H-thiapyran (tetrahydrothiopyran, thiacyclocylohexane, thiane; 2). The calculated 6-31G* and density functional theory (pBPDN**) results from the chair conformer 2 were compared with the structural parameters from other calculational methods and from microwave spectroscopic and x-ray diffraction data (Table 1).…”

“…40,41 Although steric effects are considered to be responsible for equatorial preferences in cyclohexanes, natural bond orbital analysis (NBO) 38,47 suggests that bondantibond interactions of the exocylic C(1)-C(7) bond in methylcyclohexane with s* C-C and s* C-H bonds are mainly responsible for the equatorial preference. However, bond-antibond stereoelectronic interactions may not be the sole contributor to the equatorial preference in methylcyclohexane (9). At the 6-31G* level, the C(1)-C(2), C(1)-C(7) and C(1)-H eq bond distances in 9a are 1.539, 1.534 and 1.088 Å , respectively.…”

“…21 The 6-31G* calculated C(2)-H ax and C(2)-H eq bond lengths in 2 are essentially the same (1.085 and 1.083 Å ), which suggests an absence of anomeric effects. 1,7,9 ] (the gauche interactions in cyclohexanes and thiacyclohexanes may not be comparable to those of gauche butane because the hydrogen-hydrogen interactions are minimized in the cyclic structures. [43][44][45] ) for comparison purposes with 2-methylthiacyclohexane (10; see Table 3).…”

“…At the MP2/6-31G*/ /6-31G* level, the equatorial diastereomer 9b (E = À274.160811 hartree) is more stable (ÀDG°= 1.96 kcal mol À1 ) than its axial isomer 9a (E = À274.157688 hartree). [38][39][40][41][42][43][44] An experimental ÀDG°value of 1.74 kcal mol À1 has been obtained for methylcyclohexane (9) in CFCl 3 CDCl 3 via low-temperature carbon-13 NMR spectroscopy. 40,41 Although steric effects are considered to be responsible for equatorial preferences in cyclohexanes, natural bond orbital analysis (NBO) 38,47 suggests that bondantibond interactions of the exocylic C(1)-C(7) bond in methylcyclohexane with s* C-C and s* C-H bonds are mainly responsible for the equatorial preference.…”