Ab initio molecular orbital study of the conformational energies of 3- alkyltetrahydro-2H-pyrans

F. Freeman, Marc Kasner, W. J. Hehre

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15 Scopus citations

Abstract

Ab initio HF/6-31G* and MP2/6-31G*//HF/6-31G* methods were used to calculate the relative energies of the rotamers in the chair conformations of 3-alkyltetrahydro-2H-pyrans (tetrahydropyrans, oxacyclohexanes, oxanes; CH3, C2H5, i-C3H7, t-C4H9, neo-C5H11) and 3-(trimethylsilyl)tetrahydro- 2H-pyran; Si(CH3)3). The MP2/6-31G*//HF/6-31G* conformational energies (- ΔG0 or A values, kcal/mol) of 3-alkyltetrahydro-2H-pyrans (CH3 = 0.97; C2H5 = 0.74; i-C3H7 = 0.59; t-C4H9 = 2.42; neo-C5H11 = 0.27; Si(CH3)3 = 0.86) are smaller than those calculated for alkylcyclohexanes and 2-alkyloxacyclohexanes. The carbon-oxygen bond lengths are in the narrow range of 1.399-1.403 Å with the C(2)-O(1) bond length essentially the same as the C(6)-O(1) bond length. The C(3)-H bond lengths range from 1.086 to 1.092 Å for the respective axial and equatorial conformers. The C(3)-C(7) bond length shows greater variability (1.528 to 1.640 Å) with the axial C(3)-C(7) bond generally ~0.004 Å longer than the corresponding equatorial bond. The C(2)-O(1)-C(6) bond angles range from 112.9°to 114.2°and the C(3)-C(4)-C(5) angles vary from 110.7 to 114.6°. Plots of the calculated conformational energies for the 3-alkyloxacyclohexanes versus the calculated conformational energies for the corresponding alkylcyclohexanes are linear (slope = 0.54 and r = 0.992 for HF/6-31G* and slope = 0.45 and r = 0.977 for MP2/6-31G*//HF/6-31G*).

Original languageEnglish
Pages (from-to)87-105
Number of pages19
JournalJournal of Molecular Structure: THEOCHEM
Volume487
Issue number1-2
DOIs
StatePublished - 17 Sep 1999

Keywords

  • 3-alkyltetrahydro-2H-pyrans
  • Ab initio methods
  • Conformational energies
  • Nonbonded interactions
  • Oxacyclohexanes
  • Rotamers
  • Steric effects

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