The geometries of several n-alkanes were determined by HF/6-311G**and MP2/6-311G**gradient optimization. The results make it possible to study the effects of electron correlation on non-bonded aliphatic interactions by comparing structures devoid of dispersion forces (HF/6-31 IG**) with those in which the dispersion forces are switched on (MP2/6-311 G**). Conformations with trans bonds T (C-C-C-C torsions 180°), and gauche bonds G (C-C-C-C torsions 60°) were investigated, including T and G n-butane; TT, TG, and GG n-pentane; and TTT, GTT, TGG, GTG, and GGG n-hexane. When geometries are optimized at the MP2/6-311G**level, rotamer energies do not increase with the number of individual G bonds as expected, because GGG < GTG. When MP2 energies are calculated for HF/6-31 IG**geometries, the errors connected with the lack of geometry optimization increase with the size of a system and are larger for folded conformations than stretched ones. The existence of a stabilizing cooperative energy increment associated with GG sequences in n-alkanes, previously postulated on the basis of MP4SDQ/6-31G*//HF/6-31G*calculations, is confirmed by the current study, but its magnitude (> O.3 kcal mol-1) is larger in the MP2-optimized geometries than previously allowed (0.16 kcal mol-1). In general, in all rotamers with G torsions, small contractions in torsional angles (< 5°) cause non-bonded distances in the attractive region of the van der Waals potential to be shorter in MP2 structures than HF geometries, in contrast to 1,4 interactions, which are practically invariant. Specifically, average 1,5 non-bonded distances in GG pentane, and TGG and GGG hexane shrink by 0.22 to 0.27 Å; by 0.06 to 0.08 Å in TG pentane, and GTT and GTG hexane; and they are essentially unchanged in TT sequences. The results emphasize the importance of accurate geometries in conformational analyses: when the bond lengths and angles of a molecular model are wrong, calculated energies are also wrong, because non-bonded interactions are incorrectly evaluated. © 1993.
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