ELECTRON CORRELATION-EFFECTS IN ALIPHATIC NONBONDED INTERACTIONS - COMPARISON OF N-ALKANE MP2 AND HF GEOMETRIES

The geometries of several n-alkanes were determined by HF/6-311G* and MP2/6-311G* gradient optimization. The results make it possible to s tudy the effects of electron correlation on non-bonded aliphatic inter actions by comparing structures devoid of dispersion forces (HF/6-311G *) with those in which the dispersion forces are switched on (MP2/6-3 11G*). Conformations with trans bonds T (C-C-C-C torsions 180-degrees ), and gauche bonds G (C-C-C-C torsions 60-degrees) were investigated, including T and G n-butane; TT, TG, and GG n-pentane; and TTT, GTT, T GG, GTG, and GGG n-hexane. When geometries are optimized at the MP2/6- 311G* level, rotamer energies do not increase with the number of indi vidual G bonds as expected, because GGG < GTG. When MP2 energies are c alculated for HF/6-311G* geometries, the errors connected with the la ck 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 sequenc es in n-alkanes, previously postulated on the basis of MP4SDQ/6-31G// HF/6-31G calculations, is confirmed by the current study, but its mag nitude (> 0.3 kcal mol-1) is larger in the MP2-optimized geometries th an previously allowed (0.16 kcal mol-1). In general, in all rotamers w ith G torsions, small contractions in torsional angles (< 5-degrees) c ause non-bonded distances in the attractive region of the van der Waal s potential to be shorter in MP2 structures than HF geometries, in con trast to 1,4 interactions, which are practically invariant. Specifical ly, average 1,5 non-bonded distances in GG pentane, and TGG and GGG he xane shrink by 0.22 to 0.27 angstrom; by 0.06 to 0.08 angstrom in TG p entane, and GTT and GTG hexane; and they are essentially unchanged in TT sequences. The results emphasize the importance of accurate geometr ies 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.