Effect of methyl group on the cooperativity of CH···O blue-shifted hydrogen bond in HCHO-HCHO-HCHO cyclic complex

The effect of methyl group on the cooperativity of CH···O blue-shifted hydrogen bond in HCHO-HCHO-HCHO cyclic complex has been studied with quantum chemical calculations at the MP2/6-31+G(d,p) level. We report herein the optimized geometries of the stable structures, their vibrational frequencies, NMR chemical shifts, stabilization energies, and binding energies. The cooperativity of CH···O blue-shifted hydrogen bond varies from −0.57 kcal/mol in triformaldehyde through −0.65 kcal/mol in trimethyltriformaldehyde to −0.79 kcal/mol in trifluorotriformaldehyde. The result indicates that the methyl group has a little enhancing effect on the cooperativity of CH···O blue-shifted hydrogen bond in the cyclic complex. A negative nonadditivity of methyl group is evaluated in enhancing the cooperativity of three CH···O blue-shifted hydrogen bonds. In the formation of the CH···O blue-shifted hydrogen bond, the methyl group in the proton-acceptor plays a positive contribution, whereas that in the proton-donor plays a negative contribution.