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Keywords:

  • methyl group;
  • single-electron hydrogen bond;
  • nonadditivity;
  • ab initio;
  • NBO

Abstract

The nonadditivity of methyl group in the single-electron hydrogen bond of the methyl radical-water complex has been studied with quantum chemical calculations at the UMP2/6-311++G(2df,2p) level. The bond lengths and interaction energies have been calculated in the four complexes: CH3[BOND]H2O, CH3CH2[BOND]H2O, (CH3)2CH[BOND]H2O, and (CH3)3C[BOND]H2O. With regard to the radicals, tert-butyl radical forms the strongest hydrogen bond, followed by iso-propyl radical and then ethyl radical; methyl radical forms the weakest hydrogen bond. These properties exhibit an indication of nonadditivity of the methyl group in the single-electron hydrogen bond. The degree of nonadditivity of the methyl group is generally proportional to the number of methyl group in the radical. The shortening of the C···H distance and increase of the binding energy in the (CH3)2CH[BOND]H2O and (CH3)3C[BOND]H2O complexes are less two and three times as much as those in the CH3CH2[BOND]H2O complex, respectively. The result suggests that the nonadditivity among methyl groups is negative. Natural bond orbital (NBO) and atom in molecules (AIM) analyses also support such conclusions. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009