C-H bond-shortening upon hydrogen bond formation: Influence of an electric field

Molecular orbital calculations on methane, acetylene, and HCN in electric fields of various strengths have been performed at the HF/D95** level. The molecules were oriented in the field so that one C-H bond was aligned with the field in the direction appropriate for a stabilizing polarization of that bond. Although the C-H bonds of acetylene and HCN lengthen as the field increases, that of methane shortens until the field reaches 0.02 au then lengthens as the field is further increased. Electron density analyses using three different methods (Mulliken populations, Natural Bond Orbitals, and Atoms in Molecules) all show a shift of electron density from the putative H-bonding hydrogen toward the bulk of the molecule (although they disagree with each other in several other ways). We interpret the data to suggest that the hydrogen in methane is electron rich with respect to the carbon tin contrast to those of HCN and acetylene). At small electric fields, electron density from the hydrogen moves into the C-H bond, both strengthening acid shortening it. When the electric field increases beyond 0.02 au, net electron density starts to move from the C-H bond toward the carbon causing the bond to begin to weaken and lengthen. The C-H bonds of HCN and acetylene both lengthen as the field is increased. The behavior of all three molecules in the fields is sufficient to explain their H-bonding behavior.