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

Molecular orbital calculations on methane, acetylene, and HCN in electric f ields of various strengths have been performed at the HF/D95** level. The m olecules were oriented in the field so that one C-H bond was aligned with t he field in the direction appropriate for a stabilizing polarization of tha t bond. Although the C-H bonds of acetylene and HCN lengthen as the field i ncreases, that of methane shortens until the field reaches 0.02 au then len gthens as the field is further increased. Electron density analyses using t hree different methods (Mulliken populations, Natural Bond Orbitals, and At oms in Molecules) all show a shift of electron density from the putative H- bonding hydrogen toward the bulk of the molecule (although they disagree wi th 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 co ntrast to those of HCN and acetylene). At small electric fields, electron d ensity from the hydrogen moves into the C-H bond, both strengthening acid s hortening it. When the electric field increases beyond 0.02 au, net electro n density starts to move from the C-H bond toward the carbon causing the bo nd 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.