N-H...F Improper Blue-Shifting H-Bond

摘要

The MP2/6-31G~(**) calculations reveal a different pattern of H-bonding in F_2NH...Y (Y = FH, OH_2, NH_2) complexes. Whereas the blue-shifted H-bonding was found for F_2NH...FH complex (the first known case of N-H...Y improper blue-shifting H-bonding), standard H-bonding exists for F_2NH...OH_2 and F_2NH...NH_3 complexes. A different character of H-bonding is explained by natural bond orbital analysis showing a systematic increase of electron density in the sigma~* antibonding orbital of the N-H bond as well as in the lone-electron pairs of fluorine atoms of proton the acceptor. Whereas the former effect leads to elongation of the N-H bond and a red shift of N-H stretch frequency, the latter effect gives the opposite, that is, contraction of the N-H bond and a blue shift of N-H stretch frequency. Existence of a red- or blue-shifted H-bonding is explained by the dominance of the former or latter effect.