N‐H· · ·F improper blue‐shifting H‐bond

The MP2/6‐31G** calculations reveal a different pattern of H‐bonding in F2NH· · ·Y (Y = FH, OH2, NH2) complexes. Whereas the blue‐shifted H‐bonding was found for F2NH· · ·FH complex (the first known case of NH· · ·Y improper blue‐shifting H‐bonding), standard H‐bonding exists for F2NH· · ·OH2and F2NH· · ·NH3complexes. A different character of H‐bonding is explained by natural bond orbital analysis showing a systematic increase of electron density in the σ* 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.