Diazene, a Potential New Hydrogen Bond Donor and Acceptor with Blue-Shifted N-H Stretching Modes: A Theoretical Study

The structure and vibrational spectra of hydrogen-bonded complexes formed between trans-HNNH (diazene or diimide) and the fluoromethanes CH3F, CH2F2, and CHF3 are studied at the MP2/6-311++G(2d,2p) level. For each fluoromethane, complex structures were detected in which diazene plays either the role of a hydrogen bond donor or a hydrogen bond acceptor. In addition to the well-known blue shifts of the fluoromethane C-H stretching frequencies, blue-shifted N-H stretching frequencies are predicted for the diazene molecule, irrespective of whether HNNH acts as donor or acceptor. It is shown that this finding can be interpreted as a consequence of the negative intramolecular coupling (NIC) between N-N and N-H stretching degrees of freedom in the isolated HNNH molecule which, upon complex formation, leads to a negative intramolecular response (NIR).