Blue-Shifted Hydrogen Bonding in the Gas Phase CH/DCN···HCCl Complexes.

Blue-shifting H-bonded complexes between CHCl and CH/DCN have been identified by Fourier transform infrared spectroscopy in the gas phase at room temperature. The change in FTIR peak intensity of the mixture of the two components as a function of temperature and composition provides the basis for identification of the H-bonded band in the infrared spectrum. On complex formation with CHCN and CDCN, the C-H stretching frequency of CHCl shifts to the blue by +8.7 and +8.6 cm, respectively. The molecular electrostatic potential calculation at the MP2/6-311++G** level has been used to arrive at the geometry of the complex. It has been reported in the literature that CHCl and CH/DCN form red-shifting H-bonded complex in Ar matrix. The red shifting has been verified by doing ab initio calculations in the presence of Ar atoms, which has been attributed to the matrix effect at low temperature. The interaction of Ar with CHCN makes the CHCN more basic and as a result it becomes better hydrogen bond acceptor and causes red shift. The potential energy scans and NBO analysis of the ClCH···NCCH complex have been compared with those of FCH···NCCH and ClCH···NH complexes. The change in electron density of the CHCl as a function of C-H···N distance shows that the approach of CHCN to CHCl induces a shift in electron density from the H atom to the Cl atoms of CHCl which leads to C-H bond contraction and blue shifting of C-H stretching frequency. However, in the complex ClCH···NH, where frequency shift to the red is reported, charge transfer and electrostatic interaction dominate.