Blue-Shifted Hydrogen Bonding in the Gas Phase CH/D₃CN···HCCl₃ Complexes

Blue-shifting H-bonded complexes between CHCl₃ and CH/D₃CN 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 CH₃CN and CD₃CN, the C-H stretching frequency of CHCl₃ shifts to the blue by +8.7 and +8.6 cm^-1, 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 CHCl₃ and CH/D₃CN 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 CH₃CN makes the CH₃CN 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 Cl₃CH···NCCH₃ complex have been compared with those of F₃CH···NCCH₃ and Cl₃CH···NH₃ complexes. The change in electron density of the CHCl₃ as a function of C-H···N distance shows that the approach of CH₃CN to CHCl₃ induces a shift in electron density from the H atom to the Cl atoms of CHCl₃ which leads to C-H bond contraction and blue shifting of C-H stretching frequency. However, in the complex Cl₃CH···NH₃, where frequency shift to the red is reported, charge transfer and electrostatic interaction dominate.