Blue- and Red-Shifting Hydrogen Bonding: A Gas Phase FTIR and Ab Initio Study of RR′CO···DCCl3 and RR′S···DCCl3 Complexes

Blue-shifting H-bonded (C–D···O) complexes between CDCl3 and CH3HCO, (CH3)2CO, and C2H5(CH3)CO, and red-shifting H-bonded (C–D···S) complexes between CDCl3 with (CH3)2S and (C2H5)2S have been identified by Fourier transform infrared spectroscopy in the gas phase at room temperature. With increasing partial pressure of the components, a new band appears in the C–D stretching region of the vibrational spectra. The intensity of this band decreases with an increase in temperature at constant pressure, which provides the basis for identification of the H-bonded bands in the spectrum. The C–D stretching frequency of CDCl3 is blue-shifted by +7.1, +4, and +3.2 cm–1 upon complexation with CH3HCO, (CH3)2CO, and C2H5(CH3)CO, respectively, and red-shifted by −14 and −19.2 cm–1 upon complexation with (CH3)2S and (C2H5)2S, respectively. By using quantum chemical calculations at the MP2/6-311++G** level, we predict the geometry, electronic structural parameters, binding energy, and spectral shift of H-bonded complexes between CDCl3 and two series of compounds named RCOR′ (H2CO, CH3HCO, (CH3)2CO, and C2H5(CH3)CO) and RSR′ (H2S, CH3HS, (CH3)2S, and (C2H5)2S) series. The calculated and observed spectral shifts follow the same trends. With an increase in basicity of the H-bond acceptor, the C–D bond length increases, force constant decreases, and the frequency shifts to the red from the blue. The potential energy scans of the above complexes are done, which show that electrostatic attraction between electropositive D and electron-rich O/S causes bond elongation and red shift, and the electronic and nuclear repulsions lead to bond contraction and blue shifts. The dominance of the two opposing forces at the equilibrium geometry of the complex determines the nature of the shift, which changes both in magnitude and in direction with the basicity of the hydrogen-bond acceptor.