Medium-Dependent Crossover from the Red to Blue Shift of the Donor’s Stretching Fundamental in the Binary Hydrogen-Bonded Complexes of CDCl3 with Ethers and Ketones

Abstract

Mid-infrared spectra for C–D···O hydrogen (H)-bonded binary complexes of CDCl3 with acetone (AC), cyclohexanone (CHN), diethyl ether (DEE), and tetrahydrofuran (THF) have been measured in the vapor phase at room temperature and in an argon matrix at 8 K. Remarkable matrix effect has been observed in each case with respect to the spectral shift of the donor group’s stretching fundamental (ΔνC–D). In the case of complexes with AC and CHN, the sign of ΔνC–D changes from a few wavenumbers positive (blue shift) in the vapor phase to a few tens of wavenumbers negative (red shift) in the argon matrix. For the two ether complexes, although no apparent reversal in the sign of ΔνC–D occurs, but the magnitudes of the red shifts in the matrix are manifold larger, and the bands appear with large enhancement in transition intensity. The medium effect has been explained consistently in terms of the local hyperconjugative charge transfer interaction at the H-bonding sites of the complexes and its interplay with the H-bond distance that varies with the physical conditions of the medium. Under the matrix isolation condition, νC–D bands of CHN and THF complexes depict a large number of substructures, which has been interpreted in terms of matrix site effect as well as Fermi resonance enhancement of the fingerprint combination tones and trapping of more than one isomer of the complexes in the matrix sites.