Comparative studies of blue shifting and red shifting effects in fluoroform and acetylene cryogenic solutions

Abstract

The FTIR spectra of CF 3H and C 2H 2 have been studied in liquefied Ar, N 2, and CO. The frequency blue shift of the C–H stretching vibration accompanied by a decrease in the integrated intensity of the corresponding ν 1 band was found for CF 3H, whereas in the case of C 2H 2 the usual red shift and increasing integrated intensity of the C–H stretching ν 3 band was monitored in the studied series of solvents. Stronger CH⋯BY-type interactions (BY=CD 3CN, (CD 3) 2CO, C 5D 5N, (CD 3) 3N) show first increasing frequency blue shift and then result in the gradual transfer to the usual spectroscopic effect for fluoroform. The self-consistent reaction field approach (SCRF) in the frame of gaussian 98 program could be able to reproduce qualitatively the blue shift and decreasing intensity effects observed on the ν 1 band of CF 3H as well as the conventional red shift and increasing intensity effects only at non-realistically small value of the Onsager radius a 0<2 Å. Ab initio MP2 and density functional theory (DFT)/B3LYP calculations predict weak to medium CH⋯BY complex formation in the case of both CF 3H and C 2H 2 molecules, and suggest the crucial role of the features of the dipole moment function and the intermolecular potential in the observed unconventional effects. The substantial increasing induced dipole moment, accompanied by the predomination of attractive over repulsion forces between H atom of CH group and BY subunit predicted for stronger F 3CH⋯BY interactions result in the transfer from the blue shift and decreasing intensity to conventional spectroscopic effects in accordance with experimental observations.