Blue-shift in the frequencies of the CH stretches of chloro- and fluoroform induced by C-H... pi hydrogen bonding with benzene derivatives: The influence of electron donating and withdrawing substituents

Mixed clusters of benzene derivatives and trihalogenmethanes have been prep ared in a supersonic beam and studied with infrared vibrational predissocia tion spectroscopy detected by resonant two photon ionization (IR/R2PI). We report the spectra of chloroform and fluoroform attached to the chromophore s fluorobenzene, benzene, toluene, p-xylene and their deuterated analogues. Instead of the typical H-bonding induced red-shift of the CH stretching fr equency a blue-shift is observed. From a comparison with ab initio calculat ions, we propose sandwich-like structures bonded by a C-H... jr hydrogen bo nd exhibiting an unusual partial charge transfer.