Blue shifted hydrogen bond in 3-methylindole·CHX3 complexes (X = Cl, F)

Abstract

Although the first experimental report on the blue shifted hydrogen bond in gas phase appeared about a decade ago, not many examples have been reported in the literature thereafter. Computational studies on systems exhibiting such blue shifted hydrogen bond however have been abundant since then. Many of these theoretical predictions remain to be verified experimentally. In this work we present an example of blue shifted hydrogen bond observed in the weakly bound complexes of 3-methylindole and CHX₃ (X = F, Cl). The complexes were prepared using the supersonic jet expansion method and studied using the laser spectroscopic methods. The key findings were that these complexes exhibit C–H⋯π type hydrogen bonding interaction and the CH is bound to the phenyl part of the aromatic plane. The CH stretch was found to be blue shifted by 2 and 16 cm⁻¹ in the case of CHCl₃ and CHF₃, respectively. Ab initio calculations along with atoms-in-molecule analysis and natural bond orbital analysis support the experimental findings. The computed results at the DFT/MP2 level also indicated that the IR intensity of the H-bond donor CH-stretch increases by two to three orders of magnitude for the CHCl₃ complex whereas for the fluoroform complex the same decreases by an order of magnitude, which are consistent with the trend reported in the case of C–H⋯O type of blue shifting hydrogen bonds.