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

Although the. rst 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 veri. ed 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. ndings were that these complexes exhibit C -H center dot center dot center dot p 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. ndings. 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. uoroform complex the same decreases by an order of magnitude, which are consistent with the trend reported in the case of C -H center dot center dot center dot O type of blue shifting hydrogen bonds.