Ab initio investigation on blue shift and red shift of C-H stretching vibrational frequency in NH3⋯CHnX4-n (n=1,3, X=F, Cl, Br, I) complexes

Ab initio quantum mechanics methods are employed to investigate CH⋯N intermolecular interactions between NH3 as the proton acceptor and CHnX4−n (n=1,3, X=F, Cl, Br, I) as the proton donor. For the NH3⋯CH3X complexes, C-H⋯N blue-shifting H-bonds are identified and the blue shifts are about 11-26 cm−1. For the NH3⋯CHX3 complexes, the stronger hydrogen bonds are found, but they are all classical H-bonds with red shifts of about 100-183 cm−1. The reason of the different shift directions in different halomethanes can be successfully explained by natural bond orbital analysis. The way in which the electron density is transferred for the NH3⋯CH3X complexes differs from that for the NH3⋯CHX3 complexes, which are responsible for the blue shifts in the NH3⋯CH3X complexes but the red shifts in the NH3⋯CHX3 complexes.