An insight into C-H ••• N hydrogen bond and stability of the complexes formed by trihalomethanes with ammonia and its monohalogenated derivatives

A theoretical study of the C-H center dot center dot center dot N hydrogen bond in the interactions of trihalomethanes CHX3 (X = F, Cl, Br) with ammonia and its halogen derivatives NH2Y (Y= F, Cl, Br) has been carried out thoroughly. The complexes are quite stable, and their stability increases in going from CHF3 to CHCl3 then to CHBr3 when Y keeps unchanged. With the same CHX3 proton donor, enhancement of the gas phase basicity of NH2Y strengthens stability of the CHX3 center dot center dot center dot NH2Y complex. The C-H center dot center dot center dot N hydrogen bond strength is directly proportional to the increase of proton affinity (PA) at N site of NH2Y and the decrease of deprotonation enthalpy (DPE) of C-H bond in CHX3. The CHF3 primarily appears to favor blue shift while the red-shift is referred to the CHBr3. The blueor red-shift of CHCl3 strongly depends on PA at N site of NH2Y. We suggest the ratio of DPE/PA as a factor to predict which type of hydrogen bond is observed upon complexation. The SAPT2+ results show that all C-H center dot center dot center dot N interactions in the complexes are electrostatically driven regardless of the type of hydrogen bond, between 48% and 61% of the total attractive energy, and partly contributed by both induction and dispersion energies.