Quantum-chemical study and FTIR jet spectroscopy of CHCl3–NH3 association in the gas phase

High level ab initio quantum chemical calculations have been performed on the association of chloroform with ammonia in the gas phase (counterpoise corrected MP2 and coupled-cluster CCSD(T) calculations with 6-311++G(d,p) basis functions). Minimum energy equilibrium structures have been found for CHCl3–NH3 dimer, CHCl3–(NH3)2 trimer and CHCl3–(NH3)3 tetramer. Association is characterised by a CH⋯N hydrogen bond between a chloroform and an ammonia molecule, with further ammonia units attached by hydrogen bonds to ammonia and by an electrostatic NH⋯Cl interaction to chloroform. Cooperative effects provide additional stabilisation. The complexes exhibit characteristic shifts of vibrational bands and change of IR intensity; in particular there is a pronounced progressive shift of the CH-stretching vibration towards lower wavenumber with each unit of ammonia attached in the complex. The shift is accompanied by an up to 600 fold increase in IR intensity. The experimental FTIR jet spectra have provided firm evidence of CHCl3–NH3 association, with the clearest effects seen in the region of the CH-stretching vibration. First tentative assignments have been made based on the dependence of relative intensities of cluster features on the concentration of monomers, and assignments have been corroborated by the quantum chemical calculations. The present combined ab initio and FTIR spectroscopy study reveals the structure and energetics of cluster formation and intermolecular bonding in CHCl3–NH3 association.