Quantum chemical study and infrared spectroscopy of hydrogen-bonded CHCl3–NH3 in the gas phase

Molecular association of chloroform with ammonia is studied by high-level quantum chemical calculations including correlated MP2 and CCSD(T) calculations with basis sets up to6-311++G(d,p) and counterpoise corrected energies, geometries, and frequencies. The calculations predict an eclipsed hydrogen-bonded complex of C(3v) symmetry (DeltaE(0)=-15.07 kJ mol(-1)) with 225.4 pm intermolecular CHcdots, three dots, centeredN distance. Intermolecular interactions are analysed by Kitaura-Morokuma [Int. J. Quantum Chem. 10, 325 (1976)] interaction energy decomposition. Compared to the monomer, the C-H bond is elongated, and the CH-stretching fundamental shifts to lower wave numbers and has a marked approximately 340-fold increase of its intensity. Based on these predictions, the complex is observed by infrared spectroscopy in the gas phase at room temperature. A subtraction procedure isolates its spectrum, and a dilution series confirms the presence of a 1:1 complex. The CHCl(3)cdots, three dots, centeredNH(3) complex has an experimental -17.5 cm(-1) shift of its CH-stretching vibration, and CDCl(3)cdots, three dots, centeredNH(3) a -12.5 cm(-1) shift of the CD-stretching vibration. After a deperturbation of the CH-stretching/bending mode Fermi resonance system, this indicates a "redshifting" or more appropriately, a "C-H elongating" hydrogen bond in agreement with the ab initio calculations. An estimate of the complex concentration gives the equilibrium constant K(p)=0.024 (p(theta)=10(5) Pa) at 295 K for the dimerization, providing one of the few examples where a hydrogen-bonded gas phase complex at room temperature could be quantitatively studied by infrared spectroscopy.