Structure of the Sulfuric Acid−Ammonia System and the Effect of Water Molecules in the Gas Phase

The proton-transfer reaction of sulfuric acid (H 2 SO 4 ) and ammonia (NH 3 ) and the effect of the first two water (H 2 O) molecules were studied by density functional theory and ab initio molecular orbital theory. The equilibrium structures, binding energies, and harmonic frequencies were calculated for each of the three clusters H 2 SO 4 −NH 3 −(H 2 O) n ( n = 0, 1, 2) using the hybrid density functional (B3LYP) and the second-order Møller−Plesset perturbation approximation (MP2) methods with the 6-311++G(d,p) basis set. Without water ( n = 0), the H 2 SO 4 −NH 3 system was determined to be only hydrogen bonded, with H 2 SO 4 acting as the hydrogen-bond donor and NH 3 as the acceptor. However, in the presence of one or two water molecules ( n = 1 or 2), the H 2 SO 4 −NH 3 unit exists only as the NH 4 + ·H 2 SO 4 - ion pair that results from a complete proton transfer from H 2 SO 4 to NH 3 . The analysis of selected equilibrium bond lengths, binding energies, and harmonic frequencies of the clusters provided strong support for the complete proton transfer in the presence of one or two water molecules. Atmospheric implications of the study are discussed.