Structure and energetics of the weakly bound NH3 center dot center dot center dot H2O complex

Optimal structures, interaction energies, and harmonic vibrational frequenc ies of the NH3 ... H2O complex have been determined from the second-order M oller-Plesset perturbation theory and DFT/B3PW91 calculations with the aug- cc-pVTZ basis set. Some properties of the complex have been calculated at t he MP4 and CCSD(T) levels. The tunneling motion of the water molecule aroun d its c inertial axis was studied, and the barrier to the exchange of the b ound and free hydrogen atoms was determined as 1267 cm(-1). The nature of t he intermolecular interactions in the complex was investigated by symmetry- adapted perturbation theory (SAPT). As revealed by the SAPT analysis the ma in binding contributions are the electrostatic and induction components. Th e calculations of the vibrational frequencies and infrared intensities for this complex an presented to facilitate the frequency assignments of the ex perimental spectra.