Ab initio study of (NH3)2: Accurate structure and energetics

Abstract

The equilibrium and saddle point structures of the ammonia dimer were investigated employing a series of correlation-consistent basis sets, cc-pVXZ and aug-cc-pVXZ(X=D,T,Q) at the Hartree–Fock (HF), second order and fourth order Møller–Plesset perturbation theory (MP2, MP4) levels. It is shown that the energies and geometries of the stationary structures are sensitive to the basis set and electron correlation, and the proper combination of electron correlation treatment (MP2 or better) and basis set requirement (aug-cc-pVTZ or better) is necessary to determine the equilibrium structure and ordering of the stationary structures. The equilibrium structure of the ammonia dimer is found to be an asymmetric cyclic (eclipsed) structure with Cs symmetry. Although this is generally consistent with the equilibrium structure predicted from the model potential by Olthof et al. [J. Chem. Phys. 101, 8430 (1994)], it appears to be more shifted toward the linear structure than the centrosymmetric cyclic structure, which signifies a certain degree of hydrogen bonding in this dimer. The MP2 barrier height to proton donor–acceptor interchange at MP2/aug-cc-pVQZ//MP2/aug-cc-pVTZ is 7.6 cm−1, which is in excellent agreement with the value predicted from the model potential by Olthof et al. While the barrier height to rotation of one monomer (hydrogen-bond acceptor) about its C3 axis is relatively low, ∼20 cm−1, the corresponding value of the other monomer (hydrogen-bond donor) is shown to be rather high, probably more than 450 cm−1 at the equilibrium structure. The binding energies of the ammonia dimer are computed with and without counterpoise correction for basis set superposition error. The MP2 complete basis set (CBS) limit electronic binding energy of (NH3)2 is estimated to be within 13.2±0.3 kJ/mol.