Intermolecular potential and rovibrational states of the H2O–D2 complex

Graphical abstract: H2O–D2 potential surface and pH2O–oD2 ground state wave function, for planar geometries. Highlights: ► The interaction between H2O and H2 is of great astrophysical interest. ► The rovibrational states of H2O–D2 were computed on an ab initio potential surface. ► Results are compared with the rovibrational states of H2O–H2 computed recently. ► We measured the high-resolution infrared spectrum of H2O–D2 in the H2O bend region. ► Comparison with the calculations provides information on H2O–H2 potential surface. - Abstract: A five-dimensional intermolecular potential for H2O–D2 was obtained from the full nine-dimensional ab initio potential surface of Valiron et al. [P. Valiron, M. Wernli, A. Faure, L. Wiesenfeld, C. Rist, S. Kedžuch, J. Noga, J. Chem. Phys. 129 (2008) 134306] by averaging over the ground state vibrational wave functions of H2O and D2. On this five-dimensional potential with a well depth De of 232.12 cm−1 we calculated the bound rovibrational levels of H2O–D2 for total angular momentum J = 0–3. The method used to compute the rovibrational levels is similar to a scattering approach—it involves a basis of coupled free rotor wave functions for the hindered internal rotations and the overall rotation of the dimer—while it uses a discrete variable representation of the intermolecular distance coordinate R. The basis was adapted to the permutation symmetry associated with the para/ortho (p/o) nature of both H2O and D2, as well as to inversion symmetry. As expected, the H2O–D2 dimer is more strongly bound than its H2O–H2 isotopologue [cf. A. van der Avoird, D.J. Nesbitt, J. Chem. Phys. 134 (2011) 044314], with dissociation energies D0 of 46.10, 50.59, 67.43, and 73.53 cm−1 for pH2O–oD2, oH2O–oD2, pH2O–pD2, and oH2O–pD2. A rotationally resolved infrared spectrum of H2O–D2 was measured in the frequency region of the H2O bend mode. The ab initio calculated values of the rotational and distortion constants agree well with the values extracted from this spectrum.