Rovibrational transitions of the methane–water dimer from intermolecular quantum dynamical computations

Abstract

Rovibrational quantum nuclear motion computations, with J = 0, 1, and 2, are reported for the intermolecular degrees of freedom of the methane-water dimer, where J is the quantum number describing the overall rotation of the complex. The computations provide the first explanation of the far-infrared spectrum of this complex published in J. Chem. Phys., 1994, 100, 863. All experimentally reported rovibrational transitions, up to J = 2, can be assigned to transitions between the theoretically computed levels. The deviation of the experimental and computed rovibrational transitions is 0.5 cm(-1) for the ortho and 2 cm(-1) for the para species with a variance of 0.005 cm(-1). In addition to a lower systematic error, the overall agreement of theory and experiment is also better for the ortho species (involving ortho-H2O). Most importantly, for this species all levels of the 24-fold tunneling splitting manifold corresponding to the zero-point vibration (ZPV) are involved in at least one experimentally reported transition. For the para species there are a few energy levels in the computed ZPV manifold that are not involved in the reported experimental transitions. Furthermore, computed energy levels are identified that correspond to the ZPV tunneling splitting manifold of the secondary minimum structure of the dimer, which presumably appear in rovibrational transitions in the same energy regime as the observed transitions, but have not been experimentally reported.

Bound and unbound rovibrational states of the methane-argon dimer

“…We carried out rovibrational computation on this intermolecular PES using the GENIUSH program [11,12]. GENIUSH stands for GENeral Internal-coordinate, USer-define Hamiltonians, and it has been successfully used for the computation of bound and resonance states of floppy molecular systems, including ArNO + [13], HeH + 2 [14], NH 3 [11,12], H + 5 [15], CH + 5 [16], CH 4 ·H 2 O [4,5], with various internal coordinate and frame definitions. An interesting feature, the existence of formally negative-energy rotational excitations [4,5,15,[17][18][19] had been observed in a growing number of floppy systems and led to the introduction of the notion astructural molecules in Ref.…”

“…The infrared spectrum was reported first by McKellar et al in 1994 [9], then Pak et al measured the high-resolution spectra of CH 4 ·Ar and CH 4 ·Kr complexes in the 7 µm region. Later on, further high-resolution measurements and ab initio computations were performed for the CH 4 ·Ar system [2,10].…”

Rovibrational quantum dynamical computations for deuterated isotopologues of the methane–water dimer

“…Hereby we only shortly summarize the computational parameters and explain any difference compared to our earlier work [3]. In the present study we computed the lowest-energy few dozens rovibrational eigenstates (150 states for J = 2) for the four isotopologues of the methane-water dimer with J = 0, 1, and 2 rotational angular momentum quantum numbers utilizing the QCHB15 PES [23] [37].…”

“…The same DVRs were used for the active degrees of freedom as in Ref. [3], which are listed in Table II for the sake of completeness. The global minimum (GM) structure of the QCHB15 PES is also given in Table II For the short and unambiguous referencing of the various isotopologues and their energy levels, we introduce the notation CH 4 · H 2 O (HH), CH 4 · D 2 O (HD), CD 4 · H 2 O (DH), and CD 4 · D 2 O (DD).…”

“…[29]; characterization of the nodal structure ("node counting"), see, e.g., Refs. [3,6,21]; computation of expectation values of structural parameters, see, e.g., Ref. [3]; or determination of the similarity (overlap) of the full wave function with wave functions of simple model systems as advocated, e.g., in Ref.…”