Search

We report full-dimensional and fully coupled quantum bound-state calculations of the J= 0, 1 intra- and intermolecular rovibrational states of the isotopically asymmetric HDO–CO complex. They are performed on the ab initionine-dimensional (9D) potential energy surface (PES) [Liu, Y.; Li, J.Phys. Chem. Chem. Phys.2019, 21, 24101]. The present study complements our earlier theoretical investigation of the 9D rovibrational level structure of the H2O–CO and D2O–CO complexes [Felker, P. M.; Bačić, Z.J. Chem. Phys.2020, 153, 074107]. What distinguishes HDO–CO is that, unlike the two isotopically symmetric isotopologues, it does not display hydrogen-interchange tunneling but has two distinct isomers, the lower-energy D-bonded HOD–CO and the higher-energy H-bonded DOH–CO. The highly efficient methodology employed in the present calculations derives from our earlier study referenced above, taking into account the lower symmetry of HDO–CO. The full 9D rovibrational Hamiltonian is partitioned into three reduced-dimension Hamiltonians: the 5D rigid-monomer intermolecular vibrational Hamiltonian and two intramolecular vibrational Hamiltonians, one for the HDO monomer (3D) and another for the CO monomer (1D), and a 9D remainder term. The reduced-dimension Hamiltonians are diagonalized separately, and small portions of their low-energy eigenstates are incorporated in the compact final 9D product contracted basis covering all internal, intra- and intermolecular degrees of freedom of the complex. The 9D rovibrational Hamiltonian is diagonalized in this fully contracted basis. The calculations show that the eigenstates belonging to the D-bonded and H-bonded isomers, designated as Dand H, respectively, are easy to identify, owing to the near-complete localization of their wave functions in either of the two minima on the PES. The computed intramolecular vibrational frequencies of the two monomers are either blue- or red-shifted, depending on the mode. The excitations of the intramolecular vibrational modes affect the energies of the low-lying Dand Hintermolecular vibrational states in the respective intramolecular manifolds. Comparison is made with the experimental data available in the literature.

The translation-rotation (TR) dynamics and spectroscopy of light molecules, e.g. H, HD, HF, and HO, inside nanoscale cavities such as those of fullerenes and in clathrate hydrates, is dominated by strong nuclear quantum effects (NQEs) to a degree that is without parallel among realistic molecular species. The NQEs include the large TR zero-point energy, quantisation of the translational centre-of-mass motions of the guest molecule, the coupling of various angular momenta in the system, and nuclear spin isomerism. They leave rich and intriguing fingerprints in the inelastic neutron scattering (INS) spectra arising from the transitions between the TR levels of the systems studied. Here we describe the major methodological advances made in the past decade, in both bound-state and scattering calculations that, when combined, have led to the novel and powerful approach for rigorous quantum simulations of the INS spectra a diatomic molecule, homo- and heteronuclear, inside a nanocavity of an arbitrary geometry. As illustrated by several demanding applications, these simulations have been indispensable, and very successful, for the assignment and interpretation of the measured INS spectra. Very surprisingly, this effort has also resulted in the completely unexpected, precedent-setting discovery of the INS selection rule for diatomic molecules in near-spherical nanocavities, overturning the widely accepted view that the INS has no selection rules.

We report full-dimensional and fully coupled quantum calculations of the inter- and intramolecular vibrational states of three isotopologues of the hydrogen chloride–water dimer: DCl–H2O (DH), HCl–D2O (HD), and DCl–D2O (DD). The present study extends our recent theoretical investigation of the nine-dimensional (9D) vibrational level structure of the HCl–H2O (HH) dimer [Liu, Y.; Li, J.; Felker, P. M.; Bačić, Z. Phys. Chem. Chem. Phys.2021,23,7101–7114]. It employs the same accurate 9D permutation invariant polynomial-neural network potential energy surface and the highly efficient bound-state methodology. The objective of this work is to elucidate the isotopologue variations of a range of bound-state properties of the hydrogen chloride–water dimer and compare them to those of the HH dimer. In order to achieve this, for the isotopologues considered, the rigorous 9D quantum calculations performed encompass all intramolecular vibrational fundamentals, and their frequency shifts relative to the isolated monomer values, together with the low-lying intermolecular vibrational states in each of the intramolecular vibrational manifolds of interest. Moreover, for the ground state of each isotopologue, several informative vibrationally averaged intermolecular geometric properties of the dimer are computed, as well as the three rotational constants. The energies of the intermolecular inversion and rock modes, which mainly involve the motions of the water moiety, differ greatly for H2O and D2O, but are much less sensitive to whether the hydrogen chloride isotopologue is HCl or DCl. On the other hand, the excitation of the HCl/DCl stretch changes significantly the energies of the water inversion and rock modes. The DCl stretch frequency shift computed in 9D for the DD dimer, −114.91 cm–1, agrees extremely well with the corresponding experimental value of −115.20 cm–1measured by Saykally and co-workers.