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The methodological advances made in recent years have significantly extended the range and dimensionality of noncovalently bound, hydrogen-bonded and van der Waals, molecular complexes for which full-dimensional and fully coupled quantum calculations of their rovibrational states are feasible. They exploit the unexpected implication that the weak coupling between the inter- and intramolecular rovibrational degrees of freedom (DOFs) of the complexes has for the ease of computing the high-energy eigenstates of the latter. This is done very effectively by using contracted eigenstate bases to cover both intra- and intermolecular DOFs. As a result, it is now possible to calculate rigorously all intramolecular rovibrational fundamentals, together with the low-lying intermolecular rovibrational states, of complexes involving two small molecules beyond diatomics, binary polyatomic molecule-large (rigid) molecule complexes, and endohedral complexes of light polyatomic molecules confined inside (rigid) fullerene cages. In this Perspective these advances are reviewed in considerable depth. The progress made thanks to them is illustrated by a number of representative applications. Whenever possible, direct comparison is made with the available infrared, far-infrared, and microwave spectroscopic data. [ABSTRACT FROM AUTHOR]

The interaction between HCl and H2O is of considerable theoretical and experimental interest due to its important role in atmospheric chemistry and understanding the onset of the dissociation of HCl in water. In this work, the HCl–H2O complex is quantitatively characterized in two ways. First, we report a new full-dimensional potential energy surface (PES) for the HCl + H2O system. The nine-dimensional (9D) PES is based on circa 43 000 ab initio points calculated at the level of CCSD(T)-F12a/AVTZ with the basis set superposition error correction using the permutation invariant polynomial-neural network method, which can accurately and efficiently reproduce the geometries, energies, frequencies of the complex of HCl with H2O, as well as the relevant minimum energy path. Next, we present the results of the first fully coupled 9D quantum calculations of the intra- and intermolecular vibrational states of the HCl–H2O dimer, performed on the new PES. They employ the highly efficient bound-state methodology previously used to compute accurately the rovibrational level structure of the H2O/D2O–CO and HDO–CO complexes [P. M. Felker and Z. Bačić, J. Chem. Phys., 2020, 153, 074107; J. Phys. Chem. A, 2021, 125, 980]. The 9D calculations characterize the vibrationally averaged nonplanar ground-state geometry of the HCl–H2O complex, the intramolecular vibrational fundamentals of both H2O and HCl moieties, and their frequency shifts, as well as the low-energy intermolecular vibrational states in each of the intramolecular vibrational manifolds and the effects of the coupling between the two sets of modes. The calculated properties of the HCl–H2O dimer are in excellent agreement with the available spectroscopic data. The 9D computed dimer binding energy D0 of 1334.63 cm−1 agrees extremely well with the experimental D0 equal to 1334 ± 10 cm−1 [B. E. Casterline and A. K. Mollner and L. C. Ch'ng and H. Reisler, J. Chem. Phys., 2010, 114, 9774]. Moreover, the ground-state expectation value of the out-of-plane bend angle of H2O, 33.80°, and the computed HCl stretch frequency shift, −157.9 cm−1, both from the 9D calculations, are in very good accord with the corresponding experimental values. [ABSTRACT FROM AUTHOR]

Splittings of the translation–rotation (TR) eigenstates of the solid light-molecule endofullerenes M@C60 (M = H2, H2O, HF) attributed to the symmetry breaking have been observed in the infrared (IR) and inelastic neutron scattering spectra of these species in the past couple of years. In a recent paper [Felker et al., Phys. Chem. Chem. Phys., 2017, 19, 31274], we established that the electrostatic, quadrupolar interaction between the guest molecule M and the twelve nearest-neighbor C60 cages of the solid is the main source of the symmetry breaking. The splittings of the three-fold degenerate ground states of the endohedral ortho-H2, ortho-H2O and the j = 1 level of HF calculated using this model were found to be in excellent agreement with the experimental results. Utilizing the same electrostatic model, this theoretical study investigates the effects of the symmetry breaking on the excited TR eigenstates of the three species, and how they manifest in their simulated low-temperature (5–6 K) near-IR (NIR) and far-IR (FIR) spectra. The TR eigenstates are calculated variationally for both the major P and minor H crystal orientations. For the H orientation, the calculated splittings of all of the TR levels of these species are less than 0.1 cm−1. For the dominant P orientation, the splittings vary strongly depending on the character of the excitations involved. In all of the species, the splittings of the higher rotationally excited levels are comparable in magnitude to those for the j = 1 levels. For the levels corresponding to purely translational excitations, the calculated splittings are about an order of magnitude smaller than those of the purely rotational eigenstates. Based on the computed TR eigenstates, the low-temperature NIR (for M = H2) and FIR (for M = HF and H2O) spectra are simulated for both the P and H orientations, and also combined as their weighted sum (0.15H + 0.85P). The weighted sum spectra computed for M = H2 and HF match quantitatively the corresponding measured spectra, while for M = H2O, the weighted sum FIR spectrum predicts features that can potentially be observed experimentally. [ABSTRACT FROM AUTHOR]

Symmetry breaking has been recently observed in the endofullerenes M@C60 (M = H2, HF, H2O), manifesting in the splittings of the three-fold degenerate ground states of the endohedral ortho-H2, ortho-H2O and the j = 1 level of HF. The nature of the interaction causing the symmetry breaking is established in this study. A fragment of the solid C60 is considered, comprised of the central C60 molecule surrounded by twelve nearest-neighbor (NN) C60 molecules. The fullerenes have either P (major) or H (minor) orientational orderings, and are assumed to be rigid with Ih symmetry. Only the central C60 is occupied by the guest molecule M, while the NN fullerenes are all empty. The key proposition of the study is that the electrostatic interactions between the charge densities on the NN C60 molecules and that on M inside the central C60 give rise to the symmetry breaking responsible for the measured level splittings. Using this model, the M@C60 level splittings of interest are calculated variationally and using perturbation theory, for both the P and H orientations. Those obtained for the dominant P orientation are in excellent agreement with the experimental results, with respect to the splitting magnitudes and patterns, for all three M@C60 systems considered, pointing strongly to the quadrupolar M–NN interactions as the main cause of the symmetry breaking. The level splittings calculated for the H orientation are about 30 times smaller than the ones in the P orientation. [ABSTRACT FROM AUTHOR]

The interaction between HCl and H 2 O is of considerable theoretical and experimental interest due to its important role in atmospheric chemistry and understanding the onset of the dissociation of HCl in water. In this work, the HCl-H 2 O complex is quantitatively characterized in two ways. First, we report a new full-dimensional potential energy surface (PES) for the HCl + H 2 O system. The nine-dimensional (9D) PES is based on circa 43 000 ab initio points calculated at the level of CCSD(T)-F12a/AVTZ with the basis set superposition error correction using the permutation invariant polynomial-neural network method, which can accurately and efficiently reproduce the geometries, energies, frequencies of the complex of HCl with H 2 O, as well as the relevant minimum energy path. Next, we present the results of the first fully coupled 9D quantum calculations of the intra- and intermolecular vibrational states of the HCl-H 2 O dimer, performed on the new PES. They employ the highly efficient bound-state methodology previously used to compute accurately the rovibrational level structure of the H 2 O/D 2 O-CO and HDO-CO complexes [P. M. Felker and Z. Bačić, J. Chem. Phys., 2020, 153, 074107; J. Phys. Chem. A, 2021, 125, 980]. The 9D calculations characterize the vibrationally averaged nonplanar ground-state geometry of the HCl-H 2 O complex, the intramolecular vibrational fundamentals of both H 2 O and HCl moieties, and their frequency shifts, as well as the low-energy intermolecular vibrational states in each of the intramolecular vibrational manifolds and the effects of the coupling between the two sets of modes. The calculated properties of the HCl-H 2 O dimer are in excellent agreement with the available spectroscopic data. The 9D computed dimer binding energy D 0 of 1334.63 cm -1 agrees extremely well with the experimental D 0 equal to 1334 ± 10 cm -1 [B. E. Casterline and A. K. Mollner and L. C. Ch'ng and H. Reisler, J. Chem. Phys., 2010, 114, 9774]. Moreover, the ground-state expectation value of the out-of-plane bend angle of H 2 O, 33.80°, and the computed HCl stretch frequency shift, -157.9 cm -1 , both from the 9D calculations, are in very good accord with the corresponding experimental values.

Symmetry breaking has been recently observed in the endofullerenes M@C 60 (M = H 2 , HF, H 2 O), manifesting in the splittings of the three-fold degenerate ground states of the endohedral ortho-H 2 , ortho-H 2 O and the j = 1 level of HF. The nature of the interaction causing the symmetry breaking is established in this study. A fragment of the solid C 60 is considered, comprised of the central C 60 molecule surrounded by twelve nearest-neighbor (NN) C 60 molecules. The fullerenes have either P (major) or H (minor) orientational orderings, and are assumed to be rigid with I h symmetry. Only the central C 60 is occupied by the guest molecule M, while the NN fullerenes are all empty. The key proposition of the study is that the electrostatic interactions between the charge densities on the NN C 60 molecules and that on M inside the central C 60 give rise to the symmetry breaking responsible for the measured level splittings. Using this model, the M@C 60 level splittings of interest are calculated variationally and using perturbation theory, for both the P and H orientations. Those obtained for the dominant P orientation are in excellent agreement with the experimental results, with respect to the splitting magnitudes and patterns, for all three M@C 60 systems considered, pointing strongly to the quadrupolar M-NN interactions as the main cause of the symmetry breaking. The level splittings calculated for the H orientation are about 30 times smaller than the ones in the P orientation.