Computational study of the rovibrational spectrum of (OCS)2

Computing excited OH stretch states of water dimer in 12D using contracted intermolecular and intramolecular basis functions

Due to the ubiquity and importance of water, water dimer has been intensively studied. Computing the (ro-)vibrational spectrum of water dimer is challenging. The potential has eight wells separated by low barriers, which makes harmonic approximations of limited utility. A variational approach is imperative, but difficult because there are 12 coupled vibrational coordinates. In this paper, we use a product contracted basis whose functions are products of intramolecular and intermolecular functions computed using an iterative eigensolver. An intermediate matrix F facilitates calculating matrix elements. Using F, it is possible to do calculations on a general potential without storing the potential on the full quadrature grid. We find that surprisingly many intermolecular functions are required. This is due to the importance of coupling between inter- and intra-molecular coordinates. The full G₁₆ symmetry of water dimer is exploited. We calculate, for the first time, monomer excited stretch states and compare P(1) transition frequencies with their experimental counterparts. We also compare with experimental vibrational shifts and tunneling splittings. Surprisingly, we find that the largest tunneling splitting, which does not involve the interchange of the two monomers, is smaller in the asymmetric stretch excited state than in the ground state. Differences between levels we compute and those obtained with a [6+6]D adiabatic approximation [Leforestier et al. J. Chem. Phys. 137 014305 (2012)] are ∼0.6 cm^-1 for states without monomer excitation, ∼4 cm^-1 for monomer excited bend states, and as large as ∼10 cm^-1 for monomer excited stretch states.

Computing vibrational energy levels by solving linear equations using a tensor method with an imposed rank

Present day computers do not have enough memory to store the high-dimensional tensors required when using a direct product basis to compute vibrational energy levels of a polyatomic molecule with more than about five atoms. One way to deal with this problem is to represent tensors using a tensor format. In this paper, we use the canonical polyadic (CP) format. Energy levels are computed by building a basis from vectors obtained by solving linear equations. The method can be thought of as a CP realization of a block inverse iteration method with multiple shifts. The CP rank of the tensors is fixed, and the linear equations are solved with an method. There is no need for rank reduction and no need for orthogonalization, and tensors with a rank larger than the fixed rank used to solve the linear equations are never generated. The ideas are tested by computing vibrational energy levels of a 64-D bilinearly coupled model Hamiltonian and of acetonitrile (12-D).

Fully quantum calculations of O2-N2 scattering using a new potential energy surface: Collisional perturbations of the oxygen 118 GHz fine structure line

A proper description of the collisional perturbation of the shapes of molecular resonances is important for remote spectroscopic studies of the terrestrial atmosphere. Of particular relevance are the collisions between the O₂ and N₂ molecules-the two most abundant atmospheric species. In this work, we report a new highly accurate O₂(X³Σ_g ^-)-N₂(X¹Σ_g ⁺) potential energy surface and use it for performing the first quantum scattering calculations addressing line shapes for this system. We use it to model the shape of the 118 GHz fine structure line in O₂ perturbed by collisions with N₂ molecules, a benchmark system for testing our methodology in the case of an active molecule in a spin triplet state. The calculated collisional broadening of the line agrees well with the available experimental data over a wide temperature range relevant for the terrestrial atmosphere. This work constitutes a step toward populating the spectroscopic databases with ab initio line shape parameters for atmospherically relevant systems.

Spectroscopy and Scattering Studies Using Interpolated Ab Initio Potentials

The Born-Oppenheimer potential energy surface (PES) has come a long way since its introduction in the 1920s, both conceptually and in predictive power for practical applications. Nevertheless, nearly 100 years later-despite astonishing advances in computational power-the state-of-the-art first-principles prediction of observables related to spectroscopy and scattering dynamics is surprisingly limited. For example, the water dimer, (H₂O)₂, with only six nuclei and 20 electrons, still presents a formidable challenge for full-dimensional variational calculations of bound states and is considered out of reach for rigorous scattering calculations. The extremely poor scaling of the most rigorous quantum methods is fundamental; however, recent progress in development of approximate methodologies has opened the door to fairly routine high-quality predictions, unthinkable 20 years ago. In this review, in relation to the workflow of spectroscopy and/or scattering studies, we summarize progress and challenges in the component areas of electronic structure calculations, PES fitting, and quantum dynamical calculations.

Collisional excitation of interstellar PN by H2: New interaction potential and scattering calculations

Rotational excitation of interstellar PN molecules induced by collisions with H₂ is investigated. We present the first ab initio four-dimensional potential energy surface (PES) for the PN-H₂ van der Waals system. The PES was obtained using an explicitly correlated coupled cluster approach with single, double, and perturbative triple excitations [CCSD(T)-F12b]. The method of interpolating moving least squares was used to construct an analytical PES from these data. The equilibrium structure of the complex was found to be linear, with H₂ aligned at the N end of the PN molecule, at an intermolecular separation of 4.2 Å. The corresponding well-depth is 224.3 cm^-1. The dissociation energies were found to be 40.19 cm^-1 and 75.05 cm^-1 for complexes of PN with ortho-H₂ and para-H₂, respectively. Integral cross sections for rotational excitation in PN-H₂ collisions were calculated using the new PES and were found to be strongly dependent on the rotational level of the H₂ molecule. These new collisional data will be crucial to improve the estimation of PN abundance in the interstellar medium from observational spectra.

Computational study of the rovibrational spectrum of CO2-N2

The CO2-N2 complex is formed from two key components of Earth's atmosphere, and as such, has received some attention from both experimental and theoretical studies. On the theory side, a potential energy surface (PES) based on high level ab initio data was reported [Nasri et al., J. Chem. Phys., 2015, 142, 174301] and then used in more recently reported rovibrational calculations [Lara-Moreno et al., Phys. Chem. Chem. Phys., 2019, 21, 3550]. Accuracy of about 1 percent was achieved for calculated rotational transitions of the ground vibrational state of the complex, compared with previously reported microwave spectra. However, a very recent measurement of the geared bending mode frequency [Barclay et al., J. Chem. Phys., 2020, 153, 014303] recorded a value of 21.4 cm-1, which is wildly different from the corresponding calculated value of 45.9 cm-1. To provide some insight into this discrepancy, we have constructed a new more accurate PES, and used it to perform highly converged variational rovibrational calculations. Our new results yield a value of 21.1 cm-1 for that bending frequency, in close agreement with the experiment. We also obtain significantly improved predicted rotational transitions. Finally, we note that a very shallow well, previously reported as a distinct second isomer, is not found on our new PES, but rather a transition structure is seen in that location.

Observation of the elusive "oxygen-in" OCS dimer

The carbonyl sulfide (OCS) dimer serves as a prototype system for studying intermolecular forces between nonsymmetrical linear polyatomic molecules. Here, we performed a laser spectroscopic investigation of OCS dimers embedded in helium nanodroplets and found rovibrational bands corresponding to the non-polar "sulfur-in" and parallel polar dimers that have been extensively characterized in the gas phase, as well as a new non-polar "oxygen-in" dimer that has long been predicted by theory. Frequency alternations in the rotational branches along with the absence of a Stark effect provided important clues as to its assignment.

Theoretical investigation of potential energy surface and bound states for the N2-OCS van der Waals complex

In this work we report an ab initio intermolecular potential energy surface and theoretically spectroscopic studies for N₂-OCS complex. A four-dimensional intermolecular potential energy surface (4D PES) is constructed at the level of single and double excitation coupled-cluster method with a non-iterative perturbation treatment of triple excitations [CCSD(T)] with aug-cc-pVTZ basis set supplemented with bond functions. A global minimum corresponding to a planar and nearly T-shaped structure, which has been observed experimentally, is located at R = 3.96 Å, θ₁ = 8 or 172°, θ₂ = 75°, φ = 180 or 0° with a well depth of 271.078 cm^-1 on the potential energy surface. The local minimum corresponding to a linear geometry is also found at R = 5.06 Å, θ₁ = 2 or 178°, θ₂ = 179°, φ = 0 or 180° with a well depth of 224.743 cm^-1. The bound state calculations have been performed for the complex by approximating the N₂ and OCS molecules as the rigid rotors. The calculated structural parameters and rotational transition frequencies are in good agreement with the experimental observed values. Based on the ab initio PES, the tunneling splitting is calculated to be 0.052 cm^-1 for the ground vibrational state, which can just reproduce 64% of experimental observation (0.082 cm^-1). A refined method is used to calculate the tunneling splitting, and a much better result is obtained with the value of 0.094 cm^-1.

Computational Study of the Rovibrational Spectra of CH2D+ and CHD2. J Phys Chem A 2019;123:10281-10289. [PMID: 31657568 DOI: 10.1021/acs.jpca.9b09045] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

In this paper, we present rovibrational energy levels of CH₂D⁺ and CHD₂⁺. They are computed with a large basis and the Lanczos algorithm. CH₂D⁺ and CHD₂⁺ are believed to play an important role in interstellar space, but so far, there are no definitive observations. The predictions of this paper should facilitate detection. For CH₂D⁺, two CH stretch bands have been studied at high resolution. Compared to our calculated energies, the root-mean-square error is 0.08 cm^-1. For CHD₂⁺, one CH stretch band has been studied at high resolution. Compared to our calculated energies, the root-mean-square error is 0.5 cm^-1. Errors are larger, for both isotopologues, for bend states. We attribute these errors to the potential energy surface. Wave function and probability distribution plots are used to make assignments. The ν₁ band of CHD₂⁺ is significantly perturbed, and according to our calculations, the 3ν₃ state is closest and might be the most important perturber.

Computational study of the ro-vibrational spectrum of CO-CO2

An accurate ab initio ground-state intermolecular potential energy surface (PES) was determined for the CO-CO₂ van der Waals dimer. The Lanczos algorithm was used to compute rovibrational energies on this PES. For both the C-in and O-in T-shaped isomers, the fundamental transition frequencies agree well with previous experimental results. We confirm that the in-plane states previously observed are geared states. In addition, we have computed and assigned many other vibrational states. The rotational constants we determine from J = 1 energy levels agree well with their experimental counterparts. Planar and out-of-plane cuts of some of the wavefunctions we compute are quite different, indicating strong coupling between the bend and torsional modes. Because the stable isomers are T-shaped, vibration along the out-of-plane coordinates is very floppy. In CO-CO₂, when the molecule is out-of-plane, interconversion of the isomers is possible, but the barrier height is higher than the in-plane geared barrier height.

Infrared bands of CS2 dimer and trimer at 4.5 μm

We report observation of new infrared bands of (CS₂)₂ and (CS₂)₃ in the region of the CS₂ ν₁ + ν₃ combination band (at 4.5 µm) using a quantum cascade laser. The complexes are formed in a pulsed supersonic slit-jet expansion of a gas mixture of carbon disulfide in helium. We have previously shown that the most stable isomer of (CS₂)₂ is a cross-shaped structure with D_2d symmetry and that for (CS₂)₃ is a barrel-shaped structure with D₃ symmetry. The dimer has one doubly degenerate infrared-active band in the ν₁ + ν₃ region of the CS₂ monomer. This band is observed to have a rather small vibrational shift of -0.844 cm^-1. We expect one parallel and one perpendicular infrared-active band for the trimer but observe two parallel bands and one perpendicular band. Much larger vibrational shifts of -8.953 cm^-1 for the perpendicular band and -8.845 cm^-1 and +16.681 cm^-1 for the parallel bands are observed. Vibrational shifts and possible vibrational assignments, in the case of the parallel bands of the trimer, are discussed using group theoretical arguments.

Infrared spectrum and intermolecular potential energy surface of the CO–O2 dimer

The spectrum of the weakly-bound radical complex CO–O₂ is studied for the first time.

Bonding and spectroscopic analyses of N2O-CS2 and N2O-OCS heterodimer complexes and their atmospheric consequences

Different isomers of the valence isoelectronic pairs of the heterodimers N₂O-SCS and N₂O-OCS were investigated using MP2 and CCSD(T) methods with the aug-cc-pVXZ (X = D, T) basis set with anharmonic frequency calculations. Interaction energies of both the heterodimers were estimated at the CBS limit and for CCSD(T)/aug-cc-pVTZ. One isomer was located for the N₂O-SCS heterodimer. In the case of the N₂O-OCS heterodimer, three planar slipped-parallel isomers were characterized. There was no significant change in electron density at the bond critical points (BCPs) of various intra-bonds but the presence of inter-bonds with a weak electron density at the BCPs in the complexes was found as per the results from the quantum theory of atoms in molecules (QTAIM) analysis. The interorbital interactions of the monomer in the heterodimers are discussed herein in terms of the natural bond orbital (NBO) charge transfer. Symmetry-adapted perturbation analysis was performed for all the isomers. In the case of the stable complex of N₂O-SCS, induction dominated over the dispersion energy. Among the N₂O-OCS isomers, the dispersion effect predominates for the most stable isomer, while for the other two isomers, the induction effect is dominant. Frequency calculations for these complexes showed a number of interactions induced by the low frequency modes in the far IR region. The atmospheric implications are also discussed for these complexes.

Theory vs. experiment for molecular clusters: Spectra of OCS trimers and tetramers

All singly substituted (13)C, (18)O, and (34)S isotopomers of the previously known OCS trimer are observed in natural abundance in a broad-band spectrum measured with a chirped-pulse Fourier transform microwave spectrometer. The complete substitution structure thus obtained critically tests (and confirms) the common assumption that monomers tend to retain their free structure in a weakly bound cluster. A new OCS trimer isomer is also observed, and its structure is determined to be barrel-shaped but with the monomers all approximately aligned, in contrast to the original trimer which is barrel-shaped with two monomers aligned and one anti-aligned. An OCS tetramer spectrum is assigned for the first time, and the tetramer structure resembles an original trimer with an OCS monomer added at the end with two sulfur atoms. Infrared spectra observed in the region of the OCS ν1 fundamental (≈2060 cm(-1)) are assigned to the same OCS tetramer, and another infrared band is tentatively assigned to a different tetramer isomer. The experimental results are compared and contrasted with theoretical predictions from the literature and from new cluster calculations which use an accurate OCS pair potential and assume pairwise additivity.

The vibration-rotation-tunneling levels of N2-H2O and N2-D2O

In this paper, we report vibration-rotation-tunneling levels of the van der Waals clusters N2-H2O and N2-D2O computed from an ab initio potential energy surface. The only dynamical approximation is that the monomers are rigid. We use a symmetry adapted Lanczos algorithm and an uncoupled product basis set. The pattern of the cluster's levels is complicated by splittings caused by H-H exchange tunneling (larger splitting) and N-N exchange tunneling (smaller splitting). An interesting result that emerges from our calculation is that whereas in N2-H2O, the symmetric H-H tunnelling state is below the anti-symmetric H-H tunnelling state for both K = 0 and K = 1, the order is reversed in N2-D2O for K = 1. The only experimental splitting measurements are the D-D exchange tunneling splittings reported by Zhu et al. [J. Chem. Phys. 139, 214309 (2013)] for N2-D2O in the v2 = 1 region of D2O. Due to the inverted order of the split levels, they measure the sum of the K = 0 and K = 1 tunneling splittings, which is in excellent agreement with our calculated result. Other splittings we predict, in particular those of N2-H2O, may guide future experiments.

New combination bands of N2O-CO2, N2O-OCS, and N2O-N2 complexes in the N2O ν(1) region

Spectra of the weakly bound complexes N2O-CO2, N2O-OCS, and N2O-N2 were studied in the region of the ν1 fundamental of N2O (∼2224 cm(-1)) using a tunable quantum cascade laser to probe a pulsed supersonic jet expansion with an effective rotational temperature of about 2.5 K. One new combination band was observed for each complex: a band involving an intermolecular in-plane bending mode for N2O-N2, a band involving the disrotation (in-plane geared bend) for of N2O-CO2, and a band involving the out-of-plane torsional vibration for isomer b of N2O-OCS. Small perturbations were noted for the N2O-OCS band. Because of the absence of theoretical prediction, the nature of the intermolecular bending mode for N2O-N2 has not been identified. The resulting intermolecular frequencies are 34.175(1), 17.107(1), and 22.334(1) cm(-1) for N2O-CO2, N2O-OCS, and N2O-N2, respectively. In addition, the previously known fundamental band of N2O-N2 at 2225.99 cm(-1) was analyzed in improved detail. This band exhibits very weak a-type transitions which were not detected in the first infrared observation of this complex, indicating that N2O-N2 is not exactly T-shaped. That is, the N2O molecular axis is not exactly perpendicular to the a-inertial axis, in agreement with a previous structural determination of this complex by rotational spectroscopy.

Reduced-dimensional quantum computations for the rotational-vibrational dynamics of F(-)-CH4 and F(-)-CH2D2

Variational rotational-vibrational quantum chemical computations are performed for the F(-)-CH4 and F(-)-CH2D2 anion complexes using several reduced-dimensional models in a curvilinear polyspherical coordinate system and utilizing an accurate ab initio potential energy surface (PES). The implementation of the models is made practical by using the general rovibrational code GENIUSH, which constructs the complicated form of the exact rovibrational kinetic energy operator in reduced and full dimensions in any user-specified coordinates and body-fixed frames. A one-dimensional CF stretch, 1D(RCF), a two-dimensional intermolecular bend, 2D(θ,φ), and a three-dimensional intermolecular, 3D(RCF,θ,φ), rigid methane model provide vibrational energies for the low-frequency, large-amplitude modes in good agreement with full-dimensional MCTDH results for F(-)-CH4. The 2D(θ,φ) and 3D(RCF,θ,φ) four-well computations, describing equally the four possible CH-F(-) bonds, show that the ground-state tunneling splitting is less than 0.01 cm(-1). For the hydrogen-bonded CH stretching fundamental a local-mode model is found to have almost spectroscopic accuracy, whereas a harmonic frequency analysis performs poorly. The 2D(θ,φ) and 3D(RCF,θ,φ) rotational-vibrational computations on the Td-symmetric four-well PES reveal that in most cases F(-)-CH4 behaves as a semirigid C3v symmetric top. For the degenerate intermolecular bending vibrational states substantial splittings of the rigid rotor levels are observed. For F(-)-CH2D2 the rotational levels guide the assignment of the vibrational states to either F(-)-H or F(-)-D connectivity.

Matrix-isolated hydrogen-bonded and van der Waals complexes of hydrogen peroxide with OCS and CS2

Matrix isolation spectroscopy has been combined with ab initio calculations to characterize the 1:1 complexes of H2O2 with OCS and CS2. The infrared spectra of the argon and nitrogen matrices doped with H2O2 and OCS or CS2 have been measured and analyzed. The geometries of the complexes were optimized at the MP2/6-311++G(3df,3pd) level of theory. Four structures were found for the OCS-H2O2 complex and five for the CS2-H2O2 one; every pair of the corresponding structures showed close correspondence. For every optimized structure the interaction energy was partitioned according to the SAPT Scheme and the topological distribution of the charge density (AIM theory) was performed. The SAPT analysis and AIM results indicate that only one complex among the nine optimized ones is stabilized by the hydrogen bonding, namely the OCS-H2O2 one with the OH group of H2O2 bonded to an oxygen atom of OCS. The other structures are stabilized by van der Waals interaction. The spectra analysis evidences that at least two types of the complexes are trapped in the argon matrices including the most stable ones: hydrogen bonded structure in the case of the OCS-H2O2 complex and the structure stabilized by the S···H and C···O interactions in the case of the CS2-H2O2 complex. The solid nitrogen environment triggers the formation of the structures of C2v symmetry with a sulfur atom of OCS or CS2 directed toward the center of O-O bond of H2O2, stabilized by S···O interactions.