An evaluation of existing potential energy surfaces for CO2–Ar: Pressure broadening and high‐resolution spectroscopy of van der Waals complexes

Observing the Completion of the First Solvation Shell of Carbon Dioxide in Argon from Rotationally Resolved Spectra

Widespread interest in weakly bound molecular clusters of medium size (5-50 molecules) is motivated by their complicated energy landscapes, which lead to hundreds or thousands of distinct isomers. But most studies are theoretical in nature, and there are no experimental results which provide definitive structural information on completion of the first solvation shell. Here we assign rotationally resolved mid-infrared spectra to argon clusters containing a single carbon dioxide molecule, CO₂-Ar₁₅ and CO₂-Ar₁₇. These mark the completion of the first solvation shell for CO₂ in argon. The assignments are confirmed by nuclear spin intensity alternation in the spectra, a marker of highly symmetric structures for these clusters. Precise values are determined for rotational parameters and for shifts of the CO₂ vibrational frequency induced by the argon atoms. The spectra indicate possible low-frequency (∼2 cm^-1) vibrational modes in these clusters, posing a challenge for future cluster theory.

Simulation of collision-induced absorption spectra based on classical trajectories and ab initio potential and induced dipole surfaces. II. CO2-Ar rototranslational band including true dimer contribution

This paper presents further development of the new semi-classical trajectory-based formalism described in Paper I [Chistikov et al., J. Chem. Phys. 151, 194106 (2019)]. We report the results of simulation and analysis of the low-frequency collision-induced absorption (CIA) in CO₂-Ar, including its true dimer component. Our consideration relies on the use of ab initio intermolecular potential energy and induced dipole surfaces for CO₂-Ar calculated in an assumption of a rigid CO₂ structure using the CCSD(T) method. The theory, the details of which are reported in Paper I [Chistikov et al., J. Chem. Phys. 151, 194106 (2019)], permits taking into account the effect of unbound and quasi-bound classical trajectories on the CIA in the range of a rototranslational band. This theory is largely extended by trajectory-based simulation of the true bound dimer absorption in the present paper. The spectra are obtained from a statistical average over a vast ensemble of classical trajectories restricted by properly chosen domains in the phase space. Rigorous classical theory is developed for two low-order spectral moments interpreted as the Boltzmann-weighted average of the respective dipole functions. These spectral moments were then used to check the accuracy of our trajectory-based spectra, for which both spectral moments can be evaluated independently in terms of specific integrals over the trajectory-based calculated spectral profiles. Good agreement between the spectral moments calculated as integrals over the frequency domain or the phase space largely supports the reliability of our simulated CIA spectra, which conform with the available microwave and far-infrared observations.

Symmetry breaking of the bending mode of CO2 in the presence of Ar

The weak infrared spectrum of CO2-Ar corresponding to the (0111) ← (0110) hot band of CO2 is detected in the region of the carbon dioxide ν3 fundamental vibration (≈2340 cm-1), using a tunable OPO laser source to probe a pulsed supersonic slit jet expansion. While this method was previously thought to cool clusters to the lowest rotational states of the ground vibrational state, here we show that under suitable jet expansion conditions, sufficient population remains in the first excited bending mode of CO2 (1-2%) to enable observation of vibrationally hot CO2-Ar, and thus to investigate the symmetry breaking of the intramolecular bending mode of CO2 in the presence of Ar. The bending mode of the CO2 monomer splits into an in-plane and an out-of-plane mode, strongly linked by a Coriolis interaction. Analysis of the spectrum yields a direct measurement of the in-plane/out-of-plane splitting measured to be 0.8770 cm-1. Calculations were carried out to determine if key features of our results, i.e., the sign and magnitude of the shift in the energy for the two intramolecular bending modes, are consistent with a quantum chemical potential energy surface. This aspect of intramolecular interactions has received little previous experimental and theoretical consideration. Therefore, we provide an additional avenue by which to study the intramolecular dynamics of this simplest dimer in its bending modes. Similar results should be possible for other weakly-bound complexes.

Theoretical calculation of a full-dimensional ab initio potential energy surface and prediction of infrared spectra for Xe-CS2

An effective four-dimensional (4D) ab initio potential energy surface (PES) for Xe-CS2 which explicitly involves the intramolecular Q 1 symmetric stretching and Q 3 antisymmetric stretching vibrational coordinates of CS2 is constructed. The computations are carried out employing single- and double-excitation coupled-cluster theory with a non-iterative perturbation treatment of triple excitations [CCSD(T)] method with a large basis set. Two vibrationally averaged potentials at the ground and ν 1 + ν 3 (ν 1 = 1, ν 3 = 1) excited states are obtained by integrating the 4D potentials over the Q 1 and Q 3 coordinates. The potentials have a T-shaped global minimum and two equivalent linear local minima. The radial discrete variable representation/angular finite basis representation and the Lanczos algorithm are employed to calculate the rovibrational energy levels for Xe-CS2. The infrared band origin shift associated with the fundamental band of CS2 is predicted, which is red-shifted by -1.996 cm-1 in the ν 1 + ν 3 region. In addition, we further predict the spectroscopic parameters for the ground and the ν 1 + ν 3 excited states of Xe-CS2. Compared with the previous Rg-CS2 (Rg = He, Ne, Ar, Kr) complexes, we found that the complexes of the rare gas atoms with CS2 display obvious regularities.

Theoretical studies for the infrared spectra of Ar-CO2 complex: Fundamental and combination bands

Two potential energy surfaces (PESs) were constructed for the ground and excited states of Ar-CO₂ complex at the rigid rotor approximation. Besides the notable T-shape structure, two equivalent linear structures were also found on the PES for the first time. Based on the PESs of ground and excited states, the bound state calculations were performed to determine the rotational energy levels for the ground and excited states. In combination of the experimental spectroscopic parameters of ground state and the differences of rotational energy levels, we give a theoretical prediction of infrared spectra including one fundamental band and two combination bands for two isotopomers Ar-¹²C¹⁶O₂ and Ar-¹²C¹⁸O₂ in the ν₃ region of CO₂ monomer. The predicted transition frequencies and spectroscopic parameters of excited states are in excellent agreement with the available experimental data, and these results can also be used as a guide to perform the further investigation for the infrared spectra of combination bands experimentally.

A new ab initio potential energy surface and infrared spectra for the Ar-CS₂ complex

We report a new three-dimensional potential energy surface for Ar-CS2 involving the Q3 normal mode for the υ3 antisymmetric stretching vibration of the CS2 molecule. The potential energies were calculated using the supermolecular method at the coupled-cluster singles and doubles level with noniterative inclusion of connected triples, using augmented correlation-consistent quadruple-zeta basis set plus midpoint bond functions. Two vibrationally averaged potentials with CS2 at both the ground (υ = 0) and the first excited (υ = 1)υ3 vibrational states were generated from the integration of the three-dimensional potential over the Q3 coordinate. Each potential was found to have a T-shaped global minimum and two equivalent linear local minima. The radial discrete variable representation /angular finite basis representation method and the Lanczos algorithm were applied to calculate the rovibrational energy levels. The calculated band origin shift of the complex (0.0622 cm(-1)) is very close to the observed one (0.0671 cm(-1)). The predicted infrared spectra and spectroscopic parameters based on the two averaged potentials are in excellent agreement with the available experimental data.

POTENTIAL ENERGY SURFACES AND MICROWAVE SPECTRA FOR 20Ne–13C16O2, 22Ne–12C16O2 and 22Ne–13C16O2 COMPLEXES

We report averaged potential energy surfaces for isotopic Ne–CO2 complexes (20 Ne –13 C 16 O 2, 22 Ne –12 C 16 O 2 and 22 Ne –13 C 16 O 2). According to the latest ab initio potential of 20 Ne –12 C 16 O 2 (Chen R, Jiao EQ, Zhu H, Xie DQ, J Chem Phys133:104302, 2010) including the Q3 normal mode for the υ3 antisymmetric stretching vibration of the CO2 molecule. We obtain the averaged potentials for 20 Ne –13 C 16 O 2, 22 Ne –12 C 16 O 2 and 22 Ne –13 C 16 O 2 by the integration of the three-dimensional potential over the Q3 coordinate. The averaged potential surfaces are found to have a T-shaped global minimum and two equivalent linear local minima. The radial DVR/angular FBR method and the Lanczos algorithm are applied to calculate the rovibrational energy levels. Comparison with the available observed values showed an overall excellent agreement for all spectroscopic parameters and the microwave spectra.

POTENTIAL ENERGY SURFACE, MICROWAVE AND INFRARED SPECTRA OF THE Xe–CO2 COMPLEX FROM AB INITIO CALCULATIONS

We present a new three-dimensional potential energy surface for Xe–CO2 including the Q3 normal mode for the υ3 antisymmetric stretching vibration of the CO2 molecule. Two vibrationally adiabatic potentials with CO2 in both the ground (υ3 = 0) and the first excited (υ3 = 1) states are generated by the integration of this potential over the Q3 coordinate. Each potential is found to have a T-shaped global minimum. The radial DVR/angular FBR method and the Lanczos algorithm are employed to calculate the rovibrational energy levels. The calculated band origin shifts, microwave and infrared spectra based on the two averaged potentials are in good agreement with the available experimental data.

Carbondioxide rare-gas systems: sensitivity of basis sets and double-hybrid density functionals

This study emphasizes on the performance of six newly developed double-hybrid density functionals (DHDF) in explaining the potential energy curves of different carbondioxide rare-gas systems. The basis set sensitivity has also been explored with the use of three basis sets. Our results suggest that for lighter He/Ne-CO(2) complexes, proper choice of DHDF and basis set lead to results those matches exactly with earlier calculations and also with the experiment. On the other hand, for heavier Ar/Kr-CO(2) complexes although the equilibrium separation distance matches exactly with earlier observations, the interaction energy values lie far apart. The overall investigation emphasizes on the fact that one has to tune the methods and basis sets properly to achieve good and satisfactory results.

A new potential energy surface and predicted infrared spectra of the Ar-CO(2) van der Waals complex

A new potential energy surface for Ar-CO(2) is constructed at the coupled-cluster singles and doubles with noniterative inclusion of connected triple [CCSD(T)] level with augmented correlation-consistent triple-zeta (aug-cc-pVTZ) basis set plus midpoint bond functions. The Q(3) normal mode for the v(3) antisymmetric stretching vibration of CO(2) is involved in the construction of the potential. Effective two-dimensional potentials with CO(2) in the ground and first excited v(3) vibrational states are obtained by averaging a three-dimensional potential for each case over the Q(3) asymmetric stretch vibrational coordinate. Both potentials have only a T-shaped minimum with a well depth of 200.97 and 201.37 cm(-1), respectively. No linear local minima are detected. The radial discrete variable representation/angular finite basis representation method and the Lanczos algorithm are employed to calculate the related rovibrational energy levels. The calculated band origin shift of the complex agrees very well with the observed one (-0.474 versus -0.470 cm(-1)). In addition, the predicted infrared spectra based on the two averaged potentials are in excellent agreement with the available experimental data, which again testifies the accuracy of the new potentials.

Measurements of collisional broadening coefficients by infrared polarization spectroscopy

We present measurements of collisional broadening coefficients, obtained at atmospheric pressure, by polarization spectroscopy. Using tunable single mode laser radiation at approximately 2 microm, high-resolution infrared polarization spectra were recorded for CO2-Ar and CO2-He binary mixtures. The recorded polarization spectra were fitted with a Lorentzian cubed function form to obtain the broadening coefficients. The full-width at half-maxima (FWHM) collisional broadening rates of CO2 by Ar and He, for the R14 (12 degrees1<--00 degrees0) line, have been determined to be 0.161+/-0.018 cm-1 atm-1 and 0.1823+/-0.0032 cm-1 atm-1, respectively.

Structures and energetics of CO2–Arn clusters (n = 1–21) based on a non-rigid potential model

Energetics and possible stable structures of CO2–Arn (n = 1–21) clusters are investigated by performing molecular-dynamics simulations. The pairwise-additive approximation is tested to construct the potential energy function for describing the non-rigid particle interactions in the system. A potential model by Pariseau et al. (Journal of Chemical Physics, Vol. 42, p. 2335, 1965) is used for the internal motion of the CO2 molecule and the Billing form potential (Chemical Physics, Vol. 185, p. 199, 1994) is used for all other pair interactions. The stable configurations are determined for the ground state of CO2–Arn clusters, and the growing pattern process of the clusters is determined via rearrangement collisions. Ar atoms tend to surround the CO2 molecule, and the clusters prefer to form three-dimensional compact structures. Obtained structures and energetics are in quantitative agreement with previous results (Journal of Chemical Physics, Vol. 109, p. 1343, 1998) that have used split-repulsion and ab initio potentials in which the molecule was treated as rigid.Key words: argon, CO2, cluster, potential energy function, molecular dynamics.

Collision Cross Sections, Pressure-Broadening Coefficients and Second Virial Coefficients for the Acetylene-Argon Complex: Experiments and Calculations on a New Potential Energy Surface

Integral cross sections and pressure-broadening coefficients have been measured by molecular beam scattering and by high-resolution infrared spectroscopy, respectively, for the acetylene-argon system. A new potential energy surface (PES) is proposed to describe structure and dynamical properties of this prototypical weakly bound complex. The PES has been parametrized exploiting a novel atom-bond pairwise additive scheme and has been fitted to the experimental data. Calculations of the scattering cross sections (both differential and integral), pressure-broadening, and second virial coefficients have been performed using both the present and also the most recent ab initio PES available in the literature. Analysis of the new experimental data indicates that the anisotropy of the interaction in the well region should be larger than that obtained in ab initio calculations. This is also in line with previous spectroscopic results.

Classical analysis of intermolecular potentials for Ar–CO2 rotational collisions

Classical trajectory calculations have been performed for four potential energy functions to describe Ar–CO2 collisions. A comparison is given between classical cross sections calculated using the two most recent potential surfaces and two older intermolecular potential surfaces based on the electron gas model. The two-dimensional atom ellipsoid model has also been applied for the study of multiple collisions. The model was able to predict such a phenomenon in agreement with quantum scattering results previously published for an ab initio potential surface in the region of very low collision energy. On the other hand, the two older potentials showed multiple collision effects at very high energies. The comparison of the cross sections showed some deviations from the experimental data. By introducing two parameters, a modified surface is proposed by changing the most recent intermolecular potential. In this case the agreement with experimental measurements and theoretical scattering cross sections was considerably improved. It is concluded that global potential surfaces for describing Ar–CO2 interaction are not well established. To achieve the requirement of reproducing all properties of this system, the present work suggests that one needs further experimental and theoretical investigations. Key words: classical trajectories, dynamics, cross sections, Ar–CO2 collisions, potentials.

Rotational Spectra of NeCO2 Isotopomers

Rotational spectra of seven isotopomers of the weakly bound complex NeCO2, i.e., NeCO2, 22NeCO2, Ne13CO2, 22Ne13CO2, Ne18O13CO, Ne17OCO, and 22Ne17OCO, were measured using a pulsed molecular beam Fourier transform microwave spectrometer. Rotational and centrifugal distortion constants as well as the nuclear quadrupole coupling constants of 17O in Ne17OCO and 22Ne17OCO were determined. The obtained constants were used to derive structural and force field information. The parameters obtained are discussed, and compared with those of other, previously measured, rare gas-CO2 complexes. The 17O nuclear quadrupole coupling constants were analyzed and interpreted in terms of the angular anisotropy of the intermolecular potential energy surface. Copyright 1998 Academic Press.

Diode Laser Spectroscopy in the 9.8-µm nu14 Band of Benzene

High-resolution diode laser spectroscopy was applied to measure broadening coefficients in the 9.8-µm nu14 band of benzene, C6H6. Self-broadening was investigated at 295 and 344 K for 73 lines between 1017.31 and 1045.85 cm-1 with 9 </= J" </= 66 and 1 </= K" </= 66. The measured self-broadening coefficients at room temperature vary from 0.12 to 0.22 cm-1/atm (HWHM), whereas at 344 K the self-broadening coefficients are in an interval between 0.07 cm-1/atm to 0.16 cm-1/atm. The Anderson-Tsao-Curnutte (ATC) theory was used to deduce the benzene quadrupole moment and the b0 parameter from mean values of the broadening coefficients at room temperature. It could be shown that the ATC theory is able to describe the quantum number dependence and the absolute values of the self-broadening coefficients with sufficient accuracy. The collision partner dependence of the broadening coefficients was measured for three Q-branch lines in benzene mixtures with air, N2, He, Ne, Ar, Kr, and Xe. The results are compared with similar experiments with NO2 and SO2 absorption lines with the outcome that the qualitative change of the broadening coefficients with changing collision partner is similar for all three molecular gases. Air broadening was measured for additional five lines at both temperatures mentioned above. The experimental setup and the data treatment are described in Part I of this article (J. Mol. Spectrosc., 184, 433). Copyright 1998 Academic Press.