Potential energy surface and bound states of the NH3-Ar and ND3-Ar complexes

Interaction of CH3CN and CH3NC with He: Potential Energy Surfaces and Low-Energy Scattering

Several nitrogen-bearing molecules, such as methyl cyanide (or acetonitrile, CH₃CN) and methyl isocyanide (CH₃NC) of interest here, have been observed in various astrophysical environments. The accurate modeling of their abundance requires the calculation of rate coefficients for their collisional excitation with species such as He atoms or H₂ molecules at low temperatures. In this work we compute new three-dimensional potential energy surfaces for the CH₃NC-He and CH₃CN-He van der Waals complexes by means of the explicitly correlated coupled cluster approach with single, double and perturbative triple excitation CCSD(T)/F12a in conjunction with the aug-cc-pVTZ basis set. We find a global minimum with D_e = 55.10 and 58.61 cm^-1 for CH₃CN-He and CH₃NC-He, respectively, while the dissociation energy D₀ of the complexes are 18.64 and 18.65 cm^-1, respectively. Low-energy scattering calculations of pure rotational (de-)excitation of CH₃CN and CH₃NC by collision with He atoms are carried out with the close-coupling method, and the collisional cross sections of ortho- and para-CH₃NC and CH₃CN are computed for kinetic energies up to 100 cm^-1. While the PESs for both complexes are qualitatively similar, that of CH₃NC-He is more anisotropic, leading to different propensity rules for rotational excitation. For CH₃NC-He, we find that |Δj| = 1 transitions are dominant at low kinetic energy and a propensity rule that favors odd Δj transitions is observed, whereas for CH₃CN the dominant cross sections are associated with transitions with |Δj| = 2. We expect that the findings of this study will be beneficial for astrophysical investigations as well as laboratory experiments.

Concerning the Role of σ-Hole in Non-Covalent Interactions: Insights from the Study of the Complexes of ArBeO with Simple Ligands

The structure, stability, and bonding character of some exemplary LAr and L-ArBeO (L = He, Ne, Ar, N₂, CO, F₂, Cl₂, ClF, HF, HCl, NH₃) were investigated by MP2 and coupled-cluster calculations, and by symmetry-adapted perturbation theory. The nature of the stabilizing interactions was also assayed by the method recently proposed by the authors to classify the chemical bonds in noble-gas compounds. The comparative analysis of the LAr and L-ArBeO unraveled geometric and bonding effects peculiarly related to the σ-hole at the Ar atom of ArBeO, including the major stabilizing/destabilizing role of the electrostatic interactionensuing from the negative/positive molecular electrostatic potential of L at the contact zone with ArBeO. The role of the inductive and dispersive components was also assayed, making it possible to discern the factors governing the transition from the (mainly) dispersive domain of the LAr, to the σ-hole domain of the L-ArBeO. Our conclusions could be valid for various types of non-covalent interactions, especially those involving σ-holes of respectable strength such as those occurring in ArBeO.

Intermolecular dynamics of NH3-rare gas complexes in the ν2 umbrella region of NH3 investigated by rovibrational laser jet-cooled spectroscopy and ab initio calculations

High resolution jet-cooled spectroscopy experiments have been realized to investigate the intermolecular dynamics of van der Waals (vdW) heterodimers between NH₃ and rare gas (Rg) atoms in the ν₂ umbrella mode region of NH₃. With respect to a previous study dedicated to NH₃-Ar [Asselin et al. Mol. Phys. 116, 3642 (2018)], the sensitivity and spectral resolution of our laser spectrometer coupled to a pulsed supersonic jet have been significantly improved to derive more accurate excited state spectroscopic parameters from rovibrational analyses. In addition, we calculated the ground and ν₂ excited vibration-rotation-tunneling (VRT) states of these complexes on the four-dimensional ab initio potential energy surfaces from Loreau et al. [J. Chem. Phys. 141, 224303 (2014), ibid. 143, 184303 (2015).] Transition frequencies and intensities of the allowed ν₂ = 1 ← 0 transitions obtained from the calculated energy levels and wave functions agree well with the experimental data and are helpful in their analysis. By means of a pseudodiatomic model with the assumption of weak Coriolis coupling, the rovibrational analysis of both the Π_e/f(j = 1,k = 0) ←Σ_f(j = 0,k = 0) and Σ_f(j = 1,k = 0) ←Σ_f(j = 0,k = 0) transitions in ortho NH₃-Rg (Rg = Ne, Ar, Kr, Xe) complexes enabled us to determine reliable excited state parameters and derive accurate values of the effective vdW bond length R_eff, force constant k_s, and vdW stretching frequency ν_s. Comparison between the experimental structural parameters and those from the ab initio calculated VRT levels shows good agreement for NH₃-Ne, NH₃-Ar and NH₃-Kr, and a similar variation of R_eff, k_s, and ν_s with the polarizability of Rg in the ground and ν₂ excited states. Anomalously small values of ν_s and k_s derived for NH₃-Xe in the Π_e/f(j = 1,k = 0) state suggest that the applied model is not valid in this case, due to the presence of another state coupling to the perturbed Π_f state. Such a state could not be found, however.

Explicitly correlated potential energy surface of the CH3Cl-He van der Waals complex and applications

A new 3D-potential energy surface (3D-PES) for the weakly bound CH₃Cl-He complex is mapped in Jacobi coordinates. Electronic structure calculations are performed using the explicitly correlated coupled clusters with single, double, and perturbative triple excitations approach in conjunction with the aug-cc-pVTZ basis set. Then, an analytical expansion of this 3D-PES is derived. This PES shows three minimal structures for collinear C-Cl-He arrangements and for He located in between two H atoms, in the plane parallel to the three H atoms, which is near the center of mass of CH₃Cl. The latter form corresponds to the global minimum. Two maxima are also found, which connect the minimal structures. We then evaluated the pressure broadening coefficients of the spectral lines of CH₃Cl in a helium bath based on our ab initio potential. Satisfactory agreement with experiments was observed, confirming the good accuracy of our 3D-PES. We also derived the bound rovibronic levels for ortho- and para-CH₃Cl-He dimers after quantum treatment of the nuclear motions. For both clusters, computations show that although the ground vibrational state is located well above the intramolecular isomerization barriers, the rovibronic levels may be associated with a specific minimal structure. This can be explained by vibrational localization and vibrational memory effects.

Ab initio potential energy surface and microwave spectrum of the NH3-N2 van der Waals complex

We present a five-dimensional intermolecular potential energy surface (PES) of the NH₃-N₂ complex, bound state calculations, and new microwave (MW) measurements that provide information on the structure of this complex and a critical test of the potential. Ab initio calculations were carried out using the explicitly correlated coupled cluster [CCSD(T)-F12a] approach with the augmented correlation-consistent aug-cc-pVTZ basis set. The global minimum of the PES corresponds to a configuration in which the angle between the NH₃ symmetry axis and the intermolecular axis is 58.7° with the N atom of the NH₃ unit closest to the N₂ unit, which is nearly parallel to the NH₃ symmetry axis. The intermolecular distance is 7.01 a₀, and the binding energy D_e is 250.6 cm^-1. The bound rovibrational levels of the four nuclear spin isomers of the complex, which are formed when ortho/para (o/p)-NH₃ combines with (o/p)-N₂, were calculated on this intermolecular potential surface. The computed dissociation energies D₀ are 144.91 cm^-1, 146.50 cm^-1, 152.29 cm^-1, and 154.64 cm^-1 for (o)-NH₃-(o)-N₂, (o)-NH₃-(p)-N₂, (p)-NH₃-(o)-N₂, and (p)-NH₃-(p)-N₂, respectively. Guided by these calculations, the pure rotational transitions of the NH₃-N₂ van der Waals complex were observed in the frequency range of 13-27 GHz using the chirped-pulse Fourier-transform MW technique. A complicated hyperfine structure due to three quadrupole ¹⁴N nuclei was partly resolved and examined for all four nuclear spin isomers of the complex. Newly obtained data definitively established the K values (the projection of the angular momentum J on the intermolecular axis) for the lowest states of the different NH₃-N₂ nuclear spin isomers.

Nuclear spin/parity dependent spectroscopy and predissociation dynamics in vOH = 2 ← 0 overtone excited Ne-H2O clusters: Theory and experiment

Vibrationally state selective overtone spectroscopy and state- and nuclear spin-dependent predissociation dynamics of weakly bound ortho- and para-Ne-H₂O complexes (D_0(ortho) = 34.66 cm^-1 and D_0(para) = 31.67 cm^-1) are reported, based on near-infrared excitation of van der Waals cluster bands correlating with v_OH = 2 ← 0 overtone transitions (|02^-〉 and |02⁺〉) out of the ortho (1₀₁) and para (0₀₀) internal rotor states of the H₂O moiety. Quantum theoretical calculations for nuclear motion on a high level potential energy surface [CCSD(T)/VnZf12 (n = 3, 4)], corrected for basis set superposition error and extrapolated to the complete basis set (CBS) limit, are employed to successfully predict and assign Π-Σ, Σ-Σ, and Σ-Π infrared bands in the spectra, where Σ or Π represent approximate projections of the body-fixed H₂O angular momentum along the Ne-H₂O internuclear axis. IR-UV pump-probe experimental capabilities permit real-time measurements of the vibrational predissociation dynamics, which indicate facile intramolecular vibrational energy transfer from the H₂O v_OH = 2 overtone vibrations into the VdWs (van der Waals) dissociation coordinate on the τ_prediss = 15-25 ns time scale. Whereas all predicted strong transitions in the ortho-Ne-H₂O complexes are readily detected and assigned, vibrationally mediated photolysis spectra for the corresponding para-Ne-H₂O bands are surprisingly absent despite ab initio predictions of Q-branch intensities with S/N > 20-40. Such behavior signals the presence of highly selective nuclear spin ortho-para predissociation dynamics in the upper state, for which we offer a simple mechanism based on Ne-atom mediated intramolecular vibrational relaxation in the H₂O subunit (i.e., |02^±〉 → {|01^±〉; v₂ = 2}), which is confirmed by the ab initio energy level predictions and the nascent OH rotational (N), spin orbit (Π_1/2,_3/2), and lambda doublet product distributions.

Rotationally Inelastic Scattering of Quantum-State-Selected ND3 with Ar

Rotationally inelastic scattering of ND3 with Ar is studied at mean collision energies of 410 and 310 cm(–1). In the experimental component of the study, ND3 molecules are prepared by supersonic expansion and subsequent hexapole state selection in the ground electronic and vibrational levels and in the jk(±) = 1(1) rotational level. A beam of state-selected ND3 molecules is crossed with a beam of Ar, and scattered ND3 molecules are detected in single final j′k′(±) quantum states using resonance enhanced multiphoton ionization spectroscopy. State-to-state differential cross sections for rotational-level changing collisions are obtained by velocity map imaging. The experimental measurements are compared with close-coupling quantum-mechanical scattering calculations performed using an ab initio potential energy surface. The computed DCSs agree well with the experimental measurements, confirming the high quality of the potential energy surface. The angular distributions are dominated by forward scattering for all measured final rotational and vibrational inversion symmetry states. This outcome is in contrast to our recent results for inelastic scattering of ND3 with He, where we observed significant amount of sideways and backward scattering for some final rotational levels of ND3. The differences between He and Ar collision partners are explained by differences in the potential energy surfaces that govern the scattering dynamics.