Phosphine interacting with argon: potential energy surface and line widths of PH3 perturbed by Ar.

The phosphine-Ar (PH3-Ar) van der Waals complex's three-dimensional potential energy surface (3D-PES) was mapped along the intermonomer coordinates using the explicitly correlated Coupled Clusters approach. An analytical expansion of this 3D-PES is deduced. The complex characteristics of the intermolecular interactions between PH3 and Ar, which are at the origin of the pressure broadening of the PH3 rovibrational lines, are accurately described by this 3D-PES. Afterwards, the broadening of the phosphine lines perturbed by Ar at room temperature is computed using this 3D-PES. To be able to incorporate this 3D-PES into our collisional broadening computation software, we first modified it with a new "fractional" function. Then, we used the semiclassical Modified Robert and Bonamy model improved by the so-called notion of exact trajectory. For various branches of the ν2 and ν4 PH3 rovibrational bands, room temperature data are provided. Our data agree with the experimental measurements. The precise estimation of PH3 and Ar abundances in planetary atmosphere spectra should benefit greatly from the data provided here.

IO(X2Π)-Ar cluster: ab initio potential energy surface and dynamical computations.

Iodine oxide (IO) is an important tropospheric molecule. In the present paper, we mapped the potential energy surfaces (PESs) of the doubly degenerate IO(X2Π)-Ar van der Waals system using single- and double-excitation coupled cluster approaches with non-iterative perturbation treatment of triple excitations [RCCSD(T)] extrapolated to the complete basis set (CBS) limit. In addition to bent local minima, we identified a linear Ar-IO complex as a global minimum. Afterwards, we performed scattering calculations on these PESs, considering the non-zero spin-orbit contribution and the Renner-Teller effect. The integral cross-sections exhibit an oscillatory structure vs. the final rotational state, as already observed for the NO(X2Π)-Ar system. Moreover, computations reveal that the Ar-IO complex is stable toward dissociation into IO and Ar. Therefore, it can be found in the atmosphere and participates in iodine compound physical chemical processes occurring there.

Gold with +4 oxidation state compounds: mass spectrometric and theoretical characterization of AuO2.

Using an ab initio methodology and mass spectrometric study we identify AuO2+ as a metastable species in the gas phase. This represents the first characterization of a gas phase compound of gold with the oxidation state +4. Computations show that this dication exhibits deep potential wells with long lived electronic states. Its electronic ground state is of 4∑- symmetry, which is known for very few molecular ground states. We also discussed the O + Au2+ collision dynamics, which leads mostly to charge transfer to form Au+ and O+ species. This identification may help in identifying new routes for the reactivity of gold in the gas phase, in solution and in the condensed phase.

Ab initio spectroscopic characterization of the radical CH3OCH2 at low temperatures.

Spectroscopic and structural properties of methoxymethyl radical (CH3OCH2, RDME) are determined using explicitly correlated ab initio methods. This radical of astrophysical and atmospheric relevance has not been fully characterized at low temperatures, which has delayed astrophysical research. We provide rovibrational parameters, excitations to the low energy electronic states, torsional and inversion barriers, and low vibrational energy levels. In the electronic ground state (X2A), which appears "clean" from nonadiabatic effects, the minimum energy structure is an asymmetric geometry whose rotational constants and dipole moment have been determined to be A0 = 46 718.67 MHz, B0 = 10 748.42 MHz, and C0 = 9272.51 MHz, and 1.432D (µA = 0.695D, µB = 1.215D, µC = 0.302D), respectively. A variational procedure has been applied to determine torsion-inversion energy levels. Each level splits into 3 subcomponents (A1/A2 and E) corresponding to the three methyl torsion minima. Although the potential energy surface presents 12 minima, at low temperatures, the infrared band shapes correspond to a surface with only three minima because the top of the inversion Vα barrier at α = 0° (109 cm-1) stands below the zero point vibrational energy and the CH2 torsional barrier is relatively high (∼2000 cm-1). The methyl torsion barrier was computed to be ∼500 cm-1 and produces a splitting of 0.01 cm-1 of the ground vibrational state.

Disentangling the complex spectrum of the ethynyl cation.

The ethynyl cation, C2H+, is of great importance in astrophysical media and in combustion. It is involved in the formation of larger organic compounds and in their decomposition mechanisms. Here, we investigate the low-lying electronic states of this cation using pure ab initio methodologies. The evolution of its potential energy surfaces along the stretching and bending coordinates reveals a high density of electronic states that favours mutual interactions and the mixing of wavefunctions. The ground state is of 3Π space symmetry and the lowest singlet state (1Π) is found to be a quasi-linear-quasi-linear Renner-Teller system. Our work suggests that the (spin-)rovibronic spectrum of such a molecular system is complicated, because of the contributions of multiple couplings, including Renner-Teller, vibronic and spin-orbit. We also deduced the adiabatic ionization energy of the ethynyl radical, in good agreement with recent measurements. In summary, our work shows that the ethynyl cation, in spite of its small size, still represents a challenging molecular problem to be solved.

Rotational (de-)excitation of isocyanogen by collision with helium at low energies.

Isocyanogen, CNCN, was discovered very recently in the interstellar medium (ISM). At present, the rate coefficients for the rotational (de-)excitation of CNCN by collisions with He are determined. First, we mapped the interaction potential between CNCN and He in Jacobi coordinates using highly correlated ab initio methodology. Then, an analytical expansion of the CNCN-He potential energy surface is derived. Later on, quantum dynamical treatments of nuclear motions are performed using the close coupling technique. We obtained the cross sections for the rotational (de-)excitation of CNCN after a collision by He up to 2000 cm-1 total energies. These cross sections are used to deduce the collision rates in the 10-300 K range. These data are needed for modeling the CNCN abundances in the ISM. This work should help for determining the abundance of such non-symmetrical dicyanopolyynes in astrophysical media and indirectly the symmetric one [Cyanogen (NCCN)].

Periodic Dispersion-Corrected Approach for Isolation Spectroscopy of N2 in an Argon Environment: Clusters, Surfaces, and Matrices.

Ab initio and Perdew, Burke, and Ernzerhof (PBE) density functional theory with dispersion correction (PBE-D3) calculations are performed to study N2-Arn (n ≤ 3) complexes and N2 trapped in Ar matrix (i.e., N2@Ar). For cluster computations, we used both Møller-Plesset (MP2) and PBE-D3 methods. For N2@Ar, we used a periodic-dispersion corrected model for Ar matrix, which consists on a slab of four layers of Ar atoms. We determined the equilibrium structures and binding energies of N2 interacting with these entities. We also deduced the N2 vibrational frequency shifts caused by clustering or embedding compared to an isolated N2 molecule. Upon complexation or embedding, the vibrational frequency of N2 is slightly shifted, while its equilibrium distance remains unchanged. This is due to the weak interactions between N2 and Ar within these compounds. Our calculations show the importance of inclusion of dispersion effects for the accurate description of geometrical and spectroscopic parameters of N2 isolated, in interaction with Ar surfaces, or trapped in Ar matrices.

Stereoisomers of hydroxymethanes: Probing structural and spectroscopic features upon substitution.

Ab initio studies on CHx(OH)4-x (x = 0-3) polyols are carried out to derive their structural and spectroscopic features. Several stereoisomers (both equilibrium structures and transition states) are found. Some are predicted here for the first time. We determined hence their geometrical parameters, vibrational frequencies, electronic excitation energies for the singlet manifold, and IR spectra. While the IR spectra for all polyols present similar shapes, their UV spectra exhibit however distinct band origin that are specific to each polyol and more interestingly to each diasteroisomer. Stereoelectronic effects are also noticed and discussed. It is suggested that UV spectroscopy is an efficient probe to experimentally identify polyols in mixtures involving polyols.

Understanding of matrix embedding: a theoretical spectroscopic study of CO interacting with Ar clusters, surfaces and matrices.

Through benchmark studies, we explore the performance of PBE density functional theory, with and without Grimme's dispersion correction (DFT-D3), in predicting spectroscopic properties for molecules interacting with rare gas matrices. Here, a periodic-dispersion corrected model of matrix embedding is used for the first time. We use PBE-D3 to determine the equilibrium structures and harmonic vibrational frequencies of carbon monoxide in interaction with small Ar clusters (CO-Arn, n = 1, 2, 3), with an Ar surface and embedded in an Ar matrix. Our results show a converging trend for both the vibrational frequencies and binding energies when going from the gas-phase to a fully periodic approach describing CO embedding in Ar. This trend is explained in terms of solvation effects, as CO is expected to alter the structure of the Ar matrix. Due to a competition between CO-Ar interactions and Ar-Ar interactions, perturbations caused by the presence of CO are found to extend over several Šin the matrix. Accordingly, it is mandatory to fully relax rare gas matrices when studying their interaction with embedded molecules. Moreover, we show that the binding energy per Ar is almost constant (∼-130 cm(-1) atom(-1)) regardless of the environment of the CO molecule. Finally, we show that the concentration of the solute into the cold matrix influences the spectroscopic parameters of molecules embedded into cold matrices. We suggest hence that several cautions should be taken before comparing these parameters to gas phase measurements and to theoretical data of isolated species.

Role of size and shape selectivity in interaction between gold nanoclusters and imidazole: a theoretical study.

We present a theoretical study on the structure, stability, spectra and electronic properties of imidazole (Im) adsorbed on gold nanoclusters (Aun, n = 2, 4, 6, 8, 10, and 20). These computations were performed using various density functional theories with and without inclusion of Grimme's (D3) dispersion correction. For small clusters, we also carried out wavefunction-based ab initio (MP2 and SCS-MP2) computations for comparison. Vibrational, atoms in molecules (AIM) and natural bond orbital (NBO) analyses clearly reveal the occurrence of charge transfer (CT) through covalent (N1-Au) and noncovalent interactions that play important roles in the stability of the Im@Aun complexes with anchor assisted H-bonds (Cα-H · Au). Therefore, gold clusters can act as H-bond acceptors with biomolecules for development of new materials and applications. Our study establishes also the ability and reliability of PBE0 and M05-2X functionals compared to B3LYP and PBE for an accurate description of covalent and noncovalent interactions between Im and gold clusters since they lead to close agreement with MP2. Finally, we show that the Au8 cluster may be viewed as large enough to mimic the 3D gold surface.

Characterization of gas phase WC²âº: a thermodynamically stable carbide dication.

Using an ab initio methodology, we performed a detailed theoretical study of the gas phase WC(2+) dication. These calculations were done using a multiconfigurational method in connection with a large basis set, where relativistic effects were taken into account. This dication is identified here as the first thermochemically stable doubly charged diatomic carbide in the gas phase. Our work hence confirms the stability of this dication in the gas phase and its earlier observations by atom probe mass spectrometry. Our calculations show that the shape of the potential energy curves of its lowest electronic states changes drastically upon consideration of relativistic effects. For instance, the electronic ground state possesses a Morse-like potential without spin-orbit that evolves to the usual volcanic behavior, and with a columbic 1/R evolution at large internuclear separation after inclusion of spin-orbit. We predict a set of thermochemical and spectroscopic data for this molecular species.