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-Ar_n (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 CH_x(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.

Rotational Excitation of the OH(+) Radical by Collision with H at Low Temperature

A ro-vibrationally inelastic close coupling study of the rotational excitation of OH(+)(X(3)Σ(-)) by collisions with H((2)S) is presented. The two lowest potential energy surfaces of doublet and quadruplet spin multiplicity are involved. The former is the one we developed recently, and the latter is a modified version of the quadruplet surface of Martinez et al. to include the long-range charge-induced-dipole potential. The details of the modification of this surface are presented as well as the comparison of the rotational excitation resulting from collisions with hydrogen on these two surfaces. The effect of the coupling between vibration and rotation on the rotational excitation rate is also discussed, as the potential well depth of the doublet surface is quite large and allows the coupling between many vibrational channels of OH(+). As the hydrogen exchange reaction can occur for both potential energy surfaces, we discuss the reliability of the approximation made by the calculation of the cross sections with a quantum dynamics limited to the inelastic process. The relative importance of the collisions on the doublet or quadruplet surface within a given rotational transition is also discussed.

Characterization and reactivity of the weakly bound complexes of the [H, N, S](-) anionic system with astrophysical and biological implications

We investigate the lowest electronic states of doublet and quartet spin multiplicity states of HNS(-) and HSN(-) together with their parent neutral triatomic molecules. Computations were performed using highly accurate ab initio methods with a large basis set. One-dimensional cuts of the full-dimensional potential energy surfaces (PESs) along the interatomic distances and bending angle are presented for each isomer. Results show that the ground anionic states are stable with respect to the electron detachment process and that the long range parts of the PESs correlating to the SH(-) + N, SN(-) + H, SN + H(-), NH + S(-), and NH(-) + S are bound. In addition, we predict the existence of long-lived weakly bound anionic complexes that can be formed after cold collisions between SN(-) and H or SH(-) and N. The implications for the reactivity of these species are discussed; specifically, it is shown that the reactions involving SH(-), SN(-), and NH(-) lead either to the formation of HNS(-) or HSN(-) in their electronic ground states or to autodetachment processes. Thus, providing an explanation for why the anions, SH(-), SN(-), and NH(-), have limiting detectability in astrophysical media despite the observation of their corresponding neutral species. In a biological context, we suggest that HSN(-) and HNS(-) should be incorporated into H2S-assisted heme-catalyzed reduction mechanism of nitrites in vivo.

Theoretical spectroscopic investigations of HNS(q) and HSN(q) (q = 0, +1, -1) in the gas phase

We performed accurate ab initio investigations of the geometric parameters and the vibrational structure of neutral HNS/HSN triatomics and their singly charged anions and cations. We used standard and explicitly correlated coupled cluster approaches in connection with large basis sets. At the highest levels of description, we show that results nicely approach those obtained at the complete basis set limit. Moreover, we generated the three-dimensional potential energy surfaces (3D PESs) for these molecular entities at the coupled cluster level with singles and doubles and a perturbative treatment of triple excitations, along with a basis set of augmented quintuple-zeta quality (aug-cc-pV5Z). A full set of spectroscopic constants are deduced from these potentials by applying perturbation theory. In addition, these 3D PESs are incorporated into variational treatment of the nuclear motions. The pattern of the lowest vibrational levels and corresponding wavefunctions, up to around 4000 cm(-1) above the corresponding potential energy minimum, is presented for the first time.

Theoretical spectroscopic characterization of the ArBeO complex

Using the recently developed explicitly correlated coupled cluster method in connection with the aug-cc-pVTZ basis set, we generated the three-dimensional potential energy surface (3D-PES) of the ground state of the Ar-BeO complex. This PES covers the regions of the global and local minima, the saddle point, and the dissociation of the complex. The PES is also used for the calculation of the rovibrational spectrum up to the dissociation limit. The high density of levels which is observed favors the mixing of the states and hence the occurrence of anharmonic resonances. The wavefunctions of the high rovibrational levels exhibit large amplitude motions in addition to strong anharmonic resonances. Our theoretical spectrum should be helpful in identifying the van der Waals modes of this complex in laboratory.