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 Å in 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.

Standard and explicitly correlated coupled-cluster theory computations in conjunction with large basis sets are performed to characterize [Al,P,O] isomers. Three isomers, namely, linear-AlOP, bent-AlOP, and linear-OAlP, are found to be stable species. Their optimized equilibrium geometries, harmonic vibrational frequencies, rotational constants, and relative energies are deduced. In addition, a set of spectroscopic parameters is generated from the three-dimensional potential energy surfaces of each isomer at the (R)CCSD(T)/aug-cc-pV5Z level. The linear isomers have an X 3 Σ − electronic ground state and are characterized as weakly bound systems or floppy molecules due to their lowfrequency bending modes (<150 cm −1 ). The dipole moment of linear-AlOP is calculated to be 1.48 D. By comparison, a much larger dipole moment is computed for linear-OAlP (5.01 D), indicating lower ionic character in AlOP. Both the linear-OAlP and linear-AlOP isomers are suggested to be good candidates for detection in interstellar media by radio astronomy.

Three isomers of the triatomic [Al, N, O] molecular system have been observed in a solid argon matrix by infrared absorption spectroscopy using 15N and 18O isotopic substitution. The present work provides high-level quantum chemical predictions of their spectroscopic parameters to observe this system in the interstellar medium. The spectroscopic parameters, stability, and geometries of the lowest stable isomers of its isoelectronic system [Al, N, S] were characterized using coupled-cluster CCSD(T), explicitly correlated coupled-cluster CCSD(T)-F12, and multireference configuration interaction. The three-dimensional potential energy surfaces of all isomers were computed at the CCSD(T)-F12/aug-cc-pV5Z level, and a set of spectroscopic parameters were calculated. In both systems, the most stable isomer is linear with an X3Σ− electronic ground state, and all linear isomers are characterized by small bending modes of less than 200 cm−1. Due to their large dipole moments, the high intensities of such modes, and the nonexistence of anharmonic resonance complicating their spectra, our results facilitate the detection of AlNO and AlNS in the laboratory or in the interstellar medium.

We use density functional theory in the generalized gradient approximation to study the adsorption of imidazole on the Au(111) surface and account for dispersion effect using Grimme's empirical dispersion correction technique. Our results show that the adsorption energy of imidazole depends on the slab size and on the adsorption site. In agreement with other studies, we find the largest adsorption energy for imidazole on a top site of Au(111). However, we also note that the adsorption energy at other sites is substantial.

Ab initio and Perdew, Burke, and Ernzerhof (PBE) density functional theory with dispersion correction (PBE-D3) calculations are performed to study N-Ar (n ≤ 3) complexes and N trapped in Ar matrix (i.e., N@Ar). For cluster computations, we used both Møller-Plesset (MP2) and PBE-D3 methods. For N@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 N interacting with these entities. We also deduced the N vibrational frequency shifts caused by clustering or embedding compared to an isolated N molecule. Upon complexation or embedding, the vibrational frequency of N is slightly shifted, while its equilibrium distance remains unchanged. This is due to the weak interactions between N 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 N isolated, in interaction with Ar surfaces, or trapped in Ar matrices.