New theoretical method for calculating the radiative association cross section of a triatomic molecule: Application to N2-H-

We present a new theoretical method to treat the atom diatom radiative association within a time independent approach. This method is an adaptation of the driven equations method developed for photodissociation. The bound states energies and wave functions of the molecule are calculated exactly and used to propagate the overlap with the initial scattering wave function. In the second part of this paper, this approach is applied to the radiative association of the N2H- anion. The main features of the radiative association cross sections are analysed and the magnitude of the calculated rate coefficient at 10 Kelvin is used to discuss the existence of the N2H- in the interstellar medium which could be used as a tracer of both N2 and H-

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 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

Carbon dioxide interaction with isolated imidazole or attached on gold clusters and surface: Competition between σ H-bond and π stacking interaction

Using first principle methodologies, we investigate the subtle competition between σ H-bond and π stacking interaction between CO 2 and imidazole either isolated, adsorbed on a gold cluster or adsorbed on a gold surface. These computations are performed using MP2 as well as dispersion corrected density functional theory (DFT) techniques. Our results show that the CO 2 interaction goes from π-type stacking into σ-type when CO 2 interacts with isolated imidazole and Au clusters or surface. The balance between both types of interactions is found when an imidazole is attached to a Au 20 gold cluster. Thus, the present study has great significance in understanding and controlling the structures of weakly-bound molecular systems and materials, where hydrogen bonding and van der Waals interactions are competing. The applications are in the fields of the control of CO 2 capture and scattering, catalysis and bio- and nanotechnologies. © 2014 the Partner Organisations

Periodic dispersion-corrected approach for isolation spectroscopy of N2 in an argon environment: Clusters, surfaces, and matrices

© 2017 American Chemical Society. Ab initio and Perdew, Burke, and Ernzerhof (PBE) density functional theory with dispersion correction (PBE-D3) calculations are performed to study N 2 -Ar n (n ≤ 3) complexes and N 2 trapped in Ar matrix (i.e., N 2 @Ar). For cluster computations, we used both Møller-Plesset (MP2) and PBE-D3 methods. For N 2 @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 2 interacting with these entities. We also deduced the N 2 vibrational frequency shifts caused by clustering or embedding compared to an isolated N 2 molecule. Upon complexation or embedding, the vibrational frequency of N 2 is slightly shifted, while its equilibrium distance remains unchanged. This is due to the weak interactions between N 2 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 2 isolated, in interaction with Ar surfaces, or trapped in Ar matrices

Sign reversal of the spin-orbit constant for the C³Πᵤ state of N₂

Ab initio calculations are performed at the multireference configuration-interaction level of theory on the diagonal spin-orbit functions for the lowest non-Rydberg states of ³Πᵤ symmetry in molecular nitrogen. Spin-orbit constants deduced from the ab initio results confirm the recent suggestion, based on new experimental results, that the C³Πᵤ state of N₂, long known to be regular in the region of its potential-energy curve minimum, becomes inverted at higher energies. By removing the effects of the crossing C′ ³Πᵤ state, it is shown that Av for the C state changes sign from positive to negative near v=8, corresponding to a change in principal molecular-orbital configuration from (1σg)²(1σu)²(2σg)²(2σu)(3σg)²(1πu)⁴(1πg) to (1σg)²(1σu)²(2σg)²(2σu)²(3σg)(1πu)³(1πg)² at an internuclear distance near 1.4 Å.This work was supported by the Australian Research Council Discovery Program under Grant Nos. DP0558962 and DP0773050

Low-temperature rate constants for rotational excitation and de-excitation of C3 (X 1∑g +) by collisions with He (1S)

The low-temperature rotational (de-) excitation of C3 (X 1∑g 1) by collisions with He (1S) is studied using an ab initio potential energy surface (PES). This PES has been calculated using the single- and double-excitation coupled-cluster approach with noniterative perturbational treatment of triple excitations [CCSD(T)] and the augmented correlation-consistent triple-ζ basis set (aug-cc-pVTZ) with bond functions. This PES is then incorporated in full close-coupling quantum scattering calculations for collision energies between 0.1 and 50 cm -1 in order to deduce the rate constants for rotational levels of C3 up to j = 10, covering the temperature range 5-15 K. © 2008. The American Astronomical Society. All rights reserved.The Agencia Española de Cooperación Internacional (project A/8083/07, CSIC-Université Tunis El Manar) and the Spanish Ministerio de Educación y Ciencia (project AYA 2005-00702) are thanked for financial supportPeer Reviewe

Collisional Excitation and Non-LTE Modeling of Interstellar Chiral Propylene Oxide

The first set of theoretical rotational cross sections for propylene oxide (CH3CHCH2O) colliding with cold He atoms has been obtained at the full quantum level using a high-accuracy potential energy surface. By scaling the collision reduced mass, rotational rate coefficients for collisions with para-H2 are deduced in the temperature range 5-30 K. These collisional coefficients are combined with radiative data in a non-LTE radiative transfer model in order to reproduce observations of propylene oxide made toward the Sagittarius B2(N) molecular cloud with the Green Bank and Parkes radio telescopes. The three detected absorption lines are found to probe the cold (∼10 K) and translucent (nH ∼2000 cm-3) gas in the outer edges of the extended Sgr B2(N) envelope. The derived column density for propylene oxide is Ntot ∼3 x 1012 cm-2, corresponding to a fractional abundance relative to total hydrogen of ∼2.5 x 10-11. The present results are expected to help our understanding of the chemistry of propylene oxide, including a potential enantiomeric excess, in the cold interstellar medium

Collisional Excitation and Non-LTE Modeling of Interstellar Chiral Propylene Oxide

The first set of theoretical rotational cross sections for propylene oxide (CH3CHCH2O) colliding with cold He atoms has been obtained at the full quantum level using a high-accuracy potential energy surface. By scaling the collision reduced mass, rotational rate coefficients for collisions with para-H2 are deduced in the temperature range 5-30 K. These collisional coefficients are combined with radiative data in a non-LTE radiative transfer model in order to reproduce observations of propylene oxide made toward the Sagittarius B2(N) molecular cloud with the Green Bank and Parkes radio telescopes. The three detected absorption lines are found to probe the cold (∼10 K) and translucent (nH ∼2000 cm-3) gas in the outer edges of the extended Sgr B2(N) envelope. The derived column density for propylene oxide is N tot ∼3 x 1012 cm-2, corresponding to a fractional abundance relative to total hydrogen of ∼2.5 x 10-11. The present results are expected to help our understanding of the chemistry of propylene oxide, including a potential enantiomeric excess, in the cold interstellar medium

The furan microsolvation blind challenge for quantum chemical methods: First steps

© 2018 Author(s). Herein we present the results of a blind challenge to quantum chemical methods in the calculation of dimerization preferences in the low temperature gas phase. The target of study was the first step of the microsolvation of furan, 2-methylfuran and 2,5-dimethylfuran with methanol. The dimers were investigated through IR spectroscopy of a supersonic jet expansion. From the measured bands, it was possible to identify a persistent hydrogen bonding OH-O motif in the predominant species. From the presence of another band, which can be attributed to an OH-π interaction, we were able to assert that the energy gap between the two types of dimers should be less than or close to 1 kJ/mol across the series. These values served as a first evaluation ruler for the 12 entries featured in the challenge. A tentative stricter evaluation of the challenge results is also carried out, combining theoretical and experimental results in order to define a smaller error bar. The process was carried out in a double-blind fashion, with both theory and experimental groups unaware of the results on the other side, with the exception of the 2,5-dimethylfuran system which was featured in an earlier publication