Anharmonic force field and vibrational dynamics of CH2F2 up to 5000 cm−1 studied by Fourier transform infrared spectroscopy and state-of-the-art ab initio calculations

Abstract

Difluoromethane (CH(2)F(2), HFC-32) is a molecule used in refrigerant mixtures as a replacement of the more environmentally hazardous, ozone depleting, chlorofluorocarbons. On the other hand, presenting strong vibration-rotation bands in the 9 μm atmospheric window, it is a greenhouse gas which contributes to global warming. In the present work, the vibrational and ro-vibrational properties of CH(2)F(2), providing basic data for its atmospheric modeling, are studied in detail by coupling medium resolution Fourier transform infrared spectroscopy to high-level electronic structure ab initio calculations. Experimentally a full quantum assignment and accurate integrated absorption cross sections are obtained up to 5000 cm(-1). Ab initio calculations are carried out by using CCSD(T) theory and large basis sets of either the correlation consistent or atomic natural orbital hierarchies. By using vibrational perturbation theory to second order a complete set of vibrational and ro-vibrational parameters is derived from the ab initio quartic anharmonic force fields, which well compares with the spectroscopic constants retrieved experimentally. An excellent agreement between theory and experiment is achieved for vibrational energy levels and integrated absorption cross sections: transition frequencies up to four quanta of vibrational excitation are reproduced with a root mean square deviation (RMSD) of 7 cm(-1) while intensities are predicted within few km mol(-1) from the experiment. Basis set performances and core correlation effects are discussed throughout the paper. Particular attention is focused in the understanding of the anharmonic couplings which rule the vibrational dynamics of the |ν(1)>, |2ν(8)>, |2ν(2)> three levels interacting system. The reliability of the potential energy and dipole moment surfaces in reproducing the vibrational eigenvalues and intensities as well as in modeling the vibrational and ro-vibrational mixings over the whole 400-5000 cm(-1) region is also demonstrated by spectacular spectral simulations carried out by using the ro-vibrational Hamiltonian constants, and the relevant coupling terms, obtained from the perturbation treatment of the ab initio anharmonic force field. The present results suggest CH(2)F(2) as a prototype molecule to test ab initio calculations and theoretical models.

Vibrational effects on valence electron momentum distributions of CH2F2

“…45 The calculated vibrational frequencies are presented in Table I, showing a good agreement with experimental results reported in the literature. 46 ρ f (p, Q)'s were calculated, according to Eq. (9), at several nuclear geometries distorted from equilibrium along each normal coordinate.…”

Anharmonic theoretical simulations of infrared spectra of halogenated organic compounds

“…[26][27][28][29] As far as IR spectroscopy is concerned, QM calculations carried out at a suitable level of theory allow the prediction of reliable vibrational spectra for small-to medium-sized molecules (for example see Refs. 21,[30][31][32][33][34][35][36] and references therein) In this respect, while approaches based on the vibrational perturbation theory (VPT2) 33,34,[37][38][39][40][41][42] have been shown capable of accurately calculating vibrational frequencies, comparatively less attention has been paid to infrared intensities beyond the double-harmonic approximation. With the recent implementation of the calculation of intensities at a fully anharmonic VPT2 level, 43,44 the simulation of the whole infrared spectrum become feasible, and a theoretical study of both peak positions and absorption intensities of halons can be performed.…”

“…From a computational and methodological point of view, the presence of halogen atoms is particularly challenging, since such elements show large electronegativities and, the heaviest ones, significant relativistic core-electron effects. For this reason, most of the studies performed in past years [16][17][18]20,21,[45][46][47] were carried out at the coupled-cluster level 48 employing medium-tolarge basis sets (at least of triple-ζ quality). Unfortunately, the very high accuracy which usually characterizes such calculations implies a large computational cost, and can be performed only for small-to medium-sized systems.…”

“…Indeed, the macroscopic radiation and atmospheric models, employed to understand the atmospheric chemistry as well as to model the Earth's atmosphere and its evolution, need as an input a detailed dataset. [8][9][10] Within this framework, infrared (IR) spectroscopy plays a primary role, [11][12][13][14][15][16][17][18][19][20][21] as it can provide accurate values of the relevant spectroscopic data, such as band positions and absorption cross sections. With these premises, it is not surprising that the last years have seen a renewed interest in the spectroscopic studies of halogenated organic compounds, motivated not only by their role as air pollutants, but also because these investigations are useful to improve the modeling of the atmospheric chemistry of these compounds.…”

Arbitrary order El'yashevich–Wilson B tensor formulas for the most frequently used internal coordinates in molecular vibrational analyses

“…[1][2][3] The PES is central to our understanding of reactions, 4-8 structure, 9 dynamics, [10][11][12] and a plethora of spectroscopic properties. [13][14][15][16][17][18] Given the difficulties associated with probing even the simplest of PESs experimentally, [19][20][21] the theoretical evaluation of PESs for molecular systems has become one of the most critical and widely utilized roles of computation in modern chemistry and molecular physics. The construction of PESs requires a reasonable choice of coordinates to be made.…”