The energetic of (CH2F2)(2) investigated by TDL IR spectroscopy and DFT computations: From collision induced relaxation of ro-vibrational transitions to non-covalent interactions

摘要

Difluoromethane (CH2F2) is an atmospheric pollutant presenting strong absorptions within the 8-12 mu m atmospheric window, hence it can contribute to global warming. Its dimer, (CH2F2)(2), is bound through weak hydrogen bonds (wHBs). Theoretically, wHBs are of paramount importance in biological systems, though their modeling at density functional theory (DFT) level requires dispersion correlations to be accounted for. In this work, the binding energy (3.1 +/- 0.5 kcal mol(-1)) of (CH2F2)(2) is experimentally derived from the foreign broadening coefficients of the monomer compound, collisionally perturbed by a range of damping gases. Measurements are carried out on CH2F2 ro-vibrational transitions by means of tunable diode laser spectroscopy. Six stationary points on the potential energy surface (PES) of the dimer are investigated at DFT level by using some of the last generation density functionals (DFs). The Minnesota M06 suite of functionals as well as range separated DFs and DFs augmented by the non-local (NL) van der Waals (vdW) dispersion corrections are considered. DFT results are compared to reference values at the estimated complete basis set (CBS) limit of CCSD(T) theory (coupled cluster with singles and doubles augmented by a perturbational estimate of connected triples) and to the experimental binding energy. The M06-2X, M06-HF, VV10, BLYP-NL, and B3LYP-NL DFs reproduce CCSD(T)/CBS binding energies with a mean absolute deviation <0.4 kcal mol(-1) and about the same deviation from the experimental value. The present results are of twofold relevance: (i) they show that binding energy of homodimers can be conveniently obtained from the monomer's foreign broadening coefficients and that the correct simulation of hydrogen bonds involved in (CH2F2)(2) needs non-covalent interactions to be included into DFT; (ii) O-2(-) and N-2-pressure broadening parameters represent fundamental data for exploiting the efficacy of remote sensing measurements employed to retrieve temperature and concentration profiles of our atmosphere. (C) 2015 AIP Publishing LLC.