Hybrid diatomics-in-molecules-based quantum mechanical/molecular mechanical approach applied to the modeling of structures and spectra of mixed molecular clusters Arn(HCl)m and Arn(HF)m

Abstract

A new hybrid QM/DIM approach aimed at describing equilibrium structures and spectroscopic properties of medium size mixed molecular clusters is developed. This methodology is applied to vibrational spectra of hydrogen chloride and hydrogen fluoride clusters with up to four monomer molecules embedded in argon shells Arn(H(Cl/F))m (n = 1-62, m = 1-4). The hydrogen halide complexes (QM part) are treated at the MP2/aug-cc-pVTZ level, while the interaction between HX molecules and Ar atoms (MM part) is described in terms of the semiempirical DIM methodology, based on the proper mixing between neutral and ionic states of the system [Grigorenko et al., J. Chem. Phys. 104, 5510 (1996)]. A detailed analysis of the resulting topology of the QM/DIM potential energy (hyper-)surface in the triatomic subsystem Ar-HX reveals more pronounced nonadditive atomic induction and dispersion contributions to the total interaction energy in the case of the Ar-HCl system. An extension of the original analytical DIM-based potential in the frame of the present model as well as the current limitations of the method are discussed. A modified algorithm for the gradient geometry optimization, along with partly analytical force constant matrix evaluation, is developed to treat large cages of argon atoms around molecular clusters. Calculated frequency redshifts of HX stretching vibrations in the mixed clusters relative to the isolated hydrogen-bonded complexes are in good agreement with experimental findings.

Femtisecond single-mole infrared spectroscopy of molecular clusters

“…The onset of puckering in the hydrogen-bonded ring systems reflects itself in the spectra. 66 Alcohols show closely related aggregation patterns 10 and have been studied quite systematically, 67,68 including the connection to molecular force fields. 34 The dimers lend themselves to detailed comparison with theory, 69 also in terms of relative hydrogen bond donor/acceptor strengths.…”

Matrix reorganization with intramolecular tunneling of H atom: Formic acid in Ar matrix

“…The modeling of influence of the Ng matrix on the properties of embedded molecules is usually performed by using molecular dynamics or molecular mechanics methods ͑for instance, see Ref. 33 and references therein͒, where the parameters for interatomic potentials are found by high-level quantum-chemical calculations. Such an approach was applied, for example, for modeling of halogen hydrides in Ar clusters and gave reasonable results.…”

“…Such an approach was applied, for example, for modeling of halogen hydrides in Ar clusters and gave reasonable results. 33,34 This approach seems to be rather promising; however, for more complicated polyatomic molecules the interatomic potentials usually have a rather intricate form. In the future, we plan to test this method for HCOOH embedded in large Ng clusters, but in the present work we limit ourselves to explicit quantum-chemical calculations for the HCOOH-Ar 12 clusters.…”

Matrix effect on vibrational frequencies: Experiments and simulations for HCl and HNgCl (Ng = Kr and Xe)

“…Simulations of small molecules in noble-gas (Ng) clusters lead to reasonable agreement with experimental matrix effect as, for example, for HF in Ar clusters. 5 However, this order of stretching frequencies is not general; for example, it is not valid for the CH stretching mode of formic acid; 6 thus, each case should be considered specifically.…”