Unraveling Environmental Effects on Hydrogen-Bonded Complexes:  Matrix Effects on the Structures and Proton-Stretching Frequencies of Hydrogen−Halide Complexes with Ammonia and Trimethylamine

Abstract

Anharmonicity and matrix effects play important roles in determining the proton-stretching frequencies in hydrogen-bonded complexes of HCl and HBr with NH 3 and N(CH 3 ) 3 . These effects have been investigated through ab initio calculations carried out at MP2/aug′-cc-pVDZ for complexes with HCl and at MP2/6-31+G-(d,p) for complexes with HBr. The potential surfaces of these complexes are very anharmonic, since the region surrounding the global minimum may be very broad and relatively flat, or a second region of the surface, displaced from the global minimum, can be accessed in either the ground (V ) 0) or the first excited (V ) 1) state of the proton-stretching mode. As a result, two-dimensional anharmonic frequencies, particularly for the proton-stretching vibration, can be dramatically different from the corresponding harmonic frequencies. Moreover, the zero-point energy contribution to binding enthalpies based on harmonic vibrational frequencies can be significantly overestimated in some complexes. To model the effects of matrices on the structures and spectra of these complexes, potential surfaces have been generated in the presence of external electric fields applied along the hydrogen-bonding X-H-N direction. These fields preferentially stabilize more polar hydrogen-bonded structures. The changes in anharmonic frequencies computed from these surfaces depend on the strength of the field and the nature of the equilibrium structure at zero field. Comparisons between computed frequencies for these complexes and experimental frequencies obtained in Ar and N 2 matrices provide insight into the dependence of proton-stretching frequencies on the environment. It is now possible to understand the apparently disparate effects of Ar and N 2 matrices on the spectra of closely related complexes.

Modeling Ar and Kr matrix effect on the ν s (Cl–H) and ν l (Cl–H) of Cl–H···NH3 by the IEF-PCM method

“…[1] and then by Andrews et al [2] for investigating the environmental perturbation or matrix effect on the complex. Among the 12 fundamental vibrational modes of the complex, the Cl-H stretching frequency (m s (Cl-H)) is most investigated theoretically [2][3][4][5][6] for its distinct characteristics and for the interest in the nature of the complex (hydrogen-bonded dimer or ionpair). Up to now, three approaches have been applied to modeling an argon matrix effect on the geometry and infrared spectrum of the Cl-HÁÁÁNH 3 complex, i.e., supermolecule method [4], applying external electric fields along the hydrogen bonding direction [5,6], and artificial decrease of a fixed HÁÁÁN distance [2].…”

“…A rational explanation is that such modeling was in favor of the geometry optimization of the supermolecule, which is not easy to be completed for a supermolecule, especially that containing one or more rare gas atoms. The approach of applying external electric fields requested that the complex to be investigated should have a linear hydrogen bond [5,6]. If the hydrogen bond is non-linear, the direction along which the electric field is applied will be not easily determined.…”

“…If the hydrogen bond is non-linear, the direction along which the electric field is applied will be not easily determined. In addition, the approach requested that the strength of the electric field should change continuously in a specified range and that each strength should be accompanied by a frequency calculation [5,6]; thus, the computational consumption is significant. The approach of artificial decrease of a fixed HÁÁÁN distance is rather more demonstrative [2].…”

Proton Transfer in Strong Hydrogen Bonds Revealed by Infrared Spectra - Correlations between Structure and Spectra for the BrH : NH3Complex

“…Any interaction of the hydrogen bonded complex with its surrounding may cause changes in the structure and spectra of the complex from those found in the gas phase (see recent papers and references therein [1][2][3][4][5]). In this work we consider changes in the hydrogen bonded complex that are caused by the reaction field in a dielectric medium.…”

Cooperativity and Proton Transfer in Hydrogen‐Bonded Triads

“…[33] A theoretical analysis of environmental effects on the extent of proton transfer and proton stretching frequencies for hydrogen-bonded complexes of hydrogen halides and NH 3 and N(CH 3 ) 3 has been recently published by Jordan and Del Bene. [30] Such studies have stimulated our interest in the role that a single near-neighbor can have on the ease of the PT process. In this respect, the paper of Heidrich et al [34] in which this problem was analyzed for a large set of cyclic fourand six-center systems, must be cited.…”