Anharmonic Vibrational Spectroscopy Calculations for Proton-Bound Amino Acid Dimers

Abstract

Results of anharmonic frequency calculations carried out for GlysLysH(+) and GlyGlyH(+) are presented and compared to gas phase electrospray ionization (ESI) spectroscopy experiments. Anharmonic frequencies are obtained via correlation-corrected vibrational self-consistent field (CC-VSCF) calculations. The potential used is based on the PM3 semiempirical electronic structure method, but improved by fitting to ab initio MP2 calculations at the harmonic level. The key results are as follows: (1) Hydrogens acting as intermolecular bridges have very anharmonic stretches whose frequencies cannot be reliably predicted by the harmonic approximation. An example is the carboxylate bound NH(3)(+) stretch. (2) The computed anharmonic vibrational frequencies are in good agreement with experiment and provides a very large improvement over harmonic frequencies especially for OH and NH stretches. For example the calculated CC-VSCF frequencies of GlysLysH(+) and GlyGlyH(+) have overall average deviations of 1.35% and 1.48% only, respectively, from experiment. (3) The harmonic OH bond stretching frequency deviates by 6.64% from experiments. The CC-VSCF calculations reduce this deviation by more than an order of magnitude to 0.56%. The anharmonicity of the OH stretch is intrinsic, rather than due to coupling with other modes. (4) Anharmonic coupling between the NH(3)(+) stretch and several other normal modes is strong, and provide the main contribution for the anharmonicity of this mode. Properties of the potential energy surfaces of the proton-bound complexes are briefly discussed in light of the results.

Vibrational self-consistent field calculations for spectroscopy of biological molecules: new algorithmic developments and applications

“…143 It was found that the contribution is non-negligible, but not too significant to justify the increased cost of CPU time. While this approximation must be tested on a case by case basis, it has been applied extensively for many VSCF calculations of biological molecules 56,[144][145][146][147][148][149][150][151] with satisfactory results. The VSCF algorithm was introduced in the GAMESS 152 suit of program, and later also in the MOLPRO 153 program package.…”

Ab initio molecular geometry and anharmonic vibrational spectra of thiourea and thiourea‐d4

“…15 Many successful anharmonic vibrational spectroscopy calculations based on VSCF method and its CC-VSCF variants have been reported, [13][14][15][16][17][18][19][20][21][22][23][24][25] even for molecules of considerable size (more than 10 atoms). 16,22 Nonetheless, it has been noted that PES computed on a grid derived from normal-mode displacement vectors expressed in rectilinear coordinates overestimates single-mode potentials and gives rise to artificially large mode-mode coupling potentials in the case of large amplitude vibrations, which, in turn, impedes the performance of VSCF method for such modes. 13,21 Considerable improvement in the soft mode description by the VSCF method has been achieved by using PES calculated on a grid obtained from normal-mode displacement vectors expanded in curvilinear (internal) coordinates, 23 which is devoid of factitious mode-mode coupling.…”

Theoretical Methods for Vibrational Spectroscopy and Collision Induced Dissociation in the Gas Phase

“…The group of R.B. Gerber has in particular shown that PM3 representations can be accurate enough for vibrational spectroscopy of simple molecules [82] (static calculations). DFTB (DFT Tight Binding) also shows promising results for vibrational spectroscopy; see for instance [83][84][85][86][87].…”