Estimating the carbonyl anharmonic vibrational frequency from affordable harmonic frequency calculations

Abstract

A linear correlation between harmonic and anharmonic frequencies of water calculated at B3LYP level of theory was observed with a number of basis sets. Similar relationships were found in both the gas phase and solution for several small molecules. The best correlation was found for C = O stretch mode in formaldehyde, formamide and N-methylacetamide. The average difference between B3LYP harmonic and anharmonic ν(C = O) frequencies calculated with several basis sets in these molecules was 30 cm(-1). The ad hoc correction of -30 cm(-1), added to harmonic frequencies of two different carbonyl groups present in a structure of a larger molecule was tested as a fast way of predicting anharmonic frequencies without elaborated calculations. The proposed approach was tested successfully on a larger molecule of E and Z isomers of N-acetyl-α,β-dehydrophenylalanine N',N'-dimethylamide [Ac-(E/Z)-ΔPhe-NMe(2)] and the estimated anharmonic ν(C = O) frequencies were close to directly calculated results.

Gas-phase infrared spectrum of the anionic GFP-chromophore

“…1e with ph were referred to as "phenol" modes [22], being mainly phenol CC stretches [24]. Calculated spectra of the anion in solution -generated using the polarisable continuum model (PCM), the 6-31G** and 6-31+G** basis sets and assuming frequency scale factors identical to those used for the gas-phase calculations [36] -are also presented in Fig. 1.…”

Predicting the structure and vibrational frequencies of ethylene using harmonic and anharmonic approaches at the Kohn–Sham complete basis set limit

“…In subsequent studies [ 42 ], we noticed that anharmonic frequencies showed irregular behavior in polar solvents when the PCM solvation model was used [ 43 ]. In addition, we observed [ 41 ] a simple relation between harmonic and anharmonic C=O stretching frequencies (the corresponding wavenumbers differ by about 31 cm −1 ).…”

Theoretical prediction of structural, vibrational and NMR parameters of plastic optical fiber (POF) material precursors. Cis and trans perhydro- and perfluoro-2-methylene-4,5-dimethyl-1,3-dioxolanes

“…The use of anharmonic model [44][45][46] is significantly more expensive computationally and the selection of a functional which is capable of producing wavenumbers closer to experiment (e. g. without resorting to anharmonic model) is another option. From our earlier vibrational studies on several small molecules we noticed that BLYP produces results significantly closer to experiment [47][48][49].…”