Estimating the carbonyl anharmonic vibrational frequency from affordable harmonic frequency calculations.
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.
Faculty of Chemistry, University of Opole, Opole, Poland.
Aneta
A
Buczek
Teobald
T
Kupka
Stephan P A
SP
Sauer
Małgorzata A
MA
Broda
D000081:Acetamides / Q000737:chemistry*; D000447:Aldehydes / Q000737:chemistry; D003198:Computer Simulation; D005557:Formaldehyde / Q000737:chemistry*; D005559:Formamides; D005740:Gases / Q000737:chemistry; D007659:Ketones / Q000737:chemistry; D008956:Models, Chemical; D008958:Models, Molecular; D008968:Molecular Conformation; D011789:Quantum Theory; D012997:Solvents / Q000737:chemistry; D017550:Spectroscopy, Fourier Transform Infrared; D014732:Vibration; D014867:Water / Q000737:chemistry*
D000081:Acetamides; D000447:Aldehydes; D005559:Formamides; D005740:Gases; D007659:Ketones; D012997:Solvents; D014867:Water; D005557:Formaldehyde; C031066:formamide; C018595:N-methylacetamide