A force field derived by density functional theory for 4-ethylimidazole: use of the ultraviolet resonance Raman intensities to check the vibrational analysis

Abstract

The vibrational wavenumbers of 4-ethylimidazole were obtained from ab initio studies based on the density functional theory approach. The assignments of the theoretical wavenumbers obtained after scaling of the internal force constants were performed by comparison with the observed Raman and infrared wavenumbers in histidine and 4-methylimidazole. The resonance Raman intensities were obtained from the changes in bond orders occurring during the lowest p Ç p* electronic transition. It can be seen that the change from Hartree-Fock to post-HartreeFock methods leads to revision, to some extent, of both electronic transitions and normal mode assignments.

Infrared and Raman spectroscopic study of pH-induced structural changes of l-histidine in aqueous environment

“…We partly agree on this assignment for the spectra recorded at low and high pH, but in view of the considerably increased intensity and broadening of the 1346 cm À1 component at pH 6-9, we also assume a substantial [23,27,28]. Vice versa, a contribution of the CH 2 wagging vibration to the Raman bands in the same region cannot be excluded [18,24,34,47,50], but as pointed out later, we believe that the largest intensity originates from skeletal modes.…”

“…with either N p -or N t -protonated. Indeed, as empirically demonstrated by Ashikawa and Itoh [21,22] and confirmed by others [16,18,31], the 1588 cm À1 band appears to be correlated to the N p -protonated tautomer and the 1571 cm À1 one to the N t -protonated form. From the strong intensity of the latter compared to that of the 1588 cm À1 peak it can be concluded, that protonation of N t is preferred over protonation of N p , which is in line with the results of a temperature dependency studies of histidine by means of NMR [45] and of 4-methyl-imidazole by Raman [16].…”

“…Furthermore, it can be concluded that the sensitivity of the C=C vibrational frequency for (de)protonation of N p or N t points to either an inductive effect of N-protonation or to coupling of the C=C vibration with =C-N, =C-C or =C-N-H modes. According to DFT calculations on 4-methylimidazole and 4-ethyl-imidazole [15,16,18] coupling of dN-H and nC=C indeed occurs, but only in the N t -protonated tautomer. This explains the difference in band position compared to the N p -protonated form.…”

“…The band has been reported as an N-H in-plane bending [21] and as a ring stretching combined with =C-H bending [23,29] for histidine, and to a coupled ring dN-H/nC=N mode for 4-methyl-imidazole and 4-ethyl-imidazole [15,16,18]. Coupling of vibrations undoubtedly plays a role, and in agreement with [15,16,18,21] we believe the band to be predominantly an in-plane N-H bending/C=N stretching vibration for two reasons.…”

“…However, this research concerned the conformational dependence of the pK a values www.elsevier.com/locate/vibspec Vibrational Spectroscopy 39 (2005) [114][115][116][117][118][119][120][121][122][123][124][125] and not a vibrational analysis. For that reason, only results of theoretical calculations on the model compounds imidazole [12,13], 4-methyl-imidazole [12,[14][15][16][17] and 4-ethyl-imidazole [18] have been used to support the conclusions from empirical IR and Raman studies of histidine [19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36] and closely related molecules [37][38][39][40][41]. The majority of this research has been focussed on IR or Raman bands that can be used as a marker to determine the state of protonation of the imidazole nitrogen atoms [15,16,[20][21]…”