Raman spectroscopy, surface‐enhanced Raman spectroscopy and density functional theory studies of 2‐formylfuran

Abstract

The Raman spectrum of 2-formylfuran in the liquid phase shows pairs of bands whose temperature and solvent dependence behavior can be interpreted in terms of cis-trans conformational equilibrium. Density functional theory (DFT) calculations performed at the BPW91/6-311+G * and B3LYP/6-311+G * theoretical levels show that the trans-isomer is more stable than the cis-isomer by 2.81 and 3.12 kJ mol −1 , respectively. The theoretical 1H values obtained at these theoretical levels [2.99 kJ mol −1 (BPW91) and 3.2 kJ mol −1 (B3LYP)] are very close to the experimental value 1H = 2.52 kJ mol −1 determined from the plots of logarithmic relative intensities of the band pairs 1673/1692, 1477/1465 and 1396/1371 cm −1 of these isomers against reciprocal temperature. The surface-enhanced Raman spectrum of 2-formylfuran in silver colloid suggests the chemisorption of this molecular species on the silver surface both through the ring oxygen and the oxygen atom of the substituent group, the cis-form being preferred in the adsorption state. The adsorbed molecules are oriented perpendicular or least tilted with respect to the silver surface.

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“…As can be observed from Table 7.1, the experimental wavenumber shifts (ν least stable form − ν most stable form ) are correctly reproduced by the theoretical (ν cis − ν trans ) values, thus clearly supporting the assignment of the most stable form to the trans-conformer (Iliescu et al 2003, Iliescu et al 2002c. Furthermore, the identification of the most stable form as the trans-or cisconformer was obtained by comparing the experimental and calculated wavenumber shifts of the conformers.…”

“…The total energy of the trans-and cis-isomers including zero point correction were found (Iliescu et al 2003) to be −343.40906 and −343.40799 Hartree at the BPW91 theoretical level and −343.44103 and −343.43984 Hartree at the B3LYP theoretical level. The total energy of the trans-and cis-isomers including zero point correction were found (Iliescu et al 2003) to be −343.40906 and −343.40799 Hartree at the BPW91 theoretical level and −343.44103 and −343.43984 Hartree at the B3LYP theoretical level.…”

“…The background signal present in the SERS spectrum in the range 1600-1200 cm -1 is probably due to the photo-or thermal decomposition of 2FF, which forms a carbon layer on the silver surface (Iliescu et al 1995). 7.5) further demonstrates the adsorption of 2FF molecules on the silver surface through the ring oxygen atom (Iliescu et al 2003, Iliescu et al 2002c. The selective enhancement of the vibrational modes in the SERS spectrum was used as a probe for the determination of the adsorption mode of 2FF molecules on the silver surface.…”

“…DFT calculations were performed at the BPW91/6-311 + G* and B3LYP/6-311 + G* theoretical levels to find the most stable configuration (Iliescu et al 2003, Iliescu et al 2002c). DFT calculations were performed at the BPW91/6-311 + G* and B3LYP/6-311 + G* theoretical levels to find the most stable configuration (Iliescu et al 2003, Iliescu et al 2002c).…”

“…By comparing the SERS spectrum of 2FF with the conven- tional Raman spectrum ( Fig. Therefore, one can conclude that 2FF molecules are chemisorbed on the silver surface (Iliescu et al 2003, Iliescu et al 2002c). Therefore, one can conclude that 2FF molecules are chemisorbed on the silver surface (Iliescu et al 2003, Iliescu et al 2002c).…”