Resonance Raman labels: spectroscopic studies on cis- and trans-4-benzylidene-2-phenyl-Δ2 -oxazolin-5-one and an isotopically substituted analog

Abstract

The absorption and preresonance Raman spectra of cis- and trans-4-benzylidene-2-phenyl-Δ2-oxazoIin-5-one are reported. Although steric considerations suggest that the π electron pathway in the cis isomer is considerably distorted compared to the trans isomer, the Raman and absorption spectra of the two isomers are strikingly similar. Preresonance Raman excitation profiles for the cis and trans isomers indicate that the main features in the Raman spectra owe their intensity to coupling to the 360 nm absorption band present in both isomers. It is proposed that both the electronic dipole transition responsible for this absorption and the vibrational modes giving rise to the intense Raman bands are localized in the —C=C—N=C—Ph part of the molecule. Thus the main Raman and absorption bands are insensitive to isomerization in the benzylidene portion. Support for a localized electronic transition, polarized along the —C=C—N=C—Ph long axis, comes from Raman depolarization ratio (ρ) measurements which show that ail intense Raman features in both cis and trans isomers have ρ ∼ 0.33. Further support comes from ir and resonance Raman spectra of trans-4-(4-dimethylamino-3-nitrobenzylidene)-2-phenyloxazolin-5-one substituted either with 13C in the 4 position, or with 15N, in the oxazolinone ring. These spectra indicate that the main Raman feature seen in all 4-benzylidene-2-phenyloxazolinonesat 1561 cm−1 is a symmetric stretching mode associated with the —C=C—N=C— chain and that this feature has some C=N stretching character. The substitution experiments also show that the weak 1654 cm−1 Raman band has a high degree of C=C stretching character and may represent an essentially antisymmetric mode from the C=C—N=C moiety. The preresonance Raman excitation profiles show that the intensity enhancement follows an FB2 type dependence. The utility of the Raman spectrum as a probe for the chromophore responsible for the electronic transition in a highly conjugated system is discussed.