Vibrational analysis of (E,E)-2,4-hexadienal, (E,E,E)-2,4,6-octatrienal, and (E,E,E)-3-methyl-2,4,6-octatrienal

Abstract

Attempted vibrational assignments of (E,E)-2,4-hexadienal, (E,E,E)-2,4,6-octatrienal, and (E,E,E)-3-methyl-2,4,6-octatrienal are given between 3100 and 50 cm−1. We particularly discuss spectroscopic effects of the aldehydic group presence, enlargement of the polyenal chain length, and methyl lateral substitution.

ALL‐trans RETINAL PHOTOISOMERIZATION AND PHOTOOXIDATION FROM UV LASER RADIATION. VIBRATIONAL ASSIGNMENTS OF ALL‐trans 5,8‐PEROXYRETINAL

“…Compared with its intensity for all-trans retinal, the 1337 cm-I IR band has decreased while the Raman one* has increased. Following our proposal for the u,C9C2 and u,C~~C, (see above), we assigned to this frequency both the uC' OHe and 6C14He motions coupled with the u,CyCz and u , C~~C~ ones respectively (Curry et al, 1985;Abo Aly et al, 1985a, 1986a.…”

“…The 250-1000 cm-' region. The in-plane bendings (SCCC), the out-of-plane ethylenic bendings (yCHe), the in-phase stretches (us CC2) associated with methyl substituted carbons C ' and C13 (Abo Aly et al, 1984, 1985a, 1986aCurry et al, 1985) 1981) should occur in this range. In spite of the complexity of this spectral region and the weakness of most of the bands, an accurate comparison between the IR and Raman spectra of the all-trans 5,8-peroxyretinal and of the initial and illuminated solutions may lead to some logical assignments.…”

“…(*) Values given with * are directly obtained from the spectra of the illuminated solutions, then from a mixture of alltrans and 134s and 5,8-peroxyretinal. 0) The vibrational frequencies are located by analogy with all-trans retinal, their assignments are given elsewhere (Abo Aly et al, 1985aAly et al, , 1986b. the IR band of all-trans retinal at 490 cm-' decreases and the 655 cm-' one vanishes.…”

“…This 1008 cm-I band is assigned by Curry et al (1985) to the low frequency mode of a coupled motion involving the two rllCH3 vibrations associated with the lateral methyl groups of the retinal. The band at 1020 cm-I has been assigned to the uncoupled lateral rllCH3 motions in MOAL (Abo Aly et al, 1985aAly et al, , 1986a. Consequently, we propose that in 5,8-peroxyretinal the two rllCH3 (C' and C13) both occur at 1020 cm-' and are uncoupled.…”

“…Most of the bands between 1350 and 1500 cm-' are weak in Raman and medium in IR for all-trans retinal as well as for the all-trans 5,8-peroxyretinal. They were previously assigned to SS, 6,, 6'.CH3 and sCH2 modes (Curry et al, 1985;Abo Aly et al, 1985a, 1986a. The inversion in intensity of the 136&1370 cm-' IR doublet, assigned to u,CH3 of the gem dimethyl group (p ionone) had to be mentioned and related to the NMR observations concerning the 'H and 13C chemical shifts of the Cl69l7H3 and Cl6H3 methyl groups (Tables 1 and 2).…”