Identification of the high-energy conformer of octa-1,3,5,7-tetraene detected in the one-photon excitation fluorescence spectrum

Abstract

Scaled quantum-mechanical force field (SQM-FF) vibrational analyses at the HF/6-31G//HF/6-31G computational level of the gauche, of octa-1,3,5,7-tetraene were used to identify the rotational isomer observed in the one-photon excitation fluorescence spectrum of this conjugated molecule (Ackerman et al. in J Chem Phys 80:39-44, 1984) as the gauche,Trans,trans, Trans,trans (gTtTt)-conformer. The analysis was performed by comparing the RMS deviations of the 14 fundamental wavenumbers assigned to the high-energy conformer of octa-1,3,5,7-tetraene in the fluorescence spectrum with those calculated for these conformers. Some reassignments of the wavenumbers, originally suggested by experimental observations, were required for the current analysis. The non-planar structure of gTtTt-octa-1,3,5,7-tetraene was calculated to have the terminal -CH=CH 2 moiety rotated by *30.5°with respect to the remainder of the nearly planar skeleton using both the HF/6-31G and MP2(FC)/aug-cc-pVDZ computational levels. The barriers to rotation of the -CH=CH 2 moiety at the MP2(FC)/augcc-pVDZ level were as follows: rotation from the lowestenergy planar all-trans conformer to the non-planar gTtTtconformer was 31.4 kJ/mol; rotation from the gTtTt-conformer to the planar all-trans conformer was 18.0 kJ/mol; and the barrier between the two equivalent gTtTt-octa-1,3,5,7-tetraene conformers was only 1.4 kJ/mol.Keywords Octa-1,3,5,7-tetraene Á Rotational isomerism Á Scaled quantum-mechanical force field Á Vibrational analysis Á HF/6-31G Á MP2(FC)/aug-cc-pVDZ