Flexing the muscles of m-divinylbenzene: direct measurement of the barriers to conformational isomerization.
The energy thresholds to isomerization of the three conformational isomers of m-divinylbenzene (cis-cis, cis-trans, and trans-trans) were directly measured by stimulated emission pumping-population transfer (SEP-PT) spectroscopy. The experimentally determined isomerization thresholds are Ethresh(cc --> ct, tt) = 1080-1232 cm(-1), Ethresh(tt --> ct, cc) = 1130-1175 cm(-1), Ethresh(ct --> cc) = 997-1175 cm(-1), and Ethresh(ct --> tt) = 997-1232 cm(-1). On the basis of the threshold values for X --> Y and Y --> X isomerization, the relative energies of the conformational isomers are -102 < or = E(ct) < or = +178 cm(-1) and -102 < or = E(cc) < or = +95 cm(-1) relative to E(tt) = 0. UV-hole-filling (UVHF) spectroscopy was also used to determine the effect of population returning to the ground state via fluorescence. A full set of governing equations for SEP-PT and UVHF spectroscopy is reported that will be generally useful for future studies using these methods. By comparison of these results with the computed stationary points on a calculated surface (DFT B3LYP/6-31+G*), the isomerization pathway was determined to involve sequential isomerization of each vinyl group rather than concerted motion. The energy thresholds were also combined with the ground state torsional vibrational energy levels to obtain a new fitted two-dimensional torsional potential for mDVB.
Selby
Talitha M
TM
Department of Chemistry, Purdue University, West Lafayette, Indiana 47907-2084, USA.
Zwier
Timothy S
TS