ISSN:
1089-7690
Source:
AIP Digital Archive
Topics:
Physics
,
Chemistry and Pharmacology
Notes:
The first-excited-singlet state of each of the compounds 2-vinylanthracene (2VA) and 2-(2'-propenyl) anthracene (22PA) undergoes an electronically adiabatic torsional double-well isomerization which interconverts a s-cis conformation and a s-trans conformation. The forward rate constant kf for the isomerization was measured by picosecond fluorescence spectroscopy as a function of temperature and solvent viscosity η in the range 0.23–3.25 cP, i.e., in the C5–C16 normal alkanes. For 2VA, kf is independent of η within experimental uncertainty (∼5%), while for 22PA kf(approximately-equal-to)cη−0.23. An effective torsional isomerization potential of the form V(φ)=(V1/2)(1−cos φ)+(V2/2)(1−cos 2φ) was determined from thermochemical data on the isomerization. Torsional well (ω0) and barrier (ωb) frequencies were calculated from the effective potential and the appropriate moments of inertia. With an estimate in hand for ωb, it was possible to directly evaluate theoretical predications from the (i) theory of Kramers and alternatively from the (ii) theory of Grote and Hynes [employing the Zwanzig–Bixon–Metiu–Oxtoby–Freed (ZBMOF) expression for the frequency dependent friction ζˆ(λ)]. Kramers' theory incorrectly predicts a large η dependence of kf for both 2VA and 22PA. In contrast, Grote–Hynes, with the ZBMOF ζˆ(λ), correctly predicts the absence of a significant η dependence of kf for 2VA, but underestimates the η dependence of kf for 22PA.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1063/1.449337
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