The dynamics of laser-induced infrared multiphoton isomerization in substituted cyclobutenes–butadienes

Abstract

Clean isomerization between a number of cyclobutenes and their corresponding butadienes has been induced via multiple photon infrared absorption. Appropriate choices of laser frequency allow the reaction to be pushed in either direction, even though the cyclization reaction is typically 10 kcal and 4 e.u. thermodynamically disfavored. A significant dependence of yields on fluence and added inert gas pressure is observed. A simple rate equation model is developed which reproduces the major experimental observations regarding the system. Absorption cross sections, a necessary parameter for the rate equation model, are measured both calorimetrically and photoacoutically. The absorption cross section does not vary significantly as a function of laser fluence implying that absorption cross sections for successive photon absorption processes are essentially constant. The measured absorption cross section is also very similar in magnitude to the absorption cross section taken from the infrared spectrum of the compound at the laser frequency. No pressure effects on the absorption cross section are observed over the pressure range studied.

Chemistry, spectroscopy, and isotope-selective infrared photochemistry of a volatile uranium compound tailored for 10-.mu. absorption: uranium hexamethoxide (U(OCH3)6)

“…thermore, multiphoton absorption cross sections in many systems remain constant as a function of fluence, implying an absorption cross section that does not significantly vary as a function of level of excitation. 58,59 In such systems, densities of states approach the criterion used for the onset of the quasi-continuum, ca. 100 states/cm"1, near 3000 cm"1, i.e., approximately the v = 3 level of the pumped mode.59 Uranium hexamethoxide has 87 normal modes, many of which are of substantially lower frequency than in the aforementioned hydrocarbon systems.…”