VIBRATIONAL SPECTROSCOPY AND PHOTOCHEMISTRY OF FORMYL-FLUORIDE

[摘要] We have investigated and detected the photoinduced unimolecular elimination reaction of highly vibrationally excited formyl fluoride (HFCO) prepared by direct single photon excitation to high vibrational levels of the ground electronic state. In the first step of the study we recorded the ground electronic state vibrational spectrum of formyl fluoride (HFCO) from 500 cm(;;-1) to 9500 cm(;;-1) and 11075 cm(;;-1) to 18500 cm(;;-1). The former region was recorded on an IBM FTIR spectrometer; the latter region was recorded on a home built, computer controlled, cw photoacoustic spectrometer. The vibrational spectrum has been assigned as a series of CH overtone transitions and numerous combination bands built on the CH overtones. The next step of the study entailed measuring individual rovibronic lineshapes in the 4(nu)(,CH) and 5(nu)(,CH) vibrational overtone bands under high resolution scan conditions. Additionally, the 6(nu)(,CH) and 7(nu)(,CH) overtone bands have been computer simulated as a function of average Lorentzian linewidth. The inferred lifetimes from these linewidth studies are 358, 201, 1.3 and 0.8 picoseconds for the 4(nu)(,CH), 5(nu)(,CH), 6(nu)(,CH), and 7(nu)(,CH) transitions, respectively. In the final leg of the HFCO investigation, we developed a novel experiment to detect unimolecular reactions. The novel experiment simultaneously incorporates a standing wave dye laser to pump the desired overtone or combination band and a ring dye laser to monitor the photochemical product using the photoacoustic effect. Also, we report the results of Stern - Volmer type experiments determining the specific rate coefficient, k(E), for the HFCO elimination reaction at various energies corresponding to particular HFCO overtone and combination bands. The results are compared to the calculated RRKM statistical rate coefficient and discussed in terms of mode selective vibrational photochemistry. Generally, the results of the Stern - Volmer experiments agree with statistical predictions with minor deviations. Also, tunneling through the reaction barrier is detected by pumping combination bands which lie at lower energy than the reaction threshold of 40.8 kcal/mole. We have also recorded portions of the vibrational overtone spectrum of 1,2-dibromopropane, 1-bromo-2-chloro-ethane and norbornadiene. Initial experiments attempting to induce vibrational photochemistry with these three systems have been negative.

[发布机构] Rice University