Probing the Photoisomerization of CHBr3 and CHI3 in Solution with Transient Vibrational and Electronic Spectroscopy
详细信息 查看全文
- 作者：Thomas J. Preston ; Michael A. Shaloski ; F. Fleming Crim
- 刊名：The Journal of Physical Chemistry A
- 出版年：2013
- 出版时间：April 11, 2013
- 年：2013
- 卷：117
- 期：14
- 页码：2899-2907
- 全文大小：536K
- 年卷期：v.117,no.14(April 11, 2013)
- ISSN：1520-5215
文摘
Transient infrared absorption spectroscopy monitors condensed-phase photodissociation dynamics of 30 mM CHBr3 and 50 mM CHI3 in liquid CCl4. The experiments have picosecond time resolution and monitor the C鈥揌 stretch region of both the parent polyhalomethanes and their photolytically generated isomers. The C鈥揌 stretching transitions of these isomers, in which the emergent halogen atom returns to form a C鈥揦鈥揦 bonding motif, appear about 9 ps after photolysis for iso-CHBr2鈥揃r and in about 46 ps for iso-CHI2鈥揑. These time scales are consistent with, but differ from, the time evolution of the transient electronic absorption spectra of the same samples, highlighting the subtle differences between monitoring the vibrational and electronic chromophores. The specificity of using vibrational transitions to track condensed-phase reaction dynamics permits reassessment of the transient electronic spectrum of photolysis in neat CHBr3, which has an additional prompt feature near 400 nm. Calculations show that this feature, which arises from a precursor to the isomer, is a charge-transfer transition of a contact pair between the nascent Br fragment and a nearby CHBr3 molecule. Dilution and solvent studies show that transition is independent of the solvent. The iso-CHBr2鈥揃r transition wavelength, however, shifts over the range of 400 to 510 nm depending on the solvent. Time-dependent density functional calculations faithfully reproduce these trends.