Identifying the major intermediate species by combining time-resolved X-ray solution scattering and X-ray absorption spectroscopy

Abstract

Identifying the intermediate species along a reaction pathway is a first step towards a complete understanding of the reaction mechanism, but often this task is not trivial. There has been a strong on-going debate: which of the three intermediates, the CHI2 radical, the CHI2-I isomer, and the CHI2+ ion, is the dominant intermediate species formed in the photolysis of iodoform (CHI3)? Herein, by combining time-resolved X-ray liquidography (TRXL) and time-resolved X-ray absorption spectroscopy (TR-XAS), we present strong evidence that the CHI2 radical is dominantly formed from the photolysis of CHI3 in methanol at 267 nm within the available time resolution of the techniques (similar to 20 ps for TRXL and similar to 100 ps for TR-XAS). The TRXL measurement, conducted using the time-slicing scheme, detected no CHI2-I isomer within our signal-to-noise ratio, indicating that, if formed, the CHI2-I isomer must be a minor intermediate. The TR-XAS transient spectra measured at the iodine L-1 and L-3 edges support the same conclusion. The present work demonstrates that the application of these two complementary time-resolved X-ray methods to the same system can provide a detailed understanding of the reaction mechanism.