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 (∼20 ps for TRXL and ∼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 L₁ and L₃ 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.