Nanosecond Time-Resolved Step-Scan FT-IR Spectroscopy in Conventional and Supercritical Fluids Using a Four-Window Infrared Cell

Abstract

Fast time-resolved step-scan FT-IR (s2–FT-IR) has been used to study excited states and reaction intermediates in conventional and supercritical solvents. We have developed a four-port IR cell for s2–FT-IR measurements. The generation of W(CO)5(Xe), following photolysis of W(CO)6 in supercritical Xe, has been used to optimize our s2–FT-IR measurements in supercritical fluids using the four-port IR cell. We have compared a number of different approaches for obtaining transient time-resolved IR (TR-IR) data. The IR diode-laser-based and s2–FT-IR approaches for TR-IR have been compared directly. The kinetic decay of the CpMo(CO)3 (Cp = η5–C5H5) radical in supercritical CO2 has been determined using both TR-IR approaches, and we find no significant difference in signal-to-noise between these techniques for most of our TR-IR kinetic measurements. We have attempted to compare s2–FT-IR to the scanning dispersive TR-IR method by obtaining the infrared spectrum of the triplet excited state of 4-phenylbenzophenone, which has been published previously. The importance of obtaining high spectral resolution s2–FT-IR spectra for reactions in condensed phases is investigated. The IR spectrum of the CpFe(CO)2 radical in n-heptane shows that important information regarding the structure of the radical can only be obtained by performing time-resolved s2–FT-IR experiments at high spectral resolution.