Characterization of Alkoxycarbonyl Radicals by Step-Scan Time-Resolved Infrared Spectroscopy

Abstract

A series of alkoxycarbonyl radicals has been generated by laser flash photolysis (355 nm) of fluorenone oxime alkyl oxalates in carbon tetrachloride and characterized by time‐resolved infrared spectroscopy using the step‐scan technique. The alkoxycarbonyl radicals ($\tilde {\nu}$C=O = 1802 cm−1 for R = ethyl) generally have a lifetime of several microseconds, decaying by reaction with the solvent to yield esters of chloroformic acid. In some cases, decarboxylation yielding alkyl radicals has also been observed. Thus, photolysis of fluorenone oxime tert‐butyl oxalate results in the formation of tert‐butoxycarbonyl radicals, which subsequently decay, mainly yielding CO2 and tert‐butyl radicals. The benzyloxycarbonyl radical and the acetoneiminoxycarbonyl radical both decarboxylate too rapidly to be detected with our spectrometer (25 ns rise‐time). Upon purging the solution with oxygen, the alkoxycarbonyl radicals were efficiently quenched, to yield alkoxycarbonylperoxy radicals ($\tilde {\nu}$C=O = 1845 cm−1 for R = ethyl), which again had a lifetime of the order of several microseconds. A short‐lived transient ($\tilde {\nu}$ = 1768 cm−1, τ ≈︁ 200 ns) is assigned as the carbonyloxy radical 4a on the basis of comparison with time‐resolved UV/Vis data. A further product of the photolysis of fluorenone oxime oxalates can be tentatively assigned as the 9‐fluorenylideneiminoxy radical 3 ($\tilde {\nu}$ = 1670 cm−1), which according to our DFT calculations should show a very intense $\tilde {\nu}$C=N−O,as. = 1665 cm−1. Fluorenone oxime oxalates are compounds well suited as precursors for alkoxycarbonyl radicals, since they are easily synthesized as crystalline solids, show a convenient absorption at λ = 355 nm, and exhibit a high degree of thermal stability.

Characterization of the Methoxy Carbonyl Radical Formed via Photolysis of Methyl Chloroformate at 193.3 nm

“…[16][17][18] Ab initio calculations 19 at the CCSD(T)/aug-cc-pVDZ//MP2/aug-cc-pVDZ level of theory supported the experimental results, with calculated barriers of 16.8, 17.4, 16.0, and 14.4 kcal/mol for the decarboxylations of methoxy carbonyl, ethoxy carbonyl, iso-propoxy carbonyl, and tert-butoxy carbonyl radicals, respectively. The calculations clearly indicate a correlation between the stability of the alkyl radical leaving group and the barrier height.…”

Snm as a Photolabile Protecting Group for the Cysteinyl Radical − Direct Evidence from Time‐Resolved IR and UV/Vis Spectroscopies

“…[16] Excitation of the sample is carried out by the 4th harmonic (266 nm) of an Nd-YAG laser (Spectra-Physics Quanta-Ray LAB 100) with a repetition rate of 10 Hz. [16] Excitation of the sample is carried out by the 4th harmonic (266 nm) of an Nd-YAG laser (Spectra-Physics Quanta-Ray LAB 100) with a repetition rate of 10 Hz.…”

“…[16] Since no evidence for them is found in our spectra, both fragmentation pathways can be disregarded. A cleavage of the carboxylic acid group is energetically unfavourable.…”

Free radicals in superfluid liquid helium nanodroplets: A pyrolysis source for the production of propargyl radical

“…In the infrared region of the spectrum, direct absorption methods have been used to obtain rotationally resolved spectra of a number of such systems, in both gas cells [3,4] and free jet expansions [5][6][7][8][9][10][11][12][13][14][15][16]. The radicals of interest are typically produced by pyrolysis [5,6], microwave discharge [7][8][9][10][11][12], electric discharge [13][14][15][16] or photolysis [17][18][19][20][21][22][23][24]. For example, flash pyrolysis has been used to generate radicals in cold supersonic jets, as first demonstrated by Kohn et al [5].…”