Transient infrared absorption of t-CH3C(O)OO, c-CH3C(O)OO, and alpha-lactone recorded in gaseous reactions of CH3CO and O-2

Abstract

A step-scan Fourier-transform infrared spectrometer coupled with a multipass absorption cell was utilized to monitor the transient species produced in gaseous reactions of CH3CO and O-2; IR absorption spectra of CH3C(O)OO and alpha-lactone were observed. Absorption bands with origins at 1851 +/- 1, 1372 +/- 2, 1169 +/- 6, and 1102 +/- 3 cm(-1) are attributed to t-CH3C(O)OO, and those at 1862 +/- 3, 1142 +/- 4, and 1078 +/- 6 cm(-1) are assigned to c-CH3C(O)OO. A weak band near 1960 cm(-1) is assigned to alpha-lactone, cyc-CH2C(= O)O, a coproduct of OH. These observed rotational contours agree satisfactorily with simulated bands based on predicted rotational parameters and dipole derivatives, and observed vibrational wavenumbers agree with harmonic vibrational wavenumbers predicted with B3LYP/aug-cc-pVDZ density-functional theory. The observed relative intensities indicate that t-CH3C(O)OO is more stable than c-CH3C(O)OO by 3 +/- 2 kJ mol(-1). Based on these observations, the branching ratio for the OH+alpha-lactone channel of the CH3CO+O-2 reaction is estimated to be 0.04 +/- 0.01 under 100 Torr of O-2 at 298 K. A simple kinetic model is employed to account for the decay of CH3C(O)OO.