Investigation of the Absorption Cross Section of Phenyl Radical and Its Kinetics with Methanol in the Gas Phase Using Cavity Ring-Down Spectroscopy and Theoretical Methodologies.

Abstract

Cavity ring-down spectroscopy (CRDS) was used to measure the absorption cross section of phenyl radicals (C6H5•) at 504.8 nm (2B1 ← 2A1 transition) in the nitrogen atmosphere at 40 Torr total pressure and 298 K using nitrosobenzene (C6H5NO) as the radical precursor. At 504.8 nm, the absorption cross section was measured to be σphenyl504.8nm = (5.7 ± 1.4) × 10-19 cm2 molecule-1. The absorption cross section was independent of the total pressure range (40-200 Torr) over which it was studied with a precursor concentration of (4-5) × 1013 molecules cm-3. In addition to this, the absolute rate coefficients for the reaction of phenyl radicals with methanol were measured over the temperature range of 263-298 K and at 40 Torr pressure with N2 using CRDS. The temperature-dependent rate coefficient for the title reaction over the studied temperature range was obtained to be k263-298Kexperiment (T) = (1.38 ± 0.60) × 10-11 exp [-(1764 ± 321)/T] cm3 molecule-1 s-1 with a rate coefficient of k(T) = (3.50 ± 0.32) × 10-14 cm3 molecule-1 s-1 at 298 K. The effect of pressure and laser fluence was found to be negligible within the experimental uncertainties in the studied range. In addition, to complement our experimental findings, the T-dependent rate coefficients for the title reaction were investigated using computational methods. The B3LYP/6-311 + G(d,p) level of theory was used in combination with canonical variational transition-state theory with small-curvature tunneling to calculate the rate coefficients. The T-dependent rate coefficient in the range of 200-400 K was obtained as k200-400Ktheory (T) = 2.43 × 10-13 exp[-(478.38/T)] cm3 molecule-1 s-1 with a room-temperature (298 K) rate coefficient of 4.67 × 10-14 cm3 molecule-1 s-1.