“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” K. Mondal, R. Kaipara and B. Rajakumar. J. Phys. Chem. A 2019, 123, 9682-9692.


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 of total pressure and 298 K using nitrosobenzene (C6H5NO) as the radical precursor. At 504.8 nm, the absorption cross section was measured to be: σ(phenyl@504.8nm)=(5.7±1.4)× 10^(-19) cm^2 molecule^(-1). The absorption cross section value 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 (MeOH) were measured over the temperature range of 263-298 K and at 40 Torr of pressure with N2 using CRDS. The temperature dependent rate coefficient for the title reaction over the studied temperature range was obtained to be k(263-298K_experiment)(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. B3LYP/6-311+G(d,p) level of theory was used in combination of canonical variational transition state theory with small curvature tunnelling (CVT/SCT) to calculate the rate coefficients. The T dependent rate coefficient in the range of 200-400 K was obtained as k(200-400K_theory) (T) = 2.43×10-13 exp[-(478.38/T)] cm3 molecule-1 s-1 with a room temperature (298K) rate coefficient of 4.67×10-14 cm3 molecule-1 s-1.
Figure depicting the schematic representation of the in-built CRDS experimental set-up used in our laboratory of atmospheric chemistry. [PMT- Photo Multiplier Tube; MFC- Mass Flow Controllers; DAQ- Data Acquisition Card]