First Observation Of The Spin Rotational Structure Of The Hydroxymethyl Radical (h2coh) In The Ch2 Asymmetric Mode

Rotationally-resolved direct infrared absorption spectra of hydroxymethyl radical (H$_{2}$COH) in the CH$_{2}$ asymmetric mode (\nub{2}) were observed for the first time using the Boulder difference frequency generation infrared spectrometer. Hydroxymethyl radical was formed with chemical selectivity via the reaction of Cl radical with CH$_{3}$OH in a discharge slit-jet supersonic expansion. As a result of sub-Doppler linewidth and low rotational temperature, the \textit{b}-type rotational structure and spin-rotation splitting were fully resolved. In particular, tunneling splitting was observed due to the large-amplitude COH torsional mode. Because of the feasible permutation of hydrogens in the methylenic group, nuclear spin intensity alternation was given as: 3:1 for \textit{K$_{a}$}=even, odd in the 0$^{+}$ level, and 1:3 for \textit{K$_{a}$}=even, odd in the 0$^{-}$ level of the ground vibrational state. The assignments were confirmed rigorously by four-line ground state combination differences, which agreed within the experimental frequency uncertainty (10 MHz). The identified transitions were fit with a Watson \textit{A}-reduction Hamiltonian including the spin rotational interaction, leading to unambiguous determination of asymmetric top spectroscopic constants, as well as spin rotational constants ($\epsilon$$_{aa}$, $\epsilon$$_{bb}$, $\epsilon$$_{cc}$) for the first time