The high resolution infrared spectrum for the à ((2)Π) ← ͠X ((2)Σ(+)) origin band of jet-cooled ethynyl radical (C(2)H) in the gas phase is reported, which exhibits a strong, parity-specific local perturbation in the upper (2)Π(1/2) state. Based on revised parity assignments of the levels, the perturbing state is unambiguously determined to be (2)Σ(+) symmetry, and thus coupled to the à ((2)Π) state by ΔK = ±1 Coriolis interactions. By incorporating Σ-Π Coriolis coupling into the unperturbed Hamiltonian (containing only rotational, spin-rotational, spin-orbit, and lambda-doubling contributions), we are now able to fit the observed (2)Π-(2)Σ(+) origin band to a sub Doppler experimental uncertainty of 15 MHz (0.0005 cm(-1)). In addition, the observation of pairs of transitions to mixed states permits determination of the band origin (ν(pert)) and rotational constant (B(pert)) for the "dark"(2)Σ(+) state, which prove to be in remarkably quantitative agreement with full vibronic predictions of Tarroni and Carter as well as UV dispersed fluorescence studies of Hsu et al. This represents an important benchmark in mapping out non-Born-Oppenheimer vibronic interactions and energy level structure in a polyatomic combustion radical system, an understanding of which will be key to modeling chemical reactions in both terrestrial and astronomical environments.

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High-resolution, fully rotationally resolved direct absorption spectra of hydroxymethyl radical, CH2OH, are presented in the infrared CH stretching region. As a result of low rotational temperatures and sub-Doppler linewidths obtained in the slit supersonic expansion, the Ka = 0 ← 0 band of the symmetric CH stretch for CH2OH has been unambiguously identified and analyzed. By way of chemical confirmation, hydroxymethyl radical is generated via two different slit jet discharge syntheses: (i) direct dissociation of CH3OH to form CH2OH and (ii) dissociation of Cl2 followed by the radical H atom extraction reaction Cl + CH3OH → HCl + CH2OH. The identified transitions are fit to a Watson A-reduced symmetric top Hamiltonian to yield first precision experimental values for the ground state rotational constants as well as improved values for the symmetric stretch rotational constants and vibrational band origin. The results both complement and substantially improve upon spectral efforts via previous double resonance ionization detected infrared methods by Feng et al. [J. Phys. Chem. A, 2004, 108, 7093], as well as offer high-resolution predictions for laboratory and astronomical detection of hydroxymethyl radical in the millimeter-wave region.