JILA, National Institute of Standards and Technology, and Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309, USA.
J Chem Phys. 2017 May 21;146(19):194307. doi: 10.1063/1.4982803.
The sub-Doppler CH-symmetric stretch (ν) infrared absorption spectrum of a hydroxymethyl (CHOH) radical is observed and analyzed with the radical formed in a slit-jet supersonic discharge expansion (T = 18 K) via Cl atom mediated H atom abstraction from methanol. The high sensitivity of the spectrometer and reduced spectral congestion associated with the cooled expansion enable first infrared spectroscopic observation of hydroxymethyl transitions from both ± symmetry tunneling states resulting from large amplitude COH torsional motion. Nuclear spin statistics due to exchange of the two methyl H-atoms aid in unambiguous rovibrational assignment of two A-type K = 0 ← 0 and K = 1 ← 1 bands out of each ± tunneling state, with additional spectral information obtained from spin-rotation splittings in P, Q, and R branch K = 1 ← 1 transitions that become resolved at low N. A high level ab initio potential surface (CCSD(T)-f12b/cc-pvnzf12 (n = 2,3)/CBS) is calculated in the large amplitude COH torsional and CH wag coordinates, which in the adiabatic approximation and with zero point correction predicts ground state tunneling splittings in good qualitative agreement with experiment. Of particular astrochemical interest, a combined fit of the present infrared ground state combination differences with recently reported millimeter-wave frequencies permits the determination of improved accuracy rotational constants for the ground vibrational state, which will facilitate ongoing millimeter/microwave searches for a hydroxymethyl radical in the interstellar medium.
经氯原子介导的甲醇中 H 原子的提取，在狭缝喷射超声速膨胀（T = 18 K）中形成羟甲基（CHOH）自由基，观测并分析了其亚多普勒 CH 对称伸缩（ν）红外吸收光谱。光谱仪的高灵敏度和冷却膨胀导致的光谱拥挤度降低，使得首次通过红外光谱观测到了大振幅 COH 扭转运动导致的来自 ± 对称隧道态的羟甲基跃迁。由于两个甲基 H 原子的交换导致的核自旋统计，有助于对每个 ± 隧道态中的两个 A 型 K = 0 ← 0 和 K = 1 ← 1 带的振转分配进行明确的罗维分配，从 P、Q 和 R 支 K = 1 ← 1 跃迁的自旋旋转分裂中获得额外的光谱信息，在 N 较低时这些分裂变得可分辨。在大振幅 COH 扭转和 CH 摇摆坐标中计算了高精度的 ab initio 势能面（CCSD(T)-f12b/cc-pvnzf12（n = 2,3）/CBS），在绝热近似和零点修正下，该势能面预测了基态隧道分裂，与实验结果具有良好的定性一致性。特别具有天体化学意义的是，将当前的红外基态组合差与最近报道的毫米波频率进行联合拟合，允许确定改进的基态转动常数的精度，这将有助于在星际介质中进行毫米/微波搜索羟甲基自由基。
