Department of Chemistry, Marquette University, Milwaukee, Wisconsin 53233, USA.
J Chem Phys. 2011 Sep 28;135(12):124503. doi: 10.1063/1.3640887.
Iso-polyhalomethanes are known reactive intermediates that play a pivotal role in the photochemistry of halomethanes in condensed phases. In this work, iso-bromoform (iso-CHBr(3)) and its deuterated isotopomer were characterized by matrix isolation infrared and UV/visible spectroscopy, supported by ab initio and density functional theory calculations, to further probe the structure, spectroscopy, and photochemistry of this important intermediate. Selected wavelength laser irradiation of CHBr(3) isolated in Ar or Ne matrices at ~5 K yielded iso-CHBr(3); the observed infrared and UV/visible absorptions are in excellent agreement with computational predictions, and the energies of various stationary points on the CHBr(3) potential energy surface were characterized computationally using high-level methods in combination with correlation consistent basis sets. These calculations show that, while the corresponding minima lie ~200 kJ/mol above the global CHBr(3) minimum, the isomer is bound by some 60 kJ/mol in the gas phase with respect to the CHBr(2) + Br asymptote. The photochemistry of iso-CHBr(3) was investigated by selected wavelength laser irradiation into the intense S(0) → S(3) transition, which resulted in back photoisomerization to CHBr(3). Intrinsic reaction coordinate calculations confirmed the existence of a first-order saddle point connecting the two isomers, which lies energetically below the threshold of the radical channel. Subsequently, natural bond orbital analysis and natural resonance theory were used to characterize the important resonance structures of the isomer and related stationary points, which demonstrate that the isomerization transition state represents a crossover from dominantly covalent to dominantly ionic bonding. In condensed phases, the ion-pair dominated isomerization transition state structure is preferentially stabilized, so that the barrier to isomerization is lowered.
等多卤代甲烷是众所周知的活性中间体，在卤代甲烷的凝聚相光化学中起着关键作用。在这项工作中，通过基质隔离红外和紫外/可见光谱学对异溴仿（iso-CHBr(3)）及其氘代同位素进行了表征，并辅以从头计算和密度泛函理论计算，以进一步探究该重要中间体的结构、光谱和光化学。在~5 K 下，用选定波长的激光辐照 Ar 或 Ne 基质中分离出的 CHBr(3)，生成 iso-CHBr(3)；观察到的红外和紫外/可见吸收与计算预测非常吻合，并用高水准方法与相关一致基组相结合，对 CHBr(3)势能面上各种稳定点的能量进行了计算表征。这些计算表明，虽然相应的最小值位于全球 CHBr(3)最小值之上约 200 kJ/mol，但在气相中，该异构体相对于 CHBr(2) + Br 渐近线仍被束缚约 60 kJ/mol。通过选定波长的激光辐照进入强烈的 S(0) → S(3)跃迁，研究了 iso-CHBr(3)的光化学，这导致了向 CHBr(3)的反向光异构化。内禀反应坐标计算证实了两个异构体之间存在连接它们的一阶鞍点，该鞍点的能量低于自由基通道的阈值。随后，自然键轨道分析和自然共振理论被用来对异构体和相关稳定点的重要共振结构进行了表征，这表明异构化过渡态代表了从主要共价键到主要离子键的转变。在凝聚相中，离子对占主导地位的异构化过渡态结构被优先稳定，从而降低了异构化的能垒。
