Department of Chemistry, University of Virginia, McCormick Road, P.O. Box 400319, Charlottesville, Virginia 22904, United States.
J Phys Chem A. 2011 Jun 23;115(24):6472-80. doi: 10.1021/jp200539b. Epub 2011 May 27.
Methyl formate presents a challenge for the conventional chemical mechanisms assumed to guide interstellar organic chemistry. Previous studies of potential formation pathways for methyl formate in interstellar clouds ruled out gas-phase chemistry as a major production route, and more recent chemical kinetics models indicate that it may form efficiently from radical-radical chemistry on ice surfaces. Yet, recent chemical imaging studies of methyl formate and molecules potentially related to its formation suggest that it may form through previously unexplored gas-phase chemistry. Motivated by these findings, two new gas-phase ion-molecule formation routes are proposed and characterized using electronic structure theory with conformational specificity. The proposed reactions, acid-catalyzed Fisher esterification and methyl cation transfer, both produce the less stable trans-conformational isomer of protonated methyl formate in relatively high abundance under the kinetically controlled conditions relevant to interstellar chemistry. Gas-phase neutral methyl formate can be produced from its protonated counterpart through either a dissociative electron recombination reaction or a proton transfer reaction to a molecule with larger proton affinity. Retention (or partial retention) of the conformation in these neutralization reactions would yield trans-methyl formate in an abundance that exceeds predictions under thermodynamic equilibrium at typical interstellar temperatures of ≤100 K. For this reason, this conformer may prove to be an excellent probe of gas-phase chemistry in interstellar clouds. Motivated by new theoretical predictions, the rotational spectrum of trans-methyl formate has been measured for the first time in the laboratory, and seven lines have now been detected in the interstellar medium using the publicly available PRIMOS survey from the NRAO Green Bank Telescope.
甲酸甲酯对传统的化学机制提出了挑战，这些机制被认为指导着星际有机化学。以前对星际云中甲酸甲酯潜在形成途径的研究排除了气相化学作为主要的生产途径，而最近的化学动力学模型表明，它可能通过冰表面的自由基-自由基化学有效地形成。然而，最近对甲酸甲酯和可能与其形成有关的分子的化学成像研究表明，它可能通过以前未探索的气相化学形成。受这些发现的启发，提出了两种新的气相离子-分子形成途径，并使用电子结构理论和构象特异性对其进行了特征描述。所提出的反应，酸催化的Fisher 酯化反应和甲基阳离子转移反应，在与星际化学相关的动力学控制条件下，都以相对较高的丰度产生质子化甲酸甲酯的不太稳定的反式构象异构体。气相中性甲酸甲酯可以通过电子复合反应或质子转移反应到质子亲和性较大的分子中来产生其质子化对应物。在这些中和反应中，构象的保留（或部分保留）将以超过在典型星际温度≤100 K 下热力学平衡预测的丰度产生反式甲酸甲酯。出于这个原因，这种构象可能被证明是星际云中气相化学的一个极好的探针。受新的理论预测的启发，反式甲酸甲酯的旋转光谱首次在实验室中进行了测量，并且现在已经在星际介质中使用 NRAO 绿岸望远镜的公共可用 PRIMOS 调查检测到了 7 条谱线。
