Kumar Anubhav, Mondal Supratim, Banerjee Shibdas
Department of Chemistry, Indian Institute of Science Education and Research Tirupati, Tirupati 517507, India.
J Am Chem Soc. 2021 Feb 17;143(6):2459-2463. doi: 10.1021/jacs.0c12512. Epub 2021 Feb 3.
Carbocations are short-lived reactive intermediates in many organic and biological reactions that are difficult to observe. This field sprung to life with the discovery by Olah that a superacidic solution allowed the successful capture and nuclear magnetic resonance characterization of transient carbocations. We report here that water microdroplets can directly capture the fleeting carbocation from a reaction aliquot followed by its desorption to the gas phase for mass spectrometric detection. This was accomplished by employing desorption electrospray ionization mass spectrometry to detect a variety of short-lived carbocations (average lifetime ranges from nanoseconds to picoseconds) obtained from different reactions (e.g., elimination, substitution, and oxidation). Solvent-dependent studies revealed that aqueous microdroplets outperform organic microdroplets in the capture of carbocations. We provide a mechanistic insight demonstrating the survival of the reactive carbocation in a positively charged aqueous microdroplet and its subsequent ejection to the gas phase for mass spectrometric analysis.
碳正离子是许多有机和生物反应中难以观测到的短寿命活性中间体。随着奥拉发现超酸性溶液能够成功捕获并通过核磁共振表征瞬态碳正离子，这个领域应运而生。我们在此报告，水微滴能够直接从反应等分试样中捕获转瞬即逝的碳正离子，随后将其解吸至气相进行质谱检测。这是通过采用解吸电喷雾电离质谱来检测从不同反应（如消除反应、取代反应和氧化反应）中获得的各种短寿命碳正离子（平均寿命范围从纳秒到皮秒）来实现的。溶剂依赖性研究表明，在捕获碳正离子方面，水性微滴优于有机微滴。我们提供了一种机理见解，证明了活性碳正离子在带正电荷的水性微滴中的存活及其随后喷射至气相进行质谱分析的过程。
