Photochemical Pump and NMR Probe: Chemically Created NMR Coherence on a Microsecond Time Scale
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- 作者：Olga Torres ; Barbara Procacci ; Meghan E. Halse ; Ralph W. Adams ; Damir Blazina ; Simon B. Duckett ; Beatriz Eguillor ; Richard A. Green ; Robin N. Perutz ; David C. Williamson
- 刊名：Journal of the American Chemical Society
- 出版年：2014
- 出版时间：July 16, 2014
- 年：2014
- 卷：136
- 期：28
- 页码：10124-10131
- 全文大小：394K
- ISSN：1520-5126
文摘
We report pump鈥損robe experiments employing laser-synchronized reactions of para-hydrogen (para-H2) with transition metal dihydride complexes in conjunction with nuclear magnetic resonance (NMR) detection. The pump鈥損robe experiment consists of a single nanosecond laser pump pulse followed, after a precisely defined delay, by a single radio frequency (rf) probe pulse. Laser irradiation eliminates H2 from either Ru(PPh3)3(CO)(H)2 1 or cis-Ru(dppe)2(H)2 2 in C6D6 solution. Reaction with para-H2 then regenerates 1 and 2 in a well-defined nuclear spin state. The rf probe pulse produces a high-resolution, single-scan 1H NMR spectrum that can be recorded after a pump鈥損robe delay of just 10 渭s. The evolution of the spectra can be followed as the pump鈥損robe delay is increased by micro- or millisecond increments. Due to the sensitivity of this para-H2 experiment, the resulting NMR spectra can have hydride signal-to-noise ratios exceeding 750:1. The spectra of 1 oscillate in amplitude with frequency 1101 卤 3 Hz, the chemical shift difference between the chemically inequivalent hydrides. The corresponding hydride signals of 2 oscillate with frequency 83 卤 5 Hz, which matches the difference between couplings of the hydrides to the equatorial 31P nuclei. We use the product operator formalism to show that this oscillatory behavior arises from a magnetic coherence in the plane orthogonal to the magnetic field that is generated by use of the laser pulse without rf initialization. In addition, we demonstrate how chemical shift imaging can differentiate the region of laser irradiation thereby distinguishing between thermal and photochemical reactivity within the NMR tube.