Martin-Drumel, Marie-Aline, McCarthy, Michael C., Patterson, David, McGuire, Brett A., and Crabtree, Kyle N. 2016. "Automated microwave double resonance spectroscopy: A tool to identify and characterize chemical compounds." Journal of Chemical Physics 144:124202. https://doi.org/10.1063/1.4944089
Owing to its unparalleled structural specificity, rotational spectroscopy is a powerful technique to unambiguously identify and characterize volatile, polar molecules. We present here a new experimental approach, automated microwave double resonance (AMDOR) spectroscopy, to rapidly determine the rotational constants of these compounds without a priori knowledge of elemental composition or molecular structure. This task is achieved by rapidly acquiring the classical (frequency vs. intensity) broadband spectrum of a molecule using chirped-pulse Fourier transform microwave (FTMW) spectroscopy and subsequently analyzing it in near real-time using complementary cavity FTMW detection and double resonance. AMDOR measurements provide a unique "barcode" for each compound from which rotational constants can be extracted. To illustrate the power of this approach, AMDOR spectra of three aroma compounds — trans-cinnamaldehyde, α-, and β-ionone — have been recorded and analyzed. The prospects to extend this approach to mixture characterization and purity assessment are described.