Rotational spectroscopy of methyl -butyl ether with a new band chirped-pulse Fourier transform microwave spectrometer

• Published in: Physical chemistry chemical physics : PCCP 2024-05, Vol.26 (18), p.13694-1379
• Main Authors: Crabtree, Kyle N, Westerfield, J. H, Dim, Chisom A, Meyer, Kelly S, Johansen, Sommer L, Buchanan, Zachary S, Stucky, Paul A
• Format: Article
• ISSN: 1463-9076; 1463-9084
• DOI: 10.1039/d4cp00797b

Chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy is a powerful tool for performing broadband gas-phase rotational spectroscopy, and its applications include discovery of new molecules, complex mixture analysis, and exploration of fundamental molecular physics. Here we report the development of a new K a band (26.5-40 GHz) CP-FTMW spectrometer that is equipped with a pulsed supersonic expansion source and a heated reservoir for low-volatility samples. The spectrometer is built around a 150 W traveling wave tube amplifier and has an instantaneous bandwidth that covers the entire K a band spectral range. To test the performance of the spectrometer, the rotational spectrum of methyl tert -butyl ether (MTBE), a former gasoline additive and environmental pollutant, has been measured for the first time in this spectral range. Over 1000 spectroscopic transitions have been measured and assigned to the vibrational ground state and a newly-identified torsionally excited state; all transitions were fit using the XIAM program to a root-mean-square deviation of 22 kHz. The spectrum displays internal rotation splitting, nominally forbidden transitions, and an intriguing axis-switching effect between the ground and torsionally excited state that is a consequence of MTBE's extreme near-prolate nature. Finally, the sensitivity of the spectrometer enabled detection of all singly-substituted 13 C and 18 O isotopologues in natural abundance. This set of isotopic spectra allowed for a partial r 0 structure involving the heavy atoms to be derived, resolving a structural discrepancy in the literature between previous microwave and electron diffraction measurements. The performance of a new K a band microwave spectrometer is demonstrated by investigating the rotational spectrum of methyl tert -butyl ether, including isotopologues and a new torsionally excited state.