Pulsed-nozzle Fourier-transform Microwave Research Articles

Towards a broadband chirped pulse Fourier transform microwave spectrometer

This article gives a brief review of microwave spectroscopy and the experimental techniques used for obtaining microwave spectrum of molecules and complexes since 1950s. It presents a brief summary of the pulsed nozzle Fourier transform microwave (PNFTMW) spectrometer, fabricated in our laboratory, and discusses some of the important results obtained using the spectrometer. The most significant among the results from this spectrometer is the direct structural determination of weakly bound complexes involving H2O/H2S. These have challenged the conventional wisdom on hydrogen bonding leading us to propose a modern definition for the same through IUPAC. The limitations of the PNFTMW spectrometer and the need for the new chirped pulse Fourier transform microwave spectrometer are discussed as well. Moreover, preliminary results from our laboratory on generating a 1 µs chirped pulse of 1 GHz bandwidth are given.

Microwave spectrum of the heterodimers: CH 3OH–CO 2 and CH 3OH–H 2CO

Molecular complexes, dimers and heterodimers often show interesting structures, large amplitude internal motions and orientations for reaction coordinates. These properties were the motivations for the current study of the rotational spectra of the heterodimers, CH 3OH–CO 2 and CH 3OH–H 2CO, in a pulsed nozzle Fourier-transform microwave (FTMW) spectrometer. In addition to studying the normal isotopic forms, several isotopologues containing 13C or deuterium substituted atoms of each heterodimer were analyzed in order to obtain structural data of the complexes. All species showed splittings from internal rotation of the methyl group and splittings on the b-type transitions of the CH 3OH–H 2CO species suggesting rotation of the H 2CO group between equivalent structural forms. Stark effect measurements on each of the parent species provided dipole moment components. Theoretical ab initio results are compared to the experimentally determined molecular parameters.

Pulsed Nozzle Fourier Transform Microwave Spectrometer: Advances and Applications

The pulsed nozzle Fourier transform microwave (PNFTMW) spectrometer was developed by Balle and Flygare [A new method for observing the rotational spectra of weak molecular complexes: KrHCl. J. Chem. Phys. 1979, 71 (6), 2723–2724 and 1980, 72 (2), 922–932] in 1979. The design, fabrication, and operation of this spectrometer are complicated and it has largely remained a research laboratory tool till now, though a portable spectrometer for routine analytical applications has been developed at the National Institute for Standards and Technology [Suenram, R.D.; Grabow, J.‐U.; Zuban, A.; Leonov, I. A portable pulsed‐molecular‐beam Fourier‐transform microwave spectrometer designed for chemical analysis. Rev. Sci. Instrum. 1999, 70 (4), 2127–2135]. However, the potential for extracting fundamental information about any chemical species, such as, molecules, radicals, ions, or weakly bound complexes between any of them including atoms, has been quite significant. It is evident from the fact that more than 25 laboratories around the globe have built this spectrometer, some in the recent past. Contributions from all these laboratories have widened the horizon of PNFTMW spectrometer's applications. This review summarizes the advances in design and the recent applications of this spectrometer. We also define an electrophore, as an atom/molecule that generates an electric dipole moment by forming a weakly bound complex with a species having zero electric dipole moment. The electrophore, thereby, enables structural determination using rotational spectroscopy, as in the case of Ar2–Ne, with Ne as the electrophore. Also, it can introduce a dipole moment about a principal axis where none existed before, such as in Ar–(H2O)2, enabling the observation of pure rotational transitions for several tunneling states.