New opportunities and emerging themes of research in microwave spectroscopy

Abstract

It is easy to set a frisbee spinning but hard to flip a javelin end-over-end. The properties of a rotating body are determined by its moment of inertia. Changes in the energy associated with the rotation of a single molecule are incremental, or quantized, in contrast with the everyday examples of the frisbee and the javelin. Only photons with energies that correspond to specific discrete frequencies of electromagnetic radiation can be absorbed or emitted to cause transitions between different rotational energy levels. Photons that cause rotational transitions generally have microwave or millimetre-wave frequencies. Microwave spectroscopy thus provides a basis for exploring molecular structure through studies of molecular rotation. The first experiments involving microwave spectroscopy exploited technology developed for radio detection and ranging during the Second World War. Microwave spectroscopy is now being applied to study chemical reactions significant in atmospheric chemistry and to probe superfluidity in Hen clusters. This article reviews the research themes that were the focus of the past 50 years and surveys new opportunities.

The LAM of the Rings: Large Amplitude Motions in Aromatic Molecules Studied by Microwave Spectroscopy

“…Studying the structures and internal dynamics of a molecule that are relevant to topics as diverse as astrophysics, molecular biology, and environmental sciences using microwave spectroscopy has a long tradition and is still a research field with great potential yet to be exploited. The combination of microwave spectroscopy and theoretical studies has become particularly successful in the last two decades in decoding the spectra of molecules with LAMs and has especially provided reference data for astrophysical research [62][63][64], atmospheric chemistry [65][66][67], and general applications in physical chemistry [68][69][70]. Regarding the resolution of the spectroscopic instruments, during the seventies, a typical high-resolution microwave spectrometer was to use the Stark modulated absorption technique on static gases with a resolution of about 250 kHz (corresponding to a measurement accuracy of 25 kHz) [71].…”

Probing the Chemical Nature of Dihydrogen Complexation to Transition Metals, a Gas Phase Case Study: H2–CuF

“…As a background-free technique in principle, FTMW spectroscopy only requires the sample to have a permanent dipole moment and sufficient concentration (≈10 –9 mol per gas pulse) to detect species for study. In the past few decades, FTMW spectroscopy has served as an important tool for testing the feasibility of theoretical methods of electronic structure calculations and offered valuable information on both molecular structure and energy. − The richness of the rotational spectrum of isotopic hydrogen-transition metal complexes provides us the opportunity to study, in detail, the structure and bonding involved in the adsorption of hydrogen in MOFs. In this work the authors report, to their knowledge, the first FTMW observation of a dihydrogen complex to a metal containing molecule, CuF.…”

Decoding Pure Rotational Molecular Spectra for Asymmetric Molecules

“…It has found considerable use in the field of physics and astronomy, providing an invaluable method to identify chemical species present in the interstellar medium. Microwave spectroscopy is a mature discipline and several recent review articles are available on the subject [1][2][3] . Yet recent technological advances 4 , together with lower instrument costs mean that microwave spectroscopy is more accesible to undergraduate and graduate students than it has ever been before [5][6][7][8][9][10] .…”