Microwave Spectroscopy

The study of our cosmic origins -- the evolution of molecules from the Big Bang to the molecular building blocks delivered to Earth on meteorites -- relies on observing molecules outside our solar system. Vast molecular clouds, the birthplaces of stars and planets, are unique laboratories where rich and exotic chemistry drives the molecular inventory toward life-essential molecules. More than 200 distinct molecular species have now been seen in these environment, detected through their rotational spectra: patterns of light emitted as they tumble end over end in space. These spectra are first measured in the laboratory and then matched to telescope observations, identifying molecules in space. The rotational spectrum of a molecule is a unique electromagnetic fingerprint and is the primary means by which we study molecules outside our solar system. The signal we analyze from radio telescopes contain the spectra of all molecules in the target; identifying a single molecule requires that we know its spectrum to exquisite accuracy. This necessitates laboratory study: we must make these molecules under terrestrial conditions and probe their spectra here first, then robustly match it to signals observed in the interstellar medium (ISM).

Spectrum of the source TMC-1, a cold dark molecular cloud in the constellation of Taurus, taken with the Green Bank Telescope (black). Spectrum of pure CH₂CN as measured in the laboratory on Earth (red). The exact match of the frequency pattern confirms the identification in the ISM.

Modern microwave spectroscopy is dominated by two methods: cavity-enhanced Fourier-transform microwave (FTMW) spectroscopy and the newer Chirped-Pulse FTMW (CP-FTMW) spectroscopy. Cavity-enhanced methods have been work horses of the field for the last 40 years, providing high sensitivity and very high resolution data, but over a very narrow bandwidth (~1 MHz). Most often, a single rotational transition or closely space set of transitions can be observed at once (blue spectra in the inset example spectrum). At a modest cost in sensitivity and resolution (10-100), CP-FTMW spectroscopy allows the acquisition of entire rotational spectra over a huge bandwidth (~10,000 MHz). We combine the best of both worlds, using CP-FTMW spectra to rapidly profile a molecule (or molecules), and then following up with cavity-enhanced methods to measure the spectra at the extreme precision and accuracy need to make a secure identification in the ISM.

Spectrum of a complex mixture measured first with a CP-FTMW instrument (black). The signals identified were then re-measured at high-resolution with a cavity FTMW instrument (blue). Adapted from McGuire et al. 2018 PCCP 20, 13870.

Reaction/Mixture Screening

Determining what species to target, however, requires a more innovative approach. While we can certainly make some educated guesses as to the logical next reaction products to search for, a more rigorous approach would be to attempt an unbiased reaction screening analysis on complex mixtures. In other words, we set a chemical reaction in motion using species we know to be present in TMC-1, analyze the spectrum of known products to gain chemical insight into reaction pathways, branching ratios, and so on, and then remove those lines leaving us with signals from unknown products of this new chemistry. Because we intentionally performed the experiment to generate unexpected new species, we don't know a priori how to start a spectral assignment on these unknown lines. We can leverage two new powerful techniques developed in recent years to tackle this challenge: Microwave Spectral Taxonomy (MST; Crabtree et al. 2016 JCP 144, 124201) and Automated Microwave Double Resonance (AMDOR) spectroscopy (Martin-Drumel et al. 2016 JCP 144, 124202).

These techniques work by exploiting the strengths of both cavity-enhanced and CP-FTMW spectroscopy. The broadband nature of CP-FTMW will provide the rough transition frequencies of our unknown products. We can then use the sensitivity of cavity FTMW to rapidly perform characterization tests on these molecules, grouping the lines that share a common precursor, or property such as being sensitive to magnetic fields. We then take these groups of lines and perform exhaustive microwave-microwave double resonance tests (somewhat analogous to 2D NMR techniques), attempting to deplete the signal from one line by pumping population out of it with light at another line's frequency. The result is a list of lines that must both originate from the same molecule and share an energy level. Straightforward automated fitting routines can then give us the rotational constants of the species. Combined with the knowledge of the attributes of the molecule, and its rotational constants (i.e. size and shape), assignment to new species is then straightforward and can be confirmed by quantum chemical calculations.

Exquisite Gas-Phase Molecular Structures

Microwave spectroscopy is uniquely structurally specific. Because the three rotational constants (A, B, C) are uniquely determined by the moments of inertia of the molecule about its three principle axes, if they are known the structure is uniquely determined. In practice, the spectra of many isotopically substituted isotopologues of a molecule are measured, and the resulting small shifts in the rotational constants are then used to constrain the precise location of each atom to fractions of Angstroms and degrees. We can use this structural specificity to solve problems, for instance, in molecular hydrogen bonding, by accurately determining the arrangement of the monomers with respect to each other. For example, take a look at the structure of the phenol dimer in this figure from Seifert et al. 2013 PCCP 15, 11468. The blue dots are the positions of the atoms determined using isotopic substitution microwave spectroscopy as compared to the computed structure (color) which just can't handle the bonding interactions at a modest level of theory.