Rotational Spectroscopy

Rotational spectroscopy can provide fingerprints of various molecules by distinguishing them from others in the rich chemical composition of a gas phase. Based on the rotational constants and spectroscopic parameters obtained from the analysis of spectral lines measured in the laboratory, accurate predictions of rotational spectra can be obtained. These can guide species identification in complex environments such as the atmosphere or interstellar medium.

Measurement of rotational spectra for molecules of astrophysical and astrochemical interest is critical for their observation in space. In our laboratory we are measure these spectra for unstable molecules produced by pyrolysis or discharge. The measurements are then used in collaboration with astronomers (V. Rivilla, CSIC Madrid; J. Chernicharo, CISC Madrid; P. Caselli, MPE Garching, and A. Belloche, MPE Bonn) to conduct radio astronomical observations.

Some key publications are given below.

• Gas-phase identification of (Z)-1, 2-ethenediol, a key prebiotic intermediate in the formose reaction
• Precursors of the RNA World in Space: Detection of (Z)-1,2-ethenediol in the Interstellar Medium, a Key Intermediate in Sugar Formation
• Laboratory spectroscopy of allylimine and tentative detection towards the G+0.693-0.027 molecular cloud

Spectroscopic characterization of different isotopologues for a given molecule allows for the derivation of accurate structural parameters of a molecule. These data are combined with others of computational nature, to derive the so-called semi-experimental structure. This can be used to estimate with great accuracy the rotational constants of rare isotopologues but also to derive the geometry of more complex systems using what in our lab is called the "Lego-brick" approach.

Contextually, the analysis of vibrationally excited states is used to obtain information on the anharmonic force field for a given species.

Most recent publications on this topic:

• Dipolar spin–spin coupling as an auxiliary tool for the structure determination of small isolated molecules
• Connections between the accuracy of rotational constants and equilibrium molecular structures
• Exploiting the “Lego brick” approach to predict accurate molecular structures of PAHs and PANHs

The balances between intra- and inter-molecular interactions as well as the interaction occuring with the surrounding environment (solvents, ligands, etc...) are the foundation of the study of the structure-property relationship, an important theme of the molecular approach to biology. For an accurate description of biomolecules, it is therefore essential to characterize in great detail the fundamental building-blocks, key units that, given their limited size, allow these interactions to be determined using the most accurate computational methods available. A precise description of the interactions acting in the building-blocks and their oligomers, without the perturbative intervention of the environment, is indispensable to better understand the role of the different interactions in biological activity. For this purpose, supersonic expansion rotational spectroscopy is used, which allows observation of the behavior of species in high vacuum, without interactions due to solvents or matrix effects. The use of this technique also allows the evaluation of non-bonding interactions in molecular clusters with high resolution.
Below are some key publications:

• Rotational spectra and semi-experimental structures of furonitrile and its water cluster
• Rotational Spectroscopy Meets Quantum Chemistry for Analyzing Substituent Effects on Non-Covalent Interactions: The Case of the Trifluoroacetophenone-Water Complex
• Unveiling bifunctional hydrogen bonding with the help of quantum Chemistry: The imidazole-water adduct as test case

Minor components of the atmosphere such as ozone, nitrogen and sulfur compounds, and halogenated hydrocarbons can produce important environmental effects. Monitoring of these species is done by methods based on spectroscopic techniques. For this purpose, laboratory measurement of data inherent in the line broadening due to the pressure of these species provides important information suitable for deriving quantitative data from the analysis of an atmospheric spectrum.

Lamb-dip spectroscopy improves the resolving power tenfold, allowing the identification and assignment of hyperfine structures of rotational transitions. With this technique, weak electric and magnetic interactions between rotation and nuclear spin are detectable.

• Hyperfine-resolved spectra of HDS together with a global ro-vibrational analysis
• Improved centrifugal and hyperfine analysis of ND2H and NH2D and its application to the spectral line survey of L1544
• Deuterium hyperfine splittings in the rotational spectrum of NH2D as revealed by Lamb-dip spectroscopy