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New trends in microwave spectroscopy

Final Report Summary - MWSPEC (New trends in microwave spectroscopy)

• Summary description of the project objectives
In this report is summarized the research activities performed by Dr. Luca Evangelisti during the IOF Marie-Curie at the Department of Chemistry, University of Virginia (USA) under the supervision of Prof. Brooks H. Pate and at the University of Bologna (Italy). The main aims of the proposal are summarized in the following part: the available microwave technique can supply precise information on conformation, structure, bond energy, internal motions and, sometimes, on charge distribution within molecules and molecular complexes. A lot of improvements are now possible in microwave spectroscopy with the chirped-pulse Fourier transform microwave spectrometer (CP-FTMW) invented by Prof. Brooks Pate in recent years. The objectives of this project are to progress significantly in this field. The evolutionary improvements applied to the instruments will allow to get new information about: 1) Biomolecule Microwave Spectroscopy: the topic will develop techniques that have the potential to expand the use of microwave spectroscopy as a diagnostic tool for biomolecule structure (>10 heavy atoms) and to provide additional structural characterization when the rotational spectrum is insufficient to unambiguously determine the structure; 2) Atmospheric and astrophysical molecules millimiter-wave spectroscopy: improve the sensitivity of the method by taking full advantage of recent advances in technology and develop an analytical instrument for trace gas analysis; 3) Rotational Spectroscopy Probes of Molecular Dynamics: develop a project that would use rotational spectroscopy to probe chemical reaction dynamics following laser excitation, using the line shape analysis of the dynamic rotational spectrum. All this experience will be later on used in the return phase to the final set-up molecular beam MW spectrometers combined with laser ablation, electrical discharge and 2D resonance. It will allow the study of rotational spectra of various molecules of astrophysical interest and biological interest. Moreover the main activity will be the design of a CP-FTMW spectrometer which will introduce in Italy new spectroscopic techniques.

• Description of the work performed since the beginning of the project
In the first two years, the experienced researcher carried out research on Fourier transform molecular rotational spectroscopy which were related to the main tasks of the project. He carried out: (i) developments of the CP-FTMW technique at low frequency, namely 2-18 GHz, developments of tools (i.e. laser ablation and heated nozzle) for studies of large molecules and their coupling to computational methods; (ii) Study of the performance of a chirped-pulse Fourier transform mm-wave spectrometer with a range from 260 – 295 GHz. The spectrometer has a new arbitrary waveform generator to create an excitation and detection waveform. It has been used for the development of new strategies of gas analysis; (iii) study and implementation of a new spectroscopic technique for infrared-microwave double-resonance spectroscopy where the major emphasis of the work was understanding the spectroscopy of molecules as the IVR process, and possibly reaction, occurs. In particular, in how coherent excitation of highly excited molecules can be used to influence reaction products; (iv) Pulsed-jet Fourier transform rotational spectroscopy to perform quantitative chiral analysis on molecules with multiple chiral centers. This analysis includes quantitative measurement of diastereomer products and, with the recent discovery of three wave mixing methods by Patterson, Schnell, and Doyle in 2013, quantitative determination of the enantiomeric excess of each diastereomer. In the last year he has implemented the spectrometers in Bologna. A new molecular beam spectrometers combined with laser ablation and electric discharge is now available. Moreover the design for a new CP-FTMW has been done. This will allow to introduce in Italy this new technique.

• Description of the main results achieved so far
With the research activity performed during the outgoing phase, the experienced researcher reached the largest part of the objective described in the proposal. In particular, broadband rotational spectra of several larger molecules have been measured using a chirped-pulse FT-MRR spectrometer such as measurements of cedrol (C15H26O, 5 chiral centers), ambroxide (C16H28O, 4 chiral centers), and dihydroartemisinic acid (C15H24O2, 5 chiral centers). The sensitivity of the rotational spectrum to small changes in the geometry coupled with the high spectral resolution of the measurement technique make it possible to identify and quantitate the possible conformational isomers of these diastereomers. The role that quantum chemistry calculations play in identifying structural minima and estimating their spectroscopic properties to aid spectral analysis has been investigated. In particular, methods to perform automated spectroscopic analysis by numerical algorithms using computational chemistry input has been done. Signal-to-noise ratio exceeding 1000:1 has been reached in the spectra of the broadband rotational spectroscopy. Methods to rapidly identify rotational spectra from isotopologues at natural abundance has been implemented. Finally, the implementation of three wave mixing techniques to measure the enantiomeric excess of each diastereomer and determine the absolute configuration of the enantiomer in excess has been studied. The development of Fourier transform mm-wave spectrometers using high peak power (30 mW) active multiplier chain mm-wave sources brings new measurement capabilities to the analysis of complex gas mixtures. Strategies for gas analysis based on high-throughput mm-wave spectroscopy and arbitrary waveform generator driven mm-wave sources have been analysed. As an illustration of the potential to analyze complex mixture spectra, a broadband spectrum of a leak oil extract (buchu leaf oil from A. crenulata) was measured. The ability to identify different essential oils in the leak oil extract, many of which are chiral, is demonstrated. The analysis can be performed using library spectra of sample constituents (the diastereomers of 8-mercaptomethanone, pulegone), rotational constants from spectral analyses in the literature (limonene, linalool, menthone, pinene, pinene, isopulegone), and from direct identification of previously unstudied compounds in the mixture measurement using estimates of the spectral parameters from quantum chemistry and automated spectral identification routines (1,8-cineole, diosphenol, and 4-diosphenol).

Some references
[1] Keto-Enol Tautomerism and Conformational Landscape of 1,3-Cyclohexanedione from Its Free
Jet Millimiter-Wave Absorption Spectrum J. Phys. Chem. A 117 (2013) 13712-13718 C. Calabrese, A. Maris, L. Evangelisti, L.B. Favero, S. Melandri, W. Caminati
[2] Molecular Structure and Chirality Detection by Fourier Transform Microwave Spectroscopy J. Phys. Chem. Lett. 6 (2015) 196-200 S. Lobsiger, C. Perez, L. Evangelisti, K.K. Lehmann, B.H. Pate
[3] Theory vs. Experiment for Molecular Clusters: Spectra of OCS Trimers and Tetramers J. Chem. Phys. 142 (2015) 104309-11 L. Evangelisti, C. Perez, N.A. Seifert, B.H. Pate, M. Dehghany, N. Moazzen-Ahmadi, A.R:W: McKellar
[4] On the Cl···C Halogen Bond: a Rotational Study of CF3Cl-CO Phys. Chem. Chem. Phys. 18 (2016) 17851-17855 W. Caminati, L. Evangelisti, G. Feng, B.M. Giuliano, Q. Gou, S. Melandri, J.-U. Grabow

• Expected final results and their potential impact and use (including the socio-economic impact and the wider societal implications of the project so far)
The research activities planned within the present project are connected to some of the goals of FP7 and also to the objectives of the new Horizon 2020:
1- “Excellence in science” for the technological and cultural innovation and for the development of research in Europe.
2- “Industrial leadership” to develop advance materials and for research activities connected to exploration of the interstellar space.
3- “Societal Challenges” for the impact of climate equilibria and for health preservation.
This project is highly innovative and has a potential high impact. For developing advanced materials, its contribution is given, for example, by: (i) precise determination of energetic and structural parameters characterizing several kinds of non-covalent interactions; (ii) study of enantio-selectivity of chiral molecules - some of biological interest - to understand the molecular processes. The research activity related to the space consist in the laboratory studies of the rotational spectra of chemical species which microwave signals can be identified in the interstellar space. More than 170 molecules have been localized in the interstellar space through their microwave spectra. This process requires the assignments of these spectra in laboratory. Many of this species are been directly prepared by an electrical discharge in the throat of a nozzle for a supersonic expansion and the immediately characterized in the microwave spectrometer. The partnership with the industry has the potential to improve the European competitiveness and translate directly into increased revenue. Contributions to the impact on climate equilibria concern the studies of complexes of molecules of atmospheric interest with water. The topic “health preservation” is connected with the studies on the characterization of different drugs (such as nicotinoids and analgesics) and on their interaction ability with various partners in molecular adducts. Also the effect of the substitution of hydrogen atoms with fluorine atoms on the properties of molecules of medical interest has been explored. In fact fluorine substitution is a common practice in bio-organic chemistry in order to modulate physicochemical properties and biological activity of molecules and an increasing number of drugs on the market contain fluorine, the presence of which is often of major importance to modify pharmacokinetics and pharmacodynamics properties. All these molecular systems have been studied during the project, in agreement with the achievement of the objectives in the various topics.