Conformational cooling is a phenomenon that can occur in matrix-isolation experiments when the gas phase ratio of conformers is not represented in the freshly deposited matrix-isolated sample. Gaseous methyl nitrite exists in two stable conformations: cis-CH₃ONO and trans-CH₃ONO, with cis being the more stable conformer. Previous literature does not show evidence of a change in cis to trans ratio of gas phase methyl nitrite in a supersonic beam, but there is no experimental data studying the effects of conformational cooling using an effusive source combined with matrix isolation. Our experimental results show evidence of conformational cooling, where the relative population of the cis conformer is increased compared to the ratio observed in the gas phase. We also observe an increased degree of conformational cooling when different bath gases are employed (Ar, N₂, and Xe). In addition to demonstrating this phenomenon, we investigated the photoinduced isomerization mechanism in each low-temperature matrix using 355 nm light. In this talk, we will describe the isomerization of matrix isolated CH₃ONO within the framework of caging effects and excited state dynamics, and discuss the role of conformational cooling during matrix deposition.

cis-3-Hexenal (c3H; O=CH−CH₂−CH=CH−CH₂−CH₃) is known as a grass odorant molecule, and this compound readily isomerizes to trans-2-hexenal (t2H). We have previously identified two conformers of c3H by Fourier transform microwave (FTMW) spectroscopy [1]. We also found the isomers of c3H, trans-3-hexenal (t3H) and cis-2-hexenal (c2H), from rotational spectra observed on liquid sample of c3H. In this study, the intensities of the rotational spectral lines of these conformers of 3- and 2-hexenal were measured repeatedly to obtain reliable temperature-dependent intensity ratio profiles of the isomerization reactions. The rotational spectra of c3H and t3H were observed at 30:1 in the first step in room temperature. The t3H isomer was found to be contained in the liquid c3H sample as an impurity. As the nozzle temperature was increased, the line intensities of the t2H and c2H became stronger, and finally, the ratio of t2H and c2H was 6:1. This indicates that c3H may isomerize to both t2H and c2H. We also discuss the identification of the second conformer for c3H; the structural model in which all dihedral angles of the C-C single bond are in the skew configuration, SScS, is better than the previously identified conformer [1], CScS, in which the OCCC dihedral angle is in the cis configuration. [1] R.Ozawa, N. Kuze and Y. Kawashima, ISMS, P6243 (2022).

A new instrument which combines buffer gas cooling with broadband rotational spectroscopy to detect molecules desorbed from an ice surface is described. Here, we report isomer and conformer-specific branching ratios of n- and i-propanol, the former of which contains 5 symmetry-unique conformations (Ga, Aa, Ag, Gg, and Gg’). Following deposition of these species at 4 K, temperature-programmed desorption is performed where the sublimed molecules are injected into a 25 K buffer gas cell (BGC) and detected with broadband rotational spectroscopy in the 60 – 90 GHz regime. We compare these results with direct room temperature gas-phase injection and report differences in the conformer branching ratios for species that are first deposited onto an ice at 4 K and sublimed, versus those observed following direct gas-phase injection and cooled within a BGC.

A new instrument is described which combines buffer gas cooling with broadband rotational spectroscopy to probe molecules desorbed from an ice surface (CP-ICE). Here we report isomer and conformer-specific branching ratios of n- and i-propyl cyanide, the former of which may be in either the gauche or anti configuration. Following deposition of these species onto a 4 K surface, temperature programmed desorption (TPD) experiments are performed where the sublimed gas-phase molecules are injected into a buffer gas cell (BGC) at 25 K, cooled, and detected by broadband rotational spectroscopy in the 60 – 90 GHz regime. We compare these results with room-temperature gas-phase injection of these species into the 25 K BGC and report differences observed in the conformer branching ratios.

The spectral study of isolated molecules and clusters, assisted by quantum chemical calculations, allows for predicting their three-dimensional electronic structures and gaining insight into their relative energies, non-covalent interactions, and contributions to structure stabilization. Here we will show double resonance measurements, with ionization-loss stimulated Raman spectroscopy probing single-conformation hydrates of neurotransmitters and providing their vibrational signatures in a broad frequency range for obtaining new information on the structural details and uncovering the approach potential.

The study of bioactive molecules is an important field if we want to understand how living organisms work, in this contribution we present three molecules: indole-3-carbinol (I3C), picaridin and DEET. By a combination of quantum-chemical calculations and microwave spectroscopy all molecules were interrogated. I3C was vaporized by ultrafast UV laser radiation and 1 conformers were detected. In addition, the structure could be determined by minor isotopologues species[1]. Picaridin and DEET were examined in the pulse-chirped spectrometer [2][3] and 2 and 4 species were observed respectively. Studies are in progress and higher energy conformers will be investigated and hydrated species will be analysed. References [1] E. J. Cocinero, A. Lesarri, P. Écija, J. Grabow, J. A. Fernández, F. Castaño. Angew. Chem. Int. Ed. 2012, 51, 3119 –3124 [2] E. J. Cocinero, A. Lesarri, P. Écija, J. Grabow, J. A. Fernández, F. Castaño. Phys. Chem. Chem. Phys., 2010,12, 12486-12493. [3] I. Uriarte, C. Pérez, E. Cballero-Mancebo, F. J. Basterretxea, A. Lesarri, J. A. Fernández, E. J. Cocinero, Chem. Eur. J. 2017, 23, 7238–7244.

Tetrahydro-2H-pyran-2-ol (THP) is an alcohol containing pyranose ring. The lack of substituents in THP minimizes the conformational flexibility and the non-covalent interactions within the structure, drastically simplifying the system in relation to physiologically relevant carbohydrates, such as glucose. This makes THP a prototypical system to study various phenomena and dynamics of carbohydrate chemistry. Such phenomena include the anomeric effect, which describes the preference for the axial conformation over the less hindered and more energetically favoured equatorial conformation. When in solution, THP is in equilibrium with its linear counterpart, 5-hydroxypentanal, and readily interconverts between its two chiral forms. A high resolution gas phase study of THP may not only confirm phenomena such as the anomeric affect, but also allow for the study of chiral interconversion in the gas phase. An in-depth study of THP will also lay a foundation for future gas phase carbohydrate systems with more physiological relevance. However, before we can study the various phenomena and dynamics, it is first beneficial to study the structure of THP in great detail. To elucidate the THP structure we analyzed rotational spectra measured with a chirped-pulse Fourier transform microwave spectrometer, operating in the 2-6 GHz range. In addition to the monomers, some dimer conformations were assigned to the rotational spectrum. The intermolecular interactions within the dimers were then visualized using a non-covalent interactions (NCI) analysis.

l0pt Figure Exploring the conformational landscape of multi-conformational systems, such as macrolactones, can be challenging, as their great flexibility leads to small structural changes that result in a large number of energy minima. High-resolution gas-phase microwave spectroscopy has proven to be unique technique, being able to isolate and discern conformers or clusters unambiguously without masking effects. Macrolactones are considered “privileged scaffolds” within the medicinal chemistry community, because their study led to the discovery of bioactive compounds. However, gas-phase studies on the changes in structure and intra/intermolecular interactions after solvation of these biomolecules are very scarce. Based on the previous rotational study of oxacyclotridecan-2-one (C12) and 16-hexadecanolide (C15),M. Parra-Santamaria, I. Usabiaga, A. Insausti, E. R. Alonso, F. J. Basterretxea, C. Calabrese, E.J. Cocinero, HRMS2022, Prague (Czech Republic), 2022.n this work we have studied the structural behaviour of these molecular systems after the addition of individual water (W) molecules. Using broadband chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy, five conformers have been observed so far for the C12^...W complex and the global minimum for C15^...W. This study has once again challenged the limits of rotational spectroscopy and quantum chemical calculation due to the structural complexity of these complexes. These results are the initial steps for future microsolvation studies with different solvents.

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Footnotes:

M. Parra-Santamaria, I. Usabiaga, A. Insausti, E. R. Alonso, F. J. Basterretxea, C. Calabrese, E.J. Cocinero, HRMS2022, Prague (Czech Republic), 2022.i

Aromatic rings have taken a lot of attention of the scientific community during the last years due, in part, to their detection in the interstellar medium (ISM). Benzonitrile, cyanonaphtalene, and cyanoindene are examples of the first and the few individual aromatic species detected, all of them in TMC-1 molecular cloud.McGuire B. et al., Science, 359, 202-205, 2018; McGuire B. et al., Science, 371, 1265-1269, 2021; Sita M.L. et al., ApJL, 938, L12, 2022heir presence is certain but their formation under the ISM conditions remains unclear. In the present work, we have analyzed the intermolecular interactions in small gas-phase clusters of a family of aromatic species: tolunitriles, also known as methylbenzonitriles (CH₃C₆H₄CN). Our aim is to understand the very first steps of the reactivity of small aromatic molecules by unveiling the principal binding sites for polar molecules, where the reactions might be initiated. Likewise, we have analyzed the influence of the relative position of the ring substituents (methyl and cyano groups). The rotational spectra of the complexes of ortho, meta and para tolunitriles were studied using a broadband chirped pulsed Fourier transform microwave spectrometer with an operating frequency range of 2-8GHz. We have employed water as polar probing molecule. A comparative analysis of the hydration sites of the three tolunitriles together with that of benzonitrileMelandri, S. et al., J.Chem.Phys. 111, 3874 1999as unveiled the influence of the methyl group in the water anchoring site. McGuire B. et al., Science, 359, 202-205, 2018; McGuire B. et al., Science, 371, 1265-1269, 2021; Sita M.L. et al., ApJL, 938, L12, 2022T Melandri, S. et al., J.Chem.Phys. 111, 3874 1999h

Sugars are versatile molecules that play a variety of roles in the organism. For example, they are important in energy storage processes, cellular recognition or as structural scaffolds. Here, we focus on uronic acids derivatives of the most abundant monosaccharides in the nature, concretely in glucuronic acid (GlcA) and galacturonic acid (GalA). By the combination of high resolution spectroscopy, supersonic expansions and laser ablation as vaporization technique with quantum chemistry calculations we are able to obtain valuable information in gas phase about the bare structures of both anomers of GlcA and GalA (with rotational spectroscopy) and water clusters of the 1-O-phenylated derivative of GlcA (using ion-dip spectroscopy (IR-UV)). Previous studies in the gas phase had been performed on $\alpha$/$\beta$-Glucose1,2 and $\beta$-Galactose2,3 where the importance of –CH2OH backbone in H-bond orientation and hydration was reported. In this work, we focus on how the characteristic –COOH group of uronic acids and their anomeric hydroxyl group configuration have effect in the H-bond network cooperativity/orientation and microsolvation process.

As flexible molecules with long fluoroalkyl chain and multiple internal rotations, PFOA and PFO are excellent candidates for studies of molecular helicity and resulting conformational dynamics. Additionally, the exact structure of PFOA, as man-made environmental pollutant, is highly desired for the development of new spectroscopic tools for its detection in environmental samples. We present potential energy landscapes and helical structures of PFOA and PFO that are computed at Density Functional Theory levels with B3LYP-D3BJ/6-311G basis set by scanning dihedral angles and bending angles in steps of 10 degree each. The resulting potential energy surfaces show three equivalent minima for PFO (at 70⁰, 190⁰, and 310⁰) while only two minima(at 120 ⁰ and 320⁰) are seen for PFOA. These minima correspond to stable helical conformers as results of maximizing the separation between F-atoms and rotating the carboxylic end terminal (PFOA only). The nature of helical coiling has been further explored by displacing the fluoroalkyl chain away from trans configurations back to the stable helical coiling structures. We notice that a helical angle of   14 degrees is necessary for the helical coiling to be realized and is insensitive to the end terminal. The work to evaluate the exact impact of fluoroalkyl chain on the helical coiling is underway and will be presented this talk.