The acoustic spectra of furfural were studied in the frequency range from 0.1 MHz to 150 MHz and at temperatures 303 - 333 K by the coaxial resonator method using two different cells to cover the whole frequency range. The temperature in the cells was controlled with an accuracy of 0.05 degrees. The accuracy of the determination of the attenuation of acoustic waves in liquid furfural was approximately 5%. The studies were shown that the obtained experimental data can be explained with help of two relaxation processes. One relaxation process is observed in the low-frequency range of frequencies (  0.2 MHz), and the second relaxation process is observed in the frequency range of about 2 MHz. The process with a lower relaxation frequency can be associated with the internal rotation of the furfural molecules “X0-cis” and “X0-trans”. The kinetic analysis of acoustic data about conformation processes for a low-frequency relaxation process in pure furfural was carried out by using Eyring relations. The relaxation parameters and the velocity of acoustic waves, as well as the contribution of the different factors to the attenuation of acoustic waves have been determined from experimental results.

Flavonoids constitute a family of natural compounds found in a large number of fruits and vegetables being responsible for many of their vivid colours. Flavonoids are the largest group of phytonutrients, with more than 6,000 types. Flavone (2-Phenylchromone) and flavanone (2,3-Dihydroflavone) define the basic units for whole families of these compounds, anthoxantines and flavanones respectively. Due to their biological activity, their structure–activity relationships are of interest also for medicine and biochemistry. In this work, we present the first study of the microwave spectra of both flavone and flavanone. The spectra of these two molecules have been observed using a CP-FTMW spectrometer with two vaporization methods: conventional heating and a laser ablation (LA) system recently implemented. A conformational search was done for each compound using DFT, corrected with empirical dispersion terms, and MP2 methods. We have observed only one conformer of each system and their monohydrated complexes. Flavanone is a case where DFT and MP2 methods diverge in predicting the global minimum being the experimental data in agreement with the DFT predictions.

The conformational behavior of methyl 2-methoxybenzoate has been explored using rotational spectroscopy. The two conformers identified show for each rotational transition a quadruplet attributable to two sets of splittings due to two tunneling motions, the internal rotation of the ester methyl group and the internal rotation of the entire ester group with respect to the benzene ring, which in addition interconverts both observed conformers. The analysis of such splittings allowed the determination of the potential energy barriers for the tunneling vibrations. The spectra of the complexes of methyl 2-methoxybenzoate with water and formic acid have also been analyzed. In both cases, only the global minimum structure could be identified in the spectra. In contrast with the observations in many other systems, the conformational landscape of the monomer is altered upon complexation. In the complexes the relative stability of the conformers observed in the monomer turn out to be inverted. Ester group torsion and methyl group internal rotation splittings were observed in the monohydrated complex, but not in the complex with formic acid.

This talk will describe single-conformation IR and UV spectroscopy of a series of single-turn and double-turn alpha helices as cryo-cooled, gas phase ions. Synthesized samples of tethered pentapeptides are known to form single-turn alpha helices in aqueous solution. When L-amino acids are used, a right-handed single-turn helix is formed while D-amino acids produce a left-handed helix. Due to the tether, these structures are remarkably stable in aqueous solution over a wide range of temperatures, pH, and denaturant. They can be concatenated, making LL, DD, LD, and DL double-turn forms. When a methylated arginine is placed at the C-terminal end of the tethered peptide, single-turn helices are the most stable structure, and are observed exclusively. The spectra in the NH stretch and amide I regions show distinct effects that depend on position along the helix, and the presence or absence of a kink due to concatenation of two opposite-handed helices. We will discuss these spectra and the prospects they offer as scaffolds for studying a wide range of interesting structural and dynamical problems.

r0pt Figure Bioisosteric OH-to-F replacements, concretely those produced in saccharides, have become a cornerstone in the development of novel drugs and tracers for the non-invasive imaging modalities, like (¹⁹F-MR/MRI) and (¹⁸F-PET). F and OH groups are isoelectronic and possess similar polarity and steric impact. Despite its remarkable purposes, there is a lack of knowledge about the structural non-covalent interactions involving F in their native context, which are the key in the activities that they develop. By means of high resolution rotational spectroscopy in combination with a laser ablationC. Calabrese, I.Uriarte, A. Insausti, M. Vallejo-López, F.J. Basterretxea, S.A. Cochrane, B.G. Davis*, F. Corzana*, E.J. Cocinero*. ACS Cent. Sci. 2020, 6, 2, 293-303^, C. Calabrese, P. Écija, I. Compañón, M. Vallejo-López, A. Cimas, M. Parra, F.J. Basterretxea, J.I. Santos, J. Jiménez-Barbero, A. Lesarri, F. Corzana*, E.J. Cocinero*. J. Phys. Chem. Lett. 2019, 10, 12, 3339-3345,

Following a series of previously studied silicon-containing ring molecules, the rotational spectra of 1-chloromethyl-1-fluorosilacyclopentane were observed on a CP-FTMW spectrometer in the 6-18 GHz region. Using quantum chemical computational approaches, the molecule has multiple predicted stable conformers with both trans and gauche positions of the chlorine/fluorine pair with the gauche typically being largest in dipole, but the trans being lowest in energy. This presentation will discuss the observed conformers in the spectra and attempt to make predictions on which conformers are the most stable using spectral intensity arguments and comparisons to the quantum chemical calculations. Furthermore, the structure of this molecule will be compared to the previous silicon-containing ring molecules made by this group as well as others.

The rotational spectrum of 3-furaldehyde (3-FA) was investigated in order to examine the conformational properties of the formyl group relative to the furan ring backbone. Two planar rotamers with the formyl group anti or syn to the ring oxygen atom were predicted in previous calculations, but only the anti conformer was identified previously from measurements done in a waveguide.Marstokk, K.-M. and Møllendal, H. Acta Chem. Scand. 1992, 46, 923-927.n the present work, the microwave spectrum of 3-FA was re-investigated using chirped-pulse Fourier Transform microwave (FTMW) spectroscopy to identify the syn conformer for the first time. The spectra of both conformers were then examined with the more sensitive Balle-Flygare FTMW instrument in the 6-19 GHz range to identify transitions due to singly-substituted ¹³C isotopologues for both conformers as well as singly-substituted ¹⁸O isotopologues for the anti conformer. Effective ground state structures of both conformers were derived with the aid of calculations at the MP2 level of theory with Dunning’s cc-pVTZ basis set. The relative population of the syn conformer was spectroscopically estimated to be ~12%, indicating that the anti conformer is more stable than the syn by ~5 kJ/mol which is consistent with computational estimates.

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Footnotes:

Marstokk, K.-M. and Møllendal, H. Acta Chem. Scand. 1992, 46, 923-927.I

We report the gas-phase preparation of cyanohydroxycarbene by high-vacuum flash pyrolysis of ethyl 2-cyano-2-oxoacetate and subsequent trapping of the pyrolysate in an inert argon matrix at 3 K. After irradiation of the matrix with green light for a few seconds singlet trans-cyanohydroxycarbene rearranges to its cis-conformer. Prolonged irradiation leads to the formation of cyanoformaldehyde and isomeric isocyanoformaldehyde. Cis- and trans-cyanohydroxycarbene were characterized by matching matrix IR and UV/Vis spectroscopic data with ab initio coupled cluster and TD-DFT computations. Trans-cyanohydroxycarbene undergoes a conformer-specific [1,2]H-tunneling reaction through a 33.3 kcal/mol barrier (the highest penetrated barrier of all H-tunneling reactions observed to date) to cyanoformaldehyde with a half-life of 23.5 ± 0.5 d; this is the longest half-life reported for an H-tunneling process to date. During the tunneling reaction the cis-conformer remains unchanged over the same period of time and the Curtin–Hammett principle does not apply. NIR irradiation of the O–H stretching overtone does not enhance the tunneling rate via vibrational activation. Push–pull stabilization of hydroxycarbenes through s- and p-withdrawing groups therefore is even more stabilizing than push–push substitution.