We have been observing the rotational spectra of the 1-pentanethiol (1-C₅SH; C(5)H₃C(4)H₂-C(3)H₂C(2)H₂C(1)H₂SH) by Fourier transform microwave (FTMW) spectroscopy. So far, eight conformers of the normal species for 1-C₅SH have been identified by FTMW spectroscopy and the quantum chemical calculations K. Suzuki, N. Kuze and Y. Kawashima, ISMS, WI05 (2019). They are TTTg, TTTt, TTGg, TTGg’, TGTg, TGTg’, GTTg, and GTTg’. The nomenclature of the conformer such as TTGg’ is based on the conformation of the molecular skeleton around the C(4)-C(3), C(3)-C(2), C(2)-C(1), and C(1)-S axis, respectively. This means that TTGg’ represents trans (T), trans (T), clockwise gauche (G) and anti-clockwise gauche (g’) conformers. In this work, we observed the isotopologues the 1-pentanethiol such as the deuterated species in the thiol group, 1-C₅SD. The rotational spectral lines of the seven conformers in 1- C₅SD, except for TTTt, were assigned as in 1-C₅SH. In the TTTg conformer of 1-C₅SH, characteristic splittings were observed in the rotational spectral lines of the c-type transitions. They are due to the tunneling splitting by the torsional motion of the SH group. Therefore, we focused on the line spllitings in the c-type transition of the TTTg conformer of 1-C₅SD and compared our results with data from 1-C₅SH and related compounds, 1-butanethiol Y. Kawashima, Y. Tanaka, T. Uzuyama, and E. Hirota, J. Phys. Chem. A, 125, 1166-1183 (2021). 1-propanethiol and 1-ethanethiol.

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K. Suzuki, N. Kuze and Y. Kawashima, ISMS, WI05 (2019).. Y. Kawashima, Y. Tanaka, T. Uzuyama, and E. Hirota, J. Phys. Chem. A, 125, 1166-1183 (2021).,

The rotational spectrum of acetic difluoroacetic anhydride, (CH₃COOCOCHF₂) was recorded by microwave spectroscopy and the A and E internal rotor states of the methyl group were analyzed. The A state was easily assigned using the assignment/fitting program DAPPERS, and a combined fit for the A and E state transitions was performed using XIAM. The fitted internal rotation barrier of the methyl group was determined to be 252.646(19) cm⁻¹. Using the M06-2X/6-311++G(d,p) level of theory, one non-planar cis conformer and two non-planar trans conformers of the anhydride were predicted. Agreement between theoretical and experimental rotational constants indicate that the non-planar trans conformer with the difluoromethyl alpha hydrogen participating in a hydrogen bond with a C=O oxygen is the form observed. The differences in the hydrogen bonding ability of the alpha hydrogens in the CH₃ and CHF₂ groups are a result of the differences in electron density.

l0pt Figure The 3D-arrangement of carbohydrates and, particularly, the orientation of their hydroxymethyl groups are structural features crucial for their biological activities. In this work, we investigate the influence of water on the conformational preferences of model hexopyranoses by performing a comprehensive analysis in the gas phase via microwave spectroscopyC. Calabrese, I. Uriarte, A. Insausti, M. Vallejo-Lopez, F. J. Basterretxea, S. A. Cochrane, B. G. Davis, F. Corzana and E. J. Cocinero ACS Cent. Sci., 6, 293-303, 2020.^, E. J. Cocinero, A. Lesarri, P. Ecija, F. J. Basterretxea, J.-U. Grabow, J. A. Fernandez, F. Castaño Angew. Chem. Int. Ed., 51, 3119, 2012.o

Herein we report a complete conformational analysis of caffeic acid, a relevant polyphenol^[1], and the main hydroxycinnamic acid found in humans' diet using laser ablation chirped-pulse Fourier transform microwave (LA-CP-FTMW) spectroscopy. The entire conformational space consisting of eight distinct rotameric species has been fully deciphered based on a thorough inquiry of the trend of the rotational constants supported by high-level theoretical computations. ^[1]A. M. Boudet, 2007, Phytochemistry, 68, 22-24, 2722-2735

A modular heated nozzle assembly with an open source external temperature control unit was designed for use in a new broadband microwave spectrometer under construction at Missouri University of Science and Technology. The first version of the heated source was tested using MS&T’s existing cp-FTMW. As an initial proof of concept, the microwave spectrum of molecular piperonal was collected in the 5.5 to 18.75 GHz region. The parent isotopologues of two conformers, s-cis- and s-trans-piperonal, were observed and an analysis of the ¹³C substituted species are ongoing.

A novel approach based on laser ablation of solid organic precursors has been recently proposed for the laboratory in situ generations of new chemical species.L. Kolesniková, I. León, E. R. Alonso, S. Mata, and J. L. Alonso, Angew. Chem. Int. Ed. 2021, 60, 24461.he chemical compounds generated in the laser ablation process are cooled in a supersonic expansion and probed by eyes of high-resolution microwave spectroscopy. This "micro-laboratory" enhances the scope of in situ experiments using precursors not typically accessible to traditional techniques such as electric discharge and pyrolysis. In this contribution, N,N'-Bis(hydroxymethyl)urea is chosen as a precursor. It contains a C=O functional group and two pairs of -NH and -OH groups, very appealing from the astrochemical point of view. Guided by the theoretical predictions, we assigned the precursor's rotational spectrum (four conformers), and then we focused on tuning up our experiment to achieve the experimental conditions that maximize the photo-fragmentation. A detailed analysis of the spectrum revealed the generation of hydroxymethylurea and the simultaneous formation of other species in the jet, showing that the laser ablation of solid organic precursors constitutes an innovative tool in generating new chemical species.
Acknowledgments: The authors thank the financial fundings from the Czech Science Foundation (GACR, grant 19-25116Y), Ministerio de Ciencia e Innovación (grant PID2019-111396GB-I00), Junta de Castilla y León (grants VA244P20), and European Research Council under the European Union's Seventh Framework Programme ERC-2013-SyG, Grant Agreement n. 610256 NANOCOSMOS.

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L. Kolesniková, I. León, E. R. Alonso, S. Mata, and J. L. Alonso, Angew. Chem. Int. Ed. 2021, 60, 24461.T

The class of geminal diols comprises molecules known to be intermediates in atmospheric ozonolysis and the aerosol cycle. Owing to their thermodynamic propensity to decompose into water and an aldehyde/ketone, geminal diols have proved difficult to isolate and characterize. Recently, experimental evidence was published for the existence of methanediol, the simplest member of this class, in ices of methanol and oxygen following electron bombardment.Zhu, C.; Kleimeier, N. F.; Turner, A. M.; Singh, S. K.; Fortenberry, R. C.; Kaiser, R. I. Proc. Natl. Acad. Sci. U. S. A. 2022, 119, e2111938119.o aid in future spectroscopic investigations of methanediol in the gas phase, we report fundamental frequencies and rovibrational constants for the two rotamers of methanediol using ab initio composite methods along with vibrational perturbation theory. Sensitivity of the predictions to the level of theory and the treatment of anharmonic resonances are carefully assessed. The OH stretching harmonic frequencies of both rotamers are particularly sensitive to the level of theory. The CH stretches of the C_s rotamer are sensitive to the treatment of anharmonic resonances with VPT2-based effective Hamiltonian models. Equilibrium bond distances and harmonic frequencies are converged to within   0.0001 Å and 1 cm⁻¹, respectively. The effect of tunneling on the rotational constants is investigated with a 2D variational calculation, based on a relaxed hydroxyl torsional potential energy surface. Tunneling is found to be negligible in the lower energy C₂ rotamer but should modify the rotational constants of the C_s rotamer on the order of MHz, giving rise to rotational line splittings of the same order. The rovibrational constants of the C_s rotamer are dominated by torsional effects, and here we see evidence for the breakdown of vibrational perturbation theory.

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Zhu, C.; Kleimeier, N. F.; Turner, A. M.; Singh, S. K.; Fortenberry, R. C.; Kaiser, R. I. Proc. Natl. Acad. Sci. U. S. A. 2022, 119, e2111938119.T

Non-aromatic heterocyclic molecular compounds due to their π electron orbitals have potential to be used in the surface science as coatings with good surface adhesion properties. Such properties are even more pronounced when silicon atom is present in the ring. It is due to formation of stable covalent bonding between the coating and an inorganic substrate. The objective of this study is to elucidate conformational diversity of newly synthesized five and six membered non-aromatic cyclic compounds, namely 1-chloromethyl-1-fluorosilacyclohexane, 1-chloro-1-chloromethylsilacyclohexane, 1-chloromethyl-1-fluorosilacyclopentane and 1-chlorosilacyclopentane. In order to obtain structural parameters and to perform conformational analysis of such yet unknown molecules detailed computational and experimental studies were performed. The samples were investigated by means conventional IR and Raman spectroscopy as well as matrix isolation FTIR spectroscopy, solid nitrogen acting as matrix medium. Calculations were performed using ORCA 4.1.1 software package. Geometry of different conformers, their stability, normal vibrations and the potential barriers for the conformational interconversions were calculated using density functional theory (DFT) and utilizing augmented B3LYP hybrid functional and Dunning’s augmented double zeta correlation consistent basis sets. Additionally, the Carr-Parinello Molecular Dynamics was used to mimic theoretically the matrix isolation conditions and to describe the molecular behavior in the systems with finite temperature. It was found that conformational equilibrium in the non-aromatic heterocyclic molecular compounds under study strongly depends on type of radical attached to the ring.

l0pt Figure A general method is proposed to disentangle isomeric structures by combining mass spectrometry, tunable synchrotron light source, and quantum-chemistry calculations. Reactive chemical monitoring technique is used and consists in tracking reactivity changes as a function of photoionization energy i.e., internal energy related to isomerization barriers. The capability of this technique will be illustrated with charge transfer reactions of C₆H₄⁺ isomers with allene and propyne.^a The methodology can be generalized using neutral reaction partners with ionization energies forming a chemical ruler to elucidate the ionic structure. It can also serve as a structural probe as a function of photoionization energy, unveiling isomerization routes.




^a Jacovella et al. Structural elucidation of C₆H₄⁺ using chemical reaction monitoring: Charge transfer versus bond forming reactions, ChemPhysChem (2021), doi: 10.1002/cphc.202100871
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Using time-dependent density functional theory, we probe the electron dynamics involved in the photo-chemical isomerization of 1,3-cyclohexadiene to 1,3,5-hexatriene by analyzing changes to the harmonic spectra. The reaction is often used as an analog for the photo-induced reaction of provitamin D₃ to vitamin D₃ in the skin, as that reaction proceeds through a similar scheme. Calculations are performed for 1,3-cyclohexadiene and 1,3,5-hexatriene as well as two intermediate conformers. There are distinct changes in the harmonic spectra that could be used to mark reaction progress. As expected, the breaking of the molecular symmetry leads to peaks outside of the expected odd harmonics. In addition to the harmonic spectra, we discuss the dynamical changes to the electron density and electron localization function (ELF). From these measures of electron probability, we can see ionization and recombination occur throughout the pulse, from each of the isomers.

Biliverdin (BV) is an attractive candidate for use in fluorescence microscopy applications, because it can be genetically encoded (and engineered) as a red fluorescent biomarker by binding to cysteine and including it in fluorescent proteins. An important step towards rational design of a biomarker with specific desired properties is to study the intrinsic properties of the molecule, i.e., in the absence of any chemical environment. In the present work, we focus on the singly and doubly deprotonated forms of the position isomer biliverdin IX (see Figure). The structural flexibility of this molecule makes structural identification of its conformation in vacuo rather challenging. Ion mobility and cryogenic infrared spectroscopy allow us to obtain structural information. The electronic spectrum depends strongly on the deprotonation state of the molecule. We interpret the data with density functional theory calculations. r0pt Figure