The mid-infrared spectra of helium solvated methanol and methanol-water clusters have been investigated in the symmetric CD₃ stretching band of CD₃OH and CD₃OD. We find that the position of this band provides a useful signature of the general type of hydrogen-bonded cluster it is associated with. Our results are consistent with those previously reported in the OH stretching region [1], in that methanol clusters from the trimer to the pentamer are cyclic, and that mixed clusters with one water molecule (and at least two methanol molecules) are also cyclic. We additionally provide evidence that the methanol trimer adopts a chair-like structure (as opposed to bowl-like), that mixed clusters with a larger number of water molecules are also cyclic, and that branched methanol clusters contribute to the depletion signal in larger methanol clusters. We performed DFT calculations which support these interpretations. [1] Sulaiman, M. I., Yang, S., Ellis, A. M., J. Phys. Chem. A, 2017, 121, 771-776.

The microsolvation of aromatic and heteroaromatic rings has been the subject of many microwave spectroscopy studies in recent years. In 2020, Li et al. reported the geometry of the monohydrate complex of thiazole.W. Li, J. Chen, Y. Xu, T. Lu, Q. Gou and G. Feng, Spectrochimica Acta - Part A: Molecular and Biomolecular Spectroscopy, 2020, 242, 118720ince thiazole contains multiple sites at which intermolecular bonds can form, a complex of thiazole with two water molecules was subsequently searched for. The rotational spectrum of thiazole…(H₂O)₂ was recorded over the frequency range 6.5-18.5 GHz while analysing a gaseous sample containing thiazole, water and argon by Chirped-Pulse, Fourier-Transform Microwave (CP-FTMW) spectroscopy at Newcastle University and by COBRA (coaxially oriented beam-resonator arrangement)-FTMW spectroscopy at Chongqing University. Aided by density functional theory (DFT) calculations, the spectrum of thiazole…(H₂O)₂ was assigned and rotational constants (A₀, B₀ and C₀), centrifugal distortion constants (D_J, D_JK, d₁ and d₂) and nuclear quadrupole coupling constants (χ_aa and χ_bb−cc) of nitrogen atoms were determined. The microwave spectra of four isotopologues of thiazole…(H₂O)₂ have been assigned allowing the determination of structural parameters which include intermolecular bond lengths and angles.

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Footnotes:

W. Li, J. Chen, Y. Xu, T. Lu, Q. Gou and G. Feng, Spectrochimica Acta - Part A: Molecular and Biomolecular Spectroscopy, 2020, 242, 118720S

Fenchone is a bicyclic monoterpenoid that is released to the atmosphere by natural and anthropogenic sources, where it interacts with other atmospheric molecules such as water. Here we present the investigation of the hydrates of fenchone C₁₀H₁₆O···(H₂O)_n (n = 1-6) by microwave spectroscopy, in the frequency range 2 to 20 GHz, and computational calculations¹. Several isomers of each hydrated complex have been observed and their rotational and centrifugal distortion constants determined. For fenchone···(H₂O)₁₋₄ complexes, observation of the ¹⁸O isotopologues allowed us to determine the location of the oxygen atoms of water and the configuration of water molecules around fenchone. Water binds to fenchone through O-H···O and C-H···O hydrogen bonds. In the mono-, di- and trihydrates water molecules arrange in open chains around fenchone, while for the higher order hydrates water molecules adopt distorted tetramer, pentamer and hexamer configurations. The various configurations as well as the relevant intermolecular interactions, and their modelling by computational methods, will be discussed.

¹ M. Chrayteh, E. Burevschi, D. Loru, T. R. Huet, P. Dréan, M. E. Sanz, PCCP, 23, 20686 (2021).

6-amyl-α-pyrone (6PP) is a biomass molecule that is widely used in the synthesis of industrial and pharmaceutical products. 6PP offers a green and sustainable route in the preparation of industrial chemicals such as linear ketones and hydrocarbon fuels. It is catalytically hydrogenated to produce δ-decalactone (DDL). This reaction, carried out in different solvents under similar conditions, is reported to produce significant variation in DDL yield varying from 6% to 79%.^[1] Such dramatic variations in yield during the reaction can be attributed to numerous reasons, such as the dielectric constant of solvents, or different kinds of solute-solvent interactions during the reaction.
In this work, we study the structure of 6PP and its solute-solvent interactions using chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy. This technique coupled with supersonic expansion reveals accurate structures of molecules and weakly bound complexes isolated in the gas phase. 6PP is highly flexible due to the presence of the pentyl chain. We first investigate the conformational flexibility in 6PP and then its complexes to understand the effect of the solvent molecule on the structure of 6PP. We choose two solvents, ethanol and cyclohexane one giving the best yield and the other one giving moderate results to study how the inter-molecular interactions affect the reaction yield. The observed structural changes in 6PP upon complexation, as well as the preferred intra- and intermolecular interactions, will be discussed.

1. M. I. Alam, T. S. Khan, M. A. Haider, ACS Sustainable Chemistry and Engineering 2019, 7, 3, 2894-2898.

Our characterization of the structures of gas phase heterodimers formed between haloethylenes and the three protic acids, hydrogen fluoride, hydrogen chloride, and acetylene, provided a wealth of information regarding intermolecular forces and the relative effects of electrostatic, dispersion and steric forces. By and large all of these species shared the common structural feature of a hydrogen bond formed between the protic acid donor and a halogen acceptor on the ethylene. The extension of the carbon chain by one atom via the addition of a trifluoromethyl group provides a wider variety of possible interactions and binding sites. For the heterodimer formed between (Z)-1-chloro-3,3,3-trifluoropropene and acetylene, a novel structure with no hydrogen bond but rather an interaction between the acetylenic triple bond and the two hydrogen atoms of the propene is obtained from the analysis of the microwave rotational spectra of the ³⁵Cl and ³⁷Cl isotopologues of the complex.

Understanding the interplay of the various non-covalent interactions present in biomolecular systems is important as these are the driving forces of the structure of these systems. In the condensed phase, however, it is difficult to disentangle the individual contributions of these interactions due to the plethora of structures present, which ultimately limits any systematic studies. One popular way of bypassing this is to study relevant model complexes in the gas phase where these can be mass-isolated and spectroscopically interrogated. In this way, one can obtain an in-depth understanding of the competing non-covalent interactions on a molecular level and correlate structural changes systematically, for example by increasing the system size or changing interaction partners. Due to their structural flexibility, popular model systems for such studies are small amino acids or peptides and their complexes with water or metal ions. In this contribution, we will study the complexes of three alkali metal ions (Li⁺, Na⁺, K⁺) with di- and tripeptides with cryogenic ion vibrational spectroscopy illustrating how the interaction strength changes with the size of the cation and peptide chain length by analyzing their OH, NH, and C=O stretching as well as NH bending vibrations. Of particular interest is the interplay of metal ion peptide interaction with internal NH…NH₂ hydrogen bonding of the neutral peptide itself, whose strength is modulated by the metal cation interaction.

Non-covalent interactions are vitally important for molecular recognition in many biological and chemical processes. Understanding the interplay between intra- and intermolecular forces is crucial for advancing our knowledge on these events and how they are influenced by slight changes. Here we report the interactions of the common odorant carvone with ethanol, a mimic to the amino acid side chain serine. It has been studied through combination of chirped-pulse Fourier transform microwave spectroscopy and computational calculations, including density functional theory and ab initio methods. Seven carvone-ethanol complexes have been observed showing an O…H-O primary bond between the carbonyl group of carvone, acting as a hydrogen bond acceptor, and the hydroxyl group of ethanol as the hydrogen bond donor. Secondary C-H…O dispersion interactions anchoring ethanol to carvone are also established. Changes in the conformational preferences of the monomers upon complexation will be discussed.

Released into the atmosphere by vegetation, biogenic volatile organic compounds (VOCs) contribute substantially to yearly carbon emission, amounting to approximately 1150 Tg of carbon per year.¹ α-pinene, a bicyclic monoterpene, is not only one of the most abundant biogenic VOCs released, but also plays a critical role in the generation of secondary organic aerosol. Once released, α-pinene can be photo-oxidized by atmospheric species such as ozone or various radical species.² Water is relatively abundant in the atmosphere and has therefore a high probability of a close contact with α-pinene. Complexation with water may affect the reactivity with species such as ozone, thus altering product yield, and ultimately the rate of aerosol formation. It is difficult to predict a preferred structure for the α-pinene-water cluster using chemical intuition alone, and a study of its structure and energetics can provide insights into intermolecular interactions between weakly-polar hydrocarbons and hydrogen bonding capable species, as well as data relevant to atmospheric processes. To experimentally identify α-pinene-water clusters we used a chirped pulse Fourier transform microwave spectrometer in the 2-6 GHz range³ and the experiments were supplemented with electronic structure calculations. Two potential conformers were theoretically identified, both of which involve the formation of an O-H - π bond between water and α-pinene. However, only the higher energy conformer could be assigned experimentally. From various one-dimensional energy scans along internal rotation coordinates, the absence of the lower energy conformer is most likely due to a large amplitude O-H wagging motion, which leads to a partial dipole moment cancellation. The O-H - π interaction in both complexes was then visualized and quantified using non-covalent interactions⁴ and natural bond orbital analyses⁵, respectively. 1. A. Guenther, C. N. Hewitt, D. Erickson, R. Fall, C. Geron, T. Graedel, P. Harley, L. Klinger, M. Lerdau and W. A. McKay, J. Geophys. Res. Atmos., 1995, 100, 8873–8892.; 2. J. H. Seinfeld and J. F. Pankow, Annu. Rev. Phys. Chem., 2003, 54, 121–140.; 3. N. A. Seifert, J. Thomas, W. Jäger and Y. Xu, Phys. Chem. Chem. Phys., 2018, 20, 27630–27637.; 4. J. Contreras-García, E. R. Johnson, S. Keinan, R. Chaudret, J.-P. Piquemal, D. N. Beratan and W. Yang, J. Chem. Theory Comput., 2011, 7, 625–632.; 5. E. D. Glendening, C. R. Landis and F. Weinhold, J. Comput. Chem., 2013, 34, 1429–1437.

Biomass burning greatly influences the Earth-cloud-climate system by releasing complex mixtures of organic and inorganic species into the atmosphere.¹ During biomass burning, lignin, an organic polymer and a major component of wood,² undergoes pyrolysis resulting in the direct release of substituted catechols.³ 3-methylcatechol (3MC), a common substituted catechol, may be photo-oxidized or aggregates with other atmospherically relevant molecules, such as water, forming small molecular clusters or hydrates. These hydrates play an important role in the early phases of aerosol particle formation, and can provide valuable thermodynamic data for modelling. To elucidate the conformationally complex hydrate structures we analyzed rotational spectra measured with a chirped-pulse Fourier transform microwave spectrometer in the 2-6 GHz range.⁴ To aid in spectral assignment, we used the Conformer–Rotamer Ensemble Sampling Tool (CREST)⁵ to generate an ensemble of conformers. Two monomer conformations with their respective ¹³C isotopologues were assigned, followed by the determination of substitution structures and semi-experimental effective structures. With the aid of the CREST results, transitions of several hydrates, 3MC-(H₂O)_N=1−4, were also assigned in the experimental spectrum. For the monohydrate and dihydrate, splittings of rotational transitions into quartets were observed. These splittings are a consequence of methyl internal rotation (MIR) and the proton exchange motion of water. Only MIR splittings are present for the trihydrate, while no MIR or proton exchange splitting is present for the tetrahydrate. Non-covalent interactions⁶ and natural bond orbital analyses⁷ were used to visualize and quantify the intermolecular interactions within each cluster. 1. M. Fromm et al., J. Geophys. Res. Atmospheres, 2019, 124, 13254–13272.; 2. A. V. Bridgewater, Therm. Sci., 2004, 8, 21–50.; 3. M. Asmadi et al., J. Anal. Appl. Pyrolysis, 2011, 92, 88–98.; 4. N. A. Seifert et al., Phys. Chem. Chem. Phys., 2018, 20, 27630–27637.; 5. P. Pracht et al., Phys. Chem. Chem. Phys., 2020, 22, 7169–7192.; 6. J. Contreras-García et al., J. Chem. Theory Comput., 2011, 7, 625–632.; 7. E. D. Glendening et al., J. Comput. Chem., 2013, 34, 1429–1437.

Rotationally-resolved infrared spectra of N₂O–Ar and N₂O-Kr van der Waals clusters are studied in the region of the N₂O ν₁ vibration ( ≈ 2224 cm⁻¹) using a tunable Quantum Cascade laser source to probe a pulsed supersonic jet. The N₂O-Kr ν₁ fundamental band is re-analyzed, together with previous ν₃ band data, using a unified scheme to fit the (small) observed Kr isotope splittings. This scheme is then transferred to analyze the bending combination band of N₂O-Kr near 2257 cm⁻¹where isotope effects are much larger due to stretch-bend Coriolis interactions. As a result, N₂O–Kr intermolecular bend (33.29 cm⁻¹) and stretch (34.48 cm⁻¹) frequencies are directly determined for the first time. We also report observation of weak spectra for both N₂O-Ar and -Kr corresponding to the (ν₁, ν₂^l₂, ν₃) = (1,1¹,0) ← (0,1¹,0) hot band of N₂O, located around 2209.8cm⁻¹. In the presence of Argon/Krypton atom, the doubly-degenerate ν₂ bending mode of the N₂O monomer splits into an in-plane and an out-of-plane mode. These two infrared bands are heavily linked by Coriolis interactions and their analysis yields the magnitude of the splitting of the bending modes which are significantly smaller than those observed in the analogous CO₂-containing dimers T.A. Gartner, A.J. Barclay, A.R.W. McKellar, and N. Moazzen-Ahmadi, Phys. Chem. Chem. Phys. 22, 21488-21493 (2020). The experimental results obtained here are valuable for testing the accuracy of theoretical calculation toward a better understanding of intermolecular interactions. T.A. Gartner, A.J. Barclay, A.R.W. McKellar, and N. Moazzen-Ahmadi, Phys. Chem. Chem. Phys. 22, 21488-21493 (2020)..

The present contribution focuses on the main results coming from ab initio investigations carried out on the mixed trimers containing (at least one molecule of) cyanoacetylene in combination with acetylene and/or hydrogen cyanide units. The several optimized structures corresponding to true minima on the PES of these complexes have been characterised at different levels of theory, and a set of their spectroscopic parameters relevant to rotational and vibrational spectroscopies have been determined. Besides, by employing different approaches, also the kinds and the topologies of the interactions present in each of these minima have been investigated.