The pyridine–acetone adduct generated in a supersonic expansion has been observed using chirped pulse Fourier transform microwave spectroscopy. The spectrum shows the characteristic multiplet structure due to the presence of the two equivalent methyl tops of acetone. The experimental structure shows the fingerprints of an n→π* interaction between the pyridine N atom lone pair and the acetone carbonyl group. This interaction is usually associated with the Bürgi-Dunitz trajectory of nucleophilic addition to a carbonyl compound. The observed methyl group internal rotation barrier is lower than that for free acetone. The structure and internal dynamics of the complex are compared to those of related adducts as pyridine-formaldehyde S. Blanco and J. C. López, J. Phys. Chem. Letters 2018, 9, 4632-4637nd pyridine-acetaldehyde. S. Blanco, A. Macario and J. C. López, Phys. Chem.Chem. Phys 2019, 21, 20566-20570hese complexes are an excellent benchmark for experiment and theoretical computations to explore the n→π* Bürgi-Dunitz coordinate and its interplay with the methyl group internal rotation. S. Blanco and J. C. López, J. Phys. Chem. Letters 2018, 9, 4632-4637a S. Blanco, A. Macario and J. C. López, Phys. Chem.Chem. Phys 2019, 21, 20566-20570T

The weak infrared spectrum of CO₂-Ar corresponding to the (v₁, v₂^l₂, v₃) = (01¹1) ← (01¹0) hot band of CO₂ is detected in the region of the carbon dioxide ν₃ fundamental vibration ( ∼ 2340 cm⁻¹), using a tunable OPO to probe a pulsed supersonic slit jet expansion. While this method was previously thought to cool clusters to the lowest rotational states of the ground vibrational state, here we show that under suitable jet expansion conditions, sufficient population remains in the first excited bending mode of CO₂ (1-2%) to enable observation of vibrationally hot CO₂-Ar, and thus to investigate the symmetry breaking of the intramolecular bending mode of CO₂ in the presence of Ar. The bending mode of CO₂ monomer splits into an in-plane and an out-of-plane mode, strongly linked by a Coriolis interaction. Analysis of the spectrum yields a direct measurement of the in-plane / out-of-plane splitting measured to be 0.8770 cm⁻¹. This aspect of intramolecular interactions has received little previous experimental and theoretical consideration. Therefore, we provide an additional avenue by which to study the intramolecular dynamics of this simplest dimer in its bending modes. Similar results are obtained for CO₂-Ne.

We examine the interplay of docking preference on 1-naphthol in van der Waals complexes with simple molecules. The advantage of 1-naphthol is that it allows for two main docking sites, the first on the conjugated π-system of the fused rings, and the second on the hydrogen bond donor site at the hydroxyl group. Initial complexes studied include furan, dimethylfuran, and thiophene. These three systems follow a similar structure of a heterocyclic, five-membered ring containing a chalcogen as the heteroatom, setting up a common structural motif to benchmark quantum chemical methods with experimental results. Later measurements were extended to benzene and carbon monoxide. Current progress on the ethylene and acetylene complexes will also be discussed. We compare these results with recent literature reports of the dissociation energies and computational studies, where available. The spectra for all the reported systems were recorded using a broadband microwave spectrometer in the 2-8 GHz frequency range in a cold, supersonic jet. From the spectroscopic constants of those complexes, an effective geometry has been determined for each. As is often the case in such systems, more than one docking site can be observed in the supersonic expansion, which is indeed true for the furan complex. A combination of experiment and quantum chemical calculations are used to determine the relative abundances of each furan docking site observed in the expansion.

Infrared predissociation (IRPD) spectroscopy is used to reveal and compare the microsolvation motifs of Gly and β-. The chemical structure of these amino acids differ only in the length of the carbon chain connecting the amine and carboxyl terminals, which nonetheless leads to a significant difference in the strength of the intramolecular C=O…H-N hydrogen bond in the unsolvated ions. This difference makes them useful in our studies of the competition between solvation and internal hydrogen bonding interactions. Analysis of the IRPD results reveals that the sequential addition of water molecules leads to similar effects on the intramolecular interaction in both and β-. Solvation of the - group leads to a weakening of the C=O…H-N hydrogen bond, while solvation of the carboxyl - leads to a strengthening of this bond. Additionally, we have found that for β-, the addition of a to the second solvation shell can still influence the strength of the C=O…H-N hydrogen bonding interaction. Finally, because the C=O…H-N interaction in β- is stronger than that in , more solvent molecules are needed to sufficiently weaken the intramolecular hydrogen bond such that isomers without this bond begin to be energetically competitive; this occurs at n = 5 for β- and n = 1 for .

Infrared spectra of the CO₂-CO dimer are observed in the carbon dioxide ν₃ asymmetric stretch region ( ∼ 2350 cm⁻¹) using a tunable infrared optical parametric oscillator to probe a pulsed slit jet supersonic expansion. Both C-bonded and O-bonded isomers are analyzed for the normal isotopologue as well as for ¹³CO₂-CO and ¹⁶O¹³C¹⁸O-CO, the latter being the first observation of an asymmetrically substituted form for which all values of K_a are allowed. A combination band involving the lowest in-plane intermolecular mode ( ∼ 24.5 cm⁻¹) is also studied for the C-bonded form. A weak band near 2337 cm⁻¹ is assigned to the CO₂ hot band transition (v₁, v₂^l₂, v₃) = (01¹1) ← (01¹0), yielding the splitting of the degenerate CO₂ ν₂ bend into in-plane and out-of-plane components due to the presence of the CO. The splitting, an aspect of intermolecular forces which has received little attention in the past, has rather different values of 4.56 and 1.59 cm⁻¹ for the C- and O- bonded isomers, respectively.

An exceptional and remarkable chemical event, barrierless proton transfer (PT) at the interface of weak C-H...O hydrogen bond, has been found to be occurring within the dimer cation of an atmospherically significant α,β-unsaturated aldehyde, Methacrolein, produced in the gas phase under supersonic cooling condition. The reaction has been initiated upon non-resonant multiphoton ionization by 532 nm pulses (10 ns) of a Q-switched Nd:YAG laser and the fragment ions have been probed by means of time-of-flight (TOF) mass spectrometry. Prominent signatures of the protonated molecular ion [(MC)H⁺] and dimer cation [(MC)₂]^.+ have been observed in the TOF mass spectra whereby intensity of the former is found to increase proportionately with that of the latter as the cooling condition was improved by increasing the backing pressure of the carrier gas. The observations indicate that the intermolecular PT within the dimer cation and subsequent fragmentation lead to the formation of (MC)H⁺. The laser power dependence study reveals that a 4-photon process is responsible for the photoionization of dimer and generation of the protonated molecular ion at 532 nm wavelength. The electronic structure theory calculations predict that the vertical ionization energy of the dimer is very much similar to the total four-photon excitation energy (9.32 eV) and thus it has been concluded that the dimer cations were formed with no excess energy under the experimental condition. The effect of excess vibrational energy on the PT process has also been experimentally verified by ionizing the compound with 355 nm laser light. Potential energy scans along the C-H...O coordinate of the dimer cations have revealed that the PT process occurs without encountering any effective energy barrier.

Rotational spectroscopy is often used to determine the gas-phase structures of weakly bond complexes. When combined with the power of quantum chemical calculations, relative stabilities of various conformations can be confirmed. Occasionally, complexes with more than one structure are observed, and experimentally the relative intensities of the rotational transitions lead to a nearly 50/50 estimation of the conformational population in the measurement. If it happens that the relative energies predicted computationally are nearly identical for two conformers, the question of which structure is favored is left unanswered. To this end, we present a rotational spectroscopic study on two conformations of a bromobenzene-styrene oxide complex that are nearly equivalent in energy, but structurally different. In the 2-8 GHz region, the observed nuclear hyperfine structure from the ⁷⁹Br and ⁸¹Br nuclei (I=3/2) are used to fit the nuclear quadrupole couple tensor. For each bromine isotope, the diagonalized nuclear quardrupole tensor is used to confirm the favored structure. Results are presented along side symmetry-adapted perturbation theory (SAPT) calculations of the interaction energies. A brief comparison to similar halogenated analogs is also presented.

r0pt Figure Ketone-solvent balances consisting of a hydrogen bond donor docking onto the carbonyl oxygen of an asymmetrically substituted ketone, such as an acetophenone derivative, generate two competing isomers. By experimentally probing the vibrational differences with FTIR spectroscopy in a supersonic jet, the abundance ratio of the two isomers at low temperatures is determined. This allows for the experimental benchmarking of theoretically predicted relative conformational energies on a kJ mol⁻¹ or even finer scale.H. C. Gottschalk et al., J. Chem. Phys., 2018, 148, 014301.A. Poblotzki et al., J. Phys. Chem. Lett., 2017, 8, 5656. Previous studies of acetophenone derivativesC. Zimmermann et al., Phys. Chem. Chem. Phys., 2020, 22, 2870-2877.ith methanol and tert-butyl-alcohol showed good agreement between experiment and theoretical predictions and suggest a systematic cancellation of zero point vibrational energy contributions due to the two comparable docking environments, in contrast to previously presented anisoleA. Poblotzki et al., Phys. Chem. Chem. Phys., 2016, 18, 27265.M. Heger et al., Phys. Chem. Chem. Phys., 2015, 17, 13045. and furanH. C. Gottschalk et al., Angew. Chem. Int. Ed., 2015, 55, 1921.omplexes. These ketone-solvent balances therefore allow for a more direct benchmark of different electronic structure methods with less need for an accurate vibrational treatment beyond the double harmonic approximation. The influence of London dispersion and steric hindrances on the favored docking site in acetophenone derivative/phenol systems are explored by varying the substituentes of the ketone. H. C. Gottschalk et al., J. Chem. Phys., 2018, 148, 014301.

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Footnotes:

C. Zimmermann et al., Phys. Chem. Chem. Phys., 2020, 22, 2870-2877.w A. Poblotzki et al., Phys. Chem. Chem. Phys., 2016, 18, 27265.

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Footnotes:

H. C. Gottschalk et al., Angew. Chem. Int. Ed., 2015, 55, 1921.c

The monohydrated complexes of trimethylene oxide and sulfide were investigated in the gas phase using Fourier transform microwave spectroscopy (8-18 GHz) aided by quantum mechanical calculations. In the assigned trimethylene oxide-water complex, the water subunit binds exclusively to the oxygen atom of the ring via a OH…O hydrogen bond (HB). In the sulfur containing complex, in addition to the primary OH…S HB, a secondary CH…O interaction is observed. The experimental results for the sulfur analog are consistent with the predictions from both ab initio and density functional theory (DFT) calculations with the aug-cc-pVTZ basis set whereas for the oxygen analog, both planar and puckered ring configurations are possible depending on the level of theory. Our experimental data aligns better with the results from DFT calculations which predict the trimethylene oxide ring to adopt a planar configuration in the complex. The nature and strength of the HBs are studied in detail using topological and energy decomposition analyses. Comparisons are also made computationally with the selenium analog to evaluate the characteristics of the HB along the chalcogen series. The results show that the HBs formed by these rings with water are mostly electrostatic and dispersive in nature and that the HB strength decreases from oxygen to selenium.

Present in a variety of chemical systems, π-π stacking and hydrogen bonding are intermolecular forces critical to the formation and stabilization of various chemical structures. Within DNA these two forces work in unison to stabilize the infamous double helix structure;¹ however, these forces do not always act cooperatively and often compete with each other. This competition is clearly exhibited within the phenol dimer structure, where a predominance of hydrogen bonding over π-π stacking is observed.² However, it was unclear how the competition between of hydrogen bonding and π-π stacking would change as the size of the aromatic subunit increases. We use 1-naphthol and 2-naphthol, which can be considered analogues of phenol, as a tool to further understand the complex interplay between π-π stacking and hydrogen bonding Using chirped-pulse Fourier transform microwave spectroscopy, we detected pure rotational transitions of two conformers for both 1-naphthol and 2-naphthol. We additionally observed and subsequently assigned the dimers of 1-naphthol and 2-naphthol. The experimental identifications of the observed dimers were supplemented with a conformer search enabled by dispersion corrected DFT³ and the CREST sampling program.⁴ Through this study we find, somewhat surprisingly, that the structures of 1-naphthol and 2-naphthol are dictated mainly by π-π stacking, unlike the case of the phenol dimer. Furthermore, the dominance of the π -π stacking interactions not only provides information about 1-naphthol and 2-naphthol like structures, but also insights into the complicated and intricate dynamics of these intermolecular forces and the fine balance between them. 1. Ts'o, P. O., Elsevier: 2012; Vol. 2.; 2. Seifert, N. A., Steber, A. L., Neill, J. L., Pérez, C., Zaleski, D. P., Pate, B. H., Lesarri, A., Phys. Chem. Chem. Phys. 2013, 15 (27), 11468-11477.; 3. Becke, A. D., J Chem. Phys. 1992, 96 (3), 2155-2160.; 4. Pracht, P.; Bohle, F.; Grimme, S., Phys. Chem. Chem. Phys. 2020.

Mandelic acid and its derivatives are useful as chiral synthons in the chemical and pharmaceutical industry because of their versatility. Their wide use in organic reactions make them an important case to study solute-solvent interactions. In this work, we study the structure of α-methoxy phenylacetic acid (AMPA), a mandelic acid derivative, and its solute-solvent interaction in a weakly bound complex with the solvent dimethylsulfoxide (DMSO). This system can serve as a model to characterize the non-covalent interactions of such chiral solute with different solvents. Rotational spectroscopy studies of weakly bound complexes are able to accurately reveal the structures and internal dynamics of molecules and complexes isolated in the gas phase.
We present a conformational study of AMPA and its 1:1 complex with dimethylsulfoxide (DMSO) using chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy in the 2-8 GHz range. Two conformers of AMPA were identified in the rotational spectrum. Splitting patterns arising from internal motions in the molecule are observed and will be analyzed. The experimental structures of AMPA determined from its singly-substituted ¹³C-isotopologues are in good agreement with the two lowest energy conformers obtained from quantum-chemical calculations. However, in the 1:1 lowest energy complex with DMSO, the structure of AMPA doesn't correspond to the lowest energy conformation of the monomer. The preferred intra- and intermolecular interactions, as well as the observed structural changes upon complexation, will be discussed.