Nitric oxide (NO) radicals are ubiquitous chemical intermediates present in the atmosphere and in combustion processes, where laser-induced fluorescence is extensively used on the NO (A²Σ⁺ ← X²Π) band to report on fuel-burning properties. However, accurate fluorescence quantum yields and NO concentration measurements are impeded by electronic quenching of NO (A²Σ⁺) to NO (X²Π) with colliding atomic and molecular species. To improve predictive combustion models and develop a molecular-level understanding of NO (A²Σ⁺) quenching, we report the velocity map ion images and product state distributions of NO (X²Π, v"=0,1, J", F_n, Λ) following nonreactive collisional quenching of NO (A²Σ⁺) with molecular oxygen, O₂ (X³Σ_g⁻). A novel dual-flow pulse valve nozzle is constructed and implemented to carry out the NO (A²Σ⁺) electronic quenching studies and to limit NO₂ formation. The isotropic ion images reveal that the NO–O₂ system evolves through a long-lived NO₃ collision complex prior to formation of products. Furthermore, the corresponding total kinetic energy release distributions support that O₂ collision coproducts are formed primarily in the c¹Σ_u⁻ electronic state with NO (X²Π, v"=0, J", F_n, Λ). The product state distributions also indicate that NO (X²Π) is generated with a propensity to occupy the Π(A") Λ-doublet state, which is consistent with the NO π* orbital aligned perpendicular to nuclear rotation. The deviations between experimental results and statistical phase space theory simulations illustrate the key role that the conical intersection plays in the quenching dynamics to funnel population to product rovibronic levels.

Rotationally-resolved infrared spectra of the bending modes of N₂O-Ar and N₂O-Kr van der Waals clusters have been studied in the region of the N₂O ν₁ fundamental vibration ( ≈ 2224 cm⁻¹), using a tunable quantum cascade laser source to probe complexes generated in pulsed supersonic jet expansion. The assigned vibrational transitions are hot bands (1, 1¹, 0)← (0, 1¹, 0) observed under supersonic conditions. In the presence of Argon/Krypton atom, the doubly-degenerate ν₂ bending mode of the N₂O monomer splits to two modes, in-plane and out-of-plane of the complex, with A^′ and A^′′ symmetry, respectively. These two infrared bands are heavily linked by Coriolis interaction. Analysis of the two bands for N₂O-Ar, located about 2209.8 cm⁻¹with a splitting of  ∼ 0.1 cm⁻¹between the in-plane and out-of-plane intramolecular bending modes, yields experimental values ξ_a= 0.04215and ξ_b =0.13646 for a- and b-type Coriolis interaction parameters for the lower state. For the N₂O-Kr, these values are found to be ξ_a= 0.04205 and ξ_b =0.09212. The two bands for this complex are located about 2209.5 cm⁻¹and with a splitting of  ∼ 0.4 cm⁻¹. Contrary to CO₂ containing complexes, the sign of splitting cannot be determined. This is due to lower symmetry of the N₂O-rare gas complexes. Coriolis interaction parameters found in these analyses, along with that of the previously-reported bands for CO₂-ArT.A. Gartner, A.J. Barclay, A.R.W. McKellar, and N. Moazzen-Ahmadi, Phys. Chem. Chem. Phys. 22, 21488-21493 (2020).nd CO₂-N₂A.J. Barclay, A.R.W. McKellar, and N. Moazzen-Ahmadi, J. Chem. Phys. 153, 014303 (2020).omplexes, offer important information for testing the accuracy of theoretical calculation toward a better understanding of intermolecular interactions.

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Footnotes:

T.A. Gartner, A.J. Barclay, A.R.W. McKellar, and N. Moazzen-Ahmadi, Phys. Chem. Chem. Phys. 22, 21488-21493 (2020).a A.J. Barclay, A.R.W. McKellar, and N. Moazzen-Ahmadi, J. Chem. Phys. 153, 014303 (2020).c

CO₂-CO dimer was previously observed by high-resolution microwave and infrared spectroscopy in the form of a planar T-shaped C-bonded structure.A.C. Legon, A.P. Suckley, Journal of Chemical Physics 91 4440 (1989).^,R.W. Randall, J.P.L. Summersgill, B.J. Howard, Journal Chemical Society Faraday Transactions 86 1943 (1990).ore recently, spectra of a second isomer were observed in the stretching region of carbon monoxide.S. Sheybani−Deloui, A.J. Barclay, K.H. Michaelian, A.R.W. McKellar, N. Moazzen−Ahmadi, Journal of Chemical Physics 143 121101 (2015).his isomer has also a planar T−shaped structure, but with the CO flipped by 180^. Moreover, two intermolecular frequencies were measured for each isomer. These are in very good agreement with recent high level theoretical calculations.Here, CO_2−CO dimer is observed in the carbon dioxide _3 asymmetric stretch region (2350 cm^-1). Both C−bonded and O−bonded isomers are analyzed for the normal isotopologue as well as for ^13CO_2−CO and ^16O^13C^18O−CO, the latter being the first observation of an asymmetrically substituted form for which all values of K_a are allowed. Combination bands involving the lowest in−plane intermolecular mode are also observed yielding frequencies of 24.51 cm⁻¹for the C−bonded form and 14.37 cm⁻¹for the O−bonded form. In addition, two weak bands near 2337 cm⁻¹are assigned to CO_2 hot band transitions (01^11) − (01^10), yielding the splitting of the degenerate CO_2 R.W. Randall, J.P.L. Summersgill, B.J. Howard, Journal Chemical Society Faraday Transactions 86 1943 (1990).MS. Sheybani-Deloui, A.J. Barclay, K.H. Michaelian, A.R.W. McKellar, N. Moazzen-Ahmadi, Journal of Chemical Physics 143 121101 (2015).T

Titanium cation-acetylene complexes are investigated with infrared photodissociation spectroscopy to study latent reactivity. Ti⁺(C₂H₂)_n complexes are produced via laser vaporization of a titanium rod in a supersonic expansion of argon containing acetylene. Argon-tagged complexes are mass selected in a time-of-flight mass spectrometer and their spectra are measured in the C-H stretching region with infrared laser photodissociation. These spectra are assigned with the aid of B3LYP/Def2TZVP computations. The presence of reacted structures and cation-π structures is investigated by comparing experiment to theory.

Pseudocarbynes are a new class of molecules/materials that we define as finite sp-hybridized carbon chains stabilized through non-covalent interactions with metal clusters. These exciting and innovative materials are expected to approach the unprecedented strength, elastic modulus, and stiffness of carbyne, which has defied isolation and characterization for over a century. We optimized a procedure to synthesize pseudocarbynes via the finely controlled laser ablation of a liquid/metal interface, where clusters and one-dimensional carbon chains self-assemble from colloidal solutions into new mesomaterials that bridge the molecular and materials realms. I will present our spectroscopic results demonstrating the non-covalent interaction between -C ≡ C- chains and small metal clusters as characterized by strong signatures in UV-vis, Raman, and infrared spectroscopies.

For the first time the investigation of water molecule complexed with methylamine (CH₃ NH₂) in solid neon was performed from the far to the near infrared using FTIR spectroscopy. By comparison with other hydrogen-bonded complexes and with the help of theoretical results we have identify several vibrational transitions for CH₃ NH₂ dimer, CH₃ NH₂-H₂O, and CH₃ NH₂–(H₂O)₂ complexes. Theoretical calculations at the MP2 level have been performed to obtain their equilibrium geometries and vibrational spectra at the harmonic level and comparison with experimental data allows us to give structures of observed complexes. For the CH₃ NH₂-H₂O complex, the infrared spectral changes (frequencies as well as intensities) indicate that the water plays the role of the proton donor, in agreement with the previous theoretical works. Careful examination of the far infrared allows the assignment of several intermolecular modes, confirmed by the observation of combinations of intra+intermolecular transitions. All of these results significantly increase the number of one and, especially, two quanta vibrational transitions for these complexes. This study shows the high sensibility of the solid neon isolation to the spectroscopy of the hydrogen-bonded complexes since two quanta transitions can’t be easily observed in gas phase.

Triacetone triperoxide (TATP) is a known peroxide-based explosive frequently used by terrorists in improvised devices due to its ease of synthesis and detection difficulty. Here we propose microwave spectroscopy as a highly selective detection method. The main conformer of TATP (95%) has D₃ symmetry and thus it has no dipole moment. However, TATP-water adducts are polar species and their microwave observation could open a way to detect TATP in the gas-phase. In this work we have observed the spectrum of TATP-H₂O and TATP-(H₂O)₂ which, on the other hand, have revealed the interesting properties of TATP. In TATP-H₂O water lies close to the C₃ axis and rotates around it. As a result, the ground state rotational constants average to those of a symmetric top. The complex exhibits a four center trifurcated O_w-H···O hydrogen bond further stabilized by a weak four center C-H···O_w chelated interaction. To our knowledge, those kinds of interactions have not been observed previously in the gas phase. The anomalous large O-H···O distances associated to the main hydrogen bond interaction evidences that TATP oxygen atoms are to some extent encapsulated by their hydrocarbon environment. The dihrydated cluster can be envisaged as formed from the TATP-H₂O adduct with the second water molecule acting as double hydrogen bond donor to the first water molecule and the closest TATP endocyclic oxygen atom.

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 makes 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 solutes 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 computational structures of AMPA determined from its singly-substituted ¹³C-isotopologues are in good agreement with the two lowest energy conformers observed in experiments. However, in the 1:1 lowest energy complex with DMSO, the structure of AMPA does not correspond to the lowest energy conformation of the monomer. This showed the influence of solvent over the conformational landscape of the solute. The preferred intra- and intermolecular interactions, as well as the observed structural changes upon complexation, will be discussed.

An unexpected dominating F lone pair···π_aromatic-hole interaction has been found to characterize the complex formed between benzaldehyde and tetrafluoromethane by exploring rotational spectroscopy which yields spectral feature with splittings originated from the internal rotation of -CF₃ around the F···π direction. Complemented with theoretical analyses, it pointed out irrefutably that benzaldehyde reveals an aromatic π^*character which is ready to link with F lone pair.Secondary F···π-hole_−C=O− and C σ-hole_CF4···π_aromatic interactions are also contributive to the configuration pereference. In the comparison with the complex about C₆F₆-CF₄, C₆H₆-CF₄, it implies that, the aromatic π^* antibonding orbital can be active not only by the substituents with electron withdrawing but also by the partner molecule with high electronegativity like CF₄, in participate the LP···π-hole interaction.

The rotational spectrum of the benzofuran-formaldehyde complex has been investigated by high resolution Fourier transform microwave spectroscopy. In addition to the parent species, the rotational spectra of all mono-substituted ¹³C isotopologues and the complex with H₂C¹⁸O have been also measured. This extensive set of rotational constants for isotopic species allowed an accurate structural determination exploiting the so-called “semi-experimental” approach.
In the observed isomer, the two subunits orient themselves almost parallel to each other. The non-covalent bonding distance between the carbon atom of formaldehyde and the nearest carbon atom of benzofuran has been found to be well within the sum of van der Waals radius of carbon. The joint experimental and computational study pointed out that the two moieties are linked through a π-π interaction. The interaction energy is calculated to be about 20 kJ mol⁻¹ and, according to a SAPT analysis, is dominated by dispersion.

Aryl-corrins is a class of B₁₂ derivatives that embodies “antivitamin” properties and represents a biologically important class of compounds that hence demands investigation. In our previous study, we experimentally determined bond strength in a series of aryl-corrins by the threshold CID experiments and compared these BDE’s with those calculated with DFT.[1] We found that the computed BDEs were very much dependent on the chosen structure. Given that aryl-cobinamides have many side chains that increase their conformational space, and that the question of a specific structure, measured in the gas phase was important for further evaluation of our T-CID experiment, we proceeded to analyze structural properties of arylcobinamides using CIVP spectroscopy. We explored BOMD simulations with the semi-empirical GFN-xTB methods[2-3] for interpretation of experimental spectra in these conformationally flexible systems. References: [1] A. Tsybizova, C. Brenig, C. Kieninger, B. Kräutler, and P. Chen, Chem. Eur. J., in press. (DOI: 10.1002/chem.202004589) [2] S. Grimme, C. Bannwarth, and P. Shushkov, J. Chem. Theory Comput. 13, 1989 (2017). [3] C. Bannwarth, S. Ehlert, and S. Grimme, J. Chem. Theory Comput. 15, 1652 (2019).