Chirped pulse Fourier transform microwave (CP-FTMW) spectroscopy serves as a faster-than-ever technique for structure determination that is being utilized in an ever-widening range of applications. Vacuum ultraviolet (VUV) time-of-flight mass spectrometry (TOFMS) provides a gentle and general ionization scheme for measurement of the masses, and hence molecular formulae, of the components of a gas mixture, as long as these components have ionization potentials below the threshold for single-photon photoionization (118 nm = 10.5 eV in this case). By combining these two methods, a powerful tool is created for determining the structures of the components in a mixture. By following the correlation between the microwave transition intensities and the mass-resolved signals in the TOF mass spectrum, the molecular formulae of the carriers of the microwave transitions can be determined. In this sense, the combination of CP-FTMW with VUV TOFMS produces mass-correlated broadband microwave spectra. Herein, we discuss the design, construction and initial applications of a new chamber that was designed to acquire simultaneous mass and rotational spectra. Proof of concept will be demonstrated by examining a gaseous mixture of 2-methoxyfuran, furan and furfural. The new spectrometer is also being used to characterize resonance stabilized radicals formed by a pyrolysis source. Application in this context will also be discussed.

We have combined the high-resolution provided by chirped pulse Fourier transform microwave (CP-FTMW) spectroscopy with a vacuum ultraviolet (VUV) time-of-flight mass spectrometry (TOF-MS) that allows us to find optimal conditions to detect reactive intermediates. This unique setup was used to structurally characterize the phenoxy radical. This is an important intermediate in the oxidation of many aromatic compounds. The decomposition of this radical has as primary product cyclopentadienyl radical, which is an important species in the formation of polynuclear aromatic hydrocarbons. Phenoxy radical was generated through the pyrolysis of anisole and allyl phenyl ether. The work was carried out over a 300-1500 K temperature range, using a high-temperature flash pyrolysis micro-reactor coupled with a supersonic expansion. The 2-18 GHz pure rotational spectrum was obtained concurrently with the mass spectrum and determined the resonant-stabilized radical molecular parameters. For further structural analysis spectra of ¹³C isotopomers were obtained using segmented chirps. The new structural insights and the pyrolysis setup coupled with the CP-FTMW/TOF spectrometer will be discussed.

Although the OH⁺ cation is decidedly a triplet (³Σ⁻) being over 50 kcal mol⁻¹ more stable than the corresponding singlet (¹∆), binding to an argon atom can reverse this situation. The noble gas forms a strong donor-acceptor bond to the excited state singlet cation with a bond strength of 66.4 kcal mol⁻¹ at the CCSDT(Q)/CBS level of theory. This makes the singlet 3.9 kcal mol⁻¹ more stable than the most favorable triplet Ar−HO⁺ complex. In a cold molecular beam experiment we have prepared both, singlet and triplet, isomers of this molecular ion depending on the employed ion source. Photodissociation spectroscopy in combination with messenger atom tagging reveals that the two observed spin isomers exhibit completely different spectral signatures in the infrared and the O−H stretching fundamentals differ by about 900 cm⁻¹. These findings might encourage the search for a new potential interstellar noble gas molecule.

The extended nature of the proton wave function, the shape of the ground and final state potentials in which the proton is located has been investigated in gaseous acetylacetone and three of its derivatives, benzoylacetone, dibenzoylmethaneVitaliy Feyer, Kevin C. Prince, Marcello Coreno, Sonia Melandri, Assimo Maris, Luca Evangelisti, Walther Caminati, Barbara M. Giuliano, Henrik G. Kjaergaard, and Vincenzo Carravetta, J. Phys. Chem. Lett. 9 (2018) 521–526. DOI: 10.1021/acs.jpclett.7b03175nd 3,5-heptanedione by quantum chemical calculations, microwave spectroscopy and core level photoemission study. These molecules show intramolecular hydrogen bonds, in which a proton is located in a double well potential, whose barrier height is different for the four compounds, allowing us to examine the effect of the shape of double well on photoemission and rotational spectra. For all of them, two distinct O 1s core hole peaks are observed, previously assigned to two chemical states of oxygen in the ground state. We provide an alternative assignment by taking full account of the finite temperature of the samples based on quantum chemical calculations and symmetry consideration.

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Vitaliy Feyer, Kevin C. Prince, Marcello Coreno, Sonia Melandri, Assimo Maris, Luca Evangelisti, Walther Caminati, Barbara M. Giuliano, Henrik G. Kjaergaard, and Vincenzo Carravetta, J. Phys. Chem. Lett. 9 (2018) 521–526. DOI: 10.1021/acs.jpclett.7b03175a

The hetero-oligomers formed by difluoromethane (CH₂F₂) and 1,1-difluoroethane (CH₃CHF₂) were investigated by pulsed jet Fourier transform microwave spectroscopy and theoretical calculations. For the hetero-dimer of CH₂F₂-CH₃CHF₂, three most stable conformers predicted at MP2/6-311++G(d,p) level were observed. Experimental results, ab initio calculations and quantum theory of atoms in molecules (QTAIM) analyses indicate that all the observed conformers are stabilized through a net of three weak C-H^...F-C interactions. Rotational measurements have also been extended to three ¹³C isotopologues in natural abundance for each observed conformer, which allowed precisely structural determinations. The relative populations of these three conformers in the jet were estimated by relative intensity measurements. For the hetero-trimer of (CH₂F₂)₂-CH₃CHF₂, one conformer has been observed. The conformer is stabilized through eight C-H^...F-C interactions.

The rotational spectra of the 1:1 heterodimer of difluoromethane and 1,1,1,2-tetrafluoroethane has been investigated by pulsed jet Fourier transform microwave spectroscopy and quantum chemical calculations. Three most stable conformers predicted at MP2/6-311++G(d,p) level were observed. Experimental results, ab initio calculations and quantum theory of atoms in molecules (QTAIM) analyses indicate that all the observed conformers are stabilized through a net of three weak C-H...F-C interactions.The measurements have also been extended to three or two ¹³C isotopologues in natural abundance for the conformers II and III, respectively, which allowed precisely structural determinations of these two conformers. The relative populations of these three conformers in the jet estimated to be 50/7/1.

The rotational spectra of pentafluoropyridine-waterCamilla Calabrese, Qian Gou, Assimo Maris, Walther Caminati, Sonia Melandri, Probing the Lone Pair-π-Hole Interaction in Perfluorinated Heteroaromatic Rings: The Rotational Spectrum of Pentafluoropyridine-Water, J. Phys. Chem. Lett. 2016, 7, 1513-1517 http://dx.doi.org/10.1021/acs.jpclett.6b00473nd hexafluorobenzene-waterLuca Evangelisti, Kai Brendel, Heinrich Maeder, Walther Caminati, and Sonia Melandri, Rotational Spectroscopy Probes Water Flipping by Full Fluorination of Benzene, Angew. Chem. Int. Ed. 2017, 56, 13699 –13703 https://doi.org/10.1002/anie.201707155ave shown unambiguously that substitution by fluorine atoms on the ring strongly influences the binding abilities of the aromatic ligand. Differently from their non-substituted counterparts, which form hydrogen bonds with water, both molecules interact with water forming a lone-pair-π-hole interaction between the water oxygen and the ring. We report on a series of rotational spectroscopy studies performed with a Molecular Beam Fourier Transform Microwave spectrometer in which we have tested the binding abilities of a series of fluorine substituted pyridines, namely 2,4-fluoropyridine, 3,5-fluoropyridine and 2,4,6-fluoropyridine with water. In the complexes formed by the di-substituted pyridines, the water moiety lies in the aromatic plane and forms a hydrogen bond with the heterocyclic nitrogen atom and this is by far the most stable conformation. In the tri-substituted pyridine-water complexes two isomers are possible, but the lp-π-hole form is the one observed in the rotational spectrum. More studies involving the pentafluoropyridine and hexafluorobenzene molecules and several ligands such as NH₃ and CO have also been performed and the interaction with the π-hole has been characterized.

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Camilla Calabrese, Qian Gou, Assimo Maris, Walther Caminati, Sonia Melandri, Probing the Lone Pair-π-Hole Interaction in Perfluorinated Heteroaromatic Rings: The Rotational Spectrum of Pentafluoropyridine-Water, J. Phys. Chem. Lett. 2016, 7, 1513-1517 http://dx.doi.org/10.1021/acs.jpclett.6b00473a Luca Evangelisti, Kai Brendel, Heinrich Maeder, Walther Caminati, and Sonia Melandri, Rotational Spectroscopy Probes Water Flipping by Full Fluorination of Benzene, Angew. Chem. Int. Ed. 2017, 56, 13699 –13703 https://doi.org/10.1002/anie.201707155h

l0pt Figure The conformational preferences, molecular structure and intramolecular dynamics of cyclohexanol (CHOL), the cyclohexanol dimer (CHOL₂) and cyclohexanol-water (CHOL···H₂O) have been analyzed using rotational spectroscopy in a supersonic jet expansion. The monomer conformation is controlled by ring inversion (equatorial/axial) and alcohol internal rotation (gauche+/gauche- or trans). Both equatorial trans (rigid) and equatorial gauche (tunneling) isomers were observed, but axial cyclohexanol went undetected. The monohydrate CHOL···H₂O exhibits a single hydrogen bonded equatorial gauche conformation, with tunneling splittings revealing a concerted motion of the hydroxyl and water molecules. The formation of the dimer CHOL₂ stabilizes the transient chirality of the gauche+/gauche- monomer conformations, as different chiral combinations donor-acceptor result in permanent chirality. This molecular dimer represent a larger level of complexity compared with previous studies M. S. Snow, B. J. Howard, L. Evangelisti, W. Caminati, J. Phys. Chem. A 2011, 115, 47.A. K. King, B. J. Howard, Chem. Phys. Lett. 2001, 348, 343., as 36 different donor-acceptor combinations are now possible and the relative orientation of the two rings generates multiple conformations. Finally, six different isomers were observed for the dimer. Accurate rotational parameters for the observed conformations and supporting ab initio and DFT calculations will be reported.

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M. S. Snow, B. J. Howard, L. Evangelisti, W. Caminati, J. Phys. Chem. A 2011, 115, 47.

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The rotational spectra of four conformers of urocanic acid ((2E)-3-(1H-imidazol-4-yl)prop-2-enoic acid) have been measured between 6 and 18 GHz. The molecule was prepared for study through laser vaporisation of a solid target in the presence of argon gas undergoing supersonic expansion into a vacuum chamber. The solid target was composed of urocanic acid, copper powder and a small amount of polyvinyl acetate glue used as binder. Rotational constants, B₀, centrifugal distortion constants, D_J,D_JK and the nuclear quadrupole coupling constants of the nitrogen atoms, χ_aa(N) and χ_bb−cc(N), have been determined through analysis of the assigned broadband rotational spectra. The geometry of each conformer was identified by comparison of the experimental results with geometries calculated ab initio. Isotopic substitutions at carbon and hydrogen atoms allow further insight into the molecular geometries of the two lowest energy conformers. The geometry of a complex formed between urocanic acid and water will also be described.

The CH₃CN-CO₂ complex was investigated using a Pulsed-Nozzle Fourier Transform Microwave Spectrometer. This complex offers the possibility of observing a ‘carbon-bonded’ structure. The ab initio calculations give three minima structures; a π-stacked structure, a T-shaped structure with the N end of CH₃CN interacting with the C atom of CO₂, and a linear structure with the O of CO₂ interacting with the tetrahedral C of CH₃CN. Thus, the T-shaped and the linear structures are bound with a ‘carbon-bond’. The π-stacked and T-shaped structures have similar binding energies ( -7.7 kJmol⁻¹ and -7.6 kJmol⁻¹, respectively). Many lines were observed which depend on both CH₃CN and CO₂ concentrations. Six of these lines follow a nearly prolate, ‘a’-type spectra. The K=0 (J = 3-2 to 8-7) rotational transitions have been observed. The B+C value obtained from fitting these transitions is consistent with the value predicted for the T-shaped geometry with an N-C interaction. All lines show hyperfine splitting due to quadrupole coupling of the nitrogen atom. Measurements with isotopic substitutions have been carried out to ascertain the assignment of the rotational transitions. The unassigned lines may belong to the other possible geometries of the CH₃CN-CO₂ complex.

The spectroscopic peak assignment gives barrier height of internal rotation as well as minimum angle displacement between S₁ and S₀ electronic states. The most well-known type is methyl internal rotation because most organic molecules have it. Conventional way to assign the spectra uses 1D hindered rigid rotor model. The model system is thioanisole because it engendered mode specific nonadiabatic photofragment in previous experimentJeong Sik Lim, Sang Kyu Kim, Nat. Chem. 2, 627-632(2010) Resonance enhanced multiphoton ionization (REMPI) followed by velocity-map imaging (VMI) made us be possible to assign the methyl internal rotation peaks and to see the dynamics of the photodissociation. Conformers were selectively sampled using different carrier gases in supersonic jet-expansion and their electronic transitions were confirmed by UV-UV hole-burning spectroscopy equipped with Stark-Deflector. In this presentation, trans meta-methylthioanisole (mMTA) manifests active progressions of methyl internal rotation (0a₁, 1e, 2e, 3a₁, 4e, 5e, 6a₁), whereas a few of them (0a₁, 1e, 3a₁) for cis mMTA are tangible. For cis mMTA, the rotor progressions are combined with the torsional vibration. In case of trans ortho-methylthioanisole (oMTA), not only active combinations of methyl internal rotation and torsional vibration but also couplings of them seem to be exist in S₁-S₀ transitionstrans mMTA also shows a coupling of e transition of methyl rotor and the vibronic transition of methylthio group at around 200 cm⁻¹above S₁ origin. VMI studies imply two distinguished photodissociation dynamics of oMTA and mMTA, respectively. The detailed comparison of them is to be presented.

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Jeong Sik Lim, Sang Kyu Kim, Nat. Chem. 2, 627-632(2010). trans mMTA also shows a coupling of e transition of methyl rotor and the vibronic transition of methylthio group at around 200 cm⁻¹above S₁ origin..