DOPAC (3,4-Dihydroxyphenylacetic acid) is an intermediate neural metabolite in the cycle of catecholamines, C. Cabezas, I. Peña, J. C. López and J. L. Alonso, Seven conformers of neutral dopamine revealed in the gas phase, J. Phys. Chem. Lett., 2013, 4, 486–490 and references therein.oncretely in the metabolic reaction of dopamine (DA) to produce homovanillic acid (HVA), a substance that being present in urine in a large quantity, is indicative of health problems. In this work, we present a rotational study to explore the conformational preferences of DOPAC. It has been brought into gas phase avoiding decomposition by laser ablation LA of the solid sample, and the conformers present in the supersonic expansion have been probed by CP-FTMW spectroscopy.I. Peña, S. Mata, A. Martín, C. Cabezas, A. M. Daly and J. L. Alonso, Conformations of D-xylose: the pivotal role of the intramolecular hydrogen-bonding., Phys. Chem. Chem. Phys., 2013, 15, 18243–8.s we will show, several conformers have been detected and characterized. All the data obtained is presented in this communication.

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C. Cabezas, I. Peña, J. C. López and J. L. Alonso, Seven conformers of neutral dopamine revealed in the gas phase, J. Phys. Chem. Lett., 2013, 4, 486–490 and references therein.c I. Peña, S. Mata, A. Martín, C. Cabezas, A. M. Daly and J. L. Alonso, Conformations of D-xylose: the pivotal role of the intramolecular hydrogen-bonding., Phys. Chem. Chem. Phys., 2013, 15, 18243–8.A

Methoxyphenols are emitted in the atmosphere from biomass burning and recent works have shown the potential role of these oxygenated aromatic species in the formation of secondary organic aerosols A. Lauraguais, C. Coeur-Tourneur, A. Cassez, K. Deboudt, M. Fourmentin, and M. Choel., Atmospheric reactivity of hydroxyl radicals with guaiacol (2-methoxyphenol), a biomass burning emitted compound: Secondary organic aerosol formation and gas-phase oxidation products. Atmos. Environ., 86:155-163, 2014 In fact, these semi-volatile polar aromatic species are produced from the pyrolysis of wood lignin and mainly consist in Guaiacol (2-methoxyphneol), syringol (2,6-dimetoxyphenols) and their derivatives. In this work, we carried out a complete conformational landscape of the ortho-, meta- and para- isomers of methoxyphenol (known also as 2-methoxyphenol or guaiacol, 3-methoxyphenol and 4-methoxyphenol or Mequinol, respectively) using quantum chemical calculations with GAUSSIAN program as well as room temperature millimeter wave spectrometer of the LPCA Laboratory in Dunkirk G. Mouret, M. Guinet, A. Cuisset, L. Croize, S. Eliet, R. Bocquet, and F. Hindle. Versatile sub-thz spectrometer for trace gas analysis. IEEE Sens. J., 13(1):133-138, 2013 The 70 - 330 GHz rotational spectrum is measured in this study for each isomer and analysis of their rotational signatures for all the stable conformers in ground and low-energy vibrationally excited states will be detailed and discussed. The main goal of the study is to understand the isomeric influence on the conformational landscape of the methoxy-aromatic compounds. A. Lauraguais, C. Coeur-Tourneur, A. Cassez, K. Deboudt, M. Fourmentin, and M. Choel., Atmospheric reactivity of hydroxyl radicals with guaiacol (2-methoxyphenol), a biomass burning emitted compound: Secondary organic aerosol formation and gas-phase oxidation products. Atmos. Environ., 86:155-163, 2014. G. Mouret, M. Guinet, A. Cuisset, L. Croize, S. Eliet, R. Bocquet, and F. Hindle. Versatile sub-thz spectrometer for trace gas analysis. IEEE Sens. J., 13(1):133-138, 2013.

Methyl anisole (MA), also known as methoxytoluene, exists as three isomers with the ring methyl group in ortho, meta, and para position relative to the methoxy group. Despite the similarity of their chemical properties, the effect of methyl internal rotation which often depends on the steric and electronic surroundings can be completely different. In the other hand, This species is very important as a secondary organic aerosol precursor. In our present study, we focus on the millimeter wave studies on ortho, meta and para-methyl anisole in the 70-330 GHz frequency domain in order to complete the previous microwave studies between 2-40 GHz on the ground state L. Ferres, H Mouhib, W. Stahl, and H. V. L. Nguyen, Methyl Internal Rotation in the Microwave Spectrum of o-Methyl Anisole. ChemPhysChem. 18, 1855–1859 (2017)^, L. Ferres, W. Stahl, and H. V. L. Nguyen, Conformational Effects on the Torsional Barriers in m−Methylanisole Studied by Microwave Spectroscopy, accepted in J. Chem. Phys.^, L. Ferres, W. Stahl, I. Kleiner, H. V. L. Nguyen, The effect of internal rotation in p-methyl anisole studied by microwave spectroscopy. J. Mol. Spectrosc. 343, 44–49 (2018)nd to determine internal rotation higher order parameters by analysis of the rotational structures in the excited states. Our millimeter wave spectra are measured at room temperature, which allow observing rotational structures in the ground and low-energy vibrationally excited states. Thus, we could determine precisely the V₆ potential barriers and evaluate their contributions.

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L. Ferres, H Mouhib, W. Stahl, and H. V. L. Nguyen, Methyl Internal Rotation in the Microwave Spectrum of o-Methyl Anisole. ChemPhysChem. 18, 1855–1859 (2017)\end L. Ferres, W. Stahl, and H. V. L. Nguyen, Conformational Effects on the Torsional Barriers in m−Methylanisole Studied by Microwave Spectroscopy, accepted in J. Chem. Phys. L. Ferres, W. Stahl, I. Kleiner, H. V. L. Nguyen, The effect of internal rotation in p-methyl anisole studied by microwave spectroscopy. J. Mol. Spectrosc. 343, 44–49 (2018)a

We present high-resolution rotational spectroscopy of the two conformers of 3-methylbutyronitrile. Spectra were taken between 2-24 GHz by means of Fourier transform microwave spectroscopy. Spectra between 36 and 402 GHz were recorded by means of frequency modulated (FM) absorption spectroscopy. The analysis yields precise rotational constants and higher order distortion constants, as well as a set of ¹⁴N nuclear electric quadrupole coupling parameters. In addition, quantum chemical calculations were performed assisting the assignments. Frequency calculations yield insight into the vibrational energy structure from which partition functions and vibrational correction factors are determined. These are used to determine experimentally and computationally the energy difference between the conformers, which is revealed to be negligeable. Overall, this study provides precise spectroscopic constants for the search of 3-methylbutyronitrile in the interstellar medium. In particular, this molecule is an interesting testcase to check our knowledge of (branched) molecule formation in space.

We present high-resolution rotational spectroscopy of two out of three conformers of 2-cyanobutane. Spectra were taken between 10-26 GHz by means of chirped-pulse spectroscopy. Spectra between 36 and 402 GHz were recorded by means of frequency modulated (FM) absorption spectroscopy. The analysis yields precise rotational constants and higher order distortion constants, as well as a set of ¹⁴N nuclear electric quadrupole coupling parameters. In addition, quantum chemical calculations were performed assisting the assignments. Calculations of vibrational frequencies yield insight into the vibrational energy structure from which partition functions and vibrational correction factors are determined. These are used to determine experimentally and computationally the energy difference between the conformers. Overall, this study provides precise spectroscopic constants for the search of 2-cyanobutane in the interstellar medium. In particular, this molecule appears as an interesting case to test our knowledge of (branched) molecule formation in space.

The rotational spectrum of 1-iodopentane was measured over the 7-13 GHz frequency range with a chirped pulse Fourier transform microwave spectrometer revealing rotational transitions from a number of conformers. l0pt Figure This continues the group's work on how a large substituent, in this case an iodine atom, at the terminal position will affect the dihedral angles of the alkyl carbon backbone and what influence it will exert with continuing chain length. In keeping with last year’s study of 1-iodobutane,Arsenault E.A.; Obenchain, D.A.; Blake, T.A.; Cooke, S.A.; Novick, S.E; J. Mol. Spectrosc., 2017 335 17-22.e find that the corresponding GAA conformer is the most abundant, and that while the nuclear quadrupole coupling tensor is poorly predicted by direct ab initio methods, scaling methodsAnticipated future communication with W. C. Bailey.llow very reasonable predictions to be obtained.

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Arsenault E.A.; Obenchain, D.A.; Blake, T.A.; Cooke, S.A.; Novick, S.E; J. Mol. Spectrosc., 2017 335 17-22.w Anticipated future communication with W. C. Bailey.a

The incomplete combustion of gasoline under fuel-rich conditions leads to soot formation, an unwanted pollutant and energetically wasteful product. While it is known that soot is composed of π-stacked graphitic-like structures, the incipient stages of soot formation are still something of a mystery, with conflicting evidence as to whether π-stacked polycyclic aromatic hydrocarbon (PAH) aggregates can form and survive at combustion temperatures still unresolved. The mechanism by which the initial aggregation of PAH precursors occurs is still under active investigation. This talk will describe studies of two isomeric naphthalene derivatives containing short alkyl chains: 1-ethyl naphthalene and 2-ethyl naphthalene. The structural isomers differ in the position of substitution of the ethyl side-chain on the naphthalene framework. These molecules are not only components of diesel fuel and of early stages of PAH formation in flames, but are also close analogs of a series of ethyl-bridge dinaphthyl compounds studied by our group. We will present the jet-cooled LIF excitation, dispersed fluorescence, and fluorescence-dip infrared spectra of these molecules, cooled in a supersonic free jet. The vibronic spectroscopy of these molecules is similar to that in naphthalene, modified only subtly by the ethyl side chain. Vibronic coupling between S₁ and S₂ states is pervasive, leading to textbook evidence of Herzberg-Teller coupling. We have also observed the onset of the S₀-S₂ transition near 288 nm. These results will be compared with calculations of the excited states, setting a foundation for understanding the fascinating vibronic spectroscopy of the ethyl-bridged dinaphthyl compounds.

Combustion is the main source of power worldwide. This makes understanding combustion pathways and byproducts vital to optimizing power output and minimizing the unwanted byproducts. One of the main byproducts of the combustion process is soot, which can be accurately described as a large aggregate of polyaromatic hydrocarbons (PAHs). While the general process by which soot particles grow in size have been laid out, the incipient steps by which individual PAH molecules begin to aggregate is still a mystery. Current models of soot formation require individual PAH molecules to dimerize and π stack in PAHs significantly smaller than are thermodynamically capable of doing so. One largely untested possibility is that PAHs linked together by short chemical linkages could be responsible for the initial steps of aggregation. In particular, we propose that resonance-stabilized benzylic-like radicals could recombine to form ethyl-linked PAH dimers that are responsible for the initial stages of aggregation and π stacking. These covalently-linked dimers could withstand the high energies present in the combustion flame and provide a seed for soot aggregation. This brings us to the following question: At what sized PAH will these ethyl-linked aromatics be capable of π stacking, and if they can, will conformations leading to π stacking compete with other, more extended geometries? We begin to answer this question by studying a series of ethyl-linked naphthalene dimers in which the ethyl linkage bridges the two unique sites of substitution (shown below). r0pt Figure These molecules are brought into the gas phase by heating, and cooled in a supersonic expansion. LIF excitation, IR-UV holeburning, fluorescence-dip infrared spectroscopy, and dispersed fluorescence are used to record conformer-specific UV and IR spectra. These dimers also have fascinating electronic spectroscopy associated with the presence of two UV chromophores that are in identical or nearly identical environments, leading to extensive vibronic coupling. The experimental results will be compared with a multi-mode theoretical model of near-resonant vibronic coupling.

Asparagine (Asn) and Glutamine (Gln) are the only two naturally-occuring amino acids that incorporate an amide group in the side chain, differing only by a single methylene group that lengthens the chain in Gln relative to Asn. These two amino acids appear with unusual abundance in the prion forming domain of proteins that are implicated in neurodegenerative disease pathogenesis. In a bottom-up approach towards understanding nature's preference for Asn and Gln in prion domains, we build off of our previous study of Gln-containing peptides by elucidating the inherent folding propensities of three Asn-containing peptides: Ac-Asn-NHBn, Ac-Ala-Asn-NHBn, and Ac-Asn-Asn-NHBn. These molecules are brought into the gas phase by laser desorption and cooled in a supersonic expansion before interrogation via resonant two-photon ionization (R2PI), IR-UV holeburning, and resonant ion-dip infrared (RIDIR) spectroscopies. This talk will describe the conformation-specific IR and UV spectroscopy of these three peptides, concentrating on the fundamental question of whether side-chain/side-chain, backbone/backbone, or side-chain/backbone interactions are preferred in each case. Assignment of the observed conformations are deduced from a comparison of the experimental IR spectra with the predictions of calculations. Two unique conformational isomers are assigned to the capped Asn amino acid while only one conformational isomer is present in both dipeptides. These structures will be compared with those found in the analogous glutamine-containing peptides. Best fit calculated structures reveal intriguing incipient secondary structure formation at the first possible instance.

In MALDI mass spectrometry, the role of a matrix is to transfer charge to the analyte so that it can be detected and quantified. The fundamental process of how this charge transfer actually occurs, however, is still largely unknown. Experimental evidence suggests that InterSystem Crossing (ISC) to the triplet excited state (T1) of the matrix is a crucial step in effective charge transfer to the analyte. To encourage ISC via spin-orbit coupling, we have utilized the heavy atom substitution effect, which suggests that the addition of a heavy atom to an otherwise "dead" matrix should increase the rate of ISC to the T1 state. With the addition of a halogen atom, experiments showed a visible decay in singlet lifetime and an increase in triplet lifetime, as well as improved matrix performance. Our system of interest in this study is 2,4–dihydroxybenzoic acid, which will be compared with two of its heavily substituted, halogenated (F,Cl, Br, and I) partners. Using Gaussian09 software and Density Functional Theory (DFT), all structural isomers were identified and ranked by stability using the functional B3LYP and basis set 6-31g+(d,p). Zero-Point Energies (ZPE) and IR spectra were compared for S0, S1, and T1 states of both the substituted and unsubstituted isomers. Additionally, Time Dependent (TD)-DFT calculations generated UV-Vis and fluorescence spectra. Results showed that Proton Affinity (PA) and Gas Phase Acidity (GPA) values improved with heavy atom substitution in both the 5th and 6th ring positions of 2,4-dihydroxybenzoic acid. Comparison of the S0, S1, and T1 vibrational spectra of the heavily substituted isomers revealed the weakening of bonds between the molecule and its protons, which may be partially responsible for more effective charge transfer to the analyte.

L-glutamic acid is most abundant free amino acid in brain and it is the major excitatory neurotransmitter of the vertebrate central nervous system and known to play an important role in neural differentiation process of a developing brain. Study of the rich conformational landscape of L-glutamic acid can serve as basis to understand interactions of this amino acid with other biomolecules and receptors present in central nervous system.Tapiero, H.; Mathe, G.; Couvreur, P.; Tew, K. D. Biomed. Pharmacother. 2002, 56, 446-457.-glutamic acid was trapped in an inert gas matrix by employing a heated nozzle to provide an effusive molecular beam and the various conformers of the amino acid trapped in the matrix were then characterized by FTIR spectroscopy. Ab-initio calculations were also performed, using MP2/6-311++G(d,p) and M06-2X/6-311++G(d,p) level of theories, to corroborate with experimental observations. A comprehensive scan of the potential energy surface was performed to arrive at the various conformers of L-glutamic acid, which were further classified based on their backbone structure. The tendency of lower energy conformers to adopt certain backbone structures has been pictorially represented using a ‘conformational dartboard’. Factors such as intramolecular H-bonding, delocalized orbital interactions and entropy were found to determine conformational preferences in L-glutamic acid, which will be discussed.

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Tapiero, H.; Mathe, G.; Couvreur, P.; Tew, K. D. Biomed. Pharmacother. 2002, 56, 446-457.L