Large polycyclic aromatic hydrocarbons (PAH) and polycyclic aromatic nitrogen heterocycles (PANH) have been proposed to be potential carriers responsible for the unidentified infrared (UIR) emission bands in the interstellar medium. A. Leger, J. L. Puget, Astron. Astrophys., 137, L5-L8 (1984).he nitrogen atom in PANH induces a blue shift of the CC-stretching mode from near 6.3 μm of the PAH to 6.2 μm, which might agree better with the UIB band at 6.2 μm.H. A. Galue, O. Pirali, J. Oomens, Astron. Astrophys., 517 (2010).e report the infrared (IR) spectra of the N-protonated phenanthridine (C₁₃H₉NH⁺) and seven isomers of mono-hydrogenated phenanthridine (1-, 2-, 3-, 5-, 7-, 9-, and 10-HC₁₃H₉N), produced upon electron-bombardment of a mixture of phenanthridine (C₁₃H₉N) and para-hydrogen (p-H₂) during matrix deposition at 3.2 K. Alternatively, we performed the experiment by UV/IR irradiation of a p-H₂ matrix containing Cl₂ and C₁₃H₉N to produce H atoms to generate hydrogenated phenanthridine (HC₁₃H₉N) via hydrogen tunneling reactions. Spectral groupings were achieved according to the behaviors of lines upon secondary photolysis at various wavelengths, selected according to the vertical electronic transitions of these isomers of H⁺C₁₃H₉N and HC₁₃H₉N predicted with the TD-B3LYP/6-311++G(d,p) method. The spectral assignments were achieved on comparison of the experimental results with the vibrational wavenumbers and IR intensities of possible isomers predicted with the B3LYP/6-311++G(d,p) method. All spectra of H⁺C₁₃H₉N and HC₁₃H₉N are new. Major lines for C₁₃H₉NH⁺ were observed at 2.95, 6.22, and 12.53 μm; the CC-stretching band of C₁₃H₉NH⁺ was at 6.22 μm, blue-shifted from that of the corresponding PAH, 10-C₁₄H₁₁⁺. For HC₁₃H₉N, major lines were observed at 13.43, 13.16, and 13.55 μm for C₁₃H₉NH, 9-HC₁₃H₉N, and 2-HC₁₃H₉N, respectively. Based on the overall spectral pattern, these protonated and hydrogenated species are unlikely to be the carriers of the UIR bands, characterized by emission bands near 3.3, 6.2, 7.7, 8.6, 11.2, and 12.0 μm.

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Footnotes:

A. Leger, J. L. Puget, Astron. Astrophys., 137, L5-L8 (1984).T H. A. Galue, O. Pirali, J. Oomens, Astron. Astrophys., 517 (2010).W

Silicon is the second most abundant element in the Earth’s crust and holds significant importance for industry and technology. Furthermore, due to its presence in the interstellar medium it is of interest for astrochemistry.¹ For a comprehensive understanding of the physical and chemical properties of silicon-containing molecules, a characterization of their gas-phase structures is important. The structural analysis of two silane ethers, sec-butoxytrimethylsilane ((CH₃)₃SiOCH(CH₃)C₂H₅) and sec-butoxymethyltrimethylsilane ((CH₃)₃SiCH₂OCH(CH₃)C₂H₅) will be presented. The latter species presents an extension of the first silane ether with an additional CH₂ group between the silicon and oxygen atoms, thus exhibiting higher conformational flexibility. Spectra for both samples were recorded using the chirped-pulse Fourier-transform microwave spectrometer COMPACT in the 2 – 8 GHz frequency range.² In addition, a tunneling splitting due to internal rotation of several methyl tops is partially resolved making the spectra more complex. Aided by quantum-chemical calculations, the conformational landscape of both silane ethers is studied and the measured rotational spectra were analyzed to identify the lowest energy conformers. (1) W. D. Langer, A. E. Glassgold, ApJ 1990, 352, 123 (2) D. Schmitz, V. A. Shubert, T. Betz, M. Schnell, J. Mol. Spectrosc. 2012, 280, 77-84

The conformational landscape of chalcogen-bridged ethers and thioethers changes depending on the identity of the chalcogen bridge atom and the organic side chains, and these geometric preferences reveal a great deal about the underlying intramolecular interactions that stabilize the various molecular shapes. Phenyl-substituted ethers and thioethers (C₆H₅−X−R, where R=vinyl (PVE and PVS), allyl¹ (PAE and PAS), propargyl (PPE and PPS)), have been characterized via quantum chemical calculations and Fourier transform microwave (FTMW) spectroscopy. Calculations at the B3LYP-GD3BJ/aug-cc-pVTZ level of theory were used to identify stable conformers for all six molecules. Two conformers were identified within 6 kJ/mol for PVE, PVS, and PPE; four conformers were identified in the same range for PPS and PAS, and five conformers were identified for PAE. Further calculations were performed to construct 2D potential energy surfaces to understand the topology of the energy landscape governing conformer interconversion pathways and to identify the unique underlying intramolecular interactions responsible via natural bond orbital (NBO) analysis. FTMW spectra in the region of 8 to 18 GHz were collected for prototypical compounds to verify the computational results. Assignment of the rotational transitions of the most stable conformers and their minor isotopologues is in progress. This work is combined with previous analysis of other chalcogen-bridged ethers and thioethers to work towards a unifying theory for conformational landscapes. 1. FTMW spectrum of phenyl allyl ether previously reported by G.S. Grubbs II et al. J. Mol Spectrosc. 324, 1-5 (2016).

As flexible molecules with long haloalkyl chain and multiple internal rotations, halo octanoic acids serve as excellent candidates for the studies of helicity and resulting dynamics. Additionally, the exact structure of HOA, such as perfluorooctanoic acid (PFOA), is vital for the development of new tools for its detection in environmental samples. We present potential energy surfaces and helical structure of perfluorooctanoic acid (PFOA), perchlorooctanoic acid (PClOA), and perbromooctanoic(PBrOA) acid computed at Density Functional Theory levels with B3LYP and 6-31 G basis set by scanning dihedral angles (τ6 ) at fixed dihedral angles(τ1- τ4). The resulting potential energy surfaces show different global minima at 64^° for PClOA, 114^°for PFOA, and 303^°for PBrOA. These minima correspond to the stable helical conformers as results of maximizing the separation of halogen atoms and rotating the end terminal. The nature of helical coiling has been further explored by computing helical angles in each Halo octanoic Acids. We notice that a helical angle increases with a heavy halogen atom. The work to evaluate the impact of displacing the haloalkyl chain away from trans configurations back to the stable helical coiling structure is underway.

Polyphenols represent an important class of natural products that attract research attention owing to their diverse biological activities, including antioxidant and anti-inflammatory properties, among others. Investigation of the molecular structure and dynamics of these compounds is essential to better understand their biological activities. In this work, we have investigated the conformational preferences of resveratrol using chirped pulsed Fourier transform microwave spectroscopy in combination with quantum chemical calculations. Laser ablation has been used to vaporize the sample instead of using the traditional heating system to overcome the difficulties arising from the high boiling point and low vapor pressure of this compound. The detection of four spectra with similar rotational constants gives rise to different possible interpretations related to the most stable conformers. The planar moment of inertia P_c of the detected species shows the nearly planar skeleton of resveratrol. The gas-phase data obtained in this work has been compared with structural information obtained from X-ray diffraction in crystals and complexed with proteins.

The capability to accurately identify and quantify biomarkers in the presence of a mixture of molecules is crucial for the success of future missions to search for life on ocean/icy worlds in our solar system. Mass spectrometry has been utilized on several missions; however, it alone is insufficient to unambiguously identify biomarkers due to the existence of isomers. Mass spectrometry in tandem with infrared spectroscopy allows for additional information to be extracted. Amino acids are a prime target in the search for biomarkers due to their importance in terrestrial biology. Here, we present cryogenic gas-phase infrared spectra of deprotonated valine and of its chemical analog, deprotonated aminovaleric acid throughout the fingerprint and mid-infrared regions. The structural flexibility of aminovaleric acid necessitates an extensive isomer search. We assign spectral features and determine structures using density functional theory calculations, and we discuss prospects for chemical identification. The authors gratefully acknowledge support from a JPL/NASA Strategic University Research Partnership Grant and from the National Science Foundation (CHE-2154271), as well as helpful discussions with Maddie Klumb (JILA).html:<hr /><h3>Footnotes: The authors gratefully acknowledge support from a JPL/NASA Strategic University Research Partnership Grant and from the National Science Foundation (CHE-2154271), as well as helpful discussions with Maddie Klumb (JILA).

Acriflavine has multifaceted applications in both human and veterinary medicine. Its distinctive fluorescence suggests a wide array of additional potential uses. r0pt Figure Despite having been first synthesized more than 100 years ago, a comprehensive molecular understanding of acriflavine and its solution applications suffers from the fact that commercially available acriflavine samples comprise a mixture of several different substances. These are typically thought to be proflavine, acriflavine, and doubly methylated proflavine.
In our recent study, employing high-resolution ion mobility mass spectrometry, we find that acriflavine itself is also a mixture. Contrary to the prevailing belief that the acriflavine ion comprises solely isomerically pure 3,6-diamino-10-methylacridin-10-ium, we documented a significant abundance of 3-amino-6-(methylamino)-acridin-10-ium.
To further elucidate the presence of these isomers and enhance our comprehension of acriflavine's intrinsic optical characteristics, we employed cryogenic helium tagging photodissociation spectroscopy in the visible region. This technique circumvents environmental influences, such as solvent effects, enabling us to obtain absorption spectra with highly resolved vibrational progressions. These spectra were directly compared to (TD)-DFT/Franck-Condon-Herzberg-Teller calculations, ensuring accurate identification of the isomers while circumventing inaccuracies stemming from solvent modeling. By tailoring our instrumental setup to conduct 2-color (vis/vis) hole-burning experiments, we successfully disentangled the overlapping absorption spectra of the isomers. This involved selectively removing the helium tag from the first isomer through laser irradiation at the wavelength corresponding to its 0-0 transition, while employing a second laser to measure the absorption spectrum of the second isomer.

Cryptands are three-dimensional molecular cages in which ions can be sequestered out of aqueous solution. Prototypical of this class of molecules is [2.2.2]-cryptand, which contains three alkyl ether chains (-CH₂-CH₂-O-CH₂-CH₂-O-CH₂-CH₂) that are linked together by two tertiary amines that form the endcaps for the cage. The six oxygens and two nitrogens provide an electron-rich interior binding pocket for cations. While much work has focused attention on metal ions, it is also possible to fit one or two protons into the cage. The protons are anticipated to form chemical bonds with the tertiary amine nitrogens, with single protonation taking up one half of the cage and the second proton the other half. We report on the infrared and ultraviolet spectroscopy of singly and doubly protonated [2.2.2]-benzocryptand in the gas phase under cryo-cooled conditions. A single phenyl ring is incorporated into the cage in the middle of one of the alkyl ether chains, replacing the central ethyl group. This provides a UV chromophore for ultraviolet photofragment spectroscopy. We use IR-UV double resonance to record infrared spectra of single conformers of H⁺-BzCrypt and (2H⁺)-BzCrypt in the hydride stretch region. We present spectroscopic evidence that single protonation produces several conformers due to the flexibility of the half of the cage that remains empty, while double protonation leads to a single dominant conformer and one minor conformer. We observe strong NH stretch fundamentals of the protonated amines that report directly on the strength and symmetry of the interior binding pocket. The IR spectrum in the alkyl CH stretch region is compared with the results of an anharmonic local mode theory to deduce the structures of the cages. This work was supported by the Gas Phase Chemical Physics program of the Department of Energy, Office of Science, Basic Energy Sciences.

The α-hydrofulvenyl radical is perhaps the most important intermediate in the H-atom-assisted isomerization of fulvene to benzene, a crucial process in aromatic growth in energetic environments. We have examined its Ã−X̃ spectrum using a combination of resonance-enhanced two-colour two-photon ionization (R2C2PI) and laser-induced fluorescence/dispersed fluorescence. The Ã-state lifetime is orders of magnitude shorter than the predicted f-value implies; the same is true of the similar 1,4-pentadienyl and cyclohexadienyl chromophores, suggesting that nonradiative decay is a feature of the vinylallyl π-system. The transition is fully allowed by symmetry, but weakened by transition moment interference. Intensity-borrowing among a’ modes brings about static (i.e., Condon) and vibronic (i.e., Herzberg-Teller) moments of similar size, the result being a spectrum substantially less origin-dominated than ordinarily expected of an extensively delocalized radical. Contrastingly, the diethynylallyl radical, a previously unobserved acyclic C₇H₅ isomer that can be generated efficiently from cycloheptatriene, succumbs to Franck-Condon analysis with relative ease. The radical is diagnosed in the first instance by its ionization energy, and its trans-trans and cis-trans forms further distinguished by dispersed fluorescence spectroscopy and symmetry arguments. From R2C2PI studies of more than 20 resonance-stabilized hydrocarbon radials that have been carried out in the last two decades or so, we find that the observed adiabatic ionization energy can be predicted with remarkable reliability and accuracy (within about 10 meV) using CBS-QB3 and B3LYP calculations.

To investigate the dynamical behavior of the microscopic hydrogen-bonded structures, we have observed the infrared (IR)-induced isomerizations of the hydrated phenol cations, [PhOH(H₂O)₆]⁺, in the cold ion trap. Under our experimental conditions, three isomers are observed in the ultraviolet (UV) photodissociation spectra measured at the trap temperature of 20 K. This is due to a kinetic trapping during the cooling of the ion. The isomers observed have double ring (DR), double ring with tail (DR+T), and ring with tail (RT) hydrogen-bonded structures. The isomerization was observed by IR pump-UV probe spectroscopy. We found that the DR-type isomer is promptly isomerized to a chain(C)-type structure. Basically, the C-type isomers produced are converted back to the DR-type isomer by the collision with the cold buffer gas in the trap, while a small fraction is isomerized to the DR+T and RT-type structures. We have searched theoretically for transition states in the isomerization and estimated the RRKM rates. The RRKM time constants for the DR to C isomerization are estimated to be on the order of ns, which is much faster than those for the DR to DR+T or RT isomerizations. These results are in qualitative agreement with our observations. The RRKM rates are widely distributed from ns to ms orders due to the nature of the transition states. Details of the experiments and calculations are presented in the paper.

By virtue of rotations of their respective fluoromethyl groups, both 2,3,3-trifluoropropene and allyl fluoride are found in two rotomeric forms. Each has an achiral (cis) conformer that contains a plane of symmetry and two equivalent, chiral (gauche) conformers that form an enantiomeric pair. Consequently, there is a rich variety in the possibilities for the formation of homodimers in these molecules. These include both the homochiral and heterochiral gauche-gauche homodimers, the achiral cis-cis homodimer, and the enantiomerically related pair of cis-gauche homodimers. We report on our progress in exploring this rich conformational landscape using quantum chemistry and the identification of these species via microwave spectroscopy.