Battle of the CH motions: aliphatic versus aromatic contributions to astronomical PAH emission and exploration of the aliphatic, aromatic, and ethynyl CH stretches.

Strong anharmonic coupling between vibrational states in polycyclic aromatic hydrocarbons (PAH) produces highly mixed vibrational transitions that challenge the current understanding of the nature of the astronomical mid-infrared PAH emission bands. Traditionally, PAH emission bands have been characterized as either aromatic or aliphatic, and this assignment is used to determine the fraction of aliphatic carbon in astronomical sources. In reality, each of the transitions previously utilized for such an attribution is highly mixed with contributions from both aliphatic and aromatic CH motions as well as non-CH motions such as CC stretches.

Millimeter-Wave and High-Resolution Infrared Spectroscopy of the Ground and Seven Lowest Fundamental States of 1-1,2,4-Triazole.

A combined analysis of millimeter-wave (70-700 GHz) and rotationally resolved infrared (400-1200 cm) spectra of the ground state and seven fundamental vibrational modes of 1-1,2,4-triazole is reported. While the lowest-energy vibrationally excited state (ν) is well-treated using a single-state distorted-rotor Hamiltonian, the second (ν) and third (ν) vibrationally excited states are involved in strong -type Coriolis coupling and require an appropriate two-state Hamiltonian. The oblate nature of 1-1,2,4-triazole is sufficiently close to the oblate symmetric-top limit that the analysis requires the use of A-reduced, sextic centrifugally distorted-rotor Hamiltonian models in the I representation in order to achieve low σ values.

Millimeter-Wave and High-Resolution Infrared Spectroscopy of 3-Furonitrile.

The rotational spectrum of 3-furonitrile has been collected from 85 to 500 GHz, spanning the most intense rotational transitions observable at room temperature. The large dipole moment imparted by the nitrile substituent confers substantial intensity to the rotational spectrum, enabling the observation of over 5600 new rotational transitions. Combined with previously published transitions, the available data set was least-squares fit to partial-octic, distorted-rotor A- and S-reduced Hamiltonian models with low statistical uncertainty (σ < 0.

Millimeter-wave and high-resolution infrared spectroscopy of the low-lying vibrational states of pyridazine isotopologues.

The gas-phase rotational spectrum from 8 to 750 GHz and the high-resolution infrared (IR) spectrum of pyridazine (o-C4H4N2) have been analyzed for the ground and four lowest-energy vibrationally excited states. A combined global fit of the rotational and IR data has been obtained using a sextic, centrifugally distorted-rotor Hamiltonian with Coriolis coupling between appropriate states. Coriolis coupling has been addressed in the two lowest-energy coupled dyads (ν16, ν13 and ν24, ν9).

Quantum monodromy in NCNCS - direct experimental confirmation.

Since the first confirmation of quantum monodromy in NCNCS (B. P. Winnewisser , Report No.

Millimeter-Wave and High-Resolution Infrared Spectroscopy of 2-Furonitrile─A Highly Polar Substituted Furan.

The rotational spectrum of 2-furonitrile (2-cyanofuran) has been obtained from 140 to 750 GHz, capturing its most intense rotational transitions at ambient temperature. 2-Furonitrile is one of two isomeric cyano-substituted furan derivatives, both of which possess a substantial dipole moment due to the cyano group. The large dipole of 2-furonitrile allowed over 10 000 rotational transitions of its ground vibrational state to be observed and least-squares fit to partial octic, A- and S-reduced Hamiltonians with low statistical uncertainty (σ = 40 kHz).

Vibrationally excited states of 1H- and 2H-1,2,3-triazole isotopologues analyzed by millimeter-wave and high-resolution infrared spectroscopy with approximate state-specific quartic distortion constants.

In this work, we present the spectral analysis of 1H- and 2H-1,2,3-triazole vibrationally excited states alongside provisional and practical computational predictions of the excited-state quartic centrifugal distortion constants. The low-energy fundamental vibrational states of 1H-1,2,3-triazole and five of its deuteriated isotopologues ([1-H]-, [4-H]-, [5-H]-, [4,5-H]-, and [1,4,5-H]-1H-1,2,3-triazole), as well as those of 2H-1,2,3-triazole and five of its deuteriated isotopologues ([2-H]-, [4-H]-, [2,4-H]-, [4,5-H]-, and [2,4,5-H]-2H-1,2,3-triazole), are studied using millimeter-wave spectroscopy in the 130-375 GHz frequency region. The normal and [2-H]-isotopologues of 2H-1,2,3-triazole are also analyzed using high-resolution infrared spectroscopy, determining the precise energies of three of their low-energy fundamental states.

Computational and infrared spectroscopic investigations of N-substituted carbazoles.

The carbazole moiety is a commonly identified structural motif in the high-molecular-weight components of petroleum, known as asphaltenes. Detailed characterization of carbazoles is important for understanding the structure of asphaltenes and addressing challenges in the areas of heavy oil recovery, transportation, upgrading, and oil spills, arising from asphaltene properties and composition. In this work we study carbazole and the four N-substituted carbazoles 9-methylcarbazole, 9-ethylcarbazole, 9-vinylcarbazole and 9-phenylcarbazole.

Automatic and semi-automatic assignment and fitting of spectra with PGOPHER.

Two new tools for computer assisted assignment of rotational spectra with the PGOPHER program are presented, aimed particularly at spectra where many individual lines are resolved. The first tool tries many different assignments, presenting a small number for possible refinement and a preliminary version of this has already been presented. The second tool, the nearest lines plot (a new style of residual plot) provides a clear indication as to whether a trial calculation is plausible, and allows rapid assignment of sets of related lines.

Automatic assignment and fitting of spectra with pgopher.

An initial implementation of a tool for automatic assignment of spectra within the pgopher program is presented, together with its application to rotational analysis of the ν band of cis-1,2-dichloroethene. It is based on the autofit algorithm presented by N. A.

Pursuit of quantum monodromy in the far-infrared and mid-infrared spectra of NCNCS using synchrotron radiation.

Quantum monodromy has a dramatic and defining impact on all those physical properties of chain-molecules that depend on a large-amplitude bending coordinate, including in particular the distribution of the ro-vibrational energy levels. As revealed by its pure rotational (a-type) spectrum [B. P.

Far-infrared spectrum of S(CN)2 measured with synchrotron radiation: global analysis of the available high-resolution spectroscopic data.

The high resolution Fourier transform spectrum of the chemically challenging sulfur dicyanide, S(CN)2, molecule was recorded at the far-infrared beamline of the synchrotron at the Canadian Light Source. The spectrum covered 50-350 cm(-1), and transitions in three fundamentals, ν4, ν7, and ν8, as well as in the hot-band sequence (n + 1)ν4 - nν4, n = 1-4, have been assigned and measured. Global analysis of over 21,300 pure rotation and rotation vibration transitions allowed determination of precise energies for 12 of the lowest vibrationally excited states of S(CN)2, including the five lowest fundamentals.

Initial excited-state structural dynamics of thymine derivatives.

Thymine is one of the pyrimidine nucleobases found in DNA. Upon absorption of UV light, thymine forms a number of photoproducts, including the cyclobutyl photodimer, the pyrimidine pyrimidinone [6-4] photoproduct and the photohydrate. Here, we use UV resonance Raman spectroscopy to measure the initial excited-state structural dynamics of the N(1)-substituted thymine derivatives N(1)-methylthymine, thymidine, and thymidine 5'-monophosphate in an effort to understand the role of the N1 substituent in determining the excited-state structural dynamics and the subsequent photochemistry.

pH-dependent UV resonance Raman spectra of cytosine and uracil.

Cytosine is a nucleobase found in both DNA and RNA, while uracil is found only in RNA. Uracil has abstractable protons at N3 and N1. Cytosine has only one abstractable proton at N1 but can also accept a proton at N3.

Excited-state structural dynamics of 5-fluorouracil.

5-Fluorouracil is an analogue of thymine and uracil, nucleobases found in DNA and RNA, respectively. The photochemistry of thymine is significant; UV-induced photoproducts of thymine in DNA lead to skin cancer and other diseases. In previous work, we have suggested that the differences in the excited-state structural dynamics of thymine and uracil arise from the methyl group in thymine acting as a mass barrier, localizing the vibrations at the photochemical active site.

Excited-state structural dynamics of cytosine from resonance Raman spectroscopy.

Cytosine, a nucleobase found in both DNA and RNA, is known to form photoproducts upon UV irradiation, damaging the nucleic acids and leading to cancer and other diseases. To determine the molecular mechanism by which these photoproducts occur, we have measured the resonance Raman spectra of cytosine at wavelengths throughout its 267 nm absorption band. Self-consistent analysis of the resulting resonance Raman excitation profiles and absorption spectrum using a time-dependent wave packet formalism yields both the excited-state structural changes and electronic parameters.