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 (σfit < 0.031 MHz) for the ground vibrational state. Similar to its isomer 2-furonitrile, the two lowest-energy vibrationally excited states of 3-furonitrile (ν17, ν24), which correspond to the in-plane and out-of-plane nitrile bending vibrations, form an a- and b-axis Coriolis-coupled dyad. Rotationally resolved infrared transitions (30-600 cm-1) and over 4200 pure rotational transitions for both ν17 and ν24 were fit to a partial-octic, Coriolis-coupled, two-state Hamiltonian with low statistical uncertainty (σfit rot < 0.045 MHz, σfit IR < 6.1 MHz). The least-squares fitting of these vibrationally excited states provides their accurate and precise vibrational frequencies (ν17 = 168.193 164 8 (67) cm-1 and ν24 = 169.635 831 5 (77) cm-1) and seven Coriolis-coupling terms (Ga, GaJ, GaK, Fbc, FbcK, Gb, and Fac). The two fundamental states exhibit a notably small energy gap (1.442 667 (10) cm-1) and an inversion of the relative energies of ν17 and ν24 compared to those of the isomer 2-furonitrile. The rotational frequencies and spectroscopic constants of 3-furonitrile that we present herein provide a sufficient basis for conducting radioastronomical searches for this molecule across the majority of the frequency range available to current radiotelescopes.

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). Utilizing the Coriolis coupling between the vibrational states of each dyad and the analysis of the IR spectrum for ν16 and ν9, we have determined precise band origins for each of these fundamental states: ν16 (B1) = 361.213 292 7 (17) cm-1, ν13 (A2) = 361.284 082 4 (17) cm-1, ν24 (B2) = 618.969 096 (26) cm-1, and ν9 (A1) = 664.723 378 4 (27) cm-1. Notably, the energy separation in the ν16-ν13 Coriolis-coupled dyad is one of the smallest spectroscopically measured energy separations between vibrational states: 2122.222 (72) MHz or 0.070 789 7 (24) cm-1. Despite ν13 being IR inactive and ν24 having an impractically low-intensity IR intensity, the band origins of all four vibrational states were measured, showcasing the power of combining the data provided by millimeter-wave and high-resolution IR spectra. Additionally, the spectra of pyridazine-dx isotopologues generated for a previous semi-experimental equilibrium structure (reSE) determination allowed us to analyze the two lowest-energy vibrational states of pyridazine for all nine pyridazine-dx isotopologues. Coriolis-coupling terms have been measured for analogous vibrational states across seven isotopologues, both enabling their comparison and providing a new benchmark for computational chemistry.

Probing membrane hydration in microfluidic polymer electrolyte membrane electrolyzers via operando synchrotron Fourier-transform infrared spectroscopy.

Polymer electrolyte membrane (PEM) electrolyzers are renewable energy storage systems that produce high purity hydrogen fuel from electrochemical water splitting. The PEM in particular is a key component that acts as a solid electrolyte between electrodes and separates the reactants, but despite these benefits, its internal ion transport mechanisms are not fully understood. Here, the first microfluidic PEM electrolyzer that is semi-transparent in the infrared (IR) spectrum is developed as a platform for characterizing the PEM hydration during operation. The electrochemical performance of the chip is compared to its PEM hydration, which is measured via synchrotron Fourier-transform infrared (FTIR) spectroscopy. The PEM water content is directly probed in the operating electrolyzer by measuring the transmitted light intensity at wavelengths around 10 µm. By supplying the electrolyzer with reactant starving flow rates, mass transport driven cell failure is provoked, which coincides with membrane dehydration. Furthermore, higher operating temperatures are observed to improve the stability in membrane hydration through increasing the membrane water uptake. The methods presented here prove the viability of IR techniques for characterizing membrane hydration, and future extension towards imaging and thermography would enable further quantitative studies of internal membrane transport behaviors.

Quantum monodromy in NCNCS - direct experimental confirmation.

Since the first confirmation of quantum monodromy in NCNCS (B. P. Winnewisser et al., Report No. TH07 in 60th International Symposium on Molecular Spectroscopy, Columbus, OH, (2005) and B. P. Winnewisser et al., Phys. Rev. Lett., 2005, 95, 243002) we have continued to explore its implications for the quantum structure of molecules. To confirm quantum monodromy bending-vibrational + axial-rotational quantum energy level information is needed. This was not directly available from the pure a-type rotational transitions available in 2005. The confirmation of quantum monodromy therefore had to be based on the fitting of the Generalised SemiRigid Bender (GSRB) model to the experimental rotational data. The GSRB is a physically motivated model and was able to extract the required information based on the changes of the rotational energy level structure upon excitation of the bending vibration and of the axial rotation. These results were, in some sense, predictions. Our goal here was to obtain a fully experimental and unambigous confirmation of quantum monodromy in NCNCS. This involved a series of experimental campaigns at the Canadian Light Source (CLS) synchrotron. To coax the required information out of the masses of spectral data that had been obtained a variety of techniques had to be used. The result is that we can now confirm, without recourse to a theoretical model, the existence of quantum monodromy in the ν7 bending mode of NCNCS. As a side benefit we also confirm the power of the GSRB model to extract the required information from the previously available data. The predictions previously provided by the GSRB were surprisingly accurate. Only a slight augmentation of the model was required to allow us to refit it including the new data, while maintaining the quality of the fitting for that data previously available. We also present a very basic introduction to the idea of monodromy and to how the GSRB was used.

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 ([1-2H]-, [4-2H]-, [5-2H]-, [4,5-2H]-, and [1,4,5-2H]-1H-1,2,3-triazole), as well as those of 2H-1,2,3-triazole and five of its deuteriated isotopologues ([2-2H]-, [4-2H]-, [2,4-2H]-, [4,5-2H]-, and [2,4,5-2H]-2H-1,2,3-triazole), are studied using millimeter-wave spectroscopy in the 130-375 GHz frequency region. The normal and [2-2H]-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. The resulting spectroscopic constants for each of the vibrationally excited states are reported for the first time. Coupled-cluster vibration-rotation interaction constants are compared with each of their experimentally determined values, often showing agreement within 500 kHz. Newly available coupled-cluster predictions of the excited-state quartic centrifugal distortion constants based on fourth-order vibrational perturbation theory are benchmarked using a large number of the 1,2,3-triazole tautomer isotopologues and vibrationally excited states studied.

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 (σfit = 40 kHz). The high-resolution infrared spectrum, obtained at the Canadian Light Source, allowed for accurate and precise determination of the band origins of its three lowest-energy fundamental modes (ν24, ν17, and ν23). Similar to other cyanoarenes, the first two fundamental modes (ν24, A″, and ν17, A', for 2-furonitrile) form an a- and b-axis Coriolis-coupled dyad. More than 7000 transitions from each of these fundamental states were fit to an octic A-reduced Hamiltonian (σfit = 48 kHz), and the combined spectroscopic analysis determines fundamental energies of 160.1645522 (26) cm-1 and 171.9436561 (25) cm-1 for ν24 and ν17, respectively. The least-squares fitting of this Coriolis-coupled dyad required 11 coupling terms, Ga, GaJ, GaK, GaJJ, GaKK, Fbc, FbcJ, FbcK, Gb, GbJ, and FacK. Using both the rotational and high-resolution infrared spectra, a preliminary least-squares fit was obtained for ν23, providing its band origin of 456.7912716 (57) cm-1. The transition frequencies and spectroscopic constants provided in this work, when combined with theoretical or experimental nuclear quadrupole coupling constants, will provide the foundation for future radioastronomical searches for 2-furonitrile across the frequency range of currently available radiotelescopes.

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. Experimental far- and mid-infrared spectra of these five carbazoles are measured using transmission and photoacoustic techniques. The molecular structures of the monomers and the respective dimers, optimized at the ωB97X-D/6-311++G(d,p) level of the density functional theory (DFT), are subjected to harmonic vibrational frequency calculations. The effect of changing substituents on the N-H bond, π-π stacking distances, and angles between monomers within the dimers, in addition to intermolecular interactions, is investigated. Noncovalent interaction analysis is employed to highlight the areas of attractive and repulsive interactions in the dimers. Thermochemistry calculations show that the formation of dimers of all carbazoles is spontaneous at 298 K. Comparison of the calculated vibrational spectra of these compounds with experimental spectra indicates that the existence of both monomers and dimers must be invoked to account for the observed bands in the infrared spectra. Excellent correlations between the experimentally-determined and calculated harmonic vibrational energies are obtained, with an experimental-to-calculated scaling factor of 0.95-0.96. These findings highlight the coupled computational-experimental approach for the interpretation of vibrational spectra and are essential for improving the spectroscopic characterization of N-substituted carbazoles.

High-resolution FTIR spectroscopy of benzaldehyde in the far-infrared region: probing the rotational barrier.

A discrepancy between theoretical and experimental values of the rotational barrier in benzaldehyde has been observed, which was attributed to inaccurate experimental results in part. Here, we report results on the -CHO torsion of benzaldehyde (C6H5CHO) based on a high resolution spectroscopic investigation in the far-infrared range in an effort to remove the experimental ambiguity. The rotationally-resolved vibrational spectra were measured with an unapodized resolution of 0.00064 cm-1 using synchrotron-based Fourier transform infrared (FTIR) spectroscopy at the Canadian Light Source. The torsional fundamental νt = 109.415429(20) cm-1 was unambiguously assigned via rovibrational analysis, followed by the tentative assignment of the first (2νt-νt) and second (3νt- 2νt) hot bands at 107.58 cm-1 and 105.61 cm-1, respectively, by comparison of the observed Q branch structures at high resolution with simulation based on a previous microwave study. This assignment is different from any previous low resolution infrared studies in which the intensity patterns were misleading. The key result of the assignment of the first three transitions allowed the determination of the barrier to internal rotation of (hc)1533.6 cm-1 (4.38 kcal mol-1). When compared with calculated results from vibrational second-order perturbation theory (VPT2) and the quasiadiabatic channel reaction path Hamiltonian (RPH) approach, the experimental value is still too low and this suggests that the discrepancy between theory and experiment remains despite the best experimental efforts.

Synchrotron-Based High Resolution Far-Infrared Spectroscopy of trans-Butadiene.

The high resolution far-infrared spectrum of trans-butadiene has been reinvestigated by Fourier-transform spectroscopy at two synchrotron radiation facilities, SOLEIL and the Canadian Light Source, at temperatures ranging from 50 to 340 K. Beyond the well-studied bands, two new fundamental bands lying below 1100 cm-1, ν10 and ν24, have been assigned using a combination of cross-correlation (ASAP software) and Loomis-Wood type (LWWa software) diagrams. While the ν24 analysis was rather straightforward, ν10 exhibits obvious signs of a strong perturbation, presumably owing to interaction with the dark ν9 + ν12 state. Effective rotational constants have been derived for both the v10 = 1 and v24 = 1 states. Since only one weak, infrared active fundamental band (ν23) of trans-butadiene remains to be observed at high resolution in the far-infrared, searches for the elusive gauche conformer can now be undertaken with considerably greater confidence in the dense ro-vibrational spectrum of the trans form.

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. It gives good results even for dense spectra with no obvious structure and in the presence of multiple interfering absorptions. The effectiveness of these tools is demonstrated by the analysis of high resolution IR spectra of 8 bands of cis- and trans-1,2-dichloroethene where, including hot bands and isotopologues, 31 vibrational transitions and 158 316 individual lines have been analysed, including perturbations for the higher energy states. Walkthroughs are presented to show this process is rapid.

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 ν11 band of cis-1,2-dichloroethene. It is based on the autofit algorithm presented by N. A. Seifert et al. (J. Mol. Spectrosc., 2015, 312, 13) but implemented in a more efficient and general way, allowing it to be applied to a much wider variety of spectra.

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. Winnewisser et al., Phys. Chem. Chem. Phys., 2010, 12, 8158-8189] cyanogen iso-thiocyanate, NCNCS, is a particularly illuminating exemplar of quantum monodromy: it clearly shows the distinctive monodromy-induced dislocation of the ro-vibrational energy level pattern for its low-lying bending mode. This dislocation centers on a lattice defect in the energy vs. momentum map of the ro-vibrational levels at the top of the barrier to linearity, and represents an example of an excited state quantum phase transition [D. Larese and F. Iachello, J. Mol. Struct., 2011, 1006, 611-628]. To complete the data, so far limited to ΔJ = +1 transitions, we decided to measure the high-resolution far-infrared band of the large-amplitude bending vibration ν7, and, if possible, mid-infrared bands. This Perspectives article presents our ongoing progress towards this goal, beginning with the description of how to predict line positions and intensities of the a- and b-type bands of the large amplitude bending mode using the Generalized-SemiRigid-Bender (GSRB) Hamiltonian for NCNCS and ab initio dipole moment functions [B. P. Winnewisser et al., Phys. Chem. Chem. Phys., 2010, 12, 8158-8189]. We include background information about synchrotron physics to clarify the advantages and limitations of that radiation source for our experiments. Details of the chemical preparation and sample handling, leading to the realization that NCNCS is 50 kJ mol(-1) lower in energy than its isomer S(CN)2 [Z. Kisiel et al., J. Phys. Chem. A, 2013, 117, 13815-13824] are included. We present the far-infrared and mid-infrared spectrum of NCNCS obtained at the Canadian Light Source synchrotron, using the IFS 125HR Bruker Fourier transform spectrometer. Eight of the fundamental vibrational modes of NCNCS have now been observed at high resolution. Initial analyses of the data confirm band assignments and demonstrate the accuracy of the predictions.

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. These results constitute an extensive set of benchmarks for ab initio anharmonic force field calculations and the observed and calculated vibration-rotation constants and anharmonic frequencies are compared. The semiexperimental equilibrium, r(e)(SE), geometry of S(CN)2 has also been evaluated. In the course of the measurements, new information concerning the physical chemistry of S(CN)2 has been obtained.

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. The UV resonance Raman spectrum of thymidine and thymidine 5'-monophosphate are similar to that of thymine, suggesting that large masses at N(1) effectively isolate the vibrations of the nucleobase. However, the UV resonance Raman spectrum of N(1)-methylthymine is significantly different, suggesting that the methyl group couples into the thymine ring vibrations. The resulting resonance Raman intensities and absorption spectra are self-consistently simulated with a time-dependent expression to quantitatively extract the initial excited-state slopes, homogeneous and inhomogeneous linewidths, and electronic parameters. These results are discussed in the context of the known photochemistry of thymine and its derivatives.

Photoacoustic spectroscopy using coherent synchrotron radiation: application to α-lactose monohydrate.

We produced coherent synchrotron radiation at the Canadian Light Source between about 5 and 30 cm(-1) in bursting and continuous emission modes and used it to acquire photoacoustic spectra of solids. A band was observed in the spectrum of α-lactose monohydrate at 18 cm(-1) and attributed to a rotational mode, in agreement with published data obtained using other numerical and experimental techniques.

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. The pKa values of these protons are well-known, but the effect of the change in protonation on the rest of the molecule is not well understood and is very important in base stacking, base pairing, and protein-nucleic acid interactions. In this paper, UV resonance Raman (UVRR) spectroscopy is used to probe the structures of both cytosine and uracil at varying pH to determine the structural changes that take place. The results show that cytosine has increased electronic delocalization when moving to either basic or acidic environments, whereas uracil shows no significant change in acidic environment but increases its electronic delocalization in basic environment.

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. To further test this hypothesis, we have measured the resonance Raman spectra of 5-fluorouracil at wavelengths throughout its 267 nm absorption band. The spectra of 5-fluorouracil and thymine are very similar. Self-consistent analysis of the resulting resonance Raman excitation profiles and absorption spectrum using a time-dependent wave packet formalism suggests that, at most, 81% of the reorganization energy upon excitation is directed along photochemically relevant modes. This compares well with what was found for thymine, supporting the mass barrier hypothesis.

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. From this analysis, we have been able to determine that, at most, 31% of the reorganization energy upon excitation is directed along photochemically relevant modes.