The acetylenic CH stretch (ν₁) fundamental band of propyne is known to have a "double-crossing" type perturbation that shifts the K’-subbands out of normal order. To further investigate this perturbation, we present a combined experimental and ab initio study on the jet-cooled high-resolution infrared spectra of the ν₁ fundamental band of the three mono-substituted ¹³C isotopologues of propyne: ¹³CH₃¹²C ≡ ¹²CH, ¹²CH₃¹³C ≡ ¹²CH, and ¹²CH₃¹²C ≡ ¹³CH. The experimental spectra are recorded using continuous wave cavity ringdown spectroscopy (cw-CRDS), with the isotopologues produced at natural abundance in a continuous supersonic expansion of regular propyne diluted in argon and helium. The K’ = 0 and 1 subbands of all three isotopologues are measured near 3330 cm⁻¹.K.D. Doney et al., J. Phys. Chem. A 122 (2), 582 (2018)html:<hr /><h3>Footnotes: K.D. Doney et al., J. Phys. Chem. A 122 (2), 582 (2018)

The OH stretch (ν₁) fundamental band of a non-native (cis vs trans) conformer of formic acid (HCOOH) is observed for the first time using high-resolution direct absorption infrared spectroscopy. The cis-formic acid is produced in a supersonic slit discharge plasma expansion of a trans-formic acid/H₂ mixture in 70:30 Ne:He. In this talk, we present a detailed ro-vibrational analysis of the experimentally observed OH stretch mode, including determination of accurate rotational parameters and vibrational band origin. Ab initio calculations (both at CCSD(T) and DFT levels of theory) are carried out to support the experimental assignment and to futher explore the isomerization dynamics between the trans and cis conformers. Furthermore, observation of both a- and b-type transitions in the experimental spectrum facilitate accurate determination of the direction of the transition dipole moment in the body fixed frame.

Samll conjugated dienes serve as one of the prototypical reactants in the Diels-Alder cyclization reaction commonly used in organic chemistry. These dienes exhibit an interesting conformational landscape where the lowest energy conformer is a trans conformation with a gauche conformer lying 10-15 kJ/mol higher in energy. The gauche conformer has only recently been unambiguously indentified as a non-planar structure in 1,3-butadiene (the simplest diene) by microwave spectroscopy.[1] We are interested in understanding the effect of the addition of functional groups on the conformational energy landscape of dienes. 1,3-pentadiene presents one of the simplest substitutions with the addition of a methyl group at the end of the 1,3-butadiene unit. We have measured room temperature high-resolution infrared spectra of both (E)- and (Z)-1,3-pentadiene using an external cavity quantum cascade laser (EC-QCL) in the range of 970-1010 cm⁻¹. These measurements represent the first high-resolution infrared spectra of these molecules reported to date, with individual rovibrational features exhibiting the Doppler-broadened linewidth of about 0.0016 cm⁻¹. Both (E)- and (Z)-pentadiene exhibit a strong C-type rovibrational band (due to a C-H wagging motion) in this region with a strong Q-branch. We will present our analysis of the rotational structure we have observed, which will provide experimental data to verify computational chemistry of these molecules.
[1] J.H. Baraban, M.-A. Martin-Drumel, et al., Angewandte Chemie, 57, 1821 (2018)

l0pt Figure At the last two ISMS meetings we have reported on our IR studies of various D and ¹³C substituted isotopologues of propane conducted at the Canadian Light Source synchrotron (2-¹³C, 1-¹³C, 2,2-D₂, 2-D₁). This year we are reporting on the study of the ν₉ B-type band of 1-D₁-Propane. There are only two reports of the IR spectra of this molecule both published in 1949. Neither of these studies could reach the far-IR where this band occurs. We will show the complete high resolution survey of all regions from 4000 to below 300 cm⁻¹. This will help to confirm some of the earlier assignments for this molecule. We hope to report the first modern experimental determination of the ground state constants since the 1960 MW study of Lide. This new work now includes the centrifugal distortion terms for the ground state as well as constants for the ν₉ state.

Thiophenes are one of the most promising polymeric solar cells, as in the last decade, the efficiency of thiophene-based polymer cells has increased from 1% to 11.5%.Mehmood, U., et al. (2016). "Review on recent advances in polythiophene based photovoltaic devices." Renewable and Sustainable Energy Reviews 57: 550-561. heir use is limited by several factors, including a lack of understanding about their charge-transport properties in solar cells. To this end, we used cryogenic slow electron velocity map-imaging (SEVI) to acquire vibrationally-resolved anion photoelectron spectra of a family of thiophenes, increasing in chain length, culminating with a 5-mer thiophene chain (quinquethiophene). These spectra yield electron-affinities, T₁ energies, and active vibrations in transitions from the anion to the neutral S₀ and T₁ states. Comparing experimental spectra with electronic structure calculations gives information about the delocalization of electrons on increasing length thiophene chains. This will build a model for the accommodation of electrons in polymers of thiophenes.

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Footnotes:

Mehmood, U., et al. (2016). "Review on recent advances in polythiophene based photovoltaic devices." Renewable and Sustainable Energy Reviews 57: 550-561. T

The conformers of α-proline were studied by matrix-isolation IR spectroscopy. In order to group the vibrational bands to different conformers, conformational changes were induced by NIR laser irradiations, by exciting the OH stretching overtone of a selected conformer. For identification, the single conformer spectra were compared to state-of-the-art ab initio computations. Infrared spectra and anharmonic thermodynamic corrections have been computed by means of second order vibrational perturbation theory (VPT2), with GVPT2 and HDCPT2 models, respectively. A hybrid force-field was developed by addition of the cubic and semi-diagonal quartic B3LYP-D3/SNSD force constants to the (harmonic quadratic) B2PLYP-D3/maug-cc-pVTZ results. As the result of the spectrum analysis, seven conformers could be identified. Four of these conformers were present in the as-deposited matrix, two of them were generated by the NIR laser irradiation, and found to be stable in Ar matrix. The seventh conformer could also be produced upon the NIR laser irradiation, and it decays by H-atom tunneling to a lower energy form on the sub-second and minute timescales in Ar and N₂ matrices, respectively.

Methyl anthranilate (MA) is the methyl ester of anthranilic acid. We present the jet-cooled Laser Induced Fluorescence (LIF) UV excitation spectrum, ground (S₀) and excited state (S₁) Fluorescence Dip Infrared (FDIR) spectra, IR-UV Holeburning (IR-UV HB) spectra, and Dispersed Fluorescence (DFL) spectra of the MA monomer and MA-H₂O complex. MA is a close analog of salicylic acid, a well-studied molecule thought to undergo keto-enol tautomerism involving excited state H-atom transfer. In the MA monomer, the C6 ring formed by the H_aNH_b ^... O=C hydrogen bond in the ground state undergoes an unusual increase in hydrogen bond strength following electronic excitation to the S₁ state. This is evident in the spectroscopy in several ways. The LIF excitation and DFL spectra both show long progressions in several modes involving motion of the NH₂ and COOMe groups relative to one another. Furthermore, the coupled NH₂ symmetric and asymmetric stretch fundamentals appear to uncouple upon UV excitation, giving rise to a broadened (8 cm⁻¹ FWHM), nominally “free” N-H_a stretch at 3458 cm⁻¹ and an N-H_b stretch shifted all the way down to ~2900 cm⁻¹. FDIR spectra collected in the mid-IR frequency range show the change in hydrogen bond strength from the perspective of the C=O acceptor group, where the stretch fundamental is shifted down from its S₀ position at 1721 cm⁻¹ to 1637 cm⁻¹ in S₁. Calculations at the B3LYP-D3/def2TZVP level of theory in large measure capture these frequency shifts and give insight into likely explanations for such drastic changes. Similar behavior is observed in the MA-H₂O complex, although the extent of the hydrogen bond strengthening in the S₁ state is modulated by the HOH interactions with both the NH₂ and C=O groups.

Double resonance spectroscopy has been frequently applied in state-selective excitation and to reach energy levels forbidden to single-photon transition. Due to the low cross section of two-photon transitions, usually high-power pulsed lasers are needed, which prevent high-precision measurements. Optical resonant cavity can be used to enhance the effective path length and also the laser power inside the cavity. By simultaneously locking two continuous-wave lasers with one high-finesse cavity, we established comb-locked cavity-assisted double resonance (CLCA-DR) spectroscopy of molecules. Doppler-free two-photon CLCA-DR spectroscopy of the monoxide carbon molecule were recorded by using two diode lasers with milli-watts power in the 1.5μm region. Three different types of double resonance: Λ-, V- and ladder-type, were demonstrated. By comparison to the energy difference obtained in previous high-precision single-photon spectroscopy using comb-locked cavity ring-down spectroscopy [1], we confirmed that the CLCA-DR measurement can also achieve an accuracy of kHz level. The energy of the highly excited state of ¹²C¹⁶O, V=6, J=9 was determined to be kHz level (δE/E is about 10⁻¹¹) by a “ladder-type” double resonance measurement using the V=3, J=10 level as the intermediate state. Keywords: cavity assisted; double resonance; Doppler free; Reference: [1] Wang, J. et al., J. Chem. Phys. 2017, 147: 091103.

We present an accurate simulation of the Raman spectrum of the carbon dioxide molecule in the 1150-1500 cm⁻¹ spectral range, comparing the results obtained using different polyad schemes. We first determine an optimal set of Hamiltonian interactions for the three different polyad schemes to after fit the 178 experimental term energies found in the literature. Thereafter, using a Taylor series expansion of the mean polarizability in terms of normal mode coordinates, we perform an additional canonical transformation followed by an anharmonization procedure and a fit to a subset of experimental transition moments. Once these steps are accomplished, the Raman spectrum of CO₂ is simulated and compared with the experiment.