Spectroscopy and Dynamics of the Predissociated, Quasi-linear S2 State of Chlorocarbene

In this work, we report on the spectroscopy and dynamics of the quasi-linear S2 state of chlorocarbene, CHCl, and its deuterated isotopologue using optical-optical double resonance (OODR) spectroscopy through selected rovibronic levels of the S1 state. This study, which represents the first observation of the S2 state in CHCl, builds upon our recent examination of the corresponding state in CHF, where pronounced mode specificity was observed in the dynamics, with predissociation rates larger for levels containing bending excitation. In the present work, a total of 14 S2 state vibrational levels with angular momentum ℓ = 1 were observed for CHCl, and 34 levels for CDCl. The range of ℓ in this case was restricted by the pronounced Renner-Teller effect in the low-lying S1 levels, which severely reduces the fluorescence lifetime for levels with Ka \u3e 0. Nonetheless, by exploiting different intermediate S1 levels, we observed progressions involving all three fundamental vibrations. For levels with long predissociation lifetimes, rotational constants were determined by measuring spectra through different intermediate J levels of the S1 state. Plots of the predissociation linewidth (lifetime) vs. energy for various S2 levels show an abrupt onset, which lies near the calculated threshold for elimination to form C(3P) + HCl on the triplet surface. Our experimental results are compared with a series of high level ab initio calculations, which included the use of a dynamically weighted full-valence CASSCF procedure, focusing maximum weight on the state of interest (the singlet and triplet states were computed separately). This was used as the reference for subsequent Davidson-corrected MRCI(+Q) calculations. These calculations reveal the presence of multiple conical intersections in the singlet manifold

Vapor-Phase Stabilization of Biomass Pyrolysis Vapors Using Mixed-Metal Oxide Catalysts

© 2019 American Chemical Society. Mixed-metal oxides possess a wide range of tunability and show promise for catalytic stabilization of biomass pyrolysis products. For materials derived from layered double hydroxides, understanding the effect of divalent cation species and divalent/trivalent cation stoichiometric ratio on catalytic behavior is critical to their successful implementation. In this study, four mixed-metal oxide catalysts consisting of Al, Zn, and Mg in different stoichiometric ratios were synthesized and tested for ex-situ catalytic fast pyrolysis (CFP) using pine wood as feedstock. The catalytic activity and deactivation behavior of these catalysts were monitored in real-time using a lab-scale pyrolysis reactor and fixed catalyst bed coupled with a molecular beam mass spectrometer (MBMS), and data were analyzed by multivariate statistical approaches. In the comparison between Mg-Al and Zn-Al catalyst materials, we demonstrated that the Mg-Al materials possessed greater quantities of basic sites, which we attributed to their higher surface areas, and they produced upgraded pyrolysis vapors which contained less acids and more deoxygenated aromatic hydrocarbons such as toluene and xylene. However, detrimental impacts on carbon yields were realized via decarbonylation and decarboxylation reactions and coke formation. Given that the primary goals of catalytic upgrading of bio-oil are deoxygenation, reduction of acidity, and high carbon yield, these results highlight both promising catalytic effects of mixed-metal oxide materials and opportunities for improvement

SINGLE VIBRONIC LEVEL EMISSION SPECTROSCOPY OF CHCl AND CDCl: VIBRATIONAL STRUCTURE OF THE XX1A′^{1}A^{\prime} AND aa3A′′^{3}A^{\prime\prime} STATES

Author Institution: Department of Chemistry, Marquette University, P.O. Box 1881, Milwaukee, WI 53201-1881We report on single vibronic level (SVL) emission spectra from bands in the progressions 2$_0^{n}$, 2$_0^{n}$3$_0^{m}$, 1$_0^{1}$2$_0^{n}$, and 1$_0^{1}$2$_0^{n}$3$_0^{1}$ in the $A$$^{1}A^{\prime\prime}$-$X$$^{1}A^{\prime}$ system of CH$^{35}$Cl, CH$^{37}$Cl, CD$^{35}$Cl and CD$^{37}$Cl. As in previous studies of CHF/CDF and CHBr/CDBr, the carbenes were generated using a pulsed discharge source, and SVL emission spectra obtained using a 0.3 m spectrograph in combination with a gated, intensified CCD detector. These spectra reveal rich new detail regarding the vibrational structure of the $X$$^{1}A^{\prime}$ and $a$$^{3}A^{\prime\prime}$ states, and spin-orbit induced mixing between them, up to 9000 cm$^{-1}$ above the vibrationless level of the $X$$^{1}A^{\prime}$ state. For CHCl, we observe around three times the number of $X$$^{1}A^{\prime}$ levels previously reported,}, and a number of new $a$$^{3}A^{\prime\prime}$ state levels. The results of Dunham expansion fits to the vibrational term energies, and comparisons with previous experimental and recent high quality theoretical studies}$^{,}$} will be reported. Overall, the derived vibrational parameters of the $X$$^{1}A^{\prime}$ and $a$$^{3}A^{\prime\prime}$ states are in excellent agreement with $ab$ $initio$ predictions, including our own DFT calculations

LASER SPECTROSCOPY OF A HALOCARBOCATION: OBSERVATION OF THE ELECTRONIC SPECTRUM OF CH2_{2}I+^{+}

Author Institution: Department of Chemistry, Marquette University, Milwaukee, WI 53233We report the first observation of the gas-phase electronic spectrum of a simple halocarbocation, CH$_{2}$I$^{+}$. Fluorescence excitation and emission spectra were measured under jet-cooled conditions using a pulsed discharge source with CH$_{2}$I$_{2}$ as precursor. The spectral carrier was identified by: (1) comparison of experimentally derived ground state vibrational frequencies for various isotopomers with the predictions of Density Functional Theory (DFT) calculations, and (2) simulation of the observed rotational contour in our excitation spectra, which indicated an a-type transition, consistent with an excited state of A$_{1}$ symmetry. Calculations predict a triplet A$_{1}$ excited state with transition energy in the visible region, and the observed transition is therefore assigned as $^{3}$A$_{1}$-$^{1}$A$_{1}$, with band origin at 15180 cm$^{-1}$

SINGLE VIBRONIC LEVEL EMISSION SPECTROSCOPY OF CHBr AND CDBr: VIBRATIONAL STRUCTURE OF THE XX1A′^{1}A^{\prime} AND aa3A′′^{3}A^{\prime\prime} STATES

Author Institution: Department of Chemistry, Marquette University, P.O. Box 1881, Milwaukee, WI 53201-1881We obtained single vibronic level (SVL) emission spectra following excitation of bands in the progressions 2$_0^{n}$, 2$_0^{n}$3$_0^{m}$, 1$_0^{1}$2$_0^{n}$, and 1$_0^{1}$2$_0^{n}$3$_0^{m}$ in the $A$$^{1}A^{\prime\prime}$-$X$$^{1}A^{\prime}$ system of CHBr and CDBr. The carbenes were generated using a pulsed discharge source, and SVL emission spectra obtained using a 0.3 m spectrograph in combination with a gated, intensified CCD detector. These spectra reveal rich new detail regarding the vibrational structure of the $X$$^{1}A^{\prime}$ and $a$$^{3}A^{\prime\prime}$ states, and spin-orbit induced mixing between them, up to 9000 cm$^{-1}$ above the vibrationless level of the $X$$^{1}A^{\prime}$ state. For both isotopomers we observe more than twice the number of levels previously reported,} and the results of Dunham expansion fits to the vibrational term energies, and comparisons with previous experimental and theoretical studies, will be reported. Our results lead to several revised assignments, including the $X$$^{1}A^{\prime}$ state C-H stretching fundamental. For both isotopomers we observe almost every possible $X$$^{1}A^{\prime}$ level below 4000 cm$^{-1}$. Unlike CHBr, where even the lowest bending level is strongly perturbed, every level save one below 3000 cm$^{-1}$ in CDBr is reproduced by a Dunham expansion fit to within our experimental uncertainty. However, the spin-orbit mixing is extensive above 4000 cm$^{-1}$ in both isotopomers, and many unassigned lines remain. The derived vibrational parameters of the $X$$^{1}A^{\prime}$ and $a$$^{3}A^{\prime\prime}$ states are in excellent agreement with $ab$ $initio$ predictions, including our own DFT calculations