Sub-Doppler Spectroscopy of the <i>trans</i>-HOCO Radical in the OH Stretching Mode

Rovibrational spectroscopy of the fundamental OH stretching mode of the <i>trans</i>-HOCO radical has been studied via sub-Doppler high-resolution infrared laser absorption in a discharge slit-jet expansion. The <i>trans</i>-HOCO radical is formed by discharge dissociation of H₂O to form OH, which then combines with CO and cools in the Ne expansion to a rotational temperature of 13.0(6) K. Rigorous assignment of both a-type and b-type spectral transitions is made possible by two-line combination differences from microwave studies, with full rovibrational analysis of the spectrum based on a Watson asymmetric top Hamiltonian. Additionally, fine structure splittings of each line due to electron spin are completely resolved, thus permitting all three ε_<i>aa</i>, ε_<i>bb</i>, ε_<i>cc</i> spin–rotation constants to be experimentally determined in the vibrationally excited state. Furthermore, as both a- and b-type transitions for <i>trans</i>-HOCO are observed for the first time, the ratio of transition dipole moment projections along the <i>a</i> and <i>b</i> principal axes is determined to be μ_<i>a</i>/μ_<i>b</i> = 1.78(5), which is in close agreement with density functional quantum theoretical predictions (B3LYP/6-311++g­(3df,3pd), μ_<i>a</i>/μ_<i>b</i> = 1.85). Finally, we note the energetic possibility in the <i>excited</i> OH stretch state for predissociation dynamics (i.e., <i>trans-</i>HOCO → H + CO₂), with the present sub-Doppler line widths providing a rigorous upper limit of >2.7 ns for the predissociation lifetime

Isomerization and Fragmentation of Cyclohexanone in a Heated Micro-Reactor

The thermal decomposition of cyclohexanone (C₆H₁₀O) has been studied in a set of flash-pyrolysis microreactors. Decomposition of the ketone was observed when dilute samples of C₆H₁₀O were heated to 1200 K in a continuous flow microreactor. Pyrolysis products were detected and identified by tunable VUV photoionization mass spectroscopy and by photoionization appearance thresholds. Complementary product identification was provided by matrix infrared absorption spectroscopy. Pyrolysis pressures were roughly 100 Torr, and contact times with the microreactors were roughly 100 μs. Thermal cracking of cyclohexanone appeared to result from a variety of competing pathways, all of which open roughly simultaneously. Isomerization of cyclohexanone to the enol, cyclohexen-1-ol (C₆H₉OH), is followed by retro-Diels–Alder cleavage to CH₂CH₂ and CH₂C­(OH)–CHCH₂. Further isomerization of CH₂C­(OH)–CHCH₂ to methyl vinyl ketone (CH₃CO–CHCH₂, MVK) was also observed. Photoionization spectra identified both enols, C₆H₉OH and CH₂C­(OH)–CHCH₂, and the ionization threshold of C₆H₉OH was measured to be 8.2 <i> ± </i> 0.1 eV. Coupled cluster electronic structure calculations were used to establish the energetics of MVK. The heats of formation of MVK and its enol were calculated to be Δ_f<i>H</i>₂₉₈(<i>cis</i>-CH₃CO–CHCH₂) = −26.1 ± 0.5 kcal mol^–1 and Δ_f<i>H</i>₂₉₈(<i>s-cis</i>-1-CH₂C­(OH)–CHCH₂) = −13.7 ± 0.5 kcal mol^–1. The reaction enthalpy Δ_rxn<i>H</i>₂₉₈(C₆H₁₀O → CH₂CH₂ + <i>s-cis</i>-1-CH₂C­(OH)–CHCH₂) is 53 ± 1 kcal mol^–1 and Δ_rxn<i>H</i>₂₉₈(C₆H₁₀O → CH₂CH₂ + <i>cis</i>-CH₃CO–CHCH₂) is 41 ± 1 kcal mol^–1. At 1200 K, the products of cyclohexanone pyrolysis were found to be C₆H₉OH, CH₂C­(OH)–CHCH₂, MVK, CH₂CHCH₂, CO, CH₂CO, CH₃, CH₂CCH₂, CH₂CH–CHCH₂, CH₂CHCH₂CH₃, CH₂CH₂, and HCCH

Tabletop Femtosecond VUV Photoionization and PEPICO Detection of Microreactor Pyrolysis Products

We report the combination of tabletop vacuum ultraviolet photoionization with photoion–photoelectron coincidence spectroscopy for sensitive, isomer-specific detection of nascent products from a pyrolysis microreactor. Results on several molecules demonstrate two essential capabilities that are very straightforward to implement: the ability to differentiate isomers and the ability to distinguish thermal products from dissociative ionization. Here, vacuum ultraviolet light is derived from a commercial tabletop femtosecond laser system, allowing data to be collected at 10 kHz; this high repetition rate is critical for coincidence techniques. The photoion–photoelectron coincidence spectrometer uses the momentum of the ion to identify dissociative ionization events and coincidence techniques to provide a photoelectron spectrum specific to each mass, which is used to distinguish different isomers. We have used this spectrometer to detect the pyrolysis products that result from the thermal cracking of acetaldehyde, cyclohexene, and 2-butanol. The photoion–photoelectron spectrometer can detect and identify organic radicals and reactive intermediates that result from pyrolysis. Direct comparison of laboratory and synchrotron data illustrates the advantages and potential of this approach

Chirped-Pulse Fourier Transform Microwave Spectroscopy Coupled with a Flash Pyrolysis Microreactor: Structural Determination of the Reactive Intermediate Cyclopentadienone

Chirped-pulse Fourier transform microwave spectroscopy (CP-FTMW) is combined with a flash pyrolysis (hyperthermal) microreactor as a novel method to investigate the molecular structure of cyclopentadienone (C₅H₄O), a key reactive intermediate in biomass decomposition and aromatic oxidation. Samples of C₅H₄O were generated cleanly from the pyrolysis of <i>o</i>-phenylene sulfite and cooled in a supersonic expansion. The ¹³C isotopic species were observed in natural abundance in both C₅H₄O and in C₅D₄O samples, allowing precise measurement of the heavy atom positions in C₅H₄O. The eight isotopomers include: C₅H₄O, C₅D₄O, and the singly ¹³C isotopomers with ¹³C substitution at the C1, C2, and C3 positions. Microwave spectra were interpreted by CCSD­(T) ab initio electronic structure calculations and an <i>r</i>_e molecular structure for C₅H₄O was found. Comparisons of the structure of this “anti-aromatic” molecule are made with those of comparable organic molecules, and it is concluded that the disfavoring of the “anti-aromatic” zwitterionic resonance structure is consistent with a more pronounced CC/CC bond alternation