The bending vibrational levels of the acetylene cation: A case study of the Renner-Teller effect in a molecule with two degenerate bending vibrations

Leak-out spectroscopy as alternative method to rare-gas tagging for the Renner-Teller perturbed HCCH+ and DCCD+ ions

Infrared messenger-tagging predissociation action spectroscopy (IRPD) is a well-established technique to record vibrational spectra of reactive molecular ions. One of its major drawbacks is that the spectrum of the messenger-ion complex is taken instead of that of the bare ion. In particular for small open-shell species, such as the Renner-Teller (RT) affected HCCH+ and DCCD+, the attachment of the tag may have a significant impact on the spectral features. Here we present the application of the novel leak-out spectroscopy (LOS) as a tag-free method to record the cis-bending of the HCCH+ (∼700 cm-1) and DCCD+ cations (∼520 cm-1), using a cryogenic ion trap end user station at the FELIX laboratory. We demonstrate that the obtained LOS spectrum is equivalent to a previously recorded laser-induced reactions (LIR) spectrum of HCCH+. The bending modes are the energetically lowest-lying vibrational modes targeted with LOS so far, showing its potential as a universal broadband spectroscopic technique. Furthermore, we have investigated the effect of the rare gas attachment by recording the vibrational spectra of Ne- and Ar-tagged HCCH+. We found that the Ne-attachment led to a shift in band positions and change in relative intensities, while the Ar-attachment even led to a complete quenching of the RT splitting, showing the importance of using a tag-free method for RT affected systems. The results are interpreted with the help of high-level ab initio calculations in combination with an effective Hamiltonian approach.

Thermal Decomposition of Potential Ester Biofuels. Part I: Methyl Acetate and Methyl Butanoate

Two methyl esters were examined as models for the pyrolysis of biofuels. Dilute samples (0.06-0.13%) of methyl acetate (CH₃COOCH₃) and methyl butanoate (CH₃CH₂CH₂COOCH₃) were entrained in (He, Ar) carrier gas and decomposed in a set of flash-pyrolysis microreactors. The pyrolysis products resulting from the methyl esters were detected and identified by vacuum ultraviolet photoionization mass spectrometry. Complementary product identification was provided by matrix infrared absorption spectroscopy. Pyrolysis pressures in the pulsed microreactor were about 20 Torr and residence times through the reactors were roughly 25-150 μs. Reactor temperatures of 300-1600 K were explored. Decomposition of CH₃COOCH₃ commences at 1000 K, and the initial products are (CH₂═C═O and CH₃OH). As the microreactor is heated to 1300 K, a mixture of CH₂═C═O and CH₃OH, CH₃, CH₂═O, H, CO, and CO₂ appears. The thermal cracking of CH₃CH₂CH₂COOCH₃ begins at 800 K with the formation of CH₃CH₂CH═C═O and CH₃OH. By 1300 K, the pyrolysis of methyl butanoate yields a complex mixture of CH₃CH₂CH═C═O, CH₃OH, CH₃, CH₂═O, CO, CO₂, CH₃CH═CH₂, CH₂CHCH₂, CH₂═C═CH₂, HCCCH₂, CH₂═C═C═O, CH₂═CH₂, HC≡CH, and CH₂═C═O. On the basis of the results from the thermal cracking of methyl acetate and methyl butanoate, we predict several important decomposition channels for the pyrolysis of fatty acid methyl esters, R-CH₂-COOCH₃. The lowest-energy fragmentation will be a 4-center elimination of methanol to form the ketene RCH═C═O. At higher temperatures, concerted fragmentation to radicals will ensue to produce a mixture of species: (RCH₂ + CO₂ + CH₃) and (RCH₂ + CO + CH₂═O + H). Thermal cracking of the β C-C bond of the methyl ester will generate the radicals (R and H) as well as CH₂═C═O + CH₂═O. The thermochemistry of methyl acetate and its fragmentation products were obtained via the Active Thermochemical Tables (ATcT) approach, resulting in Δ_fH₂₉₈(CH₃COOCH₃) = -98.7 ± 0.2 kcal mol^-1, Δ_fH₂₉₈(CH₃CO₂) = -45.7 ± 0.3 kcal mol^-1, and Δ_fH₂₉₈(COOCH₃) = -38.3 ± 0.4 kcal mol^-1.

Rotationally Resolved Vacuum Ultraviolet Resonance-Enhanced Multiphoton Ionization (VUV REMPI) of Acetylene via the G̃ Rydberg State

We present a 1 + 1' resonance-enhanced multiphoton ionization (REMPI) scheme for acetylene via the linear G̃ 4sσ (1)Πu Rydberg state, offering partial rotational resolution and the possibility to detect excitation in both the cis- and trans-bending modes. The resonant transition to the G̃ state is driven by a vacuum ultraviolet (VUV) photon, generated by resonant four-wave mixing (FWM) in krypton. Ionization from the short-lived G̃ state then occurs quickly, driven by the high intensity of the residual light from the FWM process. We have observed nine bands in the region between 79 200 cm(-1) and 80 500 cm(-1) in C2H2 and C2D2. We compare our results with published spectra in this region and suggest alternative assignments for some of the Renner-Teller split bands. Similar REMPI schemes should be applicable to other small molecules with picosecond lifetime Rydberg states.

The Renner-Teller effect in HCCCl(+)(X̃(2)Π) studied by zero-kinetic energy photoelectron spectroscopy and ab initio calculations

The spin-vibronic energy levels of the chloroacetylene cation up to 4000 cm(-1) above the ground state have been measured using the one-photon zero-kinetic energy photoelectron spectroscopic method. The spin-vibronic energy levels have also been calculated using a diabatic model, in which the potential energy surfaces are expressed by expansions of internal coordinates, and the Hamiltonian matrix equation is solved using a variational method with harmonic basis functions. The calculated spin-vibronic energy levels are in good agreement with the experimental data. The Renner-Teller (RT) parameters describing the vibronic coupling for the H-C≡C bending mode (ε4), Cl-C≡C bending mode (ε5), the cross-mode vibronic coupling (ε45) of the two bending vibrations, and their vibrational frequencies (ω4 and ω5) have also been determined using an effective Hamiltonian matrix treatment. In comparison with the spin-orbit interaction, the RT effect in the H-C≡C bending (ε4) mode is strong, while the RT effect in the Cl-C≡C bending mode is weak. There is a strong cross-mode vibronic coupling of the two bending vibrations, which may be due to a vibronic resonance between the two bending vibrations. The spin-orbit energy splitting of the ground state has been determined for the first time and is found to be 209 ± 2 cm(-1).