Valence Photoionization and Energetics of Vanillin, a Sustainable Feedstock Candidate

Vibrational and electronic spectra of protonated vanillin: exploring protonation sites and isomerisation

Photofragmentation spectra of protonated vanillin produced under electrospray ionisation (ESI) conditions have been recorded in the 3000-3700 cm-1 (vibrational) and 225-460 nm (electronic) ranges, using room temperature IRMPD (infrared multiphoton dissociation) and cryogenic UVPD (ultraviolet photodissociation) spectroscopies, respectively. The cold (∼50 K) electronic UVPD spectrum exhibits very well resolved vibrational structure for the S1 ← S0 and S3 ← S0 transitions, suggesting long excited state dynamics, similar to its simplest analogue, protonated benzaldehyde. The experimental data were combined with theoretical calculations to determine the protonation site and configurational isomer observed in the experiments.

A direct liquid sampling interface for photoelectron photoion coincidence spectroscopy

We introduce an effective and flexible high vacuum interface to probe the liquid phase with photoelectron photoion coincidence (liq-PEPICO) spectroscopy at the vacuum ultraviolet (VUV) beamline of the Swiss Light Source. The interface comprises a high-temperature sheath gas-driven vaporizer, which initially produces aerosols. The particles evaporate and form a molecular beam, which is skimmed and ionized by VUV radiation. The molecular beam is characterized using ion velocity map imaging, and the vaporization parameters of the liq-PEPICO source have been optimized to improve the detection sensitivity. Time-of-flight mass spectra and photoion mass-selected threshold photoelectron spectra (ms-TPES) were recorded for an ethanolic solution of 4-propylguaiacol, vanillin, and 4-hydroxybenzaldehyde (1 g/l of each). The ground state ms-TPES band of vanillin reproduces the reference, room-temperature spectrum well. The ms-TPES for 4-propylguaiacol and 4-hydroxybenzaldehyde are reported for the first time. Vertical ionization energies obtained by equation-of-motion calculations reproduce the photoelectron spectral features. We also investigated the aldol condensation dynamics of benzaldehyde with acetone using liq-PEPICO. Our direct sampling approach, thus, enables probing reactions at ambient pressure during classical synthesis procedures and microfluidic chip devices.

Isomer-Dependent Threshold Photoelectron Spectroscopy and Dissociative Photoionization Mechanism of Anisaldehyde

We studied the threshold photoionization and dissociative ionization of para-, meta-, and ortho-anisaldehyde by photoelectron photoion coincidence spectroscopy in the 8.20-19.00 eV photon energy range. Vertical ionization energies by equation of motion-ionization potential-coupled cluster singles and doubles (EOM-IP-CCSD) calculations reproduce the photoelectron spectral features in all three isomers. The dissociative photoionization (DPI) pathways of para- and meta-anisaldehyde are similar and differ markedly from those of ortho-anisaldehyde. In the para and meta isomers, the lowest-energy DPI channel corresponds to hydrogen atom loss to form the C₈H₇O₂⁺ fragment at m/z 135, which undergoes sequential dissociation processes at higher energies, such as carbon monoxide loss to C₇H₇O⁺ (m/z 107) and further, sequential CH₃, CH₂O, and CH₂CO losses to produce C₆H₄O⁺ (m/z 92), C₆H₅⁺ (m/z 77), and C₅H₅⁺ (m/z 65), respectively. Carbon monoxide loss from the parent ions, yielding C₇H₈O⁺ (m/z 108), is a subordinate dissociation channel parallel to H atom loss. At higher energies, it also gives rise to sequential formaldehyde (CH₂O) loss to produce C₆H₆⁺ (m/z 78). In the ortho-anisaldehyde cation, the vicinity of the aldehyde and methoxy groups opens up low-energy hydrogen-transfer processes, which allow for seven fragmentation channels to compete effectively with the H- and CO-loss channels. Thus, the fragmentation mechanism changes considerably, thanks to the steric interaction of the substituents. Functional group interactions, in particular H transfer pathways, must therefore be considered when predicting the isomer-specific unimolecular decomposition mechanism of cationic and neutral species, as well as mass spectra for isomers.

Aerosol mass spectrometry of neutral species based on a tunable vacuum ultraviolet free electron laser

A vacuum ultraviolet free electron laser (VUV-FEL) photoionization aerosol mass spectrometer (AMS) has been developed for online measurement of neutral compounds in laboratory environments. The aerosol apparatus is mainly composed of a smog chamber and a reflectron time-of-flight mass spectrometer (TOF-MS). The indoor smog chamber had a 2 m³ fluorinated ethylene propylene film reactor placed in a temperature- and humidity-controlled room, which was used to generate the aerosols. The aerosols were sampled via an inlet system consisting of a 100 μm orifice nozzle and aerodynamic lenses. The application of this VUV-FEL AMS to the α-pinene ozonolysis under different concentrations reveals two new compounds, for which the formation mechanisms are proposed. The present findings contribute to the mechanistic understanding of the α-pinene ozonolysis in the neighborhood of emission origins of α-pinene. The VUV-FEL AMS method has the potential for chemical analysis of neutral aerosol species during the new particle formation processes.

A plethora of isomerization processes and hydrogen scrambling in the fragmentation of the methanol dimer cation: a PEPICO study

The valence photoionization of light and deuterated methanol dimers was studied by imaging photoelectron photoion coincidence spectroscopy in the 10.00-10.35 eV photon energy range. Methanol clusters were generated in a rich methanol beam in nitrogen after expansion into vacuum. They generally photoionize dissociatively to protonated methanol cluster cations, (CH₃OH)_nH⁺. However, the stable dimer parent ion (CH₃OH)₂⁺ is readily detected below the dissociation threshold to yield the dominant CH₃OH₂⁺ fragment ion. In addition to protonated methanol, we could also detect the water- and methyl-loss fragment ions of the methanol dimer cation for the first time. These newly revealed fragmentation channels are slow and cannot compete with protonated methanol cation formation at higher internal energies. In fact, the water- and methyl-loss fragment ions appear together and disappear at a ca. 150 meV higher energy in the breakdown diagram. Experiments with selectively deuterated methanol samples showed H scrambling involving two hydroxyl and one methyl hydrogens prior to protonated methanol formation. These insights guided the potential energy surface exploration to rationalize the dissociative photoionization mechanism. The potential energy surface was further validated by a statistical model including isotope effects to fit the experiment for the light and the perdeuterated methanol dimers simultaneously. The (CH₃OH)₂⁺ parent ion dissociates via five parallel channels at low internal energies. The loss of both CH₂OH and CH₃O neutral fragments leads to protonated methanol. However, the latter, direct dissociation channel is energetically forbidden at low energies. Instead, an isomerization transition state is followed by proton transfer from a methyl group, which leads to the CH₃(H)OH⁺⋯CH₂OH ion, the precursor to the CH₂OH-, H₂O-, and CH₃-loss fragments after further isomerization steps, in part by a roaming mechanism. Water loss yields the ethanol cation, and two paths are proposed to account for m/z 49 fragment ions after CH₃ loss. The roaming pathways are quickly outcompeted by hydrogen bond breaking to yield CH₃OH₂⁺, which explains the dominance of the protonated methanol fragment ion in the mass spectrum.