Photoelectron Photoion Coincidence Spectroscopy of Biradicals

The electronic structure of biradicals is characterized by the presence of two unpaired electrons in degenerate or near-degenerate molecular orbitals. In particular, some of the most relevant species are highly reactive, difficult to generate cleanly and can only be studied in the gas phase or in matrices. Unveiling their electronic structure is, however, of paramount interest to understand their chemistry. Photoelectron photoion coincidence (PEPICO) spectroscopy is an excellent approach to explore the electronic states of biradicals, because it enables a direct correlation between the detected ions and electrons. This permits to extract unique vibrationally resolved photoion mass-selected threshold photoelectron spectra (ms-TPES) to obtain insight in the electronic structure of both the neutral and the cation. In this review we highlight most recent advances on the spectroscopy of biradicals and biradicaloids, utilizing PEPICO spectroscopy and vacuum ultraviolet (VUV) synchrotron radiation.

Threshold photoelectron spectroscopy of trimethylborane and its pyrolysis products

Trimethylborane (TMB) and its chemistry upon pyrolysis have been investigated by threshold photoelectron spectroscopy. TMB shows an unstructured spectrum and its adiabatic ionization energy (IE_ad) has been determined to be 9.93 ± 0.1 eV. Dissociative photoionization induces a methyl radical loss in TMB and the barrier to dissociation in the cation is measured to be 0.65 ± 0.1 eV. Upon pyrolysis methane loss dominates, leading to C₂H₅B, which can exist in five different isomeric structures. Quantum chemical calculations were used to investigate possible methane loss mechanisms as well as the isomerization pathways on the C₂H₅B potential energy surface. Through isomer-selective photoion mass-selected threshold photoelectron spectroscopy (ms-TPES) the two isomers CH₃BCH₂ and CH₃CHBH were identified by their ms-TPE spectra and IE_ad values of 8.55 ± 0.02 eV and 8.73 ± 0.02 eV were determined, respectively. A second channel leading to the loss of ethene from TMB forms CH₂BH, which exhibits an IE_ad value of 9.37 ± 0.03 eV. The reaction mechanism in the literature needs to be expanded by an additional methane loss from the intermediately formed ethyl methyl borane.

Bimolecular reactions of CH2CN2+ with Ar, N2 and CO: reactivity and dynamics

The reactivity, energetics and dynamics of bimolecular reactions between CH₂CN²⁺ and three neutral species (Ar, N₂ and CO) have been studied using a position sensitive coincidence methodology at centre-of-mass collision energies of 4.3-5.0 eV. This is the first study of bimolecular reactions involving CH₂CN²⁺, a species relevant to the ionospheres of planets and satellites, including Titan. All of the collision systems investigated display two collision-induced dissociation (CID) channels, resulting in the formation of C⁺ + CH₂N⁺ and H⁺ + HC₂N⁺. Evidence for channels involving further dissociation of the CID product HC₂N⁺, forming H + CCN⁺, were detected in the N₂ and CO systems. Proton-transfer from the dication to the neutral species occurs in all three of the systems via a direct mechanism. Additionally, there are product channels resulting from single electron transfer following collisions of CH₂CN²⁺ with both N₂ and CO, but interestingly no electron transfer following collisions with Ar. Electronic structure calculations of the lowest energy electronic states of CH₂CN²⁺ reveal six local geometric minima: both doublet and quartet spin states for cyclic, linear (CH₂CN), and linear isocyanide (CH₂NC) molecular geometries. The lowest energy electronic state was determined to be the doublet state of the cyclic dication. The ready generation of C⁺ ions by collision-induced dissociation suggests that the cyclic or linear isocyanide dication geometries are present in the [CH₂CN]²⁺ beam.

Photoelectron spectroscopy of low valent organophosphorus compounds, P-CH3, H-PCH2 and PCH2

We report the mass-selected slow photoelectron spectra of three reactive organophosphorus species, PCH₂, and the two isomers, methylenephosphine or phosphaethylene, HPCH₂ and methylphosphinidine, P-CH₃. All spectra were recorded by double imaging photoelectron-photoion coincidence spectroscopy (i²PEPICO) using synchrotron radiation and all species were generated in a flow reactor by the reaction of trimethyl phosphine with fluorine atoms. Adiabatic ionisation energies of 8.80 ± 0.02 eV (PCH₂), 10.07 ± 0.03 eV (H-PCH₂) and 8.91 ± 0.04 eV (P-CH₃) were determined and the vibronic structure was simulated by calculating Franck-Condon factors from optimised structures based on quantum chemical methods. Observation of biradicalic P-CH₃ isomer with its triplet ground state is surprising because it is less stable than H-PCH₂.

Photoelectron spectroscopy in molecular physical chemistry

Photoelectron spectroscopy has long been a powerful method in the toolbox of experimental physical chemistry and molecular physics. Recent improvements in coincidence methods, charged-particle imaging, and electron energy resolution have greatly expanded the variety of environments in which photoelectron spectroscopy can be applied, as well as the range of questions that can now be addressed. In this Perspectives Article, we focus on selected recent studies that highlight these advances and research areas. The topics include reactive intermediates and new thermochemical data, high-resolution comparisons of experiment and theory using methods based on pulsed-field ionisation (PFI), and the application of photoelectron spectroscopy as an analytical tool to monitor chemical reactions in complex environments, like model flames, catalytic or high-temperature reactors.

Threshold Photoelectron Spectroscopy of the CH2I, CHI, and CI Radicals

VUV photoionization of the CH_nI radicals (with n = 0, 1, and 2) is investigated by means of synchrotron radiation coupled with a double imaging photoion-photoelectron coincidence spectrometer. Photoionization efficiencies and threshold photoelectron spectra (TPES) for photon energies ranging between 9.2 and 12.0 eV are reported. An adiabatic ionization energy (AIE) of 8.334 ± 0.005 eV is obtained for CH₂I, which is in good agreement with previous results [8.333 ± 0.015 eV, Sztáray J. Chem. Phys. 2017, 147, 013944], while for CI an AIE of 8.374 ± 0.005 eV is measured for the first time and a value of ∼8.8 eV is estimated for CHI. Ab initio calculations have been carried out for the ground state of the CH₂I radical and for the ground state and excited states of the radical cation CH₂I⁺, including potential energy curves along the C-I coordinate. Franck-Condon factors are calculated for transitions from the CH₂I(X̃²B₁) ground state of the neutral radical to the ground state and excited states of the radical cation. The TPES measured for the CH₂I radical shows several structures that correspond to the photoionization into excited states of the radical cation and are fully assigned on the basis of the calculations. The TPES obtained for the CHI is characterized by a broad structure peaking at 9.335 eV, which could be due to the photoionization from both the singlet and the triplet states and into one or more electronic states of the cation. A vibrational progression is clearly observed in the TPES for the CI radical and a frequency for the C-I stretching mode of 760 ± 60 cm^-1 characterizing the CI⁺ electronic ground state has been extracted.

Kinetics of 1- and 2-methylallyl + O2 reaction, investigated by photoionisation using synchrotron radiation

The reaction kinetics of the isomers of the methylallyl radical with molecular oxygen has been studied in a flow tube reactor at the vacuum ultraviolet (VUV) beamline of the Swiss Light Source storage ring. The radicals were generated by direct photodissociation of bromides or iodides at 213 nm. Experiments were conducted at room temperature and low pressures between 1 and 3 mbar using He as the buffer gas. Oxygen was employed in excess to maintain near pseudo-first-order reaction conditions. Concentration-time profiles of the radical were monitored by photoionisation. For the oxidation of 2-methylallyl (2-MA) and with k(2-MA + O2) = (5.1 ± 1.0) × 1011 cm3 mol-1 s-1, the rate constant was found to be in the high-pressure limit already at 1 mbar. In contrast, 1-methylallyl exists in two isomers, E- and Z-1-methylallyl. We selectively detected the E-conformer as well as a mixture of both isomers and observed almost identical rate constants within the uncertainty of the experiment. A small pressure dependence is observed with the rate constant increasing from k(1-MA + O2) = (3.5 ± 0.7) × 1011 cm3 mol-1 s-1 at 1 mbar to k(1-MA + O2) = (4.6 ± 0.9) × 1011 cm3 mol-1 s-1 at 3 mbar. While for 2-methylallyl + O2 no previous experimental data are available, the rate constants for 1-methylallyl are in agreement with previous work. A comparison is drawn for the trends of the high-pressure limiting rate constants and pressure dependences observed for the O2 recombination of allylic radicals with the corresponding reactions of alkyl radicals.