H2CN+ and H2CNH+: New insight into the structure and dynamics from mass-selected threshold photoelectron spectra

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.

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.

Continuous Pyrolysis Microreactors: Hot Sources with Little Cooling? New Insights Utilizing Cation Velocity Map Imaging and Threshold Photoelectron Spectroscopy

Resistively heated silicon carbide microreactors are widely applied as continuous sources to selectively prepare elusive and reactive intermediates with astrochemical, catalytic, or combustion relevance to measure their photoelectron spectrum. These reactors also provide deep mechanistic insights into uni- and bimolecular chemistry. However, the sampling conditions and effects have not been fully characterized. We use cation velocity map imaging to measure the velocity distribution of the molecular beam signal and to quantify the scattered, rethermalized background sample. Although translational cooling is efficient in the adiabatic expansion from the reactor, the breakdown diagrams of methane and chlorobenzene confirm that the molecular beam component exhibits a rovibrational temperature comparable with that of the reactor. Thus, rovibrational cooling is practically absent in the expansion from the microreactor. The high rovibrational temperature also affects the threshold photoelectron spectrum of both benzene and the allyl radical in the molecular beam, but to different degrees. While the extreme broadening of the benzene TPES suggests a complex ionization mechanism, the allyl TPES is in fact consistent with an internal temperature close to that of the reactor. The background, room-temperature spectra of both are superbly reproduced by Franck-Condon simulations at 300 K. On the one hand, this leads us to suggest that room-temperature reference spectra should be used in species identification. On the other hand, analysis of the allyl iodide pyrolysis data shows that iodine atoms often recombine to form molecular iodine on the chamber surfaces. Such sampling effects may distort the chemical composition of the scattered background with respect to the molecular beam signal emanating directly from the reactor. This must be considered in quantitative analyses and kinetic modeling.

Thermal and Catalytic Decomposition of 2-Hydroxyethylhydrazine and 2-Hydroxyethylhydrazinium Nitrate Ionic Liquid

To develop chemical kinetics models for the combustion of ionic liquid-based monopropellants, identification of the elementary steps in the thermal and catalytic decomposition of components such as 2-hydroxyethylhydrazinium nitrate (HEHN) is needed but is currently not well understood. The first decomposition step in protic ionic liquids such as HEHN is typically the proton transfer from the cation to the anion, resulting in the formation of 2-hydroxyethylhydrazine (HEH) and HNO₃. In the first part of this investigation, the high-temperature thermal decomposition of HEH is probed with flash pyrolysis (<1400 K) and vacuum ultraviolet (10.45 eV) photoionization time-of-flight mass spectrometry (VUV-PI-TOFMS). Next, the investigation into the thermal and catalytic decomposition of HEHN includes two mass spectrometric techniques: (1) tunable VUV-PI-TOFMS (7.4-15 eV) and (2) ambient ionization mass spectrometry utilizing both plasma and laser ionization techniques whereby HEHN is introduced onto a heated inert or iridium catalytic surface and the products are probed. The products can be identified by their masses, their ionization energies, and their collision-induced fragmentation patterns. Formation of product species indicates that catalytic surface recombination is an important reaction process in the decomposition mechanism of HEHN. The products and their possible elementary reaction mechanisms are discussed.

Directed gas-phase preparation of the elusive phosphinosilylidyne (SiPH2, X2A'') and cis/trans phosphinidenesilyl (HSiPH; X2A') radicals under single-collision conditions

The reaction of the D1-silylidyne radical (SiD; X²Π) with phosphine (PH₃; X¹A₁) was conducted in a crossed molecular beams machine under single collision conditions. Merging of the experimental results with ab initio electronic structure and statistical Rice-Ramsperger-Kassel-Marcus (RRKM) calculations indicates that the reaction is initiated by the barrierless formation of a van der Waals complex (i0) as well as intermediate (i1) formed via the barrierless addition of the SiD radical with its silicon atom to the non-bonding electron pair of phosphorus of the phosphine. Hydrogen shifts from the phosphorous atom to the adjacent silicon atom yield intermediates i2a, i2b, i3; unimolecular decomposition of these intermediates leads eventually to the formation of trans/cis-phosphinidenesilyl (HSiPH, p2/p4) and phosphinosilylidyne (SiPH₂, p3) via hydrogen deuteride (HD) loss (experiment: 80 ± 11%, RRKM: 68.7%) and d-trans/cis-phosphinidenesilyl (DSiPH, p2'/p4') plus molecular hydrogen (H₂) (experiment: 20 ± 7%, RRKM: 31.3%) through indirect scattering dynamics via tight exit transition states. Overall, the study reveals branching ratios of p2/p4/p2'/p4' (trans/cis HSiPH/DSiPH) to p3 (SiPH₂) of close to 4 : 1. The present study sheds light on the complex reaction dynamics of the silicon and phosphorous systems involving multiple atomic hydrogen migrations and tight exit transition states, thus opening up a versatile path to access the previously elusive phosphinidenesilyl and phosphinosilylidyne doublet radicals, which represent potential targets of future astronomical searches toward cold molecular clouds (TMC-1), star forming regions (Sgr(B2)), and circumstellar envelopes of carbon rich stars (IRC + 10216).

Description of the Methylene Amidogene Radical and Its Anion with an Economical Treatment of Correlation Effects Using Density Functional Theory Orbitals

The ground and low-lying excited state electronic structural properties (such as equilibrium geometries, harmonic frequencies, excitation energies, barrier energy, and so on) of the methylene amidogene radical (H₂CN) and its anion (H₂CN^-) have been studied using the CASCI (complete active space configuration interaction) and SSMRPT (state-specific multireference Møller-Plesset perturbation theory) methods with density function theory (DFT) orbitals. Here, the span of the active orbitals have been obtained from Kohn-Sham DFT using B3LYP exchange-correlation functionals in the CASCI (DFT-CASCI) approximation to describe nondynamic correlation associated with electronic degeneracies. The DFT-SSMRPT protocol provides an attractive way to deal with both dynamical and nondynamical correlation effects in strongly correlated systems such as H₂CN and H₂CN^-. The present work clearly indicates that the electronic absorption band near 35,050 cm^-1 corresponds to the B̃²A₁ ← X̃²B₂ transition. DFT-SSMRPT findings are in close agreement with high-level theoretical estimates. It is concluded that the transition at 1725 cm^-1 could be due to the CN stretching of the trans-HCNH isomer which is originally assigned to the CN stretch of H₂CN in the experiment. The present results confirm most of the previous vibrational assignments. It is not possible to definitively assign a transition to the 35,600 cm^-1 band with the present estimations, suggesting further experiment is urgently called for.

Methylbismuth: an organometallic bismuthinidene biradical

We report the generation, spectroscopic characterization, and computational analysis of the first free (non-stabilized) organometallic bismuthinidene, BiMe. The title compound was generated in situ from BiMe3 by controlled homolytic Bi-C bond cleavage in the gas phase. Its electronic structure was characterized by a combination of photoion mass-selected threshold photoelectron spectroscopy and DFT as well as multi-reference computations. A triplet ground state was identified and an ionization energy (IE) of 7.88 eV was experimentally determined. Methyl abstraction from BiMe3 to give [BiMe2]• is a key step in the generation of BiMe. We reaveal a bond dissociation energy of 210 ± 7 kJ mol-1, which is substantially higher than the previously accepted value. Nevertheless, the homolytic cleavage of Me-BiMe2 bonds could be achieved at moderate temperatures (60-120 °C) in the condensed phase, suggesting that [BiMe2]• and BiMe are accessible as reactive intermediates under these conditions.

Diborene: Generation and Photoelectron Spectroscopy of an Inorganic Biradical

Diborenes, R-BB-R', are of current interest in inorganic chemistry because they offer the opportunity to tune the properties of a biradical by modifying the substituents of the diborene parent, HBBH. Here we synthesize the elusive diborene by H atom abstraction from diborane, B₂H₆, using fluorine atoms and report a vibrationally resolved photoelectron spectrum of the HBBH biradical. The spectrum is interpreted by comparison with high-level ab initio computations, taking into account the Renner-Teller splitting in the X^+ 2Π ionic ground state, which show an excellent agreement with the experimental spectrum. An adiabatic ionization energy of 9.080 ± 0.015 eV was determined, and a vibrational progression in the boron-boron stretching vibration of 0.14 eV is visible. This is due to the reduction of bond order upon ionization, accompanied by an increase of the computed boron-boron bond length, R_BB, from 1.514 to 1.606 Å.

The collision-free photochemistry of methyl azide at 157 nm: Mechanism and energy release

Synchrotron radiation VUV-photoionization based photofragment translational spectroscopy was used to identify the primary and secondary photodissociation reactions of methyl azide (CH₃N₃) at 157 nm under collision-free conditions. Two primary dissociation channels are identified, leading to CH₃ + N₃ (the radical channel) and CH₃N + N₂ (the molecular elimination channel). The last channel is the major dissociation pathway, but unlike work at longer photolysis wavelengths, here, the radical channel exclusively produces the higher energy isomer cyclic-N₃. Product time-of-flight data for both channels were obtained and compared with earlier work on methyl azide photochemistry at 193 nm based on electron impact ionization, allowing us to estimate a product branching ratio Φ_CH3-N3 Φ_CH3N-N2 =2.3%±0.6%97.7%±0.6%.

Assignment of high-lying bending mode levels in the threshold photoelectron spectrum of NH2: a comparison between pyrolysis and fluorine-atom abstraction radical sources

The bending mode progression in the photoelectron spectrum of NH₂ was observed and assigned up to υ₂⁺ ≤ 5 and K_a⁺ ≤ 3.