Photoelectron Spectroscopy of Chloro-Substituted Phenylnitrene Anions

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.

Mass spectrometry studies of nitrene anions

Nitrene anions are a class of reactive intermediates that provide a means for studying the corresponding neutral molecules via electron photodetachment spectroscopy and photoelectron spectroscopy. The added electron makes it possible for protected nitrene anions to be manipulated by external electric and magnetic fields of a mass spectrometer. Nitrene anions also display their own unique reactivities as reagents, which have been investigated using ion/molecule reactions. Mass spectrometry of negative ions has thereby provided information on the electronic states, reactivities, and thermochemical properties of nitrene intermediates. This review also includes a discussion of condensed-phase nitrene anions.

Isomer-Selective Threshold Photoelectron Spectra of Phenylnitrene and Its Thermal Rearrangement Products

The photoionization of phenylnitrene was investigated by photoion mass-selected threshold photoelectron spectroscopy in the gas phase. Flash vacuum pyrolysis of phenyl azide at 480 °C produces the nitrene, which subsequently rearranges at higher temperatures affording three isomeric cyanocyclopentadienes, in contrast to low-temperature trapping experiments. Temperature control of the reactor and threshold photoelectron spectra allows for optimizing the generation of phenylnitrene or its thermal rearrangement products, as well as obtaining vibrational information for the corresponding ions. The adiabatic ionization energies (AIE) of the triplet nitrene (³A₂) to the radical cation in its lowest-energy doublet (²B₂) and quartet (⁴A₁) spin states were determined to 8.29 ± 0.01 and 9.73 ± 0.01 eV, respectively. Vibrational frequencies of ring breathing modes were measured at 500 ± 80 and 484 ± 80 cm^-1 for both the [Formula: see text](²B₂) and [Formula: see text](⁴A₁) cationic states, respectively. The AIE differ from the values previously reported; hence, we revise the doublet-quartet energy splitting of the phenylnitrene radical cation to 1.44 eV, in excellent agreement with composite methods and coupled cluster calculations, but considerably higher than the literature reference (1.1 eV).

Photolysis of 5-Azido-3-Phenylisoxazole at Cryogenic Temperature: Formation and Direct Detection of a Nitrosoalkene

To enhance the versatility of organic azides in organic synthesis, a better understanding of their photochemistry is required. Herein, the photoreactivity of azidoisoxazole 1 was characterized in cryogenic matrices with IR and UV-Vis absorption spectroscopy. The irradiation (λ = 254 nm) of azidoisoxazole 1 in an argon matrix at 13 K and in glassy 2-methyltetrahydrofuran (mTHF) at 77 K yielded nitrosoalkene 3. Density functional theory (DFT) and complete active space self-consistent field (CASSCF) calculations were used to aid the characterization of nitrosoalkene 3 and to support the proposed mechanism for its formation. It is likely that nitrosoalkene 3 is formed from the singlet excited state of azidoisoxazole 1 via a concerted mechanism or from cleavage of an intermediate singlet nitrene that does not undergo efficient intersystem crossing to its triplet configuration.

The effect of nitro groups on N2 extrusion from aromatic azide-based energetic materials

ortho nitroaromatic azides extrude N₂ through cyclization to a benzofuroxan derivative. DFT calculations show that steric and electronic factors influence the activation barriers for extrusion in energetic materials.

A Laser Flash Photolysis Study of Azo-Compound Formation from Aryl Nitrenes at Room Temperature

The species 4-nitrenopyridine 1-oxide is known to exhibit triplet nitrene dominated chemistry to yield azo-dimer products exclusively, even at room temperature. As such, this species, and its analogue 4-nitrenoquinoline 1-oxide, are useful models to probe the mechanism of formation of azo-dimers, which is postulated to proceed by self-reaction of the nitrene or reaction of nitrene with the parent azide. A laser flash photolysis study is described where the kinetics of formation of azo-dimer were found to be most adequately modeled by competition between both mechanisms, and rate coefficients for the competing reactions were determined.

Triradicals

Consistent with the definition of diradicals, triradicals arespecies in which three electrons occupy three (nearly) degenerate orbitals, resulting in close-lying quartet and doublet states. The same concepts and rules that can be used to predict and rationalize the ground state multiplicity of diradicals also apply to triradicals, but the greater number of states in triradicals generally leads to more complex electronic structures. Most experimentally accessible triradicals are based onorganic π-systems; therefore triradicals are classified according to the σ/π symmetry of the nominally nonbonding molecular orbitals (NBMOs). The tridehydrobenzenes are prototypal σσσ triradicals with doublet ground states and significant doublet-quartet energy splittings. In all three isomers, the ordering of electronic states depends critically on the distance between the m-radical centers, which makes computational studies of these systems demanding. The experimental IR spectrum of matrix-isolated 1,2,3-tridehydrobenzene led to a revision of the previous ground state assignment based on computations. This work demonstrates the close interplay between experiment and theory in this realm of reactive intermediate chemistry. 1,3,5-Tridehydrobenzene can be isolated as its trifluoro derivative. The stabilization of dehydrophenyl nitrenes, typical members of the σσπ family of triradicals, also requires ortho-fluorination. Because of their quartet ground states, derivatives of 2-dehydrophenyl nitrene and 4-dehydrophenyl nitrene could be studied using IR or EPR spectroscopy. The zero-field splitting parameters of these systems provide direct evidence for the contribution of carbenoid resonance structures to the resonance hybrid of the high-spin systems. According to computations, the through-bond coupling of the in-plane electrons thermodynamically stabilizes the doublet ground states of m-dehydrophenyl nitrenes. But for the same reasons, these systems are prone to ring-opening reactions, which make them difficult to isolate. Remarkably, the m-phenylene unit leads to strongly antiferromagnetic coupling in σσπ triradicals, while o- or p-coupling results in high-spin systems. The more common all-π systems show the opposite pattern because the latter connectivity naturally results in closed-shell arrangements. Within the family of σππ triradicals, we could characterize 2-dehydro-m-xylylene and 4-dehydro-m-xylylene by EPR spectroscopy, whereas the 5-isomer features a doublet ground state. This observation is readily rationalized considering the nodal characteristics of the NBMOs involved and by simple spin polarization models. 1,3,5-Trimethylenebenzene strongly prefers ferromagnetic coupling and features a robust quartet ground state. We have synthesized this πππ triradical in cryogenic matrices and characterized it by IR and EPR spectroscopy. Interestingly, the triradical is photochemically much more stable than m-xylylene, a diradical that shows fascinating rearrangements upon irradiation in cryogenic matrices.

Photoelectron imaging of NCCCN(-): The triplet ground state and the singlet-triplet splitting of dicyanocarbene

The photoelectron spectra of NCCCN(-) have been measured at 355 and 266 nm by means of photoelectron imaging. The spectra show two distinct features, corresponding to the ground and first excited states of dycianocarbene. With support from theoretical calculations using the spin-flip coupled-cluster methods, the ground electronic state of HCCCN is assigned as a triplet state, while the first excited state is a closed-shell singlet. The photoelectron band corresponding to the triplet is broad and congested, indicating a large geometry change between the anion and neutral. A single sharp feature of the singlet band suggests that the geometry of the excited neutral is similar to that of the anion. In agreement with these observations, theoretical calculations show that the neutral triplet state is either linear or quasilinear (X (3)B(1) or (3)Sigma(g) (-)), while the closed-shell singlet (a (1)A(1)) geometry is strongly bent, similar to the anion structure. The adiabatic electron binding energy of the closed-shell singlet is measured to be 3.72+/-0.02 eV. The best estimate of the origin of the triplet band gives an experimental upper bound of the adiabatic electron affinity of NCCCN, EA</=3.25+/-0.05 eV, while the Franck-Condon modeling yields an estimate of EA(NCCCN)=3.20+/-0.05 eV. From these results, the singlet-triplet splitting is estimated to be DeltaE(ST)(X (3)B(1)/(3)Sigma(g) (-)-a (1)A(1))=0.52+/-0.05 eV (12.0+/-1.2 kcal/mol).