Andrew R. Dixon

Spectroscopy of Ethylenedione

The long sought-after, intrinsically short-lived molecule ethylenedione (OCCO) was observed and investigated by anion photoelectron spectroscopy. The adiabatic electron affinity of its quasi-bound (3)Σ(g)(-) state is 1.936(8) eV. The vibrational progression with a 417(15) cm(-1) frequency observed within the triplet band corresponds to a trans-bending mode. Several dissociative singlet states are also observed, corresponding to two components of the (1)Δg state and the (1)Σ(g)(+) state. The experimental results are in agreement with theoretical predictions and constitute the first spectroscopic observation and characterization of this elusive compound.

Photochemistry of fumaronitrile radical anion and its clusters

The photodetachment and photochemistry of the radical anion of fumaronitrile (trans-1,2-dicyanoethylene) and its clusters are investigated using photoelectron imaging and photofragment spectroscopy. We report the first direct spectroscopic determination of the adiabatic electron affinity (EA) of fumaronitrile (fn) in the gas phase, EA = 1.21 ± 0.02 eV. This is significantly smaller than one-half the EA of tetracyanoethylene (TCNE). The singlet-triplet splitting in fumaronitrile is determined to be ΔES-T ≤ 2.6 eV, consistent with the known properties. An autodetachment transition is observed at 392 and 355 nm and assigned to the (2)Bu anionic resonance in the vicinity of 3.3 eV. The results are in good agreement with the predictions of the CCSD(T) and EOM-XX-CCSD(dT) (XX = IP, EE) calculations. The H2O and Ar solvation energies of fn(-) are found to be similar to the corresponding values for the anion of TCNE. In contrast, a very large (0.94 eV) photodetachment band shift, relative to fn(-), is observed for (fn)2(-). In addition, while the photofragmentation of fn(-), fn(-)·Ar, and fn(-)(H2O)1,2 yielded only the CN(-) fragment ions, the dominant anionic photofragment of (fn)2(-) is the fn(-) monomer anion. The band shift, exceeding the combined effect of two water molecules, and the fragmentation pattern, inconsistent with an intact fn(-) chromophore, rule out an electrostatically solvated fn(-)·fn structure of (fn)2(-) and favor a covalently bound dimer anion. A C2 symmetry (fn)2(-) structure, involving a covalent bond between the two fn moieties, is proposed.

HOCCO versus OCCO: Comparative spectroscopy of the radical and diradical reactive intermediates

We present a photoelectron imaging study of three glyoxal derivatives: the ethylenedione anion (OCCO(-)), ethynediolide (HOCCO(-)), and glyoxalide (OHCCO(-)). These anions provide access to the corresponding neutral reactive intermediates: the OCCO diradical and the HOCCO and OHCCO radicals. Contrasting the straightforward deprotonation pathway in the reaction of O(-) with glyoxal (OHCCHO), which is expected to yield glyoxalide (OHCCO(-)), OHCCO(-) is shown to be a minor product, with HOCCO(-) being the dominant observed isomer of the m/z = 57 anion. In the HOCCO/OHCCO anion photoelectron spectrum, we identify several electronic states of this radical system and determine the adiabatic electron affinity of HOCCO as 1.763(6) eV. This result is compared to the corresponding 1.936(8) eV value for ethylenedione (OCCO), reported in our recent study of this transient diradical [A. R. Dixon, T. Xue, and A. Sanov, Angew. Chem., Int. Ed. 54, 8764-8767 (2015)]. Based on the comparison of the HOCCO(-)/OHCCO(-) and OCCO(-) photoelectron spectra, we discuss the contrasting effects of the hydrogen connected to the carbon framework or the terminal oxygen in OCCO.

Benzonitrile: Electron affinity, excited states, and anion solvation

We report a negative-ion photoelectron imaging study of benzonitrile and several of its hydrated, oxygenated, and homo-molecularly solvated cluster anions. The photodetachment from the unsolvated benzonitrile anion to the X̃(1)A1 state of the neutral peaks at 58 ± 5 meV. This value is assigned as the vertical detachment energy (VDE) of the valence anion and the upper bound of adiabatic electron affinity (EA) of benzonitrile. The EA of the lowest excited electronic state of benzonitrile, ã(3)A1, is determined as 3.41 ± 0.01 eV, corresponding to a 3.35 eV lower bound for the singlet-triplet splitting. The next excited state, the open-shell singlet Ã(1)A1, is found about an electron-volt above the triplet, with a VDE of 4.45 ± 0.01 eV. These results are in good agreement with ab initio calculations for neutral benzonitrile and its valence anion but do not preclude the existence of a dipole-bound state of similar energy and geometry. The step-wise and cumulative solvation energies of benzonitrile anions by several types of species were determined, including homo-molecular solvation by benzonitrile, hydration by 1-3 waters, oxygenation by 1-3 oxygen molecules, and mixed solvation by various combinations of O2, H2O, and benzonitrile. The plausible structures of the dimer anion of benzonitrile were examined using density functional theory and compared to the experimental observations. It is predicted that the dimer anion favors a stacked geometry capitalizing on the π-π interactions between the two partially charged benzonitrile moieties.

Heterogeneously substituted radicals and carbenes: photoelectron imaging of the FC(H)CN(-) and FCCN(-) anions.

This work represents the next step in the studies of heterogeneous substitution effects in cyanohalo radicals and carbenes. Negative-ion photoelectron imaging was used to investigate the substituted radical and carbene derivatives of fluoroacetonitrile. We report a closed-shell singlet ground state for cyanofluorocarbene, FCCN, with a directly measured adiabatic electron affinity EA = 2.081 ± 0.002 eV and a singlet-triplet gap of ΔE(S-T) = 0.42 ± 0.04 eV, estimated through a combination of experimental and theoretical results. The open-shell singlet (1)A″ state was also observed experimentally. The cyanofluoromethyl radical, FC(H)CN, was similarly estimated to have an EA of 1.53 ± 0.08 eV. This value was used to estimate the C-H bond dissociation energy (BDE) of fluoroacetonitrile, DH298 = 90.7 ± 2.8 kcal mol(-1). The results are discussed in comparison with results for other fluoro- and cyano-substituted radicals and carbenes, and in light of our recent work on the radical and carbene derivatives of chloroacetonitrile. The estimated ΔE(S-T) of FCCN agrees well with the general trend of similar carbenes. We also find that, similar to chloroacetonitrile, the low C-H BDE of fluoroaceotnitrile indicates a synergistic stabilization of the corresponding radical by a π donor (halogen) and π acceptor (CN).