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

Abstract

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).

Photodetachment anisotropy for mixed s-p states: 8/3 and other fractions

“…66 The CCl 2 − anions were synthesized via a gas-phase ion-molecule reaction within the supersonic expansion of dichloromethane seeded in N 2 O carrier gas at a backing pressure of ∼30 psi. 20,67,68 High-energy electron collisions produced slow secondary electrons, which formed O − by dissociative electron attachment to N 2 O, followed by the desired H 2 + abstraction reaction of O − with CH 2 Cl 2 , giving CCl 2 − . For NH 2 − generation, a neutral precursor of neat ammonia or 30% ammonia seeded in argon was used.…”

Spectroscopy of the breaking bond: the diradical intermediate of the ring opening in oxazole

“…These observations contrast the typical carbene-anion photodetachment scenario. 7,[29][30][31][32][33][34] The electron configuration of a canonical carbene anion is . .…”

Photoelectron angular distributions in negative-ion photodetachment from mixed sp states

“…For example, the chemistry of carbenes and the photodetachment of carbene anions are largely controlled by the non-bonding p (σ or π ) orbitals on the central carbon atom. [44][45][46] Similarly, the highest-occupied molecular orbital (HOMO) of O 2 − , a π g * (2p) molecular orbital, is well described as a d-like function (effective i = 2). 24,37 However, in other cases a single function with defined i value may not be sufficient, as for parent states that are best described as superpositions of two or more atomiclike orbitals.…”