Ethylenedione: An Intrinsically Short-Lived Molecule

Abstract

Ethylenedione C 2 O 2 is one of the elusive small molecules which have remained undetected even after numerous attempts with different experimental techniques. This is surprising, since theoretical studies predicted the triplet state of C 2 O 2 to be stable towards spinallowed dissociation and hence longlived. Here we report a comprehensive study of charged and neutral ethylenedione by means of charge reversal and neutralization ± reionization mass spectrometry. These experimental results, in conjunction with theoretical calculations, suggest that neutral ethylenedione is intrinsically short-lived rather than being elusive. Both the singlet and triplet states of C 2 O 2 are predicted to dissociate rapidly into two ground-state CO molecules, and for the triplet species, this dissociation involves facile curve-crossing to the singlet surface within a few nanoseconds.

Extensive ab Initio Study of the C2O2, C2S2, and C2OS Systems: Stabilities and Singlet−Triplet Energy Gaps

“…[15][16][17][18][19][20][21] Chen and Holmes 22 and Dawson et al 23 made claims that they could infer the existence of C 2 O 2 as a transient species in neutralization-reionization mass spectrometry experiments, but this claim is diputed by Schröder et al, who were unable to obtain any measurable evidence of its existence in a wider range of such experiments. 21 This is an extraordinary state of affairs, since the cation 24,25 C 2 O 2 + , the anion 26 C 2 O 2 -, the doubly charged anion 27 C 2 O 2 2-, and the van der Waals molecule 28 (CO) 2 , are all known, and evidence from IR spectroscopy for the existence of the doubly charged cation has been reported. 29, 30 Furthermore, there have been many theoretical investigations of neutral ethylenedione, all of which have found a stable minimum on the potential energy surface corresponding to a covalently bound species.…”

“…However, this ground-state triplet is higher in energy than two groundstate CO molecules as has been emphasized, dissociation by this pathway is spin forbidden and, unless there is an efficient potential crossing, the molecule should be obtainable in a triplet ground state analogous to the ground state of O 2 . The ease with which crossing from the singlet to the triplet surface might occur has been addressed by Schröder et al, 21 who coupled another failure of a systematic attempt to produce C 2 O 2 to a theoretical investigation of this issue. They located the minimum energy crossing point between the singlet and triplet surfaces and calculated the matrix element for the coupling between the two states at this point.…”

Communication: Low-energy free-electron driven molecular engineering: In situ preparation of intrinsically short-lived carbon-carbon covalent dimer of CO

“…Using high level ab initio quantum chemical methods, we illustrate the in situ preparation of an intrinsically shortlived ethylenedione (OCCO) molecule, i.e., the carbon-carbon 13 OCCO has been continuously pursued as a synthetic target. [14][15][16] While its higher molecular weight analogues from the oxygen family has been characterized, 17 OCCO had remained elusive for more than a century, even to observe in the transient form. Even though OCCO is not the thermodynamically stable form of the CO dimer, its ground state, 3 Σ − g , was believed to be kinetically stable with no apparent reason for its synthetic difficulty.…”

“…14 A combined experimental-theoretical study by Schröder et al has proved later that 3 Σ − g of the OCCO molecule is not kinetically stable because of the curve crossing by a dissociative singlet surface very near to its minimum energy point. 15 Due to the crossing of this dissociative singlet state, the direct chemical synthetic routes are also not viable for the preparation or characterization of the OCCO molecule. Its radical ions, however, are well known and quite stable.…”

“…Hence, forgoing the conventional preparation methods that are relied on the direct chemical synthesis of neutral OCCO, low-temperature physio-chemical methods have been pursued to prepare and characterize the OCCO molecules from its stable radical ion precursors. 15,16 The attempt to characterize OCCO from its ion precursors was accomplished very recently at the University of Arizona by Andrei Sanov group. 16,18 The OCCO molecule is created and observed as a short-lived species from its stable radical anion precursor through the photoelectron detachment experiment.…”