Is the 1,3,5-Tridehydrobenzene Triradical a Cyclopropenyl Radical Analogue?

Abstract

The gas-phase heat of formation (DeltaH(f,298)) of the 1,3,5-tridehydrobenzene triradical has been determined by using a negative ion thermochemical cycle. The first three measurements carried out were of the gas-phase acidity of 3,5-dichlorobenzoic acid, the enthalpy for decarboxylation of 3,5-dichlorobenzoate, and the enthalpy for chloride loss from 3,5,-dichlorophenide and constitute the measurement of the heat of formation for 5-chloro-m-benzyne. The last two measurements, the electron affinity of 5-chloro-m-benzyne, and the threshold for chloride loss from 5-chloro-m-benzyne, when combined with DeltaH(f,298) of 5-chloro-m-benzyne, give the heat of formation of the triradical. The 5-chloro-m-benzyne heat of formation is 116.2 +/- 3.7 kcal/mol. The heat of formation of the 1,3,5-tridehydrobenzene triradical measured in this work is 179.1 +/- 4.6 kcal/mol. This heat of formation was used to derive the bond dissociation energy (BDE) at the 5-position of m-benzyne, a third BDE in benzene. The BDE, at 109.2 +/- 5.6 kcal/mol, is ca. 4 kcal/mol lower than the first BDE in benzene (112.9 kcal/mol) and significantly higher than the BDE of phenyl radical at the meta position. The agreement between the first and third BDEs implies that the triradical is best described as a phenyl radical that interacts little with a m-benzyne moiety. The experimentally measured BDE is in good agreement with multireference configuration interaction calculations, which predict a (2)A(1) ground state for the Jahn-Teller distorted triradical. The trends in the first, second, and third BDEs of benzene are similar to those found for cyclopropane, suggesting a cyclopropenyl-like electronic structure within the six-membered ring of the 1,3,5-benzene triradical.

Fluorotrimethylsilane affinities of anionic nucleophiles: A study of fluoride-induced desilylation

“…In addition, reaction of fluoride with disilanes causes cleavage of SiOSi bonds to produce silyl anions [15,16]. Lastly, a variation of fluoride-induced desilylation involving the reaction of trimethylsilyl substituted anions has also been used to prepare open-shell anions of biradicals, triradicals, and beyond [9,[17][18][19][20][21][22].…”

5‐Dehydro‐1,3‐quinodimethane: A Hydrocarbon with an Open‐Shell Doublet Ground State

“…[20] By using the measured DH 298 along with the previously reported heat of formation of MX, 80.8 AE 2.4 kcal mol À1 , [21] the heat of formation of DMX is found to be 141 AE 5 kcal mol À1 . We have previously described the "triradical stabilization energy" (TSE) [22] as the energy required to separate a radical into radical and biradical components. The TSE for DMX corresponds to the energy change for the reaction shown in Equation (7), where DMX and MX are in their ground doublet and triplet states, respectively.…”

Trifluor‐1,3,5‐tridehydrobenzol

“…Die Tridehydrobenzole 1-3, die formal durch das Entfernen dreier Wasserstoffatome aus Benzol entstehen, sind Prototypen dieser Klasse reaktiver Zwischenstufen, deren Untersuchung jedoch erst in jüngster Zeit möglich wurde. [2][3][4] Rechnungen von Krylov und Mitarbeitern zufolge ist 1 das stabilste der drei Isomere, während 2 und 3 energetisch um 3 bzw. 17 kcal mol À1 höher liegen.…”

“…[2a] Thermochemische Daten für 3 wurden von Wenthold und Mitarbeitern in der Gasphase bestimmt. [4] Danach beträgt die Bildungsenthalpie des Triradikals (179 AE 4.6) kcal mol À1 , und es ergeben sich die in Schema 2 angegebenen Bindungsdissoziationsenergien (BDE) für die sukzessive Wasserstoffabspaltung aus Benzol.…”

“…[4] Obwohl diese Daten auf einen 2 A 1 -Grundzustand von 3 schließen lassen, sind direktere Informationen über die Eigenschaften dieses Triradikals wünschenswert, zumal einige quantenchemische Methoden den 2 B 2 -Zustand als weiteres Minimum und nicht als Übergangszustand beschreiben (siehe unten).…”