Spectroscopic Observation of the Triplet Diradical State of a Cyclobutadiene

Abstract

Tetrakis(trimethylsilyl)cyclobuta-1,3-diene (1)w as subjected to at emperature-dependent EPR study to allowt he first spectroscopic observation of at riplet diradical state of ac yclobutadiene (2). From the temperature dependent EPR absorption area we derive asinglet!triplet (1!2)energy gap, E ST ,o f1 3.9 kcal mol À1 ,i na greement with calculated values. The zero-field splitting parameters D = 0.171 cm À1 ,E= 0cm À1 are accurately reproduced by DFT calculations.T he triplet diradical 2 is thermally accessible at moderate temperatures.It is not an intermediate in the thermal cycloreversion of cyclobutadiene to two acetylene molecules.

Connecting and combining rules of aromaticity. Towards a unified theory of aromaticity

“…According to the 4 N π‐electrons Baird rule, monocyclic conjugated hydrocarbons of D nh symmetry that are antiaromatic in their singlet ground state are aromatic in their lowest‐lying triplet state, and vice versa for those that are aromatic in the ground state. Recent spectroscopic observation of the triplet diradical state of cyclobutadiene gave further support to this rule . It is worth mentioning that, as an extension to this Baird rule, it has been proved that the singlet excited state with the same electronic configuration of the lowest‐lying triplet state of annulenes with 4 N π‐electrons is also aromatic …”

Do Diradicals Behave Like Radicals?

“…Electron paramagnetic resonance (EPR) spectroscopy studies on a substituted CBD at elevated temperatures in the solid state suggest that the triplet excited state is accessible thermally. 133 As mentioned before, expressed using the delocalized e g MOs in Figure 18, the lowest singlet state of the square CBD has a "closed-shell" two-configuration wave function,…”

Bent Bond/Antiperiplanar Hypothesis and the Chemical Reactivity of Annulenes

“…14 It was also recently demonstrated by EPR studies that the triplet state of a tetrasubstituted CBD is square and ∼14 kcal/mol higher in energy than its square singlet TS. 15 The reactivity of CBD can be simply understood by the BBAH. Structure 3 corresponds to 3a (a formal bond representation of 3) and to 3b upon antiperiplanar delocalization of one of the equivalent radicals, as shown in Scheme 3.…”

“…This analysis is in agreement with the computational results, which suggests that the cycloreversion proceeds via two consecutive C−C bond cleavages. 15 Benzene (C 6 H 6 ). We previously reported 1 that the aromaticity of benzene could be rationalized by invoking antiperiplanar displacements of pyramidal diradicals with their adjacent τ bonds (Scheme 8).…”