Structural Investigations into the <i>r</i><i>etro</i>-Diels-Alder Reaction. Experimental and Theoretical Studies BirneyDavid LimTang Kuan KohJoanne H. P. PoolBrett R. WhiteJonathan M. 2002 The manifestations of the <i>retro</i>-Diels Alder reaction in the ground-state structures of a range of cyclopentadiene and cyclohexadiene cycloadducts <b>9</b>−<b>15</b> have been investigated by a combination of techniques. These include low-temperature X-ray crystallography, density functional calculations (B3LYP/6-31G(d,p)) on both the ground states and transition states, and the measurement of <sup>13</sup>C−<sup>13</sup>C coupling constants. We have found that the carbon−carbon bonds (labeled bonds <i>a</i> and <i>b</i>), which break in the rDA, are longer in the cycloadducts <b>9</b>−<b>15 </b>than in their corresponding saturated analogues <b>9s</b>−<b>15s</b>, which cannot undergo the rDA reaction. The degree of carbon−carbon bond lengthening appears to be related to the reactivity of the cycloadduct, thus the more reactive benzoquinone cycloadducts <b>5b </b>and<b> 13 </b>have longer carbon−carbon bonds. Those cycloadducts <b>14</b> and <b>15 </b>which are predicted to undergo asynchronous reactions show differing degrees of carbon−carbon bond lengthening, reflecting the differing degrees of bond breaking at the calculated transition states for the rDA.