Quadricyclane radical cation isomerization reactions: A theoretical study

Abstract

On the C 7 H 8 ⅐ϩ potential energy surface studied in this work, the electrocyclic isomerization of the quadricyclane (Q ⅐؉ ) to the norbornadiene (N ⅐؉ ) radical cations has attracted much attention. Our interest, however, concerns alternative rearrangement reactions on this surface. Using quantum chemical methods, B3LYP/6-311ϩG(d,p), MP2/6-31G(d,p), and single-point CCSD(T)/6-311ϩG(d,p), a skeletal rearrangement path leading from Q ⅐؉ to the bicyclo[3,2,0]hepta-2,6-diene radical cation (BHD ⅐؉ ) is found. The barrier for this path is 4.9 kcal/mol higher at the CCSD(T)/6-311ϩG(d,p)//B3LYP/6-311ϩG(d,p) level than the Q ⅐؉ to N ⅐؉ barrier. Moreover, this rearrangement is found to proceed through an intermediate, bicyclo[2.2.1]hepta-2-ene cation (BHE ⅐؉ ), with a conversion barrier to BHD ⅐؉ of 8.1 kcal/mol while the barrier for conversion back to Q ⅐؉ is 12.8 kcal/mol, computed with CCSD(T)/6-311ϩG(d,p)// B3LYP/6-311ϩG(d,p). Interestingly, the 1 H hyperfine coupling constants computed for this intermediate structure show an excellent agreement with the experiment by Williams and coworkers. This ESR spectrum, observed after irradiation of N and Q loaded CF 3 CCl 3 matrices, was assigned to BHD ⅐؉ ; our calculations, however, show that it is BHE ⅐؉ that gives rise to the observed spectrum.

Interatrial defect sizing by intracardiac and transesophageal echocardiography compared with fluoroscopic measurements in patients undergoing percutaneous transcatheter closure

“…The reported ESR parameters for BCP •+ (22.4 and 2.7 G) 5 agree excellently with our computed hfcc values of 24.4 G (Ha) and −2.6 G (Hb) for 1 , at the UB3LYP/6‐31G(d,p) level, which has proved successful for this purpose previously 11.…”

Ultrafast structural and isomerization dynamics in the Rydberg-exited Quadricyclane: Norbornadiene system

“…Indeed, quantum chemical calculations indicate that the activation energy for the isomerization reaction of QD into NB on the ion surface is ∼0.5 eV. 27,32 Since Rydberg electrons do not much affect the chemical bonding, the potential energy surface in the Rydberg state is similar to that in the ion state and the activation barrier calculated for QD isomerization on the ion surface is a good approximation for that on the Rydberg surface.…”

Isomerization pathways from the norbornadiene to the cycloheptatriene radical cation by opening a bridgehead-methylene bond: a theoretical investigation

“…Recently, we have undertaken computational studies 15 for the isomerization reactions of Q • + to the bicyclo[3.2.0]hepta-2,6-diene radical cation (BHD • + ), as well as for the further rearrangement 16 to CHT • + (see Scheme 1). Using quantum chemical B3LYP, MP2 and CCSD(T) methods, 17 a mechanism was proposed for the skeletal rearrangement of Q • + to BHD • + , which was found to occur via a stable intermediate, the bicyclo[2.2.1]hepta-2ene-5-yl-7-ylium radical cation (BHE • + ).…”

“…At the CCSD(T)/6-311+G(d,p)//B3LYP/6-311+G(d,p) level of calculation, 17 BHE • + was found to be 2.5 kcal mol −1 more stable than BHD • + and, moreover, the barrier of the Q • + to BHE • + reaction was found Scheme 1 to be 4.9 kcal mol −1 higher than the barrier of the facile Q • + to N • + rearrangement. 15 Also, starting from BHE • + , a multistep isomerization pathway leading to the formation of CHT • + was identified. At the above mentioned level of theory, the ratelimiting step on this path has a barrier of 16.5 kcal mol −1 above Q • + , whereas the barrier was found 16 to be higher (19.3 kcal mol −1 ) when BHE • + rearranges to CHT • + via BHD • + .…”