Isomerizations of Bicyclo[2.1.0]pent-2-ene and Tricyclo[2.1.0.0^2,5]pentane into Cyclopenta-1,3-diene:  A Computational Study by DFT and High-Level ab Initio Methods

Abstract

The thermal isomerizations of bicyclopentene (bcp) and tricyclopentane (tcp) into cyclopentadiene (cp) are investigated by a combination of DFT, CASSCF, CASSCF-MP2, and CR-CCSD(T) methods. Coupled-clusters and B3LYP methods predicted the reaction enthalpies excellently whereas the MCSCF method worked well only when dynamic correlation energy was taken into account. Both processes are concerted, and the reaction paths pass through transition states with high biradical character. Measures of biradical character in DFT and ab initio methods are discussed. The activation enthalpy in the rearrangement of bcp into cp was predicted by the CR-CCSD(T) method to be 25.5 kcal/mol, in good agreement with experiment. The UB3LYP functional also performed well in this case despite the high spin contamination that was present in the singlet biradicaloid transition state. The reaction enthalpy for the conversion of tcp into cp was predicted to be −63.7 kcal/mol. The transition state involved in the isomerization of tcp was found to be nearly degenerate with the triplet state causing less certainty in the estimated activation enthalpy of 48.3 kcal/mol.

Substituent Effects on Dynamics at Conical Intersections: Cyclopentadienes

“…Considering the nonthermal product distribution (at the CCSD(T)/6-31G(d)-level, the ground state energies of BP and TP are 46 and 63 kcal/mol above the ground state of CPD, respectively 36 ), the BP rearrangement very likely occurs via a separate channel on an excited potential energy surface. Evidence for this channel has been seen in ultrafast studies of CPD 15 and CPD-Me5.…”

Application of renormalized coupled-cluster methods to potential function of water

“…It has been demonstrated that the CR-CCSD(T) and CR-CC(2,3) approaches (particularly, CR-CC(2,3)) provide very good results for single bond breaking [10-13,29-31, 33,34,37,40-44,89,91,92], and reaction pathways involving biradicals and similar cases of electronic quasi-degeneracies [42,[46][47][48][49][50][51]59,93,94], eliminating the failures of the conventional CCSD(T) and CCSD(TQ) methods in those multireference situations. The CR-CCSD(TQ),a and CR-CCSD(TQ),b extensions of CR-CCSD(T) provide further improvements in the results for single bond breaking [11][12][13]29,31,33], while helping to obtain reasonable accuracies in cases of multiple bond stretching or breaking [11][12][13]20,[28][29][30]36,[39][40][41]44,76] (cf., also, Ref.…”

Application of the Cplex-isoelectronic theory to electrocyclisations, sigmatropic rearrangements, cheletropic reactions and antiaromaticityConsistent with Santilli's hadronic chemistry

“…The ring opening is a Woodward-Hoffmann disallowed transformation [2a]. DFT calculations do predict a concerted process with a transition state of high biradical character [15] but with no prediction of an SDSE syn FSED electronic mechanism. The ring opening of Dewar benzene to give benzene is also a candidate for a concerted SDSE process under thermal conditions.…”