Ab Initio calculations of the potential surfaces for rearrangement of methylenecyclopropane and 2,2-difluoromethylenecyclopropane. Why do the geminal fluorines have little effect on lowering the activation energy? †

Competitive kinetic processes in the thermal rearrangement of 1,1-difluoro-2-(dideuteriomethylene)-cyclopropane

“…This effective confirmation by our current experiments of the predicted ratio of rate constants for the two competing processes serves to provide excellent credibility to the kinetic model of the methylenecyclopropane energy surface that was described by Borden in the 1999 paper [7]. Thus, the final kinetic issue related to the effect of fluorine substituents on the thermal rearrangement of 1,1-difluoro-2-methylene-cyclopropane has seemingly been answered.…”

“…These issues were nicely rationalized in another paper by Borden and coworkers [7], in which it was concluded that ''the strong preference of a CF 2 radical center for a pyramidal geometry raised the enthalpies of the transition structures for both the degenerate methylenecyclopropane rearrangement of 1 and for its non-degenerate rearrangement to 2.'' This resulted in calculated activation energies for these processes that were, respectively, only 3.1 and 2.0 kcal/mol lower than that predicted for the degenerate rearrangement of the parent, non-fluorine-containing methylenecyclopropane.…”

The synergy between qualitative theory, quantitative calculations, and direct experiments in understanding, calculating, and measuring the energy differences between the lowest singlet and triplet states of organic diradicals

“…88 Therefore, although the region of the singlet potential energy surface around the planar 1 A 1 state appears to be quite flat, the CASPT2 calculations suggest that 1 A 1 may actually be a very shallow energy minimum.…”

“…51,52 The ability of the phenyl group to delocalize the nonbonding electron on nitrogen in the open-shell singlet state of PhN, thus confining this  electron to a region of space different from that occupied by the  electron of opposite spin, is responsible for all three of these calculated and observed properties of PhN. 51,52 The minimization of the Coulombic repulsion between electrons of opposite spin, by confining them to different regions of space, is also responsible a number of predictions and observations about non-Kekulé hydrocarbon diradicals These include (a) the difference between the planar geometry of triplet TMM and the preferred geometry of the singlet, which has one CH 2 group twisted out of conjugation, 1,28,29,87,88 (b) the triplet ground states calculated and found for non-Kekulé diradicals with non-disjoint NBMOs (e.g., TMM 1, 2, 28, 29, 80-83, 86, 98 and MBQDM 84,[96][97][98] ) and (c) the singlet ground states calculated and found for non-Kekulé diradicals with disjoint NBMOs (TME [105][106][107][108][109]115 and TMB 110,111,[122][123][124].…”

Methylene‐Azaphosphirane as a Reactive Intermediate

“…Indeed, at the UB3LYP/6-31G*, using a spin-projection method to obtain proper open-shell singlet energies, [52] the open-shell transition structure 37 (DE°= 22.6 kcal mol À1 ) was found together with 38 (DE°= 27.8 kca mol À1 , Figure 4), [53] which are the heteroatom analogues [5b] of the well-known trimethylenemethane (TMM) diradical intermediates for the interconversion of methylenecyclopropanes. [54,55] The B3LYP DFT method is capable of relatively economical direct comparisons of concerted and diradical mechanisms and is favored over the computational demanding multiconfiguration approach. [10,52] The N/P-allyl-radical moieties of 37 and 38 are resonance stabilized through conjugation with a phenyl substituent, [56] thereby inducing radical character on the aromatic ring carbons.…”