Ab Initio Calculations on the Formation and Rearrangement of Spiropentane

Abstract

The formation of spiropentane, by addition of singlet (1A1) methylene to methylenecyclopropane, and the unimolecular reactions of spiropentane have all been studied computationally. Benchmark calculations on two key biradicals were conducted by the multireference Mukherjee's coupled-cluster (MkCC) method. Various single-reference coupled-cluster methods and multireference second-order perturbation theory were then compared for accuracy against experimental data and the MkCC results. The object of the exercise was to get the best possible description of the potential energy surface for formation and reactions of spiropentane, as a prelude to molecular dynamics simulation of the reactions. The principal conclusions of the study were that none of the unimolecular reactions of spiropentane can be classified as pericyclic processes and that the observed stereoselectivities are probably of dynamical origin. A possible resolution of a disagreement between two studies on the dynamics of cyclopropanation reactions is also offered. Of the various approximate computational models evaluated in this study, the best fit came from a composite coupled-cluster approach in which the lower-energy result was selected from a restricted coupled-cluster and a broken-symmetry, unrestricted coupled-cluster calculation on each stationary point. However, such an approach is not strictly defensible, since coupled-cluster methods are not variational, and so further evaluation of its validity would be desirable.

Stereochemical deuterium‐labeling study on the denitrogenation of 7,7‐diethoxy‐2,3‐diazabicyclo[2.2.1]hept‐2‐ene

“…Thus, the UB3LYP calculations significantly overestimated the stability of the nitrogen‐containing, open‐shell molecules . Thus, the CCSD calculations appear to be much more reliable than the DFT calculations . The transition state TS2 (ν i 80.7 cm −1 ) for the ring flip of the five‐membered ring was found with an energy barrier of 2.9 kcal mol −1 for generation of the equatorial conformation DZc ‐eq.…”

“…[39] Thus, the CCSD calculations appear to be much more reliable than the DFT calculations. [40] The transition state TS2 (ν i 80.7 cm −1 ) for the ring flip of the five-membered ring was found with an energy barrier of 2.9 kcal mol −1 for generation of the equatorial conformation DZc-eq. The double-inversion reaction that proceeds through TS3 (ν i 449.5 cm −1 ) was found at the UCCSD/6-31G(d) level of theory.…”

Coordination Chemistry at Carbon: The Patchwork Family Comprising (Ph3P)2C, (Ph3P)C(C2H4), and (C2H4)2C

“…These data have been reproduced quite closely by quantum chemical calculations at various levels of sophistication. [85][86][87][88][89][90][91][92][93][94] The dimensions show that upon addition to carbon, each ethene molecule loses its double bond character, and four new bonds are established which are shorter owing to a better orbital overlap at close contacts.…”

The chemistry of bowtiene (tricyclo[5.3.0.02,6]decapentaene): a computational study

“…This is an adequate basis set here as shown by the fact that CCSD(T)/6-311+G**//umpw1pw91/6-311+G** single point relative energies match well experiments for the Bergman reaction (below) and that even the smaller 6-31G* basis has been shown to be widely applicable ("the cost-to-benefit ratio is optimal") with the B3LYP functional. 15 For the CASCF(4,4) calculations, convergence problems and memory requirements forced relinquishing of diffuse functions, and thus the 6-311G** basis was used for those; nevertheless, the energies and geometries fell into the "good" category. The big cc-pVTZ and 6-311+G(3df,2p) basis sets were "dictated" by the DFT functionals paired with them (below).…”