Nitrile ylide dimerization: investigation of the carbene reactivity of nitrile ylides

A series of novel hexaaryl diazatrienes 5 ("nitrile ylide dimers") were synthesized directly from the corresponding diaryl ketimines 12 and dichlorotoluenes 13 in a facile one-pot synthesis. The carbene character of the nitrile ylides was investigated by varying the substituents on the aromatic ring adjacent to the carbene center. The isolation of the corresponding carbene dimers as stable crystalline materials with absorption maxima (lambda(max)) from 363 to 422 nm was shown to be promoted by the absence of strongly electron-withdrawing substituents. The crystal structures indicate that the E-isomers were isolated when phenyl, 3-methylphenyl, and 3-chlorophenyl substituents are present at the carbene carbon; the Z-isomer was isolated when the more sterically hindered 2,4,6-trimethylphenyl substituent (Mes) is present. The (1)H NMR spectra of the E-isomers demonstrate the nonequivalence of the aromatic rings, in which two of the aromatic rings of the imine moiety are pseudoaxial and the remaining aromatic rings are pseudoequatorial. The reactions proceed via the intermediate nitrile ylides 1 generated by the base-promoted 1,1-elimination of HCl from the intermediate chloroimine 14. The nitrile ylide was also generated by 1,3-elimination of HCl from the imidoyl chloride 18, confirming common pathways via the nitrile ylide as the dimer products obtained from these different routes were identical. The strongly electron-withdrawing 4-nitrophenyl substituent promotes the linear carbanion character of the 1,3-dipole and no dimer is formed.     

Ab initio study of the hydration of carbon dioxide: Additional comments based on refined calculations

Ab initio calculations on the formation of carbonic acid from the hydration of carbon dioxide with water dimer are re-examined. Fully optimized geometries of the three stationary points (minima and transition state) with the 3-21G basis set are reported. They possess non-planar structures. The inclusion of polarization (with the 6-31G* basis) and electron correlation (via Møller-Plesset perturbation theory to second through to fourth-order using the 6-31G basis) tends to enlarge the energy barrier (35–40 kcal mol⁻¹) for the double hydrogen transfer. This suggests that the neutral hydrolysis of CO₂ could require more water molecules (an oligomer) in an autocatalytic process rather than a dimer.     

On the Dirac equation in the algebraic approximation

It is shown that the restrictive conditions of Wood et al. [1] are not necessary to reach the conclusion that the Dirac hamiltonian, projected onto the space of the large component, exhibits variational properties. The eigenvalue spectrum of matrix approximations to the partitioned hamiltonian (obtained by matrix partitioning) converges to the exact spectrum in the limit of infinite order (assuming completeness) but not necessarily from above as for true matrix representations (obtained from operator partitioning). Optimization of non-linear parameters is shown not to cause variational instabilities.Control Data Corporation PACER fellow 1984–1986     

Answers to two questions posed by Farhi concerning additive bases

Let A be an asymptotic basis for N and X a finite subset of A such that A?X is still an asymptotic basis. Farhi recently proved a new batch of upper bounds for the order of A?X in terms of the order of A and a variety of parameters related to the set X. He posed two questions concerning possible improvements to his bounds. In this note, we answer both questions.     

Can the pentazole anion (N5−) be isolated and/or trapped in metal complexes?

The 1,3-dipolar cycloreversion pathway of the pentazole anion (N₅^?) to the azide anion (N₃^?) plus dinitrogen (N₂) has been investigated using ab initio methods. At the MP4SDQ/6–31 + G* level of theory plus zero-point energy contributions, the pentazole anion is predicted to lie at 31 kcal mol^?1 above the N₃^? + N₂ system but the energy barrier for decomposition is 22 kcal mol^?1. This indicates that the pentazole anion could be isolated in an inert matrix at low temperature. Comparison between extended Hückel calculations on the (N₅)M(CO)₃ and (C₅H₅)M(CO)₃ complexes (with M = Fe²⁺, Mn⁺ and Cr) suggests that the N₅^? complexes would be formed if the fragments could be brought together. Predicted vibrational frequencies of the N₅^? anion are also reported.