On the trimerization of cyanoacetylene: mechanism of formation of tricyanobenzene isomers and laboratory detection of their radio spectra.

In support of a deeper understanding of the chemistry of cyanoacetylene--a known constituent of planetary atmospheres and interstellar space--theoretical and experimental studies address the chemical mechanism of dimerization and trimerization, and provide high-resolution rotational spectra of two of the trimeric products, 1,2,3- and 1,2,4-tricyanobenzene. Analysis of the rotational spectra is particularly challenging because of quadrupolar coupling from three (14)N nuclei. The laboratory rotational spectra provide the basis for future searches for these polar aromatic compounds in interstellar space by radio astronomy.

Effects of cyano substituents on cyclobutadiene and its isomers.

The effects of cyano substitution on cyclobutadiene are explored using density functional, coupled-cluster, CASSCF, and CASPT2 calculations. An isodesmic reaction is employed to gauge the relative stabilization (DeltaH(rxn) degrees) of cyclobutadienes with varying numbers of cyano groups. Although density functional theory predicts a relatively large stabilization for the addition of four cyano substituents to cyclobutadiene (18.5 kcal/mol), coupled-cluster theory predicts a smaller stabilization (9.3 kcal/mol). The effect of the number of cyano groups on the singlet-triplet gaps is also investigated. NBO calculations lend insight into the structural trends of the triplets, and the comparison of coupled-cluster and CASSCF calculations sheds light on the multireference electronic character in these systems. The effect of tetracyano substitution on tetrahedrane and other C(4)H(4) isomers is also explored.

Triplet 1,3-diphenylpropynylidene (Ph-C-C-C-Ph).

Photolysis (lambda>571 nm) of 1,3-diphenyldiazopropyne (9) affords triplet 1,3-diphenylpropynylidene (3), as characterized by IR, UV/vis, and EPR spectroscopy in low-temperature matrices. Two conformational isomers of triplet 3 are spectroscopically distinguishable. The initially formed, non-relaxed conformer is believed to reflect the geometry of the diazo precursor, as enforced by the rigid matrix. Annealing the matrix permits the structure to relax to the equilibrium D2d geometry. The highly symmetric equilibrium structure of 3 is best envisioned as a 1,3-allenic diradical. Density functional theory calculations suggest that the equilibrium structure does not exhibit a bond-localized structure that would be characteristic of an acetylenic carbene. Chemical trapping with O2, however, affords products that are familiar as carbene trapping products: carbonyl oxide 10, ketone 11, and dioxirane 12. Irradiation (lambda>261 nm) of triplet 1,3-diphenylpropynylidene (3) results in cyclization to singlet diphenylcyclopropenylidene (6), a process that is photochemically reversible at lambda=232 nm. Diphenyl-1,2-propadienylidene (7) was not observed under any irradiation conditions.

Ab initio molecular orbital and density functional studies on the solvolysis of sarin and O,S-dimethyl methylphosphonothiolate, a VX-like compound.

[reaction: see text] Potential energy surfaces for the alkaline hydrolysis of sarin and O,S-dimethyl methylphosphonothiolate, a VX model compound, and the perhydrolysis of the latter have been computed at the MP2/6-31+G(d)//mPW1K/MIDI! level of theory. The effect of aqueous solvation was accounted for via the integral equation formalism polarizable continuum model (IEF-PCM) at the HF/6-31+G(d) level. Excellent agreement with the experimental enthalpy of activation for alkaline hydrolysis of sarin was found. For the alkaline hydrolysis of O,S-dimethyl methylphosphonothiolate, it was found that the P-O and P-S bond cleavage processes are kinetically competitive but that the products of P-S bond cleavage are thermodynamically favored. For the perhydrolysis of O,S-dimethyl methylphosphonothiolate, it was found that P-O bond cleavage is not kinetically competitive with P-S bond cleavage. In both cases, the data support initial formation of trigonal bipyramidal intermediates and demonstrate kinetic selectivity for nucleophilic attack on the face opposite the more apicophilic methoxide ligand.