Computational studies of carbodiimide rings.

Computational studies of alicyclic carbodiimides (RN═C═NR) (rings five through twelve) at the MP2/6-31G(d,p)//MP2/6-31G(d,p) level of theory were conducted to locate the transition states between carbodiimides isomers. Transition states for rings six through twelve were found. The RNCNR dihedral angle is ∼0° for even-numbered rings, but deviates from 0° for rings seven, nine, eleven, and twelve. The even- and odd-numbered ring transition states have different symmetry point groups. Cs transition states (even rings) have an imaginary frequency mode that transforms as the asymmetric irreducible representation of the group. C2 transition states (odd rings) have a corresponding mode that transforms as the totally symmetric representation. Intrinsic reaction coordinate analyses followed by energy minimization along the antisymmetric pathways led to enantiomeric pairs. The symmetric pathways give diastereomeric isomers. The five-membered ring carbodiimide is a stable structure, possibly isolable. A twelve-membered ring transition state was found only without applying symmetry constraints (C1). Molecular mechanics and molecular dynamics studies of the seven-, eight-, and nine-membered rings gave additional structures, which were then minimized using ab initio methods. No structures beyond those found from the IRC analyses described were found. The potential for optical resolution of the seven-membered ring is discussed.

Dissolving metal reduction of acetylenes: a computational study.

The two-electron, two-proton reduction of alkynes to trans-alkenes has been studied computationally using the polarizable continuum regime to model liquid ammonia, the solvent in which such reductions are generally carried out. Two computational approaches have been used. In one, the energies of species (alkyne, radical anion, vinyl radical, vinyl anion, dianion, and alkene) that are implicated as possible participants in the reduction are obtained using high level ab initio single-point computations under the polarizable continuum model (PCM) conditions. In the other approach, the same species are surrounded by ten explicit ammonia molecules before undergoing the same single-point PCM analysis. It has been shown that the two methods provide nearly identical results in terms of relative energies. Other findings include the probable bent nature of the radical anion species in ammonia, the likelihood that the trans stereochemistry of the reduction is determined at the vinyl anion stage, and the elimination of a dianion as a possible species that determines the stereochemical result. Various observations relating the solvent effects of ammonia are made relative to known gas-phase properties of the species studied.

Electron, hydride, and fluoride affinities of silicon-containing species: computational studies.

The distance dependence of silicon substitution on the electron affinity (EA) of carbon radicals has been studied using computational methods in SiH3(CH2)nCH2 (A) and SiH2F(CH2)nCH2 (B). Large EAs result when n = 0 for both A and B. The result for A is compared with the experimental EA value of (CH3)3SiCH2. Similar comparisons with known EAs (CH3 and SiH3) establish the validity of the computational approach. Fluorine substitution in SiH2FCH2 is consistent with other fluorine substitution effects. When n > 1, the anions of both A and B cyclize to pentacoordinate structures in which silicon has trigonal bipyramidal geometry. The corresponding EA values raise important questions about computed EAs that result from profound geometry changes between radicals and anions. Anions that have not cyclized give rise to EA values more easily interpreted. Such results, combined with computations of vertical attachment energies, indicate that the EA values of A and B attenuate rapidly for n > 1, quickly approaching that of CH3. Pentacoordination effects of silicon anions were also studied for SiH4, (CH3)2SiH2, 1-silacyclopropane, 1-silacyclobutane, and 1-silacyclopentane.

Computational studies of aliphatic amine basicity.

Computational studies have been used to examine the structural and energetic effects of adding small numbers of water molecules to ammonia, methylamine, dimethylamine, and trimethylamine, and their respective ammoniums ions using the effective fragment potential method. Distinct structural effects with only a few fragment water molecules are revealed. The complexity of structures increases with the number of water fragments with the water fragments forming complex networks. Structural and energetic effects are used to probe the so-called anomalous basicity effect of ammonia and the methylamines on going from the gas phase to aqueous solution.