Steven M. Bachrach

Topological electron density analysis of halogen-substituted phosphirenes

Abstract The geometry of 1H-phosphirene (1a) and 2H-phosphirene (2a) and their fluoro-(1b and 2b) and chloro substituted (1c and 2c) analogues were optimized at the HF/6-31G∗ level. Unlike the parent system, the halogenated analogue of 1H-phosphirene is more stable than its isomer: 1b is 21.94 kcal mol−1 below 2b and 1c is 10.51 kcal mol−1 below 2c. The transition states for the reaction 1 → 2 were obtained; the barrier is 87.15 for the parent, 61.01 for the fluorinated system, and 44.24 kcal mol−1 for the chlorinated system. The relative stabilities of the isomers are explained in terms of the strong σ-acceptor properties of the halogens and the electron density distribution. Topological electron density analysis of points along the reaction coordinate for the conversion of 1a into 2a reveal some novel bond path networks.

The Diels-Alder reaction of 1,3,5-triphosphabenzene with phosphaacetylene☆

Abstract The Diels-Alder reaction between 1,3,5-triphosphabenzene and phosphaacetylene to yield tetraphosphabarrelene was examined at MP4SDQ/6-31G ∗ //M2/6-31G ∗ . The results were compared with those for the carbon analogue: benzene + acetylene to give barrelene. The phosphorus reaction is more exothermic than the carbon one, as is expected, as benzene possesses more resonance energy than triphosphabenzene. The activation barrier for the phosphorus reaction is 17.5 kcal mol − , significantly smaller than the barrier in the carbon case (44.6 kcal mol −1 ). This difference is consistent with our previous studies, which suggest a 10 kcal mol −1 decrease in activation energy for each CP involved in the Diels-Alder reaction. Topological electron density analysis suggests that both reactions are synchronous and concerted.

Cycloaddition reactions between 2 H-phosphole and phosphaketene: Ab initio examination of [2 + 2] and [4 + 2] pathways

Abstract Staudinger ([2 + 2]) and Diels—Alder ([4 + 2]) addition reactions between 2 H-phosphole and phosphaketene were examined at the MP4SDQ/6-31G*//MP2/6-31G* + ZPE level. Electronic structures were analyzed with topological electron density and NBO analyses. The Diels—Alder reaction is favored kinetically by 6 kcal mol−1 and thermodynamically by 4 kcal mol−1 over the Staudinger reaction. This is in contrast to the reaction between ketene and cyclopentadiene, for which the Diels—Alder reaction requires a 12 kcal mol−1 higher activation energy than the Staudinger reaction. In both Staudinger and Diels—Alder reactions phosphaketene reacts at P C rather than at C O, as does ketene in the parent Staudinger reaction. The transition structure of the phospha Staudinger reaction is a closed ring, but the second bond is formed employing the phosphaketene phosphorus lone pair rather than the former P C bond. The reaction mechanism is therefore not [π2s + π2a].

Ring strain energy of tetraphosphacubane. An ab initio study.

Abstract The structure of tetraphosphacubane was optimized at the HF/6-31G* level. The small ring strain energy of tetraphosphacubane (62.84 kcal mol −1 is understood in terms of geometric parameters and charge distribution.

Ab initio investigation of the diels‐alder reaction between 2H‐phosphole and phosphaethene: A model for phosphole dimerization

All four possible Diels‐Alder reactions between 2H‐phosphole and phosphaethene were examined at various theoretical levels, including HF, MP4SDQ, CCSD(T), and CASSCF. MP2/6‐31G* geometry optimizations could not be employed since the potential energy surface is qualitatively incorrect at this level of theory, due to the inherent underestimation of the activation energies (ameliorated at higher‐order MP or coupled‐cluster levels). Solvent effects were examined employing the Onsager, polarized continuum, and isodensity and surface polarized continuum models. At MP4SDQ/6‐31G*//HF/6‐31G* these reactions are exothermic by 34–38 kcal mol−1 and have very low activation energies, 5–7 kcal mol−1. The PP/CC regioisomer products are lower in energy than the CP isomers and, within each pair, the exo isomer is lower in energy. At low computational levels the smallest activation energy is for the reaction leading to the CP endo product. Larger basis sets, electron correlation, and solvent favor the transition state leading to the experimentally observed PP/CP endo isomer. The dimerization of phosphole is, therefore, kinetically controlled. Based on geometric and electron density analysis, the reactions are concerted and synchronous.

Proton affinity of SO3

Collision-induced dissociation (CID) of the radical cation H2SO4+ gives the product pairs H2O++SO3 and HO+HSO3+ with a 1:3 ratio that is essentially independent of collision energy. Statistical analysis of the two channels indicates that the proton affinity of HO is 3±4 kJ/mol lower than that of SO3. This can be used to derive PA(SO3)=591±4 kJ/mol at 0 K and 596±4 kJ/mol at 298 K. Previously, Munson and Smith bracketed the proton affinity as PA(HBr)=584 kJ/mol<PA(SO3)<PA(CO)=594 kJ/mol. The threshold of 152±16 kJ/mol for formation of H2O++SO3 indicates that the barrier to CID is small or nonexistent, in contrast to the substantial barriers to decomposition for H3SO4+ and H2SO4.

Ring strain energies of tetraphospha- and tetraarsacubanes

Abstract The ring strain energies of tetraphosphacubane and tetraarsacubane, along with their tetraoxide and tetrasulfide derivatives, were estimated at the MP2/6–31G * and MP2/LANL2DZ levels. The parents exhibit small strain energies, while the strain energy is quite large in the oxides and sulfides. These results are interpreted in terms of the hybridization at the heteroatom.

TOPOLOGICAL ELECTRON DENSITY ANALYSIS OF ORGANOSULFUR COMPOUNDS

Abstract The geometries and topological electron density analysis of 19 organosulfur compounds were determined at the HF/6-31G∗ level. This computational method provides geometries in excellent agreement with experiment. We provide a database of topological electron density values for a variety of CS bonds which can be used for comparisons with other molecules. An exponential relationship between bond order and the value of the electron density at the CS bond critical point is developed and used to investigate the degree of delocalization in prop-2-enethial, thiophene and the conjugate base of thioacetaldehyde.

PREPARATION OF 1,3-DIPHOSHAALLENE FROM 1,2-DIPHOSPHACYCLOPROPANES: A THEORETICAL INVESTIGATION

Abstract The ring opening of diphosphacyclopropane (1a), mono- (1b) and di-fluorodiphosphacyclopropane (1c) with methyllithium to give diphosphaallene is examined at the 3-21G(∗) and 6-31G∗ Hartree-Fock level. In the first step, the diphosphacyclopropane opens to give the stable Li+/diphosphaallyl anion pair. The next step, formation of the phosphaallene, is endothermic unless an ionic salt (LiF) is produced, which can be further stabilized by solvent. The overall reaction energetics are 148.7 kJ Mol−1 for 1a, -169.7 kJ mol−1 for 1b, and - 137.8 kJ mol−1 for 1c. The calculated ring strain energy for 1a is 61.8 kJ mol−1.

Conformations and relative energies of tetrahydropyridines

Abstract The ground state conformations of three tetrahydropyridine isomers (2,3,4,5-tetrahydropyridine 1 , 1,2,3,4-tetrahydropyridine 2 , and 1,2,3,6-tetrahydropyridine 3 ), optimized at MP2/6-31G*, are half chairs. The only conformation found for 2 has the nitrogen lone pair in the axial position in order to conjugate with the π-bond. The relative energies of 1 – 3 are 0.0, 6.39, and 11.9 kcal/mol. The equatorial conformer of 3 lies 0.32 kcal/mol above the axial form.