Chaitanya S. Wannere

Planar, twisted, and trans-bent: conformational flexibility of neutral diborenes.

The potassium graphite reduction of R‘BBr3 (R‘ = :C{N(2,4,6-Me3C6H2)CH}2) in Et2O led to the isolation of 3 (R‘(H)BB(H)R‘) and 4 (R‘(H)2B−B(H)2R‘), with BB double and B−B single bonds, respectively. These compounds were characterized by single-crystal X-ray diffraction, 1H and 11B NMR, and elemental analyses. Neutral diborene 3 exhibits polymorphic planar (3a), twisted (3b), and trans-bent (3c) geometries in the solid state.

A neutral Ga(6) octahedron: synthesis, structure, and aromaticity.

Potassium graphite reduction of L:Ga(Mes)Cl(2) [L: = :C{(i-Pr)NC(Me)}(2), Mes = 2,4,6-Me(3)C(6)H(2)] (1) in hexane yields the organogallium dimer L:(Mes)(Cl)Ga-Ga(Cl)(Mes):L (2), while potassium reduction of 1 in toluene affords the neutral aromatic Ga(6) octahedron L:Ga[Ga(4)Mes(4)]Ga:L (3).

4n π electrons but stable: N,N-dihydrodiazapentacenes

Despite having 4n pi electrons, dihydrodiazapentacenes are more viable than their 4n+2 pi azapentacene counterparts. Ab inito valence bond block-localized wave function (BLW) computations reveal that despite having 4n pi electrons, dihydrodiazapentacenes are stabilized and benefit substantially from four dihydropyrazine ethenamine (enamine) conjugations. Almost all of these dihydrodiazapentacenes have large negative overall nucleus independent chemical shifts NICS(0)(pizz) values even though their dihydropyrazine rings (e.g., for 6-H(2)) are modestly antiaromatic, as their paratropic contributions are attenuated by delocalization throughout the system.

The existence of secondary orbital interactions

B3LYP/6‐311+G** (and MP2/6‐311+G**) computations, performed for a series of Diels‐Alder (DA) reactions, confirm that the endo transition states (TS) and the related Cope‐TSs are favored energetically over the respective exo‐TSs. Likewise, the computed magnetic properties (nucleus‐independent chemical shifts and magnetic susceptibililties) of the endo‐ (as well as the Cope) TSs reveal their greater electron delocalization and greater aromaticity than the exo‐TSs. However, Woodward and Hoffmanns original example is an exception: their endo‐TS model, involving the DA reaction of a syn‐ with an anti‐butadiene (BD), actually is disfavored energetically over the corresponding exo‐TS; magnetic criteria also do not indicate the existence of SOI delocalization in either case. Instead, a strong energetic preference for endo‐TSs due to SOI is found when both BDs are in the syn conformations. This is in accord with Alder and Steins rule of “maximum accumulation of double bonds:” both the dienophile and the diene should have syn conformations. Plots along the IRCs show that the magnetic properties typically are most strongly exalted close to the energetic TS. Because of SOI, all the points along the endo reaction coordinates are more diatropic than along the corresponding exo pathways. We find weak SOI effects to be operative in the endo‐TSs involved in the cycloadditions of cyclic alkenes, cyclopropene, aziridine, cyclobutene, and cyclopentene, with cyclopentadiene. While the endo‐TSs are only slightly lower in energy than the respective exo‐TSs, the magnetic properties of the endo‐TSs are significantly exalted over those for the exo‐TSs and the Natural Bond Orbitals indicate small stabilizing interactions between the methylene cycloalkene hydrogen orbitals (and lone pairs in case of aziridine) with π‐character and the diene π MOs.

The geometry and electronic topology of higher-order charged Möbius annulenes.

Higher-order aromatic charged Möbius-type annulenes have been L(k) realized computationally. These charged species are based on strips with more than one electronic half-twist, as defined by their linking numbers. The B3LYP/6-311+G(d,p) optimized structures and properties of annulene rings with such multiple half-twists (C(12)H(12)(2+), C(12)H(12)(2-), C(14)H(14), C(18)H(18)(2+), C(18)H(18)(2-), C(21)H(21)(+), C(24)H(24)(2-), C(28)H(28)(2+), and C(28)H(28)(2-)) have the nearly equal C-C bond lengths, small dihedral angles around the circuits, stabilization energies, and nucleus-independent chemical shift values associated with aromaticity. The topology and nature of Möbius annulene systems are analyzed in terms of the torus curves defined by electron density functions (rho(r)(pi), ELF(pi)) constructed using only the occupied pi-MOs. The pi-torus subdivides into a torus knot for annulenes defined by an odd linking number (L(k) = 1, 3pi) and a torus link for those with an even linking number (L(k) = 2, 4pi). The torus topology is shown to map onto single canonical pi-MOs only for even values of L(k). Incomplete and misleading descriptions of the topology of pi-electronic Möbius systems with an odd number of half twists result when only signed orbital diagrams are considered, as is often done for the iconic single half twist system.

Strain and reactivity: electrophilic addition of bromine and tribromide salts to cyclic allenes.

The kinetics and the products of the bromination of several cyclic allenes, from C(9) to C(13) (1 a-e), with tetrabutylammonium tribromide (TBAT) and Br(2) have been investigated in 1,2-dichloroethane (DCE) and methanol. The first product of the interaction between the allene and Br(2) is a 1:1 pi complex. The stability constant of this complex, determined at 25 degrees C for allene 1 a, is 7.4 M(-1). The comparison of this value with those reported for several alkenes and alkynes further support the hypothesis of the existence of sizeable structural effects on the stability of these complexes. The negative values of the apparent activation energy for the reaction of allenes 1 a-e with Br(2) in DCE demonstrate the involvement of these complexes as essential intermediates along the reaction coordinate. Different stereochemical behavior was observed in the bromine addition on going from the strained 1,2-cyclononadiene to the larger compounds. Furthermore, a solvent-dependent stereochemistry has been observed for each compound. The kinetic and product distribution data have been interpreted in terms of the influence of the strain on the nature of the intermediate and by considering the competition between pre-association and ion-pair pathways on going from aprotic to nuclophilic solvents or when nucleophilic bromide ions are added. Ab initio (MP2/6-311+G**) and density functional (B3LYP/6-311+G**) computations of 1:1 Br(2) complexes showed that the association energies of allene x Br(2) and ethene x Br(2) complexes are nearly the same but are greater than that of acetylene x Br(2) complexes. Allene x 2 Br(2) complexes are more stable than their ethene x 2 Br(2) counterparts. Br(2) x allene x Br(2) structures, in which the bromine molecules interact either with a single allene double bond or individually with both double bonds, are not preferred significantly over alternatives with Br(2)...Br(2) interactions. As a result of the entropy, the association of bromine with unsaturated hydrocarbons is usually unfavorable in the gas phase (except at extremely low temperatures); complexes are observed in solution (under ambient conditions), since the entropy loss is reduced as a result of restricted translation and rotation and possible association to the solvent. The 1,2-cycloheptadiene x Br(2) > 1,2-cyclononadiene x Br(2) > 1,3-dimethylallene x Br(2) association energies increase with ring strain.

Π and σ-Phenylethynyl Radicals and Their Isomers o-, m-, and p-Ethynylphenyl : Structures, Energetics, and Electron Affinities

Molecular structures, energetics, vibrational frequencies, and electron affinities are predicted for the phenylethynyl radical and its isomers. Electron affinities are computed using density functional theory, -namely, the BHLYP, BLYP, B3LYP, BP86, BPW91, and B3PW91 functionals-, employing the double-zeta plus polarization DZP++ basis set; this level of theory is known to perform well for the computation of electron affinities. Furthermore, ab initio computations employing perturbation theory, coupled cluster with single and double excitations [CCSD], and the inclusion of perturbative triples [CCSD(T)] are performed to determine the relative energies of the isomers. These higher level computations are performed with the correlation consistent family of basis sets cc-pVXZ (X = D, T, Q, 5). Three electronic states are probed for the phenylethynyl radical. In C2v symmetry, the out-of-plane (2B1) radical is predicted to lie about 10 kcal/mol below the in-plane (2B2) radical by DFT methods, which becomes 9.4 kcal/mol with the consideration of the CCSD(T) method. The energy difference between the lowest pi and sigma electronic states of the phenylethynyl radical is also about 10 kcal/mol according to DFT; however, CCSD(T) with the cc-pVQZ basis set shows this energy separation to be just 1.8 kcal/mol. The theoretical electron affinities of the phenylethynyl radical are predicted to be 3.00 eV (B3LYP/DZP++) and 3.03 eV (CCSD(T)/DZP++//MP2/DZP++). The adiabatic electron affinities (EAad) of the three isomers of phenylethynyl, that is, the ortho-, meta-, and para-ethynylphenyl, are predicted to be 1.45, 1.40, and 1.43 eV, respectively. Hence, the phenylethynyl radical binds an electron far more effectively than the three other radicals studied. Thermochemical predictions, such as the bond dissociation energies of the aromatic and ethynyl C-H bonds and the proton affinities of the phenylethynyl and ethynylphenyl anions, are also reported.

The Design of “Neutral” Carbanions with Intramolecular Charge Compensation

Strategies to construct zwitterionic anions from the parent anions are proposed. Two principles are employed; the cationic counterpart is (a) attached as a substituent or (b) inserted as an integral part at a remote location in the assembly. The optimized geometries reveal that a striking similarity exists between the zwitterions and the respective precursor parent anion. The computed vibrational frequencies emphasize that these novel entities are minima on their respective potential energy surfaces. A substantial HOMO-LUMO gap indicates that the proposed structures do not show instability in their respective electronic states and that the higher energy configuration states do not contribute to the ground state viability. The separation of charge between the monopoles in these zwitterions is demonstrated by moderately large nonzero dipole moments. Significant large energy barriers for rearrangement to the closely related positional isomers, demonstrated in a few cases, advocate the thermal stability (associated with spectroscopic viability) of the novel molecules. The donor capacity (basicity) of the anionic subunit in these zwitterions is comparable to that of the respective parent anions. Since the qualitative and quantitative features in the designed charged compensated complexes are conserved as anions, these molecules may perhaps be employed in synthetic organic or organometallic chemistry.