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.

Structural isomerization of the gas-phase 2-norbornyl cation revealed with infrared spectroscopy and computational chemistry.

In an attempt to produce the 2-norbornyl cation (2NB(+)) in the gas phase, protonation of norbornene was accomplished in a pulsed discharge ion source coupled with a supersonic molecular beam. The C7H11(+) cation was size-selected in a time-of-flight mass spectrometer and investigated with infrared laser photodissociation spectroscopy using the method of "tagging" with argon. The resulting vibrational spectrum, containing sharp bands in the C-H stretching and fingerprint regions, was compared to that predicted by computational chemistry. However, the measured spectrum did not match that of 2NB(+), prompting a detailed computational study of other possible isomers of C7H11(+). This study finds five isomers more stable than 2NB(+). The spectrum obtained corresponds to the 1,3-dimethylcyclopentenyl cation, the global minimum-energy structure for C7H11(+), which is produced through an unanticipated ring-opening rearrangement path.

Description of aromaticity in porphyrinoids.

Like the larger nonplanar Möbius rings, porphyrinoid aromaticity is not due primarily to the macrocyclic π conjugation of the corresponding annulene perimeters. The block-localized wave function (BLW)-derived aromatic stabilization energies (ASE) of several porphyrinoids reveal that, on a per atom basis, the appended 6π electron heterocycles of porphyrinoids confer aromaticity much more effectively than the macrocyclic 4n+2 π electron conjugations. There is no direct relationship between thermochemical stability of porphyrinoids and their macrocyclic 4n or 4n+2 π electron counts. Porphyrinoids having an "antiaromatic" macrocyclic 4n+2 π electron conjugation pathway (e.g., 4) as well as those having no macrocyclic conjugation (e.g., 9) can be stabilized by aromaticity. Computed nucleus independent chemical shifts (NICS) and the anisotropy of the induced current density (ACID) disclose the intricate local versus macrocyclic circulation interplay for several porphyrinoids.

Oxidation of carbene-stabilized diarsenic: diarsene dications and diarsenic radical cations.

Oxidation of carbene-stabilized diarsenic, L:As-As:L [L: = :C{N(2,6-(i)Pr(2)C(6)H(3))CH}(2)] (1), with gallium chloride in a 1:4 ratio in toluene affords the dicationic diarsene complex [L:As═As:L](2+)([GaCl(4)](-))(2) (2(2+)[GaCl(4)](2)), while oxidation of 1 with GaCl(3) in a 1:2 ratio in Et(2)O yields the monocationic diarsenic radical complex [L:AsAs:L](•+)[GaCl(4)](-) (2(•+)[GaCl(4)]). Strikingly, complex 2(•+) is the first arsenic radical to be structurally characterized in the solid state. The nature of the bonding in these complexes was probed computationally and spectroscopically.

Aromatic transition states in nonpericyclic reactions: anionic 5-endo cyclizations are aborted sigmatropic shifts.

The transition states (TSs) of 5-endo-dig and 5-endo-trig anionic ring closures are the first unambiguous examples of nonpericyclic reactions with TSs stabilized by aromaticity. Their five-center, six-electron in-plane aromaticity is revealed by the diatropic dissected nucleus-independent chemical shifts, -24.1 and -13.7 ppm, respectively, resulting from the delocalization of the lone pair at the nucleophilic center, a σ CC bond, and an in-plane alkyne (or alkene) π bond. Other seemingly analogous exo and endo cyclization TSs do not have these features. A symmetry-enhanced combination of through-space and through-bond interactions explains the anomalous geometric, energetic, and electronic features of the 5-endo ring closure transition state. Anionic 5-endo cyclizations can be considered to be "aborted" [2,3]-sigmatropic shifts. The connection between anionic cyclizations and sigmatropic shifts offers new possibilities for the design and electronic control of anionic isomerizations.

Carbene-stabilized beryllium borohydride.

The reaction of N-heterocyclic carbene, L:, with BeCl(2) quantitatively yields L:BeCl(2)1 (L: = :C{N(2,6-Pr(i)(2)C(6)H(3))CH}(2)). The carbene-stabilized beryllium borohydride monomer L:Be(BH(4))(2)2 is prepared by the reaction of 1 with LiBH(4). Compound 3, prepared by the reaction of 2 with Na(2)[Fe(CO)(4)]·dioxane, represents an unusual "dual reduction" of the imidazole ring (i.e., hydroboration of the C═C backbone and hydrogenation of the C2 carbene center).

Why are S(n)N4 (n = 1-4) species "missing"? Answers in a broader theoretical context of binary S-N compounds.

Computational investigations of the thermochemical stability and kinetic persistence of binary S(x)N(y) compounds, SN(2), S(2)N(2), S(3)N(2), S(4)N(2), SN(4), S(2)N(4), S(3)N(4), and S(4)N(4), explain why some S(x)N(y) stoichiometries exist but not others. There is no direct link between the Hückel 4n + 2 π-electron count rule and the computed heats of formation (per atom) of the lowest-energy neutral S(n)N(4) (n = 1-4) isomers, but kinetic persistence often is paramount. Thus, the five lowest-energy S(2)N(4) minima at the B3LYP/6-311+G(3df) density functional theory level (A1-A5) all not only have high computed heats of formation [Δ(f)H°(0 K) > 131 kcal/mol or >22 kcal/mol/atom] but also have low dissociation barriers (less than 21.5 kcal/mol for the most favorable pathways). For comparison, the persistent (but potentially explosive!) cyclic S(2)N(2)-c has about the same high heat of formation (per atom) as the least unfavorable S(2)N(4) isomer, but its barrier to ring opening (51 kcal/mol) is much higher. Although aromatic, both SN(4) (6π electron) and S(3)N(4) (10π electron) have low dissociation barriers and, like S(2)N(4), are also absent from the S-N binary family.

A perfectly square-planar tetracoordinated oxygen in a tetracopper cluster-based coordination polymer.

A perfectly square-planar, D(4h)-symmetric, tetracoordinated oxygen has been observed in a tetracopper cluster-based coordination polymer that shows an unusual magnetic moment and ESR signal. The stabilization factor of the square-planar tetracoordinated oxygen has been revealed using DFT calculations.

Cleavage of carbene-stabilized disilicon.

Reaction of carbene-stabilized disilicon L:Si═Si:L (L: = :C{N(2,6-Pr(i)(2)C(6)H(3))CH}(2), 1) with BH(3)·THF results in facile cleavage of the silicon-silicon double bond and the formation of two quite different "push-pull" stabilized products with borane- and carbene-coordinated silylene moieties: 2, containing a parent silylene (:SiH(2)); and 3, containing a unique three-membered cyclosilylene.

A viable anionic N-heterocyclic dicarbene.

The first anionic N-heterocyclic dicarbene, polymeric [:C{[N(2,6-Pr(i)(2)C(6)H(3))](2)CHCLi(THF)}](n) 1, containing both normal (C2) and abnormal carbene (C4) centers in the same five-membered imidazole ring (III), has been prepared by lithiation of the imidazole monocarbene, :C{N(2,6-Pr(i)(2)C(6)H(3))CH}(2). The dicarbene nature of 1 was unambiguously demonstrated by the formation of the group 13 Lewis acid adducts (THF)(2)Li:C{[N(2,6-Pr(i)(2)C(6)H(3))](2)CHC(LA)}, where LA = AlMe(3) [2·(THF)(2)] and BEt(3) [3·(THF)(2)].

Successful computational modeling of isobornyl chloride ion-pair mechanisms.

Along with the directly related Wagner-Meerwein camphene hydrochloride-isobornyl chloride rearrangement, the racemization of isobornyl chloride involves intermediate carbocation-anion ion pairs; both processes have become mechanistic icons in organic chemistry. The two known racemization pathways, involving either a hydride transfer or a methyl migration, are observed to be concurrent. However, prior quantitative computational modeling has not been able to reproduce the fine kinetic balance of these processes. We demonstrate that a density functional approach, which includes two explicit solvent molecules embedded in a continuum solvent field, coupled with full geometric optimization using smoothed solvent cavities and free energy calculation, yields results in accord with experiment. Alternative racemization routes also have been explored.

Electrophile affinity: quantifying reactivity for the bromination of arenes.

Electrophile affinity (Ealpha), a recently proposed theoretical construct based on computed energies of arenium ion formation, rationalizes the substrate reactivity and regioselectivity of S(E)Ar bromination of three sets of available experimental arene data where closely related conditions had been employed uniformly. The Ealpha parameters (computed at B3LYP/6-311+G(2d,2p)) correlated very well (r = 0.987) with the partial rate factors (log f) for 18 regiospecific brominations of benzene and various methyl benzenes. Analysis of the bromination reactivities of 32 mono- and polysubstituted benzenes including various polar groups gave similar results (r = 0.982). The electrophile affinity treatment also accounted satisfactorily (r = 0.957) for bromination reactivities of polybenzenoid hydrocarbons. Conversely, comparisons with NBO-based charges and the electrostatic potential at nuclei (EPN) were not generally successful. The uniform effectiveness of Ealpha treatments for the cases analyzed with regard both to relative substrate reactivity (e.g., benzene vs toluene) and to regiospecificity (e.g., the positional reactivity of toluene) supports the "limiting case" conventional interpretation of the electrophilic aromatic substitution mechanism as being governed by the energy of sigma-complex formation. Although other mechanisms are possible under different conditions, the computed energies of arene-dibromine pi-complex formation for the polysubstituted benzene set examined correlated poorly with experimental reactivity data (r = 0.714) and only varied from 1.8 (for benzene) to 3.5 kcal/mol, in contrast to the 10(12) range in reactivity measured experimentally.

Communications: Infrared spectroscopy of gas phase C3H3+ ions: the cyclopropenyl and propargyl cations.

C(3)H(3)(+) ions produced with a pulsed discharge source and cooled in a supersonic beam are studied with infrared laser photodissociation spectroscopy in the 800-4000 cm(-1) region using the rare gas tagging method. Vibrational bands in the C-H stretching and fingerprint regions confirm the presence of both the cyclopropenyl and propargyl cations. Because there is a high barrier separating these two structures, they are presumed to be produced by different routes in the plasma chemistry; their relative abundance can be adjusted by varying the ion source conditions. Prominent features for the cyclopropenyl species include the asymmetric carbon stretch (nu(5)) at 1293 cm(-1) and the asymmetric C-H stretch (nu(4)) at 3182 cm(-1), whereas propargyl has the CH(2) scissors (nu(4)) at 1445, the C-C triple bond stretch (nu(3)) at 2077 and three C-H stretches (nu(2), nu(9), and nu(1)) at 3004, 3093, and 3238 cm(-1). Density functional theory computations of vibrational spectra for the two isomeric ions with and without the argon tag reproduce the experimental features qualitatively; according to theory the tag atom only perturbs the spectra slightly. Although these data confirm the accepted structural pictures of the cyclopropenyl and propargyl cations, close agreement between theoretical predictions and the measured vibrational band positions and intensities cannot be obtained. Band intensities are influenced by the energy dependence and dynamics of photodissociation, but there appear to be fundamental problems in computed band positions independent of the level of theory employed. These new data provide infrared signatures in the fingerprint region for these prototypical carbocations that may aid in their astrophysical detection.

Modeling dinitrogen activation by lithium: a mechanistic investigation of the cleavage of N2 by stepwise insertion into small lithium clusters.

Because of the inertness of molecular nitrogen, its practicable activation under mild conditions is a fundamental challenge. Nature can do it easily; chemists should be able to achieve comparable success. Lithium is exceptional among the main group elements in that it slowly reacts with N(2) at room temperature, leading finally to (NLi(3))(n), lithium nitride, a product of interest in its own right, because of its potential as a hydrogen storage medium. We explored this remarkably facile dinitrogen activation reaction by using model lithium clusters. Our extensive computations elucidate mechanisms for the ready reactions of N(2) with various model clusters, Li(2), Li(4), Li(6), and Li(8), leading to stepwise cleavage of the NN bond during dinitrogen reduction, N(2)(0) to 2 N(3-). Initial isomeric N(2)-Li(n) complexes, retaining NN triple bonds, undergo cluster insertion/reduction processes over generally low barriers. A minimum of eight lithium atoms are needed to cleave the triple bonded nitrogen completely in a highly exothermic process. Moreover, we provide an explanation for the exceptional reactivity of N(2) with Li, compared to the other alkali metals, e.g., Na and K. Li is a very strong reducing agent as its nitrides have the highest atomization energy, the shortest M-N bond distance, and the largest M-N charge separation as well as interaction energy. Our study delineates the general manner in which molecular nitrogen can be activated sequentially by electron transfer and bond elongation, to give a series of increasingly reduced complexes. We conclude that lithium incorporation into complexes might facilitate the development of nitrogen fixation catalysts.

A hierarchy of homodesmotic reactions for thermochemistry.

Chemical equations that balance bond types and atom hybridization to different degrees are often used in computational thermochemistry, for example, to increase accuracy when lower levels of theory are employed. We expose the widespread confusion over such classes of equations and demonstrate that the two most widely used definitions of "homodesmotic" reactions are not equivalent. New definitions are introduced, and a consistent hierarchy of reaction classes (RC1-RC5) for hydrocarbons is constructed: isogyric (RC1) superset of isodesmic (RC2) superset of hypohomodesmotic (RC3) superset of homodesmotic (RC4) superset of hyperhomodesmotic (RC5). Each of these successively conserves larger molecular fragments. The concept of isodesmic bond separation reactions is generalized to all classes in this hierarchy, providing a unique sectioning of a given molecule for each reaction type. Several ab initio and density functional methods are applied to the bond separation reactions of 38 hydrocarbons containing five or six carbon atoms. RC4 and RC5 reactions provide bond separation enthalpies with errors consistently less than 0.4 kcal mol(-1) across a wide range of theoretical levels, performing significantly better than the other reaction types and far superior to atomization routes. Our recommended bond separation reactions are demonstrated by determining the enthalpies of formation (at 298 K) of 1,3,5-hexatriyne (163.7 +/- 0.4 kcal mol(-1)), 1,3,5,7-octatetrayne (217.5 +/- 0.6 kcal mol(-1)), the larger polyynes C(10)H(2) through C(26)H(2), and an infinite acetylenic carbon chain.

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).

Planar hepta-, octa-, nona-, and decacoordinate first row d-block metals enclosed by boron rings.

Possible planar hypercoordinate molecules with first row d-block metal atoms in the centers of boron rings are explored comprehensively by density-functional theory (DFT) computations. Many optimized MB(n) (n = 7, 8, 9, and 10) neutral and charged clusters have local D(nh) minima, although these may not be the most stable isomers. The larger B(9) and B(10) rings are versatile in accommodating first row d-block metals, whereas the more compact B(8) ring only can enclose smaller transition metals (such as Mn, Fe, and Co) effectively. Delocalized pi and radial molecular orbitals involving boron are crucial in stabilizing these highly symmetrical planar hypercoordinate molecules. Early and middle transition metal d-orbitals participate in metal-boron covalent bonding, whereas partial ionic bonding is more important for the late d-block elements. Potential energy surface scans established several of these species to have planar hypercoordinate global minima: D(8h) FeB(8)(2-) was identified here, and D(8h) CoB(8)(-) and D(9h) FeB(9)(-) were identified in an earlier complementary study.

Ab initio study of the geometry, stability, and aromaticity of the cyclic S2N3(+) cation isomers and their isoelectronic analogues.

A theoretical study of the geometries, energies, dissociation pathways, and aromaticity of the isomeric sulfur-nitrogen S(2)N(3)(+) rings reveals that the experimentally known 1,2-isomer is only stable kinetically. A rather high barrier inhibits its dissociation into the slightly lower energy N(2) and NSS(+) fragments via a stepwise mechanism. A second possible dissociation mode, into NNS and NS(+) via a concerted [3 + 2] mechanism, is endothermic. Instead, the reverse cycloaddition reaction has a low barrier and offers an exothermic route for the formation of cyclic 1,2-S(2)N(3)(+). Despite being thermodynamically more stable, the 1,3-isomer has only fleeting existence: its facile exothermic [3 + 2] cycloreversion into N(2) and SNS(+) fragments precludes observation. Nucleus independent chemical shifts (NICS) analysis reveals considerable six pi electron aromaticity for both cyclic S(2)N(3)(+) isomers, as well as their five membered ring valence isoelectronic analogues, N(5)(-), SN(4), and S(3)N(2)(2+). The decomposition routes and the energetics of these analogues also provide comparisons along the series.

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.

Carbene-stabilized diphosphorus.

The potassium graphite reduction of L:PCl3 leads to the formation of carbene-stabilized diphosphorus molecules, L:P-P:L, 1 (L: = :C{N(2,6-Pri2C6H3)CH}2) and 2 (L: = :C{N(2,4,6-Me3C6H2)CH}2), respectively. The nature of the bonding in 1 and 2 was delineated by DFT computations.