Search

A simple symmetry adapted search algorithm (SASS) exploiting point group symmetry increases the efficiency of systematic explorations of complex quantum mechanical potential energy surfaces. In contrast to previously described stochastic approaches, which do not employ symmetry, candidate structures are generated within simple point groups, such as C2, Cs, and C2v. This facilitates efficient sampling of the 3N-6 Pople's dimensional configuration space and increases the speed and effectiveness of quantum chemical geometry optimizations. Pople's concept of framework groups [J. Am. Chem. Soc. 102, 4615 (1980)] is used to partition the configuration space into structures spanning all possible distributions of sets of symmetry equivalent atoms. This provides an efficient means of computing all structures of a given symmetry with minimum redundancy. This approach also is advantageous for generating initial structures for global optimizations via genetic algorithm and other stochastic global search techniques. Application of the SASS method is illustrated by locating 14 low-lying stationary points on the cc-pwCVDZ ROCCSD(T) potential energy surface of Li5H2. The global minimum structure is identified, along with many unique, nonintuitive, energetically favorable isomers. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The resonance energies (REs) of neutral three membered ring analogs of the cyclopropenyl cation, computed using block localized wave function (BLW) methods, reveal considerable variations. The RE's of cyclopropenes substituted with exocyclic double bonded groups CX, (X = O, NH, CH, S, PH, SiH) increase with the electronegativity of X in the same row (SiH < PH < S and CH < NH < O). The extra cyclic resonance energies (ECREs) (an energetic measure of aromaticity based on comparisons with the RE's of acyclic models) of these derivatives range from +10.5 kcal/mol for cyclopropenone (X = O) (somewhat aromatic; the benzene ECRE is 29.3 kcal/mol) to −2.4 kcal/mol (slightly antiaromatic) for X = SiH. Additional disubstitution of the CC double bond by X′ groups (X′ = CH, NH, OH, SiH, PH, SH) increases the REs considerably, but has only small effects on the ECREs. Even the ECRE of deltic acid (X = O, X′ = OH) is only increased to +13.3 kcal/mol. The conclusion based on ECRE's, that all 12 of the three membered rings are only marginally aromatic/antiaromatic, is supported by the satisfactorily plot ( R = 0.92) of ECRE against values of NICS(0) (a superior nucleus chemical independent shift magnetic index of aromaticity), which range only from −6.1 ppm (diatropic) for deltic acid (cf., −35.5 ppm for benzene and −14.2 ppm for the parent cyclopropenium ion) to +8.9 ppm (paratropic) for the silicon derivative, X = SiH, X′ = SiH. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011 [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Twenty seven new isomers, considerably lower in energy than the two previously reported structures of the novel molecule periodane, LiBeBCNOF, are found theoretically using a stochastic search methodology at the B3LYP/6–31G(d) level of theory. The global minimum is predicted to lie 122 kcal/mol lower in energy than the previously claimed lowest energy structure. The importance of an automated isomer search method to help find isomers on the potential energy surface of molecules in the gas phase is emphasized. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007 [ABSTRACT FROM AUTHOR]

Profiles two experts in the development of the aromaticity concept, Henry Edward Armstrong, professor at Imperial College London, England and Ernest C. Crocker, author of an article about the six aromatic electron structure of benzene, pyridine and five-membered ring heteroaromatics. Works and contributions to the theory of aromaticity; Pre-electronic and electronic theories of aromaticity; Development of a Benzene Model.

Presents a study which investigates the existence of covalent NF5, or the kinetic stability of the nitrogen-fluorine compound. Methods of the study; Results and discussion; Conclusions.

Focuses on stereochemistry, relating to the ring opening of substituted cyclopropylidenes to cyclic allenes, relating to organic chemistry. Details on methodology used; Further information on the cyclopropylidenes and cyclic allenes.

The reaction between benzene and a single ground state carbon atom, C([sup 3]P[sub j]), which yields a C[sub 7]H[sub 5] radical without a barrier in the exit channel, has been studied using density functional theory (B3LYP), Møller-Plesset perturbation theory, and the G2(B3LYP/MP2) and complete basis set (CBS) model chemistries. Comparing the computed reaction energies for the formation of various C[sub 7]H[sub 5] radicals with experimental data suggests that the 1,2-didehydrocycloheptatrienyl radical (15) is observed in crossed-beams experiments at collision energies between 2 and 12 kcal mol[sup -1]. The carbon atom attacks the π-electron density of benzene and forms without entrance barrier a C[sub s] symmetric complex (17T) in which the carbon atom is bound to the edge of benzene. From 17T, the insertion of the C atom into a benzene CC bond to yield triplet cycloheptatrienylidene (9T) is associated with a much lower barrier than the insertion into a CH bond to give triplet phenylcarbene (7T). As both steps are strongly exothermic, high energy vinyl carbene rearrangements on the triplet C[sub 7]H[sub 6] potential energy surface provide pathways between 9T and 7T below the energy of separated reactants. In addition, intersystem crossing in the vicinity of 17T and 9T might give rise to singlet cycloheptatetraene (12S). The monocyclic seven-membered ring compounds 9T or 12S are precursors of the 1,2-didehydrocycloheptatrienyl radical: the dissociation of a CH bond α to the divalent carbon atom proceeds without an exit barrier, in agreement with experiment. In contrast, a direct carbon-hydrogen exchange reaction pathway analogous to the aromatic electrophilic substitution followed by rearrangement of phenylcarbyne (13) to 15 involves high barriers (39 kcal mol-1 with respect to separated reactants) and is thus not viable under the experimental conditions. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

C3H3+ 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 (ν5) at 1293 cm-1 and the asymmetric C–H stretch (ν4) at 3182 cm-1, whereas propargyl has the CH2 scissors (ν4) at 1445, the C–C triple bond stretch (ν3) at 2077 and three C–H stretches (ν2, ν9, and ν1) 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. [ABSTRACT FROM AUTHOR]

C3H5+ cations are probed with infrared photodissociation spectroscopy in the 800–3500 cm-1 region using the method of rare gas tagging. The ions and their complexes with Ar or N2 are produced in a pulsed electric discharge supersonic expansion cluster source. Two structural isomers are characterized, namely, the allyl (CH2CHCH2+) and 2-propenyl (CH3CCH2+) cations. The infrared spectrum of the allyl cation confirms previous theoretical and condensed phase studies of the C2v charge delocalized, resonance-stabilized structure. The 2-propenyl cation spectrum is consistent with a Cs symmetry structure having a nearly linear CCC backbone and a hyperconjugatively stabilizing methyl group. [ABSTRACT FROM AUTHOR]

Large coupled cluster computations utilizing the Dunning weighted correlation-consistent polarized core-valence (cc-pwCVXZ) hierarchy of basis sets have been conducted, resulting in a panoply of internally consistent geometries and atomization energies for small Li[sub n] and Li[sub n]H (n=1–4) clusters. In contrast to previous ab initio results, we predict a monotonic increase in atomization energies per atom with increasing cluster size for lithium clusters, in accordance with the historical Knudsen-effusion measurements of Wu. For hydrogenated lithium clusters, our results support previous theoretical work concerning the relatively low atomization energy per atom for Li[sub 2]H compared to LiH and Li[sub 3]H. The CCSD(T)/cc-pwCVQZ atomization energies for LiH, Li[sub 2]H, Li[sub 3]H, and the most stable isomer of Li[sub 4]H, including zero-point energy corrections, are 55.7, 79.6, 113.0, and 130.6 kcal/mol, respectively. The latter results are not consistent with the most recent experiments of Wu.© 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

This article presents a study on the structural trends in transition metal cation-acetylene complexes revealed through the C--H stretching fundamentals. Gas-phase ion complexes can be studied in isolation, revealing individual molecular properties. Mass spectrometry provides bond energies and reactivities of metal ion-acetylene complexes. Electronic spectroscopy has probed excited states. Geometries and electronic structures have been investigated theoretically. IR spectroscopy is a key tool in organometallic chemistry, but the low concentration of gas-phase ions precludes conventional absorption spectroscopy.

A correction notice is presented related to "A Annulene Isomer May Be Aromatic, After All" which published in previous issue of the periodical.

The aromatic characterof fused polycyclic systems varies withthe nature of their annulated rings. Computed extra cyclic resonanceenergies (ECREs) reveal that the central six membered rings (6MRs)of the heterocyclic fused congeners 1–5are “[6]radialene-like”, but that the central 6MRsof triphenylene 9, coronene 10, and isocoronene 11are “benzene-like.” Comparisons with geometric(harmonic oscillator model of aromaticity, HOMA) and magnetic (nucleusindependent chemical shifts, NICS) criteria illustrate the multifacetednature of aromaticity in 1–11. [ABSTRACT FROM AUTHOR]

Like the larger nonplanar Möbius rings, porphyrinoid aromaticity is not due primarily to the macrocyclic p 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 6p 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. [ABSTRACT FROM AUTHOR]

Possible planar hypercoordinate molecules with first row d-block meta] atoms in the centers of boron rings are explored comprehensively by density-functional theory (DFT) computations. Many optimized MBn (n = 7,8,9, and 10) neutral and charged dusters have local Dnh, minima, although these may not be the most stable isomers. The larger B9 and B10 rings are versatile in accommodating first row d-block metals, whereas the more compact B8 ring only can enclose smaller transition metals (such as Mn, Fe, and Co) effectively. Delocalized π 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 valent 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 D8h FeB82- was identified here, and D8h CoB8- and D9h FeB9- were identified in an earlier complementary study. [ABSTRACT FROM AUTHOR]

The synthesis and X-ray characterization of two new dialkynylated diazatetracenes and the corresponding N,N-dihydrodiazatetracenes are reported. The dialkynylated heteroacenes are packed in a brick-wall motif that enforces significant overlap of their π-faces. Cyclic voltammetry indicates that the dehydrogenated forms are easily reduced to their radical anions in solution. The planarity of these species validates the discussion of their aromaticity. Nucleus Independent Chemical Shift (NICS) computations demonstrate that both of these 20 π and 24 π electron systems are aromatic. Both experimental and computational results suggest that the aromaticity of the dihydroheteroacenes is reduced. [ABSTRACT FROM AUTHOR]

Nickel cation-acetylene complexes of the form Ni+(C2H2)n, Ni+(C2H2)Ne, and Ni+(C2H2)nArm (n = 1-4) are produced in a molecular beam by pulsed laser vaporization. These ions are size-selected and studied in a time-of-flight mass spectrometer by infrared laser photodissociation spectroscopy in the C-H stretch region. The fragmentation patterns indicate that the coordination number is 4 for this system. The n = 1-4 complexes with and without rare gas atoms are also investigated with density functional theory. The combined IR spectra and theory show that π-complexes are formed for the n = 1-4 species, causing the C-H stretches in the acetylene ligands to shift to lower frequencies. Theory reveals that there are low-lying excited states nearly degenerate with the ground state for all the Ni+(C2H2)n complexes. Although isomeric structures are identified for rare gas atom binding at different sites, the attachment of rare gas atoms results in only minor perturbations on the structures and spectra for all complexes. Experiment and theory agree that multiple acetylene binding takes place to form low-symmetry structures, presumably due to Jahn-Teller distortion and/or ligand steric effects. The fully coordinated Ni+(C2H2)4 complex has a near-tetrahedral structure. [ABSTRACT FROM AUTHOR]

N2S2 is a four-membered-ring system with 6π electrons. While earlier proposals considered N2S2 to be aromatic, recent electronic structure calculations claimed that N2S2 is a singlet diradical. Our careful reexamination does not support this assertion. N2S2 is closed shell and aromatic since it satisfies all three generally accepted criteria for aromaticity: energetic (stability), structural (planarity with equal bond lengths), and magnetic (negative nucleus-independent chemical shift due to the π electrons). These characteristics as well as the electronic structure of N2S2 are compared with those for an isoelectronic π system, Li2C2H2, motivated by theoretical and recent experimental investigations that confirmed its aromaticity. However, N2S2 and Li2C4H4 are both essentially 2π-electron aromatic systems with a formal N-S (C-C) bond order of 1.25 even though they both have 6π electrons. This is because four of the six π electrons occupy the nonbonding π HOMOs and only two electrons participate effectively in the aromatic stabilization. However, wave function analysis shows relatively large LUMO occupation numbers; this antibonding effect can be said to reduce the aromatic character by approximately 7% and 4% for N2S2 and Li2C4H4, respectively. [ABSTRACT FROM AUTHOR]

2-Pyridinethione (2-mercaptopyridine, H-2mp) undergoes rapid oxidative addition with 2 mol of the 17-electron organometallic radical [sup˙]Cr(CO)[sub3]Cp* (where Cp* = C[sub5]Me[sub5]), yielding hydride H-Cr(CO)[sub3]Cp* and thiolate (η[sup1]-2mp)Cr(CO)[sub3]Cp*. In a slower secondary reaction, (η[sup1]-2mp)Cr(CO)[sub3]Cp* loses CO generating the N,S-chelate complex (η[sup2]-2mp)Cr(CO)[sub2]Cp* for which the crystal structure is reported. The rate of 2-pyridine thione oxidative addition with [sup˙]Cr(CO)[sub3]Cp* (abbreviated [sup˙]Cr) in toluene best fits rate = K[subobs][H-2mp][[sub˙]Cr]; K[subobs](288 K) = 22 ± 4 M[sup-1] s[sup-1]; ΔH[sup‡] = 4 ± 1 kcal/mol; ΔS[sup‡] = - 40 ± 5 cal/mol K. The rate of reaction is the same under CO or Ar, and the reaction of deuterated 2-pyridine thione (D-2mp) shows a negligible (inverse) kinetic isotope effect (k[subD]/k[subH] = 1.06 ± 0.10). The rate of decarbonylation of (η[sup1]-2mp)Cr(CO)[sub3]Cp* forming (η[sup2]-2mp)Cr(CO)[sub2]Cp* obeys simple first- order kinetics with k[subobs] (288 K) = 3.1 × 10[sup-4] s[sup-1], ΔH[sup‡] = 23 ± 1 kcal/mol, and ΔS[sup‡] = + 5.0 ± 2 cal/mol K. Reaction of 4-pyridine thione (4-mercaptopyridine, H-4mp) with [sup˙]Cr(CO)[sub3]Cp* in THF and CH[sub2]Cl[sub2] also follows second-order kinetics and is approximately 2-5 times faster than H-2mp in the same solvents. The relatively rapid nature of the thione versus thiol reactions is attributed to differences in the proposed 19-electron intermediate complexes, [[sup˙](S=C[sub5]H[sub4]N--H)Cr(CO)[sub3]Cp*] versus [[sup˙](H--S--C[sub6]H[sub5])Cr(CO)[sub3]Cp*. In comparison, reactions of pyridyl disulfides occur by a mechanism similar to that followed by aryl disulfides involving direct attack of the sulfur-sulfur bond by the metal radical. [ABSTRACT FROM AUTHOR]

Evaluates the aromaticity and antiaromaticity of oligocyclic annelated five-membered ring systems, and related compounds. Identification of the five-membered ring systems; Observation of pentalene upon matrix isolation at -196 degrees Celsius; Demonstration of the existence of the highly strained acepentalene with 10pi electrons.

The article presents information on perfect graphene sheets and carbon nanotubes are known to have high chemical stability and unique physical properties resulting from the strong &b.pi; interactions of their hexagonal networks. It is generally accepted that these defective sites are chemically more reactive than the perfect sites in graphenes and carbon nanotubes. Indeed, a number of recent theoretical investigations revealed the higher chemical reactivity of Stone-Wales defects than that of PS in the sidewalls of zigzag single-wall carbon nanotubes (SWNTs) and planar graphenes polycyclic graphite fragments.

New insights into [18]annulene were gained by looking more closely at its X-ray structure, revealing a close face-to-face stacking of 3.16 Å in a herringbone-like crystal packing. Hexadehydro[18]annulene was co-crystalized in a benzene matrix, demonstrating the stabilizing role of intercalated solvent molecules in solid annulenes. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Anti-aromatic compounds, as well as small cyclic alkynes or carbynes, are particularly challenging synthetic goals. The combination of their destabilizing features hinders attempts to prepare molecules such as pentalyne, an 8π-electron anti-aromatic bicycle with extremely high ring strain. Here we describe the facile synthesis of osmapentalyne derivatives that are thermally viable, despite containing the smallest angles observed so far at a carbyne carbon. The compounds are characterized using X-ray crystallography, and their computed energies and magnetic properties reveal aromatic character. Hence, the incorporation of the osmium centre not only reduces the ring strain of the parent pentalyne, but also converts its Hückel anti-aromaticity into Craig-type Möbius aromaticity in the metallapentalynes. The concept of aromaticity is thus extended to five-membered rings containing a metal-carbon triple bond. Moreover, these metal-aromatic compounds exhibit unusual optical effects such as near-infrared photoluminescence with particularly large Stokes shifts, long lifetimes and aggregation enhancement. [ABSTRACT FROM AUTHOR]

Computational investigations of the thermochemical stability and kinetic persistence of binary SxNy compounds, SN2, S2N2, S3N2, S4N2, SN4, S2N4, S3N4, and S4N4, explain why some SxNy stoichiometrics 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 SnN4 (n = 1-4) isomers, but kinetic persistence often is paramount. Thus, the five lowest-energy S2N4 minima at the B3LYP/6-311+G(3df) density functional theory level (A1--A5) all not only have high computed heats of formation [ΔρH°(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 S2N2-c has about the same high heat of formation (per atom) as the least unfavorable S2N4 isomer, but its barrier to ring opening (51 kcal/mol) is much higher. Although aromatic, both SN4 (6π electron) and S3N4 (10&960; electron) have low dissociation barriers and, like S2N4 are also absent from the S--N binary family. [ABSTRACT FROM AUTHOR]

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 cycizations 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. [ABSTRACT FROM AUTHOR]

The geometries, energies, and electronic properties of possible configurations of Meisenheimer-Wheland (M-W) complexes of sym-triaminobenzenes and 4,6-dinitrobenzofuroxan (DNBF) were investigated theoretically by MP2 and a variety of DFT methods. The p-p complex is preferred thermodynamically by more than 15 kcal/mol over the s-complexes for the unsubstituted species. However, the N-substituents of the 1,3,5-triaminobenzenes influence the relative stabilities of the alternative configurations significantly. The s-syn configuration of the M-W complex of 1,3,5-tris(N-piperidyl)benzene and DNBF has the lowest energy, followed closely by the s-anti and p-p forms. The small energy differences between different configurations are consistent with the dynamic interconversion of three homomeric structures observed experimentally by NMR. The ca. 1.63 Å C-C interring bond exchanges among three equivalent sites. Quantum theory of atoms in molecules (QTAIM) analysis provided insights into the nature of the intermonomer interactions. Charge transfer and s bonding account for the stability and remarkably large binding energies of the M-W complexes. [ABSTRACT FROM AUTHOR]

Electrophile affinity (Eα), a recently proposed theoretical construct based on computed energies of arenium ion formation, rationalizes the substrate reactivity and regioselectivity of SEAr bromination of three sets of available experimental arene data where closely related conditions had been employed uniformly. The Ea 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 monoand 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 Ect 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 π-complex formation. Although other mechanisms are possible under different conditions, the computed energies of arene-dibromine ar-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 1012 range in reactivity measured experimentally. [ABSTRACT FROM AUTHOR]

A theoretical study of the geometries, energies, dissociation pathways, and aromaticity of the isomeric sulfur−nitrogen S2N3+ 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 N2 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-S2N3+. Despite being thermodynamically more stable, the 1,3-isomer has only fleeting existence: its facile exothermic [3 + 2] cycloreversion into N2 and SNS+ fragments precludes observation. Nucleus independent chemical shifts (NICS) analysis reveals considerable six π electron aromaticity for both cyclic S2N3+ isomers, as well as their five membered ring valence isoelectronic analogues, N5−, SN4, and S3N22+. The decomposition routes and the energetics of these analogues also provide comparisons along the series. The stability of the 1,2-S2N3+ (experimental known) and its 1,3-isomer (experimental absence) has been investigated by exploring the potential energy surface. Being "thermodynamically more stable" does not necessarily mean that A also is more "kinetically persistent"; such phrases must be used with caution in the scientific context. [ABSTRACT FROM AUTHOR]

Sets of boron rings enclosing planar hypercoordinate group 14 elements (ABn(n-8); A = group 14 element; n = 6-10) are designed systematically based on geometrical and electronic fit principles: the size of a boron ring must accommodate the central atom comfortably. The electronic structures of the planar minima with hypercoordinate group 14 elements are doubly aromatic with six π and six in-plane radial MO systems (radial MOs are comprised of boron p orbitals pointing toward the ring center). This is confirmed by induced magnetic field and nucleus-independent chemical shift (NICS) computations. The weakness of the "partial" A—B bonds is compensated by their unusually large number. Although a C7v, pyramidal SiB8 structure is more stable than the D8h isomer, Born—Oppenheimer molecular dynamics simulations show the resistance of the D8h local minimum against deformation and isomerization. Such evidence of the viability of the boron ring minima with group 14 elements encourages experimental realization. [ABSTRACT FROM AUTHOR]

The geometries and relative stabilities of the singlet and triplet states of phenyl- (Cs), diphenyl- (C2), 1-naphthyl- (Cs), di(1-naphthyl)- (C2), and 9-anthryl-substituted (Cs) carbenes were investigated at the B3LYP/6-311+G(d,p) + ZPVE level of density functional theory. The singlet-triplet energy separations (ΔEST), 2.7, 2.9, 3.4, 3.7, and 5.7 kcal/mol, respectively, after including an empirical correction (2.8 kcal/mol) based on the error in the computed singlet-triplet gap for methylene versus experiment, are in good agreement with available experimental values. Consistent with literature reports, triplet di(9-anthryl)carbene has a linear, D2d symmetrical, allene structure with 1.336 Å CC bond lengths and considerable biradical character. B3LYP favors such cumulene biradical structures and triplet spin states and predicts a large (>15 kcal/mol) ‘di(9-anthryl)carbene’ singlet-triplet (biradical) energy gap. The resonance stabilization of both singlet and triplet carbenes increases modestly with the size of the arene substituent and overall, (di)arylcarbenes, both singlet and triplet, are better stabilized by bigger substituents. For example, methylene is stabilized more by a naphthyl than a phenyl group (singlets, 26.6 versus 24.4; and triplets, 20.9 versus 18.1 kcal/mol, respectively). The carbene geometries are affected by both steric effects and arene-carbene orbital interactions (σ-p and p-π). For instance, the central angles at the carbene are widened by a second arene group, which leads to increased s-character and shorter carbene bond lengths (i.e., CÄ-C, CÄ-H). In general, the aromaticity of the substituted rings in triplet carbenes is most affected by the presence of the unpaired electrons. [ABSTRACT FROM AUTHOR]

Planar tetracoordinate carbons (ptC's) can be stabilized by four-membered ring perimeters composed of four bare transition metal atoms. DFT analyses of the molecular orbitals, electronic structures, energies, and magnetic properties of these CM4 species (where M represents isoelectronic combinations of Cu, Ni, Ag, and Pd) reveal striking similarities with main group metal ptC analogues (e.g., CAl2Si2, CAl4Na-, and C5Li2). While the CCu42+, CAg42+ and CNiCu3+ ions have the largest HOMO-LUMO separations, CCu42+ is the best candidate for detection by gas-phase photoelectron spectroscopy. [ABSTRACT FROM AUTHOR]

In an important recent report, Coles and Hitchcock (Chem. Commun. 21)05, 3165) have synthesized a unique compound of the form Li9O2(hpp)7, with an O2 unit encapsulated in the Li cluster. The group hpp is the bicyclic guanidinate 1,3,4,6,7,8-hexahydropyrimido[1,2-a]pyrimidinate. Coles and Hitchcock raise the interesting possibility that the trapped species is the O2 dianion. We have theoretically investigated (with full geometry optimization) the structural, vibrational, and electronic properties of this fascinating system with 165 atoms. Every indication supports the qualitative conclusion that this system is best represented as an O2 dianion embedded in a Li cluster. Several smaller model systems, including replacement of the hpp ligands by CI atoms, are also examined and provide some interesting comparisons. [ABSTRACT FROM AUTHOR]

The first quantitative evidence for the viability of three-dimensional aromatic clusters involving d-orbitals in pseudo-octahedral coinage metal cages M6Li2 (M = Cu, Ag, Au) as well as in tetrahedral coinage metal cages M′4Li4 (M′ = Cu, Ag) was obtained computationally. These cages exhibit many features similar to those of their square planar M4Li2 analogues. The large negative nucleus-independent chemical shifts (NICS) at the cage centers indicate three-dimensional delocalization. This diatropic character arises mostly from d-orbital delocalization combined with substantial contributions from the lowest-valence orbitals. The bonding molecular orbitals of the pseudo-octahedral clusters M6Li2 (M = Cu, Ag, Au) are analogous to those in similar octahedral clusters involving p-orbital delocalization (e.g., B6H62-). The M′4Li4 clusters exhibit two isomeric forms: metal tetrahedral cages tetracapped by lithium cations on the outside [(M′4)·4Li] and lithium tetrahedra on the inside capped by coinage metal atoms on each of the four faces [(Li4)·4M]. Whereas the (M′4)·4Li type structure is preferred for copper, gold and silver favor the (Li4)·4M arrangement. NBO-NICS analysis shows that the large diatropic character in (M′4)·4Li structures is due to the favorable contribution from both s- and d-orbitals, whereas the small NICS values in the center of (Li4)·4M are due only to the diatropic contributions from the s-orbitals. [ABSTRACT FROM AUTHOR]

The adiabatic electron affinities (AEAs), vertical electron affinities (VEAs), and vertical detachment energies (VDEs) of cyclic perfluoroalkanes, c-CnF2n (n = 3-7), and their monotrifluoromethyl derivatives were computed using various pure and hybrid density functionals with DZP++ (polarization and diffuse function augmented double-ζ) basis sets. The theoretical AEA of c-C4F8 at KMLYP/DZP++ is 0.70 eV, which exhibits satisfactory agreement with the 0.63 ± 0.05 eV experimental value. The nonzero-point-corrected AEA of c-C4F8 is predicted to be 0.41 eV at the CCSD(T)/aug-cc-pVTZ//MP2/aug-cc-pVTZ level of theory, which shows a slight deviation of 0.11 eV from the KMLYP estimated value of 0.52 eV for the same. With the zero-point correction from the MP2/6-311G(d) [Gallup, G. A. Chem. Phys. Lett 2004, 399, 206] level of theory combined with the CCSD(T)/aug-cc-pVTZ//MP2/aug-cc-pVTZ result, the most reliable estimate of AEA of c-C4F8 is 0.60 eV. c-C3F6(-), c-C4F8(-), and c-C5F10(-) are unusual in preferring planar to near planar ring structures. The ZPE-corrected AEAs of c-CnF2n increase from n = 3 (0.24 eV) to n = 5 (0.77 eV), but then dramatically fall off to 0.40 eV for both n = 6 and n = 7. All of the other functionals predict the same trend. This is due to a change in the structural preference: Cs c-C6F12(-) and C1 c-C7F14- (-) are predicted to favor nonplanar rings, each with an exceptionally long C-F bond. (There also is a second, higher energy D3d minimum for C6F12(-).) The SOMOs as well as the spin density plots of the c-PFA radical anions reveal that the "extra" electron is largely localized on the unique F atoms in the larger n = 6 and n = 7 rings but is delocalized in the multiatom SOMOs of the three- to five-membered ring radical anions. The computed AEAs are much larger than the corresponding VEAs; the latter are not consistent with different functionals. The AEAs are substantially larger when a c-CnF2n fluorine is replaced by a -CF3 group. This behavior is general; PFAs with tertiary C-F bonds have large AEAs. The VDEs for all the anions are substantial, ranging from 1.89 to 3.64 eV at the KMLVP/DZP++ level. [ABSTRACT FROM AUTHOR]

The adiabatic electron affinity (EAad) of the CH3-C≡Ċ radical [experiment = 2.718 ± 0.008 eV] and the gas-phase basicity of the CH3-C:-C:- anion [experiment = 373.4 ± 2 kcal/mol] have been compared with those of their fluorine derivatives. The latter are studied using theoretical methods. It is found that there are large effects on the electron affinities and gas-phase basicities as the H atoms of the α-CH3 group in the propynyl system are substituted by F atoms. The predicted electron affinities are 3.31 eV (FCH2-C≡Ċ), 3.86 eV (F2CH-C≡Ċ), and 4.24 eV (F3C-C≡Ċ), and the predicted gas-phase basicities of the fluorocarbanion derivatives are 366.4 kcal/mol (FCH2-ĊC:-), 356.6 kcal/mol (F2CH-C≡C:-), and 349.8 kcal/mol (F3C-C≡C:-). It is concluded that the electron affinities of fluoropropynyl radicals increase and the gas-phase basicities decrease as F atoms sequentially replace H atoms of the α-CH3 in the propynyl system. The propargyl radicals, lower in energy than the isomeric propynyl radicals, are also examined and their electron affinities are predicted to be 0.98 eV (̇CH2-C≡CH), 1.18 eV (̇CFH-C≡CH), 1.32 eV (̇CF2-C≡CH), 1.71 eV (̇CH2-C≡CF), 2.05 eV (̇CFH-C≡CF), and 2.23 eV (̇CF2-C≡CF). [ABSTRACT FROM AUTHOR]

Quantitative evidence for the existence of aromaticity involving the d orbitals of transition metals is provided for the first time. The doubly bridged square planar (D4h) coinage metal clusters (M4Li4, M = Cu (1), Ag (2), and Au (3)) are characterized as aromatic by their substantial nucleus independent chemical shifts (NICS) values in the centers (-14.5, -14.1, and -18.6, respectively). Nevertheless, the participation of p orbitals in the bonding (and cyclic electron delocalization) of 1-3 is negligible. Instead, these clusters benefit strongly from the delocalization of d and to some extent s orbitals. The same conclusion applies to Tsipis and Tsipis' H-bridged D4h Cu4H4 ring (4). Canonical MO-NICS analysis of structures 1-3 shows the total diatropic d orbital contributions to the total NICS to be substantial, although the individual contributions of the five sets of filled d orbitals vary. The d orbital aromaticity of Cu4Li2 also is indicated by its atomization energy, 243.2 kcal/mol, which is larger than Boldyrev's doubly (σ and π) aromatic AI4Li2 (215.9 kcal/mol). [ABSTRACT FROM AUTHOR]

Alkyl derivatives of the ethynyl radical (•C2H) have been studied using density functional theory (DFT) with DZP++ basis sets. Adiabatic electron affinities (EAad) and ZPVE-corrected electron affinities for the alkylethynyl series •C≡C— CnH2n+1 (n=1–4) have been computed using six different DFT functionals, i.e., BHLYP, BLYP, B3LYP, BP86, BPW91 and B3PW91. These methods have been carefully calibrated for the prediction of electron affinities [J.C. Rienstra-Kiracofe, G.S. Tschumper, H.F. Schaefer, S. Nandi, G.B. Ellison, Chem. Rev. 102 (2002) 231]. The electron affinity of •C2H (2.969±0.006eV) has been compared with that of the alkylethynyl series with an attempt to determine the effect of the alkyl chain length on the electron affinities of the acetylenic species. The predicted electron affinities are 2.70eV (— C2CH3, experiment=2.718±0.008eV), 2.74eV (— C2C2H5), 2.75eV (•C2n-C3H7), and 2.75eV (•C2n-C4H9). [Copyright &y& Elsevier]

The molecular structures, relative energies, vibrational frequencies, and electron affinities for the Cl2On/Cl2O−n (n = 1–4) systems have been investigated using hybrid Hartree–Fock/density functional theories (BHLYP and B3LYP) and pure density functional theories (BP86 and BLYP). The three different types of neutral/anion energy differences reported in this research are the adiabatic electron affinity (EAad), the vertical electron affinity (EAvret), and the vertical detachment energy (VDE). The basis set used in this work is of double-ζ plus polarization quality with additional s- and p-type diffuse functions, and it is denoted DZP++. The geometries are fully optimized by all four DFT methods. We have predicted a number of possible low-lying local minima, including some that are unprecedented. Most strikingly, in several cases structures that have been observed in the laboratory turn out not to be lowest in energy for a particular chemical composition. Two structures are predicted with a formally seven-coordinate chlorine atom. The global minima for the Cl2On/Cl2O−n (n = 1–4) systems are ClOCl (C2v), ClClO− (Cs), ClOOCl (C2), OOCl−2 (C2v), ClOOOCl (C2/C1), ClO(O2)Cl− (Cs), trans-ClO(O2)OCl (Ci), and trans-ClO(O2)OCl− (Ci), respectively. The relative energies of the different minima are reported. Five of the 42 structures predicted here have been determined experimentally. Our theoretical geometries and vibrational frequencies are carefully compared with the limited available experimental results, and the BHLYP functional in general provides the best agreement. The ClClO− and ClClO−2 ground states might be regarded as Cl− … ClO and Cl− … ClO2 complexes, respectively. The adiabatic electron affinities, obtained at the favored DZP++ BLYP level of theory, are 3.12 eV for Cl2O, 3.96 eV for Cl2O2, 3.66 eV for Cl2O3, and 4.15 eV for Cl2O4. The adiabatic EAs for the first three systems are similar to those of Br2On (3.14, 3.80, and 3.46 eV with BLYP for Br2On, n = 1–3), and this may reflect the geometric similarity between these bromine and chlorine species. But, EAad for Br2O4 (1.97 eV) is different from Cl2O4 because the neutral and anionic bromine species have different global minimum geometries from those for Cl2O4. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2003 [ABSTRACT FROM AUTHOR]

The geometries of four different series of D[sub 6h]-symmetric polybenzenoid hydrocarbons (PBH) up to and including C[sub 222]H[sub 42] have been optimized at the B3LYP/6-31G(d) level of theory. Excluding C[sub 48]H[sub 24] and C[sub 138]H[sub 42], which have D[sub 3d] minima due to 1,5 H···H repulsions between adjacent perimeter rings, optimized geometries are planar D[sub 6h] minima. Nucleus Independent Chemical Shifts (NICS), at the same level, indicate the presence of individual aromatic rings, which correspond to Clar's qualitative sextets rule (Clar, E. The Aromatic Sextet; Wiley: London, 1972). NICS and the Clar valence electron topologies agree perfectly in the molecule plane; however, the NICS values computed in parallel planes further away from the molecular surface converge, indicating the presence of a uniform magnetic shielding field. For each series, PBH total NICS values (i.e., the sum of NICS values for all rings in the PBH) correlate linearly with the number of carbon atoms, indicating constant magnetic field development within a series. The C-C lengths depend on their proximity to the more olefinic rich molecular perimeters. However, the large PBH (≥C[sub 48]H[sub 24]) internal C-C distances converge to ∼1.426 Å. In agreement with Clar's rule, HF/6-31G(d)//B3LYP/6-31G(d) vertical ionization potentials and B3LYP/6-31G(d) HOMO-LUMO gaps are largest within the "fully benzenoid" series, where all carbon atoms are members of a single sextet. The largest members of the four series studied are predicted to exhibit semiconducting properties. [ABSTRACT FROM AUTHOR]

Presents information on the synthesis of two eleven-vertex azadicarbaboranes. Interest in the chemistry of eleven-vertex borane-cluster compound; Information on nido compounds; Structure of the eleven-vertex; Methodology and results of the study.

Presents a study which employed the use of the Hartee-Fock/density functional theory, second-order perturbation theory and the coupled-cluster method to examine the benzene potential energy hyperface. Mechanism for the topomerization of benzene; Thermochemical corrections and thermal topomerization of benzene; Details on theories used in study.

Presents a study to stimulate and interpret the nature of gallium, utilizing the density functional quantum methods. Details on methodology used to conduct study; Indication of results; Discussion on findings.

Reaction of carbene-stabilized diphosphorus, L:P–P:L (L: = :C{N(2,6-Pri2C6H3)CH}2, 1), with excess BH3·THF affords the boronium salt [L:P(μ-BH2)P:L]+[B2H7]−, 2, which contains a three-membered P2B ring. When 2is dissolved in THF, compounds 1and 2exist in a dynamic solution equilibrium. [ABSTRACT FROM AUTHOR]

A perfectly square-planar, D4h-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. [ABSTRACT FROM AUTHOR]

The first anionic N-heterocyclic dicarbene, polymeric [:C{[N(2,6-Pri2C6H3)]2CHCLi(THF)}]n1, 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-Pri2C6H3)CH}2. The dicar bene nature of 1 was unambiguously demonstrated by the formation of the group 13 Lewis acid adducts (THF)2Li:C{[N(2,6-Pri2C6H3)]2CHC(LA)}, where LA = AIMe3 [2·(THF)2] and BEt3[3·(THF)2]. [ABSTRACT FROM AUTHOR]