Favored Isomer Research Articles

Laser flash photolysis generation of high-valent transition metal-oxo species: insights from kinetic studies in real time.

High-valenttransition metal-oxo species are active oxidizing species in many metal-catalyzed oxidation reactions in both Nature and the laboratory. In homogeneous catalytic oxidations, a transition metal catalyst is oxidized to a metal-oxo species by a sacrificial oxidant, and the activated transition metal-oxo intermediate oxidizes substrates. Mechanistic studies of these oxidizing species can provide insights for understanding commercially important catalytic oxidations and the oxidants in cytochrome P450 enzymes. In many cases, however, the transition metal oxidants are so reactive that they do not accumulate to detectable levels in mixing experiments, which have millisecond mixing times, and successful generation and direct spectroscopic characterization of these highly reactive transients remain a considerable challenge. Our strategy for understanding homogeneous catalysis intermediates employs photochemical generation of the transients with spectroscopic detection on time scales as short as nanoseconds and direct kinetic studies of their reactions with substrates by laser flash photolysis (LFP) methods. This Account describes studies of high-valent manganese- and iron-oxo intermediates. Irradiation of porphyrin-manganese(III) nitrates and chlorates or corrole-manganese(IV) chlorates resulted in homolytic cleavage of the O-X bonds in the ligands, whereas irradiation of porphyrin-manganese(III) perchlorates resulted in heterolytic cleavage of O-Cl bonds to give porphyrin-manganese(V)-oxo cations. Similar reactions of corrole- and porphyrin-iron(IV) complexes gave highly reactive transients that were tentatively identified as macrocyclic ligand-iron(V)-oxo species. Kinetic studies demonstrated high reactivity of the manganese(V)-oxo species, and even higher reactivities of the putative iron(V)-oxo transients. For example, second-order rate constants for oxidations of cis-cyclooctene at room temperature were 6 x 10(3) M(-1) s(-1) for a corrole-iron(V)-oxo species and 1.6 x 10(6) M(-1) s(-1) for the putative tetramesitylporphyrin-iron(V)-oxo perchlorate species. The latter rate constant is 25,000 times larger than that for oxidation of cis-cyclooctene by iron(IV)-oxo perchlorate tetramesitylporphyrin radical cation, which is the thermodynamically favored electronic isomer of the putative iron(V)-oxo species. The LFP-determined rate constants can be used to implicate the transient oxidants in catalytic reactions under turnover conditions where high-valent species are not observable. Similarly, the observed reactivities of the putative porphyrin-iron(V)-oxo species might explain the unusually high reactivity of oxidants produced in the cytochrome P450 enzymes, heme-thiolate enzymes that are capable of oxidizing unactivated carbon-hydrogen bonds in substrates so rapidly that iron-oxo intermediates have not been detected under physiological conditions.

Surface‐Assisted Synthesis and Behavior of Dimetallic Mixed‐Metal Complexes [M2Cl2(μ‐Cl)4(CO)6M′(L)2] (M = Ru, Os; M′ = Fe, Co; L = CH3CH2OH, H2O)

The last column in Table 1 on page 3503 of the original article1 is incorrect. The correct Table 1 and two sentences referring to this table in lines 13–21 of the right-hand column on the same page are given below. It also seems that decomposition step II is around 40 kJ/mol more favourable with iron than with cobalt, which is also consistent with experimental observations. According to our calculations, in M′Cl2(H2O)4 the most favourable isomer is the trans(Cl) structure with both iron and cobalt, although the energy difference between the isomers is only 4–5 kJ/mol for both metal chlorides. The Authors

Binding energy and preferred adsorption sites of CO on gold and silver–gold cluster cations: Adsorption kinetics and quantum chemical calculations

We revisit the reactivity of trapped pure gold (Au(n)+, n < 26) and silver gold alloy cluster cations (Ag(m)Au(n)+, m + n < 7) with carbon monoxide as studied in a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. The experimental results are discussed in terms of ab initio computations which provide a comprehensive picture of the chemical binding behaviour (like binding energy, adsorption sites, associated vibrational frequencies) of CO to the noble metal as a function of cluster size and composition. Starting from results for pure gold cluster cations for which an overall decrease of CO binding energy with increasing cluster size was experimentally observed--from about 1.09 +/- 0.1 eV (for n = 6) to below 0.65 +/- 0.1 eV (for n > 26) we demonstrate that metal--CO bond energies correlate with the total electron density and with the energy of the lowest unoccupied molecular orbital (LUMO) on the bare metal cluster cation as obtained by density functional theory (DFT) computations. This is a consequence of the predominantly sigma-donating character of the CO-M bond. Further support for this concept is found by contrasting the predictions of binding energies to the experimental results for small alloy cluster cations (Ag(m)Au(n)+, 4 < m + n < 7) as a function of composition. Here, binding energy drops with increasing silver content, while CO still binds always in a head-on fashion to a gold atom. Finally we show how the CO stretch frequency of Ag(m)Au(n)CO+ may be used to identify possible adsorption sites and pre-screen favorable isomers.

Effects of fat source and dietary sodium bicarbonate plus straw on the conjugated linoleic acid content of milk of dairy cows

The effects of fat source (0.7 kg of fatty acids from extruded soybeans or palmitic acid), of sodium bicarbonate (0.3 kg) plus straw (1 kg) and the interaction of these treatments on the content of conjugated linoleic acid (CLA) in the milk of dairy cows were examined. During nine weeks a group of 10 cows received a ration with palmitic acid and bicarbonate plus straw (ration PAB). During three periods of three weeks a second group of 10 cows received successively a ration with extruded soybeans and bicarbonate plus straw (ration ESB), a ration with palmitic acid without bicarbonate or straw (ration PA), and a ration with extruded soybeans without bicarbonate or straw (ration ES). Rations ES and ESB increased the content of polyunsaturated fatty acids in milk, but decreased milk fat content, compared to rations PAB and PA. Ration ESB led to the greatest milk CLA content, by a synergy between the high amount of dietary fat, and the action of bicarbonate plus straw, favouring trans11 isomers of CLA and C18:1, presumably via a ruminal pH near neutrality. Ration ES favoured trans10 isomers, not desaturated in the mammary gland, so that the milk CLA content was lower than with ration ESB, and resulted in the lowest milk fat content. In conclusion, a ration supplemented with both extruded soybeans and bicarbonate plus straw, was an efficient way to increase the CLA content in the milk of dairy cows.

Stereoselectivity in Diels—Alder Reactions of Diene‐Substituted N‐Alkoxycarbonyl‐1,2‐dihydropyridines.

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Systematic search for energetically favored isomers of large fullerenes C122–C130 and C162–C180

Abstract Based on the methodology by Cioslowski et al. [J. Cioslowski, N. Rao, D. Moncrieff, J. Am. Chem. Soc. 122 (2000) 8265–8270], two empirical fit equations to predict the standard enthalpy of formation are obtained over large number of calculation results at B3LYP/6-31G* theory level for fullerene isomers, which can be used as a preliminary and second-level screening tool, respectively, for large fullerenes. By applying these equations in screening the whole isolated pentagon rule (IPR) isomers, the energetically favored isomers of large fullerenes C 122 –C 130 and C 162 –C 180 were predicted at the B3LYP/6-31G* density functional theory level for the first time. Our results show that the lowest energy isomers of C 174 (2473259: C 3 v ) and C 180 (4071832: I h ) possess much lower relative energy and larger HOMO–LUMO gaps. Moreover, the ionization energy and electron affinity of the lowest energy isomers were also investigated.

Stereoselectivity in Diels−Alder Reactions of Diene-Substituted N-Alkoxycarbonyl-1,2-dihydropyridines

Diene substituent effects on the regiochemical and stereochemical outcomes of uncatalyzed Diels-Alder reactions of N-alkoxycarbonyl-1,2-dihydropyridines with both styrene and methyl vinyl ketone (MVK) were studied. Alkyl substitution on the diene in all cases examined resulted in a kinetic preference for 7-endo isomers (7-phenyl 51-96% exo and 7-acetyl 54-96% exo). For both dienophiles, the highest stereoselectivities (>or=89% endo) were observed with 5-methyl or 6-methyl substituents in the dihydropyridine. Theoretical calculations of the energies of gas phase endo and exo transition states at the RHF/3-21G(*) predict that total entropy, DeltaStotal, considerations favor endo cycloadducts for both dienophiles with DHP, while total energy considerations, DeltaEo, favor endo cycloadducts for styrene and exo cycloadducts for MVK. At this level, favored endo-phenyl isomers are correctly predicted for styrene reactions, but the calculation of 7-acetyl exo or endo isomer dominance is diene-substituent-dependent for MVK reactions. The preference for endo addition of MVK to the parent, 5-methyl, and 6-methyl-DHPs was successfully predicted by calculations at the B3LYP/6-31G* theory level.

Asymmetric Hydrogenation of Ketones with Ruthenium Complexes of rac- and Enantiopure (S,S)-1,2-Bis((diphenylphosphino)methyl)cyclohexane: A Comparative Study with rac- and (R)-BINAP

Ruthenium(II) complexes of the type trans-[RuCl2{1,2-bis((diphenylphosphino)methyl)cyclohexane}(diamine)] based on the inexpensive and easy-to-prepare rac- and (S,S)-1,2-bis((diphenylphosphino)methyl)cyclohexane form highly active and enantioselective catalysts for the asymmetric hydrogenation of a wide range of aryl and heteroaryl ketones, in most cases giving ee's that exceed those obtained with their BINAP counterparts. Although precatalysts based on 1,2-bis((diphenylphosphino)methyl)cyclohexane slowly isomerize in solution to afford the thermodynamically favored isomer with a cis arrangement of chlorides, catalysts generated from both isomers afford similar enantioselectivities.

SASS: A symmetry adapted stochastic search algorithm exploiting site symmetry

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.

Ring-closing metathesis for the synthesis of substituted indenols, indenones, indanones and indenes: Tandem RCM-dehydrogenative oxidation and RCM-formal redox isomerization

A number of substituted indenols have been synthesized using ruthenium-mediated ring-closing metathesis (RCM) with Grubbs second generation catalyst as the key step. The required dienes were synthesized by two strategies. The first entailed the isomerization of 2-allyl-3-isopropoxy-4-methoxybenzaldehyde to its styrene derivative, isopropoxy-4-methoxy-2-propenylbenzaldehyde using [RuClH(CO)(PPh 3) 3]. This compound and 3-isopropoxy-4-methoxy-2-(1-phenyl-propenyl)-benzaldehyde were then treated with vinyl- or isopropenyl-magnesium bromide to afford four of the scaffolds required for the metathesis. As the compound 3-isopropoxy-4-methoxy-2-(1-methyl-2-propenyl)benzaldehyde proved to be difficult to isomerize, the diene substrates 1-[3-isopropoxy-4-methoxy-2-(1-methylpropenyl)-phenyl]-prop-2-en-1-ol and 1-[3-isopropoxy-4-methoxy-2-(1-methylpropenyl)-phenyl]-2-methylprop-2-en-1-ol were synthesized by the addition of the Grignard reagents to 3-isopropoxy-4-methoxy-2-(1-methyl-2-propenyl)benzaldehyde, followed by isomerization of the arylallyl group to the thermodynamically favoured isomer with potassium t-butoxide. The use of harsher conditions (higher temperature and catalyst loadings) for the metathesis reactions resulted in the formation of substituted indenones, formed by a tandem RCM-dehydrogenative oxidation in the absence of a hydrogen acceptor. Further manipulations of the reaction conditions generated two substituted indanones by way of a tandem RCM-formal redox isomerization sequence. Finally the synthesis of some substituted indenes was accomplished from their corresponding dienes by the use of RCM.

Prediction of Low-Energy Isomers of Large Fullerenes from C132 to C160

To predict energetically favored isomers, we used a topological scheme as a prescreening tool to select candidate isomers for each fullerene from C(106) to C(160). Comparison with the PM3 and tight-binding (TB) potential calculated results and few published data for the low-energy isomers of C(106) to C(130) indicates that the prescreening approach is feasible. For each fullerene from C(132) up to C(160), the selected 1000 candidate isomers were further optimized by PM3 and TB potential. The analysis of the semiempirical PM3 and TB results of C(106) to C(160) provides some qualitative features of the large fullerenes. Furthermore, calculations at the B3LYP/6-31G*//B3LYP/3-21G level of theory were carried out on the top ten PM3 and TB low-energy isomers of C(132) to C(160) to accurately predict the stable isomers, and the HOMO-LUMO gap, the ionization energy, and electron affinity of the lowest-energy isomers were also investigated at the same level.

Investigations of the clustering reactions of protonated amino acid esters by high pressure mass spectrometry and quantum chemical calculations

The equilibria involved in the formation of the proton-bound dimers of the three simplest amino acid methyl esters (glycine, alanine and valine methyl esters) were characterized by means of pulsed ionization high pressure mass spectrometry (PHPMS) experiments and density functional theory (DFT) calculations. Our results would indicate that, within the temperature range employed in these experiments, the most stable proton-bound dimer conformers are formed in the case of glycine and alanine whereas a more entropically favoured isomer would dominate in the case of valine. Various possible isomers of each of the proton-bound dimer species have been investigated computationally each exhibiting a combination of inter- and intramolecular hydrogen bonding. A system of nomenclature for these various species is proposed. The possibility of structures exhibiting 'salt-bridge’ interactions have also been explored, recognizing that such structures would necessarily result from highly energetic structural rearrangements.

Structure and Activity Peculiarities of Ruthenium Quinoline and Quinoxaline Complexes: Novel Metathesis Catalysts

Novel metathesis catalysts that are derivatives of 8-vinylquinoline, (H2IMes)(Cl)2Ru(CH-κ2(C,N)-8-C9H6N) (1a; H2IMes = 1,3-dimesityl-4,5-dihydroimidzol-2-ylidene), and 5-vinylquinoxaline, (H2IMes)(Cl)2Ru(CH-κ2(C,N)-5-C8H5N2) (2a), which possess five-membered chelate rings, were synthesized in ligand exchange reactions with (H2IMes)(PCy3)(Cl)2RuCHPh. Both 1a and 2a are formed as complexes with the trans-dichloro geometry common for Grubbs-type complexes, while on prolonged storage in dichloromethane they isomerize to the thermodynamically favored cis-dichloro isomers (1b and 2b, respectively). The structures of compounds 1a,b were confirmed by X-ray analysis. The kinetics of trans to cis isomerization were measured by 1H NMR to give the rate constants k1a→1b = 3.2 × 10-2 h-1 and k2a→2b = 5.5 × 10-3 h-1 in dichloromethane-d2 at 23 °C. Investigations of the relative activities of these catalysts in model RCM and enyne reactions showed that catalyst 1a was faster than 1b (and 2a faster than 2b) but that 2a wa...

Comprehensive Study of Okaspirodiol: Characterization, Total Synthesis, and Biosynthesis of a New Metabolite fromStreptomyces

The new spiro[4.5]acetal okaspirodiol (4) was isolated from Streptomyces sp. Gö TS 19 as a secondary metabolite in yields up to 380 mg/L. The structure of this cryptic ketotetrol was elucidated by different methods including X-ray analysis, and its equilibration under mildly acidic conditions furnishing three additional isomers was thoroughly studied. Although metabolite 4 is not the thermodynamically favored isomer, a high-yielding total synthesis was accomplished comprising a stereoselective spiroacetalization under equilibrium conditions. This approach benefits from the important influence of an intramolecular hydrogen bond on the stabilization of the spiro[4.5]acetal moiety. The biosynthesis of 4 was investigated by feeding experiments with 13C-labeled precursors proving its origin from a new type of the rare mixed acetate-glycerol biosynthetic pathway. All results are discussed on the basis of the structural diversity of spiroacetals in nature and their chemical properties.

Theoretical study of isomerism, structure, and stability of dimers of C-doped aluminide clusters (C@Al12)2 and (C@Al12)(L@Al12) (L = Si, Ge)

The geometric, energetic, and vibrational characteristics of the dimers of carbon-doped aluminum clusters (C@Al12)2, (C@Al12)(Si@Al12), and (C@Al12)(Ge@Al12) have been calculated by ab initio density functional theory B3LYP method with the basis sets 6-31G* and 6-311+G*. As distinct from the silicon and germanium analogues (Si@Al12)2 and (Ge@Al12)2 with an AA lowest-lying structure, in which the Si@Al12 and Ge@Al12 blocks retain the shape of distorted icosahedra A, the symmetric AA structure is not typical of the (C@Al12)2 dimer (itcorresponds to a transition state) and the most favorable structures are two closely spaced BB and AD isomers, in which the C@Al12 blocks have either a distorted icosahedral (A) or nonicosahedral geometry (B, a dicapped ten-vertex polyhedron; D, a marquee with a centered hexagonal base). The calculated energy of dissociation of the most favorable isomer AD into isolated C@Al12 (Ih) monomers is ∼45 kcal/mol, which is 1.5–2 times as large as the corresponding dissociation energy of (Si@Al12)2 and (Ge@Al12)2. For mixed dimers of the (C@Al12)(L@Al12) type with L = Si or Ge, three types of low-lying isomers (AA, BA, and DA) are predicted in which the Si@Al12 and Ge@Al12 blocks retain their shape A and the C@Al12 blocks have geometry A, B, and D, respectively. For all the isomers, the block-block bonds are comparable in length and energy to the bonds inside the blocks and have the same chemical nature. Specific features of the geometry, relative stability, ionization potentials, electron affinity, vibrational spectra, and other characteristics of doped aluminide dimers, which could be used for the identification of their isomers by spectroscopic methods, are discussed.

Steric and electronic effects in stabilizing allyl-palladium complexes of “P–N–P” ligands, X 2PN(Me)PX 2 (X = OC 6H 5 or OC 6H 3Me 2-2,6)

The chemistry of η 3-allyl palladium complexes of the diphosphazane ligands, X 2PN(Me)PX 2 [X = OC 6H 5 ( 1) or OC 6H 3Me 2-2,6 ( 2)] has been investigated.The reactions of the phenoxy derivative, (PhO) 2PN(Me)P(OPh) 2 with [Pd(η 3-1,3-R′,R″-C 3H 3)(μ-Cl)] 2 (R′ = R″ = H or Me; R′ = H, R″ = Me) give exclusively the palladium dimer, [Pd 2{μ-(PhO) 2PN(Me)P(OPh) 2} 2Cl 2] ( 3); however, the analogous reaction with [Pd(η 3-1,3-R′,R″-C 3H 3)(μ-Cl)] 2 (R′ = R″ = Ph) gives the palladium dimer and the allyl palladium complex [Pd(η 3-1,3-R′,R″-C 3H 3)( 1)](PF 6) (R′ = R″ = Ph) ( 4). On the other hand, the 2,6-dimethylphenoxy substituted derivative 2 reacts with (allyl) palladium chloro dimers to give stable allyl palladium complexes, [Pd(η 3-1,3-R′,R″-C 3H 3)( 2)](PF 6) [R′ = R″ = H ( 5), Me ( 7) or Ph ( 8); R′ = H, R″ = Me ( 6)].Detailed NMR studies reveal that the complexes 6 and 7 exist as a mixture of isomers in solution; the relatively less favourable isomer, anti-[Pd(η 3-1-Me-C 3H 4)( 2)](PF 6) ( 6b) and syn/ anti-[Pd(η 3-1,3-Me 2-C 3H 3)( 2)](PF 6) ( 7b) are present to the extent of 25% and 40%, respectively. This result can be explained on the basis of the steric congestion around the donor phosphorus atoms in 2. The structures of four complexes ( 4, 5, 7a and 8) have been determined by X-ray crystallography; only one isomer is observed in the solid state in each case.

Comparative theoretical study of methano- and imino-[50] fullerenes: C 50CH 2 and C 50NH

Total of seven possible isomers for both methano- and imino-[50]fullerenes have been investigated by ab initio calculations. An insight into the relationship between structures and stabilities of these isomers is presented. The results show that the [5,6]-closed structure with CH 2 group bridging across a [5,6] type of C–C bond near the equatorial belt is the ground state of C 50CH 2 while that of the C 50NH is [5,6]-open fashioned with NH group bridging across a [5,6] type of C–C bond at the site between the pole and the equatorial belt. The underlying reason of this structural difference may be the larger atomic size and the greater Pauli repulsion of carbon atom compared with those of nitrogen atom. The relative stabilities of the isomers are determined by the strain of the cage, especially the strain at the equator. And although the contribution of the cyclic phenylene substructure at the equatorial belt to the stabilization of the favored isomers probably can not be ignored, the contribution of the conjugation effect to the stabilization is not competitive. Energy difference among three favored isomers is not very large, hence it can be expected that some isomers will co-exist in a mixture once methano- and imino-[50]fullerenes are synthesized. The calculated NICSs for the isomers of C 50CH 2 and C 50NH will be useful for their identification and characterization.

Size-dependent charge-separation reaction for hydrated sulfate dianion cluster, SO42−(H2O)n, with n=3–7

The decrease in the reaction rate for the charge separation in SO(4) (2-)(H(2)O)(n) with increasing cluster size is examined by first-principles calculations of the energetics, activation barriers, and thermal stability for n=3-7. The key factor governing the charge separation is the difference in the strength of solvation interaction: while interaction with water is strong for the reactant SO(4) (2-) and the product OH(-), it is relatively weak for HSO(4) (-). It gives rise to a barrier for charge separation as SO(4) (2-) is transformed into HSO(4) (-) and OH(-), although the overall reaction energy is exothermic. The barrier is high when more than two H(2)O are left to solvate HSO(4) (-), as in the case of symmetric solvation structure and in the case of large clusters. The entropy is another important factor since the potential surface is floppy and the thermal motion facilitates the symmetric distribution of H(2)O around SO(4) (2-), which leads to the gradual reduction in reaction rate and the eventual switch-off of charge separation as cluster size increases. The experimentally observed products for n=3-5 are explained by the thermally most favorable isomer at each size, obtained by ab initio molecular-dynamics simulations rather than by the isomer with the lowest energy.

Structure and kinetics of a transient antibody binding intermediate reveal a kinetic discrimination mechanism in antigen recognition.

Induced fit is a predominant phenomenon in protein-ligand interactions, yet it is invariably attributed without establishing the existence, let alone the structure, of the initial, low-affinity encounter complex. We determined the crystal structure of the encounter complex on the pathway of ligand binding by IgE antibody SPE7. We show that this complex is formed by a wide range of ligands that initially bind with identical affinity. Nonspecific ligands rapidly dissociate, whereupon the antibody isomerizes to a nonbinding isomer. Specific ligand complexes, however, slowly isomerize to give a high-affinity complex. This isomerization involves backbone and side-chain rearrangements of up to 14 A and the formation of specific hydrogen bonds. The postbinding conformational switch, combined with the prebinding isomerization to an energetically favorable nonbinding isomer, results in a "kinetic discrimination" mechanism that mediates selective binding, by a factor of >10(3), between highly related ligands that initially bind with the same affinity. This model may apply to proteins that bind multiple ligands in a specific manner or other proteins that, although capable of binding many ligands, are activated by only a few.

An efficient approach for theoretical study on the low-energy isomers of large fullerenes C90–C140

An approach that consists of a molecular mechanics method based on the second generation reactive empirical bond order (REBO) potential and the more accurate semiempirical method PM3 (Parametric Method No. 3) was proposed to predict the energetically favored isomers of the fullerenes from C90 to C140 at the semiempirical level. All the 578,701 isolated-pentagon-rule isomers of fullerenes from C90 to C140 were enumerated from topological structures and systematically searched using an energy minimization method to select the best 100 low-energy isomers based on the REBO potential for each fullerene. Then these candidate isomers were further optimized by PM3 and ranked again to determine the top low-energy isomers. This approach was applied to calculate the energetically favored isomers of C90-C140. The results of C90-C120 are in good agreement with the published results by quantum-chemical methods. Furthermore, the top five low-energy isomers of C90-C120, as well as C122-C140 which have scarcely been systematically studied before, are also predicted with the approach. The analysis of the structures showed that the hexagon-neighbor rule is an important factor to the stability of C90-C140. The time cost for the systematical search based on the REBO potential was also discussed. It indicates that the approach proposed is efficient for predicting the energetically favored isomers of large fullerenes at the semiempirical level.