Reactions involving di-trans-[12]annulenes

The low temperature complete dehydrohalogenation of pentabromocyclododecene (C12H17Br5) with potassium tert-butoxide in THF followed by exposure to potassium metal leads to the formation of the anion radical of 1,5-di-trans-[12]annulene, which loses hydrogen and undergoes ring closure to form the anion radical of 11,12-dihydro-[8]annuleno-[6]annulene. This product can, in turn, be isolated as its neutral molecule via reoxidation with iodine. A [12]annulene obtained via the dimerization of 1,5-hexadiyne in the presence of 18-crown-6 and potassium tert-butoxide undergoes ring closure, with concomitant loss of hydrogen, to yield the heptalene anion radical. It follows that the heptalene anion radical precursor was the 1,7-di-trans isomer of [12]annulene.     

Competing isomerizations of [12]annulenes: Diels-Alder reaction versus electrocyclization

Experimentally, tri-trans-[12]annulene and tris(cyclohexeno)[12]annulene exhibit differing reactivities. Whereas the former, after isomerizing to its di-trans isomer, undergoes sequential electrocyclizations, the latter follows a Diels-Alder pathway after initial electrocyclization. B3PW91/6-31+G*//B3LYP/6-31G* calculations indicate that cyclohexenofusion simultaneously hinders the second electrocyclization and facilitates Diels-Alder reaction, primarily by inducing greater puckering in the intermediate eight-membered ring.     

Quantum mechanical investigation of the inner-sphere reorganization energy of cyclooctatetraene/cyclooctatetraene radical anion. Part I

The inner-sphere reorganization energy of the electron self-exchange of the couple cyclooctatetraene/cyclooctatetraene radical anion has been investigated by quantum mechanical calculations. The more stable Jahn Teller distorted B2g conformation of the radical anion has been used in this study. Two different theories have been applied in this first part. The harmonic approximation in the classical Marcus scheme has been modified by using projected force constants, which are obtained from the complete force constant matrix and the geometry changes of the molecule during the ET (introduced by Mikkelsen). A different approach (introduced by Nelsen) combines the different energies of the neutral and radical anion with and without relaxation corresponding to the vertical ionization potential and the vertical electron affinity. The electronic energies of the neutral molecule and the radical anion differ dramatically applying three different levels of quantum mechanical calculations (UAM1, UB3LYP, PMP2 with three different basis sets with and without diffuse functions). Nevertheless the Nelsen method gives almost consistent results for the inner-sphere reorganization energies: 120.1 kJ/mol for semiempirical UAM1 method, 159.3 kJ/mol, 156.4 kJ/mol and 158.3 kJ/mol for density functional UB3LYP/6-31G*, UB3LYP/6-31++G* and UB3LYP/AUG-cc-pVDZ calculations and 192.5 kJ/mol for ab-initio PMP2/6-31G* investigations, respectively. These values are in agreement with earlier experimental work supposing the total reorganization energy to be larger than 38 kcal/mol assuming an electron self-exchange rate of 10(4) M(-1) s(-1). The simple harmonic approximation of Marcus relation has not yet been applied for a molecule like cyclooctatetraene with large torsional geometry changes. Using the projected force constants after scaling, considerably different results for the inner-sphere reorganization energy have been calculated: 738.1 kJ/mol for the UB3LYP/6-31G*, 743.3 kJ/mol for UB3LYP/6-31++G* and 759.1 kJ/mol for UB3LYP/AUG-cc-pVDZ level of theory. Comparison with our concentration dependent EPR experiments are controversial to the earlier experimental results, but the latter supports the assumption that the electron self-exchange occurs in a time scale so that the molecules cannot complete their vibrational motions. Therefore the projected Marcus relation is not valid for cyclooctatetraene/cyclooctatetraene radical anion including a large torsional change during the electron transfer.     

A combined density functional theory and coupled cluster method investigation of the structural properties and stabilities of radical CH(2)CP and its isomers

The doublet potential energy surface of radical system [C(2), H(2), P] is investigated at the UB3LYP/6-311++G(d,p) and UCCSD(T)/6-311++G(2df,2p) (single-point) levels. Eight chainlike and three-membered ring structures are located as energy minima connected by 10 interconversion transition states. At the final UCCSD(T)/6-311++G(2df,2p)//UB3LYP/6-311++G(d,p) level with zero-point vibrational energy correction, species CH(2)CP is found to be thermodynamically the most stable isomer followed by HCCPH, H-cCPC-H, cPCC-H(H), H-cCCP-H, cis-CC(H)PH, trans-CC(H)PH, and CCPH(2) at 11.01, 12.57, 40.07, 43.63, 50.25, 56.82, and 65.36 kcal/mol, respectively. The computed results indicate that the chainlike isomers CH(2)CP and HCCPH and cyclic radical H-cCPC-H possess considerable kinetic stability at extra low pressures and temperatures. Interestingly, radical CCPH(2), whose energy is the highest in all predicted CH(2)CP isomers, can be also regarded as a kinetically stable species with the smallest isomerization barrier of 22.26 kcal/mol at extra low pressures and temperatures. Therefore, considering higher kinetic stability, in addition to the microwave spectroscopy characterized isomer CH(2)CP in previous experiments, the species HCCPH, H-cCPC-H, and CCPH(2) should be considered as excellent candidates for possible experimental observation. Furthermore, the structural nature of stable radical isomers is discussed based on bonding characteristics, single electron spin distribution, and comparison with their analogues.     

Dynamic processes in [16]annulene: Möbius bond-shifting routes to configuration change

Density functional and ab initio methods have been used to study the mechanisms for key dynamic processes of the experimentally known S4-symmetric [16]annulene (1a). Using BH&HLYP/6-311+G** and B3LYP/6-311+G**, we located two viable stepwise pathways with computed energy barriers (Ea = 8-10 kcal/mol) for conformational automerization of 1a, in agreement with experimental data. The transition states connecting these conformational minima have M?bius topology and serve as starting points for non-degenerate pi-bond shifting (configuration change) via M?bius aromatic transition states. The key transition state, TS1-2, that connects the two isomers of [16]annulene (CTCTCTCT, 1 --> CTCTTCTT, 2) has an energy, relative to the S4 isomer, that ranged from 6.9 kcal/mol (B3LYP/6-311+G**) to 16.7 kcal/mol (BH&HLYP/6-311+G**), bracketing the experimental barrier. At our best level of theory, CCSD(T)/cc-pVDZ(est), this barrier is 13.7 kcal/mol. Several other M?bius bond-shifting transition states, as well as M?bius topology conformational minima, were found with BH&HLYP energies within 22 kcal/mol of 1a, indicating that many possibilities exist for facile thermal configuration change in [16]annulene. This bond-shifting mechanism and the corresponding low barriers contrast sharply with those observed for cis/trans isomerization in acyclic polyenes, which occurs via singlet diradical transition states. All M?bius bond-shifting transition states located in [16]- and [12]annulene were found to have RHF --> UHF instabilities with the BH&HLYP method but not with B3LYP. This result appears to be an artifact of the BH&HLYP method. These findings support the idea that facile thermal configuration change in [4n]annulenes can be accounted for by mechanisms involving twist-coupled bond shifting.     

Single-electron entrapment of [8]annulyne, biannulenylenes, and an annulenoannulene

The low-temperature (-100 degrees C) dehydrohalogenation of bromocyclooctatetraene followed by immediate electron-transfer yields a stable solution of the [8]annulyne anion radical. If the unstable [8]annulyne is reacted with itself, cyclobutadiene, or benzyne, the respective bi-[8]annulenylene, [6]annuleno[8]annulene, or [6]-[8]annulenylene can be trapped as their anion radicals via one-electron transfer. These condensation products were all obtained from simple [2 + 2] cycloaddition reactions. B3LYP/6-31G geometry optimizations were carried out, and the calculated spin densities were compared to the EPR spectral results obtained for the anion radicals of [6]annuleno[8]annulene, [8]annulyne, bi[8]annulenylene, and [6]-[8]annulenylene, and excellent agreement has been realized. This simple "one-pot" approach should be applicable to a wide range of such systems.     

The Dowd-Beckwith ring expansion: a theoretical study

UB3LYP/6-311++G(d,p) and ROMP2/6-311++G(d,p)//UB3LYP/6-311++G(d,p) calculations including the effect of benzene solvent through the PCM-UAHF method render a concerted mechanism without fragmentation as the most favourable one for the Dowd-Beckwith radical ring expansion of the bromomethyl adduct of methyl cyclopentanone-2-carboxylate to yield methyl cyclohexanone-3-carboxylate. The corresponding concerted TS is a bicyclic alcoxy radical.     

Dicyclobuta[de,ij]naphthalene and dicyclopenta[cd,gh]pentalene: a theoretical study

The structures, energetics, and aromatic character of dicyclobuta[de,ij]naphthalene, 1, dicyclopenta[cd,gh]pentalene, 2, dihydrodicyclobuta[de,ij]naphthalene, 3, and dihydrocyclopenta[cd,gh]pentalene, 4, have been examined at the B3LYP/6-311++G//B3LYP/6-31G level of theory. All molecules are bowl-shaped, and the pentalene isomers, 2 and 4, are most stable. A comparison with other C(12)H(6) and C(12)H(8) isomers indicates that 2 is approximately 25 kcal/mol less stable than 1,5,9-tridehydro[12]annulene and 4 is approximately 100 kcal/mol higher in energy than acenaphthylene, both of which are synthetically accessible. The transition state structure for bowl-to-bowl inversion of 1 is planar (D(2)(h)()) and lies 30.9 kcal/mol higher in energy than the ground state; the transition state for inversion of 2 is C(2)(h)() and lies 46.6 kcal/mol higher in energy. Symmetry considerations, bond length alternations, and NICS values (a magnetic criterion) all indicate that the ground states of 1, 3, and 4 are very aromatic; however, HOMA values (a measure of bond delocalization) indicate that 3S and 4S are aromatic but that 1S is less so. NICS values for the ground state of 2 strongly indicate aromaticity; however, bond localization, symmetry, and HOMA values argue otherwise.     

Aromaticity and Homoaromaticity in Methano[10]annulenes

Aromaticity and neutral homoaromaticity have been evaluated in methano[10]annulenes systems, 1,4-methano[10]annulene (1), 1,5-methano[10]annulene (2), and 1,6-methano[10]annulene (3). C-C bond lengths indicate that 1 presents higher bond alternation than 2 and 3. The relative energies were determined at the B3LYP/6-311+G(d,p) level, and they pointed out that 3 is the most stable isomer. Strain energies, evaluated employing homodesmotic reactions, show the same order as the relative energies. Through a decomposition of strain energies, it could be concluded that the rings absorb more tension than the bridges. The changes in aromaticity were evaluated by magnetic susceptibilities, chiM, HOMA, NICS, and resonance energies, RE. HOMA, RE, and chiM indicate that 2 and 3 are strongly, and 1 is fairly, aromatic. NICS does not provide reliable results, due to interference of ring and bridge atoms. NBO analysis presents some interactions that suggest the existence of neutral homoaromaticity. GPA indices (evaluated at the B3LYP/6-31G* level) point out that homoaromaticity plays a relevant role only in 3. Moreover, this work is the first in the current literature that studies 1,4-methano[10]annulene (1).     

Energetics of oxaspirocycle prototypes: 1,7-dioxaspiro[5.5]undecane and 1,7,9-trioxadispiro[5.1.5.3]hexadecane

The relative gas-phase energetics of several low-lying isomers of 1,7-dioxaspiro[5.5]undecane and 1,7,9-trioxadispiro[5.1.5.3]hexadecane have been calculated with second-order Mller-Plesset perturbation theory and basis sets as large as aug-cc-pVQZ. Relative energies in THF, dichloromethane, acetone, and DMSO have been estimated with corrections from polarized continuum model calculations at the B3LYP/6-311+G(d) level. In the most stable conformation of 1,7-dioxaspiro[5.5]undecane, both rings adopt chair conformations, and both oxygens are axially disposed (2A). It is more than 2 kcal mol(-1) more stable than all the other conformers. In agreement with previous work, the "twist-boat" trans isomer (3A) is the most stable isomer of 1,7,9-trioxadispiro[5.1.5.3]hexadecane. However, in contrast to this earlier study, an "all-chair" conformation (3B) is found to be the most stable cis isomer of 1,7,9-trioxadispiro[5.1.5.3]hexadecane (E approximately 0.5 kcal mol(-1) in acetone and DMSO). Gauge-independent atomic orbital computations at the B3LYP/6-311+G(d) level indicate that this is the only cis isomer with (13)C NMR chemical shifts that are qualitatively consistent with the experimental spectra.     

Dehydro[12]annulenes: structures, energetics, and dynamic processes

Density functional and coupled cluster calculations on neutral monodehydro[12]annulenes (C(12)H(10)) reveal a global minimum that should be kinetically stable. At the CCSD(T)/cc-pVDZ//BHLYP/6-31G* level, the unsymmetrical CTCTC conformer 1a lies at least 3 kcal/mol below all other isomers studied. The two isomers closest in energy to 1a are Mo?bius structure 5a (CCTCC) and all-cis 6a. Isomer 1a can undergo conformational automerization with E(a) = 3.9 kcal/mol, implying that this process would be rapid on the NMR time scale, and computed (1)H NMR parameters (GIAO-B3LYP/6-311+G**//RHF/6-31G*) are presented. Cumulenic dehydro[12]annulene isomers, with 1,2,3-butatriene subunits, were found to be reactive intermediates in the interconversion of different configurations of the alkyne forms. Pathways for configuration change of 1a, and for subsequent rearrangement to biphenyl, were investigated. The 28 kcal/mol overall barrier for the lowest energy pathway connecting 1a to biphenyl suggests that 1a is kinetically stable with respect to valence isomerization.     

The [12]annulene global minimum

A new global minimum for [12]annulene has been computationally located. This mono-trans minimum 5 (CCCCCT) is computed to be 1.5 kcal/mol more stable (CCSD(T)/cc-pVDZ//BHHLYP/6-311+G**) than the known tri-trans isomer 1 (CTCTCT) and 2.4 kcal/mol lower than the di-trans isomer 4 (CCTCCT), for which there is indirect evidence. The barriers for several rearrangements of 5 were all computed to be above 15 kcal/mol, indicating that direct experimental characterization of 5 should be possible. The computed barriers for the dynamic processes (including conformational automerization) coupled with computed 1H NMR shift values should aid in the future characterization of this [12]annulene isomer.     

Structure and electronic properties for a newly synthesized red fluorescent material

The ground-state structure and electronic properties of a newly synthesized red fluorescent material, 2-[3-(2-{4-[(2-hydroxy-ethyl)-methyl-amino]-phenyl}-vinyl)-5, 5-dimethyl-cyclohex-2-enylidene]-malononitrile (A31), are investigated using a hybrid density-functional theory (DFT) approach, B3LYP, and the 6-31G* basis set. We have obtained four geometrical isomers. The theoretical infrared (IR) spectrum calculated by B3LYP/6-31G* level of theory is in very good agreement with our experimental measurement. The cation and anion are optimized to clarify the effects of the hole and electron injections and the energies needed by injections are estimated.     

Ab initio studies on isomers of macropolyhedral borane ions [B20H18]n (n = 0, -2, -4)

Various isomers of macropolyhedral borane ions [B20H18]n (n = 0, -2, -4) are investigated by using the density functional theory methods at RB3LYP/6-31+G* and RB3LYP/6-31G* levels to obtain the optimized geometries, harmonic vibrational frequencies, electron structures, and the stability order. The calculated results show that optimized bond lengths are consistent with the available experimental values and the natural populations, taking [a2 -B20H18]4- (4) as an example, are also in agreement with NMR spectra. The calculated vibrational frequencies are all real, so all of these isomers could be stable, among which [a2 -B20H18]2- (3) and [a2 -B20H18]0 (7) are considered for the first time in this paper. On the basis of the contour maps of molecular orbitals, the delocalized characteristic of molecular orbitals and the possible redox mechanism of these ions are also discussed. Moreover, the analysis on counting of skeletal bonding electrons shows that the isomers (1)-(6) obey the electronic requirement predicted by the mno rule, whereas the newly predicted isomer (7) does not match the mno rule.     

The conversion among various B4C clusters: a density functional theoretical study

Geometry optimizations and vibration frequencies of B4C clusters were performed with Becke-3LYP method using 6-31G(d) basis set. We have found 14 stable isomers, and the most stable structure among them is the five-member ring containing two three-member boron rings. We also analyzed these stable isomers in detail, and the results show that the structures containing three-member boron rings are predominant in energy for B4C clusters. In terms of MO and NBO analysis, the three-centered bond and the pi-electron delocalization play an important role in stabilizing the planar five-member rings of these B4C clusters. Our calculations suggest that isomer4 can be converted into isomer7 with only an energy barrier of 0.31 kJ mol(-1) at the B3LYP/6-311G+(3df) level. Although the planar structures of the five-member rings (isomers12-14) can be converted with each other, the conversions of isomer14 to isomer13 and isomer13 to isomer12 have high-energy barriers of 70.99 and 68.51 kJ mol(-1) at the B3LYP/6-31G(d) level, respectively.     

12]Annulynes

Only one isomer of o-benzyne ([6]annulyne or 1,2-didehydrobenzene) exists, but the dehydro analogue of the "ring-opened double benzene", [12]annulyne, was generated in several isomeric forms. 1,5-Hexadiyne undergoes self-condensation in the presence of potassium tert-butoxide to yield two isomers of [12]annulyne (3,11-di-trans-[12]annulyne and 5,9-di-trans-[12]annulyne), both of which exhibit a weak paratropic ring current in their 1H NMR spectra and are oxygen sensitive. They can be reduced to their respective dianions, which are diatropic. A third isomer (3,9-di-trans-[12]annulyne) was generated via the complete dehydrohalogenation of hexabromocyclododecene and found to be much less stable but can be tamed via one- or two-electron reduction. A tight association of the cation (K+) with the p(y)-orbitals within the alkyne moiety results in an unusually low-field resonance for an adjacent external proton.     

(CH2)2N和(CH3)2NH+的密度泛函理论计算

用密度泛函理论UB3LYP／6-31G（d,p）方法研究了二甲胺自由基（CH3）2N及其质子化离子（CH3）2NH 的构象和超精细结构．在由构象研究得到的两种自由基的最稳定结构上,用密度泛函的UB3LYP和UB3PW91方法及从头计算的UHF、UMP2（FULL）方法计算了α-质子、β-质子和N核上的超精细偶合常数A（Hα）、A（Hβ）和A（N）结果表明：两种自由基中甲基内旋转的位垒均很低,分别为0．46kJ·mol-1（（CH3）2NH ）和2．6kJ·mol-1（（CH3）2N）．UB3LYP／6-31G（d,p）和UB3PW91／6-31G（d,p）计算的A（Hα）、A（Hβ）和A（N）与ESR实验结果符合得很好,UMP2／6-31G（d,p）方法的计算值与实验值符合得也较好．     

Clar goblet and related non-Kekulé benzenoid LPAHs. A theoretical study

The results of an ab initio and semiempirical study of Clar Goblet (1), a C(38)H(18) non-Kekulé diradical LPAH, and its constitutional isomers 4 and 5 are reported. Planar D(2)(h)-1 was only 87.4 (triplet) and 83.8 (singlet) kJ/mol less stable than its planar Kekulé isomer C(2)(v)-6 (at (U)B3LYP/6-31G). Planar C(s)-4 was 63.6 (triplet) and 76.5 (singlet) kJ/mol less stable than 6. Overcrowded C(1)-5 was 80.1 (triplet) and 98.1 (singlet) kJ/mol less stable than 6. In concealed non-Kekulé 1, the singlet was more stable then the triplet by 3.6 kJ/mol, while in nonconcealed non-Kekulé 4 and 5, the triplets were more stable than the corresponding singlets by 12.9 and 18.1 kJ/mol, respectively, in accordance with theory. The spin density in 1, 4, and 5 is delocalized throughout the positions corresponding to active peri-peri coupling positions of the radical anion of naphthanthrone (2). The bond lengths in 1, 4, and 5 are in the range expected for aromatic compounds, except for the central carbon-carbon bonds, which are considerably elongated. A certain stabilization is evident in the homodesmotic reaction singlet-1 + 10 + 10 --> 11 + 3 + 3, indicating a "communication" between the two benzo[cd]pyrenyl radical (3) units of diradical 1. The HOMA indices indicate that in both singlet 1 and triplet 1 all of the rings except the central one have a significant aromatic character. The central ring is essentially antiaromatic, having negative HOMA index (-0.140 at UB3LYP/6-31+G). The stabilities of 1(2)(-) and 1(2+) are decreased relative to 3(-) and 3(+), respectively.     

An aza cyclopropylcarbinyl-homoallyl radical rearrangement-radical cyclization cascade. Synthesis of dibenzoimidazoazepine and oxazepine derivatives

The cycloaddition of the dibenzoxazepinium W-ylides, generated by heating of trans-1-aryl-7,11b-dihydro-1H-azirino[1,2-a]dibenzo[c,f]azepines, to the C═N double bond of 3-aryl-2H-azirines proceeds endo-stereoselectively giving regioisomeric cycloadducts in ca. 1:1 ratio, in good overall yields. In contrast to the dibenzoxazepinium ylides, the cycloaddition of the dibenzazepinium W-ylide proceeds regioselectively but without exo-endo-stereoselectivity. The reasons for this selectivity of the cycloaddition theoretically were studied at the DFT B3LYP/6-31G(d) level. Heating adducts, (2aRS,13SR,13aRS)-13,13a-diaryl-13,13a-dihydro-1H,2aH-azireno[1',2':3,4]imidazo[1,2-d]dibenzo[b,f][1,4]oxazepines and (2aRS,13SR,13aRS)-13,13a- diphenyl-2a,7,13,13a-tetrahydro-1H-azireno[1',2':3,4]imidazo[1,2-a]dibenzo[c,f]azepine, with an excess of AIBN in toluene gave new polyheterocyclic systems via a novel aza cyclopropylcarbinyl-homoallyl radical rearrangement-radical cyclization cascade. The energy profile of the cascade was studied at the DFT UB3LYP/6-31G(d) level. The transient imidazolinylmethyl radical was trapped by the use of other radical initiators as the corresponding peroxide or alcohol.     

咪唑氧自由基分子NLO性质的密度泛函理论研究

采用密度泛函理论(DFT) UB3LYP方法, 在6-31g(d)水平上对2,2’-(1,2-乙炔基-4,1-亚苯基)双[4,4’,5,5’-四氢]咪唑氧自由基分子及其异构体的自旋耦合性质进行分析, 并结合有限场(FF)方法计算它们的非线性光学(NLO)系数, 以探讨咪唑氧环在共轭链不同位置时体系的自旋耦合规律和NLO系数. 结果表明, 所有体系基态自旋符合自旋极化规则, 它们的极化率随自旋多重度的增加而减小; 一阶超极化率因受分子对称性影响, 对称性不同其一阶超极化率的变化也不同; 二阶超极化率呈现随着自旋多重度的增加而增加的趋势. 从理论上探讨这些自由基分子自旋耦合规律与NLO活性的关系, 为有机自由基NLO材料的分子设计与实验研究提供一定的理论依据.