Aromaticity, one of the most important concepts in organic chemistry, has attracted considerable interest from both experimentalists and theoreticians. It remains unclear which NICS index is best to evaluate the triplet‐state aromaticity. Here, we carry out thorough density functional theory (DFT) calculations to examine this issue. Our results indicate that among the various computationally available NICS indices, NICS(1)zz is the best for the triplet state. The correlations can be improved from 0.840 to 0.938 when only neutral species are considered, demonstrating the significant effect of the charge on the triplet‐state aromaticity. In addition, calculations suggest that five‐membered cyclic species with “hyperconjugative” aromaticity (and antiaromaticity) in the S0 state will become antiaromatic (and aromatic) in the T1 state, indicating an important role of hyperconjugation. Finally, a moderate correlation (r2=0.708) is identified between the NICS(1)zz values and spin distributions. 相似文献
B3LYP/6-311++G(d,p) calculations and molecular integrations from the quantum theory of atoms in molecules (QTAIM) were performed
for the purposes of studying a new class of dihydrogen-bonded hyperconjugation complexes formed by C2H5+···n(BeH2), when n = 1 (bimolecular) or n = 2 (trimolecular). Whether bimolecular or trimolecular, when the hyperconjugation on the ethyl cation (C2H5+) is taken into account, this enables the earth alkaline hydride, BeH2, to interact efficiently with the nonlocalized hydrogen (H+) of the C2H5+. In addition to computation of QTAIM topological parameters, analysis of the infrared harmonic spectrum at the B3LYP/6-311++G(d,p)
level of theory revealed the existence of red-shifts on BeH2, and this effect is explained by means of the atomic charges derived from the ChelpG approach.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
It has been shown by ESR that the addition of photochemically generated boroncentered carboranyl radicals to branched fluoroalkenes affords stable spin-adducts. It has been found that the addition of boron-centered radicals with 1-substituted 2-fluoroalkenyl-o-carborane gives stable -carboranyl radicals. Negligible variations in the values of the constants of hyperfine coupling of the unpaired electron with the nucleus of the -11B atom in radicals of various structures imply the stability of the carborane conformation with respect to the 2pz-orbital of the unpaired electron. This may be caused by hyperconjugation between the boron atom and the unpaired electron as well as steric interaction.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1316–1318, July, 1993. 相似文献
The characteristics of disulfide groups concerning the S-S dihedral angle are represented by ab initio SCF calculations using the split-valence 6-31G(*) basis set. It is shown that the hyperconjugation between the S-H bond and the electron pair on the other sulfur plays an important role in determining the characteristic S-S dihedral angle. The S 3d orbitals do not participate in such characteristics. The nature of the S-S bond is compared with that of the O-O bond. The S-S bond length varies largely depending on the S-S dihedral angle. This is related to the frequency-conformation correlation of the disulfide group. 相似文献
A series of cyclopentadienyltin(III) radicals have been prepared by the reaction: The substitution reactions of these radicals with alkyl bromides, and their addition reactions with ethylene, biacetyl, 3,6-di-t-butyl-1,2-benzoquinone, and nitrocompounds, have been studied by ESR spectroscopy. The apparent reactivity decreases as the number of Cp ligands increases, and in this, and in the ESR parameters of the Sn(IV) adducts which are formed, the Cp ligands resemble chloro groups rather than alkyl groups. This is supported by the quadrupole splitting which is observed in the Mössbauer spectra of alkylcyclopentadienyl tin compounds.In η1-Sn(IV) compounds this effect may be ascribed to carbon—metal hyperconjugation (σ—π-interaction), but as yet there is no evidence whether CpSn(III) radicals are σ(η1) or π(η2?5) bonded. 相似文献
NMR spectra of iminoxyls and the corresponding amines are used to calculate the paramagnetic shifts in the free radicals, which are used to calculate the constants ai for the hyperfine interactions with the protons in the radicals. The ai for the -protons are negative, while those for the -protons are positive. The constants are discussed in relation to a simple model based on propagation of spin density via bonds by spin polarization and hyperconjugation. The spatial structure of the radicals is also discussed. 相似文献
Unimolecular reactions of mutual isomerization of cyclopentyl and 1-penten-5-yl radicals have been investigated by chemical activation. The radicals were generated by adding energized hydrogen atoms (EH about 23 kcal mol−1) to the double bond of either cyclopentane or 1,4-pentadiene. Based on the extensive steady-state RRKM calculations employing the experimental data from this work as well as from the literature, the threshold energies for the cyclopentyl ring opening and closure are 32 ± 0.3 and 16.2 ± 0.3 kcal mol−1, respectively. The entropy of activation for the ring opening is close to zero. 相似文献
Imidazolium cations are promising candidates for preparing anion‐exchange membranes because of their good alkaline stability. Substitution of imidazolium cations is an efficient way to improve their alkaline stability. By combining density functional theory calculations with experimental results, it is found that the LUMO energy correlates with the alkaline stability of imidazolium cations. The results indicate that alkyl groups are the most suitable substituents for the N3 position of imidazolium cations, and the LUMO energies of alkyl‐substituted imidazolium cations depend on the electron‐donating effect and the hyperconjugation effect. Comparing 1,2‐dimethylimidazolium cations (1,2‐DMIm+) and 1,3‐dimethylimidazolium cations (1,3‐DMIm+) with the same substituents reveals that the hyperconjugation effect is more significant in influencing the LUMO energy of 1,3‐DMIms. This investigation reveals that LUMO energy is a helpful aid in predicting the alkaline stability of imidazolium cations. 相似文献
According to ab initio molecular orbital calculations carried out with full geometry optimization at the MP2/6–31G** level, the classical 2-fluoroethyl cation, FCH2CH2+, is a transition structure for H-scrambling in CH3CHF+. Single point MP4/6–31G** calculations at the optimized geometries predict the cyclic ethylene fluoronium ion to lie 24.2 kcal mol−1 above CH3CHF+ and 5.4 kcal mol−1 below the 2-fluoroethyl cation. ΔG‡ for ring opening of the cyclic fluoronium ion at -60° is estimated to be ca 15 kcal mol−1. This barrier is largely attributable to the powerful negative fluorine hyperconjugation in the transition state as described by Hoffmann and coworkers. When electron correlation effects are ignored a qualitatively different potential surface is obtained on which the 2-fluoroethyl cation is calculated to be a local minimum separated from the stable 1-fluoroethyl cation by an H-bridged transition state. 相似文献
In this work, the reactivities of acetonyl and benzoyl radicals in aromatic substitution and addition reactions have been compared in an experimental and computational study. The results show that acetonyl is more electrophilic than benzoyl, which is rather nucleophilic. A Hammett plot analysis of the addition reactions of the two radicals to substituted styrenes clearly support the nucleophilicity of benzoyl, but in the case of acetonyl, no satisfactory linear correlation with a single substituent-related parameter was found. Computational calculations helped to rationalize this effect, and a good linear correlation was found with a combination of polar parameters (σ+) and the radical stabilization energies of the formed intermediates. Based on the calculated philicity indices for benzoyl and acetonyl, a quantitative comparison of these two radicals with many other reported radicals is possible, which may help to predict the reactivities of other aromatic radical substitution reactions. 相似文献
A quadrupolar compound Pyr-BA with two pyrrole-type nitrogen atoms doped externally was prepared in this work. In high contrast with other π ionic radicals, its cationic radical Pyr-BA ⋅ + undergoes unusual symmetry-broken charge separation (SB-CS), generating the mixed valence complex of Pyr-BA+1−q ⋅⋅⋅ Pyr-BA+1+q , where q is the degree of charge transfer. Variable-temperature (VT) single-crystal analysis, absorption and EPR experiments all confirmed that aggregation and lower temperature would help to facilitate this SB-CS process. Gibbs energy calculations and gauge-including magnetically induced current simulation both validate that, for Pyr-BA ⋅ + , SB-CS behavior is more favorable than the conventional dimerization mode. To the best of our knowledge, this is the first study that shows solid single-crystal evidence for spontaneous SB-CS between identical ionic radicals. Such a unique phenomenon is of great significance both in terms of fundamental aspects and uncharted material science. 相似文献
To reveal the reasons for the previously found absence of end radicals upon γ-radiolysis ofn-heptane polycrystals, we performed quantum-chemical calculations (SCF-MO, RHF, 6-31G* basis set) of then-heptane molecule and its four radicals. The energies of the crystal lattice were calculated by the atom-atom potential method. Comparison of the experimental and calculated data showed that the absence of the end radicals is not related to the intermolecular interaction in the crystals. The most probable reason for the selective radical formation upon radiolysis can be a transfer of the excitation energy within then-heptane molecule occurring before the radical formation. 相似文献
Unprecedented neutral perylene‐3,4:9,10‐tetracarboxylic acid bisimide (PBI) radicals and biradicals were synthesized by facile chemical oxidation of 4‐hydroxyaryl‐substituted PBIs. Subsequent characterization by optical and magnetic spectroscopic techniques, as well as quantum chemical calculations, revealed an open‐shell singlet biradical ground state for the PBI biradical OS ‐ 2.. (〈s2〉=1.2191) with a relatively small singlet–triplet energy gap of 0.041 eV and a large singlet biradical character of y=0.72. 相似文献
The O-H bond dissociation energies (DOH) in the molecules of 2,5-dimethylhydroquinone (1) and 2,5-di-tert-butylhydroquinone (2) and in the corresponding semiquinone radicals (5 and 8, respectively) were estimated by the method of intersecting parabolas (IP) from experimental data on the rate constants for
the reactions of these compounds with N-phenyl-1,4-benzoquinonemonoimine (3) and using the density functional B3LYP/6-31+G* quantum chemical calculations. When calculating the DOH values by the IP method, solvation of reactants and transition states should be taken into account. The energies of solvation
of quinones, semiquinone radicals, and hydroquinones were evaluated by the PCM method. The results of quantum chemical calculations
obtained with inclusion of the effects of solvation and the DOH estimates obtained by the IP method are in good agreement, being equal to 337.9±1.6, 242.5±1.4, and 242.7±3.4 kJ mol−1 for molecule 1 and radicals 5 and 8, respectively.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 2244–2251, October, 2005. 相似文献
We report the synthesis and characterisation of new examples of meso‐hydroxynickel(II) porphyrins with 5,15‐diphenyl and 10‐phenyl‐5,15‐diphenyl/diaryl substitution. The OH group was introduced by using carbonate or hydroxide as nucleophile by using palladium/phosphine catalysis. The NiPor?OHs exist in solution in equilibrium with the corresponding oxy radicals NiPor?O.. The 15‐phenyl group stabilises the radicals, so that the 1H NMR spectra of {NiPor?OH} are extremely broad due to chemical exchange with the paramagnetic species. The radical concentration for the diphenylporphyrin analogue is only 1 %, and its NMR line‐broadening was able to be studied by variable‐temperature NMR spectroscopy. The EPR signals of NiPor?O. are consistent with somewhat delocalised porphyrinyloxy radicals, and the spin distributions calculated by using density functional theory match the EPR and NMR spectroscopic observations. Nickel(II) meso‐hydroxy‐10,20‐diphenylporphyrin was oxidatively coupled to a dioxo‐terminated porphodimethene dyad, the strongly red‐shifted electronic spectrum of which was successfully modelled by using time‐dependent DFT calculations. 相似文献