The new pictorial structural covariance method for qualitative quantum chemistry. II: Arenes with or without polyene side chains and polyene bridges

The aromaticity, anti-aromaticity, and two new types of behaviour of pi-hydrocarbons with one or more C _m H _m rings attached by polyene bridges and with or without several polyene side chains are readily obtainable on the blackboard directly from the structural formulae using the general pictorial method recently presented. The HOMO-LUMO types, number of non-bonding MOs, and qualitative stability are also deduced. The single pi-rings fall into four homolog classes characterized by their first members C₃H₃, C₄H₄, C₅H₅, and C₆H₆ rather than the two previous Hückel 4n, 4n + 2 types. A general rule for finding the number of NBMOs for bridged combinations of such ring types is given.     

Dynamic nuclear polarization with biradicals

Dynamic nuclear polarization (DNP) experiments in rotating solids have been performed for the first time using biradicals rather than monomeric paramagnetic centers as polarizing agents. Specifically, two TEMPO radicals were tethered with a poly(ethylene glycol) chain of variable length where the number of glycol units was 2, 3, or 4. NMR experiments show that the signal observed in DNP experiments is approximately inversely proportional to the length of the chain. Thus, the shorter chain with larger electron dipolar couplings yields larger enhancements. The size of the enhancement is a factor of 4 larger than obtained with the identical concentration of monomeric nitroxide radicals achieving a value of approximately 175 for the n = 2 chain.     

ESR spectra of biradicals with weak exchange interaction

ESR spectra are reported for a new class of aliphatic iminoxyl biradicals. The additional hyperfine-structure lines are explained in terms of a weak exchange interaction between the unpaired electrons in the two centers. This occurs during intramolecular motion, which leads to collision between the centers, not as a result of deloalization of the unpaired electrons over the bonds.     

CIDNP magnetic field dependence in biradicals

The biradical S-T_?1 CIDNP magnetic field dependence can be treated as a spectral line shape problem in which the S-T_?1 levels are tuned by the magnetic field; the shape is related to fluctuations in the through-space electron exchange interaction, J, modulated by conformational changes in the biradical. A density matrix treatment is applied to a two-conformer model of a polymethylene biradical.     

Gas-phase reactivity of charged pi-type biradicals

Four pi,pi-biradicals, 2,6-dimethylenepyridinium and the novel isomers N-(3-methylenephenyl)-3-methylenepyridinium, N-phenyl-3,5-dimethylenepyridinium, and N-(3,5-dimethylenephenyl)pyridinium ions, were generated and structurally characterized in a Fourier transform ion cyclotron resonance mass spectrometer. Their gas-phase reactivity toward various reagents was compared to that of the corresponding monoradicals, 2-methylenepyridinium, N-phenyl-3-methylenepyridinium, and N-(3-methylenephenyl)pyridinium ions. The biradicals reactivity was found to reflect their predicted multiplicity. The 2,6-dimethylenepyridinium ion, the only biradical in this study predicted to have a closed-shell singlet ground state, reacts significantly faster than the other biradicals, which are predicted to have triplet ground states. In fact, this biradical reacts at a higher rate than the analogous monoradical, which suggests that to avoid the costly uncoupling of its unpaired electrons, the biradical favors ionic mechanisms over barriered radical pathways. In contrast, the second-order reaction rate constants of the isomeric biradicals with triplet ground states are well approximated by those of the analogous monoradicals, although the final reaction products are sometimes different. This difference arises from rapid radical-radical recombination of the initial monoradical reaction products. The overall reactivity toward the hydrogen-atom donors benzeneselenol and tributylgermanium hydride is significantly greater for the radicals with the charged site in the same ring system as the radical site. This finding indicates that polar effects play an important role in controlling the reactivity of pi,pi-biradicals, just as has been demonstrated for sigma,sigma-biradicals.     

Monocyclic biradicals in the imidazolidine series

The reaction of 1-hydroxy-3-imidazoline 3-oxides with alkyl- or phenyliithium followed by oxidation gives stable biradicals with two nitroxyl groups in the imidazolidine ring. Anisotropic ESR spectra of these biradicals are discussed.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 924–927, May, 1993.     

有机双自由基体系的磁性偶合规律

采用量子化学从头算UHF方法对系列有机双自由基体系的基态自旋耦合规律进行研究, 进一步证实了自由基之间在共轭体系中出现铁磁性耦合的拓扑规则,统一了关于自由基耦合规律的几种解释, 为有机磁性材料的实验合成提供了理论指导。     

Palladium(II) chloride compounds with stable biradicals of imidazoline

Conclusions The first coordination compounds of stable nitroxyl biradical imidazolines have been synthesized. From the spectral data obtained, the paramagnetic ligand 4,4-(ethyleneiminomethyl)di-2, 2,5,5-tetramethyl-3-imidazoline-1-oxyl acts as a tetradentate bridge in a binuclear coordination compound with Pd(II), while the azine ligand 4-formyl-2,2,S,5-tetramethyl-3-imidazoline-1-oxyl acts as a bidentate bridge.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 11, pp. 2581–2584, November, 1988.The authors thank E. G. Boguslavskii, I. M. Oglezneva, and V. A. Shipachev for help in obtaining the spectra of the compounds.     

Interaction of oxygen with transient biradicals photogenerated from γ-methyl valerophenone

The 1,4-biradicals generated during the photolysis of γ-methyl valerophenone interact with oxygen at nearly diffusion controlled rates. The comparison of quantum yield studies in benzene with laser photolysis experiments in methanol allow the estimation of the biradical lifetime in benzene as 42 ns at room temperature.     

Endor investigations of biradicals in liquid crystals

It is shown that ENDOR spectroscopy of biradicals with small zero-field splitting (¦D¦ < 1 mT) can be successfully performed in liquid-crystalline solutions. An intense signal at the free-proton Larmor frequency establishes the triplet-state character. The signs of hyperfine coupling constants can be deduced from the intensity pattern.     

Rigid orthogonal bis-TEMPO biradicals with improved solubility for dynamic nuclear polarization

The synthesis and characterization of oxidized bis-thioketal-trispiro dinitroxide biradicals that orient the nitroxides in a rigid, approximately orthogonal geometry are reported. The biradicals show better performance as polarizing agents in dynamic nuclear polarization (DNP) NMR experiments as compared to biradicals lacking the constrained geometry. In addition, the biradicals display improved solubility in aqueous media due to the presence of polar sulfoxides. The results suggest that the orientation of the radicals is not dramatically affected by the oxidation state of the sulfur atoms in the biradical, and we conclude that a biradical polarizing agent containing a mixture of oxidation states can be used for improved solubility without a loss in performance.     

Does intersystem crossing in triplet biradicals generate singlets with conformational memory?

The effect of temperature, concentration, external magnetic fields and paramagnetic quenchers on the lifetime of biradicals produced in the Norrish Type II reaction has been examined. Analysis of this data and that available from earlier reports leads to the conclusion that in addition to controlling the biradical lifetimes, intersystem crossing in triplet-derived biradicals plays a critical role in determining the behaviour and products which result from singlet biradical reactions. The effect is attributed to conformational memory in the singlet biradical which reflects its short lifetime.     

含杂环并具有高自旋基态的双自由基体系的理论设计

以—·N—O—为自旋中心(SC),间苯为铁磁耦合单元(FC),苯、吡啶、哒嗪、嘧啶、吡嗪、三嗪为端基(EG),设计一系列新型稳定高自旋分子.另外以—·N—O—为SC,苯、吡啶、哒嗪、嘧啶、吡嗪、三嗪为FC,苯为EG,又设计另一系列新型稳定高自旋分子,并通过AM1—CI方法计算,研究了不同杂环作为端基或耦合单元对高自旋分子自旋多重度稳定性的影响.