The electronic structures and relative stabilities of homopolar biradicals (BR) and CT BR with significant one-electron transfer (ET) BR characters were investigated by ab initio MO calculations. The previously presented inter- and intra-molecular CT models were extended in order to elucidate possible mechanisms for decomposition reactions of dioxetane, dioxetanone, and related species. The computational results indicate that endothermic O–O cleavages, followed by charge-transfers, are operative for the chemiluminescence reactions of these peroxides with several anionic species, in contradiction to the chemically initiated electron-exchange luminescence (CIEEL) mechanism, where complete one-electron transfer (ET) is required for the formation of excited carbonyl fragments. The ionization potentials of monoanions of phenol, indole and luciferins were calculated semiempirically in order to estimate the CT excitation energies from these species to the O–O antibonding orbital. The CT excitation energies are used to distinguish between the CT induced luminescence (CTIL) mechanism and the CIEEL mechanism for chemiluminescence reactions. Orbital-interaction models are also presented to explain of the so-called odd/even selection rule for the efficiency of chemiluminescence reactions. The implications of these results are discussed in relation to recent experimental results, together with biological chemiluminescence reactions of luciferins and related species.

ABSTRACT:The reaction mechanism and relative stabilities of the intermediates and transition states in the reaction of 1 O plus ethylene molecule using ab initio molecular 2 Ž . orbital MO theories at several levels of theory with the correction of the nondynamic and dynamic electron correlation effects were systematically investigated. showing that the reaction through the 1,4-biradical intermediates initially proceeds through the gauche transition state to form the gauche minimum 1 BR state, following that the free rotation of O moiety occurs due to the energy barrier less than 4.0 2 kcalrmol. The stability of the perepoxide is surprisingly sensitive to the levels of the Ž . theory and the basis sets employed. The coupled-cluster methods, CCSD and CCSD T , gave the reasonable stabilities of 1,4-biradical intermediates, perepoxide, and dioxetane Ž . as a reaction product. From the results of the CCSD and CCSD T methods, the reaction 1 of O q H C CH proceeds by a two-step mechanism through the 1,4-biradical 2 2 2 intermediates rather than through the perepoxide.

A systematic MO-theoretical description of electronic structures of tricentric bisdesmiphiles in the ground state is presented in response to contradictory arguments on the biradical character of the species by Huisgen, Firestone, and others. The approximately spin-projected but size-consistent (AP) UHF and UMP methods remedy artifact arising from the spin contamination effect involved in the singlet UHF and UMP wavefunctions, giving reasonable a different-orbitals-for-different-spins (DODS) MO description for oxygenated dipoles such as O3 and CH2OO and 1,3-biradicals without octet stabilization such as OCH2O and CH2CH2CH2. The RHF method provides well-accepted MO description for diszonimum and nitrilium betaines having little biradical character, and the RMP method works well even for species with weak biradical character. They are successfully applied to calculating relative energies between nonradical ring- and biradical open-forms of tricentric species, singlet–triplet energy gaps of tricentric biradicals, and high- and low-spin energy gaps of polyradicals constructed of 1,3-biradical units. Reliability of the projected UHF and UMP scheme for biradical species was confirmed by the complete active space (CAS) configuration interaction (CI) by the use of UHF natural orbitals (UNO). The UNO CASCI calculations are also carried out both for the ground and excited of 1,3-biradicals. The energy gradient technique based on UNO CASSCF is applied to the full geometry optimizations of tricentric systems, followed by the UNO CASSCF MP2 calculations for dynamical correlation corrections. On the other hand, the UNO MR SDCI method by the use of the UNO CASCI reference gives a well-balanced MO-theoretical description of the ground and excited states. Comparisons of different definitions of biradical character are given in relation to theoretical explanations of 1,3-dipoles and 1,3-dipolar reactions. It is shown that the present MO-theoretical description based on both delocalized and localized natural orbitals is wholly compatible with experimental data obtained by Huisgen and others.

SUMMARYThe different orbitals for different spins molecular orbital and complete active space (CAS) SCF MO descriptions of azomethyne ylides, carbonyl ylides and related oxygenated dipoles were performed as a continuation of earlier papers. Rotational energy barriers for the C-C double bond of ethylene at the equilibrium and dissociated geometries, and the 1,3-dipolar species were calculated by UHF, APUHF, UMP, APUMPn, CASSCF and CASSCF MP2 procedures, and were discussed in relation to their biradical characters and stereospecificity of 1,3-dipolar reactions. It was found that the biradical characters of carbonyl ylides and related species are quenched by the symmetry-allowed orbital interactions with olefins, in accord with the symmetry-allowed concerted property. The rotational energy barriers for 1,5 biradicals were also examined in relation to the non-concerted mechanism of 1,3-dipolar reactions. The implications of these results are discussed in relation to the concerted and biradical mechanisms of 1,3-dipolar additions such as ozonolysis reactions.

The thermal decomposition behavior of poly ( ethylene terephthalate) (PET) fabrics containing meso -1,2,3,4-tetrabromobutane (TBB) is investigated using thermogravimetry (TG), differential thermal analysis (DTA), and pyrolysis gas chromatographymass spectrometry (Py-GC-MS). Isothermal TG analysis proves that the presence of TBB has little effect on the thermal decomposition of PET. However, isothermal DTA data reveal that the absorbed TBB participates in some reactions in the condensed phase of PET. On the other hand, Py-GC-MS analysis reveals that bromine atoms exist in the thermal decomposition products of PET. Thus, vinyl groups are generated from PET by β-CH hydrogen transfer, and they then react with hydrogen bromide, which is produced by TBB, to form bromine-containing thermal decomposition products. These facts show why TBB molecules, which have strong affinities for PET segments, are effective for Hame retardation.In our previous paper [4], we studied the relationship between flame retardation and the phase transition behavior of poly ( ethylene terephthalate ) ( PET ) fabrics treated with a bromine-containing flame retardant, me.so-1,2,3,4-tetrabromobutane (TBB), using differential scanning calorimetry. Our work revealed that the depression of the melting initial temperature is directly proportional to the effect of flame retardation. When we applied Flory's equation for depressing the melting point in polymer-diluent systems to the PET-TBB system, our results suggested that TBB molecules, which have strong affinities for PET segments, markedly retard flammability. Thus, TBB works in the condensed phase of PET fabric.In contrast, Koch et al. suggested that bromine-containing flame retardants work mainly in a gas phase and not in a condensed phase, using their thermogravimetric (TG) data from measurements at a constant heating rate [3]. They discussed the mechanism of flame retardation based on the temperatures at which flame retardants are liberated, using a comparison with the thermal decomposition temperature of PET. However, the temperature condition of TG analysis is considered to be quite different from that of actual combustion. In the case of continuous combustion, PET fibers near flames are heated rapidly, and the inflammable vapor evolving from the surfaces of molten PET is continuously supplied as a fuel. Granzow et al. measured the surface temperature of burning PET samples by means of a miniature thermocouple embedded in the polymer. They reported that the surface temperature of burning PET reaches about 380°C, and when 4% red phosphorus is added, it rises to about 450°C [ ]].Considering that the surface temperature of buming PET is constant, the mechanism of flame retardation should be investigated based on the TG data obtained from isothermal measurements rather than non-isothermal ones. Therefore, in this study, we have investigated the thermal decomposition of PET fabrics treated with TBB by comparing isothermal TG and differential thermal analysis (DATA) measurements with non-isothe...