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The thermodynamic properties of 39 polybrominated diphenyl ethers (PBDEs) in the ideal gas phase have been calculated using Gaussian 03 on the B3LYP/6-31G(d)//B3LYP/6-31G(d) level. Their thermodynamic and other physicochemical properties show a strong dependence on the bromine substitution pattern. The PBDE congeners' enthalpies of formation increase with an increasing number of bromines. The thermodynamic properties of congeners with the same number of bromines also show dependence on the bromine substitution pattern, especially for ortho-substituted congeners. PBDE congeners with one phenyl ring fully brominated, such as 2,3,4,5,6-PeBDE, 2,3,4,4‘,5,6-HxBDE, 2,2‘,3,4,4‘,5,6-HpBDE, and 2,3,3‘,4,4‘,5,6-HpBDE, were found to be the least stable among the analogues. The effects of bromine substitution pattern have been quantitatively studied by group additivity method (GAM) based on the output of the theoretical calculations. The results of the GAM were consistent with theoretical calculations, proving that theoretical calculations are reliable. Furthermore, the GAM model can be used to predict the thermodynamic properties for all of the 209 PBDE congeners.

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COMMUNICATIONSisotropy of transition structure 2 for the stepwise reaction ( -47.8) is essentially identical with that of 3a (-47.9) when the MP2 geometries are used for both. Hence, all the magnetic criteria agree that 2 and 3a do not differ significantly in their electronic character.Conclusions: The geometries and energies determined by a b initio methods, as well as the IGLO analysis of the magnetic properties, show that the parent Cope rearrangement of 1,5hexadiene occurs through a concerted, synchronous mechanism via an uromirric transition structure 3a. This is a general feature of the pericyclic reactions we have studied in this context (Table 3). There appears to be no relationship between the energies of concert or the activation energies and the magnetic susceptibility exaltations (which are in the range of the benzene value, -13.7). The cyclic electron delocalization, which is responsible for the ring current effects, also stabilizes the transition structures. but is only one of the contributing factors governing the reactivity and mechanistic pathway.Keywords: a b initio calculations . aromaticity . heterocycles a) The literature is vast. Fortunately, an excellent new comprehensive review is

[reaction: see text] Neutral homoaromaticity has been evaluated in heterocyclic systems related to the bicyclo[3.2.1]octane skeleton with replacement of CH(2) at C-2 in bicyclo[3.2.1]octa-3,6-diene with X = BH, AlH, Be, Mg, O, S, PH, NH (12); replacement of CH at C-3 in bicyclo[3.2.1]octa-3,6-dien-2-yl anion with PH, S, NH, O (13); and replacement at C-2 and C-3 with N and O (14). Stabilization energies (SE) are evaluated using density functional theory and homodesmotic equations at the B3LYP/6-311+G(3df,2p)//B3LYP/6-31G(d) level for series 12-14. Stabilization energies are compared with diamagnetic susceptibility exaltations, Lambda, CSGT-B3LYP/6-31G(d)//B3LYP/6-31G(d), and nucleus-independent chemical shifts (NICS), GIAO-B3LYP/6-311+G(2d,p)//B3LYP/6-31G(d). Analysis of frontier orbitals and geometries, B3LYP/6-31G(d)//B3LYP/6-31G(d), and proton affinities of 2-azabicyclo[3.2.1]octa-3,6-diene, pyrrole, and related model systems, B3LYP/6-311+G(2d,2p)//B3LYP/6-31G(d), provide complementary evidence supporting the division of the substrates evaluated into antihomoaromatic (12, X = BH, AlH, and Be), nonhomoaromatic (12, X = O, S, NH, PH), and homoaromatic (13, X = S, PH, NH, O and 14 X = ON), with 12 (X = Mg) appearing as transitional between anti- and nonhomoaromatic.