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Four isomers of didehydroborazine, B3N3H4, borazyne, and three isomers of azaborine, C4H4BN, are studied by DFT, CCSD, and CCSD(T) computational methods. The singlets of 1,2-borazyne (I) and 1,4-borazyne (IV) have angles about the boron of ca. 150 degrees . In 1,2-azaborine (V), the angles are ca. 140 degrees , while the N angles are ca. 112 degrees except in IV (127 degrees ) and 1,4- azaborine (VII, 120 degrees ). These geometries are almost reversed in the triplets. The 1,3-borazyne (III) shows more bicyclic character than the corresponding m-benzyne, with an N-N distance of 1.7 A. In all cases I was found to be lower in energy than the 2,4-borazyne (II), III, and IV. The order of stability of the azaborines is V > VII > 1,3- azaborine (VI). The nucleus-independent chemical shift (NICS) indicates that all isomers of borazyne are less aromatic than benzene and all isomers of azaborine are about as aromatic as benzene. Time-dependent DFT (TD-DFT) is used to study the excited states of the singlets. The significant structural differences between the singlet and triplet states suggest long phosphoresence lifetimes.

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Our recent work directed at the design, synthesis, characterization and applications of new types of polyborazylene and polyborosilazane polymers is reviewed with a focus on the use of these polymers as processable precursors to BN and SiNCB composites. A design strategy based on the controlled functionalization of preformed polymers with pendant groups of suitable compositions and crosslinking properties has been employed to yield second-generation dipentylamine-polyborazylene (DPA) and pinacolborane-hydridopolysilazane (PIN-HPZ) polymers, which, unlike the parent polyborazylene (PB) and the borazine-hydridopolysilazane (B-HPZ) polymers, are stable as melts and can be easily melt-spun into polymer fibers. Subsequent pyrolyses of these polymer fibers then provide excellent routes to BN and SiNCB ceramic fibers.

Abstract

The pathways for the reaction of aryldiazonium cations with azide anion to arylazide and nitrogen are explored using the B3LYP/6-311ϩG(d) method. CCSD(T) calculations were performed on the RN 5 (R ϭ H, OH, Cl, CN) counterparts to verify the appropriateness of this density functional theory method to cases involving NON bond breaking. As in our prior MP2/6-31G(d) study, a pathway to direct formation of aryl pentazole in a concerted reaction was not found. Transition state structures were calculated for the cyclization reaction of 24 aryl pentazenes in the E configuration and syn conformation (Es) to pentazoles and for the loss of N 2 from the Es, Ea (anti), and Za pentazenes and from pentazoles. Correlations were found between activation energies and both reaction energies and Hammett values for 24 aryl N 5 cases. The activation energies for competing cyclization and N 2 loss from Es pentazenes were both ca. 4 kcal/mol. The barriers for loss of N 2 from Ea and Za pentazenes are both ca. 20 kcal/mol. The lowering of the barriers in the Es configuration is attributed to the nucleophilic assistance of the in-plane lone pair on the N1 atom and in-plane aromaticity. Competition between N 2 loss from, and cyclization of the Es pentazene may provide for a synthesis of hitherto unknown arylpentazoles with electron withdrawing groups.

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