Resonance in compounds with multiple conjugated bonds
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ORIGINAL RESEARCH
Resonance in compounds with multiple conjugated bonds Renato P. Orenha • Se´rgio E. Galembeck Renato L. T. Parreira
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Received: 31 July 2012 / Accepted: 20 September 2012 / Published online: 9 October 2012 Ó Springer Science+Business Media New York 2012
Abstract The resonance of the compounds buta-1, 3-diyne (1), buta-1,3-diene (2), hexa-1,3,5-triyne (3), hexa1,3,5-triene (4), hexa-3-en-1,5-diyne (5), and hexa-1, 5-dien-3-yne (6) was analyzed. The molecular geometry, p molecular orbitals, and the electron density of these compounds were analyzed. The NBO, AIM, and NRT methods were used. By comparing the electronic structures of compounds 1 and 2 and by considering that the latter is a classic example of a p-conjugated compound (Org Lett 5:2373–2375, 2003; Org Lett 5:2373–2375, 2004), it was possible to conclude that the conjugation of compound 1 is larger than that of 2. Compounds 3 and 4 were also studied, in order to understand the effect of a longer conjugated chain, and it was found that the resonance also increases in the case of 3. In addition, the effect of changing the order of the central bond was investigated by comparison of compounds 5 and 6 with 3 and 4, respectively. The results indicated that changes produce small alterations in the properties of compounds 3 and 4. Keywords Conjugation Polyines Polyenes Chemical bond Introduction Compounds with conjugated multiple bonds are important in several different areas of chemistry, such as the R. P. Orenha S. E. Galembeck (&) Departamento de Quı´mica, FFCLRP, Universidade de Sa˜o Paulo, Ribeira˜o Preto, SP 14040-901, Brazil e-mail: [email protected] R. L. T. Parreira Nu´cleo de Pesquisas em Cieˆncias Exatas e Tecnolo´gicas, Universidade de Franca, Franca, SP 14404-600, Brazil
mechanism of vision, photosynthesis, and conducting polymers. Even though the electronic structure of these compounds has been studied since the early days of quantum chemistry, several questions remain unanswered. A very interesting issue is whether there is conjugation in buta-1,3-diyne, (1), and its magnitude. By using the Kistiakowsky thermochemical scheme, Rogers et al. [1] have concluded that there is no conjugation between the triple bonds in this compound, in contrast with what has been observed in the case of buta-1,3-diene, (2). Considering hyperconjugative interactions, Schleyer, Houk, and co-authors [2] have verified that the resonance of 1 is larger than that of 2. Similar conclusions have been drawn by means of energy decomposition analysis (EDA) calculations, [3, 4] and these two works have confirmed the results of a previous valence bond (VB) investigation [5]. Recently, Feixas et al. [6] used delocalization and multicenter indices to quantify the effects of conjugation and hyperconjugation, which unlike isodesmic reactions, do not depend on reference systems or on the division of molecules into different fragments. The indices were employed to compare the strengths of p-conjugation stabilization in buta-1,3-diene and buta-1,3-diyne. The result
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