ORIGINAL PAPER Borazine: to be or not to be aromatic Rafael Islas Æ Eduardo Chamorro Æ Juvencio Robles Æ Thomas Heine Æ Juan C. Santos Æ Gabriel Merino Received: 23 April 2007 / Accepted: 16 May 2007 / Published online: 31 August 2007 Ó Springer Science+Business Media, LLC 2007 Abstract Aromaticity of borazine, which has been sub- ject of controversial discussions, is addressed. Beside a short review on aromaticity of borazine we report a detailed analysis of two molecular fields, the induced magnetic field (B ind ) and the electron localization function (ELF). The induced magnetic field of borazine shows a long-range shielding cone perpendicular to the molecular plane, as in benzene, but lower in magnitude. Contrary to benzene, borazine shows two weakly paratropic regions, one of them inside the ring, and the second one enveloping the boron atoms. It is necessary to separate r and p con- tributions to identify whether borazine exhibits p-aromatic character comparable to benzene. Nucleus-independent chemical shift (NICS) isolines show that the r electrons are much stronger localized than p electrons, their local para- magnetic contributions generate a short-range response and a paratropic (deshielding) region in the ring center (similar to an anti-aromatic response). Three regions can be iden- tified as chemically meaningful domains exhibiting an internally strong electron delocalization (ELF = 0.823). Borazine may be described as a p aromatic compound, but it is not a globally aromatic species, as the electronic system is not as delocalized as in benzene. Keywords Aromaticity  Borazine  ELF  NICS  Induced Magnetic Field Introduction The study of aromaticity in borazine has a long history. In 1926, Stock and Pohland synthesized this molecule by a reaction of diborane and ammonia [1]. In 1940, Wiberg proposed the alias of ‘‘inorganic benzol’’ for borazine based on the fact that all the B–N bond lengths are equivalent [2], which is the landmark of aromaticity for hydrocarbons, and because the number of p electrons is the same as in benzene [3]. However, due to the electronega- tivity difference between boron and nitrogen, aromaticity may be expected to be lower in borazine than in benzene. Chemically, borazine has a preference for addition reactions. It seems than even if borazine may be endowed with some aromatic character, its reactivity behavior is not driven toward reassembling the p-ring system. However, Dedicated to the 70th birthday of Prof. Tadeusz Marek Krygowski. R. Islas  J. Robles  G. Merino (&) Facultad de Quı ´mica, Universidad de Guanajuato, Noria Alta s/n, Guanajuato, GTO CP 36050, Mexico e-mail: gmerino@quijote.ugto.mx E. Chamorro  J. C. Santos Departamento de Ciencias Quı ´micas, Facultad de Ecologı ´a y Recursos Naturales, Universidad Andres Bello, Republica 217, Santiago, Chile T. Heine Institut fu ¨r Physikalische Chemie und Elektrochemie, TU Dresden, 1062 Dresden, Germany 123 Struct Chem (2007) 18:833–839 DOI 10.1007/s11224-007-9229-z Electron delocalization and aromaticity