ORIGINAL PAPER Theoretical study of the solvent effect on the aromaticity of benzene: a NICS analysis Georgia M. A. Junqueira & Hélio F. Dos Santos Received: 18 September 2013 /Accepted: 21 January 2014 # Springer-Verlag Berlin Heidelberg 2014 Abstract Nucleus-independent chemical shift (NICS) quan- tities for benzene–benzene and benzene–water species were obtained and are discussed in gas phase and in solution. Besides standard polarizable continuum model (PCM) calcu- lations, sequential Monte Carlo/quantum mechanics (S-MC/ QM) were also performed. Benzene was shown to be slightly more aromatic in condensate phase when we considered the average solvent configuration (ASEC) approach with explicit molecules. Keywords S-MC/QM . NICS . Benzene Introduction Aromaticity is a fundamental concept in chemistry [1, 2]. Generally, it comprises structural, energetic, electronic and magnetic criteria. The structural criterion examines the degree of bond length alteration, normally evaluated through the harmonic oscillator model of aromaticity (HOMA) index, defined by Kruszewski and Krygowski [3, 4]. In turn, the electronic criterion evaluates the stability of the aromatic species compared to localized reference systems. The para- delocalization index (PDI) [5, 6] is obtained using the delo- calization index (DI) [7, 8] as defined in the framework of the atoms in molecules (AIM) theory of Bader [9]. The fluctuation index (FLU) [10] describes the fluctuation of electronic charge between adjacent atoms in a given ring. Solà et al. [11] reported 15 aromaticity tests to analyze the advantages and drawbacks for a group of ten aromaticity descriptors. The latter authors concluded that indexes based on the study of electron delocalization in aromatic species are more accurate among those examined. In addition, aromaticity can be de- fined as the ability to sustain a diamagnetic or diatropic ring current. If a magnetic field is directed perpendicularly to the plane of the aromatic system, a ring current is induced in the delocalized π electrons system. Since aromaticity is related to induced ring currents, magnetic properties are particularly important for its detection and evaluation [1]. Aromatic ring currents are relevant to nuclear magnetic resonance (NMR) spectroscopy since they help to distinguish these nuclear environments and are therefore of great use in molecular structure determination. A diamagnetic ring current (or diatropic ring) is associated with aromaticity whereas a paratropic ring current signals antiaromaticity. Schleyer and co-workers [2, 12, 13] have developed an aromatic index that gives results in good agreement with energetic (energy of aromatic stabilization), geometric (bond order) and magnetic (diamagnetic susceptibility) criteria: nucleus-independent chemical shift (NICS)—a computational approach that uses the absolute magnetic shieldings computed at chosen points in the molecules. Absolute magnetic shielding is computed at ring centers (non-weighted mean value of the heavy atom coordinates) and corresponds to the NMR chemical shift convention. The signs are reversed, so that negative NICS values denote aromaticity and positive NICS values denote antiaromaticity. Significantly negative (i.e., magnetically shielded) NICS values in interior positions of rings or cages indicate the presence of induced diatropic ring currents or aromaticity, whereas positive values (i.e., deshielded) at each point denote paratropic ring currents and antiaromaticity. To This paper belongs to Topical Collection QUITEL 2013 G. M. A. Junqueira : H. F. D. Santos Departamento de Química, Universidade Federal de Juiz de Fora, Juiz de Fora, MG 36036-900, Brazil Present Address: G. M. A. Junqueira (*) Departamento de Física, Universidade Federal de Juiz de Fora, Juiz de Fora, MG 36036-900, Brazil e-mail: junqueira.georgia@gmail.com J Mol Model (2014) 20:2152 DOI 10.1007/s00894-014-2152-5