Not All That Has a Negative NICS Is Aromatic: The Case of the H-Bonded Cyclic Trimer of HF Rafael Islas, † Gerardo Martı ´nez-Guajardo, † J. Oscar C. Jime ´nez-Halla, † Miquel Sola `, ‡ and Gabriel Merino* ,† Departamento de Quı ´mica, DiVisio ´n de Ciencias Naturales y Exactas, UniVersidad de Guanajuato, Col. Noria Alta s/n C.P. 36050, Guanajuato, Gto., Me ´xico, and Institut de Quı ´mica Computacional and Departament de Quı ´mica, UniVersitat de Girona, Campus de MontiliVi, 17071 Girona, Catalonia, Spain Received February 19, 2010 Abstract: In this work, we used the induced magnetic field (B ind ) to study the degree of aromaticity of the planar (HF) 3 ring. The induced magnetic field analysis shows that the degree of electron delocalization in the hydrogen-bonded cyclic trimer of HF is very low. This result is in agreement with those obtained using GIMIC and is opposite to the Rehaman’s suggestion. Our results demonstrate a clear limitation of the NICS index when a strong anisotropy is exhibited and suggest that the NICS values should be used carefully to discuss aromaticity in systems without an important p z -orbital overlap that produces the π clouds. In view of the fact that the NICS index is extensively used by computationally and theoretically oriented experimental chemists, this is an important warning. Introduction The IUPAC defines aromaticity as “The concept of spatial and electronic structure of cyclic molecular systems display- ing the effects of cyclic electron delocalization which proVide for their enhanced thermodynamic stability (relatiVe to acyclic structural analogues) and tendency to retain the structural type in the course of chemical transformations”. 1 Nevertheless, it is enough to check the last two Chem. ReV. issues dedicated to aromaticity 2 and electron delocalization 3 to realize that this definition is still controversial. In general, a quantitative and/or qualitative assessment of the degree of aromaticity is given by the chemical behavior (lower reactivity), structural features (planarity and equal bond length tendencies), 4,5 energy (stability), and spectroscopic properties (UV, proton chemical shifts, and magnetic sus- ceptibility exaltation). Recently several magnetic indices of aromaticity have been introduced and discussed. They include the famous nucleus-independent chemical shift (NICS) and related indexes, 6,7 aromatic ring-current shielding (ARCS), 8 and plotted ring-current densities. 9,10 Particularly, the concept of a ring current, introduced by Pople, has been used widely to interpret the magnetic properties of aromatic molecules. 11,12 Even though that aromaticity is not well-defined and was originally developed within the organic chemistry scheme, this concept has been extended to inorganic systems. Interestingly, all-metal clusters and inorganic compounds 13-17 have not only the conventional π-(anti)aromaticity but also the σ-, 18-22 δ- 23-25 or even -(anti)aromaticity, 26 i.e., a multifold aromaticity. 27-30 In 2005 Rehaman et al. suggested that the planar hydrogen- bonded cyclic (HF) 3 (see Scheme 1) is aromatic. 31 They claimed that the existence of aromaticity in such hydrogen- bond complexes is apparent from the NICS values (in ppm) of -2.94, -1.98, and -1.89 for (HF) 3 , (HCl) 3 , and (HBr) 3 , respectively. Similar results were also reported for water clusters (H 2 O) n . 32 Recently, one of us has studied the interplay between aromaticity and hydrogen bonding in 1,3-dihydroxyaryl-2-aldehydes. 33,34 In that case, the quasi- ring partially adopts the role of a typical aromatic ring, favoring phenomena like for proton transfer reactions, contrary to the (HF) 3 complex. Of course, the existence of “ hydrogen-bonded aromatic- ity” involving strong ring currents across hydrogen bonds is * gmerino@quijote.ugto.mx. † Universidad de Guanajuato. ‡ Universitat de Girona. J. Chem. Theory Comput. 2010, 6, 1131–1135 1131 10.1021/ct100098c  2010 American Chemical Society Published on Web 03/26/2010