On the aromaticity of the planar hydrogen-bonded (HF)3 trimer

The nuclear magnetic shieldings and magnetically induced ring currents have been calculated for the planar ring-shaped hydrogen fluoride trimer (HF)(3) at correlated ab initio and density functional theory levels. Calculations of the magnetically induced current densities using the gauge-including magnetically induced current (GIMIC) method show that, contrary to a recent suggestion, (HF)(3) has, at the MP2/TZVPP level, a very small ring-current susceptibility of 0.37 nA/T. Thus, only a weak net current is passing across the H center dot center dot center dot F hydrogen bond. An external magnetic field perpendicular to the ring plane induces strong edge currents circling around each HF molecule giving rise to a nonvanishing magnetic shielding at the center of the ring. The GIMIC results are supported by calculations of the long-range magnetic shielding function; the long-range magnetic shielding is very small, indicating that the magnetically induced ring- current is very weak. The surprisingly large nucleus-independent chemical shift (NICS) value for (HF)(3) was recently taken as an indication of H-bonded aromaticity. The NICS value calculated at the CCSD/QZ2P level is 2.77 ppm. The present GIMIC and aromatic ring- current shielding study shows that some care has to be taken when using NICS values as aromaticity indices.