On the Aromaticity of the Planar Hydrogen-Bonded (HF)₃ Trimer

Abstract

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···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.

Aromaticity of the doubly charged [8]circulenes

“…8 However, the NICS approach cannot be employed for assessing the aromatic character of more complicated molecules, 9,10 which has previously been discussed in a number of studies of the aromatic character of polycyclic aromatic hydrocarbons, 11,12 porphyrins, 13 all-metal aromatic compounds [14][15][16][17][18] and hydrogen-bonded molecular clusters. 19 A more direct approach to evaluate the aromatic or antiaromatic nature of molecules as a whole is to use the gaugeincluding magnetically induced current (GIMIC) method. [20][21][22][23][24][25][26] The GIMIC approach employs gauge-including atomic orbitals (GIAOs) in explicit calculations of the magnetically induced current density susceptibilities, current strengths and current pathways in molecules providing detailed information about electronic delocalization properties.…”

New insights into aromatic pathways of carbachlorins and carbaporphyrins based on calculations of magnetically induced current densities

“…The approach has proven to be very useful for determining current pathways in multiring molecules, where many other approaches are prone to fail. 22,[33][34][35][36][37][38][39][40][41][42][43][44][45][46][47] In this work, we have employed the GIMIC method at the density functional theory (DFT) level to investigate the aromatic character of a number of traditional carbaporphyrinoids and modified carbaporphyrinoids such as oxybenziporphyrin, [48][49][50] benzocarbaporphyrin, 51,52 azuliporphyrin 51,53,54 and tropiporphyrin. 55,56 In addition, we have also studied a number of carbachlorins that have been synthesized and characterized.…”

Aromatic pathways in conjugated rings connected by single bonds

“…The strength of the current passing selected chemical bonds is obtained by numerical integration of the current density passing through a plane perpendicularly to the investigated bond. The gimic method has previously been used in aromaticity studies of hydrocarbons [27][28][29][30][31], clusters [32][33][34][35], and hydrogen-bonded complexes [36]. The method has recently been extended for applications on open-shell species (Jusélius, Taubert, Sundholm, in preparation) rendering aromaticity studies on doublet, triplet, and higher multiplet states feasible.…”