Computation of through-space NMR shielding effects in aromatic ring π-stacked complexes

Aromatic compounds can form dimeric complexes in solution. Substituted aromatics tend to form parallel-stacked complexes, either aligned or offset. The HF-GIAO method in Gaussian 03 was employed to calculate the NMR isotropic shielding values of the proximal hydrogen of diatomic hydrogen probes above and below the center of the ring and above and below an unsubstituted ring carbon of 1,3,5-trimethylbenzene in a face-toface pi-stacked aligned complex with 1,3,5-trinitrobenzene. The calculated isotropic shielding values for the aromatic hydrogens of each of the substituted rings were subtracted from the isotropic shielding values calculated for the comparable positions in the complex. Complexation results in each aromatic ring shielding the other ring. Also, the calculated isotropic shielding values for the proximal hydrogen of a diatomic hydrogen probe over (or under) each of the individual substituted benzenes were subtracted from the isotropic shielding values calculated for the comparable positions in the complex. The difference is the shielding increment due to complexation. Complexation results in increased NMR shielding of a hydrogen probe molecule on both sides of the pi-stacked complex, with slightly more shielding due to complexation on the side nearest 1,3,5-trimethylbenzene. The results are interpreted in terms of polarization of the pi cloud of the substituted benzenes by complexation and its NMR consequences. Finally, NMR shielding calculations were done on the optimized structure of N-phenylpyrrole dimer. The data were compared to concentration-dependent NMR shift data to estimate the percent dimer present. (c) 2007 Elsevier Inc. All rights reserved.