Determination of 13C Chemical Shift Tensors in the Presence of Hydrogen Bonding and 14N Quadrupolar Coupling: p-Aminosalicylic Acid, Isoniazid, and Pyrazinamide

The 1 3 C chemical-shift tensor principal values were measured for p-aminosalicylic acid, pyrazinamide, and isoniazid. A moderate-strength hydrogen bond between adjacent p-aminosalicylic acid molecules in the crystal was simulated several different ways in attempts to find a reasonable model of the electrostatic interactions in the crystal. Crystal lattice effects on the calculated chemical-shift tensors are treated by application of the embedded ion method (EIM). Coupling between 1 4 N and several 1 3 C atoms was included in the data analysis to provide more accurate 1 3 C chemical-shift tensor principal values. The chemical-shift tensors are assigned with a permutative algorithm based upon the computed chemical shifts.