H-1 AND N-14 NMR SHIELDING CONSTANTS DIFFERENTIATE 2 POLYMORPHIC FORMS OF SOLID NH4NO3

The gauge-independent atomic orbital (GIAO) approach has been used wit hin the coupled Hartree-Fock approximation to compute H-1 and N-14 NMR shielding constants in selected ion pairs of solid NH4NO3. Assuming a dditivity of intermolecular effects the shielding constants of NH4+ io ns in two different polymorphic forms of ammonium nitrate have been de termined. According to our calculations condensation of NH4NO3 leads t o small deshielding effects for H-1 and N-14 nuclei. Geometry of the c entral NH4+ cation is the most important factor influencing results of calculations of nuclear shielding constants. This geometry reflects t he role of intermolecular interactions in the crystal lattice. Our res ults show that the GIAO approach at the Hartree-Fock level differentia tes two polymorphic forms of ammonium nitrate. Some results on tempera ture dependences for H-1 and N-14 shielding constants are included. Th ere is a linear dependence between the H-1 and N-14 shielding constant s.