LATTICE VIBRATION-SPECTRA .51. RAMAN-SPECTROSCOPIC STUDIES OF POLYMORPHIC N2H7I - PHASE-TRANSITION, ORIENTATIONAL DISORDER AND HYDROGEN-BONDING

The Raman spectra of the various polymorphs of N2H7I are presented and discussed in terms of phase transitions, orientational disorder, hydrogen bonding, and structure of the N2H7+ ion. There is a clear distinction between the vibrational modes of the NH4+ ion and the NH3 ligand even at ambient temperature, indicating a structure like H3NH⋯NH3 with strong intra-ionic hydrogen bonding. The high orientational disorder of N2H7+ in N2H7I I and II is mainly due to torsional jumps of the two NH3 units. The inherent symmetry of the N2H7+ ion is C3v in N2H7I I (cP10), II (tP10), and III (oS20), and Cs in the low-temperature polymorph N2H7I IV. The strength of the interionic NH⋯I bonds is weaker than those of the NH4I polymorphs (NH stretching modes of the isotopically dilute samples in 2346 as compared with 2295 cm-). © 1990.