Proton Tunnelling in Hydrazinium Cations: Vibrational Spectra of (N2H5)2HGaF6DT2H2O and (N2H5)2HFeF6DT2H2O and Crystal Structure of (N2H5)2HFeF6DT2H2O at Various Temperatures

• Published in: Journal of Raman spectroscopy 1997-05, Vol.28 (5), p.315-321
• Main Authors: Milicev, S, Rahten, A, Borrmann, H, Siftar, J
• Format: Article
• Language: English
• ISSN: 0377-0486
• DOI: 10.1002/(SICI)1097-4555(199705)28:5<315::AID-JRS98>3.0.CO;2-D

Infrared spectra of (N2H5)2HGaF6DT2H2O and of (N2H5)2HFeF6DT2H2O indicate weak to moderate N - HDTDTDTN hydrogen bonding. Data are interpreted in terms of a double minimum symmetrical potential well with a high barrier and statistical distribution of hydrogen in the minima on both sides of the barrier, forming an H3N+H2NDTDTDTH+NH2NH3+ hydrogen-bonded cluster with simultaneous presence of N2H5+ and N2H62+ ions. The strong Raman NN stretching band, which is characteristic of the type of cation, is split into two bands when cooled to 77 K and merged again when warmed to room temperature. This indicates that H-bridged N2H5+ ions are spectroscopically indistinguishable at room temperature, presumably because of a high rate of phonon-assisted proton tunnelling through the barrier. This process is inhibited at lower temperature, thus making the N2H5+ and N2H62+ ions distinguishable. H2O is only weakly hydrogen bonded, not protonated and therefore fairly easily released. Octahedral symmetry of GaF63- and of FeF63- ions is demonstrated. Single-crystal x-ray structure determinations on the Fe compound at three different temperatures (296, 200 and 90 K) corroborate the spectroscopic results; in particular, the statistical distribution of one H-atom within the cation was revealed. No structural phase transition was observed down to 90 K. The compound crystallizes in space group C2/c (No. 15) with a=1444.28(13) pm, b=640.09(6) pm, c=1048.09(9) pm, =118.942(6)DG and Z=4 at 90 K. The octahedron around Fe is slightly compressed with Fe - F distances along the pseudo-fourfold axis at 193.07 pm shorter than those within the plane (193.86 pm on average). The magnetic moment of the Fe(III) complex was determined as 5.7 B.