BR-81 AND I-127 NQR STUDIES OF (CH3NH3)2HGBR4 AND (CH3NH3)2HGI4

The Br-81 and I-127 NQR frequencies in (CH3NH3)2HgBr4 and (CH3NH3)2HgI4, respectively, have been recorded from above 77 K to around room temperature. The resonance lines exist in a wide range of frequencies. An unusual temperature dependence of the resonance frequencies has been observed. Small H-1-2D isotope effects have been observed between (CH3ND3)2HgBr4 and (CH3NH3)2HgBr4. Both negative and positive frequency differences between the deuterated and the nondeuterated compound have been observed at 77 K. The relatively large asymmetry parameters in the iodine compound decrease with increasing temperature, indicating the existence of H-bonds. The present results are interpreted qualitatively by considering the effects of H-bonding, N-H ... X, between the halogen atoms and the CH3NH3+ ions which may undergo large molecular motions. It is suggested that not only frequency-lowering but also frequency-rising effects may be produced, depending on the direction of the H-bond, i.e., the angle between the Hg-X bond and the X ... H bond.