Ammonium ion behaviour in feldspar: Variable-temperature infrared and ²H NMR studies of synthetic buddingtonite, N(D,H)₄ AlSi₃O₈

The behaviour of the ammonium ion in synthetic buddingtonite, N(D,H)₄AlSi₃O₈, has been studied by infrared (IR) spectroscopy from 20 K to 298 K and by ²H NMR spectroscopy from 120 K to 298 K. IR spectra were collected from 500 to 3500 cm^-1. Static ²H NMR spectra collected at 298 K and 120 K are very similar, consisting of a single sharp isotropic resonance, indicating complete averaging of quadrupolar interactions and implying that at these temperatures the ammonium ion is in rapid (<1 μs) randomised motion within the M-site cavity of the feldspar framework. NMR spectroscopy indicates that the splitting of the internal modes of the ammonium ion observed by IR spectroscopy is not due to "freezing in" of the ammonium ion. This observation rules out the formation of a preferred N-H...O hydrogen bond, with precession of the ion about it, as proposed by Kimball and Megaw (1978), because any N-H...O hydrogen bond must be very weak and transient in nature. Contraction of the cavity site upon cooling imposes a distortion upon the ammonium ion that affects vibrational modes. This distortion does not affect the motion of the ammonium ion as observed on the NMR time-scale.