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

作者：Mookherjee M; Welch MD^*; Le Polles L; Redfern SAT; Harlov DE

来源：Physics and Chemistry of Minerals, 2005, 32(2): 126-131.

DOI：10.1007/s00269-005-0455-x

• buddingtonite
• ammonium
• H-2 NMR spectroscopy
• infrared spectroscopy

The behaviour of the ammonium ion in synthetic buddingtonite, N(D,H)(4)AlSi3O8, has been studied by infrared (IR) spectroscopy from 20 K to 298 K and by H-2 NMR spectroscopy from 120 K to 298 K. IR spectra were collected from 500 to 3500 cm(-1). Static H-2 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 mu s) 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-Hcenter dot center dot center dotO hydrogen bond, with precession of the ion about it, as proposed by Kimball and Megaw (1978), because any N-Hcenter dot center dot center dotO 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.

• 出版日期2005-6