Proton tunnelling frequencies in ammonium salts from spin-lattice relaxation

Abstract

Proton reorientation tunnelling in many NH 4 + salts gives a frequency independent minimum in both T 1 and T 1 rho at the transition to classical relaxation at about 40-70K. The average tunnel frequency is approximately: f 1 approximately=4*10 9 Ti 1( rho )min, (Hz when T 1 or T 1 rho is in s). Tunnel frequencies are derived from T 1 and T 1 rho measurements on NH 4 ClO 4 , cubic and trigonal (NH 4 ) 2 SiF 6 , (NH 4 ) 2 SnCl 6 , (NH 4 ) 2 Ce(NO 3 ) 6 , (NH 4 ) 2 Cr 2 O 7 and literature data, and compared with tunnel frequencies calculated from barrier heights.