
%0 Journal Article
%A Doddrell, DM 
%A Bendall, MR 
%A O'Connor, AJ 
%A Pegg, DT 
%T Measurement, employing <sup>1</sup>H and <sup>13</sup>C spin-lattice relaxation times, of the temperature dependence of the correlation time for rotational reorientation (&#964;<sub>R</sub>)of diamagnetic transition-metal Bis- and Tris-acetylacetonate complexes
%J Australian Journal of Chemistry
%D 1977/05/01
%V 30
%N 5
%P 943-956
%K 
%R https://doi.org/10.1071/CH9770943
%U https://www.publish.csiro.au/paper/CH9770943 
%X 1H and 13C nuclear relaxation times for the ligand positions of Pd(acac)2 and Co(acac)3 (acac = acetyl-acetonate) have been measured as a function of temperature. Nuclear Overhauser enhancement factors are used to demonstrate that except for the methyl carbon nucleus internuclear dipolar interactions dominate the relaxation times. Consequently TR can be extracted from the data. It is found that TR for a 0.05 M solution in CDCl3 varies with temperature (T) as               TR = 3.92x10-13exp(1.38x103/T)      (Co)              TR = 4.42x10-13exp(1.20x103/T)      (Pd)   Activation energies for rotational reorientation calculated from the data are found to be 9-10 kJ mol-1 for both complexes. There appears to be a slight concentration dependence of ER, ER being about 12 kJ mol-1 for a 0.2 M solution of the palladium complex. Spin internal reorientation affects the T1 value of the CH3 carbon of Pd(acac)2, with T1 getting shorter as the temperature is raised.   A simple single-axis model of rotational reorientation in solution is presented whereby it can be shown that ER is the average barrier energy encountered by a molecule during a single angular reorientation. 
