Measurement, employing ¹H and ¹³C spin-lattice relaxation times, of the temperature dependence of the correlation time for rotational reorientation (τ_R)of diamagnetic transition-metal Bis- and Tris-acetylacetonate complexes

Abstract

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 T_R can be extracted from the data. It is found that T_R for a 0.05 M solution in CDCl₃ varies with temperature (T) as T_R = 3.92x10^-13exp(1.38x10³/T) (Co) T_R = 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 E_R, E_R 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 E_R is the average barrier energy encountered by a molecule during a single angular reorientation.