Nuclear Spin Relaxation in Liquids. Spheroidal Molecules

Abstract

Nuclear spin—lattice relaxation times have been measured as a function of temperature for a number of liquid hydrocarbons. These data, together with other available measurements on rigid, spheroidal molecules, are compared with the Bloembergen, Purcell, and Pound (B.P.P.) theory of spin—lattice relaxation in liquids, and it is shown that the B.P.P. calculation of the rotational contribution to the relaxation time gives a value which are much shorter than the total experimental relaxation times. It is then assumed that the time dependence of the rotational angular autocorrelation functions of these molecules is dominated by dynamical coherence, rather than by frictional forces as assumed in the B.P.P. theory. If one calculates the net spin—lattice relaxation time T1 by summing the translational contribution calculated from the B.P.P. theory and the rotational contribution calculated from an approximate autocorrelation function valid for small but nonzero friction constants, one obtains values for T1 which are in quantitative agreement with the data for nonpolar, spheroidal molecules. Furthermore, when the two calculations are compared with the data for polar spheroidal molecules, it is seen that frictional forces are the predominant factor in the rotational motion of these systems, but the values obtained for the rotational friction constant from the Stokes—Einstein equation are in error by a considerable amount.

Carbon-13 and silicon-29 spin-lattice relaxation in phenylmethanes and phenylsilanes

“…Motion relative to the laboratory frame (symbol zlL ) would be six times as fast as is indicated by ze, because of the unusual definition of z e . The value "ell = 0'05 ps is very close to the correlation time expected for rotational diffusion in the limit of inertial control (~ = 0-057 ps), as calculated from the equation given by Moniz et al [43] l ( nI ~ 1/2…”

A study of molecular re-orientation in liquid acetonitrile by nuclear spin-lattice relaxation

“…This value is small as expected for a methyl group on a linear molecule. However, it is much larger than would be predicted for the temperature dependence in the case where rotational inertial effects predominate [13]. If the inertial effects are relatively minor, the correlation times of the deuterated and protonated acetonitrile may be compared without considering the difference in molecular masses.…”

Use of a Gettering Material to obtain Oxygen-free Liquid Samples for Nuclear Spin Relaxation Studies

“…It may be pointed out that the technique described is quite different from that used by Moniz et al (7) and is a considerable improvement over the one described by Lees et al (6). Some of the features of this technique are: (I) The joint G (Fig.…”

“…It is, therefore, necessary to remove oxygen from the liquid sample in order to study T I of the pure liquid. In addition to methods involving bubbling of helium, distillation under vacuum, or freezepumpthaw techniques (1)(2)(3)(4)(5), misch metal (6) and barium (7) have been used for removing oxygen from liquid samples so that T I could be measured in pure liquids. Magnesium (1) has also been used to degas in certain instances but its use as a getter has not been investigated in any detail.…”