Relaxation times intermolecular interactions

First approaches to approximating the relaxation time on the basis of molecular parameters can be traced back to Rouse [33]. The model is based on a number of boundary assumptions (1) the solution is ideally dilute, i.e. intermolecular interactions are negligible (2) hydrodynamic interactions due to disturbance of the medium velocity by segments of the same chain are negligible and (3) the connector tension F(r) obeys an ideal Hookean force law. 

Jonas et al. measured the proton rotating frame spin-lattice relaxation time (Tip) at pressures from 1 bar to 5000 bar and at temperatures of 50 to 70 °C for DPPC and at 5 to 35 °C for POPC. If intermolecular dipolar interactions modulated by translational motion contribute significantly to the proton relaxation, the rotating frame spin-lattice relaxation rate (1/Tip) is a function of the square root of the spin-locking field angular frequency... 

In polypeptide blends, the balance of intra- and intermolecular hydrogen bond interactions in two kinds of polypeptide chains play an important role for the conformational stability and the blend miscibility. The observation of the 13 C NMR chemical shifts and relaxation times, and the two-dimensional... 

Strong intermolecular interactions such as hydrogen bonds or ion-dipole pairs restrict the motion of molecules and pertinent molecular segments. These interactions increase the correlation time zc and accelerate the 13C spin-lattice relaxation. Shorter 13C relaxation times can therefore also indicate the presence of such interactions. The Tj values of the C atoms of carboxylic acids, phenols, alcohols, and solvated molecular ions behave in this way. 

Spin-spin relaxation (time T2) can result from both intermolecular homo-nuclear exchange coupling and dipole-dipole interactions, but only the latter is observable at Mn2+ concentrations <10-2 M. Mn2+ forms both inner- and outer-sphere complexes. In symmetrical inner-sphere complexes like Mn(H20)62+, the spin-spin coupling is strongly forbidden, T2 is long, and lines remain narrow. When nonsymmetric inner-sphere complexes form, the resulting anisotropy of the electric field leads to allowed spin-spin transitions that produce very small values of T2 and very broad, perhaps even unobservable, lines (56). 

When the temperature rises, the Debye relaxation time Td and the fitted mean reorientation time xor decrease, since intermolecular interactions weaken and become more chaotic. 

Studies of intermolecular interactions by relaxation times in solution... 

However, the NMR properties of solid-phase methane are very complex, due to subtle effects associated with the permutation symmetry of the nuclear spin set and molecular rotational tunnelling.55 Nuclear spin states ltotai = 0 (irred. repr. E), 1 (T) and 2 (A) are observed. The situation is made more complicated since, as the solids are cooled and the individual molecules go from rotation to oscillation, several crystal phases become available, and slow transitions between them take place. Much work has been done in the last century on this problem, including use of deuterated versions of methane for example see Refs. 56-59. Much detail has emerged from NMR lineshape analysis and relaxation time measurements, and kinetic studies. For example, the second moment of the 13C resonance is found to be caused by intermolecular proton-carbon spin-spin interaction.60 Thus proton inequivalence within the methane molecules is created. 

The DS method occupies a special place among the numerous modern methods used for physical and chemical analysis of materials because it enables investigation of relaxation processes in an extremely wide range of characteristic times (105—10 13 s). Although the method does not possess the selectivity of NMR or ESR, it offers important and sometimes unique information about the dynamic and structural properties of substances. DS is especially sensitive to intermolecular interactions, and therefore cooperative processes may be monitored. It provides a link between the properties of the bulk and individual constituents of a complex material (see Fig. 1). 

The time scale of ET observed in these systems falls into the time scale of solvent relaxation and of nuclear motions and the reaction will be severely influenced by these dynamics. It is also the aim of the present study to investigate the role of intermolecular interaction, namely, solvent effects to the reaction as well as the effects of inter- and intramolecular dynamics. 

In intermolecular dipole-dipole relaxation, due to translational diffusion, the nuclear spins are not interacting long enough to create any spin correlation. Hence, each spin constitutes its own separate spin system and the expressions for the relaxation time only depends on the correlation function of the dipole-dipole interaction tensor. 

Furthermore, the lack of correlation of the different spins results in a decoupling of the different spin dipole-dipole interactions experience by the nuclear spin, Thus, even though there in general exists a correlation of the different dipole-dipole interaction tensors, the final expression for intermolecular relaxation time only consists of a sum of auto correlation functions of the individual interactions. 

---