Multiple quantum dynamics in linear chains and rings of nuclear spins in solids at low temperatures.

Multiple quantum spin dynamics is studied using analytical and numerical methods for one-dimensional finite systems of nuclear spins 12 coupled by dipole-dipole interactions at low temperatures. Exact expressions for intensities of multiple quantum coherences at low temperatures were obtained in the approximation of the nearest neighbor interactions. The time growth of multiple quantum coherences was analyzed numerically when all the dipole-dipole interactions in one-dimensional systems consisting of 6/8 spins were taken into account. It is shown that the growth of multiple quantum coherences gets faster when the temperature decreases, and the intensities of multiple quantum coherences can be negative at low temperatures.