Weblab considers segmental motion on a cone in powders or uni-axial ordered samples. Motion is understood either as 2-,3-,4-,6-site jumps or as rotation (only in fast motion limit) around the cone axis. Jumps are assumed to occur instantaniously. Except for 2- and 3-site jumps all sites are assumed to be distributed equally on the cone. With respect to a cone axes system (CF), whose z-axis points along the cone axis (see picture), each site i is characterized by one and the same polar angle θ, which is half the cone opening angle and the azimutal angle φi. The orientation of x(CF) does not influence the final spectrum after integration over the sphere. Hence, only the jump angle ϕ = φi+1 - φi has to be supplied by the user. See also topic ⇒ Type of Motion See also topic ⇒ Number of Cone Axes

Weblab assumes, that the interaction between spin and surrounding can be described by a symmetric and traceless 2nd rank tensor. This holds for the quadrupolar and dipole-dipole interaction but also for chemical shielding after subtracting the isotropic part. It is further assumed, that the interaction itself does not change during the motion. If the principal axis z^(PAS) coincides with the segment axis (CH² bond in the picture), the dynamics can be fully described by cone and jump angle as well as jump rate and steady state populations. If the interaction is not axial symmetric, euler angle α is additionally required. Alpha characterizes the relative orientation of both axes systems and can be considered as the angle, by which the tensor has to be rotated around z^(PAS) to place y^(PAS) along the node line, or what is the same, to move x^(PAS) into a common plane together with z^(PAS) and z^(CF).

Weblab uses euler angles according to the convention of Rose. For a description see here