  We study the self-trapping of quasiparticles (electrons, holes, excitons,
etc) in a molecular chain with the structure of a ring, taking into account the
electron-phonon interaction and the radial and tangential deformations of the
chain. A discrete system of equations is obtained and solved numerically. The
analytical solutions for the wave function of a quasiparticle and for the
molecule displacements that determine the distortion of the ring, are also
obtained and solved in the continuum approximation. The numerical solutions of
the system of discrete nonlinear equations reveals several regimes of
quasiparticle localisation in the chain which depend on the values of the
parameters of the system. It is shown that the transversal deformation of the
chain favours the formation of a soliton.
