  Motivated by the intriguing properties of magnetic molecular wheels at field
induced level crossings, we investigate the spin-Peierls instability of
antiferromagnetic rings in a field by exact diagonalizations of a microscopic
spin model coupled to the lattice via a distortion dependent
Dzyaloshinskii-Moriya interaction. We show that, beyond the unconditional
instability at level crossings for infinitesimal magnetoelastic coupling, the
model is characterized by a stronger tendency to distort at higher level
crossings, and by a dramatic angular dependence with very sharp torque
anomalies when the field is almost in the plane of the ring. These predictions
are shown to compare remarkably well with available torque and Nuclear Magnetic
Resonance data on CsFe8.
