  We investigate the role of topology and distortions in the quantum dynamics
of magnetic molecules, using a cyclic spin system as reference. We consider
three variants of antiferromagnetic molecular ring, i.e. Cr$_8$, Cr$_7$Zn and
Cr$_7$Ni, characterized by low lying states with different total spin $S$. We
theoretically and experimentally study the low-temperature behavior of the
magnetic torque as a function of the applied magnetic field. Near level
crossings, this observable selectively probes quantum fluctuations of the total
spin (''$S$ mixing") induced by lowering of the ideal ring symmetry. We show
that while a typical distortion of a model molecular structure is very
ineffective in opening new $S$-mixing channels, the spin topology is a major
ingredient to control the degree of $S$ mixing. This conclusion is further
substantiated by low-temperature heat capacity measurements.
