  Using new as well as known results on dimerized quantum spin chains with
frustration, we are able to infer some properties on the low-energy spectrum of
the O(3) Nonlinear Sigma Model with a topological theta-term. In particular,
for sufficiently strong coupling, we find a range of values of theta where a
singlet bound state is stable under the triplet continuum. On the basis of
these results, we propose a new renormalization group flow diagram for the
Nonlinear Sigma Model with theta-term.
