  The spectra of mass-11 nuclei are unusual, and so pose a challenge for
theoretical models of their structure. The set of isobars range from being well
bound to nucleon emission (11B,11C) through weakly bound (11Li, 11Be), with the
former taking the form of a two neutron halo, to being proton unstable
(11N,11O). A self-consistent approach to understand this set of nuclei is
especially important as the mirror pair 11Be-11N exhibit a parity-inverted
ground state compared to their neighbouring nuclei. Herein the Multi-Channel
Algebraic Scattering (MCAS) method has been used to describe the low excitation
spectra of those isobars in terms of nucleon-nucleus clusters. A collective
model description of the low-excitation states of the mass-10 core nuclei has
been used to form the coupled-channel interactions required in the method. For
comparison, and to understand the underlying configurations, a shell model
approach has been used to obtain those spectra with no-core (0+2+4)hw and
(0+2)hw shell-model spaces for the mass 10 and mass 11 nuclei respectively.The
results of the calculations suggest the need of a strong coupling in the
collective coupled-channel vibrational model. In particular, the strong
coupling of the collective 2+_1 state of 10Be to the valence neutron plays a
decisive role in forming the positive parity ground state in11Be.
