  We present an experimental and theoretical study of the optically detected
magnetic resonance signals for ensembles of negatively charged nitrogen-vacancy
(NV) centers in 13C isotopically enriched single-crystal diamond. We observe
four broad transition peaks with superimposed sharp features at zero magnetic
field and study their dependence on applied magnetic field. A theoretical model
that reproduces all qualitative features of these spectra is developed.
Understanding the magnetic-resonance spectra of NV centers in isotopically
enriched diamond is important for emerging applications in nuclear magnetic
resonance.
