  (Abridged) The purpose of this paper is to explore the structure of a
line-rich Class I protobinary source, Oph-IRS 67, and analyse the differences
and similarities with Class 0 and Class II sources. We present a systematic
molecular line study of IRS 67 with the Submillimeter Array (SMA) on 1 - 2"
(150 - 300 AU) scales. The wide instantaneous band-width of the SMA
observations (~30 GHz) provide detections of a range of molecular transitions
that trace different physics, such as CO isotopologues, sulphur-bearing
species, deuterated species, and carbon-chain molecules. We see significant
differences between different groups of species. For example, the CO
isotopologues and sulphur-bearing species show a rotational profile and are
tracing the larger-scale circumbinary disc structure, while CN, DCN, and
carbon-chain molecules peak at the southern edge of the disc at blue-shifted
velocities. In addition, the cold gas tracer DCO+ is seen beyond the extent of
the circumbinary disc. The detected molecular transitions can be grouped into
three main components: cold regions far from the system, the circumbinary disc,
and a UV-irradiated region likely associated with the surface layers of the
disc that are reached by the UV radiation from the sources. The different
components are consistent with the temperature structure derived from the ratio
of two H2CO transitions, that is, warm temperatures are seen towards the
outflow direction, lukewarm temperatures are associated with the UV-radiated
region, and cold temperatures are related with the circumbinary disc structure.
The chemistry towards IRS 67 shares similarities with both Class 0 and Class II
sources, possibly due to the high gas column density and the strong UV
radiation arising from the binary system. IRS 67 is, therefore, highlighting
the intermediate chemistry between deeply embedded sources and T-Tauri discs.
