  We have observed the Oph A-N6 prestellar core in the following transitions:
N2D+ J=3 to 2, DCO+ J=3 to 2 and J=5 to 4, HCO+ J=3 to 2, CS J=5 to 4 and J=7
to 6, and H13CO+ J=3 to 2 and J=4 to 3, using the James Clerk Maxwell
Telescope. We also observed the NH3 (1,1) and (2,2) inversion transitions
towards the Oph A-N6 peak with the Green Bank Telescope. We have found that the
N6 core is composed of shells of different chemical composition due to the
freezing out of chemical species at different densities. The undepleted species
N2D+ appears to trace the high-density interior of the core, DCO+ and H13CO+
trace an intermediate region, and CS traces the outermost edges of the core. A
distinct blue-red spectral asymmetry, indicative of infall motion, is clearly
detected in the HCO+ spectra, suggesting that N6 is undergoing gravitational
collapse. This collapse was possibly initiated by a decrease in turbulent
support suggested by the fact that the non-thermal contribution to the line
widths is smaller for the molecular species closer to the center of the core.
We also present a temperature profile for the core. These observations support
the claim that the Oph A-N6 core is an extremely young prestellar core, which
may have been recently cut off from MHD support and begun to collapse.
