Microstructure has been detected in Core D of TMC-1. Unless it is confined
or replenished by an as vet unexamined mechanism, this microstructure will
dissipate on the sound-crossing timescale, which is less than 10(5) yr. We
reconsider the large number of models that have been proposed to explain ch
emical Variations in TMC-1 in an effort to determine whether chemical const
raints require the microstructure to be confined or replenished for times m
uch longer than the sound crossing timescale. We explore here a chemical mo
del which, though consistent with the assumption of an age of B 10(4) yr an
d a number density of H nuclei of 2 10(5) cm(-3), shows the richness in bot
h molecular variety and abundances observed in Core D. In particular, the c
omputed HC3N fractional abundance relative to H-2 is 6 10(-8), in agreement
, with the latest observations of Ohishi & Kaifu (1998). Apparently, the ch
emistry of TMC-1 cannot be used to discard the possibility that TMC-1 Core
D is young. This model has the following characteristics: the cosmic ray io
nization rate is consistently larger than that usually assumed for dark reg
ional carbon atoms and hydrocarbons that strike grains are rapidly hydrogen
ated and promptly returned to the gas phase as methane; CO and N-2 striking
grains are immediately returned to the gas phase unaltered; other chemical
species containing at least one atom more massive than helium colliding wi
th dust grains remain frozen on their surfaces;;ind the material other than
hydrogen was initially in atomic form. For such a model to be viable, coll
apse of Core D must have been triggered by a stellar wind-driven shock of s
everal kms This speed is low enough that magnetic moderation of the shock w
ould have prevented the activation of a high temperature chemistry. The mod
el results indicate that Core D would have fractional abundances of H2O and
O-2 at a core age of g 10(4) yr consistent with the upper limits placed by
very recent observations of Core D made with SWAS. The implications of thi
s study are (1) that hydrogenation of atoms at the surfaces of dust grains
may be a significant contributor to the chemistry of dark clouds; (2) that
the special chemical nature of TMC-1 is due primarily to the exceptional yo
uth of Core D.