  Clouds of high infrared extinction are promising sites of massive
star/cluster formation. A large number of cloud cores discovered in recent
years allows investigation of possible evolutionary sequence among cores in
early phases. We have conducted a survey of deuterium fractionation toward 15
dense cores in various evolutionary stages, from high-mass starless cores to
ultracompact Hii regions, in the massive star-forming clouds of high
extinction, G34.43+0.24, IRAS 18151-1208, and IRAS 18223-1243, with the
Submillimeter Telescope (SMT). Spectra of N2H+ (3 - 2), N2D+ (3 - 2), and C18O
(2 - 1) were observed to derive the deuterium fractionation of N2H+, Dfrac
\equiv N(N2D+)/N(N2H+), as well as the CO depletion factor for every selected
core. Our results show a decreasing trend in Dfrac with both gas temperature
and linewidth. Since colder and quiescent gas is likely to be associated with
less evolved cores, larger Dfrac appears to correlate with early phases of core
evolution. Such decreasing trend resembles the behavior of Dfrac in the
low-mass protostellar cores and is consistent with several earlier studies in
high-mass protostellar cores. We also find a moderate increasing trend of Dfrac
with the CO depletion factor, suggesting that sublimation of ice mantles alters
the competition in the chemical reactions and reduces Dfrac. Our findings
suggest a general chemical behavior of deuterated species in both low- and
high-mass proto-stellar candidates at early stages. In addition, upper limits
to the ionization degree are estimated to be within 2 \times 10^-7 and 5 \times
10^-6. The four quiescent cores have marginal field-neutral coupling and
perhaps favor turbulent cooling flows.
