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 H ii 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 \mathrm { N _ { 2 } H ^ { + } } ( 3 - 2 ) , \mathrm { N _ { 2 } D ^ { + } } ( 3 - 2 ) , and \mathrm { C ^ { 18 } O } ( 2 - 1 ) were observed to derive the deuterium fractionation of \mathrm { N _ { 2 } H ^ { + } } , D _ { \mathrm { frac } } \equiv N ( \mathrm { N _ { 2 } D ^ { + } } ) / N ( \mathrm { N _ { 2 } H ^ { + } } ) , as well as the CO depletion factor for every selected core .
Our results show a decreasing trend in D _ { \mathrm { frac } } with both gas temperature and linewidth .
Since colder and quiescent gas is likely to be associated with less evolved cores , larger D _ { \mathrm { frac } } appears to correlate with early phases of core evolution .
Such decreasing trend resembles the behavior of D _ { \mathrm { frac } } 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 D _ { \mathrm { frac } } with the CO depletion factor , suggesting that sublimation of ice mantles alters the competition in the chemical reactions and reduces D _ { \mathrm { frac } } .
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 .