Context : The dense , cold regions where high-mass stars form are poorly characterized , yet they represent an ideal opportunity to learn more about the initial conditions of high-mass star formation ( HMSF ) since high-mass starless cores ( HMSCs ) lack the violent feedback seen at later evolutionary stages .

Aims : We investigate the initial conditions of HMSF by studying the dynamics and chemistry of HMSCs .

Methods : We present continuum maps obtained from the Submillimeter Array ( SMA ) interferometry at 1.1 mm for four infrared dark clouds ( IRDCs , G28.34S , IRDC 18530 , IRDC 18306 , and IRDC 18308 ) .
For these clouds , we also present line surveys at 1 mm/3 mm obtained from IRAM 30 m single-dish observations .

Results : ( 1 ) At an angular resolution of 2″ ( \sim 10 ^ { 4 } AU at an average distance of 4 kpc ) , the 1.1 mm SMA observations resolve each source into several fragments .
The mass of each fragment is on average > 10 M _ { \odot } , which exceeds the predicted thermal Jeans mass of the entire clump by a factor of up to 30 , indicating that thermal pressure does not dominate the fragmentation process .
Our measured velocity dispersions in the lines obtained by 30 m imply that non-thermal motion provides the extra support against gravity in the fragments .
( 2 ) Both non-detection of high-J transitions and the hyperfine multiplet fit of N _ { 2 } H ^ { + } ~ { } ( J = 1 \rightarrow 0 ) , C _ { 2 } H~ { } ( N = 1 \rightarrow 0 ) , HCN~ { } ( J = 1 \rightarrow 0 ) , and H ^ { 13 } CN~ { } ( J = 1 \rightarrow 0 ) indicate that our sources are cold and young .
However , the obvious detection of SiO and the asymmetric line profile of HCO ^ { + } ~ { } ( J = 1 \rightarrow 0 ) in G28.34 S indicate a potential protostellar object and probable infall motion .
( 3 ) With a large number of N-bearing species , the existence of carbon rings and molecular ions , and the anti-correlated spatial distributions between N _ { 2 } H ^ { + } / NH _ { 2 } D and CO , our large-scale high-mass clumps exhibit similar chemical features to small-scale low-mass prestellar objects .

Conclusions : This study of a small sample of IRDCs illustrates that thermal Jeans instability alone can not explain the fragmentation of the clump into cold ( T \sim 15 K ) , dense ( > 10 ^ { 5 } ~ { } cm ^ { -3 } ) cores and that these IRDCs are not completely quiescent .