The molecular evolution that occurs in collapsing prestellar cores is investigated .
To model the dynamics , we adopt the Larson-Penston solution and analogues with slower rates of collapse .
For the chemistry , we utilize the new standard model ( NSM ) with the addition of deuterium fractionation and grain-surface reactions treated via the modified rate approach .
The use of surface reactions distinguishes the present work from our previous model .
We find that these reactions efficiently produce H _ { 2 } O , H _ { 2 } CO , CH _ { 3 } OH , N _ { 2 } , and NH _ { 3 } ices .
In addition , the surface chemistry influences the gas-phase abundances in a variety of ways .
For example , formation of molecular nitrogen on grain surfaces followed by desorption into the gas enhances the abundance of this gas-phase species and its daughter products N _ { 2 } H ^ { + } and NH _ { 3 } .
The current reaction network along with the Larson-Penston solution allows us to reproduce satisfactorily most of the molecular column densities and their radial distributions observed in L1544 .
The agreement tends to worsen with models that include strongly delayed collapse rates .
Inferred radial distributions in terms of fractional abundances are somewhat harder to reproduce .
In addition to our standard chemical model , we have also run a model with the UMIST gas-phase chemical network .
The abundances of gas-phase sulphur-bearing molecules such as CS and CCS are significantly affected by uncertainties in the gas-phase chemical network .
In all of our models , the column density of N _ { 2 } H ^ { + } monotonically increases as the central density of the core increases during collapse from 3 \times 10 ^ { 4 } cm ^ { -3 } to 3 \times 10 ^ { 7 } cm ^ { -3 } .
Thus , the abundance of this ion can be a probe of evolutionary stage .
Molecular D/H ratios in assorted cores are best reproduced in the Larson-Penston picture with the conventional rate coefficients for fractionation reactions .
If we adopt the newly measured and calculated rate coefficients , the D/H ratios , especially N _ { 2 } D ^ { + } /N _ { 2 } H ^ { + } , become significantly lower than the observed values .