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(Abridged) We aim to determine the degrees of CO depletion, deuteriumfractionation, and ionisation in a sample of seven massive clumps associatedwith IRDCs. The APEX telescope was used to observe the C17O(2-1), H13CO+(3-2),DCO+(3-2), N2H+(3-2), and N2D+(3-2) transitions towards the clumps. The COmolecules do not appear to be significantly depleted in the observed clumps.The DCO+/HCO+ and N2D+/N2H+ column density ratios are about 0.0002-0.014 and0.002-0.028, respectively. The former ratio is found to decrease as a functionof gas kinetic temperature. A simple chemical analysis suggests that the lowerlimit to the ionisation degree is in the range x(e)~10^{-8}-10^{-7}, whereasthe estimated upper limits range from a few 10^{-6} up to ~10^{-4}. Lowerlimits to x(e) imply the cosmic-ray ionisation rate of H2 to lie betweenzeta_H2~10^{-17}-10^{-15} s^{-1}. These are the first estimates of x(e) andzeta_H2 towards massive IRDCs reported so far. The finding that CO is notdepleted in the observed sources conforms to the fact that they show evidenceof star formation activity which is believed to release CO from the icy grainmantles back into the gas phase. The observed degree of deuteration is lowerthan in low-mass starless cores and protostellar envelopes. Decreasingdeuteration with increasing temperature is likely to reflect the clumpevolution. On the other hand, the association with young high-mass stars couldenhance zeta_H2 and x(e) above the levels usually found in low-massstar-forming regions. On the scale probed by our observations, ambipolardiffusion cannot be a main driver of clump evolution unless it occurs ontimescales >>10^6 yr.

Deuterium fractionation and the degree of ionisation in massive clumps within infrared dark clouds