SIMBAD references

We show analytically that large abundances of NH₂D and NHD₂ can be produced by gas-phase chemistry in the interiors of cold dense clouds. The calculated fractionation ratios are in good agreement with the values that have been previously determined in L134N and suggest that triply deuterated ammonia could be detectable in dark clouds. Grain-surface reactions may lead to similar NH₂D and NHD₂ enhancements but, we argue, are unlikely to contribute to the deuteration observed in L134N.