Context. Ammonia and its deuterated isotopologues probe physical conditions in dense molecular cloud cores. The time-dependence of deuterium fractionation and the relative abundances of different nuclear spin modifications are supposed to provide a means of determining the evolutionary stages of these objects. Aims. We aim to test the current understanding of spin-state chemistry of deuterated species by determining the abundances and spin ratios of NH_(2)D , NHD_(2) and ND_(3) in a quiescent, dense cloud. Methods. Spectral lines of NH_(3) , NH_(2)D , NHD_(2) , ND_(3) and N_(2)D~(+) were observed towards a dense, starless core in Ophiuchus with the APEX, GBT and IRAM 30-m telescopes. The observations were interpreted using a gas-grain chemistry model combined with radiative transfer calculations. The chemistry model distinguishes between the different nuclear spin states of light hydrogen molecules, ammonia and their deuterated forms. Different desorption schemes can be considered. Results. High deuterium fractionation ratios with NH_(2)D/NH_(3) ~ 0.4 , NHD_(2)/ NH_(2)D ~ 0.2 and ND_(3)/ NHD_(2) ~ 0.06 are found in the core. The observed ortho/para ratios of NH_(2)D and NHD_(2) are close to the corresponding nuclear spin statistical weights. The chemistry model can approximately reproduce the observed abundances, but consistently predicts too low ortho/para- NH_(2)D , and too large ortho/para- NHD_(2) ratios. The longevity of N_(2)H~(+) and NH_(3) in dense gas, which is prerequisite to their strong deuteration, can be attributed to the chemical inertia of N_(2) on grain surfaces. Conclusions. The discrepancies between the chemistry model and the observations are likely to be caused by the fact that the model assumes complete scrambling in principal gas-phase deuteration reactions of ammonia, which means that all the nuclei are mixed in reactive collisions. If, instead, these reactions occur through proton hop/hydrogen abstraction processes, statistical spin ratios are to be expected. The present results suggest that while the deuteration of ammonia changes with physical conditions and time, the nuclear spin ratios of ammonia isotopologues do not probe the evolutionary stage of a cloud.
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