  We have observed the rotational ground-state (J = 1-0) transitions of DCO+,
HN13C and DNC with the IRAM 30m telescope toward the dark cloud LDN 1512 which
has exceptionally narrow lines permitting hyperfine splitting to be resolved in
part. The measured splittings of 50-300 kHz are used to derive nuclear
quadrupole and spin-rotation parameters for these species. The measurements are
supplemented by high-level quantum-chemical calculations using coupled-cluster
techniques and large atomic-orbital basis sets.
  We find eQq = +151.12 (400) kHz and C_I = -1.12 (43) kHz for DCO+, eQq =
272.5 (51) kHz for HN13C, and eQq(D) = 265.9 (83) kHz and eQq(N) = 288.2 (71)
kHz for DNC. The numbers for DNC are consistent with previous laboratory data,
while our constants for DCO+ are somewhat smaller than previous results based
on astronomical data. For both DCO+ and DNC, our results are more accurate than
previous determinations. Our results are in good agreement with the
corresponding best theoretical estimates. We also derive updated rotational
constants for HN13C: B = 43545.6000 (47) MHz and D = 93.7 (20) kHz.
  The hyperfine splittings of the DCO+, DNC and HN13C J = 1-0 lines range over
0.47-1.28 km/s, which is comparable to typical line widths in pre-stellar cores
and to systematic gas motions on ~1000 AU scales in protostellar cores. We
present tabular information to allow inclusion of the hyperfine splitting in
astronomical data interpretation. The large differences in the 14N quadrupole
parameters of DNC and HN13C have been traced to differences in the vibrational
corrections caused by significant non-rigidity of these molecules, particularly
along the bending coordinate.
