Context : Knowledge of the hyperfine structure of molecular lines is useful for estimating reliable column densities from observed emission , and essential for the derivation of kinematic information from line profiles .

Aims : Deuterium bearing molecules are especially useful in this regard , because they are good probes of the physical and chemical structure of molecular cloud cores on the verge of star formation .
However , the necessary spectroscopic data are often missing , especially for molecules which are too unstable for laboratory study .

Methods : We have observed the ground-state ( J = 1 - 0 ) rotational transitions of DCO ^ { + } , HN ^ { 13 } C 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 .

Results : We find eQq = +151.12 ~ { } ( 400 ) kHz and C _ { I } = -1.12 ~ { } ( 43 ) kHz for DCO ^ { + } , eQq = 272.5 ~ { } ( 51 ) kHz for HN ^ { 13 } C , 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 , which amount to eQq = 156.0 kHz and C _ { I } = -0.69 kHz for DCO ^ { + } , eQq = 279.5 kHz for HN ^ { 13 } C , and eQq ( { D } ) = 257.6 kHz and eQq ( { N } ) = 309.6 kHz for DNC .
We also derive updated rotational constants for HN ^ { 13 } C : B =43545.6000 ( 47 ) MHz and D =93.7 ( 20 ) kHz .

Conclusions : The hyperfine splittings of the DCO ^ { + } , DNC and HN ^ { 13 } C J = 1 - 0 lines range over 0.47–1.28 km s ^ { -1 } , which is comparable to typical line widths in pre-stellar cores and to systematic gas motions on \sim 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 ^ { 14 } N quadrupole parameters of DNC and HN ^ { 13 } C have been traced to differences in the vibrational corrections caused by significant non-rigidity of these molecules , particularly along the bending coordinate .