Hyperfine structure in the J=1-0 transitions of DCO+, DNC, and (HNC)-C-13: astronomical observations and quantum-chemical calculations

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+, (HNC)-C-13 and DNC with the IRAM 30 m 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 (HNC)-C-13, 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 (HNC)-C-13, and eQq(D) = 257.6 kHz and eQq(N) = 309.6 kHz for DNC. We also derive updated rotational constants for (HNC)-C-13: B = 43 545.6000 (47) MHz and D = 93.7 (20) kHz. Conclusions. The hyperfine splittings of the DCO+, DNC and (HNC)-C-13 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 similar to 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 N-14 quadrupole parameters of DNC and (HNC)-C-13 have been traced to differences in the vibrational corrections caused by significant non-rigidity of these molecules, particularly along the bending coordinate