Ab initio study of NMR shielding constants and spin-rotation constants in N, P and As diatomic molecules

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Ab initio calculations of NMR shielding constants and spin-rotation constants of six diatomic molecules XY where X, Y = N, P or As are described. We analyse the dependence of the results on the description of the correlation effects and their convergence with the extension of the basis set. Our best results, obtained at the CCSD(T)/cc-pV5Zs,p-unc level, appear to be sufficiently accurate for the smaller diatomics—N 2, PN and P 2; for the AsN, AsP and As 2 molecules large relativistic effects should be taken into account. Moreover, relativistic four-component density functional calculations of NMR shielding constants for the latter molecules demonstrate that the relativistic corrections cannot be determined at the Hartree–Fock level. The computed spin-rotation constants for PN and AsP can be compared with experimental data, the differences between theory and experiment are below 2% for PN and ≈10% for AsP.