ISOTOPE AND TEMPERATURE EFFECTS ON NUCLEAR MAGNETIC SHIELDINGS AND SPIN-ROTATION CONSTANTS CALCULATED AT THE COUPLED-CLUSTER LEVEL

The temperature dependence of nuclear shieldings as well as isotope ef fects on shieldings and spin-rotation constants have been computationa lly investigated for H-2, HF, F-2, CO, and N-2 employing the coupled-c luster singles and doubles (CCSD) method augmented by a perturbative t reatment of triple excitations (CCSD(T)) for the calculation of potent ial curves, shieldings and spin-rotation functions together with finit e-element techniques for the solution of the rovibrational problem. Ca lculated and measured temperature dependence of the isotropic shieldin gs agrees for N-2, while for CO and F-2 the computed temperature depen dence is smaller than the experimental result. Isotropic shieldings ha ve been deduced on the basis of our calculations from the measured spi n-rotation constants for four isotopomers of H-2 and agree, as require d by theory. However, calculated and measured temperature dependence o f the isotope shifts between HD and D-2 differ by up to 10% which is l arger than the estimated error bars for the experimental values. For H F and CO, calculated and measured isotope shifts agree, while for N-2 no experimental data for comparison are available. In case of spin-rot ation constants, the calculated dependence on the rotational angular m omentum quantum number are for both H-2 and F-2 in good agreement with the dependence deduced from measurements, while for HF not enough exp erimental data are available for a comparison.