The Electronic Structure of Zirconium and Hafnium Monochalcogenides

High-level ab initio CCSD(T) and spin–orbit
icMRCI+Q calculations were used to predict potential energy curves
(PECs) for the lowest-lying states of ZrO, ZrS, HfO, and HfS. The
prediction of the ground state is basis set dependent at the icMRCI+Q
level for ZrO and ZrS due to the small singlet–triplet splitting
between the lowest 1Σ+ and 3Δ states. CCSD(T) with a spin orbit correction predicted the 1Σ+ ground state in agreement with experiment.
New all-electron basis sets were developed for Hf to improve the results
over those predicted by use of effective core potentials (ECPs) that
subsume the 4f electrons into the definition of the core. The use
of the new DK-4f basis sets rather than ECPs became more important
for HfO and HfS where there is a lack of a good core–valence
separation. icMRCI+Q, CCSD(T), and DFT calculations for the spectroscopic
parameters of ZrO, ZrS, HfO, and HfS were benchmarked with available
experimental data. Bond dissociation energies (BDEs) of these four
systems were calculated at the Feller–Peterson–Dixon
(FPD) level to be 762.1 (ZrO), 543.5 (ZrS), 803.8 (HfO), and 575.1
kJ/mol (HfS), in excellent agreement with experiment. The HfS BDE
was remeasured using the R3PI method, providing an updated experimental
measurement of D0(HfS) = 5.978 ±
0.002 eV = 576.8 ± 0.2 kJ/mol. This experimental value, combined
with experimental measurements of the ionization energies of Hf and
HfS, gives the cationic BDE of D0(Hf+-S) = 5.124 ± 0.002 eV = 494.4 ± 0.2 kJ/mol.