Willard R. Wadt

Why UO/sub 2//sup 2 +/ is linear and isoelectronic ThO/sub 2/ is bent

The isoelectronic species UO/sub 2//sup 2 +/ and ThO/sub 2/ possess very different geometries, namely, UO/sub 2//sup 2 +/ is linear while ThO/sub 2/ is strongly bent (theta/sub exptl/ = 122 +- 2/sup 0/). Relativistic effective core potential (RECP) calculations using Hartree-Fock wave functions and double-zeta-plus-polarization quality basis sets were performed to determine the origin of this difference. The RECP calculations correctly predict the linear and bent geometries of UO/sub 2//sup 2 +/ and ThO/sub 2/ (theta/calcd/ = 118/sup 0/). The Th-O bond length, which is not known experimentally, is calculated to be 1.91 A. Analysis of the results shows that the difference in geometries for UO/sub 2//sup 2 +/ and ThO/sub 2/ has its origin in the relative ordering of the 5f and 6d levels. For uranium the 5f levels are lower and dominathe te back-bonding from the oxygen in UO/sub 2//sup 2 +/, while for thorium the 6d levels are lower and dominate the back-bonding in ThO/sub 2/. Finally, the 5f levels prefer linear geometries, while the 6d prefer bent geometries, hence, the difference between UO/sub 2//sup 2 +/ and ThO/sub 2/. The relative ordering of the 5f and 6d levels has a profound effect.

The low-lying excited states of water, methanol, and dimethyl ether☆

Abstract Improved virtual orbital (IVO) calculations have been performed on the low-lying excited states of water, methanol, and dimethyl ether using minimal basis sets of contracted gaussian functions augmented with diffuse s and p functions. In all three molecules the low-lying states correspond to excitations out of an oxygen lone pair orbital to an excited orbital having the basic character of a 3s or 3p Rydberg orbital centered on the oxygen. The 3s and 3pb z Rydberg orbitals in water have especially large stabilities (term values) arising from favorable interactions with the electron-deficient hydrogens. Methylation diminishes this added stability in methanol and destroys it in dimethyl ether. The IVO calculations are used to assign the low-lying transitions in the optical and electron impact spectra of methanol and dimethyl ether.

The role of dπ orbitals in spin-orbit coupling

A general framework is developed for the theoretical treatment of d-orbital contributions of heavy atoms to the spin-orbit coupling of planar molecules. Unlike pπ orbitals for which only three-centre spin-orbit terms are non-vanishing, it is shown that a number of one- and two-centre terms will contribute. The case of thiophene (C4H4S) is examined, and all one- and two-centre integrals are evaluated explicitly. Although the two-centre terms are individually only a few per cent of the one-centre term, it is found that because of the large number of two-centre terms, they usually dominate the one-centre term. In thiophene, it is found that the contribution of individual dπ terms is on the order of 0·02-2·0 cm-1, which is comparable in magnitude to other contributions. An examination of the results as a function of effective nuclear charge and internuclear distance is also made.