The electronic structure of xenon difluoride has been studied using ab initio theoretical methods. The primary goal was to determine whether current theoretical methods are capable of yielding a reasonable value of the dissociation energy of XeF2. A Slater function basis set of slightly better than "double [ plus polarization" quality was employed. Four different types of wave functions were investigated: two-configuration SCF, full valence configuration interaction (CI), the first-order wave function, and a larger 1234 configuration wave function including all double excitation from the loa, orbital. Although the TCSCF symmetric stretching potential curve has both a minimum and maximum, the minimum lies above the comparable energy of separated Xe + 2F. However, the two most complete wave functions predict dissociation energies of 1.97 and 2.14 e", in qualitative agreement with experiment, 2.78 eV. All four wave functions provide good predictions of the Xe-F equilibrium bond distance. As was the case for KrF2, the bonding in XeF2 is found to conform quite closely to Coulson's model F Xe+ F--F-Xe+ F near the equilibrium geometry, The role of the "outer orbitals" 5d and 4f appears to be a quantitative rather than qualitative one.Xenon difluoride appears to be the simplest known Xecontaining molecule, although there is still some controversy2 concerning the existence of the XeF radical. As such, XeF2 plays a special role in the chemistry of the noble gases.' XeF2 was first prepared4 in 1962, shortly after Bartlett's discovery5 of XePtF6, and several relatively simple methods of preparation are now a~a i l a b l e .~ The dissociation energy for the process XeF2 -Xe + 2F is -64 kcal/mo16 = 2.78 eV. Assuming the value' 38.8 f 2.3 kcal/mol for the dissociation energy De of F2, the molecular dissociation energy for the process XeF2 -Xe + F2 is found to be -25 kcal/mol. For comparison, the smaller KrF2 molecule is known to lie energetically aboue (by -15 kcal/mol) the analogous dissociation limit Kr + F2. This difference between KrF2 and XeF2 explains the transient nature of the former as compared to the relative stability of the latter. The geometrical structure of XeF2 is known from infrared and Raman studies to be linear and ~y m m e t r i c ,~ corresponding to point group Dmh. Reichman and Schreinerg have determined the gas-phase Xe-F bond distance to be 1.977 f 0.002

The interactiou bl~tween hydrogen atoms and Be metal clusters has been studi.cd by ?b _initio electronic structure theory. Splf-· consistcttt-field (SCF) calculations have been carried out using both minimum and larger basis sets of contract~d gaussian functions.

+ .The C + H2 10n-molecule reaction has been studied by several experimental groups and appears destined to become the focal point of much experimental and theoretical activity. Ab initio self-consistent-field and configuration interaction calculations have accordingly been carried out for this system. A double zeta basis set of contracted gaussian functions was employed and as many as 570 confiqurations included. For isosceles triangle configurations (C 2V point group) 2 2 2 the AI' B l , and B2 potential surfaces were considered, while for linear geometries (C wv ) the 2E+ and 2rr surfaces were studied. Properties reported include minimum energy paths and energy profiles for the various processes considered. The intuitive correlation diagram of Mahan and Sloane is made semiquantitative in reliability.

Ab initio calculations have been carried out to determine a potential curve for the bending motion of C3. The work was in part motivated by the disagreement between theoretical and experimental values of the entropy of C3. Three basis sets were used, with (4s2p), (4s2p1d), and (4s3p1d) centered on each carbon atom. Both self-consistent-field (SCF) and configuration interaction (CI) (656 configurations) calculations were carried out with the smallest basis. The CI wavefunctions were obtained by a method which may be viewed as an extension of the pseudonatural orbital technique of Edmiston and Krauss. Using the smallest basis, both SCF and CI calculations yield ``normal'' bending frequencies, ∼320 cm−1. It is concluded that electron correlation has little effect on the bending frequency. The larger (4s2p1d) and (4s3p1d) basis SCF calculations yield much smaller bending frequencies, the latter being 69 cm−1, in good agreement with the unusually low experimental value of Gausset, Herzberg, Lagerqvist, and Rosen. The bending potential is predicted to be quite anharmonic. These results are discussed qualitatively in terms of a Walsh diagram and the importance of d orbitals by symmetry considerations.