2ND-ORDER JAHN-TELLER DISTORTIONS IN GROUP-17 FLUORIDES EF(3) (E=CL,BR,I, AND AT) - LARGE RELATIVISTIC BOND-ANGLE CHANGES IN ATF3

Group 17 element fluorides EF(3) (E = Cl, Br, I) are well-known to und ergo second-order Jahn-Teller symmetry breaking toward a T-shaped Ct, arrangement mainly due to a(1)'(HOMO)circle times e'(LUMO) mixing at t he expected symmetric trigonal planar D-3h state. For heavy elements, the a(1)'HOMO is relativistically stabilized because of large element s-orbital participation. Hence, relativistic effects diminish the seco nd-order Jahn-Teller term. This results in a large relativistic change in the F-eq-E-F-ax bonding angle of alpha(e)(R) - alpha(e)(NR) = 5.5 degrees in the case of AtF3 and causes an anomaly in the bond angle be havior down the group 17 compounds, alpha(ClF3) > alpha(BrF3) > alpha( AtF3) > alpha(IF3). Furthermore, the difference between the symmetric D-3h and the distorted C-2v structure Of AtF3 is only 10 kJ/mol at the coupled cluster level of theory, indicating that the measured F-eq-At -F-ax angle alpha(e) will be very sensitive upon the temperature appli ed in gas phase diffraction studies. Vibrational frequencies are predi cted for all group 17 fluorides EF(3). As a consequence of the second- order Jahn-Teller distortion, the A(1) symmetric bending mode is stron gly influenced by relativistic effects and becomes much lower in frequ ency compared to the B-1 out of plane mode for the heavier elements. W ith the exception of IF3, the symmetric D-3h structure represents a (m etastable) weak local minimum at the MP2 level, rather than a transiti on state as expected. The D-3h point represents, however, a second-ord er saddle point at the HF level, and therefore, electron correlation s eems to be responsible for changing the nature of the trigonal planar structure. Extended basis sets at the MP2 level as well as coupled clu ster calculations were applied in order to obtain more accurate inform ation for the energetics and structure of CIF3. These studies show, ho wever, that the nature of the D-3h point is critically dependent upon the basis set (and the electron correlation procedure) applied.