THE TRIFLUORIDE ANION - A DIFFICULT CHALLENGE FOR QUANTUM-CHEMISTRY

The molecular structure, binding energy, and vibrational frequencies of the trifluoride anion, F3-, have been studied by a variety of quantum chemical techniques, ranging from SCF to CCSDT, using a graded selection of basis sets varying in size from DZP to TZ2Pf+ (which includes two sets of d functions and one set of both f functions and diffuse p functions). Our most sophisticated methods are sufficiently reliable to give good accounts of the structure, binding energy, and vibrational frequency of F2 and of the electron affinity of the F atom. Both the large value of the CCSD T1 diagnostic and the critical influence on the results of single and triple excitations emphasize that the trifluoride anion has very substantial multireference character, higher than for ozone or molecular fluorine. The F3- anion has remarkably long F-F bonds (1.74 angstrom) and is found to be thermodynamically more stable than (F2 + F-) by about 110 kJ mol-1. An average F-F bond energy of some 135 kJ mol-1 is implied. Calculated vibrational frequencies for the trifluoride anion are exceedingly dependent on the method used to obtain them but are less sensitive to basis set, provided that diffuse functions are included. Our best estimates are omega(1) = 440 +/- 10, omega(2) = 260 +/- 10, and omega(3) = 535 +/- 20 cm-1. These match reasonably well the frequencies of 461 cm-1 for nu(1) and 550 cm-1 for nu(3) which have been observed in inert-gas matrices and assigned to the trifluoride anion.