Structures and vibrational frequencies of vanadium (V) oligomers: An Ab initio study using effective core potentials

Ab initio molecular geometries and vibrational frequencies of various isolated vanadate species (VO43-, HVO42-, H2VO?(4-) and V2O74-) were calculated using different pseudopotentials. The relative merits of these were assessed by comparing the calculated molecular parameters with the corresponding values obtained from calculations at all-electron levels and, whenever available, from X-ray studies for the salts. The calculations were extended to higher oligomers (V3O105-, V4O136-, and V4O124-) using the pseudopotential whose basis functions are (10s5p5d)/[2s1p1d] (55/5/5) on vanadium and (4s4p)/[2s2p] (31/31) on oxygen, which yielded the best compromise between accuracy and computational effort. The results indicate a linear centrosymmetric geometry for the isolated V2O74- anion. The higher oligomers have less than 180 degrees V-O-V angles, except the noncyclic tetraoligomer which has a linear central V-O-V angle (180 degrees). The cyclic V4O124- species presents a planar structure with all the vanadium and bridging oxygen atoms in the same plane. This structure was already reported for the [(CH3)CNH3][V4O12] salt. The results suggest a lower stability of the linear V4O136- species, in agreement with previous reports. (C) 1996 by John Wiley & Sons, Inc.