The question arises as to why only one anion isomer contributes to the TiO(OH)2ˉspectrum whereas two contribute to the ZrO(OH)2ˉ spectrum, given that the relative energetics of the two isomers are very similar in the Ti and Zr complexes as are the barriers to cis-→trans conversion. The differences between the two systems was attributed to differing conditions in the laser vaporization source used to generate the clusters. For example, the bond enthalpy of Zr-O exceeds that of Ti-O by nearly 100 kJ/mol, so the additional exothermicity associated with metal oxidation might lead to a different isomer population when the hot clusters are cryogenically cooled.