The geometries of molecular complexes were optimized using the QUILD optimization routines (37) in the ADF 2017 software suite (38) at a BP86/TZP level of theory. To converge the geometry, a strict self-consistent field convergence of 10−8 was used in combination with a good numerical quality and the use of the exact density for the exchange-correlation potential (instead of using the default fitted density). As convergence criterium, a gradient convergence of 3 × 10−4 was used. Subsequently, the IR frequencies and intensities were computed using the same level of theory. For calculations on large chain complexes (10 zinc ions), the option “frozen core” was used. The IR bands were plotted using a Lorentzian function with a FWHM of 20 cm−1.

For the oxo complex, the atomic coordinates were used from the crystal structure of basic zinc pivalate (2,2-dimethylpropanoate) (26), after which the tert-butyl groups were replaced by CH2CH2CH3 to create butanoate ligands. As the chain complex is polymeric, it was necessary to create a finite chain structure as input for calculations. We took the atomic coordinates of the crystal structure of zinc crotonate (but-2-enoate) as a basis (39), added hydrogens to create butanoate ligands, and limited the structure to 10 zinc ions by capping one end with a methoxy group (CH3O) and the other with an ammonia molecule to obtain a noncharged complex. Hence, the final structure can be written as CH3O((Zn2(Bu)3)Bu)4(Zn2(Bu)3)NH3, where Bu is butanoate.

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