Structural elucidation of hydrated CuOH + complexes using IR action spectroscopy and theoretical modeling

• Complexes of CuOH+(H2O)n where n = 2–9 are examined using IRPD and theory.
• Relative energies of isomers are found for B3LYP, B3P86, MP2(full) and M06 levels.
• Relative intensities of bands agree well with relative energies at MP2(full).
• Spectra are consistent with structures having mainly a coordination number CN of 4 with some 5 evident.
• Energetic differences between 4-coordinate and 5-coordinate structures are small.

Complexes of CuOH+ (H2O)n where n = 2–9 are examined using infrared photodissociation spectroscopy (IRPD) with frequencies between ~2700 and 3900 cm−1. Structural characterization is achieved through comparison between experimental and theoretical spectra. Geometry optimizations and frequency calculations are performed on a myriad of possible low-energy structures at the B3LYP/6-311+G(d,p) level. Subsequent single- point energy calculations are performed at the B3LYP, B3P86, M06, and MP2(full) levels of theory using a 6-311+G(2d,2p) basis set to obtain relative free energies. The IRPD spectra of all complexes where n ≥ 3 are consistent with structures having a coordination number (CN) of 4 although broad features in the CuOH+ (H2O)5 and CuOH+(H2O) 6 spectra accommodate structures having both CN = 4 and CN = 5. For CuOH+(H2O) 7, spectral bands in the free-OH region narrow, revealing fine structure that confirms the presence of both CN = 4 and CN = 5 isomers. Conformational assignments are made on the basis of asymmetric stretching frequencies of free-OH water molecules specific to each CN. Relative intensities of these bands are generally in good agreement with relative energies predicted by MP2(full) and not those of the other levels. MP2(full) and M06 single-point energies for the 4 and 5-coordinate isomers are typically within 5 kJ/mol of each other suggesting possible equilibration between the two if isomerization barriers are low.

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