['The', 'O2', 'molecule', 'only', 'weakly', 'adsorbs', 'onto', 'the', 'cluster', 'at', 'relatively', 'low', 'temperatures', ',', 'in', 'the', 'range', 'of', 'several', 'tens', 'meV.', 'However', ',', 'significant', 'reaction', 'barriers', '(', '0.593–1.118', 'eV', ')', 'for', 'the', 'O2', 'molecule', 'on', 'the', 'cluster', 'are', 'identified', 'on', 'different', 'adsorption', 'sites', ',', 'nevertheless', ',', 'these', 'reaction', 'paths', 'are', 'spin', 'forbidden', 'reactions', 'according', 'to', 'Wingerʼs', 'spin', 'selection', 'rule', '.']