The peaks at 1174, 1197, 1458, and 1518 cm -1 are assigned to ring stretching modes ν9a, ν13, ν19b, and ν19a, respectively . Dimerization of 4ABA on a metal surface can produce p,p'-azodibenzoate, observed potentially in this experiment as a shoulder at 1144 cm -1 and an overlap with the ν19b band .