A Study of NO Adducts of Iron Protoporphyrin IX Adlayer on Au Electrode with in Situ ATR-FTIR Spectroscopy

In situ attenuated total reflection (ATR) Fourier transform infrared (ATR-FTIR) spectroscopy has been applied to probe the coordination of nitric oxide to iron protoporphyrin IX (FePP) adlayer on Au (FePP/Au) electrodes in 0.1 M HClO4. On the basis of potential controlled ATR-FTIR spectra on independent FePP/Au electrodes and multistep ATR-FTIR measurement on one FePP/Au electrode, for the first time, up to three IR bands corresponding to three types of nitrosyl adducts of FePP have been identified with their intensities (concentrations) varied with the potential applied. The 1915-cm-1 band, which shows up at relatively positive potentials and stabilizes in a rather narrow potential range, can be reasonably assigned to the FeIII(NO)- (OH2)PP species or to its isoelectronic format FeII(NO)+(OH2)PP. The other two bands with much lower frequencies, which can stabilize over a much wider potential range and which can exhibit nearly opposite potential-dependent intensities, are basically characteristic of nitrosyl adducts of ferrous FePP. One band at ca. 1670 cm-1 with insignificant Stark effect can be attributed to FeII(NO)PP. The other above 1705 cm-1 with significant Stark effect could be ascribed to FeII(NO)2PP. The multinitrosyl adductions may be caused by the largely inhomogeneous structure of the FePP adlayer on Au electrodes.