Measured 57Fe excitation probabilities for a series of iron porphyrins. All nitro-syl complexes have an Fe-NO stretch/bend mode in the 520 – 540 cm−1 region. Comparison among the nitrosyl complexes (c-g) reveals that peripheral groups strongly influence the vibrational frequencies and amplitudes of the central Fe. Sample temperatures were 34 K for Fe(OEP), 30 K for Fe(OEP)(NO), 80 K for Fe(TPP)(NO), 35 K for Fe(DPIXDME)(NO), 34 K for Fe(PPIXDME)(NO), and 64 K for Fe(MPIXDME)(NO). The Fe(OEP)(Cl) spectrum is an average over multiple scans with an estimated average temperature of 87 K. Error bars reflect Poisson statistics.