After graduating, Gouterman was appointed to the faculty at
Harvard University where he worked as a postdoctoral researcher with
William Moffitt. Shortly after Gouterman arrived, Moffitt died of a heart attack during a squash game. Gouterman was quickly promoted to assistant professor, and spent his time using quantum chemical calculations to understand the photophysical properties of porphyrins. He primarily made use of the
Hückel molecular orbital method to interrogate their optical spectra. Gouterman's molecular models, which included symmetry arguments and configuration interactions, were able to predict the intensity differences between the absorption bands of porphyrins. The so-called four-orbit model incorporates two, almost degenerate
highest occupied molecular orbitals and two degenerate
lowest unoccupied molecular orbitals. The
Soret and Q-bands that are visible in porphyrin spectra are the result of transitions from between these four orbitals. He described how the chemical structures of porphyrins determine whether the spectral shape was 'normal',
hyper- and
hypso-. For example, the UV-Visible absorption spectra of
hyper porphyrins contain red-shifted peaks and extra bands due to ligand-to-metal charge transfer (LMCT) transitions. Amongst the complicated structures analysed by Gouterman were cytochrome P450–carbon monoxide complexes, whose electronic spectra included a split Soret band due to LMCT transitions. == Awards and honors ==