Fe-H/D stretching and bending modes in nuclear resonant vibrational, Raman and infrared spectroscopies: Comparisons of density functional theory and experiment

Infrared, Raman, and nuclear resonant vibrational (NRVS) spectroscopies have been used to address the Fe-H bonding in trans-HFe(CO) iron hydride compound, HFe(CO)(dppe)₂, dppe = 1,2-bis(diphenylphosphino)ethane. H and D isotopomers of the compound, with selective substitution at the metal-coordinated hydrogen, have been considered in order to address the Fe-H/D stretching and bending modes. Experimental results are compared to the normal mode analysis by density functional theory (DFT). The results are that (i) the IR spectrum does not clearly show Fe-H stretching or bending modes; (ii) Fe-H stretching modes are clear but weak in the Raman spectrum, and Fe-H bending modes are weak; (iii) NRVS ⁵⁷Fe spectroscopy resolves Fe-H bending clearly, but Fe-H or Fe-D stretching is above its experimentally resolved frequency range. DFT calculations (with no scaling of frequencies) show intensities and peak locations that allow unambiguous correlations between observed and calculated features, with frequency errors generally less than 15 cm^-1. Prospects for using these techniques to unravel vibrational modes of protein active sites are discussed.