We repon here the results of a theoretical analysis of the [Ru- (NHMpy)]'+ absorption spectrum. which is known to be sensitive to media effects.' The intermediate neglect of differential overlap (INDO) model parametrized for spectroscopy at the configuration interaction (CI) level of theory has been used in these studies.? We demonstrate here that the absorption spectrum of the above complex can be reproduced only if specific outer-sphere effects are considered and, in particular. that the solvent-to-complex charge transfer is essential in lowering the energy of the metal-to-ligand charge transfer (MLCT) excitations to positions that agree with experiment.' A simple point-charge model we utilize in this work is shown to be a successful substitute for the explicit solvent modeling. The proposed procedure reproduces the experimental energies and provides a correct assignment of the absorption bands at substantially reduced computational cost.In this work we examine the spectroscopy of [Ru(NHs)s-(py)]'-. whose structure is shown in Figure 1. Although the electronic spectra of such complexes have been known for over 25 years to he sensitive to the nature of the solvent, the reasons for this sensitivity have remained elusive.'.' The bond lengths we use for the model structure shown in Figure I are taken from X-ray experiments.-' The INDO parametrization' used for the ruthenium 4d orbitals (/&) was obtained to reproduce the observed d-d spectrum of [Ru(NH&,]~+. I The j ? , , thus obtained is -13.0 eV. The value for /3 for the s and p electrons has been modified slightly from that used for the firs-row transition series, from /?. = /j --1 .O eV to , 9\ = / 3, , = -2.0 eV.The molecular orbital (MO) diagram for the frontier orbitals of [Ru(NH&,l'+ and [Ru(NH3)5(py)l'+ is given in Figure 2.The pyridine-dominated n* orbitals were found in the d-d gap as expected from the spectroscopy of similar Ru compounds and previous theoretical work.5-n The loCalion of the occupied ; r pyridine orbitals varies slightly with the geometry and is typically 0.1 au ( I au = 27.21 eV) below the occupied Ru d orbitals. labeled in Figure 2 as d,.As one can expect from the MO diagram in Figure 2. the absorption spectrum of [Ru(NHz)5(py)]'+ should contain contributions from weak d-d excitations and from the more intense d -x* MLCT and intraligand n -n* transitions. The symmeq-forbidden d-d transitions are not expected to compete 0 I Ford. P.: Rudd. De F. P.: Gaunder. R.: Tauhe. H. J . Am. Clzm#. SO<.
ABSTRACT:We follow the initial activation of the nitrogen molecule at the FeMo cofactor of nitrogenase and subsequently model the hydrogenation of N up to the fourth 2 protonation step using the intermediate neglect of differential overlap quantum-chemical model. The results obtained favor a reaction mechanism going through hydrazido intermediates on the 4-Fe surfaces, externally to the FeMo cofactor. Calculations using density functional theory on smaller model systems also support the suggested mechanism over other possible schemes that involve early release of the first molecule of ammonia as a product of the enzymatic reaction. We also demonstrate that dielectric stabilization due to the protein around the cofactor could lower markedly the barrier for the product release as an ammonium ion.
Abstract:A model for the active site of nitrogenase is suggested and examined by means of the intermediate neglect of differential overlap (INDO) quantum mechanical method. The initial steps of the nitrogen fixation process are discussed within the framework of the present model, and it is shown that of several binding sites, initial location of the nitrogen molecule inside the MoFe cofactor is favored. Possible pathways for electron and proton delivery to the active site arc also suggested on the basis of electrostatic potential calculations.
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