The unique spectroscopic behavior of the Fe(III)-nitroprusside: a DFT study of the vibronic coupling in the ground and in the lowest excited state

2002 
Abstract The adiabatic potential energy surface of the ground state (GS-APES) of the Fe III -nitroprusside anion [Fe(CN) 5 NO] 2− (Fe–NTP) has been studied by means of density functional theory (DFT). A decomposition of the total bonding energy into orbital interaction, electrostatic bonding and exchange (Pauli) repulsion terms (extended transition state method, ETS) has been carried out and the chemical bonding at all stationary points of the ground state potential surface (minima and saddle points) has been characterized. On this basis, the unique topology of the electronic ground state with minima for linear Fe–N–O (ground state, GS) and Fe–O–N (metastable state, MS1) and side-on Fe–(NO) bonded (metastable state, MS2) has been rationalized. The potential energy surface of the lowest excited [3d→π ∗ (NO)] electronic state (ES-APES) with minima at non-linear Fe–N–O and Fe–O–N arrangements has been calculated starting from higher symmetric ( C 4 v ) structures (linear Fe–N–O and Fe–O–N bond) utilizing a 1 E Jahn–Teller coupling mechanism. We found that after geometrical relaxation, the ES-APES crosses the GS-APES in the vicinity of the transition states, thus affecting the photo-isomerisation process. In the light of these results a combined ground and excited state configurational energy diagram is presented and used to discuss the mechanism of photo-isomerisation.
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