NITROGEN FIXATION VIA A TERMINAL IRON(IV) NITRIDE
4.4 Experimental Section .1 Experimental Details.1Experimental Details
4.4.4 Computational Methods
All calculations were carried out using version 3.0.3 of the ORCA package.56 Given the experimentally measured structure and ground state/excited state energy splitting for (P3B)Fe(NNMe2), this was used as a model for testing the pure exchange-correlation func- tionals BP86,57,58 M06-L,59 and TPSS.60 For the purposes of testing, gas-phase geome-
try optimizations were carried out using the def2-SVP(C,H)/def2-TZVP basis set (with atomic coordinates from XRD as inputs),61 followed by a frequency calculation at the same level of theory to ensure a true minimum. Calculations employed a fine integration grid (ORCA Grid5) during geometry optimization, as well as during the final single-point calculation (Grid6). The importance of relativistic effects were tested by inclusion of the zeroth order regular approximation (ZORA) with the BP86 functional,62 using the scalar relativistically-recontracted def2- ZORA-SVP(C,H)/def2-ZORA-TZVP basis sets and def2- SVP(C,H)/def2-TZVP auxiliary basis sets.63From these calculations, it was determined that the ZORA-BP86 method produces the most accurate geometry, as well as a singlet-triplet
∆H that agrees well with the experimental value, although the TPSS functional performed nearly as well. All subsequent geometry optimizations and single-point energy calculations employed the ZORA-BP86 method. All optimized structures are available online free of charge: DOI:10.1021/jacs.7b09364.
For the calculation of Mössbauer parameters, the hybrid functional TPSSh64was used with the def2-SVP(C,H)/def2-TZVP basis set on all non-Fe atoms and the “core properties”
CP(PPP) basis set for Fe.65 The angular integration grid was set to Grid4 (NoFinalGrid), with increased radial accuracy for the Fe atom (IntAcc 7). To simulate solid state effects, a continuum solvation model was included (COSMO) with a solvent of intermediate dielectric (methanol). To calibrate the isomer shift scale and estimate the error in the calculated quadrupole splitting using this method, the Mössbauer parameters of 8 (P3B)Fe complexes were computed from crystallographically- or computationally-determined structures; in addition, the parameters of the previously-characterized nitrido complex (PhBP3iPr)Fe≡N were computed using coordinates from the ZORA-BP86 method.17 Given the accuracy of the predicted spectroscopic parameters, all orbital analysis presented in the main text utilized the wavefunctions computed using this method.
For the calculation of XAS spectra, the TPSSh functional was used in conjunction with the def2-TZVP basis set on all non-Fe atoms and the CP(PPP) basis set for Fe. The
114 angular integration grid was set to Grid4 (NoFinalGrid), with increased radial accuracy for the Fe atom (IntAcc 7). To simulate solid state effects, a continuum solvation model was included (COSMO) with an infinite dielectric. TD-DFT transitions were calculated using the Tamm-Dancoff approximation with excitations restricted from the Fe 1s orbital.
The first 50 lowest-energy transitions were calculated, and the total intensity was computed including both dipole and quadrupole transition intensities. To calibrate the energy scale of the computed spectra, the XAS spectrum of (PhBP3iPr)Fe≡N was calculated from BP86- ZORA optimized coordinates, and compared with the experimentally-reported spectrum.37 A constant shift of 154.25 eV was determined to align the intense pre-edge transitions of the experimental and calculated spectra; subsequently, this same shift was applied to all calculated spectra. A line broadening of 1.5 eV was applied to the calculated spectra to approximate the experimentally-observed linewidth. Spectra were normalized by setting the area of the (PhBP3iPr)Fe≡N spectrum to 0.92, which is the estimated area normalized to the edge-jump (based on the related (PhBP3CH2Cy)Fe≡N variant).37 To compare with the experimental pre-edge spectra, the line-broadened TD-DFT spectrum was fit to 2 or 3 Gaussian functions, from which predicted pre-edge areas were calculated.
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