Infrared Spectra of Nickel Octaethylporphyrin and Its Isotopomers Computed via Density Functional Theory−Scaled Quantum Mechanical (DFT−SQM) Method

Revealing substituent effects on the electronic structure and planarity of Ni-porphyrins

Using density functional theory, we have studied the effects on structural and electronic consequences (including HOMO-LUMO energy gaps, vertical ionization potentials (IP_v), and vertical electron affinities (EA_v)) of the following two factors: (a) meso- and β-substituents acting as inductive donors (CH₃), inductive acceptors that are electron-donating through resonance (Br), inductive electron acceptors (CF₃), and resonance enabled acceptors (NO₂); and (b) complete replacement of pyrrole nitrogens with P-atoms. The principal results of the study are: (1) For the bare Ni-porphyrin, the solvents were found not to affect the HOMO-LUMO gaps but to change the IP_v and EA_v noticeably. (2) In the series CH₃ → Br → CF₃ → NO₂ the HOMO-LUMO energy gaps, IP_v, and EA_v increase for both meso- and β-substituents. The ruffling distortion of the porphyrin core is retained, and becomes stronger for the two acceptor groups. In general, effects of meso-substituents on the ruffling distortion of the porphyrin core is more pronounced. (3) Most significantly, complete replacement of pyrrole nitrogens in the NiP with phosphorus atoms produces the species, NiP(P)₄, with the structural and electronic features drastically different from the original NiP. This implies that NiP(P)₄ can possess interesting and unusual novel properties, including aromaticity and reactivity, leading to its various beneficial potential applications. Furthermore, NiP(P)₄ high stability both in the gas phase and different solvents was shown, implying the feasibility of its synthesis.

Defining resonance Raman spectral responses to substrate binding by cytochrome P450 from Pseudomonas putida

Resonance Raman spectra are reported for substrate-free and camphor-bound cytochrome P450cam and its isotopically labeled analogues that have been reconstituted with protoheme derivatives that bear -CD(3) groups at the 1, 3, 5, and 8-positions (d12-protoheme) or deuterated methine carbons (d4-protoheme). In agreement with previous studies of this and similar enzymes, substrate binding induces changes in the high frequency and low frequency spectral regions, with the most dramatic effect in the low frequency region being activation of a new mode near 367 cm(-1). This substrate-activated mode had been previously assigned as a second "propionate bending" mode (Chen et al., Biochemistry, 2004, 43, 1798-1808), arising in addition to the single propionate bending mode observed for the substrate-free form at 380 cm(-1). In this work, this newly activated mode is observed to shift by 8 cm(-1) to lower frequency in the d12-protoheme reconstituted enzyme (i.e., the same shift as that observed for the higher frequency "propionate bending" mode) and is therefore consistent with the suggested assignment. However, the newly acquired data for the d4-protoheme substituted analogue also support an earlier alternate suggestion (Deng et al., Biochemistry, 1999, 38, 13699-13706) that substrate binding activates several heme out-of-plane modes, one of which (gamma(6)) is accidentally degenerate with the 367 cm(-1) propionate bending mode. Finally, the study of the enzyme reconstituted with the protoheme-d4, which shifts the macrocycle nu(10) mode, has now allowed a definitive identification of the vinyl C=C stretching modes.

Density functional theory studies on the electronic and vibrational spectra of octaethylporphyrin diacid

The ground-state structure and electronic and vibrational spectra of octaethylporphyrin diacid (H4OEP2+) have been studied with the density functional theory. The geometrical parameters computed with B3LYP, PBE1PBE and mPW1PW91 functionals and 6-31G* basis sets are well consistent with the experimental values. Electronic absorption spectrum of H4OEP2+ has been studied with the time-dependent DFT method, and the calculated excitation energies and oscillator strengths are compared with the experimental results. The Raman and IR spectra of H4OEP2+ and the Raman spectrum of its N-deuterated analogue (D4OEP2+) were measured. The observed Raman and IR bands have been assigned based on the frequency calculations at the B3LYP/6-31G* level of theory.

Porphyrin distortion from resonance Raman intensities of out-of-plane modes: Computation and modeling of N-methylmesoporphyrin, a ferrochelatase transition state analog

Resonance Raman spectra of porphyrins are computed with DFT/CIS methodology to monitor out-of-plane distortions. A framework is established for assessing protein-induced distortion of porphyrin bound to a ferrochealatase antibody. Tests on undistorted porphyrins give good agreement with the experimental intensity pattern of in-plane modes for free-base porphine (FBP) and mesorporphyrin IX free base (MP). The computed spectrum of N-methylmesoporphyrin (NMP), in which the methyl-substituted pyrrole ring is tilted 32 degrees from the mean porpyrin plane, also gives reasonable agreement with experiment and reveals activation of out-of-plane (oop) vibrational modes. To model oop distortions systematically, an artificial molecule, FBP-X8, was constructed, in which the H atoms attached to the FBP pyrrole Cb atoms are replaced by heavy substituents, as in physiological porphyrins. Oop mode enhancements are computed for FBP-X8 by displacing it along the canonical distortion coordinates: doming, saddling, ruffling and waving. When FBP-X8 is constrained to the NMP porphyrin geometry, normal coordinate decomposition reveals significant contributions also from modes higher in frequency than the canonical modes, and they contribute importantly to the computed RR intensities. NMP-constrained FBP-X8 gives a reasonable facsimile of the NMP RR spectrum, but better results are obtained with a full computation of MP, constrained in the same way; thus the physiological substituents have significant influence on the RR spectra, over and above their effective masses. Attention focuses on a mode analogous to gamma 15 in 4-fold symmetric porphyrins, which is a kind of saddling mode. This mode is selectively enhanced in NMP-constrained MP; a corresponding RR band is induced upon binding MP to a ferrochelatase antibody.

Resonance Raman enhancement of FeIVO stretch in high-valent iron porphyrins: An insight from TD-DFT calculations

Density functional theory (DFT) has been applied to explain the origin of resonance Raman enhancement associated with the Fe(IV)=O stretch observed in iron(IV)oxo porphyrins. To accomplish this electronic excitations of the Im-(Por)Fe(IV)=O model were computed in the 1.5-4.0 eV spectral range using time-dependent DFT (TD-DFT). All electronic transitions having dominant pi-->pi* character were analyzed and assigned in terms of one-electron excitations. It was found that the most intense Soret band has a multi-component character, but the pi (a(2u))-->pi*(d(xz),d(yz)) and pi (a(1u))-->pi*(d(xz),d(yz)) electronic excitations are primarily responsible for observed resonance enhancement of the Fe(IV)=O stretch.

Inelastic Neutron Scattering Spectra of Free Base and Zinc Porphines: A Comparison with DFT-Based Vibrational Analysis

Inelastic neutron scattering (INS) spectra of free base (FBP) and zinc (ZnP) porphines are presented and compared with the results of density functional theory (DFT) calculations using the B3LYP functional with 6-31G(d) or 6-311G(d,p) basis sets. To obtain quantitative agreement between experiment and theory, two different scaling techniques have been applied: a scaled quantum mechanical (DFT-SQM) force field was developed for B3LYP/6-31G(d) calculations and the uniform frequency scaling technique (DFT-UFS) was applied to B3LYP/6-311G(d,p) results. The DFT-SQM calculations have been previously compared with IR and Raman spectra with good agreement, which allows for a nearly complete vibrational assignment. The results of the present study extend previous vibrational analysis to a higher level of reliability and complexity. The previous results are augmented by the comparison of calculated and observed INS intensities and the comparison of calculated modes with those observed in INS spectra but previously unobserved in optical spectra. Excellent agreement is acquired between the INS spectra and the results of both calculations, permitting a more detailed and reliable description of the vibrational properties of porphyrins.

Ab initio and density functional theory studies on vibrational spectra of palladium (II) and platinum (II) complexes of methionine and histidine: effect of theoretical methods and basis sets

The vibrational spectra of methionine and histidine-containing palladium (II) and platinum (II) complexes, cis-M(Met)X2 and cis-M(His)X2 (M = Pd and Pt; X = F, Cl, Br and I; Met = methionine, His = histidine), have been systematically investigated by ab initio Restricted Hartree-Fock (RHF) and density functional B3LYP methods with LanL2DZ and SDD basis sets. The geometries of cis-Pd(Met)Cl2, cis-Pt(Met)Cl2, cis-Pd(His)Cl2 and cis-Pt(His)I2 optimized and vibrational frequencies and IR intensities of cis-M(Met)Cl2 and cis-M(His)Cl2 (M = Pd and Pt) calculated are evaluated via comparison with the experimental values. The vibrational frequencies calculated show that the methods, rather than basis sets, affect the accuracy of the calculation. The best results that can reproduce the experimental ones are obtained at B3LYP level without any scale factor used. The vibrational frequencies of cis-M(Met)X2 and cis-M(His)X2 (M = Pd and Pt; X = F, Br and I) that have not yet been experimentally reported are predicted.

Theoretical analysis of singlet and triplet excited states of nickel porphyrins

Local density and generalized gradient approximation time-dependent density functional methods have been used for calculation of the singlet and triplet excited states of nickel-porphine, Ni-tetraphenyloporphine, and Ni-octaethyloporphyrine. Special attention is paid to metal-ligand transitions and d-d transitions. It is shown that the lowest exited singlet states of the three compounds can be described as a transfer of an electron from the porphine ring to the d(x2-y2) orbital of the nickel atom. On the other hand, the lowest excited triplet state arises from promotion of an electron between two nickel d orbitals, an occupied d(z2) and an empty d(x2-y2). It is proposed that a rapid quenching of the excited singlet states is due to an ultrafast intersystem crossing between 1Eg)and 3Eg or 3B1g states.