We have measured polarized resonance Raman spectra of nickel(II) octaethylporphyrin in CS2 and CH2Cl2 solution at different excitation wavelengths (430−580 nm) and temperatures (190−310 K). The analysis of the spectra revealed that the structure-sensitive Raman lines ν19 and ν10 can be decomposed consistently into two sublines for all excitation wavelengths and temperatures. In the resonance region of the QO and QV bands, the 0−1 and 0−0 resonances in the excitation profiles of the low-frequency (LF) sublines of ν19 and ν10 are red-shifted by (150 (30) cm-1 with respect to the sublines that are at higher frequencies (HF). In accordance with experimental and theoretical results, this indicates that the LF sublines of ν19 and ν10 result from a nonplanar conformer, whereas the HF sublines correspond to an almost planar conformer. The existence of this known conformational equilibrium in solution is further corroborated by the van't Hoff behavior of the intensity ratios I LF/I HF of the sublines of ν19 and ν10. From the straight lines in the van't Hoff plot, we calculate that the nonplanar conformer in solution is energetically favored by about 3.0 kJ/mol.

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The coexistence in solution of at least two conformers of (meso-tetraphenylporphinato)nickel(II) [Ni(TPP)] is inferred from solution and single-crystal resonance Raman spectra obtained at different temperatures (170 − 297 K) and excitation wavelengths (413.1 and 457.9 nm). The shapes of the structure-sensitive Raman lines ν8 and ν2 are clearly asymmetric and change with temperature. These broad lines can be decomposed into at least two sublines, a low-frequency (LF) and a high-frequency (HF) component. In contrast, the corresponding single-crystal Raman lines of the nonplanar structure of Ni(TPP) in the crystal are narrow and symmetric. For the line ν2, the broad LF subline results from nonplanar conformers and the narrow HF subline arises from a more planar conformer. This assignment is consistent with the observation that the LF subline of ν2 is more enhanced upon changing the excitation wavelength from 413.1 to 457.9 nm. The selective resonance enhancement is caused by the red shifts of the UV−visible absorption bands and Raman excitation profiles of the nonplanar form. The frequency assignment for the sublines of ν8 is reversed from that of ν2 (i.e., the HF subline of ν8 arises from nonplanar conformers and the LF subline results from a more planar macrocycle). This assignment is based on subline broadness and enhancement behavior using an excitation wavelength located on the red side of the B band. The assignments of the sublines to the nonplanar conformer are also in agreement with the Raman spectra of single crystals in which Ni(TPP) is known from X-ray crystallography to have a predominantly ruffled nonplanar conformation. Specifically, the frequencies of the sublines of ν8 and ν2 that are assigned to the nonplanar form in solution closely match the frequencies of Ni(TPP) in the single crystal. We propose two thermodynamic models for the interpretation of the temperature dependence of the intensity ratios of the sublines. A two-state model assumes one planar and one nonplanar conformer in solution. From the slopes in the van't Hoff plots, the nonplanar conformer is energetically favored by about 1.8 kJ mol-1 in this model. Since molecular mechanics calculations predict three conformers of Ni(TPP) in solution, we also consider a three-state model. The three structures are two nonplanar structures of purely ruffled (ruf) and purely saddled (sad) macrocyclic distortions and a planar conformer. In the calculations, the ruf conformation is the lowest-energy structure with the planar and sad conformers having almost equal higher energies. Assuming this relationship between the energies, but allowing the actual energy separation to vary, the three-state analysis gives the similar result that the ruf conformation is stabilized by about 2.3 kJ mol-1 with respect to the planar and sad conformers.