PLANAR AND NONPLANAR CONFORMATIONS OF (MESO-TETRAPHENYLPORPHINATO)NICKEL(II) IN SOLUTION AS INFERRED FROM SOLUTION AND SOLID-STATE RAMAN-SPECTROSCOPY

The coexistence in solution of at least two conformers of (meso-tetrap henylporphinato)nickel(II) [Ni(TPP)] is inferred from solution and sin gle-crystal resonance Raman spectra obtained at different temperatures (170 - 297 K) and excitation wavelengths (413.1 and 457.9 nm). The sh apes of the structure-sensitive Raman lines v(8) and v(2) are clearly asymmetric and change with temperature. These broad lines can be decom posed into at least two sublines, a low-frequency (LF) and a high-freq uency (HF) component. In contrast, the corresponding single-crystal Ra man lines of the nonplanar structure of Ni(TPP) in the crystal are nar row and symmetric. For the line v(2), the broad LF subline results fro m nonplanar conformers and the narrow HF subline arises from a more pl anar conformer. This assignment is consistent with the observation tha t the LF subline of v(2) is more enhanced upon changing the excitation wavelength from 413.1 to 457.9 nm. The selective resonance enhancemen t is caused by the red shifts of the UV-visible absorption bands and R aman excitation profiles of the nonplanar form. The frequency assignme nt for the sublines of vs is reversed from that of v(2) (i.e., the HF subline of v(8) arises from nonplanar conformers and the LF subline re sults from a more planar macrocycle). This assignment is based on subl ine 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 spect ra of single crystals in which Ni(TPP) is known from X-ray crystallogr aphy to have a predominantly ruffled nonplanar conformation. Specifica lly, the frequencies of the sublines of v(8) and v(2) that are assigne d to the nonplanar form in solution closely match the frequencies of N i(TPP) in the single crystal. We propose two thermodynamic models for the interpretation of the temperature dependence of the intensity rati os of the sublines. A two-state model assumes one planar and one nonpl anar conformer in solution. From the slopes in the van't Hoff plots, t he 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 mode l. The three structures are two nonplanar structures of purely ruffled (ruf) and purely saddled (sad) macrocyclic distortions and a planar c onformer. In the calculations, the ruf conformation is the lowest-ener gy structure with the planar and sad conformers having almost equal hi gher energies. Assuming this relationship between the energies, but al lowing the actual energy separation to vary, the three-state analysis gives the similar result that the ruf conformation is stabilized by ab out 2.3 kJ mol(-1) with respect to the planar and sad conformers.