Conformational properties of nickel(II) octaethylporphyrin in solution .1. Resonance excitation profiles and temperature dependence of structure-sensitive Raman lines

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 lints v(19) and v(10) can be decomposed consistently into two sublines for all excitation wavelengths and temperatures. In the resonance region of the Q(0) and Q(V) bands, the 0-1 and 0-0 resonances in the excitation profiles of the low-frequency (LF) sublines of v(19) and v(10) 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 v(19) and v(10) 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 v(19) and v(10). 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.