Calculation of the Vibrational Spectra of Linear Tetrapyrroles. 2. Resonance Raman Spectra of Hexamethylpyrromethene Monomers (2000)

The resonance Raman (RR) spectra of monomeric 3,3‘,4,4‘,5,5‘-hexamethylpyrromethene (HMPM) were measured upon excitation in resonance with the strong 436 nm absorption band. The experimental spectra were analyzed by comparison with calculated RR spectra that were obtained on the basis of scaled quantum chemical force fields in combination with the transform theory. The ground-state structure and force field of HMPM were calculated by density functional theory (DFT) using the B3LYP exchange functional and the 6-31G* basis set. The monomeric HMPM adopts a planar structure in contrast to HMPM dimers in which the intermolecular hydrogen-bonding interactions induce a slight torsion of the methine bridges as revealed by both the experimental and the calculated structures. The force fields were scaled by using a global set of scaling factors determined previously (Magdó, I.; Németh, K.; Mark, F.; Hildebrandt, P.; Schaffner, K. J. Phys. Chem. A 1999, 103, 289). To account for the effect of the intramolecular hydrogen bond between the pyrrolic N−H group and the pyrroleninic nitrogen in the monomeric HMPM, only the scaling factor for the N−H in-plane bending force constant required a slight adjustment. Electronic transitions were calculated by means of CNDO/S, Hartree−Fock single configuration interaction (HF-CIS), and time-dependent DFT, which all predict one strong and one or two adjacent weak transitions for the lowest electronic excitations. This pattern is in line with band fitting analyses of the 436 nm absorption band. The best agreement in excitation energies was obtained by time-dependent DFT calculations. Excited-state displacements as required for evaluating RR intensities were determined for the lowest excited singlet state S1 using the equilibrium geometries optimized for the ground and excited states by means of the HF and HF-CIS methods, respectively. For the second lowest excited state (S2), only an approximate equilibrium geometry could be used for determining the excited-state displacements as the S2 state became quasi-isoenergetic with the S1 state during the geometry optimization. Employing the transform theory, RR spectra were calculated for resonance enhancement via the S1 and S2 states. The experimental RR spectrum of HMPM excited at 413 nm agrees well with the calculated S1-RR spectrum, allowing a plausible and consistent vibrational assignment for most of the observed bands of HMPM and its isotopomer deuterated at the pyrrolic nitrogen. The root-mean-square deviations between the experimental and calculated frequencies are 7 and 5 cm-1 for nondeuterated and deuterated HMPM, respectively. Experimental RR intensities and their dependence on the excitation wavelength are reproduced in a semiquantitative manner. The only significant exceptions refer to the CC stretching and CH rocking modes of the methine bridge, ν21 and ν49. On one hand, these discrepancies may reflect intrinsic deficiencies of the HF/HF-CIS method in calculating excited-state displacements. On the other hand, the unique deviation of the experimental excitation profile of ν21 from the expected behavior suggests a destructive interference of the S1 and S2 states in the resonance enhancement specifically of this mode.