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 o...