The free-base form of tetra-tert-butyl porphine (TtBP), which has extremely bulky meso substituents, is severely distorted from planarity, with a ruffling angle of 65.5degrees. The resonance Raman spectrum of TtBP (lambda(ex) = 457.9 nm) and its d(2), d(8), and d(10) isotopomers have been recorded, and while the spectra show high-frequency bands similar to those observed for planar meso-substituted porphyrins, there are several additional intense bands in the low-frequency region. Density functional calculations at the B3-LYP/6-31G(d) level were carried out for all four isotopomers, and calculated frequencies were scaled using a single factor of 0.98. The single factor scaling approach was validated on free base porphine where the RMS error was found to be 14.9 cm(-1). All the assigned bands in the high-frequency (> 1000 cm(-1)) region of TtBP were found to be due to vibrations similar in character to the in-plane skeletal modes of conventional planar porphyrins. In the low-frequency region, two of the bands, assigned as nu(8) (ca. 330 cm(-1)) and nu(16) (ca. 540 cm(-1)), are also found in planar porphyrins such as tetra-phenyl porphine (TPP) and tetra-iso-propyl porphine (IPP). Of the remaining three very strong bands, the lowest frequency band was assigned as gamma(12) (pyr swivel, obsd 415 cm(-1), calcd 407 cm(-1) in do). The next band, observed at 589 cm-1 in the do compound (calcd 583 cm(-1)), was assigned as a mode whose composition is a mixture of modes that were previously labeled gamma(13) (gamma(CmCaHmCa)) andy gamma(11) (pyr fold(asym)) in NiOEP. The final strong band, observed at 744 cm(-1) (calcd 746 cm(-1)), was assigned to a mode whose composition is again a mixture of gamma(11) and gamma(13), although here it is gamma(11) rather than gamma(13) which predominates. These bands have characters and positions similar to those of three of the four porphyrin ring-based, weak bands that have previously been observed for NiTPP. In addition there are several weaker bands in the TtBP spectra that are also "out-of-plane" vibrations. Two of these (878 and 902 cm(-1)) correspond to the remaining 652 cm(-1) NiTPP band, gamma(17) (gamma(C-beta-H)(sym)), and are gamma(C-beta-H)(sym) vibrations centered predominantly on the pyrrolidene or pyrrole rings. Since the intensities of resonance Raman bands can be used to map the changes in geometry associated with the electronic transitions lying at the excitation wavelength, the observation that the modes which are most strongly enhanced are those which involve distortion of the C-m-pyrrole-C-m segments away from their near-planar ground-state geometries may be significant. In particular, it points to distortions in the excited state along coordinates which are different to those found in the ground state. In the ground state, each of the C-m-pyrrole-C-m units in TtBP is near-planar, even in this very sterically challenged compound, but the overall structure is ruffled because these units are tilted with respect to each other. However, the enhanced modes do not follow this distortion coordinate but are associated with twisting within the C-m-pyrrole-C-m units and this suggests that these modes are important in the excited state.