This work represents the first theoretical attempt to assign vibrational modes for methylcobalamin (MeCbl), the biologically active form of a vitamin B-12 derivative. Full vibrational analysis of MeCbl is currently beyond computational resources; thus, a simplified model, including the actual corrin ring, Im-[Co-III(corrin)]-CH3. was employed in the analysis. The scaled quantum mechanical (SQM) method has been used to refine density functional theory (DFT) based force constants calculated at the B3LYP level of theory. Normal-coordinate analysis based on a six-coordinate MeCbl model permitted assignment of the most important interligand modes. Direct comparison with experimental data showed that the DFT-SQM force field accurately predicts isotope shifts for interligand vibrations corresponding to CH3 --> CD3 and (CH3)-C-12 --> (CH3)-C-13 isotope labeling. The DFT-SQM force field for Im-[Co-III(corrin)]-CH3 gives a semiquantitative description of the corrin modes when compared to the Raman spectrum of MeCbl. The analysis of Raman data, together with DFT-computed frequencies, permitted assignment of certain modes located in the 1300-1600 cm(-1) range. The calculated modes in this spectral range are mainly composed of single/double CC and CN stretch vibrations. These modes more closely resemble vibrations of connected short linear polyenes rather than modes delocalized over the corrin ring as observed in porphyrins. This localized character most likely is responsible for lack of structure-sensitive modes such as the "core size" marker.