The normal coordinate analysis has been performed for 4-aminopyridine (4-apy) assuming C-2v molecular symmetry, A Urey-Bradley force field has been used. The force constants are adjusted to fit the observed frequencies for 4-apy and its deuterated species. The vibrational assignment has been made on the basis of the calculated frequencies and potential energy distributions. The calculated frequencies were in good agreement with the observed frequencies. The substituent effect upon the M-nitrogen (ligand) (L =4-methlypyridine, 4-ethylpyridine, 4-vinylpyridine, 4-aminopyridine and 4-cyanopyridine) and pyridine ring frequencies has also been investigated. The frequency shifts are found to be sensitive to the substituent in the 4-position of the pyridine ring. (C) 2001 Elsevier Science B.V. All rights reserved.