MOLECULAR-ORBITAL CONSIDERATIONS ON THE A MINO-IMINO TAUTOMERIZATION IN THE 2-AMINOPYRIDINE ACETIC-ACID SYSTEM IN THE EXCITED SINGLET-STATE

In order to make clear the amino-imino tautomerization mechanism in th e lowest excited singlet state and spectroscopic behavior of fluoresce nce spectrum of the 2-aminopyridine-acetic acid system, the ab initio molecular orbital calculation at the 6-31G level with a geometry-optim ization was carried out for the 2-aminopyridine-formic acid system use d as a model system for the 2-aminopyridine-acetic acid system. From c alculated results the following conclusions were drawn: (1) The lowest excited singlet state (S-1) of the optimized models of 2-aminopyridin e, 2-aminopyridine-formic acid complex (amino complex), 2-aminopyridin ium formate complex (cation complex), and 2(1H)-pyridinimine-formic ac id complex (imino complex) are of pi, pi character. (2) In the S-1 st ate the imino complex is the most stable. It is more stable than the c ation complex by 5.36 kJ mol(-1). The cation complex is more stable th an the amino complex by 13.65 kJ mol(-1) in the same state. The order of stability for these three complexes in the S-1 state is reverse to that of the corresponding one in the ground state. (3) The calculated potential energy surface suggests that the amino-imino tautomerization may proceed through the cation complex as a reaction intermediate in the S-1 state. (4) The calculated potential barrier height from the am ino complex to the cation complex is 3.78 kJ mol(-1) in the S-1 state. The potential barrier height is very low in comparing with the corres ponding one (13.10 kJ mol(-1)) in the ground state. (5) The potential barrier height from the cation complex to the imino complex is 9.23 kJ mol(-1), which corresponds to the value of 39.00 kJ mol(-1) in the gr ound state. (6) The potential barrier heights from the amino complex t o the imino complex in the S-1 state are much lower than the correspon ding ones in the ground state. (7) The order of stability among the th ree complexes in the S-1 state is consistent with that concluded from the spectroscopic behavior of the fluorescence spectrum of 2-aminopyri dine in the low concentration of acetic acid. As the concentration of acetic acid increases, the cation complex may be more stabilized in en ergy than the imino complex due to the dipole-dipole interaction betwe en the solute and solvent, because the cation complex has the largest dipole moment among the three complexes. Therefore, the order of stabi lity may be as the cation complex > the imino complex > the amino comp lex. This order of stability is consistent with that concluded from th e behavior of the fluorescence spectrum of 2-aminopyridine in the high er concentration of acetic acid.