AMMONIA AS A HYDROGEN-BOND DONOR AND ACCEPTOR IN THE GAS-PHASE - STRUCTURES OF 2-PYRIDONE-NH3 AND 2-PYRIDONE-(NH3)2 IN THEIR S0 AND S1 ELECTRONIC STATES

Rotationally resolved S1 <-- S0 electronic spectra of the hydrogen-bonded complexes of 2-pyridone (2PY) with one and two ammonia molecules have been observed. Both complexes, in both electronic states, exhibit behavior which indicates that the ammonia molecule acts as both an acid and a base. Evidence for this amphoteric behavior in the monosolvated complex (2PY-NH3) is found in the derived, effective, hydrogen-bonded structure and in the barriers to internal rotation of the NH3 group, V3(S0) = 424.3 cm-1 and V3(S1) = 274.4 cm-1. The effective structure derived for the disolvated complex [2PY-(NH3)2] shows strong hydrogen bonding, similar to that found for the dihydrated complex (Held and Pratt, J. Am. Chem. Soc., preceding paper in this issue). The donor and acceptor bonds in 2PY-(NH3)2 are strong enough to completely inhibit any internal rotation of the ammonia molecules. Like the dihydrated complex, 2PY-(NH3)2 exists in a number of different conformational forms. The structural and dynamical properties of these complexes are discussed.