Proton transfer within the NH-0-N intramolecular H bond of the title molecule in its ground and first few excited electronic states is examined by ab initio methods. The 4-31G and 6-31+G** basis sets are used at the CIS, CIS-MP2, and CASSCF levels. 3n-n* is clearly the first excited state, but the ordering of the higher states depends upon the particular level of theory. A correlation is noted in that the stronger the H bond, the lower the barrier to proton transfer. MP2 correlation lowers the barriers, such that those for all excited states are smaller than for SO. The CASSCF barriers are considerably higher than CIS-MP2 results. When permitted nonplanar distortions, each state deforms in a different manner. Because the distortions have similar energetic consequences for the equilibrium and transition state structures, the proton transfer barrier of the h-n* state is little affected by permitting such deformations; the reaction paths for the planar and nonplanar cases are nearly parallel. Some of these pattems are similar to those observed in the isoelectronic malonaldehyde molecule. The CIS transfer barriers in either case obey the order In-n* < SO < 3n-n* < 'n-n* < 3n-n*, and the barrier increments from one state to the next are remarkably similar. The barriers are uniformly higher for the intemitrogen transfers than for the OH* 0 interaction in malonaldehyde, which is attributed to the longer H bonds in 1,5-diaza-l,3-pentadiene.F or any given H-bond length, the interoxygen transfer has a slightly higher barrier than does NH - N by 2-3 kcaymol.