Infrared-optical double-resonance measurements of hydrogen-bonding interactions in clusters involving aminophthalimides

We have used infrared spectroscopy in both the ground and excited electronic states to determine the structural geometries of hydrogen-bonded clusters involving aminophthalimide molecules. This provides a higher level of structural information than available through an inertial measurement such as rotational coherence spectroscopy. Clusters of interest include the two conformers each of the water n = 1 cluster with 4-aminophthalimide (4AP) and 4-amino-N-methylphthalimide (4ANMP), which are shown to be attached by proton donation to different >C=O groups. The corresponding n = 1 water cluster of 3-aminophthalimide (3AP) is present as only a single conformer, consistently with the presence of an intramolecular hydrogen bond. Both 3AP/(H2O)(1) and 4AP/(H2O)(1) are found to form bridging structures between the imide N-H bond and an adjacent >C=O group. On the other hand, the water dimer clusters of 4AP and 4ANMP bridge the NH2 and the adjacent >C=O group. Pure dimers of the aminophthalimides have simple infrared spectra in the electronic ground states, which indicate symmetrical structures. The 4AP and 4ANMP dimers form a double hydrogen-bonded bridge across the 2,4 positions and their complex infrared spectra in the excited state show evidence of strong interactions, which may involve excited-state intermolecular double proton transfer. On the other hand, the 3AP dimer is doubly hydrogen bonded across the 1,7 positions, and appears to show evidence of excited-state single intramolecular proton transfer. Possible reasons for these differences are discussed. (C) 2002 Elsevier Science B.V. All rights reserved.