Microsolvation of similar-sized aromatic molecules: Photoelectron spectroscopy of bithiophene-, azulene-, and naphthalene-water anion clusters

We perform a comparison of electron affinities (EA) of the conjugated molec ules bithiophene, azulene, naphthalene, and their water clusters. Bithiophe ne and azulene monomers have positive EAs of +49+/-5 meV and +790+/-8 meV, but naphthalene has a negative EA. Despite their different EAs and their di fferent molecular orbital energies the three molecules show very similar mi crosolvation shifts per water unit. This is explained by similar sizes of t he pi orbitals in which the surplus electron is delocalized leading to a si milar electrostatic water to charge interaction. This qualitative dependenc e of solvation energy on anion size agrees well with classical solvation co ncepts. A comparison of our binding energies with previous calculations for other systems shows that formation of a water subcluster can be assumed. F or all three molecules the cluster EAs increase nearly linearly with the nu mber of waters. Using a linear approach and a calibration for the error in the first solvation step we extrapolated the naphthalene (H2O)(n) cluster s eries to a monomer EA-200 meV+/-50 meV, in good agreement to previous measu rements. To become new insights B3LYP/6-31++G** structures and energies hav e been calculated fur azulene, naphthalene, and their clusters with one wat er and compared with experimental EAs. (C) 2000 American Institute of Physi cs. [S0021-9606(00)00245-2].