Structural Evolution of (1-NpOH)n Clusters Studied by R2PI and IR Dip Spectroscopies

A large-size 1-naphthol cluster, (1-NpOH)(n), with n <= 30 was prepared by using a high-pressure pulsed valve. The electronic and vibrational transitions of (1-NpOH)(n) with n = 3-9 were measured by resonant two-photon ionization (R2PI) and ion-detected IR dip spectroscopies. The S-1 <- S-0 R2PI spectrum shows partially resolved structures around the origin band in the (1-NpOH)(n) cluster with n = 3-8. The (1-NpOH)(3) and the (1-NpOH)(6) clusters show relatively sharp origin bands. The structure of (1-NpOH)(3) was determined by comparison of the IR dip spectrum with the simulated one by DFT calculation, while those of (1-NpOH)(n) (n >= 4) were discussed in terms of topological geometries of a hydrogen-bonded network. Those analyses suggest that (i) the (1-NpOH)(3) cluster has the cyclic structure where three 1-NpOH monomers are linked by both the hydrogen-bonding and the pi center dot center dot center dot C-H interaction between naphthyl rings and (ii) the (1-NpOH)(n) cluster with n >= 4 is built up by attaching the 1-NpOH monomers to the (1-NpOH)(3) core.