Resonant ion-dip infrared spectroscopy of benzene-(water)(9): Expanding the cube

The techniques of resonant two-photon ionization (R2PI), UV-UV hole-burning , and resonant ion-dip infrared (RIDIR) spectroscopy have been employed alo ng with density functional theory (DFT) calculations to characterize the hy drogen-bonding topologies of three isomers of benzene-(water)(9). Isomers I and II, with R2PI transitions shifted, respectively, by +77 and +63 cm(-1) from the benzene monomer, have similar intensities in the R2PI spectrum. T he signal from the third isomer (isomer III, shifted +60 cm(-1)) is present at about one-fourth the intensity of the other two. The experimental RIDIR spectrum of isomer I bears a strong resemblance to the spectrum of the ben zene-(water)(8) D-2d-symmetry cubic structure identified in earlier work, b ut possessing an extra single-donor transition associated with the ninth wa ter molecule. Using the S-4 and D-2d symmetry forms of the water octamer as base structures to which the ninth water molecule can be added, a total of nine "expanded-cube" structures are identified for W-9 arising from two di stinct insertion points in the W-8(D-2d) cube (D1,D2) and three such points in the W-8(S-4) cube (S1-S3). DFT calculations predict these to be spread over an energy range of less than 1 kcal/mol. Given that each of the nine " expanded-cube" (water)(9) structures contains five symmetry-inequivalent fr ee OH groups, a total of 45 "expanded-cube" benzene-(water)(9) conformation al isomers are predicted. Structural and vibrational frequency calculations have been performed on seven of these to determine how the (water)(9) stru ctural type and the attachment point of benzene to the structure affect the total energy and vibrational frequencies of the cluster. Based on a compar ison of the experimental RIDIR spectrum with the calculated vibrational fre quencies and infrared intensities, isomer I is attributed to the BW9(D1) st ructure in which benzene attaches to W-9(D1) at the free OH of the water mo lecule which donates a H-bond to the ninth water. This structure has a calc ulated binding energy that is about 0.13 kcal/mol greater in magnitude than any other benzene-(water)(9) isomer studied. The experimental spectra of i somers II and III are of insufficient quality to assign them to specific BW 9 structures with confidence. However, isomer II is most consistent with an S-4-derived expanded cube structure (either S1 or S2), while isomer III sh ows characteristics consistent with a second D1-derived BW9 structure in wh ich benzene is attached at a position on the expanded cube remote from the ninth water. (C) 2000 American Institute of Physics. [S0021- 9606(00)01024- 2].