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

Abstract

The techniques of resonant two-photon ionization (R2PI), UV-UV hole-burning, and resonant ion-dip infrared (RIDIR) spectroscopy have been employed along with density functional theory (DFT) calculations to characterize the hydrogen-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. The 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 benzene–(water)8 D2d-symmetry cubic structure identified in earlier work, but possessing an extra single-donor transition associated with the ninth water molecule. Using the S4 and D2d 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 W9 arising from two distinct insertion points in the W8(D2d) cube (D1,D2) and three such points in the W8(S4) 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 free OH groups, a total of 45 “expanded-cube” benzene–(water)9 conformational isomers are predicted. Structural and vibrational frequency calculations have been performed on seven of these to determine how the (water)9 structural type and the attachment point of benzene to the structure affect the total energy and vibrational frequencies of the cluster. Based on a comparison of the experimental RIDIR spectrum with the calculated vibrational frequencies and infrared intensities, isomer I is attributed to the BW9(D1) structure in which benzene attaches to W9(D1) at the free OH of the water molecule which donates a H-bond to the ninth water. This structure has a calculated binding energy that is about 0.13 kcal/mol greater in magnitude than any other benzene–(water)9 isomer studied. The experimental spectra of isomers II and III are of insufficient quality to assign them to specific BW9 structures with confidence. However, isomer II is most consistent with an S4-derived expanded cube structure (either S1 or S2), while isomer III shows characteristics consistent with a second D1-derived BW9 structure in which benzene is attached at a position on the expanded cube remote from the ninth water.