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 along with density functional theory (DFT) calculations to characterize the hydrogen-bonding topologies of three isomers of ${\mathrm{benzene–(water)}}_{\mathrm{9}}.$ 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 ${\mathrm{benzene–(water)}}_{\mathrm{8}}$ $D_{\text{2d}}$ -symmetry cubic structure identified in earlier work, but possessing an extra single-donor transition associated with the ninth water molecule. Using the $S_4$ and $D_{\text{2d}}$ 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 ${\mathrm{W}}_{\mathrm{9}}$ arising from two distinct insertion points in the ${\mathrm{W}}_{\mathrm{8}}{\mathrm{(D}}_{\text{2d}})$ cube $\text{(D1,D2)}$ and three such points in the ${\mathrm{W}}_{\mathrm{8}}{\mathrm{(S}}_4)$ cube $\text{(S1-S3).}$ 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” ${\mathrm{(water)}}_{\mathrm{9}}$ structures contains five symmetry-inequivalent free OH groups, a total of 45 “expanded-cube” ${\mathrm{benzene–(water)}}_{\mathrm{9}}$ conformational isomers are predicted. Structural and vibrational frequency calculations have been performed on seven of these to determine how the ${\mathrm{(water)}}_{\mathrm{9}}$ 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 ${\mathrm{BW}}_{\mathrm{9}}\text{(D1)}$ structure in which benzene attaches to ${\mathrm{W}}_{\mathrm{9}}\text{(D1)}$ 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 ${\mathrm{benzene–(water)}}_{\mathrm{9}}$ isomer studied. The experimental spectra of isomers II and III are of insufficient quality to assign them to specific ${\mathrm{BW}}_{\mathrm{9}}$ structures with confidence. However, isomer II is most consistent with an $S_4$ -derived expanded cube structure (either $\text{S1}$ or $\text{S2),}$ while isomer III shows characteristics consistent with a second $\text{D1}$ -derived ${\mathrm{BW}}_{\mathrm{9}}$ structure in which benzene is attached at a position on the expanded cube remote from the ninth water.