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

Abstract

Resonant ion-dip infrared spectroscopy of the S 4 and D 2d water octamers in benzene-(water ) 8 and benzene 2 -( water ) 8The techniques of resonant two-photon ionization ͑R2PI͒, UV-UV hole-burning, and resonant ion-dip infrared ͑RIDIR͒ spectroscopy have been employed along with density functional theory ͑DFT͒ calculations to characterize the hydrogen-bonding topologies of three isomers of benzene-͑water͒ 9 . Isomers I and II, with R2PI transitions shifted, respectively, by ϩ77 and ϩ63 cm Ϫ1 from the benzene monomer, have similar intensities in the R2PI spectrum. The signal from the third isomer ͑isomer III, shifted ϩ60 cm Ϫ1 ͒ 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-͑water͒ 8 D 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 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 distinct 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'' ͑water͒ 9 structures contains five symmetry-inequivalent free OH groups, a total of 45 ''expanded-cube'' benzene-͑water͒ 9 conformational isomers are predicted. Structural and vibrational frequency calculations have been performed on seven of these to determine how the ͑water͒ 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 BW 9 (D1) structure in which benzene attaches to W 9 (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-͑water͒ 9 isomer studied. The experimental spectra of isomers 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 shows characteristics consistent with a second D1-derived BW 9 structure in which benzene is attached at a position on the expanded cube remote from the ninth water.

Solid water clusters in the size range of tens–thousands of H2O: a combined computational/spectroscopic outlook

“…Solid water clusters 383 the double resonance ion-dip infrared experiments on water clusters connected to benzene [38,108,109], and to phenol [110,111]. In particular, the stable symmetric cube structures of the octamer were reported for the (H 2 O) 8 -benzene cluster [112], and for water clusters with the phenol chromophore [113].…”

Isomer-specific spectroscopy of metal clusters trapped in a matrix:Ag9

“…For gas-phase organic molecules, e.g., uv-hole-burning spectroscopy coupled with R2PI spectroscopy has been used to differentiate the absorption spectra of different isomers [15,16]. Another promising route is to separate a gas-phase isomeric ion mixture prior to a spectroscopic probe.…”

Numerical simulation of IR absorption, reflection, and scattering in dispersed water-oxygen media

“…These clusters exhibit a high capacity for adsorbing molecules of other gases. For example, it was experimentally found that water clusters with a size of up to nine atoms are capable of capturing and holding a benzene molecule whose molecular weight is half the weight of such cluster [3]. Previously, the method of molecular dynamics in the approximation of hard TIP4P model of water was used to demonstrate [4] that, as a result of the attachment of O 2 molecules to (H 2 O) n clusters, the absorption coefficient decreases, and reflection coefficient increases.…”