Resonant ion-dip infrared spectroscopy of the S4 and D2d water octamers in benzene-(water)8 and benzene2-(water)8

Abstract

Resonant ion-dip infrared spectroscopy of benzene-(water) 9 : Expanding the cube The techniques of resonant two-photon ionization ͑R2PI͒, UV-UV ͑ultraviolet͒ hole-burning, and resonant ion-dip infrared ͑RIDIR͒ spectroscopies have been employed along with density functional theory ͑DFT͒ calculations to assign and characterize the hydrogen-bonding topologies of two isomers each of the benzene-͑water) 8 and (benzene͒ 2 ͑water) 8 gas-phase clusters. The BW 8 isomers ͑Bϭbenzene, Wϭwater͒ have R2PI spectra which are nearly identical to one another, but shifted by about 5 cm Ϫ1 from one another. This difference is sufficient to enable interference-free RIDIR spectra to be recorded. As with smaller BW n clusters, the BW 8 clusters fragment following photoionization by loss of either one or two water molecules. The OH stretch IR spectra of the two BW 8 isomers bear a close resemblance to one another, but differ most noticeably in the double-donor OH stretch transitions near 3550 cm Ϫ1 . Comparison to DFT calculated minimum energy structures, vibrational frequencies, and infrared intensities leads to an assignment of the H-bonding topology of the BW 8 isomers as nominally cubic water octamers of S 4 and D 2d symmetry surface attached to benzene through a H-bond. A series of arguments based on the R2PI and hole-burning spectra leads to an assignment of additional features in the R2PI spectra to two isomers of B 2 W 8 . The OH stretch RIDIR spectra of these isomers show them to be the corresponding S 4 and D 2d analogs of B 2 W 8 in which the benzene molecules each form a H-bond with a different dangling OH group on the W 8 sub-cluster.

Local Dense Structural Heterogeneities in Liquid Water from Ambient to 300 MPa Pressure: Evidence for Multiple Liquid–Liquid Transitions

“…The parallel intensity decrease of the sub-band (3) should be interpreted in terms of an accompanying decrease in the fraction of acyclic tetramers (whose oscillator number in overall should decrease from 16 to eight, assuming that two clusters of the type shown in Figure 4 b must join together to form the structure presented in Figure 4 c). This assignment is also in good agreement with published data for the condensed-phase cubic octamer [53][54][55][56] or the planar cyclic tetramer fragments. [50][51][52] C) We suggest, for the first time, that the two kinds of dense structures that show up under ambient and high-pressure conditions are nothing else but alternative (high-density) forms of liquid water involved in multiple liquid-liquid transitions "above" the second critical point as hypothesized in the literature.…”

“…Furthermore, these local heterogeneities (patches) should fit the "host" hexagonal structure at the maximal level of H-bonding connectivity. Among a large number of theoretically possible structures, [22][23][24][25] the planar tetramers [48][49][50][51][52] and a cubic octamer structure [53][54][55][56] seem to meet the above-mentioned requirements better (see ref. [19] for the previous conjecturing, Figure 4).…”

“…The essential difference between liquid water and its crystalline forms is that in [51] (gas phase) 2360; 2390 [53] (crystal) ! 2300 [54] 3150-3250 [55] (gas phase) 3150 [53] 2.75; 2.78; 2.80 [54] 2.64; 2.75; 2.77 [54] 5.6-6.1 [56] (average) cubic octamer doubledonor noncyclic structure (no resonance) 2500 [54] (crystal) 2600 [54] 3500-3600 [55] (gas phase) 3600 [53] 2.87; 2.90; 2.93 [54] 2.88; 2.90 [54] 5.6-6.1 [56] (average) quasiplanar tetramer singledonor cyclic struct. 2500 [51] (gas phase) 2360 [a] (liquid phase) -3230 [50] (in CCl 4 ) 3416 [52] (gas phase) 3280 [52] (scaled) 3250 [53] 2.75 [a] 2.76 [48] 2.75 [52] (average) 6.0-7.0 [48][49][50] quasiplanar tetramer double-donor noncyclic structure…”

Pople's Gaussian‐3 model chemistry applied to an investigation of (H2O)8 water clusters

“…Crystal packing forces and hydrogen bonds between the water octamer and the surrounding molecular complexes make the solid phase much different from the gas-phase environment, stabilizing these three different forms of the water octamer. In the gas-phase IR spectroscopy and DFT calculations have shown that the D 2d and S 4 isomers are present in a cold molecular beam, a finding that is supported by the stability of these two cubic structures relative to every other octamer [59,68,71,72,114]. The G3 HF/6-31G* scaled frequency of 3,696 cm Ϫ1 for the free OH stretch of the D 2d isomer is in excellent agreement with the experimental free OH stretch observed in the gas-phase benzene-D 2d structure, which is 3,713 cm Ϫ1 [72].…”

“…The solid-phase crystallographic studies of various molecules have uncovered octamers of water hydrogen-bonded with organic and inorganic complexes, including a C i cube [78], a distorted pseudo-S 4 octamer [101], and a cyclic structure [94]. In the gas-phase IR spectroscopy and density functional theory (DFT) calculations suggested that the D 2d and S 4 isomers were present [68,71], and these two species have been definitively identified associated with benzene molecules through a combination of spectroscopic methods and DFT calculations [72]. Most of the theoretical focus has been on the cubic structures, which have been observed in cold spectroscopic beams [68,71].…”