Site-specific addition of D2O to the (H2O)6− “hydrated electron” cluster: isomer interconversion and substitution at the double H-bond acceptor (AA) electron-binding site

Abstract

We report the results of an experimental study designed to establish whether, once formed, one of the isomer classes of the hydrated electron clusters, (H(2)O)(n)(-), can interconvert with others when a water molecule is added by condensation. This is accomplished in an Ar-cluster mediated approach where a single intact D(2)O molecule is collisionally incorporated into argon-solvated water hexamer anions, creating the isotopically labeled D(2)O.(H(2)O)(6)(-).Ar(n) heptamer anion. Photoelectron and infrared predissociation spectroscopies are employed both to characterize the isomers generated in the condensation event and to track the position that the D(2)O label adopts within these isomeric structures. Despite the fact that the water hexamer anion precursor clusters initially exist in the isomer I form, incorporation of D(2)O produces mostly isomers I' and II in the labeled heptamer, which bind the electron more (I') or less (II) strongly than does the isomer I class. Isomers I and I' are known to feature electron binding primarily onto a single water molecule that resides in an AA (A = H-bond acceptor) site in the network. Surprisingly, the D(2)O molecule can displace this special electron-binding H(2)O molecule such that the anionic cluster retains the high binding arrangement. In the more weakly binding isomer II clusters, the D(2)O molecule fractionates preferentially to sites that give rise to the vibrational signature of isomer II.

Isolating the Spectral Signatures of Individual Sites in Water Networks Using Vibrational Double-Resonance Spectroscopy of Cluster Isotopomers

“…The bands are grouped into four distinct regions, the OH stretches (3000−3800 cm −1 , right side of Figure A), the OD stretches (2400−2800 cm −1 , left side of Figure A), the HOH bends (1500−1800 cm −1 , right side Figure B), and the DOD bends (1100−1300 cm −1 , left side Figure B). As reported previously, there is no activity in the region of the HOD bending transitions (arrow in Figure B), which confirms that D 2 O remains intact upon Ar-cluster-mediated condensation into the anionic hexamer clusters, (H 2 O) 6 ¯·Ar p . The plethora of bands associated with the OD stretches and D 2 O bends indicates that the D 2 O molecule occupies many (if not all) of the available sites, with the overall spectrum (Figure ) appearing qualitatively similar to that of the perdeuterated isotopologue (Supporting Information).…”

“…This mixture was supersonically expanded and ionized by a counterpropagating 1 keV electron beam . The heavy water (D 2 O) was then introduced into the vacuum chamber through a remote pulsed valve and becomes entrained in the expansion, as discussed in an earlier report . The D 2 O(H 2 O) 6 ¯·Ar m clusters are formed by Ar-mediated condensation onto (H 2 O) 6 ¯·Ar p .…”

“…This is accomplished by placing the probe laser on the low-energy AA bend (D 2 O) bending transition at 1138 cm −1 , indicated in Figure A. This was the most challenging experiment carried out in this study because the laser power is quite weak at this probe frequency and D 2 O occupation of the AA site upon condensation onto the hexamer anion is relatively ineffiecient . Figure D presents the resulting dip trace, and while compromised by limited signal-to-noise, it clearly reveals the two strong bands expected for the symmetric and asymmetric stretches of the AA water molecule, as indicated by the scaled harmonic values shown in Figure B.…”

Unmasking Rare, Large-Amplitude Motions in D2-Tagged I–·(H2O)2 Isotopomers with Two-Color, Infrared–Infrared Vibrational Predissociation Spectroscopy

“…Specifically, the observation of absorption at one of these OH stretch frequencies reveals its occupation by an H atom, even if only transiently. In the case of a static system, the absorption spectrum of each isotopomer can be experimentally determined using an IR–IR population-labeling or “hole-burning” technique described in detail in earlier reports. ,,− This method exploits the photodissociation action mode (by which mass-selected ion spectra are typically obtained) as a way to selectively remove population of a particular isotopomer from an ion ensemble selected by its m / z . In an isotopomer-selective mode, two independently tuned IR lasers are employed: one of them (the probe) is fixed on a transition associated with a particular isotopomer, while the other (the pump) excites the same ion packet upstream and is scanned through the entire vibrational spectrum.…”

“…Regime I is in effect for the implementation of isotopomer-selective spectroscopy, ,,− while regime III corresponds to rapid site exchange on the time scale of the pump laser. In regime III, excitation of the strong IHB transitions can be carried out to saturation which will, in turn, completely deplete the ion ensemble by removing both isotopomers upon excitation of either one of the IHB bands.…”