Acceptor-acceptor (AA) excess electron binding motif associated with the isomer I variants of the small cluster anions. In this arrangement, a single water molecule serves as the binding site for an excess electron, pointing both free hydrogen atoms toward the charge density of the excess electron. This AA molecule is easily revealed in infrared spectra by the presence of its very redshifted HOH bending vibration. The shaded surface depicts the 0.015 isosurface contour of the wavefunction, with the darker interior lobe corresponding to the region of opposite phase.
Photoelectron spectra of the clusters. Expected electron binding energies of isomers I and II, along with the emergence of a new, higher binding isomer , are indicated by the dashed lines. The arrows indicate the energies of the infrared bleaching laser used to selectively remove isomers II and I from the population (0.372 and , respectively).
28 low-energy structures of characterized theoretically (see text). Relative energies and EBEs were calculated at the level of theory. Structure names marked with an were first identified in the Monte Carlo simulations of Defusco et al. (Ref. 28). The other structures were originally identified by Lee et al. (Ref. 27). For the latter, we have retained the labeling scheme of these authors. Numbers after the structure name refer to the dipole moment (D), vertical detachment energy (meV), and relative energy (meV, relative to Pr-b), respectively.
Calculated vertical detachment energies (VDE) of the structures shown in Fig. 3, plotted as a function of the dipole moments of their neutral scaffolds. All isomers with VDEs above exhibit the AA-type motif (filled black squares).
Vibrational spectra of in the HOH bending region. The redshifted band at indicates the presence of the AA binding motif throughout the series. The bending vibrational frequency of a free molecule is represented by the dashed line labeled as at . The appearance of features labeled , , and coincides with the emergence of isomer in the photoelectron spectrum. The band labeled by is associated with isomer II.
Vibrational predissociation spectrum of (a) before and (b) after bleaching the ion packet with an infrared laser tuned to . The bleaching laser removes isomer II from the population, leaving only isomer I for interrogation by the second infrared laser. The band labeled by is therefore associated with isomer II.
Infrared HOH bending spectrum of the isomer II variant of (trace b), determined by subtraction of trace (b) in Fig. 6 from trace (a) in Fig. 6, shown in comparison with the analogous spectra for (trace a) and (trace c) species from previous studies (Refs. 17 and 18). All three species exhibit intensity that is either at or slightly redshifted from the free bending frequency ( for and for ), indicative of similar electron binding motifs. The dashed line indicates the position of the redshifted AA band present in the type I isomers of the three species.
Comparison in the bending region of (a) the measured infrared spectrum of the isomer II variant of with (b) the corresponding calculated spectrum of Pr-b (see Fig. 3). The calculated spectra were obtained at the level of theory, with the calculated harmonic frequencies being scaled by 0.93.
Infrared spectra of the isomer I species for (top panel), (center panel), and (bottom panel). The peak labeled by AA near corresponds to HOH bend of the water molecule which is pointing both of its hydrogen atoms into the excess electron orbital, as shown in the inset. The dashed line labeled at corresponds to the bend of an isolated molecule.
Comparison of the infrared spectra of (a) isomer and (b) isomer I species in the HOH bending region. The isomer spectrum is obtained by photobleaching the ion with a laser tuned to before interaction with the second infrared laser, thereby removing both isomers I and II from the population. Peaks labeled by and are therefore associated solely with isomer .
Argon dependence of the vibrational spectra in the OH stretching region. The transition from isomer I to is most obvious at , where new peaks emerge near , labeled as . The peak labeled A is associated with isomer I, while B arises from isomer .
OH stretching spectra of (a) isomer I, (b) isomer I, and (c) isolation of the type isomer by photobleaching with a laser tuned to . The band labeled by corresponds to a tentative assignment to the overtone of the intramolecular bending vibration. The symmetric and asymmetric stretches of the AA water molecule, labeled by and , respectively, remain largely intact in all spectra.
OH stretching spectrum of (a) before and (b) after photobleaching with a laser tuned to , thereby removing isomers I and II from the population. The dashed lines indicate frequencies at which the predissociation signal was modulated by the bleaching laser, establishing that they are band positions associated with isomer I. The feature labeled C is completely removed upon photobleaching, and is therefore associated solely with isomer I.
Experimental infrared spectrum of isomer I in the HOH bending and OH stretching regions (top trace), with representative calculated structures and spectra shown in the lower traces. The most plausible structural assignments are Pf24a and Pnf-a. The calculated spectra in the HOH bending region were obtained at the level of theory with a 0.93 scaling of the frequencies, and those in the OH stretching region were produced at the level with a 0.974 scaling of the frequencies.
Experimental infrared spectrum of isomer in the HOH bending and OH stretching regions (top trace), with the calculated spectrum of the most plausible structure, Af-a, shown in the lower trace. The calculated spectrum in the HOH bending region was obtained at the level of theory with a 0.93 scaling of the frequencies, and that in the OH stretching region was produced at the level with a 0.974 scaling of the frequencies.
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