Patrick Ayotte

Electronic absorption spectra of size-selected hydrated electron clusters: (H2O)n−, n=6–50

We report photodestruction spectra for the (H2O)n− clusters (n=6, 11, 15, 20, 25, 30, 35, 40, and 50) in the range 0.7–1.6 eV. The spectra are observed to strongly redshift and narrow with decreasing cluster size, with a concomitant increase in peak intensity. The maxima occur well below the vertical detachment energies for n>20 and almost exclusively result from excitation of a bound–bound transition.

Mass-selected “matrix isolation” infrared spectroscopy of the I−·(H2O)2 complex: making and breaking the inter-water hydrogen-bond

Abstract Infrared spectra of the cold I − ·W and I − ·W 2 clusters are reported via vibrational predissociation spectroscopy of the argon solvated species, I − ·W n ·Ar m , in the OH stretching region. Several argon atoms serve to significantly simplify the spectra by collapsing complex band contours into sharp features at the vibrational origins. This effect, in addition to the substantial cooling afforded by argon solvation, dramatically change the appearance of the bare dihydrate spectrum reported earlier [P. Ayotte, C.G. Bailey, G.H. Weddle, M.A. Johnson, J. Phys. Chem. A 102 (1998) 3067]. The cold spectrum consists of a simple four line pattern anticipated by ab initio calculations for the asymmetric structure where the two waters are bound together on one side of the ion. The dramatic changes in the spectrum of the argon complex relative to that of bare I − ·W 2 are readily interpreted to be a consequence of internal energy in the latter leading to rupture of the inter-solvent H-bond.

AN INFRARED STUDY OF THE COMPETITION BETWEEN HYDROGEN-BOND NETWORKING AND IONIC SOLVATION : HALIDE-DEPENDENT DISTORTIONS OF THE WATER TRIMER IN THE X- .(H2O)3, (X=CL, BR, I) SYSTEMS

Vibrational spectra of the water trimers solvating the halide anions (Cl−, Br−, I−) have been acquired in the OH stretching region by predissociation spectroscopy of the X−⋅(H2O)3⋅Ar3 complexes. These “wet” ions display two groups of bands assigned to normal modes of the (C3) pyramidal structure. We interpret the evolution of the spectra down the halogens in the context of the rings closing up toward the structure of the bare (H2O)3 neutral. This trend is discussed in terms of the disruptive effect of the ionic H bonds on the water network.

Infrared spectroscopy of negatively charged water clusters: Evidence for a linear network

We report autodetachment spectra of the mass-selected, anionic water clusters, (H2O)n−, n=2, 3, 5–9, 11 in the OH stretching region (3000–4000 cm−1), and interpret the spectra with the aid of ab initio calculations. For n⩾5, the spectra are structured and are generally dominated by an intense doublet, split by about 100 cm−1, which gradually shifts toward lower energy with increasing cluster size. This behavior indicates that the n=5–11 clusters share a common structural motif. The strong bands appear in the frequency region usually associated with single-donor vibrations of water molecules embedded in extended networks, and theoretical calculations indicate that the observed spectra are consistent with linear “chainlike” (H2O)n− species. We test this assignment by recording the spectral pattern of the cooled (argon solvated) HDO⋅(D2O)5− isotopomer over the entire OH stretching frequency range.

Vibrational predissociation spectroscopy of the (H2O)6−⋅Arn, n⩾6, clusters

Solvation of (H2O)6− with several argon atoms suppresses the strong direct photodetachment background in the bare hexamer anion, allowing vibrational predissociation spectroscopy to be carried out in a background-free regime. In addition to the previously reported autodetaching resonances [C. G. Bailey, J. Kim, and M. A. Johnson, J. Phys. Chem. 100, 16782 (1996)] in the single donor hydrogen bonding region (∼3300u2009cm−1), the predissociation spectra reveal many weak bands scattered throughout the mid infrared (3200–3750u2009cm−1). Most of these new bands are evident in the bare hexamer spectrum after signal averaging, indicating that they are isolated using predissociation but not induced by solvation. The most intense bands display much stronger redshifts (>30u2009cm−1 by n=15) than the matrix shifts typically found for the neutral water clusters, indicating that these bands are unique to the negative ion.

Vibrational spectroscopy of the F−·H2O complex via argon predissociation: photoinduced, intracluster proton transfer?

Abstract The mid-IR vibrational spectrum of the strongly bound F − ·H 2 O complex is reported via predissociation of the size-selected F − ·H 2 O·Ar m ( m =1–3) clusters. A weak, sharp band at 3690 cm −1 confirms that this species adopts the asymmetric arrangement typical of the heavier halides, while the band arising from the ionic H-bond (IHB) is shifted very far to the red of the free H 2 O bands (shift ∼800 cm −1 ). The observed band position is actually found in the region of the predicted `overtone of the H-bonded oscillator, where the OH stretching vibration occurs in a very strongly anharmonic O–H⋯F − potential surface in which the first excited vibrational level samples the HF–OH − proton transfer configuration.

Infrared predissociation spectroscopy of I−⋅(CH3OH)n, n=1,2: Cooperativity in asymmetric solvation

Infrared spectra of I−⋅(CH3OH)n⋅Arm, n=1,2 clusters, obtained via argon and methanol predissociation, are interpreted with the aid of ab initio calculations of the OH stretching fundamentals. The spectra of the cold, argon-solvated clusters establish the coexistence of two isomeric forms of the n=2 cluster, with the asymmetric isomer displaying a dramatic (∼150 cm−1) OH red-shift relative to both the symmetric isomer and the n=1 complex. We trace this red-shift to cooperative H-bonding which is only operative in the asymmetric form. At the higher internal energies afforded by the bare (i.e., Ar-free) complexes, the spectra are radically changed. The strongly red-shifted band is suppressed, reflecting the loss of the cooperative effect as the methanol molecules are separated, while the bands assigned to the more open form are enhanced.

Infrared spectroscopic observation of the argon isomer distribution in evaporative ensembles of I−⋅ROH⋅Arm (R=methyl, ethyl, isopropyl) clusters

Vibrational predissociation spectra of argon-solvated iodide–alcohol clusters (I−⋅ROH⋅Arm, ROH=MeOH, EtOH, i-PrOH) are reported in the OH stretching region (3200–3400 cm−1). The spectra display multiple peaks associated with the ionic H-bonded OH stretching fundamental, which vary according to the extent of argon solvation. At small m, the number of peaks reflects the total number of attached argon atoms, such that peaks associated with fewer argons persist (with a small blue shift) in the spectra of the larger clusters, while new peaks appear red shifted by about 12 cm−1 with each additional argon. The effect saturates in a manner that depends on the particular alcohol (mmax=6 for MeOH, 5 for EtOH, and 4 for i-PrOH). We interpret these observations to indicate the presence of multiple isomers in the evaporative ensemble, which are distinguishable according to the different arrangements of argon atoms among two effective binding sites.

The infrared predissociation spectra of Cl−·H2O·Arn (n=1–5): experimental determination of the influence of Ar solvent atoms

Abstract We establish the argon solvent size dependence of the Cl − ·H 2 O·Ar n predissociation spectra, and discuss the discrepancies between previously reported predissociation spectra of the Cl − ·H 2 O and Cl − ·H 2 O·Ar 3 complexes [Choi et al., J. Phys. Chem. A 102 (1998) 503; Ayotte et al., J. Am. Chem. Soc. 120 (1998) 12361]. The argon-induced shift in the ∼3130 cm −1 ionic H-bonded OH stretching band, calculated to be large (>30 cm −1 /Ar red-shift) by Satoh and Iwata [Chem. Phys. Lett. 312 (1999) 522], is found to be quite small (IHB band center=3128±3 cm −1 for 1⩽ n ⩽5). We compare this result with similar behavior displayed by the bare versus argon-solvated bromide monohydrate.

Preparation and photoelectron spectrum of the CH3I− anion: rare gas cluster mediated synthesis of an ion–radical complex

We report the preparation of the bare and argon-solvated anion of CH3I, and characterize this species using negative ion photoelectron spectroscopy at 3.495 eV. The photoelectron spectrum consists of a narrow band appearing 0.11±0.02 eV above the binding energy of isolated iodide. Such behavior is similar to that displayed by iodide-(closed shell) solvent molecule complexes, indicating that photodetachment does not access the bound region of the CH3I potential. These observations suggest that CH3I− rearranges (after electron capture) to an ion-radical complex. We advance the hypothesis that this complex adopts a C2v structure where the ion is hydrogen bonded to the methyl radical.