13138
J. Phys. Chem. 1994,98, 13138-13143
Collisional Activation Dynamics of OH-(H20), Cluster Ions: Comparison to H30S(Hz0), Deron A. Wood, Kenneth A. Cowen, Bob Plastndge, and James V. Coe* Department of Chemistry, The Ohio State University, Columbus, Ohio 43210-1I73 Received: July 15, 1994; In Final Form: September 13, 1994@
The collisional activation dynamics of hydrated hydroxide clusters in a fast ion beam have been studied upon multiple glancing collisions with argon. Glancing collisions are defined as those involving detectable parent or fragment ions at the end of the beam line. Hydrated cluster ions are ideal for multiple-collision studies because one glancing collision will not on average impart enough energy to dissociate the cluster while two such collisions predominantly lead to dissociation. The absolute values of the attenuation cross section of OH-(H20),,0-4 with argon are 31.6(1.8), 29(2), 25.2(1.4), 33(2), and 50(10) A2, respectively. The absolute values of the glancing collision induced dissociation cross sections are 1.17(0.08) A2 for OH-(H20)1 and 5.7(0.4) A2 for OH-(H20)2. The fraction of single glancing collisions which do not lead to dissociation or loss from the ion beam is 0.92 for OH-(H20)1 and 0.25 for OH-(H20)2. Single-collision branching ratios of 0.88:0.12 and 0.52:0.46:0.02 have been determined for successive loss of waters from OH-(H20)2 and OH-(H20)3, respectively. Detachment gives way to dissociation upon collisional activation as several waters are clustered to OH-. Comparison of the hydrated hydroxide attenuation cross sections vs cluster size to those of hydrated hydronium and hard sphere expectations implies that the excess charge of OH- is more localized in water clusters than the proton.
Introduction Many of the unique properties of water are related to the existence of hydroxide and hydronium ions in pure water. The size of aqueous clusters of these fundamental ions associated with water can be varied to effect the degree of solvation allowing study of the transition of ion properties from the gas phase to bulk. We have previously studied the glancing collisions of hydrated hydronium clusters and are now extending these studies to hydrated hydroxide clusters to contrast and compare the dynamics of activation by glancing collisions. The potential for detachment in the negative ions and proton jumping in the positive ions result in some interesting differences. The observation of collision induced dissociation (CID) fragment ions at the end of a fast ion beam line is interesting because it indicates that a chemically significant amount of energy has been transferred without the transfer of enough momentum to knock the fragment ion out of the ion beam. These dissociative interactions must be glancing, low-angle scattering, high-impact parameter collisions. For short, we call these glancing collisions and note that in practice such collisions are defined by products which are detectable at the end of the beam line (whether as parent or fragment ions). Given that fragment ions of hydrated cluster ions are readily detectable and that dissociation requires -1 eV, it can reasonably be assumed that there are many collisions which impart
