Proton affinity water clusters

A correlation can be made between the gas phase proton affinities (PA) of the B clusters which are strongly dependent on their sizes and the propensity of the AH-B clusters to undergo proton transfer in the excited state. These proton affinities of clusters (water, methanol, ammonia, and piperidine) which are estimated (e.g., Knochenmuss and Leutwyler 1989) or deduced from experiment (Bisling et al. 1987 Ceyer et al. 1979 Kamke et al. 1988) are reported in Figure 4-16. 

In RPLC-APCl-MS, where the mobile phase consists of a mixture of water and methanol or acetonitrile, and eventually a buffer, the formation of protonated water clusters can be considered as a starting point in a series of even-electron ion-molecule reactions. The protonated water clusters transfer their proton to any species in the gas mixture with a higher proton affinity (Table 6.1). The mass spectrum of acetonitrile (MeCN)-water mixture shows protonated MeCN-water clusters, [(MeCN), (HjO) + H]", with /w-values of 1-3, and -values of 0-1. The addition of aimnonium acetate to MeCN-water results in the observation of mixed solvent clusters, e.g., [(MeCN), + and [(MeCN) , (HjO) +... 

In many cases, a protonated molecular ion (M - - H)+ is the only ion observed in a thermospray spectrum but if ammonium acetate buffer is used, depending upon the relative proton affinities of the species present, an ammonium adduct (M - - NH4)+ may be the predominant ion. In addition, clusters may be formed with components of the mobile phase. Although the thermospray ionization process involves less energy than conventional Cl, and very little intense fragmentation is usually observed, the presence of ions due to the elimination of small molecules, e.g. water, methanol and ketene, is not unknown. These latter ions are usually of relatively low intensity when compared to the protonated or... 

Figure 4-16. Gas phase proton affinities (PA in kcal mol-1) of B clusters versus /n (B = piperidine, ammonia, methanol, and water) (from Knochenmuss and Leutwyler 1989). The threshold proton affinity corresponds to the energetic limit for which excited state proton transfer occurs for 1-naphthol in small clusters.

Figure 4-17. Relation between gas phase proton affinities (in kcal mol-1) of base molecules B or B clusters and spectral shifts of the S3 <- S0 state of phenol(B ) and naphthol(B ) (Cheshnovsky and Leutwyler 1988) clusters (in cm-1). (A) water ( + ) (NH3) (x) monoethylamine (O) diethylamine ( ) trimethylamine.

For pro tic solvents with larger dielectric constants and stronger basicity, the La and 1Lb states are inverted and relaxation from xLb to xLa takes places but there is no proton transfer to the solvent. The fluorescence is then due to the 1LB state with a small Stokes shift. The intermediate sized water clusters (n = 10-20) belong in this category. The clusters with methanol for any size n < 10 (due to a weak basicity or a small dielectric constant) follow this mechanism. From the evaluated proton affinities (see Figure 4-16), it can be seen that for n k 10 molecules of methanol (PA 243 kcal mol-1 which corresponds to the limit for proton transfer evidence in 1-naphthol complexes with piperidine or ammonia), a proton transfer should be observed. The absence of such a transfer can be related to a cluster structure effect. 

From these data it can be pointed out that for a given size of the clusters the proton affinity of water is smaller than for the other solvents consequently, for fluorobenzene/methanol or para-difluorobenzene/water systems, a proton affinity of 205/215 kcal mol-1 seems to be the limit of the reaction process (it is reached for two molecules of methanol and three molecules of water). 

The observed positive ions are protonated clusters containing water and high proton affinity species such as acetonitrile in the lower stratosphere (26) or ammonia in the lower troposphere (20). Other high proton affinity species such as pyridine and picolines may enter into the positive ion chemistry of the lower troposphere (27,28). Further discussion of these studies and the experimental techniques can be found elsewhere (28,29). 

Stage two involves reactions of major trace gases, mostly H2O leading to (H20) cluster ions. It is essentially the large proton affinity of the water molecule and the strong bonding of IbO-molecules to the hydro-... 

Electronic Properties Effects of the Surrounding. The proton affinity of the zinc-bound water molecule is a key property for the enzymatic mechanism. The acidity of the zinc bound water is the result of a subtle fine tuning via hydrogen-bonded networks and electrostatic environment effects. This quantity can thus serve as a sensitive indicator of differences in the electronic structure that will have a critical influence on the enzymatic reaction. As a first attempt to quantify the effect of the electrostatic environment and the varying size of the cluster model we have therefore calculated the proton affinities for the different models. 

The chemistry of positive ions in the middle atmosphere is relatively well understood. Ions such as O.J and NO+, which dominate in the lower thermosphere, are rapidly lost below 80 km in a set of clustering reactions ending with stable proton hydrates of the type H+(H20)n. The hydration order n depends on temperature and water vapor concentration. In the stratosphere, water ligands are partly or totally replaced by other molecules such as methyl cyanide (CH3CN), whose proton affinity is larger than that of water molecules. 

Use of NH4 as the primary reactant ion can also aid compound identification. A hollow cathode ion source operating with H2O vapor produces nearly exclusively H3O+ and some H30+-water cluster ions. Similarly, when NH3 is used, only NH4 ions emerge from the source and can thus be used as primary reactant ions. While H30 ions perform proton transfer to all VOCs having a proton affinity (PA) higher than 165.2 kcalmoU NH4 only performs proton transfer to compounds with proton affinities in excess of 204.1 kcalmoU (Table 1). When air to be analyzed contains traces of two compounds with the... 

Polar samples of lower proton affinity may react by displacing a water molecule from a cluster, for example ... 

At ion source pressures on the order of 0.5 Torr and ion source temperatures of approximately 373 K, the rate constants for electron attachment and proton abstraction suggest that there are an adequate number of collisions in the ion source to permit equilibria to be sufficiently established. This is a prerequisite in order to assume a Boltzmann distribution of internal energies of the anions (or cations). Thus thermochemical data, such as electron affinities and the proton affinities of anions, can be used to calculate the energetics of these reactions (75). The cluster adduct anion [M- -C]" (Reactions 7.39-7.41) have third-order rate constants. Where comparisons can be made, the magnitude of positive- and negative-mode third-order rate constants are similar (69,103). The clustering reactions are important in NlCl spectra for polar compounds in the presence of polar molecules such as water and alcohol. 

---