Ammonia clusters

Through the use of pump-probe techniques pioneered by Zewail and coworkers,62 it is becoming possible to identify the detailed mechanisms of reactions at the molecular level and follow the actual course of a reaction. The study of ammonia clusters has provided an example of what can be accomplished using these techniques. 

The ionization of ammonia clusters (i.e. multiphoton ionization,33,35,43,70,71 single photon ionization,72-74 electron impact ionization,75 etc.) mainly leads to formation of protonated clusters. For some years there has been a debate about the mechanism of formation of protonated clusters under resonance-enhanced multiphoton ionization conditions, especially regarding the possible alternative sequences of absorption, dissociation, and ionization. Two alternative mechanisms63,64,76,77 have been proposed absorption-ionization-dissociation (AID) and absorption-dissociation-ionization (ADI) mechanisms see Figure 5. 

Figure 5. Possible reaction mechanisms leading to the formation of protonated ammonia clusters from neutral clusters. Taken with permission from ref. 65.

The possible profiles of intensity versus the delay between the pump and probe photons for the several potential processes operative in the mechanism of ionization are shown in Figures 6a, b, and c. Several different laser schemes have been employed to investigate the reaction dynamics pertaining to the formation of protonated ammonia clusters through the A and C states (the latter possibly including the B state as well). The ionization schemes employed in the present study are shown in Figure 7. 

Figure 7. Several of the ionization schemes employed in various pump-probe experiments of ammonia clusters the energy levels correspond to those of the ammonia monomer. The upper hatched region denotes the ionization limit. Taken with permission from ref. 68.

Figure 11. Average ion kinetic energies as a function of cluster size for ammonia clusters and deuterated ammonia clusters resulting from Coulomb explosion of clusters. The average kinetic energies are obtained from the splittings in the cluster ion time-of-flight peaks see Figure 10. Taken with permission from ref. 90.

An example of the usefulness of the reflectron technique discussed earlier in this chapter is evident for the case of ammonia clusters in Figures 3b and c. Upon ionization, ammonia undergoes an internal ion-molecule reaction leading to protonated cluster ions, and concomitant evaporative unimolecular dissociation. This can be viewed in the context of equations 7-9 and the following ... 

The observed H+(NH3)n and H (NH3)n(PA) clusters are thought to be formed in a two-step reaction sequence taking place after ionization of the PA(NH3) cluster. The first step is a charge transfer (CT) reaction between the resonantly ionized PA+ and the NH3 molecules in the cluster. The second step is an intracluster ion-molecule reaction (ICIMR) of the charged ammonia cluster leading to the formation of an (n - 1) protonated cluster ion this has been previously established for NH3 clusters33 and is sufficiently exothermic for fragmentation of the cluster. 

Elucidating the origin of magic numbers has been a problem of long-standing interest, made accessible through the use of the laser-based reflectron TOF technique and evaporative ensemble theory. Three test cases are considered, first protonated ammonia clusters where (NH3)4 NHj has been found to be especially prominent, and then two other cases are considered, one involving water cluster ions and another rare gas clusters. 

Ammonia also forms clusters in the gas phase and the reactions of ammonia clusters with bare metal ions have been studied (61). The ammonia clusters probed by electron impact as [(NH3) H]+ showed a monotonic decrease in intensity with increasing value of n, but the metal complex ions [M(NH3) ]+ showed intensity gaps. Thus for most of the metals the [M(NH3)2]+ ion was much more intense than the [M(NH3) ]+ ions, where n 2, and so the coordination number 2 was reported to be the favored coordination number in the first coordination sphere. The favored ions M(NH3)m]+ were n = 2 for Cr+, Mn+, Fe+, Co+, Ni+, and Cu+, and n = 4 for V+. The non-transition metal Mg+ and Al+ had the favored coordination number of 3. 

In contrast to indole-ammonia clusters, for which the different steps of the photoinduced H-transfer reaction have been analyzed in detail, we have found no hints for such a reaction in indole(H20) clusters. Probably, like for phenol(H20) complexes the endoenergetic character of the reaction H+H2O—>H30 is responsible for the missing H-transfer process in the indole(H20) clusters. Ab initio calculations of the indole-water potential energy surfaces are under way now, to elucidate this process in the heterocluster and to understand the difference with respect to the indole-ammonia complex. 

This article will deal more with naphthol-ammonia clusters but the case of phenol-ammonia can be quickly summarized here. The dynamical process observed in the excited state of phenol has long been attributed to an excited state proton transfer [1-7] ... 

In this section, we discuss the photoinduced hydrogen transfer from phenol to water and ammonia in phenol-water and phenol-ammonia clusters, respectively, as a representative model of excited-state chromophore-to-solvent hydrogen transfer reactions. 

Figure 3.33 PE profiles of the electronic ground state (circles), the lowest 1 tttt state (squares) and the lowest 17rcr state (triangles) of (a) the phenol-water cluster and (b) the phenol-ammonia cluster as a function of the hydrogen transfer coordinate, calculated with the CASPT2 method [32].

O. David, C. Dedonder-Lardeux and C. Jouvet, Is there an excited state proton transfer in phenol (or l-naphthol)/ammonia clusters , hit. Rev. Phys. Chem., 21 (2002) 499-523. 

W. Siebrand, M. Z. Zgierski, J. K. Smedarchina, M. Vener and J. Kaneti, The structure of phenol-ammonia clusters before and after proton transfer. A theoretical investigation, Chem. Phys. Lett., 266 (1997) 47-52. 

S. Ishiuchi, M. Saeki, M. Sakai and M. Fujii, Infrared dip spectra of photochemical reaction products in a phenol/ammonia cluster examination of intracluster hydrogen transfer, Chem. Phys. Lett., 322 (2000) 27-32. 

Very different proton transfer reaction efficiencies from these two potential wells have been measured, and only the clusters in which the internal energy is sufficient to cross over the barrier will lead to the reaction. The empirical potential surface and the potential barrier between the two wells have been estimated (Steadman and Syage 1991). For ammonia clusters, upper limits for the barriers were estimated to be about 1.5 eV (n = 1) to 1.0 eV (n = 4). 

Fluorobenzene with ammonia leads only to the formation of aniline+. However, the TOF mass spectra also exhibit signals due to protonated and unprotonated ammonia clusters which must be produced by dissociative electron transfer (dET). In this case, direct evidence for a 1-2 precursor for the aniline+ product with three competing channels is provided ... 

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