Clusters proton transfer reactions

In rhodium clusters, e.g., [Rh13(CO)14H5. ]" (n = 2 and 3), only one of the H atoms is encapsulated and 3H nmr spectra show that this atom interacts with all 13 Rh atoms (103Rh, spin V2) and is thus migrating within the cluster. Proton transfer reactions of interstitial hydrides have been studied72 and more complex hetero-nuclear species73 can be made by reactions such as... 

Because at equilibrium virtually all the HCl molecules have donated their protons to water, HCl is classified as a strong acid. The proton transfer reaction essentially goes to completion. The H30+ ion is called the hydronium ion. It is strongly hydrated in solution, and there is some evidence that a better representation of the species is H904+ (or even larger clusters of water molecules attached to a proton). A hydrogen ion in water is sometimes represented as H + (aq), but we must remember that H+ does not exist by itself in water and that H CC is a better representation. 

The origin of the sequence corresponding to protonated methanol peaks is a rapid intracluster proton transfer reaction following ionization of the neutral clusters. This reaction has a well-known bimolecular counterpart that proceeds at near collision rate 104... 

At the beginning of this decade, Zewail and coworkers reported a fundamental work of solvation effect on a proton transfer reaction [195]. a-naphthol and n-ammonia molecules were studied in real-time for the reaction dynamics on the number of solvent molecules involved in the proton transfer reaction from alcohol towards the ammonia base. Nanosecond dynamics was observed for n=l and 2, while no evidence for proton transfer was found. For n=3 and 4, proton transfer reaction was measured at pisosecond time scale. The nanosecond dynamics appears to be related to the global cluster behavior. The idea of a critical solvation number required to onset proton transfer... 

Since the energy to ionize PhO B is much lower than that necessary to ionize PhOH- B , these different behaviors have been interpreted as being due to the proton transfer reaction at the excited state leading to drastic changes in the ionization energies of the different clusters. The same kind of experiments have been reported for 1-naphthol, leading to the same conclusion (Kim et al. 1991b). 

The proton transfer reaction has been also studied in rare gas matrices, showing some discrepancies with the gas phase (Brucker and Kelley 1989a,b Crepin and Tramer 1991) in 1- and 2-naphthol with ammonia, the AH B — A -HB + transition takes place for AH (NH3) /Ar matrix-embedded clusters with n > 3 (Brucker and Kelley 1989a,b). 

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). 

In the mass spectrometric investigations of most hydrogen-bonded clusters, the protonated clusters form the dominant cluster ions as a result of a rapid proton transfer reaction (Mark and Castleman 1985). Castleman and coworkers have extensively investigated the neat expansions of methanol (Morgan and Castleman 1987, 1989 Morgan et al. 1989 Zhang et al. 1991) and observed the protonated cluster ions to form the most intense cluster series. In the case of methanol, the exoergic proton transfer reaction may be written as ... 

Nishi and coworkers (Shinohara et al. 1985, 1986) have reported the production of unprotonated water and ammonia cluster ions, as well as Ar X+ (X = H20 and NH3) cluster ions when argon/molecule mixed expansions were ionized using UV lines from an argon lamp. The quenching of the proton transfer reaction was attributed to the cooling of the heterocluster ions via... 

Penning ionization is a dominant reaction when nitrogen or neon is used in the DART source. Nitrogen or neon ions are effectively removed by electrostatic lenses and are not observed in the background mass spectrum. When helium is used, the dominant positive-ion formation mechanism involves the formation of ionized water clusters followed by proton transfer reactions. Negative-ion formation occurs by production of electrons by Penning ionization or by surface Penning ionization ... 

The following gas-phase reactions of anions have been studied and will be briefly reviewed in the next sections proton transfer reactions, nucleophilic displacement reactions at both aliphatic and aromatic carbon centers, elimination reactions, electron transfer (ET) reactions, reactions with carbon-carbon double bonds and carbonyl functions, and association or complex (cluster)-forming reactions of various types. 

Recent gas-phase studies of proton-transfer reactions with stepwise solvation of the reactants i.e. incremental addition of solvent molecules to form supermolecular clusters) have demonstrated that the acid/base behaviour of isolated solvent molecules can be dramatically different from their performance as bulk liquids. Water, the classical amphiprotic solvent, shall serve as an example. 

Equation 4 is not a simple proton-transfer reaction but rather involves displacement of an H2O molecule contained in an (HiOln cluster ion by a molecule C. Usually, C must have a substantially larger proton affinity than H2O (170 kcal mole ) in order to make Eq. 4 exothermic [13, 14]. This is true because Il20-molecules mostly bond more strongly to HaO than to H C. 

Electrons in the iron-sulfur clusters of NADH-Q oxidoreduetase are shuttled to coenzyme Q. The flow of two electrons from NADH to coenzyme Q through NADH-Q oxidoreduetase leads to the pumping offour hydrogen ions out of the matrix of the mitochondrion. The details of this process remain the subject of active investigation. However, the coupled electron- proton transfer reactions of Q are crucial. NADH binds to a site on the vertical arm and transfers its electrons to FMN. These electrons flow within the vertical unit to three 4Fe-4S centers and then to a bound Q. The reduction of Q to... 

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