Solvent clusters, protonated

In the latter case the dopant is ionised, interacts with the solvent and, subsequently, solvent clusters interact with the analyte. Molecular and protonated molecular ions are observed, indicating that ionisation can occur via proton (toluene) and electron transfer (acetone). 

Picosecond time regime kinetic studies of proton transfer are coming into vogue (28, 29, 30), particularly for intramolecular processes that can be very fast. Bound to play an increasingly important role in the elucidation of proton transfers are the gas phase ion-solvent cluster techniques that reveal dramatically the role played by solvent molecules in these reactions (M, 32). 

When APCI in used in combination with the normal phase LC, the nitrogen molecular ion will enter into a charge-transfer reaction with the organic solvent. Ion-molecule reactions lead to protonated solvent clusters that will react by proton transfer with the analyte molecules, forming [M + H]+ ions. 

In this chapter, we give a brief overview of several novel features of excited-state proton transfer in chromophore-solvent clusters which have been revealed by the interplay of computational chemistry and spectroscopy in supersonic jets. In the future, concerted efforts of theory and spectroscopy will be necessary to investigate the evolution of these phenomena with increasing cluster size towards liquid-phase photochemistry. 

The interpretation of the experimental data for the kinetics of photoacid-solvent clusters is complicated by the substantial fragmentation of the clusters after the excited-state reaction. The heat of reaction is often sufficient to allow the evaporation of one or several solvent molecules [14,16]. This difficulty does not arise when the H atom transfer or proton transfer occurs intramolecularly along a solvent wire attached to a bifunctional chromophore. 

We have discussed recent computational and spectroscopic results on the photoinduced hydrogen transfer and proton transfer chemistry in hydrogen-bonded chromophore-solvent clusters. The interplay of electronic spectroscopy of size-selected clusters and computational studies has led to a remarkably detailed and complete mechanistic picture... 

The 17r<7 states also dominate the photoinduced processes in hydrogen-bonded chromophore-solvent clusters. The photoinduced hydrogen transfer reaction is experimentally and computationally well documented in clusters of phenol and indole with ammonia [14,16,32], There is no clear evidence for the existence of an excited-state proton transfer process in these systems [14], The same conclusion applies to bi functional chromophores solvated in finite clusters, such as 7HQ-ammonia and 7HQ-water clusters [15]. In future work, the photochemistry of larger and biologically relevant chromophores (such as tyrosine, tryptophan, or the DNA bases) should be investigated in a finite solvent environment. 

From these experiments, it is shown that proton transfer is not only dependent on the protop affinity of the solvent or solvent cluster but also on the possibility of interaction of the solvent molecules with the nascent naphtholate anion. 

A mechanism is proposed that goes through the same proton transfer from the NH3 in the a complex toward the solvent cluster, and a capture of the halogen by the protonated cluster. For example, in the case of fluorine, the C-F bond cannot break alone, but the... 

In the second mechanism, the electron transfer from the nucleophile cluster into the aromatic ring should be facilitated by the decrease of the ionization potential (IP) of the solvent clusters as n increases. This mechanism is convincing for the ammonia or methanol clusters which show relatively low IPs when cluster size is increasing however, for water clusters, the IPs of n > 3 clusters are not known. The IPs of water and its dimer are 12.6 and 11.2 eV, respectively (Ng et al. 1977). However, these IPs are certainly higher than the one of PDFB (9.2 eV), which is not in favor of a sequential electron transfer followed by a proton transfer mechanism. This mechanism is more likely possible if one assumes, in agreement with Brutschy and coworkers, that the barrier to the reaction is lowered by a concerted electron transfer/proton transfer mechanism (Brutschy 1989, 1990 Brutschy et al. 1988, 1991, 1992, in press). 

After those first attempts to establish analytical applications of electrospray, it took more than ten years for the first bona fide electrospray mass spectrometer to emerge [14]. Yamashita and Fenn published the first electrospray MS experiment in a 1984 paper which was appropriately part of an issue of the Journal of Physical Chemistry dedicated to John Bennett Fenn [15]. They electrosprayed solvents into a bath gas to form a dispersion of ions that was expanded into vacuum in a small supersonic free jet. A portion of the jet was then passed through a skimmer into a vacuum chamber containing a quadrupole mass filter. With this setup, a variety of protonated solvent clusters as well as solvent-ion clusters (Na+, Li+) could be de-... 

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) +... 

Typical values of proton affinity for some reagent gases and mobile-phase constituents are given in Table 6.1. The proton-transfer reaction between the protonated solvent cluster SH, generated by the sequence of events indicated in Ch. 6.4.1, and the analyte molecule M is ... 

In sutmnaty, protonated solvent clusters are generated by ion-molecule reactions initiated by the corona discharge. These cluster ions act as reagent gas ions for the solvent-mediated APCl. The composition of the reagent gas is determined by the mobile-phase constituent with the highest proton affinity. 

In the above discussion, it was assumed that the PA of the solvent clusters of the type [(MeCN), (HjO) + are identical. This of comse is not really trae the various solvent cluster ions observed each have their own proton affinity, but in predicting analyte ionization in APCl-MS, these differences may be ignored. 

The critical size of the solvent cluster for detectable proton transfer is placed at n=4 as (a) Smaller species do not exhibit the broad emission of the a-Naphtolate anion, (b) The appearance of this species within the duration of the gas pulse coincides with the appearance of the a-Naphtolate emission. (c) This is the smallest species which does not exhibit a structured absorption. 

H and ENDOR spectroscopy was used to probe solvent accessibility of flie active site in mixed-valent MMOH [83-85]. There are a total of nine magnetically unique eoupled protons, and at least three of these are exchangeable in deuterated solvent. These protons exchange only slowly in H20 (over 15—25 hours), which is consistent with the hypothesis that the iron active site of MMOH is buried and not readily accessible to the solvent [84,85]. This is in contrast to the binuclear iron clusters in hemerythrin ( 8) or ribonucleotide reductase ( 3), where solvent has direct access to the active sites of these proteins [86]. 

One interesting consequence seems to be that only amines with a PA 900 kJ/mol should exhibit a sufficient PA to stabilize ion pairs with strong acids in contrast to the weaker oxygen acceptor systems (PA 750 kJ/mol). We mention that experimental studies of proton transfer reactions in neutral gas phase clusters (reaction of an acid with solvent clusters, (HOR) and (NHR2>j n l,2...) give an interesting... 

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