Reactions of Organic Ions at Ambient Surfaces in a Solvent-Free Environment
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Solvent-free ion/surface chemistry is studied at atmospheric pressure, specifically pyrylium cations, are reacted at ambient surfaces with organic amines to generate pyridinium ions. The dry reagent ions were generated by electrospraying a solution of the organic salt and passing the resulting electrosprayed droplets pneumatically through a heated metal drying tube. The dry ions were then passed through an electric field in air to separate the cations from anions and direct the cations onto a gold substrate coated with an amine. This nontraditional way of manipulating polyatomic ions has provided new chemical insights, for example, the surface reaction involving dry isolated 2,4,6-triphenylpyrylium cations and condensed solid-phase ethanolamine was found to produce the expected N-substituted pyridinium product ion via a pseudobase intermediate in a regiospecific fashion. In solution however, ethanolamine was observed to react through its N-centered and O-centered nucleophilic groups to generate two isomeric products via 2H-pyran intermediates. The O-centered nucleophile reacted less rapidly to give the minor product. The surface reaction product was characterized in situ by surface enhanced Raman spectroscopy, and ex situ using mass spectrometry and H/D exchange, and found to be chemically the same as the major pyridinium solution-phase reaction product.
Key wordsSolvent-free reactions Atmospheric pressure mass spectrometry Ion/surface reactions Surface enhanced Raman spectroscopy Ion soft landing
Organic thin films represent increasingly important materials. For example, there is growing interest in the use of such materials in electronics [1, 2, 3], as well as in catalysis and other applications [4, 5]. Moreover, thin films sequester atmospheric trace gases leading to their removal from the gas phase [6, 7], and they may also act as reactive media or as a source of reagents [8, 9]. Studies in this area have included reactions of interest in atmospheric science such as endogenous atmospheric reactions of species like N2O5 at organic-coated particles , the reactive uptake of OH and O3 by organic aerosols and films [11, 12, 13] and the little understood oligomerization reactions that lead to atmospheric organic polymers . Here, we investigate the reactions of dry, isolated organic ions with reagents at surfaces under atmospheric pressure. We chose for examination a system in which the ambient ions (pyrylium ions) are readily formed and where they undergo a characteristic reaction with functionalized organics (amines). The general goal of the study is to gain insights into the differences between reactions of organic ions on organic-coated surfaces at atmospheric pressure and the corresponding reactions occurring under bulk solution-phase conditions. This was motivated by the fact that distinctive chemical reactions are often observed within an organic layer at an interface with an aqueous phase compared to the reactivity of the same compounds under bulk solution-phase conditions [15, 16].
Electrospray ionization (ESI)  offers a simple way to generate organic ions/aerosols in ambient air and subsequently study their reactivity under vacuum using mass spectrometry (MS) [18, 19, 20] These vacuum-based experiments can employ either ion/molecule  or ion/surface reactions [22, 23]. The instrumentation used in the ion/surface reactions is similar to that used in the ion soft landing technique [24, 25, 26] except that the mass-selected polyatomic ions are given enough energy to react selectively with particular functional groups present at the surface. Studies on reactions of organic ions at atmospheric pressure have also been reported in which MS was used only to characterize reaction products under vacuum. The present study was prompted by previous studies on the interactions of ESI-derived ions with surfaces, which result in dissociation [27, 28], deposition , or reactions at the surfaces [29, 30, 31] in the open laboratory environment. In this work, we focus on using dry ions as reagents for organic reactions in a solvent-free environment. Traditionally, the study of solvent-free reactions has been motivated both by an interest in solvent effects and by considerations of waste reduction, viz. green chemistry, and it has been conducted using microwave [32, 33], mechanical ball-milling , or simply by grinding the reactants in a mortar . The current experiment allows surfaces to be modified/patterned with different organic functional groups with control of reaction time and ion spot size at atmospheric pressure. Specifically, pyrylium to pyridinium cation conversion was achieved at an ambient surface in a solvent-free environment through reaction with ethanolamine. Whereas these reaction conditions yielded a regiospecific route to the expected pyridinium product, unwanted reaction products were observed for the corresponding bulk solution-phase reaction. Possible reaction mechanisms operating under the two different reaction conditions (solvent-free vs. bulk solution-phase) and leading to the observed effects are discussed in detail.
3 Results and Discussion
The SERS results indicate that the surface reaction conditions yielded the expected reaction product (pyridinium cation), and that unreacted pyrylium cation is not detectable at the point where the SERS spectrum was recorded. The data also indicate excess ethanolamine in the solution-phase spectrum as shown by the presence of the peak at 1245 cm–1. Unfortunately, information regarding the reaction mechanism could not be inferred from the SERS data. Mass spectrometry (MS) was therefore employed to understand how the unique reaction conditions present at the surface could yield the same product as the corresponding solution-phase reaction. It was expected that by using MS the presence of low abundance isomers and reaction intermediates might be detected in addition to the main reaction products. The cation landing protocol was the same as described previously . After 1 h of cation landing at atmospheric pressure, the surface reaction products were washed and analyzed under vacuum using a linear ion trap instrument with tandem MS capabilities (MSn).
Owing to differences in reaction conditions and mechanisms, tandem MS was conducted on the reactions products (m/z 352) in order to investigate any structural differences that might exist between the surface-generated pyridinium cation and that formed in the bulk solution-phase reaction. As has already been explained, the solution-phase reaction product (m/z 352) fragments upon collision-induced dissociation (CID) to give ions at m/z 308 (via the loss of ethylene oxide) and 274 (via the loss of benzene) whereas the surface product dissociates only to give the m/z 308 ion. This striking difference strongly suggests that the precursor ions are structurally different. This is in turn rationalized by the fact that the solution-phase reaction intermediate at m/z 370 is comprised of two isomers (3 and 3′). The hydroxylamine base can act as either a N-centered or an O-centered nucleophile and it does this competitively, the O-centered reaction intermediate competing less effectively than the N-centered nucleophile. The isomeric product (6) resulting from dehydration of the O-centered reaction intermediate (3′) cannot lose ethylene oxide when subjected to CID. The gas-phase ion at m/z 352 formed upon CID of m/z 370 ion fragments predominantly to give ion at m/z 274 through the loss of a benzene molecule when collisionally activated in an MS3 experiments (Figure 4b). This gives definitive confirmation that the dehydrated product from 3′ is responsible for the fragment ion at m/z 274 observed in the MS2 product ion spectrum at m/z 352 for the solution-phase reaction. The tandem mass spectrometry (insert, Figure 2a) of the product ion (m/z 352) formed during the surface reaction, clearly suggests that only one major isomer of the solution-phase products is generated. It is assumed that only Compound 6 will give rise to the m/z 274 ion and so the absence of this fragment ion indicates the absence of Compound 6 during the surface reaction.
The presence of mixed structural isomers in the solution-phase reaction was further investigated by reaction of the pyrylium cation with n-butylamine, which contains only the N-centered nucleophile. The results of this solution-phase reaction were also compared with those for reaction at surface. Again, the 2H-pyran (m/z 382) reaction intermediate was observed only for the solution-phase reaction and the pseudobase intermediate (m/z 349) for the surface reaction (Figure S5, Supporting Information). MS/MS spectra of the pyridinium product ions at m/z 364 (both at surface and in solution) were identical, giving a major fragment at m/z 308 via 1-butene neutral loss. These findings suggest that no mixed isomers are formed in solution for n-butylamine, and support the interpretation that the presence of an O-centered nucleophile in ethanolamine caused the different fragmentation patterns observed for the solution-phase product at m/z 352. The pyrylium reaction with n-butylamine at the surface, however, yielded a second (minor) product at m/z 365 (Figure S6, Supporting Information). This is believed to have formed via ammonia loss (instead of water after pseudobase and amine reaction) leading to a transmethylation reaction. Such transmethylation reactions appear to be prominent at the surface for long chain amines (≥C4) since similar effects have been observed for D-lysine reaction with pyrylium under the same experimental conditions .
The involvement of pseudobase intermediates in pyrylium reactions with amines is not uncommon, and has been reported for bulk phase reactions of 2,4,6-triphenylpyrylium with lysine . In some cases of pyrylium reaction with amines, the type of intermediate involved (either pseudobase or 2H-pyran) is dependent on whether or not excess amine is used . In the present study, mass spectra recorded after directing solvated rather than dry pyrylium cations onto condensed phase ethanolamine showed that both the pseudobase and the 2H-pyran intermediates were formed at the surface (Figure S7, Supporting Information). (This condition was achieved by not supplying heat to the drying tube, and so positively charged droplets instead of dry ions were landed.) This observation rules out the possibility that a pseudobase intermediate might be formed on the surface because of the lack of amine. It also indicates that the 2H-pyran intermediate might be stable in the presence of solvent molecules, and hence its absence from the solvent-free surface reaction conditions.
H/D exchange experiments were also conducted to investigate the structural differences suggested by the MS data. To do this, d4-methanol/water (5:1, vol/vol) was used to rinse the ion spot from the surface. This same solution was also used as a solvent for the bulk solution-phase reaction. Up to three protons were exchanged in the both surface and solution-phase products (Figure S8, Supporting Information) owing to H/D exchange in the course of isomerization of the reaction intermediates present in each case of the reaction conditions. The results also indicate that the solution-phase reaction product underwent more H/D exchange compared with the product formed on the surface.
We have presented a methodology by which solvent-free ion chemistry and reaction mechanisms can be studied at atmospheric pressure. Insights into the mechanism of the reaction involving 2,4,6-triphenylpyrylium cations and ethanolamine have been acquired. The studies show that the atmospheric pressure surface reaction produced just one of the two isomers of the bulk phase reaction products. This is supported by the following experimental findings: (1) SERS results indicate that surface product occurred during the surface reaction not during washing prior to MS analysis, (2) the MS fragmentation pattern of the solvent-free product was found to be different from that of the solution-phase reaction product, and (3) H/D exchange results provide supporting evidence for the reaction intermediates deduced from the MS data, showing the intermediates can exchange up to three hydrogen atoms; the same results also indicate that the solution-phase reaction product readily undergoes H/D exchange compared with the product formed at the surface. The differences found in the MS and H/D exchange experiments may be accounted for by the presence of an additional minor isomer found in the solution-phase reaction. An explanation as to why the solution-phase reaction produces two distinct products has also been given: two isomers of the solution-phase reaction intermediate are proposed: N-centered and an O-centered ethanolamine-substituted 2H-pyran intermediates. The surface reaction on the other hand is proposed to proceed through the unreactive pseudobase intermediate, which presumably reacts exclusively via the more reactive N-centered nucleophile of ethanolamine. Analogously to the increase in gas-phase ion/molecule reaction rates observed upon solvent removal, here a similar increase in reaction rates can be expected under the dry surface reaction conditions although rate enhancement is not the subject of this work.
The study adds support to the observation that chemical reactions within organic layers and at interfaces between organic films and an aqueous phase or the atmosphere often exhibit markedly different reactivity to that shown by the same compounds in bulk solution phase. Unexpectedly, gas-phase 2,4,6-triphenylpyrylium ions directed onto a dried film of ethanolamine yielded distinctive and regiospecific route to 2,4,6-triphenylpyridium cation upon reaction with ethanolamine. On the other hand, some unwanted reaction products have been found to be associated with the solution-phase reaction. The procedure reported here thus allows an effective way to create and study the reactivity of organic ions at thin films and illustrates how this non-traditional way of processing materials can also provide new information on novel chemistry. Specifically, surface modification using the ambient soft landing is particularly interesting because it allows ion chemistry to be investigated at atmospheric pressure; it also provides a means to purify ions, direct them electrically, as well as to focus and deposit the ions onto a surface at a specified location. Because the ions are processed outside the mass spectrometer at atmospheric pressure, the analysis of the treated surface using surface techniques such as Raman spectroscopy is straightforward and requires no additional instrumental modifications, in much the same way as when fluorescence and atomic force microscopy are employed.
The authors acknowledge funding for this work by the National Science Foundation (CHE NSF 0848650) and U.S. Department of Energy grant DE-FG02-06ER15807.
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