Lithium formate ion clusters formation during electrospray ionization: Evidence of magic number clusters by mass spectrometry and ab initio calculations Anil Shukla and Bogdan Bogdanov Citation: The Journal of Chemical Physics 142, 064304 (2015); doi: 10.1063/1.4907366 View online: http://dx.doi.org/10.1063/1.4907366 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/142/6?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Ab initio calculations for the photoelectron spectra of vanadium clusters J. Chem. Phys. 121, 5893 (2004); 10.1063/1.1785142 Confirmation of the “long-lived” tetra-nitrogen ( N 4 ) molecule using neutralization-reionization mass spectrometry and ab initio calculations J. Chem. Phys. 120, 10561 (2004); 10.1063/1.1705571 Structure and vibrations of dihydroxybenzene cations and ionization potentials of dihydroxybenzenes studied by mass analyzed threshold ionization and infrared photoinduced Rydberg ionization spectroscopy as well as ab initio theory J. Chem. Phys. 111, 7966 (1999); 10.1063/1.480166 The formation of ArCO + ions by dissociative ionization of argon/carbonmonoxide clusters J. Chem. Phys. 107, 6667 (1997); 10.1063/1.474909 An ab initio MO study on structures and energetics of C 3 H − , C 3 H , and C 3 H + J. Chem. Phys. 106, 4536 (1997); 10.1063/1.473985

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THE JOURNAL OF CHEMICAL PHYSICS 142, 064304 (2015)

Lithium formate ion clusters formation during electrospray ionization: Evidence of magic number clusters by mass spectrometry and ab initio calculations Anil Shukla1,a) and Bogdan Bogdanov2 1

Biological Sciences Division, Fundamental and Computational Sciences Directorate, Pacific Northwest National Laboratory, Richland, Washington 99354, USA 2 Perkin Elmer, San Jose Technology Center, San Jose, California 95134, USA

(Received 21 October 2014; accepted 20 January 2015; published online 10 February 2015) Small cationic and anionic clusters of lithium formate were generated by electrospray ionization and their fragmentations were studied by tandem mass spectrometry (collision-induced dissociation with N2). Singly as well as multiply charged clusters were formed in both positive and negative ion modes with the general formulae, (HCOOLi)nLi+, (HCOOLi)nLimm+, (HCOOLi)nHCOO−, and (HCOOLi)n(HCOO)mm−. Several magic number cluster (MNC) ions were observed in both the positive and negative ion modes although more predominant in the positive ion mode with (HCOOLi)3Li+ being the most abundant and stable cluster ion. Fragmentations of singly charged positive clusters proceed first by the loss of a dimer unit ((HCOOLi)2) followed by the loss of monomer units (HCOOLi) although the former remains the dominant dissociation process. In the case of positive cluster ions, all fragmentations lead to the magic cluster (HCOOLi)3Li+ as the most abundant fragment ion at higher collision energies which then fragments further to dimer and monomer ions at lower abundances. In the negative ion mode, however, singly charged clusters dissociated via sequential loss of monomer units. Multiply charged clusters in both positive and negative ion modes dissociated mainly via Coulomb repulsion. Quantum chemical calculations performed for smaller cluster ions showed that the trimer ion has a closed ring structure similar to the phenalenylium structure with three closed rings connected to the central lithium ion. Further additions of monomer units result in similar symmetric structures for hexamer and nonamer cluster ions. Thermochemical calculations show that trimer cluster ion is relatively more stable than neighboring cluster ions, supporting the experimental observation of a magic number cluster with enhanced stability. C 2015 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4907366]

I. INTRODUCTION

Electrospray ionization (ESI) provides an easy and effective method to ionize large and labile molecules that are otherwise difficult to ionize.1–3 The ionization process requires that ions are present in the solution from which they are transferred into the gas phase by the electrospray process. ESI generates singly and/or multiply charged ions depending upon the number of possible charge sites available within the analyte molecule. The basic operation principles and associated mechanisms for the formation of ions have been discussed in detail in a number of publications even though the exact mechanism is still a matter of discussion.4–9 An interesting aspect of ESI is the formation of cluster ions along with the singly and multiply charged ions. Several publications10–23 have appeared in the last two decades that have demonstrated ESI’s capability to produce cluster ions from a variety of organic, inorganic, and biomolecules and their combinations. Thus, ESI affords an easy way to generate cluster ions from a variety of molecules compared to other a)Author to whom correspondence should be addressed. Electronic mail:

[email protected] 0021-9606/2015/142(6)/064304/13/$30.00

techniques to produce clusters, viz., supersonic expansion,24,25 ion-molecule reactions,26,27 secondary ion mass spectrometry,28 laser photo-ionization,29,30 and laser ablation,31,32 all of which require more complex instrumentation. The exact mechanism for cluster ion formation during ESI is not fully understood, just as the mechanisms for the ion formation during the ESI process are still a matter of discussion. The charge residue model33 in which ions are formed by extensive solvent evaporation and Coulomb explosions due to the Rayleigh instability from the solvated ions until only one solute molecule remains with the charge, and the ion evaporation model34 in which solvent evaporation and Rayleigh instability play similar roles until the field on the droplet surface dominates resulting in the desorption of the ion, have been proposed. The fact that very similar cluster ion spectra have been observed from ESI for a variety of substances compared to other methods of cluster formation, one might suggest that the clustering process in ESI may be an additional step similar to supersonic expansion as solvated ions are transferred into the high vacuum of a mass spectrometer from ambient pressure conditions. Meng and Fenn35 suggested that these ions are formed by the evaporation of solvent molecules from the droplets ensuing from electrospray, resulting in

142, 064304-1

© 2015 AIP Publishing LLC

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clustering of the solute via super-saturation. However, a recent study of cluster ions from alkali halides by Wang and Cole7 suggested that the charge residue model better explained the formation of larger cluster ions. If a cluster ion was generated by ion evaporation, it probably should have existed in the droplet before ionization. However, a recent NMR study of a serine solution showed that the serine octamer did not exist in solution.36 Under such circumstances, cluster ions may be formed in the final steps of the desolvation process due to the strong interactions between the monomer units and because the concentration of monomers was sufficiently high.37 The mechanism(s) aside, ESI provides an easy source to generate a variety of cluster ions that show very similar behavior like the cluster ions formed by other techniques mentioned above. For example, similar magic number clusters (MNCs) have been observed for water cluster ions from ESI15 as from supersonic expansion38,39 and secondary ion mass spectrometry studies.40 MNCs corresponding to the serine octamer41 and alkali halide clusters (13-mer)42 demonstrate the versatility of ESI for the study of cluster ions in parallel to its applications in the analysis of biological molecules. In this paper, we expand on the capability of the electrospray ionization process to produce large singly and multiply charged cluster ions of lithium formate in both the negative and positive ion modes. Lithium formate clusters have not been studied by mass spectrometry (MS) before. However, Hao et al.43 have studied sodium formate and acetate clusters that did not show the formation of MNCs. In addition, tandem MS (MS/MS) studies were not performed to explore the sizedependent fragmentation behavior that can be correlated to possible structures of these cluster ions. A recent experimental and theoretical study by Vekey and coworkers16 on small sodium formate cluster ions demonstrated the existence of certain unstable cluster sizes that was in contradiction to the MNCs observed for alkali halides as well as many other cluster ions. They observed that the tetramer ion had a lower relative abundance in the normal mass spectrum than both the trimer and pentamer cluster ions and that fragmentation of higher cluster ions did not result in the formation of the tetramer ion. For example, the pentamer ion fragmented by losing a dimer (or two monomers) resulting into the formation of a trimer ion. In the present work on lithium formate clusters, we have observed a long series of lithium formate cluster ions that showed several MNCs in both the positive and negative ion modes. We have studied the fragmentation characteristics of several small and large singly and multiply charged cluster ions in both ion modes by various MS/MS techniques. The experimental observations of magic number cluster ions and the structures of such clusters are examined by theoretical calculations as well. II. EXPERIMENTAL SECTION

Cluster ions were produced by electrospraying a 1 mM solution of lithium formate monohydrate (Sigma-Aldrich, St. Louis, MO) in 50% water/50% methanol via a 150 µm OD and 20 µm ID chemically etched silica capillary emitter44 held at 2.2 kV and using a flow rate of 1 µL/min. The ESI tip was placed at a distance of ∼3 mm from a 360 µm ID heated

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capillary interface to an LTQ Velos Orbitrap mass spectrometer (Thermo Fisher Scientific, Bremen, Germany). The heated capillary interface was maintained at a temperature of 200 ◦C. Both positive and negative cluster ions were formed in the electrospray ionization process and were detected by switching the polarity of the high voltage applied to the ESI emitter and the ion optics voltages appropriate for transporting ions of the specific charge and their detection by both the linear ion trap and the Orbitrap. Cluster ion spectra were recorded by the Orbitrap at a mass resolution of 60,000 (at m/z 400) in both the positive and negative ion modes, covering the m/z range from 50-1,000 or 150-2,000 Thomson (Th), depending upon the m/z range needed for the experiment. Collision-induced dissociation (CID) of specific cluster ions was performed in the high energy collision dissociation (HCD) collision cell by colliding mass selected cluster ions with N2 gas and resulting fragment ions were mass analyzed by the Orbitrap (at a resolution of 7,500 or higher), with an isolation width of 2-6 m/z in the ion trap. The normalized collision energy (in % according to the Thermo nomenclature) was varied to obtain energy-dependent fragmentation of cluster ions and to obtain the most informative spectra of fragment ions. Fragment ion spectra were accumulated for 2–5 min and signal averaged for a better signal-to-noise-ratio (S/N), especially for the high mass multiply charged ions. A triple quadrupole mass spectrometer (Thermo, Bremen, Germany) was used to measure fragment ion intensities as a function of the ion collision energy (rather than the % normalized collision energy used for Orbitraps which is not as informative) to generate breakdown curves for a few selected primary ions and to obtain a better understanding of the threshold dissociation processes. We also performed electron transfer dissociation (ETD) of doubly, triply, and quadruply charged clusters in the positive ion mode to explore the fragmentation behavior of these ions via electron transfer by cation/anion interactions. The negatively charged m/z 202 ions from fluoranthene (FLA) were formed by electron impact/chemical ionization of FLA vapors in a nitrogen environment and transferred to the ion trap for cation/anion reaction with the mass selected cluster ions. The resulting fragment and charge reduced ions were analyzed by the Orbitrap under similar conditions as for the HCD MS/MS. The ion interaction time was varied to obtain the most informative secondary ion spectra. III. COMPUTATIONAL METHODS

All quantum chemical computations were performed using the Gaussian 03W and Gaussian 09 suites of programs.45,46 Structures were optimized using the B3LYP47,48 density functional theory (DFT) method. For the C atoms, the 6-31G(d) basis set49,50 was used, while for the H and O atoms, the 6-31++G(d,p) basis set51,52 was used. Finally, for the Li atoms, the 6-311G basis set53,54 was used. For the Li+(HCOOLi)1−4 and the HCOO−(HCOOLi)1−4 cluster ions, the normal mode frequencies were calculated at the same level of theory used for the structure optimization to confirm the minimum structure identity of the optimized ion structure. For Li+, HCOO−, HCOOLi, (LiHCOO) Li+, Li+(HCOOLi)2, and HCOO−(HCOOLi) high-accuracy calculations at the G3,55

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G3(B3),56 and G3(MP2)57 composite method levels of theory were calculated to compare the clustering thermochemistry with the results of the B3LYP level calculations.

IV. RESULTS AND DISCUSSIONS A. Positive ion clusters

Lithium has two naturally occurring isotopes, 6Li with a mass of 6.0151 Da at 7.59% relative abundance and 7Li with a mass of 7.0160 Da at 92.41% relative abundance. The isotope distributions of ions containing larger number of lithium atoms, therefore, become quite complex due to combination of two isotopes, and the distribution may not be showing the most abundant ion at the expected mass calculated from the simple formula of that cluster. Figure 1 shows high resolution mass spectrum of lithium formate cluster cations formed by ESI that corresponds to the general formula (HCOOLi)nLi+ with n = 1 to >25. Ions corresponding to non-lithiated or protonated clusters are not observed in the spectrum at any significant abundance. There are some additional ions in the spectrum that correspond to the addition of a water molecule to some of the smaller cluster ions but these are of very low abundances and will not be discussed any further. In addition to singly charged ions, doubly, triply, and quadruply charged lithiated cluster ions, corresponding to the general formula (HCOOLi)nLimm+ were also observed in the cluster ion spectra, but only at higher masses. The first appearance of these multiply charged cluster ions is observed at n = 13 for doubly charged (m = 2) ions, n = 32 for triply charged (m = 3) ions, and n = 65 for quadruply charged (m = 4) ions, corresponding to m/z values of 344.60, 561.16, and 851.23 Th, respectively. The occurrence of MNCs has been shown to be due to a more stable structure for the specific cluster and may also be associated with some specific change(s) in the cluster structure starting from that MNC. For example, a clathrate structure for

FIG. 1. High resolution mass spectrum of lithium formate cluster ions in the positive ion mode with mass, charge (z), and number of monomer units (n) of the most abundant isotope in the distribution given on top of the peak. All singly and multiply charged cluster ions are lithiated, corresponding to the general formula, (HCOOLi)nLimm+. Ions marked with more than one charge state are due to the overlap between the charge states at that m/z value. The expanded view in the inset shows the onset for the triply charged cluster ion. Onset for the quadruply charged ion is not shown here.

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(H2O)21H+ has been shown to be responsible for its stable structure,39 and C60 is another well known example of unusual stability.58 In the case of the (NaCl)13Na+ cluster ion, it was suggested that the increased stability was due to a hexagonal structure59 that correlates with the crystal structure of sodium chloride corresponding to a 3 × 3 × 3 lattice structure60 and this was further confirmed by ion mobility measurements on negative cluster ions.61 In addition, (NaCl)4Na+ has been suggested to be a cubic structure. This suggests that (HCOOLi)4Li+ also correlates to the crystal structure of lithium formate monohydrate which has four molecules per unit cell.62 These previous results also suggest a uniquely distinct structure for the trimer MNC ((HCOOLi)3Li+) ion observed in the present study and theoretical calculations presented later demonstrate a fused three-ring structure for this ion. B. Fragmentation of positively charged cluster ions

All singly charged cluster ions dissociated first via loss of two monomer units followed by the sequential loss of a lithium formate monomer unit as the collision energy was increased. Intensity distributions of the fragment ions showed a pattern of alternating high and low intensity of the fragment ions, with the distribution shifting to lower cluster sizes with an increase in the collision energy, as expected. Fragmentation at higher collision energies tends to result in an increased intensity of the trimer ion (m/z 163), which further confirms that the m/z 163 ion is a highly stable cluster ion in this series. This cluster ion fragments further into smaller cluster ions, however, only at high collision energies (>50%). For all larger cluster cations, m/z 163 further dissociated into the m/z 111 and m/z 59 ions, but in lower abundances. Due to the lack of the instrument’s capability to detect below m/z 50, we could not explore the fragmentation of the lithiated monomer ion. Figure 2 shows this behavior for the fragmentation of the singly charged cluster ion at m/z 631 using HCD. There is a small contribution from the doubly charged 24-mer ion to the m/z 631 ion due to its matching mass and isotope distribution and fragmentation at very low energies showed a few singly charged ions at higher masses due to Coulomb fission process leading to two singly charged ions. However, their intensities are all well below 5% of the highest intensity fragment ions. Figure 3(a) shows a breakdown graph for the dissociation of the m/z 631 ions obtained using a triple quadrupole mass spectrometer which has the capability to detect ions down to m/z 10 and provides actual ion collision energy that is converted to center-of-mass collision energy by the relationship ECM = (Mneutral/(Mion + Mneutral) × Elab) for the breakdown graphs. A similar breakdown graph for the lower mass singly charged ion at m/z 319 is also shown in Figure 3(b) for comparison where there is no contribution from a doubly charged cluster ion. As can be seen from this graph, we do not observe any fragmentation of the monomer ion. It is interesting to note from Figure 3 that the intensity of alternating fragment ions goes through a maximum as the collision energy is increased. The observation of alternating low and high intensity fragment cluster ions is interesting as this behavior has not

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FIG. 2. Mass spectrum showing the fragmentation of a singly charged cluster ion at mass 631, a 12-mer, obtained at different normalized collision energies, (a) at 5%, (b) at 35%.

been observed in the intensity distribution of the cluster ions except for metallic and bimetallic cluster ions where both monomer and dimer neutral units were sequentially lost, favoring either even or odd ions.62,63 This is also different from the fragmentation of fullerene cluster ions,64,65 which fragmented via sequential loss of a carbon dimer (C2) neutral unit, thus generating only the even numbered carbon clusters. It was suggested that such behavior was due to the superior stability of fullerenes which require an even number of carbon atoms. The interesting aspect of this fragmentation property is that even numbered cluster ions result in higher intensities of even numbered cluster fragment ions and odd numbered precursor ions result in higher intensity of odd numbered cluster ions via loss of a dimer neutral from each of the precursor ions followed by the loss of monomer units. This clearly suggests that the loss of a dimer unit is dominant over the loss of a monomer unit. Figure 4 shows the fragment ion spectrum of a doubly charged cluster ion at m/z 656 (nominal) that corresponds to a cluster of 25 monomer units at a normalized collision energy of 20%. Loss of one neutral monomer unit is observed via the evaporation process along with singly charged fragment ions due to Coulomb repulsion that dominate the dissociation process. In all cases, sequential loss of a monomer unit is the prevalent fragmentation process. As the collision energy is further increased, only lower mass singly charged fragment ions survive the collision process due to secondary fragmentations leading to m/z 163 ions as the most abundant ion along with monomer and dimer ions. For some doubly charged ions, we observed several doubly charged fragment ions at higher masses than the primary ion. We discovered that these fragment ions were due to the presence of higher

FIG. 3. Breakdown graphs for the fragmentation of singly charged m/z 631(a) and m/z 319 (b) ions showing dominant loss of two neutral monomer units followed by the loss of monomer units in their fragmentation processes.

mass cluster ions from quadruply and possibly triply charged ions whose isotope distributions either matched or overlapped with the isotope distribution of the primary ion selected for fragmentation. Fragmentation of a triply charged cluster ion at m/z 873 results in both singly and doubly charged cluster ion series due to Coulomb repulsion and very little fragmentation due to the evaporation process, as shown in Figure 5. At a lower collision

FIG. 4. Fragment ion spectrum for the doubly charged ion at m/z 656 at 25% normalized collision energy.

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triply charged ions (data not shown here) at the lowest collision energy, albeit in very low abundances. This can only be possible if there are some ions of higher masses and charges present as well, such that their m/z distribution falls within the isolation window selected for the collisional dissociation of the ions. This will become clearly evident later when we present and discuss the data from electron transfer processes of these multiply charged ions with negatively charged reactant ions. Figure 6 shows the fragment ion spectra for the dissociation of a quadruply charged ion at m/z 1176 as a function of the normalized collision energy. At low collision energy, 5%, we observe two quadruply charged fragment ions at m/z 1163 and m/z 1150 due to the evaporative loss of one and two neutral monomer units, respectively, along with a few triply charged and singly charged ions in very low abundance due to Coulomb repulsion, as shown in Figure 6(a). As the collision energy is

FIG. 5. Fragment ion spectrum for the triply charged ion at m/z 873 at different collision energies showing sequential and secondary fragmentations leading to ions of lower masses and lower charges. (a) 10% and (b) 25% normalized collision energy.

energy (10%), three triply charged ions due to evaporation process are observed at m/z 856, 838, and 804 (856 and 804 ions are of very low abundance) corresponding to the loss of one, two, and four lithium formate monomers from the parent ion, as shown in Figure 5(a). It seems that loss of three monomer units is hidden under the dimer ion at m/z 821 envelop (not shown in the figure). There are several doubly charged fragment ions starting at m/z 1228 and continuing up to m/z 890 that are formed via Coulomb repulsion. The m/z 1228 ion is the highest mass fragment ion in this series and corresponds to the loss of a lithiated trimer ion (m/z 163) from the primary ion followed by the sequential loss of monomer units. It is interesting to note that the m/z 163 ion is the MNC as described above. As the collision energy is increased, first doubly charged and then singly charged fragment ions begin to dominate, most likely due to secondary dissociation processes and smaller cluster ions are formed, as shown in Figure 5(b). A new series of singly charged ions starting from m/z 179 and continuing with the addition of 52 Da (lithium formate) is observed up to m/z 439. It is doubtful if it is due to the addition of an oxygen atom to the ion, however, it could be due to the sodiated cluster ion where 179 corresponds to the sodiated trimer ion. Similar to doubly charged cluster ion fragmentation described above, there are a few triply charged ions of higher masses also present in the fragment ion spectrum of several

FIG. 6. Fragment ion spectrum for the fragmentation of a quadruply charged ion at m/z 1176 at different collision energies showing sequential and secondary fragmentations leading to ions of lower masses and lower charges. (a) 5%, (b) 15%, and (c) 25% normalized collision energy.

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TABLE I. Fragment ions, their charge state, and number of monomer units from the dissociation of quadruply charged cluster ion, (HCOOLi)90Li44+, m/z 1176.

m/z

Charge state

Monomer units

1 1 1 1 1 1 1 1 1 1

5 6 7 8 9 10 11 12 13 14

267.084 319.097 371.111 423.124 475.138 526.151 578.164 630.178 682.191 734.206

m/z

Charge state

Monomer units

708.199 734.206 760.213 786.219 812.226 838.232 864.240 890.249 916.253 942.258 967.766 993.271 1174.831 1201.325 1227.833 1266.343 1279.846 1305.352 1331.859 1357.367 1383.373 1409.38 1435.396 1461.387 1487.402 1513.406 1539.414 1565.421 1591.427 1617.938

2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2

27 28 29 30 31 32 33 34 35 36 37 38 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62

increased to 10%, mostly triply charged ions along with some doubly charged and singly charged fragment ions of very low intensity are observed. All these ions are formed as a result of Coulomb fission. As the collision energy is further increased to 20%, extensive fragmentation leading to singly, doubly, and triply charged fragment ions are observed in high abundances, as shown in Figure 6(b). As the collision energy is increased even more, two sets of doubly charged ions are observed and singly charged fragment ions are present in higher abundance and at the highest applied collision energy, the m/z 163 ion becomes the most abundant ion which is a MNC ion in the normal mass spectrum of the cluster ions and this supports the fact that this is a highly stable ion, and thereafter, m/z 163 ion dissociates into smaller singly charged cluster ions. Thus, the fragmentation of multiply charged lithium formate cluster ions can be described by the following two general pathways. A list of all fragment ions at 20% normalized collision energy is given in Table I (HCOOLi)nLimm+ → (HCOOLi)n-xLimm+ + (HCOOLi)x

(1)

↓ (HCOOLi)n-x-ym+ + (HCOOLi)y → (HCOOLi)n-xLim-y

(m−y)+

(2)

+ (HCOOLi)xLiy . y+

(3)

m/z

Charge state

Monomer units

959.264 976.601 994.273 1011.279 1028.615 1045.953 1063.289 1080.295 1097.299 1115.305 1132.643 1149.981 1166.984 1184.324 1201.325 1218.998 1236.335 1253.339 1271.01 1288.349 1305.352 1322.689 1340.368 1357.367 1375.037

3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3

55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79

In the present study, it has been observed that the lithiated trimer ion, (HCOOLi)3Li+ at m/z 163, is the most stable ion observed in the normal mass spectrum as well as in the fragmentation processes of singly, doubly, triply, and quadruply charged ions. This behavior is in sharp contrast with the observations for sodium formate cluster ions from the study of Vekey and coworkers16 where they did not find any MNCs in the normal mass spectrum as well as in the fragmentation of larger cluster ions. In fact, their study showed almost complete absence of the tetramer cluster ion when larger clusters were fragmented and it was of lower intensity than neighboring cluster ions in the normal mass spectrum and this behavior is clearly observed in our present study, as can be seen in fragment ion spectra of multiply charged ions in Figures 5 and 6 at higher collision energies. We also observe significantly lower intensity of tetramer ions in the mass spectrum for positive cluster ions. Our study, however, clearly demonstrated the presence of MNCs and also the fragmentation of ions results in alternating high and low intensity of the fragment ions. This latter effect is also evidenced from the breakdown graphs presented in the study by Vekey and coworkers. It has been shown from a number of experimental studies that fission and evaporation ratio varies linearly with z2/n, where z is the number of charges on the cluster and n is

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the number of monomer units in the cluster. For example, Brechignac et al.66 observed a value of 0.13 for doubly charged sodium clusters and Saito et al.67 determined z2/n between 0.25 and 0.5 for multiply charged Va, Nb, and Ta clusters. William and coworkers68 observed a sharp transition from dissociation by evaporation of neutral monomer to fission at a z2/n value of ∼0.47 for the protonated clusters of leucine-enkaphaline. Our studies showed z2/n to be 0.3 for doubly and triply charged protonated clusters and 0.25 for quadruply charged clusters, in line with previous several observations on multiply charged clusters. Interestingly, evaporation via neutral monomer losses is a minor process for these lithium formate clusters, as shown above. C. Negatively charged cluster ions and their fragmentations

Figure 7 shows the mass spectrum of negatively charged cluster ions which corresponds to the general formula, (HCOOLi)nHCOO−, where n values as high as 26 have been observed in the present study. There are at least three cluster ions of unusually higher intensity than their neighboring clusters that correspond to MNCs of n = 3 (m/z 201), 5 (m/z 305), and 8 (m/z 461) but these are not as dominant as the n = 3 (m/z 163.056) cluster ion in the positive ion mode spectrum. Similar to positively charged cluster ions, negatively charged cluster ions also show two distinct intensity distributions, as described earlier. Studies of negatively charged cluster ions have not been described for many systems and are limited mostly to metal oxides and metal halide clusters that were produced by several techniques for cluster formation. Only a few studies of negative cluster ions formed by ESI have been reported. In a recent study, Nanita and Cooks12 observed doubly charged negative halide adducts of serine clusters with the octamer as a MNC, like for the protonated serine octamer MNC. However, in water clusters, the negatively charged clusters did not show the same MNCs as the positively charged cluster ions. Uggerud and coworkers15 observed (H2O)55OH− as the MNC in water cluster anions formed by ESI while (H2O)21H+ has been known to be the MNC in positive ion mode in ESI as well as from

FIG. 7. High resolution mass spectrum of lithium formate cluster ions in the negative ion mode. All cluster ions correspond to the general formula, (HCOOLi)n(HCOO)mm−.

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several other modes of cluster ion formation. The stability of the negative MNC ion in water has been proposed to be due to weak stability of the corresponding neutral water clusters unlike the clathrate structure39 from shell closing in the (H2O)21H+ cluster ion. Besides singly charged cluster ions, doubly, triply, and quadruply charged cluster ions corresponding to the general formula [(HCOOLi)n(HCOO)m]m− are also observed, corresponding to n = 19 (m/z 538.6), 38 (m/z 703.5), and 73 (m/z 993.2) for the onset of the first observation of the doubly, triply, and quadruply charged ions, respectively. The fragmentation pattern for the negatively charged cluster ions is very similar to the positively charged cluster ions, i.e., loss of sequential monomer units from the singly charged clusters and Coulomb repulsion resulting in ions of lower charge states. However, no specific patterns of alternating lower and higher intensity of fragment ions were observed in any of the negatively charged cluster ions that were subjected to CID. Multiply charged cluster anions also lose monomer units via Coulomb fission. Figure 8 shows a fragment ion spectrum for a triply charged negative cluster anion at m/z 1032 at 35% normalized collision energy that fragments into singly and doubly charged fragment ions as observed for positively charged ion clusters. As the collision energy is increased, the fragmentation pattern shifts to lower mass singly charged cluster ions due to secondary and higher order fragmentations. Generally, the fragmentation behavior is very similar to that observed for the positively charged ions. D. Interaction of multiply charged positive cluster ions with negatively charged ions: Electron transfer dissociation

ETD via cation-anion reactions is a very useful technique for the dissociation of multiply charged peptides and proteins, especially for identifying post-translational modifications (PTMs) which are otherwise scrambled or lost by CID from collisions with neutrals.69,70 The process involves reacting negatively charged ions with multiply charged positive ions of interest and vice-versa that results in partial neutralization of the multiply charged ion with a large excess of energy in the internal modes of the reactant multiply charged ions, which often leads to fragmentation of the excited ions. We

FIG. 8. Fragment ion spectrum for the triply charged cluster ion at m/z 1032 at 35% normalized collision energy.

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J. Chem. Phys. 142, 064304 (2015) TABLE II. Secondary ions from electron transfer dissociation of triply charged m/z 926 ions including adduct ions with fluoranthene (FLA). Nominal mass (m/z)

FIG. 9. Mass spectrum of the product ions from the interaction of a triply charged cluster ion at m/z 925 with negatively charged fluoranthene ions (m/z 202). Chemical formula and associated charge state with these ions are given in Table II.

have utilized this method to explore the possibility of different dissociation processes for the multiply charged clusters which are mainly dissociating by the sequential loss of monomer units when subjected to collisions with neutrals, as demonstrated above. Figure 9 shows the ETD spectrum of the triply charged m/z 926 (nominal) ion, corresponding to the formula (HCOOLi)53 Li33+. Two distinct types of secondary ions are observed: (1) charge reduced ions where primary ions lose one or more charges via neutralization but leaving the ion intact and (2) loss of monomer units and charge reduction. In addition, there are several ions that correspond to higher masses, m/z 1848 (z = 2), m/z 1488 (z = 2), m/z 1232 (z = 3), and m/z 2977 (z = 1), than the primary ion (m/z 926, z = 3) selected for ETD, suggesting the possibility that higher charge clusters are present in the same mass window isolated for ETD. It is quite probable because isotope distributions of higher charged clusters will overlap with the isotope distribution of the lower charged clusters within the isolation window for the experiment. Interestingly, we do not observe any fragmentations of monomer units within the cluster ions, just as we did not observe such fragmentations from CID. A full list of the secondary ions observed along with their charge and molecular formula is given in Table II that shows a large number of charge striping processes. A very similar behavior was observed for the doubly and quadruply charged cluster ions. E. Results from theoretical calculations

Figures 10(a)-10(k) show the optimized B3LYP structures of HCOOLi and the (HCOOLi)nLi+ (n = 1-9) cluster ions. In the neutral HCOOLi monomer (Figure 10(a)), the Li+ coordinates with both oxygen atoms due to the resonance structures of HCOO− having a formal charge of − 1/2 e each. The two O–Li distances are 1.87 Å and the C–O–Li angles are 82.2◦. When a lithium ion is added to HCOOLi to generate the (HCOOLi)Li+ cluster ion (Figure 10(b)), the structure undergoes a significant transformation. The two lithium atoms coordinate with one oxygen atom each. The O–Li distances are reduced to 1.71 Å and the C–O–Li angles increased to

Charge state

Formula

872

2

(HCOOLi)33Li2+ 4

890

3

(HCOOLi)51Li3+ 3

898

2

(HCOOLi)34Li2+ 4

908

3

(HCOOLi)52Li3+ 3

915

3

(HCOOLi)52Li3+ 6

1225

3

(HCOOLi)70Li3+ 5

1232

3

(HCOOLi)70Li3+ 8

1335

2

(HCOOLi)51Li2+ 3

1362

2

(HCOOLi)52Li2+ 3

1373

2

(HCOOLi)52Li2+ 6

1387

2

(HCOOLi)53Li2+ 3

1488

2

(HCOOLi)53L2+ 3 (FLA)

1825

1

(HCOOLi)34Li3Li+5

1848

1

(HCOOLi)35Li+4

1949

2

(HCOOLi)70Li6Li2+ 2

2050

1

(HCOOLi)35Li3Li+ (FLA)

2775

1

(HCOOLi)53Li+3

2977

1

(HCOOLi)53Li+3 (FLA)

3696

1

(HCOOLi)70Li+8

3898

1

(HCOOLi)70Li+8 (FLA)

174.9◦. For the (HCOOLi)2Li+ cluster ion, two structural isomers were found. The first one (Figure 10(c)) is a true combination of HCOOLi and the Li+(HCOOLi)1 cluster ion, in which the neutral HCOOLi binds with the monomer ion through one oxygen atom with one of the lithium atoms from the Li+(HCOOLi) cluster ion. The various O–Li distances are not equal anymore and different lithium and oxygen atoms are present when considering the number of interaction of lithium with oxygen and vice versa. The second structure for the (HCOOLi)2 Li+ cluster ion is shown in Figure 10(d) and it is more stable than the first isomer. One HCOOLi is oriented such that the lithium atom of HCOOLi can bond with all four oxygen atoms in the cluster ion, although three of the O–Li distances are more than 2 Å. It is interesting that the first isomer is basically a linear addition of the two lithium formate molecules while the second isomer shows that atoms are restructuring to form a compact closed shell-like structure. The structure of the lithiated trimer ion is shown in Figure 10(e) which has three closed rings fused together. This ion has a phenalenylium-like structure71 and a remarkable thermochemical stability, which will be discussed in more detail later on. Figure 11 shows the phenalenylium structure which resembles the calculated structure for the lithiated trimer cluster ion and provides evidence for the higher stability of this cluster ion as has been experimentally observed. The central Li+ coordinates with three oxygen atoms, which in turn coordinate with one of the outer ring lithium atoms. The O–Li distances on the outside are slightly shorter (1.81 and 1.84 Å, respectively) than the O–Li distances to the central Li+ (1.95 Å). The trimer ion structure shows resemblance with alkali cationized crown ethers72 as well where the alkali cation

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J. Chem. Phys. 142, 064304 (2015)

FIG. 10. Optimized structures for the positively charged cluster ions of the general formula, (HCOOLi)nLi+: (a) neutral lithium formate, (b) lithiated monomer, (c) lithiated dimer 1, (d) lithiated dimer 2, (e) lithiated trimer, (f) lithaited tetramer, (g) lithiated pentamer, (h) lithaited hexamer, (i) lithiated heptamer.

is bonded with oxygen atoms in the crown ether and bond distances are very similar to those calculated for the lithium formate trimer ion. However, in crown ethers, all oxygen atoms are bonded to the alkali cation73 which is not the case for the trimer ion structure calculated here. The (HCOOLi)3Li+ cluster ion forms the basis for larger cluster ions that are all going to build around this structure. The (HCOOLi)4Li+ cluster ion shown in Figure 10(f) has the additional HCOOLi unit on the outside of one of the three identical sides of the (HCOOLi)3Li+ cluster ion. The ions are all pretty flat and the additional HCOOLi unit causes some minor structural changes, mainly O–Li distances getting somewhat larger or smaller compared to the (HCOOLi)3Li+ cluster ion. The extra HCOOLi unit does distort the (HCOOLi)3Li+ cluster ion to a minor extent compared with the relatively flat trimer ion as the figure shows. The (HCOOLi)5Li+ and (HCOOLi)6Li+ cluster ions have the additional one and two HCOOLi unit(s) interact similar to the extra one in (HCOOLi)4Li+ (Figures 10(g)

FIG. 11. Structure of the phenalenylium ion.

and 10(i)). Initial calculations for the (HCOOLi)5Li+ and (HCOOLi)6Li+ cluster ions resulted in two other ion structures in which one and two HCOOLi unit(s) interact differently with the (HCOOLi)4Li+ core structure. Instead of forming another six-membered ring as in the (HCOOLi)4Li+ cluster ion, the additional HCOOLi unit(s) bound by a simple Li · · · OC(H)OLi interaction (Figures 10(h) and 10(j)). For both n = 5 and 6, the first structures are more stable by approximately 8.9 kcal/mol and 19.5 kcal/mol, respectively. In the (HCOOLi)6Li+ cluster ions, all lithium atoms are interacting with three nearby oxygen atoms, which provides a very symmetric and stable structure for this ion. However, it is not obvious why the lithiated hexamer cluster ion is not a MNC but the heptamer ion is. For the (HCOOLi)7-9Li+ cluster ions, a second layer around the (HCOOLi)3Li+ core (n = 4-6 formed the first layer) is formed and (HCOOLi)9Li+ is a beautiful and almost symmetric structure (Figure 10(k)). Finally, adding a second Li+ to the nonamer, to act as a simple model for multiply charged cluster ions, shows that one of the outer six-membered rings has been transformed into a four-membered ring (2Li and 2O) and that the second Li+ is as far away as possible from the first Li+ in the (HCOOLi)3Li+ core (8.60 Å). We did not perform structure optimizations any further due to the time constraints in finding an optimized geometry. For the negative cluster ions, a very different picture emerges from theoretical calculations. The formate anion HCOO− (Figure 12(a)) has a delocalized charge on both oxygen atoms of − 1/2 e and in the (HCOOLi)HCOO− cluster ion only three oxygen atoms interact with the lithium ion (Figure 12(b)). A structure in which all four oxygen atoms interact with the lithium ion by a tetrahedral arrangement turned out to have two imaginary frequencies. The Li· · ·O distances to the HCOO− with the two interacting oxygen atoms are longer than the Li· · ·O distance to the HCOO− with the one interacting oxygen atom (2.02 Å versus 1.81 Å). In the (HCOOLi)2HCOO− cluster ion, the formate anion is

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064304-10

A. Shukla and B. Bogdanov

J. Chem. Phys. 142, 064304 (2015)

FIG. 12. (HCOOLi)n (HCOO)−: (a) HCOO−, (b) monomer, (c) dimer, (d) trimer, (e) tetramer, (f) pentamer, (g) hexamer, (h) heptamer, (i) octamer, and (j) nonamer.

in the center and both oxygen atoms interact symmetrically with two HCOOLi (Figure 12(c)). The corresponding Li· · ·O distances are almost similar to those in (HCOOLi)HCOO−. The three oxygen atoms that interact with the one lithium atom all lie in one plane (on both sides of the formate anion). Unlike the positive cluster ions that were all flat, the negative cluster ions show three dimensional structures. The (HCOOLi)3HCOO− cluster ion gave rise to a fairly dramatic structural change as shown in Figure 12(d). All atoms in the (HCOOLi)2HCOO− part end up in one plane because of rotations of the two HCOOLi molecules. The third HCOOLi molecule interacts with the central formate anion in a tetrahedral manner, although the four Li· · ·O distances are not equal (1.97 Å versus 2.10 Å). Again, the symmetrical structure for this ion may be responsible for the enhanced stability of this cluster ion, as experimentally observed. In the (HCOOLi)4HCOO− cluster ion (Figure 12(e)), the insertion of the fourth HCOOLi molecule leads to some structural changes compared to the (HCOOLi)3HCOO− cluster ion. The (HCOOLi)2HCOO− part is no longer in one plane. The two outer HCOOLi molecules have both counter rotated

out of the plane. All lithium atoms are interacting with three oxygen atoms, and so the structure has a high stability and shows features of a three dimensional network. In the (HCOOLi)5HCOO− cluster ion (Figure 12(f)), the fifth HCOOLi molecule interacts with one of the original outer HCOOLi molecules of the (HCOOLi)HCOO− part and one of the HCOOLi molecules in the lower (HCOOLi)2HCOO− part. For the following (HCOOLi)6-9HCOO− cluster ions (Figures 12(g)-12(j)), the (HCOOLi)4HCOO– core structure remained intact although distortions occurred while the structural network expanded and became more complex. For the larger systems, it is very likely that additional stable structures exist, but it is beyond the scope of this paper to apply more sophisticated computational methods to identify these. It is clear from our calculations that these relatively simple systems do show complex structural features that might serve as models for larger and more bulk-like structures and their associated chemical and physical properties. The structure and stability of (HCO2Li)3Li+ is obviously supported by the work presented here. However, the question is whether or not this structure is rather unique? In order to explore this,

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064304-11

A. Shukla and B. Bogdanov

J. Chem. Phys. 142, 064304 (2015)

nine structures of (HCOOY)3X+ were calculated (X, Y = H, Li, Na) to test this hypothesis. The results showed that only (HCOOLi)3Li+ and (HCOONa)3Li+ have the phenalenyliumlike structure. A quick experiment with sodium formate showed that the sodiated trimer ion is indeed a MNC and that the tetramer ion is of very low intensity compared with the trimer and pentamer ions, as observed by Vekey and coworkers. F. Thermochemistry

While investigating the structures of the lithium formate cluster cations and anions, it was important to obtain thermochemical information as well to explain the experimental observations of MNCs and fragmentation processes. The nature of the systems investigated prevented certain kinds of experiments prior to the introduction of ESI-MS1 and little data have been available.74 Lithium is an interesting element between hydrogen and sodium in the first group of the periodic table of elements. Hydrogen, lithium, and sodium have van der Waals radii of 120, 182, and 227 pm, respectively. Lithium is in between hydrogen and sodium and its binding properties and associated thermochemical properties might be somewhere in between as well. In Table III an overview of the calculated thermochemical data has been shown. The lithium ion affinity of HCOO− calculated at four different levels of theory are fairly similar and around 168 kcal/mol. This is much lower than the proton affinity (PA) of HCOO− that was experimentally determined at 345.2 kcal/mol using high-pressure mass spectrometry equilibrium experiments at different temperatures.75 For HCOOLi, the lithium ion affinity has been calculated around 50 kcal/mol from the three composite methods. This

is, as expected, much lower than the PA of HCOOH at 177.3 kcal/mol.76 Adding the second HCOOLi monomer to generate (HCOOLi)2Li+ led to two structures. The structure in Figure 10(e) is the most stable structure and at the B3LYP level, the standard reaction enthalpy at room temperature is 36.5 kcal/mol exothermic. This value is around 30% less than the value for binding the first HCOOLi unit, which is a common trend for electrostatically bound cluster cations and anions.77 However, adding another HCOOLi monomer to Li+(HCOOLi)2 is much more exothermic than one would have expected. Li+(HCOOLi)3 is a MNC ion and the B3LYP value of −66.1 kcal/mol is the thermochemical driving force to generate this very stable cluster ion. Adding the fourth HCOOLi monomer resulted in a value of −33.3 kcal/mol, which is fairly close to the value for (HCOOLi)2Li+. The binding energy for (HCOOLi)5Li+ is similar to the tetramer ion. However, formations of the pentamer and hexamer ions are slightly more exothermic (−36.0 and −37.3 kcal/mol, respectively). For the second layer around the magic cluster core structure (n = 7-9), the ∆REO0 K values are, surprisingly, even more exothermic (−43.6, −43.2, and −42.8 kcal/mol, respectively). It is hard to speculate on the main driving force for this, but for some reason as the cluster ion gets bigger some enhanced cooperative binding starts to become more important even though from looking at the structures it appears that the way the additional HCOOLi bind to the expanding cluster ion does not change much. In the case of doubly charged cluster ions, adding a second Li+ to (HCOOLi)9Li+ as a simplified model for multiply charged cluster ions leads to a ∆REO0 K values of −9.2 kcal/mol. For the formation of (HCOOLi)HCOO−, the standard reaction enthalpy at room temperature is more exothermic than

TABLE III. Overview of the calculated thermochemistry of positive and negative lithium formate cluster ions. ∆RHO298 K (kcal/mol) Reaction Li+ + HCOO →

LiHCOO Li+ + LiHCOO → Li+(LiHCOO) Li+(LiHCOO) + LiHCOO → Li+(LiHCOO)2 Li+(LiHCOO)2 + LiHCOO → Li+(LiHCOO)3 Li+(LiHCOO)3 + LiHCOO → Li+(LiHCOO)4 Li+(LiHCOO)5 + LiHCOO → Li+(LiHCOO)5 Li+(LiHCOO)5 + LiHCOO → Li+(LiHCOO)6 Li+(LiHCOO)6 + LiHCOO → Li+(LiHCOO)7 Li+(LiHCOO)7 + LiHCOO → Li+(LiHCOO)8 Li+(LiHCOO)8 + LiHCOO → Li+(LiHCOO)9 Li+(LiHCOO)9 + Li+ → Li22+(LiHCOO)9 HCOO− + LiHCOO → HCOO−(LiHCOO) HCOO−(LiHCOO) + LiHCOO → HCOO−(LiHCOO)2 HCOO−2 (LiHCOO)2 + LiHCOO → HCOO−(LiHCOO)3 HCOO−(LiHCOO)3 + LiHCOO → HCOO−(LiHCOO)4 HCOO−(LiHCOO)4 + LiHCOO → HCOO−(LiHCOO)5 HCOO−(LiHCOO)5 + LiHCOO → HCOO−(LiHCOO)6 HCOO−(LiHCOO)6 + LiHCOO → HCOO−(LiHCOO)7 HCOO−(LiHCOO)7 + LiHCOO → HCOO−(LiHCOO)8 HCOO−(LiHCOO)8 + LiHCOO → HCOO−(LiHCOO)9

a ∆ EO R 0 K,

∆RSO298 K (cal/mol K)

G3

G3(B3)

G3(MP2)

B3LYP

B3LYP

−168.1 −50.2

−168.2 −50.1 −34.4

−167.2 −49.7 −33.9

−26.5 −20.3 −35.8 −44.8 −35.2

−52.7

−52.6

−167.4 −52.7 −36.5 −66.1 −33.3/−34.8a −36.0a −37.3a −43.6a −43.2a −42.8a −9.2a −47.8 −38.5 −30.0 −38.0/−40.4a −38.8a −48.1a −39.0a −33.5a −52.6a

−37.4 −12.1 −45.0 −33.2

PA(HCOO−) = 345.2 kcal/mol, PA(HCOOH) = 177.3 kcal/mol.

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the formation of (HCOOH)HCOO− (−26.1 ± 3.1 kcal/mol) determined by high-pressure MS.78 The formation of (HCOOLi)2HCO−2 is, as expected, less exothermic (−38.5 kcal/mol), but the decrease is less than that for the corresponding positive cluster ions (−9.3 versus −16.2 kcal/mol). The (HCOOLi)2HCO−2 cluster ion has a very favorable structure with the two lithium ions each interacting with three oxygen atoms. The overall stability is, of course, determined by attraction and repulsion of the various nuclei of opposite and equal charge, respectively, and a quick glance at the various structures makes it clear that it is very hard to make an easy quantitative guess. The formation of (HCOOLi)3HCOO− from (HCOOLi)2HCOO− and HCOOLi is 30.0 kcal/mol exothermic. The main reason for this relatively lower value is the distance between the oxygen atoms of the central HCOO− and the lithium ion of the third HCOOLi. However, compared to (HCOOLi)HCOO−, this distance is not much different (2.10 Å versus 2.02 Å) and two oxygen atoms are interacting with the lithium ion (four in total, instead of three). In addition, the distance increase between the oxygen atoms and the lithium ions of the first two HCOOLi monomers (1.79 Å–1.86 Å) seems not responsible for the decrease of 8.5 kcal/mol. For the formation of (HCOOLi)4HCOO−, a more exothermic value of 38.0 kcal/mol was determined. This might indicate that (HCOOLi)4HCOO− is a magic number cluster ion like (HCOOLi)3Li+. The attractive interactions between the lithium ions and three oxygen atoms are the main sources of this stability. Adding additional HCOOLi units (up to the nonamer) results in the ∆REO0 K values of −38.8, −48.1, −39.0, −33.5, and −52.6 kcal/mol, respectively. Especially, the values for the hexamer and the nonamer are very exothermic and might be indicative of clusters of additional stability through very favorable cooperative binding. In addition to the enthalpy changes of the various clustering reactions, the entropy changes of the same reactions can provide additional insight into the cluster ion structures. In Table I, the calculated B3LYP standard entropy changes at room temperature for the various cluster ion reactions have been summarized. Even though it will not be possible to verify these values experimentally, the calculated results might indicate whether a structure has a high degree of order. For the positive cluster ions, the ∆RSO298 K values for the formation of (HCOOLi)2−4Li+ are much more negative (−35.8, −44.8, and −35.2 cal/mol K, respectively) compared to the value for (HCOOLi)Li+ (−20.3 cal/mol K). Especially, the value for (HCOOLi)3Li+ is indicative of the ordered structure. For the (HCOOLi)1-4 HCOO– cluster ions, there appears to be no clear trend. The ∆RSO298 K values for the formation of (HCOOLi)HCOO− and (HCOOLi)3HCOO− are much more negative (−37.4 and −45.0 cal/mol K, respectively) than for (HCOOLi)2HCOO− (−12.1 cal/mol K). The value for (HCOOLi)4HCOO− (−33.2 cal/mol K) seems too low compared to the value for (HCOOLi)3HCOO− for both structures show many similarities. It is hard to speculate on the ∆RSO298 K values for the larger clusters, but based on the calculated ∆REO0 K values that indicate potential magic number clusters, it seems reasonable that there might large ∆RSO298 K values to confirm their presence.

J. Chem. Phys. 142, 064304 (2015)

V. CONCLUSIONS

Positive and negative cluster ions were formed by electrospray ionization of lithium formate solution and their fragmentation pathways were studied by tandem mass spectrometry. In both ion modes, besides singly charged ions, multiply charged ions up to 4 and higher charges were observed above the critical cluster sizes which were dependent upon the number of charges. Magic number clusters were also observed in both modes although the trimer ion in the positive ion mode dominated the cluster ion spectrum. Fragmentation of all singly charged cluster ions is dominated by the loss of two monomer units in the first step followed by loss of single monomer units with fragment ion intensities showing a high and low intensity distributions. In the case of positive ion clusters, the trimer ion (MNC) was the most abundant fragment ion at higher collision energies although fragmentation resulted up to the lithiated monomer ions. For multiply charged ions, fragmentation via Coulomb repulsion dominated the dissociation process at all energies and resulted in the fragment ions up to the lithiated monomer ion via multi-step fragmentation processes. Cluster ion structures and thermochemical calculations confirmed higher stability of the trimer ion MNC. ACKNOWLEDGMENTS

We greatly appreciate the use of mass spectrometry facilities in the Environmental Molecular Sciences Laboratory and the Integraive Omics group at the Pacific Northwest National Laboratory and computational facilities at the University of the Pacific, Stockton, CA. A.S. would like to thank Ron Moore and Robby Robinson for availing these facilities at PNNL and Tom Fillmore for his help with the triple quadrupole mass spectrometer. We would also like to thank Professor Jianhua Ren and Dr. James A. Laramee for their critical comments and many helpful suggestions on the paper. The experimental mass spectrometry work was performed at and partially supported by the Environmental Molecular Sciences Laboratory, a DOE/BER national scientific user facility at Pacific Northwest National Laboratory in Richland, Washington and operated by Battelle for the DOE under Contract No. DE-AC05-76RLO-1830. 1M.

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Lithium formate ion clusters formation during electrospray ionization: Evidence of magic number clusters by mass spectrometry and ab initio calculations.

Small cationic and anionic clusters of lithium formate were generated by electrospray ionization and their fragmentations were studied by tandem mass ...
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