J. Phys. Chem. 1996, 100, 12235-12240
12235
Metastable Decomposition of {ROH}nH+ Cluster Ions (Where R ) CH3 or CH3CH2) Ping Xia,† Michael Hall, Thomas R. Furlani, and James F. Garvey* Department of Chemistry, Natural Science & Mathematics Building, The State UniVersity of New York at Buffalo, Buffalo, New York 14260-3000 ReceiVed: March 28, 1996; In Final Form: May 15, 1996X
We have studied the metastable decomposition of protonated alcohol cluster ions employing a reflectron time-of-flight mass spectrometer. Loss of a single alcohol molecule is observed as the dominant decomposition channel in all of these cluster ions. From the daughter ion spectra, we find experimental evidence that the {ROH}6H+ cluster ions possess a unique stability relative to the parent cluster ion. Theoretical calculations were performed to interpret the cause for this special stability and to indicate several possible isomeric structures for this cluster ion. The energetic ordering of the isomers can be interpreted in terms of hydrogen bonding and delocalization of charge.
Introduction Properties of the metastable decomposition of cluster ions have attracted great interest recently.1-6 Metastable decomposition of cluser ions is an evaporative process which reveals the evolution of a “hot” cluster ion into a smaller, cooler species (i.e., {M}nH+ f xM + {M}n-xH+). The origin of “magic number” cluster ions arising in the mass spectra of many cluster systems can often be attributed to the special stability of certain cluster sizes. The stabilities of such cluster ions can also be observed via direct observation of metastable decompositions. That is, the abundance maxima (or minima) of these cluster ions in the ordinary mass spectra are found to be correlated with anomalously low (or high) metastable fractions. Through metastable decompositions, cluster ions can therefore evolve into relatively stable structures. Thus, these magic number cluster ions will appear anomalously abundant in the spectrum of daughter ions. Normally, the intensities of daughter ions are only a few percent of the intensities of their corresponding parent ions. Because of this low signal intensity, it is therefore often difficult to detect daughter magic number cluster ions.1-3,8 However, the development of reflectron time-of-flight mass spectrometry (TOFMS-R) now provides us with a powerful tool to probe such magic number daughter cluster ions in the mass spectra of parent ions.9 In this paper, we report the observations of magic number daughter cluster ions of {CH3OH}nH+, {C2H5OH}nH+, and {CD3OH}nH+ alcohol cluster ions, generated via multiphoton ionization. In these alcohol cluster ions, the dominant metastable decomposition is loss of a single alcohol molecule (i.e., {ROH}nH+ f ROH + {ROH}n-1H+). From the daughter ion spectra, we observed anomalous peaks corresponding to a daughter ion at cluster size n ) 6, indicating that the {ROH}6H+ alcohol ions possess some inherent stability not shared by the other cluster ions. A series of theoretical calculations were then performed to help explain this unusual stability. Experimental Section A detailed description of our TOFMS-R setup has been described elsewhere.10 In brief, an inert gas (He or Ar) is bubbled through a reservoir containing the liquid alcohol at room † Present address: Department of Radiation Oncology, Box 0226, University of CaliforniasSan Francisco, San Francisco, CA 94143. X Abstract published in AdVance ACS Abstracts, July 1, 1996.
S0022-3654(96)00937-9 CCC: $12.00
Figure 1. Scheme of the experimental setup where the dotted line shows the trajectory of the daughter ions.
temperature. The resulting vapor is then supersonically expanded through a pulsed nozzle with a 0.5 mm orifice diameter. The resulting {ROH}n neutral clusters pass through a skimmer located 1.5 cm away from the nozzle and enter a second chamber containing the TOFMS-R. Upon entering the ionization region of our TOFMS-R, the clusters are irradiated by the unfocused 248 nm output generated from a Lambda Physik excimer laser (EMG 101, 50 mJ/pulse). The positive charged cluster ions are then accelerated to a nominal kinetic energy of 3975 eV by a three-plate Wiley-MeLaren assembly, as shown in Figure 1. The accelerated ions travel through a 130 cm long field-free region which terminates at a double-stage reflectron (R. M. Jordan Co.) located at the top of the flight tube. Following the © 1996 American Chemical Society
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Figure 2. Typical mass spectrum of ethanol cluster ions, taken with an Ar bath gas at a stagnation pressure of 1300 Torr. An stands for the parent n-mer cluster, while an represents the daughter cluster ion which has lost a single methanol molecule. The B series represents cluster ions of the type {C2H5OH}n{H2O}H+.
reflectron, the ions travel an additional 65 cm back to a dual microchannel plate detector. The cluster ion signals were averaged normally for 500 laser pulses at a rate of 2 Hz. By the use of the reflectron, we are then able to separate the daughter ions from their parent ions in the mass spectra. These daughter ions are formed from metastable decomposition during passage through the field-free region. Since the daughter ions have lower kinetic energies than their parent ions, we use the cutoff potential method introduced by Castleman and coworkers9 to identify the real masses of daughter ions and obtain daughter ion spectra, while the parent ions penetrate through the reflectron continuing on their original trajectory. In the present experiment, the methanol (CH3OH, 99.9%) and 1-propanol (C3H7OH, 99.9%) were obtained from Fisher Chemical. The ethanol (C2H5OH, dehydrated, 200 proof) was obtained from Pharmo. The CD3OH was obtain from Cambridge Isotope Laboratory. Results (1) Parent Ions. Figure 2 is a typical mass spectrum of ethanol cluster ions, using Ar as the bath gas at a stagnation pressure of 1300 Torr. In this spectrum, the voltage setting on the reflectron reflects both parent and daughter ions to the detector. Peaks labeled as An are clusters of the form {C2H5OH}nH+. Peaks labeled as Bn represent cluster ions of {C2H5OH}n{H2O}H+ and only appear after n > 7. Peaks labeled as an are daughter cluster ions, corresponding to the loss of an ethanol molecule from {C2H5OH}n+1H+. Peaks labeled as bn are daughter ions corresponding to the loss of an ethanol molecule from {C2H5OH}n+1{H2O}H+. Cluster mass spectra using either methanol, methanol-d3, or propanol are all similar to the spectrum in Figure 2. Similar mass spectra were also reported in a previous MPI experiment of methanol cluster ions.9 The intensity distribution of the {C2H5OH}nH+ cluster ions is plotted in Figure 3. No intensity anomalies (i.e., magic numbers) are observed, and the distribution is a smooth exponential decay curve as a function of cluster size. The origin of the protonated alcohol cluster ions is attributed to the fast intracluster proton transfer reaction following ionization of the neutral clusters.11 The mechanism of formation of alcohol-water heteroclusters was proposed as an intracluster reaction between two alcohol moieties, resulting in concurrent elimination of R2O and an alcohol monomer.9
Xia et al.
Figure 3. Intensity distribution of {C2H5OH}nH+ cluster ions from the spectrum of Figure 2.
{{ROH}nH+}* f {ROH}n-3{H2O}H+ + R2O + ROH (1) However, one may suppress this channel by introducing a trace amount of a reagent gas into the expansion which possesses a higher proton affinity than water (i.e., ammonia). By using He containing 8) are formed via the metastable decomposition channel
{CD3OH}n{H2O}H+ f {CD3OH}n-1{H2O}H+ + CD3OH (3) The daughter ions labeled as cn (n ) 4-9) are formed from the metastable decomposition channel
{CD3OH}nH+ f {CD3OH}n-3{H2O}H+ + (CD3)2O + CD3OH (4) These three decomposition channels were previously reported by Castleman and co-workers in their MPI experiment of methanol clusters with Ar as a bath gas.9 Parts b and c of Figure 5 are daughter ion spectra of {CH3OH}nH+ and {C2H5OH}nH+, respectively, taken with He/ NH3 as a bath gas. Only the decomposition channel 2 is observed, and the corresponding daughter ions are labeled as an. Since the {ROH}n{H2O}H+ cluster ions do not appear in the parent ion mass spectrum, the decomposition channels 3 and 4 are also not observed in both parts b and c of Figure 5.
Decomposition of {ROH}nH+ Cluster Ions
J. Phys. Chem., Vol. 100, No. 30, 1996 12237
Figure 4. Mass spectrum of ethanol cluster ions, taken with helium as the bath gas containing a trace of ammonia. An stands for the parent n-mer cluster, while an represent the daughter cluster ion which has lost a single methanol molecule.
However, a common feature that can be found in all of these daughter ion spectra is that the daughter ion clusters of size n ) 6 are relatiVely more abundant than their neighboring daughter ion clusters. Theory and Discussion According to the Klots evaporative ensemble statistical model,13 decomposition fractions of metastable ions usually demonstrate an increase with cluster size. On the basis of the Klots evaporative ensemble theory, the normalized population of daughter ions at time t is given by14
D ) (Cn/γ2) ln{t + (t - t0) exp(-γ2/Cn)] }
(5)
where Cn is the heat capacity of the cluster ion (in units of the Boltzmann constant kB) and γ ) 23.5 ( 1.5 is the Gspann parameter which is usually independent of cluster size. Cn is chosen as 6(n - 1)kB. This calculated fractional distribution is plotted in Figure 6 and agrees with the distribution from the experimental data. In Figure 6, the open circles represent the decay fractions D/[P + D] of ethanol cluster ions, where D and P are the intensities of daughter ions and their corresponding parent ions measured from the same spectrum. From Figure 6, it would appear that no cluster ion is especially more stable than any other cluster ion. However, from Figure 5a-c, daughter ions of {ROH}7H+ are anomalously abundant. According to RRKM theory,15 a cluster can be considered as comprised of a large number of oscillators, and the decay rate is the well-known Arrhenius equation15
k ) A exp(-Ev/Eavg)
(6)
where the temperature term kBT is replaced by the average energy Eavg per oscillator. Ev is the evaporation energy. For adjacent sizes of clusters, the corresponding values of Eavg should be similar. Thus, clusters with lower evaporation energies, Ev, should undergo larger rates of dissociation. The observed abnormal daughter ion at size n ) 6 indicates that the energy required for evaporation from the parent ions at n ) 7 is smaller than that at its neighboring sizes of cluster ions. In other words, daughter ions at size n ) 6 are stabilized during the metastable decompositions. In contrast to other magic number cluster ions, the {ROH}6H+ cluster ions do not show anomalous abundances in their normal
Figure 5. (a) Daughter ion spectrum of {CD3OH}nH+, taken with pure helium as the bath gas at a stagnation pressure of 1300 Torr. (b) Daughter ion spectrum of {CH3OH}nH+, taken with helium as the bath gas containing a trace of ammonia. (c) Daughter ion spectrum of {C2H5OH}nH+, at the same conditions as in part b. The an series represents the daughter cluster ion which has lost a single methanol molecule. The bn series represents cluster ions of the type {C2H5OH}n{H2O}H+ which have lost a single methanol. The cn series represents cluster ions of the type {C2H5OH}n{H2O}H+ which have lost a single methanol and a dimethyl ether.
mass spectra or parent ion mass spectra. The stabilities of {ROH}6H+ are exhibited only in the daughter ion spectra. A similar situation is also found in the Ar105 cluster1 where the intensity of the daughter ion cluster at Ar105+ is anomalously greater than that of its neighboring cluster ions. In order to understand this phenomena, Stace and co-workers proposed1 that magic number cluster ions can be divided into two distinct
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Figure 6. Decay fractions D/[P + D] of ethanol cluster ions. The solid line represents the calculation described in the text, while the open circles are from the experimental data.
classes. One class forms a particularly stable configuration once it is generated in an ion source (i.e., in less than a microsecond). The other class does not immediately form a stable configuration due to the fact that it possess several stable isomeric structures. The metastable decomposition of this type of cluster ion would then reduce its internal energy by evaporating a single monomer. Consequently, neighboring cluster ions would preferentially decay to this stable daughter ion, over a longer time period. Stace and co-workers suggested that the second type of magic number cluster ion can be viewed as a kinetic product. We believe that the {ROH}6H+ cluster ions observed here belong to the second class of magic number cluster ion. One possible structure for {ROH}6H+ could be a pentagon with a {ROH}H+ encaged inside. This structure is similar to the structure of the {H2O}21H+ cluster ion, proposed by Castleman and co-workers16, where the cluster ion forms a clathrate-like pentagonal dodecahedron in which H3O+ is encaged in the center. Recently, Buck and Schmidt have employed a perturbation approach to predict infrared spectra for a variety of methanol clusters.17 They considered several geometries of the hexamer cluster by calculating global minimum and found that six-member rings were the more favorable configuration. To explore this possibility, geometry optimizations were performed on five possible structural isomers of {CH3OH}6H+, including linear, branched, and cyclic structures, using the computational chemistry program Spartan.18 The goal of our calculations is to determine which of the proposed isomers was most stable and therefore a likely candidate for the observed stable daughter cluster ion. All optimizations were performed using ab initio Hartee-Fock (HF) theory as opposed to semiempirical theory because the latter failed to yield stable structures. We attribute this failure to the fact that semiempirical methods have not been extensively parametrized for charged species. Due to the relatively large size of the clusters, the geometry optimizations were performed using a minimal (STO3G) basis set. The final structures are shown in Figure 7. Note that, in each structure, the bond angle incorporating two oxygen atoms and a hydroxyl hydrogen (OHO) is almost linear, which is close to the value observed in water dimers.19 Minimal basis sets typically yield reliable geometries for molecules composed of first-row elements; however, other properties such as vibrational frequencies and ionization potentials are less well described.20 Accordingly, to more accurately determine the relative energies of the proposed structures, single-point energy calculations were performed for
Xia et al. each of the optimized structures with improved basis sets (631G* and 6-31G**) and at a higher level of theory, namely, second-order Moller-Plesset perturbation theory (MP2) and density functional theory (DFT). DFT was selected because recent calculations have shown that it can give reliable results for hydrogen-bonded and ionic clusters and because its application to cyclic clusters with multiple hydrogen bonds in promising.21 The DFT calculations were carried out with a 6-31G* basis set and the BLYP functional which contains gradient corrections for both the exchange and correlation. Similarly, a 6-31G* basis set was employed for the MP2 calculations. Gaussian9422 was utilized in both cases. As shown in Table 1, results obtained with the extended basis sets and a higher level of theory are in good accord for the linear, branched, and sixmembered ring; the relative energetic ordering of these isomers is the same for all calculations. However, the story concerning the bicyclic structures is a bit more complicated as the energy of these structures, relative to the six-membered ring, changes significantly in going from simple HF theory to methods which include the effects of electron correlation (DFT and MP2). With the exception of STO-3G calculations, HF theory predicts the two bicyclic structures to be the least stable isomers. However, the MP2 and DFT calculations change the relative ranking of the two bicyclic structures from 4th to 2nd for the 4-5 bicyclic structure, and from 5th to 4th for the 4-4 structure. On the basis of the DFT calculations, the cyclic six-membered ring is most stable, followed in order by the bicyclic structure containing both four- and five-membered rings, the linear isomer, the bicyclic isomer containing two four-membered rings, and the branched isomer. Hydrogen bond formation clearly plays a significant role in stabilization of the clusters; however, the predicted energetic order of the five structural isomers cannot be attributed solely to hydrogen bond formation as both bicyclic structures have the largest number of hydrogen bonds (eight) and yet are not the most stable isomers. Delocalization of the charge may also be important in determining the most stable structure. To determine the extent of delocalization, atomic charges in each structure were calculated from fits to the electrostatic potential. Atomic charges for a protonated methanol dimer {CH3OH}2H+ and methanol were also computed to provide a basis for comparison. Figures 7 and 8 show the computed atomic charges for the “backbone” atoms (hydroxyl hydrogens and oxygens) of the five proposed structural isomers, a protonated methanol dimer, and methanol, all of which were computed using HF 6-31G* wave functions. In the dimer, the two oxygen atoms carry a charge of -0.7 atomic units (au), while the two terminal hydrogens and the central hydrogen have charges of 0.51 and 0.65 au, respectively. Compared to methanol, all three hydrogens have acquired positive charge, indicating, not surprisingly, that the charge of the additional proton is delocalized over the hydroxyl hydrogens. The charge on the two oxygen atoms, on the other hand, is not much different from that in methanol. Comparison of the charge distribution in the linear methanol cluster to that in the protonated methanol dimer shows the distributions to be quite similar. In the linear cluster, each hydroxyl hydrogen and oxygen have a charge of about 0.5 and -0.7 au, respectively; there are no regions where the charges deviate significantly from these base line values. Clearly, as in the dimer, the charge of the additional proton is shared by all the hydroxyl hydrogens. Compare this to the bicyclic 4-4 isomer in which the oxygen and hydrogen atoms of the bridging atoms are significantly different from that of the other atoms in the ring. Further note how the negative charge is concentrated
Decomposition of {ROH}nH+ Cluster Ions
J. Phys. Chem., Vol. 100, No. 30, 1996 12239
Figure 7. Optimized structures of {ROH}6H+: (a) cyclic, (b) linear, (c) branched, (d) bicyclic 4-5, and (e) bicyclic 4-4. The computed atomic charges for the backbone atoms (i.e., the hydroxyl hydrogens and oxygens) are indicated.
TABLE 1 molecule
energysSTO-3G (hartrees)
∆E STO-3G (kcal)
∆E 6-31G* (kcal)
∆E 6-31G** (kcal)
∆E DFT-BLYP (kcal)
∆E MP2-6-31G* (kcal)
cyclic linear branched bicyclic 4-5 bicyclic 4-4
-681.9029 -681.9017 -681.8839 -681.9018 -681.8991
0 0.71 11.92 0.69 2.4
0 1.76 4.89 9.82 13.44
0 1.39 5.52 9.85 13.61
0 1.78 5.89 0.35 2.98
0 3.39 6.83 1.21 4.35
in the “corners” of the structure. Clearly, there is a greater separation of charge in this isomer when compared to the linear
isomer. The same is true for the other bicyclic structure and, to a lesser extent, for the six-membered ring. On the basis of
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Xia et al. in these cluster ions. From the daughter ion spectra of these alcohol cluster ions, we found that {ROH}6H+ cluster ions develop into stable cluster ions through metastable decompositions. Theory indicates several possible isomeric structures for this cluster ion. The energetic ordering of the isomers can be interpreted in terms of hydrogen bonding and delocalization of charge. Acknowledgment. This research was supported by the Office of Naval Research, which is gratefully acknowledged. References and Notes
Figure 8. Optimized structures of (a) methanol and (b) the protonated methanol dimer. The computed atomic charges for the backbone atoms (i.e., the hydroxyl hydrogens and oxygens) are indicated.
charge delocalization alone, we would expect the cyclic structures to be less stable than the linear isomer. Apparently, the additional hydrogen bond of the six-membered ring, relative to the linear isomer, more than compensates for the loss in charge delocalization. Given the localization of charge computed at the HF level of theory, it is easy to see why the bicyclic structures are destabilized. The energetic ordering of the isomers at the HF leVel can therefore be readily explained in terms of hydrogen bonding and delocalization of charge. We are currently computing DFT-based atomic charges to determine if the localization of charge evident at the HF level for the bicyclic structures persists at the higher level of theory. On the basis of the energetic ordering contained in Table 1 for the DFT calculations, we don’t expect that this will be the case. We are also carrying out constrained geometry optimizations using a HF 6-31G* basis set to determine if the discrepancy between the HF results and the MP2 and DFT results can be attributed to use of STO-3G-optimized structures. Conclusions We have studied metastable decompositions of alcohol cluster ions using a reflectron time-of-flight mass spectrometer. A loss of one alcohol molecule is the dominant decomposition channel
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