J. Phys. Chem. 1996, 100, 14865-14871
14865
Matrix Isolation and Cold Diffusion of Mass-Selected CS2•+ in Neon: Infrared Observation of the Asymmetric Stretch of CS2•+ and CS2•Thomas M. Halasinski, Jerry T. Godbout, John Allison,* and George E. Leroi* Department of Chemistry, Michigan State UniVersity, East Lansing, Michigan 48824-1322 ReceiVed: March 21, 1996X
Vibrational spectroscopic characterization of neon matrices in which mass-selected CS2•+ and CS•+ were deposited reveals absorptions due to (ν3) CS2•+ (1207.1 cm-1) and (ν3) CS2•- (1159.4 cm-1). The results are compared to previous CO2•+ studies from this laboratory [Godbout et al. J. Phys. Chem. 1996, 100, 2892]. We also report controlled annealing studies of the matrices in which clustering of ionic species with neutral molecules is observed.
Introduction A number of approaches have been developed for generating ionic species in inert hosts for spectroscopic studies. While photoionization of molecules in low-temperature matrices is a common and powerful approach for cation generation, it is not capable of producing the wide variety of ionic species that can be generated by mass spectrometry. In contrast, the deposition of mass-selected ions, formed in an electron-impact source, promises access to a broader range of molecular ions for spectroscopic characterization. We recently reported success in the capture of mass-selected ions in neon matrices for analysis using FTIR spectroscopy.1 We have also reported the enhancement in the efficiency of the trapping of mass-selected cations by adding small amounts of carbon dioxide to the neon matrix gas.2 Evidence was presented in support of counterion production through a mechanism whereby a portion of the incoming ion beam bombards cold metallic surfaces near the sample substrate, on which condensable molecules are present, sputtering off anions and electrons. These negatively-charged species are attracted toward the positively-charged growing matrix, thereby maintaining the required neutrality of the matrix and allowing for continued deposition. Thus, a variety of experimental conditions must be met in order for this experiment to be successful. We report here the vibrational spectroscopic characterization of neon matrices in which CS2•+ and CS•+ were deposited. Three aspects of our work make the study of CS2•+ a logical next step. First, while earlier attempts from this laboratory using mass-selected ion beams of smaller currents and lower kinetic energies were successful in accumulating sufficient numbers of CS2•+ ions for laser-induced fluorescence detection, they were unsuccessful in continued ion accumulation for infrared detection.3 Since the current experimental setup includes an ion source which produces ion currents more than an order of magnitude greater, the decision was made to return to this particular challenge. Second, through experiments involving relatively low mass ions, we hope to sufficiently understand the processes which occur during ion deposition such that data for larger ions can be understood and interpreted. The comparison of results for two similar species, CO2•+ and CS2•+, may reveal aspects that require further elucidation. There are currently two extreme views of this experiment, in which ions generated with kinetic energies above 100 eV are deposited into a growing neon matrix. The first, the ideal, would be that the X
Abstract published in AdVance ACS Abstracts, August 15, 1996.
S0022-3654(96)00862-3 CCC: $12.00
low-temperature matrix serves a simple purposesthe capture and isolation of incoming ions. Another extreme is that the system is a reactive one, in which ionic and neutral species in the outermost layers of the matrix can react. Reaction pathways of particular interest include collision-induced dissociation (CID) and cation neutralization processes. To understand which view is closer to reality, the decision was made to compare the CS2 system with the recently-studied CO2 system. The thermochemical data4 in Table 1 suggest that deposition of CS2•+ ions could yield very different results than CO2•+. While these two triatomic molecules are obviously similar, they differ in ways that could be critical in this experiment. The ionization energy (IE) of CO2 is more than 3 eV higher than that for CS2. One may expect CO2•+ to undergo charge transfer with matrix components such as H2O (IE ) 12.6 eV)4 where CS2•+ would not do so. In general, the neutral and ionic forms of CS2 are characterized by bond dissociation energies (BDEs) that are lower than the corresponding BDEs for CO2-related species. Thus, if CID occurs, it may be more extensive for CS2•+. We wish to explore possibilities such as CID in experiments with small triatomic ions rather than with larger organic ions, since the number of possible fragments from the triatomic is small, and their vibrational spectra are usually understood. The third reason why CS2 was selected is related to counterion generation in these experiments. While CO2 as a matrix additive is exceedingly useful, the linear molecule has a negative electron affinity. Thus, one goal in this work is to evaluate CS2 as a matrix additive, since, as Table 1 indicates, this triatomic species has a positive electron affinity and may thus be a more useful dopant for counterion generation. We also report here the results of time-dependent annealing experiments. Photolysis (photodetachment of electrons from matrix-isolated anions) and annealing of the matrix are two common methods used in matrix-isolation spectroscopy to provide additional information on the origin of spectral features. When ions are under study, the growth or disappearance of a spectral feature during photolysis can be connected to neutralization of ionic species. Annealing experiments are most often used to reduce matrix site distributions to simplify the spectrum. The experiments presented here were performed at matrix temperatures where diffusion of the guest species appears to occur without significant losses due to vaporization, similar to the work of Andrews et al.5 in which polymers of HF were formed during neon matrix annealing experiments. Obviously, in experiments involving ionic species, infrared spectral features could represent charged or uncharged matrix components. If © 1996 American Chemical Society
14866 J. Phys. Chem., Vol. 100, No. 36, 1996
Halasinski et al.
TABLE 1: Thermochemical Data (eV) for the Neutral and Ionic Species of CX2 (X ) O, S)a neutral EA IE BDE(CX-X) cation BDE(CX-X+) BDE(CX+-X) anion BDE(CX-X-) BDE(CX--X)
X)O
X)S
-0.6 13.8 5.5
+0.5 10.1 4.4
5.4 5.8
4.7 5.8
3.5 8.6
2.9 4.7
a
All thermochemical data listed and values used to compute the BDEs were taken from ref 4.
the temperature is raised and trapped species begin to diffuse, charged species would be expected to recombine at the highest rate. Thus, the time dependence of peaks in annealing experiments might provide a signature for the presence or absence of charge on molecules responsible for various spectral features. This possibility is evaluated here. We show that ionic species diffusing within the matrix during these cold diffusion experiments can form adducts with neutral species present and that neutralization of cationic and anionic species need not be the only process that occurs when mobilities increase. Experimental Section The instrument used to prepare and isolate mass-selected, matrix-isolated cationic species has been described previously.1 CS2•+ and CS•+ mass-selected ion currents of ∼50 and ∼10 nA, respectively, were used, and the ions were generated with kinetic energies of 130 eV. The resolution of the quadrupole mass spectrometer was degraded to less than unity to increase transmission of the mass-selected ions. Experiments involving depositions of CS2•+ used naturally occurring CS2, which contains 1.1% 13CS2; however, the amount of 13CS2•+ deposited during CS2•+ depositions is negligible. High-purity neon (AGA), 99.9995%, was used as the matrix gas. Carbon dioxide, 99.8+%, was obtained from Aldrich Chemical Co., reagent grade carbon disulfide was obtained from Fisher Scientific Co., and 99% 13CS2 was obtained from Isotec Inc. The matrix gas was premixed with CO2 or CS2. The Ne:dopant ratio was always at least 1000:1, and the gas was deposited onto the window at a rate of 0.5 mmol/h. FTIR spectra were recorded by averaging 256 scan files with 1 cm-1 resolution. The spectra taken before the deposition were used as background for correction of the absorption after matrix formation. The % ∆A values displayed in Figure 2 were obtained from base line corrected spectra. Small uncertainties in these values evolve from manual selection of the base line for each spectrum. The corrected spectra are obtained by calculating a least-squares fit polynomial curve through these selected base line regions. Ab initio calculations were performed using the 6-311+G* basis set as implemented in the GAUSSIAN 92 program6 to assist in the assignment of the observed spectral features. These calculations were carried out at the restricted closed-shell Hartree-Fock (RHF) and unrestricted open-shell Hartree-Fock (UHF) levels for the neutral and ionic species, respectively, of CS2, CS, CO2, and CO. The sample window temperature was measured with a Lakeshore Cryotronics 330 controller and a DT-470 Si diode sensor. The controller was also utilized to warm the sample window for annealing studies. In the cold diffusion experiments, the temperature of the matrix was initially ramped from 4 to 6 K in approximately 60-70 s and immediately cooled to 4 K.
Figure 1. Spectra in the 1460-1080 cm-1 region of five [ion f matrix: additive] experiments.
A spectrum of the matrix was then acquired at 4 K to analyze the matrix components. This process was repeated, successively ramping to temperatures of 6, 8, 10, and 12 K, with FTIR spectra taken after the return from each high temperature excursion. Results and Discussion Figure 1 displays the spectra in the 1460-1080 cm-1 region from five experiments in which mass-selected ions were deposited into neon matrices doped with either CO2, CS2, or 13CS . This region has been selected from the entire spectral 2 region scanned (4800-875 cm-1) because it contains the spectral features which are most pertinent to the deposited cations and the counterions. Other absorptions, such as those due to neutral CO2 and CS2, are in the regions not shown. Figure 1A displays the results following a 24 h deposition of CS2•+ into a neon matrix doped with CO2. The set of experimental variables for this experiment is denoted as [CS2•+ f Ne:CO2]. Other experiments will be designated in the same way, [ion f matrix:additive]. Unless otherwise indicated, the neutral and ionic species of CS2 will refer to the unlabeled form. As is often done in infrared studies, the experiment was repeated with an isotopomer of the ionic analyte, 13CS2•+. Figure 1B displays the results of a 17 h [13CS2•+ f Ne:CO2] experiment. Experiments were also performed in which the neon gas was doped with CS2 or 13CS2 during depositions of CS2•+. Figure 1C displays the results of a 23 h [CS2•+ f Ne: CS2] experiment, and Figure 1D displays the results of a 21 h [CS2•+ f Ne:13CS2] experiment. As a control experiment, and also to attempt to establish the vibrational frequency of CS•+ in neon, a 28 h [CS•+ f Ne:CO2] experiment was performed; the resulting spectrum is displayed in Figure 1E. A summary of the vibrational frequencies of the spectral features observed in these experiments is given in Table 2. During control experiments in which all parameters are held constant, except that the quadrupole mass filter is set at a m/z value for which the ion current is zero or negligible, only the bands due to the
Mass-Selected CS2•+ in Neon
J. Phys. Chem., Vol. 100, No. 36, 1996 14867
TABLE 2: Tabulated Vibrational Wavenumbers (cm-1) of the Spectral Bands Observed in Figure 1 A-E Figure 1A
1B
1104.4
1104.4 1122.2
1159.4
1C
1256.8 1273.7 1308.9
1E
assignment ν3 13CS2•ν3 12CS2•ν3 13CS2•+ ν3 12CS2•+
1122.5 1159.4
1167.6 1207.1 1211.5
1D
1207.1
1159.4 1167.6 1207.1
1210.5 1224.0 1237.5
1237.5 1256.8
13
CS O212C‚‚16O2•12CS 16
1273.2
1272.7 1340.9
a a ν3 12CO2•+
1386.0 1422.2
1422.2 1445.3
a
See discussion for possible assignments.
TABLE 3: Calculated Vibrational Frequencies and Intensities for the Neutral and Ionic Forms of CO2, CS2, CO, and CSa X)O triatomic species CX2•+ CX2 CX2•-
ν3 (cm-1)
X)S int (km/mol)
CX•+ CX CX•-
int (km/mol)
933.4 (906.9)b 231 (223) 1156.2 (1118.0) 362 (333) 2575.7 (2502.4) 116 (1095) 1582.1 (1529.8) 1567 (1466) 1857.8 (1807.2) 1142 (1091) 1185.2 (1146.7) 990 (933)
X)O diatomic species
ν3 (cm-1)
ν (cm-1)
X)S int (km/mol)
ν (cm-1)
int (km/mol)
2412.8 (2359.0) 8 (6) 1260.1 (1224.2) 2 (2) 2432.0 (2377.8) 163 (156) 1431.5 (1390.8) 155 (146) 1439.3 (1407.2) 2543 (2454) 1019.0 (990.0) 109 (107)
a The corresponding frequency and intensity values of each species containing a carbon-13 isotope are given in parentheses. b See Appendix.
neutral species of CS2, CO2, and H2O are observed. For the systems studied to date, neutral fragments diffusing from the ion source have not been observed in the matrix spectra. Ab initio calculations of the vibrational frequencies and the corresponding absorption intensities of both neutral and ionic species were carried out to assist in the assignment of the observed spectral bands. The results for the asymmetric stretch of the neutral and ionic CS2 and CO2, along with the vibrational frequencies of the neutral and ionic CS and CO, are listed in Table 3. Although higher level calculations on neutral and ionic CS2 and CO2 have been previously published,7 few report computed spectral intensities and all lack vibrational information for CS2•-. Thus, the decision was made to perform these calculations for the species of interest at the same computational level for comparative purposes. As Table 3 illustrates, vibrational intensities for the species involved in this experiment can vary widely. In particular, the cationic species show relatively small intensities. This decrease in the infrared intensity from the vibrational mode of a neutral species to the corresponding cation has been discussed by Chin and Person.7a Thus, with the presently available mass-selected ion currents, ions with low infrared intensities may not be spectroscopically observed, although they may be present in amounts comparable to neutral and anionic matrix guests. This may also explain the occasional experimental result in which infrared absorptions due to cationic matrix components are not observed.
The calculated results listed in Table 3 compare well with the experimentally observed values. The value for ν3 of CS2 is calculated as 1582.1 cm-1; it is observed in the gas phase8 at 1535.4 cm-1 and in a neon matrix at 1533.6 cm-1. This is typical of the difference between computed frequencies and measured values. The calculated fractional shift upon isotopic substitution of the carbon and the observed fractional shift for ν3 of CS2 are both 0.967. The discussion that follows will focus on the trends and correlations observed in the present experiments. It should be noted that many questions remain, in part due to the many experimental variables. While we can control ion energy and matrix gas flow, other important variables such as the ion beam focusing properties, ion current, and the composition of the condensed gases on the cold metal surfaces vary between experiments. CS2•+. Of the several peaks observed in the [CS2•+ f Ne: CO2] FTIR spectrum, Figure 1A, the absorption band at 1207.1 cm-1 can be readily assigned to ν3 of CS2•+. This agrees well with emission studies of CS2•+ formed in neon matrices through photoionization,9 where an emission band at 2418 cm-1 was assigned to 2ν3. A similar experiment using infrared spectroscopy10 also tentatively identified ν3 of CS2•+ at 1211 cm-1. An absorption band is observed at 1167.6 cm-1 in the [13CS2•+ f Ne:CO2] experiment, Figure 1B, which is consistent with the assignment9 of 2340 cm-1 for 2ν3 for 13CS2•+. We note that ν1 and ν2 of CS2•+ are not observed in these experiments. The bending mode ν2, which has been observed in the gas phase at 333 cm-1,11 is at too low a frequency to be observed in the spectral window provided by our present apparatus, and ν1 is not infrared-active. Previous calculations of ν3 for CS2•+ compare well with our computed result of 1156.2 cm-1. Use of the 6-31G* basis set7c results in a value of 1152 cm-1. A complete active space selfconsistent-field (CASSCF) approach7f predicts a value of 1195.0 cm-1. The observed isotopic fractional shifts also compare well with the calculated results; both are 0.967. CO2•-. As shown previously,2 CO2 can be used as a dopant to increase counterion production. Absorption bands at 1658.5 and 1665.7 cm-1, due to CO2•- and (CO2)2•-,12 respectively, were observed in all experiments which involved ion depositions into neon matrices doped with CO2. (CO2 + O2)•-. Absorption bands at 1256.8 and 1895.8 cm-1 are observed when CO2 is used as the matrix dopant, and the mass-selected ion current is high. These bands have been previously assigned to ν2 and ν1, respectively, of (CO2 + O2)•(which may be represented as CO4•-) isolated in a neon matrix.13 Three mechanisms for generation of CO4•- in the matrix are reasonable. First, CO2 and O2 adsorbed on the radiation shield might form CO4•- directly upon ion bombardment and accumulate on the window as does CO2•-. This is certainly viable, since condensable components such as oxygen will be most concentrated on the radiation shield, and oxygen molecules are present in these experiments as a constituent of the background gas present in the vacuum chamber. Since we know CO2•- is formed and deposited into the matrix, it could combine there with O2 to form CO4•- via “matrix chemistry”. It is difficult to assess this possibility directly, since the amount of O2 present in the matrix cannot be measured by infrared spectroscopy. A third possibility is that O2•- may desorb from the radiation shield upon ion bombardment and react with CO2 in the matrix. This is also a reasonable mechanism, since neutral CO2 is present in a concentration that is high relative to other matrix components. It remains to be determined whether deposited ions find a
14868 J. Phys. Chem., Vol. 100, No. 36, 1996 reactive environment in the growing matrix or whether they are simply trapped and isolated under the conditions used. A band at 1865.1 cm-1 with a shoulder at 1862.4 cm-1 has also been attributed to CO4•- isolated in a neon matrix.13 Absorption bands at 1852.9, 1855.3, 1862.5, and 1864.9 cm-1 also appear in our experiments where CO2 is present as a matrix gas dopant; however, their intensities do not correlate well with those for ν2 and ν1 of CO4•- between experiments. The 1852.9 and 1855.3 cm-1 bands were not observed in ref 13. It is possible that these four bands arise from different CO4•geometries. This would explain their variable intensities with respect to the 1256.8 and 1895.8 cm-1 absorptions over the course of several experiments in which deposition parameters and the resulting dominant mechanism for CO4•- formation may change. CS2•-. An absorption band at 1159.4 cm-1 is present in experiments in which CS2 is present in the matrix region, due either to diffusion of neutral precursors from the ion source or to direct doping of the neon matrix gas with CS2, although it is not observed in the [CS2•+ f Ne:13CS2] experiment. This band shifts to 1122.2 cm-1 when 13CS2 is used as either the neutral precursor (Figure 1B) or the matrix dopant (Figure 1D). On the basis of the ab initio calculations summarized in Table 3, we assign the 1159.4 and 1122.2 cm-1 bands to ν3 of CS2•and 13CS2•-, respectively. The calculated fractional shift for ν3 of CS2•- upon isotopic substitution of the carbon and the observed fractional shift of these bands are both 0.968. Presumably, CS2•- is formed from mass-selected ion bombardment of CS2 that has been frozen on the heat shield or other cold metal surfaces, similar to the proposed mechanism for CO2•formation.2 CS. An absorption band at 1273.7 cm-1 is observed in all experiments in which CS2 is present in the matrix region. This absorption shifts to 1237.5 cm-1 when 13CS2 is used as the matrix gas dopant. These bands are consistent with previous assignments of the vibrational frequencies of CS in argon matrices and in the gas phase. The vibrational frequency of CS has been reported at 1275.1 and 1272.2 cm-1 in an argon matrix14 and in the gas phase,15 respectively. Likewise, the vibrational frequency of 13CS has been reported at 1239.1 and 1236.3 cm-1 in an argon matrix14 and in the gas phase,15 respectively. CO2•+. A weak absorption band due to CO2•+ is observed at 1422.2 cm-1 when CO2 is present in abundance and large mass-selected ion currents have been deposited for relatively long periods of time. We have previously attributed the appearance of CO2•+ to ionization of neutral CO2 molecules in the matrix by electrons that have been formed by cation bombardment of surfaces and are accelerated toward the matrix by the potential due to the accumulated ions present.2 The computed value for ν3 in Table 3 is anomalously low for this cation. This result is discussed in the Appendix. Other Spectral Features. When CS2 is substituted for CO2 as the matrix dopant, an absorption band appears at 1386.0 cm-1, as shown in Figure 1C. This band shifts to 1340.9 cm-1 upon isotopic substitution of the carbon atom, as shown in Figure 1D. A possible assignment for this band is CS•+, as its vibrational frequency has been estimated16 at 1376.6 cm-1 in the gas phase. The calculated fractional carbon-13 shift of 0.972 is comparable to the observed shift of 0.967. However, it is more likely that the species responsible for the absorption at 1386.0 cm-1 may be neutral or charged polymers of carbon and sulfur atoms. CS2 adsorbed on the radiation shield and other cold surfaces should be expected to polymerize to a much greater extent than CO2. Matrix-isolated
Halasinski et al. polymers of CS2 may originate from the effect of ions striking the adsorbed CS2. The absorption at 1386.0 cm-1 does not appear in experiments in which Ne:CS2 1000:1 mixtures are deposited in the absence of a mass-selected ion beam and therefore is not due to polymers of CS2 formed in the gas phase. Photopolymerization studies of CS2 have been shown to yield (CS2)x polymers with a strong absorption centered around 1410 cm-1.17 This absorption shifts to 1364 cm-1 in (13CS2)x.17 The reported fractional shift of the polymer and the observed fractional shift in our experiments are both 0.967. Moreover, the large observed intensity of the bands makes it unlikely to be due to CS•+, which has a calculated absorbance coefficient of only 2 km/mol (Table 3). In support of this conclusion, when a current of 10 nA of CS•+, formed from CS2, is deposited for 28 h [CS•+ f Ne: CO2], no absorbances other than those previously assigned to CS2•- and CO2•- are observed (Figure1E). Presumably, the cation present in this experiment is CS•+, and it is not observed owing to its low extinction coefficient. Therefore, we propose that the absorption at 1386.0 cm-1 is due to oligomeric forms of CS2. In a previous publication2 we discussed a mechanism whereby anionic species, formed on metal surfaces, find their way to the matrix due to their attraction to the positive potential of the matrix. Here, it appears that oligomeric (CS2)x species may be sputtered off such surfaces and diffuse to the matrix as well. Several absorption bands are frequently observed in these experiments which remain unassigned. The most prominent of these bands are at 1104.4, 1210.5, 1224.0, 1308.9, 1445.3, and 2146.6 cm-1. The origin(s) of the low reproducibility for these bands are unknown. Of all the experimental parameters, the mass-selected ion current and ion beam focusing are the most difficult to maintain over the course of several different experiments. As discussed previously,2 the matrix potential and ion trapping efficiencies are all intimately related to these parameters. With the small signal/noise ratios encountered in these experiments, it is not surprising that some features of the spectra obtained may be difficult to reproduce. Possible candidates for these unassigned absorptions are species containing some combination of carbon, oxygen, and/ or sulfur atoms. Absorptions which have been assigned to species such as SO, SO2, S2O, OCS, C2S2, and C3S2 do not match well with these spectral features. Other candidates that we have considered are complexes of matrix guests with N2 or H2O, due to their high abundance in the background gas. H2S, HCS+, and H2CS do not have absorptions at the listed frequencies, as well. Annealing of the Matrix. Annealing experiments were performed after both the mass-selected ion beam and the matrix gas deposition were stopped to observe the behavior of the intensity of each band after matrix excursions to temperatures above 4 K. The 4 K matrix was raised to 6 K over a period of ∼1 min. Once 6 K was reached, the matrix was quickly cooled to 4 K, and a spectrum was obtained. This process was then repeated at 8, 10, and 12 K. Thus, by incrementally accessing higher temperatures, conditions could be gradually achieved at which diffusion occurs for a short period of time without considerable loss of the guest and host due to vaporization. This is reflected in absorptions for “unreactive” matrix guests like CO2 where, as shown in Figure 2A, the % ∆A due to the asymmetric stretch, normalized to the absorption in the original 4 K spectrum, never varies by more than 10% over the course of the annealing experiments. One reason for this experiment was to test the proposal that ionic species would diffuse and recombine more quickly than
Mass-Selected CS2•+ in Neon
J. Phys. Chem., Vol. 100, No. 36, 1996 14869
Figure 4. FTIR spectra in the 1650-1585 cm-1 region of a neon matrix at several annealing temperatures following a 17 h [13CS2•+ f Ne: CO2] deposition.
Figure 2. (A) Relative change of the ν3 CO2 absorption, (B) relative change in absorption of ν3 13CS2•+ and ν3 13CS2•-, and (C) relative change in the absorption representing the nonrotating and rotating ν2 bands of H2O at each annealing step, following a 17 h [13CS2•+ f Ne:CO2] deposition. In each case, % ∆A ) {A(i K) - A(4 K)}/A(4 K)} × 100, where A(i K) represents the absorption of the spectral feature taken after an annealing excursion to a temperature of i K.
Figure 3. FTIR spectra in the 1690-1640 cm-1 region of a neon matrix at several annealing temperatures following a 17 h [13CS2•+ f Ne: CO2] deposition.
neutral species as molecular mobilities increase, possibly providing clues to the charge state of the species responsible for a particular spectral feature. With a few exceptions, all bands decreased in intensity at approximately the same rate with increasing matrix temperature. The % ∆A of the peak absorptions due to (ν3) 13CS2•+ and (ν3) 13CS2•- through the annealing experiments are shown in Figure 2B. These changes in % ∆As are typical of reactive matrix components that are present at low levels. One of the exceptions was observed in the CO2 anionic monomer and dimer region of the spectra. An example is displayed in Figure 3, taken from a matrix produced from a 17 h [13CS2•+ f Ne:CO2] deposition. The front spectrum of Figure 3, which is labeled 4 K, was acquired before annealing. Each succeeding trace is labeled with the highest matrix temperature reached in that particular annealing step. In these spectra, the CO2•- absorption at 1658.5 cm-1 decreases in intensity with each annealing step. The rate at which this band decreases in intensity is comparable to the behavior of the majority of bands observed in the entire spectrum obtained. The (CO2)2•absorption at 1665.7 cm-1, however, increases in intensity with
each annealing step. A new absorption at 1670.6 cm-1 continues to increase in intensity through all annealing steps shown. These three spectral features likely demonstrate clustering chemistry in the matrix: CO2
CO2
CO2•- 98 (CO2)2•- 98 (CO2)3•On the basis of this proposal, we assign the 1670.6 cm-1 absorption to (CO2)3•-. We note that Jacox and Thompson observed12 a spectral feature at 1670.2 cm-1 in their CO2•- study and suggested that the absorption was due to a charged molecular aggregate of CO2. The (CO2)3•- assignment seems reasonable since CO2 is presumably the species with the highest matrix concentration other than neon. While CO2•- ions likely recombine with cations at some point, the cation concentration is much lower than the neutral CO2 concentration, so diffusing CO2•- anions encounter CO2 molecules more often than counterions. More generally, this demonstrates that one can deposit ions and neutrals and have them react in the matrix through controlled annealing. The absorptions at 1386.0 cm-1 [CS2•+ f Ne:CS2] and 1340.9 cm-1 [CS2•+ f Ne:13CS2] have also been observed to initially increase in intensity during the annealing process. This observation reinforces our conclusion that these absorptions are not due to CS•+ and 13CS•+, respectively, since it is unlikely that an ionic monomer would increase in abundance during the annealing process. The observation is taken as further evidence that the species responsible for these absorptions are probably oligomeric forms of CS2. While (CS2)x may originate from the effect of the mass-selected ion beam striking adsorbed CS2 on metal surfaces, the polymers might increase in abundance during the annealing process due to a reaction involving charged CS2 ions with CS2 in the matrix. Thus, while CO2•- may form oligomers upon annealing, the more chemically reactive anionic and/or cationic forms of CS2 may exhibit polymerization chemistry, as does CS, the photolysis product of CS2.18 The only other exception observed in the annealing experiments was the (ν2) H2O region of the spectrum. Figure 4 displays the 1650-1585 cm-1 region of the same spectra shown in Figure 3. The absorptions at 1595.8 and 1631.0 cm-1 have been previously assigned19 to (00,0 r 00,0) ν2 and (11,1 r 00,0) ν2 of H2O, respectively. These will be referred to as the nonrotating and rotating bands of H2O. As Figure 4 shows, the rotating water band slightly decreases in intensity and the nonrotating component increases in intensity during the annealing process. To assist in describing how the peak absorptions at 1595.8 and 1631.0 cm-1 change during the course of the annealing steps, the % ∆As are provided in Figure 2C. Their matrix-temperature dependence clearly differs from the other
14870 J. Phys. Chem., Vol. 100, No. 36, 1996
Halasinski et al.
spectral features. Ion-dipole and dipole-dipole interactions should become important as the mobilities in the matrix of ionic and polar species increase, as the matrix is heated. The abundance of nonrotating H2O in the matrix should then be expected to increase as observed due to these “clustering” interactions. We also observe the shoulder at 1599.2 cm-1, which has been previously assigned19 to (H2O)2, to increase during the annealing process. This is further evidence for the “clustering” process. We note that this behavior has not been observed in previous photoionization experiments,12,20 where the rotating H2O band increased slightly in intensity upon annealing. That behavior was attributed to the decreasing electric field as ions in the matrix are neutralized upon annealing. The reasons for the different observations in the ν2 H2O region in these two experiments are not clear; perhaps the concentration of ions in our matrices is too small to observe the opposite effect. The “rotating” and “nonrotating” labels, referring to the final rotational state in each spectral transition, can be misleading. When sites are considered in which H2O could be either held rigid or allowed to rotate, the two observed features at 1595.8 and 1631.0 cm-1 could represent a single site in which rotation is possible. Only upon annealing, when the relative peak intensities change, can we state that more than one site is available and present for H2O molecules in this experiment. Finally, we note that annealing does not result in many new spectral features. In this regard, the most substantial feature that appears upon annealing grows in at 1189 cm-1. While we have considered possible “reaction products”, the peak remains unassigned. Its annealing profile most strongly parallels the CO4•- absorption at 1852 cm-1. A Comparison of the CO2+/- and CS2+/- Systems. When incoming ions with initial kinetic energies of more than 100 eV encounter the growing matrix, is the result simple trapping or do ions undergo “matrix chemistry”? Specifically, does collision-induced dissociation (CID) of the incoming ions occur, or are the ions decelerated by the matrix potential such that CID is insubstantial? Also, do the deposited ions react with the variety of low-concentration matrix components such as H2O, N2, etc.? In our depositions of CO2•+, there was no evidence in the spectra which suggested that the CO2•+ ions fragment as they encountered the matrix.2 Table 1 indicates that the CS2•+ ions are less stable with respect to fragmentation. Thus, incoming CS2•+ ions may be more likely to undergo CID reactions such as CS2•+ + Ne(s or g)
CS•+ + S CS + S•+
(1) (2)
These reactions have been extensively studied by Futrell and co-workers.21 Prinslow and Armentrout22 show that, at low collision energies, pathway 2 is the dominant CID pathway. As shown in Figure 1, CS is observed in our experiments. However, we suggest that CID of the mass-selected CS2•+ is not the dominant source of the CS molecules detected. Comparison of many experiments in which CS2 and CO2 are used as matrix additives reveals that the CS signal is clearly much more intense when CS2 is the additive. This suggests that the CS evolves from neutral CS2 rather than from the massselected ions. It may well be another product of ion bombardment of CS2 on the heat shield, which diffuses to the matrix. Even in the [CS2•+ f Ne:CO2] experiment, there is CS2 on the heat shieldsdue to diffusion of the CS2 introduced in the ion source into the region of the matrix. Thus, although CS is observed, it is probably not indicative of the CID process. As discussed earlier, CS•+ may be present in our matrices, but its low extinction coefficient makes its detection unlikely. While
Figure 5. Visible emission detected, following a 24 h [CS2•+ f Ne: CO2] deposition, upon repetitive cycling of the temperature between 6 and 10 K, at a rate of ∼0.25 s-1.
CID may occur to some extent, it does not appear to be a dominant process. In both the CO2•+ and CS2•+ depositions, intact cations are detected, and results suggest similar trapping efficiencies for both, even though the CS2•+ ions require less energy for fragmentation. Perhaps this is indicative of high average matrix potential, which is effective in lowering the kinetic energies of the incoming ions during the deposition process. The present results also suggest that matrix chemistry in which the incoming cation is converted into another chemical species does not hinder the experiment. In these lengthy depositions, even at very low pressures, a variety of background gases condense into the matrix and are candidates for reaction. The annealing data for the (CO2)n•- species show that matrix chemistry can occur, but it does not do so in the deposition step to any great extent. The spectra suggest that the ions are quickly incorporated into a rigid host upon deposition. Ions accumulated over a 24 h period undergo very little matrix chemistry with other components, whereas 1 min at 8-10 K provides sufficient mobility for such reactions to be observed. The data obtained for the CO2 and CS2 systems suggest that larger organic ions could be successfully trapped for subsequent analysis using the methods employed here. Finally, a comment should be made on the selection of CS2 vs. CO2 as a matrix additive. On the basis of its positive electron affinity, it was thought that CS2 would be a more effective additive for the generation of counterions. The results when the neon matrix gas is doped with CS2 during cation depositions show that its use is not preferable over CO2. The use of CS2 as the dopant results in relatively complex spectra, showing a number of absorptions in addition to that attributable to CS2•-. Also, from a spectroscopic perspective, use of CO2 is preferable since CO2•- can be detected at lower levels than CS2•-. We believe that, at the temperatures sampled in these cold diffusion experiments, increased mobility is achieved without substantial losses due to sublimation. As mobility increases, recombination and other types of reactions occur. We have collected additional data related to these processes. Controlled annealing experiments have been performed where a photomultiplier detector is attached to a vacuum chamber window such that emission from the matrix can be detected and recorded. Figure 5 displays the visible emission observed during the warming of a matrix after a 24 h [CS2•+ f Ne:CO2] experiment. Unlike the earlier annealing experiments, the temperature of the matrix during this experiment was set to oscillate over an approximate range of 6-10 K. Each increase in matrix temperature is accompanied by an increase in emission. A future publication will provide the analysis of the dispersed
Mass-Selected CS2•+ in Neon emission from annealing and matrix warming experiments. A combination of the IR data and the visible emission spectra provides a more complete description of the matrix components and their chemical evolution as a 4 K neon matrix containing ions warms and is vaporized. Acknowledgment. The authors thank the MSU Chemistry Department machine shop and electronic shop for their assistance in the construction and maintenance of the equipment. This work was supported in part by National Science Foundation Grant CHE 92-04136. Appendix UHF/6-311+G* calculations using the GAUSSIAN 92 program result in four vibrational frequencies for the CO2•+ cation. These are 446.4, 626.6, 933.4, and 1363.6 cm-1. The first two values are the bending (ν2) vibrational frequencies. The nondegeneracy of these two values is a typical result for UHF calculations involving linear, open-shelled, triatomic species.23 The ordering of the fundamental frequencies for linear symmetric triatomics is commonly ν2 < ν1 < ν3, as is true for the calculated results for CS2•+, CS2, and CO2. One might be tempted to assign the 933.4 and 1363.6 cm-1 frequencies as ν1 and ν3, respectively. However, the calculated atomic displacements and IR and Raman intensities clearly show that the higher frequency vibration (1363.6 cm-1) is due to a symmetrical stretching motion (ν1) and that the lower frequency (933.4 cm-1) belongs to an asymmetrical stretching motion (ν3). This calculated ν3 frequency is anomalously low as compared to the gas phase experimental value24 of 1423.1 cm-1. The reason for this discrepancy may be similar in nature to the restricted Hartree-Fock instability attributed25 to the potential energy function for CO2•+. References and Notes (1) Halasinski, T. M.; Godbout, J. T.; Allison, J.; Leroi, G. E. J. Phys. Chem. 1994, 98, 3930. (2) Godbout, J. T.; Halasinski, T. M.; Leroi, G. E.; Allison, J. J. Phys. Chem. 1996, 100, 2892.
J. Phys. Chem., Vol. 100, No. 36, 1996 14871 (3) Sabo, M. S.; Allison, J.; Gilbert, J. R.; Leroi, G. E. Appl. Spectrosc. 1991, 45, 535. (4) Lias, S. G.; Bartmess, J. E.; Liebman, J. F.; Holmes, J. L.; Levin, R. D.; Mallard, W. G. J. Phys. Chem. Ref. Data 1988, 17 (Suppl. 1). (5) Andrews, L.; Bondybey, V. E.; English, J. H. J. Chem. Phys. 1984, 81, 3452. (6) Frisch, M. J.; Trucks, G. W.; Schlegel; H. B.; Gill, P. M. W.; Johnson, B. G.; Wong, M. W.; Foresman, J. B.; Robb, M. A.; Head-Gordon, M.; Replogle, E. S.; Gomperts, R.; Andres, J. L.; Raghavachari, K.; Binkley, J. S.; Gonzalez, C.; Martin, R. L.; Fox, D. J.; Defrees, D. J.; Baker, J.; Stewart, J. J. P.; Pople, J. A. Gaussian 92/DFT, revision F.3; Gaussian Inc., Pittsburgh, PA, 1993. (7) (a) Chin, S.; Person, W. B. J. Phys. Chem. 1984, 88, 553. (b) Wong, M. W.; Wentrup, C.; Flammang, R. J. Phys. Chem. 1995, 99, 16849. (c) Sohlberg, K.; Chen, Y. J. Chem. Phys. 1994, 101, 3831. (d) Tseng, D. C.; Poshusta, R. D. J. Chem. Phys. 1994, 100, 7481. (e) Hiraoka, K.; Fujimaki, S.; Aruga, K.; Yamabe, S. J. Phys. Chem. 1994, 98, 1802. (f) Brommer, M.; Rosmus, P. Chem. Phys. Lett. 1993, 206, 540. (g) Froese, R. D. J.; Goddard, J. D. J. Chem. Phys. 1992, 96, 7449. (8) Smith, D. F.; Overend, J. J. Chem. Phys. 1971, 54, 3632. (9) Bondybey, V. E.; English, J. H. J. Chem. Phys. 1980, 73, 3098. (10) Miller, T. A.; Bondybey, V. E. In Molecular Ions: Spectroscopy, Structure and Chemistry; Miller, T. A., Bondybey, V. E., Eds.; NorthHolland: Amsterdam, 1983; p 132. (11) Fischer, I.; Lochschmidt, A.; Strobel, A.; Niedner-Schatteburg, G.; Mu¨ller-Dethlefs, K.; Bondybey, V. E. Chem. Phys. Lett. 1993, 202, 542. (12) Jacox, M. E.; Thompson, W. E. J. Chem. Phys. 1989, 91, 1410. (13) Jacox, M. E.; Thompson, W. E. J. Phys. Chem. 1991, 95, 2781. (14) Schallmoser, G.; Wurfel, B. E.; Thoma, A.; Caspary, N.; Bondybey, V. E. Chem. Phys. Lett. 1993, 201, 528. (15) Burkholder, J. B.; Lovejoy, E. R.; Hammer, P. D.; Howard, C. J. J. Mol. Spectrosc. 1987, 124, 450. (16) Gauyacq, D.; Horani, M. Can. J. Phys. 1978, 56, 587. (17) Colman, J. J.; Trogler, W. C. J. Am. Chem. Soc. 1995, 117, 11270. (18) Cataldo, F. Inorg. Chim. Acta 1995, 232, 27. (19) Forney, D.; Jacox, M. E.; Thompson, W. E. J. Mol. Spectrosc. 1993, 157, 479. (20) Jacox, M. E. Chem. Phys. 1976, 12, 51. (21) (a) Tosh, R. E.; Shukla, A. K.; Futrell, J. H. J. Phys. Chem. 1995, 99, 15488. (b) Shukla, A. K.; Tosh, R. E.; Chen, Y. B.; Futrell, J. H. Int. J. Mass Spectrom. Ion Processes 1995, 146/147, 323. (22) Prinslow, D. A.; Armentrout, P. B. J. Chem. Phys. 1991, 94, 3563. (23) Harrison, J. F., private communication. (24) Kawaguchi, K.; Yamada, C.; Hirota, E. J. Chem. Phys. 1985, 82, 1174. (25) Brommer, M.; Chambaud, G.; Reinsch, E.-A.; Rosmus, P.; Spielfiedel, A.; Feautrier, N.; Werner, H.-J. J. Chem. Phys. 1991, 94, 8070.
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