8408
J. Phys. Chem. 1996, 100, 8408-8417
Radical Ions of Acetylene in ZSM5 Zeolites: An EPR and Theoretical Study† E. A. Piocos, D. W. Werst,* and A. D. Trifunac Chemistry DiVision, Argonne National Laboratory, Argonne, Illinois 60439
L. A. Eriksson* Department of Physics, UniVersity of Stockholm, Box 6730, S-113 85 Sweden ReceiVed: NoVember 7, 1995; In Final Form: January 23, 1996X
The cis-bent form of the acetylene radical anion (aiso(H) ) 65 G) was observed following radiolysis of acetyleneloaded HZSM5 but not NaZSM5. The acetylene radical cation is not trapped by the matrix at 77 K and undergoes ion-molecule reactions to form multimer ions, including the benzene cation. Benzene is also formed by acid-catalyzed polymerization of acetylene on HZSM5. The evidence that acetylene (IP ) 11.4 eV) is ionized places a new lower limit on the range of ionizable species in the radiolyzed zeolite. The energies, geometries, and hyperfine coupling constants were calculated for the acetylene, vinylidene, cyclobutadiene and butatriene radical anions and radical cations, and the benzene radical cation, using different levels of theory, including gradient-corrected density functional theory.
1. Introduction There is considerable interest in chemical reactivity at interfaces and in constrained spaces, both from the standpoint of understanding the fundamental interactions between surfaces and reactive intermediates and of exploring ways to gain control over chemical reactions. Zeolites have attracted much attention for reaction studies because of the unique interplay between diffusional effects (molecular sieving) and electrostatic interactions. Zeolites are useful for studies of radiation chemistry because they offer opportunities for the study of charge and energy transfer in a well-ordered solid state that can be chemically modified by changing the Si/Al ratio and/or the charge-balancing cations. A series of EPR studies in our laboratory have utilized zeolites for matrix-isolation of radiolytically generated radical cations.1-8 Zeolites are fertile test beds for studying fundamental reactions of radical cations. The range of compounds that can be ionized by matrix holes is limited by the effective ionization potential (IP) of the zeolite. Previous experiments have shown that irradiated zeolites are capable of ionizing molecules, M, with IPs(gas) as high as 10-10.5 eV, and perhaps higher. γ
Z 98 Z+ + e-(trapped)
(1)
Z+ + M f Z + M+
(2)
The low-temperature stabilization of radical cations in zeolites implies the trapping of electrons that would otherwise recombine with the positive holes (eqs 1 and 2). The nature of the electron traps varies and is not always well defined. Electrons can be trapped by lattice defects9 or as reduced sodium ion clusters, e.g., Na43+.10 Electrons can also reduce adsorbed molecules to form radical anions.11-13 Thus, both radical cations and radical anions can be generated radiolytically in zeolites and their properties and reactivities studied. Radical ion intermediates in turn are interesting probes of the microporous solid lattice and its influence on ensuing chemical reactions. † Work at Argonne performed under the auspices of the Office of Basic Energy Sciences, Division of Chemical Science, US-DOE under Contract Number W-31-109-ENG-38. X Abstract published in AdVance ACS Abstracts, April 15, 1996.
S0022-3654(95)03294-1 CCC: $12.00
The present paper illustrates many of the features and potentials of radiolysis/EPR studies in zeolites: ion-molecule reactions, observation of elusive species, and chemical insights gained by manipulation of zeolite composition. Acetylene attracted our attention because (1) it is a fundamentally important hydrocarbon molecule, the smallest alkyne, (2) its radical cation has never been observed by EPR, and (3) it has a high ionization potential (IP(gas) ) 11.4 eV),14 which helps explain the difficulty of forming the radical cation. Acetylene affords an opportunity to probe the upper bound of ionizable species in radiolyzed zeolites. Although we failed to trap the acetylene monomer radical cation in ZSM5 zeolites at 77 K, its participation in cation-molecule reactions was indicated by formation of the benzene radical cation. In HZSM5, but not in NaZSM5, we observed the radiolytic formation of the acetylene radical anion. Thus, the anion of acetylene is significantly stabilized in HZSM5 relative to the gas phase (electron affinity of acetylene in the gas phase is negative: -1.8 eV15). The understanding of the experimental results is aided by the theoretical investigation of the observed and related radical ions. 2. Experimental Section 2.1. Methods and Materials. The zeolites HZSM5-X (X ) 50, 240, 400) and NaZSM5-X (X ) 40, 170, 1000), where X denotes the Si/Al ratio, were kindly donated by Chemie Uetikon of Switzerland. Acetylene was obtained from two sources: AGA Specialty Gases and Aldrich. The former (99.9% pure) was dissolved in acetone, and the latter was diluted to 5% in nitrogen. Acetylene-d2 was obtained from Matheson and Cambridge Isotopes. Acetylene enriched to 99% 13C was also from Cambridge Isotopes. Benzene (HPLC grade) and benzened6 (99.5% deuterium) were from Aldrich. Purification was done by repeated freeze-pump-thaw cycles using liquid nitrogen. Zeolite samples were prepared on a glass vacuum manifold in 4 mm Suprasil sample tubes. The sample tube containing 50 mg zeolite powder was evacuated (e10-4 Torr) and heated to 450 °C for 4-6 h. A measured amount (0.05-1% by weight) of acetylene was condensed into the sample tube by using liquid nitrogen, and then the tube was sealed under vacuum. Each sealed sample was equilibrated at a specified temperature © 1996 American Chemical Society
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J. Phys. Chem., Vol. 100, No. 20, 1996 8409
Figure 1. EPR spectra obtained at 70 K after 77 K γ-irradiation of (a) HZSM5-400, (b) HZSM5-240, and (c) HZSM5-50 containing 0.1% acetylene.
between 77 K and room temperature for a specified time and then stored in liquid nitrogen until it was irradiated at 77 K with a 60Co γ source to a total dose of approximately 0.3 Mrad. Where equilibration conditions are not specified, the samples were equilibrated overnight at room temperature and then quenched in liquid nitrogen. No EPR signals were detected in unirradiated samples. EPR spectra of the irradiated samples were collected at temperatures between 4 K and room temperature. Spectral changes were tested for reversibility. Temperature control was accomplished with a LTR-3 liquid-helium-transfer Heli-Tran cryostat (APD) anchored in the cavity of the Varian E109 EPR spectrometer operating at a microwave frequency of 9.31 GHz. Magnetic field control and data acquisition was accomplished by a LabVIEW (National Instruments) program on a Macintosh II computer. 2.2. Results. Selected EPR spectra obtained in acetyleneloaded HZSM5 and NaZSM5 are shown in Figures 1 and 2. The spectra can be interpreted as combinations of EPR signals of two different radical ion species (Vide infra) and other unidentified species, whose relative contributions depended on the sample composition and preparation. By systematically varying the (1) acetylene concentration, (2) Si/Al ratio of the zeolite, (3) sample equilibration conditions prior to irradiation, and (4) annealing temperature (i.e., the temperature at which the EPR spectrum was obtained) for both NaZSM5 and HZSM5, we were able to identify critical trends in the EPR intensities of these two radical ions against an “interfering background” due to the unidentified species. In the following, we concentrate on the salient features of this matrix of data that support the assignments of the two radical ions. In HZSM5, two different EPR signals could be analyzed as a triplet with a spacing of approximately 65 G and another multiplet (at least five lines) with a spacing of 4.5 G, respectively. The intensity ratio of the 65 G signal to the 4.5 G signal decreased with the Si/Al ratio. The 65 G EPR signal is noticeably anisotropic, and the central line is distorted or
Figure 2. EPR spectra obtained at 70 K after 77 K γ-irradiation of (a) NaZSM5-1000, (b) NaZSM5-170, and (c) NaZSM5-40 containing 0.1% acetylene. Part d shows the EPR spectrum obtained at 130 K after 77 K γ-irradiation of NaZSM5-40 containing 0.1% acetylene.
obscured by other interfering signals. However, the intensity ratio of the low-field and high-field lines was constant as a function of temperature and acetylene concentration, as well as from sample to sample; these lines clearly belong to the same species. The spectra obtained in NaZSM5 are dominated to a greater extent by signal intensity due to two or more unidentified species (Figure 2a-c). Nevertheless, we can conclude that the 65 G triplet is never observed in NaZSM5, since there are no interfering signals at the magnetic field positions of the lowand high-field resonances. The 4.5 G multiplet is strongly masked by other signals, but it is clearly observable at the center of Figure 2a (NaZSM5-1000)seven before annealing. A search of concentration and temperature to maximize this signal relative to other interfering signals led to the EPR spectrum in Figure 2d, which very clearly shows the existence of the 4.5 G EPR signal in a NaZSM5-40 sample. To identify the 4.5 G species, we considered the radical ions of acetylene multimers. Possible C4H4 radical ions were treated theoretically (section 3). Benzene, whose radical cation in other solid matrices exhibits a seven-line EPR spectrum with hyperfine coupling constant (hfcc) of 4.3-4.5 G,16 was irradiated in NaZSM5-170 and HZSM5-50. In both zeolites the seven-line EPR spectrum of the benzene radical cation was observed. The benzene radical anion, which has a smaller hfcc of 3.75 G,17 was not observed. The EPR spectra of C6H6+ and C6D6+ generated on HZSM550 are compared to their counterparts generated from acetylene and acetylene-d2, respectively, in Figure 3. Anisotropy of the
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Figure 4. EPR spectra obtained at 70 K after 77 K γ-irradiation of HZSM5-240 containing 1% (a) acetylene-d2 or (b) H13C13CH. Part c shows a simulated powder EPR spectrum of H13C13CH-. The values of Aiso, Tx, Ty, and Tz used were (in gauss) 65, 3, 1, and -4 for H and 130, 20, -8, and -11 for C. The other parameters used were gx ) 2.0008, gy ) 2.0023, gz ) 2.0032, R(H) ) 30°, and R(C) ) 60°.
Figure 3. EPR spectra obtained at 110 K after 77 K γ-irradiation of HZSM5-50 containing (a) 0.1% acetylene, (b) 1% benzene, (c) 0.1% acetylene-d2, and (d) 0.1% benzene-d6.
C6H6+ spectrum causes a deviation from the binomial intensity distribution, and broadening of the outer lines makes them difficult to detect. Agreement of the spectral shape and the exact coincidence of the hfcc in the benzene and acetylene samples strongly support the assignment of the 4.5 G signal to the benzene radical cation. Radiolysis of acetylene-d2 in HZSM5-50 gave rise to the multiplet EPR signal with 0.72 G spacing shown in Figure 3c. Figure 3d shows the result for C6D6+. The hfcc are identical. The broad line at high field in Figure 3c is an EPR signal from another species. It is interesting that the spectrum in Figure 3c shows little sign of H/D exchange. Upon deuterium substitution, the 65 G triplet was replaced by a quintet with a spacing of approximately 10 G (Figure 4a), which confirms the assignment to a species with two equivalent hydrogens. The quintet is distorted by anisotropy and by the presence of an underlying signal. The quintet structure decayed above 150 K, leaving a broad, unresolved singlet. The 4.5(H)/ 0.72(D) G signals also decayed at approximately 150 K in the acetylene samples. The EPR spectrum obtained for a 13C-enriched sample of acetylene in HZSM5-240 showed weak new satellite lines with a spacing of approximately 60 G from the positions of the outer triplet lines in the nonenriched sample. The stronger one at low field is shown at higher gain in Figure 4b. The positions of the new satellite lines are consistent with an Aiso(C) of approximately 60 or 125 G. In either case, the positions of other satellite lines would fall very near the positions of the low-field and high-field lines of the unlabeled spectrum. A 125
G carbon hfcc would also place an extra pair of satellite lines 65 G to the outside of the observed ones. The low intensity of the satellite lines implies that the anisotropy of the carbon hyperfine coupling is large, which is consistent with theoretical results (Vide infra). Using as a guide the previous analysis of Kasai35 and the predicted anisotropy in the hyperfine couplings (section 3.2), we generated the simulated powder spectrum in Figure 4c. The simulation illustrates the diminution of the intensities of 13C satellite lines and overlap with the positions of the high-field and low-field lines of the unlabeled spectrum caused by the predicted anisotropy. If an Aiso(C) of approximately 60 G is assumed instead, anisotropy can likewise cause the 13C satellite lines to be “hidden” or very weak in the simulated powder spectrum. Owing to the low intensity in the satellite lines in the labeled EPR spectrum, the experimental value of the carbon hfcc remains uncertain. The strong similarity between the triplet hyperfine structure in the EPR spectrum observed in acetylene-loaded HZSM5 and EPR spectra of alkali metal-acetylene complexes (Table 1) prompted us to test if this species could be a radical anion. Introduction of the electron scavenger, CFCl3, together with acetylene (1% of each) in HZSM5-240 prior to radiolysis caused a marked decrease (∼80%) in the EPR intensity of the 65 G species. This is strong evidence that this paramagnetic species is generated by trapping an electron. While the 4.5 G species was still formed in the presence of the coadsorbed electron scavenger, the concomitant increase of other signals precluded a quantitative estimate of its relative intensity vs samples without scavenger. As a test for transformations of acetylene in HZSM5 prior to radiolysis, we investigated the dependence on equilibration conditions prior to 77 K γ-irradiation for HZSM5-50 samples with 1% acetylene (Figure 5). The normal, overnight equilibration at room temperature gave a very weak 65 G triplet, as in
Radical Ions of Acetylene
J. Phys. Chem., Vol. 100, No. 20, 1996 8411
Figure 5. EPR spectra obtained at 70 K after 77 K γ-irradiation of HZSM5-50 containing 1% acetylene. The sample was equilibrated at (a) room temperature overnight, (b) room temperature for 30 min and stored at 77 K overnight, and (c) 77 K overnight prior to irradiation.
TABLE 1: Literature Values of the Hyperfine Coupling Constants (in G, 1 G ) 0.1 mT) of HCCH- and Alkali Metal-Acetylene Complexes principal values species, matrix
giso, Aiso
x
y
Li+HCCH-,
2.0028 63.3 2.0023 66.5 74 ( 3 2.0023 73.0 61.2 74.4 67.0 93.5 2.0020 48 14-15
2.0039 59.6 2.0008 71.0
2.0039 66.1 2.0029 66.5
Ar g H Ar g H 13C K+HCCH-, Ar g H H Li+HCCHPWP/IGLO-III 13C H K+HCCHPWP/IGLO-III 13C HCCH•-, 3MP g H 13C Li+HCCH-,
66.3 92.3 72.4 122.3 2.0011 53 0-2
60.2 65.5 65.8 83.9 2.0023 47 42
z
geometry ref
2.0006 cis-bent 64.3 2.0032 cis-bent 62.0
46
cis-bent
35
35
57.1 cis-bent 33 65.4 62.8 cis-bent 33 83.9 2.0027 trans-bent 34 43 0-2
Figure 1c. The 4.5 G signal is superimposed on a much stronger background of other signals compared to the result obtained with only 0.1% acetylene in HZSM5-50. Shortening the sample-equilibration time before transfer to liquid nitrogen (Figure 5b, 30 min at room temperature), or lowering the equilibration temperature, caused the intensity of the 65 G triplet to be enhanced. Carried to the extreme of adsorption at 77 K and overnight storage at 77 K prior to irradiation, this approach gave the result in Figure 5c. The relative intensities of the lowfield triplet lines in Figures 5a-c are approximately 1:4:8. 3. Calculations 3.1. Theory. In order to gain further insight into the experimental results and explore structures of possible inter-
mediates, we have investigated the acetylene, vinylidene, cyclobutadiene and butatriene radical anions and cations, and the benzene radical cation at several different levels of theory by calculating geometries, spin densities, and hyperfine structures. All ab initio and density functional calculations have been performed using the IGLO-III orbital basis set,18 which is based on Huzinaga’s lls,7p primitive basis for carbon and to which two d-functions are added to describe polarization effects.19 For hydrogen, the corresponding basis is 6s + 2p. The basis sets are loosely contracted in the inner s- and spregion to have the final form [7s,5p,2d] for C and [3s,2p] for H. We have used three common gradient-corrected density functional methods: the Becke exchange,20 together with the Lee-Yang-Parr correlation correction21 (BLYP), Becke’s three-parameter exchange correction22 and the LYP correlation (B3LYP), and the exchange correction by Perdew and Wang23 together with the 1986 correlation correction by Perdew24 (PWP). In addition we have also performed geometry optimizations and spin density calculations using second order MøllerPlesset perturbation theory (MP2). The auxiliary basis set, used for the fitting of the charge density and the exchange-correlation potential in the PWP calculations, is (5,2;5,2) for carbon and (5,1;5,1) for hydrogen.25 These have previously been employed most successfully together with the IGLO bases and the PWP gradient correction for geometry optimization and hfcc calculations.26-28 All four methods (BLYP, B3LYP, PWP, and MP2) were employed in the study of the acetylene/vinylene radical anions. The acetylene cation system and the larger ions with four or six carbon atoms were investigated at the PWP/IGLO-III level of theory only. The programs employed were Gaussian 92/DFT29 (MP2, BLYP, and B3LYP calculations) and deMon30 (PWP calculations). All calculations have been performed on Dec Alpha workstations. 3.2. Acetylene/Vinylidene Anions. There are very few previous theoretical studies of the acetylene/vinylidene anion system31,32 and none where the hyperfine coupling constants of the species have been calculated. In contrast with the neutral species, where the two minimum conformers are separated by about 35-40 kcal/mol and where the barrier for the rearrangement reaction of vinylidene to acetylene (the backwards reaction) is only about 5 kcal/mol,31,32 it is found that the two anionic minima lie very close in energy. The energy barrier separating HCCH- from CCH2- is about the same as in the neutral case: 40-45 kcal/mol. In Table 2 we show the calculated optimized geometries, relative energies, and hyperfine coupling constants obtained at the PWP, B3LYP, BLYP, and MP2 levels of theory using the IGLO-III basis set. As seen, the geometries obtained with the different methods are very similar. Energetically, the transisomer of acetylene lies roughly 5 kcal/mol below the cis-isomer and is almost equal in stability to the vinylidene anion. These findings agree well with the previous ab initio HF, MPPT, and CI results obtained by various groups.32 There is also a strong similarity between the geometries and energetics of the isolated anion system and the alkali metal-HCCH/CCH2 charge-transfer complexes, supporting the anionic nature of the hydrocarbon moiety in those cases.33 In the previous theoretical study of the alkali metal-acetylene interactions, it was shown that the PWP/IGLO-III level of theory yields hyperfine couplings within a few gauss for all constituent atoms in these systems (cf. Table 1 and ref 33). We also note that there is a very good agreement between our calculated
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TABLE 2: Geometries, Isotropic, and Dipolar (PWP Only) Hyperfine Coupling Constants (in G, 1 G ) 0.1 mT) and Relative Energies of the Trans and Cis Conformers of the Acetylene Anion and the Vinylidene Anione PWP RCCb RCH AHCC Aiso(C) Adip(C)c Aiso(H) Adip(H) E(au) RCC RCH AHCC Aiso(C) Adip(C) Aiso(H) Adip(H) ∆E(kcal/mol)d RCC RC′H AHC′H Aiso(C) Adip(C) Aiso(C′) Adip(C) Aiso(H) Adip(H) ∆E(kcal/mol)d
B3LYP trans-HCCHa 1.302 1.101 123.8 29.22
1.310 1.108 123.5 29.93 12.10 48.85 51.41 3.05 -77.426 47 -77.320 48 1.291 1.117 132.4 132.15 8.39 65.00 2.52 4.98 1.338 1.117 124.3 18.60 24.15 -16.03 1.91 53.82 1.48 1.97
BLYP
MP2
1.313 1.109 123.3 29.76
1.311 1.097 123.2 14.06
50.83
48.13
-77.288 22 -77.081 51
cis-HCCH 1.284 1.107 132.0 130.14
1.292 1.116 132.3 131.19
1.292 1.103 131.8 114.33
67.50
67.23
64.07
4.58
4.91
6.67
CC′H2 1.334 1.107 124.0 22.88
1.340 1.117 124.3 20.76
1.329 1.100 123.5 10.39
-17.44
-15.06
-2.80
53.18
54.26
36.68
-0.28
2.13
4.31
a Both trans and cis-HCCH- are assumed to be planar. The dihedral angle is defined as 180° and 0° in the two cases, respectively. b Geometries in angstroms and degrees. c Adip is defined as 1/2 Tzz. d Energy relative to trans-HCCH-. e All calculations were done using the IGLO-III basis set.
hydrogen hfcc for the trans-HCCH anion (Table 2) and the matrix-isolation data by Matsuura and Muto (cf. Table 1 and ref 34). Our calculated value of 48.85 G (PWP/IGLO-III) is in excellent accord with their value of 48.7 G. Our values for the anisotropic couplings are also in good agreement with their data. Of less satisfactory agreement are the isotropic carbon couplings. Our calculated isotropic hfcc are roughly twice of that by Matsuura and Muto. The dipolar coupling, on the other hand, is in very good agreement with their value of 11 G. From the calculations we also note that there is good agreement in the isotropic hyperfine data computed using the different DFT methods, whereas the MP2 results seem less reliable. For both cis- and trans-HCCH-, the proton couplings are well described at the MP2 level, whereas the method largely underestimates the carbon hfcc in these species and on all the nuclei in CCH2-. The discrepancy between experimental and theoretical carbon couplings in the case of trans-HCCH- in alkane matrices may stem from two sources. The lack of diffuse functions in the basis sets would yield an inappropriate description of the spin density on the carbons and poor hfcc. One would expect, however, that this discrepancy would render smaller, rather than larger, theoretical hfcc compared with experimental values. A second argument against this source of deviation is that the PWP/IGLO-III method led to predictions within a few percent of experimental data for the hfcc of all atoms, including carbon, in the M+HCCH- complexes33 where the hydrocarbon moiety carries almost unit negative charge. For a small number of neutral or anionic radicals (CN, CH3, HCO, FCO, HCN-, FCN-,27 C3H5, C5H728), the agreement for the carbon hfcc
Figure 6. PWP/DZP and IGLO-III (H′-HCCH fragment only) optimized geometries of the HZSM5 model: (a) neutral zeolite; (b) anionic zeolite; (c) HZSM5-HCCH anionic complex.
TABLE 3: Atomic hfcc (in G, 1 G ) 0.1 mT) Calculated for the HZSM5-HCCH Anionic Complexa atom H′ C H (O (Al (Si
Aiso
Adip
atomic charge
-5.5 118.6 63.8 7.2 0.3 -0.2
-2.5 9.4 2.7 -2.0 0.1 0.1
-0.58 -0.46 0.03 -0.67) 0.13) 0.48)
a The structure is fully optimized at the PWP level, using DZP bases on the H3Al-O-SiH3 fragment and IGLO-III on the remaining atoms. For atomic labeling, see Figure 6c.
(isotropic and anisotropic) has in all cases ranged from good to excellent. A second, equally likely source of the low experimental 13C couplings may be from interactions with the matrix. Different matrices are known to modify the spin density distribution in radicals. Since we are here dealing with a system with a very diffuse SOMO, interactions with matrix molecules should not be ruled out. In an attempt to investigate the interaction between HCCHand the HZSM5 matrix, a cluster model consisting of HCCH and H3Al-OH-SiH3 was constructed. The HZSM5 model cluster was optimized at the PWP level of theory with and without the acetylene group present. We used the DZP basis sets in the acetylene-free optimizations and for the H3Al-OSiH3 fragment of the full complex. IGLO-III bases were used for the OH proton and acetylene in the complex. A mirror plane was retained, which contained the H-Al-O-Si-H atoms, and the HCCH group was placed symmetrically above/below this plane. The optimized geometries of the neutral and anionic acetylene-free zeolite and the full complex (anionic) are shown in Figure 6. In Table 3 we list the corresponding hfcc for the HZSM5-HCCH anionic complex along with the atomic partial charges. The local geometry of the neutral acetylene-free cluster agrees very well with previously reported structures at the semiempirical, HF, or LDA levels.36-38 The geometry of the zeolite cluster changes significantly when it is given a unit negative charge (Figure 6b). In particular, the Al-O and Si-O bonds undergo large changes. The unpaired electron resides in the HZSM5 model almost exclusively on the Si atom (F(R-β) ) 0.83). The main part of the negative charge, on the other hand, is found on the oxygen atom. When the acetylene molecule is added to the HZSM5 anion (Figure 6c), the O-H bond becomes elongated and the Si-O
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J. Phys. Chem., Vol. 100, No. 20, 1996 8413
TABLE 4: Optimized Geometries (in Angstroms and Degrees), Relative Energies, and Isotropic hfcc (in G, 1 G ) 0.1 mT) for the Acetylene/Vinylidene Cation System, Obtained at the PWP/IGLO-III Level of Theory RCC RCH AHCC ∆E (kcal/mol) Aiso (C) Adip (C) Aiso (H) Adip (H)
HCCH+
CC′H2+
1.253 1.085 180.0 0.00a 9.83 17.66 -17.61 5.03
1.391 1.108 120.8 45.6 6.67/21.94b 15.57/18.59 -16.58 5.29
a The total energy of the (linear) acetylene radical cation is -77.046 29 au. b The values for the vinylidene carbon atoms are C/C′, where C′ is the central carbon.
and Al-O bonds become more similar to those of the neutral case (Figure 6a). The angles around the central oxygen atom are also modified. The acetylene moiety accepts almost a unit of negative charge from the zeolite, 0.866 electrons, in close analogy to the M-HCCH systems (M ) Li, Na, K) studied previously.33 The HZSM5 part of the complex is hence more similar to the neutral, acetylene-free zeolite cluster than to the anionic cluster. In the complex, the unpaired electron resides almost exclusively on the acetylene fragment (F(R-β) ) 0.39 on each C, 0.10 on each H), in agreement with the charge-donation model mentioned above. The acetylene proton hfcc are changed little relative to the isolated cis-HCCH- (Table 2), but the isotropic 13C hfcc are decreased by about 14 G from 132 to 118 G. The anisotropic couplings remain essentially unaltered. We also note that the hfcc of the acetylene protons are very similar to those in the M+HCCH- complexes listed in Table 1, whereas the carbon couplings show a large dependence on the complexing system. The OH proton of the zeolite obtains a nonzero hfcc, according to the theory, although no doublet splitting of 5.5 G was observed in the experimental EPR spectrum. The DFT results then support an interpretation of the experimental spectrum with the larger value of Aiso(C) (∼125 G). The values of Aiso, Tx, Ty, and Tz used in the simulated powder EPR spectrum (Figure 4c) agree well with the DFT values: vacuum, H, 65.0, -3.7, -1.3, 5.0; vacuum, C, 132.1, -8.9, -7.9, 16.8; model cluster, H, 63.8, -4.3, -1.1, 5.4; model cluster, C, 118.6, -9.7, -9.1, 18.9. The different labeling of the coordinate axes merely reflects how the computed axes were chosen to be oriented. Our results for the HZSM5-HCCH model cluster show some interesting features and indicate the need for further study to fully understand the complicated interactions between the guest molecule and the matrix, including possible influence on the isotropic carbon couplings. Future work will involve refined matrix models as well as improved basis sets. It is probable that both of these improvements will contribute to a more accurate picture. The DFT-EPR approach employed here seems like a reasonable way to combine high accuracy with studies of large systems that otherwise would be computationally impossible to treat. 3.3. HCCH+/CCH2+ System. We have calculated the geometries and isotropic hfcc’s for the cationic acetylene/ vinylidene system at the PWP/IGLO-III level of theory only. The results are presented in Table 4. The geometry of the acetylene cation is in very close agreement with the structure obtained at the CISD/DZP level by Schaeffer and co-workers,39 and the vinylidene cation is found to lie 46 kcal/mol above the acetylene cation. In this respect, the cationic system shows large similarities with the neutral system (in which CCH2 lies 43 kcal/
TABLE 5: Optimized Geometries (in Angstroms and Degrees), Relative Energies, and hfcc (in G, 1 G ) 0.1 mT) for the Cyclobutadiene and Butatriene Radical Cations, Obtained at the PWP/IGLO-III Level of Theoryd
RCC RCC′ RC′H AHC′C DHC′C′H ∆E (kcal/mol) Aiso (C) Adip (C) Aiso (H) Adip (H)
c-C4H4+ (rectangle)
H2CCCCH2+ (planar)
H2CCCCH2+ (twisted)
1.509 1.381 1.087 134.9
1.327a 1.290 1.098 121.8 0.0 7.09 4.38/-12.01c 13.43/1.54 48.17 1.50
1.269 1.327 1.093 121.1 53.1 -5.56 -3.35/5.09 8.81/9.92 10.26 3.30
0.0b 5.23 9.09 -7.82 2.92
a C denotes the inner (dCd) carbon atoms and C′ the outer (dCH ) 2 carbons. b The total energy of the cyclobutadiene radical cation is -154.659 21 au at the PWP/IGLO-III level of theory. c The values are for the inner(C)/outer(C′′) carbon atoms. d The relative energies are with respect to the cyclobutadiene radical cation.
TABLE 6: Optimized Geometries (in Angstroms and Degrees), Relative Energies, and hfcc (in G, 1 G ) 0.1 mT) for the Cyclobutadiene and Butatriene Radical Anions, Obtained at the PWP/IGLO-III Level of Theoryd
RCC RCC′ RC′H AHC′C DHC′C′H ∆E (kcal/mol) Aiso (C) Adip (C) Aiso (H) Adip (H)
c-C4H4(rectangle)
H2CCCCH2(planar)
H2CCCCH2(twisted)
1.519 1.400 1.090 135.3
1.261a 1.328 1.088 121.6 0.0 -37.4 -5.96/5.44c 2.23/5.32 -3.49 1.61
1.261 1.347 1.089 122.2 6.7 -38.8 -5.92/4.44 2.18/5.18 -3.50 1.51
0.0b 3.48 7.09 -6.01 2.20
a C denotes the inner (dCd) carbon atoms and C′ the outer (dCH2) carbons. b The total energy of the cyclobutadiene radical anion (D2h symmetry) is -154.591 03 au at the PWP/IGLO-III level of theory. c The values are for the inner(C)/outer(C′) carbon atoms. d The relative energies are with respect to the cyclobutadiene radical anion.
mol above HCCH at the PWP/IGLO-III level) rather than the anionic one. In this work we have not considered possible reaction mechanisms or searched for transition states in the conversion between HCCH+/- and CCH2+/-. In the ionization process, the CC and CH bonds of both species lengthen somewhat, whereas the HCC angle in vinylidene is almost unchanged. For acetylene, both the neutral and the positively charged species are linear. The hfcc of the HCCH and CCH2 cations are markedly different from those of the corresponding anions and do not agree at all with the experimental couplings. 3.4. Addition Complexes: C4H4+ and C4H4-. To assess possible structures for the multimer ion (Aiso ) 4.5 G) discussed in the Experimental Section, we performed calculations for cyclobutadiene, butatriene, and benzene (benzene is found in section 3.5). Both the positive and negative ions of cyclobutadiene and butatriene were investigated using the density functional PWP/IGLO-III level of theory. The results for the cations are displayed in Table 5 and for the anions in Table 6. The geometry obtained at the PWP level for the rectangular (D2h symmetry) cyclobutadiene radical cation is in reasonable agreement with that from previous calculations, although not perfectly overlapping. The carbon-carbon bonds are 1.381/ 1.509 Å at the PWP/IGLO-III level, to be compared with 1.414/ 1.525 Å (CI/STO-3G40) and 1.385/1.497 Å (MP2/6-31G* 41). The PWP and MP2 calculations thus give very similar structures,
8414 J. Phys. Chem., Vol. 100, No. 20, 1996 with the CI results yielding more elongated bonds. The calculated isotropic proton hfcc is -7.8 G using gradientcorrected DFT. The geometry of the rectangular, planar cyclobutadiene anion is similar to that of the cation except for slightly more elongated bond lengths. Also, the hfcc are similar to those of the cation and are numerically 0.5-2 G smaller for all atoms. For the butatriene radical ion systems, two different conformers have been investigated: the fully planar form and the structure where we allow for a torsional angle to be introduced about the CH2 groups. Both of these structures have a linear carbon backbone. No systems with a bent carbon framework were considered in the present study. In the cationic case, the optimized torsional angle is 53.1°, and the relaxed (twisted) structure is found to lie 13 kcal/mol below the planar one. This energy difference is larger than what was found in the AM1 study by Kubonzo et al.42 although the torsional angle is similar (AM1 value is 45°). The experimental value for the isotropic proton hfcc of the butatriene radical cation varies from 7.5 to 11.2 G, depending on the matrix used and on the temperature.42,43 Our calculated value of 10 G falls within this range. The main geometrical parameter affecting the proton hfcc, due to the overlap between the various p-orbitals on the carbon atoms, is the HC′C′H torsional angle. Experimentally, this angle depends on the “stiffness” of the medium.43 This is also one of the more difficult geometric parameters to calculate accurately using any level of theory. Because of the flatness of the vibrational potential in the region of the minimum, the observed hfcc at the experimental temperatures used is most likely a vibrational average over a range of at least (15°. Contrary to the findings for the cyclobutadiene system, there are large differences in geometry and spin density distribution between the radical cation and the radical anion of butatriene. For the planar butatriene anion, we gain 37 kcal/mol in energy relative to the cyclobutadiene anion, and both the carbon and proton hfcc are largely modified. This energy difference can be due to two different effects: either the planar cyclobutadiene anion lies very high in enery and has a distorted or twisted (nonplanar) ground state equilibrium geometry or the energy difference is related to the larger stabilization energy when forming the vinylidene anion, as compared to the corresponding HCCH f CCH2 cation system. The butatriene radical anion does not undergo such a dramatic increase in torsional angle when releasing the planar constraint, as does the cation; the optimized torsional angle is only 6.7° at the PWP/IGLO-III level. Consequently, the planar and twisted butatriene anions lie much closer in energy (the stabilization energy is only 1.4 kcal/mol, roughly 1/10 of the cation system), and the changes in proton hfcc are very minor. 3.5. Benzene Cation. Like the cyclobutadiene system, the benzene radical cation is Jahn-Teller active. Upon removal of a π-electron from the doubly degenerate HOMOs (e1g), the cation distorts to either a 2B2g or a 2B1g state. In Table 7 we list the PWP/IGLO-III optimized geometries and hfcc of both states. The 2B2g state corresponds to a flattened structure in which two adjacent C-C bonds (C′-C′ in Table 7) are shortened, and the four remaining C-C′ bonds are elongated. Thereby, the C′-C-C′ bond angles open up slightly compared with the neutral, symmetric case from 120° to 121.4°. For the 2B state, on the other hand, the opposite effects are observed. 1g Here, the two C′-C′ bonds become elongated, the four C-C′ bonds shorten, and the C′-C-C′ bond angles decrease to 118.6°.
Piocos et al. TABLE 7: Optimized Geometries (in Angstroms and Degrees), Relative Energies, and hfcc (in G, 1 G ) 0.1 mT) for the Benzene Radical Cation, Obtained at the PWP/ IGLO-III Level of Theory state
geometry
2B 2g
RCH ) 1.089 RCC′ ) 1.433 RC′H′ ) 1.087 RC′C′ ) 1.374 AC′CC′ ) 121.4 ACC′C′ ) 119.3 AH′C′C′ ) 121.1
C(2) C′(4) H(2) H′(4)
RCH ) 1.087 RCC′ ) 1.392 RC′H′ ) 1.088 RC′C′ ) 1.456 AC′CC′ ) 118.6 ACC′C′ ) 120.7 AH′C′C′ ) 118.6
2
B1ga
a
nucleus
Aiso
Adip
13.1 -2.6 -10.5 -1.6
13.6 1.4 3.7 1.3
C (avg) H (avg)
2.6 -4.6
5.4 2.1
C(2) C′(4) H(2) H′(4)
-7.1 7.6 1.8 -7.1
1.2 9.3 0.3 2.8
C (avg) H (avg)
2.7 -4.1
6.6 1.5
The 2B1g state lies 0.21 kcal/mol above the 2B2g state.
The optimized geometries agree very well with those obtained in earlier UHF/6-31G and 6-31G(d) studies,44,45 and the computed hfcc are also in very good agreement with the previous CISD/[5s,4p/4s] data.45 In the work by Huang and Lunell,45 the 2B2g state was concluded to be the most stable one, lying 1.83, 3.43, and 0.47 kcal/mol below the 2B1g state at the MP2, MP4, and CISD levels, respectively. The PWP/IGLOIII computed energy difference is 0.21 kcal/mol. This also agrees with the MP2 data by Raghavachari et al.44 whereas their CISD data, as well as the UHF results of both previous studies, yield the opposite order. Experimental determination of the proton hfcc of the benzene radical cation, obtained by Iwasaki et al. in a freon matrix at 4 K,16 gave a(2H) ) -8.2 G and a(4H) ) -2.4 G. This agrees well with our results for the 2B2g state (a(2H) ) -10.5 G and a(4H) ) -1.4 G), whereas the data for the 2B1g state differ considerably (a(2H) ) 1.8 G and a(4H) ) -7.1 G). Our PWP/ IGLO-III values furthermore agree within 1 G with the previous CISD data.45 For the anisotropic proton hfcc our DFT results, the experimental data by Iwasaki et al.16 and the CISD data by Huang and Lunell45 are also in close accord. The slight differences between theory and experiment that nonetheless do exist can probably be attributed to matrix and/or temperature effects not taken into account by the theory. In the work by Iwassaki et al., dynamical averaging was observed above 100 K with a(6H) ) -4.32 G.16 This value lies exactly between the averaged PWP/IGLO-III data for the 2B and the 2B states, -4.6 and -4.1 G, respectively. Thus, 2g 1g for all the multimer ions considered, the strongest agreement between theory and the 4.5 G coupling observed in the present work (section 2.2) is for the benzene radical cation. 4. Discussion Assignment of the triplet hyperfine structure with 65 G splitting to the acetylene radical anion is supported by electronscavenging experiments and the agreement between experimental and calculated hyperfine coupling constants for HCCH-. The large size of the hfcc and the anisotropy of this EPR signal are consistent with previous reports of hyperfine couplings due to R-hydrogens in alkyne radical anions in general and for HCCHin particular.34,35,46 Assignment to a positive ion can be ruled out because of the small hfcc predicted for HCCH+ or CCH2+ by the calculations. Furthermore, the species with 65 G splitting is not formed on NaZSM5. This dichotomy between the NaZSM5 and HZSM5 results is in contrast with results for hydrocarbon radical cations (e.g., tetramethylethylene+, 1,3-
Radical Ions of Acetylene
J. Phys. Chem., Vol. 100, No. 20, 1996 8415
TABLE 8: Frequencies for the Carbon-Carbon Stretching Vibration, ν2, of Acetylene gas liquid solid NaX NaA HZSM5
ν2 (cm-1)
ref
1974 1961 1956 1954 1953 1950
48 48 48 49 48 47
cyclohexadiene+, toluene+) studied by us, which if stabilized in HZSM5, were also observed in NaZSM5.7,8 The 65 G splitting corresponds very well to both experimental and calculated values for alkali metal-acetylene complexes and calculated hfcc for the HCCH- in vacuum or in a model HZSM5-HCCH- complex (Tables 1-3). On the basis of the present results and previous data for the M+HCCH- (M ) Li, K) charge-transfer complexes,35,46 we conclude that the experimentally observed isotropic coupling of 65 G in the zeolite corresponds to the cis-HCCH- radical anion (PWP/IGLO-III result: 65.00 G (vacuum) and 63.8 G (model cluster)). The calculated Aiso(H) values for CCH2- differ by more than the typical error in the theory. This fact, together with the existence of a significant barrier separating HCCH- and CCH2-, leads us to rule out the vinylidene isomer. That we observe the cis and not the trans form in the experiments indicates some type of complex formation or interaction with the zeolite matrix. This is further supported by the fact that the calculated and experimental results for the proton hfcc in cis-HCCH-, the HZSM5-HCCH- complex, and the charge-transfer M+HCCH- complexes (M ) Li, K) are very similar. In the latter cases, the hydrocarbon moiety attains a cis-bent form and has a partial negative charge of 0.6-0.9 electrons. From the proton hfcc alone, we cannot tell whether we have a free radical anion or whether a charge-transfer complex has been formed. The carbon hfcc may be more sensitive to the matrix interaction. Future refinements of the theoretical model will provide additional insights. We also note the similarity in trends to the experimental work on NaHCCH,35 where it was found that no stable Na+HCCHcomplex could be isolated. The rearranged vinylidene species was, however, detected after photolysis with yellow light. Clearly, a significant stabilization relative to HCCH- in the gas phase occurs as a result of matrix-anion interactions. To understand how HCCH- might be formed upon radiolysis, we must consider the nature of adsorbed, neutral acetylene in zeolites and in HZSM5, in particular. Acetylene forms a sideon π-complex with the charge-balancing cation when adsorbed onto zeolites.47-51 The carbon-carbon stretch, ν2, becomes IRactive, which shows that there is a reduction from the D∞h symmetry when acetylene becomes associated with the cation.
The decrease in the ν2 frequency relative to the gas phase frequency is greater in zeolites than in solid acetylene (Table 8). The extra lowering has been attributed to the reduction in bond order due to subtraction of electron density from the π-orbitals. The decrease of electron density in the π-orbitals could give rise to an attractive potential for an electron in the surface-adsorbed state of acetylene. Whereas the adsorption-induced shifts of vibrational frequencies for other probe molecules are proportional to the cation-
polarizing power, the ν2 vibration of acetylene exhibits an inverse dependence in A-type and X-type zeolites exchanged with alkali metal and rare earth metal cations.48,49 This has been explained by the combination of sorbate-cation attraction and shielding of the cation by the framework oxygens. Shielding of the cation is inversely related to the cation size; smaller cations have a greater tendency to recede into the oxide framework. This is prevented in the case of H+, however, by covalent bonding of the proton to the bridging oxygen in ZSM5, which should allow interaction with acetylene with minimal shielding. The adsorption-induced shift of ν2 of acetylene in HZSM5 is slightly greater than for NaX or NaA. Direct comparison to the value of ν2 in the isomorph, NaZSM5, would be interesting. Another measure of the strength of interaction in the π-bonded complex is the NMR chemical shift of the adsorbed molecule vs the free molecule. Although this has not been measured for acetylene, the 13C chemical shift differences of adsorbed vs free isobutene (carbon-2) have been measured on zeolites NaY (10.6 ppm), HY (12.8 ppm), and HZSM5 (16.8 ppm).49 These results show that the strength of the interaction increases in the order NaY < HY < HZSM5 and reflects the greater acid strength of the Bronsted acid sites in HZSM5 vs HY. The strength of interaction in the π-complex seems to be a critical factor in the formation of HCCH- in our experiment. In light of the foregoing discussion and the EPR results, we conclude that the hydrogen-bonded π-complex of acetylene adsorbed in HZSM5 is reduced upon radiolysis to form the cisbent form of HCCH-. The absence of any detectable HCCHsignal in NaZSM5 suggests that the interaction in the surfaceadsorbed π-complex is weaker in NaZSM5 than in HZSM5. The weaker electron affinity of HCCH in NaZSM5 may be reinforced by a stronger electron affinity of NaZSM5 vs HZSM5. In any event, in NaZSM5, electrons remain trapped by the zeolite. The trend in the HCCH- signal intensity as a function of Si/Al ratio can be understood with respect to the concentration of Bronsted acid sites, which increases proportionately with Al content. The intensity of the HCCH- EPR signal varies inVersely with O-H concentration. Therefore, the decrease in the anion yield is not caused by insufficient numbers of available acid sites (in HZSM5-50, the concentration of O-H sites and acetylene are 1.9 and 2 per unit cell, respectively, at 1% loading). Neither is the trend related to the relative acidity of the O-H sites, which is constant over the dilute range of Si/Al in the different ZSM5’s studied.52 Rather, as shown by the dependence on pre-equilibration conditions, the decrease in the HCCH- signal intensity with decreasing Si/Al ratio reflects the depletion of HCCH prior to irradiation, i.e., acid-catalyzed polymerization. Studies have shown that reaction rates for acetylene polymerization on HZSM5 increase with Al content, i.e., with the extensive acidity of the zeolite.53 Although reaction studies have primarily been carried out at superambient temperatures, reaction at room temperature, albeit slow, has been reported.47 As the results in Figure 5 show, subambient temperatures were necessary to fully stop the reaction of acetylene on HZSM5-50. Acetylene adsorption was achieved even at 77 K, which shows that acetylene is able to diffuse through the ZSM5 channels at liquid-nitrogen temperature. Further evidence of the consumption of acetylene by acidcatalyzed polymerization in HZSM5 is the parallel increase in the radiolytic yield of a multimer ion with increasing Al content. The best agreement between calculated and experimental hfcc for this radical ion was obtained for benzene+, in accord with
8416 J. Phys. Chem., Vol. 100, No. 20, 1996 the excellent agreement between the EPR spectra generated from acetylene and benzene-loaded ZSM5 zeolites. At 77 K and above, dynamical averaging brings about the equivalence of the six hydrogens, and the experimental hfcc lies between the theoretical values for the 2B2g and 2B1g states of the benzene radical cation. The spontaneous formation of the tetramethylcyclobutadiene radical cation at 77 K upon exposure of 2-butyne to H-mordenite calcined in air (conditions that promote formation of Lewis acid sites55) has been observed and was attributed to the spontaneous ionization of tetramethylcyclobutadiene formed by Bronsted acid catalysis, since the parent molecule (IP(gas) ) 9.56 eV14) is not likely ionized under the conditions of the experiment.56 When it was annealed to 330 K, the EPR spectrum of hexamethylbenzene+ appeared. In another experiment, reported in a recent review,57 the benzene radical cation was observed by EPR following γ-irradiation of HZSM5 exposed to acetylene at room temperature. In the above experiments, it is ambiguous whether the trimer is formed directly as a result of catalysis or whether the trimer radical cation is formed by reaction of neutral monomers with the dimer radical cation (Vide infra). However, in our experiments the nearly total loss of the signal intensity of the acetylene anion on HZSM5-50 suggests that there is little monomer left by the time the sample is irradiated, and we conclude that benzene can be formed by Bronsted acid catalysis. The apparent absence of H/D exchange in our acetylene-d2 experiments on HZSM5 suggests that the intermediates (hydrogen-bonded complexes) involved in this catalytic reaction are such that the identity of the zeolitic hydrogen is not lost. A radiolytic mechanism must be assumed in the formation of benzene+ from acetylene in the catalytically inactive NaZSM5, and therefore, the observation of benzene+ implies ionization of acetylene. Precedent exists for the cation-molecule reaction of acetylene derivatives, MeCCH and MeCCMe, to form dimer radical cations in freon matrices.54 Based on EPR and ab initio calculations, the dimer cations were concluded to by cyclic. Similarly, we have observed the tetramethylcyclobutadiene radical cation in MeCCMe-loaded NaZSM5 following radiolysis at 77 K.58 We believe that observation of benzene+ and failure to observe a C4 radical cation intermediate or the monomer radical cation are a manifestation of the size selectivity of the matrix. Both butatriene (IP ) 9.15 eV14) and cyclobutadiene (IP ) 8.16 eV41) possess ionization potentials within the range of previously observed hydrocarbon radical cations in irradiated zeolites.1-8 However, small molecules such as acetylene are not immobilized in the ZSM5 zeolite at 77 K, and their radical cations are susceptible to ion-molecule reaction during storage at 77 K. Our studies suggest that hydrocarbons as large as C4 or C5 retain some mobility at 77 K in ZSM5.58 On the other hand, matrix constraints may hinder further expansion from benzene+ to a C8 radical cation. Trapping of HCCH- in HZSM5 shows that it is less susceptible to ion-molecule reaction than the acetylene cation. Our observation of HCCH- begs the question of why radical anions of other unsaturated hydrocarbons (olefins, diolefins, and aromatic hydrocarbons7,8) are not observed in zeolite radiolysis. At present we have no clear answer to this question. Possible factors are a steric effect on the matrix-adsorbate interaction for molecules larger than acetylene and the difficulty of ionizing acetylene because of its high IP. Further experiments to test the possible competitive nature of trapping of positive and negative charges in the zeolite matrix would be useful.
Piocos et al. 5. Conclusions We have observed the radiolytic reduction of acetylene adsorbed on HZSM5. Formation of the hydrocarbon radical anion in the zeolite radiolysis is a rare finding and contrasts with results for other unsaturated hydrocarbons adsorbed on HZSM5, for which radical cations only are formed. The EPR spectrum of HCCH- (aiso(H) ) 65 G) in HZSM5 shows the radical anion to be in the cis-bent form. The benzene radical cation was observed in both acetyleneloaded HZSM5 and NaZSM5. This radical cation can be formed via two mechanisms. On NaZSM5 it is formed by ionization of acetylene and subsequent ion-molecule reactions with neutral acetylene molecules. On HZSM5 Bronsted acid catalysis converts acetylene to trimers (benzene), which are ionized upon radiolysis. Ionization of acetylene (IP(gas) ) 11.4 eV) in NaZSM5 places a new lower limit on the range of ionizable species in the radiolyzed zeolite. Acknowledgment. We acknowledge Chemie Uetikon (Switzerland) for the gift of the zeolites, J. Gregar for constructing the glass vacuum manifold and supplying the EPR tubes, A. Svirmickas for carrying out the 60Co irradiations of our samples, P. Han for carrying out the benzene experiments, K. R. Cromack for conducting preliminary EPR studies of acetylene radiolysis in ZSM5, and P. H. Rieger (Brown University) for sharing his EPR powder spectrum simulation program. L.A.E. thanks the Swedish Natural Science Research Council (NFR) for financial support. References and Notes (1) Qin, X.-Z.; Trifunac, A. D. J. Phys. Chem. 1990, 94, 4751. (2) Barnabas, M. V.; Trifunac, A. D. Chem. Phys. Lett. 1991, 187, 565. (3) Barnabas, M. V.; Trifunac, A. D. J. Chem. Soc. Chem., Commun. 1993, 813. (4) Barnabas, M. V.; Werst, D. W.; Trifunac, A. D. Chem. Phys. Lett. 1993, 204, 435. (5) Barnabas, M. V.; Werst, D. W.; Trifunac, A. D. Chem. Phys. Lett. 1993, 206, 21. (6) Cromack, K. R.; Werst, D. W.; Barnabus, M. V.; Trifunac, A. D. Chem. Phys. Lett. 1994, 218, 485. (7) Werst, D. W.; Tartakovsky, E. E.; Piocos, E. A.; Trifunac, A. D. J. Phys. Chem. 1994, 98, 10249. (8) Werst, D. W.; Piocos, E. A.; Tartakovsky, E. E.; Trifunac, A. D. Chem. Phys. Lett. 1994, 229, 421. (9) Shih, S. J. Catal. 1983, 79, 390. (10) Liu, X.; Iu, K.-K.; Thomas, J. K. Chem. Phys. Lett. 1994, 224, 31. (11) Liu, X.; Iu, K.-K.; Thomas, J. K. J. Phys. Chem. 1994, 98, 7877. (12) Liu, X.; Iu, K.-K.; Thomas, J. K. Chem. Phys. Lett. 1993, 204, 163. (13) Kasai, P. H.; Bishop, R. J., Jr. Zeolite Chemistry and Catalysis; Rabo, J. A., Ed.; ACS Monograph 171; American Chemical Society: Washington, DC, 1976; Chapter 6. (14) In Gas Phase Ion and Neutral Thermochemistry; Lias, S. G., Bartmess, J. E., Liebman, J. F., Holmes, J. L., Levin, R. D., Mallard W. G., Eds.; Journal of Physical and Chemical Reference Data, Vol. 17, Supplement No. 1; American Chemical Society: New York, 1988. (15) Handbook of Photochemistry, 2nd ed.; Murov, S. L., Carmichael, I., Hug, G. L., Eds.; Marcel Dekker: New York, 1993; p 263. (16) Komatsu, T.; Lund, A. J. Phys. Chem. 1972, 76, 1727. Symons, M. C. R.; Harris, L. J. Chem. Res., Synop. 1982, 268. Iwasaki, M.; Toriyama, K.; Nunome, K. J. Chem. Soc., Chem. Commun. 1983, 320. Erickson, R.; Lindgren, M.; Lund, A.; Sjoqvist, L. Colloids Surf. A 1993, 72, 207. (17) Bolton, J. R. Mol. Phys. 1963, 6, 219. (18) Kutzelnigg, W.; Fleischer, U.; Schindler, M. In NMR - Basic Principles and Progress; Springer-Verlag: Heidelberg, 1990; Vol. 23, p 165. (19) Huzinaga, S. J. Chem. Phys. 1965, 42, 1293. Huzinaga, S.; Sakai, Y. J. Chem. Phys. 1969, 50, 1371. (20) Becke, A. D. Phys. ReV. A 1988, 38, 3098. (21) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785. (22) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (23) Perdew, J. P.; Wang, Y. Phys. ReV. B 1986, 33, 8800.
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