J. Phys. Chem. 1996, 100, 5245-5251
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Gas-Phase Stability and Structure of the Cluster Ions CF3+(CO)n, CF3+(N2)n, CF3+(CF4)n, and CF4H+(CF4)n Kenzo Hiraoka,* Masayuki Nasu, and Susumu Fujimaki Faculty of Engineering, Yamanashi UniVersity, Takeda-4, Kofu 400, Japan
Edgar W. Ignacio Department of Chemistry, MSU Iligan Institute of Technology, Iligan City 9200, Philippines
Shinichi Yamabe* Department of Chemistry, Nara UniVersity of Education Takabatake-cho, Nara 630, Japan ReceiVed: October 11, 1995; In Final Form: January 12, 1996X
Gas-phase thermochemical stabilities and structures of cluster ions CF3+(CO)n, CF3+(N2)n, CF3+(CF4)n, and CF4H+(CF4)n have been studied with a high-pressure mass spectrometer and ab initio calculations. The clusters examined are found to have the shell structures CF3+(CO)1(CO)3(CO)3(CO)n-7, CF3+(N2)2(N2)3(N2)n-5, CF3+(CF4)2(CF4)n-2, and CF4H+(CF4)1(CF4)n-1. The CF3+ ion is bound with CO to form the core CF3CO+ in the cluster, while the interaction between CF3+ and N2 or between CF3+ and CF4 is mainly electrostatic. The unusually small bond energy for the symmetric proton-bound dimer CF4H+‚‚‚CF4 (5.1 kcal/mol) is observed, which indicates that CF4H+ and CF4 are very poor electrophile and nucleophile, respectively.
1. Introduction The fluorine atom has several unique reactivities. For example, the F atom is bonded to almost all the elements; UF6, XeF2, XeF4, XeF6, KrF2, SiF62-, PF6-, SF6, etc. Fluorination of organic compounds increases the hydrophobicity and results in the drastic increase of transportation rates for the fluorinated compounds in the living body (lipophilicity). Because the F atom is the third smallest atom next to the He atom, the fluorinated compounds are easily taken in by the living body (mimic effect). For example, the monofluoroacetic acid is relatively more toxic than the trifluoroacetic acid. The C-F bond (average bond energy ) 116 kcal/mol) is much stronger than the C-H one (98.8 kcal/mol), and this results in the marked chemical stability of fluorinated compounds. This is known as the block effect of the F atom. Due to its high electronegativity, the F atom blocks the electrophilic attack not only on the fluorinated carbon atom but also on the neighboring hydrogenated carbon atoms. Investigations on the positive ion/molecule reactions in CF4 are rather scarce. CF4+ has not been observed by means of photon or electron impact mass spectrometry, due to the rapid dissociation of the lower electronic states of CF4+ to CF3+, and F. Dowben et al.1 and Hagenow et al.2 observed the formation of CF4+ from CF4 clusters. They detected the CF4+ and higher aggregates by electron impact ionization of a supersonic CF4 molecular beam. Morris et al.3 studied the chemistry of CFn+ (n ) 1-3) ions with halocarbons. They observed the reactions CF+ + CF4 ) CF3+ + CF2 and CF2+ + CF4 ) CF3+ + CF3, but no reaction was observed for the CF3+/CF4 system. Speranza et al. examined the CF3+-initiated ion/molecule reactions in the γ-radiolysis of CF4/n-bases gaseous mixtures.4 They found that the major products arising from attack of CF3+ on the selected nucleophiles are the corresponding carbonyl derivatives. X
Abstract published in AdVance ACS Abstracts, March 1, 1996.
0022-3654/96/20100-5245$12.00/0
In the current work, the thermochemical stabilities of cluster ions CF3+(CO)n, CF3+(N2)n, CF3+(CF4)n, and CF4H+(CF4)n have been studied by measuring the equilibria of reactions 1-4.
CF3+(CO)n-1 + CO ) CF3+(CO)n
(1)
CF3+(N2)n-1 + N2 ) CF3+(N2)n
(2)
CF3+(CF4)n-1 + CF4 ) CF3+(CF4)n
(3)
CF4H+(CF4)n-1 + CF4 ) CF4H+(CF4)n
(4)
It was found that CF3+ and CF4H+ are poor Lewis acids and the CF4 molecule is also a poor Lewis base. The structures of the cluster ions CF3+(CO)n, CF3+(N2)n, CF3+(CF4)n, and CF4H+(CF4)n have been examined by ab initio calculations. 2. Experimental and Theoretical Methods The experiments were performed with a pulsed electron beam high-pressure mass spectrometer which has been described previously.5,6 The main chamber for the ion source was evacuated using a magnetic levitation turbo molecular pump (Seiko Seiki KK, STP-2000, 1950 1/s for N2). The ions escaping from the field-free ion source into an evacuated region were mass analyzed by a quadrupole mass spectrometer (ULVAC, MSQ-400, m/z ) 1-550). The ion counts were collected in a multichannel analyzer as a function of their arrival time after the electron pulse. The collected ion counts were transferred to the microcomputer, and the time profile of the logarithm intensities was recorded with an x-y plotter. The assumption is made that the intensity of a given ion is proportional to the concentration (i.e., partial pressure) of that ion in the ion source. © 1996 American Chemical Society
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TABLE 1: Thermochemical Data, ∆H°n-1,n (kcal/mol) and ∆S°n-1,n (cal/mol K) (Standard State, 1 atm) for Gas-Phase Clustering Reactions, CF3+(CO)n-1 + CO ) CF3+(CO)n, CF3+(N2)n-1 + N2 ) CF3+(N2)n, CF3+(CF4)n-1 + CF4 ) CF3+(CF4)n, and CF4H+(CF4)n-1 + CF4 ) CF4H+(CF4)na CF3+(CO)n
CF3+(N2)n
CF4H+(CF4)n
CF3+(CF4)n
n
-∆H°n-1,n
-∆S°n-1,n
-∆H°n-1,n
-∆S°n-1,n
-∆H°n-1,n
-∆S°n-1,n
-∆H°n-1,n
-∆S°n-1,n
1
16.0 [15.60] 6.3 [6.59] 5.8 [5.89] 5.4 3.2 2.9 2.6
30 [33.04] 20
7.0 [7.74] 5.1 [6.91] 2.0 [2.81] 1.8 1.5
24 [22.49] 22
6.6 [8.38] 4.9 [6.63] 2.9
19 [19.9] 22
5.1 [6.50] 2.8 [5.05] 2.3
20 [20.81] 11
2 3 4 5 6 7
26 28 26 28 28
13 14 12
2.8
21 20
16
∼2.3
a Experimental errors for ∆H° and ∆S° may be within (0.2 kcal/mol and 2 cal/mol K, respectively. Bond energies in square brackets are theoretical ones obtained by MP4(SDQ)/6-31G*//RHF/6-31* plus RHF/6-31G* zero-point vibration energies. Entropy changes in square brackets of n ) 1 are theoretical values by RHF/6-31G* vibrational analyses.
Figure 2. n dependence of the bond energies -∆H°n-1,n (kcal/mol) of the cluster ions CF3+(CO)n, CF3+(N2)n, and CF3+(CF4)n.
Figure 1. van’t Hoff plots for the clustering reactions: (a) CF3+(CO)n-1 + CO ) CF3+(CO)n and (b) CF3+(N2)n-1 + N2 ) CF3+(N2)n. Integer numbers attached to plots denote values of n.
In the CF4/CO or CF4/N2 system, a small amount of CF4 (2040 mTorr) was introduced into the ∼3 Torr major gas (CO or N2) through a stainless steel capillary (1 m long × 100 µm diameter). For the measurements of the equilibria of reaction 3, pure CF4 at a few torrs was used as a reagent gas. In these measurements, the only ion formed was found to be CF3+. For the measurements of the equilibria of reaction 4, a known amount of CF4 (20-70 mTorr) was introduced into 3 Torr major gas of mixed H2 and O2 (H2/O2 ) 3/7). The reaction sequence shown below produces the protonated CF4, CF4H+, where M represents the third body.
H2+ + O2 ) O2H+ + H +
+
Geometries of CF3+(N2)n, CF3+(CO)n, CF3+(CF4)n (n ) 1-3), and CF4H+(CF4)n (n ) 1 and 2) were determined with ab initio calculations. The GAUSSIAN 927 program was used. The RHF/6-31G* method was used for full geometry optimization (without symmetry constraint). For the obtained geometries, vibrational analyses were carried out to assure that they are not at saddle points but at stable positions. Zero-point vibrational energies (ZPEs) were also obtained. Electronic energies are evaluated by the fourth-order Moller-Plesset perturbation energies of single, double, and quadrupole substitutions with the frozen core on the RHF/6-31G* geometries, MP4(SDQ)/ 6-31G*//RHF/6-31G*. Theoretical bond energies are, thus, obtained by the MP4(SDQ)/6-31G* electronic energies and RHF/6-31G* ZPEs. Entropy changes of only n ) 0 f 1 reactions were evaluated under the restriction of the harmonic vibration approximation. All the calculations were performed on the CONVEX C-220 computer at the Information Processing Center of Nara University of Education. 3. Experimental Results
(5)
O2H + O2 + M ) H (O2)2 + M
(6)
H+(O2)2 + CF4 ) CF4H+ + 2O2
(7)
Without the addition of the O2 gas into the major H2 gas, the major ion observed was CF3+ and the CF4H+ ion was found to be minor. This is due to the fact that the exothermicity of reaction 7 is only 6 kcal/mol, but that of the reaction H3+ + CF4 ) CF4H+ + H2 is as much as 26 kcal/mol, which induces the unimolecular dissociation of the produced CF4H+ into CF3+ and HF.
As an example, the results of the experimentally measured equilibrium constants for reactions 1 and 2 are displayed in the van’t Hoff plots in Figure 1. In Table 1, the enthalpy and entropy changes obtained from the van’t Hoff plots are summarized. In Figure 2, the -∆H°n-1,n values for reactions 1-3 are shown as a function of n. In Figure 1, the van’t Hoff plot for reaction 1 with n ) 1 is isolated in the high-temperature region, and there appears a gap between those with n ) 1 and 2. This first gap clearly indicates the covalent character in the bond of CF3+‚‚‚CO. In Figure 1a, there is also a gap between n ) 4 and 5. This second gap suggests that the core ion [CF3‚‚‚CO]+ is favorably solvated by three more CO molecules (n ) 2, 3, and 4), and the fifth
CF3+(CO)n, CF3+(N2)n, CF3+(CF4)n, and CF4H+(CF4)n
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Figure 3. Geometries of core ions CF3+ and CF4H+ optimized by RHF/ 6-31G*. CF3+ is of D3h point group. CF4H+ is an interacting system between CF3+ and hydrogen fluoride. For CF3+, its LUMO shape is shown. The C-F distance of CF4 (with Td symmetry) is 1.302 Å.
CO ligand interacts with the cluster [CF3‚‚‚CO]+(CO)3 less favorably than those with n e 4. This trend is seen more clearly in Figure 2. These irregular decreases of the -∆H°n-1,n values are observed between n ) 1 and 2 and between n ) 4 and 5. The measurements of the equilibrium constants for reactions 1-4 were studied down to the temperature just above the condensation point of the reagent CF4 gas (∼65 K for reactions 1-3). At this lowest temperature, no signals of CF3+(CO)8 or CF3+(N2)6 could be detected for reaction 1 or 2, while strong ion signals for CF3+(CO)7 or CF3+(N2)5 were observed. This indicates that there are also gaps between n ) 7 and 8 for reaction 1 and n ) 5 and 6 for reaction 2. The cluster ion CF3+(CO)n may thus be represented as the shell structure CF3+(CO)1(CO)3(CO)3(CO)n-7. As shown in Figure 2, the bond energies of CF3+(N2)n are much smaller than those of CF3+(CO)n. It is likely that the bonding in CF3+(N2)n is mainly electrostatic. Although CO and N2 are isoelectronic, the features of the van’t Hoff plots obtained for CO and N2 ligands are not similar, as shown in Figure 1a,b. In Figure 1b, the van’t Hoff plots with n ) 1 and 2 are close to each other and there appears a gap between n ) 2 and 3. As was already noted in the signal intensities, there is also a gap between n ) 5 and 6. Thus, the cluster ion CF3+(N2)n may be represented as the shell structure CF3+(N2)2(N2)3(N2)n-5. A similar trend is also observed for reaction 3, as shown in Figure 2. The bond energies for CF3+(CF4)n are found to be small and are close to those for CF3+(N2)n with n ) 1 and 2. At n g 3, the former energies are slightly larger than the latter ones. These results suggest that the cation CF3+ is sandwiched either by two CF4 molecules or by two N2 ones in the similar structure. For the higher coordination numbers (n g 3), the neutral nitrogen atom in N2 is bound more weakly to the inner, n ) 2, shell than the anionic fluorine atom in CF4. As shown in Table 1, the bond energy of the proton-bound dimer cation CF4H+‚‚‚CF4 is unusually small (only 5.1 kcal/ mol!) compared to that of the proton-bound N- or O-bases8,9 (e.g., 32 kcal/mol for H+(C2H5OH)2). This suggests that the CF4 molecule is a very poor Lewis base and/or CF4H+ is a very poor Lewis acid. In our previous paper,8 it was predicted that for the symmetric proton-bound dimers BH+‚‚‚B, the bond
Figure 4. Geometries of CF3+(N2)n, n ) 1, 2, and 3, optimized by RHF/6-31G*.
energies become stronger with decreasing proton affinity for the nitrogen or oxygen bases B. This rule of thumb does not apply to the case of CF4H+‚‚‚CF4; that is, the proton affinity of CF4 is only 126 kcal/mol.10 It is evident that the charge delocalization by the formation of the proton-bound dimer is minor and the positive charge in the complex CF4H+(CF4) is largely localized in CF4H+. This is another example of the block effect of the perfluorinated CF4 originating from the high electronegativity of the F atom. As shown in Table 1, an irregular decrease in the -∆H°n-1,n values for reaction 4 is observed between n ) 1 and 2 (i.e., 5.1 f 2.8 kcal/mol). The value of -∆H°2,3 ) 2.8 kcal/mol is unusually small. This is likely to be due to the exchange repulsion between the CF4H+(CF4)1 and n g 2 CF4 ligands (again block effect). 4. Theoretical Results and Discussion First, two core ions, CF3+ and CF4H+, are examined in Figure 3. CF3+ is planar as is expected from Walsh rules.11 The C-F distance, 1.217 Å, of CF3+ is shorter than that, 1.302 Å, of CF4. Hyperconjugation makes for a slight double-bond character in CF3+.
The LUMO shape of CF3+ shows that the carbon 2pπ orbital is the electrophilic center in spite of the hyperconjugation. The
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Figure 5. Geometries of CF3+(CO)n, n ) 1, 2, and 3.
geometry of CF4H+ is almost that of CF3+ coordinated to hydrogen fluoride. The CF3+ fragment is planar in CF4H+, which indicates that the CF3+‚‚‚FH attraction is electrostatic. In fact, the shift of electronic charge, F3C+ r F-H, is small (0.05) in spite of the moderately large F3C+-FH bond energy, 19.5 kcal/mol (the present theoretical value, MP4(SDQ)/6-31G*/ /RHF/6-31G* plus RHF/6-31G* ZPE corrections). Now, a structural question arises: is the ligand CF4 molecule bound to the cationic carbon in CF3+ or to the protonic hydrogen in HF? This selectivity will be examined later in Figure 8. Figure 4 shows the geometries of CF3+(N2)n. The n ) 1 and n ) 2 geometries are as expected. Along the electrophilic carbon 2pπ orbital of CF3+, N2 molecules are coordinated linearly. Since N2 is bound weakly to CF3+, CF3+ remains almost planar in CF3+‚N2. The weak coordination is also reflected in the high-symmetry (D3h) geometry of n ) 2. At n ) 3, the third N2 molecule is attached to CF3+ distant from two N2 ligands. The third N2 cannot be bound perpendicularly to the principal axis of N2CF3+N2 (n ) 2). This slight deviation comes from the nodal property of the LUMO in Figure 3. It is noteworthy that the orbital phase controls the orientation of the third N2 attachment to the n ) 2 cluster in spite of the extremely weak electrostatic interaction. Thus, the shell structure F3C+(N2)2(N2)3(N2)n-5 is explicable. Figure 5 displays geometries of F3C+(CO)n (n ) 1-3). For n ) 1, the CF3+ plane is distorted (to sp3) by the strong CO coordination. The C‚‚‚C distance is 1.666 Å and shows a
Figure 6. Effect of fluorination of methyl carbon on bond energies. Energies in square brackets are theoretical data of MP4(SDQ)/6-31G*. For CH3+‚‚‚CO, the energy 79.6 kcal/mol is obtained from three values11 of the heat of formation, -26.4 kcal/mol (CO), 262 kcal/mol (CH3+), and 156 kcal/mol (CH3CO+). For CF3+‚‚‚CO, the energy 15.99 kcal/ mol is the present experimental datum.
CF3+(CO)n, CF3+(N2)n, CF3+(CF4)n, and CF4H+(CF4)n
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Figure 7. Geometries of CF3+(CF4)n, n ) 1, 2, and 3.
semicovalent bond. H3CCO+ is an acylium ion used for Friedel-Crafts acylations.
F3C+(CO)1 is regarded as the trifluoroacylium ion, where the cation center is not at the CF3 moiety but at the carbonyl carbon. In this respect, two geometric isomers, (a) and (b), are compared for n ) 2 in Figure 5. The geometry (b) is confirmed to be 5.07 kcal/mol more stable than (a). The geometry of CF3+(CO)3 is also obtained to be the acylium ion type coordination. Noteworthy is the CO‚‚‚C‚‚‚CO angle, 116.3°. This is almost 120°, which allows the fourth CO molecule to be linked to the first one equivalently with the second and the third ones. The
trivalent coordination indicates the shell structure CF3+(CO)1(CO)3(CO)n-4. Since the C‚‚‚C distance is large (∼3.0 Å), the shell may be further structured as CF3+(CO)1(CO)3(CO)3(CO)n-7.
This precise shell structure was predicted in terms of changes of bond energies in Figure 2. Figure 6 shows the fluorination effect on the C‚‚‚C bond energies for acylium ions. As the number of F atoms increases, naturally, the hyperconjugation weakens the C‚‚‚C bond. But,
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Figure 8. Geometries of CF4H+(CF4)n, n ) 1 and 2.
remarkably, the C‚‚‚C bond in CF3+CO (15.60 kcal/mol) is stronger than in CHF2+CO (11.98 kcal/mol). As the number of F atoms increases, the σ-direction charge transfer (CT) decreases monotonically. On the other hand, the π-direction back CT increases. The opposite trend of the σ and π bonding gives the inversion in Figure 6. The experimental bond energy of CH3+‚‚‚CO, 79.6 kcal/mol, is too large in view of the average bond energy, ∼82 kcal/mol, and the present theoretical data (MP4(SDQ)/6-31G*, 69.31 kcal/mol). The former overestimated energy comes from the value ()156 kcal/mol) of the heat of formation for CH3CO+.12 On the basis of the theoretical energy, the value of ∆Hf° (CH3CO+), 166.3 kcal/mol, is recommended. Figure 7 exhibited geometries of CF3+(CF4)n. The clustering pattern is very similar to that of CF3+(N2)n, as was discussed in the previous section. That is, two CF4 molecules are coordinated weakly and equivalently to CF3+. The third CF4
is oriented almost in the same direction as the third N2 in Figure 4. By coordination of the third CF4 molecule, the left-sided (n ) 2) CF4 molecule is rotated by 60° to avoid ligand-ligand steric repulsion. Figure 8 shows geometries of CF4H+(CF4)n. Two geometric isomers of n ) 1 are compared: (a) a “sandwich” coordination and (b) a hydrogen bond structure. The sandwich geometry (a) is 0.81 kcal/mol more favorable than the hydrogen bond (b). The n ) 1 geometry is CF4‚‚‚CF3+‚‚‚FH. But the energy difference is so small that two isomers would practically coexist. For n ) 2, their combined structure is expected (n ) 2, (b)). This expectation is tested next. Two geometric isomers, (a) coordination to the central carbon and (b) hydrogen bond, are considered for n ) 2. The n ) 2 geometry (a) is basically the same as those of CF3+(N2)3 and CF3+(CF4)3. For n ) 2, the hydrogen bond model (b) is calculated to be 1.41 kcal/mol more stable than the (a) triple coordination to CF3+. This difference
CF3+(CO)n, CF3+(N2)n, CF3+(CF4)n, and CF4H+(CF4)n is within error, and two isomers may also coexist. However, the entropy changes for the formation of CF3+(CF4)3 is much larger than that of CF4H+(CF4)2, as shown below (two thermochemical data are taken from Table 1). CF3+(CF4)3 CF4H+(CF4)2
-∆H°n-1,n (kcal/mol) 2.9 2.8
-∆S°n-1,n (eu) 21 11
If the CF4H+(CF4)2 were of the (a) triple coordination, its ∆H°1,2 and ∆S°1,2 values should be almost equal to the corresponding ∆H°2,3 and ∆S°2,3 values of CF3+(CF4)3. Indeed, ∆H°1,2(CF4H+(CF4)n) = ∆H°2,3(CF3+(CF4)n), but the entropy changes are entirely different. The small value of -∆S°1,2 ) 11 eu indicates the less crowded and rotation-free hydrogen bond. The cation center goes back to the proton at n ) 2. In Table 1, the theoretical bond energies are slightly larger than, but in good agreement with, observed ones. 5. Concluding Remarks This work has dealt with gas-phase clustering reactions of CF3+ and CF4H+ ions with CO, N2, and CF4 molecules. Although CF4 is a polar molecule, the observed bond energies are small. CF3+(N2)n and CF3(CF4)n follow very similar clustering patterns, sandwich and triple coordinations at n ) 2 and 3, respectively. CF3+(CO)n is the trifluoroacylium ion and (n - 1) ligand CO molecules attached to the carbonyl carbon on the ion. For CF4H+(CF4)n, a hydrogen bond is recognized at n ) 2, F3C-F‚‚‚CF3+‚‚‚F-H‚‚‚F-CF3. Acknowledgment. We would like to express our appreciation for the financial support of the Morino Foundation for
J. Phys. Chem., Vol. 100, No. 13, 1996 5251 Molecular Science and the Grant-in-Aid in part for Scientific Research on Priority Area “Theory of Chemical Reactions” from the Ministry of Education. We also thank the Information Processing Center of Nara University of Education for the allotment of CPU time of the CONVEX C-220 computer. One of the authors (E.W.I.) wishes to thank the DOST-JSPS for the fellowship support. References and Notes (1) Kime, Y. J.; Driscoll, D. C.; Dowben, P. A. J. Chem. Soc., Faraday Trans. 2 1987, 83, 403. Kime, Y. J.; Dowben, P. A. J. Phys. Chem. 1989, 6881. (2) Hagenow, G.; Denzer, W.; Brutschey, B.; Baumgartel, H. J. Phys. Chem. 1988, 92, 6487. (3) Morris, R. A.; Viggiano, A. A.; Van Doren, J. M.; Paulson, J. F. J. Phys. Chem. 1992, 96, 2597. (4) Grandinetti, F.; Crestoni, M. E.; Fornarini, S.; Speranza, M. Int. J. Mass Spectrom. Ion Processes 1994, 130, 207. (5) Kebarle, P. In Techniques for the Study of Ion-Molecule Reactions; Farrar, J. M., Saunders, W. H., Eds.; Wiley: New York, 1988. (6) Hiraoka, K. J. Chem. Phys. 1987, 87, 4048. (7) Frisch, M. J.; Trucks, G. W.; Head-Gordon, M.; Gill, P. M. W.; Wong, M. W.; Foresman, J. B.; Johnson, B. G.; Schlegel, H. B.; Robb, M. A.; 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, Revision C; Gaussian, Inc.: Pittsburgh, PA, 1992. (8) Hiraoka, K.; Takimoto, H.; Yamabe, S. J. Phys. Chem. 1986, 90, 5910, and references cited therein. (9) Keesee, R. G.; Castleman, A. W., Jr. J. Phys. Chem. Ref. Data 1986, 15, 1011. (10) Lias, S. G.; Liebman, J. F.; Levin, R. D. J. Phys. Chem. Ref. Data 1984, 13, 695. (11) Walsh, A. D. J. Chem. Soc. 1953, 2260. (12) Lias, S. G.; Bartmess, J. E.; Liebman, J. F.; Holmes, J. L.; Levin, R. D.; Mallard, W. G. J. Phys. Chem. Ref. Data, Suppl. 1988, 17, 1.
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