J . Phys. Chem. 1985,89, 460-463
460
Molecular Orbital Studies of Tautomerlsm in Tetrazole Aleksander P. Mazurek* and Roman Osman Department of Pharmacology, Mount Sinai School of Medicine of the City University of New York, New York, New York 10029 (Received: September 19, 1984)
Results from ab-initio calculations of the fully optimized structures of the two tautomers of tetrazole indicate that in the gas phase the 2H tautomer is the energetically preferred form. Inclusion of polarization functions and electron correlation allowed an estimation of the energy differencebetween the tautomers as ca.2 kcal/mol. Vertical ionization potentials calculated from the difference between the energies of the radical cations and the neutral molecules agree with observed photoelectron spectra. Structure optimization of the radical cations indicates a greater susceptibility to fragmentation of the x over the u radicals. The preference of the 2H tautomer in gas phase taken together with suggested fragmentation from structure optimization explains the observed fragmentation in mass spectra of tetrazole and Smethyltetrazole.
Introduction Tetrazole can exist in the two tautomeric forms shown in Figure 1. Neither experimental nor theoretical studies were able to determine unequivocally which is the preferred tautomer. In the crystalline state’ tetrazole is clearly in the 1H tautomeric form (Figure la). Likewise, based on dipole moment measurements in dioxane and in benzene solutions, it appears that the 1H tautomer is d ~ m i n a n t . ~ ,Proton ~ magnetic resonance data4 also suggest that this form is preferred in D M F / H 2 0 solutions. in acetone or methanol However, from I4N N M R solutions no definite conclusion can be made concerning the tautomerism of tetrazole. Alkylation studies of tetrazole’ and examination of the ionization constants of substituted tetrazoles* show that both tautomeric forms exist in solution, but there is a slight preference of the 2H tautomer (Figure lb). The tautomeric equilibrium can be influenced by the solvent and therefore the nature of tetrazole tautomerism was investigated in gas phase. Results from early mass spectroscopic studies9 of substituted tetrazoles were consistent with the suggestion that both tautomeric forms are present in gas phase. Microwave spectroscopy also indicates that both tautomeric forms of tetrazole exist in gas phase.l0 However, in a recent publication Razynska et al.” claim that their mass spectrometric results, supported by semiempirical calculations, demonstrate that only the 2H tautomer occurs in gas phase. This is consistent with arguments by Palmer et al.I2 who came to the same conclusion based on the photoelectron spectra of tetrazole and N-substituted tetrazoles. They also calculated a b initio the two tautomers of tetrazole and came to the conclusion that the calculations predict the wrong tetrazole tautomer to be the more stable form. This conclusion, however, was reached based on a difference of 0.7 kcal/mol with a minimal basis set calculation. When the minimal basis set optimized geometry was used, the calculations with a double { basis set yielded an energy difference of only 0.2 kcal/mol. To investigate further the properties of tetrazole tautomers we carried out full geometry optimizations of the molecules with (1) Van der Putten, N.; Haeijdenrijk, D.; Schenk, H. Cryst. Struct. Commun. 1974, 321.
(2) Jensen, K. A,; Fredgier, A. K . Dan. Vidensk. Selsk., Mat.-Fys.Medd.
1943, 20, 1.
(3) Kaufman, M. H.; Ersenberg, F. M.; McEwan, W. S . J. Am. Chem. Soc-. 1956, 78, 4197.
(4) Moore, D. G. W.; Whittaker, A. G. J. Am. Chem. Soc. 1960,82,5007. ( 5 ) Witanowski. M.; Stefaniak. L.; Januszewski. H.; Grabowski, 2.; Webb, G . A: Tetrahedron 1972, 28,637. (6) Webb, G. A.; Witanowski, M.; Stefaniak, L. J. Magn. Reson. 1979, 36, 232. (7) Henry, R. A. J. Am. Chem. SOC.1951, 73, 4470. (8) Charton, M. J. Chem. SOC.B 1969, 1240. (9) Forkey, D. M.; Carpenter, W. R. Org. Mass Spectrom. 1969, 2,433. (10) Krugh, W. D.; Gold, L. P. J. Mol. Spectrosc. 1974, 49, 423. (1 1) Razynska, A.; Tempczyk, A.; Maslinski, E.; Szafranek, J.; Grzonka, Z . J . Chem. Soc., Perkin Trans. 2 1983, 319. (12) Palmer, M H.; Simpson, I.; Wheeler, J. R. Z. Naturforsch., A 1981, 36A, 1246.
0022-3654/85/2089-0460$01.50/0
ab-initio S C F method using two split-valence basis sets. Our calculations demonstrate that the 2H tautomer is consistently more stable than the 1H tautomer. Calculation of the ionized forms of the tautomers of tetrazole and the optimization of their structure were used to explain results from photoelectronI2 and mass spectrometry” of these compounds.
Computational Details The geometries of the neutral 1H- and 2H-tetrazoles (Figure 1) were optimized with the split valence 3-21G and 6-31G basis set^.'^,'^ To estimate the dependence of the relative energies on the quality of the basis set we performed single-point calculations of the two tautomers with the 6-31G** basis setI5 at the 3-21G and 6-31G optimized geometries. To estimate the contribution of electron correlation to the energies of the tautomers, we performed C I calculations with all double excitations on tetrazole tautomers at the 3-21G and 6-31G optimized geometries with the corresponding basis set (see Table I). Upon ionization each tautomer can form two radical cations, Le., the u radical (2A’) and the ?r radical (2A”). We performed restricted Hartree-Fock (RHF) calculations on the four possible radical cations using 3-21G and 6-31G basis sets at the 3-21G and 6-31G optimized geometries of the neutral molecules, respectively. We also optimized the structures of the four radical cations using the 3-21G basis set within the unrestricted Hartree-Fock (UHF) formalism.
Results and Discussion Energies of the optimized structures of the two tautomers of tetrazole with two basis sets are shown in Table I (lines 1 and 5). It is clear that relative energies of the tautomers are basis set dependent. With the 3-21G basis set the 1H tautomer is preferred over the 2H tautomer by 1.O kcal/mol (line 1 in Table I). But, with the 6-31G basis set the preference is reversed; the 2H tautomer is more stable than the 1H tautomer by 0.8 kcal/mol. Thus, improvement of the basis set leads to stabilization of the 2H tautomer over the 1H tautomer. This can also be seen from the comparison of the relative energies of the tautomers calculated in the 3-21G optimized geometry with the 6-31G and 6-31G** basis sets and in the relative energies of the tautomers calculated in the 6-31G optimized geometry with the 6-31G** basis set (Table I). Similar changes in preference are observed when correlation is included. In the 3-21G optimized geometry inclusion of correlation lowers the preference of 1H- and 2H-tetrazole from 1.0 to 0.3 kcal/mol. In the 6-31G optimized geometry 2H-tetrazole is preferred over 1H-tetrazole and inclusion of correlation increases this preference from -0.8 to -1.1 kcal/mol. This change, (13) Binkley, J. S.; Pople, J. A.; Hehre, W. J. J. Am. Chem. SOC.1980, 102, 939. (14) Hehre, W. J.; Dictchfield, R.; Pople, J. A. J . Chem. Phys. 1972, 56, 2275. (15) Harishnaran, P. C.; Pople, J. A. Theor. Chim. Acta 1973, 28, 213. (16) Curtiss, L.; Drapho, D. L.; Pople, J. A. Chem. Phys. Lett. 1984, 203, 437. (17) Koopmans, T. Physica (Amsterdam) 1933, 1, 104.
0 1985 American Chemical Society
MO Study of Tautomerism in Tetrazole
The Journal of Physical Chemistry, Vol. 89, No. 3, 1985 461
TABLE I: Energies and Dipole Moments of Optimized Tautomers of Tetrazole
relative energies, kcal/mol
total energies, hartrees theoretical level” 1. RHF/3-21G 2. CID/3-21G//RHF/3-21G 3. RHF/6-31G//RHF/3-21G 4. RHF/6-31G**//RHF/3-21G
1H -255.268 -255.805 -256.589 -256.751
755 499 262 630
2H -255.267 179 -255.805074 -256.590 797 -256.754 583
(2H-1H) 1.o 0.3 -1.0 -1.9
5. RHF/6-31G 6. CID/6-31G//RHF/6-31G 7. RHF/6-31G8*//RHF/6-31G
-256.591 244 -257.130 286 -256.757 350
-256.592 558 -257.1 32 051 -256.760058
-0.8 -1.1 -1.7
dipole moment, D 1H 5.84
2H 2.32
6.08 5.57
2.36 2.18
6.16
2.42
5.63
2.24
“Entries are according to the notation in ref 16. For example, RHF/3-21G means R H F optimization using the 3-21G basis set, and CID/321G//RHF/3-21G means single point CI all doubles calculation using the 3-21G basis set in the R H F optimized structure with the 3-21 basis set. TABLE 11: Optimized Structural Parameters of Tetrazole Tautomers 1H-tetrazole 2H-tetrazole structural
parameter’
1H-Tetrazole
(la)
N1C5 N2N 1 N3N2 N4N3 C5N4 H5C5 H1 N 1/H2N2 N3N2N1 N4N3N2 C5N4N3 N 1C5N4 H5C5N1 HlNlN2/H2N2N3
2H-Tetrazole
(1b)
Figure 1. Tautomeric forms of tetrazole.
albeit smaller, is in the same direction as that with the 3-21G basis set. The dipole moments of the two tautomers are presented in the last column of Table I. They should be compared to the experimental values of 5.30 and 2.19 D obtained from microwave spectra for the 1H and 2H tautomers, respectively.1° Clearly, the results with the best basis set used (6-31G**) are in excellent agreement with experiment. The optimized parameters of the tautomeric forms are presented in Table 11. The dependence of structural parameters on the basis set is rather small, contrary to that of the relative energies. The difference in the optimized bond lengths does not exceed 0.043 b; for the 1H tautomer and 0.032 A for the 2H tautomer. The largest change in optimized angles is 1.91O and 1.09O for the 1H and 2H tautomers, respectively. For the 1H tautomer there is a good agreement between the optimized structure and the one measured in the crystal’ (seeTable 11). In the crystal the hydrogen on N1 is hydrogen bonded to N4 of the adjacent molecule. This may be the reason for the deviations in H l N l bond length and the H l N l N 2 bond angle. Our calculations on the relative stability of the tautomers of tetrazole lead to the conclusion that the 2H tautomer is preferred in gas phase. Even though this conclusion depends on the specific basis set used, improvement of the basis set and inclusion of electron correlation results in a consistent preference of the 2H tautomer. Thus, at the RHF/3-21G level the 1H tautomer is predicted to be more stable than the 2H tautomer, but when better basis sets are used (e.g., 6-31G and 6-31G**) this preference is reversed. The inclusion of correlation at the 3-21G level cannot reverse the incorrect preference but reduces the energy difference between the tautomers. A similar behavior is exhibited at the 6-31G level, except for the fact that already at the RHF/6-31G level the 2H tautomer is preferred over the 1H tautomer. Here, the effect of including polarization functions (6-3 lG**) increases the preference by 0.9 kcal/mol (from -0.8 to -1.7 kcal/mol), and the effect of including correlation increases the preference by 0.3 kcal/mol (from -0.8 to -1.1 kcal/mol, Table I). The two effects have been shown to be approximately a d d i t i ~ e ; ’thus ~ J ~ we estimate (18) Raghavachari, K.; Haddon, R. C.; Starnes, Jr., W. H. J . Am. Chem. SOC.1982, 104, 5054.
a
3-21G 1.346 1.386 1.278 1.415 1.298 1.061 0.994 106.49 110.44 105.39 109.91 124.46 120.64
6-31G
exDtlb 3-21G 6-31G
1.343 1.33 1.308 1.311 1.353 1.33 1.356 1.332 1.270 1.30 1.319 1.351 1.372 1.33 1.291 1.304 1.305 1.30 1.365 1.373 1.061 0.98 1.059 1.059 1.10 0.994 0.989 0.988 106.64 107.6 112.48 113.14 110.67 107.8 105.87 106.40 106.27 109.4 107.00 106.89 108.00 106.7 112.23 111.14 125.74 115.0 124.54 124.84 120.63 131.0 123.66 123.20
Bond lengths in angstroms and bond angles in degrees. From ref
1
that the 2H tautomer is more stable than the 1H tautomer by ca. 2 kcal/mol. Thus, in gas phase the mixture would be almost completely composed of the 2H tautomer, in agreement with conclusion from microwave,I0 photoelectron,]*and mass]’ spectroscopy. The good agreement of the dipole moments estimated from microwave spectral0 and those calculated here is noteworthy. The microwave estimate of the dipole moment of the 2H tautomer is 2.19 f 0.05 D and that calculated here is 2.24 D. From the spectrum of the tetrazole-5-d Krugh and Goldlo obtained a dipole moment of 5.30 f 0.05 D and tentatively assigned it to the 1H tautomer. Our calculation gives a dipole moment for the 1H tautomer of 5.63 D and supports their assignment. The ionization potentials of both tautomeric forms of tetrazole are presented in Table 111. In section A of Table I11 the ionization potentials were calculated via Koopmans’ theorem” for ionization of u and A electrons. As can be seen, the ionization potentials in section A show very little dependence on the basis set and on the geometry of the molecule. Within Koopmans’ approximation in all instances the first ionization occurs from a A orbital and the ionization from a u orbital is approximately 1 eV higher. Also, the ionization potentials for the two tautomers are nearly identical. The vertical ionization potentials of the two tautomers, calculated as the difference in the total energies of the cation radicals is shown in section and corresponding neutral molecule (aRHF), B of Table 111. Clearly, regardless of the basis set used, the u radical cations have a lower total energy than the A radical cations. Thus, the u radical is produced at a lower ionization that the A radical cations and Koopman’s approximation does not predict the correct order of ionizations in tetrazole. The difference between the u and the A ionization potentials is smaller for the 1H tautomer than for the 2H tautomer. Calculation of the vertical ionization potentials of the 2H tautomer (Table 111) is also in agreement with the photoelectron spectra observed by Palmer et a1.’* The calculated separation (19) Lievin, J.; Breulet, J.; Verhaegen, G. Theor. Chim. Acta 1981, 60, 339.
462
The Journal of Physical Chemistry, Vol. 89, No. 3, 1985
Mazurek and Osman
TABLE I11 Ionization Potentials of Tetrazole Tautomers (in eV) 1H-tetrazole U
1. 3-21G//3-21G 2. 6-31G//3-21G 3. 6-?1G**//3-21G 4. 6-31G//6-31G 5. 6-31Ge*//6-31G
2H-tetrazole lr
lr
U
A. Ionization Potentials Calculated According to Koopmans' Theorem' 12.598 11.858 12.722 12.916 12.059 13.031 12.733 1 1.595 12.860 12.906 11.971 12.929 12.824 1 1.490 12.764
1 1.743 1 1.927 1 1.379 11.933 11.384
B. Vertical Ionization Potentialsb 6. 3-21G//3-21G 7. 6-31G//6-31G
8. UHF/3-21G//UHF/3-21G
10.290 (-254.89061 3) 10.407 (-256.208784)
10.839 (-254.870429) 11.018 (-256.186338)
C. Adiabatic Ionization Potentialsb 9.62 9.34 (-254.912579) (-254.925462)
10.439 (-254.883567) 10.534 (-256.205432)
10.997 (-254.863048) 11.077 (-256.185480)
9.86 (-254.9049 12)
C
'Calculated as the negative of the highest occupied orbital energy of that type." bCalculated as difference between the energies of the ion (in parentheses;hartrees) and the corresponding neutral molecule from Table I. CNoenergy recorded because the molecule decomposes in the course of its optimization. between the first and the second ionization for the 2H tactomer is 0.543 eV which is in fairly good agreement with the value 0.7 eV estimated from the spectrum presented in ref 12. Furthermore, the correlation correction of the closed shell molecule is approximately 1 eV greater than that of the open shell radical cation.18 When this difference is taken into account the correlated ionization potentials for the 2H tautomer are ca. 11.5 and 12.0 eV, in good agreement with observed ionizations.I2 It should be noted, however, that the results obtained for the 1H tautomer could fit the observed data as well. Thus, no assignment of the photoelectron spectrum to the corresponding tautomer can be made purely on the basis of the calculated potentials; however, taken together with the other data presented above, the assignment is consistent with a major preference of the 2H tautomer. It should also be noted that in these molecules Koopmans' appr~ximation'~ predicts the wrong ionization and is therefore of little use. The structures of radical cations were optimized with the 3-21G basis set. Adiabatic ionization potentials and total energies of the optimized radical cations are shown in section C of Table 111. For the u radical cations stable structures were obtained for both tautomers, whereas for the a radical cation a stable structure could only be obtained for the 1H tautomer. In the process of structure optimization of the ir radical of the 2H tautomer the N3N2 bond became stretched beyond 2 8,indicating a tendency to break that bond. Concominant with this process was the increase in the expectation value of ( S 2 )to 1.19, much above the expected value of 0.75 for a doublet state, indicating that states of higher multiplicity are contaminating the wave function. This behavior did not depend on the structure from which the optimization was begun. The optimization was therefore not pursued further. In general, the changes in bond length were larger for the ir radical than for the u. In the u radical of the 1H tautomer the largest changes in bond lengths are in the N2N1 (0.028 A) and in the N3N2 (0.033 A) bonds. However, in the ir radical of the same tautomer the largest changes occur in the C5N4 (0.180 A) and N2N1 (0.134 A) bonds. This difference in behavior is even more pronounced in the 2H tautomer, in which the u radical exhibits only marginal structural changes, whereas the ir radical tends to stretch the N3N2 bond as described above. Thus, the u ionization induces much smaller changes in structure than the a ionization which also occur in a different part of the ion. The optimization of the structures of the radicals suggest possible fragmentation pathways of the cations, and therefore some conclusions can be reached regarding the reported mass spectra of tetrazole." Our calculations indicate that the a radicals have lower ionization potentials than the u radicals and also exhibit larger changes in the structure compared to the parent molecules. In fact, in the a radical of the 2H tautomer the N3N2 bond length increased beyond 2 A during optimization at the UHF/3-21G level. In view of the relatively small basis set and the UHF
approximation we do not attribute much significance to the observed tendency of bond breaking during the optimization process. A multiconfigurational wave function would be required to adequately represent this process. This, however, is beyond the scope of this work. Nevertheless, it seems that the a radicals are less stable than the u radicals and will tend to decompose more readily. According to the analysis of the mass spectrum9$" of tetrazole the most abundant ions are formed due to loss of N,fragment. This agrees with the fragmentation of the ir radical of the 2H tautomer in which the N2N3 and N4C5 bonds exhibit the largest changes in bond length upon ionization and are expected to be the most susceptible to cleavage. Likewise, the most probable fragmentation of the a radical of the 1H tautomer would yield the CNH, and N 3 fragments because the most susceptible bonds for cleavage are N1N2 and N4C5. Thus, the mass spectrum results again support the conclusion that the 2H tautomner is a dominant form in gas phase. These results are also in agreement with the mass spectrum of 5-methyltetrazole. If we assume that the methyl at C5 position does not significantly affect the properties of the ions, then a similar fragmentation pattern is expected. Under such circumstances the preferred fragmentation of the 2H tautomer of 5-methyltetrazole will yield the fragments N, ( m / z 28) and C2NZC4( m / z 56). These two are in fact the most abundant peaks in the mass spectrum of 5-methyltetrazole." In contrast, the fragmentation of the 1H tautomer of 5-methyltetrazole will yield the fragments N 3 ( m / z 42) and C2NH, ( m / z 40) whose presence in the mass spectrum is much lower.]' High-resolution measurements of the mass spectrum of the ['5N]5-methyltetrazole identified that the peak at m / z 56 is composed of two ions at the ratio of 1:2. The less abundant [CH3CiSNN]+results from ejection of NzH. and the more abundant [CH3NzH]+.from the ejection of I5NN. This could only occur if the molecule existed in the 2H form as shown in I.
r3
r
1
c5
I
I
N2-&
i I
+.'
J. Phys. Chem. 1985,89, 463-465
As predicted by our calculations the cleavage of the N2N3 and 15N4C5 bonds is the most likely to occur and will yield [CH3CN2H]+-while the cleavage of the N2N3 and N1C5 bonds is less likely and will yield [CH3C15NN]+. Thus, our calculations explain reasonably the preferential bond breaking observed in the mass spectrum and confirm the large preference of the 2H tautomer. Conclusion In summary, abinitio calculations of tetrazole tautomers predict the 2H tautomer to be the dominant form in gas phase. This preference is consistent with the photoelectron spectra and is supported by the mass spectra of these molecules. The resulting
463
changes in the structure of tetrazole tautomers upon ionization help to explain the observed fragmentation in tetrazole and in 5-methyltetrazole.
Acknowledgment. The authors thank Dr. Hare1 Weinstein for his helpful comments. This work was supported by USPHS Grants DA-01875 from the National Institute on Drug Abuse, Grant BNS 83-03373 from the National Science Foundation, and by the N I H Fellowship F05 TWO3241 to A.P.M. A generous grant of computer funds from the University Computing Center of the City University of New York is gratefully acknowledged. Registry No. 1H-Tetrazole, 288-94-8.
Optically Detected Magnetic Resonance Investigation of Biacetyl Adsorption on Silica: Surface Chemistry and Effects of Overlying Media C. J. Sandroff,*t J. M. Drake,* Exxon Research and Engineering Company, Corporate Research-Science Laboratory, Annandale, New Jersey 08801
and I. Y. Chan* Department of Chemistry, Brandeis University, Waltham, Massachusetts 02254 (Received: October 4, 1984)
ODMR spectra of biacetyl adsorbed on silica supports reveal molecular interactions ranging from weak physisorption to chemical bond formation. The latter likely involves a surface silanol which adds across the carbonyl double bond. Chemisorbed biacetyl is strongly perturbed by the presence of condensed solvent overlayers.
Introduction Only a few spectroscopic techniques are capable of detecting molecular monolayers formed at the liquid/solid or solid/solid interface. This fact is easily understandable given the severe constraints to be overcome in these regimes: an experimental probe while penetrating the overlying condensed phase must also be exclusively sensitive to the small numbers of molecules at the interface. We describe here an application of optically detected magnetic resonance (ODMR)’ to the study of molecules adsorbed at these complex surfaces. Essentially a hybrid technique combining the sensitivity of optical detection with the chemical specificity of magnetic resonance, it seems surprising that only one other similar application of O D M R has found its way into the literature.2 In this work we report on the behavior of biacetyl adsorbed on high surface area silicas which were subjected to various thermal and solvent treatments. Comparisons of biacetyl ODMR spectra in several glassy matrices with those of the adsorbed dicarbonyl suggest that surface silanols are able to add a c r m the carbonyl double bond. Dynamic ODMR measurements show this silicon analogue of a hemiketal to be in equilibrium with physisorbed biacetyl. Because of this equilibrium the biacetyl is sensitive to different chemical environments: the silica surface and the overlying condensed phase. Optical Spectroscopy In Figure 1 we present phosphorescence spectra of biacetyl in a variety of bulk and surface environments. The chromatographic grade silica used in these experiments (Merck, Silica Gel 60) was characterized by a particle size distribution of 60-200 pm, pore diameters of 60 A, and a surface area of 400 m2/g. Prior to use ‘Current Address: Bell Communications Research, 600 Mountain Avenue, Murray Hill, NJ 07974.
0022-3654/85/2089-0463$01.50/0
the silica gel was hydrated and then dehydrated at 165 or 600 OC for 3 h in flow of dry helium, removing physisorbed water and some of condensable hydroxyl^.^ This treatment provided us with reproducibly activated surfaces with silica hydroxyl densities of -3-6 OH nm-2. Biacetyl was adsorbed onto the gel from solutions of the a-dicarbonyl in dried cyclohexane. From the known surface area of the silica support, we estimate that the loading ( g of biacetyl/g of support) resulted in surface coverages of biacetyl of 1 5 % . The low temperature spectra of adsorbed and bulk biacetyl show several structural differences indicating that biacetyl is strongly perturbed by its interaction with the surface. The largest perturbation is seen for biacetyl on silica surfaces which were dehydrated at 165 OC. On this surface we find a 30-nm blue shift in the phorphorescence origin relative to biacetyl in cyclohexane. This shift can be ascribed to interactions between the surface and the n orbital of the adsorbate rather than the extended ?r system! The surface sites responsible for these interactions seem to be chemically diverse as suggested by the increase in the line width of the phosphorescent emission. Further evidence for the chemical heterogeneity of the surface comes from the comparison of biacetyl behavior on silica surfaces dehydrated at different temperatures. Because of condensation of adjacent hydroxyls, dehydration of silica at 600 O C yields surfaces with lower hydroxyl densities than dehydration at 165 O C . The reduction in the number and type of hydroxyl sites capable of bonding with biacetyl gives rise to a more homogeneous distribution of adsorbed molecules. The (1) “Triplet State ODMR Spectroscopy”; Clarke, R. H., Ed.; Wiley: New York, 1982. (2) Svejda, P.; Maki, A. H. Chem. Phys. Lett. 1981, 83, 610. (3) Kondo, S.; Tomai, K.; Pak, C. Bull. Chem. SOC.Jpn. 1919, 52, 2046. (4) Sidman, J. W.; McClure, D. S.J. Am. Chem. SOC.1955, 77, 6461.
0 1985 American Chemical Society
