J. Phys. Chem. 1983, 87, 1917-1924
1917
Studies of Primary Alkyl and Aralkyl Radicals Using Electron Spin Resonance Spectroscopy and Intermediate Neglect of Differential Overlap Calculations Steven Brumby Department of Chemlstty, Natlonal Universitv of Slngapore, Kent Rldge, Slngapore 051 1 (Recelved: October 6, 1982)
It is argued that a published ESR spectrum due to n-propyl radicals in solution does not, as has been claimed, exhibit selective broadening of the lines with M = 0 for the a protons, but, rather, a broadening of the lines with M = f l for the ?,3 protons. Based on INDO calculations for the n-propyl radical, and an analysis of the restricted internal rotation, the reported variations in ay with temperature are rationalized. ESR spectra of 2-hydroxyethyl radicals in solution have been analyzed, and no line width alternation was detected at temperatures of -50 "C and above. The well-known line width alternation in the ESR spectrum of n-butyl radicals is discussed and it is suggested, partly on the basis of INDO calculations, that the preferred orientations about the C,-C, bond are dependent, to some extent, on the configuration with respect to the CB-Cybond. The ESR spectra of the species Me3CCH2CH2.,Me3CCH2CH2CH2-, and Ph3CCH2CH2*are reported and discussed. The spectrum of the former species shows no line width alternation, but the spectrum of the latter species shows a pronounced alternation, which is attributed to the chiral nature of the trityl group. ESR spectra of the species Me(CH2),CH2. and Ph(CH2),CH2.,with n = 2, 3, and 4, display particularly marked line width alternation in the latter series when n = 3 and 4. This observation, together with the results of INDO calculations, may possibly indicate that these radicals prefer conformations with the plane of the trigonal carbon atom parallel to the phenyl-group plane.
Introduction The free radicals discussed in this paper are all of the general type XCH2CH2.,and, in consequence, they all give somewhat similar isotropic ESR spectra. However, the values of the hyperfine splitting constants, and variations in line width, if any, often given insight into the preferred geometries of the radicals. Semiempirical molecular orbital calculations are sometimes of value in further elucidating these geometries. Experimental Section Bromo Compounds. Phenethyl bromide and 2-bromoethanol were available commercially. (&Bromopropyl)benzene was synthesized from cinnamyl alcohol by a literature meth0d.l A portion of (3-bromopropy1)benzene was used to prepare benzenebutanol by a modification of a literature method2 in which paraformaldehyde was used instead of tri~xymethylene.~Benzenebutanol was converted to (4-bromobuty1)benzeneby a literature method.2 l-Bromo-4,4-dimethylpentane was prepared via 4,4-dimethyl-l-pentene by using a literature m e t h ~ d . ~ 3,3,3Triphenyl-l-propanol was prepared by a modification of a literature route: in which 3,3,3-triphenylpropionicacid was reduced directly to the alcohol, by using lithium aluminum hydride, instead of being first converted to the acid chloride. 3,3,3-Triphenyl-l-propanol was treated with phosphorus tribromide and pyridinee to yield the known6 compound l-bromo-3,3,3-triphenylpropane. Carboxylic Acids. Pentanoic, hexanoic, and heptanoic acids were available commercially. .l-Phenylbutanoic, 5-phenylpentanoic, and 6-phenylhexanoicacids, all known C O ~ ~ O U were I I ~prepared S , ~ ~ from ~ ~ the appropriate bromo
compounds by malonic ester ~yntheses.~ One portion of l-bromo-4,4-dimethylpentane was converted to 4,4-dimethyl-l-pentanol by a literature procedure; the alcohol was then oxidized to the known4 compound 4,4-dimethylpentanoic acid by using potassium permanganate3 Another portion of l-bromo-4,4-dimethylpentanewas converted to the nitrile and then hydrolyzed to give 5,5dimethylhexanoic acid by a known method? but basing the details of the procedure on a published synthesis of 4-methylpentanoic acid.'O Diacyl Peroxides. A number of procedures have been described by which diacyl peroxides may be prepared.l1-l4 In this work the following procedure, in which the intermediate acid chloride is not isolated, was found to be satisfactory. The appropriate carboxylic acid ( x mol) was refluxed for 1 h with thionyl chloride (2x mol). The product was dissolved in diethyl ether and added dropwise to a mixture of 90% sodium peroxide (4x mol), crushed ice, and water, with vigorous stirring. After the addition was complete, stirring was continued for 20 min. The ether phase, combined with one ether extract, was washed with water and then dried with sodium sulfate. After the ether was removed by evaporation, the diacyl peroxide remained as a pale yellow or colorless oil or solid and was used without further purification. Samples. When cyclopropane was used as a solvent, samples were degassed on the vacuum line, and the sample tubes were sealed under high vacuum. Otherwise, sample tubes fitted with serum caps were used, and oxygen was removed by passage of helium. Equipment. The ESR spectrometer and associated equipment have been described e1se~here.l~Molecular
(1) Kharasch, M. S.; Weinhouse, S. J. Org. Chem. 1936, 1 , 209. (2) Braun, J. Ber. 1911, 44, 2867. (3) Vogel, A. I. *A Textbook of Practical Organic Chemistry"; Long mans: New York, 1955. (4) Whitmore, F. C.; Homeyer, A. J. J. Am. Chem. SOC. 1933,55,4555. ( 5 ) Wilt, J. W.; Lundquist, J. A. J. Org. Chem. 1964, 29, 921. (6) Smith, L. H. 'Organic Syntheses"; Wiley: New York, 1955; Collect. VOl. 111, p 793. (7) Schwenk, E.; Papa, D. J. Org. Chem. 1946, 11, 798.
(8) Freedman, L. D.; Doak, G.0. J. Am. Chem. SOC.1949, 71, 779. (9) Whitmore, F. C.; Homeyer, A. J.; Jones, D. M.; Trent, W. R. Chem. Zentralbl. 1936, 1 , 4328. (10) Noyes, W. A. J. Am. Chem. SOC. 1901,23, 393. (11)Fieser, L. F.; et al. J. Am. Chem. SOC.1948, 70, 3174. (12) DeTar, D. F.; Weiss, C. J. Am. Chem. SOC.1956, 78, 4296. (13) Slagle, J. R.; Shine, H. J. J. Org. Chem. 1959, 24, 107. (14) Silbert, L. S.; Swern, D. J.Am. Chem. SOC.1959, 81, 2364.
0022-3654/83/2087-1917.$01.50/0
0 1983 American Chemical Society
1918
The Journal of Fhysicai Chemistry, Vol. 87, No. 11, 1983
Flgure 1. Calculated second-order ESR spectrum for the n-propyl radical, including modulation effects (ref 21). The W m wlth M, = fl were assigned a peak-to-peak width of 0.2 0, and the lines wkh M, = 0 were assigned a width of 0.13 0. Other parameters were assigned reasonable values.
orbital calculations were carried out on Univac 1100 and IBM 4341 computer systems.
Results and Discussion n-Propyl Radical. In discussions of rotation about the C,-C, bond in the n-propyl radical, it is convenient to define a dihedral angle, 4, as in l.I6 From the magnitude
I'
H
H
H2
H.
H3
2
1
and temperature dependence of a,, determined from isotropic ESR spectra, the radical has long been thought to prefer conformations with 4 90°.17J8Krusic et al.19,20 pointed out that the a protons are inequivalent in such conformations, which might lead to selective broadening of the spectral lines with M a= 0.21 Furthermore, these author^'^^^ published a spectrum, recorded at -140 "C, which they considered showed evidence of such alternation. We have examined this spectrum carefully and compared it with second-order spectra calculated by assuming different types of line width alternation, and including modulation effects.22 Although the comparison was hampered by a lack of knowledge of the experimental conditions, especially the center of scan field setting and the modulation amplitude, we did not find evidence of selective broadening of the lines with Ma = 0. Instead, the published spectrum appears to display selective broadening of the lines with M, = f l , as assumed in the calculated spectrum shown in Figure 1. This calculated spectrum reproduces all the important features of the spectrum of Krusic et al.,19,20which is not possible on the basis of selective broadening of the lines with Ma = 0; in particular, the central multiplet then has too large a line width. The foregoing remarks are not intended to cast doubt on the longstanding conclusion that conformations are preferred in which the methyl group lies close to the plane of the trigonal carbon atom. However, even with @ = 90" the expected absolute difference between the a-proton splitting constants (Aa,) is not large. Presumably, 180"
-
(15)Lee, K. H.; Brumby, S. J. Chem. SOC.,Perkin Trans. 2 1982,1537. (16)This conforms with the definition used by: Barfield, M.J . Phys. Chem. 1970,74,621. (17)Fessenden, R. W.; Schuler, R. H. J . Chem. Phys. 1963,39,2147. (18)Fessenden, R. W.J . Chim. Phys. Phys.-Chim. B i d . 1964, 61, 1570. (19)Krusic, P. J.; Kochi, J. K. J. Am. Chem. SOC.1571,93,846. (20)Krusic, P. J.; Meakin, P.; Jesson, J. P. J . Phys. Chem. 1971,75, 3438. (21)Mzand M , represent the manetic quantum numbers for the a and protons, respectively. (22)Brumby, S.Chem. Phys. Lett. 1982,87,37.
Brumby
reorientations about the C,-C, bond occur with greater frequency than this difference, so that selective broadening of the lines with Ma = 0 is not observable. Actually, Krusic et a1.20attempted to simulate the n-propyl radical spectrum using Aa, = 1.1G, based on INDO calculations, and they conceded that an implausibly large barrier to internal rotation was implied. The suggestion that components with M , = 0 are not selectively broadened in the ESR spectrum of n-propyl radicals in solution seems to be consistent with the spectrum of the radicals in an argon matrix at 4 K, when the a protons appeared e q ~ i v a l e n t . ~Admittedly, ~ a small difference between the splittings might not be resolved under these conditions; furthermore, one cannot necessarily assume that the preferred conformations are the same in different environment^.^^ Analysis of the spectrum of the trapped radicalsz5 led to the opinion that @ = 80.2" in the preferred conformation but that considerable torsional oscillation occurred. The sample used in the experiment of Krusic et aL20 was a solution of n-butyryl peroxide in cyclopropane, cooled to -140 "C. A t this temperature the solution was significantly colder than pure cyclopropane at its melting point of -127.6 0C.26 When we used the same sample components, the lowest temperature which we could achieve in this laboratory, without solidification, was -135 "C. At this temperature, no variation in line width could be detected. One might speculate that, for the solution cooled, to -140 "C, the viscosity was sufficiently high to give rise to the observed line width variations, due to incomplete averaging of the hyperfine interactions. However, this would not explain the observed effects satisfactorily, since the anisotropic hyperfine components are smaller for the 0 protons than for the a protons.z3 A more satisfactory explanation seems to be that radical-solvent interactions modulate agand that, under the experimental conditions, the correlation time for the interactions was long enough to give rise to the observed line width variations. Molecular orbital calculations at the INDO level of approximation have sometimes been used to gain insight into the preferred geometries of free radicals. The most straightforward approach is to find the geometry which minimizes the calculated energy, but it is interesting to note that this approach fails when applied to the n-propyl radical. Thus, we found2I that the calculated energy was a minimum for the conformation with @ = OD,being 6.2 kJ mol-' less than the energy of the conformation with @ = 90". In contrast, ab initio calculations have, on at least one occasion,28been reported to predict an energy minimum when $ = 90°, in conformity with the ESR observations. On other occasions,29~30 however, no energy minimum was found for this conformation. Another approach which has been used is to find the geometry for which there is satisfactory agreement between calculated and observed (23)Adrian, F.J.;Cochran, E. L.; Bowers, V. A. J . Chem. Phys. 1973, 59, 3946. (24)It has been suggestedmthat the apparent discrepancy between the ESR spectra in solid argon and in solution might be due to radical-olvent interactions in the liquid phase. (25)Adrian, F. J.; Bowers, V. A.; Cochran, E. L. J . Chem. Phys. 1975, 63. 919. (26)Weast, R.C., Ed."Handbook of Chemistry and Physics",55th ed.; CRC Press: Cleveland, OH, 1974. (27)Phillips, A. C. QCPE 1975, 11, program 274. Standard bond lengths and bond angles were assumed. (28)Ellinger, Y.;Rassat, A.; Subra, R.; Berthier, G. J . Am. Chem. SOC. 1973,95,2373. (29)(a) Radom, L.;Paviot, J.; Pople, J. A.; Schleyer, P. v. R. J . Chem. Soc., Chem. Commun. 1974,58. (b) Pross, A.; Radom, L. Tetrahedron 1980,36,1999. (30)Pacansky, J.; Dupois, M. J. Chem. Phys. 1979,71,2095.
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The Journal of Physical Chemistry, Vol. 87, No. 11, 1983
Primary Alkyl and Aralkyl Radicals
1919
i t b+;
V
O-----
0
2rr
Tl
+@
Flgure 3. Square wave potential assumed for rotation about C,-CB in the n-propyl radical.
I
I
0
n; 2
6
n
Flgwe 2. yproton hypenine splitting constants for the npropyi radlcal, as calculated by the INDO method. The angle ,$ is defined in 1, and the distinction between the y protons H1, H2, and H3 is made in 2.
hyperfine splitting constants. y-proton splitting constants for n-propyl radicals have been estimated by using several theoretical appro ache^,^^ including the INDO method.32 It seems reasonable to suppose that staggered conformations with respect to the C,-C, bond are preferred, as shown in 2. The splittings of the y protons H1, H2, and H3 show interesting variations with the angle 4, which, based on calculations by the INDO method,27are illustrated in Figure 2. The hyperfine splitting due to the proton H1, which is trans to the CH2-group, shows particularly strong angular variations. When ,$ = O", the position of this proton corresponds to the W plan,%giving a substantial hyperconjugative, and therefore positive, contribution to the splitting constant. This conforms, qualitatively, with the results of ab initio calculations.28 The rotationally averaged y splittings which would be expected on the basis of these calculations depend on the nature of the hindered rotation about C,-C,, which will be considered next. Fessenden18 used the measured &proton hyperfine splittings for the n-propyl radical in the temperature range -180 to -50 "C to investigate the hindered internal rotation about C,-C,. The angular variation of the potential energy was assumed to be given by eq 1. The corresponding wave (1) V(,$) = f/2V0(1- cos 24) equation is of the form of Mathieu's differential equation, for which tabular solutions have been published. The proton hyperfine splittings were estimated by using the familiar equation, eq 2. The appropriate value of cos20 U B = B + A COS' 0 (2) (31) Splittings calculated by the different methods have been compared by: Underwood, G. R.; Friedman, H. S. J. Am. Chem. SOC.1974, 96.4089. (32) Underwood, G.R.; Vogel, V. L.; Iorio, J. Mol. Phys. 1973,25,1093. (33) King, F. Chem. Reu. 1976, 76, 157.
was obtained by first calculating the average value of cos2 8 for each of the rotational states and then calculating the average over the different states, each with Boltzmann population. The best results were obtained with V, = 412 cal mol-' (1.72 kJ mol-') but even then the agreement between theory and experiment was not entirely satisfactory, the calculated values of a, being too large at the higher temperature and too small at the lower temperature. It has been suggestedz0that this rather poor agreement between theory and experiment might be due to a departure from tetrahedral geometry at the &carbon atom. Another possibility is that eq 1 represents an unsatisfactory approximation. In this work, a square wave was assumed for the potential energy, as illustrated in Figure 3. Although this form may not seem particularly suitable, it is actually capable of giving closer agreement with experiment than the sine wave assumed by Fessenden, possibly because the potential is characterized by two (instead of one) adjustable parameters. The calculations are manageable because wave functions in regions of constant potential may be expressed a n a l y t i ~ a l l y . It ~ ~is possible to adjust the parameters Voand b (Figure 3) so that the calculated values of as agree with the experimental values at both extremes of temperature. However, the optimum values of Vo and b depend rather strongly on the values assumed for A and B (eq 2). We used B = 1.74 and A = 49.96 G, conforming with our INDO calculations for npropyl. With b = 126" and Vo = 2.193 kJ mol-', and using Fessenden's estimate of the reduced moment of inertia,18 the calculated values of a, were 33.22 (-180 "C) and 29.39 (-50 "C) G, in close agreement with the experimental values of 33.2 and 29.4 G, re~pective1y.l~ The rotationally averaged y-proton splittings were calculated by a similar procedure, using the angular variations of the splittings shown in Figure 2. A fixed staggered conformation about the C,-C, bond was assumed, as shown in 2, so that each calculation gave the same splitting constant for two of the y protons, which differed from that of the third. With the parameter values which seemed satisfactory in the previous calculations ( b = 126", Vo = 2.193 kJ mol-'), the calculated y splittings at -180 "C were -1.794 (two protons) and -0.404 (one proton) G and at -50 "C the splittings were -1.786 (two protons) and +0.329 (one proton) G. Experimentally, splitting by only two y protons, each with la1 = 0.69 G, was observed at -180 "C.17 On warming to -145 "C, the three y protons appeared equivalent, with la1 = 0.38 G; on further warming to -105 "C, la1 = 0.27 G.35 Nearly quantitative agreement between calculation and experiment cannot reasonably be
''
(34) Bohm, D."QuantumTheory"; Prentice Hall Englewood Cliffs, NJ,1951; Chapter 11. (35) Kochi, J. K.; Krusic, P. J. J. Am. Chem. SOC.1969, 91, 3940. (36) Stone, E. W.; Maki, A. H. J.Chem. Phys. 1962,37,1326,Figure 1. Reference 20,Figure 1.
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The Journal of Physlcal Chemktry, Vol. 87, No. 11, 1983
Brumby
TABLE I: Hyperfine Splitting Constants of Free Radicals p r o t o n hyperfine splitting constants, G radical HOCH,CH,. t-Bu-CH,CH,. t -Bu-CH,CH,CH,. Ph,C-CH,CH,. Me-(CH,),CH,. Me-(CH,),CH,. Me-( CH,),CH,. Ph-(CH,),CH,. Ph-(CH,),CH,. Ph-(CH,),CH;
temp,OC -30 -100 -60 -60 -50 -50 -50 -100 -100 -100
a,
ap
a7
21.9, 21.9, 21.9, 21.9, 21.9, 21.9, 21.9, 21.9, 21.9, 21.9,
30.2, 24.7, 28.3, 27.8, 28.4, 28.5, 28.5, 28.6, 28.8, 28.9,
0.3, 0.7, 0.7, 0.7, 0.7, 0.7, 0.7, 0.7,
expected in view of the inherent approximations of the INDO m e t h ~ d . ~ ~Nevertheless, ~~' the calculations do suggest a way of rationalizing the trends observed experimentally. These trends are attributable largely to the strong angular dependence of the hyperfine splitting of the y proton trans to the CH2-group (Figure 2). The rotational average of this splitting changes from negative at low temperatures to positive at higher temperatures. Presumably, the splitting approximates to zero at -180 "C, when rotation of the terminal methyl group is slow. At temperatures of -145 "C and above, the methyl group rotates more rapidly, so that the three y protons appear equivalent, with a hyperfine splitting constant which is a weighted average of a splitting, due to two protons, which is negative and has a relatively small temperature coefficient, and a splitting, due to one proton, which is positive, with an appreciable positive temperature coefficient. The average is a negative splitting, the absolute value of which becomes smaller at higher temperatures. This rationalization can only be acceptable if the yproton splitting constant is indeed negative. NMR measurements have provided a considerable body of information on the signs of y-proton splittings, which are sometimes negative and sometimes positive. However, considering only aromatic anions with freely rotating alkyl substituents, which might be expected to serve as reasonable models for the n-propyl radical, the signs of the splittings are usually (but not always) negative.3s Recently, hyperfine parameters for the n-propyl radical have been reported39which are difficult to explain in the light of earlier investigations. In view of this the interpretation of the spectra concerned is open to question. 2-Hydroxyethyl Radical. Krusic and Kochi reportedlg that the ESR spectrum of the 2-hydroxyethyl radical in solution displayed a "pronounced" broadening of the lines with M a= 0, but they did not state at what temperature the effect was observed. In this work, 2-hydroxyethyl radicals were generated specifically by the photolysis of solutions of 2-bromoethanol and hexamethylditin in di-tert-butyl peroxide and toluene. In this system, trimethyltin radicals are generated, which abstract bromine atoms from 2-bromoethanol.40 The method was found to give higher steadystate concentrations of 2-hydroxyethyl radicals than when triethylsilane was substituted for hexamethylditin, but the lowest temperature which could be achieved without separation of solid was -50 "C. At this temperature and (37) Sullivan, P.D.;Wright, W. L. J. Magn. Reson. 1974, 13, 232. (38) de Boer, E.;van Willigen, H. Prog. Nucl. Magn. Reson. Spectrosc. 1967,2,111. Due to a different way of labeling the protons, a, is referred to aa a &proton splitting in this review. (39) Iwasaki, M.; Torlyama, K.; Muto, H.; Nanome, K.; Fukaya, M. J. Phys. Chem. 1981,85, 1326. (40) Cooper, J.; Hudson, A.; Jackson, R. A. J. Chem. Soc., Perkin Trans. 2 1973, 1056.
Figure 4. (a) ESR spectrum of 2-hydroxyethyl radicals in solution at -30 "C. (b) Simulated secondorder spectrum, including splitting by the hydroxy proton.
above, there was no detectable indication of line width alternation. In Figure 4, a spectrum recorded at -30 "C is shown. In this spectrum the hyperfine splitting due to the hydroxy-group proton is similar in magnitude to the second-order splitting of the lines with MB= 0, so that the components with only this second-order splitting (i.e., M B = 0, Ma= f l ) appear as triplets. A simulated spectrum, in which all lines were assigned the same width, is included in the figure, and the hyperfine coupling constants used in the simulation are reported in Table I. Previous i n v e s t i g a t ~ r generated s ~ ~ ~ ~ ~ the 2-hydroxyethyl radical by photolysis of ethanol-hydrogen peroxide solutions, when the major contribution to the ESR spectrum was from 1-hydroxyethylradicals. Livingston and Z e l d e ~ ~ ~ observed structure similar to that shown in Figure 4 but did not present a detailed analysis. n-Butyl Radical. ESR spectra of n-butyl radicals in solution at low temperatures display selective broadening 3-5 represents the three of the lines with M B = 0.19,35
H 3
H
Me
4
5
conformations of the radical which are staggered with respect to the C& bond. Krusic and Kochi attributedlg the observed line width alternation to transitions between 3 and 5, which exchange inequivalent y protons. Although the y protons in 3 and 5 are technically inequivalent, it is by no means obvious why their hyperfine splitting constants should be sufficiently different in magnitude to explain the pronounced line width alternation which is observed. No explanation for this was offered by Krusic (41) Livingston, R.;Zeldes, H. J. Chem. Phys. 1966, 44, 1245.
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Primary Alkyl and Aralkyl Radicals
and Kochi, nor did they discuss the importance of conformation 4. The multiplets in the n-butyl radical spectrum which are not selectively broadened show a splitting pattern due to the y protons which is relatively insensitive to temperature% and which bears a noteable resemblance to the y-proton splitting pattern for n-propyl radicals cooled to -180 O(3.l' At this temperature it seems possible that the proton H1 of n-propyl(2) has a splitting constant which approximates to zero, while the protons H2 and H3 have a temperature-insensitive splitting constant of -0.69 G , as discused above. It is tempting to draw the conclusion that the n-butyl radical prefers the planar conformation 4, in which the terminal methyl group occupies the position of H1. However, other evidence casts doubt on this conclusion, which, incidentally, does not take account of the possibility that the terminal methyl group might actively influence the y-proton splittings. The ESR spectrum of n-butyl radicals trapped in an argon matrix at 4 K was analyzed by Adrian et al.25The 0protons appeared inequivalent under these conditions, and the radicals were thought to oscillate torsionally about the equilibrium conformation 6. The geometry of this 66.8'
k ; p E tH
H
6 conformation was based on a purely classical analysis of the internal rotation and therefore may be in error. Adrian et al. speculated on the reasons for the pronounced difference between the 0-proton splittings, corresponding to the asymmetric conformation 6, and their speculations are relevant also to the line width alternation seen in solution spectra. They postulated that nonplanar rotamers, such as 3 and 5, were favored, rather than the planar rotamer 4. In a nonplanar conformation they considered that the orientation of the a-CH2group might be influenced, sterically, by the terminal methyl group, and this might explain the preference for a conformation such as 6. We have used INDO c a l ~ u l a t o n sto~ ~investigate the stereochemistry of n-butyl radicals. Corresponding to the rotamers 4 and 5, the orientations of the a-CH2 group which minimized the total energy are shown in 7 and 8.
H ; &H
4 .1
H
H I
,
31.1
7 8 In each diagram,either a hydrogen atom or a methyl group is eclipsed by C,. The predicted &proton coupling constants are shown. Of course, the favored geometry for rotamer 3 corresponds to the mirror image of 8. Note that the orientations of the 0protons in 8 do not conform to any of the possible equilibrium conformations usually considered.% The calculations give support to the ideas of Adrian et al. inasmuch as they predict that the planar conformation 7 is relatively unstable and that in the nonplanar conformations the a-CH2-grouporientation is
No. 11,
1983
1921
influenced by the terminal methyl group. However, it is doubtful if the a-CH2-group orientations in 7 and 8 are even approximately correct: by analogy with n-propyl, one suspects that the orientation in 7 may be in error by 90°, and the orientation in 8 does not seem to be consistent with the @-protonsplittings determined at 4 K.25 Lloyd42has investigated line width alternations in the ESR spectra of two a-substituted n-butyl radicals in cyclopropane solution. The slow-exchange@-protonsplittings used in simulating the spectra were obtained from the spectrum of one of the radicals trapped in an adamantane matrix, when the 0protons appeared inequivalent. It was assumed that conformations of the radicals with planar carbon skeletons, analogous to 4, did not contribute to the spectra. It is possible to simulate n-butyl radical spectra with alternating line widths on the same assumption, but the difficulty lies in selecting suitable values for the slow-exchange 0-proton splitting constants. More precisely, the absolute difference in magnitude between the slow-exchange @-protonsplitting constants (e.g., Aa,) is difficult to ascertain, the sum of the splittings being easily determined from the observed spectra. There is reason to believe that the value of Aa, determined for n-butyl radicals in an argon matrix at 4 K (25.2 G25)is too large to be applicable at the higher temperatures used when solution spectra are recorded. Thus, torsional modulation of the @-protonsplittings was though to be significant at 4 K25and would presumably have a greater influence on the splitting at higher temperatures. Attempts in this laboratorSP3to simulate the reported spectrum of n-butyl radicals at -105 0C19935using Aa, = 25.2 G were unsuccessfu1,44but satisfactory spectra could be simulated by using smaller values of Aas. For example, with Aa, = 0.6 G, a spectrum in excellent agreement with the experimental spectrum was calculated by assuming an exchange rate of 5.5 X lo5 Hz. Me3C-CH2CHz.and Me3C-CH2CH2CH2.Radicals. The ESR spectra of these free radicals were recorded during the photolyses of the appropriate diacyl peroxides dissolved in equal parts by volume of toluene and cyclopropane, and the hyperfiie splitting constants are reported in Table I. ESR parameters have been reported previously for the former radical, but without comment.46 The spectrum of Me3C-CHzCHz.is of interest because it has two features which can be readily explained and which thereby add credence to the discussion of related radicals. First, under the conditions employed, the spectra did not show any significant variation in line width. This is as expected, since the rotamers analogous to 3-5 are indistinguishable. Second, the magnitude of the @-proton hyperfine splitting constant was smaller than that for the ethyl radical17and had a positive temperature coefficient. In these respects, the radical differs from the other radicals listed in Table I. Presumably, the relatively large size of the tert-butyl group results in stabilization of conformations in which the group is eclipsed by the p orbital of the unpaired electron. For Me3C-CH2CHzCHz.,the ESR spectrum showed alternating line widths of the same general type as shown by n-butyl radicals. However, the difference in width (42) Lloyd, R. V. J.Phys. Chem. 1981,85, 1440. (43) For details of the simulation method: Brumby, S. J.Magn. Reson. 1981, 44, 429. (44) When the exchange rate was adjusted to give exchange-broadened multiplets with approximately the correct width, their relative intensities were too small. (45) Hudson, A.; Jackson, R. A. J.Chem. SOC.,Chem. Common. 1969, 1323. (46) Edge, D. J.; Kochi, J. K. J. Am. Chem. SOC.1972, 94, 7695.
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The Journal of Physical Chemistty, Vol. 87, No. 1 7 , 7983
Scheme I
I
HA
I
,
"
8
I
Figure 5. (a) ESR spectrum of Ph3C-CH2CH,. radicals in solution at -60 OC. (b) Simulated spectrum, assuming slow enantiomerization.
between the broad and narrow lines was less pronounced, at -50 "C, than for n-butyl radicals. One can only speculate, but it seems possible that the conformation analogous to 4 is favored by the large size of the tert-butyl group. Other things being equal, this would make the line width alternation less pronounced. Ph3C-CHzCH2.Radical. This species was generated by the photolysis of solutions of 3,3,3-triphenylpropylbromide in triethylsilane, di-tert-butyl peroxide, and toluene.& The ESR spectra indicated that the Ph3C-CH2CHz-radicals formed did not rearrange significantly. This is in line with chemical evidence5 and may be contrasted with the rapid rearrangement of the homologous tritylmethyl free radi~ a l .The ~ ~lowest temperature at which measurements could be made without separation of solid was -60 "C, and an ESR spectrum recorded at this temperature is shown in Figure 5. The spectrum shows a pronounced broadening of the lines with Mg = 0, and a significant broadening of the outermost lines. I t is well-known that the trityl group is chiral, two enantiomeric propeller arrangements being possible.48 However, the trityl group differs from certain chiral groups, such as asymmetric carbon atoms, in that spontaneous Flgure 6. Portions of ESR spectra comprising seventh and eighth transitions between the enantiomers are possible.@ NMR multiplets, 10 G between NMR marks: (a) alkyl radicals, -50 "C;(b) measurements have been used to estimate rates of enanaralkyl radicals, -100 "C. tiomerization for several ortho-substituted trityl groups. about C,-C, there will generally be four rotamers correFor example, the rate of enantiomerization for trimesitylmethane at 167 "C has been estimated to be 113 H Z . ~ ~ sponding to potential minima, two of which are shown in Scheme I. If R is chiral, then q+ # &, and the protons Unfortunately, there does not seem to be any information cause splitting into eight lines in the slow-exchange limit in the literature from which one can compare the rate of and four lines in the fast limit. A point which does not enantiomerization for the unsubstituted trityl group with appear to have been mentioned by other authors is that the rate of ca. 4 X lo6 Hz, which seems to be characteristic similar remarks apply also to the a-proton ~ p l i t t i n g s .Of ~~ of the selective broadening observed in the ESR spectrum course, the a-proton splittings are not expected to vary by recorded at -60 "C. In this work, several possibilities were such large amounts as the @-protonsplittings, and one entertained regarding the order of magnitude of the rate might expect the effects on the ESR spectra to be negliof enantiomerization. If the rate of enantiomerization is relatively slow, it is gible. But, to explain the broadening of the outer lines of appropriate to interpret the ESR spectrum in the same the experimental spectrum shown in Figure 5, it is necessary to suppose that the a-proton splittings differ slightly way as for a radical such as CH2CH2CXYZ.Line width alternations arising from P-proton splittings in radicals of between the rotamers. A simulated spectrum, based on the assumption of slow enantiomerization, is shown in this type are w e l l - k n ~ w n . ~For ~ a complete revolution Figure 5 . The following parameter values (splittings in gauss) were used in the simulation: a, = 21.7, ag = 26.1, (47)Rickatson, W.;Stevens, T. S. J. Chem. SOC.1963,3960. 29.1 for one rotamer, a, = 22.3, ag = 29.6, 26.6 for the other (48)Mislow, K.Acc. Chem. Res. 1976,9, 26.
(49)Finocchiaro, P.;Gust, D.; Mislow, K. J.Am. Chem. SOC.1974,96, 2165. (50) Gilbert, B. C.; Larkin, J. P.; Norman, R. 0. C. J. Chem. SOC., Perkin Trans. 2 1972,1272. Gilbert, B. C.;Trenwith, M. Ibid. 1973,1834. Felix, C. C.;Sealy, R. C. J. Am. Chem. SOC.1981,103, 2831.
(51)The situation is not exactly equivalent, when one takes account of the two rotamers not shown in the scheme, in which the a protons are exchanged.
The Journal of Physical Chemistry, Vol. 87, No. 11, 1983 1923
Primary Alkyl and Aralkyl Radicals -20.3 -21.6
-1.8
0.1 0.8
0.6
b
a
C
Figure 7. Three conformationsof the 4-phenylbutyl radical, drawn in perspective by using a computer program (ref 46). The hyperfine splitting constants of protons, calculated by the INDO method, are shown except when they are less than 0.1 G.
rotamer; exchange rate 4 X lo6 Hz. These values are approximate only, since it is possible to find other combinations of parameter values which give equally satisfactory simulated spectra. It is possible to interpret the major features of the experimental spectrum in another way, by assuming fast rotation about the C,-C, bond, and a rate of enantiomerization of ca. 4 X lo6 Hz. Either enantiomer would display a splitting due to the @ protons of four equally intense lines, and enantiomerization would lead to exchange broadening of the central doublet. It is not clear how the selective broadening of the outermost lines of the spectrum could be explained on this hypothesis. For ortho-substituted trityl groups, such as trimesitylmethyl, the evidence suggests that the principal mode of enantiomerization is by the two ring flip mechanism.4s If a similar process occurs rapidly for an unsubstituted trityl group, it is conceivable that one of the phenyl groups might execute a number of rotations, while the other two merely oscillate in phase. On a suitable time scale, the three rotamers which are staggered with respect to the Cg-C, bond would not appear equivalent; this is illustrated in 9-1 1, where the rotating phenyl groups are shown in flat
*** C H'
9
1
10
@
11
1
profile and the oscillating phenyl groups in edge profile. A spectrum was calculated on this basis, which was in excellent agreement with the experimental spectrum shown in Figure 5, and which did not differ conspicuously from the simulated spectrum shown in Figure 5. The following parameter values were used in the simulation: a, = 21.75, ag = 25.75, 29.75 for 9; a, = 22.25, a, = 28.25 for 10; exchange rate 4 X lo6 Hz. The hyperfine splittings used for 11 were the same as for 9 except that the @-protonsplittings were exchanged. The exchange rate is the rate at which transitions occur between any pair of 9-11, and includes contributions from rotation about C,-C, as well as from spontaneous changes in the identity of the rotating phenyl group. The simulated spectra for Ph3C-CH2CHz. were calculated by the fast Fourier transform method.43The two-site problem that was formulated, corresponding to slow enantiomerization, was dealt with by using analytical expressions which have been published.43 To deal with the three-site problem, corresponding to fast enantiomerization, the eigenvalues of the 3 X 3 matrix (in+ n) corresponding to each multiplet43 were found by numerical solution of the characteristic equation.
From the foregoing discussion, it is evidently impossible to interpret the recorded ESR spectra in an unambiguous way. The spectra would be more informative if they could be recorded at sufficiently low temperatures for the exchange-broadened multiplets to be resolved into their components. Me(CHz),CH2.and Ph(CHz),CH2.Radicals (n = 2-4). The ESR spectra of these free radicals were investigated, even though all but one have been studied by other ~0rkers,3~@ because it was desired to compare the spectra in detail. The radicals were generated by irradiation of the appropriate diacyl peroxides in a mixed solvent of equal parts by volume of toluene and cyclopropane. All the spectra consisted of nine multiplets arising from splitting by the a and @ protons, and all displayed selective broadening of the components with M g= 0. To illustrate the line width alternations, the seventh and eighth multiplets are shown in Figure 6. Without line width alternation, these two multiplets would be similar in appearan~e.~~ The ESR spectrum of n-butyl radicals has been discussed above. The spectra of n-pentyl and n-hexyl radicals (Figure 6a) show only minor differences, which appear to be chiefly attributable to the influence of the effective volumes of the radicals on their transverse relaxation times.53 Although the reduced moments of inertia for rotation about the Cg-C, bonds are different for the different radicals, this does not appear to lead to significant variations in the degree of line width alternation. As regards the aralkyl radicals, Ph(CH2),CH2.,intriguing differences between the spectra of the species with n = 2 and n = 3 have been observed previously, but not explained.& When n = 2 the ESR spectrum recorded at -100 OC (Figure 6b) showed somewhat less pronounced line width alternation than the spectrum of n-butyl radicals recorded at -105 0C.35 Nevertheless, it seems reasonable to explain the line width alternation in a similar way, i.e., as a consequence of transitions between rotamers analogous to 3-5. When n = 3 and 4 the line width alternations were considerably more pronounced than for the lower homologue or for the n-butyl radical. This marked difference cannot convincingly be explained as due to differences in the effective volumes or reduced moments of inertia of the radicals. A more acceptable explanation seems to be that conformations are favored in which the plane of the phenyl substituent lies parallel to the plane of the trigonal carbon atom, due to a weakly bonding interaction between the a orbitals of the unpaired electron and the phenyl group. The difference between the 0proton hyperfine splitting constants is then relatively large, so that the enhanced line width alternation may be ex(52) But not identical, when second-order effects are taken into account. (53)Bloembergen, N.; Purcell, E. M.; Pound, R. V. Phys. Rev. 1948, 73, 679.
J. fhys. Chem. 1983, 87,1924-1928
1924
plained. Conformations of this type are possible when n = 3 and 4, but not when n = 2. This idea is supported by INDO calculation^^^ for the radical Ph(CH2)3CH2..Three conformations of this radical, drawn by are shown in Figure 7. In these conformations, the aliphatic chain is fully staggered. The proton hyperfine splitting constants predicted by the INDO calculations are shown, except when they are less than 0.1 G. Relative to conformation b, in which the interaction between the unpaired electron and the phenyl group is probably minimal, conformation a is predicted to be more stable, by 54.1 kJ mol-', whereas conformation c is predicted to be less stable, by 37.2 kJ mol-'. It appears likely that the stabilization of the conformation shown in Figure 7a is overestimated by the INDO calculation^.^^ In spite of this, it does seem possible that there may be a small but significant stabilization of this conformation. The lifetimes of the different conformations are evidently too short for splitting due to the S protons or ring protons to be resolved. It may be that the disposition of the reactive groups shown in Figure 7a favors the formation of a transition state on the reaction pathway leading to Tetralin, which is formed as a major product during the decomposition of 5-phenylpentanoyl peroxide in benzene solution at reflux temperature.12 Doyle et a1.%studied the ESR spectra of a homologous series of three free radicals, of which the most comprehensive details were reported for the species 12 and 13.
C H2CH2C H i Ph
12
13
(54) Beppu, Y. QCPE 1979, 10, program 370. (55) It is interesting to note that CNDO calculations overestimate intermolecular interactions when they are mainly of the charge-transfer type: Lochman, R.; Weller, T. Znt. J. Quantum Chem. 1976, 10, 909. (56) Doyle, M. P.; Raynolds, P. W.; Barenta, R. A.; Bade, T. R.; Danen, W. C.; West, C. T.J.Am. Chem. SOC.1973,95, 5988.
For 12 and 13, the p protons appeared inequivalent in the ESR spectra recorded at -140 and -120 "C, respectively. This behavior may be contrasted with that of n-butyl radicals, the ESR spectra of which display line width alternation involving the p protons, but not inequivalent p protons, at similar temperature^.^^ For radical 12, the inequivalence of the protons suggests that rotamers 14 and 15 are preferred. The differences between the ESR
C H; I
14
CHi I
15
spectra of 12 and n-butyl radicals are understandable, because transitions between rotamers 14 and 15 are expected to be less frequent, for steric reasons, than the analogous transitions between rotamers 3 and 5 of n-butyl radicals. A similar explanation cannot convincingly be used to explain the differences between the ESR spectra of 13 and n-butyl radicals. A possible explanation seems to be that radical 13 prefers conformations analogous to the conformation shown in Figure 7a, in which the p protons are strongly inequivalent. Acknowledgment. This work was done while I was with the University of Malaya, Kuala Lumpur, Malaysia. I am indebted to Dr. K. H. Lee for valuable discussions. My thanks are also due to Mr. T. C. Chua, who carried out some of the synthetic procedures. Registry No. HOCH2CH2.,4422-54-2; t-Bu-CH2CHz-, 20199-83-1; t-Bu-CH&H&Hy, 85250-70-0; Ph,C-CH&Hz, 85250-71-1; Me-(CHJ2CH2.,2492-36-6; Me-(CH2),CH2., 2672-01-7; Me-(CH2)4CH2., 2679-29-0; Ph-(CH2)2CH2., 25088-33-9; Ph4421-85-6; n-propyl radical, (CHJ3CH2.,4399-93-3; Ph-(CH2)4CH2., 2143-61-5.
Collisional Relaxation of Iodobenzene Ions Naomi 6. Lev and Robert C. Dunbar' Department of Chemistry, Case Western Reserve University, Clevebnd, Ohio 44 106 (Received: October 10, 1982; In Final Form: December 27, 1982)
Collisional quenching of optically excited iodobenzene molecular ions was studied by analysis of the kinetics of sequential two-photon photodissociation in the ion cyclotron resonance ion trap. When ICR line broadening was used as an independent calibration of ion-neutral collision rates, it was possible to determine the number of collisions required for internal energy quenching, giving values of 2.5 collisions (iodobenzene bath gas), 7 collisions (cyclohexane), and 50 collisions (methane). These values are consistent with complete energy equilibration in the collision complex for iodobenzene bath gas, but much less than complete equilibration for the other two bath gases. A two-laser variation of the experiment in which an infrared laser supplemented the effect of the visible laser suggested that infrared radiation has an effect kinetically similar to visible radiation in this photochemical system.
Introduction The study of two-photon dissociation processes in the ion cyclotron resonance (ICR) spectrometer provides the possibility of studying excitation and relaxation processes of highly vibrationally excited intermediates. It has been 0022-3654/83/2087-1924$01.50/0
shown that the excited iodobenzene cations created by the absorption of one visible photon' or several IR photons2 (1) Lev, N. B.; Dunbar, R. C. Chem. Phys. Lett. 1981, 84, 483.
0 1983 American Chemical Society
