2126
W. E. WENTWORTH AND WILLIAM RISTAU
dissolve in the substrate when spread on potassium bromide solution having a concentration of less than 0.2 M in potassium bromide. However, the results given here show that for A > 120 A2/molecule on 0.1 M sodium chloride the surface pressure at each A is constant with time and the a-A curve is the same for different rates of compression (40 sec as opposed to 8 min). The agreement of compression and e2pansion also negates solution of the film for A > 120 A2/molecule. For the rest of the area range shown in Figures 3 and 5 solution of the film is evident, the rate depending on a. For 80 < A < 120 A2/molecule solution is very slow (0.02 dyn/cm in the first minute at 80 A2/molecule) and consequently this part of the a-A curve is in no greater error than the quoted reproducibility. At
lower A , because solution increases markedly (0.07 or 0.36 dyn/cm in the first minute at 70 or 60 A2/molecule, respectively), there is great uncertainty in the surface pressure although it must be stressed that the discrepancies in the a-A curves of Figures 1, 2, and 4 lie well outside errors due to solution of the film. The points chosen for curve-fitting throughout this paper have always been at areas per molecule where solution is low or negligible. Acknowledgments. We extend our thanks to J. R. Brown for purification of the paraffin wax and petroleum jelly, B. Houghton for construction of the surface balances, W. K. Thompson for the infrared analysis of the CI8TAB, and Drs. J. A. G. Taylor and B. A. Pethica for discussion on several practical issues.
Thermal Electron Attachment Involving a Change in Molecular Geometry by W. E. Wentworth and William Riatau Department of Chemistry, Unhersity of Houston, Houston, Texas 77004
(Received May 8, 1968)
Thermal electron attachment to some strained molecules has been investigated using the pulse-sampling technique. The temperature dependence of the electron capture coefficient in each case showed a positive temperature dependence a t low temperatures which suggests an activation energy for electron attachment. I n cyclooctatetraene the activation energy of 1.6 =k 0.3 kcal/mol is associated with a geometrical change from the nonplanar tub form in the neutral molecule to a planar structure in the mononegative ion. I n 2,4,6-trimethylacetophenonethe carbonyl group is twisted out of the plane of the phenyl ring. The activation energy for electron attachment of 4.1 += 0.4 kcal/mol is associated with the geometrical change to a nearplanar negative ion. At higher temperatures there is a negative temperature dependence of the electron capture coefficient which is related to the electron affinity of the molecule, cyclooctatetraene 13.3 & 1.0 kcal/mol and 2,4,6-trimethylacetophenone11.3 f 1.0 kcal/mol.
Introduction In previous publications arising from thermal electron attachment studies in our laboratory the electron affinities of several molecules have been With few exceptions all of these molecules should have little or no strain. The purpose of this paper is to investigate the effect of molecular strain on the electron affinity of a molecule. As will be seen from the results of this study, the effect of molecular strain may result in a change of geometrical configuration of the errulecular negative ion. Only strain-free molecules were selected for previous studies so that the electron affinities could be correlated more readily with theoretical e ~ t i m a t e s . ~ -This ~ is especially true with simple Huckel estimates where carbon-carbon bond lengths are inherently assumed equal and independent of the calculated bond order. The Journal of Physical Chemistry
Gross changes in the geometry of the molecule and the negative ion would necessarily cause the simple Huckel estimate of electron affinity to be in error. Of the compounds reported in the previously cited references, only azulene is known to possess a significant amount of molecular strain The strain is estimated at 16 kcal/mol.e The molecule is planar but is probably distorted similar to 2-amino azulene so that the mole(1) 70, (2) (3) (4)
W. E. Wentworth, E. Chen, and J. E. Lovelock, J . Phys. Chem., 445 (1966).
W. E. Wentworth and E. Chen, ibid., 71, 1929 (1967). R . S. Becker, L. Wang, and W. E. Wentworth, in preparation. R. S. Becker and W. E. Wentworth, J . Amer. Chem. SOC.,84, 4263 (1961). (6) R. S. Becker and E. Chen, J . Chem. Phus., 45, 2403 (1966). (6) A. Streitwieser, “Molecular Orbital Theory for Organic Chemists,” John Wiley & Sons, Inc., New York, N. Y., 1961.
THERMAL ELECTRON ATTACHMENT INVOLVING A
CHANGE IN RfOLECULAR
cule is unsymmetrical.7 I n regard to thermal electron attachment studies, azulene was also unique among the compounds investigated since an activation energy was required for the electron-attachment step.' I n general, for compounds which undergo electron attachment nondissociatively, i.e., form a stable negative ion, the electron capture coefficient is independent of temperature in the lower temperature region. The capture coefficient for azulene, however, decreased by a factor of 4.5 for a corresponding decrease in temperature of 100". At that time it was suggested that the activation energy be attributed to a promotion of the azulene to an excited vibrational level to which the addition of the electron would be more probable than in the zero vibrational level. Two systems are considered in this investigation, cyclooctatetraene and 2,4,6-methylsubstituted acetophenone and benzaldehyde. Cyclooctatetraene itself is in a nonplanar tub form8 with estimates of strain energy from 15 to 30 k c a l / m ~ l . ~ On ~ ~ the ~ ' ~other hand, epr and nmr data suggest that the mononegative and dinegative anions in solution are The mononegative ion may not be symmetrical as a result of the Jahn-Teller splitting,12 but in any event there is a gross change in molecular geometry from the neutral cyclooctatetraene molecule to the negative ions. I n acetophenone and benzaldehyde the 2,6-methyl substituents should create considerable strain over the unsubstituted parent compounds. In fact there is evidence that the carbonyl group is forced out of the plane of the ring in 2,4,6-trimethylacetophenoneand we will furnish further evidence for this. 2,4,6-Trimethylbenzaldehyde is probably planar or near planar; however, there still should be considerable strain arising from the 2,6-methyl substituents. I n both 2,4,6trimethylacetophenone and 2,4,6-trimethylbenzaldehyde the negative ion should be planar or nearly planar in order to possess sufficient stability through resonance with the carbonyl group. It might be expected that strain in a molecule would decrease the electron affinity of a molecule, but this is not necessarily true. The electron affinity is a measure of the difference in energies of the gaseous negative ion and the neutral molecule. Therefore, if the strain which may exist in a molecule is relieved in the negative ion (e.g., through bond lengthening) then in fact the electron affinity can be larger than that of the corresponding unstrained molecule.
Experimental Section The equipment used and the procedures in making the electron attachment measurements were identical with those reported earlier.' A pulse width of 2 psec at 40 V was applied every 1000 psec. With the exception of 2,4,6-trimethylacetophenone the compounds were Eastman White Label quality. 2,4,6-Trimethylacetophenonewas synthesized from 1,-
GEOMETRY
2127
3,5-trimethylbenzene by a Friedel-Craft acetylation. The expected product was confirmed by ir, nmr, and uv. The infrared spectra were run on a Beckman IR10. The appearance of a band in the vicinity of 1690 cm-' is in the general region of the carbonyl stretching frequency. The exact location of this frequency in relation to that observed for other substituted acetophenones will be discussed later in regard to the specific geometrical structure of the molecule. The relative positions of the C=O stretching frequencies for the various substituted acetophenones were accurately determined by attaching a faster drive auxiliary strip chart recorder to the IR-10. The nmr spectrum of 2,4,6-trimethylacetophenone was obtained on a Varian HA-100 nmr spectrometer and showed two aromatic hydrogens and 12 hydrogens between r 7 and 8 in agreement with the expected structure. The ultraviolet spectra of the synthesized 2,4,6trimethylacetophenone and 2,4,6-trimethylbenzaldehyde were run along with the unsubstituted parent compound. The relative positions of the two uv bands for the various compounds were in good agreement with those reported by Braude and Sondheimer.16 Two gas chromatographic columns were used: a '/4 in. X 4 ft glass column packed with 4% SE 30 on 110120 ABS Chromosorb and 0.025 in. X 80 ft glass capillary coated with 3% SF-96. The column temperature was isothermal and generally at temperatures below those for which column bleed is serious for these liquid phases.
Kinetic Model A kinetic model presented earlier1s2which appears to satisfy the results of this study is now summarized. A discussion of other reaction steps, which are possible for these compounds, will be presented later. A classification into four mechanisms of thermal electron attachment to molecules has been presented.1v'6-18 Three mechanisms involve a dissociation into a negative ion and a radical which is associated (7) L. E. Sutton, "Table of Interatomic Distances in Molecules and Ions," The Chemical Society, Burlington House, London, 1965. (8) I. L. Karle, J. Chem. Phys., 20, 65 (1952). (9) C. T. Mortimer, "Reaction Heats and Bond Strengths," Pergamon Press, New York, N. Y., 1962. (10) F. A. L. Anet, J. Amer. Chem. Soc., 84, 671 (1962). (11) T. J. Katz, ibid., 82, 3784 (1960). (12) A. D. McLachlan and L. C. Snyder, J. Chem. Phys., 36, 1159 (1962). (13) H. L. Strauss, T. J. Kats, and G. K. Fraenkel, Soc., 85, 2360 (1963).
J. Amer. Chem.
(14) R. D. Allendoerfer and P. H. Rieger, ibid., 87, 2336 (1966). (15) E. A. Braude and F. Sondheimer, J . Chem. Soc., 3754 (1955). (16) W. E. Wentworth, R. 5. Becker, and R. Tung, J . Phys. Chem., 71, 1652 (1967). (17) W. E. Wentworth, E. Chen, and J. C. Steelhammer, ibid., 72, 2671 (1968). (18) W. E. Wentworth and E. Chen, J . Gas Chromatogr., 5, 170 (1967).
Volume 73, Number 7
July 1969
2128
W. E. WENTWORTH AND WILLIAM RISTAU
with a negative slope in a logarithmic graph of electron capture coefficient us. reciprocal absolute temperature. This may be a single negative slope or a negative slope in a region at higher temperatures. For the compounds considered in this study this behavior was not observed. From the nature of the molecules, dissociative electron attachment would not be expected. There is a region in which a small negative slope is observed; however, this occurs at lower temperatures and is followed by a positive slope at higher temperatures. This behavior is not in accord with any dissociative mechanism. The mechanism involving the formation of a stable mononegative anion results in the following expression, assuming that a steady-state condition exists for eand AB-
where 6 is the electron concentration before the addition of the capturing species AB; [e-] is the electron concentration in the presence of AB; a is the concentration of AB as it is eluted from the gas chromatographic column; k~ is a pseudo-first-order rate constant for the loss of electrons due to processes other than attachment to AB; k~ is a similar pseudofirst-order rate constant for the loss of AB- other than detachment; kl is the rate constant for the attachment of e- to AB; k-1 is the rate constant for the e- detachment from AB-; and K is defined as the either of the capture coefficient. In the sum ( k ~ two rate constants could predominate resulting in different expressions for K . Since b1 would be expected to have an activation energy for molecules with a positive electron affinity and k L is relatively temperature independent, kL predominates at lower temperatures (designated as p region) resulting in
+
K = kl/k~
(2)
At higher temperatures ( a region) k-1 predominates. Assuming the statistical thermodynamic equilibrium expression for an ideal gas
K = - klkL k-lkD
-
IGL -AT-‘/z exp(EA/kT)
(3)
kD
where A is a constant; E A is the electron affinity of the molecule. NIeasurement of ICD, as reported previously, as a function of temperature, shows it to be temperature independent. Therefore, the variation in K from eq 2 should reflect the temperature dependence of kl. As mentioned previously, little or no temperature dependence has been observed in the p region for the compounds investigated thus far with the exception of azulene. Although numerous compounds have been i n v e ~ t i g a t e d ~ lmany -~ compounds will not show a region. This results when the compound has a comThe Journal of Physical Chemistry
paratively low electron affinity (specifically E*-1 activation energy for detachment) and low volatility, prohibiting gas phase measurements at lower temperatures. Of the compounds in~estigatedl-~(with the exception of azulene) , four aromatic hydrocarbons and nine aromatic carbonyl compounds exhibit a p region. For four of these compounds there are sufficient data to examine the slope and it appears to be negligible within experimental error. However, in the case of azulene a negative slope corresponding to an activation energy of 0.15 eV was observed.’ At higher temperatures ( a region) it is obvious from eq 3 that a graph of In KT8’/”us. 1/T should be linear with a positive slope which is proportional to the electron affinity of the molecule. The kinetic model can further be tested by the concentration dependence which should satisfy eq 1. A complete discussion of the kinetic model, including a list of the basic assumptions and the results suporting the model, is included in a recent review.lg As will be evident from the results of this study, the previous kinetic model satisfies the observed results. However, there are alternative models which are plausible and these should be examined to see if they likewise satisfy the experimental observations. From polarographic reduction curves, the dianion is produced readily from the mononegative anion radical of cyclooctatetraene in solution.20 From nmr measurements, estimates of the equilibrium constant highly favor the dianion relative to the anion radi~a1.l~In a later paper, Allendoerfer and Rieger14 showed that the equilibrium toward the mononegative anion radical was favored by electroreduction in tetrahydrofuran. They suggested that the shift toward the dianion observed previously was caused by the presence of the alkali metal ion. The possibility of producing the dianion in the gas phase under our experimental conditions was considered. This possibility is highly unfavorable from an energy standpoint. Due to increased electron repulsion, the second electron affinity should be significantly lower than the first electron affinity. Secondly, the large excess of AB compared to AB- would make the probability of electron attachment to AB- to form the dianion extremely small. Finally a kinetic analysis involving dianion formation predicts that deviations from the linear relationship in eq 1 could occur at high percentages of electron capture. As we will see shortly, there is no experimental information to support this mechanism involving dianion formation. Finally, the possibility of the formation of a metastable negative ion, say AB-*, preceding the formation of the final stable molecular ion should be considered. The detailed development of the kinetic analysis will (19) W. E. Wentworth and J. C. Steelhammer, Advances in Chemis11,” American Chemical Society, Washington, D. C., 1968, pp 75-99. (20) R. M. Elofson, Anal. Chem., 21, 917 (1949).
try Series, No. 82, “Radiation Chemistry.
THERMAL ELECTRON ATTACHMENT INVOLVING A CHANGE IN MOLECULAR GEOMETRY
212.9
not be given. However, the analysis shows that the same concentration dependence as given in eq 1 results. For this reason, evidence for an intermediate AB-* state cannot be found from an examination of response as a function of concentration. On the other hand, the expression for K is different since it involves rate constants for the formation of the AB-* and conversion to the final AB-. For this reason, the temperature dependence of K could show up to a maximum of four linear regions. This unique response could be used t o suggest the existence of an intermediate AB-*. From our results it will be seen that a maximum of two linear regions is observed and there is no reason to complicate the mechanism or the interpretation with the additional species AB- *. For some molecules this could be a real possibility and should be considered if such a varied temperature dependence is observed.
Results In general the response of the electron capture cell as a function of concentration followed eq 1 quite satisfactorily as it has in previous s t ~ d i e s . ' - ~ ~ ~This ~~~-'~ relationship was examined critically for cyclooctateI4t traene since deviations could give evidence for dianion formation, as discussed earlier. The relationship was 0 1.0 2,o SI) f a Iddbg" examined up to 65% capture and no significant deviaFigure 1. Temperature dependence of the electron capture tions from linearity were observed. coefficient: (1) azulene; (2) cyclooctatetraene; The temperature dependence of the thermal electron (3) 2,4,6-trimethylacetophenone. Solid line () is the capture coefficients for the various compounds in this least-squares estimate of the curve. Dashed line (---- - I study is given in Figure 1. The aeulene data reported represents the electron-attachment region, the slope previouslihave been included inFigure 1 for comparison. of which is proportional to the activation energy. Cyclooctatetraene and 2,4,6-trimethylacetophenone show two linear temperature regions. The slope in the an internal rotational mode in the negative ion and abhigher temperature region should correspond to the elecsence of activation of the corresponding torsional vitron affinity of the molecule, but of particular importance brational mode in the neutral molecule. Obviously is the negative slope at low temperatures which is similar these are extreme situations and cancellation of the conto the behavior of azulene. As stated earlier, the negatributions will occur when both modes are activated or tive slope at lower temperatures is unique with respect partially activated. Within experimental error the to the compounds previously investigated.' -a Ack ~ / ratio k ~ appears to be but certainly the cording to the assumed kinetic model this negative slope "true" ratios of rate constants would be expected to be corresponds to an energy of activation for the electron different although the difference may be small. attachment step. The experimental data of In KT"I" vs. 1/T of Figure 1 Because of the somewhat limited amount of data in were fit to the general expression for K as defined in eq 1. the higher temperature region, the least-squares adArrhenius expressions A1T8/ae-E'*/kT and A-le -E- i*/k T justment of the data was carried out assuming a were replaced for kl and kz, respectively. The adjustcommon intercept.'b2 From eq 3 this will be true only ment of the data t o the general expression for K was if the ratio k ~ / kremains ~ constant among the comcarried out according t o the principle of least squares.21 pounds and the ratio of the partition functions fAB-/ The intercept in the vicinity of In KT"/' 15, which is fAB, is approximately 2, the degeneracy of the negative [ ( A I / A ~ ) ( k ~ / k is , ) poorly ] , defined by these data. In ion in the ground state.' If there is a change in conResults from other compounds appear to give a common figuration in going from the neutral molecule to the intercept in the vicinity of 15. An average value of In negative ion this would not be rigorously true since the [ (AI/A-I) ( k ~ / k ] ~ ) = 14.8 and the curve was forced rotational and vibrational contributions t o the molecthrough this point in the least-squares adjustment. ular partition functions would not cancel. We estiThe least-squares estimate of electron affinity (EA = mate the ratio should not exceed = 10, which would re1 kcal in electron affinity. This flect in an error of maximum estimate was based on complete activation of (21) W. E. Wentworth, J. Chem. Educ., 42, 96, 162 (1966).
--
Volume 79, Number 7 Julg 1969
W. E. WENTWORTH AND WILLIAM RISTAU
2130
+
E-1* El*), activation energy for e- attachment (E*), and intercept (In k l / h ) were evaluated and are given in Table I. E* refers to the activation energy for electron attachment with no preexponential temperature dependence. The T"/" term was taken into account as beforel6 and is given approximately by E* = El* - 1.1 kcal/mol. The error in EA is estimated at f 1.0 kea1 which we attribute t o the uncertainty in the assumed intercept of 14.8. Table I : Electron Affinities and Activation Energies Compound
Azulenel Cyclooctatetraene 2,4,6-Trimethylacetophenone 2,4,6-Trimethylbenzaldehyde Acetophenone2 Benzaldehydes m-Tolualdehydes p-Tolualdehydes
EA, koal/mol
E*, koal/mol
Intercept
15.1 f 1 . 0 13.3 =t1 . 0 11.3 f 1 . 0
2.9 f 0 . 6 1.6 f 0 . 3 4.1 f0.4
36.4 0 . 9 33.9 =k 0 . 5 33.3 f 0 . 6
10.18
...
...
7.69 9.90 9.57 8.86
*..
...
*.*
...
... . ...
*.. .,.
Discussion For both cyclooctatetraene and 2,4,6-trimethylacetophenone the observed activation energies for electron attachment are associated with a change in molecular geometry. As pointed out earlier, cyclooctatraene is in a strained tub configuration whereas the mononegative ion in solution is thought to be planar. There is no reason to expect the mononegative ion to have a different structure in the gas phase. The activation process is hence thought to involve the activation of some vibrational mode or combination of modes which periodically distort the neutral molecule t o a planar or near-planar structure. Electron attachment to such a vibrationally active molecule could then result in the planar negative ion. Several vibrational modes appear t o have the proper symmetry t o acquire the necessary distortion in cyclooctatetraene (Dzd).2z The most likely modes are thought to be a symmetrical AI vibration or either of two Bzvibrations. The electroreduction of cyclooctatetraene to the mononegative anion also involves an activation energy.I4 The value reported is larger than the gas-phase value reported in this work; however, the electroreduction value could involve an additional effect due to solvent reorientation in the process. The carbonyl group of acetophenone and benzaldehyde is thought to be coplanar with the phenyl group in order to allow maximum resonance or delocalization energy. However, in 2,4,6-trimethylacetophenone the two methyl groups apparently provide considerable steric interaction with the methyl and oxygen of the acetyl group. The acetyl group then rotates about The Journal of Physical Chemistry
the bond joining the phenyl group in order t o relieve the strain. This conclusion is given strong support from both uv spectral changes and dipole moment measurements upon methyl substitution.l5 I n fact from the ratio of molar extinction coefficients for the K band (-2500 A) the angle of rotation can be estimated. For 2,4,6-trimethylacetophenone the angle was estimated at 63'.15 There is considerably less strain in 2,4,6-trimethylbenzaldehyde and the estimated angle is only 22OO15 The above interpretation of the nonplanar configuration for 2,4,6-trimethylacetophenoneis supported by some infrared studies carried out in our work. The frequency of the carbonyl stretching mode was measured precisely by expanding the wavelength scale on the IR-10. The relative frequencies of substituted acetophenones varied over a range of 18 cm-l. A graph of frequency us. Taft's c value for the substituent was reasonably linear with deviations of the order of 1-2 em-' with but one exception. This exception was 2,4,6-trimethylacetophenonewhich had a negative deviation of = 19 em-'. This marked deviation has been attributed t o the steric hindrance of the 2 and 6 methyl groups resulting in the rotation of the CH3 C=O out of the plane of the phenyl group. The loss of resonance in such a geometry would thus account for the change in C=O vibrational stretching frequency. A similar correlation for substituted benzaldehyde was not carried out; however, the C=O frequency of 2,4,6trimethylbenzaldehyde was measured relative to that of benzaldehyde. The difference in frequencies agreed roughly with the change noted in the linear correlation observed for the substituted acetophenones. This agrees with a relatively small amount of strain in 2,4,6trimethylbenzaldehyde and the nearly planar geometry as deduced from the analysis of the uv spectra.15 The magnitude of the electron affinities of 2,4,6-trimethylacetophenone and 2,4,6-trimethylbenzaldehyde relative t o the unsubstituted compounds should be noted in Table I. I n general, methyl substitution lowers the electron affinity of the molecule, e.g., acetophenone and m- and p-tolualdehydes compared to benzaldehyde. Inductive and resonance parameters for various substituents have been successfully established3 within the framework of Huckel' theory and account for the electron affinities of substituted compounds. The simple inductive model for CH3 was used with good success. Using this parameter the calculated electron affinities for 2,4,6-trimethylbenzaldehyde and 2,4,6-trimethylacetophenone are 6.2 and; 3.9 kcal/mol, respectively. These calculations assume a planar, nonstrained configuration for both (22) E. R.Lippinoott, R. C. Lord, and R. 8. McDonald, J. Amer. Chem. SOC.,73, 3370 (1951).
2131
THERMAL ELECTRON ATTACHMENT INVOLVING A CHANGE IN MOLECULAR GEOMETRY the neutral molecule and the negative ion. The experimental values for both these 2,4,6-substituted compounds are considerably larger than those expected from the nonstrained molecules and are discussed below. The mechanism for electron attachment to the 2,4,6-trimethylsubstituted benzaldehyde and acetophenone will be described in detail since it can be done with reasonable precision with simple Huckel calculations. An analogous approach could be taken in the study of cyclooctatetraene; however, the system is more difficult to describe quantitatively. Since the 2,4,6-trimethylacetophenoneis expected to undergo a change from the nonplanar to a planar or near planar structure upon electron attachment, quantitative estimates of the change in potential energy for both the neutral molecule and negative ion as a function of the angle of rotation yield an estimate of the observed activation energy and electron affinity. The change in potential energy for the neutral molecule, UAB, as a function of angle is considered as the sum of change in delocalization energy and an of the energy resulting from steric interaction, SAB, -C
No
group with the 2,6 methyls on the ring.
+ SAB
=
+
XoaB1/z(i
COS
28)
f! ?p
a 5Y
to
6 -540-
&4,6 -Trimethylacetophenone E
-10
#
8
,
m
.
,
.
,
30
60
0 (deg 1
(4)
(5)
This type function has been used to represent the steric interaction as a function of rotation about the carbon-carbon bond in substituted ethanes.23 For substituted ethanes maxima in SAB should occur every cos 38). S'AB 120" and the angular function is (1 in eq 5 , the value a t 8 = O", must be assigned a value based on some external estimate or made consistent with the observed experimental electron affinity, EA,,,, as will be described shortly. The analogous potential function for the negative
+
2,4,6 -Trimethylbenzaldehydr
a 10-
l
For a convenient reference the UABat 90" has been assigned zero. A(DE) will then be negative with dewill be positive. Huckel calcucreasing 0 whereas SAB lations were used to calculate A(DE) using the inductive and resonance parameters for C=O and CH, as mentioned previously. A value of p = - 16 kcal as suggested by Streitwiesere was used for the calculation of A(DE). The A(DE) as a function of angle were evaluated by varying the resonance integral between the carbonyl carbon and the ring. The resonance integral is related to the angle of rotation by p(0) = p cos 8. The results for 2,4,6-trimethyl substituted compounds of acetophenone and benzaldehyde are shown in Figure 2 by the dashed line. The energy change resulting from the steric interaction is assumed to have the following dependence on the angle of rotation. SAB
-
90
'CH, UAB= A(DE)
1
Figure 2. Potential energy curve describing the electron-attachment mechanism.
ion consists of the terms in eq 4 in addition to the calculated electron affinity, EA,,l. The EA,,i is the energy of the lowest unoccupied molecular orbital as obtained from the Huckel calculation and necessarily neglects any steric effects. # A B - is assumed to be given by an equation of the same form as that of eq 5 , but SOAB- differs in magnitude. The change of potential as a function of the angle of rotation is given by UAB-= (DE)
- EA,,I
+ SAB-
(6)
The term A(DE) is identical in UABand in eq 6 since it involves only the four lowest occupied n-molecular orbitals. It should be noted in Figure 2 that A(DE) changes approximately 6 kcal in going from the 0" (planar) configuration to the 90" configuration. The lowest molecular orbital (MO) makes the greatest contribu0
II
tion to the n bonding from the C-CHa to the phenyl ring. This lowest MO is raised by 0.37p upon rotation from 0 to 90". This is compensated by the lowering of the second MO by 0.16p. The net effect is a rather (23) K. 8. Pitzer, "Quantum Chemistry," Prentice-Hall, Ino., Englewood Cliffs, N. J., 1953, p 240.
Volume 79, Number 7 July 1069
W. E. WENTWORTH AND WILLIAMRISTAU
2132 small, but significant, change in delocalization energy of = 6 kcal. The fifth MO, which accepts the additional electron in forming the negative ion, increases very significantly (0.29p) upon rotation from 0 to 90". This results in a prediction of a negative electron affinity (endothermic) at 90". Aside from the slight 0
I/
inductive effect of C--CH3, the fifth RiO is very similar to the lowest vacant orbital of benzene. Both theor e t i ~ a l and ~ ~ experimentalz6 l~~ estimates of the electron affinity of benzene suggest that it is negative. The functions of UABand UAB- from eq 4 and 6 have been plotted in Figure 2 for 2,4,6-trimethylbenzaldehyde with S'AB = 4 kcal/mol S'AB-= 0, and 2,4,6-trimethylacetophenone with S'AB = 15 kcal/mol, S'AB-= 0. The rationale for this choice of So,values will be given shortly. With this choice of SOi values it should be noted that both of the minima in UAB and UAB- occur at 0" for trimethylbenzaldehyde suggesting no change in geometry upon electron attachment, However, in the case of trimethylacetophenone UABhas a minimum at 90" and UAB-at 0", suggesting a change in molecular geometry. The activation energy for electron attachment according to this model is given by the energy required in going from the minimum in UABto the intersection of U A B and U A B - , designated by U*, i.e.
A negative S'AB - is physically meaningless, but a magnitude of - 1.0 is within the error and for all practical purposes can be taken as zero. The experimental error in EA,,, itself is estimated at 1.0 kea1 and the error of EAca1 will be of the order of 0.7 kcal. The error in A(DE) is difficult to estimate but considering the small magnitude of A(DE) should not exceed 1 kcal/mol. Combining these errors gives a resultant error of the order of 1.6 kcal/mol, in excess of the calculated -1.0 kcal/mol for SA^-. On this basis SABwas assigned zero not only for the 2,4,6trimethylsubstituted acetophenone but also for the substituted benzaldehyde since the steric interaction in the latter would necessarily be lower. Considering the nodal properties of the lowest energy vacant T molecular orbital for these molecules, one would expect S A B - to be smaller than SAB. Nodal planes pass through the 1,2 and 1,6 carbon-carbon bonds of the phenyl ring. Bond expansion along the bonds would relieve the methyl interference with the carbonyl group. Assigning S'AB-= 0, an estimate of S'ABfor 2,4,6-trimethylbenzaldehyde can be obtained from eq 9
EAexp - EAm1 = 10.18 - 6.16 = 4.0 kcal/mol #'AB
SOAB
=
(12)
This is the value used in Figure 2. An estimate of SOAB for 2,4,6-trimethylacetophenone E* T.J* - UAB(eABmin) (7) can be obtained by adjusting the U A B - curve until For 2,4,6-trimethylacetophenone UAB(OAB~'~) = 0 at the intersection of UABand UAB-is in agreement with e = 90" which is our arbitrary reference point, and the experimental E*. However, unfortunately, the hence E* is simply the height of the intersection as E* is not very sensitive to variations in sOAB. A designated in Figure 2. In 2,4,6-trimethylbenzaldevalue of 16 lical was used for the UAB for 2,4,6-trihyde there is no activation energy since O A B ~=~ ~ methylacetophenone in Figure 2. This gives an E* of e A B -min and no angle of rotation is required. 2.2 kcal which agrees favorably with the experimental The experimentally observed electron affinity on this value of 4.1 kcal/mol. The 15 kcal seems reasonable model is given by in comparison with a reported strain energy of 12.7 i min 1.5 kcal/mol in 4,5-dimethyl~henanthrene.~'This is EAexp= UaB(eABmin) - uAB-(oAB- ) (8) a lower limit since the 4,s hydrogens of phenanthrene where the eminrefer to the minima in UABand UAB- partially interfere. S'ABas we have used it is not and in general do not correspond to the same angle. what is generally defined as a strain energy, namely, For 2,4,6-trimethylbenzaldehydeboth UAB(eABmin) and the actual energy of the molecule above a so-called UAB-(eAB-min)OCCUr at 8 = 0" and strain-free molecule. The molecule frequently will undergo some distortion to relieve the interaction and EAexp = S'AB EAw.1 - S'AB(9) this is reflected in the actual energy of the molecule. but for 2,4,6-trimethylacetophenoneUAB(eABmin) occurs The term S'ABassumes a planar geometry and, therea t 90" and UAB-(eAB-min)at 0 " ; hence fore, S'ABwould be larger than an experimental estimate of the strain energy. For this reason S'ABof 15 EAexp = -A'(DE) EAca1 - S'AB- (10) kcal/mol seems reasonable relative to the 12.7 lical/ where Ao(DE) is the maximum value found at 0 = 0". From eq 10 we can estimate #'AB- from the ex(24) J. R. Hoyland and L. Goodman, J . Chem. Phys., 36, 21 (1962). perimental electron affinity, the calculated values of (25) R. 8. Becker and W. E. Wentworth, J . Amer. Chem. SOC.,85, Ao(DE), and EA,,1 for 2,4,6-trimethylacetophenone 2210 (1963).
+
+
- AO(DE)- EA,,, SOAB-= 3.9 + 6.4 - 11.3 = -1.0 kcal/mol S'AB= EAo,l
The Journal of Physical Chemistry
(11)
(26) R. N. Compton, L. G. Christophorou, and R. H. Huebner, Phys. Lett., 23, 656 (1966). (27) M. A. Fritsch, J. L. Margrave, C. Barber, and M. S. Newman, J. Amer. Chem. SOC.,85, 2356 (1963).
THERMAL ELECTRON ATTACHMENT INVOLVING A CHANGE IN MOLECULAR GEOMETRY mol strain energy for the 4,5-dimethylphenanthrene despite the fact that the interaction in 4,5-dimethylphenanthrene should be larger than that in 2,4,6-trimet hylacetophenone. I n the previous calculations to estimate UAB and 0
/I
as a function of rotation of the C-CH, group, the weakest part of the calculation was the assumed function for S A B in eq 5 . The general shape of the function is appropriate to the problem since it, has a minimum at 90" and a maximum at 0". However, the change of SABwith angle is too gradual. After constructing molecular models, which include atomic sizes based on van der Waals radii (Framework Molecular Models), it appears that the steric interaction changes more rapidly with angle. I n the limit of 0" the function should be exceedingly high and drop off rather dramatically with increasing 0. With such a function for SAB minima in UABand U A B -for 2,4,6trimethylacetophenone would be at angles intermediate of 90 and O " , possibly in agreement with the angle of 63" for 2,4,6-trimethylacetophenone as estimated by the uv absorption spectra. The minima for UABshould occur a t a smaller angle but probably greater than 0". Other methods of estimating XAB as a function of 0 are being investigated; however, it is not a simple problem. I n addition, the thermal electron attachment to other 2,6-substituted acetophenones and benzaldehydes is being investigated. Hopefully this work will lead to a better approximation for SAB.The SAB expression in eq 5 is satisfactory to describe the model used in this paper and can account for a geometrical change from the neutral molecule to the mononegative molecular ion. An approach similar to that described for 2,4,6-trimethyl-substituted acetophenone and benzaldehyde could be used for cyclooctatetraene. For various configurations between the tub form and the planar form, the resonance integrals between the adjacent carbons could be estimated from the angular rotation (e) of the p orbitals and k = cos 8. The EAOpland A(DE) could then be estimated by Huckel calculations. The corresponding functional dependence of the strain energy would present the greatest problem. The value of the strain in the planar relative to the equilibrium form can be taken as 13.7 kcal/mol10 from an estimate of the activation energy for alteration of the double single bonds which presumably goes through the planar structure. UAB-
2133
Azulene is analogous to cyclooctatetraene and 2,4,6trimethylacetophenone in its temperature dependence of electron capture coefficient as shown in Figure 1. This suggests that the formation of the negative ion involves a possible change in molecular geometry. The esr spectra of the negative ion have not been satisfactorily interpreted to suggest the appropriate structure.28 At this time it is difficult to be more specific as to the electron attachment mechanism.
Conclusions 1. The temperature dependence of the electron capture coefficient for cyclooctatetraene and 2,4,6trimethylacetophenone indicates an energy of activation for the initial electron-attachment step. This activation process is associated with a change in geometry in going from the neutral molecule to the negative ion. 2. The activation energy for cyclooctatetraene is 1.6 lical/mol, which involves a change in geometry from the tub form in the neutral molecule to the planar negative ion. The activation energy for 2,4,6-trimethylacetophenone is 4.1 kcal/mol, which is thought to be primarily an excitation of a torsional vibration of the carbonyl group from the nonplanar neutral molecule to a geometrical structure in which the carbonyl is planar with the phenyl ring. 3. For 2,4,6-trimethylacetophenone the electron affinity is greater than that expected for the unstrained, planar molecule. This increase in electron affinity is attributed to the strain in the neutral molecule which is apparently relieved or partly relieved in the negative ion. Similarly in 2,4,6-trimethylbenzaldehydea higher electron affinity is observed than that expected for the 2,4,6-Trimethylbenzaldehyde unstrained molecule. should be nearly planar with only a small amount of strain which again is apparently relieved in forming the negative ion. It should not be concluded, however, that strained molecules in general will have a larger electron afinity than that expected for the unstrained form.
Acknowledgments. The authors wish to express their appreciation to the Robert A. Welch Foundation for financial assistance, to Dr. Ralph S. Becker for discussions and thoughtful suggestions, and to Mr. Joe Steelhammer for assistance in the computer calculations. (28) I. Bernal, P. H . Rieger, and G . K. Fraenkel, J . Chem. Phys., 37, 1489 (1962).
Volume 78, Number 7
July 1 9 P
