T. H. SIDDALL, 111, E. L. PYE,AND W. E. STEWART
594
vs. delay time, a final straight line is obtained whose slope contains the specific reaction rate for ions of thermal kinetic energy (70" in this work). The reactions assigned to the various products, and the reaction rate constants determined, are given in Table 11. Comparison of the rate constant for the disappearance of CHBSiHf (1.4 X cm3 molecule-l sec-l) with the sum of the rate constants for product formation via CH3SiH+ reactions indicates that the reactions of CH3SiH+account for about 80% of the product formation. This mass-balance discrepancy indicates that some other ion is reacting in the system. As discussed already, from our pressure-dependence studies Tve have concluded that this ion is SiHz+, although we are not able to measure all the rate constants of its reactions. E. Energetics. The mass-spectrometric observation of ( I l ) , especially with such a large specific reaction rate, leads immediately to the conciusion that the enthalpy change of this reaction is at most zero. With
this conclusion, and the assumption that the reaction is indeed as we have written it, we may easily derive the limit : D(CHaSiH2-H) 94 kcal/mol, a value consistent with the observed silicon-hydrogen bond dissociation energy in m o n o ~ i l a n e . l ~Combining *~~ this with the appearance potential of CH38iH2+(Table I) we find the limit IZ(CH3SiHJ > 7.7 eV, a value consistent with reported estimates of the ionization potentials of SiH3l7 and (CH3)8Si19 radicals.
18.7 20.9, 21.1
.*.CAS* < 4 , 5 eu or < -5
Obsd
...
.-..
... .
1.7 1.8 2.2
0.5 0.9
$9 +9 4- 12
2.4
3
eu
On this basis, our observation that AS* 5 3 eu (Table 11) as compared to a calculated value of about 10 eu is quite significant. The small experimental values for AS* are significantly different, from the values predicted from the model in the discussion section. However, a discrimination that requires finer detail than about 5 eu will require improved accuracy, especially in signal shape analysis at high temperature. The accuracy of the infrared results is not so pertinent to the main purposes of this study. The estimated errors in A€€ are given in Figures 2 and 3. These results do establish beyond any question that I and I1 are "normal" in their ability to associate with phenol. I n fact, their behavior is quite similar to amides of simpler structure.
temperature (lOOe)----AF* - AF*o, kcal/mol AF*, koal/mol Calcd Obsd
T-High
-ue
-AS*, Celcd
0 -3 -1 $3
I
18.5, 18.8 19.3, 19.6 19.5, 19.9 20.5, 20.8
I
I
I
I
...
...
0.7 0.8 1.0
0.8 1.1 2 .o
I
T
~-
La =
-3.16 kcal/siolel
( e r r o r IC t O . 3 kcallnole) v i a l e a s t squares r e j e c t i n g t n e two e x t r e m e p o i n t s t h a t are s h o w n .
2
,
-
1 ;
Amide1 = 0 . 1 0 1 2 M $ O t ) = O.022M O.055M
en
AF,,,
L e n
cs
E
-
-
0.73
8.1 e "
1 . 0 0 - m m cells and L O W Canc. Data.
1
1
0
A Model for the Rotational Process The effect of added phenol can be used to test a model-one which seems to us to be the most obvious.
-2
I
I
I
I
2.6
2.8
3.0
3.2
L . - U 3.4
3.6
3.B
IO'IT
I 0
A
I
I
I
I
Run 1 , S a m p l e A , i O O H ) = i 5 . 5 0 ( 1 0 'IM R u n 2 , S a m p l e e , (~0H)=(5.50](10'2)M:
I
I
( A m i d e (6.80)i10ei)M (Amidei~(13.6)(10-ZlM
Figure 3. Association constants for N-methy1,N-benzyl-o-chlorothiobenzamideand phenol in o-dichlorobenzene,
This model assumes (1) that AS* = 0 for o-dichlorobenzene solutions with no phenol present and ( 2 ) that the association of phenol with the amide transition state is negligible. With phenol present, there are two kinetic routes. The first route (rate con) via rotation within free amide molecules. stant = k ~ is The second route ( k z )is via the combination of eventsrotation in the associated amide molecule and separation into transition state plus phenol. Then when a = the concentration of unassociated amide, A = the total concentration of amide (subscript zero is for kinetic quantities with no phenol present), and - AF, = the free energy of association of amide and phenol
L 2,6
J
I
I
1
I
2.8
3.0
3.2
3,4
3,6
I 3,a
10'/T
Figure 2. Associntion constants for
N-rnethy1,N-benzyl-o-chlorobenzamideand pbenol in o-dichlorobenzene. The Journal o j Phusical Chemisirzl
and
IC (the observed rate)
= 2.08 X l.O1OTe-AF*'Rx
EFFECT OF ASSOCIATION WITH PHENOL
599
but also
k = kl -l- kz theref ore = ( a / ~ + e-AFa/RT)(e-AF*o/iZT
,--AF*/RT
1
I n the limit of high phenol concentration as a/A goes to zero, the observed AF* approaches AF*Q (no phenol) plus AFa (the negative of the free energy of association). Now a t temperatures TI and Tz, in this limit AFI" - AFz* = AFRl - AF,,
but AFi*
- AFZ*
- TI)
= AS"(T2
or in the limit of high phenol concentration
but AFa,
- AFa,
( T I - T2)ASa
or AS* = - A S R = 9 . 4 e u
This analysis assumes that the experimental equilibrium constants for association and derived quantities are applicable at high phenol concentration. This neglect of activity coefficients can affect the analysis. AS* = 9.4 eu must be regarded as a representative value, not as the absolutely correct value. At lower phenol concentration, AF*o - AP* must be calculated from the relationship AF*,,
- AF*
=
RT In (a/A f
Comparison With Experiment The kinetic data for the amide, 1, are summarized in Table 11. Data for solutions of near the same composition have been averaged together to give four sets of data. The calculated and observed AP* - AF*o and entropies are listed. It is immediately obvious that the observations in no way agree with the model. One obvious modifics tion to the model is not adequate. This modification allows the transition state to also be associated with phenol, though to a lesser extent, than the ground state. I n the transition state, there is certainly less and possibly no contribution of the dipolar resonance form (E,) to tJhe molecular description. The transition 0
I1
C-N
/
0-
./
\
I
+/
(A) -+-+ c=N
/
(B)
\
state must be substantially nonplanar as cornpared to the near planar ground state with a near maximum
contribution from B. Even so, the transition state must be at least about as polar as a ketone. Ketones associate with phenol though less so than amides.I5 A polar transition state improves the prospects of a small AS* because phenol is not necessarily released during the rotation. At the same time, a polar transition state, however realistic, decreases the value of AF* - AF*Q. The fit of model and observed AS" can be improved but only at the expense of the fit of AF". Therefore, an expansion of the conceptual basis for the model seems necessary. These solutions must have phenol-amide interactions over larger domains than simple phenol-amide pairs. The phenol must stabilize the ground state more than indicated by measurement of the strength of this pair relationship alone, as is done by the infrared technique. Within this expanded concept, the shell of surrounding molecules Is not disturbed during the time of rotation. There is then no reason to expect any large AS*. The transition state is destabilized relative to the ground state by up to about 2 kcal in the presence of phenol. However, the entropy change hetween the pairs (amide, phenol-amide*, phenol) is small because of the similarity of the states and is nonexistent in the surrounding structure over the rest, of the domain. The small entropy effect on the shell may be due to (a) the shells being nearly the same on an equilibrium basis for the two states or alternatively (b) the relaxation time of this shell being long compared to the time of the rotational event itself. (The time of the rotational event is, of course, many factors of ten shorter than lifetime of the ground state.) One key requirement to the expanded model can be tested in an obvious fashion. This experimcntal test requires an amide and a ketone that distribute between two immiscible solvent phases of quite different polarity. If the ketone is assumed t o be an adequate stand-in for the transition state, the distribution measurements should predict approximately the increased barrier for rotation in the more polar solvent as compared to the less polar solvent. We are searching for a satisfactory system. It is interesting to note that CDC13 increases the barrier (sample 18), though by only about 0.5 kcal. We have observed similar effects for other amides and similar solvent pairs. The effects of phenol on I1 are much less marked and close to our experimental error. However, there does seem to be a small effect-an effect, consistent with the smaller association with phenol. These smaller effects suggest that thionamides will not be suitable for model testing.
(15) T.Gramstad, Spectrochim. Acta, 19,497 (1963). Volume 74, Number 3 February 6 , 1970
T. H. SIDDALL,111, E. L. PYE,AND W. E. STEWART
600 Rotation Around the Benzene-to-(Thio) Carbonyl Bond For rotation around the benzene-to-carbonyl bond in I in CDC13, AF*(20°) = 14.5 kcal/mol for the major isomer and 16 kcal/mol for the minor isomer. The shifts induced by benzene added to CDCL solutionsI6 yield equivocal results as to isomer assignment. Both methyl signals are shifted downjield. However, we believe the major isomer places the benzyl group cis to oxygen. The chemical shift within the AB quartet is larger for the major isomer because the anisotropic carbonyl group is closer. The barrier is slightly larger in the minor isomer for steric reasons. The CsH6CH2group is "larger" than the methyl group and increases the barrier to rotation when it is cis to the o-chlorobenzene ring. The barrier to rotation around this bond in I1 is at least as large as the barrier around the amide bond itself. As coalescence begins for the N-methyl signals, indicating intermediate exchange for rotation around the amide bond, it also begins for the two AB quartets, but not until then. Perhaps both exchange
The
J O U T ~of~ Physical
ChmbtTy
processes broaden the quartets. Rotation may occur around both of these bonds in a synchronous or sequential manner. The fast exchange region (possibly 200" and above) was not accessible in o-dichlorobenzene. The substantial increase ( 2 8 kcal/mol) for rotation around this bond in I1 as compared to I must reflect two effects. Sulfur stiffens the amide framework, as is seen from the larger barrier to rotation around the amide bond. This must be an electronic effect. However, the larger sulfur atom must also make a steric contribution to the increase. The benzene ring is out of the amide plane in the ground state. The presence of the larger sulfur atom, approximately in plane, must increase the barrier. Acknowledgment. The information contained in this article was developed during the course of work under Contract AT(O7-2)-1 with the United States Atomic Energy Commission. (16) J. V. Hatton and R. E, Richards, Mol. Phys., 3 , 253 (1960).
