TEMPERATURE DEPENDENCE OF ESRSPECTRA OF
-
29 19
CFICFz.
important effects, dispersion forces and association. It does not seem possible to include these within the framework of a classical point dipole in a cavity surrounded by a homogeneous medium of dielectric constant E. However, the model does show the importance of polar interactions and should give a qualitative basis for explaining some of the unusual phenomena of polar mixtures.
Acknowledgments. The author appreciates the encouragement and assistance of Professors H. B. Hollinger, Department of Theoretical Chemistry, and H. C. Van Ness, Department of Chemical Engineering, Rensselaer Polytechnic Institute. Acknowledgment is also made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this research.
Change with Temperature of the Electron Spin Resonance Spectra of
- CF,CF,. Trapped in Irradiated Polytetrafluoroethylene by Kazumi Toriyama and Machio Iwasaki Gonernment Industrial Research Institute, Nagoya Hirate-machi, Kita-ku, Nagoya, J a p a n (Received January 19,1060)
The change with temperature of the esr spectra of mCFZCFZ. formed by the photolysis of peroxy radicals trapped in irradiated polytetrafluoroethylene (PTFE) has been studied. Using oriented samples, the hyperfine tensors of the a-fluorine coupling were determined at 77'K and at room temperature. The principal values, A 11 and Al, are 225 and 17 G (assumed) at 77'K and 110 and 74 G at room temperature, respectively. The direction of All at; 77°K is about 45' from the molecular-chain axis and at room temperature is along the chain axis. From the principal values and their directions, it was concluded that the temperature change of the spectra is caused mainly by the molecular motion around the chain axis, which may be related to the so-called room-temperature transition of PTFE. The structure of the radicals was also determined from the hyperfine tensor. The analysis of this temperature dependence of the hyperfine tensor gave further confirmation of identificationof the radical mCFzCFz. with the 3-line spectrum trapped in irradiated PTFE.
Introduction It has been reported by a number of workers' that the esr spectra of irradiated polytetrafluoroethylene (PTFE) in vacuo at room temperature consists of a 2 X 5-line spectrum (so called double quintet), a 3-line spectrum, and some other broad component. The origin of the 2 x S i n e spectrum is well elucidated and is attributed t o the radical .-CF2. C F C F y . The 3-line spectrum with about 14-G separation and the intensity ratio of 1: 2 : 1 was initially attributed to the -CF2CFzO. radical. In the recent works by Klinshpont and Milinchuk2 and Siegel and Hedgpeth,a however, it was found that this 3-line spectrum at the center is only a part of the spectrum and that it has wing peaks due to the hyperfine anisotropy of a-fluorine coupling. The central 3-line spectrum corresponds to E M F = 0, where MFis the component of the nuclear spin of the two a-F atoms. The wing peaks are characteristic of radicals having a-F atoms as reported in our previous paper.4 From these experiments, the origin of this 3-line spectrum was attributed to the radical wCF.2-
C F 2 * . They also reported that this spectrum exhibits a remarkable temperature dependence, which was attributed to the partial averaging of the a-F hyperfine anisotropy by some molecular motion, the details of which were not elucidated. On the other hand, Iwasaki and his coworker reported that the temperature change of the esr spectrum of peroxy radicals trapped in PTFE is well interpreted in terms of the molecular motion around the chain axisS6 It was found in the present work that the temperature change of the spectrum of the mCF2CF2. (1) (a) W. B. Ard, H . Shields, and W. Gordy, J . Chem. Phys., 23, 1727 (1955); (b) Yu. D. Tsvetkov, N. N. Bonbnov, M. A. Makulskii, Yu. S. Lasurkin, and V. V. Voevodskii, Dokl. A k a d . N a u k SSSR, 122, 1053 (1958); (0) T. Matsugashita and K. Shinohara, J . Chem. Phys., 35, 1652 (1961); (d) N. Tamura, ibid., 37, 479 (1962); (e) D. W. Ovenall, ibid., 38,2448 (1963). (2) E. R. Klinshpont and V. K. Milinchuk, K h i m . V y s . Energ., 1 , 242 (1967). (3) S. Siegel and H. Hedgpeth, J. Chem. Phys., 46, 3904 (1967). (4) (a) M. Iwasaki and K. Toriyama, ibid., 46, 2852 (1967); (b) M. Iwasaki, ibid., 45, 990 (1966). (5) M. Iwasaki andY. Sakai, J . Polym. Sci., 6A-2,265 (1968).
Volume 76,Number 9 September 1069
KAZUMI TORIYAMA AND MACHIO IWASAKI
2920 radical is also well interpreted by the molecular motion around the chain axis. The geometrical structure of this radical was also determined from the hyperfine tensors at 77°K and room temperature. Our results gave further confirmation of the identification of the radical -.CF2CF2. with the 3-line spectrum. Experimental Section Samples used in the experiments were Teflon films. The oriented samples were made by stretching films up to 500% elongation at 305" and then quenching in cold water. A number of stretched films were bundled with PTFE fibers at both ends, and irradiated by e°Co y rays at room temperature after evacuation for 24 hr at mm. The total dose was about 3 X lo7 R a t a dose rate of 5 X 105 R/hr. After irradiation the samples were exposed to air for about 24 hrs and then irradiated with uv light at 77°K for 4 hr after reevacuation at room temperature. A Toshiba H-400P highpressure mercury lamp was used as the source of uv light. The esr spectra were measured with a Japan Electron Optics Model JES spectrometer operated at 9.4 Gc/sec with lOO-kc/sec modulation. The signals of Mn2+ in ZnS were used for the calibration of the magnetic field. The hyperfine splittings of Mn2+ had been calibrated previously by a side-band technique of proton magnetic resonance.
b
Figure 1. Esr spectra of the radical .wCF~CF~* trapped in oriented films of PTFE. Measurements were made at 77°K with the static magnetic field (a) parallel and (b) perpendicular to the stretched axis.
a
Results The esr spectra measured at the field directions parallel and perpendicular to the stretched axis are indicated in Figures 1 and 2. Figure 1 depicts the spectra obtained at 77°K and Figure 2 at room temperature. I n either case, the spectra consist of the central sharp peak and the weak wing peaks, the former being assigned to the EMF = 0 line and the latter to E M F = =tl lines as already reported by Siegel and Hedgpeth.3 The spectra exhibited marked angular dependence both at 77°K and at room temperature, Figures 3 and 4 indicate the angular dependence of the separation of the wing peaks corresponding to E M F = =t1 lines, The hyperfine splittings plotted in Figures 3 and 4 are half of the separation of the two wing peaks. At 77"K, the maximum coupling value of 225 G was obtained at a field direction around 45" from the stretched axis. This value of 225 G is in good agreement with the value which is half of the wingpeak separation of the unoriented samples reported by Siegel and Hedgpeth.s It is evident from theory and experiments on the line shape of randomly oriented samples having large anisotropic hyperfine tensors4 that this value corresponds to the maximum principal element of the hyperfine tensor of a-fluorine. At room temperature, however, the maximum coupling value of 110 G was obtained a t the field direction parallel to the stretched axis and the minimum The Journal of Physical Chemistry
b
A
50 G
Figure 2. Esr spectra of the radical wCF2CFZ*trapped in oriented films of PTFE. hleasurements were made at room temperature with the static magnetic field (a) parallel and (b) perpendicular to the stretched axis.
value of 74 G at the field direction perpendicular to the stretched axis. The hyperfine structure due to the two @-fluorinenuclei was barely resolved at 77°K on the spectra observed with magnetic field other than nearly perpendicular to the stretched axis. At the perpendicular direction, the central part of the spectrum showed a complicated structure at 77°K. The analysis of this structure, however, is not straightforward because the anisotropy of ,&fluorine coupling should be taken into consideration as well as the overlapping of
2921
TEMPERATURE DEPENDENCE OF ESRSPECTRA OF 43F2CF2*
an axial symmetry around the stretched axis. According to the analysis of the 9CF2CONHzradical in a
A&
100
50
200
150
AI,
250G
HYPERFINE SPLITTINGS
Figure 3. The angular dependence of the hyperfine splitting of a-fluorine nuclei observed at 77°K. The hyperfine splittings plotted are half of the separation of the wing peaks corresponding to the ZCMF = iI lines. The horizontal lines indicate the range in which the positions of Z M g = f l lines are expected to distribute. The direction of the magnetic field was measured from the stretched axis.
I
2 30'
l,d
1f
0"
0
,
50
,!',
.-
100 G
HYPERFINE SPLITTINGS
Figure 4. The angular dependences of the hyperfine splittings observed at room temperature for the radical N C F ~ C F ~ . .The hyperfine splittings plotted are half of the separation of the wing peaks corresponding to the E M F = + l lines. Black and white dots indicate the splittings due to a- and p-fluorine nuclei, respectively. The directions of the magnetic field are measured from the stretched axis.
the peaks corresponding to A , of the anisotropically broadened ~ M =F h1 lines. On the other hand, almost all the spectra showed well-resolved 3-line structure due to /?-fluorine coupling at room temperature, except the x M F = i1 lines observed when the magnetic field was applied nearly parallel to the stretched axis. The white circles in Figure 4 indicate the angular dependence of the p-fluorine coupling obtained from the hyperfine structure of the E M F = 0 line. The maximum value of 17 G was observed at the field direction perpendicular to the stretched axis. The intensity ratio of the 3-line structure was 1:2 : 1.
Analysis and Discussion The Spectra Measured at 77°K. As already mentioned in the previous paper,6 in uniaxially oriented samples one may obtain the spectral-line shape due to the two-dimensional random orientation when the magnetic field is applied along a direction not parallel to the stretched axis, unless the hyperfine tensor has
single crystal of trifluoroacetamide the hyperfine tensor is nearly axially symmetric and the direction of the maximum principal element, All, is parallel to the 2pn orbital of the radical carbon.6 I n the case of the 43F2CFZ. radical, the direction of the maximum hyperfine tensor element of the a-fluorine coupling may be perpendicular to the radical plane containing the CF2group and the adjacent carbon. Therefore, when the system is rigid a t 77"K, the symmetry axis of the hyperfine tensor, that is, the direction of Ail can not coincide with the stretched axis if one assumes the extended planar zigzag structure for this radical. Actually, the chain sturcture of PTFE is a 13s loose helix which has the internal rotational angle of only 16"28'.' Therefore, this assumption may be a good approximation for our radical, which was produced by the scission of the chain peroxy radical -CFZCF(O0 .)CFz--. Suppose a is the angle between the stretched axis and the direction of Ail, and e is the angle between the stretched axis and the field direction, as indicated in Figure 5. The direction of All may be distributed around the stretched axis in uniaxially oriented samples. Therefore, the observed hyperfine separation at the field direction B will be distributed from the minimum value Amin(e)
=
[A12
+ (A112 -
~ 1 2 COS? )
(e +
(1)
a ) ~ l / ~
to the maximum value
[AL'
Amax(8)
+ (A112 - AL')
COS'
(e - a)]"* (2)
Thus, the observed separation of the wing peaks measured from the center of the spectrum corresponds to A,,, expressed by eq 2. .When e = a, one should observe the maximum separation corresponding t o A i. As is seen in Figure 3, the maximum coupling value equal to the value of All obtained from the unoriented samples was observed at 0 = 45". This means that a is about 45" and All = 225 G. On the other hand, if
e=o
Amax(0)
=
Amin(())
=
[A,'
+ (All' AL') cos2 a ]'/% (3)
Equation 3 suggests that one can estimate the value of A L from the observed values for All, 01, and A ( 0 ) . The value of A , obtained from eq 3 is, however, quite sensitive to angle a,that is, if one assumes 44, 45, and 46" for a the values 28, 50, and 98 G, respectively, are obtained for AL. Since it is very hard to estimate the value of angle a with better accuracy than f 2 " , one can not estimate the reliable value for A , from eq 3. (6) R.J. Lontz and W. Gordy, J . Chem. Phys., 37, 1357 (1962). (7) (a) C.W.B u m , and E. R. Howells, Nature, 174,549 (1954); (b) M . Iwasaki, J . Polym. Sci., Al, 1099 (1963). Volume 79, Number 9 SeNember 1969
KAZUMI TORIYAMA AND MACHIO IWASAKI
2922
tZ
a
b
Figure 5 . Relation among the directions of All of a-fluorine nuclei, the external magnetic field, and the stretched axis. I
However in the case of room-temperature spectra, one can determine both principal values as described in the next section. If one assumes the invariance of the trace of the hyperfine tensor a t both temperatures, one can estimate the value of A l at 77°K to be 17 G. This value corresponds to a = 43.7" which is in accord with the observed angle within the experimental error involved. Besides, Al = 24 G is obtained for the similar radical .CF2COI\JH2 from the single crystal work.6 Therefore A L = 17 G for our radical seems to be quite reasonable and is tentatively assumed in the following discussion. However, this value is not so accurate and may have an ambiguity of more than *10 G. The solid curve in Figure 3 is the angular dependence of the wing separation calculated from eq 2 using the principal values and angle a thus obtained. The agreement with the observed points is fairly good. The range in which the positions of the EMF= A 1 lines are expected to distribute was calculated for various angles of 0 using eq 1 and 2. The results are indicated by the horizontal lines in Figure 3. Angle a corresponds to the direction of the 2pn orbital of the radical carbon t o the stretched axis. If one assumes the extended zig-zag structure for the radical rnCF2CF2. as shown in Figure 6a, and the tetrahedral value for the end CCC bond angle, the smallest possible angle between the half-filled 2pn orbital and the molecular chain axis is 54" at such a conformation that the p orbital lies in the zigzag plane. This position corresponds to the conformational angle of 60" as indicated in Figure 6b. If one takes into account a number of assumptions made in this computation, the agreement with the observed value a = 45" seems to be fairly good. Thus, the angular dependence of the wing separation due to the anisotropy of the a-F hyperfine tensor is well interpreted by the radical rnCF2CF2- with the structure The Journal of Physical Chemi8try
Figure 6. Conformational structure of the radical wCFZCFz* and the principal directions of the hyperfine tensor.
indicated in Figure 6. The principal values of the hyperfine tensor and their directions thus determined are tabulated in Table I. As for the p-F coupling, it was hard to analyze the poorly resolved structure.
Table I : Hyperfine Coupling Tensors for mCFd2F2. Observed a t 77°K and Room Temperature 77°K
a-F
All
225 G
45' from the
stretched axis
Ito the direction
Al
17 G"
Ao
86 Ga
Allrot
110 G
/I to the stretched
Alr~t
74 G
I to the stretched
Apt
86 G
of All
Room temp
a-F
axis
axis p-F
A ( ~ ) I I 10 G
/I to the stretched axis
A(p)l
17 G
A@),
15 G
I to the stretched axis
a
These values were estimated on the assumption that the trace
of the hyperfine tensor is unchanged at 77'K and room tempera-
ture.
The Spectra Measured at Room Temperature. As already reported, the temperature dependence of the esr spectra of the peroxy radicals trapped in PTFE is well interpreted in terms of the molecular motion around the chain axis.5 If rapid rotation around the chain axis takes place a t room temperature and the frequency of the rota-
TEMPERATURE DEPENDENCE OF ESRSPECTRA OF mCFzCF2. tion is large enough as compared with the hyperfine anisotropy, the hyperfine tensor elements should be partially averaged out giving the axially symmetric tensor in referring to the axis of rotation, although the trace of the hyperfine tensor is unchanged. All of the new tensor, which is denoted by A 1lrot, may be expressed by = AIL212
+ A2LZ; + A3LZ;
(4)
where Lzl, Lz2, and Lzs are the direction cosines of the rotational axis with respect to the principal coordinate system. A I of the new tensor, which is denoted by AIrot, is easily obtained from the trace of the tensor which is invariant with respect to the rotational motion.
AIrot := '/g[(Ai
+ Az + Aa) - A1lrOt]
(5)
The observed angular dependence shown in Figure 4 suggests that at least one of the principal axes of the hyperfine tensor of the a-fluorine coupling is parallel or perpendicular to the stretched axis. If the observed temperature dependence is due to rotation around the chain axis, one has to choose the stretched and the axis perpendicular to the stretched axis as A axis as AIrot. If this is the case, the observed value of 110 G for Allrotand 74 G for AIrot should be well explained by eq 4 and 5 using the principal values of the rigid system. The values of Allrot and A I r o t were calculated to be 121 and 69 G, respectively, from eq 4 and 5 together with the values, a = 45") All = 225 G, and AI = 17 G, observed at 77°K. The agreement of the calculated values with the observed values 110 and 74 G is fairly good. This agreement strongly suggests that our model for the motional averaging of the hyperfine tensor is correct, and that the proposed structure for the radical 4 2 F 2 C F 2 .shown in Figure 6 is very probable. I n this computation it was assumed that the end -CF2 group is rigid and no oscillation around the end C-C bond takes place at room temperature. However, if the oscillation around the chain axis is very rapid, the symmetry axis of averaged tensor is along the chain axis even though the rotation around the end C-C bond takes place simultaneously, because the invariance of the entire molecular motion is still along the chain axis. Therefore, the computation was made for the extreme case in which the rotation around the end C-C bond is also very rapid at room temperature. The result is = 69 G and AIrot = 95 G. I n this case the A smaller value has to be observed at the parallel direction, in disagreement with our experiment. Therefore, it is concluded that the rotational motion around the end C-C bond does not play an important role in averaging the hyperfine tensor at room temperature.8 If one chooses an alternative assignment of the observed principal values to A llrot and A that is, A 1lrot = 74 G and AIrot = 110 G, one has to consider that the invariance of the molecular motion is perpendicular to the chain axis. However it is hard to associate this
2923
Table I1 : Comparison of Observed and Calculated Principal Values of Partially Averaged Hyperfine Tensor, Arot
A ljrot Alrot
Apt
Obsd G
Calcd G
Direction
110 74 86
121 69 86
1 I t o chain axis J.
to chain axis
sort of motion to our radical. If one assumes that rotational motion takes place only around the end C-C bond, the principal direction of Ailrot should be 36' from the molecular chain axis. This also contradicts the experimental results. It should be mentioned here that our computation is based on the tentative value 17 G for AI a t 77°K. However, the large change for this value, say 30 G,g does not alter our conclusion. Furthermore, it has been reported that the esr spectra of the radical species, wCF2cFCF2w and wCF2CF(OO.)CF2.-, trapped in the same polymer (PTFE) show a similar temperature change, which is also attributed to molecular motion around the chain axis at room temperature.1e16 The origin of this rotational motion may be related to the well-known first-order transition of PTFE at room temperature. These facts may give further support to our interpretation of the temperature dependence of the esr spectra of the chainscission radical mCF2CF2I . It should be mentioned here that the intensity ratio of the Z M F = f1 lines to the E M F = 0 line is fairly smaller than 1/2. The reason for this may come from the following two effects. One is imperfection of the orientation of the samples, and another is that the frequency of the rotational motion is insufficient to average out the hyperfine anisotropy. Both effects cause the residual anisotropic broadening in the E M F = i l lines. I n fact, the intensity ratio of the spectra observed at higher temperature, for example, loo", becomes closer to the value '/2. As for the hyperfine coupling due to the two P-fluorine nuclei, our results ohtaiiied at room temperature indicate that the hyperfine tensor has the same principal axes as those of a-fluorine tensor at room temperature. This is quite consistent with our interpretation. The rotation around the chain axis requires the p-fluorine hyperfine tensor having the symmetry axis parallel to it. Therefore the minimum separation 10 G observed at (8) Above room temperature, anisotropy of the or-fluorine hyperfine tensor gradually decreased with increasing temperature, and during this change the principal directions were unaltered ( A l p t = 97 G and A l ' o t = 80 G a t 423OK). This may suggest that the effect of rotational motion around the end c-C bond become serious a t higher temperature. However, above 423OK the trapped radical decayed out so that we could not check our calculated value for the simulte neous motion. Besides, at higher temperature the other mode of motions may not be ignored. (9) This value gives Ailrot = 128 G and A 1 r O t = 79 G for single motion and Alpt = 79 G and A ~ r o tF 103 G for simultaneous motion.
Volume 78,Number 0 September 1060
DONALD E. LEYDENAND
2924 parallel direction is assigned to A(@) 11 and the maximum separation 17 G observed at perpendicular direction to In order to make a similar treatment to that employed in the &-fluorine hyperfine tensor, one needs to know the principal values of the P-fluorine hyperfine tensor at rigid state, and their directions. Unfortunately, they were not determined from the poorly resolved spectra a t 77°K. Therefore, the results were compared with the principal values obtained from the single-crystal work on sodium perfluorosuccinate.'O The observed principal values A (p)11 = 10 G and A (P) I = 17 G (A(@)o = 15 G ) at room temperature are very small as compared with the principal values AI = 67 G, A2 = 25 G, and A s = 21 G (Ao= 38 G ) which were obtained from the average of the two @-Fcouplings observed for *CF(COO-)CF&OO- trapped in sodium perfluorosuccinate.'O This discrepancy may be attributed to the different conformations of the C-F, bonds with respect to the half-filled 2pa orbital. As is shown in Figure 6b, the polymer radical wCF2CF2. is assumed to have a conformational angle of 60°,while
W.R.MORGAN
the radical in sodium perfluorosuccinate is considered to have a conformational angle of 30". Therefore, if the so-called cos2 6 rule is applicable to the @-fluorine hyperfine splitting, the principal values of @-fluorine coupling in our polymer radical may be reduced to about one-third of those for the radical in sodium perfluorosuccinate, resulting in the values AI = 22 G, Az = 8 G, and Aa = 7 G (Ao = 13 G). These principal values should be partially averaged by the rotation around the chain axis a t room temperature, although it is not possible to evaluate this motional effect because the principal directions in the rigid state are not known. I n the extreme case of the random motions, these values should approach the isotropic value of 13 G. Therefore the observed principal values A (@)11 = 10 G and A (@)I = 17 G ( A(/3)o = 15 G ) seem to be quite reasonable and consistent with the proposed conformational structure, and with the partial averaging due to the proposed molecular motion. (10) M. T. Rogers and D. H. Whiffen, J . Chem. Phya., 40, 2662 (1964).
Proton Exchange Mechanisms of Some Tertiary Benzylaminesl by Donald E. Leyden and W. R. Morgan Department of Chemistry, University of Georgia, Athens, Georgia $0601
(Received February 8,1969)
The rate of proton exchange in aqueous hydrochloric acid has been measured for N,N-dibenzylmethylamine, N-benzyl-N-methylethanolamine, N-benzyl-N-methyl-2-chloroethylamine,and N,Ndimethylbenzylamine. The rate constant, kn, for the breaking of the RaN . .HOH hydrogen bond was determined for each compound. It is proposed that a factor influencing the value of ka is the hydrogen bonding between the protons in the water molecule and the aromatic rings. 9
Introduction Recently the kinetic analysis of proton exchange between ammonium ions and aqueous acid has been extensively Both h i g h - r e ~ o l u t i o n ~ Jand ~~J pulsed4t637 nuclear magnetic resonance techniques have been employed, and data are available over a wide range of acid and ammonium ion concentrations. However, there is some uncertainty as to the correlation between the substituents on the nitrogen atom and the relative importance of the various mechanisms by which proton exchange may occur. In some of ,the compounds studied one mechanism appears to prevail over a wide range of acid and/or amine concentrations16whereas in the case of other compounds various exchange mechanisms are important only in very limited Apparently, the variations in the rate constants of the The Journal of Physical Chemistry
exchange reactions are not simply related to the basicity of the amines or steric factors.6 Most of the previous investigations have been limited to alkyl-substituted (1) This investigation was supported by Public Health Service Grant GM-13935 from the National Institutes of Health. (2) E.Grunwald, A. Loewenstein, and S. Meiboom, J . Chem. Phys., 27, 630 (1957). (3) A. Loewenatein and S. Meiboom, ibid., 27, 1067 (1957). (4) E.K. Ralph, 111,and E. Grunwald, J . Amer. Chem. Soc., 89,2963 (1967). (5) R. J. Day and C. N. Reilley, J . Phys. Chem., 71, 1588 (1967). (6) E.Grunwald and E. K. Ralph, 111,J. Amer. Chem. SOC.,89,4405 (1967). (7) E.Grunwald and A. Y . Ku, ibid., 90,29 (1968). (8) M.Sheinblatt, J . Chem. Phys., 36,3103 (1962). (9) M.Sheinblatt and H. 9. Gutowsky, J . Amer. Chem. SOC.,86,4814 (1964).
