Dielectric and Dynamic Kerr-effect Studies in Liquid Systems BY MARTIN S. BEEVERS CROSSLEYt, * JOHN DAVID AND WILLIAMS C. GARRINGTONGRAHAM Edward Davies Chemical Laboratories University College of Wales Aberystwyth Dyfed SY23 1NE Received 21st July 1976 Studies have been made of the dielectric relaxation and dynamic Kerr-effect of supercooled fluorenone + o-terphenyl and tri-n-butyl ammonium picrate + o-terphenyl solutions in the frequency range lo4 to Hz and over a range of temperature. One process the o! process was observed and is primarily due to the reorientational motions of the dipolar solute molecules. The dielectric relaxation times zD,and the Kerr-effect decay (relaxation) times 7K.d were found to be the same for the fluorenone + o-terphenyl system being consistent with a " fluctuation-relaxation" model for re- orientation and inconsistent with the small-angle rotational-diffusion model.The same result applies to the ion-pair + o-terphenyl system at the lower temperatures but as the temperature is raised TD and tK,d become different with at higher temperature zDN 3TK.d being indicative of a small- angle rotational diffusion situation. These data are discussed in terms of models for cooperative reorientation in viscous liquids. Reorientational motions of molecules in the liquid state may occur and be studied in the time-range 10-13s < t < 103s. While most small molecules reorientate in the short-time part of this range (<10-los) their rate of reorientation may be made as slow as is desired by working in the supercooled state.l For many systems notably viscous molecular liquids and amorphous polymers,2 motions occur in the range 10-los < t < 103s the rate being strongly dependent on temperature and applied pressure.The fact that " slow " reorientational motions lead to an absorption curve in the frequency domain which is at least one decade of frequency in half-width-or a transient decay function which is at least as slow as an exponential decay in time makes it very difficult to identify the mechanism for the reorientation process. Vari-ous mechanisms with their adjustable parameters may be applied to the results of a given experiment. Increasing the accuracy of the data from a given experiment may not lead to a substantially improved identification of the mechanism.The reasons for the difficulty in identifying reorientation mechanisms are made clear if we consider the function that describes the reorientation$ of a dipole vector in a molecule. We define @(a,tl0,O)dQ as the field-free conditional probability that the dipole vector points into the element of solid angle dQ around i2 at time t given its direction was along the z axis at t = 0. The probability function may be expanded in terms of Legendre polynominals of u = cos 8 where 8 is the polar angle and decay functionsly,(t) k # 0. 1" @(Q tl0,O) = 4n 2 (2k + 1)%4V&(~) (1) k=O * Present address Department of Physical Chemistry University of Sydney Sydney Australia. t On leave from Department of Chemistry Lakehead University Thunder Bay Ontario Canada.$ For simplicity isotropic reorientation with respect to an initial orientation is assumed. MARTIN S. BEEVERS ET AL. yo(t)= 1. The dielectric experiment measures the frequency dependent permittivity ~(co),and this is related to yl(t)in eqn (1) by the relation3 p(u)is a local field factor E~ and cooare the limiting low and high frequency pemittivi- ties respectively cr) is angular frequency and 9indicates the Fourier transformation. Thus the dielectric experiment probes only yl(t)in eqn (1). This is insufficient in- formation to define O(Q,tJ0,0) and hence the mechanism for the reorientation pro- cess. If a model for relaxation is assumed then yl(t) may be fitted using the adjust- able parameters of the model.However several models may be used in this way and this gives rise to a rather unsatisfactory situation. In order to improve the situa- tion it is essential to conduct at least two experiments which reflect different aspects of the reorientational motions. This may be achieved by making dielectric relaxation and dynamic Kerr-effect measurements over a wide range of time (or frequency) and temperature. It may be shown* that the dynamic Kerr-effect decay transient ex- cluding internal field problems is given4v5 by ty2(t)of eqn (1). If yl(t)and y2(t)are determined experimentally then O(Q,t J0,O)is partially characterized. If yz(t)decays faster thanyl(t) thenty,(t),y,(t) etc. may be estimated fromy,(t) oryz(t) usinga method based on information theory due to Berne and coworkers.6 Hence O(Q t l0,O) is then fairly well characterized at each (T,P)condition and models may be applied at that stage.Alternatively yl(t)andy2(t) may be considered in terms of models for reorien- tation since a knowledge of both acts as a considerable constraint on the applicability of a given model. Two extreme cases should be noted. (1) Reorientation by small angle steps (rotational diffusion) then4 ty,(t) = exp[-n(n + l)Dt] where D is the rotational diffusion coefficient which predicts that both tyl(t) and yz(t)are exponential in time and the relaxation time for the Kerr-effect experiment is just one third of the dielectric relaxation time. (2) Reorientation occurring by large-angle steps of ar- bitrary size (" fluctuation-relaxation " model)5 for which yl(t)= y2(t)=y,(t) = c(t) say and the relaxation function for the rise and decay Kerr-effect transients are the same as t(t).The present work describes a study of the dielectric relaxation and the dynamic Kerr-effect on two systems exhibiting slow reorientational motions and is aimed at clarifying mechanisms for reorientation in supercooled and other viscous liquids l and in solid amorphous polymers.2 Despite the obvious potential of the combined approach for cooperative low-frequency processes we have noted only one previous comparison of dielectric relaxation and Kerr-effect data. Results are given here for fluorenone + o-terphenyl and tri-n-butyl ammonium picrate + o-terphenyl in the supercooled liquid state. The former system was chosen as being representative of several solute + o-terphenyl systems1 in which the motions of solute and solvent are wholly cooperative while the latter system is non-typical being one in which the ion-pairs relax on average more slowly than the so1vent.l EXPERIMENTAL o-Terphenyl (Koch-Light Puriss Grade) was dried over zeolite fluorenone was recrystal- lized from ethanol and tri-n-butyl ammonium picrate (TBP) was prepared by the usual method." Dielectric measurements were made using a coaxial cell together with a General The pn(t)of eqn (1) are the field-free orientational correlation functions and may be defined as P"(0 = t)>= J~(Q,tIO,O)P,(u)~.40 DIELECTRIC AND DYNAMIC KERR-EFFECT STUDIES IN LIQUID SYSTEMS Radio 1620-A Assembly (lo2to lo5Hz) and a Scheiber bridge (0.6 to lo2Hz).Dynamic Kerr-effect measurements were made using an apparatus constructed in this laboratory which operated with step-pulses (<3 kV) whose plateau was variable over the range 1O-j s to lo2s. The step-pulses were applied to a Kerr-cell of optical path-length 7.45 cm and an inter-electrode spacing of 0.138 cm. The optical-transients (square-law detection) were displayed on a storage oscilloscope (Tektronix Type 7313) and photographed or recorded directly on a pen recorder. The Kern-cell was cooled using methanol circulated from a Lauda Ultra-Kryostat. Super-cooled solutions were obtained by rapidcooling of the Kern-cell from 340 K down to about 258 K. In contrast with several supercooled solute + 0-terphenyl systems,l the fluorenone + o-terphenyl and TBP + 0-terphenyl supercooled solutions were very stable and no recrystallization problems were encountered over several hours.Major problems with such low-temperature optical measurements include (i) strain- birefringence in the cell-windows (ii) water-condensation on the cell windows. Both difficulties were encountered; (i) was eliminated using well-annealed windows and checking that good optical extinctions were possible over a reasonable volume of the cell (ii) was eliminated by passing a stream of dry-air over the windows. RESULTS A. FLUORENONE IN 0-TERPHENYL Three concentrations were studied using the Kerr-effect method namely 15.1 % 22.5%and 30.4%(wt %) fluorenone in o-terphenyl. The static Kerr-constant B12= An/(hE2)for these solutions is positive and B12is far greater than the solvent Kerr- constantBl.(For example I?, for the 15.1 % solution at 260.9 K is 9.1 x VV2m and Blat 259.1 K is 1.41 x VW2m.) Fig. 1 shows representative Kerr-effect tls . tls FIG.1 .-Normalized birefringence (An/Anmax)against linear time (t) for 22.5% fluorenone in 0-terphenyl at the temperatures (K) indicated. transients at four temperatures; the fact that the rise and decay transients form reasonable normalized master curves is illustrated in fig. 2 for the 15.1 % and 30.4% solutions. Average relaxation times derived from the decay-transient (rkSd) and the rise-transient (rk,J are given in table 1. Plots of the birefringence against time may not be fitted by an exponential function of time but are fitted with reasonable a_ccuracy by the William-Watts empirical function exp -(t/.ro)B.Table 1 gives the values MARTIN S. BEEVERS ET AL. FIG.2.-Normalized birefringence(An/AnmaX)against normalized time (t/zo)for (a) 30.4% fluoren-one in o-terphenyl at 255.0 K-0 ;243.8 K-•;240.6 K-• and (b)15.1% fluorenone in o-terphenyl at 260.2 K-0 ; 254.9 K-•; 244.6 K-0. TABLE IN 0-TERF'HENYL 1.-FLUORENONE Kerr dielectric* rise decay ~~~~ ~~ ~~ 15.1 260.2 0.28 0.83 0.34 0.69 253.6 5.98 0.56 254.9 4.50 0.69 3.40 0.65 251.2 18.9 0.56 249.4 69 0.62 72 0.58 250.0 42.4 0.56 246.4 410 0.58 510 0.53 244.6 1400 0.57 1650 0.45 22.5 259.6 0.16 0.71 0.12 0.69 253.7 1.69 (0.45) 257.7 0.37 0.72 0.24 0.69 251.2 6.71 0.54 246.9 95 0.68 75 0.61 249.9 10.6 0.56 241.2 2720 0.65 2300 0.60 248.3 22.5 0.58 239.9 6200 0.59 7600 0.52 30.4 256.3 0.18 0.73 0.13 0.68 255.0 0.41 0.73 0.26 0.59 246.4 40 0.72 -243.8 175 0.70 98 0.55 240.6 960 0.60 820 0.55 239.2 2000 0.61 2020 0.50 * Shears and William~.~ 42 DIELECTRIC AND DYNAMIC KERR-EFFECT STUDIES IN LIQUID SYSTEMS for the experimental transients.Fig. 3 shows* (logf,,, T-I) plots for the data of table 1. Also included in table 1 are the dielectric relaxation results of Shears and Williams9 for this system. Inspection of table 1 and fig. 2 and 3 show that (a) the rise and decay transients (Kerr-effect) have approximately the same average relaxation time at each temperature; (b) that z (Kerr) N-z (diel.) where a comparison can be made (Le for the 15.1% and 22.5% solutions); (c) that the p values for the rise and decay transients (Kerr-effect) are similar at a given temperature and are similar to but slightly larger than the corresponding dielectric relaxation values.FIG.3.-log (fmax/Hz)against lo3KIT for (1) 15.1%; (2) 22.5% and (3) 30.4%fluorenone in o-ter- phenyl. dielectric-A ; Kerr-effect rise-0 ; Ken-effect decay-0. The dashed line represents di-electric and Kerr-effect results for pure o-terphenyl. Clearly the same motional process is being detected by the two techniques being the cooperative Brownian motions of the (dipolar) fluorenone molecules ; this process is characterized by the large apparent activation energy (Q -250 kJ mol-l) and by rather broad transients (or broad asymmetric loss curves).Note from fig. 3 that the fluorenone molecules "plasticize " the solvent implying that the glass-transition temperature T for fluorenone is lower than that of the solvent. We emphasize that the data of table 1 and fig. 1-3 relate to the supercooled liquid state and not to the glassy state. When logf,, < -4 the material may be regarded as a glass. B. TRI-N-BUTYL AMMONIUM PICRATE IN O-TERPHENYL Dynamic Ken-effect data for 1.33% 2.25% 2.91% 3.29% and 7.12% TBP in o-terphenyl (wt %) are given in table 2. In all cases B12was positive and was rather larger than the solvent value. For example B12for the 1.33% solution was 4.52 x 10"" (V-2 m) at 261.7 K. Representative transients are shown in fig.4 and in normalized form for two concentrations in fig. 5. The curves of fig. 4 are qualitatively different from those of fig. 1. At the higher temperatures the rise-transient is far * The Kerr-effect data are in the time-domain and yield to. In fig. 1 (Kerr) = 1/(2nr0). The average dielectric relaxation time (z> is obtained from the maximum loss condition in the fre- quency domain. fmar (diel). = l/(Zn<r>). We may readily show that log [fmax (Kerr)/fmaX (Gel.)] = F@) >0 and F(B) = 0.06 for = 0.75. MARTIN S. BEEVERS ET AL. TABLE Z.-nRR-EFFECT RELAXATION DATA FOR Bu3NHPiIN 0-TERPHENYL rise decay 1.33 260.6 9.6 0.83 3.95 0.90 257.1 46.0 0.78 16.7 0.96 0.86 253.8 200 0.75 108 0.71 251.7 860 0.74 400 249.0 3 100 0.68 3100 0.69 2.25 260.4 15.5 0.84 6.0 0.85 257.2 68.0 0.83 28.0 0.80 251.4 1730 0.81 1350 0.65 250.3 3100 0.69 2300 0.66 249.4 5700 0.73 3900 0.63 2.9 1 261.4 12.7 0.87 4.7 0.93 258.1 53.0 0.83 22.2 0.97 255.2 240 0.78 86.0 0.84 251.1 2900 0.78 880 0.60 250.0 5200 0.76 3100 0.73 3.29 260.7 13.3 0.91 5.6 0.90 257.7 57.0 0.90 19.0 0.96 256.4 110 0.88 46.0 0.86 254.5 260 0.86 112 0.81 251.7 1420 0.76 930 0.78 250.8 2400 0.74 1630 0.70 249.9 3950 0.75 2550 0.69 249.3 5600 0.74 3600 0.70 7.12 257.3 53.0 0.77 33.0 0.79 254.5 190 0.74 220 0.66 250.6 2200 0.59 2200 0.50 tlms tls FIO.4.Normalized birefringence (An/An,,,) against linear time (t)for 2.91%Bu3NHPi in a-terphenyl at the temperatures (K) indicated.44 DIELECTRIC AND DYNAMIC KERR-EFFECT STUDIES IN LIQUID SYSTEMS tlTg FIG.5.Normalized birefringence (An/An-) against normalized time (t/zo)for (a)3.29% BuSNHPi in o-terphenyl at 260.7 K-0 ;256.4 K-0 ;250.8 K-0 and (6) 2.25% Bu3NHPi in o-terphenyl at 260.4 K-0 ;257.2 K-0 ; 249.4 K-0.slower than the decay transient and as the temperature is lowered the rise and decay transients become more symmetrical. The data of table 2 and fig. 6 show that (zK,~/zK,~) is significantly greater than unity at the higher temperatures and tends towards unity at the lower temperatures. Table 3 gives dielectric data for this and earlier work;1° a comparison with Kerr-effect results as shown in fig. 6 reveals that the zDand z~,~ are quite similar in the region of overlap and that (Z&~,J is approxi- (a) (6) 103~1~ FIG.6.-10g(fm,,/Hz) against lo3K/Tfrom Kerr-effect rise (0) transients and Kerr-effect decay (0) for (a)1.33% and (b)3.29% Bu3NHPi in o-terphenyl and from dielectric loss curves for 1.52”/,-a ; 2.140/,4 and 2.91%-A Bu3NHPi in o-terphenyl.The dashed Iine represents Kerr-effect and dielectric results for pure 0-terphenyl. MARTIN S. BEEVERS ET AL. mately 3. Note in fig. 6 that log tfmax) for the solutions is in all cases less? than that for pure o-terphenyl; it has been suggestedlO that this arises since the cooperative mechanism applicable for fluorenone + o-terphenyl and other systems is coupled to an intrinsic relaxation mechanism for the (large) ion-pair solute. Table 2 shows that the p values obtained from the rise and decay transients are not only similar but TABLE3.-DIELECTRIC RELAXATION DATA FOR BujNHPi IN U-TERPHENYL 1.52%* 2.14%* 2.91% ~~~~~~~~ ~ ~~ ~ ~~~ ~ 266.8 0.90 0.95 268.3 0.48 0.88 268.9 0.47 0.95 262.2 5.65 0.88 263.1 5.03 0.86 267.9 0.58 0.95 258.8 20.0 0.78 259.5 21.5 0.76 266.5 1.45 0.95 260.8 12.1 0.91 259.8 24.7 0.90 257.4 63.4 0.88 255.7 130 0.85 * Davies Hains and Williams.l0 as the temperature is reduced decrease from near single relaxation time values towards values comparable with that for the fluorenone system.This behaviour closely parallels the dielectric results for this system.1° DISCUSSION While the contribution of the solvent to the dielectric and Kerr-effect relaxation in the present systems is not negligible the observed behaviour arises primarily from the dipolar solute molecules.These act as a probe on the cooperative motions between solute and solvent molecules and whereas there is little doubt that re- orientational and translational motions of the solute molecules occur on a comparable time-scale it is only the reorientational motions of the dipole vector which are being detected in both experiments. It is reasonable to neglect cross-correlation terms for the solutions studied here so wl(t) and lyz(t) are being determined; their characteristic features are summarized in tables 1 and 2 and ref. (9) and (10). The observation for the fluorenone $-o-terphenyl system (fig. 3) that z~,~ lirK,d N zDrules out a mechanism of reorientation through small-angle steps (rotational diffusion) which would require ZK,d = (1/3) Z and the Kerr-effect rise transient to be slower in time than the decay transient.The results are entirely consistent with the "fluctuation-relaxation " model5 mentioned in the Introduction. In this model a reference molecule only moves when that molecule and the local environment are subjected to a fluctuation (in local volume or energy say) exceeding a critical size. After the molecule has moved it is assumed that there is no orientational correlation on average between the initial and final directions of the dipole vector and this means that all angular correlation fuctions (P,(u,t)) have the same time-dependence c(t)say where c(t)is determined by the temporal evolution of the fluctuations.The realistic nature of this model for cooperative (or " structural ") relaxation in viscous liquids is supported by observation (using photon-correlation spectroscopy) that the These data for pure o-terphenyl refer to both dielectric relaxation and Kerr-effect relaxation. Beevers and coworkers" studied a-terphenyl and its mixtures with tri-tolyl phosphate and found z~,~ = 7K.d = Z~ for both pure liquids over a wide range of temperature. 46 DIELECTRIC AND DYNAMIC KERR-EFFECT STUDIES IN LIQUID SYSTEMS correlation time for density fluctuations is the same as that measured by dielectric or viscoelastic methods in certain viscous 1iq~ids.l~~'~ We shall consider below models which lead to the form of k(t). The observations for the TBP +o-terphenyl solutions are extremely interesting since they show that the mechanism for relaxation is changing as the temperature is varied.Fig. 6 and table 2 show that at the lowest temperatures studied z~,~ -+rK,d and although we do not have dielectric data at the lower temperatures an extra- polation of the high temperature data suggests that rD*z(Kerr) in this range. This suggests that at the lowest temperatures the ion-pairs reorientate by the "fluctuation-relaxation "mechanism applicable to the fluorenone +o-terphenyl systems. This is supported by the fact that the observed relaxation times for these fairly dilute solutions approach that of the solvent in the lower temperature region indicating that the motions of the solute and solvent on average have the same correlation time and are part of the same cooperative process.Additional confirmation of this mechanism comes from the variation of B with temperature. Tables 2 3 and the dielectric data of ref. (10) show that tends to -0.55 at lower temperatures similar to that for fluorenone +o-terphenyl and several other systems,l being a characteristic of wholly cooperative relaxation. A similar conclusion had been inferred lo from the dielectric data for TBP +o-terphenyl; the additional constraint of the Kerr-effect data makes it fairly certain. Fig. 6 shows that as Tis increased (rD/rK,d) increases to a value near 3 and that z~,~ zDat higher temperatures. This is an obvious departure from the "fluctuation-relaxation "model towards the rotational diffusion model in which the solute molecule moves through smaller angular steps.The fact that zDin this temperature range is longer than that for pure solvent implies that many "impacts "of the solvent are required in order to randomize the dipole-vector of the ion-pair-this being a condition for small-angle motions (rotational diffusion). We now briefly consider models for the relaxation in both systems. The "fluctua-tion-relaxation "mechanism may be considered to arise from the diffusion of "de-fects "through the liquid where the arrival of a "defect "at a molecule relaxes it completely on a time-scale far shorter than that required for the diffusion of "defects ". This process has been evaluated by Glarum14 and by Phillips and co-workers15 and leads to a function t(t)which has an average relaxation time governed by the defect- diffusion time 7dSdefect,and a functional form which is numerically the same as the Williams-Watts functions with D =0.514 if first and second nearest neighbour defects are inc1~ded.l~ Clearly this simple model gives an adequate representation of the "fluctuation-relaxation "mechanism for the fluorenone +o-terphenyl system and for TBP +o-terphenyl at lower temperatures.The observation that the same function c(t)characterizes both dielectric relaxation and the dynamic Ken-effect may also be rationalized* in terms of the jump-model of Anderson1'*18 for the special case18 where the probability function for reorientation through a jump-angle y is given by P(y)=C exp -(?/yo).For yo >n/4 zD fi zK,d. In physical terms this corresponds to jumps through angles of variable size and the probability of jumping through a small angle is greater than that for a large angle. The results for the TBP +o-terphenyl system at lower temperatures may be interpreted in the same manner as those for the fluorenone +o-terphenyl system. At higher temperatures a small-angle rotational diffusion situation is implied but it is essential to include the "fluctuation-relaxation "mechanism as a component to the *Ivanov16 has deduced relations for rDand ZK,d for rotational motion through steps of variable sizes and the form of these relations has been discussed by Beevers and co-workers" and by Ander- s0n.17 We note that Ivanov predicts (z&~,~)30.6 for discrete jumps through y =7r whereas the ratio should be zero in this case.17 MARTIN S.BEEVERS ET AL. overall process since there is a continuous variation in mechanism as the temperature is raised. A simple coupled scheme was previously suggested by Davies and co-workers lo for the dielectric relaxation being A 7+A * -+ C. At lower temperatures A -+ A* is the rate determining step leading to the " randomized " state C. At higher temperatures A* -+C becomes the rate determining step in the intrinsic re- orientation by the solute. Alternatively we may say that at low temperatures the solute molecules suffer infrequent but large fluctuations in their local environment leading to large-angle jumps on average when they move.At higher temperatures the frequency of the local fluctuations increases and with the increased thermal energy coupled with the large size of the solute there is a tendency to move through smaller angles (yo decreases) leading to z, == 32K,d,the small-angle rotational diffusion case. CONCLUSIONS The combined dielectric relaxation and dynamic Kerr-effect study of two super- cooled liquid systems has shown that the mechanism of dipole reorientation may be classified in a way that neither technique on its own would be able to achieve. For fluorenone + o-terphenyl reorientational motions occur by a " fluctuation-relaxation " mechanism in which the solute moves through large-angle steps. A similar mechan- ism applies to TBP + o-terphenyl at low temperatures but as the temperature is raised the mechanism appears to change to small-angle rotational diffusion.These systems may typify the behaviour of other viscous liquids and of amorphous solid polymers. We gratefully acknowledge the award of a studentship (to D. C. G.) and an equip- ment grant from the Science Research Council. G. Williams in Dielectric and Related Molecular Processes ed. M. Davies (Spec. Period. Rep. Chem. SOC. London 1979 vol. 2 p. 151. N. G. McCrum B. E. Read and G. Williams Anelastic and Dielectric Efects in Polymeric Solids (J. Wiley London 1967). G. Williams Chem. Rev. 1972 72 55. E. Fredericq and C. Houssier Electric Dichroism and Electric Birefringence (Oxford U.P. Lon-don 1974). M. S. Beevers J. Crossley D. C. Garrington and G.Williams J.C.S. Faraday ZZ 1976 72 1482. B. J. Berne P. Pechukas and G. D. Harp J. Chem. Phys, 1968,49 3125. R. Coelho and D. K. Manh Compt. rend. Cy1967,264 641. a M. M. Davies and G. Williams Trans. Favaday SOC.,1960 56 1619. M. F. Shears and G. Williams J.C.S. Faraday IZ 1973 69 608. lo M. Davies P. J. Hains and G. Williams J.C.S. Faraday IZ 1973 69 1785. l1 M. Beevers J. Crossley D. C. Garrington and G. Williams J.C.S. Faraday I& 1976 72 1482. l2 C. Demoulin C. J. Montrose and N. Ostrowsky Phys. Rev. A 1974 9 1740. C. Lai P. B. Macedo and C. J. Montrose personal communication. l4 S. H. Glarum J. Chern. Phys. 1960,33,639. l5 M. C. Phillips A. J. Barlow and J. Lamb Proc. Roy. SOC. A 1972 329 193. l6 E. N. Ivanov J. Exp. Theor. Phys. 1963,45,1509 (Sov. Phys. JETP 1964 18 1041). l7 J. E. Anderson Faraday Symp. Chem. SOC. 1972,6,82. la J. E. Anderson Faraday Symp. Chem. SOC. 1972 6 91.