J. Phys. Chem. 1994,98, 13481-13485
13481
Dipole Moments and Lifetimes of Excited Triplet States of Aniline and Its Derivatives in Nonpolar Solvents Hiroshi Shimamori* and Ayumi Sat0 Fukui Institute of Technology, 3-6-1 Gakuen, Fukui 910, Japan Received: August 17, I994@
Dipole moments have been determined for excited triplet states of aniline and its related compounds (dimethylaniline (DMA), diethylaniline (DEA), diphenylamine (DPA), and triphenylamine (TPA)) in nonpolar solvents by analyzing microwave dielectric absorption signals observed in 308-nm laser flash photolysis at room temperature. For all the compounds investigated, the dipole moment increases upon excitation; in benzene, the values are 2.1, 3.0, 3.1, 2.1, and 2.2 D for aniline, DMA, DEA, DPA, and TPA, respectively. The values for TPA and DPA in n-hexane and cyclohexane are not much different from those in benzene. The values from a semiempirical molecular orbital calculation can be compared to these experimental ones, and the polarity change upon excitation is mostly associated with the electron transfer from the N atom to the adjacent C atom in the phenyl ring. In contrast, the values in CC4 are much different and should not be referred to those for the triplet states. In dioxane, the values become higher than those in benzene, probably due to the effect of hydrogen bonding. The lifetimes of the excited triplet states have been determined, 1.2, 0.7, 1.0, 27, and 60 ,us, for aniline, DMA, DEA, DPA, and TPA, respectively, in benzene.
Introduction In the ground electronic state of aniline, the plane of the amino group is inclined at -46" to that of the phenyl ring' and is allowed no free rotation because of a partial conjugation of the lone pair electrons on the N atom with the aromatic ring.293 The increase of conjugation corresponds to the increase in the charge-transfer component along with more planar structure. For the excited states of the molecules, the excitation is primarily associated with the mc*transition with only a small intramolecular charge-transfer component! In the first excited singlet state (SI), the molecule is quasiplanar,' thus more contribution from the charge-transfer component may be involved in the S1 state than in the ground state. It has been suggested that N-alkylation of the amino group increases the charge-transfer component, but the increase in the size of the n-system decreases its c~ntribution.~ Such features should be reflected on the dipole moment of the molecule in the excited state. There are some experimental data for the dipole moment of the excited singlet state &) of aniline, but the values are diverse: 2.4 D6 for gaseous aniline corresponding to a dipole along the N-C bond axis5 and 1.8 and 5.2 D as the value for the entire molecule derived from solvatochromic effects on the absorption7and the effects on both absorption and fluorescence,8 respectively. Another compound for which ,ushas been reported is dimethylaniline (DMA): 3.6 D from solvatochromic effects on the fluorescence9 and 5.0 D from measurements of microwave dielectric loss in C-CgH12 and dioxane solvents.1° It appears that N-methylation of the amino group increases the chargetransfer component, but the data are too scarce to be compared quantitatively. The data is even limited for the excited triplet state of anilines. Although the geometry of the lowest triplet state of aniline is suggested to be nonplanar,ll no experimental data exist for its dipole moment. Only that for DMA has been measured to be 3.2 D in c-CgH12 and dioxane.1° The radiative lifetimes of the TI states of anilines at 77 K decrease markedly with N-alkylation? which can be related to the increase in the charge-transfer component. So far, however, the lifetimes at room temperature have not been established, since anilines do @
Abstract published in Advance ACS Abstracrs, November 15, 1994.
0022-365419412098-13481$04.5010
not phosphoresce at room temperature, and very few studies have been made with the triplet-triplet absorption measurements. In the present study the dipole moments and the lifetimes have been determined for the excited triplet state of aniline and its derivatives (DMA, diethylaniline, diphenylamine, and triphenylamine) in nonpolar solvents at room temperature. Our primary interest is to clarify the effects of N-alkylation and the increase of the n-system on the above excited-state properties. The time-resolved microwave dielectric absorption method has been used for the determination. As illustrated in a recent review,12the method has been applied to a variety of transients in order to determine their dipole moments and to clarify the mechanism of electron transfer processes upon photolysis. Since the present technique allows detection of transients with high sensitivity but with relatively low time resolution, it is suitable for measurements of such small dipole moment changes upon photolysis as the triplet states of anilines. Furthermore, known values of the quantum yield of the triplet state13 enable the determination of the absolute value of the excited state dipole moment. An application similar to the present study has been made recently for the determination of the dipole moments of the excited triplet state of substituted ben~ophenones.~~
Experimental Section The measurements were made as previously described.lS The sample was irradiated with a 308-nm laser pulse from an excimer laser (Lamda Physik, LPX 205). An X-band microwave circuit was used. A silica cell containing a sample solution was placed within the resonant cavity (TI5101mode, the resonant frequency = 8.8 MHz, loaded Q > 3000). The signal was averaged, mostly over 16 shots, using a Tektronix 2430 digital oscilloscope to improve the signal-to-noise ratio. In cases where too many laser pulses caused appreciable deterioration of the sample, the signals were taken with two or four shots. The laser intensity mostly used was about 2.5 mJ per pulse. The irradiation area on the sample cell was about 0.4 cm2. Aniline (99%), dimethylaniline(98%), diphenylamine (98%), and triphenylamine (98%) were purchased from Wako Chemi0 1994 American Chemical Society
Shimamori and Sat0
13482 J. Phys. Chem., Vol. 98, No. 51, 1994
cals, Inc., and diethylaniline (98%), triphenylphosphine (99%), and diphenylcyclopropenone (99%)were from Aldrich Chemical Co. Diphenylamine and diphenylcyclopropenone were purified by vacuum sublimation. Triphenylamine was recrystallized from ethyl acetate and sublimed. Other compounds were used without further purification. All the solvents (benzene, carbon tetrachloride, p-dioxane, cyclohexane, and nhexane, Wako Chemicals, Spectrograde) were dehydrated by contact with Molecular-Sieve 3A. Before irradiation all the samples were deaerated by bubbling with Ar gas for more than 20 min. Measurements were carried out at room temperature (-298 K). In determining the dipole moment of the excited species, the absolute intensity of the signal was compared with that of a reference compound (diphenylcyclopropenone) for which the data was taken under the same irradiation conditions as in the sample compound. The ratio of the initial amplitude V for a sample compound to that for the reference compound can be expressed as14,15
/l 1.5
1.oy.
'
0
20
"
"
40
"
60
" 120
"
100
80
(mM)
Concentration
Figure 1. Plot of appropriate function of reflected power as a function of concentration of solutes in benzene. Solute: aniline, DPA (diphenylamine), lTE' (triphenylphosphine), DPCP (diphenylcyclopropenone). > U
Aniline
~
\
> E
.-
where /3 is the coupling factor of the cavity, [SI is the concentration of the transient, A@*) = p; - pi bt is the dipole moment of the transient (the triplet state in the present case), pg is the dipole moment in the ground state), g ( t ) = wz/ (1 (w is the microwave radian frequency, z the dielectric relaxation time for the transient), and the subscripts s and r refer to the sample and the reference compounds. The ratio g(r)s/g(z), can be replaced with another factor determined from the "static" measurement in which the microwave power reflected from the cavity is measured as a function of the solute concentration. The results of the static measurements were analyzed using the following e q ~ a t i o n ' ~ . ' ~
+
:
I
.
I
,
I
1 ps/div
I
,
,
/
/
,
,
,
I
I
:
,
,
,
,
,
,
E
0
" t
I
500 ns/div
,
I
1
,
1
1
1
I
,
,
,
,
,
,
,
,
,
!
,
,
>
where Pr is the reflected microwave power, PO is the incident power, and C = Fg(z)p2( F is a factor related to the geometry of the cavity). This measurement was possible in aniline, dimethylaniline, diethylaniline, and diphenylamine by using these compounds themselves, but that for triphenylamine was difficult because of its very small dipole moment (0.5 D) in the ground state resulting in immeasurably low reflection power even at a high concentration. Therefore, we used triphenylphosphine (TTP) as a substitute, since the molecular structure and the size are very similar to triphenylamine, and the dipole moment is relatively high bg= 1.52 D). Examples of plots, based on eq 2, for the result of the static measurements are shown in Figure 1. Each slope gives the corresponding value of C. So the ratio g(t)s/g(t)r in eq 1 can be replaced by (C,/,u~,)/(CJp~), and we obtain
(3) From eq 3 we can determine the value of A@%) which gives the value of putby knowing pg.
Results and Discussion Dipole Moments of the Excited Triplet States. The concentration of solute molecules was chosen so that the optical
-u > -
\
>-
E -
In
10 p s / d i v 1
1
1
1
1
,
1
,
,
1
1
(
1
1
,
1
,
,
1
,
TIME Figure 2. Time dependence of the output (amplitude) of the dielectric absorption apparatus for solutions of aniline and its derivatives in benzene at room temperature. The laser intensity is about 2.5 mJ per pulse for all the compounds except DPA for which the intensity is 1.8 times higher.
density of the solution at 308 nm was unity. Microwave dielectric absorption signals observed upon laser irradiation on deaerated samples containing aniline (ANI), dimethylaniline (DMA), diethylaniline (DEA), diphenylamine (DPA), and triphenylamine ( P A ) in benzene are shown in Figure 2. Each amplitude shows a steep increases after photoabsorption followed by a gradual decrease to the corresponding base line. The initial amplitude was found to be proportional to the laser intensity employed (2-8 d per pulse), so a one-photonic process is apparent in the observed signals. Air-saturated samples show entire quenching of the signal in all cases except
. I . Phys. Chem., Vol. 98, No. 51, 1994
Excited Triplet States of Anilines in Nonpolar Solvents
w
w
0
5 ps/div
2
Diethylaniline-CCI,
>t
1
E
g t
i 500 ns/div
1
TIME Figure 3. Time dependence of the output (amplitude) of the dielectric absorption apparatus for solutions of aniline and diethylaniline in CCL at room temperature.
DPA where about a half the amplitude remains unquenched, showing a decay with a lifetime longer than that without oxygen. It is known that photolyzed DPA in the presence of oxygen produces carbazole.16 We have observed, using a spectrophotometer, an increase in the absorbance due to carbazole (a peak at 290 nm with a broad band around 320-330 nm) with the increase in the irradiation number of laser pulses. As the carbazole molecule possesses a dipole moment of 1.72 D, which is larger than that of the ground state of DPA (1.1 D), a positive signal is reasonable. Thus, we conclude that all the initial amplitudes of the signals including that for DPA correspond to the excited triplet states of respective compound and that the subsequent decays reflect the destruction of the triplet state. The positive signals in all the compounds indicate that the dipole moments of their excited triplet states are larger than those in the ground states. The values of the dipole moments determined here and the quantum yield 4 for the triplet formation used are shown in Table 1. Also in Table 1, some available values for the excited singlet states (ps)are shown for comparison. The errors in the present results come from the variation of signal heights observed for several samples for the same compound. Another uncertainty can be associated with the quantum yield 4. Measurements have been made with other solvents: n-hexane, cyclohexane, dioxane, and CC4. Since ANI, DMA, and DEA were not very soluble in n-hexane and cyclohexane, only DPA and TPA were measured in these solvents. The signals for these cases resemble those in benzene solvent, and the effect of addition of oxygen was similar: completely quenched for TPA but about a half remains for DPA. The derived dipole moments are also listed in Table 1. The values for DPA are slightly larger than that measured in benzene, and those for TPA are similar to that in benzene. The dielectric absorption signals observed in CC4 solvent suggest that some transients entirely different from those in other solvents are formed, since (i) the amplitudes of the signals are all much larger than those observed in other solvents, (ii) the time dependence of the signal is different, and (iii) there is essentially no effect of oxygen on the signals in all the
13483
compounds. Examples of the signals are shown in Figure 2 for ANI and DEA. Apparent values of the dipole moment of the transient determined from the initial amplitude by assuming the same quantum yields as in the triplet state are listed in Table 1. They are all very large and cannot be regarded as corresponding to the triplet state. We suggest them as being due to the formation of a contact ion pair such as ANI+Cl-. Boszczyk and Latowski17 have suggested that the absorption spectra of aniline in cyclohexane containing various amounts of CC4 indicate the formation of a ANI-CC4 complex with a high stability constant. The excitation of such a complex may lead to the formation of the contact ion pair if energetically possible. The formation of similar contact ion pairs has been demonstrated in photolyzed NfljV'N-tetramethyl-p-phenylenediamine (TMPD)15and NfljV'N-tetramethylbenzidine (TMB)'* in CC4 solvent. This subject will be dealt with in a separate paper, and we will not go into the details here. Dioxane was used as solvent in ANI and DEA. The value for ANI is more than three times larger than that in benzene. The dipole moment of ANI in the ground state shows a slight (0.2 D) increase when the solvent changes from benzene to dioxane. This was explained by the formation of a hydrogen bond between an oxygen atom in the dioxane molecule and a hydrogen atom in the ANI molecule, resulting in a more polar structure of ANI.19,20For the excited triplet state of ANI, the increase in the dipole moment observed here is too large to be explained similarly. Rather, formation of an exciplex with an appreciable charge separation may be responsible in this case. Not much difference in the values for DEA, and also for DMA, between that in benzene and that in dioxane is consistent with a fact that the formation of a hydrogen bond is unlikely in these cases. A semiempirical molecular orbital calculation has been made to acquire an insight into the electronic and geometrical structure of the excited states of aniline compounds. The AM1 method (in MOPAC)21 has been adopted for the calculation. Values of the dipole moments derived from this calculation are compared in Table 1. Although there are some differences in the results of the calculation between the MOPAC Ver. 6 and Ver. 5 , calculated values for the ground state (pg)and the triplet state (pt) are generally in good agreement with the experimental ones. So the results of the calculation should give a good picture of the nature of the excited triplet states of the compounds. For the excited singlet states, however, it is difficult to evaluate the reliability of the calculated values, as the experimntal ones are largely scattered and the data themselves are scarce. For the lowest excited singlet state of ANI, the transition S1 SO (1Al) is essentially n-z*, but it is regarded as (1'Bz) possessing a small intramolecular charge transfer component. In the excited triplet state T1 (3A1),the molecule tends to have more biradical nature than in the singlet state. This may cause less contribution of electronic charge transfer in the N-C direction. Thus we can expect less dipole moment in the T1 state than in the SI state. The experimental values of pt and p, for ANI and DMA seem to be in accord with this character, and the calculated values also follow this trend except in DPA. According to the results by the AMI calculations, the optimized structure of the excited triplet state of ANI shows that the electron density on the N atom of the NH2 group transfers mostly to the adjacent C atom in the benzene ring. A similar nature was apparent in DMA and DEA. In the excitation of DPA and TPA a part of electronic charge on the N atom appears to be distributed over the entire conjugated systems, but it was evident that only one of the C atoms adjacent to the N atom gained most of the charge and the remaining benzene
-
13484 J. Phys. Chem., Vol. 98, No. 51, 1994
Shimamori and Sato
TABLE 1: Dipole Moments (pt)of Triplet States of Aniline9 AMlC molecule aniline
dimethylaniline
diethylaniline diphenylamine
triphenylamine
A
2.1 f 0.2 7.5 (diox) 9.1 (CQk 3.0 f 0.3 [3.2] (c-Hex)' [3.3] (diox)' -6.0 (CCL# 3.1 f 0.3 3.6 (diox) -6.5 (CCl# 2.1 f 0.1 2.6 f 0.2 (n-Hex) 2.8 f 0.3 (c-Hex) -6.2 (CCL)k 2.2 f 0.2 1.9 f 0.2(n-Hex) 2.2 f 0.2(c-Hex) -5.8 (CCL)k
P2
2.49 1.8h 5.2'
PL 1.52
Pt
PS
Pg
0.75
2.33 (2.30)
3.02
1.53 (1.53)
1.60 1.52 1.63
0.8
3.17 (2.78)
3.30
1.43 (1.70)
1.80 1.89
0.9
3.26 (2.88)
3.32
1.44 (1.46)
1.10 1.18
0.38
2.57 (2.40)
1.30
0.76 (1.08)
0.5 0.71
0.48
2.64 (1.77)
2.45
0.46 (0.48)
1.77 [3.6y
V.OY P.OY
a Values in debye; measured in benzene solution if not specified (diox = dioxane, n-Hex = n-hexane, and c-Hex = cyclohexane). * Values for the excited singlet states (in debye). Values for the ground states (in debye), taken from McClellan, A. L. Tables of Experimental Dipole Moments, Vol. 11; Rahara Enterprises: El Cemto; 1974. Quantum yields for triplet formation, taken from ref 13. e Semiempirical molecular orbital calculation (the AM1 method, RHF) using MOPAC Ver.6.12 (ref 21). Values in the parentheses are calculated with MOPAC Ver. 5.01. f Reference 10. g Reference 6. Reference 7. Reference 8. j Reference 9. May not correspond to the triplet state, see text.
TABLE 2: Lifetimes of Excited Triplet States of Anilines in Nonpolar Solvents at Room Temperature compound lifetime (lo+ s) solvent ref aniline 1.2 f 0.3 benzene U 0.81 benzene b 0.83 ethanol dioxane 4.3 f 0.5 benzene dimethylaniline 0.7 f 0.2 benzene diethylaniline 1.0 f 0.2 benzene diphenylamine 27 f 5 n-hexane cyclohexane triphenylamine 60 ?fX benzene 416 benzene C 40 f 5 n-hexane U 28 f 5 cyclohexane a 34.5 cyclohexane c a This work Reference 22. Reference 23 I"
ring(s) is less affected. Consequently, the dipole moment change in the excitation to the triplet state is primarily determined by the transfer of the electronic charge from the N atom to the adjacent carbon atom in the benzene ring in all anilines investigated. On the other hand, in the excited singlet state of ANI, DMA, and DEA, the electronic charge on the N atom mostly flows into the C atom in the para position of the benzene ring. This may be the main reason for a larger dipole moment in the SI state compared to the T1 state in these compounds. In contrast, such a feature cannot be seen in DPA and TPA. In TPA the charge on the N atom is distributed over three phenyl rings, but in DPA the charge on the N atom unexpectedly increased upon excitation. A low dipole moment for the S1 state of DPA (1.30 D) should be the result of this effect. However, it may not be appropriate to make a quantitative evaluation of the charge density flow for such large molecules, especially on the basis of the semiempirical level calculation. It is worth noting geometrical aspects of the results of the calculation. The optimized structure derived from the calculation suggest the following. In the ground state of ANI,the plane of the NH2 group is inclined at about 45" to the plane of the
phenyl ring (a hydrogen atom rotates by 27-28" around the N-C axis), and it becomes more planar in the T1 state, Le., -35" out of the phenyl plane (-20" rotation of a hydrogen atom around the N-C axis). This is consistent with a suggestion that the lowest excited triplet state of aniline is nonplanar." In addition, the calculation supports the suggestion thatthe S1 state of aniline is quasiplanar,' only about 2" out of plane. A similar variation, Le., becoming more planar, of the plane of the dimethylamino group to that of the phenyl ring is evident in DMA in both the TI and the SIstates, but no such remarkable change can be seen in the diethylamino group in DEA. Lifetimes of Excited Triplet States. The decay of the dielectric absorption observed here can be related to the kinetics of the excited triplet state. The decays are of first order in all cases, and variations of the optical density and the laser intensity gave no significant difference in the decay constant. Therefore, the decay of the signal is directly associated with the lifetime for the destruction of the excited triplet state of each compound in a given solvent. Shown in Table 2 are the values of the lifetime derived from the decay signals that we can refer to the triplet state. The lifetime for ANI is found to be 1.2 f 0.3 ps, which is the same for both benzene and n-hexane solvents. On the other hand, in dioxane it is longer, 4.3 f 0.5 ps. This may be due to an enhancement of polar structure of the triplet aniline by forming a hydrogen bond with the solvent dioxane as in the ground ~ t a t e . The ~ ~ values , ~ ~ in benzene and n-hexane can be compared to 0.81 ps obtained by Perichet et aLZ2 from measurements of a triplet-triplet transfer process with tris(dibenzoy1methano)europium as the acceptor in cyclohexane. It is interesting that they also obtained a similar value, 0.83 ps, in ethanol. The hydrogen bond may be possible in ethanol, but the lifetime is shorter than that in dioxane. It is known that the radiative lifetimes of the T1 states of the anilines at 77 K decrease markedly with N-alkylation? The lifetimes at room temperature determined in this study show no such remarkable change with N-alkylation. The lifetime for TPA in benzene is typically 50-80 ps. This is relatively long but is still very short compared with 416 ps observed by Kemp et aLZ3based on their pulse radiolysis measurements for TPA in benzene. Although
Excited Triplet States of Anilines in Nonpolar Solvents it is possible that the present result is subject to a large error due to fluctuation of the base line or to quenching of the triplet state by residual oxygen or impurities, the difference is beyond explanation. It is interesting that they also obtained a much shorter lifetime, 34.5 ps, in cyclohexane solvent. The present result for cyclohexane solvent agrees well with this value. It is very unusual that the lifetime becomes quite long when the solvent changes from cyclohexane (or n-hexane) to benzene.
Acknowledgment. The authors are grateful to Y. Tatsumi and E. Ohshita for helpful discussion and their technical assistance. The present research is supported by a Grant-inAid for Scientific Research (C) (04640478) and is partly defrayed by a Grant-in-Aid on Priority-Area-Research “Photoreaction Dynamics” (06239257), both from the Ministry of Education, Science and Culture, Japan. References and Notes (1) Brand, J. C. D.; Williams, D. R.; Cook, T. J. J. Mol. Spectrosc. 1966, 20, 359. (2) Smith, J. W. J. Chem. SOC.1961, 81. (3) Cumper, C. W. N.; Singleton, A. J. Chem. Soc. B 1967, 1096. (4) Seliskar, C. J.; Khalil, 0. S.; M c G l y ~S. , P. In Excited State; Lim, E. C., Ed.; Academic Press: New York, 1974; p 231. (5) Malkin, J. Photophysical and Photochemical Properties of Aromatic Compounds; CRC Press: Boca Raton, FL, 1992; Chapter 7. (6) Lombardi, J. R. J. Chem. Phys. 1969, 50, 3780.
J. Phys. Chem., Vol. 98, No. 51, 1994 13485 (7) Prabhumirashi, L.S.; Narayanan, D. K.; Bhide, A.S . Spectrochim. Acta 1983, 39A, 663. (8) Suppan, P. Chem. Phys. Lett 1983, 94, 272. (9) Kohler, G.J. Photochem. 1987, 38, 271. (10) Weisenbom, P. C. M.; Varma, C. A. G. P.; de Haas, M. P.; Warman, J. M. Chem. Phys. 1988, 122, 147. (1 1) Scheps, R.;Florida, D.; Rice, S. A. J. Chem. Phys. 1974,61, 1730. (12) Shimamori, H.In Progress in Photochemistry and Photophysics; Rabek, J. F., Ed.; CRC Press: Boca Raton, FL, 1992; Vol. VI, Chapter 2. (13) CRC Handbook of Organic Photochemistry, Vol. 1; Scaiano, J. C., Ed.; CRC Press: Boca Raton, FL, 1989. (14) Shimamon, H.; Uegaito, H.; Houdo, K. J. Phys. Chem. 1991, 95, 7664. (15) Shimamon, H.;Uegaito, H. J. Phys. Chem. 1991, 95, 6218. (16) For example; Fall, C. A. T.; Diop, A.; Aaron, J. J. Bull. Soc. Chim. Belg. 1985, 95, 631. (17) Boxzczyk, W.; Latowski, T. Z. Naturforch. B 1989, 44, 1585. (18) Shimamori, H.; Tatsumi, Y. J. Phys. Chem. 1993, 97, 9408. (19) Curran, C.; Estok, G. K. J . Am. Chem. SOC.1950, 72, 4575. (20) Chitoku, K.; Higashi, K. Bull. Chem. Soc. Jpn. 1966, 39, 2160. (21) Dewar, M. J. S.; Zoebisch, E. G.; Healy, E. F.; Stewart, J. J. P. J . Am. Chem. Soc. 1985,107,3902. Stewart, J. J. P. J . Comput. Chem. Edu. 1989,62, 206. The AM1 calculations were carried out by using MOPAC Ver. 6 by J. J. P. Stewart, revised as Ver. 6.12 by R. Koch and B. Wiedel, Inst. Org. Chem., Univ. Erlangen-Numberg, for IBM-PC supplied from QCPE (program no. QCMPl13), Indiana University. Also, MOPAC Ver. 5, J. J. P. Stewart, QCPE no. 455, revised as Ver. 5.01 by T. Hirano, University of Tokyo, for HITAC and UNIX machines, JCPE News 1989, 1, 10, has been used for comparison. (22) Perichet, G.; Chapelon, R.; Pouyet, B. J. Photochem. 1980, 13, 67. (23) Kemp, T. J.; Roberts, J. P.; Salmon, G. A,; Thompson, G. F. J. Phys. Chem. 1968, 72, 1464.
