Organic Mass Spectrometry, 1970, Vol. 4, pp. 533 to 544. Heyden & Son Limited. Printed in Northern Ireland

KINETIC STUDIES IN MASS SPECTROMETRY-1x1

COMPETING [M - NO,] AND [M - NO] REACTIONS IN SUBSTI- TUTED NITROBENZENES. APPROXIMATE ACTIVATION ENERGIES

FROM IONIZATION AND APPEARANCE POTENTIALS. * PETER BROWN

Department of Chemistry, Arizona State University, Tempe, Arizona 85281, USA

(Received 29 June 1970; accepted (revised) 12 August 1970)

Abstract-Ionization (IP) and appearance potentials (AP) have been secured for apparently competing [M - NO,] cleavage and [M - NO] rearrangement reactions in a series of m- and p-X substituted nitrobenzene molecular ions. Approximate activation energies AP-IP were within the experimental reproducibility for most substituents for the [M - NOz] reaction of m- and p-X isomer pairs of compounds, but outside it for the [M - NO] process. This is interpreted as either effective sub- stituent randomization in the reacting molecular ions or as fortuitously similar activation energies for the [M - NO,] cleavage. Assuming the latter, AP-IP for both reactions gave acceptably linear correlations of negative slope when plotted against u or u+, which has been interpreted mechanistically in terms of a degree of positive charge dissipation in the rate limiting transition states. These findings are in direct contrast to the energetics of the [M - CHJ and [M - CH,O] reactions recently studied in substituted anisoles, where plots of AP-IP us. u+ displayed positive slopes. The advantages of AP-IP over peak relative abundance as a mechanistic probe are discussed, and corresponding plots of log Z/Z, us. u(a+) and log [A,]/[M,] (1 - f) us. a(u+) for the same reactions are presented and discussed. The disadvantages and limitations of IP and AP measurements are also emphasized.

I N T R O D U C T I O N

SUBSTITUENT effect studies using substrates containing substituted benzene rings attached in the proximity of the reaction site to sense electron release or demand in the transition state have long provided valuable insights into reaction mechanisms of thermal1 and photochemical2 processes. Extension of this approach to unimolecular gas-phase decompositions of ions in the mass spectrometer was pioneered by McLaf- ferty and Bur~ey,.~ using a simplified steady-state kinetic approach, and was sub- sequently utilized by other^,^ to 25 mainly in some modified form, in attempts to avoid fundamental problems in relating relative peak heights in the mass spectrum (i.e. at the collector) with reaction rates and ion yields in the source, and to recognize the theoretical implications of the Quasi-Equilibrium Theory.26

It has been suggested that approximate activation energieP obtained from ionization and appearance potential determinations (AP-IP)9,20*25 may provide a more reliable (and more soundly based theoretically) kinetic measure of the effect of substituents on reaction rate. We have uncovered approximately linear correlations of AP-IP for apparently competing pairs of reactionsa7 with u or 6t in series of m- and p-X substituted anisolesZ5 ([M - CH,] and [M - CH20] reactions) and nitrobenzenes ([M - NJ0 and [M - NO] processes), where such correlations were almost non-existent by peak relative intensity methods.4,18 Clearly a linear correlation of AP-IP with G or G+ (a linear free energy relationship) is not demanded for every

* Presented in part at the Eighteenth Annual Conference on Mass Spectrometry and Allied Topics, ASMS/ASTM Committee E-14, San Francisco, California, June, 1970.

533

534 P. BROWN

reaction, but it is felt that when such a correlation is observed, then the possibility of mechanistic significance must be at least explored.25

The justifications for relating approximate activation energy" AP-IP to an average reaction rate, and for the implications of using m- and p-X substituted benzene substrates have already been

R E S U L T S

In Table 1 are given IP's, AP's and AP-IP values in eV for the [MI -+ [M - NO2] cleavage and the [MI -, [M - NO]? rearrangement in a series of m- and p-X substituted nitrobenzenes. These were determined as before on a Varian Atlas SM-1B double focusing mass spectrometer by the use of argon as internal standard and from the extrapolation of semi-logarithmic plots,2a as previously d e s ~ r i b e d ~ ~ , ~ ' and were found to be reproducible to 10.1 eV in duplicate runs. For all compounds examined, AP-IP for the rearrangement [M - NO] was less than for the apparently competing cleavage [M - NO,], as noted before for nitrobenzene itself.27 TABLE 1 . IONIZATION AND APPEARANCE POTENTIAL DATA FOR THE [M - NO,] AND [M - NO]

REACTIONS I N SUBSTITUTED NITROBEN ZENES :

IP [M - NO21 [M - NO] X

This work LkS8 AP Lit. AP-IP AP Lit. AP-IP

m-NH , p-NHz

p-OH

m-OCH, p-OCH,

m-CH, p-CH,

H

m-F P-F

p-c1 m-C1

rn-Br p-Br

m-CN p-CN

m-NO, p-NOz

8.73 8.62

8.84

9.09 9.04

9.48 9.50

9.65

9.93 10.00

9.92 9.96

9.82 9.76

10.29 10.23

10.62 10.63

8.80 8.85

9.52

9.82

10.18

11.23 11.53

11-91

11.44 11.63

11.58 11.80

1 1.93

12.22 12.37

1200 12.30

12.01 12.19

12.25 12.42

12.34 12.50

2.50 2.91

3.07

2.35 2.59

2.10 2.30

2.28

2.29 2.37

208 234

2.19 2.43

1.96 2.19

1.72 1.87

9.12 9.56

9.90

9.39 10.03

9.98 10.34

10.35

10.25 10.64

10.3 1 10.6 I

10.26 10.55

10.45 10.80

- -

0.39 0.94

1.06

0.30 0.99

0.50 0.84

070

0.32 0.64

0.39 0.65

0.44 0.79

0.16 0.57

- -

X = OCH, [M - 301 E [M - NO] (>97%); [M - 461 [M - NO,] (>97%).

* The approximation is due to the kinetic shift, to excess energy problems, and to instrumental errors and extrapolation procedures used to obtain IPS and A P ' S . ~ * J ~ ~ ~ ~

t For X = OCHI, the [M -301 peak was shown by precise mass measurements to be t 9 7 % [M - NO], rather than [M -CH,O]. For X = NOz, AP-IP for [M - NO] was less than the reproducibility of the determinations (i.e. t 0 . 2 ev>.

Kinetic studies in mass spectrometry-IX 535

In the case of the [M - NO,] reaction, for isomeric m- and p-X substituted compounds with the same substituent, the differences between AP values (&0.1 eV) and AP-IP values (f0.2eV) are not significantly greater (Table 1) than the ex- perimental reproducibility (given in brackets). For the [M - NO] process, the corresponding values for AP and AP-IP are clearly different for m- and p-X isomers (Table l), although AP-IP is a small difference between two relatively large numbers.

DISCUSSION

The possibility of effective substituent randomization or ring cleavage in the molecular ions and/or activated complexes leading to [M - NO,] ions must, there- fore, be entertained, as discussed for [M - CH,] and [M - CH,O] reactions in

Ion. Pots. eV App. Pots

13.0

m-CN p-CN P-NOz

. . in-NO,

12.0. m-Br

in -N 0

m - m

11.0 -

Ion i za t i on and A p p e a r a n c e Potentials vs . (T : M-M-NO,

I I *cT - 1.0 0 + 0,8

FIG. 1 . IP ([XCgH4N02]+.) and AP ([XC,H,]+) us. 0. Slopes = +1.40 and +0.69 respectively.

substituted an is ole^.^^ Previous work on substituted nitro benzene^^^ produced no evidence for this, however, and recent r e p ~ r t s ~ ~ * ~ l cite positive evidence for non- rearrangement in the reacting molecular ions, although this is for the [M - NO] process,* and molecular ions of some different energylstructure might be involved in the [M - NO,] cleavage.

If for the present the assumption is made that the relevant molecular ions are not rearranging in a manner that effectively scrambles the substituent, then the activation energy data in Table 1 may be analyzed by the Hammett treatment. In Fig. 1, IP us. G is plotted, and it is apparent that the m-X substituents fall on a separate line of slightly greater slope. This even more marked in Fig. 2, where IP is plotted us. &, and

* Ref. 31 refers to metastable ions only.

536 P. BROWN

e V Ion. Pot. /

, - 1.0 0 +0.8

FIG. 2. IP ([XC,H,NO,JC.) vs. 5+. Slopes= +0.91 (p-X) and t2.20 (m-X).

two correlation lines are shown," one for p-X (slope = $0.91) and one for m-X (slope = +2-20).? The slope of +0.91 should be compared with the analogous plot for the IP of substituted anisoles us. o+, which has a slope of +0*74.,j Thus substituent effects for ionization to a positive ion are demonstrated to be more important the more electron deficient is the aromatic ring, as might be expected.

Also given in Fig. 1 is a correlation line for AP's for the [M - NO,] reaction (slope = $0.69). The energy difference AP-IP decreases with increasing electron demand by substituents, and an acceptably linear correlation (slope = -0.76) of AP-IP against 0 is obtained (Fig. 3).$ The reproducibility in all AP-IP plots is &0.2 eV,§ and it is clear that no recognizable correlation exists with number of degrees of freedom in the molecule, and hence none with effective number of oscil- lators.

In Fig. 4, AP's for the apparently competing [M -NO] reaction are plotted against u+, and again separate correlation lines can be drawn for m-X (slope = t2 .07) and p-X (slope = $0.62) substituents. As shown in Fig. 5, AP-IP again decreases with increasing electron demand by substituents (slope = -0*39), although the plot against cr+ shows some scatter. 6 Again, no correlation of AP-IP with effective number of oscillators is evident.

* A single correlation line results from a plot of IP (or AP) us. u+ (para). X = m-OH was not employed, because of the lack of a generally acceptable u+ vaIue for this

substituent. $. As indicated by vertical lines through plotted points on Figs. 3 and 5. 5 Scatter in the AP-IP plots is sufficient to obscure possible separate lines for m- and p-X sub-

stituents.@

Kinetic studies in mass spectrometry-IX

11.0.

537

X vNo2

eV A P - I P

I * u -1.0 0 + 0.8

FIG. 3. AP-IP ([XC,H,]+) us. u. Slope = -0.76.

e V App. Pots.

4

Appeoronce Potent iols vs .

M e M - N O

9.0 1

I

/ / / m-OCH,

+ I / , (J E-

- 1.0 0 + 0.6

FIG. 4. AP ([XC,H,O]+) us. u+. Slopes = +062 @-X) and $2.07 (m-X).

538 P. BROWN

eV A P - I P

r - 1.0 0 + 0.6

FIG. 5. AP-IP [XC,H,O]+ us. u+. Slope = -0.39.

For both [M - NO,] and [M - NO] reactions, the slopes of AP-IP plots us. 6' have precisely the opposite sign to the [M - CH,] and [M - CH,O] processes in substituted anis0les.2~

The [M - NO,] cleavage: The approximate activation energy AP-IP decreases with increasing electron demand (Fig. 3), and the reasonable linear correlation ob- served suggests possible mechanistic significance. Since bond cleavages are usually endothermic, application of Hammonds Postulate32 would predict a transition state more closely resembling products than reactants. In the structure at this potential energy pass, the substituent effect results suggest that a degree of positive charge is dispersed from the reaction site relative to the reacting molecular ion(s). This finding can be accommodated in terms of the resonance structures outlined in Scheme 1.

0 l+

Preferred contributor I (for -1, -R groups) suffers homolytic cleavage of the ring C-N bond, in which a formal double electron deficiency on carbon (I) is transformed into a formal single electron deficiency (ab).

Kinetic studies in mass spectrometry-IX 539

The [M - NO] rearrangement: As for the [M - NO,] reaction, AP-IP decreases with increasing electron demand in the substituents, and a similar dispersal of a degree of positive charge at the reaction site in the rate-determining transition state is proposed. As in this instance, when the overall process is that of a rearrangement accompanied by cleavage, the question arises as to the stepwise or concerted nature of the mechanism, and if the former which step is actually rate determining? The present data do not allow a firm decision on this point, and the simplifying assumption is made here that if the mechanism is indeed stepwise then the cleavage step would be expected to have a higher activation energy than the rearrangement step.27 A non- concerted mechanism can thus be envisioned, based on already postulated inter- mediates ba29830 and as in Scheme 2 :

( b d ) [M - NO]

SCHEME 2

The proposed slow step bb --f bc is, therefore, depicted with dissipation of a degree of positive charge on oxygen at the reaction site.* Convincing evidence has recently been presented3I that the decomposing [M - NO] ions (bd) of relatively low internal energy (metastable ions) from substituted nitrobenzenes (I) and the similarly endowed [M - CH,] ions from the corresponding an is ole^^^ have a common s t r u c t ~ r e . ~ ~ Thus the intriguing conclusion to be drawn from the substituent effect studies% is that this common ion (e.g. bd),l is formed by cleavage from substituted anisolesZ5 and by rearrangement from nitrobenzenes in reactions of opposite electron demand.

It must be emphasized that the substituent effect results described in this work do not prove the intermediacy of ba, bb or bc; a study of AP-IP for the [M - N O ] reaction of a series of m- and p-X substituted phenyl nitrites (XC,H,.ONO) would be illuminating in this respect.

Substituent eflects on ion relative intensities The [MI -+ [M - NO] reaction of substituted nitrobenzenes has been examined

by Bursey and McLaf fe r t~ ,~~ using Z/Zo values and energy release in flat-topped metastable ion decomposition^.^^ Substantial differences in Z/Zo between m- and p-X isomers were noted for this process, and attributed29 to preferential stabilization

* AP-IP for cleavage of NO2 from the rearranged molecular ion bb should be increased by electron withdrawing groups, just the opposite of what is observed here.

540 P. BROWN

in the [M - NO] ion (e.g. bd, Scheme 2) by +Rp-X substituents. These same +R substituents were also the only ones to show a release of kinetic energy in the further decomposition of metastable [M - NO] ions by expulsion of C0.29 No correlation of log Z/Zo with 6 values was mentioned for the [M - NO] reaction in the molecular ions.

l og z/z,

We have redetermined log Z/Z, for both [M - NO,] (Fig. 6 ) and [M - NO] (Fig. 7 ) processes at a nominal 15 eV.* It is quite evident that no linear correlation at all can be discerned for [M - NO,], but a trend of negative slope may be present for [M - NO] (dotted line in Fig. 7). If this is the case, then substituent effects on ‘rate’ data from log ZlZ, would suggest that the [M - NO] reaction is enhanced by electron donating substituents, which is opposite to the conclusion reached from AP-IP measurements.

Application of the modified kinetic treatment of Chin and Harrison18 leads one to look for linear correlations of log [Ao]/[Mo] (1 - f) us. d or d+, as depicted for the [M - NO,] cleavage in Fig. 8, and the [M - NO] rearrangement in Fig. 9. As noted previ0usly,2~ the plots of this type for two major competing reactions can be expected to have slopes of opposite sign, and this, therefore, severeIy limits the scope of mechanistic inferences that may be drawn. Data for [M - NO,] (Fig. 8) show an excellent linear correlation of positive slope, but the scatter for the [M - NO] process (Fig. 9) is considerably greater.

Recently B ~ r s e y ~ ~ has made the interesting suggestion that steric effects of ortho- substituents may affect resonance stabilization of daughter ions derived from frag- mentation of aromatic substrates in a similar way to known examples of steric

* Experimental details are given in detail in ref. 19.

Kinetic studies in mass spectrometry-IX 541

...

Op-NH,

aP-OH

0""" u+ - i.0

FIG. 7. Log Z/Zo (Z = [XC,H,0]+/[XCsH4NO2]+. at 15 eV) us. &.

log A, M,( I - f )

v t

+ l o g 2%- vs. CT

Me ( I - f )

M ----+M-NO, _7oV +

u -1.0 0 f 0.8

FIG. 8. Log [Ao]/[Mo] (1 -f) for [M - NO,] at 70 eV. vs. G+.

542

. A log vs. u+

M,(I-f)

M- M - N O m v - 2 . 5

u+

P. BROWN

A log M,o t"

l T . 5

X qNo2 - 3 "

FIG. 9. Log [A,]/[M,] (1 -f) for [M - NO] at 70 eV us. o+.

inhibition of resonance in solution chemistry. The evidence put forward involved the effect of thep-X* substituent on the [M - NO] rearrangement in 3,4,5-trisubstituted nitrobenzenes, as reflected by Z/Zo values.35 This concept was extended36 to similar comparisons of the [M - NO] reaction, again measured by Z/Zo values, in 2- and 4-halonitrobenzenes and 2,4- and 2,6-dihalonitrobenzenes (X = C1, Br, I), and the conclusion reached36 that an increase in steric hindrance at the -NO, group occurs in the activated complex for NO loss. A mechanism such as in Scheme 2 was considered

with I --+ ba being faster than the cleavage bb -+ bc. The idea of studying steric effects in mass spectrometry and comparing them to analogous solution reaction effects is an excellent one, since much may be learned about the structure and electronic states of the ions involved. In order to put any confidence in such comparisons, and their consequences, however, better methods of studying the mass spectral reactions than 2-values must be employed, such as the use of the energy selection inherent in metastable ions?' and energetic approaches such as determination of AP-IP.

CONCLUSIONS

If no loss of substituent positional identity is occurring, then approximately linear correlations of AP-IP us. 6 or a+ have now been demonstrated for apparently competing cleavage and rearrangement processes in two series of compounds, i.e. [M - CH,] and [M - CH,O] in substituted anis0les,2~ and [M - NO,] and

* X = OH, NH2, OCHI, N(CH&.

Kinetic studies in mass spectrometry-IX 543

[M - NO] in substituted nitrobenzenes. AP-IP9,25 as a measure of reaction ac- tivation energy is clearly a sounder parameter theoretically than peak relative abund- ances in a spectrum (such as Z-values and their mod i f i ca t i~ns~~ ,~~ . l*J~) are of reaction rate.

AP-IP has some obvious disadvantages also. These include (i) the usual in- strumental and procedural errors of determining and standardizing IP's and AP's; (ii) the inclusion of excess energy and kinetic shift terms in experimentally measured AP's; (iii) the insensitivity of IP's and AP's to energies < ,- k0.1 eV (when measured with a conventional ion source) ; (iv) the possibility of competitive shift errors introduced when competing processes (as studied here) have AP's which are rather different (Table 1); (v) the variation of s (number of effective oscillators) with different substituents, even though a common reaction is under scrutiny; and (vi) the need for reasonably intense ion peaks for measurement.

E X P E R I M E N T A L Mass spectra

Ionizaton and appearance potentials were determined on a Varian Atlas SM-1B double focusing mass spectrometer, as described fully in ref. 27, using argon as internal standard and the semi- logarithmic plot technique.28 As observed with nitroben~ene,~' and as originally described by Beynon et trace amounts (<0.2%) of the corresponding aniline molecular ion were detected by precise mass measurements (resolution approx. 10,000) at nominal mass [M - 301 in the spectra of the substituted nitrobenzenes taken on our instrument. This trace impurity, apparently produced catalytically in the source/inlet system,39 gave rise to a curved foot on the low eV side of the ionization efficiency curve, plotted in semi-log form. The straight line portion of the plot at higher eV was not obscured, however, and extrapolation to 001 % of the ion intensity at 50 eV could be made in the usual way:' and reproducibility in duplicate runs was still kO.1 eV. Also, the amine molecular ion contribution did not increase detectably during the 10 to 15 mins. needed for IP and AP determination.

Peak relative intensities were secured by Mr Richard Scott using a single focusing Varian Atlas CH4B instrument under conditions fully described in ref. 19.

Con pounds All compounds were commercially available samples of as high initial purity as possible. Further

purification was effected by means of an Aerograph preparative g.l.c., fitted with a 10' x f" column of 5 % XE-60 on 80-100 Gaschrom Q, at approx. 10-20" below the b.p. of the sample.

Ackno ,i ledgernents-We are greatly indebted to the National Science Foundation for funds (Grant Nos. GB-4939 and GP-6979) to purchase the Varian Atlas CH4B and SM-IB mass spectrometers, and also to the University Grants Committee at Arizona State University for a Faculty Research Grant, 1967 to 1969.

R E F E R E N C E S 1 . e.g. see (a) L. P. Hammett, Physical Organic Chemistry, McGraw-Hill, Inc, New York, 1940,

Chaps. 3, 4, 7; (b) J. Hine, Physical Organic Chemistry, McGraw-Hill, Inc, New York, 1962, Chap 4; (c) H. H. Ja%, Chem. Revs 53, 191 (1953); (d) M. Charton, J. Org. Cizem. 29, 1222 (1964).

2. e.g. see (a) E. J. Baum, J. K. S. Wan and J. N. Pitts, J. Am. Chem. SOC. 88,2652 (1966); (b) J. N. Pitts, D. R. Burley, J. C . Mani and A. D. Broadbent, J. Am. Chem. SOC. 90,5902 (1968).

3. F. W. McLafferty, Anal. Chem. 31,477 (1959). 4. M. M. Bursey and F. W. McLafferty, J. Am. Chem. SOC. 88, 529 (1966). 5. J. L. Mateos and C . PSrez, Bof. inst. Quim. Univ. Nal. Autdn MPx. 17, 202 (1965). 6. P. Brown and C. Djerassi, J. Am. Chem. Soc. 89,271 1 (1967). 7. I. Howe and D. H. Williams, Chem. Commun. 220 (1968). 8. D. G. I. Kingston and H. P. Tannenbaum, Chem. Commun. 444 (1968).

544 P. BROWN

9. R. A. W. Johnstone and D. W. Payling, Chem. Commun. 601 (1968). 10. M. M. Bursey, Org. Mass Spectrom. 1, 31 (1968). 11. 1. Howe and D. H. Williams, J. Chem. Soc. (B) 1213 (1968). 12. P. Brown, J . Am. Chem. SOC. 90,4459 (1968). 13. P. Brown, J. Am. Chem. Soc. 90,4461 (1968). 14. R. G. Cooks, I. Howe and D. H. Williams, Org. Mass Spectrom. 2, 137 (1969). 15. R. H. Shapiro and J. W. Serum, Org. Mass Spectrom. 2, 533 (1969). 16. M. A. Baldwin and A. G. Loudon, Org. Mass Spectrom. 2 , 549 (1969). 17. R. H. Shapiro and T. F. Jenkins, Org. Mass Spectrom. 2, 771 (1969). 18. M. S. Chin and A. G. Harrison, Org. Muss Spectrom. 2, 1073 (1969). 19. P. Brown, Org. Mass Spectrom. 2 , 1085 (1969). 20. T. W. Bentley, R. A. W. Johnstone and D. W. Payling, J. Am. Chem. Soc. 91, 3978 (1969). 21. R. H. Shapiro, J. Turk and J. W. Serum, Urg. Mass Spectrom. 3 , 171 (1970). 22. R. P. Buck and M. M. Bursey, Org. Mass Spectrom. 3,387 (1970). 23. M. M. Bursey and P. T. Kissinger, Org. Mass Spectrom. 3, 395 (1970). 24. P. Brown, Org. Mass Spectrom. 3, 639 (1970). 25. P. Brown, Org. Mass Spectrorn. 4, 519 (1970). 26. (a) H. M. Rosenstock, M. B. Wallenstein, A .L Wahrhaftig and H. Eyring, Proc. Natl. Acad. Sci.

US. 38, 667 (1952); (b) H. M. Rosenstock and M. Krauss, in F. W. McLafferty (Ed.) Mass Spectrometry of Organic Zons, Academic Press, New York, 1963, Chap. 1.

27. P. Brown, Org. Mass Spectrom. 3, 1175 (1970). 28. F. P. Losing, A. W. Tickner and W. A. Bryce, J . Chem. Phys. 19, 1254 (1951). 29. M. M. Bursey and F. W. McLafferty, J. Chem. Phys. 88,5023 (1966). 30. M. M. Bursey, Org. Mass Spectrom. 2 , 907 (1969). 31. B. Davis and D. H. Williams, Chem. Commun. 412 (1970). 32. G. S. Hammond, J. Am. Chem. Soc. 77, 334 (1955). 33. J. Beynon, R. A. Saunders and A. E. Williams, Znd. Chim. Belge. 29, 331 (1964). 34. (a) J. H. Beynon, R. A. Saunders and A. E. Williams, Z . Naturforsch. 20a, 180 (1965); (b) T. W.

35. M. M. Bursey, J. Am. Chem. Soc. 91,1861 (1969). 36. M. M. Bursey, Org. Mass Spectrom. 2, 907 (1969). 37. M. M. Bursey and M. K. Hoffmann, J. Am. Chem. Soc. 91, 5023 (1969). 38. G. F. Crable and G. L. Kearns, J. Phys. Chem. 66,436 (1962). 39. J. H. Beynon, J. A. Hopkinson and G. R. Lester, Znt. J . Mass Spec. Zon Phys. 2,291 (1969).

Shannon, F. W. McLafferty and C . R. McKinney, Chem. Commun. 478 (1966).