Photoelectron spectroscopy of polyhalobenzenes

Chemical Physics 326 (2006) 471–477 www.elsevier.com/locate/chemphys

Photoelectron spectroscopy of polyhalobenzenes Igor Novak b

a,*

, Branka Kovacˇ

b

a Charles Sturt University, Leeds Parade, Orange, NSW 2800, Australia Physical Chemistry Division, ‘‘Ruder Bosˇkovic´’’ Institute, HR-10002 Zagreb, Croatia

Received 12 January 2006; accepted 7 March 2006 Available online 10 March 2006

Abstract The electronic structure of several polyhalobenzenes (C6H3Br3 and C6H2XYZ2) has been investigated by HeI/HeII photoelectron spectroscopy. The spectra were assigned by Green’s functions calculations and comparison with the spectra of related halobenzenes. The careful analysis of shifts in p-orbital and halogen lone pair ionization energies vs. related halobenzenes was made. This enabled us to analyze complicated substituent effects in greater detail than is possible by performing calculations of total energies for isodesmic reactions. The use of two descriptors enables one to estimate relative magnitudes of resonance and inductive effects and to demonstrate the presence of halogen–halogen interactions via p-system relay. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Photoelectron spectroscopy; Substituent effects

1. Introduction The substituent effects (SE) in benzene derivatives have been extensively studied and reviewed although different types of effects are not easy to identify or quantify [1]. The SE contain electronic (R + F), steric (S) and solvent contributions. The electronic contributions are subdivided into two components: F component and R component. F component contains contributions due to successive polarizations of r-bonds and also due to direct, through space influence of its electric field on the reaction centre [2]. R component is due to the existence of p-bonding which acts as a relay for transmission of substituent effects. Both F and R components can be further subdivided into various mechanisms through which substituents interact with each other. However, such mechanisms may be difficult to identify experimentally and originate from theoretical calculations. Exner and Bo¨hm used DFT calculations within isodesmic reactions schemes to describe electronic part of SE [3]. One problem with quantum chemical calculations *

Corresponding author. E-mail address: [email protected] (I. Novak).

0301-0104/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.chemphys.2006.03.007

is that they are dependent on the type of Hamiltonian used and the reference molecule selected. A different, semiempirical approach to electronic SE was based on the combination of theoretically deduced indices (e.g. electrophilicity index) and experimentally derived Hammett constants rp [4]. In this work, we use only experimental parameters (orbital ionization energies) which were obtained from the photoelectron spectra of halobenzenes. The ionization energies of the two topmost p-orbitals and halogen lone pairs can be measured accurately and represent sensitive, internal probes of the electronic structure and transmission of SE. 2. Experimental and theoretical methods The compounds 1–8 were purchased from Acros Organics NV and Fluka AG and used without further purification after confirming their identity by measuring NMR and mass spectra. The polyhalobenzenes studied are shown in Scheme 1. The sample inlet temperatures were in the range 40– 60 °C. These temperatures were necessary in order to obtain sufficient vapour pressures in the ionization region.

I. Novak, B. Kovacˇ / Chemical Physics 326 (2006) 471–477

472

Br Br 1

Br

2

Cl

Cl 6

Cl

F

Br

3

4

5

Br

I Br

F

Br

Br

Br

Br F

Br Br

F

Br

Cl

Cl

Br

Br

Br

Cl

F

I

7

8

Scheme 1.

The HeI/HeII photoelectron spectra were recorded on the Vacuum Generators UV-G3 spectrometer and calibrated with small amounts of Xe or Ar gas which was added to the sample flow. The spectral resolution in HeI and HeII spectra was 25 and 70 meV, respectively, when measured as FWHM of the 3p12P3/2Ar+ Ar (1S0) line. The resolution in the HeII spectra was always inferior to HeI, which implies that certain bands that are well resolved in HeI may become unresolved in the corresponding HeII spectrum. Band intensities of chlorine, bromine and iodine lone pair ionizations depend strongly on photon energy and decrease significantly on going from HeI to HeII radiation. Such changes are caused by variations of atomic photoionization cross-sections with electron energy and are well established through theoretical [5] and UPS [6] studies. These known intensity variations are an important assignment aid and they helped us to readily identify which bands correspond to halogen lone pair ionizations as described in Section 3. The quantum chemical calculations were performed with Gaussian 03 program [7]. The full geometry optimization was performed using DFT method at B3LYP/6311G** level. Subsequently, single point Green’s functions (OVGF) type calculation implemented in the Gaussian program was performed in order to obtain ionization energies, using 6-311G(d,p) basis set for all atoms except for I for which a relativistic basis set was used [8]. GF calculations with the same basis sets as in our work were used previously to successfully assign the spectra of monohalobenzenes [9]. Calculated GF energies (whose pole strengths are >0.85 and thus indicative of the adequacy of single electron picture of ionization) are listed together with calculated ionization energies in Table 1. 3. Results and discussion The investigation of substituent effects is our primary aim, therefore we first describe the analysis of photoelectron spectra.

3.1. Spectral assignment The HeI/HeII photoelectron spectra of 1–8 are shown in Figs. 1–4 and their assignments given in Table 1. In order to obtain reliable assignments we considered, for each spectrum, relative band intensities, band profiles, the assigned spectra of corresponding bromo, chloro or iodobenzene analogues, HeI/HeII dependence of band intensities and the results of GF calculations. The band profile provides an insight into the nature of the orbital from which ionization takes place; sharp, narrow band often corresponds to the ionization from strongly localized, nonbonding orbital as suggested by the Franck–Condon principle. Furthermore, halogen lone pair bands show a particularly pronounced intensity changes with photon energy. However, not all halogen lone pair bands display intensity changes of the same magnitude. This is due to different amounts of halogen character in such orbitals and to different atomic photoionization cross-sections for different halogens [5]. Also, halogen lone pair bands sometimes overlap with other p and r orbital ionizations which may mask the expected intensity changes. The correlation with reliably assigned spectra of corresponding mono and dihalobenzenes is thus necessary. We shall briefly describe salient points in the assignment of individual spectra. The spectra of 1–2 were compared with the spectra of corresponding trichlorobenzenes [10] and triiodobenzenes [11]. Only the HeI spectrum of 1,3,5-tribromobenzene had been reported, but without the band assignment [12]. In the spectrum of 1 the bands with maxima at 8.92 and 9.54 eV correspond to ring p-ionizations (p3, p2) while the bands with maxima at 10.52, 10.77, 11.0, 11.3, 11.65 and 12.3 eV correspond to six bromine lone pair ionizations. Three of them lie in the plane and three out of the molecular plane. This analysis is in agreement with corresponding trichloro and triiodobenzenes. The spectrum of 2 can be assigned in the analogous way. The assignment shows that the band at 9.22 eV corresponds to ionization from doubly degenerate e00 p-orbital

I. Novak, B. Kovacˇ / Chemical Physics 326 (2006) 471–477 Table 1 Vertical ionization energies (EI ± 0.05 eV), empirically derived band assignments (MO), calculated ionization energies (GF/eV), pole strengthsa Compound (symmetry)

Band

EI

MO type

GF (pole strength)

1 (Cs)

X A B C D E F G

8.92 9.54 10.52 10.77 11.00 11.30 11.65 12.3

p3 (a00 ) p2 (a00 ) nBr (a 0 ) nBr (a00 ) nBr (a 0 ) nBr (a 0 ) nBr (a00 ) nBr (a00 )

8.90 9.49 10.36 10.76 10.84 11.35 11.52 12.34

2 (D3h)

X A B C D E

9.22 10.64 10.82 11.05 11.2 12.05

p3,2 (e3/2 + e1/2) nBr (e5/2) nBr (e1/2) nBr (e3/2) nBr (e5/2) nBr (e3/2 + e1/2 )

9.24 (0.91) 10.59 (0.90) 10.76 (0.91) 11.0 (0.91) 11.0 (0.90) 12.11 (0.90)

3 (Cs)

X A B C D E F G

9.10 9.60 10.63 11.15 11.37 11.7 11.9 12.61

p3 (a00 ) p2 (a00 ) nBr (a 0 ) nBr (a00 ) nBr (a 0 ) nBr (a00 ) nCl (a 0 ) nCl (a00 )

9.32 9.72 10.31 11.09 11.24 11.47 11.74 12.91

(0.89) (0.88) (0.90) (0.88) (0.90) (0.89) (0.89) (0.88)

4 (C2v)

X–A

9.40

B C D E F–G

10.65 11.00 11.27 11.93 12.2

p3 (b1), p2 (a2) nBr (b2) nBr (b1) nBr (a1) nBr (a2) nCl (b2), nCl (b1)

9.44 9.56 10.39 10.93 10.94 11.90 12.10 12.22

(0.88), (0.88) (0.90) (0.89) (0.90) (0.90) (0.88) (0.88)

5 (Cs)

X A B C D E F G

9.30 9.63 10.93 10.93 11.25 11.90 12.05 12.8

p3 (a00 ) p2 (a00 ) nBr (a 0 ) nBr (a00 ) nBr (a 0 ) nBr (a00 ) nCl (a 0 ) nCl (a00 )

9.34 9.73 10.68 10.90 10.91 11.79 11.82 12.77

(0.88) (0.88) (0.90) (0.88) (0.90) (0.89) (0.91) (0.87)

6 (Cs)

X A B C D E F G

9.23 9.68 10.93 11.28 11.78 12.0 12.13 13.05

p3 (a00 ) p2 (a00 ) nBr (a 0 ) nBr (a00 ) nCl (a 0 ) nCl (a00 ) nCl (a 0 ) nCl (a00 )

9.44 (0.88) 9.77 (0.88) 10.72(0.90) 11.26 (0.88) 11.68 (0.89) 11.86 (0.89) 11.92 (0.88) 13.18 (0.88)

7 (C2v)

X A B C D E F G

8.90 9.4 9.63 10.6 11.0 11.2 11.37 12.15

p3 (b1) p2 (a2) nI (b2) nI (b1) nBr (b1) nBr (a1) nBr (b2) nBr (a2)

8.91 9.02 9.50 10.34 10.97 11.01 11.11 12.20

(0.91) (0.93) (0.88) (0.91) (0.90) (0.88) (0.90) (0.88)

8 (Cs)

X A B C

8.8 9.55 9.85 10.45

p3 (a00 ) p2 (a00 ) nI (a 0 ) nI (a00 )

8.67 9.47 9.59 10.15

(0.92) (0.90) (0.94) (0.92)

(0.91) (0.90) (0.91) (0.90) (0.90) (0.90) (0.90) (0.90)

473

Table 1 (continued) Compound (symmetry)

Band

EI

MO type

GF (pole strength)

D E F G

10.9 11.5 11.75 12.3

nBr (a 0 ) nBr (a00 ) nCl (a 0 ) nCl (a00 )

10.90 11.54 11.83 12.25

(0.91) (0.90) (0.92) (0.91)

a Adiabatic energies only are given, they are the same as band maxima which in other cases correspond to vertical ionization energies.

Fig. 1. Photoelectron spectra of 1 and 2.

474

I. Novak, B. Kovacˇ / Chemical Physics 326 (2006) 471–477

Fig. 3. Photoelectron spectra of 5 and 6.

Fig. 2. Photoelectron spectra of 3 and 4.

while bands at 10.64, 10.82, 11.05, 11.2 and 12.05 correspond to ionizations from six bromine lone pairs. The six bromine lone pairs in 2 transform as a002 þ a02 þ e0 þ e00 and one would thus expect four bands in the spectrum with intensity ratio: 1:1:2:2 as was observed in 1,3,5-trichlorobenzene [10b]. However, in 2 five bands with ratio: 1:1:1:1:2 were observed. Furthermore, in the spectrum of 1,3,5-triiodobenzene the intensity ratio is 1:2:1:2 [11]. In order to rationalize the observed band intensities we recall that bromine and iodine atoms exhibit strong spin–orbit coupling (SOC). The final energy level pattern can be thought of as the result of competition between SOC and

heavy halogen lone pair–p-system conjugation. These effects have been described in the spectra of halobenzenes, alkyl halides and haloethynes [9,13,14]. We shall use double group notation which is appropriate for such cases. Scheme 2 shows observed energy level patterns in trihalobenzenes. SOC in 1,3,5-trichlorobenzene is weak so the symmetries of ionic states can be expressed in point group notation. Nonetheless, corresponding double group classification is also given (in brackets) for the sake of convenience. In 1,3,5,-tribromo and 1,3,5-triiodobenzene the SOC is strong and cannot be neglected. This allows strong mixing/interaction between halogen lone pairs. Scheme 2 shows how in 2 the interactions between bromine lone pair e3/2 and e5/2 spin orbitals leads to the lifting of degeneracy of the

I. Novak, B. Kovacˇ / Chemical Physics 326 (2006) 471–477

475

Cl

Cl

2E"

2A " 2

2A ' 2

I

Br

Cl (E3/2+E1/2)

(E5/2) (E1/2)

Br

Br E3/2+E1/2

E5/2

E1/2

I

I E3/2+E1/2

E5/2

E3/2+E1/2

E3/2 2E'

2E"

(E5/2+E3/2)

(E3/2+E1/2)

E5/2

E5/2

E3/2+E1/2

E3/2+E1/2

Scheme 2. Closely spaced ionic states correspond to unresolved bands.

innermost benzene p1 orbital. The point to note is that although all Cl, Br, and I lone pair bands lose intensity, the decrease is not uniform due to different interactions of lone pairs with p-system and different photoionization cross-sections for different halogens. This effect is well established from the spectra of halobenzenes which contain different halogens [15,16].

3.2. Substituent effects

Fig. 4. Photoelectron spectra of 7 and8.

parent e 0 orbital with concomitant intensity ratio 1:1:1:1:2. In 1,3,5-triiodobenzene the interaction between iodine lone pair e3/2 spin orbitals is even stronger leading to the 1:2:1:2 intensity ratio. The assignment of the photoelectron spectra of 3–8 can be readily obtained on the basis of GF calculations and changes in HeI/HeII band intensities and will not be discussed in detail. The bands corresponding to chlorine, bromine and iodine lone pair ionizations show pronounced decrease in relative intensity on going from HeI to HeII radiation. This is true of all except the lowest band (with highest ionization energy) which mixes strongly with the

The main trust of this work is the analysis of substituent effects in a complicated case where three different types of halogens are present. Halogen atoms in halobenzenes can be considered as r-acceptors and p-donors. The relative donating and accepting ability is different for different halogens. Back-of-the-envelope calculation of p-donating ability of halogens can be made from Hammett substituent parameters by using expression rp–rm. Using the tabulated values of rp and rm [2,17] we obtain 0.20, 0.15, 0.14 and 0.19 for F, Cl, Br and I substituent, respectively. The result is of only qualitative value indicating that all halogens have p-donating abilities of comparable magnitude. In order to understand substituent effects better we introduce two descriptors which are based on measured orbital ionization energies: dhpi = hpip  9.25; hpip = 0.5(p3 + p2); dhnXi = hnXip  hnXim. The descriptors correspond to the shift of average p-energy in polyhalobenzene hpip vs. benzene (whose average p-energy is 9.25 eV) and to the shift of average halogen lone pair

I. Novak, B. Kovacˇ / Chemical Physics 326 (2006) 471–477

476

F

F

F d d

F 0.26 -

0.0 0.0

Cl

Cl

Cl

Cl d d

F

F

I -0.15 0.0

0.06 0.0 Cl

F 0.50 -

0.45 Br

I

I

Br

Br

0.17 0.41

Br -0.01 0.16

0.16 0.30

I

I

Br 0.06 0.26

0.1 0.22

I

Br Cl

Cl d d

0.12 0.55

0.07 0.61 Cl

F

Br

Cl

I Br

Br

-0.02 0.33

-0.03 0.38

F Br

Br

Br

Br

Cl Br

Br Cl

0.15 0.29 0.71

I

I

Cl

I

F

Br

-0.45 0.14

-0.38 0.17

Br

Br

F

F d 0.10 d 0.29 d 0.77 d

I -0.09 0.04 I

I Br

Br

Cl

Cl

I -0.33 0.16

-0.22 -0.01

Br

Cl Cl

0.50 -

Br

Cl 0.11 0.28

Br

Cl 0.11 0.0

Cl I

0.12 0.33 0.93

0.21 0.19 0.75

-0.10 0.51 -0.04

-0.08 0.28 0.54 0.0

Fig. 5. Descriptor values for polyhalobenzenes 1–8.

energy in polyhalobenzene hnXip vs. appropriate average lone pair energy in monohalobenzene hnXim. The descriptors are given in Fig. 5. We have not calculated dhnXi values for fluorine substituents because we could not identify accurately fluorine lone pairs in the spectra of 1–8. Fluorine lone pair bands appear at high ionization energies, overlap strongly and their orbitals mix with various rorbitals. In the previous work [16], we used shifts and splittings of individual p and lone pair levels as the basis for defining descriptors. However, in polyhalobenzenes this is not practical for two reasons. Firstly, it is not easy to distinguish experimentally between in-plane and out-of-plane lone pairs (due to band overlap and orbital mixing). Secondly, the p-splitting depends on the ring position of the halogen i.e. p3 and p2 orbitals are affected differently by the same substituent [16]. This leaves only descriptors based on average ionization energies even though such descriptors contain contributions from both inductive (F-effect) and resonance effects (R-effect) i.e. they do not separate the two effects. Nonetheless, we know [16] that the R- and Feffects operate antagonistically on the p3 and p2 orbitals (R-effect destabilizes and F-effect stabilizes p3 and p2 orbi-

tals). The operation of R- and F-effects on the halogen lone pair level is usually synergistic i.e. they are both stabilizing [16]. The interpretation of substituent effects can be achieved by the following guidelines. Small dhpi value suggests that R- and F-effects are of comparable magnitude. Negative or positive value indicates that R-effect or F-effect predominates, respectively. dhnXi values are usually large and positive which can be expected from the synergistic influence of R- and F-effects. The iodine lone pairs in 7 and 8 are exceptions with small or negative values dhnIi values (Fig. 5). In this case R-effect enhances mixing of iodine with other halogens i.e. the out-of-plane iodine lone pair interacts strongly with out-of-plane lone pairs of chlorine or bromine atoms via p-system relay. Were it not the case one could expect to find large positive dhnIi values due to the synergistic stabilizing action of p-orbitals and F-effect of other halogens. This demonstrates the efficiency of psystem as an electron relay for interhalogen interactions. We shall illustrate the substituent effects by comparing descriptor values of fluorobenzenes [18], chlorobenzenes [10], bromobenzenes [19, this work] and iodobenzenes [20,11] (Fig. 5). In monohalobenezenes a clear trend from predominantly F-effect in fluorobenzene (large positive

I. Novak, B. Kovacˇ / Chemical Physics 326 (2006) 471–477

dhpi) to the predominantly R-effect (large negative dhpi) in iodobenzene can be seen. Bromine substituent exhibits both effects to an approximately equal degree. The effects in 1–8 are more complicated and require comparison with analogous dihalobenzenes. In 1 and 2 the small dhpi values indicate antagonistic mode of action of R- and F-effects on p-levels. Large positive dhnXi values reflect synergistic effects on bromine lone pairs as mentioned above. There is little variation in descriptor values between isomers 1 and 2 even though it is known that p3 and p2 orbital energies are sensitive to the substituent position [16]. Why? This is due to the presence of three halogens which obscure traditional differences between ortho, meta and para substitution by modifying localization properties of p3 and p2 orbitals. Descriptor values in isomers 3–6 exhibit little variation for the same reason. On the basis of large positive descriptor values we can conclude that substituent effects in 3–6 are of predominantly inductive nature. In 7–8 the R-effect is of importance comparable to the F-effect as indicated by dhpi values. We have already explained the appearance of small or negative dhnIi values in 7–8. As further evidence of the operation of p-relay we can point out to the dhnCli value in 8. This value is larger than in dichlorobenzenes (Fig. 5) even though 8 contains halogens which are less electronegative than chlorine so that F-effect could be expected to be less pronounced. The same argument applies to dhnBri value in 8 which is larger than corresponding values in dibromobenzenes. 4. Summary This work described complex substituent effects using only experimental ionization energies and descriptors calculated from the same. Such analysis would be difficult to perform by using methods of computational quantum chemistry alone. Our approach can be extended to other cases where the presence of several kinds of substituents leads to complex interactions. However, using photoelectron spectroscopy for analysis is impractical if substituents are large and contain numerous localized orbitals. This may lead to band overlap and the loss of accuracy with which ionization energies can be determined. Studies of benzenes with complex substitution patterns are rare and our work attempted to address this problem.

477

References [1] R.W. Taft, R.D. Topsom, Prog. Phys. Org. Chem. 16 (1987) 1. [2] A. Williams, Free Energy Relationships in Organic and Bio-organic Chemistry, Royal Society of Chemistry, Cambridge, 2003. [3] (a) O. Exner, S. Bo¨hm, J. Phys. Org. Chem. 19 (2006) 1; (b) O. Exner, S. Bo¨hm, Chem. Eur. J. 9 (2003) 4718; (c) S. Bo¨hm, O. Exner, J. Mol. Struct. Theochem. 682 (2004) 171. [4] L.R. Domingo, P. Perez, R. Contreras, J. Org. Chem. 68 (2003) 6060. [5] J.J. Yeh, Atomic Calculation of Photoionization Cross-sections and Asymmetry Parameters, Gordon and Breach, Langhorne, 1993. [6] J.K. Padden Metzker, N.E. Gruhn, D.L. Lichtenberger, J. Phys. Chem. A 106 (2002) 9999. [7] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, V.G. Zakrzewski, J.A. Montgomery Jr., R.E. Stratmann, J.C. Burant, S. Dapprich, J.M. Millam, A.D. Daniels, K.N. Kudin, M.C. Strain, O. Farkas, J. Tomasi, V. Barone, M. Cossi, R. Cammi, B. Mennucci, C. Pomelli, C. Adamo, S. Clifford, J. Ochterski, G.A. Petersson, P.Y. Ayala, Q. Cui, K. Morokuma, D.K. Malick, A.D. Rabuck, K. Raghavachari, J.B. Foresman, J. Cioslowski, J.V. Ortiz, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. Gomperts, R.L. Martin, D.J. Fox, T. Keith, M.A. AlLaham, C.Y. Peng, A. Nanayakkara, C. Gonzalez, M. Challacombe, P.M.W. Gill, B. Johnson, W. Chen, M.W. Wong, J.L. Andres, C. Gonzalez, M. Head-Gordon, E.S. Replogle, J.A. Pople, Gaussian 03, Revision B05, Gaussian Inc., Pittsburgh, PA, 2003. [8] M. Dolg, Mol. Phys. 88 (1996) 1645. [9] K. Imura, N. Kishimoto, K. Ohno, J. Phys. Chem. A 105 (2001) 4189. [10] (a) B. Rusˇcˇic´, L. Klasinc, A. Wolf, J.V. Knop, J. Phys. Chem. 85 (1981) 1486; (b) B. Rusˇcˇic´, L. Klasinc, A. Wolf, J.V. Knop, J. Phys. Chem. 85 (1981) 1495. [11] I. Novak, D.B. Li, B. Kovacˇ, J. Phys. Chem. A 106 (2002) 2850. [12] K. Kimura, S. Katsumata, Y. Achiba, T. Yamazaki, S. Iwata, Handbook of HeI Photoelectron Spectra of Fundamental Organic Molecules, Japan Scientific Societies Press, Tokyo, 1981. [13] (a) E. Heilbronner, V. Hornung, E. Kloster-Jensen, Helv. Chim. Acta 53 (1970) 331; (b) H.J. Haink, E. Heilbronner, V. Hornung, E. Kloster-Jensen, Helv. Chim. Acta 53 (1970) 1073. [14] F. Brogli, E. Heilbronner, Helv. Chim. Acta 54 (1971) 1423. [15] I. Novak, A.W. Potts, J. Phys. Chem. A 102 (1998) 3532. [16] (a) I. Novak, B. Kovacˇ, J. Phys. Chem. A 109 (2005) 3318; (b) I. Novak, B. Kovacˇ, Chem. Phys. 321 (2006) 119. [17] O. Pytela, Coll. Czech. Chem. Commun. 61 (1996) 1191. ˚ sbrink, W. von Niessen, J. Electron. Spectrosc. Relat. [18] G. Bieri, L. A Phenom. 23 (1981) 281. [19] T. Cvitasˇ, L. Klasinc, Croat. Chem. Acta 50 (1977) 291. [20] T. Cvitasˇ, H. Gu¨sten, L. Klasinc, J. Chem. Soc., Perkin Trans. 2 (1977) 962.