ROBERT E. CARTER AND JAN SANDSTROM
642
Barriers to Rotation around the Amide Bond and the Central Carbon-Carbon Bond in Tetrabenzyloxamide and Its Monothio and Dithio Analogs by Robert E. Carter* and Jan Sandstrom Divisions of Organic Chemistry 2 and 1, Chemical Center, 5-220 07, Lund 7 , Sweden
(Received August 12, 1971)
Publication costs assisted by the Swedish Natural Sciences Research Council
By a study of the temperature-dependent nmr spectra of the title compounds, the barrier t o rotation around the central carbon-carbon bond has been found to increase in the order oxamide (9.7 kcal/mol), thiooxamide (17.6 kcal/mol), and dithiooxamide (>24 kcal/mol). The barriers are predominantly steric in origin, and they result from repulsions between the oxygen or sulphur atom in one half of the molecule and the benzylic methylene group in the E position in the other. The barrier to rotation of the dibenzylamino group in the oxamide is similar to the analogous barrier in dimethylformamide,and the corresponding barriers in the thioamide parts are at least as high as in dimethylthioformamide, indicating that the molecule is free from strain in the initial state. The equilibrium conformationscalculated by a nonbonded 6-12 potential function are in good agreement with dipole moment data. The INDOR technique proved very valuable in unraveling the four overlapping AB quartets in the nmr spectrum of the monothiooxamide.
An earlier investigation of the ultraviolet spectra of mono- and dithiooxamide and their N-methyl derivativesl showed that in the N,K-dimethyl derivatives the planar trans configuration is energetically unfavorable and that the steric strain is relieved by rotation mainly around the central carbon-carbon bond (the pivot bond). From the shifts of the uv maxima on N-methylation, it was possible to calculate approximate angles of twist (e), and it was found that an -N(CH&. .O= interaction gave a smaller twist than an -N(CH&. .S= interaction, and that two --N(CH&. -S= interactions caused a larger twist than one, as exemplified by the compounds Ia-d. Such twisted molecules are chiral, and provided the H,
Ia, 8 = 50'
Ib, 8 = 58' CH,
'CH,
IC, 8 = 66'
'CH, Id, 8 = 89'
barriers to rotation around the pivot bond fall in the appropriate range, they can be estimated by a variable temperature study of the nmr spectra of suitable prochiral groups attached to the molecule. Siddall and Good2 studied tetraisopropyloxamide and observed a rotation which was slow on the nmr time scale below - 15". We have chosen to study tetrabenzyloxamide and its mono- and dithio analogs (IIa-c) in the hope of The Journal of Physical Chemistry, Vol. 76, N o . 6,1978
obtaining a quantitative estimate of the difference in steric effect between oxygen and sulfur. For comparison, the temperature dependence of the nmr spectra of tetramethyloxamide and its dithio analog (Id) were also studied.
-
IIa, X Y = 0 IIb, X = Y -S IIC, x = 0, Y = s
Experimental Section Tetrabeneyloxamide (Ira) was prepared according to Armbrecht, et al.,3 from dibenzylamine and oxalyl chloride in benzene-triethylamine. It crystallized from 1-butanol as colorless rod-shaped prisms, mp 132-132.5" (lit. 130-13 1" ) . Tetrabeneylmonothiooxamide (IIc). Tetrabenzyloxamide (10 g) was reacted with phosphorus pentasulfide (10 g) in refluxing dry toluene (200 ml) for SO hr with mechanical stirring. The solution was filtered hot, the residue extracted with hot toluene, and the combined toluene solutions were evaporated. Extraction with chloroform left an undissolved residue of phosphorus pentasulfide, and addition of light petroleum (bp 40-60") to the chloroform solution precipitated a pale yellow crystalline product (8.5 g ) , mp S28-129', consisting of a mixture of the mono- and dithioamides with unreacted starting material according to nmr anal(1) B. Persson and J. Sandstrom, Acta Chem. Scand., 18, 1059 (1964). (2) T. H. Siddall, 111,and M. L. Good, Bull. Soc. Chem. Jap., 39, 1619 (1966). (3) B. H. Armbrecht, L. M. Rise, C. H. Grogan, and E. Emmett Reid, J. Amer. Chem. SOC.,75, 4829 (1953).
TETRABENZYLOXAMIDE AND ITS MONOTHIO AND DITHIO ANALOGS ysis. Due to the very similar solubility properties of these compouinds, fractional crystallization proved to be inefficient for separation, but on chromatography of the crude product (2.5 g) on alumina, benzene eluted the dithiooxamide (0.20 g), and benzene containing 5% diethyl ether, the monothiooxamide (1.61 g), which was obtained as pale yellow prisms, mp 134-136", after recrystallization from toluene. (Calcd for CaoHzsNzOS (464.60): C, 77.6; H, 6.07; N, 6.03; S, 6.90. Found: C, 78.1; H, 6.02; N, 6.02; S, 7.03.) Tetrabenxyldithiooxamide ( I l b ) was most conveniently prepared by reacting tetrabenzyloxamide (5 g) with phosphorus pentasulfide (6 g) in refluxing dry xylene (100 rnl) for 26 hr with mechanical stirring. The solution was filtered hot, and on cooling the product separated (4.6 g, 84% yield) as pale yellow, flattened rods, mp 177-178" after recrystallization from toluene. (Calcd for C30Hz8NzSz (480.67): C, 75.0; H, 5.87; N, 5.83; S, 13.3. Found: C, 74.3; H, 5.89; N, 5.81; S, 13.4.) Tetramethyldithiooxamide was prepared according to Klopping and van der Kerk.4 Nmr spectra were recorded on Varian A60, A60-A, HA-100, and/or XL-100 instruments, equipped with variable temperature probes and V-6040 temperature controllers. I[NDOR experiments5 were performed on the HA-100 instrument operating in the frequency sweep mode and locked on the signal from internal TMS or hexamethyldisiloxane. The irradiation radiofrequency field was generated by modulation of the magnetic field using a Hewlett-Packard 200 C D Audio Oscillator. Temperature measurements were made by the replacement technique, using the Varian methanol or ethylenglycol samples, calibrated with the aid of a thermocouple as previously describeda6 Temperatures measured in this way are assumed to be accurate to within f 2 " , which is considered acceptable for the present purposes in view of the difficulty in an exact determination of the coalescence point in spectra with several overlapping AB quartets. Free energies of activation were estimated by the approximate formulas appropriate for a coalescing doublet (1)' or for a coalescing AB quartet (2),* derived from the expressions given in references 7 and 8 in conjunction with the Eyring e q ~ a t i o n . The ~ accuracy of
+
= . 57TL (9.97C log
1000
AG*
kcal/mol
log
A
500
450
Hz
Figure 1. Nmr spectra of 11% in dichlorofluoromethane at 100 MHz, methylene protons.
Results and Discussion Tetrubenxyloxamide (Ila). The 100 MHz spectra of this compound were recorded on 0.3-0.5 molal dichlorofluoromethane and deuteriochloroform solutions. At low temperatures (-90" to - 100") in dichlorofluoromethane solution, the spectrum of the benzylic protons consisted of two overlapping AB quartets (Table I and Figure l),and the signals due to the aromatic protons were two well-resolved singlets. On increasing the temperature, first the downfield and then the upfield AB quartet coalesced (at -70" and at -63", respectively), and at probe temperature the benzylic protons gave rise to two singlets at 6 = 4.27 and 4.42 (ppm from TMS). It is worth noting that the AB coupling constant in the high-field quartet (at - 100") is about 1Ha greater than that in the low-field one. This may be a (4) H. L. Klopping and G. J. M. van der Kerk, Rec. Trau. Chim. Pap-Bas, 70, 917 (1951). (5) V. J. Kowalewski in Progr. Nucl. Magn. Res. Spectrosc., 5, 1
4.57Tc X 1000 +
A
(1)
= --
('"'
643
)
Tc .t/av'~~ + ~ J ' A kcal/mol B
(2)
the AG' determinations is governed mainly by the accuracy of the temperature measurements (vide supra). The estimated limits of error are given in Table I.
(1969). (6) G. Isaksson and J. Sandstrom, Acta Chem. Scand., 24, 2565 (1970). (7) H. S. Gutowsky and C. H. Holm, J . Chem. Phys., 2 5 , 1228 (1956). (8) R. J. Kurland, M. B. Rubin, and W. B. Wise, J. Chem. Phys., 40, 2426 (1964). (9) 8. Glystone, K. J. Laidler, and H. Eyring, "Theory of Rate Processes, McGraw-Hill, New York, N. Y., 1941, p 195 ff. The Journal of PhZlsieal Chemistry, Vol. 76, No. 6, 197B
644
ROBERTE. CARTER AND JAN SANDSTROM
Table I: Nmr Parameters (100 MHz) for Aromatic Protons and AB Spectra, and Barriers to Rotation around the Pivot Bond UCHZ
7
Compound
c -
vAr, HEa
vAb
IIac 725.6;749.6 IIbd 727.7 IIbe ... IIbf 718.5;722.7 IIcd (g 721.0;722.4 h 729.7;732.1
454.7 495.7 483.7 489.2 462.1 478.8
H 2 UB
JAB
405.4 441.7 437.9 446.4 436.2 425.3
15.5 15.2 15.3 15.1 15.8 15.3
-
--To,'IC
203
>473
... ...
7 -
VAb
9.7h0.2
... ...
521.6 546.4 518.6 514.7 469.3 568.8
...
... >24
vCHz
7
AUC* koal/mol
...
...
-E
7
HZVB
381.0 492.2 500.5 508.1 446.4 449.6
JAB
14.5 14.4 14.1 14.5 14.4 14.5
Tc,OK
AG~* kcal/mol
210
9.7&0.2
,..
...
...
e . .
...
...
,..
3
373
.
.
17.9f0.5
a No assignment to specific phenyl groups has been attempted. Arbitrarily assigned to the low-field resonances. 0 I n dichlorofluoromethane a t - 100". In deuteriochloroform a t probe temperature. a In o-dichlorobenzene a t probe temperature. 1 In hexamethylphosphoric triamide a t +60°. 8 Amide part. Thioamide part.
reflection of different spatial orientations of the benzylic protons with respect to adjacent ?r bonds.lo The aromatic protons gave rise to two broad singlets at low Betemperatures (Av1lt = 6-7 Hz at ca. -100'). tween -80" and -60" the upfield signal was resolved into a broad, unsymmetrical "triplet," (Figure 2) while the downfield signal remained a singlet with AvIl2 of the order of 1.5 H Z between -50" and probe temperature. It is quite plausible that the differences in appearance of the two aromatic signals are due to different dispositions of the aryl rings with respect to the anisotropic carbonyl groups. I n deuteriochloroform solution at probe temperature, the singlets from the benzylic protons were at 6 = 4.38 and 4.51, and those due to the aromatic protons were a t 6 = 7.18 (Avl/, = 1.7 Hz) and 7.28 (Avl/%= 1.4 Hz). On increasing the temperature, the benzylic singlets broadened and then coalesced at 127" (spectra run at 100 MHz with the sample in a sealed tube). The difference in width of the aromatic proton singlets disappeared as the temperature was increased, and the signals coalesced at 121" (spectra run at 100 RiIHz). A reasonable interpretation of the appearance of the AB type spectra at low temperatures is hindered rota0 0
I/ I/
tion around the -C-C(pivot) bond. From the observed coalescence temperatures a value of 9.7 kcal/mol for AG* may be estimated by the use of eq 2. The coalescence of the benzylic and aromatic singlet pairs a t higher temperatures is of course due to rapid rotation bonds. Use of on the nmr time scale around the -C-N eq 1 allows the estimation of a value of 21 kcal/mol for AG* in this case. Solvent effects seem to be of no great consequence for the amide rotation in IIa, since the same AG* value (within the experimental error) was obtained in both hexamethylphosphoric triamide (HMPT) and o-dichlorobenzene (ODC) solution. Tetrabenxyldithiooxamide (IIb). I n deuteriochloroform solution at probe temperature (100 MHz), the benzylic protons gave rise to two well-resolved AB The Journal of Physical Chemistry, Vol. 78, NO.6 , 1978
-40'
740
730
720
H1
Figure 2. Aromatic protons of I I a in dichlorofluoromethane a t 100 MHz.
quartets (see Table I). As noted above in the case of IIa, the AB coupling constant in the high-field quartet was about 1 Hz greater than that in the low-field one. Furthermore, the line width of the downfield pair of lines in each quartet in the spectrum of IIb was greater than that of the upfield pair. This is most plausibly explained as the result of stereospecific long-range coupling between the benzylic and aromatic protons. The resonance due to the aromatic protons was a broad band consisting of at least three poorly resolved peaks. Spectra of this compound were recorded at various temperatures in HlLlPT and ODC solutions. I n both of these solvents, the internal shift of the low-field benzylic AB quartet had been considerably diminished (see Table I). As the temperature was increased, all of the resonances due to the benzylic protons slowly broadened, but even at 188", the highest temperature reached in ODC solution, the AB structure was still discernible. I n HMPT solution, at about 200" (60 M H z ) ~ a broad structureless absorption was observed. In this case, it is unfortunately impossible to distinguish between the effects of thioamide rotation and rotation around the pivot bond on the line shape. A lower limit of 24 kcal/mol for AG* for both rotations may be roughly estimated. Tetrabenxylmonothiooxamide ( I l c ). A 100-MHz spectrum in deuteriochloroform solution at probe tempera(10) M. Barfield and D. M. Grant, J. Amer. Chern. SOC.,8 5 , 1899 (1963); see S. Sternhell, Quart. Rev., Chem. Soe., 23, 236 (1969).
TETRABENZYLOXAMIDE AND ITS n ~ O N O T H I 0AND DITHIO ANALOGS
I 2 4
3
J 3 2
I
I
I 1
2
3
3
4
2
ture contained thirteen peaks between 6 = 4.1 and 5.8 ppm due to the bcnzylic protons (Figure 3), arising from four partially overlapping AB quartets (Tablc I), and four pealis due to the aromatic protons bctwecn 6 = 7.1 and 7.4 ppm. Thc four lines of each AB quartet were disclosed with the aid of the IWDOR technique;5 the resulting assignments are indicated in Figure 3 and Table I. It should be pointed out that the chemical shifts of the benzylic protons of IIa and IIc are highly temperature dependent. As the temperature was increased, a gradual broadening of all signals was observed. Due to considerable overlap of the peaks from the various AB spectra at higher temperatures, the separate coalescence of each AB system was not discernible. However, since the line shape of the lowfield half of the quartet denoted 1 (assigned to the thioamide half of the molecule; vide i n f r a ) could be observed undisturbed by the other benzylic signals, a coalescence temperature of about 100" could be estimated for this quartet. All four lines of the quartet denoted 2 (also assigned to the thioamide half of the molecule) were observable up to about 70". Between this temperature and ca. 95", the 2 quartet was transformed into a broad shoulder on the high-field side of the relatively sharp singlet arising from the coalescence of the 3 and 4 quartets (assigned t o the amide half of the molecule). It is obviouq that the line shapes of the 3 and 4 quartets must be influenced by both the rotation around the pivot bond and that around the acyl-nitrogen bond in the amide group. A coupling of these two rotations is an attractive possibility, since a rotation around the amide bond alleviates the CH2.. .S interaction (vide i n f r a ) which is responsible for the twist around the pivot bond. At ca. 130", the highest temperature reached in deuteriochloroform solution (sealed tube) , the benzylic proton resonances consisted of a strongly braodened doublet arising from the collapsed 1 and 2 quartets and, overlapping the "2-component" of the doublet, a relatively sharp singlet from the collapsed 3 and 4 quartets. The same general appearance of the spectrum wap observed at higher temperatures in HR'IPT solution, but even at 200", the 1-2 doublet had not coalesced. This allows the estimation of a rough value of 24 kcal/mol as a lower limit for the barrier to acyl-nitrogen rotation in the thioamide group. A
645
tentative aysignmcrit of the quartets 1-4 to the four methylene groups in thc monothiooxamidc may be made on the basis of the following obscrvations. (a) In the 100-AIHz spcctrum of the oxamide in dichlorofluoromcthanc and in that of the monot hiooxamidc in dcutcriochloroform solution, the protons of one of the bcnzylic methylenes show AB shifts of 140.6 Hz and 119.2 Ha, rc>spectivdy,v hcreas the AB shifts of the othcr bcnzylic protons in thew molccules fall in thc region 20-55 Hz. The large shifts may be attributed to thc proximity of a methylme group in one half of the molecule to a carbonyl group in the other half. An indication that this interpretation is correct may be obtained from the fact that both of the AB shifts in the dithiooxamide spectrum are only 54 HZ (at 100 IIHa in deuteriochloroform solution; see Tablc 1). (b) Of the four AB coupling constants in the monothiooxamidc spectrum, t x o fall in the region 14.114.5 Hz and two in the region 15.3-15.S Hz. As noted above, there is a similar difference betn cen the coupling constants in the oxamide (IIa) and dithiooxamide (IIb) spectra. With the aid of a molecular model it becomes clear that the methylene group with the large shift is most likely the E group'l in the thioamidc half of the molecule (1). From the behavior of the spctrum with increasing tempcraturc, it can be concluded that the signals of quartet 2 (in compound IIc below) 2(2) PhCH,
PhCHz 1( E )
XE)
\
S
/I
N-c-c--N
'
! I
PHZPh 'CBIPh
4V) IIc
are attributable to the thioamide Z-methylene group. The smaller of the two "thioamide" AB coupling constants is thus associated with the E-methylene group, and the larger with the Z group. It s e e m rcasonable to assume that the same relation holds in the amide half of the molecule. Accordingly, the signals of quartet 3 are assigned to the amide Z-methylene group, and those of 4 to the E group. Also for the oxamide and dithiooxamide, the AB quartet with the large coupling constant is assigned to the Z methylene group, and that with the small coupling constant to the E group. A m i d e Rotations and Equilibrium Conformations. The barrier to rotation of the dibenzylamino group in the oxamide I I a (Table 11) is close to the values given for the corresponding rotation in N,N-dimethylformamide (20.8-21.1 kcal/mol in carbon tetrachloride12). The analogous barriers in the dithiooxamide I I b and in the thioamide group of IIc are at least not signifi(11) J. E. Blackwood, C. L. Gladys, K . L. Loeninp, A. E. Petrarca, and J. E. Rush, J . Amer. Chem. SOC.,90, 609 (1968). (12) M . Rabinowite and A. Pines, J . Amer. Chem. Soc., 91, 1585 (1969).
The Journal of Physical Chemistry, Vol. 76, N o . 6 , 197'8
ROBERT E. CARTER AND JAN SANDSTROM
646
Ph /
Table I1 : Nmr Parameters (at 100 LMHZunless Otherwise
HICI
Noted) for Amide and Thioamide Rotations
\
FCHZP \
0
\\ \\
IIa
C1)ClI
HMPT HMPT HMPT
400 20.8 9.3 394 20.9 10.8 . , . 395 20.8 9.8 395 20.9 10.8 397 20.9 22.sc . . . >473 >24 47c . . . >473 >24 1.2 . . . 373 21.2
0I)C
10.2
011c HMPT IIb IIC Tetramethyloxainide Teti anielhyldithiooxamide a
PhC%-N,
14.9
Methyl and methylene protons
b
, ,.
431
Aromatic protons.
23.9 c
At
60 MISz.
cantly lower than in N,N-dimethylthioformamide (24 kcal/mol13) or N-methyl-N-benzylthioformamide (25.1 k~al/mol'~).This is to be expected, if the two halves of the molecules I1 are independent of each other in the equilibrium conformation. To clarify this point, we have calculal ed the conformational energy of the molecule as a function of the dihedral angle 8. Starting from the s-trans conformation (e = 0), we have computed the atom pair interactions that are affected by the rotation around the pivot bond, employing the technique described by Scheraga.I6 Molecular models indicate that the Isciizene rings are turned away from the opposite half of the molecule in all reasonable conformations (Figure 4), and consequently all repulsions involving these groups were neglected. An idealized geometry was assumed, with all bond angles equal to 120" and the bond lengths talien from the X-ray crystallographic studies of oxamidel'j and dithiooxamide, l7 though with the pivot bond assumed to be 1.48 A in all three systems. The atom-atom interactions were calculated by a Lennard-Jones potential (eq 3). The coefficients ell depend on the atoms involved and were
obtained using the Rater-Kirkwood equationls 1.5 efiaiaj ._ _ -- 362.32ataj .-
(4)
where ai is the polarizability of atom i (in A3) and N , is the "effective" number of outer shell electrons. N , was obtained by a graphic method devised by Scott and Scheraga.'* The coefficient for the r-12 term is obtained by minimizing the energy for a distance equal to the sum of the van der Waals radii of atoms i and j . The parameters employed are found in Table 111. The Journal of Physical Chemistry, Vol. 76, No. 6,1978
S
\
\ ,C-H PI;
Figure 4. The s-trans conformation of IIc, used for interaction calculations.
Table I11 : Parameters for Calculating Nonbonded Potentials; from Ref 15 unless Otjherwise Stated Atom or group
Nz
C
5.2
1.70
0.93
H
0.9
1.20
0.42
N
6.1
1.55
0.87
0
7.0
1.52 0.84
d,, 10-8 rmln, b
a,
ba
et3b6 Interaotion
c0 CS HN
HO HS
NN NO
S
3.90b
15.0" 1.80
CHz
7.0"
1.85'
1.77'
NS 00
os ss CHzCHi
koal/mol
A'% koal/mol
367 1409 125
205 1295 27
125 498 363
25 181 161
365 1385 367
153 979 145
1387 5405 1129
928 5883 1448
Calculated according to ref 18. b Average polarizability, 2 ai),calculated from the value for carbon disulfide (K. G. Denbigh, Trans. Faraday Soc., 36, 936 (1940)). c From D.A. Brandt, W. G. Miller, and P. J. Flory, J. Mol. B i d , 23,47 a
(all f
(1967).
The conformations of the benzylic methylene groups were chosen so that both hydrogen atoms had the same distance to the opposite oxygen or sulfur atom, and the individual H. .O(S) and C . .O(S) interactions were computed. They were found to vary in a parallel manner throughout the entire range of dihedral angles, and they were the only repulsive interactions in the e 5 100". The methylene groups were range 0 treated as united particles ("extended atoms'' according to Scheraga'j) in the CH2.. .CH2 interaction, which e
(3)
/".-z i"-f
/N1-cH2Ph
9
(13) A. Loewenstein, A. Melera, P. Rigny, and W. Walter, J . Phys. Chem., 68, 1597 (1964). (14) W. Walter, G. Maerten, and H. Rose, Justus Liebigs Ann. Chem., 691, 25 (1966). (15) H. A. Scheraga, Advan. Phys. Org. Chem., 6 , 103 (1968) (16) C. Romers, Acta Crystallogr. 6, 429 (1953). (17) P. J. Wheatley, J . Chem. Soc., 396 (1965). (18) R. A. Scott and H.A. Scheraga, J . Chem. Phys., 42,2209 (1965). I
TETRABENZYLOXAMIDE AND ITS MONOTHIO AND DITHIO ANALOGS
Figure 5 . Nonbonded interactions in compounds I1 as function of the dihedral angle 0.
was repulsive beginning at e = 100" with an energy that increased rapidly with increasing dihedral angle. Since our aim has not been to calculate the barrier to rotation around the pivot bond, we have omitted all considerations of bond bending and compression, as well as electrostatic interactions and a-electron interaction across the pivot bond. consequently, the calculated differences in strain energy between the equilibrium conformations and the transition states for rotation around the pivot bond are more or less unrealistic. 1 t is evident, however, that the transition state must have the s-trans conformation, since the
647
corresponding barriers are 12.3 kcal/mol for IIa, 116.1 kcal/mol for IIb, and 64,2kcal/mol for IIc, whereas the energies for the s-cis conformations are extraordinarily high. It also appears that the two halves of the molecule are independent of each other in the range 50' 5 e 6 110", and this conclusion is not affected by the approximations made in the calculations. Therefore, we concluded that the initial state of the amide (thioamide) rotation is free from steric strain in the equilibrium conformation. This must also apply to tetramethyldithiooxamide, and is reflected in the good agreement between the thioamide barriers in this compound (Table 11) and in dimethylthioformamide. As mentioned above, only a lower limit to the thioamide barriers in IIb and I I c could be determined. If these barriers are indeed higher than that in dimethylthioformamide, this could most reasonably be ascribed to steric strain in the transition state. The similarity of the amide barriers in IIa, tetramethyloxamide (Ti+ ble II), and dimethylformamide indicates that this effect must be small. It is worth noting (Figure 5) that the equilibrium values for the dihedral angle obtained for the oxamide (e = SO") and dithiooxamide (e = 77") systems agree well with the values found by L u m b r o ~ oby ' ~ an analysis of the dipole moments of tetramethyloxamide (0 = 85") and tetramethyldithiooxamide (e = 82').
Acknotdeclgmenls. We wish to thank Miss Eva Ericsson for able experimental assistance. This work was supported in part by the Swedish Natural Sciences Research Council. (19) H. Lumbroso in a paper presented at the Fourth Symposium on Organic Sulphur Chemistry in Venice, 1970.
The Journal of Physical Chemistry, Vol. 7'6, KO.6,197g
