1400
D. J. Nelson
The Journal of Physical Chemistry, Vol. 82, No. 72, 1978
CIDNP in Pulse Radiolysis of Aqueous Dimethyl Sulfoxideli2 D. J. Nelson Chemistry Division, Argonne National Laboratory, Argonne, Illinois 60439 (Received December 5, 1977) Publication costs assisted by Argonne National Laboratory
Variable field CIDNP in pulse radiolysis of aqueous dimethyl sulfoxide is described. The products of radiolytically produced radicals are identified, and the radical pair model of CIDNP is applied in analysis of the observed polarization. Various mechanisms of radical formation and reaction are considered.
Introduction Although dimethyl sulfoxide (MezSO) is a small molecule, its radiation chemistry comprises a multiplicity of reaction pathways and radical intermediates. In E P R studies of y-irradiatedneat MezSO (and MepSO-d6) in the solid state the methyl radical (or an anion-associated C D 3 radical) and the sulfur-centered CH3S0 and (CH3),SO+. radicals have been d e t e ~ t e d . ~Irradiation -~ of aqueous MezSO yields the C H z S ( 0 ) C H 3radical as wella3 Optical studies of the pulse radiolysis of neat MezSO showed the presence of the solvated electron and several other transients which were not identified.'+l0 Similar studies of its aqueous solutions yielded like results.'@12 In the latter an oxidizing species showing absorption a t 600 nm was tentatively identified as (CH3),SO+.,and the presence of the solvated electron was established. MezSO reacts rather slowly with e!q- and H, but reacts very rapidly with aOH.9J1J3 In addition the postulated reaction with .OH is unusual in that it is not hydrogen abstraction but addition to sulfoxide followed by rapid decomposition of the adduct.I4 Reaction of MezSO with .OH generated by the titanium(III)-H2O2 couple yields the methyl and methanesulfonyl radicals, which were identified by E P R spectro~copy.'~ A number of pathways to these various radicals by reaction of MezSO with eaq-, H , and .OH have been prop o ~ e d . ~ However, J~ the experimental data have been insufficient to substantiate these mechanisms. Furthermore, the reactions of the various radical intermediates have not been fully described. Hydrogen, ethane, and methane have been identified as radiolytic products, but their radical precursors have not been identified." Variable field CIDNP NMR study aids in the elucidation of radical reaction pathways.21 We have applied this technique to the pulse radiolysis of aqueous MezSO with the aim of clarification of the radiation chemistry of this system. Experimental Section Dimethyl sulfoxide (MezSO), reagent grade, was used as obtained from Fisher Scientific Co. The pH of its deuterium oxide solutions was adjusted by the addition of H2S04or NaOH. Solutions were degassed with helium or nitrous oxide. In He-saturated solution the primary radicals of aqueous radiolysis (and their yields) are ea;, -OH, and H (C = 3.0, 2.8, and 0.4, re~pectively).'~ At low pH values additional H is formed by reaction of eaq-and H+. In N20-saturated solution ea; reacts to yield additional .OH. Solutions were continuously recirculated from a sample reservoir using a flow system with transfer speeds of -50 mL/min. Solutions a t high pressure passed through the irradiation tube in the variable magnetic field and then 0022-3654/78/2082-1400$0 1.OO/O
TABLE I: Chemical Shifts o f Products of Aqueous Me,SO Radiolysis Ohsd 6 Lit. 6 Ref Product CH,OS(O)CH, CH,OH CH,S(O)CH,CH,S(O)CH, (CH3),SO
3.9 3.5 3.3 2.8 2.6 2.1
(CH3)1S02
(CH3)1S
CH3CH, CH,
3.7 3.5 3.3 2.6 2.6 2.1 0.9 0.2
0.9 0.2
17
18 19 18
18 18
20 20
TABLE 11: Polarization Observed in Magnetic Fields" Field strength, G CH,OS( O ) C H 3 CH,OH CH,S(O)CH,CH,S(O)CH, (CH3),SO
0
50
500
2000
E
E E E E E E E E
E
A A A A A E E AE
E
(CH3)2S02 (CH3)2S
CH3CH, CH,
E AE
E A E AE
He- or N,O-saturated solution.
to the spinning 5-mm NMR sample tube in the probe of the Varian A56/60 NMR spectrometer before return to the reservoir at lower pressure. The flow system in the NMR probe has been described.16 Solutions were irradiated with the 3-MeV electron beam from the Argonne Van de Graaff accelerator. The electron beam entered the irradiation magnet axially. During irradiation this magnetic field could be varied from 0 to 8000 G.
Results and Discussion Spectra typical of those obtained from pulse radiolysis of 0.23 M MezSO in He- and NzO-saturated solution are shown in Figures 1 and 2. The spectra display the predicted zero field effect and low field emissionQz1The high field polarization is used to characterize the mechanisms of formation of the polarized products.21 On the basis of their chemical shifts (Table I) the following products of pulse radiolysis of aqueous MezSO were identified: methyl methanesulfinate, methanol, 1,2-bis(methylsulfinyl)ethane,dimethylsulfone, dimethyl sulfide (trace contaminant in the Me2SO),ethane, and methane. The polarization observed for each of these materials in the various magnetic fields in which the radicals were generated is shown in Table 11. In general, the pH of the solution and the gas used to deaerate the solution affect only the intensity of the polarization observed. Reaction with .OH. On the basis of a kinetic analysis of the reaction of aqueous Me2S0 with .OH (from the @ 1978 American Chemical Society
CIDNP in Pulse Radiolysis of Aqueous Me,SO
The Journal of Physical Chemistry, Vol. 82, No. 12, 1978 1401
I
DMSO
methyl group of this ester displays the predicted net effect a t high field. Polarization in methanol, a very minor product, is also seen. The observation that methanol polarization is more intense in N20-saturated solution than in He-saturated solution suggests it is formed by .OH reaction. One possible formation pathway is shown in eq 6-8. Although
He
'
p H 5.3
11
CH,S(O)OCH,
,,
,
c +-'-.h"
11
1 1
I
I
+
CH,S(O)OD
(7 1
D , O + CH,OD
+
.OD
(8)
1 I
CH,O.
+
'OD
-+
OD
1 I
0
N2° pH53
I
CH,O.
I
Figure 1. Polarization in irradiated 0.2 M Me2SO: pH 5.3, He, 500-Hz sweep (H20 at 4.9 6); (A) 0-G field; (6)50-G field; (C) 2-kG field. DMSO
(6)
CH,S(O)OCH,
2
4
CH,-S(O)OCH, I OD
I /
d '1y Ic14yIIJr
,
+
+
little is known concerning the pulse radiolysis of sulfinate esters, the postulation that structurally similar sulfoxides and sulfones undergo .OH addition14 suggests that sulfinates should react in like manner.25 The mechanism of methanol polarization is unclear a t the present time. While preliminary results concerning methoxy radical involvement in polarization observed in methanol radiolysis have recently been ~ u m r n a r i z e d , ~ ~ studies in this area are incomplete. Reaction with H: Symons has considered two modes of formation of CH3S0 in y radiolysis of Me2S0, ion recombination (eq 9-11) or hydrogen atom addition to the (CH,),SO
-
(CH,),SO' t e -
(CH,),SO+ + e - + [(CH,),SO]* [(CH,),SO]* -* CH,SO + .CH,
(9)
(10) (11)
sulfoxide group (eq 12 and 13).3 (In radiolysis of aqueous D CH,
,
I
I
2
4
I/
1
0
Flgure 2. Polarization in irradiated 0.2 M Me,SO: pH 5.3, N,O, 500-Hz sweep (H,O at 4.9 6); (A) 0-G field; (B) 50-G field; (C) 2-kG field.
TABLE 111: g Factors of Radical Intermediates eaq -
H. ( D . )
.cH, .CH,S(O)CH, CH,SO CH,SO,. (CH,),SO'.
I\
g factor
Ref
2.000 3 2.00223 2.0025 2.003 2.023, 2.011, 2.003 2.00 49 2.007
22 23 23 3 3 13 3
Radical
0 CH,
TiI1I-H2O2couple) a t pH 1 Norman and colleagues proposed the reaction pathways shown in eq 1-4.14 The CH,S(O)CH, (cH,),s(O)OD .CH, .CH,
+ +
+
.OD -*
(cH,),s(O)OD
. CH, +
CH,S(O)OD CH,SO,.
-+
+
-*
(1)
CH,S(O)OD
(2)
CH,SO,. + CH,D
(3)
(CH,),SO,
(4)
polarization observed in this study supports the validity of these reactions. The products, methane and dimethylsulfone, are in enhanced absorption (AE for methane) a t high field. This polarization is predicted for products of radicals polarized by g factor differences (Table III).*1 The intensities of polarization, however, imply that formation of dimethylsulfone is a minor reaction pathway. The CIDNP results suggest that CH3- and CH3S02.also combine to give methyl methanesulfinate (eq 5 ) . The CH,.
+
CH,SO,.
-t
CH,OS(O)CH,
(CH,),SO t D -> . S I\ 0 CH, D CH, I/ .S --t CH,D + CH,SO
(5)
solutions of Me2S0 process 9 is probably not significant. However, the radical cation may be formed by chemical oxidation under these conditions.) The p H dependence of the CIDNP observed in potential products of the latter pathway may be used to probe the validity of the second mode of reaction. The pH dependence of methane and ethane polarization observed in this study suggests hydrogen atom addition is a t best a very minor reaction pathway (Figure 3). At low pH in He-saturated solution the polarization intensities of these products are very similar to those seen a t higher p H values. Thus, decomposition of an H-atom adduct apparently is not a significant source of C H 3 . However, a t low p H an increase in the polarization intensity of 1,2-bis(methylsulfinyl)ethaneis seen (Figure 4). Since this material is formed by dimerization of .CH2S(0)CH3,this observation implies that the H atom abstracts hydrogen from Me2S0. Hydrogen abstraction by H is the most probable source of -CH2S(0)CH3.Norman observed no formation of this species in reactions of aqueous Me2S0 with .0H.14 No straightforward pathway to this radical follows from reaction of e, - with Me2S0. The CH2&(0)CH3dimer, 1,2-bis(methylsulfinyl)ethane, displays unusual enhanced absorption at high field. This enhanced absorption implies a previous encounter of -CH2S(0)CH3with a radical having a higher g factor. Although any of the sulfur-centered radicals meets this
1402
The Journal
of Physical Chemistry, Vol. 82, No. 72, 1978
D.J.
Nelson
emissive intensity of ethane CIDNP. These data support the ion recombination pathway for formation of the methylsulfinyl radical (eq 10 and 11Y and suggest that decomposition of CH3S0 may yield ethane (eq 14). Thus,
_ _ _ _ _ _ I _ _
I
[(CH,),SO]*
-8
Y
I
CH,SO
+
.CH,
-+
CH,CH, ( + S O )
2
FIELD, K G
______
Flgure 3. Polarization intensity in ethane vs. magnetic field under various experimental conditions: (upper) data obtained from irradiation of He-saturated 0.2 M Me,SO; (lower)data obtained from irradiation of N,O-saturated 0.2 M Me2S0. I
I
A l l
(14)
emission in ethane is probably an example of the memory effect.21 In further support of this mechanistic scheme, we observed that the addition of 0.05 M Br- to aqueous MezSO results in decreased intensity of ethane emission. Bromide ion a t this concentration does not effectively compete with Me2S0 for reaction with .OH. However, Walker has reported that a transient radical, identified as (CH3),SOt., is reduced by Br- at this concentration.12 Because electron capture by the dimethylsulfinyl radical cation is one pathway yielding ethane, loss of this radical by reduction would be observed as decreased intensity in ethane emission. A second pathway for formation of ethane involves the methanesulfonyl radical obtained from .OH reaction with MezSO (eq 1-4). During pulse radiolysis under all experimental conditions gaseous sulfur oxides were detected. Ethane formed via reaction 15 would be in emission.21
t I?
0
+
4
953 016
CH,SO,. .CH,
-+
CH,, 'CH,
+
CH,CH,
+
SO,
(15)
Several pathways for methane formation are suggested by this study. Polarization intensities in methane also show partial dependence on eaq-. In addition to the possibility for formation of methane following the ion recombination process described above (eq 10 and ll),the dissociative electron capture mechanism also is dependent on eaq--.Symons has observed methyl radical formation in aqueous Me2S0 glasses, presumably from dissociative electron capture by Me2S0,3and this mechanism probably applies in solution as well (eq 16). Nonreactive encounter CH,S(O)CH,
+
eag--
CH,.
+
CH,SO'
(16)
of methyl and D radicals prior to methane formation would give rise to the high-field multiplet effect observed in methane. Finally, methane is probably formed in the Me2SO-hydroxyl radical reaction sequence (vide infra).
Figure 4. Polarization intensity in 1,2-bis(methylsulfinyI)ethanevs. magnetic field at values of pH indicated: (upper) data taken from irradiation of He-saturated 0.2 M Me,SO; (lower) data taken from N,O-saturated 0.2 M Me,SO.
criterion (Table 111),the greater lifetimes of the CH3SOy and CHBSO radicals3J3supports identification of these intermediates as the dimer-polarizing radicals. Reaction with eaq-. M e a 0 appears to undergo two types of reaction with eaq-. CIDNP in ethane and methane provide data supporting dual modes of reaction. The polarization observed in ethane shows some dependence on eaq- (Figure 3). polarization intensity for ethane is greater for He-saturated solutions than for N,O-saturated solutions. Although H+ concentration has little effect on the intensity of ethane polarization, the addition of nitromethane, a spur scavenger of eaq-,22to aqueous Me2S0 dramatically decreases the observed
Conclusions The variable field CIDNP data concerning pulse radiolysis of aqueous Me2S0 seem to be nicely complimentary to the other radiolytic studies of this system. Through analysis of the field dependence of polarization the proposed reaction pathways of MezSO with -OH have been given further support. In addition it has been suggested that variation in the experimental conditions may favor formation of other products via this reaction pathway. The pH dependence of polarization implies that the H atom reacts with Me2S0 by hydrogen abstraction and apparently undergoes no addition reaction. Finally, eaq-seems to show dual modes of reaction with aqueous Me2S0. Dissociative electron capture appears to occur in solution (as well as in the aqueous glasses where this process was previously reported), and ea; may also participate in the spur in an ion-recombination reaction with the dimethylsulfinyl radical cation. Acknowledgment. Stimulating discussions with Dr. A. D. Trifunac are acknowledged. Acknowledgment is also given the operators of the Argonne Van de Graaff accelerator, R. H. Lowers and A. Youngs. References and Notes (1) Work performed under the auspices of the Division of Basic Energy Sciences of the U S . Department of Energy. (2) A. D. Triinac and D.J. Nelson, J. Am. Chem. Soc., 99, 1745 (1977).
Structure and Relative Energies of C2H,Xf
Isomers
The Journal of Physical Chemistry, Vol. 82, No. 12, 1978
1403
(16) R. G. Lawler and M. Halfon, Rev. Sci. Instrum., 45, 84 (1974). (17) I.B. Douglass, F. J. Ward, and R. V. Norton, J. Org. Chem., 32, 324 (1967). (18) "High Resolution NMR Spectra Catalog", Varian Associates, Palo Alto, Calif., 1962. (19) A. L. Ternay, Jr., 0.Rep. SuNur Chem., 3, 145 (1968). (20) D. Chapman and P. D. Magnus, "Introduction to Practical High Resolution Nuclear Magnetic Resonance Spectroscopy", Academic Press, New York, N.Y., 1966. (21) (a) R. Kaptein in "Advances in Free-Radical Chemistry", Vol. 5, G. H. Williams, Ed., Academic Press, New York, N.Y., 1975; (b) G. L. Closs in "Advances in Magnetic Resonance", Vol. 7, J. S.Waugh, Ed., Academic Press, New York, N.Y., 1974. (22) E. C. Avery, J. R. Remko, and B. Smaller, J . Chem. Phys., 40, 951 (1968); F. P. Sargent and E. M. Gardy, Chem. Phys. Left., 39, 188 (1976). (23) R. W. Fessenden and R. H. Shuler, J. Chem. phys., 39,2147 (1963). (24) D. J. Nelson, C. Mottley, and A. D. Trifunac, Chem. Phys. Left., In press. (25) Reaction of methyl methanesulfinate with terf-butoxyl and trimethylsiloxyl radicals is described by W. B. Gara and B. P. Roberts, J . Chem. Soc., Perkin Trans. 2 , 1708 (1977).
(3) M. C. R. Symons, J . Chem. Soc., Perkin Trans. 2, 908 (1976). (4) Y. J. Chung, K. Nishikida, and F. Williams, J. Phys. Chem., 78, 1882 (1974). (5) K. Nishikida and F. Williams, J . Am. Chem. Soc., 96, 4781 (1974). (6) A. M. Koulkes-Pujo, L. Gilles, B. Lesigne, and J. Sutton, Chem. Commun., 749 (1971). (7) D. C. Walker, N. N. Klassen, and H. A. Gillis, Chem. Phys. Left., 10, 636 (1971). (8) T. K. Cooper and D. C. Walker, Can. J. Chem., 49, 2248 (1971). (9) A. M. Koulkes-Pujo, B. D. Michael, and E. J. Hart, Int. J . Radiat. Phys. Chem., 3, 333 (1971). (10) R. Bensasson and E. J. Land, Chem. Phys. Left., 15, 195 (1972). (11) G. Meissner, A. Henglein, and G. Beck, Z. Naturforsch. B , 22, 13 (1967). (12) T. K. Cooper, D. C. Walker, H. A. Gillis, and N. V. Klassen, Can. J . Chem., 51, 2195 (1972). (13) R. L. Willson, C. L. Greenstock, G. E. Adams, R. Wageman, and L. M. Dorfman, Int. J. Radiat. Phys. Chem., 3, 211 (1971). (14) B. C. Gilbert, R. 0. C. Norman, and R. C. Sealy, J. Chem. Soc., Perkin Trans. 2 , 303 (1975). (15) J. W. T. Spinks and R. J. Woods, "An Introduction to Radiation Chemistry", 2nd ed, Wiley, New York, N.Y., 1976.
The Structure and Relative Energies of C2H2X+ Isomers (X = F, OH, NH2, CI, and SH) Peter Kollman, Sid Nelson, Department of Pharmaceutical Chemistry, School of Pharmacy, Universify of California, San Francisco, Californb 94 143
and Steve Rothenberg Information Systems Design, Santa Clara, California (Received June 3, 1977; Revised Manuscript Received December 5, 1977) Publication costs assisted by the Petroleum Research Fund
We compare the minimum energy structure and relative stabilities of substituted carbocations C2H2X+ (X = F, OH, NH2,C1, and SH) by carrying out ab initio molecular orbital calculations with STO-3G and 431G basis sets. The minimum energy structure turns out to be the one that allows resonance stabilization of the cationic center. By calculating the energies of the reaction CzH3' + CzH3X C2HzX++ C2H4as a function of X, the relative stabilization energies are found to be in the order NH2 > SH > OH > C1> F. We then compare the relative stabilities of C2H2X+cations with those of CH2X+ cations. Second row substituents (X = C1 and SH) stabilize vinyl cations (H2C=C+-)almost as much as methyl cations (H2C+-);in contrast, first row substituents (X = F, OH, NH2) are considerably more effective in stabilizing methyl than vinyl cations. As noted by Bernardi et al. in their study of CH20H+and CH2SH+,the second row substituents are both T and u donators to cationic centers, whereas first row substituents are T donators and u withdrawers. The qualitative conclusions do not appear to be very basis set dependent.
-
Introduction Substituent effects on cationic centers have been of interest to us for some time213and some of the more interesting calculations on this problem were carried out by Denes e t al.,4 who found that the relative stabilities of classical ( I ) and nonclassical (2) vinyl cations were very '>C=;-H
C,=C,
H
H I
A\
H
1
and compared the relative stabilities of 1 and 2 for X = F7and C1.8 Radom et have calculated relative energies of isomers of C2H2X+(X = CH3) and have found the allyl cation (3) to be the most stable.
H
2
dependent on X. With X = H, they found the classical structure favored by 18 kcal/mol; with X = SH, the nonclassical structure was favored by 65 kcal/mol. I t was thus decided to carry out a systematic study of the energy of different isomers of a number of substituted vinyl cations C2H2X+,with X = F, OH, NH2, C1, and SHe5 In the interim, there have been a series of increasingly precise calculations on C2Hn+,with the currently accepted picture being t h a t the classical and nonclassical ions are roughly isoenergetic.6 In addition, since their original paper,4 Modena and co-workers have continued their studies on C2H2X+cations 0022-3654/78/2082-1403$01 .OO/O
3
4
On the other hand, Pittman et al.'O have pointed out that structures of type 4 should be of importance in our understanding of 1-Fvinyl cations. We thus attempted: (1) to systematically examine the relative energies of structures 1,2, and 4 for C2H2X+(X = F, OH, NH2, C1, and SH); (2) to compare the relative stabilities and electronic structure of these species with those of formula CH2X+ (X = F, OH, NH2, C1, and SH); and (3) to examine the basis set dependence of these results.
Computational Details and Results of Geometry Searches The ab initio quantum mechanical calculations were carried out using the program GAUSSIAN 70 (QCPE No. 236) 0 1978 American
Chemical Society
