An Esr Study of Nitrosamine Anion Radicals
61 1
An Electron Spin Resonance Study of Nitrosamine Anion Radicals Gerald I?. Stevenson,* Jesus Gilbert0 Concepci611, and Jorge Castillo Chemistry Department, University of Puerto R i m , Rio Pieras, Puerfo Rico 00931
(Received September 1, 19721
Publication costs assisted by the University of Puerto Rico
The anion radicals of a series of nitrosamines have been generated by alkali metal reduction. In tetrahydrofuran and in a mixture of tetrahydrofuran and hexamethylphosphoramide three different ion pairs were observed simultaneously in solution. For all of the nitrosamine anion radicals studied, the majority of the spin density was found on the nitroso nitrogen, and this spin density increases when the N-N bond is twisted by steric interaction of the alkyl groups with the oxygen atom. Low-temperature esr spectra of the nitrosamine anion radical solutions exhibited a single broad line superimposed upon the spectrum of the monomer radical. This single line is attributed to a "living" polymer produced from the anionic polymerization of the nitrosamine monomer anion radicals. Much of the recent interest in the chemistry of nitrosamines can be attributed to the fact that these compounds have heen characterized as powerful carcinogens that occur in tobacco smoke and may be forined in the human stomach from the interaction of nitrites, used as food preservative, with the natural amines in the b0dy.l In a preliminary communication we reported the reduction of N-methyl-N-nitrosoaniline,N-nitrosodiisopropylamine, and N-nTtrosodiethylamine in 1,2-dimethoxyethane (DME) and tetrahydrofuran (THF).2 For the case of N nitrosodiethylamine reduced by potassium metal in DME, two different ion pairs of the nitrosamine anion radical were found to exist simultaneously in solution. The two ion pairs were proposed to be the tight ion pair and the solvent separated ion pair as shown in equation 1. We now propose a number of N-nitrosamine anion radicals including those from I-VI1 together with some of the solution equilibria in which these anion radicals are involved,
,6llK+
ijK+
1 '
f
+=+=
N-N
(1)
1
I1
I11
VI
171
VI1
Experiment a1 Section
The N-nitrosamines were prepared by the method described by Vog:el.3 THF, DME, and hexamethylphosphoramide (HMPA) were distilled directly into the reaction vessel from the solvated electron under high vacuum ( 10- mm preswre).
Solutions of the N-nitrosamines (10-2-10-3 M ) were shaken on an alkali metal mirror to yield the yellow-colored anion radical solutions. The esr spectra were recorded on an X-band Varian E-3 esr spectrometer equipped with a Varian V-4557 temperature controller. Results Under low-resolution conditions (high modulation amplitude) all of the nitrosamine anion radicals, except for those of I and VI, gave esr patterns of nine lines due to two nonequivalent nitrogens. Upon high resolution more lines were observed. These extra lines are interpreted in terms of ion pairing. None of the nitrosamine anion radicals were stable enough in pure HMPA to afford esr spectra. A stable anion radical solution of N-nitrosodimethylamine could not be generated under any solvent metal conditions. The experimental s orbital spin densities were determined from spectral data using t h e standard equation p i = A ~ / 5 1 0 ,where AN = 12.6 and 2.4 G (see Table I). Calculated s orbital spin densities obtained using an INDO treatment show good agreement with the experimental values. Experimental *-electron spin densities are also shown in Table I. The results show that the odd electron resides predominantly on the nitroso nitrogen. Diethyl-N-nitrosamine (ZZ). The system 11-DME-K upon esr analysis yields an eleven-line pattern apparently resulting from the expected nine-line pattern, but with the end lines split (Figure 1). These results can be nicely interpreted in terms of two different radical anions in solution ( a and p), both having identical g values, but p having slightly larger coupling constants for both nitrogens (Table 11).This results in the m = 0,O lines superimposing, thus yielding a sharp center line. However, due to the small differences in the coupling constants, the m = 0 f 1 lines do not exactly overlap. The result is that these lines become broadened and less intense. The coupling constants differ by less than the line width. The large nitrogen coupling constants also differ by less than the line width, which results in slightly broader m = f1,0lines. Since the p anion radical has the (1) ( a ) M. F. Argus, J. C. Arcos, A . Alam, and J . H. Mathison. J. Med. Chem., 7, 460 (1964); (b) Chem. Eng. News, 49 No. 5 0 , 15 (1971). (2) G. R. Stevenson and C. J. Colon, J. Phys. Chem., 75, 2704 (1971). (3) A. I . Vogel, "Practical Organic Chemistry," 3rd ed., Wiley, New Y o r k , N . Y . , p426. The Journal of Physical Chemistry, Voi. 77, No. 5 , 1973
6 . R. Stevenson,J. G. Concepcion, and J. Castillo
612
'' T
s e p a r a t i o n treater than
Figure 2. Esr spectrum of the system I I-THF-K at -20"
'ins-width
TABLE II: Coupling Constants for Nitrosamine Anion Radicals at
Room Temperature System
Ll
ii
-
I I-THF-K radical-
Figure 1. Esr spectrum (upper tracing) of II reduced by potassium in DME and recorded at room temperature. The line width of the center line is 0.28 G. The high-field lines are broader than the low-field lines due to g tensor anisotropy. The reduction in THF gives a sirnilai spectrum, but with less obvious separation of the end lines Lower tracing shows stick diagram for the lowfield half of radicals cr and 0. TABLE I: Calculated and Experimental Spin Densities
II-THFlHMPA-K
III-THF-K
AN(nitroso1
ANtalkalatedi
12.56 f 0.05 12.5 f 0.05 12.56 f 0.05 12.4 12.1 13.0
2.26 f 0.02
2.1 i 0.02 2.29 f 0.02 1.8
1.7 5.1 2.8
12.9
2.3 2.54$: 0.02 2.7 0.02 1.29 i
12.8
IV-THF-K V-TH F-K VI-THF-K
Ion Pair
12.7 12.8 12.9
geometrical isomers or from different ion pairs. If the two or three radicals were due to a hindered internal rotation as observed by McKinney and Geske4 or some other intra-
Position
Calculated s orbital spin density
Experimental Experimental s *-electron spin orbital spin density density
I_________I___
1 2
4- 0.0002 +0.0210
0.0047 0.0247
3 4
4- O.OOB7
0.0000
5
+ K -
THF
I,
/OK+
i-
HMPA
0.16
0.84 0.00
cy
-0.0051 -Q.0030
larger coupling Constants for both nitrogens, the differences are additive in the end lines, and the resulting difference in line positions becomes slightly larger than the line width. The coupling constants for the /3 radical are 12.60 and 2.40 G. Those for the a! radical are 12.45 and 2.23 6 . As the temperature is lowered a third radical appears. This new radical exhibits only a single broad line which narrows as the temperature is lowered (see Figure 2). At about -70" only the single broad line can be observed. The fine width s 16 G, When II is reduced by potassium metal in a 1:l mixture of HMPA anld TWF, the resulting esr spectrum consists of 17 lines. This spectrum can be interpreted in terms of three radicals which are shown in eq 2, with the same g value, but with different coupling constants. The three radicals .n this case and the two radicals exhibited 4Cor the system 11--'TWF-K can result from either different The Journal of Physical Chemistry, Vol. 77, No. 5, 1973
P
Y
molecular process, an increase in the temperature should time average the two spectra. Experimentally this is not the case for at +60° no coalescing of the lines is observed. In fact the lines narrow as the temperature increases and better separation of the lines due to the different radicals is observed. At +60° the line width is 0.19 G and at +25" the line width is 0.28 61;. This leads us to believe that for both systems the different anion radicals seen simultaneously are due to different ion pairs, a tight ion pair (a),a loose ion pair (p), and an essentially free ion ( y ) . Table I1 gives the coupling constants for a , @, and y (Figure 3). N-Nitrosodiisopropylamine (111). The 111-THF-K system exhibits 17 lines at room temperature just as for the (4) T. M. McKinney and D. H. Geske, J. Chem. Phys., 44, 2277 (1966).
An Esr Study of Nitrosamine Anion Radicals
-__lli-----iL-UJ ________il_. & ' d
Figure 3. Esr spectrum (upper tracing) of I I reduced by potassium in THF: HMPA and recorded at room temperature. The line width of the center line is 0.27 G. Lower tracing shows stick diagram for the low-field half of radicals a,b, and y. system 11-THF: HMPA-K. At -40" the spectrum consists of nine equally intense hyperfine lines due to two nonequivalent nitrogens. Only a single broad line can be observed at -80". For the system 111-THF-Cs the spectrum consists of 36 lines due to A N = 12.70 G , A N = 2.73 G, and Acs = 0.89 6. When 111 is reduced in THF by Na an uninterpretably complex esr signal results with obvious metal splitting. N-Nitrosodiisobutylamine (IV). The esr spectrum of IV yields nine lines of about equal intensity at room temperature. Only the rn = -1,-1 line has a slightly lower intensity. At -40"only a single broad line is observed. ~~Nitrosopyrolrdine (V). At room temperature the system V-THF-K gives triplets of triplets superimposed upon a single broad line. The coupling constants are shown in Tabie 11. At 20" only a single broad line can be observed upon esr analysis of this system. The anion radical of V is too short lived in a mixture of THF and HMPA to observe an esr signal. N-Aiitroso-cis-2,6-dimethylpiperidine (VI) As shown in Table IT, the anion radical of Vn gives an esr spectrum resulting from two nonequivalent nitrogens, but the alkalated nitrogen gives a much smaller hyperfine splitting (1.29 6) than all of thr other nitrosamine anion radicals (Table 11). 1,2,3>4- Tetrahydroisoquinol ine-N-nitrosamine (VU). All methods of reduction of this compound lead to solutions that yield very complex uninterpretable esr spectra. This is probably due to the fact that there are two possible rotomers of this anion radical to add further complexity to the already existing ion pairs. Discussion The ion pairing interpretation of the three radicals obtained for the IT-THF: HMPA-K system is supported by the fact that sodium reduction of I1 leads to metal split-
613
ting in THF. Three different ion pairs have been previously observed simultaneously for hydrocarbon anion radicals by Hirotas and more recently by Allendoerfer and Papez.6 All of the nitrosamine anion radicals gave a single broad esr line superimposed upon the spectrum for the monomer anion radical a t low temperature. This broad line is reminiscent of the single broad line obtained for living polymers produced during anionic polymeri~ation.?-~ Since lowering the temperature increases the polymer, concentration at the expense of the monomer, and raising the temperature diminishes the concentration of the polymer radical with a subsequent increase in the monomer anion radical concentration, the enthalpy of the reaction forming the living polymer from the monomer anion radical must be less than zero. This temperature effect is completely reversible for the compounds studied except for pyrolidine. The nine-line esr signal from the N-nitrosopyrolidine anion radical reduces to a single line at low temperature, but upon warming the signal irreversibily disappears. The apparent narrowing of the esr lines upon increasing the temperature is due to the disappearance of the living polymer and the decreasing of the dielectric constant of the solvent. This latter effect affords better separation of the different ion pairs. The temperature region, where the ion pairs can be observed simultaneously, is very small. For this reason, it is impossible to obtain esr temperature data for the various ion pairs. Over the range available (about 20") the coupling constants are invariant with temperature. All attempts to reduce N-nitrosodim~thylamineto its anion radical resulted in the immediate formation of a polymeric precipitate, which can be seen forming on the metal surface. The anion radjcal of M-nitrosopyrolidine is also very unstable toward anionic polymerization. At room temperature the monomeric anion radical could be observed by esr, but a lowering of the temperature to 20" led to the observation of just the single esr line, The nitrosamine anion radicals tend toward anionic polymerization in the order I > V > I1 > IV > 111. Further, the temperature at which the single line becomes predominant follows the same order. We notice that the order shown is the same order that one would write for the acidity of the a protons. The acidity of the a protons of V is greater than those of I1 due to the presence of the small five-member ring. If steric interaction were the only consideration in the polymerization, IV would be slower to polymerize than 111.Just the opposite is observed. The anion radical of VII exhibits a coupling constant for the alkalated nitrogen of only about half of the magnitude of that for the other nitrosamine anion radicals (1.29 G). cis-1,3-Dimethylcyclohexanemust assume the diequatorial position for the methyl groups due to the steric interaction of the methyl groups when they are in the diaxial position.1° The anion radical of VI assumes the configuration shown below in order to maintain the methyl groups in the diequatorial position. However, this introduces a steric interaction between the oxygen and one of the N. Hirota, J. Phys. Chem., 71, 127 (1967). R . D. Allendoerfer and R. J. Papez, J , i3hy.s. Chem., 7 5 , 1012
11972). F. J . Smentowski and G. R. Stevenson, d. Phys. Chem., 74, 2525 (970). K. Hirotaand K. Kiuwata, J. Polym. Sci., EO, S52 (1962). H. P. Leftin and W. K. Hall, J . Phys. Chem.,'64,382 (1960). J. B. Hendrichson, D. J. Cram, and G. S. Hammond, "Organic Chemistry," McGraw-Hill, New York, N. Y . , 1970, p 214.
The Journalof Physical Chemistry, Vol. 77, No. 5, 1973
N. 6.Naahat and K.-0. Asmus
S14
trogen coupling constant increases and the ring proton coupling constants decrease as the NO2 group is twisted by steric interaction from the plane of the ring.ll ,I2 methyl groups. This interaction twists the N-0 group out of the plane of the N-N bond. This has the effect of increasing the nitroso nitrogen coupling constant and decreasing that for the alkalated nitrogen. A good analogy to this effect lies with the anion radicals of di-ortho-substituted nitrobenzenes. Both calculation and experiment show that for hindered nitrobenzene anion radicals the ni-
Acknowledgments. The authors are indebted to the Research Corporation for the financial support of this work. We also wish to thank Dr. Carlos Col6n for helpful discussion, We are grateful to Loctite of Puerto Rico for the financial support of G. ConcepciBn. (11) P. H. Riegerand G . K . Fraehkel, J. Chem. Phys., 39,609 (1961). (12) D. H. Geske, J. L. Ragle, M. A. Bambenek, and A. L. Balch, J. Amer. Chem. SOC.,86,987(1964).
ercuric Chloride by Hydrated Electrons and Reducing Radicals utions. Formation and Reactions of HgCI1
. B. hlazhat and K. -D. Asmus* Nahn-IMeitner-lnstitut fur Kernforschung, Berlin GmbH, Sektor Strahlenchemie, 1 Berlin 39, West Germany (Received September 28, 1972) Publicatton costs assisted by Hahn-Meitner-lnstitut far Kernforschung Berlin GmbH
'The reduction of HgClz to Hg2C12 in aqueous solutions has been investigated by optical absorption and conductivity pulse radiolysis. The first step is a dissociative electron capture yielding C1- and HgCl. The following rate constants were obtained: k(eaq- + HgC12),= (4.0 f 0.3) X 1010 M - l sec-I; k(H. + HgC12 =: (1.0 0.5) x loio M - l sec-l; k((CH3)zCOH + HgC12) = (2.0 0.2) = lo9 M - I see-1. HgCl absorbs in the uv with absorption maxima at 330 (€330 2.3 x lo3 M-I cm-l) and 245 nm (e244 7.5 x 103 M-1 cm -l). HgCl dimerizes to HgzClz with 2k = (8.0 f 0.5) x lo9M-l sec -l, reacts with oxygen with k = (1.0 : 0.5) x 109 M - 1 sec-1, transfers an electron to tetranitromethane with k = (4.5 f 0.3) X 109 M-1 sec-1, and undergoes a fast reaction with the hydroxyl radical, k = lolo M - I see-l. The yield of Hg2C12 precipitate has been measured in y-irradiated solutions of mercuric chloride. In the presence of OH radical scavengers G(Hg2C12) = YzG(HgC1). Much less precipitate is formed in the absence of QH radical scavengers. This is explained in terms of a reoxidation of Hg2C12 (formed by dimerization of HgCl) by hydroxyl radicals. Mechanistic details of HgCl2 reduction are discussed. Information has also been obtained on the hydrolysis and dissociation equilibrium of mercuric chloride.
*
Introduction The reduction of mercuric chloride in aqueous solution by y-irradiation leads to the formation of HgzClz precipitate as a final proc3uct.l The mechanism of this radiation chemical reduction has not yet been studied. Flash photolysis of HgCl2 solutions has been carried out, however, and the Clz- radical anion has been detected as an intermediate.2 In studies of the reduction of HgClz vapor by sodium atoms HgC1 could be identified as a short-lived intermediate with a half-life of about 10-5 sec.3 This species was found to have optical absorption bands in the UV.
Mercuric chloride is known t o be essentially undissociated in polar solvents. In aqueous solutions of 2 x 10-3 The Journal of Physical Chemistry, Vof. 77, No. 5, 1973
*
M mercuric chloride, for example, only ca. 0.5% of the HgCl2 is dissociated and ea. 1-2% is hydrolyzed according to the equilibria HgC1,
HgCl"
+
C1-
(1)
4-
2W,," i- 2C1-
(2)
and 2HgC1,
+ H,O == Hg,OCl,
respectively (at 20'). These two equilibria are only the two most important ones. Others also exist, they are con(1) G . Stein, R. Watt, and J. Weiss, Trans. Faraday Soc., 48, 1030 (1952). (2) M. E. Langmuir and E. Hayon, J. Phys. Chem., 71, 3808 (1967). (3) D. Maeder, Helv. Phys. Acta, 16,503, 520 (1943).
