J. Phys. Chem. 1980, 84, 3597-3599
TABLE VI: Calcullated L M a t r i x L Q6 --Q5
s,
s6
S,
S,
0.382 0.015 -0.217 -0.001
0.001 0.458 -0.029 -0.208
Q, -0.001 0.129 0.241 -0.133
Q, 0.000 0.023 0.007 0.1 24
relative intensities of A7 and As, which correspond to normal coordinate13 more influenced by the central metal atom, are not corirectly reproduced by using the polar tensors of Ni(C0)4 In fact these calculated values indicate that A8 is about ten times as strong as A7 whereas the experimentally merlsured intensity of v7 is about twice that of vg. Although solvent effects may complicate the situation more than our discussion here admits the above results appear to present a realistic portrait of the consequences on intensity predictions caused by variations in the values of transferred atornic polar tensoris. Acknowledgment. Stimulating discussions with Professor w. B. Person, made possible by a United Nations travel grant to R.EI.B., were extremely helpful in the execution of this project. Computer time was generously furnished by the Centro de ComputaqBo, Universidade Estadual de Campiinas and the Niicleo de ComputaCBo Electrenica, Universidade Federal do Rio de Janeiro. Partial finacial support from FINEP (Financiadora dos Estudos e Projetos) and frpm CNPq (Conselho Nacional de Desenvolvimento Cientifico e Tecnologico) is gratefully appreciated.
Appendix As mentioned earlier, the experimental atomic polar tensor elements of Ni(C0)4calculated directly from the data in ref 2 did riot satisfy the G sum rule. For this reason, a complete reanalysis of the vibrational data for Ni(C0)4was performed. The inconsistency in the G sum rule calculation Waf3 removed by the calculation of a new L matrix for this complex. A force field restrained with Fll = 17.700 mdyn A&-1, F,, = 0.527 mdyn A, Fl, = -0.084
3597
mdyn, and Fla = FTa= 0.000 mdyn (see ref 2 for the definitions of these force constants) was adjusted to give the best least-squares fit between the observed and calculated frequencies. The calculated L matrix, which is similar to the one reported in ref 2, is given in Table VI.
References antd Notes (1) S. Kh. Samvtslyan, V. T. Aleksanyan, and B. V. Lokshin, J . Mol. Specfrosc., 48, 47 (1973). (2) S.Kh. Samvolyan, B. V. Lokshln, and V. T. Aleksanyan, J. Mol. Spectrosc., 48, 566 (1973). (3) J. A. Pople arid G. A. Segal, J. Chem. Phys., 44, 3289 (1966); J. A. Pople and D. L. Beveridge, "Approximate Molecular Orbltal Theory", McGraw-Hill, New York, 1966. (4) H. L. Hase arid A. Schweig, Theor. Chlm. Acta, 31, 215 (1973). (5) (a) A. J. van Straten and W. M. A. Smlt, J . Chem. Phys., 67, 970 (1977); (b) R. E. Bruns, J. Phys. Chem., 82, 1908 (1978). (6) R. E. Bruns and R. K. Nair, J . Chem. Phys., 58, 1849 (1973). (7) W. 8. Person and D. Steele in "Molecular Spectroscopy", R. F. Barrow, D. A Long, and D. J. Millen, Ed., The Chemical Society, London, 1974, p 357. (8) D. W. Clack, hl. S. Hush, and J. R. Yandle, J . Chem. phys., 57, 3503 (1972). (9) (a) A. Serafini, J. M. Savariiut P. Cassoux, and J. F. Labarre, Theor. Chlm. Acta, 36, 241 (1975); (b) A. Serafinl, M. PBllssier, J. M. Savarlault, P. Cassoux, and J. F. Labarre, Ibid., 39, 229 (1975). (10) (a) H. J. Freurid and 0. Hohlneicher, Theor. Chlm. Acta, 51, 145 (1979); (b) G. Hohlneicher, private communication. (11) (a) R. Hoffman, J . Chem. Phys., 39, 1397 (1963); (b) R. Hoffman and W. N. Lipscomb, lbid., 36, 2179, 3489 (1962); 37, 2872 (1982). (12) L. S. Mayants and B. S. Averbukh, J. Mol. Specfrosc., 22 197 (1967). See also the Interpretative article by A. Rupprecht, J. Mol. Specfrosc., 64, 199 (1977). (13) (a) J. F. Biarge, J. Herranz, and J. Morcillo, An. R . SOC.ESP. Fls. Quim., Ser. .4,57, 81 (1961); (b) W. B. Person and J. H. Newton, J. Chent. Phjis., 61, 1040 (1974). (14) (a) B. L. Crawford, Jr., J . Chem. Phys., 20, 977 (1952); (b) W. T. Klng, G. B. Nlast, and P. P. Blanchette, Ibid., 56, 4440 (1972). (15) D. M. Sllver and K. Reudenberg, J . Chem. Phys., 49, 4301, 4306 (1968). (16) R. S. Mullikan, J. Chem. Phys., 23, 1833 (1955). (17) R. E. Bruns arid W. B. Person, J . Chem. Phys., 58, 2528 (1973). (18) W. B. Person and J. Overend, J . Chem. Phys., 66, 1443 (1977); B. J. Khhn, W. B. Person, and J. Overend IbM., 65, 969 (1976); M. Gussoni, S. Abbate, and G. Zerbi, lbid., 71, 3428 (1979). (19) B. Schurin and R. E. Ellis, J. Chem. Phys., 45, 2528 (1966). (20) N. Sheppard and T. T. Nguyen in "Advances In Infrared and Raman Spectroscopy", Vol. 5, J. H. Clark and R. E. Hester, Ed., Heydon, London, 1978, Chapter 2.
Bis(trifluofomethyl)aminyl and Bis(trifluoromethy1) Nitroxide' C. C~hatgillaloglu,2a V. Malatesta,lband K. U. Ingold" Division of Chemistry, Natlonal Research Council of Canada, Ottawa, Ontario, Canada K I A ORB (Received: April 23, 1980; In Final Form: September 10, 1980)
The EPR parameters for (CFd2N.have been determined to be g := 2.00374, uN = 13.84 G, and uF = 13.84 G in CFzC12from 150 to 220 K. The (CFJ2N0. radical has been generated from (CF3)ZN by using isotopically enriched 1702. It has ulT0equal to 23.6 G which is higher than ulT0values found for dialkyl nitroxides (19-20 G). This serves to confirm that (CF3)2NO-has a higher spin density on oxygen than other nitroxides.
Bis(trifluorometh:yl)nitroxide, (CF3)2NO., is very much more reactive in hydrogen atom abstractions, in additions, and in substitutions than are dialkyl nitr~xides.~We have measured the 0-H lbond strength in1 the hydroxylamine, (CF3)2NOH,by an electron paramagnetic resonance (EPR) spectroscopic technique and found it to be ca. 82.6 kcal/mol,4 which is cionsiderablylarger than the 0-H bond !~ strength of dialkyl nitroxides (ca. 71 4: 2 k c a l / m ~ l ) . ~The 0022-365,4/80/ 2084-3597$0 1.OO/O
increase in bond strength brought about by the two CF3 groups can be attributed4 to their electronegativity which should decrease the extent of conjugative electron delocalization onto nitrogen when compared with the delocalization which occurs in a dialkyl nitroxide. That is, canonical structure 1 is relatively more important than 2 when R = CF3, than when R = alkyl. In favor of this conclusion is the fact that the nitrogen hyperfine splitting 0 1980 Amerlcan Chemical Society
3598
The Journal of Physical Chemistty, Vol. 84, No. 26, 1980
1
2
constant (hfsc), aN,for (CF3)2N0.is 9.5 G7which is considerably smaller than the aN values found for typical dialkyl nitroxides (ca. 15 G).8 This explanation for the low aNin (CF3)2N0.was previously considered by Scheidler and Boltongbut was rejected in favor of spin transfer to the fluorine atoms by a direct p - ~interaction. However, it is difficult to see how spin delocalization is compatible with the high reactivity of (CF3)2NO-. In order to distinguish between the two explanations for the low aNvalue of (CF3)2N0.we have attempted to estimate the relative importance of canonical structures 1 and 2 by measuring a' O for this radical. In the course of this work we also generated the previously unknownlo bis (trifluoromethyl) aminyl radical, (CF3)2N.. Experimental Section Materials. Perfluoro(N-methylenemethylamine),CF3N=CF2, was prepared from trifluoronitrosomethane and tetrafluoroethylene by the method of Barr and Haszeldine." Perfluoro[bis(dimethylamino)]mercury,[(CF3)2NI2Hg, was prepared by the procedure of Young et a1.12 Trifluoromethyl hypofluorite, CF30F,was a gift from Dr. K. F. Preston and the 1702 (60 atom 90170)was a gift from Dr. J. A. Howard. Measurement of EPR Parameters. The radicals were generated directly in the cavity of a Varian E-104 EPR spectrometer. Precise field strengths and microwave operating frequencies were measured by using an NMR gauss meter and frequency counter as described by Griller and Preston.13 Corrections for the difference in magnetic field between the position of the NMR probe and that of the sample were made by using the tetracene radical cation as a standard. Results and Discussion Bis(trifluoromethy1)aminyl. The only practicable method for preparing the 170-labelednitroxide appeared to us to be via reaction of the (unknown) (CF3)2N.radical with 170-labeledgaseous oxygen.14 The most attractive approach to the desired aminyl appeared to be the procedure recently developed by Roberts and Winter,15viz. addition of the appropriate radical (a fluorine atom in the present case) to the appropriate imine. Photolysis of CF30F (a good source of F atoms) in the presence of CF3N=CF2 in CF2C12as solvent in the cavity of an EPR spectrometer gave the hoped-for (CF3)2N.radical at temperatures from 150 to 220 K. CF30F F.
--+ hv
CF30.
+ CF3N=CF2
F.
CF3NCF3
The EPR parameters for (CFJ2N- are g = 2.00374, aN = 13.84 G, and aF(6 F) = 13.84 G and are temperature independent. The g value is somewhat lower than those generally found for dialkylaminyls (ca. 2.0046).'4-17 This would be expected by, analogy with the isoelectronic secondary alkyls (CF3)2CHand (CH3)&H for which the g values are 2.00221 and 2.00267, respectively.ls The aN value for (CF3)2Nis also somewhat smaller than the values found for analogous dialkylaminyls, e.g., 14.78 and 14.2 G for (CH3)2Nand [(CH,),C],N, re~pective1y.l~Since an a-CF3 group removes less spin density than an a-CH3 group,19and since aNis very sensitive to the C-N-C bond angle,16it seems likely that the low aNvalue of bis(trifluoromethy1)aminyl is due to geometric factors.
Chatgilialoglu et al.
Bis(trifluoromethyl)Nitroxide. The spectrum from the (CF3)2Nradical generated as described above decayed immediately on cutting off the UV light and was replaced by a relatively weak EPR spectrum of the (CF3)2N0.radical. The formation of the nitroxide was not due to adventitious traces of oxygen because the deliberate addition of O2 (at low and at high concentrations) to the system gave only the persistent FOO. radical, g = 2.0038, aF = 13.7 G.20 The nitroxide's oxwen must therefore come in some way from the hypofluor%e. Photolysis of [(CF3)2N]2Hg in CF2C12or CFC13 at ambient temperaturesz1 did not give an EPR spectrum, nor were anyradicals observed in the presence of oxygen. However, when, under oxygen, the light was cut off the spectrum of (CFJ2NO- built up rather slowly with time. We presume that the nitroxide is formed during the photolysis but is then trapped, probably by the aminyl radical, to give perfluoro(2,4-dimethyl-3-oxa-2,4-diazapentane), which is thermally unstable at ambient temp e r a t u r e ~ .The ~ ~overall ~ ~ ~ process ~ ~ ~ can be represented by
-
[(CF3)2N12Hgk. 2(CF3)2N.+ Hg 2(cF3)& + 02 2(CF3)2NO. (CF3)2NO*+ (CF3)ZN (CF3)2NON(CF3)2
-
+
A
(CF3)2NON(CF3)2 (CF3)2NO*+ [(CFJ&I When these reactions were carried out with 60 atom % 170-labeled O2 additional lines having the expected intensity (10% of the corresponding parent) due to 170were readily observed. At 285 K the EPR parameters for = (CF3)'N0. are = 2.0069, aN= 9.4 G, aF = 8.3 G , 5.1 G,23 and at0 = 23.6 G. In the temperature range 245-295 K any change in the 170hfsc was I -0.1 G.z6 The 170hfsc for (CF3)2N0.and the g factor are both significantly larger than the values found for dialkyl nitroxides, which are generally in the range 19-20 G and 2.0059-2.0065, r e s p e c t i ~ e l y . ~ Both ? ~ ~ *of~ these ~ differences (plus the relatively small N hfsc, see Introduction) support the idea that canonical structure 1 is more important relative to 2 for (CF3)2NOsthan for dialkyl n i t r o x i d e ~ . ~ ~ To put this statement on a somewhat more quantitative basis, Aurich et ala2&have suggested that the T spin densities, p, at nitrogen and oxygen for nitroxides can be calculated from the McConnell relationship^:^^ pN = aN (G)/33.1 (G) po = a'70 (G)/35.3
(G)
For (CF3)2N0.our results give pN = 0.28 and po = 0.67,37 whereas typical dialkyl nitroxides in nonpolar media have pN 0.46 and po 0.54.26"That is, for dialkylnitroxides, 1 and 2 are of approximately equal significance, while for (CF,),NO. the former structure is 2 or 3 times as important as the latter. In fact, in some ways bis(trifluoromethy1) nitroxide [a'70 = 23.6 GI and its hydroxylamine [D[(CFJ2NO-HI1,; 82.6 kcal/molI4 are more similar to alkylperoxyls [ a (terminal 0) = 23.45 and alkyl hyand to didroperoxides [D[ROO-HI = 88 kcal/m01]~~ and oximes [D[R&= alkylketiminoxyls [a"O = 22.6 NO-HI = 80.9 kcal/molI6 than to dialkyl nitroxides [a"O i= 19.5 G]8,14,26 and hydroxylamines [D[R,NO-H] i= 71 k~al/mol].~,~ Finally, we note that because (CFJ2N0. is persistent we had expected to observe the 170satellite lines without the need for isotopic enrichment4' (170natural abundance = 0.037%). However, although some of the natural abundance MI = f 3 / 2 lines could be detected, they could not
-
-
EPR Study of Dialkyl Nitroxides
be assigned unambiguously because they were almost swamped by the "wings" of the main spectrum.
Acknowledgment. We thank Dr. K. F. Preston for help with the CFBOFexperiments and Dr. D. Griller for many helpful suggestions.
References and Notes (1) Issued as N.R.C.C. No. 18860. (2) N.R.C.C. Research Associate: (a) 1979-80,(b) 1977-80. (3) See, e.g., (a) Banks, R. E.; Haszeldine, R. N.; Stevenson, M. J. J. Chem. Soc. C 1966, 901. (b) Makarov, S.P.; V i i k o , A. F.; Tobolin, V. A.; Englin, M. A. Zh. Obschch. Khim. 1967, 37, 1528. (c)
Makarov, S.P.; Videiko, A. F.; Nlkolaeva, T. V.; Englin, M. A. Ibid. 1967, 37, 1975. (d) Englin, M. E.; Mel'nikova, A. V. Zh. Vses. Khim. Obshchest. 1968, 73, 594. (e) Makarov, S. P.; Engiin, M. A.; Mel'nikova, A. V. Zh. Obshch. Khim. 1969, 39,538. (f) Mel'nikova, A. V.; Baranaev, M. K.; Makarov, S. P.; Englin, M. A. Ibid. 1970. 40, 382. (9) Zh. Vses. Khim. Obschest. 1970, 75,117. (h) Banks, R. E.; Cheng, W. M.; Haszeldine, R. N.; Shaw, G. J. Chem. SOC. C 1970, 55. (i) Banks, R. E.; Haszeldlne, R. N.; Myerscough, T. IbM. 1971, 1951. (j)Banks, R. E.; Haszekline, R. N.; Justln, B. IbM. 1971, 2777. (k) Banks, H. E.; Haszeldine, R. N.; Myerscough, T. J. Chem. Soc., Perkin Trans. 7 1972, 1449. (I)IbM. 1972, 2336. (m) Banks, R. E.; Haszeldine, R. N.; Stephens, C. W. Tetrahedron Lett. 1972, 3699. (n) Banks, R. E.; Choudhury, D. R.; Haszekline, R. N. J. Chem. Soc., Perkin Trans. 7 1973, 80. (0)Ibid. 1973, 1092. (p) Banks, R. E.; Edge, D. J.; Freear, J.; Haszeldine, R. N. IbM. 1974, 721. (9) Banks, R. E.; Birchall, J. M.; Brown, A. IC; Haszeldine, R. N.; Moss, F. Ibid. 1975, 2033. (r) Haszeldine, R. N.; Rogers, D. J.; Tipping, A. E. J . Chem. Soc., Dalton Trans. 1975, 2225. (s) Ibid. 1976, 1056. (t) Coles, P. E.; Haszeldlne, R. N.;Owen, A. J.; Robinson, P. J.; Tyler, B. J. J. Chem. SOC., Chem. Commun. 1975, 340. (u) Booth, B. L.; Edge, D. J.; Haszeldine, R. N.; Holmes, R. G. G. Ibid. 1976, 2305; (v) J. Chem. SOC.,Perkin Trans. 2 1977, 7. (4) Malatesta, V.; Ingold, K. U. Unpublished results. (5) Mahoney, L. R.; Mendenhall, G. D.; Ingold, K. U. J. Am. Chem. SOC. 1973, 95,8610. (6) Caceres, A.; Lissl, E. A.; Sanhueza, E. Int. J . Chem. Kinet. 1978,
70,1167. (7) Bhckley, W. D.; Reinhard, R. R. J . Am. Chem. Soc. 1965, 87,802. (8) See Forrester, A. H. In "Landolt-BBrnstein. New series. Magnetic Properties of Free Radicals"; Vol. 9,Part c 1; Flscher, H.; Heliwege, K.-H., Ed.: Springer-Verlag, Berlin; 1979. (9) Scheidler, P. J.; Bolton, J. R. J. Am. Chem. SOC.1966, 88, 371. (10)This radical has been proposed as an intermediate on several occasions. See, e.g., Sheppard, W. A.; Sharts, C. M. "Organic Fluorine Chemistry": W. A. Benjamin, New York; 1969,Chapter 6. Fischer, R.; Haszeldine, R. N.; Tipping, A. E. J. Chem. Soc., Perkin Trans. 7 1980, 406,and references cited. (11) Barr, D. A.; Haszeldine, R. N. J . Chem. SOC.1955, 1881. (12) Young, J. A.; Tsoukalas, S. N.; Dresdner, R. D. J. Am. Chem. SOC. 1958, 80,3604. (13) Griller, D.; Preston, K. F. J. Am. Chem. SOC. 1979, 707, 1975. (14) Roberts, J. R.; Ingoid, K. U. J . Am. Chem. SOC.1973, 95,3228. (15) Roberts, B. P.; Winter, J. N. J. Chem. Soc., Chem. Commun. 1978,
960. (16) Pratt, D. W.; Dillon, J. J.; Lloyd, R. V.; Wood, D. E. J . Phys. Chem. 1971, 75,3486. (17) See Neugebauer. F. A. In "Landott-Bornstein. New series. Magnetic Properties of Free Radicals"; Vol. 9,Part c 1; Fischer, H.; Hellwege,
The Journal of Physical Chemistry, Vol. 84, No. 26, 7980 3599 K.-H., Ed.: Springer-Verlag, Berlin; 1979. (18) Chen, K. S.;Krusic, P. J.; Meakin, P.; Kochi, J. K. J. Phys. Chem. 1974, 78, 2014. (19) Krusic, P. J.; Bingham, R. C. J . Am. Chem. SOC.1976, 98,230. (20) Fessenden, R. W.; Schuler, R. H. J . Chem. Phys. 1966, 44,434. Adrian, F. J. Ibid. 1967, 46, 1543. (21) The mercurial was Insoluble at temperatures below ca. 250 K. (22) Haszeldine, R. N.; Tipping, A. E. J . Chem. SOC. C 1966, 1236. (23) Measured In natural abundance on normal (CF&NO.. This value is in agreement with the literature."
(24) Knolle, W. R.; Bolton, J. R. J. Am. Chem. Sock1969, 97,5411. (25) At 236 and 297 K, aN = 9.33and 9.46 0, and a = 8.46and 8.26 0, respectively.'
(26) (a) Baird, J. C. J. Chem. Phys. 1962, 37, 1879. (b) Hayat, H.; Silver, B. L. J . Phys. Chem. 1973, 77,72. (c) Aurich, H. G.; Hahn, K.; Stork, K.; Weiss, W. Tetrahedron1977, 33, 969. (d) Aurich, H. 0.; Czepluch, H. Tetrahedron Lett. 1978, 1187. (27) The EPR parameters for (CF,),NO- will also be affected by its geometry.28 This radical has a nonplanar conflguration with an angle between the C-N-C plane and the N-0 bond, 0, of 22 & 3OSs The available evidence for dialkyl nitroxides su ests that large values of 6' are associated with enhanced N hfs4g(which contrasts with (CF,),NO) and, possibly, enhanced g values (like (CF&NO). For example, di-tert-butyl nitroxide, 2,2,6,6-tetrarnethylpiperidinyl-l-oxy and 9azabicyclo[3.3.1]nonanyl-l-oxy In benzene at 25 OC have ad = 15.23,15.54,and 17.990, and g = 2.0060,2.0063,a 4 2.0067, res e~tiveiy.~'DI-tert-butyl nitroxide has 6' = Oo3* and a = 19.1 G,& t$ramethylpiperiiinyl-l-oxy has been shown to have 0 = 190a and a = 19.0G,'& and the bicyclic nitroxide has been shown to have a"' = 19.8 G,264while an analogous radical has 0. = 30°.33 I t is also worth noting that acyl alkyl nitroxkles, R'C(=O)N(O)R, whlch contain the e@trowwithdrawing acyl group, have 0 = '0 but have (like (CF,)'NO) small N hfs (6.5-85 GL8 enhanced g values (2.0065-2.0075),6 and enhanced ''0 hfs' (e.g., 20.3 G for R' = CH,, R = (CH,),C).'& I t would therefore be difficult to account for the EPR parameters of (CF,),NC) purely in terms of geometric effects. Polar factors would seem to be far more important. (28) Pointed out to us by a referee. (29) Glideweli, C.; Rankin, D. W. H.; Robiette, A. G.; Sheldrick, G. M.; Williamson, S. M. J. Chem. SOC.A 1971, 478. (30) For leading references, see Douady, J.; Ellinger, Y.; Rassat, A.; Subra, R. Mol. Phys. 1969, 77, 217. Underwood, G. R.; Vogel, V. L. Ibid. 1970, 79,621. Salotto, A. W.; Burnelle, L. J. Chem. Phys. 1970, 53,333. Reference 26b. (31) Malatesta, V.; Ingold, K. U. J . Am. Chem. SOC. 1973, 95,6404. (32) Andersen, B.; Andersen, P. Acta Chem. Scand. 1966, 20,2728. (33) Lajzbrowicz-Bonneteau, J. I n "Spin-Labelling Theory and Applications", Refliner, L. J., ed.; Academic Press: New York, 1976; Chapter 6. (34) Perkins, M. J., private communlcation. (35) See also, Jenkins, T. C.; Perkins, M. J.; Terem, B. Tetrahedron Left. 1978, 2925. (36) A slightly different numerical factor for po was suggested earlier by Hayat and (37) Theoretical c a i c u l a t i o n ~have ~ ~ glven pN = 0.17 and po = 0.82. (38) Morokuma, K. J . Am. Chem. SOC.1969, 97,5412. (39) Howard, J. A. Can. J . Chem. 1972, 50, 1981. (40) Mahoney, L. R ; DaRooge, M. A. J. Am. Chem. SOC.1970, 92, 4063. See also, Nangia, P. S.; Benson, S. W. J. Phys. Chem. 1979, 83, 1138;Int. J. Chem. Kinet. 1980, 72, 29. (41) Mendenhall, G. D.; Ingold, K. U. J . Am. Chem. Soc. 1973, 95,627. (42) Such was the case with Me,CONCMe,, see Woynar, H.; Ingold, K. U. J . Am. Chem. SOC. 1980, 702, 3813.
