Chemically Induced Polarization of I 9 F Nuclei
hemieaill adicab
1837
olarimation of Fluorine-19 Nuclei. Fluorinated Phenox
aplan, Marcia L. Manion, and Heinz D. Roth" Bell Laboratories, Murray Hill, New Jersey 07974 (Received January 9, 1914) ~ ~ b ~ ~ ccosts a t iassisted ~n by Bell Laboratories
wing uv irradiation of benzophenone in solutions containing fluorine-substituted phenols and anilines the I9F magnetic resonance spectra of the phenols or anilines showed strong CIDNP signals. These phenomena are explained by a mechanism involving diphenylhydroxymethyl and phenoxyl (anilinyl) radicals, The enhancement patterns indicate that the hyperfine coupling constants of ortho and para (meta) fluorine nuclei in phenoxyl and anilinyl radicals are positive (negative).
We have observed nuclear spin polarization effects in nated phenols and anilines and have derived from the absolute signs and the relative magnitudes of the lgFhyperfine coupling constants ( a ) of fluorinated phenoxyl and anilinyl radicals. The signs 0: isotropic hyperfine coupling constants (hfc) are essential for the theoretical understanding of electronnuclear interactions in free radicals. These signs can be derived experimentally by a number of techniques: from asymmetric esr line width variations, from esr data of radicals oriented icn single or liquid crystals, or by means of nmr contact shifts in paramagnetic systems.l However, none of these methods are generally applicable. In recent years a new technique, chemically induced dynamic nuclear polarization (CIDNP),2 has been developed. The CIDNP effect is a sensitive indicator for radical pair reactions; it can be applied to deeive the magnetic properties of short lived radicals, including the signs of their hfcs (cf., e.g., the pentafluorophenyl radical),3The theory underlying CIDNP relates the signa: direction to four parameters: the initial spin multiplicity of the pair ( p ) ?the mode of product formation ( e ) , the difference in the isotropic g factors of the individual radicals (As), nand the signs of their hfm4 If three of these parameters can be assigned, the fourth one can be derived from the observed signal direction.4 We have generated fluorophenoxyl and fluoroanilinyl radicals by phOtOb$Zdngbenzophenone ( I ) in the presence of f l u o r i n e - s u b ~ phenols ~ ~ ~ ~ t or ~ ~anilines. Uv irradiation of 1 initially produces an excited singlet state, l-&,which undergoes rapid intersystem crossing to a lower lying triplet state, I - T I . ~I n this state, benzophenone abstracts hydrogen atoms "rom a wide variety of substrates, including phenols (Z)s a,rd anilines (3)? The resulting radicals, for example, diphcnylhydroxymethy1, 4, and phenoxyl, 5, undergo radical reactions, such as coupling (k,) or hydrogen ; they diffuse apart to undergo free-radical return ( - k ~ ) or reactions ( k , l I ~ f ] .
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_.
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.'k;;b-% '5%
6
When we photolyzed solutions containing benzophenone and a series of fluorophenols or fluoroanilines in the probe of an nmr spectrometer, we observed strongly enhanced absorption signals for the ortho and para fluorine nuclei and less strongly enhanced absorption signals for the meta fluorine nuclei of the fluorine-substituted reaction partners (Figure 1).Multiplet effects4of varying intensities were superimposed on the net effects of most signals (e.g., Figure 2 ) . These observations indicate that the fluorine-containing substrates are involved in a degenerate radical-pair reaction with photoexcited 1. For the reactions discussed here three polarization determining parameters can be assigned, so that the sign of the hyperfine coupling constants can be derived from the CIDNP signal directions. The g factors of fluorophenoxyl radicals (e.g., g(C~F50.)= 2.0052)s,9 and fluoroanilinyl radicals should be larger than that of 4 (g4 = 2.0032;lOAg > 0). Because of the extremely rapid intersystem crossing from the initially excited singlet state, 1 should react mainly in the TI state generating radical pairs of triplet spin multiplicity ( M > 0). Two mechanisms of phenol (aniline) regeneration are compatible with the observation of CIDNP effects: in-cage return of a hydrogen atom after intersystem crossing of the radical pair ( E > 0) or degenerate exchange of a hydrogen atom between phenol (aniline) and cage-escaped phenoxyl (anilinyl) radicals (the asterisks indicate potential nuclear spin polarization; E < 0). 5" f 2 -+2* 4-5 Since we did not observe CIDNP effects that would indicate polarization of couRling products (6)and since the observation of CIDNP requires at least m e in-cage reaction of the radical pair, we assume that the in-cage return mechanism regenerates the reaction partners, benzophenone and fluorophenol (fluoroaniline). Given these parameters the observed CIDNP signal directions indicate that fluorine nuclei in the ortho and para positions of phenoxyl and anilinyl radicals have strong positive hyperfine coupling The Journal of Physical Chemistry, Voi. 78. No. 18. 1974
M. L. Kaplan, M. L. Manion, and ti. D. Roth
OH F
F
F
F
I
m
&-Figure 1. The 19F nmr spectrum (56.4MHz) of an argon deaerated
pentafluorophtsnol (0.01dM) solution in benzene containing benzophenone (0.01 M) in the dark (bottom)and of the same sample during ultraviolet irradiation (top).The spectrum was recorded on a Jeolco C-60 HL spectrometer containing a quartz probe and fitted with two mirrors to permit uv irradiation of the sample in the area of the receiver coil. I'ABLE I: Fluorine Hyperfine Coupling Constants (G)o f Plnernoxyl and Anilinyl Radicals Experimental
-----CsFsO.
CsHF40
Ortho Meta Para f
16.0° 5.8" 27.5"
Ortho Metb
G a H a F 0 . Ortho Meta Para
17.0'
Theoreticald
+mc
1-13.9
15.5b 5.2b 28.3b
+P
+21.2
16.6b
+mc -sc
1-17.1 -3.6
$mc
$20.3
5.96
17.0b
-so
-2.3
-S C
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26.5b
Figure 2. The 19F nmr spectrum (56.4MHz) of a benzene solution containing 2,3,5,6-tetrafluoroaniline (0.1 M) and benzophenone (0.1 M) in the dark (bottom)and during uv irradiation {top).
+P
mental and theoretical data that would reveal the spin density distribution of fluoroanilinyl radicals (7) are scarce, due in part to the experimental difficulti'es encountered in the generation of these radicals. To QUI" knowledge, the only formal derivatives of 7 that have been studied are the three isomeric nickel(I1) ~ , ~ ' ~ b i s ( ~ u o r o ~ h e n ~ ~ ) a ~ i n o t r o poneiminates (8). the I9F contact interaction constants of
$24.3
C ~ F J ~ N ICJrtho I Meta Para CsHF4NH Ortho Meta
G6HJ?NH Ortho Meta Para
+O .27f -0.0325f +O .506f
+mc 1- P
Determined by em.* Determined by esr.8 Sign and relative magnitude (large, medium, amail) determined by CIDNP, this publication. Calculation with optimized spin polarization constants.8 e Determined by esr.ll Contact interaclion constants of complexes 8.'% a
constants, whereas the meta fluorine substituents in these radicals have comparably weaker, negative hfcs. For fluorophenoxyl radicals, the signs and relative magnitudes of the hfcs derived on the basis of CIDNP results agree well with the magnitudes derived for these parameters from esr datas,g,ll and confirm the signs indicated by semiempirical calciilations (Table I).8 In contrast, experiThe Jorirnal of Physical Chemlisfry, Vol. 78, No. 18, 7974
8 these species have the signs and reIative magnitudes ( ~ F P> 3 1 ~ ~ one ~ would 1 ) expect ~ ~ in analo,g to the fluorophenoxy1 radicals. However, these complexes are certainly not typical representatives of f l ~ o r o a n i l i ~ y radicals; l the CIDNP enhancement patterns observed for fluoroanilines should represent the spin density d ~ s t r ~ ~of~ fluorine~ion substituted anilinyl radicals more reliably. The signal directions of polarized pentafluoro-, tetrafluoro-, as well as 0- and p-monofluoroanilines indicate that the lgF hfcs of the corresponding anilinyl radicals alternate in magnitude as well as sign (Table I). In each position the lgF hfcs have the opposite signs of the Corresponding lI-3 UFO
Eldor Spectra 01: Irradiated Malonic Acid44 Single Crystals hfcs13J4 and hhey appear to be proportional to the spin densities a t the adjacent carbon atoms.l* In conclusion, the CIDNP method allows for the first time an insight into the spin density distribution of simple fluorine-substituted anilinyl radicals. Since the hyperfine data derived by this method for fluorophenoxyl radicals essentially agree with iesr data and theoretical results, we are confident that OUR conclusions concerning fluoroanilinyl radicals are valid. Note Added in Pivof. R. V. Lloyd and D. E. Wood [J. Amer. Chem. Soe., 96, 659 (1974)] have observed the esr spectra of the three monofluoroanilinyl radicals and have calculations of the spin density distribution and the hyperfine coupling constants of these species. The magnitude of the hyperfine interactions derived from the esr spectra and the signs indicated by the calculations are in agreement with the results of the CIDNP studies reported here,
1839
References and Notes (1) A. Hudson and K. D. J. Root, Advan. Magn. Resonance, 5 , 1 (1971). (2) (a) A. R . Lepley and G. L. Closs, Ed., ”Chemically induced Magnetic Polarization,” Wiley, New York, N. Y., 1973; (b) H. D. Roth, Mol. Photochem., 5 , 91 (1973). (3) H. D. Roth and M. L. Kaplan, J. Amer. Chem. SOC.,95, 262 (1973). (4) R. Kaptein, Chem. Commun., 732 (1971). ( 5 ) See N. J. Turro in “Energy Transfer and Organic Photochemistry,” P. A. Leermakers, Jr., and A. Weissberger, Ed., Interscience, New York, N. Y., 1969, p 199. (6) H. D. Becker. J. Org. Chem., 32,2115,2124,2140 (1967). (7) S. G. Cohen, A. Parola, and G. H. Parsons, Chem. Rev., 73, 141 (1973). (8) P. V. Schastnev, G. M. Zhidomirov. and N. D. Chuvylkin, J. Struct. Chem., 10,885 (1969). (9) J. H. Marshall, personal communication. (IO) (a) K. Eiben and R. W. Fessendsn, J. Phys. Chem., 75, 1186 (1971); (b) R. Wilson, J. Chem. SOC.B, 84 (1968); (c) G. L. Closs, C. E. Doubleday, and D. R. Paulson, J. Amer. Chem. SOC., 92, 2185 (1970). (11) T. J. Stone and W. A. Waters, J. Chem. SOC.,213 (1964). (12) D. R. Eaton, A. D.Josey, W. D. Phillips, and R. E. Benson, MOL Phys., 5, 407 (1962). (13) R. V. LloydandD. E. Wood, Mol. fhys..20, 735 (1971). (14) H. G. Benson, A. Hudson, and J. W. E. Lewis, Mol. Phys., 21, 935 (1971).
Electron-Electron Double Resonance Investigations of Irradiated Malonic rystaisl Lawcell D. Kispert” and Pu Sen Wang Depa17mentof Chemistry, The Universlty of Alabama, Tuscaloosa, Alabama 35486 (Received December 2 1, 1973; Revised Manuscript Received May 16, 1974) Pubkation costs assisted by the University of Alabama
Intense deuterium electron-electron double resonance (eldor) spectra were obtained from irradiated malonic acid-d4 single crystals over a temperature range from -60 to 60O. This is in contrast to the previous investigation of partially deuterated radicals in irradiated I-alanine which showed intense eldor lines a t the proton hyperfine frequency while the intensity of the eldor lines at the deuterium hyperfine frequency was either weak or not observable. This was attributed to an increase in the nuclear relaxation time resulting from the smaller nuclear moment of deuterium. The intense deuterium eldor lines (upward of 40% of the esr height) of irradiated malonic acid-dd were found to result from the torsional motion of the CD2- group in -CU&OOD. This motion gave rise to two equivalent deuterium nuclei above room temperature and produced a cross-relaxation time between the nearly degenerate esr energy levels competitive with the electron spin-lattice relaxation time. Only weak deuterium eldor spectra were recorded for CD(C0OD)z due in part to the cross-relaxation time being longer than the spin-lattice relaxation time. This difference in cross-relaxation times permitted a separation of the overlapping spectra of CD2COOD and -CD(COOD)2.In general these results suggest that deuterated or partially deuterated radicals observed in irradiated crystals tend to give rise to weak deuterium eldor spectra unless torsional motion of a completely deuterated substituent is present. In addition, overlapping spectra of two or more radicals near room temperature can be separated by eldor techniques providing the spin-lattice relaxation time is considerably shorter than all other relaxation times for one of the two or more radicals.
Introduction Recently, two double resonance techniques, electronnuclear double resonance (endor) and electron-electron double resonance (eldor), have been used to increase the spectral resolution of poorly resolved esr spectra of irradiated organic crystals. Endor spectroscopy has been particularly useiful in resolving the small, hyperfine splittings which appear as part of a broad unresolved esr line.2 This
information has enabled various radical formation mechanisms to be d e d ~ c e d . ~ On the other hand, eldor studies have been used to separate the spectral components of radicals which possess large hyperfine splittings from the overlapping spectral components of other similar radicals. For instance, the overlapping spectral components of the two radical sites of cH2COOH in irradiated malonic acid were successfully ~eparated.~ The Journal of Physical Chemistry, Vol. 78, No. 78, 1974
