Electron spin resonance of γirradiated alkyl phosphates: The CH3ĊHOP(O)2OC2H5− radical in magnesium diethyl phosphate (MgDEP)F. S. Ezra and W. A. Bernhard Citation: The Journal of Chemical Physics 60, 1711 (1974); doi: 10.1063/1.1681264 View online: http://dx.doi.org/10.1063/1.1681264 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/60/5?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Photoluminescence and the thermal stability of color centers in γ-irradiated LiF and LiF(Mg) J. Appl. Phys. 82, 3722 (1997); 10.1063/1.365734 Electron spin resonance of γirradiated alkyl phosphates: The C2H5 radical in magnesium diethyl phosphate(MgDEP) J. Chem. Phys. 60, 1707 (1974); 10.1063/1.1681263 Electron spin resonance study of γirradiated alkyl phosphates: (C2H5O)O2 − radical in magnesium diethylphosphate (MgDEP) J. Chem. Phys. 59, 3543 (1973); 10.1063/1.1680518 Paramagnetic Resonance of Alkyl Radicals from Dissociative Electron Attachment in γIrradiated OrganicGlass J. Chem. Phys. 43, 2795 (1965); 10.1063/1.1697211 Electron Spin Resonance of γIrradiated Glycylglycine J. Chem. Phys. 35, 117 (1961); 10.1063/1.1731877

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Electron spin resonance of y-irradiated alkyl phosphates: The CHaCHOP{O)20C2HS- radical in magnesium diethyl phosphate (MgDEP)

F. S. Ezra and W. A. Bernhard*

Department of Radiation Biology and Biophysics, School of Medicine and Dentistry, The University of Rochester, Rochester, New York 14642 (Received 12 October 1973)

The free radical species observed by ESR after 'Y irradiation of magnesium diethyl phosphate (MgDEP) at 77°K and annealing to l600 K is CH 3CHOP(OhOCzlIs' The radicnl structure has been determined by deuterium and DC substitution in polycrystalline samples and by a single crystal analysis. Hyperfine principal values for the a hydrogen, {3 hydrogen, and phosphorous nuclei are: (A~o=-7.3 G, ii to C-H bond; A~o=-15.8 G, II to p orbital; A~o~- 27.5 G), (A~~= 25.2 G, II to C-C bond; A ~'jl =22.2 G; A ~jl=22.8 G), and (A ~ =4.7 G, II to p orbital: A~ = 6.1 G, ~ II to c: . . . P direction; A ~ = 2.8 G). From the temperature dependency of the phosphorous hyperfine coupling and the single crystal analysis, it is concluded that the predominant mechanism for transfer of spin to the phosphorous is hyperconjugation.

INTRODUCTION

Concurrent with the irreversible decay of the ethyl radical in Y-irradiated magnesium diethyl phosphate (MgDEP),l an ESR spectrum due to a second carboncen­tered radical CHsCHOP (0) 20C2H; is detected. Free radicals with similar structures have been previously reported as products formed in alkyl phosphates upon y irradiation2-4 or by reaction with hydroxyl radicals in aqueous solution. 5-7 In the present study substantial evidence for identification and characterization of the structure of CHsCHOP (0) 20C2H; is provided by the ESR analysis of Y-irradiated MgDEP in single crystals and polycrystalline samples labeled with deuterium or car­bon-13.

MATERIALS AND METHODS

Methods for growing single crystals, synthesizing of isotopically labeled MgDEP, 1 and recording of ESRdata have been reported. 8 Variable temperature studies at X band on single crystals of MgDEP were done either with the Varian variable temperature accessory or a version of the cryostat cavity described by Weil et al. 9

Single crystal data at Q band for temperatures other than 77 OK were recorded by changing the level of im­mersion of the cavity in a Dewar of liquid nitrogen. Ab­solute temperatures were not measured but the constan­cy ± 5 OK was monitored by recording the frequency of the cavity with each spectrum.

RESULTS AND ANALYSIS

Polycrystalline samples

The ESR spectrum of the CHsCHOP(O) 20C2H5'radical, after annealing the sample to 160 OK and returning to 77 OK, consists of a broad quintet of lines with a 23 G separation. The spectral intensity is an order of mag­nitude weaker than that due to the ethyl radical observed prior to annealing. Upon warming the sample, the lines in the spectrum become narrower and further hyperfine structure arising from the phosphorous nucleus can be resolved (Fig. 1). The phosphorous splitting (A p) in­creases with increasing temperature. At room tempera-

The Journal of Chemical Physics, Vol. 60, No.5, 1 March 1974

ture, the radical decays within one hour leaving no de­tectable ESR signal.

In order to establish the identity of the radical, deu­terium and a-13C labeled poly crystalline MgDEP were Y irradiated at 77 OK. Deuterated MgDEP (d 10), after being annealed to 160 OK and returning to 77 OK, gives rise to an eight line spectrum with a (7,16,27,35,35,27, 16, 7) intensity distribution and a splitting of 4.0 ± 0.3 G (Fig. 2). As was described in the previous paper be­cause of transesterification during synthesis, the MgDEP sample is only 5010 deuterated. The spectrum shown in Fig. 2 was therefore recorded at sufficiently low microwave power and modulation amplitude so as to diminish amplitudes of transitions due to the protonated CHsC:HOP"" radical. The even number of spectral lines results from a coupling of the unpaired electron to four equivalent deuterium nuclei and a spin t phosphorous nucleus. A ten line (1,5,14,26,35,35, 26, 14, 5, 1) spec­trum is expected but in this case the outermost lines are not observed.

A spectrum of 90% a_ 1Sc MgDEP taken at 77 OK, after having been annealed, is shown in Fig. 3. The large lSC hyperfine coupling parameters A~ = 44 G and A~ = 89 ± 2 G are indicative of the unpaired electron residing on 13C, adjacent to the ester oxygen. On the basis of the lSC and deuterated MgDEP spectra, it is concluded that the free radical structure is CHsCHOP~.

Ii i'JI, " I I ~, 'I

i,II:'1 1,"1

FIG. 1. ESR spectrum of CHi;HOP{O)20C2 H'5 in polycrystaI­line MgDEP 'Y irradiated at 77 OK. Second derivative scan re­corded at X band, ma=2.5 G, mp= 0.1 mW, and T=200 OK.

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1712 F. S. Ezra and W. A. Bernhard: ESR of C2HS radical

1/' v'

v' '

"'\./

H

FIG. 2. ESR spectrum of CD3CDOP(O)20C2Di; in 50% deuterated polycrystalline MgDEP "Y irradiated at 77 oK. Second derivative scan recorded at X band, ma= 2 G, mp= 0.01 mW, and T=77 oK.

Single crystal analysis

Single crystal data were obtained at Xband in the three crystallographic planes at 220 oK. Difficulty was en­countered in the attempt to determine principal hyper­fine values and direction cosines because of species splitting and second order transitions from both the a­hydrogen (a-H) and phosphorous nuclei. 10 Species split­ting results because hydrogen abstraction occurs from the carbon adjacent to the ester oxygen of either ethyl group giving rise to two distinct CHl:HOP-<E radicals: H3C(4)-HC(3)-O(2)-P-E (Species I) and H3C(2)-HC(1)­O(l)-PooE! (Species II).

Using Schonland's method, 11 approximate hyperfine principal values and direction cosines were determined for the phosphorous coupling in Species I. Crystals were then aligned along the three calculated principal axes and a complete set of ESR data was obtained at 210 OK in the three planes. These data were taken at Q band in order to reduce the second order effects in­volving the phosphorous nucleus. In this frequency range, the phosphorous hyperfine interaction is sufficiently less than twice the resonance frequency of the 31 p nucleus (- 43 MHz) such that the intensity of the outer transitions becomes negligible. 10

The square of the hyperfine splittings for a-H, i3-H, and 31p of Species I in the three planes in which the data were recorded are plotted in Figs. 4(a), 4(b), and 4(c), along with solid lines calculated from the principal values reported in Table I. Principal values and direction

-- \ "

, ' , ,

H

FIG. 3. ESR spectrum of CHPCHOP(O)20C2HS in 90% a_!3C polycrystalline MgDEP "Y irradiated at 77 oK. Second deriva­tivescanreeordedatXband, ma=6.3G, mp=O.OlmWand T=77°K.

~ ll. • • • .: , -, 60 1:'(1 IPC'

ANGLE Of ROTATION

FIG. 4. Square of hyperfine splittings for (a) a-H, (b) {3-H, and (c) 31 p of CH3CHOP(O)20C2 Hi; radical (Species I) in the three prinCipal planes of AP: O.L x P , • .L yP,6.L z p. Data re­corded at Q band and solid lines calculated from prinCipal val­ues are reported in Table 1.

cosines were determined using Schonland's method. 11

The analysis confirmed that the Q-band data were in­deed taken in the principal planes of the phosphorous hy­perfine tensor (A P

). Spectra, taken along the three principal axes of AP

, are shown in Figs. 5(a), 5(b), and 5(c). Although both Species I and II are formed, they can be resolved only along certain orientations. The stick diagrams show the lines that are assigned in Fig. Fig. 5(c) to each of the two species. Since the spectra were better resolved and the data were obtained in the

(0) IQg

, "

'i il

(b)

(c)

[I SPECIES I

I -- LL_ ~CL1 "Llil_LL,-~~_

II L~ __1 L __ ,lL.- __ lL 9 =20023 Ji.

FIG. 5. ESR spectra of the CH3CHOPOC2Hs radical in a MgDEP single crystal irradiated at 77 OK, taken along the three princi­pal axes of AP: (a) x P , (b) z P, (c) yp. Second derivative scans recorded atQ band ma=2 G, mp=O.5 mW and T=210oK. Stick diagram refers to Fig. 5 (c) only.

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F. S. Ezra and W. A. Bernhard: ESR of C2HS radical 1713

TABLE 1. Hyperfine principal values and direction cosines for the CH3CHOP(0)20C2H"5 radical (Species n.

Principal valuesa Direction cosinesb

gauss (± o. 5) a b c*

OI-H A~"'=-7 .3 -0.44 -0.03 0.90

A~"'=-15.8 0.08 0.99 0.06

A~'" = - 25. 6 (- 27.5°) 0.90 -0.10 0.44

J3-Ii A~a =22.2

A~B=22.8

A~a=25.2 0.98 -0.17 -0.12

Phosphorous ~=2.8 -0.40 0.00 0.92

~=4.7 -0.01 1. 00 0.00

A; = 6.1 0.92 0.01 0.40

"signs based on theory and empirical considerations (see text). bError in direction of principal axes ~ ± 5' • cMeasured at X band.

A P principal planes of I, the analysis was done for that species only. A complete g tensor analysis was not attempted; however, approximate principal values (gmin = 2.0020, glnter= 2. 0028, and gmax= 2. 0033± 0.0002) were observed to lie along A~, A~, A;, respectively. The gmin prinCipal axis is coincident with the p orbital of the lone electron (Table I).

The O!-H maximum prinCipal value A~", reported in Table I, is not a true measure of the hyperfine coupling because of second order effects; the data was not ob­tained in the AH

", principal planes (A P and A H '" are not quite coincident) and at Q band A~'" becomes comparable to twice the proton resonance frequency (-106 MHz). A more reliable value A~" = 27.5 ± 0.5 G is that read from a spectrum taken at X band along the yHa axis (±100). On the other hand, since there are no discrep­ancies in the X and Q band i3-H hyperfine data, it is assumed that no significant error due to second order effects have been introduced in i3-H prinCipal values. This is expected since the anisotropy in AHB is relatively small.

The O!- H hyperfine tensor is asymmetric, as is expect­ed for a static -CH moiety; however, the i3-H hyperfine tensor, within experimental accuracy, is axially sym­metric. The three 13 hydrogens are equivalent for all orientations of the dc magnetic field, indicating an in­ternal rotation of the methyl group about the C-C bond. With the knowledge of expected directions for the O!-H and 13-H hyperfine principal axes (see Table I) and atom­ic coordinates in MgDEP,12 the geometry of the HsC(4)-HC(3)-O(2) fragment was calculated (see Fig. 6). Distortion of this fragment from planarity is 2.50. The angle between ZHB and the original C(4)-C(3) bond direc­tion, known from crystallographic data, is - 100. This means that there is a slight but significant change in the geometry of the CHsCH20P- fragment upon abstraction of the O!-hydrogen atom.

Angles between Expected directions expected and of principal axes observed directions

II to C-H bond

II to p orbital

1 to C-H and p orbital

II to C-C bond 10'

to p orbital 5'

to C •.• P direction 18'

Temperature dependence studies

The spectra along the principal axes of AP were also recorded at a higher temperature (-270-280 OK). With­in the accuracy of measurement, no change in the O!-H or i3-H hyperfine coupling occurs; however, all three 3lp hyperfine principal values increase by an equal amount (A: = 5.2, A;= 6. 5, A~= 8.5 G).

A more detailed study of the phosphorous coupling temperature dependence was done at X band, with the magnetic field within 10 0 of the zP direction. The spec­trum at this orientation of the magnetic field is shown in Fig. 7 and the temperature dependence plotted in Fig. 8(a). The solid line is calculated from a linear least squares fit

A;=8.8 +0.042 T, where T is in DC.

Also plotted in Fig. 8(b) is the temperature depen­dence of the l3C hyperfine coupling, determined from the polycrystalline 0!_13C MgDEP. The variation in the two l3C hyperfine components may be described by straight lines

A~ = 85. 7 - O. 028 T,

A~ = 42.6 - O. 010 T,

where T is in DC. Both A;' and A~ decrease with in-

~:- 8/'

901-i 95' 0 (2) H---( W ((4)

FIG. 6. Two views of the H3C (4)- He (3) - 0(2) fragment in the CH3CHOP(0)20C2H"5 radical.

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1714 F. S. Ezra and W. A. Bernhard: ESR of C2HS radical

I' , 'I'

'\ , I ,Ii I

FIG. 7. ESR spectrum of CH3CHOP(0)20C2H5 in MgDEP single crystal irradiated at 77 oK. Second derivative scan recorded at X band, ma=2G, mp=0.05mW, andT=200oK. dcmagnetic field along the zP axis.

creasing temperature; however, the negative slope in A~ might not be significant since the total change in mag­nitude 2 G is within error of measurement.

DISCUSSION

Free radical structure

The geometry of alkyl radicals has been postulated and substantiated experimentally to be planar. IS Intro­duction of substituents results in both a loss of spin density from the central carbon to the more electro­negative substituent and distortion of the radical caus­ing an increase in the s character of the odd electron orbital. 14-18 This is evident from the ESR parameters of CHsCHX-type radicals, including CHsCHOP(OlzOC2H5", listed in Table ll. Upon introduction of substituents in the ethyl radical there is a consistent decrease in the magnitude of the a-H and (3-H isotropic hyperfine cou­plings due to loss in spin density and an increase in the 13C coupling reflecting an increase in the s contribution to the orbital.

HS • • Comparison of Aiso for CHsCH2and CHsCHOP(O)20C2HS

in MgDEP indicates that replacement of a hydrogen atom in the ethyl radical by an ethyl phosphate fragment re­sults in a 12% loss of 1T-spin density from the a carbon.

Deviation of the CHi~HOP(OhOCzH5" structure from planarity at the radical site was calculated directly from the principal axes direction cosines to be cp = 2. 50, where cp is defined as the average angle between the plane perpendicular to the lone electron p orbital sym­metry axis and the a bonds to C(3). There are other

TABLE II. Hyperfine parameters for CHsC~HX radicals.

0' protons (gauss)

Ars~ A H", z

AHa x

AHa y A~:o

CHsCH2 a 20.7 15.9 20.6 25.5 26.7 HSCHRc 19.0 8.4 16.6 32.0 25.8 (CH3)2COud 19.49

II -] -- ---I • CENTRAl DOUBLU AT -I 1 HIGH RESOLUTION I

I 0 AVERAGE OF >M

7 j DOUBLETS a I

3 ~----; ---~-- -~--~--~---,-" _____ 50 - \

~ A" I _____

;;;- r90 ~

~ 46~L b ,rss@ "'-- a ~S6 ~ ~ , ~

4 2 L,------,-----r,------,--~~-------,l8 4 -180 -140 -100 -60 -20 .20 '60

TEMPERATURE I'C)

FIG. 8. Temperature dependence of (a) phosphorous hyperfine coupling along z P (A~) in a MgDEP single crystal and (h) Ai and A~ in 90% a_ISC polycrystaUine MgDEP 'Y irradiated at 77 OK.

sources of evidence for this distortion, the most sensi­tive measure being the l3C hyperfine splitting. The 59 G ISC isotropic coupling in CHS

1SCHOP(O) 20C2H5", com­pared to 39 G for the ethyl radical, is a consequence of bending. Using the empirical equation formulated by Fessenden and Schuler l7 for methyl radicals from the data of Schrader and Karplus2o:

Afso (dJ)ooAfso (0)+1190 (2 tan2cp),

where A fso (0) = 39 G and Afso (cp) = 59 G/ O. 88 to compensate for the loss in spin density, one obtains a value of 6. 2 0

for the angle cp.

Another source of evidence for distortion is the ratio of (3-H to a-H isotropic hyperfine constants R ooA~:o / A~.~. Dobbs et al. have suggested that the deviation from R = 1. 2 can be used as a measure of bending of the radical. 16 In the present case Roo 1. 39 versus 1. 28 for CzHs trapped in MgDEP.

Phosphorous hyperfine coupling

The 12% spin density, lost from C(3), is most likely distributed on one or more of the oxygen atoms. Con­tributions of spin density on oxygen to the phosphorous coupling can be estimated by comparison with the p6~­radical. 21,22 The isotropic coupling due to spin polar­ization ofthe O-P a bonds should be between - 3 and - 4 G. In addition, a maximum (0.1, 0.1, - o. 2 G) aniso­tropic coupling is expected for the unpaired electron

(3 protons (gauss) IsCarbon ~a AHa

y AHa

z AfBo A C x A C

y A C z

26.0 26.5 27.6 39.07b

25.0 25.0 27.0 65.05

RCHOl!" 14.7-17.4 20.0-22.2 tH20Mef 47.2 CH3CHOP (0) (OR') g,h 17.4-18.1 23.8-24.6 CHstHOP(O)PC2Hi5i 16.9 7.3 15.8 27.5 23.4 22.2 22.8 25.2 59 44 44 89±2

aReference 8. c£Reference 14. gReference 6. bReference 13. "Reference 15. hReference 22. cReference 19. fReference 16. iThis work.

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F. S. Ezra and W. A. Bernhard: ESR of C2HS radical

TABLE III. Isotropic hyperfine parameters in RCHOPO (OR)(OR') radicals.

Aiso(gauss) Temp.

CH20P (0) (OR) (OR,)a 7.8-12.4 R. T. CH3CHOP(0) (OR) (OR,)a 13.8-17.4 H. T. CH20HCHOHCHOPO~- b 5.55 H. T. 6H20PO~-b 5.70 H. T. H2COPO(OMe)2 in PO(OMe)~ a 4.0 1200K H2COPO(OMe)2 in waterd 10.3 R. T. MeCHOPO(OEt)2 d 14.1 R. T. Me2COPO(H)OCHMe2 d 31. 9 R. T. CH3CHOP (0)20C 2H'5" 4.5 210 eK

6.7 270 e K (R I O)2P (O)CH(OH2)CCl2 f 60.9-68.9 R. T.

&Reference 6. dReference 5. bReference 7. "This work. cHeference 20. fHeference 23.

confined to one oxygen, with the - O. 2 G along the P-O bond direction.

Spin density on oxygen, therefore, accounts for less than 15% of the observed (+ 1. 7, ± O. 2, ± 1. 6 G) anisotrop­ic interaction.

Furthermore, there appears to be no correlation be­tween the principal axes of the A P tensor (Table I) and any of the P-O bond directions. If, in fact, the electron is localized on one oxygen atom [0(2)] either the xP or zP principal axis should be coincident with the P-0(2) bond direction. The angles between the respective princi­pal axes and P-0(2) are 70° and 29°. On the other hand, dipolar interaction of the odd electron in the carbon 2pz orbital with the phosphorous nucleus does account for the anisotropic coupling. Taking into account the r-3 dependence magnitudes of the experimentally determined values are comparable to those reported by Lucken and Mazeline 23 for the phosphoranyl radical cation. Also, as expected, the axis for the intermediate principal value of A P and the symmetry axis of the p orbital coin­cide. The angle between zP and the C··· P direction is 18°. Besides a possible reorientation of the fragment, the anisotropic interaction due to spin density on oxygen although small could contribute to noncoincidence of the zP and C· •• P directions.

Both isotropic and anisotropic components of the phos­phorous coupling must have the same sign along zP , since the principal value along that direction is maxi­mum. Because the dipolar interaction along C(3)' .• P should be positive, it is concluded that the isotropic coupling is also positive. A significant fraction of the interaction with the phosphorous nucleus must therefore occur through hyperconjugation. This means that spin density on oxygen and hyperconjugation contribute - 3. 6 G and + 8.1 G, respectively, to the net + 4. 5 G isotropic coupling.

The 3lp isotropic coupling increases, with tempera­ture, from 4.5 to 6.7 G while within the accuracy of measurement the anisotropic component remains con-

1715

stant. This can be a consequence of either a loss of spin density from the oxygen to the carbon atom or an increase in the distance of the phosphorous from the nodal plane of the odd electron, thereby increasing the contribution from hyper conjugation. The constancy of A He< and AH6 with temperature eliminates the former pos­sibility. If a dependency of the Heller-McConnell-type is assumed for alp isotropiC hyperconjugative coupling,

A~yper 0:: C cos2e , where e, calculated from the ESR data to be 78°, is the dihedral angle between the symmetry axis of the 2pz orbital on C(3) and the 0(2)-P bond. It follows that A~yper is very sensitive to e. A 2 ° decrease in e would account for the increase in 3lp isotropic coupling. This can be accomplished by a rotation of the HaC(4)-C(3) fragment about the C(3)-0(2) bond. Such a rotation is not expected to effect the dipolar component of the inter­action.

A comparison of the 3lp isotropic hyperfine interaction in phosphorous containing organic radicals (Table III) indicates that in most cases, regardless of the type of environment, those structures are preferred in which the phosphorous lies close to the nodal plane of the elec­tron. Notable exceptions are Mei;OPO(H)OCHMe~ and (Rl 0)2P(0)CH(ORz)CCI2, 24 where the in-plane configura­tion becomes sterically unfavorable. Deviation from that configuration is reflected in the large 3l p hyperfine coupling.

Radical stability and formation

Enhanced stability of the CHaCHOP-E radical, as de­scribed in the previous paper for the ethyl radical and attributed to the MgDEP crystal lattice, is observed. In the diethyl and triethyl acid phosphates, the CHaCHOP-E decays by being annealed to 210 OK while in MgDEP it can be detected at room temperature.

The mechanisms proposed by Haase et al. 2 for hydro­gen abstraction in alkyl phosphates are intermolecular ion-molecular reaction and intramolecular hydrogen transfer. One other possibility is hydrogen abstraction by the alkyl radical. With the present data, none of the three possibilities can be excluded. In order to help elucidate the mechanism of free radical formation in dialkylphosphates, this study is being extended to other salts of diethylphosphoric acid.

ACKNOWLEDGMENT

We thank K. R. Mercer for his technical assistance. This paper is based on work performed under contract with the U. S. Atomic Energy Commission at the Univer­sity of Rochester Atomic Energy Project and has been assigned Report No. 3490-387.

*Supported by Public Health Service, Research Career Pro­gram, Development Award.

IVv'. A. Bernhard and F. S. Ezra, J. Chem. Phys. 60, 1707 (1974), preceding paper.

2K. D. Haase, D. Schulte-Frohlinde, P. Kourim, and K. Vacek Int. J. Had. Phys. Chem. 5, 351 (1973).

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1716 F. S. Ezra and W. A. Bernhard: ESR of C2HS radical

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