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A paramagnetic species in irradiated (NH4)2HPO4
J. Phys. Chem. Solids
Pergamon
Press 1963. Vol. 24 pp. 209-212.
A PARAMAGNETIC
Printed in Great Britain.
SPECIES IN IRRADIATED
(NH,),HPO,
J. R. MORTON Applied Chemistry
Division, National Research Council, Ottawa, Canada (Received 10 October 1962)
Abstract-An y-irradiated
oriented paramagnetic fragment has been detected in single crystals of (NH&HP04 at 300°K. The E.P.R. spectra of the species are characteristic of a a-electron radical
centred on a nitrogen atom. The fragment is of the form H-h:’ ,, its ’ orientation probably being determined by very weak hydrogen bonds to a phosphate ion.
1. INTRODUCTION
2. EXPERIMENTAL
years considerable interest has been shown in the E.P.R. spectra of oriented n-electron radicals. The isotropic(l) and anisotropic@) hyperfine interactions of a proton attached to the radical centre were estimated by MCCONNELL and MCCONNELL and STRATHDEE CHESNUT, and respectively. The conclusions of these authors were substantiated by observations on the fragment CH(COsH)z formed by irradiation of a single crystal of malonic acid.(a) GHOSH and WHIPFEN@) also gave a semi-quantitative interpretation of the anisotropy in the a-proton hyperfine interaction of the radical CHNH$O, formed in irradiated glycine. Recent examples have corroborated the earlier work on n-electron radicals centred on carbon, and radical geometry has been inferred@>@ from the anisotropic u-proton hyperfme interaction. Anisotropic 1aC hyperfine interactions have been measured for the radicals lsCH(COsH)2 in irradiated malonic acid(Tl*) and 13CH(SO&-, in irradiated potassium methane disulphonate.@) Relatively few nitrogen centred r-radicals are known. Examples are N(SO& in potassium amine disulphonate,@) NHSO; in potassium sulphonate(l0) and NHsSOa in a single crystal of sulphamic acid.(lO) The present work describes a nitrogen centred + m-radical H-N: which is formed as an oriented paramagnetic centre in (NH&HP04 y-irradiated at 300°K. The species was discovered in the course of an investigation of radiation damage to various phosphorus oxy-salts.(llJs) IN
in the lattice
RECENT
209
Crystals of (NH&HP04 were grown by slow evaporation of aqueous solutions. The unit cell is monoclinic,(13) containing four molecules of (NH&HP04, and the space group is Z?&/a. The unit cell dimensions are a = 8.03 A, b = 6.68 A
FIG. 1. Morphology of (NH&HP04 unit-cell directions.
crystal,
showing
J.
210
R.
MORTON
and c = ll.OZa; B = 113” 38’. Most of the crystals had the form indicated in Fig. 1, which also shows the directions of the orthogonal axes a, b, c* to which all direction cosines relate. Deuterated crystals were also tabular on (001) and were grown after several recrystallizations from DsO. Suitably sized crystals were irradiated at room temperature with 1.3 MeV soCo y-rays, dosages of up to 5 Mrads being given. The E.P.R. spectra of the irradiated crystals were recorded with a Varian V-4500 X-band spectrometer having 100 kc/s modulation. A small one circle goniometer was attached to the outside of the cavity, enabling z crystal mounted on a perspex post to be rotated about one of the axes a, b or c*. The axis of rotation was perpendicular to the main magnetic field of the spectrometer, and the spectra were recorded at intervals of 10”. The magnetic field sweep was calibrated with an automatic proton magnetometer.(14) 3. INTERPRETATION
OF THE
SPECTRA
The E.P.R. spectrum of an irradiated crystal of (NH&HP04 for the magnetic field parallel to the c*-axis of the crystal is shown in Fig. 2(a) The spectrum is seen to consist of a pair of triplets, evidence of hyperfine interaction with a 14N nucleus (I = 1) and a nucleus having I = 3 (1H or s1P). The change in the spectrum on deuteration proved the nucleus having I = 4 to be a proton. Figure 2(b) shows the spectrum of an irradiated, deuterated crystal also for the direction Ho parallel to c*. This spectrum, which is a is consistent both with the triplet of triplets, spin of the deuteron (I = 1) and its lower magnetogyric ratio, as compared to the proton. The spectra were interpreted in terms of the Hamiltonian &=
involving the interaction of the nuclei with the magnetic field, the 14N quadrupole interaction, and the interaction between the nuclei. The spectrum for a particular crystal orientation gave the value of (HO * Tk . H0)1/2 and (HO * TL *Ho)liz where Ho is a unit vector in the direction of the applied field,(ls) and Ti and Ti are symmetric, second-rank tensors. The data enabled the elements of Ti and Ti to be deduced, and their principal values and directions obtained by
-~S.g.Ho+S.TN.IN+S.Tw.IH
where fi is the Bohr magneton and HO the applied magnetic field. The first term repreeents the interaction of the electron spin S with the magnetic field HO, described by the symmetric, second-rank tensor g. The second and third terms represent the interaction of the electron spin with 14N and 1H nuclear spins respectively. These interactions are described by the hyperfine tensors TN and TH. This Hamiltonian neglects terms
b
’I FIG. 2. First derivative E.P.R. spectrum of irradiated crystal for Ho parallel to c* (a) (NH&HP04, (b) (ND&DPOd.
diagonalization. Since the principal values of T are the square roots of the principal values of T2 the elements of TN and TH could be computed. The tensors TN and TH are given in Table 1, together with their principal values and principal direction cosines (referred to the abc* axis system). The relative signs of the principal values of the hyperfine tensors could not be determined experimentally. The signs of the principal values of TN have been assumed all positive, but principal values of TH have been given negative signs in accordance with the theory@) which requires that the spin density on the u-proton be negative.
A
PARAMAGNETIC
Table 1. Hypetjine
SPECIES
and g-tensors, their prim>al
IN
IRRADIATED
values and the corresponding direction cosines in the
abc* system
Tensor
TH
g
-..
.
Direction
Principal values
-
cosines
6.3
0.0 87.3 k31.5
6.3 k31.5 28.3
(+) 22.6 + 5.0 MC/S (+)lol.o (+) 10.4
(0.810, (0.030, (0.586,
TO.257, f 0.916, f 0.307,
0.528) 0.399) -0.750)
31.6 0.0 0.0
0.0 70.6 T 14.8
0.0 i 14.8 93.9
(-) 31.6 f (-) 63.4 (-_)lOl.l
(1.000, (0.000, (0x)O0,
OmO, f 0.893, TO.437,
0mo) 0.437) 0.899)
OWOO 2.0033 ~0.0013
-O+lO23 70.0013 2.0064
(0.688, (0.253, (0.680,
kO.169, kO.856, TO.489,
-0.706) 0.451) 0.546)
18.5 TN
..--.. -_._
...___...___~..~
-
-.--
211
(NHd)aHPOl
0.0
2.0066 0 WOO -0.0023
-~ ----
~.
_. _-..
I _. ~-.--
The g-tensor was assembled by remeasuring the spectra using a crystal smeared with a speck of aa-diphenyl ,S-picryl hydrazyl, assumed to have a g-value of 2.0036. 4. DISCUSSION OF THE RESULTS 4.1 General features There seems no doubt that the spectra discussed in the previous Section* are due to an oriented n-electron radical centered on a nitrogen atom. The unpaired electron in such a fragment essentially occupies a central-atom 2p-orbital directed perpendicular to the radical plane. The r4N hyperfine tensor 7’~ has the expected axial symmetry, and the directions (0.030, f 0.916, 0.399) may be taken to be perpendicular to the planes of the two magnetically distinguishable sites observed. In spite of the considerable experimental error in determining the principal directions of the g-tensor, the direction of minimum g (0.253, 2 O-856, 0.451) was approximately parallel (angle 14.9”) to the axial direction of TN. This is in accord with observations on other r-electron radicals.(sps) It is apparent that the radical contains an u-proton, i.e. a hydrogen atom bonded to the free-radical centre. The principal values of TH are similar to those obtained for a-protons in * The fragment PO:- was also detected.(“J The sip hypefine splitting was 739 + 5 G along (0.928, f 0.257, -0.268) and 573 + 5 G perpendicular to this direction.
j.OMc/s
2.0089 + OWO8 2.0026 2.0048
-
- .._... _.._. _ _~ _
.___ _._.~_
_
-
many carbon-centred n-radicals.(s~4~s~s) It is not unreasonable to expect similar anisotropy from protons a to nitrogen centred x-radicals, and quantitative estimatesus) based on the equations of MCCONNELLand STRATHDEE(~)CO~~~M~ thisview. The direction corresponding to the intermediate principal value of TH is expected(sp4) to be perpendicular to the sr-electron radicaI plane, and from Table 1 it may be calculated that this direction is only 3.0” from the unique direction of TN. This confirms that the perpendicular to the radical plane lies in bc* and + 24.8” f 1.5” from 6. The direction corresponding to the smallest, or least negative, principal value of TH is parallel to the N-H bond, and this implies that the N-H bond of both radical sites is parallel to the u-axis of the crystal. 4.2 Quantitative features The hyperfine-tensor of the central atom of a n-electron radical is expected to exhibit axial symmetry about the direction perpendicular to the radical plane, i.e. the p-orbital density direction. Resolving the tensor into its isotropic component A and an anisotropic component B, the hype&e interaction expected for the magnetic field parallel to the p-orbital density direction is A+2B, and for the magnetic field in the plane perpendicular to this direction A-B. Assuming all principal values of TN to be positive in sign the isotropic parameter A is 45 & 5 MC/S and is a measure of the unpaired spin density in the t4N 2s-orbital, this being A’A,tom if Is polarization can be
J. R.
212
%IORTON
neglected. Aatom is the hyperfine interaction to be expected for an electron in the 14N B-orbital, and may be estimated from 2_$2(0) = 4.770 a.u.(s) to be (8?rg/3~/‘3h)#~(O) = 1540 MC/S. The ratio A/&torn thus indicates a N14 2s spin population of 0.03. The anisotropy parameter 13 is a measure of the unpaired spin-density in the 14s 2p-orbital, which is given by the ratio B/Batom. The quantity &torn is equal to (2g/ly,!Sh) (r-3)1\v Mc.‘s. The value of (7-3)~” has been estimated’s) to be 3.10 a.u. for the 14N 2p-orbital, leading to Batom = 48 Mc,‘s. The value of B depends on the sign choice for the principal values of TN.The largest principal value oi TN is certainly positive, but the two smaller principal values could be of either sign. Since the two smaller principal values of TN are nearly equal, the directions to which they correspond are not accurately determined. However, the direction corresponding to the 22 MC/S principal value of TN is more nearly parallel to the N-H bond than is the direction corresponding to the 10 MC/S principal value. Provided both smaller principal values of TX are positive in sign, this observation is consistent with polarization of the N-H bond in the sense which gives rise to negative spin density at the proton. With this sign choice, the value of B is 28 RI+, corresponding to an 14N 2p spin population of 0.58. 5. THE
NATURE
OF THE
RADICAL
In the absence of atomic coordinates for the unit cell of (NH&HP04 the exact nature of the radical formed by y-irradiation of this material remains the subject of some conjecture. Since would be expected to migrate through H-N+ the crystal at 300”K, the precisely oriented nature of the fragment may be due to very weak hydrogen bonds, as in structure I. This formulation (I) has the advantage that H-O /’
0 \p/
H-N
H-I? ‘\ ‘H-0’
’
0
II
I only one N-H
\O/p\O
bond
need be broken
to form this
fragment. On the other hand it is difficult to see why the weakly bound hydrogen atoms do not contribute to the hyperfine pattern if the (dotted) hydrogen bonds are strong enough to orient the radical. Structure II does not raise this difficulty, but in this case the 31P nucleus would be expected to contribute to the hyperfine pattern. If the structure of the radical can be represented by I or II, then one would expect that the perpendiculars to the radical planes, which lie in bc* at 5 24.8” i 1.5” from 6, would also be perpendicular to OPO planes in the undamaged crystal, provided that the phosphate ion retains its original orientation.
Acknou,led~emntts-l‘he author is grateful to Dr. L. D. for discussions on the possible crystal structure of (NH&HP04, and also to Dr. J. R. ROWLANDSfor information on the radical NHSO, prior to its publication. CALVEHT
REFERENCES 1. &'ICCoNsELL II. >:I.and CIIESNCT I). B., J. che??r. Phys. 28, 107 (19%). H. M. and STRATHL)EE J., Molec. 2. MCCONNELL Phys. 2, 129 (1959). 3. MCCONNELL H. &I.. HELLER C.. COLE ‘1‘. and FESSENDENR. WT., ‘J. Amer. &II. Sac., 82, 766 (1960). 4. GIIOSII D. K. and \VHIFFEN D. H., rli’olec. Phys. 2, 285 (1959). 5. MORTON J. Ii. and HORSFIFLD .r\., J. them. Phys. 35, 1142 (1961). 6. HORSFIELDA., MORTON J. R. and WHIFFEN D. H., Molec. Phys. 4, 327 (1961). R. \v., J. them. 7. MCCONNELL H. M. and I”ESSE!SDEN Phys. 31, 1688 (1959). 8. COLE T. and HELLER C., J. them. Phys. 34, 1085 (1961). 9. HORSPIELI) A., MORTON J. R., ROWLANDS J. R. and WHIFFEN D. H., Molec. Phys. 5, 241 (1962). J. R. and \VHIFFEN1). Ii., Xuture, Land. 10. HOWLANDS 193, 61 (1962). J. R. and ~VHIFPSZ~ D. H., 11. IIOKWIELD A., MORTON Molec. Phys. 4, 475 (1961). 12 MORTON J. R., Molec. Phys. 5, 217 (1962). 13. S&tt’rH J. P., LEHH J. R. and BRO\\.SW. E., Acta. Cryst. 10, 709 (1957). 14. HOISPIELD A., MoRTON J. R. and Moss D. G., J. sci. Znsirum. 38, 322 (1961). 15. HOLMBERCIi. \V., LIVINCS'TON R. and SMITH W. T., J. them. Phys. 33, 541 (1960). 16. ROWLANDSJ. R., private communication.
Pergamon
Press 1963. Vol. 24 pp. 209-212.
A PARAMAGNETIC
Printed in Great Britain.
SPECIES IN IRRADIATED
(NH,),HPO,
J. R. MORTON Applied Chemistry
Division, National Research Council, Ottawa, Canada (Received 10 October 1962)
Abstract-An y-irradiated
oriented paramagnetic fragment has been detected in single crystals of (NH&HP04 at 300°K. The E.P.R. spectra of the species are characteristic of a a-electron radical
centred on a nitrogen atom. The fragment is of the form H-h:’ ,, its ’ orientation probably being determined by very weak hydrogen bonds to a phosphate ion.
1. INTRODUCTION
2. EXPERIMENTAL
years considerable interest has been shown in the E.P.R. spectra of oriented n-electron radicals. The isotropic(l) and anisotropic@) hyperfine interactions of a proton attached to the radical centre were estimated by MCCONNELL and MCCONNELL and STRATHDEE CHESNUT, and respectively. The conclusions of these authors were substantiated by observations on the fragment CH(COsH)z formed by irradiation of a single crystal of malonic acid.(a) GHOSH and WHIPFEN@) also gave a semi-quantitative interpretation of the anisotropy in the a-proton hyperfine interaction of the radical CHNH$O, formed in irradiated glycine. Recent examples have corroborated the earlier work on n-electron radicals centred on carbon, and radical geometry has been inferred@>@ from the anisotropic u-proton hyperfme interaction. Anisotropic 1aC hyperfine interactions have been measured for the radicals lsCH(COsH)2 in irradiated malonic acid(Tl*) and 13CH(SO&-, in irradiated potassium methane disulphonate.@) Relatively few nitrogen centred r-radicals are known. Examples are N(SO& in potassium amine disulphonate,@) NHSO; in potassium sulphonate(l0) and NHsSOa in a single crystal of sulphamic acid.(lO) The present work describes a nitrogen centred + m-radical H-N: which is formed as an oriented paramagnetic centre in (NH&HP04 y-irradiated at 300°K. The species was discovered in the course of an investigation of radiation damage to various phosphorus oxy-salts.(llJs) IN
in the lattice
RECENT
209
Crystals of (NH&HP04 were grown by slow evaporation of aqueous solutions. The unit cell is monoclinic,(13) containing four molecules of (NH&HP04, and the space group is Z?&/a. The unit cell dimensions are a = 8.03 A, b = 6.68 A
FIG. 1. Morphology of (NH&HP04 unit-cell directions.
crystal,
showing
J.
210
R.
MORTON
and c = ll.OZa; B = 113” 38’. Most of the crystals had the form indicated in Fig. 1, which also shows the directions of the orthogonal axes a, b, c* to which all direction cosines relate. Deuterated crystals were also tabular on (001) and were grown after several recrystallizations from DsO. Suitably sized crystals were irradiated at room temperature with 1.3 MeV soCo y-rays, dosages of up to 5 Mrads being given. The E.P.R. spectra of the irradiated crystals were recorded with a Varian V-4500 X-band spectrometer having 100 kc/s modulation. A small one circle goniometer was attached to the outside of the cavity, enabling z crystal mounted on a perspex post to be rotated about one of the axes a, b or c*. The axis of rotation was perpendicular to the main magnetic field of the spectrometer, and the spectra were recorded at intervals of 10”. The magnetic field sweep was calibrated with an automatic proton magnetometer.(14) 3. INTERPRETATION
OF THE
SPECTRA
The E.P.R. spectrum of an irradiated crystal of (NH&HP04 for the magnetic field parallel to the c*-axis of the crystal is shown in Fig. 2(a) The spectrum is seen to consist of a pair of triplets, evidence of hyperfine interaction with a 14N nucleus (I = 1) and a nucleus having I = 3 (1H or s1P). The change in the spectrum on deuteration proved the nucleus having I = 4 to be a proton. Figure 2(b) shows the spectrum of an irradiated, deuterated crystal also for the direction Ho parallel to c*. This spectrum, which is a is consistent both with the triplet of triplets, spin of the deuteron (I = 1) and its lower magnetogyric ratio, as compared to the proton. The spectra were interpreted in terms of the Hamiltonian &=
involving the interaction of the nuclei with the magnetic field, the 14N quadrupole interaction, and the interaction between the nuclei. The spectrum for a particular crystal orientation gave the value of (HO * Tk . H0)1/2 and (HO * TL *Ho)liz where Ho is a unit vector in the direction of the applied field,(ls) and Ti and Ti are symmetric, second-rank tensors. The data enabled the elements of Ti and Ti to be deduced, and their principal values and directions obtained by
-~S.g.Ho+S.TN.IN+S.Tw.IH
where fi is the Bohr magneton and HO the applied magnetic field. The first term repreeents the interaction of the electron spin S with the magnetic field HO, described by the symmetric, second-rank tensor g. The second and third terms represent the interaction of the electron spin with 14N and 1H nuclear spins respectively. These interactions are described by the hyperfine tensors TN and TH. This Hamiltonian neglects terms
b
’I FIG. 2. First derivative E.P.R. spectrum of irradiated crystal for Ho parallel to c* (a) (NH&HP04, (b) (ND&DPOd.
diagonalization. Since the principal values of T are the square roots of the principal values of T2 the elements of TN and TH could be computed. The tensors TN and TH are given in Table 1, together with their principal values and principal direction cosines (referred to the abc* axis system). The relative signs of the principal values of the hyperfine tensors could not be determined experimentally. The signs of the principal values of TN have been assumed all positive, but principal values of TH have been given negative signs in accordance with the theory@) which requires that the spin density on the u-proton be negative.
A
PARAMAGNETIC
Table 1. Hypetjine
SPECIES
and g-tensors, their prim>al
IN
IRRADIATED
values and the corresponding direction cosines in the
abc* system
Tensor
TH
g
-..
.
Direction
Principal values
-
cosines
6.3
0.0 87.3 k31.5
6.3 k31.5 28.3
(+) 22.6 + 5.0 MC/S (+)lol.o (+) 10.4
(0.810, (0.030, (0.586,
TO.257, f 0.916, f 0.307,
0.528) 0.399) -0.750)
31.6 0.0 0.0
0.0 70.6 T 14.8
0.0 i 14.8 93.9
(-) 31.6 f (-) 63.4 (-_)lOl.l
(1.000, (0.000, (0x)O0,
OmO, f 0.893, TO.437,
0mo) 0.437) 0.899)
OWOO 2.0033 ~0.0013
-O+lO23 70.0013 2.0064
(0.688, (0.253, (0.680,
kO.169, kO.856, TO.489,
-0.706) 0.451) 0.546)
18.5 TN
..--.. -_._
...___...___~..~
-
-.--
211
(NHd)aHPOl
0.0
2.0066 0 WOO -0.0023
-~ ----
~.
_. _-..
I _. ~-.--
The g-tensor was assembled by remeasuring the spectra using a crystal smeared with a speck of aa-diphenyl ,S-picryl hydrazyl, assumed to have a g-value of 2.0036. 4. DISCUSSION OF THE RESULTS 4.1 General features There seems no doubt that the spectra discussed in the previous Section* are due to an oriented n-electron radical centered on a nitrogen atom. The unpaired electron in such a fragment essentially occupies a central-atom 2p-orbital directed perpendicular to the radical plane. The r4N hyperfine tensor 7’~ has the expected axial symmetry, and the directions (0.030, f 0.916, 0.399) may be taken to be perpendicular to the planes of the two magnetically distinguishable sites observed. In spite of the considerable experimental error in determining the principal directions of the g-tensor, the direction of minimum g (0.253, 2 O-856, 0.451) was approximately parallel (angle 14.9”) to the axial direction of TN. This is in accord with observations on other r-electron radicals.(sps) It is apparent that the radical contains an u-proton, i.e. a hydrogen atom bonded to the free-radical centre. The principal values of TH are similar to those obtained for a-protons in * The fragment PO:- was also detected.(“J The sip hypefine splitting was 739 + 5 G along (0.928, f 0.257, -0.268) and 573 + 5 G perpendicular to this direction.
j.OMc/s
2.0089 + OWO8 2.0026 2.0048
-
- .._... _.._. _ _~ _
.___ _._.~_
_
-
many carbon-centred n-radicals.(s~4~s~s) It is not unreasonable to expect similar anisotropy from protons a to nitrogen centred x-radicals, and quantitative estimatesus) based on the equations of MCCONNELLand STRATHDEE(~)CO~~~M~ thisview. The direction corresponding to the intermediate principal value of TH is expected(sp4) to be perpendicular to the sr-electron radicaI plane, and from Table 1 it may be calculated that this direction is only 3.0” from the unique direction of TN. This confirms that the perpendicular to the radical plane lies in bc* and + 24.8” f 1.5” from 6. The direction corresponding to the smallest, or least negative, principal value of TH is parallel to the N-H bond, and this implies that the N-H bond of both radical sites is parallel to the u-axis of the crystal. 4.2 Quantitative features The hyperfine-tensor of the central atom of a n-electron radical is expected to exhibit axial symmetry about the direction perpendicular to the radical plane, i.e. the p-orbital density direction. Resolving the tensor into its isotropic component A and an anisotropic component B, the hype&e interaction expected for the magnetic field parallel to the p-orbital density direction is A+2B, and for the magnetic field in the plane perpendicular to this direction A-B. Assuming all principal values of TN to be positive in sign the isotropic parameter A is 45 & 5 MC/S and is a measure of the unpaired spin density in the t4N 2s-orbital, this being A’A,tom if Is polarization can be
J. R.
212
%IORTON
neglected. Aatom is the hyperfine interaction to be expected for an electron in the 14N B-orbital, and may be estimated from 2_$2(0) = 4.770 a.u.(s) to be (8?rg/3~/‘3h)#~(O) = 1540 MC/S. The ratio A/&torn thus indicates a N14 2s spin population of 0.03. The anisotropy parameter 13 is a measure of the unpaired spin-density in the 14s 2p-orbital, which is given by the ratio B/Batom. The quantity &torn is equal to (2g/ly,!Sh) (r-3)1\v Mc.‘s. The value of (7-3)~” has been estimated’s) to be 3.10 a.u. for the 14N 2p-orbital, leading to Batom = 48 Mc,‘s. The value of B depends on the sign choice for the principal values of TN.The largest principal value oi TN is certainly positive, but the two smaller principal values could be of either sign. Since the two smaller principal values of TN are nearly equal, the directions to which they correspond are not accurately determined. However, the direction corresponding to the 22 MC/S principal value of TN is more nearly parallel to the N-H bond than is the direction corresponding to the 10 MC/S principal value. Provided both smaller principal values of TX are positive in sign, this observation is consistent with polarization of the N-H bond in the sense which gives rise to negative spin density at the proton. With this sign choice, the value of B is 28 RI+, corresponding to an 14N 2p spin population of 0.58. 5. THE
NATURE
OF THE
RADICAL
In the absence of atomic coordinates for the unit cell of (NH&HP04 the exact nature of the radical formed by y-irradiation of this material remains the subject of some conjecture. Since would be expected to migrate through H-N+ the crystal at 300”K, the precisely oriented nature of the fragment may be due to very weak hydrogen bonds, as in structure I. This formulation (I) has the advantage that H-O /’
0 \p/
H-N
H-I? ‘\ ‘H-0’
’
0
II
I only one N-H
\O/p\O
bond
need be broken
to form this
fragment. On the other hand it is difficult to see why the weakly bound hydrogen atoms do not contribute to the hyperfine pattern if the (dotted) hydrogen bonds are strong enough to orient the radical. Structure II does not raise this difficulty, but in this case the 31P nucleus would be expected to contribute to the hyperfine pattern. If the structure of the radical can be represented by I or II, then one would expect that the perpendiculars to the radical planes, which lie in bc* at 5 24.8” i 1.5” from 6, would also be perpendicular to OPO planes in the undamaged crystal, provided that the phosphate ion retains its original orientation.
Acknou,led~emntts-l‘he author is grateful to Dr. L. D. for discussions on the possible crystal structure of (NH&HP04, and also to Dr. J. R. ROWLANDSfor information on the radical NHSO, prior to its publication. CALVEHT
REFERENCES 1. &'ICCoNsELL II. >:I.and CIIESNCT I). B., J. che??r. Phys. 28, 107 (19%). H. M. and STRATHL)EE J., Molec. 2. MCCONNELL Phys. 2, 129 (1959). 3. MCCONNELL H. &I.. HELLER C.. COLE ‘1‘. and FESSENDENR. WT., ‘J. Amer. &II. Sac., 82, 766 (1960). 4. GIIOSII D. K. and \VHIFFEN D. H., rli’olec. Phys. 2, 285 (1959). 5. MORTON J. Ii. and HORSFIFLD .r\., J. them. Phys. 35, 1142 (1961). 6. HORSFIELDA., MORTON J. R. and WHIFFEN D. H., Molec. Phys. 4, 327 (1961). R. \v., J. them. 7. MCCONNELL H. M. and I”ESSE!SDEN Phys. 31, 1688 (1959). 8. COLE T. and HELLER C., J. them. Phys. 34, 1085 (1961). 9. HORSPIELI) A., MORTON J. R., ROWLANDS J. R. and WHIFFEN D. H., Molec. Phys. 5, 241 (1962). J. R. and \VHIFFEN1). Ii., Xuture, Land. 10. HOWLANDS 193, 61 (1962). J. R. and ~VHIFPSZ~ D. H., 11. IIOKWIELD A., MORTON Molec. Phys. 4, 475 (1961). 12 MORTON J. R., Molec. Phys. 5, 217 (1962). 13. S&tt’rH J. P., LEHH J. R. and BRO\\.SW. E., Acta. Cryst. 10, 709 (1957). 14. HOISPIELD A., MoRTON J. R. and Moss D. G., J. sci. Znsirum. 38, 322 (1961). 15. HOLMBERCIi. \V., LIVINCS'TON R. and SMITH W. T., J. them. Phys. 33, 541 (1960). 16. ROWLANDSJ. R., private communication.