- Email: [email protected]
On the 31P chemical shift anisotropy in condensed phosphates
Chemical Physics 87 (1984) 339-349 . North-Holland, Amsterdam _
ON THE 3*P CHEMICAL T.M. DUNCAN
and DC.
, _ _ --
SHIFT ANISOTiOPY DOUGLASS
IN CONDiNSiii~PHO~PHA-i-E!3
_. -
’
_
AT&T Beli Ldwrarorres. Murray HdI, New Jersey 07974. USA Received 16 December 1983
In this study, the 31P chemical shielding amsotroples in six condensed phosphates (K,P20,. Na,P20,. K,PJO,,,, (NaP03),. Al,(P,O,,),. and P40,,,) and in a phosphqsilicate glass are measured with “P nuclear magnetic resonance (NMR) spectroscopy From an analysis of spectr.: of stationary s,tmples and with samplesspinnmg at the “magic angle”, the principal components of the chemical shielding ar-. determined for the multlple species m each compound_ For each condensed phosphate, the 3tP NMR spectrum is the superposItion of chaiactenstic powder patterns of the end-, mJlddle-, and branching-phosphate units. The shielding ;S interpreted in terms of the structure of each archetypai phosphate group and a correlation is presented which shows thdt the bonding of phosphate groups in solids is well characterized by the chemical shielding amsotropy and asymmetry.
1. Introduction
Condensed phosphates are composed of (P0,)3groups linked together in pairs, chains, rings, or cages [l-4]. The geometry of each (P0,)3group is determined by the size and valence of the adjacent cations, and the length of the phosphate linkages_ This study describes the relation of the bonding in phosphates to the anisotropy and symmetry of the “P chemical shielding, as detected by nuclear magnetic resonance (NMR) spectroscopy. In a sunple model of condensed phosphates, phosphorus forms five bonds to four oxygen atoms [5]. The oxygen atoms are either bridging, in that they form linkages between two phosphbrus atoms, or terminal which bond to one phosphorus atom and form a coordination shell around the cations. Bonds to bridging oxygen atoms (0,) are single bonds and the excess bonding is distributed over the bonds to terminal oxygen atoms (0,) [5]. Consequently, the lengths of the P-O,, single bonds at 1.60-1.64 A and the P-O, distances, contracted by the partial multiple bond character, range from 1.39 to 1.54 A.
- units
in
condensed phosphates are characterized by the number ‘of P-O, -and P-O, bonds and may be categorized into three types:
(P0,)3-
0301-0104/84/$03_OO 0 Elsevier Science Publishers (North-Holland Physics PubIishing Division)
end units, middle units and branching units [l-4]. For example, phosphate units at-the end of polymer chains have one P-O, bond and three P-O, bonds, which results in C,, symmetry about the P-O, bond. The bond order of each of the three P-O, bonds in end units is approximately 4/3. At the other extreme, the branching phosphate units have three P-O, bonds of bond order one and one P-O, double bond. Branching phosphate units also possess C,, symmetry, except the axis lies along P-O, bond. Branching phosphate units are present m cross-linked phosphate chains (e.g., ultraphosphates) and in _closed cages (e.g., P,O,,). The middle umts of phosphate chains have two P-O, bonds and two P,O,‘ bonds. Middle units are approximately tetrahedral, but distorted such that the 0,-P-O, bond angle is increased to = 120’ and the 0,-P-O, angle is decreased to - 100”. Middle phosphate units have approximate qv symmetry about an axis that-bisects the-0,-P-O, and O,-P.-O, bond angles. In this study, the 31P chemical shielding is measured in five types of-condensed phosphates: di-
phosphates, _a triphosphate, a polyphosphate, a ring phosphate and a cage phosphate. The simplest conden@ phosphate is ihe diphosphate (i-e., pyrophosphate) w&h contains two end units Iinked _ B-V.
340
7: M. Duncan, D-C. Douglass / “P chenucai shrfr anuorropj
through a single oxygen atom. The next homologues in the sertes. triphosphates. are three-member chains with one middle unit and two end units. are long Polyphospates (I.e.. metaphosphates) chains of middle units. terminated by end units. Phosphates also Iink into rings. usually composed of 3. 4 or 6 middle units. Cage phosphates are closed networks of branching phosphate units. Finally. the spectrum of phosphate species dispersed in a silicate lattice is measured and compared to the condensed phosphates. The utility of “P NMR in the analysis of condensed phosphates was first demonstrated durtng the initial development of the spectroscopy. It \\as shown that the types of phosphate units could be identified by the isotropic shafts observed with aqueous solutions [6]. More recently. the anisotropy of the chemical shieldmg has also been shown to contain valuable information. For example. the amsotropy of phosphate units with Cz, symmetry in condensed phosphates (end units. cage structures and monophosphates) and in POX, compounds is correlated to the P-O bond length along the axis of symmetry [7]. A more recent study reported the “P NMR spectra measured at a high magnetic field which improved the resolution of the shielding components and magic-angle spinning was employed to identify the isotropic shifts [S]. The study revealed useful trends in the 3’P chemical anisotropy of a series of condensed potassium phosphates: end and branching phosphate units have axially symmetric shielding with the (T,,principal axis downfield and upfield, respectively. from the isotropic shift, whereas the middle units have non-axtally symmetric shielding [S]. In thts study, we describe a correlation of 3’P chemical shielding anisotropy and asymmetry with phosphate chemical type in solids that provides a mere precise indicator than that obtained from 3’P rsotropic shifts. Tlus correlation is based on the “P NMR spectra of 13 phosphate species in ten compounds: we report the static md magic-angle spinning spectra of four compounds (Na,P,O,. and a phosphosiiicate (NaPO,),, A~,W,%),, glass) and confirm a study of three compounds recently reported (K4P207, K5P30,a, and PdOu,). Several of the samples exhtbit multiple, overlapping powder patterns_ In these cases, unambiguous
assignment of the chemical shielding components is made possible by analysis of the relative sideband intensities in the spectra from the spinning samples.
2. Experimental
procedures
The ‘*P NMR experiments were performed on a Bruker CXP-200 spectrometer, wtth a 47 kG cryomagnet, at a resonance frequency of 81.015 MHz. The spectra were obtained by Fourier-transforming the accumulation of. typically, a dozen quadrature-detected free-induction decays. For each sample, the decdys were observed in separate experiments as spin echoes [9], and immediately after a 90 o pulse. The 3.1 ~LS90 o pulse (50 G) was sufficiently intense to excite uniformly the spectral region studied here. The receiver dead-ttme after the 90° pulse was 8-16 ps. The delays between the pulses in the spin-echo experiment (90P-delay-180,0-delay-observe) were set at 50 us. Both methods of obtaining the spectra contain inherent errors that are particularly significant with broad spectra. For example, truncation of the initial portion of the decay during receiver saturation results in first-order phase errors and spectral distortion. as described recently [lo]. Simtlarly, the orientational inhomogeneity of dtpolar couplings may lead to spin-spin relaxation times (T,) that vary across the spectrum and consequently, to non-umform decays in the spectral intensities in the spinecho spectra. However, these systematic errors will distort the spectra differently. Therefore, consistent results with both techniques allow one to rule out most experimental artifacts. Free-induction decays were also observed while spinning the samples at the “magic angle” to decrease the broadenmg effects of dipolar interactions [ll] and to decompose the chemical shift powder pattern into a collection of sharp peaks [12]. The rotation angle was adjusted by optimizing the ‘Li spectrum of LiBr [13]. Spectra were obtained at several rotation rates in the range 2000-4000 Hz to differentiate isotropic shifts from sidebands. The 31P NMR spectra are plotted on the (I scale for chemical shifts and “downfield” (i.e. less shielded) resonances lie to the left. The
T.M_
_ 3iP spectral param et er s are -reported -relative to 85% H,PO,, such that the 31P resonance in PF, is ppm. The principal components of the at -97 chemical shielding tensor (ur 1, a,, CT& are defined such that a,,-lies downfield and a,, lies upfield. Polycrystalline samples of two diphosphates [sodium pyrophosphate (Na,P,O,) and potassium pyrophosphate (K, P,O,)], a triphosphate (potassium triphosphate (KSP30Lo) “), a po!yphosphate (sodium metaphosphate ((NaPO,),) **), and a ring phosphate [aluminum tetrametaphosphate (Ai6P4012)3)J were purchased from Alfa Products in research-grade purity (98%). The chain lengths of each condensed phosphate can be determined from the relative concentration of weakly dissociated hydroxyl groups in salt solutions, before and after hydrolysis in concentrated acrd [14]. The results of the titrations indicate that the number of phosphorus atoms per chain are 2.0 for K,P,O,; 3.1 for K,P30,,; and 25.6 for (NaPO,),. A sample of phosphorus pentoxide (P40,0) was purchased in “puratronic” purity (99.998%) from Alfa Products and was transferred to a sample tube in an inert atmosphere. The 3’P NMR spectrum of P,O,, was acquired within 1 h after the sample was sealed. The phosphate-doped silica was prepared by gas-phase reaction of POCl,, SiCl, and O2 in He at 1400 OC. During the reactton, phosphate-silicate particles nucleate and deposit in the reactor. The particles were fused at 1900 “C into a glass of density 2.2 g/c&. The relative concentration of POCl, and SiCl, was regulated to yield a silicate glass containing = 1.0 wt % phosphorus_
3. Experimental results The 31P NMR spectra of the six condensed phosphates were measured with stationary samples and with samples spinning at the “magic angle”_ Fig.1 shows the 31P NMR spectra of two diphosphates, K4Pz07 and Na4Pz07_ The centers-of-mass of the powder patterns of K,P,O, and Na,P20, * Incorrectly
labelled by supplier as sodium hexa-
metaphosphate, (NaPO,),. QC Incorrectly - labelled by phosphate, WPO,),.
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Duncan. D. C. _Dougkzss / “P~chenucal shift amsotropy
supplier as
potassium
metn-
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-2
FREQUENCY.
IN PPM. RELATIVE
TO H,PO,
hg. 1. The 31P NMR spectra of two dtphosphates at 295 K. The magic-angle spinning spectra were observed whde rota:& the sample at 4835 Hz for K, P,O, and 3930 Hz for Na., P,O,. Calculated powder patterns for K,P,O, are obtained by leastsquares fit (sohd hne) and from sldeband mtensitles (dashed line). The two methods yield the same results for Na,P,O,. mdtcated by a smgle lme through the data
are 2.7 ppm and - 1.6 ppm, respectively_ The magic-angle spinning spectra of both compounds reveal multiple crystallographic sites in the crystal. The center peak of K,P20, has a center-of-mass of 1.1 ppm and is resolved into five peaks at -0.9, 0.0, 1.8, 3.3 and 4.8 ppm, in approximate relative percentages of 25, 25, 30, 12 and 8, respectively. The center peak of Na,P?O, is split into a doublet with peaks at -1.2 and -2.4 ppm. In the magicangle spinning spectra of both diphosphates, the resolved peaks in the sidebands have the same relative peak heights as in the center peak. The 31P NMR spectra in fig. 2 of a triphosphate, K, P,O,,, contain evidence of three different phosphate species. The three species have isotropic shifts of 1.2, 4.0 and 19.5 ppm and are present in approximate equal- amounts. The center-of-mass of the powder pattern of K,P30ro is 6 ppm.
T.&f Duncan. D C. Dougfass / jrP chemical shift anisotropy
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IN PPM
RELATIVE
200
300
TO H,POI
Fig. 2. The “P NMR spectra of a triphosphatr. K,P,O,,. at 295 K obsened with a stationary sample (top) and ulth a sample spuming at 3800 Hz (bottom). The center peaks in the magic-angle spinrung spectrum are denoted by asterisks. The tunes (a), (b) and (c) are powder patterns based on the pnncipal components obtamed from the stdeband mtensittes for the center peaks at 1, 4 and 19 ppm, respectively.
Fig. 3 contains the 31P NMR spectra of a chain polyphosphate, (NaPO,),. It is observed that the widths of the peaks in the ma@c-angIe spectrum are about four times as broad as those in the two previous figures. Broadening caused by dipolar couplings to the quadrupolar 23Na nuclei is not removed completely by magic-angle spinning if the nuclei exist in lattice sites with large electrostatic field gradients [X5]. The powder pattern is centered at 18 ppm and the magic-augIe spinning spectrum reveals that the isotropic shift of the maJor species in (NaPO,), is at 19 ppm and a minor species is observed at -2 ppm. The 31P NMR spectra of the nng metaphosphate, AIJ(PdO12)3, shown in fig_ 4, are similar to those of (NaPO,),. The relatively large residual widths of the peaks in the magic-angle spinning spectrum suggest that the aluminum cations are in lattice sites with non-zero electrostatic field gradients The magic-angle spinning spectrum of A1,(P,0,2)3 reveals two isotropic shifts at 51 ppm and at 30 ppm. Smce the isotropic shift of the spectrum of the stationary sample is 48.5 ppm, the sample is composed of = 90% species with an
(NOPO,l,
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-100 =REO”ENCI
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Fig. 3. “P NMR spectra of a potyphosphate, (NaPO,),. obsened W&I a stationary sample and wble rotating the sample at 3690 Hz. The calculated hne plotted through the expcnmental data is the sum of the two theoretical pouder patterns (a) and (b).
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Fig. 4. 3’P NMR spectra of a ring metaphosphate, AI,(P,O,,),, observed with a stationary sample and whle rotating the sample at 3970 Hz. The calculated hne plotted through the expenmental data is the sum of the two theoretical powder patterns (a) and (b).
shoulder_around -kO ppm. Spectra~measured with other samples of‘ P,O,h, or= with m_ois&&added _ deliberately, reproduced precisely_ the spectrum in - _ . : ; __ fig. 5. -_Fig.-6 shows the 3tP~NMR--spectnun of phos-. phate-doped silica. The spectrum is similar_in shape and width to that of P,O,, and has a center-of-mass of 32 ppm. Because of the-irregularshape of the fused glass; it was not possible-to spin the sample.
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-200
Fig. 5. “P
NMR
4. Discussion
300 TO H,PO.
spectra of P,O,,
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* -_
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stationary sample (top) and with a samp.e +. _ug at 3750 Hz (bottom)_ The curves drawn through the data represent theoretical powder patterns obtained by a least-squares fit (solid hne) and by analysis of stdeband intensities (dashed line).
isotropic shift of 51 ppm and 10% species with a shift of 30 ppm. The intensity of the peak at 30 ppm and its sidebands are apparently attenuated in the magic-angle spinning spectrum, probably by interactions with 27Al nuclei. The chemical shift powder pattern of the cage phosphate, P,O,,, shown in fig. 5, has the opposite “directionality” of the diphosphates shown in Fig. 1. The isotropic shift determined from the magicangle spinning spectrum is 46 ppm, whereas the center-of-mass of the powder pattern is 33 ppm. The shape of the spectrum of P,O,, deviates from a theoretical powder pattern (to be discussed in the following section), particularly at the upfield
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Fig. 6. The “P NMR spectrum of phosphate dispersed in a silicate lattice. The curve drawn through the data represents a theoretical powder pattern obtained by a least-squares fit.
4.1. Interpretation
of isotropic shifts
Early in the study of the 31P NMR spectra of phosphorus compounds it was shown that the isotropic shifts of condensed phosphates measured in solutions are related to the type of phosphorus units [6]. Isolated (P0,)3groups have shifts of -6 ppm and diphosphates, (0,POP0,)4-, are typically between 5 and 7 ppm [14,16]. The average isotropic shifts of the two polycrystalline diphosphates measured in this study are 1.1 ppm for K4Pz0, and -1.6 ppm for Na,P,0,, which are shifted downfield from the solution values. The crystal structure of Na,P,O, determined from X-ray diffraction studies indicates that the phosphate groups in each pair are inequivalent [17]. Consequently, we interpret the splitting of the center peak of the spectrum of Na,P,O, as evidence of the structural inequivalence *_ A similar crystallographic inequivalence resulted in a splitting of 3 ppm between the 31P isotropic shifts of the two end groups in the diphosphr+e anion of a-Ca,P,O, [18]. A recent study of K4Pz0,- revealed two doublets in the center peak of the magic-angle spinning spectrum [8]. Although the crystal structure has not been reported, it was assumed that similar inequivalences exist, and a doublet at 0.3 and 2.2 ppm was assigned to the
* An alternative interpretation of the splitting is the 31P-31P scalar coupling. However,
we believe that this coupling,
whichis a higher-order, two-bond interaction, is too weak to bc. observed in these spectra. For example, the peak assigned to the middle group of KsP,O,, IS not split by the end groups.
344
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Duncan,
D. C. Dougkzrs
two ends in the diphosphate anions of K,P,O, and a doublet at 2.4 and 4.9 ppm was interpreted as the two end groups m the hydrated compound. K,P,O, - 3H,O [S]. Wtth respect to this interpretation, the peaks we observe at -0.9 and 0.0 ppm. and 3.3 and 4-S ppm are consistent with the previously tdentified doublets_ Howeve;. the peak at 1-S ppm n-t the K,P,O, spectrum represents ~1 third. yet unidentifted diphosphate spectes, with much less mequivalence between end groupsIn aqueous salt soluttons, there are the resonances for the triphosphate ion. u hich are assigned as end phosphate umts at 4-6 ppm and the middle phosphate unit at lS-21 ppm [14.16]. Therefore. in the spectrum of K,P:O,,, specres with isotropic shifts of 1.2 and 4-O ppm are mterpreted as end units and the peak at 19.5 ppm is attnbuted to the middle group. These shifts nre in excellent agreement with the peaks at 1.2. 4.2 and 19.4 ppm reported recently [S]. Sinnl.trly. the peaks at 19 and -2 ppm in the spectrum of (NaPO,),, are interpreted as the middle units and end units of the polymer chains. respectively. The relative intensities of the peaks m the spectrum of (NaPO;),, (92: S) agree with the average chain length of 26 measured by tttrdtton. The isotropic shifts observed m thi\ study for the end and middle units agree with the values measured for solutions_ In d previous study of polycrystailine the tsotropic values did not agree
/ A P chemicai
shaft anisorrop_b
The isotropic shift observed for P,O,, (46 ppm) compares well with a value of 46 ppm reported previously [S]. The isotropic shift (32 ppm) of the phosphate dispersed in the silicate lattice does not indicate unambiguously the type of phosphate unit. For this interpretation, we must consider the anisotropy and asymmetry of the chemical shielding. 3 2.
Prrncrpal
components
of
the chemical
shift
powder patterns
The chemical shift contribution to the NMR spectrum of a polycrystalline sample may be defmed in terms of the shie!ding along the three principal axes of the molecule [20]. The intensity at each frequency in the spectrum is determined by the relative differences between the principal shielding components. One method to determine the principal components involves a least-squares fit of the general expression for the powder pattern to the experimental data. However, this procedure may lead to systematic errors when fitting the spectra of samples with multiple species or with spectral deviations resulting from motronal averaging or inhomogeneous dipolar coupling. Alternately, the principal components can be determined from the relative intensities of the adebands in the magic-angle spinning spectra [21]. In this regard. the most straightforward approach is the graphical procedure based on the intensities at the first few sidebands [22] The isotroptc shifts of the different species in the diphosphates K,P,O, and Na,PzO, are separated by only d few ppm. which indicates similar average shielding_ Also, since the peaks corresponding to different spectes maintain the same relative heights in the sidebands, one obtains the same anisotropy and asymmetry for the powder pattern of each species. Therefore. the observed lineshapes in ftg. 1 have each been represented with a smgle theoretical powder pattern. The powder pattern for Na,P,O, derived from sideband Intensities is consistent with the results of a least-squares fit. However, the two methods yield different lineshapes for K,P20,. Since both techniques predict the same values for the relatively well-defined upfield components, the more valid results may be judged by accuracy of the isotropic
-_
TX. Duncan, D.C. Doughs
/
"P
chemical shrfz aksorripy
1
_
_
345
- > shifts, calculated from. the average of the three components. The least-squares -results-predict- an isotropic shift, that differs by 5 ppm from the actual value, and thus is in error. The _isotropic shift predicted by-magic-angle spinning is correct, by definition_ In addition, the results for K,P*O, = 43 ppm), concur with (all =82 ppm, ~,=a~~ the recently reported components_ at 84 and 42 ppm *_ The ‘IP chemical shit parameters for Na,P,O,-and K,PzO, and the other condensed phosphates are summarized in table 1. The calculated powder patterns of K,P20, and Na4Pz07 suggest axially symmetric shielding. That is, two of the components (uz-, and u,,, in this case) are degenerate_ The confirmation of degenerate components is difficult with either procedure for calculating powder patterns_ For example, with
.
a least-squares fit, -the accuracies are limited by the residual dipolar broadening-in the powder, pattern,. which blurs-any evidence of separation between u,- and u3, into a single- rounded peak; Similarb~, the graphical analysis of sideband -intensities requires one to determine the point of intersection of interpolated contour lines that‘ meet at small angles. In this study, the confidence limit on the separation between two components quoted as degenerate is approximately equal to the gaussian broadening listed in table 1. The 31P chemical shielding in the diphosphate cu-Ca,P,O,, determined by rotation patterns of a single crystal. is not axially symmetric [18]. The overall symmetry (us3 - a,,) in cz-Ca,P,O,, given in table 1, is comparable to Na,P,O, and K,P,O,. However, the upfield components of the two sites
* See footnote c of table 1. Table I “P Chemicai shielding parameters Compound
Phosphate group
Na,F$O, a-CaaPaO,
KSP30,0
WaP4L W-3),
=)
p.%o,o
(P0,)3-
m sihca
Percent of total
isotropic shift II’
25
-09
25
end end end end end end end end end end mtddle middle
50 50 50 50 33 33 33 92
-1.2 - 2.4 18 21 12 4.0 19.5 19
end middle middle ring middle ring ring
8 50 50 90 10 33 33
-2 18.5 20.7 51 30 18.2 19.9
33 100 100
21.2 46 32
ring branching branching
30 12 8
0.0 18 33
Pnnctpal components 011
a=
Broadening (kHz) b’
Ref.
Q33
-82
43
43
105
-80
37
37
1 62
-
65 67 55 51 148 160 45
0.98 098 069 160 160
WI WI
-95
32 45 55 51 -16 -13 45
-84
-30
180
-
-40 -15
20 -15
175 120
0 89 0 89
-90
-38
185
-
-66 -37
-35 -37
240 190
0.40 0.55
48
-42 -48 -106 -90 -75 -90
a’ In ppm, relative to_H,PO,; determined from magic-angle spinning spectra. b, half-width of the gaussian broadening convoluted homogeneously into the powder pattern; 10 ppm = 0.81 kHr =’ The principal components are con&ted from the data in ref. [S] with the following relations: o,, = F_- f(1 + q)Au. a, v)Au. and I+ = 5 + $Ao.
PI
I*1
= a - f(1-
_-
346
T-M
Duncan. D-C_ Doughs
in a-Ca.P,O, are separated by 33 and 22 ppm. compared -to no detectable separation in the diphosphates measured in this study. Similarly. the sphtting between the isotropic shifts is = 1 ppm for Na,P,O, and K,P,O,. and = 3 ppm for (YCa,P,O,. The basis for these differences is the geometry of the diphosphate anion in Na,P,O, and a-Ca,PzO,_ In Na,P,O,, the P-O, bond lengths are equal withm standard deviation (0.002 A). the 0,-P-O, angles differ by less than 2.5”. and the 0,-P-O, angles lie in a range less than 6 o [17] indicating near-C,, symmetry- The diphosphate anion in cu-Ca.P,O, also has an eclipsed _ _ geometry of the two (PO,)‘groups. but there is deviation from C,, symmetry [23]. The P-O, bond lengths differ by 0.02 and 0.045 A for the two end groups, and the 0,-P-O, angles vary by 6 O. Although the bond lengths and angles for K,P?O, have not been reported. the “P results suggest that the variances are small. similar to the structure of Na, Pz07. Since the magic-angle spinning spectrum of KSP,O,,) reveals three species. the spectrum of the stationary sample IS the superposition of three powder patterns. The po\\der patterns predicted from the sideband intensities are given by curves (a). (b) and (c) in fig. 2. The theoretical spectrum IS obtained by summing the pounder patterns and ddJustmg the broadening. which lieIds dn accurate fit to the experimental spectrum. Spectra (2) and (h) correspond to the isotropic shifts of - 1.2 and 4.0 ppm and therefore dre the powder patterns of the end units of KsP,O,,,. Again the components determined here are in accord with the results of a recent study [S]. We measure - 75. - 16. and 14s ppm for the components of the middle phosphate unit of KSP,O,,,, compared to - 67, - IS. and 148 ppm ‘_ The previous study did not resolve the differences betwern the end groups. but the reported values of -97. 49 and 49 pprn agree with the averages of the components for K,P,O,, end groups in table 1. As expected, the end units of the triphosphate have a shielding simiiar to the diphosphates in fig. 1. The powder pattern of the middle unit of K,P30,, has a chemical shielding remarkably similar to that of the major species of the metaphosphate (N&PO,),. * See footnote c of t.lble I.
/
“I’
chemical
shaft anirorrop_t
The theoretical chemical shielding powder pattern of the middle units of the polyphosphate (NaPO,), is given by curve (a) in fig. 3. Curve (a) is calculated from the principal components derived from the sideband intensities in the magicangle spinning spectrum_ with broadening determined by least-squared methods. It is not practical to determine the chemical shielding anisotropy of the end groups because the sidebands are weak_ Instead_ the powder pattern, curve (b), is determined by a least-squares fit to the difference between the experimental spectrum and curve (a)_ The superposition of curves (a) and (b) is given by the line drawn through the data in fig. 3. The 31P NMR spectrum of the sample of Al,(P,,0,2)3 is not as adequately represented by a superposition of theoretical powder patterns. The shoulders of the powder pattern predicted by the sideband intensities (curve (a) in fig. 4) correspond to locations of the shoulders in the experimental data. but the intensities are in error. This is to be expected since the magic-angle spinning spectrum indicates that the spectrum is the sum of two powder patterns_ However. the residual spectral intensity has a center-of-mass of = 20 ppm. which is not consistent with the isotropic shift observed dt 30 ppm_ The best fit to the residual intensity, with the restriction that the isotropic shift be 30 ppm, is given by curve (b) in fig. 4. The sum of curves (a) and (b) agrees with the overall shape of the experimental spectrum, but there is some discrepancy in intensities_ Possible causes of the inhomogeneous broadenmg include the orientational dependence of the dipolar coupling between 3’P and “Al or experimental artifacts_ The calculated principal components of the chemical shielding of P,O,, depend on which approach is taken. A discrepancy appears in the position of the upfield component, as shown by the two theoretical curves in fig. 5. The leastsquares fit yields a value of a,, = 215 ppm, whereas the graphical approach yields a value of 240 ppm. However, the isotropic shift of the least-squares results differs by 8 ppm from the center peak in the magic-angle spinning spectrum. On this basis, the principal chemical shift components obtained from the sideband intensities of several magic-angle spinning spectra are quoted in table 1.
T_M_ Duncam_D.C: Douglass / 3tP chemical :ht/ &isotropy
The spectrum of P40r0 contains anomalous, but reproducible deviations from an ideal shielding powder pattern. Unfortunately, although‘ the shielding components for P,O,, have been reported previously [7,8], no spectra have been published. Until a satisfactory model is proposed for the modulations in the spectrum of P,Ou,, it may be tenuous to interpret the results of fitting a single, unaveraged powder pattern. With this caveat, we present the following preliminary comments on the chemical shielding of P,O,,. The relatively large anisotropy of P.,O,, observed here is comparable to the anisotropy reported previously: we calculate an upfield component at 240 ppm compared to 243 ppm [S] *_ However, previous studies report that the downfield components are degenerate (e,, = oz2 = 52 ppm) [7,8] whereas we detect a splitting between CT,,and azz- That is, there is a distinct shoulder in the experimental data at -66 ppm, which implies that the branching phosphate unit in solid P,O,, does not have C,, symmetry. It is noted that electron diffraction studies of gaseous molecules of P_,O,, indicate C,, symmetry about the P-O, bond [24]. An alternate interpretation of the chemical shielding, although less hkely, is that there exists more than one phosphate species in P,O,, l&h the same isotropic shifts (_t 1 pm), but with different chemical anisotropies. The 3’P NMR powder pattern of phosphatedoped silica corresponds well with the fitted theoretical spectrum, as shown in fig. 6. Although a spectrum was not measured while spinning at the “magic-angle” to resolve the isotropic shift(s), the accuracy of the fit indicates that the phosphate exists as a single, well-defined species. That is, if there were other phosphate species with different chemical shieldings, the superposition would result in spectral aberrations. For example, the presence of the minor species in (NaPO,), and A1,(P,0,2)3 which represent = 5% of the phosphorus cause distinct effects in the experimental spectra. Finally, because of the large anisotropy and axial symmetry of the chemical shielding, we propose that the phosphate dispersed in the silica is a
* See footnote c of table 1.
_
347
branching phosphate unit. This interpretation is _ consistent with- infrared -studies- of similarly-prepared phosphbsilicate glasses that reveal a band at =_1325 cm:‘, interpreted_ as the stretch of the p--O double bond [25]. It is interesting -that the shielding anisotropy of branching phosphate in the amorphous silicate lattice is less than in the four-member cages of P,O,,. 4.3. Interpretation of the chemical shielding anisotropy The chemical shielding of the three different phosphate units measured in this study are consistent with previous results for condensed phosphates, such as (KP4),, (KPO,), [7,8] and cyCa,P,O, [18], which are included in table 1. Also, the spectra of the middle phosphate groups agree with phosphate groups in organophosphate diesters [26-291 and monoesters [29,30]. For a qualitative understanding of the relation between the bonding geometry and the chemical anisotropy, an insightful approach [7,18] is to consider each phosphate group as perturbation of a -tetrahedral (P04)3- ion. The ideal (P0,)3tetrahedron has four equal P-O, bonds, each with a bond order of = 5/4. The isotropic shift of the (P0,)3tetrahedron in K,PO, is - 10 ppm and, as expected, the chemical shift anisotropy is zero [7,8]. An end phosphate unit may be obtained by transfer of the T-bond character from one bond to the other three bonds. The data in table 1 show that this transfer increases the chemical shielding anisotropy (u3s or,) to = 120 ppm for end groups in diphosphates and to = - 150 ppm for the ends of longer linear chains. The shielding parallel to the single bond moves downfield from -10 to = -65 ppm for diphosphates and to = - 100 ppm for the ends of the longer linear chains. Similarly, the shielding perpendicular to the single bond increases from - 10 to 55 ppm for all end groups. In general, the chemical shielding is determined by the orbital angular momentum of the electrons [31], which in part is related to the electron density. For the end phosphate groups, transfer of the r-bond character from the symmetry axis to the -other bonds decreases the shielding parallel to the axis and in-
creases the perpendicular shielding. Continuing this line of analysis, a branching phosphate unit is formed from a (PO,)‘tetrahedron by transforming all the T-bond character to one P-O, bond. The anisotropy of the branching phosphate increases relative to the tetrahedron from 0 to = 300 ppm. over twice as large as that of the end units. The transfer of electron density is opposite from the end units and consequently the orientation of the -“P powder pattern has reversed. Transfer of the T-bond electrons to the P-O, bond on the symmetry rtvis increases the shielding parallel to the axis and decreases the perpendicular shieldmg. The chemical shielding of the middle phosphate unrts reflects the same trends as the end and branching groups. although the interpretation is not as straightforward_ Since there is no axial symmetry. one does not know a priori the orientation of the principal axes for the chemical shielding relative to the molecular However, analogues
coordination
the axes have been determmed for middle
umts
PHOSPHATE GROUP END
-
If
.
MIOOLE
-.
RING
.
D .
BRANCHIUNG 1 0
t 100 (ff33-ff22)
-
I 200 IN
3bO
PPM
Fl_e 7 A correlauon betueen the type of phosphategroupand the difference betueen the upfield (Q) and intermediate (CT& chemical stueldmg components.
changes in anisotropy and asymmetry as the series progresses from end phosphate to branching phosphate species_
5. Summary
frame. for three
in condensed
phos-
phates- one organophosphate diester with the for(barium diethyl phosmula M” (O,P(OR),)‘phate [25]) and two monoesters of formula (02P(OR)(OH))‘(aminoethyl phosphate [27] and deouycytidine 5’-monophosphate [30]). From rotatlon p&terns of single crystals. it was determined that the a~& of the upfield component (Use) lies in the 0,-P-O, plane, approximately ( < + 10”) bisecting the angle. and the axis of the downfield component (a,,) lies in the 0,-P-O, plane [26.27,30]. Again. the same trend prevakr sh:eldmg parallel to an axis with multiple-bonding is Increased and shielding perpendicular to the itx~s is decreased. The spectra described in ths study indicate that the type of phosphate unit cdn be identified by the positlon of the three principal components (T,~. uzz and uJJ_ Or. one may consider the anisotropy (Au) and the asymmetry parameter (17) to ascertain the geometry of the phosphate ion. However. on the basis of the compounds measured to date. it appears that the phosphate type can be correlated to the difference between two pnncipal components, us3 and uz2, as shown in fig. 7. The difference the simultaneous between uJ1 and uzr reflects
The densed
31P NMR
powder
phosphates
and phosphate
patterns
of
stx con-
dispersed
in a
silicate lattice have been measured and analyzed to extract the principal components of the chemical shielding tensors. The spectrum of each compound is the superposition of any of three characteristic powder patterns corresponding to the type of phosphate units present: end-units. middle-units. or branching-units. The end-units, which terminate linear phosphate polymer chains, exhibit axially symmetric shielding such that the shielding perpendicular to the symmetry axis is upfield. The branching phosphate groups also have axially symmetric shielding, but the shielding perpendicular to the symmetry axis is downfield. The phosphate units in the middle of chains do not possess axial symmetry and consequently three distinct shielding components are observed, with the same general trend. That is, orientations perpendicular to dn axis with multiple-bonding are deshielded and the shielding is increased parallel to the axis. The converse is observed for orientations relative to an axis along a single bond_
Early studies [6] demonstrated that the endand middle-phosphate units could be identified by the “P isotropic shifts observed by analyzing the compounds in solution_ In this study, it IS shown
_
_ _ TM_
that the end-, middle- and branching-units in solids are well characterized by their distinctive powder patterns, owing to shielding anisbtropy. Furthermore, the anisotropy of the shielding provid_a a less ambiguous distinction between the three phosphate groups, compared to the isotropic shifts_ That is, the ranges of the isotropic shifts of the three groups in solids overlap: end groups range from = 0 to 20 ppm, middle groups from = 10 to 50 ppm and branching ppm. Alternately, the principal species
components are divided
into
units
from
differences a,, and a, three distinct
=
30
to 45
between the for the three regions:
end
(O-35 ppm), middle groups (135-225 ppm) and branching groups (230-295 ppm). Furthermore, the middle groups bonded into rings lie in the upper portion of the range, from = 195 to 225 ppm. It is conceivable that future NMR studies of other condensed phosphates may expand the regions of the middle and branching phosphates shown in fig. 7, thus resulting in some overlap. However, It seems likely that any ambiguity can be resolved by considering the asymmetry parameter of the “P NMR spectrum: the powder patterns of middle units should reveal a separation between components q1 and uZZ on the order of 50 ppm, whereas these components should be more nearly degenerate (separated by less than 20 ppm) in the spectra of the branching phosphate units. If the class of phosphates to be characterized is extended to include substituted species, such as the biologically important hydroxyapatites, cross-polarization data should be added to the set of pertinent information for structural determination, as demonstrated previously [32]. groups
References [I] JR. [2] [3] [4]
[S]
_
Duncan, D C__Dotigkzs.s /
Van Wazer. Phosphorus and its compounds (Whey-Interscience. New York, 1958) D.E.C. Corbridge. Structural chemistry of phosphorus (Elsevier. Amsterdam, 1974). E Thdo. Advan. Inorg. Chem. Radmchem. 4 (1962) 1. J. Majling and F. Hanic. in: Topics in phosphorus chemistry, Vol. 10. eds. M. Grayson and E.J. Griffith (Wdey, New York, 1967) pp. 341-502 D.W J. Cruickshank. J. Chem Sot. (1961) 5486.
_
- --
-_ .
_
--._ J’P chenttcai shtft anisotropy
349
WI J R. Van_War,
,
C F.._CaiJis and -J $. Shoo!ery. J. Am. : Chem. sot. 77 (1955) 4945;. MM. Crutchfield, C.F. Calhs, R.R. Irani and G C. Roth, Inorg. Chem. 1 (1962) Si3: _ 171 A.-R. Grimmer. Spectrochim. Acta 34A (1978) 941; . A.-R. Grimmer, Proceedings of the 20th Congress Amp&e. Talhn. 1978. p_ 483. I81 A -R. Gnmmer and U Haubenreisser, Chem. Phys. Letters 99 (1983) 487. 191 E.L Hahn, Phys. Rev. 80 (1950) 580. DOI M. Rance and R A. Byrd, J. Magn. Reson. 52 (1983) 221. IllI E.R. Andrew. Progr_ Nucl. Magn. Reson Spectry. 8 (1971) 1. [121 E Lippmaa. M. Alla and T_ Tuherm, Proceedings of the 19th Congress Amp&e. Heidelberg. 1976. p- 113. EO. SteJskti, J Schaefer and R A. McKay, J Magn. Reson. 25 (1977) 569. 1131 J S Frye and G E. 1Maclel.J Magn. Reson. 48 (1982) 125. r141 S. Greenfield and M. Chft. Analytical chemistry of the condensed phosphates (Pergamon Press. New York, 1975). I151 N. Zumbuiyadls. P.M. Henrichs and R.H. Young. J. Chem. Phys. 75 (1981) 1603. 1161 M.M. Crutchfield. C.H. Dungan. J-H. Letcher. V. Mark and J R. Van Wazer. m: Topics m phosphorus chenustry, Vol 5, eds. M Grayson and U. Gnfhth (Wtley. New York. 1967) pp 319-321. 1171 K Y Leung and C. Calvo, Can. J. Chem. 50 (1972) 2519. VI S J. Kohler, J.D. Ellett Jr. and M.P. Kiem. J. Chem. Phys 64 (1976) 4451. 1191 E R. Andrew. D J. Bryant, EM. CashelI and B.A Dunell. Chem. Phys. Letters 77 (1981) 614 [20] N. Bloembergen and J A. Rowland, Acta Metall. 1 (1953) 731. 1211 M.M Maricq and J S. Waugh. J. Chem. Phys_ 70 (1979) 3300. [22] J. Herzfeld and A E- Berger, J. Chem. Phys. 73 (1980) 6021. 1231 C. Calve, Inorg. Chem 7 (1968) 1345. [24] B Beagley, D.W.J. Cruickshank and T-G. Hewitt. Trans. Faraday Sot. 63 (1967) 836. [251 AS. Tenney and M. Gheuo. J. Electrochem. Sot. 120 (1973) 1276. [26] J. Herzfeld, RG. Gnffm and R.A Haberkorn, Biochem. 17 (1978) 2711. [27] S J. Kohler and M.P. Klem. Biochem 15 (1976) 967. (281 B.T. Nail. W.P. Rothwell, J S. Waugh and A. Rupprecht, Bmchem. 20 (1981) 1881 [29] S J. Kohler and M P. Klein, B&hem. 16 (1977) 519. [30] P TutunJian, J. Tropp and J.S. Waugh. J. Am. Chem. Sot. 105 (1983) 4848. [31] C P. Slichter, Principles of magnetic resonance, 2nd Ed.
(Springer, Berlin, 1978).
[32] W P. Rothwell. J S. Waugh and J-P_ Yesinowski. J. Am_ Chem. Sot. 102 (1980) 2637; J Tropp. N.C. Blumenthal and J.S. Waugh. J. Am. Chem. Sot. 105 (1983) 22.
ON THE 3*P CHEMICAL T.M. DUNCAN
and DC.
, _ _ --
SHIFT ANISOTiOPY DOUGLASS
IN CONDiNSiii~PHO~PHA-i-E!3
_. -
’
_
AT&T Beli Ldwrarorres. Murray HdI, New Jersey 07974. USA Received 16 December 1983
In this study, the 31P chemical shielding amsotroples in six condensed phosphates (K,P20,. Na,P20,. K,PJO,,,, (NaP03),. Al,(P,O,,),. and P40,,,) and in a phosphqsilicate glass are measured with “P nuclear magnetic resonance (NMR) spectroscopy From an analysis of spectr.: of stationary s,tmples and with samplesspinnmg at the “magic angle”, the principal components of the chemical shielding ar-. determined for the multlple species m each compound_ For each condensed phosphate, the 3tP NMR spectrum is the superposItion of chaiactenstic powder patterns of the end-, mJlddle-, and branching-phosphate units. The shielding ;S interpreted in terms of the structure of each archetypai phosphate group and a correlation is presented which shows thdt the bonding of phosphate groups in solids is well characterized by the chemical shielding amsotropy and asymmetry.
1. Introduction
Condensed phosphates are composed of (P0,)3groups linked together in pairs, chains, rings, or cages [l-4]. The geometry of each (P0,)3group is determined by the size and valence of the adjacent cations, and the length of the phosphate linkages_ This study describes the relation of the bonding in phosphates to the anisotropy and symmetry of the “P chemical shielding, as detected by nuclear magnetic resonance (NMR) spectroscopy. In a sunple model of condensed phosphates, phosphorus forms five bonds to four oxygen atoms [5]. The oxygen atoms are either bridging, in that they form linkages between two phosphbrus atoms, or terminal which bond to one phosphorus atom and form a coordination shell around the cations. Bonds to bridging oxygen atoms (0,) are single bonds and the excess bonding is distributed over the bonds to terminal oxygen atoms (0,) [5]. Consequently, the lengths of the P-O,, single bonds at 1.60-1.64 A and the P-O, distances, contracted by the partial multiple bond character, range from 1.39 to 1.54 A.
- units
in
condensed phosphates are characterized by the number ‘of P-O, -and P-O, bonds and may be categorized into three types:
(P0,)3-
0301-0104/84/$03_OO 0 Elsevier Science Publishers (North-Holland Physics PubIishing Division)
end units, middle units and branching units [l-4]. For example, phosphate units at-the end of polymer chains have one P-O, bond and three P-O, bonds, which results in C,, symmetry about the P-O, bond. The bond order of each of the three P-O, bonds in end units is approximately 4/3. At the other extreme, the branching phosphate units have three P-O, bonds of bond order one and one P-O, double bond. Branching phosphate units also possess C,, symmetry, except the axis lies along P-O, bond. Branching phosphate units are present m cross-linked phosphate chains (e.g., ultraphosphates) and in _closed cages (e.g., P,O,,). The middle umts of phosphate chains have two P-O, bonds and two P,O,‘ bonds. Middle units are approximately tetrahedral, but distorted such that the 0,-P-O, bond angle is increased to = 120’ and the 0,-P-O, angle is decreased to - 100”. Middle phosphate units have approximate qv symmetry about an axis that-bisects the-0,-P-O, and O,-P.-O, bond angles. In this study, the 31P chemical shielding is measured in five types of-condensed phosphates: di-
phosphates, _a triphosphate, a polyphosphate, a ring phosphate and a cage phosphate. The simplest conden@ phosphate is ihe diphosphate (i-e., pyrophosphate) w&h contains two end units Iinked _ B-V.
340
7: M. Duncan, D-C. Douglass / “P chenucai shrfr anuorropj
through a single oxygen atom. The next homologues in the sertes. triphosphates. are three-member chains with one middle unit and two end units. are long Polyphospates (I.e.. metaphosphates) chains of middle units. terminated by end units. Phosphates also Iink into rings. usually composed of 3. 4 or 6 middle units. Cage phosphates are closed networks of branching phosphate units. Finally. the spectrum of phosphate species dispersed in a silicate lattice is measured and compared to the condensed phosphates. The utility of “P NMR in the analysis of condensed phosphates was first demonstrated durtng the initial development of the spectroscopy. It \\as shown that the types of phosphate units could be identified by the isotropic shafts observed with aqueous solutions [6]. More recently. the anisotropy of the chemical shieldmg has also been shown to contain valuable information. For example. the amsotropy of phosphate units with Cz, symmetry in condensed phosphates (end units. cage structures and monophosphates) and in POX, compounds is correlated to the P-O bond length along the axis of symmetry [7]. A more recent study reported the “P NMR spectra measured at a high magnetic field which improved the resolution of the shielding components and magic-angle spinning was employed to identify the isotropic shifts [S]. The study revealed useful trends in the 3’P chemical anisotropy of a series of condensed potassium phosphates: end and branching phosphate units have axially symmetric shielding with the (T,,principal axis downfield and upfield, respectively. from the isotropic shift, whereas the middle units have non-axtally symmetric shielding [S]. In thts study, we describe a correlation of 3’P chemical shielding anisotropy and asymmetry with phosphate chemical type in solids that provides a mere precise indicator than that obtained from 3’P rsotropic shifts. Tlus correlation is based on the “P NMR spectra of 13 phosphate species in ten compounds: we report the static md magic-angle spinning spectra of four compounds (Na,P,O,. and a phosphosiiicate (NaPO,),, A~,W,%),, glass) and confirm a study of three compounds recently reported (K4P207, K5P30,a, and PdOu,). Several of the samples exhtbit multiple, overlapping powder patterns_ In these cases, unambiguous
assignment of the chemical shielding components is made possible by analysis of the relative sideband intensities in the spectra from the spinning samples.
2. Experimental
procedures
The ‘*P NMR experiments were performed on a Bruker CXP-200 spectrometer, wtth a 47 kG cryomagnet, at a resonance frequency of 81.015 MHz. The spectra were obtained by Fourier-transforming the accumulation of. typically, a dozen quadrature-detected free-induction decays. For each sample, the decdys were observed in separate experiments as spin echoes [9], and immediately after a 90 o pulse. The 3.1 ~LS90 o pulse (50 G) was sufficiently intense to excite uniformly the spectral region studied here. The receiver dead-ttme after the 90° pulse was 8-16 ps. The delays between the pulses in the spin-echo experiment (90P-delay-180,0-delay-observe) were set at 50 us. Both methods of obtaining the spectra contain inherent errors that are particularly significant with broad spectra. For example, truncation of the initial portion of the decay during receiver saturation results in first-order phase errors and spectral distortion. as described recently [lo]. Simtlarly, the orientational inhomogeneity of dtpolar couplings may lead to spin-spin relaxation times (T,) that vary across the spectrum and consequently, to non-umform decays in the spectral intensities in the spinecho spectra. However, these systematic errors will distort the spectra differently. Therefore, consistent results with both techniques allow one to rule out most experimental artifacts. Free-induction decays were also observed while spinning the samples at the “magic angle” to decrease the broadenmg effects of dipolar interactions [ll] and to decompose the chemical shift powder pattern into a collection of sharp peaks [12]. The rotation angle was adjusted by optimizing the ‘Li spectrum of LiBr [13]. Spectra were obtained at several rotation rates in the range 2000-4000 Hz to differentiate isotropic shifts from sidebands. The 31P NMR spectra are plotted on the (I scale for chemical shifts and “downfield” (i.e. less shielded) resonances lie to the left. The
T.M_
_ 3iP spectral param et er s are -reported -relative to 85% H,PO,, such that the 31P resonance in PF, is ppm. The principal components of the at -97 chemical shielding tensor (ur 1, a,, CT& are defined such that a,,-lies downfield and a,, lies upfield. Polycrystalline samples of two diphosphates [sodium pyrophosphate (Na,P,O,) and potassium pyrophosphate (K, P,O,)], a triphosphate (potassium triphosphate (KSP30Lo) “), a po!yphosphate (sodium metaphosphate ((NaPO,),) **), and a ring phosphate [aluminum tetrametaphosphate (Ai6P4012)3)J were purchased from Alfa Products in research-grade purity (98%). The chain lengths of each condensed phosphate can be determined from the relative concentration of weakly dissociated hydroxyl groups in salt solutions, before and after hydrolysis in concentrated acrd [14]. The results of the titrations indicate that the number of phosphorus atoms per chain are 2.0 for K,P,O,; 3.1 for K,P30,,; and 25.6 for (NaPO,),. A sample of phosphorus pentoxide (P40,0) was purchased in “puratronic” purity (99.998%) from Alfa Products and was transferred to a sample tube in an inert atmosphere. The 3’P NMR spectrum of P,O,, was acquired within 1 h after the sample was sealed. The phosphate-doped silica was prepared by gas-phase reaction of POCl,, SiCl, and O2 in He at 1400 OC. During the reactton, phosphate-silicate particles nucleate and deposit in the reactor. The particles were fused at 1900 “C into a glass of density 2.2 g/c&. The relative concentration of POCl, and SiCl, was regulated to yield a silicate glass containing = 1.0 wt % phosphorus_
3. Experimental results The 31P NMR spectra of the six condensed phosphates were measured with stationary samples and with samples spinning at the “magic angle”_ Fig.1 shows the 31P NMR spectra of two diphosphates, K4Pz07 and Na4Pz07_ The centers-of-mass of the powder patterns of K,P,O, and Na,P20, * Incorrectly
labelled by supplier as sodium hexa-
metaphosphate, (NaPO,),. QC Incorrectly - labelled by phosphate, WPO,),.
341
Duncan. D. C. _Dougkzss / “P~chenucal shift amsotropy
supplier as
potassium
metn-
J
-2
FREQUENCY.
IN PPM. RELATIVE
TO H,PO,
hg. 1. The 31P NMR spectra of two dtphosphates at 295 K. The magic-angle spinning spectra were observed whde rota:& the sample at 4835 Hz for K, P,O, and 3930 Hz for Na., P,O,. Calculated powder patterns for K,P,O, are obtained by leastsquares fit (sohd hne) and from sldeband mtensitles (dashed line). The two methods yield the same results for Na,P,O,. mdtcated by a smgle lme through the data
are 2.7 ppm and - 1.6 ppm, respectively_ The magic-angle spinning spectra of both compounds reveal multiple crystallographic sites in the crystal. The center peak of K,P20, has a center-of-mass of 1.1 ppm and is resolved into five peaks at -0.9, 0.0, 1.8, 3.3 and 4.8 ppm, in approximate relative percentages of 25, 25, 30, 12 and 8, respectively. The center peak of Na,P?O, is split into a doublet with peaks at -1.2 and -2.4 ppm. In the magicangle spinning spectra of both diphosphates, the resolved peaks in the sidebands have the same relative peak heights as in the center peak. The 31P NMR spectra in fig. 2 of a triphosphate, K, P,O,,, contain evidence of three different phosphate species. The three species have isotropic shifts of 1.2, 4.0 and 19.5 ppm and are present in approximate equal- amounts. The center-of-mass of the powder pattern of K,P30ro is 6 ppm.
T.&f Duncan. D C. Dougfass / jrP chemical shift anisotropy
- 200
0
-100 FREOUENCY
qoo
IN PPM
RELATIVE
200
300
TO H,POI
Fig. 2. The “P NMR spectra of a triphosphatr. K,P,O,,. at 295 K obsened with a stationary sample (top) and ulth a sample spuming at 3800 Hz (bottom). The center peaks in the magic-angle spinrung spectrum are denoted by asterisks. The tunes (a), (b) and (c) are powder patterns based on the pnncipal components obtamed from the stdeband mtensittes for the center peaks at 1, 4 and 19 ppm, respectively.
Fig. 3 contains the 31P NMR spectra of a chain polyphosphate, (NaPO,),. It is observed that the widths of the peaks in the ma@c-angIe spectrum are about four times as broad as those in the two previous figures. Broadening caused by dipolar couplings to the quadrupolar 23Na nuclei is not removed completely by magic-angle spinning if the nuclei exist in lattice sites with large electrostatic field gradients [X5]. The powder pattern is centered at 18 ppm and the magic-augIe spinning spectrum reveals that the isotropic shift of the maJor species in (NaPO,), is at 19 ppm and a minor species is observed at -2 ppm. The 31P NMR spectra of the nng metaphosphate, AIJ(PdO12)3, shown in fig_ 4, are similar to those of (NaPO,),. The relatively large residual widths of the peaks in the magic-angle spinning spectrum suggest that the aluminum cations are in lattice sites with non-zero electrostatic field gradients The magic-angle spinning spectrum of A1,(P,0,2)3 reveals two isotropic shifts at 51 ppm and at 30 ppm. Smce the isotropic shift of the spectrum of the stationary sample is 48.5 ppm, the sample is composed of = 90% species with an
(NOPO,l,
lr,,,l,llllrllrllll1)11) -200
-100 =REO”ENCI
0 IN PPU
100
RELATIVE
200 TO H,P04
Fig. 3. “P NMR spectra of a potyphosphate, (NaPO,),. obsened W&I a stationary sample and wble rotating the sample at 3690 Hz. The calculated hne plotted through the expcnmental data is the sum of the two theoretical pouder patterns (a) and (b).
-200
-100 FREQUENCY.
0 IN PPY.
100 RELATIVE
200
‘300
TO H,PO(
Fig. 4. 3’P NMR spectra of a ring metaphosphate, AI,(P,O,,),, observed with a stationary sample and whle rotating the sample at 3970 Hz. The calculated hne plotted through the expenmental data is the sum of the two theoretical powder patterns (a) and (b).
shoulder_around -kO ppm. Spectra~measured with other samples of‘ P,O,h, or= with m_ois&&added _ deliberately, reproduced precisely_ the spectrum in - _ . : ; __ fig. 5. -_Fig.-6 shows the 3tP~NMR--spectnun of phos-. phate-doped silica. The spectrum is similar_in shape and width to that of P,O,, and has a center-of-mass of 32 ppm. Because of the-irregularshape of the fused glass; it was not possible-to spin the sample.
-100 0 loo FREQUENCY, IN PPY. RELATIVE
-200
Fig. 5. “P
NMR
4. Discussion
300 TO H,PO.
spectra of P,O,,
me’
* -_
‘CT K with a
l
stationary sample (top) and with a samp.e +. _ug at 3750 Hz (bottom)_ The curves drawn through the data represent theoretical powder patterns obtained by a least-squares fit (solid hne) and by analysis of stdeband intensities (dashed line).
isotropic shift of 51 ppm and 10% species with a shift of 30 ppm. The intensity of the peak at 30 ppm and its sidebands are apparently attenuated in the magic-angle spinning spectrum, probably by interactions with 27Al nuclei. The chemical shift powder pattern of the cage phosphate, P,O,,, shown in fig. 5, has the opposite “directionality” of the diphosphates shown in Fig. 1. The isotropic shift determined from the magicangle spinning spectrum is 46 ppm, whereas the center-of-mass of the powder pattern is 33 ppm. The shape of the spectrum of P,O,, deviates from a theoretical powder pattern (to be discussed in the following section), particularly at the upfield
I
I
I
I -200
PHOSPHATE
.
I
.
.
-xx) FREOUENCY.
.
1
.
.
DISPERSE0
1
.
.
.
IN SILICA
I
.
I
.
I
0 IN pprn, RELATIVE
TO H, PO,
Fig. 6. The “P NMR spectrum of phosphate dispersed in a silicate lattice. The curve drawn through the data represents a theoretical powder pattern obtained by a least-squares fit.
4.1. Interpretation
of isotropic shifts
Early in the study of the 31P NMR spectra of phosphorus compounds it was shown that the isotropic shifts of condensed phosphates measured in solutions are related to the type of phosphorus units [6]. Isolated (P0,)3groups have shifts of -6 ppm and diphosphates, (0,POP0,)4-, are typically between 5 and 7 ppm [14,16]. The average isotropic shifts of the two polycrystalline diphosphates measured in this study are 1.1 ppm for K4Pz0, and -1.6 ppm for Na,P,0,, which are shifted downfield from the solution values. The crystal structure of Na,P,O, determined from X-ray diffraction studies indicates that the phosphate groups in each pair are inequivalent [17]. Consequently, we interpret the splitting of the center peak of the spectrum of Na,P,O, as evidence of the structural inequivalence *_ A similar crystallographic inequivalence resulted in a splitting of 3 ppm between the 31P isotropic shifts of the two end groups in the diphosphr+e anion of a-Ca,P,O, [18]. A recent study of K4Pz0,- revealed two doublets in the center peak of the magic-angle spinning spectrum [8]. Although the crystal structure has not been reported, it was assumed that similar inequivalences exist, and a doublet at 0.3 and 2.2 ppm was assigned to the
* An alternative interpretation of the splitting is the 31P-31P scalar coupling. However,
we believe that this coupling,
whichis a higher-order, two-bond interaction, is too weak to bc. observed in these spectra. For example, the peak assigned to the middle group of KsP,O,, IS not split by the end groups.
344
T bf
Duncan,
D. C. Dougkzrs
two ends in the diphosphate anions of K,P,O, and a doublet at 2.4 and 4.9 ppm was interpreted as the two end groups m the hydrated compound. K,P,O, - 3H,O [S]. Wtth respect to this interpretation, the peaks we observe at -0.9 and 0.0 ppm. and 3.3 and 4-S ppm are consistent with the previously tdentified doublets_ Howeve;. the peak at 1-S ppm n-t the K,P,O, spectrum represents ~1 third. yet unidentifted diphosphate spectes, with much less mequivalence between end groupsIn aqueous salt soluttons, there are the resonances for the triphosphate ion. u hich are assigned as end phosphate umts at 4-6 ppm and the middle phosphate unit at lS-21 ppm [14.16]. Therefore. in the spectrum of K,P:O,,, specres with isotropic shifts of 1.2 and 4-O ppm are mterpreted as end units and the peak at 19.5 ppm is attnbuted to the middle group. These shifts nre in excellent agreement with the peaks at 1.2. 4.2 and 19.4 ppm reported recently [S]. Sinnl.trly. the peaks at 19 and -2 ppm in the spectrum of (NaPO,),, are interpreted as the middle units and end units of the polymer chains. respectively. The relative intensities of the peaks m the spectrum of (NaPO;),, (92: S) agree with the average chain length of 26 measured by tttrdtton. The isotropic shifts observed m thi\ study for the end and middle units agree with the values measured for solutions_ In d previous study of polycrystailine the tsotropic values did not agree
/ A P chemicai
shaft anisorrop_b
The isotropic shift observed for P,O,, (46 ppm) compares well with a value of 46 ppm reported previously [S]. The isotropic shift (32 ppm) of the phosphate dispersed in the silicate lattice does not indicate unambiguously the type of phosphate unit. For this interpretation, we must consider the anisotropy and asymmetry of the chemical shielding. 3 2.
Prrncrpal
components
of
the chemical
shift
powder patterns
The chemical shift contribution to the NMR spectrum of a polycrystalline sample may be defmed in terms of the shie!ding along the three principal axes of the molecule [20]. The intensity at each frequency in the spectrum is determined by the relative differences between the principal shielding components. One method to determine the principal components involves a least-squares fit of the general expression for the powder pattern to the experimental data. However, this procedure may lead to systematic errors when fitting the spectra of samples with multiple species or with spectral deviations resulting from motronal averaging or inhomogeneous dipolar coupling. Alternately, the principal components can be determined from the relative intensities of the adebands in the magic-angle spinning spectra [21]. In this regard. the most straightforward approach is the graphical procedure based on the intensities at the first few sidebands [22] The isotroptc shifts of the different species in the diphosphates K,P,O, and Na,PzO, are separated by only d few ppm. which indicates similar average shielding_ Also, since the peaks corresponding to different spectes maintain the same relative heights in the sidebands, one obtains the same anisotropy and asymmetry for the powder pattern of each species. Therefore. the observed lineshapes in ftg. 1 have each been represented with a smgle theoretical powder pattern. The powder pattern for Na,P,O, derived from sideband Intensities is consistent with the results of a least-squares fit. However, the two methods yield different lineshapes for K,P20,. Since both techniques predict the same values for the relatively well-defined upfield components, the more valid results may be judged by accuracy of the isotropic
-_
TX. Duncan, D.C. Doughs
/
"P
chemical shrfz aksorripy
1
_
_
345
- > shifts, calculated from. the average of the three components. The least-squares -results-predict- an isotropic shift, that differs by 5 ppm from the actual value, and thus is in error. The _isotropic shift predicted by-magic-angle spinning is correct, by definition_ In addition, the results for K,P*O, = 43 ppm), concur with (all =82 ppm, ~,=a~~ the recently reported components_ at 84 and 42 ppm *_ The ‘IP chemical shit parameters for Na,P,O,-and K,PzO, and the other condensed phosphates are summarized in table 1. The calculated powder patterns of K,P20, and Na4Pz07 suggest axially symmetric shielding. That is, two of the components (uz-, and u,,, in this case) are degenerate_ The confirmation of degenerate components is difficult with either procedure for calculating powder patterns_ For example, with
.
a least-squares fit, -the accuracies are limited by the residual dipolar broadening-in the powder, pattern,. which blurs-any evidence of separation between u,- and u3, into a single- rounded peak; Similarb~, the graphical analysis of sideband -intensities requires one to determine the point of intersection of interpolated contour lines that‘ meet at small angles. In this study, the confidence limit on the separation between two components quoted as degenerate is approximately equal to the gaussian broadening listed in table 1. The 31P chemical shielding in the diphosphate cu-Ca,P,O,, determined by rotation patterns of a single crystal. is not axially symmetric [18]. The overall symmetry (us3 - a,,) in cz-Ca,P,O,, given in table 1, is comparable to Na,P,O, and K,P,O,. However, the upfield components of the two sites
* See footnote c of table 1. Table I “P Chemicai shielding parameters Compound
Phosphate group
Na,F$O, a-CaaPaO,
KSP30,0
WaP4L W-3),
=)
p.%o,o
(P0,)3-
m sihca
Percent of total
isotropic shift II’
25
-09
25
end end end end end end end end end end mtddle middle
50 50 50 50 33 33 33 92
-1.2 - 2.4 18 21 12 4.0 19.5 19
end middle middle ring middle ring ring
8 50 50 90 10 33 33
-2 18.5 20.7 51 30 18.2 19.9
33 100 100
21.2 46 32
ring branching branching
30 12 8
0.0 18 33
Pnnctpal components 011
a=
Broadening (kHz) b’
Ref.
Q33
-82
43
43
105
-80
37
37
1 62
-
65 67 55 51 148 160 45
0.98 098 069 160 160
WI WI
-95
32 45 55 51 -16 -13 45
-84
-30
180
-
-40 -15
20 -15
175 120
0 89 0 89
-90
-38
185
-
-66 -37
-35 -37
240 190
0.40 0.55
48
-42 -48 -106 -90 -75 -90
a’ In ppm, relative to_H,PO,; determined from magic-angle spinning spectra. b, half-width of the gaussian broadening convoluted homogeneously into the powder pattern; 10 ppm = 0.81 kHr =’ The principal components are con&ted from the data in ref. [S] with the following relations: o,, = F_- f(1 + q)Au. a, v)Au. and I+ = 5 + $Ao.
PI
I*1
= a - f(1-
_-
346
T-M
Duncan. D-C_ Doughs
in a-Ca.P,O, are separated by 33 and 22 ppm. compared -to no detectable separation in the diphosphates measured in this study. Similarly. the sphtting between the isotropic shifts is = 1 ppm for Na,P,O, and K,P,O,. and = 3 ppm for (YCa,P,O,. The basis for these differences is the geometry of the diphosphate anion in Na,P,O, and a-Ca,PzO,_ In Na,P,O,, the P-O, bond lengths are equal withm standard deviation (0.002 A). the 0,-P-O, angles differ by less than 2.5”. and the 0,-P-O, angles lie in a range less than 6 o [17] indicating near-C,, symmetry- The diphosphate anion in cu-Ca.P,O, also has an eclipsed _ _ geometry of the two (PO,)‘groups. but there is deviation from C,, symmetry [23]. The P-O, bond lengths differ by 0.02 and 0.045 A for the two end groups, and the 0,-P-O, angles vary by 6 O. Although the bond lengths and angles for K,P?O, have not been reported. the “P results suggest that the variances are small. similar to the structure of Na, Pz07. Since the magic-angle spinning spectrum of KSP,O,,) reveals three species. the spectrum of the stationary sample IS the superposition of three powder patterns. The po\\der patterns predicted from the sideband intensities are given by curves (a). (b) and (c) in fig. 2. The theoretical spectrum IS obtained by summing the pounder patterns and ddJustmg the broadening. which lieIds dn accurate fit to the experimental spectrum. Spectra (2) and (h) correspond to the isotropic shifts of - 1.2 and 4.0 ppm and therefore dre the powder patterns of the end units of KsP,O,,,. Again the components determined here are in accord with the results of a recent study [S]. We measure - 75. - 16. and 14s ppm for the components of the middle phosphate unit of KSP,O,,,, compared to - 67, - IS. and 148 ppm ‘_ The previous study did not resolve the differences betwern the end groups. but the reported values of -97. 49 and 49 pprn agree with the averages of the components for K,P,O,, end groups in table 1. As expected, the end units of the triphosphate have a shielding simiiar to the diphosphates in fig. 1. The powder pattern of the middle unit of K,P30,, has a chemical shielding remarkably similar to that of the major species of the metaphosphate (N&PO,),. * See footnote c of t.lble I.
/
“I’
chemical
shaft anirorrop_t
The theoretical chemical shielding powder pattern of the middle units of the polyphosphate (NaPO,), is given by curve (a) in fig. 3. Curve (a) is calculated from the principal components derived from the sideband intensities in the magicangle spinning spectrum_ with broadening determined by least-squared methods. It is not practical to determine the chemical shielding anisotropy of the end groups because the sidebands are weak_ Instead_ the powder pattern, curve (b), is determined by a least-squares fit to the difference between the experimental spectrum and curve (a)_ The superposition of curves (a) and (b) is given by the line drawn through the data in fig. 3. The 31P NMR spectrum of the sample of Al,(P,,0,2)3 is not as adequately represented by a superposition of theoretical powder patterns. The shoulders of the powder pattern predicted by the sideband intensities (curve (a) in fig. 4) correspond to locations of the shoulders in the experimental data. but the intensities are in error. This is to be expected since the magic-angle spinning spectrum indicates that the spectrum is the sum of two powder patterns_ However. the residual spectral intensity has a center-of-mass of = 20 ppm. which is not consistent with the isotropic shift observed dt 30 ppm_ The best fit to the residual intensity, with the restriction that the isotropic shift be 30 ppm, is given by curve (b) in fig. 4. The sum of curves (a) and (b) agrees with the overall shape of the experimental spectrum, but there is some discrepancy in intensities_ Possible causes of the inhomogeneous broadenmg include the orientational dependence of the dipolar coupling between 3’P and “Al or experimental artifacts_ The calculated principal components of the chemical shielding of P,O,, depend on which approach is taken. A discrepancy appears in the position of the upfield component, as shown by the two theoretical curves in fig. 5. The leastsquares fit yields a value of a,, = 215 ppm, whereas the graphical approach yields a value of 240 ppm. However, the isotropic shift of the least-squares results differs by 8 ppm from the center peak in the magic-angle spinning spectrum. On this basis, the principal chemical shift components obtained from the sideband intensities of several magic-angle spinning spectra are quoted in table 1.
T_M_ Duncam_D.C: Douglass / 3tP chemical :ht/ &isotropy
The spectrum of P40r0 contains anomalous, but reproducible deviations from an ideal shielding powder pattern. Unfortunately, although‘ the shielding components for P,O,, have been reported previously [7,8], no spectra have been published. Until a satisfactory model is proposed for the modulations in the spectrum of P,Ou,, it may be tenuous to interpret the results of fitting a single, unaveraged powder pattern. With this caveat, we present the following preliminary comments on the chemical shielding of P,O,,. The relatively large anisotropy of P.,O,, observed here is comparable to the anisotropy reported previously: we calculate an upfield component at 240 ppm compared to 243 ppm [S] *_ However, previous studies report that the downfield components are degenerate (e,, = oz2 = 52 ppm) [7,8] whereas we detect a splitting between CT,,and azz- That is, there is a distinct shoulder in the experimental data at -66 ppm, which implies that the branching phosphate unit in solid P,O,, does not have C,, symmetry. It is noted that electron diffraction studies of gaseous molecules of P_,O,, indicate C,, symmetry about the P-O, bond [24]. An alternate interpretation of the chemical shielding, although less hkely, is that there exists more than one phosphate species in P,O,, l&h the same isotropic shifts (_t 1 pm), but with different chemical anisotropies. The 3’P NMR powder pattern of phosphatedoped silica corresponds well with the fitted theoretical spectrum, as shown in fig. 6. Although a spectrum was not measured while spinning at the “magic-angle” to resolve the isotropic shift(s), the accuracy of the fit indicates that the phosphate exists as a single, well-defined species. That is, if there were other phosphate species with different chemical shieldings, the superposition would result in spectral aberrations. For example, the presence of the minor species in (NaPO,), and A1,(P,0,2)3 which represent = 5% of the phosphorus cause distinct effects in the experimental spectra. Finally, because of the large anisotropy and axial symmetry of the chemical shielding, we propose that the phosphate dispersed in the silica is a
* See footnote c of table 1.
_
347
branching phosphate unit. This interpretation is _ consistent with- infrared -studies- of similarly-prepared phosphbsilicate glasses that reveal a band at =_1325 cm:‘, interpreted_ as the stretch of the p--O double bond [25]. It is interesting -that the shielding anisotropy of branching phosphate in the amorphous silicate lattice is less than in the four-member cages of P,O,,. 4.3. Interpretation of the chemical shielding anisotropy The chemical shielding of the three different phosphate units measured in this study are consistent with previous results for condensed phosphates, such as (KP4),, (KPO,), [7,8] and cyCa,P,O, [18], which are included in table 1. Also, the spectra of the middle phosphate groups agree with phosphate groups in organophosphate diesters [26-291 and monoesters [29,30]. For a qualitative understanding of the relation between the bonding geometry and the chemical anisotropy, an insightful approach [7,18] is to consider each phosphate group as perturbation of a -tetrahedral (P04)3- ion. The ideal (P0,)3tetrahedron has four equal P-O, bonds, each with a bond order of = 5/4. The isotropic shift of the (P0,)3tetrahedron in K,PO, is - 10 ppm and, as expected, the chemical shift anisotropy is zero [7,8]. An end phosphate unit may be obtained by transfer of the T-bond character from one bond to the other three bonds. The data in table 1 show that this transfer increases the chemical shielding anisotropy (u3s or,) to = 120 ppm for end groups in diphosphates and to = - 150 ppm for the ends of longer linear chains. The shielding parallel to the single bond moves downfield from -10 to = -65 ppm for diphosphates and to = - 100 ppm for the ends of the longer linear chains. Similarly, the shielding perpendicular to the single bond increases from - 10 to 55 ppm for all end groups. In general, the chemical shielding is determined by the orbital angular momentum of the electrons [31], which in part is related to the electron density. For the end phosphate groups, transfer of the r-bond character from the symmetry axis to the -other bonds decreases the shielding parallel to the axis and in-
creases the perpendicular shielding. Continuing this line of analysis, a branching phosphate unit is formed from a (PO,)‘tetrahedron by transforming all the T-bond character to one P-O, bond. The anisotropy of the branching phosphate increases relative to the tetrahedron from 0 to = 300 ppm. over twice as large as that of the end units. The transfer of electron density is opposite from the end units and consequently the orientation of the -“P powder pattern has reversed. Transfer of the T-bond electrons to the P-O, bond on the symmetry rtvis increases the shielding parallel to the axis and decreases the perpendicular shieldmg. The chemical shielding of the middle phosphate unrts reflects the same trends as the end and branching groups. although the interpretation is not as straightforward_ Since there is no axial symmetry. one does not know a priori the orientation of the principal axes for the chemical shielding relative to the molecular However, analogues
coordination
the axes have been determmed for middle
umts
PHOSPHATE GROUP END
-
If
.
MIOOLE
-.
RING
.
D .
BRANCHIUNG 1 0
t 100 (ff33-ff22)
-
I 200 IN
3bO
PPM
Fl_e 7 A correlauon betueen the type of phosphategroupand the difference betueen the upfield (Q) and intermediate (CT& chemical stueldmg components.
changes in anisotropy and asymmetry as the series progresses from end phosphate to branching phosphate species_
5. Summary
frame. for three
in condensed
phos-
phates- one organophosphate diester with the for(barium diethyl phosmula M” (O,P(OR),)‘phate [25]) and two monoesters of formula (02P(OR)(OH))‘(aminoethyl phosphate [27] and deouycytidine 5’-monophosphate [30]). From rotatlon p&terns of single crystals. it was determined that the a~& of the upfield component (Use) lies in the 0,-P-O, plane, approximately ( < + 10”) bisecting the angle. and the axis of the downfield component (a,,) lies in the 0,-P-O, plane [26.27,30]. Again. the same trend prevakr sh:eldmg parallel to an axis with multiple-bonding is Increased and shielding perpendicular to the itx~s is decreased. The spectra described in ths study indicate that the type of phosphate unit cdn be identified by the positlon of the three principal components (T,~. uzz and uJJ_ Or. one may consider the anisotropy (Au) and the asymmetry parameter (17) to ascertain the geometry of the phosphate ion. However. on the basis of the compounds measured to date. it appears that the phosphate type can be correlated to the difference between two pnncipal components, us3 and uz2, as shown in fig. 7. The difference the simultaneous between uJ1 and uzr reflects
The densed
31P NMR
powder
phosphates
and phosphate
patterns
of
stx con-
dispersed
in a
silicate lattice have been measured and analyzed to extract the principal components of the chemical shielding tensors. The spectrum of each compound is the superposition of any of three characteristic powder patterns corresponding to the type of phosphate units present: end-units. middle-units. or branching-units. The end-units, which terminate linear phosphate polymer chains, exhibit axially symmetric shielding such that the shielding perpendicular to the symmetry axis is upfield. The branching phosphate groups also have axially symmetric shielding, but the shielding perpendicular to the symmetry axis is downfield. The phosphate units in the middle of chains do not possess axial symmetry and consequently three distinct shielding components are observed, with the same general trend. That is, orientations perpendicular to dn axis with multiple-bonding are deshielded and the shielding is increased parallel to the axis. The converse is observed for orientations relative to an axis along a single bond_
Early studies [6] demonstrated that the endand middle-phosphate units could be identified by the “P isotropic shifts observed by analyzing the compounds in solution_ In this study, it IS shown
_
_ _ TM_
that the end-, middle- and branching-units in solids are well characterized by their distinctive powder patterns, owing to shielding anisbtropy. Furthermore, the anisotropy of the shielding provid_a a less ambiguous distinction between the three phosphate groups, compared to the isotropic shifts_ That is, the ranges of the isotropic shifts of the three groups in solids overlap: end groups range from = 0 to 20 ppm, middle groups from = 10 to 50 ppm and branching ppm. Alternately, the principal species
components are divided
into
units
from
differences a,, and a, three distinct
=
30
to 45
between the for the three regions:
end
(O-35 ppm), middle groups (135-225 ppm) and branching groups (230-295 ppm). Furthermore, the middle groups bonded into rings lie in the upper portion of the range, from = 195 to 225 ppm. It is conceivable that future NMR studies of other condensed phosphates may expand the regions of the middle and branching phosphates shown in fig. 7, thus resulting in some overlap. However, It seems likely that any ambiguity can be resolved by considering the asymmetry parameter of the “P NMR spectrum: the powder patterns of middle units should reveal a separation between components q1 and uZZ on the order of 50 ppm, whereas these components should be more nearly degenerate (separated by less than 20 ppm) in the spectra of the branching phosphate units. If the class of phosphates to be characterized is extended to include substituted species, such as the biologically important hydroxyapatites, cross-polarization data should be added to the set of pertinent information for structural determination, as demonstrated previously [32]. groups
References [I] JR. [2] [3] [4]
[S]
_
Duncan, D C__Dotigkzs.s /
Van Wazer. Phosphorus and its compounds (Whey-Interscience. New York, 1958) D.E.C. Corbridge. Structural chemistry of phosphorus (Elsevier. Amsterdam, 1974). E Thdo. Advan. Inorg. Chem. Radmchem. 4 (1962) 1. J. Majling and F. Hanic. in: Topics in phosphorus chemistry, Vol. 10. eds. M. Grayson and E.J. Griffith (Wdey, New York, 1967) pp. 341-502 D.W J. Cruickshank. J. Chem Sot. (1961) 5486.
_
- --
-_ .
_
--._ J’P chenttcai shtft anisotropy
349
WI J R. Van_War,
,
C F.._CaiJis and -J $. Shoo!ery. J. Am. : Chem. sot. 77 (1955) 4945;. MM. Crutchfield, C.F. Calhs, R.R. Irani and G C. Roth, Inorg. Chem. 1 (1962) Si3: _ 171 A.-R. Grimmer. Spectrochim. Acta 34A (1978) 941; . A.-R. Grimmer, Proceedings of the 20th Congress Amp&e. Talhn. 1978. p_ 483. I81 A -R. Gnmmer and U Haubenreisser, Chem. Phys. Letters 99 (1983) 487. 191 E.L Hahn, Phys. Rev. 80 (1950) 580. DOI M. Rance and R A. Byrd, J. Magn. Reson. 52 (1983) 221. IllI E.R. Andrew. Progr_ Nucl. Magn. Reson Spectry. 8 (1971) 1. [121 E Lippmaa. M. Alla and T_ Tuherm, Proceedings of the 19th Congress Amp&e. Heidelberg. 1976. p- 113. EO. SteJskti, J Schaefer and R A. McKay, J Magn. Reson. 25 (1977) 569. 1131 J S Frye and G E. 1Maclel.J Magn. Reson. 48 (1982) 125. r141 S. Greenfield and M. Chft. Analytical chemistry of the condensed phosphates (Pergamon Press. New York, 1975). I151 N. Zumbuiyadls. P.M. Henrichs and R.H. Young. J. Chem. Phys. 75 (1981) 1603. 1161 M.M. Crutchfield. C.H. Dungan. J-H. Letcher. V. Mark and J R. Van Wazer. m: Topics m phosphorus chenustry, Vol 5, eds. M Grayson and U. Gnfhth (Wtley. New York. 1967) pp 319-321. 1171 K Y Leung and C. Calvo, Can. J. Chem. 50 (1972) 2519. VI S J. Kohler, J.D. Ellett Jr. and M.P. Kiem. J. Chem. Phys 64 (1976) 4451. 1191 E R. Andrew. D J. Bryant, EM. CashelI and B.A Dunell. Chem. Phys. Letters 77 (1981) 614 [20] N. Bloembergen and J A. Rowland, Acta Metall. 1 (1953) 731. 1211 M.M Maricq and J S. Waugh. J. Chem. Phys_ 70 (1979) 3300. [22] J. Herzfeld and A E- Berger, J. Chem. Phys. 73 (1980) 6021. 1231 C. Calve, Inorg. Chem 7 (1968) 1345. [24] B Beagley, D.W.J. Cruickshank and T-G. Hewitt. Trans. Faraday Sot. 63 (1967) 836. [251 AS. Tenney and M. Gheuo. J. Electrochem. Sot. 120 (1973) 1276. [26] J. Herzfeld, RG. Gnffm and R.A Haberkorn, Biochem. 17 (1978) 2711. [27] S J. Kohler and M.P. Klem. Biochem 15 (1976) 967. (281 B.T. Nail. W.P. Rothwell, J S. Waugh and A. Rupprecht, Bmchem. 20 (1981) 1881 [29] S J. Kohler and M P. Klein, B&hem. 16 (1977) 519. [30] P TutunJian, J. Tropp and J.S. Waugh. J. Am. Chem. Sot. 105 (1983) 4848. [31] C P. Slichter, Principles of magnetic resonance, 2nd Ed.
(Springer, Berlin, 1978).
[32] W P. Rothwell. J S. Waugh and J-P_ Yesinowski. J. Am_ Chem. Sot. 102 (1980) 2637; J Tropp. N.C. Blumenthal and J.S. Waugh. J. Am. Chem. Sot. 105 (1983) 22.