- Email: [email protected]
High-resoluttin 29Si NMR in solid silicates
Volume
77, number
Zetttralmstztut
Reccwed
CHEVICAL
2
fhr attorgattrsche
7-7 August
PHSSICS
LC77-CRS
C/tetttte der Akadettrte der Wtssetrsciraftetz der DDR,
I I99-Brrlur,
DDR
1980
69H20 were measured usmg the cross “SI NhlR spectra of polycrystallmc Ca6 [SI~O,I(OH)~] and [(CH3)4N1 gS1g0x1 polarlzatmn double-resonance technique Observed shleldmg tensors are related to the l.nown SI-0 bond systems The arrangement of the four St-0 bonds III the S104 tetrahedrn 1s rcfiected by the *‘Sx shleldmp tensor TIC most shlcldcd ducctlon corresponds to the shortest Q-0 bond.
I_ Introduction
2 ExperImental
In general high-resolution N?vlR m sohds offers quantitative informatlon about the onentatlon dependence of the chemical siuft and 111this respect it Improves upon conventional lugh-resolution NMR m lsotroplc liquid phases. 31P NMR studies of sohd phosphates and phosphoryl compounds have shown the existence of correlations between the 31P shleldmg tensor and the character of the phosphorus-oxygen bond [ 1 ] . In the case of an axially symmetric stieldmg tensor, the correspondence of the most (least) shielded prmclpai axis of the tensor to the &rection of the P-O bond contaming the highest (lowest) multiple-bond character can be drawn from experimental [2] and theoretrcal [3] mvestigations. In this paper we report mvestlgations of the 29S~ shelding tensor with regard to the vahdlty of these correlations UI the field of hgh-resolution I95 NhJR m crystalhne silicates. We present the “9S~ spectra of tricalcium sticate hydrate Ca6 [S1?07 I (OH)6 ] (TCSH) and tetramethylammonium sllicate hydrate [N(CH3)4] 8Si8020- 69H,O (TMAS) and their Interpretation on the tasls of the known structures of these two compounds.
Polycrystallme samples of TCSH and TMAS were prepared and characterlLed by methods reported prevlously [4,5]. The mvestlgatlon of TCSH by the molybdate-reaction method [6] showed an lmpurlty of monoslhcate anions S10:- (10% with reference to the total SI content) m addition to the expected dlslhcate anions
0 009-26
14/S 1/OOOO-0000/S
02.50
0 North-Holland
171 The cross polarlzatlon
S&-
double-resonance eaperlments (PCW techmque [8]) were performed at room temperature on the solid-state multi-pulse spectrometer FKS 178 (Zentrum fur wissenschafthchen Geratebau der AdW der DDR) operatmg at 60 MHz for 1H and 12 MHz for 29%. A variable temperature probe-umt (-180 to +200°C) DRS l-29 contammg a crossed coil arrangement near the Helmholtz condltron was used. For 29S1 resonance excltatlon and detection, a sample co11 of 10 mm diameter was employed. The quahty factor of the ?~SI detection clrcult was 130 Efficient suppression of the 60 MHz proton excltatlon pulses (n/2, lockmg pulses: was provided by a special filter arrangement m the probe head. Measurements were carried out applying proton decouphng pulses having a field strength of 0.6 mT and Pubhslung
Company
331
Volume 77. number 2
15 January 1981
CHEMICAL PHYSICS LETTERS
width of 40 ms. In fulfilling the Hartmann-Hahn cond&on, 2981 mixpulses with a field strength of 3 mT and pulse width of 3 ms were used. The repebtlon rate of the single expenments was 6 s and 12 s for TMAS ana TCSH, respectively_ NMR stab-bon did permit data accumulation
3. Results The observed IgSi spectrum of TCSH shown in fig. 1 exhiilts a clear maxunum and two shoulders m the low-field part. tis lme shape results from the overlappmg of two components suggested by the dotted curve. The asymmetric line shape [9] of the mam component (91% of the total mtensity) indicates the clear axial symmetry of the corresponding chemical shift tensor with 6, = -109 ppm (maxmmm) and 6,, = -35 ppm (shoulder). The additional shoulder at -70 ppm is caused by the second component with nearly symmetric hne shape, mdlcatmg the small axial symmetry of the corresponding chemical stift tensor The separatlon of the two components IS possible by appropnate variations of the experimental condltlons. Due to the different reiaxation times T, of the two components, the mam component can gradually be suppressed so that determmation of the lme shape of the smaller component is easy [lo] _ Correspondmg to the composition of the sample, the more mtense component with the high stieldmg arusotropy no = u,, - u1 = -74 ppm is ascrlbed to the duner skate aruons, whereas the less intense compo-
Fig 1. 29Si Nh¶R spectrum of Gas (S12071 (OH)61 (TCSH)
332
-roe
0
b/ppm
Ftg. 2. 29S~ NMR spectrum of [(CH3)4N] sS~sOzo - 69H20 Cl-hIA.
nent with the smaller anisotropy IS attnbuted to the monomer anions. This interpretation 1s consistent with previous mvestlgatlons showmg that in the case of caicium shcates the shielding anisotropies mcrease Lvlth mcreasing branching of the SiO4 tetrahedra [ 1 I] _ Furthermore, tlus interpretation IS confirmed by the agreement of the lsotroplc chemical shft of the main component with the lsotroplc chemical shft of by Llppmaa et Ca6 [%07 1(OH)6 1 recently obtained al [12] using the magic-angle sample spinmng @US) technique (table 1) The lsotroplc chemical shift of the Impurity (6, = -72 ppm) corresponds to the isotropic value of calcium monosticate Caz [H&O4 1OH] (a,-= -72.5 ppm) [12] _ Fig. 2 shows the 7gS~ spectrum of TMAS. Even in this case the asymmetrlc line shape indicates an axially symmetric chemical shift tensor with 6, = - 73 ppm and S ,, = -152 ppm. The agreement of these data with the correspondmg values obtained by Lippmaa et al. [ 131 1s sufficrently good takmg into consideration the problems of the MAS technique for shieldmg amsotropy measurements. The low-temperature spectrum presentek by Kublcki [14] IS consistent with our room-temperature spectrum. Table 1 summarizes the observed values as well as hterature data. In both sihcates the shieldmg fensor has axial symmetry mth approximately the same value but different sign of shie1dlr.g anisotropy. In order to find correlations between the slueldmg anlsotropy and the silicon oxygen bonds, knowledge of the orientation of the principal axes of the tensor
Volume
77, number
2
CHEMICAL
PHYSICS
Table 1 “S, chemical shift results for ThIAS and TCSH a) --
_ ________
Anion
~.__
Symmetry
SlzO$-
s,so:;
__--
&II -__
nwll
-___
-35
+ 2
III relation
However
to the SI-0
be obtamed duectly
-109
oxygen
f 7-
-74
-82
6
113-l
-148
-68
-97 7 -95 -99 3
+65 +80
--
= -(a,,
effects - hl)
--_-
can
from the powder spectrum, and no
brrdgmg
oxygen
0
I131 I141 1121 _-.
_ -~
are not taken in account
IS necessary.
slueldmg components
Rcf
i 2
a\Kd a\Kd
bond duections
@
-84
+78 5 c 1
SI-0 bond system. According to W1eker [7], III TCSH there are dlsilicate anions Si,O$- consistmg of two Si01,203 end groups, precise data about the environment of the S1 atoms are not available [15]. In order to obtain more mformatlon about distances and angles, we make use of the structure data of the analogous disdicates tdleyite Ca, [S1,071C03] [16] and hemlmorphte Zn, [S1?071 (OH)2] - H,O [ 171 and assume that the geometry of the S1,07 groups m TCSH &sentlaUy do not differ from the known geometry of these two dlsiiicates. In both mmerals the oxygen enwonment of the two S1 atoms belongmg to the S1-,0,
tcrmrnal
+ 7-
c)
-99 f 2
mformation IS obtamed about the orientation of the prmcipal axes. In the present case of axlally symmetric tensors, knowledge of the symmetry of the S10, tetrahedra can be used for assIgnIng the principal axes to the
@
-_-
1981
__--ao
-73 L! 7
=) Aa = -A6
only the pnnclpal
__
______6 IS0 b,
-152 + 2
i; Values in ppm, refercncc ThlS. susceptibdlty 6,,)/3-
61
X\Kll
__--_6 so = (=1+
15 Jammy
LETTERS
groups 1s identical in so far as each S1 atom has as I-.elghbours three termmnl oxygen atoms (OT) and one bridging oxygen atom (OB) FIN 3a shows schematically the S104 tetrahedron 1n hemunorphite The mean values of bond distances and mterbond angles are given. To a good approxunat1on, the environment of each St atom m both compounds has a threefold symmetry a\1s along the bridging S1-0, direction. In the case of TMAS, the complete crystal structure has been determIned by Smohn et al. [ 181 Thus compound contains Si,O$j anions formmg a double four ring with e1ghht S103,2 0 branching groups In this cubeshaped anion the environment of all eight SI atoms 1s identical. Each S1 atom IS surrounded by three brldgng oxygen atoms and one terminal oxygen atom. The values of bond distances and interbond angles are gven 1n fig. 3b. Even 1n this bulky anion, the environment of each Si atom has a threefold symmetry axIs, however UI ths case the symmetry axis coincides with the Si-OT bond direction. Startmg from the axial symmetry of the S104 tetrahedra m these two compounds, 1t 1s possible to relate the onentation of the princtpal ales to the S1-0 bond directions. in the d1s1hcate the d1reci1on of mmunum shielding o,, lies along the S1-0~ bond, and m the double four rmg sd1cate the duectlon of maximum sJueIding a,, lies along the S1-0, bond.
sllrcon
3. Bond lengths (A) and mtcrbond angles (dcg) for the S104 umts m hemunorphltc (a) and ThlAS (b) (schematlc representitlon).
4. Discussion
Rg.
A pictorial
representation
of the shlekhng
tensor, 333
Volume
77,
number 2
CHEhIICAL PHYSICS LETTERS
15 January 1981
5. Conclusions The connection between the bond character of the Si-0 bond and the 2gS1 tielding tensor IS the same as we have observed III the case of 31P NhIR High-resolution NMR in solid phosphates and skates gives informahon about the bonding in the amon tetrahedra, and Fig 4. Plctornl representation of the orlentatlon of the *“Sl shleldmg tensor m TCSH (a) and TMAS 0~) relatwe to the
SI-0
bonds
using the shielding eilipsord, is gven m fig 4. The posltion of the prmc~pal tensor axes related to the SI-0 bond frame shows that the tielding of the 2gS~ atoms and the nature of the oxygen arrangement around the SI atom are unambiguously correlated. ‘ke pressed shielding eUipsold (Au negative) corresponds to the “pressed” tetrahedron of the S104 end groups m TCSH (fig. 4a) and the stretched shreldmg elhpsord (Au pow tlve) corresponds to the “stretched” tetrahedron of the Si04 branchmg group (fig. 4b). End groups and branchmg groups are mverse in &stances and angles (fig. 3) and therefore the mverse ngn of the amsotropy Aa is imrme&ateIy clear. Summarizing these facts, we pomt out that the short Si-0 bond corresponds to strong stieldmg and vice versa. Overlookmg the recent crrticlsm of the n-bond concept III tetrahedral oxoamons [ 151, we mterpret m the usual manner bond lengths and angles m terms of ?Tbond character [20]. in TMAS the shortemng of the Si-0, bond relative to the Si-OB bonds, as well as the widening of the angle 0B--5h-OT relative to the tetrahedron angle (fig. 3b), indicate the concentration of n-bond character m the SI-OT bond. Usmg this relationtip we can estabhsh correlation between the n-bond character in the S104 tetrahedron and the 2gS~ shieldmg arusotropy. It IS possible to make the dlstnbution of n-bond character visrble vra the shieldmg tensor. The bond direction with the h&est n-bond character co&Ides with the drrection of the observed maxLmum shielding of the Si nucleus. Startmg from this basis, future high-resolution NMR in sohds may provide quantitative information about the bondmg in silicates.
334
includes determination of the lsotropx chemical sfufts m order to identify the type of anrons as a valuable special case. The simllanty of the correlations in phosphates and sdlcates confirms the well-known analogy of these two classes of compounds and possibly reflects a general regularity whrch may be vahd for other morgamc oxygen bridged polymers
Acknowledgement The authors are grateful to Dr. D. Hoebbel for provldmg TMAS and to Professor 1%‘.Weker and Dr. K.-J. Jost for helpful dlscusnons.
References [I]
A-R
Crunmer,
[2] A -R Gnmmer,
Spectrochun Acta 34A (1978) 941 Proceedmg of the ‘20th Congress Amp&e, eds C. Kundla. C. Lrppmaa and T Saluvere
Talhnn, 1978, (1979) p 483 [31 R Radegha and A -R Grunmcr, Z. Physlk. Chcm , to be pubhshed 141 GE. Bcssey, Report of the Buildmg Research Borad, 1935 (London, 1936) p- 35 [51 D Hoebeland W Wlcber, Z Anorg Al@ Chem 384 (1971) 43 rf.51 E Thdo, W Wicker and H Stride, 2. Anorp A& Chcm 340 (1965) 26 1. 171 W Wicker, III Neuere Ergebnissc der anorgaruschen Chcmle (Deutscher Vcrlag dcr Wissenschaften, Berlm, 1974) p 238 181 A. Pmes, h1.G Gabby and J S. Waugh, BuU Am. Phys Sot. 16 (1976) 1403. 191 hf hlchrmg, in NMR basis pnncrples and progress. Vol 11, eds P DiehI. E Fluck and R Kosfeld (Sprmger, Berlm, 1976) [IO] A.-R Grunmer, D hfuller, R Peter and E rechner, to be
pubhshed. [ 1I] E. Fechner, A.-R. Grimmer and W Wicker, Z Chem (1978) 420.
18
Volume 77.
number
2
CHEMICAL PHYSICS LETTERS
[ 121 E. Llppmaa, M. hlagi, A. Samoson. C. Engelhardt and A.-R. Grunmer, J. Am. Chem. Sot. 102 (1980) 4889 i 131 E Lippmaa, hl. AlJa, T. Pehk and G. Engelhardt, J. Am. Chem Sot. 100 (1978) 1929 [ 141 N. Kubvzki. Drplomarbeit. Karl-Marx-UAersitat, Leipzig (1979). [I51 Ch S Mamedov, RP. Klebzova and N.V. Belov, Dokl. Akad. Nauk SSSR 126 (1959) 151.
15 January 1981
[ 161 S J. LouIsnathan and J K Smith, Z. Krlst. 132 (1970) 288. 1171 G.A. Barclay and E.G.Cox. Z. Krlst. 113 (1960) 23. [ 181 Yu.1. Smolm. Yu.F Shepelev, R. Pomess, D. Hoebbel and W. Wleher. KrlstallografiJa 24 (1979) 38.
[ 191 M Jansen. Angew Chem. 91 (1979) 35 [201 D W-J. Crulckshank, J Chem. Sot. (1961) 5486
335
77, number
Zetttralmstztut
Reccwed
CHEVICAL
2
fhr attorgattrsche
7-7 August
PHSSICS
LC77-CRS
C/tetttte der Akadettrte der Wtssetrsciraftetz der DDR,
I I99-Brrlur,
DDR
1980
69H20 were measured usmg the cross “SI NhlR spectra of polycrystallmc Ca6 [SI~O,I(OH)~] and [(CH3)4N1 gS1g0x1 polarlzatmn double-resonance technique Observed shleldmg tensors are related to the l.nown SI-0 bond systems The arrangement of the four St-0 bonds III the S104 tetrahedrn 1s rcfiected by the *‘Sx shleldmp tensor TIC most shlcldcd ducctlon corresponds to the shortest Q-0 bond.
I_ Introduction
2 ExperImental
In general high-resolution N?vlR m sohds offers quantitative informatlon about the onentatlon dependence of the chemical siuft and 111this respect it Improves upon conventional lugh-resolution NMR m lsotroplc liquid phases. 31P NMR studies of sohd phosphates and phosphoryl compounds have shown the existence of correlations between the 31P shleldmg tensor and the character of the phosphorus-oxygen bond [ 1 ] . In the case of an axially symmetric stieldmg tensor, the correspondence of the most (least) shielded prmclpai axis of the tensor to the &rection of the P-O bond contaming the highest (lowest) multiple-bond character can be drawn from experimental [2] and theoretrcal [3] mvestigations. In this paper we report mvestlgations of the 29S~ shelding tensor with regard to the vahdlty of these correlations UI the field of hgh-resolution I95 NhJR m crystalhne silicates. We present the “9S~ spectra of tricalcium sticate hydrate Ca6 [S1?07 I (OH)6 ] (TCSH) and tetramethylammonium sllicate hydrate [N(CH3)4] 8Si8020- 69H,O (TMAS) and their Interpretation on the tasls of the known structures of these two compounds.
Polycrystallme samples of TCSH and TMAS were prepared and characterlLed by methods reported prevlously [4,5]. The mvestlgatlon of TCSH by the molybdate-reaction method [6] showed an lmpurlty of monoslhcate anions S10:- (10% with reference to the total SI content) m addition to the expected dlslhcate anions
0 009-26
14/S 1/OOOO-0000/S
02.50
0 North-Holland
171 The cross polarlzatlon
S&-
double-resonance eaperlments (PCW techmque [8]) were performed at room temperature on the solid-state multi-pulse spectrometer FKS 178 (Zentrum fur wissenschafthchen Geratebau der AdW der DDR) operatmg at 60 MHz for 1H and 12 MHz for 29%. A variable temperature probe-umt (-180 to +200°C) DRS l-29 contammg a crossed coil arrangement near the Helmholtz condltron was used. For 29S1 resonance excltatlon and detection, a sample co11 of 10 mm diameter was employed. The quahty factor of the ?~SI detection clrcult was 130 Efficient suppression of the 60 MHz proton excltatlon pulses (n/2, lockmg pulses: was provided by a special filter arrangement m the probe head. Measurements were carried out applying proton decouphng pulses having a field strength of 0.6 mT and Pubhslung
Company
331
Volume 77. number 2
15 January 1981
CHEMICAL PHYSICS LETTERS
width of 40 ms. In fulfilling the Hartmann-Hahn cond&on, 2981 mixpulses with a field strength of 3 mT and pulse width of 3 ms were used. The repebtlon rate of the single expenments was 6 s and 12 s for TMAS ana TCSH, respectively_ NMR stab-bon did permit data accumulation
3. Results The observed IgSi spectrum of TCSH shown in fig. 1 exhiilts a clear maxunum and two shoulders m the low-field part. tis lme shape results from the overlappmg of two components suggested by the dotted curve. The asymmetric line shape [9] of the mam component (91% of the total mtensity) indicates the clear axial symmetry of the corresponding chemical shift tensor with 6, = -109 ppm (maxmmm) and 6,, = -35 ppm (shoulder). The additional shoulder at -70 ppm is caused by the second component with nearly symmetric hne shape, mdlcatmg the small axial symmetry of the corresponding chemical stift tensor The separatlon of the two components IS possible by appropnate variations of the experimental condltlons. Due to the different reiaxation times T, of the two components, the mam component can gradually be suppressed so that determmation of the lme shape of the smaller component is easy [lo] _ Correspondmg to the composition of the sample, the more mtense component with the high stieldmg arusotropy no = u,, - u1 = -74 ppm is ascrlbed to the duner skate aruons, whereas the less intense compo-
Fig 1. 29Si Nh¶R spectrum of Gas (S12071 (OH)61 (TCSH)
332
-roe
0
b/ppm
Ftg. 2. 29S~ NMR spectrum of [(CH3)4N] sS~sOzo - 69H20 Cl-hIA.
nent with the smaller anisotropy IS attnbuted to the monomer anions. This interpretation 1s consistent with previous mvestlgatlons showmg that in the case of caicium shcates the shielding anisotropies mcrease Lvlth mcreasing branching of the SiO4 tetrahedra [ 1 I] _ Furthermore, tlus interpretation IS confirmed by the agreement of the lsotroplc chemical shft of the main component with the lsotroplc chemical shft of by Llppmaa et Ca6 [%07 1(OH)6 1 recently obtained al [12] using the magic-angle sample spinmng @US) technique (table 1) The lsotroplc chemical shift of the Impurity (6, = -72 ppm) corresponds to the isotropic value of calcium monosticate Caz [H&O4 1OH] (a,-= -72.5 ppm) [12] _ Fig. 2 shows the 7gS~ spectrum of TMAS. Even in this case the asymmetrlc line shape indicates an axially symmetric chemical shift tensor with 6, = - 73 ppm and S ,, = -152 ppm. The agreement of these data with the correspondmg values obtained by Lippmaa et al. [ 131 1s sufficrently good takmg into consideration the problems of the MAS technique for shieldmg amsotropy measurements. The low-temperature spectrum presentek by Kublcki [14] IS consistent with our room-temperature spectrum. Table 1 summarizes the observed values as well as hterature data. In both sihcates the shieldmg fensor has axial symmetry mth approximately the same value but different sign of shie1dlr.g anisotropy. In order to find correlations between the slueldmg anlsotropy and the silicon oxygen bonds, knowledge of the orientation of the principal axes of the tensor
Volume
77, number
2
CHEMICAL
PHYSICS
Table 1 “S, chemical shift results for ThIAS and TCSH a) --
_ ________
Anion
~.__
Symmetry
SlzO$-
s,so:;
__--
&II -__
nwll
-___
-35
+ 2
III relation
However
to the SI-0
be obtamed duectly
-109
oxygen
f 7-
-74
-82
6
113-l
-148
-68
-97 7 -95 -99 3
+65 +80
--
= -(a,,
effects - hl)
--_-
can
from the powder spectrum, and no
brrdgmg
oxygen
0
I131 I141 1121 _-.
_ -~
are not taken in account
IS necessary.
slueldmg components
Rcf
i 2
a\Kd a\Kd
bond duections
@
-84
+78 5 c 1
SI-0 bond system. According to W1eker [7], III TCSH there are dlsilicate anions Si,O$- consistmg of two Si01,203 end groups, precise data about the environment of the S1 atoms are not available [15]. In order to obtain more mformatlon about distances and angles, we make use of the structure data of the analogous disdicates tdleyite Ca, [S1,071C03] [16] and hemlmorphte Zn, [S1?071 (OH)2] - H,O [ 171 and assume that the geometry of the S1,07 groups m TCSH &sentlaUy do not differ from the known geometry of these two dlsiiicates. In both mmerals the oxygen enwonment of the two S1 atoms belongmg to the S1-,0,
tcrmrnal
+ 7-
c)
-99 f 2
mformation IS obtamed about the orientation of the prmcipal axes. In the present case of axlally symmetric tensors, knowledge of the symmetry of the S10, tetrahedra can be used for assIgnIng the principal axes to the
@
-_-
1981
__--ao
-73 L! 7
=) Aa = -A6
only the pnnclpal
__
______6 IS0 b,
-152 + 2
i; Values in ppm, refercncc ThlS. susceptibdlty 6,,)/3-
61
X\Kll
__--_6 so = (=1+
15 Jammy
LETTERS
groups 1s identical in so far as each S1 atom has as I-.elghbours three termmnl oxygen atoms (OT) and one bridging oxygen atom (OB) FIN 3a shows schematically the S104 tetrahedron 1n hemunorphite The mean values of bond distances and mterbond angles are given. To a good approxunat1on, the environment of each St atom m both compounds has a threefold symmetry a\1s along the bridging S1-0, direction. In the case of TMAS, the complete crystal structure has been determIned by Smohn et al. [ 181 Thus compound contains Si,O$j anions formmg a double four ring with e1ghht S103,2 0 branching groups In this cubeshaped anion the environment of all eight SI atoms 1s identical. Each S1 atom IS surrounded by three brldgng oxygen atoms and one terminal oxygen atom. The values of bond distances and interbond angles are gven 1n fig. 3b. Even 1n this bulky anion, the environment of each Si atom has a threefold symmetry axIs, however UI ths case the symmetry axis coincides with the Si-OT bond direction. Startmg from the axial symmetry of the S104 tetrahedra m these two compounds, 1t 1s possible to relate the onentation of the princtpal ales to the S1-0 bond directions. in the d1s1hcate the d1reci1on of mmunum shielding o,, lies along the S1-0~ bond, and m the double four rmg sd1cate the duectlon of maximum sJueIding a,, lies along the S1-0, bond.
sllrcon
3. Bond lengths (A) and mtcrbond angles (dcg) for the S104 umts m hemunorphltc (a) and ThlAS (b) (schematlc representitlon).
4. Discussion
Rg.
A pictorial
representation
of the shlekhng
tensor, 333
Volume
77,
number 2
CHEhIICAL PHYSICS LETTERS
15 January 1981
5. Conclusions The connection between the bond character of the Si-0 bond and the 2gS1 tielding tensor IS the same as we have observed III the case of 31P NhIR High-resolution NMR in solid phosphates and skates gives informahon about the bonding in the amon tetrahedra, and Fig 4. Plctornl representation of the orlentatlon of the *“Sl shleldmg tensor m TCSH (a) and TMAS 0~) relatwe to the
SI-0
bonds
using the shielding eilipsord, is gven m fig 4. The posltion of the prmc~pal tensor axes related to the SI-0 bond frame shows that the tielding of the 2gS~ atoms and the nature of the oxygen arrangement around the SI atom are unambiguously correlated. ‘ke pressed shielding eUipsold (Au negative) corresponds to the “pressed” tetrahedron of the S104 end groups m TCSH (fig. 4a) and the stretched shreldmg elhpsord (Au pow tlve) corresponds to the “stretched” tetrahedron of the Si04 branchmg group (fig. 4b). End groups and branchmg groups are mverse in &stances and angles (fig. 3) and therefore the mverse ngn of the amsotropy Aa is imrme&ateIy clear. Summarizing these facts, we pomt out that the short Si-0 bond corresponds to strong stieldmg and vice versa. Overlookmg the recent crrticlsm of the n-bond concept III tetrahedral oxoamons [ 151, we mterpret m the usual manner bond lengths and angles m terms of ?Tbond character [20]. in TMAS the shortemng of the Si-0, bond relative to the Si-OB bonds, as well as the widening of the angle 0B--5h-OT relative to the tetrahedron angle (fig. 3b), indicate the concentration of n-bond character m the SI-OT bond. Usmg this relationtip we can estabhsh correlation between the n-bond character in the S104 tetrahedron and the 2gS~ shieldmg arusotropy. It IS possible to make the dlstnbution of n-bond character visrble vra the shieldmg tensor. The bond direction with the h&est n-bond character co&Ides with the drrection of the observed maxLmum shielding of the Si nucleus. Startmg from this basis, future high-resolution NMR in sohds may provide quantitative information about the bondmg in silicates.
334
includes determination of the lsotropx chemical sfufts m order to identify the type of anrons as a valuable special case. The simllanty of the correlations in phosphates and sdlcates confirms the well-known analogy of these two classes of compounds and possibly reflects a general regularity whrch may be vahd for other morgamc oxygen bridged polymers
Acknowledgement The authors are grateful to Dr. D. Hoebbel for provldmg TMAS and to Professor 1%‘.Weker and Dr. K.-J. Jost for helpful dlscusnons.
References [I]
A-R
Crunmer,
[2] A -R Gnmmer,
Spectrochun Acta 34A (1978) 941 Proceedmg of the ‘20th Congress Amp&e, eds C. Kundla. C. Lrppmaa and T Saluvere
Talhnn, 1978, (1979) p 483 [31 R Radegha and A -R Grunmcr, Z. Physlk. Chcm , to be pubhshed 141 GE. Bcssey, Report of the Buildmg Research Borad, 1935 (London, 1936) p- 35 [51 D Hoebeland W Wlcber, Z Anorg Al@ Chem 384 (1971) 43 rf.51 E Thdo, W Wicker and H Stride, 2. Anorp A& Chcm 340 (1965) 26 1. 171 W Wicker, III Neuere Ergebnissc der anorgaruschen Chcmle (Deutscher Vcrlag dcr Wissenschaften, Berlm, 1974) p 238 181 A. Pmes, h1.G Gabby and J S. Waugh, BuU Am. Phys Sot. 16 (1976) 1403. 191 hf hlchrmg, in NMR basis pnncrples and progress. Vol 11, eds P DiehI. E Fluck and R Kosfeld (Sprmger, Berlm, 1976) [IO] A.-R Grunmer, D hfuller, R Peter and E rechner, to be
pubhshed. [ 1I] E. Fechner, A.-R. Grimmer and W Wicker, Z Chem (1978) 420.
18
Volume 77.
number
2
CHEMICAL PHYSICS LETTERS
[ 121 E. Llppmaa, M. hlagi, A. Samoson. C. Engelhardt and A.-R. Grunmer, J. Am. Chem. Sot. 102 (1980) 4889 i 131 E Lippmaa, hl. AlJa, T. Pehk and G. Engelhardt, J. Am. Chem Sot. 100 (1978) 1929 [ 141 N. Kubvzki. Drplomarbeit. Karl-Marx-UAersitat, Leipzig (1979). [I51 Ch S Mamedov, RP. Klebzova and N.V. Belov, Dokl. Akad. Nauk SSSR 126 (1959) 151.
15 January 1981
[ 161 S J. LouIsnathan and J K Smith, Z. Krlst. 132 (1970) 288. 1171 G.A. Barclay and E.G.Cox. Z. Krlst. 113 (1960) 23. [ 181 Yu.1. Smolm. Yu.F Shepelev, R. Pomess, D. Hoebbel and W. Wleher. KrlstallografiJa 24 (1979) 38.
[ 191 M Jansen. Angew Chem. 91 (1979) 35 [201 D W-J. Crulckshank, J Chem. Sot. (1961) 5486
335