Longitudinal 29Si Nuclear Magnetic Relaxation in Aqueous Alkali-Metal Silicate Solutions Revisited

Article abstract of DOI:10.1021/jp9618845

We report the results of a wide ranging 29Si longitudinal (spin-lattice) relaxation study of aqueous alkali-metal silicate solutions.Measurement and interpretation of relaxation rates for the many chemically distinct Si centers found in these solutions is complicated by (a) Si-Si chemical exchange during the inversion-recovery pulse sequences, (b) persistent paramagnetic impurities, and (c), in the case of moderately concentrated low-temperature solutions, the inapplicability of the extreme narrowing condition.The situation is therefore complex but nonetheless amenable to analysis.Our findings indicate that the primary causes of 29Si longitudinal relaxation are interactions (dipolar and contact) with adventitious paramagnetic ions, dipole-dipole relaxation by solvent protons, and an apparent spin rotation interaction.The relative importance of these mechanisms at the individual Si centers is a function of solution composition and purity, along with the size and structure of the corresponding silicate anions.Analysis of the 29Si-1H dipole-dipole contribution to relaxation for the hydrated silicate monomer anion yielded an activation energy for isotropic tumbling (between 275 and 375 K) of 18 +/- 3 kJ mol^-1.Contrary to an earlier suggestion, there was no measurable contribution from dipole-dipole interactions with alkali-metal nuclei.

Full text of DOI:10.1021/jp9618845

J. Phys. Chem. 1996, 100, 18351-18356
18351
29
Longitudinal Si Nuclear Magnetic Relaxation in Aqueous Alkali-Metal Silicate Solutions
Revisited
Stephen D. Kinrade*
Department of Chemistry, Lakehead UniVersity, Thunder Bay, Ontario P7B 5E1, Canada
Kirk Marat
Department of Chemistry, UniVersity of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada
Christopher T. G. Knight
School of Chemical Sciences, UniVersity of Illinois at Urbana-Champaign, 505 South Mathews AVenue,
Urbana, Illinois 61801
ReceiVed: June 24, 1996; In Final Form: August 30, 1996X
We report the results of a wide ranging 29Si longitudinal (spin-lattice) relaxation study of aqueous alkali-
metal silicate solutions. Measurement and interpretation of relaxation rates for the many chemically distinct
Si centers found in these solutions is complicated by (a) Si-Si chemical exchange during the inversion-
recovery pulse sequences, (b) persistent paramagnetic impurities, and (c), in the case of moderately concentrated
low-temperature solutions, the inapplicability of the extreme narrowing condition. The situation is therefore
complex but nonetheless amenable to analysis. Our findings indicate that the primary causes of 29Si longitudinal
relaxation are interactions (dipolar and contact) with adventitious paramagnetic ions, dipole-dipole relaxation
by solvent protons, and an apparent spin rotation interaction. The relative importance of these mechanisms
at the individual Si centers is a function of solution composition and purity, along with the size and structure
of the corresponding silicate anions. Analysis of the 29Si-1H dipole-dipole contribution to relaxation for
the hydrated silicate monomer anion yielded an activation energy for isotropic tumbling (between 275 and
375 K) of 18 ( 3 kJ mol-1. Contrary to an earlier suggestion, there was no measurable contribution from
dipole-dipole interactions with alkali-metal nuclei.
Introduction
metal silicate solutions, temperature-dependent transverse re-
laxation is due mainly to rapid Si-Si chemical exchange. At
moderately elevated temperatures, the exchange process also
causes gross averaging of longitudinal relaxation rates as
measured by the inversion-recovery procedure, so that the data
for individual anions are rendered meaningless.8 Furthermore,
they demonstrated that at elevated concentrations and low
temperatures the extreme narrowing condition may not be
applicable (i.e., the rate of random molecular tumbling does
not sufficiently exceed the Larmour frequency), making con-
ventional relaxation analysis difficult.8 Nevertheless, they
determined that longitudinal relaxation is apparently influenced
by both 29Si-1H dipole-dipole and spin rotation interations as
evidenced, respectively, by a small 29Si-{1H} nuclear Over-
hauser effect (NOE) and by a rise in T1-1 as the temperature is
increased. Because the relaxation rates of individual solutions
The substantial gains made over the past 20 years in
understanding the speciation and physical chemistry of aqueous
silicate solutions have been almost entirely due to the advent
of high-resolution, Fourier transform 29Si NMR spectroscopy.1-4
Knowledge of the pertinent 29Si relaxation characteristics is
crucial to optimizing information available from NMR analysis,
and, indeed, considerable effort has been devoted over the years
to measuring longitudinal relaxation rates (T1-1) for aqueous
silicates.5-10 Reported rates span over 2 orders of magnitude
(from 0.04 to 6.71 s-1),5,6 but there is as yet no real consensus
regarding the primary mechanisms of relaxation.
The first thorough analysis of 29Si relaxation in aqueous
silicate solutions was reported by Harris and Newman6 in 1977.
Although their observations were based on limited data and, at
the time, some doubt existed as to the structures of anions giving
rise to 29Si NMR spectra, they ruled out Engelhardt’s earlier
suggestion5 that longitudinal relaxation could be caused by
proton chemical exchange and instead proposed that unknown
paramagnetic contaminants in their samples were the primary
cause of anomalously rapid longitudinal and transverse (spin-
spin) 29Si relaxation. Additionally, they noted smaller but
nonetheless significant longitudinal relaxation contributions from
29Si-1H dipole-dipole interactions.
2
were found to be roughly proportional to R ) ∑[ηγM S(S +
1)], where η is the fractional abundance of an isotope of spin S
for a given alkali-metal cation M, they suggested that 29Si-M
dipole-dipole interactions also play a significant role. (The
authors noted that proof for this tentative hypothesis could be
obtained through 29Si-{M} NOE experiments but lacked access
to the necessary equipment to carry these out.)
McCormick et al.9 reported that they were unable to find any
correlation between the cation dipole moment and 29Si longi-
tudinal relaxation rates and dismissed the possibility of a
29Si-M dipole-dipole relaxation process. By default, relax-
ation by unidentified paramagnetic impurities was invoked as
the primary cause of longitudinal relaxation. Spin rotation was
not considered, and scalar coupling was proposed as the
Kinrade and Swaddle4 later demonstrated that, if precautions
are taken to minimize paramagnetic contamination of alkali-
* Corresponding author. Tel: 807-343-8683. Fax: 807-346-7775.
E-mail: Stephen.Kinrade@lakeheadu.ca.
X Abstract published in AdVance ACS Abstracts, November 1, 1996.
S0022-3654(96)01884-9 CCC: $12.00 © 1996 American Chemical Society

18352 J. Phys. Chem., Vol. 100, No. 47, 1996
Kinrade et al.
principal mechanism governing transverse relaxation. The
applicability of this group’s findings unfortunately is limited
since the solutions used to compare cation effects varied widely
in composition (from [SiIV] ) 1.8-3 mol% and [SiIV]:[MOH]
) 1:2-1:5), were prepared with reagent grade materials, and
were placed in unprotected glass NMR tubes (a procedure
known to contaminate samples very rapidly6). Moreover, they
made no mention of the temperature at which their measure-
ments were undertaken and therefore failed to rule out the
-1
possibility of T1 exchange averaging.
The nature of longitudinal 29Si relaxation in alkali-metal
silicate solutions remains largely unresolved. This paper
describes an extensive reevaluation of the problem. Our data
reveal that, depending upon solution composition and anion
morphology, the main relaxation contributions are (a) an
apparent spin rotation mechanism, (b) 29Si-1H dipole-dipole
interactions, and (c) unpaired electrons of an unknown para-
magnetic impurity. We note moreover the lack of any measur-
able 29Si-{23Na} NOE which indicates, contrary to an earlier
suggestion,8 that dipole-dipole interactions between 29Si and
alkali-metal nuclei do not contribute appreciably to 29Si
relaxation.
Figure 1. Temperature dependence of 29Si longitudinal relaxation rates
for a solution containing both 2.2 mol kg-1 SiO2 (95 atom % 29Si) and
NaOH. (Solvent enriched 75 atom % in 2H.) The silicate anions
represented are the monomer (H4-qSiO4q-, Q0), dimer (H6-qSi2O7
Q1 ), cyclic trimer (H6-qSi3O9q-, O2 ), cyclic tetramer (H8-qSi4O12
,
,
q-
q-
2
3
Q2 ), tetrahedral tetramer (H4-qSi4O10q-, Q3 ), prismatic hexamer
4
4
(H6-qSi6O15q-, Q3 ), and cubic octamer (H8-qSi8O20q-, Q3 ).
6
8
-1
-1
precision of T1 and T2 measurements was typically (5%
Experimental Section
or better. Gated H or 23Na decoupling was used in the NOE
1
Over the course of our silicate research, we have measured
relaxation rates in approximately 120 silicate solutions of
varying composition. The solutions, in all cases, were prevented
from ever coming into contact with glass surfaces. All stock
solutions and samples were prepared directly in low-density
polyethylene bottles or specially prepared Teflon FEP/TFE
NMR tube liners. These vessels were each presoaked in
successive solutions of nitric acid, hydrochloric acid, and Na2-
EDTA so as to mitigate external contamination from paramag-
netic impurities.
experiments.
All 29Si-{23Na} experiments were conducted on the AMX
500 using a home-built, double-tuned probe head. The optimum
decoupler settings for power and duration of 23Na saturation
were directly determined by presaturating the sample under
investigationson and off the 23Na resonancesand detecting the
resultant 23Na signal using the observe channel. The 29Si-X
dipole-dipole relaxation contribution to longitudinal relaxation
rate is given by12
Amorphous silica was either prepared by the hydrolysis of
SiCl4 (Aldrich, 99.999%) or obtained directly as 29Si-enriched
SiO2 (U.S. Services, 95 atom %; Isotech, 67 atom %). Alkali-
metal hydroxides were obtained from various commercial
suppliers, with reported purities ranging from 99.9 to 99.99%
semiconductor grade. These were generally used as received,
with stock solutions being standardized against potassium
hydrogen phthalate. In some cases, the hydroxides were
analyzed by ICP and/or treated with Chelex 100 cation exchange
resin (BDH) that had been preexchanged with the appropriate
M+ alkali-metal cation.
-1
T1,DDX-1 ) [2γ(29Si)/γ(X)]TηT1
(1)
where γ is the magnetogyric ratio, T is temperature, and η is
the measured nuclear Overhauser enhancement factor which has
a maximum value of -2.52 for 29Si-{1H} and -0.66 for
29Si-{23Na}.
Precautions were generally taken to remove dissolved oxygen,
although the corresponding unpaired electron relaxation con-
tribution has been shown to be small (less than 0.01-0.04
s-1).6,8
All solutions were prepared using freshly boiled type-I
deionized water and D2O (Aldrich, 99.8 atom %) for the NMR
lock. The chelating agent 4,7,13,16,21,24-hexaoxa-1,10-diaza-
bicyclo[8.8.8]-hexacosane or “cryptand 2.2.2” (Aldrich; none
of 27 potential contaminants detected by ICP) was added to a
few sodium silicate solutions. Since cryptand 2.2.2 effectively
sequesters alkali-metal cations, its presence may be used to
determine their influence on 29Si relaxation.
Results and Discussion
We show in Figure 1 the characteristic temperature depen-
dence of the 29Si longitudinal relaxation rate for various silicate
anions in a solution containing both 2.2 mol kg-1 SiO2 and
-1
NaOH. As mentioned above, gross averaging of T1 due to
rapid Si-Si exchange during the delay time τ of the inversion
recovery sequence limits the temperature range over which
reliable data can be obtained.8 For the solution represented in
Figure 1, measurements are therefore accurate only up to about
320 K. The exchange process is slower for solutions with
[OH-]:[SiIV] > 1:1 (i.e., as pH is raised)4 and, under these
conditions, T1-1 measurements are reliable to somewhat higher
temperatures. We consider each of the possible contributions
to 29Si longitudinal relaxation, according to the following
equation:12
Silicon-29 NMR experiments were conducted on Bruker
AC-E 200, Varian XL200 (each at 39.7 MHz), Nicolet 300 (59.6
MHz), and Bruker AMX 500 (99.4 MHz) spectrometers, using
probe inserts (except the Nicolet) and sample tubes11 that had
been constructed from Si-free materials. Ninety degree pulses
were used with an interpulse delay of at least 5 times the longest
T1 value. All 29Si peak assignments were taken from previous
studies.1-3 Transverse relaxation rates were estimated from half-
-1
height line widths (T2 ) π∆ν1 2) under solution conditions
/
that excluded any possibility of line broadening caused by rapid
Si-Si exchange.4 Longitudinal relaxation rates were measured
by the 180°-τ-90°-acq inversion recovery method. The
-1
T1-1 ) T1,SA-1 + T1,SC-1 + T1,DD-1 + T1,UE-1 + T1,SR
(2)

29Si Relaxation in Aqueous Alkali-Metal Silicates
J. Phys. Chem., Vol. 100, No. 47, 1996 18353
Figure 2. Temperature dependence of the 29Si-1H dipole-dipole (DDH) contribution to the longitudinal relaxation rates for the monomer, dimer,
and cyclic trimer anions in a solution containing 2.2 mol kg-1 SiO2 (95 atom % 29Si) and 8.7 mol kg-1 MOH, where M+ ) Na+ (O), K+ (0) or
2
Rb+ (4). (Solvent-enriched 75 atom % in H.)
Shielding Anisotropy (T1,SA-1). Any significant contribution
from the shielding anisotropy mechanism would be evident by
its dependence on the square of the external magnetic field B0.12
As shown elsewhere,8 this is not the case for silicates. For
concentrated solutions at low temperature, the relaxation rates
of individual anions actually decrease upon increasing B0,
indicating that the extreme narrowing condition ceases to hold.
It is essential, therefore, to interpret relaxation data obtained
from viscous solutions with this in mind.
Scalar Coupling (T1,SC-1). For the scalar coupling mech-
anism (of the first or second kind) to affect 29Si longitudinal
relaxation efficiently, either the resonance frequency of a
coupled nucleus must be close to that of 29Si or the spin-spin
coupling constant must be very large.12 As noted elsewhere,6,8
neither of these criteria is met and therefore this mechanism
can be ignored.
Dipole-Dipole (T1,DD-1). Dipole-dipole interactions, either
Figure 3. Arrhenius plot (33 data points) of the DDH contribution to
with other nuclei or with electrons (dealt with separately below),
typically dominate the relaxation of spin-1/2 nuclei.12 The
dipole-dipole relaxation contribution of neighboring 1H nuclei
(DDH), readily determined from 29Si-{1H} NOE measure-
ments, has been reported6,8 to be of only minor significance for
alkali-metal silicate solutions. In the present study, the fractional
DDH contribution, T1,DDH-1/T1-1, ranged from zero for a fully
deuterated solution to over 60%, although the upper limit was
only reached when the overall relaxation rate was low (i.e., when
more efficient mechanisms were virtually inoperative) and at
low temperatures. Figure 2 shows that the absolute contribution,
T1,DDH-1, decreases as the temperature is raised owing to an
increase in molecular motion. Figure 3 shows an Arrhenius
the longitudinal relaxation of the silicate monomer for 10 alkali-metal
silicate solutions having a wide range of compositions (M ) Na, K, or
Rb; [SiIV] ) 0.3-2.2 mol kg-1; and [OH-]:[SiIV] concentration ratios
ranging from 1:1 to 4:1). (Solvent-enriched 75 atom % in 2H for each
solution.)
-1
plot of T1,DDH for the monomeric anion in solutions with
widely varying compositions, but with constant deuterium
enrichment. The correlation is reasonably good, with remark-
ably little scatter, despite the fact that the data are derived from
a large range of solution conditions (Na, K, Rb, and Cs silicate
solutions containing 0.3-2.2 mol kg-1 SiO2 and [OH-]:[SiIV]
concentration ratios ranging from 1:1 to 5:1). Assuming that
all DDH interactions are intramolecular, i.e., for a fully hydrated
anion, the data yield an activation energy for isotropic molecular
rotation of 18 ( 3 kJ mol-1 (between 275 and 375 K), which
is comparable to the value 16.9 ( 0.6 kJ mol-1 determined for
the isotropic tumbling of water molecules in a solution contain-
Figure 4. DDH contribution to the longitudinal relaxation rates for a
solution containing both 0.08 mol kg-1 SiO2 (95 atom % 29Si) and
MOH, where M+ ) Na+ or [Na‚cryptand]+. (Solvent-enriched 20 atom
2
% in H.)
the [OH-]:[SiIV] ratio, the anion structure, or the type of alkali-
metal cation in solution. However, the picture may change in
the presence of organic bases (such as tetramethylammonium
hydroxide, TMAOH)10,15 or when the alkali-metal cation is
chelated with an organic complexing agent such as cryptand
2.2.2.16 For instance, the DDH mechanism is a somewhat
greater contributor to T1-1 in solutions containing [Na‚cryptand]+
ing both 0.25 mol kg-1 SiO2 and KOH.13 Accordingly, the
-1 14
estimated rate of tumbling (τc
)
for the monomer is about
1.6 × 1010 s-1 at 298 K and 1.0 × 1011 s-1 at 400 K which,
being significantly greater than the 29Si Larmor frequency for
each spectrometer employed, assures the applicability of the
extreme narrowing assumption under these solution conditions.
1
(Figure 4). Selective presaturation of the H signals corre-
sponding to either free or complexed cryptand failed to perturb
the 29Si NMR spectrum, indicating that the cryptand hydrogens
are not directly responsible for the increased DDH relaxation.
-1
As demonstrated in Figures 2 and 4, T1,DDH is virtually
unaffected by variations in the silicon concentration,

18354 J. Phys. Chem., Vol. 100, No. 47, 1996
Kinrade et al.
relaxtion rates were affected. Longitudinal relaxation rates
decreased only slightly, again the effect being most pronounced
for the dimer and least for the cyclic trimer. Transverse
relaxation rates decreased rather more significantly; here, the
relative effects are reversed with the cyclic trimer being most
affected and the dimer least.
The observations above, and those reported by previous
researchers,6,9 beg the question of which paramagnetic ion or
ions are responsible. Harris and Newman6 ruled out both Cr3+
and Mn2+ as contenders since, even when added in quantities
that imparted a distinct color to their solutions, both metal ions
produced only a modest increase in relaxation rates. They also
disregarded Fe3+ on the grounds that it could not be detected
by atomic absorption analysis.
We added 0.0045 mol kg-1 CrCl3‚6H2O to the high-purity,
Chelex-treated sodium silicate solution, giving it a bright green
color. The effects on longitudinal and transverse relaxation rates
are shown in Figure 6. Although longitudinal relaxation rates
increased significantly, more or less returning to values obtained
for the sample prepared with reagent-grade NaOH, transverse
relaxation was virtually unaffected. Harris and Newman6 made
similar observations and suggested that there is insignificant
scalar coupling between 29Si and the unpaired electrons associ-
ated with Cr3+ to affect the rate of transverse relaxation. In
other words, 29Si-electron spin interactions arising from the
presence of Cr3+ are primarily dipolar (outer-sphere) in
character, whereas interactions involving the unknown para-
magnetic contaminants, which do markedly affect transverse
relaxation, appear to have significant contact character, that is,
they are directly associated with unpaired electrons delocalized
at the 29Si nucleus.18 Thus, in all likelihood, the unknown metal
ions are sequestered by silicate anions. Indeed, evidence has
been reported of silicates forming dissolved, colorless complexes
in alkaline solution with both manganese and iron, their
concentration being dependent on the sequence in which the
reagents are combined.19,20
Figure 5. 29Si-{23Na} NMR spectra at 280 K for a solution containing
0.3 mol kg-1 SiO2 and 1.5 mol kg-1 NaOD in 100 atom % D2O: (a)
23Na saturation (10 W, 25 s) 100 kHz off-resonance; (b) 23Na saturation
on-resonance; (c) difference spectraum (a) - (b).
Two remaining dipole-dipole relaxation mechanisms are pos-
2
sible: H-29Si (DDD) and M+-29Si (DDM). (The 0.037 atom
% natural abundance of oxygen-17 is too low for it to be a
factor.) It has been shown8 that DDD interactions are neces-
sarily less important than DDH interactions owing to the smaller
magnetogyric ratio of deuterium. Indeed, for a solution that is
-1
50% enriched in deuterium, T1,DDD ) 0.063T1,DDH-1. As a
definitive test for the importance of the DDM relaxation
mechanism, we conducted 29Si-{23Na} NOE measurements.
The experiments were designed to ensure that (a) the sample
had a significant sodium to silicon ratio, (b) any chance of
29Si-{1H} NOE was eliminated, and (c) the 23Na saturation
signal was of appropriate power and duration. Ultimately, we
determined that 23Na irradiation has no effect whatsoever on
the resulting 29Si NMR spectrum (see Figure 5), indicating that
DDM interactions are insignificant in these solutions. In
retrospect, this is not surprising given the lower concentrations,
smaller magnetogyric ratios and larger internuclear separations
expected for alkali-metal nuclei relative to protons.
As noted above, paramagnetic O2 dissolved in aqueous silicate
-1 6,8
solutions makes a very minor contribution to T1,UE
.
Spin Rotation Interactions (T1,SR-1). As shown in Figure
1, longitudinal relaxation rates increase with temperature
between 300 and 320 K for all Si centers in a solution containing
both 2.2 mol kg-1 SiO2 and NaOH. It is evident from the figure
itself that exchange averaging of T1-1 is insignificant over this
temperature range (calculated inverse spin site lifetimes are e10
s-1 8,13) and, as discussed above, rates of isotropic tumbling at
T g 298 K are sufficiently high to be in accordance with the
extreme narrowing condition. The observed temperature de-
pendence of T1-1 is genuine, therefore, and is consistent with a
significant contribution from the spin rotation relaxation mech-
anism, since this is the only mechanism generally considered
to have a positive activation energy.8,12 Spin rotation relaxation
has been reported for nuclei in other aqueous ions, such as 9Be
in Be(H2O)42+(aq)21 and 195Pt in PtCl42-(aq).22 McCain and
Unpaired Electron (T1,UE-1). Trace impurities of unspeci-
fied transition metal ions have been held responsible for the
wide range of 29Si relaxation rates observed in silicate solu-
tions.6,9,10 and, in particular, for anomalously high relaxation
rates in industrial solutions.5 Similarly, phosphorus-31 relax-
ation in orthophosphate solutions is dominated by paramagnetic
contaminants unless rigorous measures are taken to exclude
them.17 By substituting reagents, we determined that paramag-
netic contaminants in the present study originated principally
from the alkali-metal hydroxides. (Paramagnetic contaminants
also are leached from glass surfaces.6 We have measured as
much as a 3-fold increase in longitudinal relaxation rates for
samples stored for 48 h at room temperature in conventional
borosilicate glass NMR tubes.) Figure 6 demonstrates how
significantly the longitudinal and transverse relaxation rates
decreased in this study when reagent grade NaOH (99.9%) was
replaced with semiconductor grade (99.99%) material. The
effects appear structure specific for the three resonances
observed, being most pronounced for the dimer and least for
the cyclic trimer. Any additional relaxation contributions from
residual cationic paramagnetic contaminants in the high-purity
NaOH were removed by treating the corresponding silicate
solution with Na+ exchanged Chelex 100 resin. ICP analysis
failed to detect any change in the solution’s metal content
following this treatment. Nonetheless, Figure 6 shows that
Markley17 reported an analogous temperature dependence of 31
P
longitudinal relaxation rates for aqueous orthophosphates. They
rejected the spin rotation mechanism, however, stating that the
H2PO42- ion must be “locked into the local water structure by
numerous hydrogen bonds” and is unable therefore to spin
freely. They proposed an alternative mechanism, termed “quasi-
rotation”, in which the site of hydrogen bonding rapidly “rotates”
about the PO4 center rather than the ion itself physically rotating.
An equivalent case could be made for 29Si relaxation in aqueous
silicates, yet the data seem easier to rationalize in accordance
with the conventional mechanism. The rate of isotropic
tumbling by small silicate anions indeed is quite high, as shown

29Si Relaxation in Aqueous Alkali-Metal Silicates
J. Phys. Chem., Vol. 100, No. 47, 1996 18355
Figure 6. Rates of (a) longitudinal and (b) transverse 29Si relaxation for the monomer, dimer and cyclic trimer in solutions at 294 K containing
2.18 mol kg-1 SiO2 and 8.70 mol kg-1 NaOH. Solutions (i) and (ii) were prepared, respectively with 99.9 and 99.99% purity NaOH. Solution (iii)
is solution (ii) after treatment with Chelex 100 cation exchange resin. Solution (iv) is solution (iii) after the addition of 0.0045 mol kg-1 CrCl3‚
2
6H2O. (Solvent-enriched 75 atom % in H.)
Figure 7. Temperature dependence of longitudinal 29Si relaxation rates
Figure 8. Temperature dependence of longitudinal 29Si relaxation rates
for a solution containing 1.0 mol kg-1 SiO2 and 2.0 mol kg-1 MOH,
where M+ ) Na+ (solid symbols) or [Na‚cryptand]+ (open symbols).
for a solution containing 2.18 mol kg-1 SiO2 and 8.70 mol kg-1 NaOH.
2
(Solvent-enriched 75 atom % in H.)
2
(Solvent-enriched 20 atom % in H.)
above. In general, the apparent SR contribution correlates with
molecular size, being greater for Si centers in small ions (see
Figure 1). Yet the dimer and acyclic trimer, which are free to
rotate internally, consistently exhibit faster relaxation than the
monomer, especially at higher viscosities when whole-body
rotation would be comparatively hindered (see Figures 1, 6, 7,
and 8). This is consistent with reports of SR-enhanced 29Si
relaxation at polysiloxane chain end groups.23,24 The terminal
silicons of the acyclic trimer exhibit the highest measurable
relaxation rates in solutions that are free of paramagnetic
impurities (3.4 s-1 for a solution at 300 K with 2.2 mol kg-1
SiO2 and 8.7 mol kg-1 NaOH), presumably owing to this ion’s
greater freedom of internal rotation. Finally, in Figure 8, we
show that addition of cryptand 2.2.2 to a sodium silicate solution
can yield a marked increase in SR relaxation as revealed by
the steep rise in T1-1 as temperature is increased. Very likely,
Na+ chelation frees silicate anions from strong association with
the hydrated cation16 and thereby enhances the rate of isotropic
molecular rotation. The relative importance of this apparent
SR relaxation contribution has previously been overlooked. The
evidence is insufficient, however, to identify the mechanism
unequivocally.
variables including anion size and structure, along with overall
solution composition. These processes are as follows: (1)
relaxation by paramagnetic impurities, which are difficult to
exclude and provide very efficient relaxation, indicating that
some of the aqueous silicate anions effectively sequester metal
ions; (2) an apparent spin rotation contribution, which is
important for all species at elevated temperatures, but is of
greatest consequence for small anions that, in addition to whole-
molecule rotation, are able to rotate internally; (3) 29Si-1H
dipole-dipole relaxation, which dominates systems in which
the silicate anion is in a formal and long-lived association with
solvent molecules, such as appears to be the case in organic
base silicate solutions. This, however, is generally not of
primary importance for alkali-metal silicate solutions.
Acknowledgment. The experimental assistance of D. L. Pole
and R. T. Syvitski is gratefully acknowledged as is the technical
support of the Lakehead University Science Workshop. This
project was supported by the Lakehead University Senate
Research Committee, the Illinois EPR Research Center at the
University of Illinois at Urbana-Champaign under grants
NIHP41-RR01811 and NIHGM4-2208, and the Natural Sciences
and Engineering Research Council of Canada.
Conclusion
References and Notes
Silicon-29 longitudinal relaxation in aqueous alkali-metal
silicate solutions is governed by three main processes, the
relative proportions of which are determined by a host of
(1) Harris, R. K.; Knight, C. T. G. J. Chem. Soc., Faraday Trans 2
1983, 79, 1525, 1539.

18356 J. Phys. Chem., Vol. 100, No. 47, 1996
Kinrade et al.
(2) Knight, C. T. G. J. Chem. Soc., Dalton Trans. 1988, 1457, and
references therein.
(3) Kinrade, S. D.; Swaddle, T. W. Inorg. Chem. 1988, 27, 4253.
(4) Kinrade, S. D.; Swaddle, T. W. J. Chem. Soc., Chem. Commun.
1986, 120; Inorg. Chem. 1988, 27, 4259.
(5) Engelhardt, G. Z. Chem. 1975, 15, 1975.
(6) Harris, R. K.; Newman, R. H. J. Chem. Soc., Faraday Trans 2
1977, 73, 1204.
systems. Our experiments have revealed anomalously low fractional DDH
contributions for cage species such as the cubic octamer (Q3 ), yet the
8
corresponding net relaxation rates are anomalously high. We have suggested
elsewhere [Knight, C. T. G.; Syvitski, R. T.; Kinrade, S. D. Stud. Surf. Sci.
Catal. 1995, 97, 483] that in the presence of tetraalkylammonium cations
the cubic octamer becomes entrapped within a clathrate shell composed of
solvent molecules and the organic cations. Evidently, the shell enhances
the importance of relaxation mechanisms other than DDH (e.g., SR).
(16) Kinrade, S. D.; Pole, D. L. Inorg. Chem. 1992, 31, 4558.
(17) McCain, D. C.; Markley, J. L. J. Am. Chem. Soc. 1980, 102, 5559.
(18) Bertini, I.; Luchinat, C. NMR of Paramagnetic Molecules in
Biological Systems; Benjamin/Cummings: Menlo Park, CA, 1986.
(19) Colodette, J. L.; Rothenberg, S.; Dense, C. W. J. Pulp Paper Sci.
1989, 15, J3.
(20) Abbot, J. J. Pulp Paper Sci. 1991, 17, J10.
(21) Wehrli, F. W. J. Magn. Reson. 1976, 23, 181.
(22) Pesek, J. J.; Mason, W. R. J. Magn. Reson. 1977, 25, 519.
(23) Kosfeld, R.; Kreuzberg, C.; Krause, R. Makromol. Chem. 1988,
189, 2077.
(24) Pai, Y. M.; Weber, W. P.; Servis, K. L. J. Organomet. Chem. 1985,
288, 269.
(7) Sjo¨berg, S.; O¨ hman, L.-O.; Ingri, N. Acta Chem. Scand., Ser. A
1985, 39, 93.
(8) Kinrade, S. D.; Swaddle, T. W. J. Am. Chem. Soc. 1986, 108, 7159.
(9) McCormick, A. V.; Bell, A. T.; Radke, C. J. J. Phys. Chem. 1989,
93, 1737.
(10) Harris, R. K.; Knight, C. T. G. Magn. Reson. Chem. 1986, 24, 872.
(11) Kinrade, S. D.; Swaddle, T. W. J. Magn. Reson. 1988, 77, 569.
(12) See, e.g.: Ando, I.; Webb, G. A. Theory of NMR Parameters;
Academic Press: London, 1983. Harris, R. K. Nuclear Magnetic Resonance
Spectroscopy; Pitman: London, 1983.
(13) Kinrade, S. D. J. Phys. Chem. 1996, 100, 4760.
-1
(14) Calculated using the equation12 T1,DDH ) (µ0/4π)2γSi2γH2p2/r6,
where r is estimated to be 2.2 nm.
-1
(15) The relative DDH contribution T1,DDH-1/T1 is highly structure
specific in organic base solutions. Indeed, Harris and Knight10 have
proposed 1H-presaturation as a simple 29Si NMR assignment tool for such
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