1H Fast MAS NMR studies of hydrogen-bonding interactions in self-assembled monolayers

Article abstract of DOI:10.1021/ja029008u

The structures formed by the adsorption of carboxyalkylphosphonic acids on metal oxides were investigated by ¹H fast magic angle spinning (MAS), heteronuclear correlation (HETCOR), and ¹H double-quantum (DQ) MAS solid-state NMR experiments. The diacids HO₂C(CH₂)_nPO₃H₂ (n = 2, 3, 11, and 15) were adsorbed on TiO₂ and two types of ZrO₂ powders having average particle sizes of 20, 30, and 5 nm, respectively. Carboxyalkylphosphonic acids bind selectively via the phosphonate group, forming monolayers with pendant carboxylic acid groups. Whereas dipolar coupled P-OH protons are detected on TiO₂, there are only isolated residual P-OH groups on ZrO₂, reflecting the relative binding strengths of phosphonic acids on these two substrates. From a comparative ¹H MAS NMR study with an analogous monolayer system, HO₂C(CH₂)₇SH coated gold nanoparticles, the hydrogen-bonding network at the monolayer/air interface is found to be quite disordered, at least for SAMs deposited on nonplanar substrates. Whereas only hydrogen-bonded homodimers occur in the bulk diacids, hydrogen bonding between the carboxylic and phosphonic acid groups is present in multilayers of the diacids on the ZrO₂ nanopowder.

Full text of DOI:10.1021/ja029008u

Published on Web 03/14/2003
1H Fast MAS NMR Studies of Hydrogen-Bonding Interactions
in Self-Assembled Monolayers
†
‡
‡
‡
†
Shane Pawsey, Mark McCormick, Susan De Paul, Robert Graf, Y. S. Lee,
,
†
,‡
Linda Reven,* and Hans W. Spiess*
Contribution from the Department of Chemistry, McGill UniVersity, 801 Sherbrooke Street West,
Montreal, Quebec, Canada H3A 2K6, and Max-Planck-Institut f u¨ r Polymerforschung,
Postfach 3148, D-55021 Mainz, Germany
Received October 18, 2002; E-mail: linda.reven@mcgill.ca
Abstract: The structures formed by the adsorption of carboxyalkylphosphonic acids on metal oxides were
1
1
investigated by H fast magic angle spinning (MAS), heteronuclear correlation (HETCOR), and H double-
quantum (DQ) MAS solid-state NMR experiments. The diacids HO C(CH PO (n ) 2, 3, 11, and 15)
were adsorbed on TiO and two types of ZrO powders having average particle sizes of 20, 30, and 5 nm,
respectively. Carboxyalkylphosphonic acids bind selectively via the phosphonate group, forming monolayers
with pendant carboxylic acid groups. Whereas dipolar coupled P-OH protons are detected on TiO , there
are only isolated residual P-OH groups on ZrO , reflecting the relative binding strengths of phosphonic
acids on these two substrates. From a comparative H MAS NMR study with an analogous monolayer
system, HO C(CH SH coated gold nanoparticles, the hydrogen-bonding network at the monolayer/air
2
)
2 n
3 2
H
2
2
2
2
1
2
2 7
)
interface is found to be quite disordered, at least for SAMs deposited on nonplanar substrates. Whereas
only hydrogen-bonded homodimers occur in the bulk diacids, hydrogen bonding between the carboxylic
and phosphonic acid groups is present in multilayers of the diacids on the ZrO nanopowder.
2
1
. Introduction
to treat industrial materials and minerals containing alumina,
1
1
iron, and tin. Titanium surfaces have been modified with
phosphonic acids to improve the bone binding of medical
Self-assembled monolayers (SAMs) are widely used to
introduce different chemical functionalities on metal and metal
oxide substrates. With the exception of silanes on silica, the
modification of metal oxide surfaces with SAMs is poorly
developed. For metal oxides other than silica, long chain
carboxylic acids have been the surfactants of choice to form
SAMs, but there are relatively few studies as compared to the
1
5
implants and for photoelectrochemical cells based on poly-
16,17
crystalline titanium dioxide films.
More exotic metal oxides,
such as tantalum and niobium oxides, have been derivatized
with self-assembled monolayers of alkyl phosphonates for sensor
3
-5
development.
1
We have recently shown that phosphonic acid based SAMs
can be used to introduce polar functional groups on metal
thiol/gold or silane/silica systems. Limitations of the fatty acid
SAMs include the stability of the surface bond which is ionic
rather than covalent and the possibility of forming undesirable
looping structures for surfactants of the type X(CH2)nCO2H,
where X is some other polar functionality.1,2 Alkyl phosphonates
1
4 31
13
oxides.
P and C solid-state NMR and vibrational spectro-
scopic studies of carboxyalkylphosphonic acids, HO2C(CH2)n-
PO3H2, deposited on TiO2 and ZrO2 powders demonstrate that
the PO3H2 group binds preferentially, creating pendant car-
boxylic acid groups. This particular surfactant is an important
test case as elemental analysis indicates a high degree of surface
and phosphonic acids present an attractive alternative because
they bind more strongly than carboxylic acids to a wide range
of metal oxides.3
-14
Phosphonic acids are commonly employed
1
3
coverage, and spectroscopic data ( C NMR and FT-IR) show
that long chain carboxylic and phosphonic acids both form
as corrosion inhibitors, adhesion promoters, and flotation agents
†
McGill University.
Max-Planck-Institut f u¨ r Polymerforschung.
‡
(10) Shafi, K. V. P. M.; Ulman, A.; Yan, X.; Yang, N.-L.; Estournes, C.; White,
H.; Rafailovich, M. Langmuir 2001, 17, 5093-7.
(
(
(
1) Ulman, A. Chem. ReV. 1996, 96, 1533-54.
2) Pawsey, S.; Yach, K.; Halla, J.; Reven, L. Langmuir 2000, 16, 3294-303.
3) Brovelli, D.; Haehner, G.; Ruiz, L.; Hofer, R.; Kraus, G.; Waldner, A.;
Schloesser, J.; Oroszlan, P.; Ehrat, M.; Spencer, N. D. Langmuir 1999, 15,
(11) Van Alsten, J. G. Langmuir 1999, 15, 7605-14.
(12) Gao, W.; Dickinson, L.; Grozinger, C.; Morin, F. G.; Reven, L. Langmuir
1997, 13, 115-8.
4
324-7.
(13) Gao, W.; Dickinson, L.; Grozinger, C.; Morin, F. G.; Reven, L. Langmuir
1996, 12, 6429-35.
(
4) Textor, M.; Ruiz, L.; Hofer, R.; Rossi, A.; Feldman, K.; Haehner, G.;
Spencer, N. D. Langmuir 2000, 16, 3257-71.
(14) Pawsey, S.; Yach, K.; Reven, L. Langmuir 2002, 18, 5205-12.
(15) Viornery, C.; Chevolot, Y.; Leonard, D.; Aronsson, B.-O.; Pechy, P.;
Mathieu, H. J.; Descouts, P.; Graetzel, M. Langmuir 2002, 18, 2582-9.
(16) Zakeeruddin, S. M.; Nazeeruddin, M. K.; Pechy, P.; Rotzinger, F. P.;
Humphry-Baker, R.; Kalyanasundaram, K.; Graetzel, M.; Shklover, V.;
Haibach, T. Inorg. Chem. 1997, 36, 5937-46.
(
(
5) Hofer, R.; Textor, M.; Spencer, N. D. Langmuir 2001, 17, 4014-20.
6) Woodward, J. T.; Ulman, A.; Schwartz, D. K. Langmuir 1996, 12, 3626-
9
.
(
(
7) Woodward, J. T.; Schwartz, D. K. J. Am. Chem. Soc. 1996, 118, 7861-2.
8) Woodward, J. T.; Doudevski, I.; Sikes, H. D.; Schwartz, D. K. J. Phys.
Chem. B 1997, 101, 7535-41.
(17) Bonhote, P.; Moser, J.-E.; Humphry-Baker, R.; Vlachopoulos, N.;
Zakeeruddin, S. M.; Walder, L.; Graetzel, M. J. Am. Chem. Soc. 1999,
121, 1324-36.
(
9) Yee, C.; Kataby, G.; Ulman, A.; Prozorov, T.; White, H.; King, A.;
Rafailovich, M.; Sokolov, J.; Gedanken, A. Langmuir 1999, 15, 7111-5.
4174
9
J. AM. CHEM. SOC. 2003, 125, 4174-4184
10.1021/ja029008u CCC: $25.00 © 2003 American Chemical Society

MAS NMR Studies of Hydrogen-Bonding Interactions
A R T I C L E S
Figure 1. Schematic of some of the possible structures formed by the adsorption of HO2C(CH2)nPO3H2. (a,b) The surface binding modes may be mono-,
bi-, or tridentate in a bridging or chelating configuration. (c,d) Hydrogen-bonding interactions among the phosphate groups or with the surface hydroxyl
22
groups are also possible. (e,f) Suggested hydrogen-bonding modes of the terminal CO2H groups include acyclic linear hydrogen bonds and distorted
2
3
dimers. (g,h) The hydrogen-bonding arrangement of the acid groups in multilayers may consist of homodimers and/or heterodimers.
densely packed monolayers with ordered alkane chains on
ZrO2.2
adsorption of HO2C(CH2)nPO3H2 on TiO2 and ZrO2 are il-
,12,13
lustrated in Figure 1. The tridentate nature of the PO3H2 group
gives rise to a large range of surface bonding possibilities such
as mono-, bi-, or tridentate bonds in either a bridging or a
chelating configuration (Figure 1a and b). In the event that
P-OH groups are still present after adsorption, it would
conceivably be possible for them to engage in hydrogen bonding
with the metal oxide surface or among neighboring phosphate
groups (Figure 1c and d). Additionally, it is also probable that
remaining P-OH groups could engage in hydrogen bonding
with µ-oxo oxygen atoms on the metal oxide surface (not
shown).
Carboxylic and phosphonic acids both form strong hydrogen
bonds, and the nature of such phenomena at an interface is of
interest in fields such as adhesion, chemical sensors, and
bioseparations, among others. A great deal of effort has been
expended to characterize hydrogen bonding at the monolayer/
air interface of carboxylated organothiol SAMs on gold, which
are extensively used for applications in electrochemistry,
biology, and biomineralization. The measurements of true
surface pKa values18 and the extent of hydrogen bonding at the
air interface in these systems remain experimentally challeng-
1
9
ing. Carboxyalkylphosphonate monolayers on a variety of
metal oxide surfaces represent an analogous system to the
carboxylated organothiol/Au SAMs, thereby providing another
opportunity to study the hydrogen-bonding properties at the
monolayer/air interface.
For hydrogen-bonding interactions of the terminal acid groups
2
2
23
of SAMs, linear hydrogen bonds or distorted cyclic dimers
have both been proposed (Figure 1e and f). If more than a
monolayer is adsorbed, the possible hydrogen-bonding arrange-
ments of the excess diacid include head-to-head or head-to-tail
configurations, as illustrated in Figure 1g and h. These possible
structures were investigated using a combination of high-
Vibrational spectroscopy has been the most useful method
for detecting the presence or absence of hydrogen-bonding
interactions in self-assembled monolayers. However, vibrational
spectroscopy cannot directly give the structural details of the
hydrogen-bonding network as provided by X-ray diffraction of
bulk crystalline solids. With the development of techniques for
1
1
resolution H solid-state NMR experiments. First, H fast MAS
NMR detects the presence of acid protons and allows an estimate
of the hydrogen-bond strength based on the isotropic proton
1
chemical shift. Second, 2D H-X heteronuclear correlation
(HETCOR) experiments (X ) P, C) confirm the spectral
3
1
13
20,21
1
detecting proton-proton proximities,
solid-state H NMR
assignments and identify heteronuclear through-space proximi-
spectroscopy presents an attractive alternative for characterizing
hydrogen-bonding interactions in systems not amenable to X-ray
diffraction and can provide more structural information than
1
ties. Finally, 2D H double-quantum (DQ) MAS NMR gives
more detailed spatial information by detecting dipolar connec-
tivities and, by extension, the specific hydrogen-bonding
interactions. These semiquantitative results, when added to the
information from vibrational spectroscopy, provide a more
complete picture of both the surface bonding and the hydrogen-
bonding interactions in monolayers and multilayers on metal
oxides. A quantitative determination of these structures, derived
from density functional theory (DFT) calculations, will be
1
vibrational spectroscopy alone. H detected NMR spectroscopy
is also advantageous for surface studies. The high sensitivity
and short relaxation times of protons as compared to nuclei with
lower abundances and/or resonance frequencies enable longer
experiments, such as relaxation measurements or two-dimen-
sional NMR experiments, to be applied to surface-bound
molecules.
2
4
presented in a later publication.
In this paper, the structures formed by the adsorption of
carboxyalkylphosphonic acids on metal oxide powders are
2. Methods
1
characterized by H fast magic-angle-spinning (MAS) NMR
1
2
.1. Fast MAS H NMR. Basic information about hydrogen bonding
experiments. Some of the possible structures formed by the
1
can be gained from the solid-state H chemical shifts. The chemical
shifts of acid protons have been empirically correlated to the hydrogen-
(
18) Smalley, J. F.; Chalfant, K.; Feldberg, S. W.; Nahir, T. M.; Bowden, E. F.
J. Phys. Chem. B 1999, 103, 1676-85.
(22) Leopold, M. C.; Black, J. A.; Bowden, E. F. Langmuir 2002, 18, 978-80.
(23) Smith, E. L.; Alves, C. A.; Anderegg, J. W.; Porter, M. D.; Siperko, L. M.
Langmuir 1992, 8, 2707-14.
(24) McCormick, M.; Sebastiani, D.; Pawsey, S.; Reven, L.; Spiess, H. W.
Unpublished results.
(
19) Arnold, R.; Azzam, W.; Terfort, A.; Woll, C. Langmuir 2002, 18, 3980-
9
2.
(
20) Schnell, I.; Spiess, H. W. J. Magn. Reson. 2001, 151, 153-227.
(
21) Brown, S. P.; Spiess, H. W. Chem. ReV. 2001, 101, 4125-56.
J. AM. CHEM. SOC.
9
VOL. 125, NO. 14, 2003 4175

A R T I C L E S
Pawsey et al.
bond lengths in carboxylic and phosphonic acids.25 Strong homonuclear
dipolar couplings complicate the NMR spectroscopy of protons in rigid
organic solids due to substantial broadening of the spectral lines, which
typically obscures any chemical shift information. Previous studies of
the proton chemical shifts in solid organic acids employed multipulse
techniques that were complicated by a significant loss of signal, a
disadvantage for surface studies. These experiments also require a
tedious experimental setup to achieve maximum resolution and require
corrections to the chemical shift scale due to partial averaging of the
experiment has proven to be particularly useful in cases where single
crystals are not available and should likewise be effective for detecting
hydrogen-bonding and other intermolecular interactions of surface-
bound molecules.
3. Experimental Section
Materials. HO C(CH PO H and HO C(CH ) PO H were obtained
2
2
)
2
3 2 2 2 3 3 2
from Lancaster Synthesis Inc. and were used without any further
purification, while HO C(CH and HO C(CH were
2
2
)
11PO H
3 2
2
2 3 2
)15PO H
25
chemical shift interaction by the multipulse sequence. These obstacles
have been removed by the development of high-speed magic-angle-
spinning (MAS) technology, and high-resolution solid-state NMR
spectra of strongly dipolar-coupled spins are now routinely obtained
using spinning frequencies of 25-30 kHz. In addition to information
synthesized. All other reagents were obtained from Aldrich Chemical
Co. unless noted otherwise. Starting with either 12-hydroxydodecanoic
or 16-hydroxyhexadecanoic acid, we protected the carboxylic acid group
through the formation of an ethyl ester by refluxing in absolute ethanol
with 0.1 equiv of acetyl chloride. The resulting ester was brominated
by refluxing in a methylene chloride solution with 1.05 equiv of
N-bromosuccinimide and 1.1 equiv of triphenylphosphine. The resulting
brominated ester was phosphonated with triethyl phosphite by the
1
from the proton chemical shift, variable temperature H MAS NMR
allows the dynamics of hydrogen-bond breaking/formation to be
followed in systems undergoing thermal transitions.
1
2
.2. 2D H-X Heteronuclear Correlation (HETCOR) NMR. The
3 2 2 2 x 2
Michaelis-Arbazov reaction. The product [(H CH CO) OP(CH ) CO -
assignment of the acidic proton resonances is problematic for carboxy-
alkylphosphonic acids in both the bulk and the adsorbed states. The
proton chemical shift ranges of hydrogen-bonded carboxylic and
phosphonic acids in the solid-state overlap extensively, ranging between
CH CH ] was then hydrolyzed by refluxing in a concentrated HCl
2
3
solution for a period of several days. The samples were purified by
multiple recrystallizations.
Nonporous zirconia powder (VP zirconium dioxide, Degussa Corp.)
was calcinated at 400 °C overnight before use to remove the residual
1
2 and 20 ppm for carboxylic acids and 11-17 ppm for phosphonic
25
acids. Furthermore, assignment of the resonances of adsorbed organic
acids cannot be based on the bulk state H chemical shifts. Factors
13
organics that contaminated its C NMR spectra. The reported average
1
2
particle size is 30 nm with a BET surface area of 40 m /g. Titania
such as the amount of adsorbed water, the extent of hydrogen bonding
between the surface-bound acid groups, and the possibility of the
substrate acting as a hydrogen-bond acceptor all have an effect on the
powder (titanium dioxide P25, Degussa Corp.) with a reported average
2
particle size of 21 nm and a specific surface area of 50 m /g was used
without further modification. Nanocrystalline zirconia (ZrO
2
) powder
1
H chemical shifts that will not be observed in the bulk acid. Fortunately,
(separated equiaxial single crystals, cubic zirconium dioxide, Advanced
3
1
1
13
1
P and C NMR can be combined with fast MAS H NMR for 2D
H-X HETCOR NMR experiments to assign the CO H and PO
Powder Technology Pty. Ltd.) had a reported particle size of 5 nm and
2
H
3 2
2
a BET surface area of 140 m /g. The nanocrystalline zirconia powder
proton resonances. The 2D HETCOR experiments employed here use
was used without any further modification. The TiO
talline ZrO showed no signs of organic contamination by C NMR
and as such were not calcinated. It is anticipated that the heating of
the nonporous ZrO will lower the concentration of hydroxyl groups
2
and nanocrys-
1
simple Hartmann-Hahn H-X cross polarization which provided
13
2
3
1
sufficient sensitivity for P detection for all of the surface samples.
Proton spin diffusion, which is present to some extent even under very
fast MAS, leads to spatial equilibration of the proton magnetization
and correlations between more distant spins in the HETCOR spectra.
Therefore, short and long cross polarization contact times were used
respectively to minimize and maximize proton spin diffusion for
identifying shorter- and longer-range spatial correlations.
2
on the surface of the metal oxide, but no controlled studies were
performed to monitor its effect.
Sample Preparation. Adsorbed samples were prepared by dispersing
0
.5 g of the metal oxide substrates in 150-200 mL of acetone with
sonication. A solution of the carboxyalkylphosphonic acid containing
.5 times excess what is necessary to form a monolayer was prepared
2
.3. 1H Double-Quantum (DQ) MAS NMR. In recent years,
2
noncovalent interactions in macromolecular and supramolecular systems
have been studied by a variety of high-resolution homonuclear and
in acetone with vigorous mixing. The well-dispersed substrate was
mixed into the solution of carboxyalkylphosphonic acid and left to
reflux with stirring for 2 days followed by an additional 2 days of
annealing. The solids were separated from the solution by centrifugation
and then filtering. The solid was thoroughly washed and filtered an
additional five times in an attempt to remove any weakly bound
surfactant and finally dried under vacuum. Because of the solubility
heteronuclear multiple quantum (MQ) solid-state NMR techniques
based on through-space dipolar interactions between nuclei.2
0,21
The
largest MQ signals arise from strongly dipolar-coupled protons. Because
the dipolar interaction is averaged by fast MAS, it must be reintroduced
1
1
by a suitable pulse sequence to simultaneously detect strong H- H
1
dipolar couplings and enhance the H chemical shift resolution. Robust
of HO
phonic acids, some samples underwent additional washing steps with
hot methanol. The HO C(CH SH/Au nanoparticles were prepared as
described in an earlier study.
2 2 3 2
C(CH )11PO H as compared to the other carboxyalkylphos-
recoupling pulse sequences for the efficient generation of multiple
quantum coherences in the presence of fast MAS have been developed,
allowing semiquantitative measurement of proton-proton proximities
2
2 7
)
27
1
via 2D H double-quantum (DQ) MAS solid-state NMR experi-
Elemental Analysis. Elemental analysis and calculated coverages
for the carboxyalkylphosphonic acid/metal oxide samples that were
reported previously14 are reproduced here in Table 1. The coverages
2
0,21
ments.
The detection of a DQ signal indicates the presence of a
dipole-dipole coupling between pairs of protons, and the absence of
1
1
such a signal is due to either large H- H distances or molecular
motions on the time scale of the experiment (<100 µs). Short-range
interactions, such as hydrogen-bonding or aromatic π-π interactions,
2
were calculated by assuming that each molecule occupies 24 Å , based
3 2
on the size of the PO H group.
1
Solid-State NMR Spectroscopy. Single- and double-quantum H
MAS NMR spectra were acquired either on a Bruker ASX500 or on a
DRX700 spectrometer equipped with a 2.5 mm fast MAS probe. The
1
1
can be probed because this experiment is sensitive to H- H distances
of less than 0.35 nm. For example, the dimerization of carboxylic acid
groups in liquid crystalline hexabenzocoronene carboxylic derivatives
1
single-quantum H MAS NMR spectra were acquired with a spinning
1
26
was followed by variable temperature H DQ MAS NMR. This
frequency of 30 or 25 kHz using a Hahn echo sequence to suppress
the broad proton background signal from the probe. The back-to-back
(
25) Harris, R. K.; Jackson, P.; Merwin, L. H.; Say, B. J. J. Chem. Soc., Faraday
Trans. 1 1988, 84, 3649-72.
(26) Brown, S. P.; Schnell, I.; Brand, J. D.; M u¨ llen, K.; Spiess, H. W. Phys.
Chem. Chem. Phys. 2000, 2, 1735-45.
(27) Schmitt, H.; Badia, A.; Dickinson, L.; Reven, L.; Lennox, R. B. AdV. Mater.
1998, 10, 475-80.
4176 J. AM. CHEM. SOC.
9
VOL. 125, NO. 14, 2003

MAS NMR Studies of Hydrogen-Bonding Interactions
A R T I C L E S
1
1
31
Figure 2. 1D H DQ MAS and 2D H- P HETCOR NMR spectra of (a) HO2C(CH2)2PO3H2 and (b) HO2C(CH2)15PO3H2 acquired with MAS spinning
frequencies of 30 kHz. The DQ excitation time was 66 µs. Contact times of 1 and 3 ms are shown for the HETCOR spectra of HO2C(CH2)2PO3H2 and
HO2C(CH2)15PO3H2, respectively.
Table 1. Samples and Estimated Coverage from Elemental
Analysis
the nature of the hydrogen bonds and to test the effectiveness
of the HETCOR experiment for assignment of the acid proton
chemical shifts. Next the results for the diacids adsorbed on
two different lower surface area metal oxide powders, TiO2 and
ZrO2, which should contain mainly intramonolayer rather than
intraparticle hydrogen bonds, are described. These results are
compared to an analogous system, HO2C(CH2)7SH monolayers
surfactant
HO
2
C(CH
2
)
n
PO
3
H
2
substrate
coverage
n ) 2
n ) 3
n ) 11
n ) 15
n ) 11
n ) 15
n ) 2
n ) 3
n ) 11
n ) 15
nanoZrO2
nanoZrO2
nanoZrO2
nanoZrO2
ZrO2
ZrO2
TiO2
TiO2
TiO2
248%
182%
104/80%a
103%
90%
94%
82%
73%
90%
94%
1
on gold nanoparticles, which displays H NMR spectra in
appearance similar to that of the carboxyalkylphosphonate
monolayers. This comparison was found to be useful for
1
interpretation of the H NMR results for the adsorbed diacids
TiO2
because the possibility of interactions of the terminal CO2H
groups with the gold substrate is excluded, and yet the small
average gold particle size (∼2 nm diam) allows for intraparticle
hydrogen bonds. Finally, the hydrogen bonding in multilayers
formed by the adsorption of the diacids on a nanoparticle metal
a
Coverage after the HO2C(CH2)11PO3H2 nanoZrO2 sample was subjected
to additional washings with hot methanol.
(
BABA) recoupling pulse sequence,28,29 which efficiently generates
double-quantum coherences in the presence of very fast MAS, was
oxide substrate, ZrO nanopowder (∼5 nm diam), is compared
2
1
used to acquire 1D and 2D DQ H MAS NMR spectra with excitation
to that of the bulk diacids and the monolayers.
times of one or two rotor periods. The 2D 1H- P and H- C
heteronuclear correlation (HETCOR) NMR spectra were acquired at
spinning frequencies of 25 or 30 kHz using ramped polarization transfer
and contact times ranging from 100 µs to 5 ms.
31
1
13
4
.1. Bulk State Diacids: HO2C(CH2)nPO3H2. The 1D
1
1
31
double-quantum filter (DQF) H MAS and 2D H- P HET-
COR NMR spectra of the longest and shortest chain diacids,
HO2C(CH2)nPO3H2, n ) 2 and 15, are given in Figure 2. The
double-quantum filter effectively suppresses the adsorbed water
signals of the carboxyalkylphosphonic acids. The short chain
diacids, n ) 2 and 3, are especially hydroscopic and still showed
a water signal even after drying overnight under vacuum. An
excitation time of two rotor periods (66 µs) was used to enhance
the DQ signals of the acid protons, which have much weaker
dipolar couplings as compared to those of the methylene protons.
In addition to the broad resonance at ∼1-2 ppm for the aliphatic
protons, there are two resonances for the acidic protons. The
positions and line widths of the two acidic proton peaks varied
with the water content, approaching the shifts of 11 and 13.0
4. Results
The characterization of the hydrogen-bonding interactions of
bulk and adsorbed HO2C(CH2)nPO3H2 as a function of chain
length, substrate composition, and substrate particle size is
presented in this section. The NMR methods described above
were combined to assign chemical shifts, estimate the hydrogen-
bond lengths, and detect dipolar connectivities. The bulk diacids
are examined first as a function of chain length to determine
(
28) Feike, M.; Demco, D. E.; Graf, R.; Gottwald, J.; Hafner, S.; Spiess, H. W.
J. Magn. Reson., Ser. A 1996, 122, 214-21.
(29) Sommer, W.; Gottwald, J.; Demco, D. E.; Spiess, H. W. J. Magn. Reson.,
Ser. A 1995, 113, 131-4.
1
ppm after drying. Because the H chemical shift ranges of
J. AM. CHEM. SOC.
9
VOL. 125, NO. 14, 2003 4177

A R T I C L E S
Pawsey et al.
1
1
31
Figure 3. (a) 1D H MAS NMR spectra of ZrO2 powder and HO2C(CH2)nPO3H2, n ) 3, 11, 15, adsorbed on ZrO2. (b) 2D H- P HETCOR NMR
spectrum of HO2C(CH2)11PO3H2 adsorbed on ZrO2. A contact time of 100 µs was used for the HETCOR spectrum.
carboxylic and phosphonic acid protons overlap,16 1H-31P
HETCOR NMR spectra were acquired. Short contact times of
100 µs (not shown) were used to minimize proton spin diffusion.
At the short contact time, there is only a correlation of the
phosphate group to the peak at 11 ppm for both the short and
the long chain diacids. The broad proton resonance at 11 ppm
is thus assigned to the PO3H2 group, and the other acid proton
resonance at ∼13 ppm is assigned to the CO2H proton. The
isotropic proton chemical shifts can be empirically correlated
with the hydrogen-bond length, rO-O.25 The shifts for the CO2H
and PO3H2 protons correspond to hydrogen-bond lengths of 2.66
and 2.63 Å, respectively.
1
31
1
Figure 4. (a) 2D H- P HETCOR NMR and (b) 2D H DQ NMR spectra
of HO2C(CH2)15PO3H2 adsorbed on TiO2 acquired at MAS spinning
frequencies of 30 kHz. A contact time of 100 µs was used for the HETCOR
spectrum, and the DQ excitation time was 66 µs.
1
Note that a shoulder at 13 ppm in the H dimension of the
1
31
HETCOR spectrum is present at longer contact times for the
short chain diacid, n ) 2, but not for the long chain, n ) 15.
Apparently the distance between the two acid groups in the long
chain diacids is sufficient to prevent the development of a
correlation between the phosphonate group and the carboxylic
acid protons by spin diffusion, even for contact times as long
be ruled out from the H- P HETCOR NMR experiments on
the bulk long chain diacids, which showed that the phosphonic
acid group and CO2H protons are too far apart for a correlation
1
to develop. Furthermore, the H MAS NMR spectrum of
octadecylphosphonic acid on ZrO2 (not shown) also contains a
very weak broad peak at ∼8-10 ppm. The infrared spectra of
the HO2C(CH2)nPO3H2/ZrO2 samples all have a strong band at
1
as 5 ms. The resonances for the PO3H2 groups in the H NMR
-
1
spectra of the bulk diacids are notably broader than those of
the CO2H groups. The larger line width may reflect a more
disordered or complex hydrogen-bonding network among the
PO3H2 groups. The phosphonic acid group can act as both a
hydrogen-bond donor through the two P-OH groups and an
acceptor via the PdO oxygen. This tridentate character of the
PO3H2 group can naturally lead to more complex hydrogen-
bonded structures.
∼1710 cm , assigned to hydrogen-bonded carboxylic acid
groups, and no bands attributable to the formation of surface
1
4
1
zirconium carboxylate bonds. However, the H NMR signals
of these pendant carboxylic acid groups appear to be broadened
beyond detection.
The H MAS NMR spectra of the HO2C(CH2)nPO3H2/TiO2
samples, not shown, contain a resonance at 12 ppm in addition
to the aliphatic peak at 1 ppm. On the basis of the H- P
HETCOR NMR spectrum shown in Figure 4a, this peak is
assigned to P-OH protons rather than to the pendant CO2H
groups. The H MAS NMR spectrum of octadecylphosphonic
acid on TiO2 (not shown) also displays the 12.0 ppm peak. Like
1
1
31
4
.2. HO2C(CH2)nPO3H2 on ZrO2 and TiO2 Powders. The
1
H fast MAS NMR spectra of the nonporous ZrO2 powder (s.a.
40 m /g) and HO2C(CH2)nPO3H2, n ) 3, 11, and 15, adsorbed
on the ZrO2 powder are given in Figure 3a. The ZrO2 powder
consists of a broad resonance at ∼6 ppm, assigned to the protons
from the surface hydroxyl groups. The HO2C(CH2)nPO3H2/ZrO2
samples display the aliphatic resonance at ∼1 ppm, a weak Zr-
OH peak at 6 ppm, and a weak broad resonance centered at
2
1
≈
1
the zirconia-bound monolayers, the H NMR resonances of the
pendant CO2H groups appear to be broadened into the baseline.
A 2D H DQ MAS NMR spectrum of HO2C(CH2)15PO3H2/
TiO2 is shown in Figure 4b. 2D H DQ MAS NMR spectra
1
1
∼
8-10 ppm. The weak peaks at ∼8-10 ppm were not detected
consist of the single-quantum (SQ) dimensions plotted against
the double-quantum dimensions (DQ). Peaks that fall along the
diagonal (ω, 2ω) are autocorrelation peaks, while pairs of off-
diagonal peaks at (ωa, ωa + ωb) and (ωb, ωa + ωb) correspond
to correlations between two protons with different chemical
1
1
13
in the H DQF MAS NMR spectra. A 2D H- P HETCOR
NMR spectrum, given in Figure 3b, shows that this peak is due
to P-OH groups rather than the carboxylic acid protons. The
1
13
signal sensitivity of this sample is insufficient for a H- C
HETCOR NMR experiment. However, any possibility that the
weak 8-10 ppm H peak is due to carboxylic acid protons can
2
0
1
shifts. The 2D H DQ MAS NMR spectrum of the HO2C-
(CH2)15PO3H2/TiO2 sample shows autocorrelation peaks for the
1
4178 J. AM. CHEM. SOC.
9
VOL. 125, NO. 14, 2003

MAS NMR Studies of Hydrogen-Bonding Interactions
A R T I C L E S
1
1
Figure 6. 1D H DQ NMR spectra of HO C(CH ) PO H , n ) 2, 3, 11,
Figure 5. Variable temperature H MAS NMR spectra of HO2C(CH2)7-
2
2 n
3
2
1
3
5, adsorbed on nanoZrO2, acquired with MAS spinning frequencies of
0 kHz and DQ excitation times of 66 µs.
SH coated Au nanoparticles acquired at MAS spinning frequencies of
3
0 kHz.
aliphatic region at 2.7-2.4 ppm have chemical shifts in the same
range as the R methylene protons of disulfides (2.7 ppm) and/
or thiols (2.5 ppm). The R methylene protons are normally
aliphatic and the P-OH protons. Depending on the binding state,
the autocorrelation peak for the P-OH protons of the
HO2C(CH2)15PO3H2/TiO2 sample may be due to inter- and/or
intramolecular dipolar couplings because the adsorbed phos-
phonic acid group may contain two P-OH groups. The weak
P-OH proton peak in the HO2C(CH2)nPO3H2/ZrO2 samples on
the other hand did not give rise to a DQ signal, indicating that
only isolated residual P-OH groups are present.
1
noticeably absent in the solution H spectra of alkanethiols
3
0
bound to gold nanoparticles. We propose that the surfactant
comes off the nanoparticles primarily in the form of a disulfide
due to oxidation during heating. Only when the surfactant is
no longer surface-bound can the carboxylic acid groups rear-
range to form head-to-head dimers and a more homogeneous
bonding arrangement, giving rise to an observable CO2H
resonance. The shifting of a CO2H proton peak between 9 and
.3. Variable Temperature 1H MAS NMR of HO2C-
4
1
(
CH2)7SH/Au Nanoparticles. The assumption that the H NMR
resonances of the pendant carboxylic acid groups of the HO2C-
1
2 ppm was also observed for hexabenzocoronene carboxylic
(
CH2)nPO3H2/ZrO2 and TiO2 samples are heterogeneously
2
6
1
derivatives heated to the liquid crystalline state. In this study,
the proton chemical shift of 9 ppm in the liquid crystalline state
was attributed to free CO2H groups which re-form hydrogen-
bonded dimers upon cooling, returning to the original chemical
shift of 12 ppm. A similar process occurs for the CO2H groups
of the HO2C(CH2)7SH/Au nanoparticles after the surfactant has
been thermally desorbed from the gold particles. In contrast,
the HO2C(CH2)nPO3H2/ZrO2 monolayers are thermally stable
up to the highest temperature that the NMR probe can reach
broadened beyond detection is supported by a H fast MAS
NMR study of an analogous monolayer system, HO2C(CH2)7-
SH coated gold nanoparticles. Unlike the SAMs on metal oxide
substrates, there is no possibility for any ionic or hydrogen-
bonding interactions of the carboxylic acid groups with the gold
surface. Like the carboxyalkylphosphonate SAMs, infrared
26
spectroscopy indicates that the terminal acid groups of the gold
nanoparticles are extensively hydrogen-bonded.27 The 1H fast
MAS NMR spectrum of the HO2C(CH2)7SH/Au nanoparticles
in Figure 5 shows that this hydrogen bonding is disordered
because only a very broad shoulder at ∼8-10 ppm is visible
in addition to the methylene peak. This shoulder is suppressed
by a double-quantum filter. The broadness of the methylene
proton peak relative to those of the HO2C(CH2)nPO3H2 mono-
layers reflects the fact that the chain mobility of the carboxylic
(
∼140 °C) with only a reversible chain order/disorder process
14 31
detected. The P NMR spectra show that the chains remained
pinned to the surface during this process and no H NMR peaks
1
for the terminal CO2H groups were detected.
1
4
.4. HO2C(CH2)nPO3H2 on ZrO2 Nanopowder. (a) 1D H
1
DQ MAS NMR. The 1D H DQ MAS NMR spectra of HO2C-
(CH2)nPO3H2, n ) 2, 3, 11, and 15, adsorbed on the ZrO2
acid functionalized gold nanoparticles is quite restricted, ap-
proaching that of a rigid crystalline solid.27 The CO2H protons
nanopowder (nanoZrO2) are presented in Figure 6. In contrast
to the lower surface area ZrO2 and TiO2 powders, the acid proton
signals of the diacids adsorbed on the nanoZrO2 are visible.
Note also the splitting of the CO2H proton resonance at ∼13
of the HO2C(CH2)7SH/Au nanoparticles are clearly detected
upon heating the sample above 120 °C for a prolonged period,
as seen in Figure 5. New narrow peaks appear in both the
aliphatic and the acidic proton regions. The new carboxylic acid
proton peak broadened and shifted downfield from 9 to 12 ppm
as the sample was slowly cooled. A mobile component
remained, and examination of the sample after cooling revealed
that some decomposition of the sample had occurred, with the
presence of a white waxy solid in addition to the black
HO2C(CH2)7SH/Au nanoparticles. The new narrow peaks in the
31
ppm in the n ) 3 adsorbed diacid spectrum. The P MAS NMR
spectra of all of these samples contain narrow components
between 24.5 and 27.7 ppm in addition to the very broad peak
at ∼23-24 ppm assigned to the surface-bound phosphonate
1
4
group. Elemental analyses indicate the presence of diacid in
(
30) Badia, A.; Singh, S.; Demers, L.; Cuccia, L.; Brown, G. R.; Lennox, R. B.
Chem.-Eur. J. 1996, 2, 359-63.
J. AM. CHEM. SOC.
9
VOL. 125, NO. 14, 2003 4179

A R T I C L E S
Pawsey et al.
1
31
1
13
Figure 7. (a) 2D H- P HETCOR NMR spectrum and (b) 2D H- C HETCOR NMR spectrum of HO2C(CH2)2PO3H2 adsorbed on nanoZrO2 acquired
with MAS spinning frequencies of 30 kHz and contact times of 100 µs and 2 ms, respectively.
1
31
Figure 8. 2D H- P HETCOR NMR spectra of HO2C(CH2)nPO3H2, n ) 3, 11, and 15, adsorbed on nanoZrO2 acquired with MAS spinning frequencies
of 30 kHz and contact times of 100 µs.
excess of a monolayer (see Table 1), especially in the case of
the short chains, n ) 2 and 3, which had estimated coverages
of 248 and 182%, respectively. The excess diacid is strongly
hydrogen-bonded to the pendant carboxylic acid groups of the
surface-bound monolayer and cannot be removed by multiple
washings with acetone. The deposition solvent, acetone, does
not compete for hydrogen bonding, but when a small amount
of water or methanol was used to wash the samples, the acidic
proton resonances at 13.5 and 10.5 ppm vanished along with
spectrum of the HO2C(CH2)2PO3H2/nanoZrO2 sample, Figure
7b, which shows a correlation between the carbonyl carbon and
the H signal at 13.5 ppm. The correlation between the narrow
1
phosphorus component and the carboxylic acid proton in the
1
31
H- P HETCOR NMR spectrum must be of intermolecular
origin because no such correlation is observed for the bulk diacid
using the same short contact time of 100 µs. Therefore, the
spatial arrangement of the diacids in the adsorbed multilayers
must be rearranged from that of the bulk state such that the
CO2H and PO3H2 groups are in close proximity. The small
31
14
the narrow components in the P NMR spectra.
1
1
(
b) 2D H-X HETCOR NMR. Because the carboxylic and
changes in the isotropic H shifts of the CO2H and PO3H2
1
phosphonic acid H chemical shifts may vary in the adsorbed
protons in the multilayers (13.5 and 10.5 ppm) from the bulk
values (13.0 and 11.0 ppm) would correspond to equal
hydrogen-bond lengths of 2.62 Å for the carboxylic and phos-
1
state, 2D H-X HETCOR NMR experiments were used to
1
31
1
13
confirm the assignments. The 2D H- P and H- C HETCOR
NMR spectra of HO2C(CH2)2PO3H2/nanoZrO2 are given in
Figure 7. Only the n ) 2 adsorbed diacid on the nanoZrO2,
which had a coverage of 248%, had sufficient signal sensitivity
2
5
phonic acid groups, slightly shorter than the bulk state values.
1
31
The 2D H- P HETCOR NMR spectra of the other
carboxyalklphosphonates, n ) 3, 11, and 15, adsorbed on the
1
13
1
31
for a H- C HETCOR NMR experiment. In contrast to the
ZrO2 nanopowder are shown in Figure 8. The 2D H- P
HETCOR NMR spectrum of the other short chain diacid, n )
3, is similar to the n ) 2 diacid except that there are two narrow
1
31
H- P HETCOR NMR spectra of the bulk diacids (see Figure
), significant correlations between the phosphorus signal and
both acid protons at 10.5 and 13.5 ppm are present. These
correlations are solely associated with the narrow P component
assigned to the excess diacid. Because the correlation to the H
2
3
1
components in the P dimension, assigned to two types of
weakly bound diacid with slightly different surface hydrogen-
bonding arrangements. The HETCOR spectrum shows that the
two narrow P components are associated with the splitting in
the CO2H H NMR signal. The HO2C(CH2)11PO3H2/nanoZrO2
31
1
3
1
signal at 10.5 ppm is more intense, this peak is assigned to the
phosphonic acid group of the weakly bound excess diacid, and
the peak at 13.5 ppm is assigned to the CO2H protons. This
1
sample also contains some weakly bound diacid as seen from
1
13
31
assignment is confirmed from the H- C HETCOR NMR
the narrow component in the P dimension which shows small
4180 J. AM. CHEM. SOC.
9
VOL. 125, NO. 14, 2003

MAS NMR Studies of Hydrogen-Bonding Interactions
A R T I C L E S
1
Figure 9. 2D H DQ NMR spectra of HO2C(CH2)2PO3H2 (a) in the bulk state and (b) adsorbed on nanoZrO2 acquired with MAS spinning frequencies of
3
0 kHz and DQ excitation times of 66 µs.
1
Figure 10. 2D H DQ NMR spectra of (a) HO2C(CH2)3PO3H2 and (b) HO2C(CH2)11PO3H2 adsorbed on nanoZrO2 acquired with MAS spinning frequencies
of 30 kHz and DQ excitation times of 66 µs.
correlations to the carboxylic and phosphonic acid protons at
1 and 13 ppm. The 31P spectrum of the longest chain sample,
HO2C(CH2)15PO3H2/nanoZrO2, contains only a minuscule nar-
adsorbed water and a residual mobile component. The t1 noise
1
arises from the loss of signal coherence caused by relaxation
2
0
of the magnetization during the t1 time increment. The CO2H
autocorrelation peaks in the DQ spectra of the bulk diacids must
originate from hydrogen-bonded head-to-head acid dimers,
whereas the autocorrelation peak of the PO3H2 group may be
both inter- and intramolecular in origin. The geometry of the
hydrogen bonds of solid phenylphosphonic acid has recently
been reported and shown to consist of PO3H2 dimers linked
31
row P component which shows a correlation only to the P-OH
proton resonance at 11 ppm, indicating that all of the surfactant
is directly bound to the metal oxide surface.
1
1
(
c) 2D H DQ MAS NMR. The 2D H DQ MAS NMR
spectra of the HO2C(CH2)nPO3H2/nanoZrO2 samples and the
shortest and longest chain bulk diacids, n ) 2 and 15, are
presented in Figures 9-11. In addition to the autocorrelation
3
1
together by hydrogen bonds to form rings. In addition to
intramolecular DQ coherences between the two P-OH protons
of the PO3H2 group, several types of intermolecular DQ
coherences are possible if a similar network of linked dimers is
formed by the phosphonic acid groups of the diacids. Because
only autocorrelation peaks appear in the acid proton region, there
must be only hydrogen-bonded homodimers in the bulk diacids.
1
peaks, the 2D H DQ MAS NMR spectra of the bulk diacids
(Figures 9a and 11a) contain a single off-diagonal peak at 13,
15 ppm, corresponding to a DQ coherence between a carboxylic
acid proton and a methylene proton. Note that the companion
cross-peak which should appear at 2, 15 ppm is missing. The
absence of the companion cross-peak is commonly observed
for DQ coherences associated with strongly dipolar coupled
1
31
The H- P HETCOR spectra of the bulk diacids (Figure 2)
20
methylene protons. The column of small peaks in the spectrum
for the bulk short chain diacid, n ) 2, is due to t1 noise from
(31) Mahmoudkhani, A. H.; Langer, V. J. Mol. Struct. 2002, 609, 97-108.
J. AM. CHEM. SOC.
9
VOL. 125, NO. 14, 2003 4181

A R T I C L E S
Pawsey et al.
1
Figure 11. 2D H DQ NMR spectra of HO2C(CH2)15PO3H2 (a) in the bulk state and (b) adsorbed on nanoZrO2 acquired with MAS spinning frequencies
of 30 kHz and DQ excitation times of 66 µs.
further support the absence of hydrogen-bonding interactions
between the carboxylic and phosphonic acid end groups because
no correlations between the phosphate group and the carboxylic
acid proton are detected at short contact times.
5. Discussion
In this section, the results presented above are combined to
construct a picture of the hydrogen-bonded structures formed
by the adsorption of carboxyalkylphosphonic acids on ZrO2 and
1
As seen in Figure 9b, a dramatic change in the 2D H DQ
1
TiO2. H MAS NMR techniques provide information about both
MAS NMR spectrum of HO2C(CH2)2PO3H2 occurs upon
adsorption on the ZrO2 nanopowder. Both peaks of the off-
diagonal pairs corresponding to correlations between acid and
methylene protons are now resolved, indicating a weakening
of the methylene dipolar interactions, due to either an enhanced
mobility and/or a less dense packing as compared to that of the
bulk crystalline diacid. The autocorrelation peak for the car-
boxylic acid proton is much weaker, and that of the phosphonic
acid is no longer visible (a weak PO3H2 autocorrelation peak
was observed in some samples). A new pair of off diagonal
peaks at 13, 23.5 ppm and 10.5, 23.5 ppm dominates the acid
proton chemical shift region. These peaks correspond to a
correlation between the carboxylic and phosphonic acid protons
because the shift of 23.5 ppm in the double-quantum dimension
is equal to the sum of the chemical shifts of the two acid groups.
The appearance of the new DQ coherence between the two acid
groups concurs with the close spatial proximity between the
phosphorus group and the carboxylic acid protons detected by
the surface bonding and the intermolecular hydrogen-bonding
interactions of the adsorbed diacids. Insight into the bonding
of phosphonic acids on TiO2 versus ZrO2 is provided by the
presence or absence of dipolar couplings among the P-OH
protons. The possible sources for the disordered surface
hydrogen bonding of the carboxylic acid terminated monolayers
are examined. Last, evidence for the existence for acid hetero-
dimers in the multilayers of the diacids on the ZrO2 nanopowder
is discussed.
5.1. Surface Bonding of Phosphonic Acids. The exact nature
of the bonding state of phosphonic acids on metal oxides has
not been well characterized. The phosphonate surface-binding
mode may consist of tri-, bi-, or monodentate bonds, with the
tri- or bidentate bonds in a bridging or chelating configuration,
akin to what is shown in Figure 1a and b. Another possibility
is that the phosphonic acid may just interact with the metal oxide
surface via hydrogen-bonding interactions through the hydroxyl
groups or oxo oxygen atoms (Figure 1c). Normally, the exact
surface bond state cannot be extracted from FTIR spectroscopy
due to the broad overlapping bands in the P-O stretching band
region. The presence or absence of P-OH groups after
adsorption is also difficult to determine from vibrational
spectroscopy. Randon and co-workers detected the disappear-
1
31
the H- P HETCOR experiment. The excess diacid on the
ZrO2 nanopowder is hydrogen-bonded to the directly surface-
bound monolayer in such a way to produce acid heterodimers
rather than homodimers.
1
Weak off-diagonal signals are also detected in the H DQ
MAS NMR spectra of HO2C(CH2)nPO3H2/nanoZrO2, n ) 3 and
-
1
ance of the P-OH band at 940 cm when phenylphosphonic
11, given in Figure 10. When the HO2C(CH2)11PO3H2/nanoZrO2
32
acid was adsorbed on zirconia. However, because the adsorp-
tion band of TiO2 occurs in the same region, it was not possible
to draw any conclusions about the bonding state of the same
phosphonic acid on TiO2. In a more recent study by Guerrero
and co-workers, the broad TiO2 signal was eliminated by
subtracting a background spectrum. The attenuation of the
P-OH band was noted, and the authors proposed that surface
coordination of phosphonic acids on titania is primarily tridentate
was subjected to additional washes with warm methanol rather
than acetone, the deposition solvent, the coverage decreased
from 104 to 80%, and both the narrow 3 P NMR component
1
1
and the off-diagonal peaks in the 2D H DQ MAS NMR
1
spectrum vanished. The 2D H DQ MAS NMR spectra of the
longest chain surfactant, HO2C(CH2)15PO3H2, in the bulk and
adsorbed state are shown in Figure 11. In contrast to the other
samples, no off-diagonal peaks are observed in the acid region,
and there is only a very weak autocorrelation peak at 12.5 ppm
that is assigned to the carboxylic acid proton because this
3
3
31
with some bidentate binding modes. From the P NMR
spectra alone, one can only infer that adsorption has occurred,
1
31
peak did not appear in the H- P HETCOR NMR spectrum
Figure 8).
(
32) Randon, J.; Blanc, P.; Paterson, R. J. Membr. Sci. 1995, 98, 119-29.
(
(33) Guerrero, G.; Mutin, P. H.; Vioux, A. Chem. Mater. 2001, 13, 4367-73.
4182 J. AM. CHEM. SOC.
9
VOL. 125, NO. 14, 2003

MAS NMR Studies of Hydrogen-Bonding Interactions
A R T I C L E S
and no conclusions about the exact coordination state of the
surface bond can be made from the changes in the chemical
thiols on gold nanoparticles, where there is no possibility of
the CO2H groups interacting with the surface, also display an
extremely broad CO2H proton peak. Infrared spectroscopy
shows that the terminal CO2H groups of all of three systems
are hydrogen-bonded, but the H NMR spectra indicate that there
must be a broad distribution of slightly different hydrogen-
shift.1
2,13
The P NMR spectra of phosphonic acids adsorbed
31
31
on titania versus zirconia are quite different in that the P signal
for phosphonic acid adsorbed on ZrO2 is far more shifted and
greatly broadened as compared to that of the unbound acid, an
1
1
2,13
indication of the stronger adsorption on ZrO2.
bonding modes. A resolved CO H peak which appears in the
2
1
31
1
The H- P HETCOR NMR spectra of the adsorbed car-
boxyalkylphosphonic acids demonstrate that there are still
P-OH protons on both ZrO2 and TiO2. In the case of zirconia,
these signals are quite weak and do not survive a double-
quantum filter, indicating that only isolated residual P-OH
H NMR spectra of the gold nanoparticles after thermal
oxidation led to liberation of the surfactant from the surface.
The acid groups of the now unbound surfactant are free to form
a more uniform array of head-to-head dimers. A similar process
does not occur in the carboxyalkylphosphonate monolayers,
which are thermally stable, so the acid resonance remains
heterogeneously broadened beyond detection.
1
groups remain. By contrast, the 2D H DQ MAS NMR spectra
of carboxyalkylphosphonic acids adsorbed on TiO2 show the
presence of dipolar coupled P-OH protons. Given that two
As for the source of the disorder in the surface hydrogen
bonding, the role of the substrate geometry must be considered.
Both free and hydrogen-bonded carboxylic acid groups are
observed in the infrared spectra of carboxylic-acid terminated
thiol monolayers on flat gold substrates, but only hydrogen-
bonded groups are found for the same monolayers on the gold
protons must be less than 3.5 Å apart to produce a double-
quantum coherence,2
0,21
there are two possible sources for the
P-OH double-quantum coherences of the TiO2 samples. If a
significant fraction of the adsorbed phosphonic acid groups are
still fully protonated, intramolecular double-quantum coherences
can arise from the two P-OH protons of the PO3H2 group.
Second, hydrogen-bonding interactions between neighboring
adsorbed phosphonic acid groups may occur in such a way to
give rise to intermolecular double-quantum coherences. Because
phosphonic acids generally bind more weakly to TiO2 as
compared to ZrO2, the presence of a significant population of
P-OH groups is not surprising. Gawalt and co-workers reported
that the stability of alkylphosphonate and carboxyalkylphos-
phonate monolayers on the native oxide surface of titanium films
is greatly enhanced by annealing the monolayers after being
deposited from solution but before solvent washing.3 Without
this annealing step, the authors found that most of the surfactant
which had been deposited from solutions at ambient temperature
was removed by the solvent washing. Guerrero and co-workers
deposited phosphonic acids from solutions at ambient temper-
ature onto titania powder followed by washing and drying under
vacuum at 120 °C. From the infrared spectra, they concluded
that the phosphonic acid binds via condensation reactions with
the surface Ti-OH groups.33 Our sample preparation represents
an intermediate case in that the phosphonic acids were deposited
from hot solution, then washed with solvent, so that a certain
population of uncondensed P-OH groups may survive. The
density of these remaining P-OH groups is such that dipolar
couplings between pairs of P-OH protons are detected. Gawalt
and co-workers suggest that the initially adsorbed monolayers
assemble on the TiO2 surface through hydrogen-bonding
27
nanoparticles. In the case of nanoparticle substrates, both
intramonolayer and interparticle hydrogen bonds are possible,
which would give rise to a more extensive but also more
disordered hydrogen-bonded network. The intramonolayer
hydrogen bonding alone must be disordered because the CO2H
proton NMR peak is broad for monolayers on metal oxides with
larger particle sizes where there is a low likelihood of
interparticle hydrogen bonds. Given the surface roughness of
powder substrates and of metal oxide surfaces, in general, there
will be variation in the relative heights of the neighboring
pendant CO2H groups, and a broad distribution of slightly
different hydrogen-bonding modes can be expected. One might
likewise expect a population of non-hydrogen-bonded CO2H
groups for SAMs on rough surfaces due to this variation in
relative heights and given the fact that such a population is
detected for SAMs on flat substrates. Indeed the influence of
the gold topography on the film permeability and surface pKa
of carboxylic acid terminated SAMs has been explained by more
4
22
extensive hydrogen bonding as the substrate becomes smoother.
However, the infrared spectra of the carboxyalkylphosphonates
adsorbed on metal oxide powders indicate that only hydrogen-
bonded carboxylic acid groups are present despite the surface
14
roughness.
A mixture of hydrogen-bonding geometries, such as cyclic
and acyclic dimers or linear polymeric structures, could
contribute to the broadness of the acid proton resonances.
Although an earlier study of carboxylic acid terminated SAMs
proposed that “sideways” head-to-head dimers form by partial
rotation of the acid groups about the CR-CC)O bond,23 other
studies conclude that only laterally hydrogen-bonded configura-
3
4
interactions among the phosphonic acid headgroups. The
detection of dipolar coupled P-OH groups supports the view
that hydrogen-bonding interactions are involved in the assembly
and surface attachment of phosphonic acids on TiO2 (Figure
1a). The presence of only isolated P-OH groups on ZrO2 points
2
2
tions are possible. A recent study proposes that for steric
reasons only acyclic dimers or linear polymeric structures are
present in CO2H terminated monolayers displaying a high degree
of chain order and the presence of head-to-head dimers is
associated with monolayers with disordered chains or bilayers.19
Whereas the carboxyalkylphosphonate SAMs contain highly
disordered, lateral hydrogen bonds, the broadness of the CO2H
proton resonance of the gold nanoparticles is probably due to a
mixture of intramonolayer acyclic dimers and interparticle head-
to-head dimers. Dimerization of carboxylic acid groups should
toward a bidentate or tridentate binding mode on this substrate
through extensive condensation with the surface hydroxyl groups
(Figure 1a).
5
.2. Disordered Surface Hydrogen Bonds. Whereas the
1
P-OH groups are detected on both zirconia and titania, the H
NMR resonances for the terminal CO2H protons are completely
broadened into the baseline. Carboxylic acid functionalized
(
34) Gawalt, E. S.; Avaltroni, M. J.; Koch, N.; Schwartz, J. Langmuir 2001,
1
7, 5736-8.
J. AM. CHEM. SOC.
9
VOL. 125, NO. 14, 2003 4183

A R T I C L E S
Pawsey et al.
give rise to a double-quantum coherence, but the weak broad
shoulder at 8-10 ppm assigned to the CO2H groups in the H
favor the formation of homodimers. The preferential adsorption
of the phosphonic acid groups changes the relative stiochiometry
of free acid groups available for hydrogen bonding from equal
numbers of PO3H2 and CO2H groups to an excess of CO2H
groups. One possible configuration would have the phosphonic
acid groups of the excess diacid hydrogen-bonded to the pendant
carboxylic acid groups of the surface-bound monolayer in a
head-to-tail fashion. The details of the proposed structure of
the hydrogen-bonded multilayers of the short chain diacids, as
derived from density functional calculations in combination with
1
NMR spectrum of the carboxylic acid functionalized gold
nanoparticles did not give rise to a detectable autocorrelation
peak. Depending on the efficiency of a particular recoupling
1
pulse sequence, the DQ filter will attenuate the H NMR signal
as compared to the single-quantum spectrum without the filter.
This signal reduction, combined with the already low intensity
and broadness of the CO2H proton signal in the single-quantum
spectrum, likely precludes detection of the CO2H dimers on the
1
24
Au nanoparticles by H DQ NMR.
the NMR results, will be presented in a later publication.
5
.3. Hydrogen Bonding in Multilayers. In contrast to the
6
. Summary
monolayers, the hydrogen-bonding network formed by multi-
layers of the diacids on the ZrO2 nanopowder is more ordered
as indicated by the appearance of narrow H NMR peaks which
1
High-resolution H solid-state NMR techniques were used
1
to characterize surface hydrogen-bonding interactions in ad-
sorbed carboxyalkylphosphonates of varying chain lengths on
zirconia and titania. The detection of dipolar connectivities
among the P-OH groups when these diacids are adsorbed on
titania concurs with hydrogen-bonding interactions among the
phosphate groups and a relatively weak binding state on this
substrate. The broadening of the acid proton resonance of
carboxylic acid terminated monolayers on gold nanoparticles
and metal oxide powders points toward a highly disordered
surface hydrogen-bonding network. The hydrogen-bonding
network in multilayers of the diacids is more ordered but highly
perturbed from the bulk state with the formation of heterodimers
rather than homodimers. Finally, the results presented in this
survive a double-quantum filter. Unlike the terminal CO2H
groups of the SAMs, the excess diacid molecules can form
hydrogen bonds in a less constrained fashion among themselves
and with the terminal carboxylic acid groups of the directly
surface-bound monolayers. However, the most interesting
feature is that the hydrogen-bonded structures of the multilayers
are quite different from the bulk diacids, which only show
hydrogen-bonded homodimers, that is, CO2H-CO2H and
PO3H2-PO3H2. The combination of the heteronuclear correla-
tion and double-quantum NMR experiments conclusively shows
that the CO2H and PO3H2 groups are strongly associated in the
multilayers. This information would be difficult to obtain, even
in an indirect fashion, by other experimental means. The infrared
spectra of the short chain diacids (n ) 2 and 3) adsorbed on
the nanopowder show two CdO bands for hydrogen-bonded
carboxylic acid groups. In addition to the CdO band at 1716
1
study demonstrate that high-resolution H solid-state NMR
techniques will be highly useful for identifying specific non-
covalent interactions between organic adsorbates and function-
alized surfaces.
-
1
cm , which is also present in the monolayer samples, there is
-
1
a more intense band at 1681-1685 cm which may be
associated with the heterodimers. However, the existence of
heterodimers would not have been proposed from the infrared
spectra alone. The dominant population of CO2H-PO3H2
heterodimers is somewhat surprising given the large dimerization
Acknowledgment. The authors thank the Natural Sciences
and Engineering Research Council of Canada (NSERC) and
Le Fonds Qu e´ b e´ cois de la Recherche sur la Nature et les
Technologies (FQRNT) for financial support.
-
1
enthalpies of carboxylic acids (50-60 kJ mol ) that should
JA029008U
4184 J. AM. CHEM. SOC.
9
VOL. 125, NO. 14, 2003