Article pubs.acs.org/JPCC
Investigating Hydrogen-Bonded Phosphonic Acids with Proton Ultrafast MAS NMR and DFT Calculations John W. Blanchard,† Thomas L. Groy, Jeffery L. Yarger,* and Gregory P. Holland* Department of Chemistry and Biochemistry, Magnetic Resonance Research Center, Arizona State University, Tempe, Arizona 85287-1604, United States S Supporting Information *
ABSTRACT: Hydrogen-bonding plays a key role in the structure and dynamics of a wide range of materials from small molecules to complex biomolecules. 1H NMR has emerged as a powerful tool for studying hydrogen-bonding because the proton isotropic chemical shift exhibits a dependence on the interatomic distances associated with the hydrogen bond. In the present work, we illustrate the use of ultrafast magic angle spinning at high magnetic field (800 MHz) for resolving multiple hydrogen-bonding sites in a set of crystalline phosphonic acids that contain various functional groups (−COOH, −PO3H2, and −NH3+). Trends are observed between the proton chemical shift of the hydrogen-bonded proton and the associated hydrogen-bonding distances (O−H···X) from X-ray crystallography. Density functional theory calculations conducted on the phosphonic acid structures illustrate that the experimental proton chemical shift dependence on hydrogen-bond distance agrees with the expected theoretical trends. Further, it is shown that the chemical shift trend varies considerably depending on the functional group participating in the hydrogen bonding, albeit a −COOH, −PO3H2, or −NH3+ moiety. An improved understanding of these trends for various functional groups should be useful for determining accurate hydrogen-bond strengths from the proton chemical shift in an array of systems.
1. INTRODUCTION Hydrogen bonding is a vastly important structural feature in chemistry and biology. Hydrogen bonds link together molecules in their crystalline lattices and are directly responsible for the secondary and tertiary structure of proteins. Proton NMR spectroscopy has emerged as a premier method for obtaining hydrogen-bond strengths in both solid1−11 and solution12−14 samples from the isotropic proton chemical shift and to a lesser extent the chemical shift anisotropy (CSA).15 For solids, early 1H NMR work relied on utilizing combined rotation with multiple pulse spectroscopy16−21 (CRAMPS) techniques to average the strong 1H−1H dipolar coupling. The CRAMPS approach significantly improved the proton resolution in rigid solids and allowed the 1H chemical shift of various hydrogen-bonding sites to be extracted directly from the proton spectrum.3,5 Phosphonic acids are an important class of materials that are primarily used for modifying the surface of metal and metal oxides.22 They can be used as anticorrosion coatings23 and as capping ligands for metal and metal oxide nanoparticles.24,25 Additionally, phophonate-based hybrid materials have been applied to various types of catalysis.26,27 In all of these applications it is the phosphonic acid group that interacts strongly with the surface, thus, understanding the hydrogenbonding properties of phosphonic acids is of fundamental importance. The early 1H CRAMPS NMR work on phosphonic acids revealed considerable trends between the hydrogen-bond © 2012 American Chemical Society
length extracted from X-ray diffraction and the observed proton isotropic chemical shift of the hydrogen-bonding site. For example, a strong correlation was observed between the O···O hydrogen-bond distance from X-ray diffraction and the 1 H isotropic chemical shift where decreased chemical shielding indicated a decreased O···O distance and increase in hydrogenbond strength.3,15 Correlations between the H···O hydrogenbond length from neutron diffraction studies indicated a linear relationship between this distance and the 1H chemical shift of the hydrogen-bonded proton.2,4,8 These empirical correlations between the proton chemical shift and the hydrogen-bond strength have been corroborated by ab initio and density functional theory (DFT) calculations conducted on model systems such as water and carboxylic acid dimers.28−31 The 1H CRAMPS NMR approach has a number of drawbacks including tedious and time-consuming setup, artifacts in the spectra, an inability to detect groups undergoing motion on the time scale of the multiple pulse sequences, and a chemical shift scaling factor.3,5,21 Although the CRAMPS approach has its drawbacks, it should be noted that the combination of moderately fast or ultrafast MAS with CRAMPS has provided some of the best proton resolution reported to date for rigid solids.32−37 In the present work, we show that proton ultrafast MAS NMR with spinning Received: May 29, 2012 Revised: August 9, 2012 Published: August 11, 2012 18824
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further by Francl48 (meaning of 6-31G). The basis set also includes diffuse functions49 on all atoms, including hydrogen (meaning of ++). The basis set further includes polarized functions beyond the ** level by including 2 sets of d-functions on heavy atoms and 2 sets of p-functions on hydrogen.50 Diffuse and polarized functions for hydrogen were included to ensure that a sufficiently large basis was available to describe long-range interactions involved in hydrogen bonding. A double-ζ basis set was chosen over a triple-ζ basis function to reduce computational cost, as NMR calculations benefit more from the presence of additional polarized and diffuse functions. NMR calculations were performed using the Gauge-Independent Atomic Orbital (GIAO) method.51,52 Resulting chemical shift values were referenced to the chemical shift of TMS, calculated after geometry optimization using the same model chemistry and basis set. DFT calculations for the dependence of the proton isotropic chemical shift on O···O distance in carboxylic acids were performed on an acetic acid dimer (see Figure 1a). The O−
frequencies (νR) of 60−65 kHz, conducted at high magnetic field (800 MHz), provide sufficient resolution to resolve multiple hydrogen-bonding sites in phosphonic acids. In addition, the proton chemical shift trends observed are dependent on both the hydrogen-bonding strength and the functional group involved in the hydrogen bond. DFT proton chemical shift calculations are used to illustrate that the experimentally observed proton chemical shift trends for various functional groups match those predicted by DFT.
2. EXPERIMENTAL SECTION 2.1. Materials. The solid acids, 2-carboxyethylphosphonic acid (CEPA), phenylphosphonic acid (PPA), diphenylphosphinic acid (DPPA), 3-aminopropylphosphonic acid (APPA), and 2-aminoethylphosphonic acid (99%), were obtained from Sigma-Aldrich. All materials were used as received with the exception of CEPA that was recrystallized in DI water in a refrigerator at 4 °C over the course of 7 days to grow large single crystals for X-ray diffraction structural studies. 2.2. X-ray Diffraction. X-ray crystal structures for AEPA,38 APPA,39 DPPA,40 and PPA41 were obtained from the literature. The X-ray structure of CEPA was solved in this study. Singlecrystal data were acquired to a resolution of 0.700 Å using the Bruker APEX2 (Bruker, 2010) data collection and analysis suite employing Mo Kα radiation with a Bruker Smart-APEX CCD area detector. The data were integrated and reduced using SAINT-Plus Ver. 7.60A (Bruker, 2009). Absorption corrections were performed using the SADABS program (Bruker, 2008). The SHELXTL software package42 was utilized to solve the structure by direct methods and the refinements were performed against all F2 with anisotropic thermal parameters for all nonhydrogen atoms. Hydrogen atoms were easily located in the difference maps near the end of the refinement and were then allowed to refine as independent atoms with isotropic thermal parameters. 2.3. Solid-State NMR Spectroscopy. Proton ultrafast MAS NMR spectra were collected on a Varian VNMRS 800 MHz spectrometer equipped with a 1.2 mm triple resonance MAS probe. A one pulse sequence was utilized to collect the spectra with a 1H π/2 pulse length of 1.2 μs. The broad underlying probe background was removed from the spectra by conducting a baseline correction in the VNMRJ software. Twodimensional (2D) 1H homonuclear dipolar double quantum/ single quantum (DQ/SQ) correlation spectra were obtained with the back-to-back (BABA) pulse sequence as previously described.43,44 One rotor cycle was used for the excitation/ reconversion steps. Typical experimental parameters for the 1H 2D DQ/SQ spectra were 100 kHz sweep width in the detected dimension and a 60 kHz sweep width in the indirectly detected dimension with a 60 kHz MAS frequency. The recycle delay was 300 and 60 s for 1D and 2D experiments, respectively. All proton spectra were indirectly referenced to tetramethylsilane (TMS) by setting the single proton resonance of adamantane to 1.63 ppm. The frictional heating due to MAS at 60 kHz is approximately 30−35 °C, resulting in an actual sample temperature of 55−60 °C.45 2.4. DFT Calculations. All calculations were performed using B3LYP46 density functional theory (DFT) using the 631++G(2d,2p) basis set in Gaussian09. The basis set notation indicates the use of the Pople split valence double-ζ extended basis set with inner shells represented by a single basis function and outer shells represented by three basis functions, each given as a sum of six gaussians, as described by Hehre47 and extended
Figure 1. Hydrogen-bonding models used for DFT calculations of different functional groups: (a) COOH, acetic acid dimer, (b) P−O− H, phosphonic acid trimer, and (c) NH3+, methylammonium-methyl phosphate tetramer.
H···O distance was varied between 2.45 and 2.95 Å and the O− H distance was adjusted at each distance according to the trend determined by neutron diffraction.53 Attempts were also made to optimize the positions of the hydrogen-bonding protons, but the resulting O−H distances were generally shorter and were less effective at reproducing the experimental results compared 18825
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Figure 2. X-ray molecular structures used for DFT cluster calculations: (a) DPPA, (b) PPA, (c) CEPA, (d) APPA, and (e) AEPA. The various O− H···X distances of interest are indicated in the figure.
protons, the GaussView 5 default distance of 1.07 Å was used. Hydrogen-bonding distances were obtained from plots of O···H distance versus O···O distance and N···H distance versus N···O distance from neutron diffraction data.53 When hydrogen-bond direction was indicated by the crystal structure, only O···H or N···H distance was modified to agree with the trend shown by the neutron diffraction data. When no information was available about the direction of the hydrogen bond, the hydrogen atom was placed on a straight line between the donor and acceptor atoms, with the distance along this line taken from the neutron diffraction trend. As shown in the cluster figures (see Figure 2), the clusters were designed to be as small as possible, while encompassing the entire hydrogen-bonding environment in the vicinity of the functional group under study (phosphonic acid, carboxylic acid, or protonated amine). To reduce computational cost, portions of the molecules not involved in hydrogen bonding were typically replaced with a methyl group. The present DFT calculations do not include a full periodic description of the crystalline samples. Recent work has shown that computation of NMR chemical shifts from first-principles electronic structure calculations can be improved by including full periodic boundary conditions compared to calculation performed on isolated molecules.55−59 A full periodic boundary description was not included for the calculation of the hydrogen-bonded proton chemical shifts in the present study to reduce computational cost. In addition, very good agreement is observed between the experimentally determined proton chemical shifts and those calculated for isolated clusters. The calculated chemical shifts for the phosphonic acid hydrogen-bonding protons from the crystal structures were consistently greater than the experimental values, by about 0.8 ppm on average. Including periodic boundary conditions in the calculations could result in better agreement with the experimental shifts;55−59 however, the trend was consistent
to the calculations conducted with the neutron determined distances. All intramolecular distances were kept the same. The calculated data were then fit to an empirical equation of the form r=
a +c δ−b
(1)
where r is the O···O internuclear distance and δ is the chemical shift for the hydrogen-bonded proton. It is worth noting that this fit is extremely close to the empirical fit reported by Harris14 and is perhaps more physical across all potentially relevant distances and chemical shifts. No improvement to the empirical fit was observed by fitting to an inverse cubic function. Theoretical calculations for the dependence of the 1H isotropic chemical shift on O···O distance in phosphonic acids were conducted on a trimer based on the crystal structure of cyclohexylphosphonic acid,54 with the cyclohexane ring replaced with a single methyl group (see Figure 1b). The N···O distance-dependence of the 1H chemical shift of the hydrogen-bonding protons in protonated amines were carried out on a tetramer consisting of a methylammonium cation hydrogen-bonded to three methylphosphate anions (see Figure 1c). Similar to the carboxylic acid calculations discussed above, the O···O and O···N distances were varied between 2.45 and 3.00 Å and 2.65 and 3.10 Å for the phosphonic acid and protonated amine models, respectively. The O−H distance was varied according to the neutron trend. All other intramolecular distances were kept the same and the results of the calculations were fit to eq 1. In addition to the simple hydrogen-bonding models presented in Figure 1, larger clusters were prepared using Xray data for heavy atoms and, when available, also for nonhydrogen-bonded protons. For all other non-hydrogen-bonding 18826
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beginning to be resolved and a shoulder is observed at the higher MAS rates (60 and 65 kHz). The dependence of the full width at half-maximum (fwhm) as a function of 1/νR is shown in Figure 3b for the COOH and the CH2 resonances. A linear dependence is observed in the high speed MAS regime (νR > 25 kHz). This is similar to previous observations on adamantane and deuterated amino acid model compounds where a linear dependence on MAS speed was also reported.60,61 Interestingly, there is a stronger fwhm dependence on νR observed for the CH2 resonance compared to the acidic proton resonances. This is likely due to the higher local proton density of the CH2 groups compared to the acidic proton groups. In previous work on amino acids with varying deuteration levels, it was shown that the line narrowing had a strong dependence on the proton density.61 The assignment of the proton ultrafast MAS NMR spectrum of CEPA was based on 2D dipolar DQ/SQ correlation spectra collected with the BABA pulse sequence (see Figure 4a) and the DFT calculations presented below. The acidic protons can clearly be assigned based on their DQ correlations. The acidic proton with the largest chemical shift can be assigned to the COOH group because it displays an on-diagonal correlation indicating that it is only located spatially close to other COOH protons with no off-diagonal components observed to the other acidic protons. This agrees with the COOH environment observed in the X-ray crystal structure (see Figure 4b). The other two acidic protons display an off diagonal DQ correlation indicating that they are spatially close to each other consistent with a PO3H2 moiety. The distinct proton chemical shifts observed for the two POH resonances result from the difference in hydrogen-bond strength for the two environments. The one with the larger shift can be attributed to the stronger hydrogen-bond (i.e., shorter O···O distance from the X-ray structure). This interpretation agrees with the DFT chemical shift calculations presented below. The CEPA CH2 resonance clearly appears multicomponent in both the 1D ultrafast MAS spectrum at high MAS speeds of 60−65 kHz (Figure 3a) and the 2D DQ/SQ correlation spectrum (Figure 4a). The multicomponent nature arises from the two distinct CH2 groups of the CEPA molecule (Figure 4b). The 2D DQ/SQ correlation spectrum permits the assignment of these two CH2 components. The DQ dimension in the 2D spectrum illustrates that the high ppm component correlates with the PO3H2 groups, while the low ppm component can be assigned to the CH2 bonded to the COOH. Thus, the combination of ultrafast with DQ/SQ 2D NMR allows assignment of all proton environments in the CEPA molecule. The proton ultrafast MAS spectrum of the other solid acids is presented in Figure 5. A single POH hydrogen-bonding site is observed for the DPPA, AEPA, and APPA. This agrees with the single hydrogen-bonding site expected based on the XRD crystal structures of the molecules (see Figure 2).38−40 For the AEPA and APPA, the NH3+ moiety yields a single broad resonance because of the three site hopping motion on the μs time scale for this functional group.5 Two distinct POH hydrogen-bond sites are detected for PPA in agreement with the XRD structure.41 It should be noted that earlier proton CRAMPS work on PPA at lower magnetic field was unable to resolve these two distinct sites.3 The observed proton chemical shifts for different hydrogenbonding sites collected in this study are tabulated in Table 1 along with the O···X distances extracted from X-ray diffraction
between both the experimental and the calculated values, so the data reported in Figure 6 for the calculated values has been shifted 0.8 ppm to lower ppm. The calculated chemical shifts of the protonated amine hydrogen-bonding protons from the crystal structures were averaged, as were the N···O distances, to account for the effect of rotation of the NH3+ group. The averaged calculated shifts agreed well with the experimental results, so they were not adjusted.
3. RESULTS AND DISCUSSION 3.1. Proton Ultrafast MAS NMR. The proton MAS NMR spectrum of CEPA as a function of νR is presented in Figure 3a. The resolution of the proton spectrum clearly improves as the MAS speed is increased. At the highest spinning frequency of 65 kHz, three distinct sites are observed for the hydrogenbonded acidic protons (9−14 ppm). In the CH2 region of the spectrum (0−4 ppm), the two distinct CH2 groups are
Figure 3. (a) Proton solid-state MAS NMR spectrum of CEPA for increasing MAS frequencies (νR) and (b) the full width at halfmaximum (fwhm) for the CH2 (●) and COOH (▲) resonances as a function of 1/νR. The νR for each spectrum is indicated in the figure. The spectra were scaled to the CH2 resonance. 18827
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Figure 5. Proton MAS spectrum of (a) DPPA, (b) PPA, (c) AEPA, and (d) APPA. The spectra were collected with ultrafast MAS frequencies (νR = 60 kHz).
Table 1. Proton NMR Chemical Shifts (δ) From Proton Ultrafast MAS NMR and O···X Distance from X-ray Diffraction sample
δ (ppm)
O···X (Å)
CEPA (COOH) (POH) (POH) PPA (POH) (POH) DPPA (POH) APPA (POH) (NH3+) AEPA (POH) (NH3+)
12.5 10.8 9.9 11.9 11.3 16.0 12.1 8.4 11.8 8.2
2.656 2.578 2.597 2.554 2.608 2.468 2.521 2.818 2.543 2.837
observed for the NH3+ moiety. This can be attributed to the weaker hydrogen bonding strength (smaller N···O distances), but the moiety could also play a role in the observed proton chemical shifts. 3.2. DFT Calculations. To elucidate the impact of the different functional groups on the observed proton chemical shift trends, a series of DFT calculations were conducted on hydrogen-bonding models for the different functional groups (see Figure 1) and larger cluster calculations (see Figure 2) to confirm the trends observed for the simpler models. The results of the DFT calculations are presented in Figure 6 along with the experimental data from this study and previously reported proton chemical shift data.3 The DFT results show that the functional group significantly impacts the observed hydrogenbond trend with proton chemical shift. In addition, the simple hydrogen-bonding models yielded similar results compared to the larger cluster calculations, indicating that the simpler models are sufficient for calculating the proton chemical shifts as a function of hydrogen-bond strength (X···O distance). Based on the trends observed from the experimental and theoretical results for the proton chemical shift in hydrogenbonded solids, empirical equations were derived for calculating the hydrogen bonding strength (X···O distance) from the proton chemical shift for different moieties:
Figure 4. (a) 2D proton dipolar DQ/SQ correlation spectrum collected for CEPA with the BABA pulse sequence (νR = 60 kHz) and the (b) hydrogen-bonded X-ray structure of CEPA. Different hydrogen-bonding sites and their respective correlations are highlighted in the figure.
data. There is a clear trend observed between hydrogen bonding and the O···X distance from X-ray where higher ppm chemical shifts indicate shorter O···X distances and stronger hydrogen bonds. This is in qualitative agreement with the previous observed proton chemical shift trends for hydrogen bonding;2,3,15 however, when comparing the proton chemical shifts observed for different functional groups it becomes apparent that the trend will depend on the moiety involved in hydrogen bonding. This is most apparent for the CEPA molecule where the POH groups have considerably stronger hydrogen bonds, as judged by the O···O distance compared to the COOH functionality, but the COOH exhibits a larger proton chemical shift. This difference between the experimental COOH and POH hydrogen-bonding trends was first noted in the early 1H CRAMPS NMR work on hydrogen bonding in solid acids.3 In addition to differences between the POH and COOH functionalities, even smaller chemical shifts are 18828
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Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by grants from the National Science Foundation (CHE-1011937) and the Department of Defense Air Force Office of Scientific Research (DODAFOSR) under Award No. FA9550-10-1-0275. We thank Dr. Brian Cherry for help with NMR instrumentation, student training, and scientific discussion.
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Figure 6. Proton chemical shift as a function of X···O distance. Experimental proton chemical shifts from this study (○) and from work by Harris et al.3 (□) are shown in the figure along with chemical shift trends based on the POH (-), COOH (···), NH3+ models (---), and larger cluster DFT calculations (●). The error reported for the cluster calculations was determined from the uncertainty in the proton location from neutron diffraction data.53
r=
1.348 + 2.041, δ − 2.550
POH
r=
2.014 + 1.941, δ − 5.085
COOH
r=
4.308 + 1.291, δ − 0.435
NH3+
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where r is the internuclear distance (X···O) in Å and δ is the proton chemical shift in ppm of the hydrogen-bonded proton. These equations can be used to extract the hydrogen-bond strength for different functional groups from the measured proton chemical shift by NMR spectroscopy.
4. CONCLUSIONS Proton ultrafast MAS NMR spectroscopy at high magnetic field strength can be used to resolve multiple hydrogen-bonding sites in rigid solids. Although the proton resolution is not expected to be superior to CRAMPS methods, the ease at which these experiments are setup make them an attractive alternative. DFT calculations conducted on simple models and larger clusters show that the proton chemical shift trends depend on both the hydrogen-bond strength and the functional group involved in the hydrogen-bonding interaction agreeing with the experimental findings. These results have allowed us to fit the theoretical data and extract equations for calculating hydrogen-bond strength from the proton chemical shift for three different functional groups: COOH, POH, and NH3+. These empirical equations should be of use to researchers interested in characterizing hydrogen bonding in a diverse range of systems.
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ASSOCIATED CONTENT
S Supporting Information *
The X-ray crystal structure of 2-carboxyethylphosphonic acid (CEPA). This material is available free of charge via the Internet at http://pubs.acs.org.
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REFERENCES
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected];
[email protected]. Present Address †
Department of Chemistry, University of California, Berkeley, CA 94720. 18829
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