Solid State NMR Spectroscopy as a Precise Tool for Assigning the

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Solid State NMR Spectroscopy as a Precise Tool for Assigning the Tautomeric Form and Proton Position in the Intramolecular Bridges of o-Hydroxy Schiff Bases Magdalena Jaworska,† Paweł B. Hrynczyszyn,† Mirosław Wełniak,† Andrzej Wojtczak,† Katarzyna Nowicka,‡ Grzegorz Krasin´ski,‡ Hassan Kassassir,‡ Włodzimierz Ciesielski,‡ and Marek J. Potrzebowski*,‡ Faculty of Chemistry, Nicolaus Copernicus UniVersity, Gagarina 7, 87-100 Torun´, Poland, and Centre of Molecular and Macromolecular Studies, Polish Academy of Sciences, Sienkiewicza 112, 90-363 Lodz, Poland ReceiVed: August 26, 2010; ReVised Manuscript ReceiVed: October 5, 2010

Two analogous Schiff bases, (S,E)-2-((1-hydroxy-3-methyl-1,1-diphenylbutan-2-ylimino)methyl)phenol (1) and (S,Z)-2-hydroxy-6-((1-hydroxy-3-methyl-1,1-diphenylbutan-2-ylamino)methylene)cyclohexa-2,4-dienone (2), exist in the solid state as phenol-imine and keto-amine tautomers, respectively. Their crystal structures were solved using the X-ray diffraction method. Sample 1 forms orthorhombic crystals of space group P2(1)2(1)2(1), while 2 forms monoclinic crystals of space group P2(1). In each sample, one molecule is in the asymmetric unit of the crystal structure. One-dimensional and two-dimensional solid state NMR techniques were used for structure assignment and for inspection of the 13C and 15N δii of the chemical shift tensor (CST) values. NMR study indicates that the span (Ω ) δ11 - δ33) and the skew (κ ) 3(δ22 - δiso/Ω) are extremely sensitive to change in the tautomeric form of the Schiff bases. Theoretical calculations of NMR shielding parameters for 1 and 2 and a model compound with reduced aliphatic residue were performed using the GIAO method with B3LYP functional and 6-311++g(d,p) basis sets. From comparative analysis of the experimental and theoretical parameters, it was concluded that the position of hydrogen in the intramolecular bridge has tremendous influence on 13C and 15N CST parameters. Inspection of Ω and κ parameters allowed for the establishment of the nature of the hydrogen bonding and the assignment of the equilibrium proton position in the intramolecular bridges in the solid state. Introduction o-Hydroxy aryl Schiff bases (SB) have recently received a great deal of attention in theoretical and experimental studies.1 In particular, the geometry and energetic parameters of the intramolecular hydrogen bonding of the O-H · · · N type, formed between the hydroxyl group of the phenol ring (the proton donor) and the nitrogen atom of the imine (the proton acceptor), have been intensively investigated by several research groups.2 The proton transfer processes in the intramolecular hydrogen bond lead to two tautomeric (OH and NH) forms, presented in Scheme 1. Knowledge of the equilibrium of the proton transfer is crucial for understanding several important physicochemical properties of SB, such as photoand thermochromism and biological activity. Many analytical techniques are used for study of the structure and geometrical restraints of SB. Among these techniques, X-ray crystallography is one of the most important. To date, over 300 solved crystallographic structures have been deposited in the Cambridge Structural Database (CSD).3 Another commonly used technique is NMR spectroscopy, whose power and broad range of applications were recently reviewed exhaustively by Hansen et al.4 Recent years have witnessed incredible progress in applications of solid state NMR (SS NMR) spectroscopy as a tool for structural study of SB.5 This method can be used to obtain information on the structure of the respective tautomers, their equilibrium processes, and their electronic structure in the solid state. SS NMR also allows direct correlation between X-ray

crystallography and NMR spectroscopy for the same object and bridges two phases (liquid and solid). Determining structural parameters that could give information on the equilibrium position of the transferred proton in the intramolecular bridges of SB is still a challenge because the proton is located somewhere in the bridge (Scheme 1). Several experimental parameters, mostly based on analyses of X-ray data, have been tested so far. For example, Filarowski suggested that the CO bond length reflects the proton transfer process. The HOMA (harmonic oscillator measure of aromaticity) index increases from 0.8 to 1.00 when d(CO) increases from 1.28 to 1.35 Å.6 Very recently, Zarycz and Aucar, employing theoretical calculations, tested whether the NMR spectroscopic parameters (δiso(15N) and 1J(15NH)) can establish the position of the hydrogen atom in the bridge.7 The aim of this work is to develop new methodology that enables the assignment of the tautomeric form and equilibrium position of the proton in the intramolecular hydrogen bridges of o-hydroxy SB in the solid state. We attempt to prove that SS NMR spectroscopy is the best technique for this purpose, particularly in samples for which X-ray data are not available. We report here the full NMR assignment of compounds 1 and SCHEME 1: SB Tautomers

* To whom correspondence should be addressed, [email protected]. † Faculty of Chemistry, Nicolaus Copernicus University. ‡ Centre of Molecular and Macromolecular Studies, Polish Academy of Sciences.

10.1021/jp108104g  2010 American Chemical Society Published on Web 11/04/2010

Assigning Tautomeric Form and Proton Position SCHEME 2: Structure and Numbering System for 1 and 2

SCHEME 3: Synthesis of 1,1-Diphenylvalinol and SB 1 and 2

2 and an analysis of the 13C and 15N chemical shift tensor (CST) parameters. Our SS NMR results are supported by DFT (density functional theory) calculations of the shielding parameters and X-ray crystallography. We used chiral hydroxy SB derived from salicylaldehyde, 3-hydroxy salicylaldehyde, and (S)-1,1-diphenylvalinol as model compounds (Scheme 2). Chiral SBs derived from appropriate aldehydes and primary amines are a promising class of compounds because they can be applied as catalysts in homogeneous catalysis1 or, after immobilization on solid support in heterogeneous catalysis,8 as

Figure 1.

1

J. Phys. Chem. A, Vol. 114, No. 47, 2010 12523 biologically active compounds,9 chemical sensors in spectrophotometric method,10 and luminescent materials.11 Results and Discussion 1. Preparation of Chiral Schiff Bases. 1,1-(S)-Diphenylvalinol reacting with salicylaldehydes in ethanol formed the respective SB with 73-91% yields, as calculated after the recrystallization of the products (Scheme 3). 1,1-(S)-Diphenylvalinol was synthesized using a previously reported method12 (Scheme 3). The SB 1 had been previously obtained by Hayashi13 in the reaction of (S)-1,1-diphenylvalinol with salicylaldehyde in methanol solution after refluxing for 10 h. Our method for obtaining both SB was similar, but we used milder conditions, i.e., ethanol solutions mixed at room temperature for 24 h. 2. Assignment of 1H and 13C for 1 and 2 in Liquid and Solid Phases. Parts A and C in Figure 1 show the 1H NMR spectra for 1 and 2 at ambient temperature in toluene-d8. 1H SS NMR spectra for 1 and 2 recorded under magic angle spinning with a 60 kHz spinning rate are displayed in parts B and D of Figure 1, respectively. The differences between spectra are particularly apparent in terms of line resolution. 1 H SS NMR spectroscopy is very challenging because of extremely strong homonuclear dipolar coupling, which in many cases exceeds the range of the chemical shifts for protons.14 The broadening of the proton lines is not removed by slow magic angle spinning. Therefore, the spectra recorded under slow conditions are difficult to analyze and it is not straightforward to extract subtle structural information. The “ultrafast” (UF) regime of more than 60 kHz is obtained using commercially available 1.3 mm rotors. This frequency exceeds the strength of the homonuclear proton dipolar coupling and is therefore expected to enter a new regime for spin dynamics. 1 H SS NMR spectra for 1 and 2 recorded at 60 kHz are broad; however, signals in the diagnostic region of the OH resonances

H spectra of 1 and 2: (A) and (C) recorded in the liquid phase (toluene-d8); (B) and (D) MAS at the spinning rate of 60 kHz.

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Figure 2. Variable temperature (293-223 K) 1H NMR spectra of 2 recorded in toluene-d8.

Figure 3. 13C NMR spectra: (A, D) recorded in the liquid phase in chloroform; (B, E) solid state CP/MAS at a spinning speed of 8 kHz; (C, F) dipolar dephasing experiments for 1 and 2, respectively.

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Figure 4. (A) Molecule 2. (B) Molecule 1. The thermal ellipsoids are plotted at the 30% probability level.

TABLE 1: Hydrogen Bonds for 1 and 2 (units in Å and deg) D-H · · · A

d(D-H)

d(H · · · A)

d(D · · · A)

∠(DHA)

O2-H2O · · · N1

Structure 1 0.82

1.84

2.571(7)

147.0

O2-H2O · · · O3 O3-H3O · · · O2 N1-H1N · · · O2 O1-H1O · · · O2 [-x + 2, y + 1/2, -z + 1]

Structure 2 0.82 0.82 0.86 0.82

2.26 2.26 2.02 2.03

2.695(3) 2.695(3) 2.667(3) 2.796(3)

113.5 113.8 131.5 155.6

are well-resolved. The chemical shift of the O-H(2′) proton for 1 is in the expected region.4 For the pure OH form, δOH is in the range of 12-18 ppm and increases with increasing hydrogen bond strength. The difference in O(2)-H chemical shifts between the liquid and solid phases is noteworthy (Figure 1, spectra A and B). For sample 2, the liquid-phase (Figure 1C) H3 signals in the region of 10-18 ppm are not seen. A similar observation was reported by Pizzala and co-workers.15 For N-(3-hydroxy salicylidene)-4-methoxyaniline, the authors concluded that the H3 proton signal should be observed at 5.5-6.5 ppm as a broad line independent of concentration. It was further assumed that this chemical shift indicates a less acidic character for H3 than for H2 because H3 is involved in a weak intramolecular hydrogen bond. Analysis of the spectrum shown in Figure 1C indicates that this interpretation is not valid for 2. There are no proton signals in the specified region (5.5-6.5 ppm). On the other hand, the H3 proton is seen perfectly in the solid state at δ ) 13.2 ppm (Figure 1D). The preliminary room temperature data obtained in the liquid phase encouraged us to carry out more highly detailed NMR measurements at lower temperatures. Figure 2 shows 1H NMR spectra of 2 recorded in toluene-d8 in the temperature range of 293-223 K. The dependence of the position of the proton signal for the hydroxyl group bonded to the C9 carbon upon temperature is apparent (change from 2.8 ppm at 293 K to 5.5 ppm at 223 K). This shift does not depend upon the sample concentration, and hence, we conclude that it is the result of an intramolecular effect related to the freezing of the rotation of the O-H and a deshielding of the protons by the aromatic ring current of the adjacent phenyl groups. Freezing of rotation causes also change of position of other signals seen in aliphatic and aromatic regions. The new signal at δ ) 13.8 ppm appeared at 243 K, and its chemical shift is temperature-dependent. The fwhh (full width at half-height) of the O3-H resonance is a function of temperature. The lowest value (23.2 Hz) is reached at 223 K, which suggests that in the liquid phase at ambient temperature, the O3-H proton is in the dynamic regime and occupies different positions between O3 and O2 atoms. At 223 K, the proton position is localized. Moreover, it can be

concluded that a stable tautomer of 2 is formed in the solid state at ambient temperature. Figure 3 shows 13C NMR spectra of 1 and 2 recorded in liquid (Figure 3A,D) and solid phases. The assignment of signals in the solid state, which is based on dipolar dephasing (DD) NMR experiments (Figure 3C,F) and is supported by theoretical calculations (see section 5), is given as the top traces in spectra B and E of Figure 3 for 1 and 2, respectively. The DD pulse sequence is often used as a spectral editing technique.16 In the simplest approach, the 1H decoupler is turned off for ca. 50 µs after CP. This is sufficient time for the 13C-1H dipolar coupling to dephase the transverse magnetization of any 13C isotope with a directly bonded 1H isotope. Consequently, CH and CH2 are effectively suppressed, and quaternary signals are observed. From the analysis of the spectra (DD/MAS and CP/MAS), it can be concluded that only one molecule is an independent part of the asymmetric unit for both samples. This conclusion is consistent with XRD data. 3. X-ray Structures of 1 and 2. The molecular structures of 1 and 2 with an atom numbering scheme are presented in Figure 4. Details of the hydrogen bonds are listed in Table 1. The absolute configuration (2S) determined for both reported structures is consistent with (S)-valine, which is used as an optically pure substrate in the synthesis. A comparison revealed slight differences between 1 and 2 in the molecular geometry in the vicinity of the imine bond and the O2 phenolic hydroxyl. Also, the analysis of the electron density maps revealed peaks corresponding to the hydrogen atoms positioned near imine N1 and phenolic O1. Therefore, the proton transfer from the hydroxyl group to the imine nitrogen was detectable in 2. Similar structural effects have been reported for other hydroxy-imines17 containing two hydroxyl groups on the neighboring carbon atoms. In 2, the proton transfer between N1 and O2 is possible because of the planar conformation of the molecule, which is described by O2-C2-C1-C7-N1-C8 torsion angles of -3.2(5), 3.8(5), and -173.8(3)° (Figure 5) and results in relatively short distances between these atoms. The intramolecular H-bond formed between these groups has a N1 · · · O2 distance of 2.667(3) Å (Table 1). Similar torsion angles are

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Figure 5. 2D PASS spectra for 2 recorded with a spinning rate of 2 kHz (A) and after data shearing (B). (C) and (D) show the F1 slices for carbons C2 and C3 taken from spectrum B. The appropriate calculated spectra are presented in (E) and (F).

found in 1, with corresponding values of -4.9(10), -0.2(9), and -176.9(5)°, and an intramolecular H-bond O2-H · · · N1 with a O2 · · · N1 distance of 2.571(7) Å is also found. However, that structure reveals no keto-amine component, and no residual electron density peak is found in the vicinity of the N1 atom. A possible explanation for this phenomenon is the presence of a second hydroxyl group, O3-H, bound to the neighboring atom in the phenol ring of 2. Therefore, the keto-amine structure seems to be stabilized by the intramolecular O3-H · · · O2 hydrogen bond (O3 · · · O2 2.695(3) Å) to compensate for the hydrogen atom transferred to N1. 4. 13C 2D PASS and 15N 1D NMR Analysis of the CST Parameters for 1 and 2. In this part of the project, we were motivated by the prospect of comparative analysis of 13C and 15 N δii (ii )11, 22, 33) data for 1 and 2, inspection of anisotropic values of CSTs and correlation of the principal elements to the molecular structure and the different patterns of intramolecular hydrogen bonding. For rotating solids, 13C/15N δii parameters can be obtained from the analysis of spinning sideband intensities. Such analysis is restricted to relatively simple molecules because it is necessary to resolve the sidebands of different resonances at low spinning rates. As the number of distinct resonances increases, the 1D spectrum becomes increasingly crowded. For the samples under investigation, a spinning rate in the range of 2-3 kHz is required to obtain a spectrum with a sufficient number of 13C sidebands for further calculations of the aromatic and carbonyl regions. There are several approaches that allow the separation of the isotropic and anisotropic parts of spectra with heavy overlapped systems.18 We employed the 2D PASS sequence for analysis of 13C spectra.19 This technique offers good sensitivity compared to other methods and does not require hardware modifications or a special probehead. Figure 5A displays the 2D PASS spectra for 2 recorded at a 2 kHz spinning rate. A similar procedure was employed to study 1. In general, the carbonyl group and aromatic atoms are characterized by large CSA, and the spectra present a complex

TABLE 2: 15N δii Parameters Obtained from 1D Spectra and 13C δii Parametersa Obtained from 2D PASS Experiment, Fitted Using the TOPSPIN and WINMAS Programs δiso (ppm) δ11 (ppm) δ22 (ppm) δ33 (ppm)

Ω

κ

15

1 2

279.4 147.6

535 281

N 296 136 13

1 C7 C2 C1 2 C2 C7 C3 C1

7 26

528 255

0.09 -0.14

-0.11 0.34 0.5

C

164.9 162.2 119.2

239 236 190

159.5 181 148

96 69 20

143 167 169

168.5 166.0 148.8 112.5

213 237 210 186

211 182 163 131

80 79 74 21

132 158.5 136 166

0.91 0.30 0.32 0.33

a The 13C and 15N δii parameters are defined as follows: δ11 g δ22 g δ33. The estimated error in δ11 and δ22, and δ33 is (3 ppm; δiso ) (δ11 + δ22 + δ33)/3, span is expressed as Ω ) δ11 - δ33, and skew is expressed as κ ) 3(δ22 - δiso)/Ω.

pattern under slow sample spinning. Using proper data shearing (Figure 5B), it is possible to separate the spinning sidebands for each carbon and to use a calculation procedure to establish the 13C δii parameters. It is clear from such a presentation that the F2 projection corresponds to the TOSS20 spectrum, whereas F1 represents CSA. In this work, we concentrated only on most diagnostic C1, C2, C3, and C7 resonances that are sensitive to tautomerization equilibrium and can be unambiguously assigned. Parts C and D of Figure 5 show the F1 slices for carbons C2 and C3. The appropriate calculated spectra are presented in parts E and F of Figure 5. The values of selected 13C δii parameters for 1 and 2 are collected in Table 2. It is well-known that orientation analysis and the values of the 13C δii parameters provide detailed information about the



Figure 6. 15N CP/MAS spectra of 1 and 2 recorded at different spinning rates: (A, B) 8 kHz; (C, D) 1 and 2 kHz. The best-fitting simulated 1D spinning CSA sideband patterns obtained using the TOPSPIN program are presented in (E) and (F). The small discrepancies between experimental and calculated spectra are due to low signal-to-noise ratio which occurs during the measurements of insensitive nuclei under slow magic angle spinning.

electron distribution of each nucleus.21 The orientation of the 13 C δii principal components for aromatic compounds was exhaustively discussed by Grant.22 In principle, the δ11 elements are oriented along the C-H bonds, while the δ22 elements are aligned in the aromatic plane and δ33 is perpendicular to this plane. The changes in 13C δ22 values indicate the perturbation of aromaticity for the systems under investigation. Comparison of 13C δii values for the C2 carbon of 1 and 2 shows a dramatic difference for 13C δ22. The position of the δ22 element with respect to δiso is defined by a skew (κ) parameter expressed by the equation k ) 3(δ22 - δiso)/Ω, where span Ω is equal to difference between δ11 and δ33. The skew parameters reflect the electron distribution around the desired nucleus. For samples 1 and 2, the values of κ for the C2 carbons are dramatically different. Moreover, the Ω parameter, which indicates chemical shift anisotropy, is roughly 35 ppm larger for C2 in 1 than in 2. This difference means that the aromatic character of C2 for both tautomers varies greatly.

Figure 7. Simulated static spectra of respectively) and 2 (A, C, respectively).

13

C and

15

N for 1 (B, D,

SCHEME 4: Model Used for Calculations of the Influence of Proton Position on 13C and 15N δii Parameters

Figure 8. 15N (A) and 13C (B) line shape changes according to hydrogen position in the N · · · H · · · O(2) bridge for 2. Simulated static spectra were obtained using calculated 15N and 13C shielding parameters σii and converted into chemical shift δii parameters.

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Figure 9. Correlation between nitrogen.

13

C and

15

Jaworska et al.

N and Ω, κ parameters vs N-H distance. (A) and (C) represent carbon C2. (B) and (D) represent

The 15N spectra of 1 and 2 are relatively simple because both compounds are represented by single resonance lines (parts A and B of Figure 6, respectively). Comparison of 15N δiso values shows significant differences between 1 and 2 (∆δiso ) 132.8 ppm), which clearly proves that 1 and 2 represent different tautomers of SB. 15N δii parameters were obtained by the deconvolution of 1D spectra recorded with a 2 kHz spinning rate for 1 (Figure 6C) and a 1 kHz spinning rate for 2 (Figure 6D). The best-fitting simulated 1D spinning CSA sideband patterns obtained using the TOPSPIN program are shown in parts E and F of Figure 7 for 1 and 2, respectively. The calculated 15N δii parameter values for 1 and 2 are collected in Table 2. To better visualize the changes in the 13C and 15N line shapes related to the tautomerism of SB, we present simulated static spectra for the C2 carbon in 2 and 1 (Figure 7A,B) and spectra for nitrogen-15 (Figure 7C,D for 2 and 1, respectively). Spectra were calculated with CST parameters given in Table 2. The meaning of the δii parameters is shown pictorially in Figure 7. 5. Theoretical Calculations of 13C and 15N NMR Shielding Parameters for 1 and 2. Using the theoretical calculations, we were able to correlate NMR and X-ray data, assign the 13C and 15N chemical shifts for 1 and 2, and analyze subtle intramolecular structural effects. The X-ray coordinates of 1 and 2 were used as input files for calculations of NMR shielding parameters. The position of the hydrogen atoms directly attached to carbon was optimized because it is often difficult to locate protons accurately using X-ray diffraction. The importance of the C-H bond length optimization in the GIAO calculations of 13C NMR parameters is discussed elsewhere.23 Many methods are currently available for computing NMR parameters.24,25 In the most of them, the GIAO approach with DFT hybrid functionality is employed. Because the ab initio and DFT methods for large complexes are still very time-consuming, the proper choice of method and basis set is crucial to balance the quality of the results and the expenses. In our project, we used the commercially available Gaussian 2003 computer program.26

We used B3LYP functional and 6-311++g(d,p) basis sets for all calculations. The first analysis of 13C/15N shielding parameters revealed discrepancies between experimental and theoretical data, particularly for the atoms that are involved in intramolecular hydrogen bonding. For example, the 13C tensor of C2 carbon for 2 is axially symmetric (κ ≈ 1) according to the data shown in section 4. This result is not consistent with theoretical calculations when the hydrogen atoms are localized on nitrogen (N-H distance of 1.09 Å) and oxygen (O3-H distance of 1.09 Å). Assuming that the proton position in the intramolecular hydrogen bridge can significantly modify values of 13C δ11 and δ22 components as well as 15N δii parameters, we carried out a series of calculations of shielding parameters with different hydrogen positions in the bridges using a simplified model of SB (replacing the valinol residue by an isopropyl group), as shown in Scheme 4. The distance between nitrogen and hydrogen was gradually changed and fixed while the geometry of the N · · · H · · · O angle was optimized. The computed 13C and 15N σii parameters are attached as Supporting Information. Figure 8 displays the change of line shape for static 13C and 15N spectra to demonstrate the influence of the hydrogen position (middle column) on the NMR spectral parameters. The differences between the spectra for each nucleus, presented in columns as a stack plot, are apparent. Both span Ω and skew κ parameters are sensitive to the proton position in the hydrogen bridge. The changes in Ω and κ for 13 C and 15N are shown pictorially in Figure 9. It is interesting that an increase in N-H distance leads to a decrease in Ω for 13 C of C2 but an increase in Ω for 15N (parts A and B of Figure 9, respectively). The changes in the κ parameter are not linear. For both nuclei, the maximum N-H distance reached is in the range of 1.26-1.30 Å (parts C and D of Figure 9). Finally, we analyzed the influence of hydrogen position in the C3-O3 · · · H · · · O2-C2 bridge on the shielding parameters of C2 and C3 carbons using the model shown in Scheme 4. During this calculation, the position of the proton in the



Figure 10. Change in values of Ω and κ parameters for C2 and C3 carbons versus the hydrogen position in the C3-O3 · · · H · · · O2-C2 bridge.

15

N δii (ppm) and calculated shielding parameters σii (ppm) for 1 (A and B,

N · · · H · · · O bridge was frozen (1.26 Å between nitrogen and hydrogen), while the distance between proton and O3 in the O3 · · · H · · · O2 bridge was gradually changed from 1.0 to 1.4 Å. Figure 10 presents changes of Ω and κ for both carbons. Analysis of the plots reveals that Ω (Figure 10A) increases with increased O3-H distance for both carbons. The values of κ decrease for C2 and increase for C3. When the proton is at the halfway point between O2 and O3, κ for both carbons is equal (Figure 10B). From theoretical calculations presented, it is apparent that the position of the hydrogen in the intramolecular bridge has tremendous influence on 13C and 15N CST parameters. Thus, the analysis of these parameters allows the establishment of the nature of the hydrogen bonding and assignment of the proton equilibrium position in the intramolecular bridges in the solid state. Comparing experimental and theoretical parameters, we can conclude that the proton in C3-O3 · · · H · · · O2-C2 is aligned close to O3, while the proton in N · · · H · · · O occupies a position with a distance between the nitrogen and proton in the range of 1.1 Å-1.26 Å. Considering the sensitivity of 13C and 15N CST parameters in the position of hydrogen in the intramolecular bridges, we computed the NMR shielding parameters for 1 and 2 with

arbitrary assigned hydrogen positions. Figure 11 shows the plot of 13C δii and 15N δii CST, obtained from the measurements for 1 and 2, versus the computed 13C/15N σii parameters. A correlation of experimental versus shielding parameters is used to convert the calculated shielding numbers into calculated shift values. This approach accounts for systematic calculation errors and improves the direct comparison with the experimental data. The linear regression equations and root mean square are given in the Figure 11. In a perfect case, the slope is equal to -1.00. As seen from the corrected position of the protons in the intramolecular bridges, the correlations are of good quality. Further improvement of calculations can be achieved employing periodic systems approach as reported elsewhere.27

Figure 11. Correlation between experimental CST values respectively) and 2 (C and D, respectively).

13

C δii,

Conclusions In this paper, we proved the power and excellent diagnostic abilities of a multitechnique approach in the structural study of SB. Though this report describes results for chiral systems, which are interesting from a practical point of view, our observations are general and can be extended to achiral models. Our project was completed in relatively few steps. First, we assigned isotropic values of 1H, 13C, and 15N NMR signals to

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the molecular structures of 1 and 2. Second, we resolved the X-ray structures for both compounds. Third, we established values for principal elements of CSTs for diagnostic carbon and nitrogen atoms employing two-dimensional (2D PASS) and onedimensional NMR approaches. Finally, we performed theoretical calculations for NMR shielding parameters considering different proton localizations in the O · · · H · · · N and/or O · · · H · · · O hydrogen bridges. Comparing experimental and theoretical parameters, we have revealed that the analysis of the principal components of CSTs for 13C and 15N nuclei can provide unique information about tautomeric form and proton position in the intramolecular bridges of SB. Parameters such as κ and Ω are very sensitive to the average equilibrium position of the proton. Standard X-ray diffraction measurements are not always able to provide unambiguous information that locates the proton in the hydrogen bridges properly. Acknowledgment. The research was partially supported by the Marshal of Kujawsko-Pomorskie Voivodeships“Scholarship for Ph.D. students 2008/2009-ZPORR” no. SPS.IV-3040-UE/ 265/2009. Supporting Information Available: The synthesis and characterization of 1 and 2, 1H and 13CNMR spectra of 1 and 2, and crystallographic information files (CIF) for 1 and 2. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Katsuki, T. Chem. Soc. ReV. 2004, 33, 437–444. (2) (a) Nathan, L. C.; Traina, C. A. Polyhedron 2003, 22, 3213–3221. (b) Gilli, P.; Bertolasi, V.; Pretto, L.; Antonov, L.; Gilli, G. J. Am. Chem. Soc. 2005, 127, 4943–4953. (c) Sharif, S.; Denisov, G. S.; Toney, M. D.; Limbach, H.-H. J. Am. Chem. Soc. 2006, 128, 3375–3387. (d) Rozwadowski, Z.; Schilf, W.; Kamien´ski, B. Magn. Reson. Chem. 2005, 43, 573–577. (e) Dziembowska, T.; Jagodzin´ska, E.; Rozwadowski, Z.; Kotfica, M. J. Mol. Struct. 2001, 598, 229–234. (3) Cambridge Structural Database 3D Graphics Search System, 2010 Release. (4) Hansen, P. E.; Rozwadowski, Z.; Dziembowska, T. Curr. Org. Chem. 2009, 13, 194–215. (5) (a) Schilf, W.; Kamienski, B.; Szady-Chełmieniecka, A.; Grech, E.; Makal, A.; Wozniak, K J. Mol. Struct. 2007, 94-101, 844–845. (b) Sitkowski, J.; Stefaniak, L.; Wawer, I.; Kaczmarek, L.; Webb, G. A. Solid State Nucl. Magn. Reson. 1996, 7, 83–84. (c) Salman, S. R.; Lindon, J. C.; Farrant, R. D.; Carpenter, T. A. Magn. Reson. Chem. 1993, 31, 991–994. (d) Saitoˆ, H.; Ando, I.; Ramamoorthy, A. Prog. Nucl. Magn. Reson. Spectrosc. 2010, 57, 181–228. (6) Filarowski, A. J. Phys. Org. Chem. 2005, 18, 686–698. (7) Zarycz, N.; Aucar, G. A. J. Phys. Chem. A 2008, 112, 8767–8774. (8) Fraile, J. M.; Garcı´a, J. I.; Mayoral, J. A. Chem. ReV. 2009, 109, 360–417. (9) Rehder, D.; Santoni, G.; Licini, G. M.; Schulzke, C.; Meier, B. Coord. Chem. ReV. 2003, 237, 53–63. (10) Khatua, S.; Choi, S. H.; Lee, J.; Kim, K.; Do, Y.; Churchill, D. G. Inorg. Chem. 2009, 48, 2993–2999. (11) Binnemans, K. Chem. ReV. 2009, 109, 4283–4374. (12) Itsuno, S.; Ito, K.; Hirao, A.; Nakahama, S. J. Org. Chem. 1984, 49, 555–557. (13) Hayashi, M.; Miyamoto, Y.; Inoue, T.; Oguni, N. J. Org. Chem. 1993, 58, 1515–1522.

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