1H, 13C, and 15N Solid-State NMR Studies of Imidazole- and Morpholine-Based Model Compounds Possessing Halogen and Hydrogen Bonding Capabilities

1H, 13C, and 15N Solid-State NMR Studies of Imidazole- andMorpholine-Based Model Compounds Possessing Halogen andHydrogen Bonding Capabilities

Karim Bouchmella,† Sylvain G. Dutremez,*,† Bruno Alonso,‡ Francesco Mauri,§ andChristel Gervais*,#

Institut Charles Gerhardt Montpellier, UMR 5253 CNRS-UM2-ENSCM-UM1, Equipe CMOS,UniVersite Montpellier II, Bat. 17, CC 1701, Place Eugene Bataillon, 34095 Montpellier Cedex 5,France, Institut Charles Gerhardt Montpellier, UMR 5253 CNRS-UM2-ENSCM-UM1, Equipe MACS,8 rue de l’Ecole Normale, 34296 Montpellier Cedex 5, France, Laboratoire de Mineralogie-Cristallographie de Paris, UMR 7590, UniVersite Pierre et Marie Curie-Paris 6, 75005 Paris, France,and Laboratoire de Chimie de la Matiere Condensee de Paris, UMR 7574, UniVersite Pierre et MarieCurie-Paris 6, CC 174, 75005 Paris, France

ReceiVed October 6, 2007; ReVised Manuscript ReceiVed July 11, 2008

ABSTRACT: The halogen and hydrogen bonding interactions present in solid 1-(2,3,3-triiodoallyl)imidazole (1), morpholiniumiodide (2), the 1:1 cocrystal 1-(2,3,3-triiodoallyl)imidazole ·morpholinium iodide (3), morpholine (4), imidazole (5), and1-(3-iodopropargyl)imidazole (6) have been investigated by solid-state 1H, 13C, and 15N NMR spectroscopies. Comparison of the15N CP MAS NMR spectrum of 3 with that of 2 indicates that protonated morpholine is present in solid 3, but this conclusion mustbe taken with caution as GIPAW calculations predict a 15N chemical shift for morpholine similar to that of the morpholiniumcation. Conclusive evidence for the presence of a morpholinium cation in crystalline 3 was obtained by recording the static 15NNMR spectrum of this host-guest complex and comparing the morpholinium/morpholine part of the spectrum with the static spectraof 3 and 4 as obtained from ab initio calculations of NMR parameters based on the X-ray structures of these compounds. Concerningthe imidazolyl group, 15N NMR spectroscopy has proven quite valuable to identify changes in the bonding situation of the C-NdCnitrogen on passing from 1 to 3. In addition, slight differences are observed between the 15N chemical shifts of 1 and 6 that areascribed to differences in halogen bond strengths between the two compounds. Attempts have also been made to study halogenbonding by 13C NMR spectroscopy, but this method did not provide exploitable results as signals corresponding to the sp and sp2

carbon atoms bonded to iodine could not be observed experimentally. 1H NMR spectroscopy is a powerful tool to study hydrogenbonding interactions of moderate energies such as +NH2 · · ·X (X ) N, O, I). Indeed, we have found that the chemical shifts of theNH hydrogens were quite sensitive to the nature of X and to the N-H · · ·X distance. This is demonstrated by the fact that thechemical shifts of the +NH2 protons of the morpholinium cation in 2 and 3 are noticeably different.

Introduction and Background

The construction of wholly organic architectures through self-assembly of small building blocks is currently a topic of intenseresearch activity in the fields of supramolecular chemistry,crystal engineering, and materials science.1 This modularapproach gives access to a large variety of topologies as a resultof the various geometries (1D, 2D, 3D) of the molecular bricks,the types of donor and acceptor sites present in these bricks,and the multiplicity of the interacting sites. One motivation forthe synthesis of wholly organic architectures is their topologicalresemblance to existing inorganic systems. Indeed, whollyorganic structures are capable of assembling into 2D1-3 and3D4 networks that resemble structurally and functionally pillaredclays and zeolites. Another asset is that the organic modulescan be functional, so this methodology gives access to materialswith advanced properties.

In a large number of cases, synthons based on hydrogen-bonding or charge-assisted hydrogen bonding govern the self-assembly process, similar to what happens in biological systemssuch as DNA.1 Another attractive force that has been the focus

of much attention in recent years is halogen bonding.5 Halogenbonding is a charge transfer interaction of the n f σ* typebetween an electron-rich atom and an halogen, bonded to anelectron-deficient organic fragment or belonging to a dihalogenmolecule.6 Just like hydrogen bonding, it is a directionalinteraction whose strength depends on the distance betweenbonded atoms and on the D · · ·X-Y angle (D ) donor atom;X ) halogen; Y ) carbon, halogen). In addition, recentcomputational studies have shown that the energy of a halogenbond was comparable to that of a hydrogen bond.7

Moieties with basic sites are quite useful for the preparationof self-assembled networks. Pyridine, its analogues, and car-boxylates are probably the most frequently utilized moieties forsuch a purpose.1 In the past few years, increasing attention hasbeen devoted to another heterocycle, namely, imidazole. Justlike pyridine, imidazole can assemble via acid-base chemistrywith a large number of molecules possessing acidic sites,especially carboxylic acids, and form numerous coordinationcomplexes with many metals.8,9

Recently, we reported a structural investigation of novelmolecular assemblies constructed from imidazolyl-containinghaloalkenes and haloalkynes.10 In this work, 1-(2,3,3-triiodoal-lyl)imidazole (1) (Figure 1) was shown to self-assemble in thesolid state via N · · · I halogen bonding interactions, giving riseto polymeric chains. Furthermore, this compound was found togive a 1:1 cocrystal (3) with morpholinium iodide (2). In theX-ray crystal structure of 3, N · · · I halogen bonding interactions

* Corresponding authors. E-mail: [email protected] (S.G.D.);[email protected] (C.G.).

† Institut Charles Gerhardt Montpellier, UMR 5253 CNRS-UM2-ENSCM-UM1, Equipe CMOS.

‡ Institut Charles Gerhardt Montpellier, UMR 5253 CNRS-UM2-ENSCM-UM1, Equipe MACS.

§ Laboratoire de Mineralogie-Cristallographie de Paris.# Laboratoire de Chimie de la Matiere Condensee de Paris.

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10.1021/cg700975v CCC: $40.75 2008 American Chemical SocietyPublished on Web 09/30/2008

similar to those observed in 1 are not present, the arrangementof the molecules being governed by two interwoven hydrogenbond networks. The first network involves N-H · · ·O interac-tions between nearby morpholinium cations, and the secondnetwork is based on N-H · · ·N hydrogen bonding betweenmorpholinium cations and imidazolyl groups. The X-ray crystalstructure of morpholinium iodide (2) was also reported. It showsa completely different arrangement from that observed in 3;that is, N-H · · ·O interactions are not present. In crystalline 2,morpholinium cations are interconnected via C-H · · ·O bridges,and the NH2

+ groups interact with nearby iodide anions.Herein we wish to continue on this subject. Although the

weak interactions present in 1-3 have been well characterizedby X-ray diffraction, we wanted to study them further by solid-state NMR spectroscopy. Explicitly, we wanted to assess thesensitivity of solid-state NMR spectroscopy to detect theseinteractions and to determine to which extent the observed shiftscould be reproduced computationally. The study of weakinteractions in model crystalline materials by solid-state NMRspectroscopy is a worthy undertaking that provides informationvaluable for subsequent characterization of systems for whichsingle crystals are not available. For instance, solid-state 31PNMR spectroscopy has proven quite useful to assess the surfaceacidities of catalytic materials.11 In this work, an acidity scalewas established by measuring the difference between the 31Pchemical shift of Et3PO-loaded model systems and the 31Pchemical shift of free Et3PO. The surface acidities of variouscatalytic materials were then obtained by measuring the 31PNMR chemical shifts of these materials loaded with Et3PO andcomparing these chemical shifts with the acidity scale. Anadditional incentive for the current study is that the solid-statestructure of 3 presents an intrinsic problem: the binding of H+

to the morpholine nitrogen rather than to the imidazolyl nitrogenwas ascertained by comparing the C-N bond lengths of theimidazole ring and the C-N distances of the morpholiniumcation with known data.10 This situation agrees with the factthat morpholine is a stronger base than imidazole. However,examples in the literature report protonation schemes thatdisagree with acidity constants derived from solution.12 It wasinteresting to check whether solid-state NMR spectroscopy wasable to confirm this assignment.

Pharmaceutical science is another area where the knowledgeof the exact structure of cocrystals is of great importance. Indeed,a drug, also called active pharmaceutical ingredient (API), musthave, besides its potency, suitable physical properties (solubility,hygroscopicity, stability, dissolution rate, bioavailability), ac-ceptable mechanical properties (Young’s modulus), and goodpowder handling characteristics (particle size, flow, filterabil-ity).13,14 However, it happens sometimes that, for a given API,some of the above requirements are not met. One way ofmodifying these properties is to make a cocrystal by combiningthe API with a pharmaceutically acceptable guest molecule, quiteoften a food additive. The API and the pharmaceutically

acceptable guest molecule typically have basic and acidic sites,and so, depending on the ∆pKa (pKa(base) - pKa(acid)) value,three situations are possible.15 If ∆pKa is greater than 3,complete proton transfer occurs and the host-guest complexis a salt. If ∆pKa is less than zero, the complex is made ofnonionized components and the latter is called a cocrystal. Inthe intermediate range 0 < ∆pKa < 3, things are not as clear-cut, and the presence of a salt or a cocrystal depends on thecrystal structure of the complex. Such information is usuallyobtained from X-ray crystallography or neutron diffraction, but,in some cases, solid-state NMR spectroscopy may be of greathelp.16

Several reports have appeared in the literature concerning thestudy of hydrogen bonding in imidazole and its analogues bysolid-state NMR spectroscopy. In particular, the tautomericexchange in solid imidazole has been investigated by means of13C CP MAS and 15N MAS NMR spectroscopies: three separatesignals were observed in the 13C NMR spectrum and tworesonances in the 15N NMR spectrum.17,18 It was concludedfrom these results that, in the solid state, tautomeric exchangeis slow or nonexistent. Subsequently, a 15N CP MAS NMRstudy aimed at elucidating the conduction mechanism ofcrystalline imidazole confirmed these conclusions.19 Morerecently, the advent of fast and ultrafast MAS 1H NMRtechniques has provided new opportunities to study protonconductivity in systems possessing imidazolyl groups: Spiessand his group carried out an in-depth investigation of ethyleneoxide tethered imidazole heterocycles (Imi-nEO),20 whileGoward and colleagues focused their attention on 1,10-(1-H-imidazol-5-yl)decanephosphonic acid.21 In another connection,the crystal structures of 3-chloro- and 3-bromo-1H-1,2,4-triazolewere determined by single-crystal X-ray diffraction in order toelucidate their tautomeric structures, and the 13C and 15N CPMAS NMR spectra of these compounds could be rationalizedbased on the X-ray results.22

Interestingly, halogen bonding has not been studied by solid-state NMR spectroscopy. The only NMR studies that we knowof have been carried out in solution. It was found that thechemical shifts of the carbon atoms bonded to iodine indiiodohexatriyne and diiodooctatetrayne moved from about 1ppm in CDCl3 to approximately 15 ppm in DMSO-d6, and asimilar shift was observed for iodophenylacetylene on passingfrom CDCl3 to pyridine-d5.23 This phenomenon was attributedto the formation of a Lewis acid-base complex with the solvent.Energy calculations on the NCC≡CI ·DMSO and PhC≡CI ·DMSO complexes and comparison of these energies with thoseof the isolated molecules have confirmed that the charge transfercomplexes were likely to form.24 Also, the observed shifts weresuccessfully reproduced using different computational meth-ods.24 Additional 13C NMR studies have shown good correla-tions with various empirical models of solvent basicity thatindicate that acid-base interactions are responsible for theobserved change in chemical shift.25 But poor correlations toReichardt’s EN

T and Taft and Kamlet’s π* parameters suggestthat solvent polarity does not play a role.25 Interestingly, nosuch shifts are observed for iodoarenes on passing from anoncoordinating solvent to a coordinating one, although com-plexation has been shown to occur.26 It was proposed that thisis because the nonrelativistic complexation shifts and the change

Figure 1. Chemical structures of compounds 1-6.

3942 Crystal Growth & Design, Vol. 8, No. 11, 2008 Bouchmella et al.

in the spin-orbit induced heavy atom effect of iodine com-pensate each other.

Experimental Procedures

Sample Preparation. 1-(2,3,3-Triiodoallyl)imidazole (1), morpho-linium iodide (2), the 1:1 cocrystal 1-(2,3,3-triiodoallyl)-imidazole ·morpholinium iodide (3), and 1-(3-iodopropargyl)imidazole(6) were prepared as described elsewhere.10

Multinuclear Solid-State NMR. 1H MAS NMR spectra wereobtained at 750.16 MHz (17.6 T) on a Bruker AVANCE 750spectrometer (30° pulse with an RF field strength of 50 kHz andrecycling delay of 100 s for full relaxation) using a 2.5 mm Brukerprobe (MAS frequency ) 32 to 33 kHz). The spectra were referencedto TMS.

Standard 1Hf 13C and 1Hf 15N CP MAS NMR experiments wereperformed on a Bruker AVANCE 300 spectrometer (7.0 T) at 75.43(13C) and 30.41 MHz (15N), using 4 mm (for 13C) and 7 mm (for 15N)Bruker probes and spinning the samples at 12 and 5 kHz, respectively.15N and 13C CP MAS experiments were performed under the sameHartmann-Hahn match condition: both RF channel levels, ω1S/2π andω1I/2π, were set at about 42 kHz. 15N chemical shifts were referencedto solid NH4NO3 (δiso (15NO3) ) -4.6 ppm with respect to CH3NO2

(δ ) 0 ppm)). 13C chemical shifts were referenced to TMS using solidadamantane as a secondary standard. The spectra were simulated withthe DMFIT program.27

Static 1H f 15N CP NMR studies were carried out on a VarianInnova spectrometer (9.4 T) at 40.55 MHz (15N), using a 7.5 mm HXprobe. The Hartmann-Hahn match condition used RF channel levels,ω1S/2π and ω1I/2π, of about 31 kHz. Proton decoupling was applied.The contact time was set to 5 ms and the recycle delay time was set to10 s.

Computational Details. Calculations are performed within Kohn-Sham density functional theory (DFT) using the PARATEC code.28

PBE generalized gradient approximation29 is used and the valenceelectrons are described by norm conserving pseudopotentials30 in theKleinman-Bylander31 form. The core part for O, N and C is 1s2 and

that for I is 1s 2s 2p 3s 3p 4s 3d 4p 4d. The core radii are 1.2 au forH, 1.6 au for C, 1.45 au for N, 1.5 au for O, and 2.49 au for I. Thewave functions are expanded on a plane wave basis set with an energycutoff of 80 Ry. The crystalline structures are described as infiniteperiodic systems using periodic boundary conditions. NMR calculationsare performed as follows: partial geometry optimization is carried outfor each structure, starting from the experimental ones,10,32,33 andallowing the positions of the hydrogen atoms to relax using DFTcalculations. The new coordinates obtained after relaxation are sum-marized in Table 1. The integral over the Brillouin zone is done usinga Monkhorst-Pack 2 × 2 × 2 k-point grid34 for the charge density andthe electric field gradient calculations, and a 4 × 4 × 4 k-point gridfor the chemical shift tensor calculations. Calculations have been carriedout at the IDRIS supercomputer center of the CNRS using a parallelIBM Power4 (1.3 GHz) computer.

The shielding tensor is computed using the GIPAW approach whichpermits reproduction of the results of a fully converged all-electroncalculation.35 The isotropic chemical shift σiso is defined as σiso ) -(σ- σref), where σ is the isotropic shielding (one-third of the trace of theNMR shielding tensor) and σref is the isotropic shielding of the samenucleus in a reference system. In our calculations (Table 2), absoluteshielding tensors are obtained. To fix the scales, σref was chosen bycomparing experimental and calculated σiso values in various com-pounds,36 which leads to σref(17O) ) 261.5 ppm, σref(1H) ) 31.0 ppm,σref(13C) ) 170.9 ppm and σref(15N) ) -154.3 ppm.

It should be noted that spin-orbit (SO) effects are well-known tobe particularly important for carbon atoms directly bonded to iodine.37

Unfortunately, the computational results presented here do not takeSO corrections into account, so the calculated parameters for iodineatoms and for carbon atoms bonded to iodine will be ignored in thediscussion.

Diagonalization of the symmetrical part of the calculated shieldingtensor provides its principal components δ11, δ22 and δ33 defined as|δ33 - δiso| g |δ11 - δiso| g |δ22 - δiso| with δiso ) 1/3(δ11 + δ22 +δ33). The chemical shift anisotropy (CSA) is defined as δCSA ) |δ33 -δiso| and the CSA asymmetry parameter as ηCSA ) |δ22 - δ11|/δCSA. It

Table 1. Fractional Coordinates of the Atoms in the Structures of 1, 2, 3, and 6 after Relaxation of the Proton Positionsa

C6H5I3N2 (1) C4H10ION (2) C6H5I3N2 ·C4H10ION (3) C6H5IN2 (6)

I1 0.57252 0.35949 0.58454 I1 0.75598 0.11594 0.58493 I1 0.13512 1.03715 0.83537 I1 0.7214 0.0693 0.4477I2 0.33607 0.15191 0.57486 O2 0.7463 -0.1107 0.0176 I2 0.3563 0.89527 0.83928 N3 0.7540 0.8149 0.1658I3 0.67167 0.121246 0.35439 C3 0.7978 0.0069 0.0861 I3 0.40759 0.66961 0.77908 C5 0.6924 0.6173 0.2185C4 0.579 0.2021 0.4696 C4 0.6764 0.0252 0.1988 I4 0.69188 0.71257 0.77093 C6 0.6964 0.7620 0.2901C5 0.6755 0.2771 0.4735 N5 0.7146 -0.0863 0.2917 C5 0.476 0.7966 0.816 N7 0.7623 0.1470 0.0775C6 0.8678 0.3128 0.4021 C6 0.6797 -0.2119 0.2221 C6 0.5799 0.812 0.8234 C8 0.7039 0.8748 0.3481N7 0.7341 0.3791 0.3457 C7 0.7999 -0.2151 0.1076 C7 0.6329 0.8999 0.8591 C9 0.9082 0.0811 0.1068C8 0.8198 0.4565 0.3475 H8 0.96352 0.00811 0.12483 N8 0.6848 0.9388 0.7481 C10 0.6744 0.9832 0.1144N9 0.6581 0.502 0.2868 H9 0.76328 0.08606 0.01499 C9 0.6468 0.9419 0.6061 C11 0.9038 0.8858 0.1611C10 0.4559 0.4508 0.2432 H10 0.72529 0.11311 0.25403 C10 0.7183 0.9873 0.5426 H51 0.75776 0.40280 0.22219C11 0.4984 0.375 0.2793 H11 0.51218 0.02891 0.16110 N11 0.8036 1.013 0.6369 H52 0.57555 0.55785 0.19819H12 0.06422 0.33726 0.43482 H12 0.60313 0.92674 0.35338 C12 0.7805 0.985 0.758 H91 0.00620 0.18319 0.08566H13 0.90589 0.26589 0.35177 H13 0.86091 0.90975 0.34956 O13 1.03 0.7465 0.6658 H101 0.55143 0.97304 0.10565H8 0.29049 0.46084 0.18414 H14 0.51737 0.78317 0.18343 C14 1.0893 0.7089 0.5608 H111 0.99331 0.77780 0.19585H10 0.96050 0.48510 0.40088 H15 0.71592 0.71048 0.29045 C15 1.1451 0.7838 0.4879H111 0.37733 0.32358 0.26072 H16 0.76690 0.69475 0.05293 N16 1.0611 0.8468 0.4256

H17 0.96591 0.78874 0.14577 C17 0.9896 0.8788 0.5289C18 0.9446 0.8012 0.6009H19 0.69493 0.89119 0.95096H20 0.57022 0.94680 0.88727H21 0.57376 0.91087 0.55537H22 0.71443 0.00266 0.43169H24 0.83435 0.99103 0.85614H25 0.89522 0.82379 0.68134H26 0.89257 0.76053 0.52307H27 0.92426 0.92009 0.47496H28 0.03858 0.92047 0.60719H29 0.03280 0.67208 0.48282H30 0.14891 0.66130 0.61109H31 0.20102 0.81716 0.56933H32 0.03654 0.81417 0.32575H33 0.19111 0.75445 0.40815H34 0.10954 0.90340 0.39937

a The lattice parameters were not relaxed and the symmetry of each crystal was constrained to be monoclinic P21/n (1) or P21/c (2, 3, and 6).

Imidazole- and Morpholine-Based Model Compounds Crystal Growth & Design, Vol. 8, No. 11, 2008 3943

is noteworthy that the program outputs the orientation of the CSA tensorin the crystal axis system.

Results13C CP MAS NMR Experiments. The spectrum of 1-(2,3,3-

triiodoallyl)imidazole (1) (Figure 2a) shows four peaks at 63,119, 134 and 140 ppm that are assigned respectively to the C6(CH2), C11, C8 and C10 carbon atoms (see Figure 3 for the

atom labeling scheme). Signals corresponding to the C4 andC5 carbon atoms are not observed. This is because these atomsbear no hydrogens and, consequently, they are strongly under-estimated by the cross-polarization sequence. Attempts havebeen made to observe these carbon atoms through single-pulseand Hahn Echo MAS experiments, but these experiments werealso unsuccessful. Previous NMR studies in DMSO-d6 solutionhave shown resonances corresponding to C4 and C5 at 37 and115 ppm, respectively.10

The spectrum of morpholinium iodide (2) (Figure 2b) showstwo narrow resonances at 44.5 and 65 ppm corresponding tothe CH2N (C4 and C6) and CH2O (C3 and C7) sites of thecation (see Figure 4 for the atom labeling scheme). These valuesare in good agreement with previous liquid-state NMR studies10

Table 2. Experimental and Calculated 1H, 13C, 15N, and 17O Isotropic Chemical Shifts for Compounds 1-3

C6H5I3N2 (1) C4H10ION (2) C6H5I3N2 ·C4H10ION (3)

δiso ((1 ppm) δiso ((1 ppm) δiso ((1 ppm)

site calc exp site calc exp site calc exp

H12 (CH2) 5.0 4.4 H8 (CH2O) 4.7 4.3 H19 (CH2 Im) 6.3 6H13 (CH2) 6.0 5.8 H9 (CH2O) 5.3 4.7 H20 (CH2 Im) 5.7 5.8H8 (CH Im) 7.9 7.6 H10 (CH2N) 4.7 3.8 H21 (CH Im) 7.0 7.1H10 (CH Im) 7.4 6.9 H11 (CH2N) 3.7 3.4 H22 (CH Im) 8.0 7.2H111 (CH Im) 7.5 7.1 H12 (NH2) 9.2 7.6 H24 (CH Im) 8.8 7.3

H13 (NH2) 10.2 8.5 H25 (CH2O) 3.8 2C4 (CI2) 155.3 H14 (CH2N) 3.6 2.9 H26 (CH2O) 4.1 2.9C5 (CI) 147.6 H15 (CH2N) 3.3 2 H27 (CH2N) 4.6 4.4C6 (CH2) 51.3 63 H16 (CH2O) 4.9 5.3 H28 (CH2N) 5.3 5.3C8 (Im) 138.7 134 H17 (CH2O) 5.3 6 H29 (CH2O) 4.2 3.8C10 (Im) 140.9 140 H30 (CH2O) 4.8 5C11 (Im) 125.3 119 C3 (CH2O) 66.7 65 H31 (CH2N) 4.2 3.4

C7 (CH2O) 66.8 H32 (NH2) 9.8 8.4N7 -197.0 -207 C4 (CH2N) 42.2 44.5 H33 (CH2N) 4.5 4.3N9 -117.9 -127 C6 (CH2N) 40.6 H34 (NH2) 11.5 9.4

N5 -334.1 -339 C5 (CI2) 138.5C6 (CI) 150.7

O2 -10.3 C7 (CH2 Im) 55.5 63.3C9 (CH Im) 121.4 121.3C10 (CH Im) 130.8 133C12 (CH Im) 136.2 138.2C15 (CH2N) 44.6 44.2C17 (CH2N) 42.1C14 (CH2O) 71.0 67.3C18 (CH2O) 68.1

N8 (Im) -195.8 -204N11 (Im) -138.0 -135N16 (Morph) -341.9 -340

O13 -6.4

Figure 2. 13C CP MAS NMR spectra of 1 (a), 2 (b), and 3 (c).Figure 3. Crystal structure of 1 showing the atom labeling schemeand the N · · · I interactions between molecules.


that showed CH2N and CH2O signals at 43.8 and 64.1 ppm,respectively. It is noteworthy that the two crystallographicallyinequivalent CH2O sites are observed as a single line in thespectrum, suggesting very close chemical shift values. Thecalculated chemical shifts (Table 2) are in support of thisinterpretation. Similar observations are made for the CH2N sites.

The spectrum of cocrystal 3 (Figure 2c) is basically the sumof the previous two spectra with signals at 44.2, 63.3, 67.3,121.3, 133 and 138.2 ppm. These resonances are assigned tothe CH2N (C15 and C17, Morph), CH2 (C7, Im), CH2O (C14and C18, Morph), and CH (C9, C10 and C12, Im) sites,respectively (Figure 5).

The relative intensities of the signals will not be discussedas the CP sequence is not quantitative.

15N CP MAS NMR Experiments. The spectrum of mor-pholinium iodide (2) (Figure 6b) shows a single line at -339ppm while that of 1 (Figure 6a) exhibits two signals at -127and -207 ppm, ascribed respectively to the N9 (C-NdC)and N7 (NC3) sites (Figure 3). This assignment is based onNMR parameter calculations (Table 2) and is in goodagreement with values previously reported for crystallineimidazole.19,38

The spectrum of cocrystal 3 (Figure 6c) looks like the sumof the previous two spectra with signals at -135, -204 and-340 ppm. These resonances are assigned to the N11 (C-NdC),

N8 (NC3), and N16 (C2NH2+) nitrogens. It is noteworthy that

a significant shift (-8 ppm) of the C-NdC resonance isobserved on passing from 1 to cocrystal 3. The same phenom-enon is noticeable in the NMR calculations (Table 2).

1H MAS NMR Experiments. The 1H NMR spectra of thethree compounds (Figure 7) recorded at a moderate field (9.4T) and spinning speed (10 kHz) show broad lines due to strong1H-1H homonuclear dipolar interactions, and narrow peakscorresponding to low-level impurities trapped in the solids(water, stopcock grease). The spectra recorded at a higher field(17.6 T) and spinning speed (32 kHz) support this interpretation:a narrowing of the main signals due to averaging of the 1H-1Hhomonuclear couplings is observed while the peaks assignedto impurities remain identical.

The high-field NMR spectrum of triiodoalkene 1 (Figure 8a)shows three main resonances centered at 7, 6 and 4.4 ppm in a3:1:1 ratio. These signals are assigned to the three hydrogensof the imidazole ring and the two protons of the CH2 group,respectively (Table 2).

The spectrum of morpholinium iodide (2) (Figure 8b) showstwo large signals around 8 and 4 ppm with an intensity ratioclose to 1:4, suggesting that these signals correspond to the NH2

and CH2 protons, respectively. Calculations were initially carriedout to obtain chemical shift values for the different contributions.These values were then refined by fitting the experimental datawith lines constrained to have the same amplitude and linewidth. The final results are listed in Table 2.

The spectrum of cocrystal 3 (Figure 8c) shows three relativelynarrow signals centered at 9, 8.3, and 7.2 ppm and a broad onespanning the range 2-6 ppm. On the basis of NMR calculations,the peaks at 9 and 8.3 ppm are assigned to the NH2 protonsof the morpholinium part, and the signal at 7.2 ppm is ascribedto the three protons of the imidazole ring. The broad resonancecorresponds to the 10 CH2 protons of the 1-(2,3,3-triiodoal-

Figure 4. Crystal structure of 2 showing the atom labeling schemeand the N-H · · · I and C-H · · ·O interactions between ions.

Figure 5. Crystal structure of 3 showing the atom labeling schemeand the N-H · · ·N and N-H · · ·O interactions between fragments.

Figure 6. 15N CP MAS NMR spectra of 1 (a), 2 (b), and 3 (c).


lyl)imidazole molecule and morpholinium cation; a simulationis proposed but no precise assignment of the individualcomponents can be made due to insufficient resolution in thespectrum. It is noteworthy that the NH2 protons of themorpholinium fragment have quite different chemical shifts in2 and 3. This tendency is confirmed by the NMR calculations(Table 2).

Clearly, first-principles calculations are a powerful tool forassignment of the 1H MAS spectra of the compounds studiedherein, even though ultimate resolution is not attained experimentally.

Discussion

As mentioned in the introduction, we were interested inchecking whether solid-state NMR spectroscopy was able toconfirm the binding of H+ to the morpholine nitrogen ratherthan to the imidazolyl nitrogen in crystalline 3. Indeed, we findthat the 15N chemical shift of the morpholinium cation in 3(-340 ppm) is very close to that of 2 (-339 ppm), which fitsin nicely with the X-ray results. However, morpholine (4) andcomplexes in which morpholine is bonded to the metal centerthrough oxygen exhibit solution and solid-state 15N chemicalshift values that lie in the same range, that is, between -343and -349 ppm.39 As a check, the NMR parameters of solidmorpholine (Figure 9) were calculated using input data fromthe low-temperature crystal structure of this molecule (Table3).32 Interestingly, the calculations predict a 15N chemical shiftvalue for morpholine (-340.2 ppm) very similar to that of themorpholinium cation. Thus, it is not possible to confirm theposition of the H34 hydrogen in solid 3 on the basis of 15N CPMAS NMR results alone.

Khitrin and Fung have shown that it was possible to obtainwell-resolved 14N NMR spectra of polycrystalline samples ofKNO3, NH4NO3, and several amino acids under MAS condi-tions,40 so we checked if this technique could provide acompelling evidence for the presence of a morpholinium cationin the structure of 3. Explicitly, we wanted to obtain the 14NNMR spectrum of 3 and compare the NMR parameters (CQ

and ηQ) of the morpholinium/morpholine site with thosecalculated for 3 and 4 based on crystallographic results (Table4). A wide-spanning spectrum made of a large array of spinning

Figure 7. 1H MAS NMR spectra of 1 (a), 2 (b), and 3 (c) recorded at17.6 and 9.4 T using different spinning speeds. Narrow lines indicatethe positions of the impurities (see text).

Figure 8. Experimental (B0 ) 17.6 T, νrot ) 32 kHz) and simulated1H MAS NMR spectra of 1 (a), 2 (b), and 3 (c).

Figure 9. Crystal structure of 4 showing the atom labeling schemeand the N-H · · ·N interactions between molecules.


sidebands was obtained in the case of 2 (spectrum not shown)which could be reproduced computationally using the NMRparameters shown in Table 4. However, no measurable 14Nsignal was observed in the case of 3. Thus, these results areinconclusive and we believe that the lack of observable signalin the case of 3 is because the quadrupolar coupling constantCQ is too large as suggested by the calculations. According toKhitrin and Fung, well-resolved 14N NMR spectra can beobtained under simple MAS conditions for compounds that haveCQ values no larger than 1 MHz.40

Table 4 also gives the calculated δCSA(15N) of the morpho-linium/morpholine site of 3 and the δCSA(15N) of morpholine.The values are quite close, 22.0 and 32.2 ppm, so it wasexpected that slow magic-angle spinning experiments andexperiments performed under static conditions would not allowdifferentiation between both types of nitrogens. On the otherhand, the ηCSA(15N) parameters are quite different (0.98 vs 0.20),so this parameter was thought to be the one to focus on to tellapart a morpholinium cation from a morpholine molecule. Thestatic 15N CP NMR spectrum of 3 is shown in Figure 10 alongwith the simulated spectra of 3 and 4 based on the calculatedparameters. Clearly, the shape of the morpholinium/morpholinepart of the experimental spectrum of 3 resembles closely thatof 3 as obtained from calculations considering a morpholiniumnitrogen in the crystalline structure and does not exhibit anyasymmetry as that noted in the signal of 4. From thisresemblance it can be concluded that the N16 nitrogen of 3 isstructurally similar to a morpholinium nitrogen and not to amorpholine nitrogen.

The influence of chemical bonding on NMR parameters isanother important aspect. Molecules of 1 are connected in thesolid state through I · · ·N halogen bonds between the N9 atomof one molecule and the I3 atom of an adjacent molecule (Figure3). In cocrystal 3, I · · ·N halogen bonds are not present (Figure

5); instead, the N11 atom of the imidazolyl group is involvedin a N · · ·H-N hydrogen bond with a nearby morpholiniumcation. This difference in the bonding situations of the imidazolylgroup between 1 and 3 is possibly responsible for the -8 ppmdifference in the 15N chemical shifts. Similarly, the NH2 protons(H32 and H34) of the morpholinium cation in cocrystal 3 areinvolved in N11 · · ·H34-N16 and O13 · · ·H32-N16 hydrogen-bonding interactions. In morpholinium iodide (2), the twoprotons of the NH2 site (H12 and H13) interact with nearbyiodide anions. This difference certainly accounts for the factthat the 1H chemical shifts of the NH2 protons in morpholiniumiodide differ quite markedly from those of the morpholiniumcation in 3.

We sought to rationalize these results in a more quantitativeway. Correlations between hydrogen-bond strength and protonchemical shift have been reported for H-bonded anions, car-boxylic acids, and hydrates,41,42 and, also, for the P-OH sitesin phospho(i)nic acids43 and various hydrated calcium phos-phates.44 In all of these systems, δiso(1H) decreases withincreasing H · · ·O hydrogen bond length.

Table 3. Calculated and Experimental 1H, 13C, and 15N Isotropic Chemical Shifts for Compounds 4-6

C4H9ON (4) C3H4N2 (5) C6H5IN2 (6)

δiso (ppm) δiso (ppm) δiso (ppm)

site calc site calc exp site calc exp

H4 (NH) 5.1 H1 (NH) 15.7 H51 (CH2) 3.8H21 (CH2O) 4.8 H2 (CH) 7.7 H52 (CH2) 6.0H22 (CH2O) 4.0 H4 (CH) 6.7 H9 (CH) 7.7H31 (CH2N) 2.9 H5 (CH) 5.8 H10 (CH) 8.5H32 (CH2N) 3.2 H11 (CH) 7.4H51 (CH2N) 3.4 C2 133.6 136.3a

H52 (CH2N) 2.9 C4 127.2 126.8a C5 (CH2) 38.0 37.6H61 (CH2O) 4.1 C5 113.0 115.3a C6 (CtC) 87.7 87.3H62 (CH2O) 3.6 C8 (CtC) 85.6

N1 -202.4 -208b C9 (Im) 130.8 132.6C2 (CH2O) 68.8 N3 -133.5 -132b C10 (Im) 137.4 138.2C3 (CH2N) 44.4 C11 (Im) 116.4 118.9C5 (CH2N) 46.3C6 (CH2O) 69.1 N3 -206.2 -210.6

N7 -121.5 -129.7N4 -340.2

a From ref 17. b From ref 19.

Table 4. Calculated 15N and 14N NMR Parameters for 2, 3, and 4

compoundδiso

(ppm)δ11

(ppm)δ22

(ppm)δ33

(ppm)δCSA

(ppm) ηCSA

14N CQ

(MHz) 14N ηQ

2N5 -334.1 -313.2 -333.4 -355.7 21.6 0.94 0.98 0.833N8 (Im) -195.8 -298.6 -196.2 -92.6 103.2 0.99 2.61 0.18N11 (Im) -138.0 22.7 -65.3 -371.3 233.3 0.38 3.20 0.08N16 (Morph) -341.9 -363.6 -342.1 -319.9 22.0 0.98 1.82 0.744N4 -340.2 -359.4 -353.1 -308.0 32.2 0.20 4.81 0.25

Figure 10. (a) Simulated static 15N NMR spectrum of 3 using the CSAparameters of morpholine (4) (Table 4) for the N16 site. (b) Simulatedstatic 15N NMR spectrum of 3 using the CSA parameters of cocrystal3 (Table 4) for the N16 site. (c) Experimental 15N CP NMR spectrumof 3 recorded under static conditions.


Distances between atoms involved in hydrogen- and halogen-bonding interactions in 1, 2, and 3 are collected in Table 5. Forthe sake of completeness, interatomic distances for morpholine(4) (Figure 9) and imidazole (5) (Figure 11), which both exhibitNH · · ·N contacts, are reported. In addition, the intermolecularI · · ·N distance for 1-(3-iodopropargyl)imidazole (6) (Figure 12)is given, the structure of which is governed by I · · ·N halogenbonds.10 Table 5 shows that, for the morpholinium cation of 3and morpholine (4), δiso(1H) decreases with increasing NH · · ·N/Odistance (the distance we are referring to is that betweenunderlined atoms). Similarly, in the case of morpholinium iodide(2), δiso(1H) decreases with increasing NH · · · I distance. Regard-ing imidazole (5), the calculated δiso(1H) is 15.7 ppm for aNH · · ·N distance of 2.861 Å. This chemical shift is in directline with those measured on a sample of Imi-2EO, that is,δiso(1H) ) 16.0 and 14.7 ppm.20 In this case, the NH · · ·Ndistances are respectively 2.83 and 2.87 Å, thus confirming thetrend noted for 2, 3, and 4.

Considering the 15N chemical shift of the -Nd nitrogen incompounds 3 and 5 (Table 5), one can see that δiso(15N)increases as the NH · · ·N distance increases. Similarly, δiso(15N)for the -Nd nitrogen in 1 and 6 increases with an increase inthe N · · · I distance. Thus, apparently, stronger hydrogen- andhalogen-bonding interactions tend to make the δiso(15N) of the

-Nd nitrogen of imidazole rings more negative, but additionalexamples are needed to see if this phenomenon is a generalone.

The above discussion shows that GIPAW calculations of theisotropic 1H and 15N chemical shifts are reliable. As mentionedearlier, such calculations also give access to full tensor data,including absolute orientations. In this case we find that theCSA tensors of the protons involved in N-H · · ·N, N-H · · ·O,and N-H · · · I bonding interactions (Table 5) are characterizedby near axiality (ηCSA e 0.3) and a relatively large anisotropyδCSA ∼ 15 ppm. In addition, the unique axes (δ33) of all 1HCSA tensors are nearly collinear with the NH bonds, that is,with the hydrogen-bond directions (sketches (a), (b), and (c) inFigure 13). These results agree quite well with previouslyreported data dealing with protons involved in H-bondednetworks.43-47

The 15N CSA tensors of the nitrogen atoms involved inNH · · ·N and N · · · I bonding interactions (Table 5) are fairlysimilar, that is δCSA ∼ 240 ppm and ηCSA ∼ 0.5. It should benoted that the CSA tensor components of the 15N sites of

Table 5. Calculated and Experimental 1H and 15N Isotropic Shifts for the Sets of Atoms Involved in Hydrogen- and Halogen-Bonding Schemesin Compounds 1-6a

δiso (ppm)

compound calc exp δ11 (ppm) δ22 (ppm) δ33 (ppm) δCSA (ppm) ηCSA distancesb (Å)

2 NH · · · IH12 (NH2) 9.2 7.6 17.7 15.1 -5.3 14.4 0.2 2.698H13 (NH2) 10.2 8.5 20.2 16.8 -6.5 16.7 0.2 2.6333 NH · · ·N/OH32 (NH2) 9.8 8.4 21.1 14.8 -6.6 16.4 0.3 1.781H34 (NH2) 11.5 9.4 22.3 16.3 -4.2 15.7 0.3 1.715

NH · · ·NN11 (Im) -138.0 -135.0 22.7 -65.3 -371.3 233.3 0.4 2.7874 NH · · ·NH4 (NH) 5.1 13.1 10.4 -8.1 13.2 0.2 2.3545 NH · · ·NH1 (NH) 15.7 25.4 20.7 1.1 14.6 0.3 1.811

NH · · ·NN3 -133.5 -132c 41.1 -64.0 -377.7 244.2 0.4 2.861

-130d

6 N · · · IN7 -121.5 -129.7 53.2 -64.4 -353.4 231.8 0.5 2.7171 N · · · IN9 -117.9 -127.0 66.8 -58.6 -362.1 244.1 0.5 2.907

a Calculated chemical shift tensor components are also reported, as well as the distances between the atoms involved in these interactions. b Thegiven distance is that between the underlined atoms. c From ref 19. d From ref 38.

Figure 11. Crystal structure of 5 showing the atom labeling schemeand the N-H · · ·N interactions between molecules.

Figure 12. Crystal structure of 6 showing the atom labeling schemeand the N · · · I interactions between molecules.


imidazole (5) have been reported previously,19,38 and theexperimental values of δii for the C-NdC site (δ11 ) 19/23ppm, δ22 ) -55/-58 ppm and δ33 ) -359/-356 ppm) are ingood agreement with the calculated ones. The main feature ofthese CSA tensors is that the calculated δ22 component iscoplanar with the CNC plane of the imidazole ring, and thiscomponent lies approximately along the bisector line of the NCNangle (sketches (d), (e), (f), and (g) in Figure 13).

Conclusion

The N · · · I, N-H · · ·N, N-H · · ·O, and N-H · · · I bondinginteractions present in solid 1-(2,3,3-triiodoallyl)imidazole (1),morpholinium iodide (2), the 1:1 cocrystal 1-(2,3,3-triiodoallyl)-imidazole ·morpholinium iodide (3), morpholine (4), imidazole(5), and 1-(3-iodopropargyl)imidazole (6) have been investigatedby solid-state 1H, 13C, and 15N NMR spectroscopies.

Our efforts to demonstrate the existence of protonatedmorpholine in the X-ray crystal structure of 3 by 15N NMRspectroscopy have met with success. Comparison of the 15NCP MAS NMR spectrum of 3 with that of 2 suggests that amorpholinium cation is indeed present in the structure of 3. Yet,GIPAW calculations predict a 15N isotropic shift for morpholinevery similar to that of the morpholinium cation. Conclusiveevidence for the presence of a morpholinium cation in crystalline3 was obtained by recording the static 15N NMR spectrum ofthis host-guest complex and comparing the morpholinium/morpholine part of the spectrum with the static spectra of 3and 4 as obtained from calculations based on the X-raystructures. Concerning the imidazolyl group, 15N NMR spec-troscopy has proven quite valuable to identify changes in theimmediate environment of the C-NdC nitrogen of 1: a -8ppm difference in the 15N chemical shift was observedexperimentally on passing from pure 1 to cocrystal 3, and this

difference was reproduced computationally. Perhaps the fact thatthe C-NdC nitrogen of imidazole is sp2-hybridized and thenitrogen of morpholine sp3-hybridized makes the former moresensitive to slight modifications in its surroundings. Theenhanced sensitivity of the C-NdC nitrogen of imidazolylgroups to subtle changes is further demonstrated by the factthat the 15N isotropic shifts of 1 and 6 differ slightly, althoughthe molecules are structurally very similar and exhibit the samestructure-directing interaction. Undoubtedly, the difference inchemical shifts between the two compounds arises fromdissimilar halogen bond strengths. Also, concerning the C-NdCimidazole nitrogen, it is noteworthy that this is the δ11

component of the 15N chemical shift tensor that is most affectedby a change in the bonding situation and not the δ22 component,as might be intuitively expected since this latter component liesroughly along the N · · · I axis.

13C NMR spectroscopy is another way of studying halogenbonding. Previous NMR studies in solution have shown thatsubstantial shifts result from the interaction between haloalkynesand donor molecules, and no such shifts are detected withhalogenated aromatics.23-26 In our case, 13C NMR spectroscopydoes not provide exploitable results as signals correspondingto the sp and sp2 carbon atoms bonded to iodine are not observedexperimentally. Also, we could not study this interactioncomputationally as our calculations do not take spin-orbiteffects into account. Further work in this direction is planned.

1H NMR spectroscopy is a powerful tool to study hydrogenbonding interactions of moderate energies such as +NH2 · · ·X(X ) N, O, I). We have found that the chemical shifts of theNH hydrogen atoms were quite sensitive to the nature of Xand to the N-H · · ·X distance. This is demonstrated by the factthat the chemical shifts of the +NH2 protons of the morpho-linium cation in 2 and 3 are noticeably different. Interestingly,some differences are also observed in the chemical shifts ofthe CH2O hydrogens between 2 and 3, but currently it is unclearwhether these differences originate from the presence of low-energy C-H · · ·O interactions in 2.

Solid-state NMR spectroscopy has proven quite helpful inthe past to resolve the ambiguity between solution and solid-state structures and, also, to resolve space group ambiguities.48-52

Herein we show that solid-state NMR spectroscopy is a valuabletool to study weak interactions in wholly organic architecturesand/or systems for which an X-ray crystal structure is notavailable.53

Acknowledgment. We are indebted to Dr. DominiqueMassiot (CRMHT, CNRS UPR 4212, Orleans, France) forgiving access to the 750-MHz NMR spectrometer. We also wishto thank Prof. Christian Bonhomme (Laboratoire de Chimie dela Matiere Condensee de Paris, Universite Pierre et Marie Curie-Paris 6, Paris, France) for helpful discussions and Dr. PhilippeGaveau (Institut Charles Gerhardt Montpellier, UniversiteMontpellier II, Montpellier, France) for technical assistance.Funding for ACI project 4077 is acknowledged. Calculationswere performed at the IDRIS supercomputer center of the CNRS(Project 51461).

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1H, 13C, and 15N Solid-State NMR Studies of Imidazole- and Morpholine-Based Model Compounds Possessing Halogen and Hydrogen Bonding Capabilities