77Se Solid-State NMR of Inorganic and Organoselenium Systems: A

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Se Solid-State NMR of Inorganic and Organoselenium Systems: A Combined Experimental and Computational Study John M. Griffin, Fergus R. Knight, Guoxiong Hua, Jeanette S. Ferrara, Simon W. L. Hogan, J. Derek Woollins, and Sharon E. Ashbrook* School of Chemistry and EaStCHEM, University of St. Andrews, North Haugh, St Andrews KY16 9ST, U.K.

bS Supporting Information ABSTRACT: Experimental 77Se NMR parameters for 17 selenium-containing compounds have been determined by analysis of 77Se solid-state NMR spectra. These are compared to values obtained from first-principles gauge including projector augmented wave (or GIPAW) calculations performed on geometry-optimized crystal structures. Good agreement is observed between experimental and calculated values across a wide chemical shift range, enabling assignment of the experimental 77Se NMR spectra for compounds containing more than one crystallographically distinct selenium site. Calculations for isolated molecules extracted from the optimized structure reveal that intermolecular interactions have a relatively small effect on isotropic shifts in general, but larger effects on the chemical shift anisotropy are observed for some compounds. Further calculations for a model structure give insight into the effects of local bonding geometry on the 77Se chemical shift in a diselenide linkage. The 77Se chemical shift is found to be highly sensitive to torsional angles that define the geometry of the diselenide linkage, and this leads to an understanding of the origins of the large chemical shift differences observed between chemically equivalent selenium sites for one of the compounds studied in this work.

’ INTRODUCTION Selenium-containing compounds find important applications in many areas of synthetic chemistry, pharmaceutical and medicinal research, and materials science. For example, Woollins’ reagent is widely used in the preparation of organoselenium compounds and PSe heterocycles.1 Selenocystine, regarded as the 21st amino acid, has been shown to exhibit antitumor properties.2 Selenium-containing chalcogenide glasses have also received attention due to their favorable properties for telecommunications applications.3 As interest in selenium-containing materials increases, so does the requirement for analytical methods by which to characterize them. The high sensitivity of nuclear magnetic resonance (NMR) to local chemical environment makes it a powerful tool for structural characterization and analysis. With a natural abundance of 7.63%, 77Se (spin quantum number I = 1/2) is the only NMR-active isotope of selenium and provides a sensitive probe of molecular structure as it exhibits a large chemical shift range of over 3000 ppm.4,5 Indeed, the utility of 77Se NMR has been widely recognized and this approach is often used in the solution state as a method of structural characterization. However, applications of 77Se solid-state NMR have been less widespread.6 This may be in part due to the often long T1 relaxation times7,8 and typically large chemical shift anisotropies (CSAs) exhibited by 77Se,9 which can compromise sensitivity and complicate the acquisition and interpretation of experimental data. However, well-known experimental techniques such as cross-polarization r 2011 American Chemical Society

(CP) and magic angle spinning (MAS) can greatly facilitate the acquisition of 77Se solid-state NMR spectra,9,10 and analysis of the MAS sideband patterns that result from CSA interactions provides additional information on the local chemical environment.1117 For systems containing more than one crystallographically distinct selenium site, assignment of solid-state NMR spectra can be a challenging task. While some empirical rules exist for predicting chemical shift ranges associated with certain bonding arrangements,46,18 the sensitivity of the 77Se chemical shift to differences in local geometry and intermolecular interactions means that it can be difficult to assign 77Se solid-state NMR spectra solely on the basis of the chemical or molecular structure. In this respect, computational methods for the calculation of 77Se NMR parameters can provide valuable information for the interpretation of experimental data. Indeed, theoretical calculations have been used in a number of studies to obtain insight into experimental 77Se solid-state NMR spectra of organic and inorganic systems.12,15,17,19,20 Most theoretical studies to date, however, have been performed for single molecules, neglecting any effects due to intermolecular interactions. While these methods have shown good accuracy in comparison to experiment in some cases, for solid crystalline systems it may be more appropriate to perform calculations based on the full crystal Received: March 17, 2011 Revised: April 23, 2011 Published: May 10, 2011 10859

dx.doi.org/10.1021/jp202550f | J. Phys. Chem. C 2011, 115, 10859–10872

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Solid-State NMR. Solid-state

Figure 1. Selenium-containing compounds 117 investigated in this study.

structure. Recently, codes based on density functional theory (DFT) that utilize periodic boundary conditions have been developed, enabling efficient calculation of solid-state NMR parameters with high accuracy by exploiting the inherent periodicity associated with crystalline materials. In particular, codes using the gauge including projector augmented wave (or GIPAW) approach,21 such as CASTEP,22,23 have seen widespread application to a large range of organic and inorganic systems.2437 However, to our knowledge, there have thus far been relatively few applications of the GIPAW approach for the calculation of 77 Se NMR parameters.29,36,37 In this work, we present a combined experimental and computational solid-state NMR study of 17 selenium-containing compounds (shown in Figure 1). The accuracy of the GIPAW approach for the calculation of 77Se NMR parameters is evaluated over a wide chemical shift range by comparison with experimental data. The calculated 77Se NMR parameters are found to be in good agreement with the experimental data and enable chemical shift assignments for compounds containing more than one crystallographically distinct selenium species. By performing calculations for single molecules and for a model system, the study is then extended to obtain insight into the influence of crystal packing effects and local bonding geometry on 77Se chemical shifts in a diselenide linkage. We find that the molecular conformation can have significant effects on the observed chemical shift, leading to large differences in chemical shift between two chemically equivalent (but crystallographically distinct) selenium atoms.

’ EXPERIMENTAL AND COMPUTATIONAL DETAILS Synthesis. Compounds 15 and 7 were purchased from either Sigma Aldrich or Alfa Aesar and used without further purification, with 4 and 5 handled in a glovebox due to their high sensitivity to moisture. Compounds 6,38 8,39 9,40 10,41 11,42 12,42 13,43 14,43 15,44 and 1644 were prepared by literature methods, while 17 was synthesized by a method similar to that for the naphthalene analogue (15).

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Se NMR experiments were performed using Bruker Avance III spectrometers operating at magnetic field strengths of 9.4 and 14.1 T, corresponding to Larmor frequencies of 76.3 and 114.5 MHz, respectively. Experiments at both magnetic fields were carried out using Bruker 4-mm MAS probes, with MAS rates between 5 and 14 kHz. Chemical shifts are referenced relative to (CH3)2Se at 0 ppm. For solid-state NMR experiments, the isotropic resonance of solid H2SeO3 at 1288.1 ppm was used as a secondary reference (as discussed in the main text).9,45 For solution-state NMR experiments, compound 7 was used as a secondary reference with a chemical shift of 463 ppm. For compounds 15, experiments were performed by direct polarization of 77Se using between 4 and 240 transients separated by a recycle interval of 300 s. Static spectra were recorded using either spinecho or CarrPurcell MeilboomGill (CPMG)46,47 pulse sequences. For the spin echo experiment, a 16-step phase cycle was used, and for CPMG experiments, a 2-step phase cycle was used. For compounds 617, transverse magnetization was obtained by cross-polarization (CP) from 1H using optimized contact pulse durations of between 1 and 15 ms (ramped for 1H), and two-pulse phase modulation (TPPM) 1H decoupling during acquisition. Spectra were acquired with between 500 and 8000 transients separated by recycle intervals of between 5 and 300 s, depending on the longitudinal relaxation time of the individual samples. For all spectra, the positions of isotropic resonances within the spinning sideband patterns were unambiguously determined by recording a second spectrum at a higher MAS rate. Experimental 77Se NMR parameters were determined by line shape analysis using Bruker Topspin software. A more detailed description of the experimental parameters for individual materials is given in the Supporting Information. Calculations. NMR parameters were calculated using the CASTEP DFT code,22,23 employing the gauge including projector augmented wave (GIPAW) algorithm,21 which allows the reconstruction of the all-electron wave function in the presence of a magnetic field. The generalized gradient approximation (GGA) PBE functional48 was employed and corevalence interactions were described by ultrasoft pseudopotentials.49,50 A planewave energy cutoff of 60 Ry (816 eV) was used and integrals over the Brillouin zone were performed using a k-point spacing of 0.04 Å1. For all calculations, the initial atomic positions and unit cell parameters were taken from existing X-ray diffraction structures. However, for several structures, large forces were calculated for some atoms (up to 14 eV Å1 for 10). Therefore, prior to the calculation of the NMR parameters, geometry optimizations were performed for each structure (using cutoff energies of 5060 Ry and k-point spacings of 0.040.05 Å1 depending on the size of the unit cell). All internal atomic coordinates were allowed to vary but lattice parameters remained fixed. Calculations on isolated molecules were performed using atomic coordinates extracted from an optimized crystal structure, with a single such molecule placed inside a unit cell of dimensions 18 18 18 Å. A single k-point at (0.25, 0.25, 0.25) was found to be sufficient for accurate calculation of NMR parameters, with a planewave cutoff energy of 60 Ry. Calculations were performed using the EaStCHEM Research Computing Facility, which consists of 136 AMD Opteron 280 dual-core processors running at 2.4 GHz, partly connected by Infinipath high speed interconnects. Calculation wallclock times ranged from 1 to 24 h using 4 to 5 cores, 10860

dx.doi.org/10.1021/jp202550f |J. Phys. Chem. C 2011, 115, 10859–10872


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Figure 2. 77Se solid-state (B0 = 14.1 T) NMR spectra of (a) 1, (b) 2, (c) 3, (d) 4, and (e) 5. MAS rates of (a, c) 5 kHz, (b, e) 12.5 kHz, and (d) 14 kHz were used. Isotropic resonances in MAS spectra are indicated by *. For compounds 13 and 5, MAS and static spectra are shown. Static spectra were acquired using (a, c) spinecho and (b, e) CPMG pulse sequences.

depending on the size of the unit cell of the system being calculated. The calculations generate the absolute shielding tensor (σ) in the crystal frame. Diagonalization of the symmetric part of σ yields three orthogonal principal components, σ11, σ22, and σ33. The principal components of the chemical shift tensor, δ11, δ22, and δ33, are related to σ by δii ¼ ðσii σref Þ=ð1 σ ref Þ ðσii σref Þ

ð1Þ

where σref (assumed to be ,1) is the reference shielding. To allow a clear comparison with experimental data, here we follow the so-called Maryland convention,51 where the principal components of the chemical shift tensor are ordered such that δ11 g δ22 g δ33

ð2Þ

The isotropic chemical shift, δiso, is defined as the average of the three components of the chemical shift tensor δiso ¼ ðδ11 þ δ22 þ δ33 Þ=3

ð3Þ

The span of the chemical shift tensor represents the maximum orientation dependence of the chemical shift experienced by the nucleus and is defined by ΩCS ¼ δ11 δ33

ð4Þ

’ RESULTS Experimental 77Se Solid-State NMR. Inorganic Compounds. 77

Se solid-state NMR spectra of the inorganic compounds 15 are shown in Figure 2. These compounds were chosen for investigation as they exhibit a wide range of 77Se chemical shifts. Experimental 77Se NMR parameters are given in Table 1. Optimal experimental parameters (both for conventional MAS and for CPMAS experiments) were determined using H2SeO3

(1). The crystal structure of 1, shown in Figure 3, contains a single crystallographically distinct selenium site bonded to three oxygens. Individual H2SeO3 molecules are linked via intermolecular hydrogen bonds, resulting in a double-layered framework structure.52 The 77Se MAS NMR spectrum of 1, shown in Figure 2a, exhibits a spinning sideband pattern associated with a single isotropic resonance. In accordance with previous work by Collins et al.,9 the frequency of the isotropic resonance was set to 1288.1 ppm (relative to the standard reference, (CH3)2Se4,45). This compound was used as a secondary reference for all other compounds studied in this work. From the 77Se NMR spectrum of a static sample of 1 (also shown in Figure 2a), ΩCS is 446 ppm, in good agreement with the value of 453 ppm quoted by Collins et al. 9 As with 1, SeO2 (2) also contains a single crystallographic selenium site bonded to three oxygens, as shown in Figure 3. However, in contrast to the molecular structure of 1, the structure of 2 is composed of chains of SeO3 units parallel to the crystallographic c axis, that are covalently bonded via bridging oxygens.53 The 77Se MAS NMR spectrum of 2, shown in Figure 2b, displays a single isotropic resonance at δiso = 1357.4 ppm in addition to a manifold of spinning sidebands. We note that this chemical shift is significantly different from the value of 1600 ppm measured in an earlier study by Kemp and coworkers.18 In view of this, the crystal structure and purity of our sample was confirmed by powder X-ray diffraction (see Supporting Information). However, the chemical shift for 1 reported in the previous work was not measured directly, but quoted from ref 9, and thus there may be a discrepancy between the absolute references used in the two studies for this sample. Despite the selenium species in 2 having a local structure similar to that in 1, the 77Se static NMR spectrum of 2 shows a significantly larger span of 837 ppm, indicating a more polar environment.50 The structure of Na2SeO4 (3), shown in Figure 3, exhibits a single selenium species in a tetrahedral SeO42 environment.54 10861



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Table 1. Experimental 77Se NMR Parametersa (isotropic chemical shift, δisoexp, solution-state isotropic chemical shift, δisosoln, chemical shift tensor components, δiiexp, and span, ΩCSexp) for Compounds 117, Extracted from the Spectra in Figures 2, 4, 6 and 8 cmpd

species

δisoexp

1

Se1

1288.1(0.5)

2

Se1

3

Se1

4

Se1

124.7(1)

Se2 Se3

171.4(1) 195.1(1)

5

Se1

282.3(0.5)

273.0c

6

Se1

441.3(0.5)

441.0d

841(40)

421(40)

61(40)

Se2

455.1(0.5)

441.0d

830(40)

436(40)

99(40)

731(60)

Se3

76.5(0.5)

61.7d

179(40)

32(40)

377(40)

556(60)

Se4

67.7(0.5)

61.7d

193(40)

193(40)

182(40)

375(60)

7

Se1

423.7(0.5)

463.0e

571(10)

517(10)

183(10)

388(15)

8

Se2 Se1

348.3(0.5) 341.3(0.5)

463.0e 349.8e

515(10) 628(10)

511(10) 284(10)

18(10) 112(10)

497(15) 516(15)

Se2

376.1(0.5)

349.8e

706(10)

302(10)

121(10)

585(15)

Se1

334.1(0.5)

398.0f

519(10)

374(10)

110(10)

409(15)

Se2

383.7(0.5)

398.0f

554(10)

398(10)

202(10)

352(15)

Se1

344.0(0.5)

600(15)

457(15)

25(15)

625(20)

Se2

472.8(0.5)

760(15)

421(15)

238(15)

522(20)

Se3

424.3(0.5)

703(15)

491(15)

79(15)

624(20)

Se4 Se1

494.1(0.5) 495.1(1)

419.8g

809(15) 869(40)

423(15) 589(40)

251(15) 27(40)

558(20) 842(60)

Se2

470.5(1)

419.8g

893(40)

556(40)

37(40)

930(60)

12

Se1

586.3(0.5)

561.8g

1012(10)

592(10)

155(10)

857(15)

13

Se1

434.9(0.5)

428.6h

694(15)

411(15)

200(15)

494(20)

Se2

405.4(0.5)

428.6h

676(15)

435(15)

106(15)

570(20)

14

Se1

453.2(0.5)

455.3h

709(15)

437(15)

213(15)

496(20)

15

Se1

460.2(1)

447.8i

697(15)

444(15)

240(15)

457(20)

16 17

Se1 Se1

439.5(1) 449.6(1)

430.4i 423.7e

684(15) 746(15)

394(15) 379(15)

241(15) 223(15)

443(20) 523(20)

9 10

11

a

δisosoln

δ11exp

δ22exp

δ33exp

1300b

1474(5)

1363(5)

1028(5)

446(10)

1357.4(0.5)

1682(5)

1546(5)

845(5)

837(10)

1058.7(0.5)

1066(5)

1059(5)

1052(5)

14(10)

32(5)

24(5)

855(5)

ΩCSexp

887(10) 780(60)

All values are quoted in ppm. b Reference 45. c Reference 69. d Reference 38. e This work. f Reference 40. g Reference 42. h Reference 43. i Reference 44.

In the 77Se MAS NMR spectrum, shown in Figure 2c, a single resonance is observed at 1058.7 ppm. This chemical shift is in reasonable agreement with that of 1049 ( 3 ppm reported by Kemp et al.18 The absence of spinning sidebands at the MAS rate of 5 kHz employed in the experiment indicates a small chemical shift anisotropy (CSA), owing to the high symmetry of the Se chemical environment. This is confirmed by the static 77Se NMR spectrum where a span of approximately 14 ppm is measured. However, broadening due to 77Se23Na dipolar interactions may also contribute to the observed line width. The 77Se MAS NMR spectrum of Al2Se3 (4), shown in Figure 2d, shows three resonances at 124.9, 171.7, and 195.7 ppm. This is consistent with the crystal structure,55 shown in Figure 3, which contains three distinct Se sites; one in a divalent AlSeAl arrangement (Se1) and two in trivalent SeAl3 environments (Se2 and Se3). On the basis of the similarity in the local bonding environments of Se2 and Se3, it is reasonable to expect them to have similar chemical shits and therefore to tentatively assign the resonances at 171.7 and 195.7 ppm to these sites. However, a complete assignment is not possible solely on the basis of the crystal structure. Close inspection of the observed line shapes reveals evidence of broadened multiplet

structures for each peak. This is consistent with heteronuclear J coupling interactions between the selenium sites and the directly bonded 100% abundant 27Al (spin I = 5/2). From the width of the observed line shapes, these interactions can be estimated to be in the range 200400 Hz. However, the relatively low signal-to-noise obtained in this experiment, together with the complex multiplet structure expected for couplings between 77Se and multiple 27Al means that the exact size of the J couplings is difficult to determine. The absence of spinning sidebands at a MAS rate of 14 kHz indicates that the selenium sites have smaller CSAs than those of 1 and 2; however, the magnitude of ΩCS for each site was not measured owing to the considerably longer experimental time that would be necessary for observation of spinning sideband patterns for these broadened line shapes with reasonable signal to noise. The 77Se MAS NMR spectrum of KSeCN (5) is shown in Figure 2e. The observation of a single isotropic resonance at 282.3 ppm is consistent with the published crystal structure, shown in Figure 3, which contains a single crystallographic selenium site in a (SeCN) environment.56 The chemical shift is in good agreement with a previous study of the same compound.57 The width of the spinning sideband manifold and the 10862



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Figure 3. Crystal structures of (a) 1,52 (b) 2,53 (c) 3,54 (d) 4,55 and (e) 556. Crystallographically distinct selenium sites are indicated for 4. 77

Se static NMR spectrum show that the selenium site has a very large span of 887 ppm. The near-axial symmetry of the chemical shift tensor (as shown by the relatively small difference between the measured δ11 and δ22 values given in Table 1) is consistent with the almost linear conformation of the (SeCN) ion, which has a SeCN bond angle of 170°. Organoselenium and Organophosphine Selenide Compounds. Figures 4, 6 and 8 show 77Se CPMAS NMR spectra of organoselenium compounds 617. Expansions of the isotropic resonances for 717 are shown in the Supporting Information. These compounds were chosen to provide a range of different organic selenium environments. For all organoselenium compounds, small but noticeable changes in isotropic chemical shift with MAS rate were observed (e.g., differences of up to 3 ppm in spectra of 10 recorded at MAS rates of 5 and 14 kHz). We attribute this to the temperature change due to frictional heating of the rotor at higher spinning rates. Indeed, the high sensitivity of 77Se chemical shifts to temperature changes has been noted previously.14 All values of δiso are quoted from spectra recorded at 298 K at a MAS rate of 5 kHz, where the additional heating due to MAS is estimated to be ∼4 K from measurements performed on Pb(NO3)2.58 13C CP MAS NMR spectra for 617 are given in the Supporting Information. The 77Se CP MAS NMR spectrum of compound 6, shown in Figure 4a, exhibits a complex overlap of spinning sideband patterns, convoluted with a multiplet structure due to the large heteronuclear J couplings between the directly bonded selenium and phosphorus sites. A second spectrum, shown in Figure 4b, was therefore recorded at a higher MAS frequency of 12.5 kHz, to enable more accurate measurement of the isotropic chemical shifts and multiplet splittings. The four selenium sites within the molecule are crystallographically distinct in the structure (shown in Figure 4c), owing to the molecular geometry and crystal packing.38 Isotropic resonances observed at δiso = 441.3 and 455.1 ppm, with spans of 780 and 730 ppm, can be assigned to the endocyclic selenium sites (Se1/Se2), which have solutionstate chemical shifts of 441.0 ppm.38 The observed splittings of 430 and 480 Hz, respectively, closely agree with the value of

465 Hz measured by 77Se and 31P solution-state NMR,38 and also agree well with values determined in the solid state for compounds containing similar PSeSeP units.16 The difference in magnitude of the two splittings observed presumably reflects the small difference in geometry between the two sites. Isotropic resonances observed at chemical shifts of 76.5 and 67.7 ppm can be assigned to the exocyclic selenium sites (Se3/Se4), which have solution-state chemical shifts of 61.7 ppm. The large difference in the solid-state chemical shifts of these resonances indicates differences in local environment resulting from the crystal packing or the molecular conformation. Line shape analyses of the spinning sideband patterns reveal spans of 555 and 375 ppm for the resonances at 76.5 and 67.7 ppm, respectively. While the complex nature of the experimental spectrum means that the errors associated with these values are larger than for other compounds in this work, the large difference in span between the two sites again indicates differences in the local environments for the two sites. The observed splittings of 810 and 830 Hz for these sites agree very well with the value of 810 Hz measured in solution.38 We note that 13C CP MAS and 31 P MAS NMR spectra of 6 (given in the Supporting Information) are also consistent with the crystal structure and the observations discussed here. The structures of compounds 710, shown in Figure 5, share the common feature that selenium sites are arranged in CSeSeC diselenide linkages. In 7 and 8 the diselenide linkages are bonded on both sides to phenyl rings. In both compounds, the two chemically equivalent selenium sites in the molecule are crystallographically distinct in the crystal structure. Compound 7 adopts a bent structure with a SeSe bond length of 2.29 Å and CSeSe bond angles of 104.7° (Se1) and 107.5° (Se2).59 Compound 8 has a slightly longer SeSe bond length of 2.35 Å and the presence of an additional bond between the two phenyl groups constrains the geometry of the molecule, resulting in smaller CSeSe bond angles of 92.1° (Se1) and 92.3° (Se2).39 77Se CP MAS NMR spectra, shown in Figure 6a,b, reveal isotropic chemical shifts of 348.3 and 423.7 ppm for 7 and 341.3 and 376.1 ppm for 8. The chemical shifts for 7 in the solid 10863



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Figure 5. Crystal structures of (a) 7,59 (b) 8,39 (c) 9,40 and (d) 10.41 Crystallographically distinct selenium sites are indicated.

Figure 4. 77Se MAS (B0 = 14.1 T) NMR spectra of 6 recorded at MAS frequencies of (a) 5 kHz and (b) 12.5 kHz. Insets in (b) show expanded views of the isotropic resonances (which are indicated by * in the main spectra). The crystal structure of 638 is shown in (c).

state differ considerably from the solution-state shift of 463.0 ppm, suggesting that the conformation and packing of the molecule in the crystal structure may have a significant influence on the 77Se chemical shift. The solid-state chemical shifts for 8 are much closer to the solution-state shift of 349.8 ppm, perhaps as a result of the reduction in conformational flexibility imposed by the bond linking the two aromatic rings. Line shape analyses of the spinning sideband patterns reveal spans of 497 and 372 ppm for 7 and 516 and 585 ppm for 8. Assignment of the spectra on the basis of the crystal structures alone is not possible. However, for 7, the measured values are in good agreement with those determined in previous studies of the same compound.12,15 In particular, as in the earlier studies, we observe that the biggest contribution to the difference in δiso and ΩCS between the two selenium sites is a significant difference in the magnitude of δ33. The two selenium sites in 9 are crystallographically distinct, while in 10 the eight selenium atoms in the molecule are arranged such that sites on opposite sides of the ring-shaped molecule are crystallographically equivalent, resulting in four distinct selenium species. Both structures contain diselenide linkages that are bonded on both sides to alkyl (CH2) groups. The local

selenium bonding environment is very similar in both compounds, with SeSe bond lengths of 2.31 Å for 940 and 2.312.32 Å for 10.41 CSeSe bond angles are 103.2° (Se1) and 102.4° (Se2) for 9 and 101.0° (Se1), 99.7° (Se2), 102.0° (Se3) and 100.2° (Se4) for 10. A 77Se CP MAS NMR spectrum of 9, shown in Figure 6c, reveals isotropic resonances at 334 and 383 ppm with spans of 409 and 352 ppm, respectively, which are approximately in the same range as those observed for compounds 7 and 8. However, the resonance observed at 334 ppm is significantly different to the solution state chemical shift of 398.0 ppm. For 10, the 77Se CP MAS NMR spectrum shown in Figure 6d reveals isotropic shifts of 344.0, 424.3, 472.8, and 494.1 ppm with spans of 625, 624, 522, and 558 ppm, respectively. In general, the spans are larger than those measured for compounds 79, and the observed shifts are spread across a wider range. Given that all the sites in the molecule are chemically equivalent, the large differences between the sites indicate that crystal packing and/or the molecular geometry in the solid state play an important role in determining the chemical shifts for this system. Owing to very poor solubility, solutionstate NMR chemical shifts are not readily available for 10. Compounds 1117 are based upon peri-substituted naphthalenes, with crystal structures as shown in Figure 7. The molecular structure of compound 11 can be thought of as being similar to compound 8, except that the phenyl rings are directly linked together to form naphthalene. This results in relatively small changes to the immediate selenium bonding environments as compared to 8, with CSeSe bond angles of 91.7° (Se1) and 91.9° (Se2) and a SeSe bond length of 2.35 Å.42 However, there is a significant difference in chemical shift between 8 and 11, with isotropic resonances corresponding to the two crystallographically distinct selenium sites observed at 470.4 and 495.1 ppm in the 77Se CP MAS NMR spectrum shown in Figure 8a. These shifts are both considerably higher than the solution-state chemical shift of 419.8 ppm. The magnitudes of the CSA interactions for these sites are significantly larger than those observed for compounds 710, with spans of 960 and 870 ppm, respectively. A large reduction in CP efficiency was observed for 10864



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Figure 6. 77Se CP MAS (B0 = 9.4 T) NMR spectra of (a) 7, (b) 8, (c) 9 and (d) 10. Isotropic resonances in each spectrum are indicated by *. MAS rates of (ac) 5 kHz and (d) 8 kHz were used. Expansions around the isotropic resonances are shown in the Supporting Information.

Figure 7. Crystal structures of (a) 11,42 (b) 12,42 (c) 13,43 (d) 14,43 (e) 15,44 (f) 16 and (g) 17. Crystallographically distinct selenium sites are indicated.

this sample, for reasons not understood at this time. Use of different CP contact times resulted no obvious improvement in signal, and a 13C CP MAS NMR spectrum recorded on the same sample (see Supporting Information) confirmed that other potential factors (such as the 1H relaxation time and sample crystallinity) are not responsible for the poor spectral quality. We note that a similar observation has been made in a previous study of SeN heterocycles.17

The structure of compound 12, is related to 11 simply by the substitution of one of the selenium sites for a sulfur atom, resulting in a single crystallographic selenium site.42 The 77Se CP MAS NMR spectrum of 12 shown in Figure 8b exhibits a single isotropic resonance at δiso = 586 ppm, which is relatively close to the solution-state chemical shift of 561.8 ppm. The span of 857 ppm is similar to that observed for 11; however the higher isotropic shift indicates that the sulfur has a deshielding effect on 10865



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Figure 8. 77Se CP MAS (B0 = 9.4 T) NMR spectra of (a) 11, (b) 12, (c) 13, (d) 14, (e) 15, (f) 16, and (g) 17. All spectra were recorded at a MAS rate of 5 kHz. Isotropic resonances in each spectrum are indicated by *. Expansions around the isotropic resonances are shown in the Supporting Information.

the adjacent selenium site. Note, there is no similar reduction in CP efficiency for this compound, despite the structural similarity. The molecular structure of compound 13 is similar to that of 11, with the addition of phenyl groups attached to each selenium. This prevents the formation of a direct SeSe bond, resulting in a significantly larger SeSe distance of 3.13 Å probably due to repulsion of the lone pairs on the selenium atoms.43 The 77Se CP MAS NMR spectrum, shown in Figure 8c, reveals that the two crystallographically distinct selenium sites are more shielded than in 11, with isotropic shifts of 405.4 and 434.9 ppm. The measured spans of 570 and 494 ppm show that the addition of the phenyl groups significantly reduces the anisotropy of the shielding interaction for the two selenium sites. Similar conclusions can be made from the 77Se CPMAS NMR spectrum of 14, shown in Figure 8d. This compound is structurally related to 12, with the addition of phenyl groups to the selenium and sulfur sites.43 The observed isotropic shift of 453.1 ppm and span of 496 ppm are reduced as compared to 12. The 77Se CP MAS NMR spectra of bromo- and iodosubstituted naphthalene derivatives 15, 16, and 17 are shown respectively in parts eg of Figure 8. These compounds each have a single crystallographic selenium site and in each crystal structure the orientation of the phenyl group relative to the naphthalene moiety is very similar, as shown in Figure 7.44 The close similarity in local bonding geometry of the selenium atoms in each compound is reflected in the relatively small differences in isotropic chemical shift, with resonances observed at 439.8, 460.2, and 449.6 ppm for 15, 16, and 17, respectively. The spans measured for 15 and 16, of 443 and 457 ppm, are also quite similar, while the span of 523 ppm for 17 is somewhat larger.

Comparison of the 77Se NMR parameters for 15 and 16 indicates that the local selenium environment is not greatly affected by the nature of the heteroatom on the adjacent peri site. Indeed, ab initio calculations for 15 and 16 have shown very small Wiberg bond indices for the two systems, indicating that no significant halogenselenium weak bonding interactions take place.44 However, it is noteworthy that considerable broadening of the isotropic peak and spinning sidebands is observed for compounds 1517. This can be attributed to residual cross terms between the large quadrupolar interaction expected for the bromine and iodine atoms bonded to the adjacent peri-carbon sites, and the dipolar coupling interaction with the selenium atom.60 The solid-state chemical shifts of compounds 1317 are all relatively close to their solution-state counterparts (given in Table 1), suggesting that the steric hindrance resulting from the phenyl group bonded to the selenium atom isolates them to some extent from effects due to intermolecular interactions. Comparison of Calculated and Experimental 77Se NMR Parameters. To gain insight into the structural origins of the observed 77Se chemical shifts and CSAs, first-principles GIPAW calculations were performed on full (geometry optimized) crystal structures for all the compounds studied in this work. The calculated 77Se NMR parameters are summarized in Table 2. A plot of experimental 77Se chemical shifts against calculated chemical shieldings, shown in Figure 9a, reveals a strong correlation. The gradient (1.02) resulting from a least-squares fitting shows that there is no evidence of the need for any significant scaling factor, as has been observed in GIPAW studies of other nuclei e.g., 19F.61,62 DFT calculations for heavier nuclei may require consideration of relativistic effects for accurate prediction 10866



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Table 2. Calculated 77Se NMR Parametersa (isotropic chemical shift, δisocalc, chemical shift tensor components, δiicalc, and span, ΩCScalc) for Compounds 117, Using the Full Crystal Structure cmpd

site

δisocalc

δ11calc

δ22calc

δ33calc

1

Se1

1194.0

1346.8

1226.9

1008.3

338.5

2

Se1

1249.0

1553.6

1397.0

796.3

757.3

3

Se1

783.8

798.4

777.3

775.6

22.8

4

Se1

98.7

50.1

69.7

176.3

126.2

5 6

Se2

156.5

125.0

156.6

188.0

63.0

Se3

191.3

179.7

188.6

205.5

25.8

Se1 Se1

453.5 531.1

100.1 910.6

125.7 474.5

1134.7 208.2

1034.6 702.4

Se2

533.7

929.7

442.6

228.9

700.8

Se3

77.8

233.3

6.5

473.2

706.5

Se4

136.5

388.9

146.7

126.0

514.9

Se1

505.9

726.0

576.1

215.7

510.3

Se2

384.8

683.5

441.7

29.3

654.2

8

Se1

375.0

642.9

311.7

170.2

472.7

9

Se2 Se1

408.1 412.7

709.6 772.1

361.1 323.9

153.7 142.0

555.9 630.1

Se2

473.9

789.0

370.3

262.5

526.5

Se1

368.2

611.2

548.2

54.8

666.0

Se2

516.1

871.5

544.4

132.3

739.2

Se3

394.7

692.7

470.1

21.3

671.4

Se4

540.4

827.1

581.2

212.8

614.3

11

Se1

617.1

886.0

766.9

198.4

687.6

12

Se2 Se1

575.0 636.3

1018.5 917.2

670.9 798.5

35.8 193.1

982.7 724.1

13

Se1

381.1

605.4

377.3

160.7

444.7

Se2

348.0

590.9

410.0

43.0

547.9

14

Se1

389.0

622.5

393.6

149.6

472.9

15

Se1

412.0

625.2

405.8

205.1

420.1

16

Se1

382.0

581.0

362.9

202.2

378.8

17

Se1

400.5

673.2

346.3

181.8

491.4

7

10

a

ΩCScalc

All values are quoted in ppm. 77

of NMR parameters. Theoretical studies of Se NMR parameters in the literature have been carried out both with and without consideration of relativistic effects.12,15,17,36,57,6368 The good correlation between calculated and experimental parameters observed in the current work indicates that the nonrelativisitc GIPAW approach used here appears sufficient for the accurate calculation of 77Se NMR parameters in the systems studied. This is consistent with a previous study by Demko et al.15 which compared ab initio calculations of 77Se NMR parameters performed with nonrelativistic, scalar relativistic, and spinorbit relativistic levels of theory and which found that isotropic shifts were calculated approximately equally well with all three methods. However, in this context, it should be noted that, although experimental and calculated 13C chemical shifts are generally in very good agreement (see Supporting Information), the agreement is less good for carbon directly bonded to selenium and poorer still when a carbon has covalent bonds to two different selenium species. On the basis of the correlation between the experimental chemical shifts and calculated chemical shieldings observed in Figure 9a, it appears appropriate to assume a relationship as

Figure 9. (a) Plot of experimental 77Se chemical shifts, δisoexp, against calculated isotropic chemical shieldings, σisocalc, for compounds 117. The solid line represents a linear fit to the data. (b) Plot of experimental values, δiiexp, against calculated values δiicalc, for chemical shift tensor components δ11, δ22, and δ33 for compounds 117. (c) Plot of experimental spans, ΩCSexp, against calculated spans, ΩCScalc, for compounds 117. Dashed lines represent perfect agreement between calculated and experimental results.

described by eq 1. In this case, a suitable method for the determination of σref is to perform a linear regression of a plot of experimental shifts against calculated shieldings, fixing the gradient of the line of best fit to 1. This procedure gave a rootmean-square deviation between experimental and calculated values of 92.4 ppm, which is satisfactory compared to the ∼1600 ppm chemical shift range of the compounds studied here. From the y-intercept of the line of best fit, a value for the shielding reference, σref, of 1608 ppm, was obtained. A plot of experimental against calculated values for the individual (referenced) principal components of the chemical shift tensors is shown in Figure 9b. In this plot, good agreement between experimental and calculated values is observed for all 10867


The Journal of Physical Chemistry C three components. The data points for each component are spread approximately evenly around the line of perfect agreement (shown as a dashed line), indicating that the calculations do not lead to a systematic under- or overestimation of any particular component, as has been observed in a previous study using cluster-based calculation approaches.15 A plot of experimental against calculated spans is shown in Figure 9c. In general, the points are again evenly scattered around the line of perfect agreement, confirming that there is no overall tendency of the calculations to under- or overestimate the δ11 or δ33 tensor components. The correlation between experimental and calculated values is noticeably less good than for the tensor components themselves and the isotropic shifts; however, this is perhaps to be expected as the errors in the calculated δ11 and δ33 tensor components are combined in the calculation of ΩCScalc. The calculated 77Se NMR parameters enable chemical shift assignments of the compounds containing more than one selenium site. Of the inorganic and organophosphine systems, only 4 and 6 have multiple selenium sites. For compound 4, the calculations confirm that only a relatively small difference in δiso is expected for the two- and three-coordinate sites in the crystal structure. The three-coordinate sites Se2 and Se3 are predicted to be the most shielded by the calculation, while a larger chemical shift difference is predicted for the least shielded two-coordinate Se1 site. We note that calculated 27Al NMR parameters are also in good agreement with experimental data (see the Supporting Information for more details). For 6, the clear difference between calculated chemical shifts for the exocyclic selenium sites allows an assignment of the resonance at 76.5 ppm to Se3 and that at 67.7 ppm to Se4. Furthermore, while the calculated spans of 706 and 514 ppm appear overestimated, the smaller value predicted for the site with the higher chemical shift also supports the assignment. However, for the selenium sites Se1 and Se2 in the endocyclic positions, the calculated shifts of 531 and 533 ppm and spans of 702 and 700 ppm are almost identical, meaning that the assignment of the resonances corresponding to these sites is not unambiguous. Good agreement between experimental and calculated chemical shifts is also obtained for 31P in 6, as described in more detail in the Supporting Information. Calculated chemical shifts for the multisite organoselenium compounds, are slightly overestimated in general, with the main exception of 13, where δiso is underestimated by 53.8 and 57.4 ppm for the Se1 and Se2 sites. Calculated 13C chemical shifts also show good overall agreement with the experimental 13C CP MAS NMR spectra (see Supporting Information for more details). For 7, the difference in δiso of 75.4 ppm between the measured chemical shifts is somewhat overestimated in the calculated difference of 121.1 ppm; however, the two sites can be assigned on the basis of the large difference in δ33 which is wellreproduced by the calculation. For 8, the calculated difference in δiso of 33.2 ppm between Se1 and Se2 is in excellent agreement with the experimental difference of 34.8 ppm. The calculated difference in δiso of 49.6 ppm between the two selenium sites in 9 is also in good agreement with the experimentally observed difference of 61.2 ppm. The four resonances observed for 10 can be assigned on the basis of the relative magnitudes of the calculated δiso for the four selenium sites Se1Se4, shown in the crystal structure in Figure 6. Comparison of calculated and experimental values for the most shielded chemical shift tensor component, δ33, for the four sites gives good agreement, particularly in terms of the large

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difference between the Se1 and Se2 sites and the Se3 and Se4 sites. For 11, the calculated difference in δiso of 42 ppm is a little overestimated compared to the difference between the experimental shifts of 24.6 ppm. However, the qualitative agreement enables a tentative assignment. For 13, the calculated chemical shift difference of 33.1 ppm is in excellent agreement with the experimental chemical shift difference of 29.5 ppm. For 1517, the GIPAW calculations also allow estimations of the magnitudes of splittings that may arise from residual quadrupolardipolar cross terms between the selenium and nearby bromine and iodine nuclei.60 Using calculated 79Br and 127I quadrupolar coupling constants and Br/ISe distances extracted from the optimized structures, residual isotropic cross-term splittings of ∼222, 261, and 213 Hz are predicted for 15, 16, and 17, respectively. These values are broadly consistent with 77 Se CP MAS NMR (anisotropic) line widths of ∼300350 Hz observed for 15 and 17 in Figure 8eg. We note that any disorder or residual 77Se1H dipolar interactions will also contribute to the observed line width.

’ DISCUSSION The experimental results reported in this work show that the approximate 77Se chemical shift range can be estimated on the basis of the local bonding environment of the selenium atom; for example, selenium sites in SeO environments studied here lie within the range 10001400 ppm, while selenium sites in organic environments are found in the range 300500 ppm. The calculated results show that GIPAW calculations provide a reliable method for the prediction of 77Se NMR parameters over a wide chemical shift range. However, within a particular chemical shift range, a direct relationship between local structure and the 77Se chemical shift is not immediately obvious. In particular, for the organoselenium compounds, significant chemical shift differences are observed between chemically equivalent selenium sites in the same compound (up to 150 ppm in 10). This suggests that local effects such as intermolecular interactions and crystal packing can have a significant influence on the 77 Se chemical shift. In contrast to experimental solid-state NMR, which can only be carried out on the full crystal structure, it is possible to investigate the contributions of intermolecular interactions by comparing DFT calculations for full crystal structures and isolated molecules. To this end, calculations were carried out on isolated molecules extracted from the optimized crystal structures for the molecular inorganic and organophospine compounds 1 and 6, and all of the organoselenium compounds. Compounds 3, 4 and 5 were not included in these calculations owing to their nonmolecular structures. However, while the structure of SeO2 (2) is not molecular, it was possible to remove some of the effects of crystal packing by performing a calculation on an isolated chain extracted from the structure. The calculated results are summarized in Table 3. A plot comparing experimental and calculated isotropic shifts, shown in Figure 10a, shows that a reasonable correlation is still observed. In particular, the highest- and lowest chemical shifts are well reproduced by the single molecule calculations, while chemical shifts for the organoselenium compounds are overestimated in general. A plot comparing the chemical shift tensor components from single molecule calculations and experiment is shown in Figure 10b. This plot shows that the δ11 tensor component is overestimated in general, while the δ33 component has a 10868



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Table 3. Calculated 77Se NMR Parametersa (isotropic chemical shift, δisocalc,molecule, chemical shift tensor components, δiicalc,molecule, and span, ΩCScalc,molecule) for Single Molecules Extracted from Optimized Crystal Structures cmpd

site

δisocalc,molecule

δ11calc,molecule

δ22calc,molecule

1

Se1

1241.8

1744.2

1428.1

553.0

1191.1

2 6

Se1 Se1

1368.5 542.6

2020.0 962.8

1645.3 481.5

440.2 183.5

1579.7 779.4

Se2

520.5

958.3

410.7

192.6

765.7

Se3

90.9

226.6

11.2

488.2

714.8

7

ΩCScalc,molecule

Se4

90.3

294.3

138.5

162.0

456.2

Se1

416.0

698.3

520.0

29.8

668.5

Se2

546.4

757.8

732.6

148.9

608.9

8

Se1

396.5

690.3

333.3

165.9

524.4

9

Se2 Se1

400.7 473.9

696.4 821.2

355.2 367.9

150.4 232.4

545.9 588.8

Se2

509.6

821.8

404.4

302.7

519.0

Se1

451.7

802.0

575.6

22.4

824.4

Se2

610.0

1056.1

592.8

181.1

875.0

Se3

422.9

840.1

520.2

91.7

931.8

10

a

δ33calc,molecule

Se4

584.3

996.6

617.5

138.8

857.9

11

Se1

635.4

1337.5

778.3

209.5

1547.0

12

Se2 Se1

616.6 778.3

1268.4 1552.2

806.5 843.9

225.0 61.1

1493.5 1613.3

13

Se1

418.5

641.5

400.9

213.2

428.3

Se2

366.7

630.9

425.8

43.4

587.6

14

Se1

430.1

670.1

416.4

203.7

466.4

15

Se1

426.5

658.4

398.8

222.4

436.0

16

Se1

405.5

612.7

357.3

246.4

366.3

17

Se1

396.3

641.5

400.9

213.2

428.3

All values are quoted in ppm.

tendency to be underestimated. As shown in Figure 10c, this leads to significant overestimations of the span for some compounds, notably compounds 1, 2, 11 and 12, which are overestimated by up to 750 ppm. The overestimation of δ11 and underestimation of δ33 was also observed in ab initio calculations in the previous study by Demko et al.15 which also used a DFT approach (although not a planewave approach) applied to single molecules. This indicates that while calculations based on both the full crystal structure and single molecules show good accuracy for the calculation of isotropic shifts, calculations on the full crystal structure appear necessary for more accurate predictions of the chemical shift anisotropy. Another interesting observation from the single molecule calculations is that large differences in chemical shift are still calculated between chemically equivalent sites in the same molecule for a number of compounds. This indicates that effects due to local geometry in these compounds are at least as important as packing effects in determining the 77Se chemical shift. For 7, the single molecule calculation predicts a 130.4 ppm difference in δiso between the two sites in the diselenide linkage. The main contribution to this difference comes from a large difference in the δ33 component, mirroring both the calculated and experimental results for the full crystal structure. A number of studies have highlighted the importance of relative orientations of directly- and nondirectly bonded phenyl groups in influencing 77Se chemical shifts.12,6668 Indeed, the large difference in δiso between the two selenium sites in 7 was investigated in a theoretical study by Balzer and co-workers.12

It was suggested that the differences in the δ33 tensor component result from a β-effect of the torsional angle for the nondirectly bonded phenyl group. We may compare this with compound 8, where the experimental and calculated (full structure and single molecule) δ33 components for both selenium sites have similar magnitudes. This appears to be consistent with the hypothesis of Balzer and co-workers, as the orientations of the two phenyl groups differ by a smaller torsional angle of 7°, as compared to 26° in 7. However, effects related to the orientations of the phenyl groups do not explain the calculated chemical shifts for compound 10, which in the single molecule calculation are spread across a range of 187.1 ppm. This large shift range is somewhat surprising, given that all selenium sites are chemically equivalent, and the molecule does not contain phenyl groups. Furthermore, the largest differences in chemical shift are predicted for adjacent selenium sites, with Se1 and Se2 differing by 158.3 ppm, and Se3 and Se4 differing by 161.4 ppm. As crystal packing effects are neglected in the single molecule calculations, these differences must arise from the local bonding geometry. Computational and experimental studies of GeSe systems36 and diselenosaccharides11 have indicated that the 77Se chemical shift is very sensitive to changes in local bonding geometry. However, the large calculated chemical shift differences between the selenium sites in 10 show no obvious correlation with CSe or SeSe bond lengths in the optimized structure, all of which fall within a small range of only 0.02 Å for both types of bond. Furthermore, the calculated results do not indicate a correlation between δiso 10869



Figure 10. (a) Plot of experimental 77Se chemical shifts, δisoexp, against calculated isotropic chemical shifts for single molecules, δisocalc,molecule for compounds 1, 2, 617. (b) Plot of experimental values, δiiexp, against calculated values for single molecules δiicalc,molecule, for chemical shift tensor components δ11, δ22, and δ33 for compounds 1, 2, 617. (c) Plot of experimental spans, ΩCSexp, against calculated spans for single molecules, ΩCScalc,molecule, for compounds 1, 2, 617. Dashed lines represent perfect agreement between calculated and experimental results.

and the CSeSe bond angle; for example, the sites Se1 and Se4 have almost identical bond angles of 100.99° and 100.93°, respectively, but have a calculated chemical shift difference of 132.6 ppm in the single molecule calculation. The only structural parameters that are significantly different for the selenium sites in 10 are the CSeSeC and SeSeCSe torsional angles. To gain further insight into the structural origins of the chemical shift differences, calculations were performed to investigate the effect of changing the torsional angles of the diselenide linkage. However, due to the ring structure of compound 10, it is not possible to change one torsional angle without altering other bond angles and bond

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Figure 11. (a) Structure of the molecule used in 77Se NMR GIPAW calculations showing the torsional angles θ and φ that define the local bonding geometry around the central selenium sites Se0 and Se00 . (b) Plot of calculated isotropic shielding, σisocalc,molecule, for Se0 and Se00 as a function of torsional angle, θ, between values of 0° and 180°. (c) Plot of calculated isotropic shieldings for Se0 and Se00 , σisocalc,molecule, as a function of torsional angle, φ, between values of 0° and 180°. (d) Plot of the difference in calculated isotropic shielding between Se0 and Se00 , Δσisocalc,molecule (Se0 Se00 ), as a function of torsional angle, φ, between values of 0° and 180°, with θ fixed at 99.1°, as is the case for the Se1Se2 diselenide linkage in the crystal structure of 10.

lengths in the rest of the ring. For this reason, calculations were instead performed for a model molecule with a structure based on a fragment of 10 terminated at both ends by hydrogen atoms, as shown in Figure 11a. CSe and SeSe bond lengths and bond angles were set to be the same for each type of bond and were based on average values extracted from the crystal structure of 10. SeH bond lengths were set to 1.47 Å. All torsional angles were 10870


The Journal of Physical Chemistry C initially set to 180° so that the molecule was in a linear conformation with symmetry around the Se0 Se00 bond (as shown in Figure 11a). Calculations were then performed for values of the CSe0 Se00 C torsional angle, θ, between 180° and 0° in steps of 20°. A plot of the calculated chemical shielding for Se0 and Se00 , σisocalc, as a function of θ is shown in Figure 11b. The calculations show that the torsional angle θ has a significant effect on the 77Se chemical shielding, changing it by up to 320 ppm. This may partially explain the differences in chemical shift observed for the two crystallographically distinct diselenide linkages in 10. However, for all values of θ, both selenium atoms Se0 and Se00 in Figure 11a have the same calculated chemical shielding due to the symmetry of the molecule. The observed change in σisocalc as a function of θ does not therefore explain the large difference in isotropic shifts between adjacent selenium sites in 10. However, in the optimized crystal structure of 10, the symmetry of each diselenide linkage is broken by the different relative orientations of the directly bonded CH2 groups. It is possible to simulate this in the model structure by varying the Se0 Se00 CSe(H) torsional angle, φ. Calculations were therefore performed for values of φ between 180° and 0°, with θ set equal to 180°. A plot of the calculated chemical shielding for Se0 and Se00 as a function of φ between 0° and 180°, shown in Figure 11c, reveals that chemical shift differences of up to 150 ppm between the two sites are predicted for certain orientations. This indicates that the relative orientations of the CH2 groups surrounding the diselenide linkage (parametrized here by the torsional angles θ and φ) are a significant factor in determining the observed chemical shift differences for adjacent selenium sites in 10. However, none of the configurations of the model molecule in these simple calculations closely reflect the structure of 10. To more accurately model the difference in δiso expected in the real crystal structure, calculations were performed for a structure with the same geometry and bond lengths as the Se1Se2 diselenide linkage in the optimized crystal structure for 10. The torsional angle θ was set equal to 99.1°, as is the case in the real structure. The torsional angle φ around the CSe2 bond was then varied between þ180° and 180°. A plot of the difference in chemical shift between the selenium sites as a function of φ is shown in Figure 11d. The data shows that, at φ = þ73° (as is the case for the Se1Se2CSe3 linkage in the structure of 10), a difference in chemical shift of between 150200 ppm is predicted. This is consistent with both the calculated difference of 161.4 ppm between Se1 and Se2 predicted in the single-molecule calculation for 10 and also with the difference in experimental chemical shift of 128.8 ppm.

’ CONCLUSIONS In summary, this work shows that first-principles GIPAW calculations provide a reliable method for the prediction of 77Se NMR parameters in a range of organic and inorganic systems. The accuracy of the calculated parameters is sufficient to allow chemical shift assignment of systems containing more than one crystallographically distinct selenium site. The good agreement observed between experimental and calculated chemical shifts shows that consideration of relativistic effects is not very important for the systems studied in this work. Comparison of calculations performed for the full crystal structures and for isolated molecules shows that the accuracy of the calculated chemical shift is only slightly worse when intermolecular

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interactions and packing effects are not considered. However, this is found to result from the cancellation of larger deviations in the δ11 and δ33 tensor components, which are found to be significant for some compounds. This has the result that the agreement calculation and experiment for the span (δ11δ33) is considerably poorer when only a single molecule is considered. The single-molecule calculations also show that the 77Se chemical shift not only is affected by the local chemical environment but also provides a highly sensitive probe of local bonding geometry. In particular, for compound 10, a novel Se-containing ring compound, we have shown that large differences in chemical shift between adjacent chemically equivalent selenium sites arise mainly due to differences in torsional angles that define the geometry of the diselenide linkage.

’ ASSOCIATED CONTENT

bS

Supporting Information. Further experimental details and centerband expansions for 77Se solid-state NMR experiments, powder X-ray diffraction data for SeO2 (2), 13C CP MAS NMR spectra for compounds 617, 31P MAS NMR spectrum for 6, 27 Al NMR spectra of Al2Se3 (4). This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Telephone: þ44 (0)1334 463779. Fax: þ44 (0)1334 463808. E-mail: [email protected].

’ ACKNOWLEDGMENT We thank Dr. Mikhail Kibalchenko and Dr. Michael B€uhl for useful discussions. We are grateful to EPSRC for support (Grant No. EP/E041825/1) and to the research councils for an RCUK Academic Fellowship to S.E.A. This research has also made use of the resources provided by the EaStCHEM Research Computing Facility (http://www.eastchem.ac.uk/rcf); this facility is partially supported by the eDIKT initiative. ’ REFERENCES (1) Hua, G.; Woollins, J. D. Angew. Chem., Int. Ed. 2009, 48, 1368. (2) Ip, C.; Hayes, C.; Budnik, R. M.; Ganther, H. E. Cancer Res. 1991, 51, 595. (3) Quemard, C.; Smektala, F.; Couderc, V.; Barthelemy, A.; Lucas, J. J. Phys. Chem. Solids 2001, 62, 1435. (4) Duddeck, H. Prog. Nucl. Magn. Reson. Spectrosc. 1995, 27, 1. (5) Duddeck, H. Annu. Rep. NMR Spectrosc. 2004, 52, 105. (6) Demko, B. A.; Wasylishen, R. E. Prog. Nucl. Magn. Reson. Spectrosc. 2009, 54, 208. (7) Dawson, W. H.; Odom, J. D. J. Am. Chem. Soc. 1977, 99, 8352. (8) Odom, J. D.; Dawson, W. H.; Ellis, P. D. J. Am. Chem. Soc. 1978, 101, 5815. (9) Collins, M. J.; Ratcliffe, C. I.; Ripmeester, J. A. J. Magn. Reson. 1986, 68, 172. (10) Bryce, D. L.; Bernard, G. M.; Gee, M.; Lumsen, M. D.; Eichele, K.; Wasylishen, R. Can. J. Anal. Sci. Spectrosc. 2001, 46, 46. (11) Potrzebowski, M. J.; Michalska, M.; Bzaszczyk, J.; Wieczorek, M. W.; Ciesielski, W.; Ka_zmierski, S.; Pluskowski, J. J. Org. Chem. 1995, 60, 3139. (12) Balzer, G.; Duddeck, H.; Fleischer, U.; R€ohr, F. Fresenius’ J. Anal. Chem. 1997, 357, 473. 10871


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77Se Solid-State NMR of Inorganic and Organoselenium Systems: A

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