PAPER
www.rsc.org/pccp | Physical Chemistry Chemical Physics
Cyanide orientational ordering and copper electric field gradients in CuCNN2H4 Pedro M. Aguiarw and Scott Kroeker* Received 15th May 2008, Accepted 23rd October 2008 First published as an Advance Article on the web 10th December 2008 DOI: 10.1039/b808266a The measurement of residual dipolar coupling in 13C and 15N MAS spectra of CuCNN2H4 allows for the extraction of 63/65Cu quadrupole coupling constants (CQ(63Cu) = 26 4 MHz). Ab initio calculations were employed to determine electric field gradient and chemical shielding tensor orientations, which proved essential for the reliable analyses of residual dipolar coupled 13 C and 15N MAS spectra. The 1J(63Cu,13C) and 1J(63Cu,15N) couplings (590 and 120 Hz, respectively) indicate that the cyanide ligands are static and their magnitudes reflect the deviation of the C–Cu–N angle when compared with other copper cyanides. The observed 13C and 15N spectra are most compatible with a structural model wherein the cyanides are fully orientationally ordered: [–Cu–C–N–Cu–C–N–].
Introduction Research into metal–organic coordination polymers has grown rapidly in recent years, with an emphasis on selfassembled synthesis and functional materials.1 Metal–organic polymers allow for the tuning of properties via modification of parameters such as the coordination number, oxidation state, ligand size, and ligand flexibility.2 The ability of cyanide to bridge metal centres has been widely used in the construction of such frameworks and related materials.3–5 Structural characterization of these materials using X-ray diffraction methods encounters difficulties distinguishing between carbon and nitrogen due to their similar electron densities. The application of neutron diffraction to some metal cyanides has allowed for the distinction of C and N sites,6,7 but the large sample size requirements and the limited availability of neutron sources restrict the widespread use of this technique. Attempts to distinguish carbon and nitrogen sites based on bond lengths have been reported, but are complicated by a significant overlap between Cu–C (1.88–2.13 A˚) and Cu–N (1.92–2.20 A˚) bond lengths.8–11 The correct identification of the sites is further complicated by the possibility of head-to-tail disorder of the cyanide ligands. For example, copper(I) cyanide consists of linear (CuCN)n chains, with orientationally disordered bridging cyanide ligands.6,10,12 Copper-63/65 nuclear quadrupole resonance (NQR) suggests preferential ordering, with the CN–Cu–CN fragment being the dominant coordination ‘‘isomer’’, however, appreciable amounts of CN–Cu–NC and NC–Cu–CN are also present (see Fig. 1).10 Adducts of CuCN with nitrogenous bases form readily, and typically consist of a CuCN backbone with the nitrogenous ligands coordinated to the copper centre, giving rise to 3- and Department of Chemistry, University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2. E-mail:
[email protected]; Fax: 204-474-7608; Tel: 204-474-9335 w Current address: Commissariat a` l’E´nergie Atomique, IRAMIS, Laboratoire de Structure et Dynamique par Re´sonance Magne´tique, 91191 Gif-sur-Yvette, France.
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4-coordinated copper, and distorting the CuCN chain from linearity.3,5,8,13,14 Cyanide orientations in most of the solved structures are not definitively assigned due to the aforementioned ambiguities in distinguishing carbon from nitrogen in diffraction experiments.5,13,15 One such adduct of copper(I) cyanide with hydrazine (CuCNN2H4) was discovered decades ago while searching for a suitable re-crystallization solvent for the highly insoluble CuCN.9 The copper(I) cyanide–hydrazine adduct consists of CuCN chains possessing four-coordinate copper centres, highly distorted from tetrahedral symmetry (see Fig. 2). Each copper is bonded to two cyanide groups and two hydrazine ligands, with the hydrazine molecules linking 1D chains to form a 2D sheet. No definitive conclusions about the cyanide head-to-tail orientation could be drawn from the original X-ray diffraction work, although the final proposed structure was based on a model possessing ordered cyanides (Fig. 1a). In principle, the sensitivity of NMR to local environments would be ideal to differentiate amongst these possibilities. Copper-63/65 NMR, for example, is intuitively attractive to study nearest neighbours; however, the large quadrupole moments of 63Cu (Q = 22.0 fm2, N.A. = 69.2% and I = 3/2) and 65Cu (Q = 22.4 fm2, N.A. = 30.2% and I = 3/2)16 prohibit their direct observation except in complexes where the copper centre is of very high symmetry such as for K3Cu(CN)4.17 Nevertheless, information about the ordering may also be available from NMR of the more
Fig. 1 Possible scenarios describing the head–tail disorder of the bridging cyanide ligands; (a) is completely ordered and possesses a single copper site, (b) is also ordered, but possesses two copper sites, and (c) is disordered and possesses three copper sites.
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latter method in combination with ab initio calculations to obtain information about the magnitude and orientation of the EFG tensor. Theoretical treatments of the residual dipolar interaction between a quadrupolar nucleus and a spin-1/2 nucleus have been thoroughly investigated.18 The effect relies on the fact that the heteronuclear dipolar interaction between a spin-1/2 nucleus and a quadrupolar nucleus is not averaged out by MAS but distorts the spacing of the peaks from the unperturbed multiplet. The observed frequency displacements of the transitions, m, to second order are: 3CQ SðS þ 1Þ 3m2 2 DnðmÞ ¼ 20n s Sð2S 1Þ DJ J J fða ; b ; ZÞ ; DfðaD ; bD ; ZÞ ð2Þ 3 Fig. 2 View down the crystallographic a-axis showing the CuCN chains and the interchain linking of the hydrazine molecules. Relevant bond lengths and angles are also noted. H atoms omitted for clarity
amenable 13C and 15N nuclides through their characteristic chemical shifts and their interactions with the quadrupolar copper-63/65 nuclei. In this paper, we apply 13C and 15N MAS NMR to CuCNN2H4 to distinguish amongst the proposed X-ray structures9 and characterize the structural effects on the observed NMR parameters. The orientation of the copper63/65 electric field gradient tensor is obtained with the aid of ab initio calculations to show that the only structure consistent with the 13C and 15N NMR spectra is that of a fully ordered cyanide chain possessing a single copper centre.
Theoretical background Any deviation from cubic symmetry about the copper centre results in a non-zero electric field gradient (EFG) which couples to the nuclear quadrupole moment, giving rise to the nuclear quadrupole coupling constant, CQ: CQ ¼
eQVzz ; h
ð1Þ
where e is the elementary charge, Q is the nuclear quadrupole moment, VZZ is the largest-valued principal component of the diagonalized EFG tensor, and h is Planck’s constant. When the quadrupolar splitting energies are on the order of the Zeeman energy, it is not necessarily appropriate to treat the quadrupole interaction simply as a second-order perturbation of the Zeeman interaction as is commonly done for halfinteger quadrupoles. There are several methods of extracting quadrupolar coupling information in such cases. One approach is to use a higher static field, however, this method is inconvenient at best, and in many circumstances the highest available fields may still be too low. NQR is another method to obtain information about the quadrupolar interaction, although this too can be difficult due to challenges in locating the signals, and to very large linewidths, in some cases requiring a piecewise acquisition of spectra and limiting the resolution of sites. A third method involves the indirect observation of the quadrupolar interaction through residual dipolar coupling to a spin-1/2 nucleus. We have applied the This journal is
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where ns is the Larmor frequency of nucleus S, D is the dipolar coupling tensor, a and b are the Euler angles relating the EFG tensor to D, DJ is the anisotropy in the J tensor, Z is the asymmetry parameter of the EFG or J tensor, and f(ax,bx,Z) = 3cos2 bx 1 + Z sin2bx cos2ax, where x = J or D. In the case where J and D are co-linear, the observed dipolar coupling can be described by an effective dipolar coupling constant, Deff, such that, Deff ¼ D
DJ 3
thus eqn (2) may be reduced to: 3Deff CQ SðS þ 1Þ 3m2 2 DnðmÞ ¼ fðaD ; bD ; ZÞ 20n s Sð2S 1Þ
ð3Þ
ð4Þ
The dependence of the observed NMR frequencies on the relative tensor orientations necessitates a good understanding of the tensor orientations for the system in question in order to properly simulate the observed spectra. The possibility of cyanide orientational disorder and the limited symmetry constraints at the copper allow for multiple possible solutions for the angles a and b. If known, however, this approach can be used to obtain accurate estimates of CQ.
Conventions The principal components of the shielding tensor are assigned in the standard way: s11 r s22 r s33 and d11 Z d22 Z d33. The calculated principal components of the shielding tensor were converted to chemical shifts using the relation, dii ¼
½sref sii 106 ffi ½sref sii 106 ½1 sref
ð5Þ
where sref is the absolute shielding of tetramethylsilane, TMS or NH3(l) (188.1 and 264.54 ppm, respectively).19 These were then converted from the standard convention to that used by Herzfeld and Berger.20 The isotropic chemical shift is defined as: diso = (d11 + d22 + d33)/3
(6)
the span of the shielding is: O = d11 d33
(7)
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Spectral simulations
Fig. 3 Schematic diagram of the Euler angles a and b describing the relative position of the dipolar vector and the electric field gradient tensor.
Spinning sidebands were summed to obtain an isotropic MAS spectrum, which was simulated using the full-diagonalization variant of the residual dipolar coupling module included within the WSolids package.22 The dipolar coupling constants (D) were calculated from crystallographically determined Cu–C (1.93 A˚), and Cu–N (1.95 A˚) bond lengths. J-couplings were assumed to be isotropic and values for 1JCu,C and 1JCu,N were fit from the central J-coupled transitions. All simulations were performed assuming an axially symmetric EFG tensor, Z = 0, and accounting for coupling to both 63Cu and 65Cu isotopes. The orientation of the dipolar vector with respect to the EFG tensor was obtained using ab initio calculations (vide infra).
and the skew is:
Ab initio calculations k = 3(d22 diso)/O
(8)
The EFG tensor is symmetric and traceless, and thus may be completely described by CQ and the associated asymmetry parameter, Z Z = (VXX VYY)/VZZ
(9)
where the principal components of the EFG tensor are defined such that |VZZ| Z |VYY| Z |VXX|. The relation between the orientation of the dipolar vector with respect to the EFG tensor may be defined by two angles a and b as shown in Fig. 3.
Experimental Synthesis Isotopically enriched (37 mol%) Cu13C15NN2H4 was synthesized via precipitation from an aqueous solution of Cu13C15N in the presence of sodium thiosulfate (NaS2O3) (to solubilize the copper cyanide) upon addition of 85% N2H4(aq.).5,9 The resulting off-white precipitate was filtered and washed with de-ionized water. The product and percent enrichment were confirmed by IR spectroscopy. NMR spectroscopy Spectra were acquired on a Bruker AMX-500 (11.7 T) equipped with a 5 mm double-resonance MAS probe (Doty Scientific) and a Varian Unity-Inova 600 (14.1 T) with a 5 mm double-resonance MAS probe (Varian-Chemagnetics). 13 C{1H} and 15N{1H} CP/MAS NMR spectra were acquired with recycle delays of 6–10 s, contact times of 3–7 ms and 256–1100 transients for MAS and 3000–10 000 for nonspinning spectra. 13C chemical shifts are referenced with respect to TMS, using adamantane as a secondary reference, which has shifts of +29.5 and +38.6 ppm.21 15N chemical shifts are referenced with respect to NH3(l) at 20 1C using the secondary reference 15NH15 4 NO3, the ammonium nitrogen of which appears at +23.8 ppm. MAS spectra were obtained at spinning speeds of 4.5 to 8 kHz, and isotropic chemical shifts were determined by comparison of spectra acquired at different spinning speeds. 836 | Phys. Chem. Chem. Phys., 2009, 11, 834–840
All calculations were performed using Gaussian 98 rev. A-1123 on a Sun Fire 6800 running Solaris 8. Heavy atom positions were taken from the X-ray data using ordered CuCN chains corresponding to both cyanide orientations presented in ref. 9 (one-copper-site models, Fig. 1a, with either the nitrogen or carbon slightly closer to the copper) and a two-copper model (Fig. 1b). Hydrogens were added to the hydrazine ligands and allowed to minimize while constraining the positions of the heavy atoms at B3LYP/6-31+G*. Lacking single-crystal diffraction best-fit atomic coordinates for the two-copper model, it was generated by flipping every second cyanide group. While this simplistic approach may exaggerate the difference between inequivalent carbons due to differing Cu–C bond lengths (193 vs. 195 pm), it should be noted that the positions of the hydrazine nitrogens preclude the existence of a symmetry element that would render identical the carbons adjacent to the coppers. Chemical shielding and EFG tensors were calculated with a series of DFT methods using the 6-311++G** basis set.
Results and discussion The 13C MAS NMR spectrum is a distorted quartet arising from the large one-bond J-coupling to the spin-3/2 copper nuclei (Fig. 4). The spectrum can be simulated as a single site
Fig. 4 13C CP-MAS isotropic spectrum (obtained by summation of all spinning sidebands) and corresponding simulation at 11.7 T.
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Table 1 13C and 15N NMR data from MAS and non-spinning experiments. Errors are given in parentheses. 13
15
155 (1) 370 (30) 0.87 (0.05) 590 (15) 1117
236 (2) 450d (50) 0.82 (0.06) 120 (12) 436
C
disoa/ppm b
O /ppm kb 1 63 J( Cu,X)a/Hz Dc/Hz
N
a Determined from MAS spectrum. b Determined from non-spinning spectrum. c Calculated dipolar coupling constant from X-ray diffraction. d The position of the d33 component of the shielding tensor was obscured by the overlapping hydrazine signal.
with a chemical shift of 155 1 ppm (Table 1). This is consistent with either single-copper-site structural model (Fig. 1a), for which negligible differences in the isotropic chemical shifts, quadrupole coupling constants and tensor orientations were calculated. By contrast, the ab initio calculations for the twocopper-site ordered model (Fig. 1b) predict 13C chemical shifts differing by 6.5 ppm, which is incompatible with the experimental data. Even if the present calculational model is imperfect, previous ab initio calculations indicate that changes in the nextnearest-neighbour environment due to head–tail cyanide disorder will produce shifts ranging over about 3 ppm.10 Although the linewidths are relatively broad (ca. 500 Hz) due to unresolved spin–spin couplings such as 1J(13C,15N), 2J(13C,63/65Cu), 2 13 15 J( C, N) and 3J(13C,13C), a shift of this magnitude would be detected if present. Fig. 5 illustrates the patterns expected for various chemical shift differences. While the ordered models are essentially indistinguishable if the chemically distinct carbons have the same shift, even a very small shift difference of 0.5 or 1 ppm visibly alters the lineshape. More realistic differences of 3–6 ppm are clearly incompatible with the experimental spectrum. Furthermore, a disordered chain would necessarily possess different relative orientations of the dipolar and EFG tensors, and would result in very different residual dipolar coupling patterns. Hence, these data indicate a fully ordered 1D copper cyanide chain, in contrast to what is observed for CuCN.10 The presence of 1J(13C,63/65Cu) implies that any bond-breaking motion, such as flipping of the cyanide ligands, is on a time scale much slower than T1(63/65Cu)/J and thus dynamic disorder of the cyanides is unlikely. The significant distortion of the multiplet indicates that the copper nucleus possesses a large quadrupole coupling constant which influences the 13C peak positions via residual dipolar coupling. This distortion can be reproduced theoretically under the assumption of collinear EFG and dipolar tensors to yield an estimate of the CQ (26 4 MHz for 63Cu). This value and the assumption of coincident tensors are supported by the B3LYP calculations (Table 2). The asymmetry parameters, Z, from the various levels of theory were found to range from 0.04 to 0.22 (Table 2). Simulations incorporating the effect of a non-zero Z up to 0.3 were found to impart negligible effects on the simulated lineshapes, particularly after the application of experimentally reasonable line-broadening. Accordingly, Z was fixed at 01 for simplicity, despite not being required by the local site symmetry. The 15N MAS NMR spectrum is an overlapping quartet of roughly equal-intensity peaks located at 236 ppm, similar to other metal cyanide systems (Fig. 6).10,17,24 The overlap makes This journal is
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Fig. 5 13C MAS simulations at 11.7 T of (a) single copper model, and (b–e) two-copper model using calculated EFG tensor values and orientations with chemical shift differences of (b) 0 ppm, (c) 0.5 ppm, (d) 1.0 ppm and (e) the DFT calculated difference of 6.5 ppm. All other couplings (dipolar and J-coupling) were assumed identical.
it difficult to assess whether the multiplet arises from one or more sites. However, calculations of next-nearest neighbour effects on 15N chemical shifts suggest a 3–4 ppm difference in diso,10 which would be detectable even with the 2–3 ppm peakwidths observed here. This conclusion is consistent with evidence from 13C MAS NMR supporting a fully ordered CuCN chain. The magnitude of the J-coupling giving rise to the quartet is about 120 Hz, which is smaller than that found for CuCN (250 Hz),10 but much larger than the two-bond J-coupling of K3Cu(CN)4 (20 Hz)17 confirming chain connectivity and the lack of dynamic disorder. Spectral simulations reveal the sign of this coupling to be negative, in agreement with related systems.10 Surprisingly, the distortion from its non-quadrupole perturbed multiplet positions is very small, implying a negligible CQ, in contradiction to that measured by 13C NMR and that obtained from ab initio calculations. However, the kink in the copper chain implies that the copper EFG tensor is not likely to be collinear with the Cu–N dipolar tensor, necessitating a careful consideration of the relative orientations. Using the tensor orientation obtained from the DFT calculations (b = 511) results in a CQ which agrees well with the 13C spectral simulation. It should be noted however, that this angle is very close to the ‘‘magic’’ angle which effectively nullifies the spatial term Phys. Chem. Chem. Phys., 2009, 11, 834–840 | 837
Table 2 Ab initio chemical shift and quadrupole parameters for a single-copper-site ordered copper cyanide chain. The values in best agreement with experimental data are shown in bold 13
15
C
HF B3LYP BHandH BHandHLYP Experimental
63
N
Cu
diso/ppm
O/ppm
k
diso/ppm
O/ppm
k
CQ/MHz
Z
bCu–N/1
168.2 165.4 162.7 161.6 155
391.1 373.0 374.3 375.3 370
0.98 0.93 0.98 0.98 0.87
293.3 245.0 278.2 277.7 236
551.1 465.5 521.0 521.0 450
0.99 0.82 0.98 0.98 0.82
47.9 27.8 35.5 36.6 26
0.16 0.22 0.02 0.04 0a
49.7 51.7 49.9 51.6 (51)b
a The value of the asymmetry parameter was fixed at 01 during simulation (see text). average of the ab initio results.
(f(ax,bx,Z) = 3cos2 bx 1 + Z sin2bx cos2ax) and minimizes the influence of CQ on the spectral appearance. In such a case, the CQ obtained independently from spectral simulation possesses a large uncertainty. The 13C and 15N MAS spectra at 14.1 T were acquired nearly eight months after the initial experiments at 11.7 T, and sample degradation was noted; the sample had slightly discoloured, and a faint fishy odour was detectable. The initial X-ray diffraction studies also detected degradation, which was associated with the loss of hydrazine,9 consistent with the odour sensed for our sample. The 13C (not shown) and 15N (Fig. 6) MAS spectra at 14.1 T clearly reflect the presence of an impurity. The significant differences in the shieldings for the 15N spectrum allow for confirmation of the parameters obtained at 11.7 T, however, the 13C spectrum suffers from severe overlap of the signals.
Fig. 6 15N{1H} CPMAS spectra and simulations, at 11.7 and 14.1 T. Extra peaks at 14.1 T are from an impurity not present in the sample run at 11.7 T (see text).
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b
Value of b used in experimental simulations taken from the
The copper-63 quadrupole coupling in copper cyanide systems is highly dependent upon the molecular geometry about the copper. The CQ for the two-coordinate, linear CuCN is ca. 82 MHz,10 whereas K3Cu(CN)4 possesses a 63 Cu CQ of 1.125 MHz for its nearly tetrahedral copper site.17 The three-coordinate KCu(CN)2 has been demonstrated to have a CQ of 60.9 MHz.25 The hydrazine adduct of CuCN investigated here contains a copper coordination environment in which the linear CuCN chain has been significantly influenced by interactions with the hydrazine molecules, generating a zig-zag chain and forming a highly distorted ‘‘tetrahedral’’ geometry at copper. The measured CQ of 26 MHz is qualitatively consistent with this geometry, as it is intermediate between the linear and tetrahedral values. The deviation of the C–Cu–N bond angle from linearity results in a decrease in the observed 1J(63Cu,X) coupling: 1J(63Cu,13C) = 590 15 Hz and 1J(63Cu,15N) = 120 12 Hz are significantly smaller in magnitude than found in the linear CuCN (1J(63Cu,13C) = 725 Hz, 1J(63Cu,15N) = 250).10 The threecoordinate KCu(CN)2 was found to exhibit a 1J(63Cu,13C) coupling of B500 Hz.26 The 1J(63Cu,13C) in the hydrazine complex is also larger than that observed for the nearly tetrahedral K3Cu(CN)4, which possesses a 1J(63Cu,13C) of 300 Hz17 and that of the nearly tetrahedral copper site in [N(CH3)4][CuZn(CN)4] which was reported as 324 Hz.27 The observation of larger 1J couplings for linear arrangements in these copper cyanides is consistent with observations in other metal cyanide systems. The 1 199 J( Hg,13C) for linear HgCN is 3158,28 whereas for the tetrahedral K2Hg(CN)4 the coupling is only 1540 Hz.29 In fact, work on a series of mercury cyanides indicates that cyanide coordination may be more important than the overall metal coordination environment with respect to its impact on the J-couplings.30 The 13C NMR spectrum of a stationary sample was also recorded to learn about the impact of the geometrical distortion from linearity on the chemical shift anisotropy in CuCNN2H4 (Fig. 7). The powder pattern appears substantially like other typical cyanides, with near-axial symmetry despite the zig-zag nature of the CuCN chain, indicating that the 13C is most sensitive to nearest neighbours and little affected by the kink at Cu. Evidence of coupling to 63/65Cu is observed in the high frequency portion of the pattern and could be simulated using the 1J(13C,63/65Cu) measured in the MAS spectrum and the direct dipolar couplings calculated from the crystal structure. The quadrupole interaction was not included, however coupling to both spin-active copper nuclei was included. Comparison of the simulation with the This journal is
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Fig. 7 13C non-spinning NMR spectrum at 11.7 T along with simulation. The simulation includes 63/65Cu-13C direct and indirect dipolar couplings (D and J, respectively).
experimental spectrum (Fig. 7), shows good agreement, despite neglecting coupling to the neighbouring 15N nucleus. These couplings would be expected to result in additional splitting of each of the four subspectra. While not resolved, this additional splitting—D(13C,15N) at the high frequency end and 2*D(13C,15N) at the low-frequency edge—likely contributes to the loss of definition. The 13C shielding anisotropy obtained from simulation of the non-spinning spectrum, has a span (O) of 370 30 ppm and a skew (k) of 0.87 0.05 (Table 1). The slight deviation of the skew from axial symmetry is consistent with the Cu–C–N bond angle which deviates from linearity (y = 175.21), demonstrating the sensitivity of the anisotropic shielding to the local environment. The 15N chemical shielding was determined from a nonspinning spectrum acquired at 11.7 T (not shown). The tensor is nearly axially symmetric (k = 0.8 0.1) and possesses a span (O = 450 50 ppm) typical of metal cyanides based on simulations analogous to the 13C case. However, determination of the shielding anisotropy was hindered by overlap with the signals arising from the hydrazine ligand with the mostshielded (d33) region of the spectrum. Nevertheless, k does deviate from axial symmetry, despite a Cu–N–C angle of 179.31. This is likely due to an interaction of the cyanide nitrogen with the hydrogen atoms of the hydrazine ligand only 2.3–2.5 A˚ away.
Ab initio calculations of one single-copper-site ordered structure (orientation I in ref. 9) were carried out at X/6-311++G**, where X = HF, B3LYP,31 BHandH, or BHandHLYP. BHandH uses the LSDA32 exchange functional with the LYP33 correlation functional, whereas BHandHLYP uses the LSDA and the Becke8834 exchange functionals. Calculations at all levels provided the same results for the tensor orientations: the largest component of V is along the Cu–C bond, with deviations of no more than 41 (Fig. 8). The largest component of V was found to be at an angle of 51 31 to the Cu–N bond, providing strong evidence of the reliability of the calculated tensor orientations. The 13C and 15N isotropic chemical shifts are overestimated in all calculations, however the spans and skews calculated using B3LYP are in reasonably good agreement with the experimental data (Table 2). The calculated 63Cu CQ varied over 20 MHz, with the B3LYP value once again agreeing closely with the measured value.
Conclusions This work is a detailed investigation of the relation between anisotropic NMR parameters and subtle structural features in a hydrazine adduct of copper cyanide. The NMR data verify that the bridging cyanides are orientationally ordered within the CuCN chain as proposed in the original X-ray diffraction study, thus ruling out an alternate suggestion involving orientational disorder. However, the very subtle structural difference entailed by reversing the directionality of the bridging cyanides is undetectable by NMR, and precludes any further refinement of the X-ray structure. The zig-zag nature of the chain induced by interaction with the hydrazine molecule provides a valuable test of the sensitivity of various NMR observables to such distortions; J-couplings and quadrupole coupling constants are found to be very sensitive to these geometric modifications. The indirect measurement of CQ using residual dipolar coupling to spin-1/2 nuclei can be very effective, but requires intimate knowledge of the relative tensor orientations. The present case serves as a caution that without symmetry-constrained or otherwise known tensor orientations, the results can be misleading. Fortunately, DFT calculations at several levels provide reliable tensor orientations which can be used to simulate the experimental spectra. The use of multinuclear magnetic resonance in conjunction with theoretical calculations continues to prove a potent combination for structural studies of solids.
Acknowledgements This work is supported by the Natural Sciences and Engineering Research Council of Canada. Infrastructure support from the Canada Foundation for Innovation and the Manitoba Research Innovation Fund is gratefully acknowledged.
References Fig. 8 Calculated EFG tensor components in CuCNN2H4, looking down the crystallographic c axis. Components are designated Vii (i = x,y,z). VYY is coming out of the plane of the page.
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