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3.10 o. Nuclear magnetic resonance (E. Klaus, J. Kümmerlen, T. Schaller and A. Sebald)

Installation of the 3-channel 100 MHz NMR spectrometer has been completed and the newly-acquired 200 MHz NMR spectrometer is also fully installed and in operation. Some methodological mile-stones will be sketched briefly:

i) cross polarisation ³¹P -> ¹¹³Cd
The feasibility of this experiment on the 100 MHz instrument had to be tested, including optimisation of the hardware. The most suitable compound for these test experiments is CdSiP₂; the ³¹P -> ¹¹³Cd CP/MAS spectrum of CdSiP₂ is shown in Fig. 3.10-12 (samples of CdSiP₂ were kindly donated by H. Eckert, Münster and M. Hönle, Stuttgart).
 

Fig. 3.10-12: 31P -> 113Cd CP/MAS spectrum of CdSiP2

ii) ²⁹Si MAS, CP/MAS with simultaneous ¹H and ³¹P decoupling
The presence of more than one ³¹P nucleus directly bonded to Si, renders the corresponding ²⁹Si MAS, CP/MAS spectra hopelessly complicated (in most cases) to the point where it is not even possible to determine the number of ²⁹Si resonances present in the spectra. For such cases it is necessary to be able to fully decouple ³¹P (and ¹H) from ²⁹Si. This triple-resonance experiment has been successfully implemented and an example is shown in Fig. 3.10-13: the organometallic compound ^tBuSi-(Pchex)₃-Si^tBu has a molecular backbone of two crystallographically distinct Si-atoms, connected by three P atoms as bridging elements. Only the ²⁹Si CP/MAS spectrum obtained under simultaneous ³¹P and ¹H decoupling correctly reveals the two crystallographic Si sites (this compound was kindly donated by M. Drieß, Heidelberg).
 

Fig. 3.10-13: 29Si CP/MAS spectra of tBuSi-(Pchex)3-SitBu: left: 59.6 MHz 29Si CP/MAS, no 31P decoupling. middle: 19.8 MHz 29Si CP/MAS, no 31P decoupling. right: 19.8 MHz 29Si CP/MAS with 31P decoupling.

 
iii) The very first INEPT experiment on crystalline solids
Polarisation transfer via J-coupling is of fundamental importance in solution-state NMR, while for solids the usual mechanism of polarisation transfer occurs via dipolar interactions. J-coupling is closely related to chemical bonding and is, thus, complementary to dipolar interactions depending on geometric distances. Therefore, it is highly desirable to be able to exploit polarisation transfer paths via J-coupling also for solids. We have applied this J-coupling transfer experiment (INEPT, originally designed for isotropic liquids) to obtain the ¹⁹⁵Pt MAS spectrum of solid (NMe₄)₂[PtF₆] via ¹⁹F-¹⁹⁵Pt heteronuclear J-transfer (see Fig. 3.10-14). The possibility to exploit J-coupling polarisation transfer paths for solids under MAS conditions will be of general importance for non-¹³C-¹H applications of high-resolution solid-state NMR techniques (PtF₆^2- salts were kindly donated by P. Preetz, Kiel).
 

Fig. 3.10-14: 19F -> 195Pt INEPT/MAS spectrum of (NMe4)2[PtF6] (bottom) compared with the 19F-coupled 195Pt MAS spectrum (top) Note the septet splitting from J-coupling 1J(195Pt19F) with 6 equivalent 19F nuclei; the central transition is, principally, missing in the INEPT spectra.

iv) ¹⁷¹Yb CP/MAS
Yb in its more common formal oxidation state Yb(III) is paramagnetic. More recently, Yb(II) chemistry has attracted attention owing to the high reactivity of Yb(II) compounds. Yb(II) compounds are diamagnetic, though highly air-sensitive. The isotope ¹⁷¹Yb has NMR properties quite similar to the isotope ¹⁹⁹Hg, and we could show for the first time that ¹⁷¹Yb CP/MAS spectra of crystalline solids can be obtained by approaching an experimental ¹⁷¹Yb CP/MAS set-up from a pre-set ¹⁹⁹Hg CP/MAS set-up. The first ¹⁷¹Yb CP/MAS spectra of two Yb(II) complexes are shown in Fig. 3.10-15 (samples were kindly donated by G. Rabe, München).
 

Fig. 3.10-15: 171Yb CP/MAS spectra of (bottom) cp*2Yb(thf) · 0.5 toluene and of (top) cp*2Yb(thf)2 (cp* = C5Me5, thf = tetrahydrofuran)

v) The X-part in solid-state ABX spin systems under MAS
A solid-state spin system consisting of a homonuclear spin pair plus a heteronucleus represents an often encountered fragment in inorganic and organometallic chemistry. The fragment (³¹P)₂M in transition metal phosphine complexes constitutes such an ABX spin system where the X-part displays the complex (B_o- and ν_rot-dependent) properties of the strongly coupled AB-part. The spectral properties of the X-part of solid-state ABX spin systems under MAS have been theoretically predicted, now we have observed these properties experimentally for the first time in the ¹⁹⁵Pt CP/MAS spectra of a Pt(II) phosphine complex, depePt(S-C≡ C-^tBu)₂. The centre band region of the respective ¹⁹⁵Pt CP/MAS spectra is shown in Fig. 3.10-16 (samples were kindly donated by W. Weigand, München).
 

Fig. 3.10-16: 21.4 MHz 195Pt CP/MAS spectra (only centre band region is shown) of depePt(S-C≡ C-tBu)2 (depe = Et2P-CH2-CH2-PEt2) at different MAS frequencies; also shown calculated resonance positions and intensities