Solid-state proton orientation

X-Ray studies of the crystal structure of bis-jr-benzene chromium have been carried out by several workers [59, 60, 61]. The sandwich structure of the molecule has been amply confirmed (Figure 41) however, due to the presence of orientational disorder in the crystals at room temperature it remains uncertain whether the C-C distances of the benzene ring are the same, or whether they are alternately slightly longer and shorter. Present evidence favours a symmetrical Da molecule with all C-C distances the same within experimental error [62, 62a]. Solid-state proton magnetic resonance studies show that there is molecular motion in bis- i-benzene-chromium crystals which is frozen out at —196 [63]. Thus it may be possible to obtain more accurate X-ray data at lower temperatures. 

In the case of the smaller macrocycle [13]aneN4, six configurational isomers exist for [Ni([13]aneN4)]2+ due to the orientation of the N—H protons above or below the plane of the macrocycle. X-ray crystallography and NMR spectroscopy, respectively, revealed that the R,iS,7 ,iS -[Ni([13]aneN4)]2+ form (trans- ) is the preferred one both in aqueous solution and in the solid state.1 43... 

On the one hand, x-ray crystallography data could support this idea because the heterocycles are coplanar relative to the B-H bonds. Moreover, in the case of the compound in Strncture 5.5, this conformation remains in solution, according to NOE NMR measnrements. On the other hand, the solid-state CH- -HB distances are too long (< 2.65 A), and the HF/6-31G calculations carried ont for the componnd have shown, that where the heterocyclic rings are oriented orthogonally, the conformation is more stable. Thns, this question is still open. It is quite probable that the interaction between protonic hydrogens on the a-carbons and hydridic BH hydrogens is electrostatically attractive. 

For line shift measurements with the eight-pulse cycle between 180°K and room temperature, the reference was acetyl chloride. Its frequency was measured relative to a spherical tetramethylsilane (TMS) sample at room temperature, and all results are reported relative to this TMS on the r scale, (r = a + 10 ppm, where a is the signed chemical shift used in solid state NMR.) At lower temperatures, the reference was a single crystal of Ca(OH)2, oriented in the magnetic field such that the major axis of its proton chemical shift tensor was parallel to the external field (19). Thus, it is assumed that the proton chemical snift of the Ca(OH)2 remained unchanged as the temperature was varied. 

The 145.7 MHz proton noise decoupled 31p nmR spectra of the poly(dA-dT) duplex in 10 mM cacodylate buffer between 28° and 54°C are presented in Figure 9. A broad symmetrical unresolved resonance is observed at 28°C. By contrast, two resolved narrow resonances separated by 90.2 ppm have been observed for 150 base pair long (dA-dT)n (41). Thus, though the resolution of dT dA and dApdT phosphodiesters cannot be achieved at the synthetic DNA level in solution (18), it has been observed for the same sequence at a shorter well defined length (41). More recently, two resolved 31p resonances have also been reported in poly(dA-dT) fibers oriented parallel to the direction to the magnetic field by solid state 3lp nmr spectroscopy (42). 

For the partially deuterated benzoic acid (C6D5COOH), the solid state H NMR spectrum is dominated by the intra-dimer H- H dipole-dipole interaction. In a single crystal, both tautomers A and B are characterised by a well-defined interproton vector with respect to the direction of the magnetic field (Fig. 1). Proton motion modulates the H- H dipole-dipole interactions, which in turn affects the H NMR lineshape and the spin-lattice relaxation time. It has been shown that spin-lattice relaxation times are sensitive to the proton dynamics over the temperature range from 10 K to 300 K, and at low temperatures incoherent quantum tunnelling characterises the proton dynamics. A dipolar splitting of about 16 kHz is observed at 20 K. From the orientation dependence of the dipolar splitting, the... 

Selected solution-state 1H- and 13C-NMR, and solid-state cp/mas 13C-NMR parameters are given in Table 1. The diastereotopic protons ligated to C(3) are part of an apparent four-spin system (eight transitions), while those on C(4) are readily differentiated since they are part of an apparent five-spin system. Another feature is the Ad anisochronicity of 0.59 ppm (major species) and 2.05 ppm (minor species) for the diastereotopic protons on C(6), due to the close proximity of H(61) (endo) to its transannular 0(2) neighbor and the orientation of the A-methyl group. 

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