A. Margaret Chippendale

NMR studies of exchange between triazine rotamers

Proton and 13C solution‐state spectra were obtained for a substituted triazine and assignments made for the signals using different solvents. Site‐exchange effects were observed in 1H spectra at different temperatures below ambient for a CD3OD solution. The bandshapes were fitted for three‐site exchange and the relevant rate constants extracted. The exchange was attributed to internal rotation of the substituents about the triazine ring. Activation parameters were calculated: for exchange between one pair of sites Δ H‡ = 71 ± 4 kJ mol−1 and Δ S‡ = 33 ± 15 J mol−1 K−1, and for a second pair Δ H‡ = 72 ± 5 kJ mol−1 and Δ S‡ = 39 ± 26 J mol−1 K−1, whereas direct exchange between the remaining pair of sites is negligible. Exchange involving a fourth rotamer affects the spectra at lower temperatures, which assists in a partial assignment of the observed peaks to the rotamers. These results are discussed in relation to molecular modelling information and literature values. Solution‐state and solid‐state 13C spectra were also recorded but the latter are broad, making detailed assignment difficult. Copyright

Molecular modelling studies of side-chain rotation in substituted triazine rings

Abstract Molecular modelling computation using the gaussian 94 package of programs was carried out to estimate four different barriers to internal rotation about amine nitrogen to sp 2 carbon bonds for two compounds with triazine rings. Whereas two of the processes appear to retain a planar side-chain nitrogen environment throughout the internal rotation, the other two involve pyramidalisation. In the latter two cases, after the barrier is passed, the process may or may not include a transient inversion at nitrogen. The internal rotation for cyclotriazine systems is discussed in relation to those for aniline and formamide. The four barrier magnitudes calculated for the triazines are in the same order as (though somewhat larger than) those measured by variable-temperature NMR bandshape analysis for a relevant compound.

Magic-angle spinning phosphorus-31 nuclear magnetic resonance of polycrystalline sodium phosphates

The magic-angle spinning 31P n.m.r. spectra of a range of solid sodium ortho-, pyro-, poly-, and metaphosphates have been obtained. Counter ion, hydration state, crystalline form and inequivalence within a molecule all affect the solid-state chemical shift, which therefore deviates from the solution chemical shift. The multiplicity of 31P resonances indicates a level of structural inequivalence at least as large as that revealed by X-ray diffraction. Shielding anisotropies have also been obtained, which agree with previous studies but additionally show deviation from isotropic symmetry for some orthophosphates and deviation from axial symmetry for some terminal polyphosphate groups.

Conformational analysis by magic-angle spinning NMR spectroscopy for a series of polymorphs of a disperse azobenzene dyestuff

Polycrystalline powder samples of an azobenzene disperse dyestuff have been investigated using solid-state and solution-state 13C and 15N NMR spectroscopy in an attempt to understand the effect of polymorphism. Assignment of the 13C spectra indicates that it is possible to form an intramolecular hydrogen bond resulting in two conformations co-existing in the crystal structure of two of the polymorphs, as well as in solution, and differing by internal rotation of a side-chain acetamido group. This is clearly evident in the solid-state spectra at ambient temperature and in solution at sufficiently reduced temperatures. Each polymorph contains varying amounts of each conformer as a result of different crystal interactions, and conformer interchange can be shown to occur even in the solid state. This remarkable finding is accounted for by the open nature of the crystal structures, resulting from the steric effects of substituents, as attested by packing coefficients and molecular modelling. The conformational variations are confirmed by 15N spectra, and isotopic labelling by 15N allows tensor analysis to be applied which shows that the azo nitrogens principal shielding components are significantly influenced by the formation of the hydrogen bond in one of the conformations.

High-resolution 13C nuclear magnetic resonance spectra of some solid anthraquinone dyestuffs and related species

13 C N.m.r. spectra of the solid phase have been obtained for anthraquinone and a series of its substituted derivatives. Whilst some species show essentially similar spectra in the solid and solution phases, clear differences are apparent in other cases. The differences are attributed to the loss of molecular symmetry associated with the locking of particular conformations in the solid. In the case of 1,4-bis-(n-butylamino)-anthraquinone, the molecular asymmetry indicated by n.m.r. in the solid was confirmed subsequently by X-ray crystallographic examination.

High-resolution 13C nuclear magnetic resonance spectra of some solid trans-azobenzene and dyestuff species

13 C N.m.r. spectra of trans-azobenzene and a series of its substituted derivatives have been obtained in the solid phase. Additional effects are observed, not present in the solution spectra of the species, and these are shown to be due to the locking of the molecules into particular conformations in the solid. The first 13C n.m.r. spectrum of a reasonably complex solid dyestuff is shown and an assignment of this has been attempted, based upon the effects observed in the solid reference compounds.