NMR signal broadening

To obtain information on the reason for NMR signal broadening and insight into the chemical nature of these pre-ceramic polymers, attempts were made to follow the reaction of the sym-tetrachlorodisilane and HMDZ by ySi NMR spggtroscopy. 

Within the given limits, variation of the pH value had no influence on the results. The recommended pH value is 8.5, values above 9.5 and below 6.5 lead to hydrolysis of the phospholipids. Additionally, the line shape of the NMR signals broadened under the more acidic conditions and the chemical shift showed large variations. 

Figure B2.4.1. Proton NMR spectra of the -dimethyl groups in 3-dimethylamino-7-methyl-l,2,4-benzotriazine, as a fiinction of temperature. Because of partial double-bond character, there is restricted rotation about the bond between the dunethylammo group and the ring. As the temperature is raised, the rate of rotation around the bond increases and the NMR signals of the two methyl groups broaden and coalesce.

A higher-MHz NMR spectrometer is always a better choice, since the sensitivity of the experiment is proportional to the frequency of measurement. Moreover, with highly concentrated solutions, the presence of some solid particles can cause an increase in T) (FID will be short) and line broadening of the NMR signals will result. Therefore, an optimum concentration (say, 25-50 millimolar solution) is recommended. Of course, H-NMR spectra can be readily measured at much lower concentrations, though higher concentrations are necessary for recording - C-NMR spectra. 

The broad, featureless nature of the Si NMR signal is likely due to one of three phenomena that are well-known to cause such effects in NMR spectroscopy a multiplicity of chemical and physical environments about the silicon atoms restricted segmental motion in polycyclic or cage-like molecules or ne broadening due to ySi magnetic interactions with the numerous N quadrupoles. 

C (or 15N) NMR signals recorded by both CPMAS and DDMAS NMR, however, could be broadened or suppressed, when fluctuation motions with intermediate frequencies of 104-105 Hz interfere with... 

The reaction of 2equiv. of a pyrrole-substituted cyclopentadiene with Zn[N(SiMe3)2]2 (Scheme 19) afforded the dicyclopentadienylzinc complex 23.52 A solid-state structure of this compound was not obtained, but room-temperature 1H NMR spectroscopic studies showed two equivalent cyclopentadienyl groups, whose signals broadened on cooling. 

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