Acetone— relaxation measurements

However, when these (H)(p-H)Os3(CO),o(imine) derivatives are dissolved in polar solvents such as methanol or acetone, both syn- and anti-isomers are present. [45] Thus it is clear that the overall stereochemistry of the resulting complexes is dependent upon the competition between intra- and intermolecular hydrogen-bonding interactions. It is worth to note that H relaxation measurements show that in the syn-isomers the distance between the imine proton and the terminal hydride is significantly longer when the compounds are dissolved in the polar solvents rather than when they are dissolved in non-polar solvents (i.e. ca 4.0 A against 2.1 A). This finding has been accounted for on the basis of differences in the orientation of the imine... 

Better separation of poorly resolved signals can obviously be achieved by measuring a spectrum at higher field. However, because increased relaxation-times result in sharper lines,18 the resolution can also be improved by using a low concentration, a high temperature, and a nonviscous solvent (for example, acetone). Besides, the use of a... 

Electronic Spectrum. Acetone is the simplest ketone and thus has been one of the most thoroughly studied molecules. The it n absorption spectrum extends from 350 nm and reaches a maximum near 270 nm (125,175). There is some structure observable below 295 nm, but no vibrational and rotational analysis has been possible. The fluorescence emission spectrum starts at about 380 nm and continues to longer wavelengths (149). The overlap between the absorption and the fluorescence spectra is very poor, and the 0-0 band has been estimated to be at - 330 nm (87 kcal/mol). The absorption spectra, emission spectra, and quantum yields of fluorescence of acetone and its symmetrically methylated derivatives in the gas phase havbe been summarized recently (101). The total fluorescence quantum yield from vibrationally relaxed acetone has been measured to be 2.1 x 10 j (105,106), and the measurements for other ketones and aldehydes are based on this fluorescence standard. The phosphorescence quantum yield is -0.019 at 313 nm (105). 

Generally, the signal is weaker than the other proton signals when the spectrum is measured using a Fourier transform NMR spectrometer. The relaxation time (TO of the 5-OH is between 6-9 seconds at 28 °C in acetone- /6,4-6 seconds at 55 °C (unpublished data). When a quantitative spectrum is needed, the pulse delay time must be longer than 10 seconds. 

In terms of chemical shift there is clearly a distinction between the hydrogen bonded complex and the silyl ether addition complex, independent of cluster size. The stability of the latter is however strongly dependent on relaxation and has yet to be observed experimentally. It may however exist as a short lived intermediate in catalytic reactions that can not be observed in the time scale of the NMR. The hydrogen bonded complex has in fact been observed experimentally in the case of acetone and the measured shielding tensor is qualitatively found to be in agreement with the results for the ZOH(F) cluster, again indicating the importance of relaxation. 

Stray-field MRI was used to measure methanol ingress into 500 /xm thick PMMA pre-swollen with acetone [669]. With stray-field imaging the rigid and swollen polymer and the solvent are separately visualised with a resolution of the order of 20 /xm. The different components are distinguished on the basis of their differing spin-spin relaxation times. For a polymer partially swollen with solvent the spatial distributions of relaxation times reveal the interactions between solvent and polymer in the diffusion process. Proton NMR images of 1,4-dioxane in swollen polybutadiene rubber were reported [393]. 

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