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Linked scanning and metastable ion mapping
No full textA method has been developed for the ripid and routine acquisition of metastable ion data for all decompositions occurring in the field-free region between the accelerating and electrostatic-analyser regions of a forward-geometry double-focussing mass spectrometer. It is based on a previous idea of scanning the "B.E plane", but makes use of modern computing facilities to generate control, collect and store the data in the form of a metastable ion decomposition map. A microcomputer controls E and V, the electric-sector field strength and the accelerating voltage, respectively, whilst a commercial "data system" controls the rapid and repetitive scanning of the magnetic field B. The data are collected and stored by the commercial data system, which also performs a time-to-mass conversion, avoiding the use of a Hall probe. Although the data can be displayed as a map using standard GC-MS software, special computer programs were written to display the data in the form of more easily interpretable -fragmentation maps. Details in the map can be more readily appreciated bygenerating simulated "linked-scan" spectra from stored data. Illustrative examples are given for methanol, decanol, acetyl salicylic acid. caffeine, and a mixture. All the features of decomposition throughout the mass spectrometer up to the field-free region between the electric and magnetic sectors are seen. Their appearance is easily interpreted without recourse to complicated multiparameter plots. This technique is an efficient and rapid method of obtaining data that would normally have to be collected by many different linked-scan experiments, and hence is a potentially more powerful method not only in studies of fundamental aspects of unimolecular decomposition and structure analysis, but also in applications to the analysis of mixtures without recourse to prior separation.</p
Benzoquionones and related compounds. Part 2.<sup>1</sup> Preferred conformations of some acyl-1,4-benzoquinones in solution
No full textStudies based on polarographic reduction potentials, electronic absorption spectra, and 1H and 13C nuclear magnetic relaxation data show that in solution the preferred conformation of formyl-1,4-benzoquinone is that with the formyl and quinonoid groups coplanar, and the formyl carbonyl group anti to the 1-carbonyl, whereas that of acetyl- and pivaloyl-1,4-benzoquinone has the acyl groups approximately perpendicular to the quinonoid ring.</p
Benzoquionones and related compounds. Part 2.<sup>1</sup> Preferred conformations of some acyl-1,4-benzoquinones in solution
No full textStudies based on polarographic reduction potentials, electronic absorption spectra, and 1H and 13C nuclear magnetic relaxation data show that in solution the preferred conformation of formyl-1,4-benzoquinone is that with the formyl and quinonoid groups coplanar, and the formyl carbonyl group anti to the 1-carbonyl, whereas that of acetyl- and pivaloyl-1,4-benzoquinone has the acyl groups approximately perpendicular to the quinonoid ring.</p
Coupled relaxation in AX<sub>2</sub> spin systems. Dependence of effective relaxation times on method of measurement and application to determining internuclear distances
No full textA simple treatment of relaxation in AX2 all-proton systems shows that under certain conditions the effective spin-lattice relaxation time of the total A or X signal depends on the method of measurement. The calculations have been confirmed by measurements on 1,3-dinitro-2-chlorobenzene in CDCl 3 and 1,1,2-trichloroethane in CDCl3, (CD 3)2CO and C6D6. The experimental relaxation times, together with nuclear Overhauser enhancements, have been used to determine internuclear distances and rotational correlation times. There is good agreement with values of these parameters obtained from independent sources.</p
Coupled relaxation in AX<sub>2</sub> spin systems. Dependence of effective relaxation times on method of measurement and application to determining internuclear distances
No full textA simple treatment of relaxation in AX2 all-proton systems shows that under certain conditions the effective spin-lattice relaxation time of the total A or X signal depends on the method of measurement. The calculations have been confirmed by measurements on 1,3-dinitro-2-chlorobenzene in CDCl 3 and 1,1,2-trichloroethane in CDCl3, (CD 3)2CO and C6D6. The experimental relaxation times, together with nuclear Overhauser enhancements, have been used to determine internuclear distances and rotational correlation times. There is good agreement with values of these parameters obtained from independent sources.</p
A study of solvent motion in acetone-PMMA solutions using <sup>13</sup>C and <sup>1</sup>H spin-lattice relaxation measurements
No full textThe 13C and 1H spin-lattice relaxation times of acetone in solutions containing from 0 to 20% poly(methyl methacrylate) have been measured from -55°C to 120°C. The 13C measurements show that except at low temperature and the higher polymer concentrations, solvent rotation is little affected by the presence of polymer molecules. By assuming that the 13C relaxation time is entirely intramolecular in origin, and by progressive deuteration of the acetone, the 1H relaxation time has been separated into intramolecular, acetone-acetone intermolecular and acetone-polymer intramolecular contributions. These results show that up to 10% of polymer or so does not affect solvent diffusion, but for 20% polymer, solvent diffusion is perceptibly slowed down, and the activation energy increased by 50%. The data give no indication of polymer conformational transitions in this temperature range, as suggested by others elsewhere, but this may be because of the very short-range sensitivity of n.m.r. measurements.</p
A study of solvent motion in acetone-PMMA solutions using <sup>13</sup>C and <sup>1</sup>H spin-lattice relaxation measurements
No full textThe 13C and 1H spin-lattice relaxation times of acetone in solutions containing from 0 to 20% poly(methyl methacrylate) have been measured from -55°C to 120°C. The 13C measurements show that except at low temperature and the higher polymer concentrations, solvent rotation is little affected by the presence of polymer molecules. By assuming that the 13C relaxation time is entirely intramolecular in origin, and by progressive deuteration of the acetone, the 1H relaxation time has been separated into intramolecular, acetone-acetone intermolecular and acetone-polymer intramolecular contributions. These results show that up to 10% of polymer or so does not affect solvent diffusion, but for 20% polymer, solvent diffusion is perceptibly slowed down, and the activation energy increased by 50%. The data give no indication of polymer conformational transitions in this temperature range, as suggested by others elsewhere, but this may be because of the very short-range sensitivity of n.m.r. measurements.</p