Lock, external internal

To do this, we have unblocked the external zones using the presence of a reducing gas, while locking the internal zone using air. The results displayed (field D) lead to the supposition that gas does not cause any electric effect on tin dioxide alone. This effect is indeed limited to the difference between fields B and D. 

The proton noise-decoupled 13c-nmr spectra were obtained on a Bruker WH-90 Fourier transform spectrometer operating at 22.63 MHz. The other spectrometer systems used were a Bruker Model HFX-90 and a Varian XL-100. Tetramethylsilane (TMS) was used as internal reference, and all chemical shifts are reported downfield from TMS. Field-frequency stabilization was maintained by deuterium lock on external or internal perdeuterated nitromethane. Quantitative spectral intensities were obtained by gated decoupling and a pulse delay of 10 seconds. Accumulation of 1000 pulses with phase alternating pulse sequence was generally used. For "relative" spectral intensities no pulse delay was used, and accumulation of 200 pulses was found to give adequate signal-to-noise ratios for quantitative data collection. 

The reference nuclei may be part of the sample (internal, field-frequency stabilization, or internal lock) or contained in a separate ampule within the homogeneous volume of magnetic field (external, field-frequency stabilization, or external lock). Also, the reference nuclei may be of the same species as those whose spectra are being observed (homonuclear lock), or of a different species (heteronuclear lock). Each of these methods has advantages and disadvantages. 

Dynamic combinatorial chemistry (DCC) is founded on the study and the construction of mixtures of discrete constituents which are produced by reversible molecular or supramolecular associations [1, 2], The composition of a dynamic combinatorial library (DCL) is thermodynamically driven and, as such, is able to adapt itself to any parameter that - permanently or transiently - modifies its constitution/energy potential surface [3,4], Thus, in the presence of various internal or external parameters, the involved equilibria can be displaced toward the amplification of given products through an adaptation process that will occur through an in situ screening of these species. A schematic representation using Emil Fisher lock-and-key metaphora can be used to illustrate these concepts (Fig. 1). 

FIG. 9. Li chemical shifts in organolithium compounds, measured in three different solvents. The shieldings were originally determined relative to the proton frequency of tetramethylsilane which served as an internal lock signal, and subsequently referenced to external LiBr (0-7g/ml). (80)... 

Alkali metal NMR spectra were observed at the appropriate resonance frequencies listed in Table I, using 12-mm tubes and a Varian XL-100 spectrometer with Gyrocode Observe capability. External 19F or internal H field-frequency lock was used. Depending on the linewidth of the resonance being observed, spectral widths of 256 Hz to 12 kHz in 8192 frequency domain points were used. For 23Na and 85Rb, 90° pulses of 50/isec and a 0.1-sec repetition rate were used. For 6Li and 133Cs, the approximately 55° pulses were 30 /xsec, and the pulse repetition rates for the Nafion samples were 60 sec and 1 sec, respectively. 

ESR spectra were taken on a Bruker Scientific 418s X-band spectrometer equipped with Hall-probe stabilized magnetic field sweep. The 100 MHz proton NMR spectra were obtained on a standard Varian HA-100 spectrometer operating with internal lock using a solvent peak. All samples contained a trace of tetramethylsilane (TMS) as an internal reference. 7Li NMR spectra were obtained at 23.3 MHz on a JEOL C60H NMR spectrometer equipped with a JNM-NS-100 Nuclear Single Sideband Unit. Samples for 7Li NMR contained a capillary of 5.0M LiBr in methanol as external reference. 

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