Lock, external

The goal of this research was to shed further light on the solution chemistry of amic acid and imide materials using F-NMR. Several immediate advantages of this technique were obvious in advance. Studies could be performed in any solvent desired, with no spectral interference from the solvents. Industry has demonstrated a preference for W-methylpyrrolidinone (NMP) as a solvent for PAA and, in the case of soluble Pis, for the final polymer. Utilization of an external lock solvent and fluorinated standards would allow the investigations to be carried out in an uncontaminated NMP setting. 

Pig. 2.36. Coaxially-centered sample tubes for external lock or reference. 

NMR. The CP/MAS spectra were obtained on a single coil, double-tuned probe similar in design to that reported earlier (15) but modified (16) for the electromagnet of the Varian XL-100-15 system. A homebuilt receiver system allows single coil operation. The probe uses a D2O external lock and a special rotor and stator assembly design to provide MAS. The carbon-13 (25.16 MHz) and the proton (100.06 MHz) radio frequency fields are 17 G and 12 G, respectively for 90 watts of power. The H spin locking pulse of 90° may be varied in terms of length and amplitude while the amplitude of the - C irradiation can be controlled to within 0.1 db to match the Hartmann-Hahn (17) condition. The isolation between the two channels is in excess of... 

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. 

Spin-lattice magnetic relaxation times T. on nuclei were measured at 25.15 MHz and 62.5 MHz using Jeol PS 100 and Bruker WP 250 spectrometers respectively. DMS0-d6 was used as an external lock. Tj was obtained from the 180 -t-90 sequence with an accuracy of 1 7,. 

Figure 3-47 Analysis of fluorocarbons, 300 MHz, neat liquid, external lock,...

The measurement is done in neat liquid, and a capillaiy filled with benzene-d6 is used as an external lock. The signals of protonated benzene and water are used for qualitative and quantitative calibration. The broad signals at 5.0 and 1.8 ppm are probe characteristics. 

Figure 5. HNMR of 2-chloroethylethylsulfide (CEES top), and reaction products of CEES with Ti02 (bottom). Spectra were obtained in perfluorinated oil with a coaxial insert of D2O as an external lock.

Owing to its minor importance in spectroscopy, the external lock configuration will not be considered here. It should be emphasized that, in this case, the sample and the lock substance are not influenced by the same field. As a result the choice of Vq in Eg. (4.1) to compute the 6 values is critical. 

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