The NMR Spectrometer

The techniques and equipment necessary for solid-liquid interface surface adsórbate NMR signal acquisition are those typically used in solid-state NMR at the solid-gas interface. The frequency domain linewidths of typical 13C or 15N signals (13CO, 13CN, C15N) on platinum at 8.45 signal are between ~20 and 30 kHz (full width at half maximum), requiring fast digitization for faithful reproduction of the rapidly decaying signal in the time domain (~1 - 5 ps dwell times). At present, we are using the following equipment: for digitization, a Nicolet Explorer IIIA fast digitizer; for pulse control and data acquisition and transformation, a Nicolet 1280 computer system. The magnet is a wide-bore (89 mm) Oxford superconducting magnet. The 90° pulse width used for 13CO/Pt (electrochemical NMR cell) is -10 ps, for 13C of cyanide (mini-cell) it was 3.5 ps and 7.5 ps for 15N of cyanide (mini-cell). Signal acquisition is accomplished by applying a Hahn spin-echo sequence followed by a signal acquisition period: [2- tq-u- t\ -acquire] - where t\ < tq so that acquisition begins before the echo maximum. Typical values for To are in the range of 25 - 40 ps. The precise echo maximum is located prior to Fourier transformation.

The fact that the echo decays very rapidly in the time domain leads to a problem common in solid-state NMR work: the appearance of a „pulse feedthrong". This artifact results from the recovery of the receiver circuits and a ringdown of the transmitter and probe circuits after the high-power excitation pulses. To reduce this problem a pulse sequence used to eliminate „ringdown" in quadrupolar echo NMR was employed [10]. In this sequence, the phase of the nil and the n pulses of the Hahnecho sequence are varied in such a way that the artifacts add destructively and are essentially eliminated.

Table 1. Configuration of filter elements
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