Pulse Fourier transform spectrometer data collection

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. 

Two-Dimensional Semiquantitative NMR. Spectra were collected on a Varian Unity 400 Fourier transform NMR spectrometer at 161.90 MHz. The phosphates were prepared at 3-5 weight per cent in water and a DjO insert used for locking purposes. Homonuclear 2DJ-resolved spectra were accumulated using an 8K X 0.2X data set with an acquisition time in the F dimension of 0.946 sec, four steady state pulses, 128 transients, and 200 increments in the F domain. Spectra were analyzed with zero-filling to 16K X 0.5K and application of a sine bell or shifted sine bell weighting function on a Sun Microsystems Sparc 1+ computer. 

A microwave pulse from a tunable oscillator is injected into the cavity by an antenna, and creates a coherent superposition of rotational states. In the absence of collisions, this superposition emits a free-induction decay signal, which is detected with an antenna-coupled microwave mixer similar to those used in molecular astrophysics. The data are collected in the time domain and Fourier transformed to yield the spectrum whose bandwidth is determined by the quality factor of the cavity. Hence, such instruments are called Fourier transform microwave (FTMW) spectrometers (or Flygare-Balle spectrometers, after the inventors). FTMW instruments are extraordinarily sensitive, and can be used to examine a wide range of stable molecules as well as highly transient or reactive species such as hydrogen-bonded or refractory clusters [29, 30]. 

To meet the resonance condition of Equation 8.7, could be held constant and V varied, or v could be kept constant and changed. In continuous-wave (CW) spectrometers, the earliest form of commercial NMR instruments, the latter option is more common, and the spectrum is obtained by slowly sweeping through the range of field strengths required to produce resonance at a particular oscillator frequency. Modern instruments, called Fourier transform (FT) spectrometers, operate with a pulse technique in which all resonance frequencies are produced simultaneously while Hg is held constant this technique allows collection of spectral data in much less time than with a CW instrument. 

---