Long pulse delay

Gated decoupling and a long pulse delay time of 10 seconds were employed to obtain the spectrum. Frcxn the monomer cuid polymer peak areas, the extent of polymerization at equilibrium Ccui be determined. Measurements of chain end and the polymer peaks provide information on number-average degree of polymerization. The data collection time required to obtain this spectrum was almost three hours. 

There are two important experimental factors that must be accounted for if we are to be successful in running 15N experiments. The 15N nucleus tends to relax very slowly Tj s of greater than 80 seconds have been measured. Thus, either long pulse delays must be incorporated into our pulse sequence or, alternatively, we could provide another route for spin relaxation. A common procedure is to add a catalytic amount of chromium (III) acetylacetonate, a paramagnetic substance, whose unpaired electrons efficiently stimulate transfer of spin. In cases where Tt s are not known (and not intended to be measured), pulse delays and pulse angles must be considered carefully because the signal from one (or more) 15N resonance can accrue too slowly or be missed altogether. 

The remarkable stability and eontrollability of NMR speetrometers penults not only the preeise aeeiimulation of FIDs over several hours, but also the aequisition of long series of speetra differing only in some stepped variable sueh as an interpulse delay. A peak at any one ehemieal shift will typieally vary in intensity as this series is traversed. All the sinusoidal eomponents of this variation with time ean then be extraeted, by Fourier transfomiation of the variations. For example, suppose that the nomial ID NMR aequisition sequenee (relaxation delay, 90° pulse, eolleet FID) is replaeed by the 2D sequenee (relaxation delay, 90° pulse, delay i -90° pulse, eolleet FID) and that x is inereased linearly from a low value to ereate the seeond dimension. The polarization transfer proeess outlined in die previous seetion will then eause the peaks of one multiplet to be modulated in intensity, at the frequeneies of any other multiplet with whieh it shares a eoupling. 

Re-evaluation of pulse delay times used to record fullerene 13C NMR spectra revealed that a 16 s pulse delay, twice the value for a standard detection, allowed the observation of a weak resonance in the sp3 region at 90.4 ppm in the 13C NMR spectrum of the unlabeled heterofullerene 114. Attempts were made to optimize the NMR experimental parameters for a long 7 i, i.e. the variation of delay times and pulse angles. Various conditions were tried on the labeled material without success. This is probably due to the mixture of the labeled and unlabeled 114 which give too low S/N for signal detection. Table 49 summarizes the NMR results obtained and illustrates a distinct pattern of the azafullerenes. 

The shortest time delay in the pulsed laser experiments is limited either by the pulse width (in the picosecond time range) or by the shortest time the probe pulse can be delayed from the photolytic pulse (in the picosecond time range). The convenient long time delay could be accomplished by sending the probe pulse around the laboratory a few times (producing a delay time in the nanosecond range). 

The various probe beams can be coupled into the same singlewavelength, dual-channel pulse-probe transient optical absorption set-up. A one-meter-long optical delay line is used to control the variable time delay between the electron and the probe pulses. Approximately half of the probe beam is deflected onto a reference photodiode while the other half of the beam is slightly focused into the sample, which is placed in front of the output window of the accelerator. Subsequently, the probe beam is then transported to the sample photodiode. (Alternatively, in some laboratories the probe and reference beams are transported into the detection room by long, low-OH silica optical fibers in order to reduce electronic noise pickup on the detector signal cables.)... 

---