| dc.description.abstract | Though the variable?frequency pulsed NMR spectrometer constructed by the author is working satisfactorily, some improvements can also be done.
The present probe circuit is not a wide?band one and has to be tuned at different frequencies. The broadband transmitter/receiver circuit used by J.L. Engle (41) or a Direction Coupler (42) can also be used.
The receiver recovery time has to be improved. The recovery time can be reduced in the series and shunt chopper switch by controlling the fall time of the chopper pulses. The steep falling edge of the pulse introduces spurious oscillations in the switch and increases the recovery time. This can be avoided by controlling the fall time of the chopper pulses. Different methods are available in the literature for the reduction of the receiver recovery time (43–47).
A broadband receiver is used in the present spectrometer; the noise will also be large and can be reduced by introducing filters in between stages.
The stability of the magnetic field is achieved by regulating the current flowing through the magnet coil, i.e., current regulation. The alternative regulation scheme is field regulation. The magnetic field is sensed by a Hall probe and adjusts the current (18). The great advantage of field?regulated magnets over current?regulated magnets lies in their easy and reproducible field change (the field is changed by simply dialling) and enormous field?sweep range is possible.
As described in the literature, the pulse programmer and the box?car integrator can be replaced by a microprocessor?based system. It can generate pulse sequences and also collect the data points from the signal. The processing of the data points can also be done by the microprocessor.
The reference junction of the thermocouple can be removed by using a cold?junction compensation circuit. This circuit was constructed and tested by the author only recently and hence was not used in the actual experiment. The stability of the sample temperature can be improved using a temperature controller.
The variation of T? with temperature shows double minima. The low?temperature minimum has been explained in terms of reorientation of the NH? group relaxing all the protons via spin diffusion. The theoretically calculated T? minimum is less than the experimentally observed T? minimum. Ratcliffe (10) has considered proton–nitrogen interaction into the dipole–dipole interaction and concluded that it may lower the calculated T? minimum by about 14%. On the other hand, Johnson (11) has indicated that the partial averaging of the dipolar Hamiltonian by torsional oscillations of the –CH? groups decreases the efficiency of relaxation due to that motion. Johnson has shown that this may increase the calculated minimum by 14% to 22%. However, this correction will differ for –NH? groups, since it depends upon the activation energy for reorientation. This could be the source of discrepancy between the observed and calculated minimum.
The average values of the activation energy and pre?exponential factors are 4.9 kcal/mole and 2.7 × 10?¹³ sec respectively. The activation energy for the NH? reorientation in this compound is low compared to N?H?NO?, N?H?HC?O? and comparable with LiN?H?SO? and N?H?SO? (17).
The reduction in second moment calculated from the T? minimum, after including a correction factor of 3/5 to account for spin exchange with the –NH? protons, gives a value of 21.1 G². As presented in Chapter II.3, a reduction factor of 20 has been observed for a number of hydrazinium compounds. The relaxation constant is calculated to be 1.0 × 10?? and is in agreement with the reported value for NH? reorientation (Chapter II.3.8).
As discussed in Chapter I.18, the activation energy of the reorienting group can be calculated from the frequency?dispersion study. If the temperatures at which T? minima are observed are known (T? and T?) at two frequencies (?? and ??), it can be written as:
(VII.10)
The low?temperature T? minimum is observed at 193.2, 187.6 and 179.4 K at 15, 10 and 5.4 MHz respectively. The average activation energy calculated from two sets of frequencies is 5.2 kcal/mole. This value is in agreement with that obtained from computer fittings.
A reduction in second moment can be calculated from the T? minimum, after taking a correction factor of 2/5 to account for spin exchange with the –NH? protons and is calculated to be 1.7 G². The reduction in second moment of 2 G² has been observed for the same motion (16). Hence it may be concluded that the high?temperature T? minimum is due to a reorientation of the –NH? group about the H–N–H bisectrix by 180°.
An activation energy of 9.5 kcal/mole has been calculated from the frequency?dispersion study, using eqn. VII.10, and is the same as the computer?fitted value. | |
