Nutation experiments

The rehydrated samples were obtained by exposing dehydrated samples to water vapor at least three days over saturated NH4GI solution at room temperature. A duraction of 0.5 s between scans were allowed for nuclear spin to recover to their equilibrium magnetization. The one—dimensional Na NMR spectra were recorded by using the spin—echo technique. The strength of the radio-frequency field for the two dimensional nutation experiments was 80 kHz and 128 ti values were used (0 250 /is). Each two- dimensional experiment took about 12 hours of spectrometer time. 

Of greater relevance catalytically is that the combined use of l3C enrichment and 13C nutation NMR spectroscopy can distinguish between proposed rival mechanisms for the Ziegler-Natta catalyzed polymerization of acetylene. In the four-center insertion mechanism the enriched acetylene (HC =C H) is incorporated as shown in Scheme 6. It is to be noted that the, 3C—13C bond label is here incorporated into a carbon-carbon double bond, the length of which is significantly smaller than that of a carbon-carbon single bond, which is how the enriched acetylene would be incorporated in the two-center mechanism shown in Scheme 7. The results of nutation experiments leave little doubt that the Ziegler-Natta polymerization of acetylene proceeds by a four-center mechanism. 

Figure 1. Schematic representation of the 27A1 quadrupole nutation experiment. The rf pulse of length ti is followed by the detection of the free induction decay in die absence of rf fields. Two-dimensional Fourier transformation of the series of FIDs gives the nutation spectrum.

The 27A1 nutation spectra were measured at 104.26 MHz using a Bruker MSL-400 multinuclear NMR spectrometer with a high-power static probe-head and a 5 mm diameter horizontal solenoidal coil. Using an aluminium nitrate solution the amplitude of the rf pulse (corf/27c) was adjusted and kept at a constant value of 70 5 kHz unless otherwise stated. Nutation experiments were performed on the same amount of each sample and the same number of transients was accumulated for samples in the same series. The rf pulse length was increased in ln.s increments from 2 M-s to 65 is. The spectral width was 125 kHz, the recycle delay 0.2 s and the number of transients accumulated in each measurement was between 2000 to 4000. 

As an example, quadrupole nutation NMR of nuclei with half-integer quadrupolar spin in zeolitic materials can distinguish between nuclei of the same chemical element subjected to different quadrupole interactions, the signals of which overlap in conventional spectra. The situation is favourable for half-integer quadrupolar spins since the m=l/2 <-> m= -1/2 transition for these nuclei is broadened by the quadrupole interaction only in second-order perturbation theory. The technique can be usefully applied for the determination of the local environment of A1 in zeolitic catalysts (28). It allows discrimination between species of similar chemical shift but different quadrupolar coupling constants (see Figure 5). The main difficulty in the interpretation is the complex spectmm that results from a nutation experiment since it can consist of many overlapping powder patterns (29). 

Figure 3.20. Off-resonance nutation experiments on Mo(12)p(2)/y-Al203 with an rf field strength of 41 kHz. The normal ID MAS spectrum A. is shown and the nutation spectra of the tetrahedral B. and octahedral region C. with the tetrahedral region made up of only 7-AI2O3 but the octahedral region requiring a superposition of 7-AI2O3 and Al2(Mo04).3 from Krauss et. al. (1996) with permission of the American Chemical Society.

Early quadrupole nutation experiments concerned the chemical status of nonframework A1 in dealuminated zeolite Y of static samples or with MAS at moderate rates [137-139,154). However, it has been shown that Al quadrupole nutation with fast (12.5 kHz) MAS and radiofrequency excitation fields up to 220 kHz provides significantly better spectral resolution [155), enabling, for example, the resonances at 0, 30, 56, and 62 ppm observed in ultrastable zeolite Y to be assigned to different aluminous species. The various quadrupolar para-... 

Quadrupole nutation spectra of several aluminophosphate molecular sieves recorded with fast MAS have been compared with the DOR spectra (see below) [157], Enhanced resolution was obtained in the quadrupole nutation experiment at certain radiofrequency pulse strengths. This extra resolution can be comparable to that attainable using DOR and does not introduce spinning sidebands. 

As shown in Fig. 3 a, spin coherence is manifested in the optically detected transient nutation signal for [Rh(bpy)3] (0)04)3 the phosphorescent triplet state. In this experiment, one observes that the phosphorescence intensity becomes modulated as the pulse length of microwave pulses, resonant with the D - transition, is gradually increased. The modulation is evidence that the micro-wave excitation induces a spin coherence in the ensemble of molecules in the photoexcited triplet state [44]. Moreover, from the transient nutation experiment one obtains the information about the duration of the pulses needed in a spin echo experiment. In the case of the example, the n/2 pulse is 100 ns and the 71 pulse has a length of 200 ns. Similarly, transient nutation signals for the other zero-field spin resonances could be obtained. The optically detected spin echo decay as measured for the D - j j zero-field transition for [Rhlbpylj](004)3... 

Figure 3.60. The behaviour of an on-resonance magnetisation vector during the nutation experiment.

Several studies using Xe to probe well-known materials in order to gain additional information have also been recently published. Fraissard and coworkers " have used xenon-131 NMR to characterize the electric field gradients inside the supercages of Y zeolites. Composite pulses were used in the nutation experiments to... 

The simplest coherence experiment in the case of single spin EPR, is a (transient) nutation experiment [22]. In this experiment the spin is forced to precess perpendicular to the direction of the microwave magnetic field component in the rotating frame. This is equivalent to a periodic modulation of the population probability of the two T] sublevels in resonance with the microwaves. This periodic modulation leads to a change in the lifetime of the Ti state and thus to a corresponding modulation of the average fluorescence intensity. 

Due to experimental limitations only nutation traces up to a duration of 2 ps could be recorded. Within this time interval the modtilation amplitude of the nutation is hardly damped. This is in sharp contrast to EPR ensemble experiments, where a damping of the nutation usually is clearly visible within the first ps. The damping is caused by an inhomogeneity of the driving microwave field across the sample. This of course is absent in the single-molecule experiment, since the field is homogeneous over a molecular dimension. The nutation experiment can be described by [32, 33],... 

Wilier M, Schweiger A. 1997. Determination of g values by a new electron spin transient nutation experiment the gj value of titanimn-doped sapphire. Chem Phys Lett 264 1-8. 

Kouskov V, Sloop DJ, Liu SB, Lin TS. 1995. Pulsed transient nutation experiments on... 

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