It has been shown above that when a sample containing magnetically active nucl(·i is placed in a magnetic field, energy will be absorbed when the frequency of r lie radiation of an applied rf field corresponds to the difference between energy I('\ ('I"
In NMR experiments, either the magnetic field B0 is kept constant and the
r/
fi(·t,:is varied or vice-versa. For any particular kind of magnetic nucleus. the st n·11i~: l1
of the external magnetic field, B0 , required to provide a field at the nucleus, Bo,n, enough to cause resonance, varies slightly with the structural environment of the nucleus. This small variation in the magnetic field is called the Chemical Shift. By denoting as Bo,s, the small amount by which the external field is diminished because of the influence of the electrons in the environment of the nucleus, then:
Bo,n = Bo - Bo,s
or 27fVo = 1Bo(l - a), ( 6)
where a is the screening constant or shielding parameter. Thus, the actual frequency v0 depends both on the amount of screening (the chemical surroundings of the nucleus) and on the magnetic field under which the measurement is made. The chemical shift, 8, of a given peak is usually given with reference to a standard signal, such as the proton or carbon signal of tetramethyl silane (TA1S) and is defined by the relationship:
/j = llpeak VTMS
Vo (7)
where v0 is the operating frequency in MHz. This fractional type of unit has the advantage of being independent of the magnitude of B0 , even though shielding depends on the strength of the external field. Therefore, the use of this relative unit enables direct comparison of spectra obtained from instruments employing different magnitude external fields and thence different rf fields.
Even if the quality of energy, absorbed by various magnetically active nuclei is different, the quantity of energy absorbed is proportional to the number of nnclci responsible for the absorption. Therefore, the total area under the peak gives information about the relative number of nuclei for which resonamce occurs at different chemical shifts. With appropriate calibration of the apparatus and for the case of pure compounds, it may be possible to estimate the absolute number of nuclei responsible for the peak.
Magnetic resonance experiments usually involve several nuclei that are magnet- ically coupling each other. This creates multiplicity of resonance peaks (splitting pattern) or broadening of peaks, is independent of the size of the external field and can be eliminated by various decoupling techniques. In the case of 13C NMR, heteronuclear decoupling of protons and 13C nuclei eliminates dipolar broadening.
This method employs a second radio-frequency field, which is at or near the Larmor frequency of the protons, thereby inducing rapid transitions between the proton en- ergy levels so that the 13C nuclei see an averaged proton field. Because of the low abundance of the 13C isotope, carbon-13 homonuclear interactions are relatively small, and homonuclear decoupling is not necessary.
2.3
Basic princ'i°ples of hi"gh resolution 13 C and 1 H NMR in the soli"d state.In contrast to the high resolution achievable in liquid-state NMR, polycrystalline and amorphous solid samples produce NMR spectra that are severely broadened by direct dipole-dipole interactions as well as the anisotropy (i.e. the orientational dependence) of the chemical shift. For an isolated pair of spins the internuclear dipole-diople interaction will cause a splitting into two lines, the separation of which allows determination of the internuclear distanceH. In the more general case, how- ever, larger ensembles of nuclear dipoles interact, resulting in a structureless broad- ening of the spectrum, and a concomitant loss of structural information. However, the effect of the magnetic dipole-dipole interactions (as well as the chemical shift anisotropy) can be removed by rapid spinning or the sample at the angle of 54.7°
with respect to the static field direction15. This technique ("magic-angle-spinning
(~1AS) - \!MR") forms the basis of most high-resolution ~[viR studies in solids.
Spinning speeds required depend on the strength of the dipolar interactions to be Lime-averaged; in the case of 1 H :vl AS-.\ ;vi R. speeds of 5-8 kHz are not untypical,
but frequently insufficient16.
For solid state 13C high-resolution NMR studies the MAS-NMR technique is usually combined with crosspolarization and high-power decoupling from neighbor- ing 1 H nuclear spins17. The cross-polarization technique takes advantage of the large nuclear polarization of the abundant proton spin system, if the effective pre- cession frequencies in the rotating frame of both types of nuclei are matched. This is achieved by spin- locking the 1 H magnetization in a radio-frequency field B1,H and applying a radio-frequency field B1,c at the 13C frequency such that the Hartmann- Hahn condition 18:
(8) is fulfilled, where 1H and 1c are the gyromagnetic ratios of 1 H and 13C respectively.
At the end of this "contact period" (typically 1-5 ms), the proton radio-frequency field is left on in order to provide high power decoupling during the period in which the 13C free induction decay is acquired.
A modification of the above technique can be employed to discriminate betv1een protonated and non-protonated carbon atoms by taking advantage of the different strengths of their 1 H-13C di polar interactions. The addition of a delay time of 60-100 µs after the cross polarization period but before the decoupling and acqui- sition period results in a dephasing of all signals arising from protonated carbon atoms and, therefore, facilitates the selective observation of non-protonated carbon atoms.19 Protonated carbons can also be selectively identified by measurements em- ploying short contact times ( 100 ps or less), since the proton-carbon cross-relaxation times are greatly reduced due to the stronger dipolar coupling for these carbon nu- clei.
3 Experimental
3 .1 Sample Preparation and Characterization
The glassy carbon materials under investigation were synthesized from three basic ingredients consisting of a carbon yielding binder, a mixing agent and a pore form- ing agent. The binder was furfuryl alcohol partially polymerized with the aid of para-toluenesulfonic acid catalyst. Acetone was added to the polymer in the ratio of 2 to 1 by volume to lower the viscosity of the polymer and to facilitate mixing and subsequent atomization. Finally various organic liquids or solids were dissolved or suspended in the polymer-acetone mixture to serve as pore forming agents. The mixtures were fed through a syringe pump to an acoustically excited aerosol gen- erator and were susequently sprayed inside an externally heated thermal reactor.
The full description of this system and the thermal reactor is given elsewhere5. Fol- lowing this procedure equal sized droplets were generated, heated to a maximum temperature of 650 K in an inert atmosphere to evaporate the solvent and form solid particles, and collected by sedimentation at the bottom of the reactor. The total residence time in the reactor was approximately 4 s. Pores, in addition to those of the polymer matrix, were generated by evaporation or decomposition of the liquid pore former during the heat treatment and/ or by thermally induced stresses and cracks for the case of the solid pore formers. To eliminate sticking of the collected spherical particles all materials underwent a second st.age pyrolysis treatment at 800 K in a horizontal muffle furnace in l'\2 for 1 hr. Some samples were carbonized further in N2 or partially combusted in 0 2-l'\2 mixtures at temperatures up to 1600 E in an externally heated, laminar flow, drop tube furnace. The residence times in this last furnace varied between 2 and 3.5 s. The particle temperature was monitored by optical pyrometr/'. A typical Scanning Electron micrograph of monodisperse particles cured at 800 I\ is shown in Fig. 1.
The materials used in the present study were particles formed from PF A alone as well as particles formed from mixtures of PFA with various amounts of the following pore formers: tannic acid, polyethylene glycol (PEG), glycerol, and Triton-X 100.
Elemental analysis of the materials at various degrees of carbonization and ox- idation was performed by oxidizing samples at 1100 K in 02 and monitoring the products of combustion with the aid of a Perkin Elmer analyzer. Tables I-III give an overview of the samples studied, their carbon and hydrogen contents, and (where appropriate) their amount of "burnout", corresponding to the weight loss on high temperature oxidation. X-ray diffraction patterns were recorded on film with a Guinier powder camera using monocromatic CuKo: radiation20. Typical spectra traced by an Ultrascan XL laser densitometer are shown in Figure 2. Represen- tative TGA results, obtained on a Dupont model 920 electrodynamic balance, are shown in Figure 3. Typically, substantial weight loss occurs in the temperature range 50-60 °C; as discussed below, most of this weight loss arises from desorption of inclusion water. Total surface areas, measured by N2 BET at 77 K, were found to be a few m2 /g for the pristine materials (cured at 800 K), increasing to a few hundred m2 /g for the partially oxidized (activated) carbons.20