th  - 1988 - 51st ENC Conference

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NODIFICATIONS TO A JEOL GX270 WIDEBORE SPECTROMEI'ER
FOR NAGNETIC RESONANCE IMAGING: PETROGRAPHIC APPLICATIONS
By
National
Daryl A. Doughty and Nicida L. Maerefat
Institute for Petroleum and Energy Production
Bartlesvllle, OK 74005
ABSTRACT
Modification of an existing jEOL GX270/89 NMR spectrometer for imaging
studies is described. Inhouse modifications were made because of the
specialized nature of the petrographic samples and their restrictions on probe
geometry. Because of the wide proton linewidths measured at 270 MHz for
fluids contained in porous rock the magnetic field of the spectrometer was
reduced to 1.41T (60 MHz proton frequency) for the imaging studies. Details
concerning the probe/gradlent coil assembly and the spectrometer interface
board containing the gradient control and slice-selection circuits are
presented. Results showing the operation of the spectrometer in imaging
phantoms, brine in sandpacks, and fluids in Berea, Cottage Grove, and
Cleveland sandstone cores are also presented.
11 7 J RESONANT EFFECTS IN CP-MAS SPECTRA OF HOMONUCLEAR
DIPOLAR-COUPLED SPIN SYSTEMS ~ Thomas M. Barbara and Gerard S. Harbison:
Department of Chemistry, SUNY at Stony Brook, Stony Brook, NY 11794-5400
The CP-MAS spectra of systems which contain homonuclear dipolar couplings between inequivalent spin-I/2
sped,,, exhibit sisnlflcant broadening of the coupled resonances, which is particularly pronounced when the rotational
frequency vr is • areal/integer multiple of the isotropic chemical 8hilt difference between the nuclei ~6. This resonant
effect has been known for over 20 years but has recently attracted considerable interest. We have developed two
formalissm for underetendlng and calculating the lineshspes of these systems. The first involves a basis transformation
to • rotating frame which reduces the problem to the fsmJ/Jar double-resonance ¢~e for • two-level system. In the
simplest pmudble model system which exhibits these phenomena (two coupled spin= without • ahlelding anlsotropy
but with distinct isotropic chemical shifts) the solution of the equations of motion for the on-resonance case, where
A6 -- J0r or 2~'r, gives • splitting which is linearly proportional to the dipolar coupling and dependent on the angle
p between the dipolar and the rotor axis. This anKsdar-dependent splitting leads in unoriented samples to scaled
powder patterns, whose quite distinct shapes for A6 ----- JPr and A6 = 2vr reflect the different dependence on/~ of
these two rotor resonances. The width of these patterns is simply (~/'2D/'.~) 6n~l DI~ for Che ~, ~.d ;;r r=o-=ce~
respectively, (D being the dipolar coupling). These values, while exact only for vonishin~y small D, are correct
to within S% for D < =*r- Dipolar effects when A6 ~ =*r or 2=*r can similarly be viewed as Bloch-Siegert shifts
Jn the double rwommce formalism. These shifts are asaln an83dar-dependent and therefore also lead to apparent
line-broadening; they also causo the center of the sis~al to be ahifted away from its uncoupled frequency.
Chemical shielding anleotropy destroys these sim~le relationships between the linewidth and dipolar coupling;
it also causes resonances to appear at nvr for n y~ I or 2. The double-resonance picture is however still intuitively
useful, and gives magnitudes for the dipolar linewidth, particularly off-resonance. To calculate exact lineshapes in
these cases, we have used • numerical solution to the Liouville equation mdng a Rnnge-Kutta algorithnL This is
surprisingly modest in its use of computer time, and can be used to calculate spectra to • high degree of accuracy.
Spectra c~Iculsted ~ this method are in excellent alpreement with experimental results ond with those of the double
resommce fonnalim~ The advantages of the method are that it does not require assumptions about the ¢yc]Jcity of
the interactien= (as average Hami]tonlen theory does), their relative aises or the sdiabaticity of the perturbations,
and that it can be extended with little modification to almost any theoretical problem in NM~
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