Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
1999-07-08
2001-02-06
Oda, Christine K. (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
C324S307000
Reexamination Certificate
active
06184683
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention concerns a method of two-dimensional, heteronuclear correlation spectroscopy for the investigation of solid state samples, containing a first (
1
H) and a second (
13
C) nuclear species, in a nuclear magnetic resonance (NMR) spectrometer by means of an NMR pulse sequence, which pulse sequence comprises a preparation interval, an evolution interval, a mixing interval and a detection interval, wherein during the preparation interval the first nuclear species is excited by at least one preparation RF pulse in a first frequency band and is exposed to evolution RF pulses inside the first frequency band during the evolution interval and wherein during the detection interval the first nuclear species is exposed to at least one decoupling RF pulse inside the first frequency band while the free induction decay in the second frequency band is detected and wherein the pulse sequence (1≦p≦n) is repeated n times in a row with identical preparation interval, mixing interval and detection interval but with changed evolution interval and wherein the sample rotates with a rotation frequency greater than 1 kHz about an axis which is tilted by about 54° with respect to the axis of a homogeneous magnetic field and wherein the at least one preparation RF pulse is broad-banded with a center frequency in the center of the NMR spectrum of the first nuclear species of the sample effecting a rotation of the nuclear magnetization of the first nuclear species about an axis perpendicular to the direction of the magnetic field (X) with an angle of preferably 90° and wherein the evolution RF pulses form a so-called FSLG sequence with two successive broadband evolution RF pulses, phase shifted with respect to each other by 180° (Y, −Y), whose center frequencies are shifted in opposite directions with respect to the preparation RF pulse and which each effect a rotation of the nuclear magnetization of the first nuclear species by about 294°.
Such a method is known from EP-A 0 481 256 (U.S. Pat. No. 5,117,186).
Nuclear resonance (NMR) is a phenomenon occurring in relation to a selected group of atomic nuclei and that is based on the existence of magnetic nuclear moments of these atomic nuclei. If an atomic nucleus with nuclear spin is placed in a strong homogeneous and static magnetic field (so-called “Zeeman field”) and is excited by means of a weak radio frequency (RF) magnetic field, the nuclear spin precesses with a natural resonance frequency, the Larmor frequency, which is characteristic for each nuclear species with nuclear spin and which depends on the magnetic field strength effective at the location of the nucleus. Typical atomic nuclei with magnetic moments are e.g. protons
1
H,
13
C,
19
F and
31
P. The resonance frequencies of the nuclei can be observed by observing the transverse magnetization occurring after a strong RF pulse. Usually, the measured signal is transformed into a frequency spectrum by Fourier transformation.
Although identical nuclei show the same frequency dependence on the magnetic field, differences of the immediate chemical surroundings of each nucleus can modify the magnetic field, so that nuclei of the same sample do not see the same effective magnetic field. The differences of the local magnetic fields effect spectral shifts of the Larmor frequency between two such chemically not equivalent nuclei which are called “chemical shifts”. These chemical shifts are of interest since they yield information about the number and position of the atoms inside a molecule and about the relative arrangement of neighboring molecules inside a compound.
Unfortunately, it is not always possible to give an interpretation of the frequency spectra caused by chemical shift since there are additional and possibly dominant interactions present.
This is particularly the case in NMR spectroscopy of solids. In NMR spectroscopy of liquids, the fast molecular movements have the tendency to isolate the nuclei and to separate the nuclear interactions, so that it is much easier to recognize different nuclei in a spectrum. In solid state NMR there are many interactions between the molecules masking the result. E.g., magnetic moments of neighboring nuclei interfere with each other leading to interactions called “dipole-dipole couplings”. These couplings broaden the characteristic resonance lines and mask the “fine” resonance structure caused by chemical shift. A further problem occurring in relation to solids and which is not present in liquids, is that the orientation of molecules in solids are relatively fixed with respect to the applied Zeeman field. Therefore the chemical shifts are anisotropic so that a contribution to the resonance frequency depends on the spatial orientation of the molecules with respect to the magnetic field. Therefore, it is important to suppress some of these interactions in order to obtain meaningful results for the others. Usually this is achieved by exciting the system at selected frequencies with the consequence that undesired interactions cancel or at least that they are averaged to a reduced amplitude. For example, in solids the above-mentioned anisotropy of the chemical shift is usually largely reduced by orienting the solid sample relative to the applied magnetic field at the so-called “magic angle” (54° 44′) and by rotating it at this angle with a comparatively fast frequency. This averages the anisotropic field components to zero.
In a similar fashion, it is possible to reduce with known techniques spin-spin interactions by irradiating the nuclei with RF pulses at or close to the Larmor frequencies. By carefully selecting various polarizations and phases of the RF pulses, the magnetization of interfering nuclear spin systems in neighboring groups can be changed. Thereby spin-spin interactions can effectively be averaged out so that their contribution to the final measuring result is strongly reduced. Since for each nuclear species the Larmor frequency is different, an applied RF field will have a much greater effect on those spins with a Larmor frequency close to the applied frequency than on those spins with a significantly different Larmor frequency. In this way, applied RF fields can be used to influence one nuclear species whereas others remain unaffected.
Because of the special problems of solid state NMR, one usually applies a two-dimensional spectroscopy technique in the time domain in order to improve resolution. With this technique it becomes possible to investigate the interaction or “correlation” between two different nuclear species inside a solid—the interaction between protons and
13
C nuclei is usually of great interest in many organic solids. The basic technique of two-dimensional heteronuclear correlation in relation to solids is well-known and described in many articles, as e.g. in “Heteronuclear Correlation Spectroscopy” by P.
Caravatti, G. Bodenhausen and R. R. Ernst, Chemical Physics Letters Vol. 89, No. 5, pp. 363-367 (July 1982) and in “Heteronuclear Correlation Spectroscopy in Rotating Solids” by P. Caravatti, L. Braunschweiler and R. R. Ernst, Chemical Physics Letters Vol. 100, No. 4, pp. 305-3107 (September 1983). We explicitly refer to the contents of these articles.
As described in the above-mentioned articles, the two-dimensional heteronuclear correlation technique comprises an “experiment” in the time domain generally consisting of four different sequential time intervals. The first interval is called “preparation interval”. During this time, one of the two nuclear species under investigation is transferred into an excited, coherent nonequilibrium state, which changes or “evolves” during the following time intervals. The preparation interval can consist of irradiating of a single RF pulse or alternatively of a sequence of RF pulses. Usually, the preparation interval has a fixed length of time.
A second time interval is called “evolution interval” during which the excited nuclear spins “evolve” under the influence of the applied magnetic field, of the neighbor spins, possibly of ir
Emsley Lyndon
Lesage Anne
Sakellariou Dimitrios
Steuernagel Stefan
Bruker Analytik GmbH
Oda Christine K.
Shrivastav Brij B.
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