Weston A. Anderson

Application of Fourier Transform Spectroscopy to Magnetic Resonance

The application of a new Fourier transform technique to magnetic resonance spectroscopy is explored. The method consists of applying a sequence of short rf pulses to the sample to be investigated and Fourier‐transforming the response of the system. The main advantages of this technique compared with the usual spectral sweep method are the much shorter time required to record a spectrum and the higher inherent sensitivity. It is shown theoretically and experimentally that it is possible to enhance the sensitivity of high resolution proton magnetic resonance spectroscopy in a restricted time up to a factor of ten or more. The time necessary to achieve the same sensitivity is a factor of 100 shorter than with conventional methods. The enhancement of the sensitivity is essentially given by the square root of the ratio of line width to total width of the spectrum. The method is of particular advantage for complicated high resolution spectra with much fine structure.

Use of Weak Perturbing Radio‐Frequency Fields in Nuclear Magnetic Double Resonance

A theory is developed to explain high‐resolution nuclear magnetic resonance spectra observed in the presence of a second weak rf field at ω2. When ω2 is set on a nondegenerate transition frequency and the rest of the spectrum recorded by sweeping the frequency of the investigating rf field, any transitions that have an energy level in common with the perturbed transition will be split into doublets. If magnetic field inhomogeneities exceed the natural linewidth, the line profile of the doublets gives information about the ordering of the three energy levels involved. Good agreement with the theory has been obtained in experiments on two‐spin and three‐spin proton systems where the energy‐level diagram is already known. The method has been used to establish the ordering of the energy levels of the ABC system of protons in methyltrivinylsilane and the analysis has been completed by two independent methods. Some of the special effects which arise when double‐resonance spectra are recorded by sweeping the mag...

ELECTRICAL CURRENT SHIMS FOR CORRECTING MAGNETIC FIELDS

The design criteria of electrical current shims for high resolution nuclear magnetic resonance magnets are discussed. Specific current configurations are given for various corrections to magnetic field gradients of the first, second, third, and fourth orders.

Potential Energy Barrier Determinations for Some Alkyl Nitrites by Nuclear Magnetic Resonance

A technique for studying rapid chemical exchange and reorientation reactions by nuclear magnetic resonance techniques is discussed. A treatment for the general case relating the changes in the proton line width with changes in the frequency of rotation about the O–N bond in the alkyl nitrites is developed. Such a treatent gave fairly accurate potential energy barriers to rotation for methyl, ethyl, n‐propyl, and isopropyl nitrite. A temperature controlling device for low temperatures applicable to proton resonance techniques is discussed.

Assignment of NMR Spectra with the Aid of Double‐Quantum Transitions

The analysis of complex nuclear magnetic resonance spectra usually requires the assignment of the observed transition to an energy‐level diagram. It is shown that double‐quantum transitions can be used to determine such an assignment. Small frequency shifts of the double‐quantum transitions have been observed and their dependence upon the strength of the applied radio‐frequency field measured and compared with theory. The double‐quantum method is compared to the double resonance method in making an energy‐level assignment using the 60‐Mc/sec spectrum of trivinylphosphine for the purposes of illustration. The analysis has been completed by an algebraic method and by iteration on a computer, and the results checked by recording the 100‐Mc/sec proton spectrum and the 19‐Mc/sec P31 spectrum. The three proton—proton couplings and the three P31 proton couplings all have the same sign.

Analysis of High‐Resolution NMR Spectra

Van Vleck has shown the usefulness of moment calculations in the interpretation of magnetic interactions found in crystals. This approach has been extended to include liquids where separate resonances may be well resolved. In a simple chemical system containing only two groups of nuclei, this procedure allows the direct calculation of the spin coupling constant and the chemical shift in terms of directly measured quantities; in more complex systems, it allows some information to be obtained without finding the complete solution to a particular problem.

Applications of Modulation Techniques to High Resolution Nuclear Magnetic Resonance Spectrometers

An NMR spectrometer system is described which incorporates the use of magnetic field modulation for the simultaneous purpose of baseline stabilization and stabilization of the resonance condition. Several other modulation techniques are briefly described.

Removal of Residual Splitting in Nuclear Magnetic Double Resonance

Double resonance experiments have been used as an aid in simplifying complex nuclear magnetic resonance spectra. However, in all except the simplest cases there is a residual splitting due to the coupling to the irradiated group even at relatively large irradiating fields. It is shown that this residual splitting can be greatly reduced or eliminated by a modulation of the frequency of the irradiating field. The conditions necessary to minimize the residual splitting are derived and discussed. The technique is illustrated with two examples.

Sensitivity Enhancement in Magnetic Resonance. II. Investigation of Intermediate Passage Conditions

Intermediate passage conditions in magnetic resonance experiments are investigated to determine the optimum performance conditions for maximum sensitivity, particularly in connection with time averaging methods. The possible increase in signal height by increasing the sweep rate can be utilized in time averaging methods to enhance the sensitivity. The optimization of the conditions described here is based on numerical solutions of the Bloch equations. The results are given in graphical form and show the optimum conditions for many magnetic resonance experiments where the absorption mode signal is recorded. The improvement of the sensitivity, which is possible if instead of a slow passage, single scan experiment a fast passage, multiple scan experiment is made, is higher for a high ratio of the relaxation times T1/T2. The possible gain is limited by the maximum allowed line broadening and line shift. The conditions are nearly optimum if the sweep rate a (cps/sec) and the rf field strength γH1 (rad/sec) are...

Lens system for acoustic transducer array

A lens system for directing and focussing acoustic energy from an array of acoustic transducers on a small target can be made lightweight, low in absorption and dispersion by incorporating plastic Fresnel lenses with prisms into a monolithic tray of the same plastic with one lens for each transducer.