Applications of Magnetic Resonance Imaging to Materials Research

Abstract

This review article deals with the applications of magnetic resonance imaging (MRI) to materials. The authors have attempted to perform a thorough review of the current literature. The articles examined span the period mainly from January 1989 to June 1993. The authors used CAS on-line and Science Citation Index on CD-ROM as their main sources of references using numerous key words for the searches. The authors also assume a basic knowledge on the part of the reader of NMR and h4R.I. For the novice reader or someone who would like more background on NMR theory, the authors suggest, T. C. Farrar's book Pulse h'MR (Farragut Press) and E. D. Becker's book High Resolution M R (Academic Press). For information regarding MRI and microscopy, the reader is referred to a book by F. W. Wehrli entitled Biomedical Magnetic Resonance Imaging (VCH) and a book by B. Blumich and W. Kuhn entitled Magnetic Resonance Microscopy (VCH). A book by Ernst, Bodenhausen, and Wokaun called Principles of Nuclear Magnetic Resonance in One- and Two-Dimensions is another valuable resource.     

The Endo-Exo Isomerization of N-Phenyl-5-norbornene-2,3-dicarboximide

ENDO isomers of N-phenyl-5-norbornene-2,3-dicarboximide was converted to exo-isomers below retro-Diels-Alder temperature under various reaction conditions. The exo-endo yield ratio was studied by NMR and the ¹H and ¹³C NMR chemical shifts of the isomers are reported. While no intermediate could be isolated using several different radical trapping agents, the results show that 7,5 proton chemical shift is a possible isomerization mechanism.     

Toward an Optimal Paramagnetic Relaxation Agent for 31P NMR Spectroscopy

The dynamics of the use of relaxation agents for ³¹P NMR spectroscopy were investigated. It was found that non-lability of the associated ligand is essential, in order for line broadening to be prevented. Thus, chelates were found to be most suitable. In addition, evidence was accumulated that indicates an outer sphere coordination is significant for effective reduction of spin-lattice relaxation times (T₁'s). Finally, the magnitude of the magnetic moment, as pertains to Gd⁺³ and Fe⁺³, was examined and related to effective reduction of T₁'s.     

Studies on Cyclitols. XIV. Methyl Group Signals of gem-Dimethyldioxolanes as Indicators of Anisotropy

In connection with our interest in nmr spectroscopy of cyclopentanoid and 2, 4-dioxabicyclo[3. 3. 0] octane systems³ we have synthesized a large number of gem-dimethyldioxolanes^2,4 and have examined their spectra. The spectra of all the compounds showed two distinct singlets, representing the two methyl groups, in the region δ 1. 2-1. 5. Similar values have been reported in the case of derivatives of 1, 2-O-isopropylidene-α-D-xylohexofuranose by Abraham et al.⁵ Perlin⁶ has reported similar differences in the chemical shift of the endo- and exo- C-methyl groups of the diastereoisomeric orthoacetates of D-mannose, and Baggett et al.⁷ observed a difference of 0. 30 ppm between the chemical shifts of endo- and exo-benzyl protons of O-benzylidene sugars. Our studies permit the assignment of the lower field resonance to the endo-methyl group and the higher field resonance to the exo-methyl group of the dimethyldioxolanes.     

Peroxycarbenium-mediated C-C bond formation: applications to the synthesis of hydroperoxides and peroxides

The Lewis acid-mediated reaction of alkene nucleophiles with peroxyacetals provides an effective route for the synthesis of homologated peroxides and hydroperoxides. In the presence of Lewis acids such as TiCl(4), SnCl(4), and trimethylsilyl triflate, peroxyacetals and peroxyketals undergo reaction with allyltrimethylsilane, silyl enol ethers, and silyl ketene acetals to afford homoallyl peroxides, 3-peroxyketones, and 3-peroxyalkanoates, respectively. Reactions of peroxyacetals are Lewis acid dependent; TiCl(4) promotes formation of ethers while SnCl(4) and trimethylsilyl triflate promote formation of peroxides. Lewis acid-promoted reactions of silylated hydroperoxyacetals furnish silylated hydroperoxides, which can be deprotected to homologated hydroperoxides. Hydroperoxyketals undergo Lewis acid-mediated allylation to furnish 1,2-dioxolanes via attack of hydroperoxide on the intermediate carbocation. Lewis acid-mediated cyclization of unsaturated peroxyacetals furnishes 1,2-dioxanes, 1,2-dioxepanes, and 1,2-dioxacanes through 6-endo/exo, 7-endo/endo, and 8-endo/endo pathways. The corresponding reactions involving 6-endo/endo and 5-endo/exo pathways were unsuccessful.