Proton Magnetic Resonance Spectrometry

Nuclear magnetic resonance (NMR) spectrometry is basically another form of absorption spectrometry, akin to IR or UV spectrometry. Under appropriate conditions in a magnetic field, a sample can absorb electromagnetic radiation in the radio frequency (rf) region at frequencies governed by the characteristics of the sample. Absorption is a function of certain nuclei in the molecule. A plot of the frequencies of the absorption peaks versus peak intensities constitutes an NMR spectrum. This chapter covers proton magnetic resonance ( H NMR) spectrometry. 

With some mastery of basic theory, interpretation of NMR spectra merely by inspection is usually feasible in greater detail than is the case for IR or mass spectra. The present account will suffice for the immediate limited objective identification of organic compounds in conjunction with other spectrometric information. References are given at the end of this chapter. 

Relevant properties, including the spin number /, of several nuclei are given in Appendix H. The spin number / can be determined from the atomic mass and the atomic number as shown in the next column. 

Spectra of several nuclei can be readily obtained (e.g., H, H, JC, N, gF, P) since they have spin numbers / of 2 and a uniform spherical charge distribution (Fig. 4.1). Of these, by far the most widely used in NMR spectrometry are H (this chapter) and L,C (Chapter 5). 

Nuclei with a spin number 1 of 1 or higher have a non-spherical charge distribution. This asymmetry is described by an electrical quadrupole moment which, as we shall see later, affects the relaxation time and, consequently, the linewidth of the signal and the coupling with neighboring nuclei. In quantum mechanical terms, the spin number I determines the number of orientations a nucleus may assume in an external uniform magnetic field in accordance with the formulas 2/ + 1. We are concerned with the proton whose spin number /is 

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Derivatives of 6,7-dihydro-5//-1 -pyrindine (10) comprise the largest group of 1-pyrindines. Studies of the basic nitrogen compounds in the 120°-375° petroleum distillate fractions via chemical reactivity and infrared, mass, ultraviolet, and proton magnetic resonance spectrometry have led to the separation and identification of many derivatives of 10 2> 3> 5 8,56-66... 

A1-Badr and Ibrahim (28) described a simple, rapid and accurate method for the assay of the drug and other hypoglycemic agents using proton magnetic resonance spectrometry. The pure drug or in tablet form, can be determined using DMSO-de as solvent and maleic acid as internal standard. 

Proton magnetic resonance spectrometry has proved useful for substances whose ultraviolet spectra do not change upon ionization and which are too weak, as acids or bases, or too poorly soluble to be measured by potentiometry. In practice, a series of solutions of known pH or acidity function (see p. 47) arc prepared, and the chemical shift of a non-exchanging proton, near to the ionizing group, is plotted against pH.. As an example, the plot of oxazole (7 T) is showm in Fig. 1.2 this gave a pKj of approximately 0 8 (Brown... 

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