Magnetic Rotation Spectroscopy

Other methods for dealing with a complex spectrum include simplifying it by cooling the molecule to 5 K in a supersonic jet (Section 1.2.3) or selectively enhancing the relative intensities of certain low-7 rotational lines (Magnetic Rotation Spectroscopy, Section 1.2.5). 

No molecule is completely rigid and fixed. Molecules vibrate, parts of a molecule may rotate internally, weak bonds break and re-fonn. Nuclear magnetic resonance spectroscopy (NMR) is particularly well suited to observe an important class of these motions and rearrangements. An example is tire restricted rotation about bonds, which can cause dramatic effects in the NMR spectrum (figure B2.4.1). 

Nuclear magnetic resonance spectroscopy of the solutes in clathrates and low temperature specific heat measurements are thought to be particularly promising methods for providing more detailed information on the rotational freedom of the solute molecules and their interaction with the host lattice. The absence of electron paramagnetic resonance of the oxygen molecule in a hydroquinone clathrate has already been explained on the basis of weak orientational effects by Meyer, O Brien, and van Vleck.18... 

I. N. Levine (1975) Molecular Spectroscopy (John Wiley Sons, New York). A survey of the theory of rotational, vibrational, and electronic spectroscopy of diatomic and polyatomic molecules and of nuclear magnetic resonance spectroscopy. 

Stemhell, S. Rotation about Single Bond in Organic Molecules." In Dynamic Nuclear Magnetic Resonance Spectroscopy, Jackman, L. M. Cotton, F. A., Eds. Academic Press New York, 1975 pp. 163-201. 

Initially, stereospecific analyses were done by Pitas et al. (1967) on whole milk fat and by Breckenridge and Kuksis (1968) on a molecular distillate of butter oil. They indicated that the short chain acids were selectively associated with the sn-3 position. In the butter oil distillate, over 90% of the TGs contained two long-chain and one short-chain fatty acids. This asymmetry has been confirmed by the observation of a small optical rotation of the TGs (Anderson et al. 1970), by proton magnetic spectroscopy (Bus et al. 1976), and by nuclear magnetic resonance spectroscopy (Pfeffer et al. 1977). Pfeffer et al. found 10.3 M% 4 0 (butyric) in the oil and determined that 97% of the acid was in the sn-3 position. It is worth noting that the analysis was done without alteration or fractionation of the oil. 

Electron spin resonance (e.s.r.) spectroscopy, applied to free radicals in condensed phases, is a long established technique with several commercially available spectrometers. The gas phase applications we will describe have little in common with condensed phase studies, and are much more a part of rotational spectroscopy. However, the experimental methods used for condensed phase studies can be applied to the study of gases with rather little change, so it is appropriate first to describe a typical microwave magnetic resonance spectrometer, as illustrated schematically in figure 9.1. 

In order to assign the Zeeman patterns for the three lowest rotational levels quantitatively, one must determine the spacings between the rotational levels, and the values of g/and gr-In the simplest model which neglects centrifugal distortion, the rotation spacings are simply B0. /(./ + 1) this approximation was used by Brown and Uehara [10], who used the rotational constant B0 = 21295 MHz obtained by Saito [12] from pure microwave rotational spectroscopy (see later in the next chapter). The values of the g-factors were found to be g L = 0.999 82, gr = —(1.35) x 10-4. Note that because of the off-diagonal matrix elements (9.6), the Zeeman matrices (one for each value of Mj) are actually infinite in size and must be truncated at some point to achieve the desired level of accuracy. In subsequent work Miller [14] observed the spectrum of A33 SO in natural abundance 33 S has a nuclear spin of 3/2 and from the hyperfine structure Miller was able to determine the magnetic hyperfine constant a (see below for the definition of this constant). 

The main conclusion from these results is that the observed hyperfine splitting is determined primarily by a linear combination of the hyperfine constants corresponding to the three separate interactions. The spectrum depends upon the axial component ofthe total magnetic hyperfine interaction, which we designate / 3/2 (= a + (1 /2)(b + 2c/3)), and in a good case (a) system it is not usually possible to separate the individual contributions from the microwave magnetic resonance spectrum alone. The solution to the problem lies in the combination of these studies with pure rotational spectroscopy, as we shall see later in this chapter. 

Compared with laser magnetic resonance techniques, pure rotational spectroscopy has advanced to more complex spin and orbital systems more slowly, probably because it... 

Carbohydrates in nature are optically active and polarimetry is widely used in establishing their structure. Measurement of the specific rotation gives information about the linkage type (a or (3 form) and is also used to follow mutarotation. Nuclear magnetic resonance spectroscopy (NMR) can be used to differentiate between the anomeric protons in the a- or /3-pyranose and furanose anomers and their proportions can be measured from the respective peak areas. 

One other pyridine alkaloid has been detected in dendrobatid frogs. The structure of this minor alkaloid, noranabasamine (XIII), was established by proton and carbon-13 magnetic resonance spectroscopy (14). The ultraviolet spectrum was as follows X ,ax (CH3OH) 244 nm, e 11,000, 275 nm, e 10,000. The optical rotation, [a]o, was -14.4° (CH3OH). Anabasamine, a plant alkaloid, also is levorotatory, but it is unknown whether noranabasamine, now given a code number 239J, has the same 2S configuration. 

Chapter 14 has been modified significantly. Material has been added on the phases of atomic orbitals and the orbital art has been modified to include signs in the lobes. This approach makes it easier for students to understand how bonding and antibonding molecular orbitals result from the linear combination of atomic orbitals. Also, the treatment of spectroscopy in Chapter 14 has been greatly expanded in response to requests by users. There are new sections on electronic, vibrational, and rotational spectroscopy. A new section on magnetic resonance spectroscopy has been added. 

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