Magnetic Rotation Spectroscopy spectrum

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). 

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. 

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. 

This anisotropy also poses a challenge to the analysis of crystals by nuclear magnetic resonance spectroscopy, which is covered in Section 5.5. Each change in orientation of a crystal in the spectrometer corresponds to a different angle between the external magnetic field and the chemical bond axes, and a different spectrum. The rapid and random motions of molecules in a liquid average over these differences to yield narrow lines in the spectrum. To duplicate the same effect in crystals, the crystal is powdered and the sample is then rotated at rates of 10 kHz or faster. 

In order to combat this, the rotating frame Overhauser effect spectroscopy (ROESY) techniques can be employed. An in-depth discussion of how this technique works is outside the remit of this book but suffice to say, in the ROESY methods (1- and 2-D), NOE data is acquired as if in a weak r.f. field rather than in a large, static magnetic field and this assures that all NOEs are present and positive, irrespective of tumbling rate and magnet size. It is possible that some TOCSY correlations can break through in ROESY spectra but these will have opposite phase to the genuine ROESY correlations and so should therefore not be a problem - unless they should overlap accidentally with them. A 2-D ROESY spectrum of the naphthalene compound is shown below (Spectrum 8.6). 

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