What is Double Resonance

In Chapters 2 and 3 we described the basic components of an NMR spectrometer and how an NMR signal is generated. Before proceeding further you might want to review the relevant parts of Sections 2.3-2.5 and 3.4-3.6. To briefly summarize these sections, we can describe the key steps in signal generation as follows  

Passage of a brief but powerful pulse of rf current through the transmitter coil (whose axis lies in the xy plane) gives rise to a secondary oscillating magnetic field (Bi) perpendicular to and precessing around B0. The frequency of B is set to match the precessional frequency of the specific isotope (e.g., I3C) to be observed. 

After B, is turned off, each nonzero component of M in the x y plane (that is, precessing in the xy plane) gives rise to an induced rf signal (which is a summation of all contributing precessional frequencies) in the receiver circuit. This exponentially decaying FID sig- 

Fourier transformation of the FID signal produces the conventional frequency-domain spectrum, a plot of signal intensity versus frequency, with a line for each contributing precessional frequency of the observed nuclei. 

When the two sets of nuclei are of different isotopes (e.g.,, 3C and H), the experiment is described as heteronudear double resonance. When we observe the signal from nuclei of isotope A while irradiating nuclei of isotope B, we label the resulting spectrum with the shorthand designation A B. When the two sets of nuclei belong to the same isotope (e.g., both 1H), the technique is described as homonuclear double resonance. 

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Cyclohexatriene to benzene displays a sequence of structures from 1,3,5-cyclohexatriene (withCC single and double bonds initially set to 1.5 and 1.3 A, respectively) to benzene (witb all CC bonds set to 1.4 A) and back to cyclohexatriene. Plot energy (vertical axis) vs. CC bond length (horizontal axis). How many energy minima are there Do the minima look more like 1,3,5-cyclohexatriene or benzene What is the correct interpretation of the resonance picture ... 

The values in Table 2.4 show how resonance affects the strengths of bonds. For example, the strength of a carbon-carbon bond in benzene is intermediate between that of a single and that of a double bond. Resonance spreads multiple bond character over the bonds between atoms as a result, what were single bonds are strengthened and what were double bonds are weakened. The net effect overall is a stabilization of the molecule. 

The intramolecular n-electron donation from the remote double bond to the stannylium ion center in the stannanorbornyl cation, 62 (8 Sn = 336 in CeDg), also leads to a significant shielded tin nucleus compared to what is expected for a free trialkylstannylium ion. As already shown for the sila compounds 55-58, the coordination of the C = C double bond to the electron-deficient tin center in 62 is indicated by a low-field shift of the resonance of the vinylic carbon atoms by 11.1 ppm. Additionally, the small scalar Sn-C coupling constant J(SnC) = 26 Hz gives clear evidence for the intramolecular Ti-complexation." ... 

Second, the chemical shift can be recognized by the fact that it is directly proportional to the transmitter frequency, v. If we double v, the chemical shifts double. In contrast, the first-order spin-spin splittings remain the same. By this we mean that the magnitude (in Hz) of the spacing between the lines of a split resonance is independent of the transmitter frequency, v. This spacing corresponds to what is called the spin-spin-coupling constant, or simply the coupling constant, and is symbolized by J. 

In these experiments, there was no double-resonance tagging, so all of the HI moieties were available for photodissociation. What is observed experimentally is the resulting atomic hydrogen time-of-flight distribution which is obtained by using the HRTOF technique (Ashfold et al. 1992 Schneider et al. 1990 Wen et al. 1994). This method provides excellent resolution and S/N compared to other TOF methods. The dominant features (solid curves) are due to HI monomer. However, upon magnification, features are seen that are due to... 

We will use resonance often, so it is important to understand exactly what it means. Resonance does not mean that the structure of the molecule is changing back and forth between the structures in the case of nitromethane. It does not mean that the bond to one of those oxygens is a double bond part of the time and a single bond the rest of the time. Instead, the structure is static, an average of the individual structures. The doubleheaded resonance arrow is used only for resonance and should not be con fused with Important the equilibrium arrows, which are used to represent an equilibrium reaction in which... 

ENDOR (Electron Nuclear Double Resonance) involves the simultaneous application of a microwave and a radio frequency signal to the sample. This is a technique invented by Feher in 1956. The original studies were on phosphorous-doped silicon. A description of the experimental results and apparatus used is presented in two Physical Review articles [24, 25], An excellent treatment of EPR double resonance techniques and theory is given in the book by Kevan and Kispert [26], What follows here is the theory and application of ENDOR used the in analysis of single crystal data with the goal of identifying free radical products in DNA constituents. 

EXAMPLE 12.2 Suppose we were to reexamine the 250-MHz H spectrum of diethyl ether (Section 8.2 and Figure 8.1). But this time we will observe the methyl triplet while we irradiate the methylene quartet, (a) Does this experiment involve homo- or heteronuclear double resonance (b) What is the value of Av in this case ... 

Find the range of resonance frequencies over which a 90° pulse of 20 /is is effective (i.e., insensitive to resonance offset). If the pulse width is doubled, with B, remaining the same, what would you observe for a spectral line that is on resonance ... 

The structural formulas used to represent molecules are based on valence bond theory. Double and triple bonds simply represent additional pairs of shared valence electrons. But structural formulas, while useful, don t tell the whole story about the nature of the bonds between atoms in a molecule. Valence bond theory falls flat when it tries to explain delocalized electrons and resonance structures. To get at what is really going on inside molecules, chemists had to dig deeper. 

In this review we have attempted to give a concise account of the basic principles and applications of nuclear electron double resonance, or as it is sometimes called the nuclear electron Overhauser effect. We have also tried to illustrate to the non-specialist the progress that has been made in this subject and indicate what remains to be done. 

The lone pair on the hydroxide oxygen becomes a new carbon-oxygen bond. The octet mle requires that one of the electron pairs in the double bond be changed to a lone pair. What is the other resonance form of the product ion ... 

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