Coupling constant measuring first-order spectra

More complex spin-spin interactions. As the chemical shift difference becomes similar to the value of the coupling constant, the first-order analysis breaks down completely. It is then not possible to measure either the chemical shift or the coupling constants directly from the spectrum and resort must be made to more rigorous analytical procedures which are beyond the scope of this book. However, there are available a variety of computer programs specifically designed to analyse complex n.m.r. spectra.3 1... 

When one endeavors to measure the coupling constants from an actual spectrum, there is always some question of how to go about the task correctly. In this section, we will provide guidelines that will help you to approach this problem. The methods given here apply to first-order spectra analysis of second-order spectra is discussed in Section 7.7. What does first-order mean, as applied to NMR spectra For a spectrum to be first-order, the frequency difference (Av, in Hz) between any two coupled resonances must be significantly larger than the coupling constant that relates them. A first-order spectrum has Av/7 > 6. ... 

A) Measure the positions and amplitudes of all the lines in the spectrum and list them in order in a table (a spreadsheet program is convenient for this purpose). A well-defined measure of position in a complex spectrum is the x-axis point halfway between the maximum and minimum of the first-derivative line. The amplitude is the difference in height between the maximum and minimum. If convenient, measure the line positions in gauss if this is inconvenient, use arbitrary units such as inches, centimeters, or recorder chart boxes measured from an arbitrary zero. In your table, also provide headings for the quantum numbers (m1 m2, etc.) for each of the line positions, for the coupling constants (a, a2, etc.), and for the theoretical intensity (degeneracy) of each peak. 

The spin-lattice relaxation times of essentially all the protons in vindoline, measured by a Fourier transform method, show values reflecting the degree of steric interaction with other protons. (276) A wealth of coupling constant data has been made available by first order analysis of the 300 MHz NMR spectrum of vindolinine [458]. (277) H NMR shifts for the metabolite of vindoline [459] (278) are shown on the structure. 

Computer programs for simulation of proton NMR spectra are available. If accurate measurements of chemical shifts and coupling constants for all of the protons can be obtained, the simulated spectrum will be congruent with the actual spectrum. In many cases, at least some of the spin systems will be first order. If not, reasonable estimates of shifts and coupling constants may be made, and the iterative computer program will adjust the values until the simulation matches the actual spectrum—assuming, of course, that the identification is valid. 

In order to obtain Xu we need the first overtone of v2. This was measured for ether. HF and acetone. HF systems by Bevan, Martineau and Sandorfy41). In addition to obtaining Xn it was hoped to find subbands of the (2vj + vn) type since these must yield the same Xlo coupling constants as the subbands of the Vj fundamental. Figure 7 shows the overtone spectrum of (CH3)2O.HF and the relevant data for this and other similar systems are collected in Table 2. 

---