Multiplet system

This explains why the relative intensity of the methyl lines is 1 2 1. Spin-spin coupling is always a reciprocal process - if protons x couple to protons y , then protons y must couple to x . The possible alignments of the methyl protons (which we will call He, HD and HE) relative to the methylene protons are also shown in Spectrum 1.2. Think about the orientations of protons responsible for multiplet systems as we meet them later on. 

As was illustrated for the methyl and carbonyl signals in the 13C NMR spectrum of acetone (Fig. 1.10), 13C— H spin-spin coupling vanishes when proton broadband decoupling is applied. Proton broadband decoupling is the most important decoupling technique used in routine 13C NMR, simplifying 13C — H multiplet systems to spectra of up to z singlet lines for z nonequivalent 13C nuclei of a sample. 

The splitting (signal multiplicity) of H resonances often reveals the spatial proximity of the protons involved. Thus it is possible to identify structural units such as those which often occur in organic molecules simply from the appearance of multiplet systems and by using the + 1 rule. 

As to the correlation energy, all integrals in Eq. (11) are over non-correlated orbitals. It is true that by taking the derivative of the average of a multiplet system we are implicitly by-passing the limitation of the spin-correlated Hartree-Fock single determinantal formalism, but with so many approximations involved in the SCF—Ajj, method it is not easy to judge how important is this particular feature. 

We will now summarise the results. We start with two atoms with m and m electrons respectively the azimuthal quantum numbers are I and the multiplet systems r and r, the reflection characters w and /. For different atoms we obtain the A given in (20) for the molecule, each with each i2-value, the multiplet system of (19). For identical atoms in different states, we have double the number of terms all A from (20) appear as A+ and A.. If the atoms are in the same states, then for each combination of a A from (20 ) with an R from (19 ) we have only one term those which are A+ terms and those which are A terms can be derived from (20 a). These are the group-theoretical possible terms. 

Figure A3.13.10. Time-dependent probability density of the isolated CH clnomophore in CHF. Initially, tlie system is in a Fenni mode with six quanta of stretching and zero of bending motion. The evolution occurs within the multiplet with chromophore quantum number A = 6 = A + 1 = 7). Representations are given...

Four multiplets between Sh = 7.46 and 9.18 indicate monosubstitution of the pyridine ring, either in the 2- or 3-position but not in the 4-position, since for a 4-substituted pyridine ring an AA XX system would occur. The position of the substituents follows from the eoupling eonstants of the threefold doublet at Sh = 7.46, whose shift is appropriate for a p-proton on the pyridine ring (A). 

Hence one finds two overlapping pseudotriplets (Sn = 3.41 and 3.44) for the NC// group which appears only once and two overlapping quartets (Sm = 3.22 and 3.23) for the NC/f groups which appear twice. Since the shift differences of the CH2 protons are so small, the expected AB systems of the coupling partners approximate to A-i systems at 200 MHz thus one observes only the central multiplet signals of these AB systems. 

This type of polarization appears to be general in such systems and it has been proposed that it arises because of T i-S mixing. An interesting and important addif ional observation comes from a study of the reaction of isopropyl chloride with sodium naphthalene in a field of 60 G. Net emission in the products is predicted for T i-S mixing and multiplet polarization for Tq-S mixing. Since no multiplet polarization can be detected, it would seem that Tq-S mixing is suppressed in this system (Garst et al., 1971). Presumably, the naphthalene radical anion... 

A radical pair pathway has been identified for the conversion of 29 into 30 as indicated by the E/A multiplet due to the A2B2 system in the amine side chain at S 2-5-3-0 ppm (Baldwin and Brown, 1969 for CIDNP in the formation of a methylenecyclohexadiene, see Bethell et al., 1972c). The low enhancement factor (5-20) at maximum polarization suggests that a concerted [1,3] sigmatropic rearrangement may occur concurrently. 

A homonuclear spin-system may be excited with radiofrequency (r.f.) pulses that are so Intense (in the order of p.s), compared to the frequency width of the spectrum, that all resonances are excited essentially uniformly. This is a nonselective excitation. A homonuclear spin-system may also be excited with a relatively weak, r.f. pulse (in the order of ms), in the sense that all components of a given multiplet are inverted at time zero, whereas the other resonances in the spectrum remain essentially unperturbed this is a selective excitation. The r.f. pulse may be single-selective, that is, there is an inversion of one multiplet in the spectrum, or double-selective, triple-selective, and so on, where two, three, or more separate multiplets in the spectrum are inverted simultaneously while the remaining resonances remain unperturbed. 

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