Decoupled bonds

For na = 0, that is, the empty antibonding MO, when X-La channel is given by Scheme 1.1a, the IT bond indices correctly predict the overall IT multiplicity reflecting the six decoupled bonds in this molecular system ... 

Decoupling (Section 13 17) In NMR spectroscopy any process that destroys the coupling of nuclear spins between two nuclei Two types of decoupling are employed in NMR spectroscopy Broadband decoupling removes all the H- C couplings ojf resonance decoupling removes all H- C couplings except those between directly bonded atoms... 

The possible presence in the 4-chloro-4-hexenyl trifluoroacetate of small amounts of two cis-trans pairs of products of addition of trifluoroacetic to the triple bond without concomitant halogen shift remains speculative. In any event these compounds would be removed as ketones upon hydrolysis of the trifluoroacetate. Both the 4-chloro-4-hexenyl trifluoroacetate and the alcohol resulting from its hydrolysis have been shown to contain 9% of the (E) isomer. In the present study the hydrogen decoupled magnetic resonance spectra of the ester and alcohol were shown to contain peaks attributable to approximately 9% of E) isomer. 

Figure 2.4. CH multiplicities of a-pinene (1) [hexadeuterioacetone, 25 C, 50 MHz], (a) h broadband decoupled NMR spectrum (b) DEPT subspectrum of CH (c) DEPT subspectrum of all C atoms which are bonded to H (CH and CH3 positive, CH2 negative) (d) an expansion of a section of (c). Signals from two quaternary C atoms, three CH units, two CH2 units and three CH3 units can be seen...

The pulse sequence which is used to record CH COSY Involves the H- C polarisation transfer which is the basis of the DEPT sequence and which Increases the sensitivity by a factor of up to four. Consequently, a CH COSY experiment does not require any more sample than a H broadband decoupled C NMR spectrum. The result is a two-dimensional CH correlation, in which the C shift is mapped on to the abscissa and the H shift is mapped on to the ordinate (or vice versa). The C and //shifts of the //and C nuclei which are bonded to one another are read as coordinates of the cross signal as shown in the CH COSY stacked plot (Fig. 2.14b) and the associated contour plots of the a-plnene (Fig. 2.14a and c). To evaluate them, one need only read off the coordinates of the correlation signals. In Fig. 2.14c, for example, the protons with shifts Sh= 1.16 (proton A) and 2.34 (proton B of an AB system) are bonded to the C atom at c = 31.5. Formula 1 shows all of the C//connectivities (C//bonds) of a-pinene which can be read from Fig. 2.14. 

Figure 15 shows the normal broad-band decoupled and gated decoupled spectra of compound 1 in the latter we can see the multiplets arising from C-H coupling (across one or more bonds) and C-P coupling. The rules for the number of lines in a multiplet and their intensities are the same as for protons, since 13C and 31P are both spin-Vi nuclei. 

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