Proton coupling constants Appendix

The 500 MHz H-NMR of the primary organozinc iodides 44a and 44b have been reported . The methylenic protons a to the zinc atom occur as the AB part of an ABC spin system, indicating slow inversion rates. Applying equation 34 (see Appendix, Section IV.A) to the given chemical shifts and coupling constants, a lower limit for the free activation energy can be established as AG > 15 kcalmol" in DMF-rfv or THF-rfg at 25 °C. No further attempts to approach closer to the coalescence temperature were undertaken (equation 26). 

Values for coupling constants between protons and other atoms are given in Appendix 3, Table A3.8. 

The 31P nucleus has a natural abundance of 100% and a spin number of 1/2 (therefore no electrical quadrupole moment). The multiplicity rules for proton-phosphorus splitting are the same as those for proton-proton splitting. The coupling constants are large (/H—p 200-700Hz, and /Hc p is 0.5-20 Hz) (Appendix F) and are observable through at least four bonds. The 31P nucleus can be observed at the appropriate frequency and magnetic field (Chapter 6). 

An example of an aromatic fluorine-containing compound can be found in Figure 6.7, where we have recorded the l9F spectra (both proton-coupled and decoupled) of fluorobenzene along with the H and l3C spectra. Once again we find a singlet for the fluorine atom in the ptoton-decoupled spectrum and a complex multiplet for the fluorine atom in the proton-coupled spectrum. The fluorine atom couples differently to the ortho-, meta-, and para-protons in this mono-substituted compound. Coupling constants for proton-fluorine can be found in Appendix F of Chapter 3. 

One important question still remains Are the double bonds E (trans) or Z (cis)l This question can be answered if the olefinic proton J values can be determined. One obvious starting point is the H-2 doublet, which is the result of coupling to H-3. The J value is about 16 Hz this coupling constant falls within the range given for E-double bonds given in Appendix F, Chapter 3. 

H chemical shifts are another matter, since the signals of most protons are multiplets. Strictly speaking, spectral analysis is required to determine the chemical shifts and coupling constants of those protons whose signals display second-order behavior (Chapter 4 and Appendix 4). For those protons whose spectra are first order or even pseudo first order, chemical shifts and couplings can be determined from the spectra by inspection, but some care should be exercised in this endeavor. 

Deuterium (D or 2H) usually is introduced into a molecule to detect a particular group or to simplify a spectrum. Deuterium has a spin number of 1, a small coupling constant with protons (see Appendix G), and a small electrical quadrupole moment. The ratio of the 7 values for HH to those of HD is about 6.5. 

The magnitude of J often provides stmctural clues. For instance, one can usually distinguish between a cis olefin and a trans olefin on the basis of the observed coupling constants for the vinyl protons. Table 3.9 gives the approximate values of some representative coupling constants. A more extensive list of coupling constants appears in Appendix 5. 

Draw the structure of this compound and assign each of the protons in your structure. The coupling constants should help you to do this (see Appendix 5). 

The proton NMR spectra for methyltriphenylphosphonium halide and its carbon-13 analogue are shown in this problem. Concentrating your attention on the doublet at 3.25 ppm and the pair of doublets between 2.9 and 3.5 ppm, interpret the two spectra. You may need to refer to Appendix 5 and Appendix 9. Estimate the coupling constants in the two spectra. Ignore the phenyl groups in your interpretation. 

Determine the structure for a compound with formula C3H5CIO. The IR spectrum, NMR, NMR, DEPT, COSY, and HETCOR (HSQC) spectra are included in this problem. The infrared spectrum has a trace of water that should be ignored (region from 3700 to 3400 cm ). The HETCOR spectrum should be carefully examined, for it provides very important information. You will find it helpful to consult Appendix 5 (alkanes and cyclic alkanes) for values of coupling constants. Determine the coupling constants from the NMR spectrum, except for proton c, and compare the calculated values to those shown in Appendix 5. Draw the structure of the compound, and label the protons on the structure. 

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