Measuring coupling constants second order spectra

NMR ANALYSIS The 470 MHz HNMR spectra of benzaldehyde before and after the formation of a-cyclodextrin displayed a first-order pattern. The results are summarized in Table I. Spectra of a-cyclodextrin tefore and after inclusion of benzaldehyde cleaily showed a second-order pattern (Figure 7). A rigorous computer spin-simulation allowed us to determine their chemical shifts and coupling constants CTables I and II). The calculated values appear in reasonable agreement with those reported by Wood et al, (2 except for the H-6 protons. The discrepancy which appears is presumably due to the limited resolution of the previously measured 220 MHz HNMR spectrum. A 500 MHz spectrum of a-cyclodextrin recently reported by Yamamoto and hioue (27) is almost identical to our spectnim. However, no attempt was reported to calculate the accurate chemical shifts and coupling constants. 

Ayscough s original value for the rate constant of combination of trifluoromethyl radicals, (23) which was measured by the intermittent photolysis of hexafluoroacetone, has recently been challenged by several new determinations. Ogawa, Carlson and Pimentel (24) using flash photolysis of CFjI coupled with rapid scan infra-red detection found the combination rate constant to be both temperature and pressure dependant with values in the range 5. 9 to 9. 2 x 10 1 mol s". Basco and Hathorn (25) detected the UV absorption spectrum of CFj radicals and measured the rate constant as 3 x 10 1 mol" s at 25°C from its second order decay. Hiatt and Benson,... 

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. ... 

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