Reflection in the 1H NMR spectrum of 37Cl/35Cl isotope effects on the 19F NMR chemical shifts of 1-chloro-2,4-difluorobenzene. An isotope effect over five bonds

Abstract

In an ABX high-resolution NMR spectrum the detection of combination peaks in the X region yields only the chemical shift of X, JAX + JBX and the positive quantities [Formula: see text]. However, the presence of an additional isotopic perturbation on νA − νB, the difference between the resonance frequencies of A and B, yields two X spectra; therefore four quantities, C. Hence all quantities in the surd become available from the composite X spectrum. A generalization to ABMRX spin systems, applicable to the 1H and 19F NMR spectra of the two isotopic molecules of 1-chloro-2,4-difluorobenzene, is possible. It turns out that, from the H-5(X) spectrum alone, the following spectral quantities are extractable; the two values of νA − νB where A and B are the 19F nuclei; JAB; JAX and JBX with their relative signs; JMX and JRX where M and R are H-3 and H-6. No 37Cl/35Cl isotope effect is detectable on the 1H shielding nor on any coupling constants. F-2 undergoes an isotope shift of −1.64(3) ppb. The isotope shift, over five formal bonds, of F-4 is −0.54(3) ppb (larger shielding in the presence of 37Cl). This magnitude is three times larger than that over four formal bonds in another molecule, 2,6-dichloro-4-fluorophenol. Key words: 1H NMR, of 1-chloro-2,4-difluorobenzene, isotope effects in NMR, reflection of 37Cl/35Cl isotope effects on 19F shielding in the 1H NMR spectrum of 1-chloro-2,4-difluorobenzene; isotope effects in NMR, over three and five bonds by 37Cl/35Cl on 19F shielding in 1-chloro-2,4-difluorobenzene; 19F NMR, 37Cl/35Cl isotope effects over three and five bonds in 1-chloro-2,4-difluorobenzene.