Soren Rodmar

An NMR study of 1-indanone. Analysis of A2B2 spectra

Abstract The A 2 B 2 NMR spectra of the aliphatic protons of 1-indanone at 60 Mc/sec and at 100 Mc/sec have been analyzed with the particular aim of obtaining accurate values of the geminal spin coupling constants. It is found that these couplings are of the same sign and similar magnitude as those obtained in structurally similar molecules, in contradiction with recently published results. It has been found that the large shift A 2 X 2 approximation incorporating generalized second-order perturbation terms can provide accurate estimations of the NMR parameters even in a spectrum like that of 1-indanone at 60 mc/sec where certain lines in the two half spectra originate in transitions assigned to the nuclei of the opposite half spectrum. Variations in the spectra produced by variations in the relative chemical shift are discussed with particular reference to the noncrossing rule and to the problem of the labelling and assignment of “mixed” transitions.

Nuclear magnetic double resonance in an ABCX spin system: The sign of the long-range aldehyde coupling in benzaldehydes

Abstract The sign of the side-chain spin coupling JCHO-3 in 2-hydroxy-5-methoxy-benzaldehyde has been determined relative to that of the ring coupling constants by use of a nuclear magnetic double resonance technique. Strong non-selective double irradiation was applied in the ABC-part (the ring hydrogen band) of the spectrum to produce a sign dependent asymmetry in the X-part (the aldehyde band) of the spectrum. The results were interpreted with the aid of theoretical double resonance spectra calculated by complete numerical diagonalization of the double resonance Hamiltonian matrix. It was found that JCHO-3 carries the same sign as the ring coupling constants.

Proton magnetic resonance of symmetrical molecules

Methods have been developed for the analysis of large-shift A2B2 spectra by perturbation techniques. Modified first-order expressions for the A2X2 transition frequencies are given and two alternative types of second-order correction terms are added. For the first kind of expression the somewhat restrictive conditions pertain that all the coupling constants are to be small as compared to the relative chemical shift, whereas the second kind of expression was derived on the more general assumption that only the coupling constants between chemically shifted nuclei are small. The expressions for the relative intensities have been supplemented with first-order correction terms (i.e. terms on the order of L/(v A − v B) and N/(v A − v B)). As a useful alternative, the very simple expressions obtained in the (AX)2 approximation may be used whenever one of the coupling constants between the chemically shifted nuclei is large in comparison with the other couplings in the spin system. Expressions for the transition f...

Molecular orbital calculations of 19F N.M.R. chemical shifts in fluorothiophenes

The 19F N.M.R. chemical shifts in 4- and 5-substituted 2-fluorothiophenes and in 5-substituted 3-fluorothiophenes have been calculated from various approximate expressions for the magnetic shielding of a nucleus for a first-row atom. The different expressions correspond to different approximations of the equation for the paramagnetic tensor obtained from coupled Hartree-Fock theory by Lipscomb and others. The molecular wave functions have been calculated with the CNDO/2 method. The full perturbation expansion fails to reproduce the experimental shifts. With the average energy approximation introduced for the paramagnetic term good correlations are obtained between experimental and calculated shifts from ‘local’ contributions for the 4- and 5-substituted 2-fluorothiophenes (correlation coefficients 0·85 and 0·91, respectively), whereas the same correlation for the 3-fluorothiophenes is poorer (correlation coefficient 0·76). In a calculation including contributions from all atoms in the molecule a significa...