Ragnar A. Hoffman

Study of Moderately Rapid Chemical Exchange Reactions by Means of Nuclear Magnetic Double Resonance

A nuclear magnetic double‐resonance method for the determination of chemical exchange rates has been developed. The method is applicable to systems in which a nuclear spin is reversibly transferred between two nonequivalent sites, A and B. The lifetime (τA) and spin—lattice relaxation time (T1A) in Site A are obtained through the study of the decay to a new equilibrium value of Signal A upon the sudden saturation of Signal B. The converse experiment permits the determination of τB and T1B. A number of data for cross checks are furthermore obtained through the study of the recovery of the signals upon the release of various combinations of saturating rf fields.A simple theory based on the Bloch equations as modified by McConnell to incorporate the effects of chemical exchange is given. Experimental results on the hydroxyl proton exchange in the system salicylaldehyde and 2‐hydroxyacetophenone are well described by this simple theory.The present method, which can readily be extended to systems with several ...

Transient and Steady‐State Overhauser Experiments in the Investigation of Relaxation Processes. Analogies between Chemical Exchange and Relaxation

An exposition is given to demonstrate the close analogies that exist between chemical exchange and relaxation, and it is suggested that relaxation processes may be investigated by a procedure completely analogous to the double‐resonance method recently introduced for the study of chemical exchange. A rederivation of the Solomon equations for the time dependence of the longitudinal magnetizations of interacting spins which brings out the dependence of the relaxation times on the relative abundance of the different spins is given. Experimental results on samples of formic acid and acetaldehyde are presented.

Long range spin-spin interactions in high resolution nuclear magnetic resonance and the concept of hyperconjugation

Indirect spin-spin couplings between methyl groups attached to unsaturated organic molecules and protons directly bonded to unsaturated carbon atoms have been observed in several molecules. An attempt is made to account in a qualitative way for the observed spin-spin couplings by ascribing them largely to hyperconjugation between methyl group orbitals and π-electron orbitals. The connection of these observations with electron spin resonance spectra is mentioned.

The signs of the side-chain spin couplings in furanaldehydes as determined by nuclear magnetic triple resonance

Abstract The signs of the two long-range aldehyde couplings J CHO-4 and J CHO-5 in the 2- and 3-furanaldehydes have been determined with respect to the ring coupling constant J 45 . Only the coupling J CHO-4 in 3-furanaldehyde was found to be of opposite sign to J 45 . A triple resonance method for the determination of relative signs in weakly coupled spin systems is described and the implications of the results are discussed.

Relative signs of nuclear spin coupling constants by means of transitory selective irradiation experiments

Abstract A new method for the determination of energy level arrangements in nuclear spin systems is described. In this method, a transitory selective irradiation (TSI) of some chosen line is used to produce a redistribution of the spin level populations within a time that is short compared with the spin lattice relaxation time T 1 . These population changes give rise to intensity changes for lines which have an energy level in common with the irradiated line—and only for such lines—when the spectrum is recorded within a time that is short compared with T 1 . The application of the TSI method for the determination of the relative signs of spin coupling constants is described. As an illustration the method has been used for the determination of the relative signs of the ethylenic coupling constants in the simple three-spin systems of styrene and vinylbromide. In three-spin spectra with only two observable coupling constants the relative signs of these cannot be determined by double resonance experiments, but with an extension of the TSI method this problem can be solved. In this particular case three transitory irradiations are required. The procedure to be followed is described, and as an application we have studied the case of trans -cinnamic aldehyde where it was found that the two vicinal coupling constants in the side chain have the same sign.

On the use of partly resolved structure for the analysis of NMR spectra. Application to the determination of the signs of the hydroxyl proton-ring proton nuclear spin coupling constants in substituted phenols

Abstract A series of frequency sweep nuclear magnetic double resonance experiments have been performed on 2-hydroxy-3-methoxybenzaldehyde; the strong irradiation field was applied to various parts in the ring proton band and the effects were observed in the hydroxyl band. The transition frequencies and transition probabilities in the double resonance spectra were calculated by numerical diagonalization of the double resonance Hamiltonian of the ABCX subsystem composed of the three ring protons and the hydroxyl proton. The large number of closely spaced transitions (eight transitions within an interval of less than 0.8 cps) necessitated a more detailed evaluation of the line-shapes. Superposition of eight Lorentzian resonance lines produced theoretical spectra in very good agreement with the observed ones and it was found that changes in transition frequencies amounting to a few hundredths of a cps produced significant changes in line-shape. The fact that such marginal effects can be employed for the evaluation of the spectra is attributed mainly to the high reproducibility of the experimental line-shape as obtained with NMR spectrometers employing internal proton stabilization. On the basis of the experiments one may deduce 1. (i) that the resolved spin coupling JOH-4 = 0.62 cps carries the same sign as the ring proton couplings and 2. (ii) that there exists an additional unresolved spin coupling JOH-6 = 0.13 ± 0.05 cps which also carries the same sign as the ring proton spin couplings. The general problems of sign determination of spin couplings and of the estimation of unresolved spin couplings are briefly discussed.

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...