Effect of electron lone‐pairs on nuclear spin–spin coupling constants

Abstract

The nature and types of lone-pair effects on nuclear spin coupling constants are reviewed in the context of a localized bond description of molecular electronic structure. Emphasis is placed on the importance of residual delocalization involving the otherwise lone-pair orbital, in terms of which the effect of an X lone-pair, when compared with an isoelectronic Y-H or X-H+ group or with an X-R group, and its orientational dependence can be interpreted. One-, two-and three-bond coupling constants are considered and the importance of lone-pair effects for configurational and conformational information is stressed and illustrated. They also serve for a better understanding of substituent inductive effects on coupling constants.A large collection of illustrative examples are presented, with particular attention paid to couplings involving H, C, N, F and P nuclei, organized in a systematic manner into nine categories. The signs of the lone-pair effects on the reduced coupling constants are found to be independent of the actual nuclei under study in the same category. This is taken as an indication that the electron lone-pairs mainly affect the Fermi contact contribution to the coupling and, accordingly, an interpretation is given in terms of simple sum-over-states models. In addition, symmetry-based relationship are established involving the sign of lone-pair effects in coupling constants between nuclei which are a different number of bonds apart.KEY WORDS NMR Multinuclear spin coupling Lone-pair effects Substituent effects Stereochemistry MO Sum-over-states models Electron delocalization CONTENTS

The Effect of Substituents on Indirect Nuclear Spin-Spin Coupling Constants: Methan- and Ethanimine, Methanal- and Ethanaloxime

“…This orientational lone pair effect on geminal couplings was probably first observed in propanaloxime [88] and was later categorized as B.3 by Gil and Philipsborn [19]. It is much stronger than the corresponding effect on the one bond couplings in Table 1 angle.…”

“…For methanimine our theoretical difference, behind these changes should be the mesomeric +M effect of the OH group (see Figure 4), because the pure inductive −I effect of an electronegative substituent like OH [77] should reduce the C-H Y couplings [19]. However, the OH substituent effect is smaller, if one keeps the angles in the H 2 C moiety fixed (see Table 2), which indicates that there is also a small geometrical contribution.…”

“…In particular we were interested in the possible interplay of the stereoelectronic effects of the nitrogen lone pair [17][18][19][20][21], e.g. the so-called anomeric [22,23] or negative hyperconjugation effect [24], the positive inductive effect of the methyl group and the positive conjugation effect of the hydroxyl group.…”

Optimized basis sets for the calculation of indirect nuclear spin-spin coupling constants involving the atoms B, Al, Si, P, and Cl

“…The decrease observed in the FC term is a well known effect of the lone pairs of fluorine (see Ref. 246,247 and therein cited references). As a consequence also the total reduced coupling constants of the hydrides are positive and negative for the fluorides with the exception of ClF, because it is dominated by the large and positive PSO and SD contributions.…”

Indirect Nuclear Spin-Spin Coupling Constants 1J(17O,11B). First Observation And Calculation Using Density Functional Theory (Dft)

“…The presence of lone pairs of electrons at the oxygen atom should give rise to pronounced negative contributions to the (reduced) Fermi-contact term [37]. This is obvious in the absence of BO(pp)π interactions, e.g.…”