Ring Currents and Proton Shielding in Polycyclic Hydrocarbons

Abstract

The ring-current contribution to the shielding constant, σring, is shown to depend on (A·p)Av. In an LCAO perturbation approach σring is then determined as a sum of two terms, each term depending on the same three-center integral. The derivation further indicates that an appropriate partition of the shielding constant in an arbitrary molecule would be σ=σring+σlocal+σ′, corresponding to a current density j=jring+jlocal. The term σ′ arises from (A2)Av. Numerical calculation of the ratio σring/σring (benzene) for the two naphthalene protons gives values 1.39 and 1.21. Estimates of these ratios experimentally are 1.28 and 1.0.

The calculation of chemical shifts in conjugated molecules

“…An approximation which has been used in almost every magnetic calculation on pi electron systems (both non-empirical [4] and semi-empirical [5][6][7][8][9][10][11][12][13][14]) is the London integral approximation which we shall denote by LA (approximation A t Present address: Department of Applied Mathematics, University College of North Wales, Bangor.…”

Extended Hückel calculation of proton chemical shifts with the aid of gauge invariant atomic orbitals. Application to hydrocarbons

“…To account from them by means of simple expressions, we use Mulliken-type approximations [15]. According to the kind of ek ~ and ez ~ orbitals, we distinguished three cases We used an expression deduced from a development given by Parker and Memory [16] to make the ring current term evident. Indeed, a contribution arising from the ring currents of ~r electrons is obtained when, following McWeeny [17] the vector potential of the field induced by the magnetic moment of the proton is introduced in the gauge factor.…”

“…If, as we have done, we take into account in the gauge only the vector potential of the uniform magnetic field, the ring current effect is less obvious. Parker and Memory [16] where Skt is the projection of the signed area of the triangle formed by the proton and the nuclei A and B on the plane perpendicular to the magnetic moment direction.…”

Nuclear magnetic resonance spectrometry

“…Monomer-polymer equilibria are reported for various substituted phenols (837). The use of rigorously dried solvents for carboxylic acids has permitted reliable extrapolation to infinite dilution (1602,1601). The use of rigorously dried solvents for carboxylic acids has permitted reliable extrapolation to infinite dilution (1602,1601).…”