High-Resolution NMR Spectra of Polycyclic Hydrocarbons

Abstract

Articles you may be interested inHigh-resolution electron spectroscopy, preferential metal-binding sites, and thermochemistry of lithium complexes of polycyclic aromatic hydrocarbonsThe McWeeny theory has been used to calculate the shielding parameters for polycyclic hydrocarbons up through four rings. These results are presented along with experimental values, some of which come from new analyses of lOO-Mc/sec NMR spectra of these compounds, and semiquantitative agreement is observed. The Jonathan-Gordon-Dailey semiempirical equation for spin-spin coupling constants is also subjected to further test by new data. Absence of inter-ring spin-spin coupling is reconfirmed.

On the magnetic properties of conjugated molecules

“…It seems likely that the differences just referred to will be greatest for points near a bond, i-j, but they may not be so large for more-distant points from that bond ; Salem [26] has investigated this in the case of naphthalene, but it is of interest here to revise the results of the original McWeeny approach for a wide range of condensed, benzenoid molecules (sixteen in all) for which suitable experimental chemical shift data are available [37,38], and which have already been studied from the viewpoint of the original McWeeny method t [23][24][25] and will, therefore, afford comparison ; this we now do. Table 1 shows all the KBS(ri) (' Biot-Savart ') geometric factors required for the calculations on the sixteen molecules studied (nomenclature and protonnumbering as in [37] and [38]) together with the corresponding (S/a s) KmcW(ri) (' McWeeny') geometric factors [36,42], for comparison.…”

“…the ~" which describes the molecule in the presence of the uniform, external, magnetic field only (as, indeed, other approaches have done [10][11][12][13].) However, within the last decade, many calculations have been made [4,[14][15][16][17][18][19][20][21][22][23][24][25] based on the original McWeeny ' test-dipole ' approach, using equations (1) and (3)--involving the computation of relative 'ring current' intensities [4,[14][15][16][17][18][19][20][21] in conjugated molecules, and/or the estimation of secondary fields (and, hence, nuclear screening constants) due to these 'ring currents' [4,19,[21][22][23][24][25]. It is, therefore, of interest to consider the repercussions of this recent critique [3] on the status and validity of these calculations.…”

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.…”

“…As the calculations became theoretically sounder and largely through the influence of the pioneer work of McWeeny [8] the test dipole method has become the favoured one [8][9][10][11][12][13][14].…”

Uncoupled Hartree-Fock calculation of ring currents in substituted benzenes

“…The first attempt to evaluate the ring current contribution to the chemical shift of aromatic protons was made by Pople [2] using a point dipole model. To overcome the semiclassieal approach new theories were developed by Pople [3] and McWeeny [4] within the framework of Hiickel's method, and several calculations were performed using their suggestions [5][6][7][8][9][10]. The first SCF treatment was proposed by Hall and Hardisson [11], who used the coupled Hartree-Fock perturbation theory.…”