High-Resolution NMR Spectra of Polycyclic Hydrocarbons. II. Pentacyclic Compounds

Abstract

The McWeeny theory has been used to calculate chemical shielding parameters for all even, alternant, pentacyclic hydrocarbons, and comparison with experiment was made where data were available. The spectra of two pentacyclic molecules, 1,2,3,4-dibenzanthracene and picene, not reported previously, were obtained and analyzed, and a complete analysis of the 1,2-benzpyrene spectrum is given. Agreement between theory and experiment is satisfactory for protons not subject to large van der Waals contact, and it is found that steric effects contribute an extra downfield shift of 0.54 ppm to angular phenanthrenic type protons. Additions and new interpretations are made for previously reported results of similar data on three- and four-ring molecules.

Proton chemical shifts in NMR. Part 14. Proton chemical shifts, ring currents and π electron effects in condensed aromatic hydrocarbons and substituted benzenes

“…This was confirmed both experimentally and theoretically. In contrast it is immediately obvious from both the results of previous investigations 7, 13 and the data presented here that proton-proton steric interactions in the aromatic systems considered here give rise to deshielding effects on the proton chemical shifts. A further unambiguous demonstration that steric effects on proton chemical shifts in aromatic systems are totally different from those in saturated systems came from the observation of the proton chemical shift of the unique CH proton in the cyclophane (15).…”

“…(12) 21 for a similar set of substituents to those SCS (para) = 0.27σ I ϩ 1.25σ R 0 SCS (meta) = 0.24σ I ϩ 0.446σ R 0 (12) in Table 5 and a similar analysis of the SCS in terms of the Swain-Lupton F and R values gives eqn. (13). SCS (para) = 0.142F ϩ 0.926R SCS (meta) = 0.098F ϩ 0.376R (13) These equations are reasonably consistent implying in general a much greater resonance effect on the para proton SCS than on the meta proton SCS.…”

“…(13). SCS (para) = 0.142F ϩ 0.926R SCS (meta) = 0.098F ϩ 0.376R (13) These equations are reasonably consistent implying in general a much greater resonance effect on the para proton SCS than on the meta proton SCS. Inspection of the data in Table 5 shows a much more diverse pattern.…”

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.…”