An interpretation of ¹H and ¹³C chemical shifts in substituted benzenes on the basis of M.O. charge densities

Abstract

The results of an MOLCAO calculation on both o and T electron systems of several substituted benzenes are reported. The charge densities obtained reproduce the dipole moments of the molecules examined, provided that substituents with strong mesomeric effects are not present. It is shown that there is a satisfactory agreement between lSC and lH chemical shifts and the trend of total charge densities for all positions of substituted benzenes.

Reduction and oxidation of annulenes

“…The gradient K# amounts to about 10 ppm per unit charge. A similar correlation was later found for 13C chemical shifts with Kc « 160 ppm/e 260,263,266,267 The linear shift/ir-charge density relationship proved to be still valid when extended to various cyclic and acyclic ions. 268 It was concluded from the satisfying correlation obtained from average 1SC chemical shifts within species of nonuniform charge distribution that the local ir-charge density is, indeed, the main factor influencing the individual 13C chemical shifts.…”

Uncoupled Hartree-Fock calculation of ring currents in substituted benzenes

“…This fact would not be strictly valid in the case of nonalternant hydrocarbons, and even less so when hetero-atoms are present owing to the large charge separation. The effects of both or-and ~-electron distribution on chemical shifts of aromatic protons was previously discussed by us [24], and according to our conclusions the chemical shift of aromatic protons is mainly determined by the net charges localized on the hydrogen and the adjacent carbon atoms, while other effects should be considered to be of secondary importance. Hence, to rationalize the calculated ring current contributions, simple differences between experimental chemical shifts and these calculated values were taken into account.…”

Stable carbocations. 194. Patterns of carbon-13 chemical shift response to substituent effects in arylcarbenium ions. The phenyl, phenylethynyl, and naphthyl systems

“…[5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] In this paper we describe a procedure applicable to arylcarbenium ions, which first establishes the existence of a systematic dependence of carbon shifts on electronic effects of carbenium-type substituents, before any comparison with MO calculations is made. The success of these comparisons with theoretical quantities is highly dependent on the MO method used.…”