Intramolecular van der waals interactions and chemical shifts: A model for β‐ and γ‐effects

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A correlation has been discovered between the local steric van der Waals energy as calculated by the MM2 force field and the chemical shift of a resonant nucleus within classes of saturated aliphatic phosphines, amines, hydrocarbons and monofunctional alcohols. Root‐mean‐square errors found for 37 phosphorus, 35 nitrogen and 164 carbon nuclei are 10.9, 5.9 and 2.8 ppm, respectively. It is found that repulsive van der Waals interactions are associated with deshielding effects in the observed chemical shifts, and attractive interactions with smaller shielding effects. A simple physical picture emerges which describes both β‐ and γ‐effects under one common model. It is found that β‐effects are associated with repulsive interactions while, contrary to earlier approaches γ‐effects are due basically to attractive interactions.