A study of the N-inversion barrier and the circular dichroism spectra of 1-thia-10-aza[2.2]metacyclophane

Abstract

The inversion barriers and the excited singlet electronic states of 1-this-10-aza[2.2]metacyclophane (X = NH 1) and two isovalent compounds (X = NCH32, X = PH 3) have been calculated with semiempirical AM1 and multireference configuration interaction (MRD-CI) methods. Owing to the inversion at the heteroatom N or P, all compounds investigated exhibit two different minima which are separated energetically by less than 1 kcal mol–1. The inversion barriers are calculated to be 5.5 kcal mol–1(X = NH), 4.5 kcal mol–1(X = NCH3) and 15.6 kcal mol–1(X = PH), respectively.Good agreement between the experimental and calculated circular dichroism (CD) spectra is obtained for the X = NH compound (1). The various bands can be characterized as separate and coupled ππ* transitions localized in the aromatic rings and by valence-type excitations involving the lone-pair orbital of the sulfur atom (3p) and/or antibonding σ*-MOs of C–S bonds. The dominant CD transitions do not change significantly if the nitrogen is replaced by a phosphorus centre.From a comparison of the CD spectra calculated for both inversion minima for X = NH, it is predicted that the energetically lowest lying conformation in solution or in the gas phase is not identical with the conformer found in X-ray studies. This supports earlier conclusions which show that the conformer found in the crystal is favoured by intermolecular hydrogen bondings.