pi-cloud and non-bonding or H-bond connectivities in photochromic spiropyrans and their merocyanines sensed by C-13 deuterium isotope shifts

Deuterium induced isotopic shifts in the C-13 NMR spectra of spiropyrans and equilibrium mixtures of their TTT and TTC merocyanine form isomers give insights into rovibrational and geometric changes that occur upon isotopic substitution which also result in changes in nuclear shielding. Isotopic shifts in the C-13 NMR spectra of C-3 deuterated 1',3',3'-trimethyl-6-nitrospiro[2H-1-benzopyran-2,2-indoline], (3D-6-nitro-BIPS), are observed three bonds away from the point of deuteration in this spiropyranic form of the molecule, whereas for C-3 deuterated 1',3',3'-trimethyl-6,8-dinitrospiro[2H-1-benzopyran-2,2-indoline], (3D-6,8-dinitro-BIPS), which is stable as a merocyanine the isotope shifts have a range of five bonds. For C-4 deuterated 6,8-dinitro-BIPS (4D-6,8-dinitro-BIPS) the isotope shift range extended to four bonds.