Quadrupole Coupling Parameters of Olefinic Deuterons:  2H MAS NMR Spectroscopy of Photochromic Spiropyran and Merocyanines

Three molecules with olefinic deuterons were obtained through H−D exchange of the C3 proton on the central bridging double bond of spiropyran photomerocyanines with deuterium oxide solution. Quadrupole coupling parameters were determined by simulation of the spinning sideband manifolds in spectra obtained using solid-state deuterium MAS NMR spectroscopy. The C3-deuterated forms of the closed-ring spiropyran 1‘,3‘,3‘-trimethyl-6-nitrospiro[2H-1-benzopyran-2,2-indoline] (3D-6-nitro-BIPS) and the open-ring merocyanines 1‘,3‘,3‘-trimethyl-6,8-dinitrospiro[2H-1-benzopyran-2,2-indoline] (3D-6,8-dinitro-BIPS) and 1‘,3‘,3‘-trimethyl-6-nitro-8-bromospiro[2H-1-benzopyran-2,2-indoline] (3D-6-nitro-8-bromo-BIPS) had the same quadrupole coupling constants (QCCs) of 176 ± 4 kHz. This value, the second reported for an olefinic deuteron, is slightly less than the QCCs known for rigid aromatic deuterons (180−185 kHz). The asymmetry parameters (η) for the latter two molecules (open rings) were 0.10 ± 0.03, slightly larger than that observed for the closed-ring 3D-6-nitro-BIPS (0.00 ± 0.03). This slight asymmetry of the electric field gradient around deuterium in the merocyanines is attributed to a small deviation of the C−D bond from perfect axial symmetry, probably resulting from a through-space interaction between C3−D and C9−O.