Insight into Understanding the Inverse Isotope Effect in a One-Dimensional Spin-Peierls-Type Molecular Solid

A new deuterated spin-Peierls-type transition salt, bis(maleonitriledithiolato)nickelate radical anion with a 1-N-(4′-vinyl-benzyl)pyridinium-d₅ nonmagnetic cation, was synthesized by replacing pyridine with pyridine-d₅ and characterized. The deuterated and hydrogen parent salts, denoted as V-Py-D and V-Py-H, are isomorphic in both high- and low-temperature phases (abbreviated as HTP and LTP), respectively. The phase transition temperature (T_C) of V-Py-D is ca. 204.8 K and lower than that of V-Py-H by 3.3 K, and this finding differs from that in other members of the R-Py-H/D (R = F, Cl, Br, I, CH₃, and NO₂) family, where each deuterium salt exhibits a higher T_C than its hydrogen counterpart. A correlation was observed between the T_C and the overlap integral of two superimposed anions in an anionic stack in HTP across all members in this family, and the possible origin was investigated.