Positive Cooperativity in Binding: Spectroscopic Characterization and Dissociation Energy of the Anisole…(Ch4)2 Complex

We have previously reported on the spectroscopy and binding energy of the anisole…methane complex, which exhibits a dual mode of binding that involves both C-H/O and C-H/π interactions. In this work, we seek to examine cooperativity in binding through a study of the isolated anisole-(methane)2 complex, using a combination of experiments that include mass-selected two-color resonant two-photon ionization spectroscopy (2CR2PI), two-color appearance potential (2CAP) measurements, and velocity mapped ion imaging (VMI) augmented with a complementary theoretical characterization. Using 2CAP and VMI, we derive the dissociation energies of the complex in ground (S0), excited (S1), and cation radical (D0) states. The experimental values from the two methods are in excellent agreement and are compared with selected theoretical values calculated using DFT and ab initio methods. The data show that the dissociation energy increases by some 10% for the second methane relative to the first, with this trend being consistent across all three electronic states, indicating a cooperative binding effect where the initial solvation turns on binding of a second methane onto the opposite face. This lies in contrast to the aniline-(methane)2 complex, where recent studies have shown a negativity cooperativity, and these trends are examined.