Resonant Two-Photon Ionization and Velocity Mapped Ion Imaging Studies of Aromatic Van Der Waals Complexes

The study of van der Waals complexes provides a means for understanding the nature and strength of non-covalent interactions. Non-covalent interactions including C-H/, C-H/O, C-H/N, C-H/F, halogen and chalcogen bonding are found in important intermediates that regulate chemical and biological processes in many forefront areas of science including molecular self-assembly, drug substrate interactions, supramolecular chemistry, crystal engineering and biochemistry. To better understand these interactions, laser spectroscopic techniques that include mass selected two color resonant two photon ionization (2CR2PI) and velocity mapped ion imaging spectroscopy in combination with complementary ab initio calculations were used to probe the electronic structure, geometries, and binding strengths in aromatic van der Waals clusters. Prototypical systems of anisole...(CH4)n, aniline...(CH4)n (n=1,2) and toluene...CH3F were utilized for probing the cooperation/competition of C-H/ with C-H/O, C-H/N and C-H/F interactions. For anisole-CH4 1:1 complex, we found a dual presence of C-H/O and C-H/ interactions, consistent with the observation that the measured S0 binding energy is additive of energies determined for systems exhibiting only C-H/ and C-H/O interactions. In a follow up study on anisole-methane 1:2 complex, to probe the degree of cooperativity, the binding energies increased relative to the 1:1 complex across the three electronic states (S0, S1, D0), indicative of cooperative binding. For the aniline-methane 1:1 and 1:2 complexes the binding strength in the three electronic states were also determined. Similar to anisole-methane 1:1 complex, the aniline-CH4 was stabilized by both C-H/ and C-H/N interactions, and the S0 binding energy was additive when compared to that of systems exhibiting only C-H/ and C-H/N interactions. Contrary to anisole-methane 1:2 complex which showed an enhanced binding, the S0 binding energy required to remove one methane from aniline-CH4 1:2 complex decreased, indicating a loss in cooperative binding. The experimental results reported are in good agreement with the ab initio calculations performed. A theoretical study on Toluene...CH3F complex using Density Functional (DFT) methods benchmarked from studies of complexes of CH4 with anisole and aniline also show a dual presence in binding (C-H/ and C-H/F interactions). The predicted ground state binding energy is also additive when compared to that of systems only exhibiting C-H/ and C-H/F interactions

Unraveling a Trifecta of Weak non-Covalent Interactions: The Dissociation Energy of the Anisole-Ammonia 1:1 Complex

The anisole-ammonia 1:1 complex is a challenge for both experiment and theory. Early studies supported a non-planar structure, involving a trifecta of weak non-covalent interactions: N-H/O, N-H/π, and C-H/N. The calculated structure and binding energy of the complex proved remarkably sensitive to the level of theory employed. Here, we report the first experimental measurement of the ground state dissociation energy of the complex, and derive an excited (S1) state dissociation energy that is in excellent agreement with the cutoff observed in the experimental excitation spectrum. Results are compared with previous predictions and new calculations based on benchmarked Density Functional Theory methods