Abstract
In recent years the interaction of organophosphates and imines, which is at the core of Brønsted acid organocatalysis, has been established to be based on strong ionic hydrogen bonds. Yet, besides the formation of homodimers consisting of two acid molecules and heterodimers consisting of one acid and one base, also multimeric molecular aggregates are formed in solution. These multimeric aggregates consist of one base and several acid molecules. The details of the intermolecular bonding in such aggregates, however, have remained elusive. To characterize composition-dependent bonding and bonding dynamics in these aggregates, we use linear and nonlinear infrared (IR) spectroscopy at varying molar ratios of diphenyl phosphoric acid and quinaldine. We identify the individual aggregate species, giving rise to the structured, strong, and very broad infrared absorptions, which span more than 1000 cm–1. Linear infrared spectra and density functional theory calculations of the proton transfer potential show that doubly ionic intermolecular hydrogen bonds between the acid and the base lead to absorptions which peak at ∼2040 cm–1. The contribution of singly ionic hydrogen bonds between an acid anion and an acid molecule is observed at higher frequencies. As common to such strong hydrogen bonds, ultrafast IR spectroscopy reveals rapid, ∼ 100 fs, dissipation of energy from the proton transfer coordinate. Yet, the full dissipation of the excess energy occurs on a ∼0.8–1.1 ps time scale, which becomes longer when multimers dominate. Our results thus demonstrate the coupling and collectivity of the hydrogen bonds within these complexes, which enable efficient energy transfer.
Highlights
Aggregates formed via association of a Brønsted acid and a Brønsted base are often considered as prototypes of strong hydrogen bonds.[1]
The relevance of strong hydrogen bonds to, e.g., photochemical transfer cascades in biological systems,[2,3] enzyme function,[4] or catalysis[5−7] has triggered intense interest in the understanding of their fundamental physical and chemical characteristics.[8−17] Of particular interest are such strong hydrogen bonds for fast chemical reactions, as proton transfer along the hydrogen bond has been suggested to be an important part of the reaction coordinate.[3,18]
To elucidate hydrogen bonding of catalytically relevant phosphoric acids,[48,49] we study vibrational signatures of intermolecular bonding in acid−base mixtures consisting of diphenyl phosphoric acid and the base quinaldine dissolved in dichloromethane
Summary
Aggregates formed via association of a Brønsted acid and a Brønsted base are often considered as prototypes of strong hydrogen bonds.[1] The relevance of strong hydrogen bonds to, e.g., photochemical transfer cascades in biological systems,[2,3] enzyme function,[4] or catalysis[5−7] has triggered intense interest in the understanding of their fundamental physical and chemical characteristics.[8−17] Of particular interest are such strong hydrogen bonds for fast (photo-) chemical reactions, as proton transfer along the hydrogen bond has been suggested to be an important part of the reaction coordinate.[3,18] In asymmetric catalysis, such strong bonds can restrict the number of thermally accessible conformations of the base.[19,20] the bonding geometry has to be sufficiently flexible in order to allow the Brønsted acid catalyst to reversibly bind a wide range of substrates.[21] As such, the location and bonding dynamics of the proton, which report on the bond strength and geometry, affect the catalytic efficiency.[18] most of the current understanding of such strong hydrogen bonds stems from studies of carboxylic acids.[13,22−24] Hydrogen bonding involving phosphoric acid groups, as they occur in enzymatic transition[4] states or in asymmetric catalysis,[5] are less well studied.[25]
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