Molecular Tuning of Reactivity of Zeolite Protons in HZSM-5.

In acidic HZSM-5 zeolite, the reactivity of a methanol molecule interacting with the zeolite proton is amenable to modification via coadsorbing a stochiometric amount of an electron density donor E to form the [(E)(CH3OH)(HZ)] complex. The rate of the methanol in this complex undergoing dehydration to dimethyl ether was determined for a series of E with proton affinity (PA) ranging from 659 kJ mol-1 for C6F6 to 825 kJ mol-1 for C4H8O and was found to follow the expression: Ln(Rate) - Ln(RateN2) = ß(PA - PAN2)γ, where E = N2 is the reference and ß and γ are constants. This trend is probably due to the increased stability of the solvated proton in the [(E)(CH3OH)(HZ)] complex with increasing PA. Importantly, this is also observed in steady-state flow reactions when stoichiometric quantities of E are preadsorbed on the zeolite. As demonstrated with E being D2O, the effect on methanol reactivity diminishes when E is present in excess of the [(E)(CH3OH)(HZ)] complex. It is proposed that the methanol dehydration reaction involves [(E)(CH3OH)(CH3OH)(HZ)] as the transition state, which is supported by the isotopologue distribution of the initial dimethyl ether formed when a flow of CH3OH was passed over ZSM-5 containing one CD3OH per zeolite proton. The implication of this on the mechanism of catalytic methanol dehydration on HZSM-5 is discussed.

Generation and Implementation of Continuum Infrared Pulses for Broadband Detection in 2D IR Spectroscopy.

In recent years, new methods of generating continuum mid-infrared pulses through filamentation in gases have been developed for ultrafast time-resolved infrared vibrational spectroscopy. The generated infrared pulses can have thousands of wavenumbers of bandwidth, spanning the entire mid-IR region while retaining pulse length below 100 fs. This technology has had a significant impact on problems involving ultrafast structural dynamics in congested spectra with broad features, such as those found in aqueous solutions and molecules with strong intermolecular interactions. This study describes the recent advances in generating and characterizing these pulses and the practical aspects of implementing these sources for broadband detection in transient absorption and 2D IR spectroscopy.

Amplification of mid-IR continuum for broadband 2D IR spectroscopy.

We report the generation and characterization of microjoule level, broad bandwidth femtosecond pulses in the mid-infrared (MIR) using optical parametric amplification of continuum MIR seed pulses in GaSe. The signal (3 µm) and idler (6 µm) pulses have energies of 6 µJ and 3 µJ with bandwidths of ∼950 cm-1 and 650 cm-1 FWHM and pulse lengths of 34 fs and 80 fs. Broadband 2D IR spectra of O-H and N-H transitions are acquired with the signal beam demonstrating the capabilities of this source for cross peak and line shape measurements.

Structural Characterization of Protonated Water Clusters Confined in HZSM-5 Zeolites.

A molecular description of the structure and behavior of water confined in aluminosilicate zeolite pores is a crucial component for understanding zeolite acid chemistry under hydrous conditions. In this study, we use a combination of ultrafast two-dimensional infrared (2D IR) spectroscopy and ab initio molecular dynamics (AIMD) to study H2O confined in the pores of highly hydrated zeolite HZSM-5 (∼13 and ∼6 equivalents of H2O per Al atom). The 2D IR spectrum reveals correlations between the vibrations of both terminal and H-bonded O-H groups and the continuum absorption of the excess proton. These data are used to characterize the hydrogen-bonding network within the cluster by quantifying single-, double-, and non-hydrogen-bond donor water molecules. These results are found to be in good agreement with the statistics calculated from an AIMD simulation of an H+(H2O)8 cluster in HZSM-5. Furthermore, IR spectral assignments to local O-H environments are validated with DFT calculations on clusters drawn from AIMD simulations. The simulations reveal that the excess charge is detached from the zeolite and resides near the more highly coordinated water molecules in the cluster. When they are taken together, these results unambiguously assign the complex IR spectrum of highly hydrated HZSM-5, providing quantitative information on the molecular environments and hydrogen-bonding topology of protonated water clusters under extreme confinement.

Decoding the 2D IR spectrum of the aqueous proton with high-level VSCF/VCI calculations.

The aqueous proton is a common and long-studied species in chemistry, yet there is currently intense interest devoted to understanding its hydration structure and transport dynamics. Typically described in terms of two limiting structures observed in gas-phase clusters, the Zundel H5O2 + and Eigen H9O4 + ions, the aqueous structure is less clear due to the heterogeneity of hydrogen bonding environments and room-temperature structural fluctuations in water. The linear infrared (IR) spectrum, which reports on structural configurations, is challenging to interpret because it appears as a continuum of absorption, and the underlying vibrational modes are strongly anharmonically coupled to each other. Recent two-dimensional IR (2D IR) experiments presented strong evidence for asymmetric Zundel-like motifs in solution, but true structure-spectrum correlations are missing and complicated by the anharmonicity of the system. In this study, we employ high-level vibrational self-consistent field/virtual state configuration interaction calculations to demonstrate that the 2D IR spectrum reports on a broad distribution of geometric configurations of the aqueous proton. We find that the diagonal 2D IR spectrum around 1200 cm-1 is dominated by the proton stretch vibrations of Zundel-like and intermediate geometries, broadened by the heterogeneity of aqueous configurations. There is a wide distribution of multidimensional potential shapes for the proton stretching vibration with varying degrees of potential asymmetry and confinement. Finally, we find specific cross peak patterns due to aqueous Zundel-like species. These studies provide clarity on highly debated spectral assignments and stringent spectroscopic benchmarks for future simulations.

Strong H-bonding from Zeolite BroÌ·nsted Acid Site to Water: Origin of the Broad IR Doublet.

Hydrogen bonding between water molecules and zeolite BroÌ·nsted acid sites (BAS) has received much attention due to the significant influence of water on the adsorption and catalytic properties of these widely used porous materials. When a single water molecule is adsorbed at the BAS, the zeolite O-H stretch vibration decreases in frequency and splits into two extraordinarily broad bands peaked near 2500 and 2900 cm-1 in the infrared (IR) spectrum. This broad doublet feature is the predominant IR signature used to identify and interpret water-BAS H-bonding at low hydration levels, but the origin of the band splitting is not well understood. In this study, we used broadband two-dimensional infrared (2D IR) spectroscopy to investigate zeolite HZSM-5 prepared with a single water molecule per BAS. We find that the 2D IR spectrum is not explained by the most common interpretation of Fermi resonance coupling between the stretch and the bend of the BAS OH group, which predicts intense excited-state transitions that are absent from the experimental results. We present an alternative model of a double-well proton stretch potential, where the band splitting is caused by excited-state tunneling through the proton-transfer barrier. This one-dimensional model reproduces the basic experimental pattern of transition frequencies and amplitudes, suggesting that the doublet bands may originate from a highly anharmonic potential in which the excited state proton wave functions are delocalized over the H-bond between zeolite BAS and adsorbed H2O. Additional details about molecular orientation and coordination of the adsorbed water molecule are also resolved in the 2D IR spectroscopy.