Strained hydrogen bonding in imidazole trimer: a combined infrared, Raman, and theory study.

Thomas Forsting,Ricardo A. Mata,Julia Zischang,Martin A. Suhm,Marco Eckhoff,Benjamin Schröder

doi:10.1039/c9cp00399a

Thomas Forsting, Ricardo A. Mata + Show 4 more

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https://doi.org/10.1039/c9cp00399a

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Abstract

In this work, a careful analysis of anharmonic couplings in NH and some CH stretch modes of imidazole is carried out. This includes IR and Raman spectra of the isolated molecule and aggregates up to the trimer, together with two different theoretical approaches to the calculation of anharmonic shifts and absolute band positions. The imidazole dimer is vibrationally characterized for the first time in vacuum isolation under supersonic jet conditions, showing substantial shifts from previous helium droplet experiments and evidence for Fermi resonance for the hydrogen-bonded NH stretch. The most stable imidazole trimer structure is unambiguously shown to be cyclic with three non-equivalent, highly strained hydrogen bonds. This contrasts the helium droplet observation of a chain trimer involving two unstrained hydrogen bonds. These experimental conclusions are strongly corroborated by theory, including vibrational perturbation theory and anharmonic normal mode analysis. Systematic error compensation in some of these methods is emphasized. Intramolecular anharmonic coupling constants from perturbation theory are validated by Raman hot band jet spectroscopy of the monomer. Imidazole aggregation is shown to provide valuable benchmarking opportunities for electronic structure and in particular for anharmonic vibrational methods, covering the field of strong and strongly distorted hydrogen bonding.

Highlights

The aggregation of imidazole has frequently served to benchmark theoretical predictions of non-covalent interactions,[1,2,3,4,5,6,7,8,9] certainly motivated by its role as a building block for histidine and purine interactions
For the monomer, limited vibrational cooling of some strongly coupling modes in the hydride stretching region is detected in the Raman spectra for heated nozzles. This could be developed into a systematic tool for the elucidation of anharmonic coupling constants in medium-sized molecules which are difficult to resolve rotationally
The best results are obtained for a composite approach which uses diagonal anharmonicity at CCSD(T) level and adds anharmonic couplings at B3LYP VPT2 level

Summary

Introduction

The aggregation of imidazole has frequently served to benchmark theoretical predictions of non-covalent interactions,[1,2,3,4,5,6,7,8,9] certainly motivated by its role as a building block for histidine and purine interactions It involves a hydrogen bond donor and a hydrogen bond acceptor pointing away from each other due to their 1,3-placement in the heteroaromatic pentacycle. Small ambivalent solvent molecules have a choice between in plane solvation sites of the donor or the acceptor,[4] besides secondary C–H contacts.[10,11] Aromatic donors and acceptors can optimize secondary interactions by variation of the tilt angle in V- or T-shaped arrangements.[6,12] In contrast to the isomeric heterocycle pyrazole[13] with more aligned donor and acceptor groups, the homodimer of imidazole cannot realize both hydrogen bond functionalities at the same time and involves a single, unstrained hydrogen bond.

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