An Exploration of the Hydrogen Bond Donor Ability of Ammonia

Ammonia is an important molecule due to its wide use in the fertiliser industry. It is also used in aminolysis reactions. Theoretical studies of the reaction mechanism predict that in reactive complexes and transition states, ammonia acts as a hydrogen bond donor forming N-H⋅⋅⋅O hydrogen bond. Experimental reports of N-H⋅⋅⋅O hydrogen bond, where ammonia acts as a hydrogen bond donor are scarce. Herein, the hydrogen bond donor ability of ammonia is investigated with three chalcogen atoms i. e. O, S, and Se using matrix isolation infrared spectroscopy and electronic structure calculations. In addition, the chalcogen bond acceptor ability of ammonia has also been investigated. The hydrogen bond acceptor molecules used here are O(CH3 )2 , S(CH3 )2 , and Se(CH3 )2 . The formation of the 1 : 1 complex has been monitored in the N-H symmetric and anti-symmetric stretching modes of ammonia. The nature of the complex has been delineated using Atoms in Molecules analysis, Natural Bond Orbital analysis, and Energy Decomposition Analysis. This work presents the first comparison of the hydrogen bond donor ability of ammonia with O, S, and Se.

Setting up the HyDRA blind challenge for the microhydration of organic molecules

The procedure leading to the first HyDRA blind challenge for the prediction of water donor stretching vibrations in monohydrates of organic molecules is described. A training set of 10 monohydrates...

Matrix Isolation-Vibrational Circular Dichroism Spectroscopic Study of Conformations and Non-Covalent Interactions of Tetrahydro-2-Furoic Acid

The conformational landscape and aggregation behaviour of tetrahydro-2-furoic acid (THFA) were investigated by using matrix isolation-vibrational circular dichroism (MI-VCD). The well-resolved experimental MI-IR and MI-VCD features in an argon matrix at 10 K allow one to identify two dominant monomeric conformations as trans-THFA where the hydroxyl and carbonyl groups of COOH are at opposite sides, as well as one cis-conformer. At 24 K and 30 K deposition temperatures, the experimental IR and VCD spectral features reveal further growth of the binary THFA aggregates. Systematic conformational searches identified three vastly different binary binding topologies, resulting in a few hundred stable (THFA)₂ conformers. Interestingly, the main binary structures observed correspond to an unusual type of structure which is made of two trans-THFA subunits, in contrast to the usual double H-bonded ring binary structures, identified in a previous solution study. The present work showcases the power of MI-VCD spectroscopy in revealing unusual structures formed in a cold rare gas matrix.

Theory and algorithms for chiroptical properties and spectroscopies of aqueous systems

Chiroptical properties and spectroscopies are valuable tools to study chiral molecules and assign absolute configurations. The spectra that result from chiroptical measurements may be very rich and complex, and hide much of their information content. For this reason, the interplay between experiments and calculations is especially useful, provided that all relevant physico-chemical interactions that are present in the experimental sample are accurately modelled. The inherent difficulty associated to the calculation of chiral signals of systems in aqueous solutions requires the development of specific tools, able to account for the peculiarities of water-solute interactions, and especially its ability to form hydrogen bonds. In this perspective we discuss a multiscale approach, which we have developed and challenged to model the most used chiroptical techniques.

Chirality Induction from a Chiral Guest to the Hydrogen-Bonding Network of Its Hexameric Resorcinarene Host Capsule

The hexameric capsule of resorcin[4]arene 1 is capable of encapsulating tertiary amines, which has recently been used in the application of [(1)₆ (H₂ O)₈ ] as (co-)catalyst in various asymmetric reactions. However, not much is known about the highly asymmetric but conformationally very dynamic structure of the capsule after uptake of chiral molecules. Therefore, in this contribution, we utilize electronic circular dichroism and vibrational circular dichroism spectroscopy to investigate how several chiral guest molecules affect the structural preferences of the capsule [(1)₆ (H₂ O)₈ ]. In particular, we show that one small chiral amine encapsulated in [(1)₆ (H₂ O)₈ ] is sufficient to control and dictate the stereochemical preferences of the entire capsule. Furthermore, neither strong π-π interactions nor a significant steric bulk are required for this induction. The observation of such a chiral imprint of the guest's stereochemistry onto its host molecule is expected to have implications also for other supramolecular capsules.

The Clusters-in-a-Liquid Approach for Solvation: New Insights from the Conformer Specific Gas Phase Spectroscopy and Vibrational Optical Activity Spectroscopy

Vibrational optical activity spectroscopies, namely vibrational circular dichroism (VCD) and Raman optical activity (ROA), have been emerged in the past decade as powerful spectroscopic tools for stereochemical information of a wide range of chiral compounds in solution directly. More recently, their applications in unveiling solvent effects, especially those associated with water solvent, have been explored. In this review article, we first select a few examples to demonstrate the unique sensitivity of VCD spectral signatures to both bulk solvent effects and explicit hydrogen-bonding interactions in solution. Second, we discuss the induced solvent chirality, or chiral transfer, VCD spectral features observed in the water bending band region in detail. From these chirality transfer spectral data, the related conformer specific gas phase spectroscopic studies of small chiral hydration clusters, and the associated matrix isolation VCD experiments of hydrogen-bonded complexes in cold rare gas matrices, a general picture of solvation in aqueous solution emerges. In such an aqueous solution, some small chiral hydration clusters, rather than the chiral solutes themselves, are the dominant species and are the ones that contribute mainly to the experimentally observed VCD features. We then review a series of VCD studies of amino acids and their derivatives in aqueous solution under different pHs to emphasize the importance of the inclusion of the bulk solvent effects. These experimental data and the associated theoretical analyses are the foundation for the proposed "clusters-in-a-liquid" approach to account for solvent effects effectively. We present several approaches to identify and build such representative chiral hydration clusters. Recent studies which applied molecular dynamics simulations and the subsequent snapshot averaging approach to generate the ROA, VCD, electronic CD, and optical rotatory dispersion spectra are also reviewed. Challenges associated with the molecular dynamics snapshot approach are discussed and the successes of the seemingly random "ad hoc explicit solvation" reported before are also explained. To further test and improve the "clusters-in-a-liquid" model in practice, future work in terms of conformer specific gas phase spectroscopy of sequential solvation of a chiral solute, matrix isolation VCD measurements of small chiral hydration clusters, and more sophisticated models for the bulk solvent effects would be highly valuable.