After 50 Years of Vibrational Circular Dichroism Spectroscopy: Challenges and Opportunities of Increasingly Accurate and Complex Experiments and Computations

VCD research continues to thrive, driven by ongoing experimental and theoretical advances. Modern studies deal with increasingly complex samples featuring weak intermolecular interactions and shallow potential energy surfaces. Likewise, the combination of VCD measurements with, for instance, cryo-spectroscopic techniques has significantly increased their sensitivity. The extent to which such modern measurements enhance the informative value of VCD depends significantly on the quality of the theoretical models, which must adequately account for anharmonicity, solvation and molecular dynamics. We herein discuss how experimental advancements engage in a stimulating interplay with recent theoretical developments, pursuing either the static or the dynamic computational route. Both paths have their own strengths and limitations, each addressing fundamentally different problems. We give an outlook on future challenges of VCD research, including the possibility to combine static and dynamic approaches to obtain a full picture of the sample.

Ferrocene derivatives with planar chirality and their enantioseparation by liquid-phase techniques

In the last decade, planar chiral ferrocenes have attracted a growing interest in several fields, particularly in asymmetric catalysis, medicinal chemistry, chiroptical spectroscopy and electrochemistry. In this frame, the access to pure or enriched enantiomers of planar chiral ferrocenes has become essential, relying on the availability of efficient asymmetric synthesis procedures and enantioseparation methods. Despite this, in enantioseparation science, these metallocenes were not comprehensively explored, and very few systematic analytical studies were reported in this field so far. On the other hand, enantioselective high-performance liquid chromatography has been frequently used by organic and organometallic chemists in order to measure the enantiomeric purity of planar chiral ferrocenes prepared by asymmetric synthesis. On these bases, this review aims to provide the reader with a comprehensive overview on the enantioseparation of planar chiral ferrocenes by discussing liquid-phase enantioseparation methods developed over time, integrating this main topic with the most relevant aspects of ferrocene chemistry. Thus, the main structural features of ferrocenes and the methods to model this class of metallocenes will be briefly summarized. In addition, planar chiral ferrocenes of applicative interest as well as the limits of asymmetric synthesis for the preparation of some classes of planar chiral ferrocenes will also be discussed with the aim to orient analytical scientists towards 'hot topics' and issues which are still open for accessing enantiomers of ferrocenes featured by planar chirality.

Local Charge-Displacement Analysis: Targeting Local Charge-Flows in Complex Intermolecular Interactions

Charge-displacement (CD) analysis has recently proven to be a simple and powerful scheme for quantitatively analyzing the profile of the charge redistribution occurring upon intermolecular interactions along a given interaction axis. However, when two molecular fragments bind through complex interactions involving multiple concurrent charge flows, ordinary CD analysis is capable of providing only an averaged picture of the related charge-flow profiles and no detailed information on each of them. In this article, we combine CD analysis with a Hirshfeld partitioning of the molecular charge redistribution for a local analysis on focused portions of the molecule, allowing for a detailed characterization of one charge flow at a time. The resulting scheme - the local charge-displacement (LCD) analysis - is tested on the intriguing case of the dimethyl sulfide (DMS)-sulfur dioxide (SO2) complex, characterized by concurrent charge flows relating to a sulfur-sulfur homo-chalcogen interaction and a pair of hydrogen bonds. The LCD scheme is then applied to the analysis of multiple hydrogen bonding in the acetic acid dimer, of base-pairing interactions in DNA, and of ambifunctional hydrogen bonding in the ammonia-pyridine complex.

Spectroscopic investigation on 1,2-substituted ferrocenes with only planar chirality: How chiroptical data are related to absolute configuration and to substituents

Single enantiomers of three 1,2-substituted ferrocene derivatives, i.e. 1-methoxymethyl-2-hydroxymethylferrocene (1), 1-formyl-2-hydroxymethylferrocene (2) and 1-iodo-2-hydroxymethylferrocene (3), sharing the common hydroxymethyl substituent and the presence of planar chirality only, were investigated for their spectroscopic (IR and UV) and chiroptical (VCD and ECD) properties. Both enantiomers of 1 were obtained for the first time in optically pure form by lipase-catalyzed kinetic resolution of the corresponding racemate (±)-1 and separately converted into formyl derivatives (+)-2 and (-)-2. The experimental spectroscopic and chiroptical data were compared with DFT calculated spectra and excellent correspondence was found for all compounds, allowing one to confirm the previously assigned absolute configurations. The common features in the VCD spectra of a doublet between 940 and 965 cm^-1 and the short-wavelength (about 200 nm) doublet and the longest wavelength band in the ECD spectra were analyzed to test whether they may be taken as markers of the absolute configuration (AC). The predominance of conformers with intramolecular hydrogen bond for the first two investigated compounds is predicted by conformational analysis and also confirmed by NMR.

Charge-Flow Profiles along Curvilinear Paths: A Flexible Scheme for the Analysis of Charge Displacement upon Intermolecular Interactions

The Charge-Displacement (CD) analysis has proven to be a powerful tool for a quantitative characterization of the electron-density flow occurring upon chemical bonding along a suitably chosen interaction axis. In several classes of interesting intermolecular interactions, however, an interaction axis cannot be straightforwardly defined, and the CD analysis loses consistency and usefulness. In this article, we propose a general, flexible reformulation of the CD analysis capable of providing a quantitative view of the charge displacement along custom curvilinear paths. The new scheme naturally reduces to ordinary CD analysis if the path is chosen to be a straight line. An implementation based on a discrete sampling of the electron densities and a Voronoi space partitioning is described and shown in action on two test cases of a metal-carbonyl and a pyridine-ammonia complex.

Looking for the bricks of the life in the interstellar medium: The fascinating world of astrochemistry

The discovery in the interstellar medium of molecules showing a certain degree of complexity, and in particular those with a prebiotic character, has attracted great interest. A complex chemistry takes place in space, but the processes that lead to the production of molecular species are a matter of intense discussion, the knowledge still being at a rather primitive stage. Debate on the origins of interstellar molecules has been further stimulated by the identification of biomolecular building blocks, such as nucleobases and amino acids, in meteorites and comets. Since many of the molecules found in space play a role in the chemistry of life, the issue of their molecular genesis and evolution might be related to the profound question of the origin of life itself. Understanding the underlying chemical processes, including the production, reactions and destruction of compounds, requires the concomitant study of spectroscopy, gas-phase reactivity, and heterogeneous processes on dust-grains. The aim of this contribution is to provide a general view of a complex and multifaceted challenge, while focusing on the role played by molecular spectroscopy and quantum-chemical computations. In particular, the derivation of the molecular spectroscopic features and the investigation of gas-phase formation routes of prebiotic species in the interstellar medium are addressed from a computational point of view.

Chemical promenades: Exploring potential-energy surfaces with immersive virtual reality

The virtual-reality framework AVATAR (Advanced Virtual Approach to Topological Analysis of Reactivity) for the immersive exploration of potential-energy landscapes is presented. AVATAR is based on modern consumer-grade virtual-reality technology and builds on two key concepts: (a) the reduction of the dimensionality of the potential-energy surface to two process-tailored, physically meaningful generalized coordinates, and (b) the analogy between the evolution of a chemical process and a pathway through valleys (potential wells) and mountain passes (saddle points) of the associated potential energy landscape. Examples including the discovery of competitive reaction paths in simple A + BC collisional systems and the interconversion between conformers in ring-puckering motions of flexible rings highlight the innovation potential that augmented and virtual reality convey for teaching, training, and supporting research in chemistry.

Chemical bonding in cuprous complexes with simple nitriles: octet rule and resonance concepts versus quantitative charge-redistribution analysis

Resonance structures for six cuprous complexes with simple nitriles are interpreted by means of a quantitative analysis of charge redistribution upon copper-nitrile bonding.

Potential-Energy Surfaces for Ring-Puckering Motions of Flexible Cyclic Molecules through Cremer-Pople Coordinates: Computation, Analysis, and Fitting

Ring-puckering motion in 12 flexible cyclic molecules is investigated by calculation and analysis of two-dimensional potential-energy surfaces (PESs) using the so-called ring-puckering coordinates proposed by Cremer and Pople. The PESs are calculated by means of density-functional theory using a B2PLYP-D3BJ exchange-correlation functional with a maug-cc-pVTZ basis set, and results are compared to the available experimental and theoretical data. Special care is devoted to the aspect of symmetry in such two-dimensional PESs, which are here reported for the first time also for molecules whose planar form has symmetry lower than D_{5 h} or C_{2 v}. The issue of PES fitting and that of solving the nuclear dynamics using ring-puckering coordinates are also addressed. Analytical formulations of the computed PESs using suitable functional forms with a limited set of parameters are provided.

Two-level stochastic search of low-energy conformers for molecular spectroscopy: implementation and validation of MM and QM models

The search for stationary points in the molecular potential energy surfaces (PES) is a problem of increasing relevance in molecular sciences especially for large, flexible systems featuring several large-amplitude internal motions.