Coherent Anti-Stokes-Stokes Raman Cross-Correlation Spectroscopy: Asymmetric Frequency Shifts in Hydrogen-Bonded Pyridine-Water Complexes

Disentangling Acid-Base Chemistry through Blue Shifting Hydrogen Bond Contributions

The blue shifting of vibrational frequencies in hydrogen bonded molecules, as observed in aqueous environments, has been attributed to local partial charge transfer from solvation. Here, we extrapolate the blue shift model to the stronger ionic interactions between hydrogen bond acceptors associated with protonation through augmented pH levels and competitive interactions with counter ion pairing. The chemical model we utilize in this work is the aqueous pyridine-pyridinium equilibrium to characterize the blue shifts observed in the pyridinium chloride ionic system. The observed agreement between observed experimental and calculated spectral shifts shows that the blue shifting model can be extrapolated to stronger interactions and accurately describe the nature of the hydrogen bond.

Continuing progress in the field of two-dimensional correlation spectroscopy (2D-COS): Part III. Versatile applications

In this review, the comprehensive summary of two-dimensional correlation spectroscopy (2D-COS) for the last two years is covered. The remarkable applications of 2D-COS in diverse fields using many types of probes and perturbations for the last two years are highlighted. IR spectroscopy is still the most popular probe in 2D-COS during the last two years. Applications in fluorescence and Raman spectroscopy are also very popularly used. In the external perturbations applied in 2D-COS, variations in concentration, pH, and relative compositions are dramatically increased during the last two years. Temperature is still the most used effect, but it is slightly decreased compared to two years ago. 2D-COS has been applied to diverse systems, such as environments, natural products, polymers, food, proteins and peptides, solutions, mixtures, nano materials, pharmaceuticals, and others. Especially, biological and environmental applications have significantly emerged. This survey review paper shows that 2D-COS is an actively evolving and expanding field.

On Macroscopic Quantum Coherence with Synchronized Atoms and Molecules: Superradiance

The collective behavior of quantum particles is one of the most intriguing phenomena in quantum optics. In particular, superradiance refers to spontaneous collective emissions from a group of quantum particles behaving collectively as a whole due to the buildup of macroscopic quantum coherence. An important question is whether macroscopic quantum coherence is constructed by means of a quantum synchronization (i.e., a quantum analog of classical synchronization) or not. The purpose of this article is to draw attention to this question from the author’s perspective. A few selected studies relevant to synchronized atoms and molecules are discussed. The author concludes that collective behaviors of quantum particles may be formulated as quantum synchronizations, but extensive studies are still needed to confirm this hypothesis.

Parallelized Raman Difference Spectroscopy for the Investigation of Chemical Interactions

Raman spectroscopy provides an extremely high chemical selectivity. Raman difference spectroscopy is a technique to reveal even the smallest differences that occur due to weak interactions between substances and changes in the molecular structure. To enable parallelized and highly sensitive Raman difference spectroscopy in a microtiter-array, a diffractive optical element, a lens array, and a fiber bundle were integrated into a Raman spectroscopy setup in a unique fashion. The setup was evaluated with a microtiter-array containing pyridine-water complexes, and subwavenumber changes below the spectrometer's resolution could be resolved. The spectral changes were emphasized with two-dimensional correlation analysis. Density functional theory calculation and "atoms in molecule" analysis were performed to simulate the intermolecular long-range interactions between water and pyridine molecules and to get insight into the involved noncovalent interactions, respectively. It was found that by the addition of pyridine, the energy portion of hydrogen bonds to the total complexation energy between pyridine and water reduces. These results demonstrate the unique abilities of the new setup to investigate subtle changes due to biochemically important molecular interactions and opens new avenues to perform drug binding assays and to monitor highly parallelized chemical reactions.

Distinguishing Resonant from Non-Resonant Nonlinear Optical Processes Using Intensity-Intensity Correlation Analyses

Three-color coherent anti-Stokes Raman scattering (CARS) represents non-degenerate four wave mixing that includes both non-resonant and resonant processes, the contributions of which depend upon how the molecular vibrational modes are being excited by the input laser pulses. The scattering signal due to resonant processes builds up progressively. An advanced analytical tool to reveal this deferred resonant signal buildup phenomenon is in need. In this work, we adapt a quantitative analytical tool by introducing one-dimensional and two-dimensional intensity-intensity correlation functions in terms of a new variable (probe pulse delay) and a new perturbation parameter (probe pulse linewidth). In particular, discrete diagonal directional sums are defined here as a tool to reduce both synchronous and asynchronous two-dimensional correlation spectroscopy (2D-COS) maps down to one-dimensional plots while maintaining the valuable analytical information. Detailed analyses using the all-Gaussian coherent Raman scattering closed-form solutions and the representative experimental data for resonant and non-resonant processes are presented and compared. The present work holds a promising potential for industrial application, e.g., by extractive industries to distinguish hydrocarbons (chemically resonant substance) from water (non-resonant contaminant) by utilizing the one- and two-dimensional correlation analyses.