Measuring electric fields and noncovalent interactions using the vibrational stark effect

Caught in the Loop: Binding of the [Ru(phen)2 (dppz)]2+ Light-Switch Compound to Quadruplex DNA in Solution Informed by Time-Resolved Infrared Spectroscopy

Ultrafast time-resolved infrared (TRIR) is used to report on the binding site of the [Ru(phen)₂ (dppz)]²⁺ "light-switch" complex with both bimolecular (Oxytricha nova telomere) and intramolecular (human telomere) guanine-quadruplex structures in both K⁺ and Na⁺ containing solutions. TRIR permits the simultaneous monitoring both of the "dark" and "bright" states of the complex and of the quadruplex nucleobase bases, the latter via a Stark effect induced by the excited state of the complex. These data are used to establish the contribution of guanine base stacking and loop interactions to the binding site of this biologically relevant DNA structure in solution. A particularly striking observation is the strong thymine signal observed for the Na⁺ form of the human telomere sequence, which is expected to be in the anti-parallel conformation.

Electric-Field-Induced Effects on the Dipole Moment and Vibrational Modes of the Centrosymmetric Indigo Molecule

Intense static electric fields can strongly perturb chemical bonds and induce frequency shifts of the molecular vibrations in the so-called vibrational Stark effect. Based on a density functional theory (DFT) approach, here, we report a detailed investigation of the influence of oriented external electric fields (OEEFs) on the dipole moment and infrared (IR) spectrum of the nonpolar centrosymmetric indigo molecule. When an OEEF as intense as ∼0.1 V Å^-1 is applied, several modifications in the IR spectrum are observed. Besides the notable frequency shift of some modes, we observe the onset of new bands-forbidden by the selection rules in the zero-field case. Such a neat field-induced modification of the vibrational selection rules, and the subsequent variations of the peaks' intensities in the IR spectrum, paves the way toward the design of smart tools employing centrosymmetric molecules as proxies for mapping local electric fields. In fact, here, we show that the ratio between the IR and the Raman intensities of selected modes is proportional to the square of the local field. This indicator can be used to quantitatively measure local fields, not only in condensed matter systems under standard conditions but also in field-emitting-tip apparatus.

Combining the best of two worlds: Stimulated Raman excited fluorescence

The pursuit of a hybrid spectroscopy that combines the superb sensitivity of fluorescence and the high chemical specificity of Raman scattering has lasted for 40 years, with multiple experimental and theoretical attempts in the literature. It was only recently that the stimulated Raman excited fluorescence (SREF) process was successfully observed in a broad range of fluorophores. SREF allows single-molecule vibrational spectroscopy and imaging in the optical far field without relying on plasmonic enhancement. In this perspective, we will first review the historical efforts that lead to the successful excitation and detection of SREF, followed by the underlying physical principles, then the remaining technical challenges will be discussed, and, at last, the future opportunities in this old but yet newly emerged spectroscopy are outlined.

Catalytic Effect of Electric Fields on the Kemp Elimination Reactions with Neutral Bases

The effect of solvent reaction fields and oriented electric fields on the Kemp elimination reaction between methylamine or imidazole and 5-nitrobenzisoxazole has been theoretically studied. The Kemp reaction is the most widely used for the design of new enzymes. Our results, using the SMD continuous model for solvents, are in quite good agreement with the experimental fact that the rate of the analogous reaction with butylamine is one order of magnitude smaller in water than in acetonitrile. In the case of external electric fields, our results show that they can increase or decrease the energy barrier depending on the magnitude and orientation of the field. A duly oriented electric field may have a notable catalytic effect on the reaction. So, external electric fields and reaction fields due to the medium can contribute to the design of new enzymes. Several factors that must be taken into account to increase the catalytic effect are discussed.

Time-Resolved Vibrational Fingerprints for Two Silver Cluster-DNA Fluorophores

DNA-templated silver clusters are chromophores in which the nucleobases encode the cluster spectra and brightness. We describe the coordination environments of two nearly identical Ag₁₀⁶⁺ clusters that form with 18-nucleotide strands CCCCA CCCCT CCCX TTTT, with X = guanosine and inosine. For the first time, femtosecond time-resolved infrared (TRIR) spectroscopy with visible excitation and mid-infrared probing is used to correlate the response of nucleobase vibrational modes to electronic excitation of the metal cluster. A rich pattern of transient TRIR peaks in the 1400-1720 cm^-1 range decays synchronously with the visible emission. Specific infrared signatures associated with the single guanosine/inosine along with a subset of cytidines, but not the thymidines, are observed. These fingerprints suggest that the network of bonds between a silver cluster adduct and its polydentate DNA ligands can be deciphered to rationally tune the coordination and thus spectra of molecular silver chromophores.

Nuclear Quantum Effects Largely Influence Molecular Dissociation and Proton Transfer in Liquid Water under an Electric Field

Proton transfer in liquid water controls acid-base chemistry, crucial enzyme reactions, and the functioning of fuel cells. Externally applied static electric fields in water are capable of dissociating molecules and transferring protons across the H-bond network. However, the impact of nuclear quantum effects (NQEs) on these fundamental field-induced phenomena has not yet been reported. By comparing state-of-the-art ab initio molecular dynamics (AIMD) and path integral AIMD simulations of water under electric fields, I show that quantum delocalization of the proton lowers the molecular ionization threshold to approximately one-third. Moreover, also the water behavior as a protonic semiconductor is considerably modified by the inclusion of NQEs. In fact, when the quantum nature of the nuclei is taken into account, the proton conductivity is ∼50% larger. This work proves that NQEs sizably affect the protolysis phenomenon and proton transfer in room-temperature liquid water.

Ordered Hydrogen-Bonded Alcohol Networks Confined in Lewis Acid Zeolites Accelerate Transfer Hydrogenation Turnover Rates

The disruption of ordered water molecules confined within hydrophobic reaction pockets alters the energetics of adsorption and catalysis, but a mechanistic understanding of how nonaqueous solvents influence catalysis in microporous voids remains unclear. Here, we use kinetic analyses coupled with IR spectroscopy to study how alkanol hydrogen-bonding networks confined within hydrophobic and hydrophilic zeolite catalysts modify reaction free energy landscapes. Hydrophobic Beta zeolites containing framework Sn atoms catalyze the transfer hydrogenation reaction of cyclohexanone in a 2-butanol solvent 10× faster than their hydrophilic analogues. This rate enhancement stems from the ability of hydrophobic Sn-Beta to inhibit the formation of extended liquid-like 2-butanol oligomers and promote dimeric H-bonded 2-butanol networks. These different intraporous 2-butanol solvent structures manifest as differences in the activation and adsorption enthalpies and entropies that comprise the free energy landscape of transfer hydrogenation catalysis. The ordered H-bonding solvent network present in hydrophobic Sn-Beta stabilizes the transfer hydrogenation transition state to a greater extent than the liquid-like 2-butanol solvent present in hydrophilic Sn-Beta, giving rise to higher turnover rates on hydrophobic Sn-Beta. Additionally, reactant adsorption within hydrophobic Sn-Beta is driven by the breakup of intraporous solvent-solvent interactions, resulting in positive enthalpies of adsorption that are partially compensated by an increase in the solvent reorganization entropy. Collectively, these results emphasize the ability of the zeolite pore to regulate the structure of confined nonaqueous H-bonding solvent networks, which offers an additional dimension to modulate adsorption and reactivity.

Aromatic foldamers as scaffolds for metal second coordination sphere design

As metalloproteins exemplify, the chemical and physical properties of metal centers depend not only on their first but also on their second coordination sphere. Installing arrays of functional groups around the first coordination sphere of synthetic metal complexes is thus highly desirable, but it remains a challenging objective. Here we introduce a novel approach to produce tailored second coordination spheres. We used bioinspired artificial architectures based on aromatic oligoamide foldamers to construct a rigid, modular and well-defined environment around a metal complex. Specifically, aza-aromatic monomers having a tethered [2Fe-2S] cluster have been synthesized and incorporated in conical helical foldamer sequences. Exploiting the modularity and predictability of aromatic oligoamide structures allowed for the straightforward design of a conical architecture able to sequester the metal complex in a confined environment. Even though no direct metal complex-foldamer interactions were purposely designed in this first generation model, crystallography, NMR and IR spectroscopy concurred to show that the aromatic oligoamide backbone alters the structure and fluxional processes of the metal cluster.

Direct Observation of Carbon Dioxide Electroreduction on Gold: Site Blocking by the Stern Layer Controls CO2 Adsorption Kinetics

Directly observing active surface intermediates represents a major challenge in electrocatalysis, especially for CO₂ electroreduction on Au. We use in-situ, plasmon-enhanced vibrational sum frequency generation spectroscopy, which has detection limits of <1% of a monolayer and can access the Au/electrolyte interface during active electrocatalysis in the absence of mass transport limitations. Measuring the potential-dependent surface coverage of atop CO confirms that the rate-determining step for this reaction is CO₂ adsorption. An analysis of the interfacial electric field reveals the formation of a dense cation layer at the electrode surface, which is correlated to the onset of CO production. The Tafel slope increases in conjunction with the field saturation due to active site blocking by adsorbed cations. These findings show that CO₂ reduction is extremely sensitive to the potential-dependent structure of the electrochemical double layer and provides direct observation of the interfacial processes that govern these kinetics.

Spectroscopic Study of Sol-Gel Entrapped Triruthenium Dodecacarbonyl Catalyst Reveals Hydride Formation

Triruthenium dodecacarbonyl exhibits increased catalytic activity toward hydrogenation reactions when encapsulated in alumina sol-gels. In this study, we demonstrate structural and electronic changes induced by the encapsulation process. Fourier transform infrared (FTIR) spectroscopy reveals that the carbonyl vibrational modes dramatically red shift during aging in the sol-gel glass. These shifts are attributed to the formation of the metal hydride: [HRu₃(CO)₁₁]^-. A comparison to the FTIR spectrum of synthesized [NEt₄][HRu₃(CO)₁₁] confirms this assignment. XPS studies show that the Ru 3d_5/2 peak of [HRu₃(CO)₁₁]^- also shifts to lower binding energy, consistent with an increased electron density on the Ru nuclei compared to Ru₃(CO)₁₂ and confirmed by density functional calculations. This study should open the door to further investigations into the hydride's role in the previously observed catalytic activity. To the best of our knowledge, this is the first study to identify the presence of [HRu₃(CO)₁₁]^- in the alumina sol-gel.

Strong Electric Field Observed at the Interface of Aqueous Microdroplets

Chemical reactions in aqueous microdroplets often exhibit unusual kinetic and thermodynamic properties not observed in bulk solution. While an electric field has been implicated at the water interface, there has been no direct measurement in aqueous microdroplets, largely due to the lack of proper measurement tools. Herein, we employ newly developed stimulated Raman excited fluorescence microscopy to measure the electric field at the water-oil interface of microdroplets. As determined by the vibrational Stark effect of a nitrile-bearing fluorescent probe, the strength of the electric field is found to be on the order of 10⁷ V/cm. This strong electric field aligns probe dipoles with respect to the interface. The formation of the electric field likely arises from charge separation caused by the adsorption of negative ions at the water-oil interface of microdroplets. We suggest that this strong electric field might account in part for the unique properties of chemical reactions reported in microdroplets.

The influence of loops on the binding of the [Ru(phen)2dppz]2+ light-switch compound to i-motif DNA structures revealed by time-resolved spectroscopy

Ultrafast time resolved infrared (TRIR) is used to report on the binding site of the "light-switch" complex [Ru(phen)₂(dppz)]²⁺1 to i-motif structures in solution. Detailed information is provided due to perturbation of the local base vibrations by a 'Stark-like' effect which is used to establish the contribution of thymine base loop interactions to the binding site of 1 in this increasingly relevant DNA structure.

Vibrational Spectroscopic Map, Vibrational Spectroscopy, and Intermolecular Interaction

Vibrational spectroscopy is an essential tool in chemical analyses, biological assays, and studies of functional materials. Over the past decade, various coherent nonlinear vibrational spectroscopic techniques have been developed and enabled researchers to study time-correlations of the fluctuating frequencies that are directly related to solute-solvent dynamics, dynamical changes in molecular conformations and local electrostatic environments, chemical and biochemical reactions, protein structural dynamics and functions, characteristic processes of functional materials, and so on. In order to gain incisive and quantitative information on the local electrostatic environment, molecular conformation, protein structure and interprotein contacts, ligand binding kinetics, and electric and optical properties of functional materials, a variety of vibrational probes have been developed and site-specifically incorporated into molecular, biological, and material systems for time-resolved vibrational spectroscopic investigation. However, still, an all-encompassing theory that describes the vibrational solvatochromism, electrochromism, and dynamic fluctuation of vibrational frequencies has not been completely established mainly due to the intrinsic complexity of intermolecular interactions in condensed phases. In particular, the amount of data obtained from the linear and nonlinear vibrational spectroscopic experiments has been rapidly increasing, but the lack of a quantitative method to interpret these measurements has been one major obstacle in broadening the applications of these methods. Among various theoretical models, one of the most successful approaches is a semiempirical model generally referred to as the vibrational spectroscopic map that is based on a rigorous theory of intermolecular interactions. Recently, genetic algorithm, neural network, and machine learning approaches have been applied to the development of vibrational solvatochromism theory. In this review, we provide comprehensive descriptions of the theoretical foundation and various examples showing its extraordinary successes in the interpretations of experimental observations. In addition, a brief introduction to a newly created repository Web site (http://frequencymap.org) for vibrational spectroscopic maps is presented. We anticipate that a combination of the vibrational frequency map approach and state-of-the-art multidimensional vibrational spectroscopy will be one of the most fruitful ways to study the structure and dynamics of chemical, biological, and functional molecular systems in the future.

Why are Vibrational Lines Narrow in Proteins?

The vibrational Stark effect in proteins yields line shifts indicative of strong internal electric fields up to a few volts per angstrom. These values are supported by numerical simulations of proteins. The simulations also show a significant breadth of field fluctuations translating to inhomogeneous broadening of vibrational lines. According to fluctuation-dissipation arguments, strong internal fields should lead to broad lines. Experimentally reported vibrational lines in proteins are, however, very narrow. This disconnect is explained here in terms of the insufficient (nonergodic) sampling of the protein's configurations on the lifetime of the vibrational probe. The slow component of the electric field fluctuations in proteins relaxes on the time scale of tens of nanoseconds and is dynamically frozen on the vibrational lifetime.

Understanding the factors controlling the photo-oxidation of natural DNA by enantiomerically pure intercalating ruthenium polypyridyl complexes through TA/TRIR studies with polydeoxynucleotides and mixed sequence oligodeoxynucleotides

Ruthenium polypyridyl complexes which can sensitise the photo-oxidation of nucleic acids and other biological molecules show potential for photo-therapeutic applications. In this article a combination of transient visible absorption (TrA) and time-resolved infra-red (TRIR) spectroscopy are used to compare the photo-oxidation of guanine by the enantiomers of [Ru(TAP)₂(dppz)]²⁺ in both polymeric {poly(dG-dC), poly(dA-dT) and natural DNA} and small mixed-sequence duplex-forming oligodeoxynucleotides. The products of electron transfer are readily monitored by the appearance of a characteristic TRIR band centred at ca. 1700 cm⁻¹ for the guanine radical cation and a band centered at ca. 515 nm in the TrA for the reduced ruthenium complex. It is found that efficient electron transfer requires that the complex be intercalated at a G-C base-pair containing site. Significantly, changes in the nucleobase vibrations of the TRIR spectra induced by the bound excited state before electron transfer takes place are used to identify preferred intercalation sites in mixed-sequence oligodeoxynucleotides and natural DNA. Interestingly, with natural DNA, while it is found that quenching is inefficient in the picosecond range, a slower electron transfer process occurs, which is not found with the mixed-sequence duplex-forming oligodeoxynucleotides studied.

Efficient electron transfer requires the complex to be intercalated at a G-C base-pair. Identification of preferred intercalation sites is achieved by TRIR monitoring of the nucleobase vibrations before electron transfer.

Electrostatic Field-Induced Oscillator Strength Focusing in Molecules

The development of light-harvesting devices based on molecular materials depends critically on the ability to focus the electronic oscillator strength of molecules into the UV-vis spectral window. Typical molecular chromophores have only about 1% of their total electronic oscillator strength in this spectral region and thus perform at a small fraction of their possible effectiveness. This theoretical study finds that the electronic oscillator strength of polyenes in the UV-vis region may be enhanced by 1 order of magnitude using electrostatic fields, motivating specific experimental studies of oscillator strength focusing. We find scaling relationships between the polyene length, the intensity of the applied field, and the field-induced increase in oscillator strength that are useful for the implementation of light-harvesting strategies based on polyenes.

Directional Diels-Alder cycloadditions of isoelectronic graphene and hexagonal boron nitride in oriented external electric fields: reaction axis rule vs. polarization axis rule

The present study introduces the mechanisms for the oriented external electric field (OEEF)-participating cycloadditions of nanographene and the analogous hexagonal boron nitride (h-BN) nanoflakes. Despite the C-C and B-N pairs being isoelectronic, their different ionicities give rise to their distinct response to applied electric fields. For the nanographene models, the Diels-Alder addition obeyed the reaction axis rule and the activation barrier changed under an OEEF perpendicular to the carbon skeleton for enhanced/reduced intermolecular charge transfer, which provides a feasible strategy for the side-selective derivatization of graphene to obtain one-face-only adducts and Janus bifunctional products. By contrast, for the h-BN models, the variation of the activation barrier was pronounced when the electric field was aligned along the in-plane N-B bond rather than the well-accepted reaction axis. Electronic structure analyses indicated that, because of the opposite electron withdrawing/donating nature of the reacting sites of B/N, an OEEF along the N-B bond was capable of further enhancing the polarization via in-plane intramolecular charge transfer, resulting in a stabilized transition state and notable barrier reduction.

Electric Field Measurements Reveal the Pivotal Role of Cofactor-Substrate Interaction in Dihydrofolate Reductase Catalysis

The contribution of ligand–ligand electrostatic interaction to transition state formation during enzyme catalysis has remained unexplored, even though electrostatic forces are known to play a major role in protein functions and have been investigated by the vibrational Stark effect (VSE). To monitor electrostatic changes along important steps during catalysis, we used a nitrile probe (T46C-CN) inserted proximal to the reaction center of three dihydrofolate reductases (DHFRs) with different biophysical properties, Escherichia coli DHFR (EcDHFR), its conformationally impaired variant (EcDHFR-S148P), and Geobacillus stearothermophilus DHFR (BsDHFR). Our combined experimental and computational approach revealed that the electric field projected by the substrate toward the probe negates those exerted by the cofactor when both are bound within the enzymes. This indicates that compared to previous models that focus exclusively on subdomain reorganization and protein–ligand contacts, ligand–ligand interactions are the key driving force to generate electrostatic environments conducive for catalysis.

Mechanism of Color and Photoacidity Tuning for the Protonated Green Fluorescent Protein Chromophore

The neutral or A state of the green fluorescent protein (GFP) chromophore is a remarkable example of a photoacid naturally embedded in the protein environment and accounts for the large Stokes shift of GFP in response to near UV excitation. Its color tuning mechanism has been largely overlooked, as it is less preferred for imaging applications than the redder anionic or B state. Past studies, based on site-directed mutagenesis or solvatochromism of the isolated chromophore, have concluded that its color tuning range is much narrower than its anionic counterpart. However, as we performed extensive investigation on more GFP mutants, we found that the color of the neutral chromophore can be more sensitive to protein electrostatics than can the anionic counterpart. Electronic Stark spectroscopy reveals a fundamentally different electrostatic color tuning mechanism for the neutral state of the chromophore that demands a three-form model as compared to that of the anionic state, which requires only two forms ( J. Am. Chem. Soc. 2019, 141, 15250-15265). Specifically, an underlying zwitterionic charge-transfer state is required to explain its sensitivity to electrostatics. As the Stokes shift is tightly linked to excited-state proton transfer (ESPT) of the protonated chromophore, we infer design principles of the GFP chromophore as a photoacid through the color tuning mechanisms of both protonation states. The three-form model could also be applied to similar biological and nonbiological dyes and complements the failure of the two-form model for donor-acceptor systems with localized ground-state electronic distributions.

Electric Field Effect on Condensed-Phase Molecular Systems. X. Interconversion Dynamics and Vibrational Stark Effect of Hydrogen Chloride Clusters in an Argon Matrix

In this study, the effect of a strong (≤4 × 10⁸ V·m^-1) dc electric field on hydrogen chloride (HCl) dimers and trimers isolated in a solid argon matrix has been investigated using the ice film nanocapacitor and reflection-absorption infrared spectroscopy methods. The H-Cl vibrational bands of the HCl dimers showed a linear Stark frequency shift and an increased intensity under the applied electric field, and these changes were reversible with the electric field strength. This behavior indicated that the dimers were reoriented by the applied electric field. The reorientation occurred via tunneling inversion of individual HCl subunits of the dimer, which interconverted the proton-accepting and -donating HCl subunits, as observed for the heterodimers HCl-DCl and DCl-HCl. The interconversion of dimers could occur even at low electric field strength (∼10⁷ V·m^-1) and was almost complete above the field strength of 1.0 × 10⁸ V·m^-1. In contrast, the asymmetric H-Cl stretching bands of the HCl trimers exhibited Stark broadening under the influence of the electric field without a shift in frequency or change in intensity. This behavior indicated that the cyclic structure of the HCl trimer was stable even when subjected to a strong electric field. The Stark sensitivity factor (Δμ) of H-Cl vibrations was deduced from the Stark effect analysis of the HCl dimer and trimer bands, which gave the following: Δμ_D1 = 2.3 ± 0.2 cm^-1/(10⁸ V·m^-1) for the proton-acceptor subunit of the dimer, Δμ_D2 = 5.1 ± 0.5 cm^-1/(10⁸ V·m^-1) for the proton-donor subunit of the dimer, and Δμ_T = 4.5 ± 0.5 cm^-1/(10⁸ V·m^-1) for the asymmetric stretching vibration of the cyclic trimer.

Assessing the Location of Ionic and Molecular Solutes in a Molecularly Heterogeneous and Nonionic Deep Eutectic Solvent

Deep eutectic solvents (DES) are emerging sustainable designer solvents viewed as greener and better alternatives to ionic liquids. Nonionic DESs possess unique properties such as viscosity and hydrophobicity that make them desirable in microextraction applications such as oil-spill remediation. This work builds upon a nonionic DES, NMA–LA DES, previously designed by our group. The NMA–LA DES presents a rich nanoscopic morphology that could be used to allocate solutes of different polarities. In this work, the possibility of solvating different solutes within the nanoscopically heterogeneous molecular structure of the NMA–LA DES is investigated using ionic and molecular solutes. In particular, the localized vibrational transitions in these solutes are used as reporters of the DES molecular structure via vibrational spectroscopy. The FTIR and 2DIR data suggest that the ionic solute is confined in a polar and continuous domain formed by NMA, clearly sensing the direct effect of the change in NMA concentration. In the case of the molecular nonionic and polar solute, the data indicates that the solute resides in the interface between the polar and nonpolar domains. Finally, the results for the nonpolar and nonionic solute (W(CO)₆) are unexpected and less conclusive. Contrary to its polarity, the data suggest that the W(CO)₆ resides within the NMA polar domain of the DES, probably by inducing a domain restructuring in the solvent. However, the data are not conclusive enough to discard the possibility that the restructuring comprises not only the polar domain but also the interface. Overall, our results demonstrate that the NMA–LA DES has nanoscopic domains with affinity to particular molecular properties, such as polarity. Thus, the presented results have a direct implication to separation science.

Curved Carbon Skeleton in Oriented External Electric Fields: Modulated Curvature, Directional Bowl Inversion, and Face-Selective Cycloadditions of Corannulene

We introduced the oriented-external-electric-field-induced modification of bowl-shaped corannulene using density functional theory calculations. The results show that the electric field is capable of significantly modulating the polarization and electrostatic characteristics of the concave and convex surfaces of buckybowls. The structure-energy-reactivity relation can be precisely controlled, leading to a variety of unconventional properties for practical applications.

Internal electric fields in methanol [MeOH]2-6 clusters

Water and methanol are well known solvents showing cooperative hydrogen bonding, however the differences in the hydrogen bonding pattern in water and methanol are due to the presence of the methyl group in methanol. The presence of the methyl group leads to formation of C-HO hydrogen bonds apart from the usual O-HO hydrogen bonds. The electric fields evaluated along the hydrogen bonded donor OH and CH groups reveal that the C-HO hydrogen bonds can significantly influence the structure and energetics (by about 20%) of methanol clusters. A linear Stark effect was observed on the hydrogen bonded OH groups in methanol clusters with a Stark tuning rate of 3.1 cm^-1 (MV cm^-1)^-1 as an average behaviour. Furthermore, the Stark tuning of the OH oscillators in methanol depends on their hydrogen bonding environment wherein molecules with the DAA motif show higher rates than the rest. The present work suggests that the OH group of methanol has higher sensitivity as a vibrational probe relative to the OH group of water.

Biosynthetic Incorporation of Site-Specific Isotopes in β-Lactam Antibiotics Enables Biophysical Studies

A biophysical understanding of the mechanistic, chemical, and physical origins underlying antibiotic action and resistance is vital to the discovery of novel therapeutics and the development of strategies to combat the growing emergence of antibiotic resistance. The site-specific introduction of stable-isotope labels into chemically complex natural products is particularly important for techniques such as NMR, IR, mass spectrometry, imaging, and kinetic isotope effects. Toward this goal, we developed a biosynthetic strategy for the site-specific incorporation of ¹³C labels into the canonical β-lactam carbonyl of penicillin G and cefotaxime, the latter via cephalosporin C. This was achieved through sulfur-replacement with 1-¹³C-l-cysteine, resulting in high isotope incorporations and milligram-scale yields. Using ¹³C NMR and isotope-edited IR difference spectroscopy, we illustrate how these molecules can be used to interrogate interactions with their protein targets, e.g., TEM-1 β-lactamase. This method provides a feasible route to isotopically labeled penicillin and cephalosporin precursors for future biophysical studies.

Effect of Oriented External Electric Fields on the Photo and Thermal Isomerization of Azobenzene

Azobenzene is a prototype molecule with potential applications in molecular switches, solar thermal batteries, sensors, photoresponsive membranes, molecular electronics, data storage, and nonlinear optics. Photo and thermal isomerization pathways exhibit different charge-transfer character and dipole moments, implying that the use of electric fields can be used to modulate the reactivity of azobenzene. This article examines the differential effect of orientated electric fields on the rotation and inversion thermal and photoisomerization pathways of azobenzene to explore the feasibility of using electric fields in the design of azobenzene-based molecular devices. Our findings demonstrate that the application of orientated electric fields modifies the accessibility of the S₀/S₁ seam of electronic degeneracy, as well as changes the energetically favored relaxation pathway in the branching space to yield different photoproducts. In addition, we observed strong-field dipole-inversion effects that cause a topographical change in the response of the potential energy surface to the applied field and can result in geometric minima that do not exist under field-free conditions. On the S₀ surface, transition barriers can be modified on the order of ±10 kcal mol^-1, enabling control of thermal isomerization rates.

Bend-to-Break: Curvilinear Proton Transfer in Phenol-Ammonia Clusters

The electric field experienced by the OH group of phenol embedded in the cluster of ammonia molecules depends on the relative orientation of the ammonia molecules, and a critical field of 236 MV cm^-1 is essential for the transfer of a proton from phenol to the surrounding ammonia cluster. However, exceptions to this rule were observed, which indicates that the projection of the solvent electric field over the O-H bond is not a definite descriptor of the proton transfer reaction. Therefore, a critical electric field is necessary, but it is not a sufficient condition for the proton abstraction. This, in combination with an adequate solvation of the acceptor ammonia molecule in a triple donor motif that energetically favors the proton transfer process, constitutes necessary and sufficient conditions for the spontaneous proton abstraction. The proton transfer process in phenol-(ammonia)_n clusters is statistically favored to occur away from the plane of the phenyl ring and follows a curvilinear path which includes the O-H bond elongation and out-of-plane movement of the proton. Colloquially, this proton transfer can be referred to as a "bend-to-break" process.

A Critical Evaluation of Vibrational Stark Effect (VSE) Probes with the Local Vibrational Mode Theory

Over the past two decades, the vibrational Stark effect has become an important tool to measure and analyze the in situ electric field strength in various chemical environments with infrared spectroscopy. The underlying assumption of this effect is that the normal stretching mode of a target bond such as CO or CN of a reporter molecule (termed vibrational Stark effect probe) is localized and free from mass-coupling from other internal coordinates, so that its frequency shift directly reflects the influence of the vicinal electric field. However, the validity of this essential assumption has never been assessed. Given the fact that normal modes are generally delocalized because of mass-coupling, this analysis was overdue. Therefore, we carried out a comprehensive evaluation of 68 vibrational Stark effect probes and candidates to quantify the degree to which their target normal vibration of probe bond stretching is decoupled from local vibrations driven by other internal coordinates. The unique tool we used is the local mode analysis originally introduced by Konkoli and Cremer, in particular the decomposition of normal modes into local mode contributions. Based on our results, we recommend 31 polyatomic molecules with localized target bonds as ideal vibrational Stark effect probe candidates.

Tip-Enhanced Multipolar Raman Scattering

We record nanoscale chemical images of thiobenzonitrile (TBN)-functionalized plasmonic gold nanocubes via tip-enhanced Raman spectroscopy (TERS). The spatially averaged optical response is dominated by conventional (dipolar) TERS scattering from TBN but also contains weaker spectral signatures in the 1225-1500 cm^-1 region. The weak optical signatures dominate several of the recorded single-pixel TERS spectra. We can uniquely assign these Raman-forbidden transitions to multipolar Raman scattering, which implicates spatially varying enhanced electric field gradients at plasmonic tip-sample nanojunctions. Specifically, we can assign observations of tip-enhanced electric dipole-magnetic dipole as well as electric dipole-electric quadrupole driven transitions. Multipolar Raman scattering and local optical field gradients both need to be understood and accounted for in the interpretation of TERS spectral images, particularly in ongoing quests aimed at chemical reaction mapping via TERS.

Evaluating the Solvent Stark Effect from Temperature‐Dependent Solvatochromic Shifts of Anthracene

The solvent Stark effect on the spectral shifts of anthracene is studied with temperature‐dependent solvatochromic measurements. The Stark contribution Δv_Stark to the absorption shift Δv_p in polar solvents is measured to be Δv_Stark=(53±35) cm⁻¹, in reasonable agreement with dielectric continuum theory estimate of 28 cm⁻¹, whereas the major shift Δv_p∼300 cm⁻¹ presumably originates from the solute quadrupole. We pay attention to the accurate correction of Δv_p for the nonpolar contribution that is crucial when the shifts are modest in magnitude.

Infrared Difference Spectroscopy of Proteins: From Bands to Bonds

Infrared difference spectroscopy probes vibrational changes of proteins upon their perturbation. Compared with other spectroscopic methods, it stands out by its sensitivity to the protonation state, H-bonding, and the conformation of different groups in proteins, including the peptide backbone, amino acid side chains, internal water molecules, or cofactors. In particular, the detection of protonation and H-bonding changes in a time-resolved manner, not easily obtained by other techniques, is one of the most successful applications of IR difference spectroscopy. The present review deals with the use of perturbations designed to specifically change the protein between two (or more) functionally relevant states, a strategy often referred to as reaction-induced IR difference spectroscopy. In the first half of this contribution, I review the technique of reaction-induced IR difference spectroscopy of proteins, with special emphasis given to the preparation of suitable samples and their characterization, strategies for the perturbation of proteins, and methodologies for time-resolved measurements (from nanoseconds to minutes). The second half of this contribution focuses on the spectral interpretation. It starts by reviewing how changes in H-bonding, medium polarity, and vibrational coupling affect vibrational frequencies, intensities, and bandwidths. It is followed by band assignments, a crucial aspect mostly performed with the help of isotopic labeling and site-directed mutagenesis, and complemented by integration and interpretation of the results in the context of the studied protein, an aspect increasingly supported by spectral calculations. Selected examples from the literature, predominately but not exclusively from retinal proteins, are used to illustrate the topics covered in this review.

Globally optimal catalytic fields for a Diels-Alder reaction

In a previous paper [M. Dittner and B. Hartke, J. Chem. Theory Comput. 14, 3547 (2018)], we introduced a preliminary version of our GOCAT (globally optimal catalyst) concept in which electrostatic catalysts are designed for arbitrary reactions by global optimization of distributed point charges that surround the reaction. In this first version, a pre-defined reaction path was kept fixed. This unrealistic assumption allowed for only small catalytic effects. In the present work, we extend our GOCAT framework by a sophisticated and robust on-the-fly reaction path optimization, plus further concomitant algorithm adaptions. This allows smaller and larger excursions from a pre-defined reaction path under the influence of the GOCAT point-charge surrounding, all the way to drastic mechanistic changes. In contrast to the restricted first GOCAT version, this new version is able to address real-life catalysis. We demonstrate this by applying it to the electrostatic catalysis of a prototypical Diels-Alder reaction. Without using any prior information, this procedure re-discovers theoretically and experimentally established features of electrostatic catalysis of this very reaction, including a field-dependent transition from the synchronous, concerted textbook mechanism to a zwitterionic two-step mechanism, and diastereomeric discrimination by suitable electric field components.

Tip-Enhanced Raman Nanospectroscopy of Smooth Spherical Gold Nanoparticles

We record nanoscale-resolved chemical images of thiobenzonitrile (TBN)-functionalized smooth gold nanospheres on silicon via tip-enhanced Raman (TER) nanospectroscopy. The recorded images trace the nascence of the familiar doughnut-shaped scattering profile of nanoparticles on silicon at its origin (the particle surface), which appears as a horseshoe-shaped scattering pattern under our experimental conditions. The local optical field maps are in agreement with their simulated finite-difference time-domain analogues. Analysis of the recorded spectra with the aid of ab-initio-molecular-dynamics-based Raman spectral simulations further suggests that optical rectification and molecular charging take place throughout the course of atomic-force-microscopy-based TER nanoscale chemical imaging.

A Closer Look at Corrugated Au Tips

A series of optical and electron microscopies are utilized in concert to unravel the properties of corrugated metallic tips. While the overall microscopic shapes of the tips dictate their optical resonances and plasmonic field enhancement factors, nanometric structural details govern their tip-enhanced Raman (TER) spectra and images. Using 4-thiobenzonitrile (TBN) as a molecular reporter, spatially resolved TER spectra reveal that optical rectification and molecular charging are among the prominent observables in the tip-tip TER geometry. We show the spurious appearance of anions is driven by highly localized resonances that appear as a result of surface corrugation and their manifestation throughout the course of TER nanospectroscopy complicates spectral assignments. Overall, nanoscale spatial variations in the TERS spectra suggest that the tip-tip geometry sustains junction plasmons that appear very different from what may be expected from the hybridization of the bulk tip resonances.

Vibrational Stark Spectroscopy of Fluorobenzene Using Quantum Cascade Laser Dual Frequency Combs

We demonstrate the performance of a dual frequency comb quantum cascade laser (QCL) spectrometer for the application of vibrational Stark spectroscopy. Measurements performed on fluorobenzene with the dual-comb spectrometer (DCS) were compared to results obtained using a conventional Fourier transform infrared (FT-IR) instrument in terms of spectral response, parameter estimation, and signal-to-noise ratio (S/N). The dual-comb spectrometer provided similar qualitative and quantitative data as the FT-IR setup in 250 times shorter acquisition time. For fluorobenzene, the DCS measurement resulted in a more precise estimation of the fluorobenzene Stark tuning rate ((0.81 ± 0.09) cm^-1/(MV/cm)) than with the FT-IR system ((0.89 ± 0.15) cm^-1/(MV/cm)). Both values are in accordance with the previously reported value of 0.84 cm^-1/(MV/cm). We also point to an improvement of signal-to-noise ratio in the DCS configuration. Additional characteristics of the dual-comb spectrometer applicable to vibrational Stark spectroscopy and their scaling properties for future applications are discussed.

Vibrational Spectroscopy Reveals Effects of Electron Push-Pull and Solvent Polarity on Electron Delocalization in Radical Anions of Donor-Acceptor Compounds

The nature of excess electrons has been studied in donor-acceptor (D-A) compounds based on substituted triarylamines and a nitrile-functionalized fluorene by changing the substituents on the triarylamines and also the solvent polarity. We observed that both electron push-pull capability at the distant location in the amine donor unit and solvation in solvents of varying polarity significantly affect the nitrile ν(C≡N) vibrations of the fluorene acceptor unit in radical anions of these D-A compounds. Quantum calculations show that the push-pull capability translates the position of an excess electron while keeping its width relatively constant. On the other hand, solvation changes both, making an electron more compact in polar media. The current study points to the idea that solvation plays a more significant role in controlling the nature of excess electrons, while synthetic modification that influences electron push-pull capability enables further tuning.

Inhomogeneity of Interfacial Electric Fields at Vibrational Probes on Electrode Surfaces

Electric fields control chemical reactivity in a wide range of systems, including enzymes and electrochemical interfaces. Characterizing the electric fields at electrode-solution interfaces is critical for understanding heterogeneous catalysis and associated energy conversion processes. To address this challenge, recent experiments have probed the response of the nitrile stretching frequency of 4-mercaptobenzonitrile (4-MBN) attached to a gold electrode to changes in the solvent and applied electrode potential. Herein, this system is modeled with periodic density functional theory using a multilayer dielectric continuum treatment of the solvent and at constant applied potentials. The impact of the solvent dielectric constant and the applied electrode potential on the nitrile stretching frequency computed with a grid-based method is in qualitative agreement with the experimental data. In addition, the interfacial electrostatic potentials and electric fields as a function of applied potential were calculated directly with density functional theory. Substantial spatial inhomogeneity of the interfacial electric fields was observed, including oscillations in the region of the molecular probe attached to the electrode. These simulations highlight the microscopic inhomogeneity of the electric fields and the role of molecular polarizability at electrode-solution interfaces, thereby demonstrating the limitations of mean-field models and providing insights relevant to the interpretation of vibrational Stark effect experiments.

Electric Fields at Metal-Surfactant Interfaces: A Combined Vibrational Spectroscopy and Capacitance Study

Surfactants modulate interfacial processes. In electrochemical CO₂ reduction, cationic surfactants promote carbon product formation and suppress hydrogen evolution. The interfacial field produced by the surfactants affects the energetics of electrochemical intermediates, mandating their detailed understanding. We have used two complementary tools-vibrational Stark shift spectroscopy which probes interfacial fields at molecular length scales and electrochemical impedance spectroscopy (EIS) which probes the entire double layer-to study the electric fields at metal-surfactant interfaces. Using a nitrile as a probe, we found that at open-circuit potentials, cationic surfactants produce larger effective interfacial fields (∼-1.25 V/nm) when compared to anionic surfactants (∼0.4 V/nm). At a high bulk surfactant concentration, the surface field reaches a terminal value, suggesting the formation of a full layer, which is also supported by EIS. We propose an electrostatic model that explains these observations. Our results help in designing tailored surfactants for influencing electrochemical reactions via the interfacial field effect.