Intermolecular Vibrational Coupling Research Articles

Hydrogen-bonding and vibrational coupling of water in a hydrophobic hydration shell: Significance of alkyl chain configuration and charge on the hydrophobe

The interaction between a molecular hydrophobe and water is crucial for their mutual structure and reactivity. Here, we have investigated the hydrogen-bonding (H-bonding) structure and intermolecular vibrational coupling i.e., the collective nature of water, in the hydration shell (HS) of charged and uncharged molecular hydrophobes, using polarization-resolved Raman difference spectroscopy with simultaneous curve fitting (Raman-DS-SCF) and isotopic dilution spectroscopy. Raman-DS-SCF provides the vibrational spectrum of water pertaining to the HS of the hydrophobes. The HS-spectra reveal that the positive charge on the hydrophobe (e.g., deuterated tetramethylammonium cation, d-TMA+) as well as the branching of the alkyl chain (n-butyl alcohol (BA) vs. tert-butyl alcohol (TBA)) reinforces the intermolecular vibrational coupling of water in the HS. The water-water H-bonding in the HS of d-TMA+ and d-TBA are similar to each other, but stronger than that of the bulk water. Apart from the strongly H-bonded water, the hydrophobic hydration shells exhibit weakly interacting dangling OH (ν¯max≈3670) which is intramolecularly coupled with the H-bonded OH of the same water molecule. The prevalence of the dangling-OH decreases with chain-branching and net positive charge on the hydrophobe: BA > d-TBA > d-TMA+. Altogether, the collective nature of water and its propensity for dangling-OH in a hydrophobic hydration shell strongly depend on the net charge and alkyl chain configuration of the hydrophobe.

Vibrational Coupling and Hydrogen-Bond Structure of Water in the Extended Hydration Shell of Metal Ions.

Intra- and intermolecular vibrational coupling (VC) and hydrogen bonding (H-bonding) of water are sparsely understood in the hydration shell (HS) of a metal ion, though the corresponding knowledge for an anion is quite extensive. This is primarily due to the overwhelming effect of anions on water, which masks the subtle perturbing influence of most of the cations. Using Raman difference spectroscopy with simultaneous curve fitting (Raman-DS-SCF) in combination with isotopic dilution and polarized Raman spectroscopy, we have elucidated the VC and H-bonding of water in the HS of bi- and trivalent metal ions─Mg2+, Ca2+, La3+, Gd3+, Dy3+. Polarized Raman measurement of the HS water with VC "turned on" and "turned off" (using isotopically diluted water, HOD) reveals that water retains the intra- and intermolecular vibrational coupling in the HS of high-charge-density metal ions, which is in stark contrast to that of an anion. Hydration shell spectroscopy in HOD unambiguously shows that the average H-bonding of water becomes stronger in the HS than that of bulk water. The first HS water strongly donates two H-bonds to the second HS water (ν̅max ≈ 3200 cm-1) but weakly accepts a H-bond from the second HS water (ν̅max ≈ 3590 cm-1), which makes the HS water heterogeneous in terms of its H-bond structure. The weakly interacting OH (ν̅max 3585 cm-1 in HOD) red-shifts by ∼ 15 cm-1 while the VC is "turned on" (ν̅max ≈ 3600 cm-1 in H2O), revealing the intramolecular coupling of water in the HS of metal ions.

Spin-Based Chiral Separations and the Importance of Molecule-Solvent Interactions.

This work uses magneto-electrochemical quartz crystal microbalance methods to study the enantiospecific adsorption of chiral molecules onto a ferromagnetic substrate. The effects of solution conditions, pH, and solvent isotope composition indicate that the kinetics of the enantiomeric adsorption depend strongly on the charge state and geometry of the adsorbate, whereas no thermodynamic contributions to enantiospecificity are found. Density functional theory calculations reveal that an interplay between the adsorbate and solvent molecules is important for defining the observed enantiospecific preference with an applied magnetic field; however, it remains unclear if intermolecular vibrational couplings contribute to the phenomenon.

Characterizing Interfaces by Voronoi Tessellation.

The chemistry of interfaces differs markedly from that of the bulk. Calculation of interfacial properties depends strongly on the definition of the interface, which can lead to ambiguous results that vary between studies. There is a need for a method that can explicitly define the interfaces and boundaries in molecular systems. Voronoi tessellation offers an attractive solution to this problem through its ability to determine neighbors among specified groups of atoms. Here we discuss three cases where Voronoi tessellation combined with modeling of vibrational sum frequency generation (SFG) spectroscopy yields relevant insights: the breakdown of the air-water interface into clear and intuitive molecular layers, the study of the hydration shell in biological systems, and the acceleration of difficult spectral calculations where intermolecular vibrational couplings dominate. The utility of Voronoi tessellation has broad applications that extend beyond any single type of spectroscopy or system.

The role of resonant coupling in vibrational sum-frequency-generation spectroscopy: Liquid acetonitrile at the silica interface

Vibrational sum-frequency-generation (VSFG) spectroscopy is a versatile technique for probing molecular organization at interfaces. There is a growing recognition that dynamic phenomena, such as reorientation and intermolecular vibrational coupling, can also influence VSFG spectra. The silica/liquid acetonitrile interface is a useful system for exploring these effects in more detail. The organization of acetonitrile at this interface has been well studied, and is known to resemble that of a supported lipid bilayer in many ways. Here isotopic dilution is used to explore the influence of resonant intermolecular coupling of methyl symmetric stretches on the VSFG spectroscopy of this system. VSFG spectra in the methyl stretching region at this interface show a blue shift, a decrease in linewidth, and a higher-than-expected intensity upon dilution in deuterated acetonitrile. We demonstrate that resonant coupling influences VSFG spectral shifts through the infrared transition. Using molecular simulations, we show that our experimental observations are consistent with resonant coupling between methyl transition dipoles being a significant, but not the dominant, contribution to the observed spectral shift upon isotopic dilution. Furthermore, our molecular simulations demonstrate that resonant coupling accounts partially for changes in linewidth and intensity. These classical molecular dynamics simulations also elucidate the behavior of the isotropic Raman spectrum in the bulk liquid upon isotopic dilution. We further simulate the resonant coupling among cyano stretches in this system. These simulations match the experimental shift due to the Raman non-coincidence effect shift of the CN stretch in the bulk liquid, but suggest that the corresponding resonant-coupling-induced shift in the VSFG spectrum at the silica interface is minimal. Our experimental and simulation results indicate that proximity to an interface can cause substantial changes in the resonant coupling of vibrations in a liquid.

Isotopic dependence of intramolecular and intermolecular vibrational couplings in cooperative hydrogen bond networks: singly hydrated phenyl-α-D-mannopyranoside as a case study.

Hydrogen bonding (HB) is associated with frequency shifts, spectral broadening and intensity variation of the vibrational bands of the donor stretching modes. This is true in all systems, from the most basic molecular models, to more complex ones, and biological molecules. In the gas phase, the latter can be either fully isolated, with only intramolecular HB, or micro-solvated. The conformations of such systems are stabilized by networks of intramolecular and intermolecular HB where the donor groups can be coupled. This has been well-identified in the case of singly hydrated monosaccharides and in particular for phenyl-α-D-mannopyranoside, where the addition of a single water molecule reduces the number of observed conformations to a unique one, stabilized by such a cooperative network of intramolecular and intermolecular HB. In the present study we have re-examined this prototypical system to scrutinize subtle effects of isotopic substitution in the solvent molecule. Besides the obvious isotopic shift, coupling between intramolecular modes of sugar and water is observed, promoted by the intermolecular HB. The systematic substitution of water with heavy water, or methanol, also allowed the decomposition of the relation between HB strength and frequency shift.

Collective Mid-Infrared Vibrations in Surface-Enhanced Raman Scattering.

Surface-enhanced Raman scattering (SERS) is typically assumed to occur at individual molecules neglecting intermolecular vibrational coupling. Here, we show instead how collective vibrations from infrared (IR) coupled dipoles are seen in SERS from molecular monolayers. Mixing IR-active molecules with IR-inactive spacer molecules controls the intermolecular separation. Intermolecular coupling leads to vibrational frequency upshifts up to 8 cm–1, tuning with the mixing fraction and IR dipole strength, in excellent agreement with microscopic models and density functional theory. These cooperative frequency shifts can be used as a ruler to measure intermolecular distance and disorder with angstrom resolution. We demonstrate this for photochemical reactions of 4-nitrothiophenol, which depletes the number of neighboring IR-active molecules and breaks the collective vibration, enabling direct tracking of the reaction. Collective molecular vibrations reshape SERS spectra and need to be considered in the analysis of vibrational spectra throughout analytical chemistry and sensing.

Modular Assembly of Vibrationally and Electronically Coupled Rhenium Bipyridine Carbonyl Complexes on Silicon.

Hybrid inorganic/organic heterointerfaces are promising systems for next-generation photocatalytic, photovoltaic, and chemical-sensing applications. Their performance relies strongly on the development of robust and reliable surface passivation and functionalization protocols with (sub)molecular control. The structure, stability, and chemistry of the semiconductor surface determine the functionality of the hybrid assembly. Generally, these modification schemes have to be laboriously developed to satisfy the specific chemical demands of the semiconductor surface. The implementation of a chemically independent, yet highly selective, standardized surface functionalization scheme, compatible with nanoelectronic device fabrication, is of utmost technological relevance. Here, we introduce a modular surface assembly (MSA) approach that allows the covalent anchoring of molecular transition-metal complexes with sub-nanometer precision on any solid material by combining atomic layer deposition (ALD) and selectively self-assembled monolayers of phosphonic acids. ALD, as an essential tool in semiconductor device fabrication, is used to grow conformal aluminum oxide activation coatings, down to sub-nanometer thicknesses, on silicon surfaces to enable a selective step-by-step layer assembly of rhenium(I) bipyridine tricarbonyl molecular complexes. The modular surface assembly of molecular complexes generates precisely structured spatial ensembles with strong intermolecular vibrational and electronic coupling, as demonstrated by infrared spectroscopy, photoluminescence, and X-ray photoelectron spectroscopy analysis. The structure of the MSA can be chosen to avoid electronic interactions with the semiconductor substrate to exclusively investigate the electronic interactions between the surface-immobilized molecular complexes.

Hydration and Hydrogen Bond Order of Octadecanoic Acid and Octadecanol Films on Water at 21 and 1 °C.

The temperature-dependent hydration structure of long-chain fatty acids and alcohols at air-water interfaces has great significance in the fundamental interactions underlying ice nucleation in the atmosphere. We present an integrated theoretical and experimental study of the temperature-dependent vibrational structure and electric field character of the immediate hydration shells of fatty alcohol and acid headgroups. We use a combination of surface-sensitive infrared reflection-absorption spectroscopy (IRRAS), surface potentiometry, and ab initio molecular dynamics simulations to elucidate detailed molecular structures of the octadecanoic acid and octadecanol (stearic acid and stearyl alcohol) headgroup hydration shells at room temperature and near freezing. In experiments, the alcohol at high surface concentration exhibits the largest surface potential; yet we observe a strengthening of the hydrogen-bonding for the solvating water molecules near freezing for both the alcohol and the fatty acid IRRAS experiments. Results reveal that the hydration shells for both compounds screen their polar headgroup dipole moments reducing the surface potential at low surface coverages; at higher surface coverage, the polar headgroups become dehydrated, which reduces the screening, correlating to higher observed surface potential values. Lowering the temperature promotes tighter chain packing and an increase in surface potential. IRRAS reveals that the intra- and intermolecular vibrational coupling mechanisms are highly sensitive to changes in temperature. We find that intramolecular coupling dominates the vibrational relaxation pathways for interfacial water determined by comparing the H2O and the HOD spectra. Using ab initio molecular dynamics (AIMD) calculations on cluster systems of propanol + 6H2O and propionic acid + 10H2O, a spectral decomposition scheme was used to correlate the OH stretching motion with the IRRAS spectral features, revealing the effects of intra- and intermolecular coupling on the spectra. Spectra calculated with AIMD reproduce the red shift and increase in intensity observed in experimental spectra corresponding to the OH stretching region of the first solvation shell. These findings suggest that intra- and intermolecular vibrational couplings strongly impact the OH stretching region at fatty acid and fatty alcohol water interfaces. Overall, results are consistent with ice templating behavior for both the fatty acid and the alcohol, yet the surface potential signature is strongest for the fatty alcohol. These findings develop a better understanding of the complex surface potential and spectral signatures involved in ice templating.

Simulation of the Chiral Sum Frequency Generation Response of Supramolecular Structures Requires Vibrational Couplings.

Chiral vibrational sum frequency generation (SFG) spectroscopy probes the structure of the solvation shell around chiral macromolecules. The dominant theoretical framework for understanding the origin of chiral SFG signals is based on the analysis of molecular symmetry, which assumes no interaction between molecules. However, water contains strong intermolecular interactions that significantly affect its properties. Here, the role of intermolecular vibrational coupling in the chiral SFG response of the O-H stretch of water surrounding an antiparallel β-sheet at the vacuum-water interface is investigated. Both intramolecular and intermolecular couplings between O-H groups are required to simulate the full lineshape of the chiral SFG signal. This dependence is also observed for a chiral water dimer, illustrating that this phenomenon is not specific to larger systems. We also find that a dimer of C3v molecules predicted to be chirally SFG-inactive by the symmetry-based theory can generate a chiral SFG signal when intermolecular couplings are considered, suggesting that even highly symmetric solvent molecules may produce chiral SFG signals when interacting with a chiral solute. The consideration of intermolecular couplings extends the prevailing theory of the chiral SFG response to structures larger than individual molecules and provides guidelines for future modeling.

Enhancing the Performance of Organic Solar Cells by Prolonging the Lifetime of Photogenerated Excitons.

Exciton lifetime (τ) is crucial for the migration of excitons to donor/acceptor interfaces for subsequent charge separation in organic solar cells (OSCs); however, obvious prolongation of τ has rarely been achieved. Here, by introducing a solid additive 9-fluorenone-1-carboxylic acid (FCA) into the active layer, which comprises a nonfullerene acceptor, 3,9-bis(2-methylene-((3-(1,1-dicyanomethylene)-6/7-methyl)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2',3'-d']-s-indaceno[1,2-b:5,6-b']dithiophene (IT-M), τ is substantially prolonged from 491 to 928 ps, together with obvious increases in fluorescence intensity and quantum yield. Time-resolved transient infrared spectra indicate the presence of an intermolecular vibrational coupling between the electronic excited state of IT-M and the electronic ground state of FCA, which is first observed here and which can suppress the internal conversion process. IT-M-based OSCs display an improved short-circuit current and fill factor after the addition of FCA. Thus, the power conversion efficiency is increased, particularly for devices with a large donor/acceptor ratio of 1:4, whose efficiency is increased by 56%. This study describes a novel method, which is also applicable to other nonfullerene acceptors, for further improving the performance of OSCs without affecting their morphology and light absorption properties.

New evidence on non-covalent interactions in crystalline halo-substituted boron difluoride acetylacetonates from vibrational spectra, model calculations and visualization program tools

DFT calculations were carried out for two dimer forms and tetramers to model equatorial and axial noncovalent intermolecular interactions in the crystalline samples of F2B(AcAcX), where X = H, Cl, Br, I. Vibrational frequency shifts for dimer and tetramer models in reference to their monomer frequency values were analyzed in terms of their correlation with calculated energetic characteristics and experimental spectral and geometrical features. The role of intermolecular vibrational coupling in the νBF2 range of the parent compound was shown. RDG plots and NCI isosurfaces in combination with vibrational spectra were applied to discriminate and describe different kinds of molecular interactions in the crystalline state of the compounds. In particular, halogen bonding and van der Waals bonds were revealed.

The Bending Mode of Water: A Powerful Probe for Hydrogen Bond Structure of Aqueous Systems.

Insights into the microscopic structure and dynamics of the water’s hydrogen-bonded network are crucial to understand the role of water in biology, atmospheric and geochemical processes, and chemical reactions in aqueous systems. Vibrational spectroscopy of water has provided many such insights, in particular using the O–H stretch mode. In this Perspective, we summarize our recent studies that have revealed that the H–O–H bending mode can be an equally powerful reporter for the microscopic structure of water and provides more direct access to the hydrogen-bonded network than the conventionally studied O–H stretch mode. We discuss the fundamental vibrational properties of the water bending mode, such as the intermolecular vibrational coupling, and its effects on the spectral lineshapes and vibrational dynamics. Several examples of static and ultrafast bending mode spectroscopy illustrate how the water bending mode provides an excellent window on the microscopic structure of both bulk and interfacial water.

Probing the intermolecular coupled vibrations in a water cluster with inelastic electron tunneling spectroscopy.

The hydrogen-bonding networks of water have strong intra- and intermolecular vibrational coupling which influences the energy dissipation and proton transfer in water. Disentangling and quantitative characterization of different coupling effects in water at a single-molecular level still remains a great challenge. Using tip-enhanced inelastic electron tunneling spectroscopy (IETS) based on low-temperature scanning tunneling microscopy, we report the direct quantitative assessment of the intermolecular coupling constants of the OH-stretch vibrational bands of an isolated water tetramer adsorbed on a Au(111)-supported NaCl(001) bilayer film. This is achieved by distinguishing various coupled modes of the H-bonded O-H stretching vibrations through tip-height dependent IET spectra. In contrast, such vibrational coupling is negligible in the half-deuterated water tetramer owing to the large energy mismatch between the OH and OD stretching modes. Not only do these findings advance our understanding on the effects of local environment on the intermolecular vibrational coupling in water, but also open up a new route for vibrational spectroscopic studies of extended H-bonded network at the single-molecular level.

Vibrational Coupling on Stepwise Hydrogen Bond Formation of Amide I.

Despite the key roles of proteins and nucleic acids in biology, understanding their labile structures and hydrogen bond interactions with guest molecules has posed a critical challenge to the scientific community. In this report, I have used dimethylformamide as a model amide to account for amide hydrogen bond interactions of protein. To quantify hydrogen bond conformation and the structural change, I have monitored the amide I infrared (IR) stretching frequencies while varying the pKa of phenol derivatives. For all phenol derivatives, amide I has formed one hydrogen bond and two hydrogen bond conformation. It has been observed that the formation constant for one hydrogen bond is higher than that of two hydrogen bonds for all phenol derivatives. During the formation of hydrogen bond with amide I, IR absorbance of C═C transition is enhanced for all phenol derivatives. Enhancement of the IR absorbance of the C═C transition indicates hydrogen bond-assisted vibrational coupling between the amide I and phenol ring transition. The relative coupling constant is estimated to be higher for single hydrogen-bonded conformer than the double hydrogen-bonded conformer. This is an intriguing result as the frequency difference between the two coupled transitions predicts otherwise. Using IR absorption spectroscopy, a delicate interplay between hydrogen bonding conformations and intermolecular vibrational coupling between amide I and H-bond donor phenol molecules has been shown. This study can be used as a point of reference for understanding the structural information of proteins, peptides, and nucleosides having hydrogen bond interaction with any drug or ligand molecules. My results as well provide an insight into the vibrational coupling of carbonyl and C═C transition of nucleobases.

Signatures of Intra- and Intermolecular Vibrational Coupling in Halogenated Liquids Revealed by Two-Dimensional Raman-Terahertz Spectroscopy.

Hybrid two-dimensional (2D) Raman-terahertz spectroscopy with the Raman-terahertz-terahertz (RTT) pulse sequence is used to explore the ultrafast intra- and intermolecular degrees of freedom of liquid bromoform (CHBr3) in the frequency range of 1-8 THz. Cross peaks observed in these 2D spectra are assigned to the coupling between the narrow intramolecular modes of the molecules and the much broader intermolecular degrees of freedom of the liquid. This assignment is based on the frequency position of the cross peaks; however, it is shown that these frequency positions can be deduced accurately only when properly taking into account the convolution of the molecular response with the instrument response function of the experimental setup, the latter of which distorts the 2D spectra considerably. The assignment is supported by additional experiments on diiodomethane (CH2I2), which has only one intramolecular mode in the frequency range of the experiment, and hence excludes the possibility of intramolecular couplings.

Impact of intermolecular vibrational coupling effects on the sum-frequency generation spectra of the water/air interface

ABSTRACTWe have examined the impact of intermolecular vibrational coupling effects of the O-H stretch modes, as obtained by the surface-specific velocity-velocity correlation function approach, on the simulated sum-frequency generation spectra of the water/air interface. Our study shows that the inclusion of intermolecular coupling effects within the first three water layers, i.e. from the water/air interface up to a distance of 6 Å towards the bulk, is essential to reproduce the experimental SFG spectra. In particular, we find that these intermolecular vibrational contributions to the SFG spectra of the water/air interface are dominated by the coupling between the SFG active interfacial and SFG inactive bulk water molecules. Moreover, we find that most of the intermolecular vibrational contributions to the spectra originate from the coupling between double-donor water molecules only, whereas the remaining contributions originate mainly from the coupling between single-donor and double-donor water molecules.

OH-Stretch Raman Multivariate Curve Resolution Spectroscopy of HOD/H2O Mixtures.

Recently, in an attempt to quantify the role of intermolecular OH stretching vibrational couplings in liquid water, experimental Raman spectra of HOD/H2O mixtures were analyzed using the multivariate curve resolution (Raman-MCR) algorithm. This algorithm allowed for the separation of the HOD solute-correlated spectrum from the spectrum of bulk water. The former spectrum highlights features arising from HOD itself as well as from perturbations it induces on the surrounding H2O molecules. In this work, we apply a mixed quantum-classical methodology developed in our group to simulate the isotropic Raman-MCR spectra of HOD/H2O mixtures. Our results illustrate that intermolecular coupling leads to broadening and a red shift of the OH stretching band, in good agreement with the experiment. Our theoretical analysis provides a molecular-level interpretation of Raman-MCR experiments on HOD/H2O mixtures, suggesting that perturbations affecting the OH stretching vibrational mode of HOD result from intermolecular vibrational coupling to surrounding H2O molecules extending well beyond the first solvation shell.

A test of the significance of intermolecular vibrational coupling in isotopic fractionation

Intermolecular coupling of dipole moments is studied for a model system consisting of two diatomic molecules (AB monomers) arranged co-linearly and which can form non-covalently bound dimers.The dipolar coupling is a function of the bond length in each molecule as well as of the distance between the centers-of-mass of the two molecules. The calculations show that intermolecular coupling of the vibrations results in an isotope-dependent modification of the AB-AB intermolecular potential. This in turn alters the energies of the low-lying bound states of the dimers, producing isotope-dependent changes in the AB-AB dimer partition function. Explicit inclusion of intermolecular vibrational coupling then changes the predicted gas-dimer isotopic fractionation. In addition, a mass dependence in the intermolecular potential can also result in changes in the number of bound dimer states in an equilibrium mixture. This in turn leads to a significant dimer population shift in the model monomer-dimer equilibrium system considered here. The results suggest that intermolecular coupling terms should be considered when probing the origins of isotopic fractionation.

Interplay between Hydrogen Bonding and Vibrational Coupling in Liquid N-Methylacetamide.

Intrinsically disordered proteins play an important role in biology, and unraveling their labile structure presents a vital challenge. However, the dynamical structure of such proteins thwarts their study by standard techniques such as X-ray diffraction and NMR spectroscopy. Here, we use a neat liquid composed of N-methylacetamide molecules as a model system to elucidate dynamical and structural properties similar to those one can expect to see in intrinsically disordered proteins. To examine the structural dynamics in the neat liquid, we combine molecular dynamics, response-function-based spectral simulations, and two-dimensional polarization-resolved infrared spectroscopy in the amide I (CO stretch) region. The two-dimensional spectra reveal a delicate interplay between hydrogen bonding and intermolecular vibrational coupling effects, observed through a fast anisotropy decay. The present study constitutes a general platform for understanding the structure and dynamics of highly disordered proteins.