Noncoincidence Effect Research Articles

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.

Aggregation induced spectral splitting and Fermi resonance of Ethylene Carbonate in binary mixture

The vibration band of the ring stretching (ν14), the fundamental ring breathing (ν17) and the Fermi resonance band of carbonyl stretching mixing with the overtone of the ring breathing (ν5 + 2ν17) have been investigated in solid ethylene carbonate (EC) and EC/CH3CN and EC/CHCl3 binary mixture. Dimer structure with aggregation-induced spectral splitting model (AIS) was applied to calculate the vibration spectra using the B3LYP-D3/6-311+G (d,p) procedure. The noncoincidence effect (NCE) and concentration induced frequency shifts of the ν14 and ν5 could be well explained by AIS model based on the dimer structure. Four bands were observed with two in the isotropic and two in the anisotropic Raman spectra and their NCE value decreased with the decrease of EC volume fraction in the binary mixture, and finally disappeared. NCE value and the Fermi resonance constants of EC at different concentrations were calculated from the experimental data.

Role of Hydration in Magnesium versus Calcium Ion Pairing with Carboxylate: Solution and the Aqueous Interface

The binding of group II metal cations such as Ca2+ and Mg2+ has been largely categorized as electrostatic or ionic using carboxylate symmetric and asymmetric stretching frequency assignments that have been historically used with little regard for the solvation environment of aqueous solutions. However, given the importance of these cations and their binding mechanisms related to biological function and in revealing surface enrichment factors for ocean to marine aerosol transfer, it is imperative that a deeper understanding be sought to include hydration effects. Here, infrared reflection-absorption and Raman spectra for surface and solution phase carboxylate binding information, respectively, are compared against bare (unbound) carboxylate and bidentate Zn2+:carboxylate spectral signatures. Spectral non-coincidence effect analysis, temperature studies, and spectral and potential of mean force calculations result in a concise interpretation of binding motifs that include the role of mediating water molecules, that is, contact and solvent-shared ion pairs. Calcium directly binds to the carboxylate group in contact ion pairs where magnesium rarely does. Moreover, we reveal the dominance of the solvent-shared ion pair of magnesium with carboxylate at the air-water interface and in solution.

Concentration dependence study of non-coincidence effect (NCE) on C[dbnd]O stretching and NH2 bending modes of formamide in acetonitrile: A comparative study

The isotropic and anisotropic parts of Raman spectra corresponding to CO stretching and NH2 bending modes of formamide (HCONH2) in different acetonitrile (CH3CN) concentration were investigated in the region 1500−1800 cm−1 to study the non-coincidence effect. The experimentally obtained values of NCE with varying solvent concentrations were explained using three different models namely, Mirone-Fini model, Mirone-McHale model, and Logan model respectively. The experimentally measured values of non-coincidence splitting, Δνnce, for neat HCONH2 in the modes corresponding to CO stretching and NH2 bending are ∼21 and ∼11.38 cm−1 which then reduces to a very low value of 1.26 and 0.5 cm−1 at mole fraction, ϰf=0.1 respectively. It has been observed that Mirone-McHale model and Logan model reproduces better experimental results of NCE of the CO stretching mode whereas Mirone-Fini model gives accurate results to explain the experimental results of NCE of the NH2 bending mode.

The aggregation structure of a methanol/CHCl3 binary mixture investigated by polarized Raman spectroscopy and HNMR

The structural evolution of methanol in the protic solvent chloroform was studied by polarized Raman spectroscopy. Negative noncoincidence effects were detected for the full range of concentrations of the methanol/CHCl3 binary mixture, that is, ν⊥<ν∥ for the C-O stretching vibration. With the dilution of methanol/CHCl3 by CHCl3, the vibration frequency of the C-O bond moves to a lower wavenumber for both parallel ν∥ and vertical ν⊥ Raman spectra, contrary to the phenomenon observed for the methanol/CCl4 mixture. The absolute value of the frequency difference |ν⊥-ν∥| increased with volume fraction dilution from 1 to 0.3 and then sharply decreased from 0.3 to 0.03. Aggregation-induced split (AIS) theory was applied to explain these phenomena. PCM models of the most likely different structures of H-bond combined cyclic multimers, CHCl3 H-bonded multimers, and CHCl3 H-bonded monomers were established for different concentration stages. The DFT calculations based on these models of different concentrations can aptly reproduce the noncoincidence effect and concentration-dependent phenomenon.

Local structure of NH4Cl solution and its correlation with osmotic coefficient by molecular dynamics simulation and Raman spectroscopy

The NH4Cl aqueous solution ranging from 1.00 wt.% to 22.00 wt.% have been studied by molecular dynamic simulation, Raman spectroscopy and polarized Raman spectroscopy. Radial distribution function (RDF), diffusion property, structure of hydrogen bond network, excess Raman spectroscopy and non-coincidence effect (NCE) were obtained. The contact ion pairs of cation and anion appeared in NH4Cl solution of 15.00 wt.%, leading to fundamental changes in hydrogen bonds and the hydrated shell. The evolution of hydrogen bond with mass fraction in solution was described. The osmotic coefficient of NH4Cl aqueous solution was obtained by thermodynamic calculation, and its linear relationship with double donor single acceptor hydrogen bond (DDA-OH) was preliminarily summarized. Moreover, the experimental results are in good agreement with the simulation results.

Non-coincidence effect study of NN-Dibutyl formamide in binary liquid mixtures

Abstract The isotropic and anisotropic Raman peak frequency of C O stretching mode of N, N Dibutyl formamide (DBF) have been studied in Dimethyl Sulfoxide (DMSO) and Chlorobenzene (CLB) solvents. The shift of peak wavenumber with the increase in solute concentration has been studied. Hydrogen bonding and the difference in dielectric constant may be responsible for shifting the peak positions. The non-coincidence effect (NCE) has been studied for the CLB and DMSO solvents. The optimized geometrical structures of DBF, DBF-DMSO and DBF-CLB are determined. The peak wavenumbers of the neat solute and in binary mixtures are compared and are found consistent with the theoretical data. The relative splittings ∆ υ ∆ υ neat of the C O stretching mode of DBF for the neat liquid and in solution are determined and have been compared with three theoretical models such as Mc Hale's model, Mirone's model and Logan's model, respectively. The data obtained through theoretical models were almost consistent with the experimental data.

Argon matrix Raman spectroscopic and aggregation model density function theoretical study of methyl isothiocyanate

AbstractThe Raman spectra for methyl isothiocyanate (MTC) isolated in an argon matrix at different annealing temperatures and MTC in carbon tetrachloride at different concentrations were collected separately. Concentration‐dependent properties were characterized, and no noncoincidence effect (NCE) was observed. Argon matrix isolated Raman spectra shows that, the SCNC stretching at 6K presents as one distinct peak, it separates into two peaks at 25–35 K and merges as one peak again above 40 K, whereas its frequency shifted to a lower wavenumber. Concentration‐dependent experiments showed the peak frequency of SCNC stretching shifted to a higher wavenumber when the concentrations decreased. Aggregated dimer structures were proposed to explain the MTC SCNC and CS vibrational concentration effects with no NCE phenomenon. Density functional theories were performed to calculate the monomer and dimer of MTC; the dimer structure can explain the experimental data well. The strong antiparallel head‐to‐tail vibrational coupling of the SCNC oscillators in neighbouring molecules accounts for the frequency shifts of the SCNC stretching.

Dynamics and vibrational coupling of methyl acetate dissolved in ethanol

The structure and vibrational dynamics of methyl acetate (MA) molecules solvated in ethanol have been investigated by means of Raman and IR spectroscopies. Two distinct vibrational bands have been resolved and assigned to two different types of MA molecules, namely the “free” molecules that exist in bulk material and the “solution” molecules which interact with ethanol molecules. All the measured physical properties exhibit a characteristic change near a specific volume fraction (∼0.6) of MA. This behavior is associated to the existence of the two kinds of methyl acetate groups coexisting in the solutions. The micro-viscosity model provides reasonable agreement with the experimental vibrational relaxation times up to a characteristic volume fraction ∼0.6, but fails for higher values since it takes into account solvent instead of solute characteristics. Finally, the so-called non-coincidence effect (NCE) has been observed due to repulsive interactions and is of key importance for the vibrational relaxation process.

Raman spectra of aqueous uracil stacked dimer: First principle molecular dynamics simulation

Raman spectra of aqueous uracil stacked dimer have been studied by employing density functional theory based molecular dynamics simulation together with the maximally localized Wannier functions. The unpolarized spectra are in a reasonable good agreement with the available experimental spectrum. The discrepancies are attributed to the used density functional approximation. Static quantum chemical calculations reveal that the coupling of the carbonyl stretching modes is significantly weaker than the one of the amide modes. The noncoincidence effect of the ring breathing mode is explained in terms of repulsive steric interactions.

Raman non-coincidence effect of boroxol ring: The interplay between repulsion and attraction forces in the glassy, supercooled and liquid state

Temperature dependent Raman spectra of boric oxide have been measured in a temperature range covering the glassy, supercooled and liquid state. The shift of the isotropic band assigned to boroxol rings relative to the anisotropic component upon heating the glass is measured and attributed to the Raman non-coincidence effect. The measured shift is associated with the competition between attraction and repulsion forces with increasing temperature. The relation of dephasing and orientational relaxation times to the non-coincidence effect of the condensed phases has been examined. We discuss our results in the framework of the current phenomenological status of the field in an attempt to separate the attraction and repulsion contributions corresponding to the observed non-coincidence effect.

The structural organization of N‐methyl‐2‐pyrrolidinone in binary mixtures probed by Raman spectroscopy: Experimental and quantum chemical results

AbstractThe isotropic and anisotropic Raman spectra of N‐methyl‐2‐pyrrolidinone (NMP) in the binary mixture at a variety of volume fractions have been measured. The noncoincidence effect (NCE) of NMP was determined separately in carbon tetrachloride (CCl4) solution and acetonitrile solution. The NCE of the ν10(C=O) stretching mode versus concentration in the NMP/acetonitrile mixtures exhibits a linear plot; however, the NCE behavior in the NMP/CCl4 mixtures shows an upward (convex) curvature. The dimer structure (short‐range orientational order) of NMP may play a major role in exhibitions of different NCE behavior in the NMP/CAN and NMP/CCl4 mixtures; thus, the geometries of monomer and dimer of NMP were calculated at the B3LYP‐D3/6‐311G (d,p) level of theory. We proposed aggregated model to explain the NMP C=O vibration NCE phenomenon and its concentration effect and found it largely consistent with our experimental findings. Solvent‐dependent experiment shows the value of NCE declined with increase the solvent dielectric constant under the same condition.

Local order and vibrational coupling of the C=O Stretching Mode of \u03b3-Caprolactone in liquid binary mixtures

The isotropic and anisotropic parts Raman spectra of γ-Caprolactone in the binary mixture at different concentrations have been measured. The non-coincidence effect (NCE) of γ-Caprolactone was determined in carbon tetrachloride solution and DMSO solution. The NCE of the ν11(C=O) stretching mode in the γ-Caprolactone/DMSO mixtures exhibits a linear plot, in contrast to that in the γ-Caprolactone/CCl4 mixtures, which shows an upward (convex) curvature. The reduction and enhancement of the dimer structure (short-range orientational order) of γ-Caprolactone in the γ-Caprolactone/DMSO and γ-Caprolactone/CCl4 mixtures respectively may play a major role in shifting of peak frequencies, thus the geometries of monomer and dimer of γ-Caprolactone were calculated at the B3LYP-D3/6–311 G (d,p) level of theory. We proposed aggregated model to explain the γ-Caprolactone C=O vibration NCE phenomenon and its concentration effect and found it largely consistent with our experimental findings. Solvent dependent experiment show the value of NCE declined with the increase of the solvent dielectric constant under the same condition which is consistent with the Logan’s theory.

Study on the noncoincidence effect phenomenon using matrix isolated Raman spectra and the proposed structural organization model of acetone in condense phase

The isotropic and anisotropic Raman spectra of acetone and deuterated acetone isolated in an argon matrix have been recorded for the understanding of noncoincidence effect (NCE) phenomenon. According to the matrix isolated Raman spectra and DFT calculations, we proposed aggregated model for the explanations of the acetone C=O vibration NCE phenomenon and its concentration effect. The experimental data were in consistence with the DFT calculations performed at the B3LYP-D3/6-311 G (d,p) levels based on the proposed model. The experimental identification of the monomer, dimer and trimer are reported here, and the dynamic of the transformation from monomer to aggregated structure can be easily controlled by tuning annealing temperature.

The structural configurations of Ethylene Trithiocarbonate in the binary mixture (SCS2CH2CH2+CHCl3) investigated by polarized Raman: Experimental and quantum chemical results

The isotropic and anisotropic components of the Raman spectra of CS stretching in Ethylene Trithiocarbonate (SCS2CH2CH2, ET) shows noncoincidence effect (NCE). The calculated absolute Raman cross-section (ARCS) of CS stretching initially increases with the dilution of concentration to a maximum and then decreases. This unusual behavior indicates the existence of self-association. Thus we proposed aggregated model and the dimer structure were optimized with DFT/PCM to explain the NCE and ARCS behaviors. Vibrational spectra and depolarization-ratio were calculated for monomer and dimer individually, the dimer is in good accordance with the experimental data. The strong antiparallel CS vibrational coupling in neighboring molecules accounts for the frequency shifts of the CS stretching.

Solvation Structure around the Li+ Ion in Mixed Cyclic/Linear Carbonate Solutions Unveiled by the Raman Noncoincidence Effect

The solvation structure around the Li+ ion in a mixed cyclic/linear carbonate solution, an important factor for the performance of lithium-based rechargeable batteries, is examined by measuring and analyzing the noncoincidence effect observed for the C═O stretching Raman band. This technique has the advantage of perceiving relative distances and orientations of solvent molecules clustering around an ion in the first solvation shell and, hence, of developing information on the solvation structure along the wavenumber axis rather than along the intensity axis of the spectra. It is shown that, taking the solution of Li+ClO4– in the 1:1 mixed solvent of propylene carbonate (PC) and diethyl carbonate (DEC) as an example case, the Li+ ion is preferentially solvated by PC molecules [primarily as (PC)3(DEC)1Li+] and is totally protected from direct interaction (contact ion pairing) with the ClO4– ion. The solvation structures in neat PC and neat DEC solvents are also discussed.

Concentration-dependent frequency shifts of the CS stretching modes in ethylene trithiocarbonate studied by Raman spectroscopy

Resonant with the CS π → π* electronic transition, the intensity of CS stretching and its overtone have been greatly enhanced in the 488- and 319-nm excited resonance Raman spectra. The isotropic and anisotropic parts of the Raman spectra of CS stretching modes of ethylene trithiocarbonate (ET) at different concentrations have been analyzed in order to study the noncoincidence effect (NCE). In neat ET, the experimentally measured values of noncoincidence Δυnc are ~4.60 cm−1 for the CS stretching modes, which reduce to 1.30 cm−1 at the mole fraction χm (ET) = 0.13. Both the isotropic and anisotropic peak frequencies of CS stretching were found to shift to higher wavenumber when the concentrations are diluted, while the value of Δυnc goes on decreasing upon dilution. The absolute Raman cross section of carbonyl stretching was also measured, and their behavior was unusual (first increasing and then decreasing with the decrease of concentration). The experimental result shows that there may exist self-association in the high concentration, and the main NCE mechanism may be due to the transition dipole–transition dipole coupling between the ET molecules. Copyright © 2015 John Wiley & Sons, Ltd.

A dissociative quantum mechanical/molecular mechanical molecular dynamics simulation and infrared experiments reveal characteristics of the strongly hydrolytic arsenic(III).

This work presents a hybrid ab initio quantum mechanical/molecular mechanical simulation at the RI-MP2 level of theory investigating the hydrolysis process of arsenic(III), ultimately leading to arsenous acid (H3AsO3). A newly implemented dissociative water model has been applied to treat the interactions in the classical region, which is capable of describing non-neutral water species such as hydroxide and oxonium ions. Three stages of hydrolysis have been observed during the simulation and besides profound dynamical considerations, detailed insights into structural changes and atomic partial charge shifts are presented. In particular, the geometrical properties of H-bonds involved in each of the three proton transfer events and subsequent proton hopping reactions are discussed. A Laguerre tessellation analysis has been employed to estimate the molecular volume of H3AsO3. Estimations of pKa values of the arsenic(III)-aquo-complexes have been obtained at the G4 and CBS-Q//B3 levels of theory using a thermodynamic cycle, whereas rate constants for the final hydrolysis step have been determined via reaction path optimization and transition state theory. Newly recorded Fourier transform infrared (FT-IR) spectroscopy measurements have been compared to power spectra obtained from the simulation data, confirming its quality. The simulation findings, as well as results from computational spectroscopic calculations utilizing the PT2-VSCF methodology, proved valuable for the interpretation of the experimental FT-IR data, elucidating the particularities of the strongly observed IR Raman noncoincidence effect.

Glass transition dynamics of anti-inflammatory ketoprofen studied by Raman scattering and terahertz time-domain spectroscopy

A liquid–glass transition and a crystalline state of pharmaceutical racemic ketoprofen were studied by Raman scattering and the broadband terahertz time-domain spectroscopy (THz-TDS) in the frequency range from 9 to 260cm−1. The low-frequency Raman scattering spectra clearly shows the remarkable change related to a liquid–glass transition at about Tg=267K. After melt-quenching at liquid nitrogen temperature, a boson peak appears at about 16.5cm−1 near and below Tg and the intensity of quasi-elastic scattering related to structural relaxation increases markedly on heating. The crystalline racemic ketoprofen of “conformer A” shows the noncoincidence effect of mode frequencies below 200cm−1 between Raman scattering spectra and dielectric spectra observed by THz-TDS.

Solvent dependent frequency shift and Raman noncoincidence effect of S[dbnd]O stretching mode of Dimethyl sulfoxide in liquid binary mixtures

The isotropic and anisotropic Raman peak frequencies of SO stretching mode of Dimethyl sulfoxide (DMSO) have been discussed in different chemical and isotopic solvent molecules using different mechanisms. The shifting of peak frequency in further dilution of DMSO with solvent molecule is observed for all solvents. Transition dipole – transition dipole interaction and hydrogen bonding may play a major role in shifting of peak frequencies. The non-coincidence effect (NCE) of DMSO was determined for all the solvents and compared with four theoretical models such as McHale’s model, Mirone’s modification of McHale’s model, Logan’s model and Onsager–Fröhlich dielectric continuum model respectively. Most of the theoretical models are largely consistent with our experimental data.