Inelastic neutron scattering studies on low frequency vibrations of pentachlorophenol

Inelastic neutron scattering (INS) and DFT theoretical studies on pentachlorophenol (PCP) and d-PCP were performed. IR and Raman spectra were also measured for comparison. A special attention was focused on low frequency modes in INS spectra, which provide information about modes into which the co-ordinates of the hydrogen and chlorine atoms are involved. The intensity of respective INS bands is discussed based on the cross-sections of nuclei and calculated relative amplitudes of vibrations. The appearance of overtones and summation frequencies in INS spectra was evidenced.     

Hydrogen bonding in potassium hydrogen maleate and some simple derivatives studied by inelastic neutron scattering spectroscopy

Inelastic neutron scattering spectra (300–2500 cm^?1) of KH(CHCO₂)₂, KD(CHCO₂)₂, KH(CDCO₂)₂ and KH(CHCO₂ · CClCO₂) have been obtained and the vibrations of the hydrogen bond, with the exception of ν₂(OHO), assigned. This is the first assignment of these vibrations in a centrosymmetric intramolecularly hydrogen bonded complex. ν_as(OHO) was found to be heavily mixed and to give rise to a strong doublet in the INS spectra.     

INS,IR, RAMAN, 1H NMR and DFT investigations on dynamical properties of l-asparagine

Results of inelastic neutron scattering (INS), infra-red (IR), Raman and ¹H NMR spectroscopy used for investigations on the l-asparagine dynamics are reported. The crystallographic structure and experimental vibrational spectra are compared with those calculated by the DFT methods applied to the solid state. Very good conformity of the experimental and theoretical structures has been found. The NH₃⁺ torsional vibration mode is observed in the INS spectra at 494 cm⁻¹, while the bands assigned to the vibrations of the strong NH⋯O hydrogen bonds are observed at 2849, 2650, and 2480 cm⁻¹ in the IR spectrum. A ¹H NMR investigation has been carried out at 26.75 MHz in the temperature range 150–300 K. For l-asparagine the activation energy needed for the NH₃⁺ group reorientation is equal 5.6 kcal/mol.     

Measuring molecular force fields: Terahertz,inelastic neutron scattering,Raman, FTIR,DFT, and BOMD molecular dynamics of solid l-serine

A vibrational analysis of polycrystalline l-serine is provided using experimental terahertz, FTIR, Raman and inelastic neutron scattering (INS) spectra, calculated INS spectra – and Born–Oppenheimer molecular dynamics (BOMD) simulations from which the power spectra for the electronegative elements are compared to the THz spectra. Corrections are made to density functional theory (DFT) calculations for van der Waals interactions. Assignments and potential energy distributions are included for all 3N = 336 normal modes of an eight molecule supercell, including those for 48 non-bonded whole molecule translating and rotating vibrations, of which three are acoustic modes, usually not considered. Calculated and observed frequencies differ by an average 3 cm⁻¹ (s = 4). The INS spectrum of these modes below 100 cm⁻¹, calculated from energy second derivatives, show a remarkable similarity to the experimental 10 K spectra. The calculated low frequency modes are insensitive to small changes in cell parameters and geometry. THz intensities are represented by power spectra and not calculated explicitly. Nevertheless, power spectra of 13 ps BOMD trajectories at classical temperatures of 20 K, 400 K, and 500 K are markedly similar to the experimental terahertz spectra at 77 K and 298 K. Calculations on a serine crystal supercell 2 × 2 × 2 molecules deep appear to include, in a crude but fortuitously accurate way, enough of the principle out of phase dispersion to yield a match with experimental frequencies and intensities.     

Inelastic neutron scattering study of water in hydrated LTA-type zeolites

The vibrational dynamics of water molecules encapsulated in synthetic Na-A and Mg-exchanged A zeolites were studied versus temperature by inelastic neutron scattering (INS) measurements (30-1200 cm(-1)) as a function of the induced ion-exchange percentage by using the indirect geometry tof spectrometer TOSCA at the ISIS pulse neutron facility (RAL, UK). The experimental INS spectra were compared with those of ice Ih to characterize the structural changes induced by confinement on the H2O hydrogen-bonded network. We observed, after increasing the Mg2+ content, a tendency of water molecules to restore the bulklike arrangements together with more hindered dynamics. These results are confirmed by the analysis of the evaluated one-phonon amplitude-weighted proton vibrational density of states aimed, in particular, to follow the evolution of the water molecules librational mode region.     

The inelastic neutron scattering spectrum of chloroform in solution

The incoherent inelastic neutron scattering (INS) spectrum of CHCl₃ in liquid SO₂ (ambient temperature) in the region 0–3600 cm⁻¹ is presented. For the first time it has been demonstrated that good quality INS data on solutes can be obtained over this energy transfer range. A discussion of suitable solvents for further work is included.     

Inelastic neutron scattering studies on dichloro-1,4-benzoquinones

Inelastic neutron scattering (INS) spectra of 2,6-dichloro- and 2,5-dichloro-1,4-benzoquinone were compared with Raman and infra-red (IR) spectra and analysed in detail below 1800 cm(-1). The analysis was based on calculations tending towards simulation of spectra by using GAUSSIAN (HF, DFT/B3LYP and BLYP/6-31G(d,p)), and auntieCLIMAX programs. The correlations between calculated and experimental (either INS or Raman and IR) frequencies enabled to analyse the problem of scaling factors (SFs). The advantages of INS technique was shown in studies of low frequency vibrations with participation of H-atoms. The macroscopic lattice effect at low temperatures on INS spectra is discussed.     

Direct observation of protein structural transitions through entire amyloid aggregation processes in water using 2D-IR spectroscopy

Amyloid proteins that undergo self-assembly to form insoluble fibrillar aggregates have attracted much attention due to their role in biological and pathological significance in amyloidosis. This study aims to understand the amyloid aggregation dynamics of insulin (INS) in H₂O using two-dimensional infrared (2D-IR) spectroscopy. Conventional IR studies have been performed in D₂O to avoid spectral congestion despite distinct H–D isotope effects. We observed a slowdown of the INS fibrillation process in D₂O compared to that in H₂O. The 2D-IR results reveal that different quaternary structures of INS at the onset of the nucleation phase caused the distinct fibrillation pathways of INS in H₂O and D₂O. A few different biophysical analysis, including solution-phase small-angle X-ray scattering combined with molecular dynamics simulations and other spectroscopic techniques, support our 2D-IR investigation results, providing insight into mechanistic details of distinct structural transition dynamics of INS in water. We found the delayed structural transition in D₂O is due to the kinetic isotope effect at an early stage of fibrillation of INS in D₂O, i.e., enhanced dimer formation of INS in D₂O. Our 2D-IR and biophysical analysis provide insight into mechanistic details of structural transition dynamics of INS in water. This study demonstrates an innovative 2D-IR approach for studying protein dynamics in H₂O, which will open the way for observing protein dynamics under biological conditions without IR spectroscopic interference by water vibrations.

This study aims to understand the structural transition dynamics of INS during amyloid aggregation in H₂O using 2D-IR spectroscopy. The results show that distinct fibrillations in D₂O and H₂O originated from different quaternary structures of INS.     

Location and Vibrations of Hydrogen in La2C3H1.5

We report on the incorporation of hydrogen into La₂C₃ and on the characterisation of its position. Temperature‐dependent X‐ray investigations show that the hydrogenation process is reversible and the product La₂C₃H_1.5 is amorphous. In principle, IR and Raman spectroscopies can provide some structural information, but in the present study, they are of only limited value. Inelastic neutron scattering (INS) has advantages for measuring the H‐vibrations in La₂C₃H_1.5 and these are compared with the results of molecular dynamics simulations. A structural model based on published pair‐potentials predicts that hydrogen (or deuterium) occupies the “tetrahedral” holes in the lanthanum sublattice. This model also accounts for the observed vibrational spectra.     

A compact model system for electron–phonon calculations in discotic materials

Electron–phonon coupling underlies the unwanted rapid relaxation of electrically excited states in potential organic solar-cell materials. A compact model for the vibrational dynamics of 2,3,6,7,10,11-hexakishexyloxytriphenylene (HAT6) is derived from the combined use of inelastic neutron scattering (INS) spectroscopy and first-principles calculations. Because this model reproduces the essential features of the vibrational dynamics and electronic structure on the aromatic core of HAT6 it can be used as a basis for future calculations of the relaxation mechanisms of the electronically excited states.     

An analysis of the vibrational frequencies and amplitudes of the H5O+2 ion in H4SiW12O40·6H2O (TSA·6H2O) and H3PW12O40·6H2O (TPA·6H2O)

The infrared, Raman and inelastic neutron scattering (INS) spectra of TSA·6H₂O and TPA·6H₂O are in agreement with those expected for the presence of H₅O⁺₂ ions. Force fields for different assignment schemes are compared with the observed vibrational frequencies and the INS spectral profile. All but two schemes are eliminated. Whilst low-resolution INS spectroscopy cannot distinguish between these two schemes, the orientations of the vibrational ellipsoids for one scheme are in better agreement with those reported from low-temperature crystallographic studies of the H₅O⁺₂ ion.     

An inelastic neutron scattering study of hydrogen bonding in potassium hydrogen dichloromaleate

The inelastic neutron scattering (INS) spectrum (350–2000 cm^?1) of potassium hydrogen dichloromaleate (solid slate) has been obtained. Two of the normal modes of vibration of the hydrogen bond [γ(OHO) and δ(OHO)] were observed and assigned. No INS band v_as(OHO) was observed in the region 500–1300 cm^?1. This conflicts with expectations from infrared data.     

Inelastic neutron scattering studies of polyethylene and N-alkanes

High quality inelastic neutron scattering (INS) spectra of randomly oriented polycrystalline polyethylene, perdeuteropolyethylene and highly oriented polyethylene are presented. The instrumental resolution was significantly better than previous work and has revealed increased detail in the 0 − 600 cm⁻¹ region. For the polycrystalline sample, comparison with the best available dispersion curves shows that these qualitatively reproduce the INS spectrum, apart from the energy of the maximum in the in-plane C-C bending mode v5. For the oriented samples, comparison with calculated INS spectra show fair, but not exact, agreement with the experimental spectra.     

Inelastic neutron scattering spectra of the hydrogen tungsten bronze H0.4WO3

Inelastic neutron scattering spectra of metallic H_0.4WO₃ show only the vibrations that would be expected of a metal hydroxide. The diffusion coefficient of the proton could not be detected from quasi-elastic scattering with the best available neutron spectrometer setting an upper limit to its value of 10^?7 cm²/sec. Both these results confirm that this material is correctly described as a hydroxide.     

Inelastic neutron scattering (INS) studies on 2,5-dihydroxy-1,4-benzoquinone (DHBQ)

Inelastic neutron scattering (INS) spectra of solid 2,5-dihydroxy-1,4-benzoquinone were measured and compared with IR and Raman data. The INS spectrum is very well reproduced in the region below 1000 cm(-1) by DFT calculations on the B3LYP/6-311++G** level using Gaussian and Climax programs. To get a better agreement one should take into account additional interactions of OH groups in the solid state leading to an increase of the gamma(OH) frequency and to a decrease of frequencies for modes in which the delta(OH) participates. The studies of the deuterated compound in IR enabled to correct the assignment of gamma(OH) vibrations. Highly asymmetric nu(OH) band observed in IR spectrum with sharp maximum at about 3300 cm(-1) is discussed in terms of a stochastic approach to the analysis of hydrogen bonded systems.     

Assignment of Metal-Ligand Modes in Pt(II) Diimine Complexes Relevant to Solar Energy Conversion

This work describes a comprehensive assignment of the vibrational spectra of the platinum(II) diimine bisthiolate and chloride complexes as a prototype structure for a diversity of Pt(II) diimine chromophores. The dynamics and energy dissipation pathways in excited states of light harvesting molecules relies largely on the coupling between the high frequency and the low frequency modes. As such, the assignment of the vibrational spectrum of the chromophore is of utmost importance, especially in the low-frequency region, below 500 cm(-1), where the key metal-ligand framework modes occur. This region is experimentally difficult to access with infrared spectroscopy and hence frequently remains elusive. However, this region is easily accessible with Raman and inelastic neutron scattering (INS) spectroscopies. Accordingly, a combination of inelastic neutron scattering and Raman spectroscopy with the aid of computational results from periodic-DFT and the mode visualizations, as well as isotopic substitution, allowed for an identification of the modes that contain significant contributions from Pt-Cl, Pt-S, and Pt-N stretch modes. The results also demonstrate that it is not possible to assign transition energies to "pure", localized modes in the low frequency region, as a consequence of the anticipated severe coupling that occurs among the skeletal modes. The use of INS has proved invaluable in identifying and assigning the modes in the lowest frequency region, and overall the results will be of assistance in analyzing the structure of the electronic excited state in the families of chromophores containing a Pt(diimine) core.     

Collective dynamics in molten potassium: an inelastic x-ray scattering study

The high-frequency collective dynamics of molten potassium has been investigated by inelastic x-ray scattering, disclosing an energy/momentum transfer region unreachable by previous inelastic neutron scattering (INS) experiments. We find that a two-step relaxation scenario, similar to that found in other liquid metals, applies to liquid potassium. In particular, we show how the sound velocity determined by INS experiments, exceeding the hydrodynamic value by approximately 30%, is the higher limit of a speedup, located in the momentum region 1 < Q < 3 nm(-1), which marks the departure from the isothermal value. We point out how this phenomenology is the consequence of a microscopic relaxation process that, in turn, can be traced back to the presence of "instantaneous" disorder, rather than to the crossover from a liquid to solidlike response.     

Vibrational spectroscopy of N-phenylmaleimide

A combination of infrared, Raman and inelastic neutron scattering (INS) spectroscopies with density functional theory (DFT) calculations is used to provide a complete assignment of the vibrational spectra of N-phenylmaleimide and N-(perdeuterophenyl)maleimide. DFT is shown to give very good results for the frequencies and atomic displacements in the modes. These are used to generate INS spectra which are excellent agreement with the observed. The calculated infrared and Raman spectra are much less reliable, although this may be more of a presentation problem than a real failing.     

Coherent Vibrational Dynamics of [Au25(SR)18]- Nanoclusters?

Coherent vibrational dynamics can be observed in atomically precise gold nanoclusters using femtosecond time-resolved pump-probe spectroscopy. It can not only reveal the coupling between electrons and vibrations, but also reflect the mechanical and electronic properties of metal nanoclusters, which holds potential applications in biological sensing and mass detection. Here, we investigated the coherent vibrational dynamics of [Au₂₅(SR)₁₈]^- nanoclusters by ultrafast spectroscopy and revealed the origins of these coherent vibrations by analyzing their frequency, phase and probe wavelength distributions. Strong coherent oscillations with frequency of 40 cm^-1 and 80 cm^-1 can be reproduced in the excited state dynamics of [Au₂₅(SR)₁₈]^-, which should originate from acoustic vibrations of the Au₁₃ metal core. Phase analysis on the oscillations indicates that the 80 cm^-1 mode should arise from the frequency modulation of the electronic states while the 40 cm^-1 mode should originate from the amplitude modulation of the dynamic spectrum. Moreover, it is found that the vibration frequencies of [Au₂₅(SR)₁₈]^- obtained in pump-probe measurements are independent of the surface ligands so that they are intrinsic properties of the metal core. These results are of great value to understand the electron-vibration coupling of metal nanoclusters.