Inelastic neutron scattering on a d1-d1-2 mixed-valence trimer cluster

We have carried out the calculations for inelastic neutron scattering spectra for thermal neutrons scattering on amixed-valence trimer d¹-d¹-d² cluster, and we have investigated the selection rules. The forbidden transitions include the A₁ A₂ transitions, independently of spin, as well as the¹A₂ ¹E transition. The rest of the transitions with S = 0, 1 are allowed, and in turn are subdivided according to intensity into strong and weak transitions. The scattering cross sections of the latter include the differences in the form factors of the mixedvalence ions (the indicated form factors are close) and are small in magnitude. The scattering cross sections and the line positions in the spectrum depend considerably on the tunneling and exchange parameters of the cluster.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 25, No. 3, pp. 329–332, May–June, 1989.     

Inelastic neutron scattering from isotactic polypropylene

We used inelastic neutron scattering to probe the low‐energy excitations in semicrystalline isotactic polypropylenes with different degrees of crystallinity. The contributions from the amorphous and crystalline regions to the total scattering intensity were extracted under the assumption of a weighted linear contribution of the two regions in a simplified two‐phase system. The resulting intensity from the amorphous region showed a peak at 1.2 meV that was in good agreement with the previously determined boson peak characteristic of atactic polypropylene. The possibility of a contribution to the boson peak region by longitudinal acoustic mode modes that are characteristic of semicrystalline polymers and appear in the same low‐frequency region is discussed. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2852–2859, 2001     

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.     

Inelastic scattering of unpolarized neutrons on dimer clusters

On the basis of a microscopic calculation of the inelastic neutron scattering spectra for scattering by exchange dimer clusters, we have determined the conditions for applicability of the semiphenomenological approach to determination of the scattering cross sections based on the spin model. The scattering cross sections obtained in the spin model are distinguished from the exact values by the fact that modified form factors of the SS transitions are used in the exact calculation instead of the magnetic form factors of the individual atoms. In the case of weak overlap of the one-electron wave functions for different centers, the exact scattering cross sections coincide with the cross sections calculated on the basis of the spin model.We have investigated the inelastic neutron spectrum for scattering by a dimer d²–d¹ of mixed valency. Tunneling transfer of the extra electron leads to splitting of the exchange line corresponding to the transition with S=1 in the system with localized electrons and the appearance of new lines. The spectrum consists of a series of forbidden transitions (with conservation of parity and S=1) and allowed transitions (with a change in parity and S=0, 1). The results obtained allow us to determine the parameters of Heisenberg and double exchange in mixed-valency dimers using the inelastic neutron scattering spectra.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 24, No. 4, pp. 392–398, July–August, 1988.     

Inelastic neutron scattering from matrix isolated species

Matrix isolation is an experimental method in chemistry that is widely used for the preparation of samples for spectroscopic studies. It makes it possible to stabilize species which are unstable at room temperature, to isolate molecules in solids from each other, and to carry out molecular spectroscopy at low temperatures. Matrix isolated molecules are studied by a variety of techniques. In this paper the application of inelastic neutron scattering (INS) to matrix isolation is reviewed. Molecules that contain hydrogen atoms are diluted in inert gases such as argon, krypton and nitrogen, and then condensed inside a liquid helium cryostat of the type that is in use at many neutron spectrometers. For this work we developed a technique for the vapor deposition of inert gases with dopants that have low vapor pressure. INS focuses on some aspects of matrix isolation which are not appropriately covered by other spectroscopies, mainly solid state aspects of matrix isolated molecular aggregates and of the matrix itself. Neutron scattering is used to observe optically forbidden excitations such as methyl librations, tunnelling transitions, phonons, and rotational transitions involving a nuclear spin flip. Moreover, a direct correlation of spectroscopic data with the powder diffraction pattern of the matrix is possible with this technique. Localized modes and phonon densities of states can be observed in the same sample and may then be related to the respective diffraction pattern. The vapor deposited samples can be characterized in this way, and possible structural faults in rare-gas lattices revealed which are not usually recognized by other techniques. The structure of molecular aggregates has been elucidated by neutron spectroscopy of their low frequency internal intermolecular modes. HCN forms long linear chains, CH₃CN antiparallel dimers. Both species may, be understood as intermediates for the formation of the respective crystals. The structure of the matrix cages, in which single molecules are embedded, is explored by recording rotational and translational localized modes of these molecules. The single particle rotations of HCl, H₂O, NH₃, and CH₄ were studied directly. Tunnel splittings and librational spectra were recorded from molecules with methyl groups [CH₃CN, CH₃I, CH₃COCH₃, C₃H₈, C₄H₁₀, Sn(CH₃)₄, and others]. The mutual influence of translational modes of the guest and phonons of the host can be studied, since the neutron spectra are directly connected with the phonon density of states of the system. The librational spectrum of N₂ in Ar and a local mode of H₂ in solid D₂ are presented and compared with theoretical calculations.     

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 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.     

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.     

Two new polyoxovanadate clusters templated through cysteamine

Two new fully oxidized polyoxovanadate cluster-based solids (C₄N₂S₂H₁₄)₂[H₂V₁₀O₂₈]·4H₂O,1 and (C₄N₂S₂H₁₄)₅[H₄V₁₅O₄₂]₂·l0H₂O,2 are crystallized under self-assembly process in the presence of cysteamine. In both1 and2, cysteamines are oxidized forming disulphide linkages and occur as counter cations. The organic cations assemble around V₁₀O₂₈ cluster anions in1 whereas they aggregate around V₁₅O₄₂ clusters in 2· pH appears to be the structure determinant in the occurrence of decavanadate cluster in1 and pentadecavanadate in2, with the same counter cation.     

Inelastic neutron scattering study of electron reduction in Mn12 derivatives

We report inelastic neutron scattering (INS) studies on a series of Mn(12) derivatives, [Mn(12)O(12)(O2CC6F5)16(H2O)4]z, in which the number of unpaired electrons in the cluster is varied. We investigated three oxidation levels: z = 0 for the neutral complex, z = -1 for the one-electron reduced species and z = -2 for the two-electron reduced complex. For z = 0, the ground state is S = 10 as in the prototypical Mn12-acetate. For z = -1, we have S = 19/2, and for z = - 2, an S = 10 ground state is retrieved. INS studies show that the axial zero-field splitting parameter D is strongly suppressed upon successive electron reduction: D = -0.45 cm(-1) (z = 0), D = -0.35 cm(-1) (z = -1), and D approximately -0.26 cm(-1) (z = -2). Each electron reduction step is directly correlated to the conversion of one anisotropic (Jahn-Teller distorted) Mn3+ (S = 2) to one nearly isotropic Mn2+ (S = 5/2).     

Inelastic neutron scattering in the glassy and metastable phases of methoxy-benzylidene-butyl-aniline

The inelastic neutron scattering (INS) spectra were measured in the glassy and in the metastable phases of methoxy-benzylidene-butyl-aniline. For comparison, the spectrum of benzylidene-aniline was also recorded. Complementary information was provided by Raman scattering measurements which analyzed more accurately the vibrational motions. Strong differences between the INS and Raman spectra were evidenced. The assignment of some lines is discussed to elucidate the respective role of the core and the tail of the molecule in the appearance of the successive metastable phases.     

Inelastic neutron scattering study of methyl groups rotation in some methylxanthines

The three isomeric dimethylxanthines and trimethylxanthine are studied by neutron spectroscopy up to energy transfers of 100 meV at energy resolutions ranging from 0.7 microeV to some meV. The loss of elastic intensity with increasing temperature can be modeled by quasielastic methyl rotation. The number of inequivalent methyl groups is in agreement with those of the room temperature crystal structures. Activation energies are obtained. In the case of theophylline, a doublet tunneling band is observed at 15.1 and 17.5 microeV. In theobromine, a single tunneling band at 0.3 microeV is found. Orientational disorder in caffeine leads to a 2.7 microeV broad distribution of tunneling bands around the elastic line. At the same time, broad low energy phonon spectra characterize an orientational glassy state with weak methyl rotational potentials. Librational energies of the dimethylxanthines are clearly seen in the phonon densities of states. Rotational potentials can be derived which explain consistently all observables. While their symmetry in general is threefold, theophylline shows a close to sixfold potential reflecting a mirror symmetry.     

Inelastic incoherent neutron scattering studies of water interacting with biological macromolecules.

The interaction between water and biological macromolecules in living organisms is of fundamental importance in a range of processes. We have studied water-DNA and water-proteolipid membrane systems over a range of hydration states using inelastic incoherent neutron scattering. We find a relatively sharp transition for both systems at a water concentration above which bulk solvent can be detected. Below this concentration, bulk water is essentially absent, i.e., all the water in the system is interacting with the biological macromolecules. This water is strongly perturbed as judged by its energy transfer spectrum, with a broader and lower energy transition than bulk water in the 50-75 meV (approximately 400-600 cm(-1)) range. Taking into account the differing geometry of (cylindrical) DNA and (planar) membranes, the number of water shells perturbed by each system was estimated. A conclusion is that in living organisms a large proportion of the cellular water will be in a state quite distinct from bulk water. The data add to the growing evidence that water structure in the vicinity of biological macromolecules is unusual and that the proximal water behaves differently compared to the bulk solvent.     

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.     

Inelastic neutron scattering and infrared spectroscopic study of furan adsorption on alkali-metal cation-exchanged faujasites.

Inelastic neutron scattering (INS) as well as infrared (IR) transmission and diffuse reflection infrared Fourier transform (DRIFT) spectra of furan adsorbed on Li-LSX, NaY, NaX, K-LSX, and CsNaX zeolites have been measured in the range 2000-200 and 4000-1300 cm(-1), respectively. On the basis of an assignment of normal modes of furan taken from the literature and our own quantum chemical calculations of vibrational frequencies, the observed frequency shifts between bulk furan and furan adsorbed on the zeolites mentioned above have been interpreted in view of the interactions between furan and zeolite. For an explanation of frequency shifts of CH out-of-plane bendings, CH stretchings and some ring vibrations, it has to be assumed that in addition to the interaction between furan and the corresponding cation of the zeolite, a further interaction between the CH bonds and lattice oxygen atoms exists.     

Inelastic neutron scattering studies of the interaction between water and some amino acids

The interaction between water and some of amino acids (glycine, L-glutamine, L-threonine, L-cysteine and L-serine) was studied by inelastic incoherent neutron scattering (IINS). The vibrational spectra of dry amino acids and amino acids with a water content (e.g., 1 mol water/1 mol amino acid) were recorded. Comparing the difference spectra obtained by subtracting the spectrum of dry sample from those of wet sample with the spectra of ice Ih, we obtained that the difference spectrum for serine changed greatly from normal ice spectrum; but on the other hand, the difference spectra for the other amino acids such as glycine, glutamine, threonine, and cysteine changed slightly. The results demonstrate that serine has stronger hydrophilic character than glycine, glutamine, threonine, and cysteine. This is the first time the hydrophilic or hydrophobic character of amino acids was studied by using inelastic neutron scattering techniques, which provides important information for theoretical modeling and force field refinement for the interaction between water and the amino acids studied here.     

Double exchange in polynuclear mixed-valence clusters

A method for calculating the energies of the tunnel states of mixed-valence, clusters is proposed. It is based on the secondary quantization technique and allows one to obtain analytical expressions for all matrix elements that appear in double exchange theory as well as to determine the microstructures of the parameters of this theory. The energy spectra of trimeric systems are calculated with allowance made for the three-center transfer integrals, and the magnetic properties of these systems are investigated. Moldova State University. Translated fromZhurnal Struktumoi Khimii, Vol. 36, No. 4, pp. 644–653, July–August, 1995. Translated by I. Izvekova     

Electron delocalization in trinuclear mixed-valence clusters

Possible trapped valence states in trimeric mixed-valence clusters are considered. The adiabatic potential in the space of a doubly-degenerate e vibration is calculated. Conditions under which minima with different electron distributions exist are determined. The coexistence of localized and delocalized distributions is possible.     

Inelastic neutron scattering spectrum of cyclotrimethylenetrinitramine: a comparison with solid-state electronic structure calculations

Solid-state geometry optimizations and corresponding normal-mode analysis of the widely used energetic material cyclotrimethylenetrinitramine (RDX) were performed using density functional theory with both the generalized gradient approximation (BLYP and BP functionals) and the local density approximation (PWC and VWN functionals). The structural results were found to be in good agreement with experimental neutron diffraction data and previously reported calculations based on the isolated-molecule approximation. The vibrational inelastic neutron scattering (INS) spectrum of polycrystalline RDX was measured and compared with simulated INS constructed from the solid-state calculations. The vibrational frequencies calculated from the solid-state methods had average deviations of 10 cm(-1) or less, whereas previously published frequencies based on an isolated-molecule approximation had deviations of 65 cm(-1) or less, illustrating the importance of including crystalline forces. On the basis of the calculations and analysis, it was possible to assign the normal modes and symmetries, which agree well with previous assignments. Four possible "doorway modes" were found in the energy range defined by the lattice modes, which were all found to contain fundamental contributions from rotation of the nitro groups.     

Electron delocalization in asymmetric trimeric mixed-valence clusters

The statistical distribution of electron density in a C_2v one-electron trimeric mixed-valence cluster is considered in terms of the Piepho-Krausz-Schatz vibronic model using the adiabatic approximation. Conditions are revealed under which the lower branch of the adiabatic potential shows three minima, each corresponding to predominant localization of the electron on one of the three ionic centers. It is shown that when the symmetry of the system is disturbed, the potential has two minima, with the system localized on one of two equivalent centers at either minimum. Conditions are considered under which these minima merge into one minimum, which corresponds to delocalization of the electron over a pair of ions, i.e., partial delocalization. For a D_3h symmetric trimer, it is demonstrated that in the region of transfer parameters and vibronic couplings, where the localized and completely delocalized states coexist, even with insignificant distortions there are only three localization minima, and the central minimum, which corresponds to a completely delocalized state, vanishes. Valencia University, Spain. Institute of Chemistry, Academy of Sciences, Moldova Republic. Translated fromZhurnal Strukturnoi Khimii, Vol. 36, No. 4, pp. 654–666, July–August, 1995 Translated by I. Izvekova