Kenneth P. J. Williams

Raman Spectrum of β-Carotene Using Laser Lines from Green (514.5 nm) to Near-Infrared (1064 nm): Implications for the Characterization of Conjugated Polyenes

We have studied the Raman spectrum of β-carotene using excitation wavelengths from visible (514.5 nm) to near-infrared (1064 nm). Qualitatively, the spectrum appears independent of the choice of excitation wavelength, although this is not true in detail; in particular, the ratio of the intensities of the νC-C and νC=C fundamentals to their overtones and combinations varies with the excitation wavelength. The inelastic neutron scattering spectrum shows no evidence for unusual dynamics in this system, and the UV/vis/NIR spectrum shows that there are no allowed electronic transitions from the ground state beyond ∼700 nm. These observations provide strong evidence for the existence of a different mechanism for the enhancement of the bands. Indirect evidence suggests that, with these longer excitation wavelengths, the intensities of the νC-C and νC=C peaks are at least two orders of magnitude smaller than those from resonance enhancement with the use of excitation into the visible absorption bands. The implications for the characterization of polyenes in polymers are discussed.

Fourier Transform Raman Spectroscopy Using a Bench-Top FT-IR Spectrometer

Fourier transform Raman spectroscopy has been performed with an inexpensive bench-top FT-IR spectrometer optimized for the near-infrared. The laser excitation source was from a continuous-wave Nd: YAG laser with an output at 1.064 μm. Spectra from solid samples, ground as powders, have been obtained. Many of these are well known to fluoresce in the visible region and are thus intrinsically difficult to study by the Raman method. The FT-Raman method is described, and improvements in the technique are considered.

Vibrational spectroscopy of tetrahedral ternary metal hydrides: Mg2NiH4, Rb3ZnH5 and their deuterides

The infrared, Raman and inelastic neutron scattering spectra of the approximately tetrahedral ternary metal hydrides: Mg2NiH4, Rb3ZnH5 and their deuterides have been observed and assigned. This has enabled a comparison of some of the properties of the first row transition ternary metal hydrides.

Fourier Transform Raman and infrared spectroscopy of N-phenylmaleimide and methylene dianiline bismaleimide

Abstract We have carried out a comprehensive assignment of the infrared and Raman spectra of N-phenylmaleimide, including N-(perdeuterophenyl)maleimide. As part of this work, we have measured the depolarization ratios of N-phenylmaleimide in solution. To our knowledge this is the first time that polarization data have been recorded using FT Raman spectroscopy from a solution rather than a pure liquid. The assignments for N-phenylmaleimide have been used to assign the bands of a commercial bismaleimide, methylene dianiline bismaleimide (MDA-BMI), that change on curing. This has allowed a better understanding of the chemistry involved.

The vibrational spectrum of K2PdCl4: first detection of the silent mode ν5

Abstract The use of inelastic neutron scattering (INS) spectroscopy has allowed the determination of the infrared and Raman inactive out-of-plane bending mode ν 5 in the [PdCl 4 ] 2− ion at 136 cm −1 . To our knowledge, this is the first time this mode has been observed in any homoleptic square planar complex. An initial highly anomalous INS spectrum was shown by the use of Raman microscopy to be due to an impurity phase that is most probably a hydrate.

The vibrational spectra of norbornene and nadic anhydride

The vibrational spectra of norbornene (bicyclo[2,2,1]hept-2-ene) and nadic anhydride (cis-endo-5-norbornene-2,3-dicarboxylic anhydride) have been investigated by infrared, Raman and inelastic neutron scattering spectroscopies and density functional theory (DFT). For norbornene, the five missing fundamentals have been located. In the C–H stretch region, the infrared and Raman spectra synthesised from the DFT calculations are shown to be in poor agreement with the experimental spectra. Visualisation of the C–H stretch modes shows that assignments made on the basis of group frequency correlations are incorrect. Nadic anhydride has not been previously investigated by vibrational spectroscopy. This paper demonstrates that with state-of-the-art inelastic neutron scattering spectra it is possible to successfully analyse complex systems that have not been studied before.

Analysis of Carbon Black-Filled Polymers by Vibrational Spectroscopy

Carbon black-filled polymers are among the most challenging samples that an analyst may be called upon to identify. In this paper three novel methods of obtaining vibrational spectra from four different carbon black-filled polymers are evaluated: transmission infrared spectroscopy using a diamond anvil cell, Raman spectroscopy using 780-nm excitation, and inelastic neutron scattering. Overall, none of the three methods used in this work was totally satisfactory, but for most purposes infrared spectroscopy provides the best results in terms of spectrometer accessibility and the availability of suitable reference collections. Raman spectroscopy was successful only with one of the four polymers. Inelastic neutron scattering (INS) was successful in obtaining a spectrum with all four materials, but this success occurred only with respect to the hydrogenic part of the polymer, and, for many materials, it is the heteroelements, O, S, and halogens, that determine the important physical properties of the compound. For both Raman spectroscopy and INS, a major disadvantage is the lack of a suitable data base for identification of the materials.

The development of a sensitive near-IR Raman system

Abstract This paper describes the development of a highly sensitive near-IR laser Raman system which has been designed and built in our laboratory. The system comprises both a micro and macro sample handling facility. The approach that we have adopted uses a conventional spectrograph with a charge coupled device detector and optics optimized for the 700–1000 nm spectral region. The laser source is a tunable titanium sapphire laser which provides intense radiation in the 700–900 nm spectral region. We show that the spatial resolution and spectral sensitivity that can be obtained with the microscope attachment are excellent. A comparison between data obtained from our system and that from an FT-Raman system is given.

The effect of apodization and finite resolution on Fourier transform Raman spectra

Abstract The effects that finite resolution and choice of apodization function have on Fourier transform (FT) Raman spectra are illustrated by the 839 cm −1 (ν 1 ) and 914 cm −1 bands of KMnO 4 . FT-Raman spectra were recorded at 0.5, 1, 2, 4, 8, 16 and 32 cm −1 resolution using boxcar, Norton—Beer (strong, medium and weak) and triangular apodization functions at each resolution. The results show the dramatic changes in bandshape that occur as the ratio (resolution/true full width at half height of band) increases. The changes were measured in terms of the full width at half height of the band, the height of the band, the area of the band and the bandshape (expressed as a sum of Lorentzian and Gaussian lines). At a given resolution the degree to which each of these characteristics is affected is strongly dependent on the choice of the apodization function.

Order in nascent polyethylene

Nascent polyethylenes of the high density and linear low density types prepared by gas phase polymerization, together with a series of nascent high density polyethylenes prepared by the slurry process, have been examined by X-ray diffraction, inelastic neutron scattering and Raman spectroscopy. The X-ray diffraction data and inelastic neutron scattering spectra show that the degree of crystallinity is smaller in the nascent samples than is commonly found in melt-crystallized materials. Low frequency Raman spectroscopy shows that the lamellar thickness is also smaller than usual. Melting and resolidification of the nascent, gas phase polymerized polyethylenes result in materials that more closely resemble the melt-crystallized products. The implications of the results are discussed as regards the crystallization process.