CH Vibrations Research Articles

ATR-FTIR spectroscopy reveals involvement of lipids and proteins of intact pea pollen grains to heat stress tolerance.

With climate change, pea will be more frequently subjected to heat stress in semi-arid regions like Saskatchewan during flowering. The pollen germination percentage of two pea cultivars was reduced by heat stress (36°C) with an important decrease in cultivar ‘CDC Golden’ compared to ‘CDC Sage.’ Lipids, protein and other pollen coat compositions of whole intact pollen grains of both pea cultivars were investigated using mid infrared (mid-IR) attenuated total reflectance (ATR)–Fourier transform infrared (FTIR) spectroscopy. Curve fitting of ATR absorbance spectra in the protein region enabled estimation and comparison of different protein secondary structures between the two cultivars. CDC Sage had relatively greater amounts of α-helical structures (48.6–43.6%; band at 1654 cm-1) and smaller amounts of β-sheets (41.3–46%) than CDC Golden. The CDC Golden had higher amounts of β-sheets (46.3–51.7%) compared to α-helical structures (35.3–36.2%). Further, heat stress resulted in prominent changes in the symmetrical and asymmetrical CH2 bands from lipid acyl chain, ester carbonyl band, and carbohydrate region. The intensity of asymmetric and symmetric CH2 vibration of heat stressed CDC Golden was reduced considerably in comparison to the control and the decrease was higher compared to CDC Sage. In addition, CDC Golden showed an increase in intensity at the oxidative band of 3015 cm-1. These results reveal that the whole pollen grains of both pea cultivars responded differently to heat stress. The tolerance of CDC Sage to heat stress (expressed as pollen germination percentage) may be due to its protein richness with α-helical structures which would protect against the destructive effects of dehydration due to heat stress. The low pollen germination percentage of CDC Golden after heat stress may be also due to its sensitivity to lipid changes due to heat stress.

Surface of room temperature ionic liquid [bmim][PF6] studied by polarization- and experimental configuration-dependent sum frequency generation vibrational spectroscopy

Understanding and control of the surface properties such as molecular orientations are of great importance in numerous applications of ionic liquids. However, there remain discrepancies among the previous experimental and theoretical studies on the surface orientation and structures of room temperature ionic liquids (RTIL) systems. In this article, the orientation of 1-butyl-3-methylimidazolium ([bmin]) cation at the air/liquid interface of a characteristic RTIL, 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]), was investigated by the sum frequency generation vibrational spectroscopy (SFG-VS). Detailed polarization and experimental configuration analyses of the SFG-VS spectra showed the possibility of a small spectral splitting in the CH3 symmetric stretching region, which can be further attributed to the probable existence of multiple orientations for the interfacial [bmim] cations. In addition, the (N)-CH3 vibrations were absent, ruling out the prediction by several recent molecular dynamics simulations which state that portions of the [bmim] cations orient with a standing-up (N)-CH3 group at the ionic liquid surface. Hence, new realistic theoretical models have to be developed to reflect the complex nature of the ionic liquid surface.

Functionalization of luminescent YVO4:Eu3+ nanoparticles by sol–gel

Over the last decades, researchers have explored nanotechnological applications in different areas. The non-hydrolytic and hydrolytic sol–gel routes offer the ideal conditions to obtain materials with distinct compositions and multifunctionality, for use in such diverse areas as nanomedicine and technology. In this work, we used the modified hydrolytic sol–gel route to prepare YVO4 doped with Eu3+ ion. The YVO4:Eu3+ nanoparticles were functionalized with 3-chloropropyltriethoxysilane using the hydrolytic sol–gel process; the drug cisplatin was then added to them. The final powder was characterized by thermal analysis, infrared spectroscopy, X-ray diffraction, and photoluminescence. The powder X-ray diffraction patterns of the samples obtained before and after functionalization revealed well defined peaks ascribed to the tetragonal structure of the YVO4 phase. The thermal analysis curves evidenced mass loss relative to 3-chloropropyltriethoxysilane and cisplatin decomposition. Infrared spectroscopy showed the peaks related to the CH and NH groups vibration modes, confirming YVO4 functionalization. The excitation and emission spectrum of the Eu3+ ion did not change upon its doping into the matrix functionalized with 3-chloropropyl and cisplatin. Cytotoxicity tests conducted on normal Chinese hamster (V79 cells) and murine melanoma (B16F10) cells attested that the matrix was not toxic.

Use of the ionic liquid-tricaprylmethyl ammonium salicylate (TOMAS) as a flotation collector of quartz

Tricaprylmethyl ammonium salicylate (TOMAS), an ionic liquid has been employed as a collector for the flotation of quartz. The sign reversal of zeta potential of quartz from negative to positive values and increase in contact angle from 80 to 700 by the addition of TOMAS have substantiated the interaction of TOMAS with the quartz surface. The change in morphology as indicated in SEM and two peaks at 2933 and 2857cm−1 owing to the vibrations of CH2 and CH3 as seen in FTIR spectra support the adsorption of the reagent on quartz. The N(1s) peak as observed in the XPS spectra of the quartz treated with TOMAS, and the curve fitting data of C(1s) corroborate the reagent-mineral interaction. A comparative study of adsorption of DDA and TOMAS using classical molecular mechanics also suggests stronger adsorption of the latter on the hydroxylated quartz surface. Flotation studies of quartz at natural pH indicate ∼100% quartz recovery at a TOMAS concentration of 8*10−5M. Flotation of the natural occurring BHQ iron ore containing 38.7% Fe and 43.8% SiO2 concludes that a TOMAS concentration of 3.71*10−5M is sufficient to achieve 67.3% Fe with 66.6% iron recovery. TOMAS stands out as a better quartz collector, when compared with DDA.

Phase changes induced by guest orientational ordering of filled ice Ih methane hydrate under high pressure and low temperature

Low-temperature and high-pressure experiments were performed with filled ice Ih structure of methane hydrate under pressure and temperature conditions of 2.0 to 77.0 GPa and 30 to 300 K, respectively, using diamond anvil cells and a helium-refrigeration cryostat. Distinct changes in the axial ratios of the host framework were revealed by In-situ X-ray diffractometry. Splitting in the CH vibration modes of the guest methane molecules, which was previously explained by the orientational ordering of the guest molecules, was observed by Raman spectroscopy. The pressure and temperature conditions at the split of the vibration modes agreed well with those of the axial ratio changes. The results indicated that orientational ordering of the guest methane molecules from orientational disordered-state occurred at high pressures and low temperatures, and that this guest ordering led to the axial ratio changes in the host framework. Existing regions of the guest disordered-phase and the guest ordered-phase were roughly estimated by the X-ray data. In addition, above the pressure of the guest-ordered phase, another high pressure phase was developed at a low-temperature region. The deuterated-water host samples were also examined and isotopic effects on the guest ordering and phase changes were observed.

A Near‐Infrared Spectroscopy Study on Thermal Transitions of PMMA and PMMA/SiO2 Nanocomposites

SummaryIn this work infrared spectroscopy in the near infrared (NIR) range is used to study the dynamics of polymethylmethacrylate (PMMA) as a pure polymer and when it is modified with silica nanoparticles. Different ways of analyzing FT‐NIR spectra were explored taking the temperature as the variable of the systems: i) bands shifts in terms of the first moment of the wavenumber; ii) bands shifts in terms of an absorbance ratio method and iii) absorbance variations in terms of the integrated absorbance. The spectroscopic analysis allowed to study thermal transitions in PMMA and PMMA based nanocomposites. In particular the method based on the use of an absorbance ratio is the most sensible to analyze the thermal transitions. All bands considered could be used to detect thermal transitions reflecting how the vibrations of the groups CH2; CH3 and CO are affected by changes in the free volume of the PMMA. It seems that the PMMA dynamics is more driven by the lateral groups than the backbone of the polymer. The presence of silica nanoparticles, at least when 1% by weight is used, restrains the PMMA motion increasing the relaxation temperatures that can be associated with the glass transitions.

Infrared spectroscopy of some carbon-based materials relevant in combustion: Qualitative and quantitative analysis of hydrogen

A detailed procedure for the quantitative analysis of aromatic and aliphatic hydrogen based on infrared spectroscopy was set up and implemented on some carbon-based materials produced from organic precursors (naphthalene pitch) and/or relevant in combustion field (asphaltenes, carbon particulate matter, carbon black), spanning in the H/C atomic ratio range from 0.1 to 1. The quantitative FT-IR analysis involved the spectral deconvolution in the CH vibrations regions and the calibration factors of diverse standard species having spectral characteristics suitable for the detailed peak-to-peak analysis of the CH stretching (3100–2800cm−1) and aromatic CH bending (900–700cm−1) regions. The good agreement between the H/C atomic ratio obtained by quantitative FT-IR analysis and elemental analysis showed a reasonable reliability of the procedure. The major value of the developed FT-IR quantitative technique relies also on the capacity of discriminating between the different kinds of aliphatic and aromatic hydrogen. The quantitative and detailed analysis of hydrogen in form of CH3, CH2 and CH groups and in form of solo, duo and trio/quatro aromatic hydrogens showed to be useful also for inferring the structure of the aromatic moieties constituting the CC backbone of carbon materials.

SYNTHESIS AND CHARACTERIZATION OF C-HEXYL CALIX[4]RESORCINARENA FROM RESORCINOL-HEPTANAL CONDENSATION

<p>Synthesis of C-hexyl calix[4]resorcinarene through condensation reaction between resorcinol and heptanal with HCl catalyst has been investigated. Synthesis conducted by the ratio of resorcinol: heptanal (1:1) using reflux for 6 hours in ethanol. Structural analysis of the product was done by FTIR and <sup>1</sup>H-NMR spectroscopy, while the characterization was done by determining melting point. The product was golden brown powder with a yield 99.43% and melting point more than 300 °C. Structural analysis with FTIR and <sup>1</sup>H-NMR known vibration and signals characteristic of calix[4]resorcinarena group,the vibrations of the CH bridge. Vibration of CH bridge appearance at wave number 1445 cm<sup>-1</sup> and 1456 cm<sup>-1</sup>, while signal of CH bridge appearance at chemical shifts from 4.17 to 4.21 ppm.</p>

Investigation of aniline by high pressure Raman scattering spectroscopy and quantum chemical calculation

The unexpected redshifts of the NH and the blueshifts of the CH stretching frequencies with increasing pressure were observed in the pressure dependent Raman spectra of aniline at 0−7.7 GPa. Based on the natural bond orbital (NBO) analysis, the opposite frequency shifts of the CH and NH vibrations are attributed to the different pressure-induced change in the electronic antibonding occupancies of NH and CH. Further, the antibonding occupancy of \hbox{$\pi_{\mathrm{C{\mbox{-}}C}}^{\ast}$} π C - C ∗ and the charge of phenyl ring in aniline were evaluated, characterizing an increase of antibonding occupancy of \hbox{$\pi ^{\ast}_{\mathrm{C\mbox{-}C}}$} π C - C ∗ and a decrease of phenyl ring’s charge with increasing pressure. These results reflect that the NH-π and CH-π bonds, as the research target of intermolecular interaction, are becoming weaker as the pressure increases. The research would improve the understanding of the pressure effect on the intermolecular interactions of aniline.

Demicellization of Polyethylene Oxide in Water Solution under Static Magnetic Field Exposure Studied by FTIR Spectroscopy

FTIR spectroscopy was used to investigate the alterations of the vibration bands in the mid-infrared region of Polyethylene oxide in aqueous solution at 25 mg/mL concentration under exposure up to 4 h to a static magnetic field at 200 mT. FTIR spectroscopic analysis of PEO solution in the range 3500–1000 cm−1 evidenced the stretching vibrations of ether band, C–H symmetric-antisymmetric and bending vibrations of methylene groups, and the C–O–C stretching band. A significant decrease in intensity of symmetric and asymmetric stretching CH2 vibration bands occurred after 2 h and 4 h of exposure, followed by a significant decrease in intensity of scissoring bending in plane CH2 vibration around 1465 cm−1. Finally, the C–O–C stretching band around 1080 cm−1 increased in intensity after 4 h of exposure. This result can be attributed to the increase of formation of the intermolecular hydrogen bonding that occurred in PEO aqueous solution after SMF exposure, due to the reorientation of PEO chain after exposure to SMF. In this scenario, the observed decrease in intensity of CH2 vibration bands can be understood as well considering that the reorientation of PEO chain under the applied SMF induces PEO demicellization.

Spectroscopy of the C–H Stretching Vibrational Band in Selected Organic Molecules

The vibrational spectroscopy of C-H stretches in organic molecules is of considerable importance for the characterization of these systems and for exploration of their properties. These stretches are strongly anharmonic, and thus methods including anharmonicity have to be used. The vibrational self-consistent field (VSCF) is applied to the following organic compounds: acetone, dimethylacetylene, neopentane, toluene, ethylene, and cyclopropane. The computed spectra are compared to new experimental data, including Raman measurements of all molecules except cyclopropane and IR of acetone, neopentane, and ethylene. A high level of agreement is found for all of the molecules. The characteristic features of CH3 and CH2 groups are studied and analyzed in detail. A reliable, unambiguous assignment of vibrational modes to spectral peaks is provided. Several characteristic features of CH3 and CH2 vibrations in polyatomic molecules are clarified, providing easier assignments for different types of organic molecules.

Quantum Mechanical Interpretation of Intermolecular Vibrational Modes of Crystalline Poly-(R)-3-Hydroxybutyrate Observed in Low-Frequency Raman and Terahertz Spectra

Low-frequency vibrational bands observed in the Raman and terahertz (THz) spectra in the region of 50-150 cm(-1) of crystalline powder poly-(R)-3-hydroxybutyrate (PHB) were assigned based on comparisons of the Raman and THz spectra, polarization directions of THz absorption spectra, and their congruities to quantum mechanically (QM) calculated spectra. This combination, Raman and THz spectroscopies and the QM simulations, has been rarely adopted in spite of its potential of reliable assignments of the vibrational bands. The QM simulation of a spectrum has already been popular in vibrational spectroscopies, but for low-frequency bands of polymers it is still a difficult task due to its large scales of systems and a fact that interactions among polymer chains should be considered in the calculation. In this study, the spectral calculations with the aid of the Cartesian-coordinate tensor transfer (CCT) method were applied successfully to the crystalline PHB, which include the explicit consideration of an intermolecular interaction among helical polymer chains. The agreements between the calculations and the experiments are good in both the Raman and THz spectra in terms of spectral shapes, frequencies, and intensities. A Raman active band at 79 cm(-1) was assigned to the intermolecular vibrational mode of the out-of-plane C═O + CH(3) vibration. A polarization state of the corresponding far-infrared absorption band at ∼82 cm(-1), perpendicular to the helix-elongation direction of PHB, was reproduced only under the explicit correction, which indicates that this polarized band originates from the interaction among the polymer chains. The calculation explored that the polarization direction of this band was along the a axis, which is consistent with the direction in which weak intermolecular hydrogen bonds are suggested between the C═O and CH(3) groups of two parallel polymer chains. The results obtained here have confirmed sensitivity of the low-frequency vibrational bands to the weak hydrogen bonds among the polymer chains.

Raman spectroscopy investigation of mono- and diacyl-polyoxyethylene glycols

Physical and thermal properties of polyoxyethylene glycol glycerides (Gelucire 50/13) used as sustained release matrix forming agent in pharmaceutical applications are studied by Raman spectroscopy combined with X-ray diffraction and differential scanning calorimetry methods.At first, Raman spectroscopy was used to characterize the polymorphs and liquid state of PEG 1500, with emphasis placed on the evolution of the Raman-active CC and CO stretching region (1300–1100cm−1), along with complementary analysis of the Raman-active CH stretching modes (3000–2800cm−1) in comparison with temperature. Unique Raman signatures were obtained for all phases, with their identity confirmed using DSC and XRD. The CC and CO stretching modes, which provided insight into the trans/gauche content, permitted polymorph discrimination due to differences in the number of modes, their relative frequencies and their full-widths at half-maximum. CH stretching generally increased with polymorph stability, indicating the dominance of methylene antisymmetric CH2 vibrations as the PEG 1500 crystal lattice became more ordered. The change in the intensities of the CH stretching bands was used to probe the order–disorder transition.The second time, Raman spectroscopy of Gelucire 50/13 was performed to characterize the contribution of its each component, with emphasis placed on the evolution of the t(CH2) and ν(CC) vibrational mode regions (1300–1200cm−1), along with analysis of the Raman-active CH stretching modes (3000–2800cm−1), δ(CH2) and δ(CH3) deformation region (1500–1400cm−1), and ras(CH2) rocking region (900–800cm−1). In comparison with temperature, the changes of the ratios of I[νs(CH2)]/I[νas(CH2)] (I2850/I2890), I[νas(CH2)]/I[νs(CH3)] (I2890/I2950), I[δ(CH2)]/I[δ(CH3)] (I1444/I1490), I1296/I1282 and I[ras(CH2)]/I[t(CH2)] (I845/I1282) were directly correlated with conformational changes of the Gelucire structure. Overall, Raman spectroscopy clearly demonstrated that the different functional groups studied could be characterized independently, allowing for the understanding of their role in Gelucire polymorphism.

FT-IR spectroscopic investigation of bacterial cell envelopes from Zymomonas mobilis which have different surface hydrophobicities

Changes in the cell envelope composition of bacteria Zymomonas mobilis 113S which resulted in varied cell surface hydrophobicity (CSH) were examined using FT-IR-spectroscopy and conventional methods of biochemical analysis. Significant differences in all analyses were detected between hydrophilic and hydrophobic compounds in the envelopes of Z. mobilis. A significant positive linear relationship (P<0.01) was observed between CSH values of Z. mobilis and the ratio of absorbance band intensities assigned to the CH2 and CH3 vibrations from lipids as well as the asymmetric and symmetric stretching vibrations of these methylene and methyl groups. Differences were also seen between the ratio of band absorbances arising from Amide I and Amide II. In addition, the ratio of absorbance intensities assigned to carbohydrates was found to correlate with the CSH of Z. mobilis. Furthermore, a significant inverse relationship (P<0.01) was detected between CSH of Z. mobilis and the band position (frequency) shift of asymmetric and symmetric stretching of methylene groups, alkyl esters (>CO str) and Amide I, as well as a positive linear relationship (P<0.05) between the band position shift of Amide II and carbohydrates and CSH of Z. mobilis. Qualitative and quantitative composition of the cell envelopes was found to correlate with both the CSH of Z. mobilis and the spectral data. The observed multiple correlations between the concentration of proteins, carbohydrates and lipids of cell envelopes and selected indices of FT-IR spectra indicate the complex relationship of Z. mobilis cell envelope constituents. The results of present study suggest that FT-IR spectroscopy can be used as a convenient method to evaluate the properties of cell envelopes from Gram-negative bacteria.

Enhancement of infrared absorption of biomolecules absorbed on single-wall carbon nanotubes and graphene nanosheets

Surface-enhanced infrared absorption effect was shown for graphene for the first time. Comparative analysis of the effect was carried out for thymine (Thy), one of the bases of DNA, absorbed on single-wall carbon nanotubes (SWCNT) and graphene sheets. The enhancement factor in the case of SWCNT was generally greater in comparison with that obtained for graphene sheets. Its value for SWCNT was up to six and the largest enhancement factors were found for vibrations of CH, C−OH , and C−N bonds. The maximum enhancement factor in the case of graphene sheets equal to 3.7 was found for ring and deformational vibrations of N-H bonds. The obtained experimental data allowed us to suppose that H-bonds influence the interaction between SWCNT and Thy in Thy-SWCNT/Au, while the chemically induced charge transfer is probably dominant in the case of Thy-graphene/Au.

Multicolored stain-free histopathology with coherent Raman imaging

Conventional histopathology with hematoxylin & eosin (H&E) has been the gold standard for histopathological diagnosis of a wide range of diseases. However, it is not performed in vivo and requires thin tissue sections obtained after tissue biopsy, which carries risk, particularly in the central nervous system. Here we describe the development of an alternative, multicolored way to visualize tissue in real-time through the use of coherent Raman imaging (CRI), without the use of dyes. CRI relies on intrinsic chemical contrast based on vibrational properties of molecules and intrinsic optical sectioning by nonlinear excitation. We demonstrate that multicolor images originating from CH2 and CH3 vibrations of lipids and protein, as well as two-photon absorption of hemoglobin, can be obtained with subcellular resolution from fresh tissue. These stain-free histopathological images show resolutions similar to those obtained by conventional techniques, but do not require tissue fixation, sectioning or staining of the tissue analyzed.

Risk assessment of an old landfill regarding the potential of gaseous emissions – A case study based on bioindication, FT-IR spectroscopy and thermal analysis

Risk assessment of two sections (I and II) of an old landfill (ALH) in Styria (Austria) in terms of reactivity of waste organic matter and the related potential of gaseous emissions was performed using conventional parameters and innovative tools to verify their effectiveness in practice. The ecological survey of the established vegetation at the landfill surface (plant sociological relevés) indicated no relevant emissions over a longer period of time. Statistical evaluation of conventional parameters reveals that dissolved organic carbon (DOC), respiration activity (RA4), loss of ignition (LOI) and total inorganic carbon (TIC) mostly influence the variability of the gas generation sum (GS21). According to Fourier Transform Infrared (FT-IR) spectral data and the results of the classification model the reactivity potential of the investigated sections is very low which is in accordance with the results of plant sociological relevés and biological tests. The interpretation of specific regions in the FT-IR spectra was changed and adapted to material characteristics. Contrary to mechanically–biologically treated (MBT) materials, where strong aliphatic methylene bands indicate reactivity, they are rather assigned to the CH vibrations of plastics in old landfill materials. This assumption was confirmed by thermal analysis and the characteristic heat flow profile of plastics containing landfill samples. Therefore organic carbon contents are relatively high compared to other stable landfills as shown by a prediction model for TOC contents based on heat flow profiles and partial least squares regression (PLS-R). The stability of the landfill samples, expressed by the relation of CO2 release and enthalpies, was compared to unreactive landfills, archeological samples, earthlike materials and hardly degradable organic matter. Due to the material composition and the aging process the landfill samples are located between hardly degradable, but easily combustible materials and thermally resistant materials with acquired stability.

Raman spectra and emission of CdSe/ZnS quantum dots bioconjugated to OC125 antibodies

AbstractThis paper presents the analysis of photoluminescence (PL) and Raman scattering spectra of CdSe/ZnS quantum dots (QDs) covered by the amine (NH2)‐derivatized PEG polymer nonconjugated and bioconjugated to ovarian cancer (OC 125) antibodies and localized on the Si substrate. Commercial CdSe/ZnS QDs used are characterized by the color emission with the maxima at 525 nm (2.36 eV) and 605 nm (2.05 eV) at 300 K. PL spectra of nonconjugated 525 nm QDs can be presented as a superposition of PL bands related to exciton emission in CdSe cores (2.36 eV) and to hot electron‐hole emission via interface states (2.04, 2.20, 2.71 and 3.06 eV) localized at the CdSe/ZnS or ZnS/polymer interfaces. It is revealed that PL spectra of bioconjugated 525 nm QDs have changed dramatically with essential decreasing the hot electron‐hole recombination flow via interface states. This effect is explained on the base of the model deals with re‐charging QD interface states at the bioconjugation to OC125 antibodies. Raman scattering spectra of bioconjugated QDs can be presented as a superposition of Raman lines related to the silicon substrate. Raman lines deal with the vibration of COH, CH and OH groups of PEG polymer, which covered QDs, were detected additionally in nonconjugated QDs. It is revealed that the QD bioconjugation to OC 125 antibodies is accompanied by the changes in the intensity of all types of Raman lines. The explanation of details of bioconjugation effects has been proposed and discussed (© 2012 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Surface texture modification of spin-coated SiO2 xerogel thin films by TMCS silylation

The SiO2 xerogel thin films were deposited successfully by sol–gel technique via the acid-catalyzed hydrolysis and condensation of tetraethylorthosilicate (TEOS) with ethanol as a solvent. Further, the deposited thin films were treated wet chemically by trimethylchlorosilane (TMCS) and hexane solution with 10% and 15% volume ratio to remove the hydroxyl groups from the surface of deposited SiO2 thin films. These as deposited and surface-modified films were characterized by ellipsometer, Fourier transform infrared (FTIR) spectrometer and contact angle meter. The presence of 2900 and 850 cm−1 peaks of CH3 vibrations in FTIR spectra of surface-modified films confirms the hydrphobisation of SiO2 surface. The maximum contact angle of 108·7° was observed for the surface-modified film at 10% TMCS.

Influence of a methyl substituent on the Raman spectrum of but-3-enyl methyl ether

The Raman spectrum of but-3-enyl methyl ether, CH2CHCH2CH2OCH3 is reported and compared with those of its homologues in which a methyl group is substituted for a hydrogen atom on one of the carbon atoms of the alkenyl chain. Attention is focused on the influence of this methyl group on the bands in the spectrum associated with specific CH, skeletal stretching and bending vibrations. The use of ab initio DFT quantum mechanical calculations to assist in making these assignments reveals a high degree of mode-mixing in the skeletal vibrations. The value of model studies of this kind in refining the correlations between the presence and absence of specific bands in a Raman spectrum with molecular structure is emphasised.