Methyl Rocking Modes Research Articles

Fourier transform synchrotron spectroscopy of the in-plane methyl-rocking band of CD3OH

Infrared Fourier transform spectra of the 12CD3OH isotopologue of methanol recorded at the Canadian Light Source synchrotron have been investigated in the 750–950cm−1 region to explore the torsional energy pattern of the in-plane methyl-rocking mode. The in-plane CD3-rocking band is primarily of parallel a-type character with relatively widely spaced K-structure, and the central Q-branch region is well-resolved. Sub-bands have been assigned for the vt=0 ground torsional state from K=0 to 15 for both A and E torsional species, as well as a number of sub-bands in the vt=1 excited torsional state. A variety of perturbations due to asymmetry-induced, anharmonic Fermi and level-crossing resonances is seen in the spectra. Mapping of K-reduced torsional energies determined from the upper-state term values shows that the K-dependence is severely distorted from the usual pattern of smoothly oscillating, near-sinusoidal interlocking τ-curves. Although the K=0 levels are nominally inverted for vt=0, the torsional splitting is very small with the A level higher than the E level by only 0.625cm−1, and the anomalous behavior precludes a definite conclusion about torsional inversion. However, the range of variation with K of the vt=0 CD3-rocking τ-curves is about half that of the ground vibrational state, similar to previous observations for CH3OH isotopologues, suggesting a comparable reduction of about 25% in the effective torsional barrier height for the in-plane rocking mode.

Long-lived coherence in carotenoids

We use two-colour vibronic coherence spectroscopy to observe long-lived vibrational coherences in the ground electronic state of carotenoid molecules, with decoherence times in excess of 1 ps. Lycopene and spheroidene were studied isolated in solution, and within the LH2 light-harvesting complex extracted from purple bacteria. The vibrational coherence time is shown to increase significantly for the carotenoid in the complex, providing further support to previous assertions that long-lived electronic coherences in light-harvesting complexes are facilitated by in-phase motion of the chromophores and surrounding proteins. Using this technique, we are also able to follow the evolution of excited state coherences and find that for carotenoids in the light-harvesting complex the ⟨S2|S0⟩ superposition remains coherent for more than 70 fs. In addition to the implications of this long electronic decoherence time, the extended coherence allows us to observe the evolution of the excited state wavepacket. These experiments reveal an enhancement of the vibronic coupling to the first vibrational level of the C–C stretching mode and/or methyl-rocking mode in the ground electronic state 70 fs after the initial excitation. These observations open the door to future experiments and modelling that may be able to resolve the relaxation dynamics of carotenoids in solution and in natural light-harvesting systems.

Cationic polyfluorene: Conformation and aggregation in a “good” solvent

The conjugated polyelectrolyte (CPE) poly{9,9′-bis[6″-( N, N, N-trimethylammonium)-hexylfluorene- alt- co-phenylene] dibromide} (PFPN +Br −) demonstrates a high solubility in methanol in comparison to other more hydrophilic or hydrophobic solvents. We have employed a combination of pulsed-field-gradient-NMR, photoluminescence (PL), and Raman spectroscopy to establish the conformation and aggregation behavior of PFPN +Br − in methanol, with the aim to attain information on how to design CPEs with a high solubility in a preferred solvent. We find that the diffusion coefficient and PL spectrum of PFPN +Br −, as well as the Raman-active methyl rocking mode of methanol, all exhibit a strong dependence on PFPN +Br − concentration. We rationalize our findings with a model in which PFPN +Br − forms aggregates via π–π interactions between main-chain segments, while the ionic side chains are surrounded and electrostatically screened by the methanol solvent. Accordingly, the notably high solubility of PFPN +Br − in methanol is rationalized by favorable interactions between the ionic side chains and the methanol molecules. We propose that an appropriate design of a high-solubility CPE should consider a matching of the mixed hydrophobic/hydrophilic character of the ionic side chain with that of the preferred solvent.

Aromatic Molecules in Restricted Geometries: Pyrene Excimer Formation in an Anchored Bilayer

The galleries of an anionic clay, Mg-Al Layered Double Hydroxide (Mg-Al LDH) have been functionalized by intercalating the anionic surfactant do-decyl sulfate. Within the galleries, the alkyl chains of the surfactant adopt a bilayer structure with the sulfate headgroup anchored to the inorganic sheet. Pyrene molecules have been solubilized in the anchored bilayer by partitioning from polar solvents. The presence of pyrene molecules induces conformational disorder in the alkyl chains of the bilayer and more importantly inhibits the rotational disordering motion of the sulfate headgroup. Pyrene fluorescence indicates formation of excimers whose intensity increases with concentration of solubilized pyrene indicating that they are mobile. Pyrene solubilized in the anchored bilayer exhibits unusual phenomena; on evacuation the excimer band disappear but reappears on releasing vacuum. It is shown that this behavior arises due to the loss of water of hydration of the headgroup on evacuation and as a consequence the pyrene moves into the less polar interior of the bilayer where it is immobile and can no longer diffuse and form excimers. The motion of pyrene into the interior of the bilayer creates free space near the surfactant chain termini, which manifests in the disappearance of the methyl-rocking mode of the ordered (-tt) end-chain conformer in the Raman spectra.

Velocity-map imaging study of the photodissociation of acetaldehyde

Velocity-map imaging studies are reported for the photodissociation of acetaldehyde over a range of photolysis wavelengths (317.5-282.5 nm). Images are obtained for both the HCO and CH3 fragments. The mean rotational energy of both fragments increases with photodissociation energy, with a lesser degree of excitation in the CH3 fragment. The CH3 images demonstrate that the CH3 fragments are rotationally aligned with respect to the recoil direction and this is interpreted, and well modeled, on the basis of a propensity for forming CH3 fragments with M approximately K, where M is the projection of the rotational angular momentum along the recoil direction. The origin of the CH3 rotation is conserved motion from the torsional and methyl-rocking modes of the parent molecule. Nonstatistical vibrational distributions for the CH3 fragment are obtained at higher energies.

Vibrational spectra of trimethylsilanol: The problem of the assignment of the SiOH group frequencies

The assignment of the SiOH group vibrations of trimethylsilanol, which is still controversial, is proposed. This assignment is based on theoretical B3LYP force field scaled using the constants of the (CH 3) 3Si group optimized to fit experimental vibrational frequencies of (CH 3) 3SiF and (CD 3) 3SiF molecules as well as the OH stretching scale factor from methanol. The ab initio force field defined in this way gives a good agreement of the theoretical vibrational frequencies of trimethylsilanol with the positions of IR and Raman bands observed in the gas phase. This force field predicts the greatest contribution of the δ SiOH coordinates to the vibration with frequency of 804 cm −1. The elimination of the coupling of the SiOH deformation with methyl rocking modes by the normal coordinate treatment of (CD 3) 3SiOH gives 832 cm −1 for silanol deformation which is in a good agreement with the 834 cm −1 value proposed earlier for the bending mode of free silanol groups. The geometry and force field of the open chain H 3SiOH trimer is computed to model the change of the δ SiOH frequencies upon formation of the hydrogen-bonded polymers. This model predicts a significant shift of SiOH bending frequencies to the 1000–1200 cm −1 range while those of SiOD to the 800–850 cm −1 range. These predictions allow us to ascribe the 1087 cm −1 band observed in the IR spectrum of crystalline (CH 3) 3SiOH and the Raman 775 cm −1 band of the liquid (CH 3) 3SiOD to deformations of the hydrogen-bonded silanol groups.

Unique properties of the 11-cis and 11,11′-di-cis isomers of β-carotene as revealed by electronic absorption, resonance Raman and 1H and 13C NMR spectroscopy and by HPLC analysis of their thermal isomerization

In comparison with the all-trans and other cis isomers of β-carotene, the 11-cis and 11,11′-di-cis isomers exhibited the following unique properties. (1) The wavelengths of the Bu+â†�Ag– (0–0) absorption of these two isomers are similar to that of the all-trans isomer, and do not follow the general rule of its blue shift found in other mono-cis and di-cis isomers. Their extinction coefficients are significantly lower than those of other isomers. (2) The frequencies of the CC stretching Raman lines of these isomers are the same as that of the all-trans isomer, and do not follow the general trend of high-frequency shifts found in other mono-cis and di-cis isomers. The Raman lines due to the out-of-plane C–H wagging and methyl rocking modes appear in the 11,11′-di-cis isomer. (3) The 1H chemical shifts of these isomers indicate severe steric interaction between the methyl and the olefinic 1H atoms in the concave side of the 11-cis bend, whereas their 13C chemical shifts suggest twisting and polarization of the cis C11C12 bond. (4) The rates of thermal isomerization of these isomers are much higher than that of the 15-cis isomer, i.e. the least stable isomer previously known.The results lead us to the conclusion that the 11-cis configuration has inherent twisting around the double and single bonds in the cis bend due to the severe steric interaction between the 13-methyl and the 10-olefinic hydrogens, and that it is unstable enough to disappear thermally at room temperature.

Raman study of odd‐numbered C11C23n‐alkanes in their high‐temperature solid phases

AbstractThe Raman spectra of solid odd‐numbered n‐alkanes were analysed in the frequency ranges corresponding to the LAM‐1 and the methyl rocking modes, in order to specify the nature and the quantity of the gauche forms occurring in the high‐temperature phases. The evolution of the LAM‐1 frequency under different pressure and temperature conditions is discussed in connection with the interlamellar distances. In the high‐temperature phases, the relative quantity and the nature of the gauche forms located at the ends and near the ends of the chains was estimated as functions of the chain length and of temperature.

Reactions in mixed non-aqueous solutions containing sulphur dioxide. Part 7. A Raman spectroscopic study of adduct formation between sulphur dioxide and dimethyl sulphoxide

Details are given of the ν(SO)dmso stretching (dmso = dimethyl sulphoxide), ν1(SO2), ν3(SO2), and ν2(SO2) regions of the Raman spectra of SO2–dmso mixtures at 293 K and sub-ambient temperatures. Analysis of the ν(SO)dmso region, which contains at least seven component bands, provides evidence of several component species. Bands at 1 067, 1 057, and 1 044 cm–1 are due to monomers and associates of dmso, and a band whose frequency varies between 1 029 and 1 025 cm–1 is due to a methyl-rocking mode of dmso. Bands at 1 013, 1 006, and 997 cm–1 are attributed to the adducts SO2·2dmso, SO2·dmso, and 2SO2·dmso respectively. An example of the use of band fitting to a difference spectrum supports these assignments.

Infrared spectroscopic studies on polyethylene, 6. Linear low density polymers

AbstractThe infrared spectra of five linear low density polyethylenes, containing methyl, ethyl, butyl, isobutyl and hexyl branches, respectively, were measured. Particular interest attaches to the region 800–1200 cm−1 where the spectra, although weak, show significant differences. The findings of an earlier study on the methyl rocking mode, in the vicinity of 900 cm−1, are confirmed and extended. In particular, no band was detected for the ethyl branched polymer and it must be an order of magnitude weaker than for methyl, butyl and hexyl branches. Conversely, it is displaced in frequency and is markedly stronger for isobutyl branches. The methyl wagging mode band is at about 1 150 cm−1 with the methyl branched polymer. It is relatively broad and probably consists of two overlapping components, specific for methyl branches in crystalline and amorphous regions. This mode is not of detectable intensity for ethyl, butyl and hexyl branches but, as with the rocking mode, it is anomalously strong for isobutyl branches.

Photofragmentation dynamics of CH3I at 248 nm

The vibrational distributions of the methyl fragment resulting from the photodissociation of CH3I at 248 nm are obtained by TOF photofragment spectroscopy. Only excitation of the v 2 (umbrella) mode is observed and the distribution peaks strongly at v = 2 for the electronically adiabatic pathway (I 2 P 1/2) but extends up to ν = 8 for the minor (∼ 30 per cent) non-adiabatic pathway (I 2 P 3/2). Angular measurements show that both pathways result from an initial parallel (3 Q 0 ←N) transition followed by a curve crossing to produce the ground state I fragment. The coupling of the curves is vibronic in nature and probably involves the v 6 methyl rocking mode. The lifetime of the photodissociation is found to be ∼ 1 × 10-13 s.

Assignments of methyl rocking modes in trimethylphosphine

Analysis of the vibrational spectra of trimethylphosphine- d 3 and d 6 leads to identification of the a 2 methyl rocking mode at 779 cm −1 in the infrared crystal spectrum of the d 0 species. The second e rock is found at 828 cm −1 in the gas phase.

Infrared spectroscopic studies on polyethylene, 2. The characterisation of specific types of alkyl branches in low‐branched ethylene polymers and copolymers

AbstractBands characteristic for some specific branches have been found in two regions of the IR spectra of near linear polyethylenes made by the Phillips process. Polymers with methyl branches give a band at 935 cm−1, probably the methyl rocking mode. The intensity of this band is proportional to the methyl branch content, thus providing a quantitative method for its estimation. No band specific for ethyl branches has been found, but propyl and longer branches show one at 890 cm−1 and this is also assigned to the rocking mode. This band is observed more readily if the polymers are brominated at ambient temperature, to remove interfering unsaturation bands. The results for methyl and ethyl branched near linear polymers differ significantly from those obtained with polypropylene, poly(1‐butene), and ethylene copolymers with high concentrations of methyl and ethyl branches. This is very probably the result of the presence of blocks of the repeating unit CH2CH(R) in the latter materials. Methyl branched polymers show a band at 1150 cm−1, assigned to the methyl wagging mode of branches present in the crystalline phase. Its intensity is proportional to the methyl branch content and it may be used for quantitative purposes. Ethyl and longer branches do not give a methyl wagging band of detectable intensity.

Vibrational spectroscopic studies of internal rotation of symmetrical groups—IV. Toluene and some deuterated derivatives in the vapour phase

The vapour phase infrared spectra of C 6H 5CH 3, C 6D 5CH 3 and C 6H 5CD 3 have been measured in detail in the wavenumber regions associated with the “perpendicular” methyl (or methyl- d 3) bond-stretching, antisymmetrical bending and rocking vibrations. Although the spacings are irregular, fine-structure associated with internal rotation is clearly observed within the absorption band contours from the perpendicular stretching modes of the CH 3 and CD 3 groups. The mean features of these bands may be explained qualitatively in terms of almost-free internal rotation of the methyl and methyl- d 3 groups, together with effects associated with the splitting of the degeneracy of the “perpendicular” modes. Similar internal-rotation fine structure could not be observed with any certainty on the bands from the antisymmetrical bending or methyl rocking modes. These observations can reasonably be accounted for in terms of substantial splitting of the in-plane and out-of-plane vibrational frequencies of these modes together with overlapping from other absorption bands associated with the phenyl or phenyl- d 5 groups.

The structure of the methylamine molecule in an excited electronic state

Ultraviolet absorption spectra of four isotopic methylamines, CH 3NH 2, CH 3ND 2, CD 3NH 2, and CD 3ND 2, have been observed in their gaseous states in the spectral region of 2000–2500 Å. There is an electronic band whose 0-0 peaks are observed respectively at 2397, 2376, 2392, and 2372 Å. The vibrational structure of the band was analyzed in terms of two frequencies of the ground state and two frequencies of the excited state. The two frequencies of each state were assigned to the amino-wagging and methyl-rocking vibrations. On the basis of the observed energy levels of the amino-wagging vibration in the CH 3NH 2 and CH 3ND 2 molecules, the potential function of the excited molecule along this coordinate was determined: it has a broad minimum at the position of the planar amino group, and it is nearly harmonic up to the position where the HNH plane inclines 90° from the CN bond. On the basis of the observed vibrational frequencies, a normal coordinate treatment was made of the four methylamine molecules in the excited state. By assuming the values of two force constants ( f w and f r ), all of the seven normal frequencies observed of the four excited methylamines were explained. It was shown that the vibrational coupling of the amino-wagging and methyl-rocking modes becomes greater in the order of CH 3ND 2, CH 3NH 2, CD 3ND 2, and CD 3NH 2.

Vibrational assignments from the infra-red crystal spectrum of dimethyl ether

Infra-red spectra of dimethyl ether have been obtained between 3000 and 400 cm −1 from a crystalline film at 80°K. Some unpublished data are also quoted for the region below 400 cm −1. The methyl rocking modes are confidently assigned as follows: 1250 cm −1 ( a 1), 1181 cm −1 ( b 2), 1046 cm −1 ( a 2), 1165/1095 cm −1 ( b 1, ν CN/ϱ CH 3 ). The methyl deformation region warrants further investigation.

Vibrational Spectra of the Four Lowest Nitroparaffins

The infra-red spectra from 2 to 22μ of nitromethane, nitroethane, 1-nitropropane and 2-nitropropane have been obtained for both vapor and liquid phases. The Raman shifts have been measured for the liquids, and for nitromethane the polarizations of the bands have been determined. A satisfactory interpretation of the spectra has been obtained for nitromethane on the basis of assigned fundamental frequencies that agree with those proposed previously by Wells and Wilson, except for the B1 methyl rocking mode. The fundamental frequencies have not been determined for the other compounds, but the positions of the nitro group absorptions have been determined empirically.