Antisymmetric CH Research Articles

Orientation studies of hydrated dipalmitoylphosphatidylcholine multibilayers by polarized FTIR-ATR spectroscopy

Polarized Fourier-transform infrared-attenuated total reflection spectroscopy has been applied to explore the temperature-dependence of molecular orientations in multibilayers of 1,2-dipalmitoyl- sn-glycero-3-phosphocholine (DPPC) of various degrees of hydration. The order parameter of the hydrocarbon chain, evaluated from the dichroic ratios of the antisymmetric and symmetric CH 2 stretching bands, was drastically decreased at the main (or gel to liquid-crystalline phase) transition temperature ( T m) irrespective of the water content, suggesting that the hydrocarbon chain is in a disordered state as a result of chain-melting associated with an increase in the number of the gauche conformers. On the other hand, the dichroic ratios of the polar bands of hydrated DPPC assignable to the symmetric PO 2 − stretching and asymmetric N +(CH 3) 3 stretching modes were increased mainly at the pretransition temperature ( T p), except for less hydrated case. The dichroic ratios of both the OH stretching and OH 2 bending bands of water showed the same temperature-dependence as those of the polar bands. These results indicate that the pretransition is ascribable mainly to the reorientation of the polar groups of the DPPC and bound water, while the main transition is due to the orientational disorder of the hydrocarbon chains. For less hydrated DPPC, the reorientation of the polar groups and water did not occur around T p, but in the higher temperature region around T m. This is in accord with the previously reported observation that the pretransition disappears for less hydrated DPPC. In this case, the polar groups and water may reorient following the reorientation of the hydrocarbon chains near T m.

Infrared spectroscopy of carbo-ions. V. Classical vs nonclassical structure of protonated acetylene C2H+3

The problem of classical vs nonclassical structure of protonated acetylene (vinyl cation) C2H+3 has been studied using high resolution infrared spectroscopy. The spectrum has been observed in the 3.2 μm region in air-cooled and water-cooled plasmas using C2H2:H2:He mixtures and in liquid nitrogen-cooled plasmas using CH4:H2:He mixtures. The difference frequency spectrometer with the velocity modulation method has been used to conduct the Doppler-limited, high sensitivity spectroscopy. The observed vibration–rotation pattern with the band origin at 3142.2 cm−1 has been identified as due to the antisymmetric CH stretching ν6 band of the C2H+3 ion with the nonclassical (bridged) structure. The observed spectral pattern was anomalous, but definitive assignments could be made for a part of the spectrum using the ground state combination differences which fit to the usual asymmetric rotor pattern. The discrimination between the classical and nonclassical structures is based on the observed spectral intensity pattern due to spin statistical weights. Agreement of vibrational band patterns and the rotational constants with ab initio values gives supporting evidence. The anomaly of the spectrum is at least partly ascribed to the small energy difference between the classical and nonclassical structures and possible rearrangement between them, the idea used by organic chemists over the years in wet chemistry. Systematic splittings with the intensity ratio of 2:1 have been noticed in some parts of the spectrum indicating that the protons tunnel between the apex and the two end equilibrium positions of the bridged structure. Using a simplified internal rotation model proposed by Hougen, the barrier height of the tunneling has been estimated. Chemical kinetics in plasmas related to C2H+3 is also discussed. We conclude that (1) the nonclassical structure is lower in energy than the classical structure, and (2) the apex proton and the two end protons exchange their positions with a measurable time scale.

Effect of lateral chain-chain spacing on the Raman spectrum of dipalmitoyl phosphatidylcholine bilayers

The C—H stretching region (2800–3200 cm −1 of the Raman spectrum of dipalmitoyl phosphatidylcholine was resolved into 7 Lorentzian bands corresponding to CH 2 and CH 3 stretching fundamentals and Fermi-resonance enhanced overtones and stimulated band profiles were compared with spectra recorded in Lβ, Pβ and Lα lamellar phases. Unlike all CH 3 stretching vibration which were conformation insensitive, the CH 2 vibration displayed decreasing spectral amplitudes/intensities except the partially allowed infrared active 2910 cm −1 melting marker which gained intensity with progressing chain isomerization. The spectral change of the 2883 cm −1 band has two contributions: a linebroadening of the d a − antisymmetric CH 2 stretching fundamental (at constant intensity) which is a chain mobility effect, and an intensity decrease of the overlapping Fermi-resonance enhanced CH 2 bending overtone which is primarily a chain packing effect. Using vibrational and neutron diffraction data for the bilayer dimensions and with the assumption that the leading term in anharmonicity is the interchain interaction potential the intensity of the Fermi-resonance enhanced overtone was separated from the d a − antisymmetric CH 2 stretching fundamental.

Fourier transform infrared-attenuated total reflection spectroscopy of hydration of dimyristoylphosphatidylcholine multibilayers

The effect of hydration on the structure and molecular orientation of multibilayers of 1,2-dimyristoyl- sn-glycero-3-phosphocholine (DMPC), cast on a germanium plate, was studied by means of polarized Fourier transform infrared (FT-IR)-attenuated total reflection spectroscopy. Compared with the dry state, the antisymmetric and symmetric CH 2 stretching bands of fully hydrated DMPC in the liquid-crystalline state were shifted to the higher frequency side, indicating the increase in the number of the gauche conformers. However, the dichroism of these bands revealed that the hydrocarbon chains of DMPC were still ordered and titled. The absorption bands of the glycerol ester, phosphoryl, and choline groups were broadened upon hydration, suggesting the activation of the librational or torsional motion. Furthermore, the dichroism of the polar head group bands of DMPC indicated that these groups retained a slight orientation even in the fully hydrated and fluid multibilayers.

Fourier transform infrared-attenuated total reflection spectra of dipalmitoylphosphatidylcholine monomolecular films

Single monomolecular films of 1,2- dipalmitoyl-3-sn- phosphatidylcholine (DPPC) were transferred onto a germanium plate by the Langmuir-Blodgett technique at various surface pressures encompassing a plateau region in a surface pressure(π)-area( A) isotherm. Molecular orientation and structure of the monomolecular films were investigated by the Fourier transform infrared-attenuated total reflection spectroscopy. Appreciable changes in frequency and halfbandwidth of the antisymmetric and symmetric CH 2 stretching bands of the palmitoyl chains were found at the onset of the plateau region in the π-A isotherm . By analyzing polarized infrared-attenuated total reflection spectra, the palmitoyl chains of DPPC films were proved to be oriented vertically to the germanium surface with all-trans conformation, irrespective of the surface pressure on the film transfer. This finding suggests the existence of islands or surface micelles on the surface throughout the surface pressure examined. The changes in the spectral parameters mentioned above were explained by changes in the state of packing of these islands. The plateau is ascribable to a transition process from a close-packed island phase to a molecularly homogeneous phase.

ChemInform Abstract: INFRARED INTENSITIES AND CORIOLIS INTERACTIONS IN METHYLENE FLUORIDE

The individual intensities of the Coriolis interacting ν9, ν3, and ν7 bands of CH2F2 molecule were separately obtained through computer simulation technique. A similar analysis was carried out for the ν8 and ν2 diad, and ν9, ν3, and ν7 triad of CD2F2. The intensities of ν1 and ν6 bands of both CH2F2 and CD2F2 molecules were also reexamined. The obtained results were utilized to determine the sign of the dipole moment derivatives with respect to the normal coordinates. An evidence of a failure of the ’’CH stretching criterion’’ was found in the case of the antisymmetric CH stretching vibration of this molecule.

Infrared intensities and Coriolis interactions in methylene fluoride

The individual intensities of the Coriolis interacting ν9, ν3, and ν7 bands of CH2F2 molecule were separately obtained through computer simulation technique. A similar analysis was carried out for the ν8 and ν2 diad, and ν9, ν3, and ν7 triad of CD2F2. The intensities of ν1 and ν6 bands of both CH2F2 and CD2F2 molecules were also reexamined. The obtained results were utilized to determine the sign of the dipole moment derivatives with respect to the normal coordinates. An evidence of a failure of the ’’CH stretching criterion’’ was found in the case of the antisymmetric CH stretching vibration of this molecule.

The electronic effect of lone pairs on the nuclear spin coupling constants

There has recently been considerable interest in the electronic effect of lone pairs on the nuclear spin coupling constants (1-6). The lone pair electrons on the nitrogen or oxygen atoms have been accepted to affect the geminal protonproton spin coupling constants in adjacent CH2 fragments (4, 7). The large positive coupling constants for geminal protons in formaldehyde and imines have been interpreted as the result of delocalization of an electron from adjacent lone pair orbital to the antisymmetric CH, orbital (7). Recently it has also been reported that there are striking effects of the nitrogen lone pairs orientation on the 14N-C-H and 15N-C-H coupling constants for H&=iS-R fragments in imines (I) and oximes (II) (4-6). Q ui e recently we reported that the t directly bonded 13C-H coupling constant also depends upon the orientation of the nitrogen lone pair electrons in oxime (II) and aziridine (III) (8).

Group vibrations and the vibrational analysis of molecules containing methylene groups. Part I. The basic equations and the application of the method to methylene dichloride and cyclopropane

The vibrations of the methylene group first considered by King have been reexamined; and the group factorization procedure previously applied to molecules containing methyl groups has now been extended to molecules containing methylene groups. The vibrational analysis of such molecules can be simplified by factoring from the secular determinant those frequencies that are characteristic of the symmetric and antisymmetric CH2 stretching, and the [Formula: see text] angle bending modes. Corrections are then applied to the G and F matrices to account for the interactions with the molecular framework of the CH2 'rocking', and [Formula: see text] angle deformation modes, where the atoms X may form part of a ring system.

Raman and infrared studies of the solvent dependence of the intensities of the CH 2, fundamental stretching modes of the methylene halides

Raman intensity measurements are reported which demonstrate that the CH 2 stretching fundamentals of the methylene halides do not show in the Raman effect the high solvent sensitivity reported for the intensities of the corresponding infrared bands. This difference is interpreted as evidence for the occurrence of charge transfer during hydrogen-bond formation. Additional infrared observations are reported and a model is proposed which explains the solvent sensitivity of the infrared bands and the remarkable difference between the anti-symmetric and symmetric CH 2, modes in terms of the simple bond-moment hypothesis extended to include one bond-bond interaction term, ∂μ CX ∂ r CH .