Antisymmetric CH Research Articles

Slit Discharge IR Spectroscopy of a Jet-Cooled Cyclopropyl Radical: Structure and Intramolecular Tunneling Dynamics

High-resolution infrared spectra of a jet-cooled cyclopropyl radical are reported for the first time, specifically sampling the in-phase antisymmetric CH2 stretch (nu7) vibration. In addition to yielding the first precise gas-phase structural information, the spectra reveal quantum level doubling into lower (+) and upper (-) states due to tunneling of the lone alpha-CH with respect to the CCC plane. The bands clearly reveal intensity alternation due to H atom nuclear spin statistics (6:10 and 10:6 for even:odd Ka+Kc in lower (+) and upper (-) tunneling levels, respectively) consistent with C2v symmetry of the cyclopropyl-tunneling transition state. The two ground-state-tunneling levels fit extremely well to a rigid asymmetric rotor Hamiltonian, but there is clear evidence for both local and global state mixing in the vibrationally excited nu7 tunneling levels. In particular, the upper (-) tunneling component of the nu7 state is split by anharmonic coupling with a nearly isoenergetic dark state, which thereby acquires oscillator strength via intensity sharing with this bright state. From thermal Boltzmann analysis of fractional populations, tunneling splittings for a cyclopropyl radical are estimated to be 3.2 +/- 0.3 cm(-1) and 4.9 +/- 0.3 cm(-1) in the ground and nu7-excited states, respectively. This analysis indicates ground-state stereoracemization of the alpha-CH radical center to be a very fast process [k approximately 2.0(4) x 10(11) s(-1)], with the increase in the tunneling rate upon CH2 in-phase asymmetric stretch excitation consistent with ab initio predictions of equilibrium vs transition-state zero-point energies. Modeling of the ground-state-tunneling splittings with high level ab initio 1D potentials indicates an improved V0 = 1115 +/- 35 cm(-1) barrier height for alpha-CH inversion through the cyclopropyl CCC plane.

IRRAS Studies on Chain Orientation in the Monolayers of Amino Acid Amphiphiles at the Air−Water Interface Depending on Metal Complex and Hydrogen Bond Formation with the Headgroups

Monolayers of N-octadecanoyl-L-alanine at the air-water interface on pure water and metal ion containing subphases have been studied using polarized infrared reflection-absorption spectroscopy (IRRAS). The metal complex and hydrogen bond formation with the headgroups give rise to a change in chain order depending on metal ion in the subphase. On pure water and Ag(+)-/Pb(2+)-containing subphase, the antisymmetric CH(2) stretching band intensity [nu(a)(CH(2))] undergoes a slower increase than the symmetric one [nu(s)(CH(2))] below the Brewster angle, so the intensity ratios of nu(a)(CH(2))/nu(s)(CH(2)) are less than 1 in the cases of Ag(+) and Pb(2+). Beyond the Brewster angle, the nu(a)(CH(2)) band intensities are substantially reduced in comparison with the nu(s)(CH(2)) ones in the cases of pure water and Ag(+), but the nu(a)(CH(2)) bands still remain negative-oriented in the presence of Pb(2+). These unusual spectral features indicate that the alkyl chains take a preferential orientation with their C-C-C planes parallel to the water surface. The parallel packing of the alkyl chains results from the intermolecular hydrogen bonds C=O...H-N between the neighboring amide groups, strengthened by the metal complex of covalent interaction. On the Ca(2+)-/Cu(2+)-containing subphase, the corresponding polarized spectra display a usual behavior. The alkyl chains are roughly estimated to be inclined around 35-40 degrees from the surface normal on the assumption of chain segment orientation for the monolayers in the liquid-expanded phase. The chain conformation and tilt are closely related to the formation of intramolecular hydrogen bonds and the ionic interaction of the metal complex in the cases of Ca(2+) and Cu(2+).

Melting behavior of poly(3-hydroxybutyrate) investigated by two-dimensional infrared correlation spectroscopy

The melting behavior of a bacterially synthesized biodegradable polymer, poly(3-hydroxybutyrate) (PHB), was investigated by using generalized two-dimensional infrared (2D IR) correlation spectroscopy. Temperature-dependent spectral variations in the regions of the CH stretching (3100–2850 cm −1), CO stretching (1800–1680 cm −1), and COC stretching (1320–1120 cm −1) bands were monitored during the melting process. The asynchronous 2D correlation spectrum for the CO stretching band region resolved two crystalline bands at 1731 and 1723 cm −1. The intense band at 1723 cm −1 may be due to the highly ordered crystalline part of PHB, and the weak band at 1731 cm −1 possibly arises from the crystalline part with a less ordered structure. These crystalline bands at 1731 and 1723 cm −1 share asynchronous cross peaks with a band at around 1740 cm −1 assignable to the CO band due to the amorphous component. This observation indicates that the decreases in the crystalline components do not proceed simultaneously with the increase in the amorphous component. In the 3020–2915 cm −1 region where bands due to the asymmetric CH 3 stretching and antisymmetric CH 2 stretching modes are expected to appear, eight bands are identified at 3007, 2995, 2985, 2975, 2967, 2938, 2934, and 2929 cm −1. The bands at 2985 and 2938 cm −1 are ascribed to the amorphous part while the rest come from crystal field splitting, which is a characteristic of polymers with a helical structure.

Intermolecular interaction in an open-shell pi-bound cationic complex: IR spectrum and coupled cluster calculations for C2H2+-Ar.

The intermolecular potential energy surface (PES) of Ar interacting with the acetylene cation in its (2)Pi(u) ground electronic state is characterized by infrared photodissociation (IRPD) spectroscopy and quantum chemical calculations. In agreement with the theoretical predictions, the rovibrational analysis of the IRPD spectrum of C(2)H(2) (+)-Ar recorded in the vicinity of the antisymmetric CH stretching fundamental (nu(3)) is consistent with a vibrationally averaged T-shaped structure and a ground-state center-of-mass separation of R(c.m.) = 2.86 +/- 0.09 A. The nu(3) band experiences a blueshift of 16.7 cm(-1) upon complexation, indicating that vibrational excitation slightly reduces the interaction strength. The two-dimensional intermolecular PES of C(2)H(2) (+)-Ar, obtained from coupled cluster calculations with a large basis set, features strong angular-radial coupling and supports in addition to a global pi-bound minimum also two shallow side wells with linear H-bound geometries. Bound state rovibrational energy level calculations are carried out for rotational angular momentum J = 0-10 (both parities) employing a discrete variable representation-distributed Gaussian basis method. Effective spectroscopic constants are determined for the vibrational ground state by fitting the calculated rotational energies to the standard Watson A-type Hamiltonian for a slightly asymmetric prolate top.

Infrared study of HDTMA + intercalated montmorillonite

In this paper, FTIR spectroscopy using attenuated total reflection (ATR) and KBr pressed disk techniques has been used to characterize sorbed water and HDTMA + in organo-clay. Sorbed water content decreases with the intercalation of HDTMA +. With the decrease of the sorbed water content, the position of the ν 2 mode shifts to higher frequency dramatically while the stretching vibration shifts to lower frequency slightly, indicating that H 2O is less strongly hydrogen bonded. This might be resulted from the polarization of H 2O molecules by the changeable cations and HDTMA +. FTIR spectra show that both antisymmetric and symmetric CH 2 stretching absorption bands shift to low frequencies with increase of amine concentration within the galleries of montmorillonite, elucidating the increase of ordered conformation. Furthermore, the present study demonstrates that the antisymmetric CH 2 stretching mode is more sensitive to the conformational ordering than the symmetric stretching mode. When KBr pressed disk technique used, two well resolved absorption bands at 730 and 720 cm −1, and at 1473 and 1463 cm −1, corresponding to the methylene scissoring and rocking modes, respectively, could be observed in FTIR spectra of organo-clays with relative higher concentration of surfactant. However, the FTIR spectra using ATR technique only display singlets and they are independent of amine concentration and chain conformation. Our present study demonstrates that FTIR spectroscopy using KBr pressed disk technique is more suitable to probe the conformational ordering of surfactant in organo-clays than that suing ATR technique does.

Interaction of Sodium Dodecyl Sulfate with Dimyristoyl-sn-glycero-3-phosphocholine Monolayers Studied by Infrared Reflection Absorption Spectroscopy. A New Method for the Determination of Surface Partition Coefficients

The interaction of the surfactant sodium dodecyl sulfate (SDS) with lipid monolayers of 1,2-dimyristoyl-d54-sn-glycero-3-phosphocholine (DMPC-d54) at the air/water interface was studied by infrared reflection absorption spectroscopy. To obtain separate information about the structural changes of both the lipid and the surfactant component during the interaction, isotopically labeled DMPC-d54 was used. With increasing surfactant concentration, the slight shift of the CD2 stretching vibration of DMPC-d54 toward lower wavenumber indicates a small increase of the lipid alkyl chain order due to the incorporation of the surfactant into the lipid monolayer. The DMPC-d54 surface density stays the same. The frequency of the antisymmetric and symmetric CH2 stretching modes of the surfactant alkyl chain also shifts toward lower wavenumber with increasing amount of surfactant in the monolayer due to increased chain ordering. For the first time, we applied the isotopic substitution method for the direct calculation of surface partition coefficients describing the amount of incorporated SDS in the DMPC-d54 monolayer. The partition coefficients were determined at different SDS concentrations and surface pressures and were compared to partition coefficients of the bilayer system, obtained under similar conditions by isothermal titration calorimetry. The best agreement between monolayer and bilayer data was obtained for a monolayer held at a pressure of 29.5−33.2 mN/m, which can be considered as the bilayer−monolayer equivalence pressure.

A Molecular Cluster Study on Activated CH/π Interactions: Infrared Spectroscopy of Aromatic Molecule−Acetylene Clusters

CH stretching vibrations of jet-cooled benzene−acetylene and several aromatics−acetylene clusters were observed in order to characterize the intermolecular interaction, so-called activated CH/π interaction between an acidic CH group and π-electrons. The infrared−ultraviolet double resonance spectroscopic techniques were used for measuring their vibrational spectra. The antisymmetric CH stretching vibration of the acetylene moiety exhibits a remarkable low-frequency shift upon the cluster formation with the aromatic molecules, indicating such a cluster structure that the acetylenic CH group is bound to π-electrons of the aromatic ring. The low-frequency shifts observed for the clusters show a positive correlation with the π-electron density of the ring, as expected for an ordinary π-hydrogen bond. On the other hand, the polarizability of the proton-accepting molecules showed a weakly negative correlation with the CH frequency shifts. These results show that the intermolecular interaction between the activa...

The interaction of n-nonyl-β-d-glucopyranoside and sodium dodecyl sulfate with DMPC and DMPG monolayers studied by infrared reflection absorption spectroscopy

The interaction of the surfactants n-nonyl-β-D-glucopyranoside and sodium dodecyl sulfate with lipid monolayers of 1,2-dimyristoyl-d54-sn-glycero-3-phosphocholine and 1,2-dimyristoyl-d54-sn-3-[phospho-rac(1-glycerol)] at the air–water interface was studied by infrared reflection absorption spectroscopy. Isotopically labeled lipids were used in order to obtain separate information about the quantity and structural changes of both, the lipid and the surfactant component during the interaction. Knowing the extinction coefficients of the pure components, the number of lipid and surfactant molecules in the mixed monolayer can be calculated directly from the absorbance of the antisymmetric CD2 and CH2 stretching vibrational bands, respectively. This allows the calculation of the partition coefficient for the surfactant incorporated into the lipid monolayer. The influence of electrostatic interactions on the partition coefficient was investigated by use of the negatively charged lipid 1,2-dimyristoyl-d54-sn-3-[phospho-rac(1-glycerol)] and the surfactant sodium dodecyl sulfate. Finally, the effect of a 0.1 M NaCl subphase on the partitioning is described.

Solubilization of DMPC-d54and DMPG-d54vesicles with octylglucoside and sodium dodecyl sulfate studied by FT-IR spectroscopy

The solubilization of deuterated dimyristoylphosphatidylcholine (DMPC-d54) and dimyristoylphosphatidylglycerol (DMPG-d54) vesicles by the surfactants sodium dodecyl sulfate (SDS) and octylglucoside (OG), respectively, was studied by Fourier transform infrared spectroscopy (FT-IR) at 30 °C. In order to obtain separate information about the structural changes of both, the lipid and the surfactant component during the solubilization process, isotopically labelled lipids (DMPC-d54 and DMPG-d54) were used. With increasing surfactant concentration, the shift of the CD2 stretching mode towards higher wavenumber indicates an increasing disorder of the lipid alkyl chains during the incorporation of the surfactants into the lipid bilayer. At the same time the frequency of the antisymmetric CH2 stretching mode of the surfactant alkyl chains shifts towards lower wavenumber with increasing amount of surfactant. This occurs due to the decreasing proportion of surfactant monomers until the saturation limit is reached. In the coexistence region the frequency of the CH2 vibration stays constant, which indicates no change in the monomer concentration of the surfactant. Only the proportion of micelles increases at the expense of the vesicles. Above the coexistence region no further frequency shift is observed except for the DMPC/SDS system, which show an additional frequency decrease of the CH2 stretching mode. This is due to the decreasing amount of SDS monomers according to the decrease of the CMC for charged systems. In DMPG/SDS mixtures, prepared in 0.1 mM NaCl, the surfactant charge is screened by the salt and no frequency shift is observed. The carbonyl vibrations show a pronounced frequency shift towards lower wavenumber exclusively for the DMPG/SDS system with high net charge. This indicates an increasing degree of hydration of the lipid head-group with increasing amount of surfactant due to the effort to separate the negative charge of the lipid head-group from the negatively charged surfactant head-group.

Interaction of a synthetic peptide corresponding to the N terminus of canine distemper virus fusion protein with phospholipid vesicles: a biophysical study

The F protein of canine distemper virus (CDV) is a classic type I glycoprotein formed by two polypeptides, F1 and F2. The N-terminal regions of the F1 polypeptides of CDV, measles virus and other paramyxoviruses present moderate to high homology, supporting the existence of a high conservation within these structures, which emphasises its role in viral-host cell membrane fusion. This N-terminal polypeptide is usually termed the fusion peptide. The fusion peptides of most viral fusion-mediating glycoproteins contain a high proportion of hydrophobic amino acids, which facilitates its insertion into target membranes during fusion. In this work we report on the interaction of a 31-residue synthetic peptide (FP31) corresponding to the N terminus of CDV F1 protein with phospholipid membranes composed of various phospholipids, as studied by means of various biophysical techniques. FTIR investigation of FP31 secondary structure in aqueous medium and in membranes resulted in a major proportion of α-helical structure which increased upon membrane insertion. Differential scanning calorimetry (DSC) showed that the presence of concentrations of FP31 as low as 0.1 mol%, in mixtures with l-α-dimyristoylphosphatidylcholine (DMPC), l-α-dipalmitoylphosphatidylcholine (DPPC) and l-α-distearoylphosphatidylcholine (DSPC), already affected the thermotropic properties of the gel to liquid-crystalline phase transition. In mixtures with the three lipids, increasing the concentration of peptide made the pretransition to disappear, and lowered and broadened the main transition. This effect was slightly stronger as the acyl chain length of the phospholipid grew larger. In the corresponding partial phase diagrams, no immiscibilities or critical points were observed. FTIR showed that incorporation of 1 mol% of peptide in DPPC shifted the antisymmetric and symmetric CH 2 stretching bands to higher values, indicating the existence of an additional disordering of the acyl chain region of the fluid bilayer. FTIR studies of the CO stretching band indicated that incorporation of FP31 into phosphatidylcholine membranes produced a strong dehydration of the polar part of the bilayer. In mixtures with l-α-dielaidoylphosphatidylethanolamine (DEPE), increasing FP31 concentrations broadened and shifted to lower temperatures the lamellar to hexagonal-H II phase transition, indicating that this peptide destabilized the bilayer and promoted formation of the hexagonal-H II phase. The results are discussed in terms of lipid–peptide hydrophobic mismatch and its influence on the role of the N-terminal polypeptide of CDV F1 protein in viral membrane fusion.

Ethyne Adsorbed on CuNaY Zeolite: FTIR Spectra and Quantum Chemical Calculations

The interaction of ethyne with vacuum-dehydrated CuNaY zeolite has been investigated by means of Fourier transform infrared spectroscopy and cluster model density functional theory calculations. The spectra show three bands: two which are symmetry-forbidden for the free molecule but become allowed in the adsorbed state (the ν1 CC stretching mode at ∼1832 and 1814−1812 cm-1, and the symmetric ν2 CH stretching mode at 3271 and 3251 cm-1), and the allowed antisymmetric ν3 CH stretching mode at 3202 and 3174 cm-1. They all appear in pairs. The two components of the bands, which are all strongly shifted to lower wavenumbers relative to the free molecule, are attributed to two inequivalent Cu+ adsorption sites. On increasing C2H2 pressure, the more strongly shifted components appear first and acquire significantly higher intensity, indicating that they must correspond to the more strongly bound site. Analysis of the geometry of the complexes representing ethyne adsorbed on Cu at both sites yields a four-fold coordination of copper, including two oxygen atoms and side-on bound C2H2, with CC bonds elongated by 0.03 Å and CCH angles reduced by at least 14.7°. The calculated binding energies of the two ethyne−copper complexes are significantly different due to the energy necessary to reach the geometry within Cu(SII)−C2H2 from the relaxed Cu(SII) cluster. C2H2 is more strongly adsorbed on Cu(SIII) (in the saddle between two four-membered rings near the wall of a supercage) than on site II (above a six-ring window of a sodalite cage). Population analysis provides a net charge transfer of about 0.14e toward the ethyne. An energy decomposition method demonstrates the paramount importance of electrostatic and charge-transfer contributions to the Cu−zeolite−ethyne interaction energy. The harmonic frequency calculation suggests that the more strongly shifted band components of all stretching vibrations have to be assigned to C2H2 adsorbed on Cu+ at SIII, which is the more stable site. Apart from the overestimated Δν1 values, the vibrational frequency shifts calculated for both adsorption complexes are in excellent agreement with the experimental ones.

Raman-spectral depolarisation ratios of ions in concentrated aqueous solution. The next-to-negligible effect of highly asymmetric ion surroundings on the symmetry properties of polarisability changes during vibrations of symmetric ions.: Ammonium sulphate and tetramethylammonium bromide

Depolarisation ratios ρ have been measured for the Raman spectra of solutions of composition (NH 4) 2 SO 4 · 11H 2O and (CH 3) 4NBr · 29D 2O. Even though the former's vibration spectrum shows clear evidence of lowered ion symmetries (presence of ν 1 of SO 4 2− in the IR spectrum, IR versus R ν max shifts for ν 3 and ν 4 of SO 4 2− and ν 4 of NH 4 +) ν 1 of SO 4 2− has (apparent) ρ of only 0.014, while ν 2, ν 3 and ν 4 of SO 4 2− and ν 4 (probably also ν 2) of NH 4 + have ρ in the range 0.73–0.77; within the experimental error and base line uncertainty the latter are equal to 0.75, i.e. to ρ max with the geometry of the optics used. For (CH 3) 4NBr symmetric N +C stretching has ρ 0.012; all-in-phase CH stretching and four overtones in Fermi resonance with it have ρ in the range 0.02–0.035, but the deviation from zero here is in part due to underlying or overlapping depolarised bands. The sufficiently well isolated antisymmetric CH stretching and degenerate CH bending bands again have ρ in the range 0.74–0.76. These results show that the selection rules in respect of ρ, which apply strictly only to isolated molecules, are for practical purposes still valid for molecules in strongly symmetry-distorting external environments in the liquid phase. More specifically: (A) During vibrations in which quasi-spherical intramolecular symmetry is retained, the externally caused aspherical component of the polarizability ellipsoid does not change aspherically to a sufficient extent for an appreciably intense anisotropic Raman band to appear. (B) During intramolecularly anti-symmetric vibrations of symmetric molecules, the portion of the externally caused distortion of the polarizability ellipsoid that fails to cancel over a whole vibration period is not large enough to give rise to an appreciably intense isotropic component of the Raman band. This means in practice ρ for these Raman bands is still ρ max, even for concentrated aqueous solutions.

Picosecond vibrational energy transfer observed in the CH and OH stretching region of stearic acid dimers in liquid solution

Picosecond infrared double resonance spectroscopy was performed on stearic acid dimers in liquid solution in CCl4. The results give evidence for a redistribution of the vibrational energy among all CH stretching modes with individual time constants of a few picoseconds. In particular a redistribution time of 4±2 ps is obtained for the equilibration between the antisymmetric and symmetric CH2 stretching modes, and an effective lifetime of 14±2 ps for the whole CH stretching ensemble is derived. Furthermore the data indicate that the energy is efficiently transferred along the hydrocarbon chain, and that the CH3 endgroup has a key role in the energy transfer to the solvent; even the vibrational (OH stretching) modes in the doubly hydrogen-bonded dimer ring seem to dissipate their energy by transferring it to the CHx chains rather than directly to the solvent.

Laser-induced dispersed vibration–rotation fluorescence of acetylene: Spectra of ortho and para forms and partial trapping of vibrational energy

The laser-induced dispersed vibration–rotation fluorescence method has been developed further when compared with a previous publication [Saarinen et al., J. Chem. Phys. 110, 1424 (1999)]. More than one order of magnitude better signal-to-noise ratio has been achieved in the wave-number region 2900–3500 cm−1 by taking advantage of directionality of the fluorescence signal. The improvement has been applied to overtone spectroscopy of normal acetylene where for high CH stretching excitations separate spectra of ortho and para forms are obtained containing basically just single CH stretching vibrational quantum transitions from the pumped antisymmetric vibrational (ν1+3ν3(Σu+) and ν2+3ν3(Σu+)) and close-lying symmetric vibrational local mode (4ν3(Σg+) and ν1+ν2+2ν3(Σg+)) states. No nuclear spin conversion is observed in these spectra. Two new symmetric vibrational states (ν1+2ν2+4ν40(Σg+)(29%) and (50%)) have been observed and the precision of the spectroscopic parameters of previously published symmetric states has been improved by an order of magnitude. Unexpected fluorescence originating from the antisymmetric CH stretching fundamental state ν3 and some associated states of acetylene have also been observed. These spectra are characterized by both ortho and para forms in normal abundance and by unusual intensity patterns due to strong reabsorption of the fluorescence by ground state acetylene molecules in the sample cell. A simple collisional step-down mechanism is proposed to account for the appearance of the ν3 fluorescence band system. The excess vibrational energy in the sample volume is partly trapped in the form of ν3 mode energy and it decays from the system by radiation.

Raman spectroscopy of dimethyl sulphoxide and deuterated dimethyl sulphoxide at 298 and 77 K

Raman spectroscopy was used to determine the molecular behaviour of DMSO and DMSO-d6, and to compare it with that of DMSO in DMSO-intercalated kaolinites. For DMSO at 298 K two bands are observed at 2994 and 2913 cm−1 and are assigned to the antisymmetric and symmetric CH stretching modes. At 77 K the degeneracy of these bands is lost. Bands are now observed as antisymmetric bands at 3001, 2995 and 2988 cm−1 and symmetric bands at 2923, 2909 and 2885 cm−1, respectively. For the DMSO-intercalated low-defect kaolinite, the 2913 cm−1 band resolves into five component bands at 2882, 2907, 2917, 2920 and 2937 cm−1. The CD antisymmetric and symmetric stretching modes in the 298 K spectrum are found at 2250 and 2125 cm−1, respectively. Both bands show some asymmetry and further bands may be resolved at 2256 and 2244 cm−1 in the antisymmetric stretching region and at 2118 cm−1 in the symmetric stretching region. The spectra of the SO stretching region of DMSO and DMSO-d6 are complex with a band profile centred at 1050 cm−1. Three bands are curve resolved at 1058, 1042 and 1026 cm−1 attributed to the unassociated monomer and the out-of-phase and the in-phase vibrations of the dimer, respectively. Upon cooling to liquid nitrogen temperature, these three bands are observed at 1057, 1038 and 1019 cm−1. The spectra of the SO stretching region of DMSO-d6 are more complex because of the overlap of the DCD deformation modes with the SO stretching modes. The antisymmetric and symmetric stretching CS modes of DMSO are observed at 698 and 667 cm−1, shifting at 77 K to 705 and 672 cm−1. It is concluded that the structure of DMSO in DMSO-intercalated kaolinite is different from those of both liquid DMSO at 298 K and solid DMSO measured at 77 K. Copyright © 2002 John Wiley & Sons, Ltd.

Raman spectroscopy of dimethyl sulphoxide and deuterated dimethyl sulphoxide at 298 and 77 K

AbstractRaman spectroscopy was used to determine the molecular behaviour of DMSO and DMSO‐d6, and to compare it with that of DMSO in DMSO‐intercalated kaolinites. For DMSO at 298 K two bands are observed at 2994 and 2913 cm−1 and are assigned to the antisymmetric and symmetric CH stretching modes. At 77 K the degeneracy of these bands is lost. Bands are now observed as antisymmetric bands at 3001, 2995 and 2988 cm−1 and symmetric bands at 2923, 2909 and 2885 cm−1, respectively. For the DMSO‐intercalated low‐defect kaolinite, the 2913 cm−1 band resolves into five component bands at 2882, 2907, 2917, 2920 and 2937 cm−1. The CD antisymmetric and symmetric stretching modes in the 298 K spectrum are found at 2250 and 2125 cm−1, respectively. Both bands show some asymmetry and further bands may be resolved at 2256 and 2244 cm−1 in the antisymmetric stretching region and at 2118 cm−1 in the symmetric stretching region. The spectra of the SO stretching region of DMSO and DMSO‐d6 are complex with a band profile centred at 1050 cm−1. Three bands are curve resolved at 1058, 1042 and 1026 cm−1 attributed to the unassociated monomer and the out‐of‐phase and the in‐phase vibrations of the dimer, respectively. Upon cooling to liquid nitrogen temperature, these three bands are observed at 1057, 1038 and 1019 cm−1. The spectra of the SO stretching region of DMSO‐d6 are more complex because of the overlap of the DCD deformation modes with the SO stretching modes. The antisymmetric and symmetric stretching CS modes of DMSO are observed at 698 and 667 cm−1, shifting at 77 K to 705 and 672 cm−1. It is concluded that the structure of DMSO in DMSO‐intercalated kaolinite is different from those of both liquid DMSO at 298 K and solid DMSO measured at 77 K. Copyright © 2002 John Wiley & Sons, Ltd.

Variable Temperature FT-IR Studies of n-Alkyl Modified Silica Gels

Silica gels, chemically modified with n-alkyl chains of various lengths (CnH2n+1, n = 8, 9, 10, 12, 16, 18, 22, and 30), in the dry state were examined over a broad temperature range (123−353 K) via infrared spectroscopy. Here, for the first time a comprehensive variable temperature FT-IR study is available dealing with the conformational order of alkyl modified silica gels. The conformational state of the alkyl chains in their bound state was monitored via various conformation-sensitive vibrational bands. A qualitative statement about the conformational order was possible based on the analysis of the symmetric and antisymmetric CH2 stretching bands. Likewise, CH2 wagging bands were used to identify and determine the relative amountsi.e., integral values over the whole chainsof gauche-trans-gauche, double gauche, and end gauche conformers. The amount of gauche conformers at a specific site in the alkyl chains was accessible via the analysis of CD2 rocking bands of samples bearing selectively deuterated alkyl chains. These latter data are consistent with those obtained from a complementary analysis of the CD2 stretching vibrational bands. The present study lucidly demonstrates that the conformational state of the chemically attached alkyl chains strongly depends on temperature, actual chain length, and chain position. The total number of gauche conformers per chain of the systems examined here varies between 2.4 and 5.1, depending on the actual sample temperature and alkyl chain length. The total number of gauche conformers is found to vary linearly with temperature. Shorter alkyl chains, such as octyl and decyl chains, are found to possess a high degree of conformational disorder. The observed increase in alkyl chain length is accompanied by a decrease in conformational disorder, most probably due to a better chain packing. For the C18 modified system, selectively deuterated samples were available. Here, at room temperature the percentage of gauche conformers for positions C-4, C-6 and C-12 are 40%, 20%, and 25%, respectively, reflecting a pronounced position dependence for the degree of conformational order. The present data on the conformational alkyl chain order are discussed in conjunction with studies on other n-alkyl modified silica gels as well as related systems involving n-alkyl segments in various environments, such as pure hydrocarbons, biological membranes, and self-assembled monolayers (SAMs). On the bases of the results from the present variable temperature FT IR study, overall structures of the alkyl chains, chemically attached to the silica surface, are proposed.

Vibrational sum-frequency observation of synthetic diamonds

Sum-frequency generation (SFG) spectroscopy was applied to the study of plasma hydrogenated diamond C(100) surface of a synthetic diamond. Two vibrational resonance peaks with the intensities strongly dependent on the polarization combinations of the visible and infrared beams were observed at 2899 and 2924 cm −1. From the results of SFG measurements for various polarization combinations, the 2899 and 2924 cm −1 peaks were assigned, respectively, to the antisymmetric and symmetric CH stretching modes of HCCH group on H/C(100)-2×1 surface. In addition, a strong background signal with an oscillatory profile was observed.

The correlation between surface conductivity and adsorbate coverage on diamond as studied by infrared spectroscopy

A high-resolution analysis of CH vibrational modes on a single crystal diamond(100) surface using Fourier-transform infrared (FTIR) spectroscopy in combination with conductivity measurements is reported. On a plasma-hydrogenated diamond(100) surface, the IR spectra measured in the multiple internal reflection mode reveal three absorption lines. Two of them at 2921 and 2854 cm −1 vanish in air at an annealing temperature of 190°C and are assigned to the antisymmetric and symmetric CH 2 stretching modes of a physisorbed hydrocarbon species, respectively. The third band at 2897 cm −1 has a width of 16 cm −1, is stable up to 230°C and is associated with the stretching frequency of C 2H 2 monohydride units on the C(100) 2×1:2H surface. Upon annealing in air at temperatures lower than 200°C, the surface conductivity is reversibly reduced by up to five orders of magnitude. After cooling down to room temperature, it recovers the value of 1×10 −5 Ω −1 measured immediately after the plasma hydrogenation with a time constant of several days. Annealing at 230°C destroys the surface conductivity irreversibly and yields conductance values below the measurement limit of 5×10 −12 Ω −1. We show that the chemisorbed hydrogen in the C 2H 2 configuration, together with at least one physisorbed species, is responsible for the surface conductivity of hydrogen-terminated diamond(100).

A Fourier-transform infrared study of the interaction between germ-cell specific sulfogalactosylglycerolipid and dimyristoylglycerophosphocholine

In this study, we investigated structural and dynamic changes of sulfogalactosylglycerolipid (SGG) and dimyristoylglycerophosphocholine (DMPC) in a liposomal system (SGG+DMPC, molar ratio 2:3) by Fourier-transform infrared (FTIR) spectroscopy. Cooling of the preheated SGG liposomes (5–65°C) revealed that the liquid crystalline-to-gel phase transition was centered at 45°C. SGG+DMPC liposomes showed a single phase transition at 28°C. Spectral changes of the ester CO groups of SGG and DMPC in the mixed liposomes indicated a decrease in their interfacial hydrogen bonding intermolecularly and with water. Analysis of SGG’s symmetric and antisymmetric CH 2 stretching bands revealed that the insertion of DMPC into SGG bilayers increased the number of gauche conformers in SGG’s hydrocarbon chains. Overall, the SGG+DMPC liposomes were homogeneous, with reduced interfacial hydrogen bonding and increased orientational and conformational disorder of SGG’s hydrocarbon chains.