CH2 Rocking Modes Research Articles

Polarized Infrared Absorption Spectrum of Matrix-Isolated Methylperoxyl Radicals, CH3OO X̃ 2A‘ ‘

We have used a tandem pair of supersonic nozzles to produce clean samples of CH3OO radicals in cryogenic matrices. One hyperthermal nozzle decomposes azomethane (CH3NNCH3) to generate intense pulses of CH3 radicals, while the second nozzle alternately fires a burst of O2/Ar at the 20 K matrix. The CH3/O2/20 K argon radical sandwich acts to produce target methylperoxyl radicals: CH3 + O2 → CH3OO. The absorption spectra of the radicals are monitored with a Fourier transform infrared spectrometer. We report 10 of the 12 fundamental infrared bands of the methylperoxyl radical CH3OO, X̃ 2A‘ ‘, in an argon matrix at 20 K. The experimental frequencies (cm-1) and polarizations follow: the a‘ modes are 3032, 2957, 1448, 1410, 1180, 1109, 902, 492, while the a‘‘ modes are 3024 and 1434. We cannot detect the asymmetric CH3 rocking mode, ν11, nor the torsion, ν12. The infrared spectra of CH318O18O, 13CH3OO, and CD3OO have been measured as well in order to determine the isotopic shifts. The experimental frequencies, {ν}, for the methylperoxyl radicals are compared to harmonic frequencies, {ω}, resulting from a UB3LYP/6-311G(d,p) electronic structure calculation. Linear dichroism spectra were measured with photooriented radical samples in order to establish the experimental polarizations of most vibrational bands. The methylperoxyl radical matrix frequencies listed above are within ±2% of the gas-phase vibrational frequencies. A final set of vibrational frequencies for the CH3OO radical are recommended. See also http://ellison.colorado.edu/methylperoxyl.

The ground state of silylidene (H2C=Si), the silicon analog of vinylidene, from stimulated emission pumping and wavelength-resolved fluorescence spectroscopy

The ground state vibrational energy levels of jet-cooled H2CSi and D2CSi have been studied by a combination of wavelength-resolved fluorescence and stimulated emission pumping (SEP) techniques. By taking advantage of the vibrational selection rules and Franck–Condon factors and selectively pumping upper state single vibronic levels, readily assignable low-resolution emission spectra were obtained. Higher resolution SEP spectra were recorded to give precise measurements of the vibrational band origins of many of the lower-lying vibrational levels. All of the vibrational frequencies, except for the Franck–Condon inactive CH asymmetric stretching mode, ν5, have been determined for both isotopomers. The CH2 rocking mode (ν6) is found to have a very low 263 cm−1 frequency in the ground state. The spectra are complicated by unexpected activity in the out-of-plane bending vibration.

Matrix Isolation and Density Functional Study of the Reaction of OVCl3 with CH3SH

The matrix isolation technique has been combined with infrared spectroscopy to isolate, identify, and characterize the initial intermediates in the thermal and photochemical reactions of OVCl3 with CH3SH. The thermal reaction proceeds through formation of a 1:1 complex that is characterized by perturbations to the −VCl3, −VO, and S−H stretching modes, as well as the −CH3 rocking and bending modes. The photochemical reaction leads to HCl elimination from the initial 1:1 complex, and formation of Cl2V(O)SCH3. Additional support for the species identification and vibrational band assignments comes from isotopic substitution. Density functional calculations (B3LYP/6-311G+(d, 2p)) were carried out to confirm the experimental band assignments for this latter species. Efforts to study the thermal reaction beyond formation of the initial complex were unsuccessful, despite pyrolysis at temperatures as high as 300 °C prior to sample deposition.

Polarized Resonance Raman and FTIR Reflectance Spectroscopic Investigation of the Molecular Orientation in Industrial Poly(vinyl chloride) Specimens

The molecular orientation of uniaxially drawn commercial poly(vinyl chloride), PVC, specimens has been investigated by polarized Fourier transform infrared (FTIR) in the specular reflectance mode and by resonance Raman scattering in various combinations of sample orientation, scattering configuration and polarization geometry. The orientation was examined as a function of the specimen draw ratio. The Kramers−Kronig transformation has been used to obtain infrared absorption spectra from the FTIR reflectance data; the orientation was determined by the dichroic ratio of the C−Cl stretching and CH2 rocking modes of PVC. The resonance Raman spectra of polyene segments that result from partial thermal degradation/dehydrochlorination of PVC during processing have been utilized in order to determine the parameters of the orientation distribution function. The all-trans conjugated polyene sequences formed during degradation are considered as rodlike segments in the polymer backbone and are used as indicators of orientation within the amorphous-like polymer phase. As far as the trends are concerned, data analysis showed good agreement between FTIR and Raman results regarding the induced molecular orientation in industrial PVC specimens.

Ab Initio Study of Peroxyacetic Nitric Anhydride and Peroxyacetyl Radical: Characteristic Infrared Band of Peroxyacetyl Radical

The equilibrium geometries, the bond dissociation energies, harmonic vibrational frequencies, and infrared intensities of peroxyacetic nitric anhydride (PAN), peroxyacetyl radical (PA), and NO2 which are important species in environmental chemistry, have been studied using high level of ab initio calculations. The global minimum energy conformation of the PAN molecule is found to be s-cis-PAN. The predicted enthalpy and entropy changes at 1 atm and 298.15 K for the thermal decomposition of PAN are 25.8 kcal/mol and 42.3 cal/(mol·K), respectively. These values are in excellent agreement with the experimental values. A characteristic IR band of PAN appears at 1232 cm-1, which is a CH3 rocking mode coupled with CO and OO stretches. The corresponding band of the PA radical appears at 1216 cm-1, about 10 cm-1 lower than that of PAN. Thus, determining the concentrations of PAN based on only this band would not be so reliable. We find that the PA radical has a strong IR band at 1150 cm-1 due to the O−O stretch mode, while the PAN molecule has the corresponding band at 977 cm-1. Since PAN and the PA radical do not have other IR bands at this region, this characteristic band could be used for identification and measurement of the PA radical and thus for spectroscopic study of thermal decomposition mechanism of PAN.

On the Assignment of Optically Pumped Far-Infrared Laser Emission from CH3OH

Progress in the analysis of the infrared spectrum of CH3OH in the 930–1450 cm−1 region has led to assignments, confirmations, or new insights for a number of far-infrared laser (FIRL) transition systems optically pumped by CO2 lasers. Many of the systems involve FIRL transitions among the CO-stretching, CH3-rocking, OH-bending, and CH3-deformation vibrational modes, giving useful information on the torsion–rotation structure of the methanol vibrational energy manifold. Some anomalies and mysteries concerning the identity of the lasing levels have been resolved, but several new ones have arisen. Altogether, 45 CH3OH IR-pump/FIR-laser systems are examined in light of the new spectroscopic information; about half of the system assignments are new and half have been previously reported in the literature and are here confirmed, extended, or revised.

The electronic spectrum, molecular structure, and oscillatory fluorescence decay of jet-cooled germylidene (H2C=74Ge), the simplest unsaturated germylene

The electronic spectrum of germylidene (H2C=Ge), the simplest unsaturated germylene, has been observed for the first time. Jet-cooled H2CGe and D2CGe were produced by an electric discharge through tetramethylgermane diluted in argon at the exit of a supersonic expansion. High-resolution spectra of H2C74Ge and D2C74Ge, obtained from (CH3)474Ge prepared from isotopically enriched Ge74 metal, have been rotationally analyzed to yield the following r0 structures: r0″(CGe)=1.7908(2) Å, r0″(CH)=1.1022(5) Å, θ0″(HCH)=115.05(5)°, r0′(CGe)=1.914(4) Å, r0′(CH)=1.082(9) Å, and θ0′(HCH)=139.3(11)°. The 367–354 nm B̃1B2–X̃ 1A1 band system consists of prominent perpendicular bands involving the CGe stretching (ν3) and CH2 scissors (ν2) vibrations and a weaker series of vibronically induced parallel bands involving the CH2 rocking mode (ν6). Vibronic bands involving Δv=2 changes in ν6(b2) and ν4(b1) have also been assigned. The fluorescence decays of single rotational levels of the 000 band of H2C74Ge exhibit molecular quantum beats for about 70% of the levels surveyed. Density of states arguments reveal that most of the beats originate from interactions with high rovibronic levels of the ground state. In one case, hyperfine splittings in the Fourier transform of the beat pattern indicate an accidental coincidence with an excited triplet state level. The less frequent occurrence of quantum beats in germylidene compared to silylidene, where they are almost universal, can be attributed to the smaller density of ground state levels at the zero-point energy of the S2 state in the former.

Crystalline and Amorphous Phases in the Poly(ethylene oxide)−LiCF3SO3 System

A detailed vibrational spectroscopic study of the ionic species present in the (PEO)xLiTf system (x = 3−100) indicates that “free” ions and ion pairs are the dominant species present in the dilute salt region at room temperature. When the salt concentration is increased to an O:M ratio of about 40:1, a band due to an aggregate species first appears and is assigned to the triflate ion vibrating in the (PEO)3LiCF3SO3 compound. The amorphous phase of (PEO)3LiCF3SO3 was studied by examining the temperature dependence of the spectra as the compound was heated through the melting transition at about 176 °C. Spectral behavior of the CH2 rocking modes and the COC stretching mode indicated that disordering above the melting temperature resulted in a broad distribution of −O−C−C−O− torsional angles. However the basic lithium-polyether backbone interaction was not been significantly changed for at least 15 °C above the transition temperature. Additional effects of thermal disordering were noted in the CF3 symmetric ...

Fourier Transform Infrared Spectroscopy and Vibrational Coupling in the OH-Bending Band of13CH3OH

We present in this work a high-resolution Fourier transform infrared study of the OH-bending vibrational band of13CH3OH. We have investigated the 1070–1400 cm−1spectral region at 0.002 cm−1resolution using the modified Bomem DA3.002 Fourier transform spectrometer at the Steacie Institute for Molecular Sciences at the National Research Council of Canada in Ottawa. This study has led to (i) determination of excited-state J(J + 1) subband expansion coefficients and (ii) characterization of a variety of interactions coupling the different vibrational modes, notably a strong Fermi resonance between the OH bend and the torsionally excited CH3-rocking mode. The OH-bending band is widely spread withQsubbranches grouped in two peaks at about 1312 and 1338 cm−1. The lower levels for all assigned subbands were confirmed using closed loops of IR and FIR transitions. The subbands have been fitted toJ(J+ 1) power-series expansions in order to obtain the subband origins and the state-specific energy expansion coefficients for both the OH-bending and excited torsional CH3-rocking states. The strong interaction between the OH-bending state and the first excited torsional CH3-rocking state gives rise to several “extra” forbidden subbands due to intensity borrowing. The asymmetry splitting of the (nτK)v= (122)OHA OH-bending doublet was found to be anomalously small, and the splitting of the (122)rA CH3-rocking doublet is observed to be enhanced. We have identified a network of intermode interactions causing this unusual behavior, but a quantitative analysis of the vibrational coupling is restricted by limited knowledge of the unperturbed positions of the interacting levels. All these interactions provide relaxation channels for intramolecular vibrational redistribution among the lower vibrational modes in13CH3OH. Another important finding is that the torsion–K–rotation energy curves in the OH-bending state display an inverted pattern compared to the ground state.

Pressure and Temperature Effects on Conformational and Hydrational Properties of Lamellar and Bicontinuous Cubic Phases of the Fully Hydrated Monoacylglyceride MonoelaidinA Fourier Transform-Infrared Spectroscopy Study Using the Diamond Anvil Technique

We report on the effects of pressure and temperature on the phase behavior and structural properties of aqueous dispersions of monoelaidin (ME) using Fourier transform−infrared spectroscopy (FT-IR) in combination with the diamond anvil technique. The IR spectral parameters, such as frequencies, intensities, band shapes, and band splittings, were used to detect structural and dynamic changes upon change of pressure and temperature. Analysis of these spectral parameters yields information on conformer population, reorientational fluctuations, interchain interaction, hydrogen bonding, and phase transformations. The monoacyglyceride ME was chosen for investigation because it exhibits various mesomorphic liquid−crystalline phases of different dimensionality, including lamellar, a cubic-primitive (QIID), and a body-centered cubic phase (QIIP). The latter two are inverse bicontinuous cubic phases. We have established the excess water p,T-phase diagram of ME over the 1−23-kbar range at ∼15−95 °C. In the high-pressure region, enhanced interchain interaction leads to a correlation field splitting of the CH2 bending and rocking modes, which is expected when entering the lamellar crystalline Lc phase that has the smallest partial molar volume. As revealed by the phase diagram, the energetic degeneracy of the cubic phases is broken. With increasing temperature or decreasing pressure, the cubic phases QIIP and QIID are formed. Interpretation of the CH2 stretching and wagging modes for evaluation of conformational states in the fluidlike disordered (Lα, QIIP, and QIID) phases reveals different populations of gauche conformers and kinks in these fluidlike phases. From the analysis of the carbonyl stretching mode vibrations we have been also able to detect small but marked differences in the level of hydration of different bicontinuous cubic phases. Compared with the QIID phase of ME, the lipid chains of the body-centered cubic phase QIIP seem to contain a slightly higher population of gauche sequences and a slightly lower level of hydration of the carbonyl group. The results are compared with recent energetic models for intercubic phase transitions.

Fourier transform spectrum of the in-plane CH3-rocking fundamental and vibrational coupling in C-13 methanol

The Fourier transform infrared spectrum of the in-plane CH3-rocking fundamental of CH313OH has been investigated at 0.002 cm−1 resolution. The rocking band is principally of parallel character and has a double-peaked Q branch and relatively wide spread subbands indicative of a substantial change in torsional barrier height. All A subbands from K=0 to 11 and all but one E subband from K=0 to 9 have been assigned in the n=0 torsional state and fitted to J(J+1) power-series expansions to obtain the subband origins and excited-state energy structure. The effects of vibrational interactions between the CH3-rocking and CO-stretching modes are prominent in the spectrum. Coriolis coupling between rocking (K−1) and CO-stretching K levels is observable for K⩾6, and makes significant contributions to the subband origins and effective B values. Several J-localized perturbations due to level-crossing resonances with CO-stretch states have been observed and characterized. Two reported strong far-infrared laser lines optically pumped by the 10R(26) CO2 laser line have been found to arise through such a ΔK=3 level-crossing resonance. Modeling of the rocking-state torsion-K-rotation energies yields a height of V3r=469.2(38) cm−1 for the torsional potential barrier, a 26% increase over the ground state. The asymmetry K-doubling pattern in the excited state is qualitatively consistent with this barrier for K=2 to 4, but the K=5 rocking substate displays strongly enhanced splitting.

Hydration-Induced Gel States of the Dienic Lipid 1,2-Bis(2,4-octadecadienoyl)-sn-glycero-3-phosphorylcholine and Their Characterization Using Infrared Spectroscopy

The lyotropic phase behavior of the dienic lipid 1,2-bis(2,4-octadecadienoyl)-sn-glycero-3-phosphorylcholine (DODPC) has been investigated by means of IR spectroscopy at 25 °C. Gradual hydration has been realized exposing the lipid to an atmosphere of variable relative humidity (RH). Upon scans of decreasing RH, the liquid crystalline lipid undergoes the chain-freezing transition to a metastable gel state. By storage of the sample at low RH, the gel transforms to a crystalline subgel designated as SGI. Subsequent hydration induces the conversion to a second subgel (SGII). The subgel phases are characterized by the dense packing of the acyl chains as indicated by the correlation field splitting of the CH2 rocking and bending modes. Band shifts of phosphate group vibrations as well as the splitting of the carbonyl stretching mode are correlated with the hydration of the polar region of the bilayer given in terms of the molar ratio of water to lipid. The ν1,3(OH) absorption band of water yields qualitative i...

Lipid Domains and Orthorhombic Phases in Model Stratum Corneum: Evidence from Fourier Transform Infrared Spectroscopy Studies

A three component model for the lipid barrier of the stratum corneum (SC) consisting of ceramide III, cholesterol, and perdeuterated palmitic acid, has been characterized by Fourier transform infrared spectroscopy. At physiological temperature the CD2scissoring mode of the palmitic acid methylenes, and the CH2rocking mode of the ceramide methylenes, are each split into two components. This indicates that both components exist in separate, conformationally ordered phases, probably with orthorhombic perpendicular subcells. The magnitude of the splitting indicates that the domains are at least 100 chains in size. The thermotropic behavior of the CD2stretching vibrations demonstrates that conformational disordering of the palmitic acid commences at 42 °C with a transition midpoint of 50 °C. The CH2stretching frequency indicates the ceramide chains remain ordered until 50 °C then disorder with a midpoint of 67 °C. The results provide a molecular characterization for the complex low temperature (10-40 °C) dynamic behavior suggested by recent2H NMR experiments.

Microwave spectrum of silylidene H2CSi

Rotational spectral lines of the H2CSi molecule are observed in the millimeter-wave and submillimeter-wave regions. The molecule is produced in a glow-discharge plasma of a gaseous mixture of SiH4 and CO. Rotational constants and centrifugal distortion constants have precisely been determined from observed frequencies of 32 a-type transitions with J=5–4 to J=12–11 and Ka=0 to Ka=4. From the observed inertial defect, the vibrational frequency of the CH2 rocking mode is estimated to be as low as 331 cm−1, which is consistent with the large quartic centrifugal distortion constants, DJK and d2. Higher-order centrifugal distortion constants up to the octic terms are necessary to obtain a good fit between the observed and calculated frequencies within experimental uncertainties. The low vibrational frequency and the necessity of the higher-order centrifugal distortion terms indicate a floppy nature of the CH2 rocking mode. A preliminary radioastronomical search for H2CSi in space has been carried out toward a few astronomical sources without success.

The Spectroscopy of Formaldehyde: III. Out-of-Plane Potentials and Geometry Optimizations for Singlet States

Out-of-plane potentials and optimized geometries for excited singlet states of formaldehyde, includingn,π*, σ,π*, π,π*,n,3s, andn,3p, were obtained by multireference CI methods. The1(n,π*) and1(σ,π*) states were found to be nonplanar, with out-of-plane angles of 34.5° and 42.1°, respectively. Agreement with the experimental geometry and energy of1(n,π*) is good, and even the inversion barrier is well reproduced. The1(n,3s),1(σ,π*), and1(π,π*) (mixed withn2) states were found to have minima on different sections of theS2surface, separated from one another by low barriers of 0.2 to 0.7 eV. This causes then→ 3sRydberg transition to be perturbed by1(σ,π*) and1(π,π*). At nonplanar geometries,1(σ,π*) mixes heavily with1(π,π*); therefore, the π → π* band system, which may be observable from the higher vibrational levels of the ground state, is expected to be significantly perturbed. TheC ← Xsystem, recently reassigned ton→ 3pz, should be perturbed not only by1(n,3py), as previously suggested, but also by1(π,π*) through the ν6(CH2rocking) mode. Furthermore, theQ6potential of1(n,3pz) is expected to interact with1(n,σ*CH). TheD ← Xsystem, assigned ton→ 3py, is predicted to be perturbed by1(π,π*), as well as by1(σ,π*) through the out-of-plane bending mode.

Photoelectron Spectroscopy of the Allyl and 2-Methylallyl Anions

The electron affinities of allyl, allyl-d5, 2-methylallyl, and 2-methylallyl-d7 radicals have been determined from the 351 nm photoelectron spectra of the allylic anions. The ions were prepared in a cooled helium flow reactor by the reaction of O- with the corresponding propene. The electron affinities found for the radicals listed above are 0.481 ± 0.008, 0.464 ± 0.006, 0.505 ± 0.006, and 0.493 ± 0.008 eV, respectively. Extensive vibrational structure is observed in all the spectra, as the CCC bending and symmetric stretching modes are activated upon photodetachment. Vibrational frequencies for these modes are obtained for all of the radicals. A second harmonic of an asymmetric CH2 rocking mode is observed in the spectra of the allyl, allyl-d5, and 2-methylallyl anions. Hot bands are used to determine the CCC bending frequencies in the allylic anions. Photoelectron angular distributions were measured for the allyl and 2-methylallyl anions. The photoelectron spectrum of allyl anion is calculated using a m...

Structure, conformation, and motions of polytetrahydrofuran (PTHF) in the hexagonal form of the PTHF‐urea inclusion compound

AbstractCombination of differential scanning calorimetry, x‐ray diffraction, Fourier transform infrared, and C‐13 nuclear magnetic resonance observations made on the crystalline inclusion compound (IC) formed between polytetrahydrofuran (PTHF) and urea (U), together with their comparison to identical observations performed on bulk semicrystalline samples of PTHF, have permitted an analysis of the conformations, motions, and environments available to PTHF chains in both solid‐state phases. The isolated PTHF chains occupying the narrow channels of the PTHF‐U‐IC are highly extended, though small rotational deviations averaging 24° from the nearly all trans, planar zig zag conformation of bulk crystalline PTHF chains produce some significant differences in their behaviors. PTHF chains in PTHF‐U‐IC possess much greater mobility than bulk crystalline PTHF chains as evidenced by C‐13 spin lattice relaxation times, T1, 50 times shorter (1.5 s) than observed for bulk crystalline PTHF chains (75 s). FTIR observations are consistent with very little specific interaction between guest PTHF chains and host urea matrix molecules and result in similar spectra for bulk and IC PTHF, except for the presence of the CH2 rocking vibration band at 745 cm−1 observed for bulk PTHF. The absence of this band in the IC PTHF can be understood by considering the symmetry of the all trans, planar zig zag conformation of bulk crystalline PTHF chains, which prevents the CH2 rocking mode from coupling with skeletal stretching and bending modes as occurs in the nonplanar, helical PTHF chains in PTHF‐U‐IC. © 1995 John Wiley & Sons, Inc.

Fourier Transform Spectroscopy of CH 318OH: The In-plane CH 3-Rocking Band

The infrared Fourier transform spectrum of CH 3 18OH has been recorded in the region from 1020 to 1620 cm −1 at high resolution in order to study the low-lying bending modes, including the CH 3-rocking, OH-bending, and CH 3-deformation fundamental bands. More than 10 000 lines have now been assigned in this region. The present paper focuses on the n = 0 ground torsional subbands of the in-plane CH 3-rocking fundamental. This band is principally of parallel a-type character, due to mixing of the predominantly parallel CO-stretching and OH-bending coordinates with the perpendicular rocking coordinate. Some b-type character was also observed, with partial assignments of a number of weak perpendicular subbands. The parallel subbands have been fitted to J( J + 1) power-series expansions to obtain the subband origins, as well as compact representations of the data in terms of phenomenological state-specific expansion coefficients. The n = 0 excited state energies calculated from the subband origins follow a similar oscillatory pattern with K to that of the ground vibrational state but with significantly reduced amplitude. Analysis with our basic torsion-rotation Hamiltonian yields an effective torsional barrier height for the rocking state of 474.5 ± 2.8 cm −1, a 27% increase over the ground state value. The vibrational energy is found to be 1058.46 ± 0.62 cm −1. An interesting J-localized level-crossing resonance between the CH 3-rocking and CO-stretching modes has also been observed through perturbations in the spectrum.

Pressure-induced correlation field splitting of vibrational modes: structural and dynamic properties in lipid bilayers and biomembranes

Correlation field splittings of the vibrational modes of methylene chains in lipid bilayers, isolated lipid molecules in perdeuterated lipid bilayers, crystalline lipid, and interdigitated lipid bilayers have been investigated by pressure-tuning Fourier-transform infrared spectroscopy. The correlation field splittings of these modes are originating from the vibrational coupling interactions between the fully extended methylene chains with different site symmetry along each bilayer leaflet. The interchain-interactions of the methylene chains with the same site symmetry only contribute to frequency shift of the vibrational modes. The magnitude of the correlation field splitting is a measure of the strength of the interchain-interactions, and the relative intensities of the correlation field component bands provide information concerning the relative orientation of the zig-zag planes of the interacting methylene chains. It has been demonstrated in the present work that the correlation field splitting of the CH2 bending and rocking modes commonly observed in the vibrational spectra of lipid bilayers is the result of the intermolecular interchain-interactions among the methylene chains of the neighboring molecules. The intramolecular interchain-interactions between the sn-1 and sn-2 methylene chains within each molecule are weak. The correlation field splitting resulting from the intramolecular interchain-interactions exhibits a much smaller magnitude than that from the intermolecular interchain-interactions and is observed only at very high pressure. Interdigitation of the opposing bilayer leaflets disturbs significantly the intermolecular interchain-interactions and results in dramatic changes in the pressure profiles of the correlation field component bands of both the CH2 bending and rocking modes. The relative intensities of the correlation field component bands of these modes and the magnitude of the splitting are also altered significantly. These results provide further evidence that the correlation field splitting of the CH2 bending and rocking modes in the vibrational spectra of lipid bilayers is due to the intermolecular interchain-interactions. The present work has also demonstrated that the correlation field splitting of the vibrational modes in lipid bilayers is mainly contributed by the intermolecular interchain-interactions among the nearest neighboring molecules and that the long-range correlation interactions beyond the second neighboring molecules are insignificant.

Perturbation approach for frequency shifts in IR spectra of molecular clusters

A second order perturbation approach for the evaluation of the splitting and shifting of the infrared vibrational bands of molecular clusters, based on early publications of Buckingham, is presented. The Hamiltonian of the system comprises harmonic- and anharmonic intramolecular vibration terms, as well as the intermolecular potential. The anharmonic contributions of the intramolecular force field and the intermolecular potential are treated as a perturbation. In order to extend the applicability of the approach to homogeneous molecular clusters, the formalism of degenerate perturbation theory is employed. The new approach is applied to methanol clusters from dimer to hexamer for calculating the frequency shifts of the OH stretching mode (ω1=3681.5 cm−1), the CH3 rocking mode (ω7=1074.5 cm−1), and the CO stretching mode (ω8=1033.5 cm−1). The numerical results compare favourably with experimental and previous theoretical data (except for ω1), but with a tendency to overestimate the shifts.