Effect of surface coverage on the conformation and mobility of C18-modified silica gels

C18-modified silica gels with surface coverages of 2 to 8.2 micromol m(-2), were prepared by different synthetic pathways and characterized by Fourier Transform infrared spectroscopy (FTIR), solid-state nuclear magnetic resonance (NMR) spectroscopy, and chromatographic measurements. The effects of temperature and bonding density on the conformational order of C18-modified silica gels were studied in detail by FTIR spectroscopy. The silane functionality and degree of cross-linking of silane ligands on the silica surface were evaluated by 29Si cross-polarization magic-angle spinning (CP/MAS) NMR and the structural order and mobility of the alkyl chains were investigated by 13C CP/MAS NMR spectroscopy. CH2 symmetric and anti-symmetric stretching bands and CH2 wagging bands were used as IR probes to monitor the conformational order and flexibility of the alkyl chains in the C18 phases. Qualitative information about the conformational order was obtained from frequency shifts of the CH2 symmetric and anti-symmetric stretching bands. The relative amounts of kink/gauche-trans-gauche, double-gauche, and end-gauche conformers in the alkyl chains were determined by analysis of CH2 wagging bands. These results indicate that surface coverage plays a dominant role in the conformational order of C18-modified silica gels. The FTIR and NMR data are discussed in the context of the chromatographic shape-selectivity differences.     

Influence of synthetic routes on the conformational order and mobility of C18 and C30 stationary phases

Silica gels modified with n-alkyl chains (n = 18, 30) are prepared by two different synthetic routes and are examined by variable temperature FTIR and solid-state NMR spectroscopy. HPLC measurements of SRM 869, cis/trans ss-carotene isomers and xanthophylls isomers confirm the dependence of the separation mechanism on the alkyl chain length and the synthetic routes. The determination of the silane functionality and degree of cross-linking of silane ligands on the silica surface is achieved by 29Si CP/MAS NMR measurements. The structural order and mobility of the alkyl chains are investigated by means of variable temperature 13C CP/MAS NMR measurements. Variable temperature FTIR studies are performed where conformational order and flexibility of the alkyl chains in C18 and C30 phases are monitored through conformational sensitive CH2 symmetric, anti-symmetric stretching and wagging modes. In addition, the chromatographic properties of the C18 and C30 phases are determined. The results derived from the FTIR, NMR and HPLC measurements are discussed in the context of the applied synthetic routes and alkyl chain lengths.     

Thermotropic mesomorphic behavior of surfactant-encapsulated polyoxometalate hybrids

We investigate in detail novel organic-inorganic hybrid liquid crystalline materials, the complexes of surfactant-encapsulated polyoxometalate clusters (SECs), using thermal, X-ray diffraction, and FT-IR spectroscopic analyses. The differential scanning calorimetry measurements reveal four phase transitions under heating processes. We employ FT-IR spectroscopy to understand these phase behaviors. On the basis of vibration spectral assignments, the evidence suggests that the first two phase transitions are associated with the increase of gauche conformers and the disruption of alkyl chains packing in the heating run; the third phase transition is due to the full conformational disorder of alkyl chains covered on the polyoxometalates (PMs); no significant C-H stretching or wagging vibrations are observed with the fourth transition. We find that the fourth endothermic peak is sensitive to the charges of the PMs, and the transition temperature decreases from 185, 177, to 164 degrees C with decreasing PM charges from 13, 11, to 9, respectively. Interestingly, the temperatures of the first three phase transitions of SECs are essentially independent of the PM charges.     

Theoretical and infrared spectroscopy study of the conformational preferences for some 3-monosubstituted-2-methylpropenes

The infrared spectra of 3-X-2-methylpropenes (X = Cl, Br, I, NMe(2), NEt(2), OH, OMe, OEt, SH, SMe and SEt) have been recorded at room temperature in CCl(4) solution. The C=C stretching mode was analyzed and compared to theoretically calculated data to give insight about the conformational isomerism of these compounds. A combination band systematically appears in all spectra (except for amines); the remaining ν(C=C) band and the corresponding intensities were used to obtain the conformer populations. For second-period atoms bonded to C-3, two or more conformers are observed in CCl(4) solution, but when substitution by heavier atoms takes place, only one conformer is observed under the tested conditions, i.e., the gauche form with respect to the C=C-C-X dihedral angle. Therefore, steric hindrance between X and =CH(2) in the s-cis form is strongly dependent on the heteroatom size, while the effect of the alkyl chain bonded to the heteroatom seems to be of secondary importance.     

Normal mode analyses of methyl palmitate all-trans and disordered forms in wagging progressive region

Normal mode analyses are made for methyl palmitate molecule having all-trans or conformational disorders around the ester head group, in order to explain characteristic observed frequency shifts in the wagging progressive region between all-trans and disorder chains in triglyceride molecules. It was found that one gauche conformation at C(alpha)-C(beta) position and 90 degrees rotation of the ester head group in an alkyl chain produce frequency shifts for twisting mode as observed. For wagging modes, contamination of the disorders around the head group makes assignments change and apparent frequency shifts occur.     

Local structure at the air/liquid interface of room-temperature ionic liquids probed by infrared-visible sum frequency generation vibrational spectroscopy: 1-alkyl-3-methylimidazolium tetrafluoroborates

The air/liquid interface of 1-alkyl-3-methylimidazolium tetrafluoroborates with the general formula [C(n)mim]BF(4) (n = 4-11) was studied using infrared-visible sum frequency generation (SFG) vibrational spectroscopy. The probability of the gauche defect per CH2-CH2 bond in the alkyl chain decreases as the number of carbon atoms in the alkyl chain increases. This observation suggests that the interaction between the alkyl chains is enhanced as the alkyl chain length becomes longer. The frequencies of the C-H stretching vibrational modes observed in the SFG spectra are higher than those of the corresponding peak positions observed in the infrared spectra of the bulk liquids. This shift is consistent with a structure in which the alkyl chain protrudes from the bulk liquid into the air. A local structure, which originates from the intermolecular interaction between the ionic liquid molecules, is proposed to explain these observations.     

A study of cysteamine ionization in solution by Raman spectroscopy and theoretical modeling

Different cysteamine (H2N-CH2-CH2-SH) ionization forms have been studied by polarized Raman spectroscopy in solutions prepared with H2O and D2O and by DFT calculations at the B3LYP/6-31++G(d,p) level. To account for solvation effects, we employed the integral equation formalism polarizable continuum model (IEFPCM) option and explicit water molecules. Calculated relative energies and Raman spectra revealed that gauche rotamers around the C-C bond are the most stable conformers in solution. The experimental pKa values and Raman spectra of various ionization forms were best predicted by using a model with three explicit water molecules and the IEFPCM option. In general, the use of IEFPCM tends to lower the calculated frequencies for a few bands, but in some cases (S-H stretching mode) this effect is expressed very strongly. Potential energy distribution (PED) analysis of gauche conformers of various cysteamine ionization forms provided the possibility of discriminating spectroscopically methylene groups adjacent to sulfur, (CH2)S, and nitrogen, (CH2)N, sites. In general, stretching and scissoring modes as well as wagging and twisting vibrations of the (CH2)N group were found to be at higher frequencies. The influence of ionization of SH and NH2 groups on the vibrational spectrum is discussed, and Raman markers for further amine group ionization studies are suggested.     

A sting in the tail of flexible molecules: spectroscopic and energetic challenges in the case of p-aminophenethylamine

The neurotransmitter analogue p-aminophenethylamine (APEA) illustrates many of the pitfalls and challenges associated with spectroscopic and conformational analysis of flexible molecules. The combined experimental-theoretical study presented here resolves a long-standing controversy over its conformational energetic preferences. Jet-cooled resonance enhanced two-photon ionisation (R2PI) and IR-UV ion depletion techniques enabled conformer-specific IR spectra in the NH-CH stretch region to be measured for four distinct conformers of APEA. Comparison of spectra with theoretical calculations (including MP2, M06-2X and B3LYP with aug-cc-pVTZ basis sets) allows the two most populated conformers to be unambiguously identified as those having a gauche arrangement of the side chain, which facilitates an NH···π type hydrogen bond. The other two observed conformers are assigned to structures with an anti-side chain. A fifth gauche conformer, predicted to be least stable, is not observed. Comparison with published conformer specific IR spectra of tyramine (Makara et al., J. Phys. Chem. A, 2008, 112, 13463-13469) and Raman spectra of phenylethylamine (Golan et al., J. Chem. Phys., 2009, 131, 024305) reveals an entirely consistent pattern of spectral signatures associated with the four specific conformations of the ethylamine side chain evident in APEA, and aids assignment of the associated CH and NH stretch fundamentals, some of which have very weak IR intensities. Extensive calculations of the relative energetic trends of the five conformers have been carried out. In comparison to the highest level of theory considered, CCSD(T)-F12b/cc-pVDZ-F12, MP2 overestimate the energy difference, whereas DFT significantly underestimates the energetic preference for NH···π stabilised gauche conformers, although inclusion of dispersion (M06-2X, B3LYP-D3) improves the DFT results.     

Influence of solvents on the conformational order of C18 alkyl modified silica gels

The influence of solvents on the conformational order of C(18) alkyl modified silica gels is studied by means of variable temperature FTIR spectroscopy. Symmetric and anti-symmetric CH(2) stretching modes were utilized for getting qualitative information about the changes in alkyl chain conformational order as a function of both protonated and perdeuterated solvents. It was found that interaction between the C(18) alkyl modified silica gels and the mobile phase results in pronounced changes of the alkyl chain conformational order. Furthermore, it was observed that some perdeuterated solvents exhibit isotope effects, which again is reflected by a different alkyl chain conformational order as compared to the corresponding stationary phases with protonated solvents.     

Ab initio modeling of defect signatures in infrared reflection-absorption spectra of SAMs exposing methyl- and hydrogen-terminated oligo(ethylene glycols)

Extensive ab initio modeling has been performed to explain quantitatively the apparent shapes of characteristic bands, which are systematically observed in the fingerprint region of infrared (IR) reflection-absorption (RA) spectra of oligo(ethylene glycol) (OEG)-terminated SAMs. The presence of defects was thoroughly examined by modeling the RA spectra using the DFT method BP86/6-31G* for all-helical and all-trans conformers of HS(CH2CH2O)nR (n = 5, 6, R = H, CH3) and HS(CH2)15CONH(CH2CH2O)6H molecules and for several defect-containing conformers. These data were then used to simulate RA spectra of SAMs with different content of defects and to compare them with experiments. It is shown that for SAMs of HS(CH2CH2O)nCH3 (n = 5, 6) the pronounced asymmetry of the dominating band can be attributed to the multimode nature of COC stretching vibrations of helical conformers combined with the contribution from few percent of all-trans conformers. Arguments are presented which disprove appreciable amounts of helical conformers with single trans and/or gauche defects. Much more complex combination of factors, which can come into play in the formation of the high-frequency shoulder of COC band, is exemplified by self-assemblies of OEG-terminated amide-bridged alkanethiolates. In particular, spectral signatures of defects with inverted OH terminus are compared with other contributions to the apparent shape of COC band formation. For this family of SAMs, the presence of about 10% of all-trans conformers gives a satisfactory quantitative agreement between the calculated RA spectra and experimental observations.     

An infrared study on Langmuir–Blodgett films of 14,15-bis(hydroxyimino)-13-thiaoctacosane

The Langmuir–Blodgett (LB) films of 14,15-bis(hydroxyimino)-13-thiaoctacosane (TOC) on aluminium plated substrates were investigated using Fourier transform infrared (FTIR), grazing angle (GAIR) and horizontal attenuated total reflectance (HATR) techniques. The LB films of TOC can be transferred onto the solid substrate successfully. The molecular structure of LB films was analysis by comparing the GAIR and HTAR spectra. The intense bands at 2848 and 2918 cm⁻¹ are assigned to symmetric ν_s(CH₂) and asymmetric ν_a(CH₂) stretching vibrations of methylene groups. These peaks suggest that the alkyl chains in TOC are nearly in all-trans conformational state. The presence in the infrared spectra of several bands due to the methylene wagging and twisting modes and of the splitting of the bands due to the methylene scissoring mode at 1467 and 1459 cm⁻¹ and the CH₂ rocking mode at 720 and 731 cm⁻¹ also indicates that in films of TOC alkyl chains are in the all-trans conformation and packed in either an orthorhombic or a monoclinic structure with an orthorhombic subcell containing two mutually orthogonal molecules. Another conclusion presented in this paper that the alkyl chain tilt, which is the angle between the axis, which bisects the C–C bonds and the surface normal, was quite large by comparing the GAIR and HATR spectra.     

Infrared absorption of CH3OSO and CD3OSO radicals produced upon photolysis of CH3OS(O)Cl and CD3OS(O)Cl in p-H2 matrices

Irradiation at 239 ± 20 nm of a p-H(2) matrix containing methoxysulfinyl chloride, CH(3)OS(O)Cl, at 3.2 K with filtered light from a medium-pressure mercury lamp produced infrared (IR) absorption lines at 3028.4 (attributable to ν(1), CH(2) antisymmetric stretching), 2999.5 (ν(2), CH(3) antisymmetric stretching), 2950.4 (ν(3), CH(3) symmetric stretching), 1465.2 (ν(4), CH(2) scissoring), 1452.0 (ν(5), CH(3) deformation), 1417.8 (ν(6), CH(3) umbrella), 1165.2 (ν(7), CH(3) wagging), 1152.1 (ν(8), S=O stretching mixed with CH(3) rocking), 1147.8 (ν(9), S=O stretching mixed with CH(3) wagging), 989.7 (ν(10), C-O stretching), and 714.5 cm(-1) (ν(11), S-O stretching) modes of syn-CH(3)OSO. When CD(3)OS(O)Cl in a p-H(2) matrix was used, lines at 2275.9 (ν(1)), 2251.9 (ν(2)), 2083.3 (ν(3)), 1070.3 (ν(4)), 1056.0 (ν(5)), 1085.5 (ν(6)), 1159.7 (ν(7)), 920.1 (ν(8)), 889.0 (ν(9)), 976.9 (ν(10)), and 688.9 (ν(11)) cm(-1) appeared and are assigned to syn-CD(3)OSO; the mode numbers correspond to those used for syn-CH(3)OSO. The assignments are based on the photolytic behavior and a comparison of observed vibrational wavenumbers, infrared intensities, and deuterium isotopic shifts with those predicted with the B3P86∕aug-cc-pVTZ method. Our results extend the previously reported four transient IR absorption bands of gaseous syn-CH(3)OSO near 2991, 2956, 1152, and 994 cm(-1) to 11 lines, including those associated with C-O, O-S, and S=O stretching modes. Vibrational wavenumbers of syn-CD(3)OSO are new. These results demonstrate the advantage of a diminished cage effect of solid p-H(2) such that the Cl atom, produced via UV photodissociation of CH(3)OS(O)Cl in situ, might escape from the original cage to yield isolated CH(3)OSO radicals.     

Study of disorder in different phases of tetratriacontane and a binary alkane mixture, using vibrational spectroscopy

Raman spectroscopy has been used to investigate the monoclinic crystal --> rotator --> melt phase transitions in n-C(34)H(70), for both real-time heating and cooling runs. Changes in band intensity and frequency in the CH2 bending, CH2 twisting, skeletal C-C stretching, and CH3 rocking regions revealed both transitions, particularly when using band components related to gauche bonds. In the room temperature infrared spectrum, the CH2 rocking-twisting and CH2 wagging progressions were observed and indexed for n-C(34)H(70) and a 2:1 (w/w) mixture of C(34)H(70) and C(36)D(74). This led to best estimates for the all-trans crystal core in both cases of 33 to 34 carbon atoms, indicating that the core corresponds to almost the whole of the C(34)H(70) molecule.     

变温红外法研究聚对苯二甲酸乙二酯分子链的松弛运动及其构象转变

在30～170 ℃范围内逐渐升温过程中,用红外光谱仪原位检测无定形聚对苯二甲酸乙二酯（PET）薄膜红外光谱图的变化情况。通过特征谱带吸光度与温度的变化特点,研究了PET分子链在热变化过程中的松弛运动及冷结晶过程中分子链的构象变化。实验结果表明在冷结晶过程中,随PET结晶的不断完善,对应左右式(gauche)构象的吸收峰减弱,对应反式(trans)构象的吸收峰增强,并计算出CH2面外摇摆振动结晶前和结晶后反式构象和左右式构象的相对百分含量随温度的变化关系,以及玻璃化转变和冷结晶的温区范围。     

Resonant two-photon ionization and ab initio conformational analysis of haloethyl benzenes (PhCH(2)CH(2)X,X=Cl,F)

The S(1) <-- S(0) transitions of the gaseous (2-fluoroethyl)-benzene (FEB) and (2-chloroethyl)-benzene (CEB) have been investigated using a combination of two-color resonant two-photon ionization and UV-UV hole burning spectroscopy. Both anti and gauche conformers have been identified on the basis of rotational band contour analysis supported by ab initio calculations on the ground and electronically excited states. The gauche origin band of FEB at 37,673 cm(-1) is redshifted 50 cm(-1) relative to the corresponding anti origin, while CEB origin bands overlap at 37,646 cm(-1). Relative conformational stability and populations in the jet have been estimated for both molecules, based on the intensity ratio of S(1) <-- S(0) band origin transitions. These are compared with a range of related molecules with the structural motif PhCH(2)CH(2)X (X=CH(3),CH(2)CH(3),NH(2),OH,COOH,CCH,CN). Theory and experimental results for FEB and CEB show repulsive interactions between the halogen substituents and the pi cloud of the phenyl rings destabilizing the gauche conformers, but the preference for the anti conformers is relatively modest. The gauche conformer origins show very different hybrid character: FEB is largely b type, while CEB is an ac hybrid in keeping with theoretically computed TM "rotations" (theta(elec)) of -7 degrees and -56 degrees , respectively. This difference is attributed largely to rotation of the side chain in opposite directions about the C(1)C(alpha) bond. Spectra of FEB(H(2)O) and CEB(H(2)O) single water clusters show evidence of an anti conformation in the host molecule.     

Structures of 17,19-hexatriacontadiyne monolayers on Au(111) studied by infrared reflection absorption spectroscopy and scanning tunneling microscopy

The aggregation and reaction of 17,19-hexatriacontadiyne molecules are studied on a Au(111) surface. The molecular orientation and arrangement are elucidated by infrared reflection absorption spectroscopy (IRAS) and scanning tunneling microscopy (STM). A vapor-deposited monolayer and a multilayered film formed by adsorption from the solution provide IRA spectra with bands due to the antisymmetric and symmetric stretching of methylenes in the gauche conformation. After the adsorbed film is rinsed with the solvent, however, the spectrum loses the gauche bands and is characterized by the enhanced C-H(distal) and C-H(proximal) stretching bands, which means that all-trans molecules are laid flat. Only STM images for the rinsed film display columnar structures on the herringbones of the reconstructed Au(111) surface; the alkyl chain direction is found to be parallel to the Au atom row. The results indicate that an ordered monolayer is formed first at the liquid-solid interface, and then, disordered overlayers with the gauche conformation are grown but removed by a rinse. Upon exposure to UV light, thus obtained monomer columns are converted into oligomers with flexible backbones and an increased gauche population in the alkyl chains, which resemble red phase polydiacetylenes in LB films.     

Vibrational spectroscopic studies, conformations and ab initio calculations of 3,3,3-trifluoropropyltrichlorosilane

Infrared spectra of 3,3,3-trifluoropropyltrichlorosilane (CF3CH2CH2SiCl3) were obtained in the vapour, amorphous and crystalline solid phases in the range 4000-50 cm-1. Additional spectra in argon matrices at 5.0 K were recorded before and after annealing to 20-36 K. Raman spectra of the compound as a liquid were recorded at various temperatures between 298 and 210 K and spectra of the amorphous and crystalline solids were obtained. The spectra suggested the existence of two conformers (anti and gauche) in the fluid phases and in the matrix. When the vapour was shock-frozen on a cold finger at 80 K and subsequently annealed to 120-150 K, six weak or very weak Raman bands vanished in the crystal. Similar variations were observed in the corresponding infrared spectra after annealing and four very weak IR bands disappeared after crystallization. From intensity variations between 298 and 210 K of three Raman band pairs an average value Delta(conf)H degrees (gauche-anti)=6.1+/-0.5 kJmol-1 was obtained in the liquid. Annealing experiments indicate that the anti conformer also has a lower energy in the argon matrices. The conformational equilibrium is highly shifted towards anti in the liquid, and the low energy conformer also forms the crystal. The spectra of the abundant anti conformer and the few bands ascribed to the gauche conformer have been interpreted. Ab initio calculations at the HF/6-311G(**) and B3LYP/6-311G(**) gave optimized geometries, infrared and Raman intensities and vibrational frequencies for the anti and gauche conformers. The conformational energy differences derived were 11.8 and 9.2 kJmol-1 from the HF and the B3LYP calculations, respectively.     

Infrared intensity of 1-monolaurin-water systems in the gel phase

The infrared spectra of 1-monolaurin-water systems, where KSCN is added as the intensity standard, were observed and the infrared intensity of the bands due to acyl groups measured relatively to that of the CN stretching. The infrared intensities of the bands parallel and perpendicular to the paraffin chains decrease and increase, respectively, on going from the crystalline 1 phase to the gel phase, confirming that these intensity changes result from the long-range interaction among oscillating dipoles. The infrared spectra with no CH2 rocking and the splitting of the CH2 rocking are also discussed with the same interaction model.     

IR spectra of long-chain α,ω-alkanediols: 1,22-docosanediol and 1,44-tetratetracontanediol

The IR absorption spectra of α,ω-alkanediols with different chain lengths, HO(CH₂)₂₂OH and HO(CH₂)₄₄OH, in the spectral range of 400–5000 cm^?1 are analyzed. The assignment of numerous absorption bands to vibration modes in short methylene sequences and terminal hydroxyl groups is suggested. The splitting of IR absorption bands into doublets at 720–730 cm^?1 (rocking vibrations of CH₂ groups) and 1463–1473 cm^?1 (bending vibrations of CH₂ groups) testifies that the crystal unit subcells in the lamellae of alkanediols are orthorhombic with parameters typical of normal hydrocarbons. The specific features of absorption bands due to O-H stretching and C-O-H bending vibrations have been analyzed. These bands appear during formation of lengthy associates from hydrogen bonds formed by hydroxyl groups on the surface of elementary lamellae. A sharp increase in the intensity of the absorption bands in progression of C-C stretching and CH₂ wagging vibrations due to the anharmonic Fermi resonance with the stretching vibrations of C-O groups in the terminal hydroxyl groups has been detected.     

The observation of a large gauche preference when 2-fluoroethylamine and 2-fluoroethanol become protonated

The energies of the gauche and anti conformers of 2-fluoroethylamine, 2-fluoroethanol and their protonated analogues are calculated using density functional theory. Unlike the non protonated systems, the protonated systems show a strong gauche effect where the C-F and the C-(+)NH(3) or C-F and C-(+)OH(2) bonds are gauche rather than anti to each other. Single crystal X-ray diffraction studies of 2-fluoroethylammonium compounds identify the same conformational preference.