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Vibrational sum frequency generation (SFG) spectroscopy can specifically probe molecular species non-centrosymmetrically arranged in a centrosymmetric or isotropic medium. This capability has been extensively utilized to detect and study molecular species present at the two-dimensional (2D) interface at which the centrosymmetry or isotropy of bulk phases is naturally broken. The same principle has been demonstrated to be very effective for the selective detection of non-centrosymmetric crystalline nanodomains interspersed in three-dimensional (3D) amorphous phases. However, the full spectral interpretation of SFG features has been difficult due to the complexity associated with the theoretical calculation of SFG responses of such 3D systems. This paper describes a numerical method to predict the relative SFG intensities of non-centrosymmetric nanodomains in 3D systems as functions of their size and concentration as well as their assembly patterns, i.e., the distributions of tilt, azimuth, and rotation angles with respect to the lab coordinate. We applied the developed method to predict changes in the CH and OH stretch modes characteristic to crystalline cellulose microfibrils distributed with various orders, which are relevant to plant cell wall structures. The same algorithm can also be applied to any SFG-active nanodomains interspersed in 3D amorphous matrices.

Sum frequency generation (SFG) vibrational spectroscopy can selectively detect and analyze noncentrosymmetric components interspersed in amorphous matrices; this principle has been used for studies of nanoscale structure and mesoscale assembly of cellulose in plant cell walls. However, the spectral information averaged over a large area or volume cannot provide regiospecific or tissue-specific information of different cells in plants. This study demonstrates spatially resolved SFG analysis and imaging by combining a broad-band SFG spectroscopy system with an optical microscope. The system was designed to irradiate both narrow-band 800 nm and broad-band tunable IR beams through a single reflective objective lens, but from opposite sides of the surface normal direction of the sample. The developed technique was used to reveal inhomogeneous distributions of cellulose microfibrils within single cell walls, such as cotton fibers and onion epidermis as well as among different tissues in Arabidopsis inflorescence stems and bamboo culms. SFG microscopy can be used for vibrational spectroscopic imaging of other biological systems in complement to conventional Fourier transform infrared spectroscopy and confocal Raman microscopy.

A broadband sum frequency generation (BB-SFG) spectrometer with multimodal (MM) capabilities was constructed, which could be routinely reconfigured for tabletop experiments in reflection, transmission, and total internal reflection (TIR) geometries, as well as microscopic imaging. The system was constructed using a Ti:sapphire amplifier (800 nm, pulse width = 85 fs, repetition rate = 2 kHz), an optical parameter amplification (OPA) system for production of broadband IR pulses tunable between 1000 and 4000 cm(-1), and two Fabry-Pérot etalons arranged in series for production of narrowband 800 nm pulses. The key feature allowing the MM operation was the nearly collinear alignment of the visible (fixed, 800 nm) and infrared (tunable, 1000-4000 cm(-1)) pulses which were spatially separated. Physical insights discussed in this paper include the comparison of spectral bandwidth produced with 40 and 85 fs pump beams, the improvement of spectral resolution using etalons, the SFG probe volume in bulk analysis, the normalization of SFG signals, the stitching of multiple spectral segments, and the operation in different modes for air/liquid and adsorbate/solid interfaces, bulk samples, as well as spectral imaging combined with principle component analysis (PCA). The SFG spectral features obtained with the MM-BB-SFG system were compared with those obtained with picosecond-scanning-SFG system and high-resolution BB-SFG system (HR-BB-SFG) for dimethyl sulfoxide, α-pinene, and various samples containing cellulose (purified commercial products, Cladophora cell wall, cotton and flax fibers, and onion epidermis cell wall).

The polymorphic structures of starch were characterized with vibrational sum frequency generation (SFG) spectroscopy. The noncentrosymmetry requirement of SFG spectroscopy allows for the detection of the ordered domains without spectral interferences from the amorphous phase and also the distinction of the symmetric elements among crystalline polymorphs. The V-type amylose was SFG-inactive due to the antiparallel packing of single helices in crystal unit cells, whereas the A- and B-type starches showed strong SFG peaks at 2904 cm(-1) and 2952-2968 cm(-1), which were assigned to CH stretching of the axial methine group in the ring and CH2 stretching of the exocyclic CH2OH side group, respectively. The CH2/CH intensity ratios of the A- and B-type starches are significantly different, indicating that the conformation of hydroxymethyl groups in these two polymorphs may be different. Cyclodextrin inclusion complexes were also analyzed as a comparison to the V-type amylose and showed that the head-to-tail and head-to-head stacking patterns of cyclodextrin molecules govern their SFG signals and peak positions. Although the molecular packing is different between V-type amylose and cyclodextrin inclusion complexes, both crystals show the annihilation of SFG signals when the functional group dipoles are arranged pointing in opposite directions.

Sum-frequency-generation (SFG) vibration spectroscopy selectively detects noncentrosymmetric vibrational modes in crystalline cellulose inside intact lignocellulose. However, SFG peak assignment in biomass samples is challenging due to the complexity of the SFG processes and the lack of reference SFG spectra from the two crystal forms synthesized in nature, cellulose Iα and Iβ. This paper compares SFG spectra of laterally aligned cellulose Iα and Iβ crystals with vibration frequencies calculated from density functional theory with dispersion corrections (DFT-D2). Two possible hydrogen-bond networks A and B ( Nishiyama et al. Biomacromolecules 2008 , 9 , 3133 ) were investigated for both polymorphs. From DFT-D2 calculations the energetically favorable structures for cellulose Iα and Iβ had CH2OH groups in tg conformations and network A hydrogen bonding. The calculated frequencies of C-H stretch modes agreed reasonably well with the peak positions observed with SFG and were localized vibrations; thus, peak assignments to specific alkyl groups were proposed. DFT-D2 calculations underestimated the distances between hydrogen-bonded oxygen atoms compared to the experimentally determined values; therefore, the OH stretching calculated frequencies were ~100 cm(-1) lower than observed. The SFG peak assignments through comparison with DFT-D2 calculations will guide the SFG analysis of the crystalline cellulose structure in plant cell walls and lignocellulose biomass.

The multilayer desorption behavior of 2,5-diidothiophene and the dendritic aggregation of photochemical reaction products during the desorption of 2,5-diiodothiophene multilayers have been studied. Like many other aromatic compounds, 2,5-diiodothiophene shows a multilayer desorption behavior different from the typical zeroth-order kinetics, a metastable desorption peak growth at approximately 220 K followed by a thick multilayer peak growth at approximately 235 K. Traditionally, these desorption behaviors have been attributed to the formation of three-dimensional clusters. This paper provides the direct evidence of this clustering process by producing nondesorbing photoreaction products in the multilayer and by imaging their clusters after the multilayer desorption. Oligothiophene species are produced via photochemical reactions of 2,5-diiodothiophene during the multilayer deposition at approximately 180 K in ultrahigh vacuum (UHV). Upon heating the multilayer to room temperature, the oligothiophene species forms into fibrous aggregates with a fractal dimension varying from 1.37 to 1.81 depending on their surface concentration. Using a topographical alteration of the substrate with a repeating pattern, these oligothiophene fibers can be aligned to a certain direction. This may allow in-situ fabrication of aligned conjugated polymer fibers directly on a target substrate.

The molecular configuration of water adsorbed on a hydrophilic silicon oxide surface at room temperature has been determined as a function of relative humidity using attenuated total reflection (ATR)-infrared spectroscopy. A completely hydrogen-bonded icelike network of water grows up to three layers as the relative humidity increases from 0 to 30%. In the relative humidity range of 30-60%, the liquid water structure starts appearing while the icelike structure continues growing to saturation. The total thickness of the adsorbed layer increases only one molecular layer in this humidity range. Above 60% relative humidity, the liquid water configuration grows on top of the icelike layer. This structural evolution indicates that the outermost layer of the adsorbed water molecules undergoes transitions in equilibrium behavior as humidity varies. These transitions determine the shape of the adsorption isotherm curve. The structural transitions of the outermost adsorbed layer are accompanied by interfacial energy changes and explain many phenomena observed only for water adsorption.

A multilayer film of TiClx, model Ziegler−Natta catalyst for stereospecific polymerization, was produced by electron beam-induced deposition of TiCl4 on an Au substrate at 100 K. The deposition of solid-phase TiClx took place by dissociation of TiCl4 adsorbed at the substrate surface by electronsboth primary and secondary. The deposition kinetics, surface composition, and structure were studied with X-ray photoelectron spectroscopy and temperature-programmed desorption. Its polymerization stereochemistry was studied by analyzing the polypropylene products with infrared spectroscopy and atomic force microscopy. The deposited film was stable at temperatures lower than 450 K and had a high concentration of chemisorbed TiCl4 species. The film surface was composed of mostly a nonbasal plane (defective) structure. When activated by reactions with Al(C2H5)3, the TiClx film selectively produced isotactic polypropylene. The absence of atactic polypropylene was due to the lack of TiClx sites with the basal plane st...

The correlation between the surface structure of the Ziegler−Natta catalysts and the stereospecificity in propylene polymerization was shown experimentally using model catalysts and temperature-programmed desorption (TPD) of mesitylene as a nondestructive surface probe. Two types of titanium chloride model catalysts, with MgCl2 support (TiClx/MgCl2) and without MgCl2 (TiCly), were fabricated on an Au substrate in ultrahigh vacuum. Once activated with AlEt3 vapor, both catalysts were active for propylene polymerization in the absence of excess AlEt3 during polymerization. The TiClx/MgCl2 catalyst produced both atactic and isotactic polypropylene, while the TiCly catalyst without the MgCl2 support produced exclusively isotactic polypropylene. For the same catalysts, the mesitylene TPD revealed that TiClx/MgCl2 had two adsorption site structures, a basal plane structure and a nonbasal plane structure, while TiCly had only the latter structure. To our knowledge, this was the first direct experimental evidence...

The film structure, composition, and deposition kinetics of model Ziegler−Natta polymerization catalysts produced by chemical vapor deposition have been studied using temperature-programmed desorption (TPD), Auger electron spectroscopy (AES), and X-ray photoelectron spectroscopy (XPS). Redox reactions of metallic Mg and TiCl4 deposited from the vapor phase on a Au substrate produce the model catalysts composed of titanium chloride and magnesium chloride (TiClx/MgCl2). The low solubility of TiClx in MgCl2 leads to the formation of a TiClx monolayer on top of MgCl2 multilayers characteristic of the supported catalysts. The TiCl4/Mg flux ratio during the deposition controls the oxidation state distribution and surface coverage of TiClx species of the deposited film. The conversion of Mg to MgCl2 is completed in the codeposition process while it is incomplete in the sequential deposition process due to a kinetic barrier for chlorine diffusion. Upon heating the sequentially deposited film, the surface TiClx (T...

The chemistry of CH3 radicals on oxygen-modified Mo(100) surfaces (O/Mo(100)) has been studied using temperature-programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS). Gas-phase CH3 radicals were produced by pyrolysis of azomethane and dosed on O/Mo(100) at a surface temperature of 320 K. In TPD, O/Mo(100) with θO = 1.4 monolayer (ML) produces exclusively CH4 and CO, but O/Mo(100) with θO = 0.9 and 0.4 ML produce significant amounts of C2+ alkenes in addition to CH4 and CO. HREELS shows that the CH3 groups are bound to surface Mo atoms, not to surface oxygen. On 1.4 ML-O, the CH3 groups are stable at 320 K and have a symmetry lower than C3v. On 0.9 ML-O and 0.4 ML-O, some CH3 groups decompose to methylene groups, which react with intact CH3 groups to form surface alkyl groups. The surface species at 320 K appear to be controlled by the preadsorbed oxygen coverage, depending on whether θO 1 ML. CH4 is formed via hydrogenation of CH3 groups by surface hydr...