Silane Chain Research Articles

Structure of silane layer formed on silica particle surfaces by treatment with silane coupling agents having various functional groups

The surface treatment of spherical silica particles with silane coupling agents having various organic functional groups was conducted and the effect of the alkoxy group number on the molecular flexibility of the silane chain with multilayer coverage was investigated using 1H-pulse nuclear magnetic resonance spectroscopy. The silica particles were treated with 2-propanol solution and heated at 120 °C for 24 h after solvent evaporation to accelerate the polycondensation reaction of silanol groups. For multilayer coverage, flexible linear chain and rigid network structures were expected to form on the surface from the di- and trialkoxy structures, respectively. However, the rigid network structure was formed from both the di- and trialkoxy structures with glycidoxy, amino, and methacryloxy functional silanes. Ring opening of the epoxy group occurred, followed by reaction to form the network structure, even with the dialkoxy structure of the glycidoxy functionality. Ring opening of the epoxy group could be reduced by pH adjustment of the treatment solution and the linear chain structure was formed from the dialkoxy structure. In the case of amino and methacryloxy functional groups, hydrogen bonds were formed between the amino or methacryloxy groups and the silanol groups on the silica surface or silane molecules.

Study on the structure-properties relationship of natural rubber/SiO2 composites modified by a novel multi-functional rubber agent

Vulcanization property and structure-properties relationship of natural rubber (NR)/silica (SiO2) composites modified by a novel multi-functional rubber agent, N-phenyl- N!-(-triethoxysilane)-propyl thiourea (STU), are investi- gated in detail. Results from the infrared spectroscopy (IR) and X-ray photoelectron spectroscopy (XPS) show that STU can graft to the surface of SiO2 under heating, resulting in a fine-dispersed structure in the rubber matrix without the con- nectivity of SiO2 particles as revealed by transmission electron microscopy (TEM). This modification effect reduces the block vulcanization effect of SiO2 for NR/SiO2/STU compounds under vulcanization process evidently. The 400% modulus and tensile strength of NR/SiO2/STU composites are much higher than that of NR/SiO2/TU composites, although the crys- tal index at the stretching ratio of 4 and crosslinking densities of NR/SiO2 composites are almost the same at the same dosage of SiO2. Consequently, a structure-property relationship of NR/SiO2/STU composites is proposed that the silane chain of STU can entangle with NR molecular chains to form an interfacial region, which is in accordance with the experimental observations quite well.

Tensile properties of styrene-butadiene rubber/silica composites with mercapto functional silane coupling agents: influences of loading method and alkoxy group number

The influences of alkoxy group number and loading method of silane coupling agents on the mechanical properties of a styrene-butadiene rubber/silica composite were investigated. Mercapto functional silane coupling agents with dialkoxy and trialkoxy structures were used. The pre-treatment method and the integral blend method were compared. Both the fracture stress and modulus at 200% strain were higher in the pre-treatment than in the integral blend for dialkoxy type composites. However, they were higher in the integral blend than in the pre-treatment for trialkoxy-type composites. The interaction between the silane chains on the silica surface and the rubber molecular chains at the interfacial region was estimated by 1H pulse nuclear magnetic resonance spectroscopy using an unvulcanized silica/rubber mixture. It was found that the binding of rubber molecular chains by the silane chains was higher in the pre-treatment system for dialkoxy-type composites, whereas it was higher in the integral blend for trialkoxy-type composites. The reason is proposed as follows: in the pre-treatment for dialkoxy type, a linear silane chain formed in the case of multi-layer coverage. The silane chain entangled with the rubber chain in the interfacial region and improved the reinforcement effect. For the trialkoxy type, a network structure formed using the pre-treatment method, lowering the amount of entanglement. However, in the integral blend for trialkoxy type, the formation of the silane network and the entanglement progressed simultaneously during the preparation process. A well-entangled interfacial region was formed.

Influences of the alkoxy group number and treatment condition on the structure of glycidoxy functional silane-treated layer on silica particles analyzed by 1H pulse NMR

The surface treatment of spherical silica particles using a silane coupling agent with a glycidoxy group was conducted and the effect of the alkoxy group number on the molecular mobility of the silane chain was investigated by 1H pulse nuclear magnetic resonance (NMR). Silanes with di-alkoxy and tri-alkoxy structures were used, and the silica particles were treated with 2-propanol solution and heated at 120 °C for 24 h after solvent evaporation. The surface coverage of the silica surface was in the range from two to three layers. For multilayer coverage, linear chain and network structures were expected to form on the surface by polycondensation reaction using the di- and tri-alkoxy structures, respectively. However, the relaxation times for silane chains with both di- and tri-alkoxy structures measured by pulse NMR were short, which indicates that both silane chains formed rigid network structures. Fourier transform infrared spectroscopic analysis revealed that ring opening of the epoxy group occurred, followed by reaction to form the network structure, even with the di-alkoxy structure. Ring opening of the epoxy group could be reduced by setting the heating temperature at 80 °C. There was a significant difference in flexibility between the silane-layers with di- and tri-alkoxy structures after heating at 80 °C, as reflected by the relaxation time.

Mechanical properties of silica particle‐filled styrene‐butadiene rubber composites containing polysulfide‐type silane coupling agents: Influence of loading method of silane

AbstractPolysulfide‐type silane coupling agents containing two or four sulfur atoms were incorporated into a styrene‐butadiene rubber/silica composite by two different loading methods and the tensile properties were measured. The pretreatment method and the integral blend method were compared. To improve the 200% modulus of the composite, the combination of silane with four sulfur atoms and the pretreatment method was effective. Whereas, the combination of silane with four sulfur atoms and the integral blend method was effective for improving the fracture elongation. The fracture elongation of the integral blend using the silane with four sulfur atoms was higher than that of the untreated silica‐filled composite. The sulfur atoms in the silane should contribute to the crosslinking of rubber in the vulcanization process. The silane with much sulfur atoms strengthens the interface effectively and raises the 200% modulus in the pretreatment method. It is considered that the unreacted silane molecules in the rubber acted as a plasticizer in the integral blend method and the effect was better with four sulfur atoms. The 1H pulse nuclear magnetic resonance spectroscopy was measured for the unvulcanized rubber/silica mixture. The measured relaxation time was found to be in good correlation with the 200% modulus. It was found that the molecular mobility of rubber is lowered by the entanglement with the silane chains on the silica surface at the interfacial region, and it was more effective in the pretreatment method than in the integral blend method. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

Photografting as a versatile, localizable, and single‐step surface functionalization of silica‐based monoliths dedicated to microscale separation techniques

In this work, we developed a surface functionalization way of silica monoliths with a rapid, simple, versatile, and localizable photografting step. The elaboration of a photoreactive layer at the surface of monoliths was first optimized. The functionalization with [γ-(methacryloyloxy)propyl]trimethoxysilane at 80°C in a hydro-organic solution containing triethylamine as catalyst allows reachng the highest density of methacrylate photoactive moieties on silica surfaces. These methacrylate reactive surfaces were subsequently photografted within few minutes with acrylate monomers bearing alkyl chains (C12 and C18). The photografting efficiency was determined by monitoring the retentive properties of monoliths in the RP mode. The retention factors are of the same order of magnitude as highly retentive columns obtained by modification of silica surface with long-alkyl chain silanes or by thermal polymerization of long-alkyl chain monomers. It was also verified that such grafting neither impaired the efficiency of the monolithic stationary phase (Hmin = 6-8 μm in nano-LC) nor its permeability (about 6 × 10(-14) m(2)). Further, it was also demonstrated that photografting is localizable in nonmasked defined areas. Results obtained in anion-exchange chromatography after photopolymerization of [2-(methacryloyloxy)ethyl]trimethylammonium chloride are presented as well to demonstrate the versatility of the developed approach.

Mechanical properties of silane‐treated silica particle‐filled polyisoprene composites: Influence of the alkoxy group mixing ratio in silane coupling agent containing mercapto group

AbstractSpherical silica particles were treated with a mixture of silane coupling agents with mercapto functional groups and dialkoxy or trialkoxy groups. The surface coverage, which indicates the number of layers covering the silica surface, was in the range from 1.6 to 6.0. The molecular mobility of the treated layer was analyzed with 1H‐NMR spectroscopy. The silane chain became more flexible with increasing dialkoxy structures in the mixture and with increasing surface coverage. The stress–strain behavior was measured for a composite consisting of the treated silica and vulcanized polyisoprene rubber (PIR). There was no influence of the dialkoxy and trialkoxy mixing ratio for near monolayer coverage. In the case of the pure dialkoxy and low trialkoxy contents, a higher stress was observed at the same strain for surface coverages of 2.4–3.8. However, a longer length flexible silane chain, that is, with a surface coverage of 6.0, was disadvantageous for the reinforcement effect. The results of pulse NMR analysis of the unextracted PIR on the particle surface and swelling test indicate that the reinforcement effect was strongly affected both by entanglement and the crosslinking reaction between the silane chain and PIR at the interfacial region. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

1H pulse NMR analysis of silane-treated layers on glass fiber surfaces

The surface treatment of a glass fiber using mercapto-functional silane coupling agent having a di- or trialkoxy group has been studied. The surface of silane-treated fiber is observed by scanning electron microscopy. The treated layer looks hard like glass for the trialkoxy silane-treated, whereas it looks soft for the dialkoxy silane-treated. Molecular mobility of the treated layer is analyzed by 1H pulse nuclear magnetic resonance spectroscopy. The amount of silane loading increases with increased silane concentration in the treatment solution. The relaxation time for the surface layer is longer for the dialkoxy structure than for the trialkoxy structure. The silane chain is flexible in the dialkoxy structure, but is rigid for the trialkoxy structure, independent of the loading amount of silane. The relaxation behavior for the mixture of the di- and trialkoxy structures is between those of the pure dialkoxy and trialkoxy structures and depends on the mixing ratio. The network density of silane chains on the glass fiber can be controlled by varying the mixing ratio of the di- and trialkoxy compounds.

Novel liquid–solid adhesion superhydrophobic surface fabricated using titanium dioxide and trimethoxypropyl silane

The present paper reports the preparation of very simple and inexpensive superhydrophobic surfaces fabricated using titanium dioxide and lower alkyl chain silane (trimethoxypropyl silane) in aqueous or non-aqueous solvent. The superhydrophobic surfaces fabricated in aqueous or non-aqueous solvent had same static water contact angle, but showed different contact angle hysteresis and liquid–solid adhesion. Superhydrophobic surface fabricated in aqueous solvent showed high contact angle hysteresis (CAH). In addition, liquid–solid adhesion of superhydrophobic surface fabricated in aqueous solvent is different in which the water droplet roll on the surface but will not roll off out of the surface during tilting and fell down when turned upside down. The reason for such high contact angle hysteresis and novel liquid–solid adhesion behavior is explained on the basis of measurements on the superhydrophobic surface using water contact angle (WCA) data, profilometry, SEM images, XPS and FTIR-ATR analysis. These results are compared with the superhydrophobic surface fabricated in non-aqueous solvent which showed self-cleaning properties. Based on WCA, XPS and FTIR-ATR analysis, it is shown that the difference in the CAH is more chemical in origin than morphology.

Palladium Catalyzed Vinyl Addition Poly(norbornenes): Silane Chain Transfer Agents, Hydroxyl Group Activators and Their Impact on Optical Density at 193 nm

Palladium Catalyzed Vinyl Addition Poly(norbornenes): Silane Chain Transfer Agents, Hydroxyl Group Activators and Their Impact on Optical Density at 193 nm

Effect of grafting on microstructure, composition and surface and transport properties of ceramic membranes for osmotic evaporation

The effect of grafting with fluoroalkylsilanes on microstructure, composition and surface and transport properties of alumina membranes having different pore sizes (0.2, 0.5 and 0.8 μm) was studied. Surface and transport properties were evaluated in terms of contact angle, adhesion work and hydrodynamic membrane pore response. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDXS) were employed to evidence the induced microstructure and composition changes after grafting. Contact angle higher than 90°, in addition to low work adhesion and surface tension values confirmed the hydrophobic character of all three membranes after chemical treatment. SEM analysis showed facetted alumina grains in unmodified membranes while grains with rounded edges appeared after grafting. This effect induced compaction on internal and intermediate layers mainly on the 0.2 μm membrane. The insertion of hydrophobic chains on the outer surface and the porous structure of membranes after grafting, were evidenced by detection of fluor in the active and internal structure of treated membranes as well as by increments on the Si/Al ratios followed by EDXS. In addition, water permeability drastically decreased and intrusion pressure rose, mainly on the membrane of lower pore size tested. Hydrodynamic membrane response indicated pore size reductions of 53%, 65% and up to 82% in 0.2, 0.5 and 0.8 μm grafted membranes, respectively. However, silane chains inserted inside of 0.5 and 0.8 μm membrane pores were apparently more susceptible to flow induced deformation, limiting the potential application of these membranes on the osmotic evaporation process. Results from grafted 0.2 μm alumina membrane during OE of clarified cane juice and model sucrose solution indicate that feed and brine layer resistances contribute in less than 10% while membrane resistance contribution to the total process resistance was around 90%. Results from this work indicate that applicability of grafted alumina membranes in the OE process might further improve by using 0.2 μm < d P < 0.5 μm average pore size membranes and/or by decreasing membrane thickness.

Effect of oxidized silicon (SiOx) surfaces functionalization on real‐time PCR by Lab‐on‐a‐chip microdevices

There is a great need to improve the biocompatibility of silicon‐based lab‐on‐chip substrate materials for reliable quantitative analysis of biological solutions. These advanced microdevice surfaces need not only be biocompatible but also have surfaces of defined wettability characteristics. The inhibition of biomolecular activity due to microdevice surface interaction is common and can result in inaccurate results or decreased reaction yields. In this work we investigate different techniques for the chemical functionalization of oxidized silicon (SiOx) surfaces in order to: (i) obtain defined hydrophobic/hydrophilic surfaces; and (ii) increase the efficiency of performing Real‐Time Polymerase Chain Reaction (PCR) on a silicon‐based lab‐on‐chip. Silicon oxide surfaces are functionalized by grafting alkylic chain silanes and poly(ethylene glycol) (PEG) chains to the surfaces, rendering them hydrophobic or hydrophilic. Functionalized surfaces are characterized through contact angle and atomic force microscopy (AFM) measurements, showing stable hydrophobic surfaces with contact angles of 69–78° and layer thicknesses of 11–15 Å and hydrophilic surfaces displaying contact angles of 5–6° and thicknesses of 22–52 Å. PCR experiments carried out directly on bare silicon oxide lab‐on‐chip surfaces show low yields of DNA amplification. Hydrophobic surfaces decrease the inhibition of PCR. Hydrophilic surfaces are a major improvement on the bare silicon oxide exhibiting the same maximum reaction yield as obtained with a standard thermocycler. We have found that the best results are associated with PEG modified surfaces, which prove very suitable for the fabrication of reliable PCR silicon lab‐on‐chips. Copyright © 2011 John Wiley &amp; Sons, Ltd.

Electronic transport of the silane chain doped with phosphorus and boron atoms

Based on the nonequilibrium Green’s function method combined with the density functional theory, the properties of a silane chain doped with phosphorus and boron atoms connected to two Au electrodes are investigated. It has been found that a barrier similar to the p-n junction is formed in the molecule because of the different electronegativities of phosphorus and boron atoms. The electronic transport properties of two parallel silanes are also investigated when they are positioned between two identical electrodes. The results show that direct interactions between chains may strengthen the rectifying effect when their separation is set suitably. The rectifying effect may disappear when their separation is altered to other values due to the splitting of the frontier molecular orbitals, which contributes to the formation of new transport channels.

Investigation of sub-monolayer, monolayer, and multilayer self-assembled semifluorinated alkylsilane films

Semifluorinated self-assembled (FAS SA) films fabricated from trifunctional precursors are frequently used in myriad applications, yet an understanding of the effects of fabrication conditions, including deposition time, on adsorption mechanisms and molecular architectures is still being developed. In this work we prepared SA films based on the F(CF(2))(8)(CH(2))(2)SiCl(3) (FAS-17) precursor and characterized these films using a suite of surface analytical techniques. Contact angle, sum frequency generation (SFG) spectroscopy, X-ray photoelectron spectroscopy (XPS), and ellipsometry results are consistent with the formation of disordered sub-monolayer structures at short deposition times, well-ordered monolayers at intermediate deposition times, and inhomogeneous multilayers at long deposition times. Correlation of SFG and XPS results demonstrates a change in FAS-17 chain orientation as the deposition time increases from 2 s to 5 min. Group theory-based calculations, SFG studies, and Fourier-transform infrared (FTIR) results also afford additional evidence in support of the assignment of the SFG signals at ~1345 and ~1370 cm(-1) to the asymmetric stretching mode of the semifluorinated silane chain's terminal CF(3) group rather than to its axial CF(2) stretches. To our knowledge, this is the first report of SFG studies on semifluoroalkyl silane self-assembled films in the C-F stretching frequency region.

Effect of cerium concentration on microstructure, morphology and corrosion resistance of cerium–silica hybrid coatings on magnesium alloy AZ91D

The aim of this work was to investigate the effect of cerium concentration on microstructure, morphology and anticorrosion performance of cerium–silica hybrid coatings on magnesium alloy AZ91D. Vinyltriethoxysilane (VETO) and γ-glycidoxypropyltrimethoxysilane (GPTMS) were employed as precursors to prepare sol–gel based silica coating. Cerium nitrate hexahydrate as dopant in five different concentrations was added into the silica coatings. Fourier transform infrared (FT-IR) spectrum analysis, viscosity measurements and scanning electron microscopy (SEM) were employed to characterize the microstructure and morphology of these coatings. It was found that with the increase of cerium concentration, the degree of decomposition of silane chains in the coating network increased. The corrosion resistance of the cerium–silica hybrid coatings was estimated by electrochemical impedance spectroscopy (EIS) measurements and potentiodynamic polarization tests. The results demonstrated that corrosion resistance of coatings initially increases and then decreases as cerium concentration goes up. When the cerium concentration is 0.01 M, corrosion resistance reaches its maximum.

Modeling and prediction of compressive creep of silane-treated TiO2/high-density polyethylene

Silane-treated TiO2/high-density polyethylene (silane-TiO2/HDPE) is a potential bone substitute material with good bioactivity and mechanical properties. In this composite, 40 vol.% TiO2 particles were connected chemically with HDPE (high-density polyethylene) by silane-coupling agent. The intensive silane connection was believed to play very important role in compressive creep behavior of silane-TiO2/HDPE in our previous study. In order to deeply understand the relationship between the special structure and creep behavior, both a viscoelastic creep model named Burgers, and an empirical model called Findley power law were applied to simulate and predict the creep curves obtained in both air and saline solution. The results showed that Burgers model succeeded in simulating the creep curves but failed in long-term prediction for all dry samples, while Findley power law failed in simulating the creep curves in air but succeeded in saline solution. By analyzing the parameters obtained from Burgers model, it was believed that the structure of intense connection by silane chains resulted in the very big and gradually increased permanent viscous flow of this material with creep time. The big permanent viscous flow brought on the failure in simulating creep curves using Findley power law, and the improved permanent viscous flow during creep conduced to the positive deviation between the prediction and practical creep curve using Burgers model. The effect of saline solution around sample could not only decrease the permanent viscous flow but also stabilize it gradually, which resulted in the success in the prediction and simulation of creep curve in saline solution by the two models.

Selective Synthesis of Functional Alkynylmono‐ and ‐trisilanes

AbstractThe selective synthesis of functional alkynylsilanes RC≡C–(SiMe2)m–X (m = 1, 3) was investigated. Monofunctionalization with or without protecting groups gave moderate to good yields of alkynyldimethylmonosilanes RC≡CMe2SiX [R = Ph, X = Cl (1), NEt2 (2), OMe (3), H (4), Br (5), I (6), Cp (8), C5H4Li (10), Ph (11); R = Pr, X = Ph (12)]. Compounds 4 and 8 were converted into the (alkyne)transition‐metal complexes 4·Cp2Mo2(CO)4 (13) and 8·Co2(CO)6 (14), respectively, which were characterized by X‐ray diffraction. Stepwise extension and functionalization of the silane chain starting from 1‐chloro‐2‐(diethylamino)tetramethyldisilane (Et2NMe2Si–SiMe2Cl) yielded the trisilanes Ph–(SiMe2)3–X [X = NEt2 (18), OMe (19), Cl (20), H (21), C≡CPh (22), C≡CPr (23)]. The synthesized compounds were characterized by NMR and IR spectroscopy, 4, 11, 13, and 14 also by UV/Vis spectroscopy.

Mechanical properties of silane‐treated, silica‐particle‐filled polyisoprene rubber composites: Effects of the loading amount and alkoxy group numbers of a silane coupling agent containing mercapto groups

AbstractThe surface treatment of spherical silica particles with a silane coupling agent with mercapto groups was carried out. The treated silica particles were incorporated within polyisoprene and then vulcanized. The effects of the loading amount and alkoxy group number of silane on the stress–strain curve of the filled composite were investigated. For this purpose, silanes with dialkoxy and trialkoxy structures were used. The loading amount of silane on the silica surface was varied from 1 to 8 times the amount required for monolayer coverage. The stress at the same strain increased with the silane treatment, and it was higher in the dialkoxy structure than in the trialkoxy structure above 300% strain. There was no significant influence of the loading amount on the stress for the trialkoxy silane structure. However, the stress was influenced by the loading amount, and the maximum stress was observed at 4 times the silane amount required for monolayer coverage for the dialkoxy structure. The stress had a good relationship with the crosslinking density of silica‐filled polyisoprene rubber (measured with a swelling test). The reinforcement effect by the silane treatment of silica was found to be affected strongly both by the entanglement of the silane chain and polyisoprene rubber matrix and by the crosslinking reaction between the mercapto group of silane and polyisoprene rubber in the interfacial region. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Internal energy deposition with silicon nanoparticle-assisted laser desorption/ionization (SPALDI) mass spectrometry

The use of silicon nanoparticles for laser desorption/ionization (LDI) is a new appealing matrix-less approach for the selective and sensitive mass spectrometry of small molecules in MALDI instruments. Chemically modified silicon nanoparticles (30 nm) were previously found to require very low laser fluence in order to induce efficient LDI, which raised the question of internal energy deposition processes in that system. Here we report a comparative study of internal energy deposition from silicon nanoparticles to previously explored benzylpyridinium (BP) model compounds during LDI experiments. The internal energy deposition in silicon nanoparticle-assisted laser desorption/ionization (SPALDI) with different fluorinated linear chain modifiers (decyl, hexyl and propyl) was compared to LDI from untreated silicon nanoparticles and from the organic matrix, α-cyano-4-hydroxycinnamic acid (CHCA). The energy deposition to internal vibrational modes was evaluated by molecular ion survival curves and indicated that the ions produced by SPALDI have an internal energy threshold of 2.8–3.7 eV. This is slightly lower than the internal energy induced using the organic CHCA matrix, with similar molecular survival curves as previously reported for LDI off silicon nanowires. However, the internal energy associated with desorption/ionization from the silicon nanoparticles is significantly lower than that reported for desorption/ionization on silicon (DIOS). The measured survival yields in SPALDI gradually decrease with increasing laser fluence, contrary to reported results for silicon nanowires. The effect of modification of the silicon particle surface with semifluorinated linear chain silanes, including fluorinated decyl (C10), fluorinated hexyl (C6) and fluorinated propyl (C3) was explored too. The internal energy deposited increased with a decrease in the length of the modifier alkyl chain. Unmodified silicon particles exhibited the highest analyte internal energy deposition. These findings may suggest a role of the modifier as a moderator in the energy dissipation and relaxation process. The relatively low internal energy content of SPALDI-produced ions indicates that this is a “soft” desorption technique, with potential advantages in the analysis of labile compounds.

Tunnel Currents across Silane Diamines/Dithiols and Alkane Diamines/Dithiols: A Comparative Computational Study

Two different first-principles methods, one based on density functional theory combined with Green’s functions and the other on a configuration interaction method, are used to calculate the electronic transport properties of alkane and silane chains terminated by amine end groups in metal-molecule-metal junctions. The lowvoltage conductance is found to decay exponentially with increasing length in both systems, and decay constants are obtained from the different methods. Both methods predict smaller conductance values and steeper decay in the alkane-bridged junctions compared with the silane-bridged junctions, but quantitative differences in the decay constants obtained from the two formalisms arise. These differences are attributed to the treatment of the energy-level alignments in the tunnel junctions as well as the treatment of correlation within the molecular chains. Additionally, end-group effects for both the alkane and the silane chains are studied using both a simple tunnel barrier model and complex band-structure calculations. These results are used to explain differences observed in conductance decay constants in amine- and thiol-linked junctions obtained from the two transport methods; the results further highlight the importance of accurate energy-level alignment between the electrode and molecular states.