Ulrich Mayer

The acceptor number — A quantitative empirical parameter for the electrophilic properties of solvents

The electrophilic properties of 34 solvents have been characterized by the Acceptor Number (AN) which has been derived from31P-n.m.r. measurements of triethylphosphine oxide dissolved in the respective solvents. Relationships are found between the acceptor numbers and theZ-values,ET-values andY-values, as well as the free energies of solvation of anions and the redox potentials of the hexacyanoferrate(III)—hexacyanoferrrate(II) system in different solvents. The new parameter provides—together with the donor number—a useful guide in choosing the most appropriate solvent for a given reaction.

External Reflection Infrared Spectroscopy of Anisotropic Adsorbate Layers on Dielectric Substrates

Monolayers of octadecylsiloxane were formed on native silicon (Si/SiO2) and glass surfaces by adsorption from dilute solutions of octadecyltrichlor osilane and were investigated by polarization- and angle-dependent external reflection infrared spectroscopy. In contrast to metal substrates, both the parallel and perpendicular vibrational components of the adsorbate can be detected on these dielectric surfaces. The monolayer reflection spectra show significant changes as a function of the light incidence angle and the polarization of the infrared radiation, which contain detailed information on the surface orientation of the film molecules. Spectral simulations based on classical electromagnetic theory yield an average 10° tilt angle of the hydrocarbon chains with respect to the surface normal on both silicon and glass surfaces. Despite this apparent structural identity of the monolayer films on silicon and glass, significant differences are observed in the monolayer reflection spectra resulting from purely optical effects of the substrate.

Outer-sphere effects on the redox properties of the system Co(en)33+/Co(en)32+

Abstract The half-wave potentials for the polarographic reduction of Co(en)33+ to Co(en)32+ have been determined in the solvents: nitromethane, acetonitrile, propanediol-1,2-carbonate, acetone, dimethylformamide, dimethylacetamide, dimethylsulfoxide and hexamethylphosphoric acid triamide. A nearly linear relationship was found between the half-wave potentials E1/2 and the donor numbers of the solvents which shows that the outer-sphere solvation of Co(en)33+ and to a lesser extent of Co(en)32+ involves donor-acceptor interactions between the solvent molecules and the hydrogen atoms of the coordinated ethylenediamine ligands. The observed shift of E1/2 to more negative potential values corresponds to an increase in reducing properties of the system Co(en)33+/Co(en)32+ with increasing donor numbers of the solvents. A correlation was found between the E1/2 values and the 59Co n.m.r. chemical shifts of Co(en)3(ClO4)3 in solution which proves that the outer-sphere solvation of Co(en)33+ is accompanied by an increase in electron density at the Co atoms with increasing donor numbers of the solvents. The maximum difference in E1/2 for the solvents investigated is 0.51 V which corresponds to a free energy difference of 49.5 kJ mol−1.

Reflection-absorption infrared spectroscopy of self-assembled monolayers on gold and silicon surfaces

Abstract Reflection absorption infrared spectroscopy is used to investigate the structure of self-assembled monolayers of dioctadecyldisulfide (ODS) on gold and octadecyltrichlorosilane (OTS) on silicon surfaces and to determine the relative influence of the substrates optical properties and the molecular orientation of the adsorbate on the spectral characteristics of ordered monolayer films. Comparable sensitivity is achieved in the monolayer spectra on both surfaces despite the large difference in the absolute reflected intensities. The peak positions of the CH-stretching absorptions are identical for ODS and OTS monolayers and are characteristic of a crystalline, densely-packed film structure. The orientation of the hydrocarbon chains on the surface is determined by a quantitative spectral analysis aided by a computer simulation and fit procedure. A tilt angle of 33° between the hydrocarbon chain axis and the surface normal is found for ODS monolayers on gold. An almost perpendicular orientation (8° tilt), on the other hand, is determined from the OTS monolayer spectra on silicon. The complex absorption profiles in the latter spectra, consisting of superimposed positive and negative absorptions, are accurately reproduced in the simulated spectra. They result from the excitation of vibrational modes both parallel and perpendicular to the silicon surface, which cause reflectivity changes of the OTS/silicon adsorbate system in opposite directions. Comparatively simple, “transmission-like” spectra are obtained for ODS monolayers on gold, in which the band intensities are governed by the well-known metal surface selection rule.

Solvent effects on the redox properties of tris-(1,10-phenanthroline)iron(II/III) complexes

Abstract The redox properties of the system Fe(tmphen) 3 (II/III) (tmphen=3,4,7,8-tetramethyl-1,10-phenanthroline) have been studied in the solvents nitromethane, acetonitrile, propanediol-1,2-carbonate, dimethylformamide, dimethylacetamide, dimethylsulfoxide and of the systems Fe(phen) 3 (II/III) (phen=1,10-phenanthroline) and Fe(niphen) 3 (II/III) (niphen=5-nitro-1,10-phenanthroline) in the solvents nitromethane, acetonitrile, propanediol-1,2-carbonate and acetone. The redox potentials of Fe(tmphen) 3 (II/III) are nearly independent of the solvent suggesting that the system might be used as a reference redox couple similar to the systems ferrocene/ferricinium or bisbiphenylchromium(0/I). In contrast the redox potentials of Fe(niphen) 3 (II/III) show a significant decrease with increasing donor number of the solvent which can be explained by nucleophilic attack of solvent molecules at the iron. It is shown that such a mechanism is consistent with the known solvent and salt effects on the kinetics of dissociation of ferroin and ferriin type complexes.

IR-spectroscopic investigations on solute-solvent interactions Part 2: Solvation of triethylphosphine oxide in hydroxylic solvents

An IR-spectroscopic study has been carried out on the solvation state of the model compoundEt3PO in the solvents methanol, ethanol, propanol, 2-propanol, butanol, 1,2-ethanediol, 1,3-propanediol, water, acetic acid, and formamide. With the exception of formamide (where only one solvate species was detected) the spectra show the presence of two basic types of solvate complexes, which derive from hydrogen bonded complexes withprimary coordination numbers of one and two, respectively. In water there is evidence that higher coordinated solvates (withprimary solvation numbers of two and three) are formed. Detailed informations about the structure of these complexes were obtained by spectrophotometric titrations, which show that extensive outer-sphere interactions are taking place in the neat solvents. For solvents, where only one kind of solvate species is present, an excellent correlation was found between solvent acceptor numbers and the wavenumbersv(PO) of the P=O stretching vibration ofEt3PO. By contrast, acceptor numbers of protic solvents usually represent average values, which are determined by the relative concentrations and31P-chemical shifts of the individual solvates present in solution. By means of the wavenumbersv(PO) of these solvates and the linear correlation mentioned above, it has become possible to calculate refined acceptor number values termed “Individual Acceptor Numbers”, which characterize the acceptor properties of protic solvents as a function of the solvation numbers of the substrates under consideration. It is shown, that the use of these individual acceptor numbers in linear energy relationships allows a significantly improved unified description of solvent effects on chemical reactions and physical processes in aproticand protic solvents.ZusammenfassungEs wurden IR-spektroskopische Untersuchungen über den Solvatationszustand der ModellverbindungEt3PO in den Lösungsmitteln Methanol, Ethanol, Propanol, 2-Propanol, Butanol, 1,2-Ethandiol, 1,3-Propandiol, Wasser, Essigsäure und Formamid durchgeführt. Mit Ausnahme von Formamid (wo nur eine Solvatform festgestellt werden konnte) zeigen die Spektren die Präsenz von zwei Grundtypen von Solvatkomplexen an, die Wasserstoffbrückenkomplexen mit denprimären Koordinationszahlen eins und zwei zugeordnet werden können. Bei Wasser gibt es Hinweise für die Existenz von Solvatkomplexen mit denprimären Solvatationszahlen zwei und drei. Detaillierte Informationen über die Struktur der verschiedenen Solvatkomplexe konnten mit Hilfe spektrophotometrischer Titrationen erhalten werden, die beweisen, daß in den reinen Lösungsmitteln starke Outer-Sphere-Wechselwirkungen wirksam sind. Für Lösungsmittel, in denen nureine Solvatform vorliegt, konnte eine ausgezeichnete lineare Beziehung zwischen den Wellenzahlenv(PO) der P=O-Valenzschwingung vonEt3PO und den Akzeptorzahlen der Lösungsmittel nachgewiesen werden. Dagegen stellen die Akzeptoren hydroxylischer Lösungsmittel üblicherweise Mittelwerte dar, die durch die relativen Konzentrationen der einzelnen Solvatspezies und deren charakteristischen31P-chemischen Verschiebungen bestimmt sind. Aus den charakteristischen Wellenzahlenv(PO) der einzelnen Solvate können mit Hilfe der oben erwähnten linearen Korrelation „verbesserte“ Akzeptorzahlparameter errechnet werden, für die wir die Bezeichnung „Individuelle Akzeptorzahlen“ vorschlagen und die die Akzeptoreigenschaften protischer Lösungsmittel als Funktion der Solvatationszahlen der betrachteten Substrate beschreiben. Es wird gezeigt, daß die Verwendung dieser individuellen Akzeptorzahlen in linearen Energiebeziehungen eine wesentlich verbesserte einheitliche Beschreibung von Lösungsmitteleffekten auf chemische Reaktionen und physikalische Prozesse in aprotischenund protischen Lösungsmitteln ermöglicht.

Molecular structure of self-assembled bimolecular films on gold and silicon surfaces studied by external reflection infrared spectroscopy

Self-assembled bimolecular films were prepared by coupling homologous n-alkylsiloxane overlayers (CH 3 (CH 2 ) n SiO x , RSiO, n= 10, 13-17) to HO-terminated primer layers of HO(CH 2 ) 16 SH (HHDT) on Au(lll) and HO(CH 2 ) 11 SiO x (HUDS) on oxidized Si(100). The structure and surface orientation of the hydrocarbon chains in the RSiO overlayers were investigated by external reflection infrared spectroscopy (ERIRS) and ellipsometry, and were compared to the molecular structure of alkanethiolate monolayers (CH 3 (CH 2 ) n S, RS) on Au(111) and RSiO monolayers on oxidized Si(100). Similar structural properties were found for the RSiO overlayers on gold and silicon, despite the different structure of the primer layers HHDT and HUDS. The long-chain RSiO overlayers (n>15) show a highly ordered structure consisting of predominantly all-trans hydrocarbon chains with a uniform, close to perpendicular surface orientation of the chain axes, which is identical to the structure of the corresponding RSiO monolayers, but is markedly different to RS monolayers on Au(111), where the hydrocarbon chains are tilted by ∼30° with respect to the surface normal. Shorter-chain RSiO overlayers (n < 15) exhibit increasing disorder with decreasing chain length caused by a gradual increase of the chain tilt angles associated with a destruction of the all-trans chain geometry, which results in an isotropic, liquid-like film structure for the shortest-chain molecules in this series (n = 10). Similar order-disorder transitions also occur in the monolayer films RS/Au and RSiO/Si, whereby the transition region is shifted by at least five CH 2 groups toward shorter chains.

IR-Spectroscopic Investigations on Solute--Solvent Interactions Part 1: Solvation of Triethylphosphine Oxide in Aprotic and CH-Acidic Solvents

A systematic study has been carried out on the solvent and concentration dependence of the P=O stretching vibrationv(PO) of the model compoundEt3PO in various aprotic and several CH-acidic solvents. Evidence has been found for the existence of dipole-dipole complexes in concentrated solutions ofEt3PO in solvents of low acceptor numbers. In dilute solutions, however, the P=O stretching band generally occurs as a single symmetrical peak which shows that only one kind of solvate species is present in solution. In CH-acidic solventsEt3PO is present in the form of hydrogen bonded complexes. Informations about the structure of these complexes have been obtained by means of spectrophotometric titrations. It has been shown that the wavenumbersv°(PO) obtained by extrapolation ofv(PO) values to zero concentration are linearly related to the acceptor numbers of the solvents, previously derived from31P-chemical shift measurements. The existence of this linear relationships proves that both parameters are linearly related to the strength of the intermolecular interactions and are virtually free from unspecific contributions such as magnetic anisotropy or vibrational coupling effects. The results of the present study show that the P=O stretching vibration ofEt3PO represents an ideal probe for the investigation of solvent effects and solvation mechanisms.ZusammenfassungEs wurde die Konzentrations- und Lösungsmittelabhängigkeit der P=O-Valenzschwingungv(PO) der ModellverbindungEt3PO in verschiedenen aprotischen und CH-aciden Lösungsmitteln systematisch untersucht. In hinreichend verdünnten Lösungen tritt die P=O-Valenzschwingung als einzelne symmetrische Bande auf, was beweist, daß nur eine einzige Solvatform vorliegt. In Lösungsmitteln niedriger Akzeptorzahl konnte bei höheren Konzentrationen die Existenz von Dipol-Dipol-Komplexen nachgewiesen werden. In CH-aciden Lösungsmitteln liegtEt3PO in Form von Wasserstoffbrückenkomplexen vor. Aussagen über die Struktur dieser Komplexe konnten mit Hilfe spektrophotometrischer Titrationen erhalten werden. Die durch Extrapolation auf Konzentration null erhaltenen Wellenzahlenv° (PO) hängen linear von den aus den31P-chemischen Verschiebungen vonEt3PO bereits früher bestimmten Akzeptorzahlen der Lösungsmittel ab. Die Existenz dieser linearen Beziehung beweist, daß beide Parameter frei von störenden Einflußfaktoren (magnetische Anisotropie- und Schwingungskopp-lungseffekte) sind und tatsächlich linear mit der Stärke der intermolekularen Wechselwirkung variieren. Die Ergebnisse der vorliegenden Studie zeigen, daß die P=O-Valenzschwingung vonEt3PO eine hervorragende Sonde zur Untersuchung von Lösungsmitteleffekten und Solvatationsmechanismen darstellt.

Comment on ‘‘Infrared external reflection spectroscopy of adsorbates on dielectric substrates: Determining adsorbate orientation in Langmuir monolayers’’ [J. Chem. Phys. 98, 5825 (1993)]

In a reply to a recent paper by J. T. Buontempo and S. A. Rice [J. Chem. Phys. 98, 5825 (1993)], it is shown that incorrect expressions were used for the parallel and perpendicular components of the absorption indices k, which lead to erroneous intensities in the calculated infrared reflection spectra. Recalculation of the results with the correct equations shows that a molecular orientation analysis based on the dichroic intensity ratios, as proposed in that paper, generally does require a knowledge of the isotropic absorption coefficients.

Monolayer Formation of Organosilane Compounds on Hydroxylic Surfaces

In this study external reflection infrared spectroscopy and ellipsometry were used to investigate the formation and structure of monolayers of octadecyltrichlorosilane on native silicon surfaces (Si/SiO2). It is shown that during the initial stage of the adsorption process liquid-like films with randomly oriented hydrocarbon chains are formed. With increasing surface coverage this isotropic submonolayer structure changes into a highly ordered, crystalline monolayer phase. The overall time required for complete monolayer formation depends strongly on the water content of the adsorbate solution.