Effect of hydrogen bond formation on the elastic molecular scattering: a case study with methanol

Rayleigh optical activities of small hydrogen-bonded methanol clusters containing two to five molecules are reported. For the methanol trimer, tetramer, and pentamer both cyclic and linear structures are considered. After the geometry optimizations, the dipole moments and the dipole polarizabilities (mean, interaction, and anisotropic components) are calculated using HF, MP2 and DFT (B3LYP, B3P86 and BH&HLYP) with aug-cc-pVDZ extended basis set. The polarizabilities are used to analyse the depolarization ratios and the Rayleigh scattering activities. The variations in the activity and in the depolarization for Rayleigh scattered radiation with the increase in the cluster size for both cyclic and linear structures are analysed.

High Viscosity of Imidazolium Ionic Liquids with the Hydrogen Sulfate Anion: A Raman Spectroscopy Study

Ionic liquids based on 1-alkyl-3-methylimidazolium cations and the hydrogen sulfate (or bisulfate) anion, HSO4-, are much more viscous than ionic liquids with alkyl sulfates, RSO4-. The structural origin of the high viscosity of HSO4- ionic liquids is unraveled from detailed comparison of the anion Raman bands in 1-ethyl-3-methylimidazolium hydrogen sulfate and 1-butyl-3-methylimidazolium hydrogen sulfate with available data for simple HSO(4)(-) salts in crystalline phase, molten phase, and aqueous solution. Two Raman bands at 1046 and 1010 cm(-1) have been assigned as symmetric stretching modes nu(s)(S = O) of HSO4-, the latter being characteristic of chains of hydrogen-bonded anions. The intensity of this component increases in the supercooled liquid phase. For comparison purposes, Raman spectra of 1-ethyl-3-methylimidazolium ethyl sulfate and 1-butyl-3-methylimidazolium methyl sulfate have been also obtained. There is no indication of difference in the strength of hydrogen bond interactions of imidazolium cations with HSO4- or RSO4- anions. Raman spectra at high pressures, up to 2.6 GPa, are also discussed. Raman spectroscopy provides evidence that hydrogen-bonded anions resulting in anion-anion interaction is the reason for the high viscosity of imidazolium ionic liquids with HSO4-. If the ionic liquid is exposed to moisture, these structures are disrupted upon absorption of water from the atmosphere.

NMR studies on supramolecular aggregates in solution and the solid state

The aim of this work presented here is the characterization of structure and dynamics of different types of supramolecular systems by advanced NMR spectroscopy. One of the characteristic features of NMR spectroscopy is based on its high selectivity. Thus, it is desirable to exploit this technique for studying structure and dynamics of large supramolecular systems without isotopic enrichment. The observed resonance frequencies are not only isotope specific but also influenced by local fields, in particular by the distribution of electron density around the investigated nucleus. Barbituric acid are well known for forming strongly hydrogen-bonded complexes with variety of adenine derivatives. The prototropic tautomerism of this material facilitates an adjustment to complementary bases containing a DDA(A = hydrogen bond acceptor site, D = hydrogen bond donor site) or ADA sequences, thereby yielding strongly hydrogen-bonded complexes. In this contribution solid-state structures of the enolizable chromophor "1-n-butyl-5-(4-nitrophenyl)-barbituric acid" that features adjustable hydrogen-bonding properties and the molecular assemblies with three different strength of bases (Proton sponge, adenine mimetic 2,6-diaminopyridine (DAP) and 2,6-diacetamidopyridine (DAC)) are studied. Diffusion NMR spectroscopy gives information over such interactions and has become the method of choice for measuring the diffusion coefficient, thereby reflecting the effective size and shape of a molecular species. In this work the investigation of supramolecular aggregates in solution state by means of DOSY NMR techniques are performed. The underlying principles of DOSY NMR experiment are discussed briefly and more importantly two applications demonstrating the potential of this method are focused on. Calix[n]arenes have gained a rather prominent position, both as host materials and as platforms to design specific receptors. In this respect, several different capsular contents of tetra urea calix[4]arenes (benzene, benzene-d6, 1-fluorobenzene, 1-fluorobenzene-d5, 1,4-difluorobenzene, and cobaltocenium) are studied by solid state NMR spectroscopy. In the solid state, the study of the interaction between tetra urea calix[4]arenes and guest is simplified by the fact that the guests molecule remains complexed and positioned within the cavity, thus allowing a more direct investigation of the host-guest interactions.

Studies of pure rotational spectra of isolated molecules and molecular adducts using pulsed jet Fourier transform microwave (PJ-FTM) spectroscopy

This thesis focuses on studying molecular structure and internal dynamics by using pulsed jet Fourier transform microwave (PJ-FTMW) spectroscopy combined with theoretical calculations. Several kinds of interesting chemical problems are investigated by analyzing the MW spectra of the corresponding molecular systems. First, the general aspects of rotational spectroscopy are summarized, and then the basic theory on molecular rotation and experimental method are described briefly. ab initio and density function theory (DFT) calculations that used in this thesis to assist the assignment of rotational spectrum are also included. From chapter 3 to chapter 8, several molecular systems concerning different kind of general chemical problems are presented. In chapter 3, the conformation and internal motions of dimethyl sulfate are reported. The internal rotations of the two methyl groups split each rotational transition into several components line, allowing for the determination of accurate values of the V3 barrier height to internal rotation and of the orientation of the methyl groups with respect to the principal axis system. In chapter 4 and 5, the results concerning two kinds of carboxylic acid bi-molecules, formed via two strong hydrogen bonds, are presented. This kind of adduct is interesting also because a double proton transfer can easily take place, connecting either two equivalent or two non-equivalent molecular conformations. Chapter 6 concerns a medium strong hydrogen bonded molecular complex of alcohol with ether. The dimer of ethanol-dimethylether was chosen as the model system for this purpose. Chapter 7 focuses on weak halogen…H hydrogen bond interaction. The nature of O-H…F and C-H…Cl interaction has been discussed through analyzing the rotational spectra of CH3CHClF/H2O. In chapter 8, two molecular complexes concerning the halogen bond interaction are presented.

Solid-state NMR investigation of phosphonic acid based proton conducting materials

In this work, new promising proton conducting fuel cell membrane materials were characterized in terms of their structure and dynamic properties using solid-state nuclear magnetic resonance (NMR) spectroscopy and X-ray diffraction. Structurally different, phosphonic acid (PA) containing materials were systematically evaluated for possible high-temperature operation (e.g. at T>100°C). Notably, 1H, 2H and 31P magic angle spinning (MAS) NMR provided insight into local connectivities and dynamics of the hydrogen bonded network, while packing arrangements were identified by means of heteronuclear dipolar recoupling techniques.rnThe first part of this work introduced rather crystalline, low molecular weight ionomers for proton conducting membranes, where six different geometries such as line, triangle, screw, tetrahedron, square and hexagon, were investigated. The hexagon was identified as the most promising geometry with high-temperature bulk proton conductivities in the range of 10-3 Scm-1 at a relative humidity of 50%. However, 2H NMR and TGA-MS data suggest that the bulk proton transport is mainly due to the presence of crystal water. Single crystal X-ray data revealed that in the tetrahedron phosphonic acids form tetrameric clusters isolating the mobile protons while the phosphonic acids in the hexagon form zigzag-type pathways through the sample.rnThe second part of this work demonstrates how acid-base pairing and the choice of appropriate spacers may influence proton conduction. Different ratios of statistical copolymers of poly (vinylphosphonic acid) and poly (4-vinylpyridine) were measured to derive information about the local structure and chemical changes. Though anhydrous proton conductivities of all statistical copolymers are rather poor, the conductivity increases to 10-2 S cm-1 when exposing the sample to relative humidity of 80%. In contrast to PVPA, anhydride formation of phosphonic acids in the copolymer is not reversible even when exposing the sample to a relative humidity of 100%.rnIn addition, the influence of both spacers and degree of backbone crystallinity on bulk proton conductivity was investigated. Unlike in systems such as poly benzimidazole (PBI), spacers were inserted between the protogenic groups along the backbone. It was found that dilution of the protogenic groups decreases the conductivity, but compared to PVPA, similar apparent activation energies for local motions were obtained from both variable temperature 1H NMR and impedance spectroscopy data. These observations suggest the formation of phosphonic acid clusters with high degrees of local proton motion, where only a fraction of motions contribute to the observable bulk proton conductivity. Additionally, it was shown that gradual changes of the spacer length lead to different morphologies.rnIn summary, applying advanced solid-state NMR and X-ray analysis, structural and dynamic phenomena in proton conducting materials were identified on a molecular level. The results were discussed with respect to different proton conduction mechanisms and may contribute to a more rational design or improvement of proton conducting membranes.rn

Structure formation as studied by EPR spectroscopy: From simple solutions to nanoparticles

In this thesis, three nitroxide based ionic systems were used to investigate structure and dynamics of their respective solutions in mixed solvents by means of electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) spectroscopy at X- and W-band (9.5 and 94.5 GHz, respectively). rnFirst, the solvation of the inorganic radical Fremy’s salt (K2ON(SO3)2) in isotope substituted binary solvent mixtures (methanol/water) was investigated by means of high-field (W-band) pulse ENDOR spectroscopy and molecular dynamics (MD) simulations. From the analysis of orientation-selective 1H and 2H ENDOR spectra the principal components of the hyperfine coupling (hfc) tensor for chemically different protons (alcoholic methyl vs. exchangeable protons) were obtained. The methyl protons of the organic solvent approach with a mean distance of 3.5 Å perpendicular to the approximate plane spanned by ON(S)2 of the probe molecule. Exchangeable protons were found to be distributed isotropically, approaching closest to Fremy’s salt from the hydrogen-bonded network around the sulfonate groups. The distribution of exchangeable and methyl protons as found in MD simulations is in full agreement with the ENDOR results. The solvation was found to be similar for the studied solvent ratios between 1:2.3 and 2.3:1 and dominated by an interplay of H-bond (electrostatic) interactions and steric considerations with the NO group merely involved into H-bonds.rnFurther, the conformation of spin labeled poly(diallyldimethylammonium chloride) (PDADMAC) solutions in aqueous alcohol (methanol, ethanol, n-propanol, ethylene glycol, glycerol) mixtures in dependence of divalent sodium sulfate was investigated with double electron-electron resonance (DEER) spectroscopy. The DEER data was analyzed using the worm-like chain model which suggests that in organic-water solvent mixtures the polymer backbones are preferentially solvated by the organic solvent. We found a less serve impact on conformational changes due to salt than usually predicted in polyelectrolyte theory which stresses the importance of a delicate balance of hydrophobic and electrostatic interactions, in particular in the presence of organic solvents.rnFinally, the structure and dynamics of miniemulsions and polymerdispersions prepared with anionic surfactants, that were partially replaced by a spin labeled fatty acid in presence and absence of a lanthanide beta-diketonate complex was characterized by CW EPR spectroscopy. Such miniemulsions form multilayers with the surfactant head group bound to the lanthanide ion. Beta-diketonates were formerly used as NMR shift reagents and nowadays find application as luminescent materials in OLEDs and LCDs and as contrast agent in MRT. The embedding of the complex into a polymer matrix results in an easy processable material. It was found that the structure formation takes place in miniemulsion and is preserved during polymerization. For surfactants with carboxyl-head group a higher order of the alkyl chains and less lateral diffusion is found than for sulfat-head groups, suggesting a more uniform and stronger coordination to the metal ion. The stability of these bilayers depends on the temperature and the used surfactant which should be considered for the used polymerization temperature if a maximum output of the structured regions is wished.

The calcite(10-14) surface: a versatile substrate for molecular self-assembly

This thesis presents a detailed and successful study of molecular self-assembly on the calcite CaCO3(10-14) surface. One reason for the superior applicability of this particular surface is given by reflecting the well-known growth modes. Layer-by-layer growth, which is a necessity for the formation of templated two-dimensional (2D) molecular structures, is particularly favoured on substrates with a high surface energy. The CaCO3(10-14) surface is among those substrates and, thus, most promising. rnrnAll experiments in this thesis were performed using the non-contact atomic force microscope (NC-AFM) under ultra-high vacuum conditions. The acquisition of drift-free data became in this thesis possible owing to the herein newly developed atom-tracking system. This system features a lateral tip-positioning precision of at least 50pm. Furthermore, a newly developed scan protocol was implemented in this system, which allows for the acquisition of dense three-dimensional (3D) data under room-temperature conditions. An entire 3D data set from a CaCO3(10-14) surface consisting of 85x85x500 pixel is discussed. rnrnThe row-pairing and (2x1) reconstructions of the CaCO3(10-14) surface constitute most interesting research subjects. For both reconstructions, the NC-AFM imaging was classified to a total of 12 contrast modes. Eight of these modes were observed within this thesis, some of them for the first time. Together with literature findings, a total of 10 modes has been observed experimentally to this day. Some contrast modes presented themselves as highly distance-dependent and at least for one contrast mode, a severe tip-termination influence was found. rnrnMost interestingly, the row-pairing reconstruction was found to break a symmetry element of the CaCO3(10-14) surface. With the presence of this reconstruction, the calcite (10-14) surface becomes chiral. From high-resolution NC-AFM data, the identification of the enantiomers is here possible and is presented for one enantiomer in this thesis. rnrnFive studies of self-assembled molecular structures on calcite (10-14) surfaces are presented. Only for one system, namely HBC/CaCO3(10-14), the formation of a molecular bulk structure was observed. This well-known occurence of weak molecule-insulator interaction hinders the investigation of two-dimensional molecular self-assembly. It was, however, possible to force the formation of an island phase for this system upon following a variable-temperature preparation. rnFor the C60/CaCO3(10-14) system it is most notably that no branched island morphologies were found. Instead, the first C60 layer appeared to wet the calcite surface. rnrnIn all studies, the molecules arranged themselves in ordered superstructures. A templating effect due to the underlying calcite substrate was evident for all systems. This templating strikingly led either to the formation of large commensurate superstructures, such as (2x15) with a 14 molecule basis for the C60/CaCO3(10-14) system, or prevented the vast growth of incommensurate molecular motifs, such as the chicken-wire structure in the trimesic acid (TMA)/CaCO3(10-14) system. rnrnThe molecule-molecule and the molecule-substrate interaction was increased upon choosing molecules with carboxylic acid moieties in the third, fourth and fifth study, using terephthalic acid, TMA and helicene molecules. In all these experiments, hydrogen-bonded assemblies were created. rnrnDirected hydrogen bond formation combined with intermolecular pi-pi interaction is employed in the fifth study, where the formation of uni-directional molecular "wires" from single helicene molecules succeeded. Each "wire" is composed of heterochiral helicene pairs, well-aligned along the [01-10] substrate direction and stabilised by pi-pi interaction.

Intermolecular Clamping by Hydrogen Bonds: 2-Pyridone center dot NH3

A combined spectroscopic and ab initio theoretical study of the doubly hydrogen-bonded complex of 2-pyridone (2PY) with NH3 has been performed. The S-1 <- S-0 spectrum extends up to approximate to 1200 cm(-1) above the 0(0)(0) band, close to twice the range observed for 2PY. The S-1 state nonradiative decay for vibrations above approximate to 300 cm(-1) in the NH3 complex is dramatically slowed down relative to bare 2PY. Also, the Delta v=2,4,... overtone bands of the v(1)' and v(2)' out-of-plane vibrations that dominate the low-energy spectral region of 2PY are much weaker or missing for 2PY center dot NH3, which implies that the bridging (2PY)NH center dot center dot center dot NH3 and H2NH center dot center dot center dot O=C H-bonds clamp the 2PY at a planar geometry in the S-1 state. The mass-resolved UV vibronic spectra of jet-cooled 2PY center dot NH3 and its H/D mixed isotopomers are measured using two-color resonant two-photon ionization spectroscopy. The S-0 and S-1 equilibrium structures and normal-mode frequencies are calculated by density functional (B3LYP) and correlated ab initio methods (MP2 and approximate second-order coupled-cluster, CC2). The S-1 <- S-0 vibronic assignments are based on configuration interaction singles (CIS) and CC2 calculations. A doubly H-bonded bridged structure of C-S symmetry is predicted, in agreement with that of Held and Pratt [J. Am. Chem. Soc. 1993, 115, 9718]. While the B3LYP and MP2 calculated rotational constants are in very good agreement with experiment, the calculated H2NH center dot center dot center dot O=C H-bond distance is approximate to 0.7 angstrom shorter than that derived by Held and Pratt. On the other hand, this underlines their observation that ammonia can act as a strong H-bond donor when built into an H-bonded bridge. The CC2 calculations predict the H2NH center dot center dot center dot O distance to increase by 0.2 angstrom upon S-1 <- S-0 electronic excitation, while the (2PY)NH center dot center dot center dot NH3 H-bond remains nearly unchanged. Thus, the expansion of the doubly H-bonded bridge in the excited state is asymmetric and almost wholly due to the weakening of the interaction of ammonia with the keto acceptor group.

Modeling the Histidine–Phenylalanine Interaction: The NH···π Hydrogen Bond of Imidazole·Benzene

NH···π hydrogen bonds occur frequently between the amino acid side groups in proteins and peptides. Data-mining studies of protein crystals find that ~80% of the T-shaped histidine···aromatic contacts are CH···π, and only ~20% are NH···π interactions. We investigated the infrared (IR) and ultraviolet (UV) spectra of the supersonic-jet-cooled imidazole·benzene (Im·Bz) complex as a model for the NH···π interaction between histidine and phenylalanine. Ground- and excited-state dispersion-corrected density functional calculations and correlated methods (SCS-MP2 and SCS-CC2) predict that Im·Bz has a Cs-symmetric T-shaped minimum-energy structure with an NH···π hydrogen bond to the Bz ring; the NH bond is tilted 12° away from the Bz C₆ axis. IR depletion spectra support the T-shaped geometry: The NH stretch vibrational fundamental is red shifted by −73 cm⁻¹ relative to that of bare imidazole at 3518 cm⁻¹, indicating a moderately strong NH···π interaction. While the Sₒ(A1g) → S₁(B₂u) origin of benzene at 38 086 cm⁻¹ is forbidden in the gas phase, Im·Bz exhibits a moderately intense Sₒ → S₁ origin, which appears via the D₆h → Cs symmetry lowering of Bz by its interaction with imidazole. The NH···π ground-state hydrogen bond is strong, De=22.7 kJ/mol (1899 cm⁻¹). The combination of gas-phase UV and IR spectra confirms the theoretical predictions that the optimum Im·Bz geometry is T shaped and NH···π hydrogen bonded. We find no experimental evidence for a CH···π hydrogen-bonded ground-state isomer of Im·Bz. The optimum NH···π geometry of the Im·Bz complex is very different from the majority of the histidine·aromatic contact geometries found in protein database analyses, implying that the CH···π contacts observed in these searches do not arise from favorable binding interactions but merely from protein side-chain folding and crystal-packing constraints. The UV and IR spectra of the imidazole·(benzene)₂ cluster are observed via fragmentation into the Im·Bz+ mass channel. The spectra of Im·Bz and Im·Bz₂ are cleanly separable by IR hole burning. The UV spectrum of Im·Bz₂ exhibits two 000 bands corresponding to the Sₒ → S₁ excitations of the two inequivalent benzenes, which are symmetrically shifted by −86/+88 cm⁻¹ relative to the 000 band of benzene.

Materials properties from electron density distributions: Molecular magnetism and linear optical properties

The general goal of this thesis is correlating observable properties of organic and metal-organic materials with their ground-state electron density distribution. In a long-term view, we expect to develop empirical or semi-empirical approaches to predict materials properties from the electron density of their building blocks, thus allowing to rationally engineering molecular materials from their constituent subunits, such as their functional groups. In particular, we have focused on linear optical properties of naturally occurring amino acids and their organic and metal-organic derivatives, and on magnetic properties of metal-organic frameworks. For analysing the optical properties and the magnetic behaviour of the molecular or sub-molecular building blocks in materials, we mostly used the more traditional QTAIM partitioning scheme of the molecular or crystalline electron densities, however, we have also investigated a new approach, namely, X-ray Constrained Extremely Localized Molecular Orbitals (XC-ELMO), that can be used in future to extracted the electron densities of crystal subunits. With the purpose of rationally engineering linear optical materials, we have calculated atomic and functional group polarizabilities of amino acid molecules, their hydrogen-bonded aggregates and their metal-organic frameworks. This has enabled the identification of the most efficient functional groups, able to build-up larger electric susceptibilities in crystals, as well as the quantification of the role played by intermolecular interactions and coordinative bonds on modifying the polarizability of the isolated building blocks. Furthermore, we analysed the dependence of the polarizabilities on the one-electron basis set and the many-electron Hamiltonian. This is useful for selecting the most efficient level of theory to estimate susceptibilities of molecular-based materials. With the purpose of rationally design molecular magnetic materials, we have investigated the electron density distributions and the magnetism of two copper(II) pyrazine nitrate metal-organic polymers. High-resolution X-ray diffraction and DFT calculations were used to characterize the magnetic exchange pathways and to establish relationships between the electron densities and the exchange-coupling constants. Moreover, molecular orbital and spin-density analyses were employed to understand the role of different magnetic exchange mechanisms in determining the bulk magnetic behaviour of these materials. As anticipated, we have finally investigated a modified version of the X-ray constrained wavefunction technique, XC-ELMOs, that is not only a useful tool for determination and analysis of experimental electron densities, but also enables one to derive transferable molecular orbitals strictly localized on atoms, bonds or functional groups. In future, we expect to use XC-ELMOs to predict materials properties of large systems, currently challenging to calculate from first-principles, such as macromolecules or polymers. Here, we point out advantages, needs and pitfalls of the technique. This work fulfils, at least partially, the prerequisites to understand materials properties of organic and metal-organic materials from the perspective of the electron density distribution of their building blocks. Empirical or semi-empirical evaluation of optical or magnetic properties from a preconceived assembling of building blocks could be extremely important for rationally design new materials, a field where accurate but expensive first-principles calculations are generally not used. This research could impact the community in the fields of crystal engineering, supramolecular chemistry and, of course, electron density analysis.

Hydrogen generation from liquid phase catalytic reforming of sugar solutions using metal-supported catalysts

Most of the hydrogen production processes are designed for large-scale industrial uses and are not suitable for a compact hydrogen device to be used in systems like solid polymer fuel cells. Integrating the reaction step, the gas purification and the heat supply can lead to small-scale hydrogen production systems. The aim of this research is to study the influence of several reaction parameters on hydrogen production using liquid phase reforming of sugar solution over Pt, Pd, and Ni supported on nanostructured supports. It was found that the desired catalytic pathway for H-2 production involves cleavage of C-C, C-H and O-H bonds that adsorb on the catalyst surface. Thus a good catalyst for production of H2 by liquid-phase reforming must facilitate C-C bond cleavage and promote removal of adsorbed CO species by the water-gas shift reaction, but the catalyst must not facilitate C-O bond cleavage and hydrogenation of CO or CO2. Apart from studying various catalysts, a commercial Pt/gamma-alumina catalyst was used to study the effect of temperature at three different temperatures of 458, 473 and 493 K. Some of the spent catalysts were characterised using TGA, SEM and XRD to study coke deposition. The amorphous and organised form of coke was found on the surface of the catalyst. (C) 2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.

Transfert ultrarapide d’électron et transfert modéré d’énergie au sein d’assemblages supramoléculaires de colorants et d’un cluster de palladium

Résumé : Les transferts d’électrons photo-induits et d’énergie jouent un rôle primordial dans un grand nombre de processus photochimiques et photobiologiques, comme la respiration ou la photosynthèse. Une très grande quantité de systèmes à liaisons covalentes ont été conçus pour copier ces processus de transferts. Cependant, les progrès sont, en grande partie, limités par les difficultés rencontrées dans la synthèse de nouveaux couples de types donneurs-accepteurs. Récemment, des espèces utilisant des liaisons non-covalentes, comme les liaisons hydrogènes, les interactions [pi]-[pi], les liaisons de coordination métal-ligands ou encore les interactions électrostatiques sont le centre d’un nouvel intérêt du fait qu’ils soient plus faciles à synthétiser et à gérer pour obtenir des comportements de transferts d’électrons ou d’énergie plus flexibles et sélectifs. C’est dans cette optique que le travail de cette thèse a été mené, i.e. de concevoir des composés auto-assemblés avec des porphyrines et un cluster de palladium pour l’étude des transferts d’électrons photo-induits et d’énergie. Cette thèse se divise en quatre parties principales. Dans la première section, le chapitre 3, deux colorants porphyriniques, soit le 5-(4-carboxylphényl)-10, 15, 20-tristolyl(porphyrinato)zinc(II) (MCP, avec Na+ comme contre-ion) et 5, 15-bis(4-carboxylphényl)-15, 20-bistolyl(porphyrinato)zinc(II) (DCP, avec Na+ comme contre-ion) ont été utilisés comme donneurs d’électrons, et le [Pd3(dppm)3(CO)]2+ ([Pd32+], dppm = (Ph2P)2CH2, PF6‾ est le contre-ion) a été choisi comme accepteur d’électrons. La structure de l’assemblage [Pd32+]•••porphyrine a été élucidée par l’optimisation des géométries à l’aide de calculs DFT. La spectroscopie d’absorption transitoire (TAS) montre la vitesse de transferts d’électrons la plus rapide (< 85 fs, temps inférieurs à la limite de détection) jamais enregistrée pour ce type de système (porphyrine-accepteur auto-assemblés). Généralement, ces processus sont de l’ordre de l’échelle de la ps-ns. Cette vitesse est comparable aux plus rapides transferts d’électrons rapportés dans le cas de systèmes covalents de type porphyrine-accepteur rapide (< 85 fs, temps inférieurs à la limite de détection). Ce transfert d’électrons ultra-rapide (ket > 1.2 × 1013 s-1) se produit à l’état énergétique S1 des colorants dans une structure liée directement par des interactions ioniques, ce qui indique qu’il n’est pas nécessaire d’avoir de forts liens ou une géométrie courbée entre le donneur et l’accepteur. Dans une deuxième section, au chapitre 4, nous avons étudié en profondeur l’effet de l’utilisation de porphyrines à systèmes π-étendus sur le comportement des transferts d’électrons. Le colorant 9, 18, 27, 36-tétrakis-meso-(4-carboxyphényl)tétrabenzoporphyrinatozinc(II) (TCPBP, avec Na+ comme contre-ion) a été sélectionné comme candidat, et le 5, 10, 15, 20-tétrakis-meso-(4-carboxyphényl)porphyrineatozinc(II) (TCPP, avec Na+ comme contre-ion) a aussi été utilisé à des fins de comparaisons. TCPBP et TCPP ont, tous deux, été utilisés comme donneurs d’électrons pour fabriquer des assemblages supramoléculaires avec le cluster [Pd32+] comme accepteur d’électrons. Les calculs DFT ont été réalisés pour expliquer les structures de ces assemblages. Dans les conditions expérimentales, ces assemblages sont composés principalement d’une porphyrine avec 4 équivalents de clusters. Ces systèmes ont aussi été investigués par des mesures de quenching (perte de luminescence), par électrochimie et par d’autres techniques. Les transferts d’électrons (< 85 fs; temps inférieurs à la limite de détection) étaient aussi observés, de façon similaire aux assemblages MCP•••[Pd32+] et [Pd32+]•••DCP•••[Pd32+]. Les résultats nous indiquent que la modification de la structure de la porphyrine vers la tétrabenzoporphyrine ne semble pas influencer le comportement des cinétiques de transferts d’électrons (aller ou retour). Dans la troisième section, le chapitre 5, nous avons synthétisé la porphyrine hautement [pi]-conjuguée: 9, 18, 27, 36-tétra-(4-carboxyphényléthynyl)tétrabenzoporphyrinatozinc(II) (TCPEBP, avec Na+ comme contre-ion) par des fonctionnalisations en positions meso- et β, β-, qui présente un déplacement vers le rouge de la bande de Soret et des bandes Q. TCPEBP était utilisé comme donneur d’électrons pour fabriquer des motifs supramoléculaires avec le [Pd32+] comme accepteur d’électrons. Des expériences en parallèle ont été menées en utilisant la 5, 10, 15, 20-tétra-(4-carboxyphényl)éthynylporphyrinatozinc(II) (TCPEP, avec Na+ comme contre-ion). Des calculs DFT et TDDFT ont été réalisés pour de nouveau déterminer de façon théorique les structures de ces systèmes. Les constantes d’association pour les assemblages TCPEBP•••[Pd32+]x sont les plus élevées parmi tous les assemblages entre des porphyrines et le cluster de palladium rencontrés dans la littérature. La TAS a montré, encore une fois, des processus de transferts d’électrons dans des échelles de l’ordre de 75-110 fs. Cependant, les transferts de retour d’électrons sont aussi très rapides (< 1 ps), ce qui est un obstacle potentiel pour des applications en cellules solaires à pigment photosensible (DSSCs). Dans la quatrième section, le chapitre 6, les transferts d’énergie triplets (TET) ont été étudiés pour les assemblages MCP•••[Pd32+] et [Pd32+]•••DCP•••[Pd32+]. Les analyses spectrales des états transitoires dans l’échelle de temps de la ns-[mu]s démontrent de façon évidente les TETs; ceux-ci présentent des transferts d’énergie lents et/ou des vitesses moyennes pour des transferts d’énergie T1-T1 (3dye*•••[Pd32+] → dye•••3[Pd32+]*) opérant à travers exclusivement le mécanisme de Förster avec des valeurs de kET autour de ~ 1 × 105 s-1 selon les mesures d’absorption transitoires à 298 K. Des forces motrices non-favorables rendent ces types de processus non-opérants ou très lents dans les états T1. L’état T1 de [Pd32+] (~8190 cm-1) a été qualitativement déterminé par DFT et par la mise en évidence de l’émission S0 ← Tn retardée à 680-700 nm provenant de l’annihilation T1-T1, ce qui fait que ce cluster peut potentiellement agir comme un donneur à partir de ses états Tn, et accepteur à partir de T1 à l’intérieur de ces assemblages. Des pertes d’intensités de types statiques pour la phosphorescence dans le proche-IR sont observées à 785 nm. Ce travail démontre une efficacité modérée des colorants à base de porphyrines pour être impliquée dans des TETs avec des fragments organométalliques, et ce, même attachées grâce à des interactions ioniques. En conclusion, les assemblages ioniques à base de porphyrines et de clusters de palladium présentent des propriétés de transferts d’électrons S1 ultra-rapides, et des transferts d’énergie T1 de vitesses modérées, ce qui est utile pour de possibles applications comme outils optoélectroniques. D’autres études, plus en profondeur, sont présentement en progrès.

Analyzing the n→π* Electronic Transition of Formaldehyde in Water. A Sequential Monte Carlo/Time-Dependent Density Functional Theory

The n→π* absorption transition of formaldehyde in water is analyzed using combined and sequential classical Monte Carlo (MC) simulations and quantum mechanics (QM) calculations. MC simulations generate the liquid solute-solvent structures for subsequent QM calculations. Using time-dependent density functional theory in a localized set of gaussian basis functions (TD-DFT/6-311++G(d,p)) calculations are made on statistically relevant configurations to obtain the average solvatochromic shift. All results presented here use the electrostatic embedding of the solvent. The statistically converged average result obtained of 2300 cm-1 is compared to previous theoretical results available. Analysis is made of the effective dipole moment of the hydrogen-bonded shell and how it could be held responsible for the polarization of the solvent molecules in the outer solvation shells.

Solvation simplified

The effects of solvents on chemical phenomena is complex because there are various solute-solvent interaction mechanisms. Solvatochromism refers to the effects of solvents on the spectra of probes. The study of this phenomenon sheds light on the relative importance of the solvation mechanisms. Solvation in pure solvents is quantitatively analyzed in terms of a multi-parameter equation. In binary solvent mixtures, solvation is analyzed by considering the organic solvent, S, water, W, and a 1:1 hydrogen bonded species (S-W). The applications of solvatochromism to understand distinct chemical phenomena, reactivity and swelling of cellulose, is briefly discussed.

Electron collisions with the CH(2)O-H(2)O complex

We report cross sections for elastic collisions of low-energy electrons with the CH(2)O-H(2)O complex. We employed the Schwinger multichannel method with pseudopotentials in the static-exchange and in the static-exchange-polarization approximations for energies from 0.1 to 20 eV. We considered four different hydrogen-bonded structures for the complex that were generated by classical Monte Carlo simulations. Our aim is to investigate the effect of the water molecule on the pi* shape resonance of formaldehyde. Previous studies reported a pi* shape resonance for CH(2)O at around 1 eV. The resonance positions of the complexes appear at lower energies in all cases due to the mutual polarization between the two molecules. This indicates that the presence of water may favor dissociation by electron impact and may lead to an important effect on strand breaking in wet DNA by electron impact.