: The unpolarized absorption and circular dichroism spectra of the fundamental vibrational transitions of the chiral molecule, 4-methyl-2-oxetanone, are calculated ab initio. Harmonic force fields are obtained using Density Functional Theory (DFT), MP2, and SCF methodologies and a 5S4P2D/3S2P (TZ2P) basis set. DFT calculations use the Local Spin Density Approximation (LSDA), BLYP, and Becke3LYP (B3LYP) density functionals. Mid-IR spectra predicted using LSDA, BLYP, and B3LYP force fields are of significantly different quality, the B3LYP force field yielding spectra in clearly superior, and overall excellent, agreement with experiment. The MP2 force field yields spectra in slightly worse agreement with experiment than the B3LYP force field. The SCF force field yields spectra in poor agreement with experiment.The basis set dependence of B3LYP force fields is also explored: the 6-31G* and TZ2P basis sets give very similar results while the 3-21G basis set yields spectra in substantially worse agreements with experiment. jg

Ab initio calculation of vibrational absorption and circular dichroism spectra using density functional force fields

SIMPSON: a general simulation program for solid-state NMR spectroscopy.

Cation-π interaction: its role and relevance in chemistry, biology, and material science.

Introduction: Branched macromolecules or dendrimers have provided a rich seam of research in terms of both innovative chemistry and applications.1-14 For example, dendrimers have been studied for use as low-dielectric materials,15 as templates for the growth of single-wall carbon nanotubes,16 as catalysts,17-19 and in biological applications,20-23 including biosensors,24 magnetic resonance imaging,25-28 and drug delivery.29-33 However, it has only been more recently that such macromolecular structures have been explored in terms of their electronic and optoelectronic properties, which is the focus of this series of reviews. For example, charge-transporting dendrimers have become an important class of organic semiconducting material34 and significant effort has focused on light harvesting and energy transfer from a peripheral dye or chromophore to an emissive dye at the center or focus of the dendrimer.35-39 Organic semiconductors have become increasingly important as the active component in applications including organic light-emitting diodes (OLEDs),40-42 transistors,43,44 photovoltaic (PV) cells,45,46 optical amplifiers,47,48 and lasers.49-51 Traditionally, organic semiconductors have fallen into two main classes, small molecules and polymers, and these materials and their applications will be covered in detail by other authors. Small molecules are generally processed by evaporation techniques and have the advantages that the structure−property relationships are relatively simple to understand, the materials are mono(disperse), and they are deposited in a pure form. On the other hand, conjugated polymers are soluble and can be deposited from solution by processes such as spin-coating and ink-jet printing, which opens up the exciting prospect of simple, fast, large-area, low-temperature device manufacturing. An additional advantage for conjugated polymers is that solution processing is potentially less wasteful of material than evaporation for devices that require patterning. However, it is often difficult to control the polydispersity, molecular weight, backbone defects, and end groups of conjugated polymers reproducibly. Branched macromolecules, known as dendrimers, also have the advantage of being solution processable but by careful design can incorporate the control over the optoelectronic properties that is reminiscent of small molecules. In addition, the dendritic architecture provides a number of other attractive properties, including the ability to independently control the processing and optoelectronic properties; providing the processing power to enable simple chromophores to be deposited as stable amorphous films; dendrimer generation as a tool for controlling the intermolecular interactions that govern device performance; and the ability in well-defined dendrimers to have high chemical purity. In this review, we will focus on synthetic strategies that have been investigated for the preparation of optoelectronically active solution-processable dendritic materials and concentrate on two different applications, namely OLEDs and solar cells, in which they have been used. In the context of OLEDs, we limit the discussion to light emission, as branched macromolecules for charge transport will be discussed in the review by Shirota. We will also briefly comment on other recent light-emitting and -absorbing branched molecular materials that have been used in OLEDs and solar cells.

Development of Dendrimers: Macromolecules for Use in Organic Light-Emitting Diodes and Solar Cells

In this review, we give an overview of recent literature on the structure and stability of unimolecular G-rich quadruplex structures that are relevant to drug design and for in vivo function The unifying theme in this review is energetics The thermodynamic stability of quadruplexes has not been studied in the same detail as DNA and RNA duplexes, and there are important differences in the balance of forces between these classes of folded oligonucleotides We provide an overview of the principles of stability and where available the experimental data that report on these principles Significant gaps in the literature have been identified, that should be filled by a systematic study of well-defined quadruplexes not only to provide the basic understanding of stability both for design purposes, but also as it relates to in vivo occurrence of quadruplexes Techniques that are commonly applied to the determination of the structure, stability and folding are discussed in terms of information content and limitations Quadruplex structures fold and unfold comparatively slowly, and DNA unwinding events associated with transcription and replication may be operating far from equilibrium The kinetics of formation and resolution of quadruplexes, and methodologies are discussed in the context of stability and their possible biological occurrence

Stability and kinetics of G-quadruplex structures

Since the first report of thiol-based self-assembled monolayers (SAMs) 30 years ago, these structures have been examined in a huge variety of applications. The oxidative and thermal instabilities of these systems are widely known, however, and are an impediment to their widespread commercial use. Here, we describe the generation of N-heterocyclic carbene (NHC)-based SAMs on gold that demonstrate considerably greater resistance to heat and chemical reagents than the thiol-based counterparts. This increased stability is related to the increased strength of the gold–carbon bond relative to that of a gold–sulfur bond, and to a different mode of bonding in the case of the carbene ligand. Once bound to gold, NHCs are not displaced by thiols or thioethers, and are stable to high temperatures, boiling water, organic solvents, pH extremes, electrochemical cycling above 0 V and 1% hydrogen peroxide. In particular, benzimidazole-derived carbenes provide films with the highest stabilities and evidence of short-range molecular ordering. Chemical derivatization can be employed to adjust the surface properties of NHC-based SAMs. Thiol-based self-assembled monolayers have a huge variety of potential applications but are hampered by oxidative and thermal instability. Now, N-heterocyclic carbenes are shown to form densely packed layers on Au(111) that are considerably more stable than thiol films, resisting hot organic solvents, acid, base, oxidant and oxidative electrohemical etching.

/pdf/ultra-stable-self-assembled-monolayers-of-n-heterocyclic-ge1dlkvrdm.pdf

Ultra stable self-assembled monolayers of N-heterocyclic carbenes on gold

We describe a new approach for observation of multiple-quantum (MQ) NMR spectra of S = 3/2 nuclei with magic-angle spinning (MAS). The new method employs the Rotation-Induced Adiabatic Coherence Transfer (RIACT) that occurs between triple-quantum (3Q) and central-transition (1Q) coherences in S = 3/2 systems. In contrast to currently available coherence-transfer techniques, RIACT is relatively insensitive to the magnitude of the quadrupole interaction for e2qQ/h ≤ 4 MHz for both 3Q excitation and 3Q-to-1Q conversion. Thus, RIACT provides a means of extracting quantitative information about site populations from isotropic MQ NMR spectra. We illustrate the utility of the approach with 23Na (S = 3/2) MQ NMR spectra of a series of sodium salts exhibiting crystallographically distinct sites. The spectra provide quantitative measurements of quadrupolar parameters, chemical shifts, and relative site populations for each of the crystallographically distinct sodium sites.

Quantitative Multiple-Quantum Magic-Angle-Spinning NMR Spectroscopy of Quadrupolar Nuclei in Solids

We report a solid-state multinuclear (23Na, 15N, 13C, and 31P) NMR study on the relative affinity of monovalent cations for a stacking G-quartet structure formed by guanosine 5‘-monophosphate (5‘-GMP) self-association at pH 8. Two major types of cations are bound to the 5‘-GMP structure:  one at the surface and the other within the channel cavity between two G-quartets. The channel cation is coordinated to eight carbonyl oxygen atoms from the guanine bases, whereas the surface cation is close to the phosphate group and likely to be only partially hydrated. On the basis of solid-state 23Na NMR results from a series of ion titration experiments, we have obtained quantitative thermodynamic parameters concerning the relative cation binding affinity for each of the two major binding sites. For the channel cavity site, the values of the free energy difference (ΔG° at 25 °C) for ion competition between M+ and Na+ ions are K+ (−1.9 kcal mol-1), NH4+ (−1.8 kcal mol-1), Rb+ (−0.3 kcal mol-1), and Cs+ (1.8 kcal mol-...

Selective binding of monovalent cations to the stacking G-quartet structure formed by guanosine 5'-monophosphate: a solid-state NMR study.

We report a combined NMR and dynamic light scattering (DLS) study on the size of supramolecular structures formed by disodium guanosine 5‘-monophosphate, Na2(5‘-GMP), at pH 8. In general, two distinct types of aggregate species are present in an aqueous solution of Na2(5‘-GMP). One type consists of stacking 5‘-GMP monomers, and the other contains stacking G-quartets. Both types of aggregates can be modeled as rodlike cylinders. The cylinder diameter is 10 and 26 A for monomer aggregates and quartet aggregates, respectively. For Na2(5‘-GMP) concentrations between 18 and 34 wt %, the cylinders formed by stacking G-quartets have an average length between 8 and 30 nm, corresponding to a stack of ∼24−87 G-quartets. These nanoscale aggregates are significantly larger than what had previously been believed for Na2(5‘-GMP) self-association at pH 8. The length of both types of 5‘-GMP aggregates was found to increase with Na2(5‘-GMP) concentration but was insensitive to the added NaCl in solution. While the aggrega...

Disodium guanosine 5'-monophosphate self-associates into nanoscale cylinders at pH 8: a combined diffusion NMR spectroscopy and dynamic light scattering study.

We report a systematic solid-state (17)O NMR study of free nucleic acid bases: thymine (T), uracil (U), cytosine (C), and guanine (G). Site-specifically (17)O-enriched samples were synthesized:[2-(17)O]thymine (1), [4-(17)O]thymine (2), [2-(17)O]uracil (3), [4-(17)O]uracil (4), [2-(17)O]cytosine (5), and [6-(17)O]guanine monohydrate (6). Magic-angle-spinning (MAS) and static (17)O NMR spectra were acquired at 11.75 T for compounds 1-6, from which information about the (17)O chemical shift and electric field gradient tensors was obtained. Extensive quantum chemical calculations were performed at the B3LYP/6-311++G(d,p) level of theory for (17)O NMR properties in various molecular models. The calculated (17)O NMR tensors are highly sensitive to the description of intermolecular hydrogen-bonding interactions at the target oxygen atom. A reasonably good agreement between experimental solid-state (17)O NMR data and B3LYP/6-311++G(d,p) calculations is achievable only in molecular cluster models where a complete hydrogen-bond network is considered. Using this theoretical approach, we also investigated the (17)O NMR tensors in two unusual structures: guanine- and uracil-quartets.

Gang Wu

Papers

Ultra stable self-assembled monolayers of N-heterocyclic carbenes on gold

Quantitative Multiple-Quantum Magic-Angle-Spinning NMR Spectroscopy of Quadrupolar Nuclei in Solids

Selective binding of monovalent cations to the stacking G-quartet structure formed by guanosine 5'-monophosphate: a solid-state NMR study.

Disodium guanosine 5'-monophosphate self-associates into nanoscale cylinders at pH 8: a combined diffusion NMR spectroscopy and dynamic light scattering study.

A Solid-State 17O Nuclear Magnetic Resonance Study of Nucleic Acid Bases