Effect of convection and B1 inhomogeneity on singlet relaxation experiments

Nuclear spin singlet lifetimes can often exceed the T₁ length scales by a large factor. This property makes them suitable for polarization storage. The measurement of such long lifetimes itself can become challenging due to the influence of even very weak relaxation mechanisms. Here we show that a judicious choice of the singlet-to-triplet conversion method is highly important in order to achieve reliable singlet relaxation measurements. In particular, we identify thermal convection, in connection with B₁ field gradients, asa significant apparent decay mechanism, which limits the ability to measure the true singlet state lifetimes. Highly B₁-compensated broadband singlet excitation/detection sequences are shown to minimize the influence of macroscopic molecular motion and B₁ inhomogeneity.

Correction: Singlet lifetime measurements in an all-proton chemically equivalent spin system by hyperpolarization and weak spin lock transfers

Correction for ‘Singlet lifetime measurements in an all-proton chemically equivalent spin system by hyperpolarization and weak spin lock transfers’ by Y. Zhang et al., Phys. Chem. Chem. Phys., 2015, 17, 24370–24375.

Singlet lifetime measurements in an all-proton chemically equivalent spin system by hyperpolarization and weak spin lock transfers

Hyperpolarized singlet states provide the opportunity for polarization storage over periods significantly longer than T1. Here, we show how the singlet state in a chemically equivalent proton spin system can be revealed by a weak power spin-lock. This procedure allowed the measurement of the lifetimes of the singlet state in protic solvents. The contributions of different intra- and intermolecular relaxation mechanisms to singlet lifetimes are investigated with this procedure.

Prospects for circular dichroism detection of nonracemic extraterrestrial organic molecules

The use of circular dichroism is examined as a probe for the detection of nonracemic analytes on spacecraft explorations such as a planned mission to Titan. Advances in both instrument design and chemistry will be required to make such a method practicable. The focus of this paper is on chemical agents able to amplify the circular dichroism signal, which would simplify instrument problems and increase sensitivity. A classic phenomenon known as the Pfeiffer effect provides a conceptual strategy to achieve the necessary aims for chirality detection. Recent examples of Pfeiffer-related phenomena are discussed.

Electronic control of helical chirality

Chiral phenomena are common in living systems. Despite the fact that development of materials has often been inspired by chemistry from the biological world, materials that take advantage of inherent chirality have found relatively few applications. It is therefore probable that much remains to be gained from novel applications of molecular, macromolecular and supramolecular chirality. Among the most intriguing recent advances in studies of chiral materials is the development of mechanisms to control the shape and properties of chiral molecules. Photo-induced helical chirality inversions have been studied for several years and significant achievements have been reported. Recently, electronically triggered systems have drawn significant attention. These technologies offer the potential for development of novel materials that take advantage of photonic or electronic modulation of molecular recognition, optical or mechanical properties.

pKa Values and Geometries of Secondary and Tertiary Amines Complexed to Boronic AcidsImplications for Sensor Design

[structure in text] The pK(a) values and the geometries of secondary and tertiary amines adjacent to boronic acids were determined using potentiometric and (11)B NMR titrations. The studies showed that the secondary ammonium ion has a pK(a) similar to that of the tertiary ammonium species, which leads to the formation of tetrahedral boron centers at pH values above approximately 5.5. Therefore, secondary amines as well as tertiary amines, when placed proximal to boron centers, can be used to create tetrahedral boronic acids at neutral pH for diol complexation.

Chiroptical switches and sensors based on ligand conformational changes in labile coordination complexes

A series of molecules have been developed that give significantly differentiated circular dichroic signals upon presentation of a metal ion, exchange of metal ion, or change in oxidation state. These properties have led to prototypes of molecular sensors for metal ions and redox-mediated chiroptical switches. In some complexes, the CD signal arises by an excitonic coupling mechanism, affording further information about ligand conformation or absolute configuration.

Supramolecular detection of metal ion binding: ligand conformational control of cholesteric induction in nematic liquid crystalline phases

Tripodal tetradentate ligands may act as chemosensor molecules. Their ability to torque a nematic into a cholesteric phase increases upon complexation with copper ion. Moreover, changes in overall shape of the complexes induced by different metals and counter ions were transferred sensitively to the supramolecular level, observed by proportionate changes in the degree of twisting. Modification of the oxidation state of the metal center also gave large changes in twisting power; this suggests potential application in electrochemical molecular switches. The handedness of the induced cholesteric phase is related to the stereochemistry of the ligand: The small amount of chiral dopant needed for the LC technique (less than 2 nmol) suggests the possible determination of the absolute configuration of the parent primary amines of the ligands.

Electron-induced inversion of helical chirality in copper complexes of N,N-dialkylmethionines

Stereodynamic complexes of copper were found to undergo inversion of a helical chiral element upon oxidation or reduction. The amino acid methionine was derivatized by the attachment of two chromophores to the nitrogen atom. The resultant ligands formed stable complexes with Cu(I) and Cu(II) salts. For a derivative of a given absolute chirality, the complexes afford nearly mirror image circular dichroism spectra. The spectral changes originate from reorientation of the nitrogen-attached chromophores due to a conformation interconversion driven by the exchange of a carboxylate for a sulfide ligand. The electrically induced chirality inversion coupled with strong interactions with polarized light is unique and may lead to novel chiral molecular devices.

Absolute configurations of N,N-dialkyl alpha-amino acids and beta-amino alcohols from exciton-coupled circular dichroism spectra of Cu(II) complexes

[formula: see text] A circular dichroism technique has been applied to the absolute configurational assignment of acyclic alpha-amino acids and beta-amino alcohols with single stereogenic centers via a one-step derivitization procedure requiring microgram quantities of material. Metal ions fix the geometrical relationship of two chromophores, affording CD spectra that agree with theory on the basis of the expected conformation of the ligands in the metal complexes.

A tetrahedral zinc(II) complex of tris(2-pyridylethyl)amine

The zinc ion in the title complex, [N,N'-bis(2-pyridylethyl)-2-(2-pyridyl)ethylamine-kappa 4N]zinc(II) diperchlorate, [Zn(C21H24N4)](ClO4)2, displays a distorted tetrahedral configuration. The Zn-Npy distances range from 1.979 (5) to 1.999 (5) A, while the Zn-Namine distance is 2.028 (5) A. The Npy-Zn-Npy and Npy-Zn-Namine angles range from 111.9 (2) to 119.6 (2) degrees and from 102.3 (2) to 103.4 (2) degrees, respectively. This behavior contrasts with the five-coordinate geometry normally observed in tripodal ligands of similar structure, and can be ascribed to the presence of weakly-coordinating anions and the ligand-metal chelate-ring size.

Absolute configurational assignment of self-organizing asymmetric tripodal ligand-metal complexes

The solution configuration of labile coordination complexes may be difficult to determine, even in cases in which the solid state structure is known. We have previously synthesized a series of chiral ligands which form pseudo-C3-symmetric complexes with ZnII and CuII salts that possess an available electrophilic coordination site. Molecular modeling of ZnII complexes of the chiral ligand N,N-bis[(2-quinolyl)methyl]-1-(2-pyridyl)ethanamine (alpha-MeBQPA) showed that the spatial arrangement of the heterocyclic arms is controlled by a substituent on one methylene arm, resulting in the adoption of an enantiomeric conformation displaying a propeller-like asymmetry. In this paper we report the application of the exciton chirality method to the determination of the conformation of asymmetric metal-ligand complexes in solution. There is a good correlation between the predicted and the observed Cotton effects, demonstrating that the geometry in solution closely resembles that predicted by computational simulations and those obtained by X-ray crystallographic studies of metal complexes with racemic and enantiomerically pure ligands. The X-ray crystallographic structure of the first optically pure complex in this series is reported.