Comparing the Energetic and Dynamic Contributions of Solvent to Very Low Barrier Isomerization Using Dynamic Steady-State Vibrational Spectroscopy

Recruiting Unicellular Algae for the Mass Production of Nanostructured Perovskites

Functional capacities of lead halide perovskites are strongly dependent on their morphology, crystallographic texture, and internal ultrastructure on the nano- and the meso-scale. In the last decade, significant efforts are directed towards the development of novel synthesis routes that would overcome the morphological constraints provided by the physical and crystallographic properties of these materials. In contrast, various living organisms, such as unicellular algae, have the ability to mold biogenic crystals into a vast variety of intricate nano-architectured shapes while keeping their single crystalline nature. Here, using the cell wall of the dinoflagellate L. granifera as a model, sustainably harvested biogenic calcite is successfully transformed into nano-structured perovskites. Three variants of lead halide perovskites CH₃ NH₃ PbX₃ are generated with X = Cl^- , Br^- and I^- ; exhibiting emission peak-wavelength ranging from blue, to green, to near-infrared, respectively. The approach can be used for the mass production of nano-architectured perovskites with desired morphological, textural and, consequently, physical properties exploiting the numerous templates provided by calcite forming unicellular organisms.

Steric and electronic control of an ultrafast isomerization

Synthetic control of the influence of steric and electronic factors on the ultrafast (picosecond) isomerization of penta-coordinate ruthenium dithietene complexes (Ru((CF3)2C2S2)(CO)(L)2, where L = a monodentate phosphine ligand) is reported. Seven new ruthenium dithietene complexes were prepared and characterized by single crystal X-ray diffraction. The complexes are all square pyramidal and differ only in the axial vs. equatorial coordination of the carbonyl ligand. Fourier Transform Infrared (FTIR) spectroscopy was used to study the ν(CO) bandshapes of the complexes in solution, and these reveal rapid exchange between two or three isomers of each complex. Isomerization is proposed to follow a Berry psuedorotation-like mechanism where a metastable, trigonal bipyramidal (TBP) intermediate is observed spectroscopically. Electronic tuning of the phosphine ligands L = PPh3, P((p-Me)Ph)3, ((p-Cl)Ph)3, at constant cone angle is found to have little effect on the kinetics or thermodynamic stabilities of the axial, equatorial and TBP isomers of the differently substituted complexes. Steric tuning of the phosphine ligands over a range of phosphine cone angles (135 < θ < 165°) has a profound impact on the isomerization process, and in the limit of greatest steric bulk, the axial isomer is not observable. Temperature dependence of the FTIR spectra was used to obtain the relative thermodynamic stabilities of the different isomers of each of the seven ruthenium dithietene complexes. This study details how ligand steric effects can be used to direct the solution state dynamics on the picosecond time scale of discrete isomers energetically separated by <2.2 kcal mol-1. This work provides the most detailed description to date of ultrafast isomerization in the ground states of transition metal complexes.

Direct observation of the intermediate in an ultrafast isomerization

Using a combination of two-dimensional infrared (2D IR) and variable temperature Fourier transform infrared (FTIR) spectroscopies the rapid structural isomerization of a five-coordinate ruthenium complex is investigated. In methylene chloride, three exchanging isomers were observed: (1) square pyramidal equatorial, (1); (2) trigonal bipyramidal, (0); and (3) square pyramidal apical, (2). Exchange between 1 and 0 was found to be an endergonic process (ΔH = 0.84 (0.08) kcal mol^-1, ΔS = 0.6 (0.4) eu) with an isomerization time constant of 4.3 (1.5) picoseconds (ps, 10^-12 s). Exchange between 0 and 2 however was found to be exergonic (ΔH = -2.18 (0.06) kcal mol^-1, ΔS = -5.3 (0.3) eu) and rate limiting with an isomerization time constant of 6.3 (1.6) ps. The trigonal bipyramidal complex was found to be an intermediate, with an activation barrier of 2.2 (0.2) kcal mol^-1 and 2.4 (0.2) kcal mol^-1 relative to the equatorial and apical square pyramidal isomers respectively. This study provides direct validation of the mechanism of Berry pseudorotation - the pairwise exchange of ligands in a five-coordinate complex - a process that was first described over fifty years ago. This study also clearly demonstrates that the rate of pseudorotation approaches the frequency of molecular vibrations.

Infrared Dynamics of Iron Carbonyl Diene Complexes

The temperature dependence of the low-frequency C-O bands in the IR spectrum of [(η⁴-norbornadiene)Fe(CO)₃], reminiscent of signal coalescence in dynamic NMR, was interpreted by Grevels (in 1987) as chemical exchange due to very fast rotation of the diene group. Since then, there has been both support and objection to this interpretation. We discuss these various claims involving both one- and two-dimensional IR and, largely on the basis of new density functional theory calculations, furnish support for Grevels' original interpretation.

Experimental and theoretical studies on stability of new stabilizers for N-methyl-P-nitroaniline derivative in CMDB propellants

Although N-methyl-P-nitroaniline (MNA) was a quite effective stabilizer in composite modified double base (CMDB) propellants, it undergoes crystallization easily from nitroglycerin (NG) during storage. In order to improve its solubility in nitroglycerin (NG) and the stability in propellants, several new stabilizers including N-ethyl-p-nitroaniline (ENA), N-n-propyl-p-nitroaniline (n-PNA), N-i-propyl-p-nitroaniline (i-PNA), N-n-butyl-p-nitroaniline (n-BNA) and N-t-butyl-p-nitroaniline (t-BNA) were designed and synthesized to replace MNA by increasing the carbon chain length. The interaction between NG and different stabilizers was simulation by Materials Studio 5.5 and the stability and the high temperature stability performance of those new stabilizers in propellants were calculated by Gaussian 09. It was found that both the solubility of new stabilizers in NG and the stability and the high temperature stability performance of those in propellants were improved when the carbon chain length of substitution groups on nitrogen atom was increased. Thus, the n-BNA was a most potential stabilizer. Then all properties of the stabilizers were studied experimentally, which was agreement well with the theoretical analysis.

Reinterpretation of Dynamic Vibrational Spectroscopy to Determine the Molecular Structure and Dynamics of Ferrocene

Molecular distortion of dynamic molecules gives a clear signature in the vibrational spectra, which can be modeled to give estimates of the energy barrier and the sensitivity of the frequencies of the vibrational modes to the reaction coordinate. The reaction coordinate method (RCM) utilizes ab initio-calculated spectra of the molecule in its ground and transition states together with their relative energies to predict the temperature dependence of the vibrational spectra. DFT-calculated spectra of the eclipsed (D_5h ) and staggered (D_5d ) forms of ferrocene (Fc), and its deuterated analogue, within RCM explain the IR spectra of Fc in gas (350 K), solution (300 K), solid solution (7-300 K), and solid (7-300 K) states. In each case the D_5h rotamer is lowest in energy but with the barrier to interconversion between rotamers higher for solution-phase samples (ca. 6 kJ mol^-1 ) than for the gas-phase species (1-3 kJ mol^-1 ). The generality of the approach is demonstrated with application to tricarbonyl(η⁴ -norbornadiene)iron(0), Fe(NBD)(CO)₃ . The temperature-dependent coalescence of the ν(CO) bands of Fe(NBD)(CO)₃ is well explained by the RCM without recourse to NMR-like rapid exchange. The RCM establishes a clear link between the calculated ground and transition states of dynamic molecules and the temperature-dependence of their vibrational spectra.