M-F interactions and heterobimetallics: furthering the understanding of heterobimetallic stabilization.

Heterobimetallic complexes containing alkali, alkaline-earth, and divalent europium metals utilizing the perfluoro-tert-butoxide (PFTB) ligand following the general formula, [AM(PFTB)3(co-ligand)x] (A=Na, K; M=Mg, Sr, Ba, Eu; co-ligand=THF, toluene), have been isolated. These compounds sublime at low temperatures with low residual weight indicating their potential as metal-organic chemical-vapor deposition (MOCVD) precursors. The complexes have unique molecular architectures that are strongly influenced by M-F interactions, as was verified in the solid state by using X-ray crystallography. The significance of these interactions were further reinforced by bond-valence sums analysis and (19) F VT-NMR spectroscopy, in which rotational energies of 18.75 and 19.08 kcal mol(-1) were measured.

Terahertz spectroscopy and solid-state density functional theory simulations of the improvised explosive oxidizers potassium nitrate and ammonium nitrate.

Terahertz spectroscopy provides a noninvasive and nondestructive method for detecting and identifying concealed explosives. In this work, the room-temperature and cryogenic terahertz spectra of two common improvised explosive oxidizers, namely, potassium nitrate (KN) and ammonium nitrate (AN), are presented, along with detailed solid-state density functional theory (DFT) analyses of the crystalline structures and spectral features. At both 294 and 78 K, KN exhibits two terahertz absorption features below 100 cm(-1) that have been assigned through DFT simulations to arise from hindered nitrate rotations in the KN-II crystalline polymorph. The terahertz spectrum of AN exhibits a pronounced temperature dependence. The 294 K spectrum is free of any absorptions, whereas the 78 K spectrum consists of several narrow and intense peaks. The origin of this large difference is the polymorphic transition that occurs during cooling of AN, where room-temperature AN-IV is converted to AN-V at 255 K. The 78 K terahertz spectrum of AN is assigned here to various ion rotations and translations in the AN-V polymorph lattice. The analysis of the room-temperature AN-IV terahertz spectrum proved to be more complicated. The solid-state DFT simulations predicted that the room-temperature crystal structure of AN is not very well described using the standard Pmmn space-group symmetry as previously believed. The AN-IV polymorph actually belongs to the Pmn2(1) space group, and the perceived Pmmn symmetry results from vibrational averaging through nitrate rotations. This newly observed Pmn2(1) crystal symmetry for room-temperature AN is the reason for the absence of absorption features in the 294 K terahertz spectrum of AN and provides new insight into the polymorphic transitions of this ionic solid.

Identification and quantification of polymorphism in the pharmaceutical compound diclofenac acid by terahertz spectroscopy and solid-state density functional theory.

Polymorph detection and quantification in crystalline materials is a principle interest of the pharmaceutical industry. Terahertz (THz) spectroscopy can be used for such analytical applications since this technique is sensitive to the intermolecular interactions of molecules in the solid state. Understanding the fundamental nature of the lattice vibrational motions leading to absorptions in THz spectra is challenging, but may be achieved through computational approaches. In this study, the THz spectra of two diclofenac acid polymorphs were obtained by THz spectroscopy, and the vibrational characters of the observed absorptions were analyzed using solid-state density functional theory (DFT). The results demonstrate the quantitative capacity of THz spectroscopy and the reliability and utility of solid-state DFT in the calculation of low-frequency vibrational motions.

Application of London-type dispersion corrections to the solid-state density functional theory simulation of the terahertz spectra of crystalline pharmaceuticals.

The effects of applying an empirical dispersion correction to solid-state density functional theory methods were evaluated in the simulation of the crystal structure and low-frequency (10 to 90 cm(-1)) terahertz spectrum of the non-steroidal anti-inflammatory drug, naproxen. The naproxen molecular crystal is bound largely by weak London force interactions, as well as by more prominent interactions such as hydrogen bonding, and thus serves as a good model for the assessment of the pair-wise dispersion correction term in systems influenced by intermolecular interactions of various strengths. Modifications to the dispersion parameters were tested in both fully optimized unit cell dimensions and those determined by X-ray crystallography, with subsequent simulations of the THz spectrum being performed. Use of the unmodified PBE density functional leads to an unrealistic expansion of the unit cell volume and the poor representation of the THz spectrum. Inclusion of a modified dispersion correction enabled a high-quality simulation of the THz spectrum and crystal structure of naproxen to be achieved without the need for artificially constraining the unit cell dimensions.

Understanding the terahertz spectra of crystalline pharmaceuticals: terahertz spectroscopy and solid-state density functional theory study of (S)-(+)-ibuprofen and (RS)-ibuprofen.

The potential applications of terahertz (THz) spectroscopy in the analysis of pharmaceutical products in their crystalline state have prompted the need for a more thorough understanding of the fundamental vibrational motions contributing to the THz spectra. The detection of variations in crystal structure and the reliable assignment of observed THz absorption features can be aided by the use of solid-state density functional theory (DFT). In this study, solid-state DFT with periodic boundary conditions was used to simulate the crystalline structure and assign the experimental THz spectra (10-90 cm(-1)) of the enantiomerically pure and racemic forms of the common pharmaceutical compound ibuprofen. The results clearly demonstrate the capabilities of DFT methodologies to accurately reproduce the THz spectra of large complicated molecular systems and provide insight into the internal and external vibrational motions that form the basis of THz spectroscopy.

Investigating the anharmonicity of lattice vibrations in water-containing molecular crystals through the terahertz spectroscopy of L-serine monohydrate.

The influence of cocrystallized H(2)O molecules on the terahertz (THz) spectra and corresponding computational treatment of hydrated molecular crystals was investigated in the study of protonated and deuterium-substituted l-serine.H(2)O. The THz spectra of both solids have been measured in the range of 10 to 90 cm(-1), with simulations of the crystalline structure and THz vibrational modes performed using solid-state density functional theory. Significant and systematic overestimations of the predicted vibrational frequencies were observed in all calculations. Evidence provided by the comparison of the experimental and calculated vibrational frequencies for both the protonated and deuterated l-serine.H(2)O solids indicates the presence of significant anharmonicity in the observed lattice vibrations. The results suggest that vibrational anharmonicity may play a much larger role in the interpretation of the THz spectra of hydrates in contrast to their corresponding anhydrous forms.

Highly volatile alkaline earth metal fluoroalkoxides.

The synthetic strategies, structural comparison, and thermal gravimetric analyses for a novel family of alkaline earth metal perfluoro-tert-butoxides (PFTB) are presented. The isolated complexes follow the general formula, [Ae(PFTB)(2)(d)(x)]; (1a, Ae = Mg, d = THF, x = 2; 1b, Ae = Ca, d = THF, x = 4; 1c, Ae = Sr, d = THF, x = 4; 1d, Ae = Ba, d = THF, x = 4; 2a, Ae = Mg, d = DME, x = 1; 2b, Ae = Ca, d = DME, x = 2; 2c, Ae = Sr, d = DME, x = 2; 2d, Ae = Ba, d = DME, x = 3; 3a, Ae = Mg, d = diglyme, x = 1; 3b, Ae = Ca, d = diglyme, x = 2; 3c, Ae = Sr, d = diglyme, x = 2; 3d, Ae = Ba, d = diglyme, x = 2; THF = tetrahydrofuran, DME = 1,2-dimethoxyethane) where the crystallographically characterized compounds display either a cis or trans conformations in octahedral or pseudo-octahedral environments. Alkane elimination (Mg), direct metalation via ammonia activation (Ca, Sr, Ba), and transamination (Ca, Sr, Ba) methods yielded the target compounds in moderate to good yields. Structural studies of select compounds showed all compounds to be monomeric with varying degrees of co-ligand coordination, possibly explaining some of the observed physical properties. Thermal gravimetric analyses of the compounds shows sublimation at low temperatures, associated with minimal residue, making this family of fluoroalkoxides promising candidates for metal organic chemical vapor deposition (MOCVD).

Synthesis and stabilization-advances in organoalkaline earth metal chemistry.

The last decade has seen an impressive growth in alkaline earth metal chemistry, with applications ranging from synthetic organic and polymer chemistry to materials science. As a consequence, alkaline earth metal chemistry has made a leap from obscurity into the spotlight of modern organometallic chemistry. Much of this rapid development was made possible by the establishment of novel synthetic procedures that allowed facile access to the target compounds, as many conventional synthetic routes posed and continue to pose significant limitations. Novel approaches have allowed the preparation of a multitude of compounds, initiating progress not thought possible just five years ago. Examination of the new compounds delineates several factors responsible for their structure and function. Key elements in the coordination, aggregation behavior, and reactivity of these systems have been linked to secondary interactions, including M-Cpi, M-Npi, M-F, and M-H(agostic) interactions. This feature article will provide a very brief overview of established synthetic procedures, including a brief discussion on specific shortcomings. This will be followed by a detailed presentation of novel methodologies that are the core of the rapid development of alkaline earth metal chemistry. The second part of the article will be concerned with the analysis of various secondary interactions and their role in the further development of this rapidly emerging field of chemistry.

Synthesis of novel 2H,5H-Dihydrofuran-3-yl Ketones via ISNC reactions.

Unique 1-[2H,5H-dihydrofur-3-yl]ketones have been synthesized from propargylic nitroethers via intramolecular cycloadditions involving silyl nitronates. Various substituent groups were placed on the 2 and 5 positions of the dihydrofuran rings. We examined the scope of the long-range coupling in proton NMR of the oxo-dihydrofuran products. The identities of the diastereomers resulting from the Michael Addition/cycloaddition reactions were tentatively assigned for the first time. CAChe MNDO PM5 and CONFLEX programs were engaged to assist with the identification of these stereoisomers. The reaction times and conditions for these oxo-dihydrofurans were found to be different than that of the published dihydrofuranals, which led us to propose a different mechanism.