A photochemical mechanism for homochirogenesis. Part 2.

A theoretical investigation of the photochemistry of racemic compounds with circularly polarized light was undertaken. The exact solutions of the differential equations by numerical integration to the approximate solutions used in an earlier article were compared. The exact solutions showed that sequential reactions yield enhanced optical activities in the products. For irreversible reactions, all enantiomeric excesses are lost if the reactions are carried to completion, but appreciable resolution occurs in many cases for partial conversion. For reversible reactions, significant enantiomeric excesses are found at the photostationary state.

Ultrastable superbase-derived protic ionic liquids.

Protic ionic liquids are synthesized via proton transfer from acids to organic bases. One of the key issues associated with conventional protic ionic liquids is the thermal instability resulting from temperature-induced decomposition via reverse proton transfer. This shortcoming significantly hampers the use of these protic ionic liquids in separations, electrochemical capacitors, fuel cells, and so forth. Herein we show that it is possible to prepare protic ionic liquids with thermal stabilities approaching those of common aprotic ionic liquids. Our new class of protic ionic liquids, derived via integrated neutralization and metathesis of superbasic phosphazenes or guanidines, exhibits exceptionally low vapor pressures at 150 degrees C while being stable to strong alkali agents such as aqueous KOH, suggesting potential in energy-related applications, including electrochemical capacitors and PEM-type fuel cells.

A photochemical mechanism for homochirogenesis.

Theoretical analysis of one-step and multiple-step photoreactions initiated with circularly polarized light shows that the enantiomeric excess of a chiral reactant approaches +/- 1 as the amount of unreacted reactant approaches 0. The final product never has a large enantiomeric excess at any stage of its formation and slowly decreases to 0 at the completion of the reaction. For multiple-step reactions the behavior of the intermediate photoproducts is much more interesting. During certain stages of the overall reaction both the size of the enantiomer excess and the amount of a given intermediate photoproduct are large. Furthermore, the sign of the enantiomeric excess of an intermediate may change during the course of the reaction. Multiple-step photoreactions initiated with circularly polarized light may be a method by which the exogenous and endogenous synthesis of optically active molecules occurred in the prebiotic universe.

A study of the liquid and solvent properties of optically active and racemic alpha-methylbenzylamine.

Speed-of-sound measurements are reported for RS (racemic) and S liquid alpha-methylbenzylamine (MBA) obtained using a modified design of previously published experimental geometry. After correcting for density changes, the resulting isentropic compressibility of the S liquid is found to be 2% larger than that of the RS racemic mixture. These data, along with proton NMR chemical shifts and published partial molar volumes, suggest that the structures of the racemic and optically active liquids are subtly different. The magnitude of the compressibilities and other data such as viscosity and the Kamlet-Taft parameters are consistent with the molecules in the liquids interacting via extensive hydrogen bonding. Cyclohexane, toluene, nitrobenzene, dimethyl sulfoxide, and methanol are completely miscible in MBA, as predicted from their corresponding Hildebrand solubility parameters. Proton NMR, optical rotation, and IR studies were carried out on solutions of the five solutes in MBA as a function of mole fraction. The asymmetric NH stretch of MBA was particularly informative in this regard. Solvation models were developed for the five solutes. The NH stretch frequency in cyclohexane changed only slightly up to a mole fraction of 0.7 in the hydrocarbon at which point the NH stretch greatly increased by about 200 cm(-1) at a mole fraction of 1. The hydrogen-bonding network does not dissipate until there is at least 70 mol % cyclohexane in the MBA. Gaussian calculations were also carried out to complement the ir studies.

Spectroscopic and theoretical investigation of (R)-3-methylcyclopentanone. The effect of solvent and temperature on the distribution of conformers.

Temperature-dependent electronic circular dichroism (CD) spectra are reported for (R)-(+)-3-methylcyclopentanone (R3MCP) in 34 solvents. Analysis of these data yielded the enthalpy and entropy differences between axial methyl and equatorial methyl conformers, the dominant conformers for R3MCP. The weakly absorbing n-->pi* transition exhibited a decrease in lambdamax as the solvent polarity increased. Vibrational CD fine structure of the n-->pi* transition was observed in the gas phase in addition to several of the solvents. Vapor-phase CD spectra were compared to both the solution-phase spectra and theoretical calculations of the axial and equatorial methyl conformers. Optical rotation (OR) measurements for R3MCP in cyclohexane solution in the visible region showed excellent agreement with OR DFT calculations of the equatorial and axial methyl conformers. Density functional calculations with B3LYP and the 6-13G* and aug-cc-pVDZ basis sets, which incorporate the solvent dielectric constant, yielded trends in thermodynamic quantities as a function of polarity and solvent dipole moments that are only weakly or not observed in experiments.

Solvent effects on the optical rotation of (S)-(-)-alpha-methylbenzylamine.

The optical rotation of (S)-(-)-alpha-methylbenzylamine at 589 nm has been measured in 39 different solvents at five different concentrations: 0.25, 0.50, 1.00, 2.00, and 3.00 M. A correlation of the intrinsic rotations (i.e., extrapolation of specific rotations to zero concentration) with Kamlet's and Taft's solvent parameters (alpha, beta, and pi) is established. The polarity/polarizability, pi, and solvent acidity, alpha, terms are found to have a greater effect upon the optical rotation than the basicity of the solvent, beta. The specific rotation for (S)-(-)-alpha-methylbenzylamine has been calculated with Gaussian03 using a PCM model (B3LYP aug-cc-pVDZ) for all 39 solvated systems. Comparisons between the experimental and calculated values show the importance of hydrogen bonding on specific rotation.

Theoretical insights into photochemical reactions initiated with circularly polarized light.

A theoretical analysis of several hypothetical photoreactions initiated with circularly polarized light has been carried out to determine the factors influencing the enantiomeric excess of recovered reactant. The anistropic g value is the dominant influence, although quantum yields and F values may be important in certain cases.

Raman under nitrogen. The high-resolution Raman spectroscopy of crystalline uranocene, thorocene, and ferrocene.

The utility of recording Raman spectroscopy under liquid nitrogen, a technique we call Raman under nitrogen (RUN), is demonstrated for ferrocene, uranocene, and thorocene. Using RUN, low-temperature (liquid nitrogen cooled) Raman spectra for these compounds exhibit higher resolution than previous studies, and new vibrational features are reported. The first Raman spectra of crystalline uranocene at 77 K are reported using excitation from argon (5145 A) and krypton (6764 A) ion lasers. The spectra obtained showed bands corresponding to vibrational transitions at 212, 236, 259, 379, 753, 897, 1500, and 3042 cm(-1), assigned to ring-metal-ring stretching, ring-metal tilting, out-of-plane CCC bending, in-plane CCC bending, ring-breathing, C-H bending, CC stretching and CH stretching, respectively. The assigned vibrational bands are compared to those of uranocene in THF, (COT)2-, and thorocene. All vibrational frequencies of the ligands, except the 259 cm(-1) out-of-plane CCC bending mode, were found to increase upon coordination. A broad, polarizable band centered about approximately 460 cm(-1) was also observed. The 460 cm(-1) band is greatly enhanced relative to the vibrational Raman transitions with excitations from the krypton ion laser, which is indicative of an electronic resonance Raman process as has been shown previously. The electronic resonance Raman band is observed to split into three distinct bands at 450, 461, and 474 cm(-1) with 6764 A excitation. Relativistic density functional theory is used to provide theoretical interpretations of the measured spectra.

Lewis acid-catalyzed rearrangement of 2,2-dichloronorbornane to 1-chloronorbornane.

The mechanism for the unusual AlCl(3)-catalyzed rearrangement of 2,2-dichloronorbornane to 1-chloronorbornane in pentane has been elucidated; the reaction, which also yields four isomeric dichloronorbornanes, occurs in three steps: (1). ionization to form the 2-chloro-2-norbornyl cation, which was fully characterized by two-dimensional (1)H and (13)C NMR in SbF(5)/SO(2)ClF; (2). Wagner-Meerwein shift to yield the 1-chloro-2-norbornyl cation, which was partially characterized by (1)H NMR; and (3). hydride abstraction.

Photoreaction of a racemate proceeding through radical or ion pairs. The effect of circularly polarized light on the enantiomeric excess of recovered reactant.

A simple theory has been developed to explain how circularly polarized light will affect the outcome of a photoreaction of a racemic substrate proceeding through ion pairs or radical pairs. One can calculate the enantiomeric excess and amount of recovered substrate at any stage of the reaction.

Photochemistry of racemic and resolved 2-iodooctane. Effect of solvent polarity and viscosity on the chemistry.

The photochemistry of racemic and resolved 2-iodooctane was examined in cyclopentane, methanol, and 2-methyl-2-propanol, media with differing polarities and viscosities. The photochemistry of racemic 2-iodooctane was also examined in the gas phase. The photochemistry of 2-deuterio- and 1,1,1-trideuterio-2-iodooctane in cyclopentane and methanol was also studied. The photoreactions in cyclopentane, 2-methyl-2-propanol, and the gas phase occurred exclusively through homolytic reactions, while in methanol, they occurred predominantly (>53%) through heterolytic reactions. By comparing the disappearance of the optically active substrate with its loss of optical activity, F, the fraction of the initially formed radical pair (RP) or ion pair (IP) resulting in product was determined for the three solvents. Because F contains contributions of both escape of the partners in the RP or IP into the bulk of the solvent and reaction within the RP or IP to yield products other than the substrate, there was no correlation between F and solvent viscosity. The F values will be valuable in assessing the photochemistry of 2-iodooctane in the same media with circularly polarized light.

The guaiol color reaction.

The common and biogenetically important sesquiterpene guaiol yields a deep purple color when treated with electrophilic bromine reagents. This interesting color reaction was observed over fifty years ago, but the chemistry of this process has not been investigated. Herein, we present the chemistry of the guaiol color reaction.