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The generation of enantiopure, chiral molecules remains relevant to many scientific fields, from the study of biological systems to materials science. One critical challenge is that enantioenriched compounds are not [...], One of the most well-recognized stereogenic elements in a chiral molecule is an [sp.sup.3]-hybridized carbon atom that is connected to four different substituents. Axes of chirality can also exist about bonds with hindered barriers of rotation; molecules containing such axes are known as atropisomers (1). Understanding the dynamics of these systems can be useful, for example, in the design of single-atropisomer drugs (2) or molecular switches and motors (3). For molecules that exhibit a single axis of chirality, rotation about that axis leads to racemization as the system reaches equilibrium. Here we report a two-axis system for which an enantioselective reaction produces four stereoisomers (two enantiomeric pairs): following a catalytic asymmetric transformation, we observe a kinetically controlled product distribution that is perturbed from the system's equilibrium position. As the system undergoes isomerization, one of the diastereomeric pairs drifts spontaneously to a higher enantiomeric ratio. In a compensatory manner, the enantiomeric ratio of the other diastereomeric pair decreases. These observations are made for a class of unsymmetrical amides that exhibits two asymmetric axes--one axis is defined through a benzamide substructure, and the other axis is associated with differentially N,N-disubstituted amides. The stereodynamics of these substrates provides an opportunity to observe a curious interplay of kinetics and thermodynamics intrinsic to a system of stereoisomers that is constrained to a situation of partial equilibrium.

We deal with the issue of quantifying and optimizing the rotation dynamics of synthetic molecular motors. For this purpose, the continuous four-stage rotation behavior of a typical light-activated molecular motor was measured in detail. All reaction constants were determined empirically. Next, we developed a Markov model that describes the full motor dynamics mathematically. We derived expressions for a set of characteristic quantities, i.e., the average rate of quarter rotations or 'velocity,' V, the spread in the average number of quarter rotations, D, and the dimensionless Peclet number, Pe = VID. Furthermore, we determined the rate of full, four-step rotations ([[OMEGA].sub.eff]), from which we derived another dimensionless quantity, the 'rotational excess,' r.e. This quantity, defined as the relative difference between total forward ([[OMEGA].sub.+]) and backward ([[OMEGA].sub.-]) full rotations, is a good measure of the unidirectionality of the rotation process. Our model provides a pragmatic tool to optimize motor performance. We demonstrate this by calculating V, D, Pe, [[OMEGA].sub.eff], and r.e. for different rates of thermal versus photochemical energy input. We find that for a given light intensity, an optimal temperature range exists in which the motor exhibits excellent efficiency and unidirectional behavior, above or below which motor performance decreases. Markov model | unidirectional rotation

The restriction of a small molecule's motion on binding to a protein causes a loss of configurational entropy, and thus a penalty in binding affinity. Some energy models used in computer-aided ligand design neglect this entropic penalty, whereas others account for it based on an expected drop in the number of accessible rotamers upon binding. However, the validity of the physical assumptions underlying the various approaches is largely unexamined. The present study addresses this issue by using Mining Minima calculations to analyze the association of amprenavir with HIV protease. The computed loss in ligand configurational entropy is large, contributing ~25 kcal/mol (4.184 kJ/kcal) to [LAMBDA]G[degrees]. Most of this loss results from narrower energy wells in the bound state, rather than a drop in the number of accessible rotamers. Coupling among rotation/translation and internal degrees of freedom complicates the decomposition of the entropy change into additive terms. The results highlight the potential to gain affinity by designing conformationally restricted ligands and have implications for the formulation of energy models for ligand scoring. drug design | translation | rotamer affinity | rotation

The rotational isomerization of monomeric propionic acid isolated in solid argon is investigated. It is found that for the CH(sub 3)Ch(sub 2)COOD isotopologue, the excitation of molecules isolated in different matrix sites resulted in site-dependent photoisomerization rates for the C(sub alpha)-C and C-O internal rotations.

Research has been conducted on Zr(BH (sub)4) (sub)4 and U(BH (sub)4) (sub)4. The study of compounds' molecular structures carried out via the use of density functional theory calculations and gas electron diffraction is presented.

The relation between thermochromism in polythiophenes and the internal rotation was studied by density functional theory (DFT). It was found that the internal rotation alone was not sufficient to explain the mechanism of thermochromism.

Research has been conducted on the molecular motors. The synthesis and dynamic properties of these motors have been investigated and the results are reported.

A new study uses the DNMR method to investigate the effect of hydrostatic pressure on the internal rotation rate of cis-1,n-disubstituted[n]paracyclophanes (n=12, 13).

The analyses of rotational distribution in vibrational transfer presented develop the angular momentum (AM) model of collisional phenomena that emphasizes the commonality of mechanism. The commodity of mechanism among a variety of collision-induced processes was emphasized.

Research is done on kinetic parameters and conformational equilibrium constants characterizing the internal rotation in gaseous N-ethyl and N-methylthioamides, which are determined from exchange broadened 1H NMR spectra. In conclusion, the results are compared to solution phase values and to the corresponding oxoamides.

A theoretical study identifies nine conformers for N,N,N',N'-tetramethylenediamine and compares their relative energies.

Thee protophysical characteristics of a series of phenylacetylene dendrimers are investigated, using steady-state and time-resolved fluorescence data.

A new study determines temperature-dependent Berry pseudorotation rate constants of SF4 gas, using NMR spectroscopy.

Issues concerning orientation effects of polar molecules in a uniform electric field are discussed. This research focused on an asymmetric top molecule with a permanent dipole not parallel to a main axis.

Issues concerning theoretical transferred-echo double-resonance and rotational-echo double-resonance curves are reported for several three-spin systems.

The article discusses research methods used to study the nonrotating molecules and their unimolecular dissociation.

The problem of cooling molecular rotations was examined using optimal control theory.

An investigation of molecular vibrations in hydrogen sulfide and ammonia, and internal rotations in hydrogen peroxide, hydrogen persulfide, hydrogen thioperoxide and ethylene, is presented. It was found that conditions of maximum hardness and minimum polarizability complement the minimum energy criterion for molecular stability.

The structure and conformational equilibrium of four calix(4)arene conformers are investigated using density functional theory, and the results are compared with force field, experiment, and semiempirical molecular orbital approaches.

The torsional curves of 3,3'-dimethyl-2,2'-bithiophene and 3,4'-dimethyl-2,2'-bithiophene were examined theoretically using a density function theory approach. The curve for 3,3'-dimethyl-2,2'-bithiophene was computed at the BLYP, B3LYP and MP2 levels, while only B3LYP was used for the other isomer. Results indicated the existence of a syn-gauche and an anti-gauche conformational minimum for both isomers.

Research was conducted to determine the structure of the OCS trimer. Two additional isotopomers of the trimer were assigned, the dipole moment was measured and semiempirical calculations were added. A comparison of the structure of (OCS)3 with similar trimers involving linear triatomic molecules and with the OCS dimer was performed. Results based on the semiempirical calculations show a good qualitative agreement with the experimental structure.

A study investigated the temperature-dependent, Fourier transform-infrared spectra of liquid rare gas solutions to obtain more definitive data on the conformational stability of 3-chloropropene (allyl chloride) in the gaseous state. Ab initio calculations were also conducted using the 6-31G(d) basis set with full electron correlation by the perturbation technique to second order and using the larger 6-311++G(d.p) basis set. Experimental methods and results are presented.

Research was conducted to determine the molecular parameters of bromofulminate, BrCNO, in large-scale ab initio measurements via the coupled-cluster technique CCSD(T) and basis sets of double-through quintuple-zeta quality. The capability of various theoretical methods for accurately determining potential energy functions of the quasi-linear molecules was assessed through the use of the one-particle basis sets nearly saturated in both the polarization and valence spaces.

Research was conducted to examine the collisional deactivation of H2O by Ar by means of classical trajectory calculations with an initial vibrational energy of 50, 75 and 100 kcal/mol, translational energies corresponding to the Boltzmann distribution at 298 K and rotational temperatures in the range 1-10,000 K. The objective was to analyze the impact of internal energy on the first and second moments. The effects of internal energy on the energy transfer moments are presented.

The -COOH group of 12 bromododecanoic acid molecules has been observed for the last time to adopt into the same substrate. Scanning tunneling microscopic data have shown that 12-bromododecanoic acid molecules self-assemble on graphite so that the molecule adopts a 25 degree angle between the direction of the hydrogen bond and the hydrocarbon backbone. 2-bromoheadecanoic acid molecules orient with a 53 degree angle between the hydrogen bond direction and the long molecular axis when physisorbed on the same graphite substrate.

The decay of fluorescence anisotropy of several aromatic hydrocarbons has been measured in methylcyclohexane excited by synchrotron radiation using the time-resolved single-photon counting method. It is shown that altering the temperature from -60 to -140 degrees Celsius significantly changes the viscosity n. Rotation times are proportional to n/T for 9,10-disubstituted anthracenes, although this is not true for perylene, anthracene and triphenylene for which Dn/T increases substantially on cooling.

The decay of fluorescence anisotropy of solutions in squalane of a number of aromatic hydrocarbons has been measured using time-resolved single-photon counting in combination with synchrotron radiation source at the CCLRC's Daresbury Laboratory. The hydrocarbons under study are coronene, triphenylene, perylene, benzoperylene, anthracene and tetracene. Results show that decay rates change as a function of excitation wavelength, contrary to predictions of hydrodynamic theory. The molecular rotations undergo bifurcation in two stages, namely, slow overall motion and faster rotation over a restricted angular range.

The molecular structure and microwave spectra of seven isotopomers of tetracarbonylethyleneiron are investigated using a Pulse-Beam Fourier Transform Microwave Spectrometer. Rotational transitions for a c dipole moment with J'

The dipole emission patterns of single molecules have been studied by using noninvasive far- and near-field microscopy methods. These procedures allow the three-dimensional orientation of molecules to be determined at room temperature upon the introduction of specific aberrations. The molecular samples were prepared by spin-coating solutions of polymethylmethacrylate and 10 raised to the negative eleventh power M DiIC(sub 18)(3)(super 23) onto a quartz coverslip. The molecules were excited with the 532 nm output of a mode-locked Nd:YAG laser undergoing total internal reflection at a polymer/water boundary.

A study to investigate a group of materials known as sol-gels was conducted to monitor the rotational diffusion of two probes, namely, a small dye molecule and a typical globular protein, using time resolved fluorescence anisotropy. The rotational dynamics of sulforhodamine 101 and magnesium-substituted myoglobin were investigated in three separate gel preparations. Experimental methodology and results are presented.

The influence of translational and rotational entropy in the unfolding of a dimeric coiled-coil has been investigated in aqueous solution. To this end, the calorimetrically measured unfolding entropies of a synthetic two-stranded alpha-helical coiled-coil with and without a disulfide bond between the two polypeptide chains were compared to determine the entropy of intersubunit cross-linking of a compound. The resultant translational and rotational entropy was found to be more than an order of magnitude smaller than estimates based on ideal gas statistics.

The geometries, vibrational frequencies and relative energies of the different internal-rotation conformers of beta-hydroxyethoxy and beta-hydroxyethylperoxy radicals were investigated. The results of B3LYP-DFT/6-31G** quantum chemical studies showed that for both the peroxy and oxy radicals, the equilibrium geometries were stabilized by interactions between the hydroxy H and the (per)oxy O, thus lowering the energies by 1.5-2.5 kcal/mol. The transition structures for the transition HOCH2CH2O - CH20H + CH2O dissociation were also determined.

Experiments were performed to investigate the molecular rotation in the carbon-nitrogen (C-N) bond in the trans-N,N-dimethylaminoacrylonitrile (DMAAN) resembling a vinylogous amide. The obstruction to the C-N bond rotation in DMAAN were observed in various solvents. Gas-phase ab initio experiments were also conducted to outline the molecular rotation. The findings indicate that the variation in DMAAN barriers with solvent were similar to the pattern observed for N,N-dimethylacetamide.

The relationship between the lateral diffusion rate of the band 3 molecule and the spectrin content of the human red blood cell membrane was investigated in patients with severe autosomal recessive hereditary spherocytosis and selective spectrin deficiency. Results showed that the rotational mobility of band 3 in hereditary spherocytosis is identical to that in normal red blood cells. It was therefore concluded that the lateral mobility of band 3 is inhibited by steric interactions with the membrane skeleton.

Molecular dynamic (MD) simulation studies of the monomeric protein catechol theta-methyltransferase (COMT) showed correlated motions for the enzyme-substrate complex. These motions involving catechol, AdoMet and the Tyr68 side chain are vital to the formation of near attack conformations in COMT. The trajectory of MD affects substituent movement into a hydrophobic pocket, which conditions the preference of meta-theta-methylated products over their para-theta methylated counterparts.

Research was conducted to examine the barrier to rotation of the P-H group around the C-*P bond using a new phosphine, the diphenyldibenzobarrelenephosphine. The EPR spectrum of the resulting phosphinyl radical, trapped in a single crystal of diphenyldibenzobarrelenephosphine, was examined at 77 K and at room temperature after homolytic scission of a P-H bond by radiolysis. Results indicate that interaction between the phosphinyl hydrogen and the phenyl ring bound to the ethylenic bond explains the potential energy profile.

Research was conducted to examine the rotational spectra of the 1:1 complexes of diazines with water. The objective of the study is to establish the structure of the isolated complex and to gain an insight into the intermolecular interactions that govern the association process. Results indicate that the ground state is located either in the single global minimum with AE configuration or well above a small barrier between two possible nearby nonplanar minima.

Research was conducted to examine the selective excitation of ICN using brute force orientation. The procedure was carried out using a strong uniform electric field and supersonic expansion. The objective is to show that orientation of parent molecules can reveal signatures of not only the upper state surfaces through selectivity in photoexcitation but also the dynamics of dissociation process on each surface. Results demonstrate the possibility of selective excitation through brute force orientation and polarized excitation.

The photochemical formation of ferrimagnetic chains from a pair of manganese complexes a and b of bis(hexafluoroacetylacetonato)manganese(II) coordinated with diazodi(4-pyridyl)methane in the cis and trans arrangements as repeating units was described. The two isomeric manganese derivatives were prepared individually and were found to have helical and zigzag chain structures. Irradiation of the dilute paramagnetic manganese complexes transformed the diazo groups to the triplet carbenes and formed extended ferrimagnetic chains containing photochemically generated 2p spins.

Research was conducted to examine a theoretical study of the internal rotation of HSOH. Ab initio calculations at the HF//6-311G level reveal that HSOH is a gauche molecule presenting a double-barrier torsional potential. Good estimations of barrier heights and properties associated with the stable isomer have been obtained. An important result is that the principle of maximum hardness is verified even though the electronic chemical potential varies along the reaction coordinate.

Research was conducted to examine the rotational motion of C60 in toluene-d8 using spin-lattice relaxation measurements. The results were compared with data derived in carbon disulfide and 1,2-dichlorobenzened 4. Results indicate that the reorientational times of C60 in the solvents did not conform to conventional viscosity arguments but were better correlated to the strength of solute-solvent interactions. The fundamental observations will be useful in predicting the reorientational characteristics of other members of the fullerene family.

An experiment was conducted as part of studies into the chemistry of polynuclear complexes containing anionic ligands of the N-C-X type (where X = N, O and S). The study resulted in the first tetranuclear complex with two bridging N-C-S ligands exhibiting a mu-(1:2kappaS) coordination mode. The tetranuclear complex was found to undergo a change in the mu-(1kappaN, 2kappaS) coordination mode by replacement of the ancillary ligands.

The rates of macrocyclic ring rotation in benzylic amide [2]catenanes in solvent models were analyzed using variable-temperature nuclear magnetic resonance imaging spectroscopy and molecular mechanics calculations. Findings suggested that solvent effects and structural modifications can result to variations in the interlocked macrocycle dynamics. Other factors dictating the rate of circumrotation of benzylic amide catenanes include steric effects from the passing of bulky groups, complex hydrogen bonding, multiple inter-macrocycle pi-pi stacking and the T-herringbone interactions.

The molecular dynamics evolving on the electronic excited and ground state potential energy surfaces of iodine and bromine in the gas phase were investigated using femtosecond time-resolved coherent anti-Stokes Raman scattering spectroscopy. Fast Fourier transform calculations and a simple model outlining the rotational contribution were employed in the analysis of the coherences of the wave packet in the temporal transients of the molecules excited by the ultrashort laser pulses. It was found that the model can describe the rotational coherence for the excited state vibrations, but not the rotational structure of the ground-state transients.

The ground-state spectra of five isotopic species of a hydrogen-bonded dimer between tetrahydrothiophene and hydrogen chloride were determined in the frequency range six to 18.5 GHz using a molecular beam Fourier transform microwave spectrometer. The isotopomers are C4H8(super 32)S...H35Cl, C4H8(super 32)S...H37Cl, C4H8(super 32)D35Cl, C4H8(super 32)D37Cl and C4H8(super 34)S...H35Cl. The results showed the rotational, quartic centrifugal distortion and Cl-nuclear quadropole coupling constants for each isomer.

Research was conducted to examine the eclipsed conformation of the exocyclic N-CH2 bond in N-neophentylpiperidines and their dynamic stereochemistry. Dynamic NMR spectroscopy was employed elucidating the importance of ring inversion, nitrogen inversion and exocyclic bond rotation. Observations of minor populations of the neopentyl-axial-eclipsed conformation are reported as well as the corresponding N-alkylpyrrolidines.

Ab initio molecular orbital experiments were conducted to investigate the initiation reaction of the methyl 4-(dimethylamino)benzenesulfonate (MDBS) thermal isomerization to N,N,N,-trimethylbenzenaminium-4-sulfonate (TBS) in the crystalline state. Aside from the topochemical nature of the initiation reaction, the ionic intermediates and crystal structures of MDBS and TBS were compared. The arrangement of a pair of molecules in the MDBS crystal optimized the transition-state structure. Moreover, in-phase molecular rotation in the MDBS induced the initiation stage of thermal isomerization.