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A study on the alignment of macromolecules in the presence of liquid crystals to allow measurements of dipolar couplings reports on the identification of a respectable correlation between relaxation rates and dipolar couplings, which provide significant and unique structural information. The experiment also demonstrates that this correlation can be used to identify residues whose backbones undergo conformational exchange. The applicability of this approach is illustrated using experimental data for three proteins.

The first solid-state nuclear magnetic resonance study of the complete three-dimensional molecular structure in the tilted rotating frame was presented. Five rotational resonance in the tilted rotating frame experiments produced all of the six dihedral angles required for the construction of the three-dimensional structure using a uniformly (super 13)C,(super 15)N-labeled glycylisoleucine. The procedure can be applied to probe the structure of a restricted region essential to biological function.

Ab initio experiments were performed to investigate anomalous rotatory dispersion. Calculations were carried out using the DALTON program, an ab initio electronic structure program on a Pentium-II computer with LINUX operating system. The optical rotatory dispersion data in various molecules such as (+)-(3R)-methylcyclopentanone and 3-methylcyclohexanone were carried out. For the chain conformation, an inverse relationship between optical rotation and wavelength was observed.

The stereoselective synthesis of fully hydrogenated calixarene analogues was described. Saturated (1(n))-metacyclophane derivatives incorporating in the macrocycle cyclohexanone or cyclohexanol rings have the capability to act as host molecules and as potential synthetic intermediates for the synthesis of a novel class of calixarenes and host molecules. The rotational barrier of the product RsS SsR RsS SsR was determined using the saturation transfer method.

The molecular structure and the magnetic properties of (3(sub 6))(1,2,3,4,5,6)cyclophane, its degenerate inversion and its reaction to yield propella(3(sub 6))prismane were examined using quantum chemical methods. Geometry optimizations of C66, (3(sub 6))(1,2,3,4,5,6)cyclophane and propella(3(sub 6))prismane were conducted using classical mechanical force fields.

A combined theoretical and spectroscopic study indicates that the structures of the S2(OCH2A)2 compounds in solution are definitively dialkoxy disulfides with unusually high barriers to rotation about the S-S bond. The energies of the isomers are sufficiently close to justify considering transformation between them. However, the absolute energy difference is still not clear. The high S-S rotational barrier for ROSSOR in relation to RSSR should allow the unprecedented separation and isolation of disulfide torsional enantiomers.

The cis rotamer of formic acid (HCOOH) in the gas phase has a very small population. Its detection through infrared (IR) spectroscopy may be measured via observation of the stretching overtone of trans-formic acid isolated in solid argon. The stretching overtone lies significantly above the torsional barrier from trans to cis rotamer. Vibrational relaxation towards the torsional coordinate leads to the formation of cis-HCOOH. Interconversion back to the trans rotamer from the cis rotamer was observed in the low-temperature argon matrix.

Spin alignment research has established that the diradicals 9,9'-(m-phenylene)dixanthyl and -dithioxanthyl exhibit heteroatom dependence of the spin multiplicity in the ground state. Further research is required to establish the exact nature of the heteroatom dependence although the delocalization of lone-pair electrons of heteroatoms may be connected to the phenomenon. Semiempirical level MO calculations were used to provide full optimizations by AM1-UHF.

A study was conducted on the quasiclassical molecular dynamics on an analytical potential energy surface for the stereomutation of cyclopropane. Computation of harmonic vibrational frequencies was done to characterize the stationary points on the trimethylene potential energy surface. Then, recomputation of the energies of these stationary points were made at the CASPT2N/6-31G* level. The findings indicate that stereomutation of cyclopropane does not significantly depend on the details of the surface.

A study was conducted on stereomutation of cyclopropane-1,2-d2 using classical trajectories. In the simulations, trajectories were integrated using energies and derivatives taken from semiempirical electronic structure theory. The findings indicate that classical trajectories on the AM1-SRP PES support a mechanistic continuum encompassing concerted and nonconcerted. Hence, the decay of trimethylene is nonstatistical and intrinsically non-RRKM at low energy.

Several stereoisomers of spirophosphane featuring an apical carbon-equatorial oxygen ring were isolated and characterized for the first time. These include an apical oxygen-equatorial carbon isomer and trigonal bipyramidal stereoisomers derived from the treatment of monocyclic phosphorane with pyridine in tetrahydrofuran. Their rates of pseudorotation in toluene, which were measured via NMR spectroscopy, showed first-order kinetics behavior.

Small amounts of Cu(II) can catalyze the isomerization of amides that possess metal-binding sites. Coordination of the Cu(II) atom to N stops amide resonance and catalyzes amide isomerization. The lowering of activation energy involved in catalysis is the greatest in amides that have high rotational barriers. Preformed Cu-complexes are also able to catalyze amide isomerization. The Cu(II) atom can coordinate to the N(sub a) of tertiary amides and to the ring N(sub a) of prolyl peptides also. This indicates the ability of Cu(II) to catalyze the folding of polypeptides.

Synthesis of the acetylenic cyclophane from polyalkynyl precursors is a precise step towards the rational synthesis of endohedral metal fullerene complexes. The pathway starts with the bromination of 1,3,5-triethynylbenzene with NBS producing tris(bromoalkyne). A brief two-step sequence ends with the production of the acetylenic cyclophane. The cyclophane shows a helical chiral nature in the ORTEP plot of its crystal structure. The energy of transition state between its enantiomers indicates that, probably, racemization of the cyclophane occurs very fast.

The equilibration of 2-methyleneglutarate with (R)-3-methylitaconate ((R)-2-methyl-3-methylenesuccinate) is tested by lowering the energy barrier to rotation about the C-1/methylene bond. It is hypothesized that this would result in the equilibration of a stereospecifically deuteriated specimen of 2-methyleneglutarate with (R)-3-methylitaconate ((R)-2-methyl-3-methylenesuccinate) when the latter is incubated with 2-methyleneglutarate mutase enzyme. This hypothesis is proven to be true by the test.

The dynamics of Norrish Type II photodissociation, a chemical reaction which produces polyatomic species, is investigated. The populations of rotational states corresponding to two limiting cases of angular momentum projections are monitored. The approach is similar to the one used to assess Norrish Type II dissociation. Room temperate bulb gas experiments indicate a nonstatistical rotational state distribution with a propensity for rotation about the c-axis. Results indicate the method's potential for studying polyatomic photodissociation dynamics.

A molecular mechanics model of square pyramidal complexes of the Tc(V) oxo group is described. The model incorporates significant torsional barriers to rotation (Vi) about the Tc-L (L = donor atom) bonds. These barriers are interpreted in terms of overlap between filled p orbitals on the donor atoms and oxygen and d orbitals on the Tc.

The angle between cellular life and death, according to biochemistry textbooks, is acute. Life-giving oxygen is stored within muscle cells by a protein called myoglobin, and that storage is made [...]

The exo-anomeric effect that beta-beta-trehalose lacks cannot be assumed to be conformation defining in the C-disaccharide analog of beta-beta-trehalose, as both are demonstrated to possess similar 'zigzag' conformation in aqueous solution. A linkage conformation exists near phi,psi = 60 degrees, 60 degrees, which is evident from the NMR of c-beta,beta-trehalose. In beta,beta-trehalose, the vacuum MM3 flexible-residue molecular modeling computation, suggesting a deep energy minimum near phi,psi = 44 degrees, 44 degrees, concords with the optical rotation evident is aqueous solution.

Beam transmission characteristics of a collinear prismatic polarization rotator are given. It is found that a [TM.sub.00] laser beam is transmitted with good fidelity at an efficiency of 94.7%.

A new study focuses on the conformational disorder of the acyl chains in the highly ordered gel phase of a diacylphosphatidylcholine bilayer. IR spectroscopy is used to establish the conformation of the CH2-CH2 bonds.

The article discusses the importance of formamide and examples of research into the internal rotational aspects of intermolecular process and computational research.

The application of electron and nuclear spin relaxation theory has caused a seeming paradox to develop, even in the fast motion limit for nitroxide spin labels. Two well-resolved EPR lines are seen when using 15N nitroxide spin labels due to the interaction of the electron with the nitrogen nucleus. The spin-spin relaxation rates are related to the observed line widths. The results showed that by following the operator method of Abragam, both spin-lattice relaxation and spin-spin relaxation lead to theoretical predictions agreeing with experimental results could be developed.

The molecular structure, rotation and bonding of the S1

The potential energy surface of catenane and its pyridyl and thiophenyl derivatives was investigated for the first-ever complete theoretical description of the lowest energy pathway for the circumrotation of macrocycles in a catenane system. Molecular mechanics and calculations, using unimolecular reaction rate theory, were employed to determine the structure and dynamics of the three benzylic amide catenanes. The circumrotation processes are a concerted sequence of a number of large-amplitude motions involving rearrangements to reduce steric and electrostatic interactions through the interplay of hydrogen bonding, formation of pi-stacks, phenyl-phenyl T-shaped interactions and amide rotamer interconversions.

Reaction field theory models and Monte Carlo statistical mechanical simulation studies were carried out to investigate the solvent effects on the N,N-dimethylacrylonitrile (DMAAN) and the solvation energies in the system. The isodensity polarizable continuum model (IPCM) was utilized in the experiments. The IPCM did not predict the full solvent effects in acetonitrile. However, the Monte Carlo simulations underestimated the solvent effects by 20%, which makes it the better technique in calculating solution environments.

The impact of aglycon and its absolute configuration on the rotational population of the hydroxymethyl group was investigated. Circular dichroism and hydrogen nuclear magnetic resonance spectroscopy were performed with esterified alkyl glucopyranosides and a stereochemical experiment was conducted to determine the origin of the circular dichroism spectral differences between anomers of alkyl glucopyranosides. Results showed that the rotational populations of the hydroxymethyl group depend on the structure of the aglycon.

Partially dissociated diatomic gases near local thermal equilibrium were examined with particular focus on their rotational-vibrational properties. A final group of 11 gases whose partition functions are expected to be heavily dependent on the molecular rotational-vibrational motion was selected. Statistical mechanics were used to examine these gases to determine how the gas composition and thermodynamic functions are affected by the molecular-vibrational dynamics. The findings are discussed.

Research was conducted to examine the relative motion of the bound host-guest complexes using the time-resolved polarization spectroscopy approach. The objective of the study is to address the flexibility of intermolecular complexes with anthracene-1,3-dicarboxylate and two bis-guanidinium receptors with different site geometries as the host molecules. Results indicate that it is possible to estimate the frequency of the host-guest potential and the range of the guest chromophore's angular displacement using a simple model.

The structural properties of aromatic-aromatic van der Waals complexes of the form M-X, where M = perylene or fluorene and X = benzene or toluene, were investigated using rotational coherence spectroscopy. The structure of aromatic-aliphatic hydrocarbon dimers of the form M-Y, where M = perylene or fluorene and Y = cyclohexane or methylcyclohexane, was also characterized. Experimental results showed that the rotational constants of the perylene complexes are in good agreement with centrally bound, parallel-stacked structures in which the monomer planes are separated by a distance of 3.5-4.3 A.

The formation of twisted intramolecular charge transfer (TICT) states by rotating part of the molecule around some of the chemical bonds in a hemicyanine dye, 4-(2-(4-dimethylaminophenyl)ethenyl)-1-methylpyridinium iodide (HR), was investigated using quantum chemical calculations in combination with a cylindrical solvation model. Experimental results showed that the structure of HR is a compromise between benzenoid and quinoid resonance forms. The TICT state can be formed by rotating the dimethylamino group to the perpendicular position in polar solvents.

The usual method of predicting molecular chirality is through the inspection of a symmetry element, but achirality does not require a symmetry plane. A molecule is achiral if the process of transforming from a chemical conformation to a mirror image through chemical-bond rotation is easy to accomplish. The process of finding a chemical conformation with a rotating plane of symmetry then becomes simple.

Rotation and Solvation of Ammonium Ion RECENT NUCLEAR MAGNETIC RESONANCE (NMR) measurements (1) have shown that the rotational correlation time c of aqueous ammonium ion is 1.1 10(-12) second. This [...]