Inverse vibrational distributions of photofragments

X-ray diffraction data from a Cr₂(SO₄)₃ solution were examined. A peak at 3.25 Å in the correlation function reveals the formation of inner chromium-sulphate complexes. Least-squares refinements of the i(s) curve are consistent with a structural unit in which the CrOS angle is 136°. Each sulphate ion interacts with approximately eight water molecules.     

H-atom photofragments from H2O2 dissociated at 193 nm

Hydrogen atoms produced by photodissociation of H₂O₂ using 193 nm laser light have been observed for the first time. The H atoms are detected by VUV spectroscopy. The yield is 12 ± 1%. The H atoms are produced by a one-photon process.     

Photodissociation of HI and DI: polarization of atomic photofragments

The complete angular momentum distributions and vector correlation coefficients (orientation and alignment) of ground state I((2)P(32)) and excited state I((2)P(12)) atoms resulting from the photodissociation of HI have been computed as a function of photolysis energy. The orientation and alignment parameters a(Q) ((K))(p) that describe the coherent and incoherent contributions to the angular momentum distributions from the multiple electronic states accessed by parallel and perpendicular transitions are determined using a time-dependent wave packet treatment of the dissociation dynamics. The dynamics are based on potential energy curves and transition dipole moments that have been reported previously [R. J. LeRoy, G. T. Kraemer, and S. Manzhos, J. Chem. Phys. 117, 9353 (2002)] and used to successfully model the scalar (total cross section and branching fraction) and lowest order vector (anisotropy parameter beta) properties of the photodissociation. Predictions of the a(Q) ((K))(p), parameters for the isotopically substituted species DI are reported and contrasted to the analogous HI results. The resulting polarization for the corresponding H/D partners are also determined and demonstrate that both H and D atoms produced can be highly spin polarized. Comparison of these predictions for HI and DI with experimental measurement will provide the most stringent test of the current model for the electronic structure and the interpretation of the dissociation based on noncoupled excited state dynamics.     

Macromolecular helicity induction in a cationic polyacetylene assisted by an anionic polyisocyanide with helicity memory in water: replication of macromolecular helicity

We report the first example of the replication of macromolecular helicity. An optically active helical and anionic polyelectrolyte, the sodium salt of poly(4-carboxyphenyl isocyanide), was found to serve as the template for further helicity induction in a different polyelectrolyte with opposite charges in water, resulting in interpolymer helical assemblies with controlled helicity. The effects of the pH and salt concentration on the helicity induction were investigated.     

EPR studies of phosphine photofragments in cancrinite matrix at 77K

The far-uv photolysis of phosphine sorbed in cancrinite matrix at 77 K has been studied by epr spectroscopy. Cancrinite is demonstrated to stabilise the photolytically generated radicals ·PH₂, ·P and ·H. Computer simulation of the entire lineshapes has been employed to assess the spin Hamiltonian parameters for both ·PH₂ and ·P. The intensity distribution pattern of the ·PH₂ spectrum at 77 K is indicative of a thermally-equilibrated population of rotational states of this radical in its matrix environment. The similarity of the epr data of ·PH₂ trapped in cancrinite to that isolated in rare gas matrices, as well as theisotropic nature of the³¹P and¹H hyperfine couplings in the former situation, suggest that ·PH₂ is trapped in pseudospherical cavities of cancrinite with minimal matrix perturbations. Marked departures are observed for theg- andA- values of cancrinite-trapped P atoms as compared with the gasphase counterparts.     

Mass-analyzed velocity map imaging of doubly charged photofragments from C70

The velocity distributions of the fragments produced by dissociative photoionization of C(70) have been measured at several photon energies in the extreme UV region, by using a flight-time resolved velocity map imaging (VMI) technique combined with a high-temperature molecular beam and synchrotron radiation. Average kinetic energy release was estimated for the six reaction steps of consecutive C(2) emission, starting from C(70)(2+) → C(68)(2+) + C(2) to C(60)(2+)→ C(58)(2+) + C(2). The total kinetic energy generated in each step shows a general tendency to increase with increasing hν, except for the first and fifth steps. This propensity reflects statistical redistributions of the excess energy in the transition states for the above fragmentation mechanism. Analysis based on the finite-heat-bath theory predicts the detectable minimum cluster sizes at the end of the C(2)-emission decay chain. They accord well with the minimum sizes of the observed ions, if the excess energy in the primary C(70)(2+) is assumed to be smaller by ~15 eV than the maximum available energy. The present VMI experiments reveal remarkably small kinetic energy release in the fifth step, in contradiction to theoretical predictions, which suggests involvement of other fragmentation mechanisms in the formation of C(60)(2+).     

Calculation of adiabatic polarization of atomic photofragments under the influence of long range quadrupole-quadrupole interactions

General equations for diagonalization of the quadrupole-quadrupole interaction matrix for diatomic molecules are presented. Eigenvalue and eigenvector solutions are tabulated for atoms with levels J < or = 2. The use of the eigenvector solutions for determination of adiabatic molecule-frame photofragment polarization is illustrated, and polarization parameters a and a are tabulated. Even if knowledge of the photofragment scattering matrix is limited, we illustrate how coherent polarization parameters a0(1) and a0(2) can be calculated from the tabulated adiabatic expansion coefficients, for example, by making use of available experimental data.     

Imaging H2O photofragments in the predissociation of the HCl-H2O hydrogen-bonded dimer

The state-to-state vibrational predissociation (VP) dynamics of the hydrogen-bonded HCl-H(2)O dimer was studied following excitation of the dimer's HCl stretch by detecting the H(2)O fragment. Velocity map imaging (VMI) and resonance-enhanced multiphoton ionization (REMPI) were used to determine pair-correlated product energy distributions. Following vibrational excitation of the HCl stretch of the dimer, H(2)O fragments were detected by 2 + 1 REMPI via the C (1)B(1) (000) ← X (1)A(1) (000) transition. REMPI spectra clearly show H(2)O from dissociation produced in the ground vibrational state. The fragments' center-of-mass (c.m.) translational energy distributions were determined from images of selected rotational states of H(2)O and were converted to rotational state distributions of the HCl cofragment. The distributions were consistent with the previously measured dissociation energy of D(0) = 1334 ± 10 cm(-1) and show a clear preference for rotational levels in the HCl fragment that minimize translational energy release. The usefulness of 2 + 1 REMPI detection of water fragments is discussed.     

Determination of the crystalline and noncrystalline molecular orientation in oriented polypropylene by infrared spectroscopy

The application of orientation to enhance the properties of a wide range of macromolecular materials has stimulated an interest in the characterization of the effects of such orientation processes. In semicrystalline polymers, this requires the characterization of the orientation in both the crystalline and noncrystalline phases. An infrared dichroism method was developed to simultaneously measure the orientation in the crystalline and noncrystalline phases of polypropylene. The method was applied to a series of melt-crystallized and gel-crystallized polypropylene specimens drawn up to a maximum of about 50 ×. The Hermans orientation functions of each phase were determined. The crystalline orientation was found to increase rapidly and approach perfect orientation at relatively low draw ratio (i.e., > 10 × ). However, the noncrystalline orientation was found to increase much more slowly up to about 50 × draw ratio. The Youngs modulus was found to increase nearly linearly up to about 50 × and to correlate with the increase in the noncrystalline-orientation function and not with the crystalline-orientation function.     

Kinetic energy spectra of N2+ photofragments in the laser photodissociation of N2O+

Using a ZAB-2F double-focusing mass spectrometer together with an argon-ion laser, the kinetic energy spectra of N₂⁺ photofragments from the photodissociation of N₂O⁺ have been measured at wavelengths 514.5, 496.5, 488.0 and 476.5 nm in the visible region of the spectrum. Energies released in the centre-of-mass frame of reference are given. From the results it is deduced that the states involved in the absorption and dissociation processes ar probably N₂O⁺(B̃²Π) v ⩾ 3 and N₂O⁺ (C̃²Σ⁺) v ⩾ 0, respectively.     

Molecular orbital calculation of Mössbauer parameters for photofragments of Fe(CO)5

Molecular orbital calculations were performed to obtain electron density and electric field gradient at the iron nucleus of iron carbonyls Fe(CO)_n (n=2–5) so as to estimate the Mössbauer parameters, of these species. The calculated values were compared with the experimental values obtained in low temperature nitrogen matrices; structures of these species were optimized to fit the observed values.     

Control of absolute helicity in single-stranded abiotic metallofoldamers

Described is the design, synthesis, and characterization of abiotic, single-stranded metallofoldamers that adopt helical secondary structures upon metal complexation. The absolute helicity is determined by stereocenters at the ends of the structures and is enforced by the steric influence and hydrogen bonding ability of esters in the backbone of the foldamer. Folding of the structures is characterized in the solid state by X-ray crystallography and in solution by specific rotation, CD spectroscopy, and NMR spectroscopy.     

Synchronous helicity control in zinc bilinone trimer

In order to develop an artificial signal transmission/amplification system triggered by chiral recognition, we synthesized a series of zinc bilinone (ZnBL) trimers bearing a tripodal spacer and investigated homohelicity induction by complexation with chiral α-amino esters. Controlling the length of the peripheral alkyl groups in ZnBL moieties led to preorganization of the trimer to homohelical conformers. In addition, complexation with chiral α-amino esters induced the formation of the chiral homohelical conformer in which three ZnBL moieties adopted the same helicity.     

Enantioselective esterification of prochiral phosphonate pendants of a polyphenylacetylene assisted by macromolecular helicity: storage of a dynamic macromolecular helicity memory

A poly(phenylacetylene) bearing a phosphonic acid monoethyl ester as the pendant forms a one-handed helical structure induced by an optically active amine, and this helicity can be "memorized"after the amine is replaced by achiral diamines. The helicity memory lasts for an extremely long time but spontaneously disappears after the achiral diamines are removed by a stronger acid, indicating the dynamic nature of the helicity memory. Here we report that such a dynamic memory could be "stored" after the pendant was converted to its methyl ester with diazomethane, resulting in the generation of a phosphorus stereogenic center with optical activity. The esterification enantioselectively proceeded through chirality transfer from the induced helical conformation or the helicity memory of the polyacetylene backbone. Although the enantioselectivity was low, the pendant chirality was significantly amplified in the polymer backbone at low temperatures, resulting in higher optical activity as an excess single-handed helix than that expected from the enantiomeric excess of the pendants.     

Optical activity and helicity enhancement of diphenylmethyl molecular propellers

The diphenylmethyl group is a molecular propeller that can detect the chirality of a nearby stereogenic center through electronic circular dichroism spectra, within the short wavelength region of the phenyl ¹B transitions (190–200 nm). A positive exciton Cotton effect was associated with a positive (P) helicity of the diphenylmethyl group, while the opposite was true for M helicity. Higher helicity discrimination was observed in the presence of a hydroxy group (in hydroxydiphenylmethyl derivatives) which formed an intramolecular hydrogen bond, thus reducing the number of accessible conformers of the molecule. We found that the hydroxydiphenylmethyl groups in chiral tartaric acid TADDOL derivatives were preferably heterohelical (P, M).     

Spontaneous resolution of a racemic nickel(II) complex and helicity induction via hydrogen bonding: the effect of chiral building blocks on the helicity of one-dimensional chains

The reactions of a racemic four-coordinated nickel(II) complex [Ni(alpha-rac-L)](ClO4)2 (containing equal amount of SS and RR enantiomers) with l- and d-phenylalanine in acetonitrile/water gave two less-soluble six-coordinated enantiomers of {[Ni( f-SS-L)(l-Phe)](ClO4)}n (Delta-1) and {[Ni(f- RR-L)(d-Phe)](ClO4)}n (Lambda-1), respectively. Evaporation the remaining solutions gave two six-coordinated diastereomers of {[Ni 3(f- RR-L)3(l-Phe)2(H 2O)](ClO4)4}n (a-2) and {[Ni3(f- SS-L)3(d-Phe)2(H2O)](ClO4)4}n (b-2), respectively (L = 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane, Phe(-) = phenylalanine anion). The reaction of [Ni(alpha-rac-L)](ClO4)2 with dl-Phe(-) gave a conglomerate of c-1; in which, the SS and RR enantiomers preferentially coordinate to l- and d-Phe(-) respectively to give a racemic mixture of Delta-1 and Lambda-1, and the spontaneous resolution occurs during the reaction, in which each crystal crystallizes to become enantiopure. Removing Phe(-) from Delta-1 and Lambda-1 using perchloric acid gave two enantiomers of [Ni(alpha-SS-L)](ClO4)2 (S-3) and [Ni(alpha-RR-L)](ClO4)2 (R-3). Dissolving S-3 and R-3 in acetonitrile gave two six-coordinated enantiomers of [Ni( f-SS-L)(CH3CN)2](ClO4)2 (S-4) and [Ni( f- RR-L)(CH3CN)2](ClO4)2 (R-4), while dissolving [Ni(alpha-rac-L)](ClO4)2 in acetonitrile gave a racemic twining complex [Ni(f-rac-L)(CH3CN)2](ClO4)2 (rac-4). Delta-1 and Lambda-1 belong to supramolecular stereoisomers, which are constructed via hydrogen bond linking of [Ni( f-SS-L)(l-Phe)](+) and [Ni(f-RR-L)(d-Phe)](+) monomers to form 1D homochiral right-handed and left-handed helical chains, respectively. The reaction of S-3 with d-Phe(-) gave {[Ni(f-SS-L)(d-Phe)](ClO4)}n (5), which shows a motif of a 1D hydrogen bonded zigzag chain instead of a 1D helical chain. Compound a-2/ b-2 contains dimers of [{Ni(f-RR-L)}2(l-Phe)(H2O)](3+)/[{Ni( f- SS-L)}2(d-Phe)(H2O)](3+) and 1D zigzag chains of {[Ni(f-RR-L)(l-Phe)](+)}n /{[Ni(f-SS-L)(d-Phe)](+) n . The homochiral nature of Delta-1/Lambda-1, a-2/b-2, S-3/R-3, and S-4/R-4 are confirmed by the results of circular dichroism (CD) spectra measurements.     

Modulating helicity through amphiphilicity-tuning supramolecular interactions for the controlled assembly of perylenes

Here we show that it is possible to modulate the supramolecular assembly of designed H-bonding amphiphilic perylene-based materials through simple solvent interactions. These modulated supramolecular interactions have been translated to and observed in macroscopic properties, and provide new pathways to the preparation of switchable interfaces based on designed supramolecular interactions.