Leigh Ann Lipscomb

Resonant and nonresonant surface-enhanced hyper-Raman spectroscopy with a picosecond laser. Effect of the excitation pulse width

Abstract We report resonant and nonresonant surface-enhanced hyper-Raman scattering (SEHRS) spectra of basic fuchsin and 3-hydroxykynurenine in the Ag colloid system with a picosecond pulse laser of high repetition rate (82 MHz). It is demonstrated that, at constant average power, fiber-optic compression of the 100 ps fundamental output of a Nd:YAG laser to ≈ 5 ps increases the intensities of surface-enhanced hyper-Raman scattering and second-harmonic generation by approximately one order of magnitude. We have thus obtained high-quality SEHRS spectra for both molecules with only 0.1 W laser excitation. These results are significant in the development of SEHRS as a useful spectroscopic technique.

Surface-enhanced hyper-Raman spectroscopy with a picosecond laser: gold and copper colloids

Abstract We have obtained surface-enhanced hyper-Raman scattering (SEHRS) spectra of crystal violet, rhodamine 6G and Ru(trpy) (BPE) 3 2+ adsorbed on gold and copper colloidal surfaces (where trpy=2,2′,2″-terpyridine, BPE=trans-bis(4-pyridyl)ethylene). Our results demonstrate that the SEHRS effect is not intrinsically restricted to a Ag substrate and that surface enhancements at the emitted hyper-Raman photon frequencies are not required for observing SEHRS signals.

Surface-enhanced hyper-Raman spectroscopy

Abstract Hyper-Raman scattering (HRS), first theoretically predicted by Decius and Rauch in 1959 [l] and experimentally demonstrated by Terhune et al. in 1965 [2], is a nonlinear optical process involving two incident photons (ω0) and one emitted photon (ω). The emitted hyper- Raman photon frequencies are Raman-shifted relative to the second harmonic frequency (2ω0) of the incident laser radiation [3–6]. The energy difference (2ω0 – w) corresponds to one of the characteristicvibrational frequencies of the scattering medium or molecule. In Fig. 1 is given a schematic illustration of resonant and nonresonant HRS. The primary advantage of this nonlinear optical technique lies in its more relaxed selection rules compared with IR and Raman [7,8]. AlllR-active vibrational modes are hyper-Raman allowed, and those modes inactive in both IR and Raman (i.e., the “silent” modes) may be active in hyper-Raman scattering.

Main Group Metal Halide Complexes with Sterically Hindered Thioureas XIII. Crystallographic Study of a Unique Cross-Linked Polymeric Dichlorolead(II) Complex with 1,1′-methylenebis(3-methyl-2(3H)-imidazolethione)

Abstract Two complexes, PbCl 2 mbit and Pb(NCS) 2 mbit (mbit=1,1′-methylenebis(3-methyl-2)3 H )-imidazolethione)) have been synthesized and characterized. A single crystal X-ray study of PbCl 2 mbit revealed a unique cross-linked polymeric structure composed of very distorted PbCl 4 S 2 octahedra sharing common edges through long PbCl bridges. The bridging pairs of Cl atoms are displaced cis to each other in the coordination sphere of lead thus creating twisted polymeric PbCI strands running along the c axis. In the direction of the b axis, the strands are further linked by mbit ligands acting as bridges between neighboring twisted strands created by the lead chloride bonds. This is the first instance where mbit is shown to act a bridging ligand. There is no clear evidence for the lone pair of electrons through systematic distortions in the lead coordination sphere. A high melting point of 259–261 °C is consistent with the polymeric structure. Cell parameters are as follows for PbCl 2 mbit: space group Pbca, a =8.699(3), b =17.164(4), c =20.469(6) A, Z =8, V =3056.2 A 3 , D c =2.254 g/cm 3 , D o =2.23 g/cm 3 (λ=0.71073 A). R =0.047. Pb(NCS) 2 mbit complex was also synthesized and characterized, but crystals suitable for X-ray study were not obtained. IR stretching frequencies of the SCN group fall in the ranges observed for this ligand bonding only through N and no evidence for S bonding. The relatively high melting point (254–256 °C) gives evidence for a polymeric structure the details of which remain unknown.

Main group metal halide complexes with sterically hindered thioureas. XII: Crystallographic studies of lead(II) and cadmium(II) thiocyanate complexes with 1,3-dimethyl-2(3H)-imidazolethione

The complex, Pb(NCS){sub 2}(dmit){sub 2}, has been previously reported (dmit=1,3-dimethyl-2(3H)-imidazolethione). A new complex, Cd(NCS){sub 2}(dmit){sub 2}, has also been synthesized and characterized. Single crystal X-ray structural studies were completed for both complexes. Cell parameters are as follows, (Pb(NCS){sub 2}(dmit){sub 2}):space group C2/c, a=11.118(2), b=14.121(4), c=12.940(3){angstrom}, {beta}=108.29(2), Z=4, V=1933.4{angstrom}{sup 3}, D{sub c}=1.895 g/cm{sup 3}, D{sub o}=1.93 g/cm{sup 3} ({lambda}=0.71073{angstrom}). R=0.030. (Cd(NCS){sub 2}(dmit){sub 2}): space group C2/c, a=10.911(2), b=14.172(2), c=12.471(2){angstrom}, {beta}=107.54(1), Z=4, V=1838.7{angstrom}{sup 3}, D{sub c}=1.752 g/cm{sup 3}, D{sub o}=1.75 g/cm{sup 3} ({lambda}=0.71073{angstrom}). R=0.031. Both complexes are isostructural polymers comprised of distorted octahedra sharing common edges through bridging SCN groups. Thiourea ligands are displaced trans to each other; octahedra form a zig-zag chain through the SCN bridges with alternating N and S bonding to each metal atom. The SCN groups are almost perfectly linear. There is no clear evidence for the lone pair of electrons through systematic distortions in the lead structure although there are greater deviations from 90{degree} in the local site symmetry about Pb than about Cd.

Clathrate Hydrates Are Poor Models of Biomolecule Hydration

Clathrate hydrates form the basis of a general model of biomolecule hydration. In clathrate hydrate crystal structures, the size of hydrogen-bonded water rings is highly constrained to five members. The clathrate hydrate model predicts that the size of water rings near biomolecule surfaces is similarly constrained to five members. This report describes a test of this model of biomolecule hydration. We have demonstrated that five-membered water rings are not a general feature of protein or nucleic acid hydration. The clathrate hydrate model appears to be inappropriate for soluble biomolecules.

Asymmetry and dynamics in bis-intercalated DNA

The bis-intercalator ditercalinium (NSC 366241), composed of two 7 H-pyridocarbazoles linked by a bis(ethylpiperidinium), binds to DNA with a binding constant greater than 10(7) M-1. One distinctive aspect of the 3-D X-ray structure of a DNA-ditercalinium complex is its asymmetry. We propose here that the activity of ditercalinium may be related to structural polymorphism and dynamic conversion between conformers. It was previously reported that activity is closely related to linker composition. Activity increases with increasing conformational restraints of the linker. We suggest these conformational restraints can lead to asymmetry in DNA complexes and that this asymmetry results directly in structural polymorphism. Using the Cambridge Structural Database (CSD) as a source of information about chemical fragments that are analogous to the linker of ditercalinium, we have explored the conformational space available to ditercalinium. The results indicate that the linker is highly constrained and that the DNA complex is intrinsically asymmetric. We propose a reasonable mechanism of ring reversal that is consistent with the conformations of analogous fragments within the CSD.