51 research outputs found
Modifying Vibrational Energy Flow in Aromatic Molecules: Effects of Ortho Substitution
Ultrafast infrared (IR) Raman spectroscopy
was used to measure
vibrational energy transfer between nitrobenzene nitro and phenyl
groups, in the liquid state at ambient temperature, when ortho substituents
(−CH<sub>3</sub>, −F) were introduced. Quantum chemical
calculations were used to assign the vibrations of these molecules
to three classes, phenyl, nitro, or global. Combining transient anti-Stokes
and Stokes Raman spectra determined the energies of multiple molecular
vibrational modes, which were summed to determine the aggregate energies
in the phenyl, nitro, or global modes. In a previous study (Pein, B. C. ; Sun, Y.; Dlott, D. D., J. Phys. Chem. A 2013, 117, 6066−6072) it was shown that, in nitrobenzene, there
was no energy transfer from nitro to phenyl or from nitro to global
modes, but there was some transfer from phenyl to nitro and phenyl
to global. The ortho substituents activated energy flow from nitro-to-phenyl
and nitro-to-global and reduced phenyl-to-nitro flow. The −CH<sub>3</sub> substituent entirely shut down the phenyl-to-nitro pathway,
presumably by efficiently directing some of the phenyl energy into
methyl bending excitations. There is (inefficient) unidirectional
vibrational energy flow in nitrobenzene only in the nitro-to-phenyl
direction, whereas in <i>o</i>-nitrotoluene, vibrational
energy flows only in the nitro-to-phenyl direction
Structural Transition in an Ionic Liquid Controls CO<sub>2</sub> Electrochemical Reduction
Broad-band multiplex vibrational
sum-frequency generation spectroscopy
(SFG) was used to study CO<sub>2</sub> reduction on a polycrystalline
Ag electrode with a room-temperature ionic liquid (RTIL) electrolyte,
1-ethyl-3-methylimidazolium tetrafluoroborate (EMIM-BF<sub>4</sub>), with 0.3 mol % water. The Ag/RTIL/H<sub>2</sub>O system has been
shown to reduce CO<sub>2</sub> with low overpotential and, depending
on water concentration, with Faradaic efficiency of nearly 100% (Rosen,
B. A.; Salehi-Khojin, A.; Thorson, M. R.; Zhu, W.; Whipple, D. T.;
Kenis, P. J. A.; Masel, R. I. <i>Science</i> <b>2011</b>, <i>334</i>, 643–644). The adsorbed CO created
by CO<sub>2</sub> reduction was probed with infrared (IR) pulses tuned
to the CO stretch. Nonresonant (NR) SFG was used to probe the double
layer. SFG showed that CO binds weakly to Ag at the CO<sub>2</sub> reduction threshold of −1.33 V (vs Ag/AgCl), so CO does not
poison the surface. At potentials equal to or more negative than the
threshold, the curvature of the parabolic potential-dependent NR intensity
significantly increased, and the Stark shift of adsorbed CO, a measure
of the surface field, more than doubled. The curvature increase indicates
a potential-driven structural transition in the RTIL within the double
layer. This transition was a property of the RTIL itself since it
occurred whether or not CO<sub>2</sub> was present. Significantly,
the RTIL transition and the increased surface field occurred precisely
at the CO<sub>2</sub> reduction threshold. Thus, we have demonstrated
a close association between an electrochemically driven structural
transition of the RTIL and low overpotential CO<sub>2</sub> reduction
SURFACES AND INTERFACES OF HIGH EXPLOSIVES PROBED BY NONLINEAR OPTICAL SPECTROSCOPY
Author Institution: SCHOOL OF CHEMICAL SCIENCES, UNIVERSITY OF ILLINOIS URBANA CHAMPAIGN, 600 S MATTHEWS, URBANA, IL 61801The surfaces and interfaces of plastic explosives (PBX 9501) were studied using vibrational sum-frequency generation spectroscopy (SFG). SFG is a surface and interface selective nonlinear optical spectroscopy technique with which we study the interfaces within plastic explosive. We are interested in the explosive crystal to plastic binder interface and also the explosive crystal to explosive crystal interface. We initially studied the surfaces of the components of plastic explosives i.e. HMX crystals and a plastic binder, Estane. Our initial results showed that solution grown -HMX crystals has small deposits of the -HMX isomorphs on its surface. We also found that rapid evaporation from droplets of HMX solution produces nanocrystals of only -HMX; presumably because the polar boat conformation of -HMX is stabilize by polar solvents. A detail study of the surface vibrational modes of -HMX was also carried out on a cleaved HMX crystal
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