In operando measurements of high explosives.

In operando studies of high explosives involve dynamic extreme conditions produced as a shock wave travels through the explosive to produce a detonation. Here, we describe a method to safely produce detonations and dynamic extreme conditions in high explosives and in inert solids and liquids on a tabletop in a high-throughput format. This method uses a shock compression microscope, a microscope with a pulsed laser that can launch a hypervelocity flyer plate along with a velocimeter, an optical pyrometer, and a nanosecond camera that together can measure pressures, densities, and temperatures with high time and space resolution (2 ns and 2 µm). We discuss how a detonation builds up in liquid nitromethane and show that we can produce and study detonations in sample volumes close to the theoretical minimum. We then discuss how a detonation builds up from a shock in a plastic-bonded explosive (PBX) based on HMX (1,3,5,7-Tetranitro-1,3,5,7-tetrazocane), where the initial steps are hotspot formation and deflagration growth in the shocked microstructure. A method is demonstrated where we can measure thermal emission from high-temperature reactions in every HMX crystal in the PBX, with the intent of determining which configurations produce the critical hot spots that grow and ignite the entire PBX.

Laser pulses into bullets: tabletop shock experiments.

This article discusses tabletop high-throughput laser experiments on shock waves in solids and liquids, where the more usual laser pump pulse is replaced by a 0.5 mm diameter laser-launched bullet, a thin metal disk called a flyer plate. The hypervelocity flyer (up to 6 km s-1 or Mach 18) can have kinetic energy (∼1 J) to briefly produce extreme conditions of temperature and pressure, thousands of K and tens of GPa (1 GPa = 10 000 bar) in a small volume with a rise time <2 ns. The experiments are performed using a "shock compression microscope", a microscope fitted with the laser flyer launcher plus an optical velocimeter, a high-speed laser interferometer that measures the motion of the flyer plate or the sample material after impact. This makes it possible to generate extreme conditions at the push of a button in an intrinsically safe environment, and probe with any of the diagnostics used in microscope experiments, such as high-speed video, optical emission, nonlinear coherent spectroscopies and so on. The barrier to entering this field is relatively low since many laser laboratories already possess much of the needed instrumentation. A brief introduction to shock waves and instrumentation is presented. Then several examples of recent applications are described, including shocked water, the photophysics of fluorescent molecules under extreme conditions, shocked protein solutions, shocked metal-organic frameworks (MOFs), shocked explosives, chemical catalysis in a shocked liquid, and molecules at shocked interfaces. Since one can shoot a bullet at practically anything, there are many emerging opportunities in chemistry, biophysics, materials science, physics and hypervelocity aerodynamics.

Thermo-mechanical behavior measurement of polymer-bonded sugar under shock compression using in-situ time-resolved Raman spectroscopy.

Quantitative information regarding the local behavior of interfaces in an inhomogeneous material during shock loading is limited due to challenges associated with time and spatial resolution. This paper reports the development of a novel method for in-situ measurement of the thermo-mechanical response of polymer bonded sugar composite where measurements are performed during propagagtion of shock wave in sucrose crystal through polydimethylsiloxane binder. The time-resolved measurements were performed with 5 ns resolution providing an estimation on local pressure, temperature, strain rate, and local shock viscosity. The experiments were performed at two different impact velocities to induce shock pressure of 4.26 GPa and 2.22 GPa and strain rate greater than 106/s. The results show the solid to the liquid phase transition of sucrose under shock compression. The results are discussed with the help of fractography analyses of sucrose crystal in order to obtain insights into the underlying heat generation mechanism.

Shock Pressure Dependence of Hot Spots in a Model Plastic-Bonded Explosive.

Shock initiation of plastic-bonded explosives (PBX) begins with the formation of so-called "hot spots", which are energetic reactions localized in regions where the PBX microstructure concentrates the input shock wave energy. We developed a model PBX system to study hot spots which consists of a single crystal of the high explosive HMX (cyclotetramethylene-tetranitramine) embedded in a transparent polyurethane binder (J. Phys Chem. A, 2020, 124, 4646-4653). In the current work we use this model system to study the influence of input shock pressure (12-26 GPa) on hot spot generation using micrometer-resolved high-speed imaging and nanosecond-resolved optical pyrometry. By shocking ∼100 HMX single crystals (HMX-SC), two distinct shock pressure thresholds were observed. The threshold for producing single hot spots in some crystals was 15 GPa. At 23 GPa, hot spot density was sufficiently high to lead to rapid deflagration of the entire HMX-SC. It takes about 25 ns after the shock passes for the hot spots to appear to our visible-light detection apparatus which has a noise floor at about 2000 K. That indicates the shock produces nascent hot spots that undergo a thermal explosion that reaches temperatures >2000 K in 25 ns. The initial hot spot temperature is roughly 3800 K which settles down to 3400 K, the adiabatic flame temperature of HMX. The higher initial temperature is attributed to release of stored interfacial strain energy produced by the shock. An initial estimate for the velocity of the flame front originating at an HMX hot spot is 550 m/s.

Ethylenediamine Catalyzes Nitromethane Shock-to-Detonation in Two Distinct Ways.

Adding amines to liquid nitromethane (NM) is known to lower the threshold for the shock-to-detonation transition because amines catalyze proton transfer reactions that are the initial steps in the energy release process. We studied NM with 1 wt % ethylenediamine (NM/EDA) with 4 ns input shocks using time and space resolved diagnostics: photon Doppler velocimetry (PDV), optical pyrometry, and nanosecond video imaging. The 4 ns shocks are fast enough to time-resolve the reaction kinetics and the shock-to-detonation transition. We find that it is possible to shock ignite the NM/EDA without producing a detonation, so there is more to amine sensitization of the shock-to-detonation process than simply lowering the barrier to initial reactions. We find that although 1 wt % EDA has little effect on the ambient properties of NM, it dramatically alters the Hugoniot. The shock speed in NM/EDA is reduced, indicating that shocked NM/EDA is significantly more compressible than NM. Higher compressibility is associated with greater adiabatic heating, so EDA both lowers the barrier to proton transfer reactions and increases shock energy absorption. To explain the enhanced compressibility, we propose that shocking NM/EDA produces a reactive flow that has a much higher ionic strength than in NM. The sudden transformation from a molecular liquid to an ionic liquid with stronger intermolecular interactions is responsible for enhanced compressibility and shock heating.

Mechanochemistry of Metal-Organic Frameworks under Pressure and Shock.

ConspectusMetal-organic framework solids (MOFs) are synthetic nanoporous materials that have drawn intense efforts in synthesis and characterization of chemical properties, most notably for their ability to adsorb liquids and gases. They are constructed as "node-spacer" nanostructured materials: metal centers (ions or clusters) connected by organic linkers (commonly containing carboxylate or imidazolate groups) to form crystalline, extended, often highly nanoporous structures. MOFs exhibit a variety of advantages over conventional porous materials: rationally designed synthesis of desired crystal structures and crystal engineering become feasible; great synthetic versatility and ease of incorporating different chemical functionalities are realized; and the use of lightweight organic linkers allows for ultrahigh surface area and porosity previously not accessible to conventional materials (i.e., zeolites and porous carbon). As a consequence, MOFs show great promise for a rapidly expanding collection of applications such as gas storage, separations, catalysis, sensing, and drug delivery.The mechanochemistry of MOFs and their response to shock waves, which we discuss in this Account, have been only partially explored. Mechanochemistry, the connection between the mechanical and the chemical worlds, has ancient origins. Rubbing sticks together to start a fire is mechanochemistry. Only in the past decade or so, however, has mechanochemistry gained a notable focus in the chemical community. In the following discussion, we present a general introduction to the complex mechanochemical behavior of MOFs both under quasi-static compression and under shock loading created by high-speed impact. During elastic deformation, MOFs undergo reversible structural or phase transitions. Plastic deformation of MOFs can result in mechanochemistry and can permanently modify the crystal structure, the pore dimensions and configuration, and the chemical bonding. The large energies required to induce bond rearrangement during plastic deformation suggest an interesting potential of MOFs for shock wave mitigation applications.MOFs are promising materials for shock energy dissipation because of the high density of nanopores which can absorb shock energy as they collapse. We have recently developed a platform to assess shock wave energy attenuation by MOFs and other powdered materials. It uses a tabletop laser-driven flyer plate to impact MOF samples at velocities of up to 2.0 km/s. The pressure of the shock waves that break out from the MOF sample can be measured by photon Doppler velocimetry. By measuring the shock profiles of MOF layers with different thicknesses, we can determine the shock pressure attenuation by the MOF layer. We have identified the two-wave structure of shocks in MOFs caused by nanopore collapse. Electron micrographs of recovered shocked MOFs show distinct zones in the shocked material corresponding to shock powder compaction, nanopore collapse, and chemical bond destruction.

Observing Hot Spot Formation in Individual Explosive Crystals Under Shock Compression.

The formation of hot spots in dynamically compressed, plastic-bonded explosives is known to be the primary mechanism by which these materials ignite and initiate, but hot spots are small, fleeting, and hard to observe. Using a microscope equipped with laser-launched, miniflyer plates, we have studied hot spots in small grains of cyclotetramethylene-tetranitramine (HMX) embedded in a polyurethane binder, shocked to about 20 GPa. A nanosecond video with 4 µm spatial resolution is used to observe hot spot formation and growth, while nanosecond optical pyrometry measured temperature. Using individual ∼200 µm nominally single crystals of HMX (HMX-SC), we observed hot spots forming preferentially on corners or edges. These hot spots are about 4000 K. When there are multiple hot spots, the flame propagated along crystal edges, and the crystal is mostly combusted after about 300 ns. Using polycrystalline grains (HMX-PC), 6000 K hot spots are created near internal defects or crystal junctions. However, the thermal mass of the material at 6000 K is quite small, so after those hot spots cool down, the HMX combustion is similar to the single crystals. Comparing a HMX-based polymer-bonded explosive (PBX) to the individual polymer-bonded HMX-SC and HMX-PC grains shows that the myriad hot spots in the PBX are hotter than HMX-SC and colder than HMX-PC, but they persist for a longer time in PBX than in the individual grains.

Shock Wave Energy Absorption in Metal-Organic Framework.

Recent investigations into the mechanical properties and mechanochemical reactions of metal-organic frameworks (MOFs) have suggested the potential for energy dissipation by multiple mechanisms. Although the possibility of efficient multifunctional shock dissipation by MOFs was suggested by static high pressure studies, there is little known about MOFs under shock compression. Here, we measure the attenuation of shock wave by the MOF denoted zeolitic-imidazolate framework (ZIF-8) in its desolvated, porous state. We find that shock wave dissipation by ZIF-8 occurred by multiple processes: powder compaction, nanopore-collapse, and chemical bond-breakage. The shock energy absorbance in ZIF-8 is proportional to ZIF-8 thickness, allowing the prediction of the thickness of MOF layer needed to attenuate shock waves to a desired lower energy. Compared with PMMA, often used as a standard, ZIF-8 attenuates 7 times more shock energy per unit mass for impacts at a lower velocity of 0.75 km/s and 2.5 times more at a higher velocity of 1.6 km/s. This research illustrates how to improve the ability to attenuate shock waves for personnel and equipment protection by engineering multifunctionality into the shock wave absorbing armor material.

Drop hammer with high-speed thermal imaging.

The drop hammer test is the easiest way to assess the sensitivity of explosive materials, but drop hammer results for low-velocity impacts have not been able to explain how explosives will react to other kinds of initiating stimuli. In order to do that, we have to understand the fundamental mechanisms of drop hammer initiation and how they differ from other initiation methods. For this reason, there is interest in instrumented drop hammers that help reveal what the drop hammer does at a fundamental level. We have developed a drop hammer that combines two types of mid-wavelength infrared (MWIR) imagers that, when operated simultaneously, can detect both the rapid explosion and slower combustion from impact-initiated polymer-bonded explosives with high time (1 µs) and space (15 µm) resolution. Results are presented that show how to vibration isolate the drop hammer to minimize MWIR image shaking during impact and to quantify the noise floor for MWIR temperature determinations via optical pyrometry. Experiments were performed on polymer-encased crystals of RDX ([CH2-NNO2]3) and HMX ([CH2-NNO2]4). Our experiments showed that drop-hammer initiated explosions occur in two phases with roughly 100 µs between explosions. Drop-hammer initiation is compared to an ultrasonic hammer, which initiates explosions by rapid frictional rubbing of the explosive surfaces against the surrounding polymer. The explosion rise time is faster with the drop hammer because the drop hammer inputs energy throughout the explosive volume, whereas the ultrasonic hammer produces localized heating and much more heat at the explosive surface.

Effects of water on low-overpotential CO2 reduction in ionic liquid studied by sum-frequency generation spectroscopy.

We used vibrational sum-frequency generation spectroscopy (SFG) to investigate low-overpotential CO2 reduction on a polycrystalline Ag electrode using room temperature ionic liquid (RTIL), 1-ethyl-3-methylimidazolium tetrafluorborate (EMIM-BF4) electrolyte mixtures with 0.3 mol%, 45 mol% and 77 mol% water. Adding water dramatically increases CO2 reduction efficiency up to 87.5 mol%. We found added water reduces the (negative) threshold potential for CO2 reduction from -1.33 V to -0.9 V. Added water also moved the potentials of the nonresonant (NR) SFG minima and caused the CO Stark shift to increase in concert with the reduction threshold. In previous work (N. García Rey and D. D. Dlott, J. Phys. Chem. C, 2015, 119, 20892-20899), with nearly-dry RTIL electrolyte (0.3 mol% water), we concluded a potential-driven structural transition of RTIL in the double layer controlled CO2 reduction. At lower water concentrations, where CO2 reduction was less efficient, CO product appeared primarily on Ag atop sites. At higher water concentrations where CO2 reduction efficiency was greater, adsorbed CO was observed on multiply-bonded sites, which are likely more efficient catalytic sites.

Ultrafast Proton Transfer in Polymer Blends Triggered by Shock Waves.

We describe ultrafast proton transfer in the ground electronic state triggered by the use of shock waves created by high-speed impacts. The emission of Nile Red (NR), a polarity sensing dye, was used to probe the effects of shock compression in a series of polymers, including polymer Brønsted bases blended with organic acid proton donors. NR undergoes a shock-induced red-shift due to an increase both in density and in polymer polarity. In blends with poly(4-vinylpyridine) (PVP) and phenol, NR showed an excess shock-induced red-shift with a distinct time dependence not present in controls that are incapable of proton transfer. The excess red-shift first appeared with 0.8 km·s-1 impacts. Occurring in ca. 10 ns, this NR red-shift was caused by the formation of an ion pair created by shock-triggered proton transfer from phenol to PVP.

Shock Wave Chemistry in a Metal-Organic Framework.

Metal-organic frameworks (MOFs) have potential applications as energy absorbing materials for shock wave energy mitigation due to their nanoporosity. Here we have examined km/s laser-driven flyer plate impacts on a prototypical MOF, ZIF-8. We observed particle fragmentation and morphological changes in microcrystals of ZIF-8 at lower shock pressures (≈2.5 GPa), and amorphization and structural collapse at higher pressures (≈8 GPa). High-speed emission spectroscopy revealed that 50 ns after flyer plate impacts, an emission pulse was generated by ZIF-8 resulting from chemical bonds that were broken and subsequently reformed. MOFs may prove useful in the dissipation of shock wave energy through large structural changes (free volume collapse and endothermic bond breakage).

Multichannel emission spectrometer for high dynamic range optical pyrometry of shock-driven materials.

An emission spectrometer (450-850 nm) using a high-throughput, high numerical aperture (N.A. = 0.3) prism spectrograph with stepped fiberoptic coupling, 32 fast photomultipliers and thirty-two 1.25 GHz digitizers is described. The spectrometer can capture single-shot events with a high dynamic range in amplitude and time (nanoseconds to milliseconds or longer). Methods to calibrate the spectrometer and verify its performance and accuracy are described. When a reference thermal source is used for calibration, the spectrometer can function as a fast optical pyrometer. Applications of the spectrometer are illustrated by using it to capture single-shot emission transients from energetic materials or reactive materials initiated by km⋅s-1 impacts with laser-driven flyer plates. A log (time) data analysis method is used to visualize multiple kinetic processes resulting from impact initiation of HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine) or a Zr/CuO nanolaminate thermite. Using a gray body algorithm to interpret the spectral radiance from shocked HMX, a time history of temperature and emissivity was obtained, which could be used to investigate HMX hot spot dynamics. Finally, two examples are presented showing how the spectrometer can avoid temperature determination errors in systems where thermal emission is accompanied by atomic or molecular emission lines.

Emission Lifetimes of a Fluorescent Dye under Shock Compression.

The emission lifetimes of rhodamine 6G (R6G) were measured under shock compression to 9.1 GPa, with the dual intents of better understanding molecular photophysics in extreme environments and assessing the usefulness of fluorescence lifetime microscopy to measure spatially dependent pressure distributions in shocked microstructured media. R6G was studied as free dye dissolved in poly(methyl methacrylate) (PMMA), or dye encapsulated in silica microparticles suspended in PMMA. Thin layers of these materials in impedance-matched geometries were subjected to planar single-stage shocks created by laser-driven flyer plates. A synchronized femtosecond laser excited the dye at selected times relative to flyer plate arrival and the emission lifetimes were measured with a streak camera. Lifetimes decreased when shocks arrived. The lifetime decrease was attributed to a shock-induced enhancement of R6G nonradiative relaxation. At least part of the relaxation involved shock-enhanced intersystem crossing. For free dye in PMMA, the lifetime decrease during the shock was shown to be a linear function of shock pressure from 0 to 9 GPa, with a slope of -0.22 ns·GPa(-1). The linear relationship makes it simple to convert lifetimes into pressures. Lifetime measurements in shocked microenvironments may be better than emission intensity measurements, because lifetimes are sensitive to the surrounding environment, but insensitive to intensity variations associated with the motion and optical properties of a dynamically changing structure.

Ultrasonic hammer produces hot spots in solids.

Mechanical action can produce dramatic physical and mechanochemical effects when the energy is spatially or temporally concentrated. An important example of such phenomena in solids is the mechanical initiation of explosions, which has long been speculated to result from 'hot spot' generation at localized microstructures in the energetic material. Direct experimental evidence of such hot spots, however, is exceptionally limited; mechanisms for their generation are poorly understood and methods to control their locations remain elusive. Here we report the generation of intense, localized microscale hot spots in solid composites during mild ultrasonic irradiation, directly visualized by a thermal imaging microscope. These ultrasonic hot spots, with heating rates reaching ~22,000 K s(-1), nucleate exclusively at interfacial delamination sites in composite solids. Introducing specific delamination sites by surface modification of embedded components provides precise and reliable control of hot spot locations and permits microcontrol of the initiation of reactions in energetic materials including fuel/oxidizer explosives.

Temperature-dependent dynamic response to flash heating of molecular monolayers on metal surfaces: vibrational energy exchange.

An ultrafast nonlinear coherent laser spectroscopy termed vibrational sum-frequency generation (SFG) was used to monitor vibrational transitions of a self-assembled monolayer (SAM) of 4-nitrobenzenethiolate (NBT) on Au after Au flash heating. Ultrafast thermoreflectance measurements showed the surface temperature jumps ΔT were in the 35-250 K range. The NBT symmetric and antisymmetric nitro stretches νsNO2 and νasNO2 and a phenyl ring stretch νCC were probed. Flash heating caused these transitions to lose intensity, shift, and broaden. The time dependences all had overshoot-decay-plateau structures. In the long-lived plateau, the SAM was in thermal equilibrium with the hot Au surface. The SFG plateau intensity losses of νsNO2 and νCC, two vibrations with parallel transition moments, were identical, indicating that the SFG intensity loss was caused by thermally induced SAM orientational disorder. The T-jump-induced frequency shifts of νsNO2 and νasNO2 were identical and opposite in sign. The rise times of the shifts were identical and equal to the ∼3.5 ps time constant for the rise of Au surface temperature, which indicates that both shifts were caused by anharmonic coupling to the same lower-energy vibration. The temperature dependence of the νsNO2 shift and width indicated that this vibration was the ∼480 cm(-1) nitro bend. The νsNO2 temperature dependence was interpreted using a vibrational energy exchange mechanism between the nitro stretch and bend.

Laser-driven flyer plates for shock compression science: launch and target impact probed by photon Doppler velocimetry.

We investigated the launch and target impact of laser-driven Al flyer plates using photon Doppler velocimetry (PDV). We studied different flyer designs launched by laser pulses of different energies, pulse durations and beam diameters, that produced km s(-1) impacts with transparent target materials. Laser-launching Al flyers 25-100 µm thick cemented to glass substrates is usually thought to involve laser vaporization of a portion of the flyer, which creates many difficulties associated with loss of integrity and heating of the flyer material. However, in the system used here, the launch mechanism was surprising and unexpected: it involved optical damage at the glass/cement/flyer interface, with very little laser light reaching the flyer itself. In fact the flyers launched in this manner behaved almost identically to multilayer flyers that were optically shielded from the laser pulses and insulated from heat generated by the pulses. Launching flyers with nanosecond laser pulses creates undesirable reverberating shocks in the flyer. In some cases, with 10 ns launch pulses, the thickest flyers were observed to lose integrity. But with stretched 20 ns pulses, we showed that the reverberations damped out prior to impact with targets, and that the flyers maintained their integrity during flight. Flyer impacts with salt, glass, fused silica, and acrylic polymer were studied by PDV, and the durations of fully supported shocks in those media were determined, and could be varied from 5 to 23 ns.

Picosecond dynamics of hydrogen bond rearrangements during phase separation of a triethylamine and water mixture.

The earliest stages of phase separation in a liquid triethylamine (TEA)-water mixture were observed using a picosecond IR laser pulse to produce a temperature jump and ultrafast Raman spectroscopy. Raman spectral changes in the water OH stretching region showed that the temperature rise induced by IR pulses equilibrated within a few tens of picoseconds. Amplitude changes in the TEA CH-stretching region of difference Raman spectra consisted of an initial faster and a subsequent slower process. The faster process within 100 ps is attributed to hydrogen bond weakening caused by the temperature rise. The slower process attributed to phase separation was observed for several nanoseconds, showing the number of hydrogen bond between TEA and water gradually decreased with time. The kinetics of hydrogen bond scission during phase separation indicated a linear growth of the phase-separated component, as observed previously on the nanosecond time scale, rather than the more usual exponential growth. A peak blueshift was observed in the difference Raman spectra during phase separation. This shift implies that hydrogen bond scission of TEA-water aggregates involving very few water molecules took place in the initial stage of phase separation (up to 2 ns), and then was followed by the breaking of TEA-water pairs surrounded by water molecules. This effect may be a result from spatial inhomogeneities associated with the phase separation process: aggregates or clusters existing naturally in solution even below the lower critical soluble temperature.

Hot spots in energetic materials generated by infrared and ultrasound, detected by thermal imaging microscopy.

We have observed and characterized hot spot formation and hot-spot ignition of energetic materials (EM), where hot spots were created by ultrasonic or long-wavelength infrared (LWIR) exposure, and were detected by high-speed thermal microscopy. The microscope had 15-20 µm spatial resolution and 8.3 ms temporal resolution. LWIR was generated by a CO2 laser (tunable near 10.6 µm or 28.3 THz) and ultrasound by a 20 kHz acoustic horn. Both methods of energy input created spatially homogeneous energy fields, allowing hot spots to develop spontaneously due to the microstructure of the sample materials. We observed formation of hot spots which grew and caused the EM to ignite. The EM studied here consisted of composite solids with 1,3,5-trinitroperhydro-1,3,5-triazine crystals and polymer binders. EM simulants based on sucrose crystals in binders were also examined. The mechanisms of hot spot generation were different with LWIR and ultrasound. With LWIR, hot spots were most efficiently generated within the EM crystals at LWIR wavelengths having longer absorption depths of ∼25 µm, suggesting that hot spot generation mechanisms involved localized absorbing defects within the crystals, LWIR focusing in the crystals or LWIR interference in the crystals. With ultrasound, hot spots were primarily generated in regions of the polymer binder immediately adjacent to crystal surfaces, rather than inside the EM crystals.

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 (-CH3, -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 -CH3 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 o-nitrotoluene, vibrational energy flows only in the nitro-to-phenyl direction.