Unidirectional Vibrational Energy Flow in Nitrobenzene

Abstract

Experiments were performed on nitrobenzene liquid at ambient temperature to probe vibrational energy flow from the nitro group to the phenyl group and vice versa. The IR pump, Raman probe method was used. Quantum chemical calculations were used to sort the normal modes of nitrobenzene into three categories: phenyl modes, nitro modes, and global modes. IR wavelengths in the 2500-3500 cm(-1) range were found that best produced excitations initially localized on nitro or phenyl. Pulses at 2880 cm(-1) excited a nitro stretch combination band. Pulses at 3080 cm(-1) excited a phenyl C-H stretch plus some nitro stretch. With nitro excitation there was no detectable energy transfer to phenyl. With phenyl excitation there was no direct transfer to nitro, but there was some transfer to global modes such as phenyl-nitro stretching, so some of the vibrational amplitude on phenyl moved onto nitro. Thus energy transfer from nitro to phenyl was absent, but there was weak energy transfer from phenyl to nitro. The experimental methods described here can be used to study vibrational energy flow from one part of a molecule to another, which could assist in the design of molecules for molecular electronics and phononics. The vibrational isolation of the nitro group when attached to a phenyl moiety suggests that strongly nonthermal reaction pathways may play an important role in impact initiation of energetic materials having peripheral nitro groups.

Two-level system in spin baths: Non-adiabatic dynamics and heat transport

“…It transports heat more effectively in one direction of the temperature bias than in the reversed direction [41]. This effect has been investigated in detail experimentally and theoretically at the macro-micro scale [42,43], the nanoscale [44], and more recently in the molecular realm [45][46][47], with proposals relying on phononic, electronic, and photonic heat conduction [41]. It was also recently demonstrated that Josephson tunnel junctions (Cooper-pair condensates electrodes) can act as strong phase-tunable thermal diodes [48].…”

Quantum bottlenecks and unidirectional energy flow in molecules

“…We have carried out this analysis on one of the substituted benzenes, toluene, that was the subject of recent ultrafast studies by Dlott and coworkers [43][44][45], which reveal a preference in some substituted benzenes for energy to flow along one direction. We have shown that such vibrational energy rectification can arise from quantum bottlenecks to energy relaxation from the initially excited state, in the case of toluene from the CH fundamental of the phenyl group, which generates a preference for energy to flow in the methyl-to-phenyl direction during the time of relaxation into the surroundings.…”

“…Because it is inevitably somewhat arbitrary we avoid classifying modes as local or global, as done to describe the modes of the substituted benzenes in which energy flow was monitored experimentally [43][44][45]. To address the possibility of asymmetric energy flow we instead follow the flow of energy into the environment from each region of the molecule, following local excitation by about 3000 cm −1 in each region.…”

“…Recently, Dlott and coworkers carried out timeresolved IR-Raman studies on a series of substituted benzenes in liquids [43][44][45]. They examined point-topoint energy transport in these molecules and observed an interesting asymmetry: Breaking up the molecule, such as nitrobenzene, into two regions, phenyl and substituent, they found that by exciting one region, say phenyl, with a 3000 cm −1 IR pulse the excess energy flows into the other region more easily than energy flows into the phenyl if the substituent is initially excited with a similar amount of energy, at least over times long enough for the energy to relax into the liquid (ca.…”

“…This effect may play a role in the quasi-unidirectional flow observed in some substituted benzenes, since even though they are in liquids the rate of intramolecular energy flow may be quite slow despite coupling to the surroundings [50]. However, energy flow to other vibrational states is observed within the relatively short time of relaxation to the surroundings [43][44][45]. Even without quantum localization, bottlenecks due to a low local density of states, i.e., a sparsity of vibrational states resonantly coupled to the initially excited state, or to the first few tiers of states into which energy flows at early times, can give rise, prior to energy relaxation into the liquid, to a preference for energy to flow in one direction along the molecule.…”