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.

Ultrafast infrared (IR) Raman spectroscopy was used to study vibrational energy in ϕ-S alkylbenzenes, where ϕ = C6H5 and substituents S were CH3- (toluene), (CH3)2CH- (isopropylbenzene, IPB), or (CH3)3C- (t-butylbenzene, TBB). Using methods described previously,1 the normal modes were classified as phenyl (ϕ), substituent (S), or global (G). IR pulses were tuned to find conditions that maximized the localization of initial CH-stretch excitations on ϕ or S. Anti-Stokes Raman spectroscopy measured transient energy content of Raman-active S, ϕ, and G modes, to determine the rates of phenyl to substituent (Φ → S) or substituent to phenyl (S → Φ) transfer during the first few picoseconds, when energy transfer was mainly intramolecular. Since phenyl CH-stretches were 90-130 cm(-1) uphill in energy from substituent CH-stretches, of interest were S → Φ processes where molecular structure and local couplings were more important than energy differences. The Φ → S process efficiencies were small and about equal with all three substituents. The S → Φ transfer efficiencies could be increased by increasing substituent size. This was opposite to what would be predicted on the basis of the larger density of states of larger substituents, and it provides a path toward controlling forward-to-backward vibrational energy transfer ratios. The S → Φ transfer efficiency is understood as resulting from an increase in the local anharmonic couplings. A heavier substituent, when vibrating, transfers energy more effectively to the phenyl group.

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We introduce a novel type of three-dimensional (3D) spectroscopy to study vibrational energy transfer, where an IR pulse tunable through the CH-stretching and CD-stretching regions was used to create parent vibrational excitations in liquids and a visible probe pulse was used to generate both Stokes and anti-Stokes Raman spectra as a function of delay time. The Raman spectra determine how much vibrational excitation was present in each probed state. The three dimensions are the wavenumber of the pumped state, the wavenumber of the probed state, and the time interval. The technique was used to study nitromethane (NM) and acetonitrile (ACN) and their deuterated analogues at ambient temperature. The 3D spectra were quite complicated. Three types of artifacts due to nonlinear light scattering were observed. Along the diagonal were two fundamental CH-stretch (or CD-stretch) transitions and several weaker combination bands or overtone transitions. Because Raman spectroscopy allows us to simultaneously probe a wide wavenumber region, for every diagonal peak, there were ∼10 off-diagonal peaks. The cross-peaks at shorter delay times reveal the nature of the initial excitation by showing which lower-wavenumber excitations were produced along with the pumped CH-stretch or CD-stretch. The longer-time spectra characterized vibrational energy relaxation processes, and showed how daughter vibrations were generated by different parent excitations.

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