Ultrafast mode-specific intermolecular vibrational energy transfer to liquid nitromethane

Using ultrafast infrared-Raman spectroscopy, which permits vibrationally selective pumping and probing of different molecules in polyatomic liquid solutions, efficient direct intermolecular vibrational mode-specific energy transfer is observed between alcohols and nitromethane in weakly associated complexes. Intermolecular transfer v(OH)_MeOH → v(NO₂)_NM, where v(OH)_MeOH is an OH stretching vibration of methanol (ω ≈ 3600 cm^-1) and v(NO₂)_NM is an NO₂ stretching vibration of nitromethane (ω ≈ 1400 cm^-1), occurs with an efficiency about 45% as great as the intramolecular process v(CH)_NM → v(NO₂)_NM, where v(CH)_NM is a CH stretching vibration of nitromethane (ω ≈ 3000 cm^-1). Ethanol and tert-butyl alcohol can also be vibrational energy donors, although the transfer efficiency to nitromethane decreases with increasing donor molecular weight. Diluting alcohol-nitromethane mixtures with CCl₄ has little effect on intermolecular energy transfer. Experiments using deuterated donors and acceptors show the mechanism of intermolecular transfer involves first an intramolecular step or steps, resulting in v(OH)_MeOH → δ(CH)_MeOH transfer, where δ(CH)_MeOH is a CH bending vibration. Then the dominant intermolecular process is δ(CH)_MeOH → v(NO₂)_NM. A lesser contribution from the intermolecular process v(OH)_MeOH → v(CH)_MeOH → v(CH)_NM (or v(CD)_MeOD → v(CD)_NM) is also inferred. The likelihood that many higher energy vibrational excitations (ω > 1600 cm^-1) can undergo efficient intermolecular vibrational energy transfer to the NO₂ group of nitromethane, a powerful explosive, suggests some intriguing possibilities for understanding energy concentration mechanisms which might lead to accidental detonations and for understanding why NO₂ is ubiquitous in secondary explosives.