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.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry