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 upsilon(OH)(MeOH) --> upsilon(NO2)(NM), where upsilon(OH)(MeOH) is an OH stretching vibration of methanol (omega approximate to 3600 cm(-1)) and upsilon(NO2)(NM) is an NO2 stretching vibration of nitromethane (omega approximate to 1400 cm(-1)), occurs with an efficiency about 45% as great as the intramolecular process upsilon(CH)(NM) --> upsilon(NO2)(NM), where upsilon(CH)(NM) is a CH stretching vibration of nitromethane (omega approximate to 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 accepters show the mechanism of intermolecular transfer involves first an intramolecular step or steps, resulting in upsilon(OH)(MeOH) --> delta(CH)(MeOH) transfer, where delta(CH)(MeOH) is a CH bending vibration. Then the dominant intermolecular process is delta(CH)(MeOH) --> upsilon(NO2)(NM). A lesser contribution from the intermolecular process upsilon(OH)(MeOH) --> upsilon(CH)(MeOH) --> upsilon(CH)(NM) (or upsilon(CD)(MeOD) --> upsilon(CD)(NM)) is also inferred. The likelihood that many higher energy vibrational excitations (omega > 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.