ULTRAFAST MODE-SPECIFIC INTERMOLECULAR VIBRATIONAL-ENERGY TRANSFER TOLIQUID NITROMETHANE

Using ultrafast infrared-Raman spectroscopy, which permits vibrational ly selective pumping and probing of different molecules in polyatomic Liquid solutions, efficient direct intermolecular vibrational mode-spe cific 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 vi bration of methanol (omega approximate to 3600 cm(-1)) and upsilon(NO2 )(NM) is an NO2 stretching vibration of nitromethane (omega approximat e to 1400 cm(-1)), occurs with an efficiency about 45% as great as the intramolecular process upsilon(CH)(NM) --> upsilon(NO2)(NM), where up silon(CH)(NM) is a CH stretching vibration of nitromethane (omega appr oximate to 3000 cm(-1)). Ethanol and tert-butyl alcohol can also be vi brational energy donors, although the transfer efficiency to nitrometh ane decreases with increasing donor molecular weight. Diluting alcohol -nitromethane mixtures with CCl4 has Little effect on intermolecular e nergy transfer. Experiments using deuterated donors and accepters show the mechanism of intermolecular transfer involves first an intramolec ular step or steps, resulting in upsilon(OH)(MeOH) --> delta(CH)(MeOH) transfer, where delta(CH)(MeOH) is a CH bending vibration. Then the d ominant intermolecular process is delta(CH)(MeOH) --> upsilon(NO2)(NM) . A lesser contribution from the intermolecular process upsilon(OH)(Me OH) --> upsilon(CH)(MeOH) --> upsilon(CH)(NM) (or upsilon(CD)(MeOD) -- > upsilon(CD)(NM)) is also inferred. The likelihood that many higher e nergy vibrational excitations (omega > 1600 cm(-1)) can undergo effici ent intermolecular vibrational energy transfer to the NO2 group of nit romethane, a powerful explosive, suggests some intriguing possibilitie s for understanding energy concentration mechanisms which might lead t o accidental detonations and for understanding why NO2 is ubiquitous i n secondary explosives.