Production of the Nitromethane Aci Ion by Static High Pressure

Our observation that pressure causes an increased aci ion concentration provides an ... We present experimental evidence that very high static pressur...
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J . Phys. Chem. 1988, 92, 6815-6819

6815

Production of the Nitromethane Aci Ion by Static High Pressure Ray Engelke,* David Schiferl, C. B. Storm, and William L. Earl Los Alamos National Laboratory, Los Alamos. New Mexico 87545 (Received: March 28, 1988)

We present experimental evidence that static high pressure (=2 GPa) produces an increased concentration of the aci ion [CH2N02-]of nitromethane [CH3N02]. The presence of this form is inferred from hydrogen/deuterium isotope exchange between CH3N02and CD3N02induced by pressure. Control experiments in which isotope exchange is p r o d u d chemically and in which it is shown that temperature alone does not produce the exchange are described. The Raman spectral diagnostic used to identify the isotope-exchange process at high pressure is analyzed by use of ab initio calculations of the vibrational spectra of the four H/D nitromethane isotopomers. Ambient-pressure I3C NMR was used to establish the chemistry involved in the isotope-exchangeprocess. Our observation that pressure causes an increased aci ion concentration provides an explanation of nitromethane’s observed pressure-dependent self-explosion behavior.

I. Introduction We present experimental evidence that very high static pressure (=2 GPa) produces an increased concentration of the nitronate (enolate-like) anion of nitromethane (NM). This result allows one to rationalize previous observations of NM’s static highpressure decomposition (“self-explosion”). Nitro compounds undergo a large number of reactions that are promoted by the electron-withdrawing nature of the nitro group. In the case of nitroalkanes, these reactions are frequently discussed in analogy to keto-enol tautomerism in normal ketones.’ For N M , this tautomerism is eq 1. CH2N02-is the nitronate anion V

2CH3N02

CH2=N02-

+ CH3N02H+

?

CH2=N02H CH3N02 (1) of NM. The nitronate anion and nitronic acid [CH2N02H]are also referred to as the aci ion and aci form of NM; below we use these terms interchangeably. Nitroalkanes are weak acids. The pKa of N M is reported2 to be between 10 and 20, compared to a pKa of about 49 for methane.’ In earlier work, we have demonstrated that the aci ion of N M is an important species in the high-pressure decomposition of the liquid-phase material, e.g., liquid N M d e t ~ n a t i o n . ~This was shown through the detonation sensitization of liquid N M by organic amines (bases) and the detection of the aci ion by I3C NMR.3a*bv4We also demonstrated that the sensitization of N M by ultraviolet light correlates with the production of the N M aci i0n.3c34 Note that there is evidence that aci ion presence is important in the fast decomposition of the higher nitroalkanes also.3d For example, 1,l-dinitropropane (1,l-DNP) is known to be a significantly more sensitive explosive than 2,2-dinitropropane (2,2-DNP). 1,l-DNP has an aci ion, whereas 2,2-DNP does not. “Self-explosion” experiments by Lee et al.5 indicate that static high pressure increases the rate of N M decomposition. In their experiments, they placed samples of N M under static high pressure, heated them to a fixed boundary temperature, and measured the time to “self-explosion”. Self-explosion is determined by a very rapid rise in the sample temperature in a short-time interval (