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 (
