J. Phys. Chem. 1986, 90, 545-547 should be equal to the product of 4 M a L 2 + relative to MeOL (0.625 in MeOL)I4 and the above value (1.06) of 4 M a L relative to L20; that product is 0.66. Comparison to our value shows that 4 M a L 2 + relative to L 2 0 is 15% larger in MeOL than in L 2 0 . This solvent effect implies that the force fields which constrain motions of the exchangeable L atoms in the MeOL2+moiety are significantly different in the two solvents; those L atoms are less tightly constrained when hydrogen bonded to L20 than when hydrogen bonded to MeOL. The sign of the difference between 4 M a L 2 + and 1 also is worthy of note, since it is opposite to that observed for C H 3 substitution on uncharged 0-L and C-L sites. From the value given above, 4MeOH is -6% higher than 4L20, and Shiner’s 4 values3 for C-L sites show that a-CH3 substitution for H increases 4 values by is -17% lower than 1. -7-10%; in contrast, 4MeOL2+ This value of + M a L 2 + provides further support for our proposed mechanism for methyl transfer to L 2 0 (eq 3). Our inferences from H 2 0 / D 2 0 and ROH/ROD KIE’s depend on 4 for a positively charged 0-L+ site remaining significantly less than unity when an alkyl substituent is present.8 The observations presented here show that the methyl substituent effect makes 4 M e ~ L 2 +even smaller than is I, thus strengthening our argument.
-
Experimental Section Chemical shifts were measured at 90.5 MHz with a Bruker WH-360 spectrometer. Both I3CH30Land the internal reference (I3CH3CNor I3CH3CO2H)were 99 atom % I3C (MSD Isotopes)
545
and were present at a concentration of 0.02 M. Solutions in H20 were 100 atom % exchangeable H and solutions in D 2 0 were >99 atom % exchangeable D. Acid concentrations in the final solutions were determined by potentiometric titration. The factors which limited the reproducibility of bobsd were reproducibility and homogeneity of temperature within the sample. To optimize temperature control, protons were not decoupled, so the I3CH3resonances appeared as quartets; the value of 6obsd was taken to be the difference between the mean 6 of the four quartet components from 13CH30Hand the mean 6 of the four quartet components from the internal standard. Some measurements were made with IO-” sample tubes in the 10-mm probe and locking on solvent for the D 2 0 solutions and on D 2 0 or (CD3)2C0in a 4-mm coaxial inner tube for the H 2 0solutions (method A); other measurements were made with the 20-mm probe, mounting the 10-mm sample tube coaxially within a 20-mm tube containing D 2 0 and locking on the D 2 0 in the outer tube (method B). The data in Table I suggest that method A gave marginally better temperature control. Temperatures were measured with a 4-mm coaxial tube filled with the recommended” (CD3)2CO/CC14mixture in a 10-mm tube which was treated like a sample tube.
Acknowledgment. This material is based on work supported by the National Science Foundation under Grant CHE-8304874. (16) Rolston, J. H.; Gale, K. L. J . Phys. Chem. 1984, 88, 4394-4397.
(17) Led, J. J.; Petersen, S. B. J. Magn. Reson. 1978, 32, 1-17.
Production of the Nitromethane Aci Ion by UV Irradiation: Its Effect on Detonation Sensitivity Ray Engelke,* William L. Earl, and Celeste McMichael Rohlfing Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (Received: October 21, 1985)
Earlier workers have shown that UV irradiation of liquid nitromethane increases this explosive’s sensitivity to detonation. They thought that the increased sensitivity was caused by UV production of methyl nitrite. We have reproduced the UV detonation experiment and also found an increased sensitivity. Under conditions similar to the detonation experiment, we show that UV irradiation produces the aci ion of nitromethane and no detectable amount of methyl nitrite; 13CNMR was used to demonstrate this. This indicates that the sensitizing chemical species is the aci ion. Very small additions of organic base to liquid nitromethane also: (1) sensitize it to detonation and (2) produce the aci ion and only the aci ion. Our work suggests that both means of detonation sensitization are due to the same cause-the presence of the aci ion.
I. Introduction Earlier workers’ have shown that ultraviolet (UV) irradiation of liquid nitromethane (NM, CH3N02, species 1 of Figure 1) increases its explosive sensitivity. Their measure of explosive sensitivity was failure diameter. The failure diameter of an explosive is, by definition, the diameter of the smallest right circular cylinder of the explosive in which it is possible to propagate a steady detonation wave. These earlier workers thought that this increased sensitivity was due to production of methyl nitrite ( C H 3 0 N 0 , 3 and 4 of Figure 1) by the UV light. We show that the important new chemical species produced by the UV light is not methyl nitrite but rather the aci ion of N M (CH2N02,species 2 of Figure 1). The experimental technique used to demonstrate this is I3C N M R spectroscopy. We have also reproduced the detonation experiment and found a failure diameter reduction similar to that described in ref 1.
It is k n ~ w n l -that ~ addition of very small amounts of organic base (e.g., 0.01 wt % diethylenetriamine) also strongly sensitizes liquid N M to detonation. We have shown elsewhere4 that the only new chemical species produced by the low concentration of base in N M is the aci ion. This fact suggested to us that the UV sensitization of N M might also arise from the production of the aci ion. To test this idea, N M was subjected to strong UV irradiation and the resultant material was examined by I3C N M R spectroscopy. While the unirradiated material contains no detectable amount of the aci ion, after irradiation aci ion is present. N o methyl nitrite was observed in either the unirradiated or the irradiated material. We infer, therefore, that UV irradiation sensitizes N M to detonation by producing the aci ion. The remainder of this article is arranged as follows. Section I1 is a discussion of the experimental procedures used to obtain
(1) Kondrikov, B. N.; Kozak, G. D.; Raikova, V. M.; Starshinov, A. V.
(2) Walker, F. E. Acta Astronautica 1979, 6, 807. (3) Engelke, R. Phys. Fluids 1980, 23, 875. (4) Engelke, R.; Earl, W. L.; Rohlfing, C. M . J . Chem. Phys., in press.
Dokl. Akad Nauk SSSR 1977, 233,402 [Sou. Phys. Dokl. 1977,233, 3151.
0022-3654/86/2090-0545$0 1.5010
0 1986 American Chemical Society
546
The Journal of Physical Chemistry, Vol. 90, No. 4, 1986
Letters
TABLE I: I3C Chemical Shifts species
experimental u (ppm)"
experimental conditions
63.52 62.95 62.64 61.70 104.90 104.93 102.81 54.83 52.7 58.6
neat N M in C H 3 0 H in CHCI, in C6Hl2 N M 5.87 wt % DETA N M 11.74 wt % pyridine Na+CH2N02-in methanol methyl nitrite in N M at 298 K neat liquid methyl nitrite at 206 K neat liquid methyl nitrite at 206 K
normal form (1)
aci ion (2) methyl nitrite (3 and 4) syn-methyl nitrite (3) anti-methyl nitrite (4)
+ +
theoretical u (ppm)b,e 62.3'
98.8d
51.5' 53.OC
a Relative to Me$i (tetramethylsilane). *Values obtained from ab initio single-determinant restricted-Hartree-Fock calculations which used gauge-invariant atomic orbitals and a 4-31G basis set.
