TECHNOLOGY
Shock Sensitivity of Explosives Clarified Why some explosives are more sensitive to shock than others with similar structures may depend on the stability of carbon-carbon bonds in the aromatic rings and on hydrogen bonding. These apparent relationships resulted from studies by a team of scientists at Sandia National Laboratory, using x-ray excited Auger electron spectroscopy (XAES) to probe the electronic structures of nitroaromatic explosives. The research team's leader, chemist J. William Rogers Jr., notes that the studies involved benzene and five explosives built on the benzene ring: trinitrobenzene (TNB), trinitrotoluene (TNT), monoaminotrinitrobenzene (MATB), diaminotrinitrobenzene (DATB), and triaminotrinitrobenzene (TATB). Although the five compounds have a lot in common, their shock initiation thresholds—the shock pressure required to cause detonation 50% of the time—vary widely. The threshold is 17 kilobars for TNB, 21 for TNT, 30 for MATB, 46 for DATB, and 75 for TATB. To learn the reasons for the differences, Rogers and his Sandia colleagues—chemists Henry C. Peebles, Robert R. Rye, and J. Stephen Binkley and physicist Jack E. Houston—used XAES to measure effects of substituent groups on bond strengths, and then compared their results with theoretical calculations of molecular electronic properties. The technique allows scientists to look specifically at the energy levels involving carbon-carbon bonds, Rogers explains. In XAES, an x-ray photon removes a tightly bound inner-shell electron from an atom. The excited atom responds with an Auger transition, wherein an electron from an outer orbital moves to the vacated deep electron site. There's a good reason for using x-rays, rather than a beam of electrons, to excite the atoms, Rogers says. The electron beam is so energetic that it can cause an explosive to detonate. With the "softer" x-ray
excitation, that probability is remote. The summation of many such excitations and radiationless transitions is an Auger spectrum showing the number of transitions at different energy levels. The shapes of the spectra of the various molecules reveal subtle but unmistakable differences in the molecular bonds local to the carbon sites. For example, the spectra for TNB and TNT confirm theoretical predictions that adding nitro groups weakens the benzene ring's carbon bonds. The reason, Rogers says, is that nitro groups are strong electron withdrawers. The carboncarbon bond is formed by the sharing of electrons between two adjacent carbon atoms. When some of those electrons are partially pulled away to participate in the carbonnitro bond, the carbon-carbon bond is weakened, thus easier to break, and thus more sensitive to shock. Rogers notes that other work has identified a related effect in the explosive hexanitrostilbene (HNS), which resembles two TNT molecules joined with a carbon-carbon double bond (but which has a higher initiation threshold than TNT). Because the carbon-carbon double bond is so close to some of the nitro groups on the rings, two of the three nitro groups on each ring are twisted out of the plane. As a result, these nitro groups can't participate in resonance electron withdrawal and thus can't weaken the carbon-carbon bonds in the rings. Adding amino groups to the benzene ring decreases shock sensitivity. There appear to be two reasons, Rogers explains. First, amino groups are net electron donors. The bond-strengthening effect of the amino electron donation compensates for the nitro group electron withdrawal. More important, however, adding amino groups redistributes the charge, causing the molecule to become polarized, with the amino
Nitroaromatic explosives 02N.
CH3 1 0 2 N . s\j4Q,
2
Cr"°
V
V
1
N0 2
N0 2
TNT
TNB N0 2
N0 2 02N-
Q*
VNO2
^ N0 2
N0 2
HNS NHL 0 2 NU
N
VAY °2
NH2 1 c 02N^/ ^ N 0
V
{X f ^ N H 2
N0 2
N0 2
MATB
DATB
02N
2
NH, 1 ^ ^ N 0
H2N^\^^NH
2
2
N0 2 TATB
groups slightly positive and the nitro groups slightly negative. That leads to the formation of a strong hydrogen bonding network. TATB, for instance, consists of planar sheets of molecules, with the nitro groups on one ring connected to the amino group on another through strong hydrogen bonds. In the "special case" of TNT, the single methyl group is a weak inductive electron donor. There is also a small degree of intermolecular hydrogen bonding, absent in TNB. From their findings, the Sandia scientists infer that adding amino groups raises the initiation threshold because the resulting networks of hydrogen bonds absorb energy from a shock front and reduce the amount that goes to the ring itself. Ward Worthy, Chicago August 10, 1987 C&EN 25
