Sarin, also known by its US
designation as GB, is one of a number of toxic organo-phosphoric
compounds. The chemical name for GB is
isopropyl methylphosphonofluoridic acid (C4H10FO2P). A German scientist by the name of Dr.
Gerhard Shrader first synthesized GB in 1938 while studying the possibility of
using organo-phophorous substances as pesticides. The name Sarin was derived from the names of the researchers on
the project (Schrader, Ambros, Ritter, and Linde). Dr. Shrader soon realized that a small amount of GB can be lethal
in minutes by either inhalation or physical contact and, therefore, had
military applications as a chemical weapon.
GB kills by disrupting the nerve impulses, causing the victim to die by
suffocation. For this reason it is
referred to as a nerve agent. GB is an
odorless liquid with a relatively high volatility, when compared to other nerve
agents. Although
more toxic nerve agents have been synthesized, like the VX class of nerve agents,
GB is still the chemical weapon of choice, since it is relatively inexpensive
and easy to synthesize.
Once
released in to the environment, GB will begin to undergo hydrolysis, reacting
with water or the Hydroxyl radical (OH), resulting in the production of
isopropyl methylphosphonic acid (IMPA) and HF.
IMPA will react with a second water molecule producing other substances
like isopropynol and methylphosphonic acid.
The focus of this work will be on the degradation of GB with the
production of IMPA and HF and on the degradation of GB by reaction with the OH
radical.
There
have been many studies done with regard to the neutralization of nerve agents
by acidic, neutral, and basic solutions [47-50]. Although GB will react with the H2O molecule and the
OH radical under these conditions, it is observed that the rate of
neutralization is dependent on the pH level of the solution. Basic compounds having a high pH level, like
NaOH, will result in a higher rate of neutralization than neutral or acidic
solutions. The half-life of a solution
of GB and water is approximately 100 hours.
If a salt is added to the water to increase the pH level (becoming more
basic) the rate of degradation of GB will increase. A solution of sodium hydroxide (NaOH) is commonly used in the
neutralization or destruction of GB and other nerve agents.
There are several areas of
research with regard to GB and its precursors.
One area of study is the identification and remote sensing of GB, its
precursors, and reaction products by the detection of passive Long Wave
Infrared Radiation (LWIR) [51,52].
Jenkins et. al. [53] has investigated new ways of using polymer based
luminescent sensors to detect the presence of Sarin and Soman through hydrolysis. In 1986 Politzer et. al. [54] studied the
possibility of making Sarin less toxic by replacing the fluorine atom by
another functional group. In that work ab-initio SCF molecular orbital calculations
using small basis sets were performed on several chemical groups, including the
hydroxyl radical, OH. This work
resulted in a first step at analyzing the reactions of GB and radicals. Other work includes the study by Samuels et
al [55] on the potentials of using microwave spectroscopy in the detection of
nerve agents. In 1988 a group from
Aberdeen Proving Ground used Hartree-Fock methods and MP methods to evaluate
the potential military uses in calculating the molecular structures, harmonic
vibrational frequencies, and IR intensities for a number of chemical agents
[56,57]. The remote sensing of chemical
agents released in to the environment can be detected by using LWIR
hyperspectral sensors. The
identification and classification of the chemical agents depend on an accurate
determination of the vibrational spectra of the chemical agents. Ab-initio calculations to determine
the harmonic vibrational spectra of a molecule could be used in this
identification process. However, there
are only a few calculations. One ab-initio
calculation of the rotational spectra was performed by Walker et al. [58] for
two GB isomers.
The
use of ab-initio methods to determine the physical properties, molecular
structures, chemical reactions, energy, vibrational frequencies, etc. removes
the potential hazards of handling the substance in laboratory environments. The
study also leads to a better understanding of how GB reacts with various
constituents in the atmosphere. In
this chapter we investigate the energy surface landscape, minimum energy
structures, transition structures, and possible reaction path of GB
reacting with water and the OH radical. The study should lead to a better
understanding of how GB reacts with constituents in the atmosphere. A summary of the chemical data for GB is
given in Table 4.1.
Table 4.1:
Chemical data for GB
|
Molecular Formula |
C4H10FO2P |
|
Chemical
Name |
Isopropyl
Methylphosphonofluoridic acid |
|
U.S.
Designation |
GB |
|
Formula |
CH3
_ P (= O)(_F)(_ OCH(CH3)2) |
|
Molecular Weight |
140.09
amu |
|
Density |
1.089
g/cm3 |
|
Melting Point |
-57o
C |
|
Boiling Point |
147o
C |
|
Vapor Pressure at 20o
C |
1.48 torr (mm of Hg) |
The
optimized structures of several nerve agents using B3PW91/6-311g(d) are shown
in Fig. 4.1. All the structures of the
nerve agents include two oxygen atoms, one that has a single bond with the
phosphorus atom and the other is doubly bonded to the phosphorus atom. In addition, GB, GD, and GF have a fluorine
atom that is bonded to the phosphorus atom.
The nerve agents GA, GB, GD, and GF have stereo isomers that can be
found by interchanging the fluorine and doubly bonded oxygen atom in GB, GD,
and GF, and in GA by interchanging the C-N with the doubly bonded oxygen
atom. The nerve agent, VX, has four
related isomers that are found by interchanging the ordering of the oxygen and
sulfur groups about the phosphorus atom.
The isomer of GB studied here is GB-R, which differs from GB-S by an
exchange of the Fluorine atom and the doubly bonded oxygen atom.