ISSN 0012-5016, Doklady Physical Chemistry, 2006, Vol. 410, Part 1, pp. 260–262. © Pleiades Publishing, Inc., 2006.Original Russian Text © L.A. Gribov, A.I. Pavlyuchko, I.V. Rybal’chenko, G.I. Sigeikin, V.N. Suvorkin, B.F. Myasoedov, 2006, published in Doklady Akademii Nauk, 2006,Vol. 410, No. 2, pp. 207–210.

260

The international convention on prohibition ofchemical weapons [1] has stipulated strict analyticalcontrol of the production, stockpiling, and circulationof highly toxic chemicals. The implementation of theserules requires reliable fast methods of analysis suitablefor identification of toxic chemicals included in therestrictive lists of the convention. The major problem isthat the vast majority of the convention-controlledpotential highly toxic compounds (more than 99.99%of their total number) have never been synthesized orstudied, although they are subject to analytical control.Therefore, in this field, as in many other cases of sub-stance or materials research, development of theoreticalmethods for predicting the properties of compoundsfrom their structural formulas with an accuracy suitablefor identification is an important task.

O

-Alkyl (

≤

C10

, including cycloalkyl) alkyl(Me, Et,

n

-Pr, or

i

-Pr)fluorophosphonates are among the mosttoxic chemicals and components of chemical weapons.These chemicals (in total, 24956 compounds) areincluded in the restrictive list given in Annex 1 to theinternational convention on chemical weapons prohibi-tion. Currently, the condensed- and vapor-phase IRspectra have been recorded for only about 100 com-pounds from this list.

This paper proposes a method for simulating the IRspectra of

O

-alkyl alkylfluorophosphonates underlyingfast spectroscopic identification of these compounds.

The task of prediction of the IR spectrum of a com-pound from its structural formula was solved using anintegrated calculation technique combining the advan-tages of the quantum-chemical and fragment [2–5]methods. It is based on the fragment calculationmethod. For example, the

O

-butyl isopropylfluorophos-phonate molecule is constructed using butanol and

O

-methyl isopropylfluorophosphonate as the frag-ments. The removal of the OH group from butanol and

the

CH

3

group from

O

-methyl isopropylfluorophospho-nate and the subsequent connection of the resultingfragments along the CO bond gives the

O

-butyl isopro-pylfluorophosphonate molecule.

The first stage included quantum-chemical optimi-zation of the equilibrium geometry of the moleculestaken as fragments. All quantum-chemical calculationswere carried out at the ab initio level of theorywith inclusion of electron correlation using the6-311G(3df,3pd)/B3LYP basis set by means of theGAMESS program [6]. This was followed by quantum-chemical calculation, with the same basis set, of the ini-tial values for the parameters of the molecular potentialfunction (the second derivatives of the molecularenergy with respect to the natural vibration coordi-nates) and the parameters of the molecular electro-opti-cal function (the first derivatives of the bond dipolemoments with respect to the natural vibration coordi-nates). These and other calculations were performedusing specially developed software [7].

The parameters thus found, which characterize themolecular structures, do not provide good agreementbetween the experimental and simulated spectra. Forexample, the calculated absorption frequencies overes-timate the experimental frequencies, which is usual forquantum-chemical calculations. Therefore, the param-eters of the potential and electro-optical functions ofthe fragments are subsequently corrected in such a wayas to ensure good agreement between the experimentaland calculated absorption frequencies and intensities.

The next stage included the fragment constructionof the

O

-alkyl alkylfluorophosphonate molecule bylinking the

O

-alkyl and alkylfluorophosphonate frag-ments through the CO bond. Comparison of the simu-lated and experimental IR absorption spectra showed adifference between the absorption frequencies corre-sponding to the vibrations of this bond. This is due tothe fact that the potential function parameters of the CObond and all bending vibration coordinates involvingthe CO bond in the

O

-alkyl fragment of the

O

-alkyl

Spectral Identification of Highly ToxicOrganophosphorus Compounds

Corresponding Member of the RAS

L. A. Gribov, A. I. Pavlyuchko, I. V. Rybal’chenko, G. I. Sigeikin,V. N. Suvorkin, and

Academician

B. F. Myasoedov

Received February 22, 2006

DOI:

10.1134/S001250160609003X

Interdepartmental Center for Analytical Researchat the Presidium of the Russian Academy of Sciences,ul. Vavilova 44/2, Moscow, 117333 Russia

PHYSICALCHEMISTRY

DOKLADY PHYSICAL CHEMISTRY

Vol. 410

Part 1

2006

SPECTRAL IDENTIFICATION OF HIGHLY TOXIC ORGANOPHOSPHORUS COMPOUNDS 261

alkylfluorophosphonate molecule differ from the corre-sponding values for the alcohol molecule used to con-struct this fragment. Therefore, a number of parametersof the potential and electro-optical functions relateddirectly to the C atom in the CO bond of the

O

-alkylfragment should be corrected.

The described calculation procedure was employedto find empirical potential and electro-optical functionsof all alkylfluorophosphonate fragments (methylfluoro-phosphonate, ethylfluorophosphonate, isopropylfluoro-phosphonate, and propylfluorophosphonate) and anumber of

O

-alkyl fragments (

O

-methyl,

O

-ethyl,

O

-isopropyl,

O

-propyl,

O

-isobutyl,

O

-butyl, and

O

-cyclohexyl).In all cases, empirical potential and electro-optical

functions for the alkylfluorophosphonate fragmentswere selected using the experimental IR spectrum ofthe

O

-methyl alkylfluorophosphonate molecule.It was shown that the empirical potential and elec-

tro-optical functions of these fragments ensure, withoutadditional correction, good agreement between thesimulated and experimental IR spectra for the fragmentconstruction of all

O

-alkyl alkylfluorophosphonatemolecules (in total, 28 molecules). In the 800–4000cm

−

1

range, the average mismatch between the calcu-lated and experimental absorption frequencies was 5cm

−

1

, while the average mismatch between the calcu-lated and experimental band intensities was 5%. There-fore, the simulated IR spectra in this region can be usedfor spectroscopic identification of molecules.

The potential and electro-optical function parame-ters for

O

-alkyl fragments that are to be corrected forconnecting the fragments are different for primary, sec-ondary, and tertiary C atoms in the CO bond. In addi-tion, these parameters are soon stabilized following anincrease in the length of the

O

-alkyl fragment. This factallows predictive calculations of the IR spectra of

O

-alkyl alkylfluorophosphonate. In this case, all thenecessary parameters for the

O

-alkyl fragment can befound from the experimental IR spectrum of the corre-sponding alcohol and from the parameters for the initialmolecules in the homologous series of primary, second-ary, and tertiary

O

-alkyls.For practical use of this calculation technique, it is

important that the time needed for fragment construc-tion and for the subsequent simulation of the IR spectraof

O

-alkyl alkylfluorophosphonates from prefabricatedfragments is limited only by the operation speed of thecomputer operator and equals, on average, several min-utes.

Using the above-outlined procedure, we simulatedthe IR spectra of a series of

O

-alkyl alkylfluorophos-phonates (28 molecules). Analysis of the simulated IRspectra allows the following conclusions.

In the fingerprint region (600–1350 cm

−

1

) of the IRspectra of

O

-alkyl alkylfluorophosphonates, all strongabsorption bands belong to the alkylfluorophosphonatefragment. In this region, the integrated intensity of the

absorption bands due to the alkylfluorophosphonatefragment is two or more orders of magnitude greaterthan the integrated intensity of the absorption bandsdue to the

O

-alkyl fragment. The bands for the

O

-alkylfragment that fall in this region are either overlapped bystronger alkylfluorophosphonate bands or are involvedin the formation of the broad wings of the alkylfluoro-phosphonate bands.

The 600–1350 cm

−

1

range exhibits only a few strongabsorption bands. The strong bands in the 600–1000 cm

−

1

range are due to the P–F and P–C vibrationsand to the symmetric stretching vibrations of the P–O–Cbridge. The highly intense band at 1000–1100 cm

−

1

corresponds to the antisymmetric stretching vibrationsof the P–O–C bridge. The strong absorption band at1250–1350 cm

−

1

corresponds to the stretching vibra-tions of the multiple P=O bond.

The calculation showed that the intensity of theabsorption band at 1250–1350 cm

−

1

corresponding tothe P=O stretching vibrations changes only slightlyupon replacement of the

O

-alkyl fragment in a particu-lar alkylfluorophosphonate. Therefore, this band can beused as the internal standard for determining the inten-sities of

O

-alkyl alkylfluorophosphonate absorptionbands.

It was also found that an increase in the length of the

O

-alkyl fragment induces a low-frequency shift of theP–O–C stretches. Starting from the

O

-butyl fragment,the absorption band frequencies and intensities in the600–1350 cm

−

1

range almost stop changing. Isomeriza-tion of the

O

-alkyl fragment does not induce pro-nounced changes in the spectrum. For example, thespectra of

O

-hexyl methylfluorophosphonate isomers(

O

-hexyl methylfluorophosphonate,

O

-cyclohexylmethylfluorophosphonate, and

O

-pinacolyl methylflu-orophosphonate) prove similar to each other.

In the region 600–1000 cm

−

1

, the IR spectra of

O

-alkyl methylfluorophosphonates,

O

-alkyl ethylfluo-rophosphonates,

O

-alkyl isopropylfluorophosphonates,and

O

-alkyl propylfluorophosphonates show differ-ences related to the structure of the alkylfluorophospho-nate fragment. These differences are pronouncedenough for IR identification of these

O

-alkyl alkylfluo-rophosphonates.

Only in two IR spectral ranges do the intensities ofthe bands corresponding to the

O

-alkyl fragment mark-edly exceed the band intensities corresponding to thealkylfluorophosphonate fragment. One of these ranges(1350–1500 cm

−

1

) shows bending vibrations (H

−

C

−

Hangles) in the CH

2

and CH

3

groups. The other range(2800–3100 cm

−

1

) shows CH stretches. An essentialfeature is that many bands overlap in each spectralregion. Therefore, the experiment showed broad poorlystructured bands.

This fact precludes reliable identification of

O

-alkylalkylfluorophosphonates with isomeric

O

-alkyl frag-ments based on IR spectra. However, based on the inte-grated intensities of the absorption bands in this region,

262

DOKLADY PHYSICAL CHEMISTRY

Vol. 410

Part 1

2006

GRIBOV et al.

it is possible to draw fairly justified conclusions con-cerning the size of the

O

-alkyl fragment. As notedabove, the intensity of the band at 1250–1350 cm

−

1

,corresponding to the multiple P=O bond, can be used asthe internal standard for determining the intensities ofthese bands.

In the spectral identification of

O

-alkyl alkylfluoro-phosphonates, a challenge comes from the enormousnumbers of isomers of

O

-alkyl fragments starting from

O

-hexyl. However, the IR spectra of these

O

-alkyl alkyl-fluorophosphonates can be represented as the sums ofthe IR spectra corresponding to the

O

-alkyl fragmentand to an

O

-alkyl alkylfluorophosphonate of a smallersize.

The theoretical study of the IR spectra of

O

-alkylalkylfluorophosphonates allows one to create an algo-rithm (computer program) for fast spectral identifica-tion of these compounds. The algorithm may be con-structed as follows:

(1) Compare the experimental IR spectrum of thecompound of interest with the IR spectra included inthe database [8]. If the compound has not been identi-fied, pass to the next stages.

(2) Compare the experimental IR spectrum in the600–1350 cm

−

1

range with the IR spectra of medium-size

O

-alkyl alkylfluorophosphonates, for example,

O

-pentyl methylfluorophosphonate,

O

-pentyl ethylflu-orophosphonate,

O

-pentyl isopropylfluorophospho-nate, and

O

-pentyl propylfluorophosphonate. Compari-son of the IR spectra in this region shows whether thecompound of interest belongs to the series of

O

-alkylmethylfluorophosphonates,

O

-alkyl ethylfluorophos-phonates,

O

-alkyl isopropylfluorophosphonates, or

O

-alkyl propylfluorophosphonates.(3) If the previous step has shown that the com-

pound belongs to the

O

-alkyl alkylfluorophosphonateseries, a putative size of the

O

-alkyl fragment should bedetermined from the band intensities at 1350–1500 and2800–3100 cm

−

1. The intensity of the band at 1250–1350 cm−1, corresponding to the multiple P=O bond, isused as the internal standard for determining the bandintensities.

(4) The probable structure of the O-alkyl fragmentin O-alkyl alkylfluorophosphonate is established bycomparing the IR spectra of the compound under studywith the sum of the IR spectra of alcohols present in thedatabase and the corresponding O-pentyl alkylfluoro-phosphonate.

This algorithm gives the answer to the principalquestion, namely, whether the compound under study

belongs to the series of highly toxic O-alkyl alkylfluo-rophosphonates, and also allows one to determine themost probable structure of this compound. A moreexact structure of the compound including the isomericstate of the O-alkyl fragment can be derived from thefragment calculation of the IR spectrum for the hypo-thetical structure.

The outlined mathematical procedure and the algo-rithm for spectral identification could also be used,after appropriate correction, to identify other homolo-gous series of organic compounds.

ACKNOWLEDGMENTS

This work was supported by the Russian Foundationfor Basic Research, project no. 04–03–08074.

REFERENCES

1. Convention on the Prohibition of the Development, Pro-duction, Stockpiling and Use of Chemical Weapons andon Their Destruction, New York: Printed and Distributedby the Technical Secretariat of the Organization for theProhibition of Chemical Weapons, 2005.

2. Gribov, L.A. and Dement’ev, V.A., Metody i algoritmyvychislenii v teorii kolebatel’nykh spektrov molekul(Computational Methods and Algorithms in the Theoryof Molecular Vibration Spectra), Moscow: Nauka, 1981.

3. Gribov, L.A., Dement’ev, V.A., and Todorovskii, A.T.,Interpretirovannye kolebatel’nye spektry alkanov, alkenovi proizvodnykh benzola (Assigned Vibrational Spectra ofAlkanes, Alkenes, and Benzene Derivatives), Moscow:Nauka, 1986.

4. Gribov, L.A. and Orville-Thomas, W.J., Theory andMethods of Calculation of Molecular Spectra, Chiches-ter: Wiley, 1988.

5. Gribov, L.A., Baranov, V.I., and Elyashberg, M.E.,Bezetalonnyi molekulyarnyi spektral’nyi analiz (Refer-ence-Free Molecular Spectral Analysis), Moscow:URSS, 2002.

6. Schmidt, M.W., Baldridge, K.K., Boats, J.A., et al.,J. Comput. Chem., 1993, no. 14, pp. 1347–1363.

7. Gribov, L.A. and Pavlyuchko, A.I., Variatsionnyemetody resheniya angarmonicheskikh zadach v teoriikolebatel’nykh spektrov molekul (Variational Methods ofSolution of Anharmonic Problems in the Theory ofMolecular Vibrational Spectra), Moscow: Nauka, 1998.

8. OPCW Central Analytical Database PDF-OCAD, v. 8;e-OCAD, v.6, New York: Technical Secretariat of theOrganisation for the Prohibition of Chemical Weapons,December, 2004.