Internal Barrier of Rotation for PCBs

Letters to the Editor

Letters to the Editor The Internal Barriers of Rotation for the 209 Polychlorinated Biphenyls "If You Take Hold of Much, You Do Not Hold It" E Ulrich Biedermann, Israel Agranat

Department of Organic Chemistry, The Hebrew University of Jerusalem, Jerusalem 91904, Israel Corresponding author: Prof. Israel Agranat; e-mail: [email protected]

Anthropogenic polychlorinated biphenyls (PCBs) accumulate in the environment as ubiquitous persistent pollutants [1,2]. These highly lipophilic xenobiotics exhibit wide differences in their toxic and biological effects [2]. Many of the PCBs are non-planar due to intramolecular overcrowding and display axial chirality [3]. The helical sense of the chiral axis is maintained through hindered rotation about single bonds. There are 209 possible Cx2H10 CI" (1 _
ESPR - Environ. Sci. & Pollut. Res. 6 (2) 67 - 68 (1999) 9 ecomed publishers, D-86899 Landsberg, Germany

Our arguments are supported by the results of a comparative theoretical case study of the barriers for internal rotation of representative overcrowded PCBs with various degrees of overcrowding. The present study included 2,2',3,3',6,6'-hexachlorobiphenyl (PCB 136), 2,2',3,3',4,6'-hexachlorobiphenyl (PCB 132) and 2,2'-dichlorobiphenyl (PCB 4), which carry 4, 3, and 2 chlorine atoms, respectively, in the overcrowded regions. The semiempirical AM1 method (MOPAC 93 [7]) was compared with the ab initio DFT method B3LYP/6-31G* (Gaussian 94 [8]). The choice of B3LYP/6-31G* was based on a comparison with experiment [9]. All structures were fully optimized with appropriate point group symmetry constraints. Minima and transition states were verified by computing vibrational frequencies. We define the rotational barrier as E (global m i n i m u m ) - E (transition state) , using the lowest possible bona fide transition state for internal rotation. In the case of chiral PCBs, this rotational barrier is the enantiomerization barrier. The AM 1 heats of formation and the ab initio B3LYP/6-31 G* total energies of the global minimum conformations, the transition states for internal rotation and the planar conformations of PCB 136, PCB 132 and PCB 4, and the corresponding relative energies, are given in Table 1 along with Andersson's barriers [6] and the experimental enantiomerization barriers of PCB 132 [10] and PCB 136 (lower limit) [11]. For PCB 132, the enantiomerization barrier calculated by B3LYP/6-31G* was in excellent agreement with the experimental data [9]. The B3LYP/6-31G* calculated enantiomerization barriers of PCB 136, PCB 132 and PCB 4 are 241,185 and 70 kJ/mol, respectively. The results clearly indicate that the (E)- and (Z)-planar conformations of the highly overcrowded PCB 136 and PCB 132, and (Z)-planar PCB 4 are not bona fide transition states at AM1 and B3LYP/6-31G*. These artificially constrained planar conformations are high-energy stationary points with more than one imaginary vibrational frequency, disqualifying them as transition states. In the above cases, modeling the transition states for internal rotation as planar conformations leads to substantial overestimation of the enantiomerization barriers, for example, by -100% and -130% for PCB 136 and -30% or -43% for PCB 132 using B3LYP/6-31G* and AM1,

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L e t t e r s to t h e E d i t o r

I n t e r n a l B a r r i e r o f R o t a t i o n f o r PCBs

Table 1:AM1 and B3LYP/6-31G* results A M 1 Conformation

Point

AH~

Group

kJ/mol

'

AM1

B3LYP/6--31G*

twist

Erel

Erot b

Etot

deg

kJ/mol

kJ/mol

hartree

"

Exp

twist

Erei

t~G:

deg

kJ/mol

kJ/mol

PCB 136

twisted

C~

70 573

Min

90.0

00

-3220.85179

Min

90.0

00

(E)-anti-folded

C,

252.556

TS

180.0

182 0

-3220 75987

TS

180.0

241.4

(Z)-anb-folded

C2

252.688

TS

2.1

182 1

-3220.75978

TS

3.3

241.6

(E)-planar

C2,

488.399

5

180 0

417.8

-3220.66984

5

180 0

477.7

(Z)-planar

C2v

491.249

5

00

420.7

-3220.66654

5

0.0

486.4

Min

90.2

00

-3220 85001

Min

90 5

0.0

415

>210 [111

PCB 132

twisted

C,

69 855

(E)-anb-folded

C,

212 229

TS

167.4

142.4

-3220.77945

TS

167 6

185.2

(2)-anti-folded

C,

213.471

TS

14 2

143.6

-3220 77906

TS

14.2

186 3

(E)-planar

C~

274.724

3

180 0

204.9

-3220.75928

3

180.0

238.2

(-/')-planar

Co

275.699

3

0.0

205 8

-3220.75764

3

0.0

242.5

twisted

C=

154.652

Min

93.1

0.0

-1382.48986

Min

107.6

0.0

(E)-planar

C,,

216.365

TS

0.0

61.7

-1382.46334

TS

0.0

69.6

(Z)-antJ-fotded

C2

257 942

TS

96

103.3

- 1382 44367

TS

7.2

121.3

(Z)-planar

C2,

265 960

2

0.0

111.3

-1382.44292

2

0.0

123.3

204

182.0 [1 o]

PCB 4

60.3

Global mrnimum (Mn), Transition state (TS), or number of ~maginaryfrequencies ~n case of a h~gherorder saddle point b internal bamer of rotabon as reported by Andersson et al. [6] using AM1 and the relative energy of the (E)-planar conformation

respectively. Moreover, it can be seen from Table 1 that the semiempirical AM1 method underestimates the relative energies of transition states and higher order saddle points by 10% to 25% as compared to the ab initio DFT method B3LYP/6-31G*. This also holds true in PCB 4, where the (E)-planar conformation is a bona fide transition state. In conclusion, the study by Andersson et al. of the internal barriers of rotation for all the 209 PCBs is an illustration of the Talmudic proverb: "If you take hold of much, you do not hold it" [12]. A systematic ab initio DFT study is warranted in order to generate reliable structural physico-chemical parameters for modeling future QSARs and QSPRs of environmentally significant PCBs.

References [1] KIMBROUGH,R.D.; JENSEN,A.A. (Eds.) (1989): Halogenated Biphenyls,Terphenyls,Naphthalenes, Dibenzodioxinsand Related Products. Elsevier-North Holland, Amsterdam [2] SAFE,S.H. (1994): Polychlorinated Biphenyls(PCBs): Environmental Impact, Biochemicaland Toxic Responses, and Implications for Risk Assessment. Crit. Rev. Toxicol. 24, 87-149 [3] KAISER,K.L.E. (1974): On the Optical Activity of Polychlorinated Biphenyls. Environ. Pollut. 7, 93-101 [4] TANG,T.-H.; NOWAKOWSKA,M.; GUILLET,J.E.; CSIZMADIE,I.G. (1991): Rotational Barriers for Selected Polyfluorobiphenyl (PFB), Polychlorobiphenyls (PCB) and Polybromobiphenyl (PBB) Congeners. J. Mol. Struct. (Theochem) 232, 133-146

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[5] ER]CKSON,M.D. (1997): Analytical Chemistry of PCBs. Second Edition, Lewis Publishers, CRC, Boca Raton, FL., pp. 90-96 [6] ANDERSSON,P.L.; HAGLUND,P.; TYSKLIND,M. (1997): The Internal Barriers of Rotation for the 209 Polychlorinated Biphenyls. Environ. Sci. & Pollut. Res. - International 4, 75-81 [7] STEWARV, J.J.P; FUJITSULIMITED,ToKYO,JAPAN(1993): MOPAC 93. All Rights Reserved, Copyright [8] FRISCH,M.J.; TRUCKS,G.W.; SCHLEGEL,H.B.; GILL,P.M.W.; JOHNSON, B.G.; ROBB, M.A.; CHEESEMAN,J.R.; KEITH,T.; PETERSSON,G.A.; MO~,rrGOMERY,J.A.; RAGHAVACHARI,K.; ALLAHAM,M.A.; ZAKRZEWSm,V.G.; ORTIZ,J.V.; FORESMAN,J.B.; CIOSLOWSKi,J.; STEFANOV,B.B.;NANAYA~A~,A.; CHALt~COMBE, M.; PENG,C.Y.; AYALA,P.Y.;CHEN,W.; WONG,M.W.; ANDRES, J.L.; REPLOGLE,E.S.; GOMPERTS,R.; MARTIN,R.L.; FOX,D.J.; BINKLEY,J.S.; DEFREES,D.J.; BAKER,J.; STEWART,J.j.P.; HEADGORDON,M.; GONZALEZ,C.; POPLEJ.A. (1995): Gaussian 94, Revision E.2, Gaussian, Inc., Pittsburgh PA [9] BIEDERMANN,P.U.; SCHURIG,V.; AGRANAT,I. (1997): Enantiomerization of Environmentally SignificantOvercrowded Polychlorinated Biphenyls (PCBs). Chirality 9, 350-353 [10] SCH~aG,V.; GLAUSCH,A.; FLUCK,M. (1995): On the Enantiomerization Barrier of Atropisomeric 2,2',3,3',4,6'-hexachloroblphenyl(PCB 132).TetrahedronAsymmetry6, 2161-2164 [11] SCHURIG,V.; REICH,S. (1998): Determination of the Rotational Barriers of Atropisomeric Polychlorinated Biphenyls (PCBs) by a Novel Stopped-Flow Multidimensional Gas Chromatographic Technique. Chirality 10, 316-320 [12] A Talmudic proverb, quoted, for instance, in the Babylonian Talmud,SederMo'ed, Tractate Haggiga,17a, Abraham,I. (Translator), Epstein I. (Ed.) (1983), The Soncino Press, London

ESPR - Environ. Sci. & Pollut. Res. 6 (2) 1999