Oct. 20, 1964
hf OLECULAR
STRUCTURE O F
ethylene sites, and t h a t the subsequent ethylene adsorption displaces the adsorbed HP, but this seems unlikely Secondly, the ethylene sites are not related to the color centers T h e latter are bleached very rapidly by the addition of H L (and D z ) , and yet the C2H4subsequently adsorbs as normal Furthermore,
[COSTRIBUTION
KO.
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if the ethylene is added first, color still remains in the sample, and subsequent addition of HS, after removing the gaseous C2Hl, bleaches all the color Finally, none of the unpaired electrons which are observed in the e s r spectrum are related to the sites responsible for C 2 H 4chemisorption
CHEMISTRY, INDIANA USIVERSITY, B L o o m s c r o s , ISDIASA]
The Molecular Structure of 1,3,5Cycloheptatriene in the Vapor Phase as Determined by the Sector Electron Diffraction Method BY MARITTRXETTERERG' RECEIVED FEBRUARY 10, 1964 The sector electron diffraction method was used to determine the molecular structure of 1,3,5-cycloheptatriene in the vapor phase. The molecule possesses C, symmetry, and the most important observed structural results are as follows (for numbering of the atoms, see Fig. 5 ) : r,(C1-C2) (assumed equal to rg(C3-C4)) = 1.356, yg(C2-CB)= 1.346, ra(Cs.-Ci.) = 1.505, r,fC1-C6) = 2.511, yg(Cp-CL)= 2.792, rg(C?-C4) = 2.424, rS(C2-C;) = 2.501, rg(CI-C3) 2.510, rq(C1-Cj) 2.975, rg(C:$-Cj)= 3.064, rKiCI-C4) = 3.116, rg(Ci-HI) 1.095, yg(C;H i ) = 1.106 A , ; I,,(C1-C2) = 0.043, Ln(Cz-C2) = 0.047, Ln(C6-C;) = 0.050, L"(CI-HI) = 0.083, k ( C i - H i ) = 0.085 A , ; LC,CYCS= 127.2", LCPC3C4= 119.8", LC3CsC; = 121.8'. The molecule exists in a boat conformation with the methylene group and the opposite carbon-carbon double bond bent out of the plane determined by the carbon atoms 1, 2, 5, and 6. The methylene group is tilted 36.5' away from that plane, and the angle between the planes determined by the carbon atoms 1, 2, j, 6 and 2, 3 , 4, 5 is found to be 40 5'. The estimated standard deviations of the molecular parameters are listed in Table I.
I. Introduction During the last decade the molecular structure of 1,3,5cycloheptatriene ( C H T ) has attracted interest from many scientists in different fields of research. But, so far an unambigious over-all structure determination of this molecule has not been reported. 7
6
I
I1
I11
Three possible structural conformations of the carbon skeleton in the seven-membered carbon ring system are shown above (models 1-111). Model I with planar carbon skeleton possesses CS" symmetry, while the other two models belong to the point group C,. On the basis of the n.m.r. spectrum of C H T , Doering, et a1.,2ain 1956 suggested a pseudo-aromatic structure for the molecule with a planar carbon skeleton. The supposed aromatic structure is reflected in their use of the name tropilidene for C H T . In 1958 Abel, et suggested from n.m.r. and vibrational spectra that the six carbon atoms in C H T , other than the methylene group, must be very nearly in the same plane, and t h a t the six x-electrons form a delocalized system which bypasses the methylene group. Dunitz and Pauling3 found from X-ray analysis that C H T in crystalline C7HsMo(C0)3 has a nonplanar structure and that the methylenic carbon atom does not lie in the plane of the conjugated system. T h e close similarity between limited regions of the infrared spectra of the complex and of the free C H T makes these ( I ) ( a ) T h e a u t h o r wishes t o t h a n k t h e U n i t e d S t a t e s Air Force O f i c e of Scientific Research f o r financial assistance; ( b ) o n leave of absence f r o m S o r g e s Laererhogskole, T r o n d h e i m , S o r w a y (2) ( a ) W. von E. Doering, G L a b e r , R . Vonderwahl, N F. C h a m b e r l a i n , a n d R B . Williams, J A m . Chem .So. D u n i t z a n d P. Pauling, Heiu. Chim. A c f a , 43, 2188 (1960).
authors suppose that C, symmetry also holds for the free molecule. Evans and Lord? have studied the infrared and Kaman spectra of C H T and have satisfactorily interpreted the spectra in terms of Czvsymmetry (model 1). They admit, however, that a slight displacement of the methylene group out of the ring plane is possible, as the selection rules would not be sensitive enough to make such a slight departure apparent. Recently La Lau and De Ruyter6 reported an investigation of the vibrational spectra of C H T in order to determine the molecular symmetry. A comparative study6 of the infrared spectra of C H T and 7 - D - C H T showed that the methylenic hydrogen atoms occupy geometrically nonequivalent positions, and the authors therefore concluded that the methylene group is tilted out of the plane of the conjugated system. It should also be mentioned that Davis and Tulinsky' have studied the molecular structure of thujic acid by X-ray crystallography. They found that the C H T ring in this molecule assumes a boat conformation (model 111), with the methylene group and the CB-C4 double bond tilted 49.7 and 23.7' away from the plane determined by the carbon atoms 1 , 2 , 5, and 8. I t can be assumed that the energy of the n--electron system will decrease rapidly as the distance between carbon atoms 1 and 6 is decreased and t h a t it will increase if the ring is puckered. These two factors are closely connected with the strain energy of the molecule, since decreasing the distance between carbon atoms 1 and 6 increases the strain, especially on L C5C6C7and on L C1C7Ce,and tends to force carbon atom 7 away from the plane of the adjacent double bonds. This would twist the x-orbitals of carbon atoms 1 and (i out of parallelism of the other n-orbitals i n the molecule and there( 4 ) M \', E v a n s a n d R . C . I a r d , J . A7n. Chrm S o c , 82, 1x76 (1960) (A) C. 1.a L a u a n d H 1)e R u y t e r , .Spr
