J. Phys. Chem. 1982, 86, 117-121
117
Molecular Structure and Conformation of cis ,&-I ,5-Cyclooctadlene Kolbjorn Hagen,’ Lire Hedberg, and Kenneth Hedberg’ Department of Chemistry, Oregon State U n ~ l t yCowaiik, , Oregon 9733 I (Received: September 75, 198 1)
cis,cis-1,5-Cyclooctadienehas been investigated by gas-phase electron diffraction at a nozzle-tip temperature of 67-69 O C . The molecules have a twisted-boat conformation (symmetry C2). Values of some of the more important structural parameters with uncertainties estimated at 2u are aa follows: (rJC-H)) = 1.103 (4)A, r,(C=C) = 1.340 (3)A, (r,(CH-CHz)) = 1.514(5)A, r,(CHz-CH2)= 1.536 (9) A, (f,C-C-C) = 114.6’ (13), ( ~ ~ c 4 - C= )130.6’ (12),L,C--C-H = 110.7’(ll),LuC14z-C3-C4 = -86.5’ (44),L,C-C-C-C = 63.8’ (71),L,C~-C~-C~=C~= 8.1’ (81),and L,C-C=C-C = 6.2’ (66).Tests made for the presence of the chair form were negative, but amounts less than about 10% in an equilibrium mixture with the twisted boat cannot be ruled out.
The compound cis,cis-1,5-cyclooctadiene (hereafter COD) readily forms complexes with certain metals and salts. The structures of a number of these have been determined by X-ray methods2 and the COD residues found to have the boat conformation, slightly twisted about the single bonds (Figure l),possibly to relieve strain from eclipsed bonds and from cross-ring interactions. In these ?r complexes the two COD ?r bonds, by their extensions perpendicular to the plane of the C-C=C-C groups, tend to form a juncture near which a complexed atom is found. The structures of the ?r complexes raise questions about the structure of COD itself. Since both boat and chair forms may exist without bond or bond-angle strain, the geometry of the free molecule must depend on the importance of nonbond repulsion, torsion strain, and ?r-orbital interaction in one form relative to the other. It is known from an incomplete electron-diffraction study of the vapor? and from interpretations of the infrared and Raman spectra of the liquid and solid$6 the NMR spectra: and molecular mechanics calculations,’* that the low-energy form of the molecule is the twisted boat. One question of interest concerns the possible presence of a second conformer, the chair, in gaseous samples. Although the existence of this form has not been proved in COD itself, the COD derivative dibenzo-l,5-cyclooctadienehas a chair conformation in the crystallo and displays both chair and boat forms in solution.’l A second interesting question concerns internal motion in the COD molecule. Unlike the torsionally “rigid” chair form of Cw symmetry, the Cz twisted-boat form is flexible: combinations of appropriate torsions around the C-C single bonds offer the possibility of pseudorotation during which the molecule would pass through both a symmetric boat (C,) and a skew (Dz)form. The first of the conformational problems cited above was investigated in the original electron-diffraction work on COD. No significant evidence for the presence of the chair (1) On leave from the University of Trondheim, Trondheim, Norway. (2) See: (a) Ibers, J. A.; Snyder, R. G. Acta Crystallogr. 1962,15,923. (b) van der Hende, J. H.; Baird, W. C., Jr. J. Am. Chem. SOC.1963,85, 1009. (c) Click, M. D.; Dahl, L. F. J. Organomet. Chem. 1965,3, 200. (3) Hedberg, L.; Hedberg, K. ‘Abstracts”, American Cryatallographic Association Meeting, Bozeman, MT, July 1964; p 78. (4) Barna, G. G.; Butler, I. S. J. Raman Spectrosc. 1978, 7, 168. (5) Hendra, P. J.; Powell, D. B. Spectrochim. Acta 1961, 17, 913. (6) Anet, F. A. L.; Kozerski, L. J. Am. Chem. SOC.1973, 95, 3407. (7) Ermer, 0. J. Am. Chem. SOC.1976, 98, 3964. (8) Allinger, N. L.; Sprague, J. T. Tetrahedron 1976, 31, 21. (9) Favini, G.; Zuccarello, F.; Buemi, G. J. Mol. Struct. 1969,3,385. (10) Baker, W.; Banks, R.; Lyon, D. R.; Mann, F. G. J. Chem. SOC. 1946, 27. (11) Montacalvo, D.; St-Jacques, M.; Wasylishen, R. J. Am. Chem. SOC.1973,95, 2023. 0022-3654/82/2086-0117$01.25/0
form was found, but neither could small amounts of this form be ruled out. The second problem was not investigated because the good agreement obtained with models in which torsional motion was given no special role suggested that little improvement could be obtained by elaborating the models. We have recently become interested again in these aspects of the COD structure. Improvementa in computational methods since the early work have been substantial, and it seemed likely that some of the uncertainties in that work might now be reduced. Two items contributing to those uncertainties were, first, assumptions about the values of unrefinable amplitudes of vibration contained in the models of the structure and, second, the manner by which the background was extracted from the scattered intensities. Better estimates of the amplitudes are presently obtainable from normalcoordinate calculations based on reasonable quadratic force fields, and the backgrounds, which in the early work were constructed by drawing curves through the undulations of the scattered intensities, are presently generated analytically. Accordingly, it was decided to reinvestigate the COD structure. It was further decided to obtain new data, a simpler procedure than reconstructing the total scattered intensities from the old data as would be necessary for removal of computer-generated backgrounds. The following is an account of the work. Experimental Section The sample of COD (>98%) was obtained from Aldrich Chemical Co. and distilled before use through a 60-cm column packed with glass helixes. Diffraction photographs were made in the Oregon State University (OSU) apparatus using an 13 sector and 8 X 10 in.Kodak projector slide (medium) plates at nominal nozzle-to-plate distances of 75,30,and 22 cm (long, intermediate, and short cameras). Other experimental conditions were as follows: nozzle-tip temperature, 67-69 “C; beam currents, 0.4-0.6PA; exposure times, 35-420 s; ambient apparatus pressure during exposures, 0.4 X lod-1.1 X lod torr; electron wavelengths, 0.05780-0.057 96 A as calculated from measurements of the accelerating voltage, itself calibrated in separate experiments with gaseous C02 (r,(C=O) = 1.1646 A and r, (O...O) = 2.3244A). Four plates made at each of the three camera distances were handled as described previously12 to yield total scattered intensities (s41,) from which computer-generated13 smooth backgrounds were subtracted. The differences multiplied by s (s = 4aX-’ sin 8; 28 is the (12) Gundersen, G.; Hedberg, K. J. Chem. Phys. 1969,51, 2600. (13) Hedberg, L. “Abstracts”, 5th Austin Symposium on Gas-Phase Molecular Structure, Austin, TX,March 1974; p 37.
0 1982 American Chemical Society
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The Journal of Physical Chemistty, Vol. 86, No. 1, 1982
Hagen et al.
I
~,~-CYCLOOCTADIENE
EXPERIMENTAL
”
J \ : V V V ” THEORETICAL
- . -- Figure 1. Diagram of the cis ,cis-l,5-cyclooctadlene molecule with atomic numbering.
A “
V
-
“--\
A
DIFFERENCE - -A
A
,
.
--
h h
, I
10
20
30
40 5
Figure 3. Intensity curves. The experimental curves are averages of the curves from each camera distance. The theoretical curve is SI, for the OSU model of Table I. The difference curves are experimental minus theoretical SI,.
Formulas used in calculation of intensities and distance distributions were SI,
=
k i#i
AiAjrir1 cos (vi - vi) exp(-li?s2/2) sin (rij- q,s2)s
rD(r) = ?As n-
CeMERR
c ITS) exp(-Bs2) sin rs
s=o
(2)
with I’(s) = sI,ZC~AC-~,Ai = s2fi, and B = O.OOO9 A2. The and vi were obtainedI4 from tabled5 and the anharmonicity constants K were estimated by the diatomic approximation16to be, in units of lo* A3, 12.6, 1.39, and 2.66 for C-H, C=C, and C-C; the anharmonicity constants were assumed to be zero for all other distances. For experimental rD(r) curves, intensity data in the unobserved region s I 1.75 k1 were taken from theoretical curves. The usual corrections for the effects of vibration (“shrinkage”) were introduced via the formulas r, = rg - 12/r, = r, + K + 6r - 12/ra (3)
fi
SLiCRT CQMERP v
13
20
v
A,.-.-
w
3c
-
-
“C s
Figure 2. Experimental intensity curves (s ‘It) shown superimposed on the flnal backgrounds. The curves are magnlfled 5 times relative to the backgrounds.
scattering angle) are the molecular scattered intensities SI,. Curves of the total intensities and backgrounds are shown in Figure 2, and averages of the molecular intensities in Figure 3. The structure analysis was based on these three average SI, curves which have s ranges 2.00-12.50, 7.00-29.50, and 19.00-47.00 A-1 in steps As = 0.25 kl.The s41tcurves, calculated backgrounds, and averaged SI, curves are available as supplementary material. (See paragraph at end of text regarding supplementary material.)
where the r, values are a geometrically consistent distance set, the perpendicular amplitude K and centrifugal distortions 6r were calculated from the quadratic force field discussed below, and the amplitudes of vibration 1 were the experimental values when possible and otherwise calculated from the force field. Force Field The 54 normal modes of a C2-symmetryCOD molecule divide into the two species as 28A + 26B. Symmetry coordinates were chosen. A trial set of symmetrized force constants were then calculated from rough values of stretching, bending, out-of-plane bending, and torsional internal force constants taken from other molecules. The (14)Hagen, K.; Hedberg, K. J. Am. Chem. SOC. 1973,95,1003. (15)Elastic amplitudes and phases: SchBfer,L.; Yates, A. C.; Bonham, R. A. J. Chem. Phys. 1971,56,3056.Inelastic amplitudes: Cromer, D. T.Ibid. 1969,50,4857. (16)Kuchitsu, K. Bull. Chem. SOC.Jpn. 1967,40, 505.
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The Journal of Physical Chemistry, Voi. 86,
Molecular Structure of cis ,cis 1,B-Cyclooctadlene
No. 1,
1982
119
TABLE I: Refined Values of Structural Parameters €or 1, 5-Cyclooctadienea~
1,5-CY CLCOCTADIENE
parameter 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. RC
OSU Data
Oslo Data
(r(C-H)) 1.073 ( 4 ) r(C=C) 1.336 ( 3 ) (r(C-C)) = (rz3+ r w + r4,)/3 1.510 ( 2 ) A,r(C-C)= rM - (r,, + r Z 3 ) / 2 0.018 ( 3 6 ) A,r(C--C) = r4s - PI, [0.001] (LC=C-C) 130.6 ( 1 2 ) ALC=C-C= 0 4 5 6 - e 1 z 3 [1.61 (LC-C-C) 114.6 ( 1 3 ) ALC*+ = 0 345 - 0 234 L4.41 (LC=C-H) [121.0] (LC-C-H) 110.7 (11) 8.1 (81) L c,-C,-c ,=c6 LC,-C,=C,-C, 6 . 2 (67)
1.086 ( 7 ) 1.339 ( 4 ) 1.513 ( 3 ) 0.060 ( 3 2 ) [0.001] 129.1 ( 1 9 ) i1.61 116.1 ( 2 1 ) L4.4 1 [121.0] 109.7 (17) 17.9 (141) 1.1 (86) 0.073 0.104 Distances in angstroms, angles in degrees. Parenthesized uncertainties are 20 and include estimates of systematic error and correlation. Quantities in square brackets were assumed. R = {XiwiAiz /Ew;[s;li(obsd)l2)'', where A i = sili(obsd) - sili(ca1cd).
*
I I
-
-
v
1
2
-
-3
DIFFERENCE
4
5A
Flguro 4. Radial dlstrlbution curves. The experlmental curve is caC culated from composites of the experlmental curves in Figure 3 after multiplication by Zc2/Ac2 and with the damping coefficient B = 0.0009 A2. The theoretical curve corresponds to the OSU model of Table I. The difference curve k experimental minus theoretical.
symmetrized constants were adjusted slightly by trial and error to fit the 44 observed fundamental frequencies to within at most a few percent. Since the values of vibrational amplitudes and of vibrational corrections to distances that figure importantly in the scattered electron intensity are usually not very sensitive to the force field from which they are calculated, our rough force field is adequate for our purpose even though it has no particular spectroscopic significance. The symmetry coordinates, symmetry constants, and observed and calculated wavenumbers are available as supplementary material.
Structural Analysis The prominent features of the radial distribution curves (Figure 4) are easily interpreted. The peaks of approximately 1.10, 1.35, and 1.52 A correspond respectively to distances of the type C-H, C=C, and C-C, the peaks at 2.15 and 2.58 A to geminal C-H and C.C distances, the broad peak centered a t 3.30 A principally to cross-ring C. .C interactions but with important contributions from C*..H as well, and the broad feature at about 4.2 A to longer cross-ring C- .H interactions. These assignments are illustrated in Figure 4 by the vertical bars representing the distance distribution in our preferred model. It was known from our previous work that the principal conformer of COD in the gas was the twisted boat of symmetry Cz.(Poor agreement with experiment was obtained with the chair (C,) and boat (C2J models, and refinements begun with the boat conformation converged to the twisted boat.) This form of the structure requires a rather large number of parameters for complete specification. It is often convenient to use averages and differences of certain distances and angles rather than the distances and angles themselves. This is the case for COD where, for example, the several types of C-C single bonds make it difficult or impossible to determine accurately their individual values. By omission of certain differences which are surely small and may be set equal to zero (i.e., differences involving C-H bonds, C=C-H angles, and C-C-H angles), the number of parameters is reduced to 13. Their definitions
-
.
TABLE 11: Selected Distances (r/A) and Amplitudes ( l / A )for 1,5-Cy~looctadiene~*~ interac1 tionC r(Y ri ra 1 (calcd) C-H 1.073 ( 4 ) 1.109 1.103 0.083 ( 5 ) 0.079 C=C 1.336 ( 3 ) 1.342 1.340 0.039 ( 3 ) 0.044 0.052 C,--C, 1.504 ( 5 ) 1.515 1.513 [0.052] 0.052 C,-C, 1.505 ( 5 ) 1.516 1.514 [0.052] 0.053 C,-C, 1.522 ( 9 ) 1.538 1.536 [0.053] C;H,, 2.095 (18) 2.128 2.121 0.117 0.107 C,.H,, 2.100 ( 5 ) 2.127 2.121 0.113 0.103 C,.H,, 2.114 ( 1 8 ) 2.148 2.141 0.119 C,.H,, 2.133 ( 1 3 ) 2.178 2.170 0.128 0.118 C,.H,, 2.134 (13) 2.178 2.171 0.128 0.118 C,.H,, 2.149 (15) 2.203 2.195 0.128 0.118 C,.C, 2 . 5 1 3 ( 1 9 ) 2.521 2.518 0.089 0.097 C,.C, 2.573 ( 1 3 ) 2.579 2.577 0.073}(5) 0.081 C,.Cs 2.577 ( 1 8 ) 2.582 2.580 0.073 0.081 C,...C, 3.086 ( 4 0 ) 3.088 3.080 0.171 C,..C, 3.099 ( 2 0 ) 3.102 3.098 0.147 (15) 0.157 C;.C, 3.126 (42) 3.128 3.123 "16') 0.122 0.132 C,..C, 3.297 ( 4 6 ) 3.301 3.295 0.143 0.129 C,-.C, 3.318 (43) 3.320 3.314 0.147 ( 2 2 ) 0.133 C,.*C, 3.390 ( 3 9 ) 3.394 3.390 0.121 0.106 C,..*C, 3.446 (63) 3.448 3.441 0.152 0.138 C,..*C, 3.859 ( 2 5 ) 3.861 3.858 0.104 ( 3 9 ) 0.106
1
a Model from OSU data, Table I. Parenthesized uncertainties are 20 and include estimates of correlation and systematic error. Uncertainties for rs and ra are estimated to be the same as for ra. Quantities in square brackets were assumed; those in braces were refined as a group. Dots indicate number of bond angles across which interaction occurs.
are given in Table I. Excluding H-.*H interactions, a Cz-symmetry form of the COD molecule has 54 different interatomic distances. Some of the corresponding 54 amplitudes of vibration were chosen as separate vibration parameters, some were grouped together with fixed differences and handled as single parameters, and some were assigned values; these vibration parameters are evident from Table 11. Refinement of the above twisted-boat model was carried out by least-squares1' fitting a single theoretical intensity curve to the three average experimental curves using a unit weight matrix. As anticipated, some of the parameters could not be refined simultaneously with the others and their values had to be estimated or explored in other ways. (17) Hedberg, K.;Iwasaki, M. Acta CrystaZlogr. 1964, 17, 529.
120
The Journal of Physical Chemistry. Vol. 86,No.
I, 1982
Hagen et ai.
TABLE 111: Correlation Matrix (X100) for Parameters of 1,5-Cyclooctadiene" (
r(C-H
))
r( C= C)
(r(C-C)) a,r(C-C) (LC=C-C) ( LC - C - C , L c,-c,-c,= c, LC*-c,.- c,-c, L
c-c-Ii
1(C-H)
0.0013 0.0008 0.0005 0.0047 0.43 0.47 2.9 2.4 0.38 0.0013 0.0008 0.0024 0.0013 0.0051 0.0071 0.0137
100 23 20 -12 -7 -2 -2 8 -44 -1 4 6 -4 -12 -17 6
100 13 -35 -23 2 -4 -3 -16 8 0 13 -8 -13 -19 5
100 20 100 4 30 100 -26 - 1 7 -88 100 -15 5 -46 65 100 1 -7 -40 38 -27 -38 2 5 21 25 10 -1 17 1 6 5 4 -11 -5 30 -9 -33 6 38 40 -1 21 -56 59 35 -14 1 7 -21 32 66 -3 25 66 -55 -12 2 -9 -23 24 20
" For definitions of parameters see Table I. angstroms, and angles in degrees.
100 -14 100 -9 7 100 1 -15 -7 100 -1 -6 1 9 100 27 -8 5 3 38 100 -42 20 5 -1 30 20 100 15 3 2 -12 -31 34 100 -70 -11 -3 0 -1 3 12 7 -16 100
Standard deviations from least squares. Distances and amplitudes are in
TABLE IV: Skeletal Bond Lengths, Bond Angles, and Torsion Angles in 1,5-Cyclooctadiene and Related Molecules" COD molecular-mechanics calcn parameter r(C=C) r(c2-c3)
r(C3434) r(C,-C, 1 LC, = c , - c , L L L L
c,--c3-c, c3--c,--c s c,-c, = c, c,=c2-c,--c,
LC2-C,-C,-C, L c3-c,-c,=c,
LC,-c, =c,-C,
this workb
A=
1.340 (3) 1.513 (5) 1.536 (9) 1.514 (5) 129.3 (13) 112.4 (13) 116.8 (12) 131.4 (12) -86.5 (44) 63.8 (71) 8.1 (81) 6.2 (66)
1.332 1.498 1.524 1.499 128.4 112.3 116.7 130.0 -89.8 52.5 21.8 5.9
Bf
125.6 112.6 116.3 126.6 -88.2 37.9 41.5 0
Cg
127.5 112.7 116.1 129.6 -9 3 54 19
5
CODBr2C 1.371 (60) 1.467 (70) 1.507 (60) 1.491 (58) 130.2 (40) 113.3 (42) 118.9 (34) 128.3 (42) -82 65 11 -1
DMCODDCd 1.330 1.504 1.553 1.504 130.3 112.8 115.0 133.0 -74.9 76.0 -1.9 -3.4
a Distances in angstroms, angles in degrees. r, values. syn-3,7-Dibromo-l,5-cyclooctadiene; Mackenzie, R. K.; Perkin Trans. 2 1972, 1632. MacNicol, D. D.; Mills, H. H.; Raphael, R. A , ; Wilson, F. B.; Zabkiewicz, J. A. J. Chem. SOC., 2,6-Dimethylcycloocta-3,7-diene-l,5-dicarboxamide: Leiserowitz, L., private communication quoted in ref 5. e Reference 5. f Reference 6. 6' Reference 18.
Values of some of the unrefinable structural parameters were taken from the results of molecular-mechanics calculations' which in other respects were found to be in excellent agreement with ours. The unrefinable amplitudes and the fixed amplitude differences in refinable groups were given values calculated from the force field. The bond confiiation around the double-bonded carbon atoms was assumed to be planar, and C% local symmetry was assumed for the methylene groups. The fit provided by the twisted-boat model is very good, as may be seen by comparing the curves in Figures 3 and 4. Although it seemed unlikely that the fit would be improved by inclusion of any appreciable amounts of the chair conformer, we carried out tests for its presence. The chair form with ita higher symmetry has fewer parameters than the twisted boat: some of the difference parameters (e.g., parameters 5,7, and 9) are required by symmetry to have the value zero, and the torsions are functions of the bond angles. Apart from these modifications the parameters used for the chair form were the same as those for the twisted boat. Its possible presence was introduced by use of a refinable sample composition parameter. The results offer no evidence for the presence of any chair form, but neither can small amounts, Le., less than about 10% be ruled out. The values of the refined parameters for the twistedboat model gave no indication of large-amplitude motion such as would correspond to only slightly restricted pseudorotation. For example, were such rotation to exist,
the amplitudes of all distances affected by it would be much larger. It was therefore deemed unnecessary to include testa which specifically modeled this motion. The results of our analysis of the OSU data invite comparison with those obtained by reanalysis of the early Oslo data. The Oslo data were convenientiy accessible only as a single, composite intensity set, representing the range 2.00 Is I 45.00 A at intervals As = 0.25 A-l, in "constant coefficient" form. This form is essentially similar to the ITS)presently used in the OSU laboratory for calculation of radial distribution curves. Refinements of the twisted-boat model of COD based on these data led to the results shown in Table I.
Best Model Refinements of the twisted-boat model of COD included nine structural and seven vibrational parameters which were allowed to vary simultaneously (Tables I and 11). One of these, Alr(C-C), was found to be highly correlated with the amplitude group,,I ,I 1;, the values of Alr(C-C) in Table I reflect the assignment of values calculated from the force field to the three amplitudes. The listed uncertainties for A,r(C-C) include contributions obtained from the refinements (OSU:0.009A; Oslo: 0.005 A) as well as a contribution (0.027 A) arising from uncertainty in the assigned amplitudes. The latter estimate is based on the assumption that the calculated amplitude values are unlikely to be wrong by more than f0.003 A together with a test of the correlated behavior of the two parameters
121
J. Phys. Chem. 1982, 86, 121-126
which gave the approximate relationship 6(Alr) = -961. None of the refined values of the other parameters in Tables I and I1 were found to be very sensitive to the remaining assumptions. The bracketed values for the three difference parameters in Table I were obtained from molecular-mechanics calculations and that for C=C-H is similar to values in other molecules. Small changes in any of these quantities should not significantly affect the refinement results. The two sets of results in Table I are in pleasing agreement and are each fair statements of the COD structure. We choose the set based on the OSU data as our preferred model because of the way in which the backgrounds were removed to generate the molecular intensities on which the refinements were based: the computer-generated backgrounds used with the OSU data are presumably less subject to bias than are the hand-drawn backgrounds used with the Oslo data. Table I1 is a more complete set of distances and amplitudes for the preferred model, and Table I11 is an abbreviated correlation matrix.
Discussion As may be seen from Table IV, our structure for COD agrees well with the results of molecular-mechanics calculations, particularly those of Ermer' and Anet and Kozerski.18 Table IV also shows that the conformation of the COD ring is not affected by the presence of pseudoequatorial substituents: the ring parameters of the two COD derivatives have values very similar to those for COD itself. The carbon-carbon bond lengths in COD are quite similar to those found in aliphatic chains and in other low-strain ring systems. They require no special comment. The two types of bond angles (those adjacent and those not adjacent to the double bonds) are a few degrees larger than their open-chain counterparts, presumably mostly as a consequence of cross-ring repulsions which tend to flatten the carbon skeleton. The twisted-boat conformation adopted by the molecule is also consistent with an important role for repulsive interactions. For example, in both the chair and symmetric-boat forms vicinally situated pairs of carbon atoms are eclipsed, as are pain of hydrogen atoms on adjacent methylene groups. The strains arising from these energetically unfavorable geometries are par(18)Anet, F. A. L.; Kozerski, L., private communication.
tially relieved in the twisted boat. Quantitative evidence for this qualitative picture is available from the molecular-mechanics ~alculations,'~~ especially those of Ermer where the individual components of the potential are tabulated for several conformations of the molecule. The twisted-boat form was found to be lower in potential energy than the regular boat by 7 kcal/mol and the chair by 4 kcal/mol. The calculations are also consistent with our conclusion that large-amplitude torsional vibration is not present in COD, a crude estimate based upon the potential-energy profiles for twisting suggesting that more than 85% of the molecules are to be found within 15' of the equilibrium values of LC~-C~-C~=C~and K2-C3c4-c5.
Our final remarks concern the possible presence of other conformers in our gaseous samples. Although our testa for the chair form were negative, they can only be interpreted as ruling out substantial amounts of that form. When small amounts of chair were included in compositionconstrained calculations, the structural results and agreement with experiment did not differ significantly from those obtained with the twisted boat as the only form present. From these calculations we estimate that more than about 10% of the chair in our samples would be incompatible with the electron-diffraction data but that smaller amounts cannot be excluded. Although forms other than the chair were not tested, the same statements may be assumed to apply to them. These conclusions are consistent with the free energies of formation calculated from molecular mechanics which, for example, in the case of the chair and twisted-boat forms, corresponds to only about 1%of the former in an equilibrium mixture. Acknowledgment. We are grateful to the National Science Foundation for support of this work under Grant CHE-78-04258. K. Hagen thanks the Norges Almenvitenskapelige Forsknigsrld for a travel grant and partial support. K. Hedberg is indebted to the same organization for a stipend during which the Oslo work was carried out. Supplementary Material Available: Tables of the leveled total intensities (Table V), calculated backgrounds (Table VI), average curves from each camera distance (Table VII), symmetry coordinates (Table VIII), and observed and calculated wave numbers (Table IX)(23 pages). Ordering information is found on any current masthead page.
Rate Constant and Possible Pressure Dependence of the Reaction OH i- H02 W. B. DeMore Jet Propulsion Laboratory, Ca//forn/aInstitute of Technology, Pasadena, California 91 107 (Received: Juk 7, 1981; In f/nal Form: September 2, I98 1)
The technique of laser-induced fluorescence has been used to measure steady-state OH concentrations in the photolysis of water vapor at 184.9 nm and 298 K, with O2added in trace amounts. He or Ar was present at total pressures in the range 75-730 torr. The results were used to derive the rate-constant ratio k , / k 2 / 2 ,where kl and k6 are the rate constants for the reactions OH + HOz HzO + O2and HOz + HOz Oz,respectively. When currently available values for k5 are used, the results give k, = (1.2 f 0.4) X 10-locm3s-l at 1-atmpressure, with evidence of a decline of k, at lower pressures. No water-vapor effect on k , was observed.
-
Introduction Reliable measurement of the rate constant for the reaction 0022-3654/82/2086-0121$01.25/0
-
OH + H 0 2 -!!+ H 2 0 + O2 has been a long-standing problem in 0 1982 American Chemical Society
(1)
The
