J. Phys. Chem. 1983, 87,5381-5386
5381
Synthesis, Microwave Spectra, Electric Dipole Moment, and Molecular Structure of Cyclopentene Ozonide (6,7,8-Trioxabicyclo[ 3.2.1 ]octane) Donald G. Borseth and Robert L. Kucrkowskl” Department of Chemistry, The Universlty of Michigan, Ann Arbor, Michigan 48 109 (Received: February 1 I , 1983; In Final Form: May 4, 1983)
The ground-state rotational spectra of 14 isotopic species of cyclopentene ozonide (6,7,8-trioxabicyclo[3.2.l]octane) were assigned. These included the normal isotopic species, all single-substituted deuterium, I3C, and l8O species, and all multiple-180-substituted species. Three vibrational satellites of the normal isotopic species were also assigned. The dipole moment was found to be 2.48 (1) D, with principal axis components Ipnl = 2.30 (1)D, lpbl = 0.0 D, and IpJ = 0.91. (1)D. The ground state of cyclopentene ozonide has C, symmetry and the endo conformation. Its structure is discussed in relation to an ab initio molecular orbital calculation of the C, conformer of ethylene ozonide.
Introduction The properties of ozonides have been actively investigated by organic chemists since the 1,2,4-trioxolane structure was first proposed by Staudinger’ and established by R e i ~ h e . ~However, ,~ efforts to determine the conformation and structural details of this class of compounds have only progressed more recently. The first studies employed diffraction techniques and were directed a t ethylene ozonide4 and 3-carbomethoxy-5-anisyl-1,2,4-trioxa~yclopentane.~ Subsequently, microwave (MW) spectroscopy has been used to provide information on the ozonides from ethylene, propylene, 2-butene, vinyl fluoride, and l,l.-difluoroethylene.6 Since the first molecular mechanics calculations on ethylene ozonide,7 over 20 investigations have followed of the structure, conformation, and energetics of small ozonides and related species by semiempirical and ab initio methods.8 The recent calculations by Cremer on the primary and secondary ozonides of ethylene, propylene, and %butene have been especially n o t e ~ o r t h y . Many ~ of these efforts to provide structural insights have been prompted by the mechanistic proposals of reaction chemists which have incorporated conformational properties of transition states to rationalize the stereoselectivity observed in the ozonolysis of cis and trans alkenes.6cJ0 Presumably the ground-state conformations of primary ozonides (1,2,3-trioxolanes) and final ozonides (1,2,4-trioxolanes)will provide useful insights which can be related to the potential energy surfaces of ozonolysis (1) H. Staudinger, Ber. Dtsch. Chem. Ges., 58, 1088 (1925). (2) A. Reiche and R. Meister, Ber. Dtsch. Chem. Ges., 65, 1274 (1932). (3) A. Reiche, R. Meister, and H. Sauthoff, Justus Liebigs Ann. Chem., 553, 187 (1942). (4) A. Almenningen, P. Kolsaker, H. M. Seip, and T. Willadsen, Acta C h e n . Scand., 23, 3398 (1969). (5) P. Groth, Acta Chem. Scand., 24, 2137 (1970). (6) (a) R. L. Kuczkowski, Acc. Chem. Res., 16,42 (1983); (b) ethylene: R. L. Kuczkowski, C. W. Gillies, and K. L. Gallaher, J. Mol. Spectrosc., 60, 361 (1976); (c) propylene and trans-%butene: R. P. Lattimer, R. L. Kuczkowski, and C. W. Gillies, J. Am. Chem. SOC., 96, 348 (1974); (d) vinyl fluoride: K. W. Hillig 11, R. P. Lattimer, and R. L. Kuczkowski, ibid.,104,988 (1982); (e) 1,l-difluoroethylene: K. W. Hillig I1 and R. L. Kuczkowski, J . Phys. Chem., 86, 1415 (1982). (7) H. M. Seip, Acta Chem. Scand., 23, 2741 (1969). (8) For a review of this area: R. L. Kuczkowski in “1,3-Dipolar Cycloadditions”, A. W. Padwa, Ed., Wiley, New York, in press. (9) (a) D. Cremer, J. Chem. Phys., 70,1898 (1979); (b) ibid., 70, 1911 (1979); (c) ibid., 70, 1928 (1979); (d) J . Am. Chem. Soc., 103, 3619 (1981); (e) ibid., 103, 3627 (1981). (10) (a) N. L. Bauld, J. A. Thompson, C. E. Hudson, and P. S. Bailey, J. Am. C h e q . SOC.,SO, 1822 (1968); (b) P. S. Bailey, T. M. Ferrell, A. Rustaiyan, S.Seyhan, and L. E. Unruh, ibid., 100, 894 (1978).
reaction intermediates as well. In five of the ozonides previously studied by MW spectroscopy,6”e the ring possessed a peroxy oxygen conformation characterized by a large dihedral angle about the C-0,-0,4 bond (-42-490).11 Cremer suggested that the most significant forces stabilizing these conformations were C-0, bond dipole repulsions and oxygen p-type lone-pair interactions on both 0, and 0,.9 This motivated us to explore a bicyclic ozonide system, to determine whether the additional ring constraints might now force the ozonide ring into an 0, envelope conformation or whether some twisting about the 0,-0, bond still prevails. Further impetus was provided since this question had been raised by a photoelectron spectroscopic study of several peroxides and ozonides.12 This study suggested that a deviation from the envelope conformation probably occurs for the ozonide ring in cyclohexene ozonide. For cyclopentene ozonide the results were more ambiguous. Although a small dihedral angle was calculated for cyclopentene ozonide (- 13%), the value was not significantly large enough to rule out the possibility that it was actually zero. We have prepared both cyclopentene and cyclohexene ozonides13and examined their MW spectra. Both species have very dense spectra but only cyclopentene ozonide (hereafter, CpOz) has been successfully assigned. The double resonance technique which led to the assignment, the symmetry of the ozonide, and the techniques developed to enrich the starting materials made it possible to straightforwardly assign a large number of isotopic species. This has provided some information on how ozonide bond distances will differ between the twisted and envelope conformations. Experimental Section Instrumentation. Microwave spectra of CpOz were obtained with a Hewlett-Packard 8460A spectrometer. Radio frequency-microwave double resonance (RFMWDR) spectra were obtained by replacing the Stark modulator with a tunable amplitude modulated radio (11) The following nomenclature will be used to label the ring sites: 0, = peroxidic oxygen, 0, = ether oxygen, C = Cb = carbon in ozonide ring at bridgehead, C, = carbon adjacent to Cb C, = Cf = carbon (in flap) adjacent to C,. Hydrogen is labeled according to the carbon atom attachment. (12) P. Rademacher and W. Elling, Liebigs Ann. Chem., 1473 (1979). (13) R. Criegee, G. Blust, and G. Lohaus, Justus Liebigs Ann. Chem., 583, 2 (1953).
0022-3654/83/2087-5381$01.50/0 0 1983 American Chemical Society
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The Journal of Physical Chemistry, Vol. 87, No. 26, 1983
frequency (rf) source covering 1-60 MHz a t 50-5000-mW 0 u t p ~ t . l The ~ MW frequency measurements were accurate to f0.02 MHz, although some weak transitions were measured a t a lower resolution of *0.05 MHz. CpOz had a half-life in the gold-plated cell of 30-40 min at room temperature. Most of the lag-and all 13C-substituted and monodeuterated CpOz spectra were obtained with the sample cell temperature held in the range of -40 to -10 "C with dry ice. This raised the decomposition half-life to several hours and increased the intensity of the ground-state R-branch transitions. Synthesis. CpOz was prepared similarly to the literature pr0~edure.l~ Approximately 100-mL solution of pentane or isopentane containing 2% cyclopentane or less a t -78 "C was completely reacted with 0.1-0.3 mmol/min of O3 in O2 (determined by iodometry). All gases were dried by passing through -78 "C traps before use. The solution was then decanted (at room temperature) from the solid white polymeric material and the CpOz recovered by recrystallization or distillation of solvent on the vacuum line. CpOz was further purified by sublimation. Typical yields were 20% from 2% cyclopentene solutions and over 30% for solutions containing less than 0.5% cyclopentene. CpOz was characterized by IR, NMR, and mass spectroscopy. The results compare reasonably well with the scant NMR data in the 1 i t e r a t ~ r e . l ~The gas-phase IR spectrum at room temperature (vapor pressure = 3 torr) was obtained with a Beckman 4240 spectrometer using a Perkin-Elmer 10-m cell (in cm-l): 2870 (vs), 1450 (m), 1340 (s), 1260 (w), 1200 (w), 1095 (s), 920 (s), 800 (m), 620 (w). The lH NMR (HCC1, reference) was obtained at 360 MHz (Bruker WM-360) in CDC1,: 6 5.75 (m, 2 H (Hb)),2.19 (m, 1H 1.78 (m, 4 H (Ha,eq and Ha,,)), 1-60 (m, 1 H (Hf,eq)).The 13C NMR (D13CC13reference) was obtained a t 22.5 MHz (JEOL, FX9O-Q) in CDC1,: 6 101.4 (d, 2 C (cb)), 29.4 (t, 2 c (ca)), 14.5 (t, 1 c ( c b ) ) . The mass spectrum of a solid sample was obtained by using an AEI MS-902 mass spectrometer with the source about 20 "C above ambient temperature. Prominent peaks a t 70 eV were as follows: 116 amu (0.83), 84 (1.00), 83 (0.64), 82 (0.52), 72 (0.73), 71 (0.45), 70 (0.74), 69 (0.69). The la0species of CpOz were prepared by an ozonolysis with l80-enriched ozone. The ozone was prepared from a mixture of 25% l8OZand 75% lSO2by using the ozone generator and ozonolysis techniques described elsewhere.6d The relative intensities of the various l80species identified in the MW spectrum were consistent with the expected enrichment ratios of 42 % normal species, 42 % mono-180substituted species, 14% di-l*O-substituted species, and 2% of the tripleJa0 species (see below). Singly deuterated CpOz was prepared by the ozonolysis of nearly randomly deuterated cyclopentene-d. The cyclopentene-d was synthesized by the reduction of cyclopentanone with LiAlD4 and the subsequent addition of water.16 The 1-deuteriocyclopentanol obtained was dehydrated over 85% H3P04 at 145-150 "C to produce highly scrambled singly deuterated cyclopentene. 2H NMR combined with the microwave spectrum of the ozonide showed that it was deuterated approximately 35% a t cb, 24% a t C,, and 12% at Cf with the remainder undeuter(14) F. J. Wodarczyk and E. B. Wilson, J. Mol. Spectrosc., 37, 445 (1971). (15) R. S. Brown and R. W. Marcinko, J.Am. Chem. Soc., 100,5584 (1978). (16) The constants from the rigid rotor fit were (in MHz) A = 3035.187 (3), B = 2655.594 (4), C = 2149.477 (3). The constants from the centrifugal distortion fit were (in MHz) A = 3035.206 (l), E = 2655.616 (l), C = 2149.484 (1) and (in kHz) AJ = 0.244 (l), AJK = 0.615 (lo), AK = -0.601 (19), SJ = 0.018 (l), IS^ = 0.414 (4). The fitting program was obtained from R. H. Schwendeman.
Borseth and Kuczkowski
ated. 2H NMR (DCC1, reference) were obtained a t 55 MHz in CHC1,: 6 5.75 (%Ib), 2.17 (2Hf,,), 1.78 ('Ha, unresolved) 1.59 (2Hf,e,).
Results and Analysis Spectra. The Stark modulated spectra of CpOz between 18.0 and 40.0 GHz revealed a very dense spectrum. Very few of these transitions arose from decomposition products. A survey RFMWDR spectrum, using an approximately 5-W, 11.0-MHz rf signal, simplified matters considerably. It displayed several series of pairs of transitions that repeated every 1.1GHz. All structural models for CpOz predicted high asymmetry (-0.2 IK I0.2) and gave rise to such a pattern originating from high-J, pa-dipole, Qbranch transitions with K(ob1ate) = J/2. At these intermediate values of KO, the asymmetry splitting of levels with the same KOfell in the rf region near 11 MHz and could be modulated by a pe dipole connection. This rf modulation led to the close pair of pa,A J = 0 transitions in the MW region with AKp = 0 for the lower frequency and AK, = 2 for the higher frequency transition; the next transition pair about 1.1GHz higher had J incremented by 2 and K by 1. Several pairs of strong transitions were found in the RFMWDR spectrum which did not fit the pattern, and these arose from low-J R-branch transitions. When the origin of the patterns was recognized, a successful fit was found through trial and error. Confirmation of the assignment was also made from inspection of the Stark splittings of some of the R-branch transitions. The predicted lines from the fit accounted for all the major RFMWDR transitions observed and over 100 R-branch and Q-branch transitions were eventually assigned and measured for the ground vibrational state conformer of CpOz. A number of the assigned transitions along with the calculated spectra for both a rigid rotor and a model including centrifugal distortion corrections16are shown in Table I. The greatest deviation from the rigid rotor fit was -22.3 MHz exhibited by a J = 42 transition. The corrections to the rotational constants from centrifugal distortion were obviously small but to minimize any effect in the structural calculations a smaller number of low-J, R-branch transitions were chosen to determine the rigid rotor rotational constants for the normal species and the isotopic species. The planar second moments of inertia derived from these rotational constants are listed in Table 11. After assignment of the spectra it became apparent that only pa and p, selection rules could be observed, consistent with .C,symmetry for CpOz. Also, only the model structures which had the endo conformation for the six-membered ring (chair form or "flap down") could be reconciled with the moments of inertia. The spectra of the isotopic spectra were assigned by using RFMWDR to identify R-branch transitions involving J = 4-8. The spectra of the five l80-enriched species listed in Table I1 had the expected intensities for random isotopic distribution from a synthesis employing ozone enriched 25% in IaO in conjunction with a symmetry plane in CpOz. For example, the transitions for the species singly substituted a t the peroxy site (laOP)were about twice as intense as those for the 180species. e Likewise, the spectrum for the double-substituted laOelaOpspecies was qualitatively twice as intense as the spectrum of the 180p1a0p species. The planar second moments of inertia ( P b b ) of the species are nearly identical l8Oespecies and the 180,180p with the normal and triple-laO-substituted species, respectively (see Table 11). These observations demonstrate that the molecule possesses a plane of symmetry which passes through the midpoint of the 0,-0, bond, the 0,
The Journal of Physical Chemistry, Vol. 87, No. 26, 1983 5383
Cyclopentene Ozonide
TABLE I: Assigned Microwave Transitions of Cyclopentene Ozonide Ground Vibrational State frequency, obsd transition MHz calcdb
5(1,4) 4(1,3) 5(4,2) + 4(4,1) 5(4,1) 4(4,0) 5(2,3) 4(2,2) 5(5,0) 4(4,0) 5(5,1) 4(4,1) 6(2,5) 5(2,4) 6(1,5) 5(1,4) 6(3,4) 5(3,3) 6(5,2) 5(5,1) 6(5,1) 5(5,0) 6(4,2) 5(4,1) 6(3,3) 5(3,2) 6(6,0) 5(5,0) 6(6,1) 5(5,1) 22(8,14) 22(8,15) 22(9,14) 22(7,15) 23(9,14) 23(9,15) 23(10,14) 23(8,15) 24(9,15) 24(9,16) 24(10,15) 24(8,16) 25(10,15) 25(10,16) 25( 11,15) 25( 9,16) 26(11,15) 26(11,16) 26(12,15) 26(10,16) 27(12,15) 27(12,16) 27( 13,15) 27( 11,16) 27(11,16) 27(11,17) 27(12,16) 27(10,17) 28(13,15) 28(13,16) 28(14,15) 28(12,16) 28(12,16) 28(12,17) 28( 13,16) 28( 11,17) 29( 13,16) 29(13,17) 29( 14,16) 29( 12,17) 29( 12,17) 29(12,18) 29(13,17) 29(11,18) 30( 14,16) 30( 14,17) 30( 15,16) 30( 13,17) 30(13,17) 30(13,18) 30(14,17) 30(12,18) 31(14,17) 31(14,18) 31(15,17) 31(13,18) 31(13,18) 31(13,19) 31(14,18) 31(12,19) 32(15,17) 32(15,18) 32(16,17) 32( 14,18) 32( 1 4 3 3 ) 32( 14,19) 32(15,18) 32(13,19) +
+-
+-
+
+-
+
+
+-
+
+
+
+
+-
+
+
+
+-
+
+
+ +
+ -
+-
+
+-
+
+
+
+
+ +
+
+
+
+
+ +
+
+-
+
+
+ +
+ +
-
+
+
22 168.90 22 162.08 23 432.62 23 607.54 24 618.01 24 858.47 25 048.62 29 756.65 29 782.26 27 786.49 27 840.70 28 925.22 29 584.15 29 677.83 30 260.24 30 490.71 35 836.90 35 844.61 18 420.17 1 8 423.34 18 164.45 18 176.09 1 9 585.90 1 9 589.09 19 313.62 1 9 324.98 18 983.26 19 020.59 18 566.36 18 679.02 20 457.01 20 467.98 18 006.46 18 318.99 20 111.78 20 147.06 1 9 682.47 1 9 787.46 21 595.27 21 605.75 19 116.57 1 9 405.49 21 236.15 21 269.25 20 795.41 20 892.72 22 728.90 22 738.85 20 224.43 20 490.11 22 356.79 22 387.65
-0.02 0.03 - 0.00 -0.01
- 0.01 0.01
- 0.01
0.01 -0.03 0.02 - 0.01 0.00 0.05 -0.04 -0.03 0.00 0.05 - 0.04 -0.02 0.02 -0.02 0.01 -0.01 0.02 0.00 0.01 -0.01 0.01 -0.01 0.01 - 0.01 0.01 0.04 -0.02 0.00 0.01 0.00 0.01 - 0.01 0.02 -0.01 0.01 -0.01 0.00 -0.01 0.02 -0.02 0.02 0.00 -0.02 0.00 0.01
c
0,
obsd calcda
-0.03 0.02 0.00 -0.03 -0.08 -0.05 0.01 -0.02 -0.07 -0.03 -0.08 -0.05 -0.22 -0.28 -0.08 -0.01 -0.06 -0.16 -3.59 -3.58 -4.05 -4.13 -4.70 -4.71 - 5.20 -5.33 -5.54 - 5.94 - 5.45 -6.67 -6.56 -6.70 - 4.43 - 7.79 -6.90 - 7.37 -6.77 -8.14 -8.12 - 8.26 - 5.66 - 9.29 - 8.46 -8.99 -8.30 -9.78 -9.89 - 10.04 - 7.00 -11.02 -10.23 -10.80
a Rigid rotor model calculated with rotational constants Semirigid rotor model calculated with rotain ref 16. tional and centrifugal distortion constants in ref 16.
atom, and the carbon 0 to the bridgehead (Cf). At this stage enough isotopic data were in hand to obtain a good structural model to assist in the 13C and 2H assignments. As subsequent assignments were made and incorporated into the structural model, the ability to predict spectra of new species improved. The transitions of the five single-substituted deuterium species were observed in the enriched sample with intensities in agreement with the 2H NMR data. The assignments were also checked in a few cases from the Stark effects and from the agreement in P b b for substitution at the C, site (2Hf,qand 2Hf,ax).
axis
The three 13C species were observed in natural abundance. The spectra of the 13Cboth at the bridgehead site (C,) and at the site a to the bridgehead (C,) were approximately 2% the intensity of the normal species spec-
i
. .....-.
Ct
Y-. -.?
.. .
,
.
I
\
a axis
Flgure 1. Plane angles in a bicyclo[3.2.1] ring system with endo conformation: 4 (five-atom ring), 0 (six-atom ring), [ (seven-atom ring).
trum. The 13Cfspecies spectrum was about 1%the intensity of the normal species and P b b was the same as that of the normal species. The planar second moments for all the isotopic species derived from a rigid rotor fit of the observed transitions are listed in Table 11. The transition frequencies and rotational constants of all the species are available as supplementary material. (See paragraph a t end of text regarding supplementary material.) Vibrational Satellites. Three low-lying vibrational satellites were assigned. Over 70 transitions were measured for the most intense satellite which had (in MHz) A = 3038.62 (2), B = 2654.39 (4), C = 2147.51 (3) and centrifugal distortion constants nearly identical with those of the ground state. Relative intensity measurements indicated that this state had a vibrational frequency of 145 & 10 cm-l. A few transitions for two other excited vibrational states were also assigned with rotational constants of 3034.9 (l),2653.9 (l), 2148.9 (1) and 3033.7 (11, 2652.7 (l),and 2147.5 (1). Accurate intensity measurements were not made for these states except to estimate that they were respectively at least 220 and 280 cm-l higher than the ground state. The transition frequencies for these satellites are available as supplementary material. In summary, the minor perturbations from centrifugal distortion and the satellite data indicate an absence of any unusually lowfrequency or large-amplitude vibrational motions. Dipole Moment. The electric dipole moment of the ground state of CpOz was determined from the measurement of the second-order Stark effects of eight transitions using standard procedures. Methanol was used to calibrate the ~pectrometer.'~Several of the observed Stark coefficients were obtained as the intercept of a A v / E 2vs. E2 least-squares fit to remove any fourth-order effects. A least-squares fit of the resultant A v / E 2 values (in Table 111) vs. the calculated second-order coefficients assuming &, = 0 resulted in the dipole components in Table 111. A full fit of pa,&, and p c resulted in an imaginary value for pb. This further confirms the presence of an ac symmetry plane in CpOz. The orientation of the ac principal axes and these dipole components can be seen in Figure 1. Structure. The rotational constants of the normal and all possible single-substituted species of CpOz allow the calculation of the complete substitution structure (rJ by Kraitchman's equations,18as well as the effective structure (17)R.H. Hughes, W. E. Good, and D. K. Coles, Phys. R e a , 84,418 (1951). (18)J. Kraitchman, Am. J.Phys., 21, 17 (1953).
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The Journal of Physical Chemlstty, Vol. 87, No. 26, 1983
Borseth and Kuczkowski
TABLE 11: Planar Second Moments of the Isotopic Species of Cyclopentene Ozonide,
P,, m u A 2 129.4589 ( 2 ) b 130.1745 (6) 132.0530 (3) 132.6331 ( 7 ) 134.4216 (2) 134.9181 ( 2 0 ) 129.6052 ( 3 4 ) 130.7418 ( 3 0 ) 132.1363 ( 2 3 ) 130.3496 (4) 131.3394 ( 3 ) 132.3104 (7) 136.9245 ( 6 ) 131.1633 (10)
species
Pb, amu A z 105.6583 ( 2 ) 105.6511 (6) 106.6469 ( 3 ) 106.6618 ( 7 ) 107.8359 ( 2 ) 107.8283 (20) 106.8309 (34) 107.1606 (30) 105.6535 (23) 109.4511 ( 4 ) 109.9216 ( 3 ) 107.5614 ( 7 ) 105.6483 (6) 105.6527 (10)
P,, amu A ' 60.8483 ( 2 ) 63.9747 ( 6 ) 61.9942 (6) 65.2732 (7) 63.0805 (2) 66.4802 (20) 60.9968 (34) 60.8621 (30) 61.1997 (23) 61.4460 (4) 61.1089 (3) 61.8496 ( 7 ) 61.1799 (6) 63.4709 (10)
a A planar moment is defined from moments of inertia as I , t I b - I , = 2P,,, etc. The conversion factor from rotational constants was AI, = 505379.1 m u A'. See Table I and Table S-18 (supplementary material) for the original rigid rotor rotational constants. The uncertainty (in the last digits) represents one standard deviation of the fit.
TABLE 111: Stark Effect and Dipole Moment of Cyclopentene Ozonide Av/E2'
transit ions 4( 1,3) 5( 2,3) 5( 2,3) 6( 3,4) 6( 3,4) 6(2,4) 6(4,2) 7(5,3)
obsd 1 - 13.24
IMI
3( 1,2)b 4( 2,2)b 4( 2,2)b 5( 3,3)b 5( 3,3)b 5(2,3) 5(4,1) 6(5,2) Ip,l= 2.305 ( 2 ) D, + +
+
6
+
+
+
+
obsd calcd,
calcd
- 13.24 0.00 -4.699 -4.552 -0.146 -1.670 -1.614 0.056 7.202 7.378 0.165 27.56 -0.25 27.31 -4.444 -4.346 -0.095 -64.26 -64.31 0.05 148.5 148.5 0.0 lpcl= 0.910 ( 7 ) D, lptl= 2.478 (9) D
1 2 1 2 1 1 1
' in units of MHz cm'/kV'. ( A v l E 2)obsd obtained from the intercept of a A v / E vs. E a plot, Figure 2. Structure of cyclopentene ozonide with r o bond lengths in angstroms.
TABLE IV: ro Coordinates (Angstroms) of Cyclopentene Ozonide atom Oe 0, cb
ca
Cf Hb
Ha.w Ha+x Hf,eq Hf,a,
a0
-0.589 (2) -1.114 (1) -0.383 (1) 1.106 (1) 1.640 (1) -0.866 ( 3 ) 1.244 ( 2 ) 1.612 ( 2 ) 2.730 (1) 1.285 (2)
b0
0.000 ~ 0 . 7 3 8(1) k1.091(2) f 1.264 (1)
0.000 ~1.975 (1) k2.143 (1) rt. 1.448 (2)
0.000 0.000
CO
1.267 -0.786 0.391 0.097 -0.602 0.810 - 0.539 1.043 - 0.603 -1.641
(1) (1) (2) (2) (3) (4) (6) (3) (5) (2)
(ro)by the least-squares method.l9 The r8 and ro coordinates of each atom are essentially equal within experimental error20 and the standard deviation of the leastsquares fit is 0.006 amu A2. We prefer the ro structure and list these coordinates and parameters in Tables IV and V. This calculational procedure utilizes all the data including the redundant l*O isotopes, incorporates the plane of symmetry of the fitting process, and does not overemphasize an error for a small coordinate (such as c for CJ. I t is difficult to estimate the absolute accuracy of an ro structure without a detailed vibrational analysis. It seems likely that the r, or average structure21is within 0.01 A and 0.5' of the parameters in Tables V and VI. The structure (19) R. H. Schwendeman, "Critical Evaluation of Chemical and Physical Structural Information", D. R. Lide and M. A. Paul, Eds., National Academy of Sciences, Washington, DC, 1974, pp 74-115. (20) The only cases where Ir, - rol 2 0.005 A were for Cb (a, = 0.373 A), and C, (a, = 1.099 and c, = 0.119). (21) M. D. Harmony, V. W. Laurie, R. L. Kuczkowski, R. H. Schwendeman, D. A. Ramsay, F. J. Lovas, W. J. Lafferty, and A. G. Maki, J. Phys. Chem. Ref. Data, 8, 619 (1979).
Figure 3. Structure of cyclopentene ozonide with r o bond angles in degrees.
of CpOz generated by ORTEPZ2is illustrated in Figures 2 and 3.
Discussion The unambiguous evidence that CpOz possesses C, symmetry resolves the question whether a twist about the peroxy bond might be induced in this bicyclic system. Obviously the constraints imposed by the (CH,), moiety overcome the destabilization arising from a 0, envelope conformation in the ozonide portion. For ethylene ozonide, (22) C. K. Johnson, Report No. ORNL-3794, ORTEP, Oak Ridge National Laboratory, Oak Ridge, TN, 1965.
The Journal of Physical Chemistry, Vol. 87, No. 26, 1983 5385
Cyclopentene Ozonide
TABLE V : Bond Distances and Angles of Cyclopentene Ozonide bond angles,O deg
bond lengths, A
114.4 (2) 109.1 (2) 107.8 (2) 110.6 (3) 111.8 ( 2 ) 108.1 (3) 110.3 (2) 108.5 (1) 108.1 (3)
1.475(1)' 1.430 (2) 1.414(2) 1.528 (2) 1.540 ( 2 ) 1.091( 3 ) 1.093 (4j 1.088(3) 1.090 (2) 1.097(3)
OP-OP cb-op
Cb-Oe cb-ca
Ca-Cf Ck-Hh CG-H,, eq %-Ha ,ax Cf-Hf,eq Cf-Hf ,ax
a Parameters and uncertainties calculated from coordinates in Table 1V. molecular planes.
TABLE VI: Structural Comparison of Cyclopentene Ozonide and Bicyclo[3.2.l]octane bicyclo[ 3.2.11octane
structural tmaneter
CDOZ'
Cf-Ca, '4 ca-cb,
1.540( 2)d 1.528(2) 110.3(2) 40.8(2) 113.6(2) 136.7(2) 109.7 (1)
A
CaCfCa, deg deg 0 , deg 0 , de€! E , deg $ 9
model A b
1.543 (2) 1.543(2) 115.0 (2.3) 44.2 (2.1) 114.4 (2.1) 133.2 (1.1) 112.4 (2.4)
See Table VI for additional angles involving
TABLE VII: Comparison of ab Initio C, and C, Conformers of Ethylene Ozonide with the Experimental Ethylene Ozonide and C, Cyclopentene Ozonide Structures
model BC
1.553(57) 1.532 (71) 109.6 (2.5) 41.1 (2.1) 117.4(2.3) 132.6(4.4) 110.0(5.0)
Cyclopentene ozonide r , parameters are used. See Model A is calculated from the Figure 1for , etc. electron diffraction data assuming one average C-C distance.15 c Model B is calculated by fitting the electron diffraction data and five different C-C distances by using molecular mechanics programs. See text and ref 25. Uncertainties represent one standard deviation in the r , fit.
EtOz calcda
c-0, c-0
Op-8,
a
$J
the envelope C, form was estimated to be 3 kcal/mol higher than the peroxy twist C2 species?* The implication for transition-state modeling of the ozonolysis reaction is that envelope conformations, a t least for species similar to fiial ozonides, need not be considered prohibitively high in energy and unaccesible. Observation of the endo (chair) conformation for the six-membered ring was expected by analogy with data for the bicyclo[3.3.l]nonane systemz3and molecular mechanics calculations which place the exo conformation 5-6 kcal higher in the hydrocarbon analogue, bicyclo[3.2.l]octane." Electron diffraction data on the latter system have also identified the endo c o n f ~ r m a t i o n . ~There ~ are no significant unexplained features in the IR, NMR, or MW spectra which suggest the presence of another low-energy conformer. Of course, the MW spectra were very dense and the possibility of simply overlooking some pattern cannot be excluded. Cyclopentene ozonide is the first reported heterobicyclo[3.2.l]octane structure to be completely determined. The structure of bicyclo[3.2.l]octane determined by electron diffractionz5boffers a useful comparison to the ro structure of cyclopentene ozonide determined here. The five nonequivalent C-C distances in bicyclo[3.2.l]octane pose EI difficult problem for the accurate determination of the structure by electron diffraction. It was calculated25b (23)D. C.Webb and M. R. Becker, J.Chem. SOC.E 317 (1967);W. A. C.Brown, J. Martin, and G. A. Sim, ibid., 1844 (1965);M. Dobler and J.
D. Dunitz, Helu. Chem. Acta, 47,695 (1964). (24)E. M. Engler, J. D. Andose, and P. v. R. Schleyer,J.Am. Chem. SOC.,95,8005 (1973). (25)(a) S. Mastrynokov, E. L. Osina, 0. V. Dorofeeva, M. V. Popik, L. V. Vilkov, and N. A. Belikova,J. Mol. Struct., 62, 211 (1979);(b) R. L.Hildorbrandt,V. S. Mastryukov, E. L. Osina, and L. V. Vilkov, 'Eighth Austin Symposium on Molecular Structure, University of Texas, Austin, TX, 1980",paper A12 (Abstract).
coec
o"g8
C, C. Bond Length, A 1.426 1.415 1.433 1.439 1.467 1.476
exptb
cpoz exptb
C,
C,
1.419 1.412 1.461
1.414 1.430 1.475
105.0 105.3 99.4
101.0 104.2 104.3
Bond Angle, deg
105.8 105.4 100.2
103.4 105.8 105.0
Dihedral Angle, deg CO,CO, OeCOpbp
copo,c
15.3 38.9 47.4
37.6 23.1 0.0
16.2 40.9 49.7
45.0 27.7
0.0
a From extended basis set ab initio calculation^.^^ R. L. Kuczkowski, private communication. The ro calculation using inertial data from the 20 different isotopically substituted species.28 Uncertainties are i0.002A in bond lengths and to,5" for bond angles.
from the electron diffraction data by two methods. The first method assumed one average C-C bond length (model A in Table VI) for the molecule. The second method allowed the five different C-C bond lengths to vary by employing the molecular mechanics programs (which correct for shrinkage due to vibrations assuming the approximate bond force fields) EMIN,% E A S , ~and M M ~ .The ~ determination of the bond and plane angles of bicyclo[3.2.l]octane by this joint use of electron diffraction and molecular mechanics should be fairly accurate. Figure 3 illustrates the four plane angles $, 6, 4, and of the bicyclo[3.2.l]octane structure (which is shown projected onto the ac plane). The comparable structural parameters of cyclopentene ozonide and bicyclo[3.2.l]octane are shown in Table VII. The comparison between the structural parameters of these two molecules shows agreement within experimental error. The interchange of 0 and CH2 moieties does not markedly alter the general structural features except perhaps for a slight flattening of the envelope in the five-atom ring in the ozonide. The constraints of the bicyclo[3.2.l]octane system along with the knowledge of the r,, structure of cyclopentene ozonide should help to estimate accurate model structures of other (26)R. L. Hilderbrandt, Comput. Chem., 1, 179 (1977). (27)N.L. Allinger, J. Am. Chem. SOC.,99,8127(1977). (28)R. L. Kuczkowski, C. W. Gillies, and K. L. Gallaher, J. Mol. Spectrosc., 60,361 (1976);U.Mazur and R. L. Kuczkowski, ibid., 66,84 (1977).
5386
J. Phys. Chem. 1983, 87,5386-5388
heterobicyclo[ 3.2.11 octane systems. Cremer’s extended basis set ab initio calculations on the structure of ethylene ozonide over its pseudorotational potential surfacee”predict a lengthening of the O,-O bond by about 0.01 A when going from the C2 to the symmetry conformer. The structural parameters of the C2 and C, symmetry conformers of ethylene ozonide calculated by Cremer are shown in Table VI1 along with some experimentally determined r, structural parameters of ethylene and cyclopentene ozonides. It is interesting that a lengthening of the 0 -0 bond by about 0.01 is observed in CpOz as well 89 t i e sfight lengthening of the C-0, and shortening of the C-0, bond lengths that were predicted. These observations support Cremer’s interpretation that ethylene ozonide is forced into the Cz conformer not only by dipole-dipole repulsions but also by the delocalization of the oxygen lone-pair electrons in the C-0 Q* orbital in the twisted conformer. However, it may be unwise to place too much emphasis on the comparison between ethylene ozonide and CpOz since the bond changes are small and the additional ring constraints in CpOz complicate comparisons. This is probably the reason why the trends in
6
predicted and observed bond angles, particularly dihedral angles, are quantitatively more disparate. It is also noteworthy that the predicted plane angle 4 (Figure 1)is 136.7’ in CpOz but a flatter envelope (145.6’) is predicted for C, ethylene ozonide.
Acknowledgment. This study was supported by Grant CHE8005471 from the National Science Foundation, Washington, DC. We are grateful to Dr. Kurt Hillig for advice on many aspects of this study. Registry No. CpOz, 280-21-7; 180,-CpOz, 87801-46-5; l80,-
CPOZ,87801-47-6; 180280 -CpOz, 87801-48-7; 180~s0,-CpO~, 87801-49-8; 180~80~80,-Cp6z, 87801-50-1; 13C&pOz, 87801-51-2; 13C,-CpOz, 87801-52-3; 13CrCpOz, 87801-53-4; 2H,,eq-CpOz, 87801-55-6; 2H,,ax-CpOz,87801-56-7; 2Hf,eq-CpOz,87801-57-8; 2Hf,ax-CpOz,87801-58-9.
Supplementary Material Available: Tables Sl-Sl8 listing the transition frequencies for the 14 isotopic species, the three excited vibrational states of the normal species, and the ground-state rotational constants of the substituted isotopic species (22 pages). Ordering information is given on any current masthead page.
Energy Component Analysis Calculations on Interactions Involving I, and H I U. Chandra Singh and Peter Kollman” School of Pharmacy, Department of Pharmaceutical Chemistry, University of California, San Francisco, California 94 143 (Received: February 16, 1983; I n Final Form: April 28, 1983)
We present ab initio calculations on the complexes H2CO-HI, H2C0.-IH, H2CO-.I-CH3, H3N.-12, H3N-IH, H3N-.HI, and CH3NH2-12. Morokuma component analyses of these complexes have been carried out in order to elucidate the nature of these interactions. In particular, it is of interest to compare nitrogen and oxygen base interactions with iodine, to compare the Lewis acid properties on each “endnof H-I, and to examine the methyl substituent effect in amine-4, interactions. For HzCO--IH, we also carry out estimates of the correlation energy (MP2 level) contribution to the intermolecular attraction.
Introduction It is clear that ab initio electronic structure methods are very powerful tools for understanding the strength and directionality of intermolecular interactions. However, they still have been applied mainly to molecules containing atoms of only the first two rows of the periodic table. Although pseudopotential methods should ultimately allow routine applications to molecules containing heavier elements, it is still somewhat of a challenge to study iodinecontaining molecules. This is unfortunate, since iodinecontaining molecules often have unusual spectral properties and this atom also plays an important biological role in thyroxine and its analogues.2 Thus, a more precise understanding of the nature of alkyl and aryl iodine interactions with other molecules is of importance. We have previously carried out quantum mechanical calculations on the interactions of H3N with Iz and HI, using ab initio SCF t h e ~ r y . We ~ now extend this study ~~~
~~
in several important ways. First, we examine the methyl substituent effect on iodine interactions, by comparing the interaction of CH3NH2with 1, with that of NH, with 12. In an earlier study of alkyl-amine interactions with SO2, we have found4 a very large methyl substituent effect on this interaction and we wish to see whether this is also found in amine-I2 interactions. Secondly, we wish to use Morokuma energy component a n a l y ~ i s both , ~ to further understand the NH,-IH, NH3.-HI, and NH3-.12 systems, which were subjects of the previous study, and to see how such components are effected on changing the Lewis base from NH, to CH3NH2. Finally, we wish to examine the structure and energy of R-14=CH2 complexes, as well as the energy components for such interactions, because the nature of I-.O interactions observed in the solid state have been characterized in terms of R-1-0 distance and R-1-0 angle.6 In our earlier study,l we merely inferred the likely results of 1.-0 ab initio studies from the 1.-N results; here we carry out
~
(1)R. Mulliken, J. Am. Chem. SOC.,74,811 (1952). (2)For an analysis of the structure activity relationships of thyroxine analogues, see S. Dietrich, M. Bolger, P. Kollman, and E. C. Jorgensen, J.Med. Chem., 20,863 (1977). (3)P. Kollman, A. Dearing, and E. Kochanski, J. Phys. Chem., 86, 1607 (1982). 0022-3654/83/2087-5386$01.50/0
(4)J. Douglas and P. Kollman, J. Am. Chem. SOC., 100, 5226 (1978). (5)K. Kitaura and K. Morokuma, Znt. J . Quant. Chem., 10, 325 (1976). (6)P.Murray-Rust and W. Motherwell, J.Am. Chem. SOC., 101,4374 (1979).
0 1983 American Chemical Society
