J . Phys. Chem. 1993, 97, 10670-10673
10670
Molecular Structure and Conformational Composition of 2-Chloropropanal, a Gas-Phase Electron-Diffraction and ab Initio Investigation Kirsten Aarset and Kolbj~rrnHagen' Department of Chemistry, University of Trondheim, AVH, N- 7055 Trondheim, Norway
Gemot Frenking Fachbereich Chemie, Universitiit Marburg, 0-3550 Marburg, Germany
Arno Wehrsig Max- Planck-Institut fir Kohlenforschung, 0-4330 MheimlRuhr, Germany Received: April 26, 1993'
Gas-phase electron diffraction (ED) data obtained at 72 'C together with results from a b initio calculations have been used to determine the structure and conformational composition of 2-chloropropanal. The majority (66 f 26%) of the molecules have a conformation where the CH3 group is eclipsing the C=O bond ( C - C - C = O torsion angle ~ $ 1= 27 (7)'). Smaller amounts of a form (23 f 28%) where H is eclipsing C-0 (L$1 = 109 (20)') and a form (1 1 f 26%) where C1 is eclipsing C=O (L$l = 240') probably also is present in the gas phase. These results for the conformational composition are in fairly good agreement with the a b initio results calculated at the MP2/6-31G*//HF/6-31GS level. The results for the principal distances (rg) and angles (&) from a combined ED/ab initio study for the conformer where CH3 is eclipsing C=O, with estimated 2u uncertainties, are t'(C-H)methyl = 1.115 (10) A, r(C-cO) = 1.529 (4) A, r(C-CH3) = 1.527 (4) A, r ( C 4 ) = 1.205 (4) A, r(C-Cl) = 1.805 (6) A, LC-C-Hmethyl= 110.4' (ab initiovalue), LH-C-Cl= 108.1 (39)', LOC-C-Cl = 108.9 (7)", LC-C-C = 112.4 (15)', LC-C=O = 124.3 (18)', and LC3-C2-C1= 109.2 (7)'. n
Introduction We have earlier studied several molecules with the general formula XCH*-CY=O where X and Y have been H, CH3, or halogen atoms (see ref 1 and references therein). We have found that the conformational composition depends strongly on the nature of the substituents X and Y. In these molecules we have in some cases observed only one conformer, but for most molecules of this type a mixture of two different conformers have been found. In 2-chloropropanal, HCl(CH3)C-CH=O, the possibility of three different conformers exists, where either H, CH3, or C1 will eclipse the carbonyl bond. As part of our work on determining the structure and conformation of molecules with a C(spz)-C(sp3) bond we here report the results of an investigation of 2-chloropropanal (Figure 1) using gas-phase electron diffraction. As part of this work we have also made ab initio molecular orbital calculations for all three possible conformers and the results of these calculations have been used to assist the ED analysis.
Experimental Section and Data Reduction The sample of 2-chloropropanal was synthesized in an analogous fashion to the synthesis of 2-chlorobutanal described earlier.* The reaction product was analyzed and identified as 2-chloropropanal by elemental analysis and * H and 13C NMR spectroscopy. The sample was kept in a solution of dichloromethane to prevent polymerization and was separated from the solvent just prior to the ED experiment by distillation at reduced pressure. The purity of the sample was checked by gas chromatography (GC). The electron diffraction data were obtained with the Oregon State apparatus under the following experimental conditions: sector shape, r3; plates, 8 X 10 in. medium contrast Kodak projector slide; developed 10 min in D-19 diluted 1:2; nozzle-to-plate distances, 747.6 1 mm (long camera) and 299.24 Abstract published in Advance ACS Absrrucrs. September 15, 1993.
0022-3654/93/2097-10670$04.00/0
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Figure 1. Diagram, with atom numbering, of the conformer of 2-chloropropanal where CH3 is eclipsing C=O, = OD.
mm (middle camera); nominal electron wavelength, 0.048 A (calibrated in separate experiments with CSz: r,(C=S) = 1.557 A and ra(S-S) = 3.109 A); exposure times, 90-240 s; beam currents, 0.40-0.52 PA; ambient apparatus pressure during experiments, (5.0-7.0) X 1 V Torr; nozzle temperature, 72 'C; bath temperature 47-67 'C. Five plates from the long camera and four plates from the middle camera experiments were used in the analysis. Optical densities were measured using a JoyceLoeble double-beam microdensitometer. To reduce the noise in the data, the middle camera plates were all traced twice. The data were reduce in the usual way.3-5 The ranges of data were 2.00 Is1A-I I15.00 and 8.00 Is1A-I I35.00; the data interval was As = 0.25 A-l. A calculated background6 was subtracted from the data for each plate to yield experimental molecular intensity curves in the form sl,,,(s). The average experimental intensity curves are shown in Figure 2. The data are available as supplementary material (see paragraph at end of table). Figure 3 shows the final experimental radial distribution (RD)curve 0 1993 American Chemical Society
Structure and Composition of 2-Chloropropanal
The Journal of Physical Chemistry, Vol. 97,No. 41, 1993 10671
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Figure 2. Intensity curves s(l,,,(s))for 2-chloropropanal. The experimental curves are averages of all plates for the two camera distances. The theoretical curve was calculated from the structural parameters shown in Table I. The difference curves result from subtracting the relevant part of the theoretical curve from the experimental curves. I
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Figure 4. Theoretical radial distribution curves for the final conformational mixture and for 100%of each of the three conformers together with the experimental curve and difference curves. Only the conformationally important parts of the curves are shown.
geometry of each conformer may be defined by seventeen geometrical parameters, in our refinements taken as follows: (r(C-C)) = 0.5(r(C1-C2) + r(Cz-C3)), Ar(C-C) = r(C2-C3) - ~ ( C I - C ~ ) ,r(C3-H), Ar(C-H)l = r(C3-H) r(Cl-H), Ar(C-H)2 = r(C3-H) - r(C2-H), r(C=O), r(C-Cl), LC~-C~-H, LC-C-C, LC-C=O, LC2-CI-H, LH-C-Cl, LCl-Q-H, LCl-C2-Cl, LC3-C2-Cl, ~ q + ,the O=C-C-C torsion angle ( ~ 4 = 1 0' when C3 is eclipsing C=O), and L42, the H-C-C-C torsion angle ( ~ 4 = 2 0' when H is eclipsing C-C). C3, symmetry was assumed for the methyl group, and in the early refinements it was assumed that only the CI-CI torsion anglediffered in the different conformers. Thevibrational properties were specified by 41 amplitude parameters for each conformer, corresponding to the different interatomic distances. The structure was defined in terms of the geometrically consistent r,-type distances. These were converted to the r,-type required by the scattered intensity formula by using values of centrifugal distortion constants (6r), perpendicular amplitude corrections ( K ) and root-mean-square amplitudes of vibration (1) calculated from a harmonic force field (r, = r, - 12/r K 6r = rB- P/r). The force constants were transferred from force fields developed for related molecules.9J0 To get some more information about the structure and conformational properties of 2-chloropropanal, it was decided to do some theoretical calculations. Molecular mechanics (MM) calculations had earlier been made" for 2-chloropropanal as part of a study of halogen-substituted aldehydes. This work gave as a result that there were three minima in the C-C torsional potential function for C-C-C-0 torsional angle values of 4, 102, and 256', with relative energies of 0.0, 1.55, and 3.30 kcal/ mol, respectively. Geometries of 2-chloropropanal were also fully optimized at the a b initio HF/6-31G* level with the use of the program GAUSSIAN 9012 for the three possible conformers. These calculations indicated minima in the torsional potential function for the C - C - C 4 torsion angle L41 = 14, 103, and 240'. To get better values for the relative energies of the three conformers, additional ab initio calculations were made at the MP2/6-3 1G* level including zero-point energy corrections (ZPE) scaled by 0.89 using the HF/6-3 1G* optimized geometries. The relative energies for the conformers where CH3, H , or C1 are eclipsing C=O are 0.0, 0.60, and 2.10 kcal/mol, respectively (E(CH3 eclipsed) = -651.540 12 hartrees).l3 The ab initio calculations at the HF/6-31G* level were used to modify our model in such a way that calculated differences in corresponding distances and valence angles between the three conformers could
+ +
calculated in the usual way from the modified molecular intensity curve Z'(s) = sZm(s)ZcZc&4&&I exp(-0.002~2),where A = s2FandF i s the absolute value of the complex electron-scattering amplitudes. The scattering amplitudes and phases were taken from tables.'
Structure Analysis From the results obtained for related molecules and from the experimental R D curve, trial values for the bond lengths and valence angles could be obtained. Calculations of theoretical R D curves for the three most likely conformers where CH3, H , or C1 is eclipsing the C=O bond showed that no single conformer fit the experimental data very well. From these calculations it was obvious, however, that the majority of the molecules had a conformation where the methyl group is eclipsing the carbonyl bond. This is clearly seen from Figure 4 where the conformationally important part of the experimental R D curve is shown together with theoretical curves calculated for 100% of each of the three possible conformers and for the final mixtures of these conformers. Refinements of the structure were made by the least-squares method,8 adjusting a theoretical sZm(s)curve simultaneously to the two average experimental intensity curves, one from each of the two camera distances, using a unit weight matrix. The
Aarset et al.
10672 The Journal of Physical Chemistry, Vol. 97, No. 41, 1993 TABLE I: Structural Parameters for 2-Chloropropanal
parameter
ED" CH3 eclipsed
ab initio
MMb CHp H CI CH3 eclipsed eclipsed eclipsed eclipsed
r(G-H) &(C-Hh &(C-H)2 (r(C-C)) r(C=O)
1.081 (10) [-0).009] [0.000] 1.524 (4) 1.177(3) r(C-Cl) 1.802(6) LC2
