J . Phys. Chem. 1994,98, 2848-2852
2848
Molecular Structure and Conformational Composition of 2-Methylpropanal and 2-Methylpropionyl Chloride: A Gas-Phase Electron Diffraction and ab Initio Investigation Kirsten Aarset, Liv Curi Faksnes, Ingvild Nygird, and Kolbjarn Hagen. Department of Chemistry, University of Trondheim, AVH, N-7055 Trondheim, Norway Received: October 26, 1993; In Final Form: January 7, 1994' Gas-phase electron diffraction (ED) data obtained at 25-26 'C together with results from ab initio calculations have been used to determine the molecular structure and conformational composition of 2-methylpropanal, (CH3)2HC-C(H)=O, and 2-methylpropionyl chloride, (CH3)2HC-C(Cl)4. For 2-methylpropanal earlier published values for rotational constants for several isotopes have also been included in the analysis. For both compounds the majority of the molecules have a gauche conformation where a CH3 group is nearly eclipsing C 4 . Small amounts of a syn form where H is eclipsing C = O (C, symmetry) probably also is present in the gas phase. The results from this combined analysis for some important bond distances (re)and valence angles (La) are as follows for 2-methylpropanal: r(C-H)mcthyl = 1.109(3) A, r ( C = O ) = 1.226(3) A, r ( 0 C l - C ~ )= 1.513(2) A, r(C2-C3) = 1.526(2) A, r ( C 2 4 4 ) = 1.537(2) A, LC2-c1=0 = 123.8(9)', LCl-c2-C3 = 1 13.5(7)',~Cl-C&4 = 109.2(9)',LC3-C2-C4= 111.6(4)',andag (molefractionofthelowenergyconformer) = 0.88(14). For 2-methylpropionyl chloride the results are as follows: r(C-H),thyl = 1.108(6) A, r ( C 4 ) = 1.186(3) A, r(OCl-C2) = 1.511(3) A, r ( C r C 3 ) = 1.534(3) A, r ( C K 4 ) = 1.540(3) A, r(C-Cl) = 1.804(4) A, LC2-C1=0 = 127.3(7)', LCi-C2-C3 = 109.7(8)', LCl-C2-C4 = 109.9(8)', LC342-Cd = 113.8(27)', LC2-Cl-Cl = 113.6(5)', and ag = 0.82(12). Parenthesized values are 2a. The results are compared with the ab initio values and with results for related molecules.
Introduction As part of a program to study the structure and conformation of compounds with a C(sp3)-C(sp2) bond, we have earlier investigated several molecules with the general formula XH2C-C( Y ) = O where X and Y are H, CHp, or halogen atoms (see ref 1 and references therein). As a continuation of this work we here report the results obtained for two compounds with the general formula X2HC-C(Y)=O where X = CH3 and Y = H or C1 (Figure 1). 2-Methylpropanal(isobutyraldehyde),( C H p ) 2 H C - C ( H ) 4 , has been studied by severaldifferent methods. An earlier reported electron diffraction study2 concluded that the molecule consists mainly (90%)in the gas phase at ambient temperature of a gauche conformer where CH3 is eclipsing c-0, the other form being a syn conformer where H is eclipsingC = O . The sameconclusion about the conformationalcompositionwas reported in a microwave spectroscopy (MW) study,3 in a combined far-infrared and microwavespectroscopystudy? in a vibrational spectroscopyand ab initio study,S and from molecular mechanics (MM) calculations.6 In the solid phase only the gauche form has been observed. By combining the earlier published results from spectroscopy and theoretical calculations with new gas-phase electron diffraction data, more reliable information about the structure and conformation of 2-methylpropanal could be obtained. For 2-methylpropionylchloride (isobutyrylchloride), (CH3)zHC-C(Cl)==O, much less information about structure and conformation is available, but a study of the Raman spectra in liquid and solid phase7 has been published. It concluded that in liquid phase the gauche conformer (CH3 eclipsing C = O ) was the low-energy form, and the only form present in solid phase. MM calculations6 indicated a small energy difference between the gauche and syn conformers (0.10 kcalfmol) and a H-C-C4 torsion angle of about 150' for the gauche form. Experimental Section and Data Reduction The samples of 2-methylpropanol (99+%) and 2-methylpropionyl chloride (98%) were obtained from Aldrich Chemical
* Abstract published in Advance ACS Abstracts,
February 15, 1994.
0022-3654/94/2098-2848so4.50/0 , I
,
W
Figure 1. Diagram, with atom numbering, of the syn conformer of 2-methylpropanal (X = H) or 2-methylpropionyl chloride (X = Cl).
Co. The purity of the samples was checked by GC before use. Electron diffraction photographs were recorded with the Balzers electron-diffraction instrument at the University of Oslo899 on Kodak Electron Image plates. Voltage/distance calibration was made with benzene as reference. The nozzle-tip temperatures were 299 K for 2-methylpropanaland 298 K for 2-methylpropionyl chloride. Data were obtained at two different nozzle-to-plate distances, and 3-10 plates were recorded at each distance. Optical densities were measured using a Joyce Loeble double-beam microdensitometer, and the data were reduced in a way reported earlier.lb-" The ranges of data were 2.00 I sfA-1 I15.00 and 4.00 I sIA-1 I30.25 for 2-methylpropanal, 2.00 I sfA-1 I 15.00 and 4.00 Is1A-l I 29.75 for 2-methylpropionyl chloride; the data interval was As = 0.25 A-l. The average experimental intensity curves in the form SI&) for the two molecules are shown in Figures 2 and 3. Data for the individual curves and backgrounds, together with the average curves, are available as supplementarymaterial (see paragraph at end of paper). Figures 4 and 5 show the final experimental radial distribution (RD) curve calculated in the usual way12from the modified molecular intensity curve Z'(s) = ~l,,,(s)ZcZx(AcAx)-~ exp(-0.002s2), X = C or C1, where A = s2Fand F is the absolute value of the complex 0 1994 American Chemical Society
2-Methylpropanal and 2-Methylpropionyl Chloride
The Journal of Physical Chemistry, Vol. 98, No. 11, 1994 2849 I
I
Diff.
0
2
1
3
5
rIA
Figure 4. Radial distribution curves for 2-methylpropanal. The experimental curve was calculated from the composite of the two average intensity curves shown in Figure 2 with the use of theoretical data for the region 0 1. s1A-l 1. 1.75 and B/AZ = 0.0020. Difference curve is experimentalminus theoretical. The vertical lines indicate the interatomic distances and have lengths proportional to the distance weights.
-
Diff.
i 0
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~ 1
~
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~ 2
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' 3
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~ r/A
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Figure 5. Radial distributioncurves for 2-methylpropionylchloride. The experimentalcurve was calculatedfrom the composite of the two average intensity curves shown in Figure 3 with the use of theoretical data for the region 0 1.s/A-11 s/A-1? 1.75 and B/hZ = 0.0020. The difference curve is experimentalminus theoretical. The vertical lines indicate the interatomicdistancesand have lengthsproportionalto the distance weights. of 2-methylpropanal, some theoretical calculations were made. Molecular mechanics calculations had earlier been made: and as part of their spectroscopy investigationDurig et al.5 had made some ab initio calculations. We optimized the geometries of 2-methylpropanal at the ab initio HF/6-31G* level with the use of the program GAUSSIAN 9215 for the gauche and the syn conformers. These ab initio calculations were used to modify our electron diffraction model of the molecule in such a way that the calculated differences in the corresponding bond distances and valence angles between the two conformers could be introduced as constraints in our model. To get some information about the potential for torsion about the c 1 - C ~bond, ab initio calculations at the HF/6-31G* level were also made for several additional values of the H-C-C=O torsion angle 4. These results are shown in Table 1 and Figure 6. Finally, in an attempt to get even better values for the relative energies of the two conformers, ab initio calculations were made at the MP2/6-31GS level for the two stable conformers. The calculated gauche torsional angle was 4, = 119.95O, these calculations gave an energy difference between the syn and gauche conformers of AE = E, - EB= 0.95 kcal/mol (E, = -231.681 586 hartrees), compared to AE = 0.75 kcal/mol at the HF/6-31G* level. Values for rotational constants have been published for several
~
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Aarset et al.
2850 The Journal of Physical Chemistry, Vol. 98, No. 11, 1994
TABLE 1: Ab Initio Relative Energies (kcal/mol) for 2-Methylpropanal and 2-Methylpropionyl Chloride as a Function of the O=CI-C~--H Torsional Angle 4 b/deg
2-methylpropanaP
2-methylpropionyl chlorideb
0
0.75 1.23 1.47 0.69 0.0
1.10 2.27 3.41 2.71
30 60 90 119.1 120 136 150 180
0.54 0.0 0.23 1.01
0.89 1.73
a HF/6-31G*optimizedenergies;&,, =-230,985 893 hartrees. HF/ 6-3 1G* optimized energies; Emin= -689.901 952 hartrees.
x = CI
TABLE 2 Structural Parameters for the Gauche Conformer of 2-Methylpropanal and 2-Methylpropionyl Chloride 2-methylpropanal parameter
ED
r(C3-H) MC-H)I MC-H)2 r( C-Cl) r(C1-W MC-C)l NC-C)2 r(C=O) LC2-C1=0 LC2-CI-X LC1-Q-H LC l-C2-C3 LCI-CZ-C4 LC3
