Molecular structures and conformations of phenylglyoxal and 1-phenyl

Nov 10, 1992 - Department of Chemistry, Colgate University, 13 Oak Drive, Hamilton, ... Chemistry, AVH, University of Trondheim, N-7055 Trondheim, Nor...
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J. Phys. Chem. 1993, 97, 985-988

985

Molecular Structures and Conformations of Phenylglyoxal and 1-Phenyl-1,2-propanedione As Determined by Gas-Phase Electron Diffraction Quang Shent and Kolbj~rnHagen'*$ Department of Chemistry, Colgate University, 13 Oak Drive, Hamilton, New York 13346, and Department of Chemistry, A VH, University of Trondheim, N- 7055 Trondheim, Norway Received: August 11, 1992; In Final Form: November 10, 1992

The molecular structures of phenylglyoxal and 1-phenyl- 1,2-propanedione have been studied by gas-phase electron diffraction at nozzle temperatures of 74 and 73 'C, respectively. For both molecules the data are consistent with nonplanar models with the torsional angle 4(0=C-C=0) of about 130'. The phenyl rings are nearly coplanar with the carbonyl group (4ph = 11(9)' and 8( 1l)', respectively). The important distance (rg) and angle (L,) values for phenylglyoxal are r(C-H) = 1.120(10) A, r ( C 4 ) = 1.228(3) A, r(C-C),h = 1.400(2) A, r(Cl-C6) = 1.487(9) A, r(C1-c~) = 1.553(12) A, LCCC = 120.3(5)', LOCC = 121.1(22)', 4 = 128.6(36)O, and t#Jph = 8(11)'; theimportant values for l-phenyl-1,2-propanedionearer(C-H) = 1.120(8) A,r(C=O) = 1.228(3)A,r(C-C),h= 1.402(2)A,r(Cl-C6) = 1.486(12)A,r(C-C,)= 1.519A(assumed), r(C1-C2) = 1.565(10) A,LCCC = 119.3(1l)',LOCC = 119.6(13)',LCCCm = 121.4(17)', 4 = 129.9(24)', and $ph = 11(9)',

Introduction The most stable conformationadopted by small molecules with an u-dicarbonyl moiety is usually the planar anti form. For example, glyoxal' exists as a mixture of anti and syn forms in the gas phase with the anti form more stable than the syn form. Oxalyl chlorideZand bromide3 exist as a mixture of anti and gauche forms with the former being lower in energy. The presence of the nonplanar gauche form can be interpreted as a result of a large steric repulsive force overcoming the conjugation stabilization in the syn configuration. Only the anti form of 2,3butanedioned has been observed in the gas phase. In benzi1,s however, the only form observed was a nonplanar form with a O=C-C=O torsional angleof 117(3)" (4 = 180' for the planar anti form). The phenyl rings seem to destabilize the planar anti form. Both steric and electronic reasonings can be invoked to explain this observation. A way to further understand the influenceof the phenyl group on the conformation of a-phenyl diketones is to investigate the torsional angle of phenylglyoxal (PG) and l-phenyl-l,2-propanedione (PPD), where one of the phenyl rings in benzil is replaced by a proton and a methyl group, respectively. Leonard and Mader6 studied the n-s* absorption spectra of PPD and found that the absorption depended strongly on the torsional angle. Evans and Leermakers' estimated the torsional angle of PPD to be about 70-110°. Hoyever, no detailed structural information is available for these two molecules. We therefore initiated electrondiffraction investigations on these two molecules and are reporting our results in this article.

Experiment and Data Analysis Commercial samples of PG and PPD were obtained from Aldrich and were used without further purification. PG, which was purchased as a hydrate, was warmed up and pumped before diffraction experiments were carried out. Diffraction patterns were recorded with the Balzers apparatus8 in Oslo on Kcdak Electron Image plates at nozzle-tip temperatures of 347 K for PG and 346 K for PPD. The distance/voltage calibration was performed using benzene vapor as a standard. The nozzle-toplate distances are as follows: PG, 248.66 and 498.58 mm; and PPD, 248.82 and 498.58 mm. Six plates from the long and five +

Colgate University. University of Trondheim.

plates from the short camera were selected for analyses for each of these two molecules. Optical densities of the scattering were recorded on a single-beamphotodensitometer. The intensity data were converted to intergral q (q = (40/X) sin 8 = slO/r) units after the normal data reduction procedures. For each molecule the data were then averaged to form a long- and a short-camera experimental curve. Least squares procedures outlined by Gundersen and Hedberg9were followed using a unit weight matrix. Atomic scattering and phase factors used were the ones tabulated by Schafer et a1.IO Phenylglyoxal. The geometrical parameters chosen to define the structure of PG are as follows: r(C-H), r(Cl=O), ~ ( C I - C ~ ) ,~ ( C I - C ~ )= r(C-Cph), r(C6-C-r) = r(C-c)ph, LCI-C,=O = ~C2-Cl-0, LCZ-CI-C6, and two torsion angles 4 and 4 p h (see Figure 1 for atom numbering). 4 is the 0==CI-C2=0 torsional angle with 180' corresponding to &e planar anti form. @ph is the torsional angle cll-c6-cI-c2 which defines the deviation between the plane of the phenyl ring with respect to the C6c104 plane. The following assumptions are made: (a) all r(C-H) are identical, (b) all r(C-C) of the phenyl group are identical, (c) all LCCC valence angles of the phenyl group are identical, (d) r(C1=04) = r ( C 2 4 5 ) , and (e) LHCC- = 121'. Vibrational amplitudes (I) and perpendicular amplitudecorrections ( K ) werecalculatedusing a slightly modified force field from the bend5 investigation. Radial distribution (RD) curves (Figure 1) were calculated by Fourier transformation of the experimental intensity curves (Figure 2). The planar anti (4 = 180' and 4 p h = ) ' 0 form was tested first and poor agreement was obtained. When the value of was allowed to refine together with other geometrical parameters, it converged to a value of 45' (R = 13%) and with a poor fit in the RDcurve in the region outside 4 A. Further tests demonstrated that the anti form would not give acceptable agreement between the theoretical and the experimental data regardless theorientation of the phenyl ring. When theconstraints on 4 and 4 p h were removed, very good agreement (R = 7.0%) was obtained. The values for 4 and 4 p h converged to 129(4)' and 8( ll)', respectively. The small correlation coefficient between these two parameters (-0.21) and preliminary refinements suggested that 4 p h has little influence on the value of 4. There is no question that a nonplanar form gave better agreement with the experimental data than the planar anti form. Further tests starting with 4 = 45' were also carried out, and the value

0022-3654158/2Q91-0985fQ4.00/Q 0 1993 American Chemical Society

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986 The Journal of Physical Chemistry, Vol. 97,No. 5, I993

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Theo.

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I I I 0 1 2 3 5 6 7 8 r/A 9 Figure 1. Radial distribution curves for phenylglyoxal. The theoretical curve was calculated from the structural parameters in Table I. The difference curve is experimental minus theoretical.

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Figure 2. Intensity curves for phenylglyoxal. The theoretical curve was calculated from the structural parameters in Table I.

Figure3. Intensity curves for 1-phenyl-1,2-propanedione. The theoretical curve was calculated from the structural parameters in Table I.

converged to 4 = 50° with a R factor of 13.1%. It is clear that a nonplanar model with 4 = 129O and 4ph = 8 O is the best model to describe PG. The results of the final refinement are shown in Table I. 1-Pbenyl-lJ-prop.nedioae.. The geometrical parameters chosen to define the structure of PPD were identical to those for PG except for the following additions: r ( C A 3 ) and LCICZC~. The same assumptions as in PG were made, and the methyl group is assumed to have tetrahedral geometry. Vibrational amplitudes and perpendicular corrections were also calculated using a similar force field as in PG. Preliminaryrefinementsshowedthat parameters r(CI-C2) and r ( C 4 , ) (= r(C4m)) are highly correlated and cannot be refined simultaneously. Several different ways of handling this problem were tested, but we finally decided to keep r(C-C,) fixed at the value observed for this distance in 2,3-butanedione, 1.519 A. With this assumption the other distances could be determined without any problems. If the difference between r(CI-C2) and

r(C-C,) observed in 2,3-butanedione is used instead as a constraint in our refinement, the results for PPD were r(CI-C2) = 1.548(6) A,andr(C-C,) = 1.534(6)A. Theotherparameters have the same values in the two refinements. Models with 4 = 180°, 150°, and 50° were tested, and the model with 4 = 150° gave by far the best agreement with the experimental data (R= 13%. 896,and 11%, respectively). As expected, the major discrepancies showed up in regions with internuclear distances beyond 3.0 A. Final refinements were carried out using a model where 4 and +ph were allowed to refine. The refined values came close to 130° and 11O, respectively.The correlation coefficient between 4 and 4ph was -0.21. The final results are presented in Table I, and the corresponding intensity and RD curves are shown in Figures 3 and 4, respectively.

Discussion The electron diffraction experiments showed that in the vapor phase, both PG and PPD exist in a nonplanar form with

Phenylglyoxal and 1-Phenyl-1,2-propandione

The Journal of Physical Chemistry, Vol. 97, No. 5, 1993 987

TABLE I: Structural Results for Phenylglyoxal and 1-Phenyl-1,t-propanedione phenylglyoxal parameter"

1.120( 10) 1.228(3) 1.400(2) 1.487(9) 1.553(12)

1 -phenyl-1,2-propanedione rg,L,

181

1.120(8) 1.228(3) 1.402(2) 1.486(12) 1.565(10) 1.519(assumed) 119.6( 13) 119.3(1) 121.4(17) 129.9(24) 11(9)

0.0756 0.0389 0.045/0.0412(27) 0.0470 0.0488 0.0488

2.362 2.395 2.405 2.425 2.498 2.621 2.682 2.799 2.836 2.962 2.908 3.068 3.119 3.364 3.593 3.717 3.855 3.894 4.218 4.277 4.220 4.251 4.347 4.428 4.762 4.986 5.01 1 5.030 5.209 5.291 5.592 6.170 6.422

0.0574 0.0583 0.0577 0.0541/0.073(5) 0.0611 0.0650 0.0654 0.0590/0.060(9) 0.1301 0.0987 0.1580 0.1132 0.1132 0.0829 0.0631 0.0630 0.0861 0.0861 0.2020 0.1152 0.2020 0.1152 0.1031 0.1075 0.0678 0.0976 0.0789 0.0976 0.0904 0.1773 0.1821 0.1386 0.1622

1,:

rg' La

0.0756 0.0390 0.0452 0.0470 0.0489

121. I (22) 120.3(5) 128.6(36) 8(11) 2.342

Selected Dependent Distances 0.0577

2.410 2.420 2.500 2-635

0.0577 0.0536/0.077(6) 0.061 1 0.0650

2.794 2.794 2.978 2.982 3.108

0.0570/0.071 (10) 0.1259 0.0988 0.1551 0.1132

3.375 3.585 3.774

0.0839 0.0627 0.0635

3.905 4.178 4.271 4.286

0.0695 0.1445 0.0643 0.1441

4.363 4.468 4.745

0.1031 0.1092 0.0677

4.983 5.041 5.227 5.354

0.0789 0.0976 0.0906 0.1773

0 Distances and amplitudes in angstroms, angles in degrees. Quantities in parentheses are estimated error limits. Amplitudes of vibration calculated values/refined values with error limits. Torsional angles o=c-c=o with 180' corresponding to anti form; #pi,, torsional angle c7+6-+=04.

TABLE II: Parameter Values Obtained for PC, PPD, and Related Molecules paramete+ r(C-H) r(C=O)

McCOCOMe

PhCOCOPh

PhCOCOMc

PhCOCOH

HCOCOH

1.108(4) 1.216(2)

1.099(1 1) 1.221 (4) 1.400(2) 1.489(8)

1.120(8) 1.228(3) 1.402(2) 1.486(12) 1.519d I.565( 10) 119.3(11) 119.6(13) 121.4(7) 129.9(24) 1 1 (9) this work ED

1.120(10) 1.228(3) 1.400(2) 1.487(9)

1.132(8) 1.212(2)

1.553( 12) 120.3(5) 121. I (22)

1.526(3)

r(c6-c7) r(cI-c6)

r(C--C,) ~(CI-CZ) LCCC LOCC LCCC,

v

1.519(7) 1.533(14) 119.5(6) 116.6(2) 180

1.548(16) 118.7(9) 119.9(14)

128.6(36) 180. 8U1) this work 1 ref 4 ED ED method ED Distances are in angstroms, angles in degrees. Subscripted parameters: ph = phenyl, and m = methyl. I$,torsional angle o--C-C--O, with 180' corresponding to the anti form; +ph, torsional angle between the phenyl ring and the C6C104 plane. Assumed value, see text. The only apparent difference is the value of the c1-C~ bond O--C-C=O torsional angles, 4, close to 130°. There is no 'hh

116.9(34) 10(1) 5 ED

121.2(2)

evidence for the presence of any amount of a second conformer. The planarity of the phenyl ring relative to the carbonyl moiety is not well determined since the electron diffraction technique is not sensitive to small deviations from planarity. Only a very small structural difference was observed between PG and PPD.

length (1.553(12) A and 1.565(10) (A), but these values have rather large error limits. In addition the r(C1-Cz) value in PPD also depended on the value assumed for r(C-C,,,). Table I1 summarizes the important geometrical parameter values for 2,3-butanedione (biacetyl), benzil, glyoxal, PG,and

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Figure 4. Radial distribution curves for 1-phenyl-1,2-propanedione. The theoretical curve was calculated from the structural parameters in Table I.

PPD. Comparing the three a-phenyl diketones with glyoxal (G) and 2,3-butanedione (BDO), the following observations can be made: (1) the central CI-C~ bonds in the phenyl diketones are longer than those in G and BDO, (2) the C = O bonds in the phenyl diketones are longer than those in G and BDO, and (3) the phenyl diketones are all nonplanar while the most stable conformers in G and BDO are planar. All these observations are consistent with the establishment of electron delocalization between the benzene ring and the C=O moiety at the expense of the delocalization within the O=C-C=O moiety. Thesmall f#)ph values in the phenyl diketones also support this conclusion. Comparing the three a-phenyl diketones, the following observations can be made: (1) the more phenyl groups there are in the molecule the greater is the nonplanarity, (2) the length of the C=Obond is not sensitive to thedegreeof phenyl substitution, (3) the central CI-C2 bond is not sensitive to the degree of phenyl substitution (the error limits are large in all three determinations, however), and (4) the f#)ph values are almost identical in all three compounds. It appears that the electronic interaction between a phenyl group and the carbonyl moiety is the main cause for the nonplanarity of the phenyl diketones because once the conjugation of the O=C-C-0 moiety is disrupted by one phenyl group, further substitution (phenyl or methyl) does not change the torsional angle significantly. Steric repulsions between the substituents also play a relatively minor role as suggested by observation 1. It is unfortunate that the central CI-C~ bonds of the phenyl diketones cannot be determined more accurately since they might shed more light on the degree of interaction between the phenyl group and the C = O bond.

Acknowledgment. This work is made possible by a travel grant from NATO (No. 890475). Financial support from the Norwegian Research Council for Science and the Humanities is gratefully acknowledged. We are also very grateful to Snefrid Gundersen and Hans Vidar Volden, The University of Oslo, for their help in obtaining the electron diffraction data. Supplementary MaterialAvailable: Tables listing average total intensities and correlation matrices (3 pages). Ordering information is given on any current masthead page. Registry No. The following registry numbers were supplied by the author. Phenylglyoxal, 1074-12-0; l-phenyl-1,2-propanedione, 579-07-7. References and Notes ( 1 ) Kuchitsu, K.; Fukuyama, T.; Morino, Y.J . Mol. Srrucr. 1968, I , 463. (2) Hagen, K.; Hedberg, K. J . Am. Chem. Soc. 1973, 95, 1003. ( 3 ) Hagen, K.; Hedberg, K.J. Am. Chem. Soc. 1973, 95, 4796. (4) Danielson, D. D.; Hedberg, K. J . Am. Chem. Soc. 1979, 101,3730. (5) Shen, Q.; Hagen, K. J . Phys. Chem. 1987, 91, 1357. ( 6 ) Leonard, N. J.; Mader, P. M. J . Am. Chem. Soc. 1950, 72, 5388. ( 7 ) Evans, T. R.; Leermakers, P. A. J . Am. Chem. Soc. 1%7.89,4380. ( 8 ) Zeil, W.; Haase, J.; Wegmann, L. Z. Instrumenrenkd. 1966, 74. 84. Bastiansen, 0.;Graber, R.; Wegmann, L. Balzers High Vac. Rep. 1%9, 25, 1. (9) Gundersen, G.;Hedberg. K. J . Chem. Phys. 1969, 51, 2500. (IO) Schafer, L.; Yates, A. C.; Bonham,R.A. J . Chem. Phys. 1971,55, 3055.