J . Phys. Chem. 1987, 91, 1015-1019 choice for detecting heavy molecular weight species or for detecting complex reaction intermediates or fragments. The extension of LID techniques to high-pressure environments and liquidsolid interfaces is another important direction for future efforts. With appropriate detection techniques, LID need not be limited to low background pressures. For example, Pospieszczyk, Tagle, and co-workers have measured adsorption isotherms in the presence of modest background pressures,’ and similar experiments have been performed by Seebauer and Schmidt.Iz A number of optical detection techniques hold promise for applications at high pressure and in liquids and, at the same time, offer advantages for studies in ultra-high-vacuum. Examples of these techniques include laser-excited fluorescence,3e36 multiphoton i o n i ~ a t i o n , ~ ’and - ~ ~optical second-harmonic generati~n.’~-’~ (54) Shen, Y. R. In Structure and Chemistry at Interfaces: New Laser and Optical Investigations; Hall and Ellis, Eds.; VCH: Deerfield Beach, FL, 1986.
1015
Unquestionably, LID will find broad application in the investigation of these issues and in the study of the kinetics of interactions at surfaces.
Acknowledgment. The efforts of Dr. S . J. Bares and Mr. A. M. DeSantolo were central to the success of the experiments reported here. I a m further indebted to Drs. J. L. Gland, T. H . Upton, J. Hemminger, J. M. White, and W. Ho for many helpful discussions. All of these contributions are acknowledged with thanks . Registry No. Ni, 7440-02-0; CH,OH, 67-56-1; ethylene, 74-85-1. (55) Grubb, S.; Hall, R. B. J . Phys. Chem., submitted for publication. (56) (a) Chem, C. K.; Heinz, T. F.; Ricard, D.; Shen, Y. R. Phys. Reu. Lett. 1981,46, 1010. (b) Tom, H. W. K.; Mate, C. M.; Thu, X. D.; Crowell, J. E.; Heinz, T. F.; Somorjai, G. A.; Shen, Y. R. Phys. Reu. Lett. 1984, 52, 348. (c) Tom, H. W. K.; Mate, C. M.; Thu, X. D.; Crowell, J. E.; Heinz, T. F.; Somorjai, G. A.; Shen, Y. R. Surf. Sci., to be published.
ARTICLES Molecular Structure and Conformation of Diisopropyl Ether: A Gas Electron Diffraction Investigation Hiroshi Takeuchi, Mikio Fujii, Shigehiro Konaka,* and Masao Kimura Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo 060, Japan (Received: February 4 , 1986; In Final Form: September 16. 1986)
The molecular structure of diisopropyl ether has been investigated by gas-phase electron diffraction at 19 OC. The most stable conformer has C2symmetry. The bond lengths (r ) and angles (ra structure) with the estimated limits of error are as follows: r(C-0) = 1.431 (3) A, r(C-C) = 1.526 (3) r(C-H) = 1.119 (3) A, LCOC = 118.5 (16)O, LOCC, = 110.3 (9)O, LOCC, = 106.5 (6)O, LCCC = 113.5 (11)O, LCCH,, = 111.1 (16)O, and q+(COCH) = 39 (4)O, where the dihedral angle &(COCH) is defined to be zero when the C-O bond eclipses the C-H bond. With the help of the molecular mechanics calculation, the relative abundance and the dihedral angles 41(COCH)and &(COCH) of the next stable conformer have been determined to be 20 i= 20% and 0 & 30’ and 180 & 20°, respectively. The C-O and C-C bond lengths and the COC angle are 0.015 and 0.006 A and 7’ larger than the corresponding values of dimethyl ether and ethyl methyl ether, respectively.
A,
Introduction The conformation of diisopropyl ether was investigated by means of vibrational spectroscopy.’sZ By comparing the frequencies observed in the liquid and solid phases with the calculated ones, Snyder and Zerbil identified only one conformer with C, symmetry in both phases. Clague and Danti2 measured lowfrequency vibrational bands in the liquid and vapor phases and reported the presence of a single conformer in which two isopropyl groups rotate in opposite directions to minimize steric hindrance. These investigations showed that the molecule has C, or near-C, symmetry. In molecules with bulky substituents, nonbonded interactions influence bond lengths, bond angles, and dihedral angle^.^ In the present study, the molecular structure of diisopropyl ether has (1) Snyder, R. G.; Zerbi, G . Spectrochim. Acta, Part A 1967, 23A, 391. (2) Clague, A. D. H.; Danti, A. Spectrochim. Acta, Part A 1968,24A, 439. ( 3 ) (a) Tsuboyama, A.; Konaka, S.;Kimura, M. J. Mol. Struct. 1985,127, 77. (b) Almenningen, A.; Fjeldberg, T.; Hengge, E. J . Mol. Struct. 1984,112, 239.
0022-3654/87/2091- 1015$01.50/0
TABLE I: Exwrimental Conditions
camera distance, mm wavelength, A uncertainty in the scale factor, 7% sample pressure, Torr background pressure during exposure, Torr beam current, pA exposure time, s number of plates used range of s values, A-I
109.3 0.06283 0.05 30-32 1X 0.1 5-0.16 45-60 4 10.0-32.2
244.3 0.06298 0.06 30-32 7 x 10-5 0.07-0.09 21-29 6
2.9-17.4
been determined by gas electron diffraction (GED) with primary interest in the effect of steric hindrance between the two isopropyl groups on the molecular geometry. No assignment has been made for the bands lower than 300 cm-I. The result of a normal-coordinate analysis for the low vibrational frequencies depends on the dihedral angles. In the present study, therefore, we have used the observed molecular structure in order to make the assignment of the bands observed by Clague and Danti2 and to determine the torsional force constants by means of a normal-coordinate analysis. 0 1987 American Chemical Society
1016
The Journal of Physical Chemistry, Vol. 91, No. 5, 1987
Takeuchi et al.
-08 00
Figure 1. Diisopropyl ether with C, symmetry.
ol_
AsM(s)
20
5/p
- ~ - -- - ~ -
~
-01
TABLE II: Calculated Mean Amplitudes (lij)for Diisopropyl Ether
~
__
~
jZ8 -01
(in A)a
10
30
Figure 2. Experimental molecular scattering intensities (0) and the theoretical ones (-) for the most stable conformer of diisopropyl ether; AsM(s) = sM(s)Obsd- sM(s)qicd.
0-c c-c
C-H 0-C,
o..c, 0-c, 0.-c, C2**C5 c,..c, C2"C7 c,..cp c,..cs C,..C, c,..c, c,..c, C4"C6 c,..c,
C6"C7
0.048 0.052 0.079 0.066 0.067 0.067 0.066 0.063 0.087 0.161 0.070 0.161 0.215 0.250 0.087 0.092 0.215 0.070
1.429 1.525 1.114 2.421 2.358 2.358 2.421 2.450 3.653 3.141 2.547 3.141 4.138 4.147 3.653 4.677 4.138 2.547
0.048 0.052 0.079 0.066 0.066 0.067 0.067 0.063 0.161 0.161 0.070 0.133 0.196 0.269 0.133 0.269 0.196 0.070
1.429 1.525 1.114 2.403 2.403 2.449 2.449 2.470 3.059 3.059 2.503 3.493 4.400 3.638 3.500 3,644 4.400 2.536
"Calculated at 19 " C . Only the mean amplitudes for relatively important atom pairs are listed. bSee Figure 1 for atom numbering. 'The r, distances corresponding to the final molecular geometry given in Table 111. Experimental Section
A sample with a purity of at least 99% was purchased from Tokyo Chemical Industry Co., Ltd. and used for experiment after removing impurities by vacuum distillation. The diffraction patterns of the sample and carbon disulfide were recorded on Kodak electron image plates by using an electron diffraction apparatus equipped with an r3 sector4 at room temperature, 19 O C . The diffraction patterns of carbon disulfide were used for the determination of the wavelength of incident electron^.^ Experimental conditions are shown in Table I. Optical densities were measured at intervals of i / 3 mm on each photographic plate by microphotometry and were converted to intensities. After being corrected for the imperfection of the sector shape, the intensities from each camera distance were levelled by the theoretical background and averaged. Elastic and inelastic scattering factors were taken from ref 6 and 7, respectively.
.
n
1 1
2
r/h
'
I 5
6
Figure 3. Experimental radial distribution curve (0)and the theoretical for the most stable conformer of diisopropyl ether; Af(r) = f(r)Obsd- f(r)calcd,
one (-)
and d2. Here, Hi-Prdenotes the hydrogen atom attached to the C2 or C, carbon atom, HMedenotes the hydrogen atom in methyl groups, and 41 and 42 indicate the dihedral angles of C 5 0 C 2 Hand C20C5H, respectively. These dihedral angles are defined to be zero when C-H bonds eclipse the opposite C-0 bonds and are measured clockwise about the C-0 axes so that the molecule has C2 sym= 42. The structural parameter values were metry when determined by the least-squares method applied simultaneously to the long and short camera-distance data. In every least-squares calculation mean amplitudes l(C-0) and 1(C-H) were treated as adjustable parameters, but the other mean amplitudes and shrinkage corrections were fixed at values calculated from the force constants and structural parameter values obtained in the preceding step. Table I1 shows the mean amplitudes for relatively important atom pairs calculated by using the final structural parameter values. The details of the force constants employed will be described in the next section. Asymmetry parameters K for bonding atom pairs were calculated in a diatomic approximation by using the relation8 LOCC,, LOCC,, LCCC, LCCHM,,,LCCH,.,,,
,, = a
-14
Analysis of Data
The numbering of atoms is shown in Figure 1. The following assumptions were made to reduce the number of independent parameters: (1) two isopropyl groups have the same local geometry with C, symmetry; ( 2 ) four methyl groups have the same local geometry with C3, symmetry; (3) each methyl group takes the staggered position against the C-0 bond and has no tilt; (4) all the C-H bond lengths are equal; ( 5 ) two C-0 bond lengths are equal. Then, the following independent structural parameters were selected: r(C-O), r(C-C), r(C-H), LCOC, LOCC,, LOCC,, (4) Murata, Y.; Kuchitsu, K.; Kimura, M. Jpn. J . Appl. Phys. 1970, 9, 591. ( 5 ) Tsuboyama, A.; Murayama, A.; Konaka, S.; Kimura, M. J . Mol. Srrucr. 1984, 118, 351. ( 6 ) Kimura, M.; Konaka, S.;Ogasawara, M. J . Chem. Phys. 1967, 46, 2599. (7) Tavard, C.; Nicolas, D.; Rouault, M. J . Chim. Phys. 1967, 64, 540.
6
where a is the Morse parameter and I is the mean amplitude. By ~ , K~~ were calculated to assuming a to be 2 A-i, K ~ - K~ ~, - and 23 X and 17 X A3, respectively. be 127 X Asymmetry parameters for nonbonded atom pairs were assumed to be zero. In the initial stage, conformational analysis was carried out by assuming the existence of a single conformer. For conformers with C2 symmetry, LOCC, (=LOCC,) and LOCC, (=LOCC,) were treated as independent parameters. For conformers with C , symmetry, however, all the OCC angles were assumed to be equal since they could not be determined separately. A conformer with 4, = 42 = 3 7 O reproduced the observed molecular scattering intensities best. The molecular scattering intensities and radial distribution (RD) curves for the C2conformer are shown in Figure (8) Kuchitsu, K. Bull. Chem. SOC.Jpn. 1967, 40, 498.
The Journal of Physical Chemistry, Vol. 91, No. 5, 1987
Structure and Conformation of Diisopropyl Ether
1017
Figure 5. Plots of Hamilton's R-factor ratio for the mixture of C2 (6, = $2) and C, (dl = Oo, @2 = 180°) conformers.
3
rlA
5
6
Figure 4. Experimental radial distribution curves (0)and the theoretical ones (-) for different conformersor mixture of conformers of diisopropyl ether: (A) 100%C2 = 42= 37O); (B) 100%C I = Oo, 42= 180'); (C) 80%c2 = 62 = 390) + 20% c, (bl = oo, $9 = 1800). TABLE III: Observed and Calculated Structural Parameter Values for the Most Stable Conformer (Bond Lengths and Mean Amplitudes in anestroms. and Andes in decrees)"
observed model I b model IIc r,(C-W rg(C-0) r,(C-C) LCOC LOCC, LOCC, LCCC LCCH,, LCCHi.p, q4I
(=q42)
l(C-H) 4C-O) l(C-C) kf kk
1.119 (3) 1.431 (3) 1.526 (3) 117.9 (19) 111.5 (7) 106.9 (5) 113.4 (9) 111.8 (14) 107.5' 37 (4)' 0.076 (8) 0.051 (4) 0.052' 0.97 (2) 0.94 (5)
1.119 (3) 1.431 (3) 1.526 (3) 118.5 (16) 110.3 (9) 106.5 (6) 113.5 (11) 1 1 1 . 1 (16) 107.5c 39 (4)f 0.076 (8) 0.050 (4) 0.052' 0.97 (2) 0.95 (5)
calcdd 1.115 1.426 1.539 114.2 110.9 108.2 109.9 110.9 108.9 40h 0.079 0.048 0.052
"Values in parentheses are the limits of error attached to the last digit of the parameter values. Angles correspond to the rm structure. bThe result obtained by assuming that only the conformer with C2 symmetry exists in the vapor phase. cThe result obtained for the conformational composition of 80% C2J41= @2) + 20% C, ($1 = Oo, 62 = 180O). The parameters listed in this column should be regarded as the final results of the present study. "Bond lengths, bond angles, and dihedral angles were calculated by the molecular mechanics method. Mean amplitudes were taken from Table 11. CFixedvalue (see text). fThe observed values of the dihedral angles, C50C2C3and C50C2C4, are -82' and 153O, respectively. #The observed values of the dihedral angles, C50C2C3and C50C2C4,are -8OO and 156O, respectively. hThe calculated values of the dihedral angles, C50C2C3and C50C2C,, are -82' and 158O, respectively. 'Fixed at the calculated value. 'kl and k , are the indices of resolution for the long and short camera distances, respectively.
2 and Figures 3 and 4A, respectively. Because the CCHi.p, angle could not be determined with sufficient accuracy, it was fixed at a value for which the R factor9 becomes minimum. Determined structural parameters are listed in the 2nd column in Table 111. The limits of error were estimated from 2.58 times the standard errors and the systematic errors. The systematic errors were estimated from the following sources: (1) the errors in the scale factor, (2) the uncertainties in the asymmetry parameters, K, (3) the uncertainty in the correction curve for the sector shape, and
(4) the arbitrariness of background curves. In order to find other possible conformers, a molecular mechanics calculation was carried out employing the MM2 force fieldslo The calculation predicted the existence of three stable conformers. The most stable conformer had C2 symmetry with d1= d2 = 40' and it was 2.6 kcal mol-' more stable than the next stable conformer with d1= 12' and $2 = 177'. The third conformer had the dihedral angles of $] = 42= 165' and it was 7.4 kcal mol-] less stable than the most stable conformer. The value of 2.6 kcal mol-' corresponds to 2% population of the next stable conformer. The calculated geometry of the most stable conformer is listed in Table 111, column 4. The calculated bond angles of the next stable conformer are as follows: LCOC = 115.7', LOCC, = 109.9', LOCC, = 108.8', LOCC, = 1 12.1', LOCC, = 1 13.0', LC3C2C4 = 110.1', LC6CSC7 = 113.0', LCCH,, = 110.9', and LCC~H., = 106.6'. Bond lengths were L C C ~ H=~ 108.5', .~ insensitive to dihedral angles. Referring to the results of the molecular mechanics calculation, a conformer with dl of about 10' and d2 of about 180' was included in data analysis as the second conformer. It was assumed that the bond lengths and mean amplitudes for bonding atom pairs are independent of the conformation and that the COC angle of the second conformer is 1.5' larger than that of the most stable conformer. The other bond angles were fixed at the values calculated by molecular mechanics. Hamilton's R-factor ratios" were computed for various concentrations and dihedral angles of the second conformer. Figure 5 shows the R-factor ratios plotted of 0' and against the molar fraction of a C1conformer with $2 of 180'. Similar curves were obtained for the other dihedral angles. In the 99% confidence interval, the values of and $2, and the relative abundance of the second conformer were determined to be 0 f 30°, 180 f 20', and 20 f 20%, respectively. The calculated RD curves for a CI($] = Oo, $2 = 180') conformer and the best result are displayed in parts B and C of Figure 4, respectively. The observed structural parameter values of the most stable conformer are listed in the third column in Table 111 together with the estimated limits of error. All the structural parameter values agree with those listed in the second column in Table I11 within the limits of error. The parameter values listed in the third column were taken as the final result. Strictly speaking, the torsional motion about the C-O bond must be treated as large-amplitude motion.I2 In the usual treatment of the large-amplitude motion, bond lengths and bond angles are assumed to be independent of torsional m0ti0n.l~ However, this molecule has at least two kinds of OCC bond angles and these bond angles cannot be regarded as independent of the torsion. Therefore, it is necessary to treat pseudoconformers with CI symmetry in which four OCC bond angles have different values. This makes the treatment of the large-amplitude motion difficult. (IO) (a) Allinger, N. L. J . Am. Chem. SOC.1977, 99, 8117. (b) Allinger, N. L.; Yuh, Y . H. QCPE 1980, 12, 395. (c) Jaime, C.; Osawa, E. Tetrahedron 1983, 39, 2169. (1 1 ) Hamilton, W. C. Statistics in Physical Science; Ronald: New York,
1964. (9) R = { ~ , ~ ~ ( A s M ( s ) ~ ! ~ / ~ ~ , W ~ (where S M ( AsM(s)~ S ) ~ ~ ~ =) ~sMJ~/~, (s)iObd - S M ( S ) ~ =and ' ~ Wi is a diagonal element of the weight matrix.
(12) ter Brake, J. H. M.; Mijlhoff, F. C. J . Mol. Strucf. 1978, 77, 109. (13) ter Brake, J. H. M. J . Mol. Struct. 1984, 118, 63, 73.
Takeuchi et al.
1018 The Journal of Physical Chemistry, Vol. 91, No. 5, 1987 Alternatively, in order to examine the effect of the small-amplitude approximation on the structural parameter values, the data were analyzed by using mean amplitudes and shrinkage corrections calculated by assuming the torsional force constant about the C-O axis to be 0.035 mdyn 8, rad-2 (the force constant employed for the final analysis was 0.070 mdyn 8, rad-2, see the next section). The resulting structural parameter values coincided with the results listed in Table I11 within experimental errors except for the OCC, and COC bond angles. The values of the OCC4 and COC bond angles were 0.8' and 2.0' larger than those listed in Table 111, respectively. These changes in the two bond angles are considered to suggest the uncertainties introduced by the small-amplitude approximation. As far as the other parameters are concerned, the large-amplitude motion little influences the results.
TABLE I V Observed and Calculated Frequencies Lower Than 300 cm-I (in em-')
Assignment of Low-Frequency Modes and the Force Constants The force constants used for the calculations of mean amplitudes and shrinkage corrections were initially taken from ref 1 except for the torsional ones.I4 Some of the force constants in ref 1 were modified so as to decrease differences between the frequencies calculated for the most stable conformer and observed frequencies. The modified force constants are as follows: K, = 5.340, KR = 4.511, K, = 4.700, F, = 0.003, FB = -0.042, Hp = 0.570, Hn = 0.961, and HJ. = 0.668. The notations and units are the same as those in ref 1. The average frequency error is nearly equal to that in ref 1. Synder and Zerbi' did not observe the spectrum in the frequency range lower than 300 cm-'. Clague and Danti2 observed lowfrequency bands for various ethers, but did not mention the assignment for the present molecule. Durig and c o - ~ o r k e r sob'~ served bands at 258 and 236 cm-' for isopropylamine and assigned them to the methyl torsional modes. By referring to these results, we assigned a band at 255 cm-' observed in the liquid phase2 to the torsional mode of methyl groups of the most stable conformer and the force constant for the quadratic term of the torsional angle was determined to be 0.1 15 mdyn 8, rad-2. The COC deformation frequency calculated for the most stable conformer was 192 cm-', a value which was almost the same as the frequency of 194 cm-l observed in the vapor phase.* The calculated value of 192 cm-' is different from a value of 160 cm-l calculated by Snyder and Zerbi.' This is owing mainly to the difference in dihedral angles, + I and +2, used in the two calcuare 60°, lations: Snyder and Zerbi' assumed that and whereas we used the value of 39' for and 42which was determined finally by GED. A band around 90 cm-I observed in the liquid phase2 was assigned to the torsional mode about the C-O axis after Clague and Danti? who assigned a band at 98 cm-' observed for isopropyl methyl ether in the vapor phase to the torsional mode about the O-C(isopropy1) axis. However, the value of 90 cm-'is not definite enough to specify the peak frequency of the torsional mode in the vapor phase, since the center of a very broad, weak low-frequency band is difficult to be identified accurately and moreover, torsional frequencies in the liquid phase are known to be appreciably higher than those in the vapor phase.I6 Therefore, the force constant for the torsional motion about the C-O axis was estimated in the following manner. Using the mean amplitudes and shrinkage corrections calculated from various values of the torsional force constant, we repeated the least-squares analysis on the molecular scattering intensities, sM(s), and searched for a value of the force ~ a result, the torsional force constant minimizing the R f a ~ t o r .As constant was determined to be 0.070 f 0.010 mdyn A rad-2. From this value the torsional frequencies were calculated to be 77 f 6 and 50 f 4 cm-I. The value of 0.070 mdyn 8,rad-, is reasonable compared with the values of 0.0682 and 0.0769 mdyn A rad-2 reported by Kitagawa et a1.I6 for ethyl methyl ether. Observed frequencies of 140 and 118 cm-I are considered to be a combi-
"Reference 2. bThe present study. cSpectra of liquid phase. "Spectra of vapor phase. 'Very weak band.
+,
(14) Hilderbrandt, R. L. J . Mol. Spectrosc. 1972, 44, 599. (15) Durig, J. R.; Guirgis, G. A.; Compton, D. A. C. J . Phys. Chem. 1979, 83, 1313.
(16) Kitagawa, T.; Ohno, K.; Sugeta, H.; Miyazawa, T. Bull. Chem. SOC.
Jpn. 1972, 45, 969.
IR
observedo Raman 25SC
-
194d 14Od3'
200'
calcd* 262 258 256 256 192
77 50
descripn C-C torsion C-C torsion C-C torsion C-C torsion COC deformation combination tone (77 + 50 cm-') overtone (50 C-0 torsion C-0 torsion
X 2
cm-')
Figure 6. Newman projections: left, isopropyl methyl ether; right, di-
isopropyl ether. nation tone and an overtone, respectively. The assignment of the bands and calculated frequencies below 300 cm-' are summarized in Table IV. The normal-coordinate analysis was carried out for the second conformer with 41of about 0' and r # ~ ~of about 180'. The force constants dependent on +2, Le., fp:, fdg,fsXt, andfsXg, were taken from ref 1, while the force constants dependent on were assumed to be either the same as those dependent on 42or zero since they were not given in ref 1. Other force constants were taken to be the same as those used for the most stable conformer. The two assumptions on the force constants brought about no significant difference in the calculated frequencies. The calculated frequencies for the two conformers were significantly different from each other in the region of 300-600 cm-'. The frequencies calculated for the second conformer in this region were as follows: 580 (532), 548 (503), 439 (448), 398 (408), 362 (400), and 338 (305) cm-', where the values shown in parentheses are the calculated frequencies for the most stable conformer. No bands were observed near 580, 362, and 338 cm-l in the liquid and vapor phases.2 This shows that the molar fraction of the second conformer is not large enough to be detected by vibrational spectroscopy. However, this does not contradict the present result because of the large uncertainty accompanied with the molar fraction of the second conformer determined by GED.
Results and Discussion As is the case in diisopropylamine" and diisopropyl sulfide,'* the most stable conformer has C,symmetry. This result is consistent with those of the spectroscopic investigations'a2 and the molecular mechanics calculation. The difference between LOCC, and LOCC, indicates that the C-0 bonds are not in the symmetry planes of isopropyl groups as is the case in isopropyl methyl etherIg and gauche-isopropyl alcohol.20 The observed difference between two OCC angles in diisopropyl ether (3.8 (1 1)') is less than the corresponding differences in isopropyl methyl ether (6.0')Ig and gauche-isopropyl alcohol (4.4O).,O (17) Takeuchi, H.; Konaka, S.;Kimura, M. J. Mol. Sfruct. 1986,146,361. (18) Kondo, H.; Takeuchi, H.; Konaka, S.; Kimura, M. Nippon Kugaku Kaishi, 1986, 1458. (19) Nakagawa, J.; Imachi, M.; Hayashi, M. J . Mol. Sfruct. 1984, 112, 201. (20) Abdurakhmanov, A. A,; Elchiev, M. N.; Imanov, L. M. Z h . Strukt. Khim. 1974, 15, 42.
Structure and Conformation of Diisopropyl Ether TABLE V: Comparison of the Structures of Related Molecules (in angstroms and degrees) CH,OED (CH3)200 C2HSb (n-C3H7)20C(i-C3H7)20d rg(C-0) 1.415 (1) 1.418 (2) 1.405 (6) 1.431 (3) r&C-C) 1.520 (4) 1.526 (8) 1.526 (3) r,(C-H) 1.118 (2) 1.118 (4) 1.120 (6) 1.119 (3) L,(COC) 11 1.8 (2) 111.9 (5) 116.1 (36) 118.5 (16) L,HCH 109.2 (2) 109.0 (4) 103.V 107.8 (16)' AS 0.012 0.006
rs(C-C)' A'
1.521 (7) 0.005
1.517 (5) 0.009
1.516 (15) 0.010
Reference 22. Reference 23. Reference 21. Calculated from the ra structure. dThis work. e Calculated from LCCH(methy1). /Calculated from LCCH,,. #Difference between r (C-C) adjacent to an oxygen atom and that of propane,25 1.532 (3) hReference 24. Trans and trans-trans conformers are abbreviated as t- and tt-, respectively. 'The C-C bond length adjacent to an oxygen atom. 'Difference between r,(C-C) and that of propane,25 1.526 (2) A.
1.
In isopropyl methyl ether the CH3 group interacts with the gauche CH3 group in the isopropyl group more strongly than with the trans CH3 group. This difference in interaction causes the difference between the OCC angles for the isopropyl group. Similar interactions cause the difference between LOCC, and LOCC, in diisopropyl ether. The dihedral angle about the C-0 ' smaller than axis of the isopropyl group in diisopropyl ether is 8 that in isopropyl methyl etherIg which has one isopropyl group (see Figure 6). These facts show that the CH3-CH3 interactions between the isopropyl groups play an important role in mutual arrangement of the two isopropyl groups. Main structural parameters are compared with those of related ethers in Table V. Molecules with bulkier substituents have larger r(C-0) except for dipropyl ether.21 The r(C-0) and LCOC of diisopropyl ether are about 0.015 8, and 7' larger than the corresponding values of dimethyl ether22and ethyl methyl ether.23 The r(C-C) of diisopropyl ether is 0.006 A longer than that of ethyl methyl ether. These results reflect steric interactions between isopropyl groups. It is reasonable that the r(C-0) of ethyl methyl ether is slightly longer than that of dimethyl ether, but it seems unnatural that the r(C-0) of dipropyl ether is shorter than that (21) Astrup, E. E. Acta Chem. Scond. 1977, A31, 125. (22) Tamagawa, K.; Takemura, M.; Konaka, S.; Kimura, M. J. Mol. Strucr. 1984, 125, 131. (23) Oyanagi, K.; Kuchitsu, K. Bull. Chem. Sot. Jpn. 1978, 51, 2237.
The Journal of Physical Chemistry, Vol. 91, No. 5, 1987
1019
of ethyl methyl ether. Moreover, the H C H bond angle of dipropyl ether is considered to be too small. It seems worthwhile to reinvestigate the structure of dipropyl ether. As seen in Table 111, the observed bond lengths and angles are in satisfactory agreement with the calculated ones except for LCOC, LOCC,, and LCCC. The calculated difference between two OCC angles (2.7') is 1' smaller than the observed one. The calculated COC and CCC angles are 4.3' and 3.6' smaller than the observed ones, respectively. These comparisons show steric hindrance is not fully taken into account in the MM2 force field. The assumptions made in the data analysis regarding the tilt and torsion of methyl groups and symmetry of the isopropyl groups are consistent with the results of the molecular mechanics calculation. Hayashi and A d a ~ hdetermined i~~ the r, structures of trans-ethyl methyl ether, trans-trans-propyl methyl ether, and trans-transdiethyl ether. Two common features can be seen in these structures. First, the C-C bond length adjacent to an oxygen atom ~~ is about 0.01 A shorter than that of a normal h y d r ~ c a r b o n(see Table V). Second, alkyl groups R and R' in ROR' tilt toward the lone pair electrons on the oxygen atom. Isopropyl groups in diisopropyl ether and isopropyl methyl ether19 and methyl groups in dimethyl ether22also tilt toward the lone pair electrons. The r,(C-C) of ethyl methyl ether23is also shorter than that of a normal hydrocarbon although the tilt of ethyl and methyl groups. has not been made clear. The C-C bond of diisopropyl ether is lengthened by steric hindrance but its length is still shorter than that of a normal hydrocarbon. This shortening of the C-C bond lengths adjacent to an oxygen atom can be related to the electronegativity of the oxygen atom.24s26 It is considered that the tilt of R and R' groups is caused by the steric hindrance between R and R' groups. Acknowledgment. We are grateful to Professor Eiji Osawa for his advice on the molecular mechanics calculation. Numerical computations were performed on a HITAC M-280H of the Hokkaido University Computing Center. Registry No. Diisopropyl ether, 108-20-3.
Supplementary Material Available: Averaged total and background intensities, and the correlation matrix (3 pages). Ordering information is given on any current masthead page. (24) Hayashi, M.; Adachi, M. J . Mol. Struct. 1982, 78, 53. (25) For convenience we used the r(C-C) value of propane as the standard C-C bond length. Callomon, J. H.; Hirota, E.; Kuchitsu, K.; Lafferty, W. J.; Maki, A. G.; Pote, C. S. Structure Data of Free Polyatomic Molecules; Landolt-Bornstein,New Series; Hellwege, K.-H., Ed.; Springer-Verlag: West Berlin, 1976; Vol. II/7. (26) Gillespie, R. J. J . Chem. Educ. 1963, 40, 295; 1970, 47, 18.
