J . Am. Chem. Sot. 1989, 1 1 1 , 4821-4828
482 1
may play a role were also discussed.
discussions.
Acknowledgment. Support for this research by Allied-Signal, Inc., is gratefully acknowledged. Research a t the University of Pennsylvania was sponsored by the National Science Foundation, Grant N S F - D M R 8 5 19059, and by the Sandia National Laboratories. W e also thank Professor Robin Hochstrasser for making available equipment and laboratory space for the picosecond lifetime measurements, Dr. S. A. Aharoni for assistance with the DSC and viscosity measurements, and Dr. J. T. Yardley for helpful
Registry No. la, 34627-92-4; lb, 115977-82-7; IC, 115977-78-1; Id, I 15977-81-6; le, 1 15977-79-2; If, 1 15977-80-5; lg, 1 15977-83-8; lh, 115977-84-9;4, 3032-92-6; 5, 75867-40-2; 6, 536-74-3; 7, 34627-90-2; 9, 115977-86-1;10, 2170-06-1; 11, 3989-14-8; 12, 75867-41-3; cuprous iodide, 7681-65-4; bis(triphenylphosphine)palladium(II) chloride,
13965-03-2; iodobenzene, 591-50-4; biphenyl, 92-52-4; 1,2-diethynyltetramethyldisilane,54773-31-8;ethynylmagnesium bromide, 4301-14-8; chloropentamethyldisilane, 1560-28-7; 1,2-dichlorotetramethyldisilane, 4342-6 1-4.
Rotational Barriers. 4. Dimethoxymethane. The Anomeric Effect Revisited Kenneth B. Wiberg* and Mark A. Murckot Contribution from the Department of Chemistry, Yale University, New Haven, Connecticut 0651 1 . Receiued October 21, 1988
Abstract: The conformational energy map for the rotamers of dimethoxymethane has been calculated using both the 3-21G and 6-3 lG* basis sets with complete geometry optimization in each case. Higher level calculations including electron correlation also have been performed on selected rotamers. At larger torsional angles, the map has approximate 4-fold symmetry, and this symmetry may be used to quantify the difference in the interactions in the ( f s c , s c ) and (+sc, +sc) regions. Large changes in bond angles were found, and the dipole moment also changed considerably on C-0 bond rotation. Calculations on equatorial and axial 2-methoxytetrahydropyran show that the axial form is favored by 1.33 kcal/mol, in fair agreement with experiment. Calculations also were carried out on the two lowest energy forms of I,l-dimethoxyethane and on five forms of methyl propyl ether. Consideration of all these data indicates that the anomeric stabilization in dimethoxymethane is much greater than in sterically more congested systems, but the stabilization of 1,l-dimethoxyethane is similar to that of 2-methoxytetrahydropyran and other acetals.
Molecular modeling is of increasing importance in many areas of chemistry. The most commonly used method, molecular mechanics,] requires a knowledge of potential functions for bond length distortion, bond angle distortion, rotation about single bonds, nonbonded interactions, and atomic charges. Whereas the first two of these may be obtained from vibrational spectroscopy, the latter three frequently present experimental problems. We have begun a n analysis of rotational barriers and have presented information concerning barriers to rotation adjacent to carbonyl group^.^-^ Here, it was possible to show that the barrier had three components: (a) dipole-induced dipole stabilizing interaction between the carbonyl and a n adjacent alkyl group; (b) the 3-fold barrier characteristic of the unsubstituted compound (e.g. acetaldehyde, acetone, or acetic acid); (c) in some of the rotamers a gauche or syn repulsive interaction between the substituents attached to the carbonyl. W e now address the more general case of rotational barriers in the absence of unsaturated groups. Recently, we have studied rotational isomerism in alkanes,j 2 - h a l o e t h a n o l ~ ,and ~ 1,2-dihaloethanes,' and in each case found that good agreement with experiment could be achieved with a b initio methods. In general, it is necessary to employ split-valence basis sets augmented by polarization functions and to correct for electron correlation.* Compounds in which the anomeric effect is found are of particular interest with respect to rotational barriers. This effect was first uncovered in studies of a-methyl glucosides in which it was found that the methoxy group preferred an axial ~ r i e n t a t i o ndespite ,~ the contrary observation in the cyclohexane series.lOJl Many experimental studies have shown that this is a general effect and also have shown that the axial C-0 bond is longer than one in Present address: Merck, Sharp, and Dohme Research Laboratories, West Point. PA 19486.
0002-7863/89/ 15 1 l-482l$01.50/0
a n equatorial position by -0.1 A number of theoretical studies have been carried out in order to provide a n explanation for this effect. Some semiempirical methods do not reproduce the gauche preference and bond length alterations. For instance, it was found that extended Huckel and M I N D 0 / 2 did not predict the correct global minimum for dimethoxymethane (DMM).I3 O n the other hand, a b initio methods a t all levels do reproduce the unusual anomeric geome t r i e ~ . ' ~ . 'In ~ one of the more comprehensive studies, Pople et aLi5studied the rotamers of D M M using a 60' grid and the rigid rotor approximation with the 4-31G basis set. Their results were conveniently expressed with a contour plot showing the change (1) Burkert, U.; Allinger, N. L. Molecular Mechanics; ACS Monographs 177; American Chemical Society: Washington, DC, 1982. (2) Wiberg, K. B.; Martin, E. J. J . Am. Chem. SOC.1985, 107, 5035. (3) Wiberg, K. B. J . A m . Chem. Soc. 1986, 106, 5817. (4) Wiberg, K. B.; Murcko, M. A. J . Compuf. Chem. 1988, 9, 488. (5) Wiberg, K. B.; Murcko, M. A. J . A m . Chem. SOC.1988, 110, 8029. (6) Wiberg, K. B.; Murcko, M. A. J . Mol. Strucr. 1988, 163, 1. (7) Wiberg, K. B.; Murcko, M. A. J . Phys. Chem. 1987, 91, 3616. (8) Hehre, W. J.; Radom, L.; Schleyer, P. v. R.; Pople, J. A. Ab Initio Molecular Orbital Theory; Wiley: New York, 1986. (9) Kirby, A. J. The Anomeric Effect and Related Stereoelectronic Effects at Oxygen; Springer: Berlin, 1983. (10) Eliel, E. L.; Gianni, M. H. Terruhedron Left. 1962, 97. ( 1 I ) Eliel, E. L.; Giza, C. A. J . Org. Chem. 1968, 33, 3754. ( 12) Berman, H. M.; Chu, S . S. C.; Jeffrey, G. A. Science 1967, 157, 1576. Fuchs, B.; Schleifer, J.; Tartakovsky, E. Nouu. J . Chim. 1984,8, 275. Fuchs, B.; Ellencweig, A,; Burkert, U. Terrahedron 1984, 40, 201 1 . (13) Tvaroska, I.; Bleha, T. J . Mol. Srrucf. 1975, 2 4 , 249. (14) Wolfe, S.; Whangbo, M.-H.; Mirchell, D. J. Carbohydr. Res. 1979, 69, 1 . (15) Jeffrey, G. A,; Pople, J. A.; Binkley, J . S.; Vishveshwara, S . J . A m . Chem. Soc. 1978, 100, 373. Van Alsenoy, C.; Schaefer, L.; Scarsdale, N.; Williams, J. 0. THEOCHEM 1981, 3 , 11 1 .
0 1989 American Chemical Society
4822
J . Am. Chem. SOC.,Vol. I l l , No. 13, 1989
Wiberg and Murcko
A
0.0(F*".ii"l Erel = 2 2 j U.194
-60.+60(+%,-scI Erel = 0 0 U.052
-12lltl20 Err,=7O 11.174
-180.0
-120.0
-00.0
0.0
80.0
120.0
180.0
-i80.0
-i20.0
-60.0
0.0
60.0
120.0
180.0
180.0
B 120.0
+hO;60 (+sc,-sc) Err! = 8 3 W=22b
Figure 1. Some rotamers of dimethoxymethane. The relative energies and dipole moments were obtained using the 6-31G* basis set. Note that the global minimum was found at (67', 67') and was 0.4 kcal/mol lower in energy than (+60°, +60').
in energy as a function of the two C-0-C-0 dihedral angles, which clearly shows the preference for the rotamer having dihedral angles of (+60°, +60°) about the two CH,-0 bonds. They proposed an interaction between an oxygen lone pair and the adjacent carbon-oxygen bond, which is geometrically favored in the above rotamer. It is commonly referred to as (+sc, +sc) (sc = syn-clinal = torsional angle of 60 f 30'; Figure 1). Intuitively, one might expect that rotamers having T, = f170' and T , = f 1 7 0 ' should all have approximately the same energies since they are far from the orientations a t which the anomeric effect is found and lack close methyl-methyl interactions. Thus, at large torsional angles, a plot of energy vs torsional angles should have approximately 4-fold symmetry. However, this is not found in the published data. In order to see if this might be associated with the use of the rigid rotor approximation, we have repeated the calculations. A 30' grid for the dihedral angles was employed along with the 3-21G basis set and complete geometry optimization. The results of our calculations are presented graphically in Figure 2a. I t can be seen that the expectation of approximate 4-fold symmetry at large dihedral angles is borne out by the calculations. In order to highlight the differences between the adjacent quadrants, we have taken the upper left and lower right quadrants of Figure 2 and have rotated them by 90' to give a completely symmetrical pattern (Figure 2b). This was then subtracted from Figure 2a, giving the difference plot (Figure 2c). A strong effect is clearly localized in this plot. Either there is a stabilizing effect in the (+60°, f 6 0 ' ) region, a destabilizing effect in the (+60', -60') area, or some combination of the two. We know in other cases that the 3-21G basis set is not adequate to represent the polarization of atomic orbitals in the presence of an electric field, such as that which might be generated by C-0 dipoles.I6 When the 3-21G data were reexamined using a 60' grid, the results were essentially the same as for a 30' grid. Therefore, we have repeated the calculation using the 6-31G* basis set, but at a 60" grid. Whereas a 30' grid required optimization of the structures of 43 rotamers, a 60' grid required the study of only 13 rotamers. This smaller number made it practical to use the more flexible basis set, which usually gives results in good agreement with experimental data." (16) Reference 4 and Wiberg, K. B.; Laidig, K. E. J . Am. Chem. SOC. 1987, 109, 5935.
00.0
72
0.0
-60.0
-120.0
-100.0
.~ .
C 120.0-
60.0-
0.0-
-60.0-
-120.0-
U -180.0
j -180.0
-120.0
-60.0
0.0
60.0
120.0
80.0
Figure 2. (A) Energy of dimethoxymethane as a function of the torsional angles-3-21G basis set. The relative energies are given in kilocalories per mole. (B) Upper left and lower right quadrants rotated 90". (C) Figure 2 A minus Figure 2B showing the locus of the anomeric effect. The contours are spaced at 1 kcal/mol intervals, and the inner contour is at -8 kcal/mol relative to the plateau.
The 6-31G* energies are summarized in Table I. A contour plot of the energies as a function of dihedral angle is shown in Figure 3a. Figure 3b corresponds to Figure 2c for the 3-21G basis set. Again, it is seen that the anomeric effect is localized (17) Wiberg, K. B. J . Org. Chem. 1985, 50, 5285.
Rotational Barriers. 4. Dimethoxymethane
J . Am. Chem. SOC.,Vol. 1 I1, No. 13, 1989 4823 180.0
Table I. Calculated Energies of Dimethoxymethane Conformers,
6-31Ga Basis Set a. Relative Energies (kcal/mol)
120.0
+I
-1800 -120' -60' O0 60' 1 20' 180'
-180' 5.66 6.25 2.42 7.08 2.42 6.25 5.66
-120' 6.25 7.01 3.32
-60' 2.42 3.32
8.00
11.06 8.27 3.96 2.42
3.96 7.92 6.25
0.00
0'
7.08 8.00 11.06 22.33 11.06 8.00 7.08
60' 2.42 3.96 8.27 11.06
120' 6.25 7.92 3.96
0.00
3.32 7.01 6.25
3.32 2.42
8.00
180' 5.66 6.25 2.42 7.08 2.42 6.25 5.66
b. Energies of (+60°, +60°) Rotamer and Stationary Points (hartrees and kcal/mol) 71 72 energies 60.0 60.0 267.954 08 0.00 180.0 65.3 267.950 35 2.34 67.3 67.3 267.954 67 -0.37
80.0
72
0.0
4 . 0
-120.0
-180.0
, -180.0
-;20.0
-60.0
0.0
60.0
izo.0
i80.0
71
180.0
Figure 4. Difference between the 6-31Ga and 3-21G energy maps given in Figures 2A and 1A. The energies are given in kilocalories per mole, and positive values indicate lower 6-31G* energies as compared to 3-21G.
A 120.0
180.0
60.0
120.0
0.0
60.0 -60.0
72
0.0
-120.0
-60.0 -180.0 -180.0
-120.0
-60.0
0.0
60.0
120.0
160.0 -120.0
72 180.0
-180.0
B
, -180.0
-120.0
-60.0
0.0
60.0
120.0
180.0
120.0
71
Figure 5. 0-C-0 bond angles in dimethoxymethane as a function of the 60.0
dihedral angles.
0.0
-80. 0
-120.0
-180.0
I 10.0 -120.0
I -60.0
I 0.0
I 60.0
I 120.0
10.0
71 Figure 3. (A) Energy of dimethoxymethane as a function of the torsional angles-6-31Ga basis set. The relative energies are given in kilocalories per mole. (B) Figure 3A minus the equivalent of Figure 2B showing the locus of the anomeric effect calculated at the 6-31Ga level. The contours are spaced at 1 kcal/mol intervals, and the inner contour is at -8
kcal/mol relative to the plateau. in a small range of dihedral angles. T h e relative energies a r e in some cases significantly different from those obtained using the 3-21G basis. This is quantified in Figure 4, which gives the difference between the 3-21G and 6-31Ga relative energies. T h e effect is most pronounced a t the corners of the map, with dihedral
angles of (ISO", 180'). Here the 6-31G* relative energy is more than 4 kcal/mol lower than a t the 3-21G level. Also, at the 3-21G level, the (O", 180O) conformer is more stable than the all-trans (180', 180") species. This is difficult to reconcile with the eclipsing interaction in the (O", 180') form, which must be destabilizing. Again, the 6-31G* results are more reasonable, with the (O", 180O) form now higher in energy than the ( 180°,180")rotamer by more than 1 kcal/mol. The variation in geometrical parameters for the set of rotamers also is of interest. These data are given in Table 11. The C-0-C and 0-C-0 bond angles a r e plotted as a function of the torsional angles in Figures 5 and 6, and the CH,-0 and O-CH3 bond lengths are plotted as a function of the torsional angles in Figures 7 and 8. The C-0-C angle is taken from the side of the molecule containing T,,as is the CH3-0 bond. T h e CH2-0 bond chosen is the one about which rotation occurs in 7 , . It is clear that there are pronounced geometric changes as rotation occurs, with angles varying by up to 17O and lengths varying by up to 0.04 A. Clearly, the rigid rotor approximation leads to considerable errors in this case. W e also have relaxed the dihedral angles in order to obtain the geometry of the global minimum. For this purpose, both the 6-31Ga and 6-31+G* basis sets were used. T h e latter was included because the addition of diffuse functions (indicated by +)
4824 J . Am. Chem. SOC.,Vol. 111, No. 13, I989
Wiberg and Murcko
Table 11. Geometric Data for Dimethoxymethane, 6-31G* Basis Set TI
-1800
72
-1200
-60'
00
60'
120°
1800
1.3684 1.3769 1.3849 1.3816 1.3819 1.3767 1.3684
1.3743 1.3842 1.3885 1.3847 1.3884 1.3829 1.3743
1.3755 1.3841 1.3879 1.3856 1.3879 1.3841 1.3755
1.4011 1.3971 1.3951 1.4016 1.4002 1.3998 1.401 1
1.3966 1.3957 1.3969 1.3981 1.3969 1.3962 1.3966
1.3932 1.3926 1.3931 1.3943 1.3931 1.3926 1.3932
a. Methylene-Oxygen Bond Lengths -1 800 -1 200 -60' 00 60' 1200 1800
1.3755 1.3841 1.3879 1.3856 1.3879 1.3841 1.3755
1.3743 1.3829 1.3884 1.3847 1.3885 1.3842 1.3743
-1 soo -1 200 -60' 00 60' 1200 1soo
1.3932 1.3926 1.393 1 1.3943 1.3931 1.3926 1.3932
1.3966 1.3962 1.3969 1.3981 1.3969 1.3957 1.3966
1.3684 1.3767 1.3819 1.3816 1.3849 1.3769 1.3684
1.3816 1.3917 1.3970 1.3968 1.3970 1.3917 1.3816
b. Methyl-Oxygen Bond Lengths 1.4011 1.3998 1.4002 1.4016 1.3951 1.3971 1.4011
1.4027 1.3997 1.3955 1.3933 1.3955 1.3997 1.4027
c. C-0-C Angle Changes"
-1 soo -1200 -60' 00 60' 1200 1800
113.99 113.83 114.04 113.89 114.04 113.83 113.99
116.47 116.54 116.41 116.60 116.97 116.78 116.47
115.35 115.04 115.12 119.32 121.39 115.66 115.35
-1 800 -1200 -60' 00 60° 1200 1800
106.60 108.26 110.26 111.43 110.26 108.26 106.60
108.26 109.97 112.03 113.14 112.34 110.51 108.26
110.26 112.03 113.98 118.72 119.1 1 112.34 110.26
119.55 119.23 123.57 128.70 123.57 119.23 119.55
115.35 115.66 121.39 119.32 115.12 115.04 115.35
116.47 116.78 116.97 116.60 116.41 116.54 116.47
1 13.99 113.83 1 14.04 113.89 1 14.04 113.83 113.99
110.26 112.34 119.1 1 118.72 113.98 112.03 110.26
108.26 110.51 112.34 113.14 112.03 109.97 108.26
106.60 108.26 110.26 111.43 110.26 108.26 106.60
d. 0-C-0 Angle Changes
OT]
11 1.43 113.14 118.72 123.21 118.72 113.14 111.43
is closer to the COC angle. 180.0
120.0
60.0
z2
0.0
-60.0
-120.0
-180.0
-180.0
-120.0
-60.0
0.0
80.0
120.0
100.0
71
Figure 6. C-0-C bond angles in dimethoxymethane as a function of the dihedral angle. The C-0-C angle is taken from the side of the molecule containing T , . might improve the description of the oxygen lone pairs.'* Both geometries are given in Table 111. The C-042-0 dihedral angle opened up from 67.3 to 68.4, but other geometric parameters changed by smaller amounts. It is worthwhile to compare our calculated structure to that determined experimentally, also given in Table 111. The (60°, 120') and (60°, 180') conformers both are calculated to be at least 2 kcal/mol above the global minimum and therefore will not significantly influence the energy or geometry. Thus the calculated geometry for the global minimum (18)
Chandrasekhar,J.; Andrade, J. G.; Schleyer, P. v. R. J . Am. Chem.
SOC.1981, 103, 5609.
-180.0
-120.0
-80.0
0.0
80.0
120.0
180.0
TI
Figure 7. CH2-0 bond lengths in dimethoxymethane as a function of
dihedral angle. The C-0 bond chosen is the one about which rotation occurs in T ] . Contour 1 corresponds to 1.368 A, and the contours are spaced by 0.002 A. Table 111. Geometric Parameters of the Ground State of Dimethoxymethane 6-31+G* exptl' 6-31G* param 1.4012 1.432 1.4002 r(CH,-O) 1.3823 1.382 1.3814 r(0-CH2) 115.31 114.6 114.89 L(C-0-C) 1 13.49 114.3 113.48 L(0-c-0) 68.40 63.3 67.30 dC-0-C-0) By electron diffraction."
is directly comparable to the experimental value. An electron diffraction study found the 0-C-0 angle to be 114.3', while the
Rotational Barriers. 4 . Dimethoxymethane
J . Am. Chem. SOC.,Vol. 1 1 1 , No. 13, 1989 4825
180.0
Table IV. Effect of Electron Correlation on Calculated Energies of
Dimethoxymethane 6-31G* Basis Set
120.0
dihedral angles 60.0
,r*
67.30 67.30 64.38 117.51 65.32 180.00 180.00 180.00
0.0
MP4(SDTQ)
RHF
MP2 MP3 a. Total Energies (hartrees) 267.954 67 268.695 78 268.727 87 267.949 01 268.689 08 268.721 50 267.950 35 268.69027 268.722 72 267.94506 268.68402 268.71681
268.762 14 268.755 55 268.756 67 268.75046
b. Relative Energies (kcal/mol)
-60.0
67.30 67.30 64.38 117.51 65.32 180.00 180.00 180.00
-120.0
0.00
0.00
0.00
0.00
3.55 2.71 6.03
4.20 3.46 7.38
4.00 3.23 6.94
4.13 3.43 7.33
6-3 1 +G* Basis Set -1110.0
-iso.o
-i20.0
-60.0
0.0
io.0
ho.0
dihedral angles
TI
Figure 8. CH3-0 bond lengths in dimethoxymethane as a function of dihedral angle. The C-0 bond is taken from the side of the molecule containing T , . Contour 1 corresponds to 1.393 A, and the contours are spaced by 0.001 A.
outside C-0-C angle was 1 14.6O.I9 The inside C-0 bond length was 1.382 A while the outside C-0 bond was 1.432 A. T h e C-0-C-0 dihedral angle was 63.3O.*O The dipole moment in benzene was found to be 0.99 D. All of these agree reasonably well with our results, except for the bond lengths, which are shorter in the calculated structure. This is expected because the a b initio geometry represents a minimum energy structure and fails to account for anharmonicity a t the zero-point energy level. A question has been raised concerning the experimental C 4 - C bond angle, and a value of 1 1 1.6' has been proposed.21 However, our results provide support for the 114O experimental angle. T o see what effect electron correlation might have on the calculated relative energies, several points were chosen for extended treatment. Full M P 4 single-point calculations a t the 6-31G* and 6-31+G* levels were carried out on four structures, including the global minimum and three others with dihedral angles of roughly (64O, 117O), (65O, 180°), and (180°, 180O). These results are given in Table IV. Generally, electron correlation raises the relative energies of D M M conformers, while adding diffuse functions to carbon and oxygen (6-31 +G*) lowers their relative energies. As has already been noted, the 6-31G* relative energies themselves are significantly lower than the 3-21G results. Thus the effect of increasing basis set size appears to partially cancel the effect of adding electron correlation, and the 6-31G* basis set may provide a reasonable estimate of the relative energies in DMM. The results of the calculations confirm that the energy changes on bond rotation in D M M a r e caused by several factors. T h e changes in bond angles shown in Figures 1, 5 , and 6 must in large measure be due to steric interactions. They become large whenever the terminal groups approach each other. The changes in dipole moments such as are shown in Figure 1 will lead to changes in energy. In the gas phase, it is generally found that the rotamer with the larger dipole moment has the larger electrostatic energy, and a n increased energy.' This effect will be reduced on going to a medium with a higher dielectric constant, and as a result, the populations of various rotamers will change with solvent. Finally, the changes in bond lengths (Figures 7 and 8) probably result from a n electronic interaction, and the latter may have a n
67.30 67.30 64.38 117.51 65.32 180.00 180.00 180.00 67.30 67.30 64.38 117.51 65.32 180.00 180.00 180.00
b. Relative Energies (kcal/mol) 0.00 0.00 0.00 3.16 3.92 3.15 2.40 3.32 3.1 1 7.02 5.62 7.49
268.78292 268.776 71 268.777 63 268.77093 0.00
3.86 3.32 7.52
Table V. Calculated Geometries for 1,l-Dimethoxyethane
C2-C3 C3-07 C3-08 07-C9 08-c I O C2-C3-07 C2-C3-08 C3-07-C9 C3-08-C 10 07-C3-08 C2-C3-07-C9 C2-C3-08-C10 08-C3-07-C9 07-C3-08-C10
(+sc, +sc) 3-21G 6-31G* 1.5190 1.5175 1.4218 1.3916 1.3871 1.4207 1.4014 1.4428 1.3995 1.4412 113.88 113.98 106.39 107.33 116.59 117.45 115.10 115.24 112.05 111.23 -59.50 -58.61 171.64 168.70 60.70 63.53 63.83 65.44
(+sc, 180') 3-21G 6-31G* 1.5168 1.5155 1.4294 1.3964 1.4077 1.3760 1.4341 1.3936 1.4407 1.3992 111.33 111.82 107.51 106.77 116.29 116.32 115.65 115.71 108.43 108.74 85.46 87.59 176.09 176.28 157.37 153.85 56.03 62.48
effect on the relative energies. I t is not possible a t this time to separate the contributions of these terms to the energy differences, and this problem continues to be studied. The next larger acetal is 1,l-dimethoxyethane (DME), which differs from D M M by having one of the methylene hydrogens replaced by a methyl group. D M E , with its methyl side chain, is thus a better model for the anomeric effect in acetals, all of which are similarly substituted. The extra alkyl group will raise the steric repulsion in any conformer of D M E that has a close contact between methyl and methoxy. Thus, the global minimum (+sc, +sc) should be destabilized relative to the (+sc, trans) form.
Astrup, E. E. Acta Chem. Scand. 1973, 27, 3271. (20) It is interesting that the dihedral angle in poly(oxymethy1ene) has been estimated to be 77O, much larger than either calculation or experiment gives for DMM. Tadokoro, H.; Yasumoto, T.; Murahashi, S . ; Nitta, J. J . (19)
Polym. Sci. 1960, 44, 266. ( 2 1 ) Noerskov-Lauritsen, L.; Allinger, N. L. J . Comput. Chem. 1984, 5,
326.
MP4(SDTQ)
RHF
MP2 MP3 a. Total Energies (hartrees) 267.961 92 268.71473 268.74523 267.956 89 268.708 49 268.739 25 267.958 09 268.709 44 268.740 28 267.952 97 268.702 79 268.73404
bo.0
+IC,+\
