Ab initio investigation of conformational geometries and the torsional

J. Phys. Chem. 1993,97, 11221-1 1226

11221

Ab Initio Investigation of Conformational Geometries and the Torsional Potential Surface in Perfluorodimethoxymethane C. L. Stanton and M. Schwartz' Department of Chemistry, University of North Texas, Denton, Texas 76203 Received: May 21, 19930

A conformational potential energy contour diagram for perfluorodimethoxymethane[PFDMOM] was constructed by RHF/6-31G*//RHF/6-31GS(C,0) [the latter is the 6-31G basis set with polarization functions on C and 01 calculations a t 60° increments of the two C F f l torsional coordinates between -1 80° and 180°. Additional RHF/6-31G*//RHF/6-31GS geometriesandenergiesweredeterminedforanumber ofequilibriumand transition state stationary points. Many of the conformers exhibited a helical structure, which has been attributed to dipolar repulsions which destabilize parallel C-F bond dipoles. Several rotamers were found to contain two nonequivalent, but closely related molecular structures, corresponding to (a) the same or (b) different relative directions of helical twisting in the two halves of the molecule. The potential energy contour diagram of PFDMOM is dramatically different from that reported earlier for dimethoxymethane [DMOM], where it was found that the gauche-gauche rotamer is greatly stabilized relative to other conformations by the anomeric effect. In contrast, the most stable conformations for PFDMOM correspond to either or both of the torsional angles having values in the region near 180°. In addition, the barriers to internal rotation in PFDMOM are significantly lower than in DMOM. The basis for the differences in the energy diagrams of the two ethers and its implications on the structure of their polymeric forms [(-CF2O-)n and (-CH2O-)n] are discussed.

Introduction Perfluoropoly(alky1 ethers) (PFPAEs) represent a class of liquids with increasing applications in a number of areas,' including the development of new extended temperature and pressure lubricating fluids.2 As part of a research program designed to gain a better understanding of the conformational quilibria and rotational flexibility in PFPAEs and their relation to the bulk rheological, tribological, and thermal properties of these liquids, we are performing investigationsof the molecular geometries and barriers to internal rotation in comparatively small PFPAE homologue^.^ A general understanding of the interaction between different internal torsions is central to characterization of the structural and dynamical properties of polymers.4 Perfluorodimethoxymethane [PFDMOM], CF30CF20CF3, is the simplestlinear perfluoroether exhibiting internal rotation about multipleinternal C-O bonds. There have been a number of theoretical studies of the geometries and energies of the protonated ether, dimethoxymethanes16 [DMOM], of which the most comprehensive is the recent investigation of Wiberg and MurckoS6 It has been observed in the latter molecule that the conformationalequilibria are dominated by the anomeric effect.'J To determine the effect of fluorination on the conformational equilibria and torsional potential surfaces in poly(alky1 ethers), we have calculated a large number of the equilibrium and transition-state geometries and energies in PFDMOM. This representsthe first investigation of the rotational potential energy surface in this molecule. The results are compared to those obtained for DMOM6 and in an earlier reported study of one of the equilibrium structures of PFDM0M.S The results of this and other investigationsof perfluoroethers will be used to develop new force-field parameters in order to perform accurate molecular dynamics10 simulations of the rheological and thermal properties of PFPAE fluids. CaIculatious

Ab initio molecular orbital calculations were performed on a Cray X-MP/216 computer using the Gaussian 90 program." Abstract published in Aduonce ACT Abstracts, October 1, 1993.

0022-3654/93/2097- 11221$04.00/0

F

/

'..., "F

F

/

".,,

"F

?CF3

F

/

",+

OCF3

CF3

G F3Cjl0CF3

E1

T 9CF3

E2

Figure 1. Structure, atom numbering and conformational definitions in PFDMOM.The projections represent views from Cz to either 0 1 or 0 2 .

To obtain the potential energy surface for rotation about the two skeletal CO-CO dihedral angles, 41 and 42 [see Figure 11, in PFDMOM, geometries were gradient optimized12 for a grid of conformationswith fixed torsional angle pairs, -180° < 41,42 < 180°, in increments of 60°. As a result of symmetry, many of the 49 points on the grid are equivalent and calculations on 13 independent rotamers are sufficient to completely define the energy surface. Dixon13 has demonstrated that whereas the inclusion of polarization functions on carbon and oxygen is essential to obtaining accurate geometries in fluorocarbons and ethers, the removal of these functions from the fluorine atoms has very little effect on the calculated geometry. Therefore, because of the 0 1993 American Chemical Society

11222 The Journal of Physical Chemistry, Vol. 97, No. 43, 1993

Stanton and Schwartz

TABLE I: Selected Geometric Parameters of the Stationary Point Conformers. T-T

T-T

160.2

B 1.357 1.355 1.355 1.357 104.8 121.1 121.2 -163.5 163.5 -164.3 164.4

parameter A R(CIOI) 1.358 R(Cz0i) 1.354 R(C202) 1.354 R(C302) 1.358 401C202) 105.0 L(CIOlC2) 121.1 L(C202C3) 121.1 $ 1 ~ WIACIOICZ) 163.3 6 2 ~ @ ( F ~ A C ~ O ~ C ~163.2 ) QI = Q(C101C202) 160.2 62 = $(c302c201)

T-E2 A 1.357 1.359 1.356 1.356 106.8 121.1 122.8 163.0 -179.0 159.9 119.0

T-E2 B 1.357 1.359 1.357 1.355 106.4 121.5 122.7 -163.5 179.2 -168.3 123.5

conformation* T-G

T-G

A 1.357 1.361 1.355 1.357 108.6 121.4 121.8 163.0 -164.2 161.6 84.8

B 1.359 1.361 1.354 1.358 109.7 121.4 122.2 -163.0 157.7 -163.6 47.7

T-E 1 1.360 1.358 1.356 1.356 110.8 121.4 124.9 163.4 178.5 161.1 2.4

G-G 1.356 1.362 1.362 1.356 114.2 122.6 122.6 -171.7 -171.7 68.6 68.6

G% 3-21G 1.371 1.374 1.374 1.371 111.5 124.5 124.5 92.6 92.6

4 Bond lengths are given in angstroms and angles in degrees. The conformationsare labeled by the two dihedral angles (01and 42) with the following conventions: T($=180°); G($=60°); El(Qa0'); E2(Q=1120°). From ref 8 [Tables VI1 and VII].

large number of independent conformers necessary to define the potential energy surface, we have used the 6-3 lG*(C,O) basis set, which is the 6-3 1G basis14J5 augmented by polarization functions on the skeletal C and 0 atoms. Following the structure determinations, single point HartreeFock and second-order M~ller-Plesset~~ energies were calculated using the 6-31G* basis set with the 6-31G*(C,O) geometries. In addition to calculations at fixed dihedral angles, the geometries and energies of a number of stationary points were determined a t the HF/6-3 lG* level (following initialoptimization with the 6-31G*(CIO) basis). In the ensuing discussion, these conformations are labeled by the pair of skeletal torsions, using the notation: G, gauche (4 = f60'); T, trans (4 = 180 "); E l , first eclipsed conformation (4 = 0'); E2, second eclipsed conformation (4 = f120'; see Figure l).17 Results and Discussion Geometries. Displayed in Table I are selected geometric parameters of various stationarypoint conformers of PFDMOM, determined at the HF/6-31G* level. In this table and later discussion, the atoms are numbered in accordance with the notation in Figure 1; this figure and the table also contain definitions of the relevant dihedral angles. The most striking features in these results are that (a) the torsional angles [4l,4lA1#2,42B] in the T-T conformers differ markedly from the expected value of 180°, and (b) this and several other rotamers contain at least two nonequivalent molecular conformations of similar energy (vide infra). The first of these observations is very similar to recent results obtained by Dixonls on perfluorobutane [PFB] and in our laboratories3 on perfluoroethyl methyl ether [PFEME], where it was found that the molecular skeletons of the trans rotamers are twisted from 180' by approximately 15-20', with a similar rotation of the CFS groups about the terminal C-C (or C-0) bond. The helical structures of these molecules have been attributed1893as likely arising from repulsive interactions between parallel C-F bond dipoles on alternate carbons which destabilize the perfectly trans configuration. The helicity observed here in PFDMOM possibly arises, similarly, from C-F dipolar repulsions between Cl (or C3) and C2 [see Figure 11. An alternative explanation, suggested by one of the reviewers, is based upon the nonequivalence of oxygen lone electron pairs. Weinhold and c o - ~ o r k e r s ,using ~ J ~ natural bond orbital analysis, have demonstrated that in various bivalent oxygen molecules, one of the lone-pairs is spr ( x < 1.O),whereas the second occupies an essentially pure 'p" orbital. In the trans configuration, one expects a strong interaction of the latter, more Lewis basic, pair with antibonding C-F orbitals (vide infra). Due to the 90° orientation of the "p" orbital relative to the 04-0 skeleton, this would lead to a stable conformation with dihedral angles somewhat displaced from precisely 180'.

The tendency of the molecular skeleton to twist about the CF2-0 bonds also provides an explanation for the presence of multiple nearly equivalent rotamers. As seen in Table I, the major difference between the two T-T conformations is that 41 and are of the same sign in T-T[A], and of opposing signs in T-T[B]. In each structure, the corresponding FCOC torsion [41A or 42A] has the same sign as the skeletal COCO dihedral angle; all other parameters are nearly identical in the two rotamers. The principal difference, therefore, is that in T-T[A], there is a continuing rotation in the same direction as one traverses the molecule and the two perfluoromethoxy groups lie on opposite sides of the O1C2O2plane. Conversely, the two halves of the molecule twist in opposite directions in the T-T[B] rotamer and the C F 3 0groups are on the same side of the plane. One sees the same differences in structure in the pairs of T-E2 and T-G conformers [Table I]. Although not shown, we have also calculated the stationary point geometries with the 6-31G*(C,O) basis set. As found in earlier studies,I3 the structures are extremely close to those obtained with the 6-3 lG* basis (which also contains polarization functions on fluorine). Bond lengths agree to within 0.001 A and bond angles and torsions to within 0.5-1 .Oo. Pacansky et al.9 have recently reported the geometry of the G-G conformation of PFDMOM, using the 3-21G basis set. For comparison, their results are presented in the final column of Table I. One observes that the C-O bond lengths are greater, by 0.012-0.01 5 A, using the smaller basis. Although not shown, the 3-21G C-F bond lengths are also longer, by 0.02-0.03 A. These deviations are to be expected since one usually finds that R[3-21G] > R[6-31G*], and the effect is particularly large in bonds involving electronegative atoms such as fluorine." The COC and OCO bond angles are in quite reasonable agreement using the two basis sets, but the calculated COCOdihedral angles are markedly higher, by almost 25O, using the smaller 3-21G basis. It is of interest to compare the calculated structures of PFDMOM with that of DMOM.6 Generally, it is found that C-0 bond lengths in the fluorinated ether are shorter, by -0.020.04 A. OCO bond angles are roughly the same in the two species, but COC angles in PFDMOM are greater, by -7-go. These trends follow those found earlier in a comparison of perfluoroethyl methyl ether3 with the protonated form. The differences can be attributed to the greater bond ionicity in the perfluorinated ether [based upon Mulliken charge differences, q(C)-q(O)] ,21 which will shorten the C-0 bond, increasing the electrostatic and van der Waal's repulsions between fluorines on alternate carbon atoms and, hence, inducing an increase in the COC angle. It may be seen from the stationary point geometries in Table I that the CF3-0 [R(CIOI)and R(Cp021 and CFTo [R(C2O1) and R(C202)I bond lengths appear to be relatively insensitive to variations in the molecular skeletal conformations. This is borne C#I~

The Journal of Physical Chemistry, Vol. 97, No. 43, 1993 11223

Perfluorodimethoxymet hane 0

-lEO.C

-'73.2

-60.0

0.0

60.0

'70 0

1";.0

-'20.0

-50.0

0.0

60.0

'20.0

190.0

8

aD

9 0

N

2 $ 8 8 W I

8N 800 I

-180.0

Figure 2. Angle contour diagrams for PFDMOM: (A, top) LOCO, (B, bottom) LCOC. Black 0' IAd I2'. Dark gray: 2' C Ad I4O. Light gray: 4 O < Ad I 8 O . White: Ad > 8 O .

out by the grid point calculations, in which both 41 and 42 were varied between -1 80' and 180' [using the 6-3lG*(C,O) basis]. Although not shown, it was found that the total variation in each bond length was only about 0.01 A. In contrast,one observes a substantially greater variation in the OCOand COC bond angles. These changes are most apparent

+

when one considers the complete range of skeletal torsions, as studied in grid calculations. Plotted in Figure 2 are contour diagrams of the increase in the OCO (Figure 2A) and COC (Figure 2B) angles [above their minimum values] as a function of the two skeletal dihedrals, 61 and 42. One sees that the LOCO (Figure 2A) increases markedly, by a total of almost 20' as the

11224 The Journal of Physical Chemistry, Vol. 97, No. 43, 1993

-180.0

-120.0

-59.0

-180 0

-'23.3

-60.0

Stanton and Schwartz

0.0

60.0

120.0

180.0

60.0

'20.0

150.0

Figure 3. Potential energy contour diagrams for (A, top) DMOM6 and (B, bottom) PFDMOM [this work]. Black 0 IAE I 1 kcal/mol. Dark gray: 1 < AE I 3 kcal/mol. Light gray: 3 IAE I5 kcal/mol. White: AE > 5 kcal/mol.

two torsional angles are decreased from 180' to 0'. Not surprisingly, this increase is more rapid in the quadrants where $1 and $2 are of opposing sign, in which case both perfluoromethoxy groups are on the same side of the central OCO plane, resulting in a more congested structure. One finds the same

trend in LCOC (Figure 2B), where the total increase is 15'. For this angle, though, the increase is comparatively small until one reaches configurations for which 1$11,1$21 I-60'. Energies. It has been established both experimentally22 and theoretically5$6that the G-G [$I = $2 = 60'1 form of DMOM

Perfluorodimethoxymethane

The Journal of Physical Chemistry, Vol. 97, No. 43, 1993 11225

TABLE 11: Dihedral Angle Dependence of Conformational Energies.

TABLE 111: Calculated Conformational Energies

&(den) ~~

-180

-120

-60

0

+60

+120

+180

004 0.62 0.0 1.26 0.0 0.62 0.04

0.62 1.37 1.21 1.69 1.03 2.07 0.62

0.0 1.21 0.72 8.67 7.82 1.03 0.0

1.26 1.69 8.67 18.25 8.67 1.69 1.26

0.0

0.62 2.07 1.03 1.69 1.21 1.37 0.62

0.04 0.62

~

-180 -120 -60 0 +60 +120 +180

1.03 7.82 8.67 0.72 1.21

0.0

0.0 1.26 0.0 0.62 0.04

a Values represent HF/6-31GS//HF/6-31G*(C,0) energies and are given in kcal/mol, relative to the energy of the 18Oo,6O0 conformer.

is significantly more stable than any of the other equilibrium conformations. The stability of this rotamer has been attributed to the anomeric effect' since in this conformation, the lone-pair orbital on one of the oxygens is properly oriented to effectively contribute electron density into the u* orbital of the neighboring polar C-0 bond; in Figure 1, this would correspond to donation of electrons from 0 1 to the C2-02 antibonding orbital. The anomeric effect in DMOM is clearly evident in the potential energy contour diagram shown in Figure 3A [data taken from ref 61, where it is seen that the region $1E qh2E &60° is markedly more stable than any other equilibrium or transition-state conformation. We have calculated HF/6-31G* energies [with 6-31G*(C,O) geometries] for PFDMOM at all pairs of torsional angles, $1 and $2, ranging from -180 to 180' in 60' increments. These results are displayed in Table I1 and plotted in Figure 3B. Quite clearly, the energy contour diagram is strikingly different from that of DMOM [Figure 3A]. In contrast to the protonated ether, the most stable conformations of PFDMOM lie on the periphery of the diagram, where $1 and/or $2 is near 180'. Even the apparent low-energy region in the diagram around $1 = $2 * &60° is actually 0.7 kcal/mol above the energy of either the T-T [ 180°,180'1 or T-G [180',60'] rotamer [see Table 111. From these results, one sees that replacement of the hydrogens in DMOM by fluorine atoms dramatically alters the conformational equilibria. One can explain this effect by noting that in DMOM, the gauche conformations of the two C - O bonds are stabilized by hyperconjugative interaction of one of the lone-pair of electrons on oxygen (no) with the antibonding CO orbital (u*co). By contrast, in PFDMOM one expects the strongest interaction when the oxygen lone pair is arranged antiperiplanar to a highly polar C-F bond, due to the better overlap and closer energy match of with U*CF than with U*CO. This effect would lead to the greatest hyperconjugativestabilization, when the C-0 bonds assume the trans configuration. Indeed, this prediction is qualitatively consistent with our calculation showing E(T-T), E(T-G) < E(G-G). As discussed in the previous section and displayed in Table I, torsional angles in the equilibrium rotamers of PFDMOM can be significantlydifferent from *60° or 180'. Therefore, we have calculated HF/6-3lG1'//HF/6-31G* energies for a number of stationary point conformations; the results are shown in Table IIIA. One sees first that energies of the pairs of conformations corresponding to either the same [A] or opposite [B] direction of twist [relative to the OlC202 plane] are, not surprisingly, fairly close to each other [to within -0.3-0.4 kcal/mol]. One observes further from the table that results obtained with the grid point geometries [Table IIB] are qualitatively similar to those calculated at the stationary points. For example, in both cases, E(T-T) = E(T-G) and E(G-G) - E(T-T) F* 0.7-0.85 kcal/mol. Shown at the bottom of Table I11 [part C] are MP2 energies of thevarious grid points. One finds that the inclusion of electron correlation does induce some quantitative changes in the relative energies [e.g., E(T-T) is slightly higher than E(T-G) and E(G-

energy (hartrees)

61

+I

conforma (deg) (deg) (kcal/mol)

basis set

A. HF/6-31G*// HF/6-31G*

T-T[A] 160.2 160.2 T-T [B] -164.3 164.4 T-E2 [A] 159.9 119.0 T-E2 [e] -168.3 123.5 T-G [A] 161.6 84.8 T-G [B] -163.6 47.7 T-E1 161.1 2.4 G-G 68.6 68.6 B. HF/6-31G*// T-T 180.0 180.0 HF/6-31G*(C,O) T-E2 180.0 120.0 T-G 180.0 60.0 T-El 180.0 0.0 G-G 60.0 60.0 C. MP2/6-31G*// T-T 180.0 180.0 HF/6-31G1(C,0) T-E2 180.0 120.0 T-G 180.0 60.0 T-E1 180.0 0.0 G-G 60.0 60.0

0.00 0.34 0.94 1.33 0.33 0.W

1.70 0.85 0.04 0.62 0.W

1.26 0.72 0.28 0.82 O.O@

1.58 0.09

-1058.885 -1058.885 -1058.884 -1058.883 -1058.885 -1058.885 -1058.883 -1058.884 -1058.884 -1058.883 -1058.884 -1058.882 -1058.883 -1060.951 -1060.950 -1060.952 -1060.949 -1060.951

960 408 456 838 426 956 248 607 560 632 628 621 481 674 807 115 603 966

See text and Table I for notation. Reference energy.

35

30

25

20

! l

15

-5

I -180

, -120

40

0

60

120

180

e2

Figure 4. Torsional potential energy plots: (A) PFDMOM, $1 = 180' [this work]. (B) DMOM: 41= 180O.6 (C) PFDMOM: 61= 60' [this work]. (D) DMOM: 61 = 6OoS6

G) = E(T-G)]. However, qualitatively, the results are similar to those obtained at the HartreeFock level. In particular, all of the conformational energy differences and barriers remain relatively low and, unlike in DMOM, there is no significant stabilization of the G-G rotamer relative to the other equilibrium conformations. Finally, it is very informative to consider the potential surface for rotation about one of the dihedral angles [&], while the second torsion [&I is held constant at either 60' or 180'. These results are plotted for both PFDMOM and DMOM in Figure 4. To directly compare the two species, we have plotted the grid point energieswhich, as noted, are qualitatively similar to the stationary point results. As seen in the figure (curve A), for PFDMOM when 41 = 180°, the $2 = *60° [T-G] and $2 = 180° [T-TI rotamers are of approximatelyequal energy and the barriers to rotation between any two conformations is fairly small [ 51.3 kcal/mol (Table 11)]. By comparison, in DMOM [curve B] the two T-G rotamers [$2 = f60°] are far morestable than theT-Tstructure [by -5.7

11226 The Journal of Physical Chemistry, Vol. 97, No. 43, 1993

kcal/mol]6 and the barriers to rotation between any of the equilibrium states are quite large. For PFDMOM, when 41 = 60° [curve C], then the G-G+ 142 = +60°] rotamer is of only slightly higher energy [ -0.7 kcal/ mol (Table 11)] than the G-T conformation and there is virtually no barrier to interconversion between the two equilibrium conformers. In contrast, the only stable structure for DMOM [curve D] is for 42 = 60° [G-G+]; the T-T form is 2.4 kcal/mol higher in energya6 From the above comparison, it is quite evident that the conformational properties of PFDMOM are dramatically different from thoseof the protonated ether. One may further reach the important conclusion that, whereas one expects that a poly(methylene oxide) polymer, [-CH2O-],, will be comparatively rigid, with most bonds “frozen” in the gauche configuration, poly(perfluoromethylene oxide), [-CF2O-],, is predicted to be far more flexible, with almost free internal rotation about thevarious C-0 bonds and should have a somewhat greater than 2:l ratio of trans to gauche torsional configurations. The much greater flexibility undoubtedly contributes significantly to the unique rheological properties observed in linear perfluoropoly(alky1 ethers).2 Acknowledgment. This research was sponsored in part by the Air Force Office of Scientific Research/AFSC, United States Air Force, under Contract F49620-90-(2-0076. The authors wish also to acknowledge the Robert A. Welch Foundation (Grant B-657) and the UNT Faculty Research Fund for partial support of this project. The authors wish to thank Dr. Harvey L.Paige of the Materials Directorate, Wright Laboratory (WrightPatterson AFB), for many helpful discussions and for aid in obtaining supercomputer services. The authors also wish to thank Prof. Roderick Bates of UNT for helpful discussions on the “anomeric effect”. References and Notes (1) (a) Hennings, J.; Lotz, H. Vacuum 1977,27,171. (b) Luches, A.; Provenzano, I. J. Phys. D Appl. Phys. 1977,10, 339. (c) Lawson, D. D.; Moacanin, J.; Scherer, K. V., Jr.; Terranova, T. F.; Ingham, J. D. J . Fluorine Chem. 1978,12,221.(d) Laurenson,L.; Dennis,N. T. M.;Newton, J. Vacuum 1979,29,433. (2) (a) Snyder, C. E., Jr.; Dolle, R. E., Jr. ASLE Trans. 1976,19,171. (b) Capriccio, G.; Corti, C.; Soldini, S . Carniselli, G. Ind. Eng. Chem. Prod. Res. Dev. 1982,21,515.(c) Snyder, C. E., Jr.; Gschwender, L. J.; Tamborski, C. Lubr. Eng. 1981,37,344.(d) Snyder, C. E., Jr.; Gschwender, L. J.; Ind.

Stanton and Schwartz Eng. Chem. Prod. Res. Dev. 1983,22,383. (3) Ab Initio Study of Molecular Geometry and the Torsional Potential in Perfluoroethylmethyl Ether. Stanton, C. L.; Paige, H. L.; Schwartz, M. J. Phys. Chem., submitted. (4) Flory, P. J. S?a?is?ical Mechanics of Chain Molecules; Wiley-Interscience: New York, 1969. (5) (a) Tvardka, I.; Bleha, T. J. Mol. Struc?. 1975,24,249.(b) Wolfe, S.;Whangbo, M.-H.; Mirchell, D. J. J. Carbohydr. Res. 1979,69,1. (c) Jeffrev. G. A,: Poole. J. A.: Binklev. J. S.: Vishveshwara. S . J. Am. Chem. Soc. l978,100;37j.(d) VanAlseno~,’C.;Schaefer,L.;S~le,N.; Williams, J. 0. THEOCHEM 1981,3,1 1 1. (6) Wibern. K. B.; Murcko, M. A. J. Am. Chem. Soc. 1989,111,4821. (7) (a) Diongchamps, P. Stereoelectronic Effects in Organic Chemistry; Pergamon: Oxford, 1983,Chapter 2. (b) Kirby, A. J. The Anomeric Eflecr and Relared Stereoeleclronic Effecrs a? Oxygen; Springer: Berlin, 1983.(c) Petillo, P.; Lerner, L. In The Anomeric Effect; Thatcher, G., Ed.; American Chemistry Society: Washington, D.C., in press. (8) (a) Brunck, T. K.; Weinhold, F. J. Am. Chem. Soc. 1979,101,1700. (b) Reed, A. E.; Weinhold, F. Isr. J. Chem. 1991, 31, 277. (9) Pacansky, J.; Miller, M.; Hatton, W.; Liu, B.; Scheiner, A. J . Am. Chem. SOC.1991, 113, 329. (10) (a) Kollman, P. Annu. Reo. Phys. Chem. 1987, 38, 303. (b) McCammon, J. A.; Gelin, B. R.; Karplus, M. Narure 1977,267,585.(c) Van Gunsteren, W. F.; Berendsen, H. J. C. Angew. Chem., In?. Ed. Engl. 1990, 29,992. (1 1) Gaussian 90;Revision F.; Frisch, M. J.; Head-Gordon, M.; Trucks, G. W.; Foresman, J. B.; Schlegel, H. B.; Raghavachari,K,; Robb, M.; Binkley, J. S.;Gonzalez, C.; Defrees, D. J.; Fox, D. J.; Whiteside, R. A.; Secper, R.; Melius, C. F.;Baker, J.; Martin, R. L.; Kahn, L. R.;Stewart, J. J. P.; Tolpiol, S.;Pople, J. A,; Gaussian, Inc.: Pittsburgh, PA, 1990. (12) Pulay, P. In Applications ofElectronic Structure Theory; Schaefer, H. F., 111, Ed.; Plenum Press: New York, 1977;p 153. (13) (a) Dixon, D.A.; Fukunaga, T.; Smart, B. E. J. Am. Chem. Soc. 1986,108,4027.(b) Dixon, D. A. J. Phys. Chem. 1988,92,86.(c) Smart, B. E.; Dixon, D. A. J. Fluorine Chem. 1992,57,251. (14) (a) Hehre, W. J.; Ditchfield, R.; Pople, J. A. J . Chem. Phys. 1982, 56,2257. (b) Hariharan, P. C.; Pople, J. A. Theor. Chim. Acra 1973,28,213. (15) Frisch, M. J.; Pople, J. A.; Binkley, J. S.J . Chem. Phys. 1984,80, 3265. (16) Msller, C.; Plesset, M. S . Phys. Reu. 1934,46,618. (17) Stationary pointsfor which either or bothof thedihedral angleassume the El or E2 eclipsed configuration correspond to transition states on the potential energy surface. (18) (a) Dixon, D. A.; Van Catledge, F. A. In?. J. Supercompur. Applic. 1988,2, 52. (b) Dixon, D. A. J. Phys. Chem. 1992,96,3698. (19) Foster, J. P.; Weinhold, F. J. Am. Chem. Soc. 1980,102,7211. (20) Hehre, W. J.; Radom, L.; Schleyer, P. v. R.; Pople, J. A. Ab Initio Molecular Orbital Theory;John Wiley and Sons: New York, 1986;Chapter 6. (21) In the T-T[A] conformation of PFDMOM. q(CI) - q(O1) = +1.35 - (-0.69)= +2.04 and q(CZ).- q(Ol) = +1.38 - (-0.69)= +2.07. The equivalent charge differences in DMOM are much smaller, with q(C1) q(O1) = -0.18 - (-0.59)= +0.41 and q(C2) - q(O1) +0.35 - (-0.59) +0.94 [Stanton, C. L.;Schwartz, M., unpublished work]. (22) Astrup, E.E.Acta Chem. Scand. 1973,27,3271.

-

-