The anomeric effect in trifluoromethoxy methane, CF3OCH3

Journal of

MOLECULAR STRUCTURE ELSEVIER

Journal of Molecular Structure 376 (1996) 217-228

The anomeric effect in trifluoromethoxy Rita

methane,

CF30CH3’

Kiihn”, Dines Christena,*, Hans-Georg Mack”, Detlev Konikowskib, Rolf Minkwitzh, Heinz OberhammeP* “hrstitut fiir Physikalische und Theorrti.vche Chenrir, Universitiit Tiihingerr, 72076 Tfibirrgerr, Gurrmrr,~ “Inslim fiirAnur,yuni.xhr Chrmie, Univzrsitiit Dor/mund, 44227 Durtmund, GWFWZJ

Rcccivcd 1 June 199.5: acccptcd 29 June 1995

Abstract

The molecular structure of trifluoromethoxy methane was determined by a joint analysis of gas diffraction intensities and rotational constants, incorporating a normal coordinate analysis, in order to test the predictions of the stereoelectronic effects model. the generalized anomeric effect. Due to the internal rotation of the methyl group, some lines were split and the torsional barrier, V3(CH3) = 382 (10) cm-‘, could he determined. The rotational constants are: B, = 3069.253 (48) and C, = 3045.581 (48) MHz. Ab inilio calculations at diffefent levels of theory helped to interpret the data. The following structural parameters were derived (r; parameters in A and bond angles in degrees): O-C,,: 1.426 (9); 0-CF: 1.347 (9); C-F,: 1.318 (7); C-F,: 1.336 (5); (C-H),,,,: 1.095 (24); COC: 115.5 (4); (FCF),,,,: 107.7 (4); (HCH),,,,,,,: 111.1 (19); OCF,: 109.0(8); OCF,: 112.3 (7). The structure fully confirms the predictions of the anomeric effect model.

1. Introduction

Stereoelectronic effects arc known to have a strong influence on the conformational properties of molecules which contain the segment R-XC-Y, where X denotes an element which possesses one or more lone pairs (e.g. N, 0, P, S) and Y denotes an electronegative atom or group (e.g. F. Cl, OH). This phenomenon is called the generalized anomeric effect [l&3]. A widely accepted origin of this effect is negative hyperconjugation, i.e. orbital interaction between the lone pair orbital, n,, of X and the antibonding Q* orbital of the C-Y bond. This effect leads to a strong preference for the trans * Corresponding authors. ’Dcdicatcd to Profeswr James E. Boggs on 75th

the ~ccasmn

orientation of the C-Y bond relative to n.,. If X is oxygen, this corresponds to a gauche orientation of the C-Y bond relative to the X-R bond. Extensive ab initio calculations have been performed for fluoromethanol, CH,FOH [4-71, and methoxymethyl fluoride. CH,FOCH, [8&9]. In all cases, the gauche conformation is predicted to be stabilized relative to the trans structure by 4 5.5 kcal mol-‘, depending on the computational method. The theoretically predicted gauche structure was confirmed experimentally in the case of

of his

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0022-2860/96/$1S.O0 c, 1996 Elscvicr Science B V. All rights reserved SSDI 0022-2860(95)09040-l

,/“\c/H

/OAc

lF

A%%H

A\,

F

H

218

R

/OAc,H

R. Kiihn e, ui./Journal

f

R/

of’~~iolecular Srru-lure

6

y/n F-

%, H

CHzFOCHs by microwave (MW) spectroscopy [lo]. No rotational transitions belonging to the trans form could be identified. In addition to this influence on conformational properties, the anomerit elfect also influences bond lengths and bond angles. These structural effects are best rationalized by the mcsomeric structure for ncgative hyperconjugation. From this no + n* (C-F) orbital interaction, which leads to a double bond-no bond mesomeric structure, we expect a shortening of the 0-CF bond, a lengthening of the C-F bond and an increase in the O-C-F bond angle. Relative to the trans form, where the anomeric effect is absent. ab initio calculations (HF/4-21G) [6] for CHzFOH indeed predict a shortening of P-CF by 0.014 A, a lengthening of C-F by 0.020 A and an increase in the O-C-F angle by 4” in the gauche conformer. A much stronger inlluence of fluorination vn the O-C bond has been observed for CH2FOCH2. where the MW investigation results in a difference between the two O-C bond jengths: AOC = (0-Cu) - (O-C,) = 0.062 (8) A [lo]. This shortening of the 0-CF bond relative to 0-C” is partly due to the anomeric effect and partly due to electrostatic interactions, Since the CF atom is positively polarized an attractive Coulomb interaction with the negatively polarized oxygen atom is present for the O-Ck bond, whereas electrostatic interaction is negligible for the 0-Crt bond. The ab initio calculations for gauche- and tramCH,FOCH3 allow a discrimination of these two effects, since the anomeric effect is present only in the gauche form, whereas the electrostatic interaction is present in both conformers. Depending on the calculational method, the bond length difference AOC is predicted to be 0.04660.054 A in the gauche form and 0.02660.035 A in the trans form. From these numbers we conclude that both effects contribute about equally to the shortening of the O-CF bond.

376 (1996)

217-228

In the present study we report an experimental structure determination of CFsOCHs. Small differences between bond lengths, AOC = (O-C,)(O-C,) and ACF = (C-F,) - (C-F,), cannot be resolved by gas phase electron diffraction (GED) with the desired accuracy. Therefore, the microwave spectrum was analyzed and the geometric parameters were determined by a joint analysis of GED intensities and rotational constants. Furthermore, a harmonic force field was derived from an analysis of the vibrational spectrum. The experiments are supplemented by ab initio 90 calculations. The program system GAUSSIAN [l l] was used for full geometry optimization at the HF/3-21G, HF/6-31G** and MP2/6-3lG** levels. The results are included in Table 6. The vibrational frequencies were calculated with the MP2/6-31G* method and are given in Table 2.

2. Experimental CSOCF~ [12] was synthesized according to the literature and subsequently purified. (CHs)zSO? (Fa. Merck) was used without further purification. The syntheses were carried out on a glass vacuum line. Similarly to the description in Ref. [13]. 1 g (4.59 mmol) CsOCFs was brought into a glass tube (V = 35 ml) together with 5 ml (CH3)zS04 at 77 K. After sealing in vacua, the tube was left for 4 days at room temperature in order to react. The ether was condensed at 203 K (yield: 80%). The GED intensities were recorded with a Gasdiffractograph KD-G2 [14] at 25 and 50 cm nozzle-to-plate distances and with an accelerating voltage of N 60 kV. The sample was kept at -90°C during the experiment whereas the inlet system and nozzle were at room temperature. The photographic plates were analyzed by the usual procedures [ 151 and averaged molecular Ointensities in the scattering ranges 2-18 and 8-35 A-’ in steps of As = 0.2 A-t are shown in Fig. 1. The microwave spectrum was recorded in the spectral range 10-40 GHz using a conventional Stark spectrometer at a modulation frequency of 50 kHz. The absorption cell was cooled to -40°C and the pressure in the cell was approximately

R. Kiihn el al.lJoumal

of Molecular

Structure

219

376 (1996) 217 228

3. Rotational spectroscopy

I

0

J

5

10

15

20

25

30

35

S/k Fig. 1. Experimental (circles) and theoretical (solid line) modified molecular scattering intensities for CF,0CH3.

20 mTorr. The compound is very stable under these conditions and had only to be exchanged once or twice per full measuring day. Gas phase IR spectra were recorded with a Bomem MB-100 spectrometer at a pressure of 2 Torr at room temperature, whereas Raman spectra were recorded on the neat liquid at a temperature of -78°C. No polarisation data were collected.

The pa spectrum shows the typical traits of a near prolate rotor spectrum with widely separated K_, = 1 and K_, = 0 transitions as well as higher K_, transitions collected in the middle of this structure. Measurements at higher and lower Stark fields enabled the assignment of the Km, = 0 In order to identify the K_, = 2 transitions. transitions, a radio frequency-microwave double resonance experiment (RFMWDR) was performed, applying the r.f. signal to the Stark septum of the microwave cell. An attempt to assign pLc transitions using microwave+microwave double resonance techniques (MWMWDR), pumping already identified pL, transitions, failed, probably due to the small c component of the dipole moment. Trifluoromethoxy methane is, as already mentioned, a near prolate rotor (&,= -0.98) and the assignment was straightforward except for the complications arising because of internal rotation of the methyl group. A preliminary estimate of the torsional barriers of the methyl- and trifluoromethyl groups using the force constants derived from the normal

Ground State

ICF,

ICH,

Fig. 2. The rotational

transition /

: 2 + 1of CF30CH3

220

R. Kiihn

et ai./Journalof Molecular

Structure

376 (1996)

217-228

Stark:100V/cm Stark:800 V/cm Y RFMWDR: 2.6

ICH,

MHz.IS dbm

Fig. 3. The central part of the J 4 t 3 rotatuxnal transition. RFMWDR signals (pump frequency: 2.6 MHz, power: 15 dbm)

coordinate analysis (see purely cosine potentials:

below)

and

assuming

f,= 9v3/2 yielded the following CII,

barrier

heights:

: fr = 0.05 N cm L 7.25 kcal mall’ + P’s = 1600 cal mol-’

L 560 cm-’

CF, :.f, = 0.035 N cm & 5.07 kcal mall’

+

Vs

= 1100 cal mol-’ & 385 cm-’ Because of the large moment of inertia of the trifluoromethyl group, this second torsional barrier prohibits tunneling. The barrier for the methyl torsion is higher than that for trifluoromethyl, but

Stark

spectrum

at 100 V cm-’ and 800 V cm-’ Stark

fields and

because of the small moment of inertia for the methyl group the problem of tunneling must be addressed. Because of the arguments above, trifluoromethoxy methane can, however, be treated as a one-top tunneling problem, and theoretical calculations were performed using the program ONETOP from the microwave group at ETH, Zurich [ 161. In this simple semirigid model for the dynamical characteristics of the torsional/rotational problem, a rigid methyl top with CjV symmetry rotates against the rigid CF3-0 frame (with C, symmetry). The coordinate system chosen for the description of the kinetic energy is the principal axis system. The choice of axes is determined by the mirror plane, in which the torsional axis lies (in this case, the ac plane). In this coordinate system, the kinetic energy

R. Ktihn er al.~Journal Table 1 Measured

MW transitions

Transition

III CF,OCH;

Ground

of Molecular

state

A

E

42 - 111 202- 101 21, - 110

12206.29 12229.54 12253 37

12210.23 12229.54 12249.05

12200.29 12225.73 12250.65

313 303 322 32, 312

+ -

21: 202 221 220 211

18309.16 18343.85 18345.65 18345 65 18379.79

18310.53 18343.85 18345.65 18345.65 18378.72

414 404 432 431 42, 42: 413

+ + +

31, 301 331 33u 322 321

24412.02 24457.72 24459.69 24459.69 24459.49 24461.01 24506.47

414 404 542 + 441 541 t 440 533 + 432 57: + 431 524 42, 523 - 42: 514 + 413 616 +- 51, 606 + 505 643 + 542

5“5 -

642 +

541

64

-

533

63;

-

512

6 IS - 524 6 24 t 521 61, + 514

221

-717-228

6cot

TCF,

Tw, E

515 -

376 (1996)

(MHz)

A

312

Srrucrure

A

E

A

E

12238.14 12204.41 12242.93

12211.51 12231.19 12250.65

12215.46 12231.19 12247.46

12234.20 12258.90

12213.53 12234.20 12254 40

18300.68 18337.30 18337.30 18337.30 18375.30

18359.48 18305.97 18350.94 18350.94 18364.39

18317.75 18346.53 18348.09 1834X.09 sh

_

18313.58 18351.18 18350.94 18351.18 18389.27

18316.57 18351.18 18351.18 18351.1X 18387.99

24412.02 24457.12 24459.69 24459.69 24459.49 24459.49 24505.63

24401.31 24444.65 24451.20 2445 1.20 24450.15 2445 1.20 24500.77

24475.00 _

24423.18 24461.12

24465.54 24467.14 24467.50 24469.11 24486.63

24423.18 24461.12 _

24462.33 24464.29 24500.77

24464.29 24464.29 24500.77

24419.1 I 24467.26 24468.26 24468.26 24468.26 24469.11 245 17.09

24419.11 24465.54 24468 26 24468.26 24468.26 24468.26 24514.72

30515.43 30570.65 30575.20 30575.20 30575.20 30575.20 30573.Y2 30577.52 30632.10

30514.62 30570.65 30575.20 30575.20 30575.20 30575.20 30575.20 30575.20 30631.57

_

30528.02 30517.22

30528.02 30575.20

30625.53

30599.63 30505.64 30579.69 30584.19 30579.69 30584.69 30584.69 30587.19 30606.72

30579.87 30579.87 X1577.52 30579.87 30626.68

30579.87 30579.87 30579.87 30579.87 30625.53

36618.44 36682.63 36691.13 36691.13 36691.13 36691.13 36688.04 36694.33 36758.43

36617.10 36682.63 36691.13 36691.13 36691.13 36691.13 36691.13 36691.13 36757.70

36597.55 36668.32 36676.15 36676.15 36676.15 36676.15 36674.00 36682.63 36750.34

36722.22 36609.92 36694.33 36698.29 36698.29 36698.29 36698.29 36704.09 36730.33

36634.X9 36694.33 36694.33 36694.33 36694.33 36693.20 36698.29 36750.34

36634.08 36688.04 36688.04 36694.33 36694.33 36694.33 36694.33 36694.33 36750.34

170f30

5475.0a 3067.660 (193) 3048.052 (188) 382 mm a 62f 10

5475.0a 3069.253 (48) 3045.581 (48) 382 f 10 cm 0.0

A B C v3 (cII3) w (cm-‘)

30558.51 30563.81 30563.81 30563.81 30563.81 30561.19

-I

-

18346.53 18348.09 18348.09 18375.89

5475.0a 3070.853 (70) 3046.340 (72) 382 arm’ ’ 280 f 30

a Fixed

takes the following

form:

with gaa(a. = a, 6, c) indicating the principal moments of inertia; g, is the moment of inertia of the methyl top and the non-diagonal coupling elements are simply the products of g, with the particular direction cosines. Because the hamiltonian matrix is diagonalized (and not solved by perturbation techniques), the choice of axis system is not

222

R. Kiihn et al./Journal

Table 2 Fundamental

of Molecular

Structure

376 11996) 217-228

frequencies of trifluoromethoxy methane

Assignment

Calculated

Ab initio MP2/6-3lG**

Vib. energy dust.(%)

_

2978 28X0 1465 I326 1300

3294 3156 IS74 1536 1365

1248 w PQR 1171 YS PQR 1069 m PQR 846 w PQR

1254 w 1176~ IO60 \v X46 vs 666 In

1249 1168 106X X42 669

1312 1224 1115 X63 667

590 w PQR 447 w PQR _ 2976 m 1468 m 1173 “S 1171 YS

594 m 452 m 280 w 2988 s 1478 In 1176~ 1176 w

589 452 271 2974 1471 117R 1168

591 449 275 3259 1558 1238 1208

616 w 438 w

622 w 440 xv

614 440

620 433

CH(99.8) CH(98.0) HCH(7 1.2) OCH(79.7), HCH(48.7) CF(34.0), HCH(32.4), 0CH(26.0) CO(60.7), CF(23.2) OCH(56.0), CF(33.H) CO(59.S) CF(105.7) COC(ZS.S), CF(23.3), FC(19.4) FCF(74.1) OCF(30.3), FCF(26.1) OCF(65.0), COC(31.4) CH(l00.1) HCH(79.5) CF(86.5), 0CH(25.8) OCH(77.6), CF(28.1). HCH(21.8) 0CF(67.4) OCF(83.3)

170 63

172 65

Observed IR

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

vCH “CH kH, &HI “CF

“OC, PCH,

“OC, “CF kr? kF, PCF,

koc “CII kH3

wr f’CHx h-3 PCF,

2976 m PQR 2880 m PQR I468 m PQR I326 m PQR 1299 vs PQR

2988 s 2880 s 1478In

TCHj 7CF,

very important and the problem could just as well have been described in an internal axis system. Centrifugal distortion effects were not considered. A glance at Fig. 2 (the J : 2 + 1 transition of CF30CH;) shows to what extent vibrational excitation and torsional tunneling are encountered. The resolution is not sufficient to resolve the K_r = 0 torsional splitting, but the K_t = 1 transitions are split by several megahertz. The torsional splitting of the ground and excited states is calculated simultaneously by diagonalization of the (truncated) hamiltonian matrix. Rotational transitions of other vibrationally excited states are also present in the spectrum, most obvious at the K , = 0 transitions, where a comparison of the relative intensities yields vibrational frequencies of 63, 170, and 280 cm-’ for the three lowest energy states. The assignment of transitions belonging to these states gets increasingly difficult by way of transitions involving higher J states. because higher K_, transitions then

TCH,(9s.o) 7CF1 (97.2)

destructively overlap. The complexity of this structure is seen in Fig. 3: which shows the central part of the J : 4 + 3 transition. The K_, = 0 lines disappear at lower Stark fields. The Km, = 2 lines, however, can only be elucidated with the help of RFMWDR spectroscopy. of the ground- and CFsThe K-, = 2 transitions torsional states are readily picked out using this technique. The pump frequency was 2.6 MHz, the calculated asymmetry splitting of the K~ I = 2 levels of the J = 4 state. The remaining lines belong to the transitions of higher K_, values. The assignment of the spectrum is in excellent agreement with the theoretical model and could be conhrmed, using MWMWDR spectroscopy, by pumping the ,u,J : 3 + 2 transitions around 18.3 GHz and searching for the J : 4 + 3 transitions around 24.4 GHz. The purpose of this search was really to locate pLc transitions in order to refine the A rotational constant. It was, however, impossible to find these

R. Kiihn et nl./Journal

of Molecular

Structure

376 (1996) 217-228

223

Table 3 Force field of CH,OCF,

(N cm-‘)

CF30CHS c-o c-o-c C-H O-C-H H-C-H

5.625 2.203 4.132 0.930 a.543

TCH,

u.oso

C-F O-C-F F-C-F 7WJ CFjCF CFjOCF OCF/CF CH;OCH CFIFCF CFIFCF’ OCH/HCH OCH/HCH’ OCHiHCH OCII/OCF

CH30CH1 a

CF30CFjh

5.421 1.123 4.619

5.18 1.54

CF3CHCHzc

0.552 0.102

5.808 1.006 2.173 0.034 0.827 0.575 1.657 0.220 0.976 0.344 0.132 -0.082 -0.347 0.076

6.40

6.201 0.910 1.725 0.025 0.671 CF/CCF 0.894 CC/CF 0.475

CCF

0.86

0.584

0.183

0.14

a Ref. [18]. b Ref. [19]. ’ Ref. [20].

lines, but the pL, lines that were seen using DR techniques at least confirmed the assignment of the pa spectrum. The measured transitions are collected in Table 1

together with the resulting rotational constants, the torsional barrier heights and the frequencies of the three lowest vibrations. The A rotational constant was calculated from the derived rotational constants B and C using the inertial defect calculated from the electron diffraction model.

4. Vibrational spectrum and force field

0

1

2

R/A

3

4

5

Fig. 4. Experimental radial distribution and difference curve for CF30CH,.

In order to perform a simultaneous tit of the structural parameters to rotational and diffraction data, the vibrational corrections for interatomic distances, Ar = r, - rzr and for rotational constants, AB’ = tii - B:(B'= A, B,C),must be calculated from the harmonic force field. Therefore, the vibrational spectrum was recorded and analyzed, and a normal coordinate analysis was persued. The structural data for trifluoromethoxy methane indicate C, symmetry. One thus expects 21 fundamental frequencies that divide themselves into 13 of species a’ and eight of species a”. The lowest and highest axes of inertia lie in the mirror

224 Table 4 Interatomic

R. Kiihn et al.lJournal

distances.

C-H C-F, C-F, O-CF o-c” F, F, F,...F, O..‘F, O..‘Fs c...c C F, C F,

vibrational

amplitudes,

vibrational

Distance

Amplitude

1.08

o-075<

of Molecular Structure376 (1996) 217-228

corrections,

(ED)

133 1.35 1.34 1.44 ) 2.17 2.18 2.17 2.24 2.33 2.76 3.49

0.047(6) (a,)

0.053(8) (a>)

0.060’ 0.117(22) (az) 0.069(27) (a,,)

A?’ and corrected Amplitude

rotational

(vib.)

0.079 0.047 0.047 0.048 0.046 0.052 0.052 0.059 0.059 0.061 0.178 0.062

constant?

(233 K)

Ar = r0 - rZ U.UUY8 u.0007 0.0003 0.0024 0.0024 0.0002 0.0002 0.0002 0.0006 0.0005 -0.0027 0.0000

A?: 5471.4 &: 3067.34 (30) c,: 3047.70 (30) a Values in kgstriim. Error limits are 30 values. b Values in megaherrz. ’ Not refined.

therefore the a’ fundamentals should show IR contours that are A/C hybrids, whereas the a” modes should show B contours. The measured frequencies are collected in Table 2 with an indication of the relative intensity and the presence of PQR structure. The IR and Raman frequencies agree fairly well with one another. Below 200 cm-i no trustworthy spectra could be recorded. For these transitions (CF? and CHs torsions), the frequencies were estimated from the relative intensities of the microwave transitions of the respective excited states (see Table 1). The assignment of the vibrational spectrum was on the whole quite straightforward, only the high concentration of lines around 1200 cm-’ (CH, rockings as well as CFs and CO stretching vibrations) caused difficulty and a substantial coupling of the internal modes of the u’ species. There was no coupling of the a” internal modes. The normal coordinate calculations were carried out using valence type force constants in a basis of mass-weighted Cartesian coordinates [17]. Two fattors complicated these calculations. Most of all the lack of data: 21 measured frequencies compared with 73 different elements of the force field (assuming equivalence of both CO stretching constants, all CH and CF valence constants, as well as plane,

equivalence of different groups of deformation modes), as well as the strong mixing of internal vibrational modes, which resulted in highly significant contributions to the Jacobian matrix from several interaction constants. This mixing was also responsible for drastic changes in the vibrational energy distribution upon minute changes of the interaction constants. Thus a “reasonable” energy distribution was used as an extra criterium to decide the goodness of the fit, although even so, not all vibrational energy contributions appear quite satisfactorily. A small displacement of the normal frequencies will in most cases, however, lead to a potential energy distribution in full accordance with the assignment. For comparison, the relevant parts of the force fields of dimethyl ether [lg], perfluoro dimethyl ether [19], and 3,3,3_trifluoropropene [20] have also been listed in Table 3 together with the derived force field for trifluoromethoxy methane.

5. Structure analysis The structure determination was begun with the analysis of the GED intensities. The radial distribution function is shown in Fig. 4. In the

al 02 03 a4

(C-H), c-0-c (F-C-F), H-C-H tilt(CHJ

(C-F), AC-F

AO-C

co-Ch

Correlation

Table 5

1.000 -0.822 -0.957 0.161 po.1o9 PO.567 -0.794 -0.227 PO.294 -0.885 -0.611 po.1o4 -0.003

(O-C),

matrix

1.000 0.969 0.002 0.101 0.393 0.743 0.258 0.436 0.678 0.63 1 0.101 0.008

AO-C

1.ooo -0.059 0.097 0.469 0.821 0.218 0.357 0.794 0.586 0.102 0.004

(C-F),,,

1.000 -0.091 -0.176 -0.339 0.420 0.663 -0.362 0.064 0.027 0.013

AC-F

1 .ooo 0.095 0.021 -0.421 0.207 0.128 0.131 -0.067 0.00 1

(C-H),,,

0.258 -0.299 0.288 0.542 0.519 -0.022 0.00 1

1.000

C-O-C

1.000 0.430 0.071 0.419 0.158 0.020

0.749 0.090 0.070 -0.012

H-C-H

PO.061 -0.193

(F-C-F),,,

0.050 0.024

0.058 0.691

1.000

tilt(CF,)

z 2 ?

5

3

0.082 0.021

7

s ;

I.000 0.000

d 1.000

04

;:

03

1.000 1.ooo

u2

0.449 0.078 -0.003

u,

z ” 9 -1.

6

3

226 Table 6 Geometric

R. Kiihn et al.!Journal

parameters

ofA4olecular

Structure

376 (1996)

217-228

of CF30CH3

Parameter

ED + MW=

HF/3-21G

HF,‘6-31G**

MP2/6-31 G**

(O-C),,,,, AOC = (O-C,) - (O-C,) o-c” O-CF (C-FL,, ACF = (C-F,) - (C-F,) C-F, C-F, (C-HLw c-o-c (F-C-F),,,, AFCF = (F,-C-F,) - (Fs-C-F,) F,-C-F, F,-C-F, (H-C-H),,,, O-C-F, O-C-F, tilt (CF,) tilt (CH,)

1.386 (6) 0.079 (12) I .426 (9) 1.347 (9) 1.330 (4) 0.018 (7) 1.318 (7) 1.336 (5) 1.095 (24) 115.5 (4) 107.7 (4) 2.0 [5]b 108.4 (5) 106.4 (6) 111.1 (19) 109.0 (8) 112.3 (7) 2.2 (4) 3.3 [5]b

1.399 0.126 1.462 1.336 1.342 0.020 1.328 1.348 1.077 118.0 107.7 2.2 108.4 106.2 110.0 109.7 112.0 1.5 3.0

1.312 0 095 1419 1.324 1.318 0.016 1.307 1.323 1.080 117.2 107.7 1.7 108.3 106.6 111.0 108.9 112.4 2.3 3.3

I.391 0.093 1.438 1.345 1.346 0.020 1.333 1.353 1.084 114.2 107.8 2.2 108.5 106.3 110.3 108.3 112.5 2.8 3.5

a rz parameters in gngstrBm and bond angles in degrees. Error limits are 30 values. b Fixed value with estimated uncertainty in square brackets.

least-squares refinement the intensities were modified with a diagonal weight matrix and known scattering amplitudes and phases were used [21]. The difference between the C-F bond distances could not be resolved in this analysis and the difference between the O-C bond leqgths is not well determined (AOC = 0.077 (46) A, the error limit is the 3~ value). In the next step, the rotational constants Bi were included in the least-squares refinement. The relative weights between rotational constants and GED intensities were adjusted so as to fit the rotational constants Pi within their estimated uncertainties. These uncertainties are set to 15% of the vibrational corrections AB’. The vibrational corrections for the interatomic distances, Ar, and the corrected rotational constants, Bi, are shown in Table 4. The contributions of torsional vibrations to Ar were neglected for torsionindependent distances. These vibrations cause unrealistically large contributions in the concept of perpendicular vibrations. The geometry of the CF, group is described by the mean bond length, the difference ACF = (C-F,)(C-F),,,,,

(C-F,), the mean F-C-F angle, the difference AFCF = (F,-C-F,) - (F,-C-F,) and a tilt angle, tilt = f [(O-C-F,) - (O-C-F,)]. Because of high correlations between some of these parameters, the difference AFCF could not be refined. Based on the ab initio calculations, this difference was fixed at 2.0” with an estimated uncertainty of *0.5”. The tilt angle for the CH3 group was constrained to the ab initio result. The vibrational amplitudes were collected in groups according to the spectroscopic values and some amplitudes were not refined (see Table 4). With these assumptions nine geometric parameters and four vibrational amplitudes were refined simultaneously. The correlation matrix is shown in Table 5. The final results are collected in Table 6 (geometric parameters) and Table 4 (vibrational amplitudes and spectroscopic data).

6. Discussion The experimentally determined trifluoromethoxy methane fully

structure confirms

for the

R. Ktihn et al./Journal

of Molecular

Table 7 Relevant geometric parameters (lengths in dngstriim, angles in degrees) for dimethyl ether and fluorinated derivatives Compound CH,-0-CH, CH*F-0-CH, CF3-0-CH,’ CF,-0-CF, CF3-0-Cle CF,-O-F’

o-CH a

1.415(l) b 1.426 (3) 1.426 (9)

d

a Ref [22]. ’ Ref. (251.

-

O-CF

1.368 1.347 1.369 1.365 1.395

X-O-Y

C-F

(7) (9) (4) (7) (6)

1.395 1.330 1.327 1.325 1.319

(5) (4) (2) (3) (3)

111.8 113.5 115.5 119.1 112.9 104.9

(2) (2) (4) (8) (5) (6)

[IO].‘This work. d Ref. [23]. ’ Ref. [24]. ‘Ref.

expectations for the influence of the anomeric effect formulated in the Introduction. The largest influence occurs for the O-C bond lengths which differc by AOC = (O-C,)(O-C,) = 0.079 (12) A. This difference is well reproduced by the ab initio calculations at higher levels but is overestimated by the HF/3-21G method. This bond length difference is sljghtly larger than that in CH2FOCH1 (0.062 (8) A). Quantitatively, the expected increase of hyperconjugation in CF30CH3 due to the presence of two gauche fluorines is compensated by the energy lowering of the oxygen lone pairs relative to those in CH2FOCH3. Thus, the net structural influence of the anomeric effect should be similar in both ethers, and the observed slight increase of the bond length difference is most likely due to stronger electrostatic effects in the title compound. The predicted lengthening qf the C-F, bonds relative to C-F, (0.016-0.020 A) is fully confirmed by the experimental structure (0.018 (7) A). The observed O-C-F angles also agree well with the expectations as well as with the ab initio calculations. The discrepancy which exists between the vibrational amplitude for the 0’ ‘. F, distance derived from the GED experiment and the spectroscopic data (see Table 4) is probably due to the “rigid” model used in the GED analysis. Table 7 compares geometric parameters which are affected by the anomeric effect for dimethyl ether and various fluorinated derivatives. The 0-C” bond lengths are equal in CH2FOCH3 and CF~OCHI and slightly longer than those in the parent ether. The 0-CF bonds in all fluorinated

Structure

376 (1996)

227

217-228

compounds are shortened relative to the 0-C” bonds, with-the shortest bond being in CF,0CH3 (1.347 (9) A), where the combined influence of anomeric and electrostatic effects is expected to be strongest. The redatively long 0-CF bond in CF,OF (1.395 (6) A) probably originates from reduced electrostatic interactions. the C-F bond length in CH2FOCH3 (1.395 (5) A) is typical for a singly substituted methyl group (see e.g. 1.391 (1) A in CHIF [26]. The C-F bonds in the CF3 groups show the expected trend for CF, compounds upon increasing electronegativity X [27]. The C-O-C bond angles in the ethers increase steadily with increasing fluorination from 111.8(Z)’ in CH;OCH3 to 119.1(g)” in CF30CF3.

Acknowledgement This work was Forschungsgemeinschaft.

supported

by

Deutsche

Reference [I] R.U. Lemieux and S. Koto, Tetrahedron. 30 (1974) 1933. [2] A.J. Kirby. The Anomeric Effect and Related Stereoelectronic Effects at Oxygen, Springer Verlag, Berlin, 1983. [3] P. Deslongchamps, Stereoelectronic Eflects in Organic Chemistry, Pergamon. Oxford, 1983. [4] L. Radom, W.J. Hehre and J.A. Pople, J. Am. Chem. Sot.. 94 (1972) 2371. [5] P.v.R. Schleyer and A.J. Kos. Tetrahedron, 39 (1983) 1141. [6] B.M. Pinto, H.B. Schlegel and S. Wolfe. Can J. Chem., 65 (1987) 1658. [7] G.F. Smits, M.C. Krool and C. Altona. Mol. Phys., 65 (1988) 513. [8] G.A. Jeffrey and J.H. Yates, J. Am. Chem. Sot.. 101 (1979) 820. [9] J.R. Durig. J. Lin, GA Guirgis and B.J. van der Veken, Struct. Chem., 4 (1993) 103. [IO] J. Nagakawa. H. Kato and M. Hayashi, J. Mol. Spectrosc.. 90 (1981) 467. [I l] M.J. Frisch. M. Head-Gordon. G.W. Trucks, J.B. Foresman, H.B. Schlegel. K. Raghavachari, M. Robb, J.S. Binkley, C. Gonzalez, D.J. DeFrees, D.J. Fox, R.A. Whiteside, R. Seeger, CF. Melius, J. Baker, R.L. Martm, L.R. Kahn, J.J.P. Stewart, S. Topiol and J.A. Pople, GI\USSIAN 90, Gaussian Inc., Pittsburgh, PA, 1990. [12] M.E. Redwood and C.J. Willis, Can. J. Chem., 43 (1965) 1893. [13] G.J. Martens, Chem. Abstr.. 64 (1966) 9595.

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Structure 376 (1996)

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[21] J. Haase, Z. Naturforsch, Teil A, 25 (1970) 936. [22] K. Tamagawa, M. Takemura, S. Konaka and M. Kimura, J. Mol. Struct., 125 (19X4) 131. [23] A H. Lowrey, C. George, P. D’Antonio and J. Karle. J. Mol. Struct , 63 (1980) 243. [24] H. Oberhammer, T. Mahmood and J.M. Shreeve, J. Mol. Struct.. 117 (1984) 311. [25] F.P. Diodati and L.S. Bartell, J. Mol. Struct., 8 (1971) 395. [26] T. Egawa, S. Yamamoto, M. Nakata and K. Kuchitsu, J. Mol. Struct. 156 (1987) 213. [27] V. Typke, M. Dakkouri and H. Oberhammer, J. Mol. Struct.. 44 (1978) 85.