J . Phys. Chem. 1987, 91, 1334-1 344
1334
Low-Resolution Microwave, Infrared, and Raman Spectra, Conformational Stability, and Vibrational Assignment of 2,2,2-Trif luoroethyl Methyl Ether Ying-Sing Li, Department of Chemistry, Memphis State University, Memphis, Tennessee 381 5 2
F. 0. Cox,+ and J. R. Durig* Department of Chemistry, University of South Carolina, Columbia, South Carolina 29208 (Received: August 6. 1986)
The low-resolution microwave spectra of 2,2,2-trifluoroethyl methyl ether, CF3CHzOCH3,and 2,2,2-trifluoroethylmethyl-d, ether, CF3CHZ0CD,have been recorded from 26.5 to 39.0 GHz. From the spacings of the major transitions it is shown that the values for B + Cof 3125 and 2839 MHz for the "light" and "heavy" molecules, respectively, are consistent with the trans conformer. A second set of transitions of approximately one-fourth the intensity of the major transitions were also observed which are shown to be consistent with those for the gauche conformer. The microwave data indicate that the trans conformer is the more stable one in the gas phase. The infrared (3500 to 40 cm-I) and Raman spectra (3500 to 20 cm-l) have been recorded for both CF3CH20CH3and CF3CH20CD3in the gaseous and solid states. Additionally, the Raman spectra of both molecules in the liquid phase have been obtained and qualitative depolarization ratios have been measured. A complete vibrational assignment is presented for the trans conformer based on group frequencies, isotopic shifts, depolarization values, and band contours. Additionally,several of the fundamentals have been assigned for the gauche conformer. Furthermore, the vibrational assignment is supported by a normal-coordinate analysis utilizing a modified valence force field to calculate the normal modes and potential energy distribution for both conformers. From the far-infrared spectra of both isotopic species, it is shown that the methyl and asymmetric torsions are strongly coupled. From the relative intensities of the Raman lines of the liquid at 848 (trans) and 828 cm-' (gauche) as a function of temperature, as well as the corresponding bands for the -d3compound, the average enthalpy difference was found to be 392 f 92 cm-' (1.12 f 0.25 kcal/mol) with the trans conformer being more stable. The trans conformer is the only one present in the spectra of the solids. The threefold barriers to methyl rotation were determined to be 1077 cm-' (3.08 kcal/mol) and 640 cm-' (1.83 kcal/mol) for the trans and gauche. The value for the barrier for the CF3 group was determined to be 1321 cm-l (3.78 kcal/mol) for the trans conformer. The CF, torsional mode for the gauche conformer was not observed. These results are compared to similar quantities in related molecules.
Introduction It has been proposed that the study of the intermolecular interaction between an anesthetic and some biomolecules might provide information on the mechanism of inhalation anesthesia. It is also believed that the interpretation of this interaction will depend not only on the molecular formula but also on the knowledge of the conformation and the structure of the anesthetic. It has been found that many fluorinated hydrocarbons and ethers possess anesthetic properties in laboratory animals as well as in humans.'S2 The 2,2,2-trifluoroethyl methyl ether molecule has been evaluated as a weak anesthetic in mice by Terrell et aL3 However, to our knowledge, there is no spectroscopic information on this molecule available in the literature. Due to the internal rotation of the methoxy group with respect to the C-C bond, there are two possible nonequivalent conformers for 2,2,2-trifluoroethyl methyl ether. As shown in Figure 1, the one with the symmetry plane is referred to as the trans form (Figure la) whereas the one without any symmetry except Cl is the gauche form (Figure lb,c). Therefore, from investigations of the low-resolution microwave spectrum, as well as the vibrational spectrum, it should be possible to identify the stable conformer or conformers present in the three different phases of this molecule. The results of our spectroscopic studies are reported herein.
Experimental Section The sample of CF,CH20CH3 was prepared from 2,2,2-trifluoroethanol, sodium, and methyl bromide by the Williamson's reaction as described by Henne and S m ~ o k .The ~ product was separated and purified on a low-temperature sublimation column, and identified by its infrared and proton N M R spectra. The 2,2,2-trifluoroethanoI was purchased from PCR Research Chemicals and the methyl bromide from Matheson Gas Products. The CF3CH20CD3was prepared by the same reaction except by +Taken in part from the thesis which will be submitted to the Department of Chemistry in partial fulfillment of the Ph.D degree.
using CD,Br (Merck, Sharp and Dohme, Canada) instead of CH3Br. The microwave spectra were obtained by using a HewlettPackard Model 8460A M R R spectrometer with a Stark modulation frequency of 33.33 kHz. Sample vapor pressure was maintained around 80 pmHg. The accuracy of the reported low-resolution frequencies is estimated to be 20 MHz. The mid-infrared spectra (4000-450 cm-l) were recorded on a Digilab Fourier-transform interferometer Model FTS- 14C equipped with a Ge/KBr beam splitter, a nichrome wire source element, and a TGS detector. Spectra of the gases were obtained by using a 10-cm cell fitted with KBr windows. The corresponding spectra of the solids were recorded on the same instrument by condensing the samples onto a CsI plate held at liquid nitrogen temperature and annealed until no further changes in the spectra were observed. The spectra of the gases were recorded with an effective resolution of 0.5 cm-l while those of the annealed solid were recorded at 1.0 cm-I resolution. Far-infrared spectra of the gaseous and solid samples were recorded with a Digilab Model FTS- 15B Fourier transform interferometer equipped with a 6.25-pm Mylar beam splitter and a high-pressure Hg arc lamp source. Atmospheric water vapor was removed from the spectrometer housing by purging with dry nitrogen. The spectra of the gases were examined by using the room temperature vapor pressure of the samples contained in a 12-cm cell fitted with polyethylene windows. Interferograms were recorded 2000 times with an effective resolution of 0.25 cm-I, averaged, and transformed with a boxcar truncation function for both the sample and empty cells. Spectra of the annealed solid were obtained by condensing the sample onto a silicon plate cooled ( 1 ) Krantz, Jr., J. C.; Rudo, F.G. In Handbook of Experimenral Pharmacology; Eichler, O., Farah, A., Herken, H., Welch, A. D., Eds.; Springer: Berlin, 1966; Vol. X X / l , pp 501-564. (2) Larsen, E. R. Fluorine Chem. Rea. 1969, 3, 1. (3) Terrell, R. C.; Speers, L.; Szur, A. J.; Treadwell J.; Ucciardi, T. R. J . Med. Chem. 1971, 14, 517. (4) Henne, A. L.; Smook, M. A. J . A m . Chem. SOC.1950, 72, 4378.
0022-3654/87/2091-1334$01.50/00 1987 American Chemical Society
The Journal of Physical Chemistry, Vol. 91, No. 6. 1987 1335
Spectra of 2,2,2-Trifluoroethyl Methyl Ether
TABLE I: Calculated Rotational Constants (MHz) of 2,2,2-Trifluoroethyl Methyl Ethef orientation angle (0): deg rotational const 0 30 60 90 120 CF3CH20CH3 A
B C --K
B+C 2A-B-C
4449.8 2336.4 2150.9 0.8386 4487.3 4412.3
4459.5 2266.3 2099.1 0.8583 4365.4 4553.6
4523.5 2088.8 1967.9 0.9054 4056.7 4990.3
4271.3 2213.7 2033.5 0.8389 4247.2 4295.4
4285.6 2137.7 1'977.2 0.8610 4114.9 4456.3
4358.9 1949.5 1836.8 0.9106 3786.3 493 1.5
4676.2 1879.2 1810.6 0.9521 3689.8 5662.6
150
180
4885.2 1704.4 1674.6 0.9814 3379.0 6391.4
5069.6 1598.1 1583.0 0.991 3 3181.1 6958.1
5143.7 1565.1 1549.1 0.991 1 3114.2 7173.2
4726.7 1559.4 1533.0 0.9835 3092.4 6361.0
4908.4 1454.4 1441.5 0.9926 2895.0 6920.9
4980.9 1421.6 1408.2 0.9925 2829.8 7132.0
CF3CH20CDS A
B C --K
B+C 2A-B-C
4517.5 1734.3 1672.2 0.9564 3406.5 5628.5
"Different conformers as resulting from the internal rotation of the methoxy group around the C-0 bond.
= Oo if the CH30 eclipses the C-C
bond, i.e., cis conformer.
-
(bl
I
(C 1
Figure 1. Newman projections of the trans (a) and gauche (b and c) conformers of 2,2,2-trifluoroethyl methyl ether.
with liquid nitrogen and contained in a cell fitted with polyethylene windows. The far-infrared spectra (370-40 cm-') of the vapors from which the torsional transitions were observed were collected on a Nicolet Fourier-transform interferometer Model 200 SXV equipped with a vacuum bench, a high-pressure H g arc source, and a liquid helium cooled Ge bolometer containing a wedged sapphire filter and polyethylene windows. A 6.25-hm Mylar beam splitter was employed and the sample was contained in a 1-m cell fitted with polyethylene windows. Interferograms were recorded 256 times with an effective resolution of 0.12 cm-' and were subsequently treated as for the spectra obtained on the Digilab interferometer. The Raman spectra were recorded from 3400 to 50 cm-' with a Cary Model 82 spectrophotometer equipped with a Spectra Physics Model 171 argon ion laser tuned to the 514.5-nm line. The instrument was calibrated against the emission lines of neon or mercury. Liquid samples were sealed in a glass capillary, and the method of Miller and Harney5 was used to obtain the spectra for both the liquid- and solid-phase samples. The depolarization measurements were made using the standard Cary accessories. The temperature of the solid was maintained at 213 K. The spectra of the gases were obtained by using the standard Cary multipass accessory. Microwave Spectra and Results The microwave spectra for both CF3CH20CH3andf CF3CH20CD3are shown in Figure 2. In both cases, four major bands ( 5 ) Miller, F. A,; Harney, B. M. Appl. Specfrosc. 1970, 24, 291.
U
-
L
2ao
I
I
1
32.0
1
I
36.0
I
FREQUENCY(GHz) Figure 2. Microwave spectra of CF3CH20CH3(upper trace) and CF,-
CH20CD3(lower trace). are observed to have nearly equal spacing between them. This is typical of a nearly prolate rotor with a-type transitions or a nearly oblate rotor with C-type transitions. In order to determine the type of transitions which were being observed, the rotational constants of 2,2,2-trifluoroethyl methyl ether were calculated for both the -do and the -d3 species by assuming a combination of the corresponding structural parameters of CF3CH36and CH3CH20CH3.' In the calculation, the methyl group was assumed to be symmetric with respect to the 0-C bond, and the CH30group was rotated with respect to the 0-C bond in increments of 30'. The results of the calculation are listed in Table I. A 0-deg orientation represents an eclipsed conformation in which the OCH3 bond eclipses the C-CF3 bond in the Newman projection along the C-0 bond. From the results of the calculation, it is seen that the value of K varies from -0.83 to -0.99 with the different angles of internal rotation. It is therefore concluded that the molecule must be a nearly prolate rotor and the four major bands observed are due to a-type transitions. Also from Table I it should be noted that the values of the rotational constants are very sensitive to the angle of internal rotation of the methoxy group. Since there is more than one possible conformer for the 2,2,2-trifluoroethyl methyl ether molecule, an analysis of the rotational spectrum should give a (6) Edgell, W. F.; Miller, G. B.; Amy, J. W. J . Am. Chem. SOC.1957, 79,
2391.
(7) Hayashi, M.; Kuwada, K. J . Mol. Struct. 1975, 28, 147
1336 The Journal of Physical Chemistry, Vol. 91, No. 6, 1987
Li et al.
TABLE II: Rotational Transitions (MHz) of Trifluoroethyl Methyl Ether
--
CF3CHZOCH3 v(calcd)b ( B + C)c
transitionD conformer
J
trans
+1
u(obsd)
9-8 10-9 I1 10 12-11 13 12
28 125 31250 34375 37500
28 125 31250 34 375 37500
J4 J6 - J , 8-7 9-8 10-9 11- I O 12- I1
31 694 38740 28394 31 944 35492 39041
31 695 38738 28394 31 943 35493 39 042
J5
gauche
J
v(obsd)
28392 31 231 34070 36909 31534 38541
2839.2 2839.2 2839.1 2839.2
7043.1 7043.6
28392 31 231 34069 36909 31534 38540 29609 32898 36 188 39478
29608 32898 36 188 39478
3289.9 3289.8 3289.8 3289.8
3125.0 3125.0 3125.0 3125.0
3549.3 3549.3 3549.2 3549.2
CF3CH20CD3 v(calcd)* ( B + qd
2A - B - C
2A - B - C
7007.6 7007.3
For the Q-branch transitions, only K-, is indicated as a subscript. bCalculated from the mean value of ( B + C ) or (2A - B - C). 'Average value of B + C is 3125.0 MHz for the trans conformer and 3549.25 MHz for the gauche conformer. dAveragevalue of B + C is 2839.18 MHz for the trans conformer and 3289.83 MHz for the gauche conformer. conclusive identification to the major conformation of the molecule in the gas phase. For a nearly prolate rotor in which the asymmetry parameter K is close to -1, a-type R-branch transitions ( J 1 J) may be expressed approximately by vJfl = ( J + 1) ( B + C), and the b-type and C-type Q-branch transitions ( J J for the upper K-, = K 1 and the lower K-l = K) by vK+I = ( K + '/,)(2A - B - C ) . From these expressions the separation should be equal to B + C for the R-branch bands and 2A - B - C for the Q-branch bands; a knowledge of the separations along with the observed frequencies should give sufficient information to assign the rotational transitions J and K-,, respectively. For this reason, the four major bands observed in the microwave spectrum of CF,CH20CH3at 28 125,31250,34 375, and 37 500 MHz are assigned to the J + 1 J transitions, 9 8, I O 9, 11 10, and 12 11, respectively (see Table 11). These assignments yield 3125.0 MHz for the mean value of B + C. In comparing the experimental value of B C with the calculated B + C listed in Table I, it is concluded that the four major bands must arise from the molecule in the trans form (0 = 180'). In addition to the four major bands, there are two bands observed around 3 1 694 and 38 740 MHz. The response of these two bands to the Stark voltage is different from that of the four major bands. Consequently, these two bands are assigned as b-type or C-type AK-, = 1 Q-branch transitions, J(K-, = 5) J ( K , = 4), and J(K-! = 6) J ( K - , = 5), respectively. The bandwidth of these lines results essentially from the partial overlap of the different J transitions as well as partial overlap of transitions in the excited vibrational states. Again, the assignments give a value of 7043.3 MHz for 2A - B - C which agrees with the corresponding value calculated for the trans conformer (see Table I). Based on our structural calculations, the trans conformer has a plane of symmetry formed by the a and b axes, and no C-type transitions would be expected for the trans conformer. Therefore these Q-branch bands must belong to the b-type transitions. Additional features from the low-resolution microwave spectrum of CF3CH20CH,are four equally spaced lines located at 28 394, 3 1 944, 35 492 and 39 041 MHz. These four lines are assigned to the 8 7, 9 8, I O 9, and 11 IO transitions, respectively (Table 11), and have intensities of about one-fourth of the corresponding transitions for the trans conformer. Since their separation, Le., B C, is much different from that for the trans form, these four weak lines must arise from another conformer. In comparing the experimental value of B C with those calculated for the differnt orientation angles (see Table I), the second conformer is identified as the conformation in which the methoxy group is rotated by approximately 100' from the eclipsed position. This value of the angle should be considered as only a crude approximation since the COC angle is expected to be significantly different for this second conformer relative to the value of this angle in the trans conformer. In the microwave spectrum of CF3CH20CD3,features similar to those for CF3CH20CH3were observed. Listed in Table I1 are the rotational assignments along with the experimental values of
+
-
+
+
-
-
- -
I
i
+
+
-
-
+
+
-
-
+
+
WAVENUMBER ( C d )
Figure 3. Raman spectra of (A) gaseous, (B) liquid, and (C) solid
CF3CHZOCHS.
B + C and 2A - B - C. As expected, the values for B + C for both the trans and the gauche conformers are smaller than the corresponding values in the -doisotopic species. The study of the -d3species serves to confirm not only the assignment for the -do species but also the existence of two conformers in the gas phase for the 2,2,2-trifluoroethyl methyl ether molecule. Vibrational Assignment From the low-resolution microwave study of 2,2,2-trifluoroethyl methyl ether two conformers have been shown to exist in the gas phase. The trans conformer possesses a plane of symmetry and belongs to the C, molecular point group. Its 30 fundamental modes will span the irreducible representations: 18 A' and 12 A". The A' vibrations will give rise to polarized Raman lines, and may have type A, B, or A/B hybrid contours in the infrared spectrum of the gas. Using the calculated rotational constants listed in Table I, the P-R separation for type A, B, and C contours of CF3CH,OCH, were calculated to be 11.2, 10.5, and 10.5 cm-', respectively. Similar magnitudes of separation were obtained for the corresponding CF3CH20CD, molecule. For the gauche conformer with only C1symmetry, all of the Raman lines for the normal modes will be polarized and the infrared bands will have hybrid type A/B/C contours. The Raman spectra of gaseous, liquid, and solid 2,2,2-trifluoroethyl methyl ether are shown in Figures 3 and 4 for the -do
The Journal of Physical Chemistry, Vol. 91, No. 6, 1987
Spectra of 2,2,2-Trifluoroethyl Methyl Ether
I
I
3OOo
I
2500
I
2000 1500 WAVENUMBER (CM-l)
1
x)o
1337
500
Figure 4. Raman spectra of (A) gaseous, (B) liquid, and (C) solid CF3CHZOCD3.
3000
Figure 6. Mid-infrared spectra of solid CF3CHzOCH3(A) and CF3CHZOCD3 (B).
500
3000
2000 1000 WAVENUMBER (cm-1) Figure 5. Mid-infrared spectra of gaseous CF3CHzOCH3(upper) and CF,CHzOCD3 (lower).
and -d3isotopic species, respectively. The mid-infrared spectra of both species in the gaseous and solid phases are shown in Figures 5 and 6, respectively, and the far-infrared spectra in Figures 7 and 8, respectively. Some of the Raman lines for the liquid were measured to be depolarized which supports the existence of the trans conformer in the liquid phase. The disappearance of the polarized Raman lines at 926, 828, 460, and 320 cm-' for the -do species and the polarized Raman lines at 917, 830, 385, and 310 cm-' for the -d3 species with solidification also indicates the existence of more than one conformer in the fluid phases, the result of which is consistent with that derived from the microwave study. Since some of the depolarized Raman lines remain in the spectra of the solids, we conclude that the trans conformer remains in the solid state. The subsequent vibrational assignment for the major bands is therefore made on the basis of C, molecular symmetry utilizing infrared bands contours, depolarization data, group frequencies, and iso-
2000 1000 WAVENUMBER (cm-l)
300
100
WAVENUMBER (cm-')
Figure 7. Far-infrared spectra of gaseous CF3CHz0CH3(A) and CF3CH,0CD3 (B). Spectra recorded above 375 cm-1 were recorded on a Digilab interferometer Model FTS-15B whereas the data below 375 cm-' were recorded on Nicolet Model 200 SXV interferometer.
topic shift data. It should be noted that initial assignments were made taking account of the spectral data for the solid, which are due to the trans conformer, with subsequent identification of the corresponding bands in the fluid phases. The observed frequencies and their proposed assignments are summarized in Tables I11 and IV, for the -do and -d3 species, respectively. Carbon-Hydrogen Modes. The carbon-hydrogen stretching modes can be assigned on the basis of the assignments for similar compounds such as 2,2,2-trifluoroethanoI8 and dimethyl ether9 along with the isotopic shift data. The two CH3 antisymmetric stretches of the methyl group are assigned to the strong C-type (8) Kalasinsky, V. F.; Anjaria, H. V. J . Phys. Chem. 1980, 84, 1940. (9) Levin, I. W.; Pearce, R. A. R.; Spiker, Jr., R.C. J . Chem. Phys. 1978,
68,347 1.
1338 The Journal of Physical Chemistry, Vol. 91, No. 6, 1987
Li et al.
TABLE III: Observed' Infrared and Raman Frequencies (cm-') and Assignment for CF3CH20CH3 infrared Raman re1 re1 re1 int, re1 assignment re1 gas int solid int gas int liq depol solid int calcd ui approx descripn* 3016 R 3017 s, p 3023 s 3013 u I CH, antisym str (99%) 3 0 1 2 Q ms 3026 ms 3014 vs 3011 Y I P CH, antisym str (99%) 3006 P 2985 sh 2987 s 3013 CH, antisym str (gauche) 2952 2975 ms 2975 ms CH, antisym str (gauche) 2962 vs, p 2962 vs 2951 ~ 2 0 CHI antisym str (99%) 2960 ms 2960 s 2945 vs 2946 m 2949 vs 2945 sh 2875 CH2 sym str (gauche) 2937 s 2936 mw 2935 vs 2935 s 2890 ~2 CH3 sym str (99%) in Fermi reson with 2u5 2924 sh 2926 w 2917 ms 2915 m 2916 w 2901 mw 2900 m 2898 w 2879 s, p 2878 s 2875 u, CH, sym str (99%) in Fermi reson with 2u6 2879 mw 2874 s 2864 ms 2853 ms 2839 vs 2842 vs, p 2837 vs 2u5 in Fermi reson with CH3 sym str 2840 ms 2841 m 2834 vs 2827 s 2828 mw 2820 m 2825 s, p 2u6 in Fermi reson with CH, sym str 1475 m, p 1476 m 1479 u4 CH2 def (66%); CH, wag (26%) 1479 m 1473 m 1471 sh 1468 R 1462 bd, w 1461 m , d p 1460 sh 1461 ~ 2 1 CH, antisym def (86%); CH, rock (13%) 1 4 6 5 Q ms 1462 s 1461 Q ms 1456 P
1460 s 1455 s
1455 m , d p
1431 s 1415 R 1 4 0 9 Q ms 1404 P 1364 w 1310 1290
sh vvs
1453 ms 1450 m 1431 vw
1404 w
1467
CH, antisym def (64%); CH, rock (14%)
~5
1439 u6
CH2 wag (30%); CH, antisym def (24%); CH, def (19%); C-0-C antisym str (17%)
1418 UT
CH, sym def (96%)
1387 w 1336 vw 1285 vs
1306 b d . ~1315 ~ vw.dD 1285 w,hp'
1290 VW. sh 1283 vw
1340 1256
~ 2 2 CH2 twist ~ 2 3
(73%); CF3 antisym str (16%) CF3 antisym str (46%); CF, antisym def (19%); CH, rock
1264 w
1289
US
CF3 antisym str (39%); CF3 antisym def (16%); C-C str
(13%) 1270 vw
(1 4%) 1222 1195 1190 1177
1126
s
1250 w 1230 mw 1213 m
sh vvs vvs
1176 vs
vs
1160 vs 1148 vs 1122 s
1220 w w
1150 vw, br
1215 vw,p
1155 w, dp 1125 w, p
1212 vw
1255
~9
CH, rock (64%)
1170 vw
1188 1194
~ 1 0
CF, sym str (gauche) CF, sym str (22%); C-C str (29%); CF, sym def (24%); CF3 antisym str (14%)
1157 vw 1145 w 1126 w
. ,
1199 u24 CH, rock (81%) 1105 u l l C-0-C antisym str (44%); CH, wag (1 8%); C-0-C sym str (12%)
1118 w 979
vs
979 vs
993 w 979 w
979 m, p
C-0-C sym str (47%); CH, rock (14%); C-C-0 bend
976 mw
994
VI,
970 w
862 856 855 845
~ 2 5
(10%) 969 vs 935 w 855 sh 831 Q m 725 vvw 674 R 670 Q s 664 P 546 m 536 sh 460 w 386 w 368 sh, w 355 R 348 Q w 344 R 312Q 233 Q 216 Q 171 Q
vw mw w, sh vw
849 s
935 mw 855 m 831 s
926 m, p 848 ms, p 828 s, p
847 s
680 w 670 w
672 mw, p
673 mw
CH, CH, C-C C-C
vi3
rock (71%); CH2 twist (18%) rock (gauche) str (19%); CF, sym str (60%) str (gauche)
724 vvw 676 ms 559 m 556 s 541 w 396 mw 366 m
554 548 536 460 385 368
354 m w
349 w
355 w, p
314 w
320 vw, p
234 m 189 w
vw, sh w sh vw mw w
548 540 465 388
w, p sh, dp ww, p mw, p
559 mw 555 mw 540 mw 396 mw 366 vw 355 w
666
~ 1 4
546 ~ 1 554 Y26 477 384 365 u2,
308 223 189 vw
antisym def (65%); CF, antisym str (33%) CF, antisym def (68%); CF3 antisym str (31%) C-0-C bend (gauche) CF, rock (37%); C-0-C bend (39%); C-C-0 bend (17%) CF, rock (91%)
5 CF,
326 u17 C-0-C bend (26%); C-C str (24%); C-0-C sym str (1 7%); C-C-0 bend (1 5%)
U28
170 br, w w
CF, sym def (61%); CF, sym str (16%)
176 u I 8
CF, rock (gauche) C-C-0 bend (gauche) CH, torsion C-C-0 bend (37%); CF, rock (50%)
The Journal of Physical Chemistry, Vol. 91, No. 6, 1987
Spectra of 2,2,2-Trifluoroethyl Methyl Ether TABLE 111 (Continued) infrared re1 gas int solid 167Q 107 105 Q 102Q
~~
re1 int
gas
re1 int
liq
Raman re1 int, depol
re1 solid
int
calcd
yi
vw
sh mw mw
55
134 128 86 72 70
m m mw mw mw
uZ9
1339
~
assignment approx descripn* CH, torsion (gauche) OCH, torsion CF, torsion lattice modes
W
38 30
W
OCH, torsion (gauche) lattice modes
W
“Abbreviations used: s, strong; m, medium; w, weak; v, very; sh, shoulder; br, broad; p, polarized; dp, depolarized; P, Q, and R refer to the rotational-vibrational branches; str, stretch; def, deformation; reson, resonance. *Contributionsof less than 10% are not included.
300 100 WAVENUMBER (cm-‘) Figure 8. Far-infrared spectra of solid CF3CH20CH3(A) and CF,CHZOCDp (B). 500
band observed in the infrared spectrum of the gas at 3012 cm-I. These two vibrational modes are expected to shift into the 2300-2100-~m-~region of the spectrum upon deuterium substitution of the methoxy group and are thus assigned to the bands observed at 2259 and 2237 cm-I for the A” and A’ CD, antisymmetric stretches, respectively. This latter mode has a second band associated with it at 2135 cm-I, which is probably due to Fermi resonance with the overtone of the CD3 antisymmetric deformation observed in the Raman spectrum of the gas at 1076 cm-I. The two bands observed in the infrared spectrum of the gas at 2985 and 2975 cm-l which have no corresponding bands in the spectrum of the solid are assigned as the CH, and CH2 antisymmetric stretching modes of the gauche conformer. The CH, antisymmetric stretch, which is not expected to shift significantly upon deuteriation of the methoxy group, is assigned to the bands observed in the Raman spectrum of the gas at 2960 and 2943 cm-I for the -doand -4species, respectively. The strong peak observed at 2949 cm-l in the Raman spectrum of the gas which has no corresponding line in the spectrum of the solid is assigned to the CH, symmetric stretch of the gauche conformer. The CH, symmetric stretch for the -do compound which is in
Fermi resonance with the overtone of the CH2 wag, appears as two strong peaks in the Raman spectrum of the solid at 2878 and 2827 cm-’. The corresponding vibrational mode for the -4 compound, also in Fermi resonance with the CH, wag, is observed in the Raman spectrum of the solid at 2908 and 2830 cm-l. The CH, symmetric stretching mode, which is split into two strong bands observed at 2935 and 2839 cm-’ in the infrared spectrum of the gas because of Fermi resonance with the overtone of the A’ CH3 antisymmetric deformation, shifts upon deuteriation to the band observed at 2070 cm-I. The CH2 deformation is assigned to the bands observed in the Raman spectrum of the liquid at 1475 and 1463 cm-I for the “light” and “heavy” molecules, respectively. The CH, wag is assigned to the bands in the infrared spectrum of the solid at 1431 and 1438 cm-I for the -do and -d3 compounds, respectively. The band observed at 141 1 cm-I in the infrared spectrum of the gas of the -d3 compound which has no corresponding band in the spectrum of the solid is assigned as the CH, wag of the gauche conformer. The two CH, antisymmetric deformational modes are assigned to the strong bands in the infrared spectrum of the gas at 1465 and 1461 cm-’. These bands shift upon deuteriation and are assigned to the two bands observed in the Raman spectrum at 1076 and 1058 cm-I, respectively. The CH, symmetric deformation is assigned to the band centered at 1409 cm-’ in the infrared spectrum of the gas and appears at 11 15 cm-’ in the spectrum of the -d3 compound. The remaining CH, bending modes, the twist and rocking motions, are assigned, respectively, to the depolarized band in the Raman spectrum of the solid liquid at 1315 cm-I and the moderately weak band in the Raman spectrum of the observed at 970 cm-’ for the -docompound. The corresponding vibrations for the -d3 compound are assigned to similar bands at 1310 and 973 cm-I. Additionally, bands observed in the Raman spectrum of the liquid at 926 and 917 cm-I in the -do and -d, compounds, respectively, which have no corresponding bands in the spectrum of the solid, are assigned as the CH, rock of the gauche conformer. The two CH3 rocking modes are assigned to the bands observed in the Raman spectrum of the liquid at 1215 and 1155 cm-’. The corresponding vibrations in the -d3 compound are assigned to similar Raman bands observed at 877 and 885 cm-I. Heavy Atom Vibrations. The CF3 stretching modes are expected to be very prominent in the infrared spectrum but very weak in the Raman effect. The A” CF3antisymmetric stretching mode is therefore assigned to the very strong band observed in the infrared spectrum of the gas at 1290 cm-’ in both the -do and -d3 spectra. Another very strong band is resolved in the infrared spectrum of the solid at 1270 cm-’ in both compounds and is assigned as the A’ CF3 antisymmetric stretch. Strong bands are also observed in the infrared spectra of the gas at 1190 cm-’ in the -dospectrum and 1180 cm-’ in the spectrum of the -d3 molecule which have no corresponding bands in the spectra of the solid and are thus assigned as the CF, symmetric stretch of the gauche conformer. The CF, symmetric stretch for the trans conformer is assigned to the remaining strong band in the infrared spectrum of the gas observed at 1177 cm-’ for the -do and 1 170 cm-’ for
1340 The Journal of Physical Chemistry, Vol. 91, No. 6,1987
Li e t al.
TABLE I V Observed' Infrared and Raman Frequencies (cm-I) and Assignment for CF,CH20CD, infrared Raman re1 re1 re1 re1 int, re1
gas 2985
int mw
solid
int
2953 vw 2946 vw 2928 2903 2880
m w, sh w, sh
2815
w
2910 vw 2835 vw 2825 vw
2195 vw. sh 2265 R 2279 vw 2259 Q mw 2268 w 2256 vw 2254 P 2232 w 2219 m 2220 mw 2135 w 2136 vw 2116 vw 2075 sh 2070 m 2077 mw 2062 vw 1457 w 1468 mw 1459 vw 1443 mw 1437 w 1438 mw 1416 R 1411 Q mw 1406 P 1391 w 1376 w 1363 vw 1312 s,sh 1291 s, sh 1290 vs 1283 vs
liq
depol 2975 sh 2943 S, p
2928 s 2903 w 2878 w
2900 m, p 2880 sh
2816 mw
2817 m, p
2794 vw
2795 sh
2260 mw
2260 m, p
2267 ms
2248
~ 1 9 CD,
2231 w
2230 m
2230 w 2218 m
2253
Y,
2141 mw 2124 sh
2137 ms, p
2135 mw 2115 w
2074 vs
2073 vs, p
2075 2061 1467 1459
1455 bd,vw 1463 w, dp
solid
int
2952 s 2944 ms 2908 mw
vi ~ 2
2875 2876 u3
CH2 sym str (gauche) CH, sym str (99%) in Fermi reson with 2u6
2834 m 2830 m 2824 m
vs w mw w
1450 w,dp
1310 bd,vw 1310 vw,dp 1286 w, dp 1285 vw
1270 s
calcd 2953 2951
assignment approx descripnb CH2 antisym str (gauche) 0 CH, antisym str (99%)
gas int 2984 m 2943 m
1266 vw
2u6 in Fermi reson with CH, sym str
antisym str (98%)
CD, antisym str (97%) in Fermi reson with 2us CD, antisym str (gauche) 2us in Fermi reson with CD, antisym str
2082 w 2
CD, sym str (97%)
1479 w4
CH2 def (67%); CH, wag (26%)
1442 u6
CH2 wag (39%); CH, def (24%); C-0-C antisym str (24%)
1440
CH, wag (gauche)
1339 w2, CH2 twist (74%); CF3 antisym str (16%) 1253 ~ 2 3 CF, antisym str (47%); CF3 antisym def (20%); CH, rock (14%); CH2 twist (10%) 1288 ~g CF, antisym str (40%); CF, antisym def (17%); C-C str (14%)
1221 w 1195 1180 1170
sh vs vs
1168 vs
1181 w 1172 w
1167 vw 1160 w. br
1150 vs 1142 vs 1115
m
1118 mw 1114 m
1117 w
1114 w , p
1060
w
1063 vw 1058 w
1076 vw 1061 vw
1064 w,dp
998
s
997 s
1001 w
973 vs 969 vs
883
mw
838 sh 831 mw 669 R 664 Q ms 659 P 545 537 441 389 R 386 Q 383 375
m sh
899 889 885 840 838
w mw m mw m
670 mw 668 mw 555 w 553 mw 539 w
w w w sh
393 mw 366 ms
324 R
996 mw, p
983 vw, sh
978 w, sh
920 vw
917 w, p 885 w, sh
883 w
877 mw, p
843 832 668 665
836 ms, p 831 ms, p
m s w w
664 mw, p 547 w, p
546 w 537 w, sh 445 vw
445 w w , p
385 w 379 mw
391 sh, p 385 mw, p
1153 w 1141 vw 1113 w 1065 1058 1001 996
vw w
1199 1195
~ 1 0
CF, sym str (gauche) CF, sym str (21%); C-C str (26%); CF3 sym def (24%); CF, antisym str (13%)
1147 u I 1 C-0-C antisym str (28%); CD, sym def (33%); CH, wag (12%) 1108
CD, sym def (21%); C-0-C sym str (31%); CD, rock (1 8%); C-0-C bend (10%) 1055 U S CD, antisym def (91%) 1052 u2] CD, antisym def (92%) UT
w w
973 vw 968 vw 903 vw 893 vw 884 vw 838 s 672 sh 668 w 556 w 553 w 540 w
986 u 1 2 C-0-C sym str (11%); CD, sym def (37%); C-0-C antisym str (18%) 859 w25 CH, rock (69%); CH2 twist (16%) 869 298 w24
CH, rock (gauche) CD, rock (87%)
869
~g
CD, rock (61%); C-0-C sym str (21%)
851 845
~ 1 3C-C
str (20%); CF, sym str (61%) C-C str (gauche)
663 wI4 CF, sym def (62%); CF, sym str (1 5%) 546 w I s CF, antisym def (66%); CF, antisym str (33%) 554 W26 CF, antisym def (68%); CF3 antisym str (31%) 453 C-0-C bend (gauche)
391 w
368 w I 6 CF, rock (45%); C-0-C CF, rock (gauche) 370
366 vvw
365
u2i
CF, rock (91%)
bend (25%); C-C-0 bend (19%)
The Journal of Physical Chemistry, Vol. 91, No. 6, 1987 1341
Spectra of 2,2,2-Trifluoroethyl Methyl Ether
TABLE IV (Continued) Raman infrared re1 re1 re1 re1 int, re1 assignment gas int solid int gas int liq depol solid int calcd vi approx descripnb 320 Q vw 330 mw 320 w, br 328 w 326 vw 308 Y,, C-0-C bend (39%); C-C str (23%); C-0-C sym str (16%); C-C-0 bend (10%) 326 sh 310 R 3 0 0 Q vw 301 mw 310 vw 296 CF, rock (gauche) 296 P 275 229Q 177 Q 169Q
vvw mw
mw mw
227 vw
232
189 m 180 sh
170
ww vw
vI8 v18
C-C-0 bend (gauche) CD3 torsion C-C-0 bend (38%); CF, rock (48%)
uZ9
CD3 torsion (gauche) OCD, torsion
22 1 180 vw
170
1 4 4 0 mw 141 Q mw
9 6 Q mw
94 Q mw 50
125 120 112 85 75
m sh
sh
110 w
uj0 CF3 torsion
lattice modes
w w
vw 38 m 30 w
OCD, torsion (gauche) lattice modes
"Abbreviations used: see Table 111. the -d3 molecule. The C-0-C antisymmetric stretch is assigned to the strong band observed in the infrared spectrum of the solid of the -do compound at 1122 cm-', which upon deuteriation shifts up to 1142 cm-I. This shift to higher frequency is the result of the difference in the coupling of this mode with the carbon-hydrogen bending modes between the light and heavy molecules. The C-0-C symmetric stretch is assigned to the bands observed in the infrared spectra of the gas at 979 and 998 cm-l for the -do and -d3compounds, respectively. The C-C stretching modes for the trans conformer for the -do and -d3 species are assigned to the bands observed in the Raman spectra of the gas at 855 and 843 cm-l, respectively. The corresponding bands associated with the C - C stretch of the gauche conformer are observed at 831 cm-' in both spectra. The CF, deformations have been well characterized' and are assigned to the bands observed in the Raman spectrum of the liquid of the -do compound at 672 cm-' for the CF3 symmetric deformation and 548 and 540 cm-I for the antisymmetric deformations, the latter being the A'' mode. The corresponding vibrational modes for the 4 compound are observed in the Raman spectrum at 664, 547, and 540 cm-'. The two CF3 rocks are assigned to the bands observed in the Raman spectrum of the solid of the -do compound at 396 and 366 cm-I, and at 391 and 366 cm-' in the spectrum of the -d3 species. Two of the skeletal motions, the C-C-0 band C-0-C bends are expected to have significantly different frequencies for the two different conformers. Since deuteriation of the methoxy group should have more of an effect on the C-0-C bending motion, larger shift in frequency is expected for this mode than the CCO bend with deuteriation. The COC bend for the -do compound is assigned to the series of Q branches observed in the infrared spectrum of the gas beginning at 348 cm-', which shifts to 319.6 cm-' with deuteriation. The COC bends of the gauche conformer in the -do and -d3 compounds are observed at correspondingly higher frequencies of 460 and 441 cm-', respectively. The strong band which has a maxima of 235 cm-I with pronounced Q branches at 216 and 214 cm-' is composed of two normal modes with the 235 cm-l band being the C-C-0 bend of the gauche conformer and the lower frequency Q-branches assigned to the CH3 torsion of the trans conformer. Upon deuteriation, the C-C-0 bend of the gauche conformer shifts down in frequency to the prominent band observed in the infrared spectrum of the gas centered at 229 cm-', whereas the CH, torison of the trans conformer, expected to have a larger shift factor upon deuteriation, is assigned to the series of Q branches beginning at 177 cm-I. The C-C-0 bend for the trans conformer of the -do species is observed as a rather weak Q branch in the infrared
spectrum of the gas at 1 1.5 cm-'. The C-C-0 bend o .he trans conformer for the -d3 compound is assigned to the Q branch observed in the infrared spectrum of the gas at 169 cm-', which appears as a shoulder at 180 cm-' in the spectrum of the solid. The methyl torsions for the gauche conformer are assigned to the Q branches observed in the infrared spectra of the gases at 167 and 141 cm-' for the -do and -d3 compounds, respectively. Several Q branches are observed in the infrared spectrum of the gas of the -do compound beginning at 105.2 cm-' and falling to a lower frequency of 100.5 cm-' which we believe are due to two different modes. In the far-infrared spectrum of ethyl methyl ether the 1 0 transition for the asymmetric torsion of the trans conformer was observed at 115.4 cm-I and shifted to 106.1 cm-' with the deuteriation of the methyl group.l0 Also it should be noted that the CF3 torsion of gauche 2,2,2-trifluoroethanol, CF3CH20H, has been observed' at 106 cm-' and for trans2,2,2-trifluoroethylamine at 107 cm-' so that both the CF3 torison and asymmetric trosion of trans-2,2,2-trifluoroethylmethyl ether are expected in this region. Therefore, the pronounced broad Q branch observed in the infrared spectrum of the gas centered at 105 cm-' which shifts to 96 cm-' in the spectrum of the -d3 compound is assigned to the methoxy torsion of the trans conformer. The remaining Q-branch series beginning at 102 and 94 cm-' in the spectra of the -doand -d3 compounds, respectively, are assigned as the CF3 torsion of the trans conformer. Assuming a similar barrier to internal rotation and utilizing the calculated F numbers for the gauche conformer, the CF, torison is expected to be around 72 cm-I, but was not observed in either spectrum. The band observed at 55 cm-' in the infrared spectrum of the gas is assigned as the methoxy torison of the gauche conformer. The Teller-Redlich product rule calculation was utilized to support the proposed vibrational assignment. Generally, an agreement in the range of 3-5% between calculated and observed ratios is sifficient to support the assignment. Frequencies from the gas phase were utilized whenever possible to calculate the observed ratios. The theoretical values for the shift factors are 5.26 and 3.70 whereas the observed values for the shift factors are 5.04 and 3.55 for the A' and A" symmetry blocks, respectively. These values result in a 4.2% and 4.1% difference between the experimental and calculated values for the A' and A" symmetry species, respectively.
-
Normal-Coordinate Calculations The normal coordinates of 2,2,2-trifluoroethyl methyl ether were calculated using Wilson's F-G matrix method" and programs ~~
~~~
(10)Durig, J. R.;Compton, D.A. C. J . Chem. Phys. 1978, 69,4713.
1342 The Journal of Physical Chemistry, Vol. 91, No. 6, 1987
Li et al.
TABLE V: Symmetry Coordinates' for CF3CHzOCH3 A' Species
SI
fl + f 2
s 2
2e1 - e2 - e , e , + e2 + e3 2a3 - ul - u2 (61/2+ 2)e - (6'12 - 2)O - 01 - p2 - ~1 - ~2 f f I + ff2 + f f 3 - PI - P2 - P3 PI - W I + P l - w2 2a, - a 2 - a, a i + a2 + a, 281 - P2 - P3
s 3 s 4
ss s7
S6
S8 s 9
SI0
SI1 SI2
c-d c+d
b s14
Y1 + Y2 + Y3 - 6,- 62 - 63
s17
$J
SI5
2Y3 - Y I - 7 2 26, - 6, - 6,
+
s20
(6'12 - 2)u - (6'/2 2)O + + Y2 + Y ? + 6,+ 62 + 63 P I + p2 W I + w2 + t + 0
S2I
a1
s22
f' - f 2
SI8 s19
Yi
+ p2 + + ~2 WI
+
+ CY2 + a3 + PI + 02 + 03
CH, sym str CH3 antisym str CH, sym str CH, antisym def CH2 def CH3 sym def CH2 wag CF, antisym str CF, sym str CH, rock C-0-C antisym str C-0-C sym str C-C str CF, sym def CF, antisym def CF, rock C-0-C bend C-C-0 bend CF, redundancy CH, redundancy CH, redundancy
A" Species s23 s24
s 2 5 s26
CH, antisym str CH, antisym str CH, antisym def CH2 twist CF3 antisym str CH, rock CH2 rock CF, antisym def CF, rock
e2 - e3 Dl - f f 2 P I - UI - P2 + U2 a2 - a3
03
s21
P2 -
S28
PI - P2
s30 s29
Yl - Y 2
+ @I
- w2
62 - 6 3
Not normalized.
TABLE VI: Valence Force Constants for CF,CH,OCH," force const Kd K, K, K, Kb Kf
H7 Hm
Figure 9. Internal coordinates for CF3CH20CH,
written by Schachtschneider.'* The G matrix was calculated by using structural parameters from C F 3 C H t and CH3CH20CH3.' Thirty internal coordinates (Figure 9) were used to form the 30 symmetry coordinates listed in Table V which were used to calculate the vibrational frequencies. Initial values for the force constants were obtained from the appropriate ones determined for dimethyl ether9 and those for a series of trifluoroethyl halide^.'^ The final 15 diagonal and 11 interaction force constants, listed in Table VI, reproduced the observed frequencies within 1.2%. Frequencies of the normal modes of the gauche conformer were calculated through the use of the G matrix obtained by simply rotating the methoxy group to the gauche position with no further adjustment of the force constants. The vibrational assignment is supported by the calculation of a lower frequency for the C-C stretch of the gauche conformer relative to the trans conformer. Significant coupling was found between the CF, rocks and the skeletal bending modes, particularly for the gauche conformer. The band at 233 cm-I, which has been assigned as the C-C-0 (11) Wilson, E. B.; Decius, J. C.; Cross, P. C. Molecular Vibrations, McGraw-Hill: New York, 1955: [republished by Dover: New York, 19801. (12) Schachtschneider,J. H. "Vibrational Analysis of Polyatomic Molecules, V and VI"; Technical Report No. 231-64 and 57-65; Shell Development Co., Emeryville, CA. (13) Crowder, G. A. J . Fluorine Chem. 1973/74, 3, 125.
HB Hb Hg H, Hp H, H, Fa, Fab Fbp
Fag F,, Fda Fcd Fb,
Fad FCe
description 0-C(H3) stretch C-F stretch (H2)C-0 stretch C-H, stretch c-c stretch C-H2 stretch F-C-F bend C-0-C bend C-C-0 bend F-C-C bend H-C-0 (CH,) bend H-C-0 (CH,) bend H-C-C (CH,) bend H-C-H (CH,) bend H-C-H (CHj) bend C-F stretch/C-F stretch C-F stretch/C-C stretch C-C stretch/H-C-C (CH,) bend F-C-C bend/F-C-C bend H-C-H bend/H-C-C bend (common C-H) H-C-0 (CH,) bend/H-C-0 (CH,) bend O-C(H3) stretch/H-C-H bend (H2) C-O stretch/O-C(H,) stretch C-C stretch/F-C-F bend C-F stretch/F-C-C bend (different C-F) (H2)C-0 stretch/H-C-H bend
value" 5.30 5.28 5.19 4.8 1 4.26 4.62 2.10 I .32 1.28 0.95 0.92 0.865 0.57 0.536 0.54 0.90 0.50 0.375 0.20 0.045 -0.05 -0.45 -0.45 -0.40
-0.40 -0.45
"Stretching constants are in units of mdyn A-I; stretch-bend constants are in units of mdyn rad-'; bending constants are in units of mdyn A radT2.
bend of the gauche conformer, contains 54% CF, rock, which may explain the strong intensity of this band relative to the corresponding mode for the trans conformer. Significant coupling is also observed between the symmetric CF, stretching mode and the C-C stretching mode. The potential energy distribution indicates that the band observed at 855 cm-' in the Raman spectrum of the gas is 19% C-C stretching and 60% CF, symmetric stretching. Further refinement of the force constants failed
The Journal of Physical Chemistry, Vol. 91, No. 6,1987 1343
Spectra of 2,2,2-Trifluoroethyl Methyl Ether to increase the C-C stretching character of this vibration. However, this band appears as a conformer doublet in the Raman spectrum of both the gas and liquid phase, with significant variations in relative intensity with changing temperature as observed in a temperature study of the liquid phase. Therefore, this conformer doublet is assigned as the C-C stretching mode, which would be more likely to show conformer bands than the CF3 symmetric stretch. Similar trends were also observed in the normal-coordinate calculation for the deuteriated species utilizing the same force constants as in the calculation for the -docompound. There is some discrepancy between the observed and calculated frequencies for the C-0-C stretches and CD3 deformations for the -d3 analogue. This is mainly due to the mixing of the C - 0 - C stretching modes with the CD3 deformational modes, which of course was not present in the calculation for the -do compound. The potential energy distribution for the -d3compound shows the C-0-C antisymmetric stretch to be fairly evenly distributed among the calculated bands at 1442, 1147, and 986 cm-'. The C-0-C symmetric stretch is calculated to be fairly evenly distributed among the calculated frequencies of the 1108, 986,and 869 cm-I. The CD3 symmetric deformation is distributed among the 1147, 1108, and 986 cm-I. In the infrared spectrum of the light compound, a strong band is observed at 1122 cm-', which shifts up to 1142 cm-I upon deuteriation. A weaker band is also observed in this spectrum of the -d3compound at 1114 cm-I which is not present in the spectrum of the -do analogue and, therefore, must be associated with the methyl group which shifted down upon deuteriation. The mixing of the CD3 deformation and C-0-C stretching motions makes it difficult to specify any one band to a particular motion. A strong band is also observed in the infrared spectrum of the gas of the light compound at 979 cm-' which shifts up to 998 cm-I upon deuteriation. A weaker band is observed in the same spectrum of the -d3 compound at 883 cm-'. The potential energy distribution suggests that the 883-cm-' band is a mixture of the C-0-C symmetric stretch and CD3 rock, the latter of which is also mixed with the C-C stretch and C-0-C antisymmetric stretch. The band calculated at 986 cm-', however, contains only 11% C U C symmetric stretch and is more of the CD3 symmetric deformation and C-0-C antisymmetric stretch. Based on its relative intensity, however, it must be the band which corresponds to the 979-cm-I band in the spectrum of the -do compound, which is shown to be mainly the C-0-C symmetric stretch. Therefore, we believe the band at 998 cm-' should be more C-0-C symmetric stretch for the -d3 compound than the normal coordinates indicate.
Variable-Temperature Study Examination of the Raman spectrum of the liquid and annealed solid phases for both the -do and -d3species of 2,2,2-trifluoroethyl methyl ether reveals that several bands disappear upon annealing. Comparison of the Raman spectra of the gaseous and liquid phases in the 850-cm-' region reveals a dramatic change in the relative intensities of the higher frequency peak, assigned as the C-C stretching mode of the trans conformation, and the lower frequency band assigned as the C-C stretch of the gauche conformer. The strong intensity of these two conformer peaks in the Raman spectra of both the gaseous and liquid phases enables one to carry out a variable-temperature study to determine the energy difference, AH,between the trans and gauche conformers. According to the van't Hoff isochoric equation (eq l ) , an expression can be derived relating the Raman band intensity, Zi, of conformer i, to the temperature, T (eq 2). In eq 2, E A is an In K = -AH/(RT) + A S / R (1) In (ZA/ZB) = -AH/(RT)
+ A S / R - In ( E B / E A )
(2)
unknown constant which relates ZA to the concentration of the higher energy conformer A. If In ( E B / E A )is assumed to be insignificant, a plot of In (ZA/ZB) against 1 / T should result in a straight line having a slope of -AH/R and an intercept of M / R . A temperature study of the lines observed in the Raman spectrum of the vapor at 855 and 831 cm-I for the -do compound, and 843 and 832 cm-' for the -d3species, resulted in no discernible
TABLE VII: Calculated Total Energy, ET (bartree), and Dipole Moment Components' (debye) of CF3CH20CH3
deg of int rotatn
0 30 60 90 120 150 180
ET
-126.8986 -126.9074 -126.9093 -126.9150 -126.9148 -126.9162 -126.9173
1r.l
IWbl
Ilrcl
1/41
0.15 0.14 0.69 1.43 2.03 2.42 2.56
0.49 0.34 0.10 0.54 1.20 2.33 2.68
0.00 1.01 1.72 2.16 2.21 1.25 0.00
0.51 1.08 1.86 2.65 3.23 3.59 3.70
"Only the absolute values of the dipole moment components are given.
difference in the relative intensities of these two peaks in the 25-65 OC range investigated. The peak assigned as the C-C stretch of the gauche conformer remained approximately twice as intense as that of the trans, presumably because there are two equivalent gauche conformers as opposed to only one trans conformer. We have therfore assigned a value of 150 cm-' as the upper limit for the energy difference between the trans and gauche conformers for both the -do and -dj compounds in the vapor phase. This is the value of AH which we have found detectable over this temperature range. A temperature study was also undertaken in the liquid phase of both the -do and -d3 compounds. Upon cooling, the relative intensities of the Raman lines assigned to the trans and gauche conformers showed a dramatic change where the higher frequency band, assigned as the C-C stretch of the trans conformer, increased in intensity relative to the band associated with the gauche conformer. The pair of conformer bands was measured four times at each of six different temperatures from 23 to -40 OC for both the -do and -d3 compounds. A plot of the In (1831/1& against 1 / T for the -do compound resulted in a value for AH of 442 f 42 cm-I. In the case of the -d3compound, a plot of the ln(Z828/Z848) against 1/ T resulted in a value for AH of 342 f 29 cm-'. The calculated values of AS are 2.38 f 0.03 and 1.96 f 0.03 eu for the -doand -d3compounds, respectively. The quoted uncertainties are statistical errors and do not take into account any temperature dependence of AH or problems associated with other bands overlapping the C-C stretching modes. We have averaged the two values and spanned the two uncertainties which results in a AH value of 392 f 92 cm-' (1.12 f 0.25 kcal/mol) which should better reflect the value in the liquid than either value alone.
Discussion From the investigation of the low-resolution microwave spectra of 2,2,2-trifluoroethyl methyl ether and its -d3 analogue, two conformers, trans and gauche, have been identified in the gas phase. It has also been shown that the spectrum assigned to the trans form is about 4 times more intense than the corresponding one assigned to the gauche form. In order to estimate the relative amounts of these two conformers in the gas phase from the relative intensity data of the low-resolution microwave spectrum, the dipole moment components of the trans and gauche conformers were calculated with the molecular orbital programI4 CNDO obtained from the Quantum Chemistry Program Exchange, University of, Indiana. The program has been modified for the second row elements as recommended by Santry15 and Sabin.16 The molecular structure used in the calculation was obtained from the corresponding structural parameters of CF3CH36and CH3CH2OCH3.' The results of this calculation are listed in Table VII. It is seen that the wla dipole moment component (2.56 D) for the trans conformer is larger than that (- 1.43 D) for the gauche form but the differences in the values of the dipole moment component la are not sufficient to conclude that the gauche form is more stable than the trans conformer. From the relative intensity it is therefore believed that the trans conformer is more abundant (1 4) QCPE Molecular Orbital Program No. 141. (15) Santry, D. P. J . Am. Chem. SOC.1968, 30, 3309. (16) Sabin, J. R., private communication, 1973.
1344 The Journal of Physical Chemistry, Vol. 91, No. 6, 1987
Li et ai.
than the gauche conformer in the gas phase even taking into account the difference in the values of pa for the two conformers. This result is consistent with the total energies (ET)calculated for the conformers at different internal rotation angles (see Table VII) as well as the results derived from the Raman spectra in the different physical phases. From the relative intensity measurements of vibrational bands at different temperatures, the energy difference in the liquid phase (442 cm-l) has been found to be larger than that in the vapor phase (1150 cm-l). This increase may be rationalized on the basis of the large dipole moment for the trans conformer compared to that of the gauche form (see Table VII). The larger dipole moment is expected to cause a greater intermolecular interaction in the liquid phase than in the vapor phase and to stabilize the trans conformer relative to the other form. Ethane derivatives with an asymmetric rotor may exhibit two conformers. Such derivatives have included ethylamine (CH3CH2NH2),I7-l9ethanol (CH3CH20H),2@22ethyl hypochloride (CH3CH20Cl),23and 2,2,2-trifluoroethylamine(CF3CH2NH2)8g24 in which the more stable conformers have been identified to be the trans form. In the microwave in~estigation~~ of CF3CH2NH2, the stability of the trans form over the gauche form has been explained as being due to an intramolecular dipole-dipole interaction between the N-H and C-F bonds. Using a similar interaction in 2,2,2-trifluoroethanoI, Kalasinsky and Anjaria* were able to account for the predominance of the gauche conformer in CF3CH20H. However, upon substitution of the hydroxyl hydrogen by the methyl group, such a dipole-dipole interaction should no longer exist to any sufficient extent or certainly not predominate. For this reason, one may expect the more stable conformer in CF,CH20CH3 to be the trans form. In our recent rotoational and vibrational study of methoxyfluraneZ5it has been found that the most stable form is the trans conformer in which the methyl group is trans to the CHClz group in the Newman projection along the internal C-0 bond. This result is then in good agreement with what we have found in the present study. From the observed frequencies for the methyl torsional mode it is possible to calculate the threefold periodic barrier to internal rotation which can be written as V ( a ) = (V3/2)(l + cos 3a). 0 methyl torsional Utilizing the band at 216 cm-I as the 1 transition of the trans conformer along with the reduced internal rotation constant of 5.415 cm-' (F = h/8a2Z, where I , is the reduced moment of inertia for the internal rotation), a periodic V3barrier of 1077 cm-' (3.08 kcal/mol) is calculated. This value is nearly identical with the corresponding barrier in dimethyl ether.26 A similar calculation for the gauche conformer utilizing the methyl torsional frequency of 167 cm-l and an F number of 5.334 cm-' gives a threefold methyl barrier of 675 cm-' (1.93
kcal/mol) which is quite low and indicates that the interactions in this conformer are significantly different from those of the trans conformer. A similar calculation for the CF3rotor with an F value of 0.918 cm-l for the trans conformer gives a threefold periodic barrier of 1321 cm-I (3.78 kcal/mol) which is consistent with barriers obtained for this rotor in several similar molecules.27 Unless large variations of the structural parameters exist between the trans and gauche conformers, the threefold periodic barrier for the gauche conformer should be somewhat similar. Utilizing the 1321-cm-' barrier calculated for the trans conformer and an F number of 0.442 cm-' for the gauche conformer, we predict the CF3 torsion of the gauche conformer to be around 72 cm-I. It was not possible to obtain the potential constants for the asymmetric rotation because of the lack of definitive torsional transitions for each conformer but, from the frequency for this mode for the trans conformer, it is possible to estimate a value of 973 cm-I (2.70 kcal/mol) for the V3term. Since the AH value in the gas phase has been estimated to be less than 150 cm-I, both the VI and V2terms must be relatively small compared to the V3 term which, therefore, must be the dominant term. These conclusions are consistent with the values for the potential constants for the asymmetric torsional mode in ethyl methyl ether.I0 The fact that the CH, torsional mode only shifts by fact of 1.22 with deuteriation, whereas it is expected to shift by 1.35, indicates significant coupling of this mode with the asymmetric torsion. Further indication of this coupling is also found in the relative intensity of the methyl torsional mode in the infrared spectrum. Methyl torsions are usually quite weak in the infrared spectrum2*whereas asymmetric torsions of molecules with polar groups give rise to relatively intense infrared bandsz9 Therefore the pronounced infrared band for the methyl torsion for 2,2,2trifluoroethyl methyl ether, which increases in intensity as the mode shifts to lower frequency with deuteriation and consequently is closer in frequency with the asymmetric torison, clearly demonstrates this coupling. To properly characterize the asymmetric torsion it will probably be necessary to develop a potential function which couples the symmetric potential of the methyl group with the asymmetric potential in a two-dimensional manner. Such a potential is currently being developed in our laboratory. Since the frequencies of the skeletal bending modes are so different between the two conformers it should be possible to determine the conformation of this molecule when it is adsorbed on a surface or interacts with a bimolecule. For example, with the present state of knowledge on the conformation of 2,2,2-trifluoroethyl methyl ether, it should be possible to monitor the conformational change when this anesthetic interacts with amines, amides, or alcohols. Further studies in this area of such representative molecules could possibly provide information on the mechanism of inhalation anesthesia and such studies are planned in the future.
(17) Li, Y. S.;Laurie, V. W. Paper presented at the 24th Symposium on Molecular Structure and Spectroscopy, Columbus, OH, 1969. (18) Durig, J. R.; Li, Y. S. J . Chem. Phys. 1975, 63, 4110. (19) Tsuboi, M.; Tamagake, K.; Hirakawa A. K.; Yamagouchi, Y.; Nakagawa, H.; Manocha, A. S.; Tuazon, E. C.; Fateley, W. G. J . Chem. Phys. 1975, 63, 5117. (20) Barnes, A. J.; Hallam, H. E. Trans. Faraday Soc. 1970, 66, 1932. (21) Kakar, R.; Seibt, P. J. J . Chem. Phys. 1972, 57, 4060. (22) Durig, J. R.; Bucy, W. E.; Wurrey, C. J.; Carreira, L. A. J . Phys.
Acknowledgment. We gratefully acknowledge the financial support of this study by the National Science Foundation by Grant CHE-83-11279 and by the N I H Biomedical Research Support Grant SO7 RR07160.
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-
Registry NO. CF3CH,OCH3,460-43-5; CF3CHZOCD3, 106139-39-3.
Chem. 1975, 79, 988.
(23) Suenram, R. D.; Lovas, F. J.; Johnson, D. R. J. Mol. Spectrosc. 1978, 69, 458. (24) Warren, I. D.; Wilson, E. B. J . Chem. Phys. 1972, 56, 2137. (25) Li, Y. S.; Durig, J. R. J . Mol. Struct. 1982, 81, 181. (26) Groner, P.; Durig, J. R. J . Chem. Phys. 1977, 66, 1856.
(27) Lopata, A. D.; Durig, J . R. J . Raman Spectrosc. 1977, 6, 61.
(28) Wurrey, C. J.; Durig, J. R.; Carrerira, L. A. In Vibrational Spectra and Structure; Durig, J . R., Ed.;Elsevier: Amsterdam, 1976; Vol. 4, Chapter 4. (29) Compton, D. A. C. In Vibrational Spectra and Structure; Durig, J. R., Ed.; Elsevier: Amsterdam, 1981; Vol. 9, Chapter 5 .
