E. L. NUETTERTTES AND TI'. D. PHILLIPS
322
of the two 1-fluorine atoms are more nearly the same than if only one of the two rotational configurations alone were populated. The environments averaged over the oscillation, however, arc not identical. Increased frequency of oscillation with temperature elevation would have no effect on the chemical shift since presumably the frequency is sufficiently high even a t the lowest temperature studied to prohibit observation of fluorine resonances characteristic of the two configurations a t frequencies of 30 MC. or less. Of the four substituted gem-difluoroethanes studied, only I ,2-dichloro-l,1-difluoro-2-phenyleth-
Yol.
in
ane exhibited the temperature dependent spectra discussed above. Spectra for the other ethanes a p peared to be teniperature independent over their liquid ranges. Of the lour substituted ethanes e amined, l,'-dichloro-l, l-difluoro-"-I'heiiylethane with two small chloro substituents rather than with two large bromo substituents would be expected to exhibit the lowest barrier to the 1-30' torsional oscillation postulated in Fig. 4, and consecluently to be most likely to exhibit a temperature dcpendcnt spectrum over the nccessiblc tcinperature range. U'rr.~rvc,rov,DEL -
[CONTRIBUTION NO. 394 FROM
THE
CHEMICAL DEPARTMENT, EXPERIMENTAL S T A T I O S , E. I. I)U
P O S T DE S E M O U R S A N D c O . 1
Structure of CIF, and Exchange Studies on Some Halogen Fluorides by Nuclear Magnetic Resonance1 BY E.L. MUETTERTIES AND 117. D. PHILLIPS RECEIVED AUGUST3, 1950 The high resolution F'9 magnetic resonance spectra of CIFa at 10, 30 and 40 Mc. are presented and interpreted in terms of the structure of C,, symmetry for the molecule. Observed temperature dependencies of the spectri of CIFBand IF:, are interpreted in terms of fluorine exchange. .Activation energies for exchange of 4.8 kcal. for C1Fs and 13 kcal. for IF6 arc estimated from the n-m-r temperature-dependence results. I t is concluded that fluorine exchange occurs in these halogen fluorides through a dimer intermediate.
Introduction Electron diffraction2 and microwave3 studies of ClF3 have indicated that the molecule is planar with two long and a short C1-F bond and exhibits Ctv symmetry. Fluorine exchange between HF and ClF3 and IFs, and between Fz and CIF3 and IF5 previously has been demonstrated4 using radioactive ??IS. I n the present study, the complex F19 magnetic resonance spectrum of ClF, is analyzed and shown to be consistent with a structure possessing CzV symmetry. An observed dependence of the number, widths and positions of the F I 9 resonances of ClF? and IF5on temperature is interpreted in terms of fluorine exchange in these molecules. Xctivation energies for fluorine exchange are estimated for CIFl and IF5froin the temperature dependencies of the Flymagnetic resonance spectra. Experimental Materials.-The commercial grade of chlorine trifluoride yielded a fluorine n-m-r spectrum consisting of a single, broad, relatively temperature-insensitive peak. The fluoride was purified by a series (twelve) of bulb-to-bulb vacuum distillations in quartz equipment that had been previously dried at 400" under high vacuum. It was necessary t o exercise extreme precautions in the distillation of the fluoride in order to obtain a pure sample. Samples for analysis were obtained by distilling the fluoride into quartz capillaries (2 mm. i.d.) which were then sealed off. A sample prepared by this method was found to be "stable" (in contact ( 1 ) Presented before t h e Division of Industrial a n d Engineering Chemistry, National Meeting of t h e American Chemical Society, Atlantic C i t y , N. J., September, 19.56. ( 2 ) R . D . Rurbank and 17. N. Rensey, J . Chem. P h y s . , 21, 0 0 2 (1953). (3) D. F. S m i t h , i b i d , 21, 009 ( 1 9 3 ) . (4) M. T. Rogers a n d J . .I. K a t z , THISJOURNAL, 74, 1375 (19.52).
with quartz) indefinitely. The color of the liquid \vas :in extremely pale yellow; the solid melted sharply a t -76' (uncor.). Bromide trifluoride and bromine pentafluoride were purified in the same manner as chlorine trifluoride. Elimination (by purification) of very slow attack of the quartz by bromine trifluoride a t 25' was never achieved. The peiitafluoride was stable in quartz. Iodine pentafluoride was treated with silver( I ) fluoride t o remove elemental iodine and then distilled in a glass-platinum spinning band column. Method.-The fluorine magnetic resonance spectra were obtained using a Varian high resolution n-m-r spectrometer and electromagnet5 at frequencies of 10, 30 and 40 Mc. and fields of 2,500, 7,500 and 10,000 gauss, respectively. The spectra were calibrated in terms of displacements in cycles per second (c.P.s.) from the fluorine resonance of SFo. Positive frequency displacements indicate resonances occurring a t lower fields than the fluorine resonance of SFG. Calibration was accomplished by superimposing n i l audiofrequency on the sweep field to produce side band peaks to the SFe resonance.6
Results and Discussion 1. Spectrum and Structure of C1F3.--Slicrowave3 and electron diffraction* studies of ClFa concur in assigning the molecule the structure possessing CZv symmetry shown in Fig. 1. On the basis of this structure and t o the approximation t h a t the nuclear spin coupling between the two non-equivalent sets of fluorine atoms is much less than the chemical shift separating the two sets,7 the expected fluorine magnetic resonance spectrum of ClF3 would consist of a doublet and triplet of relative integrated intensities of 2 and 1, respectively. T h e doublet would arise from the equiva(5) Varian Associates, Palo Alto. California. (6) J . T. Arnold atid S I . E. Packard, J . Chem P h y s . , 19, GI08 (1951). (7) H. S. Gutowsky, 1) W hlcC:rll a n d C. P. S l i c h t c r , ~ h i . i . ,21, 279 (1953).
EXCHANGE STUDIESON HALOGEN FLUORIDES
Jan. 20, 1957
lent 1,3-fluorine pair and the triplet from the single 2-fluorine atom. The spacings between the doublet and triplet components would be identical and equal to the nuclear spin coupling between the non-equivalent 1,3- and 2- sets of fluorine nuclei, while the separation between the centers of the triplet and doublet multiplets would be equal to the chemical shift between the 1,3- and 2- sets of nuclei.
QI
and B2 = S 2
+ SJ + 9 P / 4
(13)
Then BZ
- A'
= 2SJ
(14)
or 6 = (B2 - A2)/2J
(15)
Substituting equations 15 and 14 into equation 12 we obtain A' = (B2
1.598A
323
- A2)*/4JZ - (B2 - A 2 ) / 2 + 9 J z / 4
(16)
Solving for J 2 ,we obtain J2
Fig. 1.-Structure
+ + h1 + hz) - J - A ] = 6/2[?(2Ho + 2hz) + A - B ] E4 - Ez &/2[7(2Ho+ 2h2) - A + B ] Ez - E1 = &/2[7(2Ho+ hi + hz) + J - BI Es - Ed = A/2[?(2Ho + hi + hz) - J + A ] Ej - E2 = A / 2 [ 7 ( 2 H o + 2/22) + A + B ] Es = &/2[~(2Ho + 2hi)] E3 - E1 = &/2[7(2Ho+ hi + hz) + J + BI - E5 - E3
= &/2[7(2Ho
E7
(2) (3) (4) (5) (6) (7) (8) (9)
where y is the gyromagnetic ratio of fluorine, Ho is the value of the constant external magnetic field, hl and hz are the absolute chemical shifts in gauss of the two non-equivalent sets of fluorine nuclei of C1F3, 6 is the chemical shift between the two non-equivalent sets of fluorine atoms of C1F3, A
(A2
+ A z ) / 9 & [(BZ + At)'
- 6J + 9J2/4)'/2and B = ( S z + SJ + 9J2/4)'/1
IO Mc FIELD+
-885
1-399 -560
B
-75
5251 560
890 1020
Fig. 2.-F19 spectrum of CIF3 a t 10 Mc. and -60".
The - 60" fluorine magnetic resonance spectra of C1F3 a t 40 and 30 Mc. are given in Figs. 3 and 4. It is noteworthy that as the ratio of J to 6 reaches
2190
-1341 1814
Fig. 3.-F19 spectrum of ClF3at 30 Mc. and -60". CI F 3 40 M
-
REF.: SFg FIELD
I
i
+
A
2835
1
2435
2835
Similarly, by subtracting equation 9 from equation 5 we obtain the expression
- (Ez - El)]/&=
9(B2 - A2)z]1/2/9 (17)
Thus, from experimental values of B and A , values of J may be obtained from equation 17 and values of 6 from equation 15.
and J is the nuclear spin coupling between the two non-equivalent sets. Subtracting equation 6 from equation 2, we obtain [(E6 - E,) - (Es - E & ) ] / & = A (10)
[(E3 - E,)
-
of C1F3.
The fluorine magnetic resonance spectrum of ClFB a t a frequency of 10 Mc. and a temperature of -60" is t h a t shown in Fig. 2 and consists of not five but eight separate resonances. It is apparent that if the spectrum of C1F3 is to be interpreted in terms of the model of Czvsymmetry, a higher approximation t o the nuclear spin-spin coupling must be utilized. Hahn and MaxwellShave treated exactly the case of a three-spin system consisting of two non-equivalent sets of nuclei, one set consisting of a single nucleus of spin = '/z and the other set consisting of two nuclei of total spin = 1. The allowed transitions for the model of ClF3 are given by the expressions E4 - E3 = &/2[?(2Ho 2hz) - A - B ] (1) E6 Es
(B2
-1320 -2144
,
I
1732
2435
(11)
From the definitions of A and B A' = S2
- SJ
+ 9Jz/4
(12)
(8) E. L. IIahn and D. E. Maxwell, P h y s . Rev., 88, 1070 (1952).
Fig. 4.-FI9
spectrum of ClF3 a t 40 Mc. and -60"
a value of -0.1 a t 40 Xc., the spectrumapproaches that of a simple doublet and triplet of relative integrated intensities of 2 and 1, respectively. Values of the nuclear spin coupling, J , and the chemical shifts, 6, for the three frequencies a t which the fluorine magnetic resonance spectrum of CIFl was examined are given in Table I.
resonance." U'hen the non-equivalent species tindergoing exchange are present in equal concentrations, the "fast exchange" single resonance appears a t the midpoint of the two "slow exchange" resonances. The average lifetime, 7 , in a given electrical (chemical) environment of a nucleus undergoing exchange is given by T = '/4a6w, where 6w is the chemical shift in cycles per second between the exchanging environments. TABLE I The 30 Mc. fluorine magnetic resonance spectra VREQUENCY DEPENDENCE OF THE CHEMICAL SHIFTA X D of ClFB a t temperatures between -30 and GO" are XUCLEAR Sprx COUPLING PARAMETERS FOR C1F3 shown in Fig. ,?. The resonance absorption of Chemical shift, Spin-spin s p l i t t i n g , C1F3 above 60" consists of a broad peak that proI:requenry, M c . 6 (c P.S.) . I(c.p.s 1 gressively sharpens with increasing temperature. 10 1113 378 The spectrum of CIFB below GO" consists of two 30 3369 39.5 peaks, one displaced to a higher and one to a lower 40 4210 435 frequency froni thc "high temperature" resonance. 'These two peaks sharpen as the teinperature is As can be seen from Table I, the values of the lowered until multiplet structures are revealed a t a chemical shift, 6, diverge slightly from a linear fre- temperature of about - 15". Thus, the behavior quency dependence, and the values of the nuclear of the magnetic resonance spectrum of ClF) as a spin coupling constants, J , apparently are not in- function of temperature is wholly consistent with dependent of frequency. However, in view of the that of a molecule uiidergoing fluorine exchange. The average lifetimes of fluorine atoms of ClFl difficulty of precise calibrations as the result of exchange broadening at temperatures above - 60" in a given environment a t - 15' where the niultiand the fact t h a t values of J and 6 must be derived plet fine structure of spacing 4-03 c.p.s. is smeared as from small differences between large numbers, the the result of exchange arc T = 1/4a(403)= 1.97 X sec. Xt 00" the two chemically shiftetl coinexperitnental values for 6 and J may be taken t o reasonably satisfy the field dependence conditions ponents merge into a single resonance, and thc for these two quantities in CIFI. These nuclear averaqe lifetime of t h e chemically exchanying magnetic resonance results confirm the structure fluorine atonis in a given environment is r = l/Jir. of C g v symmetry for ClF3. The chemical shift (3291) = 2.42 X 10-'sec. From these valucs of and nuclear spin coupling constants between the the lifetimes of fluorine in a given environment a t two non-equivalent sets of fluorine nuclei of ClF3, two temperatures, an average value of about 4.S the chemical shift being reduced to a resonance fre- kcal. over the temperature range - 1 5 O to GO" is quency of 10 Mc., are assigned the average values calculated for the activntion energy o f euchangt, for fluorine atonis in CIFJ. of 1097 and 403 c.P.s., respectively. The rooin teniperature fluorine magnetic rem2. Fluorine Exchange in C1F3 and BrFs.Exchange phenomena of sufficiently high frequency nance spectrum of J3rFI was examined and found to have been shown capable of collapsing a two-line consist of a single sharp peak. BrFs would be expected to exhibit a spectrum similar t o t h a t of ClFa spectrum into a single, concentration-dependent if i t too possessed a structure of Ctv symmetry. The absence of structure or breadth in the resonance of I BrFa, assuming the molecule t o possess a t least - 40' two non-equivalent sets of fluorine atoms, indicates that at room temperature the rate of fluorine exchange is so rapid that all spin-spin splittings and chemical shifts have been obliterated. Cnfortunately, the high melting point of BrF3 (8.S") prevents cooling, thus reducing the rate of fluorine -15' exchange in BrFJ to the point where the expected fine structure of the PI9 nlagnetic resonance spectrum of the molecule appears. Inability to observe slow exchange in RrF3 through the appeari 00 ance of structure or at least broadening in the F I 9 spectrum probably can be ascribed t o the existence of an activation energy for fluorine exchange in this molecule that is less than that of ClF?. 3. Fluorine Exchange in IFS and BrFb.-The fluorine magnetic resonance spectrum of IF6 shown in Fig. 6 is seen to consist of two lines of relative intensities 1 and 1 and multiplet structures of ,5 and 2 , respectively. The interpretation of the IF5n 7n-Y spectrum in terms of a tetragonal pyran i i d structure' confirmed an earlier structure tleter-
I
(11)
I1 S
CiitiiwSky
'in
