Epectmchimica A&a. VoL 278. pp. 125 to 129. P-n Prea 1971. Printed in Northern Ireland

The Raman spectrum of PF, gas*?

FOIL A. Mrnrxn and ROBERT J. CAP~ELL Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 16213

(Rec&ed 26 Janplary 1970; rev&d 22 April 1970)

Al&r&-The Ramen ape&nun of both gas and liquid PF, has been obtained. The long-missing fundamental v, has been identified with certainty at 174 cm-r in the gas, and 177 cm-r in the liquid. It is the second strongest band in the spectrum.

This new value makes the previously-published tables of thermodynamio properties in- correct. Good, reliable ges-phase frequencies are now known for all the fundementals of PF, and are tabulated here.

These data neither prove nor disprove Berry’s mechanism for interchange of the axial and equatorial fluorines.

THE vibrational spectrum of PF, has been the subject of a number of recent pub- lications [l-6]. An interesting feature of PF, (and of some other trigonal bipyramidal molecules) is that intramolecular exchange of fluorine atoms occurs readily. This can be represented by a symmetrical double minimum potential curve [7,4]. The

axial PF, bending mode approximates, to some extent, the coordinate responsible for the fluorine exchange. Heretofore this mode has been designated Y,, the lowest e’ fundamental.

Although v, is both infrared- and Raman-allowed, and a knowledge of it is much desired, it has been very difficult to locate. From the infrared spectrum of the gas two values have been reported, 126 [l] and 300 om-l[3]. Both are now known to be incorrect, for the former has been shown to be due to HF impurity [2] and the latter to a difference tone, probably va - va [6].

Recently LEVIN [6] obtained the Raman spectrum of solidPF,, and concluded that v7 is at 179 cm-l. He has also calculated the potential energy distribution for the e’ modes [6a], and found that contrary to earlier ideas us (533 cm-l) is a nearly pure axial PF, bending whereas v, (179 cm-l) is mainly an in-plane PF, deformation.

Our work was done before we became aware of Levin’s results. We report, for the first time, the complete gas phase Raman spectrum of PF,. The liquid was also studied. v, was easily found in both.

* From a thesis to be submitted by R. J. Capwell in partial fulilllment of the requirements for the Ph.D. degree at the University of Pittsburgh.

t This work was supported by Grant DP-9260 from the National Soience Foundation. The purchase of the Raman equipment was aided by NSF Grant GP-8287.

[1] J. E. GRIFVITES, R. P. CARTER and R. R. HOLLIES, J. C&m. P@u. 4.& 863 (1964). [2] L. C. HOSKINS, J. Chm. Phys. 42,263l (1966). [33 J. E. GRIXFITHS, J. Chem. Phya. Q&2632 (1965). [4] L. C. HOSIZINS and R. C. LORD, J. Chem. Phys. 46,2402 (1967). [6] R. M. DEITERS and R. R. HOLES, J. Chem. P&p. 48,4796 (1968). [6] I. W. LEVIN, J. Chem. P&8. M), 1031 (1969).

[6a] I. W. L~VIN, J. Mol. Spectry 33, 61 (1970). [6b] I. R. BEATTIE, K. M. S. LJ~INQST~N and D. J. Rxvxonns, J. Chm. Plya. al,4269 (1969).

[7] R. S. BEIL&Y, J. Chem. Phyu. 82, 933 (1960). 126

126 F. A. - and R. J. Cm

Very reoently BEATTIE et al. [6b] have reported finding Reman spectrum of the gas.

v, at 176 am-l in the

Phosphorous pentafluoride was obtained from the Matheson Gas Produots Co. Its purity was cheoked by its mid infrared spectrum, and detectable amounts of POF, were found. Attempts were made to purify the sample by condensing the POFs (and presumably RF) in a --78”C dry-ice bath. However a very small amount of POFa still remained.

Raman spectra were obtained for both the gas and the liquid. Both spectra were measured with a Spex Ramalog instrument desoribed elsewhere [S], but with different excitation sources. For the gas work, a Carson Model 300 argon ion laser with an output power of 1 W in the 4880 A line was used, The gas cell was a cylin- drical .ultra~olet-tie cell with a side arm. The flat windows were perpendicular to the laser beam. The liquid spectrum was excited with a Spectra Physics Node1 125 He/Ne laser giving 80 mW at 6328 8. The low temperature sample holder has been described by Mr~xa and UNXY [S]. The liquid spectrum was weak and difficult to obtain-much more difficult than for the gas.

Polarizations were measured for the gas, and for the 177 and 816 cm-1 bands in the liquid.

Figure 1 Bhows the gas phase Raman spectrum. The Raman frequencies are listed in Table 1 and all of the fundamental vibrations of PF, are summarized in Table 2.

I 314

PF5 GAS

Fig. 1. Raman spectrum of PF, gas. Pressure -710 torr. Spectral slit width 6 cm-l. Laser power -1 W at 4880 A. Time constant 1 sec. Scan speed 60

cm-+Gn.

183 K. E. RBEE and F. A. I&u.ax, &uatvoc?G~. Actu 88A, 3076 (1970). [Q] F. A. hEB and B. M. Hmmy, AppZ. Sjmctry 24,271 (1970).

The Remen spectrum of PF, gae

Table 1. Raman spectrum of PF, (cm-l)

127

Ao’

Gee Liquid

Rel. Peak Int. Polzn& This work ( - 80%) R&k. [I, 41 Nt

174 bb 0.76 177 (0.70) - 3(6’) 620 6b 0.76 513 514 (dp) v,+“)

~636 3 sh dp ~631 534 (dp) +s(e’) 848 3b 0.67 543 840 (P) “&I’) 816 100 0.12 816 (0.14) 817 (P) v&l’) 853 Cl - -

$7 z Ii8Y8)

876 tl - - POF, impurity -1029 <l,b ~1026 1026 (dp) pe(e’)

p - polarized; dp = depolarized; b = broad; ah = shoulder.

Table 2. Funtiental vibrations of PF,

A8signmenta (cm-l)

Rocom- HOSKINS Lxvr~ mended

Approximete Gxrrxrrxa end (solid) Thin v&co Specioa Activity No. description et oz. [1] LOxD [4] WI work (gm)

%’ R (P),- 1

2

%i” -# till) 3

4

8’ R(dp),ir(l) 6

6 7

e” R (dp), - 8

P-F strctoh, iIlph&M

P-F stretch, out of phese

(

PF, 8ntieym. stretch PF, sym. out of plene bond

PF, degcn. stretch

PF, bends PF, in plane

deform- ations

PF, rocking

817*

840*

945

576

1026

633

126t

514*

946.6

575.1

1024

632.5

300t

816.5 816 816

631.5 848 848

- - 946.6

- - 676.1

1016 -1029 1025

623 636 632.5

179 174 174

507.6 620 520

* Liquid state values. AU others except Levin’s are for the gas. t Scctoxt. 2 “Agrce~ quantitatively with GrifEtha et oZ.” 4 Ucvx~ [S] eugegsta this order for the desoriptiona.

DISCUSSION

Inf&ed and Raman [l, 41 and electron diffraction studiet3 [lo, 111 have ahown

PF, to be a trigonal bipyramidal molecule of point group D,. All six Ramau active fundamentals have now been observed in both the gas and the liquid states. The assignments are straightforward and only a little discussion is needed. Clearly the two polarized bands at 816 and 648 om-l are the two totally eymmetria P-F

[lo] L. 0. B~OCKWAY end J. Y. B~AOH, J. Am. Chem. Soo.60,1836 (1938). [ll] K. W. HANxoN and L. 8. BABTELL, Inorg. Oh. 4,1776 (1966).

128 F. A. MILLER and R. J. CAPWICLL

stretching vibrations (a,’ species). The depolarizstion factor for the 648 cm-l band is rather high (0.67, vs. 0.76 for a depolarized mode), but this difference is well outside experimental error. Our experimental values for lines which are known to be depolarized fall in the range 0.75 f 0.03). Also the band exhibits a oentral & branch which is characteristic of totally symmetric vibrations [12]. Although v1 (816) is very strong, vs (648) is quite weak. In vs the axis,1 and equatorial P-F bonds stretch out of phase with one another, and apparently the polarizability change is small as a result of a near cancellation from these two opposing motions.

The v, band has not been observed in the infrared spectrum of the gas despite the fact that pressures up to 2 atm. were used. In the gas phase Raman spectrum it is the second most intense band in the spectrum. We also had no trouble finding it in the liquid. One wonders, then, why GRIFXTTES et al. [l] snd HOSKINS and LORD

[4] did not detect it in their studies of the liquid. We believe that it was beosuse both used a Carry 81 Raman spectrometer with Hg 4358 A excitation. This system gives a bothersome false band near 170 cm-l. It is also rather difficult to get spectra of liquids under 200 cm-l with this instrument. One must use “single slit ” and sacrifice half the intensity.

Table 2 gives the earlier assignments, our assignments, and a reaommended set of gas phase values for sll the fundamentals.

Significant errors occur in the two previous tabulations of the thermodynamic functions because of the use of 126 [I] or 300.6 cm-l [3] for v,. Using our Raman g&8 values for vi, vg, v? and va, and infrared gas values [4] for v3, vg , v5 and vs, we calculate at 298.15OK, B” = 71.93 cal/deg mole snd -(F - H,/T) = 58.67 cal/deg mole.

The NMR spectrum of PFc reveals only a single type of fluorine atom in both gas and liquid (gas at room tempersture, liquid at -75%) [13]. There therefore seems to be some mechanism which scrambles the axial and equatorial fluorine atoms rapidly relative to the NiMR time scale snd which operstes even for isolated mole- cules. Berry suggested that this occurs via the e’ axial bending vibration [7]. If this vibrstion is csrried to large amplitude, it can produce the interchange. HOSKINS

and LORD [4] give a helpful f&ire whioh illustrates this. The vibration then has s, symmetrioal double minimum potential function, and the axial-equatorial inter- change involves going over or tunneling through the potential barrier. The potential function will be anharmonic, and one can look for any evidence of this in the spec- trum. It will be evidenced ss hot bands (1 + 2, 2 -+ 3, etc.) displaced to lower frequencies from the fundamental (0 -+ 1).

Let us assume initially that the e’ axisl bend is v, at 174 cm-l. It shows no evidence whatever of enharmonicity. The 174 cm-l band is the only one observed between 75 and 520 cm-l. Furthermore it is symmetrical in shape; there is no reason to believe that hot bands overlap it. We therefore believe that the potential well is rather deep-sufficiently so thrtt the Gret few vibrational spacings are nearly identical.

It is illuminating to consider the relative populations of the various levele relative

[IZ] G. ~~RZBERQ, Inframi and h!aman E&v&m of Polyatomk MolecuZw, p. 442. Van Nos- trand (1945).

[13] H. S. GUTOWSKY, D. W. M&ALL and C. P. SLIO~~, J. Ckm. Phye. 21, 279 (1963).

The Raman spectrum of PF, gas 129

to that of the lowest state. These are given in Table 3. It is clear that there is ample population up to w = 3 to reveal any appreciable anharmonicity. Since none is observed, we conclude that the barrier is certainly much greater than 500 cm-l. It is our belief that it must be at least 2000 cm-l. This, however, is still low enough so that the axial-equatorial interchange can occur rapidly [a].

Table 3. Relative populations of several levels of V, at 300%

V cm-l Degeneracy N,IN,

0 0 1 1.00 1 174 2 0.87 2 2 x 174 = 348 3 0.56 3 3 x 174 = 522 4 0.33 4 4 x 174 = 696 5 0.18

Now let us assume that the e’ axial bend is ys at 532 cm-l as claimed by LEVIN

[se]. In the Raman spectrum this is badly overlapped by the 520 cm-l e” band ye, so it is hard to draw conclusions about the anharmonicity. In the infrared, however, ys is forbidden and vs is therefore free of interference. It too shows no sign of anhar- monicity [a].

Unfortunately, although we can safely conclude for both assignments that the barrier must be greater than 2000 cm-l, we cannot say how much greater. Since a barrier of 2000 cm-l would allow rapid interchange, our results can neither prove nor disprove Berry’s mechanism.

Note added in poof

Dr. M. Brownstein of McMaster University has informed us th& he has examined the fluorine NMR spectrum of PF, in an inert solvent down to - 140%. It remains a sharp doublet (due to coupling with phosphorous), and shows no indication for two kinds of fluorine atoms.

We are indebted to Professor R. S. Berry for pointing out that his mechanism involves rcn admixture of both the d axial bend and the e’ inplane deform&ion. The potential surface for the interchauge has 20 identical minima, not just two. (They are divided into two sets of ten eeoh.) Each minimum is connected to three others by the large-amplitude vibration.

9