Chlorine-fluorine system at low temperatures: Infrared spectra of new chlorine-fluorine species

INORG.

NUCL.

CHEM.

LETTERS

CHLORINE-FLUORINE

Vol. 8, pp. 611-623, 1972.

Pergamon Press.

Printed in Great

Britain.

SYSTEM AT LOW TEMPERATURES: INFRARED SPECTRA OF NEW CHLORINE-FLUORINE SPECIES

M. R. Clarke, W. H. Fletcher, G. Mamantov, E. J. Vasini and D. G. Vickroy Department of Chemistry, University of Tennessee Knoxville, Tennessee, 37916 fRece ived I0 March 1972)

Recently we have been studying the photolysis of inert (N 2 or Ar) matrices (14-i7°K)

containing chlorine and fluorine.

chlorine monofluoride

aud molecular

A series of experiments

involving

fluorine led to the synthesis and charac-

terization by matrix isolation infrared spectroscopy of a new species, chlorine difluoride

radical

(CIF 2) (1,2).

Now we would like to report results

from the study of the chlorine - fluorine - matrix we have observed infrared absorptions species.

(Ar or N2) system,

in which

from at least three new chlorine-fluorine

While there is yet insufficient

cation of these compounds,

the

information for complete identifi-

the infrared data which have been obtained indicate

that these compounds likely fill some of the gaps in the series of chlorinefluorine polyhalogens

known up to now.

Experimental Reactant gases were individually mixed with matrix gases in a wellpassivated

stainless

si~Jltaneously

The two gas mixtures were then

deposited on 14-17°K infrared transpsrent windows of AgC1 or Csl

as in the previous mmoles/hour

steel vacuum system.

experiments

(1,2).

of total gas mixture.

used the Andonian AssociaUes

The rate of deposition was 5-10

Only the initial,

liquid helium dewar; most of the data were obtain-

ed using the Cryodyne Model 350 mechanical

refrigerator

the Per~Jn-Elmer Model 225 infrared spectrometer. sample temperature the low temperature

exploratory experiments

in conjunction with

With the refrigerator,

the

could be raised ~15-20°K by turning on a heater imbedded in flange to which the sample window was attached.

611

612

C H L O R I N E - F L U O R I N E SYSTEM

Vol.

8, No. 7

Samples were photolyzed after deposition with light from a General Electric BH-6 high-pressure mercury arc, filtered with one of three filter combinations.

Filter ! consisted of an ii cm quartz cell filled with water o

(transmits ~2000-9000 A) plus two Corning glass filters, CS-O-52 and CS-7-54 o

(combination

transmits ~3400-4200 A).

Filter II was the water filter in com-

bination with a i0 cm quartz cell containing (absorbs

(3) ~2700-4000 A).

Matheson research-grade 99.995%,

The water filter by itself constituted

Filter III.

argon, nitrogen and chlorine were used

(purities

99.999% and 99.965%);

from W. E. Tolberg

chlorine gas at 0.5 atm. pressure

high-purity

(99.95%)

(Stanford Research Institute).

fluorine was obtained Approximately

92% 37C12 was

prepared by the action of concentrated H2SO 4 on 96% Na37CI in the presence of MnO 2.

Chlorine monofluoride was prepared by heating an equimolar mixture of

chlorine and fluorine in a well-passivated

stainless steel vessel

(4).

Ordi-

nary cylinder grade fluorine or chlorine trifluoride were used for passivation of the gas handling system.

Results Even brief photolysis CI2-F2-Ar matrix samples

(10-15 min.) with either Filter I or II of

(molar ratios ranging from i:i0:i00 to 1:20:800)

yield-

ed a prominent new infrared absorption near 630 cm -I, along with weaker bands due to CIF and CIF 3.

Continued photolysis resulted in growth of the new band,

bands due to CIF and CIF 3 and appearance of still other new bands, much weaker, near 557 cm -I, 462 cm -I, and 270 cm -I. labelled A, B, C and D, respectively.

These four bands appear in Figure i, Samples prepared in a nitrogen matrix,

or in argon which contained an excess of CI 2 over F 2 (CI2-F2-Ar , i0:i:i00) *96% Na37CI was obtained from, and the method of preparation of 37C12 was sug**gested by, the Isotopes Division, Oak Ridge National Laboratory. In the spectrum shown (Fig. i), a band in the sample before photolysis appears at the same position as the "new" 270 cm -I band, here label~ed D. In fact, on an expanded scale it becomes clear that there are two different absorptions, separated by about 13 cm -I. The identity of this initial band is not known; it was not usually present in the initial deposit.

Vol. 8, No. 7

CHLORINE-FLUORINE SYSTEM

613

C

I.iJ (.3 Z <~

B

U

.2- i

D

CIF 3

n,-

0

U~ El <~

ClF 5

ClF5 A

80o

I

706

I

600

I

500

I

400

FREQUENCY

I

300

200

{CM-')

FIG. i Upper trace, infrared spectrum of system CI2-F2-Ar (1:10:400); lower trace, spectrum of same sample after 74 min. photolysis (Filter I) plus i0 mln. photolysis (Filter II).

yielded much weaker 630 cm -I absorptions and none of the other new bands. Photolysis of matrix samples containing only C12 or F 2 showed that both reactants were necessary for production of the new bands.

Periodically during the

experiments, regions other than those where CI-F vibrations are observed were scanned to confirm the absence of other types of vibrations, for example CI-O

614

CHLORINE-FLUORINE SYSTEM

stretches.

None of the band intensities

centration of either reactant. the new hands was a 1:20:800

could be directly correlated with con-

The optimum composition for production of

(CI2-F2-Ar)

sample;

was lower in both more and less dilute matrices interval and duration.

Vol. 8, No. 7

the intensity of the new bands for given photolysis wavelength

The use of Filter I resulted in the strongest

ties for all the new bands

intensi-

(A630 = .21, A557 ffi .045); the new species apparent-

ly were decomposed by continued exposure to the higher energy radiation passed by Filters II and III. While the growth and decay of the 557 cm -I band roughly paralleled that of the 630 cm -l band, ties at various Primarily,

there were enough differences

in their relative intensi-

times to conclude that they belonged to two different

the 557 cm

-i

species was more sensitive to photolysis,

rapidly destroyed by light filtered by water only (Filter III).

species.

and was more On the other

hand, the relative intensity of the 557 cm -I band to the 630 cm -I band was enhanced by use of Filter II, rather than Filter I. expected

Since Filter II is

to produce fluorine atoms at a greater rate than Filter I, because

of fluorine's

stronger absorption

in that spectral region

(3), this fact sug-

gests that an abundance of F atoms favors the formation of the 557 cm

-i

species.

The C and D bands could only be observed when A and B were near their maximum concentration,

hence neither could be related definitely

to either one of the

stronger bands. None of the above bands was very sensitive to an increase in temperature; they decreased somewhat when the matrix was warmed; strong,

but if initially

could be observed as long as the presence of the matrix allowed

spectra to be recorded. High resolution

scans

and 557 on -1 regions revealed structure,

(spectral slitwldth 0.3 cm -I) over the 630 cm -1 that the 630 cm -I band had a complex fine

containing up to eight overlapping

lines.

tive intensity of the various lines were sensitive photolysis

The presence and rela-

to matrix composition and

history and, in addition to isotopic splitting

(discussed

below),

Vol. 8, No. 7

CHLORINE-FLUORINE SYSTEM

615

probably represented the presence of a number of perturbed matrix sites (5). The 557 cm -1 band was simpler, containing at most two doublet pairs separated by 8 cm -I (attributed below to isotopic splitting).

The small splitting is

again attributed to site perturbations. To elucidate the nature of the fine structure in both the 630 cm

-i

and

557 cm -I bands, a sample was prepared containing approximately 92% 37CI 2 (CI2-F2-Ar, 1:10:800).

Figures 2 and 3 show the spectra obtained on photolysis

of this sample along with spectra from a photolyzed sample containing CI 2 in its natural isotopic constitution

(35C135CI:35C137CI:37C137CI, ~9:6:1).

experiment showed that the lines at 638.6, 634.2, 559.0 and 557.7 cm

-i

This are due

to two species each containing one 35CI atom* and those at 629.8, 625.5, 551.1 and 549.7 cm -I are due to the same two molecular species containing one 37CI atom.

Intensity ratios of approximately 3:1 for each pair of lines in the

natural sample and the absence of intermediate lines between the highest frequency lines in the natural sample and those in the enriched sample confirm the involvement of only a single C1 atom in each of the two vibrations observed. The splitting between isotopic pairs, as already mentioned, evidently represents some kind of difference in matrix sites induced by the proximity of other matrix components. The observed 35CI/37CI ratios for the 630 and 557 cm -I vibrations are almost the same as the ratio observed for the antl-symmetrlc CI-F stretch of CIF 3 (6) (see Table I).

It should be noted that bending the F-CI-F unit

reduces the isotopic shift, as is the case for the CIF 2 radical (2). During the course of the CI2-F2-Ar experiments, two other new bands were observed.

After lengthy photolysis at short wavelength (Filters II and III)

followed by warming of the matrix to about 25°K, a sharp new absorption appeared at approximately 302 cm -I and a broader one at 536 cm -I.

The bands

In the spectra of dilute natural isotopic samples used for the determination of exact line positions the very small matrix splittings (such as that between 634 cm-I and 635 cm-I in Fig. 2) had been removed by lengthy short wavelength photolysis.

616

%'ol. 8, No. 7

CHLORINE-FLUORINE SYSTEM

b}

I 640

i

I

I

I

I I I I 635 FREQUENCY

I

I t i 630 (CM -I)

t

t

I 625

I

FIG. 2 (630 cm

-i

region)

a) Spectrum of system C12 (natural isotopic mlxture)-F2-Ar (1:10:800) after 41.5 mln. photolysis using Filter II plus I0 mln. photolysis using Filter III (ordinate expansion by a factor of 1.5). b) Spectrum of system 37C12 (92%)-F2-Ar (1:10:800) after 25 min. photolysls using Filter I (ordinate expansion by a factor of 1.5).

Vol.

8, No.

7

CHLORINE.FLUORINE

SYSTEM

617

W

O~ I-4

..a

14

o

,u ,-I ,I-I O~

4J -,-4

14 0

O~

m

4J m

-,4

s u~

D

w

,uO t14 •

0~

I

...-~ q4 0 0 0

O v

0 .H

o 4J

°° 0 0 .~ ,-4 O oo

.H S tr~ t',,I

W

-

I~ It.

t~ 0J

O I

S U

O

q~ t~ o o oo °o o

4.1

.o

1.~ "O O ~ ~

O I

r.~

m~

I

m

o~ v ¢,,i ,-4

•

,H e4g*~ ,-4

t~

4~ 0~ 01 O,-.t O •U

I:~

o

4.1 t ~ 0 0 O) 0

S ~-~O

fJ ,.ou~

618

C H L O R I N E - F L U O R I N E SYSTEM

TABLE I.

Vol. 8, No. 7

Isotopic Shifts for Chlorine-Fluorine System

Observed Shifts (cm-I) 35CI

37CI

A

"J."/"i

CI2F 2

634.2 638.6

625.5 629.8

8.7 8.8

0.9863 0.9862

CI2F 3

557.7 559.0

549.7 551.1

8.0 7.9

0.9857 0.9859

CIF 3

683.2

672.5

10.7

0.9843

CIF 3

683.2

672.4

10.8

0.9843

CI2F 2

634.0

624.7

9.3

0.9853

Calculated Shifts (cm-I)

Only the mass of the central atom of the CI-F-CI unit in CI2F 2 was changed. Observed in an argon matrix by Frey, Redington,

and .~ljibury (6).

o

did not appear after photolysis with Filter I (3400 < ~ < 4200 A), and they were obliterated by brief photolysis using the water filter only.

Figure 4

shows the development and disappearance of both bands, labelled E and F, along with CIF2, in a particular experiment.

The symmetric stretch of CIF 2

falls (2) at 536 cm -I, but it is not nearly as intense in relation to the asymmetric stretch at 577 cm -I, as is the 536 cm -I absorption in this spectrum. Bands E and F were not obtained on UV photolysis of more concentrated samples (CI2-F2-Ar, 1:10:400), but were produced in a 1:10:800 sample. 302 em

-i

was also observed in two experiments in which CIF-CI2-N 2 (1:10:200)

samples were photolyzed. between 530 and 545 cm -i cm

A band at

-i

Unfortunately, absorption by aggregated C12 (7) precluded observation of an accompanying band at 536

Vol. 8, No. 7

CHLORINE-FLUORINE SYSTEM

619

0

B

c,%

W

E

.2 l.d (J Z ¢n

nO O0

m.4

c,~ c,~

•

I 6OO

I

I

I

soo 4oo ,3oo FREQUENCY (CM-')

2o0

FIG. 4 Spectrum of system CI2-F2-Ar (1:20:800): a) b) c) d)

25 min. photolysis using Filter II plus 20 min° photolysis using Filter III. Sample temperature raised from 17°K to 25°K, returned to 17°K. Sample temperature held at 29°K, immediately following trace b). Sample shown in (c), photolyzed for lO min. using Filter III. W is a "window" band, caused by reaction of the Csl window with chlorine or fluorine in the sample.

620

CHLORINE-FLUORINE SYSTEM

Vol. 8, No. 7

Discussion On the basis of their isotopic splittings, the 630 cm

-i

and 557 cm

-i

bands are most plausibly ascribed to the asymmetric stretching vibration of linear F-CI-F units in the two new species.

In order to consider what these

species might be, a series of possible chlorine-fluorine compounds containing increasing numbers of atoms can be written down: CIF, CIF2, CI2F , CIF3, C12F2, CI3F , CIF4, CI2F3, C13F2, ... Of these compounds, CIF, CIF 2 and CIF 3 are known.

The simplest of the other

molecules which could contain an F-CI-F unit would then be CI2F 2 which, in analogy with CIF3, would be expected to be a symmetrical, T-shaped molecule, F-C~-F . Although other isomers of CI2F 2 are conceivable, only this one would C1 exhibit a vibration characteristic of a linear F-CI-F as~mnetric stretch. This molecule should have infrared active asymmetric and symmetric F-CI-F stretches near those of CIF3, a CI-CI stretch somewhere below that of diatomic chlorine and three bends at lower wave numbers.

The 630 cm -I band is, in fact,

reasonably close to the 683 cm -I (in argon matrix (6)) asymmetric stretch of CIF 3.

If we assume that the 462 cm-I and the 270 cm -I bands go along with the

630 cm -l band, they could readily be interpreted as the CI-CI stretch and one of the bends.

Frequency calculations were performed on CIF 3 and CI2F 2 in

order to compare the observed 37CI isotope shift with the expected shift for both molecules.

These results are shown in Table I following the observed

isotope shifts.

The agreement of the observed and calculated values is

excellent for CIF 3 where the force field is adequately defined.

The agreement

is understandably poorer for CI2F 2 where not all fundamental frequencies have been observed, and the force field assumed is only approximate.

For the latter

molecule the frequency of ~5 (in-plane F-CI-CI bend) was assumed to be 250 cm in order to calculate the isotope effect.

While this is an assumed figure, it

is shown to be reasonable by using a mass of 35 for the axial fluorine atom with a CIF 3 model and force constants.

-i

The corresponding bending force

The symmetry coordinates used were the same as those employed by Frey, Redington, and Aljibury (6).

Vol. 8, No. 7

CHLORINE-FLUORINE SYSTEM

621

constants in CI2F 2 would, of course, be smaller than those of CIF 3.

There is

an observed band at 270 cm-I which has been assigned to C12F2, but the relative intensities of 93 and 95 in CIF 3 suggest strongly that this is u 3 of C12F2, not ~5"

The tentative assignment of the 630 cm -I band to CI2F 2 is consistent with

its early appearance in the Cl2-F2-matrix system, as it could readily be formed by the light-lnitlated reaction of C12 and F 2 molecules in nearby matrix sites: C12 + F2

hu~

CI2F 2

It is of interest to recall that very little of the new species is formed in a nitrogen matrix.

Milllgan and Jacox have noted that HCIH- cannot be produced

in nitrogen (8), and they have attributed this fact to greater isolation efficiency of a nitrogen matrix. To explain the presence of a second F-CI-F vibration, as in the 557 cm

-1

band, we must consider the more complex members of the chlorlne-fluorine series, namely CIF4, C12F3, C13F 2.

As the 557 cm

-i

species formation generally followed

that of the 630 cm -I species, and the 557 cm-I band was never present unless the 630 cm -1 band was strong, the 557 cm -1 species was llkely formed from the 630 cm

-1

species, tentatively identified as C12F 2.

Migration of C1 atoms

through the matrlx is unllkely (9), so the most probable reaction would involve the encounter between CI2F 2 and a migrating fluorine atom released by photolysls of F2: F-CI-F + F + F-CI-F C1

Such a process would explain the enhancement of the 557 cm -I band in those experiments where Filter II was used.

This possible product may be interpreted

as a more or less tightly bound complex between CIF 2 and CIF.

We did not

observe any band which could be attributed to the CI-F stretch of the CIF "part" of such a molecule, but the CIF stretching region contained strong bands from CIF and CiF 3 which would obscure all weak bands in that region.

No calcu-

lations were made on CI2F 3 because insufficient data are available for its

622

CHLORINE-FLUORINE

SYSTEM

Vol. 8, No. 7

rce field, but it is reasonable to assume that the near linearity of the F-CI-F unit will be maintained, and that the frequency ratio corresponding to ~5/~5 will differ only slightly from that observed in the other two cases. Considering the third new species observed in these experiments, we find support for the assignment of the 302 cm -I band to a CI-CI stretch of CI2F.

This band is very similar in its behavior to the 577 cm -I band of CIF2,

in that it does not appear directly on photolysis, but after slight warmup of the matrix, implying that it is formed as a result of increased F atom difusion It would be formed in a manner analogous to that of CIF2, in the CiF-F2-matrlx system (1,2): CIF-F2-matrix:

CIF + F + CIF 2

Cl2-F2-matrlx:

Cl 2 + F + CI2F

Sensitivity to photolysis, as demonstrated by the 302 cm -I band, would prevent the accumulation of this species during photolysis.

To test the plausibility

of this assignment, we calculated the diatomic force constant for a Cl atom vibrating against the combined mass of a CI and an F atom, with a vibrational frequency of 302 cm_l.

o This gave a force constant of k ~ 1.2 mdyne/A.

In

o

Cl 3 (i0) k is about 1.3 mdyne/A, so this is not an unreasonable value. The 536 om-i band could then be the CI-F stretch of the same molecule. While this might seem somewhat low for a CI-F stretch, by comparison with CI2F+ (ii), it is reasonable.

Adding one electron to C12 F+ is expected to

reduce the force constants by about half, as shown for CIF2+ (12,1,2) and C13+ (lO,ll).

This reduction in force constant would reduce the CI-F stretching

frequency by a factor near ~ ,

from 744 cm -I to a value near 530 cm -I.

Thus

536 cm -l is not an unreasonable value for a CI-F stretch in CI2F. Conclusion While we have not been able to identify conclusively the three new species obtained on photolysls of the Cl2-F2-matrlx system, all of the evidence is consistent with their assignment to the unknown species CI2F , C12F2, and CI2F 3.

It is interesting to note that, whereas the CIF 2 radical has a bent

Vol. 8, No. 7

CHLORINE-FLUORINE SYSTEM

623

geometry (2), the two F-CI-F units observed here appear to be linear, as in CIF 3.

The presence of a third atom bonded to the CI atom of CIF 2 apparently

favors a linear arrangement.

It should be noted that C13, formally isoelec-

tronic with CIF2, is also linear (I0).

These differences show that synthesis

and characterization of similar small interhalogen species should be of considerable interest. AcknowledKement This research was supported by the Air Force Propulsion Laboratory, Edwards Air Force Base, California and the University of Tennessee.

References i.

G. Mamantov, D. G. Vickroy, E. J. Vasini, T. Maekawa, and M. C. Moulton, Inorg. Nucl. Chem. Letters, 6, 701 (1970).

2.

G. Mamantov, E. J. Vasini, M. C. Moulton, D. G. Vickroy, and T. Maekawa, J. Chem. Phys., 54, 3419 (1971).

3.

J. G. Calvert and J. N. Pitts, "Photochemistry," John Wiley and Sons, New York, N.Y., 1966, p. 184.

4.

G. Brauer, Handbook of Preparative Inorganic Chemistry, Academic Press, New York, N.Y., 1964, 2nd ed., Vol. I, p. 153.

5.

L. Andrews, Ann. Rev. Phys. Chem., 22, 109 (1971).

6.

R. A. Frey, R. I. Redington, and A. L. K. Aljibury, J. Chem. Phys., 54, 344 (1971).

7.

M. R. Clarke and G. Memantov, Inorg. Nucl. Chem. Letters, 2, 993 (1971).

8.

D. E. Milligan and M. E. Jaeox, J. Chem. Phys., 53, 2034 (1970).

9.

M. E. Jacox and D. E. Milligan, J. Chem. Phys., 53, 2688 (1970).

i0.

L. Y. Nelson and G. C. Pimentel, J. Chem. Phys., 47, 3671 (1967).

ii.

R. J. Gillespie and M. J. Morton, Inorg. Chem., 9, 811 (1970).

12.

R. J. Gillespie and M. J. Morton, Inorg. Chem., ~, 616 (1970).