Photoelectron and vacuum ultraviolet spectra of a series of fluoroethers

435

Journal of Fluorine Chemistry, 5 (1975) 435 _ 442 0 Elsevier Sequoia%A., Lausanne-Printedin theNetherlands

Received: October6.1974

PHOTOELECTRON

AND

VACUUM

ULTRAVIOLET

SPECTRA

OF

A

SERIES

OF

FLUOROETHERS

A.H.

HARDIN

and

Department

of

C.

SANDORFY

Chemistry,

University

of

Montreal,

P.Q.,

Canada

SUMMARY

FluOroethers

of the F(~F(cF~)cF~~)~cHFcF~

have their lowest

ionization

ordinary

Their ultraviolet

ethers.

at high frequencies, are complex

the existence

about 3.5 eV higher than

absorption

at about 80000 cm

indicating

cular orbitals

potentials

-1

(n = 1-4) type

spectra

or 10 ev.

of several

also begin

Both spectra

close lying mole-

in these compounds.

INTRODUCTION

The photoelectron ethers have been examined established molecules

and ultraviolet by several

that the lowest

the electronic

concentrated

transitions

and 120 nm also originate also been studied

authors

(1-8).

(first) ionization

is due to ejection

lone pair orbital

spectra

of an electron

of saturated It is well

potential

in these

from the essentially

around the owgen

atom and that

giving rise to the bands between in that orbital.

Fluoroparaffins

(7)(9) and it is known that the presence

200 have of the

l

436 fluorine

atoms causes large hypsochromic It seemed to us interesting

uv spectra. studies

on some fluoroethers

fluorines

shifts in both the PE and then to perform

in order to examine the effect of the

on the oxygen

lone pair orbitals

The following

compounds

F(CF(CF3)CF20)2CHFCF These exchangers

requiring

and high thermal

known under the designation we shall use these symbols

in general.

: CF3CF2C@2OCHFCF3,

were studied

are increasingly

and for applications

high resistivity

and the spectra

3, F(CF(CF3)CF,O)3CHFCF3

compounds

similar

and F(CF(CF3)CF,0)4CHFCF3. used in industry

low dielectric

and chemical

constant, They are

stability.

freon E-l, E-2, . . . etc.

as heat

For convenience

in this paper.

EXPERIMENTAL

The samples 3u Pont de Nemours purities

of fluorinated

and Company

rities

of Wilmington,

were checked by infrared

nance and gas chromatography. of greater

o.d. stainless

ethers were supplied by E.I.

absorption,

(30/60 mesh acid washed 'Krytox'.

on a 6 m (1.0 cm

with 33 % 'Kel-F' oil

column of chromosorb-GHP

and treated with dimethylchlorosilans)

Impurities

are mainly

reso-

found to have pu-

than 99.2 .wt. % when separated

steel) column of chromosorb-W

Sample

proton magnetic

The samples were

No. 3 or on a 3 m (0.5 cm o.d. copper)

55

Delaware.

residual

homologues

with

of this

ether family. One sample,%1, in order to verify

was further purified

the vacuum ultraviolet

tion of the E-2 and E-3 homologues than 99.9 %.

chromatographically

spectrum.

After

separa-

from E-l, its purity was greater

437

The photoelectron Elmer ~~-16 spectrometer using the oxygen 12.08, 15.59

spectra were recorded

Spectra were calibrated

with a He1 source. 12.32,

using a Perkin-

12,54 and 12.73 eV lines, the nitrogen Sample pressures

eV line, and argon 15.75 and 15.93 eV lines.

of 0.07-0.09 recorded

Torr

typically

(at the sample inlet) were used and spectra were with a counting

of 4 set and at 1 mm/min. vertical

Ionization

potentials

absorption

7 to 95000 cm-' were recorded chromator.

Radiation

from a hydrogen

grating before

beam sampling

attachment.

pair of EMI-6256-S tronics.

Details

UV absorption

passing

discharge

spectrum

is shown with molar

Some difficulty

-1

was encountered

for the photoelectron

to record&'jphotoelectron

a very low vapour pressure

material

with a

counting

elec-

The vacuum

extinction

coefficient

with a sample pressure

-1

A slit width

and 5nbelow

in stabilizing

work.

from>:-li.o1:-)1 in the order of increasing

model 665 double

was measured

LiF windows.

to 65000 cm

by a

(10,ll).

and was obtained

used from 95000 cm

was dispersed

to a McPherson

have been given previously

-1

model 225 lm mono-

tubes and photon

of 0.15 Torr in a 9.0 cm cell having

inlet pressure

in the range 50000 cm

Linear absorbance

photomultiplier

against wavenumber

of 2O~was

spectra

using a McPherson

1200 lines/mm

Attempts

quoted are for

transitions. The ultraviolet

plotted

rate of 4 x 103, a time constant

The problem boiling

65000 cm

.

the sample increased

point and viscosity.

spectra were unsuccessful appeared

-1

since

to form at the inlet port.

438 RESULTS

AND

DISCUSSION

The first peak in the PE spectrum The band

is broad,

however,

of E-l is at 13.9 eV.

with the onset at aLdut 12.6

eV

(Fig. 1).

'5t

E4

E3 E2 El

14 I6 ELECTRON

12 Fig. 1

18 VOLTS

The photoelectron

20

spectra

of F(CF(CF3)CF20)nCHFCF3

with

n = 1,2,3 or 4 (E-l, E-2, E-3, E-4).

Since the assignment

of this band is the most important

study we will discuss a much lower value.

For dimethylether,

ether and tetrahydrofuran 9.35

(0) is stabilized

towards

explaining

E-l and similar arises whether occupied

mainly

compounds.

populated

and 12.74

diethylether, values

orbital

Thus the oxygen

stability

lone pair or-

and very low associability

of

Since the 0 IP is that high the question

in the fluoroethers

the highest

in the C-C bonds;

n-C5Fl2

diisopropyl-

are at 9.94, 9.51,

that.is the hi&<-St

or not it is still am:$inly~otbital

molecular

n-C4F10,

ethers the first IP has

by about 3.5 eV, a fact that goes a long way

the chemical

In perfluoroparaffins

13.04

the vertical

(7) and 9.L6 eV (8) respectively.

bital

C3F8'

In saturated

it first.

point of this,

and n-C6F14

eV respectively

occupied

as it is in the ethers.

orbital

is known to be

the first IP for CF are 16.20,

according

14.48,

4' C2F6'

13.70,

13.30,

to the data of Robin and his

439 It is thus quite possible

coworkers

(7).

centered

at 13.9

to ionization

eV contains

from orbitals

ethers the second

1.6

ev.

oxygen

contributions of mainly

IP is widely

from one or more

having

from the first one, by about a large population

atom but less than for the highest potential

the fluoro-ethers. C-H bond might

A MO with a significant

of C-C character. energies).

population

follow at higher

from orbitals

of mixed

many fluorine

lone pair and bonding

side of the PE

more at higher

frequencies.

In saturated

to only one distinct The PE spectra

of E-l (Table I).

chain length,

at low

energies must originate with the

contributing

more and

such orbitals

none of the apparent

peaks

of ionization.

with changes

for the dimethyl,

due to increasing

diethyl

and diisopropyl

above.

We shall present the spectra

MO is

of the first band shifts by -0.42 eV in

This is consistent

such as observed

ethers mentioned

process

in

of E-2, E-3 and E-4 are very similar to that

The maximum

going from%I toE-4.

to intervene

molecules

Probably

have IP's above 15.5 eV.

corresponds

occupied

C-O, C-C, C-H, F and C-F character C-F orbitals

in

in the single

eV for CHF 3 (12)(13);

Thus the C-H is not expected

The PE bands which

This

frequencies

such as CH -CHF2 or CH3-CF (14) the highest 3 3

molecules

around the

orbital.

to the high energy IP is at 14.8

(The lowest

band at 13.9 eV.

usually

occupied

is likely to move to much higher

also contribute

IP's due

In nonfluorinated

C-C character.

separated

It is due to an orbital

ionization

yield

that the broad band for E-l

the ultraviolet

of our other compounds

no additional

information.

is at very high frequencies

are expected

of only E-l since

to be similar

The onset of UV absorption

(Fig. 2).

compounds

and to for E-l

This is in keeping with the

high first IP as well as with the spectra among the most translucent

spectrum

of fluoroparaffins

that exist.

The bands

which are in the

440 TABLE

1

Vertical

ionization

n = 1,2,3 and 4.

potentials

of F(CF(CF3)CF20)nCHFCF3

(E-l, E-2, E-3, E-4). n=2

i-l=1

with

ev

n = 11

n=3 eV

eV

eV

13.17 sh

12.76 sh

12.96 sh

13.89

13.77

13.78

13.47

14.90

14.87

14.87

14.96

16.17

16.18

16.20

16.22

16.82 sh

16.99 sh

16.90 sh

16.81 sh

17.51

17.56

17.49

17.53

20.48

20.21

20.33

20.12

6

\

5 -r "0 ;4 TE " -_ 3 @ G PI 2r

0I-_, 60,CCQ

\

I 70.000

90,000

80.000 CM-'

Fig. 2

The vacuum (E-l). ficients

ultraviolet

Wavenumbers

absorption

in cm

in liters mole

-1

spectrum

against molar

-1 cm-1

of CF3CF2CF20C extinction

coef

441 spectrum 84500

of E-l are broad.

and 87500

We find shoulders

CIII-~ and a band maximum

case of the photoelectron single electronic sitions

transition.

might contribute

broad bands

bands,

with centers

at 92000 cm

-1

.

near 81000,

Just as in the

none of these seems to be due to a

Both Rydberg

to the observed

and valence-shell

intensity.

do not enable us to make specific

tran-

The observed

assignments,

however.

CONCLUSIONS

The coincidence spectra

of several bands

leads to the frustating

individual

bands

situation

due to given transitions

certainty.

The main conclusion

the E-l,...

type have their lowest

13 eV, and that the highest close to each other. begin

in both the PE and UV that we cannot

identify

with any degree of

is that fluorinated

ethers of

IP's at very high values,

orbitals

Correspondingly

above

of 0 and C-C character the ultraviolet

must lie

spectra

also

at very high frequencies.

ACKNOWLEDGEMENTS

We acknowledge Research

Council

sociation.

financial

of Wilmington,

from the National

of Canada and from the Manufacturing

Our special thanks

C.S. Hoffman

assistence

Chemists

are due to Dr. R.L. McCarthy,

and Dr. R.B. Ward from the Du Pont de Nemours Delaware.

and Mr. R. Routhier

We are also indebted

from our laboratory.

As-

Dr.

Company

to Mr. P. Sauvageau

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