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On the far ultraviolet absorption spectrum of ethyl fluoride
15 October
CHEMICAL PHYSICS LETTERS
Volume 22, number 3
ON THE FAR ULTRAVIOLET
ABSORPTION
SPECTRUM
OF ETHYL
1973
FLUORIDE
Tomoko YAMAZAKl and Katsumi KMURA Physical Chemisrr/
Loborarory,
Zxriture
of Applied Electricity.
Hokknido
Univexity,
Sapporo
&Or J.qmn
Received 10 July 1973
The absorption spectrum of CzHsF has been measured in the region down to 1150 A with absorption maxima at 1290, 1240 and 1180 A and has been interpreted with Rydberg transitions from the o(K) and pseudo 5r(CH3) orbitals to the carbon 3s and 3p orbitals.
Since high-resolution photoelectron spectroscopy has made it possible to detemline ionization potentials of various valence shell molecular orbit& [ 11, a Rydberg transition in an organic compound is an interest-
WAVELENGTH
Cd.1
ing subject to be investigated using these ionization potentials. A recent photoelectron study on ethyl and n-propyl fluorides by the present authors [2] has in-
dicated that the first ionization potentials of these fluorides are 12.43 and Il.96 eV, respectively, which are about 0.4 eV higher than those of the parent alkanes because of the inductive
effect of the fluorine
atom. Our photoelectron study [‘2]has also shown that the highest occupied orbitals of ethyl and ?Ipropyl fluorides are of q-C type rather than of 7r-H type, in contrast to the situation in methyl fluoride3 where it is of nCHs. The outermost orbitals of the alkyl fluorides are also different in nature from those of the corresponding alkyl chlorides, bromides and iodides which have the halogen lone pairs giving rise to the first ionization potentials [I] . It is surprising that alkyl monofluorides except CH,F lack in the literature of far ultraviolet studies, in spite of several reports on far ultraviolet spectra of fluorine-substituted alkanes [S--8]. In connection 4th
our recent
photoelectron
study
of ethyl
fluoride,
the present work we have undertaken to study its ultraviolet absorption in the region up to 87 000 cm-l, placing special emphasis on Rydberg transitions. Ethyl fluoride was synthesized from the corresponding p-toluene&lphonic acid ester according to the direction of Edge11et al. [9] and was purified by disin
.616
Fig. 1. The far ultraviolet the gas phase.
absorption
spectrum of C*Hs I: in
tillation Paice under vacuum. Absorption measurements for ethyl fluoride in the region down to 1150 A were carried out in the vapour phase at room temperature by a recording vacuum ultraviolet spectrophotometer of Seya-Namioka type? Jasco WV-LB, the apparatus containing a Bausch and Lomb concave grating with a radius of 50 con. A gas 41 of 55-mm path length with lithium fluoride windows was used. Th’e cell and the technique of handling the gaseous sample are essentially the same as those used in our earlier work [lo, 111. As seen from fig. 1, the far ultraviolet absorption spectrum of ethyl fluoride differs greatly from those of other ethyl monohalides [ 121 which show low-
Volume. 22, number 3
CHEMICAL PHYSICS LETTERS
Rydberg
transitions
Parameters Transition --
Table 1 in the far ultraviolet
of CzHsF
used
I
6
region
15 October 1973
down to 1100 A
Calc.
Exptl.
n
E WK)
h (A) -_______
E UW
h (AI
10a’ (UCC)
-+ 3s
11
1.0
3
72.5
1374
W(i-rCH3)
-+ 3s
12
1.0
3
76.4
1309
775
1290
I,
0.6
3
81.2
1232
12
0.6
3
84.8
1179
80.6 84.7
1240 1180 -.
3p 33”(ffCH3) -+ 3p
lOn'(a&
+
-
lying Rydberg bands of halogen lone-pair electrons. The ethyl fluoride spectrum in the region 1150 to 1400 8, shows broad structureless bands (fig. l), suggesting probably one band at 1290 A (emax = 5.5 X 1C3 Q mok-1
cm-l)
and
.two
shoulders
at
1180
and
1240 i% No absorption was detected in the region beyond 1400 A. Before we discuss the ethyl fluoride spectrum, let us consider Rydberg transitions in methyl fluoride and ethane in the far ultraviolet region. The far ultraviolet absorption spectrum of the simplest methyl fluoride in the region up to 110 000 cm-l has been studied by Stokes and Duncan [5] and by Brundle et al. [7] and its lowest excited state has been indicated to be of Rydberg type in character. The highest occupied orbital of CH3F has been assigned to the degenerate irCH3 orbitals, with a vertical ionization potential of 13.05 eV [7] , whereas other methyl halides hsve the outermost orbital5 due to the halogen lone pairs. The ionization potential of a lone-pair fluorine electron is much larger than those of other halogens. In fact, the first ionization potential of HF (16.05 eV) is much larger than those of HCl(12.78 eV), HBr (I 1.83, eV) and HI (10.71, eV) [13]. Sandorfy et al. [14] have pointed out in their far ultraviolet absorption study of ethane that a strong band at 1300 A
is assigned to a ‘E 6 IA, band (3s + e,& polarized perpendicular to the C-C bond direction, From the sum rule consideration of the photoelectron spectrum of ethyl fluoride [2] , four highest occupied MO’s are found to be 2a”
9a’
“F
=CH,
(14.57)
(13.96)
3a” GH3
(12.87)
IOa’
shown in parentheses and TCHa 2nd n&s denote the pseudo-n orbit& of the CH3 group. The above orbital ordering has been supported by a MIND0/2 calculation [2] . Taking the C3, and C3p orbit& of ethyl to be two lowest excited states, the Rydberg formula E = / - _!?/(n -S)l gives rise to the transition
fluoride
energies (15) shown in table 1 for the transitions from the two highest occupied orbitals. In table 1 t quantum defects
of 1 .O and 0.6 are used for the 35 and 3p-
type transitions, respectively, n = 3 being used. As seen from table 1, the calculated energies for transitions TCH, --f 3S, fJcc + 3p and irCHs + 3p are in excellent agreement with the experimental ones obtained from the 1290,124O and 1180 A bands. An absorp-
tion band corresponding to transition ucc + 3s &I,. = 1374 A) does not show any distinct band probably because of its weakness. On the other hand, calculating 6’s from the observed bands, the 1290. 1240 and 1130 A bands yield quantum defects of 0.96, 0.64 and 0.60, respectively, F_rggestingthat the transition of the 1290 A band is of s type while those of the 1240 and 1180 A bands are of p type. It should be pointed out that the low-!ying Rydberg transitions of ethyl chloride, bromide and iodide result mainly from the halogen lone-pair orbitafs whereas those of ethyl fluoride arise from the MO’s attributable
to the nC..Is and ucc.
We wish to express our thanks to Mr. H.
Wakabayashi for his help in the measurement of the far ultraviolet
Katsumata
absorption
for a valuable
spectrum
and
Aso
to Mr. S_
discussion.
=cc
(12.43)
where vertical ionization potentials in eV units are
References [l] D.W. Turner, C. Baker, A.D. Baker and CR. Brundle, Molecular
photoelectron
spectroscopy
@‘iIey, New York,
1969).
617
Volume 22. number 3 [Z] T. Yam-,
Spectry.. [3] [4 15 16
.17
S. Katsumatn
15 October 1973
CHEMICAL PHYSICS LETTERS and K. Kimura,
J. Electron
to be published.
G. Moe and A.B.F. Duncan, I. Am. Chem. Sot. 74 (1952) 3140. P. Wagner and A.B.F. Duncan, J. Am. Chem. Sot. 77 (1955) 2609. S: Stokes and A.B.F. Dunan, J. Am. Chem. Sot. 80 (1958) 6177. R.A. Spurr and T.A. Chubb, Spectrochim. Acta 10 (1958) 431. C.R. Brundle, M.B. Robin and I-I. Bnsch, J. Chem. Phys.
53 (1970) 2196. [S] R.k Boschi and D.R. Salahub, Mol. Phys. 24 (1972) 735.
191W-F.
Edge11 and L. Partz,
J. Am. Chem. Sac. 77 (195.5)
4899..
(101 H. Tsubomura, K. Kaya, K. Kimura, J. Tanaka and S. Naskura,
Bull. Chem.
Sot.
K. Kimura and S. Nagakura,
Japan
37 (1964)
417.
h9ol. Phys. 9 (1965)
117: ThTheoret.Chim. Acta 3 (1965) 164; J. Chem. Phys. 47 (1967) 2916. 112 N. Astoin, J. Granier and M. Cordier, J. Phys. Radium 19 (1958) 507. 113 H.J. Lempka, T.R. Passmore and W.C. Price, Proc. Roy. sou. A304 (1968) 53. 114 B,,\. Lombos, P. Sauvageauand C, Sandorfy, J, Mel, SpacTry. 24 (1967) 253.
CHEMICAL PHYSICS LETTERS
Volume 22, number 3
ON THE FAR ULTRAVIOLET
ABSORPTION
SPECTRUM
OF ETHYL
1973
FLUORIDE
Tomoko YAMAZAKl and Katsumi KMURA Physical Chemisrr/
Loborarory,
Zxriture
of Applied Electricity.
Hokknido
Univexity,
Sapporo
&Or J.qmn
Received 10 July 1973
The absorption spectrum of CzHsF has been measured in the region down to 1150 A with absorption maxima at 1290, 1240 and 1180 A and has been interpreted with Rydberg transitions from the o(K) and pseudo 5r(CH3) orbitals to the carbon 3s and 3p orbitals.
Since high-resolution photoelectron spectroscopy has made it possible to detemline ionization potentials of various valence shell molecular orbit& [ 11, a Rydberg transition in an organic compound is an interest-
WAVELENGTH
Cd.1
ing subject to be investigated using these ionization potentials. A recent photoelectron study on ethyl and n-propyl fluorides by the present authors [2] has in-
dicated that the first ionization potentials of these fluorides are 12.43 and Il.96 eV, respectively, which are about 0.4 eV higher than those of the parent alkanes because of the inductive
effect of the fluorine
atom. Our photoelectron study [‘2]has also shown that the highest occupied orbitals of ethyl and ?Ipropyl fluorides are of q-C type rather than of 7r-H type, in contrast to the situation in methyl fluoride3 where it is of nCHs. The outermost orbitals of the alkyl fluorides are also different in nature from those of the corresponding alkyl chlorides, bromides and iodides which have the halogen lone pairs giving rise to the first ionization potentials [I] . It is surprising that alkyl monofluorides except CH,F lack in the literature of far ultraviolet studies, in spite of several reports on far ultraviolet spectra of fluorine-substituted alkanes [S--8]. In connection 4th
our recent
photoelectron
study
of ethyl
fluoride,
the present work we have undertaken to study its ultraviolet absorption in the region up to 87 000 cm-l, placing special emphasis on Rydberg transitions. Ethyl fluoride was synthesized from the corresponding p-toluene&lphonic acid ester according to the direction of Edge11et al. [9] and was purified by disin
.616
Fig. 1. The far ultraviolet the gas phase.
absorption
spectrum of C*Hs I: in
tillation Paice under vacuum. Absorption measurements for ethyl fluoride in the region down to 1150 A were carried out in the vapour phase at room temperature by a recording vacuum ultraviolet spectrophotometer of Seya-Namioka type? Jasco WV-LB, the apparatus containing a Bausch and Lomb concave grating with a radius of 50 con. A gas 41 of 55-mm path length with lithium fluoride windows was used. Th’e cell and the technique of handling the gaseous sample are essentially the same as those used in our earlier work [lo, 111. As seen from fig. 1, the far ultraviolet absorption spectrum of ethyl fluoride differs greatly from those of other ethyl monohalides [ 121 which show low-
Volume. 22, number 3
CHEMICAL PHYSICS LETTERS
Rydberg
transitions
Parameters Transition --
Table 1 in the far ultraviolet
of CzHsF
used
I
6
region
15 October 1973
down to 1100 A
Calc.
Exptl.
n
E WK)
h (A) -_______
E UW
h (AI
10a’ (UCC)
-+ 3s
11
1.0
3
72.5
1374
W(i-rCH3)
-+ 3s
12
1.0
3
76.4
1309
775
1290
I,
0.6
3
81.2
1232
12
0.6
3
84.8
1179
80.6 84.7
1240 1180 -.
3p 33”(ffCH3) -+ 3p
lOn'(a&
+
-
lying Rydberg bands of halogen lone-pair electrons. The ethyl fluoride spectrum in the region 1150 to 1400 8, shows broad structureless bands (fig. l), suggesting probably one band at 1290 A (emax = 5.5 X 1C3 Q mok-1
cm-l)
and
.two
shoulders
at
1180
and
1240 i% No absorption was detected in the region beyond 1400 A. Before we discuss the ethyl fluoride spectrum, let us consider Rydberg transitions in methyl fluoride and ethane in the far ultraviolet region. The far ultraviolet absorption spectrum of the simplest methyl fluoride in the region up to 110 000 cm-l has been studied by Stokes and Duncan [5] and by Brundle et al. [7] and its lowest excited state has been indicated to be of Rydberg type in character. The highest occupied orbital of CH3F has been assigned to the degenerate irCH3 orbitals, with a vertical ionization potential of 13.05 eV [7] , whereas other methyl halides hsve the outermost orbital5 due to the halogen lone pairs. The ionization potential of a lone-pair fluorine electron is much larger than those of other halogens. In fact, the first ionization potential of HF (16.05 eV) is much larger than those of HCl(12.78 eV), HBr (I 1.83, eV) and HI (10.71, eV) [13]. Sandorfy et al. [14] have pointed out in their far ultraviolet absorption study of ethane that a strong band at 1300 A
is assigned to a ‘E 6 IA, band (3s + e,& polarized perpendicular to the C-C bond direction, From the sum rule consideration of the photoelectron spectrum of ethyl fluoride [2] , four highest occupied MO’s are found to be 2a”
9a’
“F
=CH,
(14.57)
(13.96)
3a” GH3
(12.87)
IOa’
shown in parentheses and TCHa 2nd n&s denote the pseudo-n orbit& of the CH3 group. The above orbital ordering has been supported by a MIND0/2 calculation [2] . Taking the C3, and C3p orbit& of ethyl to be two lowest excited states, the Rydberg formula E = / - _!?/(n -S)l gives rise to the transition
fluoride
energies (15) shown in table 1 for the transitions from the two highest occupied orbitals. In table 1 t quantum defects
of 1 .O and 0.6 are used for the 35 and 3p-
type transitions, respectively, n = 3 being used. As seen from table 1, the calculated energies for transitions TCH, --f 3S, fJcc + 3p and irCHs + 3p are in excellent agreement with the experimental ones obtained from the 1290,124O and 1180 A bands. An absorp-
tion band corresponding to transition ucc + 3s &I,. = 1374 A) does not show any distinct band probably because of its weakness. On the other hand, calculating 6’s from the observed bands, the 1290. 1240 and 1130 A bands yield quantum defects of 0.96, 0.64 and 0.60, respectively, F_rggestingthat the transition of the 1290 A band is of s type while those of the 1240 and 1180 A bands are of p type. It should be pointed out that the low-!ying Rydberg transitions of ethyl chloride, bromide and iodide result mainly from the halogen lone-pair orbitafs whereas those of ethyl fluoride arise from the MO’s attributable
to the nC..Is and ucc.
We wish to express our thanks to Mr. H.
Wakabayashi for his help in the measurement of the far ultraviolet
Katsumata
absorption
for a valuable
spectrum
and
Aso
to Mr. S_
discussion.
=cc
(12.43)
where vertical ionization potentials in eV units are
References [l] D.W. Turner, C. Baker, A.D. Baker and CR. Brundle, Molecular
photoelectron
spectroscopy
@‘iIey, New York,
1969).
617
Volume 22. number 3 [Z] T. Yam-,
Spectry.. [3] [4 15 16
.17
S. Katsumatn
15 October 1973
CHEMICAL PHYSICS LETTERS and K. Kimura,
J. Electron
to be published.
G. Moe and A.B.F. Duncan, I. Am. Chem. Sot. 74 (1952) 3140. P. Wagner and A.B.F. Duncan, J. Am. Chem. Sot. 77 (1955) 2609. S: Stokes and A.B.F. Dunan, J. Am. Chem. Sot. 80 (1958) 6177. R.A. Spurr and T.A. Chubb, Spectrochim. Acta 10 (1958) 431. C.R. Brundle, M.B. Robin and I-I. Bnsch, J. Chem. Phys.
53 (1970) 2196. [S] R.k Boschi and D.R. Salahub, Mol. Phys. 24 (1972) 735.
191W-F.
Edge11 and L. Partz,
J. Am. Chem. Sac. 77 (195.5)
4899..
(101 H. Tsubomura, K. Kaya, K. Kimura, J. Tanaka and S. Naskura,
Bull. Chem.
Sot.
K. Kimura and S. Nagakura,
Japan
37 (1964)
417.
h9ol. Phys. 9 (1965)
117: ThTheoret.Chim. Acta 3 (1965) 164; J. Chem. Phys. 47 (1967) 2916. 112 N. Astoin, J. Granier and M. Cordier, J. Phys. Radium 19 (1958) 507. 113 H.J. Lempka, T.R. Passmore and W.C. Price, Proc. Roy. sou. A304 (1968) 53. 114 B,,\. Lombos, P. Sauvageauand C, Sandorfy, J, Mel, SpacTry. 24 (1967) 253.