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Electronic spectra of aliphatic carbonyl compounds by electron impact spectroscopy
of Electron Spectroscopy and Related Phenomena, 4 (1974) 139-147 &J Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
Journal
ELECTRONIC SPECTRA OF ALIPHATIC ELECTRON IMPACT SPECTROSCOPY II. SATURATED
WING-CHEUNG Department
CARBONYL
COMPOUNDS
BY
KETONES
TAM
and C. E. BRION
of Chemistry,
University
of British Columbia,
Vancouver
V6T 1 W5
(Canada)
(First received 19 February 1974; in final form 29 March 1974)
ABSTRACT
Electron impact energy loss spectra of acetone, 2-butanone, methy isobutyl ketone, methyl isopropyl ketone, and methyl tertiarybutyl ketone have been recorded at an impact energy of 100 eV and a scattering angle of two degrees. Rydberg assignments have been made using quantum defects and term values obtained by reference to ionization potentials measured by photoelectron spectroscopy. Substituent effects on Rydberg orbital energies are discussed using Taft o* values. INTRODUCTION
The general aspects of the electron impact spectra of aliphatic carbonyl compounds have been presented in part I of this series’, where the results for saturated aldehydes were discussed. We now report the results obtained for some aliphatic saturated ketones, viz. acetone, 2-butanone, methyl isopropyl ketone, methyl t-butyl ketone and methyl isobutyl ketone. The experimental details have also been discussed in part I. RESULTS
AND
DISCUSSION
Acetone and 2-butanone The electron configurations of formaldehyde and acetaldehyde have been discussed in part I. Based on the photoelectron work of Turner et al.’ on formaldehyde and its symmetry correlation to acetaldehyde, it is suggested that the highest filled molecular orbital in these two compounds contains the non-bonding electrons on the oxygen atom and the second highest filled molecular orbital contains the electrons in the C-O x-bond. We have observed the photoelectron spectra of acetaldehyde and acetone to be very similar, at least for the first two bands, except for a shift of about
140
CH,COCH,-
)
70
2*
IOOev
8.0
9.0
CH,COC,H,-100ev -ns/3
J3 pj3
2”
14 [4
I5
I6
w
/-
,JII-nP Id
333 18
70
-
5 1
8.0
ENERGY Figure 1.
44’ ‘I”’
II
8
9.0
LOSS (eV)
Electron impa.et spectra of CH3COCH3 and CH3COCzH5 at 100 eV, 2”.
0.7 eV to lower energy when one more methyl group replaces the aIdehydic
hydrogen in acetaldehyde to give acetone. Of course, more bands are observed for acetone than acetaldehyde because more C-H bonding orbitals are available for ionization. Consequently, it is not unreasonable to believe that the ordering of the first two orbitals is the same for acetone and acetaldehyde so that the highest filled MO (til) contains the non-bonding electrons on the oxygen atom. As in the case of aldehydes, the spectra of ketones contain a broad region of weak transition maximising at about 4.3 eV. This is the n -j n* transition and has been discussed for aldehydes in part I. Following the example of aldehydes, it is possible to assign the spectra at higher energy loss to Rydberg transitions. Figure 1 shows the electron impact spectra of acetone* and 2-butanone over * Following the initial submission of this work a paper appeared (Huebner et al., J. CJzem. Whys.,59 (1973) 5434) reporting the high resolution electron impact spectrum of acetone. A similar interpretation (i.e. in terms of Rydberg transitions) to that of the present work has been given.
141 the energy loss region 6.0-9.7 eV at an impact energy of 100 eV and 2” scattering angle. The photoelectron spectra of these compounds have been studied and the first five IP’s are listed at the bottom of Tables 1 and 2. The vacuum ultraviolet spectrum of acetone was studied by Noyes et a1.3 and Duncan4. On the basis of intensity, Duncan4 selected, from the numerous electronic states, three that fit into a Rydberg series converging to a limit of 10.26 eV. This value is higher than an IP of 9.705 eV found later by Watanabe5 from photoionization studies. Subsequently Watanabe’.reanalysed the optical spectrum4 and found a different series of eleven members converging to the IP at 9.705 eV. The first member of this series occurs at 8.09 eV. Based on this we correlate the strongest peak in the electron impact spectrum at 8.09 eV (G) with this transition. Its term value is 1.62 eV with respect to our PES measurement of 9.71 eV for the first IP of acetone. This seems to fit a 4s Rydberg upper state with a calculated quantum defect of 1.09. Higher members of this til + ns series are predicted at 8.81, 9.14 and 9.32 eV respectively for n = 5, 6, 7. So the peaks, K, L and M at 8.82, 9.12 and 9.30 eV are assigned as such. The Rydberg formula then predicts the $ 1 + 3s transition to occur at 5.96 eV, which differs from the energy of the first component of the first sharp band by 0.4 eV. The average vibrational spacing of this band is 0.135 + 0.005 eV. This is comparable with the spacing between the peaks G and H. So, if we assign the first band at - 6.5 eV to $ I 4 3s plus vibrational components, H can be assigned as the vibrational component of the 4s Rydberg state. The vibrational mode excited is probably the methyl deformation as suggested by Lawson and DuncanI’ from the vibrational analysis of this band in acetone. As in the case of the spectrum of acetaldehydel, it is not unreasonable to regard the first band A in the acetone spectrum as the $i -t 3s Rydberg transition despite the 0.4 eV difference between the observed and the calculated energies. Large deviations of observed energy of the 3s Rydberg state from the calculated vaiue have also been observed in other moIecules conforming to CZv symmetry (e.g. water6, ethylene oxide’). The higher symmetry may be the cause of some perturbations which affect the energy of Rydberg states. Weiss’ has suggested the perturbation of atomic Rydberg orbitals by valence type transitions such as x + z*. This perturbation may not necessarily be localised in the immediate vicinity of the 7c+ 7t* but can be extensive. It is a well-established fact that the term values of 3d Rydberg states of compounds containing lone pair electrons remain essentially constant on alkylationl 9 6. Based on the 3d term value (1.78 eV) in acetaldehyde, we assigned the peak F at 7.87 eV in the spectrum to be t,Q1+ 3d. The term value is 1.84 eV and the calculated quantum defect is 0.28. The Rydberg formula predicts higher members at 8.73, 9.10 and 9.26 eV. So the step J at 3.69 eV is assigned a 4d Rydberg upper state and the 5d and 6d Rydberg transitions should have contributions to the peaks L and M at 9.12 and 9.30 eV respectively which have previously been assigned to 6s and 7s Rydberg upper states. The only features that are left unaccounted for up to now are the peaks D, E and I. They may belong to the $ i --+ np series. If we assume the shoulder I between the
142 TABLE
1
RYDBERG Peak
TRANSITIONS
IN ACETONE
Observed
Term
energy
value (e V) a
(e V}
A B C D E F G H I J K
6.36 6.50 6.63 7.42 7.55 7.87 8.09 8.22 8.41 8.69 8.82
3.35 3.21 3.08 2.29 2.16 1.84 1.62 1.49 1.30 1.02 0.89
L
9.12
0.59
M
9.30
0.41
Assignment
Culculated energy (e V)
I#1 + 3s? + 3s + vz -+ 3s + 2Y2 VI -+ 3P? -+ 3p’ ryl -+ 3d y1 + 4s -+ 4s + Yz Yl-+ 4P y1 + 4d y1 -+ 5s Yl --t 5P yl - 5d ~1 -+ 6s w + 6~ yl + 6d y1-+ 7s
5.96
Calculated quantum defect
6.99 0.28 1.09 0.76 8.73 8.81 8.95 9.10 9.14 9.21 9.29 9.32
a Assigned with respect to photoelectron ionization potentials of 9.71, 12.6, 13.5, 14.1, 14.6 eV.
4s and 4d transitions to be 4p, the term value and quantum defect (1.30 eV and 0.76) are comparable to those of acetaldehyde (1.25 eV and 0.77). The Rydberg formula then predicts the 3p state at 6.99 eV and the 5p and 6p at 8.95 and 9.21 eV respectively. The peak D at 7.42 eV differs from the predicted value of 3p also by 0.4 eV (cf. the case for 3s). A perturbation similar to that for the case of the 3s Rydberg state may also be responsible for the deviation of the position of the 3p state from the calculated value. If the Rydberg assignment is assumed for peak D, the shoulder E can be interpreted as another component of the 3p manifold. Table 1 gives a summary of our assignments. The spectrum of 2-butanone looks very similar to that of acetone, particularly in the 6.0-8.5 eV energy loss region. The first band exhibits four vibrational components with an average spacing of 0.135 * 0.005 eV, which is the same as that in acetone. The term value of the first vibrational component with respect to the first IP of 9.52 eV is decreased to 3.19 eV on replacing one of the a-hydrogens in acetone by a methyl group as in 2-butanone. In contrast to the case of acetone, the features in the spectrum of 2-butanone can easily be fitted into three Rydberg series r,G1---, ns, np and nd. A summary and a comparison between the calculated and observed energies is given in Figure 1 (bottom) and Table 2. The only uncertainty is the existence of the peaks K and 0. The interpretation of the 2-butanone spectrum in terms of Rydberg series seems to affirm our Rydberg assignment of the acetone spectrum. The higher
143 TABLE 2 RYDBERG TRANSITIONS IN Z-BUTANONE Peak
Observed energy (e V)
A B C D E F F’ G H I J K? L
6.33
Term value (e V) B
Assignment
3.19
$n +
6.46 6.59 6.72 7.26 7.41 7.57 7.71 7.90 8.06 8.19
3.06 2.93 2.80 2.26 2.11 1.95 1.83 1.62 1.46 1.33
8.53
0.99
M
8.68
0.84
N
8.85
0.67
9.11 9.00
0.41 0.52
O? :
Caiculated energy (e V)
3s
+ 3s f YZ --r 3s + 2ya -+ 3s + 3~2 yl -+ 3Pa’ ~1 + 3pa” yl 4 3pa’ ~1 --* 3d --, 3d’ y1+ 4s + 4s t_ YZ Wl -+ 4P + 4p’ y1 -+ 4d yr -+ 4d’ y1 --, 5s Yl + 5P -+ 5p’ yl + Sd y1 WI+ --+ 6s 6~ y’1 + 7s
Calculated quantum defect 0.94
0.55 0.46
8.07
0.26 0.10
8.38 8.43 a.55 8.63 8.70 8.83 8.86 8.92 8.99 9.07 9.15
a Assigned with respect to PES ionization potentials of 9.52, 12.3, 12.6, 13.0, 14.3 eV. 30 eV
7OeV
100 ev
a:,bj@
Figure 2. Electron impact spectra of the first two bands of CH~COCZH~at 30, 70 and 100 eV. and rising continuum in 2-butanone above 9 eV is probably due to Rydberg transitions leading to the second and higher IP’s which are more closely spaced in 2-butanone (12.3, 12.6 and 13.0 eV) than in acetone (12.6, 13.5 and 14.1 eV). It seems that the peaks in the bigger molecule 2-butanone are intrinsically broader than those in acetone since the vibrational structure in the first band is less well resolved for the same vibrational spacing. Three peaks E, F and F’ are seen in the I) 1 + 3p region instead of two as in
144 acetone. The shoulder F’ is less intense than the other two. If we assume 2-butanone to conform to C, symmetry, the three components of the 3p manifold are of a’, a’ and a” symmetry. The assignment of the three components to the different symmetries can be achieved by studying the electron impact spectra at different impact energies, 100, 70 and 30 eV. (See Figure 2.) At these impact energies, the intensity of the peak E and peak F’ relative to peak A remain quite constant (0.53, 0.51 and 0.51 for E and 0.35, 0.37 and 0.39 for F’). On the other hand, the relative intensity of peak F to peak A increases as the impact energy is lowered (0.51, 0.53 and 0.68 respectively at 100 eV, 70 eV and 30 eV impact energy). This seems to indicate that peaks E and F’ may belong to the same symmetry and so are assigned a’. The different behaviour of peak F on decreasing the impact energy suggests that it should be assigned a different symmetry (a”). Higher ketones
Figure 3a, b, c shows the electron impact spectra of methyl isobutyl ketone, isopropyl ketone and methyl t-butyl ketone respectively over the energy loss region of 6-10 eV at an impact energy of 100 eV and 2” scattering angle. Duncan9 has studied the vacuum ultraviolet spectra of methyl n-propyl ketone, methyl isopropyl ketone and diethyl ketone in the energy region 6.2-8.3 eV. The electronic transitions and their vibrational structure were discussed in comparison with acetone and 2-butanone. It is concluded that most transitions are probably of a Rydberg type, leading to IP’s in the neighbourhood of IO eV. Holdsworth and Duncan’ ’ discussed the effect
IOOeV, 2O
s
Ii
I
tJ z II.,>A?~*.~:. :I_.
I
D ~~~~s~~~_~..~,~~~~~~~”
&p
CH,
A5
(a)
I
___J
;
,I,_
I/ .‘\
‘~,~
j$~.,_L_
) r-
5..
i .f
p
i
_.~_~~~~~~‘1CH3)2CHCOCH3
;’ ‘-&_<.ke ?-\ (b)
f ;p,
.a,-
y;.‘/ 6
H
.‘i;
_j
i
L.y D
i
[
q
_p-+Lx: 1
7 r->.
c
i;il; I
I 7
ic;y”~-“ii=H3)3CCOCH3 1
1 ENERGY
kSS
1
3
,
10
(CA’)
Figure 3. Electron impactspectraof (CH&CHCH&OCH3, at 100 eV, 2”.
(CH&CHCOCH3
and (CH3)3CCOCH3
145 TABLE
3
ELECTRONIC TRANSITIONS IN METHYL KETONE AND METHYL t-BUTYL KETONE (CH3) 3CHGW3COCH3 Peak Energy Transition (eV
A B C
( CH3) 2CHCO CH3 Peak Energy Transition @VI
KETONE,
METHYL
ISOPROPYL
Term value (eVIa
(CH3) 3CCOCH3 Peak Energy Transition Term value (evj (eVJ a
E F
6.45 6.53 7.34 7.73 7.97 8.27
3s 3s -j- uz WI + 3P wi 4 3d $ur+ 4s W -+ 4P
2.97 2.89 2.08 1.69 1.45 1.15
A B c D E F
6.41 7.19 7.84 7.97 8.22 9.2
y1 -+ 3s WI -+ 3P y1 + 3d? @JJl -+ 4s Wl + 4P yl3-+3s
2.95 2.17 1.52 1.39 1.14 3.1
A B C? D E F
6.40 7.12 7.6 7.79 8.05 8.33
G
8.48
0.94
G
9.9
y3 --* 3p y4 + 4s
2.4 2.7
G
8.55
H I
9.0 9.7
~1 + Wl + qJz-+ y3 -+ w4 +
H
9.7
D
w1 4
Term value (eVa
ISOBUTYL
~1 -+ $JJ’ -+1 ~1 --f ~1 + ~1 + ~1 -+
2.81 2.09 1.6 1.42 1.16 0.88
7,112+
3s 3p 3d 4s 4p 4d 5s 3s
y3 +
3s
2.8
y1+
4d 5s 3s 3s 3s
2.9 2.9 3.2
2.83
8 Assigned with respect to PES ionization potentials (for (CH&CHCHKOCH3, 9.42, 11.4, 11.9, 13.0 and 13.6 eV; for (CH&CHCOCH3,9.36, 11.8, 12.3, 12.6 and 13.8 eV; for (CH&CCOCHz, 9.21, 11.4,12.5,13.6 and 14.6 eV.
of consecutive substitution of the a-hydrogens in acetone by methyl groups on the intensity of their electronic transitions in the vacuum ultraviolet. In Table 3, we have assigned most structures in the spectra to Rydberg transitions based on term values and the calculated quantum defects. The broad peaks at high energy loss are probably the envelope of more than one transition. It is noted that in these higher ketones, the spectra only show broad features probably because there are so many vibrational modes. In particular, the shape of the n + 3s band (- 6.4 eV) is very sensitive to the alkyl group. Sharp vibrational structure observed in acetone and 2-butanone seems to disappear for the higher ketones studied. The first band in the spectra of methyl isopropyl ketone (b) and methyl t-butyl ketone (c) appears as a continuous diffuse band while that of methyl isobutyl ketone (a) still shows some indication of vibrational structure in the first band. A detailed study of the shape of this band using optical spectroscopy has been presented by Ito et al. 13. The second band in methyl isopropyl ketone and methyl t-butyl ketone is more intense than the first band while the reverse is true for other ketones studied in which the alkyl groups are not branched at the g-carbon. This may be a useful feature to distinguish between these two kinds of ketones. Holdsworth and Duncan1 ’ have also observed that the intensity of the 6.4 eV band decreases when the a-hydrogens in acetone are replaced by methyl groups while the intensity of the transition in the region at about 7.4 eV increases. No satisfactory explanation has yet been given for this phenomenon.
146 Eflect
of alkyd
substituents
The effect of alkyl groups on term values of organic compounds has been discussed by Robin l4 . As in the case of aldehydes’, approximately straight-line plots are obtained when the first IP, the 3s, 3p and 3d term values are plotted against the sum of Taft CJ*values @a*) of the two ,alkyl groups attached to the carbonyl group (Figure 4). The behaviour of the 3s, 3p and 3d terms is the same as in aldehydesl, and the alkyl derivatives of water “. The shift of the 3s peak maxima to lower energies on alkylation is smaller than in the case of aldehydes and so the straight lines for the first IP and the 3s term value dependencies are approximateIy parallel. In optical spectroscopy’* r O a bigger shift of the peak maxima of the 7.4 eV band to lower energies has been observed as more a-hydrogens in acetone are replaced by methyl groups. In our Rydberg model this can be easily explained by the fact that the 3p Rydberg electron is less penetrating than the electrons in the highest filled molecular orbital and therefore the binding energies of the 3p Rydberg orbital decrease less rapidly than the binding energy of the highest filled molecular orbital. As a result, the transition energy to the 3p Rydberg state, which is the difference between the two binding energies, is also decreasing with increasing alkylation.
0’
I
-0.3
I
I
-0.2
xa Figure
-0.1 *
,
0.0
-
4. Effect of alkyl substitutionon Rydberg term values and first ionization
potentials.
ACKNOWLEDGEMENT The authors are indebted to Mr. Derek Yee for his assistance in running the photoelectron spectra of the compounds studied. Financial support for this work was provided by the National Research Council of Canada. One of us (W.C.T.) gratefully acknowledges the receipt of a University of British Columbia Graduate Fellowship.
147 REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14
W. C. Tam and C. E. Brion, J. Electron Spectrusc., 3 (1974) 467. D. W. Turner, C. Baker, A. D. Baker and C. R. Brundle, Molecdar Photoelectron Spectroscopy, Wiley, London, 1970, p. 132. W. A. Noyes Jr., A. B. F. Duncan and W. M. Manning, J. Chem. Phys., 2 (1934) 717. A.B. F. Duncan,J. Gem. Phys., 3 (1935) 131. K. Watanabe, f. Chem. Phy.y., 22 (1954) 1564. M. B. Robin and N. A. Kuebler, J. Electron Spectrosc., 1 (1972/73) 13. H. Basch, M. B. Robin, N. A. Kuebler, C. Baker and D. W. Turner, J. C&m. Phys., 51 (1969) 52. A. W. Weiss, Phys. Rev., 178 (1969) 82. A. B. F. Duncan, J. Chem. Phys., 8 (1940) 444. R. S. Holdsworth and A. B. F. Duncan, Chem. Rev., 41(1947) 311. W. C. Tam and C. E. Brion, J. Hectron Spectrosc., 3 (1974) 263. M. Lawson and A. B. F. Duncan, J. Gem. Whys., 12 (1944) 329. H. Ito, Y. Nogata, S. Matsuzaki and A. Kuboyama, Bull. Chem. Sot. Jap., 42 (1969) 2453. M. B. Robin, Ztzt.J. Quantum Chem., 6 (1972) 257.
Journal
ELECTRONIC SPECTRA OF ALIPHATIC ELECTRON IMPACT SPECTROSCOPY II. SATURATED
WING-CHEUNG Department
CARBONYL
COMPOUNDS
BY
KETONES
TAM
and C. E. BRION
of Chemistry,
University
of British Columbia,
Vancouver
V6T 1 W5
(Canada)
(First received 19 February 1974; in final form 29 March 1974)
ABSTRACT
Electron impact energy loss spectra of acetone, 2-butanone, methy isobutyl ketone, methyl isopropyl ketone, and methyl tertiarybutyl ketone have been recorded at an impact energy of 100 eV and a scattering angle of two degrees. Rydberg assignments have been made using quantum defects and term values obtained by reference to ionization potentials measured by photoelectron spectroscopy. Substituent effects on Rydberg orbital energies are discussed using Taft o* values. INTRODUCTION
The general aspects of the electron impact spectra of aliphatic carbonyl compounds have been presented in part I of this series’, where the results for saturated aldehydes were discussed. We now report the results obtained for some aliphatic saturated ketones, viz. acetone, 2-butanone, methyl isopropyl ketone, methyl t-butyl ketone and methyl isobutyl ketone. The experimental details have also been discussed in part I. RESULTS
AND
DISCUSSION
Acetone and 2-butanone The electron configurations of formaldehyde and acetaldehyde have been discussed in part I. Based on the photoelectron work of Turner et al.’ on formaldehyde and its symmetry correlation to acetaldehyde, it is suggested that the highest filled molecular orbital in these two compounds contains the non-bonding electrons on the oxygen atom and the second highest filled molecular orbital contains the electrons in the C-O x-bond. We have observed the photoelectron spectra of acetaldehyde and acetone to be very similar, at least for the first two bands, except for a shift of about
140
CH,COCH,-
)
70
2*
IOOev
8.0
9.0
CH,COC,H,-100ev -ns/3
J3 pj3
2”
14 [4
I5
I6
w
/-
,JII-nP Id
333 18
70
-
5 1
8.0
ENERGY Figure 1.
44’ ‘I”’
II
8
9.0
LOSS (eV)
Electron impa.et spectra of CH3COCH3 and CH3COCzH5 at 100 eV, 2”.
0.7 eV to lower energy when one more methyl group replaces the aIdehydic
hydrogen in acetaldehyde to give acetone. Of course, more bands are observed for acetone than acetaldehyde because more C-H bonding orbitals are available for ionization. Consequently, it is not unreasonable to believe that the ordering of the first two orbitals is the same for acetone and acetaldehyde so that the highest filled MO (til) contains the non-bonding electrons on the oxygen atom. As in the case of aldehydes, the spectra of ketones contain a broad region of weak transition maximising at about 4.3 eV. This is the n -j n* transition and has been discussed for aldehydes in part I. Following the example of aldehydes, it is possible to assign the spectra at higher energy loss to Rydberg transitions. Figure 1 shows the electron impact spectra of acetone* and 2-butanone over * Following the initial submission of this work a paper appeared (Huebner et al., J. CJzem. Whys.,59 (1973) 5434) reporting the high resolution electron impact spectrum of acetone. A similar interpretation (i.e. in terms of Rydberg transitions) to that of the present work has been given.
141 the energy loss region 6.0-9.7 eV at an impact energy of 100 eV and 2” scattering angle. The photoelectron spectra of these compounds have been studied and the first five IP’s are listed at the bottom of Tables 1 and 2. The vacuum ultraviolet spectrum of acetone was studied by Noyes et a1.3 and Duncan4. On the basis of intensity, Duncan4 selected, from the numerous electronic states, three that fit into a Rydberg series converging to a limit of 10.26 eV. This value is higher than an IP of 9.705 eV found later by Watanabe5 from photoionization studies. Subsequently Watanabe’.reanalysed the optical spectrum4 and found a different series of eleven members converging to the IP at 9.705 eV. The first member of this series occurs at 8.09 eV. Based on this we correlate the strongest peak in the electron impact spectrum at 8.09 eV (G) with this transition. Its term value is 1.62 eV with respect to our PES measurement of 9.71 eV for the first IP of acetone. This seems to fit a 4s Rydberg upper state with a calculated quantum defect of 1.09. Higher members of this til + ns series are predicted at 8.81, 9.14 and 9.32 eV respectively for n = 5, 6, 7. So the peaks, K, L and M at 8.82, 9.12 and 9.30 eV are assigned as such. The Rydberg formula then predicts the $ 1 + 3s transition to occur at 5.96 eV, which differs from the energy of the first component of the first sharp band by 0.4 eV. The average vibrational spacing of this band is 0.135 + 0.005 eV. This is comparable with the spacing between the peaks G and H. So, if we assign the first band at - 6.5 eV to $ I 4 3s plus vibrational components, H can be assigned as the vibrational component of the 4s Rydberg state. The vibrational mode excited is probably the methyl deformation as suggested by Lawson and DuncanI’ from the vibrational analysis of this band in acetone. As in the case of the spectrum of acetaldehydel, it is not unreasonable to regard the first band A in the acetone spectrum as the $i -t 3s Rydberg transition despite the 0.4 eV difference between the observed and the calculated energies. Large deviations of observed energy of the 3s Rydberg state from the calculated vaiue have also been observed in other moIecules conforming to CZv symmetry (e.g. water6, ethylene oxide’). The higher symmetry may be the cause of some perturbations which affect the energy of Rydberg states. Weiss’ has suggested the perturbation of atomic Rydberg orbitals by valence type transitions such as x + z*. This perturbation may not necessarily be localised in the immediate vicinity of the 7c+ 7t* but can be extensive. It is a well-established fact that the term values of 3d Rydberg states of compounds containing lone pair electrons remain essentially constant on alkylationl 9 6. Based on the 3d term value (1.78 eV) in acetaldehyde, we assigned the peak F at 7.87 eV in the spectrum to be t,Q1+ 3d. The term value is 1.84 eV and the calculated quantum defect is 0.28. The Rydberg formula predicts higher members at 8.73, 9.10 and 9.26 eV. So the step J at 3.69 eV is assigned a 4d Rydberg upper state and the 5d and 6d Rydberg transitions should have contributions to the peaks L and M at 9.12 and 9.30 eV respectively which have previously been assigned to 6s and 7s Rydberg upper states. The only features that are left unaccounted for up to now are the peaks D, E and I. They may belong to the $ i --+ np series. If we assume the shoulder I between the
142 TABLE
1
RYDBERG Peak
TRANSITIONS
IN ACETONE
Observed
Term
energy
value (e V) a
(e V}
A B C D E F G H I J K
6.36 6.50 6.63 7.42 7.55 7.87 8.09 8.22 8.41 8.69 8.82
3.35 3.21 3.08 2.29 2.16 1.84 1.62 1.49 1.30 1.02 0.89
L
9.12
0.59
M
9.30
0.41
Assignment
Culculated energy (e V)
I#1 + 3s? + 3s + vz -+ 3s + 2Y2 VI -+ 3P? -+ 3p’ ryl -+ 3d y1 + 4s -+ 4s + Yz Yl-+ 4P y1 + 4d y1 -+ 5s Yl --t 5P yl - 5d ~1 -+ 6s w + 6~ yl + 6d y1-+ 7s
5.96
Calculated quantum defect
6.99 0.28 1.09 0.76 8.73 8.81 8.95 9.10 9.14 9.21 9.29 9.32
a Assigned with respect to photoelectron ionization potentials of 9.71, 12.6, 13.5, 14.1, 14.6 eV.
4s and 4d transitions to be 4p, the term value and quantum defect (1.30 eV and 0.76) are comparable to those of acetaldehyde (1.25 eV and 0.77). The Rydberg formula then predicts the 3p state at 6.99 eV and the 5p and 6p at 8.95 and 9.21 eV respectively. The peak D at 7.42 eV differs from the predicted value of 3p also by 0.4 eV (cf. the case for 3s). A perturbation similar to that for the case of the 3s Rydberg state may also be responsible for the deviation of the position of the 3p state from the calculated value. If the Rydberg assignment is assumed for peak D, the shoulder E can be interpreted as another component of the 3p manifold. Table 1 gives a summary of our assignments. The spectrum of 2-butanone looks very similar to that of acetone, particularly in the 6.0-8.5 eV energy loss region. The first band exhibits four vibrational components with an average spacing of 0.135 * 0.005 eV, which is the same as that in acetone. The term value of the first vibrational component with respect to the first IP of 9.52 eV is decreased to 3.19 eV on replacing one of the a-hydrogens in acetone by a methyl group as in 2-butanone. In contrast to the case of acetone, the features in the spectrum of 2-butanone can easily be fitted into three Rydberg series r,G1---, ns, np and nd. A summary and a comparison between the calculated and observed energies is given in Figure 1 (bottom) and Table 2. The only uncertainty is the existence of the peaks K and 0. The interpretation of the 2-butanone spectrum in terms of Rydberg series seems to affirm our Rydberg assignment of the acetone spectrum. The higher
143 TABLE 2 RYDBERG TRANSITIONS IN Z-BUTANONE Peak
Observed energy (e V)
A B C D E F F’ G H I J K? L
6.33
Term value (e V) B
Assignment
3.19
$n +
6.46 6.59 6.72 7.26 7.41 7.57 7.71 7.90 8.06 8.19
3.06 2.93 2.80 2.26 2.11 1.95 1.83 1.62 1.46 1.33
8.53
0.99
M
8.68
0.84
N
8.85
0.67
9.11 9.00
0.41 0.52
O? :
Caiculated energy (e V)
3s
+ 3s f YZ --r 3s + 2ya -+ 3s + 3~2 yl -+ 3Pa’ ~1 + 3pa” yl 4 3pa’ ~1 --* 3d --, 3d’ y1+ 4s + 4s t_ YZ Wl -+ 4P + 4p’ y1 -+ 4d yr -+ 4d’ y1 --, 5s Yl + 5P -+ 5p’ yl + Sd y1 WI+ --+ 6s 6~ y’1 + 7s
Calculated quantum defect 0.94
0.55 0.46
8.07
0.26 0.10
8.38 8.43 a.55 8.63 8.70 8.83 8.86 8.92 8.99 9.07 9.15
a Assigned with respect to PES ionization potentials of 9.52, 12.3, 12.6, 13.0, 14.3 eV. 30 eV
7OeV
100 ev
a:,bj@
Figure 2. Electron impact spectra of the first two bands of CH~COCZH~at 30, 70 and 100 eV. and rising continuum in 2-butanone above 9 eV is probably due to Rydberg transitions leading to the second and higher IP’s which are more closely spaced in 2-butanone (12.3, 12.6 and 13.0 eV) than in acetone (12.6, 13.5 and 14.1 eV). It seems that the peaks in the bigger molecule 2-butanone are intrinsically broader than those in acetone since the vibrational structure in the first band is less well resolved for the same vibrational spacing. Three peaks E, F and F’ are seen in the I) 1 + 3p region instead of two as in
144 acetone. The shoulder F’ is less intense than the other two. If we assume 2-butanone to conform to C, symmetry, the three components of the 3p manifold are of a’, a’ and a” symmetry. The assignment of the three components to the different symmetries can be achieved by studying the electron impact spectra at different impact energies, 100, 70 and 30 eV. (See Figure 2.) At these impact energies, the intensity of the peak E and peak F’ relative to peak A remain quite constant (0.53, 0.51 and 0.51 for E and 0.35, 0.37 and 0.39 for F’). On the other hand, the relative intensity of peak F to peak A increases as the impact energy is lowered (0.51, 0.53 and 0.68 respectively at 100 eV, 70 eV and 30 eV impact energy). This seems to indicate that peaks E and F’ may belong to the same symmetry and so are assigned a’. The different behaviour of peak F on decreasing the impact energy suggests that it should be assigned a different symmetry (a”). Higher ketones
Figure 3a, b, c shows the electron impact spectra of methyl isobutyl ketone, isopropyl ketone and methyl t-butyl ketone respectively over the energy loss region of 6-10 eV at an impact energy of 100 eV and 2” scattering angle. Duncan9 has studied the vacuum ultraviolet spectra of methyl n-propyl ketone, methyl isopropyl ketone and diethyl ketone in the energy region 6.2-8.3 eV. The electronic transitions and their vibrational structure were discussed in comparison with acetone and 2-butanone. It is concluded that most transitions are probably of a Rydberg type, leading to IP’s in the neighbourhood of IO eV. Holdsworth and Duncan’ ’ discussed the effect
IOOeV, 2O
s
Ii
I
tJ z II.,>A?~*.~:. :I_.
I
D ~~~~s~~~_~..~,~~~~~~~”
&p
CH,
A5
(a)
I
___J
;
,I,_
I/ .‘\
‘~,~
j$~.,_L_
) r-
5..
i .f
p
i
_.~_~~~~~~‘1CH3)2CHCOCH3
;’ ‘-&_<.ke ?-\ (b)
f ;p,
.a,-
y;.‘/ 6
H
.‘i;
_j
i
L.y D
i
[
q
_p-+Lx: 1
7 r->.
c
i;il; I
I 7
ic;y”~-“ii=H3)3CCOCH3 1
1 ENERGY
kSS
1
3
,
10
(CA’)
Figure 3. Electron impactspectraof (CH&CHCH&OCH3, at 100 eV, 2”.
(CH&CHCOCH3
and (CH3)3CCOCH3
145 TABLE
3
ELECTRONIC TRANSITIONS IN METHYL KETONE AND METHYL t-BUTYL KETONE (CH3) 3CHGW3COCH3 Peak Energy Transition (eV
A B C
( CH3) 2CHCO CH3 Peak Energy Transition @VI
KETONE,
METHYL
ISOPROPYL
Term value (eVIa
(CH3) 3CCOCH3 Peak Energy Transition Term value (evj (eVJ a
E F
6.45 6.53 7.34 7.73 7.97 8.27
3s 3s -j- uz WI + 3P wi 4 3d $ur+ 4s W -+ 4P
2.97 2.89 2.08 1.69 1.45 1.15
A B c D E F
6.41 7.19 7.84 7.97 8.22 9.2
y1 -+ 3s WI -+ 3P y1 + 3d? @JJl -+ 4s Wl + 4P yl3-+3s
2.95 2.17 1.52 1.39 1.14 3.1
A B C? D E F
6.40 7.12 7.6 7.79 8.05 8.33
G
8.48
0.94
G
9.9
y3 --* 3p y4 + 4s
2.4 2.7
G
8.55
H I
9.0 9.7
~1 + Wl + qJz-+ y3 -+ w4 +
H
9.7
D
w1 4
Term value (eVa
ISOBUTYL
~1 -+ $JJ’ -+1 ~1 --f ~1 + ~1 + ~1 -+
2.81 2.09 1.6 1.42 1.16 0.88
7,112+
3s 3p 3d 4s 4p 4d 5s 3s
y3 +
3s
2.8
y1+
4d 5s 3s 3s 3s
2.9 2.9 3.2
2.83
8 Assigned with respect to PES ionization potentials (for (CH&CHCHKOCH3, 9.42, 11.4, 11.9, 13.0 and 13.6 eV; for (CH&CHCOCH3,9.36, 11.8, 12.3, 12.6 and 13.8 eV; for (CH&CCOCHz, 9.21, 11.4,12.5,13.6 and 14.6 eV.
of consecutive substitution of the a-hydrogens in acetone by methyl groups on the intensity of their electronic transitions in the vacuum ultraviolet. In Table 3, we have assigned most structures in the spectra to Rydberg transitions based on term values and the calculated quantum defects. The broad peaks at high energy loss are probably the envelope of more than one transition. It is noted that in these higher ketones, the spectra only show broad features probably because there are so many vibrational modes. In particular, the shape of the n + 3s band (- 6.4 eV) is very sensitive to the alkyl group. Sharp vibrational structure observed in acetone and 2-butanone seems to disappear for the higher ketones studied. The first band in the spectra of methyl isopropyl ketone (b) and methyl t-butyl ketone (c) appears as a continuous diffuse band while that of methyl isobutyl ketone (a) still shows some indication of vibrational structure in the first band. A detailed study of the shape of this band using optical spectroscopy has been presented by Ito et al. 13. The second band in methyl isopropyl ketone and methyl t-butyl ketone is more intense than the first band while the reverse is true for other ketones studied in which the alkyl groups are not branched at the g-carbon. This may be a useful feature to distinguish between these two kinds of ketones. Holdsworth and Duncan1 ’ have also observed that the intensity of the 6.4 eV band decreases when the a-hydrogens in acetone are replaced by methyl groups while the intensity of the transition in the region at about 7.4 eV increases. No satisfactory explanation has yet been given for this phenomenon.
146 Eflect
of alkyd
substituents
The effect of alkyl groups on term values of organic compounds has been discussed by Robin l4 . As in the case of aldehydes’, approximately straight-line plots are obtained when the first IP, the 3s, 3p and 3d term values are plotted against the sum of Taft CJ*values @a*) of the two ,alkyl groups attached to the carbonyl group (Figure 4). The behaviour of the 3s, 3p and 3d terms is the same as in aldehydesl, and the alkyl derivatives of water “. The shift of the 3s peak maxima to lower energies on alkylation is smaller than in the case of aldehydes and so the straight lines for the first IP and the 3s term value dependencies are approximateIy parallel. In optical spectroscopy’* r O a bigger shift of the peak maxima of the 7.4 eV band to lower energies has been observed as more a-hydrogens in acetone are replaced by methyl groups. In our Rydberg model this can be easily explained by the fact that the 3p Rydberg electron is less penetrating than the electrons in the highest filled molecular orbital and therefore the binding energies of the 3p Rydberg orbital decrease less rapidly than the binding energy of the highest filled molecular orbital. As a result, the transition energy to the 3p Rydberg state, which is the difference between the two binding energies, is also decreasing with increasing alkylation.
0’
I
-0.3
I
I
-0.2
xa Figure
-0.1 *
,
0.0
-
4. Effect of alkyl substitutionon Rydberg term values and first ionization
potentials.
ACKNOWLEDGEMENT The authors are indebted to Mr. Derek Yee for his assistance in running the photoelectron spectra of the compounds studied. Financial support for this work was provided by the National Research Council of Canada. One of us (W.C.T.) gratefully acknowledges the receipt of a University of British Columbia Graduate Fellowship.
147 REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14
W. C. Tam and C. E. Brion, J. Electron Spectrusc., 3 (1974) 467. D. W. Turner, C. Baker, A. D. Baker and C. R. Brundle, Molecdar Photoelectron Spectroscopy, Wiley, London, 1970, p. 132. W. A. Noyes Jr., A. B. F. Duncan and W. M. Manning, J. Chem. Phys., 2 (1934) 717. A.B. F. Duncan,J. Gem. Phys., 3 (1935) 131. K. Watanabe, f. Chem. Phy.y., 22 (1954) 1564. M. B. Robin and N. A. Kuebler, J. Electron Spectrosc., 1 (1972/73) 13. H. Basch, M. B. Robin, N. A. Kuebler, C. Baker and D. W. Turner, J. C&m. Phys., 51 (1969) 52. A. W. Weiss, Phys. Rev., 178 (1969) 82. A. B. F. Duncan, J. Chem. Phys., 8 (1940) 444. R. S. Holdsworth and A. B. F. Duncan, Chem. Rev., 41(1947) 311. W. C. Tam and C. E. Brion, J. Hectron Spectrosc., 3 (1974) 263. M. Lawson and A. B. F. Duncan, J. Gem. Whys., 12 (1944) 329. H. Ito, Y. Nogata, S. Matsuzaki and A. Kuboyama, Bull. Chem. Sot. Jap., 42 (1969) 2453. M. B. Robin, Ztzt.J. Quantum Chem., 6 (1972) 257.