Electronic spectra of aliphatic carbonyl compounds by electron impact spectroscopy

of

Journal

@ Elsevier

Electron

Spectroscopy

Scientific

Publishing

Phenomena, 4 (1974) 149-154 Amsterdam - Printed in The Netherlands

and Related

Company,

ELECTRONIC SPECTRA OF ALIPHATIC ELECTRON IMPACT SPECTROSCOPY III. UNSATURATED

WING-CHEUNG

of

Depavfment (First

received

TAM Chemistry,

25 February

CARBONYL

COMPOUNDS

BY

COMPOUNDS

and C. E. BRION

of British Columbia, Vancouver 1974; in final form 29 March 1974)

University

V6T I W5

(Canada)

ABSTRACT Electron impact energy loss spectra at an impact energy of 100 eV and a scattering angle of 2 degrees are presented for propenal (CH, = CH-CHO) and methyl vinyl ketone (CH, =CH-CO-CH,). The spectra are tentatively assigned using quantum defects and term values derived from ionization potentials measured by photoelectron spectroscopy. The duality of Rydberg and valence transition assignments in propenal is discussed. It is shown that a transition earlier assigned by Walsh to be x + Z* is most probably a Rydberg transition_ INTRODUCTION

In parts I and II of this series13 2, the electron impact spectra of some aliphatic saturated aldehydes and ketones have been discussed respectively. We now report the results obtained for two unsaturated conjugated carbonyl compounds, propenal (acrolein) and methyl vinyl ketone. The experimental aspects have been discussed in part Il. RESULTS

AND

DISCUSSION

Figure 1 shows the electron impact spectra of propenal and methyl vinyl ketone over the energy loss region of 2-l 3 eV at an impact energy of 100 eV and 2 o scattering angle. The photoelectron spectrum of propenal has been studied by Turner et a1.3. The first two bands possess vibrational fine structure, the first peak being the most intense. The first two vertical ionization potentials are 10.11 and 10.93 eV respectively. We have obtained the photoelectron spectrum of methyl vinyl ketone in this‘ laborat&y4. The first band is sharp and shows vibrational structure. The first

Figure 1. Electron degrees.

impact

energy

loss spectra

of propenal

and methyl

vinyl ketone

at 100 eV, 2

peak is most intense and gives a vertical IP of 9.61 eV. The second band has five vibrational components, with the second one being the most intense, giving a vertical second IP of 10.62 eV. The second sharp band in the photoelectron spectrum seems to be a characteristic of unsaturated conjugated carbonyl compounds, for it does not occur in the photoelectron spectra of saturated aldehydes and ketones. Turner et aL3 have assigned the first band in the photoelectron spectrum of propenal to the loss of an electron from the non-bonding orbital ($ 1) and the second band to the loss of an electron from the higher of the two occupied n orbitals (+2). We assume that the photoelectron spectrum of methyl vinyl ketone can be interpreted in a similar fashion. The vacuum ultraviolet spectrum of propenal has been studied by Walsh’ and the features above 7.5 eV arranged into three Rydberg series. In our spectrum we observe series III5 with members C, G, J, L at 7.58, 8.89, -9.4 and 9.65 eV respectively which have been assigned as the $I -+ np Rydberg series (n = 3,4, 5,6). The quantum defect is 0.68. The shoulder D at 7.74 eV is separated from the peak C by less than 0.16 eV and this may be associated with a vibrational quantum. The small shoulder D’ at -7.9 eV may correspond to another component of the 3p manifold (probabIy 3~’ as in acetaldehyde’). A summary of the Rydberg transitions is presented in Table 1. The first prominent feature in the electron impact spectrum of propenal is a strong diffuse peak with a maximum at about 6.35 eV. Its term value of 3.76 eV, with respect to the first IP, is consistent with a 3s upper state (cf. formaldehyde 3.78

151 TABLE

1

RYDBERG Peak

Propenal A B C D D’ E F G H I J K L M N 0

TRANSITIONS Observed energy (e V)

IN PROPENAL

3.76 3.84 2.53 2.37 2.2 1.62 2.30 1.22 0.89 1.64 0.71 0.56 0.46 0.90 0.40 3.7

Methyl vinyl ketone A 6.25 B 6.66 C 6.85 D 7.34 E 7.56 F 8.08 G 8.25 H 8.50 I 8.70 J 8.81 K 9.12

3.36 2.95 2.76 3.28 2.05 1.53 1.36 1.11 0.91 0.80 0.49

L M

9.82 10.18

METHYL _

Term Assignment value (e V) a

6.35 7.09 7.58 7.74 7.9 8.49 8.63 8.89 9.22 9.29 9.4 9.55 9.65 10.03 10.53 12.10

-

AND

y1 yz w1 y1 yr yr yz Wl y1 yz WI y1 wl yz yz y44

+ 3s -+ 3s --, 3P --t 3P + y + 3p’ + 4s + 3P + 4P -+ 5s -+ 4s + SP -+ 6s --+ 6~ - 5s -+ 7s 3s

yJ1+

Wl +

VINYL

KETONE

Calculated energy (e V)

Calculated quartturn defect

1.10 1.12 0.68

8.49 0.57 8.88 9.22 9.29 9.39 9.54 9.63 10.03 10.54

3s 3P

0.99 0.85 0.96 0.42

1.50 0.80 0.44 y2

-+

7s

8.11 8.24 8.55 8.76 8.82 9.07 9.09 9.14 9.78 lO.OB 10.2s

a. Values for propenal assigned with respect to photoelectron ionization potentials of 10.11, 10.93, 13.5, 14.8, 15.3, 16.1 eV. Values for methyl vinyl ketone assigned with respect to PES ionization potentialsof9.61, 10.62, 13.1, 13.9,14.4,15.1 eV.

eV, acetaldehyde 3.39 eV). The calculated quantum defect (I _IO) is also consistent with this assignment (cf. formaldehyde, I. 1 1 and acetaldehyde, 1 .OO). The higher members of this series I++~+ ns are then predicted to occur at 8.49, 9.22 and 9.54 eV respectively. So the features occurring at 8.49, 9.22 and 9.55 eV in our spectra (peaks E, H and K) can be assigned 4s, 5s and 6s Rydberg upper states. The sharp peak B at

152 7.09 eV has a term value of 3.84 eV with respect to the second IP at 10.93 eV. The calculated quantum defect is 1.12 and so it is assigned as a ti 2 -+ 3s Rydberg transition. The n = 4 and 5 members of the I,+~+ ns series appear at 9.29 (I) and 10.03 (M) as predicted while higher states are probably contributing to the broad envelope N. The term value of peak F at 8.63 eV with respect to the second IP at 10.93 eV is 2.30 eV and so can be assigned to the Rydberg transition from fi2 to one of the components of 3p. The other components may have contributions to peak E. The broad peak N around 10.5 eV may have contributions from Rydberg transitions leading to the second and third IP’s while the broad feature maximising at about 12.1 eV may be the envelope of Rydberg transitions from $4_ Transitions to the nd Rydberg series are probably buried under other features in the spectrum_ A weak transition is observed in our spectrum as the broad band with a maximum at 3.8 eV (cf. 4.30 eV in acetaldehyde). That this low energy transition is n + TC* was established by Inuzuka, who did theoretical calculations6 and studied the effect of solvents on this transition’. This region has also been the interest of other optical spectroscopists * - lo. Vibrational structure of this band, which is not resolved in our spectrum, has been analysed by Eastwood and Snow lo and Inuzuka*‘. CH2=CH-CHO orbital

ion -

orbital energy O.OeV

KEY -----

Rydberg orbitals

-.-.-‘-

valence orbitals

-

filled

orbitals

our assignment

expected transitions on the basis of Walsh’s assignment. ” 11predicted position of r-orbital 7 if Walsh’s assignment is correct.

Figure 2. Energy levels and electronic

transitions

in propenal.

With the exception of the $1 + np series, our interpretation of the electron impact spectrum of propenal does not agree with that for the optical spectrum given by Walsh’, who assigned the peaks A (6.35 eV) and B (7.09 eV) to intervalence transitions* 7c---,7c* and n -+ CT* respectively. From the above paragraph, the binding energy of the n* orbital is the difference between the binding energy of the n orbital (10.1 eV from PES) and the n + x * transition energy (3.8 eV). This gives a value of 6.3 eV for the binding energy of the 7c* orbital (see Figure 2). The binding energy of * This assignment was made in 1945 before the advent of photoelectron the binding energy of the 7c orbital was unknown.

spectroscopy.

Hence

153 the 7~electron is given by the second IP (10.9 eV) from the photoelectron spectrum. Therefore the transition 7~+ X* should maximise at 10.9 - 6.3 = 4.6 eV instead of Walsh’s vaIue of 6.35 eV. Therefore, we suggest that Walsh’s assignment of peak A as n: + 7~*is incorrect. Also, it is obvious that any transition in the 4.6 eV region (Figure 1) is extremely weak. Unless for some peculiar reason the n + x* transition in acetaldehyde should be much stronger than that observed in propenal, we expect the n -+ rc* transition in acetaldehyde to be weak as well, in which case the assignment of the prominent features in the spectra of carbonyl compoundsl? 2 to Rydberg transitions rather than valence transitions has a firmer foundation. In the case of acetaldehyde, the particular value (13.3 eV) of the binding energy of the n-bonding orbital leads to a similar expected energy for the valence transition 7~ + z and the Rydberg transition rz+ 3p. This chance coincidence does not occur in the spectrum of propenal. However, the evidence in our spectrum does not preclude Walsh’s assignment of peak B at 7.09 eV as n + G* since the small shoulder D’ at 7.9 eV happens to lie at the expected position for the 7~ + Q* transition. (The difference between the n 4 C* and the 7~+ C* transition energies is expected to be of similar magnitude to the difference between the n and the rc binding energies.) Although B has already been assigned as +12 + 3s and D’ as $1 + 3p’ the possibility still exists that they can also be assigned as valence transitions n + 6” and rc + cr* respectively. At the present time, we cannot tell to what extent either or both are contributing. However evidence from optical spectroscopy and photoelectron spectroscopy seems to indicate that the 6.35 eV band -+ 3s and not 7c + 7~”as suggested by Walsh (Walsh’s (A) is the Rydberg transition $ 1 assignment would require the rc orbital to have an orbital energy of - 12.7 eV whereas the PES spectrum indicates a value of - 10.9 eV). The electron impact spectrum of methyl vinyl ketone (Figure 1) in which the aldehydic hydrogen in propenal is replaced by a methyl group, looks very similar to that of propenal. The difference between the first and second IP is about 1 eV. The term value of the peak A with respect to the first IP is 3.36 eV, as compared to 3.76 eV in propenal. Since the lowering of the 3s term value on alkylation has been observed before l* ‘, we assign peak A to the $i 3 3s Rydberg transition and the caIculated quantum defect, 0.99, is consistent with an s Rydberg orbital. Using the Rydberg formula, peaks F, I and K at 8.02, 8.70 and 9.12 eV are assigned transitions $1 + ns (n = 4, 5, 6). A comparison of the observed and calculated energy for all Rydberg transitions observed is shown in Table 1. The transition t+Q2-+ 3s is expected to occur around 3.4 eV below the second vertical IP of 10.6. So the peak D at 7.34 eV is assigned to this transition and the higher members of this +12 -+ ns series are expected at 9.14, 9.78, 10.08 and 10.25 eV for n = 4, 5, 6,7 respectively. Actually, a peak K and a step L are observed at 9.12 and 9.83 eV. The broad peak at 10.18 eV may be the convolution of the $ 1 + 6s and +I -_, 7s transitions_ The only feature that can be assigned the $ 1 + 3p Rydberg transition is the shoulder B at 6.66 eV. The term value of 2.95 eV with respect to the first IP, and the calculated quantum defect (0.85 eV) are higher than those in propenal but are still comparable to those12 of formaldehyde

154 (2.81 eV and 0.83 respectiveIy). The difference in quantum defect between propenal and methyl vinyl ketone indicates the different degrees of penetration of the np Rydberg electron into the molecular core. The Rydberg formula gives the higher members as peaks G, J, K at 8.25, 8.81 and 9.12 eV respectively. The shoulder E at 7.56 eV and the small peak A at 8.50 eV are assigned $ 1 -+ 3d and 4d transitions. The calculated quantum defect of 0.42 is also comparable to that of formaldehyde. The broad peak N maximising at about 11.8 eV probably envelopes Rydberg transitions from ti5 and $/6 (fifth and sixth IP at 13.9 and 14.4 eV respectively). The maximum of the n -3 z* transition in methyl vinyl ketone (3.6 eV) is shifted to lower energy compared to that of propenal (3.8 ev). ACKNOWLEDGEMENT

The authors are indebted to Mr. Derek Yee for his assistance in running the photoelectron spectra of the compounds studied. Financial support of 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. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12

W. C. Tam and C. E. Brion, J_ Electron Spectrusc., 3 (1974) 467. W. C. Tam and C. E. Brion, J. Electron Speclrosc., 4 (1974) 139. D. W. Turner, C. Baker, A. D. Baker and C. R. Brundle, Molecular Photoelectron Spectroscopy, Wiley, London, 1970, p. 250. W. C. Tam, D. Yee and C. E. Brion, J. Electron Spectrosc., 4 (1974) 77. A. D. Walsh, Trans. Faraday Sot., 41 (1945) 49X. K. Inuzuka, Bull. Chem. Sot. Jap., 34 (1961) 6. K. Inuzuka, Bull. Chem. Sue. Jap., 34 (1961) 729. A. W. Thompson and J. W. Linnett, .7. Chem. SOL, (1935) 1452. F. E. BIacet, W. G. Young and J. G. Roof, J. Amer. Chem. Sot., 59 (1937) 608. E. Eastwood and C. P. Snow, Proc. Roy. Sot. London, 149A (1935) 446. K. Inuzuka, Bull. Chem. Sm. Jap., 33 (1960) 678. M. J. Weiss, C. E. Kuyatt and S. Mielczarek, J. Chem. Phys., 54 (1971) 4147.