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He I and He II photoelectron spectra of γ-thionobutyrolactones and γ-butyrolactones
JOURNAL OF ELECTRON SPECTROSCOPY and Related Phenomena
ELSEVIER
Journal of Electron Spectroscopy and Related Phenomena 88-91 (1998) 91-96
He I and He II photoelectron spectra of 3 -thionobutyrolactones and "y-butyrolactones J.F. Pan*, W. Huang, W.S. Chin, H.H. Huang, C.Y. Mok Department of Chemistry, National University of Singapore, Kent Ridge, Singapore 119260
Abstract The valence electronic structures of the compounds 7-thionobutyrolactone (X=S, Y=O, R=R"=H), 7-butyrolactone (X = O, Y = O, R = R"= H) and their respective 3- and 5-methyl-substituted derivatives have been investigated by photoelectron spectroscopy. Two distinct bands are observed in the ionization potential (IP) range 8-11 eV in the spectra of all these compounds, with the first ionization band exhibiting clear vibrational progressions. Variation in the He I/He II relative intensities in the spectra of the 3,-thionobutyrolactones suggests the assignment of the first band to the lone-pair ns orbital, the second band to the 7rocs orbital and the third band to the 7rocs orbital. The first two bands in the spectra of the 3,-butyrolactones, on the other hand, are ascribed to the no(c-o) and the ethereal no(r) orbitals, respectively. The assignments are aided by the results from semi-empirical PM3 and ab initio calculations using the 6-31G** basis sets. Correlation with the IPs of thioformaldehyde, cyclopentanone and tetrahydrofuran indicates that the interaction between the carbonyl group and the ring oxygen atom is mainly inductive in nature. In the "y-thionobutyrolactones, the conjugative interaction is more significant. Methyl substitution causes mainly inductive effects on the parent heterocyclics. © 1998 Elsevier Science B.V. Keywords: Thionobutyrolactones; Butyrolactones; PES; Ionization potentials; HeI/HeII spectra; MO calculations
1.
Introduction
The gas phase thermal decomposition of -y-butyrolactones of general formula (I) given below has recently been investigated [1-4].
R'~ X I Different primary pyrolysis channels have been observed for the different natures of the heteroatoms.
* Corresponding author.
Thus, whereas 3,-butyrolactone (I; where X = O , Y = O , R = R " = H ) decomposes mainly through the decarboxylation process, giving propene as the main product; decarbonylation is the major process observed for 3,-thiobutyrolactone (I; where X = O , Y = S , R = R " = H ) . "y-Thionobutyrolactone (I; where X = S , Y = O , R = R " = H ) , on the other hand, gives carbonyl sulfide as one of the main products. It was also noted that 3~-thionobutyrolactone may be converted into the corresponding thiobutyrolactone as a competing pathway [4]. The interactions between the heteroatom and the double-bonded group may have influenced the primary step in the decomposition o f these compounds. For example, it was found that the interaction between the carbonyl group and the ring sulfur atom in the 3,-thiobutyrolactones is primarily inductive in
0368-2048/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved PH S0368-2048(97)00233-8
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J.F. Pan et al./Journal of Electron Spectroscopy and Related Phenomena 88-91 (1998) 91-96
He I
He II
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IONIZATION POTENTIAL (eV) IONIZATION POTENTIAL (eV) Fig. 1. He I and He II photoelectron spectra of (A) y-thionobutyrolactone, (B) 3-methyl-y-thionobutyrolactone and (C) 5-methyl-y-thionobutyrolactone.
nature [5]. In this paper, we discuss the electronic structures of'y-thionobutyrolactone and -y-butyrolactone, through assignment of their photoelectron spectra. The thiocarbonyl and carbonyl groups are expected to give rise to a lone- pair nx(a) and a 7rcx orbital among the occupied valence orbitals [6]. The inductive or conjugative interactions between these orbitals with the 2p lone pairs of the oxygen heteroatom in the ring will be of interest. To aid in the assignment of the bands, He II spectra are also recorded, and PM3 and ab initio calculations have been performed.
2. Experimental -y-Thionobutyrolactone and its methyl-substituted derivatives were synthesized from the corresponding commercially obtained butyrolactones [7]. He I and He II spectra were recorded on a Leybold-Heraeus UPG-200 spectrometer. The spectral resolution was within 18-25 meV for the He I and 35-50 meV for the He II spectra. Calibration was carried out with the argon 2p and iodomethane 2E doublets. Ab initio 6-31G** calculations were performed with the GAUSSIAN94 programme [8] on a Cray J90
J.F. Pan et al./Journal of Electron Spectroscopy and Related Phenomena 88-91 (1998) 91-96
93
computer. PM3 semi-empirical calculations were carried out with the programme MOVAC6.0 [9] running on a Silicon Graphic 4D/20 computer. Table 1 Vertical ionization potentials (IP) and calculated eigenvalues (e) for the first few ionizations of 3"-thionobutyrolactones and 3"-butyrolactones IP (eV)
Assignment
- e (eV) PM3
6-31G**
3"-Thionobutyrolactone 8.58 9.56 11.90 12.33 13.0 13.4
9.11 10.21 11.96 13.44 14.06 14.25
8.91 9.57 13.52 13.44 14.38 15.06
ns 7rots rocs, 7rcm
acs Occ, 7rCH2 no(a), lrCH2
3-Methyl-3"-thionobutyrolactone 8.48 9.43 11.50 11.93 12.3 13.1
9.09 10.19 12.02 12.84 12.95 13.75
8.90 9.54 13.30 12.98 13.76 14.63
ns rocs 7rocs, 7rcry2, 7rcm ocs, Occ occ, 7rcH no(a), r c m , 7/'CH3
5-Methyl- y-thionobutyrolactone 9.10 10.17 12.08 12.90 13.19 13.92
8.85 9.49 13.23 13.27 13.72 14.94
ns
10.27
1 l. 12
11.69
n o~c-o>
10.97 12.48 12.8 13.4
11.26 12.31 13.76 14.01
12.22 13.61 14.00 14.68
n o(~') a'co, r c m acc, 7rCH2 no(O), a'cr~2
1 1.56 12.19 13.27 13.51 14.13
nocc=o) no(r) a-co o c c , 7rcm, 7rca3 no(a), rcH3
11.64 12.07 13.33 13.42 14.45
nof¢ o~ n o(Tr) 7rco, 7rcm 7rCH, rrcm no(a), r c m
8.40 9.38 11.62 12.1 12.5 13.2
7rocs 7rocs, 7rcr~, rcn2
acs acc, 7rcm no(a), 7rcm
"y-Butyrolactone
3-Methyl- y-butyrolactone 10.12 10.90 12.1 12.5 13.5
11.04 11.23 12.37 12.90 13.46
5-Methyl- 3"-butyrolactone 10.15 10.80 12.2 12.6 13.8
11.10 11.41 12.25 12.75 13.36
3. Results and discussion
3.1. He I and He H spectra of y-thionobutyrolactones The He I and He lI photoelectron spectra of the three 3/-thionobutyrolactones are presented in Fig. 1. Between 8 and 10.5 eV in all of these spectra, two distinct bands are detected with the first one exhibiting fine vibrational progressions. After a relatively large gap, there is a group of overlapping bands between 11 and 18 eV. The He 1/He II relative band intensities are expected to vary according to the type of orbital involved since the photoionization cross-section will change with the radiation source energy [ 10]. Thus He II excitation should result in a reduction in intensity for bands arising from orbitals localized mainly on the sulfur atom and the reverse is anticipated for bands arising from those localized on oxygen. For all three y-thionobutyrolactones, whereas the intensity ratios of the first two bands are about 1:1 in the He I spectra, the ratios change to about 1:3 in the He II spectra. A relative enhancement of intensity is also observed for the third band at 11.5 eV and a reduction in intensity for the fourth band at around 12 eV. The adiabatic IPs coincide with the vertical IPs for the first band of all three compounds. The associated vibrational frequencies are approximately 1400 cm -1, 1370 cm -1 and 1420 cm -l for 3,-thionobutyrolactone and the 3- and 5-methyl derivatives, respectively. These may be attributed to the C--S stretching vibration of the ionic states, which appears at about 13801450 cm -1 in the neutral molecules [11]. The largely reduced He II relative intensity for the first band, together with the band shape they exhibit, clearly indicates that this band is associated with a non-bonding orbital localized on the sulphur atom, i.e. an ns orbital of cr symmetry. The second and third bands are enhanced in the He II spectra, suggesting photoionization from orbitals which have a considerable oxygen character. Theoretical results from both the PM3 and the 6-31G**
94
J.F. Pan et al./Journal of Electron Spectroscopy and Related Phenomena 88-91 (1998) 91-96
Hel
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Fig. 2. He I and He II photoelectronspectra of (A) 7-butyrolactone, (B) 3-methyl-.,/-butyrolactoneand (C) 5-methyl-',/-butyrolactone. calculations predict that the second orbital consists of an antibonding combination of a'cs and the no(r) lone pair. For the third band, PM3 results suggest that it is the bonding combination of the 7rcs and the no(r) orbital, although 6-31G** calculations attribute this to the ac=s orbital. On the basis of the He I/He II intensity variation, these two bands should be assigned to the 7rocs antibonding orbital and the rocs bonding orbital, respectively. For the fourth band, the observed He I/He II intensity variation points to an orbital having mainly an S 3p character.
It is assigned to the ac=s orbital, as suggested by the PM3 calculations. The 6-31G** calculations give a reverse order for the third and the fourth orbitals, although the calculated eigenvalues in this case are very close to each other. Methyl substitution at the 3- and 5-positions seems to have little effect on the first four orbitals of 3,-thionobutyrolactone as the spectra show largely similar He I/He II intensity variations. The first few IPs of the parent compound are lowered due to the inductive effect of the methyl group, as shown in Table 1.
J.F. Pan et al./Journal of Electron Spectroscopy and Related Phenomena 88-91 (1998) 91-96
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Fig. 3. Correlation d i a g r a m o f the first few IPs o f thioformaldehyde, 3,-thionobutyrolactone, tetrahydrofuran, ~/-butyrolactone a n d cyclopentanone.
3.2. He I and He H spectra of T-butyrolactones
3.3. Correlation diagram
The He I and He II spectra of the three 3,-butyrolactones are presented in Fig. 2. The intensities for the first two bands are relatively enhanced in the He II spectra, indicating that these bands are associated with orbitals localized largely on the oxygen atoms. The narrow band envelopes of these bands also suggest that the first two orbitals are relatively nonbonding in nature. Vibrational progressions can be detected for the first bands with frequencies in the range 1560-1590 cm -1, which can be attributed to the ionic C=O stretching frequencies [12]. Both 6-31G** and PM3 calculations predict that the no,c=•) orbital is the highest occupied orbital followed by the ethereal no orbital. Other bands of 3,-butyrolactone appear at IPs higher than 12 eV and are largely overlapping. These bands do not show a prominent intensity variation from He I to He II excitation partly because of a significant mixing of orbitals, as suggested by the calculations. As shown in Table 1, methyl substitution in general has lowered the IPs slightly, but does not affect the order of the first few orbitals of the parent compound.
A correlation between the first few IPs of 3,-thionobutyrolactone and 3,-butyrolactone with those of tetrahydrofuran [13], thioformaldehyde [14] and cyclopentanone [15] is drawn in Fig. 3. For 7-thionobutyrolactone, conjugative interactions between the 7rcs and the no(r) levels give rise to an antibonding 7rocs and a bonding 7rocs orbital, although the shift is only marginal in both cases. On the other hand, both the ns and the Ocs levels in thioformaldehyde are shifted upward in -y-thionobutyrolactone. Since the oxygen atom in the ring has electron-withdrawing properties, such upward shifts suggest strong through-space interactions of the methylene group in the ring. This through-space interaction may have also coupled with the rrocs and 7rocs levels, thus resulting in only marginal shifts for the latter two levels. The first few levels in tetrahydrofuran and cyclopentanone are shifted to higher IPs in 3,-butyrolactone. These shifts suggest that interactions between the carbonyl group and the ethereal oxygen atom are essentially inductive in nature. The conjugative effect, which would have caused these levels to shift in a
96
J.F. Pan et al./Journal of Electron Spectroscopy and Related Phenomena 88-91 (1998) 91-96
different direction, is expected to be minimal in this case. In summary, the interaction between the carbonyl group with the ring oxygen atom in .y-butyrolactone is mainly inductive in nature. The observed He I/He II intensity variation, on the other hand, suggests that a conjugative interaction occurs to some extent in q/-thionobutyrolactone. The presence of a conjugative effect may have favoured the decarboxylation process in the decomposition of 3,-thionobutyrolactone and led to the formation of COS. This is apparently not the only reason since other factors, such as the formation of a stable biradical, may be important also in deciding the primary step in thermal reactions.
References [1] A. Rai-Chaudhuri, W.S. Chin, D. Kaur, C.Y. Mok, H.H. Huang, J. Chem. Soc., Perkin Trans. 2 0 (1993) 1249. [2] A. Rai-Chaudhuri, W.S. Chin, C.Y. Mok, H.H. Huang, J. Chem. Res. (S) 0 (1994) 378. [3] Y.T. Chua, C.Y. Mok, H.H. Huang, I. Novak, S.C. Ng, J. Chem. Soc., Perkin Trans. 2 0 (1996) 577. [4] J.F. Pan, W. Huang, W.S. Chin, H.H. Huang, C.Y. Mok, submitted for publication. [5] W.S. Chin, Z. P. Xu, C.Y. Mok, H.H. Huang, H. Mutoh, S. Masuda, submitted for publication.
[6] E. Schaumann, in: S. Patai (Ed.), The Chemistry of Doublebonded Functional Groups, John Wiley, Chichester, 1989, 1283. [7] S. Scheibye, J. Kristensen, S.-O. Lawesson, Tetrahedron 35 (1979) 1339. [8] M.J. Frisch, G.W. Trucks, H.B. Schlegel, P.M.W. Gill, B.G. Johnson, M.A. Robb, J.R. Cheeseman, T. Keith, G.A. Petersson, J.A. Montgomery, K. Raghavachari, M.A. AI-Laham, V.G. Zakrzewski, J.V. Ortiz, J.B. Foresman, J.Cioslowski, B.B. Stefanov, A. Nanayakkara, M. Challacombe, C.L.Y. Peng, P.Y. Ayala, W. Chen, M.W. Wong, J.L. Andres, E.S. Replogle, R. Gomperts, R.L. Martin, D.J. Fox, J.S. Binkley, D.J. Defrees, J. Baker, J.P. Stewart, M. HeadGordon, and J.A. Pople, Gaussian 94, Revision D.3, Gaussian, Inc., Pittsburgh PA, 1995. [9] J.P. Stewart, MOPAC 6.0 (QCPE455), Quantum Chemistry Program Exchange, Indiana University, Bloominton, IN, USA, 1990. [10] A. Schweig, W. Thiel, J. Electron Spectrosc. Relat. Phenom. 3 (1974) 27. [11] (a) F.F. Khouri, M. K. Kaloustian, J. Am. Chem. Soc., 108 (1986) 6683. (b) J.P. Hagen, J. Org. Chem., 58 (1993) 506. [12] M. Jinno, I. Watanabe, Y. Yokoyama, S. Ikeda, Bull. Jpn. Chem. Soc. 50 (1977) 597. [13] K. Kimura, S. Katsumata, Y. Achiba, T. Yamazaki, S. Iwata, Handbook of He(I) Photoelectron Spectra of Fundamental Organic Molecules,Japan ScientificSocieties Press, Tokyo, 1981. [14] B. Solouki, P. Rosmus, H. Bock, J. Am. Chem. Soc. 98 (1976) 6054. [15] S.H. Gerson, S.D. Worley, N. Bodor, J. Kaminskl, T.W. Flechtner, J. Electron Spectrosc. Relat. Phenom. 13 (1978) 421.
ELSEVIER
Journal of Electron Spectroscopy and Related Phenomena 88-91 (1998) 91-96
He I and He II photoelectron spectra of 3 -thionobutyrolactones and "y-butyrolactones J.F. Pan*, W. Huang, W.S. Chin, H.H. Huang, C.Y. Mok Department of Chemistry, National University of Singapore, Kent Ridge, Singapore 119260
Abstract The valence electronic structures of the compounds 7-thionobutyrolactone (X=S, Y=O, R=R"=H), 7-butyrolactone (X = O, Y = O, R = R"= H) and their respective 3- and 5-methyl-substituted derivatives have been investigated by photoelectron spectroscopy. Two distinct bands are observed in the ionization potential (IP) range 8-11 eV in the spectra of all these compounds, with the first ionization band exhibiting clear vibrational progressions. Variation in the He I/He II relative intensities in the spectra of the 3,-thionobutyrolactones suggests the assignment of the first band to the lone-pair ns orbital, the second band to the 7rocs orbital and the third band to the 7rocs orbital. The first two bands in the spectra of the 3,-butyrolactones, on the other hand, are ascribed to the no(c-o) and the ethereal no(r) orbitals, respectively. The assignments are aided by the results from semi-empirical PM3 and ab initio calculations using the 6-31G** basis sets. Correlation with the IPs of thioformaldehyde, cyclopentanone and tetrahydrofuran indicates that the interaction between the carbonyl group and the ring oxygen atom is mainly inductive in nature. In the "y-thionobutyrolactones, the conjugative interaction is more significant. Methyl substitution causes mainly inductive effects on the parent heterocyclics. © 1998 Elsevier Science B.V. Keywords: Thionobutyrolactones; Butyrolactones; PES; Ionization potentials; HeI/HeII spectra; MO calculations
1.
Introduction
The gas phase thermal decomposition of -y-butyrolactones of general formula (I) given below has recently been investigated [1-4].
R'~ X I Different primary pyrolysis channels have been observed for the different natures of the heteroatoms.
* Corresponding author.
Thus, whereas 3,-butyrolactone (I; where X = O , Y = O , R = R " = H ) decomposes mainly through the decarboxylation process, giving propene as the main product; decarbonylation is the major process observed for 3,-thiobutyrolactone (I; where X = O , Y = S , R = R " = H ) . "y-Thionobutyrolactone (I; where X = S , Y = O , R = R " = H ) , on the other hand, gives carbonyl sulfide as one of the main products. It was also noted that 3~-thionobutyrolactone may be converted into the corresponding thiobutyrolactone as a competing pathway [4]. The interactions between the heteroatom and the double-bonded group may have influenced the primary step in the decomposition o f these compounds. For example, it was found that the interaction between the carbonyl group and the ring sulfur atom in the 3,-thiobutyrolactones is primarily inductive in
0368-2048/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved PH S0368-2048(97)00233-8
92
J.F. Pan et al./Journal of Electron Spectroscopy and Related Phenomena 88-91 (1998) 91-96
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He II
(A)
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IONIZATION POTENTIAL (eV) IONIZATION POTENTIAL (eV) Fig. 1. He I and He II photoelectron spectra of (A) y-thionobutyrolactone, (B) 3-methyl-y-thionobutyrolactone and (C) 5-methyl-y-thionobutyrolactone.
nature [5]. In this paper, we discuss the electronic structures of'y-thionobutyrolactone and -y-butyrolactone, through assignment of their photoelectron spectra. The thiocarbonyl and carbonyl groups are expected to give rise to a lone- pair nx(a) and a 7rcx orbital among the occupied valence orbitals [6]. The inductive or conjugative interactions between these orbitals with the 2p lone pairs of the oxygen heteroatom in the ring will be of interest. To aid in the assignment of the bands, He II spectra are also recorded, and PM3 and ab initio calculations have been performed.
2. Experimental -y-Thionobutyrolactone and its methyl-substituted derivatives were synthesized from the corresponding commercially obtained butyrolactones [7]. He I and He II spectra were recorded on a Leybold-Heraeus UPG-200 spectrometer. The spectral resolution was within 18-25 meV for the He I and 35-50 meV for the He II spectra. Calibration was carried out with the argon 2p and iodomethane 2E doublets. Ab initio 6-31G** calculations were performed with the GAUSSIAN94 programme [8] on a Cray J90
J.F. Pan et al./Journal of Electron Spectroscopy and Related Phenomena 88-91 (1998) 91-96
93
computer. PM3 semi-empirical calculations were carried out with the programme MOVAC6.0 [9] running on a Silicon Graphic 4D/20 computer. Table 1 Vertical ionization potentials (IP) and calculated eigenvalues (e) for the first few ionizations of 3"-thionobutyrolactones and 3"-butyrolactones IP (eV)
Assignment
- e (eV) PM3
6-31G**
3"-Thionobutyrolactone 8.58 9.56 11.90 12.33 13.0 13.4
9.11 10.21 11.96 13.44 14.06 14.25
8.91 9.57 13.52 13.44 14.38 15.06
ns 7rots rocs, 7rcm
acs Occ, 7rCH2 no(a), lrCH2
3-Methyl-3"-thionobutyrolactone 8.48 9.43 11.50 11.93 12.3 13.1
9.09 10.19 12.02 12.84 12.95 13.75
8.90 9.54 13.30 12.98 13.76 14.63
ns rocs 7rocs, 7rcry2, 7rcm ocs, Occ occ, 7rcH no(a), r c m , 7/'CH3
5-Methyl- y-thionobutyrolactone 9.10 10.17 12.08 12.90 13.19 13.92
8.85 9.49 13.23 13.27 13.72 14.94
ns
10.27
1 l. 12
11.69
n o~c-o>
10.97 12.48 12.8 13.4
11.26 12.31 13.76 14.01
12.22 13.61 14.00 14.68
n o(~') a'co, r c m acc, 7rCH2 no(O), a'cr~2
1 1.56 12.19 13.27 13.51 14.13
nocc=o) no(r) a-co o c c , 7rcm, 7rca3 no(a), rcH3
11.64 12.07 13.33 13.42 14.45
nof¢ o~ n o(Tr) 7rco, 7rcm 7rCH, rrcm no(a), r c m
8.40 9.38 11.62 12.1 12.5 13.2
7rocs 7rocs, 7rcr~, rcn2
acs acc, 7rcm no(a), 7rcm
"y-Butyrolactone
3-Methyl- y-butyrolactone 10.12 10.90 12.1 12.5 13.5
11.04 11.23 12.37 12.90 13.46
5-Methyl- 3"-butyrolactone 10.15 10.80 12.2 12.6 13.8
11.10 11.41 12.25 12.75 13.36
3. Results and discussion
3.1. He I and He H spectra of y-thionobutyrolactones The He I and He lI photoelectron spectra of the three 3/-thionobutyrolactones are presented in Fig. 1. Between 8 and 10.5 eV in all of these spectra, two distinct bands are detected with the first one exhibiting fine vibrational progressions. After a relatively large gap, there is a group of overlapping bands between 11 and 18 eV. The He 1/He II relative band intensities are expected to vary according to the type of orbital involved since the photoionization cross-section will change with the radiation source energy [ 10]. Thus He II excitation should result in a reduction in intensity for bands arising from orbitals localized mainly on the sulfur atom and the reverse is anticipated for bands arising from those localized on oxygen. For all three y-thionobutyrolactones, whereas the intensity ratios of the first two bands are about 1:1 in the He I spectra, the ratios change to about 1:3 in the He II spectra. A relative enhancement of intensity is also observed for the third band at 11.5 eV and a reduction in intensity for the fourth band at around 12 eV. The adiabatic IPs coincide with the vertical IPs for the first band of all three compounds. The associated vibrational frequencies are approximately 1400 cm -1, 1370 cm -1 and 1420 cm -l for 3,-thionobutyrolactone and the 3- and 5-methyl derivatives, respectively. These may be attributed to the C--S stretching vibration of the ionic states, which appears at about 13801450 cm -1 in the neutral molecules [11]. The largely reduced He II relative intensity for the first band, together with the band shape they exhibit, clearly indicates that this band is associated with a non-bonding orbital localized on the sulphur atom, i.e. an ns orbital of cr symmetry. The second and third bands are enhanced in the He II spectra, suggesting photoionization from orbitals which have a considerable oxygen character. Theoretical results from both the PM3 and the 6-31G**
94
J.F. Pan et al./Journal of Electron Spectroscopy and Related Phenomena 88-91 (1998) 91-96
Hel
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Fig. 2. He I and He II photoelectronspectra of (A) 7-butyrolactone, (B) 3-methyl-.,/-butyrolactoneand (C) 5-methyl-',/-butyrolactone. calculations predict that the second orbital consists of an antibonding combination of a'cs and the no(r) lone pair. For the third band, PM3 results suggest that it is the bonding combination of the 7rcs and the no(r) orbital, although 6-31G** calculations attribute this to the ac=s orbital. On the basis of the He I/He II intensity variation, these two bands should be assigned to the 7rocs antibonding orbital and the rocs bonding orbital, respectively. For the fourth band, the observed He I/He II intensity variation points to an orbital having mainly an S 3p character.
It is assigned to the ac=s orbital, as suggested by the PM3 calculations. The 6-31G** calculations give a reverse order for the third and the fourth orbitals, although the calculated eigenvalues in this case are very close to each other. Methyl substitution at the 3- and 5-positions seems to have little effect on the first four orbitals of 3,-thionobutyrolactone as the spectra show largely similar He I/He II intensity variations. The first few IPs of the parent compound are lowered due to the inductive effect of the methyl group, as shown in Table 1.
J.F. Pan et al./Journal of Electron Spectroscopy and Related Phenomena 88-91 (1998) 91-96
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• "ff,ocs
f
10.27. o" no (c-o)
, •
11.76/ ~CS . . . . . . .
95
o•
0°
no(a)
no(n)
"
11.89
~'~
12.48 ~ o - * ,
•
",.,,
'l
7~c-o
'ff.c...o ~ 13.4 no(G)
C]~cs
Fig. 3. Correlation d i a g r a m o f the first few IPs o f thioformaldehyde, 3,-thionobutyrolactone, tetrahydrofuran, ~/-butyrolactone a n d cyclopentanone.
3.2. He I and He H spectra of T-butyrolactones
3.3. Correlation diagram
The He I and He II spectra of the three 3,-butyrolactones are presented in Fig. 2. The intensities for the first two bands are relatively enhanced in the He II spectra, indicating that these bands are associated with orbitals localized largely on the oxygen atoms. The narrow band envelopes of these bands also suggest that the first two orbitals are relatively nonbonding in nature. Vibrational progressions can be detected for the first bands with frequencies in the range 1560-1590 cm -1, which can be attributed to the ionic C=O stretching frequencies [12]. Both 6-31G** and PM3 calculations predict that the no,c=•) orbital is the highest occupied orbital followed by the ethereal no orbital. Other bands of 3,-butyrolactone appear at IPs higher than 12 eV and are largely overlapping. These bands do not show a prominent intensity variation from He I to He II excitation partly because of a significant mixing of orbitals, as suggested by the calculations. As shown in Table 1, methyl substitution in general has lowered the IPs slightly, but does not affect the order of the first few orbitals of the parent compound.
A correlation between the first few IPs of 3,-thionobutyrolactone and 3,-butyrolactone with those of tetrahydrofuran [13], thioformaldehyde [14] and cyclopentanone [15] is drawn in Fig. 3. For 7-thionobutyrolactone, conjugative interactions between the 7rcs and the no(r) levels give rise to an antibonding 7rocs and a bonding 7rocs orbital, although the shift is only marginal in both cases. On the other hand, both the ns and the Ocs levels in thioformaldehyde are shifted upward in -y-thionobutyrolactone. Since the oxygen atom in the ring has electron-withdrawing properties, such upward shifts suggest strong through-space interactions of the methylene group in the ring. This through-space interaction may have also coupled with the rrocs and 7rocs levels, thus resulting in only marginal shifts for the latter two levels. The first few levels in tetrahydrofuran and cyclopentanone are shifted to higher IPs in 3,-butyrolactone. These shifts suggest that interactions between the carbonyl group and the ethereal oxygen atom are essentially inductive in nature. The conjugative effect, which would have caused these levels to shift in a
96
J.F. Pan et al./Journal of Electron Spectroscopy and Related Phenomena 88-91 (1998) 91-96
different direction, is expected to be minimal in this case. In summary, the interaction between the carbonyl group with the ring oxygen atom in .y-butyrolactone is mainly inductive in nature. The observed He I/He II intensity variation, on the other hand, suggests that a conjugative interaction occurs to some extent in q/-thionobutyrolactone. The presence of a conjugative effect may have favoured the decarboxylation process in the decomposition of 3,-thionobutyrolactone and led to the formation of COS. This is apparently not the only reason since other factors, such as the formation of a stable biradical, may be important also in deciding the primary step in thermal reactions.
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[6] E. Schaumann, in: S. Patai (Ed.), The Chemistry of Doublebonded Functional Groups, John Wiley, Chichester, 1989, 1283. [7] S. Scheibye, J. Kristensen, S.-O. Lawesson, Tetrahedron 35 (1979) 1339. [8] M.J. Frisch, G.W. Trucks, H.B. Schlegel, P.M.W. Gill, B.G. Johnson, M.A. Robb, J.R. Cheeseman, T. Keith, G.A. Petersson, J.A. Montgomery, K. Raghavachari, M.A. AI-Laham, V.G. Zakrzewski, J.V. Ortiz, J.B. Foresman, J.Cioslowski, B.B. Stefanov, A. Nanayakkara, M. Challacombe, C.L.Y. Peng, P.Y. Ayala, W. Chen, M.W. Wong, J.L. Andres, E.S. Replogle, R. Gomperts, R.L. Martin, D.J. Fox, J.S. Binkley, D.J. Defrees, J. Baker, J.P. Stewart, M. HeadGordon, and J.A. Pople, Gaussian 94, Revision D.3, Gaussian, Inc., Pittsburgh PA, 1995. [9] J.P. Stewart, MOPAC 6.0 (QCPE455), Quantum Chemistry Program Exchange, Indiana University, Bloominton, IN, USA, 1990. [10] A. Schweig, W. Thiel, J. Electron Spectrosc. Relat. Phenom. 3 (1974) 27. [11] (a) F.F. Khouri, M. K. Kaloustian, J. Am. Chem. Soc., 108 (1986) 6683. (b) J.P. Hagen, J. Org. Chem., 58 (1993) 506. [12] M. Jinno, I. Watanabe, Y. Yokoyama, S. Ikeda, Bull. Jpn. Chem. Soc. 50 (1977) 597. [13] K. Kimura, S. Katsumata, Y. Achiba, T. Yamazaki, S. Iwata, Handbook of He(I) Photoelectron Spectra of Fundamental Organic Molecules,Japan ScientificSocieties Press, Tokyo, 1981. [14] B. Solouki, P. Rosmus, H. Bock, J. Am. Chem. Soc. 98 (1976) 6054. [15] S.H. Gerson, S.D. Worley, N. Bodor, J. Kaminskl, T.W. Flechtner, J. Electron Spectrosc. Relat. Phenom. 13 (1978) 421.