- Email: [email protected]
Ultraviolet absorption spectra of acetone and acetone-d6
Volume 7, -number 6
ULTRAVIOLET
CHEMICAL
ABSORPTION
PHYSICS
SPECTRA
OF
C. N. R. RAO, G. C. CHATURVEDI Department of Cliesristry,
LETTERS
Indian Institute
ACETONE
AND
ACETONE-+,
and H. S. RANDHAWA
of Tmhnology,
Kanp~~-lG,
Indin
IQceivcd 2% October 1970
Ultrnviolct absorption spectra of acetone and acetone-$ have heen photogmphetl uncier fairly high resolution and vibrational analyses carried out. The results indicate thnt the C(CO)C skeleton of the WX-
cited state of acetone is non-planar;
CX~/:!
calculations also substantiate this observation.
The high resolution ultraviolet spectrum of formaldehyde has been investigated by several workers [l-41 and the results clearly establish that both the singlet and triplet excited states are pyramidal with C, symmetry. The ultraviolet absorption spectrum of acetone was studied several years ago with a quartz spectrograph by Noyes and coworkers [51 who made tentative vibrational assignments of the upper state. We have now examined the high resoiution ultraviolet absorption spectp of ace*tone and acetone-% in the region 3300 A - 2700 A employing a 3.4 m Jarrell-Ash spectrograph in the third order. Our interest was to obtain more definitive information on the vibrational frequencies as well as the geometry of the excited state. The spectix of both acetone and acetone-d6 show the following features: (iI a long progression with a spacing of 210 cm” , (ii) a short progression with a spacing of about 1200 cm-*. and (iii) another short progression with a spacing of 1120 cm-l. The 210 cm-l spacing undoubtedly corresponds to the ground state frequency of 385 cm-l due to the C-C-C bending vibration [6]. Since the two short progressions with spacings of 1200 and 1120 cm-l are found in both acetone and acetone-de, we feel that they do not have origin in the C-H bending vibrations as suggested earlier [6]. The 1200 cm-l frequency probably corresponds to the upper state carbonyl stretching vibration; in the grou d state [S], the C=O frequency is at 1730 cm -1 In formaldehyde, the C =0 frequency decreases from 1’738 cm-’ to 1177 cm-l on excitation [1,2]. We first considered assigning the 1120 crnmd frequency to the C-C stretching vibration, but this would mean a large increase in C-C bond order in the upper state. It appears more likely that the C=O and the C-C ,
vibrations are strongly coupled in the upper state although the main Franck-Condon shift may be associated with the former. Similar mixing of the upper state modes has been observed in the case of propynal [7]. An interesting feature in the electronic spectrum of acetone is the presence of two levels separated by about 72 cm-l in some of the progressions. By analogy with formaldehyde [l-4]. we feel that the doubling of the levels in acetone may be due to the non-planarity of the C(CO)C skeleton in the excited state which gives rise to inversion. Accordingly, we find the separation of the levels is much lower (47 cm-l) in acetone-(16. These rather complex splittings suggest a flat pyramidal structure for the excited state. The presence of isotope effect indicates strong coupling with a hydrogenic mode. An rpproximate calculation
[lj shows
that the barrier
to inversion
is low (100 cm-l) compared to formaldehyde (600 cm-l). We are presently carrying out more detailed studies of the ultraviolet spectra of ncetone and its deuterated derivatives. In order to substantiate our observation that the excited state of acetone is non-planar. we have carried out CNDO/% calculations [8]. The results show that the minimum enerb? confibwration of the excited state is one where the carbonyl bond is appreciably longer (1.33 A): the C-C bond
distance
shows only a slight decrease
(0.02 A).
These variations in distances are consistent with our vibrational assignments for the upper state.
The caiculated out-of-plane bend is = 10’ in the excited state and the barrier to inversion is very small (E50 cm-l)Similar calculations on formaldehyde [9] and benzophenone [lo] show that the excited states are non-planar although the calculated barriers are small. 563
Volume
7, number 6
CHEMICAL
15 December
PHYSICSLETTERS
1970
The author& are thankful to Professor R. S. . Mulliken and Professor J. C. D. Brand for help-‘ ful comments and to Dr. p. A. Narasimham of the Spectroscopy Division, Bhahha Atomic Research . Center, for the use of the microphotometer.
[4] ,V. A. Job, V. Sethuramaqnd -K-r. Innes, J. Mol. Spectrjt, 30 (1969) 365. l5l.W.A. Noyes, A. B. F.‘.DunCan and W. ?Z..Manning, J.
REFERENCES
[a] J. A.~P&& ti.P.Santry and G.A.Segal, J. Chem. Phvs:43 (1965) S129. Sl36; 44 (1966) 3289.
[l] J.C.D.Brand, J. Chem. Sot. (1956) 858. [2] G. W. Robinson and V. E. DiGiorgio, Can. J. Chem. 3G (1.958) 31. [3] W.T.Rnynes, J. Chem. Phys. 44 (1966) 2755.
Chem. Phys. [S] GlDillepi&e
(1966) 593.
2 (1934) 717. and J;Overctid,
ci
Spectmchim.
’
Acta 23
:
[7] J. C. D. Brand, J. I-I. Collomon and J.K. G. Watson, Can. J. ~Phys:39 (1921) 1508; Discussions Faraday Sot. 35 11963) 175.. [9] H.k_Kr&o &I D.P.Santry, Ji Chem. Phys. 47 (1967) 792, 2736. [lo] R.Hoffmann and J.R.Swenson, J. Phys. Chem. 74 (1970) 415.
ULTRAVIOLET
CHEMICAL
ABSORPTION
PHYSICS
SPECTRA
OF
C. N. R. RAO, G. C. CHATURVEDI Department of Cliesristry,
LETTERS
Indian Institute
ACETONE
AND
ACETONE-+,
and H. S. RANDHAWA
of Tmhnology,
Kanp~~-lG,
Indin
IQceivcd 2% October 1970
Ultrnviolct absorption spectra of acetone and acetone-$ have heen photogmphetl uncier fairly high resolution and vibrational analyses carried out. The results indicate thnt the C(CO)C skeleton of the WX-
cited state of acetone is non-planar;
CX~/:!
calculations also substantiate this observation.
The high resolution ultraviolet spectrum of formaldehyde has been investigated by several workers [l-41 and the results clearly establish that both the singlet and triplet excited states are pyramidal with C, symmetry. The ultraviolet absorption spectrum of acetone was studied several years ago with a quartz spectrograph by Noyes and coworkers [51 who made tentative vibrational assignments of the upper state. We have now examined the high resoiution ultraviolet absorption spectp of ace*tone and acetone-% in the region 3300 A - 2700 A employing a 3.4 m Jarrell-Ash spectrograph in the third order. Our interest was to obtain more definitive information on the vibrational frequencies as well as the geometry of the excited state. The spectix of both acetone and acetone-d6 show the following features: (iI a long progression with a spacing of 210 cm” , (ii) a short progression with a spacing of about 1200 cm-*. and (iii) another short progression with a spacing of 1120 cm-l. The 210 cm-l spacing undoubtedly corresponds to the ground state frequency of 385 cm-l due to the C-C-C bending vibration [6]. Since the two short progressions with spacings of 1200 and 1120 cm-l are found in both acetone and acetone-de, we feel that they do not have origin in the C-H bending vibrations as suggested earlier [6]. The 1200 cm-l frequency probably corresponds to the upper state carbonyl stretching vibration; in the grou d state [S], the C=O frequency is at 1730 cm -1 In formaldehyde, the C =0 frequency decreases from 1’738 cm-’ to 1177 cm-l on excitation [1,2]. We first considered assigning the 1120 crnmd frequency to the C-C stretching vibration, but this would mean a large increase in C-C bond order in the upper state. It appears more likely that the C=O and the C-C ,
vibrations are strongly coupled in the upper state although the main Franck-Condon shift may be associated with the former. Similar mixing of the upper state modes has been observed in the case of propynal [7]. An interesting feature in the electronic spectrum of acetone is the presence of two levels separated by about 72 cm-l in some of the progressions. By analogy with formaldehyde [l-4]. we feel that the doubling of the levels in acetone may be due to the non-planarity of the C(CO)C skeleton in the excited state which gives rise to inversion. Accordingly, we find the separation of the levels is much lower (47 cm-l) in acetone-(16. These rather complex splittings suggest a flat pyramidal structure for the excited state. The presence of isotope effect indicates strong coupling with a hydrogenic mode. An rpproximate calculation
[lj shows
that the barrier
to inversion
is low (100 cm-l) compared to formaldehyde (600 cm-l). We are presently carrying out more detailed studies of the ultraviolet spectra of ncetone and its deuterated derivatives. In order to substantiate our observation that the excited state of acetone is non-planar. we have carried out CNDO/% calculations [8]. The results show that the minimum enerb? confibwration of the excited state is one where the carbonyl bond is appreciably longer (1.33 A): the C-C bond
distance
shows only a slight decrease
(0.02 A).
These variations in distances are consistent with our vibrational assignments for the upper state.
The caiculated out-of-plane bend is = 10’ in the excited state and the barrier to inversion is very small (E50 cm-l)Similar calculations on formaldehyde [9] and benzophenone [lo] show that the excited states are non-planar although the calculated barriers are small. 563
Volume
7, number 6
CHEMICAL
15 December
PHYSICSLETTERS
1970
The author& are thankful to Professor R. S. . Mulliken and Professor J. C. D. Brand for help-‘ ful comments and to Dr. p. A. Narasimham of the Spectroscopy Division, Bhahha Atomic Research . Center, for the use of the microphotometer.
[4] ,V. A. Job, V. Sethuramaqnd -K-r. Innes, J. Mol. Spectrjt, 30 (1969) 365. l5l.W.A. Noyes, A. B. F.‘.DunCan and W. ?Z..Manning, J.
REFERENCES
[a] J. A.~P&& ti.P.Santry and G.A.Segal, J. Chem. Phvs:43 (1965) S129. Sl36; 44 (1966) 3289.
[l] J.C.D.Brand, J. Chem. Sot. (1956) 858. [2] G. W. Robinson and V. E. DiGiorgio, Can. J. Chem. 3G (1.958) 31. [3] W.T.Rnynes, J. Chem. Phys. 44 (1966) 2755.
Chem. Phys. [S] GlDillepi&e
(1966) 593.
2 (1934) 717. and J;Overctid,
ci
Spectmchim.
’
Acta 23
:
[7] J. C. D. Brand, J. I-I. Collomon and J.K. G. Watson, Can. J. ~Phys:39 (1921) 1508; Discussions Faraday Sot. 35 11963) 175.. [9] H.k_Kr&o &I D.P.Santry, Ji Chem. Phys. 47 (1967) 792, 2736. [lo] R.Hoffmann and J.R.Swenson, J. Phys. Chem. 74 (1970) 415.