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Laser fluorescence excitation spectra of the 1,3,5-trifluorobenzene radical cation
Volume
58, number 3
PHYSICS JXTTERS
cxEM1cAL
LASER FLUORESCENCE
EXCITATION
OF THE 1,3,5-TRIFLUOROBENZENE
1 October 1978
SPECTRA RADICAL
CATION
Terry 4. MILLER and V-E_ BONDYEEY Bell Laboraron-es,
Ah-ray
HdI. New Jerse_v Oi974.
USA
Received 12 June 1978 Laser induced fluoresccncc spectra of the L3,Wrifluorobenzene radical cation have been observed in the gas phase. Vrbronic anatysis of the excitation spectra is possiile with the conclusion that a Jahn-Teller distortion is likely present.
Studies of optical spectra of gaseous molecular
ions are very limited with only a few diatomic and triatomic ions having been studied in any detail. Studies of organic ions have been essentially nonexistent until the pioneering work of Maier and co-workers [l-4] on electron beam excited fluorescence_ Recently a matrix isolated hexafluorobenzene cationspectrum has been reported [S], and a brief account of laser induced fluorescence (LIF) from that ion in the gas phase has been made [6], but no further details have been publisbed. in principle Maier’s emission studies and photoeIectron spectra can give
information
about vibrational strncture,
but in
practice the resolution is often insufficient to give much useful vibrational data. We have recently applied [7- 10 ] !..IF to the study of several simple ions_ and we now report the application of the method to the study of organic radical cations. Metastable inert gas atoms are reacted with the neutral compound and the ions formed are excited by a N2 pumped dye laser directed transversely to the flame. The nondispersed fluorescence is detected by a P-MT, amplified and digitized in a Biomation 610B waveform recorder_ Broadband interference falters or colored cut-off friters are typically used to eliminate the scattered laser light and/or background emission_ When the 13,5trifluorobenzene is reacted with metastabie helium atoms, a bright blue-green flame is observed. Resolving the emission results in a spectrum quite similar to that reported by Allan and 454
[l] and leaves little doubt that the same emitting species is involved. While the presence of au intense flame is useful in direct emission studies, in the LIF experiment it is undesirable, since it contributes to the continuous background and results in reduced signal to noise. Ar metastables lie only 11.5 eV above the ground state. While this is still sufficient to ionize the trifluorobenzene, it falls short of the ener,q needed to produce the ions in excited electronic states_ Consistent with this reasoning, except for a weak emission due to background N2 impurity, no Maier
emission is seen when the Ar metastables are reached
with the trifluorobenzene.
When one excites the
resulting “dark flame” with the laser, however, strong fluorescence is observed_ The excitation spectrum of this fluorescence is shown in fig. l_ The spectrum shows a well resolved vibrational structure and the observed bands are listed in table 1 _ Based upon cur previous work with di- and triatomic ions, we would expect the C6H3F$
to be vibrationally relaxed. The reiatively simpie spectrum we observe is consistent with this expectation_ A very similar but noisier spectrum was obtained with He metastables. In either case, the fmt strong band in the excitation spectrum coincides within the experimental error with the highest energy band in the chemiluminescence. This confirms its assignment to the O-O origin, colnsistent with expectations based on the photoelectron spectrum. Several weak bands appear quite close to the O-O band. The intervals separating them from the origin are clearly too small
CHEMICAL
volume 58. number 3
-
EXCITATION
v‘
SPE~Y
[103cm-q
l-is. 1. Laser excitation spectmm of 1,3,5-trifluorobenzcne
radical cation. The hi& frequency end illustrates the overlap between two dye ranges. to he vibrationai frequencies, and the features must be assigned as “hot” sequence bands originating in excited ground state levels. The stronger bands in the spectrum must then represent the upper state vibrational structure. The C6F3& ground state is formed by the removal of an electron from the degenerate e’ orbital and should therefore have 2E” symmetry. The electromc transition can be assigned as a z-n* transition to a nondegenerate B 2Ai electronic state_ If the molecular symmetry is unchanged in the electronic transition, only totally symmetric vibrations should appear in the electronic spectrum. In the D3h symmetry, there will be o&j fOUr Table 1 C6H& excitation spectrum (cm-l) I
AU
21652 21732 21862 21944 22010 22110 22348 22434
-210 - 130 0 82 148 248 486 572
22584
722 966 1:90 1728 1794
22528 23352 23590 23656
Assionment hot bmd hot band O-C&l
hot band hot band “6 “5 v4 vs vj
+ v6
or 2~5
v2 v2*V6
PHYSICS LETTERS
1 Octcber 1978
totally symmetric vibrations, which can be roughly described as CH, ZJ~,and CF. ~2. stretching vibrations. a trigonal distortion, ~3, and a ring breathing mode, v4 _ The CH stretch will be above 3000 cm-1 outside the range studied and is irrelevant for our discussion. The CF stretch is located near 1350 cm-l in the parent molecule [I l-131 C6H3F3 and can be expected to shift upward in the positive ion. The two ring vibrations v3 and 2~4occur at 1010 and 578 cm-l, respectively, in the neutral molecule, and there is little reason to expect them to change dramatically in the ion_ Photoelectron spectra for all the fluorobenzenes show f 1,4,14,15] two distinct vibrational frequencies, one around 1500 cm-l and another about 500 cm-l. These have commonly been assigned to the CF stretch and the symmetric ring breathing mode, respectively. The line we observe 1490 cm-l above the origin is reasonably assigned to the C-F stretch, v2_ One would thus expect only one totally symmetric C6H3F$ vibration below 900 cm-l_ fn our spectrum three strong bands occur below 600 cm-l. These bands are certainly not “hot” bands, and their strength and low frequencies make it quite unlikely that they are nominally allowed, doubly-excited, non totally symmetric vibrations. We conclude that our LIF spectra are very likely incompatible with the highest symmetry configuration in both states involved in the transition. A reasonable alternative is a Jahn-Teller distortion in the degenerate ground electronic state. For a large vibronic interaction the degenerate vibrations of the species responsible for the Jahn-Teller instability may appear in the spectrum, and one could interpret the low frequency modes as deformation vibrations. With this assumption, the ai ring deformation v4 (578 cm-l in the parent) is then assigned to either the 484 cm-1 frequency or more likely, to the 572 cm-l interval. The band at 22828 cm-1 with the 964 cm-l interval is attributed either to the ring deformation v3 or as an overtone of the 484 cm-l frequency. The 243 cm-l interval appearing frequently in the spectrum is assigned to a low frequency degenerate vibration. These assigmments are summarized in table 1. We have also observed LIF for gaseous C6Fz and with the similar assumption of a distorted ground state, one can explain its spectrum in a quite analogous way 1161.
CI-fEhfICAL
Volume 58, number 3
PHYSICS
CIeariy our data do not permit in ail cases unambiguous vibrationaI assignment. However, the present work demonstrates that the LIF technique is applicable even for larger organic ions and can provide useful data about the electronic states_ lMore extensive work drawing on comparisons between a larger set of similar compounds and studies using isotopicAly substituted samples is needed to give more complete
information
of these interesting
about
the vibrational
structure
species_
References [I] Sf_ _AIlan and J-P. hfaier, Chem. Phyr Letters 34 (1975) 442; 41 (1976) 231. [2] Jf. ABan and J-P_ Xaier, Chem. Phys. Letters 43 (1976) 94. [3] >f_ .~lan, S_ Kloster-Jensen and J-P_ Xfaier. J_ Chem- Sot. Faiadey l-mns_ II 73 (1977) 1417_ [4] &I. AIIan, 0. MarthaIer and J-P. Maler. Chem. Phys. 26 (1977) 131.
456
LEl-fERS
1 October 1978
[S] V.E_ Bondybey, J. English and T-A.-Miller, J. Am. Chem. Sac_, to be pubIished. [6] PK. Engelking and A.L. Smith, Mol&uIar Spectroscopy Symposium. Columbus, Ohio (1976); PC. EngeIklng, Ph.D. thesis, YaIe University (1976). [7] T-k hfiller and V-E. Bondybey, Chem- Phys Letters 50 (1977) 275. [S] J.M. Cook, T-2 Miller and V. E. Bondybey, J. Chem. Phys.. to be published. [9J V.E. Bondybey and T--A. LfIlIer, J. Chem. Phys 67 (1977) 1790. [lo] V.E. Bondybey and T.A. hfilfer. J. Chem. Phys. to be pubLished_ [ 1l] J.H.S. Green, D. J. H&r&on and iy. Rynaston, Spectrochhn. Acta 27A (1971) 793. [ 121 J.R. Nielsen, C. LIang and DC. Smith, Discussions faraday Sot. 9 (1950) 177. [ 131 D.A. Long and D. Steele, Spectrochfm. Acta 19 (1963) 1947. [ 141 D.G. Streets and G.P. Caesar, hfoL Phys 26 (1973) 1037. [ 151 CR_ BrundIe. MB. Robin and N-A. RuebIer, 1. Am. Chem_ Sot. 94 (1972) 1466_ [ 161 V.E. Bondybey and T.A. MIIIer, J. Chem. Phys., to be pub!ished.
58, number 3
PHYSICS JXTTERS
cxEM1cAL
LASER FLUORESCENCE
EXCITATION
OF THE 1,3,5-TRIFLUOROBENZENE
1 October 1978
SPECTRA RADICAL
CATION
Terry 4. MILLER and V-E_ BONDYEEY Bell Laboraron-es,
Ah-ray
HdI. New Jerse_v Oi974.
USA
Received 12 June 1978 Laser induced fluoresccncc spectra of the L3,Wrifluorobenzene radical cation have been observed in the gas phase. Vrbronic anatysis of the excitation spectra is possiile with the conclusion that a Jahn-Teller distortion is likely present.
Studies of optical spectra of gaseous molecular
ions are very limited with only a few diatomic and triatomic ions having been studied in any detail. Studies of organic ions have been essentially nonexistent until the pioneering work of Maier and co-workers [l-4] on electron beam excited fluorescence_ Recently a matrix isolated hexafluorobenzene cationspectrum has been reported [S], and a brief account of laser induced fluorescence (LIF) from that ion in the gas phase has been made [6], but no further details have been publisbed. in principle Maier’s emission studies and photoeIectron spectra can give
information
about vibrational strncture,
but in
practice the resolution is often insufficient to give much useful vibrational data. We have recently applied [7- 10 ] !..IF to the study of several simple ions_ and we now report the application of the method to the study of organic radical cations. Metastable inert gas atoms are reacted with the neutral compound and the ions formed are excited by a N2 pumped dye laser directed transversely to the flame. The nondispersed fluorescence is detected by a P-MT, amplified and digitized in a Biomation 610B waveform recorder_ Broadband interference falters or colored cut-off friters are typically used to eliminate the scattered laser light and/or background emission_ When the 13,5trifluorobenzene is reacted with metastabie helium atoms, a bright blue-green flame is observed. Resolving the emission results in a spectrum quite similar to that reported by Allan and 454
[l] and leaves little doubt that the same emitting species is involved. While the presence of au intense flame is useful in direct emission studies, in the LIF experiment it is undesirable, since it contributes to the continuous background and results in reduced signal to noise. Ar metastables lie only 11.5 eV above the ground state. While this is still sufficient to ionize the trifluorobenzene, it falls short of the ener,q needed to produce the ions in excited electronic states_ Consistent with this reasoning, except for a weak emission due to background N2 impurity, no Maier
emission is seen when the Ar metastables are reached
with the trifluorobenzene.
When one excites the
resulting “dark flame” with the laser, however, strong fluorescence is observed_ The excitation spectrum of this fluorescence is shown in fig. l_ The spectrum shows a well resolved vibrational structure and the observed bands are listed in table 1 _ Based upon cur previous work with di- and triatomic ions, we would expect the C6H3F$
to be vibrationally relaxed. The reiatively simpie spectrum we observe is consistent with this expectation_ A very similar but noisier spectrum was obtained with He metastables. In either case, the fmt strong band in the excitation spectrum coincides within the experimental error with the highest energy band in the chemiluminescence. This confirms its assignment to the O-O origin, colnsistent with expectations based on the photoelectron spectrum. Several weak bands appear quite close to the O-O band. The intervals separating them from the origin are clearly too small
CHEMICAL
volume 58. number 3
-
EXCITATION
v‘
SPE~Y
[103cm-q
l-is. 1. Laser excitation spectmm of 1,3,5-trifluorobenzcne
radical cation. The hi& frequency end illustrates the overlap between two dye ranges. to he vibrationai frequencies, and the features must be assigned as “hot” sequence bands originating in excited ground state levels. The stronger bands in the spectrum must then represent the upper state vibrational structure. The C6F3& ground state is formed by the removal of an electron from the degenerate e’ orbital and should therefore have 2E” symmetry. The electromc transition can be assigned as a z-n* transition to a nondegenerate B 2Ai electronic state_ If the molecular symmetry is unchanged in the electronic transition, only totally symmetric vibrations should appear in the electronic spectrum. In the D3h symmetry, there will be o&j fOUr Table 1 C6H& excitation spectrum (cm-l) I
AU
21652 21732 21862 21944 22010 22110 22348 22434
-210 - 130 0 82 148 248 486 572
22584
722 966 1:90 1728 1794
22528 23352 23590 23656
Assionment hot bmd hot band O-C&l
hot band hot band “6 “5 v4 vs vj
+ v6
or 2~5
v2 v2*V6
PHYSICS LETTERS
1 Octcber 1978
totally symmetric vibrations, which can be roughly described as CH, ZJ~,and CF. ~2. stretching vibrations. a trigonal distortion, ~3, and a ring breathing mode, v4 _ The CH stretch will be above 3000 cm-1 outside the range studied and is irrelevant for our discussion. The CF stretch is located near 1350 cm-l in the parent molecule [I l-131 C6H3F3 and can be expected to shift upward in the positive ion. The two ring vibrations v3 and 2~4occur at 1010 and 578 cm-l, respectively, in the neutral molecule, and there is little reason to expect them to change dramatically in the ion_ Photoelectron spectra for all the fluorobenzenes show f 1,4,14,15] two distinct vibrational frequencies, one around 1500 cm-l and another about 500 cm-l. These have commonly been assigned to the CF stretch and the symmetric ring breathing mode, respectively. The line we observe 1490 cm-l above the origin is reasonably assigned to the C-F stretch, v2_ One would thus expect only one totally symmetric C6H3F$ vibration below 900 cm-l_ fn our spectrum three strong bands occur below 600 cm-l. These bands are certainly not “hot” bands, and their strength and low frequencies make it quite unlikely that they are nominally allowed, doubly-excited, non totally symmetric vibrations. We conclude that our LIF spectra are very likely incompatible with the highest symmetry configuration in both states involved in the transition. A reasonable alternative is a Jahn-Teller distortion in the degenerate ground electronic state. For a large vibronic interaction the degenerate vibrations of the species responsible for the Jahn-Teller instability may appear in the spectrum, and one could interpret the low frequency modes as deformation vibrations. With this assumption, the ai ring deformation v4 (578 cm-l in the parent) is then assigned to either the 484 cm-1 frequency or more likely, to the 572 cm-l interval. The band at 22828 cm-1 with the 964 cm-l interval is attributed either to the ring deformation v3 or as an overtone of the 484 cm-l frequency. The 243 cm-l interval appearing frequently in the spectrum is assigned to a low frequency degenerate vibration. These assigmments are summarized in table 1. We have also observed LIF for gaseous C6Fz and with the similar assumption of a distorted ground state, one can explain its spectrum in a quite analogous way 1161.
CI-fEhfICAL
Volume 58, number 3
PHYSICS
CIeariy our data do not permit in ail cases unambiguous vibrationaI assignment. However, the present work demonstrates that the LIF technique is applicable even for larger organic ions and can provide useful data about the electronic states_ lMore extensive work drawing on comparisons between a larger set of similar compounds and studies using isotopicAly substituted samples is needed to give more complete
information
of these interesting
about
the vibrational
structure
species_
References [I] Sf_ _AIlan and J-P. hfaier, Chem. Phyr Letters 34 (1975) 442; 41 (1976) 231. [2] Jf. ABan and J-P_ Xaier, Chem. Phys. Letters 43 (1976) 94. [3] >f_ .~lan, S_ Kloster-Jensen and J-P_ Xfaier. J_ Chem- Sot. Faiadey l-mns_ II 73 (1977) 1417_ [4] &I. AIIan, 0. MarthaIer and J-P. Maler. Chem. Phys. 26 (1977) 131.
456
LEl-fERS
1 October 1978
[S] V.E_ Bondybey, J. English and T-A.-Miller, J. Am. Chem. Sac_, to be pubIished. [6] PK. Engelking and A.L. Smith, Mol&uIar Spectroscopy Symposium. Columbus, Ohio (1976); PC. EngeIklng, Ph.D. thesis, YaIe University (1976). [7] T-k hfiller and V-E. Bondybey, Chem- Phys Letters 50 (1977) 275. [S] J.M. Cook, T-2 Miller and V. E. Bondybey, J. Chem. Phys.. to be published. [9J V.E. Bondybey and T--A. LfIlIer, J. Chem. Phys 67 (1977) 1790. [lo] V.E. Bondybey and T.A. hfilfer. J. Chem. Phys. to be pubLished_ [ 1l] J.H.S. Green, D. J. H&r&on and iy. Rynaston, Spectrochhn. Acta 27A (1971) 793. [ 121 J.R. Nielsen, C. LIang and DC. Smith, Discussions faraday Sot. 9 (1950) 177. [ 131 D.A. Long and D. Steele, Spectrochfm. Acta 19 (1963) 1947. [ 141 D.G. Streets and G.P. Caesar, hfoL Phys 26 (1973) 1037. [ 151 CR_ BrundIe. MB. Robin and N-A. RuebIer, 1. Am. Chem_ Sot. 94 (1972) 1466_ [ 161 V.E. Bondybey and T.A. MIIIer, J. Chem. Phys., to be pub!ished.