- Email: [email protected]
Free jet absorption millimeter wave spectrum of benzophenone
5 July 1996
CHEMICAL PHYSICS LETTERS ELSEVIER
Chemical Physics Letters 256 (1996) 509-512
Free jet absorption millimeter wave spectrum of benzophenone Assimo Maris, Sonia Melandri, Walther Caminati *, Paolo G. Favero Dipartimento di Chimica "G. Ciamician' dell'Universith, Via Selmi 2, 1-40126 Bologna, Italy
Received 8 March 1996; in final form 19 April 1996
Abstract
The free jet millimeter wave spectrum of benzophenone has been investigated in the frequency range 60-73 GHz. The molecule exhibits a C 2 symmetry with the two phenyl groups tilted by 31.74 °. From the values of the quartic centrifugal distortion constants the lowest torsional frequency has been estimated to be 17 cm-~.
X-ray investigations showed that benzophenone (BPH, see Fig. 1), in the crystal phase, exists in a non-planar conformation with C 2 symmetry and an angle "r = 30 ° [1]. The C 2 symmetry and an angle -r = 35 ° [1] has been inferred also from a vibrational analysis of crystal, powder, melt and solution forms [2]. A b initio calculations gave a value ~" = 3 2 - 3 3 ° [3,4]. No gas phase data concerning the equilibrium configuration of BPH are available in the literature. Holtzclaw and Pratt reported the gas phase fluorescence excitation spectrum of BPH in a supersonic jet, but they could not estimate the 7 angle because they did not achieve the rotational resolution generally obtained in similar investigations made by this group [6]. In Ref. [5] a vibrational progression, spaced by 60 cm - l , was attributed to the totally symmetric ring torsional mode in the S 1 electronic state. The room temperature rotational spectrum o f BPH
* Coreesponding author. E-mail: [email protected]. unibo.it. Fax: + +3%51-259456.
is very difficult, if not impossible to observe, due to the high molecular weight o f the molecule and to the low energy torsions of the phenyl groups. Recently we succeeded in assigning similar complicated spectra with our free jet absorption millimeter-wave spectrometer [7] and for this reason we decided to investigate the rotational spectrum of BPH. A sample of BPH was purchased from Aldrich and used without further purification. It is solid at room temperature with m.p. 48°C and b.p. 305°C. To obtain a suitable concentration o f the sample in the carrier gas it was necessary to warm it up. After several attempts we found that the best conditions required a working temperature much lower than expected. In the experiment the sample seeded in argon at stagnation pressure of 11 kPa at 70°C was expanded to about 0.05 Pa through a 0.35 m m diameter nozzle. The heating device and the details of the spectrometer are described elsewhere [7]. Trial values of the rotational constants have been calculated from the structural parameters of Ref. [4], with T = 33 °. The differences observed in going from the ab initio 3-21G to the r o geometry o f
0009-2614/96/$12.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PH S0009-261 4(96)00467-8
510
A. Maris et al. / Chemical Physics Letters 256 (1996) 509-512
benzene [8,9] have been used to correct the C - C and C - H bond lengths of the phenyl groups. Due to the C 2 molecular symmetry only ixb type transitions were expected. Several weak transitions were observed, but the assignment was complicated by the fact that the strongest expected transitions were not modulated with the maximum Stark field available (-- 600 V / c m ) . All the experimental frequencies are listed in Table 1. They have been fitted with quartic Watson Hamiltonian [10] giving the spectroscopic constants reported in Table 2, together with some statistical parameters of the fit. While vibrational spacings due the phenyl groups torsions are available for the S l electronic state [5], little can be found in the literature for the S o state. Volovsek et al. report, for the torsions of phenyl groups, fundamentals greater than 100 cm - l [2]. They appear large when compared, for example, to the fundamental frequency of the phenyl groups torsions in stilbene ( - - 8 cm - I , [11]). Ab initio calculations suggest the saddle point geometry to be q'¿ 0 and "r2 = 90 °, with a barrier height of 4.9 =
k J / m o l , and a corresponding torsional fundamental frequency of 23 c m - 1 [4]. We were not able to observe torsional satellites, which would have been useful in describing the potential energy function of the internal rotation of the phenyl groups. Nevertheless, assuming that the centrifugal distortion parameters reported in Table 2 were mainly due to the lowest energy vibration we could obtain some information on that motion. We used Meyer's one-dimensional flexible model [12] to evaluate centrifugal distortion parameters as a function of the assumed potential. We could satisfactorily reproduce the experimental A j , A j , K and A K parameters only by assuming an in phase concerted torsion of the two phenyl groups. Owing to the scarcity of experimental data we assumed a harmonic potential, valid just at the bottom of the potential energy function, V ( ' r ) = ~k'r2, 1 and obtained the values reported in Table 3 for a comparison with the experimental data. The fundamental frequency would result to be --~ 17 cm -1. This work shows the utility of the free jet technique in studying rotational spectra of molecules
1•892• 1.0896 ~
1.3998 1.4000
1.0893
120.07
120.32 12055
1.3985 Y1
119.94
f 119"891 120'04
1.4051I 1.3981
I 1.4055 ~ 1.4757
11858
1.0899 12254~
!
Fig. 1. Sketch of BPH. The geometry is based on STO/3-21G ab initio calculations [4]. The phenyl group bond lengths [4] have been scaled with known empirical corrections (see text) and the dihedral angle ,r, C - C ( = O ) - C angle and C - C ( = O) bond length were refined to reproduce the rotational constants to within 0.1 Mhz.
A. Marls et a l . / Chemical Physics Letters 256 (1996) 5 0 9 - 5 1 2
Table 1 Experimental transition frequencies of BPH (MHz)
Table 3 Centrifugal distortion constants: observed and calculated with the flexible model
Non-degenerate transitions J r " ,.r~,
frequency
J~c~_,,r',
471o.38-469.37 471o.37-469.38
4810.39--479.38 4810.38-479.39 4910,40--489,39
J,~'_~.r',
frequency
- J~",.x'+,
60248.23 60302.63 60930.12 61008.16 61599.35
4910.39-489.4o 4411.34-4310.33
4411.33--4310,34 4511.35-441o.34 4511,34--4410,35
511
61711.33 60932.13 60933.24 61661.86 61663.56
Constant
Observed
Calculated a
A: (Hz) Aj r (Hz) Ar (Hz)
12(2) - 52(6) 462(63)
18 - 31 411
a Calculated with the parameter k= 1.4.104 J rad -2 in the function V('r) = ½k'r2.
Asymmetry degenerate K_ 1 transition doublets (only K_ 1 is given) J'v'-~JK'-'t
frequency
J,~, J~,_,
frequency
4012-3911 41 t2 -4011 4212 -4111 3613-3512 3713-3612 38t3-3712 3913--3812
60662.32 61411.29 62158.56 60292.55 61051.01 61808.51 62565.08 63320.24 64074.25 64826.87 65578.35 60640.48 61403.39 62165.18 62926.73 63687.38 64447.36 65206.66 65965.06 66722.61 67478.88 68234.56
4414-4313 2915-2814 3015-2914 3115-30t4 3215-3114 3315-3214 3415--33t4 3515-3414 3615 --3514 37t5--3614 3815--3714 3915--3814 40t5--3914 4115--4014 4215 --4114 4315--4214 4415 --4314 4515--4414 26t6 --2515 2716--2615 2816--2715 2217--21j6
68988.66 60206.30 60971.39 61736.60 62501.08 63265.36 64029.16 64792.63 65555.76 66317.89 67079.79 67840.92 68601.59 69361.37 70120.76 70878.71 71636.37 72393.07 60527.36 61293.70 62059.96 60078.04
4013 --3912 4113--4012 4213--4112 4313--4212 33t4--32j3 3414--3313 3514--3413 3614 --3513 3714--3613 3814--37j3 3914--3813 4014 --39t3 4114--4013 4214--4113 4314--4213
Table 2 Rotational and centrifugal distortion constants of BPH A (MHz) B (MHz) C (MHz) Aj (Hz) A~x (Hz) A x (Hz) b A c (u ,~2) cr (MHz) Nc
1692.916(35) a 412.620(17) 353.884(18) 12(2) -52(6) 462(63) - 95.2373 0.11 54
a Errors in parentheses are expressed in units of the last digit. b ~j and ~r were set to zero because they are not statistically significant. c Number of transitions in the fit.
which s h o w a c o n g e s t i o n a t e d spectra due to high m o l e c u l a r w e i g h t and low e n e r g y - l a r g e amplitude motions. In the specific case o f B P H a surprising low stagnation temperature, which m e a n s a v e r y low partial pressure o f B P H in the mixture with Argon, was n e e d e d to o p t i m i z e the spectrum. The need for a high dilution o f the sample in the carrier gas can be attributed to the fact that besides the rotational levels, also the closely spaced torsional levels u n d e r g o a strong relaxation.
Acknowledgements W e thank Prof. V. V o l o v s e k for pointing out the B P H p r o b l e m . T h e M i n i s t e r o d e l l ' U n i v e r s i t ~ e della R i c e r c a S c i e n t i f i c a e T e c n o l o g i c a , and the C o n s i g l i o N a z i o n a l e delle R i c e r c h e are a c k n o w l e d g e d for financial support.
References [1] E.B. Fleischer, N. Sung and S. Hawkinson, J. Phys. Chem. 72 (1968) 4311. [2] V. Volovsek, G. Baranovic and L. Colombo, Spectrochim. Acta 49A (1993) 2071. [3] T. Schaefer and G.H. Penner, J. Phys. Chem. 85 (1986) 6249. [4] K.M. Gough and T.A. Wildman, J. Am. Chem. Soc., 112 (1990) 9141. [5] K.W. Holtzclaw and D.W. Pratt, J. Phys. Chem. 84 (1986) 4713. [6] See for example the case of stilbene: B.B. Champagne, J.F. Pfanstiel, D.F. Plusquellic, D.W. Pratt, W.M. van Herpen and W.L. Meerts, J. Phys. Chem. 94 (1990) 6.
512
A. Maris et al. / Chemical Physics Letters 256 (1996) 509-512
[7] S. Melandri, W. Caminati, L.B. Favero, A, Millemaggi, P.G. Favero, J. Mol. Struet. 352/353 (1995) 253. [8] R. Benassi, private communication. [9] W. Caminati, S. Di Bemardo, L. Schiifer, S.Q. Kulp-Newton and K. Siam, J. Mol. Struct. 240 (1990) 263. [10] J.K.G. Watson, in: Vibrational Spectra and Structure, J.R.
Durig, ed., Vol. 6, Elsevier, New York/Amsterdam, 1977, pp. 1-89. [11] L.H. Spangler, R. van Zee and T.S. Zwier, J. Phys. Chem. 91 (1979) 2782. [12] R. Meyer and W. Caminati, J. Mol. Spectry. 150 (1991) 229.
CHEMICAL PHYSICS LETTERS ELSEVIER
Chemical Physics Letters 256 (1996) 509-512
Free jet absorption millimeter wave spectrum of benzophenone Assimo Maris, Sonia Melandri, Walther Caminati *, Paolo G. Favero Dipartimento di Chimica "G. Ciamician' dell'Universith, Via Selmi 2, 1-40126 Bologna, Italy
Received 8 March 1996; in final form 19 April 1996
Abstract
The free jet millimeter wave spectrum of benzophenone has been investigated in the frequency range 60-73 GHz. The molecule exhibits a C 2 symmetry with the two phenyl groups tilted by 31.74 °. From the values of the quartic centrifugal distortion constants the lowest torsional frequency has been estimated to be 17 cm-~.
X-ray investigations showed that benzophenone (BPH, see Fig. 1), in the crystal phase, exists in a non-planar conformation with C 2 symmetry and an angle "r = 30 ° [1]. The C 2 symmetry and an angle -r = 35 ° [1] has been inferred also from a vibrational analysis of crystal, powder, melt and solution forms [2]. A b initio calculations gave a value ~" = 3 2 - 3 3 ° [3,4]. No gas phase data concerning the equilibrium configuration of BPH are available in the literature. Holtzclaw and Pratt reported the gas phase fluorescence excitation spectrum of BPH in a supersonic jet, but they could not estimate the 7 angle because they did not achieve the rotational resolution generally obtained in similar investigations made by this group [6]. In Ref. [5] a vibrational progression, spaced by 60 cm - l , was attributed to the totally symmetric ring torsional mode in the S 1 electronic state. The room temperature rotational spectrum o f BPH
* Coreesponding author. E-mail: [email protected]. unibo.it. Fax: + +3%51-259456.
is very difficult, if not impossible to observe, due to the high molecular weight o f the molecule and to the low energy torsions of the phenyl groups. Recently we succeeded in assigning similar complicated spectra with our free jet absorption millimeter-wave spectrometer [7] and for this reason we decided to investigate the rotational spectrum of BPH. A sample of BPH was purchased from Aldrich and used without further purification. It is solid at room temperature with m.p. 48°C and b.p. 305°C. To obtain a suitable concentration o f the sample in the carrier gas it was necessary to warm it up. After several attempts we found that the best conditions required a working temperature much lower than expected. In the experiment the sample seeded in argon at stagnation pressure of 11 kPa at 70°C was expanded to about 0.05 Pa through a 0.35 m m diameter nozzle. The heating device and the details of the spectrometer are described elsewhere [7]. Trial values of the rotational constants have been calculated from the structural parameters of Ref. [4], with T = 33 °. The differences observed in going from the ab initio 3-21G to the r o geometry o f
0009-2614/96/$12.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PH S0009-261 4(96)00467-8
510
A. Maris et al. / Chemical Physics Letters 256 (1996) 509-512
benzene [8,9] have been used to correct the C - C and C - H bond lengths of the phenyl groups. Due to the C 2 molecular symmetry only ixb type transitions were expected. Several weak transitions were observed, but the assignment was complicated by the fact that the strongest expected transitions were not modulated with the maximum Stark field available (-- 600 V / c m ) . All the experimental frequencies are listed in Table 1. They have been fitted with quartic Watson Hamiltonian [10] giving the spectroscopic constants reported in Table 2, together with some statistical parameters of the fit. While vibrational spacings due the phenyl groups torsions are available for the S l electronic state [5], little can be found in the literature for the S o state. Volovsek et al. report, for the torsions of phenyl groups, fundamentals greater than 100 cm - l [2]. They appear large when compared, for example, to the fundamental frequency of the phenyl groups torsions in stilbene ( - - 8 cm - I , [11]). Ab initio calculations suggest the saddle point geometry to be q'¿ 0 and "r2 = 90 °, with a barrier height of 4.9 =
k J / m o l , and a corresponding torsional fundamental frequency of 23 c m - 1 [4]. We were not able to observe torsional satellites, which would have been useful in describing the potential energy function of the internal rotation of the phenyl groups. Nevertheless, assuming that the centrifugal distortion parameters reported in Table 2 were mainly due to the lowest energy vibration we could obtain some information on that motion. We used Meyer's one-dimensional flexible model [12] to evaluate centrifugal distortion parameters as a function of the assumed potential. We could satisfactorily reproduce the experimental A j , A j , K and A K parameters only by assuming an in phase concerted torsion of the two phenyl groups. Owing to the scarcity of experimental data we assumed a harmonic potential, valid just at the bottom of the potential energy function, V ( ' r ) = ~k'r2, 1 and obtained the values reported in Table 3 for a comparison with the experimental data. The fundamental frequency would result to be --~ 17 cm -1. This work shows the utility of the free jet technique in studying rotational spectra of molecules
1•892• 1.0896 ~
1.3998 1.4000
1.0893
120.07
120.32 12055
1.3985 Y1
119.94
f 119"891 120'04
1.4051I 1.3981
I 1.4055 ~ 1.4757
11858
1.0899 12254~
!
Fig. 1. Sketch of BPH. The geometry is based on STO/3-21G ab initio calculations [4]. The phenyl group bond lengths [4] have been scaled with known empirical corrections (see text) and the dihedral angle ,r, C - C ( = O ) - C angle and C - C ( = O) bond length were refined to reproduce the rotational constants to within 0.1 Mhz.
A. Marls et a l . / Chemical Physics Letters 256 (1996) 5 0 9 - 5 1 2
Table 1 Experimental transition frequencies of BPH (MHz)
Table 3 Centrifugal distortion constants: observed and calculated with the flexible model
Non-degenerate transitions J r " ,.r~,
frequency
J~c~_,,r',
471o.38-469.37 471o.37-469.38
4810.39--479.38 4810.38-479.39 4910,40--489,39
J,~'_~.r',
frequency
- J~",.x'+,
60248.23 60302.63 60930.12 61008.16 61599.35
4910.39-489.4o 4411.34-4310.33
4411.33--4310,34 4511.35-441o.34 4511,34--4410,35
511
61711.33 60932.13 60933.24 61661.86 61663.56
Constant
Observed
Calculated a
A: (Hz) Aj r (Hz) Ar (Hz)
12(2) - 52(6) 462(63)
18 - 31 411
a Calculated with the parameter k= 1.4.104 J rad -2 in the function V('r) = ½k'r2.
Asymmetry degenerate K_ 1 transition doublets (only K_ 1 is given) J'v'-~JK'-'t
frequency
J,~, J~,_,
frequency
4012-3911 41 t2 -4011 4212 -4111 3613-3512 3713-3612 38t3-3712 3913--3812
60662.32 61411.29 62158.56 60292.55 61051.01 61808.51 62565.08 63320.24 64074.25 64826.87 65578.35 60640.48 61403.39 62165.18 62926.73 63687.38 64447.36 65206.66 65965.06 66722.61 67478.88 68234.56
4414-4313 2915-2814 3015-2914 3115-30t4 3215-3114 3315-3214 3415--33t4 3515-3414 3615 --3514 37t5--3614 3815--3714 3915--3814 40t5--3914 4115--4014 4215 --4114 4315--4214 4415 --4314 4515--4414 26t6 --2515 2716--2615 2816--2715 2217--21j6
68988.66 60206.30 60971.39 61736.60 62501.08 63265.36 64029.16 64792.63 65555.76 66317.89 67079.79 67840.92 68601.59 69361.37 70120.76 70878.71 71636.37 72393.07 60527.36 61293.70 62059.96 60078.04
4013 --3912 4113--4012 4213--4112 4313--4212 33t4--32j3 3414--3313 3514--3413 3614 --3513 3714--3613 3814--37j3 3914--3813 4014 --39t3 4114--4013 4214--4113 4314--4213
Table 2 Rotational and centrifugal distortion constants of BPH A (MHz) B (MHz) C (MHz) Aj (Hz) A~x (Hz) A x (Hz) b A c (u ,~2) cr (MHz) Nc
1692.916(35) a 412.620(17) 353.884(18) 12(2) -52(6) 462(63) - 95.2373 0.11 54
a Errors in parentheses are expressed in units of the last digit. b ~j and ~r were set to zero because they are not statistically significant. c Number of transitions in the fit.
which s h o w a c o n g e s t i o n a t e d spectra due to high m o l e c u l a r w e i g h t and low e n e r g y - l a r g e amplitude motions. In the specific case o f B P H a surprising low stagnation temperature, which m e a n s a v e r y low partial pressure o f B P H in the mixture with Argon, was n e e d e d to o p t i m i z e the spectrum. The need for a high dilution o f the sample in the carrier gas can be attributed to the fact that besides the rotational levels, also the closely spaced torsional levels u n d e r g o a strong relaxation.
Acknowledgements W e thank Prof. V. V o l o v s e k for pointing out the B P H p r o b l e m . T h e M i n i s t e r o d e l l ' U n i v e r s i t ~ e della R i c e r c a S c i e n t i f i c a e T e c n o l o g i c a , and the C o n s i g l i o N a z i o n a l e delle R i c e r c h e are a c k n o w l e d g e d for financial support.
References [1] E.B. Fleischer, N. Sung and S. Hawkinson, J. Phys. Chem. 72 (1968) 4311. [2] V. Volovsek, G. Baranovic and L. Colombo, Spectrochim. Acta 49A (1993) 2071. [3] T. Schaefer and G.H. Penner, J. Phys. Chem. 85 (1986) 6249. [4] K.M. Gough and T.A. Wildman, J. Am. Chem. Soc., 112 (1990) 9141. [5] K.W. Holtzclaw and D.W. Pratt, J. Phys. Chem. 84 (1986) 4713. [6] See for example the case of stilbene: B.B. Champagne, J.F. Pfanstiel, D.F. Plusquellic, D.W. Pratt, W.M. van Herpen and W.L. Meerts, J. Phys. Chem. 94 (1990) 6.
512
A. Maris et al. / Chemical Physics Letters 256 (1996) 509-512
[7] S. Melandri, W. Caminati, L.B. Favero, A, Millemaggi, P.G. Favero, J. Mol. Struet. 352/353 (1995) 253. [8] R. Benassi, private communication. [9] W. Caminati, S. Di Bemardo, L. Schiifer, S.Q. Kulp-Newton and K. Siam, J. Mol. Struct. 240 (1990) 263. [10] J.K.G. Watson, in: Vibrational Spectra and Structure, J.R.
Durig, ed., Vol. 6, Elsevier, New York/Amsterdam, 1977, pp. 1-89. [11] L.H. Spangler, R. van Zee and T.S. Zwier, J. Phys. Chem. 91 (1979) 2782. [12] R. Meyer and W. Caminati, J. Mol. Spectry. 150 (1991) 229.