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Neutron inelastic scattering spectra and lattice vibrations of 9,10-anthraquinone crystal
Volume 33, number 1
15 May 1975,
CHEMICAL PHYSICS LETTERS
NEUTRON INELASTIC SCATTERING SPECTRA AND LATI-KE YIBRATLONS OF 9,1U-AWKRAQUINONE CRYSTAL Yoshio MIYAZM
* and Mitsuo IT0
Deportment of Chemistry, Fncuit3l of Science, Tohoh-u Received 2 January Revised manuscript
1975 received 3 February
Univetity,
Sendai,
Japan. 980
1975
Neutron inelastic incoherent scattering spectra of V,lM.ntiaqtione crystal were measuredat 20 and --144’C, and tie square amplitude spectra obtained were compaxd with the calculated frequency distribution OF the lattice vibrations. _Mean tensors of the molecule in the crystil were &so calculated at various temperatures. The zkuhted tensors agee well with the ones obtained from an X-ray diffraction study.
1. Introduction Neutron inelastic scattering spectra provide information about the frequency distribution of lattice vibrations in crystals. In an organic molecular crystal, the lattice vibrations are classified to a good approximation into intramolecular vibrations and intermolecular vibrations. The latter vibrations are appro,ximately described as rotational and translational motions of rigid molecules and they are specifica!ly called lattice vibrations of the molecular crystal. The frequency distribution of these lattice vibrations depends on the intermolecular potential, which in turn varies with temperature. Therefore, the measurement of the neutron scattering spectrum gives a critical test of the assumed intermolecular potential and its temperature dependence. The detailed crystal structure of 9,lCTanthraquinone is known at various temperatures from 20.5V to -17O’C [I]. M orecver, the mean square amplitude tensors of the molecule in the crystal are available also at various temperatures from an X-ray diffraction study 121. Since the mean square amplitude tensors are determined from the eigenvectors of the lattice vibrations, they dso provide good material for testing the assumed intermblecular potentials. * Present address: Entjneering Department, Kawape,
Saitama, Japan.
ToyE University,
In previous papers [3,4], we reported the opticallyactive lattice vibrations of 9,1C-anthraquinone crystal and it was found that the intermolecular potentials proposed by Dash,evsky explain very well the observed optically-active lattice vibrations over a wide range of temperature. In this paper, the frequency distribution of the lattice vibrations and the mean square amplitude tensors of the molecule in the crystal are calculated with the same potentials and the results obtained are compared with the neutron inelastic scattering spectrum and the mean square amplitudes obtained from the X-ray diffraction study.
2. Experimental
results
Experimenta! details of Ahe measurement of the neutron inelastic scattering spectrum have been described by one of us [5]. A 300 MeV Ljnac of the hboratory of Nuclear Science, Tohoku University, was employed as the neutron source. The scattered neutron spectra were measured by the TOF inverted futer method using beryllium oxide as a filter. Reagent-grade 9,10-anthraquinone WIS used without further purification. The powdered sample was packed in an Al cell of dimensions 20 X 20 X 0.1 cm, which was set in a cryostat to cool the sample. ‘Fig. 1 shows the neutron inelastic scattering spectra of 9,lCkanthraquinone crystal at 2O’C and -16CCnC. .,
‘.
‘121
.’
:
Volume 33, number 1
CHEMICAL PHYSICS LETPERS
_.
1.5 May 1975
100 cm-l _ Therefore, the neutron inelastic scattering spectrum in the region below x150 cm-I may be safely regarded as representing the lattice vibrations. This low frequency region is shown in detail in fig. 2. The peaks shown by the asterisks around 75 cm-l are false; due to an inadequacy in the filter used in the experiment. Thus, the only ak observed in this spectral
region is one at 45 cm- p” at 20°C. This peak
moves to 48 cm-l at -144°C. The two spectra at 20 and -1W’C show very similar features, taken as a whole.
Fig 1. Neutmn inelastic scattering spectra of 9,1C-anthraquinpne crystal at 30 and -144OC, i: observed Ramon active intramolecular viirations, !: observed infrared active intramolecular viirations.
3. Frequency
distribution of lattice vibrations
The frequency distribution of the lattice vibrations of 9,1.0-antluaquinone crystal was calculated in a rigid body approximation by the methods given by rvtiyazawa et al. [B]. In the, calculations, the X-ray determined crystal structures at 20 and -170°C [l] were used. The frequency distribution was obtained from the calculations for 13824 points of the wave vector space of the first Brillouin zone. We used the same intermolecular potentials as reported in the previous papers [3,4], that is, 6-exp type atom-atom potentials \Gth parameters proposed by Dashe-sky et al. [9]. The infrared and Raman active Jattice vibrations of k = 0 were well reproduced with these poten-
tials. Fig. 3a shows the calculated frequency
distribu-
tion
of the lattice vibrations of 9,ltSanthraquinone crystal at 20°C. It consists of two main peaks at x35 and ~60 cm -‘. From the calculated eigenvectors,
Fig 2. Neutron kehstic scattering spectra in the low energy transfa region of 9,l!Santhxquinone crystal at 20 and -144oc ‘-
.. The optically-active
intramolecular vibrations measured @th the infrared [6] and Raman spectra [7] are indicate by lines in the figure. As seen from the figure, the observed peaks of the neutron scattering spectra above 150 cm-! correspond to the opticallyactive intramolecular vibrations. According to our y :. @e&us studies. [3,4], tie apticaJy-active lattice. --:.vibtitions occur in the spectral region below
.: ..,’ . . -- -..122 ::: I:.. .:,-. ,‘.- .: ‘.‘.; .. :; ._. .. : .:. _.,-.
:
:
_: . .
: -, ._’
the frequency distribution may be resolved into the contributions from three rotational modes about the principal axes of the molecule x, y and z and three translational modes along them, as shown in fig. 4. It is found from a comparison between figs. 3a and 4 that the peak of the frequency distribution at = 35 cm-l comes mainly from the translational mode along the long axis of the molecule x. The -peak at ~60 cm-l comes from several modes including the rotational modes about the x and y‘ axes and the trans.. lational mode along the z axis. In the calculations for the crystal structures at lower temperatures, the peaks of the calculated frequency distribution move towards the h&her freguency side . For .the crystal structure at ~1’7OpC, the
,-.
:.
.,..
Volume 33, number
CHEMICAL
1
(b)
(a) Fig 3. Calculated
frequency
distributions
1.5 May 1975
PHYSICS LETTERS
of the lattice vibrations
of 9,lCkmthraquinone
crystal at 20°C (a) and at -170°C
(b).
gkk
I>\
,(>y?[k
ih 0
,
:-“r
160 crii’
; II c GD
--3x c ;
Fig. 4. Frequency distribu’5ons 2f individual kttice modes at 2WC Rx, R,, and R, are rotationd lattice modes about the long axis (x), short axis (;I) in the plane of the molecule and the axis perpendiclllar to the @me(z), respectively. TX, T’ and T, me &mshtiond lattice modes along the x, y and E ax”, respectively.
123
CHEhilCAL
PHYSICS
Table 1
.Temp.
(
( 3.05
(
-17ooc
(
0.18 0.46 I( 3.36
-0.25
0.14
-0.15
(
-112oc
-0.35 3.44
2.90
2.02 2.32
tecsors
x-my
4.25
-72’C
amplitude
CA. 5.42
-12.5”C
15 May 1975
Table 2
:.Meansquare translational qu&loI+~)
20.5”C
LETTERS
-0.11 1.56
1.33 -0.05 0.92
4.30
. 3.47
2.89
0.24 1.92
2.62
0.19 1.46
a) Suff~ves 1,2 and 3 correspond x, y -and z, rSpK&dy.
Temp.
-0.81 2.93
0.46 0.07 2.64
-0.24
0.08
-0.21
-1.23 240
H 1.94
0.07 0.11 0.08
[21
239
)(
0.08
Mean square rotational quinone a)
270
0.34 2-72 )( 0.12
for S,lh.rithra-
to molecular
1
inerti
-1.78 7.57
21.49
-1.76
14.96 )
-72°C
0.05 0.28
1.08 1
,11.32 -112~C
0.17 1.00 axes,
calculated peaks are located at 40 and 70 cmml, whkh corres+ond to the calculated. peaks at 35 and 60 cm-’ for the crystal structure at 2D”C. However, no great difference is seen between the frequency distributions at 20 and -170°C except for the whole shift of the distribution towards the higher frequency side at the lower temperature (see fig. 3b). Although the neutron inelastic scattering spectrum -catmot be compared directly: w-i’&the calculated frequency distribution, comparison will be-made between the observed peaks of the neutron scattering spectrurm and the calculated peeaks of the frequency distriiution. Frdm a comparison between fig. 2’and 3, the ” observed peak at 45 cm-’ at %‘C (the pea at 48 cm-l at --144OC) corresponds to the, calculated peak at $Fcm-’ (the &cul;lted peak at 40 cm-’ at -144°C). $nce.the Iatter is mainly due to the tram lationd mode along the x ax& the observed oeak at .45.cm-l- isassigned to this.n~o~. According to the ; ‘. j24. ,. ‘:I-,, :
(
x-ray
i5.70
-12.5”C
0.09 0.57 0.56
-: : ..
.’
:
-0.20 1.36
4.40 -0.2@ 10.14
1.65
20.45
-1.18
-1.31
-1.10 6.35
I(
-1.24 4.42
0.96 -0.68 4.46
I(
-0.86
2.08
-0.51 3.31
-i.2.5 256
0.56 0.56 4.73
16.29 -0.53
0.66
0.95
0.66 3.94
I(
I( to molecular
-0.03
-0.03 1.58 >
inertia axes
calculation, another peak is expected around 60-70 . the neutron inelastic scattering spectrum. cm -1 m Unfortunately, however, the corresponding region is obscured by the false peak due to the fdter’s inadequacy, and so the ex.pected peak could not be confirmed.
4; Mean square amplitudes The mean square translational amplitude tensor amplitude tensor (0) of the molecule in the crystal were calculated by the methods described by-Hawley [lo] from the eigenvectors and eigenvalues of the dynamical matrices for the wave vectors of .the first @Allouin zone. The calculated T and o-at various temperatures are given in tables 1 and 2, together with those obtain.ed from X-ray diffraction studies 123. It is Seen that the (T) and the mean square rotational
.
>
8.99 -0.79 -0.85
0.33 -0.28 2.04
C!.72 -0.69 7.16 > 17.90
0.68
a) Suffkes 1,2 and 3 correspond x, y and z, respectively.
;. :.- ‘‘ ...
29.26
6.29
3.36
121
1.93’ -1.32 )( 7.49
6.95 -0.58 -17wz
tensor for 9,10-anthra-
Calc
2OS”C
-0.25 416 1.35
I(
0.07 2.40
amplitude
Volume 33, number 1
CHJZh!ICAL PHYSICS LETTERS
components of the calculated tensors agree fairly well with the observed ones, except far w22_ Moreover, it is noted that the temperature changes of the tensors are reproduced very well in the calculations. As seen from table 2, the observed values of a32 do not show a monotonic change with temperature.-They are also very small compared with the corresponding values for the ant_hracene crystal (6.9 deg* at room temperature [lo] ) which has a similar crystal,structure to ; that of anthraquinone. We feel strongly that the observed values of wz2 should be re-examined. It is seen from tables 1 and 2 that the off-diagonal components of the tensors are generally much smaller than those of the diagonal components. This su=ests that the molecular motions in the 9,l O-anthraquinone crystal are C.%cribed approximately as a superposition of the rotational lattice modes around the moIecular inertia axes and the translational modes along them.
calculations. We also thank Dr. N. Niimura and Mr. A. Ozora for their assistance in the neutron scattering experiments.
References
111K_ Lonsdale, H.J. MiUedge and K EI-Sayed, Acta Cryst. 20 (1966) 1.
121K. Lonsdale, HJ. Milledge and K. El-Sayed, Acta Cryst. 20 (1966) 13.
I31 Y. hliyazaki and M. [41 [51 [61 [71 is1
Acknowledgement !91
We would like to express our thanks to Professor T. Miyazawa and Dr. T. Kitagawa for their help in the
is Ahy 1975
[lOI
Ito. Bull_ Chem Sot. Japan 46 (1973) 103. B. Wyncke, F. Brehat, k Hadni, Y. hliyazaki and !+I. ito, Chem. Phys. Letters 21 (1973) 115. A. Ozora, hf. Ito, N. Niimura and N. Watanabe, Chem. Phys. Letters 18 (1973) 306. C Pecile and B. LuneUi, J. Chem. Phys. 46 (1967) 2109. S.N. Singh and RS. Singh, Spectrochim. Acta 24A (1968) 1591. T. Miyazawa, Y. Ideguchi and K. Fukushima, J. Chem. Phys. 38 (1963) 2709: T. Kitagawa and T. hfiyazawn, Bull. (Tnem. Sot. Japan 42 (1969) 1388. V.G. Dashevsky, V.T. Struchykor and ZA. Akoppayan, Zh. Strukt- Khim. 7 (1966) 594. G.S. PawIey, Phys. Stat. SOL 20 (1967) 347.
15 May 1975,
CHEMICAL PHYSICS LETTERS
NEUTRON INELASTIC SCATTERING SPECTRA AND LATI-KE YIBRATLONS OF 9,1U-AWKRAQUINONE CRYSTAL Yoshio MIYAZM
* and Mitsuo IT0
Deportment of Chemistry, Fncuit3l of Science, Tohoh-u Received 2 January Revised manuscript
1975 received 3 February
Univetity,
Sendai,
Japan. 980
1975
Neutron inelastic incoherent scattering spectra of V,lM.ntiaqtione crystal were measuredat 20 and --144’C, and tie square amplitude spectra obtained were compaxd with the calculated frequency distribution OF the lattice vibrations. _Mean tensors of the molecule in the crystil were &so calculated at various temperatures. The zkuhted tensors agee well with the ones obtained from an X-ray diffraction study.
1. Introduction Neutron inelastic scattering spectra provide information about the frequency distribution of lattice vibrations in crystals. In an organic molecular crystal, the lattice vibrations are classified to a good approximation into intramolecular vibrations and intermolecular vibrations. The latter vibrations are appro,ximately described as rotational and translational motions of rigid molecules and they are specifica!ly called lattice vibrations of the molecular crystal. The frequency distribution of these lattice vibrations depends on the intermolecular potential, which in turn varies with temperature. Therefore, the measurement of the neutron scattering spectrum gives a critical test of the assumed intermolecular potential and its temperature dependence. The detailed crystal structure of 9,lCTanthraquinone is known at various temperatures from 20.5V to -17O’C [I]. M orecver, the mean square amplitude tensors of the molecule in the crystal are available also at various temperatures from an X-ray diffraction study 121. Since the mean square amplitude tensors are determined from the eigenvectors of the lattice vibrations, they dso provide good material for testing the assumed intermblecular potentials. * Present address: Entjneering Department, Kawape,
Saitama, Japan.
ToyE University,
In previous papers [3,4], we reported the opticallyactive lattice vibrations of 9,1C-anthraquinone crystal and it was found that the intermolecular potentials proposed by Dash,evsky explain very well the observed optically-active lattice vibrations over a wide range of temperature. In this paper, the frequency distribution of the lattice vibrations and the mean square amplitude tensors of the molecule in the crystal are calculated with the same potentials and the results obtained are compared with the neutron inelastic scattering spectrum and the mean square amplitudes obtained from the X-ray diffraction study.
2. Experimental
results
Experimenta! details of Ahe measurement of the neutron inelastic scattering spectrum have been described by one of us [5]. A 300 MeV Ljnac of the hboratory of Nuclear Science, Tohoku University, was employed as the neutron source. The scattered neutron spectra were measured by the TOF inverted futer method using beryllium oxide as a filter. Reagent-grade 9,10-anthraquinone WIS used without further purification. The powdered sample was packed in an Al cell of dimensions 20 X 20 X 0.1 cm, which was set in a cryostat to cool the sample. ‘Fig. 1 shows the neutron inelastic scattering spectra of 9,lCkanthraquinone crystal at 2O’C and -16CCnC. .,
‘.
‘121
.’
:
Volume 33, number 1
CHEMICAL PHYSICS LETPERS
_.
1.5 May 1975
100 cm-l _ Therefore, the neutron inelastic scattering spectrum in the region below x150 cm-I may be safely regarded as representing the lattice vibrations. This low frequency region is shown in detail in fig. 2. The peaks shown by the asterisks around 75 cm-l are false; due to an inadequacy in the filter used in the experiment. Thus, the only ak observed in this spectral
region is one at 45 cm- p” at 20°C. This peak
moves to 48 cm-l at -144°C. The two spectra at 20 and -1W’C show very similar features, taken as a whole.
Fig 1. Neutmn inelastic scattering spectra of 9,1C-anthraquinpne crystal at 30 and -144OC, i: observed Ramon active intramolecular viirations, !: observed infrared active intramolecular viirations.
3. Frequency
distribution of lattice vibrations
The frequency distribution of the lattice vibrations of 9,1.0-antluaquinone crystal was calculated in a rigid body approximation by the methods given by rvtiyazawa et al. [B]. In the, calculations, the X-ray determined crystal structures at 20 and -170°C [l] were used. The frequency distribution was obtained from the calculations for 13824 points of the wave vector space of the first Brillouin zone. We used the same intermolecular potentials as reported in the previous papers [3,4], that is, 6-exp type atom-atom potentials \Gth parameters proposed by Dashe-sky et al. [9]. The infrared and Raman active Jattice vibrations of k = 0 were well reproduced with these poten-
tials. Fig. 3a shows the calculated frequency
distribu-
tion
of the lattice vibrations of 9,ltSanthraquinone crystal at 20°C. It consists of two main peaks at x35 and ~60 cm -‘. From the calculated eigenvectors,
Fig 2. Neutron kehstic scattering spectra in the low energy transfa region of 9,l!Santhxquinone crystal at 20 and -144oc ‘-
.. The optically-active
intramolecular vibrations measured @th the infrared [6] and Raman spectra [7] are indicate by lines in the figure. As seen from the figure, the observed peaks of the neutron scattering spectra above 150 cm-! correspond to the opticallyactive intramolecular vibrations. According to our y :. @e&us studies. [3,4], tie apticaJy-active lattice. --:.vibtitions occur in the spectral region below
.: ..,’ . . -- -..122 ::: I:.. .:,-. ,‘.- .: ‘.‘.; .. :; ._. .. : .:. _.,-.
:
:
_: . .
: -, ._’
the frequency distribution may be resolved into the contributions from three rotational modes about the principal axes of the molecule x, y and z and three translational modes along them, as shown in fig. 4. It is found from a comparison between figs. 3a and 4 that the peak of the frequency distribution at = 35 cm-l comes mainly from the translational mode along the long axis of the molecule x. The -peak at ~60 cm-l comes from several modes including the rotational modes about the x and y‘ axes and the trans.. lational mode along the z axis. In the calculations for the crystal structures at lower temperatures, the peaks of the calculated frequency distribution move towards the h&her freguency side . For .the crystal structure at ~1’7OpC, the
,-.
:.
.,..
Volume 33, number
CHEMICAL
1
(b)
(a) Fig 3. Calculated
frequency
distributions
1.5 May 1975
PHYSICS LETTERS
of the lattice vibrations
of 9,lCkmthraquinone
crystal at 20°C (a) and at -170°C
(b).
gkk
I>\
,(>y?[k
ih 0
,
:-“r
160 crii’
; II c GD
--3x c ;
Fig. 4. Frequency distribu’5ons 2f individual kttice modes at 2WC Rx, R,, and R, are rotationd lattice modes about the long axis (x), short axis (;I) in the plane of the molecule and the axis perpendiclllar to the @me(z), respectively. TX, T’ and T, me &mshtiond lattice modes along the x, y and E ax”, respectively.
123
CHEhilCAL
PHYSICS
Table 1
.Temp.
(
( 3.05
(
-17ooc
(
0.18 0.46 I( 3.36
-0.25
0.14
-0.15
(
-112oc
-0.35 3.44
2.90
2.02 2.32
tecsors
x-my
4.25
-72’C
amplitude
CA. 5.42
-12.5”C
15 May 1975
Table 2
:.Meansquare translational qu&loI+~)
20.5”C
LETTERS
-0.11 1.56
1.33 -0.05 0.92
4.30
. 3.47
2.89
0.24 1.92
2.62
0.19 1.46
a) Suff~ves 1,2 and 3 correspond x, y -and z, rSpK&dy.
Temp.
-0.81 2.93
0.46 0.07 2.64
-0.24
0.08
-0.21
-1.23 240
H 1.94
0.07 0.11 0.08
[21
239
)(
0.08
Mean square rotational quinone a)
270
0.34 2-72 )( 0.12
for S,lh.rithra-
to molecular
1
inerti
-1.78 7.57
21.49
-1.76
14.96 )
-72°C
0.05 0.28
1.08 1
,11.32 -112~C
0.17 1.00 axes,
calculated peaks are located at 40 and 70 cmml, whkh corres+ond to the calculated. peaks at 35 and 60 cm-’ for the crystal structure at 2D”C. However, no great difference is seen between the frequency distributions at 20 and -170°C except for the whole shift of the distribution towards the higher frequency side at the lower temperature (see fig. 3b). Although the neutron inelastic scattering spectrum -catmot be compared directly: w-i’&the calculated frequency distribution, comparison will be-made between the observed peaks of the neutron scattering spectrurm and the calculated peeaks of the frequency distriiution. Frdm a comparison between fig. 2’and 3, the ” observed peak at 45 cm-’ at %‘C (the pea at 48 cm-l at --144OC) corresponds to the, calculated peak at $Fcm-’ (the &cul;lted peak at 40 cm-’ at -144°C). $nce.the Iatter is mainly due to the tram lationd mode along the x ax& the observed oeak at .45.cm-l- isassigned to this.n~o~. According to the ; ‘. j24. ,. ‘:I-,, :
(
x-ray
i5.70
-12.5”C
0.09 0.57 0.56
-: : ..
.’
:
-0.20 1.36
4.40 -0.2@ 10.14
1.65
20.45
-1.18
-1.31
-1.10 6.35
I(
-1.24 4.42
0.96 -0.68 4.46
I(
-0.86
2.08
-0.51 3.31
-i.2.5 256
0.56 0.56 4.73
16.29 -0.53
0.66
0.95
0.66 3.94
I(
I( to molecular
-0.03
-0.03 1.58 >
inertia axes
calculation, another peak is expected around 60-70 . the neutron inelastic scattering spectrum. cm -1 m Unfortunately, however, the corresponding region is obscured by the false peak due to the fdter’s inadequacy, and so the ex.pected peak could not be confirmed.
4; Mean square amplitudes The mean square translational amplitude tensor amplitude tensor (0) of the molecule in the crystal were calculated by the methods described by-Hawley [lo] from the eigenvectors and eigenvalues of the dynamical matrices for the wave vectors of .the first @Allouin zone. The calculated T and o-at various temperatures are given in tables 1 and 2, together with those obtain.ed from X-ray diffraction studies 123. It is Seen that the (T) and the mean square rotational
.
>
8.99 -0.79 -0.85
0.33 -0.28 2.04
C!.72 -0.69 7.16 > 17.90
0.68
a) Suffkes 1,2 and 3 correspond x, y and z, respectively.
;. :.- ‘‘ ...
29.26
6.29
3.36
121
1.93’ -1.32 )( 7.49
6.95 -0.58 -17wz
tensor for 9,10-anthra-
Calc
2OS”C
-0.25 416 1.35
I(
0.07 2.40
amplitude
Volume 33, number 1
CHJZh!ICAL PHYSICS LETTERS
components of the calculated tensors agree fairly well with the observed ones, except far w22_ Moreover, it is noted that the temperature changes of the tensors are reproduced very well in the calculations. As seen from table 2, the observed values of a32 do not show a monotonic change with temperature.-They are also very small compared with the corresponding values for the ant_hracene crystal (6.9 deg* at room temperature [lo] ) which has a similar crystal,structure to ; that of anthraquinone. We feel strongly that the observed values of wz2 should be re-examined. It is seen from tables 1 and 2 that the off-diagonal components of the tensors are generally much smaller than those of the diagonal components. This su=ests that the molecular motions in the 9,l O-anthraquinone crystal are C.%cribed approximately as a superposition of the rotational lattice modes around the moIecular inertia axes and the translational modes along them.
calculations. We also thank Dr. N. Niimura and Mr. A. Ozora for their assistance in the neutron scattering experiments.
References
111K_ Lonsdale, H.J. MiUedge and K EI-Sayed, Acta Cryst. 20 (1966) 1.
121K. Lonsdale, HJ. Milledge and K. El-Sayed, Acta Cryst. 20 (1966) 13.
I31 Y. hliyazaki and M. [41 [51 [61 [71 is1
Acknowledgement !91
We would like to express our thanks to Professor T. Miyazawa and Dr. T. Kitagawa for their help in the
is Ahy 1975
[lOI
Ito. Bull_ Chem Sot. Japan 46 (1973) 103. B. Wyncke, F. Brehat, k Hadni, Y. hliyazaki and !+I. ito, Chem. Phys. Letters 21 (1973) 115. A. Ozora, hf. Ito, N. Niimura and N. Watanabe, Chem. Phys. Letters 18 (1973) 306. C Pecile and B. LuneUi, J. Chem. Phys. 46 (1967) 2109. S.N. Singh and RS. Singh, Spectrochim. Acta 24A (1968) 1591. T. Miyazawa, Y. Ideguchi and K. Fukushima, J. Chem. Phys. 38 (1963) 2709: T. Kitagawa and T. hfiyazawn, Bull. (Tnem. Sot. Japan 42 (1969) 1388. V.G. Dashevsky, V.T. Struchykor and ZA. Akoppayan, Zh. Strukt- Khim. 7 (1966) 594. G.S. PawIey, Phys. Stat. SOL 20 (1967) 347.