- Email: [email protected]
Fourier transform infrared spectroscopic studies on the formation of ultrathin polyimide films using Langmuir-Blodgett techniques
Vibrational Spectroscopy, 1 (1990) 81-86 Elsevier Science Publishers B.V., Amsterdam
81
Fourier transform infrared spectroscopic studies on the formation of ultrathin polyimide films using Langmuir-Blodgett techniques H. Ancelin and J. Yarwood Department
*
of Chemistry, Durham University, Durham DHl 3LE (Great Britain)
J.H. Clint and A.J. Willatt BP Research, Sunbury Research Centre, Chertsey Road, Sunbury-on-Thames,
Mddlesex
TWl6
7LN (Great Britain)
(Received 14th June 1990)
Abstract
Fourier transform infrared attenuated total reflectance spectroscopy was used to characterize ultrathin organic polyimide films deposited on silicon using the Langmuir-Blodgett technique. Both the chemistry and the kinetics of conversion from polyamic acid salt to polyimide were elucidated with strong indications of a novel ring structure intermediate. It is concluded that the conversion may be achieved under relatively mild conditions (immersion in methanol, followed by heating to ca. 85 “C), which is likely to be of importance in an industrial context. Keywords:
Infrared spectrometry; Fourier transform; Polyimide films; Silicon electronic devices; Langmuir-Blodgett films
Polyimide materials are used extensively in industry both as a dielectric/passivation layer in electronic devices [1,2] and as separation and barrier membranes [1,3,4]. There are therefore strong reasons for investigating new methods of preparation and/or conversion, especially in connection with controlled film thickness and thermal stability. One elegant method of depositing thin organic films is that based on the Langmuir-Blodgett technique [5]. Polyamic acids, the precursors to polyimides, cannot themselves be deposited by this technique as they do not have the required amphipathic structure to spread and be com’ LARC is a trade name for a polyimide produced by the condensation of 3,3’-diaminobenzophenone with benzophenonetetracarboxylic acid dianhydride. 09242031,‘90/$03.50
@ 1990 - Elsevier Science Publishers B.V
pressed to form an ordered layer at the air/water interface. Reaction of polyamic acids with longchain alkylamines produces salts which spread and form coherent monolayers on water. The Langmuir-Blodgett deposition of such salts and subsequent conversion to polyimide has been demonstrated by Kakimoto and co-workers [6-81. However, although average molecular tilt angles have been measured [8], little attempt was made to characterize, in situ, the chemical processes that occur during the conversion process. This is essential for an understanding of the way in which chemical structure and film integrity and viability are related, and hence for future chemical design strategies. Further, it is essential to investigate alternative routes to well ordered and thermally stable films in order to maximize the efficiency of
H. ANCELIN
82
ET AL.
A 0,250
O,LlO 0.357-
0.3050 252-
,I
O-156 a 0 125 2 x b 0.09L d a 0,063
3070 3010 2950 2690 2830
1870 1810 1750 1690
Wavenumberlcml)
1630 1570 1510 IL50 1390 1330
Wavenumber
km-11
Fig. 1. Fourier transform infrared attenuated total reflectance spectra of the polyamic acid salt as a function of time of immersion in AnalaR methanol at room temperature; (A) the v,(CH,) and v~(CH,) region; (B) the v(C=O), v(COO-) and S(CH,) region. In each case the number of minutes of immersion (in methanol) are marked on the spectra.
production on an industrial scale for existing and new applications. This paper reports the preparation, deposition and conversion of polyamic acid salts based on a LARC ‘-DOA polymeric backbone (see below) using much milder (and cheaper) reagents than have been employed previously [6-81. Continuous in situ monitoring of the infrared spectrum allows an in-depth understanding of the chemical and kinetic parameters which control polyimide production.
0.125 0.109 0.093 0,078 0.062 O.OL6
EXPERIMENTAL
The polyamic acid salt was prepared using stoichiometric mixtures of LARC with dioctadecylamine (DOA) in a 1 + 1 mixture of N,N-dimethylacetamide and toluene which acted as a suitable spreading solvent. The LARC was supplied by Mitsui-Toatsu Chemicals as a 30% solution in N,iV-dimethylacetamide and the salt with
1775
1725 1675
1625 1575 1525
Wavenumberkm-'1
Fig. 2. Overlay of spectra at 0 and 10 min immersion time on the 1500-1800 cm-’ region. This shows (i) the new band arising at -1710 cm-‘; (ii) the loss of intensity in the 1570 cm-’ region due to loss of a v,(COW ) band; (iii) the shift of an amide I v(C=O) band to near 1640 cm-‘.
FT-IR
OF ULTRATHIN
POLYIMIDE
FILM
83
FORMATION
DOA had the following structure:
DOA was supplied by Hoechst and recrystallized twice from acetone before use. N,N-Dimethylacetamide (spectroscopic grade, 99+%) was obtained from Aldrich and toluene (AnalaR grade, 99.5%) from BDH. Multilayers of the salt were deposited on both sides of a silicon micro-attenuated total reflectance (ATR) crystal (0.4 X 15 X 20 mm, with end faces cut at 45”) using a Jovce-Loebl Langmuir
trough. The sub-phase was water purified with a Mill&Q system (Millipore) at pH 5.8 and 22O C. Thirty-one layers were deposited on the hydrophilic substrate at a surface pressure of 30 mN m-l and a rate of 0.2 mm s-l. The dipping record showed a deposition ratio of 1.0 for each cycle. Infrared spectra were recorded using a Mattson Sirius 100 spectrometer equipped with micro=ATR optics and a liquid-nitrogen-cooled MCI’ detector. Data collection times were of the order of 10 min at a resolution of 4 cm-‘. In situ heating was arranged using a small piezoelectric heater attached to one side of the ATR crystal (which had been cleaned to remove Langmuir-Blodgett film material). Temperatures were monitored using a thermocouple in contact with the substrate face.
0A 0’
Nt
P$yamic
acid salt (I)
Intermediate
R+
0c
-
*
N
species (II)
-H,O
Polyimide -
(III)
II
0 Fig. 3. Scheme showing the conversion of polyamic acid salt to polyimide
via a hydrogen-bonded
ring species II.
84
H.ANCELIN
ETAL.
AnalaR-grade methanol or propan-2-01 was used throughout. The samples were stored at room temperature in a desiccator. No special effort was taken to exclude moisture during the heating process.
RESULTSAND
DISCUSSION
Removal of the amine chains from the polyamic acid (LARC) salt molecules in the Langmuir-Blodgett multilayers is easily achieved using an alcohol (methanol or propan-2-01). This is demonstrated in Figs. 1 and 2, which show the Fourier transform (FT) IR spectrum of 31 layers of the salt and the effect on the spectrum of immersion in methanol (or propan-2-01) for up to 10 min. The vJCH,) (2850 cm-‘), v,,(CH,) (2920 cm-‘) and 6(CH,) (1460-1470 cm-‘) bands have all been completely removed, indicating total loss of aliphatic chains. The spectra also show intensity decreases in the regions (1360 and 1570 cm-‘) where the v,(COO-) and v,,(COO-) bands would be expected [9] to appear. This would indicate conversion of the salt species RNH:COO-R back towards the acid [unless the v(COO-) bands have
I
12
5
7
10
- tlmin)
Fig. 4. Plot of intensity of the aliphatic chain v(CH,) band intensities as a function of immersion in methanol (all three bands behave in the same way, as expected).
shifted considerably]. However, no new “acid” v(C=O) band appears in the 1720 cm-’ region. Indeed, the intensity in this region decreases owing to the loss of a small amount of acid species (1724 cm-‘) which is present in the original salt film (this band can be reduced considerably by storing the original polyamic acid under anhydrous conditions in a freezer). It is therefore clear that species II (Fig. 3B) is not the original (hydrogen-bonded)
I
I
0.165
0,165
-T=WC
,k2lh, -
3 oaa 3 $
0,065
T.150'CPOmml
f:
T= 125-C t20md I /
-0.010 Wavenumber
’
1620
Icm-1)
Fig. 5. Offset spectra showing conversion 125OC and 150°C.
,,
-0.010 1 , 1920 1860 1800 17LO 1680 1620
I
1920 1860 1800 17LO 1680
-number
of intermediate
species II to polyimide
I
km-11
at 85 o C and on increasing
the temperature
to
FT-IR
OF ULTRATHIN
t
1
I’OLYIMIDE
I
1
FILM
I
85
FORMATION
reported [11], but in a completely different context. Investigation of the kinetics of removal of the aliphatic chains (and hence the formation of the intermediate species) is demonstrated in Fig. 4, which shows that chain removal is complete in about 10 min for this particular polyamic acid salt. The “reaction” appears to follow roughly an exponential decay at least over the first few minutes. Such an observed rate is entirely feasible for an industrial-scale process using a cheap and readily available reagent (methanol). Conversion of the material left after removal of the aliphatic amine to polyimide was monitored “ in situ” during the controlled heating process. Figures 5 and 6 show the principal stages in the conversion at relatively low temperatures (3075 *C). The principal spectral markers are the disappearance of the band at 1640-1655 cm-l associated with the amide I mode, which is expected when cyclization to polyimide occurs (see Fig. l), and the appearance of two bands at ca. 1724 and ca. 1778 cm-‘, which are due to coupled Y(C=O) modes of the polyimide ring [12]. It is clear from the rate of appearance of these bands (Fig. 7) that about 50% conversion occurs relatively quickly (2 h) at about 85*C but that a temperature of about 120-150°C is necessary for 100% conversion on a silicon substrate. There is no evidence that prolonged heating at 85” C will effect complete conversion. However, the polymer film is stable up to about 200 * C in this (very thin) film and it is likely that conversion can be carried out on a polymeric substrate.
I
1
1775 1725 1675 1625 1575 1525 Wavenumber (crd) Fig. 6. Overlay spectra showing polyimide function of time at 85 *C and at 150 * C.
conversion
as a
polyamic acid [lo]. It is believed that this intermediate species contains a hydrogen-bonded ring formed between the amide group and the -COOgroup attached to the same aromatic ring. There are two pieces of spectral evidence for such a novel species. First, there is a broad (possibly doublet) band at ca. 1710 cm-’ (Fig. 2), which is probably due to a C=O---H hydrogen-bonded species. Second, there is a new band which arises at ca. 1650 cm-l and which is assigned to a shifted amide I Y(C=O) band, the shift arising from “ring” formation. It is worth noting that such a hydrogen-bonded ring species has been
polyimide
% 15o*c
100
120%
87%
50
0
we-
I
I
20
50
Fig. 7. Plot of polyimide
I
150
I
200
______--__-
I
240 t/mins
band intensity (1724 cm-‘)
,A
/
____-L
21hours
as a function of time.
H. ANCELIN
Conclusion It has been demonstrated that conversion from polyamic acid salt Langmuir-Blodgett layers to a thin, stable film of polyimide may be effected, through a novel ring structure, using a cheap reagent (methanol) and a relatively low temperature (85-12OOC.) Even more efficient conversion routes on a variety of substrates are now being sought. We are grateful to BP International for permission to publish this paper.
3 4 5
6 7 8 9 10
REFERENCES 11 1 K.L. MittaI (Ed.), Polyimides-Synthesis, Characterization and AppIications, Vols. 1 and 2, Plenum, New York, 1984. 2 R.W. Snyder and C.W. Sheen and P.C. Painter, in K.L.
12
ET AL.
Mittal (Ed.), Polyimides-Synthesis, Characterization and Applications, Vol. 1, Plenum, New York, 1984, pp. 71-78. K.C. O’Brien, W.J. Koros and G.R. Husk, Polym. Eng. Sci., 27 (1987) 211. M. Hotta, Kagaku Keizai, 35(7) (1988) 20. K.B. Blodgett and I. Langmuir, Phys. Rev., 51 (1937) 964; G.G. Roberts, Contemp. Phys., 25 (1984) 109; V. Agarawal, Phys. Today, June (1988) 40. Y. Nishikata, T. Konisbi, A. Morikawa, M. Kakimoto and Y. Imai, Polym. J., 20 (1988) 269. Y. Nishikata, A. Morikawa, M. Kakimoto and Y. Imai, Thin Solid Films, 160 (1988) 15, and references cited therein. M.A. Kakimoto, A. Morikawa, Y. Nishikata, M.A. Suzuki and Y. Imai, J. Colloid Interface Sci., 121 (1988) 599. L.J. Bellamy, Infrared Spectra of Complex Molecules, Chapman and Hall, London 1975. A. Ya. Ardasbnikov, J. Ye. Kardash and A.N. Pravedinkov, Polym. Sci. USSR, 13 (1971) 2092. G. Cassanas, E. Fabreque and L. Bardet, Spectrochim. Acta, Part A 45 (1989) 207. L.J. Bellamy, Infrared Spectra of Complex Molecular, Chapman and Hall, London, 1975, p. 247.
81
Fourier transform infrared spectroscopic studies on the formation of ultrathin polyimide films using Langmuir-Blodgett techniques H. Ancelin and J. Yarwood Department
*
of Chemistry, Durham University, Durham DHl 3LE (Great Britain)
J.H. Clint and A.J. Willatt BP Research, Sunbury Research Centre, Chertsey Road, Sunbury-on-Thames,
Mddlesex
TWl6
7LN (Great Britain)
(Received 14th June 1990)
Abstract
Fourier transform infrared attenuated total reflectance spectroscopy was used to characterize ultrathin organic polyimide films deposited on silicon using the Langmuir-Blodgett technique. Both the chemistry and the kinetics of conversion from polyamic acid salt to polyimide were elucidated with strong indications of a novel ring structure intermediate. It is concluded that the conversion may be achieved under relatively mild conditions (immersion in methanol, followed by heating to ca. 85 “C), which is likely to be of importance in an industrial context. Keywords:
Infrared spectrometry; Fourier transform; Polyimide films; Silicon electronic devices; Langmuir-Blodgett films
Polyimide materials are used extensively in industry both as a dielectric/passivation layer in electronic devices [1,2] and as separation and barrier membranes [1,3,4]. There are therefore strong reasons for investigating new methods of preparation and/or conversion, especially in connection with controlled film thickness and thermal stability. One elegant method of depositing thin organic films is that based on the Langmuir-Blodgett technique [5]. Polyamic acids, the precursors to polyimides, cannot themselves be deposited by this technique as they do not have the required amphipathic structure to spread and be com’ LARC is a trade name for a polyimide produced by the condensation of 3,3’-diaminobenzophenone with benzophenonetetracarboxylic acid dianhydride. 09242031,‘90/$03.50
@ 1990 - Elsevier Science Publishers B.V
pressed to form an ordered layer at the air/water interface. Reaction of polyamic acids with longchain alkylamines produces salts which spread and form coherent monolayers on water. The Langmuir-Blodgett deposition of such salts and subsequent conversion to polyimide has been demonstrated by Kakimoto and co-workers [6-81. However, although average molecular tilt angles have been measured [8], little attempt was made to characterize, in situ, the chemical processes that occur during the conversion process. This is essential for an understanding of the way in which chemical structure and film integrity and viability are related, and hence for future chemical design strategies. Further, it is essential to investigate alternative routes to well ordered and thermally stable films in order to maximize the efficiency of
H. ANCELIN
82
ET AL.
A 0,250
O,LlO 0.357-
0.3050 252-
,I
O-156 a 0 125 2 x b 0.09L d a 0,063
3070 3010 2950 2690 2830
1870 1810 1750 1690
Wavenumberlcml)
1630 1570 1510 IL50 1390 1330
Wavenumber
km-11
Fig. 1. Fourier transform infrared attenuated total reflectance spectra of the polyamic acid salt as a function of time of immersion in AnalaR methanol at room temperature; (A) the v,(CH,) and v~(CH,) region; (B) the v(C=O), v(COO-) and S(CH,) region. In each case the number of minutes of immersion (in methanol) are marked on the spectra.
production on an industrial scale for existing and new applications. This paper reports the preparation, deposition and conversion of polyamic acid salts based on a LARC ‘-DOA polymeric backbone (see below) using much milder (and cheaper) reagents than have been employed previously [6-81. Continuous in situ monitoring of the infrared spectrum allows an in-depth understanding of the chemical and kinetic parameters which control polyimide production.
0.125 0.109 0.093 0,078 0.062 O.OL6
EXPERIMENTAL
The polyamic acid salt was prepared using stoichiometric mixtures of LARC with dioctadecylamine (DOA) in a 1 + 1 mixture of N,N-dimethylacetamide and toluene which acted as a suitable spreading solvent. The LARC was supplied by Mitsui-Toatsu Chemicals as a 30% solution in N,iV-dimethylacetamide and the salt with
1775
1725 1675
1625 1575 1525
Wavenumberkm-'1
Fig. 2. Overlay of spectra at 0 and 10 min immersion time on the 1500-1800 cm-’ region. This shows (i) the new band arising at -1710 cm-‘; (ii) the loss of intensity in the 1570 cm-’ region due to loss of a v,(COW ) band; (iii) the shift of an amide I v(C=O) band to near 1640 cm-‘.
FT-IR
OF ULTRATHIN
POLYIMIDE
FILM
83
FORMATION
DOA had the following structure:
DOA was supplied by Hoechst and recrystallized twice from acetone before use. N,N-Dimethylacetamide (spectroscopic grade, 99+%) was obtained from Aldrich and toluene (AnalaR grade, 99.5%) from BDH. Multilayers of the salt were deposited on both sides of a silicon micro-attenuated total reflectance (ATR) crystal (0.4 X 15 X 20 mm, with end faces cut at 45”) using a Jovce-Loebl Langmuir
trough. The sub-phase was water purified with a Mill&Q system (Millipore) at pH 5.8 and 22O C. Thirty-one layers were deposited on the hydrophilic substrate at a surface pressure of 30 mN m-l and a rate of 0.2 mm s-l. The dipping record showed a deposition ratio of 1.0 for each cycle. Infrared spectra were recorded using a Mattson Sirius 100 spectrometer equipped with micro=ATR optics and a liquid-nitrogen-cooled MCI’ detector. Data collection times were of the order of 10 min at a resolution of 4 cm-‘. In situ heating was arranged using a small piezoelectric heater attached to one side of the ATR crystal (which had been cleaned to remove Langmuir-Blodgett film material). Temperatures were monitored using a thermocouple in contact with the substrate face.
0A 0’
Nt
P$yamic
acid salt (I)
Intermediate
R+
0c
-
*
N
species (II)
-H,O
Polyimide -
(III)
II
0 Fig. 3. Scheme showing the conversion of polyamic acid salt to polyimide
via a hydrogen-bonded
ring species II.
84
H.ANCELIN
ETAL.
AnalaR-grade methanol or propan-2-01 was used throughout. The samples were stored at room temperature in a desiccator. No special effort was taken to exclude moisture during the heating process.
RESULTSAND
DISCUSSION
Removal of the amine chains from the polyamic acid (LARC) salt molecules in the Langmuir-Blodgett multilayers is easily achieved using an alcohol (methanol or propan-2-01). This is demonstrated in Figs. 1 and 2, which show the Fourier transform (FT) IR spectrum of 31 layers of the salt and the effect on the spectrum of immersion in methanol (or propan-2-01) for up to 10 min. The vJCH,) (2850 cm-‘), v,,(CH,) (2920 cm-‘) and 6(CH,) (1460-1470 cm-‘) bands have all been completely removed, indicating total loss of aliphatic chains. The spectra also show intensity decreases in the regions (1360 and 1570 cm-‘) where the v,(COO-) and v,,(COO-) bands would be expected [9] to appear. This would indicate conversion of the salt species RNH:COO-R back towards the acid [unless the v(COO-) bands have
I
12
5
7
10
- tlmin)
Fig. 4. Plot of intensity of the aliphatic chain v(CH,) band intensities as a function of immersion in methanol (all three bands behave in the same way, as expected).
shifted considerably]. However, no new “acid” v(C=O) band appears in the 1720 cm-’ region. Indeed, the intensity in this region decreases owing to the loss of a small amount of acid species (1724 cm-‘) which is present in the original salt film (this band can be reduced considerably by storing the original polyamic acid under anhydrous conditions in a freezer). It is therefore clear that species II (Fig. 3B) is not the original (hydrogen-bonded)
I
I
0.165
0,165
-T=WC
,k2lh, -
3 oaa 3 $
0,065
T.150'CPOmml
f:
T= 125-C t20md I /
-0.010 Wavenumber
’
1620
Icm-1)
Fig. 5. Offset spectra showing conversion 125OC and 150°C.
,,
-0.010 1 , 1920 1860 1800 17LO 1680 1620
I
1920 1860 1800 17LO 1680
-number
of intermediate
species II to polyimide
I
km-11
at 85 o C and on increasing
the temperature
to
FT-IR
OF ULTRATHIN
t
1
I’OLYIMIDE
I
1
FILM
I
85
FORMATION
reported [11], but in a completely different context. Investigation of the kinetics of removal of the aliphatic chains (and hence the formation of the intermediate species) is demonstrated in Fig. 4, which shows that chain removal is complete in about 10 min for this particular polyamic acid salt. The “reaction” appears to follow roughly an exponential decay at least over the first few minutes. Such an observed rate is entirely feasible for an industrial-scale process using a cheap and readily available reagent (methanol). Conversion of the material left after removal of the aliphatic amine to polyimide was monitored “ in situ” during the controlled heating process. Figures 5 and 6 show the principal stages in the conversion at relatively low temperatures (3075 *C). The principal spectral markers are the disappearance of the band at 1640-1655 cm-l associated with the amide I mode, which is expected when cyclization to polyimide occurs (see Fig. l), and the appearance of two bands at ca. 1724 and ca. 1778 cm-‘, which are due to coupled Y(C=O) modes of the polyimide ring [12]. It is clear from the rate of appearance of these bands (Fig. 7) that about 50% conversion occurs relatively quickly (2 h) at about 85*C but that a temperature of about 120-150°C is necessary for 100% conversion on a silicon substrate. There is no evidence that prolonged heating at 85” C will effect complete conversion. However, the polymer film is stable up to about 200 * C in this (very thin) film and it is likely that conversion can be carried out on a polymeric substrate.
I
1
1775 1725 1675 1625 1575 1525 Wavenumber (crd) Fig. 6. Overlay spectra showing polyimide function of time at 85 *C and at 150 * C.
conversion
as a
polyamic acid [lo]. It is believed that this intermediate species contains a hydrogen-bonded ring formed between the amide group and the -COOgroup attached to the same aromatic ring. There are two pieces of spectral evidence for such a novel species. First, there is a broad (possibly doublet) band at ca. 1710 cm-’ (Fig. 2), which is probably due to a C=O---H hydrogen-bonded species. Second, there is a new band which arises at ca. 1650 cm-l and which is assigned to a shifted amide I Y(C=O) band, the shift arising from “ring” formation. It is worth noting that such a hydrogen-bonded ring species has been
polyimide
% 15o*c
100
120%
87%
50
0
we-
I
I
20
50
Fig. 7. Plot of polyimide
I
150
I
200
______--__-
I
240 t/mins
band intensity (1724 cm-‘)
,A
/
____-L
21hours
as a function of time.
H. ANCELIN
Conclusion It has been demonstrated that conversion from polyamic acid salt Langmuir-Blodgett layers to a thin, stable film of polyimide may be effected, through a novel ring structure, using a cheap reagent (methanol) and a relatively low temperature (85-12OOC.) Even more efficient conversion routes on a variety of substrates are now being sought. We are grateful to BP International for permission to publish this paper.
3 4 5
6 7 8 9 10
REFERENCES 11 1 K.L. MittaI (Ed.), Polyimides-Synthesis, Characterization and AppIications, Vols. 1 and 2, Plenum, New York, 1984. 2 R.W. Snyder and C.W. Sheen and P.C. Painter, in K.L.
12
ET AL.
Mittal (Ed.), Polyimides-Synthesis, Characterization and Applications, Vol. 1, Plenum, New York, 1984, pp. 71-78. K.C. O’Brien, W.J. Koros and G.R. Husk, Polym. Eng. Sci., 27 (1987) 211. M. Hotta, Kagaku Keizai, 35(7) (1988) 20. K.B. Blodgett and I. Langmuir, Phys. Rev., 51 (1937) 964; G.G. Roberts, Contemp. Phys., 25 (1984) 109; V. Agarawal, Phys. Today, June (1988) 40. Y. Nishikata, T. Konisbi, A. Morikawa, M. Kakimoto and Y. Imai, Polym. J., 20 (1988) 269. Y. Nishikata, A. Morikawa, M. Kakimoto and Y. Imai, Thin Solid Films, 160 (1988) 15, and references cited therein. M.A. Kakimoto, A. Morikawa, Y. Nishikata, M.A. Suzuki and Y. Imai, J. Colloid Interface Sci., 121 (1988) 599. L.J. Bellamy, Infrared Spectra of Complex Molecules, Chapman and Hall, London 1975. A. Ya. Ardasbnikov, J. Ye. Kardash and A.N. Pravedinkov, Polym. Sci. USSR, 13 (1971) 2092. G. Cassanas, E. Fabreque and L. Bardet, Spectrochim. Acta, Part A 45 (1989) 207. L.J. Bellamy, Infrared Spectra of Complex Molecular, Chapman and Hall, London, 1975, p. 247.