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The far infrared spectra of tri-acetylacetonato metal III complexes
Spcetrochlmica Acta, 1984.Vol. 20,pp. 63to 69. PergamonPressLtd. Printedin NorthernIreland
The far m&wed spectra of tris-acetylacetonato metal III complexes R. D.
GILLARD,
H.
G. SILVER
and J. L. WOOD
Chemistry Department, Imperial College of Science and Technology, London S-W.7 (Received27 June 1963) Ab&&---The infrared absorption spectra of CoIII, C@ and F&I tris 8CStyl8C0bn8hS have been recorded from 55 to 400 cmI. The vibrations of the heavy 8t4&1skeleton 8~3 digcussed on the basis of local symmetry, perturbed to give rise to the overall molecular symmetry.
A QOOD deal of attention has recently been given to the infrared spectra of the complexes formed by the acetylacetonate ion with transition metals [l-3]. From a force constant treatment of the in-plane vibrations of a single chelate ring, NAKAMOTO et al. [2] predicted absorption bands at 373, 311, 276 and 200 cm-l in the case of the Con1 complex, and at similar frequencies in the CrlI1and Fen1 complexes. The far infrared spectra of these complexes have been observed down to 55 cm-l, and do not bear out this prediction. The infrared spectra of these complexes have therefore been re-interpreted on the basis of what is believed to be a more satisfactory model.
The complexes were prepared by standard methods [4] and purified by recrystallization and sublimation. The identity of the complexes with those examined by NAKAMOTO is substantiated by the similarity of the higher frequency infrared spectra. The spectra of our samples were recorded with a PE-21 spectrometer, and agreed completely (to within 2 cm-l) with those observed by NAKAMOTO.The far infrared spectra were recorded with the single beam vacuum grating spectrometer constructed in the Department by TAWSALU et al. Samples were examined as nujol solutions enclosed in polythene bags. Spectra were compared with backgrounds of the dilute solutions or of pure nujol recorded under the same operating conditions. In all, thirty-eight runs were made. The two higher frequency bands of each substance were particularly strong, and could be established with confidence. The lowest frequency band lies at the extreme range of the instrument, and is less certain. The spectrum of Co(acac), is shown in Fig. 1; those of Cr(acac), and Fe(acac), are very similar. The observations are summarized, together with the predicted spectra for comparison, in Table 1. [l] K. N-on,
end A. E. MARTELL,J. Ohem. Phye. 83, 688 (1900). [2] K. NAXAMOTOet a& J. Am. C7um. Sot. 88, 1066 (1961). [3] K. NAKAMOTO eld., J. Am. Chma. Sot. 88, 1272 (1961). [4] B. E. BRYANT and W. C. FERNELIUS, Iwg. Syn.theaea.6, 186 (1967). 63
64
R. D. GILLARD, Trisacetylacetonato
H. G. SILVERand J. L. WOOD
cobalt III
I
1*,A JO0
5,o
J_ $50
2,oo
290
300
39
4,P
4%
Cm”Fig. 1 Table 1. The far infrared spectra of three acetylacetonate complexes (all frequenciesin cm-l) Cr(acac),
Co(acac), Obs. 386 vs
96.4 m 60 m
Fe(acac)3
Predicted [2]
Obs.
Predicted [2]
Obs.
Predicted [2]
373 311 276 200
355 vs
373 307 277 202
298 vs
373 273 265 199
103 m 60 m
111 m 60 m
INTERPRETATION OF RESULTS All spectra
related to solutions, so the low frequency bands cannot be due to lattice vibrations, and all the observed bands are considered to arise from intramolecular vibrations. The gepmetry of the complexes has been established by X-ray orystallography [5] and is shown in Fig. 2. The heavy atom skeleton contains 22 &,oms, and in order to simplify the discussion, NAKAMOTO considered the vibrations of a single chelate ring. Although this may be quite a satisfactory approach for the higher frequency modes, which do not strongly involve the M and 0 atoms, it will be far less satisfactory for the lower frequency modes, which principally involve the bending or stretching of the 0 bonds. In these modes the interactions between the rings, via the common M atom will be SOgreat as to make the “single ring” approach inappropriate. These modes may be more suitably treated as the vibrations of a MO6 octahedron perturbed and reduced to D, symmetry by the three ligand rings. The remaining ring frequencies may be expected to couple more weakly with the corresponding modes in the other rings, and the resulting splitting will probably be too small to observe in the spectra recorded. Altogether the six atoms in a single ring have eighteen modes of motion (including translation and overall rotation). Of these, nine are primarily located in the ring and will couple weakly; the remainder are associated with the MO, atoms or with the translation and rotation (Table 2)_ [6] R. B. ROOB,Ada Cryet.,9, 781 (1956),
The far inframd spectra of tris-aoetylacetonato
metal III
66
complexes
The modes associated with the Me groups will also couple weakly and can be dealt with along with the ring modes.
M (acad FICA 2 Table 2. Description
of the vibrations
of a chelate ring
For one ring
MO, modes including translation and overall rotation
3 overall translation 3 overall rotation
3 3
6 bond stretching 3 puckering (out of plane) 3 angle bending
clawedas ring modes -
2
4 3
1
2
2
4 3 2 -
-
.
(in plane) Each additional ring adds
--
6 3 3 3
bond stretching puckering angle bending “ring orientation” Total for 3 rings
-
1 3 9+0+6=21 15 vib., 3 rot., 3 trans
3(4 + 3 + 2) =
27
Application of group theory Although the vibrations of the entire molecule can be rigorously classified on the basis of the overall molecular symmetry class (II,), more information about the position and intensity of lines can be obtained by applying the concept of local symmetry perturbed by the molecular environment. !t’hus, lines that are not allowed
R.
66
D. GILLARD, H. G. SILVER end J. L. WOOD
on the local symmetry will only give rise to weak lines if these become allowed on the overall symmetry. Lines on the overall symmetry basis, which arise from a common origin on the localised symmetry, will be close together and may not be separately resolved. The local symmetry of the MO8 vibrations is Oh, the perturbed symmetry Da. The number of vibrations of each symmetry type, and their correlation, are obtained by standard methods [6] and are shown in Table 3. From this, one expects the 16 MO, vibrations to give rise to two strong doublets, which may not be resolved, and three much weaker single bands, which are probably unobserved. Table 3. Vibrations of the MO,, nucleus
Unperturbed symmetry Oh
I.R. activity
Perturbed symmetry D3
I.R. activity and probable activity
14,
no
4
no
1%
no
E
yes
2 p,,
Yes
I4
1 weak line
E
The local symmetry of a planar chelate ring is C,,. The local symmetry can be related to the overall D, molecular symmetry by the methods developed by HORNI~ [7] for the interpretation of crystal spectra. The molecular symmetry corresponds to that of the unit cell group in crystals, the local symmetry corresponds to the molecular point group, and the two are linked by their common subgroup (C, in the present case) the site group in crystals. The number of modes of each type, based on either the local symmetry or the overall symmetry, is obtained, as before, by standard methods [6]. The correlation for the ring out of plane puckering modes, shown in Table 4, indicates that these will give rise to two strong doublets, and one singlet which will be very weak, as it arises only from the inter-ring perturbation. The six in-plane ring frequencies can be dealt with entirely similarly (Table 5). Six strong bands are expected, three single and three double. The only remaining skeletal modes to be considered are those associated with the methyl groups. These are localised in the rings, and so are dealt with in a similar way. The correlation, shown in Table 6, indicates that there will be three strong [0] E. B. WILSON, J. C. DECIUS and P. C. CROSS, MoEecuZar Vibratiom, (1955). [7] D. F. HORNIO, J. Chem. Phys.
16, 1063 (1948).
McGraw-Hill,
New York
67
The far infrared spectra of tris-acetylacetonsto metal III complexes Table 4. The ring out-of-plane vibrations Molecular symmetry D8
Site Local symmetry c*.*
I.R. activity
symmetry 0,
4
no
A
no 1 weak yes 1 line
Al
(E
yes doublet yes 1 weak I 1 strong
B
Totals 3x3
2
I.R. activity and probable Spectrum
9
3x3
5
2 effective
* The l&bellingof the C,, symmetry classes in [2] is in error. Table 5. The ring in-plane vibrationa
Approximate description
Al
/“\
C
I.R. aotivity
Site Symmetry G
Yes
A
Molecular symmetry Da
I.R. activity and probable spectrum
Al
E
C bend
c-o str.
Al
A
A, E
G-O
Bl
B
As E
B1
B
Aa E
A
str.
C-G-O
bend
C-C
str.
Al
C-C
str.
Bl
Yes
3 x6
8
Totals
4
E
B
Aa E
18
no 1 etrong yeaI line
yes doublet yea 1 weak I 1 strong no letrong yes I line yea doublet yes lweak 1 1 strong
9
6 effective
R. D. GILLARD, H. G. SILVERend J. L. WOOD
68
doublets, and one weak and two strong singlets. The modes involving the H atoms may also be dealt with similarly, but are omitted from this discussion, as there is in effect no change from the previous treatment. Table 6. The Me skeletal vibrations
Approximate description
I.R. activity
Site symmetry C* A
I.R. activity and probable spectrum
(E
no lwcak yes I line
B
Aa 1E
yes yes
yes
A
Al 1E
no 1 strong yes I line
Yes
B
A2 (E
yes doublet yea 1 weak 1 1 strong
4
A
A, 1E
no 1 strong yes 1 lint
B1
B
A2 lE
yes doublet yes 1 1 weak 1 strong
18
9
0.p.
4
no
rocking i(
B,
yes
i.p.
4
rocking11
Bl
Tot&
Molecular cYmmctry *,
3x0
6
Al
doublet 1 weak I 1 strong
5 effective
Assignment From the tables, and the foregoing
section, it will be seen that the 60 skeletal bands. In the assignment, bands disallowed on the local symmetry basis are not included, and doublets are assumed to be unresolved. It is obvious that the experimental data are as yet inadequate to permit an unambiguous assignment, and force constant calculations appear unreliable. We have, however, no positive grounds for differing with the previous assignment [2] above the CH out-of-plane bending frequency. Regarding the lower frequency bands, the following comments may be made. (The bands are The 771 cm-l band is strong identified by reference to those in the ConI complex.) and constant in frequency throughout the series, and must be a fundamental located The 633 cm-l band is in the ring, possibly the second C-Me stretching frequency.* sensitive to the M atom, and is assigned as the ring frequency based on C-C-O modes will effectively give rise to very few observable
* This band has recently been grouped with the 780 cm-1 band as the C-H out of plane vibration by N-on, Infrared Spectra of Inotganic and Coordination hnpnm&, John Wiley New York (1963).
The far infrared spectra oh tris-acetyleceton&o
metal III complexes
69
bending (i.p.). This will be above the frequency in simple molecules, (~450 cm-l) and will show M atom dependence. The two MO, frequencies are readily identified at 466 cm-l and 388 cm-l from th ir strong variation with M. One of the two lowest Y observed. frequencies will be due to the Me o.p. rocking, which will be expected at a lower frequency than in nz-xylene (227 cm-l) [8]. The other very low band is assigned to one of the two ring puckering modes, which will again be expected below the corresponding frequency in m-xylene. The bands at 671 and 432 cm-l are assigned by NAKAMOTO to the Me i.p. bending modes. This seems improbable as the corresponding frequencies in m-xylene are at 404 and 206 cm-l [8]. The assignment of the lower frequencies is summarised in Table 7. Table 7. Assignment of some of the lower frequency modes
co111
C+
FeII’
771
772
770
ring mode
691
677
663
ring i.p. bend C
671 662I
658
654
/“\
C
?
L
ring i.p. bending
633
694 609
649 669
466
459 416 355
434 411 289
MO, mode
103
111
I ring o.p. end
60
60
432 386 98 60
/“\ c
c
MO, mode
r
Me o.p. rock
CONCLUSION
Quite apart from the question of the choice of an appropriate model for the basis of discussion, there is a further point in the previous treatment [2] to which attention should be drawn. When the G elements of a matrix are completely known, multiple sets of F elements can be obtained to satisfy a given set of eigenvalues. There is no a priori way of distinguishing the particular set appropriate to the molecule, and the variational procedure used in [2] can “home” on to an alternative set of force constants. This ambiguity has been discussed by FREEMAN and Ross [9], and it was therefore considered that there was little value in carrying out force constant calculations until further data, particularly regarding isotopically substituted species becomes available. Acknowledgwn.mdWe are grateful to the D.S.I.R. for a grant for the construction spectrometer, and for maintenance of one of us (H. G. S.)
[8] J. K. WILMSEURSTand H. J. BERNSTEIN,Can. J. Chmn. 85, 911 (1957). [Q] D. E. FREEMANend I. G. Ross, Spectrochim. Acta l&l393 (1961).
of the
The far m&wed spectra of tris-acetylacetonato metal III complexes R. D.
GILLARD,
H.
G. SILVER
and J. L. WOOD
Chemistry Department, Imperial College of Science and Technology, London S-W.7 (Received27 June 1963) Ab&&---The infrared absorption spectra of CoIII, C@ and F&I tris 8CStyl8C0bn8hS have been recorded from 55 to 400 cmI. The vibrations of the heavy 8t4&1skeleton 8~3 digcussed on the basis of local symmetry, perturbed to give rise to the overall molecular symmetry.
A QOOD deal of attention has recently been given to the infrared spectra of the complexes formed by the acetylacetonate ion with transition metals [l-3]. From a force constant treatment of the in-plane vibrations of a single chelate ring, NAKAMOTO et al. [2] predicted absorption bands at 373, 311, 276 and 200 cm-l in the case of the Con1 complex, and at similar frequencies in the CrlI1and Fen1 complexes. The far infrared spectra of these complexes have been observed down to 55 cm-l, and do not bear out this prediction. The infrared spectra of these complexes have therefore been re-interpreted on the basis of what is believed to be a more satisfactory model.
The complexes were prepared by standard methods [4] and purified by recrystallization and sublimation. The identity of the complexes with those examined by NAKAMOTO is substantiated by the similarity of the higher frequency infrared spectra. The spectra of our samples were recorded with a PE-21 spectrometer, and agreed completely (to within 2 cm-l) with those observed by NAKAMOTO.The far infrared spectra were recorded with the single beam vacuum grating spectrometer constructed in the Department by TAWSALU et al. Samples were examined as nujol solutions enclosed in polythene bags. Spectra were compared with backgrounds of the dilute solutions or of pure nujol recorded under the same operating conditions. In all, thirty-eight runs were made. The two higher frequency bands of each substance were particularly strong, and could be established with confidence. The lowest frequency band lies at the extreme range of the instrument, and is less certain. The spectrum of Co(acac), is shown in Fig. 1; those of Cr(acac), and Fe(acac), are very similar. The observations are summarized, together with the predicted spectra for comparison, in Table 1. [l] K. N-on,
end A. E. MARTELL,J. Ohem. Phye. 83, 688 (1900). [2] K. NAXAMOTOet a& J. Am. C7um. Sot. 88, 1066 (1961). [3] K. NAKAMOTO eld., J. Am. Chma. Sot. 88, 1272 (1961). [4] B. E. BRYANT and W. C. FERNELIUS, Iwg. Syn.theaea.6, 186 (1967). 63
64
R. D. GILLARD, Trisacetylacetonato
H. G. SILVERand J. L. WOOD
cobalt III
I
1*,A JO0
5,o
J_ $50
2,oo
290
300
39
4,P
4%
Cm”Fig. 1 Table 1. The far infrared spectra of three acetylacetonate complexes (all frequenciesin cm-l) Cr(acac),
Co(acac), Obs. 386 vs
96.4 m 60 m
Fe(acac)3
Predicted [2]
Obs.
Predicted [2]
Obs.
Predicted [2]
373 311 276 200
355 vs
373 307 277 202
298 vs
373 273 265 199
103 m 60 m
111 m 60 m
INTERPRETATION OF RESULTS All spectra
related to solutions, so the low frequency bands cannot be due to lattice vibrations, and all the observed bands are considered to arise from intramolecular vibrations. The gepmetry of the complexes has been established by X-ray orystallography [5] and is shown in Fig. 2. The heavy atom skeleton contains 22 &,oms, and in order to simplify the discussion, NAKAMOTO considered the vibrations of a single chelate ring. Although this may be quite a satisfactory approach for the higher frequency modes, which do not strongly involve the M and 0 atoms, it will be far less satisfactory for the lower frequency modes, which principally involve the bending or stretching of the 0 bonds. In these modes the interactions between the rings, via the common M atom will be SOgreat as to make the “single ring” approach inappropriate. These modes may be more suitably treated as the vibrations of a MO6 octahedron perturbed and reduced to D, symmetry by the three ligand rings. The remaining ring frequencies may be expected to couple more weakly with the corresponding modes in the other rings, and the resulting splitting will probably be too small to observe in the spectra recorded. Altogether the six atoms in a single ring have eighteen modes of motion (including translation and overall rotation). Of these, nine are primarily located in the ring and will couple weakly; the remainder are associated with the MO, atoms or with the translation and rotation (Table 2)_ [6] R. B. ROOB,Ada Cryet.,9, 781 (1956),
The far inframd spectra of tris-aoetylacetonato
metal III
66
complexes
The modes associated with the Me groups will also couple weakly and can be dealt with along with the ring modes.
M (acad FICA 2 Table 2. Description
of the vibrations
of a chelate ring
For one ring
MO, modes including translation and overall rotation
3 overall translation 3 overall rotation
3 3
6 bond stretching 3 puckering (out of plane) 3 angle bending
clawedas ring modes -
2
4 3
1
2
2
4 3 2 -
-
.
(in plane) Each additional ring adds
--
6 3 3 3
bond stretching puckering angle bending “ring orientation” Total for 3 rings
-
1 3 9+0+6=21 15 vib., 3 rot., 3 trans
3(4 + 3 + 2) =
27
Application of group theory Although the vibrations of the entire molecule can be rigorously classified on the basis of the overall molecular symmetry class (II,), more information about the position and intensity of lines can be obtained by applying the concept of local symmetry perturbed by the molecular environment. !t’hus, lines that are not allowed
R.
66
D. GILLARD, H. G. SILVER end J. L. WOOD
on the local symmetry will only give rise to weak lines if these become allowed on the overall symmetry. Lines on the overall symmetry basis, which arise from a common origin on the localised symmetry, will be close together and may not be separately resolved. The local symmetry of the MO8 vibrations is Oh, the perturbed symmetry Da. The number of vibrations of each symmetry type, and their correlation, are obtained by standard methods [6] and are shown in Table 3. From this, one expects the 16 MO, vibrations to give rise to two strong doublets, which may not be resolved, and three much weaker single bands, which are probably unobserved. Table 3. Vibrations of the MO,, nucleus
Unperturbed symmetry Oh
I.R. activity
Perturbed symmetry D3
I.R. activity and probable activity
14,
no
4
no
1%
no
E
yes
2 p,,
Yes
I4
1 weak line
E
The local symmetry of a planar chelate ring is C,,. The local symmetry can be related to the overall D, molecular symmetry by the methods developed by HORNI~ [7] for the interpretation of crystal spectra. The molecular symmetry corresponds to that of the unit cell group in crystals, the local symmetry corresponds to the molecular point group, and the two are linked by their common subgroup (C, in the present case) the site group in crystals. The number of modes of each type, based on either the local symmetry or the overall symmetry, is obtained, as before, by standard methods [6]. The correlation for the ring out of plane puckering modes, shown in Table 4, indicates that these will give rise to two strong doublets, and one singlet which will be very weak, as it arises only from the inter-ring perturbation. The six in-plane ring frequencies can be dealt with entirely similarly (Table 5). Six strong bands are expected, three single and three double. The only remaining skeletal modes to be considered are those associated with the methyl groups. These are localised in the rings, and so are dealt with in a similar way. The correlation, shown in Table 6, indicates that there will be three strong [0] E. B. WILSON, J. C. DECIUS and P. C. CROSS, MoEecuZar Vibratiom, (1955). [7] D. F. HORNIO, J. Chem. Phys.
16, 1063 (1948).
McGraw-Hill,
New York
67
The far infrared spectra of tris-acetylacetonsto metal III complexes Table 4. The ring out-of-plane vibrations Molecular symmetry D8
Site Local symmetry c*.*
I.R. activity
symmetry 0,
4
no
A
no 1 weak yes 1 line
Al
(E
yes doublet yes 1 weak I 1 strong
B
Totals 3x3
2
I.R. activity and probable Spectrum
9
3x3
5
2 effective
* The l&bellingof the C,, symmetry classes in [2] is in error. Table 5. The ring in-plane vibrationa
Approximate description
Al
/“\
C
I.R. aotivity
Site Symmetry G
Yes
A
Molecular symmetry Da
I.R. activity and probable spectrum
Al
E
C bend
c-o str.
Al
A
A, E
G-O
Bl
B
As E
B1
B
Aa E
A
str.
C-G-O
bend
C-C
str.
Al
C-C
str.
Bl
Yes
3 x6
8
Totals
4
E
B
Aa E
18
no 1 etrong yeaI line
yes doublet yea 1 weak I 1 strong no letrong yes I line yea doublet yes lweak 1 1 strong
9
6 effective
R. D. GILLARD, H. G. SILVERend J. L. WOOD
68
doublets, and one weak and two strong singlets. The modes involving the H atoms may also be dealt with similarly, but are omitted from this discussion, as there is in effect no change from the previous treatment. Table 6. The Me skeletal vibrations
Approximate description
I.R. activity
Site symmetry C* A
I.R. activity and probable spectrum
(E
no lwcak yes I line
B
Aa 1E
yes yes
yes
A
Al 1E
no 1 strong yes I line
Yes
B
A2 (E
yes doublet yea 1 weak 1 1 strong
4
A
A, 1E
no 1 strong yes 1 lint
B1
B
A2 lE
yes doublet yes 1 1 weak 1 strong
18
9
0.p.
4
no
rocking i(
B,
yes
i.p.
4
rocking11
Bl
Tot&
Molecular cYmmctry *,
3x0
6
Al
doublet 1 weak I 1 strong
5 effective
Assignment From the tables, and the foregoing
section, it will be seen that the 60 skeletal bands. In the assignment, bands disallowed on the local symmetry basis are not included, and doublets are assumed to be unresolved. It is obvious that the experimental data are as yet inadequate to permit an unambiguous assignment, and force constant calculations appear unreliable. We have, however, no positive grounds for differing with the previous assignment [2] above the CH out-of-plane bending frequency. Regarding the lower frequency bands, the following comments may be made. (The bands are The 771 cm-l band is strong identified by reference to those in the ConI complex.) and constant in frequency throughout the series, and must be a fundamental located The 633 cm-l band is in the ring, possibly the second C-Me stretching frequency.* sensitive to the M atom, and is assigned as the ring frequency based on C-C-O modes will effectively give rise to very few observable
* This band has recently been grouped with the 780 cm-1 band as the C-H out of plane vibration by N-on, Infrared Spectra of Inotganic and Coordination hnpnm&, John Wiley New York (1963).
The far infrared spectra oh tris-acetyleceton&o
metal III complexes
69
bending (i.p.). This will be above the frequency in simple molecules, (~450 cm-l) and will show M atom dependence. The two MO, frequencies are readily identified at 466 cm-l and 388 cm-l from th ir strong variation with M. One of the two lowest Y observed. frequencies will be due to the Me o.p. rocking, which will be expected at a lower frequency than in nz-xylene (227 cm-l) [8]. The other very low band is assigned to one of the two ring puckering modes, which will again be expected below the corresponding frequency in m-xylene. The bands at 671 and 432 cm-l are assigned by NAKAMOTO to the Me i.p. bending modes. This seems improbable as the corresponding frequencies in m-xylene are at 404 and 206 cm-l [8]. The assignment of the lower frequencies is summarised in Table 7. Table 7. Assignment of some of the lower frequency modes
co111
C+
FeII’
771
772
770
ring mode
691
677
663
ring i.p. bend C
671 662I
658
654
/“\
C
?
L
ring i.p. bending
633
694 609
649 669
466
459 416 355
434 411 289
MO, mode
103
111
I ring o.p. end
60
60
432 386 98 60
/“\ c
c
MO, mode
r
Me o.p. rock
CONCLUSION
Quite apart from the question of the choice of an appropriate model for the basis of discussion, there is a further point in the previous treatment [2] to which attention should be drawn. When the G elements of a matrix are completely known, multiple sets of F elements can be obtained to satisfy a given set of eigenvalues. There is no a priori way of distinguishing the particular set appropriate to the molecule, and the variational procedure used in [2] can “home” on to an alternative set of force constants. This ambiguity has been discussed by FREEMAN and Ross [9], and it was therefore considered that there was little value in carrying out force constant calculations until further data, particularly regarding isotopically substituted species becomes available. Acknowledgwn.mdWe are grateful to the D.S.I.R. for a grant for the construction spectrometer, and for maintenance of one of us (H. G. S.)
[8] J. K. WILMSEURSTand H. J. BERNSTEIN,Can. J. Chmn. 85, 911 (1957). [Q] D. E. FREEMANend I. G. Ross, Spectrochim. Acta l&l393 (1961).
of the