3380
MITSUOITO AND ROBERTWEST [CONTRIBUTION FROM
THE
VOl. 85
DEPARTMENT OF CHEMISTRY, THE UNIVERSITY OF WISCONSIN, MADISON 6, WIS.]
New Aromatic Anions. IV. Vibrational Spectra and Force Constants for C,O,-’ and C,0,-2 BY MITSUOITO AND ROBERTWEST RECEIVED FEBRUARY 22, 1963 The infrared spectra of solid K2C404 and KzCsOs and the Raman spectra of their aqueous solutions have been investigated. The spectra indicate planar symmetrical structures ( Dlh and D5h) for the ions. Vibrational assignments were made on the basis of these structures and a normal coordinate treatment was carried out using a Urey-Bradley force field. The resulting force constants support the view t h a t these ions constitute members of an aromatic series.1,2
The C,,0n-2series of ions is of interest in that i t has recently been recognized as a new class of aromatic substances. 1 , 2 In 1958, Hirata and his co-workers first suggested that the croconate ion Cb06-2 had a symmetrical resonance-stabilized structure. This conclusion was based mainly on the observation that in the infrared spectra of croconate salts, no absorption occurred in the region near 1700 cm.-l normally expected for the C=O group, but instead a very broad band was found a t lower f r e q ~ e n c y . ~Similar observations on the dianion of diketocyclobutenediol, C404-2,by Cohen, Lacher, and Park led them to propose a planar resonancestabilized structure for this anion also.* This paper reports a more complete investigation of the Raman and infrared spectra for c 4 0 4 - 2 and C606-2 ions, undertaken to provide definite evidence for the structures of these anions. A normal coordinate treatment has been carried out on the vibrational data. The resulting force constants provide evidence concerning the nature of the bonds in the two anions.
Experimental Preparation of samples used in these studies is described elsewhere.1,6,6 Raman Spectra.-A Cary model 81 automatic recording Raman spectrophotometer was used to determine the spectrum of solid K2C,0, and a by weight aqueous solution of this salt. Rhodamin 5 - G D S extra and Wratten 2A filters were used to remove ultraviolet light. The results obtained are shown in the first column of Table I .
TABLE I VIBRATIONAL FREQUESCIES OF THE C 4 0 4 - 2I O N Raman. cm - I
Infrared, em.-]
259s 294w d p
Adjustment (Dih)
v4(
.4zu) out-of-plane CO bending
in-plane co bending vI4(EU) in-plane CO bending Y I O ( Bring ~ ~ )bending vll(E,) out-of-plane CO bending uZ( AI,) ring breathing vIB(EU) CC stretching va(B1,) CC stretching u6(Blg)
350m 647s dp (662)vw“ 723s p
1090s 1 1 2 3 ~ dp s 1329vw
ut
153Ovs, v. broad 1593s dp
+
VI1
ui?(Eu)CO stretching v,(Bre) CO stretching
1700vw 1794w p
Y , ( A ~CO ~ )stretching
2200w ‘’ Observed only in the solid.
vi
+
VI.,
The Raman spectrum o f KyCsOi was obtained on a 7c/c by weight aqueous solution, usilig a spectrometer a t the Mellon Institute in Pittsburgh, I’a.7,8 Because croconate ion has a yellow (1) P.
West, H Y . N i u , D. I,. Pmvell, and X I . V. Evans, J
A m . Chem.
SOL..82, 6204 (1960). (2) R . West and D I.. P o w e l l , ibid , 86, 2.577 (1963). 1 3 ) K Yamada. N . Mizuno. and Y Hirata, BirU Chern. S O C J. a p a n , 31, 543 (1968) (4) S Cohen, J . R. 1,acher. and J D . P a r k , J A t n . Chem. Soc., 81, 3480
( I 939) (5) (6) (7) (8)
R \Vest, H Y Niu. and hl. I t o , ibid.. 86, 2584 (1963). H Y S i u , Thesis. University o f U’isciinsin, 1062. F A 3liller and R G Inskeep, J C h r f n P h y s , 18, 1519 (1950). F A hliller, I,. R Cousins, and R . B H a n n a n , i b i d . . 23, 2127 (1965).
color in solution, the 5461 A. mercury line was used for excitation. Other experimental conditions were as described by Miller and Carlson.@ Results are given in Table 11.
TABLE I1 VIBRATIONAL FREQUENCIES OF THE CsOj-2 ION Raman, em.-’
Infrared, cm. -1
248s 350vw 374x11 555s d p 637m p 795w 1lOOw 243m dp
591s d p 718w p
Assignment (Dsh)
vd(A”) out-of-plane CO bending
~ ( E I ’in-plane ) CO bending Y I I ( E*’)ring bending Y Z ( AI’) ring breathing v1
+
VI1
v6(El’)
cc stretching
vl0( E*’) CC stretching
1325w 1570vs, v . broad vz(EI’) CO stretching ~ g Ez’) ( CO stretching 1660w Yli V6 v l ( A l ’ ) CO stretching 1740w vZ Y6 2120vw V6 VI1 3150vw vj v g or 2 X v 5
+ + + +
Infr‘ared Spectra.--A Baird model 4-55 infrared spectrometer was used for the sodium chloride region, a Beckman model IR-4 with a cesium bromide prism was used for the region 660-300 cm.-’, and a small grating instrument a t the Mellon Institute was used for the region 300-185 cm.-1.10 The spectra of the solid potassium salts were observed as mulls in Kujol. The results obtained are given in the second column of Tables I and 11. The spectra of K2C,Oa and K2C505are very similar and are quite simple, showing only four strong bands over the ,entire range from 200 to 4000 cm.-l. The band near 1,500 cm.-I 1s very strong and extremely broad; this type of absorption is characteristic of salts of the C,0,-2 ions. The infrared spectra of SasC104and Sa2CaO6were also studied briefly. These spectra were generally similar to those of the corresponding dipotassium salts, but contained a few extra bands. I t therefore seemed likely that the local symmetry of the anions was lower in the sodium than in the potassium salts, and it was assumed t h a t the spectrum of the latter would most nearly represent t h a t of the anion itself. Vibrational Assignments.-Assignments were made on the basis of D4h and Djh symmetry for C404-zand csOj-2,respectively (see Discussion section). The distribution of normal vibrations among various symmetry species, their approximate vibrational modes, and the selection rules are given for CdO4-’ and CSO,-~in Tables 111 and I\‘. ( 1 ) C 4 0 J - 2Ion. AI, Species.-The totally symmetric vibrations are easily identified from the polarized Raman lines which fall a t 723 and 1794 cm.-l. T h e former is assigned to the ring breathing vibration v Z and the latter to the symmetrical CO stretching vibration v i . A totally symmetric fundamental vibration usually gives a strong Raman line because it is generally accompanied with a great change in polarizability. However, in the C404-*ion, the v1 vibration of 1794 em.-‘ was observed as a rather weak Raman line. An unusual weak Raman intensity was also observed for the corresponding totally symmetric vibration of the CsOs--2ion. BI, Species.---The vibrations in this species, v j and Y 6 , are active only in the Raman spectrum and the corresponding Raman lines should be depolarized. A strong Raman line of 1123 cm.? is reasonably assigned to the C-C stretching vibration us. The in-plane CO bending vibration, Y 6 , will be expected to give the lowest frequency in the Raman spectrum, and a Raman line of 294 c m - I was assigned to this cnode. (9) F A hliller and G. L . Carlson, Speclvochtrn. A d o , 17, 977 (1961). (10) F A Miller. G L. Cnrlson and W .B White, ibid , 16, 709 (19591
VIBRATIONAL SPECTRA FOR C404c2 AND CEO6-*
Sept. 5 , 1963
TABLE I11 NORMAL VIBRATIONS OF THE Ca04-' ION Selection rule
Number of normal vibration
(PI
VI
R
Vibrational mode
Symmetrical CO stretching Ring breathing In-plane CO bending Out-of-plane CO bending CC stretching In-plane CO bending Out-of-plane CO bending Ring twisting CO stretching Ring bending Out-of-plane CO bending CO stretching CC stretching In-plane CO bending
VP
Inactive I R R (dp)
V3
u4 YO
Yfl
Inactive
Vl
V8
R (dp)
VU Vin
R (dp) I R
VI1 u12
U13 Ul4
TABLE I\' KORMAL VIBRATIONS OF
Symmetry species (Dsh)
Selection rule
Ai '
R.(P)
A2' Ai" El'
Inactive I.R. I.R.
Vibrational mode
V3
V4 u5
V6
VI
u9 y10
u11 VI2
uI3 u14
TABLE V SYMMETRY COORDINATES FOR I N-PLANE VIBRATIONSOF c,o,- 2 10s Symmetry species ( D i h )
AI8
BZe
Sj Ss
(AS, - ASP
S3
S4
Si
E"
=
1/2(41
- $4 - 44)
According to Califano and Crawford,14 for species containing redundant coordinates, symmetry coordinates were chosen orthogonal to them. The Urey-Bradley potentials can be expressed in the form For C404-z 4
2V
4
+
[Kco(Asi)z 2
=
i = l 4
2 i = l
4
+
+
+
4
H c c o ( ~ o A p , ~ P2 i = l -
)'I
+2
4
H d t o A ~ y t ) ~2
i = l
4
KcJAti)'
Hcco'sazAPi i = l 4
+
i = l
i = l
Kcc'toAt,
4
+
Kco'soASi
+
Hcec'tozAa,
+
i = l
4
+
Foo[(A~o,cL+1)2 i = l
+
F c o ' ~ c o ~ A q +c1, o , Aqe,oi
I
+
i = l A
A
THE
Symmetry coordinate
+ ASP+ As3 + As4)/2 (AI1 + At2 + A h + At4)/2 + ABz + AB3 4(At1 At2 + At3 - At4)/2 so(APi + AB3 - AB4)/2
SI S 2
A28 Bl,
81
C j 0 5 - 2I O N
Symmetrical CO stretching Ring breathing In-plane CO bending Out-of-plane CO bending CO stretching CC stretching In-plane CO bending Out-of-plane CO bending CO stretching CC stretching Ring bending In-plane CO bending Ring twisting Out-of-plane CO bending
u1
Inactive
Out-of-Plane CO Bending Vibrations.-Two out-of-plane CO bending vibrations are expected to appear in the spectrum. One of them is vl1 of species E, and the other is V? of species .42". The former is Raman active and the latter infrared active. A very weak Raman line of 662 cm.? which was observed in the solid state is tentatively assigned as V U . In the CH3COO- ion, the corresponding out-of-plane vibration was found a t 616 cm. -'.I1 A strong infrared band of 259 cm.? which remained unassigned is tentatively assigned to u4. The assignments thus obtained are also summarized in the last column of Table I . ( 2 ) Cj0,-2 Ion.-On the bases of the close similarities in the selection rules and the spectra between the C O - z and CjOj-z ions, the vibrational assignment for C,0j-2 was carried out along the same lines as indicated above for the CdOa-2 ion. The assignments obtained are given in the last column of Table 11. Normal Coordinate Treatment of In-Plane Vibrations of the C40r-l and C,0j-2 Ions.-Normal coordinate treatments for the in-plane vibrations of these ions were carried out using the basic Urey-Bradley force field. The standard FG matrix method was used for the calculation of the normal coordinates.1z The internal coordinates are shown in Fig. 1. By using these internal coordinates, sets of symmetry coordinates were constructed on the basis of Dah and Djh symmetries. These are given in Tables V and V I , where the coordinates pi have the same meaning as defined by Crawford and Miller,I3 t h a t is 8 2 = 1/2(43
u2
E?''
THE
Number of normal fundamental
2581
Sa Sg
(AS, So(
AB1
AB4)/2
-
ABz
+ AS? - As~)/Z + Aa, - A a 4 ) / 2 (As, - A s , ) / d Z (At, - At3 + A t 4 ) / 2 4 - to(Aai lo(Aa1
- Aa2 At2
Aa2)/2
Sin
d A B z - AB4)/d2
Bzg Species.-There
are two vibrations in this species, vg and A depolarized Raman line at 1593 c m - ] is readily assigned to the CO stretching vibration u g . Only one Kaman line with a moderate intensity remains unassigned, a t 647 cm.-', and it may be assigned to the ring bending vibration u1o. E, Species.-The vibrations of U I Z , V I 3 1 and U14 belong to this species and they are active only in the infrared spectrum. The very strong and very broad infrared band centered at 1530 cm.? and a band at 1090 cm.-' should be assigned to the CO stretching vibration v I 2 and the CC stretching vibration V13, respectively. One of the two strong bands a t 259 and 350 cm.-' should be.ul4. According to normal coordinate calculations for the CaOa-' ion, v14 always has a higher frequency than ~6 for sets of reasonable values of the force constants. Therefore, the 350 cm.? band was tmtatively assigned as u14. u l o , and they also should give depolarized Ranian lines.
(11) K . Iiakamura, J . Chem. SOC.J a p a n , Pure C h e m . S e c t , 1 9 , 1411 (1958). (12) E . B. Wilson, J . C. Decius, and P . C . Cross, "Molecular Vibrations," McGraw-Hill Book C o . , Inc., New York, PI'. Y , 1955. (13) B. I>.Crawford and F . A . Miller, J . Chem P h y s . , 11, 249 (1949). ( 1 4 ) S . Califano and B . L. Crawford, Spectrcchim A c t a , 16, 889 (1960).
MITSUOTTO
2582
AND
ROBERT WEST
Vol. 85
S y m m e t r y species 1 Dah) AI'
SI
s2 Ai' El
s 3
'
S4 SS
SS
El'
s7 SS
s9 SI0
6
=
72"
n-here IC,,,, K,,, H,,,, and H,,, have the usual meanings, F,,,, F,,, and F,, are repulsion terins between nonbonded atoms, S O and to are the bond distances of the CO and CC bonds, and quo, q,,,, and qcc are the distances between nonbonded atoms. The linear terms are indicated with a prime. The potential functiqn was rewritten in terms of the internal coordinates, and the F matrix obtained was factored using t h e sanie symmetry coordinates as employed for the construction of the symmetrized G rnatrix.16
TABLE 1-111 FREQUESCIES FOR
C A L C U L A T E D AND O B S E R V E D
TIONS OF T H E
Species (Djh) A1
'
Number of normal vibration
YcnIcd.
cm.-l
em.-'
%
VI1
1718 637 1570 1100 374 1591 1243 555
VI2
..
1652 655 1605 1087 372 1584 1223 558 357
-3.8 +2.8 $2.2 -1 2 -0.5 -0.4 -1.6 +0.5 ...
VI
v5
V6 u7
E2 '
VG PI0
Fig. 1.-Internal
coordinates
In the calculation, the usual assun.ption was made t h a t linear terms are -0.1 of the corresponding quadratic Bond lengths arc not ayailable for theic ions, and so distances SO = 1.22 .4.and t i , = 1.47 A . \I rre assumed. I t was found t h a t small changes in t h e bond distanccs (10 not greatly affect the calculated freqllencies. The force coristants Mere refined by the !east-squares rnetliod.17 T h e calculated results are given in Tables VI1 and
the atoms are in the same plane. In models 11-V, all of the C-C bonds and all of the C-0 bonds are assumed to be equivalent; atoms lying above or below the plane a r e indicated by plus and minus signs. The symmetrical planar model V, of point group Ddh,has the highest
TABLE 1'11 CALCULATED A ~ ORSERVED D FREQUESCIES FOR IN-PLANE \-IBRAT I O S S OF THE
Cjoj-*
D2 L1
uohsds
%nlcd,
ern
1794 723 1123 294 1593 647 1530
ioun 3 50
m
ION
cm
-'
Difference,
%bedf
VP
E, '
I N - P L A N E VIBRA-
c ~ o sI-O~N
-1
1778 730 1128 299 1633 635 134 1070 32
Difference,
%
-0 9 +1 0 5 +l 7 1 2 5 -1 9 +1 0 -1 8 +1 1
+@
1.11I together with the obser\red frequencies. Agreement between ohseryed antl calculated results is very good, Maximum deviation o f the ca1cul:ited frinr the observed frequencies is less than 3 ( , , for C , 0 1 -and 2 about Jf,,, for CiO-.-z. The force constants used in the calculations aro liitetl in Table X .
Discussion Structures of the and C605-* Ions.-Some possible structures for C D - * are shown in Fig. 2. In model I one of the C-C bonds is shorter than the others, and two of the C-0 b o d s are shorter than the other two. This model helongs to point group CW if all ( 1 5 ) .1 R Scherer and J Ovcrcnrl. S f i ? < ~ r i i c h i nAciri, ~. 11, 7 1 9 (lTorin
