The vibrational spectra of the copper(II) formates

Journal Ekevier

THE

ofMolecu&rSbueture. 127 (1985) Science Publishers B-V., Amsterdam

VIBRATIONAL

Part II. The i&anal

SPEtXIt.A

-20 -Printed

OF THE

in Tbe Netherlands

COPPER

FORMATES

format-e and lattice modes of Cu(HCOO), - 2H20

A. M. HEYNS Department (Receded

of Chemistry.

Uniuersity

ofPretoria,

0002 Pretoria

(South

Africa)

21 June 1984)

ABSTRACT

The infrared and Raman qxctra of Cu(HCOO), - W,O and its deuterated acalogues have been recorded and the ictemal formate and lattice modes of these compounds are reported. Some of tbe internal fonnate modes reflect the existence of two crystallographically independent formate ions These ions are arranged in an anti-unti and anti-syn way in the crystal lattice hut specific internal formate ions representing the two differently bonded groups do not show kge frequency sepaationg with the exception of the asymmetric C-O stretching mode. The fact that the frequency separation between the asymmetric and symmetric C-O stretching modes is significantly larger than in Cu(HCOO), - 4H,O and other covalent metal formates is explained in terms of different C-O bond lengths within each formate group_ Many of the lattice modes could only be assigned tentatively mainfy because of very mAI isotopic shifts of the formate modes upon deuteration and also because af coupling which occurs between tbe lattice modes as a result of the low symmetry of the different groups in Cu(HCOO), - 2H,O. The librational modes of the formate groups which have a large wagging character, can be assigned with a fair amount of certainty_ INTRODUCITON

The crystal slzucture of copper(H) format-e tekahydrate and dihydrate as well as the various modifications of the anhydrous salt have been reported in the literataue [l-S]. The vibrational spectra of copper formate tetrabydrate have received a great deal attention [!3-123 rn&Qy because it shows a large antiferroelectric anomaly at low temperatures. The vibrational spectra of copper formate dihydmk amember of the isomorphous series of compounds M(HCOO), - W,O (M = Cd, Co, Cu, Fe, Mg, Mn, Ni, Zn) 131, have not been so systematically studied. Its infrared spectra have been reported as part of a study

of mixed

crystals

of formates

with the general formula

(Cu, M(lI))

(HCOO), - 2H20 [13] _ Baraldi [14] studied the thermal behaviour of the metal formates and also reported the - r tied spectra of Cu(HCOO), . W,O_ However, the farhhred and Raman spectra of this compound have not been reported before and in the present paper the complete vibrational spectra of Cu(HaDO), - 2H,O and its deuteratxzd analogues are presented with 0022-2360/05/$iO3.30

8 1985

Elsevier

Science

Puhlkhers

B-V_

10

particular reference to the internal vibrations of the formate groups and the low frequency lattice modes. In a subsequent paper the internal and librational modes of the water molecules in CU(HCOO)~ - 2Ht0 will be reported 1151. EXPERIMENTAL

Cu(HC00)z - 2H20 was prepared in two different ways. A solution of basic copper carbonate was neutralized with a concentrated (90%) formic acid solution and the resulting solution filter& into a large excess of ether [IS] _ Pale, blue-green crystals were obtained. In a second method, basic copper carbonate was neuizalized with a 30% acid solution and the crystals were allowed to separate out. This was found to be the most successful way of preparing Cu(HCOO), - 2H30 in the present investigation. AU the samples used in the spectz-oscopic measurements wer,0 analyzed for Cu;C and H and typical value are the following. Found Cu: 33.2; H: 3.10; C: 12.5. Calculated Cu: 33.6; H: 3.17; Cr 12.7. Freshly prepared samples were used for each measurement of the vibrational spectra since the crystals tend to lose water of crystalhzation on standing_ Deuterated formic acid (Merck Chemicals) and deuterium oxide were used for the preparation of Cu(DCOO), - 2HD0 and the H and D contents in HDO were calculated from the intensities of the OD and OH stretching modes of the water molecules in the intied spectra Cu(HCOO)z - 2HDO was prepared by diluting the formic acid with varying strengths of deuterium oxide before adding these to the basic copper carbonate. The mid-infmred spectra of the samples were measured on a Perkin Elmer Model 621 spectrophotometer. The spectra were obtained in the form of Nujol mulls on KBr, CsBr and AgCl windows. It has been remarked elsewhere [ 141 that CLI(HCOO)~ - 2H20 interacts with the KBr pellets; this behaviour was confu-med in the present invest-&ation and the pellet technique was therefore not used. The far-ineared spectra were recorded as Nujol mulls on-polyethylene discs on a Beckman RIIC FS 720 Fourier spectrometer and the data were transformed using a Cooley-Tukey algorithm on a NOVA 3 computer. The Raman spectra of polycrystalline samples were obtained using a Spex Model 1401 spectrometer and the green (514.5 mm) and blue (488.0 nm) lines of a Coherent Radiation CR 3 laser were used to excite the spectra All the spectra are reported with a resolution of better than 2 cm-‘. CRYST’AL

STRUCTVRE

AND SELECTION

RULES

Cu(HCOO), - 2HI0 belongs to the monoclinic space group P2,,(C&) with 2 = 4 [4] _ The structure contains two different -kinds of copper atoms, one being surrounded in an approximately octahedral arrangement by six oxygen atoms &m~ the formate groups. The other one is also s&rounded by six oxygen atoms; however, four of them belong to the water molecules in an almost square planar shucture while the approximately octahedral coordination is completed by two oxygen atoms horn formate groups. Each water

11 molecule forms two hydrogen bonds with oxygen atoms of the formate groups. The geometry of the tit bridging formate group is an anti-anti

bonding one, like that found in the tetrahydrate [l] arranged in an anti-syn configuration [4]. RESULTS

AND

while the second is

DISCUSSION

Assuming that the formate ion has a symmetrical hybrid shvcture that conforms to CzV symmetry, the internal HCOOmodes are as follows: (the infrared and Raman bands in polycrystalline NeHCOO are in parentheses [17]), u,(A,) the C-H stretching mode (IR 2830 cm-‘, R 2829 cm-l), v2(AL) the symmetrical C-O stretching mode (IR 1361 cm-‘, R 1357 cm-‘), v3(AI) the symmetrical O-C-O bending vibration (IR 775 cm-‘, R 771 cm-‘); ~.&li) the asymmetrical C-O stretching mode (IR 1607 cm-‘, R 1583 cm-‘) v5 the in-plane C-H bending mode (IR 1367, R 1368 cm-‘) and finally vg(B2) the out-of-plane C-H bending mode (IR 1068 cm-‘, R 1073 cm-‘). In Cu(HC00)2. 2H10 there are two crystalIographically inequivalent formate groups and two sets of internal HCOO- modes car, thus be expected to occur in the vibrational spectra of this compound. In addition, each HCOOmode can theoretically split into A, + B, + A, + B, components under l?&,, symmetry. However, it has been shown that the correlation field splittings in the isostructural Zn(HC00)2 - 2H10 are not resolved in the vibrational spectra of this compound [ 181. The mid-infrared and Raman spectra of Cu(HCOO)2 - 2H10 are shown in Figs. 1 and 2, respectively. The frequencies, assignments, half-widths and relative intensities of these bands are summarized in Table 1. There can be little doubt as to the correct assignment of vl, v3, p4 and v6 of the formate ions in Cu(HC00)22Hz0. The assignment of these bands can be done by analogy to previous studies on the formate ion [lO-14, 17, 181 and by noting the hequency shifts of the respective modes upon deuteration. I+ and V~ reflect the existence of the two crystallographically inequivalent formate groups and the latter mode shows two components which are separated by as much as -30 cm-’ in the Raman spectrum of Cu(DC00)2 - 2Ht0 thus reflecting a significant difference in the C-O bonds in the two formate gr0t.p~. The modes ~i and Y6, which both involve the C-H bond, show no frequency separations between the two formate groups and it can be concluded that the C-H bond lengths differ very little in the two ions. This is also the case in the iso.structuraI Mn(HC00)7 - 2Hz0, as has been found in a neutron diffraction study 151. As far as v 3 is concerned, a study of the vibrational spectra of the isostructural Zn(HCOO), - 2H20 has revealed that extensive coupling occurs between this mode and the librations of the water molecules [lS] . The behaviour of v3 in Cu(HC00)2 - 2H20, which shifts upwards upon deuteration, just as in Zn(HC00)2 - 2Hz0, seems to suggest that this mode is also extensively coupled to the librational modes of the water molecules [ 151.

leoo

1600

1200

kcocl

IMI

-cm-‘Fig. 1. (battom)

The infrared at 100 B

Fig_ 2. The Ramanspectra at 300 K.

spectra

of

Cu(HCOO),m

of Cu(HCOO),

- X-0

2H1,0

(top)

(top)

and

and Cu(DCOO),

Cu(DCOO),

- 2HD0

- 2H!!O

(bottom)

The components of vs in CU(HCOO)~ - 2H,O occur at 1399/1393 cm-’ in the infxared spectra and those of LJ=at 1340/1330 cm-‘_ The kquencies at which these modes occur agree with those determined in other studies 113, 143 _ v5 shifts to 1030 cm” upon deuteration and v2 to 1319 cm-’ in the infrared spectra of cl~(E-ICOO),‘- 2H20 at ambient conditions. p2, the

13

qnnmetkal C-O sketching mode is expected to be the most intense vibra- 2H,O but it is evident &rn tion in the Raman spectrum of Cu(HCOO)l Fig. 2 that v5 and pz are of approximately equal intensity. This is a direct consequence of a lowering of the veky of the formate groups thus ahowing y5 (of B, symmetry under Clv symmetry of the undistorted formate group) to steal intensity from I+ (A, symmetry under C,,). In Cu(DCOO), 2HD0, ys (1030 cm-‘) no longer occurs in the same frequency region as Y? and the relative intensities of its components decrease from 78/88% in the undeuterated compound to 25% in the deuterated one. It is aIso evident tirn rFig_ 2 that three components can be assigned to + in the Raman specksm of &I(DCOO)~ - 2D10. It is unlikely that the band at 1351 cm-’ represents yz of incompletely deukrated samples and it could be that the 1326/ 1351 or even the 1319/1351 cm-’ pair represents an Ag-_Bp splitting of +. In this regard it can be noted that yz showed a significant A,-$?, splitting in CU(DCOO)~ - 4Dz0 which was not reported in Cu(HC00)2 - 4H20 [ll]. Fudhezmore, it must be noted that a reverse assignment of v2 and us has been made in the case of Cu(HC00)2 - &I20 [lC+lrZ] . In this compound the formate ions have a symmetry which can be approximated by Clv and v5 is a much less intense band in the Raman spectrum than v2 [lo]. A comparison of their spectra further reveals that Y4 moves upwards in CL~(HCOO)~ 2H20 and v2 downwards with respect to the values in the tetrahydrate. The formategroupswith ananti-anti bridging arrangement in Cu(HC00)2 2H20 were reported to have C-O bond lengths of 1.21 and 1.27 A, respectively 143. These considerable differences in the C* bond lengths have been ascribed to differences in the Cu-0 bond distances, causing one Ca bond to have more double bond character than the other [4]. A later neutron diffraction study disputed these results 153 and it was stated that the differences in the C-O bond lengths may be obscured by the statistical uncertainty in the me asurements. In this study the C-O bond lengths in the anti-anti formate groups were reported to he equal to 1.26 and 1.30 A, respectively. In view of the different Cu-0 bond lengths found in both the X-ray and neutron diftiction studies there is little doubt that at least some differences occur in the C-O bond lengths in the respective formate groups in GJ(HCOO)~ - 2H20. Vibrational spectroscopy shollld be sensitive towards differences in the C-O bond lengths within one particukx formate group and has been used in the past to establish a correlation between C-O stietching modes and the strength of M-O bonds in metal@) forma& [ 191, as well as to investigate the existence of a symmetrical hybrid structure in the formate ion [20]. The frequencies at which v2 and v4 occur in Cu(HCOO), - 2H20, correspond with those in an ionic formate such as RbHCOO [19]. However, copper formate dihydrate is not an ionic formate and it is therefore much more IikeIy that the different C-O bond lengths give rise to the Wquency shifts in v2 and y4 and the fact that their kzquencies resemble some of the enes in ionic formates is a sheer coincidence. For example in HCOOH, r= 1.228

’

ZHDO

.- -

. _

.- _ -

-loo(oa

117(42 X 10) 100(100 x 13)

_ _ -

x 17)

-1bO(100 x ao) lao(02 x ie)

-2Ob(D4 x 28) -202 (a) -187(100 x 30)

-27b (111) 248(100 X 40) 294 (811)

367(07 x 90) -82o(ea x 40) 203(Bkl X 92)

-.

x X x x

1B) 24) 80) 18)

_ _ _

108(10 x a)

2fIO(4I3X 14) zoe(os x 10) 298(07 x 10) 280(84 X 10) zlo(ae x lb) 202 (oh) lel(loo x la) 177(4E x 0) le7(4e x 7) lM(O4 x 10) 148(73 x 10) laa(ao x I) 12b (ih)

411(flLl 8e!J(80 328(00 soe(ll4

0) e) 20) 88)

1070(2 X 0) 4M(24 X 20)

x x x x

_. -

20)

-.._

_ -

._

11 ame x 14) ee(lo0 x 10)

148 (Ih) 124(efl X 0)

172(47 x 19)

lDz(ae x 18)

24 2 (I) 210(21 x 0)

826(4 X 20) 2NO(32 x 20)

422(16X

_.

1320(100x 0) 13lo(eo x 0) ifleim x IO)

1097(8x ia) -11\78((1 x lfl) le4u(a x 14) 102E(25 x I)

221,!1(e2 x 8)

Rumnn 900 K

x 90)

“.

.~

113 (ah) -100 (Ih)

..,

._

148(81 x 43) 12E(Ub X 27)

-laz(loo

241(100 X 26) -280 (sh)

-JM(eO X 80) “YlB(72 X 44) 280(71 X fle)

012(12 x 4) _.

1810(02 x ab)

..,

.

. .

_”

x x x x

12) 22) 28) 17)

.

.

em

164(OC 144(83 lao(oo 122(ee 114(64 102(ril

:.

x x x x x x

_

12) 0) 20) 10) le) 10)

2ee(e2 x 10) 2B2(00 x 20) zse(a1 x 22) llM(70 x 10) lOO(O2 x 12) 188 (oh) 181(100 x e) 172(79 x lo)

411(80 902(7i 828(Del zeo(e8

012(27 X 3) 4eb(21 X 18)

182b (ah) laio(oa x 38)

1b00(100 x 80) laar(oe x eo) 1020(76 X 4)

2210(4n x lo) 2082(<1) 202ow)

2212(49 X IO)

lb711(100 x 130) 1868 (uh) 1030(OD x 8)

100 K

YOOK

lnfrnrud

_, .

ZHDO

0 2HDO”*

Cu(DCOO),’

ond CII( DCOO),

x 148)

1070(2 X 3) 47O(B x 18)

-leeQ(loo 1401(70 iaoo(Yo 1943(Ho 1880(00

x 114)

2oae,(
“,.I m 100 K

1 ZHDO

lmr(76 x 10) iflofl(7e x ii) 1340 (111) iaao(o8 x 60)

-larJo(loo

-e

lnfrarcd

14O(l30 x ltl) iao(e0 x 22)

leo(el x 21) lOB(40 x la)

200(81 x 21) lOP(79 x 17)

zao(ea x 21)

280 (sh)

82L(4 X 20) 208(33 x ia)

487(17 x 2el)

1072(13 X 8)

~oflk.ul x 90) lcm(7 x 21) 11107(4 x 2b) ifloe(78 x 0) 1804(Btl x D) laao(le x 12) i380(100 x 0)

2014(10 x 18)

Raman 800 K

Cu(HCOO),

TIIO lnlrurotl ond Ftllmlm spcctrn of Cu(HCOO),

TABLE 1

_

_

,,.

.-

v,(HCOO-)

: v&HCOO-) v&HCOO-) v&HCOO-) VC

:::::$%q v,(HCOO-) v,(HCOO’) v,(HCOO’) 1

._

%snlHaO+ VtransHCOO’

v(Ca-0)

v(Cu-0) v(Cu-0) v(cu-0)

2% 24

“I

AkQnmsnt

Ml(60 x 3) e.l(!zO x 8) 70(32 X 2) tl6(40 x 17)

Fl4(41 x 12) 78(31 x e) OO(28 x 8)

fm(40 x 4) lll(27 X a) Ofl(22 x 4)

V,(HCOcJ’) v,(IIcoo-) y(HCO0’)

PThe telatlve intcneltlca ond holf~wldths of tha bands nta glvan in pnrenthascs, bThe telatlvo lntanaltlcs nte cnlculatad uuing the 1330 cm” band In tha Rnmnn spccttn for the bnude hlghet thnn 400 cm” and uelng the 100 cm” bend for tha bnnda occuttlng below 400 cm”, In the cnac of the lnfrnrad epacttn they nta colculatcd wlth reepcct to tho 1680 cm” bond for the spcctrn nbove 400 cm” and wlth tcepcct to tho - 187 cm” bnnde for the low4tcqucncy epecttn, aThe bnnde teportod In thle tablo nrc for anmplcs contrdnlny >96% H In tha HDO molecules In Cu(HCOO), 9ZHDO und >96% D In tho HDO molecules In Cu( DCOO), 1 2HD0, The H nnd D contents were also vntlcd to locate the low.frcquency H,O modee, but tho epectra of these eamplca ate not Included in thla tnblo, dThe lnternnl nnd llbtetlonol modes ol the wntct molecules nn well 08 the v,.mode of the formnte groupa ore not lncludcd In thle tnble. Qw ‘- wag; ut - twlet; I+ - rock; ~~~~~~ - ttnnelotione,

07(88 x ItJ)

x 0) 7e(ae X a) 07(28 X E) fn(68

16 = 1.317 A [21] and in DCOOD congeners isolated in a neon andr(;o matrix, the C=O stretching mode occurs at 1725.5 cm-’ and the C-O sketch at 1170.5 cm-‘, a separation of 555 cm-’ [21] _ In other words, compared to an ionic formatesuch as NaE-COO there is an upward frequency shift of v4 and a downward shift in Ye_ This is the same trend (although on a much smaller scale) as has been observed in the comparison of the C-O vibrations in CLI(HCOO)~ - 2HIrJ with those of Cu(HCOO)l - 4H10. It is alsO interesting to note that in the isostructural Mn(HCO0) - 2Hz0 almost equal C-O bond lengths in the two formate groups have been reported [5] and if the lower &quency component-s of v2 are considered a u4-v2 separation of 220 cm-’ is observed in this compound (considering infrared muencies) compared to a value of 250 cm-’ for the corresponding separation in C~Z(HCOO)~ - 2H20_ In samples of Cu(DC00)2 - 2HD0 where ul is not extensively coupled to v5, the LJ?Y~ splittings are of the same order of magnitude as in Cu(HC00)2 - 2H10 showing that frequency shifts of I+ due to coupling effects are not mainly responsible for the comparatively large values of the v*-v, frequency separations in this compound_

Lattice

modes

Because of the low symmetry of the format-e and water groups in the crystal lattice of Cu(HCOO)II - 2H2L7, mixing of the lattice modes can be expected to occur and consequently the assignment of the low-frequency bands will be complicated_ Furthermore, there are two kinds of formate and water groups in the lattice and l & will complicate matters even further_ Isotopic substitution techniques are an invaluable aid in the assignment of such complicated spectra. Unfortunately, the isotopic shifts of the formate modes are small and in some cases virtually identic-J to those of the water translations in Cu(HCOO), - 2H20The libration modes of each set of formate groups in the lattice of Cu(HCOO)= - 2H:O form the representation rror = 3A, + 3B, + 3A, + 3B,. In other words, a maximum number of 12 librational modes of the two different formate groups can be expected to occur in either the infrared or Raman spectra of this compound_ In order to simplify the assignment of these bands it is assumed that the librations can be divided into wagging, rocking and twMing modes, although this is not-strictly true -because of the low symmetry of the formate groups. For the pure wagging modes of the formate groups (assuming CzV symmetry), Zi!,(HCOO)/(DCOO-) = 1.191 while these ratios are equal to 1.000 and 1.024 respectively for the ?zwi&ng and rocking modes. The wagging modes should therefore shift significantly

upon deuteration. but in the case of the rocking modes (l-024), the translational modes of the water molecules (1.054). the translational modes of the formate groups (1.011) and the twisting librational modes (1.00) it will be extremely difficult to interpret the frequency shifts since they are so close to unity and also differ very little from one another It must also he

17

borne in mind that if these modes are coupled, the isotopic tiquency shifts mentioned above will be modified, making an unambiguous assignment of bands even more difficult. If the isotopic shifts of the lattice modes listed in Table 1 are noted, the only bands that shift significantly upon deuteration of the formate groups are the ones at 210 and .230 cm-’ in the far-infrared spectrum of Cu(HCOOh - 2H20 at 100 K (Fig. 3). These absorption maxima shift to -190/198 cm-’ in CI.I(DCOO)~ - 2D10 and the resultant ratios are equal to 1.105 and 1.16, respectively_ These bands are not sensitive towards H20/D20 substitution and can therefore be assigned to hbrational wagging modes of the fonnate groups. The band at 191 cm-’ in the far-infrared speckum of Cu(HCOO)a - 2H20 (Fig. 3) shifts to 181 cm-’ in Cu(DCOO), - 2D,O and from the ratio of l-055 it is difficult to assign it, but since it can be identified as a for-mate vibration it is most probably also a wagging librational mode. The same applies to the two weaker absorption peaks at 167 and 177 cm-‘. In CII(HCOO)~. 4H20 [ll] the wagging modes were observed at 186/ 176 cm” and fall more or less in the same tiquency range as some of the latter wagging librational modes in CII(HCOO)~ - 2H20. However, as has already been mentioned, only one of the formate groups in the latter compound has an antianti bridging arrangement like those in the tetrahydrate

and signikant differences can be expected between the librational modes in the anti-anti and anti-syn format-e groups. Although the frequencies differ at which the librational modes of the formate groups occur in ionic formates such as LiHCOO - Hz0 [22] and NaHCOO 123,241 or in a covalent formate such as Cu(HCOO), - 4H,O [ 111, the frequency order of these bands,

Fig. 3. The far-infrared spectza of Cu(HCOO), - 2-O

at 300 K (bottom)

and 100 K (top).

18

i.e. waggkg > twisting > rocking is not changed in the different crystals. In Cu(HCOO), - 4H10 [ll] the twisting modes occur at 165/148 cm-’ and the rocking modes at 110/102/77 cm-‘. In Cu(DCOO)z - 4D20 they shifted to 161/146 and 106/95/73 cm-’ [ll]. In Fig. 3, three prominent absorption peaks at 135, 146 and 155 cm-’ can be seen in the far-infrared spectrum of CU(HCOO)~ - 2H10 and they shift to 130,144 and 154 cm-’ in Cu(DC00)2 2H=O. The latter two can be assigned to Wg librational modes since their isotopic shifts are very close to unity although there is doubt as far as the one at 135 cm-’ is concerned. Three further prominent bands appear at 70, 89 and 103 cm-’ in the far-infrared spectrum of Cu(HCOO)* - 2H,O at 100 K. If they are assigned to rocking modes of the for-mate groups, they should shift to 68,87 and 101 cm-’ respectively in Cu(DC00)2 - 2D,O. In fact they are observed at 68, 88 and 102 cm-’ which is close enough to the predicted values in order to confium their assignment to rocking modes. In the Raman speck at ambient temperatures, the isotopic shifts of the bands at 67 and 100 cm-’ are very close to the predicted value of 1.024. None of the bands observed below 200 cm-’ appears to be sensitive to H,O/D,O substitution, however, the possibility cannot be excluded that some of the bands assigned to hbrations vibrations are actually translational modes. NormaUy the translational modes of the formate groups lie at higher wavenumbers than the hbrational ones 111, 121 although formate translational modes have been observed as low as 113 cm-’ in Li(HCO0) - Hz0 1211. However, the assignment of the hbrational modes is very tentative and an unambiguous assignment does not appear to be possible at this stage. In the frequency range >200 cm-‘, the translational :nodes of the fonnate and water molecules can be expected to occur. The absorption peak at 238 cm-’ (Fig. 3) shifts to 236 cm-’ upon deuteration and the isotopic ratio of

1.013

indicates

to the 256

that this is a formate

translational

mode.

The same

applies

cm-’ band, although it is somewhat sensitive to H,O/D,O substitution and can therefore also have some H,O translational character. The band at 308 cm-’ shifts to 290 cm-’ in Cu(DCOO)_. - 2D10 and the ratio of 1.062 shows that it = be assigned to a translational mode of the water molecules, but it is most probably coupied to a translational mode of the formate groups since it shifts slightly upon HCOO-/DCOOdeuteration. Several peaks, like the ones at 326, 363, 411 and 485 cm-’ do not shift significantly upon deuteration and can be assigned to .Cu-O modes. The two features at 485 and 411 cm-’ .in the intied spectra of Cu(HCOO), - 2H20 are prominent bands at low temperatures and are most probably due to Cu.-O stretching mode. It must be remembered that in each distorted octahedral arrangement around the copper atom there are four relatively short Cu-0 bond distances (-2.0 A) and two long ones (-2.30-2.35 A) [4,5]. This can explain the two Cu-0 bonds that are separated by approximately 70 cm-’ in the _ 1 spectza of Cu(HCOO), - 2H,O. In the Reman spectra of Cu(HCOO& - 2H20 a broad diffuse peak is observed at 437 cm-’ which shifts to 422 cm-’ upon deuteration. This band alsO represents a Cu-0

19

stretching mode, but it very likely consists of several overlapping peaks. The fact that it is sensitive to H20/D20 substitution only shows that a major contribution to this- band is provided by Cu-Hz0 interactions. In bonds in the Raman spectra were Cu(HCOO)2 4H20 1111 the Cu-0 observed in the tiquency range of 316414 cm-’ and the transIationaI modes of the water molecules from 215-233 cm-‘, which agree fairly weII with the frequency ranges where the corresponding bands were observed in Cu(HCOO), - 2H,O_ l

CONCLUSIONS

The intied and Raman bands of the internal formate vibrations can be unambiguously assigned in the spectra of CIJ(HCOO)~ - 2H20 and they reflect in most of the cases the existence of two crystallographicahy distinct formate groups. The comparatively large hequency separations which have been observed between the asymmetric C-O stretching modes of different formate groups show that the C-O bonds differ in slxength in the URtiurzti and antisyn bonded formate groups. Contradictory results about differences in the C-C bond lengths in each of the two types of formate groups have been reported in the Literature 14, 53, but the frequency separations between the asymmetric-and symmetric C-O s-hztching modes point to different C-O bond lengths in each formate group. The frequency ranges where some lattice modes occur, such as the Iibrational and translational modes of the formate groups and some Cu-0 vibrations, agree wi+A those observed in Cu(HC00)2 4H,O [ 111 _ Many of the lattice modes remain unassigned in the low-frequency spectra of Cu(HCOO), - 2H,O; however, some of them, Like the Iibrational modes of the formate groups which have a large wagging character, can be essigned with a fair amount of certainty. ACKNOWLEDGEMENT

The author thanks the COLLX-IC~I of the University for financial support.

of Pretoria and the CSIR

REFERENCES 1 R. Ktiyamq H. Ibamoto and K. Matsuo. Acta Crystallogr., 7 (1954) 482. 2 K. Okada. M. L Kay, D. T. Cromer and L Almodovar, J. Chem. Phys., 44 (1966) 1648. 3 K. Os&& Y. Nakai and T. Watanabe. J. Phys. Sot. Jpn., lb (1963) 919. 4 M. Bukowska-Stnyzewska. Acta Crystallogr.. 19 (1965) 357. 5 M. I. Kay, I. Almodovar and S. F. Kapian, Acta Crystallogr., Sect. B, 24 (1968) 1312. 6 G. A. Barclay and C. H L. Kennard, J. Chem- Sot., (1961) 3289. 7 J. R. Giinter, J. Solid State Chem., 35 (1980) 43. 8 N. Burger and H. Fuw Solid State Commun., 34 (1980) 699. 9 J. Hiraishi. Bull_ Cbem. Sot. Jpn. 45 (1972) 128. 10 Part I, A. M. Heyns, J. Mol. Struct.. 18 (1973) 471. 11 J. Beger, J. Phyr C. 8 (1975) 2903.

20

12 13 14 15 16 17 18 19 20 21 22

R. P. Canterford and F. Ninio, J. Phys. C, 8 (1975) 385. T. Ogata, T_ Taga and K Osaki, Bull_ Chem. Sot_ Jpn, 50 (1977) 1674. P. Bamldi, Spestrochim. Acta. Part A, 35 (1979) 1003. Part II& A M Heyns, J. MoL Sbuct, in press R. L. Martb and H_ Watmmam, J. Chem. Sm., 1958 (1958) 1359. K. G_ Kidd and H H. Mantsch, J. Mol. Spectrosc., 85 (1981) 375_ G. D_ Tewari, V_ P_ Tayal, D. P. Rbandelwal and H D- Bkt, Appl. Spe~tros~.. 36 (1982) 441. J. D. Donaldson, J. P. Knifton and S. D. Ron. Specbocbim. Acta. Part A, 20 (1964) 847_ E_ Spinner, J. Ckrn. Sot.. 1967 (1967) 879_ R. L. Redingtsm, J. Mol. Spectrosc., 65 (1977) 171. Y_ Has. Spectrochim. Acta, Part A, 37 (1981) 276.

23 J_ P. M. Mas, Spectrn&irm. Acta, Part A., 33 (1977) 761. 24 J. P. M. Mas and F_ KeIlendo& Spectmchim_ Acb, Part A, 35 (1979)

87.