Vibrational spectra of compounds in the iodine pentoxide-water system and sodium iodate

Spectrochimic8Acta, Vol.26A,pp.1076to1992. Pe~osPrea

1070.PrIntedinNorthernIreland

Vibrational spectra of compounds in the iodine pent&de-water system and sodinm Mate P. M. A. SEEBWOODand J. J. TURNER University Chemical Laboratory, Lensfield Road, Cambridge (Recedved

28 December 1969)

Ah&&-The ix&a-red spectra of iodine pentoxide, the third hydrate (HI,O,), iodic aoid (HIO,), and sodium iodate, have been examined at room temperature and 77’K in the range 4000-40 cm-r. The Raman spectra of iodinepentoxide and the third hydrate have been assigned, and lattice modes discussed. A polymeric structure, rather than the monomeric structure previously suggested, is thought to give a better interpretationof the far infrared and Raman speotrum of iodine pentoxide, as well as fitting well into the general structural trends for this system. Some of the bands in the vibrational spectra of these compoundshave been reassigned. INTRODUCTION

THE infrared spectrum of iodine pentoxide (IaOe) has been studied by various investigators [l-4]. We have examined the Raman spectrum, and the in&-red spectrum below 280 cm -1, for the first time, and this has allowed us to make a more complete assignment of the spectrum and a further discussion of structural possibilities. The crystal structure of the third hydrate of iodine pentoxide (H&O,) has recently been determined [5], and we have examined the infrared spectrum in the range 4000-40 cm-l, as well as the Reman spectrum, which has aliowed us to make a much more detailed analysis than previous studies [2,4]. We find that an earlier Raman spectrum ascribed to I,O,, was in fact due to HI,OB. Previously published spectra of iodio acid [l-4, 0-101 show some marked differenoes, and therefore we have reexamined the infrared spectrum of iodic acid, and have compared our results with those of a recent Raman study [ll]. We have examined the infra-red ape&rum of sodium iodate in the range 4000-40 cm-l, and reexamined parts of the Reman spectrum, and compared our results with previous infra-red studies (which did not go to frequencies lower than 300 cm-l) [4, 6,12, 131, and Raman studies [ll], with particular reference to the assignment of the lattice modes. [l] C. Duvnr. and J. LECOXTE,Rea. Truv. Chim. Paye-Baa 7Q,523 (1960). [2] T. DUPUISand J. LECOMTE,Cwt. Rend. aS2, 27 (1901). [3] SadtlerStarada&Sp&ra, Sadtler Research Laboratories. [4] T. DUPUIS,Milcrochim. Actu. 157, 289 (1962). [a] Y. D. FEIKEM~and A. Vos, Acta Cryet.Xl, 769 (1966). [6] W. E. DA~ENTand T. C. WADDINGTON, J. Chem. Sot. 2429 (1960). [7] K. Nmo~o, M. MAR~OSHES and R. E. RUNDLE,J. Am. Chem. Sot. 77, 6480 (1956). [S] J. H. WISE and H. H. HANMAN,J. Imrg. Nud Chem. 23, 31 (1961). [9] L. COUTURE-MATEIEU and J. P. MATEIEU,Coqnt. Rend. @4,2185 (1952). [lo] L. COTJT~RE-MATHIEU, J. A. A. KETEZMR, W. VEDDERand J. FAERENFORT, Phyti 18 762 (1962). [l l] J. R. DURIQ,0. D. BONNERand W. H. BREASEAI~, J. Phys. C&m. 69, 3886 (1966). [12] F. A. M~LIGRand C. H. WILxINS, Anal. Chem. @, 1263 (1962). [13] C. ROCCEICCIO~L, Conapt.Rend. $350, 1232 (1960). 1976

1976

P. M. A. SHERWOODand J. J.

TURNBR

EXPERIMENTAL

The iodine pentoxide was prepared from the commercial sample by heating in an oven at 270% for some days (It has reoently [lb] been shown that the commercial sample is almost entirely HI,Os.) A sample was quickly transferred from the oven to a desicaator, and allowed to cool. The sample was then rubbed gently between caesium iodide plates which were clamped in a copper block attached to a Dewar, the whole being inserted into an evacuated vessel fitted with outer caesium iodide windows. The iodide pentoxide, which is very moisture sensitive [14-J,was thus protected from the atmosphere and could be cooled to liquid nitrogen temperatures. It was found that care had to be taken in rubbing the iodine pentoxide between the caesium iodide plates as vigorous rubbing and particularly warming resulted in the formation of caesium iodate.* Attempts to prepare a potassium bromide disc under 8 tons pressure caused complete reaction to form potassium iodate, with the liberation of iodine. Crystals of H&O, were obtaiued by reaction of AnalaR(68%)nitrio acid with iodine pentoxide [l&J (prepared as above) brought to boiling in an oil bath (at about 160%). The supernatant liquid was decanted into another beaker iu an oil bath and the solution allowed to crystal&e as the oil bath oooled. The crystals were stored in a desicoator, as they slowIy react with moisture to give iodic acid. The purity of the prepared ctompound was checked by calculation of the unit cell parameters by ala, split&ii 1161using Cu radiation, and the results gave excellent agreement with those of earlier orystallograpbic studies[6] of HI,Os. The infrared spectrum was studied using the same technique as that used for iodine pentoxide. The i&a-red spectrum of iodic acid was examined using the commercial AnalaR sample supplied by Hopkins and yearns. Iodic acid was also found to react with caesium iodide and potassium bromide,? and so the infra-red spectrum was studied using the same technique as that used for iodine pentoxide. The spectra in the range 4000-260 cm-l were run on a Perkin-Elmer 225 spectrophotometer (IaOs) and a Perkin-Elmer 521 spectrophotometer (HI,O,, HIO, and NaIO,); aud in the range 406-40 cm-1 were run on a Research and Industrial Instruments FS720 Interferometer. The Peru-Elmer 225 and 521 s~ctrome~rs were completely purged to remove moisture, and the FS720 was evacuated. When examining the far infra-red speotra (400-40 cm-l on the FS720), the samples were made up into a polythene (Rigidex) disc, and also, in the ease of iodiue pentoxide, as a suspension in para wax (heated to the decomposition temperature and allowed to cool) on s polythene disc (which became coloured by the evolved iodine). The polythene disc was clamped in a copper blockin an apparatus similar to that described above, and fitted with polythene windows to enable examination of spectra at liquid nitrogen temperatures. * The spectrum of caesium iodate has been run for comparative purposes, and is shown in Table 10. t Previous resultsusing KE%rdiscs of HIO, [3,8] end IaOa[3] are viewed with doubt since the spectra appear to correspondto HIO, + RIO, + H,O. [la] K. SXCLTB and A. KJ.XSHUS, Acta. Chma.Scmd. aS, 3309 (1968). [15] E. MOLESand A. PARTS,An&a Real Sot. E”ryKn.X.Y. Q&n. 8l, 1324 (1933). [16J N. W. ki.LOOCK and a. MM.hELDRICK, Aolcs.@yat. a8,35 (1967).

Vibrational spectra of compounds in the iodine pentoxide-water system

1977

Sodium iodate was studied by similar methods, using the commercial sample supplied by J. Ross Chemicals (formula NaIO,). The Raman spectra were obtained in a Gary Model-81 Raman spectrometer fitted with a neon-helium laser source. Spectral slit widths of 15-3.0 cm-l were used to record all Reman spectra. Deuteroiodic acid was prepared by the addition to heavy water of iodine pentoxide prepared as above. DISCUSSION 1. Sodium iodate Previous infrared studies have covered the range down to 650 cm-l [12], 400 cm-l [6] and 300 cm-l [4, 131 and 220 cm-l [17]. We have examined the infra-red spectrum down to 20 cm-l and re-examined the Raman spectrum with a laser source and under conditions of greater gain than in a previous study [ 111. Table 1 summa&es Table 1. Vibrational frequencies (4000-20 cm-l) and assignmentsfor sodium iodate Thin work 293 and 77’K (IA.)

DASENT and WADDINOTON [a] muus-20°C (I.R.)

MILL= and WILKINS [12] 20-z (I.R.)

796 774 760

800 775 767

DUXIQ et al. [ll] 20% (Reman) 817

792 777 767 392 366 333 266 244 228 203 187 169 137 88

(270) (262) (240, 231) (210) s (194) b (177) (143) s (92, 84)

66? 46,38,30?

774 764 373 366 332

Thin work 2o”c (Raman) 814 789 776 756, 741

228 220 199 I68 132 108,92 76

198 169 132 107. 92 77 66

Assignment Combination %b VI. Vl VI V4. Combina%n Rotational lattice Combination Rotational lattice Combination Rotational lattice Translational lattioe Translational lattioe Translational lattice Translational lattioe Translational lattice

Figures ( ) = at 77’K. 8, shoulder; b. broad.

the frequencies and assignments (see below), and Figs. 1 and 2 show the far i&a-red spectrum and relevant parts of the Reman spectrum. The crystal structure of sodium iodate is well known, and has the orthorhombic space group P,, * (Owls in the Schiknlies notation), with four molecules per unit cell. The character table for _D,i6 is shown in Table 2, which also lists the number of infra-red and Raman active vibrations in the unit cell. The IO, ions have site symmetry C,, and the correlation table for this system is shown in Table 3. Thus, for example, the degenerate, essentially I-O stretching mode (Ye)of the free ion is * In the original paper [18] the space group is given as P,,&. This means that in order to obtain the conventional P,,,,,, the following axis changea are necessary; a(P,,,) = c(P,,,); WPTm*) = a(P,,,): C(Pmnb)= Wmm). [17] M. PARODI, f%fn@. Rend. 265, 607 (1937). [18] I. NARAY-SZABOand J. NEUGEBAUEB, J. Am. Chm. 800.89, 1280 (1947).

1978

P. M. A. SHERWOODand J. J. TTJBNER

40

I60

280 cm-’

Fig. 1. Far in&-red spectrum of sodium iodate. Key- = room temp; --= room tamp s&e expanded; - . - ’ - = 77“K; - + . - . . = 77’K scale expanded; . . . . = background.

I

40

I

I

I20

’ ’ e!o

cm-’

Fig. 2. Raman spectrum of sodium iodate.

split by the static field into Y, and Ygb,respectively symmetric and antisymmetric in the site plane of symmetry. Coupling between the four molecules in the unit cell leads to four vibrations for each of yQa%$&(Correlation field splitting). The modes in the unit cell, additions1 to those derived from yl, Q, llg, yp are due to motions of the ions aa a whole. Comparison of the previously published Rctmrtn spectrum [ll] with infrared results causes difficulties in interpretation, so we have re-examined the Reman

Vibrational spectra of compounds in the iodine pentoxide-water system

1979

Table 2. Character table for D til6 group ad infra-red and Raman active vibrations E -4, %

1 1 1

B 10 Bw

1

-4”

u(coset)n, v(coaet)a u(coset)a-v

1 1 1 1 20 8 4

h,XB’nt

60

h,xR’T

3

B 1” B *w B I)”

h+‘T’ h,,,yR’R

%s

0,

1 1

-1

sorew screw

-1 -1

%a

screw

1 1

1

1 1 -1 -1

1 1

1 -1 1

-1 -1

-1 0 0 0

0 -1

21

1

12

0

1 0 0 0

0 0 0

0 -1

0 -1

1 0

1 1 1 1

-1 --I

-1

1 0

%a

i

-1 -1 -1 -1

glide

a,

1 1

-1

-1 -1 -1 -1

4 4 0 -12 -3 -9

1 1 -1 -1 1

1 1 0 0 0 0 1

-1

0

-4

1 8 4 4 8 1 3

-1 0

%a

glide

n,

T

702 6 0 102 1 601 -1 8 0 11014 1 813 -11014 0 0 0 0 1 -1 0

T’

1

-1 -1

1

4

R’ 14 2 1 2 2 1 2 1

nl’

Raman a a a

2 4 2 2 4 2 4

; f I a = f =

I.R.

f f f f f a a e allowed forbidden

In Table 2, v(coset) refers to the number of unshifted atoms, d refera to the number of groups and (a - v) the number of groups capable of rotation, h, the number of opexatione in each claw, XB’ the character of the operation R in the representation, and nj the total number of modes. T the number of acoustical (and therefore i&a-red inactive) modes, I’ translational lattice modes, and R’ rotational lattice modea, and n,’ the number of internal modes. Thus [21] Table 2 shows the numbers of lattice and internal modes.

Table 3. Symmetry correlationsfor Oslo factor group

Activity

unit cell (4 molecules, D,,ls)

Ram&n Rrtman

a ea

I.R. I.R.

1U

;+-

Site One 10, ion 0,

A’

Free IO,- ion c 30

Al(+

~3)

bl, %' %a~ Y4a)

3u A2

RamRaman Inactive I.R. * C,

1

30 :> u

A” 5

Jw,,

yp)

(ash,v4b)

2u

plane of symmetry is in 2%plrtne.

spectrum in this region (Fig. 2). Since only one of the correlation field components of v3bis infrared active (Table 3), this vibration is assigned to the band 792 cm-l in the infra-red. The band at 777 cm-l is probably rs, (the two correlation field components were, presumably, not resolvable). The Reman active components can be assigned as 776 cm-l [~.&4~, _&&I, and 789 [$$&&, B,,)]. The r1 vibrations are assigned as 767 [infre-red active (B,,, B,,)], and 755 [Raman active (A,, Bar)]. The bands at 741 and 814 cm-i might be assigned as combination bands. Another possible interpretation can be obtained if the correletion field splitting is assumed to be large and not small, as supposed above. If the correlation field splitting is large, each of the symmetry species listed above will give bands separsted by the magnitude of the correlation field splitting. Thus if correlation field splitting is lsrge r1 might be the two peaks at 741 and 765 (rather than a single peak st 755

P. M. A. SHERWOODand J. J. TURNER

1980

for small correlation field splitting). Ye, might be the two peaks at 776 and 755 (causing overlap with Ye),and ysbwould be expected to split into three, which might be 814,789 and 755 cm-l. If this interpretation were correct correlation field splitting could be as great as static field splitting, which has been suggested for some systems [19,20]. The intensities in this region may be complicated by intensity borrowing. The intensities in the region 400-300 cm-l were found to be very similar in both the Reman and infrared spectra, with the exception that the 355 and 332 cm-l bsnds in the Raman spectra had an inverse intensity relationship with similar bands in the i&a-red. The differences between the Raman and h&a-red values for y2 can be seen to be appreciable, a result that would be expected for appreciable correlation field splitting (since Table 3 shows that no band is both infra-red and Raman active for his compound). The peaks in this region were not resolvable, though intensity borrowing may confuse the situation. The far i&a-red spectrum is illustrated in Fig. 1. Peaks below 40 cm- l, though they may be real, could not be distinguished from noise. The assignment of transla;tory external modes is difficult, since other weak bands due to combination bands may occur, though these often form a whole series of overlapping bands. The factors that contribute to the shifting of these bands with temperature can be found discussed elsewhere [22]. The absorption intensity of an h&a-red vibration may be expressed as : I = GAMa where AM is the dipole moment change. In the case of rotation of an iodate group about its three axes, the dipole moment and polarisability change will be greater about two s,xes (y, and x)*, than about the third (z)* (which has a negligible vslue). Four out of a total of twelve rotations (Table 2, column R’) would therefore be expected to be of negligible intensity. Rotations about the x axis which are symmetrical with respect to the three screw axes will be antisymmetric with respect to inversion and must therefore belong to the infrared and Raman inective A, class. Since the remaining rotations about the x axis must be either symmetrical (Bin, &), or antisymmetrical (B,,) with respect to the inversion centre, two Raman and one in&-red mode, although symmetry allowed, are expected to be of negligible intensity. By comparison with other systems which also involve mostly movement of oxygen stoma, the sctive rotational modes are expected to be sround 200 cm-i. Translationd modes involve IO,- and Na+ ions, and the polarisability and dipole moment change will therefore be large on translation ; the intensity of translational modes of neutral molecules depends on molecular polarisability or coupling with rotational modes. The frequency of translations,1modes depends on the mass of the translating entity. An estimate for the position of the translrttionrtlmodes csn be * Refers to the P ncnospme groupin which the z axis is parallel to the IO,-

2 symmetry

Cl91D. WI C. WI

conventional molecular

axis.

A. Dows, J. Chma. Phya. 99,484 (1969). A. SWENSON,W. B. PERSON,D. A. Dows snd R. M. HEXER, J. C?um. Phys. 91,1324 (1969). S. BEAUAVANTAMand T. VE~TARAYUDU, Theory of groups and ita application to physical

problems. Andhm University, W&air A. SHEXtWOOD, to be published.

P. M. CQQI

(1948).

Vibrational spectra of compounds in the iodine pentoxide-water system

1981

obtained by comparison with the lattice mode (transverse optical) of sodium iodide which occurs at 116.5 cm-l. The translational modes of sodium iodate would thus be expected to be centred about this position. Observable modes in the Raman spectrum should number 4 (i.e. 6 less 2 of negligible intensity) rotational and 6 translational modes, and in the infrared spectrum should number 3 (i.e. 4 less 1 of negligible intensity) rotational and 11 translational modes. In the i&a-red the bands at 244,203 and 169 cm-l may be assigned to rotations, and the bands at 137,92,84,55,46,38? and 30? cm-l may be assigned to translations, overlap probably obscuring the observation of all 11. The bands at 256, 228 and 187 cm-l may correspond to combinations of rotational with translational modes. In the Raman the bands at 228,198,159 and 132 cm-l may be rotational modes, and the bands at 107, 92, 77 and 66 cm-1 may be five of the six active translational modes. While we agree with DURIQet al. [ 111, that there are six rotational and six translational Raman active vibrations, we suggest that one of these will have negligible intensity, and feel that the authors are not justified in assigning the bands at 76, 158 and 199 cm-l as rotational modes, on the assumption of a small amount of coupling, and assigning the other modes as combinations with translatory vibrations for the following reasons; (a) there appear to be the right number of infrared and Raman bands on the basis of coupling; (b) rotation about the x axis is expected to give rise to vibration of negligible intensity, thus two of the three ‘no coupling’ vibrations are expected to be more intense than the other; (c) there is some evidence for appreciable correlation field splitting in the internal vibrations. The sodium iodate vibrational spectrum illustrates the difficulties encountered in assigning the modes of a simple monomeric substance; however we will attempt to explain the spectrum of iodine pentoxide on the basis of a monomeric structure using the same approach. 2. Compounds in the iodine pentoxide-water

system

Figures 3-5 compare the spectra of &OS, H&.0, and HIO, at both room and liquid nitrogen temperatures. Tables 4-6 summa&e the frequencies and assignments. 2.1 Iodine pentoxide. Recent X-ray crystallographic studies [14] of iodine pentoxide have shown that it probably has the monoclinio space group P2lc (C&Rin Schijnflies notation), with four molecules of I,O, per unit cell. The atomic positions have not yet been determined, though by consideration of the character table of C&,*(Table 7) on the basis of the monomeric (IOBO-IO,) structure of DWAL [l], it can be seen that in order to make sense of the table (i.e. to avoid finding f&&ional numbers of vibrations), atomic positions must be constructed so that m1is a whole number. The required situation could be achieved by placing four or multiples of four atoms on inversion points (i), and/or C, axes, but his would give a crystal structure with a statistically unlikely arrangement. The more likely arrangement with no atoms being unshifted by i and a, (glide) operations has been therefore adopted in Table 7. Table 7 gives an idea of the number of i&a-red and Raman active translations and rotations (vide infra).

P. M. A.

1982

SHERWOODand J. J. TURNER

r.t. scale expd. (20°C) a

b

C

I

40

I

I

1

IA0

A0

1

I

4bo

cm-’ Fig. 3. Far infm-red spectra of (a) I,O,, (b) H&O,, and (c) HIO, at various = room temp. (20%); --= room temp. (20%) temperatures. Key-.-.-. = 77OK; _ . . _ . . _ = 77’K SC+ expmnded; . . . = scale expanded; baokground.

Vibrational SpeCtr8 of compounds in the iodine pa&oxide-water

system

20% r.t. a

Id00

rt~o’ C

I

600

IGO

400

cm-’

sbo ’

7do cm-’

’

u do

’

660 cm-’

560

I

340

’

I205

’

460

Fig. 4. Near infia-red spectra of (a) RIO,, (b) H&O,, and (c) 120, at v8rious = liquid nitrogen terntemperatures. Key- - - - = room temp. (20%); pereture (in the case of ISO6 there ~8s no difference between the liquid nitrogen temperature and room temperature spectrum in the mnge shown in Fig. 4).

1983

P. M. A.

1984

do

I

I

I

I

4bc

and J. J. TURNEB

8HEaWOOD

I

I

I

I

340

1

I

I

cm-’ Fig. 6. Expanded in&-red spectrum of I,O, at various temperatures. Key--= room temp. (20%); = 77°K. Table 4. Vibrational frequencies (4000-40 cm--l) and assignmentsfor iodine pentoxide (assuming8 polymeric structure) This work 77’K (@ureein braokets at 2OYz)

Infra-red DUPUIS and LECOBSTE [2] 2ov

DUVALEII~ LECOMTE[1] 20%

Irs(I=o) V#(I=O) vs(IC,) R&s(IO*)

688 697

690 604 630

420

428 400 374, 320 296 286, 190, 174 144 104 94 77, 42 34

vas(I0,) Vsa(ICI) Combination maxima [22] vs (ICI) vs(ICI) I=0 wagging IO, deformation IO, Jwghg IO, twisting I out-of-plane torsion (inoludss IO,, IO rooking)

834 826 800 747,737 726

860 822

676 696 610 shoulder

670 697

419 397 366, 348 329 306 ~280

Assignment

840 828 806 720, 748

832 826 800 762 broad

422 390 364, 346 328, 324 307 288. 280, 233. 212 187 166 128 (122) 112, 108 (107) 91, 86 (78) 60 (67)

Reman This work 20%

746

328

362

261 200

63. 66

:t:::; I out-of-plane torsion I out-of-plane torsion Rotational lattice Tramlational lattice

DUVAL and LECOMTE [l] consider I& &g a monomeric unit, with C, molecular symmetry, with two possible main conformations. Such a unit would have 15 vibrations, of which, on the baais of a C,, molecular unit, 13 would be it&a-red active and 15 Raman active. In the crystal, however, it can be seen (Table 7) that all 16 must be infra-red active, and will be split by correlation field splitting into 60 internal modes.

Vibrational spectra of compounds in the iodine pentoxidswater system

1986

Table 5. Vibrational frequencies (4000-40 cm-l) and aasignmentefor HIsO8 I&e-red This work 7YK (5guma in braok& at 20°C) 1090 834. 822, 806 760 640, 810 660 612. 470 462,404 360.340 311 288. 263. 247 (239) 237 219 (213) 209, 179. 169 (166) 166, 163 (141) 130 (126) 111 (107), 101 (97) 83 (SO) 62 (63)

Du~ms and LEOO= [2] 2oY!

R&man This work 20%

Aaaignment

1100.1017 822, 807 770. 745. 726 669, 646 686

624,808, 791 767. 742, 726 666, 638, 609 636

460, 398 366, 336 306

460, 392 369.346 328. 308, 300

OH bend %m(I==% MI%) v*.(IO,) vs (IO’) v&OI) Combinationa [22] vs(IOI) I=0 wagging IO, deformation

273, 264 234

IO, wagging and IO, twisting

214. 190 182 148, 109

I out-of-plane torsion

-280

&IOI)

92, 69 I out-of-plrme torsion 69 63.47

I

Translational lattice

Table 6. Vibrational frequencies (4000-40 cm-l) and assignmentsfor HIO, Thia work solid 77’K 1160 b 1110 b 8358, 776 a 800 768 748 714 642 618.696 677 620, 496. 477, 380 a 362

DUPUIS and LECO[2] solid - 2O’C

1100 1017 822, 776 807 764

DWAL and LEWMTE [l] solid - 20%

DaaENT and WADDINCS~ON [6] mula - 2O’C 1163 1101

632

836, 820 804 763 746 718 660 637

668

661

677

380 a 360 epprox. 330 feints

366

717 667 636

313

316

306 226 (221) 190, 187 (184) 167 (166) 134 (129) 118 (112) 1108 (not observed at 2OOC) 97 (93)

300

b-p

Figures ( ) = at 77’K. b, broad; B. shoulder.

Reman [ll] solid 20%

836 804 768 743 712

Aaaignment OH bend OH bend

839

Combiiationa [22]

780> 741 713 >

%a (v&IO,)

631

-1 1 378 328 296 220 192 168 124 111

91 I 72 68 1

v$b (V.10,)

Vl h

(IO')1

Combination maximum [22] Combinationa [22] v, (oq mking) vmo1+ lattice (pmibly v4 vm) VI vm (IO, deformation) Vdaaym (0’10 bending)

I out-of-plane toraiona

Rotstional

lattice

P. M. A.

1986

SHERWOOD

and

J. J.

TURNER

Table 7. Charactertable for Gzn4group and infrared and Raman active vibrations (for a monomerio structure) Oh

E A, % AU %

u(coset)n~ u(coset)s U(COS&)S-W

&xR’nl h,xR’T h&‘T’ hpXR’R’

1 1 1 1 28 4 4 84 3 9 12

C, 1

1 1

-1 1 -1 0 0 0 0 -1 1 0

glide

i

-1 -1

1 -1 -1 1 0 0 0 0 1

0 0 0 0 -3 3 0

ni

T

T’

R’

nt

Raman

I.R.

21 21 21 21

0 0 1 2

3 3 2 1

3 3 3 3

16 15 16 15

a

f f a

a

f f

a

-1 0

Abbreviations used are the same as those in Table 2.

We have circumstantial evidence [23] from a mass spectral study that supports a polymeric structure for I,O,. In our view a polymeric structure gives a better agreement with the general structural trends in the iodine pentoxide-water system (see later), and it is therefore important to find out whether a monomeric or polymeric structure gives a better interpretation of the infrared and Raman spectrum. The polymeric I,O, unit chosen is one with I=0 and IO, groups which would give better agreement with the group vibrations than a ring polymer which would have only I=0 groups. Such a polymer may have the structure*:

There would then be two polymer chains per unit cell, with 3N-4 = 38 internal vibrations (there are three translations, and one rotation about the polymer axis). Inter polymer coupling would give rise to 84 modes, as for the monomer, of which 76 are internal vibrations, 3 acoustical modes (and therefore i&a-red inactive), 3 translational lattice modes, and two rotational lattice modes (one each may be expected to be infrared and Raman active for this space group). Thus going from a monomeric to a polymeric structure reduces the number of lattice modes, and increases the number of torsional modes. * The difkence between this and the monomeric structure is that one I=0 is replaced by two I-O bonds. Since single halogen oxygen bands have more than half the double bond energy, this is a reasonablestructure thermodynamically. [23] P. M. A. SHERWOOD and J. J.

TURNER,

J. Chem. Sot. A, in press.

Vibrational spectra of compounds in the iodine pentoxide-water system

1987

For a munomem’cstructure v~(IO,), YJIO,), y,(IOI), vJIOI) 6,(10,), ~(10,) (IO, wagging), ~~~(10~)(IO, twisting) would be expected to appear in the same frequency range as similar vibrations assigned to the polymeric structure. The two IO, rocking modes &IO,) and ~~(10,) involve movement of the iodine atoms out of the plane, and thus would be expected to occur at rather lower frequency than the internal modes listed above. In addition Table 7 shows that 6 Raman active, and 3 infra-red active translations, and 6 Raman and 6 infra-red active rotational lattice modes are expected. Since no lattice mode is both infrared and Reman active (Table 7), lattice modes in the Raman aud in&a-red would be expected at different frequencies depending upon the amount of coupling. Consideration of the moment of inertia change for the 10,-O-10, monomeric units suggests that rotation about the molecular axis is expected to have a lower moment of inertia, and thus involve mainly oxygen motion, than rotation about the other two axes, which involve iodine motion. The rotational modes which involve mainly oxygen motion may be expected to occur over the same range as the iodate rotational lattice modes (the range of values being caused by the varying interactions between molecules in the four motions), and so 4 i&a-red and 4 Raman active Table 8. Comparison of CH, and IO, group vibrations (cm-l)

-6 IO,

Bending

wagging

Twisting

Rocking

1460 328,324

1330 364,345

1270 288,280

800 233 (see text)

Table 9. Fundamental frequenoiesof non-linear XY, molecules Molecule (solid) OF,(matrix at 20°K) [24] Cl,O(solid at 77“K) [26] Br,O(solid at 77’K) [26] (I,O, this work)

yBB (cm-r)

v, (cm-r)

821 671 587 687

925 631 504 420

6(cm-l) 461 296 197 156,128

modes (Ghich involve iodine motion) may be expected to be found at lower fiequenties. The translational modes of iodine have been found [27] to occur at 41 cm-l and 65 cm-l, and so all the translational modes of I,O, (which would be of low intensity for such a neutral molecule), and the rotational modes involving iodine motion would be expected in this region (i.e. below 100 cm-l). Thus in the region 100-250 cm-l one rotational lattice mode and the internal modes S(IOI), ~~(10,) and ~~(10,) would be expected (the last two would occur at similar frequencies) for the monomeric structure. The peaks at 212 and 233 cm-i (I.R.) and 190 and 200 cm-l (Raman) may be assigned as the correlation field components of ~~(10,) and y,.(IO,) (their true position will have been shifted by coupling with external modes). The peaks at [24] [26] [26] [27]

R. D. S~&~LEY, Ph.D. Thesis, University of California, Berkley (1965). M. M. Rocnxnm and G. C. PIMENTEL, J. Chma. Phye. 42, 1361 (1966). C. CAMPBELL, J.P.M. JONES and J.J.TuENEB,C~~~. Commwn. 888 (1968). S.H. WALMSLEY and A. ANDEB~ON,MOZ.Phya. 7,411 (1964).

1988

P. M. A.

&iERWOOD

and J. J. TUTWER

156 and 128 cm-l (I.R.) and 144 and 104 cm-l (Raman) may be assigned as the correlation field components of d(IO1). In the region below 100 cm-l four rotational lattice modes are expected in both the Raman and infra-red spectra, and peaks at 112, 108, 91, 85 and 60 cm-l are found in the i&a-red spectrum, and peaks at 94, 77, 63, 55 and 42 cm-l are found in the Raman spectrum, corresponding to one too many peaks in both the i&a-red and Raman spectrum. This problem could be avoided by invoking intense translational peaks (very unlikely in this system), Table 10. Infra-red frequencies (4000-250 cm-l) and spectral features of caeesiumiodate Band (cm-l)

Half width (cm-l)

142 817 shoulder 768 shoulder 595 340 360 shoulder 298

65 4 4 20 12 15 20

Relative intensity 10

o-5 6 3 6.5

combinations between translational and rotational modes (these would probably be of very low intensity), or by shifting the whole spectral assignment to lower frequencies by supposing large differences in yJI0,) and y,(IO,) vibrations (which would give 6(101) frequencies that would be lower than might be predicted by comparison with other non-linear XY, molecules- Table 9). Thus the spectrum can be made to fit the monomeric structure, but it will now be shown that the polymer% structure fits the observed spectrum more naturally. The assignment of the spectrum on the basis of a polymeric structure is given in Table 4. Comparison with iodic acid allows assignment of Y,(IOJ and ~~(10~) and Y(I=O) would be expected at higher frequencies. ~~(101) and r&101) are assigned in agreement with previous authors [2,28,29]. Table 8 compares typical CH, group vibrations with IO, group vibrations. Comparison with iodic acid allows assignment of the IO, deformation, and with the help of Table 8 (the C/H ratio approximately equals the I/O ratio) the other IOz vibrations are assigned. I=0 wagging would be expected to occur at higher frequencies. There will be a large number of I out of plane torsions (of which IO, rocking is one type), and these will explain the bands in the region 106-250 cm- l. The B(IO1) would be expected in this region by comparison with the trend in other halogen compounds (Table 9), though it is impossible to clearly distinguish between this mode and I out of plane torsions. We would assign the peaks at 156 and 128 cm-l to 6(101), corresponding to the two types of 101 group in the suggested polymer structure. The bands at 60 cm-l (i&a-red) and 42 cm-l (Raman) are assigned as the two rotational lattice modes. The broad band centred at about 200 cm-l can be resolved at liquid nitrogen temperature into four bands (the improvement is not apparent in Fig. 1 though it was quite marked in scale expansion spectra), the improvement [28] W. E. DASENTand T. C. WADDINUTON, J. Chem. Sot. 3350 (1960). [29] C. CAMPBELL, Ph.D. Thesis, University of Cambridge (1966).

Vibrational spectra of cqmpounds in the iodine pentoxid+water system

1989

caused by the removal of the large number of difference bands and the sharpenof bands on cooling. The width of the bands in the spectrum have been described elsewhere 122-J. 2.2 IO&C acid and the third hydrate. The crystal structure [30] of iodic acid is well known,’ and has the orthorhombic space group P 2,2,2, (Da’ in Schijnflies notation), with four molecules of HIO, per unit cell. The contiguration of the HIOa molecules in solid a-iodic acid has been determined by neutron diffraction [31}. There is strong hydrogen bonding in iodic acid, producing polymer-like chains, with a screw axis as the polymer axis [31,6]. It can be seen that there will be two such chains per unit cell. Each chain is capable of 3N-4 = 26 internal vibrations, which can be split in the crystal to give 52 vibrations, all of which are Raman active, those of type A being infra-red inactive. The splitting due to “interchain coupling” will presumably be much less than the splitting caused by the transition from single to polymeric HIOa units, which doubles the number of internal vibrations’ There will ‘be three acoustic modes, three translatory lattice vibrations, and two Reman active and one i&a-red active rotatory lattice modes. Since all infrared active bands are also Raman active, bands are expected to occur at the same frequency in both spectra with small differences due to the use of laser excitation: Table 6 lists the frequencies and band assignments, giving the most recent Raman work for comparison ; some of the bands of previous studies have been reassigned. Nujol and hexachlorobutadiene mulls were also run at room temperature, and these showed little difference from the room temperature spectrum of the solid. No clear band could be observed in the range of the O-H stretch even at liquid nitrogen temperatures. Previously reported O-H stretching vibrations in this range have been found to be of very low intensity. This result is not surprising, since when a hydrogen bond is short and strong the O-H stretching vibration is very broad. This result, which suggests strong hydrogen bonding, justifies our consideration of polymer-type hydrogen bonded chains. Other O-H vibrations in iodic acid and the third hydrate are very weak, a weak broad band centered at 2890 cm-l being observed for the third hydrate, and no O-H vibrations can be observed in the Raman of either compound (compare the Raman of HIO, with that of DIO, [ll]). The strong band at 677 cm-l has been assigned [2] to y(OH), and was found [4] to shift to 420 cm-r on deuteration. Since all bands are Raman active, y(OH) would be expected to be Reman active, however no band can be seen in the Raman at this frequency. Moreover we have reexamined the infrared spectrum of DIO, (prepared by adding D,O to I,O, prepared as above), and find that the spectrum is similar to that of HIO,, except that the 640 cm-l band appears to be more intense, and a weak broad band appears at about 400 cm-l, and we therefore assume that moat of the 677 cm-l band is due to combination bands, as can be seen from the band profile discussed elsewhere [22]. Hydrogen bonding leading to polymer-like chains has been shown above to double

being ing

[30] M. T. ROUEIWand L. HELMEOLZ, J. Am. Chem. SW. 68,278 (1941). [31] B. 8. GAREI-I,ORNL-1745, Oak Ridge National Laboratory, Tennessee (1954); Disa. Abe. 14, 1162 (1964). [32] D. F. HORNIQ,J. Claem. Phya. 16, 1063 (1948). 3

1990

P. M.

A.

8lTEBWOOD and

J. J.

TURNEFs

the number of internal vibrations. The separation caused by this doubling is expected to be appreciable, because of the strength of the hydrogen bonded i&era&ion. Stat& field splitting of degenerate internal modes is expected to be appreciable for the same reason, in agreement with the view of earlier workers [a]. The ~a mode of the free ion [33] is therefore expected to be appreciably split by static field splitting in the crystal into $&,(symmetric 10%stret&ing), and fao (antisymmetrio IO, stretching} respectively. In addition each of %$, and rso is expected to be apprec~bly split (into two bands) by the “polymer& coupling” destibed above. Four Raman active modes are therefore expected in this region. Raman studies [ll] have shown a-weak band at 839 cm-l, a strong band at 780 cm-i, with a shoulder at 741 cm-l, and a strong band at 713 elm-l. Our infrared studies show bands at 835 ctm-l (shoulder), 800 om-l (strong) 776 om-l (shoulder), 768 om-l (strong), 748 om-i (shoulder), and 714 em-1 (strong). In view of the requirement that all infrared active bands are also Raman aotive, and should ocm at the same frequenoy, the i&a-red band at 835 cm-l, and Raman band at 839 cm’-l may be due to a oombination band (see the band profile [22]), the i&a-red bands at 800 and 768 cm-l and the Raman band at 780 cm-1 (possibly unresolved) due to Ye, and the infrared bands at 748 and 714 cm-1 and the Raman bands at 741 and 713 cm-1 due to %b* The band at 776 is probably a ~mb~ation, possibly of a .K # 0 type, explaining its shift from the band profile value 1221. The intensities in this region are probably oomplioated by intensity borrowing. The differences between this and earlier interpretations [4,1X] should be noted. The bands between 306 and 642 cm-l, with the exception of the band at 677 cm-l are assigned in agreement with earlier infrared and Raman studies. The band at 677 cm-l is assigned as a “combination maximum” (an overlap of a number of combination bands), and has been discussed elsewhere [22]. The bands between 100 and 230 cm-i are assigned as I out of plane torsions of the polymer chain, which ooour at low frequenoy beoause of the movement of the heavy iodine atom (&de s~pro). The rotational lattice modes involve movement of the iodine atom and are thus assigned at 72 and 68 em-i. Our interpretation of the far infrared region differs from that of Du~ra et d. [ll] for the same reasons as those given in the sodium iodate ease. In our view the orientating effect of hydrogen bonding must make all rotations, except those about the polymer axis, into O-H torsions and I out of plane torsions. The band profile of iodic acid contains far fewer combination bands in the region 300-850 em-1 than that of iodine pentoxide [22], and therefore the ape&rum of HIOa is expected to be much sharper than that of I,O,, as observed. Cooling causes the removal of diflbrence bands, causing the 577 and 642 cm-l bands to be resolved [34]. The crystal structure [SJof HIsO8 has recently been determined and has the monoclinic spaoe group P 2,/n ((?,,a in SchMiies notation), with four molecules of H&O@ per unit cell. There is strong hydrogen bonding between the Ia06 and HIO, units in

[34] Since this work was submitted, a note on a single crystal study of HI08 haa bean pubM. KRATJZBUN and J. P. M~TEIEU,Oompt. Rend. 869, 1278 (1969). lished: L. CO-,

Vibrational spectra of compounds in the iodine pentoxide-wa&

system

1991

the addition compound, but note the difference between HI,O,, and RIO, where the hydrogen bonding is between IO8 units. There is also strong interaction between iodine and oxygen atoms in both the MO, and the I,O, unita since the I-O distance is 2.6 A, 1 A shorter than the sum of the van der Waal’s radii. The struoture thus oon&ts of polymer+ sheets with two sheets running through the unit cell, with sorew axes running through the sheets (this can be seen by consulting the diagram in

I

0

I

I

160

I

I

320

I

I

460

cm-’ Fig. 6. Raman spectrum of I,O, and RI&

I

I

640

i)I 1

600

I

af room temperature.

the X-ray study). This means that the two polymeric units remain invariant on the screw axis. It oan be seen from the oharaoter table that one of the correlation field components of a given mode will be i&a-red and the other Raman aotive, and so a noticeable difference between i&a-red and Raman bands is expected, and this is observed. For a sheet struoture all rotational lattice modes will be forbidden, and therefore 3N - 3 internal modes are expected, giving 69 internal modes per sheet, and therefore 69 infra-red and 69 Raman active modes (there are two sheets per unit cell). Three tranalatory lattice modes, and three acoustic modes are expected. The vibrational frequencies and spectral assignments are shown in Table 5, the spectra being assigned as an addition of the I,O, and HIO, group vibrations, with a large number of iodine out of plane torsional modes. IO, and IO modes will be effected by the I-O interactions. The band profile [22] is expected to be a combination of

1992

P; M. A.

&tERWOOD

and J. J. TURNER

the band profiles of IaO, and HIO,. HI,Os was found to be highly scattering and, as expected, the amount of scattering was found to be very dependent upon particle size. Some multiple structure was observed for the O-H stretching vibrations. Duma et aE.[ll], when trying to explain the presence of a weak band at 462 cm-l observed by WADDINOTON[6] in the infra-red spectrum of HIO,, examined the spectrum of partially dehydrated iodic acid, and this was found to have a strong band at 462 cm-l. They concluded that this Raman spectrum was that of I,O,, and that the band observed by Waddington was in fact an $0, impurity band since “this band rapidly disappeared when the sample was placed in a desiccator over HaO.” We are certain that the new spectrum was not due to I,O,, firstly since this ape&rum was identical to our H&,0, spectrum, and secondly because partial deoomposition of iodic acid leads to H&O,. CONULUSIONS Our results illustrate the importance of examination of spectra of solids at low temperatures, and over as complete a spectral range as possible. Examination of the complete spectral range allows a band analysis to be made, and this is discussed elsewhere [22]. Consideration of factor group symmetry is far preferable to the consideration of site group symmetry when lattice vibration fundamentals are considered. The analysis of only the near i&a-red spectrum, and then only by site symmetry, which takes no account of correlation field effects, or worse molecular symmetry, whiah takes no account of any solid state effects, can oause confusion; and in addition such analysis, although indicating the main internal vibrations, gives little information about combination bands. The four molecules studied in this paper provide a good example of the need for detailed studies to make any real sense of the spectra. Our results suggest that the structure of I,O, may be monomer% as discussed by Lecomte and Duval, though we feel that in view of our studies in the far infra-red, and our consideration of factor group rather than molecular symmetry a polymeric structure explains the spectrum as well, and possibly more easily. X-ray orystallographic results for H&O, which suggest the presence of a polymeric structure, and the fact that the polymeric I,O, structure can be derived from this structure, together with mass spectral studies [23] that we have carried out, lend further support for, though not conclusive proof of the polymeric structure. We await the results of the X-ray study [14] with great interest. AcknozvledgelnentgThe research was supported by a grant from the S.R.C. and one of us (P. M. A. 5.) thanks the S.R.C. for a maintenancegrant. We would like to thank the Inorganic Chemistry Department, Oxford University for the use of the Perk&Elmer 226 spectrometer, for assistance with and the Carey 81 Raman spectrometer,and Drs. G. M. and W. S. SEELDRICK the X-ray studies, and Mr. C. 5. Anaaas for running the Raman spectra.