The vibrational analysis of methylarsonic acid, trideuteromethylarsonic acid and their anions

Journal of Molecular Structure, 35 (1976) 191-200 o Elsevier Scientific Publishing Company, Amsterdam -

THE VIBRATIONAL ANALYSIS TRIDEUTEROMETHYLARSONIC

F. K. VANSANT*,

B. J. VAN

Printed in The Netherlands

OF METHYLARSONIC ACID, ACID AND THEIR ANIONS

DER VEKEN

and M. A

HERMAN

Laboratorium voor Anorganische Scheikunde, Rijksuniversitair Cenfrum Antwerpen, Groenenborgerlaan 171, B 2020 Antwerpen (Belgium) (Received

5 March 1976;

in revised form 16 June 1976)

ABSTRACT

Raman and IR spectra of methylarsonic acid, CH,AsO,H,, trideuteromethylarsonic acid, CD,AsO,H,, and their anions in aqueous solution and the solid state are discussed. Some results of a general valence force field and a Urey-Bradley force field are reported. INTRODUCTION

To continue our investigations on arsenic compounds [l-3], we have studied the vibrational spectra of methylarsonic acid and its anions, together with the CDs-derivatives. Although recently the vibrational spectra of the normal compounds have been discussed by Simon and Schumann [ 41, we reinvestigated most of these spectra, mainly because some inconsistencies arose when we extrapolated these literature data to the trideutero-compounds, for which no results have yet been published. As there is nothing to add to the group-theoretical considerations or the division in localised modes described by Simon and Schumann [4] for these compounds, we will not go into this, but confine ourselves to the discussion of the differences between our spectra and those previously observed. EXPERIMENTAL

The disodium salt of methylarsonic acid was prepared by the action of CHSI on an aqueous solution of Na3As03 [5, S]. The free acid was prepared by adding equivalent amounts of diluted sulphuric acid to an aqueous solution of the disodium salt. After evaporation, the acid was extracted with methanol and purified by recrystallisation from the same solvent. The trideutero-compounds were prepared in the same way from CD31. Infrared spectra have been recorded with a Perkin-Elmer 225 spectrometer. Aqueous phase spectra were taken with AgCl windows. For solid state spectra *Present address: BASF, Centraal Laboratorium, Scheldelaan, B 2000 Antwerpen,

Belgium.

192 the K&pellet technique was used. Raman spectra have been obtained with a coderg PHO spectrometer equipped with an OIP 181 E He-Ne laser. CH3AsO:-

and CD3AsO~-

The observed frequencies for this anion are gathered in Tables 1 and 3 for the normal compound, and Tables 2 and 3 for the deuterated Compound. For the aqueous phase spectra of the normal anion, there is general agreement

TABLE; 1 Vibrational spectraa of CH,AsO,” Raman (H,O) 270 (121 370 (17) 400 (sh) 624 p (100) 793 (sb+ b 823 p (56) 860 (sh) I 1274 p 1419 2937 p 3020

(3) (6-I (14) (2)

in aqueous sotution

Ran-Kin(D&x 275 370

(101 (17)

625 p

(93)

fR (D&0

923 w, sh 81’7 vs

830 p (loot 1220 1275 p 1419 2936 p 303.6

(4) (71 (12) (2)

854 1210 1270 1411

m vs sh w

Assignment P(AG) a,(AsW %(AsW v(As--c~ v,(~CJ Q(A-sW P(CH,) DzO $i(CW %(CH,I v&H,) ~,(CHJ

= = a; 4 e a, e 4 e 4 e

aAbbreviations used throughout: s strong, m medium, w weak, v very, sh p polarised. bin this region a band profile analysis was performed (see text). TABLE 2 Vibrational spectra of CD&O;Raman (H,O)

in aqueous solution

Raman (l&00)

250 370

(16) (25)

255 360

(sh) (17)

395

tshl

395

WI

p (100) 669 (61 790 (sh) 820~ (81) 995.P (301 1034 (7-I 570

2138 p 2262

(47) (131

571 p (67) 673 (51 805 (W 830 p (100) 996 p (24) -1037 (51 1205 214op (33) 2264 (11)

IR (D,o)

574 sh 667 sh 820 vs 989 m 1030 w 1200 vs

Assignment

shoulder,

193 TABLE

3

Vibrational

spectra of CH,AsO,Na,

IR CH,AsOaNa,

Raman CR,

and CD,AsO,Na,

AsO,Na,-6H,O

in the solid state

Raman CD,AsO,Na,-6H,O 210 255 378

IR

CD&OxNa,

(20) (15) (48)

217 (37) 370 (31) 380 (31) 603 (100)

616 m

553 (100)

562 m 569 m

786 838 856

810 vs 845 vs 870 s

785 837 660

(30) (81) (9)

818 vs 845 vs

m

(27) (5)

676 m 663 985 m 1028 w

I

992 1035

(70) (31)

1045 w 1058 w 2131 w 2250 w

I

2135 2258

(13) (56) (sh)

1269

(3)

1413

(4)

2930

3012

1264 1412 1433 1447 2925 2996

(19)

(6)

aIn the column “assignment”, indicated as CH,-vibrations. TABLE

m w w w w

Assignmenta

the fundamental

vibrations

localised

in the CDS grouping

are

4

Band profile

analysis of the Raman spectrum

4

of CH,AsO,‘-

in the region 710-920

cm-’

i,

3

Height

Au+

ii

Height

Avy2

792 823 863

17 76 6

56 42 26

795 821 857

34 51 15

53 39 27

with the frequencies reported by Simon and Schumann 143 _ These authors report a very broad band in the region of the As0 stretch frequencies at 820 cm-‘. From our spectra it became clear that an intense band was present in this region, accompanied by two weaker bands, one on the high- and one on the low-frequency side. Therefore, for the region 710-930 cm-’ of the Raman spectra of this anion, a band profile analysis, using methods described before [ 11, was performed. The results are given in Table 4. Both for the Jo and il Raman spectra three bands are found; the corresponding frequencies and half band widths in both spectra are of very comparable magnitude. From the relative intensities, it ia clear that the band at 823 cm? is polarized. Although the i///il_ratio of the intensities of the other two bands are not identical, this sboufd not be interbreted-as indicating polarization of the

194 TABLE

5

Vibrational

spectra of CH,AsO,H-

256 (18) 345 (sh) 367 (18) 633 p (100) 730P 832 p 866 p

(8) (sh)a (43)

900

(sh)

1281 1418 2937 3020

253 (11) 330 (sh) 352 (11) 632 p (100) 718 p (10)

627 sh 705 s

830 866 p

865 vs

p

(2)

890 1210 1281

p

(6) (14)

1417 2938

(2)

3021

aDisappears TABLE

(shY (43) (sh)

p

(1)

p

(4) (15) (2)

spectra of CD,AsO,H-

(H20)

Baman

(D,O)

678 732 820 852 p

678 718 p 827 p 860 p

p

2138 2266

p

a'+a"

a’

u(H)

a’

v(As-OH) See text

a'

vs(AG) v,(AsO,)

a’ a”

DzC UCH,) GCH,) UCW t&H,)

a’ a’+ a” 0’ a’+ a”

in aqueous solution

225 (11) 340 (sh) 370 (16) 580 p (100)

1034

vs sh m w w

a’+ a”

upon dilution.

234 (11) 340 (sh) 370 (17) 579 p (100)

880 1000

1210 1277 1414 2936 3018

PWO3) siwa) %tA4)

6

Vibrational Baman

in aqueous solution

(4) (6) (sh)a (35) (sh) (17)

880 1001

(3) (35)

1034 1210 2140

(10)

2269

aDisappears

p

(11) (18) (sh)a (86) (sh) (22) (4)

p

(39) (6)

IB (D,C)

Assignment

P(Aa) satAG)

a’ta” a’+ a”

us

a’

580 sh 674 sh 704 m

v(As--c) P(CDJ u(As-OH) See text

a' a’ + a” a’

865 vs

%(AsC*) va(~W UCD,) %(CDa) D@ +.(CDJ va(CQ)

a’ a” a’ a’+ a”

994 m 1029 w 1200 vs

a’ a’ta”

upon dilution.

v,(As02); the clear polarization of the central band allows unambiguous assignment. In the spectra of the normal compound, bands assignable to the p(CH,) movements have not been observed either in a former [4] or the present study. In order to clarify this, the behaviour of some of the observed methyl frequencies was investigated: the results are shown in Fig. 1. It must be remarked that for the acid and monobasic anion the methyl frequencies

195

792 cm? band, as our experience indicates that relative intensities of such calculated side bands are subject to some error. Moreover, no corrections for apparatus polarization or other instrumental influences were applied to the experimental data. In this region of the spectrum, ~,(As0& a,, z+,(As03), e and p(CH,), e are expected. Due to the clear polarization of the band at 823 cm-‘, no difficulty in assigning v,(AsO,) arises. In the-spectra of the trideuterodibasic anion, a band is observed at 670 cm-‘. As in the same region of the spectra of the normal compound no band is observed, this band must be due to a mode localised in the methyl-group, and due to its position, is assigned to p(CD3), e. Applying a harmonic oscillator approximation to this frequency makes it clear that the 863’cm-’ shoulder must be assigned to the p(CH,) mode. Also, for the Raman spectra of CD&OX=, on the band at 820 cm-’ a very clear shoulder is observed near 790 cm-‘, while on the high-frequency side of the 820 cm-’ band no shoulder can be detected. Consequently, v,(AsO,) must be assigned to the band at 792 cm-*. This places the asymmetric stretching at a lower frequency than the symmetric one; similar behaviour was observed for the tertiary arsenate anion Cl]. In the region of the deformational AsO, modes more detail is shown in our Raman spectra than previous measurements [4]. The assignments given in Tables 1 and 3 are analogous to those for CH,P0,2- 173. By comparison with the spectra of the normal compound, and using a simple harmonic oscillator formula to calculate H/D-shifts, the interpretation of the spectra of the trideutero-dibasic anion is straightforward and need not be discussed at length. In order to test the assignments, the Teller-Redlich product rule was applied to the dibasic anion. Instead of harmonic wavenumbers, mean values for the frequencies observed in the aqueous phase spectra have been used. Adopting an error of 3 cm-’ for each frequency, 0.392 f 0.009 is calculated for the wavenumber ratio; the other part of the equation was calculated with the geometry given in Table 10. This leads to a value of 0.379. As both values are sufficiently close to each other, with the difference between them in the correct order, it may be concluded that the assignments are correct. CH3AsO&l-

and CD~SO&-

Both for the normal and trideutero-monobasic anion, only aqueous phase vibrational spectra have been obtained. The observed frequencies are gathered in Tables 5 and 6. The values observed for the normal compound generally agree with those observed previously [4]. For the aqueous phase Raman spectrum a broad band near 844 cm-’ was reported by Simon and Schumann [4]. In our spectra a central band accompanied by two shoulders are observed in this region. The low-frequency shoulder disappears upon dilution. We have observed an analogous feature for the corresponding phosphonic acid [ 73, and we adopt the same explanation. The other two bands observed in this region are assigned to v,(AsO,) and

196 d8

I

mO”O

I

ocld

I

60 (CD3

1040

dt

I

mono

I

acid

I

60 (0931

-

ye::

_f-/

I di

I

mono

I acid

I dt

1 mono

I octd

Fig. 1. Comparison of CH,- with CD,-deformational

vibrations.

indicated as “asymmetric” should appear as doublets; nevertheless no splitting was observed in any of the cases. It is clear from Fig. 1 that for S, and S, the behaviour for the normal compound is analogous to the one of the trideutero-compound, and is, to a high degree of approximation, linear. It is therefore logical to accept the same behaviour for the rocking modes. In this way a value of 864 cm-’ for p(CH3) can be calculated. This frequency is almost identical to the frequency of the intense v,(AsOJ. As rocking modes usually give rise to weak bands, it can be accepted that for the monobasic anion the p(CH,) mode is masked by v,(AsO,). The other bands in the normal and deuterated compound are easily assigned; this will not be discussed.

The frequencies observed for the acid are gathered in Tables 7-9. Compared with a former study [4] on the normal acid, our spectra show a little more fine structure, but otherwise agree rather well. Some points in the interpretation, however, require some comment. In the region of the As(OH), stretches, Simon and Schumann 143 observed a single maximum at 770 cni’ in the aqueous phase Raman spectra, and consequently these authors assign

197 TABLE

7

Vibrational spectra of CH,AsO,H,

ban

(HP)

232 (17) 315 (sh) 350 (13) 638 p (100) 769 p (28) 785 (sh) 885 912 p 1287 p 1413 2940 p 3027

(sh)a (20) (1) (4) (12) (1)

-an

in aqueous solution

(QO)

230 (13) 310 (sh) 335 (11) 637 p (100) 758 p (32) 775 (sh) 890 910 p 1200 1288 p 1413 2940 p 3027

(sh1” (21)

(1) (4) (14) (2)

IR (JAO)

Assignment

P(AsO,)

a’ f a”

6a(ko3)

a’2_a”

%(~03) 755 s 875 sh

a’

v(As--C)

a’

&WOH),) UWOHM

a”

a’

PWA)

a'+ a”

v(As-0) W %.0-U WX) Q.(C%) c@-b)

a'

See text

912 1200 1282 1412 2940 3026

vs w m m m

a' a'fa" a' a'-!-a)1

aDisappears upon dilution. TABLE

8

Vibrational spectra of CDaAsO,H, in aqueous solution Baman (H,O)

Baman (D,O)

212 315 350 587 687 768 780 870 910 1004 1030

210 (13) 310 (sh) 340 (11) 588 p (100) 688 (7) 758 p (41) 775 (sh) 860 p (sh)a 912~ (27) 1005 p (10) 1032 (3) 1205 2140 p 2273

p p p p p

2139 p 2271

(13) (sh) (14) (100) (5) (34) (sh) (sh)= (23) (11) (3) (31) (7)

(30) (5)

IB (D,C)

Assignment

910 s 1000 m 1023 w

P(AsC,) 6,(AsC,) I, v(As-C) &CD,) +,(MCH),) v&WCH),) See text u(As-0) %(CDs) UC4)

a’ a' a'+a"

1200 vs 2140 sh 2270 sh

DZC +(CDJ va(CDJ

a' a'+aw

585 sh 680 sh 765 s

a'ta" a'+am a’ a' a'+a" a’ a”

aDisappears upon dilution. and v,,(As(OH)~ to this band. From our Raman and IR both v,,(As(OH)~ spectra of the aqueous solutions it is clear that in this region two bands are present: an intense band at 769 cm-*, with a shoulder at 785 cm-‘. The observed polarizations indicate that the latter should be assigned to the asymmetric and the former to the symmetric stretch. For concentrated solutions a band at 885 cm-’ is found in the Raman

198 TABLE

9

Vibrational

spectra of CH,AsO~H,

CH,AsO,H,

w w

CD&O,H,

221 w 235 w

378 m 384 sh

325 s 376 384 sh

640 m 772 vs 778 vs 871 w

589 m 769 vs 778 vs 873 m

329 s

889

in the solid state Assignmenta

Baman

IR

237 265

and CD&O$I,

m

CH,AsOaH, 155 166 228 260 296 338

(sh) (7) (25) (25) (8) (15)

116 161 212 244 291 336

(4) (23) (10) (19) (IO) (12)

386 413 642 769 780 850 870 895

(9) (20) (100) (36) (sh) (34) (sh) (3)

384 412

(7) (16)

589 769 780 849 878 686

(LOO) (30) (sh) (32) (3) (4j

666 678 929 1206 1258

sh

w vs vs vs w w

I.006 1025 1610 2380 2800 23.38 2280

s m vs vs vs

m

2940

(8)

sh

3036

(1)

=In the column

‘*assignment”

CD,-vibrations

935 1205 1255 1297 1415 1610 2380 2800 2938 3010

vs s w

m

m

CQA-Q%

6(C-As4)

! ws-C) r&WOWJ +Ws(OH),) rJ(As-0)

1

vs m w

PGW

r(OW

I 6tOH)

2Y(As-c)

1004 1030

(8) (1)

%(CB,) MCH,) \ I

are indicated

2139 2260

(17) (10)

v(OB) ?S(C%) SW%)

as CH,-vibrations.

spectra which was not observed before. As the band disappears upon dilution, it is assigned to a stretching mode of a polymeric species formed by the acid molecules in highly concentrated solutions. Although clearly present in the IR spectrum of CH~ASO~HZ published by Simon and Schumann [4], these authors fail to interpret the broad band observed at 1610 cm-‘. Clearly this band is the c‘C”-component of the “A,B,C”-pattem usually observed for this type of compound f8]. In the region $50-950 cm -l, Simon and Schumann [4] observed 3 bands in the fR spectrum of solid CH,AsO,H,: an intense band at 945 cm-’ accompanied by two shoulders on the Zow-frequency side, at 883 cm-’ and 900 cm-‘, respectively. fn the corresponding region of the Raman spectrum a band was suspected at 953 cm- ‘. In our fR spectra the same behaviour is found, where we locahse the intense band at 935 cm-‘, However, in our Raman spectra a

199

very prominent band is found at 850 cm-’ with a faint shoulder near 870 cm-l and a weak band at 895 cm-‘. In this region of the spectrum v(As=O), p(CH,) and r(0I-I) are expected. Simon and Schumann 143 assign the intense 945 cm-’ IR band to r$As=O), the weak bands to either v(OH) or to some solid state effect in connection with strong hydrogen bonding. Due to its intensity, however, the 850 cm‘-’ Raman band cannot be due to p(CH,) neither to r(OH), so it remains to assign it to Y(As=O). Then the 935 cm-l band must be due to p(CH,) or y(OH). The former possibility is excluded because in the spectrum of the trideuterocompound an intense band analogous to the 935 cm-’ band is found at 929 cm-‘, while the weak band in the 880-900 cm-l has disappeared, which indicates that the 883 cm”’ band is due to p(CH,). Consequently, the 935 cm-’ band must be assigned to r(OH). Support for this is found in the study on organolead compounds by Henry [9], where it was found that upon formation of organoleadarsonates, the 935 cm-’ band disappears, and thus must be due to y(OH), With the aid of the spectra and assignments of the normal acid, the interpretation of the spectra of CD,AsO,H, is straightforward and will not be discussed. NORMAL

COORDINATE

ANALYSIS

Although a set of valence force constants has been published for the compounds studied here [lo], we decided to reinvestigate this because the present study has seriously altered some of the assignments of (As-O) stretches. The calculations reported here cover a general valence force field and a Urey-33radley force field. In both cases, the G-matrices were constructed with the geometries given in Table 10 and with standard symmetry coordinates [7]. Both for the monobasic anion and acid molecule, the O-H groupings were treated as point masses. The GVFF was calculated with the Matrix Polynomial Expansion Method (MPEM) of Alix [ 111. The UreyBradley force field was calculated by standard least-squares procedures [ 12f _ For this, only frequencies for the normal compound have been used. The standard deviation of the experimental frequencies and those calculated with TABLE 10

Geometriesused in the calculations u

CH,As03=

CH,AsO,H-

CH,AsO,H,

C-H= As-Ca As-Ob As--o(Hp Angles: tetrahedral

1.10 1.94 1.69 -

1.10 1.94 1.65 1.71

1.10 1.94 1.66 1.73

aSingle bond values obtained from the Schomaker-Stevenson relation [13]; bthese bond lengths were obtained from a linear bondlength/mean stretchingfrequency relation [l4]_

200

the Urey-Bradley force field is under 1.5 % for the series considered. In data deposited with British Library Lending Division as SUP 26042 (15 pages) we have tabulated the symmetry coordinates, the complete F-matrices, potential energy distributions and those internal valence force constants which can be unambiguously obtained from the symmetry force constants. Further material deposited includes the Urey-Bradley force constants, a comparison of experimental frequencies for the normal compounds w.-1.tl: those calculated from the Urey-Bradley force constants, and a comparison of the experimental frequencies for the deuterated compounds with those calculated from the valence force field and from the Urey-Bradley force field. For this, standard deviations ranging from 1.4 to 2.3 % are found, depending on the compound and the force field. CONCLUSION

In the present study we have discussed the vibrational spectra of CH,AsO,H,, CD,AsO,H, and their anions. Several corrections and improvements to previous studies on the normal compounds have been proposed. Also, the principal results of a normal coordinate analysis are reported. A detailed discussion of the force constants obtained here will be given in a subsequent paper.

REFERENCES 1 F. K. Vansant,B. J. van der Veken and M. A. Herman, J. Mol. Struct., 15 (1973) 425, 439. 2 F. K. Vansant, B. J. van der Veken and M. A. Herman, Spectrochim. Acta, Part A, 30 (1974) 69. 3 F. K. Vansant, B. J. van der Veken and M. A. Herman, J. Mol. Struct., 22 (1974) 273. 4 A. Simon and H_ D. Schumann, Z. Anorg. AIlg. Chem., 393 (1972) 23; 398 (1973) 145. 5 A. J. Quick and R. Adams, J. Am. Chem. Sot., 44 (1922) 811. 6 R. Pietsch, Monatsh. Chem., 96 (1965) 138. 7 B. J. van der Veken and M. A. Herman, J. Mol. Struct., 15 (1973) 225,237. 8 J. T. Braunholz, G. E. Hall, F. G. Mann and N. Sheppard, J. Chem. Sot., (1959) 868. 9 M. C. Henry, Inorg. Chem., 1(1962) 917. 10 H. V. GrundIer, H. D. Schumann and E. Steger, J. Mol. Struct. 21 (1974) 149. 11 A. Alix, J. Mol. Struct., 20 (1974) 51. 12 D. E. Mann, T. Shimanouchi, J. H. Meal and L. Fano, J. Chem. Phys., 27 (1957) 43. 13 V. Schomaker and D. P. Stevenson, J. Am. Chem. Sot., 63 (1941) 37. 14 F. K. Vansant, unpublished results.