Electrolyte—formamide interactions studied by raman spectroscopy

of Molecular Structure, 71 (1981) 61-70 Elsevier Scientific Publishing Company, Amsterdam -

Journal

ELECTROLYTE-FORMAMIDE SPECTROSCOPY

INTERACTIONS

ALISTAIR

J. LEES* and BRIAN

Department Tyne, NE1

of Inorganic Chemistry, 7RU (Gt. Britain)

DEREK

Friited in The Netherlands

STUDIED

BY RAMAN

P. STRAUGHAN University

of Newcastle

upon

Tyne,

Newcastle

upon

J. GARDINER

Chemistry

Department,

Newcastle

upon Tyne Polytechnic,

Newcastle

upon Tyne.

NE1

8ST (Gt. Britain)

(Received 18 June 1980)

ABSTRACT The effects of dissolved electrolytes on the structure of liquid formamide have been investigated by Raman spectroscopy. The spectral features are dependent on the concentration and nature of the dissolved electrolyte and are discussed in terms of direct electrolyte formamide interactions_ Two UN-H bands, arising from ion-formamide species, have been observed superimposed on the spectrum of residual liquid formamide. The results lend further support to the interaction model for electrolytes in liquid formamide previously proposed by us on the basis of nuclear magnetic resonance and infrared spectral data. INTRODUCTION

Formamide can be regarded as a planar molecule [l-3] a substantial proportion of resonance form II.

which exists with

The C-N bond has considerable double bond character, and tends to preserve the planarity of the molecule. The Arrhenius energy of activation for internal rotation has been calculated to be 75-79 kJ mol-’ [4, 51 illustrating an absence of rotation at room temperature [ 61. Formamide is the simplest amide and therefore the simplest molecule that can be linked by hydrogen bonds between GO and N-H groups. Infrared and Raman spectroscopy have been used to study intermolecular association in the solid [ 7, 81, liquid [g-13], *Present address: Department of Chemistry, Angeles, CA 90007, U.S.A. 0022-2860/81/0000-0000/$02.50

0 1981

University of Southern California, LOS

Elsevier Scientific Publishing Company

62

solution [14--161 and gaseous [17--191 phases of formamide. Recently, we have investigated the intermolecular hydrogen bonding in the solid, liquid and solution phases of formamide by vibrational spectroscopy [20]. The N-H stretching region of formamide in each phase was indicative of the nature of intermolecular hydrogen bonding. The spectra suggest that, on melting, the linked C Zh dimers of the solid [21] break up to form smaller units of linked dimers having C, symmetry. Some recent ab initio SCF calculations [22] are compatible with this model, showing that the dominant species expected in the liquid phase of formamide comprises a cyclic dimer, cis hydrogen bonded to other formamide units. Bukowska [23] has also examined the YN-H region of liquid formamide and has suggested that Fermi resonance between a vN-H fundamental mode and an overtone of 6NH2 is responsible in part for the complexity of the vN-I-I band envelope. However, in the light of the data described here we still prefer to explain the presence of four components in the uNH region of liquid formamide in terms of our associated model. The interaction of formamide with ions is both biologically and spectroscopically interesting. Formamide molecules contain the biologically important peptide linkage (HNCO) and hence information about the nature of the interaction between ions and formamide will be of relevance to polypeptide and protein chemistry. Lithium salts in particular have been found to interact strongly with amides, the Li’ ion binding to the car-bony1 oxygen of the amide group causing a change in the geometry and spectroscopic properties of the formamide molecule [24, 251. Using IR and NMR spectroscopy we have recently presented evidence for a break up of the solvent structure when electrolytes are dissolved in liquid formamide and for direct electrolyteformamide interaction [ 261. The use of vibrational spectroscopy to study non-aqueous solvents and their solutions is well established [27]. Vibrational spectral studies of electrolyte systems in aqueous and non-aqueous solvents have been comprehensively reviewed [ 28-30]_ Although formamide is a very good solvent for many electrolytes [ 311 the area of electrolyte-formamide interaction remains relatively unexplored. This work investigates the presence of electrolyte-formamide interactions by an analysis of the Raman spectra of electrolyte solutions. EXPERIMENTAL

Formamide (BDH, AnalaR) was left for several days over a 4 a molecular sieve (BDH) to remove the majority of absorbed water. This was further purified [32] by six successive fractional crystallizations under vacuum to exclude moisture, and thereafter kept in a sealed apparatus. Fractional distillation or fractional freezing methods in the presence of air 133-351 were found to be unsatisfactory due to the very hygroscopic nature of formamide. Salts were dried by heating to above 400 K under vacuum to constant weight.

63

The samples used for spectroscopic investigations were prepared in a nitrogen-filed glove bag. Raman spectra at ambient temperature were recorded from samples held in air-tight sample tubes or thin-walled capillaries on a Cary 81 spectrometer converted to 90” geometry. This incorporated a triple prism premonochromator, a Brookdeal 9511 quantum photometer and a cooled photomultipher tube. Spectral data were punched on to paper tape using a DVM-encoder system. The spectra were excited using the 530.9 nm (- 100 mW), 514.5 nm (- 800 mW) or 488.0 nm (- 800 mW) lines from either the Coherent Radiation Model 52 krypton ion or Mode1 CR2 argon ion lasers. Spectra were calibrated using CC4 and they are believed to be accurate to * 2 cm-’ RESULTS

AND DISCUSSION

The Raman spectrum of pure liquid formamide at ambient temperature is shown in Fig. 1. Raman spectra recorded from electrolyte solutions more dilute than 1:12 electrolyte:formamide mole ratio were approximately of this form; in these cases the spectra arise from the bulk solvent. For more concentrated electrolyte solueions the Raman spectra differ to varying degrees. Raman spectra (20~3600 cm-‘) were recorded at ambient temperature for the electrolyte solutions listed in Table 1. The N-H stretching region (3000-3600 cm-l) comprises several components and we have applied standard curve-resolution procedures [36J to this part of the spectrum using an IBM 370/168 computer. The program used to arrive at this resolution uses a variable Gauss~~-~orentzi~ sum function to generate the components. However, it was found in each case, that the most satisfactory fit was obtained using pure Gaussian functions. In some cases the spectral data were smoothed using a smoothing function prior to curve-resolution. With or ~thout smooth~g, the deconvoluted bands in the N-H stretching region are considered accurate to + 3 cm-l . It is convenient to discuss the spectral results for the 200-3000 cm-l and 3000-3600 cm-’ regions separately. TABLE

I

Electrolyte concentrations ratios

in liquid formamide expressed a~ electroiyte-formamide

mole

Electrolyte

MoIe ratio

EIectrolyte

Mole ratio

Electrolyte

Mole ratio

Electrolyte

Mole ratio

LiCl

1:25 1:4 1:s 1:lO 1:3 1:4 1:s

LiCIO,

I:4 1:8 1:12 1:8 1:4 1:s 1:s

LiSCN

1:4 1:6 1:8 1:12 1:8 1:8

NaClO,

1:6 1:8 1:8 I:8 1:8

LiNO,

NaNO, NaSCN NaI

LiBr LiI

KSCN Ca(NO,)> Mg(CiO,),

64

2UU--3000

~rn-~ region

The half-widths and intensities of the solvent bands in concentrated electrolyte solutions remain approximately unaltered from those of the pure solvent but the band positions are somewhat shifted. Table 2 lists the solvent band positions observed for Mg(C104)z :8HCONHz, LiC1:2.5HCONH2 and ~iSCN:4~C~N~* solutions. These solution band frequencies are typicti of those obtained for the more concentrated electrolyte solutions. Included in Table 2 are the band frequencies for pure formamide in the liquid [ZO] and gaseous [17-19f phases. It is noticeable that three band frequencies show differences 2 40 cm“ between the gas and liquid phase of pure formamide. The other bands remain relatively unchanged. This is interpreted with consideration to the assignment of these three bands and the configuration of the molecules in the respective phases. The bands due to pure liquid formamide at 2892 cm-‘, 1669 cm-’ and 1312 cm-’ have been assigned [9-14, ZO] to C-H stretching, a mixture of mainly C--O stretching with some C-N stretching, and the C-N stretching vibration, respectively. Liquid formamide has been shown to be associated [ZO] with a substantial proportion of resonance form II present. The gas phase molecules can be regarded as monomers and the corresponding vibrational frequencies are consistent with a greater proportion of the molecules being in resonance form I. The equivalent three bands for the electrolyte solutions are appreciably

shifted from those of the gas and liquid phase. Again the other bands are little affected.

The electrolyte-formamide interactions present in solution result in the following frequency shifts from those of the monomer molecule: (i) the band at 1253 cm-* for the monomer molecule shifts to higher frequency. This is attributed to the interaction of the electrolyte increasing the double TABLE

2

Observed formamide Raman Frequencies (200-3000 cm-‘) for Mg(C10,)2:8HCONH,, LiCI: 2.5HCONHz and LiSCN:4HCONHZ solutions. Vibrational frequencies are included for pure formamide in the gaseous (IR) and liquid (Raxnan) phases Pure HCONH, G&S

Liquid

2852

2892 2768 1669 1593 1391 1312 1097 1053 606

1740 1572 1378 1253 1092 607

Mg(ClO,),:8HCONH,

LiC1:2.5WCONH,

LiSCN:BHCONH,

2933 2772 1698 1594 1394 1338 1094 1054 604

2913 2770 1698 1597 1394 1322 1094 1056 608

2914 2771 1684 1595 1395 1321 1097 1054 608

65

bond character of the C-N bond; (ii) the band at 1740 cm-’ for the monomer molecule shifts to lower frequency. This is attributed to a weakening of the C-O bond on interaction with the electrolyte; (iii) the band at 2852 cm-l for the monomer molecule is shifted to higher frequency. Self-consistent MO calculations [37-391 have shown that the electron density on the carbon atom is increased on association whereas the electron density of the aldehyde hydrogen is unchanged. This is in agreement with a strengthening of the C-H bond on interaction with electrolyte. These results imply that on addition of electrolytes to liquid formamide there is transfer of electron density through the C-NH, and C-O bonds of the molecule; this arises from the interaction of oppositely charged ions at the nitrogen and oxygen ends of the formamide molecule (see Fig. 2). The interaction of electrolytes with liquid formamide is’thus similar to the intermolecular interactions of the solvent molecules themselves; both interactions tending to favour resonance form II. The electrolyte solutions are regarded as consisting of electrolyte-formamide species but with the retention of some residual bulk solvent structure. The observed formamide band frequencies in the Raman spectra for a series of concentrations of lithium chloride in formamide are listed in Table 3. The band in the region 1676-1698 cm-l, assigned to a mixture of the

C-O and C-N stretching vibrations, v(NCO), appears to shift in frequency with increasing concentrations. This is a consequence of the lithium cation interacting with the oxygen end of the molecule and the v(NCO) band arising from this interaction becoming dominant at higher concentrations. Similar spectral changes are observed for other electrolyte-formamide solutions. Table 4 lists the observed C-N, NC0 and C-H stretching frequencies for a series of 1:4 and 1:8 electrolyte-formamide mole ratio solutions. The results

0

AI

.

AH

H2N-C ‘o-

M’

I j,:.,

4

itix

I

I&C

IECC

M’

*

;cc

cm -1

Fig. 1. Raman Fig. 2. Proposed

spectrum

of pure liquid formamide

interaction

model

for electrolytes

at ambient

temperature.

in liquid Formamide.

TABLE

3

Cbs+ervcd formamide mide

Raman

frequencies

(200-3000

cm-‘)

for LiCl solutions

in forma-

LiCl : lOHCONH,

LiCl:8HCONH,

LiCI:4HCONH2

LiC1:2.5HCONHZ

2902 2770 1676 1598 1394 1316 1095 1056 608

2903 2770 1678 1596 1393 1317 1096 1056 607

2911 2770 1683 1595 1393 1320 1095 1056 608

2914 2770 1698 1597 1394 1322 1094 1056 608

TABLE

4

Observed formamide Raman frequencies (cm-‘) for a series of 1~4 and 1:8 electrolyteformamide mole ratio solutions; C-N stretching v(CN), NC0 stretching u(NC0) and C-H stretching u( CH) frequencies Electrolyte

u(CN)

v( NCO)

v(CB)

Electrolyte

I:4 LiClO, LiSCN LiNO, LiCl NaSCN

1325 1321 1320 1320 1314

1690 1684 1684 1683 1679

2925 2914 2913 2911 2894

1338 1320 1321 1319

1698 1679 1678 1678

2933

LiNO, LiI LiBr LiCl NaCIO, NaSCN NaNO ., NaI KSCN

1-8 Mg(CIO, WNO,),

u(CN)

v(NCO)

u(CB)

1318 1318 1317 1317 1316 1313 1313 1313 1312

1680 1681 1677 1678 1677 1677 1676 1676 1677

2904 2905 2903 2903 2900

l-8

1,

LiClO, LiSCN

2913 2913 2905

2896 2897

2893 2894

show that both the cation and the anion affect these formamide band frequencies, indicating that both ions interact with formamide in solution. 3000-3600

cm-’

region

The curve-resolved Raman spectra of the N-H stretching region of pure liquid formamide and a LiN03 :8HCONH2 mole ratio solution are shown in Fig. 3. The component characteristics of the best fit (standard errors approximately 0.01) are listed in Table 5. For LiN03 : SHCONH, the bands at 3331 cm-l and -3435 cm-1 have increased in intensity, whereas the bands at -3192 cm-1 and -3258 cm-1 are reduced in intensity, compared to the corresponding bands of pure liquid formamide- Furthermore, the intensities

67

> i-2 iE

E i 3635

3iw

Fig. 3. Curve-resolved Rarnan spectra of the N-H stretching region of (A) liquid forrnamide and (B) LiNO,:BHCONH,. (-) Experimental, (....) calculated. Fig. 4. Curve-resolved Raman spectra of the N-H stretching region of 1:12,1:8 mole ratio LiCIO,:HCONH, solutions. (-) Experimental, (- * - ) calculated.

TABLS

and I.:4

5

Curve-resolved Raman spectra of the N-H (b) Li~~,:SHC~~~~ Frequency

(cm+)

Width (cm-‘)

stretching region of (a) liquid fo~~ide

Area (a)

(a)

(b)

w

(b)

(aI

(b)

3439 3335 3260 3184 3083

3435 3331 3258 3192 3089

104 116 85 100 50

99 105 61 91 53

13.0 33.9 16.5 33.3. 3.5

17.8 40.6 12.4 25.0 4.2

and

68

of these bands are increased and reduced proportionally. This is particularly reflected by the values for the percentage area of these bands (Table 5). The low intensity band at 3089 cm-l for LiN03 :8HCONH2 is assigned to a combination band 1201. Similar observations were made for all the electrolyte solutions studied. The N-H stretching vibrations for the gas phase molecules have been previously reported at 3450 cm-l (Y,) and 3545 cm-’ (zJ,) [19]. Interaction of the electrolyte with the bulk solvent will cause a break up of the intermolecular hydrogen bonding and it will shift the N-H stretching vibrations by amounts which are proportional to the strength of the interactions present. These modes are superimposed on the complex band envelope due to residual bulk formamide. This accounts for the overall intensity pattern for the electrolyte solutions. Figure 4 illustrates the curve-resolved Raman spectra of the N-H stretching region obtained for 1:12, 1:8 and 1:4 lithium perchlorate formamide mole ratio solutions. The spectral data were smoothed using a 25 point quadratic smoothing function prior to curve-resolution. The component characteristics of the best fit (standard errors approximately 0.01) are listed in Table 6. The bands assigned to the symmetric (v,) and antisymmetric (v,) stretching vibrations of the electrolyte-formamide species become more intense and are seen to emerge from the complex band envelope of the bulk solvent as the electrolyte concentration is increased. The positions of the vN-H due to the electrolyte-formamide species shift progressively to higher frequency as the electrolyte concentration is increased. This is because the curve resolved bands contain a decreasing contribution from the bulk solvent bands. Variations in the N-H stretching region for a series of electrolyteformamide solutions at any constant concentration are not readily apparent. Differences between electrolytes are hence relatively small compared to the large band widths of the curve-resolved bands. One feature in the complex band envelope spectra requires comment. The spectra recorded from electrolyte solutions containing the ClO, ion (see Fig. 4, Table 6 for example) were

TABLE

6

Curve-resolved Raman ratio LiClO,:HCONH,

spectra of the N-H solutions

)

stretching

region of 1:12,

1:s

and 1:4

mole

Frequency

(cm-’

1:12

1:8

1:4

1:12

1:8

1:4

1:12

1:8

1:4

3453 3345 3255 3181 3084

3455 3356 3266 3182 3086

3475 3371 3279 3194 3088

75 109 82 80 62

71 94 96 88 51

62 80 82 72 54

12.6 40.9 20.9 19.5 6.1

14.1 36.8 27.3 18.0 3.8

16.0 49.3 22.0 8.4 4.3

Width

(cm-l)

Area (%)

69

markedly different from other electrolyte solution spectra (of which Fig. 3(b), Table 5(b) is typical). The bands assigned to symmetric (v,) and ~tisymmet~~ (v,) stretching vibrations of these electroiyte-formamide species illustrate a greater frequency shift from pure liquid formamide than the other electrolyte solutions. For example, the high frequency N-H stretching modes of Mg(C104 )2 : 8HCONH2 are centred at - 3362 cm-* and - 3463 cm-‘, approximately 20-30 cm- 1 higher in frequency than recorded for electrolyte solutions which do not contain the Cl& ion. This large frequency shift is due to the weak ~~raction of the perchlorate ions fo~ow~g a break up of the solvent structure_ ACKNOWLEDGEMENTS

Gratitude is extended to Professor R. C. Rumfeldt (University of Windsor) for some helpful comments. A. J. L. thanks the S.R.C. for a research studentship. REFERENCES 1 R. J. Kurland and E. B. Wilson, J. Chem. Phys., 27 (1957) 585. 2 C. C. &stain and J. M. Dowling, J. Chem. Phys., 32 (1960) 158. 3 E. Hirota, R. Sugisaki, C. J. Nielsen and G. 0. Sorensen, J. Mol. Spectrosc., 49 (1974) 251. 4 B. Sunners, L. H. Piette and W. G. Schneider, Can. J. Chem., 38 (1960) 681. 5 H. Karnei, Bull. Chem. Sot. Jpn., 41(1968) 2269. 6 T. Drakenberg and S. Forsen, J. Phys. Chem., 74 (1970) 1. 7 T. Miyazawa, J. C&em. Sot. Jpn. Pure Chem. Sec., 76 (1955) 821. 8 K. Itoh and T. Shimanouchi, J. Mol. Spectrosc., 42 (1972) 86. 9 J. Lecomte and R. Freymann, Bull. Sot. Chim. Fr., 8 (1941) 612. 10 K. V. Ramiah and P. G. Puranik, Proc. Ind. Acad. Sci. A, 56 (1962) 96. 11 C.H. Smith and R. H. Thompson, J. Mol. Spectrosc., 42 (1972) 227. 12 I. Suzuki, Bull. C&em, Sot. Jpn., 33 (1960) 1359. 13 K. V. Ram&h and V. V_ Chalapathi, Proc. Ind. Acad. Sei. A, 58 (1963) 233. 14 P. G. Puranik and K. V. Ramiah. J_ Mol. Spectrosc., 3 (1959) 486. 15 P. J. Krueger and D. W. Smith, Can. J. Chem., 45 (1967) 1611. 16 E. A. Cutmore and H. E. Ha&m, Spectrochim. Acta, Part A, 25 (1969) 1767. 17 J. C. Evans, J. Chem. Phys., 31 (1959) 1435. 18 S. T. King, J. Phys. Chem., 75 (1971) 405. 19 J. C. Evans, J. Chem. Phys., 22 (1954) 1228. 20 D. J. Gardiner, A. J. Lees and B. P. Straughan, J. Mol. Struct., 53 (1979) 15. 21 J. LadelI and B. Post, Acta Crystallogr., 7 (1954) 559. 22 A_ Pullman, H. Berthod, C. Griessner-Prettre, 3_ F. Hinton and D. Harpool, J. Am. Chem_Soc.,100(1978)3391. 23 J. Bukowska, Spectrochim. Acta, Part A, 35 (1979) 985. 24 D. Balasubramanian, A. Gael and C. N. R. Rao, Chem. Phys. Lett., 17 (1972) 482. 25 D. Balasubramsnian and R. Shaikh, Biopolymers, 12 (1973) 1639. 26 A. J. Lees, B, P. Straughan and D. J. Gardiner, J. Mol. Struct., 54 (1979) 37. 27 D. J. Gardiner, Adv. 1-R. Raman Spectrosc., 3 (1977) 167. 28 D. E. Irish and M. H. Brooker, Adv. IR Raman Spectrosc., 2 (1976) 212. 29 D. E. Irish, in S. Petrucci (Ed.), Ionic Interactions, Vol. 2, Academic, New York, 1971, p_ 188.

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