Raman spectroscopy of the acetates of sodium, potassium and magnesium at liquid nitrogen temperature

Journal of Molecular Structure 526 (2000) 131–141 www.elsevier.nl/locate/molstruc

Raman spectroscopy of the acetates of sodium, potassium and magnesium at liquid nitrogen temperature R.L. Frost*, J.T. Kloprogge Centre for Instrumental and Developmental Chemistry, Queensland University of Technology, 2 George Street, GPO Box 2434, Brisbane, Queensland 4001, Australia Received 9 November 1999; received in revised form 24 January 2000; accepted 7 February 2000

Abstract A comprehensive Raman spectroscopic study of the acetates of potassium, sodium and magnesium in the solid state have been made at both 298 and 77 K. Band separation of the CyO stretching region was not achieved in the 298 K spectra but was in the 77 K spectra. The CIO stretching vibration is observed as a single band in both the 298 and 77 K spectra and its frequency is cation dependent. Single C–C stretching bands are observed for the acetates in the 77 K spectra. The OCO deformation vibrations were also cation dependent. Low frequency vibrations of magnesium acetate are observed at 338, 253 and 268 cm ⫺1 and are assigned to the MgO stretching vibration of the magnesium bisacetato complex. Low frequency bands were also observed for sodium acetate at 219, 277 and 288 cm ⫺1. 䉷 2000 Elsevier Science B.V. All rights reserved. Keywords: Potassium acetate; Sodium acetate; Magnesium acetate; Raman microscopy

1. Introduction Vibrational spectroscopic studies of acetates in both aqueous media and solid state have been comprehensive and have been studied over a long period of time [1–4]. One of the difficulties of studying the spectroscopy of acetates is the variation of data reported in the literature (Table 1). Raman spectra of aqueous acetate solutions are more easily obtained than infrared spectra and can help in the understanding of the solid state spectra of metal acetates. In recent years, spectra have been published on e.g. Mg-, Ni-, Co- and Na-acetates both in solution and in the solid state [1,5–8]. Most of the Raman spectro* Corresponding author. Tel.: ⫹61-7-3864-2407; fax: ⫹61-73864-1804. E-mail address: [email protected] (R.L. Frost).

scopy of these salts concern room temperature measurements and are not always consistent. Raman spectroscopy at liquid nitrogen temperature may help in resolving these inconsistencies. Solid phases like hydrated Mg- and Na-acetate and anhydrous Kacetate will differ in their crystal structures, therefore it can be expected that these differences are reflected in the Raman spectra. For example, in sodium acetate the CH3-group will show free rotation resulting in a higher symmetry than in Mg- or Ni-acetates where this rotation is partly or completely lost resulting in a lowering of the acetate space group symmetry from C2v to Cs [5]. However, others assume that even though rotation is limited the space group symmetry C2v is preserved [1,7]. Lowering the temperature at which the Raman spectra of the solid acetates are measured to liquid nitrogen temperature may reduce the orientational freedom of the acetate group causing

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132

Symmetry Class

Assignment

n1 n n4 n5

C–H str. Me def. C–O str. C–C str. COO def.

A2

n6

Torsion

B1

n7 n8 n9 n 10 n 11 (in plane)

C–H str. C–O str. Me def. Me rock. COO rock.

n 12 n 13 n 14 n 15

C–H str. Me def. Me rock. COO rock.

A1

B2

Acetic acid [18]

1368 892

1710 1431 1014

Aqueous solution Na-acetate [5]

Solid Na-acetate [4]

Solid Na-acetate [5]

Aqueous Aqueous solution solution Ni acetate [1] Ni acetate [5]

Solid Ni-acetate hydrate [1]

Solid Ni-acetate hydrate [5]

Solid Mg-acetate hydrate [6]

2933P 1344P 1420P 930P 653P

2936vs 1340 1414 924 664

2941 – 1426 935 650

2938 – 1430 932 654

2943 1357 1426 932 659

2936 1354 1432 962 680

2925 1350 1438 963 684

^ 1350 1436 947

–

–

–

–

–

118

–

116

3013 or 2980 1556 1429 1020 480

2989 1578 1443 1009 460

2983 1568 – – 478

– 1564 – 1020 500

2992 1575 – 1028 480

2992 1514 – 1030 502

– 1524 – 1030 508

2984 1520

2980 or 3013 1456 1050 621

– – 1042 615

3016 – 1030 617

– – 1070 620

– – – 624

3030 1454 1060 622

– 1458 1070 624

Solid Mg-acetate hydrate [7]

948 676

Solid Co-acetate hydrate [7]

952 676

Solid Co-acetate hydrate [8]

Solid K-acetate anhydrous [27]

2932 1352 1424 952 676

1402 919 644

1421 1028

1026

497

1455 1062 622

1058 628

1026 502

3020 1452 1058 620

1573

R.L. Frost, J.T. Kloprogge / Journal of Molecular Structure 526 (2000) 131–141

Table 1 Raman wavenumbers and vibrational assignments (str., stretching; def., deformation; rock., rocking; P, polarised) for the acetate group in various metal acetates in solutions and in the solid state

R.L. Frost, J.T. Kloprogge / Journal of Molecular Structure 526 (2000) 131–141

loss of degeneracy, which can be a helpful tool in interpreting and understanding of the Raman spectra of these acetates. Potassium acetate and similar molecules readily insert between kaolinite layers causing expansion along the c-axis direction [9–16]. This process is known as intercalation. Changes in both the structures of the kaolinite and the inserting molecules occur during intercalation. Often changes in the spectra of the hydroxyl-stretching region are reported but most of the times without the changes in the spectra of the inserting acetate salt. Knowledge of the interaction of the acetate or similar molecules with the kaolinite active surface sites is important to the study of the surface properties of kaolinites. The question arises as to the state of the inserted potassium acetate molecule within the kaolinite structure. Firstly all the acetate molecules are in the solid state even though the preparation route involves dissolution of the acetates in aqueous media prior to the intercalation [9– 12]. Consequently, for a further understanding of the spectroscopy of the intercalated acetates in kaolinites the spectroscopy of the acetate solid state is required. The question arises as to whether the acetates are present in the kaolinite interlayer as separate molecules or that they form some sort of crystalline complexes within the intercalated kaolinite. The situation is even further complicated due to the incorporation of water into the intercalation complex [9–12]. Therefore, the interaction of water with acetates needs to be considered and the influence of water on the spectroscopy of the acetate needs investigation. The acetate bonds to the hydroxyl surface on a 1:1 basis and therefore the acetates should probably be considered as molecular rather than being in the crystalline state. One of the difficulties of studying the spectroscopy of acetates is the variation of data reported in the literature. No satisfactory crystalline structure of sodium acetates is known [17]. The solution and solid state spectra of sodium acetate are said to be identical [18]. Thus the cation–anion interactions are weak for this acetate. This may not be the case when the acetate is intercalating kaolinite layers. Considerable variation in the literature exists for the carboxylate vibrations. For example the antisymmetric stretching vibration varies from 1574 to 1583 cm ⫺1 and the symmetric stretching vibration

133

⫺1

from 1414 to 1435 cm [19,20]. No doubt all reported values are correct and the numbers simply depend on the water content of the crystals and the degree of hydration of the sodium cation. Simple dilution in water can shift the frequencies of the antisymmetric and symmetric stretching modes by tens of wavenumbers [18]. Thus, the degree of hydration and the water content of the kaolinite intercalation complex may well determine the spectroscopic characteristics of the acetate in the intercalation complex. The objective of this work is to determine the spectral characteristics of the acetates of sodium, potassium and magnesium. In this paper, we report the Raman spectra of the acetates of sodium, potassium and magnesium at liquid nitrogen temperature. 2. Theory Vibrational spectra of the acetate ion, CH3CO2⫺, in sodium acetate have been assigned [21] on the basis of C2v molecular symmetry, assuming free rotation of the CH3 group about the C–C bond. This formula of acetate leads to a theoretical 15 degrees of vibrational freedom with the following vibrational spectroscopic activity:

G vib ˆ 5A1 ⫹ A2 ⫹ 5B1 ⫹ 4B2 in which the (5A1⫹5B1⫹4B2) modes are all Raman and infrared active; the A2 mode is a methyl torsion and is active in the Raman but inactive in the infrared. Magnesium acetate tetrahydrate is monoclinic with 5 : In this space group the site the space group C2h symmetries for Mg 2⫹, CH3COO ⫺ and H2O are Ci, C1 and C1, respectively. The Mg ion, being the inversion symmetry is surrounded by an octahedron formed from two pairs of oxygen atoms from water and one from each of the two acetate ligands. When acetate ion coordinates to a metal like magnesium, the symmetry of the acetate ligand in the resulting acetato compound is possibly lowered to Cs and in that case the vibrational activity of the Mg–CH3CO2 entity is given by:

G vib ˆ 12A 0 ⫹ 6A 00 all of which are Raman and infrared active. The three additional vibrational modes for the coordinated acetato-species of Cs symmetry arise from the n

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(Mg–O stretching, A species) and d (Mg–C deformation, A 00 species) and d (OCO deformation, A 00 species). The two n (CO2 stretching) modes of the free acetate ion (A1 ⫹ B2) will also now become n (C–O stretching, A 0 ) and n (C–O stretching, A 00 ). Essentially, other vibrations, such as n (C–C stretching) or n (CH stretching) will remain either unchanged or similarly modified by symmetry requirements. In solution, the Mg 2⫹ ion is hydrated and best represented as the hexa-aquo species, Mg (H2O)62⫹, of Oh symmetry, for which the symmetric stretching band n (Mg–O) is expected to be the strongest in the Raman spectrum. This mode is also expected in the Raman spectrum of the Mg(CH3CO)2·4H2O solid, in which there are four molecules of coordinated water per magnesium atom. In contrast to Mg- and Na-acetate, K-acetate does not contain any crystal-water, which will result in a different crystal structure in which the K cation is only coordinated by acetate groups. This will have a significant impact on especially the low frequency region where various metal–oxygen vibrations can be expected. Especially the M–O modes of the coordinated water molecules will be absent in the spectrum of K-acetate. In summary, the Raman spectra of solid sodium acetate and its aqueous solution are expected to be characteristic of the CH3CO2⫺ species, whereas those of magnesium (II) acetate should show evidence of additional vibrational features in the n (Mg–O), d (COO) and n (CO) regions if the acetato complex formation occurs. In aqueous solutions of magnesium acetate, therefore, we would expect to observe bands arising from free CH3CO2⫺ as well as from the complex. 3. Experimental The acetates of sodium and magnesium both exist as hydrates in the solid state. Sodium acetate recrystallises as the trihydrate and magnesium acetate as the tetrahydrate. Potassium acetate does not exist as a hydrate but only as an anhydrate. The acetates of sodium, potassium and magnesium were recrystallised twice and dried in a desiccator before spectroscopic measurement. 3.1. Raman microprobe spectroscopy For spectra at 298 K, acetate salts were placed on a

polished stainless steel surface on the stage of an Olympus BHSM microscope, equipped with 5 × , 20 × and 50 × objective lenses. The microscope is part of a Renishaw 1000 Raman microscope system, which also includes a monochromator, a filter system and a charge coupled device (CCD) as the detector. Raman spectra were excited by a Spectra-Physics model 127 HeNe laser (633 nm), recorded at a resolution of 2 cm ⫺1 in sections of 1000 cm ⫺1. Repeated acquisitions using the highest magnification, were accumulated to improve the signal to noise ratio in the spectra. For the 298 K spectra, data were collected at 20-s intervals for 10 min at maximum magnification (50 × ). Spectra were calibrated using the 520.5 cm ⫺1 line of a silicon wafer. Spectra at liquid nitrogen temperature were obtained using a Linkam thermal stage (Scientific Instruments Ltd, Waterfield, Surrey, England). Samples were placed in several stainless steel cups, fitted over the silver plate of the thermal stage. Samples were cooled at 20⬚ per minute until a constant temperature was achieved. Spectra were obtained using 20-s scans for 20 min using the special short 50 × (ULWD) objective. The intensity of the acetate spectra were found to be decreased in intensity by 0.3 using the ULWD objective compared with the normal 50 × objective. Spectral manipulation such as baseline adjustment, smoothing and normalisation was performed using the Spectracalc software package grams 䉸 (Galactic Industries Corporation, Salem, NH, USA). Band component analysis was undertaken using the Jandel ‘Peakfit’ software package which enabled the type of fitting function to be selected and allows specific parameters to be fixed or varied accordingly. Band fitting was done using a Lorentz–Gauss cross-product function with the minimum number of component bands used for the fitting process. The Gauss–Lorentz ratio was maintained at values greater than 0.7 and fitting was undertaken until reproducible results were obtained with squared correlations of r 2 greater than 0.995.

4. Results and discussion 4.1. The CyO stretching vibrations When the salts of carboxylic acids such as acetic

R.L. Frost, J.T. Kloprogge / Journal of Molecular Structure 526 (2000) 131–141

135

Fig. 1. The Raman spectra of the CyO stretching vibration of the acetates of: (a) sodium; (b) potassium; (c) magnesium at 298 K.

acid are formed, the CyO and C–O bonds are replaced by two equivalent CIO bonds with a bond order of 1.5 [3,22–25]. These two CIO bonds interact as in-phase and out-of-phase vibrations to

give two bands: the antisymmetric stretching vibration which is intense in the infrared spectra but weak to very weak in intensity in the Raman spectra, and a symmetric stretching band which is weak in intensity

Table 2 Raman wavenumbers and vibrational assignments (str., stretching; def., deformation; rock., rocking) for the acetate group in solid sodium acetate, potassium acetate and magnesium acetate at room temperature and at liquid nitrogen temperature Symmetry

Class

Assignment

Sodium acetate

Potassium acetate

Magnesium acetate

297 K

77 K

297 K

77 K

297 K

77 K

2935 1347 1335 1424 929 658

2931 1348 1337 1430 925 652

2930 1344

2930 1336

2933 1350

2932 1350

1437 923 650

1438 917 648

1438 945 671

1436 951 678

–

–

–

–

–

–

2980 1599

2983 1564

2981 1584

2982 1516

2984 1510

1411 1018 473

1407 1018 471

1417 1015 463

1420 1026 494

1417 1029 497

3006 1451 1045 625 635

3001 1452 1052vw 617

3006 1453 1045 622

3018 1446 1059 610

3021 1462 1061 612

n1 n2

C–H str. Me def.

n3 n4 n5

C–O str. C–C str. COO def.

A2

n6

Torsion

B1

n7 n8

C–H str. C–O str.

n9 n 10 n 11

Me def. Me rock. COO rock. (in plane)

2979 1562 1560 1410 1016 472

n 12 n 13 n 14 n 15

C–H str. Me def. Me rock. COO rock.

3012 1448 1052 619

A1

B2

136

R.L. Frost, J.T. Kloprogge / Journal of Molecular Structure 526 (2000) 131–141

Fig. 2. The Raman spectra of the CyO stretching vibration of the acetates of: (a) sodium; (b) potassium; (c) magnesium at 77 K.

in the infrared spectra but strong and polarised in the Raman spectra. These two vibrations are normally found in the 1540–1650 and 1360–1450 cm ⫺1 regions, respectively [23,24]. The Raman spectra of the CIO symmetric stretching region of the acetates of sodium, potassium and magnesium at 298 K are shown in Fig. 1. The Raman spectra of the symmetric stretching region for Na, K and Mg show bands at 1424, 1437, and 1438 cm ⫺1. The position and assignment of these bands are in good agreement with previous studies [21,25]. The figure also displays the acetate methyl HCH deformation vibration (n 2) (Table 2). This band is observed around 1350 cm ⫺1. The Raman spectra of the acetates of Na, K and Mg show very low intensity, broad complexes with maxima at 1562, 1564 and 1516 cm ⫺1 which are attributed to the antisymmetric stretching vibrations (Table 2). The positions of various bands within these complexes are difficult to determine with precision and the results differ from earlier published data probably because of this reason. Upon cooling to 77 K, the Raman spectra of the 1500–1700 cm ⫺1 region of the CIO antisymmetric stretching region does not show much change although a possible split in the antisymmetric stretching mode is observed for sodium acetate (Table 2). Like at room temperature these bands are still of very low intensity and do not show increased

splitting due to the cooling to liquid nitrogen temperature. The CIO symmetric stretching region of sodium acetate shows a double band at 1448 and 1424 cm ⫺1 in the 298 K spectrum (Fig. 1). The Raman spectrum of potassium acetate shows complexity in the CIO symmetric stretching region with three bands at 1452, 1437 and 1407 cm ⫺1. The band at 1452 cm ⫺1 may be assigned to the n 13 mode attributed to the methyl deformation vibration (Table 2). The band at 1437 cm ⫺1 is attributed to the CIO stretching vibration of the acetato group and the band at 1407 cm ⫺1 to the methyl deformation mode n 9. For magnesium acetate at 298 K, the spectral profile of the CIO stretching region also shows three bands at 1446, 1438 and 1420 cm ⫺1. Although we have a nominal band resolution of 2 cm ⫺1 for the spectrometer, the spectra for this region do not show the band separation of previously published data [6]. Nevertheless the three curve resolved bands observed in this work agree with the published data. The CIO stretching mode (n 3) lies very close to the CH3 deformation modes labelled n 9 and n 13. Thus the suggested assignments are that as for potassium acetate, the 1419 and 1424 cm ⫺1 bands represent the n 9 mode and the 1446 and 1448 cm ⫺1 bands the n 13 vibration for Mg- and Na-acetate. Three vibrations are observed at 1451, 1430, and

R.L. Frost, J.T. Kloprogge / Journal of Molecular Structure 526 (2000) 131–141 ⫺1

1411 cm for sodium acetate at 77 K (Fig. 2) with relative intensities of 16.9, 29.3 and 33.6%. The question arises as to the assignment of these bands. The band fitting shows two bands in the 298 K spectrum at 1424 and 1448 cm ⫺1. However the bandwidths of the calculated bands are twice that for potassium and magnesium. Therefore, bandfitting with three bands in the 1400–1450 cm ⫺1 region were fitted. Bands were then observed at 1446, 1424 and 1410 cm ⫺1. These bands together with the band at 1349 cm ⫺1 are symmetric in profile and no indication is given for additional bands in this region. The band at 1451 cm ⫺1 may be assigned to the n 13 mode attributed to the methyl deformation vibration (Table 2). The n 2 mode observed as a double band in the 298 K spectrum at 1349 and 1335 cm ⫺1 shifts slightly to 1348 and 1337 cm ⫺1 after cooling to 77 K. Therefore, the double band at 298 K interpreted as the n 3 and n 13 acetate vibrations for sodium acetate resolves into three bands at liquid nitrogen temperature which can be attributed to n 3, n 9 and n 13 of the acetate group. The band observed at 1424 cm ⫺1 in the 298 K spectrum of sodium acetate is attributed to ‘free’ acetate since bands are observed in similar frequencies for aqueous sodium acetate solutions. Therefore, the band at 1430 cm ⫺1 at liquid nitrogen temperature is attributed to the ‘free’ acetate similar to the CIO stretching vibration n 3 observed at 1420 cm ⫺1 for aqueous solutions of sodium acetate. In the case of potassium acetate, at liquid nitrogen temperature, three bands are observed at 1452, 1437 and 1407 cm ⫺1 (Fig. 2b). The highest wavenumber is probably the n 13 mode. The band at 1407 cm ⫺1 may be the n 9 mode. This would then leave the band at 1437 cm ⫺1 as the CIO stretching vibration n 3. The band 1439 cm ⫺1 is in a position similar to that observed for the CIO stretching vibration of nickel acetate [1,5]. Such a band has been defined as the acetate coordinating to the cation. It is therefore concluded that the 1437 cm ⫺1 band of potassium acetate is the CIO stretching vibration of the coordinating acetate. The Raman spectrum of magnesium acetate in this region is simpler. The Raman spectra of the CIO stretching region of magnesium acetate shows bands at 1446, 1438, and 1420 cm ⫺1 with relative intensities of 16.7, 28.1, and 28.5%. The 1446 cm ⫺1 band is assigned to the methyl deformation n 13 mode. The other two bands at 1438 and 1420 cm ⫺1

137

are therefore assigned to CIO stretching vibration n 3 and methyl deformation vibration n 9. 4.2. The HCH deformation and rocking vibrations The HCH bending vibrations for the three acetates studied in this work are shown in Fig. 1. The band is observed in the 298 K spectra at 1347 cm ⫺1 accompanied by a low intensity band around 1335 cm ⫺1 for sodium acetate, 1344 cm ⫺1 for potassium acetate and 1350 cm ⫺1 for magnesium acetate. The bandwidth of the HCH deformation vibration for the three acetates are 5.7, 18.1 and 10.8 cm ⫺1, respectively. The fact that the band for potassium acetate is so broad suggests the possible existence of more than one band in the spectral profile, particularly when compared with the double band observed for sodium acetate. As compared with the acetates of magnesium and sodium, potassium acetate may be obtained as the anhydrous salt. Potassium acetate is very deliquescent and may dissolve in adsorbed atmospheric water. It is therefore essential to obtain the spectra of potassium acetate under a nitrogen atmosphere. Upon cooling to liquid nitrogen temperature, the potassium acetate shows a single band in the HCH bending region (Fig. 2). For sodium acetate, the band observed in the 298 K Raman spectra at 1349 and 1335 cm ⫺1 shift slightly upon cooling to 77 K to 1348 and 1337 cm ⫺1. In contrast to sodium acetate, magnesium acetate shows only a single symmetric band at 1350 cm ⫺1 at room temperature, which does not shift upon cooling to 77 K. The HCH bending vibrations at room temperature and at liquid nitrogen temperature for the methyl groups of both magnesium and potassium acetate are identical. This suggests that the methyl groups are all identical in their structures. A low intensity band is observed at 1016 cm ⫺1 in the 298 K spectrum of solid sodium acetate. A second band is also observed at 1052 cm ⫺1. Similar bands are observed at 1020 and 1050 cm ⫺1 in aqueous sodium acetate solutions and have been assigned to the methyl rocking vibrations n 10 and n 14 [3,5]. The 1016 cm ⫺1 band shifts to 1018 cm ⫺1 at liquid nitrogen temperature. For nickel acetate(II) solution and solid, comparable bands were observed at 1030 and 1028 cm ⫺1 but the split between these two bands is much smaller [26]. The acetate in this type of solid is coordinated to the cation. For potassium acetate at 298 K, a very

138

R.L. Frost, J.T. Kloprogge / Journal of Molecular Structure 526 (2000) 131–141

Fig. 3. The Raman spectra of the C–C symmetric stretching region of the acetates of: (a) Na; (b) K; (c) Mg at 77 K.

weak band is observed at 1018 cm ⫺1. Upon cooling to liquid nitrogen temperature, this band shifts in the opposite direction compared to sodium acetate to 1015 cm ⫺1. Magnesium acetate reveals a shift similar to that of sodium acetate from 1026 to 1029 cm ⫺1 upon cooling from room temperature to 77 K. This difference in either blue or red shift may be explained by the fact that both sodium and magnesium acetate contain crystal-water influencing the freedom in bending motion of the H–C–H bond in the methyl group, whereas potassium acetate is anhydrous. 4.3. The C–C symmetric stretching vibration The 850–1000 cm ⫺1 region is quite informative for the study of the spectroscopy of solid acetates [5,25,26]. The bands observed at around 930 cm ⫺1 are assigned to the C–C symmetric stretching vibration (n 4). This mode has a high scattering intensity and is readily observed. The band is observed at 892 cm ⫺1 for acetic acid and is at 930 cm ⫺1 for aqueous solutions of sodium acetate [5]. This band shifts to higher wavenumbers upon complexation with a metal cation like Mg or Ni but not with alkali metal cations like K or Na. Fig. 3 displays the Raman spectra of the C–C symmetric stretching vibration for the acetates of sodium, potassium and magnesium at

77 K. The 298 K spectra for sodium acetate shows a single band observed at 929 cm ⫺1 which, upon cooling to liquid nitrogen temperature shifts to 925 cm ⫺1. These values are in good agreement with previously published data [5]. The C–C stretching vibration for potassium acetate also shows a shift from 923 to 917 cm ⫺1 upon cooling to 77 K. The Raman spectrum of the C–C stretching region of magnesium acetate at 77 K consists of a single vibration at 951 cm ⫺1. The value of 951 cm ⫺1 corresponds well with the value of acetate coordinated magnesium [7] and with the value of 963 cm ⫺1 observed for the C–C stretching vibration of nickel acetate [5]. The concept of magnesium acetate existing as a bisacetato complex is also supported by the observation of a peak at 1060 cm ⫺1. Peaks at these positions correspond to the methyl rocking vibrations of coordinated acetate. Such bands have been observed for nickel acetate [5]. 4.4. The O–C–O bending region of acetates Raman bands in the 600–680 cm ⫺1 region are ascribed to the OCO bending of the acetates and the 77 K spectra are shown in Fig. 4 for the three acetates studied in this work. Sodium acetate shows two bands in the 298 K Raman spectra at 658 and 619 cm ⫺1 with

R.L. Frost, J.T. Kloprogge / Journal of Molecular Structure 526 (2000) 131–141

139

Fig. 4. The Raman spectra of the O–C–O bending region of the acetates of: (a) Na; (b) K; (c) Mg at 77 K.

relative intensities of 83 and 17%. The 298 K Raman spectrum of potassium acetate has two bands at 650 and 617 cm ⫺1 with relative intensities of 68 and 32%. Magnesium acetate has only a single Raman band at 671 cm ⫺1 in the 298 K spectra. These results fit well with the observations of the symmetric stretching vibrations. For magnesium acetate, only one symmetric stretching vibration was observed at 1433 cm ⫺1. For potassium acetate two symmetric stretching modes and two bending modes were observed. Upon cooling to liquid nitrogen temperature, the Raman spectra at 77 K for sodium acetate shows complexity with three bands at 652, 635 and 625 cm ⫺1. Potassium acetate has two bands at 648 and 622 cm ⫺1. Two bands are observed in the 77 K spectra of magnesium acetate at 678 and 612 cm ⫺1. The spectrum of the OCO deformation region of magnesium acetate is significantly different from that of the sodium or potassium acetates and is probably related, as suggested above, with the difference between the coordination of acetate groups around metals like Mg, Ni or Co and alkali metals like Na and K.

bands are normally found in the 480–510 cm ⫺1 region [24]. Bands are observed at 472, 471 and 495 cm ⫺1 in the 298 K spectra of sodium, potassium and magnesium acetates, respectively. These bands shift upon cooling to liquid nitrogen temperature to 473 cm ⫺1 for NaCH3COO, 463 cm ⫺1 for KCH3COO, and 497 cm ⫺1 for Mg(CH3COO)2. The bands at 472 and 471 cm ⫺1 for the sodium and potassium acetates have been assigned to the OCO in-plane rocking mode n 11 of the free CH3COO ⫺ [21]. The band for magnesium acetate is found at a significantly higher frequency and the position is in good agreement with previously published data [6]. A comparably high frequency band has been observed for nickel(II) acetate at 508 cm ⫺1 [5]. This band was assigned to the inplane rocking of the coordinated acetato ligands. Likewise the band at 493 cm ⫺1 for magnesium acetate is assigned to the acetato ligand coordinated to the magnesium [6]. The fact that no band is observed at ⬃470 cm ⫺1 suggests that no free acetate exists and that all of the acetate is coordinated to the magnesium. 4.6. Low wavenumber region

4.5. The O–C–O in-plane rocking of acetates Additional information may be also obtained by studying the in-plane OCO rocking vibration. These

In the low-wavenumber region of the Raman spectra of magnesium acetate at liquid nitrogen temperature, bands are observed at 212, 256, 270,

140

R.L. Frost, J.T. Kloprogge / Journal of Molecular Structure 526 (2000) 131–141

Fig. 5. The Raman spectra of the low frequency region of magnesium acetate and sodium acetate at 77 K.

301 and 341 cm ⫺1 (Fig. 5). The equivalent bands are observed in the 298 K spectra at 207, 246 with a shoulder at 258, 289 and 333 cm ⫺1 (Table 3). Previous studies did not report bands in this region [6]. Bands have been reported for nickel(II) acetate at 168, 224, 272 and 379 cm ⫺1 [5]. The most intense band in this region is the band at 338 cm ⫺1 in the 77 K spectra. It is proposed that this band be ascribed to the n (MgO) stretching vibration of the aquo complex of the bisacetato tetraaquo complex. A band occurs in this position in saturated solutions of magnesium acetate and is polarised. The band observed at 246 cm ⫺1 in the 298 K spectra and which is split into two bands at 256 and 270 cm ⫺1 in the 77 K spectra is attributed to the n (MgO) vibration of the acetato magnesium

complex. Bands in this position are absent from aqueous magnesium chloride solutions. The bands at 289 cm ⫺1 in the 298 K spectrum and at 301 cm ⫺1 in the 77 K spectrum are attributed to the water librational modes of the coordinated water. The bands at 207 (298 K) and 212 (77 K) cm ⫺1 are attributed to lattice vibrations. Interestingly, bands are also observed for sodium acetate at 219, 277 and 288 cm ⫺1. The band at 219 cm ⫺1 is attributed to a lattice vibration and the other two bands to water librational modes.

5. Conclusions The Raman spectroscopy of the acetates of sodium,

Table 3 Raman wavenumbers and vibrational assignments (str., stretching; def., deformation; rock., rocking) for the acetate group in solid sodium acetate, potassium acetate and magnesium acetate at room temperature and at liquid nitrogen temperature Assignment

Lattice vibration n (M–O) stretch aceto complex n (M–O) stretch aceto complex n (H2O) libration n (M–O) stretch aquo complex

Solid Mg-acetate

Solid Na-acetate

Solid Ni-acetate [5]

298 K

77 K

77 K

298 K

207 246 258 sh 289 333

212 253 268 301 338

219

168 224

277 288

272 379

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potassium and magnesium at liquid nitrogen temperature enabled more well-defined band separation. This band separation resulted in the observation of additional bands not observed in the 298 K spectra. Definition of the bands in the CyO, C–C stretching, the CH3 rocking and the HCH deformation vibrations were obtained. It is proposed that at liquid nitrogen temperatures the bands resulting from the coordination of the acetate to the magnesium cation is more readily obtained. The use of the Raman microprobe in conjunction with the thermal stage enabled the spectra of the acetates at liquid nitrogen temperature to be obtained. Acknowledgements The financial and infra-structural support of the Queensland University of Technology, Centre for Instrumental and Developmental Chemistry is gratefully acknowledged. References [1] G.S. Raghuvanshi, M. Pal, M.B. Patel, H.D. Bist, J. Mol. Struct. 101 (1983) 7. [2] G.S. Raghuvanshi, D.P. Khandelwal, H.B. Bist, Spectrochim. Acta A37 (1981) 89. [3] K. Ito, H.J. Bernstein, Can. J. Chem. 34 (1956) 170. [4] L.H. Jones, E.J. McLaren, J. Chem. Phys. 22 (1954) 1796. [5] R.I. Bickley, H.G.M. Edwards, S.J. Rose, R. Gustar, J. Mol. Struct. 238 (1990) 15. [6] Zh. Nickolov, I. Ivanov, G. Georgiev, D. Stoilova, J. Mol. Struct. 377 (1996) 13.

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