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Vibrational study of two novel cesium saccharinates. Spectroscopic evidence for organic molecule embedded in ionic salt
Vibrational Spectroscopy 24 Ž2000. 201–211 www.elsevier.comrlocatervibspec
Vibrational study of two novel cesium saccharinates. Spectroscopic evidence for organic molecule embedded in ionic salt Pance ˇ Naumov ) , Gligor Jovanovski Institute of Chemistry, Faculty of Sciences, A SÕ. Kiril i MetodijB UniÕersity, PO Box 162, MK-91001 Skopje, Macedonia Received 21 January 2000; received in revised form 12 April 2000; accepted 17 April 2000
Abstract The FT-IR spectra of protiated and partially deuterated analogues of two novel cesium saccharinates, CsŽC 7 H 4 NO 3 S.ŽC 7 H 5 NO 3 S. P H 2 O Ž1. and CsŽC 7 H 4 NO 3 S. P 0.5H 2 O Ž2. are studied. The nŽCO. and nŽSO 2 . frequency values, along with the presence of nŽNH. absorption in the spectrum of 1, represent inevitable evidence that this compound is an adduct of molecular saccharin. Earlier spectra–structural and structural correlations in metal saccharinates are employed in conjunction with the IR spectral and thermal data to make structural inferences about the compounds. The appearance of low-frequency nŽNH. absorption continuum in the spectrum of 1 suggests certain degree of dynamical disorder of the proton within the asymmetric wN`H PPP Nxy hydrogen bonds of saccharinato ion–molecule couples. In addition, one Ž Cs . or two Ž C2v . crystallographic types of essentially non-hydrogen bonded water molecules are situated in its lattice. Structural criteria for estimation of the proton position within the adduct dimmers, based solely on various descriptors of the degree of saccharinato sulfocarboximide ring distortion, are proposed. It is shown that in addition to the diversity of bonding modes expressed by its deprotonated form, saccharin can also be incorporated in solid-state structures as a molecule, even in a typically ionic salt. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Cesium saccharinates; FT-IR spectra; Raman spectra; Saccharin adduct; Spectra–structure correlations; Thermal analysis
1. Introduction Mostly due to the suspected carcinogenic properties Že.g., see Ref. w1x. of saccharin1 ŽI in Fig. 1., its compounds have been extensively exploited for biological studies during the last several decades. Suc)
Corresponding author. Tel.: q389-91-11-70-55 ext. 527; fax: q389-91-22-68-65. E-mail address: [email protected] ŽP. Naumov.. 1 Systematic name: 1,2-benzisothiazole-3Ž2 H .-one 1,1-dioxide.
cessive structural studies have mainly evolved from the interest in the proximity of as many as three different functional groups in its structure and the possibility to employ metal saccharinates as model compounds for the metal–ligand interactions of other small-sized ring systems bearing some of these groups. The deprotonated forms of saccharin ŽII in Fig. 1., namely, shows an extraordinary variety in the type of bonding in metal salts and complexes: ionic, monodentate-ligated via the nitrogen or oxygen ŽCO or SO 2 . atoms, amidato-like bidentate bridging, or several of these modes within the same
0924-2031r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 2 0 3 1 Ž 0 0 . 0 0 0 7 2 - 2
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P. NaumoÕ, G. JoÕanoÕskir Vibrational Spectroscopy 24 (2000) 201–211
ratio were mixed, while gently heating the stirred mixture. The solution was then left at RT. Colorless, well-defined needle-shaped crystals of 1 appeared after several days. Several repetitions of this procedure yielded the same product.
Fig. 1. Formulae of saccharin ŽI. and the saccharinato anion ŽII. with the atom labeling scheme.
structure w2,3x. In spite of their wide commercial use Žparticularly of Na and K saccharinates. and in contrast with the large number of transition metal saccharinates, however, not much structural data exist on the alkali salts of saccharin. The room-temperature solid-state structures of only two alkali salts, Na Žtriclinic form. w4x and K 2 Na w5x saccharinates, have been determined and several vibrational studies have been undertaken w6–10x. In the present paper, we report the synthesis and the vibrational spectra of two novel cesium saccharinates. Similarly to a single case, wVOŽOH.Žsac.ŽHsac.ŽH 2 O.x, reported previously w11x, the preliminary spectroscopic investigation w10x indicated presence of saccharinato molecules in the structure of one of the compounds. Since the structural difference between the two saccharin forms Žionic and molecular. in this compound ŽFig. 1. consists of a single proton, it might be beneficial to employ infrared spectroscopy for its preliminary structural characterization. Since the relevant Raman spectroscopic data of the adduct were reported previously w10x, only some details of interest will be shortly outlined here. Earlier spectra–structural and structural correlations in a number of metal saccharinates are used in conjunction with the IR spectral data to make structural predictions about the compounds.
2. Experimental 2.1. Synthesis of the compounds 2.1.1. Cs(sac)(Hsac) P H2 O 1 Cs carbonate dissolved in waterrEtOH Ž1:1 vol. mixture and saccharin solution in EtOH in 1:1 mole
2.1.2. Cs(sac) P 0.5H2 O 2 Compound 2 was initially obtained as colorless prismatic crystals from the mother liquor, after removal of crystals of 1. In another preparation, large crystals of 2 were synthesized similarly as 1, using carbonate-to-saccharin mole ratio of 1:2 and reducing the volume of the reaction mixture by slow evaporation until almost entire solute was removed. It was noted that an excess of saccharin favors the formation of 1, while an excess of carbonate and concentration of the solution supports the crystallization of 2. Sometimes, however, both 1 and 2 crystallized from the solution upon cooling. Deuteration of 2 was performed by repeated recrystallization from D 2 O, whereas 1 was deuterated by exposing powdered protiated samples of the compound to D 2 O vapors in an evacuated vessel. The FT infrared spectra Žnear and middle infrared regions: 32 backgroundr64 sample or 128 backgroundr256 sample spectra; far infrared: 1028 background and 2046 sample spectra; resolution from 1 to 4 cmy1 . in the 10,000–70 cmy1 frequency range were recorded with a System 2000 interferometer ŽPerkin-Elmer.. Since no differences were observed between the spectra recorded from Nujol mulled suspensions and pressed KBr pellets, the latter technique was used for the near and middle IR. The spectra in the far infrared were recorded from Nujol mulled solid samples placed between polyethylene plates. A PrN 21525 ŽGraseby Specac. variable-temperature cell with KBr Žnear and middle IR region. windows served for the low-temperature measurements. The Raman spectra Ž1800–30 cmy1 . were recorded with a Renishaw 2000 instrument equipped with a Leica microscope, using a 514-nm argon ion or a 780-nm diode lasers. The TGrDTGrDTA analyses were recorded using Setaram TG–DTA92 instrument ŽAl 2 O 3 crucibles. in a static helium atmosphere. The DSC measurements were performed with Mettler DSC20 oven connected to a Mettler TA 400 temperature controller, using Al containers Žair, heating rate 5
P. NaumoÕ, G. JoÕanoÕskir Vibrational Spectroscopy 24 (2000) 201–211
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K P miny1 .. Na saccharinate used for the thermal studies was recrystallized from EtOH.2 Elemental ŽC, H, N. analyses were performed using standard methods.
3. Results and discussion 3.1. Vibrational spectra In spite of the similarity between certain spectral regions of 1 and 2 Žespecially below 1000 cmy1 ., their mid-infrared spectra are clearly distinct at both RT and LNT ŽFig. 2.. In this sense, the higher complexity of the spectrum of 1 is apparent, indicating lower symmetry and larger number of crystallographically non-equivalent structural units in the corresponding structure. From the results of several empirical and ab initio assignments in the vibrational ŽIR and Raman. spectra of saccharin and the corresponding anion in the solid state w7,10x, it was concluded that in addition to the water modes in the solid hydrates, the most valuable structural information can be obtained from the spectra–structural correlations concerning the stretching modes of the carbonyl and sulfonyl groups. We paid special attention, therefore, to these regions in the spectra of the studied compounds. 3.1.1. OH r OD stretching region Broad, apparently complex nŽOH. absorption is present in the 3670–3200 cmy1 region of the RT spectrum of 1 ŽFig. 2.. Two sharp bands Ž3610 and 3521 cmy1 . evolve at LNT, being superimposed on the broad absorption with a maximum at 3429 cmy1 . The initial presumptions that the broad background absorption is owing to presence of non-crystal water in the sample andror in the cell were abandoned since identical picture was observed in spectra recorded from paraffin oil suspensions of previously additionally dried sample and without utilization of the cell. Pronouncedly high frequency of one of the nŽOH. bands Ž3610 cmy1 . implies that one of the
2
As shown by Jovanovski and Kamenar w4x, different hydrates are obtained by recrystallization from H 2 O and EtOH.
Fig. 2. The 4000–500 cmy1 region in the RT Ža, c. and LNT Žb, d. FT-IR spectra of CsŽsac.ŽHsac.PH 2 O Ža, b. and CsŽsac.P 0.5H 2 O Žc, d..
water OH groups in the structure of 1 is not included in hydrogen bonding. The presence of the other nŽOH. band Ž3521 cmy1 ., in addition, indicates that the second water OH group in 1 could be weakly hydrogen bonded. The respective 2nŽOH. overtones are observed as a pair of expectedly very weak but well-defined bands at approximately doubled frequencies Ž7215 and 7040 cmy1 . in the LNT NIR spectrum ŽFig. 3., the anharmonicity of the higherfrequency band Ž5 cmy1 . being larger than that of the one at lower frequency Ž2 cmy1 .. Broad, evidently complex nŽOH. band is present in the RT and LNT IR spectra of 2 with a single maximum at somewhat lower frequency Ž3390 cmy1 at LNT. than one of the broad nŽOH. absorptions of 1 ŽFig. 2.. Regularly, being a result of extensive vibrational interactions, the appearance of the OH stretching region alone can be hardly used to obtain any precise structural information about the crystal water from the spectroscopic data. The advantages of employment of the isotopic dilution technique, on the other hand, are well known Žsee, for example, Ref. w12x. and it has been frequently used for such purposes. The presence of two nŽOD. bands Ž2644 and 2606 cmy1 . with similar intensities in the LNT difference 3 spectrum of 1 with very low deuterium
3 The difference spectra were obtained by substraction of the Žappropriately scaled. LNT spectrum of the protonated samples from the corresponding spectra of the slightly deuterated samples.
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n˜ ŽOD. vs. dŽO PPP O. correlations w13,14x, the nŽOD. frequencies of isotopically isolated HDO impurities in H 2 O or D 2 O matrices are known to have predictive value for the values of the corresponding O PPP O distances. Application of the correlation equation given for a series of solid hydrates ŽEq. 1; w13x. on the studied saccharinates resulted in the values 3.107 ˚ for 1, and 2.826 A˚ for 2. and 3.006 A
˚ s 1r Ž y3.73 . drA =ln Ž 2727 y n˜ Ž OD . rcmy1 . r8.97 = 10 6 Ž 1.
Fig. 3. The 2nŽOH. ŽA. and 2nŽNH. ŽB. overtone regions in the LNT spectrum of CsŽsac.ŽHsac.PH 2 O.
content w x ŽD. - 0.3%, Fig. 4x reveals existence of two non-equivalent OD oscillators in the corresponding structure. It is to be expected, therefore, that either a single water molecule with two non-equivalent proton sites, or possibly, two crystallographically non-equivalent C2 water molecules exist in the structure of 1. In case only one type of water molecules was present, the two nŽOD. and the corresponding LNT nŽOH. bands could be identified as the water modes n 1 and n 3 . The appearance of the doublet of nŽOD. bands, as well as of the corresponding nŽOH. and 2nŽOH. doublets, would then reflect the low degree of coupling of the two water stretching modes. The frequency split between the two nŽOD. fundamentals Ž38 cmy1 . in such case would be indicative for the low distortion of the water molecules w12x, one of the protons being nonhydrogen bonded, while the other one eventually being involved in rather weak hydrogen bonds. The presence of a single nŽOD. band of isotopically isolated HOD species in H 2 O matrix in the difference spectrum of 2 ŽFig. 4., on the other hand, reveals presence of a single type of water molecules placed on a C2 axis in its structure. The band frequency Ž2490 cmy1 . implies that the two equivalent protons are included in hydrogen bonding of weak to moderate strength. Besides being useful for qualitative discussions of the hydrogen bonding, with the means of empirical
Similar expression derived from the isomorphous series of MŽII. saccharinates ŽEq. 2; w14x. yields ˚ for 1, and 2.844 A˚ in the case of 3.156 and 3.044 A 2.
˚ s 4.47128 y 0.29754 drA =ln Ž 2727 y n˜ Ž OD . rcmy1 .
Ž 2.
3.1.2. NH r ND stretching region in the spectrum of Cs(sac)(Hsac) P H2 O An exceptional part of the spectrum of 1 is the broad absorption continuum extending from around
Fig. 4. The region of isotopically isolated nŽOD. modes in the LNT spectra of CsŽsac.P0.5H 2 O ŽA. and CsŽsac.ŽHsac.PH 2 O ŽB..
P. NaumoÕ, G. JoÕanoÕskir Vibrational Spectroscopy 24 (2000) 201–211
3200 cmy1 down to about 2200 cmy1 where it merges with the background absorption ŽFig. 2.. A maximum around 2520 cmy1 and a AwindowB of high transmittance around 2775 cmy1 Ždiscussed below. are present. Several weak bands give complex appearance to the 3100–2800 cmy1 region of the absorption. However, the spectral picture from this region is apparently reproduced in the spectrum of 2 and, therefore, such band structure of the spectrum of 1 is not inherent for the broad absorption itself, but is being resulted by other superimposed weak Ž nŽCH. and non-fundamental. bands.4 Very broad structurized nŽXH. ŽX s O, N. bands spreading over relatively low frequencies compared to what is usual for nŽXH. modes, such as the one observed in the spectrum of 1 ŽFig. 2., are wellknown characteristics of the spectra of strongly hydrogen-bonded XH groups w15x. The appearance of similar nŽNH. feature ŽFigs. 2 and 5., distinguishing for the spectrum of 1 from to-date spectroscopically characterized saccharinates, represents inevitable evidence for the existence of strongly hydrogen bonded imino groups from saccharinato molecules in its structure. The frequency of the centroid of the absorption Ž2520 cmy1 . is lower than the frequencies of all component nŽNH. bands in the spectrum of saccharin Ž3090, 2970 and 2700 cmy1 . w16x, which itself comprises rather strong hydrogen bonds of Oco PPP H`N type in the solid state Ž dŽO PPP N. s ˚ w17,18x. The imino groups in 1 are in2.796Ž5. A volved therefore in hydrogen bonds that comprise much stronger proton acceptorŽs. than the water, carbonyl or sulfonyl oxygen atoms. The only such candidate in the present system, consisted of metal cations, water molecules and saccharinato ionsr molecules, is the negatively charged nitranionic center of a neighboring saccharinato ion. There is little doubt therefore that saccharinato molecule–ion couples that comprise strong hydrogen bonds of wN`H PPP Nxy type exist in the structure of 1. What would remain unclear at this stage of the study is the precise geometry of the hydrogen bonding, i.e., the position of the proton between the saccharinato
4
The pair of weak bands around 2360 and 2340 cmy1 are uncompensated CO 2 absorptions.
205
Fig. 5. ŽA. The 3200–1850 cmy1 region in the LNT spectra of CsŽsac.ŽHsac.PH 2 O Ža. and CsŽsac.P0.5H 2 O Žb.. ŽB. Carbonyl stretching regions in the LNT spectra of CsŽsac.ŽHsac.PH 2 O Ža. and saccharin Žb..
residues. On the other hand, the infrared absorption continua, such as the one observed in the spectrum of 1, are very specific and were one of the early indications for the existence of delocalized protons in the solid state w19x. Therefore, the spectral picture of 1 might suggest that the imino protons are dynamically disordered. The protons, namely, can be visualized as AbouncingB between the saccharinato nitrogen atoms on the time-scale of the order of the stretching vibrational transition, leading to subsequent broadening of the corresponding nŽNH. absorption band. Further study, however, is to be devoted to this phenomenon in the present case. Remarkable feature of the nŽNH. absorption band in the spectrum of 1 ŽFigs. 2 and 5. is that both its branches are asymmetric and inclined towards the minimum around 2775 cmy1 . The position of the latter, moreover, is not affected by the temperature and its halfwidth seems to be reduced upon cooling, contrary to what would be expected if it was a high-transmittance region between two bands that are sharpened with the temperature decrease. Moreover, a single weak LNT band originating from the first overtone of the nŽNH. mode is present at 5203 cmy1 ŽFig. 3., giving roughly 2600 cmy1 or somewhat higher for the frequency of the corresponding nŽNH. mode, and thus a value in the vicinity of the nŽNH. minimum. It can be presumed from this that
206
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Fig. 6. Linear plot of the nŽCO. frequencies in saccharinates Žl. and saccharin ŽI.. Symbol ' denotes the highest-frequency nŽCO. band of CsŽsac.ŽHsac. P H 2 O.
there is a single structural type of strongly hydrogen bonded imino groups and therefore only one type of saccharinato molecules in the structure of 1. Having all this in mind, it seems probable that the region of high transmittance around 2775 cmy1 in the spectrum of 1 is due to Evans type Fermi resonance of the NH stretching with non-fundamental modes of low inherent intensity, rather than a minimum between two separate absorption bands. Otherwise, Evans effects have been regularly observed in the IR spectra of strongly hydrogen bonded cyclic imides in the solid state, and were also reported in case of saccharin w16x. 3.1.3. Carbonyl stretching region The carbonyl stretchings in the infrared spectra of various saccharinates have been frequently used for spectra–structure correlations Že.g., Refs. w7,10x.. A general conclusion from such studies was that the carbonyl stretching frequencies in the spectra of saccharinates are consistently lower than the corresponding value of saccharin Ž1725 cmy1 . w16x.5 A qualitative relation of the infrared shift from the saccharin nŽCO. IR frequency with the type of the metal–saccharinato bonding, moreover, was observed. Frequency shifts from about 30 cmy1 in purely covalent mercuryŽII. saccharinates to more than 80 cmy1 in typically ionic saccharinates were found w10,20,21x.
5 In some previous studies Že.g., Ref. w16x. a singlet band was prescribed to the saccharin nŽCO. mode. Our further investigations using higher resolution, however, revealed that this band is clearly a doublet, which can also be inspected from Fig. 5B. In such a case, the wavenumber of what was previously referred to as saccharin nŽCO. band Ž1725 cmy1 . corresponds to the higherfrequency component band.
Regarding the number and the intensity ratio of the bands, as well as the overall appearance, the strong doublet around 1730 cmy1 in the spectrum of 1 resembles quite close the nŽCO. doublet in the LNT spectrum of saccharin ŽFigs. 2 and 7.. Unique feature of the IR spectrum of 1 compared to all previously studied saccharinates is the position of the infrared nŽCO. doublet band ŽFig. 5.. The frequency of the higher component band Ž1732 cmy1 ., namely, is higher than that of saccharin in the solid state Ž1725 cmy1 . and even higher than the corresponding band in the RT spectrum of the only previously reported saccharin adduct, wVOŽOH.Žsac.ŽHsac.ŽH 2 O.x Ž1727 cmy1 . w11x. Being interested in this extraordinary characteristic, we have undertaken an exhaustive literature search for spectroscopic data on metal saccharinates w22x. The population of the nŽCO. frequencies of 53 saccharinates together with saccharin and its anion in DMSO solution can be inspected from the plot in Fig. 6. It can be concluded that the nŽCO. frequency of 1 is indeed the highest of all analyzed saccharinates. The corresponding nŽCO.
Fig. 7. The LNT spectra of saccharin Žc., CsŽsac.ŽHsac.PH 2 O Ža., CsŽsac.P0.5H 2 O Žd. and the solid residue Žb. after dissolution of CsŽsac.ŽHsac.PH 2 O in water and evaporation of the solvent Žb can be thought of as a sum of c and d..
P. NaumoÕ, G. JoÕanoÕskir Vibrational Spectroscopy 24 (2000) 201–211
RT Raman band of 1 is observed at 1721 cmy1 w10x and thus higher than the saccharin Raman carbonyl stretching band Ž1697 cmy1 .. The position of this infrared carbonyl band in the spectrum of 1 inevitably confirms the existence of molecular saccharin in its structure in accordance with the definitely covalent character of its N`H bond. The band maximum remains practically unshifted Ž1733 cmy1 . at LNT, but the RT shoulder on its lower-frequency side is separated into additional band Ž1726 cmy1 . resulting in an additional overlapped nŽCO. band ŽFig. 5.. The picture of this spectral region of 1 at low temperature therefore resembles that of saccharin, but the nŽCO. frequency of the first band is again significantly higher. It might be taken as indication that the carbonyl groups of saccharin molecules in 1 participate in weaker hydrogen bonds than the Oco PPP H`N bonds of solid saccharin. This, in return, goes along with the above considerations that the carbonyl oxygen atoms in 1 are not favorable candidates as imino proton acceptors. Two additional asymmetric, strong and relatively broad nŽCO. RT bands Ž1619 and 1578 cmy1 , Figs. 2 and 5. are present in the nŽCO. region of the spectrum of 1.6 On lowering the temperature, the higher-frequency band shows slight negative Žd n˜rdT - 0. shift Ž1621 cmy1 ., while the lower-frequency one undergoes appreciable positive Žd n˜rdT ) 0. shift Ž1567 cmy1 .. Pronounced frequency difference between the two lower-frequency carbonyl stretchings in 1 and saccharin Ž104 and 158 cmy1 . is expected from the presence of saccharinato ions in its structure. It is likely that the band Ž1579 cmy1 . that evolves at low temperature is not due to carbonyl stretching, but to the stretching mode that is mainly localized in the saccharinato phenylene ring and is being regularly found at roughly the same frequency in the spectra of saccharinates w10x. This sharp and practically temperature-insensitive band Žat least within the temperature range examined in this study. is probably hidden under the broader carbonyl stretching at RT. The band is insensitive to deutera-
6 Due to the decomposition of 1 upon dissolution in D 2 O, no deuteration to higher degree to account for the d ŽHOH. modes in this region was achieved.
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tion and thus cannot be prescribed to the water bending mode. Beyond any doubt, the intense RT band at 1647 cmy1 Ž1649 cmy1 at LNT. in the spectrum of 2 can be prescribed to the carbonyl stretching modes ŽFig. 2.. The frequency difference Ž76 cmy1 . between this band and the corresponding saccharin nŽCO. band is consistent with the existence of solely ionic saccharinato residues in 2. In addition, two other prominent, rather sharp bands whose positions are practically insensitive to the temperature Ž1619 and 1582 cmy1 at RT, 1620 and 1582 cmy1 at LNT. are present in the nŽCO. spectral region of 2. The insufficient intensity of the band at 1620 cmy1 , however, is in contrast with what is expected for nŽCO. bands in the spectra of saccharinates w20,21x. The presumptions based on the frequency and the appearance of the band that it is owing to the water deformational mode could not be confirmed since our attempts to obtain highly deuterated sample of 2 had failed. However, as can be seen from Fig. 8, the shoulder on the lower-frequency side of the band vanishes upon deuteration to about 10%, which might be an indication that the water bending mode in fact has certain contribution to this band. Being pronouncedly sharp and unaffected by the temperature, on the other hand, the band at 1582 cmy1 is presumably due to the above-mentioned benzenoid mode and bears
Fig. 8. The carbonyl strteching region in the spectra of protiated Žb. and deuterated Ža. CsŽsac.P0.5H 2 O.
P. NaumoÕ, G. JoÕanoÕskir Vibrational Spectroscopy 24 (2000) 201–211
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mainly nŽCC. character, as shown by the ab initio calculations w10x. The above discussion leaves as many as three nŽCO. bands in the spectrum of 1 and a single nŽCO. band in the case of 2. As it was noted earlier w20,21x, however, the number of the infrared nŽCO. bands alone, even in the case of less complicated appearance of nŽCO. region, is not reliable to make predictions on the number of structurally different saccharinato anions. 3.1.4. Sulfonyl stretching region The sulfonyl stretchings in the spectra of saccharinates have been readily exploited for structural studies w23x. The strict assignment, however, might be troubled by the presence of additional bands in this region. The bands in the spectra of 1 and 2 that are due to the nŽSO 2 . modes were assigned by consideration of the temperature sensitiveness of the bands in the region, as well as accounting for the earlier assignments w10,20x and theoretical calculations w10,24x. Three bands correspond to each of the sulfonyl stretching modes Žantisymmetric and symmetric. in the LNT IR spectrum of 1 ŽFig. 7; Table 1., while only two bands for each mode are observed in its RT Raman spectrum w10x. The pair of the highest fre-
Table 1 Sulfonyl stretching frequencies of CsŽsac.ŽHsac.PH 2 O Ž1. and CsŽsac.P0.5H 2 O Ž2. a Parameter
Saccharin w16x
1
2
n˜ w nas ŽSO 2 .x
1335
1271Ž64. 1241Ž94.
n˜ w nsŽSO 2 .x
1180
Db
155
n˜s rn˜as
0.884
1342Žy7. 1308Ž27. 1258Ž77. 1177Ž3. 1167Ž13. 1149Ž31. 165 141 109 0.877 0.892 0.913
a
1148Ž32.
123 93 0.903 0.925
All values except for those in the last row are given in cmy1 . The numbers in parentheses are the differences with the corresponding values of saccharin. b D s n˜ w nas ŽSO 2 .xy n˜ w nsŽSO 2 .x.
quency antisymmetric and symmetric infrared bands Ž1342 and 1177 cmy1 . can be prescribed to the sulfonyl stretchings of the saccharinato molecules. That is worth noting while, as expected, the antisymmetric stretching frequency is placed higher than the saccharin value, the frequency of the symmetric one is similar with, but somewhat lower Ž3 cmy1 . than the respective saccharin mode ŽTable 1.. The recent ab initio treatment of the saccharinato ion w10x, however, showed that the antisymmetric sulfonyl stretching mode is remarkably ApurerB vibration than the symmetric one and this might be the reason that it reflects the structural environment of the SO 2 group Ži.e., the existence of molecular saccharin. better than the symmetric nŽSO 2 . mode. A pair of weak Raman bands Ž1306 and 1257 cmy1 . in the spectrum of 1 were previously attributed to the nas ŽSO 2 . modes, while two somewhat stronger bands were prescribed to the nsŽSO 2 . modes Ž1175 and 1152 cmy1 . w10x. Comparison of the nŽSO 2 . data between the studied salts and saccharin ŽTable 1. shows that the frequency fluctuations of the antisymmetric stretching mode are larger than those of the symmetric, which was explained w10x by the above-mentioned vibrational purity of the antisymmetric mode. As Table 1 shows, moreover, the split between the two stretchings Žantisymmetric and symmetric. and their frequency ratio are also consistent with the type of the saccharinato species Žmolecular or ionic.. Contrary to the saccharinato ions, namely, the frequency split between the SO 2 stretchings of molecular saccharin in 1 is larger than that of pure saccharin. Nevertheless, it was noted previously w23x that one of the factors that have effect on the extent of the split between the two stretchings is the angle between the sulfonyl oxygen atoms. Thus, it can be presumed that the O`S`O angleŽs. of the saccharinato ionŽs. in 1 israre smaller whereas the corresponding angle of molecular saccharin in 1 is similar with the O`S`O angle of pure saccharin Ž117.78 w17x or 117.4Ž1.8 w18x.. Two bands Ž1271 and 1241 cmy1 . in the spectrum of 2 can be assigned as the antisymmetric sulfonyl stretchings. The strong and apparently complex band with a maximum at 1148 cmy1 , on the other hand, corresponds to the symmetric SO 2 stretching modeŽs.. It does not seem likely that the sharp band at 1119 cmy1 is originated by the stretch-
P. NaumoÕ, G. JoÕanoÕskir Vibrational Spectroscopy 24 (2000) 201–211
ing of the SO 2 groups, since a band of very similar appearance and at the same frequency is also observed in the spectrum of saccharin and was previously prescribed w10,24x to a benzenoid mode. The number of assigned nŽSO 2 . band pairs in the spectra of the of 1 and 2 suggests existence of two structural types of sulfonyl groups i.e., two crystallographically non-equivalent saccharinato ions in the corresponding structures, in addition to the saccharinato molecules in 1. 3.1.5. ConÕersion of Cs(Hsac)(sac) P H2 O into Cs(sac) P 0.5H2 O and saccharin Additional simple and fairly convincing evidence towards the existence of molecular saccharin in 1 is the simple conversion of 1 into a mixture of 2 and saccharin. Owing to the low water solubility at RT, namely, the attempt to dissolve 1 in water resulted in its partial dissolution. Subsequent evaporation of the solute yielded a mixture of 2 and saccharin that could be inspected via the vibrational spectrum of the residue Žspectrum b in Fig. 7 can be thought of as sum of the spectra c and d.. In addition to the other spectral changes, the highest-frequency nŽCO. band of 1 is shifted downwards to the saccharin value Ž1725 cmy1 .. The weak antisymmetric SO 2 stretching band Ž1342 cmy1 . shifts to the saccharin value Ž1336 cmy1 , spectrum b in Fig. 7. and two more nas ŽSO 2 . bands appear Ž1271 and 1241 cmy1 .. The latter bands were assigned above as the antisymmetric sulfonyl stretchings of 2.
209
3.2. Structural criteria The presence of characteristic Že.g., CO and SO 2 . bands for both molecular and typically ionic saccharinato entities in the adduct 1 reveals the asymmetric shape of the proton potential within the wN`H PPP Nxy hydrogen bonds of the molecule–ion pairs in its structure. The existence of both molecular and ionic forms of saccharin, however, triggers the question of structural differences between these two species. Namely, structural criteria for distinction between the saccharinato molecules and the saccharinato ions other than the presence of the proton nearer to the one or the other of them are needed. In order to predict more structural details of 1, we used the results from the recent Cambridge Structural Database ŽCSD. analysis w3x of the data on metal saccharinates to compare the structural parameters of pure saccharin and those Žaveraged. for saccharinato ion. The results are summarized in Table 2. Following the approach in Ref. w3x, the AinternalB geometry of the five-membered saccharinato ring was described by two pairs of Dieterich et al. w25x coordinates Žwhich are proportional to differences between selected angles or distances within the ring., formulated for the CCC and SN parts of the ring. A total of 24 ionic saccharinato species and two independent sets of data for saccharin were obtained from the analysis w3x. Similarly for what was shown in the statistic CSD survey of pyrazoles w26x, it was concluded that the Dieterich et al. approach has
Table 2 Structural differences between saccharinato ions and molecules based on the CSD analysis in Ref. w3x Parameter
Molecules
Ions
D aŽCC.4a D aŽSN.4a D dŽSN.4a ˚ dŽC`N.rA
f y26 f 22 f or - D dŽSN.4 Žions.
f or ) 0 - or f 16 f or ) D dŽSN.4 Žmolecules.
) 1.360
f 1.350
˚ dŽS`N.rA ˚ Žaverage. dŽS`O.rA /ŽO`S`O.r8 /ŽC`S`N.r8
f 1.660
- 1.650
- 1.430 f 118 f 93
) 1.430 - or f 117 ) 96
a The coordinates are defined as: D d ŽCC. s 100 d wCŽ2.` CŽ3.x y d wCŽ3.` CŽ4.x4, D aŽCC. s 10/ wCŽ3.` CŽ2.` NŽ1.x y /wCŽ2.`CŽ3.`CŽ4.x4, D dŽSN. s 100 dwCŽ4.`SŽ5.x y dwCŽ2.`NŽ1.x4, aŽSN. s /wSŽ5.`NŽ1.`CŽ2.x y /wCŽ4.`SŽ5.`NŽ1.x ŽThe atom labels are given in Fig. 1..
210
P. NaumoÕ, G. JoÕanoÕskir Vibrational Spectroscopy 24 (2000) 201–211
certain discriminative power for the description of distortions of the saccharinato five-membered ring. The saccharinato molecules in 1, namely, should have higher and lower values for the D aŽSN. and D aŽCC. coordinates, respectively, than the saccharinato ions. The value of D dŽSN. for the saccharinato ions should be similar or probably larger than that of the molecules. Both C`N and S`N distances in the molecules should be larger than the corresponding in ˚ being approximate limiting value the ions, 1.650 A for the latter. The structural parameters concerning the CO and SO 2 groups were considered to describe the AexternalB geometry of the ring w3x. Characteristic difference were found between the mean S`O distances, that should be lower and higher than 1.430 ˚ for molecular and ionic saccharin, respectively. A Furthermore, the O`S`O angles in the saccharinato ions should be smaller than those in the molecules, the limiting value of which would be about 1178. The value of the C`S`N angle also reflects the type of the saccharinato moiety: for ionic saccharinates, it is typically around 97.58, whereas for molecular saccharin, its value should be close to 938.
similar with the one of the ions in Na saccharinate Ž759 K..
4. Conclusions The present work shows that the compound characterized as CsŽHsac.Žsac. P H 2 O is an adduct of molecular saccharin. The spectroscopic data reveal some of the exceptional structural properties of this compound, resulted from the inclusion of relatively large organic molecule in a typically ionic salt. Moreover, the more general structural and eventually physiological significance of this work would be that it is shown that in addition to the diversity of bonding modes of its deprotonated form, saccharin can also form adducts in the solid state and even with the simple alkali saccharinato salts. This, eventually, can have some significance for understanding the mechanisms of suspected pathological action of molecular saccharin on the biological systems. Further work is to be devoted to the structure details and particularly the hydrogen bonding in the adduct.
3.3. Thermal studies The decomposition temperature of the saccharinato ion Že.g., in Na saccharinate, exothermic peak at 759 K. is for about 150 K higher than that of saccharin itself Žendothermic peak at 610 K.. The thermal decomposition of 1 starts with partial dehydration at about 368 K, resulting in CsŽsac.ŽHsac. P 0.5H 2 O that might be formally considered as a Asolid mixtureB of 2 and saccharin. The hemihydrate melts at 461 K, accompanied by complete dehydration. This is followed by melting of the saccharin, represented with the second sharp endothermic peak, at temperature Ž489 K. that is somewhat lower than the melting temperature of pure saccharin Ž502 K.. The melting is followed by three-step decomposition. Saccharinato molecules decompose during the first Žexothermic DSC peak at 525 K. and the second Žendothermic DSC peak at about 600 K. step. The temperature of the latter is very close to that of the decomposition of pure saccharin Ž608 K.. In the third, exothermic step ŽDSC peak at 751 K., the saccharinato ions are decomposed at temperature
References w1x N. Suzuki, H. Suzuki, Cancer Res. 55 Ž1995. 4253. w2x Z. Yugeng, W. Yebin, Y. Heng, Cryst. Res. Technol. 30 Ž1995. 831. w3x P. Naumov, G. Jovanovski, Struct. Chem. 11 Ž2000. 19. w4x G. Jovanovski, B. Kamenar, Cryst. Struct. Commun. 11 Ž1982. 247. w5x K.M.A. Malik, S.Z. Haider, M.A. Hossain, Acta Crystallogr., Sect. C 40 Ž1984. 1696. w6x S. Natarajan, G. Subramanian, Indian J. Phys. B 60 Ž1986. 470. ˇ w7x G. Jovanovski, O. Grupce, J. Mol. Struct. ˇ B. Soptrajanov, 219 Ž1990. 61. w8x S. Tanceva, MSc Thesis, PMF, Skopje, 1994. ˇ w9x A.H. Jubert, R. Pis Diez, S.B. Etcheverry, E.J. Baran, J. Raman Spectrosc. 23 Ž1991. 15. w10x P. Naumov, G. Jovanovski, Spectrochim. Acta, Part A, 56r57 Ž2000. 1305. w11x E.G. Ferrer, S.B. Etcheverry, E.J. Baran, Monatsh. Chem. 124 Ž1993. 355. w12x H.D. Lutz, Spectrochim. Acta, Part A 38 Ž1982. 921. w13x B. Berglund, J. Lindgren, J. Tegenfeldt, J. Mol. Struct. 43 Ž1978. 179.
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ˇ w14x Lj. Pejov, G. Jovanovski, O. Grupce, Acta ˇ B. Soptrajanov, Chim. Slov. 44 Ž1997. 197. w15x D. Hadzi, ˇ S. Bratos, ˇ in: P. Schuster, G. Zundel, C. Sandorfy ŽEds.., The Hydrogen Bond, North-Holland, Amsterdam, 1976. w16x I. Petrov, O. Grupce, ˇ T. Stafilov, J. Mol. Struct. 142 Ž1986. 275. w17x Y. Okaya, Acta Crystallogr., Sect. B 25 Ž1969. 2257. w18x J.C.J. Bart, J. Chem. Soc. B Ž1968. 376. w19x G. Zundel, Trends Phys. Chem. 3 Ž1992. 129. ˇ w20x G. Jovanovski, B. Soptrajanov, J. Mol. Struct. 174 Ž1988. 467.
211
ˇ w21x G. Jovanovski, B. Soptrajanov, B. Kamenar, Bull. Chem. Technol. Maced. 8 Ž1990. 47. w22x P. Naumov, G. Jovanovski, submitted for publication. ˇ w23x G. Jovanovski, S. Tanceva, B. Soptrajanov, Spectrosc. Lett. ˇ 28 Ž1995. 1095. w24x I.G. Binev, B.A. Stambolyiska, E.A. Velcheva, Spectrochim. Acta, Part A 52 Ž1996. 1135. w25x D.A. Dieterich, I.C. Paul, D.Y. Curtin, J. Am. Chem. Soc. 96 Ž1974. 6372. w26x A.L. Llamas-Saiz, C. Foces-Foces, J. Elguero, J. Mol. Struct. 319 Ž1994. 231.
Vibrational study of two novel cesium saccharinates. Spectroscopic evidence for organic molecule embedded in ionic salt Pance ˇ Naumov ) , Gligor Jovanovski Institute of Chemistry, Faculty of Sciences, A SÕ. Kiril i MetodijB UniÕersity, PO Box 162, MK-91001 Skopje, Macedonia Received 21 January 2000; received in revised form 12 April 2000; accepted 17 April 2000
Abstract The FT-IR spectra of protiated and partially deuterated analogues of two novel cesium saccharinates, CsŽC 7 H 4 NO 3 S.ŽC 7 H 5 NO 3 S. P H 2 O Ž1. and CsŽC 7 H 4 NO 3 S. P 0.5H 2 O Ž2. are studied. The nŽCO. and nŽSO 2 . frequency values, along with the presence of nŽNH. absorption in the spectrum of 1, represent inevitable evidence that this compound is an adduct of molecular saccharin. Earlier spectra–structural and structural correlations in metal saccharinates are employed in conjunction with the IR spectral and thermal data to make structural inferences about the compounds. The appearance of low-frequency nŽNH. absorption continuum in the spectrum of 1 suggests certain degree of dynamical disorder of the proton within the asymmetric wN`H PPP Nxy hydrogen bonds of saccharinato ion–molecule couples. In addition, one Ž Cs . or two Ž C2v . crystallographic types of essentially non-hydrogen bonded water molecules are situated in its lattice. Structural criteria for estimation of the proton position within the adduct dimmers, based solely on various descriptors of the degree of saccharinato sulfocarboximide ring distortion, are proposed. It is shown that in addition to the diversity of bonding modes expressed by its deprotonated form, saccharin can also be incorporated in solid-state structures as a molecule, even in a typically ionic salt. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Cesium saccharinates; FT-IR spectra; Raman spectra; Saccharin adduct; Spectra–structure correlations; Thermal analysis
1. Introduction Mostly due to the suspected carcinogenic properties Že.g., see Ref. w1x. of saccharin1 ŽI in Fig. 1., its compounds have been extensively exploited for biological studies during the last several decades. Suc)
Corresponding author. Tel.: q389-91-11-70-55 ext. 527; fax: q389-91-22-68-65. E-mail address: [email protected] ŽP. Naumov.. 1 Systematic name: 1,2-benzisothiazole-3Ž2 H .-one 1,1-dioxide.
cessive structural studies have mainly evolved from the interest in the proximity of as many as three different functional groups in its structure and the possibility to employ metal saccharinates as model compounds for the metal–ligand interactions of other small-sized ring systems bearing some of these groups. The deprotonated forms of saccharin ŽII in Fig. 1., namely, shows an extraordinary variety in the type of bonding in metal salts and complexes: ionic, monodentate-ligated via the nitrogen or oxygen ŽCO or SO 2 . atoms, amidato-like bidentate bridging, or several of these modes within the same
0924-2031r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 2 0 3 1 Ž 0 0 . 0 0 0 7 2 - 2
202
P. NaumoÕ, G. JoÕanoÕskir Vibrational Spectroscopy 24 (2000) 201–211
ratio were mixed, while gently heating the stirred mixture. The solution was then left at RT. Colorless, well-defined needle-shaped crystals of 1 appeared after several days. Several repetitions of this procedure yielded the same product.
Fig. 1. Formulae of saccharin ŽI. and the saccharinato anion ŽII. with the atom labeling scheme.
structure w2,3x. In spite of their wide commercial use Žparticularly of Na and K saccharinates. and in contrast with the large number of transition metal saccharinates, however, not much structural data exist on the alkali salts of saccharin. The room-temperature solid-state structures of only two alkali salts, Na Žtriclinic form. w4x and K 2 Na w5x saccharinates, have been determined and several vibrational studies have been undertaken w6–10x. In the present paper, we report the synthesis and the vibrational spectra of two novel cesium saccharinates. Similarly to a single case, wVOŽOH.Žsac.ŽHsac.ŽH 2 O.x, reported previously w11x, the preliminary spectroscopic investigation w10x indicated presence of saccharinato molecules in the structure of one of the compounds. Since the structural difference between the two saccharin forms Žionic and molecular. in this compound ŽFig. 1. consists of a single proton, it might be beneficial to employ infrared spectroscopy for its preliminary structural characterization. Since the relevant Raman spectroscopic data of the adduct were reported previously w10x, only some details of interest will be shortly outlined here. Earlier spectra–structural and structural correlations in a number of metal saccharinates are used in conjunction with the IR spectral data to make structural predictions about the compounds.
2. Experimental 2.1. Synthesis of the compounds 2.1.1. Cs(sac)(Hsac) P H2 O 1 Cs carbonate dissolved in waterrEtOH Ž1:1 vol. mixture and saccharin solution in EtOH in 1:1 mole
2.1.2. Cs(sac) P 0.5H2 O 2 Compound 2 was initially obtained as colorless prismatic crystals from the mother liquor, after removal of crystals of 1. In another preparation, large crystals of 2 were synthesized similarly as 1, using carbonate-to-saccharin mole ratio of 1:2 and reducing the volume of the reaction mixture by slow evaporation until almost entire solute was removed. It was noted that an excess of saccharin favors the formation of 1, while an excess of carbonate and concentration of the solution supports the crystallization of 2. Sometimes, however, both 1 and 2 crystallized from the solution upon cooling. Deuteration of 2 was performed by repeated recrystallization from D 2 O, whereas 1 was deuterated by exposing powdered protiated samples of the compound to D 2 O vapors in an evacuated vessel. The FT infrared spectra Žnear and middle infrared regions: 32 backgroundr64 sample or 128 backgroundr256 sample spectra; far infrared: 1028 background and 2046 sample spectra; resolution from 1 to 4 cmy1 . in the 10,000–70 cmy1 frequency range were recorded with a System 2000 interferometer ŽPerkin-Elmer.. Since no differences were observed between the spectra recorded from Nujol mulled suspensions and pressed KBr pellets, the latter technique was used for the near and middle IR. The spectra in the far infrared were recorded from Nujol mulled solid samples placed between polyethylene plates. A PrN 21525 ŽGraseby Specac. variable-temperature cell with KBr Žnear and middle IR region. windows served for the low-temperature measurements. The Raman spectra Ž1800–30 cmy1 . were recorded with a Renishaw 2000 instrument equipped with a Leica microscope, using a 514-nm argon ion or a 780-nm diode lasers. The TGrDTGrDTA analyses were recorded using Setaram TG–DTA92 instrument ŽAl 2 O 3 crucibles. in a static helium atmosphere. The DSC measurements were performed with Mettler DSC20 oven connected to a Mettler TA 400 temperature controller, using Al containers Žair, heating rate 5
P. NaumoÕ, G. JoÕanoÕskir Vibrational Spectroscopy 24 (2000) 201–211
203
K P miny1 .. Na saccharinate used for the thermal studies was recrystallized from EtOH.2 Elemental ŽC, H, N. analyses were performed using standard methods.
3. Results and discussion 3.1. Vibrational spectra In spite of the similarity between certain spectral regions of 1 and 2 Žespecially below 1000 cmy1 ., their mid-infrared spectra are clearly distinct at both RT and LNT ŽFig. 2.. In this sense, the higher complexity of the spectrum of 1 is apparent, indicating lower symmetry and larger number of crystallographically non-equivalent structural units in the corresponding structure. From the results of several empirical and ab initio assignments in the vibrational ŽIR and Raman. spectra of saccharin and the corresponding anion in the solid state w7,10x, it was concluded that in addition to the water modes in the solid hydrates, the most valuable structural information can be obtained from the spectra–structural correlations concerning the stretching modes of the carbonyl and sulfonyl groups. We paid special attention, therefore, to these regions in the spectra of the studied compounds. 3.1.1. OH r OD stretching region Broad, apparently complex nŽOH. absorption is present in the 3670–3200 cmy1 region of the RT spectrum of 1 ŽFig. 2.. Two sharp bands Ž3610 and 3521 cmy1 . evolve at LNT, being superimposed on the broad absorption with a maximum at 3429 cmy1 . The initial presumptions that the broad background absorption is owing to presence of non-crystal water in the sample andror in the cell were abandoned since identical picture was observed in spectra recorded from paraffin oil suspensions of previously additionally dried sample and without utilization of the cell. Pronouncedly high frequency of one of the nŽOH. bands Ž3610 cmy1 . implies that one of the
2
As shown by Jovanovski and Kamenar w4x, different hydrates are obtained by recrystallization from H 2 O and EtOH.
Fig. 2. The 4000–500 cmy1 region in the RT Ža, c. and LNT Žb, d. FT-IR spectra of CsŽsac.ŽHsac.PH 2 O Ža, b. and CsŽsac.P 0.5H 2 O Žc, d..
water OH groups in the structure of 1 is not included in hydrogen bonding. The presence of the other nŽOH. band Ž3521 cmy1 ., in addition, indicates that the second water OH group in 1 could be weakly hydrogen bonded. The respective 2nŽOH. overtones are observed as a pair of expectedly very weak but well-defined bands at approximately doubled frequencies Ž7215 and 7040 cmy1 . in the LNT NIR spectrum ŽFig. 3., the anharmonicity of the higherfrequency band Ž5 cmy1 . being larger than that of the one at lower frequency Ž2 cmy1 .. Broad, evidently complex nŽOH. band is present in the RT and LNT IR spectra of 2 with a single maximum at somewhat lower frequency Ž3390 cmy1 at LNT. than one of the broad nŽOH. absorptions of 1 ŽFig. 2.. Regularly, being a result of extensive vibrational interactions, the appearance of the OH stretching region alone can be hardly used to obtain any precise structural information about the crystal water from the spectroscopic data. The advantages of employment of the isotopic dilution technique, on the other hand, are well known Žsee, for example, Ref. w12x. and it has been frequently used for such purposes. The presence of two nŽOD. bands Ž2644 and 2606 cmy1 . with similar intensities in the LNT difference 3 spectrum of 1 with very low deuterium
3 The difference spectra were obtained by substraction of the Žappropriately scaled. LNT spectrum of the protonated samples from the corresponding spectra of the slightly deuterated samples.
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n˜ ŽOD. vs. dŽO PPP O. correlations w13,14x, the nŽOD. frequencies of isotopically isolated HDO impurities in H 2 O or D 2 O matrices are known to have predictive value for the values of the corresponding O PPP O distances. Application of the correlation equation given for a series of solid hydrates ŽEq. 1; w13x. on the studied saccharinates resulted in the values 3.107 ˚ for 1, and 2.826 A˚ for 2. and 3.006 A
˚ s 1r Ž y3.73 . drA =ln Ž 2727 y n˜ Ž OD . rcmy1 . r8.97 = 10 6 Ž 1.
Fig. 3. The 2nŽOH. ŽA. and 2nŽNH. ŽB. overtone regions in the LNT spectrum of CsŽsac.ŽHsac.PH 2 O.
content w x ŽD. - 0.3%, Fig. 4x reveals existence of two non-equivalent OD oscillators in the corresponding structure. It is to be expected, therefore, that either a single water molecule with two non-equivalent proton sites, or possibly, two crystallographically non-equivalent C2 water molecules exist in the structure of 1. In case only one type of water molecules was present, the two nŽOD. and the corresponding LNT nŽOH. bands could be identified as the water modes n 1 and n 3 . The appearance of the doublet of nŽOD. bands, as well as of the corresponding nŽOH. and 2nŽOH. doublets, would then reflect the low degree of coupling of the two water stretching modes. The frequency split between the two nŽOD. fundamentals Ž38 cmy1 . in such case would be indicative for the low distortion of the water molecules w12x, one of the protons being nonhydrogen bonded, while the other one eventually being involved in rather weak hydrogen bonds. The presence of a single nŽOD. band of isotopically isolated HOD species in H 2 O matrix in the difference spectrum of 2 ŽFig. 4., on the other hand, reveals presence of a single type of water molecules placed on a C2 axis in its structure. The band frequency Ž2490 cmy1 . implies that the two equivalent protons are included in hydrogen bonding of weak to moderate strength. Besides being useful for qualitative discussions of the hydrogen bonding, with the means of empirical
Similar expression derived from the isomorphous series of MŽII. saccharinates ŽEq. 2; w14x. yields ˚ for 1, and 2.844 A˚ in the case of 3.156 and 3.044 A 2.
˚ s 4.47128 y 0.29754 drA =ln Ž 2727 y n˜ Ž OD . rcmy1 .
Ž 2.
3.1.2. NH r ND stretching region in the spectrum of Cs(sac)(Hsac) P H2 O An exceptional part of the spectrum of 1 is the broad absorption continuum extending from around
Fig. 4. The region of isotopically isolated nŽOD. modes in the LNT spectra of CsŽsac.P0.5H 2 O ŽA. and CsŽsac.ŽHsac.PH 2 O ŽB..
P. NaumoÕ, G. JoÕanoÕskir Vibrational Spectroscopy 24 (2000) 201–211
3200 cmy1 down to about 2200 cmy1 where it merges with the background absorption ŽFig. 2.. A maximum around 2520 cmy1 and a AwindowB of high transmittance around 2775 cmy1 Ždiscussed below. are present. Several weak bands give complex appearance to the 3100–2800 cmy1 region of the absorption. However, the spectral picture from this region is apparently reproduced in the spectrum of 2 and, therefore, such band structure of the spectrum of 1 is not inherent for the broad absorption itself, but is being resulted by other superimposed weak Ž nŽCH. and non-fundamental. bands.4 Very broad structurized nŽXH. ŽX s O, N. bands spreading over relatively low frequencies compared to what is usual for nŽXH. modes, such as the one observed in the spectrum of 1 ŽFig. 2., are wellknown characteristics of the spectra of strongly hydrogen-bonded XH groups w15x. The appearance of similar nŽNH. feature ŽFigs. 2 and 5., distinguishing for the spectrum of 1 from to-date spectroscopically characterized saccharinates, represents inevitable evidence for the existence of strongly hydrogen bonded imino groups from saccharinato molecules in its structure. The frequency of the centroid of the absorption Ž2520 cmy1 . is lower than the frequencies of all component nŽNH. bands in the spectrum of saccharin Ž3090, 2970 and 2700 cmy1 . w16x, which itself comprises rather strong hydrogen bonds of Oco PPP H`N type in the solid state Ž dŽO PPP N. s ˚ w17,18x. The imino groups in 1 are in2.796Ž5. A volved therefore in hydrogen bonds that comprise much stronger proton acceptorŽs. than the water, carbonyl or sulfonyl oxygen atoms. The only such candidate in the present system, consisted of metal cations, water molecules and saccharinato ionsr molecules, is the negatively charged nitranionic center of a neighboring saccharinato ion. There is little doubt therefore that saccharinato molecule–ion couples that comprise strong hydrogen bonds of wN`H PPP Nxy type exist in the structure of 1. What would remain unclear at this stage of the study is the precise geometry of the hydrogen bonding, i.e., the position of the proton between the saccharinato
4
The pair of weak bands around 2360 and 2340 cmy1 are uncompensated CO 2 absorptions.
205
Fig. 5. ŽA. The 3200–1850 cmy1 region in the LNT spectra of CsŽsac.ŽHsac.PH 2 O Ža. and CsŽsac.P0.5H 2 O Žb.. ŽB. Carbonyl stretching regions in the LNT spectra of CsŽsac.ŽHsac.PH 2 O Ža. and saccharin Žb..
residues. On the other hand, the infrared absorption continua, such as the one observed in the spectrum of 1, are very specific and were one of the early indications for the existence of delocalized protons in the solid state w19x. Therefore, the spectral picture of 1 might suggest that the imino protons are dynamically disordered. The protons, namely, can be visualized as AbouncingB between the saccharinato nitrogen atoms on the time-scale of the order of the stretching vibrational transition, leading to subsequent broadening of the corresponding nŽNH. absorption band. Further study, however, is to be devoted to this phenomenon in the present case. Remarkable feature of the nŽNH. absorption band in the spectrum of 1 ŽFigs. 2 and 5. is that both its branches are asymmetric and inclined towards the minimum around 2775 cmy1 . The position of the latter, moreover, is not affected by the temperature and its halfwidth seems to be reduced upon cooling, contrary to what would be expected if it was a high-transmittance region between two bands that are sharpened with the temperature decrease. Moreover, a single weak LNT band originating from the first overtone of the nŽNH. mode is present at 5203 cmy1 ŽFig. 3., giving roughly 2600 cmy1 or somewhat higher for the frequency of the corresponding nŽNH. mode, and thus a value in the vicinity of the nŽNH. minimum. It can be presumed from this that
206
P. NaumoÕ, G. JoÕanoÕskir Vibrational Spectroscopy 24 (2000) 201–211
Fig. 6. Linear plot of the nŽCO. frequencies in saccharinates Žl. and saccharin ŽI.. Symbol ' denotes the highest-frequency nŽCO. band of CsŽsac.ŽHsac. P H 2 O.
there is a single structural type of strongly hydrogen bonded imino groups and therefore only one type of saccharinato molecules in the structure of 1. Having all this in mind, it seems probable that the region of high transmittance around 2775 cmy1 in the spectrum of 1 is due to Evans type Fermi resonance of the NH stretching with non-fundamental modes of low inherent intensity, rather than a minimum between two separate absorption bands. Otherwise, Evans effects have been regularly observed in the IR spectra of strongly hydrogen bonded cyclic imides in the solid state, and were also reported in case of saccharin w16x. 3.1.3. Carbonyl stretching region The carbonyl stretchings in the infrared spectra of various saccharinates have been frequently used for spectra–structure correlations Že.g., Refs. w7,10x.. A general conclusion from such studies was that the carbonyl stretching frequencies in the spectra of saccharinates are consistently lower than the corresponding value of saccharin Ž1725 cmy1 . w16x.5 A qualitative relation of the infrared shift from the saccharin nŽCO. IR frequency with the type of the metal–saccharinato bonding, moreover, was observed. Frequency shifts from about 30 cmy1 in purely covalent mercuryŽII. saccharinates to more than 80 cmy1 in typically ionic saccharinates were found w10,20,21x.
5 In some previous studies Že.g., Ref. w16x. a singlet band was prescribed to the saccharin nŽCO. mode. Our further investigations using higher resolution, however, revealed that this band is clearly a doublet, which can also be inspected from Fig. 5B. In such a case, the wavenumber of what was previously referred to as saccharin nŽCO. band Ž1725 cmy1 . corresponds to the higherfrequency component band.
Regarding the number and the intensity ratio of the bands, as well as the overall appearance, the strong doublet around 1730 cmy1 in the spectrum of 1 resembles quite close the nŽCO. doublet in the LNT spectrum of saccharin ŽFigs. 2 and 7.. Unique feature of the IR spectrum of 1 compared to all previously studied saccharinates is the position of the infrared nŽCO. doublet band ŽFig. 5.. The frequency of the higher component band Ž1732 cmy1 ., namely, is higher than that of saccharin in the solid state Ž1725 cmy1 . and even higher than the corresponding band in the RT spectrum of the only previously reported saccharin adduct, wVOŽOH.Žsac.ŽHsac.ŽH 2 O.x Ž1727 cmy1 . w11x. Being interested in this extraordinary characteristic, we have undertaken an exhaustive literature search for spectroscopic data on metal saccharinates w22x. The population of the nŽCO. frequencies of 53 saccharinates together with saccharin and its anion in DMSO solution can be inspected from the plot in Fig. 6. It can be concluded that the nŽCO. frequency of 1 is indeed the highest of all analyzed saccharinates. The corresponding nŽCO.
Fig. 7. The LNT spectra of saccharin Žc., CsŽsac.ŽHsac.PH 2 O Ža., CsŽsac.P0.5H 2 O Žd. and the solid residue Žb. after dissolution of CsŽsac.ŽHsac.PH 2 O in water and evaporation of the solvent Žb can be thought of as a sum of c and d..
P. NaumoÕ, G. JoÕanoÕskir Vibrational Spectroscopy 24 (2000) 201–211
RT Raman band of 1 is observed at 1721 cmy1 w10x and thus higher than the saccharin Raman carbonyl stretching band Ž1697 cmy1 .. The position of this infrared carbonyl band in the spectrum of 1 inevitably confirms the existence of molecular saccharin in its structure in accordance with the definitely covalent character of its N`H bond. The band maximum remains practically unshifted Ž1733 cmy1 . at LNT, but the RT shoulder on its lower-frequency side is separated into additional band Ž1726 cmy1 . resulting in an additional overlapped nŽCO. band ŽFig. 5.. The picture of this spectral region of 1 at low temperature therefore resembles that of saccharin, but the nŽCO. frequency of the first band is again significantly higher. It might be taken as indication that the carbonyl groups of saccharin molecules in 1 participate in weaker hydrogen bonds than the Oco PPP H`N bonds of solid saccharin. This, in return, goes along with the above considerations that the carbonyl oxygen atoms in 1 are not favorable candidates as imino proton acceptors. Two additional asymmetric, strong and relatively broad nŽCO. RT bands Ž1619 and 1578 cmy1 , Figs. 2 and 5. are present in the nŽCO. region of the spectrum of 1.6 On lowering the temperature, the higher-frequency band shows slight negative Žd n˜rdT - 0. shift Ž1621 cmy1 ., while the lower-frequency one undergoes appreciable positive Žd n˜rdT ) 0. shift Ž1567 cmy1 .. Pronounced frequency difference between the two lower-frequency carbonyl stretchings in 1 and saccharin Ž104 and 158 cmy1 . is expected from the presence of saccharinato ions in its structure. It is likely that the band Ž1579 cmy1 . that evolves at low temperature is not due to carbonyl stretching, but to the stretching mode that is mainly localized in the saccharinato phenylene ring and is being regularly found at roughly the same frequency in the spectra of saccharinates w10x. This sharp and practically temperature-insensitive band Žat least within the temperature range examined in this study. is probably hidden under the broader carbonyl stretching at RT. The band is insensitive to deutera-
6 Due to the decomposition of 1 upon dissolution in D 2 O, no deuteration to higher degree to account for the d ŽHOH. modes in this region was achieved.
207
tion and thus cannot be prescribed to the water bending mode. Beyond any doubt, the intense RT band at 1647 cmy1 Ž1649 cmy1 at LNT. in the spectrum of 2 can be prescribed to the carbonyl stretching modes ŽFig. 2.. The frequency difference Ž76 cmy1 . between this band and the corresponding saccharin nŽCO. band is consistent with the existence of solely ionic saccharinato residues in 2. In addition, two other prominent, rather sharp bands whose positions are practically insensitive to the temperature Ž1619 and 1582 cmy1 at RT, 1620 and 1582 cmy1 at LNT. are present in the nŽCO. spectral region of 2. The insufficient intensity of the band at 1620 cmy1 , however, is in contrast with what is expected for nŽCO. bands in the spectra of saccharinates w20,21x. The presumptions based on the frequency and the appearance of the band that it is owing to the water deformational mode could not be confirmed since our attempts to obtain highly deuterated sample of 2 had failed. However, as can be seen from Fig. 8, the shoulder on the lower-frequency side of the band vanishes upon deuteration to about 10%, which might be an indication that the water bending mode in fact has certain contribution to this band. Being pronouncedly sharp and unaffected by the temperature, on the other hand, the band at 1582 cmy1 is presumably due to the above-mentioned benzenoid mode and bears
Fig. 8. The carbonyl strteching region in the spectra of protiated Žb. and deuterated Ža. CsŽsac.P0.5H 2 O.
P. NaumoÕ, G. JoÕanoÕskir Vibrational Spectroscopy 24 (2000) 201–211
208
mainly nŽCC. character, as shown by the ab initio calculations w10x. The above discussion leaves as many as three nŽCO. bands in the spectrum of 1 and a single nŽCO. band in the case of 2. As it was noted earlier w20,21x, however, the number of the infrared nŽCO. bands alone, even in the case of less complicated appearance of nŽCO. region, is not reliable to make predictions on the number of structurally different saccharinato anions. 3.1.4. Sulfonyl stretching region The sulfonyl stretchings in the spectra of saccharinates have been readily exploited for structural studies w23x. The strict assignment, however, might be troubled by the presence of additional bands in this region. The bands in the spectra of 1 and 2 that are due to the nŽSO 2 . modes were assigned by consideration of the temperature sensitiveness of the bands in the region, as well as accounting for the earlier assignments w10,20x and theoretical calculations w10,24x. Three bands correspond to each of the sulfonyl stretching modes Žantisymmetric and symmetric. in the LNT IR spectrum of 1 ŽFig. 7; Table 1., while only two bands for each mode are observed in its RT Raman spectrum w10x. The pair of the highest fre-
Table 1 Sulfonyl stretching frequencies of CsŽsac.ŽHsac.PH 2 O Ž1. and CsŽsac.P0.5H 2 O Ž2. a Parameter
Saccharin w16x
1
2
n˜ w nas ŽSO 2 .x
1335
1271Ž64. 1241Ž94.
n˜ w nsŽSO 2 .x
1180
Db
155
n˜s rn˜as
0.884
1342Žy7. 1308Ž27. 1258Ž77. 1177Ž3. 1167Ž13. 1149Ž31. 165 141 109 0.877 0.892 0.913
a
1148Ž32.
123 93 0.903 0.925
All values except for those in the last row are given in cmy1 . The numbers in parentheses are the differences with the corresponding values of saccharin. b D s n˜ w nas ŽSO 2 .xy n˜ w nsŽSO 2 .x.
quency antisymmetric and symmetric infrared bands Ž1342 and 1177 cmy1 . can be prescribed to the sulfonyl stretchings of the saccharinato molecules. That is worth noting while, as expected, the antisymmetric stretching frequency is placed higher than the saccharin value, the frequency of the symmetric one is similar with, but somewhat lower Ž3 cmy1 . than the respective saccharin mode ŽTable 1.. The recent ab initio treatment of the saccharinato ion w10x, however, showed that the antisymmetric sulfonyl stretching mode is remarkably ApurerB vibration than the symmetric one and this might be the reason that it reflects the structural environment of the SO 2 group Ži.e., the existence of molecular saccharin. better than the symmetric nŽSO 2 . mode. A pair of weak Raman bands Ž1306 and 1257 cmy1 . in the spectrum of 1 were previously attributed to the nas ŽSO 2 . modes, while two somewhat stronger bands were prescribed to the nsŽSO 2 . modes Ž1175 and 1152 cmy1 . w10x. Comparison of the nŽSO 2 . data between the studied salts and saccharin ŽTable 1. shows that the frequency fluctuations of the antisymmetric stretching mode are larger than those of the symmetric, which was explained w10x by the above-mentioned vibrational purity of the antisymmetric mode. As Table 1 shows, moreover, the split between the two stretchings Žantisymmetric and symmetric. and their frequency ratio are also consistent with the type of the saccharinato species Žmolecular or ionic.. Contrary to the saccharinato ions, namely, the frequency split between the SO 2 stretchings of molecular saccharin in 1 is larger than that of pure saccharin. Nevertheless, it was noted previously w23x that one of the factors that have effect on the extent of the split between the two stretchings is the angle between the sulfonyl oxygen atoms. Thus, it can be presumed that the O`S`O angleŽs. of the saccharinato ionŽs. in 1 israre smaller whereas the corresponding angle of molecular saccharin in 1 is similar with the O`S`O angle of pure saccharin Ž117.78 w17x or 117.4Ž1.8 w18x.. Two bands Ž1271 and 1241 cmy1 . in the spectrum of 2 can be assigned as the antisymmetric sulfonyl stretchings. The strong and apparently complex band with a maximum at 1148 cmy1 , on the other hand, corresponds to the symmetric SO 2 stretching modeŽs.. It does not seem likely that the sharp band at 1119 cmy1 is originated by the stretch-
P. NaumoÕ, G. JoÕanoÕskir Vibrational Spectroscopy 24 (2000) 201–211
ing of the SO 2 groups, since a band of very similar appearance and at the same frequency is also observed in the spectrum of saccharin and was previously prescribed w10,24x to a benzenoid mode. The number of assigned nŽSO 2 . band pairs in the spectra of the of 1 and 2 suggests existence of two structural types of sulfonyl groups i.e., two crystallographically non-equivalent saccharinato ions in the corresponding structures, in addition to the saccharinato molecules in 1. 3.1.5. ConÕersion of Cs(Hsac)(sac) P H2 O into Cs(sac) P 0.5H2 O and saccharin Additional simple and fairly convincing evidence towards the existence of molecular saccharin in 1 is the simple conversion of 1 into a mixture of 2 and saccharin. Owing to the low water solubility at RT, namely, the attempt to dissolve 1 in water resulted in its partial dissolution. Subsequent evaporation of the solute yielded a mixture of 2 and saccharin that could be inspected via the vibrational spectrum of the residue Žspectrum b in Fig. 7 can be thought of as sum of the spectra c and d.. In addition to the other spectral changes, the highest-frequency nŽCO. band of 1 is shifted downwards to the saccharin value Ž1725 cmy1 .. The weak antisymmetric SO 2 stretching band Ž1342 cmy1 . shifts to the saccharin value Ž1336 cmy1 , spectrum b in Fig. 7. and two more nas ŽSO 2 . bands appear Ž1271 and 1241 cmy1 .. The latter bands were assigned above as the antisymmetric sulfonyl stretchings of 2.
209
3.2. Structural criteria The presence of characteristic Že.g., CO and SO 2 . bands for both molecular and typically ionic saccharinato entities in the adduct 1 reveals the asymmetric shape of the proton potential within the wN`H PPP Nxy hydrogen bonds of the molecule–ion pairs in its structure. The existence of both molecular and ionic forms of saccharin, however, triggers the question of structural differences between these two species. Namely, structural criteria for distinction between the saccharinato molecules and the saccharinato ions other than the presence of the proton nearer to the one or the other of them are needed. In order to predict more structural details of 1, we used the results from the recent Cambridge Structural Database ŽCSD. analysis w3x of the data on metal saccharinates to compare the structural parameters of pure saccharin and those Žaveraged. for saccharinato ion. The results are summarized in Table 2. Following the approach in Ref. w3x, the AinternalB geometry of the five-membered saccharinato ring was described by two pairs of Dieterich et al. w25x coordinates Žwhich are proportional to differences between selected angles or distances within the ring., formulated for the CCC and SN parts of the ring. A total of 24 ionic saccharinato species and two independent sets of data for saccharin were obtained from the analysis w3x. Similarly for what was shown in the statistic CSD survey of pyrazoles w26x, it was concluded that the Dieterich et al. approach has
Table 2 Structural differences between saccharinato ions and molecules based on the CSD analysis in Ref. w3x Parameter
Molecules
Ions
D aŽCC.4a D aŽSN.4a D dŽSN.4a ˚ dŽC`N.rA
f y26 f 22 f or - D dŽSN.4 Žions.
f or ) 0 - or f 16 f or ) D dŽSN.4 Žmolecules.
) 1.360
f 1.350
˚ dŽS`N.rA ˚ Žaverage. dŽS`O.rA /ŽO`S`O.r8 /ŽC`S`N.r8
f 1.660
- 1.650
- 1.430 f 118 f 93
) 1.430 - or f 117 ) 96
a The coordinates are defined as: D d ŽCC. s 100 d wCŽ2.` CŽ3.x y d wCŽ3.` CŽ4.x4, D aŽCC. s 10/ wCŽ3.` CŽ2.` NŽ1.x y /wCŽ2.`CŽ3.`CŽ4.x4, D dŽSN. s 100 dwCŽ4.`SŽ5.x y dwCŽ2.`NŽ1.x4, aŽSN. s /wSŽ5.`NŽ1.`CŽ2.x y /wCŽ4.`SŽ5.`NŽ1.x ŽThe atom labels are given in Fig. 1..
210
P. NaumoÕ, G. JoÕanoÕskir Vibrational Spectroscopy 24 (2000) 201–211
certain discriminative power for the description of distortions of the saccharinato five-membered ring. The saccharinato molecules in 1, namely, should have higher and lower values for the D aŽSN. and D aŽCC. coordinates, respectively, than the saccharinato ions. The value of D dŽSN. for the saccharinato ions should be similar or probably larger than that of the molecules. Both C`N and S`N distances in the molecules should be larger than the corresponding in ˚ being approximate limiting value the ions, 1.650 A for the latter. The structural parameters concerning the CO and SO 2 groups were considered to describe the AexternalB geometry of the ring w3x. Characteristic difference were found between the mean S`O distances, that should be lower and higher than 1.430 ˚ for molecular and ionic saccharin, respectively. A Furthermore, the O`S`O angles in the saccharinato ions should be smaller than those in the molecules, the limiting value of which would be about 1178. The value of the C`S`N angle also reflects the type of the saccharinato moiety: for ionic saccharinates, it is typically around 97.58, whereas for molecular saccharin, its value should be close to 938.
similar with the one of the ions in Na saccharinate Ž759 K..
4. Conclusions The present work shows that the compound characterized as CsŽHsac.Žsac. P H 2 O is an adduct of molecular saccharin. The spectroscopic data reveal some of the exceptional structural properties of this compound, resulted from the inclusion of relatively large organic molecule in a typically ionic salt. Moreover, the more general structural and eventually physiological significance of this work would be that it is shown that in addition to the diversity of bonding modes of its deprotonated form, saccharin can also form adducts in the solid state and even with the simple alkali saccharinato salts. This, eventually, can have some significance for understanding the mechanisms of suspected pathological action of molecular saccharin on the biological systems. Further work is to be devoted to the structure details and particularly the hydrogen bonding in the adduct.
3.3. Thermal studies The decomposition temperature of the saccharinato ion Že.g., in Na saccharinate, exothermic peak at 759 K. is for about 150 K higher than that of saccharin itself Žendothermic peak at 610 K.. The thermal decomposition of 1 starts with partial dehydration at about 368 K, resulting in CsŽsac.ŽHsac. P 0.5H 2 O that might be formally considered as a Asolid mixtureB of 2 and saccharin. The hemihydrate melts at 461 K, accompanied by complete dehydration. This is followed by melting of the saccharin, represented with the second sharp endothermic peak, at temperature Ž489 K. that is somewhat lower than the melting temperature of pure saccharin Ž502 K.. The melting is followed by three-step decomposition. Saccharinato molecules decompose during the first Žexothermic DSC peak at 525 K. and the second Žendothermic DSC peak at about 600 K. step. The temperature of the latter is very close to that of the decomposition of pure saccharin Ž608 K.. In the third, exothermic step ŽDSC peak at 751 K., the saccharinato ions are decomposed at temperature
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