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Nitrate ion reorientation in aqueous calciuim nitrate solutions: new evidence for ion association
VoIumc
95. number
2
NITRATE ION REORIENTATION
CHEMICAL
PHYSICS
17 June
LETTERS
1983
IN AQUEOUS CALCIUM NITfWTE SOLUTIONS:
NE!+ EVIDENCE FOR ION ASSOCIATZON
I& q~ertrum 01 mtr.itc *on reor~cnidtwnJ1 motton in dilute cakium nitrzzte solutions ha\ been obtsincd from the Iine41.1pc of dcpoI.~rtxxi KJ? ki# scattcrmc dercrmincd b> Fabry-Perot infrrferometr~. The results shoa the presence of at k.!\i IN,, lorentzi;ln I‘omponcniS I‘or J boktion 0 9 moI dm -3. broad .md narrot\ bands can be resolved \.hich correspond to rcl.!\.lImn ttnte\ of 3 8 .tnd 5@ ps. The fornirr is J9signed to reorrentation of NO; simkw to tIwt observed previously for ddutc Ah cll-mctJ! nitrJtc wlul10n~ Tbc slou rclawtlon probdbl) results iron1 re0rientJtion of NO; m ;Issocidtcd ion pairs (*r WIII~.I~ \truL turn:\
I_ lntroductictn
2. Experimental
\\ e recently dtscusscd the reorientational motions oiSCN- and NO; ni ~q~ie~llssolutio~ls ofsimple salts .I?; dctetniincd hy dcpol.nised R+leigh light scatlering [ i ] _ 111 tllc cdw of‘ thoc~ dIldtes both the temperature Jepenctmx anti tllc” ~(111~~11tr~111~11 drpendences of the reurIertt.ztion.4 rel~xstron time 8) conformed with the predxr~or~s ofa simple hydrodynamical model [2] dc’~eIope~~ from origmA ptopos~ls by D&ye. For the nltiafe5, ho\\e\er. the h~drodynarnic theory could 0nly be 111.& IO 1‘11 the &Id by assu111111g that the hini~lcl.~ry conditions for nitrate ion reorientation 11erc ~L~pe~i&mt 011 hot11 concentrzition and the nature of the cJtic)n prrsem II wsconchrded that there ticrr strong end specific ~atto~l-aniotl interaction effeeis 0ri rorariuns about the two-fold axes ofN0~. For cwicentrated sohrtions these interactions were ~1s~)In~n~!Pst 111 nun-lorrntzt~n lineshapes. In rhni .irrisle \\e discuss the lincshape of depoI.ttIsell li.tyls~gii sc.rttermg from calcium nitrate soluIIOIIS md ~110~~ that there is clear evidence, in this case, itir NOT zeortentattons III at least two distinct structur.d envmtnmeuts. esen in dxiute solutions.
The experimental details and data analysis were essenti,.tllp as descrtbed previously [ 11. High-resolution spectral analysis of depolarised Rayleigh scattering (0 usmg a piezoelectric to -10 cm- r) \\as performed scanning i%bry--Perot interferometer. pkate spacings produced free spectral ranges,& from I cm-t to =20 cm-t. With an operating finesse in excess of 35, the spectral resolution was thus 0.03-0.6 crt-t. Samples were prepared front Analar grade reagents and solutions were frhered repeatedly to remove as much particulate matter as possible [I]_ Data analysis was performed by fitting the depolarised scattering to a convolution of the instrument functton and either one or the sum of two lorentzian bands.
3. Results and discussion Depolarised light scattering spectra from aqueous solutions 1.8 mof dm-’ in ammonium nitrate and 0.9 rnol dm-” in calcium nitrate are shown in figs. la and 1b. A single lorentzian lineshape accounts extremely well for the ammonium nitrate spectrum but fails completely to fit the data for caIcium nitrate. The latter spectrum exhibits a strong central peak. Careful ex0 009-26 14/S3/0000-0000/S
03.00 0 1983 North-Holland
Volume 96. number 3
CHEMICAL
PHYSICS LEl-fERS
Fig. 1. Depolarised Rayleigh scattering for nitrate aqueous solutions determined by Fabry-Perot interferometry. One and a half orders of the interferogrdms are shown. The squares are data points. The frequency scales arc shown on each individual spectrum. The intensity scales are not comparable. (a) 1.8 mol drnm3 ammcnium nitrate, 384 I;, sin~lclorentzian fit.(b) 0.9 mol dmJ cakium nitrate. 293 K, single-lorentzian frt to broad component. (c) 0.9 mol dmw3 cakium nitrate, 293 K. two-lorentzian frt. An instrument function is also shown here. (d) 0.9 mol dmm3 calcium nitrate, 293 K, singlelorentzian fit to narrow component at high resolution.
periments were performed to verify that this central feature was not due to spurious polarised or general background scattering leaking through the prism polariser [ 11. Comparisons were made of the observed depolarisation ratios for calcium nitrate solutions with that obtained for a sample of carbon tetrachloride in the same sample cell, which gives extremely weak depolarised scattering. The intensity of the central peak
17 June 1983
for calcium nitrate in fig. 1b was more than an order of magnitude greater than that expected if it were spurious scattering alone. These facts. in conjunction with evidence and arguments presented previously [ I] regarding the interpretation of depolarised scattering from nitrate solutions, suggest strongly that both the narrow and broad components of the spectrum arise predominantly from reorientational motion of nitrate ions. Two different methods were used to cbaracterise further the complex structure of the depolarised spectra of aqueous calcium nitraie. Several solutions were examined, with concentrations in the range 0.3-3.0 mol dtll-3. They all showed similar brhaviour but we limit the following discussion to one solution 0.9 mol dm-3 in calcium nitrate. Firstly the best-tit single lorentzian linewidth (half-width at half height r) was obtained as a function of the free spectral range. At high resolution (2
Volume
98. number 2
CHEMICAL
PHYSICS LETTERS
tral components were much less easily resolved. It is noticeable that the width of the broad component measured at large free spectral range is considerably in excess of rt extracted from the t\%o-lorentzian fit at intermediate resolution. This may be due partly to the difficulties mentioned above but it also suggests that the t\\o-lorentzian description itself may be an oversimplification. The behaviour of the broad band is remmiscent of that found for the more concentrated solutions of alkali-metal and ammonium nitrates for which the non-lorentzian character nas revealed in a significant decrease in the “best fit” singlelorentzian linewidths as the free spectral range was increased. although in these cases no prominent central components were observed. On the basis of the above discussion we conclude that the most reasonable value for the linewidth r, of the broad component is 1.40 F 0.10 cm-t and that the hnewidth r, of the narrow component is 0.10 + 0.05 cm- 1. The-relaxatron time rl-(7) corresponding to the broad band m 0.9 mol dm-j calcmm nitrate is 3.S ps. This 1s comparable with values observed for NOi reorientatron in drlute solutions of alkali-metal nttrates and is very close to the value of 5 ps obtained for the best-fit smgle-lorentzian band for a solution 2.25 niol dm-’ in lithium nitrAe. which has essentr.ill> the sdme vrscosity [ I] _ The ndrro\\ component. accounting for =-1O% of the mtegrated intensit) of the reorientational spectrmlr at .r concentration of 0.9 mol dm-j. corresponds to .r rela_\ation time of z-50 ps \\hich is extremely long for reorientation of “free” NO; in this solution_ 11 is d f.rctor of three or more larger than values obtained for concentrated and highly viscous solutions of alkali-metal nitrates. _4 possible explanation of this feature is tltnt it arises from NO: rzorientation in a highly localised structural environment where its rotationrll motion is severely restricted_ Such an environment u ould be consistent with close association of NO< anti Cd?+ in, for instance. contact ion pairs.
It is I\nown from vibrational spectroscopic studies that there are strong cation-anion interactions in aqueous solutions of many metal nitrates [3.4] _In most cases this appears more bkely to be a solvent-separated, or outer-sphere interaction in which NO, is asso-
174
17 June 1983
ciated with a hydrated cation. Calcium nitrate is one of the few cases where the observed perturbation of the NOT vibrations is sufficiently strong for the clear characterisation of an additional set of vibrational bands and this has been interpreted in terms of the formation of contact ion pairs CaNOf [5] _ The exact structure of such a species is not certain. The CaZ+ ion could be located equidistant from two 0 atoms or, more likely [3], localised close to one 0 atom. In either case however it is unlikely that the Ca’+ is in the plane of NO3 and there will be severe restrictions on the motions of NO, about its two-fold axes (the motion to which light scattering is primarily sensitive)_ In addition rotation of the whole ion-pair structure will contribute to the spectrum of reorientational motion. The present results provide additional indirect evidence for the existence of strong ionic interactions in relatively dilute solutions of calcium nitrate. Whilst the simplest interpretation is in temis of an ion-pairfree-ion equilibrium. we suspect that the actual situation is more comples with a distribution of local structures between and beyond these two simple descriptions. It may be possible to make more quantitative conclusions concerning ionic equilibria in such systems from further more detailed studies of ionic reorientation in solutions of calcium and other multivalent metal nitrates. It is interesting to compare the value of 50 ps for r$‘) \vith other times characterisin dynamical pro& IS more than 10 cesses in nitrate solutions [ 1,6]. 7~ times longer than the reorientation time of “free” NO-7 and also the average residence times of H20 in the hydration spheres of single-valent ions, but is of the same order as the reorientation times for the hydration spheres of large ions 161. If the interpretation in terms of CaNO; is correct then we can put a lower limit of ~50 ps on its lifetime. This is comparable with the average time between ionic encounters in a solution 1 mol dmm3 in calcium nitrate as calculated from simple diffusion theory. It leaves as yet unanswered the interesting question as to whether the rotating structure is a stable species which can survive ionic “collisions” or whether it exists only benveaz such encounters.
Volume
98. number
2
CHEMICAL
References [I]
hi.Whittleand
J.H.R.Clarke. hloi. Phys44 (1981) 1435. and P-A. hfadden, Ann. Rev. Phys. Chem. 31(1980) 523. [ 31 D.E. Irish and h1.H. Brooker, in: Advances in infrared and Raman soectroscorw. Vol. Il. eds. R.J.H. Clark and R.E. Hester (Heyden. &don, 1676).
[ 21 D. Kivelson
PHYSICS
LETTERS
17 June
1983
[4] J.P. Devlin. in: Advances in infrared and Raman spectroscopy, Vol. 11. eds R.J.H. Clark and R.E. Hester (Heyden. London, 1976). [ 51 R.E. Hester and R.A. Plane, J. Chem. Phgs. 40 (1964) 411. [6] H.G. Hertz, in: Water, a comprehensive treatise. Vol. 3 ed. F. Franks (Plenum Press, Ne\\ York, 1973).
175
95. number
2
NITRATE ION REORIENTATION
CHEMICAL
PHYSICS
17 June
LETTERS
1983
IN AQUEOUS CALCIUM NITfWTE SOLUTIONS:
NE!+ EVIDENCE FOR ION ASSOCIATZON
I& q~ertrum 01 mtr.itc *on reor~cnidtwnJ1 motton in dilute cakium nitrzzte solutions ha\ been obtsincd from the Iine41.1pc of dcpoI.~rtxxi KJ? ki# scattcrmc dercrmincd b> Fabry-Perot infrrferometr~. The results shoa the presence of at k.!\i IN,, lorentzi;ln I‘omponcniS I‘or J boktion 0 9 moI dm -3. broad .md narrot\ bands can be resolved \.hich correspond to rcl.!\.lImn ttnte\ of 3 8 .tnd 5@ ps. The fornirr is J9signed to reorrentation of NO; simkw to tIwt observed previously for ddutc Ah cll-mctJ! nitrJtc wlul10n~ Tbc slou rclawtlon probdbl) results iron1 re0rientJtion of NO; m ;Issocidtcd ion pairs (*r WIII~.I~ \truL turn:\
I_ lntroductictn
2. Experimental
\\ e recently dtscusscd the reorientational motions oiSCN- and NO; ni ~q~ie~llssolutio~ls ofsimple salts .I?; dctetniincd hy dcpol.nised R+leigh light scatlering [ i ] _ 111 tllc cdw of‘ thoc~ dIldtes both the temperature Jepenctmx anti tllc” ~(111~~11tr~111~11 drpendences of the reurIertt.ztion.4 rel~xstron time 8) conformed with the predxr~or~s ofa simple hydrodynamical model [2] dc’~eIope~~ from origmA ptopos~ls by D&ye. For the nltiafe5, ho\\e\er. the h~drodynarnic theory could 0nly be 111.& IO 1‘11 the &Id by assu111111g that the hini~lcl.~ry conditions for nitrate ion reorientation 11erc ~L~pe~i&mt 011 hot11 concentrzition and the nature of the cJtic)n prrsem II wsconchrded that there ticrr strong end specific ~atto~l-aniotl interaction effeeis 0ri rorariuns about the two-fold axes ofN0~. For cwicentrated sohrtions these interactions were ~1s~)In~n~!Pst 111 nun-lorrntzt~n lineshapes. In rhni .irrisle \\e discuss the lincshape of depoI.ttIsell li.tyls~gii sc.rttermg from calcium nitrate soluIIOIIS md ~110~~ that there is clear evidence, in this case, itir NOT zeortentattons III at least two distinct structur.d envmtnmeuts. esen in dxiute solutions.
The experimental details and data analysis were essenti,.tllp as descrtbed previously [ 11. High-resolution spectral analysis of depolarised Rayleigh scattering (0 usmg a piezoelectric to -10 cm- r) \\as performed scanning i%bry--Perot interferometer. pkate spacings produced free spectral ranges,& from I cm-t to =20 cm-t. With an operating finesse in excess of 35, the spectral resolution was thus 0.03-0.6 crt-t. Samples were prepared front Analar grade reagents and solutions were frhered repeatedly to remove as much particulate matter as possible [I]_ Data analysis was performed by fitting the depolarised scattering to a convolution of the instrument functton and either one or the sum of two lorentzian bands.
3. Results and discussion Depolarised light scattering spectra from aqueous solutions 1.8 mof dm-’ in ammonium nitrate and 0.9 rnol dm-” in calcium nitrate are shown in figs. la and 1b. A single lorentzian lineshape accounts extremely well for the ammonium nitrate spectrum but fails completely to fit the data for caIcium nitrate. The latter spectrum exhibits a strong central peak. Careful ex0 009-26 14/S3/0000-0000/S
03.00 0 1983 North-Holland
Volume 96. number 3
CHEMICAL
PHYSICS LEl-fERS
Fig. 1. Depolarised Rayleigh scattering for nitrate aqueous solutions determined by Fabry-Perot interferometry. One and a half orders of the interferogrdms are shown. The squares are data points. The frequency scales arc shown on each individual spectrum. The intensity scales are not comparable. (a) 1.8 mol drnm3 ammcnium nitrate, 384 I;, sin~lclorentzian fit.(b) 0.9 mol dmJ cakium nitrate. 293 K, single-lorentzian frt to broad component. (c) 0.9 mol dmw3 cakium nitrate, 293 K. two-lorentzian frt. An instrument function is also shown here. (d) 0.9 mol dmm3 calcium nitrate, 293 K, singlelorentzian fit to narrow component at high resolution.
periments were performed to verify that this central feature was not due to spurious polarised or general background scattering leaking through the prism polariser [ 11. Comparisons were made of the observed depolarisation ratios for calcium nitrate solutions with that obtained for a sample of carbon tetrachloride in the same sample cell, which gives extremely weak depolarised scattering. The intensity of the central peak
17 June 1983
for calcium nitrate in fig. 1b was more than an order of magnitude greater than that expected if it were spurious scattering alone. These facts. in conjunction with evidence and arguments presented previously [ I] regarding the interpretation of depolarised scattering from nitrate solutions, suggest strongly that both the narrow and broad components of the spectrum arise predominantly from reorientational motion of nitrate ions. Two different methods were used to cbaracterise further the complex structure of the depolarised spectra of aqueous calcium nitraie. Several solutions were examined, with concentrations in the range 0.3-3.0 mol dtll-3. They all showed similar brhaviour but we limit the following discussion to one solution 0.9 mol dm-3 in calcium nitrate. Firstly the best-tit single lorentzian linewidth (half-width at half height r) was obtained as a function of the free spectral range. At high resolution (2
Volume
98. number 2
CHEMICAL
PHYSICS LETTERS
tral components were much less easily resolved. It is noticeable that the width of the broad component measured at large free spectral range is considerably in excess of rt extracted from the t\%o-lorentzian fit at intermediate resolution. This may be due partly to the difficulties mentioned above but it also suggests that the t\\o-lorentzian description itself may be an oversimplification. The behaviour of the broad band is remmiscent of that found for the more concentrated solutions of alkali-metal and ammonium nitrates for which the non-lorentzian character nas revealed in a significant decrease in the “best fit” singlelorentzian linewidths as the free spectral range was increased. although in these cases no prominent central components were observed. On the basis of the above discussion we conclude that the most reasonable value for the linewidth r, of the broad component is 1.40 F 0.10 cm-t and that the hnewidth r, of the narrow component is 0.10 + 0.05 cm- 1. The-relaxatron time rl-(7) corresponding to the broad band m 0.9 mol dm-j calcmm nitrate is 3.S ps. This 1s comparable with values observed for NOi reorientatron in drlute solutions of alkali-metal nttrates and is very close to the value of 5 ps obtained for the best-fit smgle-lorentzian band for a solution 2.25 niol dm-’ in lithium nitrAe. which has essentr.ill> the sdme vrscosity [ I] _ The ndrro\\ component. accounting for =-1O% of the mtegrated intensit) of the reorientational spectrmlr at .r concentration of 0.9 mol dm-j. corresponds to .r rela_\ation time of z-50 ps \\hich is extremely long for reorientation of “free” NO; in this solution_ 11 is d f.rctor of three or more larger than values obtained for concentrated and highly viscous solutions of alkali-metal nitrates. _4 possible explanation of this feature is tltnt it arises from NO: rzorientation in a highly localised structural environment where its rotationrll motion is severely restricted_ Such an environment u ould be consistent with close association of NO< anti Cd?+ in, for instance. contact ion pairs.
It is I\nown from vibrational spectroscopic studies that there are strong cation-anion interactions in aqueous solutions of many metal nitrates [3.4] _In most cases this appears more bkely to be a solvent-separated, or outer-sphere interaction in which NO, is asso-
174
17 June 1983
ciated with a hydrated cation. Calcium nitrate is one of the few cases where the observed perturbation of the NOT vibrations is sufficiently strong for the clear characterisation of an additional set of vibrational bands and this has been interpreted in terms of the formation of contact ion pairs CaNOf [5] _ The exact structure of such a species is not certain. The CaZ+ ion could be located equidistant from two 0 atoms or, more likely [3], localised close to one 0 atom. In either case however it is unlikely that the Ca’+ is in the plane of NO3 and there will be severe restrictions on the motions of NO, about its two-fold axes (the motion to which light scattering is primarily sensitive)_ In addition rotation of the whole ion-pair structure will contribute to the spectrum of reorientational motion. The present results provide additional indirect evidence for the existence of strong ionic interactions in relatively dilute solutions of calcium nitrate. Whilst the simplest interpretation is in temis of an ion-pairfree-ion equilibrium. we suspect that the actual situation is more comples with a distribution of local structures between and beyond these two simple descriptions. It may be possible to make more quantitative conclusions concerning ionic equilibria in such systems from further more detailed studies of ionic reorientation in solutions of calcium and other multivalent metal nitrates. It is interesting to compare the value of 50 ps for r$‘) \vith other times characterisin dynamical pro& IS more than 10 cesses in nitrate solutions [ 1,6]. 7~ times longer than the reorientation time of “free” NO-7 and also the average residence times of H20 in the hydration spheres of single-valent ions, but is of the same order as the reorientation times for the hydration spheres of large ions 161. If the interpretation in terms of CaNO; is correct then we can put a lower limit of ~50 ps on its lifetime. This is comparable with the average time between ionic encounters in a solution 1 mol dmm3 in calcium nitrate as calculated from simple diffusion theory. It leaves as yet unanswered the interesting question as to whether the rotating structure is a stable species which can survive ionic “collisions” or whether it exists only benveaz such encounters.
Volume
98. number
2
CHEMICAL
References [I]
hi.Whittleand
J.H.R.Clarke. hloi. Phys44 (1981) 1435. and P-A. hfadden, Ann. Rev. Phys. Chem. 31(1980) 523. [ 31 D.E. Irish and h1.H. Brooker, in: Advances in infrared and Raman soectroscorw. Vol. Il. eds. R.J.H. Clark and R.E. Hester (Heyden. &don, 1676).
[ 21 D. Kivelson
PHYSICS
LETTERS
17 June
1983
[4] J.P. Devlin. in: Advances in infrared and Raman spectroscopy, Vol. 11. eds R.J.H. Clark and R.E. Hester (Heyden. London, 1976). [ 51 R.E. Hester and R.A. Plane, J. Chem. Phgs. 40 (1964) 411. [6] H.G. Hertz, in: Water, a comprehensive treatise. Vol. 3 ed. F. Franks (Plenum Press, Ne\\ York, 1973).
175