J. Phys. Chem. 1981, 85, 1686-1692
1686
Raman and Ultrasonic Relaxation Studies of Some Nitrate Salts in N-Methylacetamide D.
E. Irish,” T. G.
Chang, S.-Y. Tang,
Guelph-waterloo Centre for Graduate Work in Chemistty, Waterloo Campus, University of Waterloo, Waterloo, Ontario, Canada N2L 3 0 1
and S. Petruccl Department of Chemistry, Polytechnic Institute of New York, Brooklyn, New York 11201 (Received: October 27, 1980; In Final Form: March 10, 1981)
Raman spectra and ultrasonic relaxation spectra of solutions of NH4N03,Mg(N03)2,Ca(N03)2,Sr(N03)2, Ba(N03)2,and Pb(N03)2in N-methylacetamide (NMA) have been obtained and correlated. Raman bands of both solvated and ion-paired anions have been identified. The ammonium and magnesium nitrates are strong electrolytes in NMA; the degree of association increases in the order Ca2+< Sr2+< Ba2+< Pb2+for the other salts. Association constants have been estimated. Ultrasonic relaxation spectra, in the frequency range 3-350 MHz, show a single Debye relaxation for both the pure solvent and the solutions. For the neat liquid this relaxation process is eliminated or shifted by the addition of water or the raising of the temperature. The addition of the electrolyte enhances the strength of the process such that it is still detectable to 70 “C. The electrolyte appears to catalyze the process by lowering both the enthalpy and energy barriers. It is suggested that ion pairs fit into and enhance the linear chain polymers and that “free” ions are centers from which chains radiate and thus “free” ions act as cross links.
Introduction In a previous paper, concerned with some sodium and lithium salts in N,N-dimethylacetamide (DMA), we demonstrated clear advantages of combining the results from vibrational spectroscopy and ultrasonic relaxation and compared and contrasted the information obtainable by the two techniques.’ The present study was undertaken to extend the objectives and seek further correlations of data from these two lines of experimentation. To contrast with DMA, N-methylacetamide (NMA), a solvent with the high dielectric constant of 178.9 at 303 K and dipole moment of 3.73 D, was chosen. The physical and chemical properties of this solvent have been rev i e ~ e d . ~Group -~ I1 metal nitrates were chosen as electrolytes because of our considerable experience with the Raman spectroscopy of nitrates6 and because knowledge of the interactions of group I1 cations with the peptide amide group is of biophysical importance. From classical theories of ionic association which involve electrostatic potentials between ions and the bulk permittivity of the solvent, significant cation-anion association would not be anticipated. Detection of associated ions suggests the occurrence of covalent forces between the ions. Elucidation of the forces and processes operative in solvents of high permittivity is of considerable interest. Experimental Section Solvent NMA was obtained from both Eastman Kodak and Aldrich. It was recystallized at least twice in a drybox under a dry nitrogen atmosphere. The portions used had a melting point of 30.5 OC. Solutions were prepared by weight in a drybox in such a way that concentrations of both the solute and solvent were obtained. “,NO3 (Baker Analyzed), Ca(N03)2-4H20(Baker Analyzed), Sr(N03)2(Fisher Certified), Ba(N03)2(Fisher (1) D. E. Irish, S.-Y.Tang, H. Talts, and S. Petrucci, J. Phys. Chem., 83, 3268 (1979). (2) R. J. Lemire and P. G. Sears, Top Curr. Chem., 74, 45 (1978). (3) L. A. Knecht, Pure Appl. Chem., 27, 281 (1971). (4) J. W. Vaughn in “The Chemistry of Non-Aqueous Solvents”,Vol. 2, J. J. Lagowski, Ed., Academic Press, New York, 1967, p 191. (5) D. E. Irish and M. H. Brooker in “Advances in Infrared and Raman Spectroscopy”, Vol. 2, R. J. H. Clarke and R. E. Hester, Ed., Heyden, London, 1976, Chapter 6.
Certified), and Pb(N03)2(Baker and Adamson) were dried by standard techniques. A solution 1M in Mg2+and 1M in NO3- was prepared from anhydrous Mg(C104)2and NH4N03. Mg(N03)2.6H20(Fisher Certified) and Mg(C104)2.6Hz0(G.F. Smith Reagent) were used as received. The Raman and ultrasonic equipment and techniques were previously described.lg6 Digitized Raman band profiles were resolved into component bands by employing the computer program CURVER.’
Results and Discussion Vibrational Spectra. The trigonal planar nitrate ion has Da symmetry and is thus expected to exhibit a four-band spectrum. In water solvent the frequencies and symmetry species are as follows: vl(Al’), 1049 cm-l, Raman-active only; v2(A2”),-830 cm-’, infrared-active only; v3(E’),a doublet centered at about -1380 cm-l, Raman and infrared active; v4(E’),-719 cm-l, Raman and infrared active. Even for dilute aqueous solution, the spectra of alkali metal nitrates contain a doublet in the v3(E’) region, with broad depolarized components at 1348 and 1406 cm-l (Raman) and 1347 and 1395 cm-’ (infrared).sie10 Because this doublet has been found to be insensitive to both the nature of the cation and the concentration for solutions less concentrated than 1M it has been proposed that the degeneracy of the v3(E’)mode has been lifted by the formation of a water-nitrate hydrogen-bonded complex.&1° It is important to note the greater sensitivity of this antisymmetric stretching mode to a perturbation than the deformation mode v4, which is also degenerate; the latter band appears as a symmetrically shaped single band. When a cation binds to the nitrate ion a second set of bands appears in the spectrum. In principle one could anticipate six new bands because the symmetry would be lowered from Da to C , C,,or C1. Two of these would arise from a lifting of the degeneracy of the antisymmetric (6) J. T. Bulmer, D. E. Irish, F. W. Grossman, G. Herriot, M. Tseng, and A. J. Weerheim, Appl. Spectrosc., 29, 506 (1975). (7) A. R. Davis, D. E. Irish, R. B. Roden, and A. J. Weerheim, Appl. Spectrosc., 26, 384 (1972). (8) D. E. Irish and A. R. Davis, Can. J. Chem., 46, 943 (1968). (9) D. J. Lockwood, J. Chem. SOC.Faraday Trans. 2,71,1440 (1975). (10) T. J. V. Findlay and M. C. R. Symons, J. Chem. SOC.,Faraday Trans. 2, 72, 820 (1976).
0022-365418112085-1686$01.2510 0 1981 American Chemical Society
The Journal of Physical Chemistty, Vol. 85,No. 12, 1981 1687
Raman Studies of Nitrate Salts in NMA
TABLE I: Raman Band Positions and Full-Widths at Half-Band-Height for Nitrate Ion in NMA (the u 4 Region) uF,, fwhh, VB,, fwhh, solute cmcm-' cmcm-' ",NO, Mg(CIO,),/NH,NO, c a w 0 3 12
SrWO,),
W N O 3 I* Pb(N03)2
712 712 709 708 708 707
16 20 14
13 14 13
732 726 720 722
I
I
I
11
io
11 15
stretch (u3);these two bands are commonly observed, e.g., 1320 and 1500 cm-' for ZnN03+ in H20/CH3CNmixed solvent.'l One band would arise in the u1 region; although it may be displaced from that of the aquated nitrate ion (e.g., 1038 cm-' for ZnN03+compared to 1049 cm-l for the aquated nitrate ion") it is also known to be virtually superimposed on the 1049-cm-l band for many nitrate salts in water such as Na+,12 Sr2+,13and Ca2+.I4 The band corresponding to u2 occurs at lower frequencies in the IR (e.g., 815 cm-l for ZnN03+ compared to 830 cm-l for the aquated nitrate ionll) and is often weakly seen in the Raman spectrum. The band arising from the deformation mode occurs at higher frequencies (e.g., 755 cm-l for ZnN03+ compared to 719 cm-l for the aquated nitrate ion'l). However, a second component of this band which would arise from the lifting of the degeneracy by interaction with the cation has not been observed and it is assumed to be essentially coincident with this higher frequency mode (755 cm-') for the ion pairs or complexes studied to this time. Thus when an equilibrium occurs between the aquated nitrate ion and the ion-paired nitrate ion experience is in keeping with observation of a maximum of ten bands, five from the aquated nitrate ion and five from the cation-bonded nitrate ion. The spectra of the Pb(N03)2/NMAsolutions contain all the bands expected for an equilibrium between solvated and ion-paired nitrate ions. In the v1 region two bands are observed at 1041 and 1033 cm-l and are assigned to the solvated and ion-paired forms of nitrate, respectively. The infrared spectrum contains two bands at 830 (shoulder, solvated) and 821 cm-l (ion paired) in the u2 region. The u4 region consists of two Raman bands at 707 (solvated) and 722 cm-l (ion paired) (Figure 1and Table I). The v3 region is obscured by bands of the solvent. For Ba(N03)2, and Ca(N03)2two bands were also observed in the u4 region (Figure 1 and Table I) but only one band was observed in the v1 region at 1043,1044, and 1045 cm-l, respectively. For these same salts in water solvent only one band is apparent in the ul region as noted above although the two classes of nitrate ion (solvated and ion paired) are evident from the v4 region. In contrast, for NH4N03and for Mg(C104)2/NH4N03 in NMA a single band was observed at 712 cm-' with a full-width at half-height (fwhh) of 16 and 20 cm-l, respectively, (Figure 1). In addition, a single band was observed in the vl region at 1043 and 1044 cm-', respectively. For these salts, therefore, it is assumed that only solvated nitrate exists in the NMA solutions. The u1 region contains several weak lines of NMA which make the baseline ill-defined for quantitative analysis of the intensity. The v4 spectral region is free of interference and was selected for quantitative study. For the Ca-, Sr-, (11) Y. K. Sze and D. E. Irish, J. Solution Chem., 8, 395 (1979). (12)J. D. Riddell, D. J. Lockwood, and D. E. Irish, Can. J. Chem., 50,
2951 (1972).
(13) J. T. Bulmer, T. G. Chang, P. J. Gleeson, and D. E. Irish, J. Solution Chem., 4,969 (1975). (14) D. E. Irish and G. Walrafen, J. Chem. Phys., 46, 378 (1967).
I
I
I
740
700
660
P/crn-'
Flgure 1. The 660-760-cm-' region of the Raman spectrum of the specified nitrate salts in NMA.
Ba-, and Pb(NO& salts, the digitized band contours in the u4 region were resolved into two components with the CURVER program and the band parameters are collected in Table I. The concentration dependence of the intensities of the two lines clearly indicates that the lower frequency line is characteristic of a solvated form of nitrate ion (note the solvent dependence of the frequency viz. 718 cm-' in water and 708 cm-l in NMA) and the higher frequency band is assigned to nitrate ions in contact with cations. All line intensities have been measured relative to the intense 883-cm-l line of NMA which serves as an internal intensity standard. The relative integrated intensity is therefore given by
Z = (area of nitrate band/area of 833-cm-l band) X concentration of NMA For ammonium nitrate these intensities were found to be directly proportional to concentration. Linear regression gave the slope, equal to the molar intensity of the "free" or solvated form of nitrate ion, JF = 0.186 f 0.004. For each salt the 1, values were fitted by a quadratic equation. These equations are z~(Ca(NO3)z)= -0.003614 + 0.18495c0~os - 0.011797C0~o,2 0.6076 5
CoNOs
5 1.8512 M (7 solutions)
I&3r(N03),) = 0.13837CoNOs - 0.011283C0~~~ 0.2140 5
CoNOs
S 2.0060 M (12 solutions)
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Irish et al.
The Journal of Physical Chemistty, Vol. 85,No. 12, 1981
IF(Ba(N03),) = 0.16926CoNO,- 0.05026coNo~ 0.2956 I C0No3I 1.0124 M (11 solutions) I~(Pb(N03)2)= 0.088570C0~os - 0.0060857C0~032 0.4326 I CoN03I 3.0920 M (11solutions)
C 0 ~ ois3 the total nitrate concentration and is equal to 2C0,db From the relation CF = I F / J F the concentrations of free nitrate ion were calculated. These were used to calculate Q, the fraction of free nitrate, and rt, the average ligand number, defined as (CoNo,- CF)/Cosalt The plot of u vs. concentration of salt is presented in Figure 2. Consistent with a qualitative inspection of the relative intensities in Figure 1, Figure 2 shows that ion association increases in the order Ca(N03), < Sr(N03)2< Ba(N03), < Pb(NO3)> For concentrations greater than 0.2 M du/dC is markedly small and constant with the exception of Ba(N03)2. Both hand calculations and the computer program GAUSS z,5J5J6which fits stability constants to rt, CFdata, were used to estimate equilibrium constants for the most important processes. For Ca(N03)2and Sr(N0J2 a single equilibrium adequately accounts for the data:
t
1
o.2 o°C!)O
0'2
0'6
0'4
1'0
0'8
1'2
1'6
1'4
'
Figure 2. The fraction of "free" nitrate, 6,vs. the total salt concentratlon for the specified salts.
M2++ NO3- + MN03+ The Kll values are collected in Table 11. For the lowest concentrations of Sr(N03)2the values of Kll are high (7.5 at 0.107 M and 4.4 at 0.200 M) but they fall and level out at the value given for concentrations greater than 0.4 M. A GAUSS z analysis gives an overall value of Kll equal to 3.4 but inspection of the fit suggests that it is preferable to consider that Kll falls to an essentially constant value. For the lowest obtainable concentration of Ca(N03)2the value of Kll was also higher (0.32 at 0.3038 M compared to 0.23 f 0.02 for the others) suggesting that at low concentrations the values may rise as for Sr(N03),. I t is interesting to note that the ion association for Ca(N03)2and Sr(N03)2is greater in NMA than in watera5,13 Two stability constants were required to fit the Ba(N0J2data. A second step Ba(N03)++ NO3-
KlZ
loo90 80
-
T = 35'C
T. 40'C T = 48-C
Ba(N03)2
T = 5 4 5'C
60 -
Kl2
ePb(N03)2
Because the concentration of free Pb2+is very small over the concentration range studied, the GAUSS z computation is unable to converge on a solution for KlP The total intensities, IF IB, for the four salts are almost directly proportional to the total concentration of nitrate ion. The slope, 0.206, is greater than the value of J F ; close inspection suggests that the slope approaches the J F value in the limit of low concentrations and that positive deviation occurs as concentrations rise. Thus the molar intensity of the bound form of nitrate, JB, is considered to be slightly larger than J F . (cf. Sr(N03)2in water s01vent.l~) The v4 band contours for Ca(N03)2and Sr(N03)2were examined for the temperature range 23-58 "C. The ratio I B / I F was virtually independent of temperature; at the highest temperature there was evidence that the degree of association was slightly greater.
+
-~~
0
70 -
has become important and accounts for the different slope in Figure 2. For Pb(N03)2rt ranges from 1.07 to 1.25. The results are consistent with a major process Pb(N03)++ NO3-
-
~
(15) R. S. Tobias and M. Yasuda, Inorg. Chem., 2, 1307 (1963). (16) F. J. C. Rossotti, H. S. Rossotti, and R. J. Whewell, J. Inorg. Nucl. Chem., 33, 2051 (1971).
50;
,
5
,
IO
1
I
20
50
,
100
1
1
200
500
f (MHz)--
Figure 3. a / f 2vs. the frequency ffor NMA at the designated tem= 0.22. peratures and for a NMA/HpO mixture XHZO
TABLE 11: Stepwise Formation Constants for 1:l and 1:2 Cation-Anion Complexes i n NMA solute
K , , , mol-' L
Ca(NO,), Sr( NO, )%
Ba(NO,),
0.23 f 0.02 2.6 f 0.2a 1.28 f 0.05
Pb(NO,),
indeterminable
K,,, mol-' L
0.04 0.11 a For concentrations > 0.4 M; below 0.4 M K , , rises. 0.66 0.28
f
i
The vibrational spectrum of the solvent was studied by Mizushima et al.17J8 They proposed the existence of linear chains of hydrogen-bonded molecules in the trans form in the liquid state,17 a conclusion supported by much evidence.19r20 Also from vibrational spectral studies it has (17) S. Mizushima, T. Simanouti, S. Nagakura, K. Kuratani, M. Tsuboi, H. Baba, and 0. Fujioka, J. Am. Chem. Soc., 72, 3490 (1950). (18) T. Miyazawa, T. Shimanouchi, and S. Mizushima,J. Chem. Phys.,
29, 611 (1958).
The Journal of Physical Chemistry, Vol. 85,No. 12, 1981 1689
Raman Studies of Nitrate Salts in NMA r
I
220 200 I80 160
t
140
N M A ; T=69.a0C. 5
20
10
50
500
100 200
IO
20
50
100
200
C
t
::I 'i 270
5
210
L
I
N M A ; T=30°C
300
I
I
5
IO
l
l
I
20
50
2o01
I
100
200
;
500
r
I
I
1
I
I
I
5
IO
20
50
100
200
I
I
5
2501
tn
230 210
190
1
0
[ 0.47M ~ in1
*
N M A ; T=30"C
N M A ; T=40°C 1
Ca 7
I
I
I
1
I
2
5
IO
20
50
I
100
I
200
500
150;
;
I
I
I
I
5
IO
20
50
100 2 0 0
e
f (MHz) + Flgure 4. a / f 2vs. the frequency f for the system Ca(NO,),/NMA
at the designated concentrations and temperatures.
been concluded that cations are solvated through interaction with the C=O group21p22and anions through the N-H group.21 Dispersed cations and anions would thus disrupt the linear chains by breaking the C=O-H-N linkages and possibly create centers of charge from which new nonparallel chains might radiate causing a cross linking.23 On the other hand, ion pairs could fit into the chains and even strengthen the intermolecular forces by virtue of their stronger dipole nature. Ultrasonic Relaxation. The ultrasonic measurements for the solvent NMA in the frequency range 3-350 MHz and the temperature range 31-54.5 "C and for solutions (19) S. J. Bass, W. L. Nathan, R. M. Meighan, and R. H. Cole, J. Phys. Clzern., 68, 609 (1964). (20) L. L. Graham and C. Y. Chang, J . Phys. Chen., 75, 776, 784 (1971). (21) M. H. Baron and C. de Lo& J . Chim. Phys., 69, 1084 (1972). (22) D. Balasubramanian, A. Goel, and C. N. R. Rao, Chem. Phys. Lett., 17, 482 (1972). (23) R. H. Wood, R. R. Wicher, 11, and R. W. Kreis, J. Phys. Chem., 76, 2313 (1971).
of Ca(N03)2,Sr(N03)2,Pb(N03)2,Mg(N03)2.6H20,and Mg(C104)2.6H20are reported in Table I11 in the form f R , A , and B parameters, p , the corresponding sound absorption per wavelength, and u , the velocity of sound: P = a e x J = (01 - W%/f In Figure 3 representative plots of the a / f " functions vs. the frequency f are shown for the solvent. In the same figure the effect of the addition of water up to a mole fraction XHzO x 0.22 is shown for a temperature of 31 "C. The relaxation process is either largely eliminated or possibly shifted to frequencies much lower than those for pure NMA at 31 "C. Also the background absorption is moved to a lower value than the B coefficient for pure NMA. From Figure 3 it can be clearly seen that the relaxation process visible a t 31 "C gradually disappears and is gone at 54.5 "C. One could infer that the equilibrium responsible for the appearance of a relaxation process must be shifted so far to one side by heating that the sound wave was unable to cause a sizeable appearance of the unfavored
1690
The Journal of Physical Chemistry, Vol. 85, No. 72, 1981
Irish et ai.
600
500 400
300
in N M A i T=30"C I
t
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I 300 T=55OC
in N M A j
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:200
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2:
N cc
\
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Y
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L:!m 200
in N M A
2ool
2
T=40'C
j
5
10
1000
20
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loo~
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2
100
000 01
600
h
5 Ib i o
io
IbO 2 0
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2oo~
2
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f ( M H z ) --b
Flgure 5. a / f 2 vs. the frequency ffor the system Sr(NO&/NMA at the designated concentrations and temperatures. The insets illustrate how p depends on ffor the same systems. The soli lines in the insets correspond to the Debye functions for a single relaxation function = pm[(f/fR)/(i + (f/fR)*)I.
molecular form participating in the equilibrium. Data for Ca(N03)2are presented in Figure 4 and Table 111. At 30.2 OC two relaxations are visible for a concentration of 0.83 M. The solid and dashed lines correspond to the sum of two Debye relaxations. The one at the upper frequency is poorly defined in view of the uncertainty in
the choice of the Bz parameter, the background absorption at frequencies larger than both the relaxation processes. The viscosity of 0.83 M Ca(N03)zis quite large and thus it is possible that the second relaxation originates from the viscoelasticity of the system; this phenomenon has been reported by Moynihan et for aqueous Ca(N0&.8Hz0.
Raman Studies of Nitrate Salts in NMA
The Journal of Physical Chemistry, Vol. 85, No. 12, 1981
1691
TABLE 111: Ultrasonic Relaxation Parameters for NMA and NMA/Salt Solutions
I O ~ ~ A , 1017~, system
t , "C
Sr(NO,), /NMA
31 35 40 48 54.5 30 30 30 30 40 55 70
NMA
p
Ca(NO,),/NMA
C, M
0.90 0.68 0.49 0.30 0.90 0.90 0.90
f R , MHz
cm-' sz
cm-' 2
10spm,
6.5 10 18 35
75 60 30 6.5
84 80 73 67.5 63.6 405 270 195 138 285 235 165
33.2 40.9 36.8 14.9
10 13 15 15 15 25 30
535 350 240 192 465 195 165
372 316 250a 200a 479 323 321
lO-'u, cm
s'l
1.361 t 0.003 1.362 i 0.005 1.363 i 0.024 1.311 t 0.004 1.282 t 0.038 1.383 i- 0.009 1.390 * 0.008 1.374 i 0.013 1.325 * 0.018 1.297 * 0.003
F r o m linear regression of densities a t C = 0.9 M = 1.0927 - 7.86 X 10-4(t- 30) g ~ m - r ~ z =; 0.99999
30 30 30 40 55 69.8
0.83b 0.64 0.47 0.83 0.83 0.83
15 13 12 17 19 25
50 48 35 90 112 80
340 232 170 24 0 183 136
53.7 43.9= 29.6 109 147 134
1.432 t 0.022 1.408 i 0.022 1.424 t 0.033 1.384 f 0.026 1.341 t 0.030
F r o m linear regression of densities a t C = 0.83 M p
= 1.0460 - 7.47 X 1 0 - " ( t - 30) g ~ m - ~ ; =r '0.99991
Pb(NO,),/NMA
40
0.51
22
216
124
314
1.323
Mg( NO,); 6H, O/NMA
40
0.67
11
420
180
332
1.441
Mg( ClO4),.6H, O/NMA
55
0.25
25
14
81
Calculated with u = 1.39 X l o 5 c m s-'. ( a l p )as t h e s u m of t w o Debye processes a
23.8
1.360
F o r this system a t t = 30 "C a second relaxation is visible. If o n e expresses
t h e n for B , = 0: A , = 50 X lo-'?, A, = 340 X lo-'' cm-' s z ; f R , = 15, f R = 650 MHz; whereas for 13, = 100: A , = 50 X A , = 240 X lo-'' cm-' s z ; f~ = 15, f~ = 500 MHz, which corresponds to t h e solid and dashed lines respectively in Calculated with u = 1.408 X l o 5 c m s-l. Figure 4. B in t h e above column $s B = B , ; A , given f R , % f R , .
Data for Sr(N03)2are presented in Figure 5 and Table 111. The data can be interpreted by a single relaxation function (solid line). Some data are also presented in Table 111 for 0.51 M Pb(N03)2a t 40 "C, 0.67 M Mg(N03)2.6H20at 40 "C, and 0.25 M Mg(C10J2-6Hz0at 55 "C. In judging the reliability of the alf" vs. f fits of the ultrasonic data for pure NMA it should be pointed out that the relaxation frequency occurs a t the lower limit attainable by pulse techniques, and the relaxation strength is small and vanishes when temperature is increased. Therefore these authors feel that the values reported for A and f R should be regarded as semiquantitative. (In view of this it is surprising that In (?l/T) correlates so well with 1/ T as presented below.) Resonator techniques, unfortunately unavailable to us, with the cell accurately calibrated with liquids of known absorption coefficients in order to evaluate its quality factor Q, would provide data which would better delineate the relaxation. On the contrary, for the solutions the present data are reliable because the total absorption is much higher. One striking feature of the results is that the relaxation occurs at the same position for the salt solutions and for the neat solvent; but it does persist up to 70 "C in the presence of the salts whereas it has disappeared at 55 "C for the neat liquid. The Raman results revealed ionic association in the salt solutions, increasing in the order Ca(N03)2C Sr(NO3I2
