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Stabilities of gas-phase NO3− · (HNO3)n, n ⩽ 6, clusters
International Journal of Mass Spectrometry and Ion Physics, 35 (1980) 39-46 @ Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
STABILITIES
S. WEODEK,
OF GAS-PHASE
Z. LUCZYlk3KI
NO;
- (HNO&,
39
n < 6, CLUSTERS
and Ii. WINCEL
Department of Radiation Chemistry, Warsrawa (Poland)
Institute of Nuclear Research, Dorodna
16, 03-195
(Received 7 September 1979)
ABSTRACT The formation of NO5 . (HNOs),,, n < 6, clusters in mixtures of IINOs and Hz and in mixtures of HNOs and CH4 is studied by means of a high-pressure mass spectrometer. The temperature and pressure ranges covered are from 160 K to 360 K and from 0.1 torr to 5 torr, respectively. The thermodynamic values A_M,-l.,z, AGE-.-l.,, and &:-I,,~ for (for n = 2 to n = 6) are dctcrthe reactions NO: - (HN03), _ 1 + HNO3 i-2 NO, - (DN03),, mined. Some solvation structure is indicated by the AH,,-I,,, and AS~-I.,~ values and by theoretical calculations of the char-go distribution in the (NOsHNOs)- (HN03),, n = 0, 1 and 2, species. The relatively strong interaction in the complex NO:-HN03 is assumed to be due to the formation of a proton-bound dimcr. INTRODUCTION
In view of the current interest in ion/neutral species bonding, as well as the practical implications about the nature of terminal ions involving oxy compounds of nitrogen in the atmosphere, we have studied the gas-phase negative ion clustering reactions NO;
- (HNOs),,_l
+ HN03
7~ NO;
- (IIN03),,
(1)
and measured the thermodynamic properties for the (n - 1, n) equilibria up to (5,6). Very few studies on clustering reactions of the NO; ion have been carried out. Kebarle and co-workers [1,2] studied the hydration of NO; and mcaFehsenfeld et al. [ 3,4] investisured the enthalpy for the (OJ) equilibrium. gated the association of NO; with HN03, NO; - (HNO&,, n G 3, and determined the thermodynamic values for the (1,2) and (2,3) equilibria. It is the purpose of the present research to extend this knowledge to higher clusters of t.hc NO;--HN09 system. EXPERIMENTAL
The experiments were eter described previously
performed [5]. Two
with the high-pressure mass spectromdifferent reaction chambers were used:
40
the original chamber used in the earlier work [6,7] (reaction path length = 4 mm, designated Chamber 1) and a new chamber whose reaction path length can be varied over the range from 4 mm to 15 mm (Chamber 2). In order to operate the apparatus at higher reactant pressures, the vacuum system was modified. The ion source was housed in a 150 mm id. by 200 mm long chamber machined from a block of stainless steel. This chamber was connected to a 2000 1 s-l oil diffusion pump by a short 150 mm i.d. tube, 40 mm in length. The diffusion pump was backed by a 60 m3 h-l mechanical pump. Pumping of the collector chamber and the analyzer tube was provided by a 300 1 s-’ oil diffusion pump with a baffle backed by a 30 m3 h-l mechanical pump. Pressures were monitored at various points in the system with a ZOPAP model Pw-11 vacuum gauge control!er. The remainder of the apparatus and technique have been described in a previous paper [S]. In most of the experiments, ionization of the samples was obtained with 1000-V electrons. Some experiments with different electron energies were also made. An ion-accelerating voltage of 2 or 3 kV and ion-optics II * were employed. The instrument was operated under field-free conditions with the repeller and electron trap maintained at the reaction chamber potential. The measurements were performed at reaction chamber temperatures between 180 K and 380 K over a pressure range of 0.1-5 torr. The mixtures contained O.l-5.0% HN03 in hydrogen or methane carrier gas. The total pressure in the reaction chamber was measured with a diaphragm-capacitance micromanometcr (MCT-Atlas Werke) connected directly to the chamber through a 5 mm id. pipe. The nitric acid used (Polish product) was distilled under low pressure and stored in a dark glass container in a dry-ice cooled trap. The carrier gas H2 was passed through a coil immersed in liquid nitrogen to remove traces of water. Methane (Polish product) was passed through a trap loaded with 4A molecular sieve and chilled with a dry-ice trap. RESULTS
AND
DISCUSSION
The nitrate ions produced from HN03 by the following reaction sequence 133 I-INO
+ e + NO;
+ OH
NO; + HN03 + NO; + HNO,
(2) (3)
..-.* “Ion-optics II” [S] - the distance from the ion exit slit to the first electrode of the focussing system is 13 mm and the voltage applied to this electrode is reduced to the minimum operating potential, 4 V. The potentials applied to the half-plates of the focussing system are 34 and 17 V, respectively. The region between the reaction chamber and the focus electrode is screened by a highly transparent wire mesh maintained at the reaction chamber potential.
41
react further with HNOJ at relatively high pressures to form clusters via the usual ion-molecule clustering reactions NO; - (HNO&_,
+ HN03 % NO; - (HN03),1
(4)
The relative concentrations of prominent ions in 1.2% HNO,-98.8% CHa gas mixture are shown in Fig. 1 as a function of the temperature. As can be seen from the figure, the formation of NO; - (HN03), clusters with n up to 6 is observed. The thermodynamic properties for the equilibria (1,2) to (5,6) have been evaluated from the temperature dependence of the measured equilibrium constants. The AE&l,, values were obtained from the least-squares lines of the van’t Hoff plot shown in Fig. 2 as an example. A number of exploratory experiments were made in order to check the equilibrium conditions in the reaction chamber. These included studies in which the total pressure was varied from 0.2 torr to 3 torr with H2 and CH.? as the carrier gas. The concentration of HNO:, in the HN03--Hz or GINO,--CI-I, mixture was varied from 0.1% to 5% and the energy of the ionizing clcc-
3 \
1
I 0'
190
22u
250
280 TEMPERATURE,
310
340
370
K
Pig. 1. Relative concentrations of the major of gas temperature at a total pressure of 1.4
ions in 1.2% HN03+8.8%1 torr (Chamber 1 )_
Cflq
as a function
K “ST,” atm-’
102
,
I
I
10 2.5
30
40
3.5 lO+T,
Fig!.
2.
Van’t
L5
5.0
5.5
K
Hoff plots for reactions NO5 - (IINOs)),-1 + EIN03 . t NO5 - (HNOJ),. Open with Chamber 1. Full points, values obtained with Chamber 2.
points, values obhincd
trons was varied over the range 700-1000 V. In Fig. 3 the equilibrium constants KnHI,* for reactions (1,2) and (2,3) arc plotted as a function of total pressure in the 1% HN03-99% H2 mixture at 351 K. At pressures in Chamber 1 above 1.5 torr the equilibrium constants were essentially that under these pressures independent of the total pressure, indicating equilibrium in the reaction chamber is established. The measurements of thermodynamic values were made within the plateau region, usually at total pressures from 1 torr to 3 torr, and satisfactory agreement was found for the corresponding equilibria. A replicate determination was made several months later and the agreement with the original determinations was good. Additionally, the attainment of thermodynamic equilibrium was examined by measuring the thermodynamic values for the (1,2) and (2,3) equilibria using Chamber 2 at a fixed reaction path length of 8 mm. Data obtained in these experiments in general corroborate those obtained with Chamber 1. In Table 1 thermodynamic values calculated from several van% Hoff plots arc given. The error limits were estimated according to the Dean-Dixon method [9] for a confidence limit of 99%. At the highest temperatures used,
43
Kn-&n atm-’ 10~
K 2,3 00
o-o-o
o.o’o %ii
10;
_A_-
1.0
0
L--
2.0 PRESSURE,
3.0
4.0
torr
Fig. 3. Plats of ICI.2 and KZ,J at constant temperature pressure of the lY0 HN03-99% H2 mixture.
(351 K) as a function of the total
the concentration of NO; was too low to allow one to measure AHO., accurately. For comparison thermodynamic values found by Fehsenfeld et al. [ 3,4] are included in the table. As may be seen, despite the different instruments and conditions employed in this study and in the work of Fehsenfeld TABLE
1
Thermodynamic (HNO3 n
--
11
values obtained
b, 1
,
for the reactions:
NO.; - (HNOs),,_l _.
_--
-AZZ,_r
.I, (kcal mol-l) .---This work Lit.
--4GO.,, , .,~ ;: (kcal mol-’ ) This work
0.1
26 lX
172 2,3
16.0 + 0.8 13.9f1.4
3,4 495 5,6
9.3 + 1.3 7.4 f 1.2 4.6 -i- 0.9
g_;
-c 1.0 1’ . c 16.1 + 1.0 b
a Standard state 1 ntm, 298 K. b Ref. 4. c Ref. 3.
Lit. 14.5 b
9.0 2 0.3 5.9 + 0.4 3.2 2 0.4 1.9 _, 1.0 2.4 f 0.7
+ HN03
(367 K) 6.9 =
-4Sz, deg-I) --
+ NO; ---
n
(Cd
mol-’
*
This work -_
23.1 * 2.4 26.7 i 4.4 19.9 * 3.4 18.6 +- 5.0 7.3 +- 5.0
-Lit. b (3:: K) 24.1 f 2 b 28.9 f 2 b
.
44
et al., the results.from the two laboratories are in reasonable agreement_ Also it should be noted that the values obtained by Fehsenfeld et al. in an earlier study 133 are in substantially better agreement with our results than is the recent report 143 from that laboratory. Figure 4 shows a plot of -AHn- l.n as a function of the number of molecules present in the cluster NO; - (HNO,),,. The results show that the decrease in the values of enthalpy change is not regular; there is a big decrease between the first two interactions AIYI~,~and AIY1,2, and an appreciable decrease between AH 2,3 and AH3,4 as well as between AI!& and AIJ5.67 while the drop-off between AH l.2 and Lv-I,_, and between AH,_, and AIYGs5is somewhat smaller. The value predicted by Fehsenfeld and co-workers [4] for AH,,, of -26 kcal mol-’ indicates relatively strong bonding in the NO;-HNOB complex. The relative stability of this species may be explained as being due to the formation of strong hydrogen bonds in the proton-bound dimer NO; .._ Hi . . . NO,. Ab initio calculations IlO] (STO-3G) for the hydrogen dinitrate anion,
0
1
2
3
4
5
”
Fig. 4. Plot of NO3 - (HNOsh,.
AH,,--L.n ins a function of n for The insert shows a plot of A..&,,,
reactions vs. n.
NO;
- (IINOs),-1
+ HN03
.L
45
H(NO,);,
suggested two configurations
0,-N
II
I
The lowest energy was obtained for structure II (tetrahedral configuration) when the proton is closer to one or other oxygen atom (along the 01-01# line), but the energy differences between this and other possible configurations are not large. Assuming that the more favourable structure is given by II, in which the equilibrium position of the proton is between four oxygen atoms of the two NO; groups, one might expect that further build-up of the cluster occurs by attachment of the HN03 molecules to the 0 atoms orientated towards the proton. In order to rationalize this assumption the CND0/2 method was used to investigate the charge distribution in the hydrogen dinitrate anion. Calculations were performed with the CNINDO program for the CNDO/B method [ll] on the CDC Cyber 73 Computer System_ The bond lengths and angles in NO; and HN03 were fixed at the experimental values [ 12,131. The results obtained for the two extreme configurations of structure II and for the clusters of this structure with one and two HN03 molecules orientated towards the 011 and Ozl atoms when the proton is closer to O1 and linked by short (2.45 A) hydrogen bonds are summarized in Table 2. As may TABLE Charge
2 distribution
Atom
01
02 01’
02’ 03
03’ Nl Nl,
H Total charge on tetrahedral osygens
in the hydrogen (NOsHNO
dinitrate
anion
)-
(see text)
-__-_
_-
(N03HNOs)-
(N03HN03)- (~iNO3)2 1-I in the minimum
H on top of barrier
H in the minimum
. HNOJ II in the minimum
-0.4732 -0.4732 -0.4732 -0.4732 -0.4379 -0.4379 0.6186 0.6186 0.5314
--0.4338 -0.4425 -0.4956 -0.4956 -0.4219 -0.4578 0.6181 0.6221 0.5119
-0.4349 -0.4434 -0.4993 -0.4673 -0.4058 -0.4269 0.6167 0.6364 0.5085
-0.4313 -0.4443 -0.4765 -0.4765 -0.3917 -0.3984 0.6154 0.6500 0.5047
-1.8928
-1.8725
-1.8449
-1.8286
--_~
46
be seen, despite the proton’s position and the presence of HN03 molecules, the negative charge in H(NO& is distributed preferentially on the four tetrahedral oxygens. This suggests that higher binding energies and more-/stable clusters would be expected when the HN03 molecules go to these atoms. The charge distribution given in Table 2 seems to be compatable with the observed changes of AH,-,.., and AS:--,., with n (Fig. 4). A distinct dropoff between AiYH2,3 and AE13,. correlates well with the relatively high negative charge located on the two oxygen atoms O1n and 021, while the appreciable decrease between Lw,, 5 and AH,,. probably corresponds to addition of the HNO, molecules to the remaining tetrahedral oxygens, thereby closing the solvent shell around the central (N03HN03)ion. The results of semi-empirical calculations for (NOsHNO&- - (HN03),, n = 1 and 2, indicate that attachment of HN03 to (N03HN03)causes only a small decrease in the total negative charge located on the tetrahedral oxygens in (N03HN03)and leads to nearly equal charge dispersion among all the outer oxygens, suggesting a nearly equal probability of binding positions of molecules forming the second solvation shell. This situation is very similar to that observed in the solvation of NIli by NH3 molecules [l4 J where a “spherical” cluster NH: - (NII& is formed, contrary to the branching structure of H30+ - (H,O),z [ 153. ACKNOWLEDGEMENTS
The authors are very grateful to Dr. M. Geller from the Department of Biophysics of Warsaw University for providing the CNINDO prognm used in this work and to Mr. K. Grabowski for technica assistance. REFERENCES 1 J.D. Payzant, R. Yamdagni and P. Kebarle, Can. J. Chcm., 49 (1971) 3308. 2 J.D. Payzant, A.J. Cunningham and P. Kebarlc, Can. J. Chcm., 50 (1972) 2330. 3 F.C. Fehsenfeld, C.J. Howard and AL. SchmcItckopf, J. Chcm. Phys., 63 (1975) 2835. 4 J.A. Davidson, F.C. Fehsenfcid and C.J. Howard, Int. J. Chem. Kin&.. 9 (1977) 17. 5 H. Wince], Int. J. Mass Spcctrom. Ion Phys., 9 (1972) 267. 6 Z. Luczyliski, S. Wioclck and H. Wincci, Radiat. Phys. Chem., 11 (1978) 55. 7 S_ Wlodek, Z. Luczyiiski and II. Winccl, Raport INR-1746/XVII/G/A (1978). 8 Z. Luczynski and H. Wincal, Int. J. Mass Spcctrom. Ion Phys., 14 (1971) 29. 9 R.B. Dean and W.J. Dixon, Anal. Chem., 23 (1951) 636. 10 R. Gunde. T. Solmajer, A. AZman and D. HadZi, J. Mol. Struct., 24 (1975) ClOti. hIolecular Orbital Theory, McGraw11 J._4. Pople and D.L. Beveridgc, Approximate Hill, New York, 1970. 12 F.A. Cotton and G. Wilkinson, Advances in Inorganic Chemistry, 2nd edn., Wiley, New York, 1966, p. 351. 33 A.P. Cox and J.M. Riveros, J. Chom. Phys., 42 (1965) 3106. 14 A.J. Cunningham, J.D. Payzant and P. Kcbarle, J. Am. Chcm. Sot., 94 (1972) 7627. 16 J.D. Payzant, A.J. Cunningham and P. Kebarlc, Can. J. Chem., 51 (1973) 3242.
STABILITIES
S. WEODEK,
OF GAS-PHASE
Z. LUCZYlk3KI
NO;
- (HNO&,
39
n < 6, CLUSTERS
and Ii. WINCEL
Department of Radiation Chemistry, Warsrawa (Poland)
Institute of Nuclear Research, Dorodna
16, 03-195
(Received 7 September 1979)
ABSTRACT The formation of NO5 . (HNOs),,, n < 6, clusters in mixtures of IINOs and Hz and in mixtures of HNOs and CH4 is studied by means of a high-pressure mass spectrometer. The temperature and pressure ranges covered are from 160 K to 360 K and from 0.1 torr to 5 torr, respectively. The thermodynamic values A_M,-l.,z, AGE-.-l.,, and &:-I,,~ for (for n = 2 to n = 6) are dctcrthe reactions NO: - (HN03), _ 1 + HNO3 i-2 NO, - (DN03),, mined. Some solvation structure is indicated by the AH,,-I,,, and AS~-I.,~ values and by theoretical calculations of the char-go distribution in the (NOsHNOs)- (HN03),, n = 0, 1 and 2, species. The relatively strong interaction in the complex NO:-HN03 is assumed to be due to the formation of a proton-bound dimcr. INTRODUCTION
In view of the current interest in ion/neutral species bonding, as well as the practical implications about the nature of terminal ions involving oxy compounds of nitrogen in the atmosphere, we have studied the gas-phase negative ion clustering reactions NO;
- (HNOs),,_l
+ HN03
7~ NO;
- (IIN03),,
(1)
and measured the thermodynamic properties for the (n - 1, n) equilibria up to (5,6). Very few studies on clustering reactions of the NO; ion have been carried out. Kebarle and co-workers [1,2] studied the hydration of NO; and mcaFehsenfeld et al. [ 3,4] investisured the enthalpy for the (OJ) equilibrium. gated the association of NO; with HN03, NO; - (HNO&,, n G 3, and determined the thermodynamic values for the (1,2) and (2,3) equilibria. It is the purpose of the present research to extend this knowledge to higher clusters of t.hc NO;--HN09 system. EXPERIMENTAL
The experiments were eter described previously
performed [5]. Two
with the high-pressure mass spectromdifferent reaction chambers were used:
40
the original chamber used in the earlier work [6,7] (reaction path length = 4 mm, designated Chamber 1) and a new chamber whose reaction path length can be varied over the range from 4 mm to 15 mm (Chamber 2). In order to operate the apparatus at higher reactant pressures, the vacuum system was modified. The ion source was housed in a 150 mm id. by 200 mm long chamber machined from a block of stainless steel. This chamber was connected to a 2000 1 s-l oil diffusion pump by a short 150 mm i.d. tube, 40 mm in length. The diffusion pump was backed by a 60 m3 h-l mechanical pump. Pumping of the collector chamber and the analyzer tube was provided by a 300 1 s-’ oil diffusion pump with a baffle backed by a 30 m3 h-l mechanical pump. Pressures were monitored at various points in the system with a ZOPAP model Pw-11 vacuum gauge control!er. The remainder of the apparatus and technique have been described in a previous paper [S]. In most of the experiments, ionization of the samples was obtained with 1000-V electrons. Some experiments with different electron energies were also made. An ion-accelerating voltage of 2 or 3 kV and ion-optics II * were employed. The instrument was operated under field-free conditions with the repeller and electron trap maintained at the reaction chamber potential. The measurements were performed at reaction chamber temperatures between 180 K and 380 K over a pressure range of 0.1-5 torr. The mixtures contained O.l-5.0% HN03 in hydrogen or methane carrier gas. The total pressure in the reaction chamber was measured with a diaphragm-capacitance micromanometcr (MCT-Atlas Werke) connected directly to the chamber through a 5 mm id. pipe. The nitric acid used (Polish product) was distilled under low pressure and stored in a dark glass container in a dry-ice cooled trap. The carrier gas H2 was passed through a coil immersed in liquid nitrogen to remove traces of water. Methane (Polish product) was passed through a trap loaded with 4A molecular sieve and chilled with a dry-ice trap. RESULTS
AND
DISCUSSION
The nitrate ions produced from HN03 by the following reaction sequence 133 I-INO
+ e + NO;
+ OH
NO; + HN03 + NO; + HNO,
(2) (3)
..-.* “Ion-optics II” [S] - the distance from the ion exit slit to the first electrode of the focussing system is 13 mm and the voltage applied to this electrode is reduced to the minimum operating potential, 4 V. The potentials applied to the half-plates of the focussing system are 34 and 17 V, respectively. The region between the reaction chamber and the focus electrode is screened by a highly transparent wire mesh maintained at the reaction chamber potential.
41
react further with HNOJ at relatively high pressures to form clusters via the usual ion-molecule clustering reactions NO; - (HNO&_,
+ HN03 % NO; - (HN03),1
(4)
The relative concentrations of prominent ions in 1.2% HNO,-98.8% CHa gas mixture are shown in Fig. 1 as a function of the temperature. As can be seen from the figure, the formation of NO; - (HN03), clusters with n up to 6 is observed. The thermodynamic properties for the equilibria (1,2) to (5,6) have been evaluated from the temperature dependence of the measured equilibrium constants. The AE&l,, values were obtained from the least-squares lines of the van’t Hoff plot shown in Fig. 2 as an example. A number of exploratory experiments were made in order to check the equilibrium conditions in the reaction chamber. These included studies in which the total pressure was varied from 0.2 torr to 3 torr with H2 and CH.? as the carrier gas. The concentration of HNO:, in the HN03--Hz or GINO,--CI-I, mixture was varied from 0.1% to 5% and the energy of the ionizing clcc-
3 \
1
I 0'
190
22u
250
280 TEMPERATURE,
310
340
370
K
Pig. 1. Relative concentrations of the major of gas temperature at a total pressure of 1.4
ions in 1.2% HN03+8.8%1 torr (Chamber 1 )_
Cflq
as a function
K “ST,” atm-’
102
,
I
I
10 2.5
30
40
3.5 lO+T,
Fig!.
2.
Van’t
L5
5.0
5.5
K
Hoff plots for reactions NO5 - (IINOs)),-1 + EIN03 . t NO5 - (HNOJ),. Open with Chamber 1. Full points, values obtained with Chamber 2.
points, values obhincd
trons was varied over the range 700-1000 V. In Fig. 3 the equilibrium constants KnHI,* for reactions (1,2) and (2,3) arc plotted as a function of total pressure in the 1% HN03-99% H2 mixture at 351 K. At pressures in Chamber 1 above 1.5 torr the equilibrium constants were essentially that under these pressures independent of the total pressure, indicating equilibrium in the reaction chamber is established. The measurements of thermodynamic values were made within the plateau region, usually at total pressures from 1 torr to 3 torr, and satisfactory agreement was found for the corresponding equilibria. A replicate determination was made several months later and the agreement with the original determinations was good. Additionally, the attainment of thermodynamic equilibrium was examined by measuring the thermodynamic values for the (1,2) and (2,3) equilibria using Chamber 2 at a fixed reaction path length of 8 mm. Data obtained in these experiments in general corroborate those obtained with Chamber 1. In Table 1 thermodynamic values calculated from several van% Hoff plots arc given. The error limits were estimated according to the Dean-Dixon method [9] for a confidence limit of 99%. At the highest temperatures used,
43
Kn-&n atm-’ 10~
K 2,3 00
o-o-o
o.o’o %ii
10;
_A_-
1.0
0
L--
2.0 PRESSURE,
3.0
4.0
torr
Fig. 3. Plats of ICI.2 and KZ,J at constant temperature pressure of the lY0 HN03-99% H2 mixture.
(351 K) as a function of the total
the concentration of NO; was too low to allow one to measure AHO., accurately. For comparison thermodynamic values found by Fehsenfeld et al. [ 3,4] are included in the table. As may be seen, despite the different instruments and conditions employed in this study and in the work of Fehsenfeld TABLE
1
Thermodynamic (HNO3 n
--
11
values obtained
b, 1
,
for the reactions:
NO.; - (HNOs),,_l _.
_--
-AZZ,_r
.I, (kcal mol-l) .---This work Lit.
--4GO.,, , .,~ ;: (kcal mol-’ ) This work
0.1
26 lX
172 2,3
16.0 + 0.8 13.9f1.4
3,4 495 5,6
9.3 + 1.3 7.4 f 1.2 4.6 -i- 0.9
g_;
-c 1.0 1’ . c 16.1 + 1.0 b
a Standard state 1 ntm, 298 K. b Ref. 4. c Ref. 3.
Lit. 14.5 b
9.0 2 0.3 5.9 + 0.4 3.2 2 0.4 1.9 _, 1.0 2.4 f 0.7
+ HN03
(367 K) 6.9 =
-4Sz, deg-I) --
+ NO; ---
n
(Cd
mol-’
*
This work -_
23.1 * 2.4 26.7 i 4.4 19.9 * 3.4 18.6 +- 5.0 7.3 +- 5.0
-Lit. b (3:: K) 24.1 f 2 b 28.9 f 2 b
.
44
et al., the results.from the two laboratories are in reasonable agreement_ Also it should be noted that the values obtained by Fehsenfeld et al. in an earlier study 133 are in substantially better agreement with our results than is the recent report 143 from that laboratory. Figure 4 shows a plot of -AHn- l.n as a function of the number of molecules present in the cluster NO; - (HNO,),,. The results show that the decrease in the values of enthalpy change is not regular; there is a big decrease between the first two interactions AIYI~,~and AIY1,2, and an appreciable decrease between AH 2,3 and AH3,4 as well as between AI!& and AIJ5.67 while the drop-off between AH l.2 and Lv-I,_, and between AH,_, and AIYGs5is somewhat smaller. The value predicted by Fehsenfeld and co-workers [4] for AH,,, of -26 kcal mol-’ indicates relatively strong bonding in the NO;-HNOB complex. The relative stability of this species may be explained as being due to the formation of strong hydrogen bonds in the proton-bound dimer NO; .._ Hi . . . NO,. Ab initio calculations IlO] (STO-3G) for the hydrogen dinitrate anion,
0
1
2
3
4
5
”
Fig. 4. Plot of NO3 - (HNOsh,.
AH,,--L.n ins a function of n for The insert shows a plot of A..&,,,
reactions vs. n.
NO;
- (IINOs),-1
+ HN03
.L
45
H(NO,);,
suggested two configurations
0,-N
II
I
The lowest energy was obtained for structure II (tetrahedral configuration) when the proton is closer to one or other oxygen atom (along the 01-01# line), but the energy differences between this and other possible configurations are not large. Assuming that the more favourable structure is given by II, in which the equilibrium position of the proton is between four oxygen atoms of the two NO; groups, one might expect that further build-up of the cluster occurs by attachment of the HN03 molecules to the 0 atoms orientated towards the proton. In order to rationalize this assumption the CND0/2 method was used to investigate the charge distribution in the hydrogen dinitrate anion. Calculations were performed with the CNINDO program for the CNDO/B method [ll] on the CDC Cyber 73 Computer System_ The bond lengths and angles in NO; and HN03 were fixed at the experimental values [ 12,131. The results obtained for the two extreme configurations of structure II and for the clusters of this structure with one and two HN03 molecules orientated towards the 011 and Ozl atoms when the proton is closer to O1 and linked by short (2.45 A) hydrogen bonds are summarized in Table 2. As may TABLE Charge
2 distribution
Atom
01
02 01’
02’ 03
03’ Nl Nl,
H Total charge on tetrahedral osygens
in the hydrogen (NOsHNO
dinitrate
anion
)-
(see text)
-__-_
_-
(N03HNOs)-
(N03HN03)- (~iNO3)2 1-I in the minimum
H on top of barrier
H in the minimum
. HNOJ II in the minimum
-0.4732 -0.4732 -0.4732 -0.4732 -0.4379 -0.4379 0.6186 0.6186 0.5314
--0.4338 -0.4425 -0.4956 -0.4956 -0.4219 -0.4578 0.6181 0.6221 0.5119
-0.4349 -0.4434 -0.4993 -0.4673 -0.4058 -0.4269 0.6167 0.6364 0.5085
-0.4313 -0.4443 -0.4765 -0.4765 -0.3917 -0.3984 0.6154 0.6500 0.5047
-1.8928
-1.8725
-1.8449
-1.8286
--_~
46
be seen, despite the proton’s position and the presence of HN03 molecules, the negative charge in H(NO& is distributed preferentially on the four tetrahedral oxygens. This suggests that higher binding energies and more-/stable clusters would be expected when the HN03 molecules go to these atoms. The charge distribution given in Table 2 seems to be compatable with the observed changes of AH,-,.., and AS:--,., with n (Fig. 4). A distinct dropoff between AiYH2,3 and AE13,. correlates well with the relatively high negative charge located on the two oxygen atoms O1n and 021, while the appreciable decrease between Lw,, 5 and AH,,. probably corresponds to addition of the HNO, molecules to the remaining tetrahedral oxygens, thereby closing the solvent shell around the central (N03HN03)ion. The results of semi-empirical calculations for (NOsHNO&- - (HN03),, n = 1 and 2, indicate that attachment of HN03 to (N03HN03)causes only a small decrease in the total negative charge located on the tetrahedral oxygens in (N03HN03)and leads to nearly equal charge dispersion among all the outer oxygens, suggesting a nearly equal probability of binding positions of molecules forming the second solvation shell. This situation is very similar to that observed in the solvation of NIli by NH3 molecules [l4 J where a “spherical” cluster NH: - (NII& is formed, contrary to the branching structure of H30+ - (H,O),z [ 153. ACKNOWLEDGEMENTS
The authors are very grateful to Dr. M. Geller from the Department of Biophysics of Warsaw University for providing the CNINDO prognm used in this work and to Mr. K. Grabowski for technica assistance. REFERENCES 1 J.D. Payzant, R. Yamdagni and P. Kebarle, Can. J. Chcm., 49 (1971) 3308. 2 J.D. Payzant, A.J. Cunningham and P. Kebarlc, Can. J. Chcm., 50 (1972) 2330. 3 F.C. Fehsenfeld, C.J. Howard and AL. SchmcItckopf, J. Chcm. Phys., 63 (1975) 2835. 4 J.A. Davidson, F.C. Fehsenfcid and C.J. Howard, Int. J. Chem. Kin&.. 9 (1977) 17. 5 H. Wince], Int. J. Mass Spcctrom. Ion Phys., 9 (1972) 267. 6 Z. Luczyliski, S. Wioclck and H. Wincci, Radiat. Phys. Chem., 11 (1978) 55. 7 S_ Wlodek, Z. Luczyiiski and II. Winccl, Raport INR-1746/XVII/G/A (1978). 8 Z. Luczynski and H. Wincal, Int. J. Mass Spcctrom. Ion Phys., 14 (1971) 29. 9 R.B. Dean and W.J. Dixon, Anal. Chem., 23 (1951) 636. 10 R. Gunde. T. Solmajer, A. AZman and D. HadZi, J. Mol. Struct., 24 (1975) ClOti. hIolecular Orbital Theory, McGraw11 J._4. Pople and D.L. Beveridgc, Approximate Hill, New York, 1970. 12 F.A. Cotton and G. Wilkinson, Advances in Inorganic Chemistry, 2nd edn., Wiley, New York, 1966, p. 351. 33 A.P. Cox and J.M. Riveros, J. Chom. Phys., 42 (1965) 3106. 14 A.J. Cunningham, J.D. Payzant and P. Kcbarle, J. Am. Chcm. Sot., 94 (1972) 7627. 16 J.D. Payzant, A.J. Cunningham and P. Kebarlc, Can. J. Chem., 51 (1973) 3242.