7488
J. Phys. Chem. 1996, 100, 7488-7493
Gas Phase Reactions of DNO3 with X-‚(D2O)n, X ) O, OD, O2, DO2, and O3 H. Wincel,† E. Mereand, and A. W. Castleman, Jr.* Department of Chemistry, The PennsylVania State UniVersity, UniVersity Park, PennsylVania 16802 ReceiVed: October 19, 1995; In Final Form: February 13, 1996X
The gas-phase reactions of DNO3 with anions X-‚(D2O)ne3, where X ) O, OD, O2, and O3, in helium at several temperatures within the range 185-298 K have been studied using a fast-flow apparatus operated at 0.28 Torr. At room temperature the ions O-‚(D2O)n)1,2, OD-‚(D2O)n)1,2, O2-‚(D2O)n)0,1, DO2-‚(D2O)n)0,1, and O3- react rapidly with DNO3 to form mainly bare product ion NO3-. At low temperatures, in the presence of larger hydrates of the reactant ions, the hydrated species, NO3-‚D2O, as well as the (O2‚DNO3)- and (DO2‚DNO3)- product ions from the O2-‚(D2O)n)1,2/DNO3 and DO2-‚(D2O)n)1,2/DNO3 systems, respectively, were seen to be efficiently produced in primary reactions. These ions react further with DNO3 to form (NO3DNO3)-. Rate constants for the observed reactions were determined. The measured rate coefficient for the association reaction of NO3- with DNO3 shows a temperature dependence and can be represented by the expression kT ) k298(298/T)3.7(0.4. The implications of the present experimental results to atmospheric ion chemistry are briefly discussed.
Introduction Nitric acid has an important role in the atmosphere. It is a temporary reservoir for both OH and NO2 species involved in reactions relevant to the ozone budget in the stratosphere.1 Moreover, HNO3 is a significant component of stratospheric aerosols which play a role in converting the less reactive chlorine species (ClONO2 and HCl) to photochemically labile ones (Cl2 and HOCl), and ultimately provide Cl for catalytic ozone destruction.2 In the Earth’s atmosphere, nitric acid is formed by a number of reactions. Among these are the association reaction of OH with NO2 and the heterogeneous reactions of N2O5 and ClONO2 with water on the aerosol stratospheric particles.2,3 Recent studies4 from our laboratory suggest that it could be also produced by reactions of N2O5 with the protonated water clusters, H+(H2O)ng5. Considering the significance of nitric acid in the atmosphere, it is of considerable interest to examine its reactions with ionic water clusters. In previous studies5 the reactions of DNO3 with the protonated water clusters, D+(D2O)n)0-30, have been reported. The present work extends this study to reactions of DNO3 with negative ion clusters, X-‚(D2O)n, X ) O, OD, O2, DO2, and O3. In situ negative ion composition measurements6 show that nitric acid containing clusters of the type NO3-‚ (HNO3)n)0-3, NO3-‚(HNO3)2‚H2O, HSO4-‚(HNO3)n)0-2, and HSO4-‚(HNO3)2‚H2O are the dominant ions throughout much of the stratosphere and the troposphere. Water is one of the important components of Earth’s atmosphere, and hydrated ions could be due to ion-molecule reactions initiated by galactic cosmic rays. Investigation of hydrated cluster ion reactions at stratospheric temperatures can contribute to a better understanding of atmospheric ion chemistry and nucleation processes and can provide insight into microscopic aspects of heterogeneous processes of aerosols and cloud particles/droplets. Additionally, the rate constants determined in this study over a large range of temperatures can be useful for mass spectrometric measurements of nitric acid in the atmosphere.7 In previous studies using a flowing afterglow apparatus Fehsenfeld et al.8 measured rate constants at room temperature for the reactions of HNO3 with various anions (O-, O2-, Cl-, I-, CO3-, SO3-, SO4-, NO2-, NO3-, NO3-‚H2O, and NO3-‚ † Permanent address: Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, Warsaw, Poland. X Abstract published in AdVance ACS Abstracts, April 1, 1996.
S0022-3654(95)03104-2 CCC: $12.00
HNO3). The rate constant for NO3- clustering to HNO3 were determined in studies by Davidson et al.9a and Viggiano et al.9b More recently Mo¨hler and Arnold7 used a flow tube and triple quadrupole mass spectrometer apparatus to determine the rate constants at 298 K for the reactions of HNO3 with OH-, O2-, O3-, CO3-, CO4-, CO3-‚H2O, NO3-, and NO3-‚H2O, and ion product distributions from the CO3-‚(H2O)n)0-2/HNO3 reactions. Recently, Viggiano et al.,10 using a selected ion flow drift tube instrument, measured the rate constants for NO3-‚ (HNO3)n)0,1/ClONO2 (at 232 K) and NO3-‚ClONO2/HNO3 (at 283 K) and for reactions of several other ions with ClONO2. Experimental Section All experiments were carried out using a temperature variable fast flow-tube (FT) apparatus. The details of this apparatus and experimental procedure have been described in previous publications,4,11 and only a brief outline is presented here. The reactant ions, X-‚(D2O)n, X ) O, OD, O2, and O3, were produced in a flow-tube ion source by discharge ionization of a D2O/He mixture containing a trace amount of O2. After formation, the ions were carried in He from the ion source into the FT, where they react with DNO3 which was diluted in He. The precursor and product ions were mass analyzed by scanning the quadrupole mass spectrometer and detected with a channeltron electron multiplier. The reaction rate constants were measured in the usual way from the slope of the relative decrease in reactant ion intensity with increasing DNO3/He mixture flow rate, using ion velocity values directly determined in pulsing experiments as described earlier.11a The flow rate of the DNO3/ He mixture was controlled with a MKS flowmeter. Typical operating conditions encompassed a total gas pressure of about 0.28 Torr, a buffer gas (He) flow rate of 7 × 103 sccm, and FT temperatures ranging from 185 to 298 K. The FT temperature was controlled with an accuracy of (1 °C. The mixture of DNO3 with He (typically ∼0.4% DNO3/He) was prepared by introducing DNO3 into one 3-L stainless steel reservoir and adding an appropriate amount of dry He. To obtain homogeneity, the mixture was also introduced into a second 3-L stainless steel reservoir. From both reservoirs, the DNO3/He mixture was introduced into the FT through a heated and temperature controlled stainless steel reactant gas inlet (RGI).4 The nitric acid was taken as the vapor from a liquid mixture of 1/3 DNO3 and 2/3 D2SO4 by volume. The D2SO4 © 1996 American Chemical Society
Gas-Phase Reactions of DNO3 with X-‚(D2O)n
J. Phys. Chem., Vol. 100, No. 18, 1996 7489
Figure 1. Mass spectra registered before (a) and after (b) addition of 22 sccm of the 0.44% DNO3/He mixture into the flow tube. The insets show the mass spectra obtained before (a) and after (b) addition of 15 sccm of the 0.47% HNO3/He mixture into the flow tube. P ) 0.28 Torr and T ) 298 K.
was used to dry the DNO3. Some experiments were also done for the X-(H2O)n/HNO3 system. Results and Discussion Reaction Mechanisms. Figure 1 shows the typical mass spectra of negative ions obtained before (a) and after (b) addition of nitric acid (∼0.4% in He) into the FT at room temperature. Under these conditions, the O-‚(D2O)n)1,2, OD-‚(D2O)n)1-3, O2-‚(D2O)n)0-2, DO2-‚(D2O)n)0-2, and O3- ions were observed to react rapidly with DNO3; the major product ion from these reactions is NO3-, which thereafter undergoes an association reaction with an additional DNO3 molecule leading to (NO3DNO3)-. As can be seen in Figure 1b, small amounts of the (O2‚DNO3)-, (DO2‚DNO3)-, and NO3-‚D2O ions are also formed. It is worthwhile pointing out that a NO3- hydrate is not detectable when only O-(H2O)ne2, OH-‚(H2O)ne2,
O2-(H2O)ne1, and HO2-‚(H2O)ne1 are allowed to react with HNO3 (see inset of Figure 1). From all these observations we deduce that at room temperature the principle primary ionmolecule reactions involving these reactant ions and nitric acid neutrals are
O-‚(D2O)n)1,2 + DNO3 f NO3- + OD + nD2O
(1)
OD-‚(D2O)n)1,2 + DNO3 f NO3- + (n + 1)D2O (2) O2-‚(D2O)n)0,1 + DNO3 f NO3- + DO2 + nD2O (3) DO2-‚(D2O)n)0,1 + DNO3 f NO3- + D2O2 + nD2O O3- + DNO3 f NO3- + OD + O2
(4) (5)
The neutral products are not detectable in the experiments;
7490 J. Phys. Chem., Vol. 100, No. 18, 1996
Wincel et al. TABLE 2: Rate Constantsa for the Reactions of DNO3 with O-‚(D2O)n and OD-‚(D2O)n at Several Temperatures O-‚(D2O)n
OD-‚(D2O)n
n
kexp
kcal
kexp
kcalb
T (K)
1
2.68 ( 0.15 2.40 ( 0.17 2.30 ( 0.10 2.40 ( 0.30 2.10 ( 0.20 2.50 ( 0.13 c 2.00 ( 0.10 1.90 ( 0.30 2.30 ( 0.30
2.35-2.60 2.52-2.76 2.06-2.28 2.21-2.43 2.35-2.57 2.48-2.70 1.91-2.12 2.05-2.37 2.18-2.51 2.30-2.63
2.68 ( 0.15 2.56 ( 0.20 2.60 ( 0.30 2.52 ( 0.22 2.25 ( 0.10 2.62 ( 0.39 2.16 ( 0.10 2.23 ( 0.12 2.12 ( 0.20 2.40 ( 0.30
2.31-2.56 2.48-2.72 2.05-2.26 2.19-2.41 2.33-2.55 2.46-2.67 1.90-2.10 2.04-2.24 2.17-2.37 2.29-2.49
298 248 298 248 211 185 298 248 211 185
2
3
b
a Units for all rate constants are10-9 cm3/s. b k cal ) theoretically calculated rate constants using the method developed by Su and Chesnavich.17 The range in kcal reflects the unknown polarizability of DNO3, which was assumed to be in the range 4-8 Å3. c At this temperature, the intensity of O-‚(D2O)3 is very small and the rate constant cannot be measured accurately.
TABLE 3: Rate Constantsa for the Reactions of DNO3 with O2-‚(D2O)n and DO2-‚(D2O)n at Several Temperatures O2-‚(D2O)n kexp 0
Figure 2. Observed variation of ion signals recorded for the addition of the 0.44% DNO3/He mixture into the flow tube at T ) 211 K and P ) 0.28 Torr.
TABLE 1: Enthalpy Changes, ∆H(kcal/mol),a for the Reactions
1
2
O-‚(H2O)n + HNO3 f NO3-‚(H2O)m + OH + (n - m)H2O OH-‚(H2O)n + HNO3 f NO3-‚(H2O)m + (n - m + 1)H2O O2-‚(H2O)n + HNO3 f NO3-‚(H2O)m + HO2 + (n - m)H2O HO2-‚(H2O)n + HNO3 f NO3-‚(H2O)m + H2O2 + (n - m)H2O O3-‚(H2O)n + HNO3 f NO3-‚(H2O)m + OH + O2 + (n - m)H2O
b
kcal
2.62 ( 0.10; 2.8c; 2.4d 2.50 ( 0.12 2.49 ( 0.20 2.52 ( 0.10 2.20 ( 0.20 2.60 ( 0.30 c 2.03 ( 0.14 1.92 ( 0.20 2.40 ( 0.20
DO2-‚(D2O)n kexp
kexp
kcalb
T (K)
2.44-2.70
e
2.39-2.65
298
2.62-2.88 2.11-2.33 2.26-2.48 2.40-2.63 2.53-2.75 1.94-2.14 2.08-2.28 2.21-2.40 2.33-2.53
e 2.50 ( 0.30 2.42 ( 0.20 1.90 ( 0.20 2.60 ( 0.30 2.20 ( 0.10 2.32 ( 0.12 2.03 ( 0.16 2.42 ( 0.20
2.57-2.82 2.09-2.31 2.23-2.46 2.38-2.60 2.50-2.73 1.93-2.13 2.06-2.27 2.19-2.40 2.31-2.52
248 298 248 211 185 298 248 211 185
a Units for all rate constants are 10-9 cm3/s. b Collision rates were calculated from the Su-Chesnavich theory.17 c Reference 8. d Reference 7b. e At this temperature, the intensities of the reactant ion are very small and the rate constant cannot be measured accurately.
SCHEME 1
m reactant ion O-‚(H2O)n OH-‚(H2O)n O2-‚(H2O)n DO2-‚(D2O)n O3-‚(H2O)n
n
0
1
2
1 2 3 1 2 3 0 1 2 0 1 2 0 1
-27 -8 12 -40 -22 -6 -29 -10 7 -51 -29 -11 -17 1
-41 -22 -3 -54 -37 -20
-36 -17 -69 -51 -35
-25 -8
-22
-43 -26
3
4
-31 -65 -49
-61
SCHEME 2 -40
-14
a
All ∆H values were estimated on the basis of the thermodynamic data cited or derived from the values listed in refs 12, 15e, and 18, assuming that D[HO2-(H2O)n-1 - H2O] ) D[OH-‚(H2O)n-1 - H2O] and D[O3-‚H2O - H2O] ) D[(O2-‚H2O) - H2O).
they are inferred from mass balance and available thermochemical information,12 which indicates that reactions 1-5 are exothermic (see Table 1). The neutral products from DO2-/ DNO3 in reaction 4 could be D2O2 and/or its dissociation fragments, D2 + O2; formation of neutral products D2O2 and (D2 + O2) in this reaction are exothermic by about 51 and 19 kcal/mol, respectively. In the case of DO2-‚D2O/DNO3 the formation of (D2 + O2) in reaction 4 is endothermic by about
4 kcal/mol, so that the neutral products from this system are most likely to be D2O2 + D2O. The curves in Figure 2 show that at temperatures of 211 K, the formation of (O2‚DNO3)-, (DO2‚DNO3)-, and NO3-‚D2O proceeds rather efficiently but at flow rates above about 20 sccm
Gas-Phase Reactions of DNO3 with X-‚(D2O)n
J. Phys. Chem., Vol. 100, No. 18, 1996 7491 SCHEME 3
Figure 3. Observed variation of the ratio [O2-‚DNO3]/([O2-‚D2O)]0 + [O2-‚(D2O)2]0) with flow rate of the 0.44% DNO3/He mixture at indicated flow tube temperatures. P ) 0.28 Torr.
Figure 4. Observed variation of the ratio [DO2-‚DNO3]/([DO2-‚D2O]0 + [DO2-‚(D2O)2]0) with flow rate of the 0.44% DNO3/He mixture at indicated flow tube temperatures. P ) 0.28 Torr.
these ions undergo further rapid reactions with DNO3. An analysis of product and reactant ion intensities suggests that at low temperatures (0.5e 1.7 ( 0.1 2.1 ( 0.1
2.14-2.38 2.31-2.54 1.82-2.01 1.95-2.15 2.08-2.27 2.19-2.38 1.81-2.00 1.94-2.01 2.07-2.26 2.18-2.37 2.01-2.17 2.16-2.31 2.29-2.45 2.41-2.57 1.88 -2.08 2.01-2.22 2.15-2.35
298 248 298 248 211 185 298 248 211 185 298 248 211 185 298 248 211
reactant ion
NO3-‚D2O
10
a Units for all rate constants are 10-9 cm3/s. b Collision rates were calculated from the Su-Chesnavich theory.17 c Reference 7b. d Reference 9b. e Reference 8.
are significantly exothermic, and elimination of one or more D2O molecules either from an intermediate complex, [X-‚ (D2O)n‚DNO3]*, and/or from hydrated product ions, NO3-‚(D2O)n, is energetically possible. This situation is similar to that found for the X-‚(H2O)n/N2O5, X ) O, OH, O2, HO2, and O3 reactions,14 as well as for many other systems.15 The species NO3- and NO3-‚D2O are seen (Figure 2) to react further with DNO3 according to known8,16 reactions 9 and 10. He
NO3- + DNO3 T [(NO3DNO3)-]* 98 (NO3DNO3)- (9) NO3-‚D2O + DNO3 f (NO3DNO3)- + D2O
(10)
Reaction 9 is termolecular at the FT conditions with He acting as the stabilizing gas.9 At low temperatures, the resulting (NO3DNO3)- from the reactions discussed above, was observed (Figure 2) to form adducts, (NO3DNO3)-‚DNO3 and (NO3DNO3)-‚(DNO3)2 (not shown), as indicated in reactions 11 and 12.
(NO3DNO3)- + DNO3 f (NO3DNO3)-‚DNO3 (11) (NO3DNO3)-‚DNO3 + DNO3 f (NO3DNO3)-·(DNO3)2 (12) Rate Constants. The rate constants determined at several temperatures are summarized in Tables 2-4. These values are averages of several runs each, and error limits show the statistical fluctuations. At temperatures within the range 248298 K the absolute error was estimated to be (25%, while at lower temperatures it might be as high as 40% due to larger uncertainty in the nitric acid concentration in the flow tube. The rate constant values for the association reaction 9 over the temperature range 211-298 K show the kT ) k298(298/T)3.7(0.4 temperature dependence (Figure 5). However, at lower temperatures the measured rate constants for this reaction were lower than the values derived from this temperature dependence. At T e 185 K, the rate constants determined for other reactions were lower than the calculated collision rates.17 Moreover, product ions were observed even when no reactant gas was added. These observations indicate that at low temperatures
Figure 5. Plot of k9 as a function of temperature: (b) this work; (4) ref 9a; (3) ref 9b; (0) ref 7b.
(T e 185 K), DNO3 condenses in the FT. Hence, the results given in Tables 2-4 do not include the rate constant values for T e 185 K. The lower limits of rate constants (Table 4) for reactions 6, 7, 9, and 10 were derived from secondary reactions in the FT. The calculated collision rate constants from the SuChesnavich parametrized trajectory theory,17 as well as the experimental values reported, are also given in Tables 2-4. The data listed in these tables indicate that with the exception of NO3-, all ions react with DNO3 at or close to the collision rates. This suggests that there is little or no activation energy for these reactions. The (NO3DNO3)- adduct ion from reaction 9 is likely to be formed predominantly by collisional stabilization in the helium-bath gas. At room temperature, a lifetime of the excited intermediate complex, [(NO3DNO3)-]*, is probably quite short with respect to dissociation back to reactants. At low temperatures, the lifetime of this complex is longer, and it can be efficiently stabilized by He in the third-body collisions. We found that at a fixed helium pressure of 0.28 Torr, the rate constant for reaction 9 varies with temperature as k9 ∝ T-3.7(0.4. It can be seen in Table 4 that at stratospheric temperatures (around 200 K), the NO3- ion reacts with DNO3 at near bimolecular collision rate, and one may expect that at stratospheric pressures this reaction will be collisional. The rate constant measured in the present study for reaction 9 at room temperature is lower than that obtained7b by Mohler and Arnold using N2 as a carrier gas. This difference could be due to a higher deactivation efficiency of N2 in comparison to He, which was used in our study. However, the reason for the discrepancy between the lower rate constants for room temperature reported in another study9b compared to the present one, is unclear. As can be seen in Figure 5, the values reported9a for elevated temperatures are close to the extrapolated line of our data. Implications for Atmospheric Chemistry. The results presented here provide data on chemical reactions of the hydrated clusters X-‚(D2O)ne3, X ) O, OD, O2, DO2, and O3, with nitric acid at temperatures 185-298 K. The observed ionic products from these reactions such as NO3-, NO3-‚H2O, and (NO3HNO3)-(HNO3)n)0-2, are also found6 in the stratosphere and troposphere. The present study reveals that different mechanisms are involved in the pathways to the product
Gas-Phase Reactions of DNO3 with X-‚(D2O)n (NO3HNO3)-. These observations should be of value in obtaining a better understanding of atmospheric ion chemistry and the formation of gaseous ions. They may also provide information relevant to the mechanisms of nitric acid reactions with solvated ions.19 Additionally, our work may provide insight into the mechanism of nitric acid incorporation into polar stratospheric cloud particles (PSCs). This latter process is important due to its role in heterogeneous reactions which form HNO3(s) as a product and more recently,20 it is believed to be included in the actual formation of PSCs. Our results also indicate that at stratospheric temperatures (∼200 K) the effective bimolecular rate constants measured for the NO3-/HNO3 association reaction approach the calculated values for the bimolecular collision rates (see Table 4). This is in contrast to the previous finding of Viggiano et al.,9b which shows that the effective second-order rate constant for this reaction saturates at a value of 2.6 × 10-10 cm3/s. This result may be important for deriving HNO3 concentrations in the atmosphere from measurements of ion compositions. The rate constants determined in the present study (Tables 2-4) are expected to be also useful for stratospheric modeling. No information has been previously available on the rate constants at stratospheric temperatures for the systems studied here. Conclusions Thermal reactions of X-(D2O)ne3, X ) O, OD, O2, DO2, and O3, with nitric acid, DNO3, in helium have been investigated by using a flow-tube apparatus. At room temperature, the reactions of the ions O-‚(D2O)n)1,2, OD-‚(D2O)n)1,2, O2-‚(D2O)n)0,1, DO2-‚(D2O)n)0,1, and O3- lead predominantly to the bare product ion NO3-. At low temperatures, in the presence of larger hydrates of these reactant ions, the efficient formation of the hydrated species, NO3-‚D2O from primary reactions is observed. Moreover, at the lowest temperatures (T ) 211 K) the (O2‚DNO3)- and (DO2‚DNO3)- ions are found to be the principal products from the O2-‚(D2O)n)1,2/DNO3 and DO2-‚ (D2O)n)1,2/DNO3 systems; the primary product ions, NO3-‚D2O, (O2‚DNO3)- and (DO2‚DNO3)-, react rapidly with DNO3 to produce in sequential reactions (NO3DNO3)- and (NO3DNO3)-‚(DNO3)n)1,2. Almost all reactions observed in the present work occur at or close to the collision rates. The measured rate coefficient for association reaction of NO3- with DNO3 shows a temperature dependence, kT ) k298 (298/T)3.7(0.4. Acknowledgment. Financial support by the Atmospheric Sciences Division of the National Science Foundation, Grant No. ATM-9321660, is gratefully acknowledged. We are also grateful to the National Science Foundation for a grant through the International Program Office, Grant No. INT-9208271. H.W. also gratefully acknowledges the Polish Committee for Scientific Research, Grant No. 208299101.
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