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Cluster reactions of surlfur trioxide and ammonia
1 April 1994
CHEMICAL PHYSICS ChemicalPhysics Letters 220 (1994) 274-280
ELSEVIER
Cluster reactions of sulfur trioxide and ammonia Z. Shi, J.V. Ford, A.W. Castleman Jr. Department of Chemistry, The Pennsylvania State University, University Park, PA 16802, USA
Received 1 November 1993;in final form 25 January 1994
Abstract
Intra-cluster reactions in the sulfur trioxide-ammonia system are investigated using a time-of-flight reflectron mass spectrometer. Mass spectra and results of metastable dissociation studies show that the products n = 2m and n = 2m + 1 of ( SOs ) m(NHs ),,H+ are especially stable, in accordance with expected structures arising from the chemical bonding between ammonia and sulfur trioxide. Based on the experimental results, possible structures for various mixed clusters are proposed and reaction mechanisms are suggested.
1. Introduction
Both sulfur trioxide and ammonia are widely used industrial materials, with interactions that play a potentially important role in atmospheric chemistry as well. Studies of sulfur-nitrogen-oxygen compounds can be dated back to the middle of the last century. During the 195Os, numerous experiments on reactions of sulfur trioxide and ammonia were carried out with the main objective of improving the industrial production of ammonium compounds. During the course of these investigations some details of the reactions between these substances were revealed [l5 ]. The products and yields of reactions between sulfur trioxide and ammonia reactions were found to depend on the reaction conditions such as temperature, mixing ratio, total pressure, and buffer gas. It is known that a variety of products such as IW(SOJ’JHa)z, NKWWb, W-U2S04, WS03NW03h
NI-bN(SOJ’J&)z, and WSO&W may
be obtained in gas-phase reactions under different reaction conditions. Liquid-phase reactions have been studied as well, and the role of the intermediate prod-
uct of H ( S03NHz) in different reactions has been discussed [ 3 1. During recent years, the molecular mechanism of fine particle formation evolved from chemically reactive gas-phase species has garnered considerable interest [ 6- 16 1. For example, within the realm of atmospheric science heterogeneous reactions involving aerosol particles are believed to play an important role in the stratospheric ozone hole and the production of reactive species in the maritime troposphere [ 17-22 1. Furthermore, studies of the dynamics of formation, energetics and structures of clusters help elucidate condensation and nucleation phenomena and the basic mechanisms of aerosol formation at the molecular level. Studies of clusters containing electrolytes are also of particular interest. These studies may be used to help determine a molecular description of solvating and ion-pair formation. The existence of sulfuric acid, and perhaps sulfur trioxide containing clusters and particles, is well known; however, there is a paucity of information on neutralization reactions such as those that might occur during the interaction of sulfur trioxide with ammonia clusters.
0009-2614/94/$07.04l 0 1994 Elsevier Science B.V. All rights reserved XSD10009-2614(94)00151-F
275
Z. Shi et al. / ChemicalPhysicsLetters220 (I 994) 2 74-280
In this Letter, the products of intra-cluster reactions, which include rearrangements and evaporations of S09/NH3 mixed clusters formed by a pickup source, are studied by a laser-based reflectron ‘IOF mass spectrometer. The magic peaks observed in mass spectra and also those arising from metastable dissociation provide information on the cluster reactions which serves to bridge studies of gas-phase and liquid-phase reactions.
2. ExperlmentaI The recently modified laser-based reflectron timeof-flight mass spectrometer used in our studies is shown in Fii. 1. A pulsed nozzle ( 150 pm) which generates a neutral ammonia cluster beam merges with four continuous sulfur trioxide beams to form the so-called pickup reaction source [ 23,241. The angle between the NH3 beam and the SO3 beams is 20”. Reactions of ammonia clusters and sulfur trioxide take place in the cross region, which is 2 cm below the pulse nozzle, and the neutral products travel down through a skimmer towards the laser ionization region which is 7 cm below the skimmer and is pumped by a turbo molecular pump. The third harmonic of a Nd: YAG laser ( 355 nm, 30 mJ/pulse, focused by a 50 cm lens) is used to ionize the products of the reactions and the ions are collected by a two-stage elec-
tric accelerating field and focused by. an Einzel lens arrangement. After they traverse a field-free distance of 160 cm, the ions are reflected by a reflectron towards a chevron microchannel plate located an additional 110 cm away. The signals are recorded and averaged by a 100 MHz transient recorder and analyzed with a personal computer. The dry ammonia gas is seeded in helium carrier gas and the mixing ratio is optimized according to the experiment. The pressure of pure dry sulfur trioxide gas which is sent into the pickup source is well controlled by a needle valve. Although the pressure of SO9 in the reaction region remains unknown, it can be qualitatively monitored by measuring the average pressure of the chamber. The reflectron, as described in a previous paper [ 25 1, is used for two purposes: obtaining high mass resolution and studying metastable dissociation. The 50-clement reflectron provides a homogenous twostage electric field and a resolution of 5000 (M/ m) soJbcan be readily achieved. The pressures of the pickup reaction chamber and ionization chamber are 5 x 10a5 and 2 x 10m6 Torr, respectively, and those for the drift region, detector chamber and reflectron chamber remain at 5 x 1O-’ Torr even during operation.
PICKUP SOURCE see detail
PULSE NOZZLE
on the left
\ I I CW REACTANT BEAMS\\\;,;
=+I!% I;/
MCP DETECTOR EINZEL LENS
TOF LENS
LASER BEAM Fig. 1. Schematic diagram of the experimental
apparatus.
276
Z. Shi et al. /Chemical Physics Letters 220 (1994) 274-280
3. Results and discussions In our experimental arrangement, (NH,), clusters are first formed and then multiple SO3 units are picked up in the reaction region. Subsequent intracluster reactions are responsible for the rearrangements and evaporations. During the process of multiphoton ionization, further evaporations, rearrangements and related ion-molecule reactions take place which gives rise to the detection of appreciable intensities of those ions which have particularly stable structures. Fig. 2 shows part of the mass spectrum of the reaction products. Besides pure protonated ammonia clusters (NI$)“H+, there are several mixed cluster series that display particularly intense mass spectral distributions. For example, the cluster sequence representedby(S03)(NHS),H+startsatn=2andthere is an intensity decrease between n=3 and n=4; (S03)z(NH3),H+ starts at n=4 and there is an intensity decrease between n= 5 and n= 6. Furthermore,the (S03)3(NHS),H+seriesbeginsatn=6a.nd the intensity drop takes place between n= 7 and 8. Thus it can be seen that (SO,),(NH,),H+ mixed clusters start at n=2m (m= 1, 2, ...) and there are regular systematic intensity drops at n = 2m + 1. Upon increasing the amount of SO3 reactant, the intensity breaks become more obvious and the relative intensities of the (S09),(NHs)nH+ mixed clusters
(n = 2m, 2m + 1) increase compared to the intensities of the (NH, ),,H+ clusters. Finally, the mixed clusters (SOS,), ( NH3 ).H+ become a double-peak series, with n=2mand n=2m+ 1 (Fig. 3). Table 1 shows the metastable dissociation channels identified in the present study. By analyzing the kinetic energies of the dissociated clusters (i.e. the daughter ions), the masses of the neutral fragments can be deduced. Thereby, the metastable dissociation mechanisms can be determined [ 71. Due to the processes of evaporative dissociation, the pure ammonia clusters (NH,),H+ always display an (NH,) loss channel [26,27]. The metastable studies conducted as part of the present investigation that the dissociation channels of reveal (SO,),(NH,),H+ depend on cluster sizes and structures as follows: (1) (SO,)(NH,),H+: the daughter ion corresponding to the ( 1, 2) species is barely observed, while there is an NH3 loss channel open to the ( 1,3 ) cluster; see Fig. 4. For n 2 4, the only dissociation observed is NH0 loss. (2) (S03)2(NHs)nH+: the daughter ion signal for the (2, 4) cluster ion is much weaker than that for ( 2,5 ) which has a major NH3 loss channel and a minor loss channel of 114 amu assigned as (SO,)(NH,),. The (2, 4) species has only one dissociation channel corresponding to a 114 amu loss. Due to the weak signals, for those species with n 3 6
-
34 41 Time of Flight (microseconds) Fig. 2. Time of flight mass spectrum of products of (S0,),(NH3)n~H+.
217
Z. Shi et al. /Chemical Physics Letters 220 (1994) 274-280
DAUGHTER
1x10-~ Ton
5X1 C-3 Torr
3s Time of Flight
IONS
45 (microseconds)
Fig. 4. The daughter ions mass spectrum of reaction products contributed mainly by the ammonia loss channel.
0
0
II
F t
T =
IXlW
S’
Torr
$ ,O0NH4
OH
‘OH
8
O@'NH2
(4
>
52.0
43.5 35.0 26.5 Time of Flight (microseconds)
18.0
(b
0 ll,0MNH4
Fig. 3. The changes of intensities with different SO, densities: the pressures are the average relative pressures of the source chamber. The absolute values in the pickup area remain unknown.
&NH
2
Table 1 Metastable dissociations Mixed clusters
NH3
( c >
+
WWmW43)nH
n=2m+ 1
n=2m
n>2m+l
not observed
-(NH,)
-w&J
m=2
-[w3)(~3Ma
4~3)
-wf3Jb
(114amu)
-[W3)(~3M’
b
-(NH31b
-wJ53)b
l Weak signal. b Too weak to observe other dissociation channels.
are only able to observe one major NH3 loss channel; see Fig. 4. (3) (SO,),(NH,),H+: the metastable dissociation patterns for these species are similar to those of
we
+ @03)2(~3)nH
.
Cd)
Fig. 5. Structure of (a) H2S04, and proposed structures of (b) (so3)(~3)2,
m=l
m=3
H+
(c)
W3)(~3)3H+
aad
(4
W3)-
(MI3)3H+WH3).
The metastable dissociation results serve to reveal the stabilities and structures of the clusters. For those mixed clusters with n, 2m+ 1, there is a weakly bonded NH3 which forms the major daughter peaks. The n= 2m mixed clusters have relatively stable structures which are evidently chemically bonded rather than being weakly bonded by van der Waals forces. The secondary dissociation channel corresponding to a loss of 114 amu from (S03)z(NH3)sH+ further reveals that (S03)(NH3)2 is a strongly bonded unit. Fig. 5a shows the well-known
278
Z. Shi et al. /Chemical Physics Letters 220 (I 994) 274-280
structure of sulfuric acid ( H2S04). A stable structure for (SO3) (NH,), can be proposed if -OH and -H in H2S04 are replaced by -NH2 and -NH4, respectively; Fig. 5b. A proton, H+, can attach onto it and form (SO3) (NH3)3H+, whereupon (NH,), as well as [ (SO3) (NH,),], can further add onto it. The absence of n<2m (m= 1, 2, ...) for (SO,),(EJH3)nH+, suchas (1, O), (1, l), (2, O), (2, l), (2, 2), (2, 3), etc., reveals that after the reaction-ionization process, only ‘saturated’ mixed clusters can survive. Here, every sulfur trioxide molecule needs two ammonia molecules to become ‘saturated whereby it forms the stable chemical bonded neutral species W3)(~3)2 or N-W (SW WH2). l-he products of protonated ammonia clusters reacting with SO3 grow through the addition of this basic unit. Fig. 5~ shows the structure of (SO,) (NH3)3H+. By adding (SO3) (NH3)2 onto it, the stable series n= 2m is formed. According to the structures of n = 2m, the mixed clusters have less chance to lose NH3 and there could be a ( S03) (NH,), loss channel open for m 3 2. These considerations account for the experimental results given in Table 1. Fig. 2 shows that the series n = 2m + 1 is more stable than those corresponding to n> 2m+2 mixed clusters; in other words, the structure of (S03),(NH3)3,H+(NH3) could be different from those mixed clusters which are richer in ammonia. Noticing that there is a charge center of -NH2H+ in (SO3)-(NH3)2H+, Fig. 5c, we suggest that additional NH3 becomes hydrogen bonded to the charge center NH2H+ as shown in Fig. 5d. By considering the addition of more (S03)(NH3)3 units onto ( S03) (NH3)2H+, the stable series corresponding to n = 2m + 1 can be readily explained. Understandably, the NH3 in ( S03) (NH3)3H+ can become lost through metastable dissociation, and secondary (SO3)(NHS)Zlossfrom (SOS)?#IH3)2,H+(NH3) mixed clusters ( m > 2 ) can be observed. Because there is only one site available for NH3 molecules to bind onto (SO,) (NH3)2H+, additional ammonia molecules can only loosely bind onto (SOS) (NH,) 3H+. Therefore, those mixed clusters are relatively more unstable than the n=2m and n= 2m + 1 series. When the number of additional ammonia molecule increases, the influence of (SO3),(NH,),,H’(NI-I,) is reduced and the observed metastable dissociation patterns are similar to
those found for pure ammonia clusters. The ammonia loss channel is observed for n 2 2m + 2 peaks and the (S03) (NH,)? loss channel is not seen due to the weak signal, see Table 1. Our studies indicate that, in the mixed clusters, H+ is attached onto a (S03)(NH3)2 unit and not the ammonia clusters. During the reaction and especially during ionization, some loosely bonded species are evaporated and the structures of mixed clusters become rearranged. The possible processes are as follows, where p, q and r refer to the clusters before ionization: ( 1) The neutral products are abundant in ammonia molecules, (SO,),(NH,),(NH,), Ih,’ (SO,),(NH,),H+(NH,),-r-1 +r(NH3)+NH2
+e- .
The number of ammonia molecules lost is governed by the available excessive energies in the cluster ions. After the fast dissociations which may take place during reaction and ionization, some ions with a small amount of excess energies undergo metastable dissociations as summarized in Table 1. Some (SO,) (NH,), molecules can also be lost during either fast or metastable dissociations, if the number of (SO,) (NH,), units is more than 2. (2) The number of ammonia molecules is less than needed to “saturate” the sulfur trioxide molecules. In this case, (SO,),(NH,),, where the number of ammonia molecules q is less than 2p, there are no extra ammonia molecules available to be ionized. The ionization potential of SO3 molecules (IP = 12.7 eV) exceeds the energy range available in our experiments. Therefore, those peaks are missing in the mass speo tra. The same reason for the absence of (S03)m(NH,)& series is that the IP of (SO,) (NH3)2 is too high to be ionized with the non-resonant technique employed in the present work. Thus, the signal intensities for the n=2m and n = 2m + 1 series with more stable structures should be stronger than those for n 3 2m + 2. When the density of the SO3 reactant increases, more ammonia molecules are needed to “saturate” them, i.e. the chance for extra ammonia molecules to survive is reduced. Therefore, the relative intensities for it = 2m
Z. Shi et al. /Chemical Physics Letters 220 (1994) 274-280
and n = 2m + 1 increase when more SO, is added into the reaction region. There are several possible channels for reactions of sulfur trioxide and ammonia under different conditions [28], S01+NI-13-+H(S03NH2), 2SO3 +NHS-rHz(HNS206)
(1) ,
(2)
=03 +4M-I3-+N’J%+[N(S03NH4)21,
(3)
SO~+2NH3-rNH4(SO~NHz),
(4)
Reaction ( 1) is a low-density gas-phase reaction where there is less chance for further reaction to take place; the others involve subsequent reactions. Under S03-rich conditions, the products of reaction ( 1) are more likely to react with additional SO3 molecules. In our experiments, the mass spectra show that such products as ( 1, 1) and (2,1) do not exist due to excess NH3 in the mixed clusters, and the small amounts of SOS in the pickup source. Gas-phase reaction ( 3 ) also gives a product of ( 1,2) stoichiometry as formed in the case of cluster reactions. However, the structures of iminodisulfonate cannot be used to explain our results of the metastable dissociation studies. In liquid ammonia where reaction (4) occurs, the initial adduct S03*NH3 is too strong an acid for the NH3 solvent and is neutralized by another ammonia molecule to form an ammonium amidosulfate salt molecule. In ammonia-rich neutral ( SOs),(NH3), mixed clusters, the aggregation of molecules and evaporative cooling lead to intracluster reactions similar to those which take place in liquid phase reactions. Because there are no n < 2m products observed, we suggest that SO3 reacts with an ammonia dimer instead of two ammonia molecules sequentially. Furthermore, the products of van der Waals bonded ammonium amidosulfate molecules can be observed in cluster reactions.
4. Conclusions Reactions which take place within mixed clusteq, e.g., reaction (5) for single S03, are similar to those which occur in the liquid phase, reaction (6), as follows:
279
(5) (6)
The relatively strong intensities for (SO,),(NH,),H+, n=2m and n=2m+ 1 reveal the formation of stable structures. The results of the present metastable dissociation studies show that chemically bonded species corresponding to ( S03),(NH3)z,H’ are the most stable among the mixed clusters. The charge center increases the strength of the hydrogen bond, whtch makes (S03)m(NH3)2m+l H+ more stable than the n > 2m + 2 series.
Acknowledgement Financial support by the US Environmental Protection Agency, Grant No. R-817437, and the National Science Foundation, Grant No. ATM-90 15855, is gratefully acknowledged.
References [l]R.AppelandW.Huber,Z.Anorg.Allg.Chem.275 (1954) 338. [ 21 H.-A. LeD. Eteyerand W. Schneider, Z. Anorg. Allg Chem. 337 (1965) 22. [3] R. Appel and W. Huber, Chem. Fter. 89 (1956) 386. [4] Y. Ito, Tokyo Kogyo Shikensho Hokoku 52 (1957) 281. [ 51 S. Uchida and Y. Ito, Rept. Govt. Chem. Ind. Res. Inst. 47 (1952) 245. [ 61 P.H. McMm-ry and SK. Friedlander, Atmos. Environ. 13 (1979) 1636. [I] EC. Wen, T. McLaughlin and J.L. Katz, Phys. Rev. A 26 (1982) 2235. [8] L.Y. ChanandV.A. Mohnen, J. AerosolSci. 11 (1980) 35. [9]D.C.MartinandH.Reiss, J.Chem.Phys.69 (1978) 1897. [lo] R. Sievert and A.W. Castleman Jr., J. Phys. Chem. 88 (1984) 3329. [ 111 D.R. Schryer, ed., Geophysical Monograph Series, Vol. 26, Heterogeneous atmospheric chemistry (American Geophysical Union, Washington, 1982). [ 121 A.W. CastIeman, Jr., in: Geophysical Monograph Series, Vol. 26, Hetemgeneous atmospheric chemistry, ed D.R Schryer (American Geophysical Union, Washington, 1982) pp. 1327. [ 131 A.W. Castleman Jr. and R.G. Keesee, J. Aerosol Sci. Technol. 2 (1983) 145.
280
Z. Shi et al. / Chemical Physics Letters 220 (I 994) 274-280
[ 141 M.A. Tolbert, M.J. Rossi, R. Malhotra and D.M. Golden, Science 238 (1987) 1258; M.J. Molina, T.-L. Tso, L.T. Molina and EC.-Y. Wang, Science 238 (1987) 1253. [ 15 ] B.J. Finlayson-Pitts and S.N. Johnson, Atmos. Environ. 22 (1988) 1107. [ 161 J.H. Seinfeld, Atmospheric chemistry and physics of air pollution (Wiley, New York, 1988) pp. 373-376. [17]D.R. Worsnop, L.E. Fox, MS. Zahniser and S.C. Wofsy, Science 259 (1993) 71. [ 181 S. Elliott, R.P. Turco, O.B. Toon and P. Hamill, J. Atmos. Chem. 13 (1991) 211. [19]P.J.CrutzenandF.Amold,Nature324(1986)651. [20] D.R. Hanson and A.R Ravinshankara, J. Phys. Chem. 96 (1992) 2682. [21]R.G. Utter, J.B. Burkholder, C.J. Howard and A.R. Ravinshankara, J. Phys. Chem. 96 (1992) 4973.
[22] D.R Worsnop, M.S. Z&miser, C.E. Kolb, J.A. Gardner,L.R Watson, J.M. Van Doren, J.T. Jayne and P. Davidovits, J. Phys. Chem. 93 ( 1989) 1159. [23] J.J.Breen,K.Kilgore,S. Wei, W.B.Tzeq,R.G.Keeseeand A.W. Castleman Jr., J. Phys. Chem. 93 (1989) 7703. [ 241 J.J. Breen, K. Kilgore, W.B. Tzeng, S. Wei, R.G. Keesee and A.W. Castleman Jr., J. Chem. Phys. 90 ( 1990) 11. [25] Z. Shi, S. Wei, J.V. Ford and A.W. Castleman Jr., Chem. Phys. Letters 200 ( 1992) 142. [26 ] S. Wei, K. Kilgore, W.B. Tzeng and A.W. Castleman Jr., J. Phys. Chem. 95 ( 199 1) 8306. (271 S. Wei, W.B. T2engandA.W. Castleman Jr., J. Am. Chem. Sot. 113 (1991) 1960. [28]N.N. Greenwood and A. Eamshaw, Chemistry of the elements (Pergamon Press, Oxford, 1984).
CHEMICAL PHYSICS ChemicalPhysics Letters 220 (1994) 274-280
ELSEVIER
Cluster reactions of sulfur trioxide and ammonia Z. Shi, J.V. Ford, A.W. Castleman Jr. Department of Chemistry, The Pennsylvania State University, University Park, PA 16802, USA
Received 1 November 1993;in final form 25 January 1994
Abstract
Intra-cluster reactions in the sulfur trioxide-ammonia system are investigated using a time-of-flight reflectron mass spectrometer. Mass spectra and results of metastable dissociation studies show that the products n = 2m and n = 2m + 1 of ( SOs ) m(NHs ),,H+ are especially stable, in accordance with expected structures arising from the chemical bonding between ammonia and sulfur trioxide. Based on the experimental results, possible structures for various mixed clusters are proposed and reaction mechanisms are suggested.
1. Introduction
Both sulfur trioxide and ammonia are widely used industrial materials, with interactions that play a potentially important role in atmospheric chemistry as well. Studies of sulfur-nitrogen-oxygen compounds can be dated back to the middle of the last century. During the 195Os, numerous experiments on reactions of sulfur trioxide and ammonia were carried out with the main objective of improving the industrial production of ammonium compounds. During the course of these investigations some details of the reactions between these substances were revealed [l5 ]. The products and yields of reactions between sulfur trioxide and ammonia reactions were found to depend on the reaction conditions such as temperature, mixing ratio, total pressure, and buffer gas. It is known that a variety of products such as IW(SOJ’JHa)z, NKWWb, W-U2S04, WS03NW03h
NI-bN(SOJ’J&)z, and WSO&W may
be obtained in gas-phase reactions under different reaction conditions. Liquid-phase reactions have been studied as well, and the role of the intermediate prod-
uct of H ( S03NHz) in different reactions has been discussed [ 3 1. During recent years, the molecular mechanism of fine particle formation evolved from chemically reactive gas-phase species has garnered considerable interest [ 6- 16 1. For example, within the realm of atmospheric science heterogeneous reactions involving aerosol particles are believed to play an important role in the stratospheric ozone hole and the production of reactive species in the maritime troposphere [ 17-22 1. Furthermore, studies of the dynamics of formation, energetics and structures of clusters help elucidate condensation and nucleation phenomena and the basic mechanisms of aerosol formation at the molecular level. Studies of clusters containing electrolytes are also of particular interest. These studies may be used to help determine a molecular description of solvating and ion-pair formation. The existence of sulfuric acid, and perhaps sulfur trioxide containing clusters and particles, is well known; however, there is a paucity of information on neutralization reactions such as those that might occur during the interaction of sulfur trioxide with ammonia clusters.
0009-2614/94/$07.04l 0 1994 Elsevier Science B.V. All rights reserved XSD10009-2614(94)00151-F
275
Z. Shi et al. / ChemicalPhysicsLetters220 (I 994) 2 74-280
In this Letter, the products of intra-cluster reactions, which include rearrangements and evaporations of S09/NH3 mixed clusters formed by a pickup source, are studied by a laser-based reflectron ‘IOF mass spectrometer. The magic peaks observed in mass spectra and also those arising from metastable dissociation provide information on the cluster reactions which serves to bridge studies of gas-phase and liquid-phase reactions.
2. ExperlmentaI The recently modified laser-based reflectron timeof-flight mass spectrometer used in our studies is shown in Fii. 1. A pulsed nozzle ( 150 pm) which generates a neutral ammonia cluster beam merges with four continuous sulfur trioxide beams to form the so-called pickup reaction source [ 23,241. The angle between the NH3 beam and the SO3 beams is 20”. Reactions of ammonia clusters and sulfur trioxide take place in the cross region, which is 2 cm below the pulse nozzle, and the neutral products travel down through a skimmer towards the laser ionization region which is 7 cm below the skimmer and is pumped by a turbo molecular pump. The third harmonic of a Nd: YAG laser ( 355 nm, 30 mJ/pulse, focused by a 50 cm lens) is used to ionize the products of the reactions and the ions are collected by a two-stage elec-
tric accelerating field and focused by. an Einzel lens arrangement. After they traverse a field-free distance of 160 cm, the ions are reflected by a reflectron towards a chevron microchannel plate located an additional 110 cm away. The signals are recorded and averaged by a 100 MHz transient recorder and analyzed with a personal computer. The dry ammonia gas is seeded in helium carrier gas and the mixing ratio is optimized according to the experiment. The pressure of pure dry sulfur trioxide gas which is sent into the pickup source is well controlled by a needle valve. Although the pressure of SO9 in the reaction region remains unknown, it can be qualitatively monitored by measuring the average pressure of the chamber. The reflectron, as described in a previous paper [ 25 1, is used for two purposes: obtaining high mass resolution and studying metastable dissociation. The 50-clement reflectron provides a homogenous twostage electric field and a resolution of 5000 (M/ m) soJbcan be readily achieved. The pressures of the pickup reaction chamber and ionization chamber are 5 x 10a5 and 2 x 10m6 Torr, respectively, and those for the drift region, detector chamber and reflectron chamber remain at 5 x 1O-’ Torr even during operation.
PICKUP SOURCE see detail
PULSE NOZZLE
on the left
\ I I CW REACTANT BEAMS\\\;,;
=+I!% I;/
MCP DETECTOR EINZEL LENS
TOF LENS
LASER BEAM Fig. 1. Schematic diagram of the experimental
apparatus.
276
Z. Shi et al. /Chemical Physics Letters 220 (1994) 274-280
3. Results and discussions In our experimental arrangement, (NH,), clusters are first formed and then multiple SO3 units are picked up in the reaction region. Subsequent intracluster reactions are responsible for the rearrangements and evaporations. During the process of multiphoton ionization, further evaporations, rearrangements and related ion-molecule reactions take place which gives rise to the detection of appreciable intensities of those ions which have particularly stable structures. Fig. 2 shows part of the mass spectrum of the reaction products. Besides pure protonated ammonia clusters (NI$)“H+, there are several mixed cluster series that display particularly intense mass spectral distributions. For example, the cluster sequence representedby(S03)(NHS),H+startsatn=2andthere is an intensity decrease between n=3 and n=4; (S03)z(NH3),H+ starts at n=4 and there is an intensity decrease between n= 5 and n= 6. Furthermore,the (S03)3(NHS),H+seriesbeginsatn=6a.nd the intensity drop takes place between n= 7 and 8. Thus it can be seen that (SO,),(NH,),H+ mixed clusters start at n=2m (m= 1, 2, ...) and there are regular systematic intensity drops at n = 2m + 1. Upon increasing the amount of SO3 reactant, the intensity breaks become more obvious and the relative intensities of the (S09),(NHs)nH+ mixed clusters
(n = 2m, 2m + 1) increase compared to the intensities of the (NH, ),,H+ clusters. Finally, the mixed clusters (SOS,), ( NH3 ).H+ become a double-peak series, with n=2mand n=2m+ 1 (Fig. 3). Table 1 shows the metastable dissociation channels identified in the present study. By analyzing the kinetic energies of the dissociated clusters (i.e. the daughter ions), the masses of the neutral fragments can be deduced. Thereby, the metastable dissociation mechanisms can be determined [ 71. Due to the processes of evaporative dissociation, the pure ammonia clusters (NH,),H+ always display an (NH,) loss channel [26,27]. The metastable studies conducted as part of the present investigation that the dissociation channels of reveal (SO,),(NH,),H+ depend on cluster sizes and structures as follows: (1) (SO,)(NH,),H+: the daughter ion corresponding to the ( 1, 2) species is barely observed, while there is an NH3 loss channel open to the ( 1,3 ) cluster; see Fig. 4. For n 2 4, the only dissociation observed is NH0 loss. (2) (S03)2(NHs)nH+: the daughter ion signal for the (2, 4) cluster ion is much weaker than that for ( 2,5 ) which has a major NH3 loss channel and a minor loss channel of 114 amu assigned as (SO,)(NH,),. The (2, 4) species has only one dissociation channel corresponding to a 114 amu loss. Due to the weak signals, for those species with n 3 6
-
34 41 Time of Flight (microseconds) Fig. 2. Time of flight mass spectrum of products of (S0,),(NH3)n~H+.
217
Z. Shi et al. /Chemical Physics Letters 220 (1994) 274-280
DAUGHTER
1x10-~ Ton
5X1 C-3 Torr
3s Time of Flight
IONS
45 (microseconds)
Fig. 4. The daughter ions mass spectrum of reaction products contributed mainly by the ammonia loss channel.
0
0
II
F t
T =
IXlW
S’
Torr
$ ,O0NH4
OH
‘OH
8
O@'NH2
(4
>
52.0
43.5 35.0 26.5 Time of Flight (microseconds)
18.0
(b
0 ll,0MNH4
Fig. 3. The changes of intensities with different SO, densities: the pressures are the average relative pressures of the source chamber. The absolute values in the pickup area remain unknown.
&NH
2
Table 1 Metastable dissociations Mixed clusters
NH3
( c >
+
WWmW43)nH
n=2m+ 1
n=2m
n>2m+l
not observed
-(NH,)
-w&J
m=2
-[w3)(~3Ma
4~3)
-wf3Jb
(114amu)
-[W3)(~3M’
b
-(NH31b
-wJ53)b
l Weak signal. b Too weak to observe other dissociation channels.
are only able to observe one major NH3 loss channel; see Fig. 4. (3) (SO,),(NH,),H+: the metastable dissociation patterns for these species are similar to those of
we
+ @03)2(~3)nH
.
Cd)
Fig. 5. Structure of (a) H2S04, and proposed structures of (b) (so3)(~3)2,
m=l
m=3
H+
(c)
W3)(~3)3H+
aad
(4
W3)-
(MI3)3H+WH3).
The metastable dissociation results serve to reveal the stabilities and structures of the clusters. For those mixed clusters with n, 2m+ 1, there is a weakly bonded NH3 which forms the major daughter peaks. The n= 2m mixed clusters have relatively stable structures which are evidently chemically bonded rather than being weakly bonded by van der Waals forces. The secondary dissociation channel corresponding to a loss of 114 amu from (S03)z(NH3)sH+ further reveals that (S03)(NH3)2 is a strongly bonded unit. Fig. 5a shows the well-known
278
Z. Shi et al. /Chemical Physics Letters 220 (I 994) 274-280
structure of sulfuric acid ( H2S04). A stable structure for (SO3) (NH,), can be proposed if -OH and -H in H2S04 are replaced by -NH2 and -NH4, respectively; Fig. 5b. A proton, H+, can attach onto it and form (SO3) (NH3)3H+, whereupon (NH,), as well as [ (SO3) (NH,),], can further add onto it. The absence of n<2m (m= 1, 2, ...) for (SO,),(EJH3)nH+, suchas (1, O), (1, l), (2, O), (2, l), (2, 2), (2, 3), etc., reveals that after the reaction-ionization process, only ‘saturated’ mixed clusters can survive. Here, every sulfur trioxide molecule needs two ammonia molecules to become ‘saturated whereby it forms the stable chemical bonded neutral species W3)(~3)2 or N-W (SW WH2). l-he products of protonated ammonia clusters reacting with SO3 grow through the addition of this basic unit. Fig. 5~ shows the structure of (SO,) (NH3)3H+. By adding (SO3) (NH3)2 onto it, the stable series n= 2m is formed. According to the structures of n = 2m, the mixed clusters have less chance to lose NH3 and there could be a ( S03) (NH,), loss channel open for m 3 2. These considerations account for the experimental results given in Table 1. Fig. 2 shows that the series n = 2m + 1 is more stable than those corresponding to n> 2m+2 mixed clusters; in other words, the structure of (S03),(NH3)3,H+(NH3) could be different from those mixed clusters which are richer in ammonia. Noticing that there is a charge center of -NH2H+ in (SO3)-(NH3)2H+, Fig. 5c, we suggest that additional NH3 becomes hydrogen bonded to the charge center NH2H+ as shown in Fig. 5d. By considering the addition of more (S03)(NH3)3 units onto ( S03) (NH3)2H+, the stable series corresponding to n = 2m + 1 can be readily explained. Understandably, the NH3 in ( S03) (NH3)3H+ can become lost through metastable dissociation, and secondary (SO3)(NHS)Zlossfrom (SOS)?#IH3)2,H+(NH3) mixed clusters ( m > 2 ) can be observed. Because there is only one site available for NH3 molecules to bind onto (SO,) (NH3)2H+, additional ammonia molecules can only loosely bind onto (SOS) (NH,) 3H+. Therefore, those mixed clusters are relatively more unstable than the n=2m and n= 2m + 1 series. When the number of additional ammonia molecule increases, the influence of (SO3),(NH,),,H’(NI-I,) is reduced and the observed metastable dissociation patterns are similar to
those found for pure ammonia clusters. The ammonia loss channel is observed for n 2 2m + 2 peaks and the (S03) (NH,)? loss channel is not seen due to the weak signal, see Table 1. Our studies indicate that, in the mixed clusters, H+ is attached onto a (S03)(NH3)2 unit and not the ammonia clusters. During the reaction and especially during ionization, some loosely bonded species are evaporated and the structures of mixed clusters become rearranged. The possible processes are as follows, where p, q and r refer to the clusters before ionization: ( 1) The neutral products are abundant in ammonia molecules, (SO,),(NH,),(NH,), Ih,’ (SO,),(NH,),H+(NH,),-r-1 +r(NH3)+NH2
+e- .
The number of ammonia molecules lost is governed by the available excessive energies in the cluster ions. After the fast dissociations which may take place during reaction and ionization, some ions with a small amount of excess energies undergo metastable dissociations as summarized in Table 1. Some (SO,) (NH,), molecules can also be lost during either fast or metastable dissociations, if the number of (SO,) (NH,), units is more than 2. (2) The number of ammonia molecules is less than needed to “saturate” the sulfur trioxide molecules. In this case, (SO,),(NH,),, where the number of ammonia molecules q is less than 2p, there are no extra ammonia molecules available to be ionized. The ionization potential of SO3 molecules (IP = 12.7 eV) exceeds the energy range available in our experiments. Therefore, those peaks are missing in the mass speo tra. The same reason for the absence of (S03)m(NH,)& series is that the IP of (SO,) (NH3)2 is too high to be ionized with the non-resonant technique employed in the present work. Thus, the signal intensities for the n=2m and n = 2m + 1 series with more stable structures should be stronger than those for n 3 2m + 2. When the density of the SO3 reactant increases, more ammonia molecules are needed to “saturate” them, i.e. the chance for extra ammonia molecules to survive is reduced. Therefore, the relative intensities for it = 2m
Z. Shi et al. /Chemical Physics Letters 220 (1994) 274-280
and n = 2m + 1 increase when more SO, is added into the reaction region. There are several possible channels for reactions of sulfur trioxide and ammonia under different conditions [28], S01+NI-13-+H(S03NH2), 2SO3 +NHS-rHz(HNS206)
(1) ,
(2)
=03 +4M-I3-+N’J%+[N(S03NH4)21,
(3)
SO~+2NH3-rNH4(SO~NHz),
(4)
Reaction ( 1) is a low-density gas-phase reaction where there is less chance for further reaction to take place; the others involve subsequent reactions. Under S03-rich conditions, the products of reaction ( 1) are more likely to react with additional SO3 molecules. In our experiments, the mass spectra show that such products as ( 1, 1) and (2,1) do not exist due to excess NH3 in the mixed clusters, and the small amounts of SOS in the pickup source. Gas-phase reaction ( 3 ) also gives a product of ( 1,2) stoichiometry as formed in the case of cluster reactions. However, the structures of iminodisulfonate cannot be used to explain our results of the metastable dissociation studies. In liquid ammonia where reaction (4) occurs, the initial adduct S03*NH3 is too strong an acid for the NH3 solvent and is neutralized by another ammonia molecule to form an ammonium amidosulfate salt molecule. In ammonia-rich neutral ( SOs),(NH3), mixed clusters, the aggregation of molecules and evaporative cooling lead to intracluster reactions similar to those which take place in liquid phase reactions. Because there are no n < 2m products observed, we suggest that SO3 reacts with an ammonia dimer instead of two ammonia molecules sequentially. Furthermore, the products of van der Waals bonded ammonium amidosulfate molecules can be observed in cluster reactions.
4. Conclusions Reactions which take place within mixed clusteq, e.g., reaction (5) for single S03, are similar to those which occur in the liquid phase, reaction (6), as follows:
279
(5) (6)
The relatively strong intensities for (SO,),(NH,),H+, n=2m and n=2m+ 1 reveal the formation of stable structures. The results of the present metastable dissociation studies show that chemically bonded species corresponding to ( S03),(NH3)z,H’ are the most stable among the mixed clusters. The charge center increases the strength of the hydrogen bond, whtch makes (S03)m(NH3)2m+l H+ more stable than the n > 2m + 2 series.
Acknowledgement Financial support by the US Environmental Protection Agency, Grant No. R-817437, and the National Science Foundation, Grant No. ATM-90 15855, is gratefully acknowledged.
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