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Experimental evidence for the gas phase existence of HSOH (hydrogen thioperoxide) and SOH2 (thiooxonium ylide)
29 April 1994
ELSEVIER
CHEMICAL PHYSICS LETTERS
Chemical Physics Letters 221 (1994) 359-362
Experimental evidence for the gas phase existence of HSOH (hydrogen thioperoxide) and SOHz (thiooxonium ylide ) Muhammad Iraqi, Helmut Schwarz Institutflr
Organische Chemie der Technischen Universitiit Berlin, StraJ3edes 17. Juni 135, D-10623 Berlin, Germany Received 3 February 1994
Abstract Experiments
are reported on the successful gas phase generation of hydrogen thioperoxide
(HSOH ) and thiooxonium
ylide
(SOHz).
1. Introduction
According to numerous calculations [ l-3 1, at least three experimentally distinguishable isomers are expected to exist on the potential energy surface of the four-atom molecules (HZ, 0, S), a system which is potentially of interest in atmospheric chemistry [ 41. The global minimum corresponds to hydrogen thioperoxide HSOH ( 1) . The sulfoxide HzSO (2 ) is predicted to be > 25 kcal/mol higher in energy (relative to 1)) and the thiooxonium ylide 3 is the least stable isomer ( > 34 kcal/mol relative to 1). The calculated barriers [ 2 ] for the unimolecular isomerization are high enough to prevent a facile isomerization of the ‘isolated’ molecules with approximate activation energies of 35.7 kcal/mol for the process H2S0 (2) +HSOH (1) and 11.9 kcal/mol for the reaction SOHz (3)+HSOH (1).
6
s-
/H
“\
HiS-O 2
s-0’
/HH
3 In spite of this favourable situation, experimental information on the potential energy surface - or at least the stationary points - of the (Hz, 0, S) system is scanty. This is the more annoying as these data are essential in assessing the importance of simple sulfur compounds in atmospheric processes [4]. Experimental evidence for the existence of HSOH (1) in an argon matrix (8 K) was reported by Smardzewski and Lin [ 5 1, where 1 is formed by ultraviolet and/or laser photolysis of a dilute argon matrix sample containing O3 and HIS, and it was suggested that 1 may be generated by the following sequence of events:
H’ 1
O+H+
0009-2614/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDIOOO9-2614(94)00293-Y
[H,SO]*+ [HSO’+H’]-,HSOH
(1) . (1)
360
M. Iraqi, H. Schwarz /Chemical Physics Letters 221 (I 994) 359-362
While the observed infrared absorptions strongly support the presence of hydrogen thioperoxide 1, the spectroscopic evidence for the initial adduct [ H,SO] is much less convincing. As stated by the authors [ 5 1, the infrared absorption at 1083.4 cm-’ is compatible with the S-O stretching mode of either a H$O molecule or a HSO’ radical. The latter radical and its isomer HOS’ which are also believed to play a key role in atmospheric chemistry, and in particular in the depletion of ozone, were recently generated and structurally characterized in the gas phase [ 6 1. While the existence of the ‘isolated’ HSOH molecule as a perfectly stable gas phase species is indirectly demonstrated by the protonation reaction of the hydroxysulfide anion (Eq. (2) below) [ 3 1, more direct evidence on the existence of the isolated HSOH species does not seem to exist, and to the best of our knowledge experimental data on the thiooxonium ylide 3 have not been reported in the literature, -SOH+HX=HSOH
(1)+X-.
(2)
In this Letter, we provide evidence that both HSOH +. ( l+ ’ ) and SOH: . (3+’ ) can be generated in the gas phase as distinct species which, when subjected to collision experiments, can be reduced in a one-electron transfer process to their neutral analogues 1 and 2, the latter are subsequently reionized and characterized. This technique of neutralizationreionization mass spectrometry (NRMS) (for reviews, see ref. [ 71) has been used to identify numerous elusive molecules in the gas phase which, due to facile intermolecular processes, are difficult if not impossible to generate in condensed phases [ 6,8 1.
lows: electron energy 100 eV, repeller voltage z 0 V, ion source temperature l OO-200°C acceleration voltage 8 kV, ion source pressure (measured in the chamber which contains the source) 5x 1O-s-1O-4 mbar. The adduct formation was optimal at T= 100°C and p= 5 x 10e5 mbar. The ions of interest were massselected by means of B ( 1) E ( 1) at a mass resolution m/Am = 3500. Collisional activation (CA) experiments were conducted by colliding the beam with He in the second of two differentially pumped collision cells located in the field-free region between E ( 1) and B (2 ). The pressure in the collision cell was such that the intensity of the primary ion beam was attenuated to 80% transmission ( T). NR experiments were conducted by colliding the beam with Xe in the first collision cell and O2 in the second. The primary ion beam was attenuated to 80% in each collision event. The deflector electrode, mounted between the two collision cells, was charged to 100 V, thus preventing any ions from entering the second collision cell in which reionization takes place. All product ions were recorded by scanning B (2)) and 15-20 spectra were accumulated using the AMD-Intectra data system. The minimal lifetime of the neutral molecules, estimated on the flight time between the two collision experiments, is of the order of 8 us.
3. Results and discussion The CA mass spectrum of the (H,, S, O)+’ ions, generated upon electron impact ionization of a mixture of H2S/N20 is given in Fig. 1. The interpreta-H’
2. Experimental All collision experiments were performed by using a modified large-scale four-sector tandem mass spectrometer of BEBE configuration (B stands for magnetic and E for electric sector). The details of the machine and its operation have been described elsewhere [ 91. The HSOH+ ’ ions 1+ * are produced from a mixture of H2S and N20 in a CI source (N20 relative pressure is dominant). The isomeric SOH,f ’ ions 3+’ were produced from a mixture of CS1 and HZ0 in a CI source. Typical ion source conditions were as fol-
+. OH+ \ r---.*nw
so+* II \I
,,B”’
x20
Fig. 1. CA mass spectrum
of HSOH+ . (He).
361
h4. Iraqi, H. Schwarz/Chemical PhysicsLetters221(1994) 359-362
tion of this spectrum is straightforward, and the spectrum is best compatible with the connectivity of HSOH+ - (l+ ’ ). Structurally indicative fragment ions are due to HS+ (m/z 33) and OH+ (m/z 17). The large intensity difference between these two ions most likely reflects the much higher heat of formation of the product combination (OH+/SH’; CHB = 342 kcal/mol) as compared with 282 kcal/ mol for the pair HS +/OH’ [ lo]. We note the absence of a signal at m/z 34 ( H2S+. ) which would be indicative of the presence of the sulfoxide ion H,SO+ ’ (2+’ ). Oxidation of neutral H2S with 0 in an argon matrix [ 51 as well as H2S with N20 under electron impact ionization does not result in detectable amounts of the sulfoxides 2 and 2+‘. It is interesting to recall that dimethyl sulfenate ion (CH$OCH: *) is the primary species formed when dimethyl sulfoxide ( (CH3)*SO) is subjected to electron impact ionization [ 111; a facile rearrangement occurs. Reduction of HSOH+ ’ ( 1+ ’ ) in a NR experiment yields an intense recovery signal (Fig. 2 ) . In view of the vertical nature [ 12 ] of the electron transfer processes HSOH+ ’ +HSOH+HSOH+ ’ we conclude that the neutral HSOH (1) molecule retains the connectivity of its cationic precursor. Isomerization of 1 to either H2S0 (2) or 0SH2 (3) is not indicated as occurring in the time scale of the experiment, in line with the ab initio calculations [ 2 1, according to which 1 is separated by significant barriers from its isomers 2 and 3. In the CA mass spectrum of the ( H2 ,S ,O ) + * ions generated from a mixture of CS2 and Hz0 (Fig. 3 ), we observe a fragmentation pattern quite distinct HSOH+’ Recovery Signal
HSO+ S+’ HS+
Fig. 2. NR mass spectrum of HSOH+’ (Xe//Od.
S
+*
-H
so+.
\ [ oHHy+a x10 I ItI/=-
Fig. 3. CA mass spectrum of SOH: ’ (He).
S+’
-D D20+’ OD+
/ DS+
so+. \,
m/z_ Fig. 4. CA mass spectrum of SOD: * (He).
from that reported in Fig. 1. The absence of m/z 32 ( H2S+ ) again suggests that H,SO+ ’ is also not present, which is perhaps not too surprising. In the comparison of Fig. 1 and Fig. 3, there are two structureindicative differences. (i) In the low-mass region of Fig. 3 we note a signal at m/z 18 (HzO+’ ) which is not present in Fig. 1. (ii) In the high-mass region, the intensity ratio [ S+ ’ ] / [ HS ] + is much larger in Fig. 3 (4.8) than for the ion HSOH+’ in Fig. 1 (1.7). These findings clearly suggest that in the reaction of CS: ’ with Hz0 the hitherto unknown cation of the thiooxonium ylide SOH$ ’ (3+ a) is generated, which may have undergone partial isomerization to HSOH+’ (l+ ’ ). We cannot rule out that this isomerization is collision induced. In Fig. 4, the CA mass spectrum is given for SOD: ., generated from CS,’ * and D20, and
362
M. Iraqi, H. Schwarz /Chemical Physics Letters 221(1994) 359-362 SOH;’ Recovery Signal
S+’
I
SOH’
m/zFig. 5. NR mass spectrum of SOH:’ (Xc//G,).
the fragmentation pattern is much alike to that reported in Fig. 3 for SOHZ ’ . Reduction of SOH$ * (3+ ’ ) in a NR experiment gives rise to the spectrum shown in Fig. 5. Again, we note the presence of a recovery signal which, together with the fragmentation pattern, provides experimental evidence that the neutral thiooxonium ylide SOHz is indeed a viable molecule in the gas phase as predicted by MO calculations [ 2 1.
Acknowledgement
The generous financial support of our work by the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie is appreciated. HS is grateful to the Alexander von Humboldt-Foundation for support ( Max-Planck-Research-Award ).
References [ 1 ] H. Wallmeier and W. Kutzelnigg, J. Am. Chem. Sot. 101 ( 1979) 2084; E. Magnusson, J. Comput. Chem. 5 ( 1984) 612; J. Am. Chem.Soc. 106 (1984) 1185; T.J. Lee, NC. Handy, J.E. Rice, A.C. Schemer and H.F. Schaefer III, J. Chem. Phys. 85 ( 1986) 3930; M. Sol&, C. Gonzales, G. Tunachini and H.B. Schlegel, Theoret. Chim. Acta 77 ( 1990) 28 1.
[2] S. Wolfe and H.B. Schlegel, Gazz. Chim. Ital. 120 ( 1990) 285. [ 3] R.A.J. O’Hair, C.H. DePuy and V.M. Bierbaum, J. Phys. Chem. 97 (1993) 7955. [4] C.F. Cuilis and M.M. Hirschler, Atmos. Environ. 14 (1980) 1263; D. M811er,Atmos. Environ. 18 ( 1984) 19; R.R. Friedl, W.H. Brune and J.G. Anderson, J. Phys. Chem. 89 (1985) 5505; E.R. Lovejoy, N.S. Wang and C.J. Howard, J. Phys. Chem. 91 (1987) 5749; V.P. Bulatov, S.I. Vereshchuk, EN. Dzegilenko, O.M. Sarkisov and V.N. Kbabarov, Khim. Fiz. 9 ( 1990) 1214; N.S. Wang and C.J. Howard, J. Phys. Chem. 94 (1990) 8787; N. Balucani, L. Beneventi, P. Casavecchia, D. Stranges and G.G. Volpi, J. Chem. Phys. 94 (1991) 8611; G.S. Tyndall and A.R. Ravishankara, Intern. J. Chem. Kinetics 23 (1991) 483; R.P. Wayne, Chemistry of atmospheres, 2nd Ed. (Clarendon Press, Oxford, 199 1), and references therein. [ 51 R.R. Smardzewski and M.C. Lin, J. Chem. Phys. 66 ( 1977) 3197. 1M. Iraqi, N. Goldberg and H. Schwarz, J. Phys. Chem. 98 ( 1994) 000, and references therein. 171H. Schwarz, Pure Appl. Chem. 61 (1989) 685; J.L. Holmes, Mass Speetrom. Rev. 8 ( 1989) 5 13; J.K. Terlouw, Advan. Mass Spectrom. 11 (1989) 984; F.W. McLafferty, Science 247 (1990) 925; Intern. J. Mass Spectrom. Ion Processes 118/ 119 ( 1992) 22 1. [ 8 ] N. Goldberg, D. Sillzle and H. Schwarz, Chem. Phys. Letters 213 (1993) 593; N. Goldberg, J. HruSak, M. Iraqi and H. Schwarz, J. Phys. Chem. 97 ( 1993) 10687, and references therein. [ 91R. Srinivas, D. Stllzle, T. Weiske and H. Schwarz, Intern. J. Mass Spectrom. Ion Processes 107 ( 199 1) 368; R. Srinivas, D. Sillzle, W. Koch, C.H. DePuy and H. Schwarz, J. Am. Chem. Sot. 113 ( 199 1) 5970; R. Srinivas, D.K. Bohme, D. Sillzle and H. Schwarz, J. Phys. Chem. 95 (1991) 9836. [lo] S.G. Lias, B.E. Dartman, S.F. Liebman, J.L. Holmes, R.D. Levin and W.G. Maillard, J. Phys. Chem. Ref. Data 17 Suppl. 1 (1985). [ 11I R.A. Kmel’nitski and Y.A. Efremov, Russian Chem. Rev. 46 (1977) 46. [ 12I P. Foumier, J. AppeIl, EC. Fehsenfeld and J. Dump, J. Phys. B 5 (1972) L58; J.C. Lorquet, B. Leyh-Nihaut and F.W. McLatferty, Intern. J. Mass Spectrom. Ion Processes 100 (1990) 465.
ELSEVIER
CHEMICAL PHYSICS LETTERS
Chemical Physics Letters 221 (1994) 359-362
Experimental evidence for the gas phase existence of HSOH (hydrogen thioperoxide) and SOHz (thiooxonium ylide ) Muhammad Iraqi, Helmut Schwarz Institutflr
Organische Chemie der Technischen Universitiit Berlin, StraJ3edes 17. Juni 135, D-10623 Berlin, Germany Received 3 February 1994
Abstract Experiments
are reported on the successful gas phase generation of hydrogen thioperoxide
(HSOH ) and thiooxonium
ylide
(SOHz).
1. Introduction
According to numerous calculations [ l-3 1, at least three experimentally distinguishable isomers are expected to exist on the potential energy surface of the four-atom molecules (HZ, 0, S), a system which is potentially of interest in atmospheric chemistry [ 41. The global minimum corresponds to hydrogen thioperoxide HSOH ( 1) . The sulfoxide HzSO (2 ) is predicted to be > 25 kcal/mol higher in energy (relative to 1)) and the thiooxonium ylide 3 is the least stable isomer ( > 34 kcal/mol relative to 1). The calculated barriers [ 2 ] for the unimolecular isomerization are high enough to prevent a facile isomerization of the ‘isolated’ molecules with approximate activation energies of 35.7 kcal/mol for the process H2S0 (2) +HSOH (1) and 11.9 kcal/mol for the reaction SOHz (3)+HSOH (1).
6
s-
/H
“\
HiS-O 2
s-0’
/HH
3 In spite of this favourable situation, experimental information on the potential energy surface - or at least the stationary points - of the (Hz, 0, S) system is scanty. This is the more annoying as these data are essential in assessing the importance of simple sulfur compounds in atmospheric processes [4]. Experimental evidence for the existence of HSOH (1) in an argon matrix (8 K) was reported by Smardzewski and Lin [ 5 1, where 1 is formed by ultraviolet and/or laser photolysis of a dilute argon matrix sample containing O3 and HIS, and it was suggested that 1 may be generated by the following sequence of events:
H’ 1
O+H+
0009-2614/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDIOOO9-2614(94)00293-Y
[H,SO]*+ [HSO’+H’]-,HSOH
(1) . (1)
360
M. Iraqi, H. Schwarz /Chemical Physics Letters 221 (I 994) 359-362
While the observed infrared absorptions strongly support the presence of hydrogen thioperoxide 1, the spectroscopic evidence for the initial adduct [ H,SO] is much less convincing. As stated by the authors [ 5 1, the infrared absorption at 1083.4 cm-’ is compatible with the S-O stretching mode of either a H$O molecule or a HSO’ radical. The latter radical and its isomer HOS’ which are also believed to play a key role in atmospheric chemistry, and in particular in the depletion of ozone, were recently generated and structurally characterized in the gas phase [ 6 1. While the existence of the ‘isolated’ HSOH molecule as a perfectly stable gas phase species is indirectly demonstrated by the protonation reaction of the hydroxysulfide anion (Eq. (2) below) [ 3 1, more direct evidence on the existence of the isolated HSOH species does not seem to exist, and to the best of our knowledge experimental data on the thiooxonium ylide 3 have not been reported in the literature, -SOH+HX=HSOH
(1)+X-.
(2)
In this Letter, we provide evidence that both HSOH +. ( l+ ’ ) and SOH: . (3+’ ) can be generated in the gas phase as distinct species which, when subjected to collision experiments, can be reduced in a one-electron transfer process to their neutral analogues 1 and 2, the latter are subsequently reionized and characterized. This technique of neutralizationreionization mass spectrometry (NRMS) (for reviews, see ref. [ 71) has been used to identify numerous elusive molecules in the gas phase which, due to facile intermolecular processes, are difficult if not impossible to generate in condensed phases [ 6,8 1.
lows: electron energy 100 eV, repeller voltage z 0 V, ion source temperature l OO-200°C acceleration voltage 8 kV, ion source pressure (measured in the chamber which contains the source) 5x 1O-s-1O-4 mbar. The adduct formation was optimal at T= 100°C and p= 5 x 10e5 mbar. The ions of interest were massselected by means of B ( 1) E ( 1) at a mass resolution m/Am = 3500. Collisional activation (CA) experiments were conducted by colliding the beam with He in the second of two differentially pumped collision cells located in the field-free region between E ( 1) and B (2 ). The pressure in the collision cell was such that the intensity of the primary ion beam was attenuated to 80% transmission ( T). NR experiments were conducted by colliding the beam with Xe in the first collision cell and O2 in the second. The primary ion beam was attenuated to 80% in each collision event. The deflector electrode, mounted between the two collision cells, was charged to 100 V, thus preventing any ions from entering the second collision cell in which reionization takes place. All product ions were recorded by scanning B (2)) and 15-20 spectra were accumulated using the AMD-Intectra data system. The minimal lifetime of the neutral molecules, estimated on the flight time between the two collision experiments, is of the order of 8 us.
3. Results and discussion The CA mass spectrum of the (H,, S, O)+’ ions, generated upon electron impact ionization of a mixture of H2S/N20 is given in Fig. 1. The interpreta-H’
2. Experimental All collision experiments were performed by using a modified large-scale four-sector tandem mass spectrometer of BEBE configuration (B stands for magnetic and E for electric sector). The details of the machine and its operation have been described elsewhere [ 91. The HSOH+ ’ ions 1+ * are produced from a mixture of H2S and N20 in a CI source (N20 relative pressure is dominant). The isomeric SOH,f ’ ions 3+’ were produced from a mixture of CS1 and HZ0 in a CI source. Typical ion source conditions were as fol-
+. OH+ \ r---.*nw
so+* II \I
,,B”’
x20
Fig. 1. CA mass spectrum
of HSOH+ . (He).
361
h4. Iraqi, H. Schwarz/Chemical PhysicsLetters221(1994) 359-362
tion of this spectrum is straightforward, and the spectrum is best compatible with the connectivity of HSOH+ - (l+ ’ ). Structurally indicative fragment ions are due to HS+ (m/z 33) and OH+ (m/z 17). The large intensity difference between these two ions most likely reflects the much higher heat of formation of the product combination (OH+/SH’; CHB = 342 kcal/mol) as compared with 282 kcal/ mol for the pair HS +/OH’ [ lo]. We note the absence of a signal at m/z 34 ( H2S+. ) which would be indicative of the presence of the sulfoxide ion H,SO+ ’ (2+’ ). Oxidation of neutral H2S with 0 in an argon matrix [ 51 as well as H2S with N20 under electron impact ionization does not result in detectable amounts of the sulfoxides 2 and 2+‘. It is interesting to recall that dimethyl sulfenate ion (CH$OCH: *) is the primary species formed when dimethyl sulfoxide ( (CH3)*SO) is subjected to electron impact ionization [ 111; a facile rearrangement occurs. Reduction of HSOH+ ’ ( 1+ ’ ) in a NR experiment yields an intense recovery signal (Fig. 2 ) . In view of the vertical nature [ 12 ] of the electron transfer processes HSOH+ ’ +HSOH+HSOH+ ’ we conclude that the neutral HSOH (1) molecule retains the connectivity of its cationic precursor. Isomerization of 1 to either H2S0 (2) or 0SH2 (3) is not indicated as occurring in the time scale of the experiment, in line with the ab initio calculations [ 2 1, according to which 1 is separated by significant barriers from its isomers 2 and 3. In the CA mass spectrum of the ( H2 ,S ,O ) + * ions generated from a mixture of CS2 and Hz0 (Fig. 3 ), we observe a fragmentation pattern quite distinct HSOH+’ Recovery Signal
HSO+ S+’ HS+
Fig. 2. NR mass spectrum of HSOH+’ (Xe//Od.
S
+*
-H
so+.
\ [ oHHy+a x10 I ItI/=-
Fig. 3. CA mass spectrum of SOH: ’ (He).
S+’
-D D20+’ OD+
/ DS+
so+. \,
m/z_ Fig. 4. CA mass spectrum of SOD: * (He).
from that reported in Fig. 1. The absence of m/z 32 ( H2S+ ) again suggests that H,SO+ ’ is also not present, which is perhaps not too surprising. In the comparison of Fig. 1 and Fig. 3, there are two structureindicative differences. (i) In the low-mass region of Fig. 3 we note a signal at m/z 18 (HzO+’ ) which is not present in Fig. 1. (ii) In the high-mass region, the intensity ratio [ S+ ’ ] / [ HS ] + is much larger in Fig. 3 (4.8) than for the ion HSOH+’ in Fig. 1 (1.7). These findings clearly suggest that in the reaction of CS: ’ with Hz0 the hitherto unknown cation of the thiooxonium ylide SOH$ ’ (3+ a) is generated, which may have undergone partial isomerization to HSOH+’ (l+ ’ ). We cannot rule out that this isomerization is collision induced. In Fig. 4, the CA mass spectrum is given for SOD: ., generated from CS,’ * and D20, and
362
M. Iraqi, H. Schwarz /Chemical Physics Letters 221(1994) 359-362 SOH;’ Recovery Signal
S+’
I
SOH’
m/zFig. 5. NR mass spectrum of SOH:’ (Xc//G,).
the fragmentation pattern is much alike to that reported in Fig. 3 for SOHZ ’ . Reduction of SOH$ * (3+ ’ ) in a NR experiment gives rise to the spectrum shown in Fig. 5. Again, we note the presence of a recovery signal which, together with the fragmentation pattern, provides experimental evidence that the neutral thiooxonium ylide SOHz is indeed a viable molecule in the gas phase as predicted by MO calculations [ 2 1.
Acknowledgement
The generous financial support of our work by the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie is appreciated. HS is grateful to the Alexander von Humboldt-Foundation for support ( Max-Planck-Research-Award ).
References [ 1 ] H. Wallmeier and W. Kutzelnigg, J. Am. Chem. Sot. 101 ( 1979) 2084; E. Magnusson, J. Comput. Chem. 5 ( 1984) 612; J. Am. Chem.Soc. 106 (1984) 1185; T.J. Lee, NC. Handy, J.E. Rice, A.C. Schemer and H.F. Schaefer III, J. Chem. Phys. 85 ( 1986) 3930; M. Sol&, C. Gonzales, G. Tunachini and H.B. Schlegel, Theoret. Chim. Acta 77 ( 1990) 28 1.
[2] S. Wolfe and H.B. Schlegel, Gazz. Chim. Ital. 120 ( 1990) 285. [ 3] R.A.J. O’Hair, C.H. DePuy and V.M. Bierbaum, J. Phys. Chem. 97 (1993) 7955. [4] C.F. Cuilis and M.M. Hirschler, Atmos. Environ. 14 (1980) 1263; D. M811er,Atmos. Environ. 18 ( 1984) 19; R.R. Friedl, W.H. Brune and J.G. Anderson, J. Phys. Chem. 89 (1985) 5505; E.R. Lovejoy, N.S. Wang and C.J. Howard, J. Phys. Chem. 91 (1987) 5749; V.P. Bulatov, S.I. Vereshchuk, EN. Dzegilenko, O.M. Sarkisov and V.N. Kbabarov, Khim. Fiz. 9 ( 1990) 1214; N.S. Wang and C.J. Howard, J. Phys. Chem. 94 (1990) 8787; N. Balucani, L. Beneventi, P. Casavecchia, D. Stranges and G.G. Volpi, J. Chem. Phys. 94 (1991) 8611; G.S. Tyndall and A.R. Ravishankara, Intern. J. Chem. Kinetics 23 (1991) 483; R.P. Wayne, Chemistry of atmospheres, 2nd Ed. (Clarendon Press, Oxford, 199 1), and references therein. [ 51 R.R. Smardzewski and M.C. Lin, J. Chem. Phys. 66 ( 1977) 3197. 1M. Iraqi, N. Goldberg and H. Schwarz, J. Phys. Chem. 98 ( 1994) 000, and references therein. 171H. Schwarz, Pure Appl. Chem. 61 (1989) 685; J.L. Holmes, Mass Speetrom. Rev. 8 ( 1989) 5 13; J.K. Terlouw, Advan. Mass Spectrom. 11 (1989) 984; F.W. McLafferty, Science 247 (1990) 925; Intern. J. Mass Spectrom. Ion Processes 118/ 119 ( 1992) 22 1. [ 8 ] N. Goldberg, D. Sillzle and H. Schwarz, Chem. Phys. Letters 213 (1993) 593; N. Goldberg, J. HruSak, M. Iraqi and H. Schwarz, J. Phys. Chem. 97 ( 1993) 10687, and references therein. [ 91R. Srinivas, D. Stllzle, T. Weiske and H. Schwarz, Intern. J. Mass Spectrom. Ion Processes 107 ( 199 1) 368; R. Srinivas, D. Sillzle, W. Koch, C.H. DePuy and H. Schwarz, J. Am. Chem. Sot. 113 ( 199 1) 5970; R. Srinivas, D.K. Bohme, D. Sillzle and H. Schwarz, J. Phys. Chem. 95 (1991) 9836. [lo] S.G. Lias, B.E. Dartman, S.F. Liebman, J.L. Holmes, R.D. Levin and W.G. Maillard, J. Phys. Chem. Ref. Data 17 Suppl. 1 (1985). [ 11I R.A. Kmel’nitski and Y.A. Efremov, Russian Chem. Rev. 46 (1977) 46. [ 12I P. Foumier, J. AppeIl, EC. Fehsenfeld and J. Dump, J. Phys. B 5 (1972) L58; J.C. Lorquet, B. Leyh-Nihaut and F.W. McLatferty, Intern. J. Mass Spectrom. Ion Processes 100 (1990) 465.