Excited-neutral-metastable SO2 formation in the , O3-olefin-SO2, and active nitrogen-SO2 systems

VoIume 72, number 1

15 Hay E98Q

CHEWCAL PHYSICS LETTERS

EXCITED-NE~~~MET~TA~LE LNTHE 0,-C!,

SO, FORMATION

O,-OLEFIN-SO,,

Richard I. MARTINEZ

AND ACTIVE NITROGEN-SO,

SYSTEMS

and John T. HERRON

N~rionnl BElreau of Srun&rds,chemwd Kinencs Wvisron, Centerfor ‘17hermodpamics andMo~ecukr Science.

i(Wzmg~otz.DC 20234. USA Received 20 January 1980

Long-h\ed (T > 10m3s), electromcaliyeWited (E* > 4 eV) neutral metastables,whose forma&ionwas attributed to the

presence of SO2, were observed in the three Wed systems Their possible identity as colbnonatly~c~d of the normal (OKsNO), cychc (O’S’O), or srrperoude(We-O) isomers of SO2 is discussed.

1. Introduction

A recent mass-spectrometric study of the mechamsm of the gas-phase reaction of thGrane (Cm) ~nth excess wzcme was complicated by an extraordinary artifact - an intense “noise” spike wkch was mdependent of mass settmg and which was observed when the concentration of the primary products, C2HI and SO,, approached a maximum [I 3 _We concluded that @us “noise” sp&e was due to long-bed, electronically-exclted neutral metastable species produced

metastablestates

in the nascent OS--olefmSO2 system by some WIdetermined mechanism. We report here experimental observations which led to this conclusion and which are relevant to the identification of the metastable species-

2. Experimental

The apparatus (shown in tig. I), experimental! prucedures, and data-reduction techniques were the sa.mz MICROWAVE DISCHARGE REGOWAWCEIA

PREMIX

GOLENOID VALVES f REACTANTS OUT

DETECTOR

TRIGGER

Fii 1. Apparatus used m all e-qenmenfs and se&on 2 herem

COPPIJTER

for a detaded desfnption of the apparatus and the experimental procedures, see ref. 131

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as those prevrously described [2,3], unless otherwike noted. Bnefly, a stopped-flow reactor was coupled to a beam-samphng mass spectrometer utrhzmg a quadrupole mass filter, a Daly-type Ion-electron scint~ation detector, and mert-gas (Ar, Ne, or He) resonance lamps as photoionizatron sources. AU experiments \sere conducted at 296 K and at an imtial total pressure of 1 .I kPa (8 Torr). When added, SO, was m.ected at a pomt approxlmately 60 cm upstream from the onfice of thestoppedflow reactor (refer to fig. I) (For the OJ-olefm-SO7 expenments, the SO, and olefin uere thus pre-mixed prior to encountermg the O,.) Actrve mtrogen and active oxygen were both generated at a pomt appro?llmately 1 .I m upstream from the onfice of the stoppedflow reactor (50 cm upstream from the point ofSO, ejection) by a 2450 MHz mrcrowave drscharge in pure mtrogen (at 8 Torr total pressure) and m 2%-16% O,/Ar matures (at 2-S Torr total pressure), respectively. Unless othenvtse noted. signals due to neutral metastables were measured by usmg the Daly detector wrthout any Ionization source, wrth the mass filter turned off, and wrth an Ion-stopprng potentral apphed to both the entrance and the exrt lenses mounted on the mass filter housmg.

3. Resutts

and discussion

3 I T1ze ozone-trzztratze

slsrenz

Frg. 2a shows an example of the 0,-C“noise” sprke for the concentration-versus-tune profile obtaed at rzz/e = 18 (HzO) with the xontzatton source turned on.

A series of tests were performed whrch led us to believe that the “norse” sprkes were due to the productron of some long-lived (T > 1W3 s), exerted neutral metastable. The vanous components of the detection system shown m fig. I - resonance lamps, mass fdter, entrance and e.xrt eon lenses. Daly “doorknob”, and photomultrpher - were turned on and off in various combmattons to estabhsh that the “norse” sprkes were not due erther to ions (lamp off, ronstopping potentral apphed to Ion lenses) or to luminescence (Daly ~‘doorkno~ set to ground pot~ntlal, photomuttjp~er turned on), but may be due to a highly polar or po78

LE-ITERS

Fig 2 (a) Sqnal observed at m/e = 18 1~1th the He resonance lamp Expenmental condltlons uere those of fig 1 in ref [ 11 (b) Excited neutral metastable sgnaf observed for the 03/cw 2-butenefSOz system under stopped-flou conditzons at 296 K and 1 I Wa (8 Torr) tnttialtotal pressure [Os ]e = I 5 x 10m8 ==8 x 1O-*o mol cmm3 . [02 j&IO3 lo = 27 8, [ns-2-butenej* mol cms3; [SOz]o = 2.9 x 10mQ mol cms3 (c) Excited neutral metastablesqgm.d obserred for the actwe rutrogen-SO2 system under stopped-flow condlhons at 296 K and 1 I kPa (8 Torr) nxtlal total pressure fN2 ju = 4 3 X lo-’ mot cmm3, ISO1 lo = 7.9 X IO+ mol cmm3

lanzable species, as Indicated by the partral attenuatton of the “metastable” srgnal by the field wrthrn the quadrupole mass ftter. And, smce apertures on both ends of the quadrupole mass filter define a collimated, cyfindricaI beam of drameter < 6 mm, the ~urn~umcoated Daly “doorknob” is out of the lure of s&t of any neutral spectes whrch travels from the onfice of the stopped-flow reactor all the way through the mass filter. Under our condrtrons of high vacuum [< 1.3 X 104 Pa ( 10m6 Ton)], the only way a neutral specres could reach the doorknob ESto be reflected off the rear chamber wall, a total ff rght distance of approw-

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mately 1 .l m, correspondmg at room temperature to a mean molecular flight tune of approxunately 3 5 ms for SO? 3 2. The ozone-olefin-SO2

system

Smce the ozone-ttiirane “nose” spike is observed when [CZHa] and [SO-,] are approaching a mmmum [ 11, it seemed hkely that the phenomenon would be as readily observed by reacting ozone wth ethene m the presence of SO,. IIus was found to be the case. In fact, spikes were also observed wrth the O,-propene and Oj-ca-2-butene systems m the presence of added SO,. Fig. 2b shows the metastable signal obtamed with the OS-cis-2-butene-SO2 system. No metastables were observed for these ozone-olefii systems m the absence of added SO,. The Identity (and the source) of the metastables, whose formation probably constitutes only a very rnitzor reaction channel. is dlscussed m section 3.4

3 3. T#Qeactwe wrtroger~-SO2 and actwe o_rygen-SO2 systems

It was also possible to generate a metastable signal by mlxmg SO2 with active nitrogen ([Nl]o/[SOZ]O > loo), as shown m fig .2c. Metastable production was very sensitive to the nature of the reactor Also, the metastable signal was drastically

surface.

reduced, close to background levels, when the total pressure was decreased from = 1.1 kPa (8 Torr) to =0.3 kPa (2 Torr) by reducmg the N2 flow-rate. No metastables were detected (I) in the active oxygen-SO, system, or(u) in the absence of added SO2 m either the actwe rutrogen or the active oxygen. However, the presence of excited states of O2 [a 1 Ag(’ == 2700 s, E’ = 0.98 eV) and/or b 1 Cl<, =Z 7 s, E = 1.63 ev)] III the ac-

tive oxygen was confirmed by photolomzation with an Ar lamp. Active nitrogen provides a source of the N, metastable states A 32z(~ > 2 s, E* > 6.2 ev), a’ 12;(~ = 0.7 s, E* > 8.4 ev), B’ 3Z;(~ = 10M2 s, E’ > 8.2 ev), W 3AU(E * > 7.4 eV) and 5 $(E * > 9.6 ev), which result from the recombmation of two nitrogen atoms in the very long-hved (tl,l of the order of mmutes) Lewis-Rayleigh nitrogen afterglow [4]. The behavior observed in the active nitrogen-SO2 system IS consistent with what is known about both (I) the gross

LETTERS

15 May 1980

effect of pressure in active mtrogen and (ii) the recombmation of nitrogen atoms in the afterglow (the rate of the nonlummescent heterogeneous component is hrghly dependent on the nature of the surface, so surface effects are not unexpected) [4] _Thus, fig. 2c probably reflects the temporal dependence of the nitrogen atom concentration, and hence the concentration of excited N, metastables, N;, entrapped in the stopped-flow reactor. Since no metastable was observed in the absence of added S02, the signal observed III fig. 2c must be due to some unidentified, long-lived (7 > 10m3 s), excited-neutral-metastable state of a sulfur-containmg species formed by energy transfer to SO, from an N2 metastable state (cf. ref. [5]). Moreover, smce SO, does not react with active nitrogen [6], It is presumed (see section 3.4) that the unidentified species is some metastable state of S02, SO; _ N; was not detected in the absence of added SO,, suggestmg that the NZ may be deactrvated within the glass sampling orifice of the reactor (which was inevitably contaminated by reaction products of the chemical systems examined in this study, viz., 03olefm-SO, and 03-C2HaS) and/or by Its collision \wth the rear chamber wall, so that the N; never reaches the Daly doorknob. Ni could also be quenched by collisions with N and/or N, in the stopped-flow reactor {quenchmg by N atoms dominates the removal of N,(A 3 Ci) [4] 1. However, m the presence of added S02,energy transfer from N; to SO,, to form an apparently long-lived SOS, competes with the quenching of Nr by N and/or N,. Hence. in thrs case. the longlived SO; serves as a “tracer” for the “invinble” Nf, pro-ding a means for detectmg and monitoring N1 metastables. On the other hand, in the case of active oxygen, the long-hved excited states of 0, are not sufficiently energetic either to be detected by direct Auger de-excitation (refer to section 3.4) or to form SO; by energy transfer to SO,. 3 4. Formatton and nature of the tnetastable species The precise nature of the species giving rise to the metastable signals cannot be determined with our present instrument. Consequently, it is not known whether or not the metastable signals are due to more than one species for the five systems studied. Nonetheless, It is known that for al! five systems, metastable production can be drectly attributed to the presence 79

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PHYSICS

of SO?. Moreover, for the five systems stuled, it IS highly improbable that the metastable signal could be due to the detection of an 101-1whch IS formed by either (1) or (II). (I) By pre-ionization of a metastable in the vlcmtty of the Daly “doorknob”. Even if this were true, the oniy effect would be to decrease our estimate for the lower limit of the metastable hfetune from approximately7>35mstor>1.3ms. (in) By high-energy (< 30 keV) electron-impact lonizatron of background gas (== 10m6 Torr) by photoelectrons generated by the phosphorescence (Jw > 4 eV. h < 3 10 nm) from metastable molecules in the vlcmlty of the Daly doorhnob. No loruzation of background gas was noted when either the fluorescent room lights (360 nm < h < 700 nm) or an mcandescent lamp (h > 380 nm) were shone into the vacuum chamber contammg the Daly knob at -30 kV. Among several alternative explanations, it IS improbable, on the basts of energetics and kmetlcs conaderatlons. that the source of the metastable signals IS an excited metastable state of sulfur-contaming specles such as S. SO. (SO)?. or $0. Presumably. m the case of the active N , -SO, system, such sulfur-contairung species might be formed (m the absence of reactants such as 02 and OS) [7,8 J If SO? were to dissociate [D(OS-0) Z= 5.6 eV] subsequent to Its colhslonally-mduced excitation (cf. ref. [5 J with reference to collisronal dlssociatlon of CO, by energy transfer from NG to CO,). However, It seems most probable that the net result m each system studied IS the formation of an excited SO,, SO; The discussion that follows examines the possible IdentIty of such an SO;. Although the Daly detector system IS effecLveIy “blind” to photons emitted by phosphorescent species. the metastable signals could be due to the detection by direct Auger de-excltatlon of a long-lived phosphorescent species ‘. One obvious can&date IS SOZ(X 3B,) (here designated 3S0,). the lowest-lying triplet state of SOa, whose colhslon-free hfetune is * The detetion

of elated neutral metastables by duect Auger deexatauon of an exated molecule [9]. which results m the eJection of secondary electrons from metti surfaces. has been well documented but LSnot well understood [9-l 11 Whtie the Daly detector can dlscnmmate between photons and eons. It cannot dncnmmate between eons and thzse exated neutral metastables whose elatatlon energy, E , IS at least 4 eV, the work function of Al

so

LJXTERS

15 May 1980

now beheved to be 8.1 f 2.5 ms [12]. However, the observed metastable signals cannot be attnbuted to 3S02 for the following reasons: (I) The excitation energy of 3SO? (-3 3 eV above ground-state SO?) 1s msufficient for-it to be detected m our system by direct Auger de-excltatlon smce the work function of the aluminized Daly doorknob is =4 eV. (u) Smce 3S0, 1s efficiently quenched by surfaces [ 13 1, most of the 3SOZ would probably not survive either Its passage through the frustum-shaped, contanunated glass samplmg orifice of the stopped-flow reactor or its collision with the rear vacuum-chamber wall. ?lus IS supported by the observation that the long-lived, highly excited N? metastables of active mtrogen could not be detected. (III) Under our experimental conltlons, 3S02 would be strongly quenched in the reactor by the olefins, 03, SO,, O,, and N,, which are extremely efficient quenchers of 3SOZ [ 141. Another possible ekplanatron IS that we are formmg some metastable state of the normal (ORsXO), cychc (O&O), or superoxlde (SHoLO) Eomers of SO2 accessible only by colhslonal excitation. For example, it has been demonstrated, by using electron energyloss spectroscopy under large momentum-transfer conditions wluch enhance the excitation of optically forbldden states, that there exist one or more optically forbldden excited states of the Isoelectronic ozone molecule which are stable against drssocratron [ 151. Also, the shape of the electron-Impact excitation spectrum of SO, [9] provides support for the existence of more than one SO? metastable, one of which is SO@ 3B,) (observed under isolated-molecule con&tlons). In addition, Vlkesland and Stnckler [ 161 deduced that the (Z3BI)-(XIA1) vlbronic transition moment m SO, decreases rapidly as the O-S-O bond angle decreases. Hence, one may argue that a transltion between the ground state and an excited, highly bent (or possibly cyclic) state of SO, would probably be strongly forbidden. Therefore, If colhslonally accessed, such states would etibit metastabibty. The metastable “thermal isomer” of ground-state SO, can be ruled out on the basrs of energetics l [8]. The ldentlfication of an excited, metastable state of SO, could have important photochemical conse* For footnote see next page

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quences [ 12,19,20]. The discovery of dioxirane (H2CZ) [21,22], the ring isomer of dioxymethylene (the &radical H,COO -) which is isoelectronic wrth ozone and SO,, lends credence to the possrble existence of the analogous cyclic and superoxlde isomers of SO,, and provrdes support for the ab untie calculations of Harding and Goddard [23] who recently estimated that droxirane lies 1.56 eV beCow dioxymethylene, the pnmary product in the reaction of ozone wrth terminal alkenes [21,22 ] . Moreover, recent ab mitro calculattons mdtcate that the cyclic and superoxlde isomers of SO2 he 4.12 eV and 4.29 eV, respectively, above the normal ground state [24], whrle cyclic 0, hes = 1.2 eV above the symmetric bent ground state (O-O-O angle of 116.8O) [25-27 1. Hence, it IS quite hkely that cychc or superoxrde SO, may also exist, and that the metastable SO; observed m the active nitrogen-SO2 system (and presumably also m the 03-thrirane and 03-olefin-SO, systems) may be an excited state of normal, cyclic, or superoxide SO,. Whereas one can really ratlonahze the formatron of SO; by energy transfer in the active mtrogen-SO2 system, stnce there are several plausible metastable nitrogen species with E* > 4 eV, one cannot as readrly rationalize such energy transfer in the other four systems studied, smce their reactron mechamsms have not been fully characterized,, and the energy transfer agents are unknown. Nonetheless, one may reasonably assume that the Criegee mtermedrates (the pnmary dn-adrcal products, RCHOO -, of ozone-olefii reactions) are probably the energy transfer agents [28]. Then one can estrmate for the ozone-olefin-SO2 systems and for the 0,-C! reaction system, smce the latter effectively becomes an 03-C2H4-SO2 systern [ 11, that the available excrtatron energy is at * Bottenheim and Calvert [8] recently provided a cntrque on the nature of the very long-hved transrent whrch has been observed m the gas-phase flash photolysrs (2500-3500 A) of SOa [8,17 1. They suggested, among several possrbltres, that the transient could conceivably be the hrghly bent “thermal roomer” of SOa [O-S-O bond angle of about 72” (cf. 119” for ground state), somewhat extended S-O bond distance] postulated by Hayes and Pferffer [18] nhe eustence of such a metastable could make rt an important intermediate m photochemrcal reactions of SO2 117 j, and nught profoundly affect the equihbnum composrtron of flame gases containmg sulfur [7].

15 May I980

LETTERS

least 1.9 eV (182.0

kJ mol-I)

-+ H#OO-

0, + C2H4 + [Cm] AIf= -182.0

[23]

k.J moF1

f H,CO,

,

01

which is not exoergic enough to account for SO; formation. However, recent observations of aerosol formation m ozone-olefinS02 systems suggest that while H,O catalyzes the rearrangement of the Criegee interim+ diate vra the formation of some complex between the diradrcal and H20, SO, presumably reacts with it (see ref. [28 ] and references therein): RCHOO - + Hz0 + RC=O

•t-H,O,

‘OH LUI = -615.0 RCHOO - + SO1

kJ mol-’ 4

AH =Z-410.0

,

G)

RCHO f SO,, kJ mol-l

_

(3)

(Reaction (2) presumably rationalizes the marked inhrbitton by HZ0 of the SO2 oxidation [reaction (3)l observed in the 0, -olefm-SO, system [28 ] .) However, It is quite possrble that SO? may also catalyze the rearrangement vra intermediate complex formation with the diradrcal or with its isomer, R

methylenebrs(oxy),

H

RCHOO

O-

‘/c(

_ 0.

+S02-


--cRC~-

OH + SO2

.

(4)

AH=--615OWmoc’

thereby providing a very ex,oergic minor pathway for SO; formation in the O3-C2bS and 03-olefm-SO2 systems [reactions (I) and (4) could provide as much as 8.3 eV of excitation energy] _

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Reacttons (2) and (4) may also ratronahze the observatron m the 03-C? reactlon that the metastable spike occurs as the prrmary products C,H, and SO, approach tlieir maximum concentration [ 11. SOS production vra reaction (4) would probably become expenmentally evrdent only when the rate of production of HCHOO - vra reaction (1) becomes significant. And. as evrdenced by the mduction penod observed for CO, and H,CO [I], and Hz0 (see Jig. 2a), the rate of formatton of these three products - and consequently also that of H3cOO - [3] - [vra the secondary 03-C,H, reaction (I)] does not become srgmfrcant until the pnmary products SO, and C2H_t approach theu mavtmum concentratron. Furthermore, as the secondary 03-C3H, reactton proceeds m time, [H,O] increases, and the dominant reaction (2) probably begms to compete effectively with the mmor reactlon (4), thereby “quenching“ the production of metastable SO; _Thus. reactions (2) and (4) may partly account for the temporal dependence of the metastable spike observed m the 03-Cm reactron. Thts dtscussron has necessardy been speculative, and it IS hoped that our observattons wrJJ stimulate addtttonal experimental studies mto the nature of the SO2 metastables produced in the systems described herem.

Acknowledgement Thts work was supported by the Office of Au and Water Measurement, Natronal Bureau of Standards, Washmgton. DC.

References I11 R 1. Martmrz and J_T Herron, Cbem Phys. Letters 72 (1980)

74

131 R I Martmez and J-T. Herron. Intern J. Chem Kmetxs 10 (1978) 433. 131 J T Herron and R E Hue,

J. Am Chem

Sot. 99 (1977)

5530. III h1 F Golde and B A. Thrush. Rept Progr Phys 36 (1973)

1285.

LETTERS

1.5 May 1980

[5 J W.T. Ravvlins and F. Kaufman, J. Chem. Phys 64 (1976) 1128 [6] J J. Srmth and W L. Jolly, Jnorg Chem. 4 (1965) 1006 [7 J C F. Culhs and M F-R Mulcahy, Combustron Flame 18 (1972) 225 [8] J-W Bottenherm and J G Calvert, J Phys Chem 80 (1976) 782 [9J J C Hemnunger, B G Wicke and W. Klemperer. J Chem. Phys 65 (1976) 2798 [lOJ R-D Rundel and R F. Stebbrngs. in: Case studies m atomrc colhsron ohysia, Vol 2. eds. E W. McDaniel and M R C hIcDoweU (North-Holland, Amsterdam, 1972) p. 547. IllI J B Hasted, Physra of atomtc coUis~ons (Butterworths, London, 1964) J131 F. Su, J W. Bottenheim, D L Thorsell. J G. Culvert and E K Damon, Chem Phys Letters 49 (1977) 305 1131 K Otsuka and J G CaJvert. J. Am Chem. Sot. 93 (1971) 2581 t141 K.L Demerpan, J G CaJvert and D L ThorseU, Jntem. J Chem. Kmetrcs 6 (1974) 829. 1151 N. Swanson and R J. Celotta, Phys Rev. Letters 35 (1975) 783 I161 J P. Vikesland and S J StnckJer, J. Chem. Phys. 60 (1974) 660 I171 C Hellner and R A Keller. J An PoUut. Control Assoc. 22 (1972) 959 1181 E F Hayes and G V Pferffer. J. Am Chem. Sot. 90 (1968) 4773. 1191 S J. StrrckJer, J P. Vihesland and H D Boer, J Chem. Phys 60 (197-l) 664 t201 H-W. SIdebottom. CC. Badcock. J.G Calvert, G.W Remhardt. B R Rabe and E.K. Damon, J Am Chem Sot. 93 (1971) 2587. Ptl F J. Lovas and R D. Suenram, Chem. Phys. Letters 51 (1977)453 [37] R I hiartmez, R E Hure and J T. Herron, Chem Phys Letters 51 (1977) 457 [231 L.B. Hardmg and W A Goddad Ill, J. Am. Chem Sot. 100 (1978) 7180 12-t I T H. Dunnmg Jr and R C. Raffenettr, to be published. 1381 P J Hay, T H. Dunrung Jr. and W 4 Goddard 111, J. Chem Phys. 62 (1975) 3912. t761 P J Hay and T H DUnmng Jr , J Chem. Phys. 67 (1977) 2290 P7J L B Harding and WA Goddard 111. J Chem. Phys 67 (1977) 2377. J78 1 J G Calvert. F Su, J-W. Bottenherm and 0 P. Strausz. Atmos Enwon 12 (1978) 197.