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Kinetic study of the reactions Sn+N2O→SnO+N2
Volume 42,
CHEMICAL PHYSICS LETTERS
number 7
KINETIC STUDY OF THE REACTION J.R. WIESENFELD
L September L976
Sn + N,O + Sn0 f N,
and M.J. YUEN
Department of Chemistry, cOrnel University. Ithacn.New York 14853, USA Received 7
June
1976
The rate constant for the rcaction SII(~P,,) + N,O(X ‘Zi) -+ SnO(?) + N,(X ‘Cg) has ‘Jeen dctermined using the techniques of flash photolysis and time-resolved atomic absorption analysis. Over the temperature range 341-377 K, thïs rate + 2OO)jRTJ cm3 molecule-’ s-l. The role of adiabatic constant may be expressed by kT= (5.0 t 1.0) X 10-13 expj_(4000 and diabatic spin and orbital symmetry corrdation in thc reaction may be invoked to explain the observed energy. The smal1 magnïtude of the rate constant implies that maintenance of a population inversion on the ëlectronic transition from excited Sn0 would be diffîcult.
1. Introduction
tion
Recent attempts to develop a chemical laser operating in the visible region of the spectrum have ïnvolved consideration of the reactions of Group IV atoms with various oxidizers, the overall process being
Sn(5 3Po) f N2(X ‘ZZ’) + SnO(a’ 311) i N,(X ‘Zz) (2a) AH = -24.9
kcal/mole
+ SnO(X ‘2’) f N2(X ‘Zi)
M(n 3Po) f N,O(X ?Z+) 4 M0(31-í, 3Z+) f N2(X ’ $-) .
(2b)
(1) As the ground states of the Group IV monoxides
are lx+ [l] , this reaction would be very attractive as a potential candidate for chemically producing a population inversion on an electronic transition, namely M0c311 3Z”) + MO(X lZ+) . A number of experimental observations of emission from electronïcally excited MO following (1) have been reported [2,3] for the cases where M = Si, Ge and Sn. These investigations suggested that a variety of electronically excited states were populated with varying yields, but that the overall yield of excited MO* molecules was very high (up to 50% in the case [3] of SnO*). In general, these experiments were carried out in high temperature fast flow reactors designed for spectroscopic measurements, so reIativeIy little is known about the kinetic parameters associated with these reactions. One measurement carried out at 1000 K suggests that the rate constant for reaction (1) is 5 % 10-13 cm3 molecule-l s-1 for M = Sn [3] _ Such a low rate constant for a highly exothermic reac-
AH = may, at first, seem somewhat sïmilar reaction 0(2
-84.5
kczl/mole
surprising, but the
‘P/) + N,O(X ‘Z*) + N2(X ‘2;)
f 0,(X
3Z;) (3)
AH = -79.2
kcal/mole
has also been found to be very slow, possessing an activation energy of ca. 25 kcal [4] in spite of being adiabatic with respect to conservation of spin and orbit symmetry 151. Such slow processes must be compared to fast reactions such as Ba(6 ‘So) f N,O(X l,+)
+ BaO(X ‘Z+) f N2(X lx;) AH= -93.6
kcaljmole
,
appear to proceed very rapidly even near room temperature 161. Ln the experiments reported below, the rate constant for the overall removal of Sn(3Po) by reaction with N20 was deP.ermiaed ín the temperature range which
293
V&më 12, number 2
CHEMICAL PHYSICS LETTERS
340-377 K in order to obtain the kinetic parameters for this process. The results are considered in terms of simple symmetry arguments; the implications for development of a chemica! laser based cn such reactions wil! also be discussed.
2. Experimental 7’he apparatus used in these experiments permitted the photolytic generation of gas phase tin atoms in a temperature-controiled reaction vessel. The photolytic source was a 400 J discharge tiougb a quartz flash lamp fii!ed with 30 torr of Kr. Mounted collinearly with the lamp was a quartz reaction vessel (I = 3c) cm, d = 3.8 cm) which was fìtted (using Techkits E-7 epoxy) with two optica1 quality quartz windows. The vessel and lamp were mounted in an insuhrted box containing two 250 W heaters. These were switched using a simple controller which maintaìned the temperature in the box constant to t0.S K. The actual temperature of the premixed gases inside the reaction vesse! was monitored through the use of a thermocouple and potentiometer. Following admission of the gases to t!re previously evacuated vesse!, temperature equilibration required ca_ 2 minutes and was reproducible to 9.1 K. 7he photolytic pulse dissociated the source gas, Sn(CII&, producing Sn(S 3Pc). The pressure of Sn(CH3)1 was typically (1.04.0) X 10-4 torr. The presence of a great excess (105 - 106 X) of unphotolyzed gases, N,O and Ar, at a tota! pressure of 30 torr prevented adiabatic heating of the reaction mixture. The Sn(5 3P0) was detected by monitoring the time-resolved attenuation of the atomic resonance line, SIS + SP’(~P,,), at 286.33 run. The source of this rad$tion was an electrodeless discharge lamp (Fkll, Ltd.) operating at an incident power of 10-15 W. The lamp was heated in order to enhance its output intensity [7]. After passing through the reaction vessel and a set of baffles to minimize interference by scattered light from the flashlamp, the resonance radiation was dispersed by a 0.5 m monochromator and monitored with a lP28 photomultiplier. The output of this device was displayed on an oscilloscope screen and photographed for subsequent analysis. The transmission signal obtained in this fashion was related to the concentration of 294
1
September 1976
Sn(5 3Pt-,) in the reaction vessel by the modified r-ambert expression !n(10/1r) = E CllSn(5
3P&P
,
Beer-
(9
which has been discussed previously [S] . The value of y for the 286 run transition was determined to be 0.60 +-0.05, over the range of transmission observcd in these investigations. Purification of Sn(CH3)4 (Ventron) was accomplished by repeated in vacuo distillation from room temperature to liquid nìtrogen. Argon (Airco Grade 6) and nitrous oxide (Airco Grade 4.5) were used as supplied. In prelimïnary experiments carried out using medical grade N,O puritìed by repeated (as many as 8) distillations from pentane slush to liquid nitrogen, it was found that trace impurities, probably 0,, played a very significant role in contributing to the overall removal rate of Sn(5 3Pu) Via Sn(5 3P,) t 0,(X
3L$
-3 SnO(’ 2’) + 0(2 3Po) . (4)
As the rate constants for (2) measured in these experiments fa!! in the region of ca. !O-15 cm3 molecule-’ s-1 and k4 = 3.5X 10M1lcm3 molecule-1 s-1, it is clear that the vahres of k, reported here may represent upper limits for this reaction. It should, however, be noted that much lower activation energies were obtained in experiments carried out with relatively impure N,O, suggesting that the kinetic parameters obtained in these investigations are, in fact, associated with reaction (2) and not (4).
3. Results and discussion Representative oscilloscope traces demonstrating the type of data collected in these experiments have been presented previously [9]. When the data is reduced through the use of(i), the reaction of Sn(5 3P0) with a large excess (ca. 10 6 : 1) of N,O is seen to be pseudo-first order with respect to the concentration of tin atoms (fig. 1). The sensible linearity of the plots of In !n(lO/lt) versus time suggests that any deactivation of higher lying metastable states of the atoms, such as Sn(S 3PZ , or 5 lD2), does not affect the concentration of the 5 3P0 state. Thin, is due both to a relatively smal1 yield of the excited states in the photodissociative process and relatively fast deactivation of these excited species by N,O and undissociated Sn(CH& Cl01 -
-05
[
0
H -fO
4
s -1.5
September 1976_
Tab& 1 Bimolecukir rate constants for reaction of tin atoms witb N,O
0
í2 .
1
CHEMiCIAt PHYSICS l+EITERS
Volume 42. number 2
l
-20 0
3 Time
io.9 tarc 4
341.0 351.0 362.3 367.3 372.0 377.3
(1.4 1.6 1.9 2.I 2.2 2.5
usual farm of an iisrhenius
\ i 2
kT(cm3
molecule-’
s-l j
c_0.1 j x 10-zs * 0.1 f 0.1 % 0.1 * 0.1 + 0.2
‘0 oton
2 3 iorr
1 l
T(K)
t 5
J 6
(msec)
Fig. 1. Semîiogarithmic plot of absorbance at 286 nm followkg puked photolysis of Sn(CH& in presence of various ~6;ss3=;” of N2 0. Ptotaf (~5th Ar) = 40 torr. Temperature:
‘Ihe sïopes of the pseudo-fïrst order plots, -yk’, are obtained by a lìnear least-squares fit. The true bimoiecular rate constant at a given temperature is then computed from a plot of k’ versus [N2O] where k’ = kszo [N20] + KT, where KT represents the rate constant for reaction of Sn(5 3Po) with Sn(CH3)4 at the temperature of the gas(K3uu = 1.t x 10-1’ cm3 molecute-l s-l). Such plots are presented in fig. 2 for al1 of the data taken in this study (table_1). The values of k&o may now be tast (fig. 3) in &he
kTNaO = A exp(-&#T)
expression,
.
From the present data, values ofA = (5.0 2 1.0) X 10-’ 3 and Ea = 4.0 1+0.2 kcal/mole are derived. The value of thc: rate constant at 1000 K calculated from these kínetic parameters (and corrected for the differente between collision rates at 1000 and 350 K) is k”
= (8.5 kO.3) X 10WL4 cm3 moleculeU
s-r
.
‘Ris is somewhat smaller than that reported in the high Eemperature flow experiments f3], k = 5 X 10-13 (with an estimated rms error of ?O%), but the agreement is not unreasonable considering the different techniques and the length of the extrapolation. The reaction of Sn with N20 is thus clearly seen to be quite slow. The possibihty of termolecular processes was eliminated by carrying out a series of runs at 355 K and ptoti = 80 torr; the rate constant, k&O, obtained was identical, within experimental error, to that measured at the lower total pressure of 40 torr. Thïs slow reaction rate may be compared to the similar process (3) which has been shown 141
3.Or
1.01
2.6
Fig. 2. Pseudo-first order rate constants of Snf3Poj remoti as a function of N,O densïty. Temperature: v 341 .Q K, o 351 .O K, * 362.3 K, A 367.3 K, 0 372.0 K, = 377.2 K.
,
1
*
2.7
1
8
2.8
1
2.9
t
1
30
íO3 x I /3- (“K-‘1 Fig. 3. Arrhenius plot of k& 377 K.
over temperature range 34l-
295
1 September 1976
in shock tube experiments to have an activation energy of 25 km!/mole. The similarity in electroriic configuration and chemical reactivity between the reactions of Sn(S 3Po) and 8(2 3PJ) is striking and may be interpreted in terms of adiabatic correlation rules and diagrams fS,I IJ _ The latter have previously been
,2
IA’+
‘AM
published for (3) and the parallel reaction to form tbvo NO mokcules fl 11. Process (33) is both thermo-
chemically
accessible and adiabatic in terms of the of spin and orbital angular momenturn. The CorreIation diagram (in IJ coupling) for reaction (2) is presented in fig. 4~3,Only the lower lying states of SnO are included here 111 as the states at higher energies have not yet been identified, It may be seen from thîs diagr?m, in which the connecting Eines correlate reactants and productc whose electronic states have spin and symmetry species in common with states of some arbitrary Intermediate complex [51 fiere a C, planar mofecule), tbi-it the reaction of Sn(S 3Po) with N,O is expected to yield eiectronically axcited tripiet states, Sn0(3ZI!C, 311), via adiabatically allowed BA’ and 3A” parhways. Tbe observation of high chemihlminescence yields does indeed suggest that such adìabatic processes represent a significant product charme1 for the reaction of SnfS 3P0) with $0. Xlowever, the presence of a 4 kcal activation energy implies that the avaiiability of exoergic adiabatic routes is not sufficient to resuit in rapid reactíon. In order to better understand the details of(Z), it is necessary to consider the shape of tbe po~en~ia~ hypersurfa~e upon which it takes place. The initial encounter of Sn atoms with N,O must take place on a hypersurface which correlates with atomic oxygen in the 2 ‘D, state as it is from this state that ground state N,O(X IF) arises. The state of Sn0 which would be formed from Sn[S 3Po) f O(2 lDz) iu not the electronic ground state nor, on the basis of the non-crossing rufe, wauld ít be one of the fower excited states. &Qther, as is shown schematically in fig. 4b, the initial diabatic correlation must be with a relatively highIy excited stare of SnO, labelled SnO<*). The abscissa of fig. 4b represents the reaction coordinate of reaction (2). The hypersurface [really several surfaces correlating with the various low-lying Sn0 tripIet states and Sn(S 3Pe) + N,O(X lx+)] undergoes an ‘avoided” crossing with that emanating from the reactants Sn(5 3Po> + Nz0(3Zb), the ìowest conservation
296
Fig_4. Correlation diagrams for thc reaction Sn + N,O - Sn0 + N,. RusseU-Saunders coupliig and an intermediate complex of Cs symmetry are assumed. (a) Adìabatic correlations. &b) Schematic representation of reaction wordinate illustrating effects of diabatic conclations anä avoided crossìngs. Many states have been omitted in order to demonstrate the energetics of tbe low-lying surfaces. The ïntroduction of more states wil1 dramatinliy alter thc appearance of the surf;ices at higher energie% SnO(*) represcnts an Sn0 molecule correlating With Sn{5 3Po) f O(2 II),). SnOf**) correlates with Sn15 ‘D,):
f O(2 ‘D,l.
excited state of N,O which arises + a2 3P~). The crossing following along an endoergic pathway Ieads an activation energy for the Sn +
from N2(X ‘El) the initial approach to the prediction of N20 resction- Indeed,
Volume 42, number 2
CliiEhttCAt
PUYSK!S
Table 2 Quenching af Sn(S 2P2,r ) by N,O attd CO,
{cm3 mdecule-r
k~
s-r)
Q
3p,
3%
co, 4
(3.2 ir 0.2) x Lo-‘3
N,O w
(i-1
(6.2 + 0.6) X 1O-‘2 (3.5 i 0.7) x 10‘”
a) Ref. [X2].
F 0.1)
x x0-12
b) This work.
a similar prediction might be made for the Sn(5 *Dz) + NzO reaction which has also been reported to be relatively slow (k= 5.0X 1W12 cm3 molecule-1 s-I). The detailed consideration of the potential hypersurface could, of course, take into account different symmetries of the intermediate complexes as well as the rob of spin-orbit coupling in facilitating non-adiabatic transitions between surfaces. The basic conclusion drawn above, i.e., that a barrier exists in the adiabatic channels leading to products, does not change. It should be further noted that simiIar arguments may be invoked to explain the large activation energy characterizing reaction (‘3) Finally, the overall deactivation rate coefficients for removal of Sn(S 3P2) and Sn(5 3P,) by collisions with N,O(X lx+) were measured at 300 !Sasdescribed previously [ 121. The results are displayed in table 2 along with those obtained earlier I121 for deactivation by CO,. Due to the unavailability of adiabatic, energetically accessible routes for Sn(3Pz) and Sn(3P1) reaction with CC2 to form SnO f CO, deactivation of these atomic states by C07_ probably proceeds via energy transfer to the vibrational modes of the CO, molecule. As the rates of St@ 3P2) and Sn(5 3Pr) deactivation by NzO are quite similar to those characterizing deactivation by CO2, physical quenching to a lower-lying state and concomitant energy transfer to N20 is likely here. The marked difference in quenching rates may either be due to a selective excitation of the u1 (N-N stretch) mode of N,O at 2224 cm--l which is accessible to the 5 3P, state 3Po) = 3428 cm-“) but not to 5 3Pt 3Po) = 1692 cm-‘). Mternatively, the
1
I_.tTI’~~S
greater
deactivation
effciency
Septemliler1976
of SnfS sPz) may be
due to favorabIe coupiing between the J = I and J = 2 levels and the l$Of% ‘Ez). ms would suggest that the Sn(5 3P,,g) f N,O(X ‘2;) interaction occurs on an essentially repul&e hypersurface as was indicated in the analysis of the Sn(3Po) + N$J reaction dynamics. The relatively low rate for reaction @a) will hinder the development of a chemical laser pumped by this reaction. The major problem would be the maintenance of a population inversion via process (2a) in the presence of potentially efficient deactivation of electronicaliy excited SnO molecules either by physical quenching or chemical reaction.
Acknowledgement This work was supported by the General Electric &ompany and the U.S. Air Force Office of Scientific Research.
References [I] [Z] [3j [41 [S) [6 ] [?I &I [91 f IO] [ll] [12]
KC. Oldenborg, C-R. Dickson and R.N. Zare, J. Mot, Spectry. 58 (19’76) 283. G. Hager, L-E. Wtson and S.G. Hadley, Chcm. Phys. Letters 27 (1974) 439. IV. Felder and A. Font@, Chem. Phys. Letters 34 (1975) 398. ES. Fishburnc and R. Edse, J. Chem. Phys. 44 (1966) 515. K.E. Shulcr, J. Chem. Phys. 21(1953) 624. R.H. Obenauf, C.3. Hsu and H.B. Palmer, J. Chem Phys. 57 0972) 5607; 58 (1973) 2674. J.R. Wicsenfcid, Chem. Phys. Letters 21 (1973) 517, P.D. Foe, T. Lohman, 3. Podolske and J.R. Wiesenfeld, J. Phys. Chem. 79 (1975) 414, P.D. Foo, 3.R. Wiesenfeldand D. Husain, C&em.Phys. Letters 32 (1975) 443. A. Brown and D. Husain, Intern. 3. Ghem. Kinetics 7 (1975) 77. R.J. Donovan and D- Husain, Chem_ Rev. 70 (1970) 483. P-D. Foe. J.R. ~Vies~nfeid, M.J. Yuen and D. Husain, J. whys. Chem. 80 (1976) 91.
297
CHEMICAL PHYSICS LETTERS
number 7
KINETIC STUDY OF THE REACTION J.R. WIESENFELD
L September L976
Sn + N,O + Sn0 f N,
and M.J. YUEN
Department of Chemistry, cOrnel University. Ithacn.New York 14853, USA Received 7
June
1976
The rate constant for the rcaction SII(~P,,) + N,O(X ‘Zi) -+ SnO(?) + N,(X ‘Cg) has ‘Jeen dctermined using the techniques of flash photolysis and time-resolved atomic absorption analysis. Over the temperature range 341-377 K, thïs rate + 2OO)jRTJ cm3 molecule-’ s-l. The role of adiabatic constant may be expressed by kT= (5.0 t 1.0) X 10-13 expj_(4000 and diabatic spin and orbital symmetry corrdation in thc reaction may be invoked to explain the observed energy. The smal1 magnïtude of the rate constant implies that maintenance of a population inversion on the ëlectronic transition from excited Sn0 would be diffîcult.
1. Introduction
tion
Recent attempts to develop a chemical laser operating in the visible region of the spectrum have ïnvolved consideration of the reactions of Group IV atoms with various oxidizers, the overall process being
Sn(5 3Po) f N2(X ‘ZZ’) + SnO(a’ 311) i N,(X ‘Zz) (2a) AH = -24.9
kcal/mole
+ SnO(X ‘2’) f N2(X ‘Zi)
M(n 3Po) f N,O(X ?Z+) 4 M0(31-í, 3Z+) f N2(X ’ $-) .
(2b)
(1) As the ground states of the Group IV monoxides
are lx+ [l] , this reaction would be very attractive as a potential candidate for chemically producing a population inversion on an electronic transition, namely M0c311 3Z”) + MO(X lZ+) . A number of experimental observations of emission from electronïcally excited MO following (1) have been reported [2,3] for the cases where M = Si, Ge and Sn. These investigations suggested that a variety of electronically excited states were populated with varying yields, but that the overall yield of excited MO* molecules was very high (up to 50% in the case [3] of SnO*). In general, these experiments were carried out in high temperature fast flow reactors designed for spectroscopic measurements, so reIativeIy little is known about the kinetic parameters associated with these reactions. One measurement carried out at 1000 K suggests that the rate constant for reaction (1) is 5 % 10-13 cm3 molecule-l s-1 for M = Sn [3] _ Such a low rate constant for a highly exothermic reac-
AH = may, at first, seem somewhat sïmilar reaction 0(2
-84.5
kczl/mole
surprising, but the
‘P/) + N,O(X ‘Z*) + N2(X ‘2;)
f 0,(X
3Z;) (3)
AH = -79.2
kcal/mole
has also been found to be very slow, possessing an activation energy of ca. 25 kcal [4] in spite of being adiabatic with respect to conservation of spin and orbit symmetry 151. Such slow processes must be compared to fast reactions such as Ba(6 ‘So) f N,O(X l,+)
+ BaO(X ‘Z+) f N2(X lx;) AH= -93.6
kcaljmole
,
appear to proceed very rapidly even near room temperature 161. Ln the experiments reported below, the rate constant for the overall removal of Sn(3Po) by reaction with N20 was deP.ermiaed ín the temperature range which
293
V&më 12, number 2
CHEMICAL PHYSICS LETTERS
340-377 K in order to obtain the kinetic parameters for this process. The results are considered in terms of simple symmetry arguments; the implications for development of a chemica! laser based cn such reactions wil! also be discussed.
2. Experimental 7’he apparatus used in these experiments permitted the photolytic generation of gas phase tin atoms in a temperature-controiled reaction vessel. The photolytic source was a 400 J discharge tiougb a quartz flash lamp fii!ed with 30 torr of Kr. Mounted collinearly with the lamp was a quartz reaction vessel (I = 3c) cm, d = 3.8 cm) which was fìtted (using Techkits E-7 epoxy) with two optica1 quality quartz windows. The vessel and lamp were mounted in an insuhrted box containing two 250 W heaters. These were switched using a simple controller which maintaìned the temperature in the box constant to t0.S K. The actual temperature of the premixed gases inside the reaction vesse! was monitored through the use of a thermocouple and potentiometer. Following admission of the gases to t!re previously evacuated vesse!, temperature equilibration required ca_ 2 minutes and was reproducible to 9.1 K. 7he photolytic pulse dissociated the source gas, Sn(CII&, producing Sn(S 3Pc). The pressure of Sn(CH3)1 was typically (1.04.0) X 10-4 torr. The presence of a great excess (105 - 106 X) of unphotolyzed gases, N,O and Ar, at a tota! pressure of 30 torr prevented adiabatic heating of the reaction mixture. The Sn(5 3P0) was detected by monitoring the time-resolved attenuation of the atomic resonance line, SIS + SP’(~P,,), at 286.33 run. The source of this rad$tion was an electrodeless discharge lamp (Fkll, Ltd.) operating at an incident power of 10-15 W. The lamp was heated in order to enhance its output intensity [7]. After passing through the reaction vessel and a set of baffles to minimize interference by scattered light from the flashlamp, the resonance radiation was dispersed by a 0.5 m monochromator and monitored with a lP28 photomultiplier. The output of this device was displayed on an oscilloscope screen and photographed for subsequent analysis. The transmission signal obtained in this fashion was related to the concentration of 294
1
September 1976
Sn(5 3Pt-,) in the reaction vessel by the modified r-ambert expression !n(10/1r) = E CllSn(5
3P&P
,
Beer-
(9
which has been discussed previously [S] . The value of y for the 286 run transition was determined to be 0.60 +-0.05, over the range of transmission observcd in these investigations. Purification of Sn(CH3)4 (Ventron) was accomplished by repeated in vacuo distillation from room temperature to liquid nìtrogen. Argon (Airco Grade 6) and nitrous oxide (Airco Grade 4.5) were used as supplied. In prelimïnary experiments carried out using medical grade N,O puritìed by repeated (as many as 8) distillations from pentane slush to liquid nitrogen, it was found that trace impurities, probably 0,, played a very significant role in contributing to the overall removal rate of Sn(5 3Pu) Via Sn(5 3P,) t 0,(X
3L$
-3 SnO(’ 2’) + 0(2 3Po) . (4)
As the rate constants for (2) measured in these experiments fa!! in the region of ca. !O-15 cm3 molecule-’ s-1 and k4 = 3.5X 10M1lcm3 molecule-1 s-1, it is clear that the vahres of k, reported here may represent upper limits for this reaction. It should, however, be noted that much lower activation energies were obtained in experiments carried out with relatively impure N,O, suggesting that the kinetic parameters obtained in these investigations are, in fact, associated with reaction (2) and not (4).
3. Results and discussion Representative oscilloscope traces demonstrating the type of data collected in these experiments have been presented previously [9]. When the data is reduced through the use of(i), the reaction of Sn(5 3P0) with a large excess (ca. 10 6 : 1) of N,O is seen to be pseudo-first order with respect to the concentration of tin atoms (fig. 1). The sensible linearity of the plots of In !n(lO/lt) versus time suggests that any deactivation of higher lying metastable states of the atoms, such as Sn(S 3PZ , or 5 lD2), does not affect the concentration of the 5 3P0 state. Thin, is due both to a relatively smal1 yield of the excited states in the photodissociative process and relatively fast deactivation of these excited species by N,O and undissociated Sn(CH& Cl01 -
-05
[
0
H -fO
4
s -1.5
September 1976_
Tab& 1 Bimolecukir rate constants for reaction of tin atoms witb N,O
0
í2 .
1
CHEMiCIAt PHYSICS l+EITERS
Volume 42. number 2
l
-20 0
3 Time
io.9 tarc 4
341.0 351.0 362.3 367.3 372.0 377.3
(1.4 1.6 1.9 2.I 2.2 2.5
usual farm of an iisrhenius
\ i 2
kT(cm3
molecule-’
s-l j
c_0.1 j x 10-zs * 0.1 f 0.1 % 0.1 * 0.1 + 0.2
‘0 oton
2 3 iorr
1 l
T(K)
t 5
J 6
(msec)
Fig. 1. Semîiogarithmic plot of absorbance at 286 nm followkg puked photolysis of Sn(CH& in presence of various ~6;ss3=;” of N2 0. Ptotaf (~5th Ar) = 40 torr. Temperature:
‘Ihe sïopes of the pseudo-fïrst order plots, -yk’, are obtained by a lìnear least-squares fit. The true bimoiecular rate constant at a given temperature is then computed from a plot of k’ versus [N2O] where k’ = kszo [N20] + KT, where KT represents the rate constant for reaction of Sn(5 3Po) with Sn(CH3)4 at the temperature of the gas(K3uu = 1.t x 10-1’ cm3 molecute-l s-l). Such plots are presented in fig. 2 for al1 of the data taken in this study (table_1). The values of k&o may now be tast (fig. 3) in &he
kTNaO = A exp(-&#T)
expression,
.
From the present data, values ofA = (5.0 2 1.0) X 10-’ 3 and Ea = 4.0 1+0.2 kcal/mole are derived. The value of thc: rate constant at 1000 K calculated from these kínetic parameters (and corrected for the differente between collision rates at 1000 and 350 K) is k”
= (8.5 kO.3) X 10WL4 cm3 moleculeU
s-r
.
‘Ris is somewhat smaller than that reported in the high Eemperature flow experiments f3], k = 5 X 10-13 (with an estimated rms error of ?O%), but the agreement is not unreasonable considering the different techniques and the length of the extrapolation. The reaction of Sn with N20 is thus clearly seen to be quite slow. The possibihty of termolecular processes was eliminated by carrying out a series of runs at 355 K and ptoti = 80 torr; the rate constant, k&O, obtained was identical, within experimental error, to that measured at the lower total pressure of 40 torr. Thïs slow reaction rate may be compared to the similar process (3) which has been shown 141
3.Or
1.01
2.6
Fig. 2. Pseudo-first order rate constants of Snf3Poj remoti as a function of N,O densïty. Temperature: v 341 .Q K, o 351 .O K, * 362.3 K, A 367.3 K, 0 372.0 K, = 377.2 K.
,
1
*
2.7
1
8
2.8
1
2.9
t
1
30
íO3 x I /3- (“K-‘1 Fig. 3. Arrhenius plot of k& 377 K.
over temperature range 34l-
295
1 September 1976
in shock tube experiments to have an activation energy of 25 km!/mole. The similarity in electroriic configuration and chemical reactivity between the reactions of Sn(S 3Po) and 8(2 3PJ) is striking and may be interpreted in terms of adiabatic correlation rules and diagrams fS,I IJ _ The latter have previously been
,2
IA’+
‘AM
published for (3) and the parallel reaction to form tbvo NO mokcules fl 11. Process (33) is both thermo-
chemically
accessible and adiabatic in terms of the of spin and orbital angular momenturn. The CorreIation diagram (in IJ coupling) for reaction (2) is presented in fig. 4~3,Only the lower lying states of SnO are included here 111 as the states at higher energies have not yet been identified, It may be seen from thîs diagr?m, in which the connecting Eines correlate reactants and productc whose electronic states have spin and symmetry species in common with states of some arbitrary Intermediate complex [51 fiere a C, planar mofecule), tbi-it the reaction of Sn(S 3Po) with N,O is expected to yield eiectronically axcited tripiet states, Sn0(3ZI!C, 311), via adiabatically allowed BA’ and 3A” parhways. Tbe observation of high chemihlminescence yields does indeed suggest that such adìabatic processes represent a significant product charme1 for the reaction of SnfS 3P0) with $0. Xlowever, the presence of a 4 kcal activation energy implies that the avaiiability of exoergic adiabatic routes is not sufficient to resuit in rapid reactíon. In order to better understand the details of(Z), it is necessary to consider the shape of tbe po~en~ia~ hypersurfa~e upon which it takes place. The initial encounter of Sn atoms with N,O must take place on a hypersurface which correlates with atomic oxygen in the 2 ‘D, state as it is from this state that ground state N,O(X IF) arises. The state of Sn0 which would be formed from Sn[S 3Po) f O(2 lDz) iu not the electronic ground state nor, on the basis of the non-crossing rufe, wauld ít be one of the fower excited states. &Qther, as is shown schematically in fig. 4b, the initial diabatic correlation must be with a relatively highIy excited stare of SnO, labelled SnO<*). The abscissa of fig. 4b represents the reaction coordinate of reaction (2). The hypersurface [really several surfaces correlating with the various low-lying Sn0 tripIet states and Sn(S 3Pe) + N,O(X lx+)] undergoes an ‘avoided” crossing with that emanating from the reactants Sn(5 3Po> + Nz0(3Zb), the ìowest conservation
296
Fig_4. Correlation diagrams for thc reaction Sn + N,O - Sn0 + N,. RusseU-Saunders coupliig and an intermediate complex of Cs symmetry are assumed. (a) Adìabatic correlations. &b) Schematic representation of reaction wordinate illustrating effects of diabatic conclations anä avoided crossìngs. Many states have been omitted in order to demonstrate the energetics of tbe low-lying surfaces. The ïntroduction of more states wil1 dramatinliy alter thc appearance of the surf;ices at higher energie% SnO(*) represcnts an Sn0 molecule correlating With Sn{5 3Po) f O(2 II),). SnOf**) correlates with Sn15 ‘D,):
f O(2 ‘D,l.
excited state of N,O which arises + a2 3P~). The crossing following along an endoergic pathway Ieads an activation energy for the Sn +
from N2(X ‘El) the initial approach to the prediction of N20 resction- Indeed,
Volume 42, number 2
CliiEhttCAt
PUYSK!S
Table 2 Quenching af Sn(S 2P2,r ) by N,O attd CO,
{cm3 mdecule-r
k~
s-r)
Q
3p,
3%
co, 4
(3.2 ir 0.2) x Lo-‘3
N,O w
(i-1
(6.2 + 0.6) X 1O-‘2 (3.5 i 0.7) x 10‘”
a) Ref. [X2].
F 0.1)
x x0-12
b) This work.
a similar prediction might be made for the Sn(5 *Dz) + NzO reaction which has also been reported to be relatively slow (k= 5.0X 1W12 cm3 molecule-1 s-I). The detailed consideration of the potential hypersurface could, of course, take into account different symmetries of the intermediate complexes as well as the rob of spin-orbit coupling in facilitating non-adiabatic transitions between surfaces. The basic conclusion drawn above, i.e., that a barrier exists in the adiabatic channels leading to products, does not change. It should be further noted that simiIar arguments may be invoked to explain the large activation energy characterizing reaction (‘3) Finally, the overall deactivation rate coefficients for removal of Sn(S 3P2) and Sn(5 3P,) by collisions with N,O(X lx+) were measured at 300 !Sasdescribed previously [ 121. The results are displayed in table 2 along with those obtained earlier I121 for deactivation by CO,. Due to the unavailability of adiabatic, energetically accessible routes for Sn(3Pz) and Sn(3P1) reaction with CC2 to form SnO f CO, deactivation of these atomic states by C07_ probably proceeds via energy transfer to the vibrational modes of the CO, molecule. As the rates of St@ 3P2) and Sn(5 3Pr) deactivation by NzO are quite similar to those characterizing deactivation by CO2, physical quenching to a lower-lying state and concomitant energy transfer to N20 is likely here. The marked difference in quenching rates may either be due to a selective excitation of the u1 (N-N stretch) mode of N,O at 2224 cm--l which is accessible to the 5 3P, state 3Po) = 3428 cm-“) but not to 5 3Pt 3Po) = 1692 cm-‘). Mternatively, the
1
I_.tTI’~~S
greater
deactivation
effciency
Septemliler1976
of SnfS sPz) may be
due to favorabIe coupiing between the J = I and J = 2 levels and the l$Of% ‘Ez). ms would suggest that the Sn(5 3P,,g) f N,O(X ‘2;) interaction occurs on an essentially repul&e hypersurface as was indicated in the analysis of the Sn(3Po) + N$J reaction dynamics. The relatively low rate for reaction @a) will hinder the development of a chemical laser pumped by this reaction. The major problem would be the maintenance of a population inversion via process (2a) in the presence of potentially efficient deactivation of electronicaliy excited SnO molecules either by physical quenching or chemical reaction.
Acknowledgement This work was supported by the General Electric &ompany and the U.S. Air Force Office of Scientific Research.
References [I] [Z] [3j [41 [S) [6 ] [?I &I [91 f IO] [ll] [12]
KC. Oldenborg, C-R. Dickson and R.N. Zare, J. Mot, Spectry. 58 (19’76) 283. G. Hager, L-E. Wtson and S.G. Hadley, Chcm. Phys. Letters 27 (1974) 439. IV. Felder and A. Font@, Chem. Phys. Letters 34 (1975) 398. ES. Fishburnc and R. Edse, J. Chem. Phys. 44 (1966) 515. K.E. Shulcr, J. Chem. Phys. 21(1953) 624. R.H. Obenauf, C.3. Hsu and H.B. Palmer, J. Chem Phys. 57 0972) 5607; 58 (1973) 2674. J.R. Wicsenfcid, Chem. Phys. Letters 21 (1973) 517, P.D. Foe, T. Lohman, 3. Podolske and J.R. Wiesenfeld, J. Phys. Chem. 79 (1975) 414, P.D. Foo, 3.R. Wiesenfeldand D. Husain, C&em.Phys. Letters 32 (1975) 443. A. Brown and D. Husain, Intern. 3. Ghem. Kinetics 7 (1975) 77. R.J. Donovan and D- Husain, Chem_ Rev. 70 (1970) 483. P-D. Foe. J.R. ~Vies~nfeid, M.J. Yuen and D. Husain, J. whys. Chem. 80 (1976) 91.
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