- Email: [email protected]
Intersystem crossing in methylene
CHEMlCAL PHYSICS LETTER6
Volume 3, number 7
INTERSYSTEM THOMAS
W. EDER,
Department
CROSSING ROBERT
IN
W. CARR
July 1969
METHYLENE
Jr. and JOHN W. KOENST *
of Chemical Engineering, Univet-sity Minneapolis, Minnesota, US.4
of Minnesota,
Received 20 May 1969
The rate of singlet-triplet intersystem crossiug in methylene is a second-order process requiring collisional perturbation to couple with the triplet manifold so that the transition can occur. The efficiency of the process increases with increasin,= mass of the collision partner, but nevertheless is relatively slow, requiring in excess of 200 gas-kinetic collisions on the average.
There is convincing evidence that the ground electronic state of methylene is (3Zgj and that the first excited state (lA1) can be converted to the triplet ground state in the presence of species toward which it is chemically inert [l-4]. We have obtained kinetic data on the rate of singletto-triplet intersystem crossing of methylene prodticed from ketene photolysis relative to the rate of reaction of singlet methylene (lCH2) with propane. The data show that the intersystem crossing in kinetically second order, first order with respect to ‘CH2 and first order with respect to inert species. The second order kinetics are interpreted to arise from a bimolecular mechanism for intersystem crossing in methylene, wherein the singlet-triplet transition results from a collisional perturbation of the stationary siates of ICH2 in the very coarsely spaced lower portior of the vibrational manifold. Intersystem crossing in methylene can be characterized as a case of what Robinson has called the (Y limit 151. Photolyses at 2659A, 3139& 3349A and 366OA were done on mixtures of ketene (CH2CO) in propane !C3H2) and 02 at varying dilutions with CF4, N2, Xe, Ar and He at constant total pressure usin the technique recently described for obtaining f CH2 yields in systems not containing inert gas f6]. The yields of C2H4, i-C4HI6, and n-C4Hl9 were used to account for all of the methylene reacting as ‘cifi. ‘CH2 + C3H8 lCH2 -+ CH2CO
%_p - ik
and
gk C2H4 + CO.
n-C4HH-J (1) (2)
* Science Foundation Undergraduate Research ParticiPam,
The yields of butanes and ethylene decreased monotonically with increasing dilutions of inert g=, 1CH2+M23CH2tM.
(3)
Singlet methylene is also removed by 02 either by a reaction to form unknown products, or by collision-induced intersystem crossing to 3CH2, or both k lCH2 + 02 2’ unknown products. (4) We are unable to distinguish kinetically between the two possible processes involving 02, and they have been combined
A steady-state yields: 1 1 K=R&+AI
in reaction
(4).
analysis of reactions
'[h(02)+ ksp x
(1) - (4)
$pO.Q]
c
(c3~8)
+ %(CH2COj ksp
1
-1
(5)
R, is rate of formation of C2H4 and C4H16, R1 is rate of formation of lCH2 in the CH2CO primary photochemical process. Under conditions where Rl is constant, eq. (5) predicts that a plot of l/Rs versus [(C3H6) + + <&Sk& ) (CH CO)]-1 will be linear if all species composin! (Mj fi ave similar efficiencies in reaction (3). Since the amount of inert gas used in any one photolysis series ranged from about 60% of (M) to > 90% of (M), the total efficiency of (M) toward reaction (3) is very nearly equal to the efficiency of the inert gas used.
Volume 3, number i
CHEMICAL PHYSICS LETTERS
Fig. 1. Photolysis of ketene-oxygen-propane mixtures with added inert gas at 3130A and 300 mm Hg total pressure.
Photolysis series at different wavelengths, total pressures, and with different inert gases all gave straight-line plots in accordance with eq. (5). Fig. 1 is a modified type of plot in which eq. (5) was divided by l/al (determined from the intercept of the former plots) to normalize the intercepts for purposes of presentation. Values of ksk/ksp for use in eq. (5) were determined for each photolysis series, and values of kso/ks were determined in separate experiments. 7?hus values for ki/ksp were extracted from plots such as fig. 1, and for the data of fig. 1, ki/k,p = 0.05, (Xe); 0.04'7,(CF4); 0.037, (Nz); 0.025, (Ar); 0.009, (He). The interpretation of ki/k_spfor N2 is precarious since N2 is not entirely mert toward lCH2 [7], and the value for He must be regarded as an upper limit since eq. (4) is the major contributor to the slope in He experiments and the determination of ki/ksp may involve systematic errors. Nevertheless, there is a clearly distinguishable dependence of ki/ksp upon the mass of inert species used. The values of ki/ksp are independent of total pressure over the range of pressure (80 - 760 Torr) covered in the experiments, supporting the bimolecular mechanism of eq. (3). Also, there is no detectable wavelength dependence of ki/‘ksp althougn lCH2 formed at wavelengths shorter than 3660A contams increasing amounts of internal energy [8]. Since reaction of lCH2 with C-H bonds may proceed wi’th activation energies c 2 kcal/mole [9], the value of ksp may be insensitive to wavelength, or perhaps be slightly larger at short wavelengths The ki/ksp values reported here would then be either a true reflection of the relative ki values
.
JuLy 1969
in the former case, or be slightly attenuated due to a wavelength dependence of ks in the latter case. A wavelength dependence of kt cannot be determined with certainty from these experiments, although if i’_ exists, the trend must be an increase of ki with decreasing waveleneth l3ader and Generosa [2b] have used the LandauZener [lo] formula to catclllate a unimob2cuLzfr rate constant for methylene intersystem crossing near the crossing of the lowest singlet and triplet potential energy surfaces. They used the potentill energy surfaces calculated by Jordan and LonguetHiggins [ll] to calculate a rate constant which is about ten times the collision frequency at NTP for lCH2 (v = I). They suggested that the role of the inert species used in their experiments was to collisionally deactivate the initiaI!y highly-exci:ed 1CH2 from CH2N2 photolysis to low-lying vibrational levels of lCH2 where intersystem crossing could occur as a rapid, intramolecular process. The model used to derive the Landau-7ener formula is not applicable to methylene intersystem crossing since the formula was derived for transitions between a discrete state and a continuum, whereas the transition in methylene must be between two discrete states. The applicability of the theory to systems containing other than s orbit& (such as the present one) must also be questioned, as was pointed out by Bates [12]. Coulson and Zalewski have furnished further criticism of the conventional Landau-Zener formula [13]. The results of this investigation show that methylene intersystem crossing from the lower vibrational energy levels of lCH2 is a relatively slow bimolecular process rather than a rqid, intramolecular process. If the collision yield for reaction of 1CH2 with C3H6 is assumed to be 0.1, then methylene intersystem crossing will onIy occur after an average of more than 200 collisions. Moreover, the relative ki values which are reported here cannot be due to rate-limiting vibrational relaxation since the initial energy distribution in the lCH2 vibrational manifold has been probed
Volume
3. number 7
CHEMICAL
PHYSICS LETTEPS
perturbation which is a rather inefficient process with the chemical reactivity of kH2. Work is in progress to further elucidate the methylene intersystem crossing, and complete details will be reported later.
RE FERSNCES [l] G. Herzberg. Proc. Roy. Sot. (London) A262 (1361) 231. [2] a. F. A. L. Anet. R. F. W. Bader and A. ILL Van der Auwera. J. Am. Chem. Sot. 82 (1960) 3217; b. R F. W. Bader and J. I_ Geccrosa.
Xan. J. Chem.
43 (1965) 1631. [a] KM_ prey. .J_em_ them_ Sot_ a9 (1960) 5947_ [4] D. F. Ring and B.S. Rabinovitch, Can. J. Chem. 46 (1968) 2~35; J. Phys. Chem.
522
72 (1368) 191.
July 1363
J. Chem. Phys. 47 (1367) 1967. r51 G. W.Robinson. I6] T.?I. Eder and R.W. Carr Jr., J. Phys. Chem.. to be published. 173 A. k. Shilov, A. A. Shteinman and M. B. Tjabtn. Tetrahedron Letters (1368) 4177. 161 M. G. Topor and R. k. C&r Jr., Paper presented at 157th National A.C.S. meeting, Minneapolis (1963). Progr. Phys.Org. Chem. 2 (1364) 1. PI j.A.Bell, 1101L. D. Landau and E. M. Lifshitz, Quantum mechan-
WI [I21 Cl31
ics. non-relativistic theory (Addison-Wesley, Readkg, Mass.. 1358). P. C. H. Jordan. and H. C. Longuet-Higgins. Mol. Phys. 5 (1962) 121. D. R. Bates. Proc. Roy_ Sot_ (London) A257 (1966)
22. C_ A_ Coulson rrnd I-L Zdewslii. (London) A268 (1962) 437.
Proc-
Roy-
Sot-
Volume 3, number 7
INTERSYSTEM THOMAS
W. EDER,
Department
CROSSING ROBERT
IN
W. CARR
July 1969
METHYLENE
Jr. and JOHN W. KOENST *
of Chemical Engineering, Univet-sity Minneapolis, Minnesota, US.4
of Minnesota,
Received 20 May 1969
The rate of singlet-triplet intersystem crossiug in methylene is a second-order process requiring collisional perturbation to couple with the triplet manifold so that the transition can occur. The efficiency of the process increases with increasin,= mass of the collision partner, but nevertheless is relatively slow, requiring in excess of 200 gas-kinetic collisions on the average.
There is convincing evidence that the ground electronic state of methylene is (3Zgj and that the first excited state (lA1) can be converted to the triplet ground state in the presence of species toward which it is chemically inert [l-4]. We have obtained kinetic data on the rate of singletto-triplet intersystem crossing of methylene prodticed from ketene photolysis relative to the rate of reaction of singlet methylene (lCH2) with propane. The data show that the intersystem crossing in kinetically second order, first order with respect to ‘CH2 and first order with respect to inert species. The second order kinetics are interpreted to arise from a bimolecular mechanism for intersystem crossing in methylene, wherein the singlet-triplet transition results from a collisional perturbation of the stationary siates of ICH2 in the very coarsely spaced lower portior of the vibrational manifold. Intersystem crossing in methylene can be characterized as a case of what Robinson has called the (Y limit 151. Photolyses at 2659A, 3139& 3349A and 366OA were done on mixtures of ketene (CH2CO) in propane !C3H2) and 02 at varying dilutions with CF4, N2, Xe, Ar and He at constant total pressure usin the technique recently described for obtaining f CH2 yields in systems not containing inert gas f6]. The yields of C2H4, i-C4HI6, and n-C4Hl9 were used to account for all of the methylene reacting as ‘cifi. ‘CH2 + C3H8 lCH2 -+ CH2CO
%_p - ik
and
gk C2H4 + CO.
n-C4HH-J (1) (2)
* Science Foundation Undergraduate Research ParticiPam,
The yields of butanes and ethylene decreased monotonically with increasing dilutions of inert g=, 1CH2+M23CH2tM.
(3)
Singlet methylene is also removed by 02 either by a reaction to form unknown products, or by collision-induced intersystem crossing to 3CH2, or both k lCH2 + 02 2’ unknown products. (4) We are unable to distinguish kinetically between the two possible processes involving 02, and they have been combined
A steady-state yields: 1 1 K=R&+AI
in reaction
(4).
analysis of reactions
'[h(02)+ ksp x
(1) - (4)
$pO.Q]
c
(c3~8)
+ %(CH2COj ksp
1
-1
(5)
R, is rate of formation of C2H4 and C4H16, R1 is rate of formation of lCH2 in the CH2CO primary photochemical process. Under conditions where Rl is constant, eq. (5) predicts that a plot of l/Rs versus [(C3H6) + + <&Sk& ) (CH CO)]-1 will be linear if all species composin! (Mj fi ave similar efficiencies in reaction (3). Since the amount of inert gas used in any one photolysis series ranged from about 60% of (M) to > 90% of (M), the total efficiency of (M) toward reaction (3) is very nearly equal to the efficiency of the inert gas used.
Volume 3, number i
CHEMICAL PHYSICS LETTERS
Fig. 1. Photolysis of ketene-oxygen-propane mixtures with added inert gas at 3130A and 300 mm Hg total pressure.
Photolysis series at different wavelengths, total pressures, and with different inert gases all gave straight-line plots in accordance with eq. (5). Fig. 1 is a modified type of plot in which eq. (5) was divided by l/al (determined from the intercept of the former plots) to normalize the intercepts for purposes of presentation. Values of ksk/ksp for use in eq. (5) were determined for each photolysis series, and values of kso/ks were determined in separate experiments. 7?hus values for ki/ksp were extracted from plots such as fig. 1, and for the data of fig. 1, ki/k,p = 0.05, (Xe); 0.04'7,(CF4); 0.037, (Nz); 0.025, (Ar); 0.009, (He). The interpretation of ki/k_spfor N2 is precarious since N2 is not entirely mert toward lCH2 [7], and the value for He must be regarded as an upper limit since eq. (4) is the major contributor to the slope in He experiments and the determination of ki/ksp may involve systematic errors. Nevertheless, there is a clearly distinguishable dependence of ki/ksp upon the mass of inert species used. The values of ki/ksp are independent of total pressure over the range of pressure (80 - 760 Torr) covered in the experiments, supporting the bimolecular mechanism of eq. (3). Also, there is no detectable wavelength dependence of ki/‘ksp althougn lCH2 formed at wavelengths shorter than 3660A contams increasing amounts of internal energy [8]. Since reaction of lCH2 with C-H bonds may proceed wi’th activation energies c 2 kcal/mole [9], the value of ksp may be insensitive to wavelength, or perhaps be slightly larger at short wavelengths The ki/ksp values reported here would then be either a true reflection of the relative ki values
.
JuLy 1969
in the former case, or be slightly attenuated due to a wavelength dependence of ks in the latter case. A wavelength dependence of kt cannot be determined with certainty from these experiments, although if i’_ exists, the trend must be an increase of ki with decreasing waveleneth l3ader and Generosa [2b] have used the LandauZener [lo] formula to catclllate a unimob2cuLzfr rate constant for methylene intersystem crossing near the crossing of the lowest singlet and triplet potential energy surfaces. They used the potentill energy surfaces calculated by Jordan and LonguetHiggins [ll] to calculate a rate constant which is about ten times the collision frequency at NTP for lCH2 (v = I). They suggested that the role of the inert species used in their experiments was to collisionally deactivate the initiaI!y highly-exci:ed 1CH2 from CH2N2 photolysis to low-lying vibrational levels of lCH2 where intersystem crossing could occur as a rapid, intramolecular process. The model used to derive the Landau-7ener formula is not applicable to methylene intersystem crossing since the formula was derived for transitions between a discrete state and a continuum, whereas the transition in methylene must be between two discrete states. The applicability of the theory to systems containing other than s orbit& (such as the present one) must also be questioned, as was pointed out by Bates [12]. Coulson and Zalewski have furnished further criticism of the conventional Landau-Zener formula [13]. The results of this investigation show that methylene intersystem crossing from the lower vibrational energy levels of lCH2 is a relatively slow bimolecular process rather than a rqid, intramolecular process. If the collision yield for reaction of 1CH2 with C3H6 is assumed to be 0.1, then methylene intersystem crossing will onIy occur after an average of more than 200 collisions. Moreover, the relative ki values which are reported here cannot be due to rate-limiting vibrational relaxation since the initial energy distribution in the lCH2 vibrational manifold has been probed
Volume
3. number 7
CHEMICAL
PHYSICS LETTEPS
perturbation which is a rather inefficient process with the chemical reactivity of kH2. Work is in progress to further elucidate the methylene intersystem crossing, and complete details will be reported later.
RE FERSNCES [l] G. Herzberg. Proc. Roy. Sot. (London) A262 (1361) 231. [2] a. F. A. L. Anet. R. F. W. Bader and A. ILL Van der Auwera. J. Am. Chem. Sot. 82 (1960) 3217; b. R F. W. Bader and J. I_ Geccrosa.
Xan. J. Chem.
43 (1965) 1631. [a] KM_ prey. .J_em_ them_ Sot_ a9 (1960) 5947_ [4] D. F. Ring and B.S. Rabinovitch, Can. J. Chem. 46 (1968) 2~35; J. Phys. Chem.
522
72 (1368) 191.
July 1363
J. Chem. Phys. 47 (1367) 1967. r51 G. W.Robinson. I6] T.?I. Eder and R.W. Carr Jr., J. Phys. Chem.. to be published. 173 A. k. Shilov, A. A. Shteinman and M. B. Tjabtn. Tetrahedron Letters (1368) 4177. 161 M. G. Topor and R. k. C&r Jr., Paper presented at 157th National A.C.S. meeting, Minneapolis (1963). Progr. Phys.Org. Chem. 2 (1364) 1. PI j.A.Bell, 1101L. D. Landau and E. M. Lifshitz, Quantum mechan-
WI [I21 Cl31
ics. non-relativistic theory (Addison-Wesley, Readkg, Mass.. 1358). P. C. H. Jordan. and H. C. Longuet-Higgins. Mol. Phys. 5 (1962) 121. D. R. Bates. Proc. Roy_ Sot_ (London) A257 (1966)
22. C_ A_ Coulson rrnd I-L Zdewslii. (London) A268 (1962) 437.
Proc-
Roy-
Sot-