J . Phys. Chem. 1990, 94, 7969-7972 The present results are found to be in fair agreement with a number of previous measurements. In general, Stern-Volmer collisional quenching rate constants are found to be strongly dependent on experimental conditions, such as bulb size, filters used, duration of observations, the wavelength sensitivity of the detector, and the excitation wavelength. These instrumental perturbations of the quantity measured are so great that most Stern-Volmer quenching rate constants in the literature are strongly affected by them, including those of this article. The present results on k, are found to be in disagreement (sometimes high and sometimes low) with a number of previous measurements, but these differences are interpreted to be caused by the sensitivity of the quenching "constants" to detailed experimental method. For near-nascent excited NOz, we have observed these quenching constants to increase with
Theoretical Study of the O('P)
7969
decreasing excitation energy (except near the 398-nm dissociation threshold); this increase is attributed mostly to the long-wavelength cutoff of the detection system. Near the dissociation limit, the apparent quenching rate increases with increasing excitation energy due to fast collisional conversion of rotational energy (present in the ground-state NO2 before excitation) to energy in the dissociation coordinate. Acknowledgment. This project was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Chemical Sciences Division of the U.S. Department of Energy under Contract No. DE-AC03-76SF00098. We thank C. E. Miller for providing NO, absorption spectra for the range 700-800 nm and for many helpful discussions. Registry No. NO2, 10102-44-0.
+ Allene Reaction
B. L. Hammond, S.-Y. Huang, W. A. Lester, Jr.,* Department of Chemistry, University of California, Berkeley, and Materials and Chemical Sciences Division, Lawrence Berkeley Laboratory, I Cyclotron Road, Berkeley, California 94720
and M. Dupuis IBM Research Laboratory, Neighborhood Road, Kingston, New York I2401 (Received: February 5, 1990)
Ab initio complete active space SCF calculations have been performed to determine the structures and energetics of the electrophilic addition of O(3P) to allene (propadiene). The long-established channel for this reaction has been 0-atom attack of the central carbon atom (CCA). However, recent experimental studies have suggested that terminal carbon attack (TCA) is a significant process. In this study the classical barrier height for the CCA channel was found to be only 1.8 kcal/mol lower than that for the TCA channel. I n addition, the triplet diradical species and the experimentally observed product of the TCA channel, allenyloxy radical, are characterized. Allenyloxy radical is predicted to have low-lying near-IR and UV transitions to electronic excited states 0.525 and 3.70 eV above the ground state. Similar transitions for the analogous vinoxy radical have previously been predicted theoretically and confirmed experimentally.
Introduction
The combustion of small unsaturated hydrocarbons plays a key role in the understanding of combustion processes of practical interest. A number of simple unsaturated hydrocarbon reactions with O(3P) have been investigated e~perimentally.'-~The motivation for the present effort is the crossed molecular beam study of the reaction of O(3P) with allene6 for reasons described below. The reaction of allene (propadiene) with O(3P) to form C O and ethylene has long been accepted' to proceed by addition of oxygen to the central carbon atom followed by a tripletsinglet transition to form vibrationally excited cyclopropanone. The vibrationally hot cyclopropanone then dissociates to form final products. The spin-forbidden triplet-singlet crossing has been explaineds by coupling the twisting of the allene A electrons to the electron spin flip to conserve angular momentum. Several experimental studies have indicated this as the dominant ~ h a n n e l . ~ * ~ J ~ ( I ) Sibener, S.J.; Buss, R. J.; Casavecchia, P.; T. Hirooka, Lee, Y . T. J . Chem. Phys. 1980, 72, 4341. (2) Hunziker, H. E.; Kneppe, H.; Wendt, H. R. J . Phofochem. 1981, 17, 317. (3) Buss, R. J.; Baseman, R. J.; He, G.; Lee, Y. T. J . Photochem. 1981, 17, 389. (4) Schmoltner, A. M.; Chu, P. M.; Lee, Y. T. J . Chem. Phys. 1989, 91, 5365; Schmoltner, A. M.; Chu, P. M.; Brudzynski, R. J.; Lee, Y. T. J . Chem. Phys. 1989, 91.6926. (5) Kleinermanns, K.; Luntz, A. C. J . Chem. Phys. 1982, 77, 3533. (6) Schmoltner, A. M.; Huang, S. Y.; Brudzynski, R.; Chu, P. M.; Lee, Y.T., private communication. (7) Havel, J . J . J . Am. Chem. SOC.1974, 96, 530. (8) Chiu, Y.-N.; Abidi, M . S. F. A. J . Phys. Chem. 1982, 86, 3288. (9) Lin, M. C.; Shortridge, R. G.; Umstead, M. E. Chem. Phys. Leu. 1976, 37, 279.
0022-3654/90/2094-7969$02.50/0
The formation of cyclopropanone, however, is not the exclusive channel; experiments have detected minor products of acrolein7 and allene oxide." Recent molecular beam experiments: as well as earlier flow tube experiments,I0 have observed final products of H 2 C C C H 0 (1-allenyloxy) and H atom, suggesting that 0 can attack a terminal C as well as the central C. A previous molecular beam experiment3 and subsequent theoretical investigation1*on the closely related O(3P) ethylene reaction have shown that the formation of the analogous radical H 2 C C H 0 (and H) is consistent with the conditions of the experiment. Of interest in the theoretical study was the characterization of the diradical intermediate as well as the final product, vinoxy radical, formed by H-atom ejection. The vinoxy radical was computed to have an electronic transition at 3.22 eV, in good agreement with experiment, and a predicted near-IR transition between 0.9 and 1.25 eV, which was subsequently observed at 0.99 eV.I3 It is the goal of this work to study the addition of O(3P) both to the central carbon (CCA channel) and to a terminal carbon (TCA channel) on the triplet surface. We shall not attempt here to account for the singlet-triplet intersystem crossing (ISC). Of particular interest are the relative barrier heights encountered in the CCA and TCA pathways. We characterize the diradical
+
(IO) Aleksandrov, E. N.; Arutyunov, V. S.; Kozlov, N. Kiner. Katal. 1980, 21, 1327. ( 1 1 ) Singmaster, K. A.; Pimentel, G.C. J . Mol. Sfruct. 1989. 194, 215. ( I 2) Dupuis, M.; Wendoloski, J. J.; Takada, T.; Lester, W. A. Jr. J . Chem. Phys. 1982, 76, 481. Dupuis, M.; Wendoloski, J . J.; Lester, W. A. Jr. J . Chem. Phys. 1982, 76, 488. ( I 3) Hunziker, H. E.; Kleinermanns, K.; Kneppe, H.; Luntz, A. C.; Wendt, H. R. Abstract to 15th International Conference on Free Fadicals (June 1981).
0 1990 American Chemical Society
7970 The Journal of Physical Chemistry, Vol. 94, No. 20, I990
Hammond et al.
7)4q-4 .,,,,“ttN~
5
HH
6 3
2
Figure 1. Schematic diagram of O(3P) + allene reaction TABLE I: Geometries and Energies along the CCA Reaction Path for the DZP Basis Set: Reactants (C3H,(A) + O(”)), Transition State (C3H,-.0(3A”)), and Diradical H2CCOCH,(3A”) CiH, + 0 C,H,***O H,CCOCH, -190.737223 -190.713058 - 1 90.809008 -45.05 0 15.16 1.496 1.371 1.324 1.454 1.331 1.324 1.246 2.022 1.076 1.073 1.079 1.076 1.075 1.079 151.1 119.2 180.0 120.6 104.7 120.0 120.9 120.2 121.3 120.9 120.5 14.0 21.7 0.0
“Bond lengths in angstroms, angles in degrees. b a is the angle between the CHH plane and the C-C bond.
intermediate of the TCA channel (which is not known to arise in the CCA reaction). Finally, the structure and energetics of the allenyloxy radical are discussed. The existence of a low-lying near-infrared electronic excited state and of a UV excited state are predicted.
Computational Method For the addition of O(3P) to allene, complete active space (CAS) S C F calculations were carried out with six active electron distributed in six orbitals. The active electrons are chosen to be all those involved in bond formation and bond breaking, corresponding initially to the two unpaired electrons on O(’P) and the four K electrons of allene. The role of electronic correlation is expected to be large in this system because of the 0 linkage to adjacent C-C double bonds.14 The basis set used for results on the CCA and TCA channels was Dunning’s double-zeta plus polarization16 (DZP). For the study of the allenyloxy radical, Dunning’s split valenceI6 (DZ) basis set was used. All structures correspond to fully optimized geometries obtained with analytic gradients. The CCA Channel Figure 1 shows a schematic diagram of the O(jP) + allene reaction. Table I lists the geometries and total energies of critical points along the reaction pathway. The favored orientation of attack is of C, symmetry with one unpaired oxygen electron in the symmetry plane, poised to form a C - O u bond, and the other unpaired electron of oxygen out of the plane with favorable interaction with the other C=C A bond. In this conformation there are three active a” electrons, the out-of-plane C=C A electrons, and three active a’ electrons, two forming the C - 0 u bond and one unpaired electron on the terminal methylene group. In ad(14) Kello, V.; Urban, M.; Noga, J.; Diercksen, G.H. F. J . Am. Chem.
Figure 2. Optimized geometries for the (a) CCA and (b) TCA intermediate diradicals. TABLE 11: Geometries and Energies along the TCA Reaction Path for the DZP Basis Set: Reactants (C3H,(A) + O(3P)), Transition State (CqH4.-O(3A”)),and Diradical H2CCCHIO(A’’) C3H, 0 C3H4-+0 H2CCCH10 energy, a u -190.737223 -190.710105 -190.747527 AE, kcal/mol 0 17.02 -6.47 R(C‘1)C‘2’)“ 1.324 1.379 1.492 R( C‘2’C(])) 1.324 1.325 1.332 R(C(’)O) 1.960 1.43I R(C(1)H(4 1.079 1.077 1.085 R(C(1)H(6)) 1.079 1.076 1.078 R( C(])H(’)) 1.079 1.077 1.075 i(cc~ )c(2)cc39 180.0 155.1 135.0 L(OC(1)C‘2’) 108.9 113.3 L(C”C(’’H(4~5’) 120.9 117.7 1 1 1.2 I ( C ( ~ ) C ( ~ ) H ( ~ ) ) 120.9 121.0 120.9 L(C(~)C(~)H(T)) 120.9 120.7 121.0 a(C(2)C(I)H(4)H(S))b 0.0 29.8 52.4
+
Bond lengths in angstroms, angles in degrees. * a is the angle between the CHH plane and the C-C bond.
dition, there is an oxygen electron pair inactive in an a’ orbital. The molecule has therefore ’Aff symmetry. As the oxygen atom approaches allene, a C-0 u bond is formed as the in-plane allene K bond is broken. At the transition state, the in-plane C=C A bond has been partially broken and the terminal CH, group has bent 14’ backward from the C-C bond. Both terminal carbons retain nearly sp2 hybridization. The dihedral angle between the terminal CH, groups is 90’. Beyond the transition state the reaction proceeds on the 3Affsurface down to a local minimum structure of 3A” 2-allenylidoxy (2-propadienylidoxy) diradical (Figure 2a). As suggested in the transition state, one of the two terminal methylene groups has an unpaired electron and is rotated 90’ with respect to the symmetry plane. Since C=O A bonds are stronger than C=C T bonds, the electron originally unpaired on the oxygen atom in the a” orbital recouples with one of the two C = C .rr bond electrons to form a C=O A bond, leaving an unpaired electron on the in-plane methylene group in an a” orbital. Thus the two C H z groups, in a perpendicular orientation, have one unpaired electron each. In this conformation, the out-of-plane terminal methylene group may undergo nearly free rotation about the C-C bond. The lowest energy structure has the two CH, groups in the plane of the molecule, and the unpaired electrons on CH, are coupled into a triplet spin and occupy orbitals of a, and b, symmetry ( C , point group), yielding a species of ’B2 symmetry. The energy of the C , conformation is 8.5 kcal/mol below the twisted C, structure, the C=O bond distance lengthens slightly to 1.264 A, while the two C-C bonds both shorten to 1.451 A and the CCO angle is 119.6’. This C = O bond length is to be compared to the 1.19-A’’ distance observed for cyclopropanone. In addition to the C, triplet structure, the rotation of the out-of-plane methylene can couple
SOC.1984, 106, 5864. ( I 5 ) Deleted in proof.
(16) Dunning, T. H. Jr.; Hay, P. J . Modern Theoretical Chemistry; Schaefer 111. H F . Ed.; Plenum: New York, 1976: Vol. 2, Chapter I .
(17) Pochan, J. M.; Baldwin, J. E.; Flygare, W. H. J . A m . Chem. SOC. 1969. 91. 1896.
The O(jP)
+ Allene Reaction
The Journal of Physical Chemistry, Vol. 94, No. 20, 1990 7971
TABLE 111: Natural Orbital Occupation and Populations for the TCA Channel (jA”) of the Three Chemically Active Orbitals4 dl
reactants
transition state
products
occupation population occupation population occupation population
1.92 C(l) 0.476 (32) 0.524 0 0 1.87 C(’) 0.271 C(2) 0.416 0.3 13 0 1.97 0.018 C‘2’ 0.300 0.682 0
0 0
0 0.132 0.303 0.493 0.204 0.027 0.018 0.527 0.455
Initially 6 , and @2 correspond to the in-plane x and allene, and 63 to the 0 in-plane unpaired electron. In figuration, the x bond has been transferred to the C - 0 unpaired electron resides in an sp2 hybrid orbital of the a
6, I .oo
d7
0.080 0.539 0.461
1 1
.oo
0.303 0.00 1 0.696 1 .oo
0.9 I5 0.04 1 0.043
orbitals of the final conbond, and the central C .
TABLE IV: Geometries and Energies of Allenyloxy Radical, Ground and Two Excited States
X2A” -190.126019 0 R(C(’)C(2))“ 1.386 R(C(’)C(3)) 1.326 R( C(*)O) 1.290 R(c(~)H) 1.076 R(C(?)H) 1.074 L(C(*’C(’)C‘3’) 180.0 L(C(”C(2)O) 123.0 L(C”)C(2)H) 118.3 L(C‘1)C(3)H) 121.3
energy, au AE, e V
Bond lengths in
A2A“ -189.990076 3.699 1.386 1.326 1.290 1.076 1.074 180.0 123.0 118.3 121.3
a2A’ -190.092301 0.5245 1.323 1.328 1.404 1.071 1.074 180.0 123.2 124.4 121.3
A.
x*
X 2A”
to the a” unpaired electron angular momentum facilitating the ISC to form cycIopropanone.8
The TCA Channel Table I1 summarizes calculations on the terminal-carbon attack pathway. Here the attacked carbon is taken to be C(I). One of the unpaired 0 electrons is poised to attack the C(’)-Cc2)T bond in the C, plane, while the other unpaired electrons is positioned out of the plane. Note, however, that no C-0 A bond can be formed as was the case in the CCA product. Repulsion of 0 electrons by the central C and the H’s causes 0 to approach at an angle. As 0 approaches, the C(1)-C(2)A bond is broken and the C(’)H2group bends away. The C(2)-C(3)x bond remains mostly unaffected. The angle of 0 approach is 113’. At the transition state the C(1)-C(2)x bond has been broken, allowing the carbon backbone to bend and the C(I)-C(2)bond to lengthen. The C(2)-C(3)bond has not changed substantially in length or character. Following the transition state, the diradical 1-allenylidoxy (I-propadienylidoxy) is formed (Figure 2b). The 0 has moved to its final distance, the C(I)-C(2)bond has stretched to a single bond, and the C(2)-C(3)has weakened only slightly. The two unpaired electrons have become localized on the 0 p orbital (a’’) and central C sp2 hybrid (a’). Even though six MO’s, two A and A* orbitals of allene and the unpaired p electrons of 0 are included in the CASSCF wave function, only the C(’)-C(2)x and x* orbitals and the 0 in-plane unpaired electron change significantly over the course of the reaction. The C(2)-C(3)A and x* orbitals do not change character from reactant to product and change occupation by only -0.02. The out-of-plane radical electron is inert throughout the reaction. In Table 111 we give the natural orbital occupation and atomic n and x* orbitals and the 0 in-plane populations for the C(1)-C(2) unpaired electron, denoted d,, d2,and d3, respectively. Orbitals 4’ and (b2 begin as C(l)-C(2)x and A* and become C(’)-O CT and u* orbitals. The characters and occupations of 4, and d3 are consistent with the description of oxygen-lefin reactions proposed by Bader et a1.I8 First the C(I)-C(2)x electrons decouple while C(2)takes on partial radical character at the transition state. In the products the C(I)electron has recoupled with 0, and the radical electron is localized on the central C. These results reflect exactly our findings for O(jP) + ethylene,’* and we conclude that for terminal attack of olefins only the (Y and 0 carbon atoms are affected, leaving the rest of the molecule relatively unchanged. The Allenyloxy Radical The observed final products of the TCA channel6 are H and HzCCCHO (1-allenyloxy). The allenyloxy radical has also been proposed as an intermediate in several other reaction^.^^-^^ In
a 2A
A ZA’’
Figure 3. Electronic structure of the allenyloxy radical
+
the O(jP) ethylene reaction, a similar product, vinoxy radical, was found to have several low-lying electronic states, corresponding to resonance between the C=C and C=O bonds and the orientation of the unpaired electron. In the present case, the allenyloxy radical has the same linear carbon backbone and perpendicular a system as allene, but with one H replaced by 0. Thus, we expect a close correlation between the C2H30 states and the C3H30states since the additional CH2 group plays little role in the three lowest states. Energies and geometries have been determined for the three lowest electronic states by MCSCF with a DZ basis. For the A’’ states a CASSCF wave function including five electrons in 3a” and 2a’ MO’s is sufficient to account for the major correlation and resonance effects. For the A’ state, seven electrons are distributed in 3a” and 3a’ MO’s. Results of these calculations are given in Table IV. Similar to the vinoxy radical,I2 there are two resonance structures of A” symmetry, allenyloxy, H2C= C=CH-0, and formylethyl, H2C=C-CH=O (Figure 3), where the unpaired electron is perpendicular to the CHO plane. Also shown in Figure 3 is the lowest state of A’ symmetry, H2C=C=CH-0, where the unpaired electron lies in the CHO plane. At the DZ level the X2A” state is a combination of the two resonance structures, with H2C=C-CH=O having greater weight, resulting in a C - 0 bond distance somewhat longer than a typical aldehyde bond. The excited A2A” state is dominantly H2C=C=CH-0 and the C - 0 bond distance is slightly shorter than a typical C - 0 single bond. For the two A” states there are three electrons in the extended CCO a system. For the a2A‘state, however, there are four electrons in the a system, causing the dominant state to be C=C-0:, and the C - 0 bond length to be a shortened single bond. From Table IV we see that the bond lengths and angles of the terminal CH2 group are unaffected by electron delocalization on the C-C-0 backbone. The symmetry and occupation of the natural orbitals support the bonding picture indicated by the bond lengths. Besides the linear backbone structure there are two local minima2’ for bent geometries with
~~
(18) Bader, R. F. W.; Stephens, M . E.; Gangi, R. A. Can.J . Chem. 1977, 5 5 , 2156. (19) Chan, T.H.; Ong,B. S. J . Org. Chem. 1978, 43, 2994
(20) Sclove. D. 9.; Pazos, J. F.; Camp, R. L.; Greene, F. D. J . Am. Chem. Soc. 1970, 92, 7488. (21) Huang, S.-Y. Ph.D. Thesis, University of California, Berkeley, 1989.
7972 The Journal of Physical Chemistry, Vol. 94, No. 20, 1990
1
Hammond et al.
In the crossed molecular beam experiments: allenyloxy radical is observed for collision energies less than 8 kcal/mol and a
20
I 4
, HzCCCH20 ,
Terminal carbon attack
I
-20
1 I
I
I 1
I I I I
(tripleti 1 I
I I
Central carbon attack
previous experiment indicated a barrier of no greater than 5.6 kcal/mol,*' as compared to our predicted value for the classical barrier height of 17 kcal/mol. In the previous CASSCF study of the O(3P) ethylene reaction1, a similar overestimation of the barrier was obtained. For that case the authors noted that a large basis set and more complete CI treatment could reduce the barrier by =I5 kcal/mol. A similar decrease in barrier height in our present results would bring us in good agreement with experiment. Finally, we note that the CASSCFI2 electronic transitions in the vinoxy radical (0.9-1.25 and 3.22 eV), occur very near those for allenyloxy radical (0.52 and 3.70 eV). This is due to the relatively weak interaction with the CH, group. Similar transitions have been computed for a number of different molecules containing the CCHO' radical moiety.22
+
Conclusions I
Figure 4. Electronic correlation diagram for the O(jP) + allene reaction. Energies (in kcal/mol) are from present CASSCF computations.
sp2 hybridization on the central carbon which may play a role in the dynamics. Discussion
The computed correlation diagram linking reactants to products for both channels is presented in Figure 4, showing that the CCA pathway has lower energy throughout. In the CCA product, because the 0 attacks the central atom its electrons may recouple with the ?r electrons of the allene moiety to form a C=O double bond. In the TCA product, however, this recoupling is not possible, because the allene moiety is on the 0-carbon from the attacked carbon. The lack of a C = O double bond accounts for the relative stabilities of the TCA and CCA products (-6.47 and -45.05 kcal/mol, respectively, relative to O(3P) allene). In addition, the relative stabilities and barrier heights are in accord with Hammond's postulate, the more exothermic reaction having the smaller barrier ( 1 5.16 kcal/mol CCA, 17.02 kcal/mol TCA) and the earlier transition state (the transition-state C-0 distance is 2.022 8, for CCA, and 1.960 8, for TCA).
+
Using a CASSCF wave function we have characterized the potential energy surfaces for the central and terminal carbon attack of O(3P)on allene. The CCA channel is found to have a smaller classical barrier to reaction. Both of our computed barriers are larger than those implied from experiment, but further improvement of basis set and correlation treatment should produce agreement. We do not expect these improvements in the wave function to change the qualitative conclusions reached here. The TCA channel is found to be very similar to the reaction of O(3P) and ethylene, both in energetics and geometries. Electronic transitions in the near-IR and UV regions at 0.52 and 3.70 eV are predicted for the allenyloxy radical close to the transitions previously found for the vinoxy radical.I2 Acknowledgment. We dedicate this paper to Professor Kenneth Pitzer in recognition of his numerous contributions to chemistry and his significant advances to the area of theoretical chemistry. This work was supported in part by the Director, Office of Energy Research, Office of Basic Energy Sciences, Chemical Sciences Division of the US.Department of Energy under contract No. DE-AC03-76SF00098. (22) Mougenot, P.; Dupuis, M. Cbem. Pbys. Lett., submitted for publication.
