12186
J. Phys. Chem. 1993,97, 12186-12188
A CI-A1I-Single-ExcitationsStudy of Norrish Type I Process in Acetyl Chloride Xavier Martin, Miquel Moreno, and J d M. Lluch’ Departament de QuImica, Universitat Autbnoma de Barcelona, 08193 Bellaterra (Barcelona),Catalunya, Spain Received: April 12, 1993; In Final Form: September 14, I9930
The photocleavage of a bond alpha to the carbonyl group, the Norrish type I process, in acetyl chloride has been studied by means of ab initio methods. A recently developed methodology that makes use of configuration interaction among all single-substituted determinants using a spin-restricted HartreGFock reference state (RCIS) is used in order to study the carbon-carbon and carbon-chlorine bond cleavages in both the first excited singlet state and the first triplet state. In good agreement with experimental results it is found that the C-Cl bond cleaves preferentially over the C-C bond. Due to the evolution of the excited states toward a np(C1) u*(C-Cl) configuration, the C-Cl bond cleavage involves a low energy barrier in both the triplet and singlet excited states, though as intersystem crossing in not likely to occur, the experimentally observed behavior can be attributed to the singlet excited state. These results confirm the ability of the RCIS methodology to deal with chemical reactions in excited states.
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Introduction In saturated carbonyl compounds, the dominant dissociation pathway upon photoexcitation in thelow-lyingn(0) +r*(c-O) transition is cleavage of a bond alpha to the carbonyl group. This is known as Norrish type I process.I4 Because in most systems the n ( 0 ) ?r*(C=O) electronic state does not correlate directly to ground-state dissociation products, the usual mechanism for alpha bond fission involves intersystem crossing to the lowest triplet potential energy s ~ r f a c e ,although ~?~ internal conversion to the ground electronic state can also play a Butler’s group5-6has measured the photofragment velocity and angular distributionsfrom the photodissociationof acetyl chloride at 248 nm with a crossed laser-molecular beam a p p a r a t u ~ . ~ J ~ The anisotropic angular distribution measured showed that dissociation occurs on a time scale of less than a rotational period, resulting in primary C - C I bond fission with nosignificant C - C bond f i ~ s i o n . ~The . ~ highly anisotropic angular distribution of the chlorine atom from acetyl chloride precluded the contribution of internal conversion or intersystem crossing to a triplet state, so that Butler’s g r o u ~ ~proposes - ~ J ~ a new pathway that results from the presence in the acetyl chlorideof a configuration crossing with a np(C1) u*(C-Cl) repulsive electronic configuration. Recently Pople’s groupI2 has considered the excited states of several organic molecules, showing that configuration interaction (CI) among all single-substituted determinants using a spinrestricted Hartree-Fock reference state (RCIS) is an adequate treatment to obtain satisfactory descriptions of potential energy surfaces and electronic properties of excited states. A special attractive point of this theory developed by Pople’s groupI2is that it is able to consider RCIS as a zeroth-order treatment, a secondorder Mdler-Plesset perturbation theory (RCIS-MP2) being introduced to take into account some effects of the electronic correlation. The aim of this paper is twofold. From a methodological point of view, we will test the ability of the RCIS methodology to deal with chemical reactions in excited states. From a chemical point of view, the relative fission of the C-Cl bond over the C - C bond in acetyl chloride will be analyzed in order to see the role of the np(C1) a*(C-Cl) electronic configuration in the cleavage process as proposed by Butler’s group.
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Methodology All calculations have been performed with the 6-31G* basis setI3-l6 as implemented in the GAUSSIAN-90 series of pro-
* Abstract
published in Aduance ACS Absrracfs, October IS, 1993.
0022-3654/93/2097-12186$04.00/0
grams.” A basis set with polarization functions has been chosen because although it often decreases the accuracy of adiabatic and vertical transition energies, the excited-statepotential energy surfaces are more accurate.’2 For the ground state of acetyl chloride, the C - C and C - C l bond fission processes have been analyzed by means of a reaction coordinate methodology, progressively enlarging the bond to be broken and fully optimizing at each point the rest of the geometrical parameters. An unrestricted Hartree-Fock level (UHF)18 has been initially used and then the energy has been recalculated by introducing the correlation through the MprllerPlesset perturbation theory up to the second order (MP2),19-20 the UHF geometry being kept frozen. For theexcited states, we have taken the Hartree-Fock groundstate minimum-energy structure of the acetyl chloride. The bond fissions have been analyzed by enlarging the cleaving bond and keeping frozen the rest of the geometrical parameters. CI-allsingle-excitationswith a spin-restricted Hartree-Fock reference ground state (RCIS) and spin-restrictedCI-all-single-excitations with the MP2 correction (RCIS-MP2) have been used in order to obtain the energy profile of the bond-cleavage processes. Note that taking into account Brillouin’s theorem, the Hartrec-Fock ground-state calculations are equivalent to CI among singlesubstituted determinants calculations. All the computations described in this paper use the frozen-core approximation.
Results and Discussion First of all, we will analyze the C - C l and C - C bond cleavage in the electronic ground state of acetyl chloridewhich is the singlet state SO.Afterwards the first singlet excited state St and the first triplet state TI will be considered. A full geometry optimization of acetyl chloride in the ground state gives a nearly planar structure for the CCOCl skeleton and optimized values for the Cl-Cl and C - C bond distances of 1.78 and 1SOA,respectively (Figure 1). the bond-cleavage processes have been studied by means of the reaction coordinate method by taking the breaking bond distance as fixed reaction parameter. For both cases, the energy profile along the reaction coordinate monotonically increases when the bond distance increases, so that the final products are found at the highest energy along the process. The dissociation energies obtained at the MP2 level are 83.3 and 95.2 kcal/mol for the C - C l and C - C bond cleavages, respectively. Whereas the value for the C - C l bond cleavage is in very good concordance with the experimental result (83 kcall
0 1993 American Chemical Society
The Journal of Physical Chemistry, Vol. 97, No. 47, 1993 12187
Norrish Type I Process
TABLE 11: Energy and Characteristics of the First Excited Singlet State (SI)for the C-CI Cleavage Process
127.07'
Eb
.08' 1.78
rMlo
(RCIS-MP2)
truncated RCIS wave function
1.78
6.80
1.90
6.89
2.05
7.1 1
0.20(a1- r3*) 0.14(nl- *3*) - 0.94(n3 r3*) - 0.24(u1 ~ 3 * -) O.17(03 *3*) - O.ZO(nl- n3*) 0.91(n3 r3*) 0 . 2 0 ( ~ 1 - ~ 3 * )-0.17(02-*3*) - 0.24(03 *a*) - 0.28(nl ~ 3 * ) + 0.87(n3 r3*) O.l8(a2-~3*) + 0.26(~3-r3*) + 0.28(nl- r3*) - 0.36(n2 u4*) - 0.80(n3 r3*) 0.99(nz u4*)
--
+
+
+
Figure 1. Main geometrical parameters of acetyl chloride in the electronic ground-state minimum-energy structure.
TABLE I: Relevant Molecular Orbitals for the Acetyl Chloride Minimum-Energy Structure orbital energya -0.68 -0.67 -0.65 -0.59 -0.56 -0.47 -0.46 -0.45 0.14 0.19 0.24 0.29 a
tvpe
symmetry type
2.15
7.17
2.38
5.76
a" a' a" a'
a" a'
Energy in electronvolts.
mo1),21the C-C dissociation energy seems somewhat exaggerated as compared with the experimental result for acetone of ca. 80 kcal/mol.22 As a matter of fact, a t our level of calculation the dissociation energy for the acetone is 90.6 kcal/mol. In order to analyze the excited states Table I shows a classification of the more relevant molecular orbitals for the acetyl chloride minimum-energy structure. We have considered the most important contribution in order to label the orbitals. The different kinds of orbitals ( T , Q, and n) bear a subscript index that labels the relative energetic position of theorbital. An asterisk is used as an index to indicate that the orbital is a virtual one in theground state at theequilibriumgeometry. Taking intoaccount that the molecule is close to having a plane of symmetry, the approximate symmetry of each orbital is also indicated in the third column of Table I. The first u orbital, q,is mainly located between carbon and chlorine, u2 is mostly of bonding character between carbon and oxygen whereas 0 3 can be considered as carbon-carbon bonding. The u4*is mainly antibonding between carbon and chlorine with a small C-C antibonding character. us* and u6* are mainly C-C antibonding character. As for the T orbitals, AI has the main component between both carbons whereas ~2 and ~ g are * the bonding and antibonding C-0 pair. Finally, the three lone pair n orbitals present a major contribution of the np orbitals of the chlorine atom, the nonbonding oxygen pair having a small participation in the nz orbital and a more important contribution to the n3 orbital. The C-Cl and C-C bond cleavage processes have been studied in both the first triplet and first singlet excited states by means of a reacion coordinate procedure by progresively enlarging the breaking bond and keeping frozen the rest of the geometrical parameters. A large set of points has been calculated at the RCIS level for each reaction coordinate. Some selected points along the reaction coordinate have been chosen in order to evaluate the MP2 energy correction. Along the reaction coordinates the orbitals are labeled by their symmetry correlation with respect to the minimum-energy structure ones. In Tables I1 and 111, results along some of those selected points
A" A" A"
A"
TABLE III: Energy and Characteristics of the First Triplet State (TI) for the C-CI Cleavage Process Eb
rc.cIa
(RCIS-MP2)
truncated RCIS wave function
sym
1.78
6.49
A"
1.90
6.63
2.02
6.85
2.05 2.38 4.78
6.82 5.49 4.00
0.22(01- ~ 3 * )+ 0.14(nl- r3*) - 0.93(n3 r3*) 0.24(01- T3*) O.20(~3 r3*) O.ZO(nl r3*) - 0.90(n3 r3*) 0.21(~1- *3*) - 0.15(~2' *3*) - 0.26(u3 r3*) - 0.25(nl *3*) + 0.24(n3 u4*) 0.87(n3 a,*) 0.28(nl- u4*) - 0.91(n3 ud*) - 0.22(nl u,*) 0.96(n3 ad*) 0.99(n3 ud*)
a"
a' a' a'
A"
C-CI distance in angstroms. Energy in electronvolts.
a' a'
--
sym
+
--
--
+
--+ -
A"
A"
MC A' A'
C-C1 distance in angstroms. Energy in electronvolts. Mixing of approximately A' and A" configurations.
for the first excited singlet state SIand first triplet state TI are given in order to analyze the evolution of the C - C l bond cleavage. In the first column the reaction coordinate value is given. The second column gives the electronic transition energies coming from the ground state at the equilibrium geometry. The third column gives the major contributions of each state. Only those configurations that have a coefficient of significant weight in the CI expression are shown. These functionscorrespond to the spinadapted configurations obtained by taking appropriate linear combinations of the Slater determinants used in the RCIS calculation. The vertical electronic transition energy at the minimum-energy structure, 6.80eV, corresponds to a wavelength of 182 nm which is a low value as compared with the experimental result of 248 nm11.5*6 However, it has been shown12that the RCISMP2 vertical transition energies are greater than their RCIS results counterparts. In our case the RCIS value for the vertical transition wave number is 208 nm. The energy barrier along the C-Cl bond cleavage process in the first singlet excited state SIis 8.5 kcal/mol. Table I1 shows that along the reaction coordinate the major contribution to the excited state evolves adiabatically from the n3 ~ 3 excitation * to the n2 u4*one, in good agreement with the results of Butler's group.S.6 Note that both n2 and n3 are lone pair orbitals with an important contribution of the chlorine atom, though the oxygen has an appreciable contribution in the n3 orbital. For the triplet stateTI,the energy barrier for the bond cleavage is 8.3 kcal/mol. In Table I11 it is seen that along the reaction coordinate the triplet state evolvesfroma majoritary participation of the n3 ~ 3 excitation * to a mainly n3 a4* contribution. The n2 u4*excitation which was dominant in the first excited state does not appear here though it is the dominant configuration in the first excited triplet state, which appears over 1 eV above TI. Therefore, given the small energy barriers, the C-CI bond cleavage process may take place quite easily in both the SIand TI surfaces. Anyway, given that experimentally the chlorine atom presents a highly anisotropic angular distribution, the bond
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12188 The Journal of Physical Chemistry, Vol. 97, No. 47, 1993
TABLE I V Energy and Characteristics of the First Excited Singlet State (SI) for the C-C Cleavage Process Eb
rc.ca (RCIS-MP2) 1S O
6.80
1.90
7.89
2.05
8.48
2.38
9.59
2.72 3.45 4.78
9.88 9.67 8.10
truncated RCIS wave function
sym
0.20(01- r 3 * ) + 0.14(nl- z3*) - 0.94(n3 r 3 * ) - 0.20(ul r3*) + 0.22(nl r3*) - 0.92(n3 r3*) - O . I ~ ( U I r3*) 0.15(~2 r3*) 0.23(nl r3*)- 0.93(n3 r3*) 0.17(u1 r3*)- 0.20(n1 r3*) + 0.94(n3 r3*) 0.82(n3 u4*) 0.52(n3 r3*) 0.97(n3 a*) 0.97(n3 u4*)
A“
---- + --- + -
+
--
~
A” A” A” MC A‘ A‘
C-C distance in angstroms. Energy in electronvolts. Mixing of approximately A‘ and A” configurations.
TABLE V Energy and Characteristics of the First Triplet State (TI) for the C-C Cleavage Process Eb
rc.ca 1.50
(RCIS-MP2)
truncated RCIS wave function
sym
6.49
+
A”
1.90
7.64
1.95
8.08
2.05
7.68
2.38 3.45 4.78
6.58 5.63 5.47
0.22(u1- r3*) 0.14(nl- r3*) - 0.93(n3 r3*) 0.22(u1- z3*) - 0.22(nl- r3*) 0.90(n3 r3*) 0.18(uz u4*) + 0.80(n3 u4*) 0.44(n3 r3*) 0.23(uz u4*) + 0.92(n3 0 4 * ) -0.16(n3 u6*) 0.20(uz a*)+ 0.95(np a*) 0.99(n3 u4*) 0.99(n3 u4*)
+ +
- -----
-
A” MC A’
A‘ A‘ A‘
C-C distance in angstroms. Energy in electronvolts. Mixing of approximately A’ and A” configurations.
cleavage through the triplet excited state, which should involve a previous intersystem crossing, can be ruled out. The C-C bond cleavage process has also been analyzed in both the first singlet excited state and the first triplet state. Tables IV and V show for both states the evolution of the process along the C-C bond distance reaction coordinate. The energy barriers are now of 71.0 kcal/mol for the singlet state SIand 36.7 kcal/ mol for the triplet state. The evolution of the two states along the reaction coordinateis now quite similar,both showing a change from a dominant n3 r3* configuration to an almost exclusively n3 u4* character as the bond distance is enlarged. It is seen that, as the C-C distance increases, the a*orbital evolves from being of mainly C - C l antibonding character to an almost exclusively C-C antibonding character. It is noteworthy that the barriers are quite high when compared with the ones corresponding to the C-Cl bond cleavage, previously analyzed. This is specially true for the SI state, experimentally the state which is supposed to be involved in the photodissociation process of acetyl chloride. At this point, it has to be noted that the reaction coordinates have been obtained by enlarging the cleaving bond and keeping frozen the rest of the geometrical parameters. This methodology is expected to overestimate the energy barriers, particularly for the C - C bond cleavages where, for instance, the CH3 fragment is forced to remain pyramidal whereas its optimized geometry is almost planar. Neverthelessthe differencesin barriers obtained are large enough, so that our RCIS-MP2 calculations are in agreement with the fact that for acetyl chloride the C - C l bond cleaves preferentially over the C - C one. In the last column of Tables 11-V the symmetry labels of the
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Martin et al. corresponding excited states are given. It has to be remembered that this symmetry assignation is not absolute as the molecule is only close to having a plane of symmetry. This is why in the crossing region (~C-CI = 2.02, 2.72, and 1.95 A for the cases of Tables 111-V, respectively) there is a mixing of configurations that in a truly planar geometry would be of A’ and A” type. It is noteworthy that only for the C - C l bond cleavage in the first excited singlet state the symmetry of the state is conserved along all the rection coordinate (Table 11). From the results obtained in this paper we can conclude that our theoretical study is in good agreement with the fact that in the Norrish type I process of acetyl chloride, the C-Cl bond cleaves selectively over the C-C bond. The C - C l bond cleavage is a process which involves a low energy barrier both in the first triplet state and in the first excited singlet state, though experimentallythe cleavage through the triplet state can be ruled out because intersystem crossing is not expected to occur. Also in agreement with the experimental prediction, a np(C1) a*(C-Cl) electronic configuration is seen to be dominant in the C - C I bond cleavage process. Finally, we have to remark that, in spite of the fact we have just roughly determined the energy barriers for the different cleavages by making use of a simplereaction coordinate procedure, the RCIS approximationis seen to be a very suitable methodology in order to study chemical reactions taking place in excited states.
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Acknowledgment. We thank the European Center for Parallelism of Barcelona, CEPBA, for providing computational time in a CONVEX (2-3480 computer. References and Notes (1) Calvert, J. G.; Pitts, J. N., Jr. Photochemistry; Wiley: New York, 1966, p 379. (2) Michl, J.; BonaEiC-Koutecky, V. Electronic Aspecrs of Orgunic Photochemistry; Wiley: New York, 1990, p 378.
(3) Chandra, A. K. J. Mol. Strucr. (THEOCHEM) 1988,181,255. (4) Reinchs, M.; Klessinger, M. J . Phys. Org. Chem. 1990, 3,81 and extensiue references within. (5) Person, M. D.; Kash, P. W.; Butler, L. J. J. Phys. Chem. 1992.96, 2021. (6) Person, M.D.; Kash, P. W.; Butler, L. J. J. Chem. Phys. 1992, 97, 355.
(7) Ho, P.; Bamford, D. J.; Buss,R. J.; Lee, Y. T.; Moore, C. B. J. Chem. Phys. 1982, 76, 3630. (8) Chuang, M.; Foltz, M. F.; Moore, C. B. J . Chem. Phys. 1987, 87, 3855. (9) Lee, Y.T.; McDonald, J. D.; LeBreton, P. R.; Herschbach, D. R. Reu. Sci. Instrum. 1969, 40, 1402. (10) Person, M.D. Ph.D. Thesis, Department of Chemistry, University of Chicago, 1991. (1 1) Person, M. D.; Kash, P. W.; Schofield, S.A.; Butler, L. J. J. Chem. Phys. 1991, 95, 3843. (12) Foresman, J. B.; Head-Gordon, M.; Pople, J. A.; Frisch, M.J. J . Phys. Chem. 1992, 96, 135. (13) Hehre, W. J.; Ditchfield, R.; Pople, J. A. J. Chem. Phys. 1972, 56, 2257. (14) Hariharan, P. C.; Pople, J. A. Theor. Chim. Acru. 1973, 28, 213. (IS) Gordon, M. S.Chem. Phys. Left. 1980, 76, 163. (16) Frisch, M. J.; Pople, J. A.; Binkley, J. S. J. Chem. Phys. 1984.80, 3265. (17) GAUSSIAN 90; Frisch, M. J.; Head-Gordon, M.;Trucks. G. W.; Foresman, J. B.;Schlegel, H. B.; Raghavachari, K.; Robb, M.; Binkley, J. S.; Gonzalez, C.;Defreca. D. J.; Fox, D. J.; Whiteside, R. A.; Seeger. R.; Melius, C. F.;Baker, J.;Martin,R.L.;Kahn,L.R.;Stewart, J. J. P.;Topiol.S.;Pople, J. A. Gaussian Inc.: Pittsburgh, PA. (18) Pople, J. A.; Nesbet, R. K. J. Chem. Phys. 1959, 22, 571. (19) Msller, C.; Plesset, M. S.Phys. Reu. 1934,16, 618. (20) Pople, J. A.; Seeger, R.; Krishnan, R. Inr. J . Quunrum. Chem.Symp. 1977, 11, 149. (21) Devore. J. A.; ONeal, H. E. J . Phys. Chem. 1969, 73, 2644. (22) Rosenstock, H. M.; Draxl, K.;Steiner, B. W.; Herron, J. T. J . Phys. Chem. Ref. Dura 1977,6 (Suppl. 1). 1-774.
