J. Phys. Chem. 1996, 100, 16045-16052
16045
ARTICLES Photolysis of the Dichlorocyclobutenedione in Rare Gas at 10 K. Infrared Spectral Analysis and ab Initio Calculations of Vibrational Frequencies. First Identification of Two New Species (Dichloro-Substituted Bisketene and Dichloropropadienone). Kinetics and Reaction Mechanism I. Mincu and M. Hillebrand Department of Physical Chemistry, UniVersity of Bucharest, Bd. Republicii 13, Bucharest, RO-70034, Romania
A. Allouche,† M. Cossu, P. Verlaque, J. P. Aycard, and J. Pourcin* Laboratoire des Interactions Ioniques et Mole´ culaires, URA 773, UniVersite´ de ProVence, Centre de Saint Je´ roˆ me, Case 232, 13397 Marseille, Cedex 20, France ReceiVed: February 27, 1995; In Final Form: July 11, 1996X
The photochemistry of 3,4-dichlorocyclobutene-1,2-dione (A) embedded in rare gas matrices and irradiated by a filtered broad band source (λ > 335 nm) has been studied by FTIR spectroscopy. The new IR absorption bands were assigned to the photoproducts by ab initio calculations of theoretical vibrational spectra, at the MP2/6-31G* level. A new highly reactive intermediate bisketene, 2,3-dichloro-1,3-butadiene-1,4-dione (OdCdC-Cl)2 (B), was formed; the experimental infrared band activities of the symmetric and antisymmetric vibrational modes of B in the CdO spectral range corroborate its calculated twisted structure. B is decomposed by further irradiation and leads to the formation of carbon monoxide (D), dichlorocyclopropenone (E), and dichloropropadienone (Cl2CdCdCdO) (F), by two concurrent pathways (branching ratios: 93% for E, 7% for F in an argon matrix). Photolysis of E and F by the full light of a mercury lamp (λ > 230 nm) led to the infrared characterization of dichloroacetylene (G), formed via a simple primary dissociation process. A study of the integrated absorbances versus time permitted us to characterize the kinetic behavior of the process and to establish the reaction mechanism.
Introduction In this paper we are following our interest in the identification, at cryogenic temperatures, of highly reactive intermediate species arising from photolytic processes.1,2 We report now an analysis by FTIR spectroscopy of reaction products obtained by photolysis of 3,4-dichloro-3-cyclobutene-1,2-dione (A in Figure 1) embedded in an inert gas matrix at 10 K and irradiated by the filtered or full light of a high-pressure mercury lamp. In the text we usually use I.U.P.A.C. nomenclature (independently from the atom numbering used in Figure 1), as well as for the photoproducts. Argon or xenon was chosen as matrix material in order to avoid any reaction of the studied species with the environment. Indeed, the photolysis at 240 nm, of difluoropropadienone (a parent molecule of the species labeled F in the Figure 1) in a nitrogen matrix at 12 K, which has been reported by Brahms and Dailey,3 leads to the formation of difluorodiazoethene as an intermediate, by the replacement of the CO group of the precursor by one molecule of nitrogen coming from the matrix. By systematic recording of FTIR spectra at different times during the photolytic process, we have assigned the infrared bands that presented the same kinetic behavior to the same compound. The identification of the products from their infrared spectra was supported by comparison with the frequencies and intensities of their ab initio computed vibrational spectra (Tables 2-5). Infrared absorbance bands are minutely deconvoluted † X
E-mail:
[email protected]. Abstract published in AdVance ACS Abstracts, September 1, 1996.
S0022-3654(95)00576-4 CCC: $12.00
Figure 1. Atom numbering of compounds involved in photolysis (we use a numbering that conserves the number of each atom during the process).
and analyzed in order to establish the kinetic behavior and the reaction mechanism. The precursor, labeled A in Figure 1, was the first known of the synthesized dihalogenocyclobutenediones. It presents a fourmembered trapezoidal structure containing a double bond within the ring and two external double bonds. Its equilibrium geometry has C2V point group symmetry. The calculated geometrical parameters using the MNDO method4 or ab initio quantum calculations5 are in good agreement with the experimental ones measured in the gas phase.6 When the matrix is irradiated in the spectral range above 335 nm, the photodecomposition (Scheme 1) of the precursor (A) leads to the appearance of new infrared bands assigned to a first intermediate that we © 1996 American Chemical Society
16046 J. Phys. Chem., Vol. 100, No. 40, 1996
Mincu et al.
TABLE 1: Comparison between Experimental and Calculated (MP2/6-31G*) Optimized Geometric Molecular Parameters of Compounds Involved in the Photolysis (Bond Lengths, Å; angles, deg)b compounda A exptc R(C1-C2) R(C1-C4) R(C2-C3) R(C1-Cl5) R(C2-Cl5) R(C2-Cl6) R(C3-O7) R(C4-O8) ∠(C2-C1-C4) ∠(C1-C2-C3) ∠(Cl5-C1-C2) ∠(Cl6-C2-C1) ∠(O7-C3-C2) ∠(O8-C4-C1) ∠(Cl5-C2-C1) ∠(C3-C2-C1-C4)
1.358 1.516 1.516 1.683
B MP2 6-31G*
MP2 6-31G*
1.369 1.507 1.507 1.680
1.441 1.336 1.336 1.749
MP2 DZ+P 1.449 1.342 1.342 1.745
1.683 1.191 1.191 94.1 94.1 133.5 133.5 135.7 135.7
1.680 1.208 1.208 93.8 93.8 133.8 133.8 135.8 135.8
1.749 1.177 1.177 118.7 118.7 123.2 123.2 180e 180e
1.745 1.180 1.180 118.4 118.4 122.9 122.9 180e 180e
0.0
0.0
79.2
80.0
E
F
MP2 6-31G*
MP2 6-31G*
1.351 1.459
1.328 1.309
G
1.689 1.689
1.722 1.738
1.205 62.4
1.181 137.6f
145.0 145.0
exptd
MP2 6-31G*
1.1920(8)
1.217
1.6410(5)
1.644
1.6410(5)
1.644
125.6 170.4g 121.8
152.4
a
Precursor A and photoproducts B, E, F, and G. b Experimental and ab initio calculated parameters. c Reference 5 and references cited therein. d Reference 26. e Fixed parameter. f Respectively 137.7° and 130.1° for propadienone and difluoropropadienone, ref 18. g Respectively 166.5° and 170.9° for propanedione and difluoropropadienone, ref 18.
TABLE 2: Comparison of ab Initio Calculated (MP2/6-31G*) Harmonic Vibrational Frequencies (cm-1) and Relative Infrared Intensities with Corresponding Experimental Data for Compound B experimental νi
sym
Ar ν a
ν1
A
ν2 ν3 ν4 ν5 ν6 ν7 ν8 ν9 ν10 ν11
A A A A A A A A A B
ν12 ν13 ν14 ν15 ν16 ν17 ν18
B B B B B B B
2158.1 2152.3a 1075d
2130.9a 2128.6a 1246d 831d 578d
theoretical (MP2/6-31G*)
intb
Xe ν
sh 50
2153.4a
s vw s s s s s s s 100 vw vw vw s s s s
2150.7a 2149.1a s s s s s s s s s 2123.4a 2120.6a 1237.0 834 s s s s s
νharm
νscalc
intb
2225
2152
45
1428 1137 619 517 493 390 139 96 48
1384 1102 619 515 491 390 139 96 48
2 11 ,1 2 ,1 ,1 ,1 ,1 ,1
83 (C-C); 14 (CdC) 53 (C-C); 35 (C-Cl) 96 (CdCdO) 73 (tors C-C) 20 (C-Cl); 77 (tors C-C) 21 (C-C-Cl); 68 (o-o-p Cl) 40 (CdC-Cl); 35 (o-o-p Cl) 36 (CdC-C); 26 (CdC-O); 29 (o-o-p Cl) ≈100 (tors C-C)
2207
2134
100
83 (CdO); 17 (CdC)
1277 827 595 508 398 196 108
1241 825 594 508 398 196 108
4 4 2 3 335 nm), is illustrated in Figure 2. The evolution of the system in the 1700-2300 cm-1 range of the infrared spectrum is presented for three different irradiation times. The lower trace presents the spectrum of the precursor A, after its deposition from the argon gas-phase mixture and before irradiation (time t ) 0 min). The middle trace, recorded after 21 min of irradiation, shows the appearance of the photoproduct B, and the upper trace, taken after 200 min of irradiation, illustrates the presence of photoproducts D, E, and F. The infrared spectrum of A in the range of the carbonyl stretching mode appears as a well-resolved triplet at 1817.6, 1822.3, and 1829.6 cm-1. The intensity of the first peak depends on the history of the matrix. It may almost completely disappear for lower deposition temperature, but the other two infrared bands are then split into multiple sites around the main bands centered at 1822.3 and 1829.6 cm-1, as often observed in matrix isolation spectroscopy. On the basis of a preliminary vibrational analysis,5 these two bands are assigned to the pure symmetric and antisymmetric CdO stretching vibrational modes. The peak at 1817.6 cm-1 is assigned to a site splitting of the antisymmetric mode, which appears to be more sensitive
16048 J. Phys. Chem., Vol. 100, No. 40, 1996
Mincu et al.
Figure 3. Experimental integrated intensities (in cm-1), in the carbonyl and cumulenic stretching ranges, during photolysis (λ > 335 nm), at different reaction times (in min), compared with a first-order behavior according to Scheme 1. The solid lines are fits of the experimental points by the theoretical curves, calculated from the rate constants evaluated in the text. The rise curve of D is equated that of E+F.
Figure 2. Infrared spectral evolutions versus irradiation time, in the 2300-1700 cm-1 spectral range, of precursor A and photoproducts B, E, and F, in a dilute argon matrix (1/1000): (a) at 10 K after deposition and without annealing; (b) after 21 min of irradiation, at 10 K, with the filtered light of a high-pressure lamp (λ > 335 nm); (c) after 200 min of irradiation, at 10 K, with the filtered high-pressure lamp.
to the cage interaction than the symmetrical one. The other modes of A are well resolved and almost free of site splitting. The first appearing photoproduct (labeled B on the middle trace of Figure 2) exhibits mainly, in a dilute matrix, two strong bands around 2153 and 2129 cm-1. The multiplet structure of these bands is interpreted as site splitting. For example, the wellresolved doublet around 2129 cm-1 depends on the physical conditions of formation of the matrices: in comparison with the spectral feature in a dilute matrix, in a concentrated matrix the component at 2128.6 cm-1 is much more intense than that at 2130.9 cm-1 (Figure 1S in the supporting information). After 21 min of irradiation, the intensities of these new bands reach maximum values, while the peaks attributed to E, F, and D, by their kinetic behavior upon irradiation, are arising above the spectral noise. Weaker vibrational modes belonging to B have been observed in a very concentrated matrix (initial concentration 1/100) (Table 2 and Figure 1S). After 200 min, A and B had almost disappeared, while groups of bands attributed to E, F, and D increased in intensity (Figure 2c): E is characterized by a triplet with its strongest peak located at 1896.3 cm-1, D by a broad peak (5 cm-1 half-width) centered at 2138.0 cm-1, and F by two doublets at 2200.0-2260.0 cm-1 and 1757.71779.9 cm-1. Upon warming, the peak at 1779.9 cm-1 irreversibly collapses into the background, while the last one, at 1757.7 cm-1, increases in intensity and then would correspond to the more thermodynamically stable trapping site. Additional peaks have been observed for F in the more concentrated matrices (Table 4 and Figure 1S). Frequencies, assigned to carbonyl or cumulenic stretching modes, are significantly higher than those of other dicarbonyl compounds. For instance, s-cisoxalyl chloride, generated from the s-trans form by photoisomerization in an argon matrix, presents two infrared active symmetric and antisymmetric vibrational modes8 at 1733.1 and 1787.4 cm-1. The infrared spectrum of B was identified when its absorption
bands reached their highest values. Those of the products E and F have been identified when B had almost completely disappeared. We report in Figure 3 the kinetic behavior of the integrated absorbances in the whole spectral range covered by each vibrational mode corresponding to the infrared bands labeled A, B, E, and F in Figure 2. The temporal evolution of D was the same than that of E+F, and this infrared band was not plotted. Among the groups of bands that disappear or grow with the same rate, only the strongest antisymmetric carbonyl or cumulenic stretching modes of A or B and the sum of the integrated cumulenic absorbances corresponding to F are reported in order to simplify the presentation. Matrices of A, B, E, and F in xenon display spectra (Figure 2S) that undergo temporal change similar to those in argon matrices. The experimental frequencies are reported in Tables 2-4 for comparison with those in argon matrices. The photoproducts E and F are stable to prolonged irradiation by the filtered light (λ > 335 nm), but they disappear when they are irradiated with the full mercury lamp (λ > 230 nm) and lead to the formation of new infrared bands, whose frequencies match those of carbon monoxide and dichloroethyne, labeled D and G in Figure 1. Discussion Product Growth and Product Yield. For matrices having thickness l, the integrated absorbance Ai over the frequency range ∆ covered by a fundamental infrared band is defined, from the Beer-Lambert law, as
Ai ) ∫∆ln(I0/I) dν ) icil where i and ci are the absolute integrated extinction coefficient and the concentration of species i, respectively; like the frequencies νi, Ai is expressed in cm-1. The dependence on the irradiation time of the experimental integrated absorbances of the A, B, D, E, and F species is reported in Table IS in the supporting information. The concentrations ci are functions of the irradiation time, and the kinetic behavior of the integrated absorbances (see Figure 3) suggests the chemical mechanism presented in Scheme 1. At time t, the sum of different products’ concentrations is equal to the initial concentration of the precursor A:
Photolysis of Dichlorocyclobutenedione in Rare Gas
J. Phys. Chem., Vol. 100, No. 40, 1996 16049
cA(t) + cB(t) + cE(t) + cF(t) ) cA(0)
(1)
In terms of the integrated absorbances, eq 1 becomes
AA(t)/lA + AB(t)/lB + AE(t)/lE + AF(t)/lF ) AA(0)/lA (2) which can be written as
∑i xi(t) ) 1,
i ) {A, B, E, F}
(3)
where xi(t) ) Ai(t)/Ri and Ri ) AA(0) i/A. The different unknown Ri have been calculated from the measured integrated absorbances of the carbonyl or cumulenic stretching modes (Table IS). The RA coefficients for the symmetric and the antisymmetric carbonyl stretching modes are equal to the corresponding integrated absorbances before irradiation, at t ) 0 min (3.51 and 5.86 cm-1, respectively, as shown in Table IS); the RB coefficients, respectively equal to 14.1 and 26.2 cm-1 for the symmetric and antisymmetric cumulenic stretching modes, have been calculated from the first rows of Table IS, before the formation of the E and F products. Their ratio is in good agreement with that experimentally obtained from the linear dependence of the integrated absorbances (columns 4 and 5 in Table IS). Note that this ratio is also in good agreement with the ratio of the theoretical intensities calculated at either the HF/4-31G* level (1590/818) or the MP2/ 6-31G* level (797/360). The RE and RF coefficients, equal to 6.8 and 27.0 cm-1, have been calculated assuming their ratio equal to that of the calculated intensities at the HF/4-31G* level of theory. The functions xi(t) have been fitted to exponential behavior following the postulated chemical Scheme 1, and the first-order rate constants have been calculated by least squares analysis, using the Macintosh “Curve Fit” program, which provided
Figure 4. Linear plots of the yields of E and F during the photolysis.
being known to provide more extensive and helpful information on the physical understanding of the problem, we described a straightforward means for calculating, from the (3N × 3N) f matrix, an F matrix in the complete set of internal coordinates, including the redundancies, by the use of the pseudoinverse B-1 of the non square augmented B matrix, which transforms Cartesian coordinates into internal ones. The term “complete” is used following the definition given by Boatz and Gordon:10 the double matrix transformation, using B-1 to convert first f into F and then to get from F back to f, must lead to an f matrix in Cartesian coordinates identical with that obtained by quantum mechanical calculations. The internal coordinate sets for compounds B, E, F, and G are reported in Table IIS. The pseudoinverse B-1 matrix can be calculated from the following relation:
B-1 ) M-1BtG-1
k1 ) (2.42 ( 0.05) × 10-4 s-1, k2 ) (15.8 ( 0.3) × 10-4 s-1, and -4 -1
k3 ) (1.2 ( 0.2) × 10
s
These values are in good agreement with the steady state of B arising after 21 min of irradiation. The formation of E appears to be 13 times faster than that of F. The k2 and k3 rate constants can be checked by monitoring in detail the CO product, whose kinetic analysis gives a rate constant equal to 17.5 × 10-4 s-1, in excellent concordance with the sum of k2 and k3. The RD coefficient, equal to 0.69 cm-1, is obtained from the linear relationship between AD and (1 - xA). The product yield of the species E or F is simply defined as the ratio xi/(xE + xF), where i ) E or F, and can be measured from the linear fit of the experimental data, as can be seen in Figure 4. Product Identification. As was often shown in the literature, structural information on a molecular species can be deduced from the theoretical simulation of its infrared spectrum by vibrational analysis. This includes the accurate determination of the equilibrium structure, harmonic vibrational frequencies, and infrared intensities. From the empirically corrected theoretical approach described by Fogorasi and Pulay9 and the analysis of Boatz and Gordon,10 we recently proposed a procedure in which the molecular ground state potential surface and the derived harmonic force field f, in the working set of Cartesian coordinates, are evaluated by quantum ab initio calculations performed, for instance, by the Gaussian 88 or HONDO 8 program using a standard Gaussian type orbital set.11 Then, the problem expressed in terms of internal coordinates
where M is the diagonal matrix of the atomic masses and G-1 the diagonalized kinetic energy matrix restricted only to the nonzero eigenvalues of G. We used here the extended MP2/6-31G* level of theory, for which most of the ab initio computed frequencies are quite close to the experimental ones. To reproduce the experimental infrared spectra, the computed vibrational force fields in internal coordinates have been refined by a scaling procedure that takes into account not only the wellknown overestimation of the diagonal force constants but also the anharmonic effects.9 A generally good agreement between the experimental frequencies and the scaled ones is achieved by using different scaling factors for different types of internal coordinates for the different compounds (see Tables 2-5). Ab Initio Optimized Geometries. The geometry optimization calculations were performed by the ab initio method, using the MP2/6-31G* level of theory. The relevant calculated geometrical parameters for the investigated compounds in their equilibrium structures, together with some known experimental data, are listed in Table 1. As the experimental values are not always available for all the related compounds, it is not possible to discuss in all cases the accuracy of the calculated geometries. Nevertheless, some general features can be outlined: (i) The CdO bond lengths are somewhat higher than the usually quoted values; as expected, these bonds are shorter in the cumulenic compounds (B and F) than in the classical carbonyl compounds (A and E); (ii) The CdC bond lengths are varying within the 1.309-1.369 Å limits; the lowest value was found for compound F, reflecting the
16050 J. Phys. Chem., Vol. 100, No. 40, 1996 special features of the cumulenone chain; (iii) With the exception of 1,2-dichloroacetylene (G), the C-Cl bond lengths have lower values in the cyclic compounds (A and E) than in the open chain ones (B and F). Special mention is due to bisketene B (2,3-dichloro-1,3butadiene-1,4-dione), whose ab initio predicted geometry appears to be twisted rather than planar. Recent ab initio calculations by Tidwell et al. (ref 12 and references cited therein) found the same result for different bisketenes, among them some symmetrical bisketenes (2,3-R,R-1,3-butadiene-1,4-dione; R ) H, F, SiH3). Our quantum ab initio calculation of the torsional potential for the bisketene was performed at a high level of theory using the GAMESS ab initio computer system.13 The chosen orbital basis set is the valence double-ζ basis,14 augmented with a polarization d function on each atom. The SCF Hartree-Fock calculation was completed with a secondorder Møller-Plesset perturbation including all the molecular eigenfunctions (58 orbitals, 138 primitive Gaussian functions) for every point of the potential energy surface. For each value of the Cl-C-C-Cl torsional parameter, all the other geometrical parameters were optimized at the same level of computation (MP2), but the CdCdO group was restrained to be linear. The molecular point group is C2V for the s-cis form, C2 for the s-gauche form, and C2h for the s-trans form. The torsional potential shows a minimum for a dihedral angle of 80°; no other local minimum was found. The torsional energy barriers are 52.0 kJ/mol for the s-gauche to s-cis transition and 30.3 kJ/mol for the s-gauche to s-trans transition. The inequality of these energies suggests that the CdO/Cl repulsion is less important than that of Cl/Cl or CdCdO/ CdCdO. Compared to the oxalyl chloride,15 all the bond lengths (calculated using the same basis set and the MP2 perturbation) are noticeably shorter in the bisketene C4O2Cl2, particularly the central C-C bond (1.449 Å in C4O2Cl2, 1.558 Å in the oxalyl chloride). The CdCdO group is much longer than the CdO group, and the attenuation of the repulsion between the negative charges favors the s-gauche form to be less sterically hindered. This hypothesis is confirmed by the C-C-Cl and C-CdC angle values. The important shortening of the single C-C bond is also due to negative hyperconjugation, which operates more efficiently on the s-gauche form. The relaxation amplitude of the geometrical parameters with respect to the torsional angle is relatively large for the C-C bond (0.041 Å) and C-Cl bond (0.021 Å), but remains moderate for CdC (0.007 Å) and CdO (0.006 Å); nevertheless, these last two bond lengths change in opposite ways. As concerns compound F, there are, to our knowledge, no reports on its structure, but experimental and theoretical data reported on the related difluoropropadienone suggest a planar but strongly bent equilibrium geometry.16,17 Previous ab initio calculations confirm the distorted geometry of this compound, both in the HF/6-31G* basis set16 and at the MP2/6-31G* level.18 It is not the same for the unsubstituted propadienone: previous experiments19-21 clearly demonstrated that the cumulenone chain is bent, but calculations at the Hartree-Fock level did not reproduce this feature;22,23 on the contrary, using electron correlation (MP2/6-31G*), the distorted form of propadienone could be accurately described.24 Moreover, Brown and Dittman interpreted the unusual nonsymmetrical geometry of propadienone by taking into account the possible influence of one of its low-energy excited electronic configurations.25 The geometry of compound F, computed at the MP2/6-31G* level, is also strongly bent. The CdCdC and CdCdO angles are given in Table 1, in comparison with those of propadienone and difluoropropadienone, calculated at the same level of
Mincu et al. theory.18 Although experimental geometry data are not available, the CdO bond length may be overestimated, as in propadienone or difluoropropadienone.18 The molecular geometry of G was optimized at the MP2/631G* level, and the structural parameters are reported in comparison with the experimental values deduced from the infrared spectrum26 (Table 1). While the computed C-Cl bond lengths are in good agreement with experiment, the CtC distance seems to be overestimated. Product B. As already stated, ring opening of the dichlorocyclobutenedione (A) gives rise to the product (B) (see Scheme 1), whose molecular structure was identified as a bisketene, the 2,3-dichloro-1,3-butadiene-1,4-dione, from its infrared spectrum in cryogenic matrices collected after 21 min of irradiation. Although being energetically less stable than the precursor, this last conformation could be stabilized by the crystal force field, which would act against the closing of the ring and partially prevent the reverse reaction, at least in a spontaneous manner; nevertheless, we found in the literature an example of a photoreversible reaction that regenerates benzocyclobutenedione from the bisketene,27 but the authors mention that the reverse reaction occurs only under irradiation conditions different from those used for the direct reaction. Thus, some bisketenes analogous to B, mainly obtained at low temperature under photochemical conditions, have already been observed. Zhao et al.28 have synthesized a silyl-substituted bisketene that is still stable and persistent at room temperature. They also established that the more stable conformer is twisted rather than planar. Very recently, Werstiuk et al.29 published more experimental proof (UV photoelectron and dipole moment studies) of the characteristic nonplanarity of two silyl-substituted bisketenes (2,3-bis(trimethylsilyl)buta-1,3-diene-1,4-dione and 2,3-bis(dimethylsilyl-tert-butyl)buta-1,3-diene-1,4-dione). Therefore, it is all the more interesting that the hypothesis of the twisted structure is supported by the measured infrared band activities, in the cumulenic stretching region, of the two bands around 2153 and 2129 cm-1, assigned to the ν1 (A) and ν11 (B) modes of the photoproduct B, assumed to be in an s-gauche conformation (Table 2): indeed, the linear dependence of their growing integrated absorbances (from Table IS) suggests a gauche form, pertaining to the C2 point group, rather than a mixture of C2V and C2h conformations. The quadratic force field, harmonic frequency, and IR intensity calculations were performed at the MP2/6-31G* and MP2/DZ+P levels. The valence force field was calculated in the complete set of 20 internal coordinates, as reported in Table IIS. The theoretical spectra were then scaled (Table 2). Products E and F. According to Scheme 1, the photodecomposition of the bisketene (B) leads to the formation of a carbon monoxide molecule, labeled D in Figure 1, and of two new species that we shall assign, in this section, to dichlorocyclopropenone and dichloropropadienone, labeled E and F in Figure 1. In a dilute argon matrix about 84% of the intermediate B was decomposed after 200 min of irradiation. In Table IS, the linear dependence of the integrated absorbances of E with those of F clearly demonstrates the concomitant formation of these two products from the photodecomposition of B by the elimination of D. This result suggests a different chemical pathway from that observed by Brahms and Dailey,30 in which the propadienone is obtained as a minor but stable component by photolysis of cyclopropenone irradiated in an argon matrix (λ > 185 nm). The slopes of the linear variation of xE or xF versus (xE + xF) respectively equal to 0.97 and 0.07 give a good estimate of the branching ratio of the photodissociation processes giving rise to E or F (Figure 4). Presumably, the reaction path proceeds through the formation of a ketene carbene (labeled C
Photolysis of Dichlorocyclobutenedione in Rare Gas in Figure 1), from which the formation of the dichloropropadienone (F) is disfavored by the necessary chlorine atom shift in the trapping cage, while the folding of the transient species C to yield the less bulky ring-closed structure E should be easier because of the constraints exerted by the surrounding matrix. The dichlorocyclopropenone (E) is a three-membered ring molecule containing a double bond within the ring and an external double bond. It belongs to the C2V symmetry point group and has 12 fundamental vibrational modes, 11 of them being infrared active.31 The assignment of the infrared absorption was made by using the ab initio calculated spectra at the MP2/6-31G* level. Observed and calculated frequencies are reported in Table 3, together with the description of the fundamentals in the last column. The CdO and CdC stretches are clearly assigned to the bands at 1896 and 1630 cm-1, as previously done by Mitchell et al.,31 but there are some discrepancies in the assignment of the other modes. The peak at 1118 cm-1 was assigned to the antisymmetric C-Cl and that at 628 cm-1 to the CdO out-of-plane bending. For the dichloropropadienone (F), a single publication26 announced its possible presence as an intermediate in dichloromaleic anhydride pyrolysis, which gives dichloroacetylene as the final product. However the authors assumed, only by analogy with earlier work by Brahms and Dailey,30 that some unassignable transient infrared bands in their pyrolysis mixture were due to dichloropropadienone. No concrete values were given. From the optimized geometry of F reported in Table 1, the vibrational spectrum, calculated at the MP2/6-31G* level of theory and given in the complete set of 15 internal coordinates (which are reported in Table IIS), predicts the frequency of the stretching cumulenic mode at 2241 cm-1. Experimentally, this mode appears, however, as a doublet of equal intensities, located around 2200 and 2260 cm-1 in argon and at 2193 and 2253 cm-1 in xenon matrices; the identical kinetic behavior of these two components supports their assignment to the single cumulenic mode split into two spectral lines. This anomalous spectral feature of the cumulenic mode does not depend on the host material or concentration; it is still present after matrix annealing. Temperature cycling just induces a reversible red shift of the two components. In the xenon matrix, even after annealing to 80 K, despite a significant diffusion of the carbon monoxide and a partial but irreversible redistribution of the intensities between the two peaks, the doublet, at 10 K, is always found at the same frequencies. A multiplet structure in the 2125-2105 cm-1 range was already observed by Chapman et al.32 for propadienone isolated in either argon or nitrogen matrices, but this structure changed with deposition temperature and disappeared on annealing. These authors assigned the multiplet to the same vibrational mode of the molecule embedded in different physical environments, corresponding to different states of complexation with the carbon monoxide trapped in the same crystalline cage. Rather than a distortion of the equilibrium geometry of the trapped molecule, induced by the neighboring rare gas atoms or molecules, Fermi resonance between the fundamental ν1 and a combination band could be the reason for the doublet structure. Then, the observed distribution of the experimental intensities suggests an exact resonance between the two interacting levels, which seems to be the case for ν1 and (ν2 + ν6), almost at the theoretical level, but an experimental proof would be necessary. We are not in a position to synthesize the isotopic species; nevertheless preliminary calculations show that the easier substitution of O16 by O18 would not remove the problem since the gap between ν1 and ν2 + ν6 frequencies (11 instead of 6 cm-1) is probably too small to modify strongly the postulated
J. Phys. Chem., Vol. 100, No. 40, 1996 16051 Fermi resonance. Further work on 13C-substituted compounds would be necessary. Note that such a splitting was not clearly observed in the vapor-phase infrared spectrum of difluoropropadienone.16 Nevertheless, although the origin of this splitting has not been elucidated, we believe that the coherence of the process, leading to the dichloropropadienone formation, rests on a reasonable assumption. The averaged value of the frequencies of the two peaks is in very good agreement with our scaled calculated frequency of the dichloropropadienone cumulenic stretching mode. For the other modes, the experimental spectrum of F matches well enough the calculated one for dichloropropadienone, as can be seen in Table 4. Product G. The dichloroethyne, labeled G in Figure 1, is a linear molecule of D∞h symmetry, whose infrared spectrum in the gas phase has been recently reported by McNaughton.26 In concentrated argon matrices (Figure 3S) we observed weak bands at 2090 and 846 cm-1, respectively assigned to (ν1-ν5) and (2ν4+ν5), and an intense doublet at 990.2 and 993.6 cm-1 assigned to the antisymmetric C-Cl stretching for the two 35ClCC-35Cl and 35Cl-CC-37Cl isotopomers (spectra are given in the supporting information). We also observed a very weak signal at 2232 cm-1, which might be assigned to the forbidden ν1 symmetric triple-bond stretching vibration, becoming active due to partial loss of symmetry in the surrounding cage.33 Following this assumption, from the combination bands, ν4 and ν5 can be calculated at 352 and 141 cm-1, respectively, in good agreement with the gas-phase values.26 The attribution is supported by the calculated vibrational spectrum in comparison with the experimental ones; results are reported in Table 5. Conclusion The photochemistry of 1,2-dichlorocyclobutene-3,4-dione isolated in cryogenic matrices has been studied using highresolution FTIR spectroscopy. The results prove the formation of unstable species, which could not be observed at room temperature on the experimental time scale. The reaction products have been identified by theoretical analysis using ab initio calculations. The lowest energy conformation of the first reaction intermediate, identified as the bisketene 2,3-dichloro1,3-butadiene-1,4-dione, has been calculated using a secondorder Møller-Plesset perturbation including all the molecular eigenfunctions, for every point of the potential energy surface. The calculations corroborate the twisted structure of this species. The decomposition of the bisketene leads to the concomitant formation of dichlorocyclopropenone and dichloropropadienone by two concurrent pathways. The amounts of reaction products were carefully examined, and coherent kinetic data support the assumed chemical scheme. A detailed examination of the dissociation mechanism, by determining the geometries and energies of the transition states and by estimation of the activation energies, is presently in progress. A modeling by quantum mechanical calculations will be published later. Acknowledgment. The French Government is gratefully acknowledged for its financial support. The calculations were carried out at the Centre Re´gional de Calcul et de Te´le´communications Scientifiques (Re´gion Provence, Alpes, Coˆte d’Azur). Supporting Information Available: Figure 1S: Spectra of A in a concentrated argon matrix after 0, 26, and 270 min of irradiation. Figure 2S: Spectra of A in a xenon matrix before and after irradiation. Figure 3S: Spectrum of G in an argon matrix after irradiation of A. Table IS: Experimental integrated intensities in the carbonyl and cumulenic stretching range at
16052 J. Phys. Chem., Vol. 100, No. 40, 1996 different irradiation times for A, B, D, E, and F. Table IIS: Internal coordinate sets for B, E, F, and G (6 pages). Ordering information is given on any current masthead page. Registry Numbers. A: 2892-63-9; D: 630-08-0; E: 2043410-0; G: 7572-29-4. References and Notes (1) Bachman, C.; N'Guessan, T. Y.; Debu, F.; Monnier, M.; Pourcin, J.; Aycard, J. P.; Bodot, H. J. Am. Chem. Soc. 1990, 112, 7468. (2) Davidovics, G.; Monnier, M.; Allouche, A. Chem. Phys. 1991, 150, 395. (3) Brahms, J. C.; Dailey, W. P. J. Am. Chem. Soc. 1990, 112, 4046. (4) Lunelli, B.; Orlandi, G.; Zerbetto, F.; Giorgini, M. G. J. Mol. Struct. (THEOCHEM) 1989, 201, 307. (5) Mincu, I.; Allouche, A.; Cossu, M.; Aycard, J. P.; Pourcin, J. Spectrochim. Acta 1995, 51A, 349. (6) Hagen, K.; Lunelli, B. J. Phys. Chem. 1989, 93, 1326. (7) De Selms, R. C.; Fox, C. J.; Riordan, R. C. Tetrahedron Lett. 1970, 781. (8) Schroeder, W.; Monnier, M.; Davidovics, G.; Allouche, A.; Verlaque, P.; Pourcin, J.; Bodot, H. J. Mol. Struct. 1989, 197, 227. (9) Fogarasi, G.; Pulay, P. Vibrational Spectra and Structure; Elsevier: Amsterdam-Oxford-New York-Tokyo, 1985; Vol. 14, Chapter 3. (10) Boatz, J. A.; Gordon, M. S. J. Phys. Chem. 1989, 93, 1819. (11) Allouche, A.; Pourcin, J. Spectrochim. Acta 1993, 49A, 571. (12) McAllister, M. A.; Tidwell, T. T. J. Am. Chem. Soc. 1994, 116, 7233. (13) Schmidt, M. W.; Baldridge, K. K.; Boatz, J. A.; Jensen, J. H.; Koseki, S.; Gordon, M. S.; Nguyen, K. A.; Windus, T. L.; Elbert, S. T. QCPE Bull. 1990, 10, 52.
Mincu et al. (14) Krishnan, R.; Binkley, J. S.; Seegere, R.; Pople, J. A. J. Chem. Phys. 1980, 72, 650. (15) Hassett, D. M.; Hedberg, K.; Marsden, C. J. J. Phys. Chem. 1993, 97, 4670. (16) Brahms, J. C.; Dailey, W. P. J. Am. Chem. Soc. 1989, 111, 3071. (17) Tam, H. S.; Harmony, M. D.; Brahms, J. C.; Dailey, W. P. J. Mol. Struct. 1990, 223, 217. (18) Scott, A. P.; Radom, L. Chem. Phys. Lett. 1992, 200, 15. (19) Brown, R. D.; Godfrey, P. D.; Champion, R.; McNaughton, D. J. Am. Chem. Soc. 1981, 103, 5711. (20) Brown, R. D.; Godfrey, P. D.; Champion, R.; McNaughton, D. J. Am. Chem. Soc. 1982, 104, 6167. (21) Brown, R. D. J. Mol. Struct. 1983, 97, 293. (22) Radom, L. Aust. J. Chem. 1978, 31, 1. (23) Komornicki, A.; Dykstra, C. E.; Vincent, M. A.; Radom, L. J. Am. Chem. Soc. 1981, 103, 1652. (24) Farnell, L.; Radom, L. Chem. Phys. Lett. 1982, 21, 373. (25) Brown, R. D.; Dittman, R. G. Chem. Phys. 1984, 83, 77. (26) McNaughton, D. Struct. Chem. 1992, 3, 245. (27) Mosandl, T.; Wentrup, C. J. Org. Chem. 1993, 58, 747. (28) Zhao, D.; Allen, A. D.; Tidwell, T. T. J. Am. Chem. Soc. 1993, 115, 10097. (29) Werstiuk, N. H.; Ma, J. J. Chem. Soc., Faraday Trans. 1994, 90, 3383. (30) Brahms, J. C.; Dailey, W. P. J. Am. Chem. Soc. 1989, 111, 8940. (31) Mitchell, R. W.; Merrit, J. A. Spectrochim. Acta 1971, 27A, 1643. (32) Chapman, O. L.; Miller, M. D.; Pitzenberger, S. M. J. Am. Chem. Soc. 1987, 109, 6867. (33) The significant regions of the 1,2-dichloroacetylene infrared spectrum, as all experimental data, are available on simple request.
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