UV-Induced Unimolecular Photochemistry of Diketene Isolated in

Article pubs.acs.org/JPCA

UV-Induced Unimolecular Photochemistry of Diketene Isolated in Cryogenic Inert Matrices S. Breda,* I. Reva,* and R. Fausto Department of Chemistry, University of Coimbra, 3004-535 Coimbra, Portugal S Supporting Information *

ABSTRACT: Diketene (C4H4O2) monomers were isolated in cryogenic Ar (15 K) and Xe (30 K) matrices. The infrared (IR) spectra of the freshly deposited matrices show that diketene monomers exclusively adopt the 4-methylene-oxetan2-one form. In situ photochemical transformations of diketene were induced by tunable UV laser light. Diketene was found to be photostable when exposed to near-UV irradiations (λ> 300 nm). Irradiations in the middle-UV domain showed different types of photochemical reactivity occurring upon irradiations with 280 > λ > 240 nm and λ = 225 nm. The photoproducts were characterized by IR spectroscopy supported by B3LYP/6-311++G(d,p) calculations. Upon irradiation in the 280 > λ > 225 nm range, diketene was found to decompose in two ways: (i) with production of two parent ketene molecules (OCCH2), and (ii) with production of cyclopropanone (CP) plus carbon monoxide. For irradiations in the 280 > λ > 240 nm range, diketene exhibited two additional reactions: (iii) decomposition to allene (H2CCCH2) and carbon dioxide, and (iv) isomerization into cyclobutane-1,3-dione (CB). Of the above photoproducts, CP and CB were consumed by the same UV irradiations that resulted in their generation. Positive spectroscopic identification of CP and CB turned out to be possible with near-UV irradiations: CP decomposes to ethylene and carbon monoxide upon irradiation with λ = 345 nm; CB decomposes exclusively to two parent ketene molecules, without isomerization back to diketene or decarbonylation, upon irradiation with λ = 330 nm. Natural bond orbital (NBO) analysis showed that the two lowest excited singlet states of diketene are almost degenerate in energy and correspond to π* orbitals of CC and CO moieties. The NBO calculations helped to establish that the third excited singlet state, in terms of energy, has σ*(3s) Rydberg character, in accord with the literature. molecule,5,6 and in addition to the chemical reactions that were used to determine the structure of the “ketene dimer”, a variety of physical measurements was undertaken. Prior to 1940, these measurements included determinations of the molecular weight, the refractive index,7 and the ultraviolet spectrum;8 the dipole moment was measured,9 and the Raman spectrum was analyzed.6,9 An infrared spectrum, first obtained in 1946 by Whiffen and Thompson,10 established that the “ketene dimer” contained a four-membered ring, and the structure was unequivocally established in 1952 by the X-ray study4 that showed that crystalline DK had the structure of 4-methyleneoxetan-2-one (Scheme 1), which had been suggested as one of several alternatives in the previous infrared studies.10,11 This structure was later corroborated by 1H NMR studies12−14 showing that, for both crystalline material and liquid between room temperature and 120 °C, this is the only significant form. In 1957, Miller and Carlson15 reinvestigated the infrared spectrum of the DK vapor and concluded that DK, in all its states, exists in the same single form. They concluded that the five strong bands in the double bond stretching region, which had long been a stumbling block to understanding DK structure, are due to two fundamentals and three combination

1. INTRODUCTION Diketene (4-methylene-oxetan-2-one or DK; Scheme 1) is employed in preparation of acetoacetic acid derivatives and Scheme 1. Diketene (4-Methylene-oxetan-2-one): Structure and Numbering of O and C Atoms Used in this Study

heterocycles, and its derivatives have versatile applications in manufacturing agrochemicals, dyes, pigments, pharmaceuticals (including vitamins) and stabilizers for polyvinyl chloride (PVC) and polyesters. In 1986, in a review entitled “Diketene”,1 Clemens strived to catalogue every type of synthetic transformation that had been effected with DK and to provide a comprehensive guide on the literature published from 1907 until early 1985 that described this chemistry. The elucidation of the structure of DK required 45 years, from the time of discovery of the “ketene dimer” (as the molecule was called at that point in time)2,3 until an X-ray diffraction4 study was completed in 1952. During that period, five different formulas were suggested for the C4 H4 O 2 © 2012 American Chemical Society

Received: November 22, 2011 Revised: January 20, 2012 Published: January 24, 2012 2131

dx.doi.org/10.1021/jp211249k | J. Phys. Chem. A 2012, 116, 2131−2140

The Journal of Physical Chemistry A

Article

tones. Later, Durig and Willis, Jr.16 studied the vibrational spectra and structure of DK and DK-d4. The infrared spectra of the gaseous DK and DK-d4 and of solutions in carbon tetrachloride and carbon disulfide have been reinvestigated, and the measurements extended from 4000 to 250 cm−1. The Raman spectra of DK were also reinvestigated and band depolarization values measured. Assignments of the fundamentals based on position, band type, and depolarization values were given. They agree with the choices of combination and overtone frequencies given by Miller and Carlson15 for the assignment of three of the five bands in the double bond stretching region in the spectrum of DK, and that the intensities of these overtones and combination bands result from Fermi resonances with the two fundamentals lying in the same region. In 1969, Carreira and Lord17 reported the farinfrared spectra of DK under high resolution. They concluded that the ring is planar, according with previous X-ray4 and midinfrared16 evidence. DK is usually obtained by dimerization of two parent ketene (H2CCO) molecules,1 but it can also be produced from carbon dioxide and allene by catalytic synthesis.18 According to these studies, other structural isomers having a common C4H4O2 formula: 3-methylene-2-oxetanone (3MO) and 2-methylene-3-oxetanone (2MO, Scheme 2) can be formed in the process of the syntheses, besides DK.

(N50, Air Liquide), and argon (N60, Air Liquide). The concentrations of the premixtures were controlled manometrically and were chosen to be 1:2000 (DK:Xe) and 1:6000 (DK:Ar). The premixture was introduced into the cryostat chamber through a needle valve. A cold CsI window mounted on the tip of an APD Cryogenics DE-202A closed-cycle helium refrigerator was used as the optical substrate. Its temperature was kept at 15 and 30 K in the experiments with argon and xenon, respectively. The matrices were irradiated through the outer quartz window of the cryostat, with narrow-band UV radiation provided by a frequency-doubled signal beam of the QuantaRay MOPO-SL pulsed (10 ns) optical parametric oscillator (fwhm ∼0.2 cm−1, repetition rate 10 Hz, pulse energy ∼3 mJ) pumped with a pulsed Nd:YAG laser. Alternatively, the matrices were irradiated with filtered or unfiltered light from a 500 W Hg(Xe) lamp (Spectra-Physics, model 66142) adjusted to provide 200 W output power. All the infrared spectra, in the 4000−400 cm−1 region, were recorded with 0.5 cm−1 resolution using a Nicolet 6700 FT-IR spectrometer, equipped with a KBr beamsplitter and a DTGS detector.

Scheme 2. Structures of 3MO and 2MO

The equilibrium geometries for all studied species were fully optimized at the density functional theory (DFT) level of theory with the standard 6-311++G(d,p) basis set, and the three-parameter density functional abbreviated as B3LYP, which includes Becke’s gradient exchange correction,27 the Lee, Yang, and Parr correlation functional28 and the Vosko, Wilk, and Nusair29 correlation functional. No restriction of symmetry was imposed on the initial structures. The harmonic vibrational frequencies were calculated at the optimized geometries using the same theoretical method. The nature of the obtained stationary points on the potential energy surfaces of the respective systems was checked through the analysis of the corresponding Hessian matrix. A set of internal coordinates was defined, and the Cartesian force constants were transformed to the internal coordinates space, allowing ordinary normal-coordinate analysis to be performed as described by Schachtschneider and Mortimer.30 The internal coordinates used in this analysis (listed in Tables S01−S08 of the Supporting Information) were defined as recommended by Pulay et al.31 Potential energy distribution (PED) matrices32 have been calculated, and the elements of these matrices greater than 10% are given in Table 1 and Tables S09−S15 (Supporting Information). Optimized structures and atom labeling of DK, 3MO, 2MO, ketene, cyclopropanone (CP), cyclobutane-1,3-dione (CB), allene, and ethylene molecules are given in Scheme S01 (Supporting Information). All calculations in this work were done using the Gaussian 03 program.33 The specific nature of the electronic structures in the studied compounds was characterized by natural bond orbital (NBO) analysis,34,35 using NBO version 3, as implemented in Gaussian 03. The calculated harmonic frequencies were also used to assist the analysis of the experimental spectra (scaled with a factor of 0.978) and to account for the zero-point vibrational energy (ZPVE) corrections (nonscaled).36,37

3. COMPUTATIONAL

The theoretically predicted stabilities and IR spectra of DK and its isomers were calculated at the MP2/6-311++G(d,p) level by Dobrowolski and coauthors.19−21 Comparison of the zero-point corrected total energies of the studied isomers clearly showed DK to be the most stable structure, while 3MO and 2MO isomers are less stable than DK, by 3.86 and 20.1 kcal mol−1, respectively. Summarizing, the energies and vibrational spectra of DK and its positional isomers are well-documented based on theoretical calculations,19−25 as well as early experimental works from the 1960s.10,11,15,16 Surprisingly enough, among all the abundant literature dedicated to DK, there is still a blank space concerning its photochemical reactivity. To the best of our knowledge, there is only one study in this field reporting 13 C-selective infrared multiple photon decomposition of DK, yielding CO2 and allene, using a tunable IR laser (in the 936− 967 cm−1 range) as the source of irradiation.26 No peaks due to decomposition products other than CO2 were detected in that study.26 In the present work, we aim to address the photochemistry of DK induced by a tunable ultraviolet laser source.

2. EXPERIMENTAL SECTION The compound used in this study was a commercial product supplied by Aldrich. Because of the high reactivity of DK, special care was taken to prevent its contact with the atmosphere. Before usage, DK was additionally purified from the volatile impurities by multiple freeze−pump−thaw cycles. Vapors of degassed liquid DK were premixed in a 3 L glass reservoir with a large excess of the host matrix gases, xenon 2132



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Table 1. Experimental Wavenumbers ν (cm−1), Integral Intensities I (Relative) of the Infrared Bands of Diketene Compared with Theoretical Wavenumbers ν (cm−1), Absolute Intensities Ath (km mol−1) and Potential Energy Distributions (%) experimental Ar matrix (1:6000; 15K) ν̃

experimental Xe matrix (1:2000; 30K)

I

ν̃

a

3037.8

exp.16 ν̃

a

sym.

PED c (%)

3183.3 3094.3

0.5 0.7

A′ A′

ν(CH2)(sp2)as(100) ν(CH2)(sp2)s(99)

3076 2979 1910 1879 1770 1715 1673 1416 1392 1373

3060.7 3010.1 1907.3

0.3 2.2 424.1

A″ A′ A'

1721.1

336.6

A′

1411.3 1385.9

12.1 4.7

A′ A′

ν(CH2)(sp3)as(100) ν(CH2)(sp3)s(100) ν(C2=O5)(81) FR1 [ν(C2=O5)] with [ν(O1-C2) + ν(C4-O1)] FR2 [ν(C4=C8)] with [2 × ν(C4-O1)] ν(C4=C8)(70) FR2 [ν(C4-C8)] with [ν(C4-O1) + ν(C3-C4)] δ(CH2)(sp3)scis(87) + δ(CH2)(sp2)scls(11) δ(CH2)(sp2)scis(81) + δ(CH2)(sp3)scls(12)

1244

1230.1

92.9

A′

δ(CH2)(sp2)rock(23) + ν(C4-Ol)(20) + δ(C4=C8)(15) + ν(O1-C2)(13) + ν(C3-C4)(10) + δ(C2=O5)(10)

1180.5

0.5

A′

δ(CH2)(sp3)wag(87)

1096

1091.2

1.5

A″

δ(CH2)(sp3)twist(91)

1006

994.4

134.9

A′

ν(O1-C2)(32) + δ(CH2)(sp2)rock(24) + δ(C2=O5)(13) + ν(C3-C4)(12) + δ ring(11)

965.5

11.6

A′

961.3

3.4

A″

ν(C2-C3)(37) + δ ring(21) + δ(CH2)(sp2)rock(13) + ν(C3-C4)(13) δ(CH2)(sp3)rock(58) + τ ring(18) + γ(C2=O5)(15) ν(C4-O1)(55) + ν (O1-C2)(20) + δ(CH2)(sp2)rock(19) δ(CH2)(sp)2wag(83) + γ(C4=C8)(15) ν(C3-C4)(30) + ν(C2-C3)(23) + ν(Ol-C2)(18) + δ(CH2)(sp2)rock(10) δ(CH2)(sp2)twist(90) δ ring(58) + ν(C4=C8)(12) δ(C2=O5)(57) + δ(C4=C8)(18) + ν(C2-C3)(15) γ(C2=O5)(67) + δ(CH2)(sp3)rock(25) γ(C4=C8)(50) + δ(CH2)(sp3)rock(15) + δ(CH2)(sp2)wag(12) + τ ring(11) δ(C4=C8)(65) + δ(C2=O5)(11) τ ring(69) + γ(C4=C8)(18)

3137 3018 R

2967.5 1908.6 FR1 1875.0 FR2 1765.6 FR2 1714.7 FR2 1673.6 1413.1 1395.2 1376.0 FR1

1241.8, 1239.5

1221.6 1207.1 1181.0 1160.2 1129.5 1095.0 1050.2 1031.2 1017.8, 1008.2

336.3

264.9 5.1 2.3 3.5 65.6

1.6 0.7 0.9 0.5 1.6 1.4 1.0 0.7 149.9

1005.2 996.9 980.3

1.2 5.3

960.4

3.2

2967.4 1900.3 FR1 1866.8 FR2 1751.7 FR2 1704.5 FR2 1667.0 1407.8 1389.8 1370.7 1272.8 1237.7 FR1

382.5

279.7 8.6 7.7 6.5 1.0 91.5

1228.7 1222.5 1196.7 1181.1 1154.6, 1149.2 1125.4 1116.0 1095.4

1.0 1.1 1.4 0.9 2.3 2.3 0.8 2.1

1011.1, 1006.1

162.0

1003.7 991.6 983.4 979.2 958.9, 957.8

0.8 1.5 1.2 4.1

ν

th

I

3017.2 3008.3

calculated B3LYP/6-311++G(d, p) b

A

1123

R

984

959

884.0, 878.1

179.9

875.7

185.3

885

870.0

195.4

A′

837.3 805.4, 802.7

53.8 31.1

834.5 803.4

63.8 37.5

838 803

849.1 797.9

61.1 25.2

A″ A′

715.1 661.8 517.5 499.7 445.9

0.2 0.7 2.9 4.7 7.6

A″ A′ A′ A″ A″

313.2 128.8

0.7 0.7

A′ A″

R

525.1 507.3, 505.9 446.3, 444.8

1.9 4.6 5.8

525.6 502.8

2.6 2.9

673 525 506 444

R

324 144

R

a

Relative integrated experimental intensities, normalized in such a way that their sum is equal to the sum of the corresponding calculated intensities. Theoretical positions of absorption bands were scaled by a factor of 0.978. cPEDs lower than 10% are not included. Bold numbers show the strongest contribution. Definition of symmetry coordinates is given in Table S01 (Supporting information). See Scheme 1 for atom numbering. R Raman frequencies (Ref 16). FR1Fermi resonance #1; FR2Fermi resonance #2; the experimental intensities for FR1 and FR2 are provided for the entire multiplets. b

4. RESULTS AND DISCUSSION 4.1. Infrared Spectra of DK Monomers Isolated in Ar and Xe Matrices. The experimental infrared spectra of DK monomers isolated in argon and xenon matrices are presented

in Figure 1a,b. They agree nicely with the theoretical spectra (Figure 1c) in the region below 1500 cm−1. According to the previous reports,1,18 the synthesis of DK might result in the contamination of the target compound with 2133



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Figure 1. Experimental infrared spectra of DK monomers isolated in (a) argon matrix at 15 K (ratio DK:Ar is 1:6000) and (b) xenon matrix at 30 K (ratio DK:Xe is 1:2000) compared with the theoretical infrared spectra of (c) DK calculated at the B3LYP/6-311++G(d,p) level. Theoretical wavenumbers were scaled by 0.978. Baselines of the experimental spectra were corrected.

uniformly. The band assignments for the fingerprint region of the spectra of DK isolated in argon and xenon matrices are provided in Table 1. Besides the bands ascribable to the fundamental vibrations, the experimental spectrum of DK monomers also exhibits a large number of bands due to anharmonic effects (overtones and combination tones), which are particularly evident in the 2300−1930 cm−1 spectral range (Figure 2), where harmonic

its structural isomers, 3MO and 2MO (see Scheme 2). In order to check the matrices for the presence of these possible contaminants, the infrared spectra of DK, 3MO, and 2MO were theoretically predicted at the B3LYP/6-311++G(d,p) level (Figure S01). We could only identify, in both Ar and Xe matrices, bands corresponding to the DK form, although at a first glance 3MO and 2MO isomers seemed to explain the “excessive” high intensity bands observed in the 1950−1650 cm−1 region that the theoretical spectrum of DK does not predict (see Figure S01). However, this possibility is quickly put apart when the fingerprint (1500−500 cm−1) region is analyzed. If these isomers were present in the sample, one should observe at least their most intense bands in the fingerprint region: at 1047.3 cm−1 (νO1−C2) and 921.4 cm−1 (νC4−O1), for 3MO (Table S09), and at 1182.5 cm−1 [δCH2(sp2)rock + ν(O1−C2)] and 950.5 cm−1 (νC4−O1), for 2MO (Table S10). Figure S01 clearly shows that at those frequencies the experimental spectra do not show any bands. Additionally, the relative intensities of the theoretically predicted DK spectrum (Table 1), suggest that the band due to the C2O5 stretching, at 1907.3 cm−1, should be twice as intense as that predicted at 870.0 cm−1. However, this can only be true if the two bands at 1908.6 and 1875.0 cm−1 observed in the Ar matrix (1900.3, 1866.8 cm−1 in Xe) are due to the νC2O5 mode. This doublet can be explained by Fermi resonance between the νC2O5 fundamental mode, predicted at 1907.3 cm−1, and the 994.4 + 870.0 = 1864.4 cm−1 combination tone (νO1−C2 + νC4−O1), as previously suggested by Durig and Willis, Jr.16 The same reasoning can be made for the assignment of the νC4C8 mode, where the observed triplet at 1765.6, 1714.7, and 1673.6 cm−1 in the Ar matrix (1751.7, 1704.5, and 1667.1 cm−1 in Xe) is due to Fermi resonance between the νC4C8 fundamental (predicted at 1721.1 cm−1), the overtone 2× 870.0 = 1740 cm−1 (2× νC4−O1), and the combination 870.0 + 797.9 = 1667.9 cm−1 (νC4−O1 + νC3−C4). The characteristic shape of the infrared spectrum of matrixisolated DK, with five strong peaks in the 2000−1600 cm−1 range (see Figure 1) also agrees well with the previously reported spectrum for the compound in gas phase,38 thus providing further support to the assignment of all these bands to the DK monomer. Additional evidence of the monomeric nature of these absorptions is provided by the photochemical experiments (vide infra). During the photochemical transformations of DK, all these bands decrease proportionally and

Figure 2. Experimental infrared spectra of DK isolated in (a) an Ar matrix at 15 K and (b) a Xe matrix at 30 K, in the 2300−1930 cm−1 region (combination bands and overtones).

calculations do not predict any band. As it could be expected, the average intensities of bands in this region are considerably lower than in the fingerprint region. The fact that all these bands proportionally decrease in intensity during the UVinduced transformations of DK, concomitantly with the bands due to the fundamental vibrations, confirms the origin of these features. The frequencies of the combination bands and overtones appearing in the 2300−1930 cm−1 region of the infrared spectrum of DK are provided in Table S16. 4.2. Irradiation of the Matrices (Photochemical Experiments). Monomers of DK isolated in Ar and Xe matrices were irradiated with UV laser light. The photochemistry of DK isolated in these matrix environments was found to be similar. The difference between the results obtained in the two matrices consisted only in the existence 2134



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molecular orbital (HOMO) is occupied by the lone electron pair of the carbonyl oxygen atom of DK and is of σ-type. Interestingly, the two lowest virtual orbitals have very close calculated energies, especially at the MP2 level. These two orbitals correspond to the antibonding π* orbitals of the CC and CO bonds. The calculations thus point to the simultaneous engagement of both the lowest unoccupied molecular orbital (LUMO) and LUMO+1 in the lowest energy electronic band of DK. The irradiation of the matrices at λ = 280 nm was followed by irradiations at shorter wavelengths. All irradiations with 280 > λ > 240 nm were found to produce similar spectral changes. The second transition in the electronic spectrum of DK was reported at 5.89 eV (∼210.5 nm) and assigned to a π → σ*(3s) Rydberg transition.39 This assignment is again in agreement with the present NBO calculations that predict the LUMO+2 to be of the Rydberg type. Furthermore, it was gratifying to find that when one reduced the wavelength of the UV-tunable laser to λ = 225 nm (in a separate experiment), the photochemical behavior of DK resulted in different relative amounts of photoproducts (compared to the longer wavelength excitations described above), thus clearly indicating the involvement in the phototransformations of a new excited electronic state of DK. Further increase of the excitation energy in our experiments was not possible, since the output of our laser equipment is limited by 220 nm. Two typical, quantitatively different outcomes of UVirradiations of DK, obtained in experiments with λ = 250 nm and λ = 225 nm, are shown in Figure 4.

of band splitting, due to the site-effects, in the case of argon. This band splitting (on the order of 1−2 wavenumbers) was characteristic both of the DK reagent and its photoproducts. For simplicity, the following text discusses only the photochemistry of DK isolated in xenon, where site-splitting effects were found to be unimportant. Irradiation of the matrices was performed at different wavelengths. For the first irradiation, UV light with λ = 345 nm was chosen. Shorter wavelengths were used for subsequent irradiations of the matrix. After each irradiation (with duration of 1 min), the matrix was monitored by taking its IR spectrum. The cycles of UV irradiation and IR monitoring continued, with UV wavelength reduced every time by 5 nm. Irradiations in the range 345 > λ > 290 nm did not induce any transformation in matrix-isolated DK. Minor spectral changes occurred upon irradiation with λ = 285 nm. When irradiation at 280 nm was applied, the IR spectrum of the DK monomer started to decrease noticeably, while new bands due to photogenerated species appeared in the spectrum. The energy of 4.36 eV (∼284.4 nm) was earlier tentatively assigned to an n → π* transition of monomeric DK, in an experimental study of the electronic spectrum of DK by the technique of variable-angle, electron energy loss spectroscopy.39 The NBO calculations performed in the present study confirm the assignment reported before.39 The energies of the frontier and several adjacent molecular orbitals calculated at the DFT and MP2 levels of theory are presented in Figure 3. The highest occupied

Figure 4. Experimental difference spectra obtained upon 2.5 min irradiation of DK monomers isolated in xenon matrices at 30 K with UV light: (a) λ = 250 nm; (b) λ = 225 nm. Positive bands are due to photoproducts, and negative bands (truncated) are due to consumed reagent (DK). The bands designated with X and Y are discussed in the text.

The observed different relative intensities of the IR bands due to the photoproducts in the experiments with different incident UV light can be considered as evidence of generation of several groups of photoproducts. Identification of some of the photoproducts was quite obvious. The strongest photoproduct band, centered at 2138.5 cm−1 (see Figure 4), indicates the formation of ketene, OCCH2. The spectrum of this molecule has a very characteristic strong infrared absorption due to the antisymmetric OCC stretching vibration in this range.40−44 Another photoproduct band, centered at 2335.6 cm−1 (see Figure 4a) also has a straightforward assignment, corresponding to the antisymmetric OCO vibration of matrix-isolated CO2.45

Figure 3. Energy diagram of selected NBOs calculated for DK at the DFT and MP2 levels of theory with the 6-311++G(d,p) basis set. 2135



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Table 2. Experimental Wavenumbers ν (cm−1) of the Infrared Bands of the Observed Photoproductsa Compared with Literature Data and with Theoretical Wavenumbers ν (cm−1), and Absolute Intensities Ath (km mol−1) Calculated at the B3LYP/6-311++G(d,p) Level approximated description (% PED) carbon dioxide v(CO2)as(100) δ(CO2)(100) ketene v(CH2)(sp2)as(100) v(CH2)(sp2)s(99) v(CCO)as(98) v(CC18O)as v(C13CO)as δ(CH2)(sp2)scis(79) v(CCO)s(76) δ(CH2)(sp2)rock(75) δ(CH2)(sp2)wag(92) γ(CCO)(92) carbon monoxide v(CO)(100) allene v(CC)as(99) δ(CH2)(sp2)asscis(100) δ(CH2)(sp2)2rock(66) δ(CH2)(sp2)1rock(66) δ(CH2)(sp2)1wag(100) δ(CH2)(sp2)2wag(100)

experimental Xe matrix (1:2000; 30 K)

experimental literature

v

v

sym.

vb

Ath

ref 45 (Xe matrix) 2334.5

D∞h Σu Πu

2367 654

712 66

refs 40−44 (Ar matrix) 3153 3062 2142

C2v B2 A1 A1

3198 3107 2173

10 30 693

A1 A1 B2 B1 B1

1375 1144 966 588 555

19 6 8 107 35

refs 65 and 66 (Xe matrix) 2133

C∞v Σ

2163

89

ref 46 (Ar matrix) 1954 1392 998 855 851

D2d B2 B2 E E E E

1998 1391 994 994 851 851

89 4 0.2 0.2 65 65

refs 63 and 64 (Xe matrix)

C2v

1886 1401 1382 1038 985 930

374 5 8 13 25 107

2335 659/654

3158/3147 3061/3051 2138 2113 2083 1375/1374 1110 977/976 589 569/565

2085 1379 1114 974 590

2131

1949

841 839

cyclopropanone 1989/1983 1978

1979 1901 1867 1820/1817

calculated B3LYP/6-311++G(d, p)

1878 1816 1386 1371 1041 967/965 933/930

965/959 934/931/929

cyclobutane-1,3-dione v(CO)as(94) 5(CH2)(sp3)asscis(100) V ringasas(64)

ref 67 (soln. CHCl3) 1765 1340 1158

D2h

1766 1339 1153

B2u B2u

1792 1347 1138

667 61 166

ethylene v(CH2)(sP2ras(100) v(CH2)(sp2)sas(100) 5(CH2)(sp2)asscis(100) 5(CH2)(sp2)swae(100)

3097/3089 2987/2975 1445/1438/1434 942

refs 63 and 64 (Xe matrix) 3106 2994 1440 947

D2h B2u Blu Blu B3u

3151 3053 1439 953

26 17 10 112

v(CO)(82) δ(CH2)sscis(96) δ(CH2fscis(100) δ(CH2)astwis(71) δ(CH2)stwis(86) δ(CH2)stwis(86)

1372

A1 A1 B2 B2 A1 B2

a Photoproducts formed during irradiation of DK in Xe matrix. Definitions of symmetry coordinates are given in Tables S04−S08 (Supporting Information). Complete lists of calculated vibrational frequencies, intensities and PEDs are presented in Tables S11−S15 (Supporting Information). b Theoretical positions of absorption bands were scaled by a factor of 0.978.

the DK precursor occurred upon irradiation of the matrix. These reactions correspond to the simultaneous cleavages of

Appearance of the absorptions due to OCCH2 and CO2 clearly shows that at least two different photocleavages of 2136



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the two pairs of opposite single bonds in the four-membered ring of DK: DK → OCCH2 + OCCH2

(1)

DK → OCO + H2CCCH2

(2)

In one of these fragmentation reactions (1), the DK precursor is transformed into two ketene (K) molecules, and this outcome is observed at any of the UV irradiations applied. The second fragmentation reaction (2) occurs predominantly with irradiations of lower energy (i.e., λ= 250 nm, Figure 4a). Detailed analysis of the IR spectra of the irradiated matrices, revealed that the formation of carbon dioxide from DK is accompanied by production of allene (H2CCCH2), whose much weaker characteristic infrared absorptions at 1960−1950 and 841−839 cm−1 could also be observed experimentally (see Table 2).46 The energetics of the two reactions described in eqs 1 and 2 has been the subject of numerous theoretical works,21,24,25,38,47−49 and photochemical production of ketene, carbon dioxide, and allene from DK could be anticipated. The spectra of the irradiated matrices exhibited additional absorptions due to other photoproducts, whose identification was not so straightforward. Especially characteristic bands of these photoproducts appear centered at 1816.6 and 1766.0 cm−1. These bands are labeled in Figure 4 as X and Y, respectively. The identification of the carriers of these bands turned to a bottleneck of this study. Absorptions at such frequencies are characteristic of a carbonyl stretching vibration. Initially we considered a possible DK fragmentation with the formation of different aliphatic aldehydes or ketones. However, for the plausible structures of carbonyl-containing photoproducts generated from DK, the carbonyl stretching vibrations were reported previously to occur at significantly lower frequencies: 1740−1738 cm−1 for formaldehyde,50−54 1727 cm−1 for acetaldehyde,55 1721 cm−1 for acetone,56 and 1737/1731 cm−1 for hydroxyacetone.57 Therefore, the aliphatic carbonyls were ruled out as the possible carriers of the absorptions observed at 1816.6 and 1766.0 cm−1, and we turned our attention to species containing carbonyl groups attached to cyclic backbones. There is an empirical rule describing the behavior of the carbonyl stretching frequency in heterocyclic carbonyl compounds as a function of the ring size. When the ring strain decreases (i.e., the ring size increases) the carbonyl stretching frequency also decreases.58 According to this rule, the CO stretching vibration was observed in six-membered rings at 1757.4 cm−1 (α-pyrone in Xe matrix)59 and 1761.3 cm−1 (4,6-dimethyl-α-pyrone in Ar matrix),60 in a five-membered ring at 1796.8/1793.5 cm−1 (2(5H)-furanone in Ar matrix),61 and in a four-membered ring at 1814 cm−1 (for gaseous cyclobutanone).62 Despite 2(5H)-furanone sharing the common C4H4O2 formula with DK, expansion of the four-membered DK ring to a five-membered ring seems unlikely in the present case: the observed bands of the photoproducts X and Y generated from DK do not fit the IR spectrum of 2(5H)-furanone.61 Isomerization of DK into a six-membered ring such as α-pyrone or cyclohexanone is impossible. However, formation of a four-membered ring could not be excluded a priori, and the apparent proximity of the observed frequencies 1816.6 cm−1 (X) and 1814 cm−1 in cyclobutanone62 suggested the tentative assignment of X to a carbonyl group linked to a four-membered ring. However, this also turned out to be wrong. A series of additional experiments was necessary to clarify the identities of the carriers of X and Y. The results of these experiments are presented in Figure 5. First of all, it must be noted

Figure 5. Fragments of the experimental spectra of DK isolated in a xenon matrix at 30 K (a) immediately after deposition of the matrix (dotted line), (b) after a series of consecutive short UV irradiations (150 s, λ = 250 nm) + (30 s, λ = 245 nm) + (60 s, λ = 240 nm), (c) after subsequent 25 min UV irradiation with λ = 345 nm, and (d) after subsequent 25 min UV irradiation with λ = 330 nm. The spectra are shifted for clarity. Note that the band of the precursor designated DK does not change from b to d.

that the photoproducts X and Y, just as they are produced with UV-irradiations with 280 > λ > 240 nm, are also consumed with these irradiations. Figure 5b shows the stage of the experiment where X and Y reach their maximum intensities. At that stage, the UV excitation light was tuned to a wavelength of 345 nm and the irradiation continued with intermediate monitoring of the IR spectra. At λ = 345 nm DK was found to be photostable, but the products X and Y were slowly consumed, exhibiting different kinetics. The product X was completely consumed upon 25 min of irradiation at λ = 345 nm (Figure 5c), while approximately half of Y (comparing to the preceding irradiation stage, Figure 5b) was still present in the sample. This permitted us to establish a set of bands disappearing from the IR spectrum along with the band at 1816.6 cm−1 (X), as well as another set of bands appearing in the IR spectrum at the expense of product X. In order to better establish the set of experimental IR bands belonging to X, a dedicated experiment was conducted. In this experiment, the monomers of matrix-isolated DK were initially irradiated with UV light tuned at λ = 225 nm. Under these conditions, photoproduct Y does not show in the spectra of the irradiated samples, while the yield of X increases (cf. Figure 4b). Such a sample, enriched with X and devoid of Y, was subjected to UV irradiations with 345 > λ > 300 nm (which do not consume DK). As a result, X was consumed, and further photoproduct was generated. This experiment allowed us to doubtlessly establish that, besides 1816.6 cm−1, also much weaker absorptions at 1386, 1371, 1041, 967/965, 933/930 cm−1 belong to X. This set of the experimental bands is in excellent agreement with the spectrum of matrix-isolated cyclopropanone.63,64 Formation of CP from DK must be accompanied by decarbonylation and concomitant generation of carbon monoxide.65,66 In turn, CP appears to undergo additional decarbonylation, with production of a second molecule of carbon monoxide and ethylene (H2CCH2). Indeed, decarbonylation 2137



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The above-mentioned points have long hampered the assignment of Y. After the assignment of Y to CB, we scrutinized the bibliography with the aim to find experimental reports on its infrared spectra. We found only one original report on the IR absorptions of CB,67 providing very similar frequencies to those observed by us. A possible mechanism of isomerization from DK to CB requires cleavage of only one single CO bond followed by one internal rotation and ringclosure. The intermediates in such isomerization process must be open shell biradicaloids (see Scheme 3). The adequate Scheme 3. Putative Isomerization Mechanism of DK into CB

theoretical description of such processes requires modeling by multiconfigurational CASSCF methods, followed by multireference configuration interaction calculations,68 which extend beyond the scope of the present study. Having assigned a structure to Y, it is also interesting to comment on its photochemical behavior. During irradiations consuming Y (UV light λ = 330 nm), it was reliably verified that bands due to CP do not increase. This means that CB does not undergo decarbonylation (unlike DK and CP). It does not isomerize back to DK either; the only bands growing upon consumption of CB were the bands due to matrix-isolated ketene. Therefore, CB decomposes with the formation of two ketene molecules upon irradiation at λ = 330 nm. For irradiations with other wavelengths, the photochemistry of CB is overlapped with the reach photochemistry of DK, and no conclusions about CB can be extracted. Scheme 4 summarizes the structures of the identified products. Interpretation of the bands in the 2400−1600 cm−1 region proved to be the key to solve the photochemistry of DK. It became possible to validate our assignments of the photoproducts by comparison with the experimental spectra available in the literature for the respective compounds isolated in low-temperature inert matrices, in solution, or in the gas phase. Interpretation of the experimental data was further assisted by theoretical calculations at the DFT(B3LYP)/6-311+ +G(d,p) level (Table 2). All the calculations on the photoproducts were performed for the monomers (see also Tables S09−S15). It should be noted here that the cycloaddition reaction of two ketene molecules (C2H2O) yielding adducts having the general C4H4O2 formula has been extensively addressed in the literature.24,47−49,69,70 The opposite reaction, decomposition of the C4H4O2 system, has been addressed as well.38,71−73 However, all these studies refer to the ground-electronic-state chemistry of DK formation or decomposition, where the typical reaction barriers are of the order of 50 kcal mol−1. The abovementioned studies do not elucidate much the results obtained in the present work. Here, the isomerizations or cleavages of the C4H4O2 system occur in the excited electronic states, with the typical energies of 100 kcal mol−1 (∼285 nm) initially involved. We hope that this experimental work will stimulate the relevant theoretical research on this subject.

Figure 6. (a) Experimental difference spectrum. Negative bands are due to the consumed photoproduct X generated from DK using UV light λ = 225 nm for 150 s. Positive bands are due to photoproduct generated from X using subsequent UV irradiation with λ = 330 nm for 10 min; (b) Theoretical infrared spectra calculated at the B3LYP/ 6-311++G(d,p) level for CP (▼) and ethylene (Et, ◊) photoproducts. In the theoretical spectrum of CP, intensities were scaled by (−1). The symbols ″∼″ indicate truncated bands.

of CP yielding ethylene corresponds well with the theoretical predictions (see Figure 6). Now, after the nature of photoproduct X is established as CP, let us return to the identification of product Y. In the experiments with initial decomposition of DK using UV light tuned at λ = 250 nm, and subsequent bleaching of X using λ = 345 nm, there is still a non-negligible amount of product Y (see Figure 5c). The remaining amount of Y was completely consumed by applying UV light λ = 330 nm. This allowed us to establish a set of infrared absorptions having the same photochemical behavior as the band at 1766 cm−1 (Y). Surprisingly, we were able to find in the experimental spectra only two such bands, at 1339 and 1153 cm−1. After a long wrestling with the problem of fragmentation of DK, we could not find any molecular fragments whose absorption spectra would match these three observed infrared bands. The solution, however, floated to the surface: there is no decomposition but, instead of that, isomerization of DK took place! The spectrum consisting only of three medium and strong infrared absorptions is indeed characteristic of cyclobutane-1,3-dione. The molecule of CB possesses an overall D2h symmetry, and only a few vibrations of this species are infrared-active. The three observed vibrations are those that have the highest predicted infrared intensities (667, 61, and 166 km mol−1). All the remaining vibrations are either IR inactive or have very low predicted IR intensities (7 km mol−1 or less). Interestingly, the band observed at 1766 cm−1 corresponds to the antisymmetric stretching vibration of two carbonyl groups attached to a fourmembered ring. This vibration occurs at a frequency much lower than that of cyclobutanone (observed at 1814 cm−1).62 Also interestingly, the antisymmetric stretching vibration of CB occurs at a lower frequency than the corresponding symmetric stretching vibration in the same molecule (the theoretically predicted difference between these two modes is ∼85 cm−1) (Table 2). For the majority of symmetric molecules, the ordering of antisymmetric and symmetric vibrations is the opposite. 2138



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singlet state, in terms of energy, has σ*(3s) Rydberg character, in accord with the literature.

Scheme 4. Transformations of Diketene Induced by UV Irradiations of the Matrix-Isolated Compound and UVInduced Secondary Transformations of the Photoproducts Generated from Diketene Observed Experimentally in the Present Study

■

ASSOCIATED CONTENT

S Supporting Information *

Figure S01 shows the experimental infrared spectra of DK monomers isolated in argon matrix at 15 K and xenon matrix at 30 K compared with the theoretical infrared spectra of DK, 3MO, and 2MO calculated at the B3LYP/6-311++G(d,p) level. Scheme S01 provides the optimized structure and atom labeling for all considered molecules. Tables S01−S08 provide internal coordinates used in the normal-mode analyses. The theoretical spectra of 3MO and 2MO are given in Tables S09 and S10. Tables S11−S15 contain the theoretical spectra of ketene, CP, CB, allene, and ethylene. Table S16 shows the experimental infrared combination bands and overtones appearing in the 2300−1930 cm−1 region of the infrared spectrum of DK. This material is available free of charge via the Internet at http:// pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (S.B.), [email protected] (I.R.). Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS These studies were funded by the Portuguese “Fundaçaõ para a Ciência e a Tecnologia” (FCT) Projects PTDC/QUI/71203/ 2006-No. FCOMP-01-0124-FEDER-007458, PTDC/QUIQUI/111879/2009, and PTDC/QUI-QUI/118078/2010, FCOMP-01-0124-FEDER-021082, co-funded by QREN-COMPETE-UE. S.B. acknowledges FCT for Grant SFRH/BPD/ 65712/2009.

5. CONCLUSIONS

■

Infrared spectra of diketene (DK) isolated in Ar and Xe matrices were recorded and interpreted. In agreement with previous evidence, DK monomers were found to adopt exclusively the 4-methylene-oxetan-2-one form. Upon UV irradiation, matrix-isolated DK was found to behave differently depending on the excitation wavelength. When exposed to near-UV irradiations (λ > 300 nm), the compound was photostable, whereas irradiation in the 280 > λ > 225 nm range led to fragmentation of DK molecule in two ways: (i) with production of two parent ketene molecules (OCCH2), and (ii) with production of cyclopropanone (CP) plus carbon monoxide. For irradiations in the 280 > λ > 240 nm range, DK exhibited two additional reactions: (iii) decomposition to allene (H2CCCH2) and carbon dioxide, and (iv) isomerization into cyclobutane-1,3dione (CB). Among the photoproducts, CP was found to decompose to ethylene and carbon monoxide upon irradiation with λ = 345 nm, while CB decomposes exclusively to two parent ketene molecules, without isomerization back to DK or decarbonylation, upon irradiation with λ = 330 nm. All photoproducts could be doubtlessly characterized by IR spectroscopy supported by B3LYP/6-311++G(d,p) calculations and comparison with previously reported data. NBO analysis showed that the two lowest excited singlet states of DK are almost degenerate in energy and correspond to π* orbitals of CC and CO moieties. The NBO calculations helped also to establish that the third excited

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UV-Induced Unimolecular Photochemistry of Diketene Isolated in

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