Experimental and theoretical studies of the photoisomerization of

The sample, isolated in rare gas matrices (argon, krypton, and xenon), has been irradiated ... species are unstable, and only a very few studies exist...
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5917

J. Phys. Chem. 1993,97, 5917-5925

Experimental ana Theoretical Studies of the Photoisomerization of Malonaldehyde Isolated in Rare Gas Matrices T. Chiavassa, P. Verlaque, L. Pizzala, A. Alloucbe, and P. Roubin' Service de Spectroscopie IR- TF, Laboratoire de Physique des Interactions Ioniques et MolEculaires, URA CNRS 773, UniversitE de Provence, 13 397 Marseille Cedex 20, France Received: January 4, 1993; In Final Form: March 9, 1993

The photochemistry of malonaldehyde (propanedial) has been studied using high-resolution FT-IR spectrometry. The sample, isolated in rare gas matrices (argon, krypton, and xenon), has been irradiated with broad band UV light. Experimental results have been compared to ab initio calculations to identify the created species. These comparisons prove the formation of some of the unstable stereoisomers of malonaldehyde. A detailed characterization of the new species has been performed.

1. Introduction Malonaldehyde (MA) or propanedial is the simplest of j3-dicarbonyl compounds (CHO-CH2-CHO). Figure 1 shows the tautomeric equilibrium between the dialdehydic form and the enol form. The latter is stabilized by the existenceof a strong hydrogen bond and i%predominant in the gas phase. The main interest of MA lies in the proton transfer between the two oxygen atoms. During the past 15 years, MA has led to a considerableamount of experimental (X-ray photoelectron,l,2 and infrared8-' spectroscopies) as well as theoretical work (semiemp i r i ~ a l , 'ab ~ ,initio,' ~ ~ 1,14-18 local spin density functional theory" calculations). The important electronic delocalization due to the hydrogen bonding and to the conjugation between the double bonds leads to specific infrared features. The vibrational characteristics of the molecule have been recently well described by local spin density functional computation^.^ I Figure 2 shows the seven planar stereoisomersofthemolecule (MA1 to MA7). These nonchelate species are unstable, and only a very few studies exist for these molecules. AM 1 calculation^^^ give their geometries and their relative stabilities. If the photoreactivity of nonenolizable j3-dicarbonylcompounds is now well-known,19 only a few results concerning enolizable j3-dicarbonyl compounds have been published. Flash photolysis experiments20.21 have been performed at room temperature for a dilute solution of acetylacetone (CH3COCH2COCH3). They have led to the photoisomerization of the molecule and to the formation of some of the nonchelate enol forms. The authors show that the keto form also appears with continuous irradiation. Infrared studies of the irradiation of acetylacetone isolated in rare gas matriceshave confirmed the existence of the isomerization process.22923 However, no precise identification of the new species has been reported up to now. In our previous work we have shown23 the same behavior for MA isolated in a xenon matrix. However, additional experimental data and theoretical calculations proved then to be necessary to give a valid interpretation of the experiments. In this work, we perform a detailed study of the photochemistry of MA isolated in rare gas matrices (argon, krypton, and xenon). The small size of the molecule allows accurate ab initiocalculations giving valuable information for the complete identification of the species created. The crystalline environment hinders the association between molecules which would take place in solution. Frequency shifts can occur because of the local environment of the molecule, but they are usually small (a few cm-I). The theoretical calculations concerning the isolated molecules are nevertheless a good tool for spectral identification. In addition, 0022-3654/93/2097-5917%04.00/0

Figure 1. Keto-enol equilibrium of MA. The labeled atom numbers are referred to in the text.

when unstable species are created, the low temperatures prevent a back reaction. High-resolution FT-IR spectroscopy allows characterization of the evolution of the species induced by the irradiation and differentiation of the chelate and the nonchelate enol forms. Irradiations of the isotopic l80derivatives of the molecule have also been performed to obtain more information on the vibrational species of the infrared bands. The experimentalapparatus and the preparation of the samples arediscussedin section 2. Section 3 gives theexperimental results. The identification of the new species is discussed in section 4 by comparing experimental results with ab initio calculations. Kinetic studies are reported in section 5.

2. Experimental Section MA and its l 8 0 isotopicderivativeswere prepared as described previously.3 The samples were obtained by mixing sublimated MA with the rare gas (argon, krypton, and xenon) in a matrix/ sample molar ratio equal to 500 and by depositing the gas onto a CsBr window cooled at 20 K in the case of argon, 25 K in the case of krypton, and 30 K in the case of xenon. The deposition time was about 100 min, and the matrix thickness was about 250 pm. All of the infrared spectra were recorded in the range 4004000 cm-I, using a FT-IR Nicolet 7000 spectrometer at a 0.12-cm-1 resolution while the temperature of the sample was 10

K. Broad band UV irradiations of MA and of its isotopicderivatives were performed using an Oriel high-pressure 200-W mercury lamp (A > 230 nm) to promote the chelate enol a T* electronic transition located at 263 nm.24 MA also has a n T * transition located at 354 nm,25 but its excitation does not lead to the isomerization of the molecule.23The temperature of the sample during the irradiation was 10 K. The experimental conditions have been kept constant for the different experiments.

--

3. Results

Photoinduced Modifications of the Infrared Spectra. Figures 3,4, and 5 display the infrared spectra recorded for MA isolated in argon, krypton, and xenon matrices, respectively, in the spectral 0 1993 American Chemical Society

5918 The Journal of Physical Chemistry, Vol. 97, No. 22, 1993

Chiavassa et al.

MA4

MA

1610

1

Wavenumber (cm.1)

a MA1

H0

MAS

E‘“r“ E k

“O

MA2

A

I sw

(A) i

Wavenumber (em-’)

Y

A

Figure 3. FT-IR spectra of MA isolated in an argon matrix, irradiation time 84 min, temperature 10 K: (A) before irradiation; (B) after UV irradiation. (a) YO-H region, 3670-3535 cm-I; (b) Y C ~ VC+ , region, 1730-1550 cm-I; (c) 1480-430 cm-I.

“Po A

1

MA6

I

(a)

1

(b)

si

P

I

MA3 Figure 2. Labels of the stereoisomers of MA.

MA7

ranges: 3670-3535 (a), 1730-1550 (b), and 1480-430 cm-I (c). The lower curve (A) is the spectrum before irradiation, and the upper curve (B) is the spectrum after UV irradiation. The irradiation times are 84 (Figure 3), 30 (Figure 4), and 19 min (Figure 5). Lower curvesshow low-frequencyshifted vibration bands which are characteristic of the cyclic structure of the chelate form. This shift reflects the bond softening due to the existence of a strong hydrogen bond and to the conjugation between double bonds. This is the case for the 0-H stretching vibration band (Y-H 2870 cm-1)899 and for the c--O and C = C stretching vibration bands ( Y C ~1650cm-I = a n d u c e - 1 5 9 5 ~ m - ’ ) . ~Moreover, ?~J~ thevibrational analysisof the 1480-430-~m-~ region has revealed that the related internal coordinates are strongly mixed.” Our results are similar for the three cases. Upper curves show that free 0-H stretching vibration bands clearly appear in the 3670--3535-~m-~range. New carbonyl stretchingvibrations also appear, shifted up to 1700cm-1. These modifications are consistent with the formation of nonchelate enol isomers of MA. Confirming this hypothesis, new intense features appear in the 1480430-cm-I range that we attribute to nonchelate C - 0 vibration bands. A few tens of minutes were sufficient to eliminate all of the MA chelate enol form. Table I summarizes the absorption band frequencies measured for the irradiated sample isolated in the three matrices. The frequencies corresponding to doublet structures that will be attributed to site effects are linked by a dash.

-

3670

I

3.580 Wavenumkr (em.!)

1730

Wavenumber 1610(em.!)

(A)

I

I

180

1

Wavenumber (em-!)

Figure 4. Same as Figure 3 but for a krypton matrix and an irradiation time of 30 min.

Extended irradiations lead to the appearance of small peaks which are due to the presence of carbon monoxide. The latter is probably created by the decarbonylation of the molecule. On the other hand, very weak absorption bands at about 1720-1730 cm-I are detected for the three matrices. These bands are due to the C=O symmetric and antisymmetric stretching vibration bands of the dialdehydic tautomer. This reveals the existence of the photoinduced enol-dialdehyde tautomerization. These two processes are almost negligible and thus will not be taken into account in the following. All of these results show that the dominant process is the photoinduced stereoisomerization of the chelate MA molecule.

Photoisomerization of MA in Rare Gas Matrices

The Journal of Physical Chemistry, Vol. 97, No. 22, 1993 5919

TABLE k Infrared Absorption Band Frequencies (cm-1) Measured after W Irradiation of MA Isolated in Argon, Krypton, and Xenon Matricesa

(a) I

-i

I

0

I

argon

I 1

LLL) A& J

r

3670

I

I

3580

I

1730

2

I

4

3642.3-3637.1 3629.5 3584.8-3582.6 3580.9-3580.7

3616.5-3607.1 3595.9 3551.1 3564.6

2862.612842.112833.1 2823.3/2780.5/2770.7 2752.0/2743.0/2735.8 2727.9127 15.412696.7

2870.212863.612859.5 2836.2/2829.7/2820.0 277 1.812767.912747.0 2736.0/2725.8/2694.2 2688.3-2680.0

2860.0/2844.0/2828.8 2826.0/2784.5/2764.0 2746.5/2740.5/2730.2 2724.0 f 2695.8

1729.6-1725.4

1722.2

1732.4

1715.1-17 11.711700.3 1697.71 1697.2 1678.8 1662.5 1648.8-1645.5 1633.8-1627.7 1614.2

17 13.7-1708.21 1696.4 1695.2-1695.0 1694.111693.9 1675.9 1660 1647.0- 1643.4 1626.0 1610.6

1716.0-17 11.511702.3 1700.6 1699.2 1683.0-1681.8 1667.8- 1663.3 1646.2- 1641.2 1631.8 1614.0-1610.1

1439.4-1436.8 1428.2-1426.6 1410.611407.3

1437.0-1435.4 1426.7-1425.5 1408.711398.8

1440.0-1436.5 1430.0- 1428.1 1417.111407.9

1364.3 1352.5 1323.111320.2 1312.9-1311.6 1303.811299.4 1264.21 1263.7

1362.6 1351.8 1318.4 13 10.5- 1309.6 1304.211297.6 1265.611263.5

1362.9 1356.9 1322.9 1315.0 1305.5/1300.2 1263.211262.6

1221.0 1215.1 1206.1- 1204.2

1222.7 1213.0- 1212.6 1204.5

1225.4 1222.3-1220.4 1209.3-1208.2

1144.7-1137.4 1130.7/1126.0

1143.2 1131.0/1125.5

1141.2 1133.1/1129.0

1110.711109.2

1109.5/1 106.7

1114.2/1112.0

1004.6- 1002.81 1001.4 984.1198 1.21979.5 959.8-958.7

1003.5- 1OOO.8 982.21977.01975.3 958.9-956.7

1013.5-1010.5-1003.2 989.5 1979.51978.7 958.7-956.9

I

Wsvenumber (cm-I)

Wavenumber (an.')

Figure 5. Same as Figure 3 but for a xenon matrix and an irradiation time of 19 min.

The question is now to identify the new species among the seven isomers of Figure 2. Characterizationof the InfraredBands. The YO+ region shows three major features: one doublet and one singlet. In argon and krypton matrices, the doublet is located at a lower frequency than the singlet, and the opposite location is observed for the xenon matrix. An important red shift of the whole V+H signal of about 20 cm-' is also measured from argon to krypton and from krypton to xenon (Figures 3-5). We will designate respectively by H and L the high-frequency and low-frequency structures. The H and L frequencies are distant by about 50 cm-l for the three raregases. The questions ariseif these structures are related to the same species in the three matrices and if the doublet structure of H or L is due to different isomers or to different trapping sites. The characterization of the H- or L-related peaks in the whole spectral region will allow their identification. Though their Y-H signals are different, we believe that the new species are the same in the three matrices. The experimental arguments are the following: (i) Very similar band positions are observed for the three matrices in the range 400-2000 cm-I. (ii) H and L have a similar evolution during the irradiation for the three matrices; H and L both increase until MA has totally disappeared, while H decreases and L still increases for a longer irradiation. a. Evolution of the Spectra with the Irradiation Time. This evolution allows the unambiguous correlation of all the absorption bands to H or to L. Figure 6 shows the spectrum recorded after 84 min of irradiation (A), the spectrum recorded after 290 min of irradiation (B), and the difference spectrum (C) = (B) - (A), in the case of the argon matrix. The H-related peaks appear with negative intensities in (C) while the L-related peaks appear with positive intensities. Figure 6b concerns the V C ~Y , C region. ~ It is clear, for example, that the peaks located at 1697,1679, and 1663 cm-' are correlated with H. Other studies, such as the evolution with the temperature, the effect of an extended irradiation, and the irradiation of the isotopic derivative, will allow the full determination of the correlations between all of the absorption bands.

xenon

3659.1-3655.2 3639.3 3601.6-36.1 .O-3598.0 3595.3-3594.5-3593.8

1610

Wavenumber (cm.')

krypton

849.9 836,31829.2-828.7

847.8 836.0-827.7

825.3-822.2

799.9 782.5

799.0 782.7

801.0 784.8

737.8 680.6

734.5 679.8

740.2 686.5

503.9-501.8 497.1-496.41492.5 472.2147 1.9

504.51501.2-497.8 491.9 471.5

503.0 48831486.7 477.1

Frequencies separated by a dash will be attributed to a multiplet structure due to site effects, while frequencies separated by a slash will be attributed to different forms.

The YC-H region presents many weak bands attributed to nonfundamentalbands. The correlationof these bands with other features in the experimental spectrum has not been completely achieved. Because their assignment remains too uncertain, they will not be discussed in the following. b. Evolution of the Spectra with Temperature. Analysisof the infrared spectra recorded at different temperatures (argon, 1030 K; krypton, 1040 K, xenon, 10-75 K) reveals that the H line is only weakly broadened when the temperature is raised. On the contrary, important modificationsare measured for L in the case of argon and krypton matrices. Figure 7A shows the infrared spectrum of MA isolated in an argon matrix at 10 K after 290 min of irradiation. Figure 7B shows the spectrum for the same sample when the temperature is raised to 15 K, and Figure 7C shows the difference spectrum (B) - (A). The doublet L has two components: L1(high-frequency peak) and L2 (low-frequency peak). The L1component has completely vanished at 15 K while

5920

Chiavassa et al.

The Journal of Physical Chemistry, Vol. 97, No. 22, 1993

+!kz+=$lh*? Wavenumber

Wavenumber (em.')

(em-1)

3670

I

(C)

3665

3643

I

I

3621 Wavenumber (em.') (b)

r

r -

I

3599

(C)

Lz

Wavenumber (cm-9

Figure 6. Evolution of the FT-IR spectra with irradiation time, MA isolated in an argon matrix, temperature 10 K (A) after 84 min of UV irradiation; (B) after 290 min of UV irradiation;(C) difference spectrum (B-A). (a) YO+ region; 3670-3580 cm-I; (b) Y C ~ YC-C , region, 17351600 cm-I; (c) 1450-460

cm-I.

)A& 3670

3630

3590 Wavenumber (cm.9

Wavenumber (em.))

1

I

L.

I,

I .I

I

L'A'

Wavenumber (em-')

Figure 8. Evolution of the FT-IRspectra with extended irradiation,MA isolated in an argon matrix, temperature 10 K: (A) S84;(B) difference ; difference spectrum (S16~- S290). (S, spectrum (S290- S S ~ )(C) designates the spectrum recorded after n min of irradiation.) (a) YO+ region, 3665-3588 cm-I; (b) VC+, YC+ region, 1735-1595 cm-I.

permit the conversion of MA to one of the seven stereoisomers. Note that the sites are entirely recreated by a further irradiation. This temperature effect allows the correlation of the bands in the whole spectral region either to L1 or to L2 (Figure 7b,c). c. Evolution of the Spectra with Extended Irradiation. In the case of an extended irradiation (Le., when the sample is irradiated much longer than necessary for the vanishing of MA), a photostationary equilibrium between H and L is observed for the three matrices. We will note S,, the spectrum recorded after n min of irradiation for MA isolated in an argon matrix. Figure 8A shows Sg4. Figure 8B is the same as Figure 6C and shows the difference spectrum, s290 - S84,summarizing the evolution of H and L. Figure 8C shows the difference spectrum SI600 s290. A slight decrease of the low-frequency side of L2 is observed on (C). It can be correlated with a slight increase of the carbon monoxide signal in the 2131-2139-cm-1 region. This is thus probably due to the decarbonylation of the corresponding molecule. These observations lead to the following hypothesis. The high-frequencyside of LZcorresponds to the same isomer as LI. It corresponds to the site effect discussed in the preceding section and will be noted LI in the following. The low-frequency side of L2 correspondsto another isomer and will further be noted L2. As we shall see later, this hypothesis is justified by the calculations. The analysis of (B) and (C) spectra gives the correlation of the bands to L1 and Lz on the whole spectral range. d. Irradiation of the '80 Isotopic Derivatives of MA. The analysis of the spectra recorded after the irradiation of the I 8 0 isotopic derivative of MA is complicated by the presence of the fourisotopomers: 16011605,1 6 0 1 1 8 0 5 , 1*011605, lSOli 8 0 5 . 1 1 Each new isomer created by the irradiation can thus be found for the four different isotopic molecules. The infrared bands exhibit a doublet, a triplet, or a quadruplet structuredependingon whether one or two types of oxygen atom are involved and depending on their relative influence on the frequency values. The measured shifts will be characterized in the following by their largest value:

Photoisomerization of MA in Rare Gas Matrices

1\,A 3665

3625 Wavenumber (em-')

-

The Journal of Physical Chemistry, Vol. 97, No. 22, I993 5921

, (A) 3585 Wavenumber (cm-9

II

&07' (8)

0

11.093

1.348

0

1.091

1.072

b.071 1310

1230

'

iiso

.

Wavenumbel

1070 (cm-I)

990

Figure 9. Irradiation of the I8O derivatives of MA; molecules isolated in an argon matrix, temperature 10 K: (A) after 84 min of UV irradiation; (B) isotopicderivatives after 55 min of UV irradiation. The arrows show the shift AWO for some bands. (a) U+H region, 3665-3575 cm-I; (b) V C ~ UCK , region, 1735-1600 cm-I; (c) 1310-950 cm-I.

AVIQ= v(MA) - V ( I ~ O ~ ~ O MFigure A ) . 9A shows the spectrum of MA isolated in an argon matrix after 84 min of irradiation. Figure 9B shows the spectrum of the isotopicderivativesisolated in an argon matrix after 55 min of irradiation. In agreement with the previous attribution of L and H to the VO-H vibration bands of the nonchelate molecule, a shift of about 12 cm-I is observed for the three signals: H, LI, and Lz (Figure 9a). In the 1735-1600-~m-~region (Figure 9b), the structures exhibiting shifts from 25 to 28 cm-I (at 1711 and at about 1700 cm-I) will be attributed to VC+ vibration bands and the structures with shifts from 2 to 12 cm-l (at 1679, 1663, 1646, and 1614 cm-I) will be attributed to V C vibration ~ bands. The values of these shifts reflect, as for MA,'' the mixing of the internal coordinates in the vibrational modes. Nevertheless, due to the loss of the hydrogen bond, this mixing is less marked. The peaks at 1679 and 1663 cm-I have been related to H. Being both attributed to VC=C vibration bands, they are probably due to two different isomers related to H. The corresponding V C vibration ~ bands are both located in the multiplet at about 1697 cm-I. The C - 0 stretching vibration modes (vc-) are well characterized in the 1310-950-~m-~region (Figure 9c) by the large intensities and the large isotopic shifts (15-1 8 cm-I) of their bands. Four major bands relative to this vibration mode are measured, confirming the hypothesis of the formation of principally 4 isomers (two are related to H and two are related to L). A definite assignment of the new species to the isomer MA1, MA2, ..., or MA7 remains difficult on the basis of experimental arguments only, and it is necessary to compare these results with vibrational frequency calculations.

4. Vibrational Assignment Calculations. We have calculated the vibrational frequencies of the seven nonchelate enol isomers (MA1-MA7) by a selfconsistent field theory. The ab initio computations were performed using the Hondo 8 program.26 The Gaussian basis consists of 68 Duning double-zeta plus polarization (DZP) functions p on H6 and d on the oxygen atoms. The electron correlation is

r

b

1.070

Figure 10. Calculated geometries of MA and its stereoisomers. Results are given by DZP/MP2 ab initio calculations. The bond lengths are in angstroms, and the angles are in degrees. Oxygen atoms are black, carbon atoms are shaded, and hydrogen atoms are white. (a) MA; (b) MA1; (c) MA2;(d) MA3;(e) MA4;(f) MA5; (g) MA6;(h) MA7.

TABLE Ik Relative Total Energies of the Seven Stereoisomers of MAa ~~

AE

~~

MA1

MA2

MA3

MA4

MA5

MA6

MA7

52.0

38.5

37.7

31.4

34.4

27.7

35.2

Results are given by DZPIMP2 calculations. The differences AE with the formation energy of MA are displayed (kJ/mol-I). (1

taken into account by second-order Maller-Plesset perturbation (MP2) theory. Figure 10 and Table I1 summarize the optimized geometries and the total energies obtained for MA and its seven stereoisomers. In agreement with the previous AM 1 calculat i o n ~ ,these ' ~ results show that MA1 is the least stable isomer. A geometry giving results close to the experimentalones has been calculated by Frisch et al. for MA.17 The description of the chelate molecule indeed requires the use of a more extended basis set. Local spin density functional computations also take into account correctly the electronic delocalization.' The values of Figure 10 are not the best ones for the chelate molecule, but they can directly be compared with the values obtained for the stereoisomers. It is clear that the nonchelate 0-H bonds (0.943 and 0.947 A) are shorter than the chelate one (0.956 A). This is due to the strength of the hydrogen bond that enhances the conjugation between the bonds. Consistently, C = O and C=C bonds are longer for the chelate molecule (respectively, 1.217 and 1.353 A) than for the nonchelate molecules (respectively, 1.202-1.206 and 1.336-1.348 A). The molecules present 15 A' modes (in-plane vibrations) and 6 A'' modes (out-of-plane vibrations). The same set of internal coordinates has been chosen to describe the seven isomers (eight bonds lengths, 0-H, C-0, C-C, C-C, C-0, C-H7,

Chiavassa et al.

5922 The Journal of Physical Chemistry, Vol. 97, No. 22, 1993

TABLE III: Calculated Vibrational Frequencies (cm-l) of the Seven Stereoisomers of MA’ MA1 A’ 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 A” 16 17 18 19 20 21

MA2

MA3

MA4

MA5

MA6

MA7

a

b

a

b

a

b

a

b

a

b

a

b

a

b

3654 2861 2821 2668 1744 1646 1434 1398 1279 1246 1073 878 849 435 183

13 0 0 0 31 5 1 7 11 6 19 5 9 13 10

3640 2887 2864 2637 1740 1626 1426 1379 1303 1216 1116 957

3652 2878 2840 2720 1736 1663 1421 1365 1276 1231 1111 969 668 455 192

13 0 0 0 30 7 7 2 12 1 5 18 8 14 12

3612 2893 2832 2688 1730 1610 1395 1373 1294 1235 1136 968 722 43 1 200

14 0 0 0 29 5 9 6 1 8 20 6 12 13 8

3655 2864 2853 2686 1727 1640 1394 1340 1318 1216 1139 955 725 426 199

14 0 0 0 29 8 9 4 0 17 11 5 12 15 7

3610 2866 2842 2665 1741 1640 1404 1348 1302 1200 1135 1095 55 1 47 1 212

14 0 0 0 35 5 10 6 0 4 20 9 10 15 7

3654 2872 2822 2654 1744 1669 1402 1326 1312 1186 1132 1093 542 472 213

14 0 0 0 33 7 9 4 5 8 18 5

452 21 1

13 0 0 0 34 6 8 1 6 4 11 18 8 15 8

1013 995 763 434 29 1 118

2 0 2 0 3 3

1010 988 790 467 280 135

0 2 1 5 1 3

1007 994 784 413 247 142

2 0 1 1 2 3

1024 1000 822 449 257 173

0 6 1 4 3 2

1021 985 836 311 259 176

0 5 1 1 4 3

1012 974 836 452 295 144

1 0 1 9 0 4

1005 963 842 330 250 141

2 1 1 2 3 4

-

667

Results are given by DZP/MP2 calculations. shifts.

UC-0vibration

-

-

10

15 6

frequencies are underlined. (a) MA frequencies; (b) Au1.0 = u(MA) - u(I80l80MA)

C-H8, C-H9; 10 in-plane angles, 0-C-H7, C=C-H7, C-C-HI, C-C-H8,0=C-H9, C-C-H9, H-0-C, 0-C=C, C-C-C, C - C 4 ; and six out-of-plane coordinates, the three torsions around the C-C, C - C , and C-0 bonds and the three out-of-plane bending angles of the C-H bonds). Standard scaling factors have been introduced so that the calculated frequencies may be directly compared with the experimental frequencies. These scaling factors were equal to 0.75 for YO++, 0.70 for Y C - H , 0.80 for the other stretching coordinates and in-plane angular deformation coordinates, and 0.77 for out-of-plane angular deformation coordinates. The scaling of the frequenciesusually allows an accuracy better than 50 cm-I. The main arguments that we will use for the comparison with the experimental frequencies will involve frequency differences larger than this value. Table 111 displays the calculated frequencies for the seven isomers (column a), MA1-MA7. The isotopic shift is indicated in column b. Its knowledge is the key allowing the characterization of the VC+ modes. As they will be crucial for the vibrational assignment, the VC+ frequencies have been underlined. Identification of the Isomers. a. YO-H Region. MA isomers are clearly divided in two groups: MAl, MA2, MA3, MAS, and MA7, whose YO+ frequencies (mode 1) are between 3640 and 3654 cm-I (high frequencies), and MA4 and MA6, whose UO-H frequencies are between 3610 and 3612 cm-I (low frequencies). This frequency difference (=40 cm-I) is well correlated to the frequency difference measured for the structures H and L (-50 cm-I). H is thus probably related to MA1, MA2, MA3, MAS, or MA7, while L is related to MA4 and MA6. b. YC+, VC+ Region. The important differencesbetween the calculated YC-C frequencies (mode 6) are a key ingredient to the spectral identification. Experimental YC+ frequencies of the H-related peaks (1663 and 1679 cm-1 for the argon matrix) are in good agreement with MA3 and MA7 YCN frequencies. These two C = C vibrations are the high-frequency ones. On the other hand, MA4 has a low-frequency YC=C mode at 1610 cm-1. It is well correlated with the L2-related peak (at 1614 cm-I for the argon matrix). LI-related YC-C frequency can also be compared to MA6 VC=C frequency, in agreement with the conclusions concerningthe mode. To summarize, H is probably related to MA3 and MA7, while it is confirmed that L is related to MA4 and MA6.

c. VC- Region. As can be seen in Table 111,UC+ frequencies arescatteredin therange957-1216 cm-I. As previouslydiscussed, they are also well characterized experimentally. This mode thus allows a reliable test for the assignment. The VC+ experimental frequencies of LI- and L2-related isomers (1 1 1 1 and 1109 cm-I for the argon matrix) are in agreement with the MA6 and MA4 theoretical frequencies (mode 11). The two H-related YC+ experimentalfrequenciesareverydifferent(1 13 1 and 1005 cm-I), and they are also in good agreement with the calculated values for MA7 (mode 11) and MA3 (mode 12). This YC+ spectral region thus corroboratesthe previous assignment. It will be further confirmed for the whole spectral region. d. Complete Assignment. Tables IV-VI1 show the comparison between the rounded-off frequencies that have been measured for the three rare gas matrices (columns 5 , 8 , and 10) and those calculated (column 3) for MA3, MA4, MA6, and MAI. In the case of a multiplet structure, only the frequency of the largest peak has been listed. The internal coordinates that contribute to each mode for more than 20%are indicated in the first column. The experimental isotopic shifts Aut80 are also listed in column 6 for the molecule isolated in an argon matrix. Column 4 shows the calculated values. The identification of MA3, MA4, MA6, and MA7 is strongly supported by the close agreement existing between the experimental and the calculated values. Columns 7, 9, and I 1 show the relative differences Av/u between experimentaland calculatedvalues. Except for the low-frequency modes 13 of MA7 and 19 of MA4 and MA6, these differences are less than 5%. Only a few weak bands have not been assigned to one of these four isomers. They are probably related to the presence of a small amount of another isomer. Since the signal is very weak, the assignment remains uncertain. To summarize, this study reveals that the stereoisomerization is the major process involved in the photochemistry of MA. The careful analysis of the infrared spectra shows without ambiguity that the species created are principally the isomers MA3, MA4, MA6, and MA7 (Figure 2). Among those, MA4 and MA6 correspond to an s-cis position of the 0-H bond while MA3 and MA7 correspond to an s-trans position. This may be related to the important differenceexistingbetween their Y+H frequencies.

Photoisomerization of MA in Rare Gas Matrices

The Journal of Physical Chemtsrry, Vol. 97, No.22, 1993 5923

TABLE IW Assignment of the Infrared Bands of MAP experiments calculations assignment

a

argon b

b

a

krypton C

a

a

C

3637 1694 1660 1426 1363 1266 1223 1126 1004 680

0 2 0 0 0 1 1 -1 -2

3607 1699 1665 1430 1363 1263 1225 1129 1012 687

1 2 0 -1 0 1 0 -1 -4 -3

982 783

1 0

989 785

1 0

~~

YO-H VC-0

vc-c k-H73 k-H9, b-H. bC-HLI9

k-H9 k-Hl vC-0 b-H

vc-c vc-0

ring deformation YC-H7,

YC-H9

YC-HO,

YC-Hl

A‘ 1 5 6 7 8 9 10 11 12 13

3652 1736 1663 1421 1365 1276 1231 1111 969 668

13 30 7 7 2 12 1 5 18 8

3655 1697 1663 1427 1364 1264 1221 1126 1005 68 1

12 25 12 6 3 11 5 5 16 12

0 2 0 0 0 1 1 -2

A“ 17 18

994 784

0 1

984 783

2 1

1 0

4 -2

xenon C

~

-4

The calculated values are the results of DZPIMP2 ab initio computations. The experimental values are those observed for the molecule isolated in argon, krypton, and xenon matrices at 10 K. (a) frequencies (cm-I); (b) isotopic shifts Am0 (cm-I); (c) Av/u = (ualc - ucrp)/uale ((no).

TABLE V

Assignment of the Infrared Bands of MA4’ experiments calculations

v c 4 vc-c k-H9 b-H,

k-HO

&-H7 k-Hh

vC-0

vC-0,b-H

vc-c

ring deformation YC-H91 YC-H7 YC-HLI YO-H

krypton

a

b

a

b

C

a

A’ 1 5 6 7 8 9 10 11 12 13

3612 1730 1610 1395 1373 1294 1235 1136 968 722

14 32 5 9 6 1 8 20 6 12

3594 1700 1614 1407 1353 1320 1264 1109 980 738

12 25 8 9 7 0 11 15 5 12

0 2 0 0 1 -2 -2 2 -1 -2

3581 1696 1611 1399 1352 1318 1264 1107 975 735

A” 17 18 19

lo00 822 449

0 2 4

1001 836 493

1 1 4

0 2 -7

lo00 836 492

assignment YO-H

argon

xenon a

C

-1 -2

3565 1701 1612 1408 1357 1323 1263 1112 979 740

1 2 0 -1 1 -2 -2 2 -1 -2

0 2 -7

487

-8

C

1

2 0 0 1 -2 -2

3

Sce footnote a of Table IV.

TABLE VI. Assipnment of tbe Infrared Bands of MA@ experiments calculations

argon

krypton a

a

C

1 2 0 0 3 0 -1 2

3565 1712 1641 1417 1315 1300 1220 1114

1 2 0 -1 2 0 -2 2

47 1

0

477

-3

957 828 498

2

959 822 489

2 2 -8

a

b

a

b

C

14 35 5 10 6 0 4 20 9 10 15

3601 1712 1646 1411 1312 1299 1215 1111

12 28 5 9

8 9 10 11 12 13 14

3610 1741 1640 1404 1348 1302 1200 1135 1095 551 47 1

4 2 15

0 2 0 0 3 0 -1 2

3585 1708 1643 1409 1310 1298 1213 1110

472

11

0

A” 17 18 19

974 836 452

1 1 9

959 829 497

0 1 4

2

assignment

xenon C

A‘

1

YO-H

5 6

VC-0

vc-c

I

k-H9

6 0 - H ~ k-HO &-H7 &-He, VC-0 VC-0 yc-c

ring deformation ring deformation YC-HP, YC-HLI

YC-Hl

YO-H 0

1

-10

1

-10

Sce footnote u of Table IV.

5. Kineticstudy

The evolutionof M A as a function of irtadiation time has been studied in detail in the case of argon and xenon matrices. The experimental setup was identical in both cases. As discussed in section 3, the H and L structures each show a specific evolution.

The different peaks of each structure will be considered as a single feature. This is valid because we do not consider extended irradiations or temperature effects. Figure 11 shows the experimental p i n t s giving the integrated absorbance relative to YD-H for MA,H, and L. M A totally disappears with a characteristic

Chiavassa et al.

5924 The Journal of Physical Chemistry, VoL 97, No. 22, 1993

TABLE MI: Assignment of the Infrared Bands of MA7' calculations assignment

experiments krypton

argon

a

b

a

b

C

a

3654 1744 1669 1402 1326 1312 1186 1132 1093 542 471

14 33 6 9 1 7 10 17 5 9 15

3655 1697 1679

12 25 2

0 3 -1

3637 1694 1676

1304 1299 1204 1131

2 3 3 18

2 1 1 0

ring deformation ring deformation

5 6 7 8 9 10 11 12 13 14

502 472

4 11

YC-H'I,YC-HP YC-H8, YC-H7

A" 17 18

963 842

1 1

981 850

4 1

A' 1

YO-H YC=O

vc=c 6C-H9 &-H7

60-H ~C-HBI60-H YC-0 YC-c

xenon a

C

0 3 0

3617 1699 1682

1 3 -1

1304 1298 1205 1131

2 1 1 0

1306 1300 1209 1133

2 1 2 0

7 0

505 472

7 0

503 477

7 -1

-2 -1

977 848

990

-3

C

-1 -1

See footnote a of Table IV.

----_ - ---

A

1

0

(

l

m

1

0

(

a

4

s

L

Irradiation time (mla)

Figure 12. Modeling of the kinetic processes for the photoisomerization of MA.

TABLE WI:Kinetic Constants for the Photoisomerization of MA Isolated in Areon and Xenon Matrices' ~

a

m

Y

LO

a

I too

110

110

IU

lrradlation time (mln) Figure 11. Kinetics of the photoisomerization of MA: (a) argon matrix; (b) xenon matrix. Points are the experimental values, and the continuous lines correspond to the kinetic modeling.

time I'. 'I is also about the time for which L is maximum. For an irradiation time much longer than 7 , a photostationary equilibrium between H and L is observed. Moreover, it is clear that 'I is larger in the case of xenon than in the case of argon. The evolution of the species has been modeled by the kinetic diagram presented in Figure 12. The values of the kinetic constants are summarized in Table VIII. Calculated curves are the continuous lines of Figure 11. The k ~ / ( k k~ ~values ) are about the same for the two matrices and show that the process leading to H is faster than the process leading to L. Kinetic constants for the xenon matrix are about 5 times those for the argon matrix. This can be explained by a heavy atom effect. If the relaxation process involves a triplet state, the intersystem coupling is enhanced in the case of a heavy atom environment. The same process has been invokedto explain the photodissociation of 1,2-dichloroethene isolated in rare gas matrices.*'

+

6. Conclusion The photochemistry of MA isolated in the three rare gas matrices (argon, krypton, and xenon) has been studied using high-

kinetic constants ki kz k3 kti/ki

argon 3 x 104 1 x 104 5 x 10-5 0.8

~

~

~~~~~~

xenon

1.5 X lo-' 4.7 x 10-4 2 x 104 0.7

kl, k ~k,, (s-I), and kH are referred to in Figure 12.

resolution FT-IR spectrometry. The experimental results prove the formation of some of the unstable stereoisomers of MA. This study is the first detailed characterization of unstable isomers of such H-bonded (3-dicarbonyl compounds. The photolysis of the molecule and the formation of the dialdehydic compound are only minor processes that appear for extended irradiations. The 21 vibrational frequencies of the seven possible isomers have been calculated by an ab initio method. Comparison of the experimental results and the calculated frequencies led to the identification of the four isomers which are principally formed as MA3, M A 4 MA6, and MA1 (Figure 2). Kinetic studies show that a triplet state is probably involved in the relaxation processes. Nevertheless, no simple arguments are available to explain the formation of those four isomers. Two points can be emphasized: (i) Important site effects are observed in the case of argon and krypton matrices. Unstable sites totally disappear when the temperature is raised. This occurs for MA6 only. The reverse evolution is not observed when the temperature is lowered, but it is observed with a further irradiation. The irradiation seems thus to strongly disturb the environment of the molecule and to lead to a specific trapping site around each isomer.

Photoisomerization of MA in Rare Gas Matrices (ii) As mentioned in a previous paper," the hydrogen bond of MA leads to specific infrared relaxation mechanisms. The line widths measured for the molecule isolated in matrices are indeed very large (up to 10 cm-I). This is especially true for the vibrational modes involving internal coordinates that are strongly coupled to the hydrogen bond. This study clearly confirms this fact: the correspondingline widths measured for the nonchelate isomers are smaller by a factor 5-10. The analysis of the broadening processes is currently in progress.

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