Article pubs.acs.org/JPCA
Cite This: J. Phys. Chem. A XXXX, XXX, XXX−XXX
UV and IR Photochemistries of Malonaldehyde Trapped in Cryogenic Matrices A. Trivella,† T. N. Wassermann,‡,∥ C. Manca Tanner,§ N. O. B. Lüttschwager,‡ and S. Coussan*,∥ †
Département Génie Biologique, UMR EPOC (5805)-LPTC, IUT de Bordeaux, site de Périgueux, Rue du Doyen Joseph Lajugie, 24000 Périgueux, France ‡ Institüt für Physikalische Chemie, Universität Göttingen, Tammannstrasse 6, 37077 Göttingen, Germany § Laboratory of Physical Chemistry, ETH Zurich, Vladimir-Prelog-Weg 1-5/10, 8093 Zurich, Switzerland ∥ CNRS, PIIM, Laboratoire des Interactions Ioniques et Moléculaires, Aix Marseille Universite, 13397 Marseille Cedex 20, France ABSTRACT: UV and IR photochemistries of malonaldehyde, the simplest molecule exhibiting an intramolecular proton exchange, have been studied in four cryogenic matrices at 4.3 K, N2, Ne, Ar, and Xe. Samples have been irradiated using a UV and IR OPO type tunable laser, and with a broad band UV mercury lamp. UV and IR spectra have been recorded and compared with theoretical calculations carried out at the SAC−CI/6-31++G(d,p) (UV transitions) and B3LYP/6-311+ +G(2d,2p) (IR spectra) levels of theory. After deposition, the intramolecularly H-bonded form is found exclusively, while several open forms are formed upon UV irradiation. These open forms show ability to interconvert upon UV irradiation too. Some of them are also able to isomerize upon selective IR irradiations. The whole set of results allowed us to identify seven isomers among the eight postulated. The photodynamics of the electronic relaxation of malonaldehyde have also been investigated. By following the decay or rise of suited specific vibrational bands in the IR spectra, and by comparing the results with an earlier study of the homologous acetylacetone, we deduced that the electronic relaxation of malonaldehyde proceeds through singlet states, most probably through a 3-fold conical intersection, as postulated from theoretical calculations. In contrast with acetylacetone, malonaldehyde does not show fragmentation after UV excitation.
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has been measured by Baughcum et al.5 to be of ≈21 cm−1 with a tunneling period of ≈1.54 ps.6 We ourselves performed experiments in supersonic jets to elucidate the coupling between vibrational excitation and proton transfer motion, and we compared them to matrix experimental results,7−9 concluding that in cryogenic matrices the proton transfer is totally quenched. This experimental fact is of a great interest because it simplifies vibrational spectra, avoiding band splittings. Our main goal in this work is to study UV and IR photochemistries of malonaldehyde trapped in four cryogenic matrices, taking advantage of our observations in case of acetylacetone.10−12 Cryogenic matrices present two other advantages: they allow the observation of nonchelated (open) forms, for which experimental data are so far very scarce, and they prevent against major fragmentation and relaxation toward the stable CCC form. As we have reported in our former work,12 depending on the calculation level, H-bond strength is calculated to range between 12.4 and 15.8 kcal mol−1 (51.9 and 66.1 kJ mol−1) for MA against 12.0 and
INTRODUCTION Malonaldehyde (MA) and acetylacetone (AA) are the two most simple intramolecularly H-bonded molecules which exhibit a proton tranfer between the two oxygen atoms. The intramolecularly hydrogen bonded form can be formed from the diketostructure via keto−enol tautomerization. Electronic conjugation and the resulting planar six-atoms ring organization stabilize the enol structure and thus make it the ground state form. Due to this structure, the molecules are rather rigid. In Figure 1 the 8 possible isomers of MA, labeled XYZ, are displayed, these letters being C or T, standing for Cis and Trans configurations with respect to C−C, CC, and CO bonds, respectively. MA and AA stand as prototypes for the entire family of β-diketones, which are of broad interest for their strong internal H-bonds and their ability for intramolecular proton transfer. For experimental investigations, MA has to be synthesized and purified,1 since it cannot be stored stable for long. Maybe also for this reason, the majority of studies has so far been of theoretical nature. For example, if one searches in the Web Of Science database, three of the most recent articles are theoretical dealing with the intramolecular proton transfer.2−4 The ground state tunneling splitting due to this proton transfer © XXXX American Chemical Society
Received: December 5, 2017 Revised: February 2, 2018 Published: February 8, 2018 A
DOI: 10.1021/acs.jpca.7b11980 J. Phys. Chem. A XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry A
Figure 1. Schematic picture of the chelated (CCC) and the seven possible open structures of malonaldehyde. C and T stand respectively for the Cis and Trans characters relative to C−C, CC, and C−O bonds. Relative energies to CCC isomer, calculated at the B3LYP/6-311++G(2D,2p) level of calculation, are given in kcal mol−1 and kJ mol−1.
18.5 kcal mol−1 (50.2 and 77.4 kJ mol−1) for AA. Malonaldehyde can be converted into open forms upon UV irradiation. These open forms, also called nonchelated forms, are then able to interconvert upon IR irradiations. The only structural difference between MA and AA is the presence of the two methyl rotators (AA), and their weak coupling.13 In this work, we want to address the following questions: is the UV photochemistry of MA totally different from that of AA as theoretically predicted?14 In other words, does MA electronic relaxation proceed through a triple conical intersection between S2/S1/S0 states? What could be the effects on the photoproducts? Will we observe more nonchelated forms than for AA, considering TTT and TTC species do not present the destabilizing syn-1,3 interaction any more?10 Will we see a clear effect on the isomerization kinetic? Will we be able to induce, as in the case of AA, IR interconversions between nonchelated forms, mainly by rotation around single bonds? Will this IR photochemistry be different between cryogenic matrices as already observed in the case of 1-propanol?15 We have tried to answer all these questions following a subtle logical reasoning in order to identify unambiguously the isomers present in the matrices, and to suggest an approach of the electronic relaxation pathway of MA.
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For UV spectroscopy we injected 20 mbar of mixture in N2 and Xe, and 10 mbar in Ar. Deposition temperatures were 20 K in N2 and Ar against 35 K in Xe. We did not record UV spectra in Ne because the cryogenic head was not able to reach temperatures lower than 11 K (no Ne condensation). For IR spectra we injected 30 mbar of mixture in Ne, Ar, and Xe, and 60 mbar in N2. Deposition temperatures were 4.3 K in Ne, 20 K in Ar and N2, and 35 K in Xe. The mixtures were sprayed onto either a CsI windows (UV spectroscopy) or a highly polished Au-platted copper cube maintained at the deposition temperatures (see above) by closed-cycle cryogenerators (CTI-cryophysics or Cryomech PT-405). O2 (Air Liquide, N45 grade) was also used to dope matrices with ratii MA/O2/MG = 2/2/1000 or 2/4/1000. Fourier transform IR (FTIR) spectra were all recorded at 4.3 K in the transmission-reflection mode using a Bruker IFS 66/S spectrometer (resolution: 0.12 cm−1) equipped with a MCT detector. UV spectra were recorded at 12 K in the transmission mode using a SAFAS 190 DES spectrometer (resolution: 2 nm). Selective UV and IR irradiation were performed using a pulsed (10 Hz, 15 ns) tunable UV−visible−IR OPO BMI-Thalès system (average power, UV, 8 mW; IR, 10 mW; FWHM, UV, ≈ 1 nm, IR ≤ 4 cm−1). To ensure a reliable assigment of the MA isomers, many experiments were performed at different excitation wavelengths. Broad band UV irradiation was carried out with an Oriel high-pressure 500 W mercury lamp (average power, 300 mW, no optical filter, no water filter). Kinetics were recorded at a power of 100 mW. Indeed, we noticed that at 300 mW, MA kinetics, namely CCC consumption, was a lot faster than that of AA. Lowering power to 100 mW, we thought that it would slow down these processes (MA and AA), allowing an easier study. The fact that such a type of UV lamp does not emit below ≈220 nm allows us to think that the main effect we should observe after deposition is CCC consumption.
EXPERIMENTAL SECTION
MA synthesis: in the first step MA sodium salt is obtained by acidic hydrolysis of 1,1,3,3-tetramethoxypropane and purified by recrystallization in methanol/ether. In the second step, MA is synthesized from its sodium salt using hydrochloric acid dissolved in ether at −40 °C. MA is finally purified by pumping at temperatures below 0 °C and extracted by sublimation.1 MA vapor was then mixed in vacuum line with matrix gases (MG), N2 and Ar: N60 grade, Ne: N50 grade and Xe: N48 grade in partial pressure-ratii MA/MG = 5/1000 for IR and UV spectroscopies. B
DOI: 10.1021/acs.jpca.7b11980 J. Phys. Chem. A XXXX, XXX, XXX−XXX
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UV Experimental Spectra. Those experimental spectra are displayed in Figure 2. After deposition, uniquely one broad band centered at 269, 270, and 275 nm, in Ar, N2 and Xe, respectively, is observed. It is assigned to the CCC isomer, the only one present after deposition. These CCC wavelengths are close to that observed in gas phase at 263 nm,17 and to that predicted theoretically, 258 nm. Upon broad band irradiation, as displayed in the same Figure, CCC decreases while new bands at 229, 228, and 236 nm, in Ar, N2 and Xe, respectivly, increase and shift slightly toward lower wavelengths at 224 and 231 nm, in Ar and Xe, respectively. No more effects are observed for irradiation times longer than 11 min in Ar and N2 and 3 min in Xe. These latter bands are due to open conformers. Their absorptions are found at lower wavelengths, since the breaking of the pseudocycle reduces the degree of conjugation compared to that of CCC. One has also to note that those experimental wavelengths are in good agreement with the calculated ones at the SAC-CI/ 6-31++G(d,p) level. For CCC the difference between theoretical and experimental transitions is 15 nm while it is up to 25 nm in the case of the open forms. Moreover, the theoretical average gap between CCC wavelength and those of the open forms is 41 nm against 45 nm observed experimentally. However, while theoretical calculations led us to discriminate between three groups, we experimentally observe only two bands. To further elucidate the conformational isomerism of MA, we have performed UV selective irradiations and followed their effects by IR spectroscopy.
UV SPECTROSCOPY Theoretical Calculations. We have carried out UV transitions theoretical calculations in the SAC−CI/6-31++G(d,p) formalism using the Gaussian 03 suit of programs.16 These results are gathered in Table 1. As in the case of our work on acetylacetone,11 it is convenient to discriminate between three groups: Table 1. Theoretical UV Transitions Calculated for the Four First Excited Electronic States of Each MA Isomers at the SAC-CI/6-31++G(d,p) Level of Theorya isomers
transition
wavelength (eV)
wavelength (nm)
oscillator strengths
CCC
S1 ← S0 S2 ← S0 S3 ← S0 S4 ← S0 S1 ← S0 S2 ← S0 S3 ← S0 S4 ← S0 S1 ← S0 S2 ← S0 S3 ← S0 S4 ← S0 S1 ← S0 S2 ← S0 S3 ← S0 S4 ← S0 S1 ← S0 S2 ← S0 S3 ← S0 S4 ← S0 S1 ← S0 S2 ← S0 S3 ← S0 S4 ← S0 S1 ← S0 S2 ← S0 S3 ← S0 S4 ← S0 S1 ← S0 S2 ← S0 S3 ← S0 S4 ← S0
3.856 4.809 6.196 6.902 3.936 6.062 6.106 6.687 3.932 5.837 6.004 6.546 3.668 5.455 5.914 6.662 3.691 5.432 5.813 6.673 3.681 5.620 6.114 6.402 3.634 5.757 5.904 6.789 3.512 5.601 5.657 6.640
322 258 200 180 315 205 203 185 315 212 207 189 338 227 210 186 336 228 213 186 337 221 203 194 341 215 210 183 353 221 219 187
0.0004 0.2531 0.0268 0.0083 0.0001 0.5466 0.0106 0.0988 0.0001 0.0042 0.4989 0.1752 0.0001 0.3602 0.0083 0.0170 0.0001 0.3500 0.0013 0.0153 0.0002 0.4131 0.0059 0.0773 0.0001 0.0057 0.4075 0.0142 0.0002 0.3359 0.0083 0.0188
TTC
TTT
CTC
CTT
TCC
TCT
CCT
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IR SPECTROSCOPY Theoretical Calculations. Calculations have been performed at the B3LYP/6-311++G(2d,2p) level of calculation, using the GAUSSIAN 03 suit of programs16 and the 6-311 ++G(2d,2p) basis set of Pople et al.18,19 Becke’s20,21 threeparameters functional was used, including the gradient-exchange correction and the non local correlation function of Lee, Yang, and Parr.22 We have calculated all the structures of MA isomers (Figure 1) with their sets of harmonic frequencies. We then have chosen seven regions in which one can find intense and nonoverlapping bands which can be used to discriminate between isomers. These bands are gathered in Table 2. We normalized all the intensities to the νCC mode of CTC (most intense of the calculated IR bands, in the selected regions). νOH Region. CCC νOH mode is calculated at 3134.8 cm−1 with a normalized intensity of 0.33. However, as in the case of acetylacetone,10,11 this band is certainly spread over hundreds of wavenumbers because of the H-bond strength. We do not expect to observe it. On the other hand, for the nonchelated forms, we can clearly discriminate between two groups: (a) A high frequency (HF) group containing TCT, TTT, CTT, and CCT forms with bands centered at 3862.0, 3849.4, 3847.7, and 3847.1 cm−1, respectively. (b) A low frequency (LF) one containing TCC, TTC, and CTC forms with bands centered at 3799.7, 3795.6, and 3791.6 cm−1, respectively. This HF/LF discrimination reveals that the HF group contains all the isomers which are Trans with respect to the C−O bond (XXT), while the LF one contains those which are Cis (XXC). As a postulated result, if νOH irradiations induce rotations around the C−O bond, we should observe exchanges between HF and LF groups of bands. The two most intense bands of each group are separated by ≈56 cm−1.
a
Wavelengths are given in eV and nm. First allowed transitions are written in bold.
(a) CCC with an allowed calculated transition at 258 nm. (b) CTT, CTC, TCC, and CCT with allowed calculated transitions at 228, 227, 221, and 221 nm, respectively. (c) TCT, TTT, and TTC with allowed calculated transitions at 210, 207, and 205 nm, respectively. In the case of acetylacetone,11 two types of UV transition calculations had been performed: one at the high coupled-cluster level of theory, including EOMCCSD and CR-EOMCCSD(T) level of calculation, and another at the levels used in the present work. Because this last set of results was shown to be really reliable in the acetylacetone case, we confidently use them in the present case as well. We thus guess that using both, broad band and selective UV irradiations with sources able to deliver photons between ≈225 and 400 nm, we should be able to discriminate between MA isomers. C
DOI: 10.1021/acs.jpca.7b11980 J. Phys. Chem. A XXXX, XXX, XXX−XXX
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Figure 2. Broadband UV irradiation (laser power, 300 W) of malonaldehyde (MA) trapped in N2, Ar and Xe cryogenic matrices. All the ratios are MA/MG = 5/1000 (MG stands for matrix gas). In case of N2 and Xe, 20 mbar have been deposited against 10 mbar in case of Ar. Spectra have been recorded at 12 K. The irradiation times are in N2 and Ar, from top to bottom (bands at 270, 269, and 275 nm): after deposition, 30 s, 1 min, 2 min, 4 min, 7 min, 11 min, 16 min, 26 min, and 46 min. In Xe, from top to bottom: after deposition, 5 s, 15 s, 25 s, 35 s, 45 s, 1 min, 1 min 33 s, 2 min, 3 min, and 5 min.
Table 2. Theoretical IR Frequencies (Not Scaled) of all MA Isomers Calculated B3LYP/6-311++G(2d,2p) Level of Theorya vibrational modes νOH νCO νCC Δνb δOH νC−O νC−C τOH
CCC
TTC
TTT
CTC
CTT
TCC
TCT
CCT
3134.8 0.33 1692.2 0.35 1627.0c 0.40 65.2 1390.8 0.17 1287.2 0.25 1001.1 0.07 936.7 0.09
3795.6 0.12 1746.6 0.51 1679.6 0.58 67.0 1137.1 0.58 1240.2 0.09 1162.9 0.00 537.4 0.12
3849.4 0.22 1747.5 0.53 1710.5 0.41 37.0 1340.0 0.25 1209.9 0.37 1147.2 0.13 418.4 0.15
3791.6 0.11 1757.4 0.19 1640.1 1.00 117.3 1159.4 0.23 1290.8 0.02 998.5 0.17 551.2 0.13
3847.7 0.21 1762.0 0.18 1664.6 0.77 97.4 1178.4 0.00 1246.9 0.35 992.8 0.05 435.7 0.15
3799.7 0.06 1742.7 0.53 1663.9 0.40 78.8 1241.9 0.26 1011.6 0.16 1152.8 0.18 559.9 0.16
3862.0 0.23 1736.4 0.50 1699.6 0.25 36.8 1295.6 0.41 1017.3 0.21 1153.7 0.05 284.6 0.18
3847.1 0.21 1767.2 0.25 1666.1 0.52 101.1 1275.8 0.13 1310.9 0.30 920.2 0.12 489.6 0.04
Frequencies are given in cm−1. Intensities (in italics) are normalized to the νCC mode of CTC. bΔν = |νCO − νCC|. cThe mode which contributes the most to this vibration is δOH (58%).
a
(a) The first group contains CTC, CCT, and CTT isomers with respective Δν of 117.3, 101.1, and 97.4 cm−1. The bands of this group are supposed to surround those of the others nonchelated isomers. (b) The second group contains TCC and TTC isomers with respective Δν of 78.8 and 67.0 cm−1. (c) The last group contains TTT and TCT isomers with respective Δν of 37.0 and 36.8 cm−1. The combination of observations in this region together with findings in the OH region should be most distinctive to
νCO /νCC Region. CCC νCO and νCC modes are calculated respectively at 1692.2 and 1627.0 cm−1 with normalized intensities of 0.35 and 0.40. For the nonchelated forms, this region presents some strong similarities with that of acetylacetone:10,11 it is in this region that signals are the most intense, and the νCC CTC mode is supposed to be the most red-shifted and intense band. A total of 16 bands spread over 150 cm−1 are expected which could result in experimental overlapping. We can separate them conveniently by their Δν (Table 2) and the relative positions of their bands. D
DOI: 10.1021/acs.jpca.7b11980 J. Phys. Chem. A XXXX, XXX, XXX−XXX
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between groups like we did above, but there is one fact which could be used to identify TTC. Indeed, this isomer is supposed to display the most intense signal in this region located around 1137.1 cm−1.
discriminate between the different nonchelated isomers after UV and IR irradiations. δOH /νC−O /νC−C Region. In this region (1400−900 cm−1) many intense bands are calculated. It is difficult to discriminate
Figure 3. Broad band UV irradiation (lamp power, 300 W) of malonaldehyde (MA) trapped at 4.3 K, in Ar, N2, Ne, and Xe. In Ar: (a) black spectrum, after deposition; red spectrum, after 20 min irradiation; (b) difference spectrum (after 20 min irradiation − after deposition); (c) difference spectrum (after 130 min irradiation − after 20 min irradiation). In N2: (a) black spectrum, after deposition; red spectrum, after 10 min irradiation; (b) difference spectrum (after 10 min irradiation − after deposition); (c) difference spectrum (after 105 min irradiation − after 10 min irradiation). In Ne: (a) black spectrum, after deposition; red spectrum, after 23 min irradiation; (b) difference spectrum (after 23 min irradiation − after deposition); (c) difference spectrum (after 98 min irradiation, after 23 min irradiation). In Xe: (a) black spectrum, after deposition; red spectrum, after 8 min irradiation; (b) difference spectrum (after 8 min irradiation − after deposition); (c) difference spectrum (after 90 min irradiation − after 8 min irradiation). HF/LF: High/Low νOH Frequencies. E
DOI: 10.1021/acs.jpca.7b11980 J. Phys. Chem. A XXXX, XXX, XXX−XXX
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which leads to Δνexp of ≈116 and 90 cm−1. From UV theoretical transitions, the only isomers able to be isomerized by a prolongated 270 nm irradiation (irradiation on the ”red foot” of their UV absorption bands) are CTC, CTT, TCC and CCT. Therefore, the isomer correlated to these decreasing bands should present the following characteristics: (a) A “medium” UV absorption band: CTC, CTT, TCC, and CCT (b) A LF νOH frequency: CTC, TCC, and TTC (c) A Δνtheo > 90 cm−1: CTC, CTT, and CCT The only species which meets all of the above criteria is CTC (Table 4). Concerning the decreasing bands, we cannot at this stage suggest an unambiguous identification. A prolongated broad band UV irradiation (Figure 3) induces the same effects as those observed upon a 240 nm selective one (Figure 4). One observes the decrease of the HF 3655.6 cm−1 band while those LF at 3602.8, 3601.0, and 3594.2 cm−1, increase. That of CTC, at 3598.0 cm−1, decreases also upon those irradiations. In the νCO/νCC region, CTC bands (1729.9, 1704.5, and 1614.4 cm−1) decrease together with those centered at 1697.7, 1679.2, and 1662.5 cm−1. These three latters are thus correlated to the HF 3655.6 cm−1 one. Therefore, the eight increasing bands between 1715.3 and 1628.0 cm−1 are correlated to the LF ones. Upon UV selective irradiation at 230 nm (Figure 4), all the LF νOH bands decrease except that of CTC at 3598.0 cm−1. These decreases are correlated to five multiplets in the νCO/νCC region whose the most intense bands are 1712.1, 1697.7, 1679.2, 1662.5, and 1645.6 cm−1. Reciproquely, one observes the increase of the CTC bands and of three other signals centered at 1656.8, 1592.2, and 1588.8 cm−1. Those three latters correspond to a
We do not detail τOH region because we could not record experimental spectra in this region.
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IR EXPERIMENTAL SPECTRA Argon Matrix. UV Irradiations. Results of a broad band or of selective UV irradiation on MA trapped in Ar matrix are shown in Figures 3 and 4, respectively. Before irradiation, one observes no νOH CCC band (for reason evocated above), while two intense and broad doublets are observed at 1657.7−1649.0 and 1592.2− 1588.8 cm−1 in the νCO/νCC region. After 20 min of broad band, or 12 min of 270 nm selective UV irradiation, one observes new band groups growing in the νOH region: one, HF, ranging from 3670.0 to 3630.0 cm−1, displays bands at 3655.5 and 3640.1 cm−1, the other, LF, ranging from 3605.0 to 3580.0 cm−1, displays bands at 3601.0, 3598.0, 3594.2, and 3584.5 cm−1. The growing bands are due to nonchelated isomers. In the νCO/ νCC region, one observes four weak bands growing at 1729.9, 1704.5, 1633.9, and 1628.0 cm−1 while six intense multiplets also grow, with their most intense bands located at 1712.1, 1697.7, 1679.2, 1662.5, 1645.6, and 1614.4 cm−1. For longer broad band (Figure 3) and 270 nm irradiations (Figure 4), i.e. longer than 20 and 12 min, respectively, one observes not only CCC toward nonchelated forms isomerization but also conversion between nonchelated forms (Table 3). Conversion between nonchelated forms is also observed upon 240 and 230 nm selective irradiations (Figure 4). For a more than 12 min 270 nm irradiation, νOH LF band located at 3598.0 cm−1 decreases counterbalanced by the increase of HF bands located at 3655.5 and 3640.1 cm−1, and of LF ones at 3602.8 and 3601.0 cm−1. The 3598.0 cm−1 decrease is correlated to those of 1729.9, 1704.5, and 1614.4 cm−1 ones in νCO/νCC region,
Figure 4. Selective UV irradiations (laser power ≤1 mW) of MA trapped in Ar at 4.3 K. (a) black spectrum, before irradiation; red spectrum, after 51 min irradiation at 270 nm; (b) difference spectrum (after 51 min irradiation at 270 nm − after deposition); (c) difference spectrum (after 280 min irradiation at 240 nm − before irradiation at 240 nm); (d) difference spectrum (after 425 min irradiation at 230 nm − before irradiation at 230 nm). HF/LF: High/Low νOH Frequencies. F
DOI: 10.1021/acs.jpca.7b11980 J. Phys. Chem. A XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry A Table 3. Theoretical (B3LYP/6-311++G(2d,2p)) and Experimental IR Frequencies of CCCa CCC νExp νTheo
Ar
Xe
Ne
N2
νOH νCH
3134.8 (0.33) 2979.7 (0.16)
− 2877.7 2855.3
νCO
1692.2 (0.35)
− 2902.1 2876.6 2861.2 1657.6 1649.8 1640.1
− 2879.6 2871.3 2864.1 1656.3 1652.0 1641.6
νCC
1627.0 (0.40)
1599.8 1593.7
1592.9 1560.2
Δν δHCC/νC−C/δCCH
65.2 1482.8 (0.08)
− 2900.2 2877.7 2866.8 1657.7 1656.8 1649.0 1643.5 1592.2 1588.8 1532.2 1530.8 1494.5 65.5 1459.7 1449.3 1446.6
56.1 1463.1 1447.8 1424.0
59.1 1448.5 1446.9 1445.0 1443.6
δOCH/δHCC
1408.8 (0.03)
54.8 1461.9 1445.9 1442.9 1440.6 1438.3 1390.2 1386.2 1380.9
1383.0
1386.4 1381.0 1371.8
δOH/νC−O/νC−C
1390.8 (0.17)
1355.1
−
νC−O/δHCO/δCCH/δOH
1287.2 (0.25)
1375.4 1364.1 1360.1 1273.6 1265.9 1254.7
δCCH/δHCC/νC−O
1119.0 (0.02)
oop/τCC/τCO νC−C
1044.2 (0.02) 1001.1 (0.07)
vibrational modes
τOH
936.7 (0.09)
δCCC/δOCC/δCCO
896.2 (0.02)
1393.0 1389.6 1385.4 1382.5 1377.7 1360.0 1274.8 1267.3 1262.3 1256.6 1253.6 1252.0 1248.2 1246.4 1228.2 1225.0 1098.6 1095.1 1093.5 1034.0 991.8 990.1 988.4 986.5 984.7 983.1 982.1 981.3 899.8 892.4 884.1 877.8 877.0 874.8 871.3 867.7 852.0 851.1 G
1659.2 1655.1 1649.4 1643.4 1604.3 1594.6
1286.9 1273.0 1268.6 1261.1 1249.0 1241.2
1101.7 1093.1
1095.5
996.2 983.5 978.9 972.1 967.9
984.9
919.3
−
885.1 879.1 876.6 857.8
885.8 879.0 870.5
1269.2 1266.3 1259.7 1257.0 1253.5 1251.6 1248.4 1238.9 1229.4 1097.6 1096.2 1093.5 1043.8 986.0 983.0 981.9 978.9
901.9 900.4 888.0 881.7 879.0 873.8 871.4
DOI: 10.1021/acs.jpca.7b11980 J. Phys. Chem. A XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry A Table 3. continued CCC νExp vibrational modes oop/torsions
νTheo
Ar
Xe
Ne
N2
786.5 (0.06)
776.1 774.0 773.0 771.6 769.8 768.5 767.6
775.2 772.7 770.6
769.3
782.7 772.7
Frequencies are given in cm−1. Intensities (in italics) are normalized to CTC νCC mode. In the case of a multiplet, the most intense signal is bold. oop: out of plane. Δν = |νCO − νCC|.
a
Table 4. Theoretical (B3LYP/6-311++G(2d,2p)) and Experimental IR Frequencies of CTCa CTC νExp νTheo
Ar
Xe
Ne
N2
νOH
3791.6 (0.11)
3598.0
3565.0
3589.5
νCH
2902.2 (0.22)
2749.9
−
νCO
1757.4 (0.19)
1732.2 1702.9
1727.4
−
νCC
1640.1 (1.00)
1614.2 1610.7 1601.9
1614.1
Δν δCCH/δHCO/νCC
117.3 1326.3 (0.09)
118.0 1300.5
113.3 −
1613.1 1610.8 1608.3 1607.0 − −
δOH/νC−O
1159.4 (0.23)
2748.4 2744.4 1729.9 1725.7 1704.5 1618.7 1615.5 1614.4 1612.5 115.5 1304.0 1299.8 1126.4 1125.7 984.8 980.7
3583.0 3581.3 3579.2 −
1128.8 1127.7 991.0 987.8 979.4 978.5 − 740.9 739.3
−
−
−
−
− 741.4 736.7
− 737.6
vibrational modes
νC−C
998.5 (0.17)
oop δOCC
825.1 (0.05) 749.5 (0.12)
800.5 738.0 737.3
a Frequencies are given in cm−1. Intensities (in italics) normalized to CTC νCC mode. In the case of a multiplet, the most intense signal is bold. oop: out of plane. Δν=|νCO − νCC|.
The LF isomers are CTC (already identified), TCC and TTC. In addition to νOH and νCO/νCC regions, these two latter isomers should present intense bands separated by 104.8 cm−1 in the δOH/νC−O/νC−C region (Tables 6 and 9). It allows us to assign the 1214.9 and 1110.8−1108.9 cm−1 bands, separated by ≈104 cm−1, to TCC and TTC isomers, respectively. At the same time, the intense and broad νOH HF band at 3655.6 cm−1 and the weak LF one at 3598.0 cm−1 decrease. Knowing that this latter is a CTC one, it allows us to suggest the presence of also the TCT and TTT isomers. Indeed, despite HF groups contains TCT, TTT, CTT and CCT isomers, the changes observed in the νCO/νCC region are only compatible with TCT and TTT. The presence of TCT is comforted by the observation of intense signals at 1264.1 and 1005.0 cm−1, in the δOH/νC−O/νC−C region. Concerning TTT, the situation is less clear, but the observation of the 1679.2 cm−1 band is what is theoretically expected.
weak reconversion of the nonchelated forms toward CCC. However, this recovering is not the major relaxation channel. Here, contrary to what we observed for acetylacetone,10,11 the CCC conversion rate is almost 100%. Table 5 shows data for CTT, Table 6 shows data for TCC, Table 7 shows data for TCT, Table 8 shows data for TTT, and Table 9 shows data for TTC. The main insights brought by these UV irradiations are (a) The 270 nm irradiation provokes CCC conversion toward open forms and then conversion between those latter forms, allowing us to identify CTC (Table 4). (b) The main result of an 230 nm irradiation is a partial regeneration of CCC and CTC. (c) Upon 240 nm irradiation the only νOH bands which increase are those LF. H
DOI: 10.1021/acs.jpca.7b11980 J. Phys. Chem. A XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry A
blocked while we irradiated. We describe below only irradiations at 3598 and 3594 cm−1 because they are the most salient (Figure 5). The effects of a 3598 cm−1 irradiation (after a UV irradiation at 240 nm) are presented in Figure 5. In the νOH domain one observes a weak CTC decrease at 3598.0 cm−1 correlated to a weak increase in the HF set of bands, at 3639.5 cm−1. In the νCO/νCC region, these changes are correlated to CTC bands decrease and the increase of two bands centered at 1637.2 and 1628.0 cm−1. Therefore, the photoproduct presents a HF νOH frequency which could be TCT, TTT, CTT, or CCT. However, TCT and TTT have been already identified and present νCC bands which do not match the experimental observations. The remaining candidates are thus CTT and CCT. If we consider the photons energy (
