Vibrational studies of methyl derivatives of malonaldehyde: Determination of a reliable force field for β-dicarbonyl compounds

UV photoreaction pathways of acetylacetaldehyde trapped in cryogenic matrices

The broadband UV photochemistry kinetics of acetylacetaldehyde, the hybrid form between malonaldehyde and acetylacetone (the two other most simple molecules exhibiting an intramolecular proton transfer), trapped in four cryogenic matrices, neon, nitrogen, argon, and xenon, has been followed by FTIR and UV spectroscopy. After deposition, only the two chelated forms are observed while they isomerize upon UV irradiation toward nonchelated species. From previous UV irradiation effects, we have already identified several nonchelated isomers, capable, in turn, of isomerizing and fragmenting; even fragmentation seems to be most unlikely due to cryogenic cages confinement. Based on these findings, we have attempted an approach to understand the reaction path of electronic relaxation. Indeed, we have demonstrated, in previous studies, that in the case of malonaldehyde, this electronic relaxation pathway proceeds through singlet states while it proceeds through triplet ones in the case of acetylacetone. We observed CO and CO₂ formations when photochemistry is almost observed among nonchelated forms, i.e., when the parent molecule is almost totally consumed. In order to identify a triplet state transition, we have tried to observe a "heavy atom effect" by increasing the weight of the matrix gas, from Ne to Xe, and to quench the T₁ state by doping the matrices with O₂. It appears that, as in the case of acetylacetone, it is the nonchelated forms that fragment. It also appears that these fragmentations certainly take place in the T₁ triplet state and originate in an Π* ← n transition.

UV Photochemistry of Acetylacetaldehyde Trapped in Cryogenic Matrices

The broad band UV photochemistry of acetylacetaldehyde, the hybrid form between malonaldehyde and acetylacetone (the two other most simple molecules exhibiting an intramolecular proton transfer), trapped in four cryogenic matrices, neon, nitrogen, argon, and xenon, has been studied by IRTF spectroscopy. These experimental results have been supported by B3LYP/6-311G++(2d,2p) calculations in order to get S₀ minima together with their harmonic frequencies. On those minima, we have also calculated their vibrationally resolved UV absorption spectra at the time-dependent DFT ωB97XD/6-311++G(2d,2p) level. After deposition, only the two chelated forms are observed while they isomerize upon UV irradiation toward nonchelated species. From UV irradiation effects we have identified several nonchelated isomers, capable, in turn, of isomerizing and fragmenting, even if this last phenomenon seems to be most unlikely due to cryogenic cages confinement. On the basis of these findings, we have attempted a first approach to the reaction path of electronic relaxation. It appeared that, as with acetylacetone, the path of electronic relaxation seems to involve triplet states.

Quantum Effects for a Proton in a Low-Barrier, Double-Well Potential: Core Level Photoemission Spectroscopy of Acetylacetone

We have performed core level photoemission spectroscopy of gaseous acetylacetone, its fully deuterated form, and two derivatives, benzoylacetone and dibenzoylmethane. These molecules show intramolecular hydrogen bonds, with a proton located in a double-well potential, whose barrier height is different for the three compounds. This has allowed us to examine the effect of the double-well potential on photoemission spectra. Two distinct O 1s core hole peaks are observed, previously assigned to two chemical states of oxygen. We provide an alternative assignment of the double-peak structure of O 1s spectra by taking full account of the extended nature of the wave function associated with the nuclear motion of the proton, the shape of the ground and final state potentials in which the proton is located, and the nonzero temperature of the samples. The peaks are explained in terms of an unusual Franck-Condon factor distribution.

Luminescence and Raman Spectra of Acetylacetone at Low Temperatures

Raman spectra of acetylacetone were recorded for molecules isolated in an argon matrix at 10 K and for a polycrystalline sample. In the solid sample, broad bands appear superimposed on a much weaker Raman spectrum corresponding mainly to the stable enol form. The position of these bands depends on the excitation wavelength (514.5 and 488.8 nm argon ion laser lines were used), sample temperature, and cooling history. They are attributed to transitions from an excited electronic state to various isomer states in the ground electronic state. Laser photons have energies comparable to energies of a number of excited triplet states predicted for a free acetylacetone molecule (Chen, X.-B.; Fang, W.-H.; Phillips, D. L. J. Phys. Chem. A 2006, 110, 4434). Since singlet-to-triplet photon absorption transitions are forbidden, states existing in the solid have mixed singlet/triplet character. Their decay results in population of different isomer states, which except for the lowest isomers SYN enol, TS2 enol (described in Matanović I.; Doslić, N. J. Phys. Chem. A 2005, 109, 4185), and the keto form, which can be detected in the Raman spectra of the solid, are not vibrationally resolved. Differential scanning calorimetry detected two signals upon cooling of acetylacetone, one at 229 K and one at 217 K, while upon heating, they appear at 254 and 225 K. The phase change at higher temperature is attributed to a freezing/melting transition, while the one at lower temperature seems to correspond to freezing/melting of keto domains, as suggested by Johnson et al. (Johnson, M. R.; Jones, N. H.; Geis, A; Horsewill. A. J.; Trommsdorff, H. P. J. Chem. Phys. 2002, 116, 5694). Using matrix isolation in argon, the vibrational spectrum of acetylacetone at 10 K was recorded. Strong bands at 1602 and 1629 cm(-1) are assigned as the SYN enol bands, while a weaker underlying band at 1687 cm(-1) and a medium shoulder at 1617 cm(-1) are assigned as TS2 enol bands.

UV and IR Photoisomerization of Acetylacetone Trapped in a Nitrogen Matrix

UV- and IR-induced photoisomerization of acetylacetone trapped in a nitrogen matrix at 4.3 K have been carried out using a tunable optical parametric oscillator type laser, or a mercury vapor lamp, coupled with Fourier Transform IR and UV spectroscopies. After deposition, the main form present in the cryogenic matrix is that chelated (enol). Upon UV irradiation, the intramolecular H bond is broken leading to nonchelated isomers among seven possible open forms. These forms have then been irradiated by resonant pi* <-- pi UV irradiation, or by resonant nuOH irradiation. The selective UV irradiation allows us to suggest a first vibrational assignment while the nuOH irradiation leads us to observe interconversions between the nonchelated isomers. In order to support our vibrational assignment, we have carried out theoretical calculations at the B3LYP/6-311++G(2d,2p) level of theory. This study shows that only five isomers are observed among eight postulated.

Theoretical Studies of the Photochemical Dynamics of Acetylacetone: Isomerzation, Dissociation, and Dehydration Reactions

The potential energy surfaces of the C-O cleavage, rotational isomerization, keto-enolic tautomerization, and dehydration reactions of acetylacetone in the lowest triplet and ground states have been determined using the complete active space self-consistent field and density functional theory methods. The main photochemical mechanism obtained indicates that the acetylacetone molecule in the S(2)((1)pipi*) state can relax to the T(1)((3)pipi*) state via the S(2)-S(1) vibronic interaction and an S(1)/T(1)/T(2) intersection. The C-O fission pathway is the predominant dissociation process in the T(1)((3)pipi) state. Rotational isomerization reactions proceed difficultly in the ground state but very easily in the T(1)((3)pipi*) state. Keto-enolic tautomerization takes place with little probability for acetylacetone in the gas phase.

Experimental and Theoretical UV Characterizations of Acetylacetone and Its Isomers

Cryogenic matrix isolation experiments have allowed the measurement of the UV absorption spectra of the high-energy non-chelated isomers of acetylacetone, these isomers being produced by UV irradiation of the stable chelated form. Their identification has been done by coupling selective UV-induced isomerization, infrared spectroscopy, and harmonic vibrational frequency calculations using density functional theory. The relative energies of the chelated and non-chelated forms of acetylacetone in the S0 state have been obtained using density functional theory and coupled-cluster methods. For each isomer of acetylacetone, we have calculated the UV transition energies and dipole oscillator strengths using the excited-state coupled-cluster methods, including EOMCCSD (equation-of-motion coupled-cluster method with singles and doubles) and CR-EOMCCSD(T) (the completely renormalized EOMCC approach with singles, doubles, and non-iterative triples). For dipole-allowed transition energies, there is a very good agreement between experiment and theory. In particular, the CR-EOMCCSD(T) approach explains the blue shift in the electronic spectrum due to the formation of the non-chelated species after the UV irradiation of the chelated form of acetylacetone. Both experiment and CR-EOMCCSD(T) theory identify two among the seven non-chelated forms to be characterized by red-shifted UV transitions relative to the remaining five non-chelated isomers.

Infrared Spectroscopy of the Intramolecular Hydrogen Bond in Acethylacetone: A Computational Approach

The intramolecular hydrogen bond in the enol-acethylacetone (ACAC) is investigated by performing reduced-dimensional quantum calculations. To analyze the shared proton vibrations, two sets of coordinates were employed: normal mode coordinates describing the motion in the vicinity of the most stable configuration, and internal coordinates accounting for the double minimum proton motion. It is proved that the extreme broadness of the OH-stretch band in ACAC is a consequence of the coexistence of two enol-ACAC structures: the global minimum and the transition state for rotation of the distal methyl group. Further, a ground-state tunneling splitting of 116 cm(-1) is found, and it is shown that the inclusion of the kinematic coupling is mandatory when treating large-amplitude proton motion. In the OH-stretch direction a splitting of 853 cm(-1) was predicted.

Fourier transforms infrared spectra and structure of triformylmethane. A density functional theoretical study

Molecular structure and vibrational frequencies of triformylmethane have been investigated by means of density functional theory (DFT) calculations. The geometrical parameters and vibrational frequencies obtained in the B3LYP, B3PW91, BLYP, BPW91, G96LYP and G96PW91 levels of DFT and compared with the corresponding parameters of malonaldehyde (MA). Fourier transform infrared spectra of triformylmethane and its deuterated analogue were clearly assigned. Theoretical calculations show that the hydrogen bond strength of triformylmethane is stronger than that of MA, which is in agreement with spectroscopic results.