Double deuterated acetylacetone in neon matrices: infrared spectroscopy, photoreactivity and the tunneling process

Abstract

The effect of deuteration of acetylacetone (C5O2H8) is explored by means of IR spectroscopy of its single and double deuterated isotopologues trapped in neon matrices. The whole vibrational spectra of chelated enols are very sensitive to the H-D exchange of the hydrogen atom involved in the internal hydrogen bond. UV excitation of double deuterated acetylacetone isolated in neon matrices induces the formation of four open enol isomers which can be divided into two groups of two conformers, depending on their formation kinetics. Within each group, one conformer is more stable than the other: slow conformer interconversion due to a tunneling process is observed in the dark at low temperature. Moreover, IR laser irradiation at the OD stretching overtone frequency is used to induce interconversion either from the most stable to the less stable conformer or the opposite, depending on the excitation wavelength. The interconversion process is of great help to assign conformers which are definitively identified by comparison between experimental and calculated IR spectra. Kinetic constants of the tunneling process at play are theoretically estimated and agree perfectly with experiments, including previous experiments with the totally hydrogenated acetylacetone [Lozada García et al., Phys. Chem. Chem. Phys., 2012, 14, 3450].

Multiple-State Nonadiabatic Dynamics Simulation of Photoisomerization of Acetylacetone with the Direct ab Initio QTMF Approach

“…Some of them have been identified in different kinds of matrices by the combined infrared spectroscopy and electronic structure calculations. 31,32,42,43,59 With respect to the E-S 0 minimum, however, these non- Once E-S 0 is excited to the S 2 state, the subsequent relaxation processes were observed to have a multi-exponential character in the previous femtosecond pump-probe experiment. 41 A simple linear kinetic model was used to fit the time dependencies of the Theory and Computation 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 18 observed signals, which is a sequential decay involving four transients,…”

Selective photoisomerisation of 2-chloromalonaldehyde

“…Within these four pairs, conformers differ only by the position of the OH group, and we assume that a tunneling process through the OH rotational barrier occurs to deplete the less stable form, as it was observed in acetylacetone. 44 This tunneling effect is then efficient in the three matrices, whereas the tunneling process for the H transfer in the CCC seemed to depend strongly on the host, with an efficient quenching in Ne. 18 The 2-ClMA conformers stabilized in argon are different from those of MA detected under similar experimental conditions.…”

“…It has been explained by a rapid tunneling through the OH rotational barrier making stable the only conformer of lower energy in each pair (XYT-XYC), this tunneling is slowed down by deuteration of the H of the hydroxyl group. 44 We assume the same process to explain the observation of four conformers (CCC, TTC, CTC, and TCT) corresponding to the most stable conformers in each of the four pairs. In particular, it means that in 2-ClMA, TCT is more stable than TCC, in agreement with the calculations which give rather close energies for both conformers.…”

“…18 Briefly, one cryogenic setup (Air Products Displex DE202FF) reached the minimal temperature of 7 K and was used for argon and neon matrices (as described in Ref. 44). The other (ICE Oxford ICES3731) was devoted to experiments in para-hydrogen (pH 2 ) solids at 3 K and was also used for complementary experiments in neon.…”

“…The matrix induced band shifts between Ar, Ne, and pH 2 samples should be small, following the results on the π * ← π transition of MA in different matrices 29 and those on a similar electronic transition of AcAc in various matrices including Ar, Ne, and pH 2 . 32,44,46 One can expect a slight blue shift from the argon matrix to the neon matrix, the latter generally inducing the smallest gas-to-matrix shifts.…”