2-Chloromalonaldehyde, a model system of resonance-assisted hydrogen bonding: vibrational investigation

Abstract

The chelated enol isomer of 2-chloromalonaldehyde (2-ClMA) is experimentally characterized for the first time by IR and Raman spectroscopies. The spectra are obtained by trapping the molecule in cryogenic matrices and analyzed with the assistance of theoretical calculations. Experiments were performed in argon, neon and para-hydrogen matrices. The results highlight puzzling matrix effects, beyond site effects, which are interpreted as due to a tunneling splitting of the vibrational levels related to the proton transfer along the internal hydrogen bond (IHB). 2-ClMA is thus one of the very few molecules in which the H tunneling has been observed in cryogenic matrices. The comparison with its parent molecule (malonaldehyde) shows experimentally and theoretically the weakening of the IHB upon chlorination, with a reduced cooperative effect in the resonance assisted hydrogen bond. In addition, the Cl substitution induces an important stabilization of two open enol conformers. These two open forms appear in the spectra of as-deposited samples, meaning that, in contrast with other well-studied molecules of the same family (β-dialdehydes and β-diketones), they are present in the gas phase at room temperature.

Theoretical study of “trapping sites” in cryogenic rare gas solids doped with β-dicarbonyl molecules

“…In Xe, one crystallographic plane is much more occupied than the other and the first experimental data could only show the spectra in the most populated trapping site. The comparison between MA and 2ClMA in terms of tunneling effect due to the proton transfer in the intramolecular hydrogen bond can be tentatively related to the kind of trapping sites occupied by the molecule: similar sites in Ne with large deformations of the crystallographic ordering around the impurity, to be related to the absence of tunneling splitting in MA/Ne and 2ClMA/Ne samples on one hand, and different sites in Ar with MA mainly trapped in (111) crystallographic planes and 2ClMA trapped both in (001) and (111) planes, to be related to the observation of a tunneling splitting of vibrational levels only in the case of 2ClMA/Ar on the other hand [10].…”

“…On the other hand, vibrational spectroscopies (FT-IR and Raman) of 2ClMA in rare gas matrices clearly show the presence of sites effects in Ar matrices and not in Ne [10]. IR spectra in Kr and Xe matrices have been recently recorded (not published).…”

“…Site effects play an important role in the spectroscopy of atomic and molecular impurities trapped in cryogenic solids [1][2][3][4][5][6][7][8][9]. Their influence is connected with the observation for a single vibrational mode or a single vibronic transition, of several bands in absorption and emission and with the shapes of the bands [1][2][3]10]. In presence of different trapping sites it is a difficult task to separate the individual spectroscopic contribution of each site.…”

“…In presence of different trapping sites it is a difficult task to separate the individual spectroscopic contribution of each site. Several experimental and theoretical methods have been developed and used for this purpose [1][2][3][4][5][6][7][8][9][10]. Among them, molecular dynamics (MD) represents an useful tool to describe cryogenic crystals.…”

“…In these four systems, the comparison with spectro-scopic experimental data has confirmed the helpfulness of this simulation method. A brief discussion on the effect of trapping sites in the spectroscopy of malonaldehyde (MA) in comparison with 2chloromalonaldehyde (2ClMA) when they are embedded in Ar matrices has been previously presented [10]. A similar deposition model was used in that recent study.…”