Structure and Photochemistry of N-Salicylidene-p-carboxyaniline Isolated in Solid Argon

Abstract

Infrared matrix isolation spectroscopy and DFT/B3LYP/6-311++G(d,p) calculations have been used to characterize the conformational space of the enol-imine and keto-amine tautomers of N-salicylidene-p-carboxyaniline (SCA) in both their E and Z isomeric forms. Monomers of SCA were isolated in an argon matrix (15 K), which was shown to contain only the most stable conformer of the E-enol isomer of the compound. The matrix-isolated E-enol was then subjected to in situ UV irradiation (λ = 335; 345 nm, provided by a laser/MOPO system, or λ > 235 nm, provided by a Hg(Xe) broad-band source), and the photoinduced processes probed by infrared spectroscopy. Two photoreaction channels were observed, with a branching ratio of ∼1:1, corresponding to E-enol → Z-enol isomerization and E-enol → E-keto tautomerization. Both processes were found to be rather effective, with practically complete consumption of the reactant after broad-band irradiation by 1 min only. Identification among the photoproduced species of the Z-enol conformer that differs from the reactant only by E-to-Z isomerization suggests the initial photoproduction of this conformer, which subsequently decays into the lowest energy Z-enol conformer (also identified experimentally). The E-enol → E-keto tautomerization requires an excited state intramolecular proton transfer and twisting about the exocyclic CC bond of the molecule. These processes most probably take place sequentially. However, in the present study the Z-keto isomer, which should act as intermediate in this sequence of processes, could not be detected, most probably due to its short lifetime under the used experimental conditions. On the contrary, the detailed structural and vibrational characterization of the photoproduced E-keto form was successfully achieved.

Structural Relevance of Intramolecular H-Bonding in Ortho-Hydroxyaryl Schiff Bases: The Case of 3-(5-bromo-2-hydroxybenzylideneamino) Phenol

“…The C–C=N–C dihedral stays in the range ~5–8° in the case of the Z -enol-imine conformers, where the repulsions between the two phenol moieties dominate, and between ~2–4° in the case of the E -enol-imine conformers. This structural feature has already been pointed out before for other aryl Schiff bases [ 26 , 28 , 29 , 30 , 47 , 48 ].…”

“…The four most stable conformers of the E -enol-imine isomer (conformers I, II, V and VI) are more than 38 kJ mol −1 lower in energy than the next conformer in the increasing order of energy (conformer XV). These conformers exhibit an intramolecular hydrogen-bond interaction of the O–H···N type (see Figure 2 ), which leads to the formation of a nearly planar pseudoaromatic six-membered ring where the dihedral angles ϒ and δ are both ~0°, and that is responsible for their stabilization [ 47 ]. Within the two pairs of conformers (I, II) and (V, VI), the main structural difference is the orientation of the non-hydrogen bonded phenolic OH group (the α dihedral angle being near 0 or 180°), while the orientation of the two aromatic rings is the same (i.e., the dihedrals β and ϒ are similar in each pair of conformers).…”

Polymorphism and Conformational Equilibrium of Nitro-Acetophenone in Solid State and under Matrix Conditions

“…For deeper insight into the hydrogen-bonding effect on the polymorphic states and isomerization we applied a wide variety of research methods (DFT, X-ray, DSC, NQR, IINS, IR and Raman) in different environments and over a wide temperature range. It is worth noting that the method of matrix isolation was unique in its ability to trace metastable states and help interpret complex reactions [ 11 , 12 , 13 , 14 ]. Moreover, investigations of objects with hydrogen bonding by IINS [ 15 , 16 , 17 , 18 ] and NQR [ 19 , 20 ] techniques are useful.…”

Infrared Spectrum and UV-Induced Photochemistry of Matrix-Isolated Phenyl 1-Hydroxy-2-Naphthoate

“…The excellent reproduction of the experimental data by the calculations shall be highlighted, because frequently the DFT method does not describe hydrogen bonding precisely, which is an essential intramolecular interaction in the case of the experimentally-relevant conformer of the studied molecule. Nevertheless, the functional/basis set used in the present study has been shown to be reliable for infrared spectra predictions for many other intramolecularly-bonded molecules formed exclusively by atoms of the first and second row of the periodic table [26][27][28]. In here, only the assignments for the most prominent characteristic vibrations of PHN will be briefly addressed.…”