Low temperature IR spectroscopy and photochemistry of matrix-isolated α-pyridil

Abstract

2 ] has been isolated in low temperature argon and xenon matrices and studied by FTIR spectroscopy, supported by DFT(B3LYP)/6-311++G(d,p) calculations. Calculations predicted the existence of three different conformers exhibiting skewed conformations around the intercarbonyl bond and the two C 5 H 4 N C( O) fragments nearly planar. The two higher energy forms, TCG and CCSk were estimated theoretically to be, respectively, 21.0 and 35.1 kJ mol −1 higher in energy than the most stable form, TTG. In consonance with the relatively high energies predicted by the calculations for the two less stable conformers of ␣-pyridil, only the most stable conformer was found spectroscopically to be present in the studied matrices. Infrared spectra obtained for the neat low temperature amorphous and crystalline states reveals that the TTG conformer is also the sole conformer present in these phases. UV irradiation ( > 235 nm) of matrix-isolated ␣-pyridil led to its isomerization into unusual molecular species bearing Hückel-type pyridine (aza-benzvalene) rings.

Structure, Vibrational Spectra, and Cryogenic MatrixPhotochemistry of 6-Bromopyridine-2-carbaldehyde: From the Single Molecule of the Compound to the Neat Crystalline Material

“…The spectra of the irradiated matrices do not show any evidence of occurrence of other photo-induced reactions, in particular pyridine ring valence isomerization [ 9 , 10 , 11 ] and Br atom extrusion [ 12 , 13 ].…”

“…From the structural point of view, BPCA appears as an interesting compound due to the following characteristics: (i) the molecule of the compound possesses an internal rotational degree of freedom (the torsion around the exocyclic C–C bond), so that different conformers can be expected to exist; (ii) the location of the aldehyde substituent at the ortho position to the pyridine nitrogen atom can be anticipated to lead to relevant conformation-dependent intramolecular interactions between these two moieties; (iii) the presence of the voluminous, strongly polarizable bromine substituent in the molecule opens expectations regarding its importance in determining fundamental characteristics of the crystal structure of the compound; (iv) the unimolecular photochemistry of the isolated molecules of the compound appears as an interesting topic to be investigated, considering the a priori possibility of occurrence of both UV-induced conformational isomerization [ 4 , 5 , 6 ] and photofragmentation reactions, specifically decarbonylation [ 6 , 7 , 8 ], pyridine ring valence isomerization [ 9 , 10 , 11 ], and Br atom extrusion [ 12 , 13 ]. The investigation of all these characteristics appears as a fundamental step for the detailed understanding of the physicochemical behavior of BPCA, bridging the properties of the isolated molecule to those of the neat crystalline compound and potentiating its use in supramolecular chemistry, in synthesis, and other practical applications, as those mentioned above.…”