Two almost isoenergetic conformers of 4-methoxybenzaldehyde (p-anisaldehyde), O-trans and O-cis, are nearly equally populated in gas phase at room temperature. The existence of these two conformers of similar energy makes p-anisaldehyde an attractive molecule for conformational investigations, in which the relative populations of the two forms might be subjected to optical control. In the present study, monomers of the compound were trapped from the room-temperature gas phase into cryogenic argon and xenon matrices. The initial relative amount of the two conformers present in the freshly deposited matrices is shifted slightly in favor of the O-trans conformer. The ratio of the two forms could be reversibly varied by irradiating the sample with UV light in different wavelength ranges or by using the temperature variation. Increasing the temperature of the xenon matrix up to ca. 57 K led to conversion of the less stable O-cis form into the O-trans conformer, shifting the O-cis/O-trans ratio to ca. 1:7. A series of UV irradiations with different long-pass cutoff filters was carried out. UV excitation induced transformation of O-cis and O-trans conformers into each other. These transformations were leading to the UV-wavelength-specific photostationary equilibria characterized by the O-cis/O-trans ratios of about 1:2.2, 1:1.4, 1:1.1, and 1:0.89 for λ > 328, 295, 288, and 234 nm cutoff filters, respectively. The isomerization processes were probed by infrared spectroscopy and supported by quantum chemical calculations. The absorption bands observed in the infrared spectra of p-anisaldehyde isolated in argon and xenon matrices were assigned to the theoretically predicted normal modes.

β-aminoisobutyric acid (BAIBA) has been studied in isolation conditions: in the gas phase and trapped into a cryogenic N2 matrix. A solid sample of the compound was vaporized by laser ablation and investigated through their rotational spectra in a supersonic expansion using two different spectroscopic techniques: broadband chirped pulse Fourier transform microwave spectroscopy and conventional molecular beam Fourier transform microwave spectroscopy. Four conformers with structures of two types could be successfully identified by comparison of the experimental rotational and (14)N nuclear quadruple coupling constants with those predicted theoretically: type A, bearing an OH⋯N intramolecular hydrogen bond and its carboxylic group in the trans geometry (H-O-C=O dihedral ∼180°), and type B, having an NH⋯O bond and the cis arrangement of the carboxylic group. These two types of conformers could also be trapped from the gas phase into a cryogenic N2 matrix and probed by Fourier transform infrared (IR) spectroscopy. In situ irradiation of BAIBA isolated in N2 matrix of type B conformers using near-IR radiation tuned at the frequency of the O-H stretching 1st overtone (∼6930 cm(-1)) of these forms allowed to selectively convert them into type A conformers and into a new type of conformers of higher energy (type D) bearing an NH⋯O=C bond and a O-H "free" trans carboxylic group.

DFT(B3LYP)/6-311++G(d,p) calculations on 2-fluorobenzoic acid (2FBA) show that the molecule has four conformers: two low energy conformers (forms I and II) with the carboxylic acid group assuming the cis configuration (O=C-O-H dihedral equal to 0°) and two higher-energy conformers (III, IV) with a trans carboxylic group configuration. Isolation of 2FBA monomers in argon or nitrogen matrices allows for the efficient trapping of both low-energy conformers. Narrowband selective near-IR(NIR) excitation of the 2νOH mode of I in both argon and N matrices leads to its efficient conversion into conformer III, which is stabilized by an intramolecular O-H···F interaction. On the other hand, upon identical selective vibrational excitation of II no changes could be noticed in the argon matrix spectra, while experiments carried out on N matrices showed conversion of II into III. In conformer IV (the expected direct product resulting from NIR excitation of II), the stabilizing O-H···F interaction existing in III is replaced by an O-H···H repulsive interaction, which leads to a barrier separating this form from II that is about one third of that separating III from I. Under these circumstances, once formed by vibrational excitation of II, conformer IV can easily convert to the reactant species by fast tunneling, justifying the apparent inefficiency of the II→IV conversion upon vibrational excitation of II in an argon matrix. On the other hand, the stabilization of the initially formed conformer IV by the N matrix (due to OH···N interactions) allows this species to survive long enough to allow the IV→III over-the-barrier conversion to be competitive with the IV→IItunneling, justifying the observed net conversion of the NIR excited conformer II into form III in N matrix. These results demonstrate as the intramolecular local topology may decisively influence the intrinsic kinetic stability of different conformers of the same molecule. They are also a clear additional evidence of the stabilization of higher-energy trans conformers of carboxylic acids by the N matrix medium that has been found before for other molecules of the same family.