Monomers of pyruvic acid (PA) isolated in cryogenic argon and nitrogen matrixes were characterized by mid- and near-infrared spectroscopy. Interpretation of the experiments was aided by fully anharmonic calculations of the fundamental modes, overtones, and combinations up to two quanta, including their infrared intensities. The initially dominating PA conformer (Tc) has a cis CCOH arrangement and is stabilized by a strong intramolecular H-bond. Selective near-infrared excitation of Tc at the first OH overtone (6630 cm(-1) in Ar, 6643 cm(-1) in N2) induced a large scale conformational conversion to the higher-energy conformer (Tt) with trans CCOH arrangement. Tt was then converted back to Tc by selective NIR irradiation at the first Tt OH overtone (6940 cm(-1) in Ar, 6894 cm(-1) in N2). In N2 matrix, the Tt form was stabilized due to interaction between the OH group and the matrix molecules. This stabilization manifested itself in the absence of Tt → Tc relaxation and in a considerable change of the vibrational Tt signature upon going from argon to nitrogen matrix. In argon, the Tt form spontaneously decayed back to Tc in the dark (characteristic lifetime +16 h). In the presence of broad-band near-infrared light, the Tt → Tc relaxation speed considerably increased. The decay mechanisms are discussed.

This paper describes the pyrolysis of parent isoxazole and of its 5-methyl and 3,5-dimethyl derivatives by the high-pressure pulsed pyrolysis method, where activation of the precursor molecules occurs predominantly by collisions with the host gas (Ar in our case), rather than with the walls of the pyrolysis tube, where catalyzed processes may occur. The products were trapped at 15 K in Ar matrices and were characterized by vibrational spectroscopy. Thereby, hitherto unobserved primary products of pyrolysis of isoxazole and of its 5-methyl derivative, 3-hydroxypropenenitrile or 3-hydroxybutenenitrile, respectively, were observed. E-Z photoisomerization could be induced in the above hydroxynitriles. On pyrolysis of isoxazole, ketenimine and CO were observed as decomposition products, but this process did not occur when the 5-methyl derivative was pyrolyzed. Instead, the corresponding ketonitrile was formed. In the case of 3,5-dimethylisoxazole, 2-acetyl-3-methyl-2H-azirine was detected at moderate pyrolysis temperatures, whereas at higher temperatures, 2,5-dimethyloxazole was the only observed rearrangement product (next to products of dissociation). These findings are rationalized on the basis of quantum chemical calculations. Thereby it becomes evident that carbonyl-vinylnitrenes play a pivotal role in the observed rearrangements, a role that had not been recognized in previous theoretical studies because it had been assumed that vinylnitrenes are closed-shell singlet species, whereas they are in fact open-shell singlet biradicaloids. Thus, the primary processes had to be modeled by the multiconfigurational CASSCF method, followed by single-point MR-CISD calculations. The picture that emerges from these calculations is in excellent accord with the experimental findings; that is, they explain why some possible products are observed while others are not.

Triplet 2-formyl phenylnitrene was generated by photolysis of 2-formyl phenylazide isolated in Ar, Kr, and Xe matrixes and characterized by IR, UV-vis, and EPR spectroscopies. Upon generation at 10 K, the triplet nitrene spontaneously rearranges in the dark to singlet 6-imino-2,4-cyclohexadien-1-ketene on the time scale of several hours. The intramolecular [1,4] H atom shift from the nitrene to the imino ketene occurs by tunneling, on the triplet manifold, followed by intersystem crossing. This case constitutes the first direct evidence of a tunneling reaction involving a nitrene.

Quantum mechanical tunneling (QMT) of heavy atoms like carbon or nitrogen has been considered very unlikely for the longest time, but recent evidence suggests that heavy-atom QMT does occur more frequently than typically assumed. Here we demonstrate that carbon vs nitrogen heavyatom QMT can even be competitive leading to two different products originating from the same starting material. Aminosubstituted benzazirine was generated in solid argon (3−18 K) and found to decay spontaneously in the dark, with a half-life of 210 min, to p-aminophenylnitrene and amino-substituted ketenimine. The reaction rate is independent of the cryogenic temperature, in contradiction to the rules inferred from classical transition state theory. Quantum chemical computations confirm the existence of two competitive carbon vs nitrogen QMT reaction pathways. This discovery emphasizes the quantum nature of atoms and molecules, thereby enabling a much higher level of control and a deeper understanding of the factors that govern chemical reactivity.

We discovered a 1,3-dipolar species co-existing in two different structures. Photolysis of matrix-isolated 5-phenyltetrazole generates two forms of phenylnitrile imine: propargylic and allenic. They are not resonance structures but correspond to different energy minima, representing bond-shift isomers. These distinct species were characterized spectroscopically and confirmed by calculations up to the CASSCF(14,12) theory level.