Structures of cycloserine and 2-oxazolidinone probed by X-ray photoelectron spectroscopy: theory and experiment

Dehydrogenation of Ammonia Borane Impacts Valence and Core Electrons: A Photoemission Spectroscopic Study

Ammonia borane (H₃BNH₃) is a promising material for hydrogen storage and release. Dehydrogenation of ammonia borane produces small boron-nitrogen hydrides such as aminoborane (H₂BNH₂) and iminoborane (HBNH). The present study investigates ammonia borane and its two dehydrogenated products for the first time using calculated photoemission spectra of the valence and core electrons. It is found that a significant decrease in the dipole moment was observed associated with the dehydration from 5.397 D in H₃BNH₃, to 1.942 D in H₂BNH₂, and to 0.083 D in HBNH. Such reduction in the dipole moment impacts properties such as hydrogen bonding, dihydrogen bonding, and their spectra. Dehydrogenation of H₃BNH₃ impacts both the valence and core electronic structure of the boron-nitrogen hydrides. The calculated valence vertical ionization energy (VIE) spectra of the boron-nitrogen hydrides show that valence orbitals dominated by 2p-electrons of B and N atoms exhibit large changes, whereas orbitals dominated by s-electrons, such as (3a₁4a₁5a₁/3σ4σ5σ) remain less affected. The first ionization energy slightly increases from 10.57 eV for H₃BNH₃ to 11.29 eV for both unsaturated H₂BNH₂ and HBNH. In core space, the oxidative dehydrogenation of H₃BNH₃ affects the core electron binding energy (CEBE) of borane and nitrogen oppositely. The B1s binding energies increase from 194.01 eV in H₃BNH₃ to 196.93 eV in HBNH, up by 2.92 eV, whereas the N1s binding energies decrease from 408.20 eV in H₃BNH₃ to 404.88 eV in HBNH, dropped by 3.32 eV.

Experimental and Theoretical Soft X-ray Study of Nicotine and Related Compounds

The valence and core electronic structure of nicotine, nicotinic acid, and nicotinamide have been studied by photoelectron and soft X-ray absorption spectroscopy, supported by theoretical calculations, which take into account conformational isomerism. The core-level photoionization spectra of all molecules have been assigned, and theory indicates that the effects of conformational differences are small, generally less than the natural line widths of the core ionic states. However, in the case of nicotinamide, the theoretical valence ionization potentials of cis and trans conformers differ significantly in the outer valence space, and the experimental spectrum is in agreement with the calculated outer valence cis conformer spectrum. In addition, the C, N, and O K edge near-edge absorption fine structure spectra are reported and interpreted by comparison with reference compounds. We find evidence at the N and O K edges of interaction between the delocalized orbitals of the pyridine ring and the substituents for nicotinic acid and nicotinamide. The strength of the interaction varies because the first is planar, while the second is twisted, reducing the extent of orbital mixing.

Electronic structure and conformational flexibility of D-cycloserine

The first comprehensive investigation of the effect of conformational flexibility of gaseous D-cycloserine on the valence and core electronic structures is reported here. The seven most stable conformers among the twelve structures calculated at the MP2/6-311++G** level of theory were assumed to properly describe the properties of the investigated compound. Taking into account the contribution of these isomers, the valence photoelectron spectrum (UPS) was simulated by the Outer Valence Green' s Function (OVGF) method. A different sensitivity towards the conformational flexibility of the outermost photoelectron bands was exhibited in the simulated spectrum. The comparison of the theoretical UPS with the experimental one allowed a detailed assignment of the outermost valence spectral region. The composition and bonding properties of the relevant MOs of the most stable conformers were analyzed in terms of leading Natural Bond Orbital (NBO) contributions to the HF/6-311++G** canonical MOs. The C1s, N1s, and O1s photoelectron spectra (XPS) were theoretically simulated by calculating the vertical Ionization Energies (IEs) of the relevant conformers using the ΔSCF approach. The different IE chemical shift spread of the XPS components associated with various conformers, which is expected to affect the experimental spectra, could be evaluated by simulated XPS, thus providing a new insight into the core electronic structure. The comparison of the theoretical results with the experimental ones unraveled that the atomic XPS components are not mixed by conformational flexibility of D-cycloserine, and that the specific vibronic structure of different spectral components should play a crucial role in determining different relative intensities and band shapes observed in the experiment.