The N-benzyloxycarbonyl-l-alanine molecule is a derivative of the l-α-alanine amino acid with a benzyl carbamate protecting group at the N-terminus, more commonly denoted Cbz-Ala or Z-Ala. In this computational investigation, we sought to determine the available isomers of Z-Ala and their distinguishing spectroscopic signatures via quantum-chemistry methods. Sixty-five total isomers were obtained, and coupled-cluster- and perturbation-theory-based relative energies were computed. The two nearly degenerate, lowest-energy isomers were found to differ in their configuration than the lowest-energy form of isolated alanine, suggesting that the protecting group changes the dominant form of Ala. Through comparisons to exhaustive sampling of Ala and benzyl formate isomers, nearly all of the Z-Ala structures could be ascribed to fragment-paired structural motifs, with a few outliers exhibiting new intramolecular interactions between the constituent fragments. Based on this observation, an assessment of the additivity of the two fragments’ relative energies was performed for Z-Ala energies. Many of the isomers’ energies were reasonably described by such considerations, although backbone strain and hydrogen-bonding interactions altered this energy landscape and led to nonadditive effects for several of the isomers. Comparison to experimental REMPI-based UV/IR ion-dip vibrational spectra in the 90–1822 cm–1 region indicated that two isomers are dominantly present at the experimental conditions, although signatures of other isomers from the ensemble were also observed. Clear assignments of structural motifs were possible through this experimental comparison. Computed coupled-cluster benchmarks allowed for methodology assessments in this study. The modified opposite-spin MP2 method (MOS-RI-MP2) was found to be particularly accurate, relative to these benchmarks, after minor adjustment of the range-separation parameter. Density functional theory (DFT) methods were found to be variable in their accuracy for both energies and spectra, although a few key functionals performed particularly well for this system in the low-frequency region of the vibrational spectrum. These methodology constraints provided recommendations for similar systems and subsequent anharmonic analyses.

Transition state modeling is a powerful tool for unraveling the mechanistic intricacies of chemical reactions. It plays a pivotal role in the design of innovative catalytic systems, facilitating progress in the field of catalysis. A carbon-neutral chemical hydrogen battery is the most relevant approach for the storage and transportation of hydrogen fuel. Herein, with the aid of transition state models, we decipher the mode of activity of the pincer-ligated manganese complex that enables the reversible hydrogenation and dehydrogenation process for the efficient H2 storage and release. We identified the critical contributions from the basic amino acids, specifically lysine and its potassium salt, as well as the influence of solvent, counterions, and water, in governing the reversibility. In addition, we have probed into the role of noncovalent interactions during the capture and release of CO2 by potassium lysinate, thereby enabling the realization of a carbon-neutral chemical hydrogen battery system.

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We present a study of static polarizabilities of organic molecules in aqueous environments using projection-based coupled-cluster in density functional theory quantum embedding. We propose two methods...