In the present work, the chirality recognition of the enantiomers of a chiral molecule (1-phenyl-1-propanol) interacting with a nanotubular cyclic peptide (E-type cyclic decapeptide) was investigated by their ionization in the gas phase, theoretically. The absolute energy difference between the interaction of the S- and R-enantiomer with the cyclic peptide, calculated at the M06-2X/6-311++G(d, p) level of theory, was 4.70 kcal·mol(-1). Two different schemes of "Our own N-layered Integrated molecular Orbital and molecular Mechanics (ONIOM)" method such as (quantum mechanics (QM):molecular mechanics (MM)) and (QM:QM) were employed to study the effect of the interaction on the gas-phase ionization energies of the enantiomers and cyclic peptide, separately. The symmetry-adapted cluster/configuration interaction (SAC-CI) methodology was used for the calculation of the ionization energies. It was found that the difference between the interactions of R- and S-enantiomer with the cyclic peptide caused different changes in the photoelectron spectrum of each enantiomer so that these changes could be used for the chirality discrimination of the enantiomers in the gas phase. Similarly, the photoelectron spectrum of the cyclic peptide interacting with the R and S-enantiomer were calculated, separately, and it was observed that the difference in the interaction with the R- and S-enantiomer created different changes in the spectrum of cyclic peptide. Finally, it was shown that the difference in the interaction of cyclic peptide with the enantiomers of a chiral molecule in the gas phase can be used for the identification of enantiomers in the gas phase by the direct ionization.
Cyclic peptides, because of their unique spatial conformations, simplicity, and limited conformational freedom, are widely used as model molecules for larger peptides in chemistry and biochemistry. In this work, the ionization energies and photoelectron spectra of different conformers of the cyclic peptides (n = 2–15) were calculated using the symmetry‐adapted cluster‐configuration interaction (SAC‐CI) method and D95 + (d,p) basis set in the gas phase. The calculated photoelectron spectra were used to study the electronic structures of the cyclic peptides. It was observed that the first ionization energy of the cyclic peptides decreases with the ring size, reaches a minimum, and then increases. In addition, the first ionization band of the cyclic peptides was assigned to the ionization of the lone electron pairs of the nitrogen atoms, although there are π electrons of the CO bond and the lone electron pairs of oxygen atoms in the structure of the peptides.
The bonding in lithium high-spin clusters contradicts the usual chemical bonding concept since there are no electron pairs between the atoms, and they are bound with parallel spin electrons. Quantum theory of atoms in molecules and interacting quantum atom analysis (IQA) were used to investigate the nature of bonding in the high-Our findings demonstrate that the non-nuclear attractors (NNAs) are an essential component of the high-spin lithium clusters and play a key role in keeping them stable. Based on IQA energy terms, an electrostatic destabi-
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Different quantum chemical topology (QCT) methods were used to study the boron triel bonded complexes between BX3 (X=H, F, Cl, and Br) and some Lewis bases (NH3, CO and NCH). The Laplacian of electron density distribution indicates that the weak boron‐Lewis base complexes can be classified as lump‐hole interactions. On the other hand, in the covalent‐type complexes, the lump in the valence shell of the Lewis base is polarized toward the boron atom, leading to tetravalent borons. Natural atom in molecule (NAIM) analysis demonstrates that a bonding NBO (natural bond orbital) is the primary source of electron density at the triel bond critical points in the covalent‐type complexes. The interacting quantum atom (IQA) approach shows that in both closed‐shell and covalent‐type complexes, the exchange‐correlation terms are the dominant inter‐fragment interactions, indicating the importance of electron sharing in the formation of complexes. Natural adaptive orbital (NAdO) analysis reveals that donation and back‐donation channels contribute almost equally to inter‐fragment electron delocalization. Interestingly, in the closed‐shell systems, the electrostatic inter‐fragment interactions are negligibly small, and the exchange‐correlation terms are large enough to compensate for the unstabilizing deformation energies. In contrast, in the covalent‐type complexes both exchange‐correlation and electrostatic terms are essential to overcome deformations.
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