Interacting Quantum Atoms (IQA) and Interacting Quantum Fragments (IQF) analyses are used to study normalF3C-X⋯NnormalH3 (X=Cl and Br) model complexes in order to determine the origin of halogen bond directionality. IQA allows for the calculation of intra‐ and interatomic classical and exchange‐correlation energies, which can be used to determine the energetic nature of the changes that occur when deviating from the preferred halogen bond approach. The Relative Energy Gradient (REG) method is also applied to rank the IQA energies and reveal which energy contributions best describe the total behavior of the system. Indeed, all the pairwise interactions and atomic self‐energies are angularly dependent; some terms favor the linear structure and some tend toward nonlinear arrangements. For instance, when the C−X−N angle is altered, the halogen‐nitrogen interaction energy behaves like the total energy of the system while the carbon‐nitrogen interaction works against the total energy profile. Furthermore, the REG values reveal that the contribution of the halogen‐nitrogen interaction to the total behavior of the system is small. Instead, the secondary interactions (e. g., fluorine‐nitrogen and carbon‐hydrogen interactions) and atomic self‐energies are mainly responsible for the angular preference of these halogen bonds. Finally, IQF calculations followed by REG analysis reveal the importance of the self‐energy of the fragments.

The effect of the core atom type of the M@Au 12 nanocluster (M = Au, Ag, Pt, and Pd) on the normal (NR) and charge-transfer surface-enhanced Raman spectroscopy (CT-SERS) of the zwitterion form of L-cysteine (ZWCYS) adsorbed on two different sites (D1 and D2) of the nanocluster is investigated separately in the gas phase and water. Because SERS requires the calculation of the absorption spectrum, the effect of the core atom type on the absorption spectrum of M@Au 12 and its complex with the ZWCYS has also been investigated. The vibrational bands that show the intensity enhancement in the CT-SERS of the ZWCYS interacting with the D1 site of M@Au 12 nanocluster in water are O�C�O asymmetric stretching (M = Au and Ag), NH 2 bending (M = Ag), S−H stretching (M = Ag, Pt, and Pd), CH 2 bending (M = Pt), and CH 2 symmetric stretching (M = Pt and Pd). The ZWCYS at the D 2 site of the M@Au 12 nanocluster in water exhibits intensity enhancement for O�C�O asymmetric stretching (M = Pt), NH 3 wagging (M = Au), and S−H stretching (M = Pd). The intensity of the vibrational bands of ZWCYS does not increase for M = Ag but decreases for O�C�O asymmetric stretching, S−H stretching, CH 2 symmetric stretching, CH 2 asymmetric stretching, and especially NH 2 symmetric stretching.

Different computational methods are used to investigate the nature of interaction in the NCFÁ Á ÁNH 3 model complex, in which the fluorine atom acts as a Lewis acid and forms a noncovalent bond with the ammonia (Lewis base). Symmetry-adapted perturbation theory based on density functional theory (SAPT(DFT)) indicates that the noncovalent interaction in the NCFÁ Á ÁNH 3 complex is mainly electrostatics. However, dispersion and induction terms also play important roles. Although fluorine noncovalent interactions are typically classified as halogen bonds, they are somewhat different from the well-known halogen bonds of iodine, bromine, and chlorine. The halogen bonds of NCClÁ Á ÁNH 3 and NCBrÁ Á ÁNH 3 are directional and the C X N (X = Cl or Br) angle tends to be linear. In contrast, the fluorine interaction in NCFÁ Á ÁNH 3 is not directional; the interaction energy shows no sensitivity to the angular (C F N) distortions, and the energy profile is flat over a wide angular range (from 180 to about 140 ). However, for the angles less than 130 , the energy curve shows a clear angular dependence and the interaction between NCF and NH 3 becomes stronger as the C F N angle decreases. It seems that at the tighter angles, a tetrel-bonded NCFÁ Á ÁNH 3 complex is preferred. Moreover, interacting quantum atoms (IQA) analysis shows that the competition between different intraatomic and interatomic interactions determines the geometry of NCFÁ Á ÁNH 3 complex.

Thermodynamic stability of neutral carbon nanoclusters of C m, up to the size of C 50 have been investigated using density functional theory. Four different symmetry point groups, C (m/2)h, acetylenic D (m/2)h, cumulenic D (m/2)h and D mh, have been considered for the clusters and the C (m/2)h has been found to be the most stable isomer for the studied clusters. Peierls transition is also studied for acetylenic D (m/2)h and cumulenic D (m/2)h point groups. It has been revealed that the relative stability of clusters converges to a constant value for clusters larger than C 38. The changes of calculated structural parameters for these clusters, such as bond lengths, have been in agreement with their relative stability.

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