DFT investigation on the structural and vibrational behaviours of the non-protein amino acids in hybrid explicit/continuum solvent: a case of the zwitterions γ-aminobutyric and α - aminoisobutyric acids.

BACKGROUND: The influence of hybrid solvation models on the molecular structures and vibrational characteristics of g-aminobutyric acid (GABA) and a-aminoisobutyric acid (AIB) zwitterions was assessed by employing a variety of Density Functional Theory (DFT). The quantum chemical methods included the B3LYP and B3PW91 hybrid functionals and the 6­311++G(d,p) basis set. METHODS: The most stable conformation derived from the potential energy surface (PES) scans using the B3LYP/6-311++G(d,p) model chemistry for each studied molecule was predicted within a continuum environment represented by the COSMO and SMD solvation models. The stable structures were subsequently immersed in explicit/COSMO and explicit/SMD hybrid solvation models, where 10 and 8 water molecules were explicitly positioned around the functional groups of the GABA and AIB zwitterions, respectively. The number of water molecules chosen was sufficient to prevent proton transfer among the carboxylate group (COO-) and the ammonium group (NH3+) within each molecule under investigation. After optimizing the geometry of each hydrated complex, the normal vibrational modes were determined. The scaled theoretical frequencies obtained from the various model chemistries were then compared to available experimental data from infrared (IR) and Raman spectroscopy. RESULTS: In the case of GABA and AIB molecules, the comparisons revealed that the B3LYP/6-311++G(d,p) model chemistry yielded wavenumber values that closely matched the experimental IR and Raman data, particularly when the explicit/SMD solvent was employed. The computed results indicate deviations of less than 4% when compared to the experimental data for the two molecules under investigation.

Influence of the d orbital occupation on the structures and sequential binding energies of pyridine to the late first-row divalent transition metal cations: a DFT study.

The ground-state structures and sequential binding energies of the late first-row divalent transition metal cations to pyridine (Pyr) are determined using density functional theory (DFT) methods. Five late first-row transition metal cations in their +2 oxidation states are examined including: Fe(2+), Co(2+), Ni(2+), Cu(2+), and Zn(2+). Calculations at B3LYP, BHandHLYP, and M06 levels of theory using 6-31G* and 6-311+G(2d,2p) basis sets are employed to determine the structures and theoretical estimates for the sequential binding energies of the M(2+)(Pyr)x complexes, where x = 1-6, respectively. Structures of the Ca(2+)(Pyr)x complexes are compared to those for the M(2+)(Pyr)x complexes of Fe(2+), Co(2+), Ni(2+), Cu(2+), and Zn(2+) to further assess the effects of the d-orbital occupation on the preferred binding geometries. The B3LYP, BHandHLYP, and M06 levels of theory yield very similar geometries for the analogous M(2+)(Pyr)x complexes. The overall trends in the sequential BDEs for all five metal cations at all three levels of theory examined are highly parallel, and are determined by a balance of the effects of the valence electronic configuration and hybridization of the metal cation, but are also influenced by repulsive ligand-ligand interactions. Present results for the M(2+)(Pyr)x complexes are compared to the analogous complexes of the late first-row monovalent transition metal cations, Co(+), Ni(+), Cu(+), and Zn(+) previously investigated to assess the effect of the charge/oxidation state on the structures and sequential binding energies. Trends in the sequential binding energies of the M(2+)(Pyr)x complexes are also compared to the analogous M(2+)(water)x, M(2+)(imidazole)x, M(2+)(2,2'-bipyridine)x, and M(2+)(1,10-phenanthroline)x complexes.

Energy-resolved collision-induced dissociation studies of 1,10-phenanthroline complexes of the late first-row divalent transition metal cations: determination of the third sequential binding energies.

The third sequential binding energies of the late first-row divalent transition metal cations to 1,10-phenanthroline (Phen) are determined by energy-resolved collision-induced dissociation (CID) techniques using a guided ion beam tandem mass spectrometer. Five late first-row transition metal cations in their +2 oxidation states are examined including: Fe(2+), Co(2+), Ni(2+), Cu(2+), and Zn(2+). The kinetic energy dependent CID cross sections for loss of an intact Phen ligand from the M(2+)(Phen)3 complexes are modeled to obtain 0 and 298 K bond dissociation energies (BDEs) after accounting for the effects of the internal energy of the complexes, multiple ion-neutral collisions, and unimolecular decay rates. Electronic structure theory calculations at the B3LYP, BHandHLYP, and M06 levels of theory are employed to determine the structures and theoretical estimates for the first, second, and third sequential BDEs of the M(2+)(Phen)x complexes. B3LYP was found to deliver results that are most consistent with the measured values. Periodic trends in the binding of these complexes are examined and compared to the analogous complexes to the late first-row monovalent transition metal cations, Co(+), Ni(+), Cu(+), and Zn(+), previously investigated.

Energy-Resolved Collision-Induced Dissociation Studies of 2,2'-Bipyridine Complexes of the Late First-Row Divalent Transition-Metal Cations: Determination of the Third-Sequential Binding Energies.

The third-sequential binding energies of the late first-row divalent transition-metal cations with 2,2'-bipyridine (Bpy) are determined using guided-ion-beam tandem mass spectrometry (GIBMS) techniques. The metal cations investigated include the late first-row divalent transition-metal cations, Fe2+ , Co2+ , Ni2+ , Cu2+ , and Zn2+ . The kinetic-energy-dependent cross sections for collision-induced dissociation (CID) of the M2+ (Bpy)3 complexes are analyzed to extract absolute 0 and 298 K bond dissociation energies (BDEs) for the loss of an intact Bpy ligand. Theoretical electronic structure calculations at the B3LYP, BHandHLYP, and M06 levels of theory are performed to determine stable geometries and sequential BDEs of the M2+ (Bpy)x complexes (x=1-3). BDEs computed using the M06 functional are the largest, BHandHLYP values are intermediate, whereas B3LYP produces the smallest values. Very good agreement between the B3LYP theoretically calculated and threshold collision-induced dissociation experimentally determined BDEs is found, which suggests that the B3LYP functional is capable of accurately describing the binding in these M2+ (Bpy)3 complexes. Periodic trends in the binding of the M2+ (Bpy)x complexes are examined and compared to the analogous complexes with 1,10-phenanthroline (Phen), M2+ (Phen)x . Comparisons are also made to the analogous Bpy complexes, M+ (Bpy)x , with the late first-row monovalent transition-metal cations, Co+ , Ni+ , Cu+ , and Zn+ investigated previously.