Quantum mechanical analysis of donor-acceptor interactions in organometallic complexes and comparative analysis of class size and teacher experience on student satisfaction and learning

Density functional theory;agostic complexes;quantum mechanics;donor-acceptor complexes;class size

Flener, Charity E.

关键词: Density functional theory;    agostic complexes;    quantum mechanics;    donor-acceptor complexes;    class size;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/14607/Flener_Charity.pdf?sequence=2&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

The first five chapters of this thesis uses modern density functional and ab initio methods to analyze donor-acceptor complexes of transition metals. Chapter 1 gives background for the theories used in this work. In chapter 2, density functional theory (DFT) is used to investigate the geometries and metal-ligand bonding in nickel complexes of bidentate phosphines, Ni(CO)2(R2P(CH2)nPR2) and NiH2(R2P(CH2)nPR2), where n = 1, 2, or 3; and R = H, Me, CF3, Et, i-Pr, t-Bu, Ph, OMe, or F.The net donor/acceptor properties of the phosphine ligands can be deduced from the computed frequency of the symmetric CO stretch of the Ni(CO)2(R2P(CH2)nPR2) complexes.This frequency can be estimated from the empirical expression ν(CO) = 1988 + Σ χB – 4 n, where the sum is over the four phosphorus bound substituents, χB is a substituent-dependent parameter, and n is the number of carbon atoms in the backbone (1 ≤ n ≤ 3).The deduced values of χB (in units of cm-1) – t-Bu (0.0), i-Pr (0.8), Et (3.0), Me (4.0), Ph (4.3), H (6.3), OMe (10.8), CF3 (17.8), and F (18.3) – are generally similar to Tolman’s electronic parameter χ derived from nickel complexes of unidentate phosphines, except that OMe is significantly less electron releasing when incorporated into a bidentate phosphine vs. a unidentate phosphine. The calculated frequencies also show that the phosphine appears to be a better donor (or weaker acceptor) as the number of carbon atoms in the backbone increases: increasing the number of carbons by one has about the same effect as changing all four substituents from iso-propyl to tert-butyl (or changing one of the four substituents from methyl to tert-butyl).For the NiH2(R2P(CH2)nPR2) complexes, the global minimum is a non-classical dihydrogen structure irrespective of the nature of the phosphine. For bidentate phosphines that are strongly donating, a classical cis-dihydride structure lies 2 kcal mol-1 or higher in energy than the global minimum.For phosphines that are less electron donating, this structure is no longer a local minimum, but instead is an inflection point on the potential energy surface.Atoms in molecules and natural bond order analyses confirm that the Ni-H2 interaction is a three-center two-electron bond in the dihydrogen tautomer. Energy decomposition analysis of these complexes is used to differentiate between the π and σ donor interactions in these complexes.There is a very good linear correlation between the calculated strength of the [M]←(H2) σ donation and [M]→(H2) π backdonation which clearly shows thatthe π backdonation has a much stronger effect on stretching the H-H distance than the σ donation.In Chapter 3, density functional theory and ab initio methods have been used to calculate the structures and energies of minima and transition states for the reactions of methane coordinated to a transition metal. The reactions studied are reversible C-H bond activation of the coordinated methane ligand to form a transition metal methyl/hydride complex, and dissociation of the coordinated methane ligand. The reaction sequence can be summarized as LxM(CH3)H →← LxM(CH4) →← LxM + CH4, where LxM is the osmium-containing fragment (C5H5)Os(R2PCH2PR2)+ and R is H or CH3.Three-center metal-carbon-hydrogen interactions play an important role in this system. Both basis sets and functionals have been benchmarked in this work, including new correlation consistent basis sets for a third transition series element, osmium. Double zeta quality correlation consistent basis sets yield energies close to those from calculations with quadruple zeta basis sets, with variations that are smaller than the differences between functionals. The energies of important species on the potential energy surface, calculated by using ten DFT functionals, are compared both to experimental values and to CCSD(T) single point calculations.Kohn-Sham natural bond orbital descriptions were used to understand the differences between functionals. Older functionals favor electrostatic interactions over weak donor-acceptor interactions, and therefore are not particularly well suited for describing systems – such as σ-complexes – in which the latter are dominant. Newer kinetic and dispersion-corrected functionals such as MPW1K and M05-2X provide significantly better descriptions of the bonding interactions, as judged by their ability to predict energies closer to CCSD(T) values.Kohn-Sham and natural bond orbitals are used to differentiate between bonding descriptions. Our evaluations of these basis sets and DFT functionals lead us to recommend the use of dispersion corrected functionals in conjunction with double zeta or larger basis sets with polarization functions for calculations involving weak interactions, such as those found in σ-complexes with transition metals.In Chapter 4, M05-2X and BB1K were used to analyze the effect of ancillary ligands on the hydrogen exchange reaction of (C5HxR5 x)Os(Y2PCZ2PY2)(CH3)H+ where R = Me, F, CF3, SiH3, or SiMe3, or H; x = 1-5; Y = H, Me, Ph, or F; and Z = H or F. The ligands. Three points on the potential energy surface are studied, the methyl hydride 1, the methane tautomer 2, the transition state between 1‡, and the fragment molecule, 3. We find that the steric and electronic effects of the ligands affect the relative energies ofthese structures on the potential energy surface. Electron withdrawing ligands such as CF3 or F decrease the energy of 1‡ and stabilize 2 relative to 1 while electron donating ligands such as SiMe3 increase the energy of 1‡ and destabilize 2 relative to 1. The energy of 3 relative to 1 was found to be correlated to the steric bulk of R or Y.In Chapter 5, we use M05-2X, B3LYP, and PBE0 methods to analyze a possible agostic interaction in the that compound Ti2Cl6[N(t-Bu)2]2.The crystal structure of Ti2Cl6[N(tBu)2]2shows a very close contact (2.634 Angstroms) between the electron poor titanium atom and a methyl group. Short distances between an electron deficient metal center and carbon-hydrogen bonds are often assigned to be agostic, i.e., attractive interactions involving 3-center-2-electron bonds. To ascertain whether or not this close contact is due to an agostic interaction between Ti and the C-H atoms, the gas phase structure of the complex and related model compounds were optimized with dispersion corrected density functional methods, which have been shown to be capable of accurately describing agostic interactions. These calculations reveal that decreasing the steric bulk of the amido ligand (by replacing the non-interacting tert-butyl ligand with a smaller alkyl group)caused the Ti-H distances to increase significantly. Natural bond order (NBO) analysis of the gas phase structure showed that there are no bonding interactions between titanium and hydrogen.We conclude that close contacts between electron deficient metal centers and nearby C-H bonds are not always attractive, and that some agostic interactions are repulsive and consequences of steric repulsions between the ligands in the inner coordination sphere.Chapter 6 discusses the results of a year long research project studying student perceptions and success in two types of general chemistry courses. Studies have shown that college students perform better in courses with 30 or fewer students, but in large public universities such as University of Illinois Urbana-Champaign (UIUC) economic and space constraints dictate larger class sizes.In an effort to provide a small, intimate classroom setting for first semester students, the department created a general chemistry course consisting of discussion classes of 30 students taught by graduate teaching assistants (TAs).The 2,000 students who enrolled in the course were given the choice to participate in a traditional lecture section of 350 students taught by teaching staff or the smaller TA-led discussion course. This study uses three ways to determine if student performance and satisfaction were different between the two types of sections; exam scores, attrition rates, and student happiness in the class (as measured by effectiveness ratings). On average, students in the small discussion performed worse on exams, left the course in higher numbers than large lecture students and rated their instructor less effective than students in large lecture course.However, students in sections with experienced TAs performed significantly better than students in the large lecture course, suggesting that small classes with TAs can be effective provided that TAs are trained and motivated. These results show clearly that the quality of instruction matters much more than the quantity of students instructed.