John Morrison Galbraith

Concerning the applicability of density functional methods to atomic and molecular negative ions

There is concern within theoretical chemistry that density functional methods are fundamentally in error for negative ions. We have tested this hypothesis by evaluating electron affinities for F and F2 with a variety of density functionals and extremely large, diffuse basis sets. In addition, we have observed the behavior of a known unbound system Ne−. We have found no convincing evidence to support the claims of negative ion instability with density functional methods.

π Bonding in Second and Third Row Molecules: Testing the Strength of Linus's Blanket

The flexibility of valence bond (VB) theory provides a new method of calculating pi-bond energies in the double-bonded species H(m)A=BH(n), where A, B = C, N, O, Si, P, S. This new method circumvents the problems usually associated with obtaining pi-bond strengths by targeting only the pi bond, while all other factors remain constant. In this manner, a clean separation between sigma- and pi effects can be achieved which highlights some expected trends in bond strength upon moving from left to right and up and down the Periodic Table. Intra-row pi bonds conform to the classic statement by Pauling [L. Pauling, The Natiure of the Chemical Bond, Cornell University Press, Ithaca, 1960, 3rd edition] regarding the relationship of heteronuclear bond strengths to their homonuclear constituents whereas inter-row pi bonds do not. This variance with Paulings statement is shown to be due to the constraining effect of the underlying sigma bonds which prevents optimal p(pi)-p(pi) overlap. While Paulings statement was based on the assumption that the resonance energy (RE) would be large for heteronuclear and small for homonuclear bonds, we have found large REs for all bonds studied herein; this leads to the conclusion that REs are dependent not only on the electronegativity difference but also the electronegativity sum of the constituent atoms. This situation where the bond is neither covalent nor ionic but originates in the covalent-ionic mixing has been termed charge shift (CS) bonding [S. Shaik, P. Maitre, G. Sini, P. C. Hiberty, J. Am. Chem. Soc. 1992, 114, 7861]. We have shown that CS bonding extends beyond single sigma bonds in first row molecules, thus supporting the idea that CS-bonding is a ubiquitous bonding form.

A Valence Bond Study of the Bergman Cyclization: Geometric Features, Resonance Energy, and Nucleus-Independent Chemical Shift (NICS) Values

The Bergman cyclization of (Z)-hex-3-ene-1,5-diynes (1, enediynes), which produces pharmacologically important DNA-cleaving biradicals (1,4-benzyne, 2), was studied by using Hartree-Fock (HF) and density-functional theory (DFT) based valence bond (VB) methods (VB-HF and VB-DFT, respectively). We found that only three VB configurations are needed to arrive at results not too far from complete active space [CASSCF(6 x 6)] computations, while the quality of VB-DTF utilizing the same three configurations improves upon CASSCF(6 x 6) analogous to CASPT2. The dominant VB configuration in 1 contributes little to 2, while the most important biradical configuration in 2 plays a negligible role in 1. The avoided crossing of the energy curves of these two configurations along the reaction coordinate leads to the transition state (TS). As a consequence of the shape and position of the crossing section, the changes in geometry and in the electronic wavefunction along the reaction coordinate are non-synchronous; the TS is geometrically approximately 80% product-like and electronically approximately 70% reactant-like. While the pi resonance in the TS is very small, it is large (64.4 kcal mol(-1)) for 2 (cf. benzene=61.5 kcal mol(-1)). As a consequence, substituents operating on the sigma electrons should be much more effective in changing the Bergman reaction cyclization barrier. Furthermore, additional sigma resonance in 2 results in unusually high values for the nucleus-independent chemical shift (NICS, a direct measure for aromaticity). Similarly, the high NICS value of the TS is due mostly to sigma resonance to which the NICS procedure is relatively sensitive.

The monochlorine fluorides (ClFn) and their anions (ClFn-) n = 1-7: structures and energetics

Predictions of molecular structure and energies have been made for each of the members in the ClF n /ClF n - (n = 1-7) series of molecules using density functional theory (DFT). The three different types of prediction for electron affinities determined in this work are: the adiabatic electron affinity (EAad), the vertical electron affinity (EAvert), and the vertical detachment energy (VDE) of the anion. Also reported are the first Cl-F dissociation energies for both the neutral and the anion (D CIFn ). Self-consistent Kohn-Sham orbitals were obtained using four different functional forms and a double-ζ plus polarization (DZP) basis set. When diffuse s- and p-type functions were added to the basis set, the anions were stabilized significantly with respect to the neutrals, and the electron affinity values were increased significantly. Overall, the best agreement with experimental structures was obtained with the DZP++ BHLYP method based upon Beckes half-and-half exchange functional and the Lee-Yang-Parr co...

Some new structures of C28

Abstract Twenty-four distinct structures of C28 are considered theoretically using Hartree–Fock theory, hybrid Hartree–Fock/density functional theory (B3LYP), and second-order perturbation theory. The tetrahedral 5 A 2 state is confirmed as the electronic ground state isomer. However, a second fullerene structure, a singlet state of D2 symmetry, is predicted to lie less than 2 eV above the expected global minimum. The relative energies of the various C28 isomers can be rationalized in terms of their structural features.

A Valence Bond Study of Three-Center Four-Electron π Bonding: Electronegativity vs Electroneutrality†

Three-center four-electron (3c4e) pi bonding systems analogous to that of the ozone molecule have been studied using modern valence bond theory. Molecules studied herein consist of combinations of first row atoms C, N, and O with the addition of H atoms where appropriate in order to preserve the 3c4e pi system. Breathing orbital valence bond (BOVB) calculations were preformed at the B3LYP/6-31G**-optimized geometries in order to determine structural weights, pi charge distributions, resonance energies, and pi bond energies. It is found that the most weighted VB structure depends on atomic electronegativity and charge distribution, with electronegativity as the dominant factor. By nature, these systems are delocalized, and therefore, resonance energy is the main contributor to pi bond energies. Molecules with a single dominant VB structure have low resonance energies and therefore low pi bond energies.

Isomerization pathway of the aluminum monocarbonyl/isocarbonyl pair, AlCO/AlOC: Evidence of a cyclic minimum

The isomerization pathway between AlOC and AlCO has been explored at the self-consistent field, configuration interaction, and coupled-cluster levels of theory. Five stationary points on the Al+CO potential energy surface were located and show that the path of Al migration from the isocarbonyl to the monocarbonyl involves a very small barrier to a perhaps unexpected cyclic minimum structure followed by a second barrier to the AlCO isomer. A quantitative analysis of the relative stabilities of the isomers as well as the ZPVE-corrected isomerization barriers are presented and compared to the boron carbonyl analogs. At the coupled-cluster level with single, double, and perturbatively applied connected triple substitutions [CCSD(T)] using a TZ2P+f basis set, the cyclic minimum is 9.4 kcal/mol higher in energy than AlCO but is 11.4 kcal/mol more stable than AlOC. The barriers from AlOC to the cyclic isomer and to the dissociation products 2P Al and X 1Σ+ CO are only 3.5 and 1.0 kcal/mol, respectively, and leav...

Equilibrium geometry of isocyanomethylene (HCNC) and comparison to the troublesome isomer cyanomethylene (HCCN)

Inspired by the recent experimental study of the radical anions HCCN− and HCNC− and by earlier examinations of HCCN, the equilibrium geometry of the HCNC molecule has been investigated using both self‐consistent field (SCF) and configuration interaction methods including single and double excitations (CISD). The largest basis set used was a triple‐ζ plus double polarization with diffuse functions and higher angular momentum functions appended to each atom [TZ2P(f,d)+diff]. Using this basis, the H–C–N equilibrium angle is predicted to be 128.5° at the CISD level of theory. Additionally, the zero point vibrational energy (ZPVE) corrected energy separation of the bent and linear conformations was predicted to be 10.1 kcal mol−1 at the CISD level of theory with the largest basis set employed. The barrier to linearity is 7.7 kcal mol−1 at the CCSD level of theory and 6.9 kcal mol−1 at the CCSD(T) level of theory, employing the CISD optimized geometries with a basis that was comprised of triple‐ζ plus double po...

The vibrational frequencies of borane (BH3): A comparison of high level theoretical results

Abstract Using the borane molecule, BH 3 , as an example, a detailed comparison is made among the results achieved using many levels of theory for the treatment of electron correlation. In particular, with three different basis sets ranging from a simple double-ζ plus polarization (DZP) to the very flexible triple-ζ plus two sets of polarization functions plus higher angular momentum functions (TZ2P(f,d)), total energetic, geometrical and harmonic vibrational frequency results from several of the more complete single reference methods of electron correlation are compared to the full configuration interaction (FCI) results. Emphasis is placed on comparison of augmented coupled cluster (CCSD(T)) theory and the CI method including all singles, doubles, triples and quadruples (CISDTQ) methods with FCI. These high-level results may also provide some additional insight into the ongoing controversy surrounding the experimentally assigned fundamental vibrational frequencies of BH 3 . The TZ2P(f,d) CCSD(T) harmonic vibrational frequencies predicted in this work were ω 1 = 2567, ω 2 = 1163, ω 3 = 2696 and ω 4 = 1223 cm −1 implying experimental misassignments by Kaldor and Porter [J.Am.Chem.Soc.,93 (1971) 2140] of the third and fourth vibrational frequencies, in agreement with previous theoretical works.

NMR CHEMICAL SHIELDING SURFACE OF N-ACETYL-N'-METHYLALANINAMIDE : A DENSITY FUNCTIONAL STUDY

The five energetically lowest minima on the potential energy surface of N‐acetyl‐N′‐methylalaninamide were optimized at the Becke3LYP/DZd level of theory to compare these density functional theory results with the literature findings at restricted Hartree‐Fock/3‐21G. While the relative energies are very similar, the amide moiety is predicted to be much more flexible at Becke3LYP/DZd. As a consequence, the three minima that favor a nonplanar amide group differ by up to 14° in their ϕ and ψ values between the two levels. To compare the change in the density functional NMR chemical shifts with respect to ϕ and ψ with experimental results, Becke3LYP/DZd was employed to optimize a structure for N‐acetyl‐N′‐methylalaninamide at each 30° interval on the (ϕ, ψ) surface in the regions that correspond to the α helix and the β‐pleated sheet and at each 60° interval elsewhere. The corresponding NMR chemical shielding surface was computed with the density functional program deMon. The resultant NMR chemical shielding surfaces for N and Cβ are in good agreement with the experiment, while the change in the NMR chemical shielding of C′ and Cα cannot be described only in terms of ϕ and ψ. The chemical shifts for those atoms also depend on the nonplanarity of the amide moiety. We evaluated this dependence for N‐methylacetamide as a model system. Estimates of the parameters derived from N‐methyl‐acetamide allowed the NMR‐shielding surfaces of C′ and Cα to be corrected for the nonplanar nitrogen influence. Although the effect is less pronounced with lower level theoretical geometries, due to the smaller degree of pyramidalization of the amide nitrogen, the (ϕ, ψ) NMR chemical shielding surfaces will need to be corrected. The agreement with the experiment was much better for the corrected surface of C′ when the nitrogen in the α helix had a nonplanar environment.