Dennis S. Marynick

Localized molecular orbitals and electronic structure of buckminsterfullerene

Abstract The method of PRDDO is employed to calculate the optimized geometry, energy of formation, ionization potential, bond orders, and localized molecular orbitals for Buckminsterfullerene (C 60 ).

Theoretical estimates of the η6 bonding capability of buckminsterfullerene in transition metal complexes

Abstract In order to provide a quantitative description of the relative ability of C 60 to bond as an η 6 ligand, arene exchange energies for tricarbonyl(arene)chromium systems were calculated using the method of partial retention of diatomic differential overlap (PRDDO). This represents the first theoretical estimate of the relative affinity of C 60 as an η 6 ligand, and results indicate that C 60 is an inferior ligand compared to benzene. The arenes considered included benzene, chlorobenzene, p -difluorobenzene, and buckminsterfullerene. The exchange of benzene for C 60 as an arene ligand is calculated to require 19 kcal/mol, but this can be reduced by modifying the exchanging ligand. Reduced overlap from C 60 hybrid orbitals is found to be partly responsible for less favorable η 6 bonding, which suggests that larger metals such as tungsten may be better able to bond in an η 6 fashion to C 60 .

The inversion barriers of NF3, NCl3, PF3, and PCl3. A theoretical study

The inversion barriers of NF3, NCl3, PF3, and PCl3 have been calculated from ab initio molecular orbital theory using large basis sets including polarization functions on the central atom. The calculated inversion barriers (in kcal/mole) are 78.5(NF3), 22.9(NCl3), 121.5(PF3), and 90.8(PCl3). The NF3 inversion barrier is clearly larger than the experimental first bond dissociation energy (∼57 kcal/mole), indicating that planar NF3 is not bound relative to NF2+F. For PF3 and PCl3, bond dissociation is energetically competitive with inversion, since the experimental first bond dissociation energies are 130±14 and 81±5 kcal/mole, respectively. The lowest energy closed shell electronic configuration for planar PF3 is shown to have the phosphorus nonbonding (lone pair) electrons in a A1′ type orbital, rather than the pure p orbital of A2″ symmetry usually assumed for molecules of this type. Limiting the description of inner shells to a minimum basis set is shown to have relatively small (0–3 kcal/mole) effects ...

Modified extended Hückel band calculations on conjugated polymers

In order to more accurately predict band gaps, corresponding to π–π* transitions of one‐dimensional conducting polymers, the formula for the off‐diagonal elements, Hαβij in the extended Huckel (EH) band calculation method was modified according to the form Hαβij=K1(Hααii +Hββjj)exp(−K2Rαβ) Sαβij. Parametrizations for the constants K1 and K2 were performed so as to yield reasonable band gaps for the pure hydrocarbon polymers trans‐polyacetylene, poly(para‐phenylene), and poly(phenylene vinylene). Since there is a large difference in bond alternations along polymeric chains between ab initio and modified neglect of diatomic overlap optimized geometries, especially for heterocyclic polymers, the valence orbital exponents of oxygen, nitrogen, and sulfur were separately adjusted, depending on the chosen geometry, to reproduce the band gaps of polyfuran, polypyrrole, and polythiophene. It is found that geometrical relaxations in the presence of heteroatoms strongly affect the C1–C4 interactions as well as bond ...

Computational estimates of the gas-phase acidities of dihydroxybenzoic acid radical cations and their corresponding neutral species

Abstract The gas-phase acidities (GAs) of the radical cations of all six isomers of dihydroxybenzoic acid ( x , y -DHB), which are prototypical matrices used in matrix-assisted laser desorption/ionization (MALDI) mass spectrometry have been calculated using density functional theory (DFT). The GAs vary from 815.3 kJ/mol for the most acidic species (3,4-DHB) to 862.7 kJ/mol for the least acidic one (2,5-DHB). The reported GAs of the x , y -DHB radical cations are in excellent agreement with previous experimental measurements. The results indicate that deprotonation, in all six radical cations, takes place on the phenol sites. In addition, the GAs of the six neutral DHB isomers have been similarly calculated.

An SCF–CI study of the structures, inversion barriers, and inversion frequencies of SiH3, PH3+, and SH3++

Large basis set SCF–CI calculations have been employed to determine the structures, inversion barriers, and inversion frequencies of SiH3, PH3+, and SH3++. All three species are nonplanar; however, the general trend is toward planarity as the positive charge increases. Values of the inversion barriers are 4.4 kcal/mol (SiH3), 2.9 kcal/mol (PH3+), and 0.5 kcal/mol (SH3++). Calculated frequencies of the vibrational transitions of the ν2 modes of these molecules are consistent with experiment for PH3+, but not for SiH3.

Computational estimates of the gas-phase basicities, proton affinities and ionization potentials of the six isomers of dihydroxybenzoic acid

The gas-phase basicities (GBs), gas-phase proton affinities (PAs) and ionization potentials (IPs) of all six isomers of dihydroxybenzoic acid have been calculated using density functional theory at the B3LYP/6-311++G(2df,p)//B3LYP/6-31+G** level. A detailed conformational analysis of each isomer was performed, and the calculated thermodynamic properties were Boltzmann averaged over all conformations. Respectively, the GBs and the gas-phase PAs vary from 803.8 and 832.5 kJ mol−1 for the least basic species (3,5-DHB) to 830.1 and 861.4 kJ mol−1 for the most basic isomer (2,4-DHB). The reported GBs and gas-phase PAs of 2,3-DHB and 2,4-DHB, are in excellent agreement with previous experimental measurements. Agreement for the 2,5-DHB and 3,4-DHB isomers are not as good, but still close to or within the experimental error estimates. The calculated values for the GB and gas-phase PA of 2,6-DHB and especially 3,5-DHB are significantly outside the experimental error brackets. Repeating these calculations on the lowest energy conformation of each isomer at the MP2/6-311++G(2df,p)//MP2/6-31+G** level yielded significantly worse results. Our results indicate that protonation in all isomers takes place on the carboxylic sites. The vertical IPs vary from 8.14 eV for 2,5-DHB to 8.56 eV for 2,4-DHB.

On the structure and electron photodetachment spectra of Ga3P− and Ga3As−

Abstract The main features of the Ga 3 P − photodetachment spectrum reported by Taylor, Asmis, Xu and Neumark [Chem. Phys. Lett. 297 (1998) 133] have been assigned. Ga 3 P − has a 2 B 2 ( C 2 v ) ground state. A near degeneracy is found between the 1 A 1 ( C 2 v ) and the 1 A 1 ( C 3 v ) states of Ga 3 P. The observed peaks are assigned to Ga 3 P states with C 2v structure. In line with Ga 3 P, the 1 A 1 ( C 2 v ) and 1 A 1 ( C 3 v ) states of Ga 3 As are nearly degenerate. Electron detachment processes are presented and discussed for the Ga 3 As − /Ga 3 As system on the basis of a C 2v ground state geometry for the anion and the neutral molecule.

Electrochemistry of thioxanthene, thioxanthone and related compounds in acetonitrile: Substituent effects and correlation of electrochemical behavior with molecular orbital calculations

Abstract This paper presents the synthesis, electrochemistry, and molecular orbital (MO) picture of a series of conformationally-restricted diaryl sulfur compounds. The primary electrooxidative and electroreductive pathways of these compounds are compared with model systems including dibenzothiophene, thiochroman-4-one, and benzothiophene. The oxidation of these compounds is invariably irreversible (with the exception of thianthrene) and involves rapid dismutation of the radical cation which is formed in the primary electron transfer step. In the presence of “electrophoric” groups such as CO (e.g., thioxanthone) and SO 2 (e.g., dibenzothiophene sulfone), characteristic reversible electrochemical reduction responses are observed, which involve the radical anion in each case. The combined use of cyclic voltammetry and chronoamperometry permitted computation of the number of electrons involved in the electrochemical reaction and the diffusion coefficient for each compound. A series of C2-substituted thioxanthones was examined to probe the electronic influence of the substituent on the electrooxidation and electroreduction sites (i.e., on the electron densities at the 10- and 9-positions) respectively. These substituent effects are presented in terms of correlations of oxidation (or reduction) potentials with the substituent (Hammett) constants, and the highest occupied molecular orbital (HOMO), or lowest unoccupied molecular orbital (LUMO) energies respectively. The orbital eigenvalues were computed using the RPDDO method. Finally, the influence of varying bridging atoms in the above structure is discussed in terms of the ease of oxidation and the disposition of the HOMO orbital.

Equilibrium conditions in laser-desorbed plumes: thermodynamic properties of α-cyano-4-hydroxycinnamic acid and protonation of amino acids

The equilibrium nature of a plume of laser desorbed material is explored through the application of a simple equilibrium model to the ion signals observed in 355 nm laser desorption/ionization mass spectra of mixtures of the MALDI matrix α-cyano-4-hydroxycinnamic acid (αCHCA) with the amino acids glycine, alanine, valine, isoleucine, and phenylalanine. In these studies it is found that there are systematic and predictable increases in the relative yield of protonated amino acid with increases in amino acid gas-phase basicity. In addition, the thermodynamic values extracted from the equilibrium plot are shown to be in good agreement with values obtained from computational investigation of plausible αCHCA proton donor species. These results are supportive of a picture wherein the laser-desorbed material is viewed as a dense plume in which facile charge transfer occurs leading, ultimately, to a thermodynamically equilibrated distribution of proton donor and proton acceptor species.