Edward S. Chen

Classification of organic molecules to obtain electron affinities from half wave reduction potentials: The aromatic hydrocarbons

Adiabatic absolute electron affinities (EA) of about 80 aromatic hydrocarbons are calculated from literature values of half wave reduction potentials in aprotic solvents. Solution energy differences are estimated by grouping the molecules based on experimental and theoretical values, chemical logic and statistical analysis. The electronegativity values, (IP+EA)/2, calculated using literature data for ionization potentials, are constant for many of the compounds. Others vary in a systematic manner, such as benzene, 4.26±0.05; naphthalene, 4.14±0.02; anthracene, 4.06±0.02; tetracene, 4.03±0.03; pentacene, 3.98±0.03, in eV. Theoretical values, recalculated from the beginning, compare favorably with the literature values. Three independent methods for obtaining absolute EA’s are summarized and verified; the calibration of reduction potentials, the combination of ionization potentials with electronegativities and the use of semiempirical methods.

CLASSIFICATION OF ORGANIC MOLECULES TO OBTAIN ELECTRON AFFINITIES FROM HALF-WAVE REDUCTION POTENTIALS : CYTOSINE, URACIL, THYMINE, GUANINE AND ADENINE

A procedure for obtaining the adiabatic electron affinities (AEA) of organic molecules from half-wave reduction potentials in aprotic solvents is presented. Molecules are placed into groups according to their structure. Each group has a different solution energy difference. Calculations of AEA and charge distributions with AM1-multiconfiguration configuration interaction are used to support the intuitive classification of the molecules. The procedure is illustrated for Vitamins A and E, riboflavin, the azines, polyenes, hydroxy-pyrimidine, oxo-guanine, the hydrogen bonded cytosine-oxo-guanine as well as the AEA, and vertical EA (VEA) of Cytosine (C), Uracil (U), Thymine (T), Guanine (G) and Adenine (A). The latter values are: (VEA) G, 0.10; A, -0.49; U, 0.33; T, 0.31; C, -1.48 and (AEA) G, 1.51 +/- 0.05; A, 0.95 +/- 0.05; U, 0.80 +/- 0.05; T, 0.79 +/- 0.05; C, 0.56 +/- 0.05 in eV.

A proposed model for electron conduction in DNA based upon pairwise anion π stacking: electron affinities and ionization potentials of the hydrogen bonded base pairs

Abstract The values of the electron affinities (EAs) and ionization potentials (IPs) of the Watson–Crick hydrogen-bonded base pairs were calculated using the AM1-multiconfiguration configuration interaction (AM1-MCCI) method. This procedure was previously used to verify the electron affinities of the monomers determined from half wave reduction potentials. The electron affinities of the DNA base pairs differ by only 180 meV (4 kcal/mol). This makes rapid electron transport in DNA thermodynamically feasible in biological systems. During anion formation, calculations show geometry changes which affect the noncovalent hydrogen bonds and π stacking interactions. A model for rapid electron transfer through the π-system inherent in the base pairs is postulated. This mechanism is based upon the separation of the system into two distinct portions: a tunneling down the backbone and a pairwise donor–acceptor process through the π-way.

Electron-capture detector and multiple negative ions of aromatic hydrocarbons.

Multiple electron affinities are identified in the temperature dependence of the electron-capture detector: naphthalene, 0.16, 0.13+/-0.01, anthracene, 0.69, 0.60, 0 53+/-0.01; tetracene 1.1, 0.88+/-0.03, 0.53+/-0.05; pyrene, 0.61, 0.50+/-0.02; azulene 0.90, 0.80, 0.70+/-0.02, 0.65, 0.55+/-0.05; acenaphthylene, 0.80, 0.69, 0.60, 0.50+/-0.05; and c-C8H8, 0.80, 0.70, 0.55+/-0.02; (all in eV). These are obtained from a rigorous least squares procedure incorporating literature values and uncertainties. The adiabatic electron affinities for about 40 hydrocarbons listed in the US National Institute of Standards and Technology (NIST) tables are evaluated. The adiabatic electron affinity values not listed in NIST are biphenylene, 0.45+/-0.05 eV and coronene. 0.8+/-0.05 eV. Morse potential energy curves in the C-H dimensions illustrate multiple states for benzene and naphthalene.

Comment on “Ionization potentials and electron affinities from the extended Koopmans’ theorem applied to energy derivative density matrices: The EKTMPn and EKTQCISD methods” [J. Chem. Phys. 107, 6804 (1997)]

In the subject article, it is stated that “the EKTMP2 formalism is certain to replace the Koopmans’ theorem as the ‘black box’ estimator of ionization potentials” and by inference, for electron affinities. It is shown that semiempirical MINDO/3 multiconfiguration interaction ionization potentials for the five aromatic hydrocarbons considered in the article are as accurate as the EKT values. No theoretical EKT electron affinities are presented in the subject article. Experimental and theoretical electron affinities for a number of aromatic hydrocarbons calculated using MINDO/3 with MCCI are presented to be matched by any other estimator.