Base pair ionization potentials

Some recent calculations,9 based on ionization potentials derived from photoelectron spectroscopy, favour the high lone-pair to lone-pair repulsions in the molecule as being very significant in the weakness of the bond. 

Steric effects and lone pair-lone pair repulsions appear to the most important contributors to the barrier to rotation about the P—N bond in the aminophosphines Me2NP(CF3)2, Me2NPCl(CF3), and Me2NPCI2.35<> These conclusions are based on the results of recent lone-pair ionization-potential data obtained from photoelectron spectroscopy. 

Charge-Transfer Compounds. Similat to iodine and chlorine, bromine can form charge-transfer complexes with organic molecules that can serve as Lewis bases. The frequency of the iatense uv charge-transfer adsorption band is dependent on the ionization potential of the donor solvent molecule. Electronic charge can be transferred from a TT-electron system as ia the case of aromatic compounds or from lone-pairs of electrons as ia ethers and amines. 

Pyridine has practically the same ionization potential as benzene it does not, however, necessarily follow that the flrst ionization potentials (Table 9) relate to an electron of the highest occupied 7r-orbital. These compounds are bases, though weaker than aliphatic amines, and parallels have been noted between base strength and ease of ionization (Nakajima and Pullman, 1958 Krishna and Chowdhury, 1963), which might indicate that the flrst ionization potential is that of a lone-pair... 

Density of states weighted Franck-Condon factor Deoxyribonucleic acid Barrier height for the adiabatic hole motion Difference in ionization potentials of adenine-thymine and guanine-cytosine base pairs... 

Here AB is the difference in ionization potentials of AT and GC base pairs, b is the transfer integral, c, and c, are the creation and annihilation operators for a hole at the f-th site, respectively, index i labels DNA base pairs in the sequence, and the sum E is taken over GC sites only. 

As expected, DABCO is a much stronger base than triethylamine, has two ionization potentials separated by several electron-volts, and is more easily oxidized than trimethylamine. What is unusual about DABCO is that the in-phase combination of the nonbonded pairs is the HOMO, the lowest IP is lower than that of trimethylamine, and the next higher IP is higher than that of trimethylamine. Explain. 

Equation 27 is a useful tool in chemistry for assigning spectra. The slope and intercept of such relationships depend on the nature of the basic site, whether charge is localized in orbitals of Tt or cr symmetry and whether the system is closed or open shell. But also caution must be exercised in developing relationships between ionization potentials and molecular properties such as base strength the correct ionization potential must be chosen and molecules in which the presence of equivalent lone-pairs leads to resonance stabilization of the radical cation should be regarded as potential exceptions. 

Russo N, Toscano M, Grand A (2000) Theoretical determination of electron affinity and ionization potential of DNAand RNA bases. J Comput Chem 21 1243-1250 Sagstuen E, Hole EO, Nelson WH, Close DM (1998) Radiation damage to DNA base pairs. II. Paramagnetic resonance studies of 1 -methyluracil. 9-ethyladenine complex crystals X-irradiated at 10 K. Radiat Res 149 120-127... 

Table 20-2. Ionization potential (IP) of GC and AT base pairs in gas-phase and in hydrated system... 

The availability of the electron pair in the isolated ligand can be directly measmed by photoelectron spectroscopy. The comparative ionization potentials show a reverse of the proton aftrnities in aqueous systems. For example, the gas-phase basicity of PPhs (lower ionization energy) is greater than that of PMc3 a selection of values is given in Table 6. These differences between solution based gas-phase basicities are ascribed to solvation effects. 

---