Gas-Phase Experimental and Theoretical Studies of Adenine, Imidazole, Pyrrole, and Water Non-Covalent Complexes

We present both experimental and theoretical gas-phase studies of several noncovalent complexes of elementary molecules of biological interest: adenine, imidazole, pyrrole, and water. By means of charge-transfer collisions between those complexes and laser-excited atoms, dipole-bound cluster anions are observed. This Rydberg electron transfer (RET) spectroscopic technique is used to experimentally determine the very weak excess electron binding energies of the complex anions. Theoretical calculations which rely on a homemade semiempirical intermolecular force field allow for the determination of the structures of the low-lying equilibrium configurations of the neutral complexes. The electrostatic properties of these configurations (dipole moments, quadrupole moments, etc.) lead to predicted excess electron binding energies which are compared to the experimental values. This comparison provides a test of the validity of the employed methods, as discussed in the case of the five studied complexes: adenine−imidazole, adenine−pyrrole, adenine−water, pyrrole−water, and imidazole−water.