Distinct photophysical behavior of nucleobase adenine and its constitutional isomer, 2-aminopurine, has been studied by using quantum chemical methods, in particular an accurate ab initio multiconfigurational second-order perturbation theory. After light irradiation, the efficient, ultrafast energy dissipation observed for nonfluorescent 9H-adenine is explained here by the nonradiative internal conversion process taking place along a barrierless reaction path from the initially populated 1 (* La) excited state toward a low-lying conical intersection (CI) connected with the ground state. In contrast, the strong fluorescence recorded for 2-aminopurine at 4.0 eV with large decay lifetime is interpreted by the presence of a minimum in the 1 (* La) hypersurface lying below the lowest CI and the subsequent potential energy barrier required to reach the funnel to the ground state. Secondary deactivation channels were found in the two systems related to additional CIs involving the 1 (* Lb) and 1 (n*) states. Although in 9H-adenine a population switch between both states is proposed, in 7H-adenine this may be perturbed by a relatively larger barrier to access the 1 (n*) state, and, therefore, the 1 (* Lb) state becomes responsible for the weak fluorescence measured in aqueous adenine at Ϸ4.5 eV. In contrast to previous models that explained fluorescence quenching in adenine, unlike in 2-aminopurine, on the basis of the vibronic coupling of the nearby 1 (*) and 1 (n*) states, the present results indicate that the 1 (n*) state does not contribute to the leading photophysical event and establish the prevalence of a model based on the CI concept in modern photochemistry.conical intersections ͉ DNA photophysics ͉ fluorescence quenching ͉ quantum chemistry ͉ ultrafast decay S tudies on the photostability of DNA and RNA bases after absorption of UV light have flourished since the early 1970s, when quenched DNA fluorescence at room temperature was first reported (1). Knowledge about the mechanisms that control nonradiative decay is fundamental, not only because of the extreme importance of photodamage in systems that form the genetic code but also because they have been used to establish the paradigm of the essentials of radiationless decay processes in nonadiabatic photochemistry (2). Adenine (6-aminopurine), the most studied nucleobase, maintains photostability by efficiently quenching its fluorescence, whereas a close constitutional isomer, 2-aminopurine, displays strong emission, and it is commonly used to substitute adenine in DNA as a fluorescent probe to detect protein-induced local conformational changes (3-7). Former proposals to explain low quantum yields of fluorescence and excited singlet deactivation in nucleobases by means of excited-state photoreactions or phototautomerisms were basically ruled out because of the absence of photoproducts and deuterium isotope effects in different solvents (2). Apparently more successful was the hypothesis known as proximity effect (8, 9), which explained ultrafast internal conver...

New basis sets of the atomic natural orbital (ANO) type have been developed for the lanthanide atoms La-Lu. The ANOs have been obtained from the average density matrix of the ground and lowest excited states of the atom, the positive ions, and the atom in an electric field. Scalar relativistic effects are included through the use of a Douglas-Kroll-Hess Hamiltonian. Multiconfigurational wave functions have been used with dynamic correlation included using second-order perturbation theory (CASSCF/CASPT2). The basis sets are applied in calculations of ionization energies and some excitation energies. Computed ionization energies have an accuracy better than 0.1 eV in most cases. Two molecular applications are included as illustration: the cerium diatom and the LuF3 molecule. In both cases it is shown that 4f orbitals are not involved in the chemical bond in contrast to an earlier claim for the latter molecule.

The bond order and in particular the possibility of multiple bonding between atoms in a molecule have been highlighted in two recent articles. [1,2] Theoretical and experimental work have challenged old chemical paradigms concerning the possible multiplicity that can be achieved in a chemical bond. On the other hand, the concept of a multiple bond is not clearly defined and there is a need for a more quantitative measure. In this contribution we attempt to introduce such a measure and apply it to a number of multiply bonded systems. As a result of the analysis, we show that the highest multiplicity that can be achieved in a bond between two equal atoms is six. The multiplicity of a chemical bond is determined by the number of electron pairs that occupy the region between the two bonded atoms in bonding molecular orbitals. The hydrogen molecule has, for example, a single bond with two electrons in one orbital formed from the 1s orbitals on each atom. The nitrogen molecule, N 2 , has a triple bond; the three unpaired 2p electrons on each atom combine to form this very strong bond. Before 1964, the triple bond was assumed to be the highest multiplicity that a chemical bond can have. We show here, through a systematic study of the covalent chemical bond covering the entire periodic system, that the maximum bond multiplicity is six. The maximum value is reached by the tungsten diatom, W 2 . No other pair of atoms in the periodic system (atomic numbers smaller than about 100) reaches a higher bond order.A single covalent chemical bond between two atoms is, in simple molecular orbital (MO) theory, described by a bonding orbital occupied by two electrons. This is, however, an oversimplified picture of bonding that only works for strong bonds and near the equilibrium geometry. None would say that there is a chemical bond between two hydrogen atoms that are at a distance of 100 from each other. However, this is the picture that emerges from the simple theory. A more accurate description uses two orbitals to describe the bond: a bonding orbital and the corresponding antibonding orbital. Both orbitals are occupied in the true wave function of the molecule. Let us assume that the occupation of the bonding orbital is h b = 2Àx. The occupation of the antibonding orbital will then be close to h a = x, such that the sum is two. When the molecule is close to equilibrium, x will be small for a normal chemical bond, but when the molecule dissociates, x will increase to become one. In this case, there is no chemical bond and the wave function describes a system of two radicals with one electron on each of them. We can use this property of the molecular orbitals to define an effective bond order (EBO) for a single bond as (h b Àh a )/2, which is then close to one for a normal chemical bond but goes to zero when the bond weakens. For multiply bonded molecules we add up the contributions from each bond to obtain the total EBO. The EBO is non-integer and in naming the multiplicity of a bond one may then use the lowest integer value larg...

An ab initio theoretical study at the CASPT2 level is reported on minimum energy reaction paths, state minima, transition states, reaction barriers, and conical intersections on the potential energy hypersurfaces of two tautomers of adenine: 9H- and 7H-adenine. The obtained results led to a complete interpretation of the photophysics of adenine and derivatives, both under jet-cooled conditions and in solution, within a three-state model. The ultrafast subpicosecond fluorescence decay measured in adenine is attributed to the low-lying conical intersection (gs/pipi* La)(CI), reached from the initially populated 1(pipi* La) state along a path which is found to be barrierless only in 9H-adenine, while for the 7H tautomer the presence of an intermediate plateau corresponding to an NH2-twisted conformation may explain the absence of ultrafast decay in 7-substituted compounds. A secondary picosecond decay is assigned to a path involving switches towards two other states, 1(pipi* Lb) and 1(npi*), ultimately leading to another conical intersection with the ground state, (gs/npi*), with a perpendicular disposition of the amino group. The topology of the hypersurfaces and the state properties explain the absence of secondary decay in 9-substituted adenines in water in terms of the higher position of the 1(npi*) state and also that the 1(pipi* Lb) state of 7H-adenine is responsible for the observed fluorescence in water. A detailed discussion comparing recent experimental and theoretical findings is given. As for other nucleobases, the predominant role of a pipi*-type state in the ultrafast deactivation of adenine is confirmed.

The nonadiabatic photochemistry of the guanine molecule (2-amino-6-oxopurine) and some of its tautomers has been studied by means of the high-level theoretical ab initio quantum chemistry methods CASSCF and CASPT2. Accurate computations, based by the first time on minimum energy reaction paths, states minima, transition states, reaction barriers, and conical intersections on the potential energy hypersurfaces of the molecules lead to interpret the photochemistry of guanine and derivatives within a three-state model. As in the other purine DNA nucleobase, adenine, the ultrafast subpicosecond fluorescence decay measured in guanine is attributed to the barrierless character of the path leading from the initially populated 1(pi pi* L(a)) spectroscopic state of the molecule toward the low-lying methanamine-like conical intersection (gs/pi pi* L(a))CI. On the contrary, other tautomers are shown to have a reaction energy barrier along the main relaxation profile. A second, slower decay is attributed to a path involving switches toward two other states, 1(pi pi* L(b)) and, in particular, 1(n(O) pi*), ultimately leading to conical intersections with the ground state. A common framework for the ultrafast relaxation of the natural nucleobases is obtained in which the predominant role of a pi pi*-type state is confirmed.