An electronic time scale in chemistry

Abstract

Ultrafast, subfemtosecond charge migration in small peptides is discussed on the basis of computational studies and compared with the selective bond dissociation after ionization as observed by Schlag and Weinkauf. The reported relaxation could be probed in real time if the removal of an electron could be achieved on the attosecond time scale. Then the mean field seen by an electron would be changing rapidly enough to initiate the migration. Tyrosine-terminated tetrapeptides have a particularly fast charge migration where in <1 fs the charge arrives at the other end. A femtosecond pulse can be used to observe the somewhat slower relaxation induced by correlation between electrons of different spins. A slower relaxation also is indicated when removing a deeper-lying valence electron. When a chromophoric amino acid is at one end of the peptide, the charge can migrate all along the peptide backbone up to the N end, but site-selective ionization is probably easier to detect for tryptophan than for tyrosine.attosecond lasers ͉ charge transfer ͉ protein mass spectrometry A chemical rearrangement occurs when atoms in a molecule change their specific arrangement. The time scale of chemistry is therefore the time scale for the motion of atoms (1, 2). The forces operating on the atoms determine the path of this motion. In the Born-Oppenheimer approximation, it is the averaging over the much faster motion of the electrons that specifies the potential energy and hence the forces. The physical picture for this approximation is that the electrons instantly adjust to the current position of the nuclei so that the equilibrium arrangement of the bonded atoms is determined as a minimum of the electronic energy. In the Born-Oppenheimer approximation, the system is confined to be in a single stationary electronic state, and if this confinement is strict then there is no electronic time scale that is relevant to chemistry. During a chemical rearrangement, the motion of the atoms can be accompanied by a reorganization of the electronic structure, and recent experimental progress allows for probing this change (3) in real time. However, it is the motion of the nuclei that sets the time scale because the electronic reorganization exactly tracks the shifts in the positions of the nuclei. As the heavier nuclei move, the light electrons immediately adapt.It is recognized that the energy of the same stationary electronic state can have two distinct minima, where the electronic charge distribution has a quite different character. At the immediate vicinity of either of the two configurations of the nuclei, the motion is bounded, but larger-amplitude motion can connect the two hollows. Major intramolecular charge reorganization follows upon a change in nuclear geometry (4-8). The time scale of the charge shuffling is that of the motion of the nuclei relocating between the two configurations.Optical excitation can prepare a nonstationary state, and such a state is often known as the optically bright state. The optically bright state is not sta...

Computational Studies on Structural, Excitation, and Charge-Transfer Properties of Ureidopeptidomimetics

“…This procedure of probing hole-migration dynamics was first introduced by Levine and Remacle, and we have closely followed their procedure. 9 The molecular orbitals obtained using all three functionals for cationic and neutral states are alike. Moreover, the wave functions of both charge states are calculated using CASSCF methods, with an active space of 6 (neutral) or 5 (cation) electrons and 10 orbitals to validate the wave functions obtained using DFT methods.…”

“…6,7 Peptidomimetics and their applications have been extensively studied using various experimental and theoretical methods. 8,9 Ureidopeptidomimetics (UPs) are one among synthetic peptidomimetics, 10 in which a peptide bond is replaced with a ureido group. The presence of a ureido group has proven to enhance the metabolic stability and probereceptor specificity for drug binding.…”

“…24 Specifically, in some bi-and tripeptide models, hole-migration dynamics have been studied and proven to occur within a 1.5 fs time scale. 9 Although the phenomenon of CT has been known for a long time, experimental and computational realization of this phenomenon in biomolecules like proteins is a difficult task. This is due to the complexity in the structures of the bioentities and the ultrafast time scale in which charge is transfered.…”

Reconstruction and control of a time-dependent two-electron wave packet

“…The same Coulomb force, however, acts repulsively between the electrons. This electron-electron interaction represents a major challenge in the understanding and modelling of atomic and molecular states, their structure and in particular their dynamics 2,3,14 . Here, we focus on the 1 P sp 2,n+ series 15 of doubly-excited states in helium below the N = 2 ionization threshold.…”

MAKING OPTICAL WAVES, TRACING ELECTRONS IN REAL-TIME: THE ONSET OF THE ATTOSECOND REALM (Invited Review)

“…In pure analogy to excitation of coherences among vibrational levels, the phase along with the amplitude of the excited wavefunction and its precise evolution plays a key role in manipulating the intramolecular forces [8]. Coherent excitation of several electronic states gives rise to attosecond or few-femtosecond evolution of these forces and may provide new routes to control the chemical reactivity [9][10][11][12]. But in order for optical pulses to attain the onset and manipulation of electronic coherences in matter they would have to drive the excitation or control of quantum systems ten to hundred times faster than that intuitively implied by their typical femtosecond duration.…”