Narrow Results

A great deal of theoretical work has been carried out to investigate the properties of the six lowest singlet electronic states of N₂O molecule: the ground state X¹A′; the excited states 1¹A′′, 2¹A′, 2¹A′′, 3¹A′ and 3¹A′′. Multireference configuration interaction (MRCI) approach has been used to compute the full-dimensional potential energy surfaces of the six lowest states employing aug-cc-pVQZ minus g orbital basis set. It was found that such of highly accurate potential yields excellent results of bond dissociation and vertical excitation energies in comparison with the experimental values. Several important symmetry and nonsymmetry related conical intersections in linear and bent geometries have been discussed. Of particular interest is the location of conical intersections between the 2¹A′(¹Δ) and 3¹A′(¹Π) states, and between the 1¹A′′(¹Σ^-) and 3¹A′′(¹Π) states in linear geometry, as well as conical intersection between the X¹A′ and 2¹A′ states in bent geometry. The corresponding transition dipole moment surfaces have also been computed, connecting the ground electronic state to the lowest five excited states. Detailed discussion on the vector properties of the dipole transition has been presented specifically in the vicinity of the conical intersections.

Two routes of hydrogen transfer between two heterocyclic moieties or two carbonyl groups were detected for different identity exchange reactions X_1• + H − X₃ → X₁ − H + _•X₃. The weak X₁ − X₃ interaction is a sequel of X-heteroatom incorporation in a conjugated π-system. Quasidegenerate transition state (TS) — two TSs (1²A and 1²B) on the same energy level — will be produced in the case of negligible X₁–X₃ coupling between hydrogen-bonded identical fragments. Both components have similar structures, identical electronic and spin distributions; both of them are of proton-coupled electron transfer (PCET) origin. Minor hydrogen-atom transfer (HAT) contribution (2σ structure of ²B symmetry) to the 1²B state is indicated by the order of PCET components — lowest 1²A and highest 1²B. The couple of ground-state (GS) TSs — lowest 1²A and highest 1²B — or quasidegenerate TS 1²A/1²B is a PCET label that allows to distinguish between HAT and PCET mechanisms for the hydrogen-atom identity exchange reactions. Single GS 1²B TS is a marker of HAT mechanism. Quasidegenerate TS is a nonadiabatic hydrogen-transfer funnel. Splitting of degenerate TS leads to the adiabatic path through the lowest PCET TS (1²A) and second PCET nonadiabatic route (1²B) in the vicinity of electronic degeneracy (1²A/1²B) for all studied carbonyl systems. The hydrogen bond is a natural stabilizing factor required for PCET mechanism.

This article presents a modern perspective on E. Teller, "The Crossing of Potential Surfaces," J. Phys. Chem. 41, 109–116 (1937), in which the "conical intersection" was first described. It also provides a generalization of the Landau-Zener-Stückelberg formula for the probability of nonadiabatic transitions at avoided crossings or conical intersections, also called "mode conversion."

The following sections are included:

• Introduction
• Theoretical Background
• Avoided Crossing versus Conical Intersection
• Ultrafast Processes Involving Conical Intersections
• Ultrafast Dynamics through Avoided-Crossing of NaI
• Ultrafast Dynamics Through Conical Intersection of Ethylene
• References