Direct Observation of Resonance Motion in Complex Elimination Reactions: Femtosecond Coherent Dynamics in Reduced Space

Nonadiabatic Photodissociation and Dehydrogenation Dynamics of n-Butyl Bromide Following p-Rydberg Excitation

Femtosecond time-resolved mass spectrometry, correlation mapping, and density functional theory calculations are employed to reveal the mechanism of C═C and C≡C formation (and related H₂ production) following excitation to the p-Rydberg states of n-butyl bromide. Ultrafast pump-probe mass spectrometry shows that nonadiabatic relaxation operates as a multistep process reaching an intermediate state within ∼500 fs followed by relaxation to a final state within 10 ps of photoexcitation. Absorption of three ultraviolet photons accesses the dense p-Rydberg state manifold, which is further excited by the probe beam for C─C bond dissociation and dehydrogenation reactions. Rapid internal conversion deactivates the dehydrogenation pathways, while activating carbon backbone dissociation pathways. Thus, unsaturated carbon fragments decay with the lifetime of p-Rydberg (∼500 fs), matching the growth recorded in saturated hydrocarbon fragments. The saturated hydrocarbon signals subsequently decay on the picosecond time scale as the molecule relaxes below the Rydberg states and into halogen release channels.

Ion-Pair Formation in n-Butyl Bromide through 5p Ryberg State Predissociation

The ultrafast photodynamics of n-butyl bromide are explored with femtosecond time-resolved mass spectrometry. Absorption of two UV (400 nm) pump photons induces the direct dissociation of the C-Br bond from the A state within 160 fs. Absorption of three UV pump photons excites the molecule into the 5p Rydberg state which undergoes several relaxation pathways including to the ion-pair state. Relaxation to the ion-pair state is tracked through the transient of the C₄H₉⁺ fragment and suggests an E state lifetime of 10.8 ± 0.5 ps, in close agreement with the tunneling time of smaller molecules. Predissociation from the 5p Rydberg states leads to the β-elimination of H-Br and formation of C₄H₈⁺ within 3.0 ± 0.25 ps. A portion of the excited parent molecule avoids the ion-pair formation and instead relaxes through the Rydberg excited state manifold into the D state within 30.2 ± 0.21 ps.

Nonadiabatic couplings from time-dependent density functional theory: Formulation in the Casida formalism and practical scheme within modified linear response

We present an efficient method to compute nonadiabatic couplings (NACs) between the electronically ground and excited states of molecules, within the framework of time-dependent density functional theory (TDDFT) in frequency domain. Based on the comparison of dynamic polarizability formulated both in the many-body wave function form and the Casida formalism, a rigorous expression is established for NACs, which is similar to the calculation of oscillator strength in the Casida formalism. The adiabatic local density approximation (ALDA) gives results in reasonable accuracy as long as the conical intersection (ci) is not approached too closely, while its performance quickly degrades near the ci point. This behavior is consistent with the real-time TDDFT calculation. Through the use of modified linear response theory together with the ground-state-component separation scheme, the performance of ALDA can be greatly improved, not only in the vicinity of ci but also for Rydberg transitions and charge-transfer excitations. Several calculation examples, including the quantization of NACs from the Jahn-Teller effect in the H3 system, have been given to show that TDDFT can efficiently give NACs with an accuracy comparable to that of wave-function-based methods.

Reflection on medium effects on photochemical reactivity

In reexamining medium effects on photochemical reactions, we have emphasized those on unequilibrated excited species such as the Franck-Condon species. Despite recent advances in femtochemistry, such a discussion in molecular photochemistry is uncommon, and the problem remains challenging on account of the extremely short-lived excited species. However, in such cases, a small perturbation resulting from, for example, weak guest-host interactions could turn into a determining factor in dictating the course of a photochemical channel of deactivation. Examples of medium-directed diabatic processes have been examined with this idea in mind. A modified view on rhodopsin photoisomerization is presented along with the consideration that confinement does not necessarily lead to inhibition of reactions of the trapped substrate.