Are Conical Intersections Responsible for the Ultrafast Processes of Adenine, Protonated Adenine, and the Corresponding Nucleosides?

• Published in: Chemphyschem Vol. 6; no. 7; pp. 1276 - 1281
• Main Authors: Nielsen, Steen Brondsted, Solling, Theis I
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 11.07.2005; WILEY‐VCH Verlag; Wiley
• Subjects: adenine; Adenine - analogs & derivatives; Adenine - chemistry; Atomic and molecular physics; Calculations and mathematical techniques in atomic and molecular physics (excluding electron correlation calculations); conical intersections; density functional calculations; Electronic structure of atoms, molecules and their ions: theory; Exact sciences and technology; Models, Chemical; molecular dynamics; Molecular dynamics and other numerical methods; nucleosides; Nucleosides - chemistry; Photochemistry; Physics; Quantum Theory; Surface Properties; Time Factors; Density functional; density functional calculations, molecular dynamics ,nucleosides; Molecular dynamics method; Nucleosides; Adenines; Purine base; adenine, conical intersections
• ISSN: 1439-4235; 1439-7641
• DOI: 10.1002/cphc.200400644

Excited‐state potential energy surfaces of adenine, protonated adenine, and their N9‐methylated analogs are explored by means of a complete active space (CAS) and time‐dependent density functional theory (TD‐DFT) study to understand the dynamics associated with internal conversion. After photoexcitation of the ground‐state molecules to the S1 state, the nuclear motions that are responsible for taking the wavepacket out of the Franck–Condon region are either an HN9/CN9 stretch or a ring‐puckering motion that leads to pyramidalization. These motions lead to accessible conical intersections with the ground‐state surface. The results are used to successfully interpret previous measurements on the photodissociation of adenosine 5′‐monophosphate nucleotide anions and cations, where the latter react in a highly nonstatistical manner. A dynamic conversion: Ultrafast internal conversion from the first excited state to the ground state in adenine and its protonated form occurs by either activation of an NH stretch or via ring puckering that leads to pyramidalization. The potential energy surface topology directs the internal energy in a nonstatistical manner and the conical intersection is thereby responsible for the nonstatistical decomposition of adenosine monophosphate (see picture).