QM/MM Nonadiabatic Decay Dynamics of 9H‐Adenine in Aqueous Solution

Abstract

The photoinduced nonadiabatic decay dynamics of 9H-adenine (hereafter, adenine) in aqueous solution were investigated by surface-hopping simulations within a quantum mechanical/molecular mechanical (QM/MM) framework. The QM subsystem (adenine) was treated at the semiempirical OM2/MRCI level, whereas the MM solvent (water) was described by the TIP3P force field model. Classical molecular dynamics (MD) simulations were used to generate snapshots with different solvent configurations and geometries. For a representative number of these snapshots, the energy minima of the lowest electronic states and the most important conical intersections were located by QM/MM geometry optimization. Surface-hopping QM/MM MD simulations were performed for all selected snapshots to study the nonadiabatic dynamics after photoexcitation, including the two lowest excited singlet states, which are both populated in the initial photoexcitation due to strong vibronic coupling in the Franck-Condon region. The simulations yield ultrafast S(2)-S(1) decay within 40 fs and S(1)-S(0) internal conversion to the ground state within 410 fs, which is consistent with recent experimental results from time-resolved spectroscopy.

On the Participation of Photoinduced N–H Bond Fission in Aqueous Adenine at 266 and 220 nm: A Combined Ultrafast Transient Electronic and Vibrational Absorption Spectroscopy Study

“…We speculate that this ultrafast spectral evolution may be due to either: (i) rapid motion of the excited state wavepacket relaxing towards a conical intersection geometry with S0; 25,26 or (ii)…”

“…The biologically relevant 9H tautomer of Ade (Scheme 1, blue) has received considerable theoretical attention. 9,[12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30] A number of conical intersections can potentially facilitate repopulation of S0 from the initially excited 1 ππ* state(s), depending on the initial excitation energy. There has been some debate regarding the dominant pathway for re-accessing S0 after UV excitation in 9H-Ade, although the general consensus is that out-ofplane ring distortion at either the C2 or C6 sites (see Scheme 1) leads to conical intersections which facilitate ultrafast (and near barrierless) internal conversion (IC) back to S0; we note for completeness that recent work suggests direct 1 ππ* → S0 IC is likely to be the dominant process, 9,21,26,31 rather than a sequential 1 ππ* → 1 nπ* → S0 process.…”

Mapping the Ultrafast Dynamics of Adenine onto Its Nucleotide and Oligonucleotides by Time-Resolved Photoelectron Imaging

“…18,31 On the other hand, in solution, strong mixing of state character often prevents a strict diabatic label from being applied. 33 This makes definitive assignment of mechanism difficult.In our experiments on dAMP -, we cannot determine the amount of mixing of the 1 nπ* state along the decay pathway, although it is worth noting that we observe no changes in the photoelectron anisotropy during the decay, which is consistent with dynamics occurring on a single excited state. 23 Our tentative conclusion that the dynamics do not directly involve the 1 nπ* state is consistent with those reached for solvated deoxyadenosine, 31 and with certain high-level calculations on solvated Ade: 18,19 the biexponential dynamics observed are a consequence of motion away from the Franck-Condon region towards conical intersections followed by internal conversion.…”

Generalized trajectory surface-hopping method for internal conversion and intersystem crossing

“…In our implementation, only the QM components of the derivative coupling vectors are used in QM/MM GTSH dynamics simulations because the MM components are expected to be very small; in addition, they are available only numerically so that their evaluation is expensive. Similar approximations have been used in previous QM/MM FSSH applications 7,12,14,24,[76][77][78] and in QM/MM optimizations of conical intersections. 79,80 In the same spirit, the spin-orbit couplings between electronic states I and J with different spin are determined from the QM wavefunctions obtained in the QM/MM calculations with electronic embedding,…”

“…[4][5][6][7][8][9][10][11][12][13][14][15][16] It has been applied to study many ultrafast photophysical and photochemical processes in the gas phase and the condensed phase. [17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33] Despite the growing popularity and success of the trajectory-based FSSH method, it remains approximate and has some well-known drawbacks. [34][35][36][37] One major limitation is that the original FSSH method has been formulated with a focus on nonadiabatic internal conversion processes, disregarding intersystem crossing events that are often encountered in photochemistry and may even occur on an ultrafast timescale in systems with non-negligible spin-orbit couplings.…”