Temperature effects on the internal conversion of excited adenine and adenosine

This work aims to elucidate the dependence of the excited-state lifetime of adenine and adenosine on temperature. So far, it has been experimentally shown that while adenine's lifetime is unaffected by temperature, adenosine's lifetime strongly depends on it. However, the non-Arrhenius temperature dependence has posed a challenge in explaining this phenomenon. We used surface hopping to simulate the dynamics of adenine and adenosine in the gas phase at 0 and 400 K. The temperature effects were observed under the initial conditions via Wigner sampling with thermal corrections. Our results confirm that adenine's excited-state lifetime does not depend on temperature, while adenosine's lifetime does. Adenosine's dependency is due to intramolecular vibrational energy transfer from adenine to the ribose group. At 0 K, this transfer reduced the mean kinetic energy of adenine's moiety so much that internal conversion is inhibited, and the lifetime elongated by a factor of 2.3 compared to that at 400 K. The modeling also definitively ruled out the influence of viscosity, which was proposed as an alternative explanation previously.


SI-1. Excited-state characterization
Table S1.Excitation energies, oscillator strengths (f), and state characterization given by natural transition orbitals and their respective contributions (in percentage) calculated for 9H-adenine and anti-adenosine at the S0 minimum at ADC(2)/SV(P) level.Adenosine vertical excitation energies resemble the ones from adenine, as shown in our ADC(2) calculations and also experimentally 11 (see Tables S2 and S3).Similarly to adenine, the S1 state of adenosine has a n character at the Franck-Condon region.For the syn-isomer, CASPT2 calculations also identify a S0 MECI with a puckered geometry lying lower than the S1 state at the Franck-Condon region.EDPT reaction pathway from the bright  (La) state to this intersection is barrierless, but from the S1 (n) to the S0 MECI, a barrier of 0.3 eV is observed. 3Reaction pathways computed at both ADC(2) and CASPT(2) levels for adenosine (Figure 15

SI-2. Nuclear-ensemble spectra
Figure S3.Computed absorption spectra based on 500 points nuclear ensemble for adenine and adenosine at 0 and 400 K calculated at ADC(2)/SV(P) level.The grey area represents the excitation window (5.1 ± 0.1 eV in all cases).

SI-3. Excited-state occupation
Figure S4.Adiabatic state occupation evolution for adenine and adenosine at 0 and 400 K calculated at ADC(2)/SV(P) level.

SI-4. Electronic density differences
Figure S5.Electronic density difference between S1 and S0 states at the S0 minimum, S1 minimum, C2-puckered and C6-puckered intersections.Green and orange regions represent a decrease and an increase in electron density, respectively.

SI-5. Normal mode analysis
Normal modes are linear combinations of cartesian coordinates, typically obtained as the eigenvectors of the mass-weighted Hessian matrix of the energy.They are handy in identifying relevant nuclear vibrational motions of polyatomic molecules.The normal mode analysis is a way to project the molecular motion of a trajectory in terms of the corresponding normal modes displacement of the molecule. 13,14  Taking an average over all trajectories, the average displacement  ̅ () for each normal mode can be obtained to monitor the average motion (nuclear vibrations) of the molecule as a function of time.
Moreover, the standard deviation of this average displacement over time boils down to a single number per normal mode to represent its coherent activity: In this equation,  ̂ℎ 2 shows the coherent activity of a normal mode during the dynamics.  and   are the first and last time steps considered, and Δ is the constant time interval of the dynamics.
Alternatively, one may compute the total standard deviation over time steps and trajectories without prior averaging over the trajectories.Hence, it will quantify the total motion observed along the normal modes.
= (  |  | ⋯ | − ) be the matrix containing the 3 − 6 normal modes at a reference geometry  0 in Cartesian coordinates, where  is the number of atoms in a molecule.(, ) is the Cartesian coordinates of the  th trajectory at time .The displacement vector in normal mode coordinates (i.e., normal mode amplitudes) with respect to the reference geometry can be written as (, ) =  −1 [(, ) −  0 ] Figure S6.Main normal modes contributing to vibrational energy transfer in adenosine.

Table S2 .
[2][3][4]excitations and potential energy surfaces of adenine were explored at different levels of theory (see TableS2and supplementary Ref. 1).Reaction pathways connecting ground state minimum and conical intersections for adenine at the multiconfigurational level are also explored in previous works.[2][3][4]Ithas been shown that ADC(2) can satisfactorily reproduce both vertically excited states and reaction pathways for adenine.Comparison of excitation energies and oscillator strengths (f) obtained at different levels of theory for 9H-adenine.Lb and La states denote, respectively, the weakly absorbing  state and the bright  state.

Table S3 .
Comparison of excitation energies and oscillator strengths (f) obtained at different levels of theory for adenosine.Lb and La states denote, respectively, the weakly absorbing  state and the bright  state.