Characterizing Conical Intersections in DNA/RNA Nucleobases with Multiconfigurational Wave Functions of Varying Active Space Size

Abstract

We characterize the photochemically relevant conical intersections between the lowest-lying accessible electronic excited states of the different DNA/RNA nucleobases using Cholesky decomposition-based complete active space self-consistent field (CASSCF) algorithms. We benchmark two different basis set contractions and several active spaces for each nucleobase and conical intersection type, measuring for the first time how active space size affects conical intersection topographies in these systems and the potential implications these may have toward their description of photoinduced phenomena. Our results show that conical intersection topographies are highly sensitive to the electron correlation included in the model: by changing the amount (and type) of correlated orbitals, conical intersection topographies vastly change, and the changes observed do not follow any converging pattern toward the topographies obtained with the largest and most correlated active spaces. Comparison across systems shows analogous topographies for almost all intersections mediating population transfer to the dark ¹n_O/Nπ* states, while no similarities are observed for the "ethylene-like" conical intersection ascribed to mediate the ultrafast decay component to the ground state in all DNA/RNA nucleobases. Basis set size seems to have a minor effect, appearing to be relevant only for purine-based derivatives. We rule out structural changes as a key factor in classifying the different conical intersections, which display almost identical geometries across active space and basis set change, and we highlight instead the importance of correctly describing the electronic states involved at these crossing points. Our work shows that careful active space selection is essential to accurately describe conical intersection topographies and therefore to adequately account for their active role in molecular photochemistry.

Figure 1

Chemical structure and atom labeling of the DNA and RNA pyrimidine-based (upper half) and purine-based (lower half) nucleobases.

Figure 2

Classification of the conical intersections in terms of and parameters with a three-dimensional representation of how are the two potential energy surfaces in the branching space.

Figure 3

and parameters of (¹ππ*/S₀)_CI using multiple different active spaces (see Section 2) for (a) cytosine, (b) uracil and (c) thymine. Active space size is denoted by both marker size and the contour gradient color provided in the right-hand side of each panel. A picture with the superimposed geometries of all optimized conical intersections are provided as insets, with the colored structures representing the outlier intersections marked with a square. An analogous picture with the results obtained using a triple-ζ basis set can be found in the SI.

Figure 4

and parameters of (¹n_Oπ*/¹ππ*)_CI using multiple different active spaces (see Section 2) for (a) cytosine, (b) uracil, and (c) thymine. Active space size is denoted by both marker size and the contour gradient color provided in the right-hand side of each panel. A picture with the superimposed geometries of all optimized conical intersections are provided as insets, with the colored structures representing the outlier intersections marked with a square. An analogous picture with the results obtained using a triple-ζ basis set can be found in the SI.

Figure 5

and parameters of the (¹n_Oπ*/S₀)_CI using multiple different active spaces (see Section 2) for (a) cytosine, (b) uracil, and (c) thymine. Active space size is denoted by both marker size and the contour gradient color provided in the right-hand side of each panel. A picture with the superimposed geometries of all optimized conical intersections are provided as insets, with the colored structures representing the outlier intersections marked with a square. An analogous picture with the results obtained using a triple-ζ basis set can be found in the SI.

Figure 6

and parameters of the (a) (¹n_Nπ*/S₀)_CI and (b) (¹n_Nπ*/¹ππ*)_CI of cytosine. Active space size is denoted by both marker size and the contour gradient color provided in the right-hand side of each panel. Pictures with the superimposed geometries of all optimized conical intersections are provided as insets, with the colored structures representing the outlier intersections marked with a square. An analogous picture with the results obtained using a triple-ζ basis set can be found in the SI.

Figure 7

and parameters of (L_a(¹ππ*)/S₀)_CI for (a) guanine and (b) adenine using multiple different active spaces (see Section 2). Active space size is denoted by both marker size and the contour gradient color provided in the right-hand side of each panel. Pictures with the superimposed geometries of all optimized conical intersections are provided as insets, with the colored structures representing the outlier intersections marked with a square. An analogous picture with the results obtained using a triple-ζ basis set can be found in the SI.

Figure 8

and parameters of (L_a(¹ππ*)/L_b(¹ππ*))_CI using multiple different active spaces (see Section 2) for guanine (a) and adenine (b). Active space size is denoted by both marker size and the contour gradient color provided in the right-hand side of each panel. Pictures with the superimposed geometries of all optimized conical intersections are provided as insets.

Figure 9

and parameters of (L_b(¹ππ*)/¹n_Nπ*)_CI (a) for adenine and (L_a(¹ππ*)/¹n_Oπ*)_CI (b) and (L_b(¹ππ*)/¹n_Nπ*)_CI (c) for guanine using multiple different active spaces (see Section 2). Active space size is denoted by both marker size and the contour gradient color provided in the right-hand side of each panel. A picture with the superimposed geometries of all optimized conical intersections is provided as an inset, with the colored structures representing the outlier intersection marked with a purple square.

Figure 10

and parameters of photochemically relevant conical intersections (¹ππ*/S₀)_CI and (L_a(¹ππ*)/S₀)_CI (a) and the different (¹n_O/N/¹ππ*)_CI (b) for all DNA/RNA nucleobases, obtained using the largest active space feasible for each system, except for guanine where (18,13) was used in subpanel (b).