Ultrafast radiationless decay mechanisms through conical intersections in cytosine: Computational insight and topological analysis of the charge density distributions
27-34<span style="font-size:9.0pt;mso-fareast-font-family:Calibri;mso-bidi-font-family:
Calibri;mso-ansi-language:EN-US;mso-bidi-language:AR-SA" lang="EN-US">The quantum theory of
atoms-in-molecules (QTAIM) in conjunction with the DFT/B3LYP/6-311++G(2d,2p)
wave function are used to compute the atomic, bonded and non-bonded interactions,
distributions of the charge density, (r)<span style="font-size:9.0pt;mso-fareast-font-family:Calibri;mso-bidi-font-family:
Calibri;mso-ansi-language:EN-US;mso-bidi-language:AR-SA;mso-bidi-font-weight:
bold" lang="EN-US">, and its Laplacian, 2(r), for the ground
equilibrium structure of cytosine. The study has been further extended to
include two conical intersection (CI) structures that underlie the
radiationless decay of cytosine. Complete Active Space Multi-configuration SCF
level of theory with the 6-311++G** basis set are used to identify,
characterize and to optimize the geometrical structures of the conical
intersections between So and S1. In the
case of cytosine, all ring bond critical points show 2(r)<span style="font-size:9.0pt;
mso-fareast-font-family:Calibri;mso-bidi-font-family:Calibri;mso-ansi-language:
EN-US;mso-bidi-language:AR-SA" lang="EN-US"> <span style="font-size:
9.0pt;font-family:Symbol;mso-ascii-font-family:" times="" new="" roman";mso-fareast-font-family:="" calibri;mso-hansi-font-family:"times="" roman";mso-bidi-font-family:calibri;="" mso-ansi-language:en-us;mso-bidi-language:ar-sa;mso-char-type:symbol;="" mso-symbol-font-family:symbol;mso-bidi-font-weight:bold"="" lang="EN-US"><<span style="font-size:9.0pt;mso-fareast-font-family:Calibri;mso-bidi-font-family:
Calibri;mso-ansi-language:EN-US;mso-bidi-language:AR-SA;mso-bidi-font-weight:
bold" lang="EN-US"> 0<span style="font-size:9.0pt;mso-fareast-font-family:Calibri;mso-bidi-font-family:
Calibri;mso-ansi-language:EN-US;mso-bidi-language:AR-SA" lang="EN-US"> <span style="font-size:9.0pt;mso-fareast-font-family:Calibri;mso-bidi-font-family:
Calibri;mso-ansi-language:EN-US;mso-bidi-language:AR-SA" lang="EN-US">indicating covalent
binding and accumulation of the electron density in the bonding regions. On the
other hand, the C1-O11 bond shows, depletion of charge
density, indicating the increased ionic character of this bond. This point
might very well underlie the reactivity and the low keto-enol barrier in
cytosine. Contour plots and relief maps have been analyzed for regions of
valence shell charge concentrations and depletions in the ground state and the
three CI structures of cytosine. NBO analysis reveals that the conformational
and overall stability of the studied cytosine conformations is facilitated by
the competitive conjugative and the lone-pair interactions. In cytosine, there
is a delicate balance between these two forces, whereas, in each of the two CI
configurations there is one dominant force that underlies the stability of the
cytosine structure. The present work indicates that the QTAIM provide not only
a graphical presentation of very important critical points on the PES but also
unique quantitative descriptors of CI's that characterize it.
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