3 research outputs found
Protonation and deprotonation enthalpies of alloxan and implications for the structure and energy of its complexes with water: a computational study
<div><p>The optimized geometries, harmonic vibrational frequencies, and energies of the structures of monohydrated alloxan were computed at the DFT/ÏB97X-D and B3LYP/6â311++G** level of theory. Results confirm that the monohydrate exists as a dipolar alloxanâwater complex which represents a global minimum on the potential energy surface (PES). Trajectory dynamics simulations show that attempt to reorient this monohydrate, to a more favorable orientation for H-bonding, is opposed by an energy barrier of 25.07âkJ/mol. Alloxan seems to prefer acting as proton donor than proton acceptor. A marked stabilization due to the formation of NâHâOH<sub>2</sub> bond is observed. The concerted proton donorâacceptor interaction of alloxan with one H<sub>2</sub>O molecule does not increase the stability of the alloxanâwater complex. The proton affinity of the O and N atoms and the deprotonation enthalpy of the NH bond of alloxan are computed at the same level of theory. Results are compared with recent data on uracil, thymine, and cytosine. The intrinsic acidities and basicities of the four pyrimidines were discussed. Results of the present study reveal that alloxan is capable of forming stronger H-bonds and more stable cyclic complex with water; yet it is of much lower basicity than other pyrimidines.</p></div
Electronic structure and acidâbase properties of Kojic acid and its dimers. A DFT and quantum topology study
<p>Kojic acid is a polyfunctional heterocyclic compound, with several important reaction centres; it has a wide range of applications in the cosmetic, medicine, food, agriculture and chemical industries. The present study aims at better insight into its electronic structure and bonding characteristics. Thus, density functional theory at the M06-2x /6-311++G<sup>**</sup> level of theory is used to investigate its ground state electronic and acidâbase properties. Protonation and deprotonation enthalpies are computed and analysed. The ability of Kojic acid to form both water complexes and dimers is explored. Several different complexes and dimer structures were examined. Natural bond order and quantum topology features of the charge density were analysed. The origin of the stability of the studied complexes and dimer structures can be traced to hydrogen bonding, Ï-conjugative and non-covalent dispersive interactions.</p
Nonadiabatic Dynamics of Cycloparaphenylenes with TD-DFTB Surface Hopping
We
implemented a version of the decoherence-corrected fewest switches
surface hopping based on linear-response time-dependent density functional
tight binding (TD-DFTB), enhanced by transition density analysis.
The method has been tested for the gas-phase relaxation dynamics of
two cycloparaphenylene molecules, [8]ÂCPP and [10]ÂCPP, explaining some important features of their nonadiabatic
dynamics, such as the origin of their long fluorescence lifetimes
(related to the slow radiative emission from the S<sub>1</sub> state)
and the trend of increasing the fluorescence rate with the molecular
size (related to an increase in the S<sub>1</sub>âS<sub>0</sub> energy gaps and oscillator strengths in the larger molecule). The
quality of the TD-DFTB electronic structure information was assessed
through four quantities: excitation energies; charge-transfer (CT)
numbers, which estimate the charge transfer character of states; participation
ratio (PR), which describes delocalization of electronic density;
and participation ratio of natural transition orbitals (PRNTO), which
describes the multiconfigurational character of states. These quantities
were computed during dynamics and recomputed for the same geometries
with the higher-level long-range-corrected TD-LC-DFTB and a lower-level
single-determinant approximation for the excited states, SD-(LC)-DFTB.
Taking TD-LC-DFTB as the standard, TD-DFTB underestimates the excitation
energies by âŒ0.5 eV and overestimates CT and PR. SD-DFTB underestimates
excitation energies and overestimates CT to the same extent that TD-DFTB
does, but it predicts reasonable PR distributions. SD-LC-DFTB leads
to an extreme overestimation of the excitation energies by âŒ3
eV, overestimates the charge transfer character of the state, but
predicts the PR values very close to those obtained with TD-LC-DFTB