Milena Spassova

Ab initio investigation of doping-enhanced electronic and vibrational second hyperpolarizability of polyacetylene chains

The effect of charging on the longitudinal second hyperpolarizability of polyacetylene (PA) chains containing up to nearly 70 carbon atoms has been investigated ab initio by characterizing chains with and without an explicit alkali atom (Li, Na, K) as dopant. Whereas charging dramatically enhances the static electronic and vibrational hyperpolarizabilities, γLe(0) and γLv, of an isolated chain at intermediate chain lengths, the presence of an alkali atom counterion substantially reduces this effect. As the size of the alkali atom increases, most properties, including the hyperpolarizabilities, approach those of the isolated chain. Detailed analysis shows that the behavior of γLe(0) is most simply explained in terms of a reduced electrostatic pinning potential due to increased distance between chain and counterion. At all chain lengths studied γLe(0) of PA is enhanced by alkali doping. For chains containing 50 carbon atoms (NC=50), the increase due to K doping is about 9×107 a.u., which more than doubles t...

Assessing long-range corrected functionals with physically-adjusted range-separated parameters for calculating the polarizability and the second hyperpolarizability of polydiacetylene and polybutatriene chains

The use of physically-adjusted range-separated parameter (μ) together with long-range corrected exchange-correlation functionals is shown not to be reliable for calculating the polarizability (α) and the second hyperpolarizability (γ) of large π-conjugated systems. This statement has been substantiated by calculating the polarizability and the second hyperpolarizability of increasingly large polydiacetylene and polybutatriene chains in comparison with reference data evaluated at the coupled-cluster singles and doubles with perturbative estimation of the triples [CCSD(T)] level. Further comparisons highlight that long-range corrected exchange-correlation functionals with conventional and larger range-separated parameters perform better for α (μ = 0.47) and γ (μ = 0.33 and 0.47) but for polybutatriene chains none can describe satisfactorily the chain length evolution of γ. On the other hand, Møller-Plesset second- and fourth-order values are in closer agreement with CCSD(T), except for γ of polybutatriene, which is strongly overestimated.

Theoretical investigation on H1 and C13 NMR chemical shifts of small alkanes and chloroalkanes

Using density functional theory (DFT) with the B3LYP, PBE, and PBE0 exchange-correlation functionals as well as the Moller-Plesset second-order perturbation theory (MP2) combined with a series of rather extended basis sets, 1H and 13C chemical shifts of small alkanes and chloroalkanes (with different numbers of chlorine atoms on specific positions) have been simulated and compared to experimental data. For the 1H chemical shifts, theory tends to reproduce experiment within the limits of the experimental errors. In the case of 13C chemical shift, the differences between theory and experiment increase monotonically with the number of chlorine atoms and exhibit a deviation from additivity. This behavior is related to the saturation of the experimental 13C chemical shifts with the number of chlorine atoms, whereas the evolution is mostly linear at both DFT and MP2 levels of approximation. This difference has been traced back to the relativistic spin-orbit coupling effects, which are exalted as a result of the enhancement of the s character of the C atom when increasing the number of linked Cl atoms. Thus, it was demonstrated that not only electron correlation but also relativistic effects have to be considered for estimating the 13C chemical shifts when several Cl atoms are directly attached to the C atom. Linear (theory/experiment) regressions have then been performed for the different types of C atoms, i.e., bearing one, two, and three Cl atoms, with excellent correlation coefficients. The linear correlation relationships so obtained can then serve to predict and facilitate the interpretation of the nuclear magnetic resonance spectra of more complex compounds. Furthermore, by investigating the basis set effects, the correlation between the chemical shifts calculated using the 6-311 + G(2d,p) basis set and the more extended 6-311 + G(2df,p) and aug-cc-pvtz basis sets is excellent, demonstrating that the choice of the 6-311 + G(2d,p) basis set for calculating the 1H and 13C chemical shifts is relevant.

Structural properties of doped polyacetylene chains: a comparative theoretical investigation using Hartree–Fock, Møller–Plesset second-order perturbation theory, and density functional theory approaches

The effect of doping on the geometrical structure of polyacetylene chains containing up to 101 carbon atoms has been investigated theoretically by using the Hartree–Fock approach, the second-order Moller–Plesset perturbation theory as well density functional theory with the hybrid B3LYP exchange–correlation functional. In all cases, the transfer of charge associated with doping induces important geometrical modifications as a result of the large electron–phonon coupling characterizing these π-conjugated systems. The geometrical modifications are mostly described by variations of the bond length alternation following a hyperbolic tangent relation. For chains bearing a positive charge, the Hartree–Fock scheme overestimates the defect localization with respect to the Moller–Plesset scheme where the soliton spans a 36 CH unit region. On the other hand, the B3LYP approach predicts the soliton to be excessively delocalized. When a counterion is added, the soliton defect is more localized. The soliton width goes from 5 ± 2 CH units when the counterion is a Li atom to 9 ± 2 CH units when it is a Cl atom. Moreover, in the presence of a counterion, the three approaches provide much similar bond length alternation and charge distribution patterns.

Aggregation of 2‐Aminobenzimidazole—A Combined Experimental and Theoretical Investigation

An investigation of 2-aminobenzimidazole was carried out by calculations at HF, MP2, and DFT levels of theory and also by UV and IR spectroscopy. The quantum chemical calculations predict a full shift of the equilibrium towards the amino form, but the absorption spectra in different solvents distinctly show a two-component equilibrium system. Examination of possible equilibria in solution shows that an equilibrium between two dimeric forms of the amino tautomer of 2-aminobenzimidazole explains the spectral observations.