Wojciech Bartkowiak

A new theoretical insight into the nature of intermolecular interactions in the molecular crystal of urea

The nature of interactions in the molecular crystal of urea is analyzed in terms of the interaction energy decomposition. The influence of the electron correlation effects was estimated on the basis of the calculated second order Moller–Plesset corrections and their analysis. In the crystal, the urea molecules form infinite ribbons which reveal strong cooperative effects. The hydrogen-bonded interactions of the orthogonal ribbons do stabilize the crystal, whereas interactions between parallel tapes are repulsive. The stability of the crystal structure is determined by a subtle balance between these two types of interactions. Although, the electron correlation effects are stabilizing, their contribution is rather small in comparison with the total interaction energy.

Conformation and solvent dependence of the first and second molecular hyperpolarizabilities of charge-transfer chromophores. Quantum-chemical calculations

Abstract The results of semiempirical quantum-chemical calculations of the first- and second-order hyperpolarizabilities (β and γ, respectively) for molecules in the gas phase and in chloroform and aqueous solvents are presented. The finite-field formalism and the sum-over-states method was used to calculate individual components of the β and γ tensors. The solvent effect was included via the discrete quantum-mechanical Langevin dipoles/Monte Carlo approach and the continuum virtual charge method. The calculations demonstrate the existence of large geometry/conformation and solvent effects on the hyperpolarizabilities of the charge-transfer type chromophores. The calculated β and γ values including the solvent effect are compared to respective experimental data determined in solution phase.

Electronic and vibrational contributions to first hyperpolarizability of donor-acceptor-substituted azobenzene.

In this study we report on the electronic and vibrational (hyper)polarizabilities of donor-acceptor-substituted azobenzene. It is observed that both electronic and vibrational contributions to the electric dipole first hyperpolarizability of investigated photoactive molecule substantially depend on the conformation. The contributions to the nuclear relaxation first hyperpolarizability are found to be quite important in the case of two considered isomers (cis and trans). Although the double-harmonic term is found to be the largest in terms of magnitude, it is shown that the total value of the nuclear relaxation contribution to vibrational first hyperpolarizability is a result of subtle interplay of higher-order contributions. As a part of the study, we also assess the performance of long-range-corrected density functional theory in determining vibrational contributions to electric dipole (hyper)polarizabilities. In most cases, the applied long-range-corrected exchange-correlation potentials amend the drawbacks of their conventional counterparts.

Solvent effect on static vibrational and electronic contribution of first-order hyperpolarizability of π-conjugated push–pull molecules: quantum-chemical calculations

Abstract Results of ab initio quantum-chemical calculations of the first-order static electronic (βe) and vibrational (βv) hyperpolarizabilities for the prototype push–pull conjugated molecules 4-nitro-aniline, 4-nitro-4′-aminostilbene, 4-amino-4′-nitrobiphenyl, and 4-amino-4′-nitrodiphenylacetylene in the gas phase and in chloroform and aqueous solvents are presented. The coupled perturbed Hartree–Fock method and the sum-over-modes formalism was used to calculate individual components of βe and βv tensors. The solvent effect has been included via the continuum self-consistent reaction field model. The calculations demonstrate the existence of larger solvent effect on the βv compared with βe for molecules investigated in this paper.

Relation between bond-length alternation and two-photon absorption of a push–pull conjugated molecules: a quantum-chemical study

Abstract The results of the semiempirical study of the structure/property relationships for the two-photon absorption cross-section ( δ ) of a series of prototypical π-conjugated push–pull molecules are presented. The calculations of δ for the first charge-transfer (CT) excited state were performed as a function of the bond length alternation (BLA). The molecular hyperpolarizabilities ( β and γ ) were calculated using the finite-field (FF) method. The obtained data were analyzed based on the simple two-state models. A strong dependence of δ on the BLA parameter was noticed.

New theoretical insight into the thermal cis–trans isomerization of azo compounds: Protonation lowers the activation barrier

The detailed mechanism of thermal cis–trans inversion of protonated diazenes and azobenzenes has been investigated for the first time using ab initio Hartree–Fock (HF), density functional theory (DFT), second order Mo/ller–Plesset (MP2), and complete active space self consistent field (CASSCF) approaches. Protonation of one of the nitrogens at the N=N bond leads to a considerable (5–20 kcal/mol) reduction of the activation barrier. The magnitude of this effect depends on the electron withdrawing nature of the substituents and the differential stabilization of the transition state by positively charged functional groups. In the case of 4-phenylazopirydine protonated at pyridine nitrogen, the barrier is dramatically reduced to just 7 kcal/mol. This result establishes the lowest (close to an experimental estimate) ab initio value for the cis–trans isomerization for a substituted azobenzene. The structures of neutral and protonated transition states are similar, and the correlation effects for molecular param...

Intermolecular interactions in solution: Elucidating the influence of the solvent

A new approach for the analysis of intermolecular interactions in a solution is proposed. The changes in the interaction energy components due to the solvent effects are estimated on the basis of the interaction energy calculated in the presence of the electric field induced in a polarizable medium, or in the field of the effective fragment potentials. Obtained results indicate a significant increase in stabilization resulting from electrostatic interactions as a result of the cooperative interactions between interacting subsystems and solvent molecules.

Electronic Structure, Bonding, Spectra, and Linear and Nonlinear Electric Properties of Ti@C28

The potential energy surface (PES) of Ti@C(28) has been revisited, and the stationary points have been carefully characterized. In particular, the C(2v) symmetry structure considered previously turns out to be a transition state lying 2.3 kcal/mol above the ground state of C(3v) symmetry at the MP2/6-31G(d) level. A large binding energy of 181.3 kcal/mol is found at the ROMP2/6-31G(d) level. Topological analysis of the generalized Ti@C(28) density reveals four bond paths between Ti and carbon atoms of the host. The character of all four contacts corresponds to a partially covalent closed shell interaction. UV-vis, IR, and Raman spectra are calculated and compared with C(28)H(4). The dipole moment and the static electronic and double harmonic vibrational (hyper)polarizabilities have been obtained. Distortion of the fullerene cage due to encapsulation leads to nonzero diagonal components of the electronic first hyperpolarizability β, and to an increase in the diagonal components of the electronic polarizability α and second hyperpolarizability γ. However, introduction of the Ti atom causes a comparable or larger reduction in most cases due to localized bonding interactions. At the double harmonic level, the average vibrational β is much larger than its electronic counterpart, but the opposite is true for α and for the contribution to γ that has been calculated. There is also a very large anharmonic (nuclear relaxation) contribution to β which results from a shallow PES with four minima separated by very low barriers. Thus, the vibrational γ (and α) may, likewise, become much larger when anharmonicity is taken into account.

Molecular Structure – Optical Property Relationships for a Series of Non-Centrosymmetric Two-photon Absorbing Push-Pull Triarylamine Molecules

This article reports on a comprehensive study of the two-photon absorption (2PA) properties of six novel push-pull octupolar triarylamine compounds as a function of the nature of the electron-withdrawing groups. These compounds present an octupolar structure consisting of a triarylamine core bearing two 3,3′-bis(trifluoromethyl)phenyl arms and a third group with varying electron-withdrawing strength (H < CN < CHO < NO2 < Cyet < Vin). The 2PA cross-sections, measured by using the femtosecond open-aperture Z-scan technique, showed significant enhancement from 45 up to 125 GM for the lowest energy band and from 95 up to 270 GM for the highest energy band. The results were elucidated based on the large changes in the transition and permanent dipole moments and in terms of (i) EWG strength, (ii) degree of donor-acceptor charge transfer and (iii) electronic coupling between the arms. The 2PA results were eventually supported and confronted with theoretical DFT calculations of the two-photon transition oscillator strengths.

Performance of density functional theory in computing nonresonant vibrational (hyper)polarizabilities

A set of exchange‐correlation functionals, including BLYP, PBE0, B3LYP, BHandHLYP, CAM‐B3LYP, LC‐BLYP, and HSE, has been used to determine static and dynamic nonresonant (nuclear relaxation) vibrational (hyper)polarizabilities for a series of all‐trans polymethineimine (PMI) oligomers containing up to eight monomer units. These functionals are assessed against reference values obtained using the Møller–Plesset second‐order perturbation theory (MP2) and CCSD methods. For the smallest oligomer, CCSD(T) calculations confirm the choice of MP2 and CCSD as appropriate for assessing the density functionals. By and large, CAM‐B3LYP is the most successful, because it is best for the nuclear relaxation contribution to the static linear polarizability, intensity‐dependent refractive index second hyperpolarizability, static second hyperpolarizability, and is close to the best for the electro‐optical Pockels effect first hyperpolarizability. However, none of the functionals perform satisfactorily for all the vibrational (hyper)polarizabilities studied. In fact, in the case of electric field‐induced second harmonic generation all of them, as well as the Hartree–Fock approximation, yield the wrong sign. We have also found that the Pople 6–31+G(d) basis set is unreliable for computing nuclear relaxation (hyper)polarizabilities of PMI oligomers due to the spurious prediction of a nonplanar equilibrium geometry.