arXiv subjects

Viktorija Gineityte

Publications and source records attributed to Viktorija Gineityte.

7 recordsLinked to original sources

arXiv2023

An image of the heteroatom influence in acyclic polyenes

The study is devoted to development of the theory of the heteroatom influence in acyclic polyenes from a local perspective. The charge redistribution due to introduction of heteroatom(s) (X) is decomposed into additive and meaningful contributions undergoing an extinction with growth of the substructure embraced. Each of these contributions, in turn, is traced back to specific additional interactions of bond orbitals (BOs) of C=C bonds of the parent hydrocarbon. The expansion starts with local and transferable dipoles of individual X=C (or C=X) bonds that are proportional to direct interactions of BOs within respective parent C=C bonds. Thereupon, populations transferred between first-neighbouring double bonds follow along with parallel and anti-parallel secondary (induced) dipoles of the latter. These contributions are determined by an interplay of two indirect interbond interactions of BOs and consequently depend on positions of heteroatom(s) inside the bonds concerned.

physics.chem-ph↗

arXiv2022

Elementary conjugated fragments in acyclic polyenes with heteroatoms

The study is aimed at revealing the most important substructures (fragments) of polyenes with heteroatoms determining the alteration in the conjugation energy of the whole compound due to substitution and the relevant charge redistribution. The systems are modelled as sets of weakly-interacting formally-double bonds, where the formally-single bonds represent the interaction. Expressions for total energies and populations of basis orbitals are then derived in the form of power series with respect to two small parameters. Analysis of these series shows that conjugated substructures consisting of two connected formally-double bonds and containing at least s single heteroatom play the role of fragments being sought. Nine potential fragments of this type are considered separately that differ one from another in the number of heteroatoms and/or their relative positions inside and are called elementary conjugated fragments.

physics.chem-ph↗

arXiv2019

A local perspective on conjugation of double bonds in acyclic polyenes

The study is devoted to elaboration of an alternative image of conjugation in acyclic polyenes as a weak and essentially local delocalization of initially-localized pairs of electrons ascribed to individual double bonds (instead of formation of a completely delocalized electron system as usual). To this end, polyenes are modelled as sets of weakly interacting formally-double bonds, where the single bonds represent the interaction between the former and are treated as a perturbation. Mathematically, the above-formulated goal is realized by means of a particular version of the non-canonical method of molecular orbitals (MOs) based on the Brillouin theorem and yielding the expressions both for total energies and for non-canonical (localized) MOs (NCMOs) directly without any reference to usual (canonical) MOs. In addition, total energies and NCMOs are interrelated explicitly in the approach applied, viz. the former are representable via the so-called delocalization coefficients of the latter. Adaptation of these general results to the above-specified model of polyene yields coincidence between the conjugation energy (CE) and the total delocalization energy of all pairs of electrons contained. Moreover, a local relation follows between constitution of the nearest environment of a certain bond, delocalization pattern of the respective pair of electrons and contribution of just this pair to the total CE of the given polyene. As a result, different stabilities of distinct polyenes (e.g. of isomers) prove to be accompanied by variable extents of delocalization of separate pairs of electrons. Linear and cross-conjugated polyene chains are comparatively analyzed.

physics.chem-ph↗

arXiv2018

Perturbational non-canonical theory of molecular orbitals and its applications

The article contains a summary of fundamentals of the perturbational non- canonical molecular orbital (PNCMO) theory formerly developed by the author. In some respects, the PNCMO theory is a generalization of the well-known simple PMO theory: First, the usual diagonalization problem (and/or the eigenvalue equation) for a certain model Hamiltonian matrix ($\mathbf{H}$) is now replaced by two interrelated non-canonical one-electron problems, namely by the block-diagonalization problem for the matrix $\mathbf{H}$\ following from the Brillouin theorem and determining non-canonical (localized) MOs (NCMOs) and by the commutation equation for the respective one-electron density matrix (charge-bond order (CBO)) matrix. Second, perturbative solutions of the above-specified alternative problems are sought in terms of entire submatrices (blocks) of the matrix $\mathbf{H}$\ instead of usual matrix elements (e.g. of Coulomb and resonance parameters). Third, a generalized version of the perturbation theory (PT) is used in place of the standard Rayleigh-Schrödinger PT (RSPT), wherein non-commutative quantities stand for the usual (commutative) ones (cf. the so-called non-commutative RSPT (NCRSPT)). As a result, algebraic expressions are derived for the principal quantum-chemical characteristics (including the CBO matrix, the NCMO representation matrix and the total energy) that embrace definite classes of Hamiltonian matrices and thereby of molecules. To illustrate the point, saturated and conjugated hydrocarbons are taken as examples. Arguments are given that the PNCMO theory possibly forms the basis of a novel way of qualitative chemical thinking.

physics.chem-ph↗

arXiv2017

Perturbational perspective on Kekulè valence structures of biphenylene and related hydrocarbons

Individual Kekule valence structures of biphenylene and related hydrocarbons are treated perturbatively by modelling them as sets of weakly-interacting uniform double bonds. Total pi-electron energies of these structures are then expressed in the form of power series with respect to the resonance parameter of uniform single bonds. On this basis, the Kekule structures concerned are ordered according to their relative stabilities and thereby importances when building up the actual electronic structures. To rationalize the results, interrelations are sought between separate members of the power series, on the one hand, and presence of definite substructures in the given Kekule structure, on the other hand. It is shown that monocycles containing two and four exocyclic methylene groups participate in the formation of energy corrections of the relevant Kekule valence structures along with the usual rings consisting of uniform double and single bonds alternately and known as conjugated circuits. An extension of the empirical Fries rule to the case of biphenylene-like hydrocarbons is consequently formulated that embraces monocycles of the above-specified types. The reasons are also discussed why the results of the usual theory of conjugated circuits are less satisfactory for phenylenes as compared to benzenoids.

physics.chem-ph↗

arXiv2016

Supplementary Conjugated Circuits for Biphenylene and related hydrocarbons

Individual Kekule valence structures of biphenylene and related hydrocarbons are comparatively studied in respect of their total pi-electron energies and thereby relative stabilities. These structures are modeled as sets of weakly-interacting initially-double (C=C) bonds. The relevant total energies are represented in the form of power series, wherein the averaged resonance parameter of initially-single (C-C) bonds underlies the expansion. To rationalize the resulting distinctions in total energies, interrelations are sought between separate members of the series, on the one hand, and presence of definite substructures in the given Kekule valence structure, on the other hand. It is shown that monocycles S_1 and S_2 correspondingly containing two and four exocyclic methylene groups (like 3,4-dimethylene cyclobutene and [4]radialene) participate in the formation of energy corrections of the relevant Kekule valence structures along with the usual conjugated circuits of the 4n+2 and 4n series (R_n and Q_n, n=1,2,3...). Thus, the cycles S_1 and S_2 are deductively predicted to play the role of supplementary conjugated circuits for biphenylene-like hydrocarbons. Moreover, the S_2- and S_1- containing structures are shown to be the most stable ones among all Kekule valence structures of the given hydrocarbon. Meanwhile, the lowest stability is predicted for structures in which either the neighboring hexagonal rings are connected by two C=C bonds or two exocyclic C=C bonds are attached to the same hexagonal ring.

physics.chem-ph↗

arXiv2015

On Relative Stabilities of Distinct Polyenes. An Extension of the Concept of Conjugated Paths

The study continues the previous development [MATCH, 72 (2014) 39-73] of the perturbative approach to relative stabilities of pi-electron systems of conjugated hydrocarbons modeled as sets of weakly-interacting initially-double (C=C) bonds. Distinct isomers of acyclic hydrocarbons (polyenes) are now under focus. The relevant total pi-electron energies (E) are expressed in the form of power series containing members (E_(k)) of even orders (k=0,2,4,...) with respect to the averaged resonance parameter of initially-single (C-C) bonds. Terms to within the sixth order (k=6) inclusive are shown to be of importance for discrimination between similar isomers. In this connection, missing expressions for corrections E_(6) are originally derived. Conjugated paths of various lengths (i.e. linear chains consisting of C=C and C-C bonds alternately) are shown to be the most important (but not the only) fragments contributing to stabilization of any acyclic pi-electron system. Again, new types of fragments (substructures) are revealed (viz. the so-called composite conjugated paths) that contribute to destabilization of the system concerned. As a result, formation of the total energy of an acyclic pi-electron system is concluded to be governed by an interplay between stabilizing and destabilizing factors. Accordingly, the perturbative approach applied offers us an extension of the concept of conjugated paths. Particular isomers containing four, five and six C=C bonds are considered in a detail as examples.

physics.chem-ph↗