**1. Introduction**

Chemistry have stepped in the last 40–50 years to the incredible mountain. Nowadays the scientists can synthesize large composite organic molecules including important biological active species and apply physicochemical instruments with digital operating and rapid registration to the insight of pure and technological processes.

However, the current formation mechanism theory and organic liquid structure models can be questioned. Even in the late 50s–early 60s, attempts were made, basically by spectral methods, to study the nature of interactions in liquid phase of organics that lead to its high resistance to external actions and at the same time preserve its mobile properties. After numerous studies on this topic, at one point there was a pause. However, no intermolecular forces, except for classical hydrogen bonds, have been discovered.

The hope arose that with the discovery of new diffraction wave scattering methods with Fourier transformation for studying liquid substances, this problem would be clarified. Nevertheless, back in 2005, S. Ballint, studying liquid dichloromethane, concluded that the internal parameters of individual molecules that form a liquid-phase system are well defined, but their intermolecular arrangement is ambiguous.

This is strange, but we have not found a single comprehensive review that would include a comparative analysis of different approaches, including *ab initio* calculations, in a unified vision. The theory of dipole–dipole coupling, in other words Coulomb interactions, is generally accepted, although in this theory there are many contradictions with the modern theoretical and experimental findings. At the same

time, the concept of specific intermolecular interactions existence in organic liquids is verified by structural, thermodynamic and spectroscopic studies. These studies are presented and discussed in Section 2.

Ten years ago, when conducting the IR investigation of simplest organic solvents to account their role in the reactivity of some organic substrates, we have paid attention to the IR bands, which could not be assigned to any known internal mode of substance. In terms of classical vibrational spectra theory these bands can be interpreted as overtones or combination bands, if any, or Fermi resonance. However, we have suggested another assignment that was combined with DFT calculations and allowed us to make the assumption that these bands are a manifestation of specific interactions in organic liquids. In first regard, our arguments were based on the principle that mutually exclusive approaches of classical and quantum mechanics cannot be mixed in the same approach. The results of our investigations are presented and discussed in detail across Sections 2–3.

The analysis of current theoretical and experimental methods exhibits the value of DFT in the knowledge progress in the field of organic liquid phase molecular arrangements. We have presented the traditional DFT calculations and our approach to this problem as well in Section 3. Unlike quoted literature that use the paired interactions model for description of condensed phase, which arises, as a rule, under hydrogen bond, in our approach, the mechanism of liquid phase formation is considered in terms of molecular transformations. This insight applies not only to the substances with hydrogen bond, but also to the systems of identical molecules without hydrogen bonds. The conclusions of our research are formulated in Section 4.
