**2. Theoretical and experimental approach to the molecular arrangement in organic liquids**

In this section the fundamentals of molecular structure and its formation mechanism in liquid organic phase are presented, including the experimental data and theoretical study (dynamic simulation theory) in combination with the authors analysis of this approach.

Although there are numerous papers and books devoted to this problem, none of them fully resolved the problem of organic liquids molecular arrangement, and the character of long-range molecular interaction in liquid bulk is still ambiguous. Therefore, in the first part of this chapter we tried to present the current state of considered problem in terms of theoretical and experimental approach to evaluate the role of DFT calculation in the solution of the presented problem.

However, at the beginning of this consideration we would like to review the dipole–dipole interaction concept briefly because it is the oldest and most common theory of liquid formation. In fact, this theory is based on the model of Coulomb interactions between polar systems or temporary dipoles caused by fluctuation in electronic distribution.

#### **2.1 Classical dipoles interaction model**

Cooled and pressed real gases become liquid due to the intermolecular interactions called van der Waals. This intermolecular interaction energy can be defined as the heat of a liquid evaporation or rather the difference between the vaporization temperature and the work of expanding one mole of gas at the atmospheric pressure. Value of this interaction at the boiling point of gases is 1–3 kJ/mol. As it was shown by the quantum mechanical calculations, the energy of the van der Waals interactions consists of electrostatic, inductive, and dispersive components [1].

#### *The Formation Mechanism and Structure of Organic Liquids in the DFT Challenges DOI: http://dx.doi.org/10.5772/intechopen.100429*

The so-called orientational interaction of polar molecules is the most important part of the electrostatic interaction. Its essence, known as the Keesome effect, consists in the orientation of two interacting polar molecules with identical dipole moments that leads to the minimization of the system energy. In this case, the head-to-tail orientation becomes the most advantageous configuration. The energy of the orientational binding can be calculated as the sum of the Coulomb attractions and repulsion of the pole charges in the dipoles, expressed in terms of the dipoles [2].

Inductive interaction, or the Debye effect [3], is the interaction of the constant dipole moment and the induced dipole moment, arising due to an additional charge separation. Inductive interactions occur with the formation of noble gases hydrates with the dissolution of polar substances in the non-polar liquids and they are valuable only for the molecules with significant polarizability (ex. molecules with conjugated bonds) [3].

The dispersion interaction, or the London effect [4], arises between the electrons in the interacting molecules. There are molecules that have no dipole moment – for example, homoatomic molecules of noble gases. The additivity of dispersion forces is manifested in adsorption, in the processes involving gas condensation, etc. Dispersion forces play the important role not only in the individual molecules, but also in the macroscopic particles (ex. colloids). However, these forces are relatively weak.

The intermolecular interaction is the summarizing action of attraction and repulsion forces. At large distances, the attraction prevails, and at very short distances the repulsion is the main contributor. The attractive forces of Van der Waals are long ranged [5, 6], and the attraction energy rather decreases with distance. Van der Waals interaction forces at the equilibrium distance is small: ~ 1–5 kJ/mol, which is considerably less than the chemical bond energy.

The main conclusion is that Coulomb and Van der Waals interactions can relate to the formation of liquid from the gaseous substances at the cooling or pressing. However, this theory is very limited for organic liquids. There are at least three fundamental reasons for that: i) the intermolecular forces in organic liquids are sufficiently stronger, accounting their vaporization enthalpy [7]; ii) there is no direct dependence of liquid stability on dipole and molecular volume of consisting molecules in many cases [7]; iii) the interaction of dipoles in organic fluids should have a certain orientation, while the molecules in liquid do not have any anisotropy [5, 6]; iv) this conception ignores the specific non-valence interaction in liquid systems [7].

## **2.2 Thermodynamic data source**

The concept of the specific interaction in liquid systems based on the thermodynamic data is developed in [7]. The regularities in the homological series exhibit no correlation between the molecular mass and the vaporization enthalpy in many cases. The numerous thermodynamic data indicate the existence of specific nonvalence interaction in organic liquids. In case of substances with the different variant of hydrogen bond (ex. with OH–, NHi–, C=O– or COOH–group) the specific interaction providing molecular structure can be explained by classical H–donor– H–acceptor interaction [7].

However, for the liquid saturated hydrocarbons, alkenes, and alkynes this approach is not applicable. The author of [7] has suggested the scheme of hydrocarbon interactions. The main idea is that the carbon atoms in chains have a different negative charge owing the shift of 2 *s*–electronic pair between the carbon atoms. The electron density of carbon with a larger negative charge transfers partially to the carbon atom with the reduced negative charge.

In other words, the alkane molecules can be bound in the liquid state so that the carbon atoms with an enhanced negative charge would be closer to the carbon atom with a reduced charge. The carbon atoms of the methyl groups in hydrocarbons that have the enhanced negative charge can participate in the specific interaction between molecules under the donor–acceptor mechanism resembling the hydrogen bond.

In author's opinion, the non-equivalence charges distribution in hydrocarbon chains leads to the appearance of charge shift not only for the carbon atom, but for the hydrogen atom as well. It means that the hydrogen atom of the neighboring molecules can form the donor–acceptor C‧‧‧H bond and even the dihydrogen bond. The comparison of thermodynamic data indicates the relatively high contribution of similar pair interactions in vaporization energy of organic liquids. These specific interactions are the nature of liquid saturated hydrocarbons formation mechanism and their stability without involving of the specific hydrogen bon. The evaluation of specific interaction energy is given in [7] and relates to the range 3–10 kJ/mol. These values correspond to the weak hydrogen bond [1, 2, 6].

The thermodynamic analysis of the vaporization enthalpy of unsaturated hydrocarbons leads to the conclusion that the presence of a double or a triple bond in the molecule results in a stronger electron density shift from one or two hydrogen atoms to the carbon atoms of neighboring molecule [7]. For example, the hydrogen atoms of the CH2–group in propene and CH–group in propyne possess enhanced positive charges, and the carbon atoms acquire higher negative charges. Therefore, the acceptor and donor properties of these substances are more pronounced than in the propane molecule (this statement are cited fully in accordance with [7]).

#### **2.3 Molecular light scattering (MLS) and X-ray data source**

In the 70s – 80s the professor at Moscow University M.I. Shachparonov has published a work, which was, unfortunately, limitedly known in the scholar community [8, 9]. In this paper he has predicted the existence of the specific interaction in the non-polar liquids, in which the hydrogen bond lacks. For benzene, the existence

**Figure 1.** *The stack of aromatic rings in liquid benzene.*

#### *The Formation Mechanism and Structure of Organic Liquids in the DFT Challenges DOI: http://dx.doi.org/10.5772/intechopen.100429*

of a new specific interaction in liquids – the formation of molecular π-complexes (molecules stacks) between aromatic rings – was suggested (**Figure 1**).

At present, this concept was confirmed and developed in the works of I.A. Abramovich and the coauthors [10–17], devoted to the structure of the liquid system in benzene and its substituted species. The specific interaction with different spatial geometry can form in a liquid bulk of molecules without hydrogen bonds. For example, in solution of *di*-chlorobenzene, there are strong interactions in neighboring molecules between chlorine atom and carbon in aromatic ring and between carbon atoms.

Since the arrangements of molecules in a crystal always correspond to the potential energy minima of the system, it can be used for the verification of the obtained data in LS experiments combined with the modeling procedure. This approach allowed to reveal the structure and characteristics of the mutual arrangement in molecules as well as the types of intermolecular contacts in the liquid phase [18, 19].

For the determination of intermolecular arrangement in liquid *di*-chlorobenzene and other compounds with chlorine atoms, the Cambridge crystallographic database for these organics in solid state were used. The intermolecular distances between molecules, obtained by MLS study, are 4–5 Å, which is considerably longer than the internal distances in molecules. However, the liquid system has a quite certain spatial geometry realized by this long-ranged molecular binding.

X-ray studies allow to define the internal parameters of molecules, but the intermolecular structure of organic liquids stays in an ambiguous conclusion: on one hand, in the liquid phase the distances between nearest molecules are longer than 4 Å, and on the other hand, the peak of distribution function was observed in dichloromethane at distance about 2 Å. However, in these experiments it was confirmed that the organic liquids have certain intermolecular composition and labile binding between identical molecules. However, this does not exclude the spatial transformation in liquid bulk [20–22].

#### **2.4 IR-data source**

The IR spectra of organic liquids in the middle- and high-frequency regions have "additional" bands [23] that cannot be expected by the normal coordinates' analysis [24–26]. The observed spectral phenomenon was interpreted in literature as a manifestation of the vibration anharmonicity or Fermi resonance (ex. [27, 28]). Such a version has a few contradictories; firstly, it is non-logic to mix the approach using the vibrational theory based on the classical mechanics of vibration (normal coordinates' analysis) and the quantum mechanical interpretation; secondly, many systems that are considered in this conception include heavy molecular weight atoms and their vibration anharmonicity is negligible; thirdly, the overtones and combination bands should have considerably lower intensities than the basic bands. At last, these bands remain in the solid state (in a low-temperature spectra), and they are observed for homological analogs [20]. We have suggested another variant of assignment based on the conception observing spectral features relating to the manifestation of specific interaction in liquids.

In the spectra of liquid CCl4, in accordance with the selection rules for the Td symmetry in IR spectra, only one stretching band should be active, but for the C3V symmetry, two bands of E– and A1–symmetry species are permitted. In the IR spectrum of liquid carbon tetrachloride two overlapping bands at 786 and 761 cm−1 having an approximately equal intensity were observed. These bands are well-resolved in IR spectrum of the low-temperature film recorded at 20 K [29].

In the gas phase two bands were also observed (at 795 and 776 cm−1), but they have a different counter structure: the first of them consists of overlapped components and another one is a single band. Therefore, we can conclude that carbon tetrachloride exists in gas phase not only in the single molecular shape that has Td symmetry, but also in the transformed shape having C3V symmetry. The first one is manifested by the band at 776 cm−1 and another one – by the band at 795 cm−1. The spectra recorded for different temperatures of gaseous CCl4 confirm this assumption [29].

It is reasonable to resume that the pyramidal structure relates to the cluster shape, in which the chlorine atom provides the binding between molecules. The shift of the chlorine atom in the cluster can occur in the condensed phase owing to the association of the molecules [29]. It leads to transformation of the molecular geometry to the almost planar D3h symmetry, in which A1 stretching band is forbidden in IR spectra.

In the high-frequency region of chloroform in the liquid phase, the bands at 3020 cm−1 (stretching vibration of the CH bond) and at 2401 cm−1 with a shoulder at 2435 cm−1 are detected (**Figure 2**, left side, spectrum 2) [30]. A similar spectrum is observed for bromoform: the bands at 3021 и 2256 cm−1 are shown (**Figure 2**, left side, spectrum 1). The bands in 2400–2200 cm−1 range cannot be assigned to overtones 2δ (bending of angle CHalH, where Hal-halogen), because the band have the isotopic D/H shift (**Figure 2**, right side), corresponding to the theoretical values (1.32–1.34) and their intensities relative to CH stretching band at 3020–3021 cm−1 are considerably stronger than it could be expected for anharmonic components. Besides, in bromoform molecule, the contribution of anharmonic components should decrease due to a significant increase of molecule mass. However, the opposite picture is observed in the spectra: the relative intensity of the band at 2256 cm−1 for bromoform is stronger than the band at 2401 cm−1 of chloroform [30].

The similar spectral picture is detected for water associates in liquid phase: the band of OH stretching vibration in 3400–3600 cm−1 region is combined with the band in 2200–2400 cm−1 range, assigned to stretching vibration of hydronium ion. This band manifests the hydrogen atom transfer in hydrogen bonded structures [31, 32].

The presented results indicate that the specific interaction between molecules exists in liquid haloforms due to the proton binding and its intermolecular shift, leading to the transformation of initial geometry.

The geometry of single benzene molecule is taken as a planar ring due to the conjugation of pz-orbitals and π-aromatic configuration appearance. This state corresponds to D6h symmetry point group. According to selection rules, only one stretching CH band (E-specie) should be active in IR spectra. However, in the real spectra of liquid benzene there are three bands (3092, 3071, 3036 cm−1) and in the

#### **Figure 2.**

*Fragments of FTIR spectra of chloroform and bromoform (left side) and chloroform-d in liquid phase (right side).*

#### *The Formation Mechanism and Structure of Organic Liquids in the DFT Challenges DOI: http://dx.doi.org/10.5772/intechopen.100429*

solid benzene (spectra were recorded at 20 K) there are four bands (3090, 3071, 3034, 3005 cm−1) (**Figure 3**, right side) in CH stretching region [33].

Three bands that have isotopic H/D shift (**Figure 3**, left side) close to the theoretical prediction also were detected in benzene-d6 spectrum. This spectral picture can be assigned to the existence of two molecular shapes existing in the liquid phase: planar, in which one IR band is active (D6h symmetry), and shape with two IR bands (A1- and E-species), corresponding to the C3V symmetry.

For the interpretation of the observed effect, we can assume that the benzene molecule exists in the liquid state as a cluster system. In this shape CH bonds deviate from the ring plane to the neighboring molecule (**Figure 4**). This leads to a distortion of the symmetry of the stretching vibrations of the CH bond.

In the middle IR range two bands at 1952, 1814 cm−1 were revealed (**Figure 5**, bands A), which cannot be assigned to the internal vibration modes. They also cannot be assigned to combination modes, because these bands have an isotopic shift close to the CH bond vibrations of aromatic ring, and the same bands are observed in substituted homologs of benzene as well (**Figure 5**). Besides, their amount in C6H5X spectra corresponds to the non-equivalent *o-, m-* and *p-*position in molecules [23]. These data confirm the idea that the benzene can form the π-stacks structure in the liquid phase (see section 2.3). The hydrogen atom in this system, as it was mentioned above, can interact with the carbon atom of the neighboring molecule in the stacks. Therefore, CH stretching band can shift to the middle IR interval due to the mixing of ν(C–C) and ν(C–H) stretching vibrations in the intermolecular bond (C–C–H‧‧‧C–C).

#### **Figure 3.**

*Fragment of liquid benzene-d6 FTIR spectrum at 295 K (left side) and solid benzene film at 20 K (right side) in CH/CD stretching region.* 

**Figure 4.** *Hydrogen atom shift in molecular stacks of liquid benzene.*

**Figure 5.** *Fragment of liquid benzene (left side) and benzene-d6 (right side) FTIR spectrum in middle IR region.*

#### **2.5 Dynamic simulation data source**

This approach was developed in numerous works and their complete citation was beyond the scope of this study. Therefore, we would like to highlight the studies that present the molecular arrangement of organic systems as a supramolecular structure, forming not only under hydrogen bond, but also existing in the shape of identical molecular associates [34–37].

The obtained data allow to discuss the thermodynamic and kinetic aspects of the processes in liquid phase, in terms of the supramolecular organization. These investigations give the knowledge about the composition of the aggregates as well as the general approach to their transformation due to the intermolecular binding. In author's opinion, the dynamic simulation model combined with the experimental findings allows to explain the supramolecular formation mechanism under the long-ranged interactions. The model of the homogeneous molecules' association is a key to manifold conclusions of the liquid properties' nature [37].

This approach gives the complementary information about the structure of aggregates, which is not possible by other methods, especially when long-ranged molecular binding is present. The application of this method is useful to interpret some regularities of organic liquids, although it does not detail the specific interactions' appearance [34, 37].

#### **2.6 Section conclusion**

As we have shown in the short review of the modern approaches to the problem of molecular arrangement in organic liquids, all data sources agree that the structure of liquid phase can be described as a supramolecular system with non-polar or weak polar long-ranged interactions. This binding arises at the distances close to 4–5 Å, but its energy contribution is comparable with vaporization enthalpy of liquids [7, 17, 18, 20]. The thermochemical, wave scattering, X-Ray and dynamic simulation data interpret well the lability of the system, but do not explain its high stability.

Unlike the mentioned methods, IR data predict that the initial molecular geometry transformation in comparison to a single molecule occurs in the liquid system. These changes are caused by the intermolecular forces between neighbors in bulk, even if they do not correspond to the hydrogen bond [23, 29, 30].

Applied to this problem, the DFT calculation could be useful for explaining the reasons of the transformation under weak interactions in terms of general electronic distribution. Therefore, in the following section we consider the essential results of the DFT study referring to the problems discussed in this paper.

*The Formation Mechanism and Structure of Organic Liquids in the DFT Challenges DOI: http://dx.doi.org/10.5772/intechopen.100429*
