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Influence of the Structure of Azole and Aliphatic Alcohol Molecules on the Effectiveness of Van der Waals Interactions with Carbon Tetrachloride

N. L. Lavrik*
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Corresponding author (Address):
N. L. Lavrik, V.V. Voevodsky Institute of Chemical Kinetics and Combustion, Siberian Branch of Russian Academy of Science, Novosibirsk, Russian Federation, E-mail: lavrik@kinetics.nsc.ru

Received Date: May 20, 2025 Accepted Date: June 20, 2025 Published Date: June 23, 2025

doi:10.17303/jocs.2025.3.103

Citation: N. L. Lavrik (2025) Influence of the Structure of Azole and Aliphatic Alcohol Molecules on the Effectiveness of Van der Waals Interactions with Carbon Tetrachloride. J Org Chem Chem Sci 3: 1-5

Annotation

The literature data on the frequencies of N-H vibrations of azole molecules and O-H vibrations of aliphatic alcohol molecules in the gas phase and in the CCl4 medium are analyzed. It is shown that the relative spectral shifts β of the considered molecules in a condensed medium can vary by a factor of 3 depending on their structure. The difference in the values of β is explained by the difference in the efficiency of the Van der Waals interactions VVI between the CCl4 molecule and the solute molecule. The effectiveness of the VVI is determined by the action of orientation, dispersion and induction forces. The action of these forces affects the structure of the electron shell of the solute molecule. The deformation of the electron shell of this molecule leads to a change in the force constants of the chemical bonds of the molecule and, accordingly, to a shift in the vibrational frequencies of these bonds. The degree of deformation of the electron shell of a solute molecule depends on its polarizability α and the magnitude of the dipole moment d. Thus, the established difference in the values of β is a consequence of the individuality of the values of α and d of the solute molecules. It was found that fluorination of alcohol molecules reduces the effect of VVI, which is explained by the increase in the size of alcohol molecules during fluorination.

Keywords: Carbon Tetrachloride; Frequency of NH; OH Vibrations of Azole and Alcohol Molecules, Van Der Waals Interaction

Introduction

In vibrational spectroscopy of liquid solutions, carbon tetrachloride CCl4 is widely used in the study of universal and specific interactions as a standard neutral solvent [1,2]. Apparently, the use of the CCl4 molecule as a neutral solvent is due to the fact that the dipole moment of this molecule is dCCl4 = 0. However, the partial dipole moment of the C-Cl bond dpCCl4 is 1.87 D [2]. The presence of a dipole moment indicates that there is an electric charge on the chlorine atom of the C-Cl bond. As a result, the CCL4 molecule, which has side chlorine atoms, creates a local electric field around itself, which, due to the Van der Waals interactions VVI, is able to influence the electronic shell of the solute molecule. The deformation of the electron shell of a molecule can lead to a change in the force constants of the chemical bonds of this molecule and, accordingly, to a shift in the vibrational frequencies of these bonds. VVI is caused by the electrostatic interaction, which has three components: orientation Ed-d (1), induction Eind (2) and dispersion Edisp (3) [2].

where di, dCCl4 are the dipole moment of the i-th solute molecule, the partial dipole moment of the C-Cl bond in the CCl4 molecule; k is the Boltzmann constant; T is the absolute temperature; αi is the polarizability of the solute molecule; r is the distance between the dipoles of different molecules; I is the ionization potential of the i-th molecule. Thus, the change of the frequency of the chemical bond of a molecule of a substance dissolved in CCl4 may depend on the individual electronic structure of the i-th molecule, and, accordingly, on the magnitude of the polarizability i and the intrinsic dipole moment di. The mechanism of shifts in vibrational frequencies as a result of the explosive force of a solute molecule with a dipole moment of a solvent molecule was discussed in detail earlier in [1-4].

Despite the long history of work on the spectroscopy of intermolecular interactions, in which CCl4 was widely used as a neutral solvent, there is no quantitative systematic information on the effect of the CCl4 medium itself on the change in the vibrational frequencies of solute molecules relative to the vibrational frequencies of molecules in the gas phase. A priori, it can be assumed that since solute molecules have individual electronic structures, the magnitudes of changes in shift frequencies that are directly related to the structure in the CCl4 medium will differ for different molecules.

The purpose of this report was to compare the previously obtained frequencies of valence N-H vibrations of azole molecules and O-H vibrations of aliphatic alcohols in the CCl4 medium and the frequencies of vibrations of these molecules in the gas phase (IR spectra). The choice of the azole and alcohol molecules considered was due to the availability of data on the frequencies of N-H and O-H vibrations of these molecules in the CCl4 medium and in the gas, since we did not have such information for other compounds. The study of the effect of VVI on the vibrational spectra of molecules of selected classes is of interest, since alcohols are widely used in organic chemistry as reagents and solvents [5], and molecules of the azole class are currently one of the most sought-after compounds in the synthesis of pharmaceutical products [6].

The effect of changes in the frequencies of N-H and O-H vibrations in the CCl4 medium relative to the gas phase was studied in relative units. The value i, determined by (4), was taken as the value of the relative frequency shift of these vibrations.

In (4) gas,i, CCl4,i frequency N-H, O-H vibrations of the i-th molecule azole or alcohol in the gas and in CCl4. Thus, the value i was a descriptor of the effectiveness of the VVI effect on the frequencies of N-H and O-H vibrations in molecules of azoles and alcohols.

Results and Discussion

Table 1 shows the frequencies of N-H vibrations of azole and alcohol molecules in the gas gas,i, in CCl4 CCl4,i and the relative shifts of the frequencies of N-H and O-H vibrations of the i-th molecule in CCl4 i. As can be seen from the above data, each i-th molecule of azole or alcohol has an individual value of i The difference in values of i is more than 3 times. The difference in the values of i confirms the position that the effectiveness of the van der Waals interaction of the CCl4 molecule with the electronic structure of azole and alcohol molecules individually affects the frequency of N-H and O-H vibrations of these molecules.

From the data in the Tab.1 it follows that fluorination of alcohols leads to a decrease in the effectiveness of VVI: 13 < , 12 < 11. During fluorination, a hydrogen atom (radius 0.116 nm) is replaced by a fluorine atom (radius 0.14 nm) [16]. As a result, the Onsager distance H...Cl 0.297 nm becomes 0.326 nm for the distance F...Cl [16]. According to (1) - (3), an increase in the value of r should lead to a decrease in the value of VVI, which is observed in the experiment. Thus, the decrease in the values of i during fluorination can be explained by an increase in the intermolecular distance between the dipoles of solute molecules and the partial dipole moment of the C-Cl bond of the CCl4 molecule.

The results obtained on the difference in the relative shift frequencies of the solute molecules in CCl4 relative to the gas mean that correlations such as "spectral parameters of a molecule in a CCl4 medium – enthalpy of H-bond formation" [17-19] are not entirely correct. The approach is not correct, which is based on the fact that the data of the spectral frequency shift of the A-H vibration of the proton donor molecule in the H complex A-H…B relative to the frequency in CCl4 are compared with the total value of the enthalpy of formation of the H bond obtained thermochemically. In this approach, part of the shift due to the VVI, which also determines the total frequency shift of the A-H vibration of the proton donor molecule in the H-complex, is ignored. The correlation "Spectral parameters of a molecule in a gas – enthalpy of H-bond formation" should be considered more correct.

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Tables at a glance

CommentsTable 1