Gas chromatographic measurements of the retention times of alkyl naphthalenes on packed columns with polar and non-polar stationary phases have proven that the logarithm of the relative retention time increases bi-linearly (not linearly) with the number of carbon atoms in a molecule. This is caused by a strong inclination of alkyl side chains toward intramolecular cyclization. A FTIR spectral analysis has shown that longer alkyl side chains of alkyl naphthalenes are cyclized through an interaction between the terminal CH<sub>3</sub> group and the aromatic ring. Conventional aromatic-aliphatic molecules thus become new molecules with quasi-alicyclic rings. This, however, alters the effect of non-covalent van der Waals attractive forces both inside and outside the molecules, which is reflected in an exponential increase of the retention times of alkyl naphthalenes with a side chain longer than propyl and in the bi-linearity of the logarithmic dependence of the relative retention times on the number of carbons in the molecule.
Classic considerations on the dependence of retention times (Rt,rel) on the number of carbons in a molecule (z) in different homologous series of gas-chromatographically separated substances always focus on purely linear dependence without any divergence, be it for isotherm or non-isotherm separations. However, the bi-linearity detected in the dependence log Rt,rel = az + b [
In the mentioned studies [
The work [
In the case of the formed aromatic molecules with quasi-alicyclic rings, the effect of van der Waals forces thus increases not only intramolecularly but also intermolecularly. In the case of n-alkyl phenols and n-alkyl benzenes, this effect is strong with the molecules having the number of carbons higher than nine. This results in bi-linearity in the retention characteristics of these compounds, observed in the dependence of the logarithm of the relative retention time on the total number of carbons in the molecule. Yet it is not clear whether the cyclization of the side chains of alkyl phenols and alkyl benzenes is their inherent feature or it occurs only during their motion through the column.
The mentioned phenomena can be evaluated and compared using methods of computational chemistry. In order to assess cyclization, association and different interactions, one can compute the energies of the covalent bonds and non-covalent, mainly van der Waals, interactions. For the purpose of this assessment, one can also estimate molecular conformations which are realistic and energetically advantageous or possible and are in correlation with the ascertained chromatographic data. In our case, on the basis of experience with the evaluation of aromatic, aromatic-aliphatic, aromatic-alicyclic and phenolic structures in terms of energy using the methods of molecular mechanics [
In this work, the question of the cyclization of the alkyl side chains as an inherent property of aromatic-aliphatic molecules is resolved on the basis of the FTIR spectra interpretation of n-alkyl phenols as model compounds.
Another aim of the presented work is to explain the bi-linearity in the logarithmic dependence of the relative retention times on the number of carbons in the molecule in the case of n-alkyl naphthalenes by molecular mechanics modeling and to clarify the behavior of n-alkyl naphthalenes in the environment of different stationary phases of packed GC columns. For the calculations, the potential energies of the bonds and non-covalent interactions have been considered.
For the FTIR spectral study, 2-n-ethyl phenol and 2-n-butyl phenol (analytical grade, Sigma Aldrich) were used. For the GC measurements of n-alkyl naphthalenes, (a) 1-methyl-, 1-ethyl-, 1-n-propyl-, 1-n-butyl- and 1-n-hexyl- naphthalenes, (b) 2-methyl-, 2-ethyl-, 2-n-propyl-, 2-n-butyl- and 2-n-pentyl-naphthalenes (GC grade, Lach-Ner SRO, Czech Rep.) were utilized. Where necessary, naphthalenes were diluted with isooctane.
Gas chromatographic measurements were carried out on packed columns (1.2 m, an internal diameter of 2 mm) with stationary phases of 2.5% liquid crystals BMBT on 100/120 Chrom W-HP (Alltech Associates) (polar phase [
Samples of 2-ethyl phenol and 2-n-butyl phenol were measured using the capillary thin film method while liquid samples were placed between two NaCl windows. FTIR spectra were acquired on a FTIR spectrometer Nicolet 250 (Nicolet Instruments Co., USA). A KBr beam splitter and a DTGS detector were used. 132 scans were collected for each measurement over the spectral range of 400 - 4000 cm−1 with a resolution of 4 cm−1; for apodization, a Happ-Genzel function was used. The spectra were processed using Omnic 6.1 software (Nicolet Instruments Co., USA).
Calculations were done with molecular mechanics methods [
- bond stretching (Ebond), which is associated with the deformation of a bond from its standard equilibrium length;
- bond angle bending (Eangle), which is associated with the deformation of an angle from its normal value;
- stretch-bend (Estretch-bend)—the bond-stretching and angle-bending cross term, which includes coupling between the bond stretching and angle bending;
- dihedrals (Edihedral)—torsional energy, which is associated with the tendency of dihedral angles to have a certain n-fold symmetry and to have minimum energy;
- van der Waals (EvdWaals), which describes the repulsive forces keeping two non-bonded atoms apart at close range and the attractive forces drawing them together at long range;
- electrostatic (Eelst), which describes non-bonded electrostatic interactions, particularly dipole-dipole interations.
These energetic terms were calculated using both the MM+ and AMBER methods, with the exception of Estretch-bend, which was calculated only by means of the MM+ method.
The mentioned potential energies of the covalent bonds and non-covalent interactions were calculated for common (conventional) n-alkyl naphthalenes as well as for the models of the cyclized forms of these compounds. The concepts of the cyclized forms were formulated on the basis of the study of the distribution of electron densities (atomic charges in a molecule, [
The following considerations presented are based on gas chromatographic measurements on packed columns with both polar and nonpolar stationary phases. The measurements of the relative retention times (Rt,rel) of n-alkyl naphthalenes C11 - C16 on these columns showed that the logarithms of these retention times increase bi-linearly with the number of carbon atoms in a molecule (z) (Figures 3-6). Two linear areas in the consecutive intervals C11 - C13 and C13 - C16, with the slopes of each line being different, were proven in the relation log Rt,rel = az + b (where a and b are constants), although only one line had been anticipated to be found. The complete dependence was thus of a bi-linear character. The turning point in the bi-linearity was always observed when the propyl was a substituent. The same phenomenon was observed with n-alkyl benzenes and n-alkyl phenols [
It arises from Figures 3-6 that the chromatographic behavior of the alkyl naphthalenes studied is independent of the polarity of the stationary phase, because the polar and nonpolar phases provided the same bi-linear dependence. This implies that the significant increase in the retention times of alkyl naphthalenes with a chain longer than propyl is rather connected with the physical properties of the molecules than with their chemical structure. Therefore, the potential energies of the bonds and of non-covalent, namely van der Waals, interactions were computed and compared. It was also taken into account that the considered cyclization leads to a decrease in molecular size, which may facilitate the mixing of alkyl naphthalene molecules with the stationary phase by reducing the enthalpy of mixing (ΔHmix), in an ideal case even to the thermodynamic condition ΔHmix = 0 (Consequently, ΔGmix = ΔHmix – TΔSmix < 0, when the mixing and hence also the solubility of alkyl naphthalenes in the stationary phase reach their maximum). The longer a side chain is, the more significantly its cyclization contributes to mixing. The stationary phase-cyclized alkyl naphthalene system is thermodynamically more stable, which may be a reason for the preference of cyclized structures over conventional ones.
The computed potential energies of covalent bonds and van der Waals interactions between two non-bonded atoms in molecules of 1-n-alkyl naphthalenes and 2-n-alkyl naphthalenes have been summarized in Tables 1-4. The energies of electrostatic, specifically dipole-dipole, interactions were always zero as expected. The energies of the bonds and interactions were calculated for conventional molecules and molecules cyclized into the ortho position with respect to the alkyl substituent. The cyclization was considered to have occurred on the terminal methyl group.
With conventional molecules of the alkyl naphthalenes studied, neither an increase in the number of carbons nor the growing length of the alkyl chain led to a significant change in potential energies. Minor changes were recorded in Ebond and EvdWaals values; all the other values remained constant or almost constant. With cyclized molecules, on the other hand, the values of all potential energies and interactions with the exception of Eelst
1-n-alkyl naphtalenes | Ebond | Eangle | Estretch-bend | Edihedral | EvdWaals | Eelst |
---|---|---|---|---|---|---|
(kJ/mol) | ||||||
Naphtalene | 6.78 | 0.00 | 0.00 | −64.48 | 26.63 | 0 |
Common: Methyl naphtalene Ethyl naphtalene n-Propyl naphtalene n-Butyl naphtalene n-Pentyl naphtalene n-Hexyl naphthalene | 9.13 11.01 12.90 14.82 16.71 18.63 | 0.25 0.25 0.25 0.25 0.25 0.25 | −0.13 −0.13 −0.13 −0.13 −0.13 −0.08 | −64.19 −64.77 −64.77 −64.77 −64.77 −64.77 | 36.09 60.75 56.06 66.57 69.76 72.94 | 0 0 0 0 0 0 |
Cyclized on CH3: Methyl naphtalene Ethyl naphtalene Propyl naphtalene Butyl naphtalene Pentyl naphtalene Hexyl naphthalene | 9.71 30.57 14.61 14.24 16.37 18.05 | 949.23 137.67 16.20 1.30 2.22 0.75 | −8.00 −0.42 −0.33 −0.17 −0.08 −0.21 | 126.03 −51.58 −48.11 −62.47 −47.15 −34.79 | 23.03 22.07 28.60 56.86 117.03 514.46 | 0 0 0 0 0 0 |
1-n-alkyl naphtalenes | Ebond | Eangle | Edihedral | EvdWaals | Eelst |
---|---|---|---|---|---|
(kJ/mol) | |||||
Naphtalene | 0.00 | 0.00 | 0.00 | 24.54 | 0 |
Common: Methyl naphtalene Ethyl naphtalene n-Propyl naphtalene n-Butyl naphtalene n-Pentyl naphtalene n-Hexyl naphthalene | 0.13 0.38 0.63 0.88 1.13 1.42 | 0.00 1.63 1.63 1.63 1.63 1.63 | 0.00 0.00 0.00 0.00 0.00 0.00 | 31.74 52.97 52.04 53.84 54.60 55.39 | 0 0 0 0 0 0 |
Cyclized on CH3: Methyl naphtalene Ethyl naphtalene Propyl naphtalene Butyl naphtalene Pentyl naphtalene Hexyl naphthalene | 1.42 2.18 3.85 1.09 1.30 1.72 | 626.21 348.90 40.74 2.68 4.23 4.65 | 58.41 10.68 19.18 1.80 14.99 28.30 | 23.24 23.41 23.24 46.22 156.09 6188.39 | 0 0 0 0 0 0 |
2-n-alkyl naphtalenes | Ebond | Eangle | Estretch-bend | Edihedral | EvdWaals | Eelst |
---|---|---|---|---|---|---|
(kJ/mol) | ||||||
Naphtalene | 6.78 | 0.00 | 0.00 | 24.64 | 26.63 | 0 |
Common: Methyl naphtalene Ethyl naphtalene n-Propyl naphtalene n-Butyl naphtalene n-Pentyl naphtalene n-Hexyl naphthalene | 9.13 11.01 12.90 14.82 16.71 18.63 | 0.25 0.25 0.25 0.25 0.25 0.25 | −0.13 −0.13 −0.13 −0.13 −0.13 −0.13 | −67.49 −68.08 −68.08 −68.08 −68.08 −68.08 | 28.93 53.80 56.82 59.79 62.97 66.15 | 0 0 0 0 0 0 |
Cyclized on CH3: Methyl naphtalene Ethyl naphtalene Propyl naphtalene Butyl naphtalene Pentyl naphtalene Hexyl naphthalene | 8.67 30.98 14.65 14.28 16.12 18.00 | 955.35 137.17 16.20 1.26 2.01 1.42 | −7.49 −0.59 −0.33 −0.17 −0.04 −0.04 | 121.59 −54.89 −51.42 −65.74 −51.33 −48.49 | 24.03 24.62 28.01 41.58 78.00 96.59 | 0 0 0 0 0 0 |
2-n-alkyl naphtalenes | Ebond | Eangle | Edihedral | EvdWaals | Eelst |
---|---|---|---|---|---|
(kJ/mol) | |||||
Naphtalene | 0.00 | 0.00 | 0.00 | 24.54 | 0 |
Common: Methyl naphtalene Ethyl naphtalene n-Propyl naphtalene n-Butyl naphtalene n-Pentyl naphtalene n-Hexyl naphthalene | 0.13 0.38 0.63 0.88 1.13 1.42 | 0.00 1.63 1.63 1.63 1.63 1.63 | 0.00 0.00 0.00 0.00 0.00 0.00 | 25.71 47.19 47.61 48.19 48.95 49.78 | 0 0 0 0 0 0 |
Cyclized on CH3: Methyl naphtalene Ethyl naphtalene Propyl naphtalene Butyl naphtalene Pentyl naphtalene Hexyl naphthalene | - 19.18 3.85 1.00 1.84 1.72 | 620.35 347.86 40.61 2.68 5.78 6.24 | 57.53 10.68 19.13 1.80 12.31 14.57 | 24.12 24.79 22.69 28.47 72.31 107.52 | 0 0 0 0 0 0 |
changed. Considerably different Eangle values were exhibited by cyclized methyl- and ethyl naphthalenes, which is connected with a significant strain in three- and four-membered hypothetical quasi-alicyclic rings. Since such cyclization reactions do not occur in reality, these values do not have to be taken into account. In the cases of 1- and 2-methyl naphthalenes, the Edihedral values calculated using the MM+ method were also substantially different (
The increase of van der Waals forces acting inside a molecule must necessarily be reflected also intermolecularly. Between the cyclized molecules, there are hence attractive forces, which increase with the length of an alkyl chain and whose effect in the case of quasi-alicyclic rings from butyl, pentyl and hexyl substituents is strong. This leads to an exponential increase in retention times with the length of the alkyl chain and to the bi-linearity in the logarithmic dependence of the relative retention times with a steeper connecting line of log Rt,rel for butyl, pentyl and hexyl substituents.
A key issue in the study of the cyclization of side chains is whether it occurs during their motion through the dense medium of the chromatographic column or it is an inherent feature of alkyl chains attached to an aromatic structure.
The possibility of the cyclization of alkyl side chains as an inherent feature of the chains was studied with n-alkyl phenols, whose infrared spectra should reflect well the interactions of these chains with the aromatic ring. As mentioned, the interaction should take place on the terminal CH3 group [
The methyl-group interaction being considered is also supported by a shift of the absorption band of the stretching vibration of C=C in the aromatic ring observed in the region of 1680 - 1530 cm−1 (
Another difference was observed in the region of the deformation vibrations of C=C-C in the plane of the aromatic ring of 680 - 505 cm−1. For 2-n-butyl phenol, a band was discovered here at 543 cm−1 (
In addition, dilution experiments were performed with the aim to prove that the interaction in question is intramolecular. The relative intensity of the absorption bands in the spectral region of 3150 - 2900 cm−1 (i.e. in the region of the symmetric and asymmetric stretching vibrations of C-H in CH3 and CH2 groups attached to an aromatic ring) was monitored in dependence on the dilution of alkyl phenol by carbon disulfide. The spectra of the solutions were studied in measuring cells of 0.1 and 1 mm. In the case of neither 2-ethyl phenol nor 2-n-butyl phenol did the spectra obtained show any significant changes after the samples were diluted. It can thus be clearly concluded that the interaction of a butyl chain with an aromatic ring is intramolecular; otherwise, the relative intensity of the studied absorption bands would have changed with increasing dilution. The cyclization of alkyl side chains by means of an intramolecular reaction of the terminal methyl group with an aromatic ring hence seems to be an inherent feature of alkyl-substituted aromatic compounds.
In summary, intramolecular cyclization is accompanied by a decrease in the effective size of molecules, which is significant for C13 and larger molecules. Aromatic quasi-alicyclic molecules of a reduced size are more easily mixed with the dense stationary phase, and the formed system is, in comparison with a system with common aromatic-aliphatic molecules, more homogenous and thus thermodynamically more stable. The change in the size of the molecules is then accompanied by changes in van der Waals interactions and subsequently by a change in the relative retention time, because an increase in the van der Waals interactions inside the cyclized molecules
must also be reflected in an increase of these forces between the molecules as a more intense effect of the attraction forces between the molecules.
The alkyl side chains of n-alkyl naphthalenes become cyclized, as demonstrated using FTIR spectroscopy. Cyclization is made possible because of the intramolecular interaction between the aromatic ring (−δ and a hydrogen of the terminal methyl group (+δ of the alkyl chain. With the formed aromatic molecules with a quasi-alicyclic ring, the effect of the van der Waals forces increases not only intramolecularly but also intermolecularly. In the case of n-alkyl naphthalenes, this effect is strong with molecules having the number of carbons in the molecule higher than thirteen. This results in bi-linearity in the retention characteristics of these compounds, observed in the dependence of the logarithm of the relative retention time on the total number of carbons in the molecule in both the polar and nonpolar stationary phases of packed columns. The role of van der Waals forces was demonstrated using the potentials energies of the covalent bonds and non-covalent interactions for 1-n-alkyl naphthalenes and 2-n-alkyl naphthalenes, both common and cyclized into the ortho position with respect to the substituent.
This work was carried out thanks to the support of the long-term project for the conceptual development of the research organization No. 67985891.