Aromatic systems like phenol, diphenol, cyano benzene, chloro benzene, aniline etc shows effective π-π stacking interactions, long range van der Waals forces; ion- π interactions etc. and these forces of interactions play an crucial role in the stability of stacked π-dimeric system. On the other hand, substituents and conformational change in the stacked dimmers of aromatic system may also change the stability of different stacked dimers. In this current study, stacked phenolic dimmers (both phenol and diphenol) have been taken for investigation of the stacking π-π interaction. But, the stacking interactions are also greatly affected by the conformational change with internal rotation ( i.e. dihedral angle, φ) between the stacked dimers. It is generally accepted that larger basis sets are required for the highly accurate calculation of interaction energies for any stacked aromatic models. But, it has recently been reported that M062X/6-311++G(d,p) basis set is effectively better than that of B3LYP/6-311++G(d,p) for determining the interaction energies for any kind of long range interaction in aromatic systems. Therefore, all the calculations were carried out by using M062X/6-311++G(d,p) basis set. However, in most of the cases the calculated π-π stacking interaction energies show almost same result for both DFT and ab initio methods.
Non covalent interactions in phenolic dimers refer π-π staking interaction, which is basically an attractive interactive force between the aromatic rings. Just like an electrostatic interaction, where a region of negative charge interacts with a positive charge, the electron rich π-system can interact with a metal (cationic or neutral) or an anion with π-system [
In this current research work, we have performed a comprehensive study on the π-π stacking interaction energy of phenolic systems by using DFT methods. During the last few years, there has been a significant progress in the method of calculating interaction energies in many aromatic complexes [
In this current research comprehensive computational investigations have been carried out for determining the actual π-π stacking interactions of different stacked dimers of phenolic systems such as phenol-phenol, diphenol-diphenol and phenol-diphenol models. The π-π stacking interactions in the stacked models of phenolic dimers directly effect on the proper molecular geometry, atomic charge distribution, spin densities and intermolecular rotation or dihedral angles of the stacked models [
Density functional Method (DFT) of calculation found to be very effective for studying non-bonded long range interactions in aromatic system. Nowadays, these methods have become an important tool for calculating all kinds of π-π stacking interaction. The Gaussian09 program has been widely used for DFT method to calculate the interaction energies of the molecular systems. The use of DFT is known for the applicability in medium and large molecular system. It is limited in system where dispersion part is considered as the dominant part and in that case the calculated interaction energy values are always under estimated. On the other hand, if a reasonably larger basic set is used then these method of calculations account well for determining the interaction energies as well as the electronic correlation energy of the molecules in the gas phase. The commonly used B3LYP method fails to predict dispersion energy, therefore in such cases inclusion of dispersion energy is very difficult for a large system. All the electron correlation energies for phenol and diphenol system have been calculated by DFT level of theory. All the geometrics of the studied molecules were optimized by using M062X/6-311++G(d,p), basic sets. All single point calculation were carried out by using M062X, B3LYP with 6-311++G(d,p) basis set. But, the minimized stacked structures were obtained by M062X method which shows the most favoured geometry. All the calculations were performed with the Gaussian 09 software Package and the visualization was done by GaussView5.0 [
The interaction energy for the stacked geometry can directly be calculated by the following equation:
Interactionenergies = E st − 2 × E us
Here, Est = energy of stacked model, Eus = energy of unstacked model.
Phenols are generally packed with its neighbours through weak vander waals forces to form dimer, trimer tetramer etc. Generally in normal conditions, one phenol ring could directly stack parallelly with other phenol ring by different ways either in eclipsed or staggered conformation. The eclipsed conformation of stacked phenolic dimers show exact sandwich form at a dihedral angle of 0˚. On the other hand, the staggered conformation for stacked phenolic dimers may also suitably stacked at different dihedral angles, viz. 60˚, 120˚ and 180˚. The individual models in a stacked conformation of stacked phenolic dimers are separated at a fixed vertical separation of 3.6 Å, which is found to be the most favoured separation between two phenol rings. The −OH group of the phenol ring also plays an important role in the stability of stacking interaction in the stacked phenolic dimers. Therefore, during the construction of stacked models, −OH group of the two phenol rings may be placed either in same or opposite direction. Here, we have constructed the stacked models to study the stacking interactions of following phenolic systems:
In this investigation, the π-π stacking interactions of phenolic dimers such as Phenol-Phenol, Diphenol-Diphenol and Phenol-Diphenol, have been studied for different conformations in gas phase. All the phenolic systems were optimized by using DFT method with M062X/6-311++G(d,p) basis set (
Same procedure was carried out to construct the stacked models for Diphenol-Diphenol and Phenol-Diphenol stacking. We have carried out all computation calculations by DFT method using B3LPY/6-311++G(d,p) and
M062X/6-311++G(d,p) basis set, but M062X is more reliable to calculate stacking interaction energy values as it gives more negative values. During the construction of stacked phenol-phenol model, one phenol ring has been horizontally shifting along X, Y or Z-axis (from positive to negative end), keeping the other ring at constant position. This process was carried out to get the most favored minimized stacked model with minimum repulsion as well as the highly repulsive stacked model. In this case, the horizontal shifting for the stacked model was investigated along X-axis from −3 to +3 Å. Similarly, all the staggered conformations for the stacked models of phenol-phenol stacking systems were also prepared with dihedral angles 60˚, 120˚ and 180˚ respectively. Same procedure was carried out to construct the stacked models for Diphenol-Diphenol and Phenol-Diphenol stacking. We have carried out all computation calculations by DFT method using B3LPY/6-311++G(d,p) and M062X/6-311++G(d,p) basis set, but M062X is more reliable to calculate stacking interaction energy values as it gives more negative values.
The direction of −OH group of the phenolic ring in a stacked model always makes a significant impact on the overall stability of any stacked phenolic system. The −OH groups of a phenolic stacked model, with two phenolic rings may be either facing in same direction or opposite direction as shown in
The relative changes for the π-π stacking interaction energies in gas phase, with M062X methods for different stacked phenolic systems are shown in
or minima of the interaction energy plots results the more stable stacked model. Here, we can clearly see the differences in stacking interaction energies of phenolic systems with different dihedral angle as well as change in the direction of ?OH groups of the stacked models (
minimum repulsion (
Stacked Models | Dihedral Angles (φ) | Interaction Energies (kcal/mol) | |
---|---|---|---|
−OH same side | −OH opposite side | ||
Phenol-Phenol Stacking | 0˚ | −3.6292 | −4.7468 |
60˚ | −4.4238 | −3.3999 | |
120˚ | −3.9786 | −3.8103 | |
180˚ | −4.5439 | −4.0456 | |
Diphenol-Diphenol Stacking | 0˚ | −3.8986 | −6.9298 |
60˚ | −5.3295 | −5.2118 | |
Phenol-Diphenol Stacking | 0˚ | −4.3055 | −12.3070 |
60˚ | −5.1123 | −4.8308 |
Similar procedure was followed to calculate the stacking interaction energies for diphenol-diphenol and phenol-diphenol stacked system. In these systems, interaction energies of the stacked models with dihedral angles 0˚ and 60˚ have only been computed as 0˚ - 180˚ and 60˚ - 120˚. For diphenol-diphenol and phenol-diphenol systems, where the −OH group of the two phenol rings facing
each other, stacking interactions for 0˚ conformation is found to be more stable than that of 60˚ conformations. The interaction energies of minimized stacked model for diphenol diphenol and phenol-diphenol systems were computed as −5.3295 and −5.1123 kcal/mol respectively. On the other hand, when the −OH groups of the two phenol rings face opposite to one another, the stacked systems with dihedral angle 0˚ is much more stable than 60˚ conformations. The interaction energies are also found to be more negative than that of other conformers, the minimized stacked interaction energy values for diphenol-diphenol and phenol diphenol stacked models were computed as −6.9298 and −12.3070 kcal/mol respectively. Generally, in any stacked model of phenolic system at 0˚ intermolecular rotation (i.e. 0˚ dihedral angle) the stacked models are found to be more repulsive due to the strong repulsion between the −OH groups of the phenol rings. But, in this investigation it has been observed that when the −OH of the two phenol rings of the stacked models are facing opposite to each other then 0˚ conformation can also give the much more stable stacked models (Figures 10-13). To observe the effective change in stacking interaction energies within the phenolic dimer, we can also compare the Mullikan charges density of −OH groups for unstacked and stacked phenolic systems, Mullikan charge density shows a significant change for minimized stacked models as compared to the unstacked model (Tables 2-4).
Stacked Models | Dihedral Angles (φ) | Mulliken Charges (kcal/mol) | |||||
---|---|---|---|---|---|---|---|
Unstacked Phenol | Stacked Phenol | ||||||
−OH of Ring1 | −OH of Ring2 | ||||||
O | H | O | H | O | H | ||
−OH Same direction | 0˚ | −0.2370 | 0.2650 | −0.2094 | 0.2726 | −0.2105 | 0.2669 |
60˚ | −0.2370 | 0.2650 | −0.2240 | 0.2698 | −0.2095 | 0.2681 | |
120˚ | −0.2370 | 0.2650 | −0.2190 | 0.2683 | −0.2168 | 0.2720 | |
180˚ | −0.2370 | 0.2650 | −0.2030 | 0.2598 | −0.2022 | 0.2596 | |
−OH opposite direction | 0˚ | −0.2370 | 0.2650 | −0.2135 | 0.2711 | −0.1950 | 0.2659 |
60˚ | −0.2370 | 0.2650 | −0.2198 | 0.2678 | −0.1896 | 0.2577 | |
120˚ | −0.2370 | 0.2650 | −0.2060 | 0.2616 | −0.2049 | 0.2608 | |
180˚ | −0.2370 | 0.2650 | −0.1957 | 0.2608 | −0.1942 | 0.2591 |
Stacked Models | Dihedral Angles (φ) | Mulliken Charges of Stacked Diphenol | |||||||
---|---|---|---|---|---|---|---|---|---|
−OH of Ring1 | −OH of Ring2 | ||||||||
O | H | O | H | O | H | O | H | ||
−OH same direction | 0˚ | −0.2113 | 0.2632 | −0.2094 | 0.2661 | −0.2010 | 0.2635 | −0.2302 | 0.2665 |
60˚ | −0.2236 | 0.2686 | −0.2194 | 0.2690 | −0.2281 | 0.2722 | −0.2307 | 0.2671 | |
−OH opposite direction | 0˚ | −0.2269 | 0.2749 | −0.2175 | 0.2720 | −0.2178 | 0.2722 | −0.2266 | 0.2748 |
60˚ | −0.2289 | 0.2674 | −0.2035 | 0.2611 | −0.2272 | 0.2701 | −0.2168 | 0.2690 |
*MC for unstacked diphenol: O = −0.2494 and H = 0.2640.
Stacked Models | Dihedral Angles (φ) | Mulliken Charges of Stacked Phenol-Diphenol system | |||||
---|---|---|---|---|---|---|---|
−OH of Ring of phenol | −OH of Ring of diphenol | ||||||
O | H | O | H | O | H | ||
−OH same direction | 0˚ | −0.1971 | 0.2646 | −0.2310 | 0.2668 | −0.2173 | 0.2635 |
60˚ | −0.1850 | 0.2603 | −0.2305 | 0.2689 | −0.2125 | 0.2629 | |
−OH opposite direction | 0˚ | −0.2049 | 0.2724 | −0.2267 | 0.2736 | −0.2178 | 0.2638 |
60˚ | −0.2187 | 0.2719 | −0.2273 | 0.2655 | −0.2061 | 0.2584 |
*MC for unstacked diphenol: O = −0.2494, H = 0.2640 and Phenol O = −0.2370, H = 0.2650.
From the above investigations, it has been observed that the phenolic systems are well stacked within themselves. But, the intermolecular rotation and direction of −OH groups of the phenolic systems play an important role in the stability of the stacked models. We can conclude that when the −OH groups of the stacked models are facing same direction then 180˚ or 60˚ conformations gets more stabilized. Whereas, when the −OH groups are facing opposite to one another then the eclipsed conformation with 0˚ dihedral angle gives more stable stacked model.
Authors are highly grateful to the AICTE-TEQIP-3 fund and Ministry of Human Resource Development (MHRD), New Delhi, for providing research assistance.
The authors declare no conflicts of interest regarding the publication of this paper.
Ali, I., Sharma, S. and Bezbaruah, B. (2018) Quantum Mechanical Study on the π-π Stacking Interaction and Change in Conformation of Phenolic Systems with Different Intermolecular Rotations. Computational Chemistry, 6, 71-86. https://doi.org/10.4236/cc.2018.64006