Initio Study of Atropisomers of Derivatives of N-Benzyl-2-phenylpyridinium Ions

Ab initio calculation at RHF/6-31G level of theory for geometry optimization of conformers of N-benzyl-2-phenylpyridinium ions are reported. The series of electron-withdrawing and electron-donating groups have been replaced by 3-H phenyl and benzyl of N-benzyl-2-phenylpyridinium ions and the energy difference between the synand antiforms discussed in terms of π-π stacking.


Introduction
Atropisomers are stereoisomers resulting from hindered rotation about single bonds where the barrier to rotation is high enough to allow the isolation of the conformational isomers 1 .Among the non-covalent weak interactions, π-π interactions between aromatic rings have attracted much attention due to their importance in many fields of science such as chemistry, biology and material science 2 .This interaction holds molecules together in natural and artificial supra molecules including DNA 3 and other biological systems [4][5][6][7][8][9][10] .In continuation of our work on π-π interaction in atropisomers 11,12 , in the current work, the structural optimization of conformers of N-benzyl-2-phenyl pyridinium ions (models 1-7) are investigated (Scheme 1).Scheme 1.The selected models: (a) Model 1: X, Y=H; Model 2: X, Y= Cl; Model 3: X, Y= OMe; Model 4: X, Y= CN; Model 5: X, Y= CF 3 ; Model 6: X=CN, Y= OMe; (b) Model 7 It was found in solution and solid phases that these compounds preferred the conformation in which the benzyl groups would stack center to edge to the phenyl rings.This preference was relatively insensitive to electronic substituent effects on the benzyl groups 13 .The models exist as a mixture of syn-and anti-isomers.Interconversion of the isomers requires the phenyl ring rotate about the C 2 -C 7 (sp 2 -sp 2 ) bond.Thus the phenyl ring will be resting as the same plane as pyridine in transition state (Figure 1).

Experimental
Ab initio molecular orbital calculations were carried out using the Gaussian 1998 program 14 The restricted Hartree-Fock calculations with the split-valence 6-31G * basis set, which include a set of d-type polarization functions on all non-hydrogen atoms, were used 15 .The frequencies were scaled by a factor of 0.9135 16 and were used to compute zero-point vibrational energies.

Results and Discussion
At the first the structure of N-benzyl-2-phenyl pyridinium ion (model 1) was studied.For model 1, we have found two minima, one splayed and one stacked conformer (Figure 2) 11 .By replacing the various substitutions as electron-donating (-OMe), electronwithdrawing (-CF 3 , -CN), hindered (naphthyl) and halogen (-Cl), we studied the created models.These different substitutions cause some conflicts in electronic characteristics of the rings.It can be determined whether electrostatic interaction or dispersion forces are merely responsible to the effects on the distributions of the conformers.Rotation of aryl ring around the sp 2 -sp 2 bond connecting the pyridine ring generates the transition states.In the all transition states the substituted benzene ring is placed on with pyridine ring on the same plane and the substituted benzyl rings goes as far as possible (Scheme 2).

Anti
Ts Syn The values of dihedral angles of stacked and splayed conformers are summarized in Table 1.
Table 1 Our studied showed that in these models, dihedral angles of N1-C2-C7-C8 change between +73 to +289 º (or -71º) in stacked anti-conformers and +100 to 109º in stacked synconformers to make π-π interaction possible between two rings.The values of this angle have been faced between 1 to 5º in splayed conformers.Model 3 (with -OMe as an electrondonating substitution in both rings) does not stack in anti-conformer.Phenyl and benzyl ring polarized by electron-donating substituent (-OMe) which caused increasing the π-electron repulsion 2 .
The values of dihedral angles of C2-N1-C13-C14 in stacked syn-and anti-models are between +287 to +249º (or -73 to -111º).Due to positioning of substituted benzyl rings at splayed models (in both syn-and anti-conformers) the value of this angle is ranged between +230 to +241, except model 1 (Table 1).
According to calculations performed, splayed conformers have a little lower energy comparing to stacked conformers.The achieved result shows a clear indication of the presence FIROUZEH NEMATI et al.
of π-π interaction in stacked conformers.Otherwise, there must be much more difference between the energy of splayed and stacked conformers.The difference in stability in studied models with electron-withdrawing (Models 4 and 5) is less than models with electrondonating (Models 2 and 3) and non-substituted models (Models 1 and 7).The lowest difference in energy is in syn-conformer model 6; which proves there is greater π-π interaction at this model than the others.The energies are given in Table 2.

Absolute values are in hartree and relative values are in kcal mol -1 , b There is no anti or syn conformer. c There is no anti stacked conformer
In Table 3 absolute and relative energies between splayed and stacked conformers are given.According to gained data, in model 7 with naphthyl groups, the relative energy of stacked conformers is greater than splayed.Naphthyl has an area larger than benzyl and phenyl.So it showed the much stronger interaction than the other models.Moreover, in all models, except model 6, the anti-isomers are more stable than syn-isomers.In model 6, the syn-stacked isomer (+0.33 kcal mol -1 ) is more stable than the anti-isomer.

Table 3. Relative energies of stacked and splayed conformers a Absolute values are in hartree and relative values are in kcal mol -1 , b There is no anti-or synconformer. c There is no anti-stacked conformer
The rotational energy barriers for stacked isomers show that rapid interconversion of syn-and anti-atropisomers 17 can take place at room temperature (Table 4).Model 7 is the most hindered of the isomers studied and it is reasonable to assume that the bulk of the groups cause enough π-π interaction to increase the rotational energy barrier and hence make the isomer separation possible at room temperature.
In models with electron-withdrawing groups, we can see the decrease of energy barrier (Models 2, 4 and 5) (Table 4).In model 3 the presence of electron-donating group (-OMe) caused increasing electron density on benzyl and phenyl rings.It also indicated increased the repulsion of π-electrons of ring.This repulsion causes the substituted benzyl and phenyl ring far away from each other, so the only stable anticonformer of model 2 is splayed (Scheme 3

Scheme 3. Three conformers of N-(3-methoxy benzyl)-2-(3-methoxy phenyl)pyridinium ion
In all studied stacked conformers (except model 6), the anti-isomer is more stable than syn-isomer.The amount of this energy difference in model 5 is the least (0.01 kcal/mol) and in models 3 and 7 is the most (1.92 and 1.69 kcal/mol).The presence of electron-donating group (-OMe) in model 3 caused the anti-conformer be existing as splayed form.Consequently the difference in stability has been increased between antisplayed isomer and syn-stacked isomer.The presence of bulky group (naphthyl) in model 7 increased the π-π repulsion; therefore, in syn-isomer where the contact surface of naphthyl ring is more, the resulted conformer stability is less.Due to decreasing of electron density on benzyl and phenyl rings in models with electron-withdrawing groups, it causes a little difference in energy level.In model 6, the syn-isomer (0.0 kcal/mol) is more stable than the anti-isomer (0.33 kcal/mol).The syn-isomer has more stacked area relative to anti-isomer (Figure 3).The computed gas phase energies of conformers of models 2-7 are reported in Table 4.

Conclusion
The obtained results showed that, these models subsist in stacked and splayed conformers.
While the splayed conformer has the priority over, the very low difference is indicated between stacked and splayed conformers.Whereas, there is low difference in energy barriers between stacked conformers of studied models, as a result the electrostatic interaction has small contribution on conformer's distribution compared to dispersion interactions.The effective π-π interaction was proved by the less difference between energies of two conformers of stacked and splayed.

Figure 3 .
Figure 3. Stereo view of substituted benzyl and phenyl rings in stacked conformers of model 6The computed gas phase energies of conformers of models 2-7 are reported in Table4.

Table 2 .
Energies of stacked and splayed conformers