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Ionic Liquids Project

Ionic liquids (ILS) have been an emerging subject in the past years, as evident by the increasing number of publications in the field. They are particularly important in industry as their low vapour pressure makes them more energy efficient and environmentally friendly than conventional volatile solvents. ILS are generally composed of bulky, delocalized cations and anions that do not allow strong electrostatic interactions often found in conventional ionic salts. By carefully modifying the different functional groups in the constituting ions, one can achieve desired changes in their physical properties, such as boiling point, polarity and viscosity to complement their reaction of choice.

An ambitious step further would be to use computational chemistry as a simulating tool in order to predict their physical properties and hence allow engineering of their structure to fit any reaction of interest. This project has two aims; to compare and contrast the properties of the first three tetramethyl -onium cations of group V and to investigate the influence that different functional groups have on their molecular properties.

Optimization of Methyl -onium cations

The optimization of [N(CH3)4]+, [P(CH3)4]+ and [S(CH3)3]+ was carried out using the moderate level basis set 6-31G(d,p). The nosymm keyword was used for all optimizations as to avoid constraining the cations into a fixed geometry and hence provide a better optimization of their structure.


Compound:
[N(CH3)4]+
[P(CH3)4]+
[S(CH3)3]+
File Type: .LOG .LOG .LOG
Calculation Type: FOPT FOPT FOPT
Calculation Method: RB3LYP RB3LYP RB3LYP
Charge: 1 1 1
Basis Set: 6-31G(d,p) 6-31G(d,p) 6-31G(d,p
E(RB3LYP) (au): -214.18127261 -500.82700296 -517.68326034
Gradient (au): 0.00002101 0.00002565 0.00001046
Dipole Moment (D): 16.7855 8.8765 2.4417
Point Group: C1 C1 C1
Calculation Time: 3 minutes, 24.8 seconds 5 minutes, 33.3 seconds 4 minutes, 22.7 seconds
Bond Distance (C-X) (Å): 1.509 1.817 1.823
Bond Angle (C-X-C): 109.45o 109.51o 102,7o
Dspace link: DOI:10042/25403 DOI:10042/25407 DOI:10042/25413

The bond length of [N(CH3)4]+ as determined using this computation (1.509 Å) was close to the experimental values[1] (1.492 Å). The bond angle C-N-C was determined to be 109.45o which is in accordance with that of a perfect tetrahedral's 109.5o. This implies that a tetrahedral geometry around the Nitrogen atom is established.

Similarly, for [P(CH3)4]+, the bond length was determined to be 1.817 Å whereas the literature[2] suggests an experimental value of 1.841 Å. This difference is within the error of this calculation. The C-P-C bond angle was computed to be 109.51o, again indicating that there is a tetrahedral geometry around the Phosphorus atom.

For [S(CH3)3]+, while the bond lengths computed (1.823 Å) are in agreement with experimental values[3] (1.82 Å), the bond angle (102.7o) shows deviation from trigonal pyramidal geometry. This is owed to the less 'hybridized' character of the participating orbitals. The valence orbitals of Sulfur are more s and p like rather than sp3, as evidenced by the NBO analysis. The 3s orbital wavefunction has a non-zero amplitude close to the region of the nucleus and is thus more stabilized than the corresponding 3p orbital. This results in a larger 3s-3p energy gap. As a consequence there is less mixing of states, resulting in more 'p' character of the orbitals and hence smaller bond angles. This effect is not seen in Phosphorus as it has one less proton in its nucleus and hence the s-p gap is not as pronounced.

Convergence verification of structures
[N(CH3)4]+ [P(CH3)4]+ [S(CH3)3]+ 
Item Value Threshold Converged?
Maximum Force 0.000074 0.000450 YES
RMS Force 0.000017 0.000300 YES
Maximum Displacement 0.000668 0.001800 YES
RMS Displacement 0.000227 0.001200 YES
Predicted change in Energy=-4.876111D-08
Optimization completed.
-- Stationary point found.
 
Item Value Threshold Converged?
Maximum Force 0.000148 0.000450 YES
RMS Force 0.000033 0.000300 YES
Maximum Displacement 0.000901 0.001800 YES
RMS Displacement 0.000308 0.001200 YES
Predicted change in Energy=-1.807900D-07
Optimization completed.
-- Stationary point found. 
Item Value Threshold Converged?
Maximum Force 0.000096 0.000450 YES
RMS Force 0.000040 0.000300 YES
Maximum Displacement 0.001009 0.001800 YES
RMS Displacement 0.000310 0.001200 YES
Predicted change in Energy=-1.663661D-07
Optimization completed.
-- Stationary point found.

Frequency analysis -onium cations

Using the optimized structures, a frequency analysis was carried out using the same basis sets (6-31G(d,p)) and method (B3LYP) in order to examine as to whether the optimized structure has indeed reached a minimun.

Compound:
[N(CH3)4]+
[P(CH3)4]+
[S(CH3)3]+
File Type: .LOG .LOG .LOG
Calculation Type: FREQ FREQ FREQ
Calculation Method: RB3LYP RB3LYP RB3LYP
Charge: 1 1 1
Basis Set: 6-31G(d,p) 6-31G(d,p) 6-31G(d,p
E(RB3LYP) (au): -214.18127261 -500.82700296 -517.68327910
Gradient (au): 0.00002103 0.00002565 0.00001040
Dipole Moment (D): 16.7855 8.8765 2.4417
Point Group: C1 C1 C1
Calculation Time: 8 minutes, 0.2 seconds 7 minutes, 41.1 seconds 3 minutes, 47 seconds
Dspace link: DOI:10042/25406 DOI:10042/25410 DOI:10042/25415
IR Spectrum:
Low frequencies table
Compound Result
[N(CH3)4]+ 
Low frequencies --- -13.0579 -0.0003 0.0006 0.0010 6.1350 11.9430
Low frequencies --- 179.8801 278.8600 285.6917
[P(CH3)4]+ 
Low frequencies --- -16.5874 0.0019 0.0026 0.0033 4.6239 16.1084
Low frequencies --- 153.3248 183.0564 190.9732
[S(CH3)3]+ 
Low frequencies --- -13.0456 -8.0899 -0.0031 0.0015 0.0045 22.8850
Low frequencies --- 158.4008 194.0307 198.5783

As it can be inferred from the low frequency table, all external degrees of freedom are within a ± 20 cm-1 range. Furthermore, no imaginary frequencies were present. The above two suggest that the optimized structures had indeed reached a minimun value on the potential energy surface and hence further calculations can now be done.



Population Analysis of Methyl -onium cations

An energy calculation was made using the same basis sets '6-31G(d,p)' and method 'B3LYP'. The keyword pop=(nbo, full) was used in order to generate the Natural Bond Order orbitals of the molecules. The molecular orbitals were generated using the formatted checkpoint file and were examined. 5 representative molecular orbitals of [N(CH3)4]+ were characterized in order to provide further insight to the orbital interactions present in these cations.

Compound:
[N(CH3)4]+
[P(CH3)4]+
[S(CH3)3]+
File Type: .fch .fch .fch
Calculation Type: SP SP SP
Calculation Method: RB3LYP RB3LYP RB3LYP
Charge: 1 1 1
Basis Set: 6-31G(d,p) 6-31G(d,p) 6-31G(d,p
E(RB3LYP) (au): -214.18127261 -500.82700296 -517.68327910
Gradient (au): 0.00000000 0.00000000 0.00000000
Dipole Moment (D): 16.7859 8.8767 2.4417
Point Group: C1 C1 C1
Calculation Time: -- -- --
Dspace link: DOI:10042/25426 DOI:10042/25434 DOI:10042/25435





Molecular Orbital Analysis of [N(CH3)4]+
Orbital order Molecular Orbital Comments
6
This is the lowest energy non-core orbital of the molecule. Bonding interactions are present over the whole molecular framework and all internuclear distances. Furthermore, weak through space interactions are present between the Hydrogen atoms of adjacent methyl groups.
10
This orbital is characterized by strong, through bonding interactions of all -CH3 fragments with the central Nitrogen being in opposite phase. As a consequence there are 4 nodes between the C-N internuclear regions contributing to strong anti-bonding interactions. It should be noted however that there are many through space bonding interactions present between the adjacent -CH3 fragments. The orbital should therefore be characterized as weakly bonding.
13
This orbital is characterized by two nodal planes found in the internuclear distance of two C-N bonds. There is a significant amount of bonding interactions spanning across the nitrogen and the other two methyl groups. A strong bonding interaction is observed from the sp3 orbital of the top methyl carbon with the H3 fragment. Furthermore, strong through space interactions are present in the H3 fragment composed from one axial Hydrogen of each remaining methyl groups. This orbital should then be considered as weakly bonding.
15
This molecular orbital features two nodal planes, perpendicular to eachother passing through both C-N-C axis. The nodes effectively lie on 4 carbon atoms, constituting a weak anti bonding interaction but also to 4 C-N bonds and 4 C-H bonds which contribute to strong anti-bonding interactions. Despite the strong through space interactions of the methyl hydrogens the orbital is anti-bonding for the reasons mentioned above.
21
While there are three nodes in this molecular orbital, all of them lie on atoms rather than internuclear distances. There are strong through space bonding interactions between the axial H3 fragment of the adjacent methyl groups and a strong bonding interaction of the remaining -CH3 group. Furthermore, there is a sp3-sp3 interaction between the top methyl Carbon and the Nitrogen resulting in a σ bond with a significant amount of electron density located in the internuclear distance. Overall this orbital is weakly bonding.






Natural Bond Order Analysis of Methyl -onium cations

Using the .log file of the population analysis calculation, the NBOs of the compounds were generated and their charge distribution was determined.

Charge distribution of [N(CH3)4]+
Atom Charge (e)
Charge distribution of [N(CH3)4]+. Charge limit ranges from -1 to 1 e.
N -0.295
C -0.483
H +0.269


Charge distribution of [P(CH3)4]+
Atom Charge (e)
Charge distribution of [P(CH3)4]+. Charge limit ranges from -1 to 1 e.
P +1.667
C -1.060
H +0.298
Charge distribution of [S(CH3)3]+
Atom Charge (e)
Charge distribution of [S(CH3)3]+. Charge limit ranges from -1 to 1 e.
S +0.917
C -0.846
H (2, 3, 5,
7, 10, 11)		 +0.297
H (4, 8, 12) +0.279

The charge distribution of [N(CH3)4]+ shows that it is the Hydrogen atoms that bear the positive charge on the molecule. This is in contrast to how chemists tend to represent the ammonium cation with the positive charge on Nitrogen. The electronegative N atom, despite having donated its lone pair it still retains the majority of the electron density of the C-N bond formed by the lone pair (66.35% is still retained by Nitrogen). This is what inorganic chemists would describe as a tightly bound lone pair. Furthermore, the N atom also draws most of the electron density of the N-C bonds towards itself thus making it overall negatively charged. This is backed by the NBO analysis of the C-N bond where the majority of the contribution is localized on the N atom.

Carbon is also negatively charged due to it being more electronegative than Hydrogen, pulling electron density towards itself via the inductive effect. This leaves the Hydrogen atoms to possess a positive charge. The overall charge of the molecule can be found by summing the charges of all constituent atoms, giving a +1 e charge overall.




For [P(CH3)4]+ the central atom Phosphorus is less electronegative than Nitrogen and hence it is better at donating its lone pair. Furthermore, Carbon is more electronegative than Phosphorus. This results in a positively charged Phosphorus atom, surrounded by negatively charged Carbon atoms. This can be seen in the NBO analysis below which shows that the Carbon is the atom that gets the bigger share of the sp3 orbitals.

The Hydrogen atoms again bear a positive charge as electron density is drawn out of them via the inductive effect of Carbon. Since Carbon uses more electron density from the sp3 orbitals of both the Hydrogens and Phosphorus, its negative charge is more pronounced than in the case of [N(CH3)4]+.






Sulfur has really similar electronegativity to Carbon. However it is less electronegative than Nitrogen and hence it is still better at donating its lone pair, resulting in it being positively charged. The Carbon atoms are again negatively charged because they pull out electron density from the more electropositive H atoms via the inductive effect.

A really interesting feature of the charge distribution is the differing positive charge of the synclinal and antiperiplanar Hydrogen atoms, with respect to the lone pair. The antiperiplanar ones show a less positive charge than the synclinal Hydrogens. This is because when an antiperiplanar conformation is established there is maximun overlap between the filled σ orbital of the Sulfur's lone pair and the σ* orbital of the C-H bond. This donation of electron density is therefore stronger for the aforementioned Hydrogen atom and hence it becomes less positive.



As one can see in the table below, the Second Order Perturbation Theory Analysis of the Fock Matrix in a NBO Basis shows an interaction of the lone pair with only the antiperiplanar Hydrogen atoms. While this interaction is weak, amounting to only 2.31 kcal mol-1, it could be responsible for the charge difference observed. The additional electron density donated to the H atoms renders them less positive than their synclinal counterparts. 

Donor NBO        (i) Acceptor NBO (j)   kcal/mol a.u. a.u.
========================================================
21. LP ( 1) S 13 /100. BD*( 1) C 1 - H 4 2.31 1.02 0.043
21. LP ( 1) S 13 /104. BD*( 1) C 5 - H 8 2.31 1.02 0.043
21. LP ( 1) S 13 /108. BD*( 1) C 9 - H 12 2.32 1.02 0.043

Orbital contributions of the C-X bond:

A Second Order Perturbation Theory Analysis of the Fock Matrix in a NBO Basis was performed to determine the orbital contributions that the Carbon and Heteroatom have on the C-X bond.

Orbital contributions of the C-X bond
Compound Result
[N(CH3)4]+ 
 (Occupancy) Bond orbital/ Coefficients/ Hybrids
---------------------------------------------------------------------------------
 4. (1.98453) BD ( 1) C 1 - N 17
(33.65%) 0.5801* C 1 s( 20.77%)p 3.81( 79.06%)d 0.01( 0.16%)
                      -0.0003 -0.4551 0.0237 -0.0026 -0.2958
                      -0.0125 -0.4189 -0.0178 -0.7254 -0.0308
                      -0.0110 -0.0191 -0.0271 0.0039 -0.0203


(66.35%) 0.8146* N 17 s( 25.00%)p 3.00( 74.97%)d 0.00( 0.03%)
                       0.0000 -0.5000 0.0007 0.0000 0.2886
                       0.0000 0.4081 -0.0001 0.7070 -0.0001
                      -0.0048 -0.0084 -0.0119 0.0017 -0.0089
[P(CH3)4]+ 
 (Occupancy) Bond orbital/ Coefficients/ Hybrids
---------------------------------------------------------------------------------
4. (1.98030) BD ( 1) C 1 - P 17
(59.57%) 0.7718* C 1 s( 25.24%)p 2.96( 74.68%)d 0.00( 0.08%)
                      0.0002 0.5021 0.0171 -0.0020 0.2887
                     -0.0053 0.4068 -0.0075 0.7054 -0.0129
                      0.0079 0.0137 0.0193 -0.0028 0.0145


(40.43%) 0.6359* P 17 s( 25.00%)p 2.97( 74.15%)d 0.03( 0.85%)
                      0.0000 0.0001 0.5000 -0.0008 0.0000
                      0.0000 -0.2872 0.0005 0.0000 -0.4058
                      0.0006 0.0000 -0.7031 0.0010 0.0251
                      0.0436 0.0615 -0.0089 0.0461
[S(CH3)3]+ 
 (Occupancy) Bond orbital/ Coefficients/ Hybrids
---------------------------------------------------------------------------------
 4. (1.98631) BD ( 1) C 1 - S 13 
(48.67%) 0.6976* C 1 s( 19.70%)p 4.07(80.16%)d 0.01( 0.14%)
                      0.0003 0.4436 0.0140 -0.0033 -0.3636
                     -0.0098 -0.4088 0.0032 0.7086 -0.0055
                      0.0120 -0.0208 -0.0230 -0.0017 0.0163


(51.33%) 0.7165* S 13 s( 16.95%)p 4.86( 82.42%)d 0.04( 0.63%)
                      0.0000 0.0001 0.4116 -0.0075 0.0012
                      0.0000 0.4039 0.0259 0.0000 0.4057
                     -0.0179 0.0000 -0.7032 0.0309 0.0310
                     -0.0537 -0.0428 0.0080 0.0240


The orbital contribution table correlates well with the charge distribution arguments made above. For [N(CH3)4]+, the orbital contributions to the C-N bond arise primarily from the Nitrogen atom. NBO analysis splits the electron density of the molecule to its constituents AOs, forming 2e--2c bonds. In a completely covalent bond, all atoms would make a 50% contribution to the bond and hence Nitrogen would bear a +1 positive charge. However, since 66.35% of the bond's electron density is localized on Nitrogen its charge will deviate towards lower values. Indeed the N atom's charge was negative and hence this verifies the electronegativity arguments made above in the charge distribution section.


For [P(CH3)4]+ the case is reversed. The NBO analysis suggests that only 40.43% of the C-P bond's electron density is localized on Phosphorus. Hence, this explains why Phosphorus bears a positive charge larger than +1 in the cation( +1.667). This is because not only does it get the smaller share of electron density, but also only retains 40.43% of the electron density it donated via its Lone Pair.


Similar arguments to Phosphorus can be raised for the charge distribution of [S(CH3)3]+. The C-S bond contributions suggest a 51.33% contribution from Sulfur, hence the S atom's charge should be in close agreement with a 50% covalent system which would suggest a +1 charge on Sulfur. In reality the Sulfur atom bears a +0.917 e charge and hence it is more in accordance to the completely covalent bond than the case of Phosphorus.







Functional Group Variation

The second objective of this project was to determine how ionic liquids can be engineered in order to tune their properties. One proton of one of the methyl groups of [N(CH3)4]+ was replaced with an electron donating group (EDG) in one study and an electron withdrawing group (EWG) in another. The different charge distributions and orbital interactions that arised were then compared and contrasted. The EDG group of choice was the hydroxyl group, -OH, whereas the EWG group chosen was the nitrille group -CN.

Optimization

The optimization of both structures was done using the moderate level basis sets 6-31G(d,p) and the method was B3LYP. The keyword nosymm was used to allow for flexibility in the molecule's geometry and the optimization was done under tight convergence criteria. This extra level of sophistication was added to the calculations as the structure of the -OH derivative would otherwise fall into a transition state, resulting in a poorly optimized structure. As a consequence, the [N(CH3)4]+ compound was re-optimized to the same level of sophistication of the other calculations as to allow comparisons to be drawn.

Compound:
[N(CH3)3(CH2OH]+
[N(CH3)3(CH2CN]+
[N(CH3)4]+
File Type: .LOG .LOG .LOG
Calculation Type: FOPT FOPT FOPT
Calculation Method: RB3LYP RB3LYP R3BLYP
Charge: 1 1 1
Basis Set: 6-31G(d,p) 6-31G(d,p) 6-31G(d,p)
E(RB3LYP) (au): -289.39471340 -306.39377725 -214.18127268
Gradient (au): 0.00000161 0.00000098 0.00000148
Dipole Moment (D): 8.2384 16.2109 12.0467
Point Group: C1 C1 C1
Calculation Time: 57 minutes, 1.1 seconds 3 minutes, 36 seconds 1 minute, 28 seconds
Log file: DOI:10042/25481 Log File Log File


Convergence verification of structures
[N(CH3)3(CH2OH]+ [N(CH3)3(CH2CN]+ [N(CH3)4]+ 
Item Value Threshold Converged?
Maximum Force 0.000009 0.000450 YES
RMS Force 0.000002 0.000300 YES
Maximum Displacement 0.000182 0.001800 YES
RMS Displacement 0.000048 0.001200 YES
Predicted change in Energy=-1.603319D-09
Optimization completed.
-- Stationary point found.
 
Item Value Threshold Converged?
Maximum Force 0.000001 0.000015 YES
RMS Force 0.000000 0.000010 YES
Maximum Displacement 0.000044 0.000060 YES
RMS Displacement 0.000012 0.000040 YES
Predicted change in Energy=-2.993868D-11
Optimization completed.
-- Stationary point found.
 
 Item Value Threshold Converged?
Maximum Force 0.000001 0.000015 YES
RMS Force 0.000000 0.000010 YES
Maximum Displacement 0.000033 0.000060 YES
RMS Displacement 0.000010 0.000040 YES
Predicted change in Energy=-2.094334D-11
Optimization completed.
-- Stationary point found.

Frequency Analysis

Using the optimized structures, a frequency analysis was carried out using the same basis sets 6-31G(d,p) and method (B3LYP) in order to examine whether the optimized structure has indeed reached a minima.

Compound:
[N(CH3)3(CH2OH]+
[N(CH3)3(CH2CN]+
[N(CH3)4]+
File Type: .LOG .LOG .LOG
Calculation Type: FREQ FREQ FREQ
Calculation Method: RB3LYP RB3LYP RB3LYP
Charge: 1 1 1
Basis Set: 6-31G(d,p) 6-31G(d,p) 6-31G(d,p)
E(RB3LYP) (au): -289.39471340 -306.39377725 -214.18127268
Gradient (au): 0.00000165 0.00000107 0.00000148
Dipole Moment (D): 8.2385 16.2109 12.0467
Point Group: C1 C1 C1
Calculation Time: 5 minutes, 0 seconds 3 minutes, 53 seconds 2 minutes, 14 seconds
Log file: Log file Log file Log file
IR Spectrum:
Low frequencies table
Compound Result
N(CH3)3(CH2OH]+ 
Low frequencies --- -8.5468 0.0012 0.0013 0.0013 11.8643 15.2318

Low frequencies --- 132.1322 214.3664 256.0423
N(CH3)3(CH2CN]+ 
Low frequencies --- -10.1793 -0.0009 -0.0007 -0.0007 15.6034 16.2108

Low frequencies --- 91.3061 154.3329 208.9445
N(CH3)4]+ 
Low frequencies --- -14.9896 -4.1878 -0.0010 -0.0009 0.0004 9.2904

Low frequencies --- 179.1845 278.4764 285.3303

As it can be inferred from the low frequency table, all external degrees of freedom are within a ± 20 cm-1 range. Furthermore, no imaginary frequencies were present. Hence, the above two suggested that the optimized structures had indeed reached a minimun value of the potential energy surface.

Population Analysis

An energy calculation was made using the same basis sets '6-31G(d,p)' and method 'B3LYP'. The keyword pop=(nbo, full) was used in order to generate the Natural Bond Order orbitals of the molecules. The molecular orbitals were generated using the formatted checkpoint file and were examined.

Compound:
[N(CH3)3(CH2OH]+
[N(CH3)3(CH2CN]+
[N(CH3)4]+
File Type: .LOG .LOG .LOG
Calculation Type: SP SP SP
Calculation Method: RB3LYP RB3LYP RB3LYP
Charge: 1 1 1
Basis Set: 6-31G(d,p) 6-31G(d,p) 6-31G(d,p)
E(RB3LYP) (au): -289.39471340 -306.39377725 -214.18127268
Gradient (au): 0.00000000 0.00000000 0.00000000
Dipole Moment (D): 8.2385 16.2109 12.0467
Point Group: C1 C1 C1
Calculation Time: 0 minutes, 18 seconds 0 minutes, 26 seconds 0 minutes, 14 seconds
Log file: Log file Log file Log file

NBO Charge Distribution Analysis

NBO charge distribution table
Labeled atoms of [N(CH3)3(CH2OH]+
Nitrogen Carbon Hydrogen Oxygen
Charge distribution of [N(CH3)3(CH2OH]+. Charge limit ranges from -1 to 1 e.
(16) -0.322 (1) -0.494 (2) 0.271 (17) -0.726
(5) 0.088 (3) 0.272
(8) -0.491 (4) 0.252
(12) -0.492 (6) 0.249
(7) 0.237
(9, 10, 13) 0.266
(11) 0.282
(14) 0.269
(15) 0.274
(18) 0.521

The introduction of the donating group -OH over one of the H atoms has had a significant impact on the overall charge distribution of the molecule. The first thing to notice is that the Nitrogen atom is now more negative than in the case of [N(CH3)4]+. This indicates that there is some degree of delocalization between the N-C-O fragment which allows donation of the Oxygen's lone pair to the Nitrogen atom.

Furthermore, while the methyl carbons still share a negative charge of approximately the same value as for [N(CH3)4]+, the carbon adjacent to the -OH group has a slight positive charge on it. This is owed to Oxygen being more electronegative withdrawing electron density from the carbon via the inductive effect. Lastly, there is a distinguish of charge distribution in the Hydrogen atoms. In particular, the ones attached to the CH2OH carbon are less positively charged than the ones of the methyl groups. Second Order Perturbation Theory Analysis of the Fock Matrix in an NBO Basis sheds some light into this peculiar phenomenon: 

Donor NBO      (i) Acceptor NBO (j)    kcal/mol a.u. a.u.
=========================================================
24. LP ( 1) O 17 /138. BD*( 1) C 5 - H 6 1.21 1.04 0.032
25. LP ( 2) O 17 /138. BD*( 1) C 5 - H 6 2.99 0.77 0.044
24. LP ( 1) O 17 /139. BD*( 1) C 5 - H 7 3.98 1.02 0.057
25. LP ( 2) O 17 /139. BD*( 1) C 5 - H 7 0.82 0.75 0.023

As one can tell, there is significant lone pair donation to both Hydrogen atoms (6) and (7). It can be seen however that there is more donation to (7), owed to higher orbital overlap. This results in (7) to be less positive than its (6) counterpart as more electon density is donated to it.

The same analysis was also done on Nitrogen and it turns out that there is a strong interaction with the Oxygen's lone pair, amounting to 18.92 kcal mol-1. This indicates a strong delocalization of the lone pair via the N-C-O fragment which can be also seen in the LUMO of the molecule. 

Donor NBO        (i) Acceptor NBO (j)     kcal/mol a.u. a.u.
=========================================================
25. LP ( 2) O 17 /140. BD*( 1) C 5 - N 16 18.92 0.51 0.088



Charge distribution of [N(CH3)3(CH2CN]+
Labeled atoms of [N(CH3)3(CH2CN]+
Nitrogen Carbon Hydrogen
Charge distribution of [N(CH3)3(CH2CN]+. Charge limit ranges from -1 to 1 e.
(16) -0.289 (1) -0.489 (2) 0.274
(18) -0.186 (5) -0.485 (3,10) 0.282
(9) -0.486 (4) 0.270
(13) -0.358 (6) 0.277
(17) 0.209 (7, 8) 0.271
(8) 0.266
(11) 0.269
(12) 0.274
(14, 15) 0.309





















The substitution of an -H with -CN, an electron withdrawing group, has also introduced a lot of variability in the charge distribution of the molecule, with respect to its [N(CH3)4]+ counterpart. While the central Nitrogen's negative charge has not been affected by much, there has been a significant impact to the Carbon and Hydrogen environments. While the methyl carbons bear nearly the same negative charge as in the case of [N(CH3)4]+, the CH2CN carbon now has a less negative charge. This is because of the electron withdrawing effects of the -CN group which pulls out electron density via its π orbitals, a phenomenon known as the mesomeric effect. This effect is less pronounced than the -OH group's inductive effect that propagates through the σ framework as the Carbon still remains negatively charged.

The nitrille Carbon was determined to be positive while the Nitrogen was negative. That is because the N atom is more electronegative and hence a higher proportion of the bond's electron density is localized into Nitrogen.

While there's not much variability in the positive charges of the methyl Hydrogens, the CH2CN Hydrogen atoms have a more pronounced positive charge. This is again owed to the electron withdrawing effects of the -CN group.

Interestingly, there's a strong interaction between the nitrille Nitrogen's lone pair and a Rydberg orbital, presumably the 3p orbital, of the nitrille Carbon which amounts to 16.69 kcal mol-1. 

Donor NBO   (i) Acceptor NBO (j)    kcal/mol a.u. a.u.
=======================================================
 27. LP ( 1) N 18 /122. RY*( 1) C 17 16.69 1.20 0.127



HOMO - LUMO Comparison

caption
[N(CH3)4]+ [N(CH3)3(CH2OH]+ [N(CH3)3(CH2CN]+
Molecular Orbital Energy (au) Molecular Orbital Energy (au) Molecular Orbital Energy (au)
HOMO orbital of [N(CH3)4]+
-0.57931
HOMO orbital of [N(CH3)3(CH2OH]+
-0.48758
HOMO orbital of [N(CH3)3(CH2CN]+
-0.50049
LUMO orbital of [N(CH3)4]+
-0.13303
LUMO orbital of [N(CH3)3(CH2OH]+
-0.12460
LUMO orbital of [N(CH3)3(CH2CN]+
-0.18184

As one can tell, both the shape and energy of the HOMO and LUMO changes significantly with the addition of different functional groups. The HOMO of [N(CH3)3(CH2OH]+ no longer features a bonding interaction between the H3 fragment of the three methyl groups. Furthermore, the bonding of the Me-N-Me fragment that featured a delocalized bonding interaction is significantly weakened with the CH2 Hydrogens being non-bonding. Both of this are justified when considering the inductive effect of Oxygen which withdraws electron density from this fragment. The orbital also shows anti-bonding interactions between the Oxygen's lone pair and the adjacent CH2 fragment. Overall the energy of the HOMO has increased as a consequence of the increased anti-bonding interactions.

In [N(CH3)4]+, the LUMO orbital shows delocalized bonding interaction through the molecular framework with the central Nitrogen atom being in opposite phase. For [N(CH3)3(CH2OH]+ however, an apparent N-CH2-O overlap is observed that disturbs the bonding interaction of the molecular framework. As a consequence the LUMO is shifted higher in energy than the initial tetramethyl ammonium cation. From that, one can infer that the molecule will be a weaker acceptor and hence will not interact as strongly with anions, resulting in a lower boiling point ionic liquid.


The HOMO of [N(CH3)3(CH2CN]+ barely shows any interactions in the main body of the molecule. The electron density is primarily located in the CH2CN fragment which demonstrates the electron withdrawing properties of the -CN functional group. There is a bonding interaction between the 2p orbitals of C-N. This π bond in turn interacts in an anti-bonding fashion with the 2p orbital of the CH2 Carbon. Overall, the HOMO has moved higher in energy than its [N(CH3)4]+ counterpart but its still lower in energy than that the HOMO of the electron donating group's cation.

The LUMO orbital of [N(CH3)3(CH2CN]+, albeit similar to [N(CH3)4]+, also features a π* bond between the C-N group. This orbital interacts in a bonding manner with one of the 2pz orbitals of the adjacent methyl Carbon. The main body electron cloud is also more deficient, owed to the withdrawing effects of -CN which decreases its anti-bonding interaction with the 2pz orbitals of the Carbon atoms. As a consequence the LUMO is more stabilized and hence it is deeper in energy than that of the tetramethyl ammonium cation. From that, one can expect that this cation will be a far better acceptor of electron density and hence will interact more strongly with anions, resulting in a higher boiling point liquid.

HOMO-LUMO gap
Compound Energy gap (au)
[N(CH3)4]+ 0.44628
[N(CH3)3(CH2OH]+ 0.36298
[N(CH3)3(CH2CN]+ 0.31865


As one can tell the HOMO-LUMO gap of the functionalized cations has significantly decreased with respect to tetramethyl ammonium cation. The HOMO-LUMO energy difference generally provides a good insight to the cation's stability. The smaller the energy gap the less its structure will be destabilised by addition of electron density from an anion.

As a consequence, both functional groups will lead to more stable ion pairs than what tetramethyl ammonium would.





Conclusion

In this computational study the properties of the tetramethyl -onium cations of the first three group V elements were succesfully reproduced and their charge distributions were rationalized using NBO analysis and second order perturbation theory of the Fock matrix in an NBO basis. Different functional groups were introduced to the tetramethyl ammonium cation in order to examine the difference in properties that would arise. Not surprisingly, when the electron withdrawing group -CN, was introduced, the LUMO orbital of the cation was shifted lower in energy and therefore its electron accepting properties were enhanced. The opposite was true when an electron donating group, -OH, was added. This demonstrated the ability of ionic liquids to be engineered as to compliment a reaction of interest.

References

  1. ↑ 1. E. Trush, O. Shishkin, V. Trush, I. Konovalova, T. Sliva, Acta Crystallographica, 2012. DOI:10.1107/S1600536811055024
  2. ↑ 2. D. Lide, D. Mann, The Journal of Chemical Physics, 2004, 29, p. 914. DOI:10.1063/1.1744611
  3. ↑ 3. L. Brockway, H. Jenkins , Journal of the American Chemical Society, 1936, 58, p. 2037.
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