Hamiltisla2

Lab1 Marking

You have a very good working wiki. It would be good if you report values of wavenumber in your answers. Overall a very good attempt. If you have any specific questions, do email Prof. Hunt

NH₃ Molecule

Calculation Data

Name of submitted log file	hamiltisla_nh3_optf.log
Molecule	NH3
Method	RB3LYP
Basis set	6-31G(d,p)
Final energy	-56.557769
RMS gradient	0.000000
Point group	C3v

Logfile: Media:HAMILTISLA_NH3_OPTF_POP.LOG

Convergence Confirmation

Item Table

         Item               Value     Threshold  Converged?
 Maximum Force            0.000000     0.000015     YES
 RMS     Force            0.000000     0.000010     YES
 Maximum Displacement     0.000003     0.000060     YES
 RMS     Displacement     0.000001     0.000040     YES

Low Frequency Analysis

Confirmation that the stable minimum was reached during optimisation is seen as: -20 < low frequency row 1 < 20

Low frequencies ---   -5.6864   -3.6131   -3.6124    0.0017    0.0048    0.0162
Low frequencies --- 1089.3674 1693.9284 1693.9284

Optimised NH₃ Molecule Image

3D Rotateable NH₃ Molecule

Optimised NH₃ Molecule

Important Geometric Parameters

Optimised bond distance for NH₃
r(N-H)= 1.02Â
Optimised bond angle for NH₃
θ(H-N-H)= 106°

Vibrational Analysis

Vibrations Table

Mode	1	2	3	4	5	6
Wavenumber (cm-1)	1089	1694	1694	3461	3590	3590
Symmetry	A1	E	E	A1	E	E
Intensity (arbitrary units)	145	14	14	1	0	0

IR Spectrum of optimised NH₃

Charge Analysis

Charge distribution of Nh₃

Atom	Charge
N	-1.13
H	0.38

Project Molecule: N₂F₂

Calculation Data

Name of submitted log file	hamiltisla_n2f2_optimisation.log
Molecule	N2F2
Method	RB3LYP
Basis set	6-31G(d,p)
Final energy	-309.012413
RMS gradient	0.000000
Point group	C2v

Logfile: Media:HAMILTISLA N2F2 OPTIMISATION.LOG

Convergence Confirmation

Item Table

         Item               Value     Threshold  Converged?
 Maximum Force            0.000001     0.000015     YES
 RMS     Force            0.000000     0.000010     YES
 Maximum Displacement     0.000001     0.000060     YES
 RMS     Displacement     0.000001     0.000040     YES

Low Frequency Analysis

Confirmation that the stable minimum was reached during optimisation is seen as: -20 < low frequency row 1 < 20

Low frequencies ---    0.0005    0.0005    0.0013    3.2233    4.3533    5.0998
Low frequencies ---  347.8772  561.2472  771.6105

Optimised N₂F₂ Molecule Image

The molecule from the log file did not have bonds between the F and N atoms due to the bond distance of 1.40Â exceeding the bond distance limit Gauss view will draw bonds for. Thus in the above image the N-F bonds were added using the modify bond tool, and the bond distance was not changed (consistent with the optimised value).

3D Rotateable N₂F₂ Molecule

Optimised N₂F₂ Molecule

Important Geometric Parameters

Optimised bond distances for N₂F₂
r(N-F)= 1.40Â
r(N=N)= 1.22Â

Optimised bond angles for N₂F₂
θ(F-N-F)= 114°
Dihedral angle θ(F-N=N-F)= 0°

Vibrational Analysis

Vibrations Table

Mode	1	2	3	4	5	6
Wavenumber (cm-1)	348	561	772	949	987	1637
Symmetry	A1	A2	B2	A1	B2	A1
Intensity (arbitrary units)	1	0	75	75	81	21

IR spectrum of optimised N₂F₂

Vibrational Analysis Questions

How many vibrations are expected from the 3N-6 rule?

• N=4, thus from the 3N-6 rule, 3(4)-6=12-6=6, thus 6 vibrational modes are expected. It can be seen that 6 vibrational modes are indeed found.
  

Why are there only 4 peaks in the IR spectrum?

• Vibrational modes 1 & 2 do not have peaks corresponding to them in the IR spectrum, as they have essentially zero intensity of their vibrational modes due to nearly no net dipole moment. Vibrational modes 3,4,5 and 6 are represented by the 4 peaks in the spectrum.
  

Which vibration is the asymmetric N-F stretch?

• The vibration that is the asymmetric N-F stretch is mode 3 in the table. Mode 3 has wavenumber 772 cm^-1, and intensity 75.
  

What is the nature of the highest energy vibration?

• The highest energy vibrational mode is mode 6, the N=N bond stretching mode. It has wavenumber 1637 cm^-1, and intensity 21.
  

Charge Analysis

Charge Distribution of N₂F₂

Atom	Charge
N	0.22
F	-0.22

Molecular Orbital Analysis

Which MOs are core orbital MOs?

• The core MOs are MOs 1, 2, 3, and 4.
  

Molecular Orbital 9 of N₂F₂

LCAO of Molecular Orbital 9 of N₂F₂

Molecular orbital 9 of cis N₂F₂ is the in phase linear combination of p_z orbitals of the N and F atoms

BH₃ Molecule

Calculation Data

Name of submitted log file	hamiltisla_bh3_optimisation.log
Molecule	BH3
Method	RB3LYP
Basis set	6-31G(d,p)
Final energy	-26.615324 Hartree
RMS gradient	0.000002
Point group	D3h

Logfile: Media:HAMILTISLA BH3 OPTIMISATION.LOG

Optimisation Confirmation

Item Table

        Item               Value     Threshold  Converged?
 Maximum Force            0.000004     0.000015     YES
 RMS     Force            0.000003     0.000010     YES
 Maximum Displacement     0.000017     0.000060     YES
 RMS     Displacement     0.000011     0.000040     YES

Low Frequency Analysis
Confirmation that the stable minimum was reached during optimisation is seen as: -20 < low frequency row 1 < 20

Low frequencies (cm^-1)---  -12  -12   -7    0    0   0
Low frequencies (cm^-1)--- 1160 1210 1210

Optimised BH₃ Molecule image and Jmol

3D Rotatable BH₃ Molecule

Optimised BH₃ Molecule Image

Important Geometric Parameters

Optimised bond distance for BH₃
r(B-H)= 1.19Å
Optimised bond angle for BH₃
θ(H-B-H)= 120°

Vibrational Analysis

Vibrations Table
Mode	1	2	3	4	5	6
Wavenumber (cm-1)	1163	1213	1213	2583	2716	2716
Symmetry	A2' '	E'	E'	A1'	E'	E'
Intensity (arbitrary units)	93	14	14	0	126	126

IR Spectrum of optimised BH₃

3 peaks are seen in this spectrum consistent with the modes 2 and 3 having identical wavenumber (1213 cm^-1), modes 5 and 6 having identical wavenumber (2716 cm^-1) and mode 1 having wavenumber 1163 cm^-1. Mode 4 has intensity of 0, thus does not show a peak in the IR spectrum.

NH₃BH₃ Molecule

Calculation Data

Name of submitted log file	hamiltisla_nh3bh3_optimisation.log
Molecule	NH3BH3
Method	RB3LYP
Basis set	6-31G(d,p)
Final energy	-83.224689 Hartree
RMS gradient	0.000001
Point group	C1

Logfile: Media:HAMILTISLA NH3BH3 OPTIMISATION.LOG

Optimisation Confirmation

Item Table

 Item               Value     Threshold  Converged?
 Maximum Force            0.000001     0.000015     YES
 RMS     Force            0.000001     0.000010     YES
 Maximum Displacement     0.000043     0.000060     YES
 RMS     Displacement     0.000019     0.000040     YES

Low Frequency Analysis
Confirmation that the stable minimum was reached during optimisation is seen as: -20 < low frequency row 1 < 20

Low frequencies (cm^-1)---   -5   -3   0   0   0   1
Low frequencies (cm^-1)---  263  633  638

Optimised NH₃ Molecule image and Jmol

3D Rotateable NH₃BH₃ Molecule

Optimised NH₃BH₃ Molecule Image

Important Geometric Parameters

Optimised bond distance for NH₃BH₃
r(N-B)= 1.67Å
r(B-H)= 1.21Å
r(N-H)= 1.02Å
Optimised bond angle for NH₃BH₃
θ(H-B-H)= 114°
θ(H-N-H)= 108°
θ(H-B-N)= 105°
θ(H-N-B)= 111°

Vibrational Analysis

Vibrations Table
Mode	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18
Wavenumber (cm-1)	263	633	638	638	1069	1069	1196	1204	1204	1329	1676	1676	2472	2532	2532	3464	3581	3581
Symmetry	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A	A
Intensity (arbitrary units)	0	14	4	4	41	41	109	3	3	114	28	28	67	231	231	3	28	28

IR Spectrum of optimised NH₃BH₃

8 peaks are seen in the IR spectrum of NH₃BH₃, consistent with modes 1, 3, 4, 8, 9, and 16 having intensities too low to be seen on the spectrum. The peaks correspond to mode 2 (633 cm^-1), modes 5 and 6 (1069 cm^-1), mode 7 (1196 cm^-1), mode 10 (1329 cm^-1), modes 11 and 12 (1676 cm^-1), mode 13 (2472 cm^-1), modes 14 and 15 (2532 cm^-1), and modes 17 and 18 (3581 cm^-1).

Association Energy

E(NH₃)= -56.557769 au
E(BH₃)= -26.615324 au
E(NH₃BH₃)= -83.224689 au

Association Energy:
ΔE = E(NH3BH3) - [E(NH3) + E(BH3)]
=> ΔE= -83.224689 - [ -56.557769 - 26.615324] = -0.0516 au (3sf)
=> ΔE= -136 kJ/mol

This association energy shows the stabilisation that occurs when NH₃ and BH₃ form the adduct NH₃BH₃. The negative value of ΔE (−136 kJ/mol) indicates that formation of the adduct is energetically favourable relative to the separated fragments, meaning the bonded system is more stable than NH₃ and BH₃ alone.

Me₃NH-Cl Molecule

Calculation Data

Name of submitted log file	hamiltisla_Me3NHCl_optimisation.log
Molecule	Me3NH-Cl
Method	RB3LYP
Basis set	3-21G
Final energy	-632.162083 Hartree
RMS gradient	0.000016
Point group	C1

Logfile: Media:HAMILTISLA ME3NHCL OPTIMISATION.LOG

Optimisation Confirmation

Frequency Item Table

          Item               Value     Threshold  Converged?
 Maximum Force            0.000043     0.000450     YES
 RMS     Force            0.000010     0.000300     YES
 Maximum Displacement     0.001476     0.001800     YES
 RMS     Displacement     0.000355     0.001200     YES

Low Frequency Analysis
Confirmation that the stable minimum was reached during optimisation is seen as: -10 < low frequency row 1 < 10

Low frequencies (cm^-1)---   -5   0    0    0   3   9
Low frequencies (cm^-1)---   55  57  190

Optimised Me₃NH-Cl Molecule image and Jmol

3D Rotateable Me₃NH-Cl Molecule

Optimised Me₃NH-Cl Molecule Image

Optimised bond distances for Me₃NH-Cl

r(N-H)= 1.16Å
r(H-Cl)= 1.74Å
r(N-Cl)= 2.90Å

Scan of Total Energy

Rigid Scan log file: File:HAMILTISLA ME3NHCL RIGIDSCAN THURS7.LOG

Processed PES plot of Me₃NH-Cl

The scanned coordinate corresponds to the N–H bond length, starting at 0.8 Å and increasing in 0.1 Å increments up to 2.1 Å. This tracks the gradual shift of the proton between the ionic ion-pair (Me3NH+ - - Cl-) and a neutralised form (Me3N - - HCl). The PES shows that, for the scan with N–Cl distance fixed at 3.2 Å, the most stable configuration corresponds to the ion-pair form (Me3NH+ - - Cl-), with a clear energy minimum in this region.

At longer N–H distances, a broad shelf is observed rather than a distinct second minimum, indicating that proton transfer towards a neutral Me3N - - HCl pair is not energetically favoured. As the proton is moved towards chloride, the energy increases smoothly, consistent with a gradual loss of stabilising ion-pair interactions rather than formation of a different stable structure.

Raw PES data table

Scan Coordinate (Å)	Total Energy (Hartree)	Relative Total Energy (kJ/mol)
0.8	-632.0662556	229.9
0.9	-632.122424	82.4
1.0	-632.1460522	20.4
1.1	-632.1535102	0.8
1.2	-632.1538035	0
1.3	-632.1515778	5.8
1.4	-632.1490207	12.6
1.5	-632.1470189	17.8
1.6	-632.1457428	21.2
1.7	-632.1447861	23.7
1.8	-632.1429362	28.5
1.9	-632.1375943	42.6
2.0	-632.1238571	78.6
2.1	-632.093225	159.0

I have made the choice to report the relative energies in kJ/mol to 1dp instead of round to the nearest whole number to ensure they are easily comparable when reading the graph.

Ionic liquid pair 1-methyl-imidazolium chloride (HMim-Cl)

Molecule a)

Log File: File:IH IONPAIR A OPTIMISATION.LOG

Calculation Data

Name of submitted log file	IH_ionpair_a_optimisation.log
Molecule	HMim-Cl
Method	RB3LYP
Basis set	3-21G
Final energy	-722.687898
RMS gradient	0.000028
Point group	C1

Frequency Item table

  Item               Value     Threshold  Converged?
 Maximum Force            0.000053     0.000450     YES
 RMS     Force            0.000018     0.000300     YES
 Maximum Displacement     0.001533     0.001800     YES
 RMS     Displacement     0.000443     0.001200     YES

Low Frequency analysis
Confirmation that the stable minimum was reached during optimisation is seen as: -10 < low frequency row 1 < 10

Low frequencies (cm^-1)---   -4   -1   0   0    0    3
Low frequencies (cm^-1)---   36   64   80 

Important Geometric Parameters
Atom relevant to bond is indicated by number 1-14 in labeled diagram below
[7-14] r(H-Cl) = 1.72Å;
[9-7] r(N-H) = 1.18Å
[1-4] r(C-H) = 1.07Å
[3-6] r(C-H) = 1.07Å

HMim-Cl Molecule a Energy Scan

N-H Rigid scan of molecule a log file: File:IH IONPAIR A RIGIDSCAN THURS7.LOG

Gaussview raw Potential Energy Surface plot of HMim-Cl molecule a

Processed Potential Energy Surface of HMim-Cl molecule a) rigid scan

This rigid scan shows how the energy of HMim–Cl changes as the N–H distance is varied from 0.8Å to 2.1Å, while the N–Cl distance is fixed at 3.2Å. The lowest energy occurs at 1.1Å, which corresponds to the most stable N–H - - Cl interaction is the equilibrium for the ion pair along this coordinate.

At shorter distances (<1.0Å), the energy rises steeply due to strong repulsion between the nuclei as they are forced too close together. At longer distances (>1.2Å), the energy increases more gradually as the N–H - - Cl interaction weakens and the system moves towards dissociation.

Raw Data of rigid energy scan of HMim-Cl Molecule a

Scan Coordinate (Å)	Total Energy (Hartree)	Relative Total Energy (kJ/mol)
0.8	-722.595816	219.4
0.9	-722.6504012	76.1
1.0	-722.6727673	17.3
1.1	-722.6793742	0
1.2	-722.6792901	0.2
1.3	-722.6772109	5.7
1.4	-722.675321	10.6
1.5	-722.6744387	13.0
1.6	-722.6746233	12.5
1.7	-722.6753492	10.6
1.8	-722.6753453	10.6
1.9	-722.672138	19.0
2.0	-722.6613138	47.4
2.1	-722.6354852	115.2

Molecule b)

Log File: File:IH IONPAIR B OPTIMISATION.LOG

Calculation Data

Name of submitted log file	IH_ionpair_b_optimisation.log
Molecule	HMim-Cl
Method	RB3LYP
Basis set	3-21G
Final energy	-722.666200 Hartree
RMS gradient	0.000018
Point group	C1

Frequency Item table

 Item               Value     Threshold  Converged?
 Maximum Force            0.000192     0.000450     YES
 RMS     Force            0.000040     0.000300     YES
 Maximum Displacement     0.001419     0.001800     YES
 RMS     Displacement     0.000215     0.001200     YES

Low Frequency analysis
Confirmation that the stable minimum was reached during optimisation is seen as: -10 < low frequency row 1 < 10

Low frequencies (cm^-1)---   -6   -4   -2   0   0   0
Low frequencies (cm^-1)---   45  162  199

Important Geometric Parameters
Atom relevant to bond is indicated by number 1-14 in labeled diagram below
[4-15] r(H-Cl) = 2.13Å
[12-14] r(H-Cl) = 2.28Å
[2-5] r(C-H) = 1.10Å
[10-12] r(C-H) = 1.11Å

Molecule c)

Log File: File:IH IONPAIR C OPTIMISATION.LOG

Calculation Data

Name of submitted log file	IH_ionpair_c_optimisation.log
Molecule	HMim-Cl
Method	RB3LYP
Basis set	3-21G
Final energy	-761.779525
RMS gradient	0.000014
Point group	C1

Frequency Item table

  Item               Value     Threshold  Converged?
 Maximum Force            0.000050     0.000450     YES
 RMS     Force            0.000009     0.000300     YES
 Maximum Displacement     0.001308     0.001800     YES
 RMS     Displacement     0.000306     0.001200     YES

Low Frequency analysis
Confirmation that the stable minimum was reached during optimisation is seen as: -10 < low frequency row 1 < 10

Low frequencies (cm^-1)---   -4   -3   0   0   0    2
Low frequencies (cm^-1)---   52  103  107

Important Geometric Parameters
Atom relevant to bond is indicated by number 1-17 in labeled diagram below
[4-17] r(H-Cl) = 2.03Å
[16-17] r(H-Cl) = 2.42Å
[1-4] r(C-H) = 1.12Å
[13-16] r(C-H) = 1.10Å
[3-5] r(C-H) = 1.07Å
[9-12] r(C-H) = 1.09Å

Ionic Liquid Analysis

Bond Length Comparison Table

Ion Pair Molecule	H-Cl bond length (Å)	H-Cl bond length additional (Å)
HMim-Cl a	1.72
HMim-Cl b	2.13 [Cis alkene H to Cl]	2.28 [Methyl H to Cl]
HMim-Cl c	2.03
Me3NH-Cl	1.74

How do the H-Cl bonds of Me3NH and HMim compare and how do the H-bonds of the N-H and C-H H-bonds compare?

• The H-Cl bonding in both Me₃NH–Cl [1.74 Å] and HMim–Cl molecule a [1.72 Å] is quite strong, with very similar H-Cl distances (Difference of 0.02 Å). In both of these cases the proton involved is attached to a nitrogen, producing a highly polarised N–H bond that strongly attracts the chloride. The other HMim–Cl molecules, b and c, show longer H-Cl distances than those of molecule a and Me₃NH–Cl. Molecule b has a 2.13 Å bond from its cis alkene proton to the chloride and a 2.28 Å bond from the methyl protons to the chloride. Molecule c has a 2.03 Å bond from its CH proton to the chloride. These longer distances correspond to weaker hydrogen bonding interactions in the molecules. In structures b and c the chloride interacts with C-H protons rather than N-H protons. Since C–H bonds are less polarised than N–H bonds due to the greater electronegativity difference for N-H bonds than C-H bonds, the hydrogen atoms carry a smaller partial positive charge in C-H bonds and therefore form weaker hydrogen bonds with chloride.

Are these distances representative of a H-bond?

• Van der Waals radius of Cl = 1.75 Å
  
• Van der Waals radius of H = 1.20 Å
  
• Sum of the Van der Waals radii of H and Cl = 1.75 + 1.20 = 2.95 Å
  
• All the observed H-Cl distances for the studied ion pairs are shorter than 2.95 Å. Distances shorter than the sum of the van der Waals radii indicates attractive interactions that are stronger than simple intermolecular interactions thus this consistent with the hydrogen bonding seen in the studied ion pairs. The much shorter distances for Me₃NH–Cl [1.21 Å shorter] and HMim–Cl molecule a [1.73 Å shorter] are consistent with partially ionic hydrogen bonds.

Will the ionic nature of the ions effect a distance based assessment of H-bonding?

• Yes the ionic nature of the ions will effect a bond distance based assessment. In the ion pairs the positive and negative ions attract each other, which can shorten the H-Cl bond distance. This implies that bond distance alone shouldn't be used to assess the strength of the H-bonding.

Association Energy Comparison of HMim–Cl isomers

Molecule a and b cation HMim+
The cation cluster of a and b are the same, therefore using the same optimisation of HMim+ in 3-21G basis set:

Log file: File:IH A B CATION CLUSTER OPTIMISATIN.LOG

Optimised HMim+ Molecule a and b cation

Calculation Table

Name of submitted log file	IH_a_b_cation_cluster_optimisation.log
Molecule	Hmim+
Method	UB3LYP
Basis set	3-21G
Final energy	-264.589131 au
RMS gradient	0.000011
Point group	C1

Frequency Item Table

  Item               Value     Threshold  Converged?
 Maximum Force            0.000030     0.000450     YES
 RMS     Force            0.000006     0.000300     YES
 Maximum Displacement     0.001212     0.001800     YES
 RMS     Displacement     0.000357     0.001200     YES

Low Frequency Analyis

Low frequencies ---   -5   -4   0   0   0    4
Low frequencies ---   91  155  336

Molecule 3 cation cluster Hmim+ with extra methyl Log file: File:IH A B CATION CLUSTER OPTIMISATIN.LOG

Optimised HMim+ Molecule c cation

Calculation Table

Name of submitted log file	IH_c_cation_optimisation.log
Molecule	Hmim+
Method	UB3LYP
Basis set	3-21G
Final energy	-303.687395 au
RMS gradient	0.000006
Point group	C1

Frequency Item Table

  Item               Value     Threshold  Converged?
 Maximum Force            0.000014     0.000450     YES
 RMS     Force            0.000003     0.000300     YES
 Maximum Displacement     0.000247     0.001800     YES
 RMS     Displacement     0.000076     0.001200     YES

Low Frequency Analysis

Low frequencies ---   -4   0    0    0   3   4
Low frequencies ---   83   98  147

Chlorine anion optimisation
Logfile: File:IH CHLORINE ANION OPTIMISATION.LOG

Name of submitted log file	IH CHLORINE ANION OPTIMISATION.LOG
Molecule	Cl-
Method	UB3LYP
Basis set	3-21G
Final energy	-457.945733 au
RMS gradient	0.000000
Point group	C1

Frequency Item Table

 Item               Value     Threshold  Converged?
 Maximum Force            0.000000     0.000450     YES
 RMS     Force            0.000000     0.000300     YES
 Maximum Displacement     0.000000     0.001800     YES
 RMS     Displacement     0.000000     0.001200     YES

Frequency Analysis table

Low frequencies ---    0    0   0

Optimised HMim+ Molecule c cation

Association Energy Table

ΔE = E(Products) - [E(Reactants)] = E(Ion pair) - [E(Cation) + E(Anion)]

HMim–Cl isomer	Total Energy (au)	Association Energy ΔE (au)	Association Energy ΔE (kJ/mol)
a)	-722.687898	-0.153034	-402
b)	-722.666200	-0.131336	-345
c)	-761.779525	-0.146397	-384
HMim+ cation a and b	-264.589131
HMim+ cation c	-303.687395
Chloride anion	-457.945733

Relative Energy between HMim–Cl isomers a) and b)
ΔE = E_b - E_a = (-722.666200) - (-722.687898) = 0.021698 au
=> ΔE = 56.97 kJ/mol (4sf)
Therefore, isomer b is 57 kJ/mol higher in energy than isomer a. This indicates that isomer a is more stable and lower in energy than isomer b. This is consistent with isomer a forming a strong N–H-Cl hydrogen bond with a short H-Cl distance of 1.72 Å. The N–H bond is highly polarised, so the hydrogen carries a partial positive charge and interacts strongly with the chloride anion. In comparison isomer b forms weaker C–H-Cl interactions with longer H-Cl distances of 2.13 Å and 2.28 Å. The C–H bonds in b are less polarised than the N–H bond in a, resulting in weaker electrostatic attraction to chloride and therefore less stabilisation. As a result, isomer b is higher in energy than isomer a.

Discuss the dissociation energy of (c) relative to (a) and (b). What does the comparison tell us about the H-bonding?
The association energy of HMim–Cl isomer c (−384 kJ/mol) is intermediate between isomer a (−402 kJ/mol) and isomer b (−345 kJ/mol). Since more negative association energies correspond to stronger stabilisation and larger dissociation energies, this suggests that isomer c forms a stronger ion-pair interaction than b, but a weaker interaction than a.

This trend is consistent with the H - - Cl bond distances observed for the three isomers. Isomer a has the shortest H - - Cl distance (1.72Å), consistent with a strong N–H - - Cl hydrogen bond. In contrast, isomer b contains longer C–H - - Cl contacts (2.13Å and 2.28Å), which are weaker and result in a less stabilised ion pair. Isomer c has an intermediate H - - Cl distance of 2.03 Å and correspondingly intermediate stabilisation.

Chemical diagram of the two protonation states for HMim-Cl

HMim-Cl and Me3NH-Cl PES Comparison

MeNH-Cl and HMim-Cl PES plot

• Both the HMim–Cl and Me3NH–Cl PES plots show a clear minimum around 1.1–1.2 Å, which corresponds to the lowest energy and therefore most stable N–H-Cl hydrogen bond interaction. In both cases, the energy rises sharply at distances shorter than 1.1 Å due to strong repulsion when the nuclei are forced too close together, and increases more gradually at distances longer than 1.2 Å as the N–H interaction with chloride is reduced and the system moves towards dissociation.

• The overall shapes of the two curves are very similar, but HMim–Cl shows a slightly deeper and broader minimum compared to Me3NH–Cl. This suggests that HMim–Cl is more stabilised and less sensitive to small changes in N–H distance near the scan coordinate equilibrium than Me3NH-Cl. This difference is also reflected at the extremes of the scan. Me3NH–Cl is higher in energy at both short (0.8 Å) and long (2.1 Å) N–H distances, and its energy increases more steeply after 1.2 Å, indicating a less stabilised interaction when the hydrogen bond is stretched.

• One possible reason for these differences in the PES of the ion pairs is the difference in electronic structure of their respective cations. In HMim-Cl, the positive charge is delocalised over the imidazolium ring, which may increase the polarisation of the N–H bond compared to the more localised charge in Me3NH–Cl. This may leads to HMin-Cl having a stronger interaction with chloride than Me3NH-Cl.