Hydrogen Bonding Study on Protic Ionic Liquids Composed of N-Alkyl Ethylenediaminum Cations with Trifluoroacetic Anion†

doi:10.7503/cjcu20180309

Abstract

Abstract:

All possible molecular structures of chelating mono-protic ionic liquids(PILs) composed of N-alkyl(hexyl, octyl and 2-ethylhexyl) ethylenediaminum cations with trifluoroacetic acid(TFA) anion that form [HHex][TFA], [HOct][TFA] and [HEtHex][TFA] were studied by the density functional theory(DFT) at M06-2X/6-311G(d,p) level. Hydrogen bond and stabilites of five possible structures S1—S5 were investigated. The obtained data show that the hydrogen atom has been transferred from N—H of [HAlkyl]⁺ to O of [TFA]^- in the ion pairs, and O—H…N hydrogen bond was formed. The obtained inter-molecular interaction energies(Δ $E int BSSE$ ) corrected by BSSE for all the structures are in the region of -449.91—-522.87 kJ/mol. The IR results show that N—H vibration spectra is disappeared. In the meantime, the stronger O—H vibration spectra was appeared in the region of 2200—2700 cm^-1since stronger O—H…N hydrogen bond was formed between [Alkyl] and [HTFA]. The natural bond orbital(NBO) and the atoms in molecules(AIM) analysis were applied to quantify the strength of hydrogen bond interactions and the effects of hydrogen bonding interactions on the stability of molecular configurations. NBO analysis shows that the stabilization energy mainly caused by the interaction between the lone pair electron LP(N) in the [HAlkyl] and the anti-bonding orbital σ^*(H—O) in the [TFA]-LP(N)→σ^*(H—O). The protonation site of [HAlkyl]⁺, and the effect of the alkyl chain length and side chain on the strength of the hydrogen bonding between [HAlkyl] and [TFA] were also discussed.

Key words: Protic ionic liquid, Density functional theory, Inter-molecular hydrogen bonding, Natural bond orbital(NBO), Atoms in molecules theory(AIM)

CLC Number:

O641

TrendMD:

XU Yu,HUA Er. Hydrogen Bonding Study on Protic Ionic Liquids Composed of N-Alkyl Ethylenediaminum Cations with Trifluoroacetic Anion^†[J]. Chem. J. Chinese Universities, 2018, 39(9): 1954.

Figures/Tables 5

Fig.1 Electrostatic potential surface of [HAlkyl]+ cations and [TFA]- anion at the M06-2X/ 6-311G(d,p) level N: Blue; O: red; H: white; C: gray; F: light blue.

Fig.2 Optimized structures of [HAlkyl][TFA] ion pairs at the M06-2X/6-311G(d,p) level The bond lengths and angles are in nm and degree, respectively.

Table 1 Interaction energy(ΔEint) and H-Bonding energies(ΔEHB) calculated for [HAlkyl][TFA] ion pairs at the M06-2X/6-311G(d,p) level*

Structure	ΔE_int/(kJ∙mol^-1)	BSSE/(kJ∙mol^-1)	Δ $E int BSSE$ /(kJ∙mol^-1)	ΔE_HB/(kJ∙mol^-1)
[HHex][TFA]S1	-539.40	83.85	-519.36	-81.55
[HHex][TFA]S2	-528.86	113.80	-501.66	-71.00
[HHex][TFA]S3	-501.62	123.64	-472.04	-75.23
[HHex][TFA]S4	-501.16	131.17	-469.82	-74.81
[HHex][TFA]S5	-510.74	107.32	-485.09	-84.35
[HOct][TFA]S1	-534.84	84.31	-514.72	-77.28
[HOct][TFA]S2	-521.70	126.11	-491.58	-64.14
[HOct][TFA]S3	-497.02	124.22	-467.31	-74.98
[HOct][TFA]S4	-496.77	130.79	-465.51	-74.73
[HOct][TFA]S5	-506.18	106.69	-480.70	-84.14
[HEtHex][TFA]S1	-539.65	70.21	-522.87	-81.55
[HEtHex][TFA]S2	-529.07	113.26	-502.00	-70.92
[HEtHex][TFA]S3	-481.45	132.01	-449.91	-69.79
[HEtHex][TFA]S4	-490.62	144.47	-456.06	-78.91
[HEtHex][TFA]S5	-499.78	115.60	-472.16	-88.12

Table 1 Interaction energy(ΔEint) and H-Bonding energies(ΔEHB) calculated for [HAlkyl][TFA] ion pairs at the M06-2X/6-311G(d,p) level*

Structure	ΔE_int/(kJ∙mol^-1)	BSSE/(kJ∙mol^-1)	Δ $E int BSSE$ /(kJ∙mol^-1)	ΔE_HB/(kJ∙mol^-1)
[HHex][TFA]S1	-539.40	83.85	-519.36	-81.55
[HHex][TFA]S2	-528.86	113.80	-501.66	-71.00
[HHex][TFA]S3	-501.62	123.64	-472.04	-75.23
[HHex][TFA]S4	-501.16	131.17	-469.82	-74.81
[HHex][TFA]S5	-510.74	107.32	-485.09	-84.35
[HOct][TFA]S1	-534.84	84.31	-514.72	-77.28
[HOct][TFA]S2	-521.70	126.11	-491.58	-64.14
[HOct][TFA]S3	-497.02	124.22	-467.31	-74.98
[HOct][TFA]S4	-496.77	130.79	-465.51	-74.73
[HOct][TFA]S5	-506.18	106.69	-480.70	-84.14
[HEtHex][TFA]S1	-539.65	70.21	-522.87	-81.55
[HEtHex][TFA]S2	-529.07	113.26	-502.00	-70.92
[HEtHex][TFA]S3	-481.45	132.01	-449.91	-69.79
[HEtHex][TFA]S4	-490.62	144.47	-456.06	-78.91
[HEtHex][TFA]S5	-499.78	115.60	-472.16	-88.12

Table 2 Significant natural bond orbital interactions of the ion pairs and their second order stabilization energies E(2) calculated at the M06-2X/6-311G(d,p) level

Structure	Charge transfer	E⁽²⁾/(kJ∙mol^-1)
[HHex][TFA]S1	LP(N1)→BD^*(Hp—O2)	256.23
[HHex][TFA]S2	LP(N1)→BD^*(H2—O2)	233.89
[HHex][TFA]S3	LP(N2)→BD^*(H7—O2)	271.58
[HHex][TFA]S4	LP(N2)→BD^*(H7—O2)	240.50
[HHex][TFA]S5	LP(N2)→BD^*(H7—O1)	288.78
[HOct][TFA]S1	LP(N1)→BD^*(Hp—O2)	254.64
[HOct][TFA]S2	LP(N1)→BD^*(H1—O1)	213.55
[HOct][TFA]S3	LP(N2)→BD^*(Hp—O2)	273.26
[HOct][TFA]S4	LP(N2)→BD^*(Hp—O2)	246.44
[HOct][TFA]S5	LP(N2)→BD^*(H7—O2)	285.85
[HEtHex][TFA]S1	LP(N1)→BD^*(Hp—O2)	243.76
[HEtHex][TFA]S2	LP(N1)→BD^*(H1—O1)	233.68
[HEtHex][TFA]S3	LP(N2)→BD^*(H7—O2)	301.75
[HEtHex][TFA]S4	LP(N2)→BD^*(H7—O2)	283.21
[HEtHex][TFA]S5	LP(N2)→BD^*(H7—O1)	328.70

Table 3 Properties of the electron density of bond critical points for the intermolecular interactions in configurations [HAlkyl][TFA] calculated at the M06-2X/6-311G(d,p) level*

Structure	BCP	ρ_c/a.u.	▽²ρ_c/a.u.	G(r)/a.u.	V(r)/a.u.	H(r)/a.u.
[HHex][TFA]S1	O2—Hp…N1	0.0743	0.0878	0.0486	-0.0753	-0.0267
[HHex][TFA]S2	O2—H2…N1	0.0699	0.0905	0.0459	-0.0693	-0.0233
[HHex][TFA]S3	O2—H7…N2	0.0834	0.0758	0.0526	-0.0863	-0.0337
[HHex][TFA]S4	O2—H7…N2	0.0754	0.0820	0.0481	-0.0757	-0.0276
[HHex][TFA]S5	O1—H7…N2	0.0857	0.0690	0.0530	-0.0888	-0.0358
[HOct][TFA]S1	O2—Hp…N1	0.0745	0.0843	0.0483	-0.0756	-0.0273
[HOct][TFA]S2	O1—H1…N1	0.0635	0.0910	0.0420	-0.0612	-0.0192
[HOct][TFA]S3	O2—Hp…N2	0.0838	0.0752	0.0528	-0.0868	-0.0340
[HOct][TFA]S4	O2—Hp…N2	0.0768	0.0804	0.0487	-0.0774	-0.0286
[HOct][TFA]S5	O2—H7…N2	0.0852	0.0671	0.0524	-0.0881	-0.0357
[HEtHex][TFA]S1	O2—Hp…N1	0.0743	0.0841	0.0482	-0.0753	-0.0271
[HEtHex][TFA]S2	O1—H1…N1	0.0698	0.0905	0.0459	-0.0692	-0.0233
[HEtHex][TFA]S3	O2—H7…N2	0.0888	0.0633	-0.0923	-0.0885	-0.1808
[HEtHex][TFA]S4	O2—H7…N2	0.0840	0.0705	0.0519	-0.0861	-0.0343
[HEtHex][TFA]S5	O1—H7…N2	0.0937	0.0501	0.0555	-0.0985	-0.0430

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