1. Introduction

2. Materials and Methods

3. Results and discussion

3.3. KEMS

The background subtracted electron ionization mass spectra of the most intensive ions under different evaporation conditions are shown in Figure 5. The full-scale mass spectra of the EMImPF₆ can be found in Supplementary Materials (Figures S1–S4). One can see that spectra under OC conditions are temperature independent. Under equilibrium conditions, a slight temperature trend of increasing intensity of ions corresponding to the fragmentation of C₆N₂H₁₀PF₅ (see below) was revealed. During the analysis of the mass spectra presented in Figure 5 and Figure 6 the intensity of the parent cation C₆N₂H₁₁⁺ was corrected by the intensity of the second isotope of the ion C₆N₂H₁₀⁺ since the latter makes a significant contribution to the peak with m/z = 111. The ion with m/z = 82 is dominating under equilibrium conditions while under OS and OC conditions the parent cation with m/z = 111 is the most intensive. In all spectra except OS the ions corresponding to fragmentation of C₆N₂H₁₀PF₅ are present. The peak with m/z = 217 corresponds to fluorine detachment (C₆N₂H₁₀PF₄⁺) and that with m/z = 169 corresponds to a cleavage of C₆N₂H₁₀PF₄⁺ onto C₂H₅F and C₄N₂H₅PF₃⁺. All these ions are fingerprints of the formation of the substituted imidazole-2-ylidene. Similar ions were observed in the homologous BMImPF₆ in our work [4].

Under OC conditions, two interesting ions appear in the spectrum. One of them with m/z = 130 corresponds to the fluorine attachment to the parent cation. The same kind of ions were previously registered in similar BMImPF₆ and BMImBF₄ ILs [2,3,4,18]. The scheme of the formation of this ion is given in [18]. Another ion with m/z = 367 registered at 524 K (0.2% of the parent cation intensity) corresponds to the formula C₂A⁺, where C is the parent cation and A is the PF₆^– anion. This fact indicates the formation of a small amount of NIP dimer. Previously, dimerization was observed only for the ILs evaporated without decomposition [19].

The mass spectrum recorded under OS conditions contains only ions with m/z = 82, 110, 111. As it is shown earlier [2], evaporation in nonequilibrium condition suppresses the decomposition processes and the OS spectrum corresponds to that of NIP.

The mass spectrum of the distillate (Figure 6) corresponding to that of C₆N₂H₁₀PF₅ contains the following major ions (relative to intensity of ion with m/z = 82): 82 (100%), 217 (60%), 110 (50%), 169 (17%), 107 (15%), 81 (12%). The full-scale mass spectrum of the distillate is given in Supplementary Materials (Figure S5).

Ionization efficiency curves (IEC) for the most intensive ions were measured under different conditions and temperatures (Figure 7). The energy scale was calibrated by the ionization energy of H₂O (IE = 12.62 eV [20]). Appearance energies (AE) of the ions were determined by the vanishing current method (Table 1).

The AE values of the ions with m/z = 82 and 110 under both EC-I and EC-II conditions are temperature independent. Comparison of their AEs between EC and OC reveals a slight decreasing tendency against temperature. Both of these ions are also present in the OS mass spectrum and can originate from NIPs; the low intensity of the ion currents did not allow measuring their IECs under OS conditions. The values AE(82) = 9.9±0.3 eV and AE(110) = 8.6±0.1 eV being the average among EC-I, EC-II, OC (464 K), and OC (484 K) decrease by 2.0 and 0.6 eV, respectively under OC (524 K) conditions. This behavior can be explained as follows. Under equilibrium conditions, the major contribution into the current of these ions is due to decomposition products. The transition from the Knudsen conditions to the Langmuir conditions is accompanied by a decrease in the amount of decomposition products, therefore the contribution from NIPs becomes greater leading to decreasing of the AE values. The AE values of the ions with m/z = 130 and 367 under OC conditions at 524 K are 10.3±0.5 eV and 10.9±0.5 eV, respectively.

IECs for the IL distillate measured under EC-II conditions at 374 K are shown in Supplementary Materials (Figure S6). AEs of the main ions are collected in Table 1. One can see that they are close to those obtained on equilibrium conditions, confirming our suggestion that the C₆N₂H₁₀PF₅ is the main molecular precursor of the ions with m/z = 82 and 110. The AE(217) = 13.7±0.5 eV for distillate agrees with that measured for IL under EC-II conditions at 484 K (13.1±0.5 eV).

The temperature dependencies of the ion currents were measured under different evaporation conditions and their slope tangents are collected in Table 2. One can see that the slope of the temperature dependence of the ion with m/z = 111 is near under all conditions. The slopes for ions with m/z = 110, 111, and 130 under OC and EC-II conditions are very close to each other indicating NIPs as the common source of their origination. The slope for an ion with m/z = 217 corresponding to the detachment of fluorine from C₆N₂H₁₀PF₅ is rather high (-16.4). As expected, the ions with m/z = 82 and 110 have two sources of origination: NIPs and C₆N₂H₁₀PF₅. Hence, their slopes are intermediate values as it is shown in Table 2.

To identify the molecular precursors of the observed ions, the dependencies of the relative ion currents on the evaporation conditions are studied (Figure 8). One can see that the difference between the EC-I and EC-II conditions is negligible. The absence of a trend of the 130/111 ratio confirmed the suggestion that NIP is their common source. The same situation occurs for the 169/217 ratio pointing out the formation of both these ions from C₆N₂H₁₀PF₅. An ion with m/z = 110 under equilibrium conditions is primarily formed from substituted imidazole-2-ylidene, while under nonequilibrium conditions NIP is its main source. The main change in the considered ratios is observed at the transition from equilibrium (EC-I and EC-II) to nonequilibrium and intermediate (OC and OS) conditions. One can see that the ratios 82/111 and 110/111 are very close for OC and OS conditions indicating that in OC conditions the intensities of these ions are primarily determined by the contribution from NIPs.

Based on the above conclusions the following scheme of fragmentation of the vapor species is suggested (Figure 9). Some ions having only one molecular precursor are fingerprints of NIPs (m/z = 111 and 130) and C₆N₂H₁₀PF₅ (m/z = 169 and 217), while the others have two precursors.

The quantitative interpretation of the mass spectra was performed on a basis of the fragmentation coefficients derived from the OS mass spectrum of IL (NIP is the only molecular precursor) and the spectrum of the distillate (C₆N₂H₁₀PF₅ is the only molecular precursor). The contributions into the ion intensities from different sources were found by the following equations:

I(82/NIPs) = k(82/NIPs)·I(111),

(1)

I(110/NIPs) = k(110/NIPs)·I(111),

(2)

I(130/NIPs) = k(130/NIPs)·I(111),

(3)

I(82/C₆N₂H₁₀PF₅) = k(82/C₆N₂H₁₀PF₅)·I(217),

(4)

I(96/C₆N₂H₁₀PF₅) = k(96/C₆N₂H₁₀PF₅)·I(217),

(5)

I(107/C₆N₂H₁₀PF₅) = k(107/C₆N₂H₁₀PF₅)·I(217),

(6)

I(110/C₆N₂H₁₀PF₅) = k(110/C₆N₂H₁₀PF₅)·I(217).

(7)

The fractions of the vapor components (Figure 10) were calculated. One can see from Figure 8 that the vapor composition is almost temperature independent. Under equilibrium conditions, the substituted imidazole-2-ylidene C₆N₂H₁₀PF₅ dominates. The transition to Langmuir conditions leads to a significant increase of the NIP fraction in vapor.

The value of thermodynamic activity of IL is an important question to perform thermodynamic calculations of sublimation enthalpy. Decomposition products are high volatile and do not accumulate in the condensed phase as shown by NMR and IR spectroscopy of the IL residue after KEMS (see above). Hence the IL composition is practically unchanged and its activity is close to unity.

The partial pressures of the vapor species in the Knudsen cell were obtained using their fractions (Figure 10) and the total vapor pressure found by the Knudsen effusion method. The procedure was described earlier [21] The experiment was carried out at T = 540 K during 19 h using EC-I. The effective area of the effusion orifice, s = (3.40±0.66)·10^–8 m², was determined by sublimation of pure zinc, see [21] for details. The average molar mass of the EMImPF₆ IL vapor, M = 191.9 kg·kmol^–1, was calculated from the molar masses of the effusion products and their fractions (Figure 10). The found total vapor pressure (0.67 Pa) was used to obtain the sensitivity constant of the mass spectrometer. The experimentally measured ion currents were recalculated into the partial vapor pressures (Table 3) according to a conventional mass spectrometric procedure described elsewhere in details [19]. The temperature dependencies of the vapor pressures (Figure 11) in the form ln(p) = f(1000/T) were approximated by a linear equation ln(p/Pa) = a·10³/T + b with the a and b coefficients given in Table 4. One can see from Figure 11 that the vapor pressure of NIPs obtained in this work is close to that from [7].

The thermodynamic functions of the EMImPF₆ IL in the condensed state needed for thermochemical calculations were obtained from the temperature dependencies of the molar heat capacity. The available literature data [22,23,24] on heat capacity are restricted to T = 355 K and have only four points above melting. Since heat capacity of liquids usually poorly depends on temperature, the c_p° values above experimental temperatures were taken as a constant equal to c_p°(355 K) = 374 J·mol^–1·K^–1. S°(EMImPF_6,cr, 298.15 K) = 353 J·mol^–1·K^–1 was assessed by the method proposed by Glasser and Jenkins [25]. The density of EMImPF₆ in the condensed state needed for the entropy assessment was taken from [26]. The thermodynamic functions H°(Т)–H°(298.15) found in this way are in good agreement with those obtained by an alternative method given in [7] based on the thermal expansion coefficient, isothermal compressibility, and molar volume. The thermodynamic functions used in this work were approximated by polynomial

(8)

the coefficients of which are given in Table 5.

The sublimation enthalpies (Table 6) found in this work by the methods of the second and third law (see Table S2) agree to each other and to the values from [7] as well. Taking into account large uncertainties of the thermodynamic functions, the second law data are preferable. The value Δ_sH°(298.15 K) = 157±6 kJ·mol^–1 being an average between our data and those from [7] was recommended. Using this value and Δ_fH°(EMImPF_{6, s}, 298.15 K) = −2098.9±4.7 kJ·mol^–1 measured by solution calorimetry [7], the formation enthalpy of EMImPF₆ NIPs was found as –1942±8 kJ·mol^–1.

Δ_fH°(EMImPF_{6, g}, 298.15 K) was also calculated by the isodesmic reaction approach with the use of the composite G4 method. The required experimental data on the formation enthalpies of the reactants are taken from [27,28]. A full list of reactions used as well as the obtained formation enthalpies can be found in Supplementary Materials (Table S1). All values were treated by the Student’s method and the average Δ_fH°(EMImPF_{6, g}, 298.15 K) = –1937±3 kJ·mol^–1 was evaluated. This enthalpy is in good agreement with the experimental one.

The enthalpy of the gas-phase reaction (I) was calculated using experimentally determined partial pressures by the method of the second and third law (Table 6 and Figure S7). One can see large discrepancies in the obtained enthalpies and entropies. Theoretical calculation of the reaction enthalpy using G4 method gives Δ_rH°(298.15 K) = 75 kJ·mol^–1, which is much below both experimental values. It means that the experimental partial pressures of the decomposition products do not reach their equilibrium values. This phenomenon can be explained as the partial equilibrium caused by the kinetically hindered decomposition. Such behavior was previously observed for the similar BMImPF₆ IL [3,4].

To test the kinetic factor influence, the time dependencies of the intensity of the ions with m/z = 111 (corresponding to NIPs) and m/z = 110 (corresponding to C₆N₂H₁₀PF₅) were measured at cooling and heating between 484 and 504 K under EC-II conditions with 20 s step (heating rate is 5 K/min). The relative gain/loss of intensity were calculated according to following formula:

$$∆I_i=\frac{\left|I_i-I_0\right|}{I_{max}}\ast 100\%,$$

(9)

where I₀ is the ion current at the starting temperature, I_max the maximal ion current.

ΔI of an ion with m/z = 111 rises and falls faster than that with m/z = 110 (Figure 12). A rate of the intensity changes was found as a first derivative of the time dependence of the relative ion current. The rate of reaching a steady-state for an ion with m/z = 111 is higher than that with m/z = 110 both at heating and cooling. This fact indicates that the attainment of equilibrium in decomposition reaction (I) is kinetically hindered and any thermodynamic calculations for the decomposition products (C₆N₂H₁₀PF_{5, g} and HF, _g) are impossible.

The temperature dependencies of the ion currents measured for the IL distillate (90% of C₆N₂H₁₀PF₅, Figure S8) show that in the temperature range 332-374 K all ions are characterized by the close slopes indicating a single source of their origination. The determined vapor pressure of C₆N₂H₁₀PF₅ was fitted by a linear equation with coefficients given in Table 4. The plot of the temperature dependence of the vapor pressure is given in Supplementary materials (Figure S9). The assessed activity a(C₆N₂H₁₀PF₅) ≤ 2·10^–4 at an average temperature of the KEMS study of EMImPF₆ confirms our assumption that the activity of IL is close to 1. The upper limit of the assessed C₆N₂H₁₀PF₅ activity is determined by the fact that the studied distillate is not pure substituted imidazole-2-yidene. The sublimation enthalpy and entropy of C₆N₂H₁₀PF₅ at T = 350 K were 86±1 kJ·mol^–1 and 120±4 J·mol^–1·K^–1, respectively. The value of sublimation enthalpy is close to those of substituted alkylimidazoles [29]. The enthalpy of the C₆N₂H₁₀PF_{5, g} formation at 298.15 K (–1594±6 kJ·mol^–1) was calculated from the theoretical enthalpy of the reaction II and the found value Δ_fH°(EMImPF_{6, g}, 298.15 K).

5. Conclusions

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