2023 年 12 巻 1 号 p. A0124
The negative ion at m/z 20 observed at atmospheric pressure corona discharge ionization mass spectra has been identified by supplying the vapors of deuterium oxide (D2O) and H218O. From the mass shifts of the ion at m/z 20 observed with D2O and H218O, it was suggested that the chemical composition of the ion at m/z 20 is to be H4O. Further mass shift from m/z 20 to 22 was observed by supplying the vapor of perfluorokerocene, suggesting the chemical composition of H3F. The chemical compositions of the negative ions H4O− and H3F− were consistence with the dipole-bound complex states between hydrogen H2 and polar molecules such as H2O and hydrogen fluoride (HF) having dipole moments beyond a critical dipole moment of 1.625 D, theoretically proposed by Skurski and Simons. The ionic chemical compositions and structures of H4O− and H3F− obtained with density functional theory calculations implied that both dipole-bound complex H2O−…H2 and HF−…H2 can be formed by exothermic reactions by which H2 molecule is complexing with negative ions H2O− and HF−, respectively.
We and Nagato et al. have previously reported that atmospheric pressure corona discharge ionization (APCDI) of ambient air resulted in various kinds of negative ions Y− (Y=COx, HCOx, NOx, HNOx, Ox, HOx) and water clusters Y−(H2O)n.1–4) Although almost negative ions generated by APCDI have been identified, a small mass ion at m/z 20 remains unknown to date.3) The negative ion at m/z 20 can be observed at high voltage conditions such as −2.7 and −3.5 kV applied to the corona needle, while at low voltage conditions such as −1.9 kV, the hydroxide HO− and its water clusters HO−(H2O)n can be observed by accompanying a magic cluster of HO−(H2O)3 at m/z 71.2,3) The hydroxide HO− can be formed by an ion molecule reaction between O− and H2O1,5) or by the attachment of electron to hydroxyl radicals ·OH due to its positive electron affinity (EA, 1.83 eV),6) while it is believed that the hydroxyl radicals ·OH are generated via dissociation of water molecules on the tip of needle. The dissociation of water molecules into hydroxyl and hydrogen radicals (·OH + H·) may occur on the tip with high electric field strength such as 108–109 V/m resulting in over 100 eV kinetic energy of electrons.3,7) Regarding the dissociation of water molecules on the steel surface, Takahashi et al. showed that water molecules attached to the steel surface heated easily dissociate into ·OH and H· radicals.8) From this, it is expected that some kind of negative ions originated from hydrogen radical H· and/or hydride H− can be observed, because the hydrogen radical has a positive value of EA 0.75 eV.6)
Here we identify the negative ion at m/z 20 as a dipole-bound complex ion H4O− between hydrogen, water, and electron, proposed by Skurski and Simons.9) Another negative ion at m/z 22 corresponding to H3F− produced by supplying hydrogen fluoride (HF) gas is also identified as the dipole-bound complex. The stability and structures of H4O− and H3F− are discussed from the point of quantum chemical calculations.
All the mass spectra were obtained with a reversed geometry double-focusing mass spectrometer JMS-BU30 (JEOL, Tokyo, Japan) attached to a home-build ion source of APCDI. The schematic illustration and main experimental conditions have been reported elsewhere.2,3) The discharge gap d between the electrodes and the needle angle α with respect to the orifice axis were adjusted to 3 mm and 0 rad, respectively. An angle of 0 rad is defined as the needle being located on the orifice axis. The needle was located perpendicular to the orifice plate as a plane electrode and could be shifted parallel of 0 and 1 mm to the orifice plate. The DC voltage of −2.0 kV was applied to the needle relative to the orifice plate. It is of importance to recognize that the conditions of the angle at 0 rad and the center location at 0 mm of the needle give high electric field strength even at the DC voltage of −2.0 kV.2,3,7) The orifice was heated at 40°C to generate hydrated clusters Y−(H2O)n. The room temperature of 298 K and relative humidity of 30–68% were controlled by a standard commercial air conditioner. For evaluating the correlations between the ion at m/z 20 and the negative ions at m/z 16 (O−) and m/z 33 (HO2−), the DC voltage of −2.0 to −3.4 kV and the needle location of 1 mm were employed under room temperature and 54% humidity. HF gas was generated by using a home-build reaction system made up of ultraviolet light, polytetrafluoroethylene (PTFE), and hydrogen gas.10) Using the system, the HF gas was generated by which fluorine was abstracted by hydrogen radicals from the surface of PTFE due to the difference of bond dissociation energy for HF and carbon–fluorine (CF) bonds.
2.2. ReagentsPerfluorokerocene (PFK, low boiling) was purchased from Tokyo Chemical Industry (Tokyo, Japan). D2O (99.9 atom%) and heavy oxygen water (H218O, 99 atom%) were purchased from Sigma-Aldrich (St. Louis, MO, USA).
2.3. CalculationsAll the calculations reported in this paper have been performed using the Gaussian 16 suite of programs,11) and the initial molecules and ionic structures of non-covalent complex ions (H2…H2O)− and (H2…HF)− were generated by means of visual inspection using the GaussView program 6.0.11) The geometry optimization and vibration frequency analysis of all mentioned species were performed with the M06-2X hybrid functional12) level of theory and 6-31G+(d,p) basis set.
Figure 1 shows negative ion APCDI mass spectra of ambient air obtained with three different humidity conditions, at low electric field strength. The spectra obtained at 50 and 68% in humidity showed ion peaks at m/z 20 and/or 38, as well as the peaks corresponding to hydroxide HO− at m/z 17 and its water clusters OH−(H2O)n (n=1–4) at m/z 35, 53, 71, and 89, while the humidity 30% did not result in the ion at m/z 20. Figure 2 shows the spectra obtained at high electric field strength, under three different humidity conditions. As already reported,3) the high electric field resulted in various kinds of marked negative ions such as O−, HCN−, CO−, HCO−, HO2−, N2O−, NO2−, NO3−, HNO3−, and NO2−(NO2) as shown in Fig. 2. Interestingly, the spectra with high electric field strength showed the definite ion at m/z 20 together with its water clusters at m/z 38, 56, and 74 (Fig. 2C). The results obtained above indicate that the conditions of high humidity and high electric field strength are favorable for the formation of the negative ion at m/z 20. Especially, the influence of the high humidity in Figs. 1C and 2C suggests that the negative ion at m/z 20 might be composed from a water molecule H2O, and this means that the ion is made up of H2O and H2.
As was reported in the previous paper,3) the ion at m/z 20 could be observed under relatively high electric field conditions. This may be due to that the high electric field results in efficient ionization of O2, abundant dissociation of O2− and H2O, and the sequential progress of the ion-molecule reactions.2–4) Especially, the negative ions of O−, HO−, and HO2−, and radical species such as HO2· and HO· are generated by higher kinetic energy of electrons.4) At the same time, the hydrogen radical H· and/or hydride H− may be generated from water molecules by the high kinetic energy electrons and/or by the dissociation of water molecules on the tip of the needle, although the H· and H− could not be detected by the mass spectrometer used. As a result, it is considered that the ion at m/z 20 is generated by which water molecules interact with H· and/or H−. Here we show the data of positive correlation of the formation of the ion at m/z 20 with the formation of the ions corresponding to O− at m/z 16 and HO2− at m/z 33 (Fig. 3). The correlations showed in Fig. 3 were made from the numerical data (Table 1) obtained with the DC voltage of −2.0 to −3.4 kV and the needle location of 1 mm. These correlations indicate that the high electric field conditions are favorable for the formation of H4O−, O−, and HO2− ions, although the involvement of atomic hydrogens is not clear.
| Voltage (−kV) | m/z 20 H4O− | m/z 16 O− | m/z 33 HO2− |
|---|---|---|---|
| 2.0 | 0.00983 | 0.01584 | 0.06371 |
| 2.1 | 0.01461 | 0.01997 | 0.07096 |
| 2.2 | 0.02714 | 0.01863 | 0.08135 |
| 2.3 | 0.03645 | 0.03217 | 0.10844 |
| 2.4 | 0.05937 | 0.03510 | 0.13313 |
| 2.5 | 0.07044 | 0.03698 | 0.15986 |
| 2.6 | 0.08807 | 0.04649 | 0.16809 |
| 2.7 | 0.10572 | 0.04297 | 0.16862 |
| 2.8 | 0.09747 | 0.03398 | 0.13351 |
| 2.9 | 0.07126 | 0.03227 | 0.09174 |
| 3.0 | 0.06305 | 0.02695 | 0.09268 |
| 3.1 | 0.06306 | 0.02515 | 0.09783 |
| 3.2 | 0.05185 | 0.03076 | 0.09702 |
| 3.3 | 0.05082 | 0.02610 | 0.08291 |
| 3.4 | 0.04602 | 0.02358 | 0.07341 |
To examine the favorable conditions for appearance of the ion at m/z 20, the influence of the vapor of water on the spectral patterns was studied by using D2O and H218O. The application of the high humidity condition at 68% resulted in the definite peaks at m/z 20, 38, 56, and 74 (Fig. 2C), while lower humidity at 30% did not result in such ion peaks (Fig. 2A). This suggests that the ion at m/z 20 and its water clusters 20−(H2O)n (n = 1–3) are expedited by supplying the vapor of water. Therefore, it is expected that the mass shift of the ion at m/z 20 would be observed via the H/D or 16O/18O exchange by supplying the vapor of D2O or H218O. To confirm the influence of the water molecules on the mass shift of the ion at m/z 20, the vapors of D2O and H218O were supplied into the region of corona discharge under the conditions of high electric field strength, room temperature, and 57% humidity. The spectra obtained with supplying the vapor of D2O showed the mass shift n at m/z m + n (m = 20, 38, 56), i.e., n = 1–4 for m/z 20, n = 1–4 for m/z 38, and n = 1–2 for m/z 56, as shown in Fig. 4. The mass shift n at m/z 20 + n (n=1–4) indicates that the H/D exchange to form the ions at m/z 21 (= 20 − H + D), m/z 22 (= 20 − 2H + 2D), m/z 23 (= 20 − 3H + 3D), and m/z 24 (20 − 4H + 4D) takes place by supplying the vapor of D2O. This suggests that unknown species of the ion at m/z 20 has a composition of 16 + 4H.
To obtain further information about the chemical composition of the ionic species at m/z 20, the vapor of H218O was supplied in expectation of the mass shift by the 16O/18O exchange. The resulting spectra showed the peaks at m/z 19, 22, 30, 37, and 40, as shown in Fig. 5. The ion at m/z 19 represents H18O− ion produced by the 16O/18O exchange of HO− ion at m/z 17. The ion at m/z 22 may be corresponding to the m/z 22 (=20 − 16O + 18O). From the results obtained by the experiments with D2O and H218O described above, it is strongly suggested that the chemical composition of the ion at m/z 20 has H4O, although this is an unusual chemical composition.
The results obtained above suggest that the ion at m/z 20 has ionic chemical compositions of H2−(H2O), H2(H2O−), H−(H3O), and HO−(H3) or an electron delocalized composition H4O−. Although the values of EA of H2, H2O, H, and HO are all positive, i.e., 0.9 eV,13) 1.3 eV,14) 0.75 eV,6) and 1.83 eV,6) respectively, it is known that the negative ion H2− has short lifetimes 8–11 μs and rapidly dissociates into H− and H·.15) The ion at m/z 20 observed in Figs. 1 and 2 was measured through the length of 3 mm under ambient air and the flight length of ca.1000 mm in a double-focusing mass spectrometer. This indicates that the ionic chemical composition of H2−(H2O) consisted of H2− as the core ion would be deleted from the candidates described above. Regarding the unusual chemical composition H4O of the negative ion at m/z 20 formed of hydrogen and water molecules, there is an interesting report that unusual molecular anions in their dipole-bound ground state are produced via charge exchange between polar molecules and high Rydberg atoms.16) A possibility has been shown that such dipole-bound anions made of polar molecules beyond a critical dipole moment of 1.625 D will form bound anion states.17) Indeed, some polar molecules such as H2O and HF have dipole moments of 1.857 and 1.827 D, respectively.18) Skurski et al. have reported that using a theoretical concept of molecular trap, unstable or metastable anions such as N2− and H2− can be stabilized by trapping with polar molecules,9,19) although it is unclear whether the electron is localized on such unstable ions or on the polar molecules. Therefore, here we suppose that the anion at m/z 20 is produced as a dipole-bound complex between hydrogen H2, water H2O, and electron e−.
3.3. The formation of dipole-bound complex ion at m/z 22 by supplying HFTo elucidate the proposed ionic composition of the ion H4O− at m/z 20, here we performed the experiments with adding another polar molecule HF in expectation of the formation of H3F− ion at m/z 22. The negative ion APCDI mass spectra were obtained with supplying HF gas under several conditions. Here we used an HF generator10) and used another method for generating HF molecules, i.e., by supplying the vapor of a calibrant reagent PFK, because it is expected that hydrogen radicals generated by dissociation of water molecules on the tip of corona needle extract fluorine from PFK molecules. Figure 6 shows negative-ion APCDI mass spectra of ambient air obtained with supplying HF gas or the vapor of PFK molecules. It is noteworthy, as expected, that the spectra showed the ions at m/z 22 and its water clusters that may be assigned as H3F−(H2O)n (n = 1–4) at m/z 40, 58, 76, and 94. The spectra showed other ions at m/z 19 originating from F−, the hydroxide HO−, and its water clusters HO−(H2O)n (n = 1–4) at m/z 17, 35, 53, 71, and 89, and the H4O− and its water cluster ions H4O−(H2O)n (n = 1–3) at m/z 20, 38, 56, and 74. As already described above, high humidity and high electric field conditions gave the negative ion at m/z 20, as shown in Fig. 6A and 6B, indicating that direct supply of the vapor of PFK seems to be more favorable for the formation of the ion at m/z 22 than the use of the HF generator. The high electric field combined with low humidity gave better conditions for the formation of the ion at m/z 22, although the ions at m/z 19 and 20 were slightly observed in Fig. 6C.
The definite observation of the unusual ionic species H4O− and H3F− implies that unstable or metastable H2− ion might be stabilized by the dipole-bound complexing with polar molecules such as H2O and HF, as was theoretically proposed by the Skurski and Simons group.9,16,19) Although the detailed processes for the formation of H4O− and H3F− ions are unclear at present, those ions may be formed by the ion–molecule reactions of H2O, HF, H, H−, and/or electrons under ambient conditions.
3.4. Stability and structure of the ions H4O− and H3F−Next we calculate the stability and structure of the ions of H4O− and H3F− by using density functional theory (DFT) calculations to elucidate the ionic chemical compositions. The calculations for H4O− and H3F− could be successfully converged and gave appropriate ionic structures. The ionic structures and highest occupied molecular orbitals (HOMOs) of the complex ions of H4O− and H3F− are shown in Fig. 7. The Mulliken charge and spin density of the H4O− ion indicate that negative charge and attached electron (spin) are localized on the oxygen atom (O1) of the H2O molecule, although the tail-end hydrogen (H5) of the H2 molecule has slightly negative charge (Table 2). In case of the H3F− ion, the negative charge is distributed on the fluorine (F1) of HF and the tail-end hydrogen (H4) of H2, while the attached electron (spin) is merely localized on the fluorine (F1) of the HF molecule (Table 3). The results calculated above indicate that the ions of H4O− and H3F− have non-covalent bonding ionic structures or the dipole-bound complex structures of H2O−…H2 (or H2O…H2−) and HF−…H2 (or HF…H2−), respectively. From the calculated results, it is suggested that the ions of H4O− and H3F− are formed by the interaction of neutral molecule of H2O or HF with H2 and electron, and that electron is delocalized on the complex (H2O…H2) or (HF…H2).
| 1 | 2 | ||
|---|---|---|---|
| 1 | O | −1.649629 | 1.017438 |
| 2 | H | 0.329641 | −0.016132 |
| 3 | H | 0.329641 | −0.016132 |
| 4 | H | 0.171328 | 0.005307 |
| 5 | H | −0.180982 | 0.009519 |
| 1 | 2 | ||
|---|---|---|---|
| 1 | F | −1.515993 | 1.262231 |
| 2 | H | 0.591796 | −0.310891 |
| 3 | H | 0.748428 | 0.035705 |
| 4 | H | −0.824230 | 0.012955 |
To estimate the stability of the dipole-bound complex ions described above, the free energy changes ΔG for the reactions of (H2 + H2O)− → (H2O−…H2) or (H2O…H2−) and (H2 + HF) − → (HF−…H2) or (HF…H2−) were calculated using the same functional level of theory and basis set. The results obtained are summarized in Table 4. The complexing reactions of neutral hydrogen H2 with negative ions of H2O− and HF− showed slightly negative values of ΔG, which mean exothermic, while the reactions of negative hydrogen H2− with neutral molecules were largely endothermic. The exothermic reactions of neutral hydrogen with negative polar molecules indicate that the dipole-bound complex states of H2O−…H2 and HF−…H2 are thermodynamically stable and consistent with the calculated ionic structures shown in Fig. 7.
| Reaction | Free energy change (kJ/mol) |
|---|---|
| H2 + H2O− → H2O−…H2 | −6.97 |
| H2− + H2O → H2O…H2− | 131.99 |
| H2 + HF− → HF−…H2 | −2.90 |
| H2− + HF → HF…H2− | 190.50 |
HF, hydrogen fluoride.
The negative-ion at m/z 20 produced by APCDI of ambient air was identified as the chemical composition of H4O by supplying the vapor of D2O and H218O. The abundance of the ion at m/z 20 increased with increasing the humidity in ambient air at high electric field condition. The supply of HF gas or the vapor of PFK as a source of HF gas resulted in a negative ion at m/z 22, suggesting the formation of H3F−, whereby HF is complexing with H2 and electron. The formation of the negative ions H4O− and H3F− was discussed from the viewpoint of the dipole-bound complex between electron e−, hydrogen H2, and polar molecules such as H2O and HF having the dipole moment beyond the critical dipole moment, theoretically proposed by the Skurski and Simons group.9,16,19) According to the proposition, the hydrogen anion H2− can be stabilized by complexing with the polar molecules such as H2O and HF. However, it is difficult to say that the complex ions H4O− and H3F− have sufficient stability in air and even in the mass spectrometer, because the negative ions H2−, H2O−, and HF− are metastable or unstable with short lifetimes in itself. The DFT calculations suggested that the ions H4O− and H3F− have the dipole-bound complex structures H2O−…H2 and HF−…H2 and also that electrons are delocalized over the whole of the complex H2O…H2 and HF…H2. It was shown, furthermore, by the DFT calculations that the ionic structures H2O−…H2 and HF−…H2 are produced by exothermic reactions, while the formation of the ions involving negative hydrogen, H2O…H2−, and HF…H2−, is produced by endothermic reactions.
Mass Spectrom (Tokyo) 2023; 12(1): A0124
