Quantum Chemical Analysis of the Molecular and Fragment Ion Formation of Butyrophenone by High-Electric Field Tunnel Ionization at Atmospheric Pressure

Abstract

The molecular ion M^+· was observed when the liquid sample of butyrophenone was supplied using atmospheric pressure corona discharge (APCD). In contrast, the vapor supply resulted in the formation of the protonated molecule [M+H]⁺. The mass spectrum obtained with the liquid supply showed two distinctive fragment ions at m/z 105 and 120, resulting from α-cleavage and McLafferty rearrangement (McLR), respectively. The APCD spectrum showed peaks of M^+· and the characteristic two fragment ions that were the same as the field ionization mass spectra of butyrophenone as reported by Chait et al. and Beckey et al. The formation of the molecular and fragment ions strongly indicated that high-electric field tunnel ionization (HEFTI) occurs by the HEF strength exceeding 10⁸ V/m at the tip of the corona needle in APCD. The charge and spin density distributions of the molecular and fragment ions were analyzed by quantum chemical calculations using time-dependent density functional theory (TDDFT) and natural bond orbital (NBO) analysis.

1. INTRODUCTION

Butyrophenone serves as a model compound for the McLafferty rearrangement (McLR) reaction in mass spectrometry (MS). Chait et al.¹⁾ first reported a field ionization (FI) mass spectrum of butyrophenone utilizing a high-resolution mass spectrometer. They have also highlighted that a fragment ion at m/z 120 originates from the loss of ethylene (C₂H₄), not of CO, from the molecular ion M^+· at m/z 148. The FI mass spectrum of butyrophenone yielded only two fragment ions at m/z 105 and 120, with relatively low intensities of approximately 5.0% and 1.8%, respectively. Beckey et al.²⁾ also presented the fragment intensity in both the FI and electron ionization (EI) mass spectra of butyrophenone. The fragment intensity in the FI mass spectrum was m/z 105 (1.1%) and m/z 120 (0.5%), while the EI mass spectrum exhibited m/z 105 (100%) and m/z 120 (12.5%). These findings were obtained using a single magnetic sector-type mass spectrometer. The fragment ions at m/z 105 and 120 can be attributed to α-cleavage, and the McLR reaction involved keto/enol transformation and C–C bond cleavage (Scheme 1). In a recent report,³⁾ Valadbeigi and Causon presented an atmospheric pressure chemical ionization (APCI) mass spectrum of butyrophenone. Although the APCI mass spectrum did not show the McLR ion at m/z 120, it did display a lot of fragment ions resulting from cleavage of the protonated molecule [M+H]⁺ at m/z 149.

Scheme 1. Characteristic fragment ions at m/z 105 and 120 were observed in FI mass spectra reported by Chait et al.¹⁾ and Beckey et al.²⁾ FI, field ionization.

It is established that APCI, atmospheric pressure corona discharge (APCD), and direct analysis in real-time (DART) can typically yield protonated molecules (M+H)⁺,⁴⁾ whereas APCI frequently produces dehydride molecules (M−H)⁺.³⁾ In our previous report, we demonstrated that APCD mass spectra of benzene and its derivatives, butylamines, and hexane exhibited analyte ions such as [M+H]⁺, M^+·, and [M−H]⁺, which were dependent on the analyte molecule ionization energy (IE), proton affinity (PA), and the method of analyte molecule supply, namely vapor supply and liquid smear supply methods.⁵⁾ The use of the liquid supply method proved particularly effective for the formation of molecular ions M^+· of benzene and its derivatives, which possess relatively low PA values (750–882 kJ/mol) compared to butylamines (PA 901–998 kJ/mol). However, the IEs of benzene and its derivatives (7.7–9.2 eV) do not necessarily exhibit a lower value than those of butylamines (7.3–8.7 eV). It is noteworthy that the APCD mass spectrum of n-hexane obtained with the liquid supply method showed a peak corresponding to the M^+· ion, displaying a relative intensity of 19%. As previously reported,⁵⁾ the formation of [M+H]⁺ and [M−H]⁺ ions is attributed to the proton transfer from the hydronium ion (H₃O⁺) to analyte molecules (M) and to the hydride abstraction by H₃O⁺ from M, respectively, via gas-phase collisional interactions. It is crucial to acknowledge that both [M+H]⁺ and [M−H]⁺ ions can be generated through ion/molecule (chemical ionization, CI) reactions (1) and (2) in both vapor and liquid supply methods. Conversely, we have proposed that molecular ions M^+· are generated by high-electric field tunnel ionization (HEFTI), also known as FI,⁵⁾ through the application of high-electric fields (HEFs) E > 10⁸ V/m at the apex of the corona needle.^6,7) The use of the liquid supply method in APCD can facilitate the approach of analyte molecules to the vicinity of the needle tip prior to the CI reactions (1) and (2), as illustrated in Fig. 1. With regard to the formation of M^+·, it is important to consider the charge transfer between the analyte molecules M and the reactant ions R^+·, which occurs through reaction (4).³⁾

  

$$\begin{array}{ll}\text{M}+{\text{H}}_3{\text{O}}^{+}\to {\left[\text{M}+\text{H}\right]}^{+}+{\text{H}}_2\text{O}& \left(1\right)\text{ proton transfer in CI}\\ \text{M}+{\text{H}}_3{\text{O}}^{+}\to {[\text{M}-\text{H}]}^{+}+{\text{H}}_2+{\text{H}}_2\text{O}& \left(2\right)\text{ hydride abstraction in CI}\\ \text{M}+\text{ high-electric field}\to {\text{M}}^{+·}+{\text{e}}^{-}& \left(3\right)\text{ tunnel ionization}\\ \text{M}+{\text{R}}^{+·}\to {\text{M}}^{+·}+\text{R}& \left(4\right)\text{ charge transfer}\end{array}$$

Fig. 1. The formation of molecular ions M^+· via HEFTI with the liquid supply method and the formation of the protonated and dehydride molecules [M+H]⁺ and [M−H]⁺ via CI with the vapor supply method. CI, chemical ionization; HEFTI, high-electric filed tunnel ionization.

This paper focuses on the formation of the molecular ion M^+· and fragment ions at m/z 105 and the McLR ion at m/z 120 of butyrophenone, employing the APCD with the liquid supply method. The appearance of the M^+· and characteristic fragment ions m/z 105 and 120 is in good agreement with the FI mass spectra of butyrophenone as reported by Chait et al.¹⁾ and Beckey et al.²⁾

2. EXPERIMENTAL

2.1. Mass spectrometry

APCD mass spectra were obtained with an orthogonal acceleration time-of-flight (oaTOF) mass spectrometer, the JMS-T100LC (JEOL, Tokyo, Japan), which was attached to a home-built ion source for APCD. The acceleration voltage and mass resolution in the oaTOF were 7 kV and 6000 (full width at half maximum), respectively. The ions were detected with a microchannel plate detector. A schematic illustration of the ion source is provided in Fig. 2. The distance between the needle tip and the orifice plate was 3 mm. A direct current (DC) voltage of +3.5 kV was applied to the needle in relation to the orifice plate. The orifice temperature and room relative humidity were 328 K and 40%–56%, respectively.

Fig. 2. Schematic illustration of the ion source combined with the APCD. RP and TMP represent rotary pump and turbo molecular pump, respectively. APCD, atmospheric pressure corona discharge.

2.2. Reagents

Butyrophenone (99.0%) was purchased from the Tokyo Chemical Industry (Tokyo, Japan).

2.3. Calculations

The calculations presented in this paper were conducted using the Gaussian 16 suite of programs,⁸⁾ and the initial molecules and ionic structures were generated through visual inspection using the GaussView program 6.0.⁸⁾ The structural optimization of all neutral and charged molecules in the gas phase was conducted using density functional theory (DFT) with the M06-2X hybrid functional⁹⁾ level of theory and 6-31+G(d,p) basis set. The excited states of neutral molecule M and the charge and spin density of molecular ion M^+· were estimated by time-dependent DFT (TDDFT) and natural bond orbital (NBO) methods, respectively.

3. RESULTS AND DISCUSSION

3.1. The formation of molecular and fragment ions of butyrophenone with HEF tunnel ionization

The APCD mass spectra of butyrophenone obtained with the method of analyte vapor supply coupled with nitrogen (N₂) and argon (Ar) gas are presented in Fig. 3. The spectra showed peaks that corresponded to hydronium ions H⁺(H₂O)_n at m/z 19 and 37, oxygen ions (O₂⁺) at m/z 32, and propyl ions (C₃H₇⁺) at m/z 43 in the low mass region. However, the spectra did not show the molecular ions of nitrogen (m/z 28) and argon (m/z 40). The absence of the N₂^+· and Ar^+· ions may be attributed to the higher IE of N₂ (15.6 eV) and Ar (15.8 eV) in comparison to those of O₂ (12.1 eV) and C₃H₇ (8.09 eV).¹⁰⁾ Moreover, all the spectra exhibited a lack of a definitive peak corresponding to the molecular ion M^+· at m/z 148. However, the spectra displayed similarities to the APCI mass spectrum in analyte ions such as [M+H]⁺ and [M−H]⁺ and fragment ions.³⁾ In consideration of the IEs for butyrophenone (9.1 eV) and O₂ (12.1 eV), as referenced in the NIST Book, it is plausible that a charge transfer may occur, resulting in the formation of the M^+· ion of butyrophenone. However, the APCD mass spectra of butyrophenone obtained with the vapor supply method did not yield the definite M^+· ion at m/z 148. This suggests that the primary gas-phase reactions in the APCD are proton transfer (1) and hydride abstraction (2) reactions involving hydronium ions H⁺(H₂O)_n.

Fig. 3. APCD mass spectra of butyrophenone obtained by the vapor supply method with (A) ambient air, (B) N₂ gas, and (C) Ar gas. APCD, atmospheric pressure corona discharge.

Subsequently, a comparison was conducted between the APCD mass spectra of butyrophenone obtained with the vapor supply and liquid supply methods (Fig. 4). The spectrum obtained with the liquid supply showed peaks that corresponded to the molecular ion at m/z 148 and the McLR ion at m/z 120, as well as an increment of the fragment ion at m/z 105 (Fig. 4B). These two fragment ions are characteristic of the HEFTI mass spectrum of butyrophenone,¹⁾ as well as the EI mass spectrum in the NIST book.¹⁰⁾ They were first identified by Chait et al.¹⁾ and Beckey et al.²⁾ in the HEFTI mass spectrum. As illustrated in the magnified APCD spectra (Fig. 5), the spectrum obtained with the vapor supply and at a relatively high humidity (50%) exhibited the presence of hydronium ions H⁺(H₂O)_n (Fig. 5A), whereas the spectrum obtained with the liquid supply and at a lower humidity (40%) did not display the hydronium ions (Fig. 5B).

Fig. 4. APCD mass spectra of butyrophenone obtained with the methods of (A) vapor supply and (B) liquid supply. APCD, atmospheric pressure corona discharge.

Fig. 5. Magnified APCD mass spectra of butyrophenone obtained with (A) vapor supply and (B) liquid supply methods. The molecular and fragment ions at m/z 148, 105, and 120 remarkably enhanced in intensity when the liquid supply method was used. APCD, atmospheric pressure corona discharge.

As reported by Smalø et al.¹¹⁾ Davari et al.¹²⁾ and Åstrand,¹³⁾ the HEF resulted in the reduction of the IE of organic compounds, relative to the field-free ionization energy (IE₀). To illustrate, the IE₀ of benzene (9.24 eV) decreases to approximately 8, 7, and 6 eV at HEF values of 1 × 10⁸, 5 × 10⁸, and 3 × 10⁹ V/m, respectively. Similarly, the IE₀ of methyl butyrate (10.07 eV) decreases to approximately 9, 8, and 6 eV at HEF values of 1 × 10⁸, 5 × 10⁸, and 3 × 10⁹ V/m, respectively. In consideration of the compound butanal (HCOC₃H₇) as a model of the butyl ketone moiety of butyrophenone, it can be postulated that the IE₀ (9.82 eV) may drop to approximately 9, 8, and 7 eV at the HEF values of 1 × 10⁸, 5 × 10⁸, and 3 × 10⁹ V/m, respectively. In contrast, our previous findings indicate that a local field strength of 3 × 10⁸ V/m at the tip apex of the corona needle resulted in the emission of field-generated electrons with a kinetic energy of 116 eV under the conditions utilized in the present study.⁷⁾ Two possibilities for molecular ion formation at the tip of the needle have been identified: the EI at 116 eV and the HEFTI at 3 × 10⁸ V/m. However, it should be noted that N₂ and Ar gas could not be ionized (Fig. 3) and that Fig. 5B did not show an EI characteristic fragment ion at m/z 77 corresponding to the phenyl ion (C₆H₅⁺).¹⁰⁾ The aforementioned evidence indicates that the molecular ion M^+· of butyrophenone can be formed by the HEFTI with a field strength of >10⁸ V/m. To examine the formation of M^+· and fragment ions, it is necessary to obtain information regarding the highest occupied molecular orbital (HOMO) of the neutral molecule M and the singly occupied molecular orbital (SOMO) and lowest unoccupied molecular orbital (LUMO) of the M^+· ion.

3.2. Charge and spin density distributions of the molecular ion of butyrophenone

To obtain the HOMO of M and the SOMO and LUMO orbitals of the M^+· ion of butyrophenone, quantum chemical calculations (QCCs) were performed with the M06-2X/6-31+G(d,p) level of theory. The HOMO orbital of the neutral molecule M indicates that the π-electrons in the phenyl group and the n-electrons in the carbonyl group are susceptible to being released from the molecule (Fig. 6A). The SOMO orbital indicates that excess electrons remain on the exterior of the phenyl group (Fig. 6B), while the LUMO orbital demonstrates a deficiency of electrons on the interior of the phenyl group (or on a side of the carbonyl group) (Fig. 6C). The LUMO orbital indicates that the positive charge is localized on the carbons (C3 and C4) of the phenyl group.

Fig. 6. The non-excited orbitals of (A) HOMO of M, (B) SOMO of M^+·, (C) LUMO of M^+·, calculated at the M06-2X/6-31+G(d,p) level. HOMO, highest occupied molecular orbital; LUMO, lowest unoccupied molecular orbital; SOMO, singly occupied molecular orbital.

To estimate the charge and spin (radical) density distributions of the M^+· ion, an NBO analysis was conducted at the M06-2X/6-31+G(d,p) level. The positive charge and spin of the M^+· ion were primarily localized on the carbonyl carbon (C12) and carbonyl oxygen (O13), respectively. In addition, the positive charge exhibited slight distribution on the C3 and C6 carbons of the phenyl group, while the spin was slightly distributed on the C3, C14, and C6 carbons (Fig. 7). Here it should be noted that negative values at C3 (−0.13) and C6 (−0.18) carbons do not necessarily mean a negative charge because there is no quantitatively correct value for the charge density.¹⁴⁾ A positive charge may be judged based on whether the negative value is relatively close to the positive value. The fragment ions at m/z 105 and 120 can be successfully explained by the principal charge (C12) and spin (O13) densities, as illustrated in Scheme 1 and Fig. 7. As previously outlined in Section 3.1, it can be postulated that the phenyl and butyl ketone moieties of butyrophenone may undergo ionization at the energy levels found to be approximately 7 and 8 eV, respectively, under the 3 × 10⁸ V/m HEF condition. In light of the IE₀ value for butyrophenone (9.1 eV), this estimation appears to be reasonable. However, the principal charge and spin density distributions in Fig. 7 could not be explained by the molecular orbitals (C3 and C4) in Fig. 6. This inconsistency may be resolved by considering the field-induced excitation states of butyrophenone. Subsequently, the excitation energies of butyrophenone were calculated using the TDDFT method¹⁵⁾ to ascertain the excitation energies and the transitions of the energy levels that are responsible for the ionization of butyrophenone molecules.

Fig. 7. Principal charge (C12) and spin (O13) density distributions of the molecular ion M^+· of butyrophenone, obtained with the NBO analysis at the M06-2X/6-31+G(d,p) level. The negative values at C3 and C6 atoms do not necessarily represent negative charge because there is no universally agreed upon best procedure for computing the charge for atoms in molecules.¹⁴⁾ NBO, natural bond orbital.

3.3. Tunnel ionization of butyrophenone molecules excited with a HEF

The TDDFT method can be employed to calculate the excitation energies of molecules of interest. This approach was first proposed by Smalø et al.¹¹⁾ The HEF excites the molecular orbitals, thereby facilitating the ionization of molecules via a quantum tunneling effect. TDDFT calculations were performed to obtain the 20 singlet excitation energies of butyrophenone molecules. The calculated excitation states, leading to tunnel ionization, along with the symmetry, oscillator strength, and excitation energy, are presented in Table 1. The ionization of the phenyl moiety is evident in the transitions of HOMO-1 (39)→LUMO (41), HOMO-1 (39)→LUMO+1 (42), HOMO-1 (39)→LUMO+3 (44), and HOMO-2 (38)→LUMO+4 (45). The excited HOMO (40), HOMO-1 (39), and HOMO-2 (38) orbitals can be represented schematically in Fig. 8A–8C, respectively. The calculated excitation energies, ranging from 6.62 to 7.74 eV, are sufficient to ionize the phenyl moiety of butyrophenone under the condition of 3 × 10⁸ V/m HEF. In contrast, the ionization of the carbonyl group can be attributed to the transition of HOMO-4 (36) to LUMO (41) with an excitation energy of 7.83 eV. This process is illustrated in Fig. 8D, which depicts the excited HOMO-4 (36) orbital. The ionization of the phenyl moiety would yield stable molecular ions devoid of fragmentation, whereas the ionization of the carbonyl group results in the characteristic fragment ions at m/z 105 and 120, as illustrated in Scheme 1 and Fig. 5B.

Table 1. The excitation energy of neutral butyrophenone calculated with the TDDFT at the M06-2X/6-31+G(d,p) level.

Excitation states (MO)	Symmetry	Oscillator strength	Energy (eV)
HOMO (40)→LUMO (41)	Singlet-A	0.2699	5.57
HOMO-1 (39)→LUMO (41)	Singlet-A	0.2405	6.62
HOMO-1 (39)→LUMO+1 (42)	Singlet-A	0.5075	6.68
HOMO-1 (39)→LUMO+3 (44)	Singlet-A	0.0300	7.37
HOMO-2 (38)→LUMO+4 (45)	Singlet-A	0.0299	7.74
HOMO-4 (36)→LUMO (41)	Singlet-A	0.0479	7.83

TDDFT, time-dependent density functional theory; HOMO, highest occupied molecular orbital; LUMO, lowest unoccupied molecular orbital.

Fig. 8. The excited molecular orbitals of (A) HOMO, (B) HOMO-1, (C) HOMO-2 in the phenyl moiety, and (D) HOMO-4 in the carbonyl moiety of butyrophenone at the M06-2X/6-31+G(d,p) level. HOMO, the highest occupied molecular orbital.

The fragment ions at m/z 105 and 120, which originate from the principal charge (C12) and spin (C13) density molecular ion (Fig. 7), were produced by endothermic reactions, as illustrated in Scheme 2. The reason for the markedly lower intensity of the McLR fragment ion at m/z 120 relative to that of the fragment ion at m/z 105 (Fig. 5), despite the former’s lower endothermicity (0.59 eV), can be attributed to the observation that the fragment ions originating from rearrangement reactions are often relatively weak or even undetected. This phenomenon has been documented in previous studies, including those by Beckey.^2,16) In a recent report by Stamm et al.¹⁷⁾ on the other hand, it was suggested that using strong-field tunnel ionization, the McLR reaction may occur in two steps, with the first step keto/enol transformation being relatively fast (10⁻¹³ s) and the second step C–C bond cleavage being relatively slow (10⁻¹¹ s) (see Scheme 1). It may be the case that the difference in intensity of the ions at m/z 105 and 120 can be explained by the difference in time scales between the α-cleavage (10⁻¹³ –10⁻¹² s) and the McLR reaction (10⁻¹¹ s). It seems reasonable to suggest that the intensity of these fragment ions can be explained by this kinetic consideration, although quantitative considerations about the peak intensity need further study. The NBO analysis yielded consistent results regarding the sites of the charge and spin in the fragment ions at m/z 105 and 120 in Scheme 2, aligning with the ion structure of the principal charge and spin density molecular ion M^+·, as illustrated in Fig. 7.

Scheme 2. The energy diagram for fragmentation to form fragment ions at m/z 105 and 120 represents the endothermic reactions at the M06-2X/6-31 + G(d,p) level.

4. CONCLUSION

The observation of the molecular ion M^+· and the characteristic two fragment ions at m/z 105 and 120 in the APCD mass spectrum of butyrophenone obtained with the liquid supply method indicated that the ionization process occurs by HEFTI. The proposed HEFTI process was strongly supported by the FI mass spectral data reported by Chait et al.¹⁾ and Beckey et al.²⁾ The high-electric field strength of >10⁸ V/m at the tip of the corona needle in APCD can reduce the IE₀ of organic molecules (9–10 eV) to 7–8 eV, and the subsequent ionization occurs via a quantum tunneling effect. The TDDFT calculations indicated that an excitation energy of 7.83 eV is sufficient to ionize the carbonyl moiety of butyrophenone molecules through a transition involving the HOMO-4→LUMO orbitals. The NBO analysis indicated that the majority of the charge and spin density of the molecular ion were localized on the carbonyl carbon and carbonyl oxygen, respectively. The sites of the charge and spin density of the fragment ions were found to be consistent with those of the molecular ion.

ACKNOWLEDGMENTS

The author thanks K. Nagasawa and Y. Onaga, who were undergraduate students in the mass spectrometry laboratory at YCU, for their experimental assistance.

Notes

Mass Spectrom (Tokyo) 2024; 13(1): A0156

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