Mass spectrometric determination of Morse

Research Article Received: 19 January 2016

Revised: 9 March 2016

Accepted: 10 March 2016

Published online in Wiley Online Library

Rapid Commun. Mass Spectrom. 2016, 30, 1384–1390 (wileyonlinelibrary.com) DOI: 10.1002/rcm.7564

Mass spectrometric determination of Morse parameters for the fifty-four superoxide states dissociating to the lowest limit Edward S. Chen1,2*, Edward C. M. Chen2,3, Reece Rosenthal2,4, Spencer Chang2,4 and Charles Herder2† 1

Baylor College of Medicine, Molecular and Human Genetics, One Baylor Plaza, Houston, TX 77030, USA The Wentworth Foundation, 4039 Drummond, Houston, TX 77025, USA 3 University of Houston Clear Lake, Natural Science and Mathematics, 2700 Bay Area Blvd., Houston, TX 77059, USA 4 Rice University, 6100 Main Street, Houston, TX 77005, USA 2

RATIONALE: Superoxide is the most significant homonuclear diatomic anion in biochemistry. Theory predicts 12 doublet (X, A–K) and 12 quartet (a–l) electronic states split by spin orbital coupling into 54 states dissociating to the 3P(O) + 2P(O–) limit. Dissociation energies for the 27 bonding states with positive electron affinities have been determined from mass spectrometric data. However, the 27 antibonding states with negative electron affinities have not been experimentally characterized. METHODS: The electron affinity of the hydrogen atom per electron, the Hylleraas, is the fundamental measure of electron correlation. It has been used to assign and evaluate experimental electron affinities of atoms and diatomic molecules. The 27 negative electron affinities of oxygen are estimated from the 27 positive values and the Hylleraas. These values are used to determine frequencies and internuclear separations by fitting theoretical electron impact distributions to the gas-phase mass spectrometric atomic oxygen anion distribution peaking at about 6.5 eV. RESULTS: The dissociation energies, internuclear distances and frequencies giving the first complete set of Morse potential energy curves for the 54 superoxide states dissociating to the lowest limit are reported from mass spectrometric data. The potentials are compared to theoretical and empirical literature curves. CONCLUSIONS: The existence of the 27 bonding and 27 antibonding spin orbital coupling superoxide states dissociating to 3P(O) + 2P(O–) is established from mass analyzed thermal, photon, and electron ionization data. There are electron affinities from 0 to 0.15 eV, and onsets and peaks for dissociative electron attachment that cannot be explained by the 54 states. These support the existence of the 36 superoxide spin states dissociating to [1D(O) + 2P(O– )] and [1S(O) + 2P(O– )] predicted by quantum mechanics. Copyright © 2016 John Wiley & Sons, Ltd.

Superoxide is the most significant homonuclear diatomic anion in biochemistry. Before 1950, Massey postulated potential energy curves with adiabatic electron affinities, AEa(O2): (eV) X 2Πg, 0.9; a 4Σu–, 0.2 or –0.8; A 2Δu, –1.5 or –2; B 2Δu, –2.2 or –2.8.[1] The AEa values are the energy differences between the ground state neutral and ground and excited state anions in their stable geometries. The National Institute of Standards and Technology (NIST) website[2] cites experimental values AEa(O2): 0.15(5) to 1.12(7) eV. Theoretical AEa(O2) from NIST: –8.1 to 1.05 eV are compared only with the experimental AEa(O2) value of 0.451(7) eV.[2] In 1928, Mulliken identified 54 spin orbital coupling states of BO dissociating to [2P(B) + 3P (O)] from the product of 9[3P(O)] and 6[2P(B)] states.[3] In 1981, Michels listed 12-[Σ, Π, Δ] doublet states and 12-[ Σ, Π, Δ]

* Correspondence to: E. S. Chen, Baylor College of Medicine, Molecular and Human Genetics, One Baylor Plaza, Houston, TX 77030, USA. E-mail: [email protected] †

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Current address: Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139-4307, USA

Rapid Commun. Mass Spectrom. 2016, 30, 1384–1390

quartet states dissociating to [3P(O) + 2P(O–)], 18-[ Σ, Π, Δ, Φ] doublet states dissociating to [1D(O) + 2P(O– )] and 4-[ Σ, Π] doublet states dissociating to [1S(O) + 2P(O– )].[4] In 2004, Chen and Chen summarized experimental AEa(O2) values from electron capture detector data from 1971 and 2003 (ECD-1971 and ECD-2003, respectively).[5] In 2010, Chen et al. identified 54 spin orbital coupling states dissociating to [3P(O) + 2P(O–)], 30 states dissociating to [1D(O) + 2P(O– )] from 5[1D(O)] times 6[2P(O–)] and 6 states to [1S(O) + 2P(O], and reported 27 AEa(O2) values of 0.15 to 1.07 eV from atmospheric pressure ionization mass spectra (APIMS-2010) and atmospheric pressure ionization electron capture detector data (APIECD-2010) determined simultaneously.[6] Freeman determined AEa: (eV) O2 [0.45(5), 0.9]; NO [0.15(2), 0.8] from the ECD-1971 data.[7] The AEa(O2) values of 0.725005 eV from the ECD-2003 data is the only new value cited by NIST this century.[2,8] We have reported 27 AEa(O2) values from pulsed discharge electron capture detector (PDECD-2006) data,[9] cyclic voltammetry (CV-2012) data,[10] and the published negative ion photoelectron spectra (NPES-1995) used to report only the AEa(O2) values of 0.450(2), 0.430(2) eV.[11] The present 27 AEa(O2 ) values of 0.148(1) to 1.07(1) eV shown in Table 1 are now the most precise values available. The 2015 R matrix

Copyright © 2016 John Wiley & Sons, Ltd.

Morse parameters for the 54 O2– states dissociating to the lowest limit Table 1. Oxygen activation energies, E1, dNHyl and electron affinities, AEa a

State 2

X1 Π3/2 X2 2Π1/2 a1 4Σ3/2 a2 4Σ1/2 b1 4Δ7/2 b2 4Δ5/2 b3 4Δ3/2 b4 4Δ1/2 c1 4Σ3/2 c2 4Σ1/2 A12Δ5/2 A2 2Δ3/2 B 2Σ1/2 C1 2Π3/2 C2 2Π1/2 D 2Σ1/2 E 2Σ1/2 d1 4Π5/2 d2 4Π3/2 d3 4Π1/2 d4 4Π%1/2 e1 4Σ3/2 e2 4Σ1/2 f1 4Π5/2 f2 4Π3/2 f3 4Π1/2 f4 4Π%1/2

E1 (eV) 0.89 0.87 0.84 0.82 0.68 0.68 0.68 0.66 0.68 0.68 0.48 0.48 0.27 0.13 0.13 0.12 0.12 0.12 0.10 0.10 0.10 0.08 0.08 0.08 0.08 0.07 0.07

b

expAEa (eV)

1.070(1) 1.050(1) 0.960(1) 0.940(1) 0.787(1) 0.751(1) 0.725(1) 0.705(1) 0.755(1) 0.735(1) 0.601(1) 0.561(1) 0.515(1) 0.450(1) 0.430(1) 0.415(1) 0.355(1) 0.312(1) 0.280(1) 0.260(1) 0.248(1) 0.252(1) 0.232(1) 0.212(1) 0.180(1) 0.160(1) 0.148(1)

c

thAEa (eV) 1.047 1.047 1.008 1.008 0.693 0.693 0.693 0.693 0.693 0.693 0.597 0.564 0.473 0.458 0.429 0.429 0.350 0.317 0.286 0.286 0.286 0.286 0.195 0.195 0.195 0.150 0.150

† †

dNHyl

[%1.00] %1.08 %1.33 %1.38 %1.78 %1.88 %1.94 [%2.00] %1.87 %1.92 %2.28 %2.38 [%2.50] %2.67 %2.73 %2.78 %2.93 [%3.00] %3.13 %3.18 %3.21 %3.20 %3.26 %3.31 %3.39 %3.44 [%3.50]

a

estAEa (eV)

State 2

F1 Π3/2 F2 2Π1/2 g1 4Σ3/2 g2 4Σ1/2 h1 4Δ7/2 h2 4Δ5/2 h3 4Δ3/2 h4 4Δ1/2 i1 4Σ3/2 i2 4Σ1/2 G1 2Δ5/2 G2 2Δ3/2 H 2Σ1/2 I1 2Π3/2 I2 2Π1/2 J 2Σ1/2 K 2Σ1/2 j1 4Π5/2 j2 4Π3/2 j3 4Π1/2 j4 4Π%1/2 k1 4Σ3/2 k2 4Σ1/2 l1 4Π5/2 l2 4Π3/2 l3 4Π1/2 l4 4Π%1/2

%1.56 %1.58 %1.68 %1.70 %1.85 %1.89 %1.91 %1.93 %1.88 %1.90 %2.04 %2.08 %2.12 %2.19 %2.21 %2.22 %2.28 %2.33 %2.36 %2.38 %2.39 %2.39 %2.41 %2.43 %2.46 %2.48 %2.49

c

thAEa (eV)

Activation energy for thermal electron attachment from thermal data, E1, from the literature.[5–10,27–33] Weighted average adiabatic electron affinities from methods dating from the 1950s. c Values from NIST[2] and Michels[4] closest to the present values. d Differences in the NHyl: dNHyl (Z, Z2) = (AEa(Z2) – AEa(Z))/Hyl. e The AEa (ab, O2) = AEa (bd, O2) – 2.639 eV.

%1.55 %1.55 %1.60 %1.70 %1.96 %1.96 %1.96 %1.96 %1.96 %1.96 %2.09 %2.09 %2.09 %2.09 %2.09 %2.09 %2.30 %2.30 %2.37 %2.37 %2.37 %2.37 %2.37 %2.39 %2.42 %2.47 %2.47

†

dNHyl

[%8.00] %8.08 %8.23 %8.38 %8.78 %8.88 %8.94 [%9.00] %8.87 %8.92 %9.28 %9.38 [%9.50] %9.67 %9.73 %9.78 %9.93 [%10.00] %10.13 %10.18 %10.21 10.20 %10.26 %10.31 %10.39 %10.44 [%10.50]

b

potential AEa(O2): (eV) 0.45, –2.03, –2.74 and –2.84 eV values were assigned by Laporta et al. to the X(2Πu), a(4Σu–), A (2Πu) and B (2Σu–) states.[12]

UðZ2 Þ ¼ De ðZ2 Þ–2 De ðZ2 Þ expð–βðr–re ÞÞ þDe ðZ2 Þ expð–2βðr–re ÞÞ

EXPERIMENTAL

(1)

Figure 1. Morse potential energy curves for superoxide from parameters in Table 2.


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Recently, Chen, Chang, Rosenthal, Chen and others postulated[9,13–15] that the ground state electron affinity of the hydrogen atom per electron, the Hylleraas: Hyl = gs-Ea(H)/2 = 0.7542/2 = 0.3771 eV/electron, is the fundamental measure of electron correlation and assigned positive values to all atoms, Z, from NHyl = AEa(Z)/Hyl. Experimental and theoretical AEa(Z, Z2) values were evaluated and assigned to predicted states from the differences in the NHyl: dNHyl (Z, Z2) = (gs-AEa(Z2) – AEa(Z))/Hyl. The dNHyl values for O2, S2 and F2 are –1.[13–16] The gs-AEa(O2) = –0.377 + 1.46 = 1.08 eV is generalized to AEa(O2) = 0.377 dNHyl + 1.46 eV. If the bonding (bd) and antibonding (ab) energies are symmetrically split about the dissociation energy of the neutral De[O2]/2 = 7Hyl, then AEa(ab,O2) = AEa(bd,O2) – 7Hyl = AEa(bd,O2) – 2.639 eV and De[ab,O2–] = De[bd,O2–] – 2.639 eV. In 1987, Herschbach recalled his 1960s classification of diatomic anions based on the signs of De[Z2–], vertical electron affinity, VEa, and the energy for dissociative electron


attachment, Edea.[17] The term De[Z2–] was replaced by re to define 23 = 8 Herschbach Ionic Morse Person Electron Curves (HIMPEC).[5–11,13–15] The neutral and anion curves are:

E. S. Chen et al. UðZ2 – Þ ¼ De ðZ2 Þ–2kA De ðZ2 Þ expð-kB βðr–re ÞÞ þkR De ðZ2 Þ expð–2kB βðr–re ÞÞ–Ea ðZÞ

(2)

kA, kR and kB are dimensionless constants; β = me(2π2Ma/De [(Z2])1/2 where me is the electron mass, M is the atomic weight, and Ea the equilibrium internuclear separation. The HIMPEC

Figure 2. Atomic oxygen distributions from the 27 antibonding curves in Fig. 1 compared with literature experimental curves: Locht and Momigny (1970 L&M);[21] Nandi et al. (2006 NP&K)[25] and Rapp and Briglia (1065 R&B).[22] The calculated distributions are projections from the 0 and 1 vibrational levels of the neutral.

are Morse potentials with: De((Z2–)/De((Z2) = [kA2/kR]; re (Z2–) – re (Z2) =[ln (kR/kA)]/[kBβ(Z2)]: ω (Z2[–])/ω (Z2) = kAkB/kR1/2. The HIMPEC in Fig. 1 and the O– distributions in Fig. 2 are calculated from the parameters in Table 2. The 27

Figure 3. Morse potential energy curves for comparison with the present curves. The 2Πg curves are from Massey[1] and Michels,[4] the two 2Πu curves are from Locht and Momigny[21] and Michels.[4] All the remaining curves are from Michels.[4] Only two curves dissociate to [1D(O) + 2P(O– )] and two to [1S(O) + 2P(O– )] whereas theory predicts 36 curves dissociating to these limits.

Table 2. Superoxide Morse parameters

State X1 2Π3/2 X2 2Π1/2 a1 4Σ3/2 a2 4Σ1/2 b1 4Δ7/2 b2 4Δ5/2 b3 4Δ3/2 b4 4Δ1/2 c1 4Σ3/2 c2 4Σ1/2 A12Δ5/2 A2 2Δ3/2 B 2Σ1/2 C1 2Π3/2 C2 2Π1/2 D 2Σ1/2 E 2Σ1/2 d1 4Π5/2 d2 4Π3/2 d3 4Π1/2 d4 4Π%1/2 e1 4Σ3/2 e2 4Σ1/2 f1 4Π5/2 f2 4Π3/2 f3 4Π1/2 f4 4Π%1/2

a De (eV)

b re (pm)

c ω (cm%1)

4.81 4.79 4.70 4.68 4.52 4.48 4.45 4.43 4.48 4.46 4.33 4.29 4.24 4.17 4.15 4.14 4.09 4.04 4.00 3.98 3.97 3.97 3.95 3.94 3.91 3.89 3.88

132 132 133 133 134 134 134 134 134 134 134 134 134 135 135 135 135 135 135 135 135 135 135 135 135 135 135

1145 1145 1145 1145 1145 1145 1145 1145 1145 1145 1145 1140 1140 1108 1108 1108 1108 1108 1108 1108 1108 1108 1108 1108 1108 1108 1108

a

State F1 2Π3/2 F2 2Π1/2 g1 4Σ3/2 g2 4Σ1/2 h1 4Δ7/2 h2 4Δ5/2 h3 4Δ3/2 h4 4Δ1/2 i1 4Σ3/2 i2 4Σ1/2 G12Δ5/2 G22Δ3/2 H 2Σ1/2 I1 2Π3/2 I2 2Π1/2 J 2Σ1/2 K 2Σ1/2 j1 4Π5/2 j2 4Π3/2 j3 4Π1/2 j4 4Π%1/2 k1 4Σ3/2 k2 4Σ1/2 l1 4Π5/2 l2 4Π3/2 l3 4Π1/2 l4 4Π%1/2

De (eV)

re (pm)

ω (cm%1)

2.17 2.15 2.06 2.04 1.89 1.85 1.82 1.79 1.84 1.82 1.69 1.65 1.60 1.53 1.51 1.50 1.45 1.40 1.36 1.34 1.33 1.33 1.31 1.30 1.27 1.25 1.24

188 188 188 188 188 188 188 188 188 188 188 188 188 188 188 188 188 188 188 188 188 188 188 188 188 188 188

520 520 510 510 500 500 500 500 500 500 490 490 490 480 480 480 480 465 465 465 465 465 465 460 460 460 460

De, Morse potential equilibrium dissociation energy. re, Morse potential equilibrium internuclear separation. c ω, Morse potential vibrational frequency. b

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Morse parameters for the 54 O2– states dissociating to the lowest limit bonding states are defined by the average AEa(O2) in Table 1; E1 from the thermal data in Fig. 5 and re, and the frequency, ω, from the photoelectron spectra in Fig. 6 and alkali metal superoxide data.[5–11,13–15,18] The 27 antibonding states are defined by De, and re and ω from fits to O– distributions.

Figure 4. R matrix potential energy curves from Laporta et al.,[12] designated the X, A, B and a states, and the present curves for the X-C, I and K and the a, b and i states. The X, C, I, k and i curves have the same symmetries as the R matrix curves.

RESULTS AND DISCUSSION The assignment of the AEa(O2) values of 1.07 eV to 0.15 eV in Table 1 to the 27 predicted spin orbital bonding superoxide states confirms the existence of 54 states dissociating to the lowest limit and resolves the difference between the gs-AEa(O2) value of 0.15 eV preferred by Mulliken from the molecular orbital bond order of 0.75 and the gs-AEa(O2) value of about 0.9 eV from Pauling’s valence state three electron bond order of 0.875. Mulliken noted in 1959: "It is shown, using molecular orbital theory and experimental data, that 0.15 eV is much the more probable of two alternative values for the electron affinity of O2 which have been considered. Thermochemical evidence favoring a value of 0.9 eV is rather uncertain and should be given less weight."[19] In 1979, Pauling published "The discovery of superoxide" with this abstract: "Earlier this year Irwin Fridovich and H. Moustafa Hassan wrote of the toxic effects of the superoxide radical. Here Linus Pauling describes how this radical may have been the first important substance whose existence was predicted through arguments based on the theory of quantum mechanics. "[20] The estimate of antibonding dissociation energies from the constant separation of the 27 bonding and 27 antibonding states includes electron correlation in the two simple theories.




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Figure 5. Least-squares fits to benchmark thermal data from the literature[5–10,13–15,27–33] that yield electron affinities included in the average values in Table 1. The 2010 atmospheric pressure ionization negative ion mass spectral data (2010 APIMS) were determined along with the atmospheric pressure ionization electron capture detector data (2010 APIECD). The mass spectrometric flame data 30 demonstrates that superoxide exists at these temperatures so that the magnetron data (MGN) from Meyer and Vier[28] and Page[29] are for the formation of superoxide not the oxygen atom anion as was assumed in the original articles. We note that the NIST[2] does not list the atomic oxygen electron affinities from the magnetron method.

E. S. Chen et al. Only one m/z 16 distribution, peaking about 6.5 eV, has been reported from the gas-phase electron ionization of O2 at room temperature. Locht and Momigny assigned their distribution at 526K to the A 2Πu state and used it to construct a potential with De = 2.14 eV, re = 167 pm and ω = 800 cm–1.[21] In 1965, Rapp and Briglia published a room temperature O– distribution that has become the benchmark.[22] O’Malley[23] proposed a theory of dissociative electron attachment to O2 based on the A 2Πu state with De =1(5) eV and re = 155 to 210 pm to explain the dramatic temperature dependence reported by Henderson et al. in 1979.[24] In 1981, Michels calculated ab initio curves for 14 superoxide states dissociating to three limits with first order parameters: De = 2.14 to 0.38 eV, re = 188 to 275 pm and ω = 346 to 605 cm–1.[4]

Until recently, it was believed that the gas-phase O– distribution peaking at about 6.5 eV at room temperature was due only to the 2Πu state. In 2006, Nandi et al. published angular data suggesting that two states contribute to their distribution.[25,26] Here we assume that the 27 antibonding superoxide states contribute to this peak. The antibonding De[O2(–)] and an internuclear distance of 188 pm, calculated by Michels,[4] are used to fit the literature distributions by adjusting the frequencies to the values in Table 2. Figure 3 shows the four predicted literature 2Π curves dissociating to the lowest limit: X from Massey;[1] C from Scheidt and Weinkauf;[11] F from Locht and Momigny;[21] and I from Michels and 11 other curves from Michels.[4] Freeman presented X and C curves from the ECD-1971 data.[7] The

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Figure 6. The line diagrams in the NPES-1995 data[11] and the superoxide distribution from the electron ionization of NO2[35] are at the 27 average bonding electron affinities. The inset in the electron ionization data shows the HIMPEC for these states. The large peaks in the NPES-95 data were assigned to transitions from the v = 0 vibrational level of the C superoxide state to the three vibrational levels of the ground state of O2 in Schiedt and Weinkauf.[11] The structures below 0.15 eV are unassigned. The absolute values of the derivatives of the NPES-1995 data with respect to energy indicate an additional hyperfine structure for superoxide. wileyonlinelibrary.com/journal/rcm



Morse parameters for the 54 O2– states dissociating to the lowest limit 2015 R matrix curves incorrectly assigned to X, A, B, and a states[12] are compared with the present X to C, I, K, a, b and i curves in Fig. 4. In Fig. 5 the non-linear least-squares fits to the APIMS-2010 and APIECD-2010 data; the ECD-2003, 1972, and 1971 data; the 1981 microwave swarm data; the 1979 mass spectrometry/flame data; the 1966 swarm data; and the 1944 and 1966 magnetron (MGN-1944 and MGN-1962, respectively) data are used to determine the E1 in Table 1.[5–10,13–15,27–33] The APIMS-2010 and APIECD-2010 data provided both ECD and mass spectrometry data.[6] The AEa(O2), 0.45(5) from the ECD-1971 data and AEa(O2), the average of AEa(O2) 0.700 and 0.750 eV from the ECD-2003 data are listed by the NIST.[2,7,8] The microwave swarm-1981 data are the lowest temperature thermal data. The AEa(O) values of 3.07 and 1.38 eV were reported from the MGN-1944 and MGN-1962 surface ionization data assuming dissociation of O2.[28,29] The mass spectrometric observation of m/z 32 at flame temperatures demonstrates that the MGN data are for the formation of superoxide states with AEa(O2) 0.70 to 1.07 eV, as illustrated in Fig. 5.[30] Pack and Phelps determined the first thermal AEa(O2) value of 0.43(2) eV listed by the NIST from the swam-1966 data.[31] The AEa(O2) value 0.5(1) eV from the ECD-1972 data of Van de Wiel and Thomassen is not included in the NIST webbook.[7,32] The unpublished PDECD-2003 data collected by Herder are used to determine AEa(O2) values of 0.15 to 0.25 eV.[9,33] Our 27 values from thermal data, the NPES-1995 data and the m/z 32 distribution from the electron ionization of NO2 (Fig. 6) were included in the weighted averages in Table 1.[11,34]

CONCLUSIONS AND FUTURE WORK


Acknowledgements The identification of the 90 superoxide states and the collection of the PDECD data were carried out by Herder in 2003 when he was the first Wentworth Scholar. Chang and Rosenthal were the tenth and eleventh Wentworth Scholars who helped identify the Hylleraas as the fundamental measure of electron correlation. Rosenthal was a HYPERCHEM and Herder Scholar. Some of their work was presented in Gordon Conferences in Galveston each February between the years 2011 and 2015. The paper has been examined by all the authors. The support of the Herder research grant at Kinkaid High School, Hypercube and the Wentworth Foundation is appreciated.

REFERENCES [1] H. S. W. Massey. Negative Ions. Cambridge University Press, Cambridge, 1950. [2] National Institute of Standards and Technology. Available: http://cccbdb.nist.gov/: http://webbook.nist. [3] R. S. Mulliken. The assignment of quantum numbers for electrons in molecules. Phys. Rev. 1928, 115, 1225. [4] H. H. Michels. Electronic structure of excited states of selected atmospheric systems. Adv. Chem. Phys. 1981, 45, 225. [5] E. C. M. Chen, E. S. Chen. The Electron Capture Detector and the Study of Reactions with Thermal Electrons. Wiley Online Library, 2004. [6] E. S. Chen, C. Herder, H. Keith, E. C. M. Chen. Hund’s strong field states of superoxide and NO–. J. Chem. Theory Comput. 2010, 9, 393. [7] R. R. Freeman. I. Electron attachment to small selected molecules; II. The development and characterization of a photoionization/electron capture detector for use in gas chromatography systems. Doctoral Dissertation, University of Houston, Houston, TX, 1971. [8] E. S. Chen, E. C. M. Chen. Semiempirical characterization of homonuclear diatomic ions: group VI and VII. J. Phys. Chem. A 2003, 107, 169. [9] E. C. M. Chen, C. Herder, S. Chang, R. Ting, E. S. Chen. Experimental determination of spin-orbital coupling states of O2–. J. Phys. B: At. Mol. Opt. Phys. 2006, 39, 2317. [10] E. S. Chen, E. C. M. Chen, F. C. Anderson, S. Pai. Paradigms and paradoxes: what are the 54 electron affinities of O2. Struct. Chem. 2012, 23, 407. [11] J. Schiedt, R. Weinkauf. Spin orbital coupling of O2. Z. Naturforsch. 1995, 50, 1041. [12] V. Laporta, R. Celiberto, J. Tennyson. Dissociative electron attachment and electron-impact resonant dissociation of vibrationally excited O2 molecules. Phys. Rev. A: At. Mol. Opt. Phys. 2015, 91, 012701. [13] E. S. Chen, E. C. M. Chen. The Hylleraas binding energy of hydride and electron affinities. J. Chem. Theory Comput. 2013, 12, 1350016. [14] E. S. Chen, S. Pai, E. C. M. Chen. Hyperfine electron affinities of superoxide. Comp. Theor. Chem. 2014, 1050, 89. [15] E. S. Chen, H. Keith, T. Lim, D. Pham, R. Rosenthal, C. Herder, S. Pai, R. A. Flores, E. C. M. Chen. Hylleraas



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The HIMPEC for the 54 superoxide spin states in Fig. 1 are improvements over the previous literature curves, and support the theoretical predictions of the 90 superoxide states and recent communications in this journal presenting HIMPEC for multiple anion states of SF6 and C7F14 from the temperature dependence of our APIMS data and literature surface ionization, beam and flowing afterglow data.[35,36] The dramatic temperature dependence of the O– distributions cannot be explained by the 54 states shown in Fig. 1. In 1967, O’Malley attributed the onset of O– at about 1 eV at 2100K to an increased survival factor of the A2Πu state.[23] In 1970, Van Brunt and Kieffert supported this conclusion with angular distributions of O– but noted the possibility of two or more states contributing to this peak.[37] In 2006, Nandi et al. stated: "In conclusion, our measurements show the presence of the 4Σu– resonance in the dissociative electron attachment channel in O2. The results indicate that the lifetime of this state against autodetachment is much larger than assumed until now. This may have a wider implication to the studies on O2 in condensed form as well as in clusters."[25] The 18 bonding states dissociating to [1D(O) + 2P(O– )] and [1S(O) + 2P(O– )] will contribute to the increased lifetimes of superoxide states. In a future publication we will use the Hylleraas and the unassigned positive AEa(O2) value of less than 0.15 eV to estimate anion dissociation energies and obtain HIMPEC from electron ionization distributions in the gas phase and on surfaces and films such as curves dissociating to the higher limits presented by Sambe and Ramaker using condensed-phase distributions.[11,34,38] The

27 electron affinities in the NPES-95 data (Fig. 5) were identified using absolute values of derivatives of NPES, abs(dNPES-95). We have also identified hyperfine electron affinities from the structure in the abs(dNPES-95).[14] In a future study we will attempt to identify additional hyperfine electron affinities for the 54 states.

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