Electron affinities from gas chromatography electron capture detector and negative ion mass spectrometry responses and complementary methods

Accepted Manuscript Title: Electron affinities from gas chromatography electron capture detector and negative ion mass spectrometry responses and complementary methods Authors: Edward C.M. Chen, Edward S. Chen PII: DOI: Reference:

S0021-9673(18)31061-6 https://doi.org/10.1016/j.chroma.2018.08.041 CHROMA 359637

To appear in:

Journal of Chromatography A

Received date: Revised date: Accepted date:

20-5-2018 9-8-2018 19-8-2018

Please cite this article as: Chen ECM, Chen ES, Electron affinities from gas chromatography electron capture detector and negative ion mass spectrometry responses and complementary methods, Journal of Chromatography A (2018), https://doi.org/10.1016/j.chroma.2018.08.041 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Electron affinities from gas chromatography electron capture detector and negative ion mass spectrometry responses and complementary methods Edward C. M. Chen 1 and Edward S. Chen2 University of Houston Clear Lake, 2700 Bay Area Blvd. Houston Tx., 77059, U.S.A. Phone 713.667.3001 [email protected] Baylor College of Medicine, One Baylor Plaza, Houston Tx, 77030, U.S.A. [email protected]

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Contact author: [email protected]

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Highlights  Electron Affinities from gas chromatography electron capture detectors are reviewed  New electron affinities from complementary methods are reported.  The accuracy and precision of electron affinities from three methods are established  Optimized procedures for analytical and physical measurements are presented

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Abstract:

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The use of the electron-capture detector, ECD, to measure molecular electron affinities and kinetic parameters for reactions of thermal electrons with molecules at atmospheric pressure separated by chromatography and the sensitive and selective quantitative analysis of certain classes molecules are reviewed. The evaluated ground state electron affinities of the main group elements and diatomic molecules from slightly positive, 0+, to 3.6 eV are summarized. The electron affinities of twenty-seven superoxide states determined from pulsed discharge ECD and other methods are used to calculate one dimensional potential energy curves in agreement with theory. Advances in literature searches have uncovered ECD data in dissertations and theses and in the Russian and Japanese literature. These data, unpublished radioactive and pulsed discharge ECD thermal data from the University of Houston laboratories are used to report and evaluate electron affinities. The accuracy and precision of ECD electron affinities of organic molecules are identified and tabulated so that they can be added to compilations. A procedure for calculating the temperature dependence of electron molecule reactions is presented using kinetic and thermodynamic data. These are used to select the most appropriate equipment and conditions for ECD analyses and physical determinations.

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Keywords: Electron Capture Detector; Negative Ion Mass Spectra;Temperature Dependence; Selective Detection; Electron Affinities

1. Introduction More than a half century ago Lovelock and Lipsky identified peaks in gas chromatography with the electron-capture detector (ECD). [1] It is now used in many

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quantitative analyses and to determine molecular electron affinities, EA and kinetic parameters for thermal electron reactions. [2-5] The energy differences between a neutral N electron system and an N+1 anion are: adiabatic electron affinities, EAa in their most stable forms; vertical

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electron affinities, EAv in the neutral geometry; photodetachment energy, EApd in the anion geometry. The limiting ground state EAa is zero as the T approaches zero Kelvin. The gas phase EA of many atoms and small molecules were first measured using the magnetron (MGN) method while those for large organic molecules were first determined with the ECD. [2-6]

In the 1960’s, Lovelock, and Wentworth and Becker described the reactions in the ECD

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as non-dissociative AB + e(-) = AB(-) + EAa, and dissociative AB + e(-) -> A + B(-) + Edea; the

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energy for dissociative electron attachment: [Edea= EAa(A or B ) – D0(AB)]. [7,8] Herschbach

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classified these reactions for diatomic halogens with anion potentials based on the signs of EAv,

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Edea and De(AB(-)). Replacing the always positive De(AB(-)) with EAa, defined 2m+1 = 16 classes; m is the number of positive metrics. These (b) bonding (n) nonbonding and (a)

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antibonding curves are now called Herschbach Ionic Morse Person Empirical Curves (HIMPEC).

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[5,9] Theoretical EA and De(AB) used to calculate HIMPEC can be obtained from a

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multiconfiguration configuration interaction (MCCI) procedure, CURES-EC: “Configuration interaction or Unrestricted orbitals to Relate Experimental quantities to Self-consistent field

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values by estimating Electron Correlation.” It is a systematic method of varying the number of MCCI orbitals to minimize the difference between the experimental and theoretical values to

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cure the electron correlation problem. [2,5] The ECD, MGN, and swarm equilibrium methods are based on the temperature

dependence of the equilibrium constant for the reactions of thermal electrons at higher pressures and provide EAa from fundamental constants. The thermal charge transfer (TCT) and collisional

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ionization (CI) methods give relative EA based on anion intensities often calibrated to ECD values. The thresholds for reactions with alkali metal beams (AMB) are combined with ionization potentials to obtain EA. The photodetachment (PD), photoelectron spectroscopy

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(PES), and photoabsorption methods obtain EA from thresholds. The TCT [10], CI(11), AMB [12] and photon [13] studies at lower pressures have been reviewed. The determination of EA from negative ion lifetimes [NILT] in anion mass spectrometers was simplified in 2015 by Russian scientists. [14]

The experimental methods and calculation procedures used in the University of Houston,

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(UH) and complementary procedures will be briefly presented. Then the “best” (most precise and

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accurately assigned) EA of the atoms and molecules are selected from those listed in the

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National Institute of Standards and Technology website, NIST. [15] These data and additional

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molecular EA will be incorporated into the procedure for estimating the temperature dependence of thermal electron attachment reactions and the selection of detectors to optimize analytical and

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physical chemistry measurements. The emphasis will be on newly discovered literature data and

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data obtained using the pulsed discharge electron capture detector (PDECD) recently compared

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to radioactive ECD by Poole in a review of ionization detectors. [16-27]

2. Experimental methods, ECD Model and Morse Potentials. 2.1 Experimental Methods.

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Briefly, the procedures for collecting the ECD data at the UH are as follows: solutions of one or more compounds and an internal standard with a known temperature dependence are injected sequentially at one temperature and then the temperature of the detector is changed after

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the elution of the last compound. The quantities measured in the ECD are the electron current in the absence of the test material (b) and in the presence of the test material (e). The solid samples are weighed on a Mettler H54AR balance while the liquid solutions and the dilutions are

prepared with high-quality volumetric equipment. Nanograde hexane was used to prepare the

solutions and was checked for electron- capturing impurities. In order to ensure sample purity,

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the components were separated in a gas chromatograph fitted with a bonded-phase capillary

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column (J & W DB-5 fused silica, 1-micron thickness, 30 m by 0.322 mm 75C flow rate 3.5

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mL/min). The effluent from the column was simultaneously detected by a flame ionization

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detector and a scantium tritide or Ni-63 electron-capture detector or since the 1990’s a pulsed discharge ECD, PDECD, and a pulsed discharge photoionization detector, PDPID to identify the

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major components. In separate experiments, the compounds were examined in an atmospheric

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pressure negative ionization mass spectrometer, (APINIMS) monitored for the free-electron

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concentration as in the ECD. [2, 5, 16-19, 21-24, 28-33] ECD Model

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In the ECD, a constant rate of thermal electrons is produced and a steady state value obtained by recombination with positive ions. The rate constant for the recombination reaction in

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the absence of a capturing species is: e(-) + P(+) -> neutrals: kD = constant

(1)

A major problem in the ECD is the effect of impurities on the standing current where the electrons can be captured by impurities. If the effect of the impurities is constant, this will only reduce the standing current and give an effective kD so that kD = kD(electrons) +kI[I]. If however 4

the effect of the impurities changes with temperature, the effect of the “test” material will be attenuated. In the UH studies, the samples were not injected until the baselines were constant. In the presence of a capturing species, AB, the molecular negative ion is stabilized to a ground or

AB + e(-) -> AB(-)

:k1 = A1T-1/2(exp(-E1/RT))

AB(-) -> A + B(-)

:k2 = A-1T(exp(-E2/RT))

(2) (3)

AB + e(-) -> A + B(-) :k12 = A12T-1/2(exp(-E12/RT)) AB(-)

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excited state from which detachment, dissociation or recombination occurs with rate constants .

-> AB + (e-) :k-1= A-1T(exp(-E-1/RT))

(4) (5) (6)

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AB(-) + P(+) -> neutral :kN = constant

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The A1 is the pre-exponential term for electron attachment while A-1 is the pre-exponential term

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for the rate constant for electron detachment. The A1/A-1 is determined from the neutral and

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anion partition functions. The steady state solution of these rates gives the ECD response: = KECD [ a]

(7)

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[b-e] k1(kN + k2) k12 ____ = { __________ + ______} [a] [e] 2kD(k-1 + k2+ kN ) 2kD

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where [a] is the concentration of the capturing species, [b] is the electron concentration in the absence of capturing species and [e ] is the electron concentration in the presence of the

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capturing species. At higher temperatures,T> 400K for EAa > 0.6 eV when k-1 >> kN and k2 <<

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kN = kD then the ECD response, KECD = (k1/2k-1) = KEq/2 = (A1/2A-1)T-3/2exp(EAa/RT) (alpha region). The molecules with EAa < 0.6 eV and Edea > 1 eV will have only alpha regions. For the

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compounds with only an alpha region, the EAa is obtained from the positive slope of a plot of Ln(KT3/2) vs 1000/T. For these compounds, one point and the theoretical intercept will give the positive slope, EAa. When kN >> k-1 at lower temperature for molecules with EAa > 0.6 eV, the response levels off to a negative slope due to the activation energy, E1 and KECD = k1/2kD (the beta region). For some compounds, there are multiple excited states with small dissociation

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energies so that when k2 >>kN <
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and the Edea can be determined. 2.3 Miscellaneous Methods

Streitweiser summarized the determination of gas phase EAa for aromatic hydrocarbons from reduction potentials (ERED) and the Nernst equation: EAa = KRED + ERED in 1961 where KRED is a constant for molecules with similar electron affinities and/or charge distributions.[34]

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The UH laboratories have used accurate and precise EAa from gas phase ECD values to establish

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values of KRED to obtain gas phase EAa for molecules. For example, the KRED for the

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aldehydes and ketones with a greater charge distribution on the oxygen atoms is different from

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the aromatic hydrocarbons. We have also used solution charge transfer complex data to estimate molecular EAa, for aromatic hydrocarbons. [2,5,16,34,35]

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In the (TCT) method the electron affinity of a molecule is determined by bracketing the

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electron affinity of the test material between two species. The general procedure has been to

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develop a ladder of bracketing reactions and to reference that ladder to one accurate and precise electron affinity, that of SO2, 1.107(8) eV. These reactions can be studied by observing the

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direction of charge transfer or by measuring the equilibrium concentrations of the anions and neutrals. The error in the measured electron affinity is no smaller than the errors in the electron

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affinity of the bracketing species. The determinations can be completed at one temperature or multiple temperatures. [10] In 1989, Mock and Grimsrud devised an ECD/photodetachment device that could be used to measure onsets for photodetachment of aromatic nitro compounds at

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atmospheric pressures. The onsets listed in NIST as PD values are the same or higher than the thermal ECD EAa. The values for some molecules were 1 eV higher. [36,37] In 2015, Asfandiarov and co-workers reported ground state AEa of eleven methyl,

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fluoro and chloro substituted nitrobenzenes from negative ion lifetimes (NILT, a ) determined using mass spectrometry and the following equation: AEa

[Ln(a/ 0 )] [NkT +] = ___________________ [N- Ln(a/ 0)]

(8)

where N = 3n-6 is the internal number of degrees of freedom, NkT is the vibrational energy

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storage of the target molecule with k the Boltzman constant, T the absolute temperature; , the

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kinetic energy of the incoming electron and 0, the typical time required for anion motion on the

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reaction coordinate from the anion to the neutral; on the order of 100 to 500 fs. The 0 was

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described as follows: “The choice of 0 requires experimental or reliable theoretical evaluations for each molecule. Unfortunately, up till now the availability of these data is very limited. ” Thus

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they considered o an adjustable constant.[14]

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2.4 Herschbach Ionic Morse Person Electron Curves, HIMPEC

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In 1987, Herschbach recalled his 1960s classification of diatomic anions based on the signs of De[Z2-], vertical electron affinity, EAv, and the energy for dissociative electron

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attachment, Edea. [2,5,9] The De[Z2-] was replaced with the AEa to define 23 = 8 Herschbach Ionic

U(Z2) = De(Z2)-2 De(Z2) exp(-(r - re)) + De(Z2)exp(-2 (r - re))

(9)

U(Z2-) = De(Z2)-2kA De(Z2) exp(-kB (r-re)) + kR De(Z2)exp(-2kB (r-re)) –EAa(Z)

(10)

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Morse Person Electron Curves, HIMPEC. The neutral and anion curves are:

r is the internuclear separation, re = r at the U(Z2) minimum,  = me(22/ U(Z2))1/2,, me electron mass, , reduced mass, The HIMPEC are Morse potentials with : De((Z2-)/De((Z2); = [kA2/kR ];

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re (Z2-)-re (Z2) = [ln (kR/kA)]/[kB  (Z2)]: (Z2-)/ (Z2) = kAkB /kR1/2. Three data points such as Ea, VEa and Edea and /or other data give the dimensionless constants kA , kB and kR. 3.0 Evaluation procedures for electron affinities.

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The accuracy of a value is determined by systematic uncertainties. According to Deming “A systematic uncertainty or bias is never discovered, nor has any meaning, unless two or more distinct methods of observation are compared.” [38] The EA in NIST are tabulated without

evaluation. [15] In 2007, we reported EAa(SF6 ) 2.60(10) to 0.10(10) eV. The search of the NIST database for the EA(SF6) returned: (eV) 3.16, 1.49(22), 1.39(13), 1.15(15),1.10, 1.07(7),

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1.05(10), >0.93, 0.95(52), 0.8(1), 0.75(10), >0.7(1), >0.6(1), 0.54, 0.54(17), 0.53(10), 0.46(20),

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0.43, 0.32(15) eV. The UH EA(SF6) were (eV) ECD, 1994, 1.07(7), 1968 and 1983, > 0.7;

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NIMS 1988, 1.15(15). The three most recent values in NIST are the UH NIMS 2.60(10) eV; and

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1.03(5) and 0.91(7) eV reported by others in 2014. We have also reported the ground state AEa(SF6.), 3.00(15) eV from high temperature NIMS data. [15,39-41]

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If there are two or more values determined by different techniques that agree within the

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random uncertainty, the weighted average value is the accurate value and the precision is

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determined by the random uncertainties. Both precision and accuracy are characteristics of the measurement procedure, not the value, except at the extremes. [2,5,38] When the random

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uncertainty in one value is much smaller than the others, the average and uncertainty will be dominated by these values. Five EAa for nitrobenzene are in NIST [values (yi), uncertainties (si),

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method]: (eV) 1.00(1), NPES; 1.01(10), TCT; 1.00(6), NIMS; 1.02(5) TCT; and 1.00(2), ECD. The weighted average is 1.000(8) eV and the uncertainty is slightly reduced. If there are significantly different values, the gs-EAa is the largest most precise value. The uncertainty in the average will never be larger than the smallest uncertainty (10 meV). [15]

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Recently, we postulated 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 examined the periodicity in the ground state values for atoms

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from NHyl = EAa(Z)/Hyl. [42,43] The NHyl for the group 0 and II elements with filled shells are about zero; the NHyl of the group III elements are about one; of group I elements about two; and of the halogens are greater than eight. The gs-EA for the main group elements in Fig. 1 are the same magnitude as in 2004 but are more precise. The selected values for some of the d and f

atoms are different because the previous values have been assigned to different states. [2,5,15]

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The only ECD AEa for diatomic molecules in NIST are NO, O2, Cl2, Br2, and I2. The

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dNHyl (Z, Z2) = (gs-EAa(Z2)-EAa(Z))/Hyl, a measure of electron correlation, were used to assign

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gs-EAa( Z2). The largest value is dNHyl (C, C2), 5.30 and the smallest value dNHyl (Cl,Cl2), -3.08 .

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The dNHyl (Br, Br2) is -1.31 and for iodine is -1.42. The dNHyl for the adjacent O2, S2 and F2 are 1 and for Se2, Te2, and the alkali metal dimers are zero. [2,5,15]

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The ECD EA listed in NIST are: (eV) F2, 3.08; Cl2, 2.33; Br2, 2.42 and I2, 2.33 from an

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empirical anion bond order of 0.5 that generally predict dissociative thermal electron attachment.

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[2,5,42-45] The EA(Cl2) in NIST are: (eV) 3.20(20), 2.52(17), 2.50(20), 2.46(14), 2.45(15), 2.40(20), 2.32(10) , 2.38(10), 1.02(5). The 2.32(10) to 2.52(17) eV are the most precise

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overlapping values. The weighted average of these, 2.40(8) eV is assigned to gs-EAa. The 1.02(5) eV is an EAv while the 3.20(20) eV is systematically high. The largest most precise EA

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in NIST (eV) : F2, 3.12(7); Br2, 2.87(14) and I2, 2.524(5) are assigned to the gs-EAa. In Fig. 2A thermal ECD data and NIMS data for O2 from the UH laboratories and from

other laboratories reported since 1966 are plotted. In Fig. 2B are the 1970’s ECD data for O2, NO and N2O collected by Freeman ; the 1981 ECD data for Cl2, Br2 and I2 collected by Ayala ; the

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2003 ECD data for O2 collected by Chen and Chen and recent flowing afterglow Langmuir probe (FALP) data for Cl2 and F2. The data for O2 and NO represent stable negative ion formation since the Edea are larger than 1 eV. [33,42,44,45] The FALP data for Cl2 from 200 to 1100K

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overlap the 1981 ECD data from 300 to 700K while that for F2 from 300 to 800K are above that for Cl2. [46,47] The structure indicates multiple activation energies for the formation of the halogen atomic ions via multiple states. These values and the EAv in NIST were used to

calculate HIMPEC for the 6x2 =12 spin orbital coupling states predicted for the diatomic

halogen anions. The EA for N2O in NIST range from 0.22 eV to >0.76 eV. The 1971 ECD data

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for N2O gave the EAa = 0.27(17) eV. [15,33,44] Here we publish PDECD data from 227 to 400

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C (1000/T = 2 to 1.5) with an activation energy of 0.8-1 eV, the same as for NO and O2. The best

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temperature for the determination of N2O is the highest temperature as suggested in 1973. [44]

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We concentrate on the simple ECD but the use of an N2O or O2 doped ECD was summarized by Poole in 2015.[27] Results

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Diatomic oxygen, nitric oxide

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Freeman determined EAa: (eV) O2 [0.45(5), 0.9]; NO [0.15(2), 0.8] from the ECD data in Fig 2B in 1971 but did not publish the data or results in the open literature because the two

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positive values could not be understood.[33] The NIST database lists twenty values for O2; 0.15(5) to 1.12(7) eV including 1983 ECD values of 0.451(52) and 2003 0.725 eV and sixteen

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values for NO; 0.02 to 0.91 eV including the ECD value of 0.10(10) eV. [15] Unaware to us at the time, Michels had predicted 2x3= 6 spin orbital coupling states dissociating to the first and 9x9 = 81 to the second giving 27 + 3 = 30 [b] bonding, 27 [n] non-bonding and 30 [a] antibonding states. [48] Thus we have analyzed the ECD and magnetron data for NO in Fig.2in

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terms of states dissociating to [N(-) +O] and [N +O(-)]. The fits to the ECD data (not shown) in Fig 2 were used to obtain the 30 positive EAa. In Fig. 3 are previously published HIMPEC for the bonding states and approximate curves for the non-bonding and antibonding states of the

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nitric oxide anion. [49] Superoxide is the most important homonuclear diatomic anion, and a large body of data has been produced for it since 1958. [33,48-66 ] Most of the published thermal data are shown in Fig.2A: (1,2) UH APIMS and ECD data that show a positive slope with an AEa 1.07 eV [49];

(3,4) high pressure and low temperature microwave swarm data in O2 and N2 in the beta region

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[61]; (5) high temperature scantium tritide ECD data with multiple positive and negative slopes

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[50]; (6) swarm data from 1966 with a positive slope [56]; (7) ECD data from Van der Wiel and

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Tomassen [58]; (8) m/z =32 data from flames, (9) magnetron data, (10) Herder’s unpublished

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PDECD data in pure Helium. [53]

In 2002, Chen, Wentworth and Chen used slopes in the (5) scantium tritide data to report

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multiple ECD EAa., including values of 0.70 and 0.75 eV that are listed in NIST as an average

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value although we now know there are multiple states in that range.[50] Then, Chen and Chen

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combined these peaks in the photoelectron spectra published by Scheidt and Weinkopf in 1995 to report the ground state EAa of O2, 1.07 eV listed in the 2004 review. [2,51,52] The twenty seven

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curves in Fig. 2 are least squares fits to the highest sets of data that coincide with positive slopes

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in the lower data.[63-66] The most frequently cited AEa(O2), 0.448(6) eV, the value returned from a search of

NIST for oxygen was reported by Erwin and co-workers in 2003 in an article entitled “The only stable state of O2- is the X 2g ground state and it (still!) has an adiabatic electron detachment energy of 0.448 eV.”[63] In 2004 Herder, Chen and Chen followed Michels and identified 54

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theoretical spin orbital coupling states of superoxide dissociating to [ 3P(O) + 2P(O-)] from 9[3P(O)] times 6[2P(O-)], half of which are positive.[53 ] This is in contrast to the assumption of only one stable state of superoxide. Multiple sets of PDECD, and NIMS were collected by Chen,

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Herder, Chang, Ting and Chen to report the 27 positive EAa(O2), activation energies and Morse parameters listed in Table 1. The positive EAa(O2 ) are supported by the 1958, 0.15(5) eV PD

value; the 1969, 1.1 eV from EI data on NO2 ; and the 1972, 1989, 1995, and 2003 photon data. [52,54-66] Cyclic voltammetry data were analyzed by Chen, Keith, Lim, Pham, Rosenthal,

Herder, Pai, Flores and Chen in terms of the positive EAa(O2) in 2015. [42] The 27 negative

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electron affinities were estimated from electron impact data and the Hylleraas. The unpublished

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PDECD data collected by Herder in Fig. 2 are representative of multiple sets of PDECD data

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published by Chen, Herder, Chang, Ting and Chen in 2006.

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The 2006 article stated: “Electron affinities, EAa, E1 and A1 are reported for the 12 primary X, A–K (27 spin) states of O2(− ): These are obtained from pulsed discharge electron

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capture detector data by rigorously including literature values and uncertainties in a global non-

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linear least-squares adjustment.[…] The plans for the PDECD experiments were made in

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December 2003 in collaboration with Professor Wayne Wentworth prior to his death in March 2004.” The HIMPEC calculated from the Morse parameters in Table 1 are shown in Fig. 3. [54]

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The analysis of the ECD temperature dependence of reactions of thermal electrons with O2 led to the identification of multiple negative ion states of larger molecules in ECD data in this review.

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4.2

Aromatic hydrocarbons, heterocyclics, esters, OH, CH3C=O and HC=O compounds Wentworth and Becker first measured electron affinities using the ECD at the suggestion

of Lovelock. [3,4,67] The 1981 Journal of Chromatography Library volume “Electron Capture Theory and Practice in Chromatography” began with Lovelock’s odyssey with the ECD: “When

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I first came to Houston in 1958 […] I was met at the airport by Al Zlatkis […] In no time we were running samples from the Houston Petrochemical Industry that gave the most glorious and unbelievably excellent chromatograms. In 1961, I came to Houston again to work at Baylor

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College of Medicine. This provided at last an opportunity to spend full time in discovering how the ECD really worked. I was fortunate to have nearby both Dr. Zlatkis and Drs Wentworth and Chen of the University of Houston. This fruitful collaboration led to the first plausible kinetic

model of the ECD. Unlike so much theoretical modeling, this led to practical improvements in the use of the detector one of which was its determination of atmospheric N2O by using the

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detector at a temperature of 300 C or higher.”[44,67]

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R. S. Becker and W. E. Wentworth used the relative ECD responses of aromatic

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hydrocarbons at one temperature to report AEa by postulating that the response was related to the

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equilibrium constant for the reaction of thermal electrons. The ECD AEa from their 1962 JACS paper are 0.1 to 0.2 eV low because of the low value for the reference compound (anthracene).

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[3,4] Wentworth Chen and Lovelock used the kinetic model and the thermal data to determine

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AEa of nine aromatic hydrocarbons in 1966. [28] Becker and Chen reported these values and

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values for seven additional aromatic hydrocarbons from positive slopes but did not publish the data that are plotted in Fig. 4. Some of these values were low because of the limited temperature

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range that resulted in a high intercept. [68] Lyons, Morris and Warren reported ECD electron affinities of pyrene, 0.50(5),

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anthracene, 0.57(2), tetracene, 0.88(4) eV in 1968. [69] Wojnarovits and Foldiak reported ECD EA for 23 hydrocarbons in 1981 from 0.05 eV to 0.52 eV. Many of these are the only ones in NIST for these molecules. In a report that has now been made available Wojnarovits reported the following values: (eV) naphthalene, <0.14(4); phenanthrene, 0.30(4), anthracene, 0.52(4) and

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azulene, 0.60(1); while the values in NIST are: (eV) naphthalene, < 0.134(40); phenanthrene, <0.27(4), anthracene, <0.481(39) and azulene, <0.529(13). The temperature dependence could be used to identify the isomeric naphthalene and azulene. He also reported data for mono, di and tri-

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alkyl benzenes that were not published in the open literature. We have estimated limiting values for these molecules and include them in Table 2. [70] Also in 1981, Grimsrud published relative ECD responses for aromatic hydrocarbons at a constant temperature of 523K that support

published values. Here additional 1981 ECD responses are used to calculate EAa for heterocyclic hydrocarbons containing N, O, S not in NIST. These are basic molecules for substitutions.[71]

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In 1995 the ECD EAa for AHC were compared to EAa from reduction potentials and

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donor acceptor complexes and the EAa of anthracene, tetracene, and benz(a)pyrene measured by

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TCT. [72-75]. Lower values were assigned to excited states. Excited-state EAa have also been

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observed for pyrene, cyclooctaterene, acenaphthylene, fluoranthene, and azulene. [75,76]. The NIST database lists about fifty EA for H,C molecules, the majority of which are ECD values.

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The largest value is the TCT 1.39(4) eV for pentacene, that has recently been reported using PES

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as 1.43(3) eV averaging 1.41(2) eV. [77] In Table 2 are the ECD EAa for AHC, heterocyclics,

EP

esters, acetic anhydride, and OH, CH3C=O, HC=O compounds. [68-86] The major gas phase method confirming the ECD values before 2017 was the TCT

CC

method. [10,74] In 2011 and 2017, Khatymov, Muftakhov, Shchukin published the lifetimes of 17 polycyclic aromatic hydrocarbons PAH and described the literature values for electron

A

affinities, “For the PAH molecules one can find numerous literature data on experimental EA, which are often poorly consistent with each other […] In view of lack of reliable data, the adiabatic EA for PAH molecules can be assessed using simple regularity EA + IE ≈ 8 eV (consequence of nearly equal electronegativity of PAH molecules ) where IE is the vertical

14

ionization energy of the molecule.” [15,78,79] The poor consistency was resolved by assigning gs AEa: (eV) pentacene, 1.41(2); tetracene, 1.058(5); benz(a)pyrene, 0.83(3); benz(a)anthracene, 0.69(2); dibenz(a,h)anthracene, 0.69(3); anthracene, 0.68(2); dibenz(a,j)anthracene, 0.68(3);

SC RI PT

benz(e)pyrene, 0.60(3); picene, 0.59(3); pyrene, 0.58(2); chrysene 0.42(2) and excited state AEa: (eV) tetracene: 0.88(4); anthracene 0.55(1) ; pyrene, 0.41(1); benz(a)anthracene, 0.39(10);

chrysene, 0.32(1); and phenanthrene, 0.12(2). The lifetime of benz(a)pyrene. the carcinogenic

isomer, was predicted to be larger than 150 sec and for benzo(c)phenanthrene and picene about 40 sec.

U

In Table 3 are the evaluated ionization energies in NIST which when combined with the

N

ground state AEa give the constant electronegativity. The excited state values for pyrene,

A

chrysene and phenanthrene were from photon data while that for benzanthracene was from

M

collisional ionization data.[15,80] In 2017, Asfandiarov and co-workers reported gs-EAa : (eV)

D

pyrene.0.80, benzo(e)pyrene, 0.70, anthracene, 0.79; tetracene, 1.07 and pentacene, 1.13 eV. [81]

TE

Here we use different empirical lifetimes to calculate gs-AEa of aromatic hydrocarbons in Table 3 from lifetimes in agreement with the values in Table 2. [68-81]

EP

Here two ECD AEa (eV) for anthracene 0.68(2), 0.55(2); tetracene 1.06(1), 0.88(1); acenaphthylene 0.80(1) and 0.40(5); and one for pyrene 0.58(1) are reported from unpublished

CC

UH high temperature tritium ECD data. The ECD data for the pentacyclic aromatic hydrocarbons and earlier data for anthracene, pyrene and other AHC; the PDECD data for pyrene, the tritium

A

data for tetracene, and anthracene are shown in Fig. 4B. The electron attachment to anthracene shows a transition from an excited state at 0.55 eV to the ground state at 0.68 eV. The electron attachment to tetracene is in the beta region at low temperatures where KECD = k1/2kD and is relatively temperature independent. It transitions to an alpha region where KECD = k1kN/2(k-1KD)

15

and the positive slope times R is equal to the electron affinity, EAa , 0.88 eV. The tritium ECD data in Fig. 2B transitions to a second alpha region for the ground state with a different activation energy and then to a second positive slope that gives the ground state adiabatic electron affinity,

SC RI PT

1.06 eV. The ECD data for the AHC are compared to the earliest ECD data for acetophenone and benzaldehyde in Fig. 4B that indicate the range of temperatures for these types of data (300500K). These data were obtained at high and low frequencies.

In 1968 Kuhn, Levy and Lilley reported single point ECD EAa: (eV) dimethyl phthalate, 0.55; diethyl phthalate, 0.54 ; dimethyl isophthalate, 0.55 ; dimethyl terephthalate, 0.64 ;

U

dimethyl tetrachloroterephthalate, 0.77; terephthalaldehyde, 0.56; methyl benzoate, 0.18 listed in

N

NIST that are lower limits. [15,82]. In 1978, Hattori, Kuge and Asada reported ECD EAa for

A

esters from positive slopes. [83] We have obtained values by fixing the intercepts:(eV) dimethyl,

M

diethyl, dipropyl, diheptyl and dioctyl phthalate 0.65(5); dibutyl, diisopropyl, diisobutyl and bis(2-ethylhexyl) phthalate, 0.70(5); diallyl phthalate,0.80(5); dimethyl isophthalate, 0.70(5);

D

dimethyl terephthalate, 0.82(5) eV.[83] The EAa dimethyl tetrachloroterephthalate is estimated

TE

as 0.82 + 4 x 0.15 =1.4 eV. ECD data from the UH give EAa, 1,4 diacetylbenzene, 1.05(10) eV

EP

terephthalaldehyde, 1.25(10) eV consistent with TCT. The ECD EAa ethyl benzoate, 0.18 eV can be compared with ECD EAa for pentafluorobenzoates of hexanol, cyclohexanol and phenol

CC

about 1.05(5) eV from positive slopes in data presented by Poole and Zlatkis in 1981. [84,85] Other oxygen containing molecules in Table 2 consist of smaller molecules. The ECD

A

electron affinity of biacetyl EAa 0.68(2) eV has been used as a reference. This value is not listed in NIST which cites the TCT value of 0.69(10) and an electron scattering value of 1.10 eV. The ECD value supports the TCT value. [2,5,10] The ECD values for ethyl acetate, benzyl acetate, and acetic anhydride are also not in NIST. The activation energies and bond dissociation

16

energies for these three compounds were used to report one of the first EAa of the acetate radical in 1968 by Wentworth, Chen and Steelhammer. The ECD value in NIST, 3.18(5) eV overlaps the most recent photoelectron spectroscopy value in NIST, 3.250(10) eV. [86,87]

SC RI PT

In 1965, Walter Hirsch, Wentworth’s first MS student determined unpublished ECD data for 1 and 2 naphthol, salicylaldehyde, phenol, and butanol in Figure 5B. The positive slopes in these data were adjusted to an EAa for acetophenone 0.338(2) eV. The adiabatic electron

affinities for the naphthols and salicyaldehyde are larger than for benzaldehyde, 0.437(2) eV while that for phenol is the same as for benzaldehyde and that for butanol is lower than for

U

acetophenone. To our knowledge, the values listed in Table 2 are the only EAa for OH

N

substituted molecules that have been measured. These values are consequently included in Table

A

2 for convenience. [29]

M

4.3 Species containing Carbon, Hydrogen, Oxygen, Halogens, CH3C=O and HC=O. 4.3.1 Introduction.

D

The ECD is routinely used to quantitatively analyze for compounds containing halogens.

TE

Although the determination of the temperature dependence of the ECD response to these

EP

compounds is a simple method of determining EAa, there are few ECD values in NIST due to the absence of positive slopes. In Figs. 5-7 are representative ECD data and in Tables 4 and 5 are

CC

ECD EAa of halogenated, OCH3 substituted and carbonyl compounds. [84,85,88-108]

A

4.3.2 Fluoro benzenes, anisoles, cycloalkanes and alkanes. The NIST database contains ECD EAa of only few molecules containing CF or CHF.

[15] In 1987 the UH laboratory reported the largest and most precise EAa (C6F6), 0.86(3) eV using the ECD and confirmed this value in 1994 using NIMS data. [88,89] There are two PES values 0.7(1) eV and 0.8(1) eV in NIST. Three other values average 0.53(5) eV. Two earlier

17

outliers are 1.8(3) eV and 1.20(7) eV. The ECD value is assigned to the ground state and the lower value to an excited state based on the data in Fig. 5A. In 1987, an ECD EAa(C6HF5), 0.73(8) eV was reported by the UH group but a search of NIST returns the 1989 TCT value of

SC RI PT

0.43(9) eV but also lists the larger ECD value. The UH group also obtained ECD data for fluorobenzene, p-difluorobenzene and 1,2,3,4-tetra fluorobenzene but did not report electron

affinities. Here the ECD EAa, 0.13(5); 0.25(5); and 0.52(5) eV are reported from the data in Fig. 5A plotted with the data for C6F6, C6HF5, and for tetracene, azulene, chrysene, naphthalene, and other compounds from Fig. 4 as references for slopes determined from a fixed intercept and as

U

few as four data points for chrysene.[15,88-93]

N

Also listed in NIST are TCT electron affinities of c-C7F14 that has been studied in both

A

the ECD and in the photo detachment ECD. In the 1980’s, the Wentworth group measured c-

M

C7F14 and c-C7F13 anions from 343 to 598K and the c-C6F11 anion above 563K giving an EAa(C7F14), 1.4(2) eV. In 1985, Grimsrud and co-workers independently determined an

D

EAa(C7F14) 1.06(13) eV from thermal charge transfer studies. [94] In 2015 we concluded the

TE

EAa: (eV) c-C6F11-CF3, 1.06, c-C6F10-CF3, 3.9, c-C6F11, 3.5 and D(R1-CF3), 3.8; D(R-F), 4.3 are

EP

supported by thermal NIMS data. The gs-AEa c-C6F10-CF3 is 3.0(1) eV from photon data. Additional EA for c-C6F11-CF3, from 0.5 to 1.5 eV observed by thermal NIMS were assigned to

CC

excited states. [15,93-96]

The NIST database lists ECD EAa of pentafluoro and 2,3,5,6-tetraflouroanisole and the

A

3-F phenoxybenzyl ether from 0.21 to 0.55 eV and the fluorophenoxy radicals 2.61(8) to 3.07(8) reported by the Wentworth laboratory in 1984.[98] The EAa of 2,4,6-tribromoanisole and 2,4,6trichloroanisole on the order of 1 eV were reported using the NILT method at low temperatures in 2017. [99] The lower ECD electron affinities for the fluoroanisoles were determined from

18

limited data and a high intercept. Here we use ECD data similar to the unpublished data for the pentafluoroanisole shown in Figure 5B to report adiabatic ground state electron affinities: (eV) pentafluoroanisole, 0.80(5); 3-F-phenylbenzyl ether, 0.65(5); tetrafluorophenyl anisole, 0.60(5);

SC RI PT

and the tetrafluorophenoxy radical, 2.95(10). The latter is scaled to the radical EA: 3-F-phenoxy, 2.61(5) and pentafluorophenoxy, 3.08(5) eV determined by ECD and acidity data in NIST. [15] The ECD EAa for the fluorobenzenes and the fluoroanisoles are listed in Table 4.

The ECD AEa CF3CHF2, 0.45(15) eV and CF3CH2F, 0.38(15) eV listed in Table 4 were determined from limited positive slopes in ECD data reported by Sousa and Bialkowski in 1997

U

but are not in NIST. [101] These values are consistent with CURES-EC calculations. The larger

N

uncertainties in these values is due to the limited alpha data. Since electron capture by

A

fluoroalkanes is non dissociative, the analysis is best carried out at the lowest temperature in the

M

alpha region. [101-104] The TCT decafluorobiphenyl AEa 0.824 eV, and pentafluoro(trifluoromethyl) benzene 0.859 eV in NIST are listed here for comparison to the

D

ECD values for the halogenated compounds listed in Table 4 that range from 0.10 to 1.05(5) eV.

TE

[15,93] The EA for trifluoromethylbenzene, 0. 002 eV, the lowest value for a CHF molecules in

CC

EP

NIST, is for a dipole bound state. [15]

A

4.3.2 Compounds containing CHCl, Thermal NIMS EAa for 1,1-di, tri and tetra chloro ethylene: (eV) 0.10(2); 0.40(20); and

0.64(5) listed in NIST are selected as the gsEAa. [2,5,15,89]. The ECD EAa for tetra, tri and di chlorobenzenes: (eV) 0.45; 0.34 and 0.1 in NIST are lower limits estimated from low temperature data in the dissertation of Shen Nan Lin. As shown in Fig 6, we fit the low

19

temperature ECD data with a non-linear least squares to the common intercept. We report EAa for all of the isomers of the chlorobenzenes and also the three chlorotoluenes from the DC-ECD data reported by Hattori, Kuge and Nakagawa that does not show a positive slope by calibrating

SC RI PT

to the UH data. For example, the UH 1,3,5 trichloro benzene EAa is 0.48 eV so we report values for the other trichlorobenzene isomers based on the relative values at the low temperatures.

[2,5,97,105] The data in Fig. 6 are for the different isomers. The ECD EAa for C6Cl6 , 1.05(5) eV and for C6HCl5 , 0.88(5) eV from unpublished UH-PDECD data and the DC-ECD data reported by Hattori , Kuge and Nakagawa overlap the TCT values, 0.92(10) eV for C6Cl6 and the TCT

U

value 0.73(9) eV for C6HCl5 in NIST.

N

Lin also measured the dissociative electron impact spectrum of these compounds and

A

C6F5X that led to the ground state-EAa of C6F5Cl, 1.00(5) eV and the C6F5 radical, between that

M

of I and Br or between, 3.07 and 3.37 eV. The EAa in NIST for C6F5Cl, are 0.75(8) and 0.82(11), eV and for C6F5, 3.18(13). NIST also lists EAa C6F5Br, 1.15(11) and C6F5I, 1.41(11)

D

eV. The UH- ECD data for bromo and iodobenzene in Fig. 6A have only negative slopes. The

TE

ECD EAa of 1 chloro-naphthalene, 0.35(2) eV reported from the positive slope in ECD data is

EP

larger than the value in NIST, 0.277(3) eV. [31] The ECD EAa for the chloroanthracenes about 0.87 eV reported from the 1981 Grimsrud ECD data agree with those from TCT data and the

CC

CURES-EC values as shown in Table 4. [71, 106] The temperature dependence of the I, Br, and Cl alkanes are the easiest to predict. Iodo

A

compounds have the largest response and smallest temperature dependence, about zero similar to that of iodobenzene shown in Figure 6A. The chloroalkanes have the smallest response and largest activation energies about 0.6 eV similar to that of chlorobenzene. The activation energy for dichloromethane and methylbromide are about 0.3 eV; for chloroform is about 0.12 eV and

20

for carbon tetrachloride, dibromomethane and methyl iodide are about zero. Thus the temperature dependence can be calculated assuming a nominal value for the preexponential term. Since there are no positive slopes it is not possible to obtain electron affinities for these

SC RI PT

compounds from ECD data. However, it is noted that the reduction potentials for the halogenated benzenes give positive electron affinities consistent with the values in NIST. [21,24,30,101-104] 4.4 .4 Compounds containing Carbon, Hydrogen, Oxygen and Halogens

Besides the aromatic hydrocarbons, alcohols and esters, the EAa of a number of halogen substituted acetophenones, benzaldehydes, and benzophenones have been determined using the

U

ECD. In Table 5 are the EAa for these compounds. Some of the thermal ECD data are plotted in

N

Fig. 7. The EAa of acetophenone and benzaldehyde have also been used as internal standards for

A

ECD work as in Figs. 4-6. These were chosen because their temperature dependence is well

M

established. The ECD EAa for acetophenone in the NIST table, 0.334(4) eV, and for benzaldehyde 0.425(8) eV are slightly lower than the complete averages, 0.338(2), 0.437(5) eV.

D

NIST also lists an EAa of acetophenone, 2.55(5) eV that is clearly an outlier.[15,109-117] The

TE

preferred ECD value for o-CF3 -acetophenone, 0.71(5) eV is higher than the earlier ECD value,

EP

0.643 eV listed in NIST. The evaluated value is the average of the TCT and ECD values, 0.74(5) eV. The preferred ECD EAa values for m-F acetophenone and p-F-benzaldehyde are also higher

CC

than the values in NIST because of the fit to the common intercept. The ECD EAa for p-diacetyl-benzene, 1.05(10) eV terephthalaldehyde, 1.25(10) eV

A

listed in Table 4 and m- and p-CF3 –acetophenones, and other aromatic compounds in Table 5 agree with the TCT EAa in NIST. [15, 112-117] The ECD EAa for the 1 and 2 naphthaldehydes, are larger than for 1-acetonaphthone. The ECD EAa for 9 phenanthrenealdehyde is larger than that for phenanthrene and the ECD EAa for cinnamaldehyde is larger than that for benzaldehyde.

21

[2,5,15] The evaluated ECD EAa for benzophenone, 0.68(5) eV, is the average of the ECD and the two most precise TCT values. In 1972, Vessman and Hartvig reported ECD data for the benzophenone and benzophenone substituted by halogen, alkyl, methoxy, CF3, and NO2 in the 4

SC RI PT

position using a Ni-63 detector. [112] In 1981, we used their data to report approximate EA, 0.64(10) to 1.0(2) eV from positive slopes. [2,5,16] subsequent TCT values agree with the 1981 values for benzophenone and the Cl and CF3 substituted benzophenones. Here we use the nonlinear least squares procedure to obtain the more precise values given in Table 5. 4.4 Aromatic and aliphatic nitrocompounds.

U

The ECD temperature dependence of nitro compounds have been determined by the UH

N

laboratories under Wentworth, Chen and Zlatkis, by the Montana State group under Grimsrud

A

and the Japanese groups. However, the UH ECD-EAa of the nitrotoluenes, nitrobenzene,

M

pentafluoronitrobenzene, nitromethane and alpha nitrotoluene are the only ECD-EAa listed in NIST although ECD-PD values from Grimsrud and Mock are listed as PD values. [36,37] The

D

UH-ECD data for nitromethane shows a positive slope at low temperatures and a negative slope

TE

at high temperatures. There is no positive slope for alpha nitrotoluene but the magnitude is much

EP

larger than for nitromethane. The UH ECD EAa of nitromethane, 0.50(2) eV supported by TCT and alkali metal beam data is larger than the PD values in NIST. 0.172 eV, nitromethane; 0.191

CC

eV nitroethane; 0.223 eV nitropropane; 0.240 eV nitrobutane. [15] The ECD activation energies and EAa for aliphatic and aromatic nitro compounds were

A

published by Kojima and Satouchi in the 1970’s. They reported EAa for the nitroalkanes from 0.2 to 0.3 eV with a transition to dissociative electron capture taking place at 423K. They reported thermal electron attachment data of nitrobenzene , chloro, bromo and

22

alkylnitrobenzenes that transitioned to a positive slope at 573K indicating similar EAa from 0.95 to 1.2 eV.[114] Hattori, Kuge and Nakagawa reported thermal ECD DC data for CH3, Cl and Br

SC RI PT

substituted nitrobenzenes and nitrobenzene. [97] In 1989, Christopher Jones, a graduate student in the Grimsrud laboratory reported thermal ECD data for nitrobenzene, F, Cl and Br substituted nitrobenzenes. Jones also reported approximate ECD responses at 473 and 573K for 36 aromatic nitro compound, some of which have EA greater than 1.6 eV. [14,36,37,98,115-123]

In Table 6 are the values for the ECD-EAa for the aromatic nitrocompounds; the TCT and

U

ECD-PD values in NIST and values from the negative ion lifetime studies. The ECD-PD values

N

from Grimsrud and Mock are upper limits. The UH ECD-EAa of nitrotoluenes listed in NIST are

A

lower limits from early data. The ECD values are weighted averages of the recent UH; the Jones

M

and Grimsrud; and the Japanese values for m,o,p nitrotoluene; 0.98(2),0.94(2), 0.96(2) and m,o,p F-nitrobenzene; 1.22(2);1.10(2);1.14(2). Jones also obtained ECD data for Cl and Br

D

nitrobenzenes indicating higher EAa than for the F-nitrobenzenes. The UH EAa for m and p Cl-

TE

nitrobenzene are 1.25(5) eV and those for m,o,p Cl-nitrobenzene from ECD-DC data of Hatori,

EP

Kuge, and Nakagawa are: (eV) 1.25(5),1.15(5),1.28(5) eV. Their bromo responses are larger than for the chloro compounds giving larger EAa. The UH laboratory finds a slightly positive

CC

slope in the temperature dependence of the ortho-dinitrobenzene at the highest temperature that

A

suggests an EAa of 1.6(1) eV. [14,36,37,98,115-123] In Fig. 8 are least squares fits to some of the data for the halonitrobenzenes that show

multiple states. There are many other nitro-compounds listed in NIST studied using the TCT method and /or the NILT method. The latter has been used to report an EAa for p-dinitro-benzene

23

of 1.96 eV in agreement with the TCT method. Thus, the accuracy and precision of the ECD, TCT and NILT methods for the determination of AEa of these compounds is established. In 2006, Collins, Jackson and co-workers presented a paper “Fast Gas Chromatography

SC RI PT

of Explosive Compounds Using a Pulsed-Discharge Electron Capture Detector” . [123] They observed “the main reasons for using an ECD for the analysis of explosives are threefold: (1) low detection limits; (2) selectivity of electron-capturing compounds over non-electron-capturing

background species; (3) low cost and relative ease of use compared to more elaborate detectors such as mass spectrometers.”

U

This paper was the first to apply the PDECD to explosive compounds, including nitrate

N

esters, nitroaromatics, and nitramines. “It is a fast screening procedure for the detection of a

A

mixture of nine components: ethylene glycol dinitrate (EGDN), 4-nitrotoluene (4-NT),

M

nitroglycerine (NG), 2,6-dinitrotoluene (2,6-DNT), 2,4-dinitrotoluene (2,4-DNT), 2,4,6trinitrotoluene (TNT), pentaerythritol tetranitrate (PETN), cyclotrimethylenetrinitramine (RDX),

D

and 2,4,6-trinitrophenylmethylnitramine (Tetryl) in less than 140 sec. The method employs a

TE

microbore capillary gas chromatography (GC) column in a standard GC oven to achieve on-

EP

column detection limits between 5 and 72 fg for the nine explosives studied. The fast separation time limits on-column degradation of the thermally labile compounds and decreases the peak

CC

widths, which results in larger peak intensities and a concomitant improvement in detection limits.” They demonstrated limits of detection better than the previous radioactive ECD methods.

A

They reported temperature dependencies for TNT, NG, and RDX that had positive slopes in the order NG> RDX > TNT. The lower detector temperatures would be best for these three compounds and would be advantageous for the thermal stability of all the compounds. 5.0 Selection of ECD and NIMS analytical and physical conditions.

24

The ECD has two unique applications. A small temperature dependence is desired for the quantitation of a specific compound while a large temperature dependence is desired for the determination of physical properties. The small temperature dependence and large response for

SC RI PT

the diatomic halogens in Fig 2 and the compounds with the largest responses in Figs 4-8. The thermal data for O2, NO, and N2O in Fig. 2 illustrate the greatest amount of structure and provide the largest amount of physical data that characterize the anion states predicted by theory. The

higher temperature data for the halogens and NO were obtained in a prototype pulsed discharge detector.

U

The data for the aromatic hydrocarbons with AEa less than about 0.5 eV, acetophenone

N

and benzaldehyde illustrate compounds with only an alpha region where the theoretical common

A

intercept can be used with the data to establish more precise values of the AEa. The data for

M

acetophenone and benzaldehyde in Figs 4, 5 and 7 have the largest range of positive slopes for determining electron affinities and the common intercept. The scantium tritide data for

D

anthracene and tetracene in Fig. 4 enable the determination of activation energies and AEa. These

TE

data are included in Fig. 5 to establish positive slopes in the data for the fluorobenzenes and

EP

azulene. As noted above, the ECD data for bromo and iodo benzene in Fig. 6 do not have positive slopes. The data for the acetates and acetic anhydride in Fig. 7 illustrate transitions from

CC

alpha to gamma dissociation regions that give AEa of radicals from bond dissociation energies.

A

The data for o-dinitrobenzene in Fig. 8 is a prototype for pentacene. The ideal temperature dependence and magnitude of electron capture is illustrated by the

data for pentafluorobenzyl ether in Fig. 5 where the LnKT3/2 is about 38 and the slope is near zero. The ideal temperature dependence for determining both kinetic and thermodynamic data for organic molecules is the data for tetracene in Figs 4 and 5 or for pentaflouroanisole in Fig.5.

25

The compounds with LnKT3/2 less than about 28 or with EAa less than about 0.4 eV are best analyzed using a non-selective detector. The compounds with EAa > 0.5 eV could be analyzed by ECD. However, as shown in Figures 2, 4-7, the temperature dependence would be large at the

SC RI PT

higher temperatures, so that a lower temperature would be best used. In the cases where the molecules undergo dissociative electron attachment with large activation energies, the highest temperatures would be best. In the cases where both non-dissociative and dissociative capture take place, the higher temperature and the DC systems would be best.

As another example, we use unpublished ECD data for three mono thiocyanate

U

substituted alkanes; 1,2-dithiocyano ethylbenzene; dithiocyanomethane and two SCN

N

naphthalenes collected by Ristau in 1967. There were no positive slopes for these compounds.

A

The electron affinity of SCN is 3.537(5) eV, which is between Br and Cl. The activation energies

M

for the three mono substituted alkanes are 0.18 eV, between the values for the Br and I compounds. The activation energy for the dithiocyanomethane is 0.05 eV and for the 1,2-

D

dithiocyano ethylbenzene is 0.1 eV. These correspond to C-SCN dissociation energies of 4.0 to

TE

4.2 eV about the same as for C-Cl bonds. The activation energies for the SCN naphthalenes, 0.25

EP

eV is smaller than for chloronaphthalene. The AEa of the naphthalenes from the slopes from the lowest temperature point to the common intercept is about 0.5 eV. Clearly, mass analysis is

CC

required to establish the mechanism for these compounds but they could be quantitated by GC-

A

ECD. If the SCN naphthalenes were to be analyzed, the GC/MS method would be best.[124] In a 1988 book “Electron Capture Negative Ion Mass Spectra of Environmental

Contaminants and Related Compounds” Stemmler and Hites published atmospheric pressure mass spectra at 373K and 523K. These support the AEa in Tables 2-6. For, C6F6, C6Cl6, C6Br6, C6Cl5H ,C6F5Cl , C6F5Br, m,o,p-C6H5CH3NO2 , C6H5NO2, m,o,p-C6H5ClNO2 and three

26

polycyclic aromatic hydrocarbons, the molecular anions are dominant at both temperatures indicating an EAa greater than 0.5 eV. For the C6HnX6-n, with n = 3 for X =Cl, Br, and the phthalates the molecular anions are observed at 373K but not at 523K indicating an EAa between

SC RI PT

0.3 and 0.5 eV and for C6HnX6-n n=1,2, no parent negative ions are observed indicating an EAa between 0.0 and 0.3 eV. Spectra for compounds substituted by Cl, Br, I, OH, CN, HC=O, CH3C=O, CF3 and NH2 are also reported. [125]

In1980, Hattori, Kuge and Asada reported the temperature dependence of the DC ECD

responses of benzene hexachloride C6H6Cl6 and observed a small positive slope that was greater

U

than for hexachlorobenzene giving an AEa greater than the 1.05 eV for C6Cl6. [126] The NIMS

N

data shows three anion peak clusters, M-Cl, Cl2 and Cl with the dominant peak being M-Cl. This

A

suggests dissociative electron attachment in two steps and that the electron affinity of M-Cl is

M

greater than that of Cl or 3.65 eV. It also suggests that the EA(M-2Cl) is less than EA(Cl2) or 2.4 eV. The CURES-EC calculations support all of the above energetics. Clearly, these compounds

D

could be profitably studied using the mass spectral lifetimes of the anions to obtain EA and also

EP

6.0 Conclusions:

TE

the ECD where the temperature dependence could give the Edea.

This review provides examples of the uses of the ECD for the analysis of specific

CC

compounds and the determination of kinetic and thermodynamic properties of thermal electron attachment so that others can apply them to specific situations. The analytical chemist requires a

A

large response and a low temperature dependence while the physical chemist requires sufficiently large positive and negative temperature dependence to define the kinetic and thermodynamic data.

27

The accuracy and precision of ECD EAa for large organic molecules published by the UH laboratory and other laboratories and ECD EAa from unpublished data or data in the Russian and Japanese literature that are not currently in NIST are examined. The electron affinities of the

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main group elements, homonuclear diatomic molecules, NO, N2O, SF6 , aromatic hydrocarbons, heterocyclic compounds, nitrocompounds, carbonyl compounds and halogenated aromatic

molecules have been considered. Some of the ECD EAa in NIST are lower limits because they were obtained from limited data in the positive slope regions or are for excited states.

The ECD, TCT, and NILT procedures have been applied to the determination of EAa.

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The CURES-EC calculations and half wave reduction potentials support the gs-ECD-EAa and are

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the only confirmations for the ECD EAa from 0.05 to 0.3 eV. The largest confirmed value or the

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weighted average is assigned to the gs-AEa. In the case of tetracene, hexafluorobenzene,

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anthracene and others there are multiple precise ECD EAa as has been observed for superoxide. The revised ground state ECD EAa for the organic molecules are listed in Tables 2 to 7. The TCT

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and lifetimes methods give AEa between 0.5 and 1.6 eV that agree with the ECD values and

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supports the accuracy and precisions of the values from the three procedures.

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The use of the ECD in the environmental, forensic and pharmaceutical areas of analytical chemistry is due to its high sensitivity to some electronegative compounds, such as polycyclic

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aromatic hydrocarbons, diketones, esters, halogenated hydrocarbons, and nitro compounds especially when multiply substituted. The methods of carrying out these analyses can be found in

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the literature by searching for the electron capture detector.

Acknowledgements.

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We thank the reviewers for their constructive comments. This contribution is both a review and a presentation of new results. The review is of the publications and dissertations of the University of Houston data collected under the pioneering direction of R. S. Becker, J. E.

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Lovelock, W. E. Wentworth, and Albert Zlatkis without whom this work would not been possible. We have reexamined these data and adjusted the results with the data published by

others in the American, Japanese, and Russian literature. The revised electron affinities and the

critical evaluation of values from unpublished data from the University of Houston laboratories and the Montana State laboratory under Grimsrud establish the accuracy and precision of the

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overlapping electron affinities of the ECD, TCT, and NILT methods.

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Figure Captions;

Figure 1 Electron affinities of atoms and electron affinities and bond dissociation energies of homonuclear diatomic molecules selected from values in NIST.

SC RI PT

Figure 2: Ln KT3/2 vs 1000/T A. Least Squares fits to 10 sets of O2 data. The flame and MGN data are at about 2000 K. The PDECD data are unpublished. B. Data for 1,2,4,5 diatomic halogens, T= 200K to 1000K; 3. unpublished PDECD N2O ;T = 500-700K; 6. 7. ECD O2;300700K 8. Magnetron NO about 2000K: 9. ECD NO T = 330 -900K 10. ECD N2O T =220-500K from references [2,5,44-47,49-54] Figure 3: A. Fifty-four curves for the superoxide anions and B. Eighty-seven potential energy curves for NO(-).

N

U

Figure 4. Least squares fits to Ln KT 3/2 vs 1000/T for aromatic hydrocarbons, benzaldehyde and acetophenone with a common intercept. The data for tetracene and anthracene in 4B are from a high temperature scandium tritide detector data. T = 300 to 500K

M

A

Figure 5 Least squares fits of Ln KT 3/2 vs 1000/T for fluorobenzenes, aromatic hydrocarbons, benzaldehyde and acetophenone ethers, OH substituted compounds with a common theoretical intercept. . T = 300 to 700K

TE

D

Figure 6 Least squares fits to Ln KT 3/2 vs 1000/T for monohalogenated benzenes and polychlorobenzenes with a common intercept. T = 250 to 700K. The different sets of data correspond to different isomers. . Only one curve is shown for each of the isomers. The higher temperature data in Fig 5A are from the UH with a pulsed discharge ECD. The low temperature ECD data were collected by Shen Nan Lin in 1969. The data in Fig 5B are from a DC-ECD reported by Hattori Kuge and Nakagawa.

CC

EP

Figure 7 Least squares fits to Ln KT 3/2 vs 1000/T for acetophenone, benzaldehyde, esters, aldehydes, and acetic anhydride with a common intercept. The curves with no data are calculated from data in Figure 4. The data for the fluorobenzoates are from Zlatkis and Poole. . T = 250 to 700K

A

Figure 8 Least Squares fits for Ln KT3/2 vs 1000/T for nitrobenzene and CH3, F and Cl nitrobenzenes with a common intercept. T = 400 to 700K

37

38

D

TE

EP

CC

A

SC RI PT

U

N

A

M

39

D

TE

EP

CC

A

SC RI PT

U

N

A

M

40

D

TE

EP

CC

A

SC RI PT

U

N

A

M

41

D

TE

EP

CC

A

SC RI PT

U

N

A

M

42

D

TE

EP

CC

A

SC RI PT

U

N

A

M

43

D

TE

EP

CC

A

SC RI PT

U

N

A

M

44

D

TE

EP

CC

A

SC RI PT

U

N

A

M

45

D

TE

EP

CC

A

SC RI PT

U

N

A

M

Table 1 Activation energies, adiabatic electron affinities for Superoxide Morse potentials (references 49-54;63-66)

b1 b2 b3 b4 c1 c2 A1 A2 B C1 C2















       

 

        

      

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

CC

D



0.84

E

A

d1 d2 d3 d4 e1

    



 



   

4.79 4.70 4.68 4.52 4.48 4.45 4.43 4.48 4.46 4.33 4.29

0.12

pm 132 132 133 133 134 134 134 134 134 134 134

4.24 4.17 4.15 4.14 4.09



EAa

c

e

cm-1

(eV)

1145 1145 1145 1145 1145 1145 1145 1145 1145

State

1145 1145

1.070(1) 1.050(1) 0.960(1) 0.940(1) 0.785(1)

135 135 135 135

1108 1108 1108 1108

g1 g2 h1

i1

0.735(1)

i2

0.755(1)

0.601(1)

0.450(1) 0.430(1) 0.415(1) 0.355(1)

G1 G2 H I1 I2

510

1.89

188

500

1.85

188

500

188

500

188

500

1.84

188

500

1.82

188

500



1.69

188

490

  

1.65

188

490

1.60

188

490

1.53

188

480



 

0.260(1) 0.248(1) 0.252(1)



 



 



 



 



 

 



 



 

1.50

188

480

K

1.45

188

480

1.40

188

465

1.36

188

465

j2

1108

188

  

0.280(1)

135

2.04



 

J

1108

3.97

510

480

135

0.08

188

188

4.00

1108

2.06

 

1.51

0.10

135

520



j1

3.97

188

  

0.312(1)

0.10

2.15

1.79

1108

1108



  

h4

0.705(1)

135

135

520

1.82

h3

4.04

3.98

188

0.725(1)

0.12

0.10

2.17



 

h2

0.515(1)

1140

F2

 cm-1



0.751(1)

134

1140

F1

re pm

(eV)

0.561(1)

134

De

SC RI PT

A2

0.87

4.81

re

U

A1

   

0.89

b

N

X2



 

(eV)

EP

X1

De

A

(eV) 

a

M

E1

D

d

TE

State

j3

 

      



1.34

188

465

j4

  

1.33

188

465

k1

  

1.33

188

465



46

f1 f2 f3 f4





       





0.08

3.95

135

1108

0.232(1)

k2

0.08

3.94

135

1108

0.212(1)

l1

0.08

3.91

135

1108

0.180(1)

l2

0.07

3.89

135

1108

0.160(1)

l3

0.07

3.88

135

1108

0.148(1)

l4







 

        



1.31

188

465

1.30

188

460

1.27

188

460

1.25

188

460

1.24

188

460

SC RI PT

e2



a. De, Morse potential equilibrium dissociation energy b. re Morse potential equilibrium internuclear separation c. ω Morse potential vibrational frequency d. E1 Activation Energy

A

CC

EP

TE

D

M

A

N

U

e. AEa Adiabatic Electron Affinities

47

Table 2 Aromatic Hydrocarbons, Heterocyclics F, OH, CH3C=O and HC=O compounds Molecule

EAa (eV)

SC RI PT

_____________________________________________________________________________ ________ a Toluene 0.01(1) Benzene(m,o,p dimethyl)

0.02(2)

a

Benzene(n,iso,sec,tertbutyl)

0.04(4)

a

Benzene(trimethyl)

0.04(4)

Benzene, 1,2,4,5-tetramethyl Benzene, 1,2,3,5-tetramethyla Styrene

0.05(16) 0.11(2) 0.10(5)

Benzene, hexamethylBiphenyl Naphthalene, 2-methylNaphthalene, 1-ethylNaphthalene Naphthalene, 1-methylDiphenylmethane Naphthalene, 2,6-dimethylNaphthalene, 2,3-dimethylIndene Benzene, pentamethylMethyl benzoate Naphthalene, 2-ethyl a Ethyl acetate

0.12(9) 0.13(3) 0.14(7) 0.14(6) 0.15 (1) 0.16(12) 0.16(4) 0.16(4) 0.17(13) 0.17(3) 0.18(5) 0.18(2) 0.19(8) 0.20(2)

Acetic anhydride

a

N A M D

TE

Benzyl acetate

a

U

a

0.21(2) 0.22(5) 0.28(3) 0.29(2) 0.29(5)

CC

EP

Naphthalene, 1,4-dimethylFluorene Triphenylene a 1-Butanol

0.20(5)

Dibenzofuran

a

0.30(8)

Dibenzothiophene

0.30(8)

Phenanthrene Diphenylethyne Stilbene Ethylene-1,1-diphenyl aPhenol a Xanthane

0.31(2) 0.32(7) 0.35(5) 0.39(6) 0.40(5) 0.40(8)

a

7,8 Benzoquinoline

0.40(8)

a5,6 Benzoquinoline a Chrysene

0.40(8) 0.42(3)

A

a

48

1-Naphthol

0.45(2)

a

2-Naphthol

0.45(2)

Benzo[c]phenanthrene a Pyrene

0.54(2) 0.58(2)

a

Picene

0.59(3)

Benz[e]pyrene

0.60(2)

1-2-dimethyl phthalate

0.65(5)

Anthracene,

0.68(2)

Dibenz[a j]anthracene

0.68(3)

Biacetyl

0.68(2)

Anthracene, 9-methyl-

0.68(3)

Dibenz[ah]anthracene

0.69(3)

Dibenz[ac]anthracene

0.69(3)

Benz[a]anthracene

0.69(2)

Carbazole

0.70(5)

1,3-Dimethyl isophthalate

0.70(5)

a a

b

SC RI PT

a a a a a a a a

U

a

Anthracene, 2-methyl-

0.71(5)

a

Acenaphthylene

0.80(2)

N

a

1,3,5,7-c-C8H8

0.80(5)

a

1,4-dimethyl terephthalate

A

a

Fluoranthene

M

a

Benz[a]pyrene

a a

Azulene Acridine Hexapentafluorobenzoate

a

D

a

0.82(5) 0.82(4) 0.83(2) 0.84(2) 0.90(8) 0.95(5) 1.00(5)

Phenylpentafluorobenzoate

1.05(10)

1,4-diacetylbenzene

1.05(10)

a a

EP

Tetracene a Terephthaldehyde Pentacene

1.06(2) 1.25(10) 1.41(2)

CC

c

TE

Cyclohexapentafluorobenzoate

a

A

a. These ECD values are not listed in NIST b. See text for other alkyl phthalates with values from 0.65(5) to 0.80(5) eV. Excited state values are: (eV) tetracene, 0.88(1);azulene, 0.65(1) and 0.52; anthracene, 0.55(2); acenaphthylene, 0.40(5); pyrene 0.41(1); benz(a)anthracene, 0.39(10); chrysene, 0.32(1); phenanthrene, 0.12(2). c. This is not an ECD value d. references 2,5,15,68-86

49

Table 3 Electron Affinities, Ionization Energies, Electronegativities, Lifetimes,

Compound

AEa(GS)

(eV)

IP

EN

Tal(zero)

(eV)

(eV)

microsec

Lifetime s

T

K microsec ________________________________________________________________________________ ________________________ 1 Naphthalene 0.15(3 8.14 4.15 [<< 1] ) 2 Triphenylene 0.29(2 7.87 4.08 <1 364 ) 3 Phenanthrene 0.31(2 7.89 4.10 <1 368 ) 4 Chrysene 0.41(2 0.43 7.60 4.01 1.E-03 40 391 ) 5 Benzo(c)phenath 0.54(2 0.55 7.60 4.07 20 rene ) 6 Pyrene 0.58(2 0.55 7.43 4.01 2.E-05 29 391 ) 7 Picene 0.59(3 0.59 7.51 4.05 3.E-06 40 ) 8 Benz(e)pyrene 0.60(3 0.61 7.43 4.02 3.E-06 40 354 ) 9 Anthracene 0.68(2 0.68 7.44 4.06 1.E-06 41,25 363 ) 1 9-Me-anthracene 0.68(5 0.72 7.31 4.02 1.E-06 45 391 0 ) 1 Dibenz(a,c)anthr 0.68(3 0.70 7.39 4.04 1.E-06 42 315 1 acene ) 1 Dibenz(a,h)anthr 0.69(3 0.69 7.39 4.04 1.E-06 44 383 2 acene ) 1 Benz(a)anthrace 0.69(2 0.70 7.45 4.07 1.E-06 44 368 3 ne ) 1 2-Me-anthracene 0.71(5 0.68 7.37 4.03 1.E-06 38 345 4 ) 1 9-Phenyl0.80(2 0.84 7.25 4.03 1.E-07 145 458 5 anthracene ) 1 Benz(a)pyrene 0.82(3 0.84 7.12 3.97 1.E-07 [>150] 6 ) 1 Tetracene 1.058( 1.00 6.97 4.02 5.E-07 >12,000 371 7 5) 1 Pentacene 1.41(2 1.39 6.63 4.02 1.E-08 411 8 ) >17,000

A

CC

EP

TE

D

M

A

N

U

SC RI PT

(eV)

AEa(Tal)

50

Table 4 Electron Affinities of Halogenated and OCH3 Compounds (in eV) Molecule

EAa(NIST)

EAa CURESEC _____________________________________________________________ _________________ 1,1 Di-Cl-ethylene 0.10(5) 0.10(5) 0.10 p-C6H4ClCH3 0.12(5) 0.13 C6H5F

0.13(5)

-

o-C6H4ClCH3

0.14(5)

-

m-C6H4ClCH3

0.15(5)

-

C6H5Cl

0.17(5)

-

p-C6H4F2

0.25(5)

-

1-Cl-Naphthalene p-dichlorobenzene o- dichlorobenzene m- dichlorobenzene CF3CH2F

0.35(5) 0.30(5) 0.31(5) 0.32(5) 0.38(15)

0.28(5) 0.10(5) -

Tri-Cl-ethylene CF3CHF2 1,2,4-Trichlorobenzene 1,3,5- Trichlorobenzene 1,2,3- Trichlorobenzene 1,2,3,4- Tetrafluorobenzene Tetrafluoroanisole Tetra-Cl-ethylene 3F-phenylbenzylether 1,2,4,5- Tetrachlorobenzene 1,2,3,5- Tetrachlorobenzene 1,2,3,4- Tetrachlorobenzene Pentafluorobenzene Pentafluoroanisole Hexafluorobenzene 1-Chloroanthracene 9-Chloroanthracene 2-Chloroanthracene Pentachlorobenzene Choloropentafluorobenzene Hexaachlorobenzene C6F5Br

0.40(10) 0.45(15) 0.47(5) 0.48(5) 0.50(5) 0.52(5) 0.55(5) 0.64(3) 0.65(5) 0.66(5) 0.66(5) 0.70(5) 0.72(5) 0.80(5) 0.86(2) 0.87(5) 0.87(5) 0.88(5) 0.88(5) 1.00(5) 1.05(5) -

0.40(22) 0.34(5) 0.22(9) 0.64(3) 0.29(9) 0.45(5) 0.43(10) 0.54(9) 0.54(5) 0.83(10) 0.86(10) 0.80(10) 0.88(5) 1.00(5) 1.05(5) -

0.40 0.50 0.49 0.47 0.45 0.55 0.50 0.65 0.60 0.71 0.70 0.72 0.72 0.75 0.84 0.85 0.90 0.85 0.90 1.01 1.10 1.20

0.10 0.13 0.15 0.17

N

U

0.20

A

M

D

TE

EP

CC

A

SC RI PT

ECD EAa

0.33 0.30 0.32 0.34 0.40

C6F5I

-

-

1.50

3-F-phenyl-O 2,3,5,6-tetraF-PhenylO C6F5O

2.61(8) 2.90(8) 3.06(8)

2.61(8) 2.75(8) 3.11(8)

2.60 2.90 3.10

C6F5

3.22(15)

3.18(13)

3.20

51

A

CC

EP

TE

D

M

A

N

U

SC RI PT

References 88-93,96-99

52

Table 5 Electron Affinities of acetophenones, benzaldehydes and benzophenones Molecule

EAa

NIST EAa

CURES-EC EAa

0.43(2)

0.41

Benzaldehyde-2,4,6 triMe Benzaldehyde Benzaldehyde-m-OCH3

0.44(4) 0.437(2) 0.48(4)

0.44 0.429(5) 0.43

Acetophenone-2,4,6 triMe Acetophenone-o-F Acetophenone-p-F Benzaldehyde-p-F Acetophenone-m-F 1-Acetonaphthone Benzophenone-4-methoxy Benzophenone-4-methyl Benzophenone-4-ethyl 2-Naphthaldehyde Acetophenone-p-Cl Benzaldehyde-o-F Benzaldehyde-m-F Acetophenone-m-Cl Benzophenone 1-Naphthaldehyde Benzophenone-p-F

0.49(4) 0.49(3) 0.52(3) 0.55(5) 0.58(3) 0.60(3) 0.61(5) 0.64(5) 0.64(5) 0.64(5) 0.64(5) 0.66(3) 0.67(3) 0.67(5) 0.68(5) 0.69(5) a 0.70(5)

0.49 0.44 0.40 0.49 0.58 0.60 0.64 0.59 0.64 0.67 0.62 0.62 0.68 b 0.72

TE

D

M

A

N

U

Benzaldehyde-m-CH3

SC RI PT

_____________________________________________________________________ (eV) (eV) (eV) _____________________________________________________________________ Acetophenone 0.338(2) 0.334(4) 0.34 Propiophenone 0.35(2) 0.35 0.36 Benzaldehyde-p-CH3 0.39(2) 0.37 0.40 0.45 0.47 0.45 0.45 0.47 0.45 0.50 0.55 0.59 0.55 0.60 0.64 0.64 0.68 0.62 0.62 0.68 0.66 0.68 0.73 0.72

0.72(5) 0.79(5)

0.72 0.64

0.75 0.83

Acetophenone-m-CF3

0.77(2)

0.77

0.80

Cinnamaldehyde Benzophenone-p-Cl

0.82(4) a 0.78(5)

0.82 0.85

0.80 0.81

Acetophenone-p- CF3

0.90(10)

0.90

1.00

Benzophenone-p-Br Benzophenone-p-CF3

0.90(10) 1.00(10)

1.07

0.88 1.20

Benzophenone-p-I Benzophenone-p-NO2

1.10(10) 1.57(10)

1.10 1.57

1.10 1.57

A

CC

EP

9-Phenanthraldehyde Acetophenone-o-CF3

b

b

b

a. ECD values not in NIST b. TCT value in NIST with an uncertainty of 0.1 eV. References (2,5,15,109-112,115)

53

54

D

TE

EP

CC

A

SC RI PT

U

N

A

M

TABLE 6 Electron Affinities (in eV) for Nitroaromatics from TCT, ECD, NILT,ECD-PD Methods Molecule EAa (tw)

NIST

ECD PDNIST

ECD

TCT

NILT

0.78(3)

0.81

-

0.77(6)

0.78(5)

-

0.85(3) 0.91(3) 0.92(5) 0.94(2)

0.85 0.92 0.92

1.21 1.24

0.85(3) 0.90(3) 0.94(3)

0.87(5) 0.92(5) 0.94(5)

p-Nitrotoluene m-Nitrotoluene

0.96(2) 0.98(2)

0.95 0.98

1.18 1.10

0.96(3) 0.98(3)

0.95(5) 0.98(5)

Nitrobenzene NB Nitrobenzene-mF

1.000(8) 1.22(2)

1.00 1.24

1.18 1.24

1.00(1) 1.22(3)

1.01(5) 1.18(5)

Nitrobenzene-oF

1.10(2)

1.08

1.24

1.10(3)

1.09(5)

Nitrobenzene-pF

1.14(2)

1.12

1.12

1.14(3)

1.09(5)

Nitrobenzene-oCl

1.13(5)

1.16

1.34

-

1.13(5)

Nitrobenzene-mCl

1.25(5)

1.28

1.38

1.25(5)

1.26(5)

Nitrobenzene-pCl

1.26(4)

1.26

1.27

1.25(5)

1.23(5)

1-nitronaphthalene

1.24(5)

1.23

1.24

1.23(10)

1.24(5)

1.18

1.24

1.18(10)

1.13(5) 1.29(5) 1.29(4) 1.48(5)

1.16 1.32 1.29 1.45

1.42 1.34

1.16(10) 1.28(5) 1.28(5) 1.50(10)

1.23(10 ) 1.18(10 ) 1.13(5) 1.26(5) 1.23(5) 1.45(5)

1.01(5 ) 0.98(5 ) 1.22(5 ) 1.11(5 ) 1.15(5 ) 1.11(5 ) 1.21(5 ) 1.21(5 ) -

1.65(5)

1.65

1.77

1.65(10)

1.65(5)

-

U

N

CC

o-Dinitrobenzene

A

M

D

EP

Nitrobenzene-oBr Nitrobenzene-mBr Nitrobenzene-pBr Nitrobenzene-F5

TE

2-nitronaphthalene

SC RI PT

2,6Dimethylnitrobenzene 2,3-Dimethylnitrobenzene 3,4-Dimethylnitrobenzene 3,5-Dimethylnitrobenzene o-Nitrotoluene

A

_____________________________________________________________ ____________ References 14,15,98,115,117

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