Assigning Oxidation States to Organic Compounds via Predictions

Dec 31, 2013 - We also list the typical (IUPAC) rules for assigning oxidation states, review recent suggestions of Loock and Steinborn that are based ...
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Assigning Oxidation States to Organic Compounds via Predictions from X‑ray Photoelectron Spectroscopy: A Discussion of Approaches and Recommended Improvements Vipul Gupta,† Hasitha Ganegoda,‡ Mark H. Engelhard,§ Jeff Terry,‡ and Matthew R. Linford*,† †

Department of Chemistry and Biochemistry, Brigham Young University, Provo, Utah 84602, United States Department of Physics, Illinois Institute of Technology, Chicago, Illinois 60616, United States § EMSL Environmental Molecular Science Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99354, United States ‡

S Supporting Information *

ABSTRACT: The traditional assignment of oxidation states to organic molecules is problematic. Accordingly, in 1999, Calzaferri proposed a simple and elegant solution that is based on the similar electronegativities of carbon and hydrogen: hydrogen would be assigned an oxidation state of zero when bonded to carbon. Here, we show that X-ray photoelectron spectroscopy, a core electron spectroscopy that is sensitive to oxidation states of elements, generally agrees with his suggestion. We also list the typical (IUPAC) rules for assigning oxidation states, review recent suggestions of Loock and Steinborn that are based on Pauling’s earlier approach, discuss the traditional (IUPAC and Pauling−Loock−Steinborn) assignments of oxidation states to organic molecules, review Calzaferri’s suggestion, introduce X-ray photoelectron spectroscopy (XPS), show the general agreement of Calzaferri’s suggestion with XPS results, provide supporting examples from the literature, and discuss the limitations of Calzaferri’s recommendation vis-à-vis XPS results. We conclude by recommending that either (i) Calzaferri’s suggestion be implemented into the current IUPAC rules or (ii) the Loock definition be explanded to deal specifically with atoms with similar electronegativities. KEYWORDS: First-Year Undergraduate/General, Second-Year Undergraduate, Upper-Division Undergraduate, Analytical Chemistry, Organic Chemistry, Physical Chemistry, Misconceptions/Discrepant Events, Textbooks/Reference Books, Oxidation State

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species, the IUPAC and Pauling−Loock approaches typically give oxidation states that are in reasonable agreement with observed chemistry. However, when applied to organic molecules, these rules often lead to unusual, and sometimes unchemical, predictions. A few years ago, Calzaferri recognized this problem and made a simple suggestion:19 because of the similar electronegativities of carbon and hydrogen, hydrogen would be assigned an oxidation state of zero when bonded to carbon. Here, we show that results from X-ray photoelectron spectroscopy, a core electron spectroscopy that is sensitive to oxidation states of elements,20,21 generally agree with Calzaferri’s recommendation. We then provide supporting examples from the literature, discuss some of the limitations of the Calzaferri approach, and end with one of two recommendations to the community, which are to (i) incorporate the Calzaferri suggestion into the IUPAC rules or (ii) consider a modification of Loock’s suggestion that encompasses and broadens Calzaferri’s recommendation. We favor the latter of these two approaches.

xidation states are almost universally taught in general chemistry. They are a useful bookkeeping tool for recognizing oxidation−reduction (redox) reactions and for identifying the species that are oxidized or reduced in them. Oxidation states are often applied in inorganic chemistry. For example, hydride (H−), hydrogen (H2), and the hydrogen ion (H+) have oxidation states of −1, 0, and +1, respectively, where these numbers correlate and can be viewed as being consistent with the very different chemistries of these three types of hydrogen. A number of articles have been published in this Journal on the topic of oxidation states, including discussions of the rules for assigning them.1−18 Clearly this is a topic with significant impact on chemical educators and students of the discipline. As noted by Steinborn:11 “The concept of oxidation states is one of the most powerful heuristic concepts in chemistry.” Herein, we first list the traditional (IUPAC) rules for assigning oxidation states. We then note a recent suggestion by Loock to both generalize them and make them more chemically reasonable and acceptable to students, where his recommendation builds on the earlier approach by Pauling. For inorganic © 2013 American Chemical Society and Division of Chemical Education, Inc.

Published: December 31, 2013 232

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Figure 1. Oxidation states of different (A) alkanes, (B) alcohols, and (C) an aldehyde, a ketone, and two carboxylic acids. The first oxidation state in brackets was obtained using the traditional rules. The second number in brackets was derived from Calzaferri’s suggestion.

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IUPAC AND PAULING−LOOCK−STEINBORN DEFINITIONS OF OXIDATION STATE

The oxidation number of an atom is a number which represents the electrical charge which the atom would have if the electrons in a compound were assigned to the atoms in a certain way. About a paragraph later he states: The rules [for assigning oxidation states] are given in the following sentences. 1. The oxidation number of a monatomic ion in an ionic substance is equal to its electrical charge. 2. The oxidation number of atoms in an elementary substance is zero. 3. In a covalent compound of known structure, the oxidation number of each atom is the charge remaining on the atom when each shared electron pair is assigned completely to the more electronegative of the two atoms sharing it. A pair shared by two atoms of the same element is split between them. 4. The oxidation number of an element in a compound of uncertain structure may be calculated from a reasonable assignment of oxidation number to the other elements in the compound. In 2011, Loock13 used Pauling’s third rule to suggest a definition for oxidation states that encompasses the IUPAC definition but which he argues is more pedagogically effective: The oxidation state of an atom in a compound is given by the hypothetical charge of the corresponding atomic ion that is obtained by heterolytically cleaving its bonds such that the atom with the higher electronegativity in a bond is allocated all electrons in this bond. Bonds between like atoms (having the same formal charge) are cleaved homolytically. Writing in this Journal in 2004, Steinborn11 quoted Pauling’s suggestion and proposed essentially the same approach as Loock to assign oxidation states. In particular, he recommended assigning “oxidation numbers of atoms in molecules on the basis of their Lewis structures”, noting that: This system of splitting shared electrons in covalent bonds heterolytically, according to certain principles..., is less frequently explained in textbooks. Thus, from the Lewis structure of a molecule, with knowledge of the electronegativities of elements, oxidation numbers can be assigned.

The International Union of Pure and Applied Chemistry (IUPAC) has provided the widely accepted definition of oxidation state: A measure of the degree of oxidation of an atom in a substance. It is defined as the charge an atom might be imagined to have when electrons are counted according to an agreed-upon set of rules: (l) the oxidation state of a free element (uncombined element) is zero; (2) for a simple (monatomic) ion, the oxidation state is equal to the net charge on the ion; (3) hydrogen has an oxidation state of 1 and oxygen has an oxidation state of −2 when they are present in most compounds. (Exceptions to this are that hydrogen has an oxidation state of −1 in hydrides of active metals, e.g., LiH, and oxygen has an oxidation state of −1 in peroxides, e.g., H2O2); (4) the algebraic sum of oxidation states of all atoms in a neutral molecule must be zero, while in ions the algebraic sum of the oxidation states of the constituent atoms must be equal to the charge on the ion. For example, the oxidation states of sulfur in H 2 S, S 8 (elementary sulfur), SO2, SO3, and H2SO4 are, respectively: −2, 0, +4, +6, and +6. The higher the oxidation state of a given atom, the greater is its degree of oxidation; the lower the oxidation state, the greater is its degree of reduction.22 The rules for assigning oxidation states found in most general chemistry textbooks are based on these rules23−27 and apply reasonably well to most inorganic compounds. They can be expanded upon and remembered by mnemonics (see Supporting Information for an example). Other mnemonics used to help teach redox chemistry include: “OIL RIG” (oxidation is loss of electrons; reduction is gain of electrons), “LEO says GER” (loss of electrons is oxidation, gain of electrons is reduction), and “ROLR” (right-oxidation-leftreduction).16 In Pauling’s book General Chemistry, he gave the following definitions and comments on oxidation states:28 233

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Because of the similar Pauling electronegativities30 of carbon (2.55) and hydrogen (2.20), Calzaferri recommended that hydrogen be assigned an oxidation state of zero when bonded to carbon. He also recommended an oxidation state of zero for hydrogen bonded to silicon (1.90), germanium (2.01), and boron (2.04). Calzaferri’s suggestion resolves the problems in the examples above. If the hydrogen atoms in the alkanes in Figure 1A are assigned oxidation states of zero, then the carbon atoms also have oxidation states of zero and they can be represented as C(0). This result is clearly consistent with the identical hybridizations, similar reactivities, and nonpolar natures of these hydrocarbons. That is, we do not expect hydrogen bonding from C−H groups but traditionally and inconsistently assign the same oxidation state to hydrogen whether it is bonded to C, N, O, or F, where the latter three atoms do exhibit hydrogen bonding. For the alcohols in Figure 1B, all the carbon atoms bonded to hydroxyl groups have oxidation states of +1: C(I), which is chemically reasonable, and all the other carbon atoms in these alcohols have oxidation states of zero. Of course, Calzaferri’s suggestion does not change the assignment of +1 to hydrogen when it is bonded to oxygen. For the carbonyl and carboxyl groups in Figure 1C, the carbonyl carbons in both compounds have oxidation states of +2, C(II), and the carbon atoms in both carboxylic acids have oxidation states of +3, C(III). Interestingly, the number of oxygen−carbon bonds on a carbon atom in these examples (alkane: 0, alcohol: 1, carbonyl: 2, carboxyl: 3) is the oxidation state of that carbon atom, where an increase in the number of oxygen−carbon bonds at a carbon atom should increase its oxidation state. These results are also consistent with organic redox chemistry: alcohols are oxidized to aldehydes or ketones, and aldehydes undergo oxidation to carboxylic acids. This approach also makes the reasonable predictions that carbon atoms bonded to oxygen in ethers should be (like alcohols) C(I), and for carbon in carbonates or carbon dioxide, which have four oxygen−carbon bonds, C(IV).

Loock compares the Pauling approach to the IUPAC definition noting: This expanded definition of oxidation states builds on the knowledge of electronegativity, whereas the IUPAC definition builds on a set of rules.13 Loock also reminds us that, according to IUPAC, the term “oxidation number” only refers to the charge that would exist on a single metal atom in a coordination compound if all the ligands with their electrons were removed. The IUPAC definition of “oxidation state” is broader and can apply to any atom, molecule, or ion.13 Although we will adhere to the IUPAC convention in this work, we note that the terms are used interchangeably by many chemists.

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CALZAFERRI DEFINITION OF OXIDATION STATE AND EXAMPLES In his article from 1999 in this Journal,19 Gion Calzaferri noted some of the unusual results that stem from assigning oxidation states to organic molecules using the IUPAC or Pauling approaches. In particular, the problems arise from assigning hydrogen an oxidation state of +1 when bonded to carbon. Some of these consequences are presented in the examples below in a little more detail than Calzaferri originally gave, and the structures of the molecules discussed below are given in Figure 1, with the carbon atoms in question in bold. When assigned in the traditional fashion, the oxidation states of the central carbon atoms in a series of alkanesmethane (CH4), ethane (CH3CH3), propane (CH3CH2CH3), 2methylpropane (CH3CH(CH3)2), and 2,2-dimethylpropane (CH3C(CH3)3)are −4, −3, −2, −1, and 0, respectively (see Figure 1A). Here, we see a wider range of oxidation states for five chemically similar carbon atoms (all are in alkanes, sp3 hybridized, and bonded only to C or H), compared to a narrower range of oxidation states (−1 to +1) for the three very different types of hydrogen listed above (H−, H2, and H+). When assigned in the traditional fashion, the oxidation states for the carbon atoms bonded to oxygen in the series of alcohols: CH3OH, CH3CH2OH, CH3CHOH(CH3), and CH3COH(CH3)2, are −2, −1, 0, and +1, respectively (see Figure 1B). A tenet of modern organic chemistry is that a given functional group behaves similarly in different organic molecules. Thus, it is hard to see how the carbon atoms bonded to oxygen in these alcohols should be so chemically different as to merit such different oxidation states. When assigned in the traditional fashion, the oxidation states of the carbon atoms bonded to oxygen in CH3C(O)H (acetaldehyde) and CH3C(O)CH3 (acetone) are +1 and +2, respectively (see Figure 1C). Although aldehydes and ketones do have somewhat different chemistries, they are based on the carbonyl group, and so it is questionable whether these functional groups should show different oxidation states. Also using the conventional rules, the carbon atoms in the carboxyl groups in formic acid (HCOOH) and acetic acid (CH3COOH) have different oxidation states: +2 and +3, respectively (see Figure 1C). Further problems with these examples arise when one considers that methyl (−CH3) groups are electron donating, so the carbon atoms in the carbonyl group of acetone and in the carboxyl group of acetic acid might be expected to have a slightly lower oxidation states than acetaldehyde and formic acid, respectively, and not higher as predicted by the traditional rules. In 1969 Jorgensen also discussed the anomalies of assigning oxidation states to homopolar bonds.29

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OVERVIEW OF X-RAY PHOTOELECTRON SPECTROSCOPY X-ray photoelectron spectroscopy (XPS), also known as electron spectroscopy for chemical analysis (ESCA),31 utilizes the photoelectric effect and analysis of the kinetic energy distribution of the emitted photoelectrons to probe the electronic states and chemical compositions of materials (see Figure 2). The depth of the analyzed region varies with the electron mean free path (vide infra). For a conductive sample in good electrical contact with the analyzer the kinetic energy of

Figure 2. General schematic of X-ray photoelectron spectroscopy (XPS). 234

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some electron density to atom B. Due to this interaction, the remaining electrons in atom A will “feel” the nuclear charge to a greater extent and be more strongly attracted to the nucleus. Accordingly, the binding energies of the electrons on atom A will increase. Depending on the number and electronegativity of the atoms bonded to another atom, the resulting chemical shifts may be 1−5 eV. Thus, the effects of oxidation and reduction in XPS are clear. When an atom is oxidized (or reduced), it loses (or gains) electron density, and its photoelectrons are chemically shifted to higher (or lower) binding energies. Peak fitting is commonly performed on XPS narrow scans to help identify the oxidation states of a given atom. Spectra from organic materials are usually fit with Gaussian or Gaussian−Lorentzian (pseudo-Voigt) lineshapes.41,42 Different oxidation states of an atom are typically assigned based on literature precedent, which is considerable, or using prior experience with a sample. Occasionally, another shift is seen due to changes in the final state, which has a hole in the core shell, but such final state effects are not significant in the C 1s core level in XPS.39

the ejected photoelectron (Ek) is given by the following equation: E k = hν − E b − ØA

(1)

where hν is the energy of the incident X-rays, Eb is the binding energy of the photoelectron, and ØA is the analyzer work function, which is generally quite small compared to Ek and Eb. Note the conservation of energy in eq 1: the energy of the Xray (hν) is equal to the sum of the binding and kinetic energies of the photoelectron, with a small correction for the spectrometer. XPS is generally performed under high vacuum due to the limited mean free paths of photoelectrons in the air and the need to preserve many samples in a pristine state for analysis. Volatile molecules similar to those in Figure 1 are incompatible with high vacuum conditions and, thus, difficult to analyze using conventional XPS. Nevertheless, XPS is extremely valuable for the analysis of many organic materials, including polymers,32 monolayers,33 and organic materials and nanomaterials,34 for example, carbon nanotubes.35,36 The X-rays employed in most standalone XPS instruments, that is, Mg Kα = 1253.6 eV and Al Kα = 1486.6 eV, have energies that greatly exceed those in covalent bonds (ca. 4 eV) and are sufficient to eject core electrons from atoms. Using eq 1, binding energies of atoms are calculated from EK values measured by an electron energy analyzer. XPS is a nearly universal analysis technique as it can detect all elements except hydrogen and helium. But even though H cannot be detected per se, its bonding often strongly affects XPS spectra.37 No two atoms yield photoelectrons with exactly the same pattern of binding energies, so each atom can be uniquely identified. XPS is also surface sensitive because the photoelectrons that are generated by the impinging X-rays cannot travel more than ca. 3 mean free paths (MFP) in a material,38 which is about 10 nm at the kinetic energies expected for the photoelectrons generated in conventional XPS. The X-rays themselves penetrate much more deeply. Two types of spectra are typically taken in XPS analyses: survey and narrow. Survey spectra cover a wide range of binding energies, for example, 0−1100 eV, and are used to identify the various elements present in a sample. Narrow scans generally focus on a smaller, for example, ca. 20 eV, region of a spectrum and allow information about the chemical (oxidation) states of materials to be determined (see below). Accordingly, survey scans are typically taken at low energy resolution, whereas narrow scans are obtained at high resolution. It is of significance here that narrow scans enable the detection of changes in electron binding energy (chemical shifts) due to different oxidation states of the elements.39 The C 1s narrow scan provides important information about the oxidation states of organic materials (see below). The binding energy of a C 1s electron ejected from an sp3 carbon atom bonded only to other carbon and hydrogen atoms (C−C,H) is ca. 285 eV.40 Two primary effects and a secondary one determine the binding energies of photoelectrons. The first two are the shell structure of the atom and the nuclear charge, that is, electrons will have higher binding energies if they are (i) closer to the nucleus and (ii) attracted by a higher Z nucleus.39 Although smaller in magnitude, the second effect, the chemical shift, is chemically more important. This effect is a result of changes in electron density around an atom that arise because of its chemical bonding.39 For example, if atom A is bonded to a more electronegative atomatom Bthen atom A will lose

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GENERAL CONSISTENCY OF CALZAFERRI’S RECOMMENDATION WITH RESULTS FROM CORE ELECTRON SPECTROSCOPY Because XPS has been around for decades and the functional groups in the molecules in Figure 1 are common, sufficient data and experience exist in the field to allow C 1s narrow scans to be reasonably predicted for these molecules. Predicted C 1s Binding Energy Shifts of the Molecules in Figure 1A

Carbon atoms bonded only to carbon or hydrogen, such as the sp3 carbons in Figure 1A, show essentially the same C 1s binding energy by XPS.43−45 In other words, all the carbon atoms in Figure 1A would be experimentally found to be in essentially the same oxidation state, which is consistent with Calzaferri’s recommendation and not with the conventional approach for assigning oxidation states to organic molecules. Predicted C 1s Binding Energy Shifts of the Molecules in Figure 1B

When carbon is bonded to oxygen, the electron density around the carbon atom decreases because of oxygen’s higher electronegativity. Accordingly, it is well-known, and consistent with Calzaferri’s suggestion, that each oxygen bond to carbon will increase the C 1s binding energy by ca. +1.2−1.5 eV (for an alcohol the shift is ca. 1.5−1.8 eV).46 Accordingly, one would expect one C 1s signal from methanol that would be chemically shifted ca. +1.5 eV from a C−C,H signal, by which we again mean carbon bonded only to other carbon and hydrogen atoms (the C(0) signal). The other alcohols in Figure 1B would show two signals: one from carbon atoms bonded only to other carbon or hydrogen atoms and one from carbon atoms bonded to oxygen, which would be shifted to higher EB by ca. +1.5 eV versus C(0). The ratios of the peak areas from these signals would be representative of the number of carbon atoms in each oxidation state. For CH 3 CH 2 OH, (CH 3 ) 2 CHOH, and (CH3)3COH, the ratios of the signals from C−C,H (C(0)) and C−O (C(I)) would be 1:1, 2:1, and 3:1, respectively. Predicted C 1s Binding Energy Shifts of the Molecules in Figure 1C

A carbonyl group (CO) has two carbon−oxygen bonds and a carboxyl group (COOH) has three. Accordingly, the C 1s 235

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bonded to two fluorine atoms, and ca. +7.5 eV versus C(0) when bonded to three fluorine atoms.56−59 Other elements bonded to carbon show similar series of chemical shifts. The limitation in this analysis is that the XPS C 1s peak position for a given oxidation state of carbon bonded to oxygen (and hydrogen and carbon) may not be the same as the peak position for the same oxidation state of carbon when it is bonded to a different atom (and hydrogen and carbon). Absolute binding energy does not correlate to unique oxidation state but within a series of the same elements it does.

signal from a carbonyl carbon, whether from an aldehyde or a ketone, is shifted ca. +2.4−3.0 eV47 from C(0), and the C 1s signal from a carboxyl carbon is shifted ca. +3.6−4.5 eV.48 These observations are consistent with the +2 and +3 oxidation states predicted by Calzaferri for carbonyl and carboxyl carbons, respectively. For the carbonate group, ROC(O)OR’, the chemical shift continues to increase versus C(0); it is ca. +5.5−6.7 eV.49−51 This is consistent with the four C−O bonds to carbon in this functional group, the +4 oxidation state for this carbon predicted by Calzaferri, and the increased electron withdrawal expected from more oxygen around a carbon atom.

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First Possible Recommendation

ADDITIONAL EXAMPLES FROM THE LITERATURE The literature contains many examples of C 1s spectra of organic materials that confirm the analysis presented herein. A few examples follow. XPS was performed on different polyethers, including polyethylene glycol (PEG, (CH2CH2O)n), polypropylene glycol (PPG, (CH2CHCH3O)n), and polytetramethylene glycol (PTMG, (CH2CH2CH2CH2O)n).41 According to Calzaferri’s recommendation, PEG should show carbon in a single oxidation state: +1, PPG should show carbon atoms in two oxidation states: 0 and +1 in a 1:2 ratio, respectively, and PTMG should also show carbon atoms in the C(0) and C(I) oxidation states but in a 1:1 ratio. All of these predictions are confirmed by XPS. PEG showed one signal by XPS at the expected peak position. For PPG, the peak positions were 285.0 eV for C(0) and 286.5 eV for C(I), showing the expected splitting of ca. 1.5 eV with the expected area ratios. The same peak splitting was observed for the two signals in the C 1s spectrum of PTMG, which had essentially equal areas. In contrast, the more traditional approach (IUPAC or Pauling−Loock−Steinborn) for assigning oxidation states would put the three carbon atoms in the PPG repeat unit into three different oxidation states, which is chemically unreasonable and inconsistent with experimental results. A few more of the many examples in the literature of C 1s spectra of organic materials include the C 1s spectra of poly(methyl methacrylate) and poly(ether ether ketone),52 suberoyl chloride chemisorbed onto scribed silicon,53 oxidized carbon nanotubes,54 oxidized graphene,55 and bisphenol A polycarbonate50 (a discussion of this spectrum is in the Supporting Information). All of these examples are consistent with Calzaferri’s suggestion. Also in accord with his suggestion is a C 1s spectrum of polyethylene terephthalate from our research that is similarly discussed in the Supporting Information.

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TWO POSSIBLE RECOMMENDATIONS

It does not appear that Calzaferri’s recommendation has been widely adopted; his paper from 1999 has only been lightly cited, we are unaware of the implementation of his recommendation in any textbooks, and there appears to have been limited application of his suggestion in classroom teaching. He suggested the following changes to the traditional rules for assigning oxidation states: For hydrogen [an oxidation state of]: 0 in combination with C, Si, Ge, and also B, +1 in combination with nonmetals, −1 in combination with metals. We believe that these changes to the IUPAC rules would allow the more reasonable use of oxidation states in organic chemistry and biochemistry. Calzaferri’s recommendation makes sense on the basis of atomic electronegativities, and it is also consistent with photoelectron spectroscopy results. This closer representation of reality should better resonate with students. Second Possible Recommendation

Modify Loock’s definition to read: (omissions to his original statement are struck through and suggested changes are in bold): The oxidation state of an atom in a compound is given by the hypothetical charge of the corresponding atomic ion that is obtained by heterolytically cleaving its bonds such that the atom with the higher electronegativity in a bond is allocated all electrons in this bond. Bonds between like atoms (having the same formal charge with similar electronegativities) are cleaved homolytically. General and organic chemistry students are often taught that atoms that differ in Pauling electronegativities by 0.5 units or less form quite nonpolar, covalent bonds. This criterion might be applied here as the definition of “similar electronegativities”, although it would clearly need to fit the particular definition of electronegativity employed. This modification of Loock’s suggestion would then encompass Calzaferri’s suggestion and apply even more generally. It is a simple statement that can be implemented if atomic electronegativities are known.

LIMITATIONS OF THE CALZAFERRI APPROACH

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As shown herein, Calazaferri’s assignment of oxidation states applies nicely to carbon atoms bonded to hydrogen, oxygen, and other carbon atoms. This approach also works well when a different heteroatom is bonded to carbon. For example, fluorinated organic materials show different oxidation states for C−C,H (C(0)), CF, CF2, and CF3 groups. That is, carbon bonded to 0, 1, 2, or 3 fluorine atoms, but otherwise bonded only to carbon and hydrogen, will have Calzaferri oxidation states of 0, +1, +2, and +3. As expected, steadily increasing chemical shifts are observed by XPS for these different carbon atoms; carbon is chemically shifted ca. +3 eV versus C(0) when bonded to one fluorine atom, ca. +6 eV versus C(0) when

SUMMARY We favor the second of these possible recommendations (the modification of Loock’s definition) although the implementation of the first would be a step in the right direction. These approaches follow Woolf’s statement that oxidation states not be ...treated as a purely numerical exercise, but, rather, that contact [be] maintained with chemical reality.5

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(18) Woolf, A. A. Assigning oxidation numbers. J. Chem. Educ. 1993, 70 (8), 691. (19) Calzaferri, G. Oxidation Numbers. J. Chem. Educ. 1999, 76 (3), 362. (20) Kötz, R.; Neff, H.; Stucki, S. Anodic Iridium Oxide Films: XPS− Studies of Oxidation State Changes and. J. Electrochem. Soc. 1984, 131 (1), 72−77. (21) Wang, S.; Chia, P.-J.; Chua, L.-L.; Zhao, L.-H.; Png, R.-Q.; Sivaramakrishnan, S.; Zhou, M.; Goh, R. G. S.; Friend, R. H.; Wee, A. T. S.; Ho, P. K. H. Band-like Transport in Surface-Functionalized Highly Solution-Processable Graphene Nanosheets. Adv. Mater. 2008, 20 (18), 3440−3446. (22) McNaught, A. D.; Wilkinson, A.; Nic, M.; Jirat, J.; Kosata, B.; Jenkins, A. IUPAC. Compendium of Chemical Terminology. Blackwell Scientific Publications: Oxford, 1997; http://goldbook.iupac.org, (accessed Dec 2013). (23) Stoker, H. S., Introduction to Chemical Principles. Prentis Hall: NJ, 1999. (24) Shriver, D.; Atkins, P. Shriver and Atkins Inorganic Chemistry, 4th ed.; W.H. Freeman and Company: New York, 2006. (25) Brown, T. E.; LeMay H. E. H.; Bursten, B. E.; Murphy, C. J.; Woodward, P. M. Chemistry The Central Science, 12th ed.; Prentice Hall: Glenview, IL, 2012. (26) Liard, B. B. University Chemistry; McGraw-Hill Higher Education.: New York, 2009. (27) McMurry, J. E.; Fay, R. C. Chemistry, 4th ed.; Prentice Hall: Upper Saddle River, NJ, 2004. (28) Pauling, L. General Chemistry; an introduction to descriptive chemistry and modern chemical theory; W.H. Freeman: San Francisco, CA, 1970. (29) Jorgensen, C. K., Oxidation Number and Oxidation States. Springer Verlag Inc.: New York, 1969. (30) Pauling, L., The Nature of the Chemical Bond. 3rd ed.; Cornell University: New York, 1960. (31) Siegbahn, K. Electron Spectroscopy for Chemical Analysis (E.S.C.A.). Philos. Trans. R. Soc., A 1970, 268 (1184), 33−57. (32) Saini, G.; Yang, L.; Lee, M. L.; Dadson, A.; Vail, M. A.; Linford, M. R. Amino-Modified Diamond as a Durable Stationary Phase for Solid-Phase Extraction. Anal. Chem. 2008, 80 (16), 6253−6259. (33) Zhang, F.; Sautter, K.; Larsen, A. M.; Findley, D. A.; Davis, R. C.; Samha, H.; Linford, M. R. Chemical Vapor Deposition of Three Aminosilanes on Silicon Dioxide: Surface Characterization, Stability, Effects of Silane Concentration, and Cyanine Dye Adsorption. Langmuir 2010, 26 (18), 14648−14654. (34) Zhang, F.; Gates, R. J.; Smentkowski, V. S.; Natarajan, S.; Gale, B. K.; Watt, R. K.; Asplund, M. C.; Linford, M. R. Direct Adsorption and Detection of Proteins, Including Ferritin, onto Microlens Array Patterned Bioarrays. J. Am. Chem. Soc. 2007, 129 (30), 9252−9253. (35) Jensen, D. S.; Kanyal, S. S.; Vail, M. A.; Dadson, A. E.; Engelhard, M. H.; Linford, M. R. Multiwalled Carbon Nanotube Forest Grown via Chemical Vapor Deposition from Iron Catalyst Nanoparticles, by XPS. Surf. Sci. Spectra 2013, 20, 62−67. (36) Jensen, D. S.; Kanyal, S. S.; Madaan, N.; Hancock, J. M.; Dadson, A. E.; Vail, M. A.; Vanfleet, R.; Shutthanandan, V.; Zhu, Z.; Engelhard, M. H.; Linford, M. R. Multi-instrument characterization of the surfaces and materials in microfabricated, carbon nanotubetemplated thin layer chromatography plates. An analogy to ’The Blind Men and the Elephant. Surf. Interface Anal. 2013, 45 (8), 1273−1282. (37) Kerber, S. J.; Bruckner, J. J.; Wozniak, K.; Seal, S.; Hardcastle, S.; Barr, T. L. In The nature of hydrogen in x-ray photoelectron spectroscopy: General patterns from hydroxides to hydrogen bonding; AVS: Mineapolis, MN, 1996; pp 1314−1320. (38) Tanuma, S.; Powell, C. J.; Penn, D. R. Calculations of electorn inelastic mean free paths. II. Data for 27 elements over the 50−2000 eV range. Surf. Interface Anal. 1991, 17 (13), 911−926. (39) X-ray photoelectron spectroscopy; an introduction to principles and practices. Reference & Research Book News 2012/06, 2011; p 264. (40) Terry, J.; Linford, M. R.; Wigren, C.; Cao, R.; Pianetta, P.; Chidsey, C. E. Determination of the bonding of alkyl monolayers to

ASSOCIATED CONTENT

S Supporting Information *

A list of rules for assigning oxidation states based on the IUPAC recommendation with accompanying mnemonic, the XPS C 1s narrow scan of polyethylene terephthalate collected by MHE, a discussion of a literature C 1s narrow scan of bisphenol A polycarbonate, and experimental conditions for MHE’s data collection.This material is available via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*M. R. Linford. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the Department of Chemistry and Biochemistry and the College of Physical and Mathematical Sciences at Brigham Young University for their support of this work. We similarly acknowledge EMSL, a national scientific user facility sponsored by the Department of Energy’s Office of Biological and Environmental Research located at Pacific Northwest National Laboratory.

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REFERENCES

(1) Birk, J. P. The computer as student: An application of artificial intelligence. J. Chem. Educ. 1992, 69 (4), 294. (2) Eggert, A.; Middlecamp, C.; Kean, E. CHEMPROF: An intelligent tutor for general chemistry. J. Chem. Educ. 1991, 68 (5), 403. (3) Holleran, E. M.; Jespersen, N. D. Elementary oxidation-number rules. J. Chem. Educ. 1980, 57 (9), 670. (4) Packer, J. E.; Woodgate, S. D. Assigning oxidation numbers. J. Chem. Educ. 1993, 70 (8), 691. (5) Woolf, A. A. Oxidation numbers and their limitations. J. Chem. Educ. 1988, 65 (1), 45. (6) Kauffman, J. M. Simple method for determination of oxidation numbers of atoms in compounds. J. Chem. Educ. 1986, 63 (6), 474. (7) Stonestreet, R. H. Oxidation number for fluorine (the author responds). J. Chem. Educ. 1972, 49 (4), 300. (8) Halkides, C. J. Assigning and Using Oxidation Numbers in Biochemistry Lecture Courses. J. Chem. Educ. 2000, 77 (11), 1428. (9) Feinman, R. D. Ethanol Metabolism and the Transition from Organic Chemistry to Biochemistry. J. Chem. Educ. 2001, 78 (9), 1215. (10) Holder, D. A.; Johnson, B. G.; Karol, P. J. A Consistent Set of Oxidation Number Rules for Intelligent Computer Tutoring. J. Chem. Educ. 2002, 79 (4), 465. (11) Steinborn, D. The Concept of Oxidation States in Metal Complexes. J. Chem. Educ. 2004, 81 (8), 1148. (12) Chirik, P. J. Preface: Forum on Redox-Active Ligands. Inorg. Chem. 2011, 50 (20), 9737−9740. (13) Loock, H.-P. Expanded Definition of the Oxidation State. J. Chem. Educ. 2011, 88 (3), 282−283. (14) Shibley, I. A.; Amaral, K. E.; Aurentz, D. J.; McCaully, R. J. Oxidation and Reduction Reactions in Organic Chemistry. J. Chem. Educ. 2010, 87 (12), 1351−1354. (15) Parkin, G. Valence, Oxidation Number, and Formal Charge: Three Related but Fundamentally Different Concepts. J. Chem. Educ. 2006, 83 (5), 791. (16) Cox, A. L.; Cox, J. R. Determining Oxidation-Reduction on a Simple Number Line. J. Chem. Educ. 2002, 79 (8), 965. (17) Menzek, A. A New Approach to Understanding OxidationReduction of Compounds in Organic Chemistry. J. Chem. Educ. 2002, 79 (6), 700. 237

dx.doi.org/10.1021/ed400401c | J. Chem. Educ. 2014, 91, 232−238

Journal of Chemical Education

Article

(58) Weng, L.-T.; Ng, K.-M.; Cheung, Z. L.; Lei, Y.; Chan, C.-M. Quantitative analysis of styrene-pentafluorostyrene random copolymers by ToF-SIMS and XPS. Surf. Interface Anal. 2006, 38 (1), 32−43. (59) Ganegoda, H.; Jensen, D. S.; Olive, D.; Cheng, L.; Segre, C. U.; Linford, M. R.; Terry, J. Photoemission studies of fluorine functionalized porous graphitic carbon. J. Appl. Phys. 2012, 111 (5), 053705− 053705−6.

the Si (111) surface using chemical-shift, scanned-energy photoelectron diffraction. Appl. Phys. Lett. 1997, 71 (8), 1056−1058. (41) Hearn, M. J.; Ratner, B. D.; Briggs, D.; SIMS, X. P. S. studies of polyurethane surfaces. 1. Preliminary studies. Macromolecules 1988, 21 (10), 2950−2959. (42) Navaneetha Pandiyaraj, K.; Selvarajan, V.; Deshmukh, R. R.; Gao, C. Adhesive properties of polypropylene (PP) and polyethylene terephthalate (PET) film surfaces treated by DC glow discharge plasma. Vacuum 2008, 83 (2), 332−339. (43) Ton-That, C.; Shard, A. G.; Teare, D. O. H.; Bradley, R. H. XPS and AFM surface studies of solvent-cast PS/PMMA blends. Polymer 2001, 42 (3), 1121−1129. (44) Liston, E. M.; Martinu, L.; Wertheimer, M. R. Plasma surface modification of polymers for improved adhesion: a critical review. J. Adhes. Sci. Technol. 1993, 7 (10), 1091−1127. (45) Gerenser, L. J. XPS studies of in situ plasma-modified polymer surfaces. J. Adhes. Sci. Technol. 1993, 7 (10), 1019−1040. (46) López, G. P.; Castner, D. G.; Ratner, B. D. XPS O 1s binding energies for polymers containing hydroxyl, ether, ketone and ester groups. Surf. Interface Anal. 1991, 17 (5), 267−272. (47) Ameen, A. P.; Ward, R. J.; Short, R. D.; Beamson, G.; Briggs, D. A high-resolution X-ray photoelectron spectroscopy study of trifluoroacetic anhydride labelling of hydroxyl groups: demonstration of the β shift due to -OC(O)CF3. Polymer 1993, 34 (9), 1795−1799. (48) Akhter, S.; Zhou, X. L.; White, J. M. XPS study of polymer/ organometallic interaction: Trimethyl aluminum on polyvinyl alcohol polymer. Appl. Surf. Sci. 1989, 37 (2), 201−216. (49) Heuer, J. K.; Stubbins, J. F.; An, X. P. S. characterization of FeCO3 films from CO2 corrosion. Corros. Sci. 1999, 41 (7), 1231− 1243. (50) Muir, B. W.; Mc Arthur, S. L.; Thissen, H.; Simon, G. P.; Griesser, H. J.; Castner, D. G. Effects of oxygen plasma treatment on the surface of bisphenol A polycarbonate: a study using SIMS, principal component analysis, ellipsometry, XPS and AFM nanoindentation. Surf. Interface Anal. 2006, 38 (8), 1186−1197. (51) Descostes, M.; Mercier, F.; Thromat, N.; Beaucaire, C.; GautierSoyer, M. Use of XPS in the determination of chemical environment and oxidation state of iron and sulfur samples: constitution of a data basis in binding energies for Fe and S reference compounds and applications to the evidence of surface species of an oxidized pyrite in a carbonate medium. Appl. Surf. Sci. 2000, 165 (4), 288−302. (52) Tsougeni, K.; Vourdas, N.; Tserepi, A.; Gogolides, E.; Cardinaud, C. Mechanisms of Oxygen Plasma Nanotexturing of Organic Polymer Surfaces: From Stable Super Hydrophilic to Super Hydrophobic Surfaces. Langmuir 2009, 25 (19), 11748−11759. (53) Lua, Y.-Y.; Fillmore, W. J. J.; Yang, L.; Lee, M. V.; Savage, P. B.; Asplund, M. C.; Linford, M. R. First Reaction of a Bare Silicon Surface with Acid Chlorides and a One-Step Preparation of Acid Chloride Terminated Monolayers on Scribed Silicon. Langmuir 2005, 21 (6), 2093−2097. (54) Xing, Y.; Li, L.; Chusuei, C. C.; Hull, R. V. Sonochemical Oxidation of Multiwalled Carbon Nanotubes. Langmuir 2005, 21 (9), 4185−4190. (55) Mattevi, C.; Eda, G.; Agnoli, S.; Miller, S.; Mkhoyan, K. A.; Celik, O.; Mastrogiovanni, D.; Granozzi, G.; Garfunkel, E.; Chhowalla, M. Evolution of Electrical, Chemical, and Structural Properties of Transparent and Conducting Chemically Derived Graphene Thin Films. Adv. Funct. Mater. 2009, 19 (16), 2577−2583. (56) Moulder, J. F.; Chastain, J.; King, R. C., Handbook of X-ray photoelectron spectroscopy: a reference book of standard spectra for identification and interpretation of XPS data; Physical Electronics: Eden Prairie, MN, 1995. (57) Castner, D. G.; Lewis, K. B.; Fischer, D. A.; Ratner, B. D.; Gland, J. L. Determination of surface structure and orientation of polymerized tetrafluoroethylene films by near-edge x-ray absorption fine structure, x-ray photoelectron spectroscopy, and static secondary ion mass spectrometry. Langmuir 1993, 9 (2), 537−542. 238

dx.doi.org/10.1021/ed400401c | J. Chem. Educ. 2014, 91, 232−238