On the determination of atomic charge via ESCA including application to organometallics

JOURNAL OF ELECTRON SPECTROSCOPY and Related Phenomena

ELSEVIER

Journal of Electron Spectroscopyand Related Phenomena77 (1996)41-57

On the determination of atomic charge via ESCA including application to organometallics Christopher Sleigh 1, A.P. Pijpers, Alex Jaspers, Betty Coussens, Robert J. Meier* D S M Research, P.O. Box 18, 6160 MD Geleen, The Netherlands

Received 18 May 1995; acceptedin final form 11 August 1995

Abstract

A consistent set of relative atomic charges for several light elements has been obtained employing the semiempirical AM 1 method. Following a method previously proposed by Folkesson and Larsson, linear relationships between charges and experimentally determined core-electron ESCA shifts obtained from model compounds were established. The relations revealed for metal ions allow subsequent characterisation of changes in the charge on metal ion sites, thereby enabling the study of the relation between metal ion charge and catalytic activity. Keywords: Atomic charge; Electron spectroscopy for chemical analysis; Organometal

1. Introduction

X-ray photoelectron spectroscopy (XPS) has proved its importance as an analytical tool for chemists, which is why it is often referred to as electron spectroscopy for chemical analysis (ESCA). After the pioneering work of Siegbahn and co-workers [1,2], one of the further developments involved the application of ESCA to polymers [3,4]. Although ESCA can be applied to organic materials as well as to organometallic species, the former has been the objective of the majority of related publications. However, characterisation of organometallics by ESCA is possible, recognising that ESCA is essentially looking at atoms in a molecule, rather than entities as probed * Corresponding author. IOn leave from the Chemistry Department, University of York, York YO1 5DD, UK.

by vibrational spectroscopy. In this context, the potential application to catalysis involving metalcontaining compounds has been known for some time; see, for example, Refs. [5] and [6]. For organic elements, it has already been proposed by Siegbahn et al. [1] that a linear relationship can be established between the ESCA shift (shift with respect to a chosen reference binding energy E0) and a quantum chemically calculated atomic charge, q, i.e. Eb = E 0 + k q

Although the relations established are essentially linear over the normal range of charges of the elements, evidence has been presented for carbon [7-10] suggesting non-linear behaviour in the series CHnFa_n, n = 0-4. Problems are encountered when trying to establish similar relations for metal elements. It is difficult to obtain an accurate charge distribution

0368-2048/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSD1 0368-2048(95)02392-5

42

C. Sleigh et al./Journal o f Electron Spectroscopy and Related Phenomena 77 (1996) 41-57

in transition metal complexes using quantum chemical calculations owing to the complex bonding involving diffuse d- and f-orbitals. Moreover, referring to the possible application to catalytically active species, quantum chemical calculations can only be carried out on well-defined species. For this reason, an experimental approach to the calculation of atomic charge, based on the work of Folkesson and Larsson, has been undertaken. These authors have reported an impressive amount of work, which has culminated in the publication of a large series of papers [11-22] dealing with the relation between experimental ESCA shifts and calculated atomic charges for both light elements and metal elements. The approach to calculating the charge on a transition metal ion suggested by Folkesson and Larsson involves, as a first step, an attempt to relate experimentally determined binding energies to the charge on atoms of light elements, such as C, N and O. These types of atoms are common in ligands associated with metal ions in many organometallics. In the Folkesson and Larsson approach, the charge on the metal is then assumed to have the same magnitude as, but opposite sign to, the sum of charges residing on the ligands. Despite the extensive work of Folkesson and Larsson, and although many of the relationships put forward do in fact suggest linearity, we have noticed some difficulties concerning accuracy and consistency. Firstly, calculated charges have been taken from different literature sources. Although all the charges were derived from ab initio calculations, a closer inspection reveals that different levels of ab initio calculation were used (STO-nG, DZP, etc.). There is some doubt as to the validity of these charges, as different values are obtained depending on the level and type of calculation employed [8]. Secondly, in a number of publications (see, for example, Ref, [13]), Folkesson and Larsson have adopted the aromatic carbon associated with the tetraphenylphosphonium ion Ph4P+ as an internal standard for the calibration of spectra. This has the advantage of providing a large counter-ion that is non-polar and therefore not thought to be susceptible to undesirable effects such as charge transfer. Their use of Ph4 P+ systems, however, may not be totally justified as

they assume a total charge of +1. We have performed quantum chemical calculations (unpublished ab initio and semiempirical calculations) which revealed that this is not correct for several compounds containing the Ph4 P÷ moiety. As an example, we mention the value of +0.75 on PhPH3 + in PhPH3Br obtained using a 3-21G basis set and full geometry optimisation. The actual number is obviously basis set dependent, but it illustrates the point, also corroborated from semiempirical calculations, that Ph4P + does not have a charge of + 1. Finally, in the course of their investigations, Folkesson and Larsson have modified some of the earlier established relations for light elements. However, the counter-effect on the relations for metals was often not re-evaluated. The above highlights some of the possible origins of the uncertainties that currently exist, and stresses the need to acquire a new and consistent set of results. This is the aim of the present work. However, the basic approach proposed by Folkesson and Larsson was found to be very fruitful and was therefore retained. In brief, the current study aims to determine a new set of parameters for the linear equation (binding energy versus atomic charge) of several light elements. Using only one type of calculational method, i.e. the semiempirical AM 1 method, we aim to arrive at a consistent set of results which is regarded as essential for the successful determination of the charge on a transition metal following the Folkesson and Larsson approach. The AM 1 method was chosen as it is a well-tested semiempirical model for normal organic molecules. Experimental electron binding energies for transition metals such as chromium, titanium, zirconium, palladium and rhodium compounds along with their ligand atoms are taken as the necessary additional experimental data.

2. Experimental details 2.1. The ESCA instrument

ESCA spectra were recorded on a Leybold MAX 200 photoelectron spectrometer equipped with a HP A400 computer and a Leybold DSl00 data system.

C. Sleigh et al./Journal of Electron Spectroscopy and Related Phenomena 77 (1996) 41-57

Turbo-molecular pumps kept the pressure at a level of less than 10-9 mbar. Mg K s and A1 Kc~ radiation from a twin anode X-ray tube operating at 13 kV and 20 mA was used. A glove box (MBraun) was connected to the instrument. The water concentration was less than 0.05 ppm and oxygen levels were below 0.2 ppm. The calibration of the instrument was accomplished using Cu, Ag and Au [23]. A quadrupole mass spectrometer detected gaseous molecules in the analysing chamber. Calibration was accomplished using the natural hydrocarbon contamination. The Cls line was set to 284.8 eV [4,24]. Some samples were run with a thin layer of silicon oil on the surface, i.e. ((CH3)2SiO)n, and the silicon 2p3/2 peak was set equal to 102.1 eV which is the Si 2p3/2 binding energy corresponding to the normal aliphatic

carbon at 284.8 eV. We need to be very explicit and precise here to make clear the procedure that was followed. All carbon values were referenced against 284.8 eV, implying that some values quoted in the literature have been corrected by us in order to arrive at a consistent set of relative values. This applies, for example, to the quoted values from Pireaux et al. referred to in Table 3. Further, when not referenced to the carbon line set at 284.8 eV, the Si 2p3/2 line was used set at 102.1 eV.

2.2. Samplepreparation All the samples handled were solids, normally powders, and were usually mounted on doublesided sticky tape. Hygroscopic and toxic

Table 1 ESCA chemical shift of C Is electrons from 284.8 eV and partial charge from AM1 calculations Molecule

Shift (eV)

Charge (a.u.)

-(CaH2CaH2NH)n -

Ca

0.56

-0.078

- (CCH2 (CaH2)3CbH2 Cd (O)O)n-

Ca Cb Cc Cd

0.00 0.55 1.54 4.08

-0.159 -0.157 -0.012 0.303

- ( c b H 2 (CaHE)2CbH20)n -

Ca Cb

0.00 1.35

-0.177 -0.020

Ca Cb Cc

0.00 1.91 4.23

-0.167 0.053 0.317

o-CbH2CaH(CaH3)2

Ca Cb

0.00 1.30

-0.187 0.005

Cc(O)CbH2CaH3

Ca Cb Cc

0.00 0.38 2.81

-0.176 -0.192 0.233

CCOOH

Ca Cb Cc

0.00 0.42 4.18

-0.158 -0.100 0.310

C d (O)OCCn3

Ca Cb Cc Cd

0.00 0.72 1.79 4.03

-0,185 -0,026 -0,017 0,307

CCONH2

Ca Cb Cc

0.00 0.33 3.59

-0.145 -0.143 0.323

-(C"H2CbHX)n - where X is Oc(o)CbH2(CaH2)gCaH3

43

44

C. Sleigh et al./Journal o f Electron Spectroscopy and Related Phenomena 77 (1996) 41-57 290

289 7288

~ 287 ._~

~ 286 r..) 285

284 --0.3

-0.2

-0.1

0 0.1 charge (a.u.)

0.2

0.3

0.4

Fig. 1. C ls electron binding e n e r g y - A M l atomic charge relation.

compounds were handled in the glove box under a nitrogen atmosphere. Because the glove box was directly attached to the ESCA instrument, the samples could be directly transferred into the ESCA instrument under inert conditions. In order to obtain a clean surface some samples were crushed to expose new surfaces. Metals were cleaned in alcohol, acetone and alkane solutions. Some extremely reactive metals were argon etched to remove a surface layer at a rate of 1 nm min -~. Samples containing water of crystallization usually lose most of the water at low pressures. Therefore, whenever possible, the pressure was lowered slowly

so that not all the H 2 0 left at once, thus minimising disturbance of the sample structure. TiCI4 was measured at - 140°C.

2.3. Quantum mechanical calculations Semiempirical AM1 calculations were carried out on structures of organic molecules in order to obtain a relation between binding energy and atomic charge for light elements. All structures were energy minimised (e.g. geometry optimised) using the AM1 method. Atomic charges as obtained from a Mulliken population analysis

Table 2 O ls electron binding energies along with the AM I calculated charges Molecule

E b O ls (eV)

Charge (a.u.)

-(CH2CH(CH3) (c(oa)obCH3))n -

0a Ob

532.01 533.57

-0,335 -0,276

-(CH2(CH2)3CHzC(Oa)Ob)n -

Oa Ob

532.04 533.34

-0.354 -0.282

Oa Ob

532.13 533.42

-0.362 -0.277

Oa Ob

531.99 533.33

-0.365 -0.309

- ( C H 2 C H X ) , - where X is o b c ( o a)cHz (CH2)9CH3 c(oa)obH

c. Sleighet al./Journalof ElectronSpectroscopyand RelatedPhenomena77 (1996) 41 57

539f / /

45

X

538 7 537 f

~

1

/

536

/"

535 ~"~ 534 533 ! 5321

~

531 -0.4

_

~

-0.3

.

L

l

-0.2 -0.1 charge (a.u.)

±

±

_

0

~_

0.1

Fig. 2. O ls electron bindingenergy-AM1 atomiccharge relation. were used to establish these correlations. Calculations were run with the AM1 method [25], employing the MOPAC6.0program and Insight 2.2.0 from Biosym Technologies [26] implemented on a Silicon Graphics 4D35 GT workstation. Each calculation was performed on a molecule typically comprising five monomer units of the polymer. Therefore, in this procedure there is no explicit allowance for intermolecular effects. 2.4. Collection of literature ESCA data A selection of ESCA binding energies were taken from the literature. Only data from what we considered to be unambiguously well-calibrated spectra were used, i.e. we selected data from Refs. [3], [27] and [28]. This does not imply that we consider all the other published data to be unreliable; however, consistency with regard to referencing is essential for establishing the binding energyatomic charge relationships. 2.5. Final data handling producing binding energy-charge relationships Relations of the form E b = kq + Eo have been obtained for a series of light elements, i.e. C, N, O, F and CI, and for a series of metals, i.e. Ti,

Cr, Rh, Pd and Zr, using a least-squares fit and regression analysis.

3. Results of establishing binding energy versus atomic charge relations Following the method discussed in Sections 1 and 2, binding energy-atomic charge relations were established for both light elements (C, O, F, N, CI) and a series of catalytically important metals (Ti, Cr, Rh, Pd, Zr). Calculated and measured data have been collected in the Tables and Figures, and will now be shown and briefly discussed for each of the aforementioned elements.

3.1. Light elements (C, O, F, N, Cl and H) Carbon All experimental binding energies were taken from Beamson and Briggs [3]. Experimental ESCA shifts and calculated atomic charges are collected in Table 1, and the relation obtained from linear regression analysis is displayed in Fig. 1. The analytical form of the relation is E b C ls (eV) --- 8.0qc (a.u.) + 286.2 (eV)

46

C. Sleigh et al./Journal of Electron Spectroscopy and Related Phenomena 77 (1996) 41-57

We have not included in Table 1 the C 1s binding energies for the fluorinated polyethylenes; the corresponding F ls values will be presented in the section on the element F (see below). This is because we have found that, when using the semiempirical AM1 method, the secondary or next-nearest neighbour shifts on carbon induced by the fluorines appear to be of opposite sign compared to experiment. Whereas the nearest neighbour effect shows an increasing C ls binding energy upon increasing fluorination, from the AM 1 calculations it was inferred that the next-nearest neighbour effect of the fluorines on the C ls Table 3 F ls electron binding energies and AM 1 calculated partial charges for fluorine Molecule

Eb F ls (eV)

Charge (a.u.)

(A) 1,2,3,4,5,6-Fluorobenzene 1,3,5-Fluorobenzene 1-Fluorobenzene 1,4-Fluorobenzene 1,2,3,5-Fluorobenzene 1,2-Fluorobenzene 1,3-Fluorobenzene 1,2,3-Fluorobenzene 1,2,3,4-Fluorobenzene 1,2,4,5-Fluorobenzene 1,2,3,4,5-Fluorobenzene

690.6 690.0 689.3 689.5 690.2 689.5 689.5 690.0 690.2 690.1 690.4

-0.058 -0.093 -0.107 -0.103 -0.072 -0.092 -0.100 -0.085 -0.075 -0.080 -0.070

686.74 687.95 689.47

-0.171 -0.180 -0.107

689.6 688.1 687.9 690.1

-0.119 -0.171 -0.180 -0.107

689.1 689.4 689.9 690.0

-0.17 l -0.180 -0.119 -0.107

689.4

-0.107

(B) -(CH2CHF),- (CH2CF2), -(CF2CF2),-

(c) -(CHFCF2), a_ -(CH2CHF),-(CH)2CF2),- ( C F 2CF 2).-

(D) -(CH 2C H F ) . -(CH2CF2) . -(CHFCF2). -a -(CF2CF2) n-

(E) -(CF2CF2) . -

Key: data set (A) Clark et al. [27]; (B) Beamson and Briggs [3]; (C) Pireaux et al. [28]; (D) Clark et al. [32]; (E) this work. a Only one F Is peak is observable. Hence, the calculated charge is averaged.

binding energies is negative rather than positive (increasing), the latter being in accordance with experimental observation. Both the AM I set of data for the fluorinated polyethylenes and the experimental data lead to a parallelogram of data points [29,30], but the AM1 set gives the wrong sign to the angle c~ describing the skewness of the parallelogram. This was also observed when using the semiempirical MNDO-PM3 method, as well as for the chlorinated polyethylenes. We therefore conclude that whenever significant secondary shifts are present, one should be cautious as to the employment of AM1 results in the context of the present investigation.

Oxygen Experimental binding energies and calculated charges are shown in Table 2 and Fig. 2. The oxygen binding energies and atomic charge values are not spread as widely as for carbon, i.e. for typical organic polymers the range of charges on oxygen is -0.28 to -0.36 in comparison to -0.2 to +0.33 for carbon. Therefore, an initial attempt to establish a correlation between experimental binding energies and calculated atomic charges involving many more species than those collected in Table 2 results in a very poor correlation. The major cause of this is the problem of proper referencing of the experimental binding energies, so as to obtain a consistent set of mutually comparable experimental data. When the binding energy range is small, the inherent scatter introduced by the referencing problem of the experimental data leads to very significant scatter in a plot relating experimental binding energies and calculated charges, ultimately resulting in a poor correlation. The ideal solution to this problem would be to locate oxygen in extreme charge environments. However, one of the few situations in which oxygen is expected to be positively charged is when it is attached to fluorine. This is not a common environment and AM 1 calculations are not expected to describe such species well. It is also unlikely that the corresponding ESCA electron binding energies could be found for such a species. In order to avoid these problems, and to obtain an accurate slope for the correlation shown in Fig. 2, only polymers with two oxygen atoms in differ-

C. Sleigh et al./Journal of Electron Spectroscopy and Related Phenomena 77 (1996) 41-57

47

692

691 > ,a 690 u

._= ._=

689

.,o

_~ 688 ~687 t

686 L -0.2

+ J

I

__

I

I

-0.15

I

-0.1

I

-0.05

charge (a.u.) Fig. 3. F I s electron binding e n e r g y - A M l atomic charge relation.

ent chemical environments were chosen. In Fig. 2 we have taken into account a series of literature values from Beamson and Briggs [3]. On the basis of data on four polymers (eight data points), a regression line was determined as Eb O ls (eV) = 18.0qo (a.u.) + 538.5 (eV) In addition, we have taken the gas-phase oxygen O 1s binding energy [1] and adjusted this value for the workfunction in order to be able to compare the gas-phase value with the solid-state data on polymers. From various sources in the literature (see, for example, Ref. [31]) it is known that the

workfunction is roughly 5 eV. When we apply this correction to the gas-phase value for 02, the resulting O Is electron binding energy lies practically (see Fig. 2) on the regression line determined on the basis of the polymer data. This provides additional support for the validity of the relation established on the basis of the given polymer data. Fluorine Data for fluorine have been collected from Beamson and Briggs [3], Clark et al. [27] and Pireaux et al. [28]. Table 3 shows the collected and calculated values and Fig. 3 graphically

Table 4 Binding energy of N 15 electron with charge calculated by AM 1 Molecule

Eb N Is (eV)

Charge (a.u.)

Ref.

-(CH2CH(CONH2)). -(CH2CH2NH).-(CHE(CH2)3CH2C(O)NH2) n-

399.63 398.87 399.57

-0.451 -0.303 -0.384

[3] [3] [3]

N2

404.9

NH 3 N(CH3) 3 N2H4

400.9 400.0 401.4

-0.396 -0.267 -0.279

[33] [33] [33]

Melamine a

398.8

-0.340

[34]

0.000

[1], [33]

a Melamine contains two nitrogen atoms in different chemical environments, but a broad N ls peak is observed and the charge (AMI) is almost equivalent.

48

C. Sleigh et al./Journal of Electron Spectroscopy and Related Phenomena 77 (1996) 41-57

406 405 I 404 ~ 403

401 400 Z 399

J

i

398 ~-

+ --

js J

z_~ / J

j -

+

+

/ j j/

397 396 . . . . -0.55

~- ~-0.45

-0.35

-0.25 charge (a.u.)

-0.15

-0.05

0.05

Fig. 4. N ls electron binding energy-AM1 atomic charge relation.

shows the relation, which is given by EbF

ls (eV) = 19.6qF (a.u.) + 691.7 (eV)

It may be noted that the various published F ls binding energies of these polymers mutually show a relatively large variation. Observed differences between the experimental values reported by Beamson and Briggs [3] and Pireaux et al. [28] tend to increase upon decreasing level of fluorination, with a difference of 1.4 eV for the F is binding energy in -(CH2CHF)n-. We note that this difference is not observed (compare the corresponding data in Refs. [3] and [28]) for the C l s binding energies in these molecules. Further, following the discussion on secondary

shifts on carbon atoms as discussed in the subsection on the element C with reference to fluorinated polyethylenes, an effect on the correlation found for fluorine might be anticipated. Although we certainly do not claim to have found a completely satisfactory answer to this point, we believe that, for the present time, a reasonable reply can be formulated. With respect to the above-mentioned large differences in experimental F 1s binding energies, the data from Pireaux et al. (see Table 3) were considered the most reliable because of the excellent agreement between the relative ESCA shifts from these experimental data and high level ab initio calculations [30]. Referring to Pireaux's data on -(CH2CHF)n- and -(CH2CF2)n- (see Table

Table 5 CI 2p3/2 electron binding energies and AM1 calculated partial charges for chlorine Molecule

Eb CI 2p3/2 (eV)

Charge (a.u.)

Ref.

Polyvinylchloride Polyvinylidene chloride Poly(2-chlorostyrene) Poly(3-chlorostyrene) Poly(4-chlorostyrene) Ph4PCI KCIO3 KCIO4

200.44 200.58 200.24 200.30 200.33 196.4 206.3 208.5

-0.122 -0.042 -0.024 -0.017 -0.013 -0.879 0.910 1.020

[3] [3] [3] [3] [3] This work [2], [24] [2], [24]

C. Sleigh et al./Journal of Electron Spectroscopy and Related Phenomena 77 (1996) 41-57

49

210,

+

208

¢" 206 i/"

204 /

.3 202

/

J

/

J

/ J

m ¢q

200

t~ ¢q

U 198 196

j"

L

194 -1.25

I

-0.75

t

i

I

-0.25 0.25 charge (a.u.)

h_

k

J

0.75

1.25

Fig. 5. C1 2P3/2 electron binding energy-AM1 atomic charge relation.

3(C)), the close values of the F ls binding energies and therefore of the charges on the fluorine atoms suggests the absence of a secondary shift on the fluorine due to the other fluorine. Consequently, we may consider the fluorine shifts to be pure primary shifts that can be directly used to set up the charge-binding energy relation. A problem may arise from the requirement that the total charge of the molecule is zero, and when calculated carbon charges are inaccurate there will be an obvious effect on the other atoms. It is not possible to verify this owing to the inherent problems concerning the experimental determination of H ls binding energies (see later subsection on hydrogen). Because of the apparent insensitivity of fluorine shifts to secondary substituents, e.g. another fluorine, it is more likely that the fluorine data are pure primary shifts, which is particularly reflected by the closeness of the calculated AM 1 charges for fluorine (see Table 3(C)). Finally, there is additional support for the validity of our F 1s charge-binding energy relation from the consistency of the relation between the slope of this linear relation and the Pauling electronegativity for the C Is, N Is, O ls and F ls data, as will be illustrated later (Fig. 11). We therefore believe we have arrived at a satisfactory consistent set of data regarding fluorine.

Nitrogen N ls electron binding energies were taken from Beamson and Briggs [3] unless stated otherwise. Table 4 shows the results obtained and Fig. 4 gives a plot of binding energy versus atomic charge. Although numerous experimental ESCA data are available for N, the limited applicability of the AM1 method to many inorganic nitrogen-containing species limits the number of ESCA data we can use in the present study. As a consequence, the small range in binding energy or, alternatively, in q, covered by the polymers studied, necessitated the use of the gas-phase N 2 data point (qN = 0, see Ref.

Table 6 AM1 calculated charge on hydrogen in phenyl rings. Average charges were taken over all hydrogens in each species. Variation between individual hydrogens in one species is also of the order of 0.01-0.03 a.u. Molecule

Average charge on hydrogen atoms in phenyl rings (a.u.)

C6HsNO 2 C6HsCHO C6HsCOCH 3 C6HsOH C6HsOCH3 C6HsCOCI

0.156 0.140 0.140 0.136 0.136 0.147

50

C. Sleigh et al./Journal of Electron Spectroscopy and Related Phenomena 77 (1996) 41-57

Table 7 AM I calculated atomic charges for hydrogen in CH 3 groups Molecule

Charge (a.u.)

CH(CH~) 2 CH2CH3 C(O)CH2CH 3 OCH2CI-I 3 CH2C(CH3)(COOC..)CH2 OCH3

0.074 0.073 0.083 0.086 0.089 0.091

Chlorine

Experimental binding energies for the chlorine 2p level were collected by ourselves as well as taken from the literature [3]. Table 5 contains the numerical data, and Fig. 5 shows the plot of binding energy versus atomic charge for chlorine. E b CI 2p3/2 (eV) = 6.3qc 1 (a.u.) + 201.0 (eV)

Hydrogen

[1]) in order to establish the relation shown in Fig. 4. To allow comparison with the polymer data on solid samples, a correction has to be applied relating to the workfunction of the polymer. The magnitude of this correction is approximately 5 eV as was discussed in the subsection on oxygen. Despite this complication, the relation finally established for N on the basis of the aforementioned data seems satisfactory for the moment. A linear regression with the binding energy for N2 fixed at the experimental value for gaseous N2, corrected for the workfunction, yields Eb N ls (eV) = 14.1qN (a.u.)+ 404.9 (eV)

All atoms other than hydrogen can be detected and identified using ESCA. Further, we have not used any model compounds for which we actually need a charge-binding energy relation for hydrogen, as the only hydrogen-containing subgroups in any of the compounds involved H20 and NH3 as ligands, which turned out to have a total charge of zero. This was shown from ab initio calculations [35]. It nevertheless seems important to understand how the charge on hydrogen varies from molecule to molecule, as hydrogen is one of the more likely atoms to appear as a constituent in ligands. Averaged AM 1 calculated charges for hydrogen in phenyl rings are collected in Table 6. These data

Table 8 AM 1 calculated atomic charges for hydrogen in CH 2 groups Molecule

Charge (a.u.)

-(CH2CH2NH) -OCH2CH(CH3) 2 -CH2CH__2OCH_H_2CH2CH 2-(CH2CH2)-(CH2CH2CH2CH2CH2COO)-CH2CH2OCH2CH_H~CH2- (CH2CH2CH2CH__2CH2COO)-C(O)CH__2CH 3 -(CH2CH2CH2CH2CH_2COO) -CI-I~CH2CH2CH2CH2COO)-C(O)OCH__2CH2

0.074 0.072 0.078 0.081 0.084 0.092 0.097 0.109 0.1 i 8 0.092 0.099

- ( C H 2 C H X ) , - where X is COOH OC(O) CI-I~(CH2)9CH3

0.115 0.121

C(O)CH__2CH3

0.104

OCH_H_2CH(CH3)2 C(O)OCH_H_2CH3

0.100 0.107

-(CH2CHCI).- (CH2 CHCI). -(CH___2CCI2)n-(CHCICCI2)n-

0.107 0.142 0.117 0.165

CH 2 directly attached to either oxygen or nitrogen CH2, simple aliphatic

CH2 secondary (to O) CH2 next to CO or COO CH2 next to -OC(O)

C. Sleigh et al./Journal of Electron Spectroscopy and Related Phenomena 77 (1996) 41-57 Table 9 Electron binding energies for Ti 2p3/2 (eV) and ligand atoms (ligands: ls core electron binding energies for O and N, 2P3/2 for Cl) and the calculated charge on the Ti atom. The number in parentheses denotes the number of independent measurements on this system taken into account. The binding energy presented is the average over this number of data points, whereas in the linear regression analysis of binding energy versus atomic charge the weight of this data point was increased to the number of independent data taken into account Compound

Eb Ti 2p3/2 (eV)

Eb ligand (eV)

Charge on Ti (a.u.)

Ref.

Ti Ti TiN TIN0.95 TiC13 TiCI4 Ti203 TiO2 TiO2 TiO2 TiO 2 TiO2 TiO2

454.0 (5) 454.0 456.9 455.8 458.5 459.8 457.8 458.6 458.5 458.7 458.6 458.9 458.6

397.3 396.9 199.5 199.2 529.6 529.8 529.7 530.0 530.1 530.1 530.0

0 0 0.55 0.57 0.72 1.16 0.74 0+97 0.97 0.95 0.93 0.93 0.95

[4l This work This work [36] This work This work [37] This work [38] [38] [39] [40] This work

show that the average charge on the hydrogen atom does not vary substantially. Variation between individual hydrogens in a single molecular species is of the order of 0.01-0.03 a.u. Some effects might be noted explicitly, e.g. NO2 is electron withdrawing and as a consequence an increase of the positive charge on the hydrogen atoms is observed, as for COCI. For aliphatic molecular species, typical AM1 calculated charges are collected in Table 7 for C H 3 group hydrogens and in Table 8 for CH2 group hydrogens. Note that we have excluded cases where a specific interaction between the hydrogen and the metal is present, as for metal hydrides or hydrogens exhibiting agostic interaction. Although the charge variation is small for the CH3 hydrogen atoms, there appear to be three categories: (i) simple aliphatic with no influence from oxygen (q=0.073); (ii) CH 3 under the influence of oxygen two or three atoms away (q -- 0.086); (iii) CH 3 directly bonded to oxygen (q = 0.091). A n i n v e s t i g a t i o n i n t o w h e t h e r the c h a r g e o n h y d r o g e n is solely d e p e n d e n t o n t h e c h a r g e o f the a t o m to w h i c h it is b o n d e d s h o w e d n o c o r r e l a t i o n . Therefore, the only possible prediction of the a t o m i c c h a r g e o n h y d r o g e n a t o m s c a n be m a d e f r o m the a c t u a l d a t a in T a b l e s 6 - 8 .

461

460 459 458 .9 457

._m .o

456 455 454 453 -0.1

m

~

0.1

I

I

0.3

--

m

51

-J

~

J

0.5 0.7 charge (&u.)

I

I

0.9

L

t

1.1

Fig. 6. Ti 2p3/2 electron binding energy-AM1 atomic charge relation.

k

1.3

52

C. Sleigh et al./Journal of Electron Spectroscopy and Related Phenomena 77 (1996) 41-57

Table 10 Electron binding energies for Cr 2p3/2 (eV) and ligand atoms (ligands: I s core electron binding energies for O, N and F, 2p3/2 for CI) and the calculated charge on the Cr atom. The number in parentheses denotes the number of independent measurements on this system taken into account. The binding energy presented is the average over this number of data points, whereas in the linear regression analysis of binding energy versus atomic charge the weight of this data point was increased to the number of independent data taken into account Compound

Eb Cr 2p3/2 (eV)

Eb ligand (eV)

Charge on Cr (a.u.)

Ref.

Cr Cr CrN CrN CrN0.98 Cr2N CrCI 3 CrF3 CrF3 Cr203 Cr203 Cr203 Cr203 Cr203 CrO3 CrO 3 CrO 3

574.3 (10) 574.1 575.6 575.7 575.8 576.1 577.7 579.2 579.1 576.8 576.7 576.8 576.8 576.6 579.6 579.7 580.1

396.6 396.8 396.7 397.4 199.4 685. I 685.5 530.4 530.5 530.5 530.8 530.3 530.6 530.6 530.8

0.00 0.00 0.59 0.57 0.58 0.27 0.76 0.93 0.87 0.68 0.67 0.67 0.64 0.68 1.32 1.32 1.29

[4] This work This work [41] [36] [41] This work This work [42] This work [42] [43] [44] [45] This work [45] [42]

581

÷

580 579

o=

578 577

,.D

576 ¢o ('4

575 574 573 -0.1

I

0.1

0.3

0.5

0.7 0.9 charge (a.u.)

1.1

1.3

Fig. 7. Cr 2p3/2 electron binding energy-AM 1 atomic charge relation.

1.5

C. Sleigh et al./Journal of Electron Spectroscopy and Related Phenomena 77 (1996) 41-57 Table 11 Electron binding energies Rh 3d5/2 (eV) and ligand atoms (ligands: ls core electron binding energies for O, 2p3/2 for C1) and the calculated charge on the Rh atom. The number in parentheses denotes the number of independent measurements on this system taken into account. The binding energy presented is the average over this number of data points, whereas in the linear regression analysis of binding energy versus atomic charge the weight of this data point was increased to the number of independent data taken into account Compound

Eb

Eb Rh 3d5/2 ligand (eV) (eV)

Charge on Rh (a.u.)

Ref.

Rh [Rh(NH3)6]C13 [Rh(NH3)6]CI3 RhC13 • 3H20 RhCI3 • 12H20 RhCI3 RhC13 Rh(NH3)3C13 [Rh(NH3)sCI]CI2 RhzO 3 Rh203 Rh203

307.2 (6) 310.1 310.5 310.0 310.1 310.1 310.2 310.3 310.2 309.1 308.7 308.9

0.00 1.19 1.38 1.14 0.85 0.81 1.14 1.38 1.19 0.64 0.68 0.68

[4] This work [46] [47] [48] [49] [50] [46] [50] This work [51] [48]

198.5 198.1 198.6 199.2 199.3 198.6 198.1 198.5 530.8 530.4 530.3

53

E S C A d a t a are m e n t i o n e d in T a b l e 9, a n d the relation o b t a i n e d f r o m linear regression analysis is d i s p l a y e d in Fig. 6. A n a l y t i c a l l y , the relation reads E b Ti 2p3/2 (eV) = 5.0qT i (a.u.) + 454.0 (eV)

Chromium T h e references f r o m which we t o o k e x p e r i m e n t a l E S C A d a t a are m e n t i o n e d in T a b l e 10, a n d the relation o b t a i n e d f r o m linear regression analysis is d i s p l a y e d in Fig. 7. A n a l y t i c a l l y , the relation reads Eb Cr 2p3/2 (eV) = 4.2qcr (a.u.) + 574.2 (eV)

Rhodium The references f r o m which we t o o k e x p e r i m e n t a l E S C A d a t a are m e n t i o n e d in T a b l e 11, a n d the relation o b t a i n e d f r o m linear regression analysis is d i s p l a y e d in Fig. 8. A n a l y t i c a l l y , the relation reads Eb R h 3d5/2 (eV) -- 2.5qR h (a.u.) + 307.3 (eV)

3.2. Model compounds containing metal ions ( Ti, Cr, Rh, Pd and Zr)

Palladium The references from which we took experimental ESCA data are mentioned in Table 12, and the relation obtained from linear regression analysis

Titanium T h e references f r o m which we t o o k e x p e r i m e n t a l 311

+ 310

.~_

309 / /

J

308

J /J

307

L ~

-0.1

I

_1

0.1

J

/J

I ~ _ _ l

0.3

I__

0.5

I

I

I

~

0.7 0.9 charge (a.u.)

k

J

1.1

b

I

1.3

Fig. 8. Rh 3d5/2 electron binding energy-AM 1 atomic charge relation.

1.5

54

C. Sleigh et al./Journal of Electron Spectroscopy and Related Phenomena 77 (1996) 41-57

Table 12 Electron binding energies for Pd 3d5/2 (eV) and ligand atoms (ligands: Is core electron binding energies for O, 2P3/2 for CI) and the calculated charge on the Pd atom. The numbers in parentheses denote the number of independent measurements on this system taken into account. The binding energy presented is the average over this number of data points, whereas in the linear regression analysis of binding energy versus atomic charge the weight of this data point was increased to the number of independent data taken into account

Table 13 Electron binding energies for Zr 3d5/2 (eV) and ligand atoms (ligands: ls core electron binding energies for O and F, 2p3/2 for CI) and the calculated charge on the Zr atom. The number in parentheses denotes the number of independent measurements on this system taken into account. The binding energy presented is the average over this number of data points, whereas in the linear regression analysis of binding energy versus atomic charge the weight of this data point was increased to the number of independent data taken into account

Compound

Compound

Pd PdCI 2 PdCI2 PdCI2 Pd(NH3)2C12 [Pd(NH3)4]CI 2 PdO

Eb Pd 3d5/2

Eb ligand

Charge on Pd

(eV)

(eV)

(a.u.)

335.1 (18) 338.0 337.8 337.8 338.5 338.7 336.3 (2)

199.0 199.3 198.9 198.9 198.1 529.3

0.00 0.63 0.54 0.66 0.66 0.92 0.51

Ref.

[4] This work [52] [53] [53] [53] [54]

Zr Zr ZrC14 ZrF 4 ZrF4 ZrO2 ZrO2 ZrO 2 ZrOC12

EB Zr 3dv2

Eb ligand

Charge on Zr

(eV)

(eV)

(a.u.)

179.0 178.8 (5) 183.7 185.6 185.0 182.3 182.2 182.0 183.0

199.3 685.9 685.1 530.3 530.2 529.9 530.5/198.5

0.00 0.00 1.07 1.08 1.23 0.91 0.92 0.96 1.24

Ref.

This work [4] This work This work [55] This work [40] [56] This work

is displayed in Fig. 9. Analytically, the relation reads reads

EDPd 3d5/2 (eV)= 4.1qpd (a.u.)+335.1 (eV)

Eb Z r 3d5/2 (eV) = 4 . 4 q z r (a.u.) + 178.8 (eV)

Zirconium

The references from which we took experimental ESCA data are mentioned in Table 13, and the relation obtained from linear regression analysis is displayed in Fig. 10. Analytically, the relation

4. Final discussion and conclusions

Following the approach proposed by Folkesson

340

339

+

~ 338 ~ 337 .~_ '~ 336 335 334 -0.1

i

I

0.1

~

I

0.3

i

I

0.5

L

I

0.7

i

I

0.9

charge (a.u.) Fig. 9. Pd 3d5/2 electron binding energy-AM 1 atomic charge relation.

i

1.1

55

C. Sleigh et al./Journal of Electron Spectroscopy and Related Phenomena 77 (1996) 41-57

186

÷ ÷

185 184 183 182 "z, 181 180 179 178 -0.1

1

i

0.1

0.3

~

L

0.5

a

~ _ _ ~

I

~

I

0.7 0.9 charge (a.u.)

n

I

1.1

n

1.3

1.5

Fig. 10. Zr 3d5/2 electron binding energy-AM 1 atomic charge relation. and Larsson, we have established a consistent and well-defined set of linear relations between ESCA shifts and atomic charge for both a series of light elements and a set of metals which are of particular importance to catalysis. The method is general and can be extended to other elements. The present results allow the implementation of studies aiming

at the characterisation of catalysts through changes in the valence of the metal centre as determined from the ESCA shift. In the course of our investigations, a linear relation was observed between the slope of the c h a r g e binding energy relation and the electronegativity according to Pauling [57]. These relations are

25

20

o,s / /

"~ 15

N ls-~

Cl~

0

......... ~ ................

L

0.8

1.8

i

2.8

J

3.8

1

4.8

electronegativity(Pauling) Fig. 11. The slopesof the charge-binding energy relations plotted versusPauling electronegativities. For further commentssee text. Key" +, datapoints related to ESCA ls binding energies; O, data points related to 2p binding energies; 4, data points related to 3d binding energies.

56

C. Sleigh et al./Journal of Electron Spectroscopy and Related Phenomena 77 (1996) 41-57

compiled in Fig. 11. It is observed that the values for C, N, O and F corresponding to the 1s levels lie on a straight line, and the same seems to hold for the 2p levels (Ti, Cr and C1), although the slope is much smaller than for the ls levels. For the data relating to 3d-orbitals the ranges are so narrow that no conclusion can be drawn. However, the fact that an almost perfectly linear relation is observed on basis of the C Is, N Is, O ls and F ls data further strengthens the proposed relation charge-binding energy for N, which was prone to error owing to the limited number of useable data (see discussion in corresponding section above). Finally, it should be noted that the calculated charges are not to be considered as 'absolute' charges. For any practical purpose, such as employing the relations for revealing changes in atomic charges during reaction and between various species, this is no problem. Acknowledgements Dr. Brrje Folkesson and Professor Ragnar Larsson are gratefully acknowledged for useful discussions before starting our investigations, and even more for critically reading the manuscript of this paper and for making very many valuable suggestions and comments. We wish to emphasise how deeply we appreciated their cooperation while we were working on a reappraisal of their work. The EC is thanked for partial financial support of one of us (C.S.) through the COMETT scheme. Professor Bruce Gilbert and Dr. Gordon Fettis of the University of York are thanked for their help in obtaining this support. The management of DSM Research is thanked for giving permission to publish this work. References [1] K. Siegbahn, C. Nordling, G. Johansson, J. Hedman, P.F. Heden, K. Hamrin, U. Gelius, T. Bergmark, L.O. Werne, R. Manne and Y. Baer, ESCA Applied to Free Molecules, North-Holland, Amsterdam/London, 1969. [2] K. Siegbahn, C. Nordling, A. Fahlman, R. Nordberg, K. Hamrin, J. Hedman, G. Johansson, T. Bergmark, S.E. Karlsson, I. Lindgren and B. Lindberg, ESCA - - Atomic, Molecular and Solid State Structure Studied by Means of

Electron Spectroscopy, Uppsala, 1967, Nova Acta Regiae Soc. Sci. Upsaliensis Ser IV, V.20, 1967. [3] G. Beamson and D. Briggs, High Resolution XPS of Organic Polymers: The Scienta ESCA300 Database, Wiley, New York, 1992. [4] D. Briggs and M.P. Seah, Practical Surface Analysis, 2nd edn., Wiley, Chichester, 1990. [5] State-of-the-art techniques for catalyst characterisation, in New Catalytic Materials, Volume XI, Catalytica Associates, Inc., 430 Ferguson Drive, Mountain View, CA 94043, USA, 1984. [6] S.C. Avanzino, H.-W. Chen, C.J. Donahue and W.L. Jolly, Inorg. Chem., 19 (1980) 2201. [7] P. Sundberg, R. Larsson and B. Folkesson, J. Electron Spectrosc. Relat. Phenom., 46 (1988) 19. [8] R.J. Meier and A.P. Pijpers, J. Electron Spectrosc. Relat. Phenom., 49 (1989) cl. [9] R.J. Meier and A.P. Pijpers, J. Electron Spectrosc. Relat. Phenom., 50 (1990) 129. [10] B. Folkesson and R. Larsson, J. Electron Spectrosc. Relat. Phenom., 50 (1990) 251. [11] R. Larsson, J. Electron Spectrosc. Relat. Phenom., 24 (1981) 37. [12] B. Folkesson and P. Sundberg, Spectrosc. Lett., 20 (1987) 193. [13] R. Larsson and B. Folkesson, Phys. Scr., 16 (1977) 357. [14] R. Larsson and B. Folkesson, Chem. Scr., 9 (1976) 148. [15] B. Folkesson and R. Larsson, Chem. Scr., 110 (1976) 105. [16] R. Larsson, B. Folkesson and R. Lykvist, Chem. Scr., 13 (1978/1979) 178. [17a] R. Larsson and B. Folkesson, Chem. Scr., 19 (1982) 31. [17b] R. Larsson and B. Folkesson, Acta Chem. Scand., 45 (1991) 567. [18] B. Folkesson, Chem. Scr., 20 (1982) 108. [19] B. Folkesson and R. Larsson, J. Electron Spectrosc. Relat. Phenom., 26 (1982) 157. [20] B. Folkesson, P. Sundberg, L. Johansson and R. Larsson, J. Electron Spectrosc. Relat. Phenom., 32 (1983) 245. [21] P. Sundberg, C. Andersson, B. Folkesson and R. Larsson, J. Electron Spectrosc. Relat. Phenom., 46 (1988) 85. [22] B. Folkesson and R. Larsson, J. Electron Spectrosc. Relat. Phenom., 50 (1990) 267. [231 M.P. Seah, Surf. Interface Anal., 14 (1989) 488. [24] J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben and J. Chastown, Handbook of X-ray Photoelectron Spectroscopy, Perkin-Elmer Corporation, Physical Electronics Division, Eden Prairie 1992. [25] M.S. Dewar, E.G. Zoebisch, E.F. Healy and JJ.P. Stewart, J. Am. Chem. Soc., 107 (1985) 3902. [26] Models developed using software programs from BIOSYM Technologies of San Diego - - computed with the incorporated version of MOPAC6.0 and displayed using InsightlI. [27] D.T. Clark, D. Kilcast, D. Adams and W. Musgrave, J. Electron Spectrosc. Relat. Phenom., 1 (1972/1973) 227. [28] J.J. Pireaux, J. Riga, R. Caudano, J.J. Verbist, J.M.

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[45] A. Cimino, B.A. DeAngelis, A. Luchetti and G. Minelli, J. Catal., 45 (1976) 316. [46] V.I. Nefedov, E.F. Shubochkina, I.S. Kolomnikov, I.B. Baranovskii, V.P. Kukolev, M.A. Golubnichaya, L.K. Shubochkin, M.A. Porai-Koshits and M.E. Vol'pin, Zh. Neorg. Khim., 18 (1973) 444. [47] S.L.T. Andersson and M.S. Scurrell, J. Catal., 59 (1979) 340. [48] J.P. Contour, G. Mouvier, M. Hoogewys and C, Leclere, J. Catal., 48 (1977) 217. [49] Y. Okamoto, N. Ishida, T. Imanaka and S. Teranishi, J. Catal., 58 (1979) 82. [50] S.L.T. Andersson and M.S, Scurrell, J. Catal., 71 (1981) 233. [51] V.1. Nefedov, M.N. Firsov and I.S. Shaplygin, J. Electron Spectrosc. Relat. Phenom., 26 (1982) 65. [52] B.M. Choudary, K.R. Kumar, Z. Jamii and G. Thyagarajan, J. Chem. Soc., Chem. Commun., (1985) 931. [53] V.I. Nefedov, 1.A. Zakharova, I.I. Moiseev, M.A. PoraiKoshits, M.N. Vargaftik and A.P. Belov, Zh. Neorg. Khim., 18 (1973) 931. [54] K.S. Kim, A.F. Gossman and N. Winograd, Anal. Chem., 46 (1974) 197. [55] V.1. Nefedov, Yu.V. Kokunov, Yu.A. Buslaev, M.A. Porai-Koshits, M.P. Gustyakova and E.G.IL'In, Zh. Neorg. Khim., 18 (1973) 3264. [56] D. Sarma and C.N.R. Rao, J. Electron Spectrosc. Relat. Phenom., 20 (1980) 25. [57] C.D. Wagner, D.E. Passoja, H.F. Hillery, T.G, Kinisky, H.A. Six, W.T. Jansen and J.A. Taylor, J. Vac. Sci. Technol., 21 (1982) 933.