Carbon Alloys part 9 pptx

Etching of the surface carbon atoms by oxygen at room temperature is unexpected because both carbon and oxygen atoms are strong adsorbates on molybdenum metal surfaces.. Imag- ing of sur

a h

h

Fig 5 Selective etching reaction of surface C atoms by exposing to 0, gas (1.3 x 10“ Pa) at RT in the same areavisualized by STM; (a) 0 L (before exposure, I,: 2.0 nA, V,: 10 mV); (b) 2.5 L (I,: 3.0 nA, V,: 20 mV), (c)

5.0 L (I,: 3.0 nA, V,: 20 mV), (d) 10.0 L (Z,: 2.0 nA, V,: 20 mV) 7.3 x 7.3 nm2 The STM tip was retracted

during exposure to oxygen gas Each image was measured under UHV without oxygen ambient (e) A

magnified image of the square region in (c) with lines of the (1 x 1) lattice 3.6 x 3.6 nm2 ( f ) Model structure

of (e) Empty circles and shaded circles represent Mo and C atoms, respectively

Figure 5d indicates that 100 carbon atoms were etched by -2.3 x lo3 molecules of oxygen impinging on the area

Etching of the surface carbon atoms by oxygen at room temperature is unexpected because both carbon and oxygen atoms are strong adsorbates on molybdenum metal surfaces The carbon and oxygen atoms could not be removed from the molybdenum surfaces below 1200 K, when adsorbed as a single element [24,25] If carbon and oxygen atoms co-exist on molybdenum metals, they desorb as carbon monoxide at temperatures of -1000 K [24,25] An air-exposed Mo,C catalyst desorbs carbon monoxide and carbon dioxide at temperatures above 800 K [26] Thus, stable adsorbed oxygen atoms cannot etch carbon atoms on a Mo,C(0001) surface On the other hand, a dissociative adsorption of 0, on Al( 11 1) [27] or Pt( 11 1) [28] produces

‘hot’ oxygen atoms with translational energies parallel to the surface Thermo- induced or photo-induced ‘hot’ oxygen atoms on Pt(ll1) react with adsorbed carbon monoxide to form carbon dioxide at temperatures below 150 K [29] Such energetic-

ally excited oxygen atoms formed on the surface should be responsible for the etching

reactions reported above

The STM tip does not contribute to the etching reaction The tip was retracted from the tunneling region during exposure to oxygen gas Each frame of Fig 5 is a sequential STM image measured under ultra-high-vacuum (UHV) conditions The etched area was unchanged in the sequential images

Atomic-scale Structure and Reactivity of Metal Carbide Surfaces 265

The etching reaction does not occur homogeneously but starts from specific points and expands the (1 X 1) areas Energetic oxygen atoms migrate and react with the carbon atoms at the edges of the ordered domains The reaction rates (collision efficiency) are lower for carbon atoms at the edges of the (&x&)R3O0-honeycomb domains and at the c(2 x4)-zigzag row domains Thus, preferential etching is probably due to different adsorption energies of carbon atoms in the two structures, i.e the adsorption energy of carbon atoms in the ( A x &)R3O0-honeycomb domain is higher than that of carbon atoms in the c(2x4)-zigzag row domains

Certain atomic protrusions were found in the ( A x &)R30°-honeycomb region after exposure to oxygen Consecutive STM imaging showed that they occupied three-fold hollow sites at the center of the triangles formed by three carbon atoms, and movement (hopping) between equivalent sites, frame by frame, was observed, assigned to oxygen atoms On the other hand, such oxygen atoms were not found on the (1x1) and the c(2x4)-zigzag row regions, suggesting high mobility of oxygen atoms in those regions rendering them invisible to STM This contrasts with immobile oxygen atoms on molybdenum metal surfaces The high mobility of oxygen atoms explains the high etching rate of carbon atoms on the surface

A mild oxygen treatment of Mo,C or WC catalysts improves their catalytic activities [1,13,30,31], probably because of the formation of oxycarbide phases Other oxide phases formed by more extensive oxidation inhibit catalysis

5 Conclusions and Future Prospects

STM is a powerful tool to image atomic arrangements of metal carbide surfaces Each surface carbon atom of C-terminated Mo,C(OOOl) surfaces can be identified as a shallow sombrero protrusion using a low tunneling resistance less than 1 MR Imag- ing of surface carbon atoms by STM elucidates chemical reactions on metal carbide surfaces The etching by oxygen of surface carbon atoms of C-terminated Mo,C(0001) surfaces was observed This etching reaction selectively occurs on the c(2 x 4)-zigzag row structure even at room temperature, leading to exposure of the underlying (1 x 1) molybdenum layer This reaction may be an important intermediate in the under- standing of improvements to catalytic activity of Mo,C or WC catalysts by mild oxidation treatments

STM images indicate the local electronic density of states near to the Fermi level of

a surface and relate to applied bias voltages between the tip and the surface Local mapping of the electronic states of metal carbide surfaces is essential to the under- standing of their physical properties and chemical reactivities STM images indicate that molybdenum atoms in the (&x~)R3O0-honeycomb structure and the ~ ( 2 x 4 ) - zigzag row structure have continuous electronic states near to the Fermi level and that the (1 x 1) molybdenum atoms without carbon atoms have localized electronic states Combinations of high resolution STM mapping with scanning tunneling spectroscopy (STS) should clarify the causes of high activities of Mo,C compared with noble metal catalysts

3 S Ramanathan and S.T Oyama, New catalysts for hydroprocessing: transition metal car- bides and nitrides J Phys Chem., 9 9 16365-16372,1995

4 L.I Johansson, Electronic and structural properties of transition metal carbide and nitride surfaces Surf Sci Rep., 21: 177-250,1995

5 A.T Santhanam, Application of transition metal carbides and nitrides in industrial tools In: S.T Oyama (Ed.), The Chemistry of Transition Metal Carbides and Nitrides, pp 28-52 Blackie Academic and Professional, Glasgow, 1996

6 J.G Chen, Carbide and nitride overlayers on early transition metal surfaces: preparation, characterization and reactivities Chem Rev., 96: 1477-1498,1996

7 J.-K Zuo, R.J Warmack, D.M Zehner and J.F Wendelken, Periodic faceting on TaC( 110): Observations using high-resolution low-energy electron diffraction and scan- ning tunneling microscopy Phys Rev B, 47: 10743,1993

8 J.-K Zuo, D.M Zehner, J.F Wendelken, R.J Warmack and H.-N Yang, TaC(ll0): a pe- riodic facet reconstruction studied by LEED and STM Surf Sci., 301: 233, 1994

9 R.M Tsong, M Schmid, C Nagl, P Varga, R.F Davis and I.S.T Tsong, Scanning tunnel- ing microscopy studies of niobium carbide (100) and (110) surfaces Surf Sci., 366: 85-92,

1996

10 M Hammer, C Tornevik, J Rundgren, Y Gauthier, S.A Flodstrom, K.L Hakansson, L.I Johansson and J Haglund, Surface atomic structure of reconstructed VC,,,,( 111) studied with scanning tunneling microscopy Phys Rev B, 45: 6118,1992

11 E Pathe and V Sadagopan, The structure of dimolybdenum carbide by neutron diffraction technique Acta Crystallogr., 1 6 202-205,1963

12 R.-L Lo, K Fukui, S Otani, S.T Oyama and Y Iwasawa, C-termicated reconstruction and C-chain structure on Mo2C(0001) surface studied by LEED and STM Jpn J Appl Phys., 38: 3813-3815,1999

13 M.J Ledoux, C Pham-Huu, H Dunlop and J Guille, n-Hexane isomerization on high spe- cific surface Mo,C activated by an oxidative treatment Proc 10th Int Cong Catal., pp

14 J Ahn, H Kawanowa and R Souda, S T M study of oxygen-adsorbed TiC(111) surface

Surf Sci., 429: 33&344,1999

15 R.-L Lo, K Fukui, S Otani and Y Iwasawa, High resolution images of Mo,C(OOOl)-(

& x&)R30° structure by scanning tunneling microscopy Surf Sci., 440: L857-L862,1999

16 K Fukui, R.-L Lo, S Otani and Y Iwasawa, Novel selective etching reaction of carbon at- oms on molybdenum carbide by oxygen at room temperature visualized by scanning tunnel- ing microscopy Chem Phys Lett., 325: 275-280,2000

17 R Young, J Ward and F Scire, Metal-vacuum-metal tunneling, field emission, and the transition region Phys Rev Lett., 27: 922-924, 1971

18 R Young, J Ward and F Scire, The Topografiner: An instrument for measuring surface microtopography Rev Sci Instrum., 43: 999-1011,1972

19 G Binnig, H Rohrer, Ch Gerber and E Weibel, Tunneling through a controllablevacuum gap Appl Phys Lett., 40: 178-180, 1982

20 G Binnig, H Rohrer, Ch Gerber and E Weibel, Surface studies by scanning tunneling mi- 955-967,1993

Atomic-scale Structure and Reactivity of Metal Carbide &$aces 267

croscopy Phys Rev Lett., 49: 57-61, 1982

J Cryst Growth, 154: 202-204,1995

Iwasawa, Characterization of a-Mo,C(OOOl) Surf Sci., 426: 187-198,1999

21 S Otani and Y Ishizawa, Preparation of Mo2C single crystals by the floating zone method

22 T.P St Clair, S.T Oyama, D.F Cox, S Otani, Y Ishizawa, R.-L Lo, K Fukui and Y

23 NIST Surface Structure Database, ver 3.0

24 E.I KO and R.J Madix, Adlayer effects on adsorption/desorption kinetics: N2, H,, C,H,,

and CO on Mo(lOO)-C Surf Sci., 1 0 0 L449-U53,1980

25 K Fukui, T Aruga and Y Iwasawa, Chemisorption of CO and H2 on clean and oxy-

gen-modified Mo(112) Surf Sci., 281: 241-252,1993

26 G.S Ranhotra, A.T Bell and J.A Reimer, Catalysis over molybdenum carbides and ni-

30 E Iglesia, J.E Baumgartner, F.H Ribeiro and M Boudart, Bifunctional reactions of al- kanes on Tungsten carbide modified by chemisorbed oxygen J Catal., 131: 523-544,1991

31 A Muller, V Keller, R Ducros and G Maire, Catalytic activity and XPS surface determi-

nation of tungsten carbide for hydrocarbon reforming Influence of the oxygen Catal Lett., 35: 65-74,1995

269

Chapter 17

Infra-Red Spectra, Electron Paramagnetic Resonance,

and Proton Magnetic Thermal Analysis

Osamu Ito", Tadaaki Ikoma" and Richard Sakurovsb

"Institute of Multidisciplinary Research for Advanced Materiak, Tohoku University, Katahira, Sendai 980-8577, Japan; "CSIRO, Division of Energy Technology, Noah Ryde 1670, Australia

Abstract: This chapter describes the characterization of carbon alloys using infra-red

spectroscopy (IR spectra), electron paramagnetic resonance (EPR) and proton magnetic resonance thermal analysis (PMRTA) The broad absorption bands of IR spectra observed using diffuse reflectance methods provide information about the ring size of aromatic molecules within a sample and the extent to which these are ordered The sharp C-H stretching peaks are quantitatively compared with peaks using solid-state NMR Two magnetic resonance methods employed pulse techniques where relative hydrogen contents are evaluated as ratios

to carbon contents PMRTA, which measures the temperature dependence of signal intensities and relaxation times of proton magnetic resonance, provides information about molecular motion in heat-treated carbons and in coals These spectroscopic techniques give information about the composition of carbon precursors prepared at temperatures below 500400°C

Keywords: IR, UV, Vis, Near-IR, NMR, EPR, PMRTA

1 Infra-Red (IR) Spectra

I I D i m e Rejkctance Absorption (DR4) Spectroscopy

IR spectra of carbons from the KBr pellet transmittance method contain a broad background (from scattering ofthe monitoring light) that needs to be subtracted from the spectrum With decreasing particle size these broad backgrounds are reduced and must be subtracted to obtain reliable IR spectra An example is shown in Fig l a for a bituminous coal [l] For IR spectra from the diffuse reflectance absorption (DRA)

method, the absorption intensities, represented as a Kuberka-Munk function (f(R,)

or F(R,)) show weak broad bands in the higherwave-number region (Fig lb) By the DRA method, high rank coals, heat-treated organic materials and coal-tar pitches (CTP) also show broad absorptions attributable to the absorption tail of electronic transitions extending from the W N i s regions [2] Such broad bands are observed by

Wavenumber (om-')

Fig 1 IR spectra of a bituminous coal (sample/KBr = 1/150): (a) transmittance KBr pellet method; (b)

diffise reflectance absorption method [l]

DRA and increase with decreasing particle size of the sample [2] Thus, it is not necessary to subtract these broad absorptions in the entire region measured by the DRA method

FT-IR spectra observed by DRA for various coals are shown in Fig 2 [2] The baseline of a low rank coal (Monvell coal: C = 67.4 wt%) is flat in the region of

4000-6000 cm-' For bituminous coal (Oyubari coal: C = 85.4 wt%), small absorp- tions were observed at 4000-6000 cm-' with a decrease in the IR-band intensities of

hydrogen bonds at 3000-3500 cm-' and carbonyl groups at 1700 cm-' With a further increase in coal rank, Moura (C = 85.6 wt%) and Hongei (C = 93.7 wt%), the absorption tails extend to lower wave-numbers with a decrease in intensity of alkyl group band at 2940 cm-' For active carbon, only the broad band was observed, the absorption edge extending to wave-numbers less than 1000 cm-', indicating a narrow band gap in the active carbon Absorptions in the near-IR region are used as a measure of coal rank [2]

For a bituminous coal (Shin-Yubari: C = 86.9 wt%), the intensity of the broad band increases with increasing heat treatment temperature (HTT) to 400°C

accompanied by a decrease in the intensities of the alkyl bands At an HTT of 6OO0C,

the steep rise of the near-IR absorption ceases when it then resembles the absorption spectrum of activated carbon as in Fig 2 These changes in absorption spectra in the near-IR region are used to study effects of HTT on organic materials

On heat-treating decacyclene to 5OO0C, the broad band in the near-IR region appeared after 1 h increasing in intensity after 2 h, but decreased after 3 h At an HTT

IR Spectra, Electron Paramagnetic Resonance, and Proton Magnetic Thermal Analysis 271

Fig 2 IR/near-IR spectra of several coal samples [2]

of 550°C, the absorption spectrum in the near-IR region is flat showing decreases in the intensities of the C-H bands These changes in the near-IR region relate to absorption bands in the Vis regions (see below)

1.2 Relutionship of IR spectra with UVlEslNear-IR Spectra

On heat-treatment of aromatic hydrocarbons, polycondensation reactions occur resulting in formation of mesophase which is the precursor of graphitizable carbon fibers and carbons Changes in the molecular structures and molecular arrangements during this process are monitored as electronic transitions in the Vis and near-IR region

I

Fig 3 Correlation of observed p-band wave-number with calculated energy gap (A&) between LUMO and

HOMO in p-unit for several hydrocarbons [3]

The absorption spectra of solid carbon precursors are measured using the KBr-CsI pellet transmittance method At an HTT of 5OO0C, decacyclene first forms mesophase spheres which coalesce into anisotropic flow textures The absorption band of parent decacyclene is only slightly broader than when in solution, because of intermolecular interactions in a concentrated solution and the solid state With heat-treatment, the absorption bands at wavelengths longer than 500 nm increase in intensity Inter- mediates of the carbonization reactions of decacyclene are zethrene derivatives, with absorption bands beyond 550 nm The relationship between the energy gap from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) and the absorption maximum at longest wavelength of the aromatic hydrocarbons is shown in Fig 3 By comparing the observed absorption bands with Fig 3, it is seen that a zethrene dimer is a main component in pyrolyzed decacyclene

[3] For coal-tar pitch, the diffusion reflectance method gives more reliable absorp- tion spectra in Vidnear-IR region compared with the transmittance method

At 500"C, coal-tar pitch forms mesophase spheres [4], a process described by the absorption spectra in Vis/near-IR/IR region as measured by the DRA (Fig 4) Here, the absorption intensities of the Vis region increase with heat-treatment time (1 h) The absorption intensity in the 300-500 nm region reaches a maximum at -3 h and then decreases The initial increase in absorption intensity is attributed to the aroma- tization of the coal-tar pitch, the later decrease in intensities being due to the broadening of absorption bands because of inter-aromatic interactions The

I R Spectra, Electron Paramagnetic Resonance, and Proton Magnetic Thermal Analysis 273

wavenumber x1~3cm-1

Fig 4 Separation of the observed spectra into intensities due to lamella (L-part) and to isolated aromatic

hydrocarbons (A-part) Inset: Shaded parts are plots of Eq (1) [4]

absorption band of the lamellar aromatic hydrocarbons is described by the following

equation, which has been applied to amorphous semiconductors [5]:

where AE is the band gap energy and B to a proportional factor increasing with extents of ordering of the lamellar aromatic hydrocarbons The straight line of the

inset in Fig 4 shows values as calculated from Eq (1) The hatched area in Fig 4

results from absorption by lamella (L-part), the remaining white area being due to absorption by single aromatic molecules (A-part) After heat treatment for 0.5 h, the L-part increases, but the A-part decreases, indicating increased interactions between aromatic molecules due to the improved alignment and ordering of the aromatic molecules

1.3 Relationship with Solid-state NMR

13C-NMR spectra of solid carbon samples are measured using magic-angle NMR However, the NMR signals for carbons and coals samples that are rich in quaternary

carbon atoms surrounded by other carbon atoms exhibit ‘side-bands’ In the

13C-NMR signals from coronene, the main signal is at 125 ppm, but with side-bands in the wings of the 125 ppm signal (Fig 5a) [l] The positions and relative intensities of

the side-bands change with the rotation speed of the sample probe NMR spectra of

coals observed under the same conditions are composed of signals and side-bands (Fig 5b), the principal signal, 100-150 ppm, being assigned to aromatic carbon atoms

(C) corrected spectrum

sideband sideband

To distinguish further aromatic carbon atoms and carbon atoms in alkyl groups, the technique of dipolar dephasing is used Thus, from a spectrum observed at Tdd =

0, in which the signal has not decayed, signals are obtained (Fig 6), in which

side-bands have already been subtracted At Tdd = 40 ps, that is the decay time of components with shorter relaxation times, carbon atoms with long relaxation times such as quaternary C, CH,, mobile CH,, and polar carbon atoms, remain The difference between the total signal and the signal with long relaxation times is assigned to signals of short Tdd such as aromatic CH, aliphatic CH and rigid CH,

In order to determine the absorption coefficients of the FT-IR bands corres- ponding to these isolated NMR signals, the IR bands need to be curve-resolved For

IR Spectra, Electron Paramagnetic Resonance, and Proton Magnetic Thermal Analysis 275

fraction of carbon per total carbon [l]

the same coal, the FT-IR spectra are shown in Fig 1 [l] Figure lb, observed by the DRA method, shows the aromatic C-H band at 3030 cm-' and aliphatic groups in the 2800-3000 cm-' region The aromatic C-H band at 3030 cm-' is hidden in the broad scattering bands of the IR spectra obtained by the KBr transmittance method (Fig la) The shadowed signals in the region of 2700-3500 cm-' in Fig l b are curve- resolved as shown in Fig 7 The aromatic C-H has two peaks at wave-numbers higher than 3000 cm-' The CH, group also has two peaks at 2850 cm-' and 2930 cm-' The

CH, group has two peaks at 2880 and 2960 cm-' The alkyl C-H has one band at 2870 cm-' The sum of the band areas of each group is plotted against the corresponding NMR signal areas The same plots are made for several coals and model compounds Then, linear relationships between the NMR signal intensity and the IR peak area were obtained By using the gradients of these plots, IR intensities obtained by the DRA method are converted directly to a weight percentage of different carbon atoms

in coals [l]

For the coal-tar pitch, it is more difficult to separate the solid NMR signals from the side-bands in the alkyl C-H region because of strong side-bands and weak alkyl C-H signals It is also not easy to evaluate alkyl C-H bands from the IR spectra, because of quite weak alkyl C-H bands relative to the strong aromatic C-H bands Coal-tar pitches require different treatments

Fig 7 Curve-resolution of the C-H stretching region of the IR spectrum of a bituminous coal [l]

2 EPR

Amorphous carbons usually give single symmetric signals when using continuous wave (CW)-EPR Spin concentrations are evaluated by comparing the signal intensity with that of a standard sample The magnetic field position of the signal corresponds

to the ‘g-value’ of the paramagnetic species which derives from the free electron value

of 2.00023 as a result of increases in spin-orbit interactions Because heteroatoms induce strong spin-orbit interactions, g-values measure the presence of heteroatoms

included in the radical content of carbon The line-width (AHpp) of an EPR signal is related to the spin relaxation times ( T , and T2), which are strongly influenced by the presence of adsorbed oxygen The relative relaxation time (TJ is evaluated, quanti-

tatively, from the dependence of signal intensity upon incident microwave power (P)

Measurements of these EPR parameters for a coal-tar pitch are given in Fig 8 [6]

The plot of the peak-peak signal height (hpp) against log P is normally a parabolic

curve The microwave power at the maximum signal intensity (P,,,=) and that at half the maximum signal intensity (P,J are known The P,, and P,n parameters closely

relate to spin relaxation times

IR Spectra, Electron Paramagnetic Resonance, and Proton Magnetic Thermal Analysis 277

against log P; P,, and P,, are shown by arrow [7]

EPR parameters obtained by in-situ measurements of coals and a coal-tar pitch

change with heat-treatment temperature [7] On cooling immediately after heating to

500"C, the log (1/Pma) values return to almost the same values as observed before heating the sample, indicating that the mobility of the molecules in the coal-tar pitch does not change appreciably after heat-treatment at 500°C for a short time On the other hand, when the coal-tar pitch is kept at 500°C for 2 h, UP,,,, changes drastically, indicating a significant change in the mobilities of the constituent molecules For

AHpp, irreversible changes were observed for a sample maintained at 500°C for 2 h, indicating that chemical changes took during the heat-treatment The observed increase in the signal intensity with increasing temperature suggests that chemical reactions, producing free radicals, took place at relatively low temperatures P,, and

P,,* differ for coals of different rank and maceral contents [6]

2.2 Pulsed-EPR

Many microwave pulse sequences are developed for pulsed EPR, by which new EPR and electron nuclear double resonance (ENDOR) spectra are measured This section introduces two examples of pulsed EPR applications to carbonaceous solids The nutation spectrum for a coal sample is shown in Fig 9a, which uses a pulse sequence due to two-pulse echo detection (Fig 9b) [8,9] The nutation is a rotational motion of

a magnetization vector caused by the interaction with microwave pulse (B,) in a rotation axis system For the weak limit of B , compared with zero-field splitting due to interactions between spins, the nutation frequency (v,), for EPR allowed transitions,

is analytically written as follows:

v, =y,B,.JS(S+l)-M,(M, -1)

Here, ye is the magnetogyric ratio for electron spin; v, depends on the spin multiplicity

( S ) and the spin sublevels (M,) resonating with the applied microwave Therefore, this

10.01 MHz, respectively) estimated from a comparison with DPPH (doublet species) and (b) pulse

sequences for 2D-nutation in which microwave power (= 1 kW) was adequately attenuated to create 90"

pulses [SI

IR Spectra, Electron Paramagnetic Resonance, and Proton Magnetic Thermal Analysis 279

nutation spectrum is useful to distinguish between signals from doublet (free radical) species and triplet species As indicated by arrows in Fig 9a, the signal detected for the coal sample is almost entirely due to the doublet species There is little evidence for triplet species in this coal Similar spectra, lacking in triplet states, are also observ-

ed for the coal-tar pitch

Another pulsed EPR method is hyperfine sublevel correlation spectroscopy

(HYSCORE) which is two-dimensional electron spin echo envelope modulation

(ESEEM) that allows the detection of a broad hyperfine spectrum from disordered

carbons Figure 10a illustrates an electron spin ( e ) and a nuclear spin (n) in an

external magnetic field (Bo) The e-spin is almost quantized along Bo, but the n-spin orients to the effective field (Be,,) composed of Bo and the magnetic dipole field (Bhf)

due to the e-spin B,, reverses its direction, leading to a sudden change inB,,, when the

resonance with the e-spin takes place due to a strong microwave pulse (Fig lob) As a

consequence, the n-spin starts to precess along the new Befp This periodic motion of

the n-spin induces an oscillating local field (B,,,,,(t)) that modulates the intensity of

electron spin echo (ESE) This coherent interaction between the electron and nuclear spins is a basic mechanism for ESEEM and is called a nuclear modulation effect

Hence, the spectrum in the frequency domain obtained by Fourier transformation of

I \

._*'

Y

Bhf

Fig 10 Vector model of the electron-e and nuclear-n spins: (a) in thermal equilibrium; and (b) after

applying microwave (MW) pulse

Fig 11 (a) HYSCORE spectra of 'H and "C for a coal-tar pitch; 7 = 128 ns and measurement temperature

of 298 K, (b) pulse sequences for HYSCORE [lo]

ESEEM is attributed to an alternative of ENDOR that enhances spectral resolution substantially

The spectra of CTP observed by HYSCORE are shown in Fig l l a [lo] and were measured using the four pulses shown in Fig l l b The nuclear Zeeman frequencies

(v, and v,) of 'H and I3C are 14.7 and 3.7 MHz, respectively, at the external magnetic filed for the usual pulsed EPR experiment The ENDOR frequencies of v, and v, in Fig l l a indicate the energy separation among the nuclear spin sublevels, which include Zeeman and hyperfine interactions The broad ridge-type signals crossing at the diagonal point of (v,, v,) run perpendicular to the diagonal v, = v2 Such broad ridge-type signals are identified as the hyperfine spectrum due to 'H The other broad ridge around (v,, v,) is assigned to 13C-hyperfine spectrum The signals on the v, = v,

axis are mainly due to noise From a comparison with various model radicals, the

intensity ratio of the I3C/'H of HYSCORE spectrum affords a good measure of the molecular size of the radical The intensity ratio of I3C/'H of CTP is larger than that of the cation radical of coronene [SI This indicates that the average size of molecules included in coal-tar pitch is larger than that of coronene The I3C/'H ratio of CTP increases on heat treatment, indicating that the radical size increases on heating On heating a coal-tar pitch with iodine, the 13C/*H ratio in the HYSCORE spectrum also increases, indicating an acceleration of dehydrogenation at relatively low tempera- tures from the charge transfer complexes formed between iodine and aromatic