Complementary use of cw-EPR, HYSCORE and pulsed ENDOR spectroscopies for scanning the environment of...

Complementary use of cw-EPR, HYSCORE and pulsedENDOR spectroscopies for scanning the environment of

unpaired states in a- and c-B2O3

George Kordas *

Institute of Materials Science, National Center for Scienti®c Research, Demokritos, Aghia Paraskevi Attikis, 15310 Athens, Greece

Received 14 December 1998; received in revised form 14 July 1999

Abstract

Paramagnetic states in glass (a-) and crystal (c-) B2O3 were induced by c-irradiation and studied by cw-EPR,

HYSCORE and pulsed-ENDOR spectroscopies at 20 K. A `four-line-plus shoulder' spectrum was detected by cw-EPR

in a- and c-B2O3. The cw-EPR spectra were simulated assuming AI a-B2O3� (39.44, 43.45, 22.00 MHz) and

AI c-B2O3� (20.44, 40.00, 32.00 MHz). Pulsed ENDOR revealed a strong coupling AI-ENDOR� (ÿ25, ÿ36, ÿ46 MHz) for

a- and c-B2O3. A weaker coupling was detected by HYSCORE and pulsed ENDOR spectroscopies with

AII a-B2O3 and c-B2O3� (5.5, 5.6, 6.4 MHz). HYSCORE spectroscopy determined two weak hyper®ne couplings of

AII a-B2O3-1� 5.8 MHz and AII a-B2O3-2� 2 MHz when recorded with a s-value of 248 and 168 ns, respectively. Computer

simulation of the spectra together with SCF±HF and MNDO calculations determined that the unpaired electron is

trapped by an oxygen-dangling bond attached to three-fold coordinated boron. The hfs tensors AII a-B2O3-1 and AII a-B2O3-2

for a-B2O3 originate from an adjacent 11B atom within two di�erent boroxol rings. The hfs tensor AII c-B2O3was at-

tributed to the interaction of the unpaired electron with the next 11B atom that is three-fold coordinated. We report here

the complementary use of the cw-EPR, HYSCORE, and pulsed-ENDOR spectroscopies in detecting hyper®ne cou-

plings in a glass and crystal. Ó 1999 Elsevier Science B.V. All rights reserved.

1. Introduction

B2O3 glass was studied in the literature usingNMR [1], NQR [1±3], IR [4], Raman [5], andneutron di�raction [6] techniques. These spectros-copies provided considerable evidence for the ex-istence of rings in the form of boroxol (B3O3)although their concentration was resolved lately[7]. There is a consensus among the variety of thespectroscopic techniques that the fraction of boron

in boroxol rings amounts to 0.65 and so the frac-tion of boroxol rings themselves is 0.38. The re-maining boron is in the non-ring BO3 units servingas connectors between boroxol rings. Contrary toa-B2O3, the boroxol rings are absent in c-B2O3.NMR [8] and X-ray [9] data showed that in c-B2O3

the boron atoms have planar trigonal coordina-tion. The BO3-units are connected over the cornersin the form of long chains [9]. Theoretical evidencewas provided by MO calculations for IR [10,11],Raman [11] and quadrupole coupling constant[12].

cw-EPR spectroscopy was used for the investi-gation of the paramagnetic states induced in bo-

Journal of Non-Crystalline Solids 260 (1999) 75±82

www.elsevier.com/locate/jnoncrysol

* Tel.: +30-1 65 03301; fax: +30-1 65 47690.

E-mail address: [email protected] (G. Kordas).

0022-3093/99/$ - see front matter Ó 1999 Elsevier Science B.V. All rights reserved.

PII: S 0 0 2 2 - 3 0 9 3 ( 9 9 ) 0 0 5 6 7 - 0

rate glasses and crystals to determine their localstructure [13]. Two centers were isolated in theborate glasses ± one dominates below 25 mol%alkali (center I) and the other above 25 mol% al-kali (center II) (see Table 1).

Center I was named as Boron Oxygen HoleCenter (BOHC), the signal of which consisted of`®ve lines plus shoulder' and is inhononeneouslybroadened due to distribution of the g-values andthe anisotropic hyper®ne coupling [13] (see Table2). c-B2O3 exhibits the same cw-EPR spectrumincluding the low-®eld shoulder [14].

The cw-EPR spectrum of the a-B2O3 exhibits ahyper®ne coupling Aiso� 35 MHz due to the in-teraction between the electron spin and a single 11B(I� 3/2) nucleus [13±18]. MO calculations for theB2O3 glass led to the assignment that the unpairedelectron is localized at non-bridging oxygen at-tached to a three-fold coordinated boron [14]. TheMO calculations revealed negative sign for the hfsdue to a spin-polarization mechanism [14]. Thesimilarity of the cw-EPR spectra between c- and a-B2O3 suggested an analogous model for c-B2O3

[14].In recent years, signi®cant advances occurred in

theory and instrumentation, enhancing the abilityof the EPR spectroscopy to provide detailed de-scription not only of the short-range order but alsoof the long-range order of glasses. These new EPRspectroscopies including the Electron Spin EchoEnvelop Modulation (ESEEM) spectroscopy [19±21] and four-pulse two-dimensional Hyper®neSublevel Correlation Spectroscopy (HYSCORE)[22±24] are well adapted for determining weakhyper®ne interactions in a-B2O3 [14] which are not

resolved by cw-EPR [13]. The combination ofmolecular orbital calculations, the ESEEM andHYSCORE data led to a detailed structural modelfor the defects in B2O3-glass [14]. Time domainanalysis of electron spin echo modulation enve-lopes in lithium silicate glasses revealed the num-ber of ions surrounding the paramagnetic states upto distances of 7 �A [25].

The cw-ENDOR technique was introduced byFeher [26] in order to determine the hfs of 13P inphosphorus doped silicon. This technique waswidely used to resolve the structure of organome-tallic complexes [27,28], of proteins and enzymes[29], and of organic radicals [30]. Besides the cw-ENDOR technique, basically two-pulsed ENDORtechniques were devised, one by Mims [31] and theother by Davies [32]. For both, the ENDOR re-sponse is expressed as a change in the intensity ofthe electron spin echo due to shifts of electron spinpackets caused by nuclear resonance transitions.There are several monographs reporting the ap-plication of the pulsed ENDOR techniques inseveral materials [33,34]. Until now, there is not asingle paper dealing with pulsed ENDOR spec-troscopy in glasses.

In the present work, HYSCORE and pulsedENDOR spectroscopies were used to study thedefects occurring in c- and a-B2O3. The cw-EPRdata were analyzed elsewhere [14] and will not bediscussed here. The spin densities of the variousunits were calculated by the SCF±HF methodusing the Gaussian 98W program [35, 36]. Prior toG98W calculations, the structures were optimizedusing the AMPAC [37], MOPAC [38], andSPARTAN [39] commercial packages. The HY-

Table 1

Glass composition g1 g2 g3 A1 (G) A2 (G) A3 (G) Aiso (G)

Center I <25% 2.0020 2.0103 2.0350 12.1 14.2 10.0 12.1

Center II >25% 2.0049 2.0092 2.0250 11.2 12.9 8.0 10.7

Table 2

cw-EPR g1 g2 g3 A1 (MHz) A2 (MHz) A3 (MHz) LW (G) Aiso

c-B2O3 2.0000 2.0107 2.0415 20.44 40.00 32.00 10.00 30.81

a-B2O3 2.0025 2.0118 2.0370 39.44 43.45 22.00 10.00 34.96

76 G. Kordas / Journal of Non-Crystalline Solids 260 (1999) 75±82

SCORE and pulsed ENDOR spectra were simu-lated using our own code described extensively inprevious publications [14,40,41].

2. Experimental

B2O3 glasses were prepared by melting H3BO3

in a platinum crucible at 1200°C. The single-phaseB2O3 crystal was produced by a published proce-dure [9].

The cw-EPR spectra were recorded at 20 Kusing a 300 E Bruker instrument. The pulsed EPRwork was carried out with a Bruker ESP380spectrometer and with a Bruker ESP380-1078 INecho-integrator. The dead time of the instrumentwas about 100 ns. The pulsed ENDOR measure-ments were done with a Bruker ESP360 DICEENDOR system and an RF ampli®er. A counterwas used for microwave frequency measurements.The three-pulse ESEEM data were measured atmaximum ®eld determined by ®eld swept spectrumas follows:

p=2�16 ns�±s�120±600 ns�±p=2�16 ns�±T �40� dT �� 8 ns��±p=2�16 ns�:

The HYSCORE spectra were recorded usingthe sequence

p=2�16 ns�±s�104; 168; 240 ns�±p=2�16 ns�±t1�56� dt�� 16 ns��±p�32 ns�±t2�56� dt�� 16 ns��±p=2�16 ns�±echo:

Phase cycling was employed to remove the un-wanted echoes in the Bruker Pulse Spel library.

Pulsed-ENDOR was carried out using the Da-vies sequence [32]

p�80±320 ns�±T1�1000 ls�±p�rf 6000 ls 6 dB�±T2�3000 ls�±p=2�64 ns�±�s � 144 ns�±p�128 ns�±s±echo:

Fig. 1 shows the electronic structure on whichthe calculations were based.

3. Results

3.1. Hyscore spectroscopy

Fig. 2 gives the HYSCORE spectrum of the c-B2O3 recorded with s� 248 ns. Fig. 3 gives theHYSCORE spectra of a-B2O3 with s� 168 and

Fig. 1. Energy level diagram for S� 1/2 and I� 3/2 at constant

®eld.

Fig. 2. HYSCORE spectrum (contour plots) recorded at

s� 248 ns for the crystal. Position of the cross peaks (6.7 MHz,

2.7 MHz) and (2.7 MHz, 6.7 MHz).

G. Kordas / Journal of Non-Crystalline Solids 260 (1999) 75±82 77

248 ns, respectively. The HYSCORE spectraconsist of cross peaks at Larmor frequencies of 11B(4.8 MHz) in the (+, +) quadrant. The cross peakswere assigned to jm1 � �1=2 > () jm1 � ÿ1=2> �2() 3� transitions (Fig. 1) [14]. This wasveri®ed again in the present paper. Fig. 4 shows atheoretical HYSCORE spectrum using theparameters of Table 3. The program allowscalculating speci®c transitions individually.Speci®cally, the cross peaks of Fig. 4(A) corres-pond to transitions 2() 3 of Fig. 1. Fig. 4(B)results from the calculation of all transitionstogether. Only Dm1 � 1 (single quantum) contrib-uted to the cross peaks of the HYSCORE spectra.Other transitions (e.g. Dm1 � 2) occur outside thisregion with negligent intensity. Based on thistheoretical work, the cross peaks of theHYSCORE spectra around the 11B Larmorfrequency are mainly generated by 2() 3 tran-sitions. Thus, the Aiso values can be obtaineddirectly from the spectra.

Taken the frequency coordinates, Aiso is equalto 5.6±6.0 MHz for the crystalline material

(Fig. 2). For the glass, two cross peaks (Fig. 3)were observed corresponding to Aiso values equalto 2 and 6 MHz.

Fig. 4. Calculated HYSCORE spectrum allowing only transi-

tions between 2() 3 state (A) and all states (B).

Fig. 3. (A) HYSCORE spectra (contour plots) recorded at s� 168 ns for the glass. (B) HYSCORE spectra (contour plots) recorded at

s� 248 ns for the glass. Position of the cross peaks (1) (7.2 MHz, 2.2 MHz) and (2.2 MHz, 7.2 MHz) (2) (5.8 MHz, 3.5 MHz) and (3.5

MHz, 5.8 MHz).


3.2. Pulse ENDOR spectra

Fig. 5 gives the pulse ENDOR spectra ofthe crystal and glass. There are two sets ofpeaks centered above and below 9.6 (2 vI) MHz.These peaks correspond to a strong (Aiso > 2 vI)and weak coupling (Aiso < 2 vI), respectively. Thecouplings belong to the interaction of the electronspin with the 11B-nucleus. The pulse ENDORspectra were simulated using the parameters ofTable 4. The results of the simulation are displayedin Fig. 5.

Due to small transition probability, low fre-quency peaks are very di�cult to detect. There-fore, the result of the simulation for this pair ofpeaks was left out from Fig. 5. The results of Table4 must be treated with caution for the low fre-quency pair.

3.3. SC±HF calculations

The G98W program was used for the Aiso cal-culations. The calculations were performed using

Fig. 5. Davies ENDOR spectrum of crystalline and amorphous

B2O3 (dashed lines) and calculated (solid lines).

Table 3

Parameters used for the HYSCORE simulation

T (ns) I g1 g2 g3 v (MHz) Q (MHz) g T Aiso

168 3/2 2.0025 2.0118 2.0370 4.7820 2.70 0.06 0.4 6.0

Fig. 6. (A) Electronic and equilibrium structure of four OBO2

triangles in a chain calculated by G98W. (B) Electronic and

equilibrium structure of an oxygen dangling bond associated

with a boroxol ring calculated by G98W. (C) Electronic and

equilibrium structure of an oxygen dangling bond associated

with an OBO2 triangle attached to two boroxol rings calculated

by G98W.


the 6-311G-basis set at the B3LYP approximationlevel providing high accuracy of the estimatedvalues. The calculations started using a simpleBO3H2 unit. This unit gives an Aiso of ÿ36.80MHz. In the crystal, two distinct di�erentBm�1;2Om�1;2

n�1;2;3 triangles compose the structure of c-B2O3 [9]. Thus, six di�erent Om�1;2

n�1;2;3-sites can berealized at which the unpaired electron can be lo-calized. Table 5 gives the Aiso values derived by thesix di�erent sites inserted into the program. Table5 gives the geometry inserted into the program.

Fig. 6(A) presents the structure of the crystalconsisting of four BO3 triangles. The Aiso values arelisted in the ®gure. The unpaired electron is local-ized nearly 100% at the non-bridging oxygen site.

Fig. 6(B) and (C) shows the optimized structuresof two boroxol ring variants that might be present inthe glass. Again, the unpaired electron is localized atthe oxygen-dangling bond. The expected hyper®neconstants are displayed in Fig. 9(B) and (C).

4. Discussion

The g-values deduced from the simulation ofthe cw-EPR spectra detected in a- and c-B2O3 arethe same within the error of measurements asthose reported in various silicate, titanate, ger-manate, and aluminate glasses [14]. In these oxideglasses, nearly all spin density was localized at the

oxygen-dangling bond. Therefore, we suggest thatthe unpaired electron be localized at the oxygen-dangling bond in both glass and crystal [14]. This®nding agrees very well with our SCF±HF calcu-lations.

XRD study of c-B2O3 demonstrated that onlyBO3 units are present in the crystal [9]. Based onthis result, the structural parameters were insertedin G98W program and the spin densities werecalculated. The calculations revealed the Aiso

values of ÿ31.5, 5.7, and <1 MHz for the ®rst,second, and the third neighbor, respectively. Thesevalues were veri®ed by cw-EPR, HYSCORE andpulsed ENDOR spectroscopies.

Speci®cally, the simulation of the cw-EPRspectrum gave Aiso-I-B2O3

� 30.81 MHz [14]. Thisvalue is the same within the error of measurementsto the unit of Fig. 6(A). The simulation of thepulse ENDOR spectrum gave A1�ÿ25.00 MHz,A2 � ÿ36:00 MHz, A3 � ÿ46:00 MHz andAI-ENDOR � ÿ35:7 MHz. This value is predicted forsingle boron oxide triangle that gives ÿ35.02 � 4MHz. Furthermore, the pulsed ENDOR spectrumconsists of a low frequency spectrum. Althoughthe simulation cannot be considered as accuratedue to the missing low frequency peak, an Aiso

coupling of 5.8 MHz can be suggested for thisinteraction. The theory gives for the second 11Bneighbor Aiso� 5.7 MHz (Fig. 6(A)). This is anexcellent agreement between the SCF±HF

Table 5

Aiso values for the six di�erent Om�1;2n�1;2;3-sites of the two Bm�1;2Om�1;2

n�1;2;3 triangles occurring in c-B2O3 [9]

Aiso (average) (MHz) ÿ35.02

Om�1;2n�1;2;3 m� 1, n� 1 m� 1, n� 2 m� 1, n� 3 m� 2, n� 1 m� 2, n� 2 m� 2, n� 3

Aiso (MHz) ÿ34.78 ÿ37.91 ÿ31.91 ÿ32.30 ÿ39.04 ÿ34.31

m n� 1 n� 2 n� 3 n� 1, l� 2 n� 2, l� 1 n� 3, l� 1

1 1.337 1.366 1.404 120.1 119.0 114.7

2 1.384 1.336 1.401 124.1 121.5 113.8

Table 4

Parameters used to simulate the pulse ENDOR spectra

Strong coupling Weak coupling

A1 (MHz) A2 (MHz) A3 (MHz) A1 (MHz) A2 (MHz) A3 (MHz)

5.5 5.6 6.4 ÿ25.00 ÿ36.00 ÿ46.00


calculation and the pulse ENDOR spectroscopy.This result was also veri®ed by HYSCORE spec-troscopy. HYSCORE spectroscopy givesAiso� 5.4±6.0 MHz. This coupling correspondsagain to the second 11B neighbor based on thetheory. Furthermore, HYSCORE detects a peakwith Aiso < 1 MHz. This value corresponds to theexpected Aiso for the third neighbor (Fig. 6(A)).The overall experimental data are consistent withthe theoretical calculations and literature obser-vations [9].

The simulation of the cw-EPR spectra gave forthe hfs couplings the values A1� 39.44, A2� 43.45,A3� 22.00, and Aiso-I-a-B2O3

� 34.96 MHz [14]. ThisAiso value can be attributed to an unpaired electroninteracting with the nucleus of a three-foldcoordinated 11B (Fig. 6(B) and (C)). This valuewas also observed by pulse ENDOR spectroscopyin the same glass. The simulation of the pulsedENDOR spectra gave AI-ENDOR of ÿ35.7 MHz(ÿ25, ÿ36, ÿ46 MHz). This suggests that theunpaired electron is trapped by an oxygen dan-gling bond attached to a tree-fold coordinated 11B.Besides the strong coupling, two-week hfscoupling were observed for the B2O3 glass. The®rst coupling was AII-a-B2O3-2� 1.9 MHz and thesecond around AII-a-B2O3-1� 5.5±6.0 MHz. In orderto explain the recorded AII-a-B2O3-1 and 2-values,various units were optimized involving boroxol-ring structures. In conclusion, the AII-a-B2O3-1 and

2-values can be reproduced assuming thetwo structures of Fig. 6 (B) and (C). These twocouplings can be attributed to the interaction be-tween the unpaired electron and the nucleus of 11Bwithin the boroxol ring structures. The twodi�erent boroxol ring structures account for thestrong and weak couplings determined byHYSCORE and pulse ENDOR spectroscop-ies. Furthermore, HYSCORE gives a third hfc withAiso < 1 MHz. The theoretical analysis reproducesthis value assuming an interaction of the unpairedelectron with the third 11B neighbor.

The nuclear quadrupole parameter was takenequal to 2.7 MHz for the simulations of bothHYSCORE and pulsed ENDOR spectra. Thisresult is in good agreement between our work andthe work reported by NQR [8]. The NQRspectrum demonstrated a quadrupole parameter of

2701.10 kHz. The asymmetry parameter, g,was taken to be equal to 0.0667 as in the NQRstudy.

5. Conclusions

cw-EPR, HYSCORE, and pulse ENDORspectroscopies mapped the structure of amorphousand crystalline B2O3. This occurred through anevaluation of the Aiso parameters using simulationcodes developed in our group. The experimentalresults were supported by SCF±HF calculations.The combined use of advanced EPR techniquescan lead to new opportunities in materials scienceto study the structure of defects. The advancedmethods extend the sensitivity to energy, spatial,and time resolution of the cw-EPR techniques.This paper also demonstrates for the ®rst time thatpulse ENDOR can be used in glasses contrary tocw ENDOR spectroscopy.

Acknowledgements

The author thanks the Greek General Secre-tariat for Research and Technology and the Eu-ropean Community for funding under EGET II296 and STRIDE 130 and 115 programs. Theauthor thanks Dr R.A. Smith for providing thecrystal.

References

[1] D. Kline, P.J. Bray, H.M. Kriz, J. Chem. Phys. 48 (1990)

5277.

[2] D.L. Lee, S.J. Gravina, P.J. Bray, Z. Naturforsch. 45a

(1990) 268.

[3] P.J. Bray, in: A.C. Wright, S.A. Feller, A. Hannon (Eds.),

Borate Glasses, Crystals, and Melts, vol. 1, The Society of

Glass Technology, She�eld, 1997.

[4] E.J. Kamitsos, M.A. Karakassides, J. Non-Cryst. Solids

182 (1989) 19.

[5] M. Massot, S. Souto, M. Balkanski, J. Non-Cryst. Solids

182 (1995) 49.

[6] A.C. Hannon, D.I. Grimley, R.A. Hulme, A.C. Wright,

R.N. Singlair, J. Non-Cryst. Solids 177 (1994) 299.

[7] J.W. Zwanziger, R.E. Youngman, in: A.C. Wright, S.A.

Feller, A. Hannon (Eds.), Borate Glasses, Crystals, and


Melts, vol. 21, The Society of Glass Technology, She�eld,

1997.

[8] P.J. Bray, J.O Edwards, J.G. OÕKeefe, V.F. Ross, I.

Tatsuzaki, J. Chem. Phys. 32 (2) (1961) 455.

[9] G.E. Gurr, P.W. Modgomery, C.D. Knudson, B.T.

Gorrese, Acta Crystallogr. B 26 (1970) 906.

[10] T. Ushino, T. Yoko, in: A.C. Wright, S.A. Feller, A.

Hannon (Eds.), Borate Glasses, Crystals, and Melts, vol.

417, The Society of Glass Technology, She�eld, 1997.

[11] J.A. Tossel, J. Non-Cryst. Solids 183 (1995) 307.

[12] L.C. Snyder, G.E. Peterson, C.R. Kurkjian, J. Chem. Phys.

64 (4) (1976) 1569.

[13] D.L. Griscom, P.C. Taylor, D.A. Wake, P.J. Bray, J.

Chem. Phys. 48 (11) (1968) 5158.

[14] Y. Deligiannakis, L. Astrakas, G. Kordas, R.A. Smith,

Phys. Rev. B 58 (17) (1998) 11420.

[15] D.L. Griscom, J. Non-Cryst. Solids 13 (1973) 251.

[16] M.C.R. Symons, J. Chem. Phys. 53 (1970) 468.

[17] G. Kordas, Phys. Chem. Glasses 38 (1997) 21.

[18] G. Kordas, in: A.C. Wright, S.A. Feller, A. Hannon (Eds.),

Borate Glasses, Crystals, and Melts, vol. 148, The Society

of Glass Technology, She�eld, 1997.

[19] L.G. Rowan, E.L. Hahn, W.B. Mims, Phys. Rev. A 138

(1965) 4.

[20] L. Kevan, in: L. Kevan, R.N. Schwartz (Eds.), Time

Domain Electron Spin Resonance, Wiley, New York, 1979,

p. 279.

[21] S.A. Dikanov, Y.D. Tsvetkov, ESEEM Spectroscopy,

CRC, Boca Raton, FL, 1992.

[22] P. H�ofer, A. Grupp, H. Nebef�uhr, M. Mehring, Chem.

Phys. Lett. 132 (1986) 279.

[23] P. H�ofer, J. Magn. Reson. Ser. A 111 (1994) 77.

[24] F. Gemperle, A. Schweiger, Chem. Rev. 91 (1991) 1481.

[25] L. Astrakas, G. Kordas, J. Non-Cryst. Solids (244) (1999)

205.

[26] G. Feher, Phys. Rev. 103 (1956) 834.

[27] A. Schweiger, Structure Bonding 51 (1982) 1.

[28] M.M. Dorio, J.H. Freed (Eds.), Multiple Electron Reso-

nance Spectroscopy, Plenum, New York, 1992.

[29] B.M. Ho�man, Acta. Chem. Res. 24 (1992) 164.

[30] H. Kurreck, B. Kirste, W. Lubitz, ENDOR Spectroscopy

of Radicals in Solution, VHC, New York, 1998.

[31] W.B. Mims, Proc. Royal Soc. London 283 (1965) 452.

[32] E.R. Davies, Phys. Lett. A 47 (1974) 1.

[33] L.J. Berliner, J. Reuben, EMR of Paramagnetic Molecules,

Biological Magnetic Resonance, vol. 13, Plenum, New

York, 1993.

[34] C. Gemperle, A. Schweiger, Chem. Rev. 81 (1991) 1481.

[35] M.J. Frisch, G.W. Trucks, H.B. Schlegel, P.M.W. Gill,

B.G. Johnson, M.A. Robb, J.R. Cheeseman, T. Keith,

G.A. Peterson, J.A. Montgomery, K. Raghavachari, M.A.

Al-Laham, V.G. Zakrewski, J.V. Ortiz, J.B. Foresman, J.

Cioslowski, B.B. Stefanov, A. Nanayakkara, M. Challa-

combe, C.Y. Peng, P.Y. Ayala, W. Chen, M.W. Wong, J.L.

Andres, E.S. Replogle, R. Gomperts, R.L. Martin, D.J.

Fox, J.S. Binkley, D.J. Defrees, J. Baker, J.P. Stewart, M.

Head-Gordon, C. Gonzalez, J.A. Pople, Gaussian 98,

Revision D. 2, Gaussian, Inc., Pittsburgh, PA, 1995.

[36] J.B. Foresman, A. Frisch, Exploring Chemistry with

Electronic Structure Methods, second ed., Gaussian, 1996.

[37] AMPAC 5/6 UserÕs Manual, Semichem, 7128 Summit,

Sawnee, KS 66216, USA.

[38] MOPAC, Chem3D, Molecular Modeling and Analysis,

Cambridge Soft, Cambridge, MA 02139-9586, USA.

[39] SPARTAN, Wavefunction, Inc. 18401 Von Karman Av-

enue, Suite 3700, Irvine, CA 92612, USA.

[40] L. Astrakas, Y. Deligiannakis, G. Mitrikas, G. Kordas, J.

Chem. Phys. 109 (19) (1998) 8612.

[41] L. Astrakas, PhD thesis, NCSR Demokritos, IMS, Athens,

1999.


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