XPS-investigation of intramolecular charge transfer in polynitro-aminobenzenes

Journal of Electron Spectroscopy and Related Phenomena, 46 (1988) 363-372 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

XPS-INVESTIGATION OF INTRAMOLECULAR TRANSFER IN POLYNITRO-AMINOBENZENES

WANG JIANQI*, WU WENHUI,

363

CHARGE

ZENG MINXIU and LANG HENGYUAN

Department of Chemical engineering,

Beijing I~titu~

of Technology,

Beijing (China)

(Received 6 April 1988)

ABSTRACT It has been shown that the shake-up energy AL!?, which depends on the ionization potential (IP) of the neutral (poly)nitro-~inobenzenes, vanishes for IP z 8.4eV. The R values, which measure the contribution of intramolecular charge-transfer configurations in the molecules-in-molecules models correlate rather well with the difference $, - E,, i.e., the difference of the peak positions corresponding to lower and higher binding energies.

INTRODUCTION

Polynitro-amino-aromatics are heat resistant compounds, which have been applied in many area, e.g., petroleum exploration, space travel and nuclear technology [l]. It is known that introducing in amino-group into a nitro- or polynitro-aromatic system improves the heat resistance of the compound [l]. A typical example is 1,3,5-triamino-2,4,6_trinitrobenzene (TATB). The importance of intermolecular hydrogen bonding in such molecules has been demonstrated [l]. However, not much is known about electronic aspects which could be relevant for improved heat resistance. The simultaneous presence of electron-donating NH, and electron-accepting NO, groups gives rise to intramolecular charge-transfer (CT). Although intramolecular CT is usually investigated by UVjVIS spectroscopy, we have used XPS spectroscopy for our work. The reasons for this are: (1) The intensity of shake-up satellites in XPS spectra shows a significant dependence on the IP of the donor atom [2]. This is a more sensitive probe than UV spectroscopy because of the sudden perturbation when a hole is created on the acceptor atom. (2) Due to their extreme insolubility, molecules such as TATB can not be investigated by UV/VIS spectroscopy. (3) Finally, we are interested in the study of the mechanism and the stability of energetic solids under heating, i.e., in the surface chemistry of such compounds [ 31.

*Author

for correspondence.

0368.2048/88/$03.50

Q 1988 Elsevier Science Publishers

B.V.

364

Pignataro and Distefano [4] showed that, in the XP spectra off-nitroaniline, the multipeak of Nls and 01s of -NC& in XPS can be ascribed to intramolecular CT interaction. Since then this effect has been investigated repeatedly [5-141, although only for mononitro aromatics. The aim of this paper is to extend the investigation to polynitro-benzenes. EXPERIMENTAL

Specimens: 15 nitro-compounds containing NH, groups were used in this work: TATB (l), 1,3-diamino-2,4,6trinitrobenzene DATB (2), l-amino-2,4,6trinitrobenzene MATB (3), 1,3,5trinitrobenzene (4), p-nitroaniline (5), mnitroaniline (6), 2,4_dinitroaniline (7), 3,5-dinitroaniline (S), 2-amino-!$nitropyridine (9), Z-amino-3,5-dinitropyridine (lo), 4-amino-3,5-dinitropyridine (ll), 1,3,5-trinitro-l.,3,5-triaza-cyclohexane RDX (12), 1,3,5,7-tetranitro-1,3,5,7tetraaza-cyclooctane HMX (13), 1,5-dinitroso-3,3’-7,7’-tetranitro-1,5-diazaoctane HNDACO (14), bis-trinitroethylene-dinitramine BTNEDA (15). All compounds, except (5), (6) and (7) which were commercial reagents, were synthesized by the authors. Instrumentation: The XP spectra (Mgfiror) were recorded on a PHI 5300 ESCA SYSTEM (PERKIN-ELMER) instrument at 250 W (12.4 KV x 20 m&. Calibration was done by assuming a binding energy (BE) of adventitious carbon equal to 285.0eV. Data massage (smoothing, deconvolution, curvefitting, X-ray satellite subtraction, background subtraction, normalization etc.) was done on a PE 7500 professional microcomputer. The vacuum system was kept at z 2.0 x lo-’ Torr. The samples (in powder form) were fixed on copper-based Scotch tape. Calculations: CNDO/B calculations were performed for molecules l-11 on a FUJITSU MS340S computer, assuming a planar structure and the validity of Pople’s standard structure parameters [ 151. EXPERIMENTAL

RESULTS

The Nls spectra of 1,2 and 3 are presented in Fig. la and their curve-fits in Fig. lb. The corresponding 01s spectra and their curve-fits are shown in Figs. 2a and Zb, as listed in Table 1. The Nls spectra of 5 and of 7 -10 are collected in Fig. 3. The trend of all compounds, except 9-11, are displayed graphically in Fig. 4. DISCUSSION

The description

of intramolecular

C.T.

For simplicity the molecules-in-molecule model of Longuet-Higgins and Murrell [16, 171 was used to rationalize the XPS data. Limiting ourselves to only two configurations, the ground (G) configuration II/( and the CT

365

(a)

(b)

MATB

MATB

JL

L

DATB

kn

TATB

. ,I1

409 407405

403 401 399

Bindmg Energy

_LIbJ!l 410 408 406 404 402 400 398 396

391

Bmdmg

e”

Fig. la. The Nls spectra of compounds

Energy

ey

1 and 2.

Fig. lb. The curve-fits of Fig. la.

configuration $(A- D’ ), and assuming resulting states are $1 =

(1 + A”)-““{@(AD)

&

(1 + 2”))““{$(A-D+)

=

the zero-overlap

approximation,

+ I$(A-D’)} -

the (1)

@(AD)}

(2)

with

;1 =

-28

AE+$i%-i$

In eqns. (3) and (4) the symbols have the following AE

=

E(A- D’)

=

IP(D) -

-

(3)

> meaning

E(AD)

EA(A)

-

C

(5)

where IP(D) is the vertical ionization energy of the donor, EA(A) the electron affinity of the acceptor, and C the Coulombic interaction. The exchange integral between the bonded 2p AOs of the ring carbon atom and the nitrogen

(b)

A MATB

A TATB

TATB

&x!L

539 537 535 533 531 529 527

Binding

Energy

Fig 2a. The 01s spectra Fig. 2b. The curve-fits

539 537 535 533 531 529527

e\ of compounds

Binding Energy

eV

1-3.

to Fig. 2a.

atom of the NO, group in eqn. (4) is BCNoz, and a, d are the A0 coefficients respectively. Exactly the same description holds for the ion, which can be envisaged as a composite molecular ion consisting of a donor group and an acceptor group with a core hole, i.e. by considering the interaction between $(AeD) and $(A@- D’ ), where 0 refers to the core hole and the + to a valence hole. The above formalism is still valid, but one has to use the parameters IP(D)*, EA(A)*, c*, AE*, 8*, A* etc. for the CT and ground configurations of the ion, instead of those without a star valid for the CT and G configurations of the neutral molecule. The relaxation energy which accompanies the creation of the core hole causes an orbital shrinking which increases the interaction between the HOMO and the LUMO. This in turn leads to a strengthening of the shake-up satellites, at the expense of the main photoelectron peak. It is obvious that I and A* can be used as a criterion which measures the amount of the charge

367

NOz NOz NOz

NOn I

I

410

406

I

I

402

398

Binding

Energy

eV

Fig. 3. The Nls spectra of compounds TABLE

5 and 7-10.

I

Et and intensity ratio derived from Nls and 01s spectra of compounds

1, 2 and 3

BE (eV) 01.9

Nls Compound

Eb* = I&, - ExI

-%I-%

MATB (3) DATB (2) TATB (1)

1.28 2.10 2.84

0.45

-%/A,

Eb* = IE,, - E,l

A, IA,,

1.88 2.60 3.37

0.3

2.8 3.6

AI, IA,

3.5 6.1

368 408

406

407

(6)

405

404

403

402

401

400

3oo

I

zj

(::::m\

BE/eV

-y

-NO2 !Satellite

I

. i

(7)

: ’ _- -:

(4) (3)

. __.- - -___

(2) (1) (12) (13)

* ' I-'

1

! I

1

___------

i

“A'~

’

1

I_

(14)

I

(15)

..’

! -NO

1

i

Fig. 4. The correlation and 12-15.

diagram of the Nls binding energies (ionization

transfer. The following satellites [7, 81 A,,/AI

=

I

(2 -

relationship

L*)2/(1 + U*)2

is obtained

energies) for compounds l-8

for Nls and 01s shake-up (6)

where AI1 and A, are the measured intensities of the peaks at E,, and E,, respectively. Pignataro and Distefano [4] implicitly assumed that all nitroanilines and related compounds show a positive shake-up. This is by no means always the case. Domcke et al. [6] and Katrib and El-Rayyes [14] proposed that under certain premises the energy gain through the screening of the Nls (NO,) core-hole is large enough to make the shake-up energy negative, i.e., BE* < 0 (cf. also refs. 8 and 14). However, as discussed by Agren et al. and by Ford and Hillier [13], the situation may not be quite so simple, but there is now hardly any doubt that in more complicated molecules and in molecules adsorbed on surfaces the shake-up energies can be negative. It is evident from the result of Nakagaki et al. [8] that AE* varies with IP(D) of the neutral molecule, i.e., it is determined mainly by the initial-state parameter IP. From the literature [8] we found that the border line for AE* = 0 is IP(D) z 8.4eV. Based on the trends of the ionization energies of neutral molecules, the trends of the shake-ups can be easily obtained. However, the ionization energies of the molecules 1, 2 and 3 are missing. They can be estimated using the additivity rule [18] in the following manner: From known data we have from which we see that the increments for -NO, and -NH, are 0.55 and - 1.52 eV, respectively, from which the IP = 9.44 eV for MATB (3) can be

369

r

+0.55 i

6

@NO2

NO2

-1.51

(9.44)

8.89['g1

8.34["]

+a55cf@N02

NO2

NO2

-1.54

9.85L20'

+0.58

\

C-1.52)

10.43['~

&No2

lO.96i2o1

+0.53 izNdNo2

derived. Based on the data for aniline, m-diaminobenzene and 1,3,5triaminobenzene, the IPs for DATB (2), TATB (1) and their corresponding donors are obtained as follows

Compound

tie, IP/eV

10.96[201

I -0.55

I

Ii02

-0.55

(9.44)

(8.68)

-0.55

(8.30)

NO2

Donor (D)

NO2 NH2 NO2

IP(D)/eV

(8.14)

NO2 (7.76)

In p-nitroaniline, the nitro group and the aniline moiety were defined by Pignataro and Distefano [4] and by Nakagaki et al. [8, 111 as acceptor and donor, respectively. It is difficult to use the same approximation in the case of polynitroaminobenzenes. In the latter case -NO, can still be selected as an acceptor and the remaining moiety as a whole as the donor. But in doing this there is more than one possibility. The one with maximum Id( has been chosen in this paper (cf. Table 2, as shown by the bold lines). Using the data of Nakagaki et al. [8] the sequence of A-values so obtained is (cf. Table 2) n(1)

>

;1(2) >

A(5) >

L(7)

>

A(3)

(7)

This is consistent with the sequence of the CT interaction. The coefficient Id(D)] of the carbon atom marked with a dot are also listed in Table 2. From our data we find that the quantity Et( = E,, - E,), as measured in XPS, shows a fairly

370 TABLE 2 Data for the evaluation

IW)

(eV)

AE* i Id( E*b(eV)

of CT interaction

in compounds

1, 2, 3, 5 and 7

(8.14) 8.10 negative shake-up 0.50 0.34 0.4841 0.4689 2.10 1.90

(7.76) ‘ 0.58 0.5773 2.84

8.34 8.89 positive shake-up0.23 .0.21 0.4471 0.4206 1.49 1.28

good correlation with I (cf. Fig. 5). This provides us with a convenient approach for estimating intramolecular CT interaction, i.e. to use E;: to characterize the intramolecular CT interaction instead of il. From the shape of the shake-ups in Fig. 3, compound (8) appears to be the one without any appreciable intramolecular CT interaction. The comparison of the pairs (5), (9) and (7), (10)leads to the conclusion that the presence of an aza atom in the ring exerts an unfavourable effect upon CT interaction. In contrast to previous work, our data show that the 01s spectra of the molecules l-2 display negative shake-ups (cf. Fig. 2a, 2b and Table 1). The binding energies of Nls

of the -NO, groups of polynitro-aminobenzenes

There is a great deal of information in the literature concerning the binding energies of Nls of NO, groups, especially in mononitrocompounds [4, 8, 121. A value of 405.9 eV (reference to Cls = 285.0 eV) is an average obtained from the Nls spectra of mononitro-compounds, in the absence of observable CT interactions [ll]. It is recognized that the factors affecting the BEs are rather inovlved in polynitrocompounds, due to the presence of CT interaction. This is shown by the data in Fig. 4.(The value of 6 is from ref. 11). NO

NO2

NO2

I H2C -N NO2

02N-N

02N

H2fN’CH2 I I /N\C/NlN02 HZ

(12)

I H2C -

H2C -ti

-cti2

1

I

N-NO2 I N -CH2 NO2

(13)

2(02Nb-C

-CH2

I

I H2C-

CH2-

N-CH2-UN0213

CH2-

N -CH2-C(N02)3

I

YN0212 N -CH2 NO (14)

NO2

(15)

371

Fig. 5. The correlation

between the parameter /I and q.

The molecules 1215 are exempt of CT interaction. From Fig. 4 it is obvious that the BE values are getting larger with increase of the number of aza-atoms. This is due to the inductive effect, caused by the greater electronegativity of the nitrogen atom. The binding energies of amino groups present a similar trend within the experimental error. It is worth noting that the Nls BE of the NO group in 14 is 403.26 eV, i.e., dependent on CT interaction in the same way as that of the NO2 group [22]. To summarize, as far as the Nls BEs of NOz groups are concerned one must take into consideration the following three effects: (1) Intramolecular charge transfer interactions; (2) Inductive effects of the substituents or/and the hetero-atoms; and (3) The negative or positive shake-ups. ACKNOWLEDGEMENTS

The authors gratefully acknowledge the financial Natural Science Foundation Committee. We also Professor Chen Boren for his help.

support by the Chinese express our thanks to

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