Journal of Electron Spectroscopy
and Related
Elsevier Science Publiihers B.V., Amsterdam
Phenomena,
57 (1991)
165-187
165
A valence- and inner-shell electronic and photoelectron
spectroscopic study of the frontier orbitals of 2,1,3benzothiadiazole, C6H4SNz, 1,3,2,4_benzodithiadiazine,
CBH4S2N2,and 1,3,5,2,4-benzotrithiadiazepine, C,H,S,N,
A.P. Hitchcock”, R.S. DeWitte”, J.M. Van Esbroeck”, P. Aebi”, C.L. Frenchb,
R.T. Oakleyb and N.P.C. Westwoodb
“Department
of Chemistry, McMuster University, Hamilton, Ont. LAS 4Ml (Canada)
bGueEph Waterloo Centre for Graduate Work in Chemistry,
Guelph Campus, Department
Chemistry and Biochemistry,
University
of Guelph. Guelph, Ont. NlG 2Wl (Canada)
(Received
of
6 May 199 1)
Abstract
He1 photoelectron spectroscopy, inner-shell electron energy loss spectroscopy involving the S2p,
528, Cls and Nls edges, and Sls synchrotron radiation photoabsorption spectroscopy have been
used to probe the occupied and unoccupied valence levels of 2,1,3-benzothiadiazole,
1,3,2,4&nzodithiadiazine, and 1,3,5,2,4-benzotrithiepine.
The term values obtained fromthe Sls, 52s
and Nls oscillator strength spectra are sensitive to the aromatic/anti-aromatic
character in the
thiazyl ring, whereas the first ionization potentials for the latter two molecules remain relatively
constant. The combination of valence photoelectron and core excitation results, aided by semiempirical (MNDO ) molecular orbital calculations, provides a useful probe of the frontier molecular orbitals of these aromatic/anti-aromatic
molecules.
INTRODUCTION
All planar binary sulphur nitride rings have 4n + 2 n-electron counts [ 11.
This observ&ion has sparked a continued debate as to whether the concept of
aromaticity is pertinent to the study of these systems. Topological resonance
energies (per electron) for various known and unknown systems have been
calculated 121, and the variations between 4n and 4n+ 2 systems rival those
found in organic chemistry; a planar 12 n-electron S,N, molecule, for example
[3 1,is predicted to be strongly anti-aromatic. The absence, however, of any
experimental probe for measuring the degree of aromatic character has left
these theoretical ideas untested. The rapid growth in heterocyclic sulphur nitrogen chemistry during the last decade has provided novel examples of both
aromatic and anti-aromatic benzo-SN molecules. For example, in addition to
the long-known 10 n-electron 2,1,3-benzothiadiazole,
1, the 12 x-electron,
0368-2048/91/$03.50
0 1991 Elsevier Science Publishers B.V.
All rights reserved.
166
1,3,2,4-benzodithiadiazine, 2 [4 J, and 14 x-electron 1,3,5,2,4benzotrithiadiazepine, 3 [4,5], have recently been characterized. Of the three, 2 is both reduced and oxidized most easily, and is also thermodynamically most susceptible to addition of norbornadiene.
A
= N>
a
1
N
B
1
acN>s
N
While the above results are qualitatively useful, their interpretation in terms
of analytical models of aromaticity is hampered by the fact that 2 and 3 are
not members of an homologous series. For example, the molecular structure of
1 reflects a heavy contribution of the quinoidal valence formulation (B ) [ 61,
while the structures of 2 and 3 both favour the sulphur diimide representations
(A).The ground state electronic structures of l-3 have been examined [ 51,
and, consistent with the observed molecular geometries, the results of MNDO
calculations on the two molecules reveal only minor interaction (especially for
3 ) between the organic and inorganic subunits. More recently the excited state
manifold of 3 has been probed by MCD spectroscopy [7]. The molar ellipticities of the B and L bands are consistent with somewhat restricted electron
delocalization around the 11-atom perimeter.
In order to probe more fully the electronic structure of these molecules, in
particular the distributions and energies of the high-lying occupied and Iowlying unoccupied molecular orbitals, i.e. those of chemical importance, we have
carried out studies of these molecules using two forms of electron spectroscopy.
Experimental investigations of the occupied valence level structure of moiecules is best achieved with ultraviolet photoelectron spectroscopy (UPS ). This
has been done for a variety of sulphur-nitrogen molecules and radicals, which
often have extremely rich electronic structure [S-11]. In many cases the ordering of energy levels is often supported by the use of ab initio calculations,
or for larger systems, semi-empirical methods such as MNDO; Koopmans’
theorem is assumed to hold, given the infeasibility of performing calculations
beyond this level on molecules of this size. As it turns out, semi-empirical
MNDO calculations, with their limitations acknowledged, can provide reasonable estimates of the occupied orbital energies [ lOa, 12 1. Electron transmis-
167
sion (ET) spectroscopy, or inner shell excitation by electron energy loss (ISEELS ) or X-ray absorption (XAS ) spectroscopies provide probes for the
unoccupied levels. Indeed, localized core excited states act as a site-specific
probe of unoccupied electronic structure. In principle, core excitation can give
information on the spatial distribution of the final orbital, i.e. orbital mapping
[ 131. Recently, an ISEELS investigation of borazine, the aromatic analogue
of benzene, provided direct evidence that it consist of a delocalized x-system
rather than a proposed, alternative, localized ionic structure [ 141.
EXPERIMENTAL
Starting materials
2,1,3-Benzothiadiazole, 1, was obtained commercially (Aldrich), 1,3,2,4benzodithiadiazine, 2, and 1,3,5,2,4_benzotrithiadiazepine, 3, were prepared
according to literature methods [ 5 1.
ISEBLS
The inner-shell electron energy loss spectra were obtained using techniques
and apparatus described previously [ 15 1. Briefly, the spectra are the result of
inelastic electron scattering through a small angle ( x 2” ) by vapour phase
samples, with a high velocity incident electron beam (&= 2.5 keV plus the
energy loss). Under these electron scattering conditions primarily electric dipole transitions are excited, with intensities (after minor correction for kinematic factors [ 161) that are very similar to those of the corresponding soft Xray photoabsorption spectra. Because of the very low vapour pressure of 3 it
was placed directly in the collision chamber of the spectrometer, which was
then heated by circulating hot water. Care was taken in recording the Nls
spectra to ensure there were no contributions from the strong Nk-+rt* transition (401.1 eV) of Nz.
The Cls spectra were calibrated with the ti feature of CO, at 290.74 eV [ 171.
The Nls spectra were calibrated with respect to the it* feature of N, at 401.1
eV [ 18 1. In these cases the calibration was carried out on a stable mixture of
the gas and the sample. The S2p/S2s spectra were calibrated internally to the
Cls spectra. All results were converted to absolute optical oscillator strengths
(fvalues) by normalization of the isolated continuum core ionization signal to
atomic oscillator strengths [ 19 ] using a procedure described and tested previously [ 16 J.
X-ray absorption spectra
The Sls spectra were recorded using gas ionization and total ionization yield
168
2.0
7ro
=2 -
TV
0.0
I
-5.0
bla2-
__---
-2.0
bl
.._
-_
_-
-------m
-
4
-7.0
1
WI
-8.0
_G__a2-_-_----
bl----____
-10.0
-12.0
-14.0
{@VI
Fig. 1. Computed (MNDO) n-orbital energies for HNSNH and benzene, along with the a- and xorbital energies of 1 (a-levels are offset to right). On the right are shown the average term values
(TVs) of the core excitations to virtual z*NS and X* (CC) orbitala, as well as the experimental
IPs. The term value scale is calibrated from the LUMO.
detection at the double crystal beam line of the Canadian Synchrotron Radiation Facility located at the Synchrotron Radiation Centre (SRC ) , Stoughton,
WI. The monochromated X-ray flux was passed through a thin Mylar window
into a 23 cm long ionization chamber which contained the sample at its equilibrium vapour pressure. For 1 and 2 the room temperature vapour pressure
( x 50 m Torr ) gave adequate ionization signals. For 3 the vapour pressure was
insufficient, so the spectrum shown was measured as the sample current induced by the X-ray beam incident on the solid material as a coating on a conducting carbon tape. The energy scale was calibrated using the a,, line at 2486.0
eV in the gas phase spectrum of SFs [ 201. The oscillator strength was set at
25 eV above the estimated Sls ionization potential (IF) to the literature value
for Sls ionization of atomic sulphur (9.6 x 10e4 eV_l) [ 191. {Note that the
units for the vertical scale in Fig. 9 are one-tenth that of the other scales on
the core excitation figures in the paper. )
169
2.0
TV
-5.0
0.0
-10.0
-12.0
-14.0
W
--__
--__
--_
-
Exp’t
Fig. 2. Computed (MNDO) n-orbital energies far HNSNSH and benzene, along with the IT-and
n-orbital energies of 2 (u-levels are offset to right). See Fig. 1 for details.
HeI photdectron
spectra
Two photoelectron spectrometers were used. 1 and 2 were vapourized at 25
and 40”C respectively, into the ionization chamber of a home-built instrument
[ 211 with digital data acquisition via a PC/XT
[ 221. Resolution was 40 meV
during the experiments, and spectra were calibrated with the known (IPs of
CHJ and Ar [23]. 1 has been studied previously by UPS [24-261, but it is
included here for completeness. The more involatile compound 3 was investigated using a PS 16/18 spectrometer with a probe temperature of 60” C. Resolution was k: 40 meV with calibration of IPs using CHBI and Nz [ 221.
Computation
Semi-empirical calculations employing the MNDO hamiltonian for heats of
formation, optimized structures, and eigenvalues of 1,2 and 3 were performed
170
7ru
2.tl-
bi
I
TV
I
OS)-
*-
.’
-5.0
#0’
baI m-------m
-___
_______b,-------
-
I-
-8.1)-
-10.1D-
-12.1D-
G
_-________a*-______
b,-____--
cc-
)
W)
---
m
a*-___---
--._
I.---__
-_ -__
-bt m
-
--a_
it:>
-
-14.tD(W
-7.0
i
I_-___
---_
c
0
b
I-
-
Exp’t
N-Y/\
P\\
N-5
‘/
Al
Fig. 3. Computed (MNDO) n-orbital energies for HSNSNSH and benzene, along with the 6- and
z-orbital energies of 3 (a-levels are offset to right}. See Fig. 1 for details.
on a 20 MHz 386 PC running MOPAC (Version 5.0) [ 27 ] a. Pull optimization
was performed within C,, symmetry for 1 and 3, and within Cs symmetry for
2.
RESULTS
AND DISCUSSION
Any discussion of the valence electronic configurations of 1,2 and 3 is in
part predicated upon a consideration of the occupied and unoccupied valence
manifolds. Figures l-3 show these energy levels obtained from the MNDO
calculations for the three molecules of interest. Also shown are the a-orbital
energies of HNSNH, HNSNSH, HSNSNSH and C6H6, these being the molecular building blocks from which the three molecules are constructed. These
calculations are new versions of those previously described [ 5 ], and employ
the new parameters for sulphur [ 281 which give much improved heats of for*This version of MOPAC, coppiled with NDP F&ran
386, running under the PharLap
extender, gives a performance comparable to an Apollo DN4500 without an FPA.
DOS
171
TABLE 1
Experimental and
calculated (MNDO, in parentheses) bond lengths (A> for 1,2 and 3”
Bond
lb
2c
C&b
1.46
(1.453)
1.29
(1.368)
1.46
(1.458)
1.41
(1.487)
1.387(N),
(1.414(N),
1.407(N),
(1.411(N),
1.381
(1.398)
1.394
(1.432)
1.796
(1.711)
1.423
(1.395)
1.693
(1.607)
1.544
(1.553)
1.546
(1.542)
cb-cc
G-G
G-C6
c-s
C-N
1.34
(1.327)
N-S(C)
S-N(C)
-
N-S-N
bv. 1
1.60
(1.626)
3d
1.387(S)
1.397(S))
1.403(S)
1.413(S))
1.421
(1.438)
1.349
(1.387)
1.396
(1.417)
1.387
(1.408)
1.731
(1.690)
1.609
(1.572)
1.538
(1.519)
*Carbon atoms lettered alphabetically from the quaternarycarbons.
bFkf. 6.
‘Ref. 4.
dR4?f.5.
mation. The principal structural conclusions remain the same, i.e. the calculated bond lengths and angles follow those obtained by X-ray diffraction (Table l), and support the dominance of the quinoidal formulation (B) for 1 and
the sulphur diimide structure (A) for 2 and 3 [4-61.
Although the absolute magnitudes of the eigenvalues for the occupied and
(especially) the unoccupied levels must be taken cum grano salis, the trends
evident in Figs. l-3 illustrate some essential features of relevance to this work.
Of particular importance is the perturbation of the frontier x-orbitals of the
three sulphur diimides HNSNH, HNSNSH and HSNSNSH occasioned by the
attachment of the benzene ring. Taken collectively, we view these differences
as reflecting the relative merits of C (Pplr)-N (2pn) versus either C (2px)S (3~7~) or N (2plt)-S (3pa) overlap. Accordingly, fragment orbital mixing is
heaviest, and structural changes the most pronounced, in 1,least in 3 and,
intermediate in 2. Both the relative and absolute values of these calculated
orbital energies will be analysed in the light of the results of the inner shell and
valence level spectroscopy to be described below. The effect of the NSN chromophore has been discussed before in the context of the UV-visible spectrum
of several organic sulphur diimides [29 1.
172
I
.
.
.
.
I
.
300
200
Energy
Loss
.
.
.
1..
400
<eV>
Fig. 4. Electron energy loss spectra of 1 (C,H,SN, ), 2 ( CBH,SzN, ) and 3 (C&H,S,Nz) recorded
with 2.5 keV final electron energy, 2” scattering angle and 0.7 eV fwhm resolution. The intensity
of the Cls edge jump (I (300 eV)-I(280
eV)) has been assigned a value of one in each case to
visually demonstrate that the S2p intensity scales with the number of S atoms in these species.
The approximate locations of the S2pz,z, S2s, Cls and Nls ionization thresholds are indicated by
the bars at the beginning of the hatched regions.
Inner-shell excitation
Overviews of the energy loss spectra of 1, 2 and 3 spanning the S2p, S2s,
Cls and Nls excitation regions (Fig. 4) indicate the true signal-to-background
(S : B ) ratio at each core edge and illustrate the basic sensitivity of ISEELS to
elemental composition. The S2p ionization intensity relative to the Cls and
Nls ionization intensities scales approximately with the number of sulphur
atoms in each molecule (1:2 :3 for the molecules 1,2 and 3) as expected from
the elemental aspects of inner-shell excitation.
The Cls oscillator strength spectra {Fig. 5, the energies, estimated term
values (TV) and proposed assignments in Table 2) of 2 and 3 are similar to
each other with the main Cls-,n* peaks occurring at nearly the same energy.
Both Cls spectra are generally similar to that of benzene and other aromatic
ring compounds [ 14,301. The features around 285.2 and 287.6 eV in both molecules (identified as llry and 27r*in Table 2 ) can be attributed to core hole
173
l”~-.‘-.*~‘..‘.‘*.l
280
290
Energy
300
310
Cd0
Fig. 5. Oscillator strengths for Cls excitation derived from electron energy loss spectra of 1,2 and
3 (see Fig. 4 for details). The hatched lines indicate estimates of the Cls IPs (see Tables).
states with considerable benzene-based composition. In the orbital picture obtained from the MNDO calculations the lx*-2n* separation is x 2.2 eV (Figs.
2 and 3). However, the Cls spectrum of 1 is markedly different (Fig. 5 and
Table 3 ), as it contains two strong low-lying resonances with the second peak
of even greater intensity than the first, rather than a single dominant low-lying
lr peak as seen in 2 and 3. This makes it difficult to specify the W-2n*
separation in 1 which is calculated to be smaller, 1.6.eV, than those of 2 and
3. The melting point, IR data, mass spectrum and UPS (below) all indicate
that the ISEELS spectrum was obtained on a pure sample. Thus the most
probable explanation for the apparently unusual spectrum of 1 is that the quinoid form (A) dominates and that, in contrast to the benzene-like spectra of
2 and 3, that of 1 consists of lower energy Cls (C-C) + Ir*(C-C ) and higher
energy Cls(C=N) -+n*( C-N) transitions which differ in energy largely because of an appreciable chemical shift between the two Cls levels but also,
because of the difference in energy of the optical orbitals of n* (C-C) and
n* (C-N) character. Although the Cls XPS spectrum of 1 has not been reported to our knowledge (the Nls spectrum of 1 in the solid state is known
[31] ), the chemical shift for a ring carbon attached to N is 1.3 eV in phenylamines and 1.6 eV in phenyl isocyanides [32 1. Thus, based on the shift be-
174
TABLE
2
Energies, term values and proposed assignments for features in the carbon la spectra of 2 and 3
2
Feature
Assignment
3
E
Feature
E (eV)
(eW
1 sh
2
284.44”
285.34”
6.1
5.2
5.7
6.2
3
4 sh
5
287.5
289.6
290.0
3.0
0.9
0.5
1.4
1.0
4.5
2.4
2.0
IPb
IPb
IPb
290.5
291.0
292.0
6sh
7
8
293(l)
294.0 (5)
301(l)
- 2.5
-3.5
-11
1 sh
2
3 sh
4
5
6
284.3
285.1”
285.6
287.6
289.3
290.7
IPb
lPb
290.5
291.0
7
8
294.3(5)
301(l)
Z’cH
Tcs
(eV)
(eV)
6.2
5.4
4.9
2.9
1.2
5.9
5.4
3.4
1.7
0.3
a*(N-S)?
17ic
&(C-S)
27?, a* (C-S)
Rydberg
Rydberg
Estimated
Estimated
Estimated
-2.8
-11
CH IP
CS IP
CN IP
@(C-N)
ti (C-C)-lC
dc(c-c)-2=
“Calibration: 2 (-5.40(5)
eV relative to Cls+z*
in CO, (290.74 eV); 3 (-5.64(6)
eV relative
to Cls++fl in CO,).
bIPs estimated from experimental
XPS values of related compounds: CsHe, -290.3
eV;
C,H,NH,(CH),
-290.2 eV; CN, -291.3 eV, &H,NC,
-290.9 eV, -292.5 eV) [32].
‘These continuum features correspond to those characteristic
of Cls+ti(C-C)
transitions in
benzene and other 6 n-electron aromatics [ 14,301.
tween the two main peaks in the Cls spectrum of 1,we believe that there is a
chemical shift of up to 2.4 eV between the two carbons attached to the heteracyclic ring and the other four carbons. A smaller chemical shift is then likely
to occur for the C-N and C-S carbons in 2 and 3. Based on XPS IPs of related
molecules [ 321 (see footnotes to tables) we estimate ( t 0.5 eV) the C-H IP
to be 290.5 eV in all three molecules, the C-N IP in 1 and 2 to be 292.0 eV and
the C-S IP in 2 and 3 to be 291.0 eV. A similar pattern of two very strong
resonances, with the higher energy peak of greater intensity, has been observed
in the Cls spectrum of benzoquinone [ 34 ] , a molecule which is the paradigm
of the quinoid structure. As for 1, distinct Cls-t~*(C-C)
and
Cls( C-O ) + 7~*(C=O) transitions, separated primarily by the Cls chemical
shift, have been used to explain the spectrum. Interestingly, the low energy
region of the Cls spectrum of acrylic acid [35] is also similar to that of 1 and
benzoquinone, for similar reasons.
There are other differences in the Cls spectra of these three species in the
288-294 eV region. They can be explained in terms of the different numbers
175
TABLE 3
Energies, term values and proposed assignments for features in the carbon Is spectra of 1
Feature
1
2 sh
3 sh
4
5 sh
IP(CH)
6
IP(CN)
7
8 sh
9w
10 br
E
(eVJ
TCH
(eV)
284.44”
234.8
286.5
286.9
6.1
5.7
4.0
3.6
290.2
290.5b
290.8
292.0b
293.4
294.8
298.3
301.4 (5)
0.3
TCN
(eV)
- 2.9
- 4.3
CH
CN
5.5
1x*
In*
5.1
1.8
271’
w*(C-N)
Rydberg
1.2
Continuum onset
Rydberg
la*(CC)
Continuum onset
-1.4
-2.8
7
10
Assignment (final orbital)
17P
la* (CC)
Double electron excitation
Zdc(CC)
“Calibration: -6.29(5) eV relative to Cls+rt* in COz.
“IPs estimated from experimental XPS values of related compounds: C6H6, - 290.3 eV; C&,NH2
(CH), - 290.2 eV, CN, -291.3 eV, G&NC, - 290.9 eV, 292.5 eV [32 1. Note that the solid phase
Cls XPS spectrum of naphthabis(thiadiazole) exhibits a chemical shift of 1.3 eV between the
Cls (CN) and Cls (CH) peaks, in good agreement with this estimate [33].
1
Fig. 6. Schematic diagrams of the ground state HOMOs and LUMOs of 1,2 and 3 predicted by
MNDO.
176
Nls
390
400
410
Energy
420
(eV)
430
Fig. 7. Oscillator strengths for Nls excitation derived from electron energy loss spectra of
1, 2
and 3 (see Fig. 4 for details).
of C-N and C-S bonds in the three compounds. Thus, according to the bond
length correlation [36] one expects Cls+a*(C-N)
transitions close to the
core ionization threshold (around 290 eV ) . This is probably the origin in 2 of
the additional breadth and intensity on the low energy side of the 294 eV feature relative to the corresponding feature in 3. Similarly, Cls-+b* (C-S) transitions are expected around 286-287 eV [ 371. Thus the presence of twice as
many C-S bonds in 3 compared with 2 gives rise to the stronger dt (C-S)
signal observed as the high energy shoulder (285.6 eV) on the Cls+lt* transition in 3. We note that there is a peak with a TV of 6.2 eV observed for the
lower symmetry 2; from the Nls and S2s/2p spectra (below) this is assigned
to a Cls+ K* (N-S ) transition, terminating in a state which has some contribution from the benzene ring, unlike the corresponding LUMO in 3 which is
strongly localized on the thiazyl fragment (Fig. 6 ) .
The Nls, S2s and Sls spectra are the most interesting with regard to the
question posed in initiating this research since they should be sensitive to the
aromatic/anti-aromatic
character of the thiazyl ring .through their sampling
of the N2p and S3p characters of the I1L(N-S) LUMO. The Nls spectra of 1,
2 and 3 are comparea in Fig. 7 with the energies, estimated term values and
177
TABLE 4
Energies, term values and proposed assignments for features in the nitrogen 1s spectra of 1,2
3
1
2
Feature
E
T
1
398.6”
6.9
2
3
4
IPb
5
401.3
402.7
404.2
405.5
409.5
4.2
2.8
1.3
-4.0
Assignment
3
Feature
E
1
2 sh
3
4
397.76”
400.1
401.1
402.8
IPb
5
405.5
407.4
T
7.8
5.4
4.4
3.7
-1.9
and
Feature
E
T
1
2 sh
3
398.6”
406.7
401.6
6.9
4.8
3.9
4
IPb
405.3
405.5
0.2
a*(N-S)
3s Rydberg
@(N-S)
3p Rydberg
Rydberg
Estimated
dc (N-C)
“Cabbration: 1, -2.53(6)
eV relative to Nls-+R* in Nz (401.1 eV); 2, -3.43(5)
eV relative to
Nls+x*
in N,; 3, -2.57(6)
eV relative to Nls+TE* in N,.
bIPs estimated from experimental XI’S values of related compounds: Nr, - 469.9 eV; N&, -465.3
-405.6 eV [32]. In the solid state the Nls IP of 1 (399.5 eV) is 0.4 eV greater than
eV; N&
that of solid pyridine 133 1. (Other aromatic thiadiazoles show similar Nls IPs in the solid state
[33].) Based on the gas phase Nls IP of pyridine (404.9 eV 13211, this predicts the gas phase
Nls IP of 1 to be 405.3 eV.
proposed assignments summarized in Table 4. The Nls+ Ir* (N-S) transition
occurs at 0.84 eV lower energy in the Nls spectrum of 2 when compared with
1 or 3; the calculated (MNDO) difference in the LUMO energies is 0.73 and
0.71 eV respectively. This result is consistent with the greater aromatic character, larger t~,ti) splitting and thus higher energy fl orbital in the more
aromatic 1 and 3 species. Another characteristic of the Nls spectra (Fig. 7) is
the systematic decrease in the intensity of the broad peak around 409 eV attributed to Nls-, a* (N-C > transitions. This scales in proportion to the number of C-N bonds in these species (2 : 1: 0 in 1,2 and 3).We assign the relatively strong feature observed around 402 eV in each species to Nls+b*
(N-S ) transitions.
The S2s and the near-edge region of the S2p spectra of 1,2 and 3 are presented in Fig. 8 (energies, term values and proposed assignments are compiled
in Tables 5 and 6). The main feature in the S2s spectrum of 2 is attributed to
an unresolved overlap of S2s+7r*(N-S) and S2s-+dr (N-S) transitions. This
occurs about 0.4-0.8 eV lower than the corresponding feature in the S2s spectra
of 1 and 3, in accord with the calculated LUMO energies, within our assumption that the S2s IP of each species is the same. Although the background
slopes, and thus the intensities, of the S2s spectra presented in Fig. 8 are sensitive to the details of the difficult background subtraction, it appears that the
intensity of the S2s+dc (N-S) transitions increases in the order 1,2,3,consistent with the LUMO S3p character predicted by the cahzulations (Fig. 6).
178
S2s
b
E
II
12
I
3
Enargy
Fig. 8.Oscillator strengths for the S25 spectrum and the discrete region of the S2p excitation
spectrum derived from electron energy loss spectra of 1,2 and 3 (see Fig. 4 for details),
The Sls spectra are presented in Fig. 9 while energies, estimated term values
and proposed assignments are given in Table 7. Sls excitation should access
similar final states to those in S2s excitation. Relative to the S2s spectra, the
shape and detailed structure of the Sls spectra are much better defined. This
is due both to the greater edge jump and to the smaller natural linewidth. Even
so, comparison of the Sls and S2s spectra shows that they are very similar,
particularly in the discrete structure below the respective IPs. At each edge, 1
has a low energy tail and a small peak/large peak pattern; 2 has a large peak/
smaller peak pattern; while 3 has a single broad peak. The similarity of the
Sls and S2s spectra indicates that there were no problems with photo- or surface-induced modifications of the sample in the sealed ionization chamber.
The low energy shoulder of the Sls spectrum of 1 is attributed to transitions
to the LUMO which has mainly Ir*(C-C) character but also considerable delocalization onto the S-N ring (see Fig. 5). This feature is in effect the signature of the strong quinoid character of 1 as seen from the heteroatom ring. The
second peak is assigned to Sls + n* (S-N) transitions. Its estimated term value
(6.8 eV) is quite similar to that for the first peak in the Nls spectrum of 1. A
179
TABLl35
Energies, term values and proposed assignmenta for features in the sulphur 2p and 2sspectra of 1 and 3
Sulphur 2p
Feature
1
2
3
4
5
IP (3/Z)
IP (l/2)
6
7
8
9
1
Assignment
3
E (eV)
T(3/2)
(eV)
164.73
166.3
167.7’
168.7
169.9
170.5b
171.6b
175.4
178.2
188(l)
201(2)
5.8
4.2
2.8
1.8
0.7
-4
-7
-17
-30
T(1/2)
(eV)
5.2
3.8
2.8
1.6
E
sh 165.1
166.48’
ah 168.0
169.5
171.2
171
172
173.2
174.8
176
192(2)
T(3/2)
(eV)
W/2)
(eV)
- 1.2
-2.8
-5
-20
l/2
n+(N-S)
5.9
4.5
3.0
1.5
3/2
5.5
4.0
2.5
0.8
dC(S-N)
4s (R, )
%
Rs
Estimated
n*(S-N)
o+‘(S-N)
4s (R,)
Rz
3d resonance
3d resonance
3d resonance
EXAFS
Sulphur 2s
Feature
1
E (eV)
1
IP (estimated)
2br
229.2
236
239(l)
3
T(eV)
6.8
-3
E (eV)
229.6(3)
236
238( 1)
Assignment
T (eV)
6.4
-2
n*(N-S)
a+(N-C)?
YX!p calibration: 1,- 116.7 (1) eV relative to the 284.44 eV Cls peak; 3, -124.3(l) eVrelativetoCls-rn*
in CO,. The 52s calibration of 3 wae taken from a spectrum containing both S2p and S2s sign&.
bin the solid state the S2p3,2 IP of 1 is 0.6 eV above that for Solid thiophene [33 1, Based on the gas phase
S2p of thiophene (170.0 eV) [ 32 1, this predicta the gas phase S2p,,, IP of 1 tobe 170.5 eV.
strong transition is found at similar energy in the Sls spectra of all three species. The third peak in 1,which is the strongest feature in the spectrum, is
tentatively assigned as dc (S-N) throughout the three species. Excitations to
a second orbital of ti (S-N) character may also make important contributions, thus explaining the very strong intensity at 2474.4
eV in 1.
The intense, sharp, first feature in the Sls spectrum of 2 is attributed to
Sls-t ti (S-N ) transitions. It is followed by a broader feature attributed to the
overlap of dc (S-N) resonances. Previous studies have shown that a* (S-C )
resonances occur with term values of 4.2-4.8
eV, with very little dependence
on bond length [ 383.cP(S-N)resonances would be expected to occur at slightly
higher term values and to show a similar independence of bond length.
The Sls spectrum of 3, which was recorded for the solid rather than the gas,
is less structured than those of 1 or2.Inpart this may be attributed to intermolecular interactions in the solid state. In addition we believe the first broad,
asymmetric peak to arise from the overlap of n* (S-N), dr(S-C)and a* (S-N)
180
TA3LE
6
Energies, term values and proposed assignments for features in the sulphur 2p and 2s spectra of
2
Sulphur 2p
Feature
E (cV)
T(3/2)
(eV)
T(W)
(eV)
Assignment
3/2
1 ah
2
3
4
5
6
IPb
IP
7
8
9
163.8
164.8
166.1
167.68
169.1
170.6
7.2
.
7.2
5.1
l/2
a*(N-S)
R*(N-S)
@(N-S)
5.4
b*(N-S)
Rydberg
0.4
Rydherg
Estimated
171
172
174.2
178.2(5)
191(l)
-3
-6
-20
Double excitation
S3d
S3d
Feature
E (eV)
T (eV)
Assignment
1
2 sh
IP
3
228.8(2)
231(l)
236
239(l)
7.2
5
Sulphur 2s
-3
ti(N-S)
ti (N-S)
bc (N-C)?
“SZp calibration: -123.2(2)
eV relative to Cls+n* in COx.
bIPs estimated from experimental XPS values of related compounds [ 321; S2N2, - 172.44 eV
( 2paIz), 236.6 eV (2s). The S2paf2 IP of 1 is estimated to be 170.5 eV (see Table 5).
resonances and that this overlap results in a single broad band without clear
shoulders in contrast to the more structured spectra of 2 and 3 which have
fewer transitions expected in this energy range. A number of broad features
are observed at higher energy above 2479 eV, the estimated Sls IP [ 39 1. These
probably arise from a combination of Sls-+ S3d resonances, double excitations
and extended X-ray absorption fine structure features. More detailed assignments of these continuum structures probably require core hole decay studies
of the sort recently carried out for SF, [ 401.
The S2p spectra should be dominated by excitations to the d”(N-S) orbital,
since this will have a larger S3s character than n* (N-S). In all three species
the discrete features are a complex overlap of two sets of S2p excitations to
orbitals with large S3s character, separated by the 1.1 eV spin-orbit splitting.
The S2p spectra (Fig. 6) also reflect the trends discussed above for the Nls,
S2s and Sls spectra in that the overall shape of the S2p spectra of 1 and 3 are
181
4-
3-
O-
1....1....1....1....~~..‘1
2460
2470
2480
Photon
2490
2500
Energy
GaV)
251
Fig. 9. Oscillator strengths for Sls excitation of 1,2 and 3 derived from X-ray total ionization
yield spectra recorded at the OCMR-CSR.F double crystal beam line at SRC, Stoughton, WI. The
hatched lines indicate the estimated Sls II’s [39].
similar and significantly different from that of 2. The S2p discrete region of 2
is considerably more structured than that of 1 or 3. The lesser amount of
structure in 3 may simply reflect the larger number of sulphurs and the presence of two sites of different symmetry (in 2 and 3 ) . The latter should give
rise to greater spectral congestion owing to a larger number of transitions from
possibly chemically shifted S2p levels to various unoccupied orbitals with S2s
character. The fact that a rather sharp, low-lying feature is seen in 1,but does
not have a counterpart in 2 or 3, may reflect the clearer character of S2p
excitation in this species which cannot have any complications from chemical
shifts of the core levels. As with the Nls spectrum, the similarity of the 1 and
3 spectra and their contrast with 2 would seem to be related to the aromatic/
anti-aromatic character of these heterocyclic rings.
Thus the picture of the unoccupied levels of 1,2 and 3 that emerges from
the core excitation spectra is one of z* orbitals with decreasing TVs from ti
(N-S), through the (split) la* benzene-like level to the 2ti benzene-like level;
the 7t*(N-S) orbital in 2 lies below that in 3 by about 1 eV. Interspersed between these benzene-like levels are dr (N-S) and a* (C-S) levels, the latter
182
TABLE 7
Energies, estimated term values and proposed assignments for features in the Sls spectra of 1,2
and 3
2
1
Feature
E
(eV)
1 sh
2
2471.1
2472.7
3
4 sh
5
2474.4
2475.7
2478.8
IP
2479.0
6
7
8
2484.9
2493.1
2504.4
Feature
G)
3
(eV)
8.9
6.3
TeV)
E
T
(eV)
(cV)
7r* (C-C)
1
2sh
4.6 3 br
3.3 4 sh
0.2 5
-6
-14
-24
Feature
E
Assignment
2472.9
2474.0
2475.6
2476.3
2478.0
IP
2479.0
6
7
8
2484.1
2493.3
2496.8
6.1 1
5.0
3.4 2 sh
2.2
1.0
-5
- 13
-18
2473.2
5.8
2474.9
4.1
IP
2479.0
3
4
5
6
2484.0
2483.1
2489.3
2495.7
x* (S-N)
a* (S-C)
dc (S-N)
4~ (Rydberg)
5~ (Rydbecd
Estimatedb
-1
-4
-10
-17
Double excitation
S3d?
S3d?
EXAFS?
“T= IP - E. Note that the accuracy of these values is dependent on the accuracy of the estimate
of the Sls IPs.
bValueestimated from the Sls IPs ofother species, e.g. RSH, - 2477.8 eV; RSR, 2478 eV (R = CH,
or C2H,; Me,SO, 2480.4 eV [39]. There do not appear to be any data for SN compounds so the
accuracy of this estimate is difficult to evaluate.
being identified by their greater intensity in 3. The Cls spectrum of 1 provides
clear evidence for distinct ti (C-C ) and R*(C-N ) upper levels, consistent with
a strong quinoid character of this molecule. The calculated eigenvalues for the
ground state unoccupied orbitals (Table 8) are generally consistent with these
trends. Table 8 also includes the eigenvalues of the ground state occupied orbitals which are of relevance to the He1 spectrum.
Hdphotoekctron
spectra
The photoelectron spectra of 1,2 and 3 are shown in Fig. 10, with the experimental IPs compiled in Table 9. As is evident from both Table 8 and Fig.
10 only distinct bands up to about 12 eV can be assigned with any certainty.
The UPS spectrum of the 10 n-electron malecule 1 has been observed before
[ 24-261, and in the last instance was assigned with the aid of ab initio calculations. The sharp first IP at 8.95 eV with some associated vibrational structure, relatively high in binding energy, is associated with the a,, z orbital (node
at S), which is N-N antibonding. The second IP arising from an orbital (b,)
183
TABLE 8
Calculated (MNDO)
2
1
Unoccupied
3.33 b,
1.96a2
1.69aI
1.22b,
0.50 bl
0.17 a2
- 1.35b,
Occupied
-9.53 a2
- 9.90 b,
- 11.99 al
- 12.15b,
-
occupied/unoccupiedorbital energies (eV) for 1,2 and 3”
12.28bI
12.33a2
13.71aI
13.93bz
14.15aI
15.43bI
15.55b2
3
3.40 a’
2.31 a”
1.85a’
1.16a’
0.46a’
0.36 a”
- 0.07 a”
- 2.06an
3.55 b2
2.10 a2
1.96bz
1.36a,
0.86 aI
0.14 b2
-0.02 b,
-0.32 a2
- 1.33bI
- 8.05 arr
- 9.50 an
- 10.57a”
- 10.90a’
- 12.26a’
- 12.66a’
- 12.83a’
- 13.15a”
- 13.39a’
- 13.88a’
- 14.72a’
- 15.47a“
- 15.73a’
- 8.15 a2
- 8.93b,
- 10.01a2
- 10.54b2
-11.04 b,
- 12.17a1
- 12.65a,
- 13.02a2
- 13.24bz
- 13.32a,
- 13.50b,
- 13.91bz
- 15.27b,
- 15.45b,
“HOMO and LUMO distributions are shown specifically in Fig. 6. Correlation lines in Figs. l-3
indicate approximate orbital constitution of remaining z-orbit&.
comprising principally the S lone pair, is close in energy (9.50 eV), with no
vibrational fine structure. Beyond this there is some disagreement with the
assignments [24-261,
but in the absence of other information we are inclined
to follow the ab initio results [ 241.
For 2 and 3 the three uppermost occupied levels in each case are x as indicated by the relatively sharp first few IPs, although the distribution of atomic
orbital contributions is different in each case (Fig. 6), reflecting the addition
of successive sulphur atoms, and the additional z electrons associated with
these increasingly electron rich systems. Thus the simple N-N antibonding
combination in the 10 x-electron molecule 1 now has some S3p character mixed
in for the 121~system 2, and even more for the 141~3. In the case of 2, the
HOMO is an asymmetric distribution of N-S a-antibonding, whereas for 3 the
HOMO is a unique a2 orbital comprising two separated, antibonding N-S frag-
3
8
IAr
IO
I2
lONlZAT,ON
14
16
ev
POTENTIAL
Fig. 10. He1 photoelectron
spectra of 1,2 and 3.
ments, resulting in a sharper photoelectron band with some vibrational structure (1070 + 450 cm-l). For 3, this IP is 0.7 eV lower than that in the parent
1,3,5,2,4_benzotrithiadiazepine molecule [ 411, indicating the destabilizing effect of attaching a benzene ring relative to a simple olefmic linkage.
The second IP of 3 (8.54 eV) is associated with a large S p7c contribution,
whereas the corresponding band of 2 has a larger contribution from the benzene ring, as reflected in the higher IP (9.30 eV) , and slightly broader band.
The third distinctive band in both 2 and 3 with a maximum at 10.3 and 9.9 eV
respectively, contains two IPs (low energy shoulders are observed on both
bands), attributable to a 0 plus a R orbital, the latter having a large benzene
component. The higher IPs then become somewhat more cluttered, and the
listing/correlation of experimental and calculated IPs in Tables 8 and 9 simply
reflect the best match; by and large the distribution of experimental IPs follows
the calculated values fairly closely, albeit giving IPs a little too high. This is
quite typical for these kind of molecules as illustrated by an analysis of MNDO
calculated and observed IPs for some 12 SN molecules with the NSN moiety,
which have been unambiguously assigned [ lOa,12].
185
TABLE
9
Experimental
IPs (eV) for 1,2 and 3”
1
2
3
8.95
7.84
7.88
9.50
9.30
8.54
10.66
11.32
11.7 (sh)
10.1 {ah)
10.33
12.74
13.53
11.65
12.22 (sh)
12.65
14.76
15.75
16.53
9.6(sh)
9.87
11.0
11.4
13.94
12.5
13.0 {sh )
15.10
15.87
14.0
14.7
“The spacing in the columns reflects the distribution of IPs in the He1 photoelectron
CONCLUDING
spectra.
REMARKS
As mentioned initially, the structural parameters of 1,2 and 3 reveal varying degrees of quinoidal involvement which can be traced to the extent of interaction between the benzene ring and the N2S, (x= 1,2,3
) fragment. These
same factors also influence the orbital energies of the three molecules, offsetting, for example, the oscillation expected for the first IPs of an homologous
series of 10,12 and 14 x-perimeters. The similarity of the first IPs of 2 (7.84
eV) and 3 (7.88 eV) can be understood within this context. On the basis of
aromaticity one would expect the first IP of 3 to be increased relative to that
of 2 [421b.However, the absence of the ideal homologous pattern makes such
an expectation unrealistic. By virtue of its capability to incorporate some quinoidal stabilization (it can form one C=N bond), the ground state of 2, and
hence its HOMO, is stabilized relative to that of 3. This result is in accord with
the calculated IPs for 2 and 3, which show a spread of only 0.1 eV (Figs. 2 and
3, and Table 8). The aromaticity of 3 is therefore not immediately apparent
from the occupied levels. In contrast to 3, the quinoidal structure of 1 stabilizes
bA classical example of the oscillation in first IPs expected for an homologous series of 4n and
4n+ 2 z-systems is observed in the fluorophosphazenes (F2PN),
(n = 3-6) [42 1.
186
the orbital energies relative to that expected from the aromatic structure, and
leads to a low-lying HOMO. The virtual levels appear to retain the relative
ordering expected from a perimeter model. The LUMO energy of 3, as determined by ISEELS and predicted by calculation, lies well above that of 2. The
strong contribution of the quinoid form of 1 and the predominant benzenoid
form of 2 and 3 is unambiguously demonstrated by the Cls spectra.
From the analysis of the UPS, ISEELS and synchrotron XAS data it is
apparent that the experimental results, and the trends shown therein, are followed reasonably closely by the semi-empirical calculations. As these molecules are not part of an homologous series, both the quinoidal versus nonquinoidal character and the aromatic/anti-aromatic concepts need to be considered in the analysis. These results refine the picture of frontier orbital energies based on simple perimeter model notions.
ACKNOWLEDGEMENTS
We thank the Natural Sciences and Engineering Research Council of Canada for financial support in the form of operating grants (to A.P.H., R.T.O.
and N.P.C.W.) and an NSERC Summer Research Scholarship (to J.M.vE.).
A.T. Wen is thanked for his assistance with spectral acquisition. We thank the
CSRF team, particularly RX. Yang, for their assistance with the new double
crystal beamline used for the Sls measurements. This facility is funded by the
Ontario Centre for Materials Research and operated by the Canadian Synchrotron Radiation Facility. The Synchrotron Radiation Facility in Stoughton, WI is funded by NSF and operated by the University of WisconsinMadison.
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