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The calculated spectra of the azanaphthalenes
JOVRNAL
OF MOLECULAR
SPECTROSCOPY
50, 457-473
The Calculated J. E. fkpartrtzent
(1974)
Spectra of the Azanaphthalenes RIDLEV
c$ Chemistry,
AND M.
C. ZERNER
L’nioersity of Guelph, Guelph, Ontario, Canado
X modified ISDO (intermediate neglect of differential overlap) method is used to calculate the electronic spectra of naphthalene and some mono-, di-, and tetraazanaphthalenes. The technique is capable of reproducing the better classified bands of this series within a rms error of -1000 cm-‘. The four lowest n-x* bands of naphthalene are well represented; a fifth band reported may be iBs,(s-x*) borrowing intensity. at -52,600 cm-‘, and generally assigned ‘B 2U( or*), The lowest excited singlet state calculated for quinoline is n + K*, estimated nearly degenerate with the lowest V-K*, while that for isoquinoline is calculated B--?T*; experimental evidence supports the P-T* assignment in both cases, but the corresponding absorption in quinoline appears complex. Of the diazanaphthalenes examined (with two nitrogens in one benzenoid ring) all are calculated to have one n-r* transition before the first F?T* except phthalazine, in which two n-r* transitions are calculated to be the lowest lying. This is in accord with experimental evidence to date, although the nature of the observation in phthalazine is reinterpreted as one ‘&(n-a). 1,4,&S-tetrazanaphthalene is predicted to have a group theoretically forbidden na* excited state as low as 21,000 cm-‘, and should prove interesting experimentally. Even though n-rr* excited states are often calculated to be the lowest lying, none of these compounds are predicted to have an “a” orbital as homo. I:urther interpretations calculations. Theoretical
of the spectra of the azanaphthalenes are made in view of the limitations of the method employed for this study are discussed.
INTRODUCTION It is the purpose of this paper to examine the spectra of the more common azanaphthalenes. We employ for this purpose a modified “intermediate neglect of differential overlap” technique (INDO) previously developed and parameterized on the spectra of benzene and the azabenzenes (1). When applied to the compounds of this study the model seems capable of reproducing the energies of the better understood bands within a rms error of - 1000 cm-l. In addition to reproducing known spectra, these calculations strongly suggest explanations for bands not yet classified, point to the possible presence of excitations not yet observed, and suggest perturbing influences on bands reported as (‘diffuse.” We examine the azanaphthalenes for two reasons. First is the observation that there are many interesting ambiguities and uncertainties in the experimental spectra of these molecules that are central to the understanding of heterocyclic spectra. Second, preliminary investigations with double excitations lead us to believe that theories that onl! include single excitations in the “configuration interaction” (CI) may begin to degenerate in accuracy with molecules much larger than naphthalene. This is a problem to 4.57 Copyright 0 1974 by Academic Press, Inc. All rights of reproduction in any form reserved.
458
RIDLEY
_UD
ZERNER
Y
i
Y
&
Y
ck?
N
N
x
x
N
N
QUi”WOli”e
I;IG. 1. The Azanaphthalenes .4ctn tryst. 10, 504 (1957).
PteIidine
of This Study.
References
which we give only cursory examination
refer to Coordinates.
here, remarking
(a) D. IV. J. Cruickshank,
only when our results might
be affected by inclusion of higher excitations. Several other investigators
have examined the spectra of azanaphthalenes
within the
LCAO-MO-SCF-CI framework. The most extensive of these studies are perhaps the works of Favini, Vandini, and Simonetta (2) and Baba and Yamazaki (3). Both of these groups worked within cluded core polarization
the Pariser-Parr-Pople
“pi-electron-only”
effects in a fairly systematic
spectra do not lead to very different but, of course, such calculations
conclusions
cannot
framework,
but in-
fashion. Their results for the r~*
than does the present investigation,
be used to estimate
the location
of the n-u*
transitions. Some of the theoretical and experimental information on the n-r* states of the azanaphthalenes has been summarized in a paper by Jordon, Ross, Hoffmann, Swenson, and Gleiter (4). METHODS
The model that has been used in these calculations has been described elsewhere (I). The computer program performs the LCAO-MO SCF calculations from an input of molecular geometry and atomic numbers. A ground state calculation is followed by a configuration interaction calculation which results in the desired spectroscopic transition energies and oscillator strengths. These oscillator strengths are estimated from the dipole length operator.
Table I: Naphthelene (x1000 .uI-~) Calculated
Observed= oscillaror Strength
Energy
Oscillator Strength
Energy
SyllZllletryTYPO
synrmetry
Type
lB2u
ll-TI*
32.0
0.002
1 B2u
n--n*
32.5
0.002(x)
ll--T*
37.5h
0.102
1 Bl"
*-lT*
37.7
0.148(y)
R?
45.0
11-v*
44.8
0.0
?l-Ti*
47.5
Tr--TI*
45.8
1.866(x)
1B3B
ll-ll*
46.7
0.0
lBlu
1
Bzu
IA R 1 B2u
1.0
IL3 1U
lt-?T*
49.5
0.3
1 Bl"
TI-Il*
48.6
0.672(y)
1 B2u
Ti--il*
52.6
0.1
1Blg
ll-a*
52.7
0.0
1B3B
ll-n*
52.7
0.0
lA S
TI-ll*
56.0
0.0
1B28
a-IT*
51.2
0.0
1B28
n-o*
57.4
0.0
1 B2"
71-11*
60.5
0.115(x)
R
>54.0
1 Bl" a)
-59.a=
Ref. 5
b) vmmaxvalue
The molecular referenced estimated
coordinates
c) Ref. S
used
in these
in Fig. 1. When such data from those of representative
save naphthalene, in geometry
there
can affect
is some uncertainty the calculated
prove
possible
to improve
were
in the atomic
spectra.
changes, however, to reverse the order etries we have assumed are reasonable it ma\.
calculations
obtained
from
the data
was not available, bond lengths and angles were compounds. In all the compounds esamined, It appears
coordinates.
Modest
to take rather
of energy leveis. One indication that is our agreement with esperiment.
our results
by adjusting
assumed
changes
large geometr!the geomSimilari!-
coordinates,
and
thus also gather geometric information. This we have not attempted. The parameters that have been chosen for this study are based solely on the results obtained for benzene and pyridine (I). Ko attempt was made to optimize eters over the series of molecules studied. C’onsequentl!,, some adjustment
these paramof parameters
may result in a better over-all agreement with experiment. In general, our calculated results are insensitive to modest parameter change. For the configuration interaction calculation we have included all single excitations that lie within -65,000 cm-’ of the ground state, as well as some additional selected configurations
of interesting
symmetry
type
(generally
50-70 configurations).
Including
singly excited configurations much bevond 65,000 cm-l is of questionable value for two reasons. R\-dberg-type transitions are estimated to begin to occur at -50,000 cm-‘, and
460
RIDLEY
AND ZERNER
we have no mechanism for their inclusion or their mixing with valence states. Second, we estimate double excitations to begin to occur at -65,000 cm-‘. Unlike singles that have the predictable
behavior
of mixing with other configurations
of proper symmetry
type within a range of, say, ~1~10,000 cm-‘,
doubles appear to affect calculated
within a much larger energy range. Rather
than include the large number
excitations content
states
of double
which can occur once we admit them, it is best to first examine the physical
of such semiempirical
parameterization
methods
as employed
here. That
such as we have chosen includes electron
is, it is expected
correlation,
that
and to include
doubles would be to include this effect twice. We avoid this question here, and admit that the utility of our answers may degenerate
much above 50,000 cm-‘.
We have chosen to compare our results with experimentally than ~00values where possible for reasons previously
determined
vrnaxrather
discussed (1).
RESULTS a. Waphlhalene The calculated
spectrum for naphthalene
is shown in Table I along with experimental
values reported by George and Morris (5). The calculated transition energies of the four lowest R-*
transitions
are in excellent
agreement with the experimental values. The calculated oscillator strengths for these bands are also close to those observed. The calculations indicate forbidden r-u* bands at 44,759 cm-l bands reported as Rydberg,
(‘A,) and 46,707 cm-l (lL&). centered
at 45,000
cm-l.
since they are not observed
and nearly fit into a Rydberg
One, or both, of these may be observed as
These bands have tentatively in solution,
sequence with the bands at -54,000
number of additional forbidden bands are calculated cm-’ and 62,000 cm-‘. George and Morris report Hummel and Ruedenberg
are too intense
a band at 52,650
cm-‘.
two forbidden
cm-’
(5). A large
to be in the region between 58,000 On the basis of calculations
(6) and on the basis of the intensity
as ‘Bt,. In this region we calculate
been assigned
to be hot bands,
by
of the band, it is assigned
bands, ‘BI, (T--B*) and lBpg (a-r*).
The next band of lBz, (T-T*) we find at 60,500 cm-‘, to be compared with Pariser’s (7) estimate at 64,700 cm-l. Extensive singly excited configuration interaction of ‘Bw states does not lower our calculated value significantly.
From this calculation
we suggest
than the band observed at 52,650 cm-’ is ~BQ, borrowing from the lBlr band at 48,630 cm-’ through one of the bt,, modes. We cannot, however, rule out the possible influences of higher excitations on the lBzu band we calculate at 60,500 cm-‘. In an early work of Klevens and Platt (8) a transition is noted at -59,800 cm-‘. We have found no recent mention of this transition: if real, then the observed singlet spectrum of naphthalene to 60,000 cm-r is nicely specified by our model if the band observed at 52,650 cm-’ is assigned to lBsg (r-r*), and the band at 59,800 to ‘Bzu (r-x*). In Table II we present the highest occupied and lowest virtual molecular orbital energies. Three ionization potentials of naphthalene are readily obtained from photoelectron spectroscopy (9); these are in good agreement with our eigenvalues (Koopmann’s Approximation). Beyond the third band the observed (and the calculated) structure becomesmore complex. Two ab initio SCF calculations on naphthalene have been reported in the literature (10). Our calculation compares well with the more complete of these (IfIb). In general,
SPECTRA
461
OF AZANAPHTHALENES
Table II: The Highest Occupied and Lowest Unoccupied Orbital Energies (eV). Naphthalene
Quinoline
Isoquinoline
Pteridine
b2,d(s*) 0.58
TI*
0.59
ll*
0.35
TI*
0.23
blg(n*)
71*
-0.40
ll*
-0.33
TI*
-1.45 -9.27
0.04
a"(n)
-7.95 (8.15)a
?I
-8.29
n
-8.05
lr
bgU(')
-8.60 (8.9ja
II
-8.74
n
-9.24
74%n -9.56
-9.96
66Xn -10.01
Ti
TI
78Xn -10.58
68Xn b2g(il) -10.12 aO.Oja
lr
-10.69
-10.36
-10.01
blg(fl) -11.77
ll
-12.08
Ti
-12.46
76Xn -11.56
b3g(u) -11.80
CI
-12.09
0
-12.19
88%n -11.90
ag(o)
-11.96
(I
-12.83
4
-12.81
*
-12.26
b2,W
-13.25
0
-14.28
D
-13.99
71
-13.54
bluW
-14.46
(J
-14.70
Is
-15.11
0
-14.28
b3"('T) -14.49
ll
-15.00
71
-14.97
ci
-14.89
b2"W
-14.77
Phthalazine
Quinoxaline
Cinnoline
a2m*j
0.02
0.24
71*
0.38
bl@*)
-0.51
bl('T*)
-0.85
71*
-0.79
71*
-0.60
a,(n)
a2w*1
0.50
7*
Quinazoline
-8.39
a,(a)
-8.60
TI
-8.41
TI
-8.52
b2(82%n) -9.19
bl(n)
-8.82
71
-9.32
ll
-9.39
bIW
-9.64
al(64%n) -9.78
7EXn -9.72
77%n -9.71
-10.56
64Xn -10.97
b2(90%n) -11.50
71
-10.73
a1(52%n)-11.29
a,(a)
-11.68
0
-11.34
lr
-11.18
a,(o)
b1W
-12.29
61%n -12.39
TI
-12.68
a,(a)
-12.65
bl(Tf) -13.13
b2W
-12.65
n
-13.09
0
-12.79
b2@)
a,(o)
-12.94
u
-13.52
0
-13.44
-13.17
b2W)
-15.10
a1(0)
-15.15
0
-14.48
0
-14.34
a,(n)
-15.51
b2(o)
-15.26
v
-15.27
0
-15.48
a)
Ref. 9
our highest occupied m.o.‘s have eigenvalues greater than the corresponding ab initio values; while our lowest lying unoccupied m.o.‘s lie at lower energy (decreasing the orbital energv gap by - 0.1 a.u.). In the ten highest occupied m.o.5 there are two energ). reversals, but these are between m.o.‘s calculated nearly degenerate in both techniques. In both calculations the tertiary carbon C, is most positive (+O.Olg), while the C,( -0.026) and C,( -0.022) populations are nearly identical (the numbers in parentheses refer to our values). Our pi bond orders (C-C@, 0.77; (Z-C,, 0.57; C&B, 0.55; (Z-C,, 0.52) agree in order with the results obtained in this ab initio calculation; both calculations disagree with most other results reported (IOU, II). b. Quinoline
The first band calculated for quinoline is an nlr* at 32,500 cm-r (Table III). There is no experimental evidence either in vapor or solution to support the existence of an n?r* state lying below the lowest K-T* (the latter observed with onset at 31,900 cm-r),
462
RIDLEY
AND ZERNER
Table III: Quinoline Observed Energy
T?W
n--71* 71-lT*
31.9a (onset) -33.oc (VCW) 36.2a
+-n*
*-TI*
44.3a
49.3a
Calculated Oscillator Strength
Energy
Oscillator Strength
Xl--Tl*
32.5
0.008 (2.)
il--il*
33.4
0.04
(-11')
TYPO
cxb, O.llP cyb,
Ti-TI*
36.6
0.16
(72")
0.517*(xb)
Ti-ll*
44.1
0.67
(-4')
n-ll* ?I-ll*
44.7 47.1
0.00 0.02
(2) (-55")
o.7438
I
IT-71* T-71* n-71*
ll--71* 71-Z*
1.15 (-2") 0.57 (87") 53.7 54.2 55.1
0.0004 (2) 0.15 (-87O) 0.0003 (2)
a) Ref. 3 b) Ref. 13 c) Vapor spectrum taken for this work:
bend center
although such a state would certainly affect emission. We calculate the maximum of the lowest ?T-?T*band at 33,400 cm-’ (Expt. -33,000 cm-l), -900 cm-’ above the n-?r*. This apparent energy reversal might be a shortcoming of the model itself, or the geometry we assume for quinoline. Within the model, additional configuration interaction might reverse the order of two bands calculated so close in energy. Such an interchange might also be attributed to the existence of two excited states with quite different equilibrium geometries (1). In this case one would expect calculated energy values to be greater than the observed vmax with greater deviation in geometry from the ground state. This n-n* transition is, however, noted for quinoline in naphthalene host crystals (4). Since in isoquinoline (Table IV) we calculate the rzl* transition 900 cm-’ above the first 7r-7r*, and there is no experimental evidence of this band in host crystals (4), the corresponding two states in quinoline must, in any case, lie very close in energy [see, however, (IZ)]. A second ?T-?T*transition is calculated at 36,550 cm-l and observed at 36,216 cm-‘. A third ?T-?F*calculated at 44,122 cm-’ is again in excellent agreement with the strong band reported at 44,282 cm-‘. A broad intense system is observed at 49,283 cm-l which we correspond to two calculated ?T--?T* bands at 47,907 cm-l and 49,638 cm-‘. A weak r-x* band is calculated at 47,103 cm-‘; it may be buried under the following intense structure. Zimmerman et al. (13) have assigned polarizations to the three lowest lying ?rq* transitions which are in accord with the dipole directions we calculate. c. Isoquinoline
The lowest electronic transition of isoquinoline is reported with onset at 31,540 cm-’ (3) and is assigned ?T--?T*.The band extends to -35,000 cm-’ with center at
SPECTRA
463
OF AZANAPHTHALENES Tsoquinoline
IV:
Table
Observed
Calculated
Energy
TYPO
Oscillator Strength
31.58 (onset) 32.5= (L)ctr)
X-T*
n-71*?
35.5?b
T-T*
37.6a
Energy
TYPE
0.02
n-e
I
0.09 (-56")
II-TI* 0.09
n-lT*? ‘r-71* ?
"43.5c
T-T* TI-Tl*
46.3a 47.3a
il-lT*
51.0 (s)C.d
3
Oscillator Strength
0.93
34.2
0.007 (1)
n-RI
37.2
0.16 (69")
"-lT*
42.9
0.001 (z)
71-n*
44.8
0.12 (19")
T--71* n-71*
46.7 47.5
0.89 (9") 0.81 (-8")
K-TT*
51.2
0.52 (86')
n-IT* T-T* TI-lT*
52.2 52.3 55.2
0.000 (7.) 0.08 (-110) 0.01 C-310)
a) Ref. 3 b) Vapor, this work. A feature on the following ?r+r*that disappears in CA30H: possibly part of the preceding system. c) Vapor, this work d) Ref. 14
-32,500
cm-r;
(Table
IV).
accord
with
bands
be compared
rq*
the calculated
pall\- y (short analogous
this ma;
A second asis)
value.
polarized.
to the lLb (t&J
as more y than
Between
these
with
is observed Both
with
our calculated result of the same value Y,,,,~ at 37,600 cm-‘, again in excellent
of those
transitions
The first of these
of naphthalene.
might
Yamazaki
are calculated
be expected and Baba
to be princi-
to be x polarized,
(3) also estimate
these
x (79”).
two
The vapor
P-?T* bands
34,2X)
cm-‘.
spectrum
readily
tit into the progression
we calculate
shows
a weakly
a maximum
of the previous
band,
allowed
at -35,5DO and is difficult
cm-r
n-a*
band
which
does not
to associate
at
as the
onset of the following structure. This weak maximum disappears in alcoholic solution. A more diffuse maximum is noted at -43,530 cm-r, where we again calculate an n-r* transition; the calculated intensity is too small. The band we calculate at 44,800 cm--’ is either buried in the subsequent intense band, or, perhaps, is associated with the peak at -43,500 cm-l, further obscuring our calculated n-a*. Two intense peaks are observed at -46,300 cm-r and 47,300 cm-r and are associated with two electronic transitions calculated which we assign *-a*
to lie 800 cm-’ (my).
apart.
A shoulder
is reported
at 51,000 cm-r
(Z-0
tl. Quinomline The
observed
spectrum
of quinosaline
(Table
V) contains
a ‘HI (n-r*)
transition
at 27$)71 cm-’ (16) which is in good agreement with our calculated value of 26,382 cm-‘. The first ‘-11 (VT*) band is observed and calculated at 31,950 cm-’ (v,,,:,~ value). The spectrum reported by Baba and Yamazaki (3) shows a slow decay of this band
464
RIDLEY Table
AND ZERNER V:
Quinoxaline
Observed Symetry=
1 B1
Calculated
Typea
Energy
II-n*
27.1a
1 Sl
ll--TI* 31.gb -35.oc
1 *I 1' B2
Oscillator Strength
Symmetry
0.13b
TYPE
Energy
II--TI* 26.4
0.013 (x)
?T-ll* 31.9
0.156 (2)
ll-n*
0.105 (y)
34.4
0.0 0.0
Il-ll* 36.2 n-l+ 42.3 .1
71-n*
R2
43.ga, 43.2b
0.30b
51.0apb
v-ii*
42.1
0.565 (2)
1R2
n-ll*
47.2
0.092 (y)
:::
n-n*
l*1
4a.3=, 47.5b 0.61b
1 *1
a)
Oscillator Strength
49.5
1.017 (2)
ll-TI* 50.2
0.549 (y)
n-n*
0.000 (x)
50.9
Ref. 16
b) Ref. 3 c) Interpretationof Ref. 3, estimated vmx
(extending to -40,000 allowed r--rr* transition
cm-‘) which we would attribute to the presence of a second of ‘Bs symmetry calculated to lie at ~34,400 cm-‘.
The second well-defined reasonable however, maximum -51,000
value
structure
is seen at 43,150 cm-’
accord with the ‘A1 (?T-?T*) we calculate is changed at -47,500
from
that
previously
we assign ‘Bz
(r-rr*).
suggested The
(3) (vmax value) and is in
at 42,670 (16).
intense
cm-l.
The
The assignment
broad
structureless
band with maximum
at
cm-’ we again calculate as two nearly degenerate allowed bands.
e. Phthalazine In 1968 Hochstrasser
and Marzzacco
(15) reported two n-?r* transitions
at 25,359 cm-’ and 27,016 cm-r, both assigned ‘A2 (forbidden).
Phthalazine
with origins would then
differ from the other diazines of this study not only in possessing a forbidden n+r* transition as the lowest transition, but also in the observation of two such states before the onset of the ?~-rr*. Noting
the uniqueness
of this latter observation,
Hochstrasser
and Wiersma (17) used the Stark effect on dominant lines of the “two” features, determining a change in dipole moment between the ground state and both of these excited states as 3.05 f 0.10 D. Arguing that two different electronic states are unlikely to have such similar dipole moments, Hochstrasser and Wiersma reassigned all the structure before the first ?T-?T* to a single ‘AZ (n-r*) state with a characteristic, if somewhat “mysterious:” 1657 cm-l totally symmetric vibrational model. (This vibronic ‘A2 state is forbidden, but all major features of the first system are repeated at 1657 cm-‘). The vibration is “mysterious” in that its frequency increases upon deuteration. Our calculation yields ‘Br (nlr*) and ‘AZ (n-r*) below the lowest ‘A1 (K-T*) state (Table VI). These nl* states stem principally from excitation of the highest “n”
SPECTRA
01’ AZANAPH’l-HALEYES Table
VI:
Phthalazine Calculated
Observed
Type
symetry
Oscillator Strength
Energy
lB2
1A
symetry
Type
Energy
Oscillator Strength
lBI
n-Tr*
28.5
0.02
II-n*
31.4
0.00
25.4a
n-l!*
27.0’
il-lT*
33 .ob 34.oc
NO-O) ) (V Dlax
0.01
lA1
ll-il*
34.0
0.06
(2)
ll-ll*
38. .5b (Urnax)
0.10
IB 2
71-ll*
37.7
0.17
(Y)
lA2
II-n*
46.3
0.000
IBl
II-l!*
46.8
0.003
lA1
r-71*
47.4
0.958
1
47.3b
1
1
1.18
A2
B2
IAl IB 2 1
a) Ref. 15,
single
b)
Ref.
c)
Interpretation
d, e)
(P)
*--1T*
28.5d
lA1
46.5
crystal
“O_o
values,
see
B,
(2)
TI-TI*
48.1
.noz
(Y)
ll--71*
4A.5
0.84
(2)
ll-7i*
w.5
0.66
(y)e
v-71*
52.3
.no2
(Yje
text
3
for
of
these
umax The relative
published
two features,
intensities
of
spectra, see these
Ref.
3
text two
1
R2 states
is
verv
geometry
dependent
orbital &(a_) (Table II) to the four lowest unoccupied A* m.o.‘s, contradicting the previous assumption that these states arise from excitations between bn(n_) and ar(n+) orbitals to the lowest unoccupied ?r* m.o., br(r*) (4). Excitations from a*(%+) seem to play only a minor role in the formation of these states. Noting that our calculations generally yield nlr* states with oscillator strengths 5 times larger than experiment, and ?T-* oscillator strengths 2 times larger, we predict a lowest lying weakly allowed 0.003), a forbidden ‘A I (a~*) - 3000 cm-’ higher in energy, and then ‘Hl (a**) (j = a close lying ‘A 1 (T-T*), (f - 0.03). In comparing with experiment we are led to examine two major possibilities. (1) Our calculated order is correct. The ‘A 1 state would then probably be covered by the preceding ‘Br (n-r*) and following ‘A r (?T-?T*)systems. (2) The calculated order of the two nlr* transitions is reversed. This could be effected by, a poorly assumed geometry for the molecule, additional configurations in the configuration interaction selectively depressing the I.42 (n-r*), or a IA 2 (~-IT*) state of much different geometry than the ground state. In such a case the weakly allowed ‘Br (n-r*) might be expected to make its presence felt before or in the following IA 1 (FT*). Dipole moments are, of course, sensitive to our assumed geometry, but the differences of dipole moments between states might be less dependent. The dipole moments of excited states, however, are also very sensitive to configurational mixing. We calculate a ground state dipole moment of -6.59 D. Our ‘Br (nlr*) has a calculated dipole moment of -2.86 D. To indicate how sensitive this calculated dipole is to configurational mixing, the most dominant ‘RI [b2(n_) - ar(?r*)] configuration has a dipole moment
466
RIDLEY
FIG. 2. Spectrum of Phthalazine line weight schematically indicates
of +3.2
D. We estimate
configurations the latter,
AND
ZERNER
vs N-N Bond Length. Only the major greater oscillator strength.
that “reasonably”
features
different combinations
are shown.
of the two dominant
could lead to a dipole moment between this value, +3.2
of course, being considerably
the ground state and the ‘B1 (n-r*)
D, and -3.6
closer to the value we calculate.
is thus estimated
(generously)
Heavier
D,
Ap between
to lie between 9.8 D
and 3.0 D (our calculated value, 3.7 D), and includes the Hochstrasser and Wiersma value of 3.05 D. The ‘AZ (n-a*) has a calculated dipole moment of -0.14 D, although different
mixtures
of the two dominant
configurations
yield a dipole moment
from
+5.6 D to -1.2 D. For the ‘AZ (n-n*) Ap = 12.1 to 5.4 D, and does not include the experimental value 3.05 D. We conclude that it is very unlikely that both these states have the same dipole moment,
and that only our lB1 (nl*)
could have the observed
dipole. The above analysis favors the first hypothesis
that only one n-?r* state is observed,
and that it is of ‘B1 symmetry. Together
these two features,
now assigned
(vln*x appears to lie slightly beyond two quanta with our calculated value.
as one lB1, have a vrnax -28,500
cm-l
of the 1647 cm-l mode), in agreement
In examining the second hypothesis, we have studied the effect of ground state geometry on the calculated positions of these two n-?r * transitions. Figure 2 summarizes the effect of the N-N bond length on the major features of our calculated spectrum. Bond length vs bond order considerations, as well as extrapolation along the series benzene (known), pyridine (known), pyridazine (not known) suggest RN__N= 1.30 A. Reasonable deviations from this geometry do not change the order of the ‘BI and ‘AT states. A shortening of this bond length in the excited state, as suggested by Hochstrasser and Wiersma (l?), and supported by an analysis of our ‘At state would cause us to overestimate the transition energy to ‘At by a masimum amount equal to the zero point energy of the modes which describe the distortion (1). This could lower the ‘At relative
to the lB1 by an amount
sufficient to reverse the levels.
probably
no larger than -1000
cm-‘,
which is in-
SPECTRA
467
OF AZANAPHTHALENES
Table VII: Cinnoline Observed
Calculated
I
TYPO
Energy
n-rr*
22.7a
II-lT*
24.8
0.01
TI-TT*
31.5a
[2ROOlb'=
8-T*
32.7
0.17 (50')
TI--il*
35.3b
[25001d’C
7T-TI*
35.8
0.17 (37")
n-l+
44.3b
[420001d'=
oscillator Strength
TYPO
Energy
Oscillator Strength (2)
n-TI*
39.2
0.002 (2)
ll-?T* iT-1T*
42.4 43.9
0.27 (-10') 0.41 C-24')
U-T*
45.8
0.003 (2)
IL-T* T-IT*
49.4 49.8
0.007 (2) 0.41 (69')
TI-IT* U-71* 71-l!*
51.5 52.2 52.2
0.18 (39") 0.002 (2) 0.74 C-9")
a) Ref. 19, band origins b) Ref. 20
d) average from references 19 and 20
We can, of course, never eliminate the possibility of additional configurations in the configuration interaction reversing the energy of these two states. It is unlikely that single ercitations will effect such a reversal : we hesitate at this time to speculate on the role of the distant but far reaching doubles. .4n ‘A 1 (r~*) band is observed with origin at 32,990 cm-’ and v,,,,, at -34,000 cm-’ ; the calculated value is in excellent agreement. A band of ‘Bz (VT*) symmetry is observed at 38,636 cm-l and calculated at 37,700 cm-‘. .A very intense band is reported at 47,267 of ‘A1 (r-r*) type; three ?T-?T*transitions are calculated in this region, including two intense *Ar (a-*) separated by 1100 cm-‘. The observed band appears asymmetric, suggesting the possible presence of this second intense band. The corresponding structure in naphthalene appears as a shoulder at -49,500 cm-‘. This shoulder was also calculated as a separate electronic band, but again within the “shadow” of the more intense preceding one. Of the azanaphthalenes we have investigated, the ?r electronic spectrum of phthalazine bears the greatest resemblance to the spectrum of naphthalene. From the molecules of this study, it would appear that the presence of an LYazanitrogen is a much greater perturbation on the spectrum of naphthalene than is the presence of a p azanitrogen. That phthalazine resembles more the spectrum of naphthalene than does isoquinoline is probably the influences of phthalazine’s greater symmetry. f. Cinnoline
The lowest observed band in the spectrum of cinnoline is an nlr* with origin reported at 22,711 cm-’ (19). This is in good agreement with the calculated value of 24,797 cm-’ (Table VII). The second band (~a*), observed at 31,542 cm-l (origin) and calculated
RIDLEY
468
AND ZERNER
Table VIII: QUinaZOline Observed
Calculated
I
TYPe
Energy
II-ri*
27.@
r-n*
32.7b
[25001=
VT*
36.8b
[2500]=
n-n*
45.0b
[400001c
Oscillator StrenPth
Energy
Oscillator Strength
n-F+
30.6
0.01 (2)
T--B*
33.5
0.08 (81")
II-T*
36.1
0.0001 (2)
ll-lT*
38.0
0.08 (79')
I-&-?I*
42.1
0.001 (z)
n--71* ll-ll* 71-n* n-71*
49.9
50.4 51.8 51.8
0.006 (2) 0.34 (45-j 0.46 C-63') 0.002 (2)
ll-IS* T-lT*
54.5 56.2
0.0005 (2) 0.19 (54')
IT-71* T-lT*
a) Ref. 21, band origin b) Ref. 20
at 32,687 cm-‘, is reported by Wait and Grogan (19) to be a continuum 2200 cm-’ Simple analysis of our calculated state does not wide and to lead to “predissociation.” indicate the nature of this predissociation. Two additional maxima are observed in water solution: one at 35,300 cm-‘, the other, an asymmetric band, at 44,300 cm-’ (20). These we assign to r-?r* transitions calculated at 35,780 cm-l and to two allowed transitions calculated at 42,380 cm-’ and 43,890 cm-‘. The published spectrum of (20) suggests a further maximum > 50,000 cm-l. From 50,000 cm-’ to 52,000 cm-* we predict several transitions, including three intense P-T*. g. Quinazoline Hasegawa et al. (21) report an n-n* band with origin at 27,581 cm-’ in the vapor spectrum of quinazoline (Table VIII). We calculate an nlr* band with vrnax at 30,648 cm-‘. From cinnoline to phthalazine to quinazoline (and approximately quinoxaline) this nq* is observed shifted 2650 cm-l and 2220 cm-l, respectively, compared to calculated shifts of 2350 cm-’ and 3500 cm-*. Three additional bands are reported for quinazoline in Hz0 (ph 7) with maxima -32,700, -36,800 and -45,000 cm-l (20). These we correlate with r~* transitions calculated at 33,500, 38,100 and two bands at 45,600 and 47,100 cm-‘. Experimental oscillator strengths are not available for quinazoline, but the calculated oscillator strengths seem in good accord with the reported tmax and the general shapes of the bands. h. Pteridine
The calculated electronic spectrum of pteridine is compared are predicted Mason (22) in Table IX. Two nlr * transitions
with that reported by before the first ?T--?T*.
SPECTRA
OF AZANAPHTHALENES
Table IX:
Pteridlne Calculated
Observed TYpe
Energy
OSCillatOI screngthb
TY!Je
Energy
Oscillatot Strength
Xl--Ti*
25.8
[S41
II-*
23.4
0.003 (2)
30.4= ?
[6311
II-n*
30.0
0.020 (2)
33.2
174901
il-Tl*
Ti-TI*
42.6
n-71*
47.6
r29101
[110001
11-n*
32.6
0.280 (-26')
n-R* n--71*
35.1 35.9
0.000 (2) 0.000 (2)
II-n*
V-V*
39.4 39.8
0.001 (2) 0.089 (82")
II-n* TI-TI* II-ll* ll-TI*
45.9 46.2 46.7 50.0
0.006 0.336 0.000 0.644
II-lT*
52.1 52.4
0.001 (2) 0.800 (-110)
n--t*
(2) (35") (2) (26')
a) Ref. 22 b) An average 6, see ref. a) c) Shoulder on spectrum published in ref. a)
Although the second of these is calculated to lie within the observed system of the neighboring T--B*, it is calculated to have intensity of its own and its effect might be observed. (The published spectrum of Mason shows a shoulder at -30,400 cm-’ where we calculate this second nlr*). The lowest nlr* calculated at 23,400 cm-’ is in good agreement with that observed at 25,800 cm-‘. The lowest lying ?r-u* is both calculated and observed at -33,000 cm-‘. A second rq* band is seen at 42,600 cm-’ and calculated at 39,800 cm-‘. In the region between 35,100 and 46,700 cm-l five transitions of n-s* type are calculated, none of which are estimated to have much intensity. Following these we find three intense sq* bands at 46,200 cm-‘, 50,000 and 52,400 cm-‘; an intense band is observed with maximum at 47,600 cm-r (22). Again, the calculated oscillator strengths nicely reflect the trend observed in the values of extinction coefficients that Mason reports (22). i. 1,4,5,8_Tetrazanaphthalene A comparison of the calculated spectra of 1yvs P azanaphthalenes indicates that the tetra 01aza compound should possess a very low lying nlr* forbidden excitation (4). The results of a calculation on this compound are presented as Table X. 1,4,5,8-Tetrazanaphthalene has DPh symmetry, and the axes are defined as in naphthalene (Fig. 1). There are two n-n* transitions predicted before the first ?T--?T*:as with pteridine the second of these lies very close in energy to the r+ *. The prediction of a forbidden low lying vz--?~*transition (at 465 rncl) is verified and ought to prove interesting experimentally. The only experimental information we have located on this molecule is rather cursory (2.3). The x-rr* states observed at 31,100 cm-’ and 44,600 cm-’ (solution C6H1J are in
470
RIDLEY Table X:
AND ZERNER
1,4,5,S_Tet+azanaphthalene
Observeda Type
Energy
n-71*
31.1
symetry
[2301b)
24.9 ?
TI-‘TI*
Calculated
Oscillator Strength
IB IS 1 B3u
[160001b)
lB2"
lB 2S l* u TI-‘TI*
44.6
[55001b)
lB23 1
Bill IA
Rlu
lA " 1 % IB IS lB" a) vmx
Energy
Oscillator Strength
n-7+
21.5
0.0
n-n*
29.1
0.026 (2)
?I--TT*
30.6
0.432 (x)
n-V*
34.2
0.0
n-71*
35.5
0.0
II-n*
39.6
0.0
1T-IT*
40.5
0.126 (y)
II-IT*
43.2
0.0
II-n*
45.1
0.0003 (2)
n-IT*
49.3
0.642 (y)
II-ll*
50.1
0.0
g
lB3u 1
TYPO
II-r*
52.6
0.0
n-77*
53.3
0.0
n-71*
53.4
1.552 (x)
in C6H12, Ref. 23
b, Emax
reasonable
accord with our calculated
values,
and our calculated
oscillator
strengths
reflect nicely the observed emax. DISCUSSION a. The Calculations
The model we have employed
seems equally
accurate
spectra of azanaphthalenes as it was for azabenzenes. of double excitations, estimated to appear at -85,000 -65,000 calculated
cm-l for naphthalene, transition
energies.
in offering rationale
for the
Omission from these calculations cm-’ for benzene, but as low as
has not seemed to affect greatly
the reliability
It could be argued that our parameterization
of our includes
some of the influences of ignored higher excitations, and, no doubt, this is correct. Nevertheless, our parameters were chosen for the azabenzenes where the influences of higher excitations might be expected to do considerably less damage to the calculated transitions below -60,000 cm-’ than they would for the azanaphthalenes. It has been observed that the calculated intensities, however, are considerably
more
sensitive to the inclusion of higher excitations than the transition energies themselves, and most sensitive are the transition directions (24). With this in mind, we view our predicted directions, where they are not determined by symmetry, as merely suggestive. This uncertainty would apply, in particular, to the higher lying transitions. We have suggested that the band observed at 52,600 cm-’ in naphthalene is lBQg (?T-?T*), borrowing
intensity
from a lower lying transition.
That
no vibrational
SPECTRA
structure
is reported
excitations another
for this
calculated possible
band
might
be a ramification
in close proximity.
explanation
471
OF AZANAPHTHALENES
is that
As discussed
this observed
of the presence
in the section
band
correlates
of other
on naphthalene,
with
our calculated
band at 60,500 cm-’ this model. Extensive
(Table I). If so, this is the worst ‘error’ that we encounter using CI studies on the ‘NzU states does not change this conclusion.
Admission
excitations
of higher
onI>. a reparameterization to make the calculation observed
at -52,600
in understanding have employed b. The Speclra
cm-‘,
the here.’
might
lower
this
transition
energy,
but
requires
not
of the model, but a difficult configurational selection process feasible. A better experimental understanding of the band then,
phvsical
would
content
not only be of experimental of such
quantum
interest,
mechanical
but help
methods
as we
_
oj dzanapkthalenes
We have found to be misleading,
the apparent similarities in the spectra of the azanaphthalenes often and as such have discussed each molecule separately. Nevertheless,
the urge to draw
general
conclusions
is inescapable,
and we have
attempted
to do so.
1. The highest occupied molecular orbital (homo) of the azanaphthalenes which we have esamined is always of ?r symmetry (Table II). This is so regardless of whether the first electronic
transition
,7. As might nitrogens. better
is nlr*
be expected,
Generally
nitrogen
or r-r*.
there
(although
character
not always)
(Table
5’. In the diazanaphthalenes 1. Examining u substitution. to be a lowering naphthalene
occupied Table
of this
orbital
little
(Z?)].
m.o.‘s
that
n m.o.‘s
as there
are calculated
are principally
rr molecular
more cy character
Examining has very
these
by at least 2 eV (Table
the highest
it has slightly
however, identifiable
are aza.-
to have
60yo or
II). the m.o.‘s
lone pairs are split in energy find that
[Note,
are as manv
orbital
(hoamo)
than 0: it should
be slightly
II, this is seen to be true, b!
density
-0.30 eV/nitrogen at the 01 positions
the f
((2 uinazoline
II).
combinations
of
is the exception.) of naphthalene
we
more sensitive
but the major
effect
to
seems
atom. The second hormo of and should be relatively stable
to 01substitution. This is demonstrated : each p substitution lowers the orbital -0.55 eV, each (Y substitution, -0.15 eV. In a similar fashion the third hoamo is more sensitive to /3 substitution clusions
of naphthalene again
and the fourth
are reached appears
to CL By the perfect
for the lowest
is slighti?.
more
to be an energy
empty
sensitive lowering,
r m.o.‘s
pairing
theorem
(le?rmo’s).
to cr substitution
(29) similar
For example,
than
in this case of -0.35
p, but the major eV/nitrogen,
con-
the lermo effect
etc.
The highest occupied u symmetry orhital has -7Cy, nitrogen character and is located at -9.8 f 0.2 eV for all of these azanaphthalenes except phthalazine, which has 84% nitrogen character, and lies at -9.2 eV (Table II). 5. Quinoline
is calculated
to have
has not been observed. It ma\transitions before the first a-2, least one n-r*
transition
before
an n-s*
be that
transition the
below
the first ~--1r*, but
monoazanaphthalenes
show
whereas all of the polyazanaphthalenes the onset of the first ?T-?T*.
’ The orizin of the second transition in benzene is another example. ploying only singly excited configurations predict this state as *B1,‘ (I)
Semiempirical
this
no n--T* show
calculations
at
em-
: inclusions of double excitations
(either within an approximate or ah initio framework) suggest that this state might be I& Vi, 26)]. Shavitt reports a more extended CI, in which he restores this state to IB,, (27).
[e.g.
RIDLEY
472
AND ZERNER
6. The two lowest ~z--?T*states are mainly transitions between the highest occupied n m.o. and the two lowest empty ?r m.o.‘s, respectively. The calculated position of these two states can be obtained approximately from the orbital energy differences of the appropriate m.o.‘s minus a constant (i.e., --J •l- 2K = 44,500 cm-l). Since the lowest ?r-7r* transition in all of these compounds has onset at -32,000 cm+ (another constant), the number of n-n* transitions before the first ?T-?T* can be estimated in this model by the number of times the condition At(n-?r*) - 76,500 5 0 is met. Any reasonable quantum mechanical model should also be useful in this estimate with, perhaps, an adjusted constant. 7. The lowest rr-lr* band in these compounds is observed and calculated to lie between 32,000-34,000 cm-l. This observation is not simply interpreted; for naphthalene, quinoline, isoquinoline, phthalazine, and quinoxaline (called Group I for subsequent discussion) this structure is caused by configurational mixing between f+ h and e 3 g with some, if symmetry permits, ,f-+ g. (Platt notation for ?r orbitals only (30), f = honmo, g = lenmo, etc.) For cinnoline, quinazoline (Group II) and pteridine, this band is calculated to be principally f --+ g. 8. The second ?T-?T*state of the azanaphthalenes is calculated to lie between 34,!)0040,000 cn-I, and is, except for cinnoline, short axis polarized. For Group I (see discussion under point 7) the transition is principally f -+ g; for Group II and pteridine, f-t h, e --+ g with some f + g. Within Group I each (Ysubstitution lowers the transition energy relative to the corresponding state of naphthalene, while B substitutions have little effect. (For quinoxaline with two (Yazanitrogens, the first two r-a* states are calculated only one band is observed.) This behaviour is very near in energy. Experimentally predictable from the nature of the f’ and g ?r m.o.‘s (point 4). 9. At least two intense transitions are calculated to lie in the region between 42,00052,000 cm-‘. For naphthalene and /3 substituted azanaphthalenes, the two are close in energy and appear not to be completely resolved. o-substitution is a greater perturbation on the naphthalene spectrum; two maxima replace the single, although asymmetric, peak of naphthalene found at 47,530 cm-‘. 10. How closely the ?T--?T* spectra of the azanaphthalenes resemble naphthalene seems to be simply reflected in how closely the two lermo’s resemble those of naphthalene. For example, the orbital energy difference between g and h in phthalazine, at 0.54 eV, is the same as that observed for naphthalene (Table 11). The spectrum of phthalazine closely resembles that of naphthalene. Isoquinoline, in which this difference is 0.68 eV is the only other azanaphthalene with a spectrum “obviously” resembling that of naphthalene. These differences for the other molecules of this study are greater than 1 eV. The occupied ?r m,o.‘s do not reflect this simple correspondence. ACKNOWLEDGMENT This work was supported in part- by a research grant from the National Research Council of Canada.
RECEIVED: September
20, 1973 REFERENCES
1. M. ZEKNEK AND J. RIDLEY, Theor. Chim. Acte 32, 111 (1973). 2. G. FAVINI, I. VANDONI, AND M. SIMONETTA, Theor. Chiwr. Acta (Bed.) 3, 45 (1965).
SPECTRA
01: AZAXAPHTHALENES
473
.?. H. BABA AND 1. YAMAZAE;I, J. Mol. Speclrosc. 44, 118 (1972). f. A. D. JORDAN, I. G. Ross, R. HOFFMANN, J. R. SWENSON, AND K. GLEITEK, C’hern. P&w. Lett. 10. 572 (1971). 5. G. A. GEORGEEND G. C. MORKIS, J. Mol. Spectrosc. 26, 67 (1968). 6. R. I,. HUMMEL AND K. RUEDENBERG, J. Phvs. Ckern. 66, 2334 (1962). 7. R. PAIIISEK, 1. Chem. Phys. 24, 250 (1956). X. H. KLEVENS AND J. R. PLATT, J. Cheni. Pizys. 17, 470 (1949). Y. D. W. TUWNEK, “Molecular Photoelectron Spectroscopy,” Wile?- Interscience, London, 1970. IO. a. H. PKEUSS, Inlernat. J. @uwztutrz Cheru. 2, 651 (1968). I). R. J. BUEN~EK AND S. D. PEYEKIMHOPF, Cherlt. Phys. Lett. 3, 3i (1969). I/. a. R. PAKISEK, J. Chem. Phys. 24, 250 (1956). t). I:. MOMICCHIOLI AND A. RASTELLI, J. Mol. Spectrosc. 22, 310 (1967). 12. J. P. BVKNE AND I. G. Ross, Am-tralian J. Chettt. 24, 1107 (1971). 13. H. ZIMMEKMANN AND N. JOOP, Ber. Bunsenges. Physik. Cke~t. 65, 61 (1961). If. G. FAVINI AND I. R. BELLOHONO, Institute Lo~~tbardo (Rend. Sci.) A99, 380 (1965). 1.:. R. H~CHSTKASSEK AND C. MAKZZACCO, J. Chem. Phys. 48, 4079 (1968). 16. R. W. GLASS, 1,. C. ROBEKTSON, AND J, A. MEKKITT, J. Chew. Phys. 53, 3857 (1970). 17. R. M. HOCHSTKASSER AND D. A. WIEKSMA, J. Chew. Phys. 56, 528 (1972). 1X. H. LL~THEKAND H. J. DREWITZ, 2. IJectrochet,t. 66, 546 (1962). lY. S. C. WAIT ANU F. M. GROGAN, J. Mol. Spectrosc. 24, 383 (1967). 20. A. R. OSBORNE, K. SCHOFIELD, AND L. N. SHORT, J. Chem
Sot., 4191 (1956). 41, 2608 (1968).
31. Y-. HASEGAWA, Y. AMAKO, AND H. AZUMI, BuZl. Cherrl. Sot. Japan
172. S. I;. MASON, J. Chem. Sot. 2336 (1955). 23. S. I:. MASON, “Physical Academic
Methods
Press, New York,
in Heterocyclic
Chemistry”
(A. R. Katritzky,
Ed.), Vol. II, p. 24,
1963.
Zf. J. MICHL, private communications. Xi. J. E. BLOOK, J. LEE, AND S. GARTSIDE, Proc. Chevr. Sot. 413 (1960). 26.
R. BUENKEK, J. L. WHITTEN, AND J. D. PETIE, J. Che~z. Phys. 49, 2261 (1968).
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at the First International
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lY73.
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Z
Menton,
Prance,
OF MOLECULAR
SPECTROSCOPY
50, 457-473
The Calculated J. E. fkpartrtzent
(1974)
Spectra of the Azanaphthalenes RIDLEV
c$ Chemistry,
AND M.
C. ZERNER
L’nioersity of Guelph, Guelph, Ontario, Canado
X modified ISDO (intermediate neglect of differential overlap) method is used to calculate the electronic spectra of naphthalene and some mono-, di-, and tetraazanaphthalenes. The technique is capable of reproducing the better classified bands of this series within a rms error of -1000 cm-‘. The four lowest n-x* bands of naphthalene are well represented; a fifth band reported may be iBs,(s-x*) borrowing intensity. at -52,600 cm-‘, and generally assigned ‘B 2U( or*), The lowest excited singlet state calculated for quinoline is n + K*, estimated nearly degenerate with the lowest V-K*, while that for isoquinoline is calculated B--?T*; experimental evidence supports the P-T* assignment in both cases, but the corresponding absorption in quinoline appears complex. Of the diazanaphthalenes examined (with two nitrogens in one benzenoid ring) all are calculated to have one n-r* transition before the first F?T* except phthalazine, in which two n-r* transitions are calculated to be the lowest lying. This is in accord with experimental evidence to date, although the nature of the observation in phthalazine is reinterpreted as one ‘&(n-a). 1,4,&S-tetrazanaphthalene is predicted to have a group theoretically forbidden na* excited state as low as 21,000 cm-‘, and should prove interesting experimentally. Even though n-rr* excited states are often calculated to be the lowest lying, none of these compounds are predicted to have an “a” orbital as homo. I:urther interpretations calculations. Theoretical
of the spectra of the azanaphthalenes are made in view of the limitations of the method employed for this study are discussed.
INTRODUCTION It is the purpose of this paper to examine the spectra of the more common azanaphthalenes. We employ for this purpose a modified “intermediate neglect of differential overlap” technique (INDO) previously developed and parameterized on the spectra of benzene and the azabenzenes (1). When applied to the compounds of this study the model seems capable of reproducing the energies of the better understood bands within a rms error of - 1000 cm-l. In addition to reproducing known spectra, these calculations strongly suggest explanations for bands not yet classified, point to the possible presence of excitations not yet observed, and suggest perturbing influences on bands reported as (‘diffuse.” We examine the azanaphthalenes for two reasons. First is the observation that there are many interesting ambiguities and uncertainties in the experimental spectra of these molecules that are central to the understanding of heterocyclic spectra. Second, preliminary investigations with double excitations lead us to believe that theories that onl! include single excitations in the “configuration interaction” (CI) may begin to degenerate in accuracy with molecules much larger than naphthalene. This is a problem to 4.57 Copyright 0 1974 by Academic Press, Inc. All rights of reproduction in any form reserved.
458
RIDLEY
_UD
ZERNER
Y
i
Y
&
Y
ck?
N
N
x
x
N
N
QUi”WOli”e
I;IG. 1. The Azanaphthalenes .4ctn tryst. 10, 504 (1957).
PteIidine
of This Study.
References
which we give only cursory examination
refer to Coordinates.
here, remarking
(a) D. IV. J. Cruickshank,
only when our results might
be affected by inclusion of higher excitations. Several other investigators
have examined the spectra of azanaphthalenes
within the
LCAO-MO-SCF-CI framework. The most extensive of these studies are perhaps the works of Favini, Vandini, and Simonetta (2) and Baba and Yamazaki (3). Both of these groups worked within cluded core polarization
the Pariser-Parr-Pople
“pi-electron-only”
effects in a fairly systematic
spectra do not lead to very different but, of course, such calculations
conclusions
cannot
framework,
but in-
fashion. Their results for the r~*
than does the present investigation,
be used to estimate
the location
of the n-u*
transitions. Some of the theoretical and experimental information on the n-r* states of the azanaphthalenes has been summarized in a paper by Jordon, Ross, Hoffmann, Swenson, and Gleiter (4). METHODS
The model that has been used in these calculations has been described elsewhere (I). The computer program performs the LCAO-MO SCF calculations from an input of molecular geometry and atomic numbers. A ground state calculation is followed by a configuration interaction calculation which results in the desired spectroscopic transition energies and oscillator strengths. These oscillator strengths are estimated from the dipole length operator.
Table I: Naphthelene (x1000 .uI-~) Calculated
Observed= oscillaror Strength
Energy
Oscillator Strength
Energy
SyllZllletryTYPO
synrmetry
Type
lB2u
ll-TI*
32.0
0.002
1 B2u
n--n*
32.5
0.002(x)
ll--T*
37.5h
0.102
1 Bl"
*-lT*
37.7
0.148(y)
R?
45.0
11-v*
44.8
0.0
?l-Ti*
47.5
Tr--TI*
45.8
1.866(x)
1B3B
ll-ll*
46.7
0.0
lBlu
1
Bzu
IA R 1 B2u
1.0
IL3 1U
lt-?T*
49.5
0.3
1 Bl"
TI-Il*
48.6
0.672(y)
1 B2u
Ti--il*
52.6
0.1
1Blg
ll-a*
52.7
0.0
1B3B
ll-n*
52.7
0.0
lA S
TI-ll*
56.0
0.0
1B28
a-IT*
51.2
0.0
1B28
n-o*
57.4
0.0
1 B2"
71-11*
60.5
0.115(x)
R
>54.0
1 Bl" a)
-59.a=
Ref. 5
b) vmmaxvalue
The molecular referenced estimated
coordinates
c) Ref. S
used
in these
in Fig. 1. When such data from those of representative
save naphthalene, in geometry
there
can affect
is some uncertainty the calculated
prove
possible
to improve
were
in the atomic
spectra.
changes, however, to reverse the order etries we have assumed are reasonable it ma\.
calculations
obtained
from
the data
was not available, bond lengths and angles were compounds. In all the compounds esamined, It appears
coordinates.
Modest
to take rather
of energy leveis. One indication that is our agreement with esperiment.
our results
by adjusting
assumed
changes
large geometr!the geomSimilari!-
coordinates,
and
thus also gather geometric information. This we have not attempted. The parameters that have been chosen for this study are based solely on the results obtained for benzene and pyridine (I). Ko attempt was made to optimize eters over the series of molecules studied. C’onsequentl!,, some adjustment
these paramof parameters
may result in a better over-all agreement with experiment. In general, our calculated results are insensitive to modest parameter change. For the configuration interaction calculation we have included all single excitations that lie within -65,000 cm-’ of the ground state, as well as some additional selected configurations
of interesting
symmetry
type
(generally
50-70 configurations).
Including
singly excited configurations much bevond 65,000 cm-l is of questionable value for two reasons. R\-dberg-type transitions are estimated to begin to occur at -50,000 cm-‘, and
460
RIDLEY
AND ZERNER
we have no mechanism for their inclusion or their mixing with valence states. Second, we estimate double excitations to begin to occur at -65,000 cm-‘. Unlike singles that have the predictable
behavior
of mixing with other configurations
of proper symmetry
type within a range of, say, ~1~10,000 cm-‘,
doubles appear to affect calculated
within a much larger energy range. Rather
than include the large number
excitations content
states
of double
which can occur once we admit them, it is best to first examine the physical
of such semiempirical
parameterization
methods
as employed
here. That
such as we have chosen includes electron
is, it is expected
correlation,
that
and to include
doubles would be to include this effect twice. We avoid this question here, and admit that the utility of our answers may degenerate
much above 50,000 cm-‘.
We have chosen to compare our results with experimentally than ~00values where possible for reasons previously
determined
vrnaxrather
discussed (1).
RESULTS a. Waphlhalene The calculated
spectrum for naphthalene
is shown in Table I along with experimental
values reported by George and Morris (5). The calculated transition energies of the four lowest R-*
transitions
are in excellent
agreement with the experimental values. The calculated oscillator strengths for these bands are also close to those observed. The calculations indicate forbidden r-u* bands at 44,759 cm-l bands reported as Rydberg,
(‘A,) and 46,707 cm-l (lL&). centered
at 45,000
cm-l.
since they are not observed
and nearly fit into a Rydberg
One, or both, of these may be observed as
These bands have tentatively in solution,
sequence with the bands at -54,000
number of additional forbidden bands are calculated cm-’ and 62,000 cm-‘. George and Morris report Hummel and Ruedenberg
are too intense
a band at 52,650
cm-‘.
two forbidden
cm-’
(5). A large
to be in the region between 58,000 On the basis of calculations
(6) and on the basis of the intensity
as ‘Bt,. In this region we calculate
been assigned
to be hot bands,
by
of the band, it is assigned
bands, ‘BI, (T--B*) and lBpg (a-r*).
The next band of lBz, (T-T*) we find at 60,500 cm-‘, to be compared with Pariser’s (7) estimate at 64,700 cm-l. Extensive singly excited configuration interaction of ‘Bw states does not lower our calculated value significantly.
From this calculation
we suggest
than the band observed at 52,650 cm-’ is ~BQ, borrowing from the lBlr band at 48,630 cm-’ through one of the bt,, modes. We cannot, however, rule out the possible influences of higher excitations on the lBzu band we calculate at 60,500 cm-‘. In an early work of Klevens and Platt (8) a transition is noted at -59,800 cm-‘. We have found no recent mention of this transition: if real, then the observed singlet spectrum of naphthalene to 60,000 cm-r is nicely specified by our model if the band observed at 52,650 cm-’ is assigned to lBsg (r-r*), and the band at 59,800 to ‘Bzu (r-x*). In Table II we present the highest occupied and lowest virtual molecular orbital energies. Three ionization potentials of naphthalene are readily obtained from photoelectron spectroscopy (9); these are in good agreement with our eigenvalues (Koopmann’s Approximation). Beyond the third band the observed (and the calculated) structure becomesmore complex. Two ab initio SCF calculations on naphthalene have been reported in the literature (10). Our calculation compares well with the more complete of these (IfIb). In general,
SPECTRA
461
OF AZANAPHTHALENES
Table II: The Highest Occupied and Lowest Unoccupied Orbital Energies (eV). Naphthalene
Quinoline
Isoquinoline
Pteridine
b2,d(s*) 0.58
TI*
0.59
ll*
0.35
TI*
0.23
blg(n*)
71*
-0.40
ll*
-0.33
TI*
-1.45 -9.27
0.04
a"(n)
-7.95 (8.15)a
?I
-8.29
n
-8.05
lr
bgU(')
-8.60 (8.9ja
II
-8.74
n
-9.24
74%n -9.56
-9.96
66Xn -10.01
Ti
TI
78Xn -10.58
68Xn b2g(il) -10.12 aO.Oja
lr
-10.69
-10.36
-10.01
blg(fl) -11.77
ll
-12.08
Ti
-12.46
76Xn -11.56
b3g(u) -11.80
CI
-12.09
0
-12.19
88%n -11.90
ag(o)
-11.96
(I
-12.83
4
-12.81
*
-12.26
b2,W
-13.25
0
-14.28
D
-13.99
71
-13.54
bluW
-14.46
(J
-14.70
Is
-15.11
0
-14.28
b3"('T) -14.49
ll
-15.00
71
-14.97
ci
-14.89
b2"W
-14.77
Phthalazine
Quinoxaline
Cinnoline
a2m*j
0.02
0.24
71*
0.38
bl@*)
-0.51
bl('T*)
-0.85
71*
-0.79
71*
-0.60
a,(n)
a2w*1
0.50
7*
Quinazoline
-8.39
a,(a)
-8.60
TI
-8.41
TI
-8.52
b2(82%n) -9.19
bl(n)
-8.82
71
-9.32
ll
-9.39
bIW
-9.64
al(64%n) -9.78
7EXn -9.72
77%n -9.71
-10.56
64Xn -10.97
b2(90%n) -11.50
71
-10.73
a1(52%n)-11.29
a,(a)
-11.68
0
-11.34
lr
-11.18
a,(o)
b1W
-12.29
61%n -12.39
TI
-12.68
a,(a)
-12.65
bl(Tf) -13.13
b2W
-12.65
n
-13.09
0
-12.79
b2@)
a,(o)
-12.94
u
-13.52
0
-13.44
-13.17
b2W)
-15.10
a1(0)
-15.15
0
-14.48
0
-14.34
a,(n)
-15.51
b2(o)
-15.26
v
-15.27
0
-15.48
a)
Ref. 9
our highest occupied m.o.‘s have eigenvalues greater than the corresponding ab initio values; while our lowest lying unoccupied m.o.‘s lie at lower energy (decreasing the orbital energv gap by - 0.1 a.u.). In the ten highest occupied m.o.5 there are two energ). reversals, but these are between m.o.‘s calculated nearly degenerate in both techniques. In both calculations the tertiary carbon C, is most positive (+O.Olg), while the C,( -0.026) and C,( -0.022) populations are nearly identical (the numbers in parentheses refer to our values). Our pi bond orders (C-C@, 0.77; (Z-C,, 0.57; C&B, 0.55; (Z-C,, 0.52) agree in order with the results obtained in this ab initio calculation; both calculations disagree with most other results reported (IOU, II). b. Quinoline
The first band calculated for quinoline is an nlr* at 32,500 cm-r (Table III). There is no experimental evidence either in vapor or solution to support the existence of an n?r* state lying below the lowest K-T* (the latter observed with onset at 31,900 cm-r),
462
RIDLEY
AND ZERNER
Table III: Quinoline Observed Energy
T?W
n--71* 71-lT*
31.9a (onset) -33.oc (VCW) 36.2a
+-n*
*-TI*
44.3a
49.3a
Calculated Oscillator Strength
Energy
Oscillator Strength
Xl--Tl*
32.5
0.008 (2.)
il--il*
33.4
0.04
(-11')
TYPO
cxb, O.llP cyb,
Ti-TI*
36.6
0.16
(72")
0.517*(xb)
Ti-ll*
44.1
0.67
(-4')
n-ll* ?I-ll*
44.7 47.1
0.00 0.02
(2) (-55")
o.7438
I
IT-71* T-71* n-71*
ll--71* 71-Z*
1.15 (-2") 0.57 (87") 53.7 54.2 55.1
0.0004 (2) 0.15 (-87O) 0.0003 (2)
a) Ref. 3 b) Ref. 13 c) Vapor spectrum taken for this work:
bend center
although such a state would certainly affect emission. We calculate the maximum of the lowest ?T-?T*band at 33,400 cm-’ (Expt. -33,000 cm-l), -900 cm-’ above the n-?r*. This apparent energy reversal might be a shortcoming of the model itself, or the geometry we assume for quinoline. Within the model, additional configuration interaction might reverse the order of two bands calculated so close in energy. Such an interchange might also be attributed to the existence of two excited states with quite different equilibrium geometries (1). In this case one would expect calculated energy values to be greater than the observed vmax with greater deviation in geometry from the ground state. This n-n* transition is, however, noted for quinoline in naphthalene host crystals (4). Since in isoquinoline (Table IV) we calculate the rzl* transition 900 cm-’ above the first 7r-7r*, and there is no experimental evidence of this band in host crystals (4), the corresponding two states in quinoline must, in any case, lie very close in energy [see, however, (IZ)]. A second ?T-?T*transition is calculated at 36,550 cm-l and observed at 36,216 cm-‘. A third ?T-?F*calculated at 44,122 cm-’ is again in excellent agreement with the strong band reported at 44,282 cm-‘. A broad intense system is observed at 49,283 cm-l which we correspond to two calculated ?T--?T* bands at 47,907 cm-l and 49,638 cm-‘. A weak r-x* band is calculated at 47,103 cm-‘; it may be buried under the following intense structure. Zimmerman et al. (13) have assigned polarizations to the three lowest lying ?rq* transitions which are in accord with the dipole directions we calculate. c. Isoquinoline
The lowest electronic transition of isoquinoline is reported with onset at 31,540 cm-’ (3) and is assigned ?T--?T*.The band extends to -35,000 cm-’ with center at
SPECTRA
463
OF AZANAPHTHALENES Tsoquinoline
IV:
Table
Observed
Calculated
Energy
TYPO
Oscillator Strength
31.58 (onset) 32.5= (L)ctr)
X-T*
n-71*?
35.5?b
T-T*
37.6a
Energy
TYPE
0.02
n-e
I
0.09 (-56")
II-TI* 0.09
n-lT*? ‘r-71* ?
"43.5c
T-T* TI-Tl*
46.3a 47.3a
il-lT*
51.0 (s)C.d
3
Oscillator Strength
0.93
34.2
0.007 (1)
n-RI
37.2
0.16 (69")
"-lT*
42.9
0.001 (z)
71-n*
44.8
0.12 (19")
T--71* n-71*
46.7 47.5
0.89 (9") 0.81 (-8")
K-TT*
51.2
0.52 (86')
n-IT* T-T* TI-lT*
52.2 52.3 55.2
0.000 (7.) 0.08 (-110) 0.01 C-310)
a) Ref. 3 b) Vapor, this work. A feature on the following ?r+r*that disappears in CA30H: possibly part of the preceding system. c) Vapor, this work d) Ref. 14
-32,500
cm-r;
(Table
IV).
accord
with
bands
be compared
rq*
the calculated
pall\- y (short analogous
this ma;
A second asis)
value.
polarized.
to the lLb (t&J
as more y than
Between
these
with
is observed Both
with
our calculated result of the same value Y,,,,~ at 37,600 cm-‘, again in excellent
of those
transitions
The first of these
of naphthalene.
might
Yamazaki
are calculated
be expected and Baba
to be princi-
to be x polarized,
(3) also estimate
these
x (79”).
two
The vapor
P-?T* bands
34,2X)
cm-‘.
spectrum
readily
tit into the progression
we calculate
shows
a weakly
a maximum
of the previous
band,
allowed
at -35,5DO and is difficult
cm-r
n-a*
band
which
does not
to associate
at
as the
onset of the following structure. This weak maximum disappears in alcoholic solution. A more diffuse maximum is noted at -43,530 cm-r, where we again calculate an n-r* transition; the calculated intensity is too small. The band we calculate at 44,800 cm--’ is either buried in the subsequent intense band, or, perhaps, is associated with the peak at -43,500 cm-l, further obscuring our calculated n-a*. Two intense peaks are observed at -46,300 cm-r and 47,300 cm-r and are associated with two electronic transitions calculated which we assign *-a*
to lie 800 cm-’ (my).
apart.
A shoulder
is reported
at 51,000 cm-r
(Z-0
tl. Quinomline The
observed
spectrum
of quinosaline
(Table
V) contains
a ‘HI (n-r*)
transition
at 27$)71 cm-’ (16) which is in good agreement with our calculated value of 26,382 cm-‘. The first ‘-11 (VT*) band is observed and calculated at 31,950 cm-’ (v,,,:,~ value). The spectrum reported by Baba and Yamazaki (3) shows a slow decay of this band
464
RIDLEY Table
AND ZERNER V:
Quinoxaline
Observed Symetry=
1 B1
Calculated
Typea
Energy
II-n*
27.1a
1 Sl
ll--TI* 31.gb -35.oc
1 *I 1' B2
Oscillator Strength
Symmetry
0.13b
TYPE
Energy
II--TI* 26.4
0.013 (x)
?T-ll* 31.9
0.156 (2)
ll-n*
0.105 (y)
34.4
0.0 0.0
Il-ll* 36.2 n-l+ 42.3 .1
71-n*
R2
43.ga, 43.2b
0.30b
51.0apb
v-ii*
42.1
0.565 (2)
1R2
n-ll*
47.2
0.092 (y)
:::
n-n*
l*1
4a.3=, 47.5b 0.61b
1 *1
a)
Oscillator Strength
49.5
1.017 (2)
ll-TI* 50.2
0.549 (y)
n-n*
0.000 (x)
50.9
Ref. 16
b) Ref. 3 c) Interpretationof Ref. 3, estimated vmx
(extending to -40,000 allowed r--rr* transition
cm-‘) which we would attribute to the presence of a second of ‘Bs symmetry calculated to lie at ~34,400 cm-‘.
The second well-defined reasonable however, maximum -51,000
value
structure
is seen at 43,150 cm-’
accord with the ‘A1 (?T-?T*) we calculate is changed at -47,500
from
that
previously
we assign ‘Bz
(r-rr*).
suggested The
(3) (vmax value) and is in
at 42,670 (16).
intense
cm-l.
The
The assignment
broad
structureless
band with maximum
at
cm-’ we again calculate as two nearly degenerate allowed bands.
e. Phthalazine In 1968 Hochstrasser
and Marzzacco
(15) reported two n-?r* transitions
at 25,359 cm-’ and 27,016 cm-r, both assigned ‘A2 (forbidden).
Phthalazine
with origins would then
differ from the other diazines of this study not only in possessing a forbidden n+r* transition as the lowest transition, but also in the observation of two such states before the onset of the ?~-rr*. Noting
the uniqueness
of this latter observation,
Hochstrasser
and Wiersma (17) used the Stark effect on dominant lines of the “two” features, determining a change in dipole moment between the ground state and both of these excited states as 3.05 f 0.10 D. Arguing that two different electronic states are unlikely to have such similar dipole moments, Hochstrasser and Wiersma reassigned all the structure before the first ?T-?T* to a single ‘AZ (n-r*) state with a characteristic, if somewhat “mysterious:” 1657 cm-l totally symmetric vibrational model. (This vibronic ‘A2 state is forbidden, but all major features of the first system are repeated at 1657 cm-‘). The vibration is “mysterious” in that its frequency increases upon deuteration. Our calculation yields ‘Br (nlr*) and ‘AZ (n-r*) below the lowest ‘A1 (K-T*) state (Table VI). These nl* states stem principally from excitation of the highest “n”
SPECTRA
01’ AZANAPH’l-HALEYES Table
VI:
Phthalazine Calculated
Observed
Type
symetry
Oscillator Strength
Energy
lB2
1A
symetry
Type
Energy
Oscillator Strength
lBI
n-Tr*
28.5
0.02
II-n*
31.4
0.00
25.4a
n-l!*
27.0’
il-lT*
33 .ob 34.oc
NO-O) ) (V Dlax
0.01
lA1
ll-il*
34.0
0.06
(2)
ll-ll*
38. .5b (Urnax)
0.10
IB 2
71-ll*
37.7
0.17
(Y)
lA2
II-n*
46.3
0.000
IBl
II-l!*
46.8
0.003
lA1
r-71*
47.4
0.958
1
47.3b
1
1
1.18
A2
B2
IAl IB 2 1
a) Ref. 15,
single
b)
Ref.
c)
Interpretation
d, e)
(P)
*--1T*
28.5d
lA1
46.5
crystal
“O_o
values,
see
B,
(2)
TI-TI*
48.1
.noz
(Y)
ll--71*
4A.5
0.84
(2)
ll-7i*
w.5
0.66
(y)e
v-71*
52.3
.no2
(Yje
text
3
for
of
these
umax The relative
published
two features,
intensities
of
spectra, see these
Ref.
3
text two
1
R2 states
is
verv
geometry
dependent
orbital &(a_) (Table II) to the four lowest unoccupied A* m.o.‘s, contradicting the previous assumption that these states arise from excitations between bn(n_) and ar(n+) orbitals to the lowest unoccupied ?r* m.o., br(r*) (4). Excitations from a*(%+) seem to play only a minor role in the formation of these states. Noting that our calculations generally yield nlr* states with oscillator strengths 5 times larger than experiment, and ?T-* oscillator strengths 2 times larger, we predict a lowest lying weakly allowed 0.003), a forbidden ‘A I (a~*) - 3000 cm-’ higher in energy, and then ‘Hl (a**) (j = a close lying ‘A 1 (T-T*), (f - 0.03). In comparing with experiment we are led to examine two major possibilities. (1) Our calculated order is correct. The ‘A 1 state would then probably be covered by the preceding ‘Br (n-r*) and following ‘A r (?T-?T*)systems. (2) The calculated order of the two nlr* transitions is reversed. This could be effected by, a poorly assumed geometry for the molecule, additional configurations in the configuration interaction selectively depressing the I.42 (n-r*), or a IA 2 (~-IT*) state of much different geometry than the ground state. In such a case the weakly allowed ‘Br (n-r*) might be expected to make its presence felt before or in the following IA 1 (FT*). Dipole moments are, of course, sensitive to our assumed geometry, but the differences of dipole moments between states might be less dependent. The dipole moments of excited states, however, are also very sensitive to configurational mixing. We calculate a ground state dipole moment of -6.59 D. Our ‘Br (nlr*) has a calculated dipole moment of -2.86 D. To indicate how sensitive this calculated dipole is to configurational mixing, the most dominant ‘RI [b2(n_) - ar(?r*)] configuration has a dipole moment
466
RIDLEY
FIG. 2. Spectrum of Phthalazine line weight schematically indicates
of +3.2
D. We estimate
configurations the latter,
AND
ZERNER
vs N-N Bond Length. Only the major greater oscillator strength.
that “reasonably”
features
different combinations
are shown.
of the two dominant
could lead to a dipole moment between this value, +3.2
of course, being considerably
the ground state and the ‘B1 (n-r*)
D, and -3.6
closer to the value we calculate.
is thus estimated
(generously)
Heavier
D,
Ap between
to lie between 9.8 D
and 3.0 D (our calculated value, 3.7 D), and includes the Hochstrasser and Wiersma value of 3.05 D. The ‘AZ (n-a*) has a calculated dipole moment of -0.14 D, although different
mixtures
of the two dominant
configurations
yield a dipole moment
from
+5.6 D to -1.2 D. For the ‘AZ (n-n*) Ap = 12.1 to 5.4 D, and does not include the experimental value 3.05 D. We conclude that it is very unlikely that both these states have the same dipole moment,
and that only our lB1 (nl*)
could have the observed
dipole. The above analysis favors the first hypothesis
that only one n-?r* state is observed,
and that it is of ‘B1 symmetry. Together
these two features,
now assigned
(vln*x appears to lie slightly beyond two quanta with our calculated value.
as one lB1, have a vrnax -28,500
cm-l
of the 1647 cm-l mode), in agreement
In examining the second hypothesis, we have studied the effect of ground state geometry on the calculated positions of these two n-?r * transitions. Figure 2 summarizes the effect of the N-N bond length on the major features of our calculated spectrum. Bond length vs bond order considerations, as well as extrapolation along the series benzene (known), pyridine (known), pyridazine (not known) suggest RN__N= 1.30 A. Reasonable deviations from this geometry do not change the order of the ‘BI and ‘AT states. A shortening of this bond length in the excited state, as suggested by Hochstrasser and Wiersma (l?), and supported by an analysis of our ‘At state would cause us to overestimate the transition energy to ‘At by a masimum amount equal to the zero point energy of the modes which describe the distortion (1). This could lower the ‘At relative
to the lB1 by an amount
sufficient to reverse the levels.
probably
no larger than -1000
cm-‘,
which is in-
SPECTRA
467
OF AZANAPHTHALENES
Table VII: Cinnoline Observed
Calculated
I
TYPO
Energy
n-rr*
22.7a
II-lT*
24.8
0.01
TI-TT*
31.5a
[2ROOlb'=
8-T*
32.7
0.17 (50')
TI--il*
35.3b
[25001d’C
7T-TI*
35.8
0.17 (37")
n-l+
44.3b
[420001d'=
oscillator Strength
TYPO
Energy
Oscillator Strength (2)
n-TI*
39.2
0.002 (2)
ll-?T* iT-1T*
42.4 43.9
0.27 (-10') 0.41 C-24')
U-T*
45.8
0.003 (2)
IL-T* T-IT*
49.4 49.8
0.007 (2) 0.41 (69')
TI-IT* U-71* 71-l!*
51.5 52.2 52.2
0.18 (39") 0.002 (2) 0.74 C-9")
a) Ref. 19, band origins b) Ref. 20
d) average from references 19 and 20
We can, of course, never eliminate the possibility of additional configurations in the configuration interaction reversing the energy of these two states. It is unlikely that single ercitations will effect such a reversal : we hesitate at this time to speculate on the role of the distant but far reaching doubles. .4n ‘A 1 (r~*) band is observed with origin at 32,990 cm-’ and v,,,,, at -34,000 cm-’ ; the calculated value is in excellent agreement. A band of ‘Bz (VT*) symmetry is observed at 38,636 cm-l and calculated at 37,700 cm-‘. .A very intense band is reported at 47,267 of ‘A1 (r-r*) type; three ?T-?T*transitions are calculated in this region, including two intense *Ar (a-*) separated by 1100 cm-‘. The observed band appears asymmetric, suggesting the possible presence of this second intense band. The corresponding structure in naphthalene appears as a shoulder at -49,500 cm-‘. This shoulder was also calculated as a separate electronic band, but again within the “shadow” of the more intense preceding one. Of the azanaphthalenes we have investigated, the ?r electronic spectrum of phthalazine bears the greatest resemblance to the spectrum of naphthalene. From the molecules of this study, it would appear that the presence of an LYazanitrogen is a much greater perturbation on the spectrum of naphthalene than is the presence of a p azanitrogen. That phthalazine resembles more the spectrum of naphthalene than does isoquinoline is probably the influences of phthalazine’s greater symmetry. f. Cinnoline
The lowest observed band in the spectrum of cinnoline is an nlr* with origin reported at 22,711 cm-’ (19). This is in good agreement with the calculated value of 24,797 cm-’ (Table VII). The second band (~a*), observed at 31,542 cm-l (origin) and calculated
RIDLEY
468
AND ZERNER
Table VIII: QUinaZOline Observed
Calculated
I
TYPe
Energy
II-ri*
27.@
r-n*
32.7b
[25001=
VT*
36.8b
[2500]=
n-n*
45.0b
[400001c
Oscillator StrenPth
Energy
Oscillator Strength
n-F+
30.6
0.01 (2)
T--B*
33.5
0.08 (81")
II-T*
36.1
0.0001 (2)
ll-lT*
38.0
0.08 (79')
I-&-?I*
42.1
0.001 (z)
n--71* ll-ll* 71-n* n-71*
49.9
50.4 51.8 51.8
0.006 (2) 0.34 (45-j 0.46 C-63') 0.002 (2)
ll-IS* T-lT*
54.5 56.2
0.0005 (2) 0.19 (54')
IT-71* T-lT*
a) Ref. 21, band origin b) Ref. 20
at 32,687 cm-‘, is reported by Wait and Grogan (19) to be a continuum 2200 cm-’ Simple analysis of our calculated state does not wide and to lead to “predissociation.” indicate the nature of this predissociation. Two additional maxima are observed in water solution: one at 35,300 cm-‘, the other, an asymmetric band, at 44,300 cm-’ (20). These we assign to r-?r* transitions calculated at 35,780 cm-l and to two allowed transitions calculated at 42,380 cm-’ and 43,890 cm-‘. The published spectrum of (20) suggests a further maximum > 50,000 cm-l. From 50,000 cm-’ to 52,000 cm-* we predict several transitions, including three intense P-T*. g. Quinazoline Hasegawa et al. (21) report an n-n* band with origin at 27,581 cm-’ in the vapor spectrum of quinazoline (Table VIII). We calculate an nlr* band with vrnax at 30,648 cm-‘. From cinnoline to phthalazine to quinazoline (and approximately quinoxaline) this nq* is observed shifted 2650 cm-l and 2220 cm-l, respectively, compared to calculated shifts of 2350 cm-’ and 3500 cm-*. Three additional bands are reported for quinazoline in Hz0 (ph 7) with maxima -32,700, -36,800 and -45,000 cm-l (20). These we correlate with r~* transitions calculated at 33,500, 38,100 and two bands at 45,600 and 47,100 cm-‘. Experimental oscillator strengths are not available for quinazoline, but the calculated oscillator strengths seem in good accord with the reported tmax and the general shapes of the bands. h. Pteridine
The calculated electronic spectrum of pteridine is compared are predicted Mason (22) in Table IX. Two nlr * transitions
with that reported by before the first ?T--?T*.
SPECTRA
OF AZANAPHTHALENES
Table IX:
Pteridlne Calculated
Observed TYpe
Energy
OSCillatOI screngthb
TY!Je
Energy
Oscillatot Strength
Xl--Ti*
25.8
[S41
II-*
23.4
0.003 (2)
30.4= ?
[6311
II-n*
30.0
0.020 (2)
33.2
174901
il-Tl*
Ti-TI*
42.6
n-71*
47.6
r29101
[110001
11-n*
32.6
0.280 (-26')
n-R* n--71*
35.1 35.9
0.000 (2) 0.000 (2)
II-n*
V-V*
39.4 39.8
0.001 (2) 0.089 (82")
II-n* TI-TI* II-ll* ll-TI*
45.9 46.2 46.7 50.0
0.006 0.336 0.000 0.644
II-lT*
52.1 52.4
0.001 (2) 0.800 (-110)
n--t*
(2) (35") (2) (26')
a) Ref. 22 b) An average 6, see ref. a) c) Shoulder on spectrum published in ref. a)
Although the second of these is calculated to lie within the observed system of the neighboring T--B*, it is calculated to have intensity of its own and its effect might be observed. (The published spectrum of Mason shows a shoulder at -30,400 cm-’ where we calculate this second nlr*). The lowest nlr* calculated at 23,400 cm-’ is in good agreement with that observed at 25,800 cm-‘. The lowest lying ?r-u* is both calculated and observed at -33,000 cm-‘. A second rq* band is seen at 42,600 cm-’ and calculated at 39,800 cm-‘. In the region between 35,100 and 46,700 cm-l five transitions of n-s* type are calculated, none of which are estimated to have much intensity. Following these we find three intense sq* bands at 46,200 cm-‘, 50,000 and 52,400 cm-‘; an intense band is observed with maximum at 47,600 cm-r (22). Again, the calculated oscillator strengths nicely reflect the trend observed in the values of extinction coefficients that Mason reports (22). i. 1,4,5,8_Tetrazanaphthalene A comparison of the calculated spectra of 1yvs P azanaphthalenes indicates that the tetra 01aza compound should possess a very low lying nlr* forbidden excitation (4). The results of a calculation on this compound are presented as Table X. 1,4,5,8-Tetrazanaphthalene has DPh symmetry, and the axes are defined as in naphthalene (Fig. 1). There are two n-n* transitions predicted before the first ?T--?T*:as with pteridine the second of these lies very close in energy to the r+ *. The prediction of a forbidden low lying vz--?~*transition (at 465 rncl) is verified and ought to prove interesting experimentally. The only experimental information we have located on this molecule is rather cursory (2.3). The x-rr* states observed at 31,100 cm-’ and 44,600 cm-’ (solution C6H1J are in
470
RIDLEY Table X:
AND ZERNER
1,4,5,S_Tet+azanaphthalene
Observeda Type
Energy
n-71*
31.1
symetry
[2301b)
24.9 ?
TI-‘TI*
Calculated
Oscillator Strength
IB IS 1 B3u
[160001b)
lB2"
lB 2S l* u TI-‘TI*
44.6
[55001b)
lB23 1
Bill IA
Rlu
lA " 1 % IB IS lB" a) vmx
Energy
Oscillator Strength
n-7+
21.5
0.0
n-n*
29.1
0.026 (2)
?I--TT*
30.6
0.432 (x)
n-V*
34.2
0.0
n-71*
35.5
0.0
II-n*
39.6
0.0
1T-IT*
40.5
0.126 (y)
II-IT*
43.2
0.0
II-n*
45.1
0.0003 (2)
n-IT*
49.3
0.642 (y)
II-ll*
50.1
0.0
g
lB3u 1
TYPO
II-r*
52.6
0.0
n-77*
53.3
0.0
n-71*
53.4
1.552 (x)
in C6H12, Ref. 23
b, Emax
reasonable
accord with our calculated
values,
and our calculated
oscillator
strengths
reflect nicely the observed emax. DISCUSSION a. The Calculations
The model we have employed
seems equally
accurate
spectra of azanaphthalenes as it was for azabenzenes. of double excitations, estimated to appear at -85,000 -65,000 calculated
cm-l for naphthalene, transition
energies.
in offering rationale
for the
Omission from these calculations cm-’ for benzene, but as low as
has not seemed to affect greatly
the reliability
It could be argued that our parameterization
of our includes
some of the influences of ignored higher excitations, and, no doubt, this is correct. Nevertheless, our parameters were chosen for the azabenzenes where the influences of higher excitations might be expected to do considerably less damage to the calculated transitions below -60,000 cm-’ than they would for the azanaphthalenes. It has been observed that the calculated intensities, however, are considerably
more
sensitive to the inclusion of higher excitations than the transition energies themselves, and most sensitive are the transition directions (24). With this in mind, we view our predicted directions, where they are not determined by symmetry, as merely suggestive. This uncertainty would apply, in particular, to the higher lying transitions. We have suggested that the band observed at 52,600 cm-’ in naphthalene is lBQg (?T-?T*), borrowing
intensity
from a lower lying transition.
That
no vibrational
SPECTRA
structure
is reported
excitations another
for this
calculated possible
band
might
be a ramification
in close proximity.
explanation
471
OF AZANAPHTHALENES
is that
As discussed
this observed
of the presence
in the section
band
correlates
of other
on naphthalene,
with
our calculated
band at 60,500 cm-’ this model. Extensive
(Table I). If so, this is the worst ‘error’ that we encounter using CI studies on the ‘NzU states does not change this conclusion.
Admission
excitations
of higher
onI>. a reparameterization to make the calculation observed
at -52,600
in understanding have employed b. The Speclra
cm-‘,
the here.’
might
lower
this
transition
energy,
but
requires
not
of the model, but a difficult configurational selection process feasible. A better experimental understanding of the band then,
phvsical
would
content
not only be of experimental of such
quantum
interest,
mechanical
but help
methods
as we
_
oj dzanapkthalenes
We have found to be misleading,
the apparent similarities in the spectra of the azanaphthalenes often and as such have discussed each molecule separately. Nevertheless,
the urge to draw
general
conclusions
is inescapable,
and we have
attempted
to do so.
1. The highest occupied molecular orbital (homo) of the azanaphthalenes which we have esamined is always of ?r symmetry (Table II). This is so regardless of whether the first electronic
transition
,7. As might nitrogens. better
is nlr*
be expected,
Generally
nitrogen
or r-r*.
there
(although
character
not always)
(Table
5’. In the diazanaphthalenes 1. Examining u substitution. to be a lowering naphthalene
occupied Table
of this
orbital
little
(Z?)].
m.o.‘s
that
n m.o.‘s
as there
are calculated
are principally
rr molecular
more cy character
Examining has very
these
by at least 2 eV (Table
the highest
it has slightly
however, identifiable
are aza.-
to have
60yo or
II). the m.o.‘s
lone pairs are split in energy find that
[Note,
are as manv
orbital
(hoamo)
than 0: it should
be slightly
II, this is seen to be true, b!
density
-0.30 eV/nitrogen at the 01 positions
the f
((2 uinazoline
II).
combinations
of
is the exception.) of naphthalene
we
more sensitive
but the major
effect
to
seems
atom. The second hormo of and should be relatively stable
to 01substitution. This is demonstrated : each p substitution lowers the orbital -0.55 eV, each (Y substitution, -0.15 eV. In a similar fashion the third hoamo is more sensitive to /3 substitution clusions
of naphthalene again
and the fourth
are reached appears
to CL By the perfect
for the lowest
is slighti?.
more
to be an energy
empty
sensitive lowering,
r m.o.‘s
pairing
theorem
(le?rmo’s).
to cr substitution
(29) similar
For example,
than
in this case of -0.35
p, but the major eV/nitrogen,
con-
the lermo effect
etc.
The highest occupied u symmetry orhital has -7Cy, nitrogen character and is located at -9.8 f 0.2 eV for all of these azanaphthalenes except phthalazine, which has 84% nitrogen character, and lies at -9.2 eV (Table II). 5. Quinoline
is calculated
to have
has not been observed. It ma\transitions before the first a-2, least one n-r*
transition
before
an n-s*
be that
transition the
below
the first ~--1r*, but
monoazanaphthalenes
show
whereas all of the polyazanaphthalenes the onset of the first ?T-?T*.
’ The orizin of the second transition in benzene is another example. ploying only singly excited configurations predict this state as *B1,‘ (I)
Semiempirical
this
no n--T* show
calculations
at
em-
: inclusions of double excitations
(either within an approximate or ah initio framework) suggest that this state might be I& Vi, 26)]. Shavitt reports a more extended CI, in which he restores this state to IB,, (27).
[e.g.
RIDLEY
472
AND ZERNER
6. The two lowest ~z--?T*states are mainly transitions between the highest occupied n m.o. and the two lowest empty ?r m.o.‘s, respectively. The calculated position of these two states can be obtained approximately from the orbital energy differences of the appropriate m.o.‘s minus a constant (i.e., --J •l- 2K = 44,500 cm-l). Since the lowest ?r-7r* transition in all of these compounds has onset at -32,000 cm+ (another constant), the number of n-n* transitions before the first ?T-?T* can be estimated in this model by the number of times the condition At(n-?r*) - 76,500 5 0 is met. Any reasonable quantum mechanical model should also be useful in this estimate with, perhaps, an adjusted constant. 7. The lowest rr-lr* band in these compounds is observed and calculated to lie between 32,000-34,000 cm-l. This observation is not simply interpreted; for naphthalene, quinoline, isoquinoline, phthalazine, and quinoxaline (called Group I for subsequent discussion) this structure is caused by configurational mixing between f+ h and e 3 g with some, if symmetry permits, ,f-+ g. (Platt notation for ?r orbitals only (30), f = honmo, g = lenmo, etc.) For cinnoline, quinazoline (Group II) and pteridine, this band is calculated to be principally f --+ g. 8. The second ?T-?T*state of the azanaphthalenes is calculated to lie between 34,!)0040,000 cn-I, and is, except for cinnoline, short axis polarized. For Group I (see discussion under point 7) the transition is principally f -+ g; for Group II and pteridine, f-t h, e --+ g with some f + g. Within Group I each (Ysubstitution lowers the transition energy relative to the corresponding state of naphthalene, while B substitutions have little effect. (For quinoxaline with two (Yazanitrogens, the first two r-a* states are calculated only one band is observed.) This behaviour is very near in energy. Experimentally predictable from the nature of the f’ and g ?r m.o.‘s (point 4). 9. At least two intense transitions are calculated to lie in the region between 42,00052,000 cm-‘. For naphthalene and /3 substituted azanaphthalenes, the two are close in energy and appear not to be completely resolved. o-substitution is a greater perturbation on the naphthalene spectrum; two maxima replace the single, although asymmetric, peak of naphthalene found at 47,530 cm-‘. 10. How closely the ?T--?T* spectra of the azanaphthalenes resemble naphthalene seems to be simply reflected in how closely the two lermo’s resemble those of naphthalene. For example, the orbital energy difference between g and h in phthalazine, at 0.54 eV, is the same as that observed for naphthalene (Table 11). The spectrum of phthalazine closely resembles that of naphthalene. Isoquinoline, in which this difference is 0.68 eV is the only other azanaphthalene with a spectrum “obviously” resembling that of naphthalene. These differences for the other molecules of this study are greater than 1 eV. The occupied ?r m,o.‘s do not reflect this simple correspondence. ACKNOWLEDGMENT This work was supported in part- by a research grant from the National Research Council of Canada.
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01: AZAXAPHTHALENES
473
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Z
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Prance,