914
KICHISUKE NISHIMOTO AND LESLIES. FORSTER
SCFMO Calculations of Heteroatomic Systems with the Variable-p Approximation. 111. Electronic Spectra of Anions of Hydroxyaromatics' by Kichisuke Nishimoto and Leslie S. Forster Department of Chemistry, University of Arizona, Tucson, Arizona
(Received August 21 I 1967)
The singlet transition energies of the anions of phenol, a- and 8-naphthol, and the quinolinols have been computed. Good agreement with experiment is obtained with la= 27.83 eV and yoo = 19.43 eV, when is assigned as the core charge for oxygen.
-
Introduction The SCFMO method in the Pariser-Parr-Pople
O [ ~ ( S Pl(sp312 ~) 1(sp3)2 i(sP31, v2i
approximation has proven to be very useful in computing singlet transition energies. A particular parameterization (the variable-p approximation) has been applied, with considerable success, to a wide variety of neutral molecules containing nitrogen and oxygen atom^.^,^ Extension of the method to ionized species has proven to be troublesome. The central problem in any semiempirical method is the parameter choice. We now describe a suitable parameterization for the anions of hydroxy-substituted molecules, phenol, a- and @-naphthol,and the quinolinols.
The appropriate quantities are IO = 24.39 and TOO= 18.28 eVa4 Again the core charge is assumed to be +1. It can be seen (Table I) that the phenolate spectrum is not well reproduced by Model I. Model I1 is considerably better for phenolate, but not very satisfactory for the naphtholates (Table 11). Two bands of moderate intensity (3.58 and 4.40 eV) appear in the pnaphtholate s p e c t r ~ r n . ~In a-naphtholate, the higher energy transition becomes a shoulder on the lower energy band. Model I1 is conspicuously deficient in reproducing this behavior. Model III. In this model the effect of changing the ROH core (02+[u(sp2)u(sp2)l(sp2)2,V2]) to R0,-is assumed to correspond to reducing Zeffby 0.35 (Slater's rules). The core charge is then +"3 and the parameters are calculated by the weighted averages
Method With the exception of the parameters associated with 0-, the method was identical with that previously d e s ~ r i b e d . ~The ? ~ Nishimoto-Xataga integrals were used throughout. Three approaches to the anion parameterization were explored. Model I . The removal of the proton is assumed to leave the valence state essentially undisturbed. Thus ROH 3 RO- is represented by O[a(sp2)2 l(sp2)2 i r 2 , V,] + O-[a(spZ) l(sp2)2 l(sp2)2 79,Vll
The two electrons are then successively removed and the valence-state ionization energy corresponds to O[a(sp2) l(sp2)Z 1(sp2)2 ir, V21 + O+[a(sp)2 l(sp2)2 l(sp2)2, VI]
This is analogous in every respect to the situation encountered when a carbonyl group donates one ir electron to the conjugated system and the normal carbonyl parameters are used, Io = 17.7 and YOO = 15.23 eV.2 The core charge is +l. Model II. I n this approach, the removal of the proton from O H is accompanied by rehybridization to 0-[u(sp3) l ( ~ p ~~ () S~ P ~~( )S ~P ~ VI]. ) ~ , The valencestate ionization energy is then computed for T h e Journal of Physical Chemistry
O + [ ~ ( S Pi(sp3)2 ~) 1(~p3)2,v1i
Io Yo0
=
+ 17.7
2 X 32.9 3 2 X 21.53 3
= __
=
27,83 eV
+ 15.23 = 19.43 eV
=
0.741
It is evident that this model yields the best results of the three schemes. The spectra of the anions of phenol and a- and 0naphthol resemble the spectra of the corresponding amines, aniline, and the naphthylamines.6 The C-N bond orders in the amines are about 0.4. The model (1) Supported by the U. S. Public Health Service, Grant No. GM13218. (2) K. Nishimoto and L. S. Forster, Theor. Chim. A c t a , 4, 155 (1966). (3) K. Nishimoto and L. S.Forster, J . P h y s . Chem., 71,409 (1967). (4) J. Hinze and H. H. Jaff6, J . Am. Chem. Soc., 84, 540 (1962). (5) G. W. Ewing and E. A . Steck, ibid., 6 8 , 2181 (1946). (6) H. Baba and S. Suzuki, Bull. Chem. SOC. J a p a n , 34, 76, 82 (1961).
915
SCFMO CALCULATIONS OF ANIONSOF HYDROXYAROMATICS Table I: Data for the Phenolate Ion Model I
___---
Model I1 Model 111 Transition energies, eV, and intensities, f--------
4.48 (0.082) 5.55 (0.307) 6.61(0.660) 6.?7(1.073)
4.56 (0.127) 5.48(0.262) 6.71 (0.633) 6.86 (1.118)
3.90 (0.146) 4.99 (0.514) 6.32 (0,079) 6.73 (0.881)
y-Exptla---Polarization$
Energy
i o -4tmRx
4.32 5.30
0.32 1.1
2
Y X
Y
_________-__- Charge densities---------91 93 P4
97
0.913 1.082 1.009 1.074 1.830
0.789 1,076 1.042 1.125 1.849
0.858 1.120 1.039 1.164 1.660
P2
Bond orders---
_ I I _ _
PlZ P2s Pa4 PI7
0.583 0.692 0.648 0.601
0.517 0.712 0,632 0.615
------
0.599 0.682 0.657 0.427
Energy of highest occupied orbital, eV-
-3.97 a
-5.06
-7.61
American Petroleum Institute Research Project 44.
' The y axis coincides with the C-0
direction.
Table 11: Transition Energies, eV and Intensities, f for the Naphtholate Ions
-Model Energy
'L1 'Lp
3.52 3.83 5.00 5.28 5.66 5.95
-a-Naphtholate --Model 11j Energy
(0,204) (0.130) (0.582) (0.197) (0.071) (0.632)
3.80 3.84 5.05 5.34 5.58 5.77
-@-Naphtholate------c--Modsl 1 1 1 - 7
,
e,
--Exptl'Energy 10-4tmsx
deg
(0,228) (0.080) (0,570) (0.156) (0.021) (0.923)
60 133 1 130 138 0
---Model Energy
3.73
0.76
'LI
5.04
2.6
lLZ
-7.22
American Petroleum Institute Project 44.
8,
Energy
3.65 4.19 5.09 5.26 5.42 3~90
f
deg
(0.123) (0.096) (0.209) (1.553) (0.046) (0.595)
125 145 39 15 141 128
---Exptla-Energy 10-'rmax
3.59 4.40
0.30 0.62
5.20
5.8
-5.08
* Measured
11-f
3.38 (0.178) 3.99 (0.041) 5.03 (0.123) 5.11 (0,160) 5.43 (0.501) 6.02 (0.259)
Energy of highest oocupied orhitsl, eV
7
-4.83
1
111----
-7.41
from the long axis toward the oxygen atom.
I11 parameterization yields approximately the same value for the C--0 bond order in phenolate.
Results and Discussion Phenolate. The ILB,bbenzene transition energies are progressively lowered in energy by OH and 0- substitution (Figure 1). The 'E, degeneracy is removed but the spiitting is probably insufficient to detect spectrally. The ionization potential of phenol should be reduced about 1.7 eV (model 111) by removal of the proton. Although the benzene spectrum is strongly affected by the 0- substituent, little departure from hexagonal symmetry is predicted.
a-Naphtholate. There is configuration mixing between 6 + 7, 6 4 8, and 5 --t 7, as in a-naphthol,' but this mixing is so marked that the 'L,and 'Lb designations are no longer and the transitions are labeled 'L1 and 'Lz in order of increasing energy. The 'L1 and 'Lz transitions are predicted to be at nearly equal energies. The weak lL1 would then be obscured by the stronger 'LZ. This is consistent with the observation of one broad absorption band.6 The computed polarizations are 'L1 (SO') and 'L, (133"), the angle being measured from the long molecular axis toward (7) L. S. Forster and K. Nishimoto, J. Am. Chem. SOC.,87, 1459 (1965).
Volume 78. Number S
March 1968
KICHISUKE NISHIMOTO AND LESLIES. FORSTER
916
i i
I
l3
li
jJS f
Ir:
/n v
The Journal of Physical Chemistry
917
SCFMO CALCULATIONS OF ANIONS OF HYDROXYAROMATICS
0.11
Benzene
0.11
0,11
.a
-0OS5
Phenol
IO*
Phenolate
Figure 1. Variation in transition energies with structure.
7 and 6 -P the oxygen atorn. The combination of 5 8 that leads to the lBb transition in naphthalene contributes strongly to two states (5.05 and 5.77 eV) in anaphtholate. The spectrum of a-naphthylamine does exhibit two strong bands in this region.'j The ionization potential, computed from the energy of the highest occupied orbital (corrected by adding 1.3 e V ) , is 6.28 eV. This species should be a good electron donor in solution. From the molecular diagram (Figure 2), it can be seen that most of the 0.17 unit of charge that has migrated into the ring system is concentrated in the ring to which the substituent is attached. P-Naphtholate. As indicated above, two moderately strong transitions are expected at wavelengths greater than 2500 A. The failure of the usual variable /? parameterization (assuming either 1 or + 2 core) in this computation provided the impetus for the proposal of the fractional charge model. The configuration mixing in LY- and p-naphtholate is nearly the same, in contrast to the situation in the neutral species.' The lL1 and lL2 transitions are polarized at 125 and 145", respectively. The calculated and observed intensity ratios for 'L1 and 'L2 are inverted, but all other calculations yield much smaller intensities for 'LZ. The 'L1 transition is polarized nearly perpendicular to the 6-0 bond axis, as is the corresponding transition in /3-naphthol and p-naphthylamine, Experimental inflormation about the polarizations is lacking. Quinolinolates. The spectra of the anions of hy-
+
Figure 2. Molecular diagrams for a- and p-naphtholate ions, model 111.
droxy-substituted quinolines provide a good test for the parameterization. The calculations follow the observed substitutional shifts quite well (Table 111). Noteworthy is the corroboration that 'La and 'Lb are nearly degenerate in the 4-OH derivative, but not in the 5- and 8-quinolinolates (the validity of the Platt symbols is restored by the uxa substitution). The calculated intensity ratios for the two lowest states of the anions derived from P-naphtholate are not satisfactory. Nevertheless, the applicability of the fractional charge model to the calculation of singlet transition energies has been established. With the exception of the near degeneracy in 4quinolate, the 'La states are lower than 'Lb, a reversal of the order in quinoline. A similar inversion was obtained in the quinolinol calculation^.^ The marked lowering in lL, compared to quinoline is largely due to the reduction in the energy difference between the highest occupied and lowest empty orbitals. The effect of the 0- substituent on 'La can be treated as a simple perturbation, and a Huckel scheme may be used for approximate calculations. The substituent effect 'Lb is more complicated; both the configuration energies and the interconfiguration matrix elements are altered by substitution. The references to 'La in this paragraph are applicable to the 'L1 transitions and the remarks about 'Lb are pertinent to 'Lz.
Volume 78,Number 3 March 1968
