EARLM. EVLETH AND ROBERT J. Cox
4082
Assignments of the Electronic Transitions in the Methoxy-Substituted Benzenediazonium Cations
by Earl M. Evleth’ and Robert J. Cox I B M Research Laboratory, San Jose, California
(Received May 17, 1967)
The visible-ultraviolet absorption spectra of benzenediazonium, 2-, 3-, 4methoxybenzenediazonium, 2,4-, 2,5-,2,6-,3,4-dimethoxybenzenediazonium,and 2,3,5-, 2,4,5-, 2,4,6-, 3,4,5-trimethoxybenzenediazonium cations were measured in the 200-450-mp region. The assignments of the electronic transitions of these materials were made possible through the use of molecular orbital theory. An excellent quantitative correlation between the calculated and observed values of the transition energies is obtained. The assignments of the ‘L, and ’Lt, transitions are in qualitative agreement with those previously assigned for other “strongly substituted” benzenes.
Introduction Although a number of experimental studies have been reported on the spectroscopic properties of diazonium and diazo compounds,2-22 a systematic theoretical treatment of the electronic structures of these materials has not been reported. The diazonium moiety ( - N b N ) is reported t o be the most electronwithdrawing substituent known;2* an observation in agreement with expectations considering both the net positive charge of the substituent and the interaction of it’s 7 electrons with the aromatic nucleus. The diazonium moiety represents a strongly perturbing substituent on aromatic systems and in this manner resembles the nitro group.23 Experimentally, aromatic diazonium cations do not exhibit the n-r* transitions which complicate the analysis of the spectra of aromatic nitro compound^.^^ The strong interaction of electron-donating groups (methoxy, dimethylamino) with benzenediazonium cation (I) is shown by an inordinately high shift in the N=N stretching frequency on s u b s t i t u t i ~ n . ~ This, * ~ , ~ ~in combination with the high photosen~itivity~*-~~ and variable chemical reactivity,22b28has prompted us to investigate the electronic absorption spectra of a number of structurally related aryldiazonium cations. We have analyzed the observed spectral properties of these materials within several theoretical frameworks. The Journal of Physical Chemistry
Experimental Section The syntheses of the aryldiazonium fluoroborates investigated here are reported elsewhere.22 The vis~~~
(1) University of California at Santa Cruz, Santa Crur, Calif. (2) E. A. Boudreaux, H. B. Jonassen, and L. J. Theriot, J . A m . Chem. Soe., 85, 2039, 2896 (1963). (3) R. H. Nuttall, E. R. Roberts, and D. W.A. Sharp, Spectrochim. Acta, 17, 947 (1961). (4) A. Wohl, BuU. SOC.Chim. France, 6, 1319 (1939). (5) K. B. Whetsel, G. F. Hawkins, and F. E. Johnson, J . A m . Chem. Soc., 78, 3360 (1956). (6) A. F. Gremillion, H. B. Jonassen, and R. J. O’Connor, ibid., 81, 6134 (1959). (7) L. S. Gray, V. A. Fassel, and R. N. Kniseley, Spectrochim. Acta, 16, 514 (1960). (8) E. S. Lewis and H. Suhr, J . A m . Chem. Sos., 80, 1367 (1958). (9) E. 5. Lewis and M. D. Johnson, ibid., 82, 5399 (1960). (10) A. Hantzsch and J. Lifschitz, Chem. Ber., 45, 3011 (1912). (11) L. C. Anderson and J. W. Steedly, Jr., J . A m . Chem. SOC.,76, 5144 (1954). (12) L. C. Anderson and B. Manning, ibid., 77, 3018 (1955). (13) M. Aroney, R. J. W. Le Fevre, and R. L. Werner, J . Chem. SOC.,276 (1955). (14) R. J. W. Le Fevre, J. B. Sousa, and R. L. Werner, ibid., 4686 (1954). (15) W. von E. Doering and C. H. De Puy, J . A m . Chem. SOC.,75, 5955 (1953). (16) M. J. Amrich and J. A. Bell, ibid., 86, 292 (1964). (17) J. D. C. Anderson, R. J. W. Le Fevre, and I. R. Wilson, J . Chem. SOC.,2082 (1949). (18) L. C. Anderson and M. J. Roedel, J . A m . Chem. Soc., 67, 955 (1945).
ELECTRONIC TRANSITIONS IN THE BENZENEDIAZONIUM CATIONS
ible-ultraviolet absorption spectra of the diazonium salts were determined in spectral grade acetonitrile (Matheson Coleman and Bell) on a Cary Model 14 spectrophotometer. All solutions were prepared in actinic glassware, and subsequent dilutions and handling were done in such a way as to protect the materials from photodecomposition. Beer’s law was obeyed in all cases over the concentration ranges tested. Theoretical Computation. Initial attempts at SCFCI calculations on benxenediaxonium cation (I) (uide infra) proved unsuccessful.2s As will be shown, Huckel molecular orbital calculations were successful. These calculations were conducted on an IBM 7094 using a FORTRAN IV program described elsewhere.” The computational application of Huckel theory is well discussed in standard text^.^^^^^ The major problem in the application of the theory to polyatomic systems containing heteroatoms is the assignment of the Coulomb and resonance parameters, h and k, as defined by Streitwie~er.~~ The values of the h and k parameters used here are listed in Table I. These Table I : Coulomb and Resonance Parameters Coulomb Atom
parameter
type
h
N (diazo, 8) N (diazo, 7)
c (1)
0 (methoxy)
1.0 2.5 0.4
2.0
Bond type
N-N C-N C-C C-0
(7-8) (1-7) (all) (methoxy)
Resonance parameter k
2.0 0.7 1.0 0.7
parameters arbitrarily assigned with the exception of the h and k values for the -N+=N moiety. These were assigned by conducting several calculations using various values for h7, hs, k78 until a reasonable pair of relative excitation energies were obtained for the first two electronic transitions in benzenediaxonium cation (1).
Discussion and Results Table I1 contains the me sured sp ctral characteristics of compounds I-XII. Graphical representations of the visible-ultraviolet spectra of these materials are given in Figures 1, 2, and 3. Also listed in Table I1 are the calculated energies for the lowest
2.0
4083
1
200
\ 300 Wavelength, mp.
400
Figure 1. Ultraviolet-visible spectra of the unsubstituted and monosubstituted methoxybenzenediazonium cations: --- benzenediazonium fluoroborate; . . . 3-methoxybenzenediazonium fluoroborate; - . - . 2methoxybenzenediazonium fluoroborate; - - - -, 4methoxybenzenediasonium fluoroborate.
.
two electronic transitions and their respective transition moments, squared. The symmetry assignments given in Table I1 will be discussed later. Visual comparison of the spectral features of compounds I-XI1 shows tha.t all but two of the materials possess two absorption maxima in the region above 250 mp. Ad(19) C. B. Moore and G. C. Pimentel, J . Chem. Phys., 40, 342, 329, 1529 (1964). (20) L. L. Leveson and C. W. Thomas, Tetrahedron, 2 2 , 209 (1966). (21) E. 5. Lewis and >I, D. Johnson, J. Am. Chem. Soc., 81, 2070 (1959). (22) R. J. Cox and J. Kumamotc, J . Org. Chem., 30, 4254 (1965). (23) For a discussion, see J. N. Murrell, “Theory of Electronic Spectra of Organic Molecules,” John Wiley and Sons, Inc., New York, N. Y., 1964, pp 186, 187. (24) J. deJonge, R. Dijkstra, and G. L. Wiggerink, Ree. Trav. Chim., 71, 846 (1952). (25) C. F. Goodeve and L. J. Wood, Proc. Roy. SOC.(London), A166, 342 (1938). (26) R. Moraw and J. Munder, “Kolliquim uber Wiss. Photgraphie,” Section IV, I1 Zurick 1961, J. Kosar, “Light-Sensitive Systems,” John Wiley and Sons, Inc., New York, N. Y., 1965, pp 194-320. (27) J. G . Calvert and J. N. Pitts, Jr., “Photochemistry,” John Wiley and Sons, New York, N. Y., 1966, pp 472,473. (28) H. Zollinger, “Azo and Diazo Chemistq,” Interscience Publishers Inc., New York, N. Y., 1961. (29) E. &I. Evleth, unpublished results. (30) E. M. Evleth, J. A. Berson, and S. L. Manatt, Tetrahedron Ldtefa, 42, 3087 (1964); J. A . Berson, E. &I. Evleth, and S. L. Manatt, J. Am. Chem. SOC.,87, 2901 (1965). (31) A. Streitwieser, Jr., “Molecular Orbital Theory for Organic Chemists,” John Wiley and Sons, Inc., New York, N. Y., 1961. (32) K. B. Wiberg, “Physical Organic Chemistw,” John Wiley and Sons, Inc., New York, N. Y., 1964, pp 65-103. (33) Reference 31, Chapter 4.
Volume 7 1 , Number 12 November 1967
EARLM. EVLETH AND ROBERT J. Cox
4084
Table II : Ultraviolet-Visible Spectra of Methoxy-Substituted Diazonium Cations" -ExperimentalBeneenediszonium derivative
I
Unsubstituted
I1
2-Methoxy
I11
3-Methoxy
IV
4-Methoxy
V
2,4-Dimethoxy
VI VI1
2,6-D ime thoxy
VI11
3,CDimethoxy
IX
X
2,4,5-Trimethoxy
XI
2,4,6-Trimethoxy
XI1
3,4,5-Trimethoxy
Calculated
Amax
(mr)
Log
296 261 356 264 350 275 313
3.28 4.09 3.69 4.09 3.36 3.80 4.39
23 1 350 sh 300 232 206 405 272 220 380 296 348 305 240 212 410 307 231 377 308 240 218 340 30 1 208 sh
3.89 4.02 4.34 3.86 4.22 3.69 3.93 4.26 3.66 4.16 4.07 4.02 4.15 4.04 3.48 3.77 4.25 3.96 4.19 4.02 4.09 3.69 4.56 4.27
...
e
...
...
...
356 220 202
4.11 4.23 4.21
E@
#TPO
1.337 1.466 1.233 1.464 1.211 1.441 1.324 1.347
0.19 1.08 0.29 1.00 0.27 0.95 1.19 0.15
1.239 1.335
0.37 0.99
...
...
1.121 1.447
0.36 0.89
... ...
.
.
I
1.170 1.435 1.183 1.333
Symmetryd
...
...
AB A AB A
... 0.22 1.06
'Bi
0.56 0.75
A A
1.062 1.358
0.27 0.90
B A
1.111 1.329
0.51 0.82
... ...
...
... ...
... ...
..
*
...
0.17 1.17
1.162 1.204
0.16 1.08
... ...
...
'Lb
'La
A A
...
1.180 1.320
...
'Ai
...
...
.*.
Fluoroborate salts in acetonitrile. * Calculated transition energy in 6 units, first two calculated trtmsitions. c Square of the computed transition moment, angstrom units. d Czvsymmetry symbol and Platt nomenclature assignment, see Discussion for an explanation of the symbols, A, B, and AB, upper state symmetry given in all cases.
ditional bands appear in the region below 250 mg; these will not be discussed further. In Figure 4 is shown a plot of the observed versus the calculated transitions. An adequate correlation (coefficient = 0.97) is obtained from a least-squares plot of this data. The slope yields a spectroscopic value for p (the resonance parameter) of 33,650 cm-'. The reasonable agreement between the observed and calculated spectral features of compounds I-XI1 is not particularly enlightening. Correlation curves of this type do not present a systematic understanding of the effect of structure on spectra but possibly demonstrate the adequacy of the theoretical method. The Jmrrml of Physical Chemistry
The first question to answer, however, is why the Huckel calculations are successful at all in quantitatively correlating the spectral features of these materials. It is generally accepted, although most certainly not true,34that Huckel calculations are unable to yield adequate spectral correlations. The methods developed by Pariser and Parr (ASMOCI)3a and Pople (SCF-CI)3e~37 have yielded adequate (34) Reference 31,Chapter 8. (35) R. Pariser and R. G. Parr, J . C h . Phys., 21, 466,767 (1953). (36) J. A. Pople, Trans. Faraday Soc., 49, 1375 (1953). (37) J. A. Pople, Proc. Phys. SOC.(London), 68A, 81 (1955).
ELECTRONIC TRANSITIONS IN THE BENZENEDIAZONIUM CATIONS
4085
,
1.50
a S 1.40 I 4 1.30 .
.j
9 1.20 1.10
1.00
1I
,
1
20
I
2.0
230
300
400
Wavelength, mp.
Figure 2. Ultraviolet-visible spectra of the disubstituted methoxybenzenediazonium cations: , 2,6-dimethoxybenzenediazonium fluoroborate ; . . . , 2,4dimethoxybenzenediazonium fluoroborate; ., 2,5-dimethoxybenzenediazoniumfluoroborate; , 3,4-dimethoxybenzenediazonium fluoroborate.
. .---Lo
I
2.0 1
230
300
400
Wavelength, mfi.
Figure 3. Ultraviolet-visible spectra of the trisubstituted methoxybenzenediazonium cations: , 3,4,5-trimethoxybenzenediazonium fluoroborate; . . . ,2,4,6-trimethoxybenzenediazoniumfluoroborate; - . - ., 2,4,5-trimethoxybenzenediazoniumfluoroborate; - - - -, 2,3,5-trimethoxybenzenediazonium fluoroborate.
.
results in the analysis of the spectral features of planar organic molecules. The essential accomplishment of these methods has been due to the introduction of configuration interaction under appropriate parameteri~ation.~~-*The major consequences of the inclusion of electron repulsion terms are that (i) a separation of the singlet and triplet states is predicted and (ii) degenerate or closelying states of the same symmetry are split. This latter phenomena is particularly important under conditions where the pairing rule is ~ b e y e d . ~The * , ~inadequacy ~ of the Huckel method
25
30
35
40
ET,kk., obsd.
\
Figure 4. Correlation plot of the first two calculated electronic transitions vs. the observed electronic transitions of the aryldiazonium salts shown in Table 11.
in correlating spectral properties is particularly evident where the pairing rule is obeyed;38 Le., the polyacenes. If configuration interaction is not of importance in determining the relative intensities and positions of the electronic transitions from the ground state to the first or second excited states, simple Huckel theory may be useful in spectral interpretati~n.~~ Preliminary SCF-CI calculations on benzenediazonium cation (I) were carried out but yielded poor quantitative predictions of the first two transition^.^^ It was determined that configuration interaction was of little importance in lowering the calculated SCF energies of the first two excited states. Both transitions were nearly “pure,” Le., the configurational weights were over 90% of a single excited-state SCF wave function. I n addition the symmetries of the first two transitions were in the same order as predicted from the Huckel calculations (Table 11). In the Huckel calculation the degree of the splitting of the transitions of a substituted benzene over the identical case for benzene (degenerate in the Huckel approximation) is dependent on the magnitude of the Coulomb and resonance parameters of the s ~ b s t i t u e n t . Under ~~ such circumstances configuration interaction plays a diminished role and adequate spectral correlations can be anticipated using simple Huckel theory.42 This is ap~
(38) R. Pariser, J . Chem. Phys., 24, 250 (1956). (39) Y. L’Haya in “Advances in Quantum Chemistry,” P. 0. Lowdin, Ed., Academic Press, New York, N. Y.,1964, Vol. 1, pp 203-240. (40) I. Fischer-Hjalmars in “Molecular Orbitals in Chemistry, Physics, and Biology,” P. 0. Lowdin and B. Pullman, Ed., Academic Press, New York, N. Y., 1964,pp 361-383. (41) A. D.McLochlan, Mol. Phys., 2 , 361 (1959). (42) N. M. Atherton and J. N. Murrell, Ann. Rept. Chem. SOC.,61, 197 (1964). (43) C. R.Sandorfy, “Electronic Spectra and Quantum Chemistry,” Prentice-Hall, Englewood Cliffs, N. J., 1964,p 217.
Volume 71. Number 12 November 1967
4086
EARLM. EVLETH AND ROBERT J. Cox
parently the case with the substituted benzenediazonium cations. The main difficulty with a purely computational approach to spectral analysis is that it is unable to provide a systematic understanding of the spectral properties of a number of structurally related materials. Systematic spectral analyses have been provided from three main sources : (i) resonance the0ry,4~146 (ii) free electron theory,46 and (iii) charge-transfer calculation^.^^^^^ Too few valence-bond calculations have been done on organic molecules with regard to spectral analysis45 to test the validity of resonance theory. As a consequence resonance arguments have no computational support. Of the few charge-transfer resonance structures shown below for benzenediazonium cation (I), Ia, Ib, and IC can be considered “traditional.” There is no compelling reason to exclude Id and Ie from a truncated set of important valence-bond structures. I n any case there is no noncomputational method of either assigning the weights or relative importance of these and other structures in the ground and particularly the excited states of I.49
I
Ia
Ib
IC
Id
Ie
In the case of the monosubstituted methoxybenzenediazonium cations both I1 and 111 absorb at lower energies than IV. Traditional resonance arguments would compel using structures IIa, IIIa, and IVa to rationalize this observation. The chargetransfer caclulations of Murrelld7 on aminonitrobenzenes indicate that charge-transfer interactions of the donor or acceptor with the benzene ring are of more importance than with one another, especially in the case of IV. In addition, although the weaker intensity of I1 vs. IV can be explained on the basis of the shorter transition dipole, the weak intensity of 111 is anamolous in resonance terms. Also traditional
N-
N-
Nt
N+
/I
I1
IIIa
IIa
IVa
N
VI = 356 mp
350 mp
The JOUTW~ of Physical Chemistry
313 mp
spectral resonance arguments usually disregard other low energy transitions. The systematic application of free electron theory has lead to the assignments of the two lowest energy transitions in benzene and substituted benzene~.4‘3~W,bl The weak 254-mp band in benzene has been assigned as being ‘Lb ‘A. The stronger 204-mp band has been assigned as being ‘La ‘A. Visual comparison of the known absorption spectra for the monosubstituted benzenes shows that the labeling of the two lowest energy electronic transitions as being La or Lb has general validity. For monosubstituted benzenes the molar extinction coefficient of the La transitions are in a region between 6000 and 13,000.W~62 The L,, transitions, when observed, are much weaker, in the region between several hundred and 3000. For mildly perturbed systems, such as anisole, the energy shifts of the La and Lb transitions from their values in benzene are ~mall.~OAs the number and chargetransfer characteristics of the substituents change, the La and Lb purity of the observed transitions become tainted by contributions from higher benzene transitions and charge-transfer structures of appropriate symmetry.47 Stevenson has showna that the spectral features of 191 substituted benzenes, previously experimentally investigated by Doub and Vandenbelt,63are adequately treated as perturbations of the La and Lb transitions in benzene. In all but ten cases the La and Lb transitions could be assigned on the basis of their different intensities. Table I1 contains the assignments of the electronic transitions for compounds I-XII. Of these materials, compounds I, IV, VII, XI, and XI1 have Czvsymmetry.54 For molecules having Czv symmetry the r-r* electronic transitions are polarized in the plane of the molecule and either along the molecular axis (‘Al ‘A1) or perpendicular to it (’B1 ‘A1). +
+
+
(44) G. W. Wheland, “Resonance in Organic Chemistry,” John Wjley and Sons, Inc., New York, N.Y . , 1955. (45) Reference 43, Chapter 9. (46) J. R. Platt, J . Chem. Phys., 17, 484 (1949); see also “Systematics of the Electronic Spectra of Conjugated Molecules,” J. R . Platt, Ed., John Wiley and Sons, Inc., New York, N.Y., 1964. (47) M. Godfrey and J. N. Murrell, Proc. Roy. SOC.(London), AZ78, 67 (1964). (48) See K. Kimura and S. Nagakura, Mol. Phys., 8, 117 (1965), and references cited therein. (49) For a recent discussion read E. Heilbronner, ref 39, pp 329359. (50) P . E.Stevenson, J . Mol. Spectry., 15, 220 (1965). (51) P. E.Stevenson, J . Chem. Educ., 41, 234 (1964). (52) J. Petruska, J . Chern. Phys., 34, 1120 (1961). (63) L. Doub and J. M. Vandenbelt, J . Am. Chem. SOC.,69, 2714 (1947); 71, 2414 (1949); 77, 4535 (1955). (64) Neglecting the symmetry of the 0-CHJ group.
ELECTRONIC TRANSITIONS IN THE BENZENEDIAZONIUM CATIONS
These transitions correspond to, respectively, the La and Lb transitions of the perturbed benzene model. The other molecules shown have C, symmetry and strictly speaking all excited *-electron states have the same symmetry. We have taken the liberty of assigning these transitions as being A, AB, or B. A and B are assigned on the basis of whether the computed squared transition moment acquired 70% or more of its strength from the x or y component of the transition moment. Transitions having approximately equal contributions of the x and y components were assigned AB. In this way a direct comparison of the molecules having Czvand C , symmetry can be made. Based on their weak intensities the low-energy transitions of compounds I, 11, 111, VI, VII, IX, and XI are assigned as being Lb. This is agreement both with the polarization (B or AB) and weak computed squared transition moments obtained from the Huckel calculations. Conversely the second electron transitions of these materials are significantly stronger and are assigned as being L, transitions. Compounds IV and XI1 only exhibit a single strong transition in the region above 250 mp. These transitions are assigned as being La transitions on the basis of their intensities. The Huckel calculations indicate that the La and Lb transitions of these materials are nearly degenerate. The rationalization is that the weaker Lb transition is buried beneath the L, transition. In some strongly perturbed systems, particularly the para-disubstituted benzenes, Lb transitions have not been o b ~ e r v e d ~and ~ > are ~ * presumably buried beneath the stronger La transitions. The charge-transfer calculations by Murre114' on the 2-, 3-, and 4-aminonitrobenzenes and t,he SCF-GI calculations by Labhart55 on 4-dimethylaminonitrobenzene also indicate a computed near degeneracy of the La and Lb transitions. The three remaining materials, V, VIII, and X, exhibit two transitions in the region above 250 mp having reasonably strong intensities (e > 9000). On the basis of intensity it is not possible to assign these as being either perturbed L, or Lb transitions. The computed weaker intensity of the low-energy transitions (and certainly the AB character of the transition for V) would lead toward the Lb assignment for V, VIII, and X. The theoretical expectation is that the La and Lb interpretation will lose its meaning a t a strongly perturbed stage. This is apparently the case for these latter three compounds. In Figure 5 is shown a comparison of the computed and observed positions of the L, and Lb transitions for compounds I-XII. This figure graphically demonstrates that the low-energy transitions of V, VIII, and X are
4087
CALCULATED Lb La .... -
i 3
OBKRVED
*s
.I
Lb
El
a
>:
p 40
m
0
.I
2
38 36 a 34 .* 2 32 2 38 28 5 26 24 g 22
g
1.45
0
0
1.35 .$ .*
$ ?j
