J. Phys. Chem. 1993,97, 9613-9617
9613
Ab Initio Structures of Para-Substituted Phenoxy Radicals (XC&IdO*: X = H, F, CI, OH, NH2, and 0-)and Substituent Effects on Molecular Vibrations Ruifeng Liu' and Xuefeng Zbou Department of Chemistry and Biochemistry, University of Arkansas, Fayetteville, Arkansas 72701, and Computational Center for Molecular Structure and Design, Department of Chemistry, University of Georgia, Athens, Georgia 30602 Received: June 16. 1993'
The structures of the ground-state phenoxy and para-substituted phenoxy radicals (XCaH40': X = H, F, C1, OH, NH2, and 0-) were calculated by the unrestricted natural orbitakomplete active space (UNO-CAS) method using the 6-31G* basis set. The results indicate that the p-aminophenoxy radical is not a special phenoxy radical; its structure is more like those of the other neutral phenoxy radicals rather than the p-benzosemiquinone radical anion. Presumably, the difference between the structural features of the p-aminophenoxy radical and the p-benzosemiquinone radical anion is due to significant delocalization of the unpaired electron of the latter onto the substituent, which results in equal CO distance of the radical anion. Qualitative molecular orbital analysis indicates that it is the CC instead of the CO stretching modes of the phenoxy radicals which should be most strongly resonance enhanced with the Z B ~ 2Az transition. On the basis of p~ donating abilities of the substituents, the CO stretching frequencies of phenoxy and para-halogensubstituted phenoxy radicals were concluded to be lower than those of the p-aminophenoxy radical and p-benzosemiquinone radical anion. The qualitative analysis indicates the necessity for reexamination of the vibrational spectra, and the l80 isotope experiment is recommended for resolving the dispute on the assignment of the CO stretching modes.
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Introduction Since the first spectroscopicobservation of the phenoxy radical (C&O') 30 years ago,' there has been tremendous interest in its structure and vibrations.2-1' However, due to the unusual reactivity, this radical does not lend itself to many of the usual kinds of structural and spectral investigations. As a result, only a few vibrational bands were observed, and the spectral features are not fully understood. During the past decade, quantum mechanical calculations on molecular structure and force fields have been quite successful for closed-shellorganic molecules.12-14For this radical, however, there has been no report of successful quantum mechanical investigations. The difficultiesof theoretical studies on this radical were understood recently.15 First, the size of this radical is too large for very high-level quantum mechanical calculations. Second and more importantly, electron correlation is extraordinarily important in this radical. Mainly due to strong electron correlation, the restricted HartreeFock procedure gives a CO single bond and a ring structure similar to that of benzene.3Js This is contrary to results of ESR studies which concluded that the unpaired spin of the radical is highly delocalized onto the ring, resulting in a partial CO double bond and a quinoid ring skeleton.1618 To understand the structure and substituent effects on the phenoxy radicals, Tripathi and Schuler carried out time-resolved resonance Raman studieson a number of para-substituted phenoxy radicals,19-22 XC&O', where X = H, CH3, F, C1, Br, OCH3, OH, NH2, and 0-. The Raman spectra of the phenoxy radicals in aqueous solution,excited in resonance with the -400-nm (ZB1 2A2) electronic transition, are characterized by a highly resonance-enhanced band at about 1500 cm-l for X = H, CH3, F, C1, Br, another highly resonance-enhanced band at about 1600 cm-1 in addition to the one at about 1500 cm-l for X = OCH3 and OH, and a highly resonance-enhanced band at about 1630 cm-1 for X = 0-and NH2. The strong Raman bands at about
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To whomcomspondmceshouldbeaddreasedattheUniversityofGeorgia. Abstract published in Advance ACS Absrracr3, September 1, 1993.
1500 cm-l were assigned by Tripathi and SchulerZ2to the CO stretching modes. (One should be aware that, in polyatomic molecules such as the subject molecules, there is almost no pure CO stretching mode. The phase T O stretching modes" used in this paper means modes composed mainly of the CO stretching by an energy distribution criterion.) The structural implication of resonance enhancement of these bands was explained to be that in the relevant excited state (2A2);the CO bond acquires a nearly single-bond character while the ring structure remains largely intact.22 For the p-aminophenoxy radical and p-benzosemiquinone radical anion, the strongest Raman bands cannot be reasonably assigned to the CO stretching modes, because frequencies above 1600 cm-l are simply too high for a partial CO double bond. On the other hand, ab initio calculation at the Hartree-Fock level on the p-benzosemiquinone radical anion20.23indicated that the in-phase CO stretching is at 1435 cm-l, and the observed deuterium isotope shifts of the p-aminophenoxy radical19clearly indicated that the in-phase coupled CO/C-N stretching mode is at 1434cm-l. As a result, the strong bands at above 1600cm-l The differences in were assigned to C=C stretching~.~~**~*~3 spectral features and vibrational assignments between these two radicals and the rest of the phenoxy radicals were explained by Tripathi and Schuler to be due to the p r electron-donating effect of the amino and 0- groups. They concluded that the structure of thep-aminophenoxy radical is different from those of the other phenoxy radicals but rather close to that of the p-benzosemiquinone radical anion.22 The experimental assignment of the vibrational spectra and the empiricalexplanation of the structural consequences were however based on spectroscopists' intuition. Since the experimental information is far from enough for the derivation of accurate force fields, we carried out some ab initio calculations to examine the structures and vibrational spectra of these interesting radical~.~5J~.~5 This paper presents some qualitative analysis of the calculated molecular structures as well as substituent effects on structures and vibrational spectra, which preludes the necessity for further studies on these radicals. Detailed analysis of the calculated
0022-3654/93/2091-9613$04.00/00 1993 American Chemical Society
9614
Liu and Zhou
The Journal of Physical Chemistry, Vol. 97, No. 38, 1993
force fields and interpretation of the observed spectra are subjects of subsequent publications.
TABLE I: CAS-SCF and UNO-CAS Geometrical Parameters. of the Ground-State Phenoxv Radical 6-31G*
3-21G
Method of Calculation It has been a general consensus that multiconfigurational selfconsistent-field (MC-SCF) methods are the most promising techniques of calculating wave functions for systems with strong electron correlation. The major objection against these methods is the arbitrariness associated with the selection of configurations and orbitals in the trial wave function. Much of these objection against early MC-SCF methods has been resolved by including all configurations within a limited set of orbitals, the active space, in the wave function. This complete active space self-consistentfield (CAS-SCF) method has been shown to be efficient in recovering nondynamical correlationso as to produce qualitatively correct descriptions of strongly correlated molecules. However, there is still some arbitrary element in the selection of active space in the general CAS-SCF method. For small molecules, it has been practiced that all the valence orbitals are included in the active space. This approach becoma impractical for molecules consisting of more than a few heavy atoms. To this problem, a method based on the occupancy of the unrestricted HartreeFock (UHF) natural orbitals was proposed recently. According to Pulay and Hamilton,26 the natural active space should be consisted of the fractionally occupied UHF natural orbitals. In fact, for most molecules, UHF natural orbitals were found to be a good approximation to the natural orbitals of the CAS-SCF wave function.26~2’ To reduce the expenses of CAS-SCF calculations, Pulay and Bofill further proposed to use the UHF natural orbitals in full CI calculations in the fractionally occupied UHF natural orbital subspace. Such a calculation was found to produce results equivalent to those of CAS-SCF but with only a fraction of the cost of the latter. This method is called the unrestricted natural orbitakomplete active space (UNO-CAS) method.27 It has been found that both electron correlationand polarization functions in the basis set are extraordinarily important for the correct description of the molecular structures of phenoxy radical~;l~ these *~~ factors - ~ ~ make the phenoxy radicals relatively large molecules for post-Hartree-Fock treatments. To minimize the computational costs without losing significance, we used the UNO-CAS method and the medium size 6-31G* basis set28 in present study. This level of theory has been found to be reliable for quantitative reproduction of the geometries of many conjugated organic molecules.29J’J At around the equilibrium geometries, the ground states of all the neutral phenoxy radicals were found to have seven fractionally occupied (occupancy between 0.02 and 1.98) UHF natural orbitals, while the p-benzosemiquinone radical anion has only three. According to the UHF natural orbital occupancy criterion, the multiconfigurational wave function for the neutral phenoxy radicals should include all the configurations that result from distributing seven electrons in the seven fractional occupied orbitals, while that of the p-benzosemiquinone radical anion is a linear combination of all the configurations which result from distributing three electrons in three fractional occupied orbitals. All the geometrical parameters of these radicals were fully optimized by the analytic gradient technique at the UNO-CAS/ 6-31G* level. For most of the radicals, quadratic force fields were calculated at the optimized geometries by numerical differentiationof the UNO-CAS gradients with either the 6-3 1G* or the 4-2 1G3I basis sets. Normal-mode calculations indicate that these structures are minima on the potential energy surfaces. For comparison, we also calculated the structure of aniline by UNO-CAS/6-3 lG* and the structure of p-benzosemiquinone radical anion by UHF with the 6-31G* basis set. The significantly smaller active space of p-benzosemiquinone radical anion indicates that it is a much less strongly correlated
UNO-CAS RCIC~ R c ~ Rc~c~ Rco Ran RC~H RC~H L123 L345 L712 L821 L934 L932
1.429 1.381 1.406 1.294 1.070 1.072 1.071 120.7 120.0 121.0 117.9 119.7 120.1
CAS-SCFb
UNO-CAS
CAS-SCF‘
1.430 1.380 1.406 1.292 1.070 1.072 1.07 1 120.7 120.0 121.0 117.8 119.6 120.1
1.452 1.378 1.413 1.239 1.075 1.075 1.074 120.5 120.2 121.2 117.5 119.5 120.0
1.455 1.377 1.414 1.236 1.075 1.075 1.074 120.6 120.3 121.2 117.4 119.4 120.0
Bond length in A and angles in deg; the numbering of atoms is given in Figure 1. b Reference 4. Reference 15. j7
I I
x 10 Figure 1. Numbering of atoms of the phenoxy radicals.
molecule. Therefore, the Hartree-Fock treatment is likely to give a qualitatively correct description. Indeed, for this radical anion, a UHF calculation using the 3-21G basis set qualitatively reproduced the observed spectral features, and the observed resonance Raman bands were accordingly a ~ s i g n e d . ~On ~ .the ~~ other hand, all the Hartree-Fock calculations failed to reproduce the observed spectral features of the rest of the phenoxy radicals.
Structures and Discussions Ground-State Geometries. The unsubstituted phenoxy radical is the prototype of this class of radicals. Consequently, it has been the subject of detailed experimental and theoretical investigations. Table I compares the UNO-CAS and the CASSCF geometrical parameters of the ground state of this radical. The numbering of atoms referred to in this table is given in Figure 1. As is shown, the UNO-CAS results are in excellent agreement with those of CAS-SCF, which corroborates the observation that the UNO-CAS potential energy surface is closely parallel to the CAS-SCF one. The dramatic difference in the results obtained by using the 3-21G and the 6-31G* basis sets indicates that the former is insufficient to describe the structures of this class of molecules. For phenoxy, p-fluorophenoxy, and p-benzosemiquinone radicals, we also carried out UNO-CAS geometry optimizations using the much larger 6-3 1 1G(2d,p) basis set. The structures obtained by the larger basis set are essentially the same as the 6-31G* results. The only difference is the slight shortening of all the bond distances with the larger basis set.lfQ4.2* The UNO-CAS/6-3 lG* geometrical parameters of the parasubstituted phenoxy radicals are presented in Table 11. No symmetry restriction was imposed in the geometry optimization. The optimization converged to CZ, symmetry for X = H, F, and C1, C, symmetry for X = OH and NH2, and DZh symmetry for p-benzosemiquinone radical anion.
Structures of Para-Substituted Phenoxy Radicals
The Journal of Physical Chemistry, Vol. 97, No. 38, 1993 9615
TABLE II: UNO-CAS/6-31G* Geometrical Parametersa of the Para-Substituted Phenoxy Radicals X C A O . X = H X = F X=C1
RClCz 1.452 R c ~ c ~ 1.378 Rcac4 1.413 Rco 1.239 RCZH 1.075 R C ~ H 1.075 R C ~ X 1.074 L123 L345 L712 L821 L934 L932
120.5 120.2 121.2 117.5 119.5 120.0
1.452 1.378 1.403 1.240 1.073 1.074 1.328 120.8 122.8 121.2 117.6 119.1 121.9
LCOH
1.452 1.377 1.412 1.238 1.074 1.073 1.736 120.9 121.6 121.3 117.9 119.5 120.9
X=OHb X=NHz X = O 1.450(2) 1.373(9) 1.408(14) 1.238 1.074 1.077(4) 1.351 120.9(1.0) 120.6 121.3(6) 117.7(5) 119.7(8.3) 120.1(1.5) 111.1
1.452 1.374 1.417 1.237 1.074 1.076 1.395 121.0 119.1 121.6 117.6 119.0 120.0
1.442 1.356 1.442 1.248 1.078 1.078 1.248 122.6 114.8 122.6 116.6 116.6 120.8
v
U
2.9 42.7 0.997 110.8
Y1 72
RNH LHNH
0 Bond length in A and angles in deg; the numbering of atoms is given Figure 1, b For details of thep-benzosemiquinone radical, see ref 24. The molecule is planar; the geometrical parameters which are equivalent to the ones given under CZ,pseudosymmetry are indicated by the numbers in parentheses; e.g., 1.408(14) means one bond distance is 1408 A while the other one is 1.414 A. y1 is the angle between C-N bond with the C5C& plane; y2 is the angle between the C-N bond and the NHz plane.
As is shown in Table 11, the structures of phenoxy, p-fluorophenoxy, p-chlorophenoxy, and p-benzosemiquinone radicals are very similar. The structure of the p-aminophenoxy radical closely resembles the other neutral phenoxy radicals, but the amino group is nonplanar with the 6-31G* basis set. (When a basis set without polarization functions was used, the optimized structure is planar, which is similar to the SCF structures of aniline.32J3) For aniline at the same theoretical level (UNO-CAS/6-3 1G*), the C-N bond is 2.2O out of the C3C4Cs plane and 45.0° out of the NH2 plane. The same angles in p-aminophenoxy are 2.9' and 42.7O, respectively. The ring skeleton of thep-aminophenoxy radical is only slightly nonplanar with the largest torsional angle being smaller than 1O . The fact that the p-benzosemiquinone radical anion is not as strongly correlated as the neutral phenoxy radicals is borne out by thesimilaritybetween the UNO-CAS/6-3 lG* bond distances (1.248,1.442,and 1.356A for CO, C - C , and C=C, respectively) and the UHF/6-31GS bond distances (1.245, 1.444, and 1.357 A, respectively). The CO distance obtained by Chipman and Prebenda23using the UHF/3-21G method, 1.274 A, is significantly longer. This is clearly due to the deficiency of the 3-21G basis set. In all neutral phenoxy radicals, the CO distances range from 1.237 to 1.240 8, with the shortest one in the p-aminophenoxy radical and the longest one in thep-fluorophenoxy radical. The CO distance of the p-benzosemiquinone radical anion, 1.248 A, is on the other hand significantly longer. These results clearly indicate that the structure of thep-aminophenoxy radical is much more similar to the neutral phenoxy radicals than to the p-benzosemiquinone radical anion. This is contrary to Tripathi and Schuler, who, in order to explain the spectral similarities between the p-aminophenoxy radical and p-benzosemiquinone radical anion, proposed that the structure of thep-aminophenoxy radical looks more like the p-benzosemiquinone radical anion instead of the other neutral phenoxy radicals.22 Structural Change upon Excitation to the z A ~State. As concluded from resonance Raman studies and confirmed by theoretical calculations, the -400-nm electronic transition in the resonance Raman experiments of the phenoxy radicals corresponds to ZB1 2A2 excitation. Sketches of the 17 orbitals of the ground-state (zB1)phenoxy radical obtained from UHF natural orbital calculation are given in Figure 2. In the ground
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0
0
0
Figure 2. Sketches of the 7 orbitals of the ground (ZB&state phenoxy radical.
state, two bl and one a2 orbital (orbitals labeled rl, 172, and 173 in Figure 2) are doublyoccupied,while the third bl orbital (orbital a4in Figure 2) is singly occupied. There are two possible ZB12A2 transitions. One corresponds to exciting one electron from r3to 174 (a2*bl a2bl2),and the other corresponding to exciting one electron from the singly occupied bl (174) to the LUMO (175 in Figure 2). The excitation energy for the former is lower than that for the latter. Since the p orbitals of Cz and C3 (and also CSand (26) in 173 are in phase, this a2 orbital is purely bonding to the CZCSand c5c6 bonds. On the other hand, the p orbitals of C2 and C3 (and also CS and C6) are out of phase in 174; this bl orbital makes antibonding contribution to the CzC3 and CsCa bonds. Thus, one reasonably expects that there would be significant increase in the C2C3 and C5c6 bond lengths accompanying the a2bl azb? excitation. As 173 is nonbonding to the CO bond, while 174 is antibonding for the CO bond, it is expected that the CO distance will also increase in the excitation. However, since this excitation results in the change from bonding to antibonding for the C2C3 and C5Cs bonds, while it is nonbonding to antibonding for the CO bond, the increase of the C2C3 and C5c6 bond lengths would be more significant than the change in the CO bond length. Thus,
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-.
9616 The Journal of Physical Chemistry, Vol. 97, No. 38, 19'93
according to Ziegler and AlbrechPand Heller et ~ l . , ~should ~~it be the in-phase CzC3 and c5cS stretching vibration which is most significantly resonance enhanced by the a22b1-., a2bI2excitation. Therefore, the strongest resonance Raman band of the phenoxy radical should be attributed to the CC stretching instead of the CO stretching vibration. Since 7r5 is purely antibonding to the C2C3 and CSCS bonds, while 7r4 also has components along the C2 axis, it is expected that exciting an electron from 7r4 to ~5 (bl a2) will also be accompanied by an increase in the C2C3 and c5cS bond lengths. On the other hand, since ASis nonbonding to the CO bond, while r4is antibonding to the CO bond, the bl ( ~ 4 ) az (175) excitation will result in decrease in the CO bond length. As the CO distance of the ground state is close to a CO double bond length (for example, the CO distance inp-quinone is 1.225 A as determined by gas-phase electron diffraction'd), it is reasonable to expect that the CO distance will shrink only slightly with the bl a2 excitation. The above qualitative analysis indicates that, on going from the ground state to the 2 A excited ~ state via both the a22b1 a2blZ and the bl a2 excitations, the change of the CO bond length will not be as large as the change of the CC bond lengths. This is contrary to the experimental assignment of the strongest resonance Raman bands to the CO stretching modes of the phenoxy radicals, indicating the necessity for reexamination of the vibrational spectra of the phenoxy radicals. SubstituentEffects. In the experimental studies, the strongest resonance Raman bands at about 15OOcm-l of the neutral phenoxy radicals were assigned to the CO stretching modes. However, in the p-benzosemiquinone radical anion and p-aminophenoxy radical, the strongest bands shift to above 1600 cm-1, which are too high to be reasonably assigned to the partial CO double bond stretching modes. On the other hand, there is evidence from ab initio calculation on the p-benzosemiquinone radical anion20923 and observed deuterium isotope shifts of the p-aminophenoxy radical19 indicating that the CO stretching modes in these two radicals are at about 1430 cm-*. Tripathi and Schuler explained that the PA donating effect of the amino and 0- groups is responsible for thesignificant redshifr of theCO stretchingmodes. Such an explanation is, however, contrary to the following qualitative consideration. The para-substituted phenoxy radicals can be viewed as being formed by cleavage of the OH bonds of para-substituted phenols. The delocalization of the unpaired spin onto the rings and formation of partial CO double bonds ca be qualitatively viewed as partial localization of the delocalized A electrons of the ring in the CO bond region. Thus, there is some degree of bonding A electron deficiency in the rings of the phenoxy radicals compared to benzene and phenol. The result of this bonding electron deficiency is the weakening of the overall CC bond strength of the ring skeletons. The CO bond in the phenoxy radicals can also be viewed as a bonding electron-deficientCO double bond. Due to the bonding electron-deficientcharacter, one expectsthat when there is strong PA electron-donating substituent such as OH, OCHs, NH2, and 0- at the para position, both the CC and CO bond strengths will be enhanced and therefore both the CC and CO stretching frequencies will increase. This, however, disagrees with the empirical assignments of Tripathi and Schuler, who placed the CO stretching mode of the phenoxy radical about 100 cm-1 higher than those of the p-aminophenoxy radical and p-benzosemiquinone radical anion. According to Wiberg et a1.,36halogens are not PA donating substituents to C=C and benzene rings; their effect is mainly electrostatic. Thus, the approximate ordering of the PA electron donating abilities of the substituents is -+
-
-
-
F < C1- Br
- H
CH3
