J. Phys. Chem. 1992, 96, 5344-5350
5344
Resonance Raman Spectra and Structure of Phenylthiyl Radical' G.N. R. Tripathi,* Qun Sun, David A. Armstrong, Daniel M. Chipman, and Robert H. Schuler Radiation Laboratory and Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, and Department of Chemistry, University of Calgary, Calgary, Alberta, Canada (Received: March 2, 1992)
Time-resolved resonance Raman spectroscopic and theoretical studies of phenylthiyl radical (PhS') show that the CS bond in this radical is essentially a single bond with the unpaired electron localized on the sulfur atom. This structure contrasts with that of phenoxy radical (PhO') where the CO bond is close to a double bond and the unpaired spin is largely delocalized onto the ring. The vsa ring stretching and vga CH bending frequencies in the PhS' (1 5 5 1 and 1180 cm-I) and in PhO' (1552 and 1157 cm-I) radicals are comparable, but the substituent-sensitive modes v , ~ , v12, and V6a in PhS' (1073, 724, and 436 cm-I) are at much lower frequencies than in PhO' (1 505, 840, and 528 cm-I). While a drop of 92 cm-' in the v6a frequency from PhO' to PhS' is almost entirely due to increased mass of sulfur atom, a downward shift of more than 400 cm-I in the qafrequency indicates a drastically reduced CS bond strength. The structural differences between the two radicals account for their distinctly different reactivities, e.g., 4 times faster second-order decay of PhS' radicals (2k = 9.6 X lo9 M-l s-I 1 as compared to PhO', and combination at the sulfur site in PhS' in contrast to reaction at the ring carbon sites in PhO'. It has been found that the PhS' absorption in the 400-510-nm region involves contributions from at least two electronic transitions, 2A2 2B, and 2B, 2B,, which are vibronically coupled via a non-totally symmetric ring distortion mode (V6b).
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Introduction Structural information on aromatic sulfur radicals is rare. In early flash photolysis studies in the gas phase phenylthiyl radical (C6H5S'; hereafter PhS') exhibited a broad band at -290 nm which showed no vibronic featuresS2 A number of other flash photolysis studies on PhS' in solution have been reported, and the photophysical properties of the excited states have been examined.,-I2 Recent laser-induced fluorescence (LIF) studies of this radical in the gas phase by Shibuya et al.13 have shown an electronic band origin at 517.4 nm and vibrational progressions which are interpreted in terms of ground-state fundamental frequencies of 430,610, and 1165 cm-I. Pulse radiolysis studies of the radical in aqueous solutionI4J5show a broad moderately intense absorption band at -460 nm, but there is disagreement with certain of the results from the photochemical experiments. Proton hyperfine splittings have not been resolved in ESR studies either in glassy matrices at low temperatures16J7 or in solution1*so that detailed information on the distribution of unpaired spin population in this radical is not available. While one might expect PhS' radical to have an electronic structure very similar to that of phenoxy radical (PhO'), in which ESR studies show the unpaired spin to be distributed over the aromatic system19 and Raman studies that the CO bond has large doublebond character,20such a description does not adequately account for the distinctively different modes of reaction of these two radicals. In the present study we apply time-resolved resonance Raman spectroscopy to examine in detail the vibrational structure of PhS' and find this structure to be very different from that of PhO'. In particular, the highest frequency mode with significant CS stretching character at 1073 cm-' is very little shifted with respect to the parent molecule, as compared to the upward shift of -250 cm-l in the predominantly CO stretching mode at 1505 cm-l in PhO'. Similar frequencies in PhS' and PhSH indicate that the CS bond in the radical is, in fact, close to a single bond and implies that the unpaired electron is largely localized on the sulfur atom. Ab initio calculations reported here confirm this description and provide detailed information on the CS stretching contribution to the normal modes. Recent ESR studies of Jeevarajan and Fewndenl8 show that the radical has a very high g factor (2.0252), also indicating that unpaired spin is highly localized on the sulfur atom in contrast to the delocalization noted above for PhO'. These structural differences account for the differences in the modes of reaction of these two radicals and provide an understanding *Towhom correspondence should be addressed at the University of Notre Dame.
of their comparative redox properties which we will treat in a separate paper. Experimental Section The experimental procedures used in this laboratory for timeresolved resonance Raman studies of transient intermediates have been described previously in detail.2*23 A Van de Graaff accelerator was used to generate 100-ns electron pulses of 2-MeV electrons at a dose sufficient to produce an initial radical concentration of -lo4 M. A Lambda Physik excimer pumped dye laser system was used to excite Raman spectra in the appropriate absorption bands. Detection used as PAR OMA I1 system with an intensified diode array gated in synchrony with the 10-ns laser pulse. Time resolution was attained by a controlled delay between the accelerator and laser pulses. The Raman frequencies were measured by reference to ethanol bands and are accurate to 1 cm-' for the sharp bands and - 5 cm-I for the weak and shoulder bands. Optical absorption measurements were carried out by timeresolved absorption spectrophotometry at the Laboratory's LINAC pulse radiolysis facility, as described elsewhere.24 The initial concentration of the radicals in this latter case was -2 X 10" M, as determined from thiocyanate dosimetry with e( (SCN),-) taken as 7580 cm-' M-I a t 472 11311.~~ Thiophenol (PhSH) used in this study was from Aldrich. Solutions were 0.2-2 mM in substrate, purged of dissolved air, and saturated with N 2 0or N2. The pH was adjusted with KOH or HC104. The PhS' radical was prepared in basic solutions by the oxidation of thiophenolate anion (PhS-) with N,' radical produced pulse radiolytically in a 0.1 M N3- solution saturated with N 2 0 . In acidic solutions the radical was prepared by hydrogen abstraction from PhSH either by acetone ketyl radical (1-hydroxymethylethyl radical) produced by OH abstraction of H from 1 M 2-propanol or by C02'- produced from 2 M formate.25*26In both the spectrophotometric and Raman studies a flow system was used to refresh the solution between pulses. Product analysis was by HPLC using a Waters 990+ chromat~graph.~' Theoretical studies were carried out on a Convex C120 system using the GAUSSIAN 90 and MELDF program packages. Details are described below.
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Results and Discussion A. Absorption Spectrum. The absorption spectrum of phenylthiyl radical observed in basic solution by oxidizing thiophenolate anion with pulse radiolytically produced N3*is shown in Figure 1. The spectrum of phenoxy radical obtained similarly
0022-3654/92/2096-5344%03.00/0 Q 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, No. 13, 1992 5345
Spectra and Structure of Phenylthiyl Radical I
I
-n
I
L!
h
N e
0
0-
e
n
/I
c
I
"
ji, A
0
280
370
460
550
Wavelength, nm
-
Figure 1. Transient absorption spectra observed 1 after pulse radiolysis of N,O-saturated aqueous solutions (pH 11) containing (A) 1 mM thiophenol and (B) 2 mM phenol.
by oxidizing phenolate anion is given for comparison. At the azide ion concentration used, H atoms are removed by reaction with azide2*so that there is no contribution from H atom adducts that otherwise would contribute somewhat in the 340-380-nm region. An identical absorption band at 460 nm (e = 2590 M-' cm-') was found in the oxidation of thiophenol by COT a t pH 4.5. However, in the latter case a narrow band with an extinction coefficient of 11 OOO M-'cm-I, obscured by the absorption of thiophenolate in basic solution, was also observed at 295 nm. These absorptions are very similar to those reported for PhS' radical by Bonifacic et al.Is It will be noted in Figure 1 that the absorption spectrum of the PhS' is much broader than that of the PhO' radical and lacks the well-defined vibrational structure of the latter. It is additionally rather asymmetric and has a modestly intense sideband on the low-energy side of the main 460-nm band at -483 run and a much weaker one at -508 nm. The shape of the absorption does not change over the temperature range 10-70 "C so the contributions from these sidebands do not likely represent absorption by a low-lying excited electronic state of the radical. The oscillator strength is also considerably greater than the absorption of PhO' radical in the 340-420-nm region. It is clear that the nature of the electronic transition in the 460-nm region is rather different than the transition in the 400-nm region in PhO' radical. Since it is known that aliphatic thiyl radicals are strongly complexed by thiylate ions and that the complexes absorb strongly in the 390-400-nm r e g i ~ n , ~it ~is"important ~ to examine for the possibility that the spectrum of Figure 1 might be attributable to an analogous aromatic complex. In fact, in an early flash photolysis study of thiophenol in aqueous solution" the band observed at -460 nm was attributed to PhSSPh'-. Our finding that the spectrum observed in basic solution, where the substrate is present as the thiophenolate anion (PhS-), is identical to that observed in acidic solution, where the substrate is thiophenol (PhSH), indicates that complex formation cannot be important. We also find that the spectrum is not dependent on the substrate concentration, showing that the radical does not complex significantly at substrate concentrations of M or less. In summary, there is no evidence in any of our absorption or Raman studies that PhS' complexes to any significant extent. The reason for this difference between the aliphatic" and aromatic systems remains to be determined. Comments on the Absorption Spectra in Nonaqueous Media. The absorption spectra in the literature attributed to the PhS' radical produced photolytically in nonaqueous media show a wide variation in absorption maxima. While many of these previous studies report absorptions in the 400-460-nm region, in a number of cases bands were also reported at -380 nm33799J0where PhS' in aqueous solution exhibits little absorption. Most of the photochemical studies in nonaqueous media involved photolysis of thiophenol or diphenyl disulfide where it was assumed that SH
(C) p
l
y
I
I
y -
2!
1
le00
I
1
1
1
1400
1
1000
Raman Shift
1
- cm-'
1
1
1
1
1
600
Figure 2. Raman spectra (A, B) of phenylthiyl radical obtained 1 p s after pulse radiolysis of 1 mM thiophenol in aqueous solution containing 0.1 M NaN, at pH 11 (NzO saturated): (A) spectrum excited at 480 nm; (B) spectrum excited at 460 nm;(C) Raman spectrum of phenoxy radical, excited at 400 nm (see ref 20). The bands marked with asterisks N
are spurious. or SS bond scission produced the PhS'. In this context it is noted that a transient absorption similar to that of Figure 1, but with a maximum at -440 nm, has recently been observed in the photodissociation of diphenyl disulfide in decalin.4 This absorption decays on the picosecond time scale as a result of geminate combination of PhS' pairs. Complications in the photochemistry rather than solvent effects on the absorption very likely explain the poor agreement between the spectra reported in the various previous photolytic studies. B. Raman Spectra and Structure. The resonance Raman spectra of PhS' reproduced in Figure 2 were recorded 1 ps after the pulse radiolytic oxidation of thiophenolate by N3*,No significant differences in the Raman features were observed when this radical was prepared by hydrogen abstraction from thiophenol by acetone ketyl radical a t pH 3 or in basic solutions by oxidation of thiophenolate directly by OH radicals or by electron transfer to phenoxy radical. These spectra excited at 480 nm (Figure 2A) and 460 nm (Figure 2B) are compared with the 400-nm Raman spectrum of PhO' produced by N3' oxidation of phenolate ion (Figure 2C). The 460- and 480-nm Raman spectra of PhS' differ principally in the relative resonance enhancement of the bands, with the 724-cm-l band being relatively weaker with excitation a t 460 nm. Since the PhS' absorption is reasonably strong, only totally symmetric (al) vibrations are expected to be prominent in the resonance Raman spectra. There are eight such vibrations which may appear in the frequency region below 1700 cm-I: v8a ( c c stretch), uIga (CC stretch and C H bend), vga (CH bend), u~~ (CS and ring stretch), vlga (CH bend), vI (ring breathing), v I 2 (CCC bend), and v6a (CCC bend). Five Raman bands at the frequencies as reported in Table I (1551, 1180, 1073, 724, 436 cm-') are observably enhanced with excitation at 460 and 480 nm and can be assigned from the spectroscopic observations by the following arguments. These assignments are corroborated by the ab initio calculations reported below. An additional weak band is also observed at 991 cm-' with excitation at 480 nm. Frequencies for comparable vibrations are also given in Table I for PhO' radical and for PhOH and PhSH. The prominent band at 1551 cm-l can be readily assigned to the VBa ring stretching vibration commonly observed for most aromatic radical systems in this region.23 Although this vibration is not observed in the 400-nm Raman spectrum of PhO' radical, it is found for the latter at 1552 cm-I with excitation at -250 nm.j2 The 1551-cm-I band of PhS' is clearly not related to the
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5346 The Journal of Physical Chemistry, Vol. 96, No. 13, 1992
Tripathi et al.
TABLE I: Comparison of the Vibrational Frequencies (cm-I) in Phenylthiyl Radical with the Frequencies in Thiophenol, Phenoxy Radical, and Phennl
phenylthiyl radical"
thiophenol*
phenoxy radicalC
phenold
1551 s 1180 m 1073 m 991 wf 724 s 436 m
1582 (1574) 1182 (1181) 1093 (1079) 1002 (998)' 697 (694) 412 (418)
1552' 1157 w 1505 s 990 w-m 840 vw 528 w-m
1604 1167 1259 1026 810 526
assignment*qd %a
b a
Va YIBaIYI VI2 Y6a
C C stretch C H bend C X stretch C H bend or C X stretch C X stretch
+ ring distortion ring breathing
+ ring distortion + ring distortion
'Present work. Relative intensities from 480-nm spectrum (s = strong, m = medium, w = weak). *References 34 and 35. Frequencies in parentheses are for phenylthylate ion (ref 36). CReference20. dReference 34. 'Enhanced by -250-nm excitation but not observable in 400-nm excitation (see ref 32). fcorrelation doubtful.
1505-cm-I band of PhO' assigned to the u7a (CO and ring stretch) vibration20since, because of the increased mass, the CS stretching frequency must be well below the CO frequency. It was shown in an earlier paper that the uga vibration of aromatic radicals observed in the 1160-cm-l region, which also is very weak in the PhO' spectrum, is enhanced parallel to the us, band, as this vibration involves a similar CC stretching c~mponent.'~The band a t 1180 cm-' is therefore assigned to this mode. It is obvious that there is no band in the region above 1100 cm-l in the PhS' spectrum which can be correlated with the most intense band of PhO' radical observed at 1505 cm-I. If the molecular structures of the two radicals were similar, one would expect the frequency of this mode to be shifted to the 1200-13OO-~m-~ region because of the increased mass of the sulfur atom. The 1441-cm-I band in Figure 2A is at too high a frequency and clearly represents the first overtone of the 724-cm-l band. It is not observed in the 460-nm spectrum where the 724-cm-' band is much weaker. The v7a CS stretching vibration in the PhS' radical must, therefore, be assigned to a band in the frequency region below 1100 cm-I. The likely correlation is with the band at 1073 cm-'.In thiophenol and thiophenolate anion similar frequencies have been assigned ~ * ~ ~ of its low frequency, the CS to the u7a v i b r a t i ~ n . ~Because stretching motion is expected to be strongly coupled with phenyl skeleton motions. As a result of this coupling, several vibrations in the 400-1 100-cm-' region may also involve a CS stretching component, and none of the normal modes should represent a mainly CS stretching vibration. For making a structurally meaningful correlation between the PhO' and PhS' frequencies, it is important to recognize that the v7a, vI2, and vk vibrations are substituent-sensitive vibrations and should be assigned by their ring distortion components, similar to the approach adopted by Green3s and Scott et al.34for the parent molecules. For example, the prominent bands at 724 and 436 cm-', which we assign to the u I 2and Y6a modes, appear to involve an important C S stretching component as these frequencies are significantly lower than the corresponding vibrations in phenoxy radical (840 and 528 cm-1).20 The weak 991-cm-l band can be tentatively assigned either to the Yga CH bending or to the u1 ring breathing mode. The absence of a fundamental band in the 1450-1500-cm-1 region also indicates that the uIgamode is not observably enhanced, although it could be masked by the 724-cm-I first overtone. While one might be surprised by the similarity between the vibrational frequencies of the PhS' and those in the parent molecule (Table I), this similarity is borne o u t by the theoretical work reported below. The downward shift of the uSafrequency of 1581 cm-' in PhSH to 1552 cm-I in PhS' results in part from elimination of the SH bending component, as this frequency drops to 1574 cm-I in PhS- ion.36 The low-frequency bands in the radical (436, 724, 1073, and 1180 cm-I) all have frequencies similar to those in the parent molecular anion. This similarity leads to an important conclusion, Le., that the double-bond character of the CS bond in the radical, if any, has to be small. The structure of the PhS' radical is thus very unlike that of the PhO' radical where the CO bond acquires almat doublebond character.20 This difference in turn implies that the unpaired electron in the PhS' is largely localized on the sulfur atom, in contrast to its delocalization onto the ring in the case of PhO'. In early ESR studies on matrix-isolated radical, the localization of the unpaired spin
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on the sulfur atom was held responsible for the asymmetric g factor and broadening of the ~pectrum.'~J'Very recent studies of this radical in solutionis show only a broad line (-5 G half-width) and an isotropic g factor of 2.0252. The very high value of the latter indicates that the unpaired spin is highly localized on the sulfur atom, in agreement with the conclusion from the Raman experiments. Comments on the LIF Studies. In their LIF studies Shibuya et al.I3 report a number of vibronic transitions of the PhS' radical with an uncertainty of f 4 0 cm-I in frequencies. They analyzed the LIF spectra in terms of ground-state fundamentals 430,610, and 1165 cm-' (estimated error f 2 0 cm-I). Only two of these fundamentals are observed in the present Raman studies, and their frequencies are more accurately determined to be 436 and 1180 cm-'. The assignment of a 610-cm-' frequency to a totally symmetric vibration would appear to be erroneous since the work of Scott et al.34and Green35on the parent molecule and the theoretical studies reported below indicate that there should be no totally symmetric modes in this region. As pointed out in the theoretical section, it is likely that this frequency is assignable to the non-totally symmetric v6b vibration. The ground-state frequency 750 (f40) cm-l in the LIF spectrum, which was left unassigned, probably corresponds to the 724-cm-' fundamental observed in the Raman experiments. Shibuya et al. assigned the 1070 (f40)-cm-l frequency as a combination of 430- and 610-cm-' fundamentals. In the present study any such combination would be too weak to interfere with the fundamental at 1073 cm-'.The most intense band in the LIF spectrum involves a frequency of 1590 (fa) cm-'.This frequency was assigned as a combination of the 430- and 1165-cm-l fundamentals. However, it seems more likely that this vibration is related to the intense vsa fundamental found in the present study a t 1551 cm-I. In view of the additional vibrational features and frequency precision which the present study provides, the nature of vibronic transitions in the LIF spectra should now be able to be ascertained more reliably. C. Theoretical Studies. Ab initio theoretical calculations were carried out on the ground state of phenylthiyl radical using the standard 3-21G(*) basis set3' that consists of a split-valence representation for all atoms supplemented with d-functions on sulfur. The GAUSSIAN 90 program package38 was used for ground-state SCF and analytic derivative calculations. Spinunrestricted Hartree-Fock (UHF) calculations showed very serious spin contamination for the ground state, with ( S 2 ) 1.3 as compared to 0.75for a pure doublet state. Therefore, all calculations were based instead on the spin-restricted open-shell HartreeFock method (ROHF), which produces a pure doublet single-determinant wave function. The ground state was found to have 2B1symmetry (molecular plane Y Z ) in the C , point group corresponding to the electron configuration ...(15aJ 2...(9b2)2...(3b,)2(4bl)( la$ where only the highest orbitals of each symmetry type are explicitly shown. The singly occupied 4bl molecular orbital (SOMO)is primarily a sulfur p ( ~ atomic ) orbigal, with only a slight admixture of ring T atomic orbitals. The equilibrium geometry, as determined by analytic energy first derivative methods, is reported in Table 11. For comparison, the RHF/3-21G(*) geometry of the thiophenolate
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The Journal of Physical Chemistry, Vol. 96, No. 13, 1992 5341
Spectra and Structure of Phenylthiyl Radical
1088 an-' 11ucm-' 1470 cm-' 1557 cm-' Figure 3. Ab initio description of atomic displacements in the totally symmetric normal modes of phenylthiyl radical (PhS') in the 400-1600-cm-~ region.
TABLE 11: Comprison of ROHF/IZlG( *) Relative Energies" and Equilibrium Geometries Calculated for the Ground States of Pheaylthiyl Radical (*BJand of Thiopheaolnte Anion ('Al) radical anion Relative Energy (eV) (0.00) -0.82 vertical adiabatic (0.00) -0.87 c6s
c6cI
ClC2
c2c3 C1H C2H C3H c5c6c I c6cIc2
clc2c3
c2c3c4 C6CIH CIC2H
Bond Distance 1.750 1.393 1.381 1.385 1.071 1.072 1.072
(A) 1.756 1.406 1.381 1.387 1.072 1.076 1.074
Bond Angle (deg) 119.1 120.4 119.9 120.2 119.1 119.9
115.8 122.0 121.0 118.2 117.9 119.4
"Energies are given relative to -624.4903 au, the calculated total energy of the 2Bl ground state of phenylthiyl radical at its calculated equilibrium geometry.
anion (PhS-) is also given. The latter corresponds to adding a second electron to the 4bl orbital, producing a closed-shell 'Al state with MscF vertical electron affinity of -0.8 eV. Since this orbital is largely nonbonding, one would expect the neutral and anion species to have very similar geometries, and this is borne
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out in Table 11. The CC bond lengths of 1.4A and CCC bond angles of -120O imply a typical aromatic ring structure. In particular, we note in Table I1 that the calculated CS bond lengths are nearly the same and typical of experimentally determined aromatic carbon to sulfur single bonds.39 These calculations indicate very clearly that the CS bond in PhS' must be essentially a single bond. The force field of the radical ground state was obtained from finite first differences of analytic energy first derivatives, using atomic displacements of 0.005 A from the calculated equilibrium structure. Diagonalization of the mass-weighted force constant matrix produced the fundamental vibrational normal modes and frequencies. Frequencies were multiplied by the empirically determined factor@ of 0.89 to correct approximately for the combined errors due to basis set incompleteness, neglect of electron correlation, and anharmonicity. Calculated frequencies41of the totally symmetric modes (excluding the high-frequency C H stretches, which are not observed experimentally) are compared to experiment in Table 111. Calculated and experimental frequencies for the structurally very similar thiophenolate anion are also given in parentheses. The calculated frequencies for Phs' are in quite good agreement with experiment, all deviations being less than 25 cm-I, indicating that the standard scaling factor of 0.89 is nearly optimal for this study. Five of the experimentally observed bands can be immediately assigned on the basis of their close proximity to the calculated frequencies. The weak band observed at 991 cm-l in the 460-nm Raman spectrum could correspond to either the 980- or 1017-cm-I calculated mode. On the basis of comparison to the thiophenolate anion frequencies, we assign it to the calculated 980-cm-I mode. The mode calculated at 1470 cm-' (and found
TABLE III: Frequencies and Composition of Normal Modes Calculated for Phenylthiyl Radical" frequency (cm-I) theoretical composition 0.89' calcb exptC CS str C C str CCC bend 409 (405) 708 (692) 980 (965) 1017 (1004) 1068 (1072) 1184 (1172) 1470 (1453) 1557 (1546)
436 (418) 724 (694) 991 (998) - (1020) 1073 (1079) 1180 (1181) - (1475) 1551 (1574)
" Experimental frequencies are given for comparison. (see also ref 41). bFrequencies calculated
0.5 1 0.25 0.03 0.00 0.21 0.00 0.01 0.00
0.10 0.17 0.84 0.40 0.30 0.31 0.25 0.62
0.39 0.56 0.09 0.46 0.40 0.00 0.00 0.10
C H bend
0.00 0.02 0.03 0.13 0.09 0.70 0.74 0.28
Calculated and experimental frequencies for thiophenolate anion are also given in parentheses
for PhS' and (in parentheses) for PhS-. CExperimentalfrequencies measured for PhS' in this work. In
parentheses, experimental frequencies measured for PhS- in ref 36.
5348 The Journal of Physical Chemistry, Vol. 96, No. 13, 1992
near there in thiophenolate anion)36is also not experimentally observed in this work. It may be obscured by the first overtone of the 724-cm-l fundamental. To gain a better appreciation of the calculated normal modes, diagrams of the atomic excursions are given in Figure 3, and Table I11 provides a population analysis of the participation of each internal coordinate in each normal mode as determined by the "M matrix" The low-frequency 409- and 708-cm-' modes are primarily combinations of CS stretching and ring bending motions. The 980-cm-I mode corresponds to ring breathing, and the nearby 1017-cm-I mode is mainly a ring distortion mixed with C H bend. The 1068-cm-I mode is the highest frequency mode having appreciable CS stretch character. However, it also contains significant contributions from ring distortions. The 1184- and 1470-cm-I modes are primarily C H bending motions. Finally, the 1557-cm-' mode is mostly CC stretching in nature. These descriptions of the various vibrations, which are fully in accord with the conclusions from the spectroscopic arguments given above, particularly emphasize that the CS stretching motion is highly mixed with ring stretching and bending motions. The excellent agreement between the observed and calculated frequencies of the PhS' radical encouraged us to calculate the frequencies of a hypothetical PhS' isotope in which the mass of sulfur is replaced by that of oxygen. This PhS' isotope (PhI6S') is essentially a phenoxy radical with a PhS' structure. Comparison of the observed frequencies of the PhO' radical (528, 840,990, 1157, 1505, and 1552 cm-I) with the corresponding calculated frequencies of Ph'6S' (500,742,1017,1184,1083, and 1557 cm-I) then allows to distinguish the vibrations which are sensitive to the CO/CS bond strength. For example, the 92-cm-l drop in the experimental v6, frequency from 528 cm-I in PhO' to 436 cm-I in PhS', when compared to the calculated drop of 91 cm-I from Ph16S*to PhS' (500 to 409 cm-I), shows that it is mostly due to the increased mass of sulfur. Although the calculated 1083-cm-l mode of the Ph%' has a large (2% (or CO) stretching component, the shift from the analogous frequency of the PhS' radical (1068 cm-I calculated, 1073 cm-I experimental) is small. Therefore, for comparison of the CO and CS bond strengths the 1505-cm-I v,, C O stretching frequency in PhO' should be correlated with the 1073-cm-I f r e q ~ e n c yand ~ ~ not ? ~ ~with the 436cm-l frequency in PhS'.I3 The experimental PhO' to PhS' shift of more than 400 cm-I (from 1505 to 1073 cm-I) in the v~~ vibration reflects a change from nearly double-bond character of the CO bond in the PhO' radical to single-bond character of the CS bond in PhS'. The vga ring stretching frequency ( 1550 cm-') is almost identical in the two radicals, implying that the ring structure remains largely unchanged. Discussion of the Absorption Spectrum. As pointed out above, the visible absorption of PhS' is broad and asymmetric with an absorption maximum at 460 nm (2.7 eV), a moderately intense sideband at ~ 4 8 nm 3 that is -0.13 eV lower in energy, and a very weak sideband that is -0.25 eV lower. In an attempt to gain some insight into the excited states potentially involved, several low-lying vertical excited states of PhS' radical were characterized by means of single excitation configuration interaction (SCI) calculations using the MELDF package.43 Since the molecular orbitals used are optimal only for the 2Bl ground state, this procedure would be expected to generally overestimate the excitation energies. Valence states may be too high by 1 eV or more, and due to the lack of diffuse functions in the basis set any states with Rydberg character should have even larger errors. SCI dipole-length oscillator strengths have also been determined, but due to use of the small 3-21G(*) basis set these should be regarded as providing only qualitative guidance. Those excited states found to be dominated by a single configuration were also characterized by ROHF calculations to obtain MScF estimates of the excitation energies. All excited states found in the calculations to be within 5 eV of the ground state are summarized in Table IV. The lowest excited state is of 2B2symmetry and is dominated by a single configuration that corresponds to a 9b2 4bl pro-
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Tripathi et al. TABLE IV: Theoretical Characterization of Low-Lying Excited States of Phenylthiyl Radical" symmetry
mSCF
0.3
2B2
2B,
3.4 4.6 4.7
2A2 2A,
4B,
" 0.5 3.2 3.8 4.7 3.3
f 0.000 0.010
0.005 0.01 1 0.000
"Vertical excitation energies from the 2B,ground state are given in electronvolts.
motion; Le., the unpaired electron is in the sulfur p-orbital that is parallel to the molecular plane and transverse to the symmetry axis. The vertical excitation energy is calculated to be quite small, only -0.4 eV, but the transition is not dipole allowed. The temperature independence of the PhS' absorption from 10 to 70 OC clearly indicates that the 2B1to ,B2 excitation energy is much larger than kT (0.025 eV). For these reasons, the low-lying ZB2 excited state is not expected to have any significant effects on the experiments reported here. Three dipole allowed doublet excited states with similar oscillator strengths are calculated in the range -3-5 eV. The first is of 2B1symmetry and cannot be well described by any single configuration: significant contributions come from configurations corresponding to 3bl 5bl, 3bl 4bl, 4bl 5bl, and la, 2a2 promotions, where the 3bl, 5bl, la2, and 2az orbitals are all a orbitals localized on the ring. Since this state has the same symmetry as the ground state, the transition has z-polarization (Le,, parallel to the symmetry axis). Next is a state with ZA2 symmetry that is dominated by a single configuration corre4bl promotion and has x-polarization (Le., sponding to a la, perpendicular to the molecular plane). Finally is a state with 2Al symmetry that is dominated by a single configuration corresponding to a 15al 4bl ( u T ) promotion and has y-polarization (Le., in the molecular plane and perpendicular to the symmetry axis). Consideration was also given to quartet spin states, which could conceivably attain oscillator strength through spin-orbit coupling induced by the heavy sulfur atom. Considerable experimentation indicated that the lowest quartet state has 4B1symmetry with the electron configuration ...(15a1)2...(9 b2)2...(3bJ2(4bl)( 1a2)(2a2). Using this as a reference state, SCI calculations (using the ,B1 molecular orbitals) were carried out to search for any other low-lying quartet states, but all others were calculated to be more than -5 eV above the 2Bl ground state. The 4B1state was found to be largely composed of the single-reference configuration indicated above, but the large difference between the SCF and SCI excitation energies indicates that other configurations also make significant contributions. The state is probably the major cause of the serious spin contamination found in U H F calculations on the 2Bl ground state. However, it is unlikely to influence the absorption or resonance Raman experiments of this study because the transition is forbidden; i.e., consideration of symmetry transformation properties indicates that the spin-orbit Hamiltonian matrix element connecting the 4B, excited state with the 2B1 ground state vanishes. All three of the dipole-allowed ,B1,,A2, and IA, valence excited states described above have appreciable oscillator strength and, in view of the significant uncertainties in such crude calculations, appropriate energy to be considered as candidates for influencing the absorption and resonance Raman spectra observed in this work. They are likely to occur in the order indicated, but we cannot absolutely rule out the possibility that the first two might be interchanged. The PhS' absorption in the 440-510-nm region can possibly be attributed to a single electronic transition, with the weak shoulder band at -508 nm as the 0,Oband and the strong -483and 460-nm bands as the vibronic progressions of a 1000-cm-l excited-state vibration. However, the frequency separations between the absorption bands in decalin4 are different than in aqueous soiution. While the weak -508-nm band in aqueous solution is shifted to higher frequency by 600 (f100)cm-I, the
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The Journal of Physical Chemistry, Vol. 96, No. 13, 1992 5349
Spectra and Structure of Phenylthiyl Radical
I
I
100
300
500
700
so0
Time, ps
Figure 4. Decay kinetics of the phenoxy absorbance at 400 nm ( 0 )and the phenylthiyl absorbance at 460 nm (0). Second-order rate constants for these decays are respectively 2.3 X lo9 and 9.6 X lo9 M-'s-'.
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prominent bands at -483 and -460 nm are shifted respectively by 1300 and loo0 (f100) cm-I in decalin,4 which shows that the three bands cannot belong to the same electronic band system. This conclusion is further evident from the gas-phase LIF and LIF-excitation spectra where the 0,O band at 517.4 nm is the most intense band and the vibronic transitions involvingvibrational levels of the ground or excited electronic states are relatively weaker. The intensity pattern of the Raman spectrum excited in resonance with the main peak a t 460 nm is significantly different from the spectrum excited at 480 nm (Figure 2), which also suggests that more than one excited electronic state is accessible. In the resonance Raman spectra the substituent-sensitive modes v7a (1073 cm-I), uI2 (724 cm-l), and v6a (436 cm-I) are very prominently enhanced with excitation a t 480 nm, as compared to the enhancement at 460 nm where the vga (1551 cm-')mode dominates the spectrum. The Raman spectrum in resonance with the 508-nm absorption band of the radical could not be obtained because of enormous background emission; therefore, information on the vibrational modes enhanced by this transition is not available. However, some insight into the electronic origin of the 508-nm absorption band can be gained by examination of the LIF spectra.13 As commented on earlier, the 610 (f20) cm-I fundamental indentzed in the LIF studies cannot be assigned to a totally symmetric vibration, but it can be readily assigned to a non-totally symmetric vibration, Y6b (CCC bend), having bz symmetry. The corresponding frequency is observed a t 617 cm-' in thiophenol and 615 cm-l inthiophenylate anion36and is calculated here to be at 630 cm-' in the radical.41 From symmetry considerations, a vibration of bz symmetry can be observed in LIF* only via vibronic coupling between the ZBI zBland 2A2 ZB,transitions. In the LIFexcitation spectrum (equivalent to absorption spectrum) the vibronic transition involving the Y6b vibration should be much stronger than in the LIF spectrum, since the energy gap between the *Azand 2BIexcited states is much smaller than the energy gap between the ZAzexcited and ZBlground electronic state. The vibronic transition involving the excited-state counterpart (483 cm-') of the 610-an-] vibration is indeed very strong, the intensity being comparable to that of the 0.0 band,13 which provides strong evidence for the Ysb mode induced vibronic coupling. The above considerations support the conclusion from the theoretical calculations that the lowest excited state is 2B,, with 'A2 higher in energy, and that both states contribute to the observed broad absorption in the 440-510-nm region. The calculations further suggest that a ZA, excited state may also occur in this region. These states may be the cause of the structure observed in the absorption spectrum. Since the candidate 2B,, ZAz,and 2AIexcited states have transition moments in different directions, polarization studies of oriented radicals in low-temperature matrices or polymer films should be capable of settling this issue. +
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D. Chemical Consequencesof Electron Localization. One can effectively gain some further insight into the chemical consequences of localization of the unpaired electron on the sulfur atom by restating the above theoretical descriptions in a more qualitative empirical molecular orbital language. In the parent molecule the nonbondmg electron lone pair on the sulfur (S)atom of thiophenol (3~:) has lower energy than the electron pair (2p,Z) on the heteroatom of the analogous oxygen or nitrogen compounds." Therefore, its conjugation with the ring p ( ~ electrons ) should lead to a relatively lower ionization potential, larger spectral shifts, and stronger ortho and para directivity for the SH group as compared to that for OH and NH2 groups. These effects are, however, smaller than expected and have been explained in terms of a CS bond distance which is much larger than the CO and CN bond The high electronegativity of 0 and N atoms, as compared to that of the S atom, and small orbital overlap between the 2p electrons on phenyl C atom with the 3p electrons on S atom, as compared to the overlap with the 2p electrons on 0 or N atoms, are responsible for the longer CS bond. As a result, the extent of conjugation of the sulfur electron lone pair p(): with the ring is reduced so that the physicochemical properties of thiophenol become similar to those of phenol." These arguments can be extended to the corresponding radicals where the p electrons on the substituent group (X) can attain two configurations, i.e. p,p? and p,'p,,, with equal probability when interaction with the ring is negligible. However, the codiguration pxpy' will be strongly favored when the X(p,) orbital overlaps effectively with the adjacent C(p,) orbital, as it does in the phenoxy radical, thus stabilizing the structure due to a large resonance energy for electron exchange and hence formation of a covalent K bond. In a situation where the electron exchange energy is small or comparable to the X(p,)-C(p,) repulsion energy, as appears to be the case for phenylthiyl radical, the C-X K bond, if formed, will be very weak, and the unpaired electron in both configurations will be largely on the substituent (X) group. The localization of the unpaired spin on the sulfur atom is manifest in the rapid second-order decay of PhS' radicals. In Figure 4 we compare the decay of PhS' and PhO' radicals prepared at similar concentrations under essentially identical pulse radiolysis conditions. In both cases the decay kinetics is second order in radical concentration. It is seen that PhS' radicals decay -4 times more rapidly than PhO' radicals. In the latter case the second-order rate constant has been determined by both absorption (in acidic media) and Raman methods to be 2.3 X lo9 M-' s-l. The decay of PhS' radical in Figure 4 corresponds to a rate constant of 9.6 x lo9M-' s-' . Presumably, because of localization of the electron on the sulfur atom, spin relaxation makes all radical encounters effective whereas reaction occurs only with singlet encounters in the case of PhO' and, therefore, with 4 times lesser probability. Spin localization is also directly manifest in the formation of diphenyl disultide (PhSSPh) as the dominant combination product. The yield of diphenyl disulfide, determined chromatographically in the pulse irradiation of thiophenolate anion, shows that virtually all radicals combine a t the sulfur position. This result contrasts with the combination of phenoxy radicals which occurs mainly at the ortho and para positions of the ring.27 Spin localization is also manifest in the electron-transfer kinetics and equilibria of phenylthiyl radical as will be discussed in a separate paper. Acknowledgment. We thank Professor R. W. Fessenden and Dr. A. Jeevarajan for providing us their unpublished ESR data on phenylthiyl radical in aqueous solution. Registry No. Phenylthio, 4985-62-0.
References and Notes (1) The research described herein was supported by the Office of Basic Energy Sciences of the Department of Energy. This is Contribution No. NDRL-3385 from the Notre Dame Radiation Laboratory. (2) Porter, G.; Wright, F. J. Trans. Faraday Soc. 1955, 51, 1469. (3) Franz, J. A.; Bushaw, B. A.; Alnajjar, N. C. J . Am. Chem. SOC.1989, 111,26a. (4) Scott, T. W.; Liu, S. N. J . Phys. Chem. 1989, 93, 1393. (5) Burkey, T. J.; Griller, D. J . Am. Chem. SOC.1985, 107, 246.
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J . Phys. Chem. 1992, 96, 5350-5355
(6) (a) Mamoru, J.; Imamura, T.;Kinichi, 0.;Tanaka, 1. Chem. Phys. Lett. 1984, 109, 31. (b) J. Am. Chem. SOC.1979, 101, 1815. (c) J. Am. Chem. SOC.1979, 101, 5732. (7) Osamu, I.; Matsuda, M. J . Am. Chem. SOC.1983, 105, 1937; 1979, 101, 1815, 5732. (8) Cohen, S. G.; Rose, A. W.; Stone, P. G.; Ehret, A. J . Am. Chem. SOC. 1979, 101, 1827. (9) Russel, P. G. J . Phys. Chem. 1975, 79, 1353. (10) Thyrion, F. C. J . Phys. Chem. 1973, 77, 1478. ( 1 1 ) Caspari, G.;Granzow, A. J . Phys. Chem. 1970, 74, 836. (12) Ernsting, N. P. Chem. Phys. Lett. 1990, 166, 221. (13) Shibuya, K.; Nemoto, M.; Yanagibori, A,; Fukushima, M.; Kinichi, 0. Chem. Phys. 1988, 121,237. The ula (CS stretch) mode assigned by these authors is designated as the substituent-sensitive mode in this paper. As the ring distortion and CS stretching motions are mixed, both representations can be considered equally valid. However, for comparing the vibrational frequencies in PhS' and PhO' and making structural inferences from the comparison, it is more appropriate to assign the lowest frequency a l mode in PhS' and PhO' to the v6a vibration (see text). (14) Anderson, R. F.; Patel, K. B.; Stratford, M. R. L. J . Biol. Chem. 1990, 265, 1952. (15) Bonifacic, M.; Weiss, J.; Chaudhary, S. A.; Asmus, K.-D. J . Phys. Chem. 1985, 89, 3910. (16) Zandstra, P. J.; Michaelsen, J. D. J . Chem. Phys. 1963, 39, 933. (17) Schmidt, V . Angew. Chem., Int. Ed. Engl. 1964, 3, 602. Schmidt, V.; Miller, A. Angew. Chem., Int. Ed. Engl. 1963. (18) Jeevarajan, A,; Fessenden, R. W. Private communication. (19) Neta, P.; Fessenden, R. W. J . Phys. Chem. 1974, 78, 523. Stone, T. J.; Waters, W. A. J . Chem. SOC.1964, 213. (20) Tripathi, G. N. R.; Schuler, R. H. J . Chem. Phys. 1984,81, 113. The 840-cm-' frequency has been more recently assigned to u I 2 (ref 23). (21) Tripathi, G. N. R. Chem. Phys. Lett. 1985, 118, 271. (22) Tripathi, G. N. R. In Multichannel Image Detector3 II; Talmi, Y., Ed.; ACS Symposium Series No. 236; American Chemical Society: Washington, DC, 1983; p 171. (23) Tripathi, G. N. R. In Advances in Spectroscopy: Time-Resolved Spectroscopy; Clark, R. J. H., Hester, R. E., Eds.; John Wiley: New York, 1989; p 157. (24) Schuler, R. H.; Patterson, L. K.; Janata, E. J . Phys. Chem. 1980,84, 208 8. (25) Adams, G. E.; McNaughton, G. S.; Michael, B. D. Trans. Faraday SOC.1968, 64, 902. (26) Akhlaq, M. S.; Schuchmann, H.-D.; von Sontag, C. Int. J . Radiat. Biol. Relat. Stud. Phys., Chem. Med. 1987, 51, 91. (27) Schuler, R. H. Int. J . Radiat. Phys. Chem. 1992, 39, 105. (28) Ye, M.; Madden, K. P.; Fessenden, R. W.; Schuler, R. H. J . Phys. Chem. 1986, 90, 5397.
(29) Adams, G. E.; McNaughton, G. S.; Michael, B. D. In The Chemistry of Ionization and Excitation; Taylor and Francis: London, 1967; p 281. (30) Karman, W.; Granzow, A.; Henglein, A. Int. J . Radiat. Phys. Chem. 1969, l 9395. (31) Packer, J. E. In The Chemistry of the Thiol Group; Patel, S., Ed.; Wiley: London, 1974; Part 2, Chapter 11. Von Sontag, G.; Schuchmann, H.-P. In Chemistry of Ethers, Crown Ethers, Hydroxyl Groups and Their Sulfur Analogues; Patel, S., Ed.; Wiley Chichester: England, 1980; Vol. 2. (32) Johnson, C. R.; Ludwig, M.; Asher, S. A. J . Am. Chem. SOC.1986, 108, 905. (33) Tripathi, G. N. R.; Schuler, R. H. J . Phys. Chem. 1988, 92, 5129. (34) Scott, D. W.; McCullough, J. P.; Hubbard, W. N.; Miserly, J. F.; Hossenlopp, I. A.; From, F. R.; Waddington, G. J . Am. Chem. Soc. 1956, 78, 5463. (35) Green, J. H. S. Specrrochim. Acra 1968, 24A, 1627; 1962; 18, 39. (36) Joo, T. H.; Kim, M. S.; Kim, K. J . Raman Spectrosc. 1986, 57. (37) Binkley, J. S.; Pople, J. A.; Hehre, W. J. J . Am. Chem. SOC.1980, 102, 939. Gordon, M. S.; Binkley, J. S.; Pople, J. A,; Pietro, W. J.; Hehre, W. J. J . Am. Chem. SOC.1982, 104, 2797. Pietro, W. J.; Francl, M. M.; Hehre, W. J.; Defrees, D. J.; Pople, J. A.; Binkley, J. S. J . Am. Chem. SOC. 1982, 104, 5039. (38) Frisch, M. J.; Head-Gordon, M.; Trucks, G. W.; Foreman, J. B.; Schlegel, H. B.; Raghavachari, K.;Robb, M. A.; Binkley, J. S.; Gonzalez, C.; Defrees, D. J.; Fox, D. J.; Whiteside, R. A.; Seeger, R.; Melius, C. F.; Baker, J.; Martin, R. L.; Kahn, L. R.; Stewart, J. J. P.; Topiol, S.; Pople, J. A. Gaussian 90; Gaussian, Inc.: Pittsburgh, PA, 1990. (39) Allen, F. H.; Kennard, 0.;Watson, D. G.; Brammer, L.; Orpen, A. G.; Taylor, R. J . Chem. SOC.,Perkin Trans. 2 1987, S1. (40) Pople, J. A.; Schlegel, H. B.; Krishnan, R.; DeFrees, D. J.; Binkley, J. S.; Frisch, M. J.; Whiteside, R. A.; Hout, R. F.; Hehre, W. J. Int. J . Quantum Chem. 1981, S I 5 269. (41) The totally symmetric CH stretch modes are calculated at 2988, 3007, and 3021 cm-I but are not expected to be resonance enhanced in the present experiments. In addition, b2 modes are calculated at 290, 630, 1055, 1105, 1198, 1326, 1431, 1544,2997, and 3015 cm-', bl modes are calculated at 177, 478, 713, 788, 991, and 1068 cm-l, and a2 modes are calculated at 403,884, and 1041 cm-I. The b2, bl, and a2 modes are non-totally symmetric and so in general would not be expected to be resonance enhanced. (42) Pulay, P.; Torok, F. Acta Chim. Acad. Sci. Hung. 1965, 47, 273. (43) MELDF was originally written by L. McMurchie, S. Elbert, S. Langhoff, and E. R. Davidson. It has been substantially modified by D. Feller, R. Cave, D. Rawlings, R. Frey, R. Daasch, L. Nitzche, P. Phillips, K. Iberle, C. Jackels, and E. R. Davidson. A public version is available in: Davidson, E. R. Quantum Chemistry Program Exchange 9, QCPE 1989, No. 580. (44) Matsen, F. A.; Robertson, W. W. J . Am. Chem. Soc. 1950,72,5248. (45) Price, C . C.; Shigeru, 0. Sulfur Bonding, The Ronald Press: New York, 1962.
Theoretical Study of Metal Ions Bound to He, Ne, and Ar Harry Partridge,* Charles W. Bauschlicher, Jr., and Stephen R. Langhoff NASA Ames Research Center, Moffett Field, California 94035 (Received: October 10, 1991)
The spectroscopic constants for the ground and selected low-lying electronic states of the transition metal-noble gas ions have been determined at the modified coupled-pairfunctional level of electron correlation treatment. There is a strong correlation between the binding energy and bond length, since the bonding is predominantly electrostatic (charge-induced dipole). In general, our calculated binding energies are about 20% less than the experimental values due to limitations in the level of theory used. The ions with an occupied s orbital (Mg', Sc+, Ti', Mn', and Fe+) have much smaller binding energies than those that have a 3d"+' occupation. Those ions with a n occupied metal valence s orbital are more weakly bound, because the small noble-gas polarizabilities do not lead to a sufficiently strong interaction to significantly polarize the metal s orbital. This leads to binding energies and bond lengths that follow the trends Ar > Ne > He and He > Ne > Ar, respectively. For the metal ions without an occupied valence s orbital, the binding energies involving Ar are largest, but whether MHe' or MNe+ is more strongly bound depends on the details of the interaction.
1. Introduction
There has been considerable interest in transition-metal ion chemistry in the past decade.' Recently, Bowers and ~ w o r k e r s * . ~ have determined the binding energies for a variety of transitionmetal positive ion ligand systems from both mobility and equilibrium clustering experiments. With these experimental techniques they have been able to obtain binding energies for both
ground and excited states. Binding energies have been reported for Cr+, Co+, and Ni+ bound to the noble-gas atoms He, Ne, and Ar. For CrAr' their Do value is in very good agreement with the accurate spectroscopic determination of Lessen et a1.4 Interestingly, the work of Bowers and co-workers3 showed that while the binding energies of Cr' with the noble gases increased from He to Ar following the polarizabilities ( a )of the noble-gas atoms,
0022-365419212096-5350$03.00/0 0 1992 American Chemical Society
