J. Phys. Chem. 1983, 87, 1648-1652
1648
TABLE 111: Effect of Static Pressure on the Color Temperature Sonoluminescence of Water color static color static press., atm temp, K press., atm temp, K _-.__I1.0 1.o 1.0
2.4 2.7 3.7 5.1
5300 5600 5300 5200 5600 4800 5200
5.1 6.5 7.8 9.2 11.9 11.9 14.6
observation that the increase in the total intensity of sonoluminescence was due to the change in the upper and lower bounds for cavitation nuclei, leading to an increase in the number of cavitation bubbles.
53 00 5600 5200 5500 6100 6200
NIA
estimate for the color temperature. The color temperatures of the sonoluminescence of aqueous solutions at various static pressures are given in ‘Fable 111. The total intensity at 14.6 atm was suppressed signifirnntly and hence, the spectral distribution and the color temperature at that pressure could not be determined. When the color temperatures were correlated with .itat ic pressure, no statistical correlation could be found a t the 99% confidence interval, indicating that the color temperatures were indepenent of the static pressure. The mean color temperature was determined to be 5500 f 350 K. The fact that the overall sonoluminescence intensity increased by over 600% without a change in the spectral distribution is indicative of a change in the number of luminescing bubbles. This conclusion supports the earlier
Conclusions The sonoluminescence intensity of nitrogen-saturated water increases with static pressure, reaching a maximum a t about 6 atm and subsequently decreases with further increase in the static pressure. For pressures of about 15 atm, the sonoluminescence is suppressed. No correlation could be found between the color temperature of sonoluminescence and the static pressure. The color temperature obtained over the 1-15-atm pressure range was 5500 f 350 K. The lack of correlation between the color temperature of sonoluminescence and static pressure indicated that the spectral distribution is not appreciably affected by a change in static pressure. The change in totalsonoluminescence intensity without a change in the spectral distribution is explained by a change in the number of cavitation bubbles. The increase in the number of cavitation bubbles at the higher static pressure predicted from the bubble collapse equation agrees with the observed increased in the sonoluminescence. Registry No. Water, 7732-18-5; nitrogen, 7727-37-9.
Methyl Formate Cation Radical: Electron Spin Resonance Evidence for a CT* Radical Formed by Strong Matrix-Solute Cation Interaction in Frozen CFCI, Solutions David Becker,‘ Kevln Plank, and Mlchael
D. Sevllla’
Deparfment of Chemistry, Oakland Universiv, Rochester, Michigan 48063 (Received: October 28, 1982)
Unequivocal evidence for strong matrix-solute cation interaction in ?-irradiated frozen (77 K) CFC13solutions of methyl formate is presented. It is suggested that a new species results from the formation of a u* bond between a chlorine in the CFC13and the methyl formate cation. Experiments with deuterated methyl formate confirm the formation of the complex cation. The experimental chlorine hyperfine splittings show evidence for a large spin density on one chlorine and near axial symmetry. CNDO calculations for the complex cation give a spin density distribution in agreement with that suggested from experiment, and suggest the unpaired electron is essentially localized to the C1-0 bond between the CFC13and HCOzCH3molecules. When the complex cation is irradiated with visible light or warmed to 140 K the isolated methyl formate cation is formed. This species i s predicted to be a n-type radical by INDO calculations of the spin density. A discussion is presented which suggests that the formation of such complex cations in CFC13depends on the proximity of the ionization potentials of solute and solvent and the nature of the spin density distribution in the solute cation.
Introduction In a continuing ESR study of the radiation chemistry of lipids and related model compounds,2 we have investigated the effects of y-irradiation on frozen solutions of various esters in trichlorofluoromethane (Freon-11). It is well-established by a number of workers that many solutes in Freon-11 and related solvents form stable cationic radicals through a resonant charge transfer process when irradiated at 77 K.3-10 Many of these radicals undergo a (1)Department of Physical Sciences, Oakland Community College, Tarmington Hills, MI 48018. ( 2 ) (a) Sevilla, M. D.; Sevilla, C. L.; Swarts, S. Radiat. Phys. Chem. 1982,20, 141. (b) Sevilla, M. D.; Swarts, S.; Bearden, R.; Morehouse, K. M.; Vartanian, T. J . Phys. Chem. 1981, 85, 918. (c) Sevilla, M. D.; Morehouse. K. M.; Swarts, S., Ibid. 1981, 85, 923.
characteristic reaction when the sample is warmed, in which a neutral radical is formed through a deprotonation r e a ~ t i o n . For ~ , ~ the systems studied up to now, there has (3) (a) Wang, J. T.; Williams, F. J . Phys. Chem. 1980, 84, 3156. (b) Snow, L. D.; Wang, J. T.; Williams, F. J. Am. Chem. SOC. 1982,104,2062. ( c ) Wang, J. T.; Williams, F. Ibzd. 1981, 103, 6994. (d) J. Chem. Soc., Chem. Commun. 1981, 667. (e) Ibid. 1981, 1184. (0 Hasegawa, H.; Shiotani, M.; Williams, F. Faraday Discuss. 1977, No. 63, 157. (4) Shida, T.; Egawa, Y.; Kubodera, H. J. Chem. Phys. 1980, 73,5963. (5) Kubodera, H.; Shida, T.; Shimokoshi, K J Phys Chem. 1981,85, 2583. (6) Grimison, A,; Simpson, G. A. J . Phys. Chem. 1967, 72, 1776. (7) Shida, T.; Iwata, S. J. Am. Chem. SOC. 1973, 95, 3473. (8) Shida, T.; Shida, T. J. Am. Chem. SOC.1979, 101, 6869. (9) Shida, T.; Kubodera, H.; Egawa, Y. Chem. Phys. Lett. 1981, 79, 179. (10) Symons, M. C. R.; Boon, P. J. Chem. Phis. Lett. 1982,89, 516
0022-3654/83/2087-1648$01.50/00 1983 American Chemical Society
The Journal of Physical Chemistry, Vol. 87,
No. 9, 1983 1049
TABLE I : Parallel Hyperfine Parameters and g Values for t h e Methyl Formate-CFC1, Cationugb
methyl formate-CFC1, cation deuterated methyl formate-CFC1, cation
B ~
I'
100 0
i
Figure 1. (A) Flrstderivatlve ESR spectrum found at 77 K after the y-irradation of methyl formate (2 mol % ) in CFCI,. (6) ESR spectrum at 77 K of y-irradiated deuterated methyl formate (DC02CH,) 2 mol % in CFCI,. The spectra In A and B are attributed to complex cations formed between the methyl formate and CFCI, (see text). The stick diagram between the s ectra gives a reconstruction of the parallel hyperfine structure for 'CI, %I, and 'H (formate proton) couplings. The loss of the proton coupling in B in unambiguous evidence for complex cation formation. The three reference marks in the center of each spectrum are separated by 13.09 G. The central mark is at g = 2.0056.
A 1136c1, A 1137c1, A IIH, G G G 84.4 70.3 17 84.0
70.6
-C
g /I
2.0032 2.0032
T h e nuclear quadrupole coupling for C1 is ignored in our analysis. This effect is expected t o b e small in comparison t o the spectral line width in the parallel orientation. Since the center of t h e experimental spectrum is strongly overlapped by a matrix background signal as well as a quartz signal, it is not possible t o accurately analyze for the perpendicular components. However, first-order computer simulations of the spectrum limit the value of A135c1t o be n o more than 20 G. The deuterium splitting which is expected t o be 2.6 G is n o t resolved in the spectrum.
with a E-4531 dual cavity was employed. Hyperfine splittings and g values were measured vs. Fremy's salt with AN = 13.09 G and g = 2.0056.2
Results and Discussion Methyl Formate-CFC1, Cation. The ESR spectrum of irradiated methyl formate in CFC13, taken at 77 K, is shown in Figure 1A. The spectrum of deuterated methyl formate (DC02CH3),in which the formate proton has been replaced by deuterium, is shown in Figure 1B. Both spectra were taken under the same cmditions. The ESR been no report, to the best of our knowledge, of any strong spectrum of the deuterated species shows hyperfine interactions between the cationic radicals and the solvent, structure which is characteristic of a large anisotropic although a solvent effect on the electronic spectra of some chlorine coupling. An analysis of the spectrum for hyradicals has been reported.'s We would like to report that, perfine and g-factor parameters (Table I) indicates that for methyl formate in CFCl,, the ESR spectra give unthese spectral parameters show approximate axial symequivocal evidence of a strong radical-matrix interaction metry with A,13 >> A136C1. The fact that the spectra of for the original cationic species formed upon irradiation. both 35Cland 37Clare present further confirms that the We suggest that a u* bond is formed between a chlorine spectra shown in Figure 1are largely due to an interation in the CFC1, and the methyl formate cation, forming a between the unpaired electron and chlorine. A calculation metastable bimolecular cation complex. This complex is of the ratio of the measured hyperfine splittings of the two an intermediate in the production of the methyl formate isotopes, AW/AnC1= 1.20 f 0.01, agrees well with the ratio cation in this solvent and is the first observation of a cation calculated from the known magnetic moments of the two complex with CFC13. A number of u* radicals have been nuclei, p36C1/p37C1= 1.20. characterized in the past in various chemical ~ y s t e m s . ~ ~ J ~ - 'isotopic ~ The fact that we are observing a solute radical which is In these radicals, the unpaired electron occupies an anstrongly interacting with a chlorine and not simply a tibonding u* molecular orbital, resulting in a three-electron chlorine-containing radical resulting from the CFC1, matrix bond with bond order 1/2. far removed from a solute molecule is indicated by comExperimental Section paring spectrum 1A with 1B. When the deuterium is Samples were prepared from commercially available replaced by a proton, a coupling to the formate proton (AllH methyl formate (Eastman Spectrograde), Freon-11 (PCR = 17 G ) appears in the spectrum. Thus, in spectrum lA, Research Chemicals, Inc.), and deuterated methyl formate, we observe a large coupling to a chlorine atom and to the DC02CH3(Sigma Chemical Corp.). Degassing the sample formate hydrogen. This could occur only if a solute or removing dissolved oxygen by bubbling N2through the molecule is strongly interacting with the solvent. The stick sample had no effect on the resulting spectra. Samples diagram in Figure 1shows the parallel couplings for both were irradiated in Spectrosil quartz tubes at 77 K for doses chlorine isotopes and the hydrogen coupling from methyl ranging from 0.016 to 0.25 Mrd. The only effect increasing formate. dose had on the resulting spectra was the expected one of Previous work in freon solvents would suggest that the a linear increase in radical concentration with dose. initial species formed upon irradiation at 77 K should be Varying concentrations of methyl formate from 1/25 mol Ia. However, with methyl formate the initial observed ratio to 1/10oO mol ratio also had no effect on the resulting spectra except that higher concentrations of methyl formate gave higher concentrations of radical. An initial ESR spectrum at 77 K was typically taken within 1-4 h of irradiation. A Varian Century Series EPR spectrometer H' Ia (11)Symons, M.C. R.Pure Appl. Chem. 1981,53, 223. (12)Sevilla, M.;Swarts, S.J.Phys. Chem. 1982, 86, 1751. (13)Lindsay, D.M.;Symons, M. C. R.; Herschbach, D. R.; Kwiram, A. L. J.Phys. Chem. 1982,86, 3789.
species formed upon irradiation at 77 K is not Ia, but a species in which Ia has associated with a chlorine from the matrix.
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Becker et al.
The Journal of Physical Chemistry, Vol. 87, No. 9, 7983
There are two possible structures (11,111) which would
”.
H\
H m.C -0.
Hv
HYlC-0
H
TABLE 11: CNDO Spin Densities for t h e Methyl Formate-CFCI, Cation Complexa,b
\
@ c=o
/
‘c=o H
H
Y
‘;Cl
111
I1
.&’ F
account for the observed ESR spectra. The first (11) results from the association of Ia with a chloride ion. Chloride is produced upon irradiation by the scavenging of radiation-produced electrons by solvent molecules by reaction The second (111) is the result of the association l.4v587
CFC13
+ e-
-
CFC1,-
-
CFC12. + C1-
0.002 0.002
S
Cl-C,rCr
PX PY PZ d,2->,2 d2
0.005 0.000
0.038 0.124 0.009 0.000
0.000
0.026 0.560 0.008
0.013 0.016 0.002 0.000 0.000
0.000 0.075 0.071
0.000 0.037 0.009 0.007 0.001
0.000 0.000 0.000 0.000
0,000
Standard bond lengths and angles were employed for the CFCl, and methyl formate portions of the complex. The C-0-Cl and 0-C1-C bond angles employed were 1 2 0 and l o g ” , respectively. The numbering scheme and axis system used in t h e calculation are as follows: a
(1)
between cation Ia and a chlorine in a solvent molecule. In both cases, we assume that a three-electron bond (discussed below) would form between the carbonyl oxygen in the methyl formate cation and a chlorine. However, on the basis of a dose vs. concentration study, we have eliminated I1 as a candidate for the species being formed. In this study, the concentration of radicals formed was linear with dose, even for very low doses or irradiation in very dilute solutions of methyl formate. This linear relationship should not occur if I1 is the radical species being formed, for the following reason. The production of C1- should occur randomly in the matrix, and it is unlikely that C1- would diffuse through the solid matrix at 77 K. As cation I is formed, also randomly, the probability that I will always form near a C1- is therefore exceedingly low. Particularly at low doses, where very few chlorides and very few cations I are formed, we expect to find only rare instances where association between the cation and chloride occurs. Hence, if I1 were being formed, we would not see a linear but a quadratic dose vs. concentration relationship, with a sharp drop off of concentration at low doses. We are alert to the possibility that the presence of a nearby chloride ion may stabilize the formation of 11. However there are two facts that we believe eliminate even this possibility. First we detected the relevant radical at very low concentrations (1/1000) of methyl formate. Second, the matrix anion CFC1,- is likely stable at 77 K,3f resulting in little C1- formation until warmer temperatures are reached. One argument against structure I11 is that we might expect to find hyperfine couplings to the three halogen atoms in CFC1, which are not associated with the methyl formate cation. In our spectra, we do not observe any such couplings. We do not consider this too troublesome since we are working with a polycrystalline material in which the ESR spectra inherently contain broad lines that limit resolution. It is possible that these couplings are present but are too small to be seen. The results of CNDO calculations on 111, discussed below, support this view since the spin densities found on these halogens are quite small (Table 11). CNDO calculations were done on I11 in order to see if the large anisotropic chlorine interaction we observe is predicted as well as to check for agreement with other spectral features.’* In these calculations, standard bond lengths and bond angles were used for the methyl formate and Freon molecules. The 0421 bond length, through trial calculations, was energy minimized at 1.58 A. The C-O-Cl bond angle was assumed to be 120° and the C-C1-0 bond angle logo. The overall results of these calculations were (14) Pople, J. A.; Beveridge, D. L. ‘Approximate Molecular Orbital Theory”; McGraw-Hill: New York, 1970.
TABLE 111: CNDO Total Valence Electron Densities in t h e Methyl Formate-CFC1, Cation Complex
atom
H,
H2 H3 c 4
0.
c
bS
0,
H* total charge
electron density in methyl formate portion 0.928 0.962 0.961 3.878 6.116 3.502
6.096
0.923 23.366 0.63+
atom
c1,
C110 C1,l F,, C,, ’., total charge
electron density in CFC1, portion 6.889 6.966 6.950 7.118 3.711 31.734 0.37--
not very sensitive to moderate changes in these bond angles, nor were they very sensitive to rotations around the C1-0 bond. In Table 11, the calculated spin densities for six of the atoms in structure 111 are given. The axis system used oriented the x axis along the C1-0 bond, so that the bonding which occurs between the C1 and 0 is due primarily to overlap of px orbitals from each atom. There are a number of interesting features in these spin density distributions. First, most of the spin density of I11 is in the bonded chlorine px orbital with quite small densities in the pr and p2 orbitals. This spin density distribution predicts axial symmetry for the hyperfine tensor, in agreement with the observed spectra. Qualitatively, the calculated distribution also predicts that the unpaired electron is predominantly located on chlorine, again in agreement with our observed results. From the measured A1*’ of 84 G and the estimated maximum value of ALSC1 of *20 G (Table I), we find the anisotropic coupling (20) to be in the range 40-70 G. Since 20 is 100 G for a full unpaired electron in a p orbital,’, the range found for 20 suggests a spin density of 0.4 to 0.7 in the chlorine px orbital. This is in good agreement with the results from the CNDO calculation (0.56). Second, the sum of the spin densities in the bonded oxygen px orbital and chlorine px orbital is 0.68, indicating that much of the unpaired electron is in these orbitals in the C1-0 bond. In fact the CNDO calculation predicts that 0.911 of the overall spin density is localized on the bonded chlorine and oxygen atoms. The essential u nature of this species is further indicated by noting that the spin densities in the various
Methyl Formate Cation Radical
Flgure 2. (A) ESR spectrum of the methyl formate (HC02CH,) cation in CFCi,, formed by warming the complex whose spectrum is shown in Fgwe 1A to 140 K. The spectrum shows coupling to ail four protons in the radical cation. (B) ESR spectrum of the deuterated methyl formate (DCO,CH,) cation in CFCi, formed by warming the complex whose spectrum is shown in Figure 1B to 140 K. Comparison of the spectra in A and B shows the loss of a 5 . 6 4 spling from the formate proton. The remaining couplings in B (23, 23, and 4.0 G) arise from the methyl group which is not rotating on an ESR time scale. Note that the reference marks which are 1 3 . 0 9 4 separated are not the same spacing in A and B.
pn orbitals are extremely small, and zero in the bonded oxygen and chlorine pn orbitals. Table 111, which gives total valence electron densities, is also rather informative. Specifically, it indicates that the methyl formate molecule has lost a total of 0.63 valence electron density and the Freon molecule a total of 0.37 electron density when I11 forms. This results in a 0.63+ charge on the methyl formate moiety and a 0.37+ charge on the freon. As a final check on our model, a calculation was performed in which the distance between the chlorine and the oxygen was set at 2.58 A, rather than the energy minimized 1.58 A. This calculation gave the following results: (1)The total energy of the system was considerably higher than when the distance was 1.58 A, suggesting bond formation occurs at 1.58 A. (2) A t 2.58 A the positive charge is located solely on methyl formate as is the spin density. At this distance, this calculation predicts an isolated methyl formate cation radical. Methyl Formate Cation. We find the cation radical Ib is formed when I11 is warmed to approximately 110 K, or when I11 is briefly photobleached with an incandescent lamp at 77 K. The ESR spectrum of Ib, taken at 140 K, is shown in Figure 2A for the normal methyl formate and in Figure 2B for the deuterated methyl formate (DC02CH3). It is of interest to note that the spectra are both virtually isotropic in nature, suggesting only small anisotropic couplings. Comparison of the spectrum arising from the undeuterated sample with the deuterated one shows that a 5.6-G splitting arises from the formate proton. For both spectra, two of the three methyl protons contribute a 23.34 splitting, and the third a 4.0-G splitting. This accounts for all of the protons in the methyl formate cation Ib. The methyl group shows no evidence of rotating as the temperature is raised even to 160 K. This is evidence for a significant barrier (>5 kcal) to internal rotation. The cation radical Ib is lost at 160 K, which is near the melting
The Journal of Physical Chemistry, Vol. 87, No. 9, 1983 1651
point of the Freon matrix.15 A number of INDO calculations using standard bond lengths and angles were performed for various possible molecular conformations for the cation Ib. A close fit to experimental splittings was found only for structure Ib. This INDO calculation for Ib resulted in a spin density of 0.57 in the carbonyl oxygen pn orbital (O,), 0.78 in the O5 pz orbital, and -0.42 in the C6 pn orbital, with much smaller spin densities elsewhere. Thus the radical is predicted to be a ?r type radical and the couplings to the methyl protons arise from the typical @-protoninteraction with ?r spin density on O5(A = @a" cos2e).& Since @-proton couplings have little anisotropy, this calculation explains in part the near isotropic nature of the experimental spectrum. The experimental equivalence of the two methyl protons and the small value of the third are in agreement with a @-protoncoupling model with the dihedral angle 0 equal to 30' for H1and H, and 90° for H3. The INDO calculated isotropic couplings are 23 G (HJ, 20 G (H2),and 0.4 G (H3)for the three @-protonsand 8.2 G for the formate proton (H8). These values are in good agreement with the respective experimental values of 23, 23, 4.0, and 5.6 G. In related work Wang and Williams report that the dimethyl ether cation in Freon is a oxygen-centered 7~ radical showing near isotropic couplings to the methyl protons; however, they report a larger g value anisotropy for the dimethyl ether cation than is apparent in our spectra for the methyl formate cation.3c The INDO calculation for Ib shows an unusual feature upon rotation of the methyl group. As the methyl group is rotated, the 7~ character of the radical is rapidly lost and, after 60' rotation of the methyl group, the radical is predicted to be u in nature, lower in total energy, with only small hyperfine couplings (
