Paul D. Sullivan and Nick A. Brette
47 4
tional frequencies, and only by observing these vibrational frequencies directly can the product rule be satisfied. This explanation, of course, is very tentative.
Acknowledgement. The authors gratefully acknowledge the financial support of this work by the National Science Foundation by Grant No. GP-42907X. Miniprint Material Available. Full-sized photocopies of the miniprinted material (Tables I-V) from this paper only or microfiche (105 X 148 mm, 24X reduction, negatives) containing all of the miniprinted and supplementary material for the papers in this issue may be obtained from the Journals Department, American Chemical Society, 1155 16th St., N.W., Washington, D.C. 20036. Remit check or money order for $3.00 for photocopy or $2.00 for microfiche, referring to code number JPC-75-468. References a n d Notes (1) Part VII: J. D. Odom, V. F. Kalasinsky, and J. R. Durig, J. Mol. Struct., 24. - 139 . (1975). (2) Taken in'part irom the thesis of V. F. Kalasinsky, to be submitted to the Department of Chemistry in partial fulfillment of the requirements for a Ph.D. Degree. (3) J. R. Durlg, S. Riethmiller, V. F. Kalasinsky, and J. D. Odom, Inorg. Chem., 13, 2729 (1974). (4) J. D. Odom, S.Riethmiller, J. D. Witt, and J. R. Durig, lnorg. Chem., 12, 1123 (1973). (5) J. D. Odom, 9. A. Hudgens, and J. R. Durig, J. Phys. Chem., 77, 1972 (1973).
(6) J. D. Odom, S. Rlethmlller, S. J. Meischen, and J. R. Durlg, J. Mol. Struct., 20, 471 (1974). (7) J. R. Durig, Y. S. LI, L. A. Carreira, and J. D. Odom, J. Amer. Chem. SOC.,95, 2491 (1973). (8) P. S.Bryan and R. L. Kuczkowski, lnorg. Chem,, 11, 553 (1972). (9) R. L. Kuczkowski and D. R. Lide, Jr., J. Chem. Phys., 46, 357 (1967). (IO) J. P. Pasinski and R. L. Kuczkowskl, J. Chem. Phys., 54, 1903 (1971). (11) L. J. Maloneand R. W. Parry, inorg. Chem., 6, 176 (1967). (12) E. C. Evers, E. H. Street, Jr., and S.L. Jung, J. Amer. Chem. SOC.,73, 5088 (1951). (13) J. A. Lannon and E. R. Nixon, Spectrochim. Acta, Part A, 23, 2713 (1967). (14) A. D. Norman and W. L. Jolly, lnorg. Syn., 11, 15 (1968). (15) D. F. Shriver, "The Manipulation of Air-Sensitive Compounds," McGraw-Hill, New York, N.Y., 1969. (16) W. J. Lehman, C. 0. Wilson, J. F. Ditter, and I. Shapiro, Advan. Chem. Ser., No. 32, 139 (1961). (17) I. Shapiro, C. 0. Wilson, J. F. Ditter, and W. J. Lehman, Advan. Chem. Ser., No. 32, 127 (1961). (18) A. B. Burg and R. i. Wagner, J. Amer. Chem. SOC.,75, 3872 (1953). (19) F. A. Miller and B. M. Harney, Appl. Spectrosc., 24, 291 (1970). (20) R. N. Jones and A. Nadean, Spectrochim. Acta, 20, 1175 (1964). (21) R. T. Hall and J. M. Dowling, J. Chem. Phys., 47, 2459 (1967); 52, 1161 (1970). (22) F. G. Baglin, S. F. Bush, and J. R. Dtirig, J. Chem. Phys., 47, 2104 (1967). (23) J. D. Odom, S. Riethmiller, and J. R. Durig, J. lnorg. Nucl. Chem., 36, 1713 (1974). (24) J. H. Schachtschneider, Technical Report No. 231-64 and 57-65, Shell Development Co. (25) See paragraph at end of paper regarding miniprinted material. (26) V. W. Laurie and K. K. Lau, personal communication. (27) E. C. Tuazon and W. G. Fateiey, J. Chem. Phys., 54, 4450 (1971). (28) J. R. Durig, C. M. Player, Jr., J. Bragin, and Y. S.Li, J. Chem. Phys., 55, 2895 (1971). (29) J. R. Durig and Y. S.Li, J. Mol. Struct., 13, 459 (1972). (30) R. W. Rudolph and R. W. Parry, J. Amer. Chem. Sac., 89, 1621 (1967). (31) W. H. Fink and L. C. Allen, J. Chem. Phys., 46, 2261 (1967). (32) T. Kogima, E. L Breig, and C. C. Lin, J. Chem. Phys., 35, 2139 (1961).
Temperature-Dependent Splitting Constants in the Electron Spin Resonance Spectra of Cation Radicals. V.' CH Protons in Some Tetrasubstituted Benzenes Paul D. Sullivan* and Nick A. Brette Department of Chemlstry, Ohio University, Athens, Ohio 4570 1 (Received October 2, 1974) Publication costs assisted by the Ohio University Research lnsfitute
Measurements of the temperature coefficients of the CH protons in a series of 1,2,4,5-tetrasubstitutedbenzene cation radicals are reported. The compounds studied fall into two groups depending on the sign of the CH temperature coefficient. A representative compound from each group, 1,2,4,5-tetramethoxybenzene and 1,4-dimethoxy-2,5-dimethylbenzene, was studied in detail in order to evaluate the aCHH/aCUD ratios. Qualitative observations on line width asymmetries are also reported. The results are interpreted in terms of theories previously proposed for temperature coefficients and line width asymmetries to show that when the sign of a CH proton splitting is positive one observes a positive temperature coefficient, ~ c H ~ / ~> c H 6.514 and low-field broadened lines, or when the sign of the splitting is negative one observes negative temperature coefficients, aCHH/aCDD < 6.514 and high-field broadened lines.
Introduction Our investigations into the temperature dependence of the splitting constants of various substituted aromatic cation radicals have led to the evaluation of the temperature coefficients of a number of CH protons (Table I). These results are of particular interest for several 1,2,4,5-tetrasubstituted benzene cation radicals since the spin density on the The Journal of Physical Chemistry, Vol. 79, No. 5, 7975
unsubstituted carbon atom can be positive or negative depending upon the 1,2,4,5 substituents. Previous theoretical treatments have indicated that the sign of the temperature coefficient,2 the aH/aD splitting constant ratio,2 and the line width asymmetry parameters3 should be related to the sign of the spin density a t the proximate carbon atom and hence to the sign of the splitting constant. It is the purpose of this paper to show how a simple qualitative treatment of
~
Temperature Dependence of Esr Spectra of Cation Radicals
475
TABLE I: Experimental Temperature Coefficients and Experimental and Calculated Relative Temperature Coefficients for the CH Protons of 1,2,4,5-TetrasubstitutedBenzene Cation Radicals ( l/aH)(daH/dT) l o i
Substituents at positions 1
2
4
5
Compd no.
daH/dT, mG/deg
Exptl
Calcd'
Ref
*
0.27 + 0.05 30 f 5 27 4 13 i 4 37 * 5 0.12 i 0.03 0.27 0.03 4 32 3 23 + 15 4 0.21 -i: 0.14 32 5 47 + 6 5 0.37 i 0.05 67 i 15 0.52 0.11 6 SCZH5 SC2H5 -129 i 20 -56 i- 6 7 -0.75 0.10 CH3 CH, -0.63 0.08 -66 i 9 -30 rt 4 a CH3 CH3 -83 + 8 9 -28 i 4 -0.83 + 0.08 t-C,Hg t-CdH, 10 -0.62 f 0.10 -16 + 3 -43 i 10 t-CdH, t-CdH, -103 * 11 11 -0.62 + 0.06 CH3 CH, -72 11 12 -0.76 + 0.11 t-CdH, t-CdHg a Calculated from Reddoch's equation (l/aH)(daH/dT)105 = (-3.8 f 1 . 2 ) ~- ~(3.23 f O.Bl)(Zu~/ap). This work.
OCH, OH OCA OCH, OCZH5 OCZH5 OCH, OH OCH, OH OCZH5 OCZH5
OCH, OH OCZH5 OH SCH,
OCH, OH OCZH5 OCH, OCZH5 OCZH, OCH, OH OCH, OH OCZH5 OCZH5
OC H3 OH OCZH5 OH SCH,
1
2
*
*
* *
*
these effects leads to consistent assignments of the signs of the splitting constants of the CH protons. Experimental Section The compounds studied were either commercially available or were prepared according to previously described meth0ds.l The cation radicals were prepared by treating the neutral compounds with aluminum chloride, sulfuric acid, or sulfuric acid-d2 in nitromethane or nitroethane. The esr spectra were measured on a Varian E-15 spectrometer in a dual cavity using a sample of the perylene radical anion as a secondary standard. The least-squares analysis4 of the experimental spectra were carried out as previously de~cribed.~ Results The splitting constants and temperature coefficients for compounds 3, 5, 6, 11, and 12 have been previously publishedl and are quoted directly from that work. The temperature coefficients and splitting constants of 1,2,4,5-tetrahydroxybenzene+, 2, were recently reported by Bullock and Howard6 and are taken from their work. The splitting constants of compounds 1 and 7-10 were previously obtained7,s but the temperature coefficients have not been reported; complete data for these compounds are given in Table 11. Neither the splitting constants nor the temperature coefficients of compound 4 have been previously obtained and these are also given in Table 11. Some specific comments are appropriate to compounds 1 and 7. 1,2,4,5-Tetramethoxybenzene(TMB, 1). The esr spectrum of this compound consists of 13 groups of well-spaced triplets from the 12 methoxyl protons and 2 ring protons. Because of the well-spaced lines it was possible to observe simultaneously the spectra of TMB+, TMB+-d and TMB+-d2 in a mixture of H2S04-D2S04 in nitromethane (see Figure 1). The only overlap which occurs is between the MCH= f%,MCD= ~1 lines of TMB+-d and the f i c ~ = f 2 lines of TMB+-d2. These lines were therefore neglected in our analysis but the rest of the lines from the M O C H=~fl, 0 groups were used to evaluate the splitting constants of TMB+, TMB+-d, and TMB+-d2 at each particular temperature. 1,4-Dimethoxy-2,5-dimethylbenzene (DMDMB, 2). The technique used above for TMB to obtain the splittings
*
I
1 -
H
2G
b 6 1 b 1 1 b b b b 1 1
I
1
Figure 1. Esr spectrum of TMB in a 5050 mixture of H2S04 and D2S04 in nitromethane at -40'. The spectrum is a summation of TMB+, TMB+-d, and TMB+-d2. Only the & c ~= ~f l , 0 groups are
shown, note the low-field broadening of the methoxyl splittings.
from the deuterated compounds was not suitable for DMDMB because of the large amount of overlap between the spectra involved. The spectra of DMDMB+ and DMDMB+-d2 were therefore measured separately in H2SO4 and D2SOr-nitromethane mixtures, the data obtained are shown in Table 11. Line Width Asymmetries. Qualitative observations on the line width asymmetries were made on solutions at the lowest possible temperatures attainable before freezing. The observations were made on lines which were nonoverlapped and which were from only one type of equivalent nuclei. Taking 2,5-dimethylhydroquinoneas an example, the & ~ c H=~ f l , f2, OH = 0, & f c=~0 lines were compared to determine the asymmetry associated with the = f l ,f i = ~ 0, f ~ i = 0~to methyl protons, the lines f i o ~ determine th_easymmetjy for the OH protons, and the lines &CH = f l , MOH = 0, M C H = ~ 0 for the asymmetry associated with the CH protons. The observed asymmetries are given in Table 111. Discussion The experimental temperature coefficients (Table I) and line width asymmetries (Table 111) of the CH protons fall very obviously into two groups. The first group comprises compounds 1-6 and is composed of all compounds containThe Journal of Physical Chemistry, Vol. 79, No. 5, 1975
~
~
476
Paul D. Sullivan and Nick A. Brette 3 3 3
01 N
“3
3
0 0 0 0
0 0 0 0
0 0 0 0
0 0 0 0
0 0 0 0
0 0 0 0
0 0 0 0
0 0 0 0
d000dd0d
-E8
8
TABLE 111: Qualitative Observations on Line Width Asymmetriesa Compd
no.
OH
OCH2CH3b
OCH,
1 2 3
HFB
4
HFB
LFB
A
N W O O W C U
3 3 * N 0 3
o o o o o o 3 m 0. 0. 0 0. 0. 0 0 0 0 0 0 0 0 0 0 9
0 0 iiiiiiii++
B El
e ^
+
c ~ r l m o m w
c - w w ~ m t - o mm t - m m m m l n 3 ~I
09109“950“.
LFB
0 0 0
0 0
909
00
LFB
NA NA HFB HFB HFB
HFB LFB
NA
LFB LFB
NA LFB
LFB LFB
NA
HF B HFB
a HFB = high-field broad, LFB = low-field broad. Asymmetry of P-CHz protons only. Not available.
eE &$
.1
’ i
m a
C,H,
LFB LFB
LFB LFB LFB
0 0 0 0 0 0 ~ d
m a w
CH,
LFB LFB
7
CH
NAC
5 6 8 9 10 11 12
tSCH,
g..m;
’/4 O
O
b4
I ”
1/,2$1/12 1/12
1/12
113
t
Figure 2.
3 4
1. 3 a a .
g 8
u
The Journal of Physical Chemistry, Vol. 79, No. 5, 1975
ing four alkoxy or alkylthio substituents, the temperature coefficients of the CH protons of this group are all positive and the line widths (where observable) are all low-field broad. The second group (7-12) comprises all compounds with two alkyl and two alkoxy or alkylthio substituents and the temperature coefficients of the CH protons are all negative and the line widths are all high-field broad. A qualitative explanation for this behavior is obtained by considering the results of McLachlan modified HMO calculations or by simply considering the compounds in terms of benzene-like molecular orbitals. Thus, four approximately equal electron-donating substituents (alkoxy or alkylthio) in the 1,2,4,5 positions will result in the highest occupied molecular orbital being of the antisymmetric type (Figure 2). This orbital has a node a t the unsubstituted ring positions, however, one expects electron correlation effects to produce a negative spin density at this position and the ring proton splitting constant arising via spin polarization should be of positive sign. For unequally electron-donating substituents one expects a different situation. If one pair (in the 1,4 positions) of substituents have a much greater effect one would expect the highest occupied molecular orbital to be of the symmetric type (Figure 2). However, as the other pair of substituents (in the 2,5 position) become more electron donating one should see a gradual transition from the symmetric type orbital into the antisymmetric type orbital. The spin density at the unsubstituted position should thus vary, depending upon the strength of the substituent, between 1/12 and some small negative value. The splitting constant of the ring proton may therefore be positive or negative and may be a sensitive indicator of the relative electron-donating abilities of the two sub~tituents.~
Temperature Dependence of Esr Spectra of Cation Radicals
477
TABLE IV: Possible Conditions which Might Exist for a CH Proton and Predicted Temperature Coefficients for Those Conditionsa Conditionno. aCHH 1 2 3
-
4
-
5
6
PP
pA
+
+ + +
+ +
-
+ +
+
-
-
Additional constraints
Temp coeff -
PA
PA PA PA
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