NOTES
208 Table I : Molecular Parameters of the Alkali Metal Hydroxides0 Vibration frequenoiea
P
Molecule
Y1
Y I
LiOH NaOH
(800) 43 1 (400) 354 336
(410) 337 (340) 309 310
KOH RbOH CsOH
va
-
(3800) (3650) (3610) (3610) (3610)
Partition functions at 2476'K 10%.4-
0
(1.52) (1.94) (2.27) 2.31 2.40
10881
fv ib
10 -afrot
0.22 0.66 1.09 1.34 1.55
68 161 171 226 238
1.36 4.1 6.7 8.2 9.5
a Vibration frequencies are in om-', internuclear distances rA-o in ern and molecular moments of inertia I in g em2. Vibration frequencies and moments of inertia in parentheses are estimates;a those not in parentheses stem from recent experiments. 4 - g
Table I1 : Calculated Zero-Point Standard Enthalpy Changes" K1,247p5
Metal
Li Na
K Rb cs a
ml molecule-1
7.7 x 8.3 x 2.2 x 4.0 X 2.2 x
10-18 10-18 10-17 10-1' 10-16
AXoa (bent),
AH"a (linear),
kJ mol-]
kJ mol-1
-423 - 322 - 338
-437 - 325
-348 - 382
- 339 - 347 -380
K1 is calculated from equilibrium constants for the reaction
+ H20 Ft AOH + H given in ref 3, the equilibrium constant for the reaction HzO Ft OH + H being taken to be 3.0 X lo1'
A
molecule ml-l a t 2475°K (JANAF' data).
mol-' (dispersion due approximately equally to uncertainties in K1 and in fvibfrot) appears to be the best compromise until reliable experimental values for the molecular parameters of LiOH become available.
Acknowledgment. The support of the Office of Naval Research under Contract Nonr-3809 (00) is gratefully acknowledged.
n-T
* Transition in the Dimethylthioacetamide-
Iodine and Thioacetamide-Iodine Complexes
by Arthur F. Grand and Milton Tamres Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48104 (Received September 3, 1969)
Donor-acceptor interactions can be investigated by measuring either a property of the complex,or a perturbed property of one of the components. In the specific case of the iodine interaction with thioacetamide, CHG(S)NHZ, thermodynamic data for the complex in dichloromethane' have been obtained from a study of the charge-transfer (CT) band at 286 mp. In addition, on the low wavelength side of the CT band, the n + n* band for thioacetamide was found shifted from 269 to 248 mp. It was located in the spectrum of the complex because of its high intensity which was comparable to that of the unperturbed transition. The iodine complex of dimethylthioacetamide, CH&The Journal of Physical Chemistry
(S)N(CH&, in carbon tetrachloride has been investigated utilizing the blue-shifted iodine band in the visible region.2 The same complex has also been characterized by studying the charge-transfer band in the ultraviolet region. Unlike other thione-iodine complexes, however, a pronounced shoulder was observed on the high wavelength side of the CT band. This shoulder is attributed to a blue-shifted n -+ n* transition in the donor, with an intensity greatly enhanced over that in the free donor. A similar band is also found to exist in the thioacetamide-iodine system that was not recognized previously. Experimental Section The purification of carbon tetrachloride, iodine, and dimethylthioacetamide have been described previously. Thioacetamide was obtained from Eastman Organic Chemicals and was recrystallized from anhydrous ethyl ether. Its ultraviolet spectrum agreed with that in the literature.1 Reagent grade dichloromethane was dried and distilled prior to use. The procedure to obtain spectrophotometric data and the mathematical treatment to determine spectral and thermodynamic properties also have been described. Results and Discussion The composite band of the dimethylthioacetamideiodine complex in carbon tetrachloride is shown in Figure 1 (curve A). It was obtained by subtracting the absorbances of the equilibrium concentrations of free donor and of free acceptor from the total absorbance of the mixture. The band has a maximum at -310 mp and a distinct shoulder at -340 mp. In addition, the curvature at the low wavelength indicates a contribution from the tail of another band, possibly the perturbed R 3 T * band of the donor. One way to test for the presence of more than one species in a complex band is to use the Liptay m e t h ~ d . ~ When this was done for dimethylthioacetamide-iodine (1) R. P. Lang, J . Amer. Chem. SOC.,84, 1185 (1962). (2) R. J. Niedzielski, R. S. Drago, and R. L. Middaugh, ibid., 8 6 , 1694 (1964). (3) A. F. Grand and M.Tamres, Inorg. Chem., 8,2495 (1969). (4) W. Liptay, 2.EZeMrochem., 65, 375 (1961).
209
NOTES I
I
I
I
I
I
I
I
I
I
Table I: Data and Results for the Resolution of the Dimethylthioacetamide-Iodine Ultraviolet Absorption in Carbon Tetrachloride a t 20'
x 106, mol/l.
Do" v, 0
m
a 02 01 L
I
36 I
280
I
I I I l l I I I ' X I 34 32 30 20 26 FREQUENCY .X I O ~ . ( C ~ - ~ ) 1
I
300
I
I
I
I
I
I
I
I
I
I
320 340 360 380
WAVELENGTH (mp)
Figure 1. Resolution of dimethylthioacetamide-iodine ultraviolet spectrum in carbon tetrachloride. (A) absorption of complex (corrected for free donor and acceptor), (B) extrapolation of major band (symmetrical with high-energy side), (C) curve A minus curve B; concentrations in 1-cm cell a t 20'. Initial donor = 6.70 X M , initial iodine = M , complex at equilibrium = 1.37 x M. 1.44 X
over the wavelength range 300-360 mp no systematic variation in the Liptay matrix as a function of wavelength was found. I n accord with this, calculations of the thermodynamic functions from the absorbances a t 340 mp matched those at 310 mp3 (in parentheses) : - A H o = 9.29 f 0.14 (9.77 f 0.37) kcalmol-'; -AG02g8 = 4.34 (4.32) kcal mol-', -AXo = 16.6 f 0.5 (18.3 f 1.3) eu, which are in reasonably good agreement with those obtained from a study of the blue-shifted iodine band.2 These observations suggest that the peak at -310 mp and the shoulder at -340 mp must both be attributed to a single complex. The overlapping bands were resolved qualitatively by making two assumptions; (1) the absorbance of the minor band is negligible below 310 mp, and (2) the major band is symmetrical with respect to frequency. Subtracting the major band (curve B, Figure 1) from the total absorbance (curve A) produced the minor band (curve C). The second assumption is an oversimplification because CT bands are generally a ~ y m m e t r i c ,but ~ the presence of the unresolved tail on the high-frequency side partially compensates for the neglect of asymmetry. I n spite of the approximations, the evidence is good that two bands exist, a more intense one a t -306-308 mp and a minor one at -340-350 mp. Six dimethylthioacetamide-iodine solutions were resolved in a similar manner, and the equilibrium constants and extinction coefficients were determined for the resolved bands at the wavelengths 307 and 340 mp. The data and results are shown in Table I. The equilibrium constants for the two bands are in reasonable agreement with one another and with those obtained prior to resolution.6 The earlier report on thioacetamide-iodine in dichloromethane did not indicate any complexity of the
6.70 7.81 6.70 5.70 5.70 2.01
zo*x
104, mol/l.
1.44 6.95 2.88 4.63 2.32 8.63
A D ,307 mp
do,340 mp
0.605 1.640 0.864 0.987 0.636 0.470
0.130 0.389 0.207 0.233 0.162 0.105
a Initial concentration of donor. Initial concentration of acceptor. 'Absorbance of the resolved band. For 307 mlt, K = 1960 f 140 l./mol; E 37,500 l./mol-cm. For 340 mu, K = 2760 =t580 l./mol; e 7570 l./mol-cm.
CT band.' That study was repeated to see if the same resolution procedure would show a second band. The method was applied to four solutions at 20". Figure 2 and Table I1 show the results. A small but apparent absorption band was located with a maximum a t -320-325 mp, in addition to the main band at -295296 mp. The equilibrium constants are in good agreement with each other and with the data of Lang' ( K 2 p calculated from his data is 11,100 1. mol-'). The extinction coefficient of the 295-mp band also is in reasonable agreement with that of Lang' (-48,000 mol-' cm-l). The extinction coefficient of the second band at the higher wavelength is of similar general magnitude as that found for dimethylthioacetamideiodine. Table 11: Data and Results for the Resolution of the Thioacetamide-Iodine Ultraviolet Absorption in Dichloromethane a t 20' Doa x 106, mol/l.
zob x 106, mol/l.
A C ,295 mp
A C ,325 mp
6.47 6.47 6.47 6.47
6.92 9.69 13.8 27.7
1.035 1.295 1.590 0.479
0.130 0.145 0.200 0.059
a Initial concentration of donor. Initial concentration of acceptor. 'Absorbance of the resolved band. For 295 mlt, K = 11,500 i: 1600 l./mol; E 45,500 l./mol-cm. For 325 mM, K = 12,300 =t7380 I./mol; E 5340 l./mol-cm.
A, and emax for the n ---t T* transition of dimethylthioacetamide in carbon tetrachloride are -360 mp and -70 1. mol-' cm-', respectively, and of thioacetamide in dichloromethane' are -356 mp and -40 1. mol-' cm-', respectively. The minor bands in Figures 1 and 2 are most likely blue-shifted n --t T* transitions of greatly enhanced intensity. The factor of -100-fold increase (5) G. Briegleb, "Elektronen Donator-Acceptor-Komplexe," Springer-Verlag, Berlin, 1961, p 46. (6) The complex in dichloromethane as solvent had the same general absorption pattern as in carbon tetrachloride, with a slight blue shift of the two bands (to ~ 3 0 and 4 -335 mp) ,
Volume 74, Number 1 January 8,1070
2 10
NOTES I
I
I
I
I
I
I
I
I
I
I
IS-
-
W V
z a
m
LL 0 v) m
a
Figure 2. Resolution of thioacetamide-iodine ultraviolet spectrum in dichloromethane. (A) absorption of complex (corrected for free donor and acceptor), (B) extrapolation of major band (symmetrical with high energy side), (C) curve A minus curve B; concentrations in 1-cm cell a t 20”. Initial donor = 6.47 X lod6 M , initial iodine = 1.38 X 10-4 M , complex at equilibrium = 3.41 X 10-6 M .
in E for the shifted band may be off appreciably, considering the possible errors in the resolution. There is no question, however, that the intensity must be greatly increased because, for the concentration of donor used, the absorbance of the unperturbed n + a* transition would not be observable. The enhancement may be due to the transition being more allowed for the perturbed case, or intensity might be acquired from the mixing of states, particularly those of the weak intramolecular n + n* band and the highly intense intermolecular CT band.’ It should be pointed out that since both the C T and perturbed n --t T* bands are the result of the same complexation, calculations of thermodynamic values rereported in the literature from ultraviolet studies on related complexes still are valid. However, in those cases where the two bands overlap too closely, the emax calculated for the CT band may be high.
Aclcnowledgment. This work was supported in part by a grant from the National Science Foundation, GP6429. We wish to acknowledge helpful discussions with Dr. H. Hosoya and Professor S. Nagakura. (7) J. M. Murrell, J . Amer. Chem. SOC.,81,5037 (1959).
Nuclear Magnetic Resonance Spectral Correlation of Symmetrically Substituted 1,2-Diols and 1,3-Dioxalanes
by M. Gianni, J. Saavedra, R. Myhalylr, and I(.Wursthorn St. Michael’s College, Winooski Park, Vermont (Received J u l y 1 , 1969)
Ob404
I n a previous communication,l a method for an nmr structural correlation of symmetrically substituted The Journal of Physical Chemistry
epoxides and olefins was presented. The method depends on the observation of the chemical shift for the methine (C-H) hydrogen and requires that both cis and tmns isomers be available for comparison. I n the course of work on another problem, it became necessary to assign structures to a series of symmetrically substituted 1,Zglycols. Since only one isomer was available for some of these glycols, the previous method was not applicable. We now wish to report a method of unequivocal assignment of structure for disubstituted 1,2-glycols and l,&dioxalanes which obviates the necessity of observing the nmr spectra of both isomers. The synthetic sequence is illustrated below.
Previously, the best method of unequivocal structural assignment for the 1,2-glycols was optical resolution. The method is tedious and often difficult with tertiary alcohols. Dioxalane formation is easily accomplished and avoids difficulty with tertiary alcoho1s.2*3 The glycols are treated in benzene solvent with paraformaldehyde and catalytic amounts of p-toluenesulfonic acid employing a Dean-Stark apparatus for separation of the water from the reaction mixturen4 The nmr spectra of the dioxalanes are then used to scrutinize the methylene hydrogens. Dioxalanes of type 1 give an AB nmr spectrum due to the diastereotopic relationship5 of the methylene hydrogens. Dioxalanes of type 2 give an Az spectrum as a consequence of the enantiotopic relationship of the methylene hydrogens. Table I lists a series of such nmr values6 and the type of spectra observed. The nmr values for the methine hydrogens in the 1 ,&dioxalanes are consistent with the previous correlation rule: the methine hydrogen for the meso (cis) isomer absorbs at lower field than for the d,Z racemate (1) M.H.Gianni, E. L. Stogryn, and C. M. Orlando, J . Phys. Chem., 67, 1385 (1965). Summarized in this work were the cis and trans2,l-butanediol carbonates of F. A. Anet, J . Amer. Chem. Soc., 84, 747 (1962),and the cis and trans isomers of symmetrically substituted cyclic hydrocarbons of D. Curtin, Chem. Ind. (London) 1205 (1958). (2) This is a consequence of the mechanism of acetal formation; M. Kreevoy and R. W. Taft, J . Amer. Chem. Soc., 77,5590 (1955). (3) E. L. Eliel, ibid., 84, 2377 (1962). (4) The method of synthesis for all compounds reported is that of K. C. Branncock and G. Lappin, J . Org. Chem., 1366 (1956). All compounds gave satisfactory analytical data including ir and nmr. (5) For the terminology diastereotogic and enantiotopic as applied to these systems, see K. Mislow, “Topics in Stereochemistry,” Vol. I, John Wiley and Sons, New York, N.Y., 1967. (6) Caution must be exercised in extending the method t o conforma tionally mobile systems since a rapid inversion may cause the methylene hydrogens to give an AZspectrum due to averaging.
