NOTES
1142
are reconverted in the hot acetic anhydride. The solid complexes appear quite resistant to hydrolysis so that no special precautions during handling are necessary. All spectra were determined on a Cary 14 spectrophotometer, maintained at 25 lo,using cells of 1-cm or 10-cm path lengths. Absorption maxima for a few cases, as indicated in Table I, were determined with an equivalent concentration of donor in the reference cell to compensate for partial overlap from neighboring absorption bands. The formation constants and molar absorptivities were calculated by means of the BenesiHildebrand equation. Where there was interference from neighboring bands, as indicated in Table 11, calculations were carried out on data obtained from the long-wavelength sides of the absorption bands.'
Results and Discussion The maxima of the charge-transfer spectra for the PMDA-aromatic hydrocarbon complexes in six solvents are listed in Table I. Despite an apparent general trend, benzene solutions excepted, of the charge-transfer maxima toward shorter wavelengths with decreasing refractive index of the solvent, no unambiguous correlation with this solvent property is possible owing to a distinct and discontinuou s change in the shift of tht: charge-transfer maxima in going from the group of low dielectric constant solvents to the group with high dielectric constant. Variation in the dielectric constant of the solvent cannot solely account for the present results either as then the observed blue shift from acetic anhydride to 2-butanone should be reversed. I n view of the limited successes of any general interpretations of solvent effects on molecular complexes, we would prefer to forego any assessment of the significance of the present observations except to point out that both the dielectric constant and refractive index of the solvent appear to be significant factors in determining the charge-transfer spectra, a t least for the complexes of this study. Benzene clearly seems to form an exception to the general trends observed in other solvents. Competitive equilibria between complex-forming solvents and acceptors have been considered for other complexes* but appear to be a negligible factor in the present case as we have found no evidence in the form of benzenePMDA charge transfer, which should be detectable considering the large concentration of benzene present. Investigation in the region of the expected benzenePNDA charge transfer is complicated by the overlapping absorption of the donors and acceptor, as well as benzene itself, so presence of a competitive equilibrium cannot be ruled out absolutely. However, the observation that the formation constants in benzene are The Journal of Physical Chemistry
comparable to those in methylene chloride and chloroform, as shown in Table 11, lends further support to the idea of a negligible competitive equilibrium. The relative blue shift in the charge-transfer maxima in benzene with respect to methylene chloride and chloroform, despite a higher refractive index, is best accounted for by assuming a specific interaction of the type donor-acceptor-benzene. Finally, it, may be of interest to note that in the present case a rough parallelism exists between the shifts of the charge-transfer maxima and the variation of formation constants in the four solvents in which the constants have been measured. Owing to the many additional factors that may affect an equilibrium, it is doubtful whether a similar trend may be expected in other systems.
Acknowledgments. This work was supported in part by a grant from the University of Connecticut Research Foundation. P. R. wishes to thank the United Aircraft Corp., East Hartford, Conn., for the use of some of their facilities. (7) W. B. Person, J . Am. Chem. Soc., 87, 167 (1965). (8) R. E.Merrifield and W. D. Phillips, ibid., 80,2778 (1958).
Nitrogen-14 Chemical Shifts in Nitro Compounds by C. F. Poranski, Jr., and W. B. Moniz Nawal Research Laboratory, Washington, D. C. 80890 (Received September 6 , 1966)
I n spite of the W 4quadrupole moment, the use of high field strengths and internal reference standards provides a capability for accurately measuring relatively small variations in the W4 chemical shift of nitro compounds. Witanowski, Urbanski, and Stefaniak,' working at 4.3 MHz (-14,000 gauss), were able to characterize the nitro groups in nitramines and primary, secondary, and tertiary nitroalkanes by their N14chemical shifts. Additionally, they observed in the series RP CNOz that if an R group has a negative inductive effect, Le., withdraws electrons, the N14 chemical shift moves to higher fields. For nitro compounds this appears to be a secondary effect superimposed on the primary factor influencing nitrogen (W4or N16) chemi(1) M. Witanowski, T. Urbanski, and L. Stefaniak, J . Am. Chem. Soc., 86,2569 (1964).
NOTES
1143
cal shifts. The primary factor is currently b e l i e ~ e d ~ , ~ to be the paramagnetic term of Saika and Slichter4 and 40 its effect can be stated as follows: as the electronegativity of the atom directly bonded to the nitrogen increases, the nitrogen resonance moves to lower field. g 20 The secondary effect is evident in the data from the present work presented in Table I and in Figure 1, which is a plot of the available NL4chemical shifts us. the Taft polar substituent constant,6 u*, for the R group in RNO2.
7 016
156 014
013
R GROUPS IN
-“I
-401
’
icnlin
c;n,,
CH
4,
lCH,l2 CCI CH,CH2 cH3icn2i, 7. C H ~ C H Z C H Z 8. C ~ H ~ C H Z 9. HOCH9CH.
5,
e.
-30 A1
Table I: N1*Chemical Shifts and Line Widths of Aliphatic Nitro Compounds
7 CH,
2. 3.
1.0
I
I
2.0
3.0
cn2 cn CI
12. 13, CH3 ClNO& 14. HO CHp CINO212 IS.
H ClN021~
16. CH,CH*O 17. CINOd,
I 4.0
0’
width, Hz-
--Line Compound
6,‘ ppm
b
C
CHaCHzNOz CH3CH2CH2NO2 HOCHzCHzNOz CHaCHzCHClNOz CHaNOz CHaC(N0zh HOCHzC(N0z)a CH3CHzONOz
-10.8 f O . 1 -9.5 f 0.3 -5.5 f 1 . 4 -2.6 f 1 . 5 0 26.2 f 0.2 29.9 f 0.3 37.5f0.1
3 0 f 1.5
20 30
...
...
... 18fl.O 11 f 1 . 0 10 f 1 . 0 1 2 f 1.0
14
7
a A positive chemical shift indicates resonance a t higher field than CH3N02. The error is the standard deviation. * This work. Calculated from ref 6.
Besides showing the upfield shift which occurs upon substituting polar groups for H or CH,, the data also suggest that the N14 chemical shifts fall on lines whose slopes and displacements depend on the types of substituents in R. For example, the points for the mononitroalkanes (and nitrocyclohexane) fall on a straight line. Additional data for other classes of R groups might allow the separation of anisotropic, inductive, and hybridization effects. The line width of nitromethane in a 1:1 mixture with ethyl nitrate was 17.6 Hz. This is close to the value of 14.3 Hz obtainod from TIdata.6 With nitromethane as solvent for 2,2,2-trinitroethanol, line widths of both compounds were dependent on concentration. As the concentration of the alcohol was decreased from 3.7 to 1.0 M the nitromethane and alcohol line widths changed from 37 to 23 Hz and from 12.5 to 10.2 Hz, respectively. No change in the chemical shift of the alcohol was observed. Addition of ethyl nitrate to one of the solutions caused no change in line widths. The cause of the broadening has not been established, but possible explanations are hydrogen bonding or viscosity changes. It may be significant that the chemical shift of ethyl nitrate was 0.6 ppm to higher field in the ternary solution than in the CH~N02-C2H60N02 system, for this
Figure 1. Plot of N14 chemical shift us. T a f t polar substituent constant, u*, for R in RNO2: 0,this work; A, ref 1.
could be interpreted as a downfield shift of the nitromethane resonance. However, the shift of 2,2,2-trinitroethanol remained constant, relative to nitromethane, at all concentrations measured.
Experimental Section l,l,l-Trinitroethane and 2,2,2-trinitroethanol were supplied by Dr. T. N. Hall of the Naval Ordnance Laboratory, Silver Spring, Md. The other compounds were used as received from commercial sources. Measurements were made at 6.1 MHz in the HR side-band mode of a Varian HA-100 spectrometer system at 24”. Calibrations were made using the audio side band method. All shifts were measured from an internal. reference of CHINOz or C H ~ C H ~ O N O and Z related to ~ C H ~ N=O ~0. Sampleswere prepared in standard 5-mm0.d. nmr tubes. The larger line widths obtained in this work compared to those calculated from 7’1 data6are believed to be due to the sweep rates and filtering employed, since resolution was found to be -6 Hz from measurements on the quintet of an acidified NH&03 solution. Sample spinning improved S I N . For sharp signals, N14 magnetic resonance can be fairly sensitive. Johnson’ has recorded the 7.22-MHz spectrum of 1.0 M NH4N03 (acidified) in which the S I N of the center peak of the NH4+quintet was 5 : 1. (2) D. Herbison-Evans and R. E. Richards, Mol. Phys., 8, 19 (1964). (3) D. T. Clark and J. D. Roberts, J . Am. Chem. Soc., 88, 745 (1966),and references cited therein. (4) A. Saika and C. P. Slichter, J . Chem. Phys., 2 2 , 26 (1954). (5) R. W.Taft, Jr., “Steric Effects in Organic Chemistry,” M. S. Newman, Ed., John Wiley and Sons, Inc., New York, N. Y.,1956, p 619. (6) W. B. Monis and H. S. Gutowsky, J . Chem. Phys., 38, 1155 (1963). (7) L. Johnson, private communications.
Volume 71 Number 4
March 1967
