1324
ANALYTICAL CHEMISTRY, VOL. 51, NO. 8, JULY 1979
Pyridine Nitrogen- 15 Nuclear Magnetic Resonance Chemical Shift as a Probe of Medium Effects in Aprotic and Hydrogen Bonding Solvents Sir: As a part of an earlier literature review on molecular probes of solvent polarity, it was shown that published data for the l9F-NMR chemical shifts for the fluoropyridines along with the 14N-ESRhyperfine splitting constants for the alkyl nitroxide free radicals correlate particularly well with the solvatochromic shifts for a number of bathochromic indicators in polar aprotic solvents ( I , @ . However, in hydrogen bonding solvents, single regression correlations between parameters derived from a variety of spectral probe species are usually unsatisfactory. Correlations improve with a linear multiple regression treatment of hydrogen bonding contributions to electronic transition energies, although ambiguities do arise within the apparent order of increasing solvent polarity for the very weak hydrogen bond donor solvents ( 2 ) . Thus, it is necessary to examine the responses of other model probes in order to increase the reliability of the better scales of solvent polarity. This communication summarizes the results of a quantitative treatment of data for the 15N-NMR chemical shift of pyridine in eleven solvents ranging from the nonpolar cyclohexane to the strong hydrogen bond donor, trifluoroethanol. The NMR shift values for 15N-pyridinehave been reported recently by Duthaler and Roberts ( 3 ) ,and the corrected solvent shifts are given with respect to the gas phase as the reference state. The trends in solvent effects were analyzed using the “solvatochromic comparison method” of Kamlet and Taft (4-7); and new experimental values were determined for the a parameters of all of the hydrogen bond donors (HBD) used in this study. At the outset it should be noted that the solvent-probe interactions will not be the same a t the molecular level when 15N-pyridine is substituted for a fluoropyridine as the NMR probe species, Giam and Lyle (8) have demonstrated that the HBD solvent influence upon the deshielding effect in 3fluoropyridine requires the positive end of a solvent dipole to be oriented toward the fluorine atom; and in a nonpolar solvent the dipolar probe polarizes the surrounding solvent molecules with the fluorine atom again determining the orientation of that induced dipole. On the other hand, for I5N-pyridine the shielding effect by the solvent is localized on the nitrogen atom as the acceptor site directed toward a HBD solvent molecule and as the negative end of the dipolar probe species in dipole-dipole interactions with polar aprotic solvents. In spite of these differences in the molecular responses of the probes, a regular though nonlinear correlation is observed when the NMR shift of (19F)3-fluoropyridine is compared to that of (15N)pyridine in Figure 1, and the single continuous curve includes data points for both aprotic and hydrogen bonding solvents. For both the lgF-fluoropyridines and 15N-pyridine, the solvent-induced NMR shifts of the probe fail to correlate with functions involving the dielectric constant of the solvent (3, 8). Similarly, even though approximately linear correlations have been observed between the NMR shifts and the empirical Dimroth Et or Kosower 2-values for the solvents, the scattering of data points is extensive and exceeds the reliability of the experimental data ( 3 , 8 ) . An improved statistical treatment of the solvent dependence of the pyridine nitrogen-NMR shifts is now being proposed, using some of the concepts incorporated by Kamlet and Taft ( 4 , 5 )in their analysis of hydrogen bond donor-acceptor effects on solvatochromic indicators. The net solvent-induced NMR shift for 15N-pyridinecan be resolved into a minimal set of two distinctive interactions: 0003-2700/79/0351-1324$01 .OO/O
Table I. Solvent-Induced Spectral Shift Data “N-A6
shifta ppm-
solvent 1. benzene 2. carbon tetrachloride 3. chloroform 4 . cyclohexane 5. dichloromethane 6. dimethylsulfoxide 7 . methanol
(solvgas)
4.9 4.3 12.5
1.5 9.1 6.9 24.9
Kamlet-Taft 19F
shiftb 13.71 13.96 12.81 14.32 13.28 13.40
ir*c
0.588 0.294 0.760 0,000 0.802 1.000
.d
0.0 0.0
0,220 0.0 0.121 0.0
0,586 0,984 0,990‘ 6.3 13.40 0.867 0.0 8. pyridine 2.9 --0.277 0.0 9. tetrachloroethane 10, trifluoroethanol 39.9 .-- 1.018 1.51 11. water 28.1 .-- 1.090 1.02 1.017‘ a Values obtained by Dutnaler and Roberts 13). Shift values for 3-fluoropyridine reported by Giam and Lyle (8). ‘ Solvatochromic parameters computed by Kamlet, Taft, and Abboud (5, 6 ) . New experimental 0-values with u n certainties of iO.01-0.03 (SD). 12.08
(a) purely polar effects arising from dipoledipole orientations determined by the basic nitrogen atom of the probe; and (b) hydrogen bonding effects caused by the acceptor behavior of the pyridine nitrogen toward HBD solvents. By analogy to solvatochromic shifts in the UV-visible region, the energy contributions to the total interaction are assumed to be additive and in the absence of hydrogen bonding by the solvent the (b) influence must go to zero (9). The appropriate Kamlet-Taft parameters for quantifying the solvent contributions to these interactions are: the P* scale of solvent polarity for influence (a) since it has been shown that T* values correlate well with the dipole moments of aprotic solvents (7); and the (Y values representing the HBD strengths (or HBD acidities) for the pure donor solvents as measures of influence (b). Literature and new experimentally derived a values for the HBD solvents are listed in Table I. The latter were obtained from data for the solvatochromic shifts of 4-nitroaniline and 4-nitroanisole, following the procedures of Kamlet and Taft (4, 5). Two essential tests in the Kamlet-Taft method for differentiating between polar and hydrogen bonding influences by the solvent are that pairwise comparisons of the solvent-dependent probe responses should yield a single regression line for polar aprotic and nonhydrogen bonding solvents, and all data points for HBD solvents should be displaced from that line in a common direction ( 4 ) . The graph in Figure 2 shows that these conditions are met when the 15N-NMR shifts (solvent-gas) for pyridine are compared to the solvent polarity parameter (a*)for the solvents in Table I. Likewise, the magnitudes of the displacements of the points for the HBD solvents from the line parallel the usual qualitative order for increasing hydrogen bond donor behavior: CH2C12 < CHC13 < CHBOH < HzO < F3CCH20H. The general form of the linear regression model relating the spectroscopic response of the probe (R,) to the polarity (P,) and HBD (AHBD)paramaters of the solvent is given in Equation 1. R , = (Rp)o + bP, + ~ A H B D (1) 1979 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 51, NO. 8, JULY 1979
1325
I
-
LtO
Z L3
a
w-
I
0
Figure 1. Scatter diagram of solvent-induced chemical shifts for the NMR probes: (1) "F in 3-fluoropyridine; vs. ( 2 ) 15N in pyridine
40,
v .
1.0
0
1
TT"
Figure 2. Comparison of A615N (pyridine) with the Kamlet-Taft T * parameter. ( 0 )Nonhydrogen bonding solvents; (0)hydrogen bond donors. Numbered points refer to the solvents in Table I
If the probe response in a specified reference state is designated as (Rp)o,then the shift in the property becomes: AI?, = R, - (R,)o. This general form can now be re-stated as Equation 2 to describe the particular function for the 'jNpyridine NMR shift in a given solvent compared to the 15N chemical shift in gaseous pyridine. A6 = 6, - ( 6 J o = br* + u CY (2) The final multiple regression in Equation 3 was established from the data in Table I by iterative procedures and is shown by the plot in Figure 3 as well.
+
+
A6 = 5 . 9 2 ( ~ * 3 . 3 ~ ~ )1.5
T r * + act Figure 3. Multiple regression for A6 15N(pyridine)as a function of the Kamlet-Taft parameters, T * and a
(3)
The linear correlation coefficient is 0.993 including all eleven solvents. As a predictive equation, the maximum uncertainty in the calculated A6 is f1.3 (SD). The uncertainty in the coefficient of a is f0.05 (SD); and the nonzero intercept of Equation 3 arises from the condition that A6 = 0 for the gas phase reference state whereas the T * scale goes to zero for cyclohexane. Duthaler and Roberts have concluded that the solvent shielding effects found in the 15N-NMR spectrum of pyridine are determined largely by hydrogen bonding from the HBD solvents ( 3 ) . The weighting coefficient of CY in Equation 3 is consistent with that conclusion when one considers the relative importance of the two parameters for the stronger hydrogen
bond donors. However, for the weaker HBD solvents (Le., dichloromethane and chloroform) as well as the nonhydrogen bonding solvents, the dominant mechanism for the medium effect clearly has its source in probe-solvent dipolar interactions within the cybotactic region ( I O ) . Because of the magnitude of the slope for the regression line in Figure 3, the measured 15N-NMRchemical shifts for pyridine should provide an alternate route to the evaluation of a-scale values for the weaker HBD solvents. This is potentially significant since it has been shown by Taft and Kamlet ( 5 ) that not only are their experimental uncertainties in the a-values for the weak HBD solvents much larger (=k20%) than for the stronger HBD solvents but also the solvatochromic comparison method appears to be close to its lower limit of resolution for HBD influences when applied to the weaker donors. On the other hand, the uncertainty in the T * values is reported to be f O . l l kK based upon forty-seven regressions derived from many solvatochromic indicators (6) and that uncertainity is uniform over the total range of the T* scale. For the empirical analysis of medium effects upon reaction kinetics, it appears that the Kamlet-Taft scales will have broad applications. Therefore, alternative methods for evaluating the solvent hydrogen bonding parameters need to be examined in order to increase the reliability of both a and 0 values over the total ranges of those scales.
LITERATURE CITED Kolling, 0. Anal. Chem. 1977, 49,591. Kolling, 0.Anal. Chem. 1978, 50, 212. Duthaler, R.; Roberts, J. D. J . Am. Chem. SOC. 1978, 700, 4969. Karnlet, M.; Taft, R. W. J . Am. Chem. SOC.1978, 98,377. Taft, R.; Kamlet, M. J . Am. Chem. SOC. 1976, 98,2886. Karnlet. M.; Abboud. J.; Taft, R. J . Am. Chem. SOC.1977, 99,6027. Abboud, J.; Karnlet, M.; Taft, R. J . Am. Chem. SOC. 1977, 99,8325. Giarn, C.;Lyle, J. J . Am. Chem. SOC.1973, 95,3235. Figueras, J. J. Am. Chem. SOC.1971, 93,3255. (IO) Knauer, 6 . ;Napier, J. J . Am. Chem. SOC. 1976, 98, 4395. (1) (2) (3) (4) (5) (6) (7) (8) (9)
Orland W. Kolling Chemistry Department Southwestern College Winfield, Kansas 67156
RECEIVED for review January 8,1979. Accepted April 4,1979.
Radiative Auger Effect in X-ray Fluorescence Analysis Sir: In 1969 Aberg and Utriainen (I) found evidence for the existence of a K-L2 radiative Auger transition. They 0003-2700/79/0351-1325$01 .OO/O
observed a broad X-ray emission structure on the low-energy side of the K a lines of Mg, Al, Si, and S. 0 1979 American
Chemical Society
