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ANALYTICAL CHEMISTRY, VOL. 50, NO. 2, FEBRUARY 1978
Ensman, Nick Loy, and Steve Williams for their advice in the design and construction of the electronic circuitry.
American Chemical Society, New York, N.Y., April 1976; taken in part from the Ph.D. thesis of T. W. Hunter, Indiana University, Bloomington, Ind., 1976. Support of this work by Grant PHS GM 17904-04A1 from the National Institutes of Health is gratefully acknowledged.
RECEIVED for review September 16, 1977. Accepted October 31, 1977. Presented in part at the Centennial Meeting of the
Comparisons between Hydrogen Bond Donor-Acceptor Parameters and Solvatochromic Red Shifts Orland W. Kolling Chemistry Department, Southwestern College, Winfield, Kansas 67 156
Transltlon energles were tabulated for three bathochromlc lndlcators In representatlve nonpolar, polar aprotlc, and amphlprotlc solvents. Nonlinear statlstlcal correlation functlons prevlously used for comparing the dye transltlon energy with a second solvent polarity parameter were resolved Into llnear equatlons for polar aprotlc solvents and a devlatlon variable for hydrogen bond donor solvents. I n these comparlsons, the Gutmann acceptor number was useful as a second emplrlcal parameter responslve to solvent polarity. For the lower aicohols, acetlc acld, chloroform, and acetonltrlle, the qualltatlve trend In the devlatlon varlable parallels the order obtained wlth the Kamlet-Taft ac-scale for hydrogen bond donor solvents. Slmllar correletlons between transltlon energles for the bathochromlc dyes and enthalpy-based measures of donor-acceptor lnteractlons generally felled.
Solvatochromic indicators having chromophoric groups absorbing in the UV-visible region have been used t o define empirical scales of solvent polarity. The most commonly used scales deduced from blue-shift chromophores are the Kosower 2 values and the Dimroth-Reichardt numbers. These have been found to be most adequate for representing hydrogen bonding and dielectric characteristics of polar nonaqueous solvents, but are less satisfactory as measures of solvent basicity for nonpolar and aprotic solvents ( I ) . A series of red-shift (bathochromic) dyes have been developed which are more capable of representing distinctions among polar aprotic and nonpolar solvents (2-4) and their transition energies can be used t o quantify medium effects in some instances where the Kosower and Dimroth values fail. Recently, Taft and Kamlet have devised a double scale for hydrogen bond donor and acceptor solvents which is derived from the solvatochromic shifts of nitroanilines and nitrophenols (5, 6). In previous investigations on the solvatochromism of three bathochromic dyes (Phenol Blue, Nile Blue A oxazone, and Brooker's dye VII) a general nonlinear correlation was found between the transition energy ( E T )of the indicator and the "F NMR shift and the 14NESR hyperfine splitting constant for selected model basic probes of solvent polarity (7). A purely statistical treatment of all of the data for both hydrogen bonding and nonhydrogen bonding solvents produced the rational correlation function given in Equation 1
e =cE, E,-
-a
b 0003-2700/78/0350-0212$01.OO/O
where P, is the probe variable, ET the transition energy (kcal/mol) for the solvatochromic indicator, and a , b, and c are constants. The present paper is concerned with the results of a more detailed examination of bathochromic shifts by the three model dyes in pure hydrogen bond donor solvents with the objective of resolving the influence of hydrogen bonding from the correlated parameters of solvent polarity incorporated into Equation 1. For the latter, the Gutmann donor (DMand acceptor (AM numbers were selected for the P, variables. The Figueras concept of a n additive perturbation energy contribution was extended to the response to the probe in hydrogen bond donor solvents ( 4 ) .
EXPERIMENTAL Solvents. Seven solvents were included in this study for which bathochromic shifts of the dyes had not been determined in previous work. These are: benzonitrile, carbon tetrachloride, absolute ethanol, n-hexane, methylene chloride (re-determination), nitrobenzene, and 2-propanol. In all cases, the liquid was initially dried for 1week over anhydrous calcium sulfate. Subsequent steps in purification are those specified below. Anhydrous ethanol was prepared by the usual refluxing over fresh CaO, followed by fractional distillation. Isopropyl alcohol was treated with clean magnesium ribbon prior to fractional distillation. Carbon tetrachloride, dichloromethane, and n-hexane were further dried by passing through a chromatographic column of alumina and then re-distilled. Benzonitrile was passed through two alumina columns, but not re-distilled. Purified nitrobenzene was obtained by reduced pressure (7 Torr) fractional distillation. Solvatochromic Dyes. Purified samples of Phenol blue and Nile Blue A oxazone were prepared by the procedures for recrystallization and column chromatography reported earlier (8, 9). Literature melting points and thin layer chromatography were again used as criteria for purity of the products. Spectra. Measurements of the absorption maxima for specific dye solutions in the visible region were made with a PerkinElmer-Coleman 111 spectrophotometer, following the methods previously described (8). RESULTS AND DISCUSSION Additional experimental values for the transition energies of Phenol blue and Nile Blue A oxazone in nonaqueous solvents are given in Table I, along with published data for Brooker's dye VII. Representative parameters measuring solvent basicity which were used in previous work on red shift vs. basicity correlations were re-examined with special attention being given to the bathochromic shifts in hydrogen bond donor solvents. Possible relationships to enthalpy-based 0 1978 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 50, NO. 2 , FEBRUARY 1978
Table I. Spectral Data for Solvent Polarity Probes (at 25 "C) Electronic ET (kcalimol) XR a PBb NBAOC Solvent 1. Acetic acid 2. Acetone 3. Acetonitrile 4. Benzene 5 . Benzonitrile 6. Carbon disulfide 7. Carbon tetrachloride 8 . Chloroform 9 . Diethyl ether 10. N,N-Dimethyl
... 45.7 45.7 46.9 43.3
...
48.7 44.2 48.3 43.7
I4N ESR AN radical Id
Gutmann A Ne
...
52.9 12.5 19.3 8.2 15.5
15.289 15.331 15.863 15.334 15.635
8.6 23.1 3.9 16.0
47.14 49.14 48.97 49.73 47.90 50.87 50.61 48.13 51.58 48.06
51.24 53.64 53.74 55.87 52.77 55.46 55.09 53.24 56.39 52.85
16.420 15.527 15.666 15.404
47.26 50.07 47.20 51.96 47.03 48.46 47.26 48.39 47.96 47.98 48.72
52.17 55.19 52.10 56.50 51.98 53.52 52.36 52.85 52.94 52.55 53.31
15.692 15.452 16.030 15.134 16.210 15.752
213
...
formamide 11. Dimethyl sulfoxide 12. 1,4-Dioxane 13. Ethanol 14. n-Hexane 15. Methanol 16. Methylene chloride 17. Nitrobenzene 18. Nitromethane 19. 2-Propanol 20. Pyridine 21. Tetrahydrofuran 22. Water
42.0 48.4 43.9 50.9 43.1 44.9 42.6 44.0 44.5 43.9 46.6 68.9
...
15.759 15.973 15.608 15.373 17.175
19.3 10.8 37.1 0.0 41.3 20.4 14.8 20.5 33.5 14.2 8.0 54.8
a Values from Brooker's mercyanine dye (VII), L. G. S. Brooker et al. ( 2 ) . Re-evaluated and new data for Phenol blue as well as earlier values: J. Figueras ( 4 ) ;0. Kolling and J. Goodnight (8);0.Kolling ( 9 ) . Literature values and new experiHyperfine splitting constants for mental results for Nile Blue A oxazone: M. M. Davis and H. Hetzer (3);0. Kolling (9). di-tert-butyl nitroxide radical: B. Knauer and J. Napier, J. A m . Chem. SOC.,98, 4395 (1976). e Solvent acceptor numbers: V. Gutmann ( I 0).
scales for hydrogen bond donor-acceptor interactions were included as well. Gutmann Donor and Acceptor Numbers. The donor number (DN)of Gutmann is defined as -AH (in kcal/mol) for the reaction between a given Lewis base and SbC& as the reference acid, and the numbers are evaluated in very dilute solutions of the reactants in 1,2-dichloroethane as the solvent (10). The broad scale of donor numbers ranges from 0.1 for benzene to 61.0 for triethylamine; and extensive empirical correlations between D N values and the effects of donor solvents upon redox and ligand substitution reactions have been developed by Gutmann. In an important critical review of the data for enthalpies of formation for hydrogen bonded adducts between different donors and common acceptors, Arnett, Mitchell, and Murty noted that the uncertainty in enthalpy changes predicted from DN values is i0.4 kcal/mol and that the DN scale provides a fairly adequate measure of the hydrogen bond acceptor capacities of a variety of Lewis bases (11). Among the various Lewis acids which have been used as reference acceptors, antimony(V1 chloride appears to be among the stronger if one compares the electrostatic contributions to adduct formation from the acidic reactants. For example, on the DragoWayland E-C scale the EA value of 7.38 for SbC15 is comparable to that of BF3 and is well above the values for phenol and p-fluorophenol (12). Thus, positive correlations between donor numbers and solvent parameters would be expected for interactions characterized by significant electrostatic (or Coulombic) contributions. The Kosower 2 scale derived from pyridinium iodides as blue shift chromophores has been successfully used as a measure of solvent polarity in polar and hydroxylic solvents (13). Gutmann has noted a seemingly linear correlation for 2 vs. DN (10);however, there is extensive scattering of points along the linear plot with no visible pattern to deviations when comparing hydrogen bonding to nonhydrogen bonding solvents. When similar plots are made for DN vs. ETfor the data of the three model red shift dyes in Table I, no clear trend is observed among either the hydrogen bond donor or acceptor
solvents. (The figures are not included herein.) Pairwise comparisons between bathochromic shifts and other enthalpy parameters for hydrogen bond donor-acceptor interactions (11) do not exhibit any significant correlations. By contrast to the donor scale, the Gutmann acceptor number (AN) values are spectral parameters derived from the 31PNMR shift for triethylphosphine oxide as the reference base dissolved in single solvents or in mixed solvents (10). However, AN-scale values are normalized shift numbers based upon an assigned value of 100 for the 1:ladduct, SbC16:Et3P0, in 1,2-dichloroethaneand for n-hexane as zero for the reference solvent. A connective function between D N and AN has been developed (14). Because of the structural similarity of the basic functional group in triethylphosphine oxide to the dialkyl nitroxide radical, the interaction of a given solvent with the oxygen atom of the probe species would be expected to be comparable, as well. As noted in the earlier review (7), hyperfine splitting constants (AN)for di-tert-butyl nitroxide exhibit a regular though nonlinear trend when related to the red shift of solvatochromic indicators. Therefore, the initial new correlation examined was that between AN (triethylphosphine oxide) and the A N of dibutyl nitroxide, using the data from Table I. The resulting graph in Figure 1conforms to the linear regression function of Equation 2.
A N = 44.36(AN - 15.23) Here, the regression coefficient is 0.986 based upon 17 solvents, including hydrogen bond donors as well as aprotic solvents. If the 14N ESR measurements are used with Equation 2 as a predictive function for AN values, the uncertainty in the computed results is kO.9 (std dev). A similar regression function can be established for AN vs. 19Fchemical shift for 3-fluoropyridine in the same set of solvents; however, the regression coefficient is lower (0.956) because of the greater experimental uncertainties in NMR shifts for the fluoropyridines (7). Gutmann has noted a correlation (with negative slope) between AN and 13CNMR shifts for the carbonyl group of acetone in polar and hydrogen bond donor solvents (10).
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 2, FEBRUARY 1978
Table 11. Derived Parameters for Hydrogen Bonding Solvents (at 25 "C) A(AN) from: Solvent Acetic acid Acetonitrile Chloroform Ethanol Methanol Methylene chloride Nitromethane 2-Propanol
PB 33.7 7.0
XR
... 7.7 8.3
7.6
18.1
21.8
NBAO
OKT'
EAb
SBC
31.8 6.3 8.3 18.3 22.4 6.7 4.5 17.8
0.79 0.29
... ... 3.02 3.41 3.91
0.005 -0.104
0.0
0.850
-0.200 0.000
24.0 21.7 0.990 0.050 ... 7.4 6.1 ... 1.66 ... ... ... 5.3 5.9 19.4 16.8 0.687 2.59 -0.041 Alpha scale values from R . W. Taft and M. Kamlet ( 6 ) . Data from R. Drago and B. Wayland, J. A m . Chem. SOC.,87, 3571 (1965);R. Drago, G. Vogel, and T. Needham, ibid., 93, 6014 (1971);D.Martire, et al., ibid., 98,3101 (1976). S. Brownstein, Can. J. C h e m . , 38, 1590 (1960).
J l
L
45
35
16
0
kh
Correlation diagram for Gutmann acceptor numbers (AN) ESR hyperfine splitting constants (AN) di-terf-butylnitroxidefor the solvents listed in Table I Figure 1.
with
I4N
Hydrogen Bond Donor-Acceptor Parameters. From the preceding investigations on red shift indicators, linear free energy functions were found between ET and AGf for the phenol-DMSO complex in polar aprotic solvents (15); and detailed treatment of the specific case of Phenol blue has demonstrated that ET is linearly related to the McRae two-variable equation when restricted to pure aprotic solvents (4). Therefore, one may infer that the transition energies of each of the three solvatochromic dyes included in this study will, in general, be linearly correlated to a second idealized solvent polarity parameter for those nonaqueous media in which hydrogen bonding and other specific solvent-solute interactions are absent. If the above deduction is correct, then one approach to the resolution of hydrogen bonding effects from the intrinsic solvent polarity included within empirical but nonlinear correlations is to employ the assumption of Figueras, Le., that the deviation in the measured parameter is proportional to the perturbation energy caused by hydrogen bonding (4). The usefulness of this assumption has been demonstrated by Taft and Kamlet in their "solvatochromic comparison method" ( 5 , 6). The Gutmann AN data appear to conform to the Figueras concept and the graphs of A N vs. ET for the three model bathochromic indicators in Figure 2 were prepared from the data summarized in Table I. Regression functions for the best-fit lines within the data points for the nonhydrogen bonding and basic solvents are given by Equations 3-5. Curve A: A N = - 2.12 (ET- 51.1)
(3)
- 3.61 (ET- 52.46) Curve C : A N = - 3.43 (ET - 57.31)
(4)
Curve B: A N =
(5)
The enhanced response or perturbation in the acceptor number for each hydrogen bond donor solvent is represented
50
55
ET
Figure 2. Comparison of Gutmann ANvalues with the transition energies ( E T )for three bathochromic indicators: (A) Brooker's dye (VII); (B) Phenol blue; (C) Nile Blue A oxazone. (0)Aprotic solvents, (0)hydrogen
bonding solvents
as the displacement (AA1Cq from the linear function. Numerical AAN values for the eight donor solvents are listed in Table I1 for the curves of the three bathochromic dyes. A sequence of decreasing AAN for the nonaqueous hydrogen bond donors is visible from the data in Table 11: HOAc > CH3OH > CPHBOH > i-CSH70H > CHC13 > CHZC12 CH3CN > CH3N02. The deviation in A N yields a linear function of positive slope when correlated with the Kamlet-Taft cy values for the alcohols and chloroform (with acetonitrile and acetic acid as outlying points); and the same sequence is observed with the Brownstein S parameter as well. It should be emphasized in this regard that AAN does not relate directly with vmax values for the nitroanilines used by Kamlet and Taft as the solvatochromic indicators but qualitatively relates only to the derived cy values. However, there is seemingly no correspondence between AAN and the Drago EA-CAparameters or their ratio. Infrared frequency shifts for the free -OH bond in reference solute-donor solvent systems follow the order noted above with the exception of nitromethane: HOAc > CH30H > C2H50H> i-C3H70H> CH3N02 > CHC13 (16). The enhanced response shown by the Gutmann acceptor number for hydrogen bond donors appears to parallel other measures of donor capacity (6) and can be identified within parameter correlations to solvent polarity data derived from the solvatochromic dyes. The same conclusion applies also to other correlations between the bathochromic ET and the 19FNMR shifts of the fluoropyridines or 14NESR splitting constants of the alkyl nitroxide radicals; however, the exact placement of nitromethane in the lower portion of the resulting qualitative sequence remains uncertain. When the McRae equation is used to calculate an intrinsic solvent polarity contribution to ET of the bathochromic dye as was done by Figueras ( 4 ) , the difference between that calculated ET and the experimental value (as a AET)can be viewed as one measure responsive to hydrogen bonding. The
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 2, FEBRUARY 1978
\
I,
I I
47
50
ET
Flgure 3. Correlation of x' scale (Kamlet-Taft) with E, of Phenol blue in polar and nonpolar aprotic solvents. Both coordinates in units of kcal/mol. Data from literature sources ( 4 , 7, 9 , 77)
limited data reported on Phenol blue derivatives seem to parallel the above order for ST in several donor solvents ( 4 ) ; however, when the more complete data on Phenol blue are examined (including those solvents in Table I), only the gross form of the sequence among the stronger donors is the same: HOAc > CH30H > C2HjOH > i-C3H70H>> CH&N. The ambiguity in the order among the weaker hydrogen bond donors in this case appears to reflect the statistical limits of reliability within the McRae equation itself as well as small from specific solvent effects nonadditive contributions ST (8).
A further test for the resolution of the data points as shown in Figure 2 was obtained from the recent proposal of Kamlet, Abboud, and Taft (17) for representing solvatochromic effects by the summation in Equation 6.
Here, a, b, and s are empirical slope coefficients, a and 6 the previously defined hydrogen bond acidities and basicities, respectively ( 5 , 6 )and , x* the parameter depicting the intrinsic solvent polarity-polarizability. For aprotic solvents in which
215
the contributions of a and fl are essentially zero, the probe response would be expected to be a simple linear function of H* with intercept Pro. The plot of ET data for Phenol blue in polar aprotic solvents vs. x* (converted to kcal/mol) shown in Figure 3 confirms this expectation. The correlation coefficient is 0.97 for 19 solvents (excluding CS2as an outlying point) and a value of -0.52 is obtained for s in Equation 6. In the absence of any single general parameter which can be used as a quantitative measure of solvent polarity, it is important to examine correlations between the empirical responses of a variety of probes for medium effects upon reactions. Comparisons among various spectroscopic shifts, including those of a number of solvatochromic indicators and NMR and ESR probe species, seem to give evidence for common and consistent responses in polar aprotic media. The extensive investigations of Taft and Kamlet and of Figueras, as well as findings reported here, indicate that abnormal spectral shifts observed in the stronger hydrogen bonding solvents can be treated as additive perturbation energy contributions induced by the donor solvents.
LITERATURE CITED (1) (2) (3) (4) (5) (6) (7)
F. Fowler, A. Katritzky, and R. Rutherford, J . Cbem. SOC.B , 1971, 460. L. G. S . Brooker et al., J . Am. Cbem. Soc., 87, 2443 (1965). M. M. Davis and ti. Hetzer, Anal. Cbem., 38, 451 (1966). J. Figueras, J . Am. Chem. Soc., 93, 3255 (1971). M. Kamlet and R . Taft, J . Am. Cbem. Soc., 98, 377 (1976). R. Taft and M. Kamlet, J , Am. Chem. Soc., 98, 2886 (1976). 0. Kolling, Anal. Cbem., 49, 591 (1977). (8) 0. Kolling and J. Goodnight, Anal. Cbem., 45, 160 (1973). (9) 0. Kolling, Anal. Cbem., 48, 884 (1976). (10) V. Gutmann, Cbem. Tech., 7, 255 (1977). (1 1) E. Arnett, E.Mitchell, and T. Murty, J . Am. Cbem. Soc., 96, 3875 (1974). (12) R. Drago. G. Vogel, and T. Needham, J . Am. Cbem. Soc., 93, 6014 (1971). (13) E. M. Kosower, J . Am. Cbem. Soc., 80, 3253(1958). (14) U. Mayer, V. GuhMnn, and W. Gerger, Monatsh. Cbem., 106, 1275(1975). (15) 0 . Kolling, Anal. Cbem., 48, 1814 (1976). (16) A. Lemley, in "The Chemistry of Nonaqueous Solvents", Vol. IV. J. Lagowski, Ed., Academic Press, New York, N.Y., 1976, 19-43. (17) M. Kamlet, J. Abboud, and R . Taft, J . Am. Chem. Soc., 99, 6027 (1977).
RECEIVED for review September 12,1977. Accepted November 7 , 1977.
