J. Org. Chem. 1996, 61, 6404 6406
UV/Vis Spectroscopic Evaluation of 4-Nitropyridine N-Oxide as a
Solvatochromic Indicator for the Hydrogen-Bond Donor Ability of
Anthony F. Lagalante,* Ryan J. Jacobson, and Thomas J. Bruno National Institute of Standards and Technology, Chemical Sciences and Technology Laboratory, Physical and Chemical Properties Division, 325 Broadway, Boulder, Colorado 80303 The potential of 4-nitropyridine N-oxide to act as a solvatochromic indicator of the hydrogen-bonddonor ability of solvents has been evaluated. A linear free-energy relationship has been establishedthat is predominantly dependent on the Kamlet Taft to the previously reported results obtained for pyridine N-oxide, 4-nitropyridine N-oxide possessesa solvatochromic effect that is located in the long wavelength ultraviolet region (λ of the spectrum, making it a viable probe for hydrogen-bond donation assessment.
of a dissolved small organic probe, such as 4-nitroanisole.
Similarly, by measuring the π f π* transition maximum UV/vis spectroscopic measurement of charge-transfer of a second small dissolved organic probe, such as maxima of probe solutes in solution is known to provide 4-nitrophenol, in conjunction with the peak maximum for numerical values for the intermolecular interactions between solute and solvent. The most extensively ap- be calculated. The relationship can be understood in plied method of generating values for intermolecular terms of a LFER, in that the transition maximum of solute/solvent interactions is the method of Kamlet and 4-nitroanisole will not include contributions from the R The Kamlet Taft parameters are , the hydro- terms of eq 1. Replacement of the methoxyl group gen bond donation ability of the solvent, , the hydrogen- of 4-nitroanisole by the hydroxyl group of 4-nitrophenol bond acceptance ability of the solvent, and π*, a param- results in a probe solute that is capable of hydrogen-bond eter that describes the dipolarity and polarizability of the solvent. Using linear free-energy relationships (LFER), the Kamlet Taft parameters can effectively model pro- cesses in solution according to the general expression ally determined using large organic or organometallicprobes,3 due to the extensive use of these probes as polarity indicators of solvents. Often these large probesare insoluble in fluorinated solvents.7,8 In our laboratory, where XYZ is the value of the solvent-dependent process we are in the process of determining the Kamlet Taft to be modeled, XYZ°, s, d, a, and b are the coefficients parameters for alternative solvents which may be useful determined from the LFER analysis, and δ is a polariz- as replacements for chlorinated solvents. Many of the ability adjustment term. The δ term is dependent on the alternative solvents possess a high degree of fluorination class of solvent to be studied; for aromatic solvents δ (but no chlorine or bromine) resulting in zero ozone 1, for polyhalogenated solvents δ depletion potential by currently acceptable mechanisms.
As a potential solution to the solubility problem encoun- success in modeling solution processes as diverse as tered when using conventional UV/vis spectroscopic solubility,4 partition coefficients,5 and chromatographic acidity probes in fluorinated solvents, the replacementof the methoxyl group of 4-nitroanisole with a group that retention.6 Analysis of the coefficients of the LFER is capable of hydrogen-bond acceptance is desirable.
provide insight into the dominant solute/solvent interac- Such an approach was undertaken using pyridine N- tions involved in a particular solvent-dependent process.
oxide;9 however, the π f π* transition maxima observed As suggested by Kamlet and Taft, the determination 283 254 nm) resided in the absorption region of and π* parameters using solvatochromic peak many solvents themselves, thus detracting from the maxima of select probe solutes is relatively straightfor- spectroscopic utility of the probe. Recognizing the po- ward. A value of π* for a particular solvent can be tential utility of pyridine N-oxide as an acidity probe, the directly calculated from the π f π* transition maximum 13C NMR chemical shift of pyridine N-oxide was used toestablish a LFER that was related solely to a dependence * To whom correspondence should be addressed.
X Abstract published in Advance ACS Abstracts, August 1, 1996.
A probe that would more closely resemble the nitroaro- (1) Kamlet, M. J.; Taft, R. W. J. Am. Chem. Soc. 1976, 98, 377.
(2) Kamlet, M. J.; Abboud, J.-L. M.; Taft, R. W. J. Am. Chem. Soc.
matics typically used in the Kamlet and Taft approach, 1977, 99, 6027.
(3) Taft, R. W.; Kamlet, M. J. J. Am. Chem. Soc. 1976, 98, 2886.
(4) Taft, R. W.; Abraham, M. H.; Doherty, R. M.; Kamlet, M. J.
(7) Reichardt, C. Chem. Rev. 1994, 94, 2319.
Nature 1985, 313, 384.
(8) Reichardt, C.; Asharin-Fard, S.; Blum, A.; Eschner, M. et al. Pure (5) Kamlet, M. J.; Doherty, R. M.; Abraham, M. H.; Marcus, Y.; Taft, Appl. Chem. 1993, 65, 2593.
R. W. J. Phys. Chem. 1988, 92, 5244.
(9) Vorkunova, E. I.; Levin, Y. A. Zh. Obshch. Khim. 1984, 54, 1349.
(6) Park, J. H.; Jang, M. D.; Kim, D. S.; Carr, P. W. J. Chromatogr. (10) Schneider, B. H.; Badrieh, Y.; Migron, Y.; Marcus, Y. Z. Physik. 1990, 513, 107.
Chem. 1992, 177, 143.
4-Nitropyridine N-Oxide as a Solvatochromic Indicator J. Org. Chem., Vol. 61, No. 18, 1996 Maximum of the π f π* Transition of
Measurement of Solution Spectra.
4-Nitropyridine N-Oxide in the 48 Solvents Studied and
available dual-beam high-resolution UV/vis spectrophotometer the π*, a, b and d parameters of the solvent taken from
was used to determine the peak maximum of the transition reference 16
for 4-nitropyridine N-oxide in the solvents. The neat solvent was placed in a 1 cm quartz reference cuvette, and a small amount of 4-nitropyridine N-oxide was placed in the matched 1 n-heptane
sample cuvette and filled with solvent. The solution in the 2 n-hexane
sample cuvette was shaken until a constant absorbance value 3 n-pentane
4 cyclohexane
was obtained. The 4-nitropyridine N-oxide solution was then 5 triethylamine
either diluted with the solvent or several more crystals were 6 diethyl ether
added to the solution to adjust the absorbance value to between 7 tetrachloroethene
0.2 and 1.8 absorbance units. The spectrum of 4-nitropyridine 8 carbon tetrachloride
N-oxide in the solvent was scanned at a resolution of 0.05 nm 9 1-chlorobutane
per data point. The peak maximum was determined both by 10 p-xylene
a peak detection algorithm of the spectrophotometer software 11 mesitylene
12 m-xylene
package and by visual confirmation by the operator using an 13 1,1,1-trichloroethane
unsmoothed spectrum. Five spectra were measured for 4-ni- 14 trichloroethene
tropyridine N-oxide in each solvent and the average value of 15 toluene
16 1,4-dioxane
17 ethyl acetate
18 p-difluorobenzene
19 tetrahydrofuran
Results and Discussion
20 benzene
21 methyl acetate
22 fluorobenzene
Results of the π f π* transition of 4-nitropyridine 23 cyclohexanone
24 1,2-dichloroethane
N-oxide in the solvents studied are given in Table 1. The 25 pyridine
max value is expressed in kilokaysers (1 kK 26 N,N-dimethylformamide
along with the standard uncertainty, σ, multiplied by a 27 dimethyl sulfoxide
28 sec-butyl alcohol
29 octanol
classes of solvents studied. Within the framework of eq 30 isobutyl alcohol
1, multiple LFER equations were computed to examine 31 hexanol
the data collected. It was concluded that the experimen- 32 pentanol
33 isopentyl alcohol
tal νmax value in solvents 47 and 48 would not be included
34 tert-butyl alcohola
in any further regression equations because the values 35 decanol
deviated significantly (greater than three standard de- 36 n-butyl alcohola
viations) from the best LFER using all the data. A 37 isopropyl alcohola
38 ethanol
possible explanation for the poorer correlation of the 39 chloroform
transition maxima in solvents 47 and 48 is that 4-nitro-
40 methanol
pyridine N-oxide may not be sufficiently basic to offset 41 2-butanone
42 acetone
the self-association of these solvents,12 or 4-nitropyridine 43 aniline
N-oxide may be simply protonated in solvent 47.
44 acetonitrilea
The following LFER set was computed for the remain- 45 dichloromethane
46 benzyl alcohol
ing 46 solvents using different combinations of the 47 acetic acid
Kamlet Taft parameters as independent variables.
yet still retain the desirable functionality of the pyridine N-oxide probe, is 4-nitropyridine N-oxide. Addition of anitro group to the previously investigated pyridine N- oxide would produce a bathochromic shift of the peak maximum due to increased ring conjugation, resulting in a greater spectroscopic utility for the probe. It is thepurpose of this study to measure the peak maxima of 4-nitropyridine N-oxide in various solvents and optimize the LFER using Kamlet Taft parameters as the depend- Examination of the standard error and t-value of the term showed that it was not a statistically significant Experimental Section
variable in the regression equation.
expected from a probe, such as 4-nitropyridine N-oxide,that is incapable of hydrogen-bond donor abilities. The Chemicals.
4-nitropyridine N-oxide (purity 97%) was obtained from a commercial supplier and was vacuum desic-cated over CaSO (11) Taylor, B. N.; Kuyatt, C. E. Guidelines for Evaluating and 4 prior to use due to the hygroscopic nature of the compound. Solvents were obtained from commercial Expressing the Uncertainty of NIST Measurement Results, NationalInstitute of Standards and Technology, U.S. Government Printing suppliers and were of spectroscopic purity or better and were (12) Chmurzynski, L. J. Chem. Soc., Faraday Trans. 1991, 87, 1729.
J. Org. Chem., Vol. 61, No. 18, 1996 According to the Franck Condon principle, although the dipole moments of the excited state, µe, and groundstate, µg, are different, the positions of the nuclei of theexcited state solute and the nuclei of the surroundingsolvent molecules should not change on the time scale ofthe electronic transition. The dipole moment of 4-nitro-pyridine N-oxide in the ground state was calculated tobe 0.09 D.14 The hypsochromic band shift observed insolvents capable of hydrogen-bonding can be attributedto the increased stabilization of the electronic groundstate relative to the excited state of 4-nitropyridineN-oxide (µg Figure 1. Linear correlation of the experimental UV/vis
Considering all LFER equations, we recommend that absorption maxima and the predicted values according to eq 3. Points for solvents 47 and 48 are included in the plot, but
N-oxide as a probe. Both the π* and not in the regression equation. The inset shows the chemical to the measured transition maxima in solution; however, structure of 4-nitropyridine N-oxide.
transition maxima appears to also depend on solvent positive charge at the pyridinium nitrogen is resonance class, addition of the δ term appears to correct for the delocalized about the ring for poorer electron acceptor polarizability of the solvent classes and is not an ability of the probe. A LFER using solely additional experimentally determined quantity. In com- the independent variables, as in eq 4, results in separate parison to the aforementioned results obtained for pyri- LFERs among the solvent classes measured. Therefore, dine N-oxide, 4-nitropyridine N-oxide possesses a solva- inclusion of the δ term in eq 3 was deemed necessary to tochromic effect that is located in the long wavelength account for the variation in polarizability among the various solvent classes. The correlation is graphically making it a viable probe for hydrogen-bond donation The a/s coefficient ratio in eq 3 is 9.42 indicating that the hydrogen-bond donor ability of the solvent is the Acknowledgment. A.L. wishes to acknowledge the
predominant solute/solvent interaction on the solvato- financial support of the Professional Research Experi- chromic activity of 4-nitropyridine N-oxide.
ence Program at the National Institute of Standards and manifest in the relative insensitivity of the position of the peak maximum of the π f π* transition13 to the solvents incapable of hydrogen-bond donation. In fact,a good correlation exists for the solvents capable of (14) Lazzeretti, P.; Malagoli, M.; Turci, L.; Zanasi, R. J. Mol. Struct. 1993, 288, 255.
(15) Reichardt, C. In Solvent and Solvent Effects in Organic (13) Pierre, M.; Baldeck, P. L.; Block, D.; Georges, R.; Trommsdorff, Chemistry, 2nd ed.; VCH: Weinheim, Germany, 1988.
H. P. Chem. Phys. 1991, 156, 103.
(16) Marcus, Y. Chem. Soc. Rev. 1993, 22, 409.

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