The results of these runs are shown in Table I. The three (or two) data sets obtained for each sample have been obtained on different chromatographic units. T B is the observed average boiling point; s shows the standard deviation of each set of measurements which is computed from
where n is the number of data points. The data show that consistent TB'Scan be obtained under different operating conditions with different units. The spread in values of the temperatures corresponding to +1 u and -1 u is much larger than that observed for T g . This is understandable since the spread is a function of a number of factors, such as the efficiency of the chromatographic separation and the degree of overloading. The average values for the standard deviation of these three points are given a t the bottom of Table I. The average standard deviations for all three temperatures are considerably smaller than the deviations obtained by the ASTM simulated distillation method. Although very little has been published on interlaboratory reproducibility, we estimate from our experience that the 50% point variability in the ASTM method is a t least 10 "F or more. This graphical approach is, therefore, a more consistent and reproducible method to obtain boiling range
information on narrow, high boiling petroleum hydrocarbon fractions.
CONCLUSION The described method does not require equipment as sophisticated as what is necessary for regular simulated distillation. It appears to yield quite precise values for the average boiling point of narrow boiling, high molecular weight hydrocarbon fractions. The boiling range is subject to somewhat larger deviations because it is dependent on the operating parameters. The method is obviously also applicable to lower molecular weight fractions; in this case, it would probably be more appropriate to carry out a complete component analysis by gas chromatography. The described approach should not be regarded as a replacement or alternative technique to the ASTM D 2887-T method; it should be considered as a complementary method which is very useful in specific applications. Received for review August 14, 1972. Accepted December 27, 1972.
Ionization Sequences in the Ground and Lowest Electronically Excited Singlet States of 3-Hydroxy-2-Naphthoic Acid Peter J. Kovi and Stephen G. Schulman College of Pharmacy, University of Florida, Gainesville, Fla. 32601
Since the pioneering study of Weller ( 1 ) which demonstrated intramolecular proton transfer in the lowest electronically excited singlet state of salicylic acid, the spectroscopy and photochemistry of 0-arylhydroxycarboxylic acids have been subjects of interest. In a study of the pH and Hammett acidity dependences of salicylic acid and methyl salicylate, it was shown that in aqueous solutions the phototautomerism of methyl salicylate in the lowest excited singlet state is bimolecular rather than intramolecular ( 2 ) . Hirota ( 3 ) in a solvent dependence study of the fluorescence of 3-hydroxy-2-naphthoic acid, found that an anomalous low frequency fluorescence appeared in basic solvents and in aprotic solvents containing small amounts of basic solvents. From this, he concluded that intramolecular proton transfer occurred during the lifetime of the lowest excited singlet state of 3-hydroxy-2-naphthoic acid. Recently, the hypothesis of true intramolecular proton transfer in the lowest excited singlet state of 3-hydroxy-2naphthoic acid has been refuted by Ware et al. ( 4 ) who employed fluorescence quenching and singlet state lifetime measurements, in a variety of solvents to show that the phototautomerism of 3-hydroxy-2-naphthoic acid involves intermolecular hydrogen bonding with the solvent in the ground state. Although the studies of the fluorescence of 3-hydroxy-2-naphthoic acid seem to be preoccu(1) A. Weller, 2. Elektrochem., 60, 1144 (1956). (2) P. J. Kovi, C. L. Miller, and S.G. Schulman. Anal. Chim. Acta., 61, 7 (1972). (3) K.'Hirota, 2. Physik. Chem. N.F., 35, 222 (1962). (4) w. R. Ware, P. I%.Shukla, P. J. Sullivan, and R. V. Bremphis, J. Chem. Phys., 55,4048 (1971).
pied with the singlet state phototautomerism of the neutral molecule, several prototropic forms can be derived from this compound (as shown in Scheme I) and not only can the neutral molecule ( N ) phototautomerize to the zwitterion (Z), but also the singly charged anion (A) can phototautomerize, subsequent to excitation, to the singly charged anion (AI). Moreover, it is known that in the ground states of salicylic acid and of 3-hydroxy-2-naphthoic acid, the ionization sequence (C) (N) (A) (D) is followed while in the lowest excited singlet state of (Z) (AI) (D) is folsalicylic acid the sequence (C) lowed. The ionization sequence in the lowest excited singlet state of 3-hydroxy-2-naphthoic acid has never (to our knowledge) been studied. In order to elucidate the relative importances of protolytic dissociation, tautomerization, and hydrogen bonding phenomena in ground and lowestOexcitedsinglet states of 3-hydroxy-2-naphthoic acid, the present absorption and fluorescence spectrophotometric study of the acid-base titrimetry of this compound was undertaken.
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EXPERIMENTAL 3-Hydroxy-2-naphthoic acid was purchased from Aldrich Chemical Co., Milwaukee, Wis., and purified by multiple recrystallization from ethanol. 3-Methoxy-2-naphthoic acid was prepared by the method of Werner and Seybold ( 5 ) .The commercial apparatus and solvents employed in this study have been previously described ( 2 ) .
(5) A. Werner and W. Seybold, Ber. Deutsch. Chem. Ges., 37, 3661 (1904).
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li (Z)
Scheme I.
Table I. Low Frequency Absorption Y ( l L;) and 8(1L ~ and ) Fluorescence (5f) Maxima of 3-Hydroxy-2-naphthoic Acid in Aqueous and Sulfuric Acid Media. Spectral Maxima Are Reported in cm-' X
Vel,,, V(lLb)
V(f
I
Ho-IO
Ho-0.7
pH 6.0
pH 10.0
pH 14.0
3.11 2.49 1.96
3.48 2.78 2.17
3.52 2.84 1.96
3.52 2.84 2.36i1.96
3.50 2.82 2.05
RESULTS AND DISCUSSION The low frequency ('La and 'Lb) absorption and fluorescence maxima of the various prototropic species derived from 3-hydroxy-2-naphthoic acid are presented in Table I. Because both substituents in the naphthalene ring are in positions where they affect most the longitudinally polarized 'Lb transition of naphthalene and because the 'Lb state is the lowest excited singlet state in naphthalene, the fluorescences of all prototropic species derived from 3-hydroxy-2-naphthoic acid are presumed to originate from the 'Lb state. The spectral shifts accompanying prototropic dissociation in ground and lowest excited singlet states are related to the charge-transfer acceptor properties of the carboxyl group and the charge-transfer donor properties of the hydroxyl group and have been previously discussed (2). Dissociation from a carboxyl group produces a shift to higher frequencies while dissociation from a phenolic group produces a shift to lower frequencies. However, in the 0 arylhydroxycarboxylic acids the absorption spectral shifts are anomalously small compared with ordinary carboxylic acids and phenols because of the presence of intramolecular hydrogen bonding between the carboxyl group and phenolic group of the neutral molecule ( N ) and between the carboxylate group and phenolic group of the singly charged anion (A) in the ground and Franck-Condon excited states (6). Dissociation of the protonated 3-hydroxy-2-naphthoic acid molecule (C) produces a shift to higher frequencies in the 'Lb absorption and fluorescence bands, the midpoint of the absorptiometric titration occurs a t Ho - 8.0 while that of the fluorimetric titration occurs a t Ho - 4.7. These results indicate that dissociation of the cation occurs from the protonated carboxyl group in both ground and 'Lb states, the pKa in the ground state being -8.0 and the PKa* in the 'Lb state -4.7. The anomalously negative pKa and PKa* values relative to the 2-naphthoic acidium cation can be attributed to difficulty of protonation of the carboxyl group as a result of stabilization of the neutral molecule by intramolecular hydrogen bonding in (6) S.G. Schulman and H. Gershon,J. Phys. Chern., 72, 3297 (1968)
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ground and excited states. That excited 3-hydroxy-2-naphthoic acid fluoresces from the neutral rather than the zwitterionic form, shows that intramolecular hydrogen bonding is not a sufficient condition for phototautomerism to occur. In the region between p H 1 and pH 4, there is a small shift to higher frequency of the lLb absorption band and a substantial shift to lower frequency of the fluorescence of 3-hydroxy-2-naphthoic acid. Absorptiometric and fluorimetric titrations both yield a pKa of 2.6 which is assigned to the ground state pKa of the equilibrium between the neutral molecule (N) and the singly charged anion (A). However, the blue shift of the absorption spectrum indicates that in the ground state, the proton is lost from the carboxyl group (as expected) while the red shift of the fluorescence, upon dissociation, indicates that in the 'Lb state, the proton is lost from the hydroxyl group forming the excited singly charged anion (A'). That the ground state pKa is obtained even for'the latter process, suggests that proton exchange with the solvent is static ( i e . , too slow to compete with fluorescence for deactivation of the excited state) but that subsequent to excitation of (A) intramolecular proton transfer from the hydroxyl group to the carboxyl group must occur within the lifetime of the 'Lb state so that emission originates from (AI). Supporting this hypothesis, the fluorescence of 3-methoxy-2-naphthoic acid, which has no hydroxylic proton, fluoresces a t 2.36 X l o 4 cm-1 a t p H 1 and a t 2.53 X l o 4 cm-1 a t p H 6, thereby showing the anticipated shift of the fluorescence to higher frequency upon dissociation of the neutral molecule from the carboxyl group. In the pH region 7 to 11, a high frequency fluorescence band appears a t 2.36 X l o 4 cm-1, in addition to the fluorescence band arising from (A') a t 1.96 X l o 4 cm-1. No change in the absorption spectrum is observed in this p H interval. Further increase in pH to 1.5.2 results in the disappearance of the bands a t 1.96 X l o 4 cm-1 and 2.36 X l o 4 cm-1 and a new intense fluorescence band a t 2.05 X l o 4 cm-1 appears. The midpoint of the fluorimetric titration of the latter band lies a t pH 13.0 and coincides with the absorptiometrically observed conversion of the singly charged anion (A) to the doubly charged anion (D) in the ground state. The high frequency fluorescence band a t 2.36 x l o 4 cm-1 is assigned to the excited singly charged anion (A) while the fluorescence band at 2.05 X l o 4 cm-1 is assigned t o the excited doubly charged anion (D). That the conversions of the fluorescences of the excited singly charged anions (A) and (A') to the fluorescence of (D) occur in the same region as the absorptiometrically observed conversion of the ground state of (A) to (D) indicates that excited (A) and (A') are formed only by excitation of ground state (A) and the pKa of 13.0 corresponds
to the ground state conversion of (A) to (D). However, the absence of absorption spectral changes between p H 7 and 11 suggests that the appearance of the excited anion (A) is a result of some change in the reactivity of the excited anion (A) as a result of high pH. This might be the result of intermolecular hydrogen bonding in the excited state, of the hydroxylic proton with the hydroxide ion, preventing the transfer of the hydroxylic proton to the carboxyl group. However, the absorption spectral shifts accompanying dissociation of the carboxyl and hydroxyl groups are small so that absorption shifts accompanying ground state intramolecular hydrogen bonding may be too small to be observed. Thus the possibility that ground state intermolecular hydrogen bonding of the hydroxide ion with the hydroxyl proton may be kinetically responsible for the failure of some of the anion (A), which is excited, to phototautomerize cannot be immediately ruled out. In n-hexane, n-hexane containing 2% (v/v) trifluoroacetic acid, and n-hexane containing 2% (v/v) morpholine, 3-hydroxy-2-naphthoic acid fluoresces a t 2.47 X 104 cm-1, 2.25 X 104 cm-1, and 1.90 X l o 4 cm-1, respectively. These emissions are believed to be due to the neutral molecule (N), the cation (C), and the singly charged anion (A1), respectively. The differences in emission frequencies of these species in hexane relative to water are comparable to the solvent dependences of the fluorescences of the corresponding species derived from salicylic acid ( 2 ) . Regarding the possibility that the long wavelength emission in basic hexane might originate from the excited zwitterionic species (z) as a result of intermolecular hydrogen bonding [as proposed by Ware et al. ( 4 ) ] ,the fluorescence maximum of 3-hydroxy-2-naphthoic acid in n-hexane containing 2% (v/v) 1,4-dioxane lies a t 2.43 X 104 cm-1, only slightly lower than that in neutral hexane. Dioxane is a hydrogen bond acceptor solvent (as are morpholine and pyridine) and has been found to affect the fluorescence spectra of phenolic compounds (7). However, dioxane, unlike morpholine (or pyridine), is not sufficiently basic to completely accept a proton from 3-hydroxy-2naphthoic acid in the ground state. Consequently, it is proposed that the static dependence of the fluorescence of 3-hydroxy-2-naphthoic acid in low-dielectric media on added base ( 4 ) is the result of formation of the excited anion (A1) by phototautomerization of the excited anion (A) formed as a result of the ground state dissociation of the neutral 3-hydroxy-2-naphthoic acid molecule. This conclusion is consistent with the spectroscopic behavior of 3-methoxy-2-naphthoic acid and methyl 3-hydroxy-2naphthoate as reported by Hirota ( 3 )and Ware et al. ( 4 ) . These experiments show that there are substantial differences between the excited state prototropic behaviors of salicylic acid ( 2 ) and 3-hydroxy-2-naphthoic acid
even though the ground state properties of both molecules are similar, reflecting intramolecular hydrogen bonding in the low PKa values of the neutral molecules (relative to benzoic acid and 2-naphthoic acid), the high pKa values of the singly charged anions (A) (relative to phenol and 2-naphthol), and the small p H dependences of the absorption spectra (5). The small p H dependences of the absorption spectra of both molecules also indicate that the properties of the Franck-Condon excited states of the corresponding prototropic species derived from salicylic acid and 3-hydroxy-2-naphthoic acid are similar. The differences in excited state behavior between salicylic acid and 3-hydroxy-2-naphthoic acid may be summerized as follows: (1) dissociation of the excited salicylic acidium cation from the phenolic group and the excited 3-hydroxy-2naphthoic acidium cation from the protonated carboxyl group; (2) phototautomerism of excited salicylic acid but not of 3-hydroxy-2-naphthoic acid to the excited zwitterion; (3) complete phototautomerism of the excited salicyIate anion (A) to the excited anion ( A I ) and partial phototautomerism of the excited 3-hydroxy-2-naphthoate anion (A) to the excited anion (A1); (4) dynamic dissociation of the excited salicylate anion ( A I ) to the excited doubly charged anion (D) and static conversion of the 3-hydroxy2-naphthoate anion (A) to the doubly charged anion (D). The differences between the excited state prototropic reactivities of salicylic acid and 3-hydroxy-2-naphthoic acid must obviously originate with the presence of the linearly annellated ring of the naphthalene derivative. However, we find it difficult to account for these differences on a thermodynamic basis. For example, the observed acidity dependences of the fluorescence maxima clearly indicate that in the excited state, the carboxyl group is more basic and less acidic and the hydroxyl group more acidic in both compounds. The cations of both molecules have comparable ground state pK, values, yet the excited cation derived from salicylic acid dissociates from the phenolic group while that derived from 3-hydroxy-2-naphthoic acid dissociates from the protonated carboxyl group. Moreover, the spectral shifts observed in aprotic solvents indicate that these modes of dissociation are independent of solvent. Perhaps the most dramatic difference, structurally, between salicylic acid and 3-hydroxy-2-naphthoic acid is in the vibrational composition of the excited states. If it is considered that the transition states through which the participants in the various prototropic reactions pass may be thought to be derived from vibrationally excited states of the lowest electronically excited singlet states, this may account for the differences in prototropic reactivity between salicylic acid and 3-hydroxy-2-naphthoic acid. In this regard, kinetic studies of these reactions should be enlightening.
(7) S. Nakagura and H. Baba, J. Arner. Chern. SOC.,74, 5693 (1952).
Received for review July 14, 1972. Accepted January 31, 1973.
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