3639
J . Phys. Chem. 1990, 94, 3639-3643
these products also correlate with the lowest lA” surface. In the case-of NH(a) N2 the analogous chemical channel to form N3(X) H is endothermic. At higher pressures or in the condensed phase, HNCO(%) can be expected to be the main reaction product of NH(a) + CO. HNCO was indeed isolated as the major product after photolysis of HN3 in a CO matrix at 4, 14, and 24 K.I9 The thermal decomposition of HNCO is reported to lead to NH(X) + CO with an activation energy of E = 402 kJ/mol, which is noticeably greater than the heat of the reaction HNCO NH(’Z) + C0.20 It can be concluded that the reaction NH(a) + C O proceeds via an HNCO(’A’,’A’’) complex which is formed without any activation barrier and decomposes mainly into NCO(X) H and which, to a minor fraction, undergoes intersystem crossing to form NH(X) + CO. The reactions of the isoelectronic species O(ID) and CH2(3) with CO have been studied intensively. For the depletion of O(lD) by CO the absolute rate constant was found to be k = 3.5 X 1013 cm3/(mol.s) at 300 KS2’ An isotope-labeled experiment
+
+
-
+
- - 3co,* -+
I ~ O ( I D+) c ~ * o ( x ) t o 2 *
-
c16o(v)
cl8o(v)
+ 160(3~)
lso(3p) (50%)
(50%)
and the vibrational population distribution of CO(v) clearly indicate that the quenching takes place via a complex mechanism, which involves a singlet ( 1C02*)-triplet (3C02*) crossing2* whereby ICO2* may be formed via the ‘A’ surface in its electronic IZ: ground state or in four other electronic singlet states. +he temperature dependence of the rate of the reaction O(lD) with C O was examined by Davidson et al.,2’ who found an Ar+
(17) Bradley, J . N.; Gilbert, J. R.; Svejda, P. Trans. Faraday SOC.1968, 64, 91 I . (18) Spiglanin, T. A.; Perry, R. A.; Chandler, D. W. J. Chem. Phys. 1987, 87, 1568, and references therein. (19) Milligan, D. E.; Jacox, M. E. J . Chem. Phys. 1964, 41, 2838. (20) Kajimoto, 0.;Kondo, 0.; Okada, K.; Fujikane, J.; Fueno, T. Bull. Chem. SOC.Jpn. 1985, 58, 3469. (21) Davidson, J. A.; Schiff, H. I.; Brown, T. J.; Howard, C. J. J. Chem. Phys. 1978,69, 1216. (22) Shortridge, R. G.;Lin, M. C. J . Chem. Phys. 1976, 64, 4076.
rhenius expression with a small negative activation energy in the range 113 I T/K I333: k ( T ) = 2.8 X 1013 exp(527/RT) cm3/(mol.s); ( E Ain J/mol). These isoelectronic reactants behave in a way very similar to the reactants NH(a) + CO studied in this work. The reaction of CH2(B) with CO has been intensively studied.23-2s It proceeds with k = 3.0 X lo1’ cm3/(mol.s), which is even faster than NH(a) CO. From the deviation of the data from a linear Parmenter-Seaver correlation plot for the collisional removal rates, the authors suggest that a main fraction of the CH2(1) depletion is due to chemical reaction.24 In the reaction of 14CH2(Z)with CO, an isotope carbon atom exchange to form I4CO was observed.26 From that the authors conclude that an oxirene adduct is formed as an intermediate product. Approximately half of the initially formed excited oxirene complex is stabilized at a CO pressure of 1120 mbar.26 In a flow syste-m experiment at low pressures, in which the formation of CH2(X) in the reaction CH2(B) + CO was followed by LMR, it was found that physical quenching was the main channel under those condition~.~’ The reaction of the imino radical in the higher excited NH(b’Z+) state with C O is slower than the reaction NH(a) C O and has k = 7.8 X lo8 ~ m ~ / ( m o l . s ) .This ~ ~ is not surprising, taking into consideration that the reactions of NH(b) are in general slower ( lo3) than those of NH(a).
+
+
N
Acknowledgment. We are greatly indebted to Prof. Dr. H. Gg. Wagner for his generous support and stimulating interest. Financial support of the Deutsche Forschungsgemeinschaft SFB 93 is acknowledged. Registry No. NH, 13774-92-0; CO, 630-08-0. (23) Langford, A. 0.;Petek, H.; Moore, C. B. J . Chem. Phys. 1983, 78, 6650. (24) Ashfold, M. N. R.; Fullstone, M. A.; Hancock, G.;Ketly, G.W. Chem. Phys. 1987, 55, 245. (25) Laufer, A. H.; Bass, A. M. J. Phys. Chem. 1974, 78, 1344. (26) Montague, D. C.; Rowland, F. S . J . Am. Chem. SOC.1971,93,5381. (27) Koch, M. MPI fur Stromungsforschung Gottingen, Report 22, 1988. (28) Gelernt, B.; Filseth, S. V.; Carrington, T. Chem. Phys. Left. 1975, 36, 238. (29) Hofzumahaus, A,; Stuhl, F. J. Chem. Phys. 1985, 82, 3152.
Ground-State Lactim-Lactam Equilibrium and Excited-State Proton Transfer of Methyl 2-Hydroxy-6-met hyinicotinate Pi-Tai Chou,* Marty L. Martinez, and Shannon L. Studer Department of Chemistry, University of South Carolina, Columbia, South Carolina 29208 (Received: August 31, 1989; In Final Form: November 2, 1989) The excited-stateintramolecularproton-transfercompound methyl 2-hydroxy-6-methylnicotinate (MHMN) has been investigated. The ortho position of the hydroxyl group with respect to the pyridine nitrogen for MHMN results in significantly different molecular properties than the molecule that has the hydroxyl group in the meta position. The ground-state lactim-lactam equilibrium constant for MHMN is calculated to be 0.065 f 0.006 in n-hexane by the absorption measurement. Multiple fluorescences at room temperature were recorded consisting of lactam-form normal emission and lactim-form normal and tautomer emissions. An excited-state intramolecular proton-transfer rate of >>1.0 X 10” s-I for the lactim form is deduced from picosecond fluorescence measurements at room temperature.
Introduction The multiple fluorescences of methyl salicylate (MS) and its derivatives have been studied extensively by both steady-state and time-resolved spectroscopic techniques.”-16 Both in the gas phase ( I ) Weller, A. Z . Elecfrochem. 1956, 60, 1144. (2) Beens, H.; Grellmann, K. H.; Gurr, M.; Weller, A. H . Discuss. Faraday Sor. 1965, 39, 183. (3) Sandros, K . Acta Chem. Scand., Secf. A 1976, 30, 761.
0022-3654/90/2094-3639$02.50/0
and in nonpolar solvents, the fluorescence spectrum consists of two bands: one exhibiting a normal Stokes shift with a band (4) Smith, K. K.; Kaufmann, K. J. J . Phys. Chem. 1978,82, 2286. ( 5 ) Kloepffer, W.; Kaufmann, G.J . Lumin. 1979, 20, 283. (6) Acuna, A. U.; Amat-Guerri, F.; Catalan, J.; Gonzalez-Tablas, F. J . Phys. Chem. 1980,84, 629. (7) Ford, D.; Thistlethwaite, P. J.; Woolfe, G. J. Chem. Phys. Letf. 1980, 69. 246.
0 1990 American Chemical Society
3640 The Journal of Physical Chemistry, Vol. 94, No. 9, 1990
Chou et al. SCHEME I: Schematic Diagram of4he Dynamics of Each Conformer for MHMN in n-Hexane Solution in the Ground and Excited States II
H
B
A
/
\
II
,,/'\R
H
b
e
o
\
-
H
lactam'
hu,'
D
hu' (385
!I"
Figure 1. (a) Conformers A-C: Ro = H,R I= C , methyl salicylate; Ro = CH3, R, = N, methyl 2-hydroxy-6-methylnicotinate.(b) Conformer
D: 3-hydroxypicolinamide.
maximum at -340 nm; the second has a large fluorescence shift and a band maximum at -450 nm. Weller'v2 first correctly concluded that the long-wavelength fluorescence arises from a tautomeric form of the molecule that results from a proton-transfer process in an electronically excited state. On the basis of the different excitation spectra for the 450- and 340-nm emissions, Sandros first suggested that the dual luminescence was the consequence of a ground-state equilibrium involving two configurational isomers that do not equilibrate in the singlet excited-state m a n i f ~ l d .This ~ interpretation has been supported by numerous experiment^,^-^ and it is now generally concluded that in the gas phase and in hydrocarbon solvents conformer A (Figure la), in which the hydroxyl proton is H bonded to the carbonyl oxygen, undergoes a rapid, excited-state intramolecular proton transfer, giving rise to the long-wavelength fluorescence. In contrast, conformer B is incapable of executing proton transfer in the excited state and is responsible for the normally Stokes-shifted emission. In H-bonding solvents, a more complex equilibrium involving externally H-bonded species analogous to conformer C must be considered. Most recently, Nagaoka et aI.,l7 using the symmetry properties of benzene-like S (nn*) wave functions, have successfully demonstrated that the excited-state intramolecular proton transfer takes place in the S l ( m * ) state for o-hydroxybenzaldehyde, whereas proton transfer is energetically unfavorable in the S2 (m*) state. Since the n-conjugated system for MS and related molecules is analogous to that of o-hydroxybenzaldehyde, similar behavior is expected. It is thus of theoretical and experimental interest to study the mechanism of the excited-state intramolecular proton transfer based on a heteromolecular n-conjugated system. Recently, we reported the spectroscopy and dynamics of the excited-state intramolecular proton-transfer reaction of 3-hydroxypicolinamide (3HP, Figure 1b)'* in which the O H group is in the meta position with respect to the nitrogen atom in the pyridine ring. The results show that the nitrogen in the pyridine ring has a significant effect on the proton-transfer spectroscopy of 3HP. A rapid, temperature-independent, intramolecular proton transfer was observed (8) Catalan, J.; Toribio, F.; Acuna, A. U. J . Phys. Chem. 1982,86, 303. (9) Heimbrook. L. A.; Kenny, J. E.; Kohler, B. E.; Scott, G . W. J . Phys. Chem. 1983, 87, 280. (IO) Goodman, J.; Brus, L. E.J . Am. Chem. SOC.1978, 100, 7472. ( 1 1 ) Thistlethwaite, P. J.; Woolfe, G. J. Chem. Phys. Left. 1979, 63, 401. (12) Woolfe, G. J.; Thistlethwaite, P. J. J . Am. Chem. SOC.1980, 102, 6917. (13) Barbara, P. F.; Rentzepis, P. M.: Brus, L. E. J . Am. Chem. Soc. 1980, 102, 2786. (14) Toribio. F.: Catalan, J.; Amat, F.; Acuna, A. U. J . Phys. Chem. 1983, 87, 817. ( 1 5) Sanchez-Cabezudo, M.; G. De Paz, J. L.; Catalan, J.; Amat-Guerri, F. J. Mol. Srrucr. 1985. 131, 277. (16) Nishiya. T.; Yamauchi, S . ; Hirota, N.; Baba, M.; Hanazaki, I . J. Phys. Chem. 1986. 90, 5730. (17) Nagaoka, S.; Nagashima, U.; Ohta, N.; Fujita, M.; Takemura, T . J . Phys. Chem. 1988, 92, 166. (18) Studer. S. L.; McMorrow. D.;Chou, P. T . Chem. Phys. Left.,in press.
Y .
actim tautomer
in the excited state. Conformer A of 3HP, which is the precursor for the excited-state proton transfer, is stabilized by two intramolecular hydrogen bonds, the hydroxy-carbonyl H bond and a H bond involving the pyridine nitrogen and one of the amide protons. Thus, a unique tautomer emission is observed in aprotic solvents at room temperature as well as at 77 K. It is noted that unlike methyl salicylate where only one position of the hydroxyl group is possible, the hydroxyl proton in 3HP can be in the ortho, para, or meta position with respect to the pyridine nitrogen, possibly resulting in a significant difference in the mechanism of excited-state proton transfer. In this report, the compound methyl 2-hydroxy-6-methylnicotinate (MHMN) has been investigated by steady-state absorption, emission, and picosecond time-resolved fluorescence spectroscopies. A strongly solvent-dependent ground-state equilibrium between the lactam and lactim forms was observed. Multiple fluorescence spectra were recorded in n-hexane, consisting of lactam-form normal emission and lactim-form normal and tautomer emissions. In pure methanol, only lactam-form fluorescence was observed. In the excited state, the interconversion between lactim and lactam forms in nonpolar solvents is concluded to be negligible. Experimental Section Steady-state absorption spectra were recorded by a H P Model 8452A spectrophotometer. A Shimadzu spectrofluorometer (Model RF5000U) was used to record the steady-state fluorescence spectra. For measuring the phosphorescence at 77 K by using a continuous wave (CW) light source, a mechanical chopper operated at 10-250 Hz was placed outside the sample tube with two windows at 90". Alternatively, when the light source is the YAG laser (266 nm), an intensified photodiode array (EG&G Model 1420R) coupled with a Spex Model 1870 polychromator was applied. Quantum yield measurements were performed by comparing the emission intensity with that of methyl salicylate (a = 0.022 in cy~lohexanel~).Picosecond experiments were performed using the fourth harmonic of a Quantel active/passive mode-locked ND:YAG laser operating at 10 Hz. The fluorescence was collected through a CS, optical Kerr shutter activated by the fundamental laser output at 1064 nm, which was passed through a variable optical delay. The transmitted fluorescence was passed through a monochromator (5-nm band-pass) and detected with a 1 P28 photomultiplier tube. The fluorescence rise and decay curves presented here are typically the average of ten measurements. Data analyses were manipulated by an IBM AT with ASYST software.
Lactim-Lactam Equilibrium and Proton Transfer of MHMN
c
a
0.08
The Journal of Physical Chemistry, Vol. 94, No. 9, 1990 3641
4
a,
2
0.05
2
s:
nm
200
300
400
500
WAVELENGTH , nm Figure 3. Room-temperature luminescence spectra of MHMN (2.1 X M) in n-hexane at different wavelengths: (a) 290 and (b) 330 nm.
0.0
Excitation spectra of MHMN in n-hexane monitored at different emission wavelengths: (a’) 450, (b’) 385, and (c’) 325 nm at room temperature.
Figure 2. Room-temperature UV absorption spectra of MHMN in nhexane with varying methanol concentrations: (a) no methanol; (b) 0.04, (c) 0.06.(d) 0.08, (e) 0.1, (f) 0.14, and (g) 1.0 M.
MHMN and methyl 2-methoxy-6-methylnicotinate (MMMN) were synthesized according to ref 19. All solvents used were of spectroscopic quality; methylcyclohexane (MCH, Aldrich) and n-hexane (Fisher) were dried by fractional distillation from lithium aluminum hydride; ether (Fisher) and methanol (Fisher) were fractionally distilled and used immediately.
Results An equilibrium between lactim and lactam forms (see Scheme I) of MHMN was easily observed in the UV absorption spectrum. In a nonpolar hydrocarbon solvent such as n-hexane, the absorption maximum was observed at 300 nm, and a small, nonnegligible long-wavelength absorption tail appeared around 330-360 nm (Figure 2a). This absorption tail gradually increased as the degree of external hydrogen bonding from the solvents was increased. Upon incremental addition of small amounts of methanol (0.01-0.1 4 M, Figure 2b-f), the long-wavelength absorption intensity gradually increased and the 3Wnm band decreased. Above a concentration of 1.O M methanol, the change in the absorption spectrum is negligible with a maximum at 330 nm. An isosbestic point at 3 12 nm appeared during titration, indicating the presence of two species in a ground-state equilibrium. In pure methanol, MHMN showed a single absorption maximum at 330 nm. As the next step in the investigation, MMMN was examined. Since the hydroxyl group has been substituted by a methoxyl group, only the lactim form exists in the solution. The So Si (m*) absorption maximum of 292 nm and spectral profile (fwhm 3800 c d ) are very similar to that of the MHMN absorption spectrum in n-hexane. Thus, it is reasonable to suggest that the 300-nm maximum band observed in n-hexane is attributed to the absorption of the MHMN lactim-form and the 330-nm band is assigned to the MHMN lactam-form absorption. The energy difference of -913 cm-’ for MMMN in comparison to MHMN (difference of -8 nm in the absorption maximum) is indicative . of the existence of a strong, intramolecular H bond between the hydroxyl proton and carbonyl oxygen in MHMN. Multiple fluorescences were observed for MHMN in n-hexane. The fluorescence profile and maximum were changed significantly when excited at different wavelengths (Figure 3a,b). To further characterize the emitting species, excitation spectra were examined (Figure 3a’-c’). The excitation spectra monitored at the longwavelength emission region (450 nm, Figure 3a’) and the margin of the short-wavelength emission region (325 nm, Figure 3c‘) were found to be similar except for a slight difference between the
-
-
-
~~~
~~
~
(19) Shimizu, K.; Sakamoto, 1.; Fukushima, S . Yakugaku Zasshi 1967, 87, 672. (20) Katritzky, A. R. Physic. Methods Heterocycl. Chem. 1962, 1 (21) Albert, A.; Phillips, J. N. J . Chem. SOC.1956, 1294.
5 z w
+ m
z W
5 w
[I
0
300
500
400
WAVELENGTH, nm Figure 4. Room-temperature fluorescence of MHMN in ethyl ether with varying methanol concentrations: (a) no methanol, (b) 0.01 M, (c) 0.05 M, and (d) 0.1 M. Excitation spectra of MHMN (2.4 X M) in ethyl ether monitored at different emission wavelengths: (a’) 390, (b’ ) 460, and (c’) 327 nm.
excitation maxima, 305 and 297 nm, respectively. Excitation maxima at 308 and 335 nm were observed when the emission intensity was monitored at 385 nm (Figure 3b’), consistent with the absorption maxima of the lactim and lactam forms, respectively. Since the compound MMMN, which is considered to have no proton-transfer reaction in the excited state, only gives a very weak emission maximum at -326 nm in MCH, the observed short-wavelength emission at 320-350 nm in MHMN is attributed to the lactim-form normal emission (B*,Scheme I, asterisk denotes the excited state). Due to the similarity of the excitation spectra between the 450-nm emission and the lactim normal emission (B*), the emission around 450 nm is concluded to be mainly derived from the lactim-form species, which undergoes proton transfer in the excited state, giving a lactim tautomer emission. On the basis of the excitation spectrum, it is apparent that the emission in the region of -385 nm is attributed to the combination of lactim tautomer and lactam emissions. When monitored at 460 nm, the lifetime of the lactim tautomer emission is calculated to be 158 f 20 ps (Aex = 266 nm), the rise time ultrafast and beyond the system response time, which is measured to be -10 ps. An attempt to measure the lifetime of lactam form in n-hexane excited at 355 nm was not successful due to the low absorbance at this wavelength. In addition, the Kerr cell solvent CS2 has an absorption cutoff at 350 nm. This makes the kinetic measurement of the normal emission at 330 nm impossible. In methanol solution MHMN exhibited a single, strong lac0.15). The tam-form fluorescence maximum at 385 nm (@ lifetime of the emission is measured to be -2.3 ns while the rise time is unresolved (>>I .O X 10” s-l). In ethyl ether solvent, the absorbance ratio of lactim (300 nm) to lactam (330 nm) is approximately 3.1. Figure 4a shows the emission spectrum of
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3642
The Journal of Physical Chemistry, Vol. 94, No. 9, 1990
MHMN in ethyl ether when excited at 300 nm. Since the lactam form has a much higher quantum yield than the lactim form, it is difficult to resolve the lactim tautomer emission from the lactam-form emission. However, the excitation spectral features still showed a slight difference when the emission was monitored at shorter (390 nm) and longer (460 nm) wavelengths (Figure 4a',b'). The emission spectrum monitored at 460 nm gives more contribution to the excitation spectrum at -300 nm (region I) than that monitored at 390 nm, indicating the existence of lactim tautomer emission. By methanol being incrementally added into the ethyl ether solution, gradual diminution of the 327-nm emission band was observed accompanied by an increase of the lactam-form emission (Figure 4b-d). Hence, the 327-nm emission is apparently derived from B*. When MHMN was excited at the absorption region of 330 nm in 77 K MCH glass a fluorescence maximum at 385 nm as well as a phosphorescence maximum at 480 nm with a lifetime of 1.8 s was observed. The excitation spectra at 385 and 480 nm are identical with the excitation maximum at 330 nm. Since the same fluorescence and phosphorescence were observed in 77 K MeOH glass in which the lactam form dominates, the fluorescence and phosphorescence observed in 77 K MCH are apparently derived from the lactam species. The lactim tautomer emission that was observed at 417 nm (Aex = 290 nm) at room temperature cannot be resolved at 77 K. It is noted that a 410-nm phosphorescence with a lifetime of 1 .O s is also observed when excited at C300 nm. This emission is significantly reduced by further careful purification. Since a strong phosphorescence maximum at 410 nm with a lifetime of 1.2 s was observed for MMMN in 77 K MCH glass, the 410-nm phosphorescence observed in MHMN is likely due to the emission from MMMN. This can be rationalized since the esterification of 2-hydroxy-6-methylpyridine-3-carboxylic acid may be accompanied by methylation of the hydroxyl group during synthesis.
-
-
-
Discussion Ground-State Lactim-Lactam Tautomerism. In our previous paper,'* it was shown that in 3HP, which is similar to 3hydro~ypyridine,~~-~~ no keto form exists in either aprotic or protic solvents. In contrast to 2 - h y d r o ~ y p y r i d i n e which , ~ ~ ~ ~appears ~ to exist predominantly in the keto form in both polar and nonpolar solvents, equilibrium between the lactim and lactam forms of MHMN is strongly solvent dependent. Our results show that MHMN mainly exists as the lactim form in nonpolar solvents, indicating stabilization of the lactim form (A and B) by the formation of an intramolecular H bond. In H-bonding solvents such as methanol, the intramolecular H bond is superseded by an external H bond formed with methanol. Since the lactam form is proven to be the thermally stable species by the low-temperature fluorescence study, the lactim-lactam tautomerization catalyzed by intermolecular H-bonding solvent favors the lactam form. The lactim-lactam equilibrium in n-hexane catalyzed by methanol can be expressed as ki
MHMNlactim(C,) + n[MeOH] Cr. k
MHMNIactim[MeOHIn(C2) 1. k
M H M N I ~ , , ~ ~ [ M ~ O H I , (A C,) MHMNlactam(C4) + n[MeOHI where k, = C2/(C,[MeOH]") k2 =
c,/c2
(1) (2)
and k3 = (C4[MeOH]")/C3
(3)
(22) Kitagawa, T.; Mizukami, S.; Hirai, E. Chem. Phorm. Bull. (Tokyo) 1974, 22, 1239. (23) Lardenois, P.; Selim, M. Bull. Sot. Chim. Fr. 1971, 5 , 1858. (24) Bauer, L.; Wright, G. E.; Mikrut, B. A,; Bell, C. L. J . Heterocycl. Chem. 1965, 2, 447.
Chou et al.
a
c k'
2.7 0
.
9
0.9
F
I
,
,
,
2.7
4.5
6.3
8.1
MeOH2 x 102 Figure 5. Plot of A / ( & - A ) versus [MeOHI2at 340 nm with varying methanol concentrations from 0.01 to 0.28 M: (a) the best fit curve from 0.01 to 0.08 M using eq 6 and (b) the best fit curve from 0.01 to 0.28 M using eq 7 .
This expression is similar to that proposed by Kitagawa et al.25 for the compound 2-hydroxynicotinate (2HN). However, their assumption that only 2HN (lactam-methanol) complex exists is not applied in our equation. Instead, we propose the existence of an equilibrium between the lactim-methanol and lactammethanol associated forms. Since only one isosbestic point at 312 nm was observed, the molar extinction coefficients for the lactim, lactim-methanol complex pair as well as the lactam, lactammethanol complex pair are proposed to be identical. Thus, the measured absorbance at a selected wavelength is given by [(C,
+ C&, + (C, + C4)€2]l= Cod
(4)
where the optical path length I is 10 mm and t l and ez are the molar extinction coefficients of the lactim and lactam forms, respectively. t is the apparent molar extinction coefficient for the solution and Co is the total concentration of the solution. If [I] = C, + C2 and [II] = C3 + C4,the ratio of the lactam form ([II]) to that of the lactim form ([I]) can be expressed by [III/[Il = ( A - AI)/(AII - A ) = [k, + (k,/k3)[MeOHln1/[(1/k2k3)(l + k,[MeOHI"l (5) where AI is the absorbance when the equilibrium is completely shifted to the lactim form (I), AII is the absorbance when it is completely shifted to the lactam form (11), and A is the apparent absorbance. Since the equilibrium favors the lactam form upon addition of methanol, it is reasonable to assume that k2 > k l . When the amount of added methanol is small (e&, CO.1 M), k,[MeOH]" = C2/C, is assumed to be
