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Spectrochimica Acta Part A 69 (2008) 350–360
Amino–imino tautomerization reaction of the 4-aminopyrimidine/acetic acid system Teruyoshi Kitamura a , Masahiro Okita a , Yutaka Sasaki b , Hironori Ishikawa b , Akira Fujimoto a,∗ b
a Department of Environmental Materials Science, Tokyo Denki University, Kanda, Chiyoda-ku, Tokyo 101-8457, Japan Mitsubishi Chemical Group Science and Technology Research Center, Inc., Kamoshida-cho, Aoba-ku, Yokohama 227-8502, Japan
Received 19 December 2006; accepted 12 April 2007
Abstract The amino–imino tautomerization of the 4-aminopyrimidine (4APM)/acetic acid (AcOH) system through dual hydrogen bonding in n-hexane at room temperature was investigated using UV absorption and fluorescence spectroscopies, fluorescence time-profile measurements, and molecular orbital calculations, with those of the imino-model compound of 3-methyl-4(1H)-pyrimidinimine (3M4PMI). From the experimental results, it was confirmed that the imino-tautomer was formed in the first excited singlet state (S1) state through the double-proton transfer of the dual hydrogen-bonded complex of 4APM with AcOH. The fluorescences of the free 4APM monomer (band maximum at 353 nm), imino-tautomer (at 414 nm), and 3M4PMI monomer (at 437 nm) exhibit the single-exponential decays of 98, 73, and 19 ps time constants, respectively. The shorter decay time of the imino-tautomer fluorescence compared with the free monomer one is suggestive of the low activation energy process in the S1 state. From the result of the shortest decay time of the 3M4PMI fluorescence, it can be deduced that 3M4PMI has a non-planar structure in the S1 state. The theoretical calculations on the S0 and S1 state double-proton transfer for the 4APM/AcOH dual hydrogen-bonded system were performed with the use of formic acid (FoOH) in place of AcOH for the sake of simplicity. Only one peak of transition state was resolved in the S0 and S1 states. The energy barrier for the S1 state double-proton transfer of the 4APM/FoOH complex → 3H-4(1H)-pyrimidinimine/FoOH tautomer was estimated to be ∼2 kJ mol−1 using the CIS/6-31G(d) methods. On the other hand, the energy barrier for the S0 state reverse proton transfer of the 3H-4(1H)-pyrimidinimine/FoOH tautomer → 4APM/FoOH complex was estimated to be almost zero kJ mol−1 at B3LYP/6-31G(d) level. © 2007 Elsevier B.V. All rights reserved. Keywords: 4-Aminopyrimidines; Amino–imino tautomerization; Proton transfer; Fluorescence spectra
1. Introduction Proton transfer reaction is one of the simplest and most fundamental reactions whose processes include an acid–base reaction—the hydrogen atom exchange reaction of water in chemistry [1]. Moreover, this elementary reaction plays a crucial role in biology [2]. The excited state proton transfer [3–6] is especially interesting from the viewpoint of photochemistry. The 7-azaindole (7AI) dimer system has been studied experimentally [7–17] and theoretically [18–20] by many researches, as a prototype of excited state double-proton reaction. The 7AI dimer system has been regarded as a model system of the photoinduced
∗
Corresponding author. Tel.: +81 3 5280 3397; fax: +81 3 5280 3570. E-mail address: [email protected] (A. Fujimoto).
1386-1425/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2007.04.006
mutation of the DNA base pairs [7]. This assumption is in need of attention as it is connected with the threat to life as resulting from the ozone layer depletion; one of the major causes of environmental destruction on the earth [21]. This kind of excited-state double-proton transfer reaction process has been observed in many other molecule systems, for example, excited state double-proton transfer reaction controls the catalyst mechanism caused by alcohol or ammonia [22,23]. Particularly, biochemical interests are processes involving nitrogen-containing heterocycles molecules. Amino–imino tautomerization processes facilitated by proton transfer reaction in heterocycles molecules have been the subjects of a number of studies [24–30]. These processes are relevant to many areas of biological chemistry. The physicochemical properties of aminopyridines and aminopyrimidines have received considerable attention from both experimental [30] and theoretical
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research [29], due to their importance in understanding many fundamental biochemical processes. 4-Aminopyrimidines are constituents a part of vitamin B1 thiamine molecules [31,32]. Recently, Baykal et al. noted that the 4-aminopyrimidine moiety of thiamine diphosphate undergoes tautomerization of 1,4-iminomeric form during the catalytic cycle of the enzymes. However, it may be considered that 3,4iminomeric form produced by the hydrogen-bonded interaction of the 4-aminopyrimidine moiety with the carboxyl group of glutamic acid and/or aspartic acid is also important for the catalytic cycle of the enzymes in the first singlet excited (S1) state. Therefore, we thought that investigating the amino–imino tautomerization reaction of the simplified 4-aminopyrimidine molecules would also give the key to understanding one step of enzyme reaction activity in the mechanism of the vitamin B1. However, the tautomeric reaction of the 4-aminopyrimidine (4APM)/acetic acid (AcOH) system in which the aminopyrimidine molecular formation of cyclic complexes with AcOH, lead to S1 state tautomerizaton involving the movement of two protons and the formation of the imino tautomeric form, has not yet been reported. We investigated the amino–imino tautomerism reaction of 4APM using the bifunctional catalyst [28,30] of an AcOH molecule and compared it with the amino- and iminomodel compounds of 4-dimethylaminopyrimidine (4DMAPM) and 3-methyl-4(1H)-pyrimidinimine (3M4PMI) in non-polar solvent, respectively. In this study, we will report the first investigation of the 4APM/AcOH system in n-hexane. The following sections are organized according to a sequence of steps. We first performed absorption spectroscopies of the 4APM hydrogen-bonded complex with AcOH and determined the equilibrium constants. The second step involved steady-state fluorescence measurements to examine the excited state proton transfer properties. The proton transfer species of 4APM has been determined through syntheses and spectral characterization of their corresponding methylated derivatives. The third step, the experimental results were picosecond fluorescence time-profile measure-
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ments. Finally, the theoretical calculations on the ground (S0) and S1 double-proton transfer through the 4APM/AcOH dual hydrogen-bonded system were performed to make clear the amino–imino tautomerization process in the S1 state. 2. Experimental 2.1. Materials The 4APM (Aldrich Co.) was recrystallized several times from spectrograde chloroform and subsequently dried in a vacuo before use. Purities were checked. The fluorescence excitation spectrum of 4APM in n-hexane was measured for several excitation wavelengths. The AcOH (KANTO Chemical) was made of atomic absorption spectragrade and used without further purification because no fluorescence was detected in the wavelength region of interest. The n-hexane (KANTO Chemical) was of spectragrade and used without further purification. Twice distilled, deionized water was used for the preparation of aqueous solution. Sulfuric acid (KANTO Chemical) was special grade and used without further purification. 4-Dimethylaminopyrimidine (4DMAPM) was synthesized according to the methods described by Brown and Short [33]. The products were purified by twice vacuum distillation. The preparation of 3-methyl-4(1H)-pyrimidinimine (3M4PMI) was performed using a modified version of the Brown and Lenega method [34], in the following manner: as shown in Scheme 1, malononitrile (Acros Organics) dissolved in a mixture (4:1) of dry ether (KANTO Chemical) and dry tetrahydrofuran (THF; KANTO Chemical) was reduced to 3-aminoacrylonitrile (red oily material) by lithum aluminium hydride (KANTO Chemical) at 0 ◦ C. The mixture was stirred for 2 h, after water and sodium hydroxide solution (30%) was added carefully, and the mixture was stirred for a further 6 h. The mixture was then filtered. The filtrate was dried using CaCO3 (KANTO Chemical) for 24 h, and the solvent was removed. The crude 3-aminoacrylonitril with triethyl orthoformate (Acros
Scheme 1.
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Organics) and acetic anhydride (KANTO Chemical) was heated under reflux for 15 min, after that the refluxed mixture was evaporated. The black oily material was distilled in a vacuo to give a colorless crystalline ethoxymethyleneaminoacrylonitrile. The colorless crystalline material was twice distilled in a vacuo. The material was added to methanolic methylamine (40%, Tokyo Kasei). After 10 min the solution was evaporated in a vacuo. The yellow oily residue was purified using a triple vacuum sublimation followed by triple recrystallized from spectra grade n-hexane, and subsequently dried for 12 h in a vacuo. 3M4PMI was obtained as colorless crystal. The 1 H NMR spectroscopic data of the 4DMPM and 3M4PMI are given below. - 4DMAPM: 1 H NMR (CDCl3 ) δ = 3.10 (6H, s, CH3 ), 6.41 (1H, d, J = 6 Hz, ring H), 8.15 (1H, d, J = 6 Hz, ring H), 8.58 (1H, s, ring H). - 3M4PMI: 1 H NMR (CDCl3 ) δ = 3.14 (3H, s, CH3 ), 6.14 (1H, d, J = 7 Hz, ring H), 6.20 (1H, s, NH), 7.28 (1H, d, J = 7 Hz, ring H), 7.75 (1H, s, ring H).
Fig. 1. A block diagram of fluorescence decay profiles setup.
files were fitted to the convolution of an instrumental response (Gaussian of 25 ps fwhm) and single-exponential functions.
2.2. Measurements 2.3. Theoretical method Absorption spectra were recorded on a Hitachi U-3410 UV spectrophotometer. Samples were measured in a quartz cell of path length of 1 cm. The temperature dependent study at 293–323 K was performed using an insulated cell holder with an electronic circulator (Tokyo Rikakikai, model CTP-101). For the measurements dry air was flushed around the cell to avoid the deposit of atmospheric water vapor on the surface of the cell. The temperature was measured by use of a copper-constantan thermocouple junction. The fluorescence and excitation spectra were recorded on a Hitachi F-4010 spectrofluorometer. The excitation light source of the fluorometer has been corrected by the Rhodamine B spectrum. The 1 H NMR spectra (300 MHz) were measured on a Bruker DPX-300 spectrometer. Chemical shifts (δ in ppm) were measured in CDCl3 referred to tetramethylsilane (TMS) as internal standard. The fluorescence quantum yields (Φf ) were measured with 1-methyl-2(1H)-pyridone (Φf = 0.006 [35]) in alcohol (ethanol containing 10% methanol) mixture and corrected for the solvent’s refractive index. The fluorescence time-profile experiments were performed with a setup schematically shown in Fig. 1. The excitation source was a wavelength-variable sub-picosecond laser system composed of an optical parametric generator/amplifier (Light Conversion, TOPAS) pumped with an amplified mode-locked Ti:sapphire laser (Quantronix, Integra, 800 nm, 100 fs, 1 kHz, 1.7 W), and some harmonic generation/mixing crystals. The excitation wavelength was 260 or 320 nm. The pump pulses were focused on a sample solution in a 1 cm quartz cell. Picosecond time-resolved fluorescence spectra were measured by using a streak scope (Hamamatsu, C4334) equipped with an imaging spectrograph (Chromex, 250is). The time-resolution was about 25 ps. A polarizer in front of the spectrograph was set at the magic angle for acquiring fluorescence decay signals free from the rotational relaxation. Several streak images were averaged for achieving a high S/N ratio. The fluorescence decay pro-
The ab initio molecular orbital calculation was performed by using the package of Gaussian 03 W. Rev. B. 05 [36]. The geometry optimizations for all molecular systems in the S0 state were carried out with the 6-311++G(d,p) basis set at the restricted B3LYP level. All structures were vibrationally characterized; checking for the absence of imaginary frequencies in the energy minimum. The formation energies for the hydrogen-bonded complex, −Hcalc were calculated from the energy difference between the hydrogen-bonded complex and two different monomers. The geometry optimization of the excited state was carried out at the configuration interaction with single excitations (CIS) with the 6-311++G(d,p) basis set. Electronic vertical transition energies on the UV absorption and fluorescence spectra were also obtained by using the CIS/6-311G++G(d,p) basis set. 3. Results and discussion 3.1. UV absorption spectra Fig. 2a shows the UV absorption and fluorescence spectra of the 4APM with the addition of various concentrations of AcOH in n-hexane at 20 ◦ C. From the UV absorption spectra, its band maximum of the 4APM was observed at 264 nm. The absorption band maximum shifted to a longer wavelength with increasing AcOH concentration and was observed at 271 nm (7 nm). Two isosbestic points were observed at 246 and 268 nm, respectively. In comparison, 4DMAPM is considered as an amino model in which the amino–imino tautomerism is prohibited due to its lack of amino group hydrogen. Fig. 2b shows the UV absorption spectra of the 4DMAPM with addition of AcOH in n-hexane. The UV absorption band of the 4DMAPM having the structural vibronic progression was observed at ca. 282 nm. The absorption spectrum of this compound occurred in the same region as that
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of the 4APM/AcOH system may not form in the S0 state. The molecular extinction coefficient of the absorption band maximum of 4APM at 264 nm is about 6.7 × 103 dm3 mol−1 cm−1 in n-hexane. This implies that it should be assigned to the –* transition. The red shift of the –* absorption spectra may be due to the formation of the 4APM hydrogen-bonded complex with AcOH in n-hexane. 3.2. Determination of equilibrium constants and enthalpy changes
Fig. 2. (a) The UV absorption spectra (left axis) and fluorescence spectra (right axis, excitation at 270 nm) of the 4APM/AcOH system in n-hexane at 20 ◦ C. Concentration of 4APM: 2.5 × 10−5 mol dm−3 ; concentration of AcOH (mol dm−3 ) (1) 0, (2) 2.0 × 10−4 , (3) 5.0 × 10−4 , (4) 6.0 × 10−4 , (5) 1.0 × 10−3 , (6) 2.0 × 10−3 , (7) 4.0 × 10−3 , (8) 1.0 × 10−2 , and (9) 1.5 × 10−2 . (b) The UV absorption and fluorescence spectra (excitation at 270 nm) of the 4DMAPM/AcOH system in n-hexane at 20 ◦ C. Concentration of 4DMAPM: 2.5 × 10−5 mol dm−3 ; concentration of AcOH (mol dm−3 ) (1) 0, (2) 1.0 × 10−5 , (3) 1.0 × 10−4 , and (4) 1 × 10−3 . (c) The UV absorption and fluorescence spectra (excitation at 310 nm) of the 3M4PMI in n-hexane at 20 ◦ C. Concentration of 3M4PMI: 2.5 × 10−5 mol dm−3 . (d) The UV absorption and fluorescence spectra (excitation at 270 nm) of the 4APM in H2 SO4 aqueous solution at 20 ◦ C. Concentration of 4APM: 2.5 × 10−5 mol dm−3 ; concentration of H2 SO4 : 5.0 × 10−5 mol dm−3 .
of the 4APM, except for a relatively small red shift and structural vibronic progression, due to the perturbing action of methylation of the amino group. In addition, the spectra of the 4DMAPM did not change with the addition of various concentrations of AcOH because of the lack of the amino group hydrogen and prevented the formation of any hydrogen-bonded complexes. This indicates that the shift of the spectra for the 4APM/AcOH system in Fig. 2a is related to the amino group of 4APM. On the other hand, the 3M4PMI is the ring N3-methyl compound stabilized as an imino-tautomer model of the 4APM. The UV absorption spectrum of 3M4PMI in n-hexane is also shown in Fig. 2c, in which the band maximum with vibronic structure was observed at ca. 323 nm. The band maximum had a much longer wavelength than that of the 4APM/AcOH system. Therefore, the imino-tautomer
The equilibrium constants (K) and enthalpy changes (−H; hydrogen bond energies) accompanying hydrogen bond formation between 4APM and AcOH are estimated as well as the 2APM/AcOH in previous paper [30]. The 4APM has the possibility to form hydrogen-bonded complex with two AcOH, because the pyrimidine ring has two nitrogen atoms. The stoichiometry of the complex was analyzed using Job’s plot method [37]. Job’s plot can be obtained by plotting A − A0 versus C4APM /(C4APM + CAC ). Here, A0 and A are the absorbances of 4APM in n-hexane and in a mixed solvent of AcOH and n-hexane, respectively. C4APM and CAC are the initial concentrations of 4APM and AcOH, respectively. The Job’s plot for the 4APM/AcOH system in n-hexane is shown in Fig. 3. The plot in Fig. 3 clearly provides evidence for the formation of 1:1 complex. The 4APM/AcOH (1:1) complex formation is considered to compete two equilibriums, since AcOH forms a number of selfdimerization in non-polar solvent (Scheme 2 and Eqs. (1) and (2)). 4APM + AcOH 4APM/AcOH complex
(1)
2(AcOH) AcOH dimmer
(2)
Fig. 3. The Job’s plots of A − A0 vs. C4APM /(C4APM + CAC ) for the 4APM/AcOH system in n-hexane. The UV absorption wavelength for the analysis was 285 nm.
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Scheme 2.
The association constant K for the AcOH monomer and dimer is expressed as CD , (CM )2 √ −1 ± 1 + 8K CAC = 4K
K =
(3)
CM
(4)
where CM is the concentration of the monomer, and CD is that of the dimer, CAC is the initial concentration of AcOH (CAC = CM + 2CD ). Eq. (1) of the equilibrium constant (K) between 4APM and AcOH can be written as given below: K=
CAC
CC , (C4APM − CC )(CAC − CC )
(5)
2 A−A0 1 = 2K +1 εc −ε0 (C4APM −(A−A0 )/(εc −ε0 ))K 1 A − A0 +1 + εc − ε0 (C4APM − (A − A0 )/(εc − ε0 ))K (6)
where Cc denotes the concentration of hydrogen-bonded complex. ε0 and εC are the molar extinction coefficients of 4APM and the hydrogen-bonded complex, respectively. Eq. (6) is performed by fitting to the plots of CAC versus A − A0 , and K and K can be estimated. Fig. 4 shows the best fitting curves for the 4APM/AcOH system in n-hexane at 20 ◦ C. From the
Fig. 4. The plots of CAC vs. A − A0 for the 4APM/AcOH system in n-hexane at different temperatures and the best non-liner least-squares fitting curves (RoseDrago method). The UV absorption wavelength for the analysis was 285 nm.
curves K = 1200 mol−1 dm3 and K = 7600 mol−1 dm3 were estimated. The value of K for the monomer and dimer of AcOH is reported to be 3200 mol−1 dm3 in CCl4 at 298 K [38]. The value of K (7600 mol−1 dm3 ) in n-hexane at 293 K is in fair agreement with that of 3200 mol−1 dm3 in CCl4 at 298 K. In order to obtain the enthalpy and entropy changes for 4APM/AcOH hydrogen-bonded complex formation, the temperature dependence of the equilibrium constant was determined, i.e., the absorption spectra of 4APM were measured in n-hexane containing various amounts of AcOH at various temperatures (293–323 K), as shown in Fig. 5. Plots of ln K versus 1/T gave a straight line, whose slope yielded 55.0 ± 1.8 kJ mol−1 for the enthalpy change (−H). The values of −H for the 2AP/AcOH and 2APM/AcOH systems have been reported to be 59.0 ± 3.4 kJ mol−1 and 51.0 ± 2.6 kJ mol−1 in n-hexane, respectively. These values are almost the same as that of −H for the 4APM/AcOH system. On the other hand, the value of −H for the 1:1 hydrogen bond formation between the ring nitrogen atom of the 4APM and the O–H group of phenol by IR spectrum is reported to be 25 kJ mol−1 [39]. Accordingly, the 4APM/AcOH system can be considered to have taken the structure which carried out the dual hydrogen bond formation between 4APM (ring nitrogen atom and amino group) and AcOH (OH and C O groups). 3.3. Fluorescence spectra Fig. 2a shows remarkable changes of the fluorescence spectra of 4APM with addition of AcOH in n-hexane at room tempera-
Fig. 5. The plots of ln K vs. 1/T for 4APM/AcOH system.
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Fig. 6. The excitation spectra of the 4APM (2.5 × 10−5 mol dm−3 ) and the 4APM (2.5 × 10−5 mol dm−3 )/AcOH (2.5 × 10−5 mol dm−3 ) system in nhexane at room temperature. The emission wavelength of 4APM was 334 nm (1). The emission wavelengths of 4APM/AcOH system were 334 nm (2) and 437 nm (3), respectively.
ture, compared with the UV absorption spectral behavior. The fluorescence peak (F1) of 4APM in n-hexane was observed at 353 nm in all concentrations. With the addition of AcOH, however, F1 decreased its intensity without a shift of the band and another large Stokes shifted emission band (F2) appeared at 414 nm. Furthermore, a clear isoemissive point was observed at 381 nm. These observations indicate the existence of a dual emitting species. The assignments of the dual emitting species can be resolved using the excitation spectra in Fig. 6. In the region of the F1 band wavelength (e.g. 353 nm), the excitation band maxima (264 nm) were independent of the monitored emission wavelengths. In conformity with the absorption band maximum of 4APM monomer, it was concluded that the F1 band results originate exclusively from the amino-form monomer of the excited 4APM. On the other hand, when the emission wavelength was monitored in the region of the F2 band wavelength (e.g. 414 nm), the excitation band shifted to 13 nm red and appeared at 277 nm. The excitation spectral feature at the 277 nm band resembles the growing portion of the absorption spectrum upon increasing AcOH in n-hexane. Accordingly, these are thus tentatively ascribed to the excitation spectrum of the 4APM/AcOH hydrogen-bonded complex. Alternatively, the dual fluorescences should originate from different ground-state precursors; as the free 4APM monomer and the 4APM/AcOH hydrogen-bonded complex. Consequently, the precursor of F2 band should be associated with the 4APM/AcOH hydrogen-bonded complex. Due to the high acidity of the carboxylic acid proton and the drastic increase of the basicity of the pyrimidal nitrogen, the fluorescence band at 414 nm may result from a single proton transfer from AcOH to the 4APM pyrimidal nitrogen, giving rise to the 4APM cation emission described below. Such a possibility has been eliminated by the sequence of experiment. Fig. 2d shows the fluorescence
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spectrum of 4APM in 5.0 × 10−5 mol dm−3 H2 SO4 aqueous solution at room temperature. The broad fluorescence spectrum was observed at 358 nm. The protonated 4APM cation exhibits a single fluorescence band with large intensity. This fluorescence peak at 358 nm was different from those of F1 and F2, and it did not show the vibronic structure band. This indicates that the F2 for the 4APM/AcOH system did not emit from the 4APM cation species. Furthermore, the fluorescence spectra of 4DMAPM were little affected by the addition of AcOH in n-hexane at room temperature and did not show the emission corresponding to the F2 (Fig. 2b). The broad fluorescence spectrum of 4DMAPM was observed at 334 nm. The fluorescence spectrum of this compound showed almost the same region as that of 4APM monomer, except for a relative increase of intensity in AcOH concentration. This indicates that the fluorescence spectrum of the 4DMAPM/AcOH system is an emission from the free 4DMAPM molecule in the S1 state. On the other hand, the F2 band with an unusually large Stokes shift of the 15,400 cm−1 relative to the absorption band of the free 4APM leads to the proposition that a possible occurrence of the excited double-proton transfer reaction, resulted from the formation of the imino-tautomer complex. To further verify this viewpoint, the model compound of the imino-tautomer complex, 3M4PMI possessed similar -electronic properties with respect to the imino-tautomer complex of 4APM. The fluorescence spectrum of 3M4PMI is also shown in Fig. 2c. This spectrum had the vibronic structure emission bands (ca. 1550 cm−1 ) near 437 nm. It was noted that the vibronic bands and band width of the fluorescence spectrum of 3M4PMI were very similar to the F2 of the 4APM/AcOH system near the same wavelength region (near 414 nm). This similarity supports the occurrence of AcOH-catalyzed excited amino–imino tautomerization reaction in the 4APM/AcOH 1:1 hydrogen-bonded complex. The results clearly indicate that the amino–imino tautomerization reaction for the hydrogen-bonded complex of the 4APM/AcOH system takes place in the S1 state, giving rise to the fluorescence from the imino-tautomer complex as shown in Scheme 3. The reaction is similar to the doubleproton transfer reaction of the 7AI/AcOH [40], 2AP/AcOH [24,29], and 2APM/AcOH [30] systems. In the present study of the steady-state measurement, the emissions corresponding to the hydrogen-bonded complexes of the 4APM/AcOH system was not observed. This suggests that the rate of the double-proton transfer must be much faster than the decay rate of the 4APM/AcOH amino-hydrogen-bonded complex in the S1 state.
Scheme 3.
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Table 1 The fluorescence quantum yields (Φf ) of 4APM, 4APM/AcOH imino-tautomer, cation, 3M4PMI, 2APM, 2APM/AcOH imino-tautomer, cation, 1M2PMI, 2AP, 2AP/AcOH imino-tautomer, cation, and MPI in n-hexane at room temperature Compound
Φf
4APM monomer 4APM/AcOH imino-tautomer Cation 3M4PMI
(1.1 ± 0.4) × 10−3 (8.8 ± 0.5) × 10−4 (3.7 ± 0.4) × 10−3a (9.3 ± 0.6) × 10−6
2APM 2APM/AcOH imino-tautomer Cation 1M2PMI
(1.3 ± 0.2) × 10−3b (4.2 ± 0.4) × 10−4b (3.3 ± 0.2) × 10−3b (3.1 ± 0.7) × 10−6b
2AP 2AP/AcOH imino-tautomer Cation MPI
(4.3 ± 0.4) × 10−2c (9.4 ± 0.3) × 10−3c (2.8 ± 0.0) × 10−1c (3.7 ± 0.9) × 10−5c
a b c
In 0.1 N-H2 SO4 . From Ref. [30]. From Ref. [24].
3.4. Fluorescence quantum yields The measurements of the fluorescence quantum yields (Φf ) were carried out to further clarify the results of the fluorescence intensities. The values of Φf of various 4APM, 4APM/AcOH imino-tautomer complex, cation, and 3M4PMI in n-hexane are listed in Table 1, compared with the corresponding 2APM and 2AP. The Φf values of 4APM, 4APM/AcOH imino-tautomer complex, cation, and 3M4PMI in n-hexane are roughly similar to those of the corresponding 2APM in isooctane [30], respectively. However, these Φf values are smaller 10−1 than those of the corresponding 2AP in isooctane except decrease in the Φf value of cation of 10−2 times. The pyrimidine molecule has two nitrogen atoms from the ring. Therefore, we can consider that the Sn, * is approaching very near the S, * as compared with the 2AP. The weak fluorescence decrease of the Φf values for the pyrimidines than the pyridines may suggest that strong mixing between Sn, * and S, * enhances the non-radiation transition as pseude-Jane-Teller effect and couple with the Tn, * as intersystem crossing. This is actually understood from the increase in the Φf values of cation. In Table 1, it is noted that the Φf values of the imino-tautomers are larger than those of the corresponding imino-model compounds. This can be explained by the fact that the Sn, * state of the imino-tautomer shifted to a higher energy level than that of the imino-model compound through hydrogen bond formation. 3.5. Fluorescence decay time profiles Previously, fluorescence decay times of 2AP, 2AP/AcOH system in n-hexane at room temperature monitored at F1 and F2, with that of the imino-model compound (1-methyl-2(1H)pyridinimine; MPI) were reported to be 1.4 ns, 3.2 ns, and 43 ps, respectively [28]. We have investigated the fluorescence decay time profiles of the 4AMP/AcOH system in n-hexane at room temperature
Fig. 7. (a) The fluorescence decay profile of the 4APM (2.5 × 10−5 ) in n-hexane monitored at the band ranging 325–380 nm (τ = 98 ps). Excitation wavelength: 260 nm. (b) The fluorescence decay profile of the 4APM (2.5 × 10−5 )/AcOH (1.0 × 10−2 ) in n-hexane monitored at 400–500 nm (τ = 73 ps). Excitation wavelength: 260 nm. (c) The fluorescence decay profile of the 3M4PMI only (2.5 × 10−5 ) in n-hexane monitored at 450–525 nm (τ = 19 ps). Excitation wavelength: 320 nm. All curves fit assuming a single-exponential decay.
monitored at F1 and F2 with that of the imino-model compound. The fluorescence decay profiles monitored at the F1 band (Fig. 7a) of the 4APM monomer in n-hexane was fitted to a single-exponential function with time constant of 98 ps. The rate constant of the fluorescence decay was kf1 , is of 1.02 × 1010 s−1 . On the other hand, we found that the tautomer fluorescence (F2) of the 4APM/AcOHsystem exhibits a wavelength-independent single-exponential decay of 73 ps in a n-hexane solution (Fig. 7b) with the decay rate, kf2 of 1.37 × 1010 s−1 . The excited-state double-proton transfer reaction of the 4APM/AcOH system in n-hexane clarified in the present fluorescence decay time measurements is summarized in Fig. 8. The fluorescence decay time of the F2 for the 4APM/AcOH system was shorter than that of the 4APM monomer (F1) in n-hexane. On the other hand, the decay time (3.2 ns) of the F2 for the 2AP/AcOH was longer than that (1.4 ns) of the 2AP monomer (F1) in n-hexane. The double-proton transfer in the 4APM/AcOH system occurs in a very short time, compared with the 2AP/AcOH system. It is thought that the double-proton
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Fig. 8. Schematic energy diagram illustrating the dynamics of the S1 state double-proton transfer reaction of the 4APM/AcOH system with 3M4PMI.
transfer for the 4APM/AcOH system in n-hexane at room temperature occurs very rapidly because the activation energy for the amino → imino tautomer reaction is very small or almost zero in the S1 state. Further, the fluorescence decay profile of 3M4PMI was fitted to a single-exponential function with time constant of 19 ps (Figs. 6c and 7). The rate constant, kfi is of 5.2 × 1010 s−1 and this reaction is particularly rapid. It is considered that 3M4PMI has the electronic flexible structure in the S1 state (see theoretical approaches in S1 state), the excited state, Sn, * state, of 3M4PMI is to a lower energy level than that of the hydrogenbonded imino-tautomer complex with AcOH, and strong mixing between Sn, * and S, * enhances the non-radiation transition, as described previously in the MPI molecule [41]. In addition, the Φf value of 3M4PMI in n-hexane is very small (Φf = 9.3 × 10−6 ), as given in Table 1. As the result of the strong mixing, the fluorescence decay time (19 ps) of 3M4PMI in nhexane at room temperature may become very short due to the flexible structure and low Φf value, as well as that of MPI in n-hexane (43 ps). 3.6. Theoretical approaches in the S0 state In order to have a better understanding of the above experimental results, we performed theoretical calculations. Fig. 9 shows the optimized geometries of various proton transfer isomers of 4APM; E- and Z-3H-4(1H)pyrimidinimines (3H4PMI-E and 3H4PMI-Z) and E- and Z1H-4(1H)-pyrimidinimines (1H4PMI-E and 1H4PMI-Z) at the B3LYP/6-311++G(d,p) level of theory, with those of E- and Z-3methyl-4(1H)-pyrimidinimines (3M4PMI-E and 3M4PMI-Z).
These molecules have almost planar structures, though the amino group of 4APM takes pyramidal. The total energies (ET ) of these compounds and the energy differences (ET ) between 4APM and tautomer are given in Table 2. The direct proton-transfer tautomer forms of the 4APM monomer, i.e., 3H4PMI-E (see Fig. 9 and Table 2) were calculated to be 55.3 kJ mol−1 higher in energy than the normal form. Among the possible four isomers (tautomers) of the direct proton transfer, 3H4PMI-E form was the most stable. However, as described above in Section 2, the presence of the 3H4PMI-E was not found from the result of the UV spectral measurement of 4APM in n-hexane at room temperature. The optimized geometries and the ET of 3M4PMI-E and 3M4PMI-Z as the tautomer model compounds are also given in Fig. 9 and Table 2, respectively. In this Table 2, E-form is stable in energy than Zform by 14.4 kJ mol−1 . Accordingly, the synthesized 3M4PMI is considered to be the E-form. Fig. 10 shows the optimized geometries of the 4APM/AcOH and 3H4PMI/AcOH complexes at the S0 states by B3LYP/6Table 2 The S0 state calculated total energies (ET ) of 4APM, tautomers, and tautomer model compounds by B3LYP/6-311++G(d,p) level Compounds
ET (a.u.)
ET (kJ mol−1 )
ET (kJ mol−1 )
4APM 3H4PMI-E 3H4PMI-Z 1H4PMI-E 1H4PMI-Z 3M4PMI-E 3M4PMI-Z
−319.693009 −319.671961 −319.667348 −319.657422 −319.663861 −358.963564 −358.958062
0 55.3 67.4 93.4 76.5 – –
– – – – – 0 14.4
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T. Kitamura et al. / Spectrochimica Acta Part A 69 (2008) 350–360 Table 3 The S0 state B3LYP/6-311++G(d,p) level calculated total energies (ET ), energy differences (ET ), and hydrogen bond energies (−Hcalc ) of 4APM and 3H4PMI/AcOH system Compounds
ET (a.u.)
ET (kJ mol−1 )
−Hcalc (kJ mol−1 )
4APM/AcOH 3H4PMI/AcOH
−548.817796 −548.805430
0 32.5
56.0 79.0
311++G(d,p) level of theory. Strong dual hydrogen bond formation in the 4APM/AcOH complex is indicated by the short ˚ and 1.730 A, ˚ H(1)· · ·O(1) and N(2)· · ·H(2) distances of 1.880 A respectively. In comparison to the 4APM monomer (Fig. 9) ˚ significant changes in N(2)· · ·C(1) 1.353 − 1.342 = +0.011 A ˚ and in C(1)–N(1) 1.347 − 1.366 = −0.019 A, respectively, were found in the formation of the 4APM/AcOH complex. There results correlate well with the concept of conjugated dual hydrogen bond effect [29,42], in which the bond distances relevant to the n-electron delocalization are subject to change. Table 3 shows that hydrogen bond energy (−Hcalc ) was calculated at the B3LYP/6-311++G(d,p) level of theory to be 56.0 kJ mol−1 for the 4APM/AcOH complex and 79.0 kJ mol−1 for the 3H4PMI/AcOH complex. The value for the 4APM/AcOH complex is smaller than that for the 3H4PMI/AcOH complex, however, the normal form complex was more stable in energy than the tautomer complex by 32.5 kJ mol−1 at the B3LYP/6-311G++(d,p) level. This result indicates that the 4APM/AcOH complex is more easily formed than the 3H4PMI/AcOH complex in the S0 state. The −Hcalc value (56.0 kJ mol−1 ) for the 4APM/AcOH complex formation was in fair agreement with the experimental data of −H (55.0 ± 1.8 kJ mol−1 ) in n-hexane. On the other hand, it is noted that the presence of the 3H4MPI/AcOH complex in the S0 state was not found from the results of UV absorption spectra. 3.7. Theoretical approaches in the S1 state
Fig. 9. The optimized geometries of normal 4APM monomer, tautomer, and tautomer model compounds at the S0 states by B3LYP/6-311++G(d,p) level. ˚ unit. The values indicate the bond distances in A
The CIS method, which has been proven to be a relatively useful method to obtain the approximate wave function and molecular geometry of electronic excited states, was applied in this study. Fig. 11 shows the optimized geometries of the 4APM, 3M4PMI, 4APM/AcOH, and 3H4PMI/AcOH. In the S1 state, the amino group of 4APM become near planar and the 4APM, 4APM/AcOH, and 3H4PMI/AcOH have almost near planar structures, but the ring skeleton of 3M4PMI has a non-
Fig. 10. The optimized geometries of 4APM/AcOH and 3H4PMI/AcOH at the S0 states by B3LYP/6-311++G(d,p) level. The values indicate the bond distances in ˚ unit. A
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359
Fig. 11. Calculated stationary point geometries of for 4APM, 3M4PMI, and normal and proton-transfer tautomer of the 4APM/AcOH complex in the S1 states by ˚ unit. CIS/6-311++G(d,p) level. The values indicate the bond distances in A
planar structure like the twist-boat form of cyclohexane, as shown in Fig. 11 in detail. The total energies (ET ) of these compounds are given in Table 4. Table 4 shows that the ET value for the 4APM/AcOH complex was smaller than that for the 3H4PMI/AcOH complex, and the 3H4PMI/AcOH complex was more stable in energy than the normal form complex by 19.7 kJ mol−1 . This indicates that 3H4PMI/AcOH complex is more easily formed than the 4APM/AcOH complex in the S1 state. 3.8. Potential energy surface for S0 and S1 states
The energy barrier for the 4APM/FoOH complex → transition state (TS) process in the S1 state was calculated to be 2 kJ mol−1 using the TD-B3LYP/6-31G(d) method. The barrier value was less than that in the S0 state by 33 kJ mol−1 for the B3LYP/6-31G(d) method. Particularly, the energy barrier for the 4APM/FoOH complex → TS process in the S1 state is very small. This indicates that in the S1 state 3H4PMI/FoOH complex may be easily formed through the dual proton transfer of the 4APM/FoOH complex. In the S0 state, the energy barrier of reverse proton transfer (3H4PMI/FoOH complex → TS) was calculated to be almost zero kJ mol−1 for B3LYP/6-31G(d) method. This reverse proton transfer in S0
The potential energy surfaces along the proton transfer reaction in the S0 and S1 states were calculated to make clear the amino–imino tautomerization process. For the sake of simplicity, the formic acid (FoOH) complex was used for the AcOH complex. The N(1)–H(1) and O(2)–H(2) H-bonding distances ˚ in each step as shown in Fig. 12. The were scanned by 0.02 A rest of the bond angles and distances were fully optimized. Table 4 The S1 state CIS/6-311++G(d,p) level calculated total energies (ET ) of 4APM, tautomer, tautomer model compounds, 4APM/AcOH, and 3H4PMI/AcOH systems Compounds
ET (a.u.)
ET (kJ mol−1 )
ET (kJ mol−1 )
4APM 3H4PMI-E 3M4PMI-E 4APM/AcOH 3H4PMI/AcOH
−317.512635 −317.458081 −356.525487 −545.345638 −545.353139
0 143.2 – – –
– – – 0 −19.7
Fig. 12. Potential energy surfaces of 4APM/FoOH system in the S0 and S1 states byB3LYP and TD-B3LYP methods.
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state is so rapid that the presence of the S0 state 3H4MPI/FoOH complex could not be found by the measurement of the UV absorption spectra. As described above, it is considered what must be a fundamental difference between the FoOH complex and the AcOH complex. Our experimental and/or theoretical results of UV absorption and fluorescence spectra, decay times, and theoretical calculations should serve as useful guides to the amino–imino tautomerization for the 4-aminopyrimidine moieties in the vitamin B1 coenzyme and DNA base pairs. Acknowledgements This work was supported in part by grants from the Research Institute for Technology of Tokyo Denki University. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
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[19] V. Guallar, V. SBatista, W.H. Miller, J. Chem. Phys. 110 (1999) 9922. [20] J. Catalan, J.C. del Valle, M.J. Kasha, Proc. Natl. Acad. Sci. U.S.A. 96 (1999) 8338. [21] P.R. Cantor, P.R. Schimmel, Biophysical Chemistry, 2, W.H. Freeman, San Francisco, CA, 1980 (Chapters 7 and 8). [22] S. Kohtani, A. Tagami, R. Nakagaki, Chem. Phys. Lett. 316 (2000) 88. [23] C. Tanner, C. Manca, S. Leutwyler, Science 302 (2003) 1736. [24] K. Inuzuka, A. Fujimoto, Spectrochim. Acta 42A (1986) 929. [25] J. Konijnenberg, A. Ekelmans, C.A.G.O. Varma, J. Chem. Soc., Faraday Trans. 2 (85) (1989) 1539. [26] K. Inuzuka, A. Fujimoto, Bull. Chem. Soc. Jpn. 63 (1990) 971. [27] A. Fujimoto, K. Inuzuka, Bull. Chem. Soc. Jpn. 64 (1991) 3758. [28] H. Ishikawa, K. Iwata, H. Hamaguchi, J. Phys. Chem. A 106 (2002) 2305. [29] F.T. Hung, W.S. Hu, T.H. Li, C.C. Cheng, P.T. Chou, J. Phys. Chem. A 107 (2003) 3244. [30] T. Kitamura, A. Hikita, H. Ishikawa, A. Fujimoto, Spectrochim. Acta 62A (2005) 1157. [31] A. Schellenberger, G. Hubner, H. Neef, Methods Enzymol. 278 (1997) 131. [32] A.T. Baykal, L. Kakalis, F. Jordan, Biochemistry 45 (2006) 7522. [33] D.J. Brown, L.N. Short, J. Chem. Soc. (1953) 331. [34] D.J. Brown, K.J. Lenega, J. Chem. Soc. Perkin Trans. 1 (1974) 372. [35] K. Kimura, R. Nagai, Bull. Chem. Soc. Jpn. 49 (1976) 3343. [36] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, J.A. Montgomery Jr., T. Vreven, K.N. Kudin, J.C. Burant, J.M. Millam, S.S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G.A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J.E. Knox, H.P. Hratchian, J.B. Cross, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, P.Y. Ayala, K. Morokuma, G.A. Voth, P. Salvador, J.J. Dannenberg, V.G. Zakrzewski, S. Dapprich, A.D. Daniels, M.C. Strain, O. Farkas, D.K. Malick, A.D. Rabuck, K. Raghavachari, J.B. Foresman, J.V. Ortiz, Q. Cui, A.G. Baboul, S. Clifford, J. Cioslowski, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R.L. Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, M. Challacombe, P.M.W. Gill, B. Johnson, W. Chen, M.W. Wong, C. Gonzalez, J.A. Pople, Gaussian 03, Revision B. 05, Gaussian, Inc., Pittsburgh, PA, 2003. [37] P. Job, Compt. Rend. 180 (1925) 928. [38] H.E. Affsprung, S.D. Christain, A.M. Mecnick, Spectrochim. Acta 20 (1964) 285. [39] O. Kasende, T. Zeeger-Huyskens, J. Phys. Chem. 88 (1984) 2636. [40] P.T. Chou, C.Y. Wei, C.P. Chang, M.S. Kuo, J. Phys. Chem. 99 (1995) 11994. [41] K. Inuzuka, Nippon Kagaku Kaishi (1997) 393. [42] P.T. Chou, T.H. Liao, C.T. Wei, W.S. Yu, T.H. Chou, J. Am. Chem. Soc. 122 (2000) 986.
Spectrochimica Acta Part A 69 (2008) 350–360
Amino–imino tautomerization reaction of the 4-aminopyrimidine/acetic acid system Teruyoshi Kitamura a , Masahiro Okita a , Yutaka Sasaki b , Hironori Ishikawa b , Akira Fujimoto a,∗ b
a Department of Environmental Materials Science, Tokyo Denki University, Kanda, Chiyoda-ku, Tokyo 101-8457, Japan Mitsubishi Chemical Group Science and Technology Research Center, Inc., Kamoshida-cho, Aoba-ku, Yokohama 227-8502, Japan
Received 19 December 2006; accepted 12 April 2007
Abstract The amino–imino tautomerization of the 4-aminopyrimidine (4APM)/acetic acid (AcOH) system through dual hydrogen bonding in n-hexane at room temperature was investigated using UV absorption and fluorescence spectroscopies, fluorescence time-profile measurements, and molecular orbital calculations, with those of the imino-model compound of 3-methyl-4(1H)-pyrimidinimine (3M4PMI). From the experimental results, it was confirmed that the imino-tautomer was formed in the first excited singlet state (S1) state through the double-proton transfer of the dual hydrogen-bonded complex of 4APM with AcOH. The fluorescences of the free 4APM monomer (band maximum at 353 nm), imino-tautomer (at 414 nm), and 3M4PMI monomer (at 437 nm) exhibit the single-exponential decays of 98, 73, and 19 ps time constants, respectively. The shorter decay time of the imino-tautomer fluorescence compared with the free monomer one is suggestive of the low activation energy process in the S1 state. From the result of the shortest decay time of the 3M4PMI fluorescence, it can be deduced that 3M4PMI has a non-planar structure in the S1 state. The theoretical calculations on the S0 and S1 state double-proton transfer for the 4APM/AcOH dual hydrogen-bonded system were performed with the use of formic acid (FoOH) in place of AcOH for the sake of simplicity. Only one peak of transition state was resolved in the S0 and S1 states. The energy barrier for the S1 state double-proton transfer of the 4APM/FoOH complex → 3H-4(1H)-pyrimidinimine/FoOH tautomer was estimated to be ∼2 kJ mol−1 using the CIS/6-31G(d) methods. On the other hand, the energy barrier for the S0 state reverse proton transfer of the 3H-4(1H)-pyrimidinimine/FoOH tautomer → 4APM/FoOH complex was estimated to be almost zero kJ mol−1 at B3LYP/6-31G(d) level. © 2007 Elsevier B.V. All rights reserved. Keywords: 4-Aminopyrimidines; Amino–imino tautomerization; Proton transfer; Fluorescence spectra
1. Introduction Proton transfer reaction is one of the simplest and most fundamental reactions whose processes include an acid–base reaction—the hydrogen atom exchange reaction of water in chemistry [1]. Moreover, this elementary reaction plays a crucial role in biology [2]. The excited state proton transfer [3–6] is especially interesting from the viewpoint of photochemistry. The 7-azaindole (7AI) dimer system has been studied experimentally [7–17] and theoretically [18–20] by many researches, as a prototype of excited state double-proton reaction. The 7AI dimer system has been regarded as a model system of the photoinduced
∗
Corresponding author. Tel.: +81 3 5280 3397; fax: +81 3 5280 3570. E-mail address: [email protected] (A. Fujimoto).
1386-1425/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2007.04.006
mutation of the DNA base pairs [7]. This assumption is in need of attention as it is connected with the threat to life as resulting from the ozone layer depletion; one of the major causes of environmental destruction on the earth [21]. This kind of excited-state double-proton transfer reaction process has been observed in many other molecule systems, for example, excited state double-proton transfer reaction controls the catalyst mechanism caused by alcohol or ammonia [22,23]. Particularly, biochemical interests are processes involving nitrogen-containing heterocycles molecules. Amino–imino tautomerization processes facilitated by proton transfer reaction in heterocycles molecules have been the subjects of a number of studies [24–30]. These processes are relevant to many areas of biological chemistry. The physicochemical properties of aminopyridines and aminopyrimidines have received considerable attention from both experimental [30] and theoretical
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research [29], due to their importance in understanding many fundamental biochemical processes. 4-Aminopyrimidines are constituents a part of vitamin B1 thiamine molecules [31,32]. Recently, Baykal et al. noted that the 4-aminopyrimidine moiety of thiamine diphosphate undergoes tautomerization of 1,4-iminomeric form during the catalytic cycle of the enzymes. However, it may be considered that 3,4iminomeric form produced by the hydrogen-bonded interaction of the 4-aminopyrimidine moiety with the carboxyl group of glutamic acid and/or aspartic acid is also important for the catalytic cycle of the enzymes in the first singlet excited (S1) state. Therefore, we thought that investigating the amino–imino tautomerization reaction of the simplified 4-aminopyrimidine molecules would also give the key to understanding one step of enzyme reaction activity in the mechanism of the vitamin B1. However, the tautomeric reaction of the 4-aminopyrimidine (4APM)/acetic acid (AcOH) system in which the aminopyrimidine molecular formation of cyclic complexes with AcOH, lead to S1 state tautomerizaton involving the movement of two protons and the formation of the imino tautomeric form, has not yet been reported. We investigated the amino–imino tautomerism reaction of 4APM using the bifunctional catalyst [28,30] of an AcOH molecule and compared it with the amino- and iminomodel compounds of 4-dimethylaminopyrimidine (4DMAPM) and 3-methyl-4(1H)-pyrimidinimine (3M4PMI) in non-polar solvent, respectively. In this study, we will report the first investigation of the 4APM/AcOH system in n-hexane. The following sections are organized according to a sequence of steps. We first performed absorption spectroscopies of the 4APM hydrogen-bonded complex with AcOH and determined the equilibrium constants. The second step involved steady-state fluorescence measurements to examine the excited state proton transfer properties. The proton transfer species of 4APM has been determined through syntheses and spectral characterization of their corresponding methylated derivatives. The third step, the experimental results were picosecond fluorescence time-profile measure-
351
ments. Finally, the theoretical calculations on the ground (S0) and S1 double-proton transfer through the 4APM/AcOH dual hydrogen-bonded system were performed to make clear the amino–imino tautomerization process in the S1 state. 2. Experimental 2.1. Materials The 4APM (Aldrich Co.) was recrystallized several times from spectrograde chloroform and subsequently dried in a vacuo before use. Purities were checked. The fluorescence excitation spectrum of 4APM in n-hexane was measured for several excitation wavelengths. The AcOH (KANTO Chemical) was made of atomic absorption spectragrade and used without further purification because no fluorescence was detected in the wavelength region of interest. The n-hexane (KANTO Chemical) was of spectragrade and used without further purification. Twice distilled, deionized water was used for the preparation of aqueous solution. Sulfuric acid (KANTO Chemical) was special grade and used without further purification. 4-Dimethylaminopyrimidine (4DMAPM) was synthesized according to the methods described by Brown and Short [33]. The products were purified by twice vacuum distillation. The preparation of 3-methyl-4(1H)-pyrimidinimine (3M4PMI) was performed using a modified version of the Brown and Lenega method [34], in the following manner: as shown in Scheme 1, malononitrile (Acros Organics) dissolved in a mixture (4:1) of dry ether (KANTO Chemical) and dry tetrahydrofuran (THF; KANTO Chemical) was reduced to 3-aminoacrylonitrile (red oily material) by lithum aluminium hydride (KANTO Chemical) at 0 ◦ C. The mixture was stirred for 2 h, after water and sodium hydroxide solution (30%) was added carefully, and the mixture was stirred for a further 6 h. The mixture was then filtered. The filtrate was dried using CaCO3 (KANTO Chemical) for 24 h, and the solvent was removed. The crude 3-aminoacrylonitril with triethyl orthoformate (Acros
Scheme 1.
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Organics) and acetic anhydride (KANTO Chemical) was heated under reflux for 15 min, after that the refluxed mixture was evaporated. The black oily material was distilled in a vacuo to give a colorless crystalline ethoxymethyleneaminoacrylonitrile. The colorless crystalline material was twice distilled in a vacuo. The material was added to methanolic methylamine (40%, Tokyo Kasei). After 10 min the solution was evaporated in a vacuo. The yellow oily residue was purified using a triple vacuum sublimation followed by triple recrystallized from spectra grade n-hexane, and subsequently dried for 12 h in a vacuo. 3M4PMI was obtained as colorless crystal. The 1 H NMR spectroscopic data of the 4DMPM and 3M4PMI are given below. - 4DMAPM: 1 H NMR (CDCl3 ) δ = 3.10 (6H, s, CH3 ), 6.41 (1H, d, J = 6 Hz, ring H), 8.15 (1H, d, J = 6 Hz, ring H), 8.58 (1H, s, ring H). - 3M4PMI: 1 H NMR (CDCl3 ) δ = 3.14 (3H, s, CH3 ), 6.14 (1H, d, J = 7 Hz, ring H), 6.20 (1H, s, NH), 7.28 (1H, d, J = 7 Hz, ring H), 7.75 (1H, s, ring H).
Fig. 1. A block diagram of fluorescence decay profiles setup.
files were fitted to the convolution of an instrumental response (Gaussian of 25 ps fwhm) and single-exponential functions.
2.2. Measurements 2.3. Theoretical method Absorption spectra were recorded on a Hitachi U-3410 UV spectrophotometer. Samples were measured in a quartz cell of path length of 1 cm. The temperature dependent study at 293–323 K was performed using an insulated cell holder with an electronic circulator (Tokyo Rikakikai, model CTP-101). For the measurements dry air was flushed around the cell to avoid the deposit of atmospheric water vapor on the surface of the cell. The temperature was measured by use of a copper-constantan thermocouple junction. The fluorescence and excitation spectra were recorded on a Hitachi F-4010 spectrofluorometer. The excitation light source of the fluorometer has been corrected by the Rhodamine B spectrum. The 1 H NMR spectra (300 MHz) were measured on a Bruker DPX-300 spectrometer. Chemical shifts (δ in ppm) were measured in CDCl3 referred to tetramethylsilane (TMS) as internal standard. The fluorescence quantum yields (Φf ) were measured with 1-methyl-2(1H)-pyridone (Φf = 0.006 [35]) in alcohol (ethanol containing 10% methanol) mixture and corrected for the solvent’s refractive index. The fluorescence time-profile experiments were performed with a setup schematically shown in Fig. 1. The excitation source was a wavelength-variable sub-picosecond laser system composed of an optical parametric generator/amplifier (Light Conversion, TOPAS) pumped with an amplified mode-locked Ti:sapphire laser (Quantronix, Integra, 800 nm, 100 fs, 1 kHz, 1.7 W), and some harmonic generation/mixing crystals. The excitation wavelength was 260 or 320 nm. The pump pulses were focused on a sample solution in a 1 cm quartz cell. Picosecond time-resolved fluorescence spectra were measured by using a streak scope (Hamamatsu, C4334) equipped with an imaging spectrograph (Chromex, 250is). The time-resolution was about 25 ps. A polarizer in front of the spectrograph was set at the magic angle for acquiring fluorescence decay signals free from the rotational relaxation. Several streak images were averaged for achieving a high S/N ratio. The fluorescence decay pro-
The ab initio molecular orbital calculation was performed by using the package of Gaussian 03 W. Rev. B. 05 [36]. The geometry optimizations for all molecular systems in the S0 state were carried out with the 6-311++G(d,p) basis set at the restricted B3LYP level. All structures were vibrationally characterized; checking for the absence of imaginary frequencies in the energy minimum. The formation energies for the hydrogen-bonded complex, −Hcalc were calculated from the energy difference between the hydrogen-bonded complex and two different monomers. The geometry optimization of the excited state was carried out at the configuration interaction with single excitations (CIS) with the 6-311++G(d,p) basis set. Electronic vertical transition energies on the UV absorption and fluorescence spectra were also obtained by using the CIS/6-311G++G(d,p) basis set. 3. Results and discussion 3.1. UV absorption spectra Fig. 2a shows the UV absorption and fluorescence spectra of the 4APM with the addition of various concentrations of AcOH in n-hexane at 20 ◦ C. From the UV absorption spectra, its band maximum of the 4APM was observed at 264 nm. The absorption band maximum shifted to a longer wavelength with increasing AcOH concentration and was observed at 271 nm (7 nm). Two isosbestic points were observed at 246 and 268 nm, respectively. In comparison, 4DMAPM is considered as an amino model in which the amino–imino tautomerism is prohibited due to its lack of amino group hydrogen. Fig. 2b shows the UV absorption spectra of the 4DMAPM with addition of AcOH in n-hexane. The UV absorption band of the 4DMAPM having the structural vibronic progression was observed at ca. 282 nm. The absorption spectrum of this compound occurred in the same region as that
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of the 4APM/AcOH system may not form in the S0 state. The molecular extinction coefficient of the absorption band maximum of 4APM at 264 nm is about 6.7 × 103 dm3 mol−1 cm−1 in n-hexane. This implies that it should be assigned to the –* transition. The red shift of the –* absorption spectra may be due to the formation of the 4APM hydrogen-bonded complex with AcOH in n-hexane. 3.2. Determination of equilibrium constants and enthalpy changes
Fig. 2. (a) The UV absorption spectra (left axis) and fluorescence spectra (right axis, excitation at 270 nm) of the 4APM/AcOH system in n-hexane at 20 ◦ C. Concentration of 4APM: 2.5 × 10−5 mol dm−3 ; concentration of AcOH (mol dm−3 ) (1) 0, (2) 2.0 × 10−4 , (3) 5.0 × 10−4 , (4) 6.0 × 10−4 , (5) 1.0 × 10−3 , (6) 2.0 × 10−3 , (7) 4.0 × 10−3 , (8) 1.0 × 10−2 , and (9) 1.5 × 10−2 . (b) The UV absorption and fluorescence spectra (excitation at 270 nm) of the 4DMAPM/AcOH system in n-hexane at 20 ◦ C. Concentration of 4DMAPM: 2.5 × 10−5 mol dm−3 ; concentration of AcOH (mol dm−3 ) (1) 0, (2) 1.0 × 10−5 , (3) 1.0 × 10−4 , and (4) 1 × 10−3 . (c) The UV absorption and fluorescence spectra (excitation at 310 nm) of the 3M4PMI in n-hexane at 20 ◦ C. Concentration of 3M4PMI: 2.5 × 10−5 mol dm−3 . (d) The UV absorption and fluorescence spectra (excitation at 270 nm) of the 4APM in H2 SO4 aqueous solution at 20 ◦ C. Concentration of 4APM: 2.5 × 10−5 mol dm−3 ; concentration of H2 SO4 : 5.0 × 10−5 mol dm−3 .
of the 4APM, except for a relatively small red shift and structural vibronic progression, due to the perturbing action of methylation of the amino group. In addition, the spectra of the 4DMAPM did not change with the addition of various concentrations of AcOH because of the lack of the amino group hydrogen and prevented the formation of any hydrogen-bonded complexes. This indicates that the shift of the spectra for the 4APM/AcOH system in Fig. 2a is related to the amino group of 4APM. On the other hand, the 3M4PMI is the ring N3-methyl compound stabilized as an imino-tautomer model of the 4APM. The UV absorption spectrum of 3M4PMI in n-hexane is also shown in Fig. 2c, in which the band maximum with vibronic structure was observed at ca. 323 nm. The band maximum had a much longer wavelength than that of the 4APM/AcOH system. Therefore, the imino-tautomer
The equilibrium constants (K) and enthalpy changes (−H; hydrogen bond energies) accompanying hydrogen bond formation between 4APM and AcOH are estimated as well as the 2APM/AcOH in previous paper [30]. The 4APM has the possibility to form hydrogen-bonded complex with two AcOH, because the pyrimidine ring has two nitrogen atoms. The stoichiometry of the complex was analyzed using Job’s plot method [37]. Job’s plot can be obtained by plotting A − A0 versus C4APM /(C4APM + CAC ). Here, A0 and A are the absorbances of 4APM in n-hexane and in a mixed solvent of AcOH and n-hexane, respectively. C4APM and CAC are the initial concentrations of 4APM and AcOH, respectively. The Job’s plot for the 4APM/AcOH system in n-hexane is shown in Fig. 3. The plot in Fig. 3 clearly provides evidence for the formation of 1:1 complex. The 4APM/AcOH (1:1) complex formation is considered to compete two equilibriums, since AcOH forms a number of selfdimerization in non-polar solvent (Scheme 2 and Eqs. (1) and (2)). 4APM + AcOH 4APM/AcOH complex
(1)
2(AcOH) AcOH dimmer
(2)
Fig. 3. The Job’s plots of A − A0 vs. C4APM /(C4APM + CAC ) for the 4APM/AcOH system in n-hexane. The UV absorption wavelength for the analysis was 285 nm.
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Scheme 2.
The association constant K for the AcOH monomer and dimer is expressed as CD , (CM )2 √ −1 ± 1 + 8K CAC = 4K
K =
(3)
CM
(4)
where CM is the concentration of the monomer, and CD is that of the dimer, CAC is the initial concentration of AcOH (CAC = CM + 2CD ). Eq. (1) of the equilibrium constant (K) between 4APM and AcOH can be written as given below: K=
CAC
CC , (C4APM − CC )(CAC − CC )
(5)
2 A−A0 1 = 2K +1 εc −ε0 (C4APM −(A−A0 )/(εc −ε0 ))K 1 A − A0 +1 + εc − ε0 (C4APM − (A − A0 )/(εc − ε0 ))K (6)
where Cc denotes the concentration of hydrogen-bonded complex. ε0 and εC are the molar extinction coefficients of 4APM and the hydrogen-bonded complex, respectively. Eq. (6) is performed by fitting to the plots of CAC versus A − A0 , and K and K can be estimated. Fig. 4 shows the best fitting curves for the 4APM/AcOH system in n-hexane at 20 ◦ C. From the
Fig. 4. The plots of CAC vs. A − A0 for the 4APM/AcOH system in n-hexane at different temperatures and the best non-liner least-squares fitting curves (RoseDrago method). The UV absorption wavelength for the analysis was 285 nm.
curves K = 1200 mol−1 dm3 and K = 7600 mol−1 dm3 were estimated. The value of K for the monomer and dimer of AcOH is reported to be 3200 mol−1 dm3 in CCl4 at 298 K [38]. The value of K (7600 mol−1 dm3 ) in n-hexane at 293 K is in fair agreement with that of 3200 mol−1 dm3 in CCl4 at 298 K. In order to obtain the enthalpy and entropy changes for 4APM/AcOH hydrogen-bonded complex formation, the temperature dependence of the equilibrium constant was determined, i.e., the absorption spectra of 4APM were measured in n-hexane containing various amounts of AcOH at various temperatures (293–323 K), as shown in Fig. 5. Plots of ln K versus 1/T gave a straight line, whose slope yielded 55.0 ± 1.8 kJ mol−1 for the enthalpy change (−H). The values of −H for the 2AP/AcOH and 2APM/AcOH systems have been reported to be 59.0 ± 3.4 kJ mol−1 and 51.0 ± 2.6 kJ mol−1 in n-hexane, respectively. These values are almost the same as that of −H for the 4APM/AcOH system. On the other hand, the value of −H for the 1:1 hydrogen bond formation between the ring nitrogen atom of the 4APM and the O–H group of phenol by IR spectrum is reported to be 25 kJ mol−1 [39]. Accordingly, the 4APM/AcOH system can be considered to have taken the structure which carried out the dual hydrogen bond formation between 4APM (ring nitrogen atom and amino group) and AcOH (OH and C O groups). 3.3. Fluorescence spectra Fig. 2a shows remarkable changes of the fluorescence spectra of 4APM with addition of AcOH in n-hexane at room tempera-
Fig. 5. The plots of ln K vs. 1/T for 4APM/AcOH system.
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Fig. 6. The excitation spectra of the 4APM (2.5 × 10−5 mol dm−3 ) and the 4APM (2.5 × 10−5 mol dm−3 )/AcOH (2.5 × 10−5 mol dm−3 ) system in nhexane at room temperature. The emission wavelength of 4APM was 334 nm (1). The emission wavelengths of 4APM/AcOH system were 334 nm (2) and 437 nm (3), respectively.
ture, compared with the UV absorption spectral behavior. The fluorescence peak (F1) of 4APM in n-hexane was observed at 353 nm in all concentrations. With the addition of AcOH, however, F1 decreased its intensity without a shift of the band and another large Stokes shifted emission band (F2) appeared at 414 nm. Furthermore, a clear isoemissive point was observed at 381 nm. These observations indicate the existence of a dual emitting species. The assignments of the dual emitting species can be resolved using the excitation spectra in Fig. 6. In the region of the F1 band wavelength (e.g. 353 nm), the excitation band maxima (264 nm) were independent of the monitored emission wavelengths. In conformity with the absorption band maximum of 4APM monomer, it was concluded that the F1 band results originate exclusively from the amino-form monomer of the excited 4APM. On the other hand, when the emission wavelength was monitored in the region of the F2 band wavelength (e.g. 414 nm), the excitation band shifted to 13 nm red and appeared at 277 nm. The excitation spectral feature at the 277 nm band resembles the growing portion of the absorption spectrum upon increasing AcOH in n-hexane. Accordingly, these are thus tentatively ascribed to the excitation spectrum of the 4APM/AcOH hydrogen-bonded complex. Alternatively, the dual fluorescences should originate from different ground-state precursors; as the free 4APM monomer and the 4APM/AcOH hydrogen-bonded complex. Consequently, the precursor of F2 band should be associated with the 4APM/AcOH hydrogen-bonded complex. Due to the high acidity of the carboxylic acid proton and the drastic increase of the basicity of the pyrimidal nitrogen, the fluorescence band at 414 nm may result from a single proton transfer from AcOH to the 4APM pyrimidal nitrogen, giving rise to the 4APM cation emission described below. Such a possibility has been eliminated by the sequence of experiment. Fig. 2d shows the fluorescence
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spectrum of 4APM in 5.0 × 10−5 mol dm−3 H2 SO4 aqueous solution at room temperature. The broad fluorescence spectrum was observed at 358 nm. The protonated 4APM cation exhibits a single fluorescence band with large intensity. This fluorescence peak at 358 nm was different from those of F1 and F2, and it did not show the vibronic structure band. This indicates that the F2 for the 4APM/AcOH system did not emit from the 4APM cation species. Furthermore, the fluorescence spectra of 4DMAPM were little affected by the addition of AcOH in n-hexane at room temperature and did not show the emission corresponding to the F2 (Fig. 2b). The broad fluorescence spectrum of 4DMAPM was observed at 334 nm. The fluorescence spectrum of this compound showed almost the same region as that of 4APM monomer, except for a relative increase of intensity in AcOH concentration. This indicates that the fluorescence spectrum of the 4DMAPM/AcOH system is an emission from the free 4DMAPM molecule in the S1 state. On the other hand, the F2 band with an unusually large Stokes shift of the 15,400 cm−1 relative to the absorption band of the free 4APM leads to the proposition that a possible occurrence of the excited double-proton transfer reaction, resulted from the formation of the imino-tautomer complex. To further verify this viewpoint, the model compound of the imino-tautomer complex, 3M4PMI possessed similar -electronic properties with respect to the imino-tautomer complex of 4APM. The fluorescence spectrum of 3M4PMI is also shown in Fig. 2c. This spectrum had the vibronic structure emission bands (ca. 1550 cm−1 ) near 437 nm. It was noted that the vibronic bands and band width of the fluorescence spectrum of 3M4PMI were very similar to the F2 of the 4APM/AcOH system near the same wavelength region (near 414 nm). This similarity supports the occurrence of AcOH-catalyzed excited amino–imino tautomerization reaction in the 4APM/AcOH 1:1 hydrogen-bonded complex. The results clearly indicate that the amino–imino tautomerization reaction for the hydrogen-bonded complex of the 4APM/AcOH system takes place in the S1 state, giving rise to the fluorescence from the imino-tautomer complex as shown in Scheme 3. The reaction is similar to the doubleproton transfer reaction of the 7AI/AcOH [40], 2AP/AcOH [24,29], and 2APM/AcOH [30] systems. In the present study of the steady-state measurement, the emissions corresponding to the hydrogen-bonded complexes of the 4APM/AcOH system was not observed. This suggests that the rate of the double-proton transfer must be much faster than the decay rate of the 4APM/AcOH amino-hydrogen-bonded complex in the S1 state.
Scheme 3.
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Table 1 The fluorescence quantum yields (Φf ) of 4APM, 4APM/AcOH imino-tautomer, cation, 3M4PMI, 2APM, 2APM/AcOH imino-tautomer, cation, 1M2PMI, 2AP, 2AP/AcOH imino-tautomer, cation, and MPI in n-hexane at room temperature Compound
Φf
4APM monomer 4APM/AcOH imino-tautomer Cation 3M4PMI
(1.1 ± 0.4) × 10−3 (8.8 ± 0.5) × 10−4 (3.7 ± 0.4) × 10−3a (9.3 ± 0.6) × 10−6
2APM 2APM/AcOH imino-tautomer Cation 1M2PMI
(1.3 ± 0.2) × 10−3b (4.2 ± 0.4) × 10−4b (3.3 ± 0.2) × 10−3b (3.1 ± 0.7) × 10−6b
2AP 2AP/AcOH imino-tautomer Cation MPI
(4.3 ± 0.4) × 10−2c (9.4 ± 0.3) × 10−3c (2.8 ± 0.0) × 10−1c (3.7 ± 0.9) × 10−5c
a b c
In 0.1 N-H2 SO4 . From Ref. [30]. From Ref. [24].
3.4. Fluorescence quantum yields The measurements of the fluorescence quantum yields (Φf ) were carried out to further clarify the results of the fluorescence intensities. The values of Φf of various 4APM, 4APM/AcOH imino-tautomer complex, cation, and 3M4PMI in n-hexane are listed in Table 1, compared with the corresponding 2APM and 2AP. The Φf values of 4APM, 4APM/AcOH imino-tautomer complex, cation, and 3M4PMI in n-hexane are roughly similar to those of the corresponding 2APM in isooctane [30], respectively. However, these Φf values are smaller 10−1 than those of the corresponding 2AP in isooctane except decrease in the Φf value of cation of 10−2 times. The pyrimidine molecule has two nitrogen atoms from the ring. Therefore, we can consider that the Sn, * is approaching very near the S, * as compared with the 2AP. The weak fluorescence decrease of the Φf values for the pyrimidines than the pyridines may suggest that strong mixing between Sn, * and S, * enhances the non-radiation transition as pseude-Jane-Teller effect and couple with the Tn, * as intersystem crossing. This is actually understood from the increase in the Φf values of cation. In Table 1, it is noted that the Φf values of the imino-tautomers are larger than those of the corresponding imino-model compounds. This can be explained by the fact that the Sn, * state of the imino-tautomer shifted to a higher energy level than that of the imino-model compound through hydrogen bond formation. 3.5. Fluorescence decay time profiles Previously, fluorescence decay times of 2AP, 2AP/AcOH system in n-hexane at room temperature monitored at F1 and F2, with that of the imino-model compound (1-methyl-2(1H)pyridinimine; MPI) were reported to be 1.4 ns, 3.2 ns, and 43 ps, respectively [28]. We have investigated the fluorescence decay time profiles of the 4AMP/AcOH system in n-hexane at room temperature
Fig. 7. (a) The fluorescence decay profile of the 4APM (2.5 × 10−5 ) in n-hexane monitored at the band ranging 325–380 nm (τ = 98 ps). Excitation wavelength: 260 nm. (b) The fluorescence decay profile of the 4APM (2.5 × 10−5 )/AcOH (1.0 × 10−2 ) in n-hexane monitored at 400–500 nm (τ = 73 ps). Excitation wavelength: 260 nm. (c) The fluorescence decay profile of the 3M4PMI only (2.5 × 10−5 ) in n-hexane monitored at 450–525 nm (τ = 19 ps). Excitation wavelength: 320 nm. All curves fit assuming a single-exponential decay.
monitored at F1 and F2 with that of the imino-model compound. The fluorescence decay profiles monitored at the F1 band (Fig. 7a) of the 4APM monomer in n-hexane was fitted to a single-exponential function with time constant of 98 ps. The rate constant of the fluorescence decay was kf1 , is of 1.02 × 1010 s−1 . On the other hand, we found that the tautomer fluorescence (F2) of the 4APM/AcOHsystem exhibits a wavelength-independent single-exponential decay of 73 ps in a n-hexane solution (Fig. 7b) with the decay rate, kf2 of 1.37 × 1010 s−1 . The excited-state double-proton transfer reaction of the 4APM/AcOH system in n-hexane clarified in the present fluorescence decay time measurements is summarized in Fig. 8. The fluorescence decay time of the F2 for the 4APM/AcOH system was shorter than that of the 4APM monomer (F1) in n-hexane. On the other hand, the decay time (3.2 ns) of the F2 for the 2AP/AcOH was longer than that (1.4 ns) of the 2AP monomer (F1) in n-hexane. The double-proton transfer in the 4APM/AcOH system occurs in a very short time, compared with the 2AP/AcOH system. It is thought that the double-proton
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Fig. 8. Schematic energy diagram illustrating the dynamics of the S1 state double-proton transfer reaction of the 4APM/AcOH system with 3M4PMI.
transfer for the 4APM/AcOH system in n-hexane at room temperature occurs very rapidly because the activation energy for the amino → imino tautomer reaction is very small or almost zero in the S1 state. Further, the fluorescence decay profile of 3M4PMI was fitted to a single-exponential function with time constant of 19 ps (Figs. 6c and 7). The rate constant, kfi is of 5.2 × 1010 s−1 and this reaction is particularly rapid. It is considered that 3M4PMI has the electronic flexible structure in the S1 state (see theoretical approaches in S1 state), the excited state, Sn, * state, of 3M4PMI is to a lower energy level than that of the hydrogenbonded imino-tautomer complex with AcOH, and strong mixing between Sn, * and S, * enhances the non-radiation transition, as described previously in the MPI molecule [41]. In addition, the Φf value of 3M4PMI in n-hexane is very small (Φf = 9.3 × 10−6 ), as given in Table 1. As the result of the strong mixing, the fluorescence decay time (19 ps) of 3M4PMI in nhexane at room temperature may become very short due to the flexible structure and low Φf value, as well as that of MPI in n-hexane (43 ps). 3.6. Theoretical approaches in the S0 state In order to have a better understanding of the above experimental results, we performed theoretical calculations. Fig. 9 shows the optimized geometries of various proton transfer isomers of 4APM; E- and Z-3H-4(1H)pyrimidinimines (3H4PMI-E and 3H4PMI-Z) and E- and Z1H-4(1H)-pyrimidinimines (1H4PMI-E and 1H4PMI-Z) at the B3LYP/6-311++G(d,p) level of theory, with those of E- and Z-3methyl-4(1H)-pyrimidinimines (3M4PMI-E and 3M4PMI-Z).
These molecules have almost planar structures, though the amino group of 4APM takes pyramidal. The total energies (ET ) of these compounds and the energy differences (ET ) between 4APM and tautomer are given in Table 2. The direct proton-transfer tautomer forms of the 4APM monomer, i.e., 3H4PMI-E (see Fig. 9 and Table 2) were calculated to be 55.3 kJ mol−1 higher in energy than the normal form. Among the possible four isomers (tautomers) of the direct proton transfer, 3H4PMI-E form was the most stable. However, as described above in Section 2, the presence of the 3H4PMI-E was not found from the result of the UV spectral measurement of 4APM in n-hexane at room temperature. The optimized geometries and the ET of 3M4PMI-E and 3M4PMI-Z as the tautomer model compounds are also given in Fig. 9 and Table 2, respectively. In this Table 2, E-form is stable in energy than Zform by 14.4 kJ mol−1 . Accordingly, the synthesized 3M4PMI is considered to be the E-form. Fig. 10 shows the optimized geometries of the 4APM/AcOH and 3H4PMI/AcOH complexes at the S0 states by B3LYP/6Table 2 The S0 state calculated total energies (ET ) of 4APM, tautomers, and tautomer model compounds by B3LYP/6-311++G(d,p) level Compounds
ET (a.u.)
ET (kJ mol−1 )
ET (kJ mol−1 )
4APM 3H4PMI-E 3H4PMI-Z 1H4PMI-E 1H4PMI-Z 3M4PMI-E 3M4PMI-Z
−319.693009 −319.671961 −319.667348 −319.657422 −319.663861 −358.963564 −358.958062
0 55.3 67.4 93.4 76.5 – –
– – – – – 0 14.4
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T. Kitamura et al. / Spectrochimica Acta Part A 69 (2008) 350–360 Table 3 The S0 state B3LYP/6-311++G(d,p) level calculated total energies (ET ), energy differences (ET ), and hydrogen bond energies (−Hcalc ) of 4APM and 3H4PMI/AcOH system Compounds
ET (a.u.)
ET (kJ mol−1 )
−Hcalc (kJ mol−1 )
4APM/AcOH 3H4PMI/AcOH
−548.817796 −548.805430
0 32.5
56.0 79.0
311++G(d,p) level of theory. Strong dual hydrogen bond formation in the 4APM/AcOH complex is indicated by the short ˚ and 1.730 A, ˚ H(1)· · ·O(1) and N(2)· · ·H(2) distances of 1.880 A respectively. In comparison to the 4APM monomer (Fig. 9) ˚ significant changes in N(2)· · ·C(1) 1.353 − 1.342 = +0.011 A ˚ and in C(1)–N(1) 1.347 − 1.366 = −0.019 A, respectively, were found in the formation of the 4APM/AcOH complex. There results correlate well with the concept of conjugated dual hydrogen bond effect [29,42], in which the bond distances relevant to the n-electron delocalization are subject to change. Table 3 shows that hydrogen bond energy (−Hcalc ) was calculated at the B3LYP/6-311++G(d,p) level of theory to be 56.0 kJ mol−1 for the 4APM/AcOH complex and 79.0 kJ mol−1 for the 3H4PMI/AcOH complex. The value for the 4APM/AcOH complex is smaller than that for the 3H4PMI/AcOH complex, however, the normal form complex was more stable in energy than the tautomer complex by 32.5 kJ mol−1 at the B3LYP/6-311G++(d,p) level. This result indicates that the 4APM/AcOH complex is more easily formed than the 3H4PMI/AcOH complex in the S0 state. The −Hcalc value (56.0 kJ mol−1 ) for the 4APM/AcOH complex formation was in fair agreement with the experimental data of −H (55.0 ± 1.8 kJ mol−1 ) in n-hexane. On the other hand, it is noted that the presence of the 3H4MPI/AcOH complex in the S0 state was not found from the results of UV absorption spectra. 3.7. Theoretical approaches in the S1 state
Fig. 9. The optimized geometries of normal 4APM monomer, tautomer, and tautomer model compounds at the S0 states by B3LYP/6-311++G(d,p) level. ˚ unit. The values indicate the bond distances in A
The CIS method, which has been proven to be a relatively useful method to obtain the approximate wave function and molecular geometry of electronic excited states, was applied in this study. Fig. 11 shows the optimized geometries of the 4APM, 3M4PMI, 4APM/AcOH, and 3H4PMI/AcOH. In the S1 state, the amino group of 4APM become near planar and the 4APM, 4APM/AcOH, and 3H4PMI/AcOH have almost near planar structures, but the ring skeleton of 3M4PMI has a non-
Fig. 10. The optimized geometries of 4APM/AcOH and 3H4PMI/AcOH at the S0 states by B3LYP/6-311++G(d,p) level. The values indicate the bond distances in ˚ unit. A
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Fig. 11. Calculated stationary point geometries of for 4APM, 3M4PMI, and normal and proton-transfer tautomer of the 4APM/AcOH complex in the S1 states by ˚ unit. CIS/6-311++G(d,p) level. The values indicate the bond distances in A
planar structure like the twist-boat form of cyclohexane, as shown in Fig. 11 in detail. The total energies (ET ) of these compounds are given in Table 4. Table 4 shows that the ET value for the 4APM/AcOH complex was smaller than that for the 3H4PMI/AcOH complex, and the 3H4PMI/AcOH complex was more stable in energy than the normal form complex by 19.7 kJ mol−1 . This indicates that 3H4PMI/AcOH complex is more easily formed than the 4APM/AcOH complex in the S1 state. 3.8. Potential energy surface for S0 and S1 states
The energy barrier for the 4APM/FoOH complex → transition state (TS) process in the S1 state was calculated to be 2 kJ mol−1 using the TD-B3LYP/6-31G(d) method. The barrier value was less than that in the S0 state by 33 kJ mol−1 for the B3LYP/6-31G(d) method. Particularly, the energy barrier for the 4APM/FoOH complex → TS process in the S1 state is very small. This indicates that in the S1 state 3H4PMI/FoOH complex may be easily formed through the dual proton transfer of the 4APM/FoOH complex. In the S0 state, the energy barrier of reverse proton transfer (3H4PMI/FoOH complex → TS) was calculated to be almost zero kJ mol−1 for B3LYP/6-31G(d) method. This reverse proton transfer in S0
The potential energy surfaces along the proton transfer reaction in the S0 and S1 states were calculated to make clear the amino–imino tautomerization process. For the sake of simplicity, the formic acid (FoOH) complex was used for the AcOH complex. The N(1)–H(1) and O(2)–H(2) H-bonding distances ˚ in each step as shown in Fig. 12. The were scanned by 0.02 A rest of the bond angles and distances were fully optimized. Table 4 The S1 state CIS/6-311++G(d,p) level calculated total energies (ET ) of 4APM, tautomer, tautomer model compounds, 4APM/AcOH, and 3H4PMI/AcOH systems Compounds
ET (a.u.)
ET (kJ mol−1 )
ET (kJ mol−1 )
4APM 3H4PMI-E 3M4PMI-E 4APM/AcOH 3H4PMI/AcOH
−317.512635 −317.458081 −356.525487 −545.345638 −545.353139
0 143.2 – – –
– – – 0 −19.7
Fig. 12. Potential energy surfaces of 4APM/FoOH system in the S0 and S1 states byB3LYP and TD-B3LYP methods.
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state is so rapid that the presence of the S0 state 3H4MPI/FoOH complex could not be found by the measurement of the UV absorption spectra. As described above, it is considered what must be a fundamental difference between the FoOH complex and the AcOH complex. Our experimental and/or theoretical results of UV absorption and fluorescence spectra, decay times, and theoretical calculations should serve as useful guides to the amino–imino tautomerization for the 4-aminopyrimidine moieties in the vitamin B1 coenzyme and DNA base pairs. Acknowledgements This work was supported in part by grants from the Research Institute for Technology of Tokyo Denki University. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
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