The hydrogen bonding and amino–imino tautomerization of the alkoxy-aminopyridines and amino-methoxypyrimidines with acetic acid

Spectrochimica Acta Part A 68 (2007) 979–991

The hydrogen bonding and amino–imino tautomerization of the alkoxy-aminopyridines and amino-methoxypyrimidines with acetic acid The effects of the methoxy group Teruyoshi Kitamura a , Naomi Mochida a , Masahiro Okita a , Mistuya Motohashi b , Hironori Ishikawa c , Akira Fujimoto a,∗ a

Department of Environmental Materials Science, Tokyo Denki University, Kanda, Chiyoda-ku, Tokyo 101-8457, Japan b Department of Materials Science and Engineering, Graduate School of Engineering, Tokyo Denki University, Kanda, Chiyoda-ku, Tokyo 101-8457, Japan c Mitsubishi Chemical Group, Science and Technology Research Center Mitsubishi Chemical Corporation, Kamoshida-cho, Aoba-ku, Yokohama 227-8502, Japan Received 23 August 2006; accepted 13 January 2007

Abstract The hydrogen bonding and amino–imino tautomerization of the systems of 2-amino-3-methoxypyridine (2A3MOP), 2-amino-6-methoxypyridine (2A6MOP), 2-amino-6-n-propoxypyridine (2A6NPOP), 2-amino-6-iso-propoxypyridine(2A6IPOP), 2-amino-4-methoxypyrimidine (2A4MOPM), 4-amino-2-methoxypyrimidine (4A2OPM), 4-amino-6-methoxypyrimidine (4A6MOPM), 2-amino-4-methoxy-6-methylpyrimidine (MMPM), and 2-amino-4,6-dimethoxypyrimidine (DMOPM), with acetic acid (AcOH) in n-hexane at room temperature were investigated by means of the UV absorption and fluorescence spectroscopy. From the UV absorption spectra the presence of the dual hydrogen-bonded complexes that linked by a 1:1 molar ratio with AcOH were found, since the enthalpy changes accompanying the hydrogen bond formation between 2A3MOP, 2A4MOPM, 4A2MOPM, 4A6MOPM, or MMPM, and AcOH were ca. 42.8–61.1 kJ mol−1 in n-hexane. The fluorescence spectra of the 2A3MOP/AcOH, 2A4MOPM/AcOH, 4A6MOPM/AcOH, and MMPM/ AcOH systems revealed that the imino-tautomers were produced through double proton transfer in the amino hydrogen-bonded 1:1 complexes in the S1 state, but the imino-tautomer formation for the 4A2MOPM/AcOH system was not found on account of the steric hindrance due to the inversion of the methoxy group in the S1 state. The imino-tautomer for the MMPM/AcOH system fluoresces most intensely among these systems investigated. On the other hand, not only the formation of the corresponding amino dual hydrogen-bonded complex and but also that of imino-tautomer were prevented for the 2A6MOP/AcOH, 2A6NPOPM/AcOH, 2A6IPOP/AcOH, and DMOPM/AcOH systems, because of the steric hindrance of the methoxy group in both the S0 and S1 states. The theoretical approaches by an ab initio molecular orbital calculation were in accord with the experimental results. © 2007 Elsevier B.V. All rights reserved. Keywords: Alkoxy-aminopyridines; Amino-methoxypyrimidines; Amino–imino tautomerization; Fluorescence spectra; Proton transfer

1. Introduction Amino–imino tautomerization facilitated hydrogen bonding in the excited state of heterocyclic compounds is an elementary process which plays an important role in organic and biochemical reactions [1–5]. The UV irradiation to the compounds causes DNA changes and these changes are responsible for the mutation [5–7]. It is well known that thymine forms the dimer in DNA [7].

∗

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.2007.01.011

On the other hand, the formation of tautomer species through the proton transfer in hydrogen bonding occurs in the excited state [8–16]. This is suggested is to be responsible for the mutation. The amino–imino tautomerization is of interest in connection with the DNA damage. The amino–imino tautomerization of the system of the 2-aminopyridine (2AP), 2-aminopyrimidine (2APM), or these methyl derivatives with acetic acid (AcOH) had been was already reported [9,15]. 2AP and 2APM having methyl group at 4- and/or 6-positions were increase in the imino-tautomer formation, because of the electron donating methyl group. It is well known that the electron donating property of methoxy group is stronger than that of methyl group.

980

T. Kitamura et al. / Spectrochimica Acta Part A 68 (2007) 979–991

Accordingly, the imino-tautomer formation for the methoxy derivatives of 2AP and 2APM could be expected to be greater than that for the methyl derivatives. On the other hand, the methoxy group adjacent to the ring nitrogen atom of the pyridine and pyrimidine having the amino group may serve as the steric hindrance, and it may contribute to prevent the imino-tautomer formation. In the present study, the amino–imino tautomerization for the systems of 2-amino-3-methoxypyridine (2A3MOP), 2-amino-6-methoxypyridine (2A6MOP), 2-amino-6-n-propoxypyridine (2A6NPOP), 2-amino-6-iso-propoxypyridine (2A6IPOP), 2-amino-4-methoxypyrimidine (2A4MOPM), 4amino-2-methoxypyrimidine (4A2MOPM), 4-amino-6-methoxypyrimidine (4A6MPM), 2-amino-4-methoxy-6-methylpyrimidine (MMPM), and 2-amino-4,6-dimethoxypyrimidine (DMOPM), with AcOH in n-hexane at room temperature were investigated by means of the UV absorption and fluorescence spectroscopy. Furthermore, the theoretical consideration by the molecular orbital calculation will also make more clearly the role of the methoxy group for the amino–imino tautomerization of aminopyridines and aminopyrimidines. As the results, when there is the alkoxy group adjacent to the ring nitrogen atom of aminopyridines or aminopyrimidines, it is considered to serve to as the steric hindrance by the partially possible conformation. This situation will result that the fluorescence of the imino-tautomer for the alkoxyaminopyridine/AcOH and amino-methoxypyrimidine/AcOH systems cannot be recognized. In the case of no steric hindrance, the ordinary fluorescence of the imino-tautomer will be able to be perceived clearly; the corresponding imino-tautomer is formed through the double proton transfer in the amino hydrogen-bonded complex in the S1 state. Fig. 1 shows the molecular structures related to the effects of methoxy group for the amino–imino tautomerization of aminopyridines and aminopyrimidines.

2. Experimental 2.1. Materials 2A3MOP, 2A6MOP, 2A6NPOP, and 2A6IPOP were prepared by the methods of Mase et al. [17]. 2A4MOPM and 4A2MOPM were prepared by the method of Karlinskaya and Khromov-Borisov [18]. 4A6MOPM, MMPM, and DMOPM were purchased from Aldrich Co. Ltd., were recrystallized several times from n-hexane, and were sublimed twice in vacuo at 80 ◦ C before use. The preparations of 4-methoxy-1-methyl-2(1H)-pyrimidinimine (4OPMI) and 1,6-dimethyl-4-methoxy-2(1H)-pyrimidinimine (MMPMI) are described in the literature [19]. AcOH (KANTO Chemical) was of atomic absorption spectrograde and was used without further purification because no fluorescence was detected in the wavelength region of interest. n-Hexane (KANTO Chemical) was of spectrograde and was used without further purification. 2.2. Measurements The UV absorption and the fluorescence spectra with the excitation spectra were measured using a Hitachi model U3410 and F-4010 spectrophotometers, in a 1 cm quartz cuvette at room temperature, respectively. The 1 H-NMR spectra measurements were carried out with a Brucker FT-NMR spectrometer (Model DPX300) in 5 mm sample tubes at 25 ◦ C. DMSO-d6 (99.6% D) from Aldrich was used for the NMR measurements. TMS was used as internal for chemical shift. The 1 H-NMR spectral data of the 2A3MOP, 2A6MOP, 2A6NPOP, 2A6IPOP, 2A4MOPM, 4A2MOPM, 4OPMI, and MMPMI were given below. 2A3MOP; 1 H-NMR(DMSO-d6 ) δ = 2.50 (3H, s, OCH3 ), 5.58 (2H, br, NH2 ), 6.49 (1H, q, J = 5 and 7 Hz, C5 -H), 6.99 (1H, d, J = 7 Hz,C4 H), 7, 49(1H, d, J = 5 Hz, C6 H).

Fig. 1. Molecular structures of alkoxy-aminopyridines, amino-methoxy-pyrimidines, and imino-model compounds: 2-amino-3-methoxypyridine (2A3MOP), 2-amino-6-methoxypyridine (2A6MOP), 2-amino-6-n-propoxypyridine (2A6NPOP), 2-amino-6-iso-propoxypyridine (2A6IPOP), 2-amino-4-methoxypyrimidine (2A4MOPM), 4-amino-2-methoxypyrimidine (4A2MOPM), 4-amino-6-methoxypyrimidine (4A6MOPM), 2-amino-4-methoxy-6-methylpyrimidine (MMPM), 2amino-4, 6-dimethylpyrimidine (DMOPM), 4-methoxy-1-methyl-2(1H)-pyrimidinimine (4OPMI), and 6-methoxy-3,4-dimethyl-2(1H)-pyrimidinimine (MMPMI).

T. Kitamura et al. / Spectrochimica Acta Part A 68 (2007) 979–991

2A6MOP; 1 H-NMR (CDCl3 ) δ = 3, 72 (3H, s, OCH3 ), 5.81 (2H, br, NH2 ), 5.92 (1H, d, J = 7 Hz, C3 H), 5.99 (1H, d, J = 7 Hz, C5 H), 7.27 (1H, t, J = 7 Hz, C4 H). 2A6NPOP; 1 H-NMR (DMSO-d6 ) δ = 0.93 (3H, t, J = 7 Hz, CH3 ), 1.66 (2H, q, J = 7 Hz, –CH2 –), 4, 06 (2H, t, J = 6Hz, –CH2 –), 5.76 (2H, br, NH2 ), 5.84 (1H, d, J = 7 Hz, C3 H), 5.96 (1H, d, J = 6Hz, C5 H), 7.25 (1H, t, J = 7Hz, C4 H). 2A6IPOP; 1 H-NMR (DMSO-d6 ) δ = 1.21 (6H, d, J = 6 Hz, CH3 × 2), 5.13 (1H, seven-fold, J = 6 Hz, –CH ), 5.73 (2H, br, NH2 ), 5.79 (1H, d, J = 8 Hz, C3 H), 5, 94 (1H, d, J = 8 Hz, C5 H), 7.23 (1H, t, J = 8 Hz, C4 H). 2A4MOPM; 1 H-NMR (DMSO-d6 ) δ = 3.77 (3H, s, CH3 ), 5.97 (1H, d, J = 6 Hz, C5 H), 6.53 (2H, s, NH2 ), 7.93 (1H, d, J = 6 Hz, C6 H). The nuclear overhauser effect (NOE) was observed between the signals of the O CH3 and the C5 H of 2A4MOPM from the measurement of two-dimensional NOESY spectrum. This means that the OCH3 group is introduced to the pyrimidine ring at the 4-position. 4A2MOPM; 1 H-NMR(DMSO-d6 ) δ = 3.73(3H, s, CH3 ), 6.05 (1H, d, J = 6 Hz, C5 H),6.84 (2H, br, NH2 ), 7.84 (1H, d, J = 6 Hz,C6 H). The nuclear overhauser effect (NOE) was observed between the signals of the NH2 and the C5 H of 4A2MOPM from the measurement of two-dimensional NOESY spectrum. This means that the NH2 group is introduced to the primidine ring at the 4-position. 4OPMI; 1 H-NMR(DMSO-d6 ) δ = 2.13(3H, s, O–CH3 ), 3.27(3H, s, NCH3 ), 5.47(1H, d, J = 6 Hz, C5 H), 5.96 (1H, br, NH), 7.27 (1H, d, J = 6 Hz,C6 H). MMPMI; 1 H-NMR (DMSO-d6 ) δ = 2.18 (3H, s, C6 CH3 ), 3.29 (3H, s, N1 CH3 ), 3.68 (3H, s, OCH3 ), 5.43 (1H, s, C5 H), 5.93 (1H, br, NH). The nuclear overhauser effect (NOE) was observed between the signals of the N1 CH3 and the C6 CH3 of MMPMI from the measurement of two-dimensional NOESY spectrum. This means that the CH3 group is introduced to the pyrimidine ring at the 1-position of MMPI, rather than at the 3-position.

981

Fig. 2. The UV absorption and fluorescence spectra of the 2A3MOP/AcOH system in n-hexane at 20 ◦ C. Concentration of 2A3MOP: 1 × 10−4 mol dm−3 ; concentration of AcOH (mol dm−3 ) (1) 0; (2) 2 × 10−5 ; (3) 5 × 10−5 ; (4) 1 × 10−4 ; (5) 2 × 10−4 ; (6) 5 × 10−4 ; (7) 1 × 10−3 ; (8) 2 × 10−3 ;(9) 5 × 10−3 . Excitation wavelength was 311 nm.

3. Results and discussion 3.1. The UV absorption spectra of the alkoxy-aminopyridine/AcOH systems Fig. 2 shows the UV absorption spectra of the 2A3MOP with the addition of various concentrations of AcOH in hexane at 20 ◦ C. From the UV absorption spectra, its band maximum of the 2A3MOP was observed at 294 nm. The absorption band maximum shifted to a longer wavelength with increasing AcOH concentration and become to be observed at 301 nm (7 nm). Three isosbestic points were observed at 233 nm, 256 nm, and 288 nm, respectively. This result indicates that the spectral changes are due to the following chemical equilibriums (Scheme 1 and Eqs. (1) and (2)): 2A3MOP + AcOH monomer  2A3MOP − AcOH complex (1) 2(AcOH)  AcOH dimmer

2.3. Methods of calculation The ab initio molecular orbital calculation was performed by use of the package program of Gaussian 0.3 W.Rev.B.05 [20]. The geometry optimizations for all molecular systems in the S0 state were carried out with the 6–31G(d, p) basis set [21] at the restricted B3LYP and Hartree-Fock (RHF) levels, respectively. All structures were vibrationally characterized, checking for the absence of imaginary frequencies in the energy minimum. The enthalpy changes −Hcalc accompanying hydrogen-bonded formation were calculated from the energy difference between the hydrogen-bonded complex and two different monomers. The geometry optimization of the excited (S1) state was carried out at the configuration interaction with single excitations (CIS) with the 6-31G(d) basis set level [22].

(2)

The equilibrium constant K of Eq. (1) and the association constant K of Eq. (2) can be estimated using an equation proposed by Rose and Drago [23], Chou and coworkers [13] and Chou et al. [24]. Then the association constant K for the AcOH

Scheme 1.

982

T. Kitamura et al. / Spectrochimica Acta Part A 68 (2007) 979–991

Fig. 3. The plots of CAC versus A–A0 for the 2A3MOP/AcOH system in nhexane at different temperatures and a best non-liner least-squares fitting curves (Rose-Drago method). The UV absorption wavelength for the analysis was 315 nm.

monomer and dimmer in expressed as CD (CM )2

K = CM =

−1 ±

(3) √

1 + 8K CAC 4K

(4)

where CM is the concentration of AcOH monomer, and CD is that of AcOH dimer. Here, CAC is the initial concentration of AcOH; CAC = CM + 2CD . Eqs. (5) and (6) of the equlibrium constant K between the 2A3MOP and AcOH in n-hexane can be written as follows. K=

CC (C2A3MOP − CC )(CAC − CC )

CAC

(5)

 2 A−A0 1 +1 εc −ε0 (C2A3MOP −(A−A0 )/(εc −ε0 ))K   1 A−A0 + +1 (6) εc −ε0 (C2A3MOP −(A−A0 )/(εc − ε0 ))K

=2K



where Cc denotes the concentration of hydrogen-bonded complex, C2A3MOP is the initial concentration of 2A3MOP in n-hexane. A0 and A are the absorbances of 2A3MOP in n-hexane and in a mixed solvent of AcOH and n-hexane, respectively. ε2A3MOP and εc are the molar extinction coefficients of the 2A3MOP and the hydrogen-bonded complex, respectively. Eq. (6) is performed by fitting to the plots of CAC versus A–A0 , and K, K and εc can be estimated. Fig. 3 shows the best fitting curves for the 2A3MOP/AcOH in n-hexane. From the curves, K = 6600 mol−1 dm3 and K = 7600 mol−1 dm3 were estimated. The value of K for the monomer and dimmer of AcOH is reported to be 3200 mol−1 dm3 in CCl4 at 298 K [25]. The value of K (7600 mol−1 dm3 ) in n-hexane is in fair agreement with that of 3200 mol−1 dm3 in CCl4 at 298 K. Next, assuming that the enthalpy changes (−H) are independent of temperature.

Fig. 4. The Plots of ln K versus 1/T for 2A3MOP/AcOH system.

The enthalpy changes (−H) accompanying hydrogen bond formation can be obtained from a knowledge of the variation of K with temperature, according to the well-known equation of van’t Hoff. Fig. 4 shows the plots of ln K versus 1/T for the 2A3MOP/AcOH system in n-hexane. The value of −H between the 2A3MOP and AcOH was estimated to be 42.8 kJ mol−1 . The −H value (42.8 kJ mol−1 ) is similar to those of 2-aminopyridine (2AP)/AcOH system (32.6 kJ mol−1 in cyclohexane) [13] and 2-aminopyrimidine (2APM)/AcOH system (41.2 kJ mol−1 ) in isooctane (2,2,4-trimethylpentane) [15]. This value (42.8 kJ mol−1 ) is also smaller to that of 3-methyl2-aminopyridine (3MAP)/AcOH system (58.2 kJ mol−1 ) in isopentane [9]. Accordingly, the 2A3MOP/AcOH system can be considered to have taken the structure which carried out the dual hydrogen bond formation between 2A3MOP (ring nitrogen atom and amino group) and AcOH (OH and C O groups) (Scheme 1). Further, the free energies (−G) and entropy changes (−S) at 293 K were calculated using the K and −H. The K, −H, −G, and −S for the 2A3MOP/AcOH system is summarized in Table 1, with the thermodynamic properties estimated from the UV spectral data for the other systems of the methoxy derivative and AcOH in the present study. Figs. 5–7 how the UV absorption and fluorescence spectra of 2A6MOP, 2A6NPOP, and 2A6IPOP with addition of various concentration of AcOH in n-hexane at 20 ◦ C, respectively. The UV absorption spectra of free 2A6MOP, 2A6NPOP, and 2A6IPOP in n-hexane were similar to each other, and were little affected by the addition of AcOH, compared with the 2A3MOP/AcOH system in Fig. 2. That is, even n-propoxy or iso-propoxy group was used in the place of methoxy group at the 6-position, a large difference in the UV absorption spectra of the 2A6NPOP/AcOH and 2A6IPOP/AcOH systems could not be detected, compared with the 2A6MOP/AcOH system. Therefore, the UV absorption spectra of these three 6alkoxy substituted-2-aminopyrimidine/AcOH systems were not

T. Kitamura et al. / Spectrochimica Acta Part A 68 (2007) 979–991

983

Table 1 Observed values of K, −H, −G and −S for the various systems of alkoxy-aminopyridines and aminomethoxy pyrimidines with AcOH in n-hexane, with calculated values of −Hcalc Systems

K × 10−3 (dm3 mol−1 )a

−H (kJ mol−1 )

−G (kJ mol−1 )a

2A3MOP/AcOH 2A6MOP/AcOH 2A6NPOP/AcOH 2A6IPOP/AcOH 2A4MOPM/AcOH 4A2MOPM/AcOH 4A6MOPM/AcOH MMPM/AcOH DMOPM/AcOH

6.60 ± 0.14 – – – 12.4 ± 0.3 6.80 ± 0.15 3.84 ± 0.09 17.2 ± 1.0 –

42.8 ± 3.6 – – – 60.0 ± 2.4 58.3 ± 2.4 48.3 ± 3.1 61.1 ± 4.4 –

21.4 ± 0.1 – – – 23.0 ± 1.4 21.5 ± 0.3 20.1 ± 0.1 23.8 ± 1.3 –

a

−S (J K−1 mol−1 )a 72.9 ± 12.0 – – – 126 ± 8.0 126 ± 6.0 96.0 ± 10.7 127 ± 15.8 –

−Hcalc (kJ mol−1 ) 46.65 – – – 43.31 46.16 42.44 42.04 –

At 20 ◦ C.

Fig. 5. The UV absorption and fluorescence spectra of the 2A6MOP/AcOH system in n-hexane at 20 ◦ C. Concentration of 2A6MOP: 1 × 10−4 mol dm−3 ; concentration of AcOH (mol dm−3 ) (1) 0; (2) 2 × 10−5 ; (3) 5 × 10−5 ; (4) 1 × 10−4 ; (5) 2 × 10−4 ; (6) 5 × 10−4 ; (7) 1 × 10−3 ; (8) 2 × 10−3 ; (9) 5 × 10−3 . Excitation wavelength was 275 nm.

analyzed because these UV spectra were not so effective change as expected. However, in the case of the 2A3MOP/AcOH system the methoxy group introduced at the 3-position of 2AP did not interfere with the formation of amino-form hydrogen-bonded complex with AcOH by the direction on the methoxy-methyl group.

Fig. 6. The UV absorption and fluorescence spectra of the 2A6NPOP/AcOH system in n-hexane at 20 ◦ C. Concentration of 2A6NPOP: 1 × 10−4 mol dm−3 ; concentration of AcOH (mol dm−3 ) (1) 0; (2) 2 × 10−5 ; (3) 5 × 10−5 ; (4) 1 × 10−4 ; (5) 2 × 10−4 ; (6) 5 × 10−4 ; (7) 1 × 10−3 ; (8) 2 × 10−3 ; (9) 5 × 10−3 . Excitation wavelength was 275 nm.

Fig. 7. The UV absorption and fluorescence spectra of the 2A6IPOP/AcOH system in n-hexane at 20 ◦ C. Concentration of 2A6IPOP: 1 × 10−4 mol dm−3 ; concentration of AcOH (mol dm−3 ) (1) 0; (2) 2 × 10−5 ; (3) 5 × 10−5 ; (4) 1 × 10−4 ; (5) 2 × 10−4 ; (6) 5 × 10−4 ; (7) 1 × 10−3 ; (8) 2 × 10−3 ; (9) 5 × 10−3 . Excitation wavelength was 290 nm.

On the other hand, when the methoxy group at the 6-position of pyridine ring is presence, the 2-aminopyridines have two conformations with respect to the hydrogen-bonded ring nitrogen atom, as shown in Scheme 2. One (conformer (A)) of them has steric hindrance for hydrogen bond formation with AcOH. The other (conformer B) makes the hydrogen bonds without this situation. As described in the later section (see Section 3.3, theoretical approaches), the 2A6MOP/AcOH, 2A6NPOP/AcOH, and 2A6IPOP/AcOH systems indicate that the conformer (A) is preferable to the conformer (B) in the S0 state. This indicates that the methoxy group of the conformer (A) may act as a marked inhibition for the formation of the amino dual hydrogen-bonded complex.

Scheme 2.

984

T. Kitamura et al. / Spectrochimica Acta Part A 68 (2007) 979–991

3.2. The fluorescence spectra of the alkoxy-aminopyridine/AcOH systems In Fig. 2, the fluorescence spectra of 2A3MOP shows a remarkable changes with addition of AcOH in n-hexane at room temperature, compared with the corresponding UV absorption spectral behavior. The fluorescence peak (F1) of 2A3MOP was observed at 340 nm in any concentrations. With the addition to AcOH, however, F1 decreases its intensity without shift of the band and another large Stokes shifted emission band (F2) appeared at 410 nm. Furthermore, a clear isoemissive point appeared at 375 nm. These indicate the existence of two emitting species. In the previous papers [9,15], the similar result to the 2A3MOP/AcOH system was observed for the 2AP/AcOH and 2APM/AcOH systems. In the region of F1 band wavelength (e.g. 350 nm), the excitation band maxima were independent of the monitored emission wavelengths. In conformity with the absorption band maximum of 2A3MOP monomer, it is concluded that F1 band results exclusively to originate from the monomer of the 2A3MOP. On the other hand, the F2 band with the large Stokes shift leads to propose a possible occurrence of the excited double proton transfer reaction, resulting the formation of the iminotautomer, as well as the 2AP/AcOH and 2APM/AcOH systems reported in the previous papers [9,15]. It is noted that the F2 fluorescence intensity for the 2A3MOP/AcOH system is similar to those of the 2AP/AcOH and 3MAP/AcOH systems. The results is clearly explained that the amino–imino tautomerization reaction through the dual hydrogen bonds of the 2A3MOP/AcOH system takes place in the S1 state, giving rise to the iminotautomer, as shown in Scheme 3. In the present study of the steady-state measurement, the emission corresponding to the amino-form hydrogen-bonded complex of the 2A3MOP/AcOH system was not almost observed, because the rate of the double proton transfer must be much faster than the decay rate of the amino hydrogen-bonded complex in the S1 state. In addition, the UV absorption of the imino-tautomer complex for this system was not almost found, because the S0 state imino-tautomer is considered to be less stable than the S0 state amino hydrogen-bonded complex, so the back proton transfer takes place forming the original amino hydrogen-bonded complex S0 state.

Scheme 3.

As shown in Figs. 5–7, the fluorescence spectral changes of the 2A6MOP/AcOH, 2A6NPOP/AcOH, and 2A6IPOP/AcOH systems occurred very slightly, compared with that of the 2A3MOP/AcOH system in n-hexane at room temperature. The fluorescence spectral changes for these systems were slightly larger than those of the UV spectral changes. A new emission band corresponding to the imino-tautomer complex for the 6alkoxy-2-aminopyridine/AcOH systems appeared near 420 nm as a very weak broad shoulder band. This indicates that the alkoxy group at the 6-position may act as a steric hindrance for the hydrogen-bonded complex formation with AcOH in the S0 and S1 states. The consideration on the steric hindrance due to the alkoxy group at the 6-position will be made more detail in the following theoretical section. 3.3. Theoretical approaches in the S0 and S1 states In order to have a better understanding the above experimental results, we have performed the theoretical approaches in this section. Table 1 shows the calculated value (−Hcalc ) of the enthalpy change, with the observed value (−H) estimated from the UV absorption spectra for the 2A3MOP/AcOH system. The calculated enthalpy change (−Hcalc ) for the complex formation was ca. 46.7 kJ mol−1 for this system. From the calculated value (−Hcalc ) and the observed value (42.8 kJ mol−1 ) near inclined to the similarity with each other. On the other hand, when the methoxy group is adjacent to the heterocyclic ring nitrogen atom having the close-neighboring amino group, it may act as the sreric hindrance with the conformation in certain cases, as already shown in Scheme 2. The optimized geometries of two conformers (A) and (B) for alkoxy2-aminopyridine were calculated by RHF/6-31G(d, p) method and CIS/6-31G(d) method in the S0 and S1 states, respectively, with those of 2-methoxypyridine (2MOP). The calculated total energies (ET ), energy differences (ET (A − B)), and dipole moments (μS0 and μS1 ) were summarized in Table 2 for the S0 state and in Table 3 for the S1 state. The conformers (A) of the 2MOP and alkoxy-2aminopyridine, which alkoxy group is directed to the pyridine Table 2 Total energies (ET ), energy differences (ET (A − B)), and dipole moments (μS0 ) of the optimized conformers(A) and (B) of alkoxy-aminopyridine and amino-methoxypyrimidine in the S0 state calculated by the RHF/6-31G(d, p) method

Conformer

ET (a.u.)

2MOP(A)a

−360.472694 −360.463842 −415.482359 −415.481921

2MOP(B)a 2A3MOP(A)b 2A3MOP(B)b

ET (A − B) (kJ mol−1 ) 23.24 1.15

μS0 (D) 0.86 3.33 3.13 1.62

T. Kitamura et al. / Spectrochimica Acta Part A 68 (2007) 979–991 Table 2 (Continued ) Conformer

ET (a.u.)

2A6MOP(A) 2A6MOP(B) 2A6NPOP(A) 2A6NPOP(B) 2A6IPOP(A) 2A6IPOP(A) 2A4MOPM(A) 2A4MOPM(B) 4A2MOPM(A) 4A2MOPM(B) 4A6MOPM(A) 4A6MOPM(B) MMPM(A)c MMPM(B)c DMOPM(A) DMOPM(A )d DMOPM(B)

−415.500567 −415.493261 −493.520947 −493.514023 −493.518715 −493.516629 −431.522375 −431.514756 −431.517868 −431.519042 −431.501201 −431.493265 −470.533641 −470.525883 −545.389612 −545.383375 −545.375150

ET (A − B) (kJ mol−1 )

μS0 (D) 1.87 2.40 1.69 2.15 1.76 2.21 1.68 2.09 4.33 2.76 1.78 4.77 1.29 2.40 2.28 1.66 3.44

19.18 18.18 5.48 20.00 −3.08 20.84 20.38 16.38 37.97

a

.

b

.

985

tively and these were very small. Consequently, the amino hydrogen-bonded complex formation for the 2A3MOP/AcOH system is not responsible for the direction of methoxy group for this system in n-hexane. The hydrogen-bonded complex of the conformer (A) of 2A3MOP with AcOH should proceed to produce into the imino-tautomer complex in the S1 state, as already shown in Scheme 3. On the other hand, the amino hydrogen-bonded complex formation for the 6-alkoxy2-aminopyridine/AcOH system was absence or very small amounts of presence, because of the steric hindrance of the alkoxy group at the 6-position. As describe above, the calculated results are in agreement with the experimental results. 3.4. The effects of methoxy group for some amino-methoxypyrimidine/AcOH systems The concept as described above will be extended so as to include some amino-methoxypyrimidines to make more clearly the role of the methoxy group. In the next stage, the above knowledge can be applied to determine the main structure of the amino dual hydrogen-bonded 1:1complex correlated with the presence or absence of the imino-tautomer for the following five aminomethoxypyrimidine/AcOH systems. Figs. 8–12 show the UV absorption and fluorescence spectra of the 2A4MOPM/AcOH, 4A2MOPM/AcOH, 4A6MOPM/AcOH, MMPM/AcOH, and DMOPM/AcOH systems in n-hexane at room temperature, respectively. The observed K, −H, −G, and −S for the these systems were also given in Table 1, with the values of −Hcalc . Furthermore, the total energies (ET ), energy differences (ET (A − B)), and dipole moments (μS0 and μS1 ) of the optimized conformers (A) and (B) for aminomethoxypyrimidines in the S0 and S1 states calculated by the RHF/6-31G(d, p) and CIS/6-31G(d) methods, were also summarized in Tables 2 and 3, respectively. 3.4.1. The 2A4MOPM/AcOH system As shown in Fig. 8, the addition of a small amount of AcOH to 2A4MOPM in n-hexane perturbed the absorption spectra. The large band-shift to the longer wavelength and appearance of clear

c

. Table 3 Total energies (ET ), energy differences (ET (A − B)), and dipole moments (μS1 ) of the optimized conformers(A) and (B) of alkoxy-aminopyridine and amino-methoxypyrimidine in the S1 state calculated by the CIS/6-31G(d) method

d

.

nitrogen atom, were energetically more stable than the conformers (B) by 5.48–23.24 kJ mol−1 for the S0 state and 11.71–26.68 kJ mol−1 for the S1 state, respectively, except the 2A3MOP/AcOH system. In the case of the 2A3MOP/AcOH system, the ET (A − B) energy differences were 1.15 kJ mol−1 for the S0 state, −0.01 kJ mol−1 for the S1 state, respec-

Conformer

ET (a.u.)

2MOP(A)a

−360.243882 −360.233719 −415.262630 −415.262635

2MOP(B)a 2A3MOP(A)b 2A3MOP(B)b

ET (A − B) (kJ mol−1 ) 26.68 −0.01

μS1 (D) 0.88 3.39 2.72 1.76

986

T. Kitamura et al. / Spectrochimica Acta Part A 68 (2007) 979–991

Table 3 (Continued ) Conformer

ET (a.u.)

2A6MOP(A) 2A6MOP(B) 2A6NPOP(A) 2A6NPOP(B) 2A6IPOP(A) 2A6IPOP(A) 2A4MOPM(A) 2A4MOPM(B) 4A2MOPM(A) 4A2MOPM(B) 4A6MOPM(A)c 4A6MOPM(B)c MMPM(A) MMPM(B) DMOPM(A) DMOPM(A )d DMOPM(B)

−415.276104 −415.266841 −493.290001 −493.281197 −493.288459 −493.283998 −431.289572 −431.284364 −431.287239 −431.283919 −431.280937 −431.274603

ET (A − B) (kJ mol−1 )

3.29 3.29 2.51 1.44 2.54 1.42 2.74 0.64 3.17 1.32 2.05 4.94 – 1.29 3.57 2.64 –

24.32 23.11 11.71 13.67 8.72 16.63

e

−470.525883 −545.136186 −545.135902 e

– 0.75 –

a

.

b

.

c

Fig. 8. The UV absorption and fluorescence spectra of the 2A4MOPM/AcOH system in n-hexane at 20 ◦ C. Concentration of 2A4MOPM: 1 × 10−4 mol dm−3 ; concentration of AcOH (mol dm−3 ) (1) 0; (2) 2 × 10−5 ; (3) 5 × 10−5 ; (4) 1 × 10−4 ; (5) 2 × 10−4 ; (6) 5 × 10−4 ; (7) 1 × 10−3 ; (8) 2 × 10−3 ; (9) 5 × 10−3 ; concentration of 4OPMI: 1 × 10−4 mol dm−3 . Excitation wavelength; 260 nm for the 2A4MOPM/AcOH system and 340 nm for the 4OPMI.

23.0 kJ mol−1 , and 126 J K−1 mol−1 , respectively (Table 1). These values for the 2A4MOPM/AcOH system were appreciably similar to those for the 2A3MOP/AcOH system, respectively, The −H value (60.0 kJ mol−1 ) was also considerably similar to that of 2APM/AcOH system (41.2 kJ mol−1 )[15]. The observed and calculated −H values (60.0 kJ mol−1 and 43.31 kJ mol−1 ) in Table 1 indicates that dual hydrogen bonds are present between the 2A4MOPM and AcOH in n-hexane. It is considered that the 2A4MOPM makes two kinds of the dual hydrogen-bonded complexes with AcOH in n-hexane. As shown in Scheme 4, one of them is the hydrogen-bonded complex (A) and the other is the hydrogen-bonded complex (B). However, as given in Table 2, the calculated result in regard to the direction of the methoxy group for 2A4MOPM, showed

.

d e

μS1 (D)

. Devergence on the calculation.

isosbestic points at 215, 225, 245, and 275 nm were attributed to the formation of a hydrogen-bonded 1:1 complex between 2A4MOPM and AcOH. The values of K, −H (−Hcalc ), −G, and −S for the 2A4MOPM/AcOH system were to be 12.4 × 103 dm3 mol−1 , 60.0 kJ mol−1 (43.31 kJ mol−1 ),

Fig. 9. The UV absorption and fluorescence spectra of the 4A2MOPM/AcOH system in n-hexane at 20 ◦ C. Concentration of 4A2MOPM: 1 × 10−4 mol dm−3 ; concentration of AcOH (mol dm−3 ) (1) 0; (2) 2 × 10−5 ; (3) 5 × 10−5 ; (4) 1 × 10−4 ; (5) 2 × 10−4 ; (6) 5 × 10−4 ; (7) 1 × 10−3 ; (8) 2 × 10−3 ; (9) 5 × 10−3 . Excitation wavelength was 260 nm.

T. Kitamura et al. / Spectrochimica Acta Part A 68 (2007) 979–991

Fig. 10. The UV absorption and fluorescence spectra of the 4A6MOPM/AcOH systems in n-hexane at 20 ◦ C. Concentration of 4A6MOPM: 1 × 10−4 mol dm−3 ; concentration of AcOH (mol dm−3 ) (1) 0; (2) 2 × 10−5 ; (3) 5 × 10−5 ; (4) 1 × 10−4 ; (5) 2 × 10−4 ; (6) 5 × 10−4 ; (7) 1 × 10−3 ; (8) 2 × 10−3 ; (9) 5 × 10−3 . Excitation wavelength was 270 nm.

that the conformer (A) was energetically more stable than the conformer (B) by 20 kJ mol−1 . Consequently, this indicated that the formation of the hydrogen-bonded complex (A) was preferable to that of the hydrogen-bonded complex (B) in the S0 state (Scheme 4). On the other hand, the fluorescence spectra of 2A4MOPM in n-hexane changed drastically upon the addition of AcOH (Fig. 8). On the disappearance of the emission band (F1) of free 2A4MOPM molecules at 320 nm with an increase in the AcOH concentration, a new emission band (F2) appeared near 420 nm. In Fig. 8, a clear isoemissive point was observed at 380 nm in the fluorescence spectra. The spectra near 420 nm had the vibronic structure bands (ca. 1550 cm−1 ), though the bands were broad. In order to ascertain an experimental interpretation

Fig. 11. The UV absorption and fluorescence spectra of the MMPM/AcOH system in n-hexane at 20 ◦ C. Concentration of MMPM: 1 × 10−4 mol dm−3 ; concentration of AcOH (mol dm−3 ) (1) 0; (2) 2 × 10−5 ; (3) 5 × 10−5 ; (4) 1 × 10−4 ; (5) 2 × 10−4 ; (6) 5 × 10−4 ; (7) 1 × 10−3 ; (8) 2 × 10−3 ; (9) 5 × 10 −3 ; concentration of MMPMI: 1 × 10−4 mol dm−3 . Excitation wavelength; 260 nm for the MMPM/AcOH system and 315 nm for the MMPMI.

987

Fig. 12. The UV absorption and fluorescence spectra of the DMOPM/AcOH system in n-hexane at 20 ◦ C. Concentration of DMOPM: 1 × 10−4 mol dm−3 ; concentration of AcOH (mol dm−3 ) (1) 0; (2) 2 × 10−5 ; (3) 5 × 10−5 ; (4) 1 × 10−4 ; (5) 2 × 10−4 ; (6) 5 × 10−4 ; (7) 1 × 10−3 ; (8) 2 × 10−3 ; (9) 5 × 10−3 . Excitation wavelength was 250 nm.

of the emission bands near 420 nm, the model compound of the imino-tautomer complex, 4OPMI was synthesized. The imino-model compound possessed similar ␲-electronic properties with respect to the imino-model compound of 2A4MOPM. The fluorescence spectrum of 4OPMI is also shown in Fig. 8. This spectrum had the vibronic structure emission bands (ca. 1550 cm−1 ) near 450 nm, though the bands were also broad. It was noted that the vibronic bands and bandwidth of the fluorescence spectrum of 4OPMI were very similar to the F2 of the 2A4MOP/AcOH system near the same wavelength region (near 420 nm). From the similarity of the F2 and imino-model compound fluorescence, it is revealed that the F2 correspond to the emission from the 4-methoxy-1(2H)-pyrimidinimine/AcOH complex (imino-tautomer) formed through the dual hydrogen-bonds of the 2A4MOPM/AcOH system (Scheme 4). 3.4.2. The 4A2MOPM/AcOH system As shown in Fig. 9, the addition of a small amount of AcOH to 4A2OPM in n-hexane perturbed the UV absorption spectra. The large band-shift to the longer wavelength and appearance of clear isosbestic points at 210, 225, 245, and 280 nm were attributed to the formation of the hydrogenbonded 1:1 complex between 4A2MOPM and AcOH. The absorption spectra of the 4A2MOPM/AcOH system were similar to those of the 2A3MOP/AcOH and 2A4MOPM/AcOH systems. The UV spectra were analyzed by the same method as the 2A3MOP/AcOH system. The values of K, −H (−Hcalc ), −G, and −S for the 4A2MOPM/AcOH system were 6.80 × 103 dm3 mol−1 , 58.3 kJ mol−1 (46.16 kJ mol−1 ), 21.5 kJ mol−1 and 126 JK−1 mol−1 , respectively, as shown in Table 1. The −H and −Hcalc values (58.3 kJ mol−1 and 46.16 kJ mol−1 ) indicates that the dual hydrogen bonds are also present between the 4A2MOPM and AcOH. As shown in Table 2, it is noted that the calculated result in regard to the direction of methoxy group for 4A2MOPM shows that conformer (B) was more stable than conformer (A) by 3.08 kJ mol−1 in the

988

T. Kitamura et al. / Spectrochimica Acta Part A 68 (2007) 979–991

Scheme 4.

S0 state. This calculated result implies that the dual hydrogenbonded complex formation is possible between the conformer (B) of 4A2MOPM and AcOH in the S0 state, as shown in Scheme 5. On the other hand, as shown in Fig. 9, the fluorescence for the 4A2MOPM observed at 340 nm decreases its intensity without a shift on the band with the addition of the AcOH but the emission corresponding to the imino-tautomer complex did not appear near 420 nm. The calculated result on the conformation of methoxy group for 4A2MOPM in Table 3 indicates that conformer (A) is more stable than conformer (B) by 8.72 kJ mol−1 in the S1 state. The dual hydrogen-bonded 1:1 complex is formed between the ring nitrogen atom and amino group of the 4A2MOPM and OH and C O groups of AcOH, respectively, in the S0 state. However, the inversion of the methoxy group of the 4A2MOPM in the complex occurs in the S1 state. At least, one of the dual hydrogen bonds may be broken. As the result of the inversion, it is considered that the 4A2MOPM/AcOH imino-tautomer is not formed in the S1 state, as shown in Scheme 5. With the inversion, the internal conversion process of the 4A2MOPM from the S1 state to the S0 state may become to predominate.

3.4.3. The 4A6MOPM/AcOH system As shown in Fig. 10, the addition of a small amount of AcOH to 4A6MOPM in n-hexane perturbed the UV absorption spectra. The large band-shift to the longer wavelength and appearance of clear isosbestic points at 215 nm and 257 nm were attributed to the formation of a hydrogen-bonded 1:1 complex between 4A6MOPM and AcOH in n-hexane. The values of thermodynamic properties for the system of 4A6MOPM with AcOH were also shown in Table 1. The −H value of the formation of the hydrogen-bonded complex was 48.3 kJ mol−1 in n-hexane. This indicates that dual hydrogen-bonded 1:1 complex is formed between 4A6MOPM and AcOH, as shown in Scheme 6. In this system, the −H value (48.3 kJ mol−1 ) was similar to almost same as that of 2APM/AcOH system (41.2 kJ mol−1 ) [15]. In the case of 4A6MOPM, the conformation of the methoxy group at the 6-positin did not interfere with the formation of the dual hydrogen-bonded complexes, which was similar to that of methoxy group at the 3-position of 2A3MOP. From the fluorescence spectra in Fig. 10, with the disappearance of the weak emission of the free 4A6MOPM monomer at 335 nm with an increase in the AcOH concentration in nhexane, another large Stokes shifted emission band appeared at

Scheme 5.

T. Kitamura et al. / Spectrochimica Acta Part A 68 (2007) 979–991

989

Scheme 6.

440 nm. From the similarity of the band position (420 nm) of the 2A4MOPM/AcOH imino-tautomer fluorescence, the emission band at 440 nm may be assigned due to the imino-tautomer of the 4A6MOPM/AcOH system. The relative fluorescence intensity at 440 nm was larger than that of the free monomer at 335 nm, though the relative fluorescence intensity at 440 nm was also small. 2A4MOPM, 4A2MOPM, and 4A6MOPM are isomers with each other. It is particularly interesting that the presence or absence of the imino-tautomer fluorescence can be used to distinguish clearly these three isomers: the 2A4MOPM and 4A6MOPM systems with AcOH show the imino-tautomer fluorescence, and the former relative fluorescence intensity was larger than the latter one (Figs. 8–10). On the other hand, the 4A2MOPM/AcOH system does not show the imino-tautomer fluorescence. Furthermore, it is noted that both 4A2MOPM and 4A6MOPM is very similar to an 4-aminopyrimidine moiety of adenine in DNA, though adenine has no methoxy group, and the 4A2MOPM does not form the imino-tautomer with AcOH in the S1 state but 4A6MOPM forms easily the imino-tautomer in the S1 state.

3.4.4. The MMPM/AcOH system As shown in Fig. 11, the addition of a small amount of AcOH to MMPM in n-hexane perturbed the UV absorption spectra. The large band-shift to the longer wavelength and appearance of clear isosbestic points at 210, 230, 245, and 270 nm were attributed to the formation of a hydrogen-bonded 1:1 complex between MMPM and AcOH. The values of K, −H, −G and −S for the MMPM/AcOH system in n-hexane were also shown in Table 1. In this system the −H value (61.1 kJ mol−1 ) was experimentally the largest among the present systems. This value shows the presence of the dual hydrogen-bonded 1:1 complex between MMPM and AcOH in n-hexane. The hydrogen-bonded complexes (C) and (D) would be considered as the structure of the dual hydrogen-bonded 1:1 complex, as shown in Scheme 7. The calculated result in regard to the direction of the methoxy group for MMPM as given in Table 2, showed that the conformer (A) was more stable than the conformer (B) by 20 kJ mol−1 . The methoxy group of the conformer (A) for MMPM could be considered to interfere with the hydrogenbonded complex formation with AcOH. In addition, the methyl group at the 6-position of 2-amino-6-methylpyridine (2A6 MP)

Scheme 7.

990

T. Kitamura et al. / Spectrochimica Acta Part A 68 (2007) 979–991

and 2-amino-4,6-dimethylpyrimidine (2ADMPM) enhanced the hydrogen-bonded complex formation between 2A6MP and AcOH [9] and between 2ADMPM and AcOH [15], respectively. Consequently, these indicate that the formation of the hydrogenbonded complex (C) is preferable to that the hydrogen-bonded complex (D) in the S0 state (Scheme 7). For the sake of the no steric hindrance of the methoxy group and the electron-donating characteristics of both the methyl and methoxy groups, the fluorescence spectra of MMPM in n-hexane changed drastically upon the addition of AcOH in nhexane (Fig. 11). With addition of AcOH, the emission (F1) of the free MMPM monomer near 325 nm decreased its intensity without a shift of the band and another large Stokes shifted intense-emission (F2) appeared near 410 nm. Furthermore, a clear isoemissive point was observed at 350 nm. The emission near 410 nm had the vibronic bands (ca. 1550 cm−1 ), though the bands were broad. In order to ascertain an experimental interpretation of the F2 near 410 nm. The model compound of the imino-tautomer complex, MMPMI was synthesized and its fluorescence spectrum in n-hexane is also shown in Fig. 11. The emission of MMPMI was observed near 440 nm and had the vibronic bands (ca. 1550 cm−1 ), though the bands were appreciably broad. From the similarity of the vibronic bands and band width, the F2 band of the MMPM/AcOH system near 410 nm may be considered to correspond to the fluorescence of the 4-methoxy-6-methyl-2(1H)-pyrimidinimine/AcOH complex (tautomer) formed through the hydrogen-bonded complex formation of the MMPM/AcOH system in n-hexane, as shown in Scheme 7. The fluorescence of the MMPM/AcOH imino-tautomer was found to be the largest among the four imino-tautomers in n-hexane. Therefore, the conformation of the methoxy group at the 4-position did not interfere with in the formation of the tautomer, which was similar to that of methoxy group at the 3-position of 2A3MOP. In addition, the relative fluorescence intensity of the MMPM/AcOH imino-tautomer is larger than that of the 2ADMPM /AcOH imino-tautomer reported previously [15]. This must be due to the strong electrondonating characteristic of the methoxy group, compared with the methyl group at the 4-position. 3.4.5. The DMOPM/AcOH system As can be seen from Fig. 12, the UV absorption and fluorescence spectra of DMOPM in n-hexane were little affected by the addition of AcOH, compared with other four pyrimidine systems, as described above. These spectral results of the DMOPM/AcOH system were rather appreciably similar to those of the 2-amino-6-alkoxypyridine/AcOH system in nhexane (Figs. 5–7). It is considered that the formation of the dual hydrogen-bonded complex does not almost occur for the DMOPM/AcOH system. Accordingly, the UV spectra of the DMOPM/AcOH system were not analyzed, as well as the 2amino-6-alkoxypyridine/AcOH systems (Table 1). In addition, as shown in Figs. 9 and 12, the fluorescence of DMOPM in n-hexane was observed at 340 nm and the wavelength and intensity of the fluorescence band for the DMOPM free molecule were very similar to those of the free 4A2MOPM in n-hexane. The emission corresponding to the imino-tautomer

Scheme 8.

does not be appeared near 420 nm (Fig. 12). The no iminotautomer fluorescence would be due to the steric hindrance methoxy groups in the S0 and S1 states. According to the calculated results, the conformer (A) of DMOPM was more stable than the conformer (A ) by 16.38 kJ mol−1 and the conformer (B) by 37.97 kJ mol−1 in the S0 state, as given in Table 2. The ET (A − B) value of 37.97 kJ mol−1 for DMOPM was the largest among these amino-methoxypyrimidines in the present study (Table 2). Further, the conformer (A) was more stable than the conformer (A ) by 0.75 kJ mol−1 in the S1 state, as shown in Table 3. Consequently, two methoxy groups in DMOPM direct toward the ring N1 and N3 atoms, respectively, in both S1 and S2 states as shown in Scheme 8. These calculated results support that DMOPM does not almost form only the amino hydrogenbonded complex, but also the imino-tautomer complex with AcOH, because of the steric hindrance of the methoxy groups. As described above, it is concluded that the methoxy group adjacent the ring nitrogen atom having the closeneighboring amino group in alkoxy-aminopyridines and amino-methoxypyrimidines prevents the imino-tautomer formation with AcOH in the S1 state. Acknowledgements This work was supported in part by grants from the Research Institute for Technology of Tokyo Denki University. References [1] P.R. Cantor, P.R. Schimmel, Biophysical Chemistry, vol. 2, W.H.Freeman, San Francisco, CA, 1980 (Chapters 7 and 8). [2] M. Kasha, J.Chem.Soc., Faraday Trans. 2 (82) (1986) 2379. [3] K.C. Ingham, M.A. El-Bayoumi, J. Am. Chem. Soc. 96 (1974) 1674. [4] P.F. Barbara, P.K. Walsh, L.E. Brus, J. Phys. Chem. 93 (1989) 29. [5] S. Sheiner, J. Phys. Chem. A 104 (2000) 5898. [6] R.H. Simic, L. Grossman, A.C. Upton, Mechanism of DNA Damage and Repair, Plenum Press, NY, 1986, p. 29. [7] R. Beukers, W. Berends, Biochim. Biophys. Acta 41 (1960) 550. [8] H. Xingbang, L. Haoran, L. Wanchun, H. Shinjun, J. Phys. Chem. B108 (2004) 12999. [9] K. Inuzuka, A. Fujimoto, Spectrochim. Acta 42A (1986) 929. [10] K. Inuzuka, A. Fujimoto, Bull. Chem. Soc. Jpn. 63 (1990) 971. [11] K. Inuzuka, A. Fujimoto, Bull. Chem. Soc. Jpn. 64 (1991) 3758. [12] H. Ishikawa, K. Iwata, H. Hamaguchi, J. Phys. Chem. A 106 (2002) 2305.

T. Kitamura et al. / Spectrochimica Acta Part A 68 (2007) 979–991 [13] F. Hung, W. Hu, T. Li, C. Cheng, P. Chou, J. Phys. Chem. A 107 (2003) 3244. [14] K. Inuzuka, Nippon Kagaku Kaishi (1997) 393. [15] T. Kitamura, A. Hikita, H. Ishikawa, A. Fujimoto, Spectrochim. Acta 62A (2005) 1157. [16] H. Sekiya, K. Sakota, Bull. Chem. Soc. Jpn. 79 (2006) 373. [17] N. Mase, T. Ohno, H. Morimoto, F. Nitta, H. Yoda, K. Takabe, Tetrahedron 46 (2005) 3213. [18] R.S. Karlinskaya, N.V. Khromov-Borisov, J. Gen. Chem. USSR 27 (1957) 2170. [19] E.C. Taylor, R.O. Kan, J. Am. Chem. Soc. 85 (1963) 776. [20] 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.

[21] [22] [23] [24] [25]

991

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, in: Revision B. 05, Gaussian, Inc., Pittsburgh, PA, 2003. W.J. Hehre, L. Radom, P.V.R. Schleyer, J.A. Pople, Ab initio Molecular Orbital Theory, John Wiley & Sons, New York, 1986. F.B. Foresman, M. Head-Gordon, J.A. Pople, M.J. Frisch, J. Chem. Phys. 96 (1992) 135. J.N. Rose, R.S. Drago, J. Am. Chem. Soc. 81 (1958) 6138. P. Chou, C. Wei, K. Meng-shin, J. Phys. Chem. 99 (1995) 11994. H. Affsprung, S.D. Christain, A.M. Mecnick, Spectrochim. Acta 20A (1964) 285.