Solvent Dependence of 7-Azaindole Dimerization - The Journal of

Article pubs.acs.org/JPCB

Solvent Dependence of 7‑Azaindole Dimerization Hideaki Shirota,*,†,‡,§ Takao Fukuda,‡,§ and Tatsuya Kato† †

Department of Nanomaterial Science, Graduate School of Advanced Integration Science, ‡Department of Chemistry, Faculty of Science, and §Center for Frontier Science, Chiba University, 1-33 Yayoi, Inage-ku, Chiba 263-8522, Japan S Supporting Information *

ABSTRACT: We have investigated 7-azaindole (AI) in a variety of solvents including CCl4, CHCl3, CH2Cl2, acetone, CH3CN, and DMSO by femtosecond Raman-induced Kerr effect spectroscopy. In differential low-frequency Kerr spectra between the solutions and the respective neat solvents, vibrational bands of the AI hydrogen-bonding (HB) dimer have been observed at ca. 90 and 105 cm−1 in CHCl3 and CH2Cl2, as well as CCl4: the standard solvent for the AI dimer. In contrast, a broad monomodal band at ca. 80 cm−1 characterizes an HB mode between the AI monomer and solvent in acetone, CH3CN, and DMSO. The overdamped Kerr transients in the picosecond region show evidence of both the AI monomer and dimer reorientations in CHCl3, CH2Cl2, acetone, and CH3CN, but only the monomer reorientation has been confirmed in DMSO. The clear intermolecular HB bands have not been observed in acetone, CH3CN, and DMSO because these solvents are sufficiently strong HB acceptors, which form HB AI−solvent complexes, thus preventing quantitative AI dimerization. In addition, it is plausible that the HB band of between AI and solvent obscures the intermolecular bands of the AI dimer when the concentration of the AI dimer is much lower than the AI monomer. For comparison, we have employed NMR to study the concentration-dependent chemical shift of the proton attached to the N at the 7-position of AI and to estimate the dimerization constant: 356, 13.3, 14.7, 0.727, and 0.910 M−1 in CCl4, CHCl3, CH2Cl2, acetone, and CH3CN, respectively. The femtosecond Raman-induced Kerr effect spectroscopy and NMR results are in good agreement.

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INTRODUCTION 7-Azaindole (AI) dimerizes in the gas phase and in nonpolar solvents via cooperative hydrogen bonds (HBs, Chart 1).

polarity has not been extensively studied. In one example, the AI monomer in CHCl3 (0.15 M) served as reference for the AI dimer in CCl4 (0.35 M).14 Kwon and Zewail estimated the rate constants for proton transfer in the AI dimer in a wide variety of solvents including n-heptane (0.02 M), diethyl ether (0.1 M), dichloromethane (0.1 M), and acetonitrile (0.1 and 0.5 M).12 On the other hand, Catalan pointed out that no AI dimer existed in diethyl ether, dichloromethane, and acetonitrile (0.1 M).18 Hence, whether AI exists as monomer or dimer or both in solution is very important to understand not only the behavior of AI in the ground state in solution but also the double-proton-transfer mechanism of AI in solution. Low-frequency vibrational spectroscopic techniques are very useful to confirm the presence of the AI dimer in solution because intermolecular HB modes in HB molecular systems often appear in the low-frequency region around 90 cm−1, where the AI monomer shows no signature. In 2006, Fedor and Korter first directly observed an IR-active low-frequency vibrational mode of the AI dimer in cyclohexane at a concentration of 0.05 M at ca. 76 cm−1 using terahertz time-domain spectroscopy.19 We also successfully observed Raman-active lowfrequency vibrational bands at ca. 90 and 105 cm−1 due to the AI dimer in CCl4 at various concentrations by femtosecond Ramaninduced Kerr effect spectroscopy (RIKES).20

Chart 1

Because of its HB structure, the AI dimer has been studied as a model for nucleic acid base pairs. In 1969, Kasha and coworkers reported excited-state double proton transfer in the AI dimer in solution.1 After this pioneering work, the AI dimer has become a popular research target in chemistry. In particular, the mechanism of the primary step of the photoinduced double proton transfer in the AI dimer was extensively investigated.2−17 Studies on AI in solution have mostly focused on nonpolar solvents such as alkanes and CCl4 probably because AI easily dimerizes in nonpolar solvents. In contrast to nonpolar solvents, the behavior of AI in solvents of medium and high © 2013 American Chemical Society

Received: August 12, 2013 Revised: October 30, 2013 Published: November 5, 2013 16196

dx.doi.org/10.1021/jp408040s | J. Phys. Chem. B 2013, 117, 16196−16205

The Journal of Physical Chemistry B

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Table 1. Shear Viscosities η at 292 K of Solvents and 0.75 M AI Solutions, Dielectric Constant ε, Refractive Index nD, Donor Number DN of Solvents, and Dipole Moments μ of Solvent Molecules ηa,b (cP)

a

solvent

neat

solution

μc (D)

εc

nDc

F(ε,nD)d

DNe

CCl4 CHCl3 CH2Cl2 acetone CH3CN DMSO

0.909 0.544 0.421 0.344 0.380 2.148

1.072 0.634 0.486 0.412 0.432 2.529

0.00 1.04 1.60 2.88 3.92 3.96

2.24 4.81 8.93 21.01 36.64 47.24

1.4601 1.4459 1.4242 1.3588 1.3422 1.4793

0.019 0.293 0.470 0.650 0.712 0.655

0 4 1 17.0 14.1 29.8

292.0 ± 0.2 K. b±5%. cData at 293 K from ref 54. dEstimated by eq 1. eData from ref 56.

In this study, we explore the solvent dependence of the AI dimerization in solution through direct probing of vibrational modes due to the cooperative HB AI dimer by femtosecond RIKES. Solvents chosen here include CHCl3, CH2Cl2, acetone, CH3CN, and DMSO. Although we investigated CCl4 previously,20 we also select CCl4 as a good control solvent for the AI dimer in this study. Femtosecond RIKES is a time-domain Raman spectroscopy technique that can capture low-frequency motions in a frequency range of approximately 0.1−700 cm−1 in condensed phases.21−33 This spectroscopic technique is thus suitable for directly observing intermolecular HB modes in HB molecular systems. To date, intermolecular HB vibrational bands of some HB molecular systems in the low-frequency region were observed by femtosecond RIKES.34−37 This spectroscopic technique can also detect the collective orientational dynamics of the solute and solvent that is related solute size and solvent viscosity. Furthermore, because femtosecond RIKES is a nonresonant spectroscopic technique, we can obtain information on the electronic ground state of AI in solution and ignore the effect of its photochemistry accordingly. We have observed clear HB bands at ca. 90 and 105 cm−1 due to the AI dimer in CCl4, CHCl3, and CH2Cl2, but a broad monomodal band at ca. 80 cm−1 in acetone, CH3CN, and DMSO. In addition to the RIKES experiments, we have employed NMR measurements to estimate the dimerization constant of AI in solution. The solvent dependence of the AI dimerization estimated by NMR is reasonably in good agreement with the RIKES results.

of the titanium sapphire laser was approximately 430 mW. The typical temporal response, which was estimated as the cross correlation between the pump and probe pulses measured using a 200 μm thick KDP crystal (type I), was 35 ± 3 fs (full width at half-maximum). Scans with a high time resolution of 3072 points at 0.5 μm/step were performed for a short time window (ca. 10 ps). Long time window transients were recorded with a data acquisition of 5.0 and 20.0 μm/step for the neat solvents and solutions, respectively. Pure heterodyne signals were obtained by combining the transients recorded with a quarter-wave plate rotated by both ca. +1.5° and −1.5° in the probe beam path to eliminate the residual homodyne signal. Prior to the femtosecond RIKES measurements, the samples were injected into a 3 mm optical-path-length quartz cell (Tosoh Quartz) using 0.2 and 0.02 μm Anotop filters (Whatman). All RIKES measurements were performed at 292 ± 1 K under dark conditions. The details of the analytical procedure to obtain the differential Kerr spectra from the measured Kerr transients were reported previously.20 In brief, the Kerr transients were first analyzed by the standard Fourier-transform deconvolution method, originally developed by McMorrow and Lotshaw.40,41 After normalization of the Fourier-transformed low-frequency Kerr spectra of the solution and solvent via an intramolecular vibrational mode of the solvent, the contribution of the neat solvent was subtracted from the obtained spectra of the AI solutions. The lowfrequency spectra of the neat solvents measured in this study were similar to those reported previously (CCl4,23,28,39,42−44 CHCl3,28,45,46 CH2Cl2,28,35 acetone,28 CH3CN,28,47−49 and DMSO25,28,50). Furthermore, the line-shape analyses for the differential spectra were based on a model function: a sum of Ohmic51 and antisymmetrized Gaussian functions.52 Ab initio quantum chemistry calculations at the B3LYP/6311++G(d,p) level of theory were performed to obtain the optimized structures of the AI monomer, AI dimer, and the interaction energy of the AI dimer using the Gaussian 03 program suite.53 The effect of each dielectric medium was determined using the default IEF-PCM model in the Gaussian 03 program. The atomic coordinates of the optimized structures are summarized in the Supporting Information. NMR measurements were performed with a 400 MHz JEOL NMR spectrometer (JNM-ECS400) at 295 ± 1 K in the dark. Because the purpose of the NMR measurements in this study was to reveal the solvent dependence of the chemical shift of the proton bonded to the N at the 7-position of 7-azaindole in solution, a Shigemi external coaxial NMR tube system (SC-0010) was used. The lock solvent DMSO-d6 and sample solutions were introduced into the outer and inner tube, respectively.

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MATERIALS AND METHODS AI (Acros Organics), CCl4 (Kanto Chemical, atomic absorption analysis grade), CHCl3 (Kanto Chemical, dehydrated), CH2Cl2 (Kanto Chemical, dehydrated), acetone (Kanto Chemical, dehydrated), CH3CN (Kanto Chemical, dehydrated), and DMSO (Kanto Chemical, dehydrated) were used as received. The AI concentration of the sample solutions was 0.75 M for the RIKES experiments. The concentrations for the NMR experiments were adjusted to obtain dimerization constants (K, vide infra) within the solution ranges. Shear viscosities (η) of the sample solutions and neat solvents were measured using a reciprocating electromagnetic piston viscometer (Cambridge Viscosity, ViscoLab 4100) equipped with a circulating water bath (Yamato, BB300) at 292.0 ± 0.2 K. Details of the lab-built femtosecond optical heterodynedetected RIKES setup used in this study were reported previously.28,38,39 Briefly, the light source was a titanium sapphire oscillator (KMLabs Inc., Griffin) pumped by a Nd:VO4 diode laser (Spectra Physics, Millennia Pro 5sJ). The output power 16197



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Figure 1. Differential Kerr spectra of (a) AI/CCl4, (b) AI/CHCl3, (c) AI/CH2Cl2, (d) AI/acetone, (e) AI/CH3CN, and (f) AI/DMSO. Gray lines denote the experimentally obtained spectra; red lines denote the entire fits, blue areas denote Ohmic functions, and green and purple areas denote antisymmetrized Gaussian functions.

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RESULTS AND DISCUSSION Solvent Properties. Table 1 summarizes the shear viscosities η of the neat solvents and AI solutions measured at 292 K, which are reasonably similar to tabulated values at 298 K.54 The dipole moment μ, dielectric constant ε, refractive index nD,54 polarity parameter F(ε,nD), defined as55 F(ε , nD) =

n 2−1 ε−1 − D2 ε+2 nD + 2

shows a unique spectral shape in the low-frequency region below 10 cm−1: AI/CCl4 shows a strong positive peak, but AI/CHCl3, AI/CH2Cl2, AI/acetone, and AI/CH2Cl2 show a negative intensity, and AI/DMSO displays a smooth spectral shape. Before we discuss the spectral shape of the differential Kerr spectrum, we recall the results from a previous study on the concentration dependence of the differential Kerr spectrum for AI and 1-benzofuran (BF) in CCl4.20 As in the present study (Figure 1), clear bands were observed at 89 and 105 cm−1 in AI/CCl4, whereas no such band was found in BF/CCl4. By means of ab initio quantum chemical calculations of the AI and BF monomers and AI dimer, the observed bands in AI/CCl4 were assigned to the overlap of stagger and wheeling modes and the intermolecular stretching mode of AI dimer. Moreover, the strong band in the low-frequency region below 10 cm−1 was less intense in AI/CCl4 than in BF/CCl4. This feature was attributed to the less active intermolecular translational motion in AI/CCl4 than that in BF/CCl4 due to dimerization-induced increases in mass and moment of inertia.20 On the basis of the previous work,20 the present results further elucidate the behavior of AI in a variety of solutions. Figure 1 shows two bands at ca. 90 and 105 cm−1 in AI/CCl4,

(1)

and the donor number DN56 are also listed in Table 1. F(ε,nD) is a scale for the dielectric polarity and DN is a measure of the Lewis basicity. Thus, the latter one is a scale for HB acceptor. Though other solvent polarity scales have been also proposed,57−62 we focus the two scales, F(ε,nD) and DN in this study for a simplicity. These solvent properties will be used to discuss the RIKES results in the following section. Vibrational Modes of Cooperative HB Dimer. Figure 1 shows the differential Kerr spectra of AI solutions and their line-shape analysis, whose fit parameters are summarized in Table 2. The shape of the differential spectra exhibits two noticeable features. First, all the AI solutions display a spectral band or bands at about 80−110 cm−1. Second, each AI solution 16198


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2.10 ± 0.02 −0.0821 ± 0.0072 5.65 ± 0.48 0.101 ± 0.057 13.6 ± 0.8 2.09 ± 0.04 0.0390 ± 0.0006 9.37 ± 1.16 0.0408 ± 0.0127 15.6 ± 4.7

31.6 ± 5.2 0.358 ± 0.003 56.7 ± 1.3 86.8 ± 1.1 0.0840 ± 0.0027 90.0 ± 0.5 25.0 ± 0.6 39.8 ± 3.0 0.279 ± 0.004 60.5 ± 1.7 81.6 ± 1.6 0.0674 ± 0.0050 92.4 ± 0.4 35.7 ± 0.9

0.0258 ± 0.0032 111 ± 1 20.8 ± 1.0

AI/CHCl3, and AI/CH2Cl2, but these bands are hardly distinguishable in AI/acetone, AI/CH3CN, and AI/DMSO. Accordingly, AI dimerizes via cooperative HBs in CCl4, CHCl3, and CH2Cl2 at a concentration of 0.75 M, as evidenced by their virtually identical peaks of the two bands. Conversely, a single broad peak at ca. 80 cm−1 is observed in AI/acetone, AI/CH3CN, and AI/DMSO, which is in contrast to the result of BF in solution showing no dimerization via cooperative HBs.20 In fact, intermolecular HB bands are often observed at ca. 90 cm−1 in the low-frequency Kerr spectrum for HB molecular systems.34−37,63−65 In the gas phase, vibrational modes of AI complexes with some HB solvent molecules were also observed in the low frequency region.66,67 For example, Brause et al. studied AI and water clusters in gas phase by dispersed fluorescence spectroscopy and observed 120 cm−1 mode of the cluster.66 Chakraborty and co-workers found an intermolecular vibrational mode at 86 cm−1, as well as 28, 47, 56, and 148 cm−1 modes and combination band at 114 cm−1, of AI−formamide cluster in the gas phase by laser-induced fluorescence spectroscopy.67 Therefore, it is natural to think that the broad band at ca. 80 cm−1 for AI in acetone, CH3CN, and DMSO is attributed to intermolecular HB vibrations involving the solvent. However, because the broad band at ca. 80 cm−1 possibly obscures the HB bands at 90 and 105 cm−1, if the AI monomer is dominant, we cannot rule out a possibility of the existence of the AI dimer in these solutions at this stage. We further understand from the differential Kerr spectra of AI in CCl4, CHCl3, and CH2Cl2 that the bands at ca. 90 and 105 cm−1 become more ambiguous and are broader in the former solutions than that of the latter solution. Thus, the existences of both the AI monomer and dimer and the weaker HB strength in the AI dimer might be expected in CHCl3 and CH2Cl2 than in CCl4. For deeper insight, we consider the quantum chemical calculation results. HB distance (NH···N) of the optimized dimer, the calculated energies of the optimized AI monomer Em and dimer Ed, and the interaction energies Ei

E i = 2Em − Ed

(2)

are estimated and summarized in the Supporting Information. Figure 2 plots Ei versus F(ε,nD) within the solvent region

−0.610 ± 0.008 0.114 ± 0.001

Figure 2. Plots of interaction energy Ei vs solvent parameter F(ε,nD) for AI solutions. Ei is estimated by quantum chemistry calculations at the B3LYP/6-311++G(d,p) level of theory with the IEF-PCM model. F(ε,nD) is based on the dielectric continuum model.

(excluded the gas-phase condition) to focus on the effect of solvent. Ei linearly decreases with F(ε,nD): the interaction energy of the AI dimer decreases with increasing polarity of the dielectric medium. However, Ei in polar media including

AI/CH3CN AI/DMSO

0.2 0.0635 ± 0.0005 107 ± 1 14.1 ± 0.2 0.4 0.0263 ± 0.0004 108 ± 1 14.2 ± 0.2 0.2 0.00680 ± 0.0011 107 ± 1 17.7 ± 0.2 0.3 ± ± ± ± 13.7 16.5 16.5 34.0 0.1 0.2 0.2 0.1 ± ± ± ± 0.3 0.0443 ± 0.007 89.9 0.2 0.0244 ± 0.0004 90.2 0.5 0.0662 ± 0.0010 88.9 1.2 0.356 ± 0.007 89.1 ± ± ± ± 67.7 73.7 73.9 87.4 0.3 0.2 0.1 1.6 ± ± ± ± 68.8 66.9 68.5 49.9 ± ± ± ± AI/CCl4 0.228 ± 0.002 AI/CHCl3 −0.0976 ± 0.0025 AI/CH2Cl2 −0.236 ± 0.001 AI/Acetone −0.523 ± 0.017

1.32 0.79 1.79 2.67

± ± ± ±

0.54 0.0727 ± 0.02 0.0173 ± 0.01 0.00947 ± 0.06 0.108 ±

0.0015 0.0003 0.00163 0.018

5.42 7.29 12.1 7.27

± ± ± ±

0.18 0.53 1.7 0.78

0.0688 0.0160 0.0375 0.0683

± ± ± ±

0.0091 0.0043 0.0207 0.0143

17.2 21.6 14.1 25.5

2.1 1.8 5.0 0.7

30.2 22.4 28.3 16.1 ± ± ± ±

2.0 2.4 5.4 1.3

0.193 0.119 0.280 0.884

± ± ± ±

0.001 0.002 0.021 0.011

ωG2 (cm−1) aG2 ΔωG1 (cm−1) ωG1 (cm−1) aG1 ωO2 (cm−1) aO2 ωO1 (cm−1) aO1 sample

Table 2. Fit Parameters Including the Standard Deviations for Differential Kerr Spectra of AI Solutions

ΔωG2 (cm−1)

aG3

ΔωG3 ωG3 (cm−1) (cm−1)

aG4

ωG4 (cm−1)

ΔωG4 (cm−1)


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acetone, CH3CN, and DMSO still favors AI dimerization if only the effect of the dielectric medium is considered. In addition to the pure dielectric medium effect, the effectiveness of the solvent as HB acceptor would affect the AI dimerization because the HB formation between AI and solvent competes with the AI dimerization. While it is not the case in the solvent itself, the formation enthalpies of the complexes of AI with CH3CN and acetone in CCl4 were reported by Dunken and Fritzsche and they are −6.15 and −11.1 kJ/mol which are quite similar to the values of Ei calculated in this study.68 Table 1 reveals that DN is tiny for chlorinated methanes but quite large for acetone, CH3CN, and DMSO. Therefore, AI potentially interacts with the solvent molecules of acetone, CH3CN, and DMSO even in neat solvent but with high concentration. We will further see the solvent effect on the diffusive reorientation and dimerization constant in more detail later. Regarding the low-frequency region below 10 cm−1, each solution shows a unique spectral line shape in the differential Kerr spectrum, as seen in Figure 1. In this low-frequency region, the translational motion (interaction-induced motion) significantly influences the shape of the intermolecular vibrational spectrum in many liquids.69−72 In the case of CCl4, however, the signal intensity is very small owing to its tetrahedral structure and increases with increasing interaction of CCl4 with anisotropic AI. In contrast, the signal intensity of the other solvents in this frequency region is large compared to that of neat CCl4. Because each differential Kerr spectrum is obtained from the difference between the Kerr spectra of the AI solution and its neat solvent normalized by the intramolecular vibrational mode of the solvent, the intensity of the differential spectrum is possibly negative if solvent−solute vibrations give rise to weaker signals than solvent−solvent vibrations. The intensity of the Kerr spectrum in the low-frequency region below 10 cm−1 is much larger for neat CH3CN than for other neat solvents including CCl4.28 This finding supports our consideration of the uniqueness of the low-frequency region of below 10 cm−1 in the differential Kerr spectra of the three studied solutions. A broad and strong spectral band at ca. 60 cm−1 (second Gaussian component IG2(ω)) is required to fit the spectra for all the solutions. Although the motion of the band cannot be simply assigned because of the cross section or coupling of translational and reorienttaional vibrational motions, the major contribution of this band is likely the aromatic ring libration like neat benzene.72 The width of the band in the solutions of acetone, CH3CN, and DMSO seems to be broader than that in the solutions of chlorinated methanes. The results suggest that the inhomogeneity of the librational motion of AI is larger in acetone, CH3CN, and DMSO than in the chlorinated methanes. Orientational Dynamics. Figure 3 shows the logarithmic plots for the Kerr transients of AI/CHCl3, AI/CH3CN, AI/ DMSO, and the corresponding neat solvents. The Kerr transients over 3 ps are fitted by a multiexponential function, and the fit parameters are summarized in Table 3. As clearly shown in Figure 3, the diffusive relaxation process is much slower in the AI solutions than in the neat solvents. In the case of neat CCl4 at 292 K, diffusive relaxation is not observed because of the symmetry of the molecule. Thus, the slow relaxation in AI/CCl4 is due to the solute, and we previously attributed it to the AI−dimer reorientation.20 Table 3 unveils that the time constants of the two fast relaxation processes (τ1 and τ2) are similar in AI/CHCl3, AI/CH2Cl2, AI/CH3CN, and AI/DMSO and the respective neat solvents.

Figure 3. Logarithmic plots for the Kerr transients of (a) AI/CHCl3 and neat CHCl3, (b) AI/CH3CN and neat CH3CN, and (c) AI/DMSO and neat DMSO. AI solutions and neat solvents are shown in blue and red, respectively. Multiexponential fits are also shown by the respective light color lines.

In the case of AI/acetone, the time constant of the fast relaxation process matches that in neat acetone that has only a singleexponential relaxation component. The remaining slow components of the relaxation processes in AI solutions are thus attributed to the neat or free solvent. Two extra relaxation processes are observed in AI/CHCl3, AI/CH2Cl2, AI/acetone, and AI/CH3CN, but one extra component is found in AI/DMSO. These extra components are assigned to relaxation processes of the solute. In the simple Stokes−Einstein−Debye (SED) hydrodynamic model, the rotation time of a solute in solution is given by73,74 τr =

Vη kBT

(3)

where V is the solute volume, η is the shear viscosity of the medium, kB is the Boltzmann constant, and T is the absolute temperature. On the basis of the SED model, taking τ2 in AI/CCl4 as reference, the slowest additional relaxation component in AI/CHCl3, AI/CH2Cl2, AI/acetone, and AI/CH3CN can be accounted for by the relaxation of the AI dimer because 16200


3.01 ± 0.61 2.29 ± 0.02

2.39 ± 0.11 7.39 ± 0.13 0.02715 ± 0.00061 0.00264 ± 0.00007

a2

0.02573 ± 0.01790 0.04874 ± 0.00846

13.12 ± 0.35 0.01292 ± 0.00104

1.21 1.79 0.17 1.69 0.83 ± ± ± ± ±

η/η(CCl4) is rather similar to τslow/τslow(CCl4): η/η(CCl4) = 0.59 and τ4/τ2(CCl4) = 0.67 for AI/CHCl3, η/η(CCl4) = 0.45 and τ4/τ2(CCl4) = 0.55 for AI/CH2Cl2, η/η(CCl4) = 0.38 and τ3/τ2(CCl4) = 0.38 for AI/acetone, and η/η(CCl4) = 0.40 and τ4/τ2(CCl4) = 0.34 for AI/CH3CN. The faster extra relaxation process in AI/CHCl3, AI/CH2Cl2, AI/acetone, and AI/CH3CN, however, can be expected to be due to the AI monomer. As mentioned above, we previously investigated BF in CCl4 as a control for AI monomer and found that the ratio of the slow relaxation times between BF/CCl4 and AI/CCl4 is about 0.3.20 This value is in good agreement with the ratio between the fast and slow relaxation times due to the solute in AI/CHCl3, AI/CH2Cl2, AI/acetone, and AI/CH3CN. Therefore, the long-time Kerr transients clearly indicate the existence of both the AI monomer and dimer in CHCl3, CH2Cl2, acetone, and CH3CN. In the case of AI/DMSO, the slowest relaxation time can be attributed to the monomer reorientation because the ratio {τ3/η}/{τ2(CCl4)/η(CCl4)} is 0.33, which is close to the ratio of the slow relaxation times between BF/CCl4 and AI/CCl4.20 Moreover, this relaxation component is the slowest observed in the present study. We now compare the results from the low-frequency Kerr spectra with those of the diffusive relaxation processes. In the low-frequency Kerr spectra, clear vibrational bands at ca. 90 and 105 cm−1 due to the AI dimer are observed in AI/CHCl3 and CH2Cl2, whereas a broad band at ca. 80 cm−1, which is likely due to the HB interaction of AI with the solvent, is found in AI/acetone, AI/CH3CN, and AI/DMSO. The interpretation of the AI/CHCl3 and CH2Cl2 cases is straightforward. Because the low-frequency Kerr spectra clearly display the above-mentioned distinct intermolecular vibrational bands, the AI dimer is surely present. However, unlike AI/CCl4, the Kerr transients in the picosecond region show evidence for monomer and dimer reorientations in these solutions. Thus, AI exists as monomer and dimer in CHCl3 and CH2Cl2. AI in DMSO clearly only exists as monomer because neither the low-frequency Kerr spectrum nor the diffusive relaxation processes detected the AI dimer. The cases of AI/acetone and AI/CH3CN are a little puzzling. No clear intermolecular HB bands prevail in the lowfrequency Kerr spectra for these solutions, but evidence for both monomer and dimer reorientations stems from the picosecond Kerr transients. It is plausible that the HB band arising from the AI−solvent interaction obscures the intermolecular HB bands of the AI dimer and/or that the concentration of the AI dimer is lower than that of the AI monomer. In the following section, we quantitatively address AI dimerization in solution. Dimerization Constant. Before we discuss NMR results, we would like to emphasize that the low-frequency vibrational bands at ca. 90 and 105 cm−1 in the Kerr spectra are the direct evidence of the presence of the AI dimer in chlorinated methane solutions. Also, the results of the picosecond Kerr transients showed the indirect evidence of the presence of the AI dimer in CCl4, both the AI monomer and dimer in CHCl3, CH2Cl2, acetone, and CH3CN, and the AI monomer in DMSO. On the basis of the RIKES results, we can attribute the chemical shift of a proton of AI to the either AI monomer, dimer, or AI−solvent complex. Walmsley estimated the dimerization constants of AI in CCl4 and benzene by NMR spectroscopy.75 We employ Walmsley’s method to estimate the dimerization constants of AI in a series of solutions studied herein. Figure 4 shows the concentrationdependent 1H NMR spectral band of the proton attached to

± ± ± ± ±

3.02 0.19 0.01 0.05 0.07 ± ± ± ± ± ± ± ± ± ±

1.31 0.58 1.25 1.33 1.56

τ1 (ps)

τ2 (ps)

a1

± ± ± ± ± ±

a2

0.00019 0.00429 0.00225 0.00021 0.01220 0.00024

39.10 ± 2.00 2.89 ± 0.92 2.25 ± 0.26 4.50 ± 0.10 2.11 ± 0.93 7.11 ± 0.94 (B) Neat Solvents

0.00135 0.00645 0.00772 0.02788 0.00230

a3

0.00053 0.00371 0.00020 0.00483 0.00011

7.33 5.70 14.91 3.96 30.40

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0.00098 0.07748 0.03541 0.11148 0.00569

0.01200 0.12000 0.00028 0.00336 0.00035

0.00403 0.02314 0.03788 0.01621 0.01414 0.00233 ± ± ± ± ± ±

0.54 0.56 0.31 0.04 0.82 0.13

τ1 (ps)

± ± ± ± ± ±

CHCl3 CH2Cl2 acetone CH3CN DMSO

sample

0.00441 0.00134 0.09294 0.02951 0.06863 0.00715

0.00045 0.00046 0.19700 0.00168 0.06000 0.00027

3.32 1.80 0.57 1.10 1.31 1.79

a1 sample

AI/CCl4 AI/CHCl3 AI/CH2Cl2 AI/Acetone AI/CH3CN AI/DMSO

(A) AI Solutions

Table 3. Multiexponential Fit Parameters for Picosecond Kerr Transients in (A) AI Solutions and (B) Neat Solvents

τ3 (ps)

a4

τ2 (ps)

26.16 ± 6.11 21.31 ± 0.54 0.00645 ± 0.00382 0.00796 ± 0.00046

τ4 (ps)


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Figure 4. Concentration-dependent NMR spectral band of the proton attached to the N at the 7-position of AI in (a) CCl4, (b) CH3CN, and (c) DMSO.

the N at the 7-position of AI (H7N) in (a) CCl4, (b) CH3CN, and (c) DMSO. The chemical shift of H7N in CCl4 and CH3CN shifts to the low magnetic field side with increasing concentration, but that in DMSO slightly shifts to the high magnetic field side. The peaks were estimated via fits to a Lorentzian function. The solvent dependence of the chemical shift of H7N in solution, displayed in Figure 5, reveals that the

effect of the aromatic ring of AI coming from the higher concentration.76 Each solvent-dependent chemical shift of H7N in solution, except for AI/DMSO, is analyzed by a simple aggregation model,75 which considers only the AI monomer and dimer, not larger aggregates, in equilibrium. The equilibrium constant (dimerization constant K) is thus given by K = [AI 2]/[AI1]2

(4)

where [AI2] and [AI1] are the concentrations of the AI dimer and monomer, respectively. The mass balance is [AI] = [AI1] + 2[AI 2]

(5)

and [AI] is 0.75 M in the present RIKES study. The observed chemical shift of a proton in a rapid-exchange system is the weighted average of the respective chemical shifts of the two different species.76 The present system obeys this condition, and the observed chemical shift thus reads δobs =

[AI1] 2[AI 2] δ1 + δ2 [AI] [AI]

(6)

where δ is the chemical shift in ppm, and the subscripts obs, 1, and 2 denote observed, monomer, and dimer, respectively. Accordingly, each solvent-dependent chemical shift of H7N can be fitted by

Figure 5. Chemical shift of the proton bonded to the N at the 7-position of AI vs concentration. Fits according to eq 7 are also given by lines.

chemical shift of H7N increases with increasing concentration in all solutions except for AI/DMSO. The slight high magnetic field side shift in AI/DMSO could be due to the shielding

δobs = 16202

1 + 8K[AI] − 1 ( 1 + 8K[AI] − 1)2 δ1 + δ2 4K[AI] 8K[AI]

(7)



Article

Table 4. Dimerization Constants K and Chemical Shift Parameters δ1 and δ2 Obtained by the Concentration-Dependent Chemical Shift of the Proton at 7-Position N of 7-Azaindole in Solutions by NMR solutions AI/CCl4 AI/CHCl3 AI/CH2Cl2 AI/acetone AI/CH3CN AI/DMSO a

K (M−1) 356 13.3 14.7 0.727 0.910 −

± ± ± ± ±

47 0.8 1.2 0.064 0.056

δ1 (ppm) 7.85 7.93 8.02 11.07 9.90 ∼11.61a

± ± ± ± ±

0.20 0.06 0.09 0.01 0.01

δ2 (ppm)

Xmonomer(0.75 M)

± ± ± ± ±

0.04 0.20 0.19 0.60 0.57 −

13.13 12.73 12.74 13.07 13.13 −

0.03 0.02 0.03 0.06 0.07

Value was estimated from the solution of 0.01 M.

CH3CN, and CCl4 but not in DMSO by RIKES and to estimate the dimerization constants of AI in these solutions by NMR.

As seen in Figure 5, each curve except for AI/DMSO is well described by eq 7. The fit parameters and the mole fraction of the AI monomer, Xmonomer = [AI1]/[AI] ([AI] = 0.75 M), in each solution are summarized in Table 4. It seems that K of AI solutions is better correlated with DN of the solvents than with F(ε,nD) (Table 1). We will discuss this feature later. More importantly, we can now quantitatively describe AI dimerization in these solutions. Under the condition of the RIKES experiments ([AI] = 0.75 M), the mole fraction of the AI monomer increases from ca. 20% in CHCl3 and CH2Cl2 over more than 50% in acetone and CH3CN (although up to 40% of AI exist as dimer) to almost 100% in CCl4. The dimerization constant in CCl4 has been reported3,75,77 to be in the range of ca. 100−400 M−1, and the value estimated in this study is in reasonably good agreement with the reported values. We next discuss the features of δ1 and δ2. As seen from Table 4, δ2 is ca. 13 for all the cases, but δ1 varies. In the case of chlorinated methanes, δ1 is ca. 7.9, but δ1 is 9.9 in AI/CH3CN and 11.1 in AI/acetone. This feature is most likely due to HB of AI with the solvent. In fact, a trace of specific HB interaction/ complex of AI monomer with solvent molecules (e.g., benzene) was reported by Walmsey.75 In this study, δ1 is attributed to the free or isolated AI monomer because chlorinated methanes do not interact with AI via HB specifically. Conversely, acetone, CH3CN, and DMSO can accept HB. Accordingly, it appears natural that the δ1 values in acetone and CH3CN solutions are higher than in chlorinated methane solutions. It is also plausible that the chemical shift of H7N in DMSO remains almost unchanged at ca. 11.5 owing to the HB complex of AI and DMSO. Therefore, it is rather reasonable that the tendency of δ1 values of AI in acetone, CH3CN, and DMSO agrees well the trend of DN, which is a quantitative measure of Lewis basicity: DN(acetone) = 17.0, DN(CH3CN) = 14.1, and DN(DMSO) = 29.8 (Table 1).56 The present results thus indicate that AI dimerization and the chemical shift of H7N (δ1) cannot be simply explained by the dielectric scale but also need to take the ability of the solvents to act as HB acceptor into consideration. This trend in the dimerization constant estimated by the NMR measurements supports the present RIKES experiments. The low-frequency spectra exhibit clear HB bands at ca. 90 and 105 cm−1 arising from the AI dimer in AI/CCl4, AI/CHCl3, and AI/CH2Cl2 but a monomodal broad band at ca. 80 cm−1 attributed to the HB cluster between the solvent and AI monomer in AI/acetone, AI/CH3CN, and AI/DMSO. Moreover, the diffusive orientation dynamics contain contributions to the relaxation processes from the monomer and dimer in AI/CHCl3, AI/CH2Cl2, AI/acetone, and AI/CH3CN, whereas only the dimer reorientation process has been observed in AI/CCl4 and the orientational dynamics of the monomer in AI/DMSO. Accordingly, we surely succeed in spectroscopically observing the AI dimerization in CHCl3, CH2Cl2, acetone,

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CONCLUSIONS In this study, we have investigated AI in solvents of varying polarity, CCl4, CHCl3, CH2Cl2, acetone, CH3CN, and DMSO, by means of femtosecond RIKES. Differential low-frequency Kerr spectra of AI in CH2Cl2, CHCl3, and CCl4, the reference solvent for the AI dimer, display unique vibrational bands arising from the AI dimer at ca. 90 and 105 cm−1. In contrast, the spectra of the solutions of acetone, CH3CN, and DMSO solutions show a broad modal band at ca. 80 cm−1, which is attributed to intermolecular vibrations arising from the HB between the AI and solvent molecule. The overdamped Kerr transients in the picosecond region in the AI solutions show extra relaxation components compared to that in the respective neat solvents. Two extra components have been observed in CHCl3, CH2Cl2, acetone, and CH3CN. On the basis of the SED hydrodynamic model and the previous study of AI and BF in CCl4,20 the faster and slower relaxation processes have been assigned to the reorientations of the AI monomer and dimer, respectively. In DMSO, only one extra relaxation component has been confirmed, which is attributed to the reorientation of the AI monomer. Accordingly, we conclude that AI at a concentration of 0.75 M exists as both monomer and dimer in CHCl3, CH2Cl2, acetone, and CH3CN, but only as monomer in DMSO. Together with the quantum chemistry calculations and solvent parameters, the RIKES results have been rationalized in terms of the solvent DN. We have also performed NMR measurements to evaluate the concentration dependence of the chemical shift of the proton attached to the N at the 7-position in AI in solution. The NMR results are in good agreement with the RIKES results. The solutions of AI in CCl4, CHCl3, and CH2Cl2 display traces of the free AI monomer and AI dimer. However, in acetone and CH3CN, the AI dimer and AI−solvent complex, and AI in DMSO only exhibit signatures of the AI−solvent complex. On the basis of a simple monomer−dimer equilibrium model, the dimerization constants have been estimated as 356 M−1 in CCl4, 13.3 M−1 in CHCl3, 14.7 M−1 in CH2Cl2, 0.727 M−1 in acetone, and 0.910 M−1 in CH3CN.

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ASSOCIATED CONTENT

S Supporting Information *

Atomic coordinates of AI monomer and dimer based on the quantum chemistry calculations at the B3LYP/6-311++G(d,p) level of theory with gas phase and IEF-PCM conditions, the calculated energies and interaction energies and hydrogenbonding lengths for the AI monomer and dimer, and complete author list for ref 53. This material is available free of charge via the Internet at http://pubs.acs.org. 16203



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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Mr. Shohei Kakinuma, Miss Atsuko Awata, and Dr. Katsuhiko Moriyama (all Chiba University) for their kind help with NMR measurements. This work was partially supported by the Inamori Foundation.

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REFERENCES

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