Chapter IV IR electroabsorption spectroscopic study of 4(3H)-pyrimidinone in
IV- 3. Results and discussion
IV-3-1. Concentration dependent FT-IR spectra
First, to confirm the presence of both monomer and the dimer of 4(3H)-Pyr in our p-dioxane solution, we measured concentration-dependent FT-IR spectra of 4(3H)-Pyr in p-dioxane in the 1740–1640 cm−1 region at 12 different concentrations (5, 10, 15, 20, 25, 31, 35, 40, 45, 48.95, 55, and 62mM) [Figure IV-1(a)].
Two IR bands are observed in this wavenumber region, and the intensities of both bands increased with increasing the concentration of 4(3H)-Pyr. However, the extent of increase of these two bands is different. Figure IV-1(b) shows that the absorbance ratio (defined as the ratio of the absorbance of the lower-wavenumber band to that of the higher-wavenumber band) has a linear dependence with a positive slope on solute concentration. This result indicates that the increase of the lower-wavenumber band is greater than that of the higher-wavenumber band. Considering that the dimer is more abundant at higher concentrations, and that the intermolecular hydrogen bonding in the 4(3H)-Pyr dimer weakens the C=O bond, resulting in a red-shift of C=O stretch peak, this observation leads to a conclusion that the higher-wavenumber band arises from the monomer, and the lower wavenumber band from the dimer. This conclusion is consistent with the literature [40].
In addition, we developed a model describing the equilibrium between the monomer and dimer of 4(3H)-Pyr, in which the equilibrium concentration of the monomer (β ) and dimer
where c is the total concentration, K is the equilibrium constant defined as
( )
2 2K α
= β .
P and Q are the coefficients that account for the path length and the extinction coefficient.
To determine K, we carried out a least-squares fitting analysis on the area intensities of the monomer band and dimer band using Eqs. IV-5 and IV-6. Figure IV-2 plots the area intensities of both bands as a function of total concentration and the fitted results. The value of the equilibrium constant K is determined as 0.031±0.005, indicating the predominance of the monomer in the concentration range (5-62 mM).
IV-3-2. Electric field strength dependence of IREA spectra
Figure IV-3 shows the absorption spectrum and a series of ∆A spectra of 4(3H)-Pyr in p-dioxane measured with 45, 55, 65, 75 volts for the wavenumber region of 1740–1640 cm−1. These voltages are equal to the strength of the external electric field of 9, 11, 13, and 15 MVm−1 respectively. The ∆A spectrum of pure p-dioxane was also measured at each voltage as blank experiment [dashed line in Fig. IV-3(a)] and turned out to be negligible, ensuring the spectral features seen in ∆A spectra of the solution (solid line) result from 4(3H)-Pyr. The absorption spectrum [Fig. IV-3(b)] has been solvent-subtracted and measured with a 50 µm path-length cell to achieve a higher S/N. The two IR bands obseved at 1706 cm−1 and 1675 cm−1 are assigned to the C=O stretching mode of the monomer and dimer of 4(3H)-Pyr as already discussed in Chapter IV-3-1 and in previous studies [40].
The ∆A spectra were measured with normally incident IR light (i.e., χ = 90°). In the ∆A spectra, two negative features at ~1706 cm−1 and ~1675 cm−1 are observed, apparently corresponding to the absorption maxima of the monomer and dimer bands. However, the origin of a positive feature at around 1695 cm−1 is unclear because there seems no absorption band at this wavnumber. The origin of those ∆A signals will be discussed later (see Chpater IV-3-4).
As the external electric field strength increases, the ∆A signals become stronger while the overall shape remains unchanged. Figure IV-4 plots the band area of the ∆A signals estimated in the interval 1722–1700, 1700–1682, and 1682–1658 cm–1 , as a function of the square of
the external electric field strength, Fext2
. The experimental data are fitted well to a straight line, ensuring that what we observed is the second-order Stark effect as mentioned in Chapter III.
IV-3-3. Angle χχχχ-dependent IREA spectra
To clarify the origins of the distinct ∆A features, we measured a series of χ dependent ∆A spectra of 4(3H)-Pyr in p-dioxane. Figure IV-5 shows three independent sets of the angle χ-dependent IREA spectra of 4(3H)-Pyr in p-dioxane (external field strength ~ 12.5 MVm−1) measured at five different angles χ = 55°, 66°, 76°, 83°, and 90° (normal incidence). The angle χ was calculated by taking into account the refractive index of the solvent (n=1.43 for p-dioxane). All the angle χ-dependent spectra shown in this thesis have been baseline-corrected. The variation in optical path length with angle χ was also corrected. Mere inspection of the three sets of ∆A spectra in Fig. IV-5 does not show any appreciable χ dependence. To examine the χ dependence of the spectra more closely, we performed SVD analysis of the data.
Before performing SVD, three sets of the data were first normalized to an averaged value of the ∆A signal at 1708 cm−1 to account for signal variation arising from different measurements and then averaged to achieve higher S/N. The averaged χ-dependent IREA spectra are displayed in Fig. IV-6(a). A plot of the singular values obtained from the SVD of this averaged dataset and the spectral components associated with the largest three singular values are shown in Figs. IV-6(b) and (c), respectively. Because the spectral component 3 is dominated by noise, we focus on the spectral components 1 and 2 when we reconstruct the physically meaningful spectral components (see Chapter IV-2-2). The χ dependences we obtained from Eq. IV-4 (ua and ub) as well as the expected χ dependences (constant and 1−3cos2χ) are shown in Fig. IV-7(a). The corresponding spectral components (va and vb in Eq.
IV-4) are shown in Fig. IV-7(b). In Figure IV-8, the ∆A spectra reconstructed according to our model with Eq. IV-4 are compared with the observed spectra. The good agreement between the observed and reconstructed spectra verifies that the above SVD analysis is correct.
Hereafter, we discuss the least-squares fitting analysis of the IREA spectra on the basis of these decomposed spectra components.
IV-3-4. Least-squares fitting analysis of the IREA spectra
To decompose the χ-independent and χ-dependent ∆A spectra [Fig. IV-7(b)] into the distinct contributions described in Chapter III, we carried out a least-squares fitting analysis.
First, the absorption spectrum [Fig. IV-3(b)] was fitted to a superposition of two Gaussian functions plus a baseline represented by an offset a:
1 or 2). The best fit is displayed in Fig. IV-9(a), and the peak assignments, the peak positions, and the bandwidths of the two bands so determined are summarized in Table I. With those bandwidths and peak positions fixed, the ∆A spectra were subsequently fitted to a linear combination of the zeroth, first, and second derivatives of each absorption band:
4(3H)-Pyr. Figure IV-9(b) shows the fitted results of the χ-independent and the χ-dependent
∆A spectra. It is clear from Fig. IV-9(b) that the model function (Eq. IV-8) reproduces well both χ-independent and the χ-dependent ∆A spectra The decomposition of the fitted results into the zeroth, first, and second derivative components is shown in Fig. IV-10. The coefficients determined by the fitting are summarized in Table II. They are related to the coefficients, Aχ, Bχ, and Cχ in Eq. III-23 via the following equations:
aχ =F A2 χ (IV-9)
bχ =15Fhc2 Bχ =
(
3.356 10× 23)
F B2 χ (IV-10) cχ =30Fh c2 22 Cχ =(
8.447 10× 47)
F C2 χ (IV-11) where F is the internal electric field strength. From the parameters aχ, bχ, and cχ,the values of the molecular properties such as ∆αααα, ∆µµµµ, and A can, in principle, be obtained using Eqs III-24-26. To do so, however, the local-field correction needs to be estimated (see below).The IREA spectra of 4(3H)-Pyr show that the contribution of the χ-independent component is dominant over the χ-dependent component. This may not be surprising because we expect to observe equilibrium change (see Chapter III-4) between the monomer and dimer of 4(3H)-Pyr as the χ-independent component. If the equilibrium between the monomer and the dimer exists, an external electric field applied to the 4(3H)-Pyr molecule would cause a shift in the equilibrium in favor of the polar monomer. In that case, we should observe a significant positive ∆A signal for the monomer and a significant negative ∆A signal for the dimer. However, this prediction contradicts our IREA spectra, which exhibit a negative ∆A signal for both monomer and dimer [see zeroth derivative contribution shown in Fig IV-10(a)]
This result implies that the stabilization due to the dipolar interaction between the dipole moment of the monomer and the external field (~10MVm−1) may not be strong enough to dissociate the intermolecularly hydrogen-bonded dimer. Another possible origin of the zeroth derivative contribution to the χ-independent component is the electronic polarization. The significant contribution of the electronic polarization to the aχ of the monomer is most likely a consequence of a large field dependence of the transition moment, i.e., transition polarizability A. In other words, the C=O stretching vibration of 4(3H)-Pyr is highly susceptible to the surrounding electrostatic environment, which is different from the case of acetone. The coefficients bχ and cχ of the 4(3H)-Pyr monomer and dimer can be attributed to electronic polarization. The bχ term depends mainly on the interaction of the molecular
polarizability and the applied electric field. The cχ term describes the interaction of ∆µµµµ and the applied field. Detailed analysis of those electronic polarization contributions will require higher resolution measurements and it is left for future studies.
The most surprising result we found is that the orientational polarization, which is usually dominant for polar molecules [15, 17, 19], does not contribute much to the ∆A signal.
Recall that the orientational polarization signal is χ-dependent (Eq. III-22). As discussed above, no appreciable χ-dependence was observed in a series of χ-dependent ∆A spectra, indicating that the contribution from the orientational polarization signal is small. We attribute this observation to angle α that is very close to the magic angle 54.7°. This hypothesis is correction factor, which is taken from the previous IREA work [24]. The assumption may be justified because the materials of our home-made sample cell and the solvent (p-dioxane) are the same as that work, and a very similar local-field correction factor should apply as well.
The coefficient aχ of the monomer obtained with the fitting analysis for the χ-dependent component is aχ = (∆A/A)ori = 5.7×10−7.It is obvious from this value of aχ that the angle α of 4(3H)-Pyris neither 0° nor 90°. Using (∆A/A)ori = 5.7×10−7 with Eq.III-22, we obtain α = 55.23°. The calculated result is consistent with our hypothesis that the angle α is very close to the magic angle 54.7° if the zeroth derivative contribution in the χ-dependent spectral component comes solely from orientational polarization. Figure IV-11 shows the optimized structure of 4(3H)-Pyr with a calculated dipole moment of 2.45 D and the direction of the vibrational transition moment predicted from our experimental results. The structural
optimization of 4(3H)-Pyr was performed using the DFT calculations with RB3LYP functional and the 6-31+G(d,p) basis set on Gaussian09.
For the dimer, too, no significant orientational polarization signal is observed as well [Fig IV-10(b)], suggesting that the structure of the dimer may resemble the base pair-like hydrogen bonded structure (Figure I-1.) as argued in the previous study [40].