Solvent Dependence of Absorption Intensities and Wavenumbers of

Feb 3, 2011 - Quantum chemical calculations of the wavenumbers and absorption intensities of NH stretching bands by using the 1D Schrödinger equation...
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Solvent Dependence of Absorption Intensities and Wavenumbers of the Fundamental and First Overtone of NH Stretching Vibration of Pyrrole Studied by Near-Infrared/Infrared Spectroscopy and DFT Calculations Yoshisuke Futami,† Yasushi Ozaki,‡ Yoshiaki Hamada,§ Marek J. Wojcik,^ and Yukihiro Ozaki†,* †

Department of Chemistry, School of Science and Technology, Kwansei Gakuin University, Sanda, Hyogo 669-1337, Japan Department of Chemistry, Faculty of Science, Josai University, Sakado, Saitama 350-0295, Japan § The Open University of Japan, Wakaba, Mihama-ku, Chiba, 261-8586, Japan ^ Faculty of Chemistry. Jagiellonian University, 30-060 Krakow, Ingardena 3, Poland ‡

ABSTRACT: Near-infrared (NIR) and IR spectra were measured for pyrrole in CCl4, CHCl3, and CH2Cl2 to study solvent dependence of absorption intensities and wavenumbers of the fundamental and first overtone of NH stretching vibration. It was found that the wavenumbers of the NH fundamental and its first overtone decrease in the order of CCl4, CHCl3, and CH2Cl2, which is the increasing order for of the dielectric constant of the solvents. Their absorption intensities increase in the same order, and the intensity increase is more significant for the fundamental than the overtone. These results for the solvent dependence of the wavenumbers and absorption intensities of NH stretching bands of pyrrole are quite different from those due to the formation of hydrogen bonds. Quantum chemical calculations of the wavenumbers and absorption intensities of NH stretching bands by using the 1D Schr€odinger equation based on the self-consistent reaction field (SCRF)/ isodensity surface polarized continuum model (IPCM) suggest that the decreases in the wavenumbers of both the fundamental and the overtone of the NH stretching mode with the increase in the dielectric constant of the solvents arise from the anharmonicity of vibrational potential and their intensity increases come from the gradual increase in the slope of the dipole moment function.

1. INTRODUCTION In recent years, near-infrared (NIR) spectroscopy has made a marked progress in its applications to basic sciences as well as applied sciences.1-5 NIR spectroscopy treats overtones and combination modes, which are the forbidden transitions in essence. Unique properties of NIR spectroscopy such as nondestructive analysis and bulk analysis arise from the forbidden transitions, which yield only weak signals, in other words, superior transmissibility. Studies of overtones, combinations, and anharmonicities stretch back more than 60 or 70 years,1,4,5 but their studies are still insufficient. Recent rapid developments of NIR spectrometers, particularly FT-NIR spectrometers, spectral analysis methods like chemometrics, and 2D correlation analysis, and quantum chemical calculations have stimulated novel studies on the overtones and combination modes, and their anharmonicities.1-5 This article is concerned with absorption intensities and wavenumbers of the fundamental and first overtones of NH stretching vibration of pyrrole studied by NIR/IR spectroscopy and DFT calculations. Functional groups including a hydrogen atom such as CH, NH, and OH play important roles in chemical bondings and chemical reactions such as hydrogen bondings, hydrogen transfer, r 2011 American Chemical Society

and tunneling effects. Molecular vibrations of these substituents are greatly reflected on the changes in molecular structures and intermolecular interactions. NIR spectroscopy is powerful in exploring chemical bondings and interactions of XH groups because NIR bands due to XH groups appear much more strongly than those due to the other groups.1-5 Moreover, the wavenumbers shifts of the molecular vibrations of XH stretching modes caused by hydrogen bondings and molecular interactions are larger for the overtone transitions than the fundamental transitions. The observation of an overtone transition may enable one to identify structural isomers that cannot be differentiated by the observation of a fundamental transition. For example, one can differentiate the structural isomers of halogenated phenols by the observations of overtones.6,7 Moreover, it is possible to estimate a vibrational potential function from the observations of a series of overtones.4,5 Effects of inter- and intramolecular interactions appear clearly not only in wavenumber shifts of vibrational bands but also in Received: September 7, 2010 Revised: December 29, 2010 Published: February 3, 2011 1194

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The Journal of Physical Chemistry A their absorption intensities. For example, the vibrational wavenumbers of fundamentals of OH and NH stretching modes shift to lower wavenumbers and their absorption intensities become stronger upon the formation of hydrogen bondings. The vibrational wavenumbers of the first overtone shifts to lower wavenumbers as in the case of the fundamentals, but their absorption intensities become weaker.3,5,8 Howard and Kjaergaard compared an IR spectrum of hydrogen-bonded methanol-trimethylamine complex in a gas state with a result of quantum chemical calculation.9 It was found that the fundamental of OH stretching mode of methanol becomes strong upon the formation of hydrogen bonding with trimethylamine, whereas its first overtone becomes weak. The calculated result reproduced the experimental result. On the basis of the result of quantum chemical calculation, they ascribed the intensity changes to the effect of dipole moment function of the OH stretching mode.10 Futami et al. found similar results for the NH stretching mode of pyrrole-pyridine complex.11 They measured NIR/IR spectra of pyrrole, pyridine, and pyrrole-pyridine complex in CCl4 solutions. The first overtone of the NH stretching vibration band of free pyrrole was observed at 6856 cm-1, but that of the pyrrole-pyridine complex was missing or extremely weak. Theoretical calculations of molecular vibrational potentials and dipole moment functions of the NH stretching modes of free pyrrole and pyrrole-pyridine complex elucidated that the transition dipole moment derived from the dipole moment function of the N-H distance becomes much smaller upon the formation of the complex, resulting in the remarkable intensity decrease in the overtone of the hydrogen-bonded NH group. Takahashi et al.12 calculated the absorption intensities of the fundamental and the first overtone of CH stretching mode of 1,2-dichloroethylene in gas and solution states by using Onsager model of self-consistent reaction field (SCRF). They reported that the intensities of both the CH fundamental and its first overtone are stronger in the solution state than in the gas state. One can expect from the above three studies that effects of hydrogen bondings and solvent effects induce different changes in absorption intensities of fundamentals and first overtones of the XH stretching modes. Preat et al.13 investigated dependences of dielectric constant on the wavenumbers and IR absorption intensity of CO stretching mode and electronic excitation energy of courmarin by using a polarized continum model (PCM). According to their calculation results together with IR measurement of coumarin, the frequency of CO stretching mode decreases while its intensity increases with the increase in the dielectric constant up to ε = ∼25, and they stay almost constant above ε = ∼25. Polovkova et al.14 reached similar conclusion that the frequency and the intensity of a NH stretching band change with little above ε = ∼25. for 3-dimethylamino-2-acetyl propenenitrile and 3-dimethy amino-2-methylsulfonyl propenenitrile by use of PCM and a static isodensity surface polarized continuum model (IPCM). In the present study, we have investigated solvent dependences of the wavenumbers and absorption intensities of the fundamental and the first overtone of NH stretching mode of pyrrole in CCl4, CHCl3, and CH2Cl2 by using both IR and NIR spectroscopy and quantum chemical calculation based on a SCRF/IPCM model. It is of note that the dielectric constants of CCl4 (2.2), CHCl3 (4.8), and CH2Cl2 (8.9) are quite different from each other. We used 1D Schr€odinger equation for the analysis of molecular vibration because the NH stretching mode of pyrrole has an A1 symmetry. The results of quantum chemical calculations based

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on SCRF/IPCM/DFT are in good agreement with the experimental results for the solvent dependences of absorption intensities and wavenumbers. The experimental results have been reproduced very well by the model of molecular vibration in the induced electric field due to the different dielectric constant. Because there is little study that compares experimental results with quantum chemical calculations for solvent dependences of absorption intensities of the overtones of XH stretching vibrations, the present study should have significant novelty in this point.

2. EXPERIMENTAL AND CALCULATION METHODS Pyrrole (Tokyo Kasei Kogyo, 98% purity) was used without further purification. CCl4, CHCl3, and CH2Cl2 (Wako, 98% purity) were treated with molecular sieves before use. The concentrations of pyrrole in CCl4, CHCl3, and CH2Cl2 were 0.04 mol L-1. NIR/IR spectra of the solutions in the region of 15 0002500 cm-1 were measured with an FT-NIR/IR spectrophotometer (PerkinElmer Spectrum One NTS FT-NIR/IR spectrometer). The spectral resolution used was 0.5 cm-1, and the number of spectral accumulations was 16. The spectra were measured at room temperature by use of a rectangular cell with a pass length of 1 mm. The DFT calculations were carried out by using the Gaussian 03 program15 with three basis sets (6-311þþG(3df,3pd), 6-31þG**, 6-31G*). Becke’s three-parameter hybrid density function in combination with the Lee-Yang-Parr correlation functional (B3LYP) was used for the optimization of geometrical structures and the calculations of normal coordinates, vibrational potential curves, and dipole moment functions.16,17 The quantum chemical calculations based on a SCRF/IPCM model were carried out by using the chemical structures in the gas phase. In the present study, we solved the Schr€odinger equation of 1D NH stretching vibration " # p2 d2 þ V ðqÞ ψv ðqÞ ¼ Ev ψv ðqÞ ð1Þ Hψv ðqÞ ¼ 2μ dq2 where q, μ, and V(q) are the normal coordinate, the reduced mass and the potential energy function, respectively. We calculated the integrated absorption coefficient (km mol-1, base e) A(v) for each NH stretching transition by Z 8NA π3 F 2 jμ j ~vv0 ð2Þ AðvÞ ¼ ln10 εð~v Þd~v ¼ 3  105 hc v0 where ε(v~) is the molar extinction coefficient (base 10), ~vv0 is the Fv0|2 is the sum of the squared transition energy in cm-1, and |μ transition dipole moments of the x, y, and z components in (debye)2 units.18 |μFv0|2 is given by Fz 2 jμFv0 j2 ¼ jμFv0xj2 þjμFv0yj2 þjμ v0j Z Z ¼ j ψv ðqÞμFv0xψ0 ðqÞdqj2 þj ψv ðqÞμFv0yψ0 ðqÞdqj2 Z þj ψv ðqÞμFv0zψ0 ðqÞdqj2

ð3Þ

Here, |μF xv0|2 = 0 and |μF yv0|2 = 0, because these are the A1 mode. We obtained the numerical results for the energy levels and the wave functions using the method given by Johnson.19 The 1195

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Figure 1. FT-NIR/IR spectrum of pyrrole in CCl4. Concentration of the solution was 0.04 M.

potential energy curves used for the calculations cover the range from -0.7 to 1.0 q0 in 0.02 q0 steps around the equilibrium position, where q0 is the unit for the normal coordinate corresponding to the NH stretching mode; they are represented by the displacement vectors of atoms (Figure 1) in Å unit as follows: q0 ¼ fN1 ð0, 0,-0:07Þ,H1 ð0, 0, 1:00Þ,H2 ð0:01, 0, 0Þ,H3 ð-0:01, 0, 0Þg

The very wide range of q = -0.4 to 0.8 shown in Figure 4 causes only slight errors in the calculations of transition probabilities because the wave functions have large amplitude only in the small q region, q = -0.2 to 0.2. The number of calculation points is sufficient for the numerical calculations without such assumption like the Morse function. This ensures converged energy levels with precision higher than 0.001 cm-1.

3. RESULTS AND DISCUSSION 3.1. Near-Infrared Spectra of Pyrrole. Figure 1 shows an NIR/IR spectrum in the 9000-3000 cm-1 region of pyrrole in CCl4 (0.04 M). The fundamental and the first overtone of NH stretching mode of pyrrole are observed at 3497 and 6856 cm-1, respectively. It can be seen from Figure 1 that the intensity of the overtone is much weaker than that of the fundamental. Parts a and b of Figure 2 show IR spectra in the 35503400 cm-1 region and NIR spectra in the 6950-6700 cm-1 region of pyrrole in CCl4, CHCl3, and CH2Cl2, respectively. It should be pointed out that for both the NH fundamental and its first overtone, the wavenumbers slightly decrease in the order of CCl4, CHCl3, and CH2Cl2. The NH fundamental and its first overtone become broader in the order of CCl4, CHCl3, and CH2Cl2; the absorption intensities show significant solvent dependences. In the case of the NH fundamental, the relative area intensities are 1.00 1.40, and 1.65 for the CCl4, CHCl3, and CH2Cl2 solutions, respectively. They are 1.00, 1.11, and 1.13 respectively for the CCl4, CHCl3, and CH2Cl2 solutions for the first overtone. Because the NH and OH bands show large intensity changes and small frequency shifts due to the formation of hydrogen bonds,3,7 the present intensity change and frequency shift might be ascribed to the formation of a weak hydrogen bond between NH and Cl. However, in the case of hydrogen bondings, the intensities of OH and NH fundamentals increase while those of

Figure 2. Solvent dependence of (a) the fundamental and (b) the first overtone of the NH stretching mode of pyrrole. Solvents used were CCl4, CHCl3 and CH2Cl2. Concentrations of the solutions in each case were 0.04 M for all.

their first overtones decrease.9,11 Hence, the observed solvent dependence is clearly different from the results of the hydrogen bondings. The dielectric constants of CCl4, CHCl3, and CH2Cl2 are 2.2, 4.8, and 8.9, respectively. Parts a and b of Figure 2 reveal that both the NH fundamental and its first overtone show the downward wavenumber shifts and their intensities increase with the increase in the dielectric constants of the solvents. Therefore, to investigate the relation between the wavenumbers and absorption intensities of the NH fundamental and its first overtone and the dielectric constant of the solvent, we calculated the potential of NH stretching mode and its dipole moment function of pyrrole in dielectrics with various dielectric constants by using the SCRF/ IPCM method. Parts a and b of Figure 3 depict the calculated wavenumbers and relative intensities versus the dielectric constant for the NH fundamental and its first overtone of pyrrole, respectively. The NH fundamental shifts from 3540 to 3520 cm-1 and the first overtone shifts from 6944 to 6905 cm-1 when the dielectric constant increases from 1 to 80. Most of the changes occur between ε = 1 and ε = 20 for both the fundamental and the first overtone. The absorption intensity increases by 2.4 and 1.3 times for the NH fundamental and the overtone respectively with the increase in the dielectric constant from 1 to 80. The following observations in the wavenumbers and intensities of the NH band in the solutions are well reproduced by this calculation: (1) The wavenumbers show larger red shift in the solvents with larger dielectric constants for both fundamental and first overtone. 1196

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(2) The shift is larger for the fundamental than for the first overtone. (3) The intensities increase as in the solvents with the larger dielectric constants for both fundamental and first overtone. (4) The intensity change is more pronounced for the fundamental than the first overtone. The present results for the band shifts and the intensity changes are in good agreement with those by Takahashi et al.12 and Preat et al.13 Henry and co-workers20,21 investigated systematically the X-H vibrations of gas-phase molecules on the basis of the local mode model. In their reports, the observed frequencies and intensities of the fundamental and overtone bands of

X-H are well reproduced by quantum chemical calculations. They mainly studied the C-H vibrations but reported a little about the N-H vibrations.20,21 Table 1 compares the observed and calculated wavenumbers and intensities of the NH fundamental and the overtone of pyrrole in CCl4, CHCl3, and CH2Cl2. The calculated results are in good agreement with the experimental results. Particularly, the absorption intensities are reproduced very well. Both the experimental and calculated results show that the intensity increase is more pronounced for the fundamental than the overtone. One cannot see a significant solvent dependence of anharmonic constant. The experimental and calculated results for the intensity changes suggest that the solvent dependence of the NH stretching band can be explained by the dielectric constant of the solvent but not by the hydrogen bonding between pyrrole and the solvents. Overtone bands, which are forbidden on the harmonic oscillation model, appear due to the anharmonicity of molecular vibrations and the intensity of a fundamental would decrease, whereas that of the corresponding overtone would increase with the increase of the anharmonicity of molecular vibrational potential on the premise that the dipole moment function is a linear function of the NH distance. This indicates that the increase of the anharmonicity does not cause the observed changes in the fundamental and first overtone bands from the fact (3) and (4). The calculated potential functions also support the above findings. Part a of Figure 4 shows the vibrational potential energy curve and first few vibrational wave functions, as well as the dependences of dipole moment function on the standard coordinate of NH stretching mode of pyrrole. Part b of Figure 4 shows the difference of the vibrational potential energy for the different dielectric constant, from ε = 1 to 10. Part c of Figure 4 shows the difference of dipole moment for the different dielectric constant, from ε = 1 to 10. As shown in parts b and c of Figure 4, the dependences of the vibrational potential and dipole moment on the dielectric constant are extremely small. The intensities of the vibrational bands also depend on the dipole moment function when it is not a linear function of the NH distance. Parts b and c of Figure 4 shows the dependences of the vibrational potential and the dipole moment function on the dielectric constant ε. It can be clearly seen in part c of Figure 4 that the slope of the dipole moment function becomes larger with the dielectric constant and that the minimum of vibrational potential curve shifts to the positive direction on the normal coordinate and the anharmonicity of the potential increases gradually. The results in Figure 4 suggest that the calculated red shift of wavenumbers with the increase in the dielectric constant arises from the anharmonicity of vibrational potential and that the intensity increases come from the gradual increase in the slope of the dipole moment

Figure 3. Dependence on dielectric constant of the wavenumber and intensity ratio of (a) the fundamental and (b) the first overtone of the NH stretching mode of pyrrole calculated at the IPCM//DFT/B3LYP/ 6-311þþG(3df.3pd) level.

Table 1. Observed and Calculated Wavenumbers (cm-1) and Relative Absorption Intensities of the Fundamentals and the First-Overtones of NH Stretching Bands of Pyrrolea First Overtone Obs. solvent

a

ε0

v

Calc. int.

Anharmonicity Constant χ

Fundametal

v

Obs. int.

v

Calc. int.

v

Obs.

Calc.

int.

CCl4

2.2

6855.6

1.00

6932.5

1.00

3496.6

1.00

3533.2

1.00

-68.8

-66.9

CHCl CH2Cl2

4.8 8.9

6837.3 6822.4

1.11 1.13

6919.4 6913.8

1.10 1.13

3486.0 3477.2

1.40 1.65

3526.6 3523.7

1.35 1.52

-67.3 -66.0

-66.8 -66.8

The intensities are normalized against those of bands of CCl4 solutions of pyrrole. 1197

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constant of the solvents. We also carried out quantum chemical calculations of the wavenumbers and absorption intensities of NH stretching bands of pyrrole by using the 1D Schr€odinger equation based on the SCRF/IPCM model. The calculation results have reproduced very well the experimental results. Our calculations also have shown that the wavenumbers and absorption intensities change largely in the range of 1 < ε < 25 but change with little above ε = ∼25. This tendency is very similar to that of the energy calculation by Polovkova et al.14 The present results for the solvent dependences of the wavenumbers and absorption intensities of NH stretching bands are quite different from the results for NH hydrogen bondings.

’ AUTHOR INFORMATION Corresponding Author

*Fax: þ81- 79-565-9077. E-mail: [email protected].

Figure 4. (a) Dependences on the dielectric constant of the potential energy curve, dipole moment function (ε = 1 to 10) and wave function (ε = 1) of NH stretching mode, (b) difference of the potential energy curve between the calculation result for dielectric constant of 1 and a variety of dielectric constants, (c) difference of the dipole moment function between the calculation result for dielectric constant of 1 and a variety of dielectric constants.

function. It is also clear that the variation in the vibrational potential affects little the absorption intensity changes. The change in the dipole moment function caused by the dielectric constant is large, and this large change in the dipole moment function induces the increase in the absorption intensity with more significant change in the fundamental. The variation in the dipole moment function suggests that the deviation of electric change in the molecule by the molecular vibration increases with the increase in the induced electric field. The inversion of the direction of vector of dipole moment function upon the formation of hydrogen bonding observed for pyrrole-pyridine complex [3] did not occur by increasing the electric constant from 1 to 80. Therefore, we conclude that the solvent dependences of the wavenumbers and absorption intensities of NH stretching bands of pyrrole are quite different from the variations in the wavenumbers and absorption intensities caused by the formation of an NH hydrogen bonding. At last, it is noted again that the above model well reproduces the red shifts of the vibrational wavenumbers, the origin of which is the change of the vibrational potential functions arising from the change in the dielectric constants of the solvents as shown in parts a and b of Figure 4.

4. CONCLUSIONS We measured NIR/IR spectra of pyrrole in CCl4, CHCl3, and CH2Cl2 to investigate solvent dependences of the wavenumbers and absorption intensities of the NH fundamental and its overtone. The wavenumbers of the NH fundamental and its first overtone decrease in the order of CCl4, CHCl3, and CH2Cl2 and their absorption intensities increase in the same order. The intensity increase is more remarkable for the fundamental than the overtone. The above order is the order for the dielectric

’ ACKNOWLEDGMENT The authors thank Dr. Saeko Shin, Ms. Nakako Tokita, and Mr. Ei-ichi Masuko (The Open University of Japan) for their kind help in the calculations and discussion. This study was partly supported by “Open Research Center” project for private universities; matching fund subsidy from MEXT (Ministry of Education, Culture, Sports, Science and Technology), 2001-2008. ’ REFERENCES (1) Near-Infrared Spectroscopy; Siesler, H. W.; Ozaki, Y.; Kawata, S.; Heise, H. M., Eds.; Wiley-VCH: Weinheim, 2002. (2) Near-Infrared Spectroscopy in Food Science and Technology; Ozaki, Y.; McClure, W. F.; Christy, A. A., Eds.; Wiley-Intescience: Hoboken, NJ, USA, 2007. (3) Siesler, H. W. in ref 1, p 213. (4) Bokobza, L. in ref 1, p 1 (5) Sandorfy, C.; Buchet, R.; Lachenal, G., in ref 2, p 11. (6) Pauling, L. J. Am. Chem. Soc. 1936, 58, 94. (7) Wulf, O. R.; Jones, E. J. J. Chem. Phys. 1940, 8, 475. (8) Iwamoto, R.; Matsuda, T.; Kusanagi, H. Spectrochim. Acta, Part A 2005, 62, 97. (9) Howard, D. L.; Kjaergaard, H. G. J. Phys. Chem. A 2006, 110, 9597. (10) Kjaergaard, H. G.; Low, G. R.; Robinson, T. W.; Howard, D. L. J. Phys. Chem. A 2002, 106, 8955. (11) Futami, Y.; Ozaki, Y.; Hamada, Y.; Wojcik, M. j.; Ozaki, Y. Chem. Phys. Lett. 2009, 482, 320. (12) Takahashi, K.; Sugawara, M.; Yabusita, S. J. Phys. Chem. A 2002, 106, 2676. (13) Preat, J.; Loos, P.-F.; Assfeld, X.; Jacquemin, D.; Perpete, E. A. Int. J. Quantum Chem. 2007, 107, 574. (14) Polovkova, J.; Gatial, A.; Milata, V.; Cernuchova, P.; Pronayova, N.; Liptaj, T.; Matejka., P. J. Mol. Struct. 2006, 785, 85. (15) Frisch, M. J. et al. GAUSSIAN 03, Rev. B.05; Gaussian, Inc.: Pittsburgh, PA, USA, 2003. (16) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (17) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785. (18) Takahashi, K.; Sugawara, M.; Yabushita, S. J. Phys. Chem. A 2005, 109, 4242. (19) Johnson, B. R. J. Chem. Phys. 1977, 67, 4086. (20) Niefer, B. I.; Kjaergaard, H. G.; Henry, B. R. J. Chem. Phys. 1993, 99, 5682. (21) Kjaergaard, H. G.; Daub, C. D.; Henry, B. R. Mol. Phys. 1997, 90, 201.

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