- Email: [email protected]
Theoretical study of the vibrational spectra of 2-chloropyridine metal complexes II. Calculation and analysis of the IR spectra of Cd– and Ni–2-Chloropyridine complexes
Journal of Molecular Structure 476 (1999) 21–26
Theoretical study of the vibrational spectra of 2-chloropyridine metal complexes II. Calculation and analysis of the IR spectra of Cd– and Ni– 2-Chloropyridine complexes M. Bakiler a,*, I.V. Maslov b, Sevim Akyu¨z c a
Department of Physics, Mimar Sinan University, Bes¸iktas¸ 80690, Istanbul, Turkey V.I. Vemadsky Institute of Geo- and Analytical Chemistry, RAS, Kosygin str. 19, 117972 Moscow, Russia c Department of Physics, Faculty of Science, Istanbul University, Vezneciler 34459, Istanbul, Turkey
b
Received 19 January 1998; revised 15 May 1998; accepted 15 May 1998
Abstract The vibrational IR spectra of a Cd complex of 2-chloropyridine, was calculated on the basis of a parameter set determined in our previous study on the 2-chloropyridine molecule. The Cd–N bond strength was determined by the variation of the force ˚ ). Calculated IR intensities indicate the presence of field, and the corresponding force constant is found to be (1.064 mdyne/A some changes in electron distribution of the 2Cl-pyridine molecule in a complex formation, with respect to the free molecule. The distortion of the electro-optical parameters occurs around the N atom. The interpretation of the normal vibrations and IR intensities of the Cd–2Cl-pyridine complex is given. Comparison with the corresponding shifts for the case of the Ni complex of 2Cl-pyridine, ensures that the force field of the free 2Cl-pyridine molecule should be altered in a complex formation, in order to represent experimental data. 䉷 1999 Elsevier Science B.V. All rights reserved. Keywords: 2-Chloropyridine: IR spectrum; IR intensity; Force field refinement; Transition metal complexes
1. Introduction Investigation of the metal–organic complexes plays a significant role in moderm chemistry. Metal complexes are used in various chemical technologies, such as catalysis and photochemical processes. Vibrational spectroscopy is a powerful tool for studying the structure and physical properties of the complexes. Along with the vibrational frequencies, the IR intensities contain rich inforfnation about molecular electronic properties. Electron charge dis* Corresponding author. Fax: +90-2126370216; E-mail: [email protected]
tribution or atomic charges can be obtained from the vibrational spectra. The metal ligand charge transfer effect can then be studied thoroughly by the analysis of vibrational spectra. The electrostatic potential is determined directly from the atomic charges obtained with the help of IR spectra. As was pointed out in our previous paper [1], experimental and theoretical studies of IR intensities are rare. The theory for calculation of IR intensities was developed [2] and approved in a number of calculations [3]. In the course of the investigation of the vibrational spectra of 2-halopyridine tetracyanonickelate complexes [4], it has been found that several modes of coordinated halopyridines (2Cl-py and 2Br-py) have upward shifts in frequency,
0022-2860/99/$ - see front matter 䉷 1999 Elsevier Science B.V. All rights reserved. PII: S 00 22 - 28 6 0( 9 8) 0 04 9 2- X
22
M. Bakiler et al. / Journal of Molecular Structure 476 (1999) 21–26
compared with those of the free molecule, and the shifts are metal dependent. These are thought to occur as a result of kinematic interaction of the vibrational modes of the ligand molecule by metal–ligand bond vibrations [5]. To check this idea and investigate thoroughly metal–ligand coupling, the vibrational IR spectra of the Cd 2Cl-pyridine complex was calculated and reported here. In our previous study, we have determined the sets of parameters (force constants and electro-optical parameters) for the free 2Cl-pyridine molecule [1]. This key step allows us now to calculate metal complexes of 2Cl-pyridine.
2. Methods and calculation The geometry optimization by quantum chemical methods is not available for molecules containing the Cd atom. We chose planar geometry for the complex ˚ . Determined and the N–Cd distance was set to 2.0 A sets of parameters for the 2Cl-pyridine molecule [1] were transferred without any changes to the Cd–2Clpyridine complex. The further parameter refinement is discussed below. The details of the Cd–ligand force field are not well known, and we started with more or less standard values for metal–nitrogen interactions. The initial value of the N–Cd bond stretching force ˚ , the C–N–Cd constant was taken as 2.564 mdyne/A ˚ , and valence angle force constant as 2.665 mdyne A
the out-of-plane, ring-Cd bending force constant was ˚ . The calculated IR spectra taken as 0.076 mdyne/A with unchanged 2Cl-pyridine force field, qualitatively represent correct experimental shifts of wave numbers with respect to the 2Cl-pyridine spectra (see Table 1). The variation of the force field was then performed in order to represent experimental shifts of wave numbers with better accuracy. We did not directly fit the experimental spectrum of the Cd–2Cl-pyridine complex, but added experimental shifts (coordinated 2Cl-pyridine wave numbers — free 2Cl-pyridine wavenumbers) to the theoretical values for 2Clpyridine [1]. At the first step, we tried to represent the shifts by variation of the force constants related to the Cd atom: N–Cd stretching, C–N–Cd and C–N–C valence angle bendings, and interactions of these force constants with each other. Unrestricted variation of these force constants leads to a very small value of the N–Cd bond stretching force constant equal to ˚ , a decrease of the C–N–Cd force 1.064 mdyne/A ˚ ), and a sigconstant (from 2.665 to 1.142 mdyne A ˚ nificant decrease by 0.135 mdyne A of the C–N–C ˚ ). In this force constant (initial value 0.902 mdyne A case, we were able represent experimental shifts of wave numbers with good accuracy (see Table 1). The calculated value of the N–Cd stretching force constant corresponds to those obtained for the Cd–pyr˚) idine and Cd–bipy–Cd complexes (0.94 mdyne/A
Table 1 Experimental and theoretical wavenumber shifts, (cm −1) of coordinated 2-chlropyridine with respect to free molecule 2-Clpy n experimental 312 426 617 724 991 1045 1083 1117 1149 1286 1365 1420 1453 1568 1577 a
(n Cd–2Cl-py − n 2Cl-py) Experimental 21 —a 12 7 15 7 5 15 4 8 8 −1 8 −2 16
Theor.model 1 21 0 12 5 5 5 7 6 5 1 0 −1 14 2 9
Theor.model 2 21 8 11 7 5 5 1 7 8 8 4 2 13 1 12
Could not be observed because of overlapping with Ni(CN) 4 group band.
(n Ni–2Cl-py − n 2Cl-py) Experimental 22 17 14 io 17 12 8 19 8 12 30 2 io −7 19
Theoretical 25 0 18 6 11 5 7 6 5 1 0 0 14 3 9
23
M. Bakiler et al. / Journal of Molecular Structure 476 (1999) 21–26
[5,6]. We have also made a determination of the altemative force field model. The method of the force field refinement implemented in program LEV allows the use of additional restrictions during variational procedure. The method is based on the least-square minimization of the functional F with a penalty function, with respect to parameters x (force constants): exp 2 F = ∑ [ncalc i (x) − ni ] + P,
(1)
i
where n i is the theoretical and experimental values of wave numbers, and P is a penalty function defined by: " # " #10 xj − x0j xj − x0j +b ∑ , (2) P=a ∑ D D j j
where a and b are coefficients, D is the desired variation interval, and x j and x j 0 are the current and initial values of the parameters, respectively. When we add this additional potential (penalty function) in order to restrict the N–Cd force constant inside the ˚ (D = 1.0), the change interval 2.564 ⫾ 0.641 mdyne/A of the 2Cl-pyridine force field became necessary, to represent shifts with an accuracy greater than 6 cm −1 for all wave numbers. The changes of the force field involve all ring stretching, ring angles, and C–H bending force constants. In order to obtain a good accuracy for the wave number shifts, we had to add to our force field model, the interaction between C– C–C and C–C–H valence angles. In the initial force field, the values of corresponding force constants were zero. The values of resulting shifts are shown in Table
Table 2 ˚ ) and normal modes (in internel coordinates) for Cd 2Cl-pyridine complex. Experimental values Wave numbers (cm −1), intensities (D/AMU*A and assignments are taken from Ref. [1]. Seven theoretical wave numbers with intensity values that do not correspond to the experimental ones are marked in bold Experimental — — 333 m — 629 s 731 vs 1006 s 1052 s 1088 s 1132 vs 1153 s 1294 s 1373 vw 1419 vs 1461 s 1566 s 1593 s 3069 m — 3079 sh 3103 m — — 418 s 481 m 692 m 759 vs — 920 w 961 m
Refined force field
X-sens X-sens d-ring X-sens n-ring d-CH d-CH X-sens d-CH d-CH n-C-C n-C-C n-C-C n-C-C n-C-C n-C-H n-C-H n-C-H n-C-H g X-sens g-ring g-ring g-ring g-C-H g-C-H g-C-H
64(0.14) 191(0.25) 333(0.70) 429(6.88) 626(7.78) 728(0.28) 997(13.97) 1039(0.38) 1082(13.83) 1127(13.24) 1166(13.77) 1280(1.13) 1361(1.13) 1420(0.16) 1466(15.84) 1575(10.92) 1603(8.61) 3002(2.27) 13070 (3.28) 3070(7.10) 3074(2.71) 57(0.00) 193(0.34) 411(4.74) 482(0.75) 675(2.29) 780(13.73) 882(0.26) 923(0.01) 969(1.93)
d X-sens (Cd) X-sens (Cd) d X-sens (Cl + Cd) X-sens (Cl) + d-ring d-ring d-ring + X-sens (Cl) ring breadth d-ring + d-CH d-CH + n-ring d-CH X-sens (Cl) d-CH + n-ring d_CH + n_ring d-CH + n-ring d-CH + n-ring d-CH + n-ring n-ring n-ring n-C-H n-C-H n-C-H n-C-H g X-sens(Cd) g X-sens(Cl) g-ring g-ring g-C-H g-C-H + g-ring C-H C-H C-H
24
M. Bakiler et al. / Journal of Molecular Structure 476 (1999) 21–26
1. Attempts to achieve a greater accuracy may lead to inadequate results, because unharmonical effects may change wave numbers calculated in harmnonical approximation by 5–10 cm −1. Moreover, interactions with Ni(CN) 4, in the Cd(2Clpy) 2Ni(CN) 4 complex, that experimental wave numbers of coordinated 2Cl-pyridine were taken [4], were not included in our model. These interactions could be important for calculations with high accuracy. From the former discussion, it seems reasonable to restrict ourselves to a study on the qualitative and semi-quantitative level. Approximate normal coordinate analysis for the Cd complex of 2Clpyridine is given in Table 2. The details of the method used in the calculation of vibrational intensities are given in Ref. [1]. The charge of the Cd atom should not be changed during the vibrations, since any significant charge transfer from N to Cd is hardly possible. In this situation, the N–Cd bond dipole moment and all its derivatives, with respect to the vibrational coordinates, is naturally set to zero. The influence of Cd on the electron
distribution is also unknown. In the model of the unaltered 2Cl-pyridine force field, the variation of the electro-optical parameters should be small, and IR intensities calculated for complex with 2Cl-pyridine parameters should then represent experimental spectral intensities. For the second model, analysis of the IR intensities can also be performed on the basis of electro-optical parameters of 2Cl-pyridine. We did not perform the refinement of the parameters, since the tendency of the changes in parameter values cannot be estimated by quantum chemical methods for the case of transition metal complexes, and because the experience of analogous calculations is quite poor. The variation of the electrooptical parameters in order to fit the experimental data will be rather speculative in this situation. Therefore, for the interpretation of IR intensities of the complex we have used electro-optical parameters of free 2Clpyridine, without any variations. The details of the calculation of electro-optical parameters of the 2Clpyridine are given in our previous study [1].
3. Discussion
Fig. 1. Experimental (top) spectra of Cd (2Cl-pyridine) 2Ni(CN) 4 complex. IR lines corresponding to Ni(CN) 4 group vibrations, are marked by dots. Three bottom curves correspond to the theoretical spectra of the Cd–2Cl-pyridine complex calculated for various force field models. Spectrum 1 corresponds to the spectra with initial values of Cd ring force constants (values are given in the text). Spectrum 2 represents the spectra after refinement of Cd–N interactions. Spectrum 3 shows the curve resulting from variation of the whole 2Cl-pyridine force field.
In our force field calculations, we have used two alternative models of the force field. Both models lead to satisfactory agreement between calculated and experimental upward shifts of the frequencies. Now we are interested in the possible variation of the electron density on the 2Cl-pyridine, caused by the presence of a metal atom in the coordination compound. It is difficult to extract the necessary information by analysis of the wave numbers. In this situation, calculated IR intensities can assist in the solution of our problem. As is seen from Fig. 1, the intensities for the experimental lines 626 and 1466 cm −1 (617 and 1453 cm −1, respectively, in the IR spectrum of free 2Cl-pyridine) have wrong values in the calculated spectrum of Cd–2Cl-pyridine, for the first force field model (spectra 1 and 2 in Fig. 1). However, corresponding values improve after force field refinement of 2Cl-pyridine (spectrum 3 in Fig. 1). This is a sign that ring force field and normal modes should be altered in the presence of the Cd atom with respect to the free 2Cl-pyridine molecule. There are qualitative physical reasons for such a force field alternation. In the 2Cl-pyridine molecule, all nitrogen valence elec-
M. Bakiler et al. / Journal of Molecular Structure 476 (1999) 21–26
25
Fig. 2. IR intensities interpretation for the Cd 2Cl-pyridine molecule. Curve 1 corresponds to the total spectrum, on curve 2 the Cl contribution is removed, on curve 3 hydrogen contributions are removed, on curve 4 Cl and hydrogen contributions are removed, and on curve 5, C—N contributions are removed.
26
M. Bakiler et al. / Journal of Molecular Structure 476 (1999) 21–26
trons are involved in the ring bonding. It is clearly seen from the quantum calculations of the force fields of pyridine and 2Cl-pyridine molecules. For each of the molecules, ring stretching force constants correspond to the conjugated ring force field. In the metal complexes, the distribution of the nitrogen valence electrons should be changed, and this is reflected in the whole ring force field. For each of the force field models there is qualitative disagreement of IR intensities for seven calculated normal vibrations (see Table 2). For five of these, calculated in plane normal vibrations which correspond to experimental lines at 333, 73l, 1052, 1294 and 1419 cm −1, the theoretical intensities are too small, and for the line 1373 cm −1 theoretical calculation gives a higher intensity than that detected on experimental spectra. Vibration at 333 cm −1 involves some Cd stretching and bending movements, and for the other lines normal vibrations contain some part of n-ring and d-ring vibrations. In the vibration at 1373 cm −1, (H)C–N stretching is involved. Vibrations at 1294 and 1419 cm −1 are related partially to (Cl)C– N stretching vibrations. The line 481 cm −1 corresponds to the g-ring vibration and also involves the C–N bond. The poor accuracy of our theoretical IR intensities for the former vibrations, means that at least the C–N bond electro-optical parameters should be corrected. Thus, from the analysis of the IR intensities it can be concluded that electron distribution and possibly force field of the coordinated 2Cl-pyridine, are altered with respect to the free molecule. At least there should be alteration of the electron distribution in the C–N bond region. As is seen from Fig. 2, the C–N bond electro-optical parameters contribute significantly in the IR intensities at the whole spectral interval. In Refs. [4–6], the definite and regular dependence of the shifts in frequencies on the metal was indicated. The shifts were interpreted in Refs. [5,6], in tenns of the M–N force constant value only, and the force field of the free ligand molecule was kept unchanged in coordination. We used our first force field model to calculate frequency shifts for the Ni complex of 2Cl-
pyridine. Note that if all force constants of the Cd complex of 2Cl-pyridine is being kept unchanged, and that only the atomic mass of Cd is substituted by the mass of Ni, we should observe tipward shifts in frequencies. This situation is adequate to isotopic substitution. Since the mass of the Ni atom is almost half that of the Cd atom, the shift of the line 626 cm −1 (at the IR spectrum of the Cd–2Cl-pyridine complex), which includes the M–N stretching vibration, should be quite large. The values of the shifts for Ni calculated in the frames of this model, are given in Table 1. If the value of the Ni–N stretching force constant is taken as larger than that for the Cd–N(2Cl-Pyridine) case (the larger the value of the force constant follows from the chemical properties of the Ni atom), we should obtain a larger shift in this frequency for the Ni complex, with respect to the Cd complex. However, the experimental difference between the two corresponding lines is only 2 cm −1. We again have a sign that the ring force field should be altered for the correct representation of the frequency shifts.
Acknowledgements We thank the Turkish Scientific and Technical ¨ BI˙TAK) for their financial supResearch Council (TU ¨ BI˙TAK and NATO fellowship proport through TU grammes, which make the current study possible. S.A. wolud also like to thank the Research Fund of The ¨ -293). University of Istanbul (project number O
References [1] M. Bakiler, I.V. Maslov, S. Akyiiz, Theoretical study.of 2-Cl pyridine metal complexes vibrational spectra. I. Calculation and analysis of 2-Cl pyridine IR spectra, J. Mol. Struct. (submitted). [2] L.A. Gribov, S.V. Kotov, J. Mol. Struct. 198 (1989) 93. [3] L.A. Gribov, K.A. Zinovyev, J. Mol. Struct. 268 (1992) 223. [4] S. Akyilz, M. Bakiler, J.E.D. Davies, J. Coord. Chem. 37 (1996) 1. [5] S. Suzuki, W.J. Orville-Thomas, J. Mol. Struct. 37 (1977) 321. [6] A. Topac¸li, S. Akyu¨z, Spectrochim. Acta 51 (A) (1995) 663.
Theoretical study of the vibrational spectra of 2-chloropyridine metal complexes II. Calculation and analysis of the IR spectra of Cd– and Ni– 2-Chloropyridine complexes M. Bakiler a,*, I.V. Maslov b, Sevim Akyu¨z c a
Department of Physics, Mimar Sinan University, Bes¸iktas¸ 80690, Istanbul, Turkey V.I. Vemadsky Institute of Geo- and Analytical Chemistry, RAS, Kosygin str. 19, 117972 Moscow, Russia c Department of Physics, Faculty of Science, Istanbul University, Vezneciler 34459, Istanbul, Turkey
b
Received 19 January 1998; revised 15 May 1998; accepted 15 May 1998
Abstract The vibrational IR spectra of a Cd complex of 2-chloropyridine, was calculated on the basis of a parameter set determined in our previous study on the 2-chloropyridine molecule. The Cd–N bond strength was determined by the variation of the force ˚ ). Calculated IR intensities indicate the presence of field, and the corresponding force constant is found to be (1.064 mdyne/A some changes in electron distribution of the 2Cl-pyridine molecule in a complex formation, with respect to the free molecule. The distortion of the electro-optical parameters occurs around the N atom. The interpretation of the normal vibrations and IR intensities of the Cd–2Cl-pyridine complex is given. Comparison with the corresponding shifts for the case of the Ni complex of 2Cl-pyridine, ensures that the force field of the free 2Cl-pyridine molecule should be altered in a complex formation, in order to represent experimental data. 䉷 1999 Elsevier Science B.V. All rights reserved. Keywords: 2-Chloropyridine: IR spectrum; IR intensity; Force field refinement; Transition metal complexes
1. Introduction Investigation of the metal–organic complexes plays a significant role in moderm chemistry. Metal complexes are used in various chemical technologies, such as catalysis and photochemical processes. Vibrational spectroscopy is a powerful tool for studying the structure and physical properties of the complexes. Along with the vibrational frequencies, the IR intensities contain rich inforfnation about molecular electronic properties. Electron charge dis* Corresponding author. Fax: +90-2126370216; E-mail: [email protected]
tribution or atomic charges can be obtained from the vibrational spectra. The metal ligand charge transfer effect can then be studied thoroughly by the analysis of vibrational spectra. The electrostatic potential is determined directly from the atomic charges obtained with the help of IR spectra. As was pointed out in our previous paper [1], experimental and theoretical studies of IR intensities are rare. The theory for calculation of IR intensities was developed [2] and approved in a number of calculations [3]. In the course of the investigation of the vibrational spectra of 2-halopyridine tetracyanonickelate complexes [4], it has been found that several modes of coordinated halopyridines (2Cl-py and 2Br-py) have upward shifts in frequency,
0022-2860/99/$ - see front matter 䉷 1999 Elsevier Science B.V. All rights reserved. PII: S 00 22 - 28 6 0( 9 8) 0 04 9 2- X
22
M. Bakiler et al. / Journal of Molecular Structure 476 (1999) 21–26
compared with those of the free molecule, and the shifts are metal dependent. These are thought to occur as a result of kinematic interaction of the vibrational modes of the ligand molecule by metal–ligand bond vibrations [5]. To check this idea and investigate thoroughly metal–ligand coupling, the vibrational IR spectra of the Cd 2Cl-pyridine complex was calculated and reported here. In our previous study, we have determined the sets of parameters (force constants and electro-optical parameters) for the free 2Cl-pyridine molecule [1]. This key step allows us now to calculate metal complexes of 2Cl-pyridine.
2. Methods and calculation The geometry optimization by quantum chemical methods is not available for molecules containing the Cd atom. We chose planar geometry for the complex ˚ . Determined and the N–Cd distance was set to 2.0 A sets of parameters for the 2Cl-pyridine molecule [1] were transferred without any changes to the Cd–2Clpyridine complex. The further parameter refinement is discussed below. The details of the Cd–ligand force field are not well known, and we started with more or less standard values for metal–nitrogen interactions. The initial value of the N–Cd bond stretching force ˚ , the C–N–Cd constant was taken as 2.564 mdyne/A ˚ , and valence angle force constant as 2.665 mdyne A
the out-of-plane, ring-Cd bending force constant was ˚ . The calculated IR spectra taken as 0.076 mdyne/A with unchanged 2Cl-pyridine force field, qualitatively represent correct experimental shifts of wave numbers with respect to the 2Cl-pyridine spectra (see Table 1). The variation of the force field was then performed in order to represent experimental shifts of wave numbers with better accuracy. We did not directly fit the experimental spectrum of the Cd–2Cl-pyridine complex, but added experimental shifts (coordinated 2Cl-pyridine wave numbers — free 2Cl-pyridine wavenumbers) to the theoretical values for 2Clpyridine [1]. At the first step, we tried to represent the shifts by variation of the force constants related to the Cd atom: N–Cd stretching, C–N–Cd and C–N–C valence angle bendings, and interactions of these force constants with each other. Unrestricted variation of these force constants leads to a very small value of the N–Cd bond stretching force constant equal to ˚ , a decrease of the C–N–Cd force 1.064 mdyne/A ˚ ), and a sigconstant (from 2.665 to 1.142 mdyne A ˚ nificant decrease by 0.135 mdyne A of the C–N–C ˚ ). In this force constant (initial value 0.902 mdyne A case, we were able represent experimental shifts of wave numbers with good accuracy (see Table 1). The calculated value of the N–Cd stretching force constant corresponds to those obtained for the Cd–pyr˚) idine and Cd–bipy–Cd complexes (0.94 mdyne/A
Table 1 Experimental and theoretical wavenumber shifts, (cm −1) of coordinated 2-chlropyridine with respect to free molecule 2-Clpy n experimental 312 426 617 724 991 1045 1083 1117 1149 1286 1365 1420 1453 1568 1577 a
(n Cd–2Cl-py − n 2Cl-py) Experimental 21 —a 12 7 15 7 5 15 4 8 8 −1 8 −2 16
Theor.model 1 21 0 12 5 5 5 7 6 5 1 0 −1 14 2 9
Theor.model 2 21 8 11 7 5 5 1 7 8 8 4 2 13 1 12
Could not be observed because of overlapping with Ni(CN) 4 group band.
(n Ni–2Cl-py − n 2Cl-py) Experimental 22 17 14 io 17 12 8 19 8 12 30 2 io −7 19
Theoretical 25 0 18 6 11 5 7 6 5 1 0 0 14 3 9
23
M. Bakiler et al. / Journal of Molecular Structure 476 (1999) 21–26
[5,6]. We have also made a determination of the altemative force field model. The method of the force field refinement implemented in program LEV allows the use of additional restrictions during variational procedure. The method is based on the least-square minimization of the functional F with a penalty function, with respect to parameters x (force constants): exp 2 F = ∑ [ncalc i (x) − ni ] + P,
(1)
i
where n i is the theoretical and experimental values of wave numbers, and P is a penalty function defined by: " # " #10 xj − x0j xj − x0j +b ∑ , (2) P=a ∑ D D j j
where a and b are coefficients, D is the desired variation interval, and x j and x j 0 are the current and initial values of the parameters, respectively. When we add this additional potential (penalty function) in order to restrict the N–Cd force constant inside the ˚ (D = 1.0), the change interval 2.564 ⫾ 0.641 mdyne/A of the 2Cl-pyridine force field became necessary, to represent shifts with an accuracy greater than 6 cm −1 for all wave numbers. The changes of the force field involve all ring stretching, ring angles, and C–H bending force constants. In order to obtain a good accuracy for the wave number shifts, we had to add to our force field model, the interaction between C– C–C and C–C–H valence angles. In the initial force field, the values of corresponding force constants were zero. The values of resulting shifts are shown in Table
Table 2 ˚ ) and normal modes (in internel coordinates) for Cd 2Cl-pyridine complex. Experimental values Wave numbers (cm −1), intensities (D/AMU*A and assignments are taken from Ref. [1]. Seven theoretical wave numbers with intensity values that do not correspond to the experimental ones are marked in bold Experimental — — 333 m — 629 s 731 vs 1006 s 1052 s 1088 s 1132 vs 1153 s 1294 s 1373 vw 1419 vs 1461 s 1566 s 1593 s 3069 m — 3079 sh 3103 m — — 418 s 481 m 692 m 759 vs — 920 w 961 m
Refined force field
X-sens X-sens d-ring X-sens n-ring d-CH d-CH X-sens d-CH d-CH n-C-C n-C-C n-C-C n-C-C n-C-C n-C-H n-C-H n-C-H n-C-H g X-sens g-ring g-ring g-ring g-C-H g-C-H g-C-H
64(0.14) 191(0.25) 333(0.70) 429(6.88) 626(7.78) 728(0.28) 997(13.97) 1039(0.38) 1082(13.83) 1127(13.24) 1166(13.77) 1280(1.13) 1361(1.13) 1420(0.16) 1466(15.84) 1575(10.92) 1603(8.61) 3002(2.27) 13070 (3.28) 3070(7.10) 3074(2.71) 57(0.00) 193(0.34) 411(4.74) 482(0.75) 675(2.29) 780(13.73) 882(0.26) 923(0.01) 969(1.93)
d X-sens (Cd) X-sens (Cd) d X-sens (Cl + Cd) X-sens (Cl) + d-ring d-ring d-ring + X-sens (Cl) ring breadth d-ring + d-CH d-CH + n-ring d-CH X-sens (Cl) d-CH + n-ring d_CH + n_ring d-CH + n-ring d-CH + n-ring d-CH + n-ring n-ring n-ring n-C-H n-C-H n-C-H n-C-H g X-sens(Cd) g X-sens(Cl) g-ring g-ring g-C-H g-C-H + g-ring C-H C-H C-H
24
M. Bakiler et al. / Journal of Molecular Structure 476 (1999) 21–26
1. Attempts to achieve a greater accuracy may lead to inadequate results, because unharmonical effects may change wave numbers calculated in harmnonical approximation by 5–10 cm −1. Moreover, interactions with Ni(CN) 4, in the Cd(2Clpy) 2Ni(CN) 4 complex, that experimental wave numbers of coordinated 2Cl-pyridine were taken [4], were not included in our model. These interactions could be important for calculations with high accuracy. From the former discussion, it seems reasonable to restrict ourselves to a study on the qualitative and semi-quantitative level. Approximate normal coordinate analysis for the Cd complex of 2Clpyridine is given in Table 2. The details of the method used in the calculation of vibrational intensities are given in Ref. [1]. The charge of the Cd atom should not be changed during the vibrations, since any significant charge transfer from N to Cd is hardly possible. In this situation, the N–Cd bond dipole moment and all its derivatives, with respect to the vibrational coordinates, is naturally set to zero. The influence of Cd on the electron
distribution is also unknown. In the model of the unaltered 2Cl-pyridine force field, the variation of the electro-optical parameters should be small, and IR intensities calculated for complex with 2Cl-pyridine parameters should then represent experimental spectral intensities. For the second model, analysis of the IR intensities can also be performed on the basis of electro-optical parameters of 2Cl-pyridine. We did not perform the refinement of the parameters, since the tendency of the changes in parameter values cannot be estimated by quantum chemical methods for the case of transition metal complexes, and because the experience of analogous calculations is quite poor. The variation of the electrooptical parameters in order to fit the experimental data will be rather speculative in this situation. Therefore, for the interpretation of IR intensities of the complex we have used electro-optical parameters of free 2Clpyridine, without any variations. The details of the calculation of electro-optical parameters of the 2Clpyridine are given in our previous study [1].
3. Discussion
Fig. 1. Experimental (top) spectra of Cd (2Cl-pyridine) 2Ni(CN) 4 complex. IR lines corresponding to Ni(CN) 4 group vibrations, are marked by dots. Three bottom curves correspond to the theoretical spectra of the Cd–2Cl-pyridine complex calculated for various force field models. Spectrum 1 corresponds to the spectra with initial values of Cd ring force constants (values are given in the text). Spectrum 2 represents the spectra after refinement of Cd–N interactions. Spectrum 3 shows the curve resulting from variation of the whole 2Cl-pyridine force field.
In our force field calculations, we have used two alternative models of the force field. Both models lead to satisfactory agreement between calculated and experimental upward shifts of the frequencies. Now we are interested in the possible variation of the electron density on the 2Cl-pyridine, caused by the presence of a metal atom in the coordination compound. It is difficult to extract the necessary information by analysis of the wave numbers. In this situation, calculated IR intensities can assist in the solution of our problem. As is seen from Fig. 1, the intensities for the experimental lines 626 and 1466 cm −1 (617 and 1453 cm −1, respectively, in the IR spectrum of free 2Cl-pyridine) have wrong values in the calculated spectrum of Cd–2Cl-pyridine, for the first force field model (spectra 1 and 2 in Fig. 1). However, corresponding values improve after force field refinement of 2Cl-pyridine (spectrum 3 in Fig. 1). This is a sign that ring force field and normal modes should be altered in the presence of the Cd atom with respect to the free 2Cl-pyridine molecule. There are qualitative physical reasons for such a force field alternation. In the 2Cl-pyridine molecule, all nitrogen valence elec-
M. Bakiler et al. / Journal of Molecular Structure 476 (1999) 21–26
25
Fig. 2. IR intensities interpretation for the Cd 2Cl-pyridine molecule. Curve 1 corresponds to the total spectrum, on curve 2 the Cl contribution is removed, on curve 3 hydrogen contributions are removed, on curve 4 Cl and hydrogen contributions are removed, and on curve 5, C—N contributions are removed.
26
M. Bakiler et al. / Journal of Molecular Structure 476 (1999) 21–26
trons are involved in the ring bonding. It is clearly seen from the quantum calculations of the force fields of pyridine and 2Cl-pyridine molecules. For each of the molecules, ring stretching force constants correspond to the conjugated ring force field. In the metal complexes, the distribution of the nitrogen valence electrons should be changed, and this is reflected in the whole ring force field. For each of the force field models there is qualitative disagreement of IR intensities for seven calculated normal vibrations (see Table 2). For five of these, calculated in plane normal vibrations which correspond to experimental lines at 333, 73l, 1052, 1294 and 1419 cm −1, the theoretical intensities are too small, and for the line 1373 cm −1 theoretical calculation gives a higher intensity than that detected on experimental spectra. Vibration at 333 cm −1 involves some Cd stretching and bending movements, and for the other lines normal vibrations contain some part of n-ring and d-ring vibrations. In the vibration at 1373 cm −1, (H)C–N stretching is involved. Vibrations at 1294 and 1419 cm −1 are related partially to (Cl)C– N stretching vibrations. The line 481 cm −1 corresponds to the g-ring vibration and also involves the C–N bond. The poor accuracy of our theoretical IR intensities for the former vibrations, means that at least the C–N bond electro-optical parameters should be corrected. Thus, from the analysis of the IR intensities it can be concluded that electron distribution and possibly force field of the coordinated 2Cl-pyridine, are altered with respect to the free molecule. At least there should be alteration of the electron distribution in the C–N bond region. As is seen from Fig. 2, the C–N bond electro-optical parameters contribute significantly in the IR intensities at the whole spectral interval. In Refs. [4–6], the definite and regular dependence of the shifts in frequencies on the metal was indicated. The shifts were interpreted in Refs. [5,6], in tenns of the M–N force constant value only, and the force field of the free ligand molecule was kept unchanged in coordination. We used our first force field model to calculate frequency shifts for the Ni complex of 2Cl-
pyridine. Note that if all force constants of the Cd complex of 2Cl-pyridine is being kept unchanged, and that only the atomic mass of Cd is substituted by the mass of Ni, we should observe tipward shifts in frequencies. This situation is adequate to isotopic substitution. Since the mass of the Ni atom is almost half that of the Cd atom, the shift of the line 626 cm −1 (at the IR spectrum of the Cd–2Cl-pyridine complex), which includes the M–N stretching vibration, should be quite large. The values of the shifts for Ni calculated in the frames of this model, are given in Table 1. If the value of the Ni–N stretching force constant is taken as larger than that for the Cd–N(2Cl-Pyridine) case (the larger the value of the force constant follows from the chemical properties of the Ni atom), we should obtain a larger shift in this frequency for the Ni complex, with respect to the Cd complex. However, the experimental difference between the two corresponding lines is only 2 cm −1. We again have a sign that the ring force field should be altered for the correct representation of the frequency shifts.
Acknowledgements We thank the Turkish Scientific and Technical ¨ BI˙TAK) for their financial supResearch Council (TU ¨ BI˙TAK and NATO fellowship proport through TU grammes, which make the current study possible. S.A. wolud also like to thank the Research Fund of The ¨ -293). University of Istanbul (project number O
References [1] M. Bakiler, I.V. Maslov, S. Akyiiz, Theoretical study.of 2-Cl pyridine metal complexes vibrational spectra. I. Calculation and analysis of 2-Cl pyridine IR spectra, J. Mol. Struct. (submitted). [2] L.A. Gribov, S.V. Kotov, J. Mol. Struct. 198 (1989) 93. [3] L.A. Gribov, K.A. Zinovyev, J. Mol. Struct. 268 (1992) 223. [4] S. Akyilz, M. Bakiler, J.E.D. Davies, J. Coord. Chem. 37 (1996) 1. [5] S. Suzuki, W.J. Orville-Thomas, J. Mol. Struct. 37 (1977) 321. [6] A. Topac¸li, S. Akyu¨z, Spectrochim. Acta 51 (A) (1995) 663.