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Halomethylpyrroles as candidate monomers for conducting polymers: a theoretical study
Chemical Physics 306 (2004) 105–113 www.elsevier.com/locate/chemphys
Halomethylpyrroles as candidate monomers for conducting polymers: a theoretical study Hassan Sabzyan
a,*
, Hossein Nikoofard
b
a b
Department of Chemistry, University of Isfahan, Isfahan 81764-73441, Iran Department of Chemistry, Shahrood University of Technology, Shahrood, Iran Received 27 March 2004; accepted 22 July 2004 Available online 20 August 2004
Abstract Structural, electronic, thermochemical and electrical properties of mono-, di- and trihalomethylpyrroles (HMPys), NC4H4– CHnX3 n; X = F, Cl, Br; n = 0, 1, 2, 3, and their radical cations have been studied using DFT-B3LYP method with 6-31G(d,p) basis set. Vibrational frequencies and NMR shielding constants of these compounds have also been calculated and analyzed. HMPys are proposed in this research as candidate monomers for conducting polymers with modified characteristics compared to polypyrrole and polymethylpyrrole. Stability of HMPy radical cations have been studied in detail and compared with available experimental data, including oxidation potentials. Results of the present study show that bromomethylpyrroles have the highest thermochemical stability and have higher characteristics for electropolymerization compared to fluoro- and chloromethylpyrroles. Stability of HMPys increases with increasing number of substituted halogen atoms. Ó 2004 Elsevier B.V. All rights reserved. Keywords: Halomethylpyrroles; Conducting polymers; DFT-B3LYP; Electric charge; Polarizability
1. Introduction Electrochemical synthesis of electrically conducting organic polymers, first described in detail for polypyrrole [1–3], has proven important roles in allowing development of new polymeric materials with electrical properties similar to metals. The reversibility of charging/discharging processes and high specific capacitance of the polymers in their oxidized states has stimulated suggestions for a variety of applications such as rechargeable batteries, super-capacitors and electrochromic devices, etc. [4–13]. The main objective of the design and preparation of new materials is to improve the desired electrical and electrochemical properties. It is of high industrial importance to increase the conduc*
Corresponding author. Tel.: +98 311 793 2749; fax: +98 311 668 9732. E-mail address: [email protected] (H. Sabzyan). 0301-0104/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.chemphys.2004.07.025
tivity of polymers and their solubility in certain solvents. Conducting polymers with modified solubility in some industrially important solvents were recently synthesized by introducing alkyl and other substituents on the monomers [14,15]. Since a conducting polymer has a p-conjugate system along the polymer chain, both electronic and structure of a substituent contribute to the delocalization of the polymer p-conjugate system [16– 18], while its steric factor (which is important for substituents like n-alkyls) is usually not determining [19,20]. Substitution of pyrrole monomers with appropriate 3-substituents induces a push–pull effect on the p-electrons and alters the electrical conductivity of the corresponding polymers compared with that of polypyrrole [2,21]. Halomethylpyrroles (HMPys) have been considered in this work as potential monomers for the synthesis of conductive polymers with modified physical and electrical characteristics compared to those of polypyrrole.
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13
12
X 11
X
X
9
5
H10
6
selected for the present study of HMPys. Therefore, B3LYP/6-31G(d,p) method was used throughout this work.
3. Results and discussion 3.1. Electronic properties 7
H
2
3
N1
H8
H 4
Fig. 1. Structure and numbering scheme used for halomethylpyrroles, HMPys, in this work.
The halomethyl substituents can effectively change packing parameter and density of the polymer blend as well as its pyrolysis temperature and its environmental recycle period. Obviously, a halomethyl substitution on the pyrrole ring affects electrical conductivity of the corresponding polymers. In the present work we have carried out a density functional theory (DFT) study on HMPys and their radical cations using the most popular BeckeÕs threeparameter hybrid functional, B3 [22], with non-local correlation of Lee–Yang–Parr, LYP [23], abbreviated as B3LYP, method. Calculations on the highest members of this series of compounds, iodomethylpyrroles, could only be carried out with smaller basis sets such as 3-21G**. Therefore, results of the calculations for iodomethylpyrroles could not be compared directly to those of other HMPys obtained with the higher quality basis set 6-31G(d,p). In this study, pyrrole (Py) and methylpyrrole (MPy) were used as reference compounds for all comparative studies. Structure and numbering scheme used for HMPys in this study are introduced in Fig. 1. All of the calculations in this work were carried out using Gaussian 94W program [24].
2. Computational procedures The preliminary studies showed that 6-31G(d,p) [25,26] is the best basis set considering computational times and our available hardware facilities. Furthermore, in the preliminary studies, MP2 calculations could not be carried out for some representative HMPys, mainly due to the software and hardware limitations, and thus, a comparative study was not applicable based on MP2 calculations. Furthermore, HF method failed to predict reasonable values for the thermochemical stabilities of the representative HMPys. Based on these preliminary studies, DFT-B3LYP level of theory was
Energy gap between the highest occupied and the lowest unoccupied molecular orbitals (HOMO and LUMO, respectively) known as the HOMO–LUMO gap or simply HLG, is a critical parameter determining molecular electrical transport properties and electric admittance (being charged under applied electric filed) because it is a measure of the electron density hardness [27]. A reasonable estimate of HLG values can be obtained from both HF and DFT calculations [28,29]. The HLG values determine also the voltage produced in a photovoltaic cell as well as the electromotive force (EMF) of an electrochemical cell when the polymer is used as its negative or as its positive electrode. Increase in the chain length of a conductive polymer (or the length of the p-conjugated system) decreases the HLG values of the polymer. Therefore, it seems that HLG values of a polymer cannot be determined exactly from those of their monomers. However, the study of HLG values of monomers may be used to predict the comparative band gap behavior of the corresponding polymers. The HLG values of HMPys defined in Fig. 2, are calculated and listed in Table 1. It can be seen from this table that the HLG values for the chloro- and bromomethylpyrroles, decrease when the number of halogen atoms increases. In addition, among each NC4H4– CH3 nXn series with fixed n, the HMPy with X = Br has the lowest HLG values. Therefore, we can predict that bromomethylpyrrole polymers may have the highest electrical conductivity compared with other HMPy polymers. It should be noted here that HLG values are not the only parameters that determine electric conductance of a polymer film. Orientation and alignment of the monomers in the polymer chain are two other determining
LUMO L4
L1 L2
HOMO L5
L3
L6
Fig. 2. The energy gaps, Li, including HOMO–LUMO gaps (HLG), L1, studied in this research for HMPys.
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107
Table 1 The B3LYP/6-31G(d,p) calculated values of energy gaps Li (defined in Fig. 2) including HOMO–LUMO gap (HLG) L1, and ionization potentials (IP), both in eV, for HMPys and their reference molecules, pyrrole and methylpyrrole HMPy
L1
L2
L3
L4
L5
L6
IP
Pyrrole PyCH3 PyCH2F PyCHF2 PyCF3 PyCH2Cl PyCHCl2 PyCCl3 PyCH2Br PyCHBr2 PyCBr3
6.85 6.82 4.84 6.84 6.86 6.07 5.81 5.38 5.64 5.19 4.46
7.69 7.51 5.60 7.64 7.50 6.80 6.61 6.01 6.25 6.00 5.30
11.26 10.67 7.54 9.70 10.61 7.74 7.67 7.04 6.67 6.33 5.47
7.69 7.65 7.54 7.63 7.66 7.03 6.18 5.93 6.95 5.54 5.23
8.52 8.34 8.30 8.43 8.30 7.76 6.98 6.56 7.57 6.35 6.07
12.10 11.50 10.25 10.49 11.41 8.70 8.04 8.44 7.98 6.68 6.24
7.81 7.53 7.82 8.11 8.35 7.88 8.03 8.18 7.75 7.82 7.88
characteristics that play important roles in the electrical conductance of a polymer. Large substituents on the pyrrole ring may prevent the polymer chain to adopt a planar structure and hence increase the resistance against alignment and packing of the polymer layers. This causes a decrease in the density of the polymer and consequently, reduces the electrical conductance of the polymer [21,30,31]. The large substituents, on the other hand, increase inter-chain distance and thus decrease their electrostatic repulsive interactions during the electric charge transfer process in the bulk of the polymer. 3.2. Ionization potentials It is known that the first step in the electopolymerization of conductive polymers is the formation of intermediate radical cations from the monomers, which can be considered as ionization reaction [1,2] M ! M þ þ e ð1Þ in which M and M+ denote monomer and its radical cation, respectively. Hence, stability of intermediate radical cations has important role in the electropolymerization process. Therefore, it is useful to calculate electronic energy deference between the neutral monomer (as the initial species) and the positively charged monomer (as the intermediate). This energy difference is proportional to the ionization potential (IP) of the monomer. The geometries of the HMPy cations were optimized at the same level of theory with the same basis set as used for neutral HMPys prior to the calculation of other properties. The calculated values of IP for HMPys based on reaction (1) are presented in Table 1. According to the results listed in this table, methylpyrrole monomer has the lowest IP, which may be due to the electron donor character of the substituent –CH3 on the pyrrole ring, and so its radical cation in the gas phase has the highest
stability among all HMPY radical cations. This is in agreement with the lower oxidation potential measured experimentally for methylpyrrole compared to pyrrole monomer [32]. In this series of HMPys, the PyCF3 monomer has the highest IP corresponding to the highest electronegativity of its halogen substituents. Results reported in Table 1 also show that the IP of these compounds increases when the electron-pull effect of the substituent (CX3) increases, that is, the oxidation potential of NC4H4–CHnX3 n HMPY series increases in the order of X = Br < Cl < F. In general, it can be concluded that electropolymerization of HMPys becomes more difficult as the number and electronegativity of the substituted halogen atoms increase. 3.3. Charge and spin density distributions In the growth step of the polymerization of pyrrole, radical cations link to the polymer chain from the ring position with the most positive charge (a and a 0 positions, or C2 and C3 positions in Fig. 1). Also, all of HMPys can undergo polymerization via their a and a 0 carbons due to their positive charges [18–20]. Using Mulliken population analysis, the net atomic electric charges and spin density distribution in the HMPy cations are calculated and summarized in Table 2, respectively. As mentioned above, geometries of the cations were optimized at the same level of theory with the same basis set prior to the calculation of charges. According to the results presented in Table 2, a and a 0 carbons (C2 and C3 positions in Fig. 1) in pyrrole monomer have equal charges and spin densities, but with attachment of a halomethyl substituent to the pyrrole ring, this equality is altered. The difference between a and a 0 positions depends on the type and number of halogen atoms. For most cations, the positive charge on the a-carbon (C2 position in Fig. 1), which is close to the substituent position (C5), is higher than that on the a 0 -carbon (C3). Conversely, spin density is
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Table 2 Electric charge (spin) distributions calculated for the B3LYP/6-31G(d,p) optimized structures of HMPy radical cations (X denotes the halogen atoms) Cation
N1
C2(a)
C3(a 0 )
C5
C6
X
Pyrrole+ PyCHþ 3 PyCH2F+ PyCHFþ 2 PyCFþ 3 PyCH2Cl+ PyCHClþ 2 PyCClþ 3 PyCH2Br+ PyCHBrþ 2 PyCBrþ 3
0.46(0.12) 0.47(0.08) 0.47(0.10) 0.47(0.12) 0.47(0.12) 0.47(0.10) 0.47(0.10) 0.48(0.10) 0.47(0.08) 0.47(0.08) 0.47(0.08)
0.20(0.54) 0.20(0.52) 0.20(0.53) 0.21(0.53) 0.22(0.53) 0.21(0.50) 0.23(0.48) 0.23(0.48) 0.20(0.44) 0.21(0.43) 0.22(0.43)
0.20(0.54) 0.18(0.48) 0.18(0.50) 0.21(0.54) 0.21(0.53) 0.18(0.49) 0.18(0.49) 0.19(0.49) 0.17(0.44) 0.17(0.43) 0.17(0.44)
0.76(0.05) 0.08(0.21) 0.01(0.15) 0.04(0.07) 0.08(0.00) 0.09(0.17) 0.11(0.18) 0.13(0.17) 0.08(0.21) 0.08(0.21) 0.11(0.20)
0.76(0.05) 0.10(0.11) 0.09(0.08) 0.09(0.02) 0.09(0.00) 0.10(0.09) 0.10(0.09) 0.06(0.08) 0.10(0.11) 0.09(0.12) 0.09(0.11)
– – 0.27(0.01) 0.26(0.00) 0.23(0.00) 0.04(0.07) 0.09(0.05) 0.13(0.03) 0.03(0.15) 0.05(0.09) 0.09(0.61)
distributed mainly on the a 0 -carbon. This behavior could be expected, as the distributed charge is positive while the spin density is carried by electrons which have negative charge. Larger differences between the electric charges at a and a 0 positions (which are propagation centers in the electropolymerization process) will result in higher selectivity of the tacticity of the polymer chain. The syndiotactic structure of the HMPy polymers corresponds to the lower steric effects between halomethyl groups on the neighboring monomers, and thus higher thermal stability of the polymer. It can, therefore, be concluded that halomethyl substituents increase selectivity of the syndiotactic order of the polymer chain. A set of PM3 semi-empirical computations was carried out on the trifluoromethylpyrrole pentamer with different tacticities, shown in Fig. 3, to investigate the
tacticity issue. The results of this study showed that for this oligomer, the syndiotactic order is 12.2 kcal/ mol more stable than the atactic order. Furthermore, as can be seen from Table 2, symmetry of the halomethyl group with respect to the molecular plane has a noticeable effect on the charge and spin distributions. Because the charge density of the a-carbon for HMPy cations is higher than that for pyrrole cation, the initiation step of the polymerization is faster for these monomers, but the length of their polymer chains may be shorter due to the large substituent effects. The atomic charge distributions in the neutral HMPys, which are not reported here for the sake of brevity, show, more or less, the same behavior as is found for their cation radicals. Analysis of the NBO atomic charges calculated for the optimized geometries of the HMPy cations, not reported here for brevity, resulted in exactly the same trends observed for the Mulliken charges, although the absolute values of the atomic charges were different. From electric charge distribution point of view, there is no significant preference for the tacticity of the HMPy polymer chains as the distributed electric charge on the two a and a 0 carbon atoms are identical for a couple and are just slightly different for other HMPy cations. Therefore, electric charges cannot be accounted for any possible preferences (if any) of the tacticity of the HMPy polymer chains that might be observed experimentally. 3.4. Electric dipole moments and polarizabilities
Fig. 3. The PM3 optimized geometries of two different tacticities of the trifluoromethylpyrrole pentamer.
An important factor in the chemical and electrochemical synthesis of conducting polymers is the choice of proper solvent and support electrolyte [10]. Orientation of the electric dipole moment vector of the monomers with respect to the direction of the polymer chain determines electrochemical characteristics of the polymer on the electrode surface. In addition to their orientations, sizes of the electric dipole moment vector of the monomers and their interactions with the solvent and support
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electrolyte are the key rule in their selection for electropolymerization. Furthermore, structure of the electrical double layer at the surface of electrodes which determines the kinetics of the diffusion-controlled electrode reactions, depends on the dipole moment of the solute molecules. Relative conformations of the neighboring monomers that determine the growth rate of the polymer chain in the propagation step depend on both size and orientation of the monomer electric dipole moment vector. Thus, to study this feature of polymers, a detailed analysis of the electric dipole moment vector is needed. The calculated sizes of the electric dipole moment vectors for HMPys have been listed in Table 3. The data presented in this table show that both type and number of halogen atoms affect the size of the electric dipole moment vector. The comparative sizes of the dipole moment vectors follow the orders: (a) l (PyCH2X) > l (PyCHX2) > l (PyCX3); X = F, Cl, Br (b) l (PyCBr3) > l (PyCCl3) > l (PyCF3) The same order is found for the dipole moment vectors of the HMPY radical cations. Because of larger electric dipole moments, bromomethylpyrroles are expected to be more soluble in polar solvents. Diagonal components of the electric polarizability tensor, axx, ayy and azz, calculated for HMPys and their radical cations are presented in Table 3. This table shows that values of axx, ayy and azz depend on the type and number of halogen atoms according to the following trends: (a) aqq (Py–CH3 nBrn) > aqq (Py–CH3 nCln) > aqq (Py–CH3 nFn); q = x, y, z (b) aqq (PyCX3) > aqq (PyCHX2) > aqq (PyCH2X); X = F, Cl, Br Table 3 shows also that polarizabilities of these HMPys increase with the size and number of substituted
109
halogen atoms. A comparison between the calculated electric polarizabilities of the neutral and positively charged HMPys shows that, generally, axx and ayy are slightly smaller for the HMPy cations, while azz is slightly larger. 3.5. Structural analysis It is known that electrical conductivity of polymer films depends on planarity, orientation and alignment of their monomers in the polymer chain backbone. Thus, in this section, we have calculated the corresponding parameters and studied these aspects of the HMPys structures. Some geometrical parameters corresponding to the optimized structures of HMPys and their radical cations obtained at DFT-B3LYP/6-31G(d,p) level of theory are presented in Table 4. The geometrical parameters used in this study are defined in terms of the numbering scheme introduced in Fig. 1 as: Ri,j, the bond length between atoms i and j; Aijk, the bond angle formed by atoms i, j and k, and Dijkl, the dihedral angle formed by atoms i, j, k and l. It is already found that branching of polypyrroles in the polymer growth step, is initiated on C2 (a) and C3 (a 0 ) ring positions [16–18]. Obviously, a and a 0 carbon atoms are identical for pyrrole (i.e., R21 = R31 and R83 = R72), but they may be different for HMPys. As can be seen from Table 4, for all HMPys, R21 is generally smaller than R31, while their variations with the halomethyl group are not significant. Introducing the halomethyl group has almost no effect on the values of R83 and R72. Values of the dihedral angle defining torsion of the halomethyl groups –CH2X, CHX2 and –CX3 (where X denotes the halogen atoms) with respect to the molecular plane, referenced to the atom X for the –CH2X and –CX3, and to the atom H for –CHX2 are very close to 0°, with a maximum deviation of 2.88° for Py–CHCl2. This means that the referenced atom (X and H for the two sets of groups, respectively) lies in the ring plane, and the other two atoms of the halomethyl group lie
Table 3 Electric dipole moments (D) and diagonal elements of the electric polarizability tensor (bohr3) for the B3LYP/6-31G(d,p) optimized structures of neutral (radical cations) of pyrrole, methylpyrrole and HMPys HMPy
l
Pyrrole PyCH3 PyCH2F PyCHF2 PyCF3 PyCH2Cl PyCHCl2 PyCCl3 PyCH2Br PyCHBr2 PyCBr3
1.90 1.71 3.46 3.68 4.42 4.14 4.26 4.83 4.23 4.16 4.51
axx (1.79) (2.54) (6.30) (8.15) (9.77) (6.81) (8.61) (10.23) (7.96) (9.56) (10.82)
49.2 58.1 64.6 63.4 63.0 79.4 85.0 91.4 91.1 98.2 111.0
ayy (45.5) (56.0) (57.4) (56.6) (56.8) (67.2) (72.4) (79.4) (77.5) (88.9) (102.3)
50.7 65.6 57.1 56.8 56.7 60.8 73.6 84.3 65.6 77.0 104.1
azz (49.4) (57.4) (54.9) (55.7) (55.7) (59.5) (71.0) (83.6) (66.4) (81.6) (108.8)
18.1 27.9 28.9 28.5 27.6 39.6 46.8 58.4 48.9 73.9 81.1
(20.3) (29.4) (30.5) (30.4) (30.2) (41.0) (49.7) (58.7) (49.2) (71.9) (84.5)
110
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Table 4 ˚ and degrees), defined in Fig. 1, and quinoid coefficient defined as f = 2R56/(R25 + R36) for The B3LYP/6-31G(d,p) optimized geometric parameters (A HMPys and their radical cations R21
R31
R83
R72
A259
f
HMP Pyrrole PyCH3 PyCH2F PyCHF2 PyCF3 PyCH2Cl PyCHCl2 PyCCl3 PyCH2Br PyCHBr2 PyCBr3
1.375 1.377 1.371 1.371 1.368 1.369 1.367 1.366 1.369 1.366 1.363
1.375 1.374 1.377 1.376 1.378 1.377 1.377 1.378 1.378 1.377 1.377
1.080 1.080 1.080 1.080 1.079 1.079 1.079 1.079 1.080 1.079 1.079
1.080 1.080 1.080 1.079 1.078 1.080 1.078 1.078 1.080 1.080 1.079
125.7 126.5 126.0 125.7 125.4 126.0 127.7 127.1 125.9 127.4 124.3
1.0342 1.0376 1.0358 1.0359 1.0362 1.0371 1.0373 1.0378 1.0376 1.0367 1.0354
HMPy+ Pyrrole+ PyCHþ 3 PyCH2F+ PyCHFþ 2 PyCFþ 3 PyCH2Cl+ PyCHClþ 2 PyCClþ 3 PyCH2Br+ PyCHBrþ 2 PyCBrþ 3
1.363 1.344 1.349 1.358 1.357 1.345 1.343 1.344 1.342 1.340 1.341
1.363 1.386 1.380 1.368 1.366 1.381 1.382 1.380 1.386 1.387 1.386
1.083 1.082 1.083 1.083 1.083 1.082 1.082 1.082 1.082 1.082 1.082
1.083 1.083 1.083 1.083 1.083 1.083 1.083 1.082 1.082 1.082 1.081
124.8 124.9 126.3 124.2 123.9 123.9 123.6 122.3 123.5 123.1 122.2
0.9583 0.9729 0.9669 0.9589 0.9567 0.9683 0.9689 0.9670 0.9778 0.9787 0.9764
symmetrically above and below the pyrrole ring plane. It can thus be said that the local dipole moment of the halomethyl groups –CH2X and CHX2 are aligned in the molecular plane. This allows one to determine interactions between neighboring HMPy polymer chains based on the planar structure of the pyrrole rings. In other words, halomethyl group contribution appears as a pure steric effect only. Obviously, thermal rotation of the halomethyl group alters this picture. However, at sufficiently high temperatures, time averaging of the free fast rotation of the halomethyl group results in an average (group) dipole moment which is directed towards the carbon atom of the halomethyl group located in the pyrrole ring plane. A critical geometrical parameter determining the ease of cationic electropolymerization is the quinoid coefficient defined as f = 2R56/(R25 + R36) [16,33–35], the closer the quinoid coefficient f to 1, the easier the cationic polymerization. The calculated values of quinoid coefficient f for HMPys which are listed in Table 4, show that within less than ±3% deviation, all HMPys have the same quinoid character as pyrrole and methylpyrrole have. Compared to fluoropyrroles, HMPys have larger quinoid coefficients [16]. Table 4 also shows that an extra positive charge on the halomethylpyrrole monomers significantly reduces the quinoid coefficient. Variation of the quinoid character
with the HMPy cation is larger than that of the neutral HMPy. Among the HMPy cations, bromomethypyrrole cations (and among them PyCHBrþ 2 ) have the closest quinoid coefficient to 1. Thus, the bromomethylpyrroles can be said to undergo easier electropolymerization. This is while values of the quinoid coefficients for other HMPys are even farther from 1 compared to the reference molecule methylpyrrole itself. This indicates that, based on the quinoid character only, and compared to methylpyrrole, larger entropic effects of the backbone (chain of the C–C bonds) of the fluoro- and chloromethypyrroles reduces the rate of the propagation step of the electropolymerization of these monomers. Therefore, bromine substitution on the methyl group of methylpyrrole results in easier or faster electropolymerization. 3.6. Vibrational analysis For all of the systems studied in this work, vibrational frequency calculations were carried out to confirm the optimized structures as the local minima on the potential energy surfaces (PES) and to calculate contribution of thermal degrees of freedom to the thermochemical properties. The fundamental harmonic vibrational frequencies for all monomers were calculated using optimized structures at DFT-B3LYP/6-
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111
vibrational states via the translational and rotational stairways. Therefore, in the long time, thermal treatment (under environmental conditions) of the polymeric systems with lower vibrational spacings can result in faster decomposition and shorter environmental recycle time.
31G(d,p) level of theory. In all calculations we obtained only real frequencies confirming that the structures correspond to equilibrium points on the molecular potential energy surface. Characteristics of the three vibrational modes with the highest IR transition intensities for all monomers are presented in Table 5. From the analysis of these data, one can draw the following regularities.
3.7. Thermochemistry Thermochemical properties of HMPy monomers, including standard gas phase molar energy, enthalpy and Gibbs free energy of formation, heat capacity and zero-point energy (ZPE) have been calculated for the B3LYP/6-31G(d,p) optimized structures based on the generalized formation reaction (2) at 298.15 K and 1 atm
(I) In a series of NC4H4–CH3 nXn HMPys with different values of n, both the frequency and intensity of the most intense IR transitions increase when the number of halogen atoms, n, increases. (II) In a series of NC4H4–CH3 nXn HMPys with different halogen atom X, positions of the most intense IR transitions displace to higher frequencies in the order of X = F < Cl < Br, while their intensities decrease in the order of X = F > Cl > Br, respectively. (III) Compared to the methylpyrrole, distribution of the vibrational modes and IR absorption bands of the HMPys shifts towards lower frequencies in the order of X = F < Cl < Br, and with increasing number of substituted halogen atoms. This indicates that thermal and environmental degradations of the HMPys and their environmental recycle time is reduced in the same order.
1 7n n N2 ðgÞ þ 5CðgraphiteÞ þ H2 ðgÞ þ X2 ðl; gÞ 2 2 2 ! NC4 H4 –CH3n Xn ðgÞ
ð2Þ
In this reaction, X = F, Cl and Br, and n = 0, 1, 2Pand 3. For example, DGf is calculated using DGf ¼ i mi Gi , where mi is the stoichiometric factor of substance i in the formation reaction (2) (which is positive for products and negative for reactants) and Gi is the standard Gibbs free energy of substance i at 298.15 K and 1.00 atm. All of the thermochemical properties for the stable forms of the constituting elements are calculated at the same level of theory with the same basis set except for C(graphite) and Br2(l), which have been taken from the literature [36]. The calculated values of the thermodynamic properties for this series of monomers are not presented here for brevity and are available from the authors upon request. The calculated values of DH f and DGf show that fluoromethylpyrroles have less thermodynamic stabilities than chloro- and bromomethylpyrroles. This stability is also affected by the number of substituted halogen atoms on the halomethyl group. Furthermore, thermochemical stability of fluoromethylpyrroles decreases with increasing the number of fluorine atoms, while the stability of chloro- and bromomethylpyrroles
As the number of low frequency vibrations increases in a molecular system, possibility of thermal activation to higher (dissociative) vibrational states and further activation to higher (dissociative and reactive) electronic states will be more feasible. This is because the smaller gaps between vibrational states can be filled easier with the corresponding translational and rotational substates. This trend is similar (but countercurrent in the sense of energy flow) to the radiationless deactivation of the electronically excited molecules in the fluorescence and phosphorescence processes. For larger vibrational spacings, higher temperature variations are needed to connect the population of the lower to that of the higher
Table 5 Frequencies (cm1) and intensities (km/mol) of the three most intense IR transitions for pyrrole, methylpyrrole and HMPys calculated at B3LYP/631G(d,p) level of theory Molecule
m1
Intensity
m2
Intensity
m3
Intensity
Pyrrole PyCH3 PyCH2F PyCHF2 PyCF3 PyCH2Cl PyCHCl2 PyCCl3 PyCH2Br PyCHBr2 PyCBr3
734.4 431.6 1035.7 1123.5 1176.0 670.2 203.2 762.4 1229.8 580.9 693.1
110.7 91.1 103.4 124.5 281.0 94.8 719.8 254.5 79.1 111.4 128.6
461.3 3690.4 479.2 1094.4 1165.0 485.3 112.9 827.0 3686.7 699.9 705.4
77.5 57.6 88.2 105.3 182.8 81.01 736.4 138.2 77.6 102.5 106.2
3688.8 764.1 3686.9 1411.6 1190.8 3685.2 3686.7 780.1 480.0 3682.4 3686.9
54.7 50.7 64.0 107.8 176.6 72.9 83.4 103.1 75.0 82.2 94.0
112
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Table 6 NMR chemical shifts of different nuclei in ppm (for H and C referenced to TMS, and for N referenced to nitromethane) calculated at B3LYP/631G(d,p) level of theory for HMPys using IGAIM method HMP
N1
C2
C3
C5
C6
H4
H7
H8
PyCH3 PyCH2F PyCHF2 PyCF3 PyCH2Cl PyCHCl2 PyCCl3 PyCH2Br PyCHBr2 PyCBr3
163.7 167.1 163.2 162.9 162.8 161.8 159.8 152.9 162.8 157.1
114.9 114.8 117.7 115.4 116.7 118.0 118.0 120.6 117.6 120.5
114.9 116.2 117.1 117.1 117.5 116.7 115.5 113.9 117.4 113.5
107.2 117.0 119.7 120.9 118.7 117.7 122.2 136.6 119.6 127.2
107.2 108.3 110.0 106.7 107.2 110.0 107.3 106.3 109.9 106.8
4.94 4.75 4.99 5.01 5.00 4.93 4.92 5.40 4.87 5.02
4.25 3.96 4.16 4.20 4.26 4.02 4.16 4.20 4.01 4.44
4.25 4.26 4.34 4.31 4.31 4.19 3.98 3.65 4.21 3.86
See Fig. 1 for the numbering scheme.
increases with increasing the number of Cl and Br atoms, respectively. Calculated values of S 298 and CV for HMPys depend on the type and number of halogen atoms. Comparison of the ZPE values for all monomers shows that fluoromethylpyrroles have the largest ZPE. As is expected, in this series of molecules, bromomethylpyrroles have the smallest value of ZPE due to their largest reduced masses. 3.8. NMR chemical shifts NMR study can be used to predict ring currents and estimate the aromaticity and conjugation in these compounds. NMR chemical shielding of different nuclei of HMPys have been calculated for the B3LYP/631G(d,p) optimized structures using the same level of theory based on the IGAIM, SGO and CSGT methods. Analysis of the results showed that IGAIM and CSGT methods predict equal shieldings, while shielding values obtained by SGO method are considerably different. We selected results of IGAIM method for the analysis of NMR properties. The 1H and 13C shieldings for TMS and 15N shielding for nitromethane, calculated with the same level of theory and the same basis set, were used as references for a comparative analysis of the chemical shieldings. In this way, relative shielding constant or chemical shift, Drn, for the nucleus n is defined as Drn ¼ rn ðreferenceÞ rn ðcompoundÞ;
ð3Þ
in which rn(reference) and rn(compound) are chemical shielding of the nucleus n in the reference and in the HMPy molecule, respectively. The calculated values of NMR chemical shifts for the selected nuclei in these series of monomers are listed in Table 6. These results show that the halomethyl substitution affects the chemical shieldings mainly via inductive effects rather than changing the ring current. Introduction of the halomethyl group induces a positive charge on the C5 carbon which results in higher chemical
shifts compared to the C6 carbon. This is evident from the data reported in Table 6. The shielding data obtained for all of the nuclei also show that orientation of the –CHnX3 n group has a significant effect even larger than that of its pull effect on the pyrrole ring. As electropolymerization of HMPy will proceed via a and a 0 positions, the H7 and H8 protons cannot be used as probes or as references for the study of the synthesized polymer. However, the chemical shifts of H4 and pairs of (C2,C3) and (C5,C6) nuclei are excellent probes of the extent of polymerization and the tacticity and packing of the synthesized polymers. A detailed analysis of this could be possible only after ab initio or DFT study of oligomers of HMPys which is beyond the scope of this work. Note that since the reference used for the nitrogen nuclei, nitrogen in nitromethane, is less shielded than the nitrogen in HMPys (which is opposite to what used for H and C nuclei), the trends for the nitrogen shieldings is opposite to those of other nuclei.
4. Conclusion This study started with a set of test calculations upon which B3LYP level of theory and 6-31G(d,p) basis set were found to be the most appropriate methods and basis set for the study of the structure, spectroscopic properties, charge and spin distributions, electrical characteristics such as electric polarizabilities and HOMO–LUMO gaps (HLG), and electric dipole moment for halomethylpyrroles, HMPys. Analysis of the results of this study showed that HMPys can be regarded as possible candidates for the synthesis of corresponding conducting polymers with modified properties compared to the polymers of methylpyrrole, and pyrrole itself. It was also found that characteristics of the halomethyl substituent (number and type of the halogen atoms) in these molecules have important role in the polymerization process and their polymer products. In this series of HMPY monomers, bromomethylpyrroles with two or three bromine atoms have higher capa-
H. Sabzyan, H. Nikoofard / Chemical Physics 306 (2004) 105–113
bility as building blocks for conducting polymers because they have lower ionization potentials, lower HLG values, higher dipole moment vectors, and appropriate charge and spin densities, which are all in agreement with their electrochemical experimental data. Furthermore, based on the analysis of the formation reaction thermochemical properties, bromomethylpyrroles have higher thermodynamic stability, which is also in agreement with what predicted by structural and electronic study. Analysis of the vibrational frequencies also showed that bromomethylpyrrole polymers will have shorter environmental (thermal- and photo-degradations) recycle times. Acknowledgement We thank the University of Isfahan and Sharood University of Technology for research facilities. References [1] [2] [3] [4] [5] [6] [7]
[8] [9] [10] [11] [12]
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[13] C.K. Chiang, Polymer 22 (1981) 1545. [14] X.-L. Wei, Y.Z. Wang, S.M. Long, C. Bobeczko, A.J. Epstein, J. Am. Chem. Soc. 118 (1996) 2545. [15] M. Tsuchiya, S. Matsui, K. Sano, T. Kojima, J. Appl. Polym. Sci. 70 (1998) 471. [16] H. Sabzyan, A. Omrani, J. Phys. Chem. A 107 (2003) 6476. [17] H. Sabzyan, A. Omrani, Theochem, submitted. [18] D.D. Cunningham, L.L. Davidson, H.B. Mark, C.V. Pham, H. Zimer, J. Chem. Soc. Chem. Commun. 1021 (1987). [19] M. Charton, J. Am. Chem. Soc. 97 (1975) 1152. [20] R.W. Taft, Steric Effects in Organic Chemistry, Wiley, New York, 1956 p. 556. [21] R.J. Waltman, J. Bargon, A.F. Diaz, J. Phys. Chem. 87 (1983) 1459. [22] A.D. Becke, J. Chem. Phys. 98 (1993) 5648. [23] C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 785. [24] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, V.G. Zakrzewski, J.A. Montgomery Jr., R.E. Stratmann, J.C. Burant, S. Dapprich, J.M. Millam, A.D. Daniels, K.N. Kudin, M.C. Strain, O. Farkas, J. Tomasi, V. Barone, M. Cossi, R. Cammi, B. Mennucci, C. Pomelli, C. Adamo, S. Clifford, J. Ochterski, G.A. Petersson, P.Y. Ayala, Q. Cui, K. Morokuma, D.K. Malick, A.D. Rabuck, K. Raghavachari, J.B. Foresman, J. Cioslowski, J.V. Ortiz, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. Gomperts, R.L. Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, C. Gonzalez, M. Challacombe, P.M.W. Gill, B. Johnson, W. Chen, M.W. Wong, J.L. Andres, C. Gonzalez, M. Head-Gordon, E.S. Replogle, J.A. Pople, Gaussian94W, Gaussian Inc., Pittsburgh PA, 1994. [25] W.J. Hebre, L. Radon, P.V.R. Schleyer, J.A. Pople, Ab Initio Molecular Orbital Theory, Wiley, New York, 1986. [26] F. Jensen, Introduction to Computational Chemsitry, Wiley, Chichester, 1999. [27] U. Salzner, J.B. Lagowski, P.G. Pickup, R.A. Poirier, Synth. Met. 98 (1998) 177. [28] M. Holb, P. Ehrilich, J. Polym. Sci. 15 (1977) 627. [29] Y.W. Park, M.A. Druy, C.K. Chiang, A.G. MacDiarmid, A.J. Heeger, H. Shirakawa, S. Ikeda, J. Poly. Sci. 17 (1979) 195. [30] R.J. Waltman, A.F. Diaz, J. Bargon, J. Electrochem. Soc. 131 (1984) 1452. [31] R.G. Parr, R.G. Pearson, J. Am. Chem. Soc. 105 (1983) 7512. [32] M.A. De Oliveira, H. Duarte, J. Pernaut, W.B. de Almeida, J. Phys. Chem. A 104 (2000) 8256. [33] R. Colle, A. Curioni, J. Phys. Chem. A 104 (2000) 8546. [34] S. Lunell, L.A. Erikson, M.-B. Huang, J. Mol. Struct. (Theochem) 230 (1991) 263. [35] M.-B. Huang, Z.-X. Wang, J. Chem. Phys. 109 (1998) 8953. [36] Available from:.
Halomethylpyrroles as candidate monomers for conducting polymers: a theoretical study Hassan Sabzyan
a,*
, Hossein Nikoofard
b
a b
Department of Chemistry, University of Isfahan, Isfahan 81764-73441, Iran Department of Chemistry, Shahrood University of Technology, Shahrood, Iran Received 27 March 2004; accepted 22 July 2004 Available online 20 August 2004
Abstract Structural, electronic, thermochemical and electrical properties of mono-, di- and trihalomethylpyrroles (HMPys), NC4H4– CHnX3 n; X = F, Cl, Br; n = 0, 1, 2, 3, and their radical cations have been studied using DFT-B3LYP method with 6-31G(d,p) basis set. Vibrational frequencies and NMR shielding constants of these compounds have also been calculated and analyzed. HMPys are proposed in this research as candidate monomers for conducting polymers with modified characteristics compared to polypyrrole and polymethylpyrrole. Stability of HMPy radical cations have been studied in detail and compared with available experimental data, including oxidation potentials. Results of the present study show that bromomethylpyrroles have the highest thermochemical stability and have higher characteristics for electropolymerization compared to fluoro- and chloromethylpyrroles. Stability of HMPys increases with increasing number of substituted halogen atoms. Ó 2004 Elsevier B.V. All rights reserved. Keywords: Halomethylpyrroles; Conducting polymers; DFT-B3LYP; Electric charge; Polarizability
1. Introduction Electrochemical synthesis of electrically conducting organic polymers, first described in detail for polypyrrole [1–3], has proven important roles in allowing development of new polymeric materials with electrical properties similar to metals. The reversibility of charging/discharging processes and high specific capacitance of the polymers in their oxidized states has stimulated suggestions for a variety of applications such as rechargeable batteries, super-capacitors and electrochromic devices, etc. [4–13]. The main objective of the design and preparation of new materials is to improve the desired electrical and electrochemical properties. It is of high industrial importance to increase the conduc*
Corresponding author. Tel.: +98 311 793 2749; fax: +98 311 668 9732. E-mail address: [email protected] (H. Sabzyan). 0301-0104/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.chemphys.2004.07.025
tivity of polymers and their solubility in certain solvents. Conducting polymers with modified solubility in some industrially important solvents were recently synthesized by introducing alkyl and other substituents on the monomers [14,15]. Since a conducting polymer has a p-conjugate system along the polymer chain, both electronic and structure of a substituent contribute to the delocalization of the polymer p-conjugate system [16– 18], while its steric factor (which is important for substituents like n-alkyls) is usually not determining [19,20]. Substitution of pyrrole monomers with appropriate 3-substituents induces a push–pull effect on the p-electrons and alters the electrical conductivity of the corresponding polymers compared with that of polypyrrole [2,21]. Halomethylpyrroles (HMPys) have been considered in this work as potential monomers for the synthesis of conductive polymers with modified physical and electrical characteristics compared to those of polypyrrole.
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13
12
X 11
X
X
9
5
H10
6
selected for the present study of HMPys. Therefore, B3LYP/6-31G(d,p) method was used throughout this work.
3. Results and discussion 3.1. Electronic properties 7
H
2
3
N1
H8
H 4
Fig. 1. Structure and numbering scheme used for halomethylpyrroles, HMPys, in this work.
The halomethyl substituents can effectively change packing parameter and density of the polymer blend as well as its pyrolysis temperature and its environmental recycle period. Obviously, a halomethyl substitution on the pyrrole ring affects electrical conductivity of the corresponding polymers. In the present work we have carried out a density functional theory (DFT) study on HMPys and their radical cations using the most popular BeckeÕs threeparameter hybrid functional, B3 [22], with non-local correlation of Lee–Yang–Parr, LYP [23], abbreviated as B3LYP, method. Calculations on the highest members of this series of compounds, iodomethylpyrroles, could only be carried out with smaller basis sets such as 3-21G**. Therefore, results of the calculations for iodomethylpyrroles could not be compared directly to those of other HMPys obtained with the higher quality basis set 6-31G(d,p). In this study, pyrrole (Py) and methylpyrrole (MPy) were used as reference compounds for all comparative studies. Structure and numbering scheme used for HMPys in this study are introduced in Fig. 1. All of the calculations in this work were carried out using Gaussian 94W program [24].
2. Computational procedures The preliminary studies showed that 6-31G(d,p) [25,26] is the best basis set considering computational times and our available hardware facilities. Furthermore, in the preliminary studies, MP2 calculations could not be carried out for some representative HMPys, mainly due to the software and hardware limitations, and thus, a comparative study was not applicable based on MP2 calculations. Furthermore, HF method failed to predict reasonable values for the thermochemical stabilities of the representative HMPys. Based on these preliminary studies, DFT-B3LYP level of theory was
Energy gap between the highest occupied and the lowest unoccupied molecular orbitals (HOMO and LUMO, respectively) known as the HOMO–LUMO gap or simply HLG, is a critical parameter determining molecular electrical transport properties and electric admittance (being charged under applied electric filed) because it is a measure of the electron density hardness [27]. A reasonable estimate of HLG values can be obtained from both HF and DFT calculations [28,29]. The HLG values determine also the voltage produced in a photovoltaic cell as well as the electromotive force (EMF) of an electrochemical cell when the polymer is used as its negative or as its positive electrode. Increase in the chain length of a conductive polymer (or the length of the p-conjugated system) decreases the HLG values of the polymer. Therefore, it seems that HLG values of a polymer cannot be determined exactly from those of their monomers. However, the study of HLG values of monomers may be used to predict the comparative band gap behavior of the corresponding polymers. The HLG values of HMPys defined in Fig. 2, are calculated and listed in Table 1. It can be seen from this table that the HLG values for the chloro- and bromomethylpyrroles, decrease when the number of halogen atoms increases. In addition, among each NC4H4– CH3 nXn series with fixed n, the HMPy with X = Br has the lowest HLG values. Therefore, we can predict that bromomethylpyrrole polymers may have the highest electrical conductivity compared with other HMPy polymers. It should be noted here that HLG values are not the only parameters that determine electric conductance of a polymer film. Orientation and alignment of the monomers in the polymer chain are two other determining
LUMO L4
L1 L2
HOMO L5
L3
L6
Fig. 2. The energy gaps, Li, including HOMO–LUMO gaps (HLG), L1, studied in this research for HMPys.
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107
Table 1 The B3LYP/6-31G(d,p) calculated values of energy gaps Li (defined in Fig. 2) including HOMO–LUMO gap (HLG) L1, and ionization potentials (IP), both in eV, for HMPys and their reference molecules, pyrrole and methylpyrrole HMPy
L1
L2
L3
L4
L5
L6
IP
Pyrrole PyCH3 PyCH2F PyCHF2 PyCF3 PyCH2Cl PyCHCl2 PyCCl3 PyCH2Br PyCHBr2 PyCBr3
6.85 6.82 4.84 6.84 6.86 6.07 5.81 5.38 5.64 5.19 4.46
7.69 7.51 5.60 7.64 7.50 6.80 6.61 6.01 6.25 6.00 5.30
11.26 10.67 7.54 9.70 10.61 7.74 7.67 7.04 6.67 6.33 5.47
7.69 7.65 7.54 7.63 7.66 7.03 6.18 5.93 6.95 5.54 5.23
8.52 8.34 8.30 8.43 8.30 7.76 6.98 6.56 7.57 6.35 6.07
12.10 11.50 10.25 10.49 11.41 8.70 8.04 8.44 7.98 6.68 6.24
7.81 7.53 7.82 8.11 8.35 7.88 8.03 8.18 7.75 7.82 7.88
characteristics that play important roles in the electrical conductance of a polymer. Large substituents on the pyrrole ring may prevent the polymer chain to adopt a planar structure and hence increase the resistance against alignment and packing of the polymer layers. This causes a decrease in the density of the polymer and consequently, reduces the electrical conductance of the polymer [21,30,31]. The large substituents, on the other hand, increase inter-chain distance and thus decrease their electrostatic repulsive interactions during the electric charge transfer process in the bulk of the polymer. 3.2. Ionization potentials It is known that the first step in the electopolymerization of conductive polymers is the formation of intermediate radical cations from the monomers, which can be considered as ionization reaction [1,2] M ! M þ þ e ð1Þ in which M and M+ denote monomer and its radical cation, respectively. Hence, stability of intermediate radical cations has important role in the electropolymerization process. Therefore, it is useful to calculate electronic energy deference between the neutral monomer (as the initial species) and the positively charged monomer (as the intermediate). This energy difference is proportional to the ionization potential (IP) of the monomer. The geometries of the HMPy cations were optimized at the same level of theory with the same basis set as used for neutral HMPys prior to the calculation of other properties. The calculated values of IP for HMPys based on reaction (1) are presented in Table 1. According to the results listed in this table, methylpyrrole monomer has the lowest IP, which may be due to the electron donor character of the substituent –CH3 on the pyrrole ring, and so its radical cation in the gas phase has the highest
stability among all HMPY radical cations. This is in agreement with the lower oxidation potential measured experimentally for methylpyrrole compared to pyrrole monomer [32]. In this series of HMPys, the PyCF3 monomer has the highest IP corresponding to the highest electronegativity of its halogen substituents. Results reported in Table 1 also show that the IP of these compounds increases when the electron-pull effect of the substituent (CX3) increases, that is, the oxidation potential of NC4H4–CHnX3 n HMPY series increases in the order of X = Br < Cl < F. In general, it can be concluded that electropolymerization of HMPys becomes more difficult as the number and electronegativity of the substituted halogen atoms increase. 3.3. Charge and spin density distributions In the growth step of the polymerization of pyrrole, radical cations link to the polymer chain from the ring position with the most positive charge (a and a 0 positions, or C2 and C3 positions in Fig. 1). Also, all of HMPys can undergo polymerization via their a and a 0 carbons due to their positive charges [18–20]. Using Mulliken population analysis, the net atomic electric charges and spin density distribution in the HMPy cations are calculated and summarized in Table 2, respectively. As mentioned above, geometries of the cations were optimized at the same level of theory with the same basis set prior to the calculation of charges. According to the results presented in Table 2, a and a 0 carbons (C2 and C3 positions in Fig. 1) in pyrrole monomer have equal charges and spin densities, but with attachment of a halomethyl substituent to the pyrrole ring, this equality is altered. The difference between a and a 0 positions depends on the type and number of halogen atoms. For most cations, the positive charge on the a-carbon (C2 position in Fig. 1), which is close to the substituent position (C5), is higher than that on the a 0 -carbon (C3). Conversely, spin density is
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Table 2 Electric charge (spin) distributions calculated for the B3LYP/6-31G(d,p) optimized structures of HMPy radical cations (X denotes the halogen atoms) Cation
N1
C2(a)
C3(a 0 )
C5
C6
X
Pyrrole+ PyCHþ 3 PyCH2F+ PyCHFþ 2 PyCFþ 3 PyCH2Cl+ PyCHClþ 2 PyCClþ 3 PyCH2Br+ PyCHBrþ 2 PyCBrþ 3
0.46(0.12) 0.47(0.08) 0.47(0.10) 0.47(0.12) 0.47(0.12) 0.47(0.10) 0.47(0.10) 0.48(0.10) 0.47(0.08) 0.47(0.08) 0.47(0.08)
0.20(0.54) 0.20(0.52) 0.20(0.53) 0.21(0.53) 0.22(0.53) 0.21(0.50) 0.23(0.48) 0.23(0.48) 0.20(0.44) 0.21(0.43) 0.22(0.43)
0.20(0.54) 0.18(0.48) 0.18(0.50) 0.21(0.54) 0.21(0.53) 0.18(0.49) 0.18(0.49) 0.19(0.49) 0.17(0.44) 0.17(0.43) 0.17(0.44)
0.76(0.05) 0.08(0.21) 0.01(0.15) 0.04(0.07) 0.08(0.00) 0.09(0.17) 0.11(0.18) 0.13(0.17) 0.08(0.21) 0.08(0.21) 0.11(0.20)
0.76(0.05) 0.10(0.11) 0.09(0.08) 0.09(0.02) 0.09(0.00) 0.10(0.09) 0.10(0.09) 0.06(0.08) 0.10(0.11) 0.09(0.12) 0.09(0.11)
– – 0.27(0.01) 0.26(0.00) 0.23(0.00) 0.04(0.07) 0.09(0.05) 0.13(0.03) 0.03(0.15) 0.05(0.09) 0.09(0.61)
distributed mainly on the a 0 -carbon. This behavior could be expected, as the distributed charge is positive while the spin density is carried by electrons which have negative charge. Larger differences between the electric charges at a and a 0 positions (which are propagation centers in the electropolymerization process) will result in higher selectivity of the tacticity of the polymer chain. The syndiotactic structure of the HMPy polymers corresponds to the lower steric effects between halomethyl groups on the neighboring monomers, and thus higher thermal stability of the polymer. It can, therefore, be concluded that halomethyl substituents increase selectivity of the syndiotactic order of the polymer chain. A set of PM3 semi-empirical computations was carried out on the trifluoromethylpyrrole pentamer with different tacticities, shown in Fig. 3, to investigate the
tacticity issue. The results of this study showed that for this oligomer, the syndiotactic order is 12.2 kcal/ mol more stable than the atactic order. Furthermore, as can be seen from Table 2, symmetry of the halomethyl group with respect to the molecular plane has a noticeable effect on the charge and spin distributions. Because the charge density of the a-carbon for HMPy cations is higher than that for pyrrole cation, the initiation step of the polymerization is faster for these monomers, but the length of their polymer chains may be shorter due to the large substituent effects. The atomic charge distributions in the neutral HMPys, which are not reported here for the sake of brevity, show, more or less, the same behavior as is found for their cation radicals. Analysis of the NBO atomic charges calculated for the optimized geometries of the HMPy cations, not reported here for brevity, resulted in exactly the same trends observed for the Mulliken charges, although the absolute values of the atomic charges were different. From electric charge distribution point of view, there is no significant preference for the tacticity of the HMPy polymer chains as the distributed electric charge on the two a and a 0 carbon atoms are identical for a couple and are just slightly different for other HMPy cations. Therefore, electric charges cannot be accounted for any possible preferences (if any) of the tacticity of the HMPy polymer chains that might be observed experimentally. 3.4. Electric dipole moments and polarizabilities
Fig. 3. The PM3 optimized geometries of two different tacticities of the trifluoromethylpyrrole pentamer.
An important factor in the chemical and electrochemical synthesis of conducting polymers is the choice of proper solvent and support electrolyte [10]. Orientation of the electric dipole moment vector of the monomers with respect to the direction of the polymer chain determines electrochemical characteristics of the polymer on the electrode surface. In addition to their orientations, sizes of the electric dipole moment vector of the monomers and their interactions with the solvent and support
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electrolyte are the key rule in their selection for electropolymerization. Furthermore, structure of the electrical double layer at the surface of electrodes which determines the kinetics of the diffusion-controlled electrode reactions, depends on the dipole moment of the solute molecules. Relative conformations of the neighboring monomers that determine the growth rate of the polymer chain in the propagation step depend on both size and orientation of the monomer electric dipole moment vector. Thus, to study this feature of polymers, a detailed analysis of the electric dipole moment vector is needed. The calculated sizes of the electric dipole moment vectors for HMPys have been listed in Table 3. The data presented in this table show that both type and number of halogen atoms affect the size of the electric dipole moment vector. The comparative sizes of the dipole moment vectors follow the orders: (a) l (PyCH2X) > l (PyCHX2) > l (PyCX3); X = F, Cl, Br (b) l (PyCBr3) > l (PyCCl3) > l (PyCF3) The same order is found for the dipole moment vectors of the HMPY radical cations. Because of larger electric dipole moments, bromomethylpyrroles are expected to be more soluble in polar solvents. Diagonal components of the electric polarizability tensor, axx, ayy and azz, calculated for HMPys and their radical cations are presented in Table 3. This table shows that values of axx, ayy and azz depend on the type and number of halogen atoms according to the following trends: (a) aqq (Py–CH3 nBrn) > aqq (Py–CH3 nCln) > aqq (Py–CH3 nFn); q = x, y, z (b) aqq (PyCX3) > aqq (PyCHX2) > aqq (PyCH2X); X = F, Cl, Br Table 3 shows also that polarizabilities of these HMPys increase with the size and number of substituted
109
halogen atoms. A comparison between the calculated electric polarizabilities of the neutral and positively charged HMPys shows that, generally, axx and ayy are slightly smaller for the HMPy cations, while azz is slightly larger. 3.5. Structural analysis It is known that electrical conductivity of polymer films depends on planarity, orientation and alignment of their monomers in the polymer chain backbone. Thus, in this section, we have calculated the corresponding parameters and studied these aspects of the HMPys structures. Some geometrical parameters corresponding to the optimized structures of HMPys and their radical cations obtained at DFT-B3LYP/6-31G(d,p) level of theory are presented in Table 4. The geometrical parameters used in this study are defined in terms of the numbering scheme introduced in Fig. 1 as: Ri,j, the bond length between atoms i and j; Aijk, the bond angle formed by atoms i, j and k, and Dijkl, the dihedral angle formed by atoms i, j, k and l. It is already found that branching of polypyrroles in the polymer growth step, is initiated on C2 (a) and C3 (a 0 ) ring positions [16–18]. Obviously, a and a 0 carbon atoms are identical for pyrrole (i.e., R21 = R31 and R83 = R72), but they may be different for HMPys. As can be seen from Table 4, for all HMPys, R21 is generally smaller than R31, while their variations with the halomethyl group are not significant. Introducing the halomethyl group has almost no effect on the values of R83 and R72. Values of the dihedral angle defining torsion of the halomethyl groups –CH2X, CHX2 and –CX3 (where X denotes the halogen atoms) with respect to the molecular plane, referenced to the atom X for the –CH2X and –CX3, and to the atom H for –CHX2 are very close to 0°, with a maximum deviation of 2.88° for Py–CHCl2. This means that the referenced atom (X and H for the two sets of groups, respectively) lies in the ring plane, and the other two atoms of the halomethyl group lie
Table 3 Electric dipole moments (D) and diagonal elements of the electric polarizability tensor (bohr3) for the B3LYP/6-31G(d,p) optimized structures of neutral (radical cations) of pyrrole, methylpyrrole and HMPys HMPy
l
Pyrrole PyCH3 PyCH2F PyCHF2 PyCF3 PyCH2Cl PyCHCl2 PyCCl3 PyCH2Br PyCHBr2 PyCBr3
1.90 1.71 3.46 3.68 4.42 4.14 4.26 4.83 4.23 4.16 4.51
axx (1.79) (2.54) (6.30) (8.15) (9.77) (6.81) (8.61) (10.23) (7.96) (9.56) (10.82)
49.2 58.1 64.6 63.4 63.0 79.4 85.0 91.4 91.1 98.2 111.0
ayy (45.5) (56.0) (57.4) (56.6) (56.8) (67.2) (72.4) (79.4) (77.5) (88.9) (102.3)
50.7 65.6 57.1 56.8 56.7 60.8 73.6 84.3 65.6 77.0 104.1
azz (49.4) (57.4) (54.9) (55.7) (55.7) (59.5) (71.0) (83.6) (66.4) (81.6) (108.8)
18.1 27.9 28.9 28.5 27.6 39.6 46.8 58.4 48.9 73.9 81.1
(20.3) (29.4) (30.5) (30.4) (30.2) (41.0) (49.7) (58.7) (49.2) (71.9) (84.5)
110
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Table 4 ˚ and degrees), defined in Fig. 1, and quinoid coefficient defined as f = 2R56/(R25 + R36) for The B3LYP/6-31G(d,p) optimized geometric parameters (A HMPys and their radical cations R21
R31
R83
R72
A259
f
HMP Pyrrole PyCH3 PyCH2F PyCHF2 PyCF3 PyCH2Cl PyCHCl2 PyCCl3 PyCH2Br PyCHBr2 PyCBr3
1.375 1.377 1.371 1.371 1.368 1.369 1.367 1.366 1.369 1.366 1.363
1.375 1.374 1.377 1.376 1.378 1.377 1.377 1.378 1.378 1.377 1.377
1.080 1.080 1.080 1.080 1.079 1.079 1.079 1.079 1.080 1.079 1.079
1.080 1.080 1.080 1.079 1.078 1.080 1.078 1.078 1.080 1.080 1.079
125.7 126.5 126.0 125.7 125.4 126.0 127.7 127.1 125.9 127.4 124.3
1.0342 1.0376 1.0358 1.0359 1.0362 1.0371 1.0373 1.0378 1.0376 1.0367 1.0354
HMPy+ Pyrrole+ PyCHþ 3 PyCH2F+ PyCHFþ 2 PyCFþ 3 PyCH2Cl+ PyCHClþ 2 PyCClþ 3 PyCH2Br+ PyCHBrþ 2 PyCBrþ 3
1.363 1.344 1.349 1.358 1.357 1.345 1.343 1.344 1.342 1.340 1.341
1.363 1.386 1.380 1.368 1.366 1.381 1.382 1.380 1.386 1.387 1.386
1.083 1.082 1.083 1.083 1.083 1.082 1.082 1.082 1.082 1.082 1.082
1.083 1.083 1.083 1.083 1.083 1.083 1.083 1.082 1.082 1.082 1.081
124.8 124.9 126.3 124.2 123.9 123.9 123.6 122.3 123.5 123.1 122.2
0.9583 0.9729 0.9669 0.9589 0.9567 0.9683 0.9689 0.9670 0.9778 0.9787 0.9764
symmetrically above and below the pyrrole ring plane. It can thus be said that the local dipole moment of the halomethyl groups –CH2X and CHX2 are aligned in the molecular plane. This allows one to determine interactions between neighboring HMPy polymer chains based on the planar structure of the pyrrole rings. In other words, halomethyl group contribution appears as a pure steric effect only. Obviously, thermal rotation of the halomethyl group alters this picture. However, at sufficiently high temperatures, time averaging of the free fast rotation of the halomethyl group results in an average (group) dipole moment which is directed towards the carbon atom of the halomethyl group located in the pyrrole ring plane. A critical geometrical parameter determining the ease of cationic electropolymerization is the quinoid coefficient defined as f = 2R56/(R25 + R36) [16,33–35], the closer the quinoid coefficient f to 1, the easier the cationic polymerization. The calculated values of quinoid coefficient f for HMPys which are listed in Table 4, show that within less than ±3% deviation, all HMPys have the same quinoid character as pyrrole and methylpyrrole have. Compared to fluoropyrroles, HMPys have larger quinoid coefficients [16]. Table 4 also shows that an extra positive charge on the halomethylpyrrole monomers significantly reduces the quinoid coefficient. Variation of the quinoid character
with the HMPy cation is larger than that of the neutral HMPy. Among the HMPy cations, bromomethypyrrole cations (and among them PyCHBrþ 2 ) have the closest quinoid coefficient to 1. Thus, the bromomethylpyrroles can be said to undergo easier electropolymerization. This is while values of the quinoid coefficients for other HMPys are even farther from 1 compared to the reference molecule methylpyrrole itself. This indicates that, based on the quinoid character only, and compared to methylpyrrole, larger entropic effects of the backbone (chain of the C–C bonds) of the fluoro- and chloromethypyrroles reduces the rate of the propagation step of the electropolymerization of these monomers. Therefore, bromine substitution on the methyl group of methylpyrrole results in easier or faster electropolymerization. 3.6. Vibrational analysis For all of the systems studied in this work, vibrational frequency calculations were carried out to confirm the optimized structures as the local minima on the potential energy surfaces (PES) and to calculate contribution of thermal degrees of freedom to the thermochemical properties. The fundamental harmonic vibrational frequencies for all monomers were calculated using optimized structures at DFT-B3LYP/6-
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111
vibrational states via the translational and rotational stairways. Therefore, in the long time, thermal treatment (under environmental conditions) of the polymeric systems with lower vibrational spacings can result in faster decomposition and shorter environmental recycle time.
31G(d,p) level of theory. In all calculations we obtained only real frequencies confirming that the structures correspond to equilibrium points on the molecular potential energy surface. Characteristics of the three vibrational modes with the highest IR transition intensities for all monomers are presented in Table 5. From the analysis of these data, one can draw the following regularities.
3.7. Thermochemistry Thermochemical properties of HMPy monomers, including standard gas phase molar energy, enthalpy and Gibbs free energy of formation, heat capacity and zero-point energy (ZPE) have been calculated for the B3LYP/6-31G(d,p) optimized structures based on the generalized formation reaction (2) at 298.15 K and 1 atm
(I) In a series of NC4H4–CH3 nXn HMPys with different values of n, both the frequency and intensity of the most intense IR transitions increase when the number of halogen atoms, n, increases. (II) In a series of NC4H4–CH3 nXn HMPys with different halogen atom X, positions of the most intense IR transitions displace to higher frequencies in the order of X = F < Cl < Br, while their intensities decrease in the order of X = F > Cl > Br, respectively. (III) Compared to the methylpyrrole, distribution of the vibrational modes and IR absorption bands of the HMPys shifts towards lower frequencies in the order of X = F < Cl < Br, and with increasing number of substituted halogen atoms. This indicates that thermal and environmental degradations of the HMPys and their environmental recycle time is reduced in the same order.
1 7n n N2 ðgÞ þ 5CðgraphiteÞ þ H2 ðgÞ þ X2 ðl; gÞ 2 2 2 ! NC4 H4 –CH3n Xn ðgÞ
ð2Þ
In this reaction, X = F, Cl and Br, and n = 0, 1, 2Pand 3. For example, DGf is calculated using DGf ¼ i mi Gi , where mi is the stoichiometric factor of substance i in the formation reaction (2) (which is positive for products and negative for reactants) and Gi is the standard Gibbs free energy of substance i at 298.15 K and 1.00 atm. All of the thermochemical properties for the stable forms of the constituting elements are calculated at the same level of theory with the same basis set except for C(graphite) and Br2(l), which have been taken from the literature [36]. The calculated values of the thermodynamic properties for this series of monomers are not presented here for brevity and are available from the authors upon request. The calculated values of DH f and DGf show that fluoromethylpyrroles have less thermodynamic stabilities than chloro- and bromomethylpyrroles. This stability is also affected by the number of substituted halogen atoms on the halomethyl group. Furthermore, thermochemical stability of fluoromethylpyrroles decreases with increasing the number of fluorine atoms, while the stability of chloro- and bromomethylpyrroles
As the number of low frequency vibrations increases in a molecular system, possibility of thermal activation to higher (dissociative) vibrational states and further activation to higher (dissociative and reactive) electronic states will be more feasible. This is because the smaller gaps between vibrational states can be filled easier with the corresponding translational and rotational substates. This trend is similar (but countercurrent in the sense of energy flow) to the radiationless deactivation of the electronically excited molecules in the fluorescence and phosphorescence processes. For larger vibrational spacings, higher temperature variations are needed to connect the population of the lower to that of the higher
Table 5 Frequencies (cm1) and intensities (km/mol) of the three most intense IR transitions for pyrrole, methylpyrrole and HMPys calculated at B3LYP/631G(d,p) level of theory Molecule
m1
Intensity
m2
Intensity
m3
Intensity
Pyrrole PyCH3 PyCH2F PyCHF2 PyCF3 PyCH2Cl PyCHCl2 PyCCl3 PyCH2Br PyCHBr2 PyCBr3
734.4 431.6 1035.7 1123.5 1176.0 670.2 203.2 762.4 1229.8 580.9 693.1
110.7 91.1 103.4 124.5 281.0 94.8 719.8 254.5 79.1 111.4 128.6
461.3 3690.4 479.2 1094.4 1165.0 485.3 112.9 827.0 3686.7 699.9 705.4
77.5 57.6 88.2 105.3 182.8 81.01 736.4 138.2 77.6 102.5 106.2
3688.8 764.1 3686.9 1411.6 1190.8 3685.2 3686.7 780.1 480.0 3682.4 3686.9
54.7 50.7 64.0 107.8 176.6 72.9 83.4 103.1 75.0 82.2 94.0
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Table 6 NMR chemical shifts of different nuclei in ppm (for H and C referenced to TMS, and for N referenced to nitromethane) calculated at B3LYP/631G(d,p) level of theory for HMPys using IGAIM method HMP
N1
C2
C3
C5
C6
H4
H7
H8
PyCH3 PyCH2F PyCHF2 PyCF3 PyCH2Cl PyCHCl2 PyCCl3 PyCH2Br PyCHBr2 PyCBr3
163.7 167.1 163.2 162.9 162.8 161.8 159.8 152.9 162.8 157.1
114.9 114.8 117.7 115.4 116.7 118.0 118.0 120.6 117.6 120.5
114.9 116.2 117.1 117.1 117.5 116.7 115.5 113.9 117.4 113.5
107.2 117.0 119.7 120.9 118.7 117.7 122.2 136.6 119.6 127.2
107.2 108.3 110.0 106.7 107.2 110.0 107.3 106.3 109.9 106.8
4.94 4.75 4.99 5.01 5.00 4.93 4.92 5.40 4.87 5.02
4.25 3.96 4.16 4.20 4.26 4.02 4.16 4.20 4.01 4.44
4.25 4.26 4.34 4.31 4.31 4.19 3.98 3.65 4.21 3.86
See Fig. 1 for the numbering scheme.
increases with increasing the number of Cl and Br atoms, respectively. Calculated values of S 298 and CV for HMPys depend on the type and number of halogen atoms. Comparison of the ZPE values for all monomers shows that fluoromethylpyrroles have the largest ZPE. As is expected, in this series of molecules, bromomethylpyrroles have the smallest value of ZPE due to their largest reduced masses. 3.8. NMR chemical shifts NMR study can be used to predict ring currents and estimate the aromaticity and conjugation in these compounds. NMR chemical shielding of different nuclei of HMPys have been calculated for the B3LYP/631G(d,p) optimized structures using the same level of theory based on the IGAIM, SGO and CSGT methods. Analysis of the results showed that IGAIM and CSGT methods predict equal shieldings, while shielding values obtained by SGO method are considerably different. We selected results of IGAIM method for the analysis of NMR properties. The 1H and 13C shieldings for TMS and 15N shielding for nitromethane, calculated with the same level of theory and the same basis set, were used as references for a comparative analysis of the chemical shieldings. In this way, relative shielding constant or chemical shift, Drn, for the nucleus n is defined as Drn ¼ rn ðreferenceÞ rn ðcompoundÞ;
ð3Þ
in which rn(reference) and rn(compound) are chemical shielding of the nucleus n in the reference and in the HMPy molecule, respectively. The calculated values of NMR chemical shifts for the selected nuclei in these series of monomers are listed in Table 6. These results show that the halomethyl substitution affects the chemical shieldings mainly via inductive effects rather than changing the ring current. Introduction of the halomethyl group induces a positive charge on the C5 carbon which results in higher chemical
shifts compared to the C6 carbon. This is evident from the data reported in Table 6. The shielding data obtained for all of the nuclei also show that orientation of the –CHnX3 n group has a significant effect even larger than that of its pull effect on the pyrrole ring. As electropolymerization of HMPy will proceed via a and a 0 positions, the H7 and H8 protons cannot be used as probes or as references for the study of the synthesized polymer. However, the chemical shifts of H4 and pairs of (C2,C3) and (C5,C6) nuclei are excellent probes of the extent of polymerization and the tacticity and packing of the synthesized polymers. A detailed analysis of this could be possible only after ab initio or DFT study of oligomers of HMPys which is beyond the scope of this work. Note that since the reference used for the nitrogen nuclei, nitrogen in nitromethane, is less shielded than the nitrogen in HMPys (which is opposite to what used for H and C nuclei), the trends for the nitrogen shieldings is opposite to those of other nuclei.
4. Conclusion This study started with a set of test calculations upon which B3LYP level of theory and 6-31G(d,p) basis set were found to be the most appropriate methods and basis set for the study of the structure, spectroscopic properties, charge and spin distributions, electrical characteristics such as electric polarizabilities and HOMO–LUMO gaps (HLG), and electric dipole moment for halomethylpyrroles, HMPys. Analysis of the results of this study showed that HMPys can be regarded as possible candidates for the synthesis of corresponding conducting polymers with modified properties compared to the polymers of methylpyrrole, and pyrrole itself. It was also found that characteristics of the halomethyl substituent (number and type of the halogen atoms) in these molecules have important role in the polymerization process and their polymer products. In this series of HMPY monomers, bromomethylpyrroles with two or three bromine atoms have higher capa-
H. Sabzyan, H. Nikoofard / Chemical Physics 306 (2004) 105–113
bility as building blocks for conducting polymers because they have lower ionization potentials, lower HLG values, higher dipole moment vectors, and appropriate charge and spin densities, which are all in agreement with their electrochemical experimental data. Furthermore, based on the analysis of the formation reaction thermochemical properties, bromomethylpyrroles have higher thermodynamic stability, which is also in agreement with what predicted by structural and electronic study. Analysis of the vibrational frequencies also showed that bromomethylpyrrole polymers will have shorter environmental (thermal- and photo-degradations) recycle times. Acknowledgement We thank the University of Isfahan and Sharood University of Technology for research facilities. References [1] [2] [3] [4] [5] [6] [7]
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