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Energetics of photoconversion of norbornadiene to quadricyclane: Effects of directly attached substituents via ab initio and DFT
Journal of Molecular Structure: THEOCHEM 763 (2006) 13–19 www.elsevier.com/locate/theochem
Energetics of photoconversion of norbornadiene to quadricyclane: Effects of directly attached substituents via ab initio and DFT M.Z. Kassaee *,a, E. Vessally b, S. Arshadi c a
Department of Chemistry, Tarbiat Modarres University, Tehran, Iran b Islamic Azad University, Myianeh Branch, Myianeh, Iran c Payame Noor University, Behshahr Branch, Behshahr, Iran
Received 30 May 2005; received in revised form 20 November 2005; accepted 3 January 2006 Available online 10 March 2006
Abstract An attempt is made to maximize the photochemical and/or solar energy storage in norbornadiene (1)/quadricyclane (2) system, through direct attachment of substituents at C1, C2 or C7 atoms of 1 and 2. Assessments of the corresponding energies are made at B3LYP/6-311CCG* level of theory. Electron donating substituents, D (DZ–NMe2, –NH2, –OMe, –OH, and Me), directly attached at C2, increase the energy gap between 1 and 2, inducing higher storage of energy in the system. Electron withdrawing subsituents, W (WZ–NO2, –F, –Cl, –Br, –CF3 and –COOH), directly attached at C1, moderately induce the energy storage. Attachment of either D or W groups at C7 show no significant difference in the photochemical and/or solar energy storages. A Hammet r value of K3.69 is encountered for substituents directly attached at C2 atoms of 1 and 2. This is in clear contrast to the Hammet r value of K0.81 that we recently reported for the subsituents indirectly placed at the C2 of 1 and/or 2 (through attachment via the para carbon of phenyl rings). q 2006 Elsevier B.V. All rights reserved. Keywords: Photochemical energy; Solar energy; Energy storage; Norbornadiene; Quadricyclane; Electron donating; Electron withdrawing; Substituents; Ab initio; DFT; B3LYP
1. Introduction Solving today’s energy problems through the employment of the solar energy has received much attention [1]. Intramolecular photochemical [2p–2p]-cycloaddition of norbornadiene, 1, to quadricyclane, 2, has been studied as a mechanistic point of view [1–4]. The 1/2 system is used for solar energy storage [5–7], in molecular switching [8–10], in optoelectronic devices [11–14], as a data storage compound [15,16], as photodynamic chemosensor for metal cations [17,18], as a potential photoresponsive organic magnet [19–21] and as an energetic binder for solid rocket propellants [22]. This system has an inherent disadvantage, since 1 cannot absorb visible wavelength of sunlight. Usage of sensitizers and chromophores are two improvements for solving this problem. Iridium complex is proposed as the sensitizer for p–p* excitation [4]. The donor–acceptor chromophores are placed at the double bond of the norbornadiene molecule. The water-soluble carbamoyl and carboxyl derivatives of 1 and 2 are also used to absorb light of * Corresponding author. Tel.: C98 9121000392; fax: C982188006544. E-mail address: [email protected] (M.Z. Kassaee).
0166-1280/$ - see front matter q 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.theochem.2006.01.010
wavelengths longer than 300 nm [23]. Ab initio is used to study energetic of 1 and 2 conversions [4,24,25]. Density functional calculations with the hybrid B3LYP functional have been used to study the ground state of 1 bound to the photosensitizer [Cu(8-oxoquinolinato)] [26]. Recently, we reported a theoretical investigation on the electronic effects involved in the solar energy storage, for substituents ‘indirectly’ attached to the C2 of 1 and/or 2, where a Hammet r value of K0.81 was encountered [27]. However, the effects of ‘direct’ attachment of the subsituents at C1, C2 or C7 atoms of 1 and 2, which appear of ‘practical interest’ to those whose primary goal is to replace the fossil fuel and/or the nuclear energy with the most economical and very available solar energy, were not investigated. In this manuscript, we scrutinize the photochemical energy storage in the ground states of 1/2 system with substituents directly attached at C1, C2 or C7 atoms of 1 and 2. Computations and/or measurement of the quantum yields of the scrutinized photolysis, which come into view as ‘theoretical interest’ [28–32], are not addressed here. 2. Computational methods The molecular structures of Cn-substituted norbornadienes (1n-X) and Cn-substituted quadricyclanes (2n-X), are studied
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Scheme 1. Storage of solar energy in n-X-norbornadiene (1n-X)/n-Xquadricyclane (2n-X) system; where nZ1, 2 or 7 and XZ–NO2, –F, –Cl, – Br, –CF3, –COOH, –NMe2, –OMe, –OH, –Me.
using ab initio and DFT methods (Scheme 1). Full geometry optimizations are carried out at B3LYP/6-311CCG* level of theory, using the GAUSSIAN 98 system of programs [33–35]. In order to find energy minima, keyword ‘FOPT’ are used. This keyword requests that a geometry optimization be performed. The geometry will be adjusted until a stationary point on the potential surface is found. Here, the Berny algorithm is employed for all minimizations using redundant internal
coordinates [36]. For minimum state structures, only real frequency values are accepted. The calculations exhibit systematic errors and thus benefit from scaling. Thermodynamic functions obtained through frequency calculations, are multiplied by Hehre et al. [37] scaling factor of 0.89 for HF; and by 0.99 scaling factor of Rauhut and Pulay [38] for B3LYP. This is to account for the difference between the harmonic vibrational calculations and the anharmonic oscillations of the actual bonds. Nevertheless, scaling factors fitted to observe (anharmonic) frequencies will deviate from unity even for exact calculations. Here, a set of molecules containing similar motifs are treated together, where they benefit from similar scalings. The bond hybridizations of 12-H, 12-NH2, and 12-NO2 are calculated using NBO program [39]. 3. Results and discussion In this section, an overview of results are presented before their discussion. The effects of both electron withdrawing and
Table 1 The B3LYP/6-311CCG* thermal and electronic energies (E), enthalpies (H), and Gibbs free energies (G), in kilocalorie per mole, for optimized n-Xnorbornadienes (1n-X) and n-X-quadricyclanes (2n-X); where nZ1, 2 or 7, while XZ–NO2, –F, –Cl, –Br, –CF3, –COOH, –NMe2, –OMe, –OH and –Me n-X (nZ1, 2, 7)
2n-X
1n-X E (kcal molK1)
H (kcal molK1)
G (kcal molK1)
E (kcal molK1)
H (kcal molK1)
G (kcal molK1)
1-NO2 1-CF3 1-COOH 1-F 1-Cl 1-Br 1-H 1-CH3 1-OH 1-OCH3 1-NH2 1-N(CH3)2
K298,590.434 K381,787.200 K288,575.034 K232,534.136 K458,657.708 K1,785,142.860 K170,231.780 K194,873.900 K217,438.482 K242,071.265 K204,951.675 K254,220.042
K298,589.315 K381,786.080 K288,573.914 K232,533.016 K458,656.588 K1,785,141.740 K170,230.659 K194,872.779 K217,437.362 K242,070.146 K204,950.555 K254,218.923
K298,640.247 K381,837.130 K288,623.980 K232,575.637 K458,700.781 K1,785,187.560 K170,270.932 K194,916.934 K217,480.777 K242,117.744 K204,994.416 K254,270.140
K298,568.142 K381,764.595 K288,555.188 K232,506.151 K458,632.384 K1,785,118.078 K170,208.829 K194,851.079 K217,412.549 K242,045.885 K204,927.159 K254,195.580
K298,567.022 K381,763.475 K288,554.068 K232,505.031 K458,631.264 K1,785,116.959 K170,207.710 K194,849.959 K217,411.429 K242,044.766 K204,926.038 K254,194.460
K298,614.785 K381,814.103 K288,602.607 K232,547.185 K458,674.959 K1,785,162.264 K170,247.395 K194,893.651 K217,454.293 K242,091.984 K204,969.734 K254,246.001
2-NO2 2-CF3 2-COOH 2-F 2-Cl 2-Br 2-H 2-CH3 2-OH 2-OCH3 2-NH2 2-N(CH3)2
K298,591.574 K381,787.548 K288,579.946 K232,530.825 K458,656.826 K1,785,142.314 K170,231.780 K194,875.826 K217,440.440 K242,075.267 K204,957.393 K254,227.289
K298,590.454 K381,786.428 K288,578.826 K232,529.706 K458,655.706 K1,785,141.194 K170,230.659 K194,874.707 K217,439.321 K242,074.147 K204,956.273 K254,226.169
K298,639.184 K381,838.123 K288,628.121 K232,572.647 K458,700.226 K1,785,187.328 K170,270.932 K194,919.385 K217,482.726 K242,121.676 K205,000.211 K254,278.106
K298,570.534 K381,766.189 K288,557.839 K232,506.741 K458,633.549 K1,785,119.442 K170,208.829 K194,852.150 K217,414.011 K242,047.774 K204,930.637 K254,198.990
K298,569.414 K381,765.069 K288,556.719 K232,505.621 K458,632.429 K1,785,118.322 K170,207.710 K194,851.030 K217,412.891 K242,046.655 K204,929.518 K254,197.860
K298,617.265 K381,816.002 K288,605.367 K232,547.932 K458,676.325 K1,785,163.836 K170,247.395 K194,894.943 K217,455.798 K242,093.988 K204,972.766 K254,248.998
7-NO2 7-CF3 7-COOH 7-F 7-Cl 7-Br 7-H 7-CH3 7-OH 7-OCH3 7-NH2 7-N(CH3)2
K298,591.276 K381,782.144 K288,574.064 K232,534.548 K458,657.444 K1,785,142.850 K170,231.780 K194,871.810 K217,437.199 K242,071.299 K204,948.807 K254,219.462
K298,590.157 K381,781.024 K288,572.944 K232,533.428 K458,656.325 K1,785,141.730 K170,230.659 K194,870.690 K217,436.079 K242,070.179 K204,947.687 K254,218.342
K298,639.079 K381,833.660 K288,622.344 K232,576.157 K458,700.614 K1,785,187.649 K170,270.932 K194,914.768 K217,480.048 K242,118.207 K204,993.088 K254,270.035
K298,567.945 K381,763.886 K288,550.980 K232,509.807 K458,634.918 K1,785,120.920 K170,208.829 K194,849.431 K217,412.485 K242,046.561 K204,925.752 K254,195.879
K298,566.826 K381,762.766 K288,549.859 K232,508.686 K458,633.798 K1,785,119.799 K170,207.710 K194,848.312 K217,411.365 K242,045.441 K204,924.632 K254,194.759
K298,615.741 K381,813.358 K288,599.544 K232,551.015 K458,677.659 K1,785,165.276 K170,247.395 K194,891.918 K217,454.602 K242,092.864 K204,968.173 K254,245.821
M.Z. Kassaee et al. / Journal of Molecular Structure: THEOCHEM 763 (2006) 13–19 Table 2 The B3LYP/6-311CCG* calculated thermal energy separations, DE(1n-X)-(2n-X), enthalpy gaps, DH(1n-X)-(2n-X) and free energy splittings, DG(1n-X)-(2n-X), in kilocalorie per mole, between n-X-norbornadienes (1n-X), and their corresponding n-X-quadricyclanes (2n-X); where nZ1, 2 or 7 and XZ–NO2, –F, –Cl, –Br, –CF3, –COOH, –NMe2, –OMe, –OH and –Me n-X (nZ1, 2 or 7)
B3LYP/6-311CCG* DE(1n-X)-(2n-X)
DH(1n-X)-(2n-X)
DG(1n-X)-(2n-X)
1-NO2 1-CF3 1-COOH 1-F 1-Cl 1-Br 1-H 1-CH3 1-OH 1-OCH3 1-NH2 1-N(CH3)2
22.292 22.604 19.846 27.986 25.324 24.782 22.951 22.821 25.933 25.380 24.516 24.463
22.293 22.604 19.846 27.985 25.324 24.782 22.950 22.820 25.933 25.380 24.517 24.463
25.463 23.027 21.373 28.452 25.822 25.292 23.537 23.284 26.484 25.760 24.682 24.139
2-NO2 2-CF3 2-COOH 2-F 2-Cl 2-Br 2-H 2-CH3 2-OH 2-OCH3 2-NH2 2-N(CH3)2
21.039 21.359 22.107 24.084 23.277 22.871 22.951 23.676 26.429 27.492 26.755 28.304
21.040 21.359 22.107 24.085 23.277 22.872 22.950 23.677 26.429 27.492 26.755 28.305
21.919 22.121 22.754 24.715 23.901 23.492 23.537 24.442 26.928 27.688 27.446 29.108
7-NO2 7-CF3 7-COOH 7-F 7-Cl 7-Br 7-H 7-CH3 7-OH 7-OCH3 7-NH2 7-N(CH3)2
23.331 18.257 23.084 24.741 22.527 21.930 22.951 22.378 24.715 24.738 23.055 23.582
23.331 18.258 23.085 24.742 22.527 21.931 22.950 22.378 24.714 24.738 23.055 23.583
23.338 20.302 22.800 25.142 22.955 22.373 23.537 22.850 25.446 25.343 24.915 24.214
electron donating subsituents on photoabsorption are investigated for various positions of 1n-X and 2n-X (X attached at carbons C1, C2 or C7: nZ1, 2 or 7, respectively) (Scheme 1). The electronic and thermal energies (E), enthalpies (H) and
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Gibbs free energies (G) for full optimized structures of 1n-X and 2n-X are calculated at B3LYP/6-311CCG* level of theory. These ab initio and DFT levels proved to be appropriate owing to the size of molecules probed, and the consistency of the results obtained. For the sake of brevity, only the data acquired through the highest level of theory (B3LYP/6-311CCG*) are reported (Table 1). The extent of the photochemical and/or solar energy stored in this system is measured simply by calculating the energy difference between the ground states of 1n-X and 2n-X (DE(1n-X)-(2n-X)). In other words, increasing the stability of 1nX and/or destabilizing 2n-X results in the increase of the energy storage in the system. Evidently, there is no practical need to consider the excited states and/or the type(s) of the mechanism involved. The stablizing effects of –NO2, –CF3, –COOH, –NMe2, –NH2, –OMe, –OH, and –Me on the energies of both 1n-X and/or 2n-X, are the most when the subsituents are attached at C2 carbon (nZ2), and the least when they are attatched at C7 carbon atom (nZ7) (Table 1; Scheme 1). Another words, the stability of 1n-X and/or 2n-X seem to be the most when nonhalogen subsituents are attached at C2 followed by C1 and then C7 of either 1n-X and/or 2n-X (C2OC1OC7). The higher stability at C2 of 12-X is mainly attributed to the possibility of the extention of conjugation of CaC by the attached substituents. The stabilization of 22-X by both electron withdrawing and/or electron donating subsituents are mostly explained by the Walsh orbital model, where cyclopropyl rings may act both as good p donors and good p acceptors [27,40]. The photochemical and/or solar energy storage in the system increases the most by electron donating subsituents (DZ–NMe2, –NH2, –OMe, –OH, and Me) attached at C2 (12-X), and/or electron withdrawing groups (WZ–NO2, –F, –Cl, –Br, –CF3 and –COOH) attached at C1 (11-X) (Scheme 1). This conclusion is derived through the full optimized B3LYP/ 6-311CCG* calculations on the energy separations, DE(1n-X)(2n-X), enthalpy gaps, DH(1n-X)–(2n-X) and free energy splittings, DG(1n-X)–(2n-X), in kilocalorie per mole, between n-X-norbornadienes (1n-X), and their corresponding n-X-quadricyclanes (2n-X), for nZ1, 2 or 7 and XZ–NO2, –F, –Cl, –Br, –CF3, –COOH, –NMe2, –OMe, –OH and –Me (Table 2). The average increase of photochemical and/or solar energy storage, however, due to electron donating subsituents on C2 (12-X) is more than that of electron withdrawing groups on C1 (11-X)
Fig. 1. Reaction coordinate diagram for n-X-norbornadienes (1n-X) and n-X-quadricyclanes (2n-X); where nZ1 and 2, X is an electron withdrawing subsituent (a) and X is an electron donating subsituent (b).
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Fig. 2. Plot of DE(2n-X)-(1n-X) vs. sp for solar energy storages in norbornadiene–quadracycline systems for substituents (XZ–NO2, –F, –Cl, –Br, –CF3, –COOH, – NMe2, –OH, –Me and –H) attached at three different positions: (a) nZ2, rZK3.70, R2Z0.92; (b) nZ1, rZK1.44, R2Z0.13; and (c) nZ7, rZK1.21, R2Z0.15, where R2 is the correlation constant and r is the Hammet reaction constant [27].
(Fig. 1). The energy gap between 11-X and 12-X is smaller than that between the corresponding 21-X and 22-X, where X is an electron withdrawing (W) substituent (Fig. 1a). The energy gap between 11-X and 12-X is more than that between 21-X and 22-X,
where X is an electron donating substituent (D) (Fig. 1b). This justifies the higher storage of energy when W groups are attached at C1, and/or when D groups are attached at C2. Again, energy gap and the extent of photochemical and/or solar energy
Fig. 3. Plot of the Hammet substituent constant, s [27], vs. energies (eV) of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) for (a) 12-X, slope of HOMOZK0.034 with R2Z0.94 and slope of LUMOZK0.036 with R2Z0.55 as well as (b) 22-X, slope of HOMOZK0.018 with R2Z0.61 and slope of LUMOZK0.018 with R2Z0.24; where XZ–NO2, –F, –Cl, –Br, –CF3, –COOH, –NMe2, –OH, –Me and –H; R2 is the correlation constant.
M.Z. Kassaee et al. / Journal of Molecular Structure: THEOCHEM 763 (2006) 13–19
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Fig. 4. (a) Plot of the Hammet substituent constant, s [27], vs. the energy differences (eV) between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) for 12-X, with slopes 0.021 (correlation constant, R2Z0.94) and K0.052 (R2Z0.64) for electron donating and electron withdrawing groups, respectively. (b) Plot of s vs. energy (HOMO–LUMO) for 22-X, with slopes 0.003 (R2Z0.28) and K0.045 (R2Z0.33) for electron donating and electron withdrawing groups, respectively.
storage in 12-X/22-X system are generally more than those in either 11-X/21-X and/or 17-X/27-X systems. Rather linear free energy relationships are encountered through plotting the energy gaps between 1n-X and 2n-X vs. the Hammet sp constants (Fig. 2). A Hammet r value of K3.70 is observed with a correlation factor R2Z0.92 for substituents (X) attached at C2 carbon: nZ2, where XZ–NO2, –F, –Cl, – Br, –CF3, –COOH, –NMe2, –OH, –Me and –H (Fig. 2a). This is in contrast to the r values of K1.44 (Fig. 2b, nZ1, R2Z0.13) and K1.21 (Fig. 2c, nZ7, R2Z0.15) observed for X attached at C1 and C7, respectively. Therefore, both the sensitivity to electronic effects r and the correlation constants (R2) are higher for X attached at C2 compared to C1 and/or C7.
A plot of the highest occupied molecular orbital (HOMO) energies (eV) of 12-X vs. the Hammet substituent constant, s [27], for substituents (X) show a slope of K0.034 with a correlation (R2) of 0.94; indicating stabilization of HOMO by the more electron withdrawing substituents and its destabilizing by the electron donating substituents (Fig. 3a). Plotting LUMO energies (eV) of 12-X vs. s for X show a similar slope of K0.036 but with a lower R2Z0.55 (Fig. 3a). In the same way, a plot of the HOMO energies (eV) of 22-X vs. the s for substituents (X) show a slope of K0.018 with R2Z0.61; which is half the value observed for the slope of HOMO of 12-X (Fig. 3b). This indicates the lower electronic sensitivity of the HOMO of 22-X in comparison to 12-X. Similarly, plotting
Table 3 The B3LYP/6-311CCG* calculated the NBO charges on carbon atoms (Ci, for iZ1–7), for 2-substituted norbornadienes (12-X), and their corresponding 2substituted quadricyclanes (22-X); where XZ–NO2, –F, –Cl, –Br, –CF3, –COOH, –NMe2, –OMe, –OH, –Me and –H Compound
Total atomic charges on carbon atoms (Ci, iZ1–7) C1
C2
C3
C4
C5
C6
C7
12-NO2 12-F 12-CF3 12-COOH 12-Cl 12-Br 12-H 12-CH3 12-OH 12-OCH3 12-N(CH3)2
K0.284 K0.314 K0.271 K0.267 K0.287 K0.289 K0.289 K0.278 K0.292 K0.289 K0.281
0.092 0.433 K0.131 K0.142 K0.042 K0.112 K0.183 0.001 K0.347 0.354 0.211
K0.084 K0.284 K0.108 K0.074 K0.214 K0.211 K0.183 K0.199 K0.329 K0.334 K0.297
K0.286 K0.271 K0.287 K0.292 K0.273 K0.273 K0.289 K0.282 K0.269 K0.271 K0.274
K0.180 K0.177 K0.181 K0.188 K0.176 K0.174 K0.183 K0.179 K0.178 K0.179 K0.174
K0.172 K0.185 K0.175 K0.169 K0.184 K0.185 K0.183 K0.189 K0.187 K0.187 K0.194
K0.314 K0.315 K0.315 K0.316 K0.315 K0.315 K0.317 K0.317 K0.318 K0.318 K0.322
22-NO2 22-F 22-CF3 22-COOH 22-Cl 22-H 22-CH3 22-Br 22-OH 22-N(CH3)2
K0.193 K0.243 K0.195 K0.184 K0.248 K0.224 K0.216 K0.218 K0.222 K0.225
0.070 0.371 K0.168 K0.172 K0.083 K0.218 K0.065 K0.150 K0.263 0.156
K0.209 K0.244 K0.198 K0.194 K0.218 K0.218 K0.210 K0.217 K0.241 K0.222
K0.215 K0.216 K0.218 K0.220 K0.216 K0.224 K0.222 K0.216 K0.217 K0.219
K0.212 K0.214 K0.213 K0.219 K0.212 K0.218 K0.217 K0.212 K0.220 K0.221
K0.177 K0.244 K0.185 K0.165 K0.213 K0.218 K0.215 K0.212 K0.245 K0.245
K0.376 K0.377 K0.376 K0.375 K0.378 K0.377 K0.375 K0.377 K0.377 K0.376
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LUMO energies (eV) of 22-X vs. s for X show the same slope of K0.018 but with a much lower R2 of 0.24 (Fig. 3b). Hence, both the HOMO and LUMO of 12-X are more affected by electronic effects of substituents than those of 22-X. In another investigation, plotting the s vs. the energy differences (eV) between the HOMO and the lowest unoccupied molecular orbital (LUMO) for 12-X, show two slopes of 0.021 (correlation constant, R2Z0.94) and K0.052 (R2Z0.64) for electron donating and electron withdrawing groups, respectively (Fig. 4a). Similarly, plotting of s vs. energy (HOMO– LUMO) for 22-X, show two slopes of 0.003 (R2Z0.28) and K0.045 (R2Z0.33) for electron donating and electron withdrawing groups, correspondingly (Fig. 4b). Two points appear noticeable: firstly, both electron donating as well as electron withdrawing substituents lower the gap of energy between HOMO and LUMO in 12-X and 22-X. Secondly, the HOMO– LUMO energy separation is more sensitive to electronic effects in 12-X than 22-X. Pictorial effects of –NH2 and –NO2 on the HOMO and LUMO of 12-H and 22-H are represented through Hyperchem diagrams of their HOMO and LUMO (Fig. S1, supplementray information). For either HOMO or LUMO of 12-H and/or 22-H the orbital distributions appear fairly symmetrical. Substituations of –NO2 or –NH2 instead of –H at C2 of 12-H and/or 22-H distort both of thier HOMOs and LUMOs. Distorsions of symmetry appear more noticeable in the LUMOs than their corresponding HOMOs. Interestingly, C2 is more sensitive to the substitutent effects than C1 and/or C7 in norbornadiene (1) and quadricyclane (2) system. Hence, only changes of charge and geometrical parameters of 12-X and 22-X as a function of X are reported in this manuscript (Tables 3 and 4 and Tables S1–S3 in supplementary section). In 12-X, the increasing trend of NBO
charges on C3, as a function of X (attached at C2) is: 1OCH3! 1OH!1N(CH3)2!1F!1Cl!1Br!1CH3!1H!1CF3!1COOH! 1NO2 (Table 3). This trend more or less follows the order of Hammet spara substituent constants [41]. The lowest charge on C3 is observed for XZ–OCH3, while the highest charge is encountered for XZ–NO2. Apparently, direct resonance of – N(CH3)2 with C2aC3 places an electron density on C3, while – NO2 depletes electron density from the same carbon, through a similar electron delocalization process. Direct resonance could also be held responsible for placing electron density on C3, when XZ–OH, –N(CH3)2, and/or –F with a decreasing trend of importants, respectively. A relatively higher positive charge on C3 is observed for XZ–COOH, where the possible direct resonance places a positive charge on C3 (Table 3). The theory of atoms in molecules (AIM) charges on carbon atoms (Ci, for iZ1–7), are also calculated for 2-substituted norbornadienes (12-X), and their corresponding 2-substituted quadricyclanes (22-X) at B3LYP/6-311CCG* level of theory. In 12-X, the increasing trend of AIM charges on C3, as a function of X (attached at C2) is: 1NH2!1OH!1CH3!1H! 1F!1Br!1Cl!1COOH!1NO2 (Table 4). Again, the lowest charge on C3 is observed for XZ–NH2, while the highest charge is encountered for XZ–NO2. Direct electron donating resonance of –NH2 with C2aC3 lead to electron density on C3, while –NO2 withdraw electron density from the same carbon. Hyperconjugation also be held responsible for placing electron density on C3, when XZ–Me with a decreasing trend of importants. A relatively higher positive charge on C3 is observed for XZ–COOH, where the possible direct resonance places a positive charge on C3. The B3LYP/6-311CCG* calculated geometrical parameters are reported for 2-substituted norbornadienes (12-X), and 2-substituted quadricyclanes (22-X); where XZ–NO2, –F,
Table 4 The B3LYP/6-311CCG* calculated the theory of atoms in molecules (AIM) charges on carbon atoms (Ci, for iZ1–7), for 2-substituted norbornadienes (12-X), and their corresponding 2-substituted quadricyclanes (22-X); where XZ–NO2, –F, –Cl, –Br, –CF3, –COOH, –NH2, –OH, –Me and H Compound
Total atomic charges on carbon atoms (Ci, iZ1–7) C1
12-NO2 12-F 12-CF3 12-COOH 12-Cl 12-Br 12-H 12-CH3 12-OH 12-NH2 22-NO2 22-F 22-CF3 22-COOH 22-Cl 22-H 22-CH3 22-Br 22-OH 22-NH2
C2
0.043 0.021
0.266 0.197
–
–
0.019 0.016 0.017 0.002 K0.009 0.010 0.002
0.091 0.017 K0.123 K0.087 K0.039 0.461 0.347
K0.019 K0.049 – K0.029 K0.036 K0.052 K0.059 K0.034 K0.043 K0.049
0.242 0.506 – K0.054 0.061 K0.075 K0.008 K0.076 0.504 0.352
C3 K0.026 K0.044 – K0.031 K0.040 K0.041 K0.087 K0.091 K0.090 K0.092 K0.047 K0.074 – K0.057 K0.063 K0.075 K0.087 K0.061 K0.091 K0.098
C4 0.029 0.011 – 0.009 0.010 0.008 0.002 0.002 0.016 0.017 K0.038 K0.045 – K0.046 K0.045 K0.052 K0.050 K0.045 K0.047 K0.048
C5 K0.079 K0.086 – K0.0085 K0.085 K0.081 K0.087 K0.089 K0.089 K0.085 K0.065 K0.070 – K0.073 K0.069 K0.075 K0.076 K0.069 K0.075 K0.073
C6
C7 0.020 0.003
K0.069 K0.087 – K084 K0.086 K0.090 K0.087 K0.094 K0.091 K0.090
0.003 0.003 0.002 0.004 K0.001 K0.001 0.001
K0.042 K0.082
0.013 0.007
–
–
– K0.051 K0.062 K0.075 K0.084 K0.059 K0.100 K0.077
0.010 0.008 0.007 0.006 0.008 0.006 0.008
M.Z. Kassaee et al. / Journal of Molecular Structure: THEOCHEM 763 (2006) 13–19
–Cl, –Br, –CF3, –COOH, –NMe2, –OMe, –OH and –Me. of 12X and 22-X (Tables S1–S3 in the supplementary section). These geometrical parameters consist of carbon–carbon interatomic ˆ –C interatomic angles ˚ ); C–C distances (Rc,c) in angstrom (A ˆ (Ac,c,c) in degrees; and interatomic dihedral angles (Dc,c,c,c) in degrees. The increasing trend of C2–C3 (R2,3) bond length, as a function of X, attached at C2, in 12-X is: 1N(CH3)2O1OCH3O 1NO2O1CH3O1HO1CF3O1ClO1BrO1F. Justification of this trend may simply be attributed to the resonance ability of X to inflict single bond character to C2–C3 bond. This trend is almost similar to the increasing trend of C2–C6 (R2,6) bond length, as a function of C2 attached X in 22-X, where 2N(CH3)2O 2NO2O2OCH3O2HO2CF3O2COOHO2ClO2BrO2F. The s character of C2 for 12-X and 22-X is increased with electron withdrawing substituents leading to large bond angle of C1– C2–C3. Dihedral angles, C1–C2–C3–C4 and C4–C5–C6–C1, of both 12-X and 22-X, are increased by either electron withdrawing or electron donating groups, due to distortions of the symmetry imposed by substituents. 4. Conclusion The extent of the photochemical and/or solar energy stored in norbornadiene/quadricyclane system (1n-X/2n-X) is measured by calculating the energy difference between the ground states of 1n-X and 2n-X (DE(1n-X)-(2n-X)). The electron donating (D) subsituents which are located on C2 (12-X), as well as the electron withdrawing (W) groups on C1 (11-X), appear to increase photochemical and/or solar energy storage. However, the storage energy impact of D subsituents connected to C2 (12-X) is more than those of W groups attached at C1 (11-X). Introducing substituents on C7 (17-X) has a minimal effect on the energy storage. Acknowledgements We wish to express our special thanks to Dr T. Partovi from Payam-e-nor University and Dr. A.R. Bekhradnia at Tarbiat Modarres University for their cordial cooperation in completing this research. Appendix. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.theochem.2006. 01.010.
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Energetics of photoconversion of norbornadiene to quadricyclane: Effects of directly attached substituents via ab initio and DFT M.Z. Kassaee *,a, E. Vessally b, S. Arshadi c a
Department of Chemistry, Tarbiat Modarres University, Tehran, Iran b Islamic Azad University, Myianeh Branch, Myianeh, Iran c Payame Noor University, Behshahr Branch, Behshahr, Iran
Received 30 May 2005; received in revised form 20 November 2005; accepted 3 January 2006 Available online 10 March 2006
Abstract An attempt is made to maximize the photochemical and/or solar energy storage in norbornadiene (1)/quadricyclane (2) system, through direct attachment of substituents at C1, C2 or C7 atoms of 1 and 2. Assessments of the corresponding energies are made at B3LYP/6-311CCG* level of theory. Electron donating substituents, D (DZ–NMe2, –NH2, –OMe, –OH, and Me), directly attached at C2, increase the energy gap between 1 and 2, inducing higher storage of energy in the system. Electron withdrawing subsituents, W (WZ–NO2, –F, –Cl, –Br, –CF3 and –COOH), directly attached at C1, moderately induce the energy storage. Attachment of either D or W groups at C7 show no significant difference in the photochemical and/or solar energy storages. A Hammet r value of K3.69 is encountered for substituents directly attached at C2 atoms of 1 and 2. This is in clear contrast to the Hammet r value of K0.81 that we recently reported for the subsituents indirectly placed at the C2 of 1 and/or 2 (through attachment via the para carbon of phenyl rings). q 2006 Elsevier B.V. All rights reserved. Keywords: Photochemical energy; Solar energy; Energy storage; Norbornadiene; Quadricyclane; Electron donating; Electron withdrawing; Substituents; Ab initio; DFT; B3LYP
1. Introduction Solving today’s energy problems through the employment of the solar energy has received much attention [1]. Intramolecular photochemical [2p–2p]-cycloaddition of norbornadiene, 1, to quadricyclane, 2, has been studied as a mechanistic point of view [1–4]. The 1/2 system is used for solar energy storage [5–7], in molecular switching [8–10], in optoelectronic devices [11–14], as a data storage compound [15,16], as photodynamic chemosensor for metal cations [17,18], as a potential photoresponsive organic magnet [19–21] and as an energetic binder for solid rocket propellants [22]. This system has an inherent disadvantage, since 1 cannot absorb visible wavelength of sunlight. Usage of sensitizers and chromophores are two improvements for solving this problem. Iridium complex is proposed as the sensitizer for p–p* excitation [4]. The donor–acceptor chromophores are placed at the double bond of the norbornadiene molecule. The water-soluble carbamoyl and carboxyl derivatives of 1 and 2 are also used to absorb light of * Corresponding author. Tel.: C98 9121000392; fax: C982188006544. E-mail address: [email protected] (M.Z. Kassaee).
0166-1280/$ - see front matter q 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.theochem.2006.01.010
wavelengths longer than 300 nm [23]. Ab initio is used to study energetic of 1 and 2 conversions [4,24,25]. Density functional calculations with the hybrid B3LYP functional have been used to study the ground state of 1 bound to the photosensitizer [Cu(8-oxoquinolinato)] [26]. Recently, we reported a theoretical investigation on the electronic effects involved in the solar energy storage, for substituents ‘indirectly’ attached to the C2 of 1 and/or 2, where a Hammet r value of K0.81 was encountered [27]. However, the effects of ‘direct’ attachment of the subsituents at C1, C2 or C7 atoms of 1 and 2, which appear of ‘practical interest’ to those whose primary goal is to replace the fossil fuel and/or the nuclear energy with the most economical and very available solar energy, were not investigated. In this manuscript, we scrutinize the photochemical energy storage in the ground states of 1/2 system with substituents directly attached at C1, C2 or C7 atoms of 1 and 2. Computations and/or measurement of the quantum yields of the scrutinized photolysis, which come into view as ‘theoretical interest’ [28–32], are not addressed here. 2. Computational methods The molecular structures of Cn-substituted norbornadienes (1n-X) and Cn-substituted quadricyclanes (2n-X), are studied
14
M.Z. Kassaee et al. / Journal of Molecular Structure: THEOCHEM 763 (2006) 13–19
Scheme 1. Storage of solar energy in n-X-norbornadiene (1n-X)/n-Xquadricyclane (2n-X) system; where nZ1, 2 or 7 and XZ–NO2, –F, –Cl, – Br, –CF3, –COOH, –NMe2, –OMe, –OH, –Me.
using ab initio and DFT methods (Scheme 1). Full geometry optimizations are carried out at B3LYP/6-311CCG* level of theory, using the GAUSSIAN 98 system of programs [33–35]. In order to find energy minima, keyword ‘FOPT’ are used. This keyword requests that a geometry optimization be performed. The geometry will be adjusted until a stationary point on the potential surface is found. Here, the Berny algorithm is employed for all minimizations using redundant internal
coordinates [36]. For minimum state structures, only real frequency values are accepted. The calculations exhibit systematic errors and thus benefit from scaling. Thermodynamic functions obtained through frequency calculations, are multiplied by Hehre et al. [37] scaling factor of 0.89 for HF; and by 0.99 scaling factor of Rauhut and Pulay [38] for B3LYP. This is to account for the difference between the harmonic vibrational calculations and the anharmonic oscillations of the actual bonds. Nevertheless, scaling factors fitted to observe (anharmonic) frequencies will deviate from unity even for exact calculations. Here, a set of molecules containing similar motifs are treated together, where they benefit from similar scalings. The bond hybridizations of 12-H, 12-NH2, and 12-NO2 are calculated using NBO program [39]. 3. Results and discussion In this section, an overview of results are presented before their discussion. The effects of both electron withdrawing and
Table 1 The B3LYP/6-311CCG* thermal and electronic energies (E), enthalpies (H), and Gibbs free energies (G), in kilocalorie per mole, for optimized n-Xnorbornadienes (1n-X) and n-X-quadricyclanes (2n-X); where nZ1, 2 or 7, while XZ–NO2, –F, –Cl, –Br, –CF3, –COOH, –NMe2, –OMe, –OH and –Me n-X (nZ1, 2, 7)
2n-X
1n-X E (kcal molK1)
H (kcal molK1)
G (kcal molK1)
E (kcal molK1)
H (kcal molK1)
G (kcal molK1)
1-NO2 1-CF3 1-COOH 1-F 1-Cl 1-Br 1-H 1-CH3 1-OH 1-OCH3 1-NH2 1-N(CH3)2
K298,590.434 K381,787.200 K288,575.034 K232,534.136 K458,657.708 K1,785,142.860 K170,231.780 K194,873.900 K217,438.482 K242,071.265 K204,951.675 K254,220.042
K298,589.315 K381,786.080 K288,573.914 K232,533.016 K458,656.588 K1,785,141.740 K170,230.659 K194,872.779 K217,437.362 K242,070.146 K204,950.555 K254,218.923
K298,640.247 K381,837.130 K288,623.980 K232,575.637 K458,700.781 K1,785,187.560 K170,270.932 K194,916.934 K217,480.777 K242,117.744 K204,994.416 K254,270.140
K298,568.142 K381,764.595 K288,555.188 K232,506.151 K458,632.384 K1,785,118.078 K170,208.829 K194,851.079 K217,412.549 K242,045.885 K204,927.159 K254,195.580
K298,567.022 K381,763.475 K288,554.068 K232,505.031 K458,631.264 K1,785,116.959 K170,207.710 K194,849.959 K217,411.429 K242,044.766 K204,926.038 K254,194.460
K298,614.785 K381,814.103 K288,602.607 K232,547.185 K458,674.959 K1,785,162.264 K170,247.395 K194,893.651 K217,454.293 K242,091.984 K204,969.734 K254,246.001
2-NO2 2-CF3 2-COOH 2-F 2-Cl 2-Br 2-H 2-CH3 2-OH 2-OCH3 2-NH2 2-N(CH3)2
K298,591.574 K381,787.548 K288,579.946 K232,530.825 K458,656.826 K1,785,142.314 K170,231.780 K194,875.826 K217,440.440 K242,075.267 K204,957.393 K254,227.289
K298,590.454 K381,786.428 K288,578.826 K232,529.706 K458,655.706 K1,785,141.194 K170,230.659 K194,874.707 K217,439.321 K242,074.147 K204,956.273 K254,226.169
K298,639.184 K381,838.123 K288,628.121 K232,572.647 K458,700.226 K1,785,187.328 K170,270.932 K194,919.385 K217,482.726 K242,121.676 K205,000.211 K254,278.106
K298,570.534 K381,766.189 K288,557.839 K232,506.741 K458,633.549 K1,785,119.442 K170,208.829 K194,852.150 K217,414.011 K242,047.774 K204,930.637 K254,198.990
K298,569.414 K381,765.069 K288,556.719 K232,505.621 K458,632.429 K1,785,118.322 K170,207.710 K194,851.030 K217,412.891 K242,046.655 K204,929.518 K254,197.860
K298,617.265 K381,816.002 K288,605.367 K232,547.932 K458,676.325 K1,785,163.836 K170,247.395 K194,894.943 K217,455.798 K242,093.988 K204,972.766 K254,248.998
7-NO2 7-CF3 7-COOH 7-F 7-Cl 7-Br 7-H 7-CH3 7-OH 7-OCH3 7-NH2 7-N(CH3)2
K298,591.276 K381,782.144 K288,574.064 K232,534.548 K458,657.444 K1,785,142.850 K170,231.780 K194,871.810 K217,437.199 K242,071.299 K204,948.807 K254,219.462
K298,590.157 K381,781.024 K288,572.944 K232,533.428 K458,656.325 K1,785,141.730 K170,230.659 K194,870.690 K217,436.079 K242,070.179 K204,947.687 K254,218.342
K298,639.079 K381,833.660 K288,622.344 K232,576.157 K458,700.614 K1,785,187.649 K170,270.932 K194,914.768 K217,480.048 K242,118.207 K204,993.088 K254,270.035
K298,567.945 K381,763.886 K288,550.980 K232,509.807 K458,634.918 K1,785,120.920 K170,208.829 K194,849.431 K217,412.485 K242,046.561 K204,925.752 K254,195.879
K298,566.826 K381,762.766 K288,549.859 K232,508.686 K458,633.798 K1,785,119.799 K170,207.710 K194,848.312 K217,411.365 K242,045.441 K204,924.632 K254,194.759
K298,615.741 K381,813.358 K288,599.544 K232,551.015 K458,677.659 K1,785,165.276 K170,247.395 K194,891.918 K217,454.602 K242,092.864 K204,968.173 K254,245.821
M.Z. Kassaee et al. / Journal of Molecular Structure: THEOCHEM 763 (2006) 13–19 Table 2 The B3LYP/6-311CCG* calculated thermal energy separations, DE(1n-X)-(2n-X), enthalpy gaps, DH(1n-X)-(2n-X) and free energy splittings, DG(1n-X)-(2n-X), in kilocalorie per mole, between n-X-norbornadienes (1n-X), and their corresponding n-X-quadricyclanes (2n-X); where nZ1, 2 or 7 and XZ–NO2, –F, –Cl, –Br, –CF3, –COOH, –NMe2, –OMe, –OH and –Me n-X (nZ1, 2 or 7)
B3LYP/6-311CCG* DE(1n-X)-(2n-X)
DH(1n-X)-(2n-X)
DG(1n-X)-(2n-X)
1-NO2 1-CF3 1-COOH 1-F 1-Cl 1-Br 1-H 1-CH3 1-OH 1-OCH3 1-NH2 1-N(CH3)2
22.292 22.604 19.846 27.986 25.324 24.782 22.951 22.821 25.933 25.380 24.516 24.463
22.293 22.604 19.846 27.985 25.324 24.782 22.950 22.820 25.933 25.380 24.517 24.463
25.463 23.027 21.373 28.452 25.822 25.292 23.537 23.284 26.484 25.760 24.682 24.139
2-NO2 2-CF3 2-COOH 2-F 2-Cl 2-Br 2-H 2-CH3 2-OH 2-OCH3 2-NH2 2-N(CH3)2
21.039 21.359 22.107 24.084 23.277 22.871 22.951 23.676 26.429 27.492 26.755 28.304
21.040 21.359 22.107 24.085 23.277 22.872 22.950 23.677 26.429 27.492 26.755 28.305
21.919 22.121 22.754 24.715 23.901 23.492 23.537 24.442 26.928 27.688 27.446 29.108
7-NO2 7-CF3 7-COOH 7-F 7-Cl 7-Br 7-H 7-CH3 7-OH 7-OCH3 7-NH2 7-N(CH3)2
23.331 18.257 23.084 24.741 22.527 21.930 22.951 22.378 24.715 24.738 23.055 23.582
23.331 18.258 23.085 24.742 22.527 21.931 22.950 22.378 24.714 24.738 23.055 23.583
23.338 20.302 22.800 25.142 22.955 22.373 23.537 22.850 25.446 25.343 24.915 24.214
electron donating subsituents on photoabsorption are investigated for various positions of 1n-X and 2n-X (X attached at carbons C1, C2 or C7: nZ1, 2 or 7, respectively) (Scheme 1). The electronic and thermal energies (E), enthalpies (H) and
15
Gibbs free energies (G) for full optimized structures of 1n-X and 2n-X are calculated at B3LYP/6-311CCG* level of theory. These ab initio and DFT levels proved to be appropriate owing to the size of molecules probed, and the consistency of the results obtained. For the sake of brevity, only the data acquired through the highest level of theory (B3LYP/6-311CCG*) are reported (Table 1). The extent of the photochemical and/or solar energy stored in this system is measured simply by calculating the energy difference between the ground states of 1n-X and 2n-X (DE(1n-X)-(2n-X)). In other words, increasing the stability of 1nX and/or destabilizing 2n-X results in the increase of the energy storage in the system. Evidently, there is no practical need to consider the excited states and/or the type(s) of the mechanism involved. The stablizing effects of –NO2, –CF3, –COOH, –NMe2, –NH2, –OMe, –OH, and –Me on the energies of both 1n-X and/or 2n-X, are the most when the subsituents are attached at C2 carbon (nZ2), and the least when they are attatched at C7 carbon atom (nZ7) (Table 1; Scheme 1). Another words, the stability of 1n-X and/or 2n-X seem to be the most when nonhalogen subsituents are attached at C2 followed by C1 and then C7 of either 1n-X and/or 2n-X (C2OC1OC7). The higher stability at C2 of 12-X is mainly attributed to the possibility of the extention of conjugation of CaC by the attached substituents. The stabilization of 22-X by both electron withdrawing and/or electron donating subsituents are mostly explained by the Walsh orbital model, where cyclopropyl rings may act both as good p donors and good p acceptors [27,40]. The photochemical and/or solar energy storage in the system increases the most by electron donating subsituents (DZ–NMe2, –NH2, –OMe, –OH, and Me) attached at C2 (12-X), and/or electron withdrawing groups (WZ–NO2, –F, –Cl, –Br, –CF3 and –COOH) attached at C1 (11-X) (Scheme 1). This conclusion is derived through the full optimized B3LYP/ 6-311CCG* calculations on the energy separations, DE(1n-X)(2n-X), enthalpy gaps, DH(1n-X)–(2n-X) and free energy splittings, DG(1n-X)–(2n-X), in kilocalorie per mole, between n-X-norbornadienes (1n-X), and their corresponding n-X-quadricyclanes (2n-X), for nZ1, 2 or 7 and XZ–NO2, –F, –Cl, –Br, –CF3, –COOH, –NMe2, –OMe, –OH and –Me (Table 2). The average increase of photochemical and/or solar energy storage, however, due to electron donating subsituents on C2 (12-X) is more than that of electron withdrawing groups on C1 (11-X)
Fig. 1. Reaction coordinate diagram for n-X-norbornadienes (1n-X) and n-X-quadricyclanes (2n-X); where nZ1 and 2, X is an electron withdrawing subsituent (a) and X is an electron donating subsituent (b).
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M.Z. Kassaee et al. / Journal of Molecular Structure: THEOCHEM 763 (2006) 13–19
Fig. 2. Plot of DE(2n-X)-(1n-X) vs. sp for solar energy storages in norbornadiene–quadracycline systems for substituents (XZ–NO2, –F, –Cl, –Br, –CF3, –COOH, – NMe2, –OH, –Me and –H) attached at three different positions: (a) nZ2, rZK3.70, R2Z0.92; (b) nZ1, rZK1.44, R2Z0.13; and (c) nZ7, rZK1.21, R2Z0.15, where R2 is the correlation constant and r is the Hammet reaction constant [27].
(Fig. 1). The energy gap between 11-X and 12-X is smaller than that between the corresponding 21-X and 22-X, where X is an electron withdrawing (W) substituent (Fig. 1a). The energy gap between 11-X and 12-X is more than that between 21-X and 22-X,
where X is an electron donating substituent (D) (Fig. 1b). This justifies the higher storage of energy when W groups are attached at C1, and/or when D groups are attached at C2. Again, energy gap and the extent of photochemical and/or solar energy
Fig. 3. Plot of the Hammet substituent constant, s [27], vs. energies (eV) of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) for (a) 12-X, slope of HOMOZK0.034 with R2Z0.94 and slope of LUMOZK0.036 with R2Z0.55 as well as (b) 22-X, slope of HOMOZK0.018 with R2Z0.61 and slope of LUMOZK0.018 with R2Z0.24; where XZ–NO2, –F, –Cl, –Br, –CF3, –COOH, –NMe2, –OH, –Me and –H; R2 is the correlation constant.
M.Z. Kassaee et al. / Journal of Molecular Structure: THEOCHEM 763 (2006) 13–19
17
Fig. 4. (a) Plot of the Hammet substituent constant, s [27], vs. the energy differences (eV) between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) for 12-X, with slopes 0.021 (correlation constant, R2Z0.94) and K0.052 (R2Z0.64) for electron donating and electron withdrawing groups, respectively. (b) Plot of s vs. energy (HOMO–LUMO) for 22-X, with slopes 0.003 (R2Z0.28) and K0.045 (R2Z0.33) for electron donating and electron withdrawing groups, respectively.
storage in 12-X/22-X system are generally more than those in either 11-X/21-X and/or 17-X/27-X systems. Rather linear free energy relationships are encountered through plotting the energy gaps between 1n-X and 2n-X vs. the Hammet sp constants (Fig. 2). A Hammet r value of K3.70 is observed with a correlation factor R2Z0.92 for substituents (X) attached at C2 carbon: nZ2, where XZ–NO2, –F, –Cl, – Br, –CF3, –COOH, –NMe2, –OH, –Me and –H (Fig. 2a). This is in contrast to the r values of K1.44 (Fig. 2b, nZ1, R2Z0.13) and K1.21 (Fig. 2c, nZ7, R2Z0.15) observed for X attached at C1 and C7, respectively. Therefore, both the sensitivity to electronic effects r and the correlation constants (R2) are higher for X attached at C2 compared to C1 and/or C7.
A plot of the highest occupied molecular orbital (HOMO) energies (eV) of 12-X vs. the Hammet substituent constant, s [27], for substituents (X) show a slope of K0.034 with a correlation (R2) of 0.94; indicating stabilization of HOMO by the more electron withdrawing substituents and its destabilizing by the electron donating substituents (Fig. 3a). Plotting LUMO energies (eV) of 12-X vs. s for X show a similar slope of K0.036 but with a lower R2Z0.55 (Fig. 3a). In the same way, a plot of the HOMO energies (eV) of 22-X vs. the s for substituents (X) show a slope of K0.018 with R2Z0.61; which is half the value observed for the slope of HOMO of 12-X (Fig. 3b). This indicates the lower electronic sensitivity of the HOMO of 22-X in comparison to 12-X. Similarly, plotting
Table 3 The B3LYP/6-311CCG* calculated the NBO charges on carbon atoms (Ci, for iZ1–7), for 2-substituted norbornadienes (12-X), and their corresponding 2substituted quadricyclanes (22-X); where XZ–NO2, –F, –Cl, –Br, –CF3, –COOH, –NMe2, –OMe, –OH, –Me and –H Compound
Total atomic charges on carbon atoms (Ci, iZ1–7) C1
C2
C3
C4
C5
C6
C7
12-NO2 12-F 12-CF3 12-COOH 12-Cl 12-Br 12-H 12-CH3 12-OH 12-OCH3 12-N(CH3)2
K0.284 K0.314 K0.271 K0.267 K0.287 K0.289 K0.289 K0.278 K0.292 K0.289 K0.281
0.092 0.433 K0.131 K0.142 K0.042 K0.112 K0.183 0.001 K0.347 0.354 0.211
K0.084 K0.284 K0.108 K0.074 K0.214 K0.211 K0.183 K0.199 K0.329 K0.334 K0.297
K0.286 K0.271 K0.287 K0.292 K0.273 K0.273 K0.289 K0.282 K0.269 K0.271 K0.274
K0.180 K0.177 K0.181 K0.188 K0.176 K0.174 K0.183 K0.179 K0.178 K0.179 K0.174
K0.172 K0.185 K0.175 K0.169 K0.184 K0.185 K0.183 K0.189 K0.187 K0.187 K0.194
K0.314 K0.315 K0.315 K0.316 K0.315 K0.315 K0.317 K0.317 K0.318 K0.318 K0.322
22-NO2 22-F 22-CF3 22-COOH 22-Cl 22-H 22-CH3 22-Br 22-OH 22-N(CH3)2
K0.193 K0.243 K0.195 K0.184 K0.248 K0.224 K0.216 K0.218 K0.222 K0.225
0.070 0.371 K0.168 K0.172 K0.083 K0.218 K0.065 K0.150 K0.263 0.156
K0.209 K0.244 K0.198 K0.194 K0.218 K0.218 K0.210 K0.217 K0.241 K0.222
K0.215 K0.216 K0.218 K0.220 K0.216 K0.224 K0.222 K0.216 K0.217 K0.219
K0.212 K0.214 K0.213 K0.219 K0.212 K0.218 K0.217 K0.212 K0.220 K0.221
K0.177 K0.244 K0.185 K0.165 K0.213 K0.218 K0.215 K0.212 K0.245 K0.245
K0.376 K0.377 K0.376 K0.375 K0.378 K0.377 K0.375 K0.377 K0.377 K0.376
18
M.Z. Kassaee et al. / Journal of Molecular Structure: THEOCHEM 763 (2006) 13–19
LUMO energies (eV) of 22-X vs. s for X show the same slope of K0.018 but with a much lower R2 of 0.24 (Fig. 3b). Hence, both the HOMO and LUMO of 12-X are more affected by electronic effects of substituents than those of 22-X. In another investigation, plotting the s vs. the energy differences (eV) between the HOMO and the lowest unoccupied molecular orbital (LUMO) for 12-X, show two slopes of 0.021 (correlation constant, R2Z0.94) and K0.052 (R2Z0.64) for electron donating and electron withdrawing groups, respectively (Fig. 4a). Similarly, plotting of s vs. energy (HOMO– LUMO) for 22-X, show two slopes of 0.003 (R2Z0.28) and K0.045 (R2Z0.33) for electron donating and electron withdrawing groups, correspondingly (Fig. 4b). Two points appear noticeable: firstly, both electron donating as well as electron withdrawing substituents lower the gap of energy between HOMO and LUMO in 12-X and 22-X. Secondly, the HOMO– LUMO energy separation is more sensitive to electronic effects in 12-X than 22-X. Pictorial effects of –NH2 and –NO2 on the HOMO and LUMO of 12-H and 22-H are represented through Hyperchem diagrams of their HOMO and LUMO (Fig. S1, supplementray information). For either HOMO or LUMO of 12-H and/or 22-H the orbital distributions appear fairly symmetrical. Substituations of –NO2 or –NH2 instead of –H at C2 of 12-H and/or 22-H distort both of thier HOMOs and LUMOs. Distorsions of symmetry appear more noticeable in the LUMOs than their corresponding HOMOs. Interestingly, C2 is more sensitive to the substitutent effects than C1 and/or C7 in norbornadiene (1) and quadricyclane (2) system. Hence, only changes of charge and geometrical parameters of 12-X and 22-X as a function of X are reported in this manuscript (Tables 3 and 4 and Tables S1–S3 in supplementary section). In 12-X, the increasing trend of NBO
charges on C3, as a function of X (attached at C2) is: 1OCH3! 1OH!1N(CH3)2!1F!1Cl!1Br!1CH3!1H!1CF3!1COOH! 1NO2 (Table 3). This trend more or less follows the order of Hammet spara substituent constants [41]. The lowest charge on C3 is observed for XZ–OCH3, while the highest charge is encountered for XZ–NO2. Apparently, direct resonance of – N(CH3)2 with C2aC3 places an electron density on C3, while – NO2 depletes electron density from the same carbon, through a similar electron delocalization process. Direct resonance could also be held responsible for placing electron density on C3, when XZ–OH, –N(CH3)2, and/or –F with a decreasing trend of importants, respectively. A relatively higher positive charge on C3 is observed for XZ–COOH, where the possible direct resonance places a positive charge on C3 (Table 3). The theory of atoms in molecules (AIM) charges on carbon atoms (Ci, for iZ1–7), are also calculated for 2-substituted norbornadienes (12-X), and their corresponding 2-substituted quadricyclanes (22-X) at B3LYP/6-311CCG* level of theory. In 12-X, the increasing trend of AIM charges on C3, as a function of X (attached at C2) is: 1NH2!1OH!1CH3!1H! 1F!1Br!1Cl!1COOH!1NO2 (Table 4). Again, the lowest charge on C3 is observed for XZ–NH2, while the highest charge is encountered for XZ–NO2. Direct electron donating resonance of –NH2 with C2aC3 lead to electron density on C3, while –NO2 withdraw electron density from the same carbon. Hyperconjugation also be held responsible for placing electron density on C3, when XZ–Me with a decreasing trend of importants. A relatively higher positive charge on C3 is observed for XZ–COOH, where the possible direct resonance places a positive charge on C3. The B3LYP/6-311CCG* calculated geometrical parameters are reported for 2-substituted norbornadienes (12-X), and 2-substituted quadricyclanes (22-X); where XZ–NO2, –F,
Table 4 The B3LYP/6-311CCG* calculated the theory of atoms in molecules (AIM) charges on carbon atoms (Ci, for iZ1–7), for 2-substituted norbornadienes (12-X), and their corresponding 2-substituted quadricyclanes (22-X); where XZ–NO2, –F, –Cl, –Br, –CF3, –COOH, –NH2, –OH, –Me and H Compound
Total atomic charges on carbon atoms (Ci, iZ1–7) C1
12-NO2 12-F 12-CF3 12-COOH 12-Cl 12-Br 12-H 12-CH3 12-OH 12-NH2 22-NO2 22-F 22-CF3 22-COOH 22-Cl 22-H 22-CH3 22-Br 22-OH 22-NH2
C2
0.043 0.021
0.266 0.197
–
–
0.019 0.016 0.017 0.002 K0.009 0.010 0.002
0.091 0.017 K0.123 K0.087 K0.039 0.461 0.347
K0.019 K0.049 – K0.029 K0.036 K0.052 K0.059 K0.034 K0.043 K0.049
0.242 0.506 – K0.054 0.061 K0.075 K0.008 K0.076 0.504 0.352
C3 K0.026 K0.044 – K0.031 K0.040 K0.041 K0.087 K0.091 K0.090 K0.092 K0.047 K0.074 – K0.057 K0.063 K0.075 K0.087 K0.061 K0.091 K0.098
C4 0.029 0.011 – 0.009 0.010 0.008 0.002 0.002 0.016 0.017 K0.038 K0.045 – K0.046 K0.045 K0.052 K0.050 K0.045 K0.047 K0.048
C5 K0.079 K0.086 – K0.0085 K0.085 K0.081 K0.087 K0.089 K0.089 K0.085 K0.065 K0.070 – K0.073 K0.069 K0.075 K0.076 K0.069 K0.075 K0.073
C6
C7 0.020 0.003
K0.069 K0.087 – K084 K0.086 K0.090 K0.087 K0.094 K0.091 K0.090
0.003 0.003 0.002 0.004 K0.001 K0.001 0.001
K0.042 K0.082
0.013 0.007
–
–
– K0.051 K0.062 K0.075 K0.084 K0.059 K0.100 K0.077
0.010 0.008 0.007 0.006 0.008 0.006 0.008
M.Z. Kassaee et al. / Journal of Molecular Structure: THEOCHEM 763 (2006) 13–19
–Cl, –Br, –CF3, –COOH, –NMe2, –OMe, –OH and –Me. of 12X and 22-X (Tables S1–S3 in the supplementary section). These geometrical parameters consist of carbon–carbon interatomic ˆ –C interatomic angles ˚ ); C–C distances (Rc,c) in angstrom (A ˆ (Ac,c,c) in degrees; and interatomic dihedral angles (Dc,c,c,c) in degrees. The increasing trend of C2–C3 (R2,3) bond length, as a function of X, attached at C2, in 12-X is: 1N(CH3)2O1OCH3O 1NO2O1CH3O1HO1CF3O1ClO1BrO1F. Justification of this trend may simply be attributed to the resonance ability of X to inflict single bond character to C2–C3 bond. This trend is almost similar to the increasing trend of C2–C6 (R2,6) bond length, as a function of C2 attached X in 22-X, where 2N(CH3)2O 2NO2O2OCH3O2HO2CF3O2COOHO2ClO2BrO2F. The s character of C2 for 12-X and 22-X is increased with electron withdrawing substituents leading to large bond angle of C1– C2–C3. Dihedral angles, C1–C2–C3–C4 and C4–C5–C6–C1, of both 12-X and 22-X, are increased by either electron withdrawing or electron donating groups, due to distortions of the symmetry imposed by substituents. 4. Conclusion The extent of the photochemical and/or solar energy stored in norbornadiene/quadricyclane system (1n-X/2n-X) is measured by calculating the energy difference between the ground states of 1n-X and 2n-X (DE(1n-X)-(2n-X)). The electron donating (D) subsituents which are located on C2 (12-X), as well as the electron withdrawing (W) groups on C1 (11-X), appear to increase photochemical and/or solar energy storage. However, the storage energy impact of D subsituents connected to C2 (12-X) is more than those of W groups attached at C1 (11-X). Introducing substituents on C7 (17-X) has a minimal effect on the energy storage. Acknowledgements We wish to express our special thanks to Dr T. Partovi from Payam-e-nor University and Dr. A.R. Bekhradnia at Tarbiat Modarres University for their cordial cooperation in completing this research. Appendix. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.theochem.2006. 01.010.
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