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Solar energy storage in norbornadiene–quadricyclane system: electronic effects via ab initio computations
Journal of Molecular Structure: THEOCHEM 716 (2005) 159–163 www.elsevier.com/locate/theochem
Solar energy storage in norbornadiene–quadricyclane system: electronic effects via ab initio computations M.Z. Kassaee*, E. Vessally Department of Chemistry, Science Faculty, Pole-Ghisha, Chamran High way, Tarbiat Modarres University, Tehran 141554838, Iran Received 1 November 2004; revised 29 November 2004; accepted 3 December 2004 Available online 30 December 2004
Abstract Electronic effects involved in the solar energy storage in norbornadiene (1)/quadricyclane (2) system, are investigated using ab initio methods at HF/6-31G* and DFT/6-31G* levels of theory. Substituents X, with various Hammett s constants, are placed at the para carbon of a phenyl ring, attached to the C2 of 1 and/or 2. A Hammett r value of K0.81 is encountered; indicating that electron donating substituents (–NMe2, –NH2, –OMe, –OH and –Me) induce storage of higher quantities of solar energy, than electron withdrawing groups (–NO2, –F, –Cl, –Br, –CF3 and –COOH). q 2004 Published by Elsevier B.V. Keywords: Ab initio; Norbornadiene; Quadricyclane; Solar energy; Electronic effect
1. Introduction Solar energy storage in norbornadiene (1)/quadricyclane (2) system, has attracted much attention, particularly as a mechanistic point of view [1–7]. Highly strained 2 has a potentially high energy, for containing a cyclobutane and two cyclopropane rings [8]. Conversion of 2 to 1 occurs through a radical-cation chain reaction route, initiated by chemical, electrochemical and photosensitized single-electron oxidations. The stored energy of 2 is released thermally [9]. The rearrangement of 1 to 2 occurs via the triplet state of 1 [9,10]. Low quantum yield (F) of this photo-conversion turns out to be the major drawback of this system [6,11]. Also, the 1/2 system has an inherent disadvantage of 1 not absorbing visible wave length of sunlight. Electron donating and/or electron withdrawing substituents, attached to the double bond of 1, may provide a red shift of absorption, and increase the F value [12–14]. The water soluble carbamoyl and carboxyl derivatives of 1 and 2 are also used to absorb light of wavelengths longer than 300 nm [15]. Besides the usage of chromophores, using sensitizers is another method * Corresponding author. Tel.: C98 11 9891 2100 0392; fax: C98 11 982 1800 6544. E-mail address: [email protected] (M.Z. Kassaee). 0166-1280/$ - see front matter q 2004 Published by Elsevier B.V. doi:10.1016/j.theochem.2004.12.006
for solving the problem of not absorbing the visible light. Iridium complex is proposed as the sensitizer for p–p* excitation [4]. Density functional calculations with the hybrid B3LYP functional have been used to study the ground state of 1 bound to the photo-sensitizer [Cu(8-oxoquinolinato)] [16]. Ab initio is used to study energetic of 1 and 2 conversions [17–19]. Up to date, no systematic ab initio calculations of donor-acceptor effects on 1/2 system are reported. In this work, electronic effects on the solar energy storage of substituted norbornadiene (1p-X)/substituted quadricyclane (2p-X) are reported using HF and B3LYP methods, for XZ–NO2, –F, –Cl, –Br, –CF3, –COOH, –NMe2, –NH2, –OMe, –OH, –Me (Scheme 1).
2. Computational methods A quest for higher storage of the solar energy is carried out, for substituted norbornadienes (1p-X) and quadricyclanes (2p-X) system, via ab initio (Scheme 1). Geometry optimizations are performed by HF and B3LYP [20,21] methods using 6-31G* basis set of the GAUSSIAN 98 system of programs [22]. The HF/6-31G* optimized geometrical outputs are used as inputs for the B3LYB/6-31G* calculations. This is for obtaining more accurate values of
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Scheme 1. The solar energy storage system of substituted norbornadiene (1p-X)/substituted quadricyclane (2p-X); where XZ–NO2, –F, –Cl, –Br, –CF3, –COOH, –NMe2, –NH2, –OMe, –OH and –Me.
activation electronic energies (DE), enthalpies (DH) and Gibbs free energies (DG). In order to find energy minima, keyword ‘FOPT’ is 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 [23]. 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 the scaling factor of 0.89 suggested by Hehre et al. [24] for HF/6-31G*; and by 0.99 scaling factor of Rauhut and Pulay [25] for B3LYP/6-31G*. 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 the observed (anharmonic) frequencies will deviate from unity even for exact calculations [26]. Here, a set of molecules containing similar motifs are treated together, where they benefit from similar scalings. The Hammet spara substituent constants are employed in order to figure out the Hammet r value for the conversion of 1p-X to 2p-X [27].
3. Results and discussion The objective of this study is to determine the electronic effects, on the solar energy storage in norbornadiene (1)–quadricyclane (2) system, using ab initio methods at HF/6-31G* and DFT/6-31G* levels of theory. Taking into consideration the size of molecules probed, and the consistency of the results obtained, these ab initio calculation levels seem to be suffice. The extend of the solar energy stored in this system is measured simply by calculating the energy difference between the ground states of 1 and 2, without having any need to consider the excited states and/or the type of the mechanism involved. Both electron withdrawing and electron donating substituents are investigated. These substituents (X) are situated only at para carbons of phenyl rings, that are in turn attached to the C2 of 1p-X and/or 2p-X (Scheme 1). Substituents attached at the ortho and/or meta positions are excluded because of showing the concurrent involvement of both steric and electronic effects. The C2 position is reported in this
manuscript, since C2 proved to be more sensitive to the substituent effects than carbons C1 and/or C7. Nevertheless, an account of substituent effects at C1 and C7 as well as substitution at the meta positions will be presented in the near future. The thermal and electronic energies (E), enthalpies (H) and Gibbs free energies (G) for optimized structures of 1p-X and 2p-X are presented at HF/6-31G* and B3LYP/6-31G* levels of theory, where thermodynamic functions obtained through frequency calculations, are multiplied by the scaling factor of 0.89 suggested by Hehre et al. [24] for HF/6-31G*; and by 0.99 scaling factor of Rauhut and Pulay [25] for B3LYP/6-31G* (Table 1). The calculated harmonic force constants and frequencies are usually higher than the corresponding experimental quantities, at the ab initio SCF level, due to a combination of electron correlation effects and basis set deficiencies. The overestimation of the frequencies becomes more severe if the calculated harmonic frequencies are compared with the observed fundamentals, as anharmonicity usually lowers the frequencies [26,28]. Here, a set of molecules containing similar motifs are treated together, where they benefit from similar scalings. Both electron withdrawing groups (XZ–NO2, –F, –Cl, –Br, –CF3 and –COOH) and electron donating substituents (XZ–NMe2, –NH2, –OMe, –OH and –Me) appear to stabilize both 1p-X and 2p-X. Stabilization of 1p-X may be attributed to the possibility of extension of conjugation through conglomeration of p-X-phenyl with the CaC of norbornadiene. The stabilization of 2p-X may be explained using the Walsh orbital model, where the cyclopropyl ring may act both as a good p donor and a good p acceptor [29]. Energy gaps between 1p-X and 2p-X, which are measures of solar energy storages in the system, are calculated (Table 2). Solar energy storage increases the most by electron donating substituents attached at phenyl-C2 of 1p-X and 2p-X. A linear free energy relationship is observed upon plotting the energy gaps between 1p-X and 2p-X (DE1x–2x) vs. the Hammet spara substituent (X) constants [27]; showing a Hammet r value of K0.81 with a correlation factor R2Z0.97 (Chart 1). In other words, The Hammet r value is obtained through plotting spara/DE(1p-X)–(2p-X) for: –NO2 (0.78/22.065), –CF3 (0.54/22.303), –COOH (0.45/22.231), –F (0.06/22.667), –Cl (0.23/22.515), –Br (0.23/22.496), –H (0.00/22.598), –Me (K0.17/22.760), –OH (K0.37/22.800) and –NMe2 (K1.23/23.814) (Chart 1). The Mulliken charges on carbon atoms of 1p-X and 2p-X are presented (Table 3). The increasing trend of charge on C3 in 1p-X, as a function of p-X is: 1p-N(CH3)2!1p-OCH3! 1p-OH ! 1p-CH3 ! 1p-Br ! 1p-Cl ! 1p-F ! 1p-H ! 1p-COOH! 1p-CF3!1p-NO2 (Table 3). This trend more or less follows the order of Hammet spara substituent constants [27]. The lowest charge on C3 is observed for XZp-N(CH3)2, while the highest charge is encountered for XZp-NO2. Apparently, direct resonance of p-N(CH3)2 with C2]C3 places an electron density on C3, while p-NO2 depletes electron density from the same carbon, through a similar electron
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Table 1 The thermal energies (E), thermal enthalpies (H), and thermal free energies (G), in kcal/mol, for HF/6-31G* (B3LYP/6-31G*) optimized substituted norbornadienes (1p-X) and quadricyclanes (2p-X); where XZ–NO2, –F, –Cl, –Br, –CF3, –COOH, –NMe2, –NH2, –OMe, –OH and –Me Substituents (X)
p-NO2 p-CF3 p-COOH p-H p-F p-Cl p-Br p-CH3 p-OH p-OCH3 p-N(CH3)2 p-NH2
1p-X
2p-X
E
H
G
E
H
G
K440646.672 (K443388.494) K523567.001 (K526550.209) K430684.876 (K433377.263) K312974.650 (K315069.125) K375013.058 (K377347.292) K600945.170 (K603476.399) K1925225.948 (K1928448.390) K337433.622 (K339704.515) K359939.165 (K362260.782) K384389.528 (K386888.052) K396383.252 (K399036.401) K347481.611 (K349780.870)
K440645.553 (K443387.315) K523565.882 (K526549.030) K430683.756 (K433376.084) K312973.530 (K315067.946) K375011.938 (K377346.113) K600944.051 (K603475.220) K1925224.827 (K1928447.211) K337432.502 (K339703.336) K359938.046 (K362259.602) K384388.409 (K386886.873) K396382.132 (K399035.222) K347480.491 (K349779.691)
K440706.201 (K443453.051) K523631.055 (K526619.088) K430744.949 (K433442.501) K313026.516 (K315125.286) K375067.286 (K377406.141) K601000.922 (K603536.907) K1925283.218 (K1928510.528) K337491.699 (K339767.202) K359994.130 (K362320.275) K384448.440 (K386951.802) K396446.122 (K399105.791) K347537.084 (K349840.895)
K440624.607 (K443367.113) K523544.698 (K526528.572) K430662.645 (K433355.666) K312952.051 (K315047.126) K374990.391 (K377325.780) K600922.655 (K603454.441) K1925203.452 (K1928426.438) K337410.872 (K339683.512) K359916.365 (K362237.945) K384366.645 (K386865.111) K396359.439 (K399012.947) K347458.707 (K349758.734)
K440623.488 (K443365.933) K523543.578 (K526527.392) K430661.525 (K433354.486) K312950.931 (K315045.947) K374989.271 (K377324.602) K600921.535 (K603453.261) K1925202.332 (K1928425.258) K337409.753 (K339682.333) K359915.245 (K362236.766) K384365.524 (K386863.932) K396358.319 (K399011.768) K347457.587 (K349757.555)
K440684.028 (K443430.982) K523608.729 (K526596.780) K430722.648 (K433420.302) K313004.235 (K315102.984) K375045.770 (K377384.203) K600978.733 (K603514.654) K1925261.005 (K1928488.226) K337469.774 (K339742.702) K359972.139 (K362297.336) K384426.178 (K386929.048) K396424.127 (K399083.617) K347515.101 (K349815.350)
delocalization process. Direct resonance could also be held responsible for placing electron density on C3, when XZ p-OH and p-OCH3. A relatively higher positive charge on C3 is observed for XZ–COOH, where the possible direct resonance places a positive charge on C3. The decreasing trend of charge on C2 in 2p-X as a function of p-X is: 2p-N(CH3)2O2p-OCH3O2p-OHO2p-FO2p-ClO2P-HO2p-CH3O 2p-BrO2p-COOHO2p-CF3O2p-NO2. Hence, electron withdrawing groups stabilize the observed negative charge on C2 and positive charge on C6 (Table 3), as anticipated by the Walsh orbital model [29]. In contrast electron density Table 2 The thermal energy separations, DE(1p-X)-(2p-X), enthalpy gaps, DH(1p-X)-(2p-X) and free energy splittings, DG(1p-X)-(2p-X), in kcal molK1, between substituted norbornadienes (1p-X), and their corresponding quadricyclanes (2p-X); calculated at HF/6-31G* (B3LYP/6-31G*) levels of theory where XZ –NO2, –F, –Cl, –Br, –CF3, –COOH, NH2, –NMe2, –OMe, –OH and –Me Substituent (X)
DE1x–2x
DH1x–2x
DG1x–2x
p-NO2 p-CF3 p-COOH p-F p-Cl p-Br p-H p-CH3 p-OH p-OCH3 p-N(CH3)2 p-NH2
22.065 (21.381) 22.304 (21.637) 22.231 (21.597) 22.667 (21.511) 22.515 (21.958) 22.496 (21.952) 22.598 (21.999) 22.749 (21.003) 22.800 (22.837) 22.884 (22.942) 23.814 (23.454) 22.904 (22.137)
22.065 (21.381) 22.304 (21.638) 22.231 (21.598) 22.667 (21.511) 22.516 (21.959) 22.495 (21.953) 22.599 (21.999) 22.749 (21.003) 22.800 (22.837) 22.884 (22.942) 23.814 (23.454) 22.904 (22.136)
22.173 (22.069) 22.326 (22.308) 22.301 (22.199) 21.517 (21.937) 22.189 (22.253) 22.213 (22.301) 22.281 (22.302) 21.925 (24.500) 21.991 (22.938) 22.262 (22.754) 21.994 (22.174) 21.983 (25.545)
distributions on the carbons of cyclopropyl ring C3C4C5 (Scheme 1) do not differ due to the absence of the electron withdrawing and/or electron donating groups at these positions (C3, C4 and C5). The geometrical parameters of 1p-X and 2p-X are calculated (Table 4 and Tables 5–7 in the supplementary section). The increasing trend of C2–C3 (R2,3) bond length, as a function of p-X in 1p-X is: 1p-NO2O1p-N(CH3)2O1p-CF3Z 1p-COOHO1p-OCH3O1p-OHO1p-CH3Z1p-BrZ1p-ClO1p-FZ 1P-H (Table 4). This may be attributed to the ability of X to inflict single bond character to C2–C3, through direct resonance. The size of the bond angle C1 –C 2–C 6
Chart 1. Hammet r value found as the slope of the curve (yZK0.81x C 22.66; correlation factor, R2Z0.97) for DE(1p-X)-(2p-X) vs. spara; where XZ–NO2, –F, –Cl, –Br, –CF3, –COOH, –NMe2, –OH and –Me.
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Table 3 B3LYP/6-31G* calculated total atomic Mulliken charges on carbon atoms, for substituted norbornadienes (1p-X), and their corresponding quadricyclanes (2p-X); where XZ–NO2, –F, –Cl, –Br, –CF3, –COOH, –NH2, –NMe2, –OMe, –OH and –Me Compound
Total atomic Mulliken charges on carbon atoms C1
C2
1p-NO2 1p-CF3 1p-COOH 1P-H 1p-F 1p-Cl 1p-Br 1p-CH3 1p-OH 1p-OCH3 1p-N(CH3)2
K0.10513 K0.10694 K0.10558 K0.10852 K0.10988 K0.10878 K0.10849 K0.10894 K0.11049 K0.11104 K0.11172
0.10706 0.10951 0.10999 0.11129 0.11220 0.11130 0.11085 0.11286 0.11371 0.11390 0.11511
2p-NO2 2p-CF3 2p-COOH 2P-H 2p-F 2p-Cl 2p-Br 2p-CH3 2p-OH 2p-OCH3 2-NH2 2p-N(CH3)2
K0.03055 K0.03534 K0.03377 K0.04001 K0.04032 K0.03784 K0.03771 K0.04045 K0.06159 K0.06239 K0.05386 K0.05486
K0.03400 K0.02660 K0.02784 K0.02014 K0.02033 K0.02265 K0.02342 K0.01912 K0.00356 K0.00200 0.00002 0.00051
C3
C4
C5
K0.0079 K0.0189 K0.0171 K0.0298 K0.0319 K0.0260 K0.0254 K0.0338 K0.0414 K0.0401 K0.0493
K0.07056 K0.07304 K0.07344 K0.07575 K0.07469 K0.07381 K0.07383 K0.07633 K0.07650 K0.07677 K0.07850
0.04241 0.03898 0.03832 0.03467 0.03636 0.03772 0.03774 0.03391 0.03370 0.03365 0.03085
0.04098 0.03703 0.03807 0.03329 0.03349 0.03529 0.03538 0.03247 0.03176 0.03037 0.02859
0.04132 0.03637 0.03594 0.03030 0.03197 0.03419 0.03424 0.02915 0.02841 0.02805 0.02475
K0.03965 K0.04342 K0.04379 K0.04771 K0.04614 K0.04476 K0.04471 K0.04838 K0.05175 K0.05241 K0.05290 K0.05362
0.01836 0.01401 0.01376 0.00892 0.01063 0.01233 0.01245 0.00798 0.00656 0.00530 0.00165 0.00089
0.03035 0.02245 0.02504 0.01533 0.01516 0.01834 0.01885 0.01377 0.01804 0.01693 K0.0028 K0.0038
0.07606 0.07008 0.07027 0.06350 0.06497 0.06757 0.06772 0.06215 0.06029 0.05933 0.05337 0.05242
0.01867 0.01499 0.01576 0.01205 0.01115 0.01296 0.01317 0.01125 0.01486 0.01611 0.02383 0.02428
(:A1,2,6) as a function of p-X is: 2p-N(CH3)2 O 2p-NH2 O 2p-OCH3 O 2p-OH O 2p-CH3 O 2 P-H O 2p-F O2p-ClO2p-Br O 2p-CF3O2p-COOHO2p-NO2 in 2p-X (Table 4). Decreasing of :A1,2,6 with the electron withdrawing groups may be due to the increasing of the s character of C2 in cyclopropyl ring through the direct resonance of C2 with the p-X-phenyl. Again, the analogous bond angles in C4–C3–C5 cyclopropyl ring (Scheme 1) do not vary as much, since no perturbations due to p-X-phenyl occurs in this ring. The size of dihedral angle :D3,2,8,13 in 1p-X is, as a function of p-X: 1p-N(CH3)2!1p-NH2!1p-Br!1p-NO2! 1p-OCH3!1p-OH!1p-CH3!1p-Cl!1p-COOH!1p-F!1p-CF3! 1P-H (Table 4). This trend may also be justified by considering the possibility of extension of conjugation through conglomeration of p-X-phenyl with CaC. Finally, this research is to serve those whose primary interest is to replace the fossil fuel and/or the nuclear energy with the most economical and very available solar energy. Hence, the scrutiny of the photochemical energy storage in the ground states of norbornadiene (1)/quadricyclane (2) system is carried out. Therefore, computations and/or measurement of the quantum yields of the scrutinized photolysis, which appears of interest to many workers [30–34], will be addressed elsewhere.
4. Conclusion The effects of both electron withdrawing and electron donating substituents are investigated on various positions
C6
C7
Table 4 Selected B3LYP/6-31G* calculated carbon-carbon bond (R2,3)/angstrom ˆ c,c,c)/degrees and dihedral angle (:D3,2,8,13)/ ˚ ), interatomic angles (A (A degree for substituted norbornadienes (1p-X), and quadricyclanes (2p-X); where XZ–NO2, –F, –Cl, –Br, –CF3, –COOH, –NH2, –NMe2, –OMe, –OH and –Me Compound
1p-NO2 1p-CF3 1p-COOH 1p-F 1p-Cl 1p-Br 1P-H 1p-CH3 1p-OH 1p-OCH3 1p-NH2 1p-N(CH3)2 2p-NO2 2p-CF3 2p-COOH 2p-F 2p-Cl 2p-Br 2P-H 2p-CH3 2p-OH 2p-OCH3 2p-NH2 2p-N(CH3)2
˚ ), interatomic Carbon–carbon bond (R2,3)/angstrom (A ˆ c,c,c)/degrees and dihedral angle (:D3,2,8,13)/ angles (A degree R2,3
A1,2,6
A4,3,5
D3,2,8,13
1.3484 1.3471 1.3478 1.3467 1.3469 1.3469 1.3467 1.3469 1.3470 1.3472 1.3477 1.3479 1.5528 1.5540 1.5536 1.5552 1.5546 1.5545 1.5551 1.5550 1.5599 1.5600 1.5655 1.5657
– – – – – – – – – – – – 58.685 58.702 58.747 59.126 59.043 59.014 59.046 59.094 59.458 59.522 59.989 59.999
– – – – – – – – – – – – 60.117 60.132 60.140 60.140 60.156 60.159 60.178 60.176 60.224 60.224 60.226 60.201
19.6608 20.7023 20.3748 20.61 20.1619 19.3027 21.0778 19.9491 19.7289 19.6682 18.0806 17.4645 16.8759 17.0823 17.0724 20.0759 19.7678 19.3027 19.4807 20.7477 32.3201 37.0113 93.8217 83.1235
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of norbornadiene (1p-X)/quadricyclane (2p-X) system. Electron donating substituents (p-X), attached at phenyl-C2 carbon, increase the solar energy storage. A Hammet r value of K0.81 is found for this process at HF/6-31G* and DFT/6-31G* levels of theory.
Acknowledgements We wish to express our special thanks to Dr Mahjoob, Mr Arshadi, Mr A.R. Bekhradnia, Ms M. Koohi and Mr S.M. Musavi, at Tarbiat Modares University, for their cordial cooperation in completing this research. Technical assistances of Mr A. Sayadi (at the Chemistry Department of Imam Hossein University), Mr M. Amanbaee and Ms Z. Farahani are most appreciated.
Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.theochem. 2004.12.006
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Solar energy storage in norbornadiene–quadricyclane system: electronic effects via ab initio computations M.Z. Kassaee*, E. Vessally Department of Chemistry, Science Faculty, Pole-Ghisha, Chamran High way, Tarbiat Modarres University, Tehran 141554838, Iran Received 1 November 2004; revised 29 November 2004; accepted 3 December 2004 Available online 30 December 2004
Abstract Electronic effects involved in the solar energy storage in norbornadiene (1)/quadricyclane (2) system, are investigated using ab initio methods at HF/6-31G* and DFT/6-31G* levels of theory. Substituents X, with various Hammett s constants, are placed at the para carbon of a phenyl ring, attached to the C2 of 1 and/or 2. A Hammett r value of K0.81 is encountered; indicating that electron donating substituents (–NMe2, –NH2, –OMe, –OH and –Me) induce storage of higher quantities of solar energy, than electron withdrawing groups (–NO2, –F, –Cl, –Br, –CF3 and –COOH). q 2004 Published by Elsevier B.V. Keywords: Ab initio; Norbornadiene; Quadricyclane; Solar energy; Electronic effect
1. Introduction Solar energy storage in norbornadiene (1)/quadricyclane (2) system, has attracted much attention, particularly as a mechanistic point of view [1–7]. Highly strained 2 has a potentially high energy, for containing a cyclobutane and two cyclopropane rings [8]. Conversion of 2 to 1 occurs through a radical-cation chain reaction route, initiated by chemical, electrochemical and photosensitized single-electron oxidations. The stored energy of 2 is released thermally [9]. The rearrangement of 1 to 2 occurs via the triplet state of 1 [9,10]. Low quantum yield (F) of this photo-conversion turns out to be the major drawback of this system [6,11]. Also, the 1/2 system has an inherent disadvantage of 1 not absorbing visible wave length of sunlight. Electron donating and/or electron withdrawing substituents, attached to the double bond of 1, may provide a red shift of absorption, and increase the F value [12–14]. The water soluble carbamoyl and carboxyl derivatives of 1 and 2 are also used to absorb light of wavelengths longer than 300 nm [15]. Besides the usage of chromophores, using sensitizers is another method * Corresponding author. Tel.: C98 11 9891 2100 0392; fax: C98 11 982 1800 6544. E-mail address: [email protected] (M.Z. Kassaee). 0166-1280/$ - see front matter q 2004 Published by Elsevier B.V. doi:10.1016/j.theochem.2004.12.006
for solving the problem of not absorbing the visible light. Iridium complex is proposed as the sensitizer for p–p* excitation [4]. Density functional calculations with the hybrid B3LYP functional have been used to study the ground state of 1 bound to the photo-sensitizer [Cu(8-oxoquinolinato)] [16]. Ab initio is used to study energetic of 1 and 2 conversions [17–19]. Up to date, no systematic ab initio calculations of donor-acceptor effects on 1/2 system are reported. In this work, electronic effects on the solar energy storage of substituted norbornadiene (1p-X)/substituted quadricyclane (2p-X) are reported using HF and B3LYP methods, for XZ–NO2, –F, –Cl, –Br, –CF3, –COOH, –NMe2, –NH2, –OMe, –OH, –Me (Scheme 1).
2. Computational methods A quest for higher storage of the solar energy is carried out, for substituted norbornadienes (1p-X) and quadricyclanes (2p-X) system, via ab initio (Scheme 1). Geometry optimizations are performed by HF and B3LYP [20,21] methods using 6-31G* basis set of the GAUSSIAN 98 system of programs [22]. The HF/6-31G* optimized geometrical outputs are used as inputs for the B3LYB/6-31G* calculations. This is for obtaining more accurate values of
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Scheme 1. The solar energy storage system of substituted norbornadiene (1p-X)/substituted quadricyclane (2p-X); where XZ–NO2, –F, –Cl, –Br, –CF3, –COOH, –NMe2, –NH2, –OMe, –OH and –Me.
activation electronic energies (DE), enthalpies (DH) and Gibbs free energies (DG). In order to find energy minima, keyword ‘FOPT’ is 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 [23]. 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 the scaling factor of 0.89 suggested by Hehre et al. [24] for HF/6-31G*; and by 0.99 scaling factor of Rauhut and Pulay [25] for B3LYP/6-31G*. 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 the observed (anharmonic) frequencies will deviate from unity even for exact calculations [26]. Here, a set of molecules containing similar motifs are treated together, where they benefit from similar scalings. The Hammet spara substituent constants are employed in order to figure out the Hammet r value for the conversion of 1p-X to 2p-X [27].
3. Results and discussion The objective of this study is to determine the electronic effects, on the solar energy storage in norbornadiene (1)–quadricyclane (2) system, using ab initio methods at HF/6-31G* and DFT/6-31G* levels of theory. Taking into consideration the size of molecules probed, and the consistency of the results obtained, these ab initio calculation levels seem to be suffice. The extend of the solar energy stored in this system is measured simply by calculating the energy difference between the ground states of 1 and 2, without having any need to consider the excited states and/or the type of the mechanism involved. Both electron withdrawing and electron donating substituents are investigated. These substituents (X) are situated only at para carbons of phenyl rings, that are in turn attached to the C2 of 1p-X and/or 2p-X (Scheme 1). Substituents attached at the ortho and/or meta positions are excluded because of showing the concurrent involvement of both steric and electronic effects. The C2 position is reported in this
manuscript, since C2 proved to be more sensitive to the substituent effects than carbons C1 and/or C7. Nevertheless, an account of substituent effects at C1 and C7 as well as substitution at the meta positions will be presented in the near future. The thermal and electronic energies (E), enthalpies (H) and Gibbs free energies (G) for optimized structures of 1p-X and 2p-X are presented at HF/6-31G* and B3LYP/6-31G* levels of theory, where thermodynamic functions obtained through frequency calculations, are multiplied by the scaling factor of 0.89 suggested by Hehre et al. [24] for HF/6-31G*; and by 0.99 scaling factor of Rauhut and Pulay [25] for B3LYP/6-31G* (Table 1). The calculated harmonic force constants and frequencies are usually higher than the corresponding experimental quantities, at the ab initio SCF level, due to a combination of electron correlation effects and basis set deficiencies. The overestimation of the frequencies becomes more severe if the calculated harmonic frequencies are compared with the observed fundamentals, as anharmonicity usually lowers the frequencies [26,28]. Here, a set of molecules containing similar motifs are treated together, where they benefit from similar scalings. Both electron withdrawing groups (XZ–NO2, –F, –Cl, –Br, –CF3 and –COOH) and electron donating substituents (XZ–NMe2, –NH2, –OMe, –OH and –Me) appear to stabilize both 1p-X and 2p-X. Stabilization of 1p-X may be attributed to the possibility of extension of conjugation through conglomeration of p-X-phenyl with the CaC of norbornadiene. The stabilization of 2p-X may be explained using the Walsh orbital model, where the cyclopropyl ring may act both as a good p donor and a good p acceptor [29]. Energy gaps between 1p-X and 2p-X, which are measures of solar energy storages in the system, are calculated (Table 2). Solar energy storage increases the most by electron donating substituents attached at phenyl-C2 of 1p-X and 2p-X. A linear free energy relationship is observed upon plotting the energy gaps between 1p-X and 2p-X (DE1x–2x) vs. the Hammet spara substituent (X) constants [27]; showing a Hammet r value of K0.81 with a correlation factor R2Z0.97 (Chart 1). In other words, The Hammet r value is obtained through plotting spara/DE(1p-X)–(2p-X) for: –NO2 (0.78/22.065), –CF3 (0.54/22.303), –COOH (0.45/22.231), –F (0.06/22.667), –Cl (0.23/22.515), –Br (0.23/22.496), –H (0.00/22.598), –Me (K0.17/22.760), –OH (K0.37/22.800) and –NMe2 (K1.23/23.814) (Chart 1). The Mulliken charges on carbon atoms of 1p-X and 2p-X are presented (Table 3). The increasing trend of charge on C3 in 1p-X, as a function of p-X is: 1p-N(CH3)2!1p-OCH3! 1p-OH ! 1p-CH3 ! 1p-Br ! 1p-Cl ! 1p-F ! 1p-H ! 1p-COOH! 1p-CF3!1p-NO2 (Table 3). This trend more or less follows the order of Hammet spara substituent constants [27]. The lowest charge on C3 is observed for XZp-N(CH3)2, while the highest charge is encountered for XZp-NO2. Apparently, direct resonance of p-N(CH3)2 with C2]C3 places an electron density on C3, while p-NO2 depletes electron density from the same carbon, through a similar electron
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Table 1 The thermal energies (E), thermal enthalpies (H), and thermal free energies (G), in kcal/mol, for HF/6-31G* (B3LYP/6-31G*) optimized substituted norbornadienes (1p-X) and quadricyclanes (2p-X); where XZ–NO2, –F, –Cl, –Br, –CF3, –COOH, –NMe2, –NH2, –OMe, –OH and –Me Substituents (X)
p-NO2 p-CF3 p-COOH p-H p-F p-Cl p-Br p-CH3 p-OH p-OCH3 p-N(CH3)2 p-NH2
1p-X
2p-X
E
H
G
E
H
G
K440646.672 (K443388.494) K523567.001 (K526550.209) K430684.876 (K433377.263) K312974.650 (K315069.125) K375013.058 (K377347.292) K600945.170 (K603476.399) K1925225.948 (K1928448.390) K337433.622 (K339704.515) K359939.165 (K362260.782) K384389.528 (K386888.052) K396383.252 (K399036.401) K347481.611 (K349780.870)
K440645.553 (K443387.315) K523565.882 (K526549.030) K430683.756 (K433376.084) K312973.530 (K315067.946) K375011.938 (K377346.113) K600944.051 (K603475.220) K1925224.827 (K1928447.211) K337432.502 (K339703.336) K359938.046 (K362259.602) K384388.409 (K386886.873) K396382.132 (K399035.222) K347480.491 (K349779.691)
K440706.201 (K443453.051) K523631.055 (K526619.088) K430744.949 (K433442.501) K313026.516 (K315125.286) K375067.286 (K377406.141) K601000.922 (K603536.907) K1925283.218 (K1928510.528) K337491.699 (K339767.202) K359994.130 (K362320.275) K384448.440 (K386951.802) K396446.122 (K399105.791) K347537.084 (K349840.895)
K440624.607 (K443367.113) K523544.698 (K526528.572) K430662.645 (K433355.666) K312952.051 (K315047.126) K374990.391 (K377325.780) K600922.655 (K603454.441) K1925203.452 (K1928426.438) K337410.872 (K339683.512) K359916.365 (K362237.945) K384366.645 (K386865.111) K396359.439 (K399012.947) K347458.707 (K349758.734)
K440623.488 (K443365.933) K523543.578 (K526527.392) K430661.525 (K433354.486) K312950.931 (K315045.947) K374989.271 (K377324.602) K600921.535 (K603453.261) K1925202.332 (K1928425.258) K337409.753 (K339682.333) K359915.245 (K362236.766) K384365.524 (K386863.932) K396358.319 (K399011.768) K347457.587 (K349757.555)
K440684.028 (K443430.982) K523608.729 (K526596.780) K430722.648 (K433420.302) K313004.235 (K315102.984) K375045.770 (K377384.203) K600978.733 (K603514.654) K1925261.005 (K1928488.226) K337469.774 (K339742.702) K359972.139 (K362297.336) K384426.178 (K386929.048) K396424.127 (K399083.617) K347515.101 (K349815.350)
delocalization process. Direct resonance could also be held responsible for placing electron density on C3, when XZ p-OH and p-OCH3. A relatively higher positive charge on C3 is observed for XZ–COOH, where the possible direct resonance places a positive charge on C3. The decreasing trend of charge on C2 in 2p-X as a function of p-X is: 2p-N(CH3)2O2p-OCH3O2p-OHO2p-FO2p-ClO2P-HO2p-CH3O 2p-BrO2p-COOHO2p-CF3O2p-NO2. Hence, electron withdrawing groups stabilize the observed negative charge on C2 and positive charge on C6 (Table 3), as anticipated by the Walsh orbital model [29]. In contrast electron density Table 2 The thermal energy separations, DE(1p-X)-(2p-X), enthalpy gaps, DH(1p-X)-(2p-X) and free energy splittings, DG(1p-X)-(2p-X), in kcal molK1, between substituted norbornadienes (1p-X), and their corresponding quadricyclanes (2p-X); calculated at HF/6-31G* (B3LYP/6-31G*) levels of theory where XZ –NO2, –F, –Cl, –Br, –CF3, –COOH, NH2, –NMe2, –OMe, –OH and –Me Substituent (X)
DE1x–2x
DH1x–2x
DG1x–2x
p-NO2 p-CF3 p-COOH p-F p-Cl p-Br p-H p-CH3 p-OH p-OCH3 p-N(CH3)2 p-NH2
22.065 (21.381) 22.304 (21.637) 22.231 (21.597) 22.667 (21.511) 22.515 (21.958) 22.496 (21.952) 22.598 (21.999) 22.749 (21.003) 22.800 (22.837) 22.884 (22.942) 23.814 (23.454) 22.904 (22.137)
22.065 (21.381) 22.304 (21.638) 22.231 (21.598) 22.667 (21.511) 22.516 (21.959) 22.495 (21.953) 22.599 (21.999) 22.749 (21.003) 22.800 (22.837) 22.884 (22.942) 23.814 (23.454) 22.904 (22.136)
22.173 (22.069) 22.326 (22.308) 22.301 (22.199) 21.517 (21.937) 22.189 (22.253) 22.213 (22.301) 22.281 (22.302) 21.925 (24.500) 21.991 (22.938) 22.262 (22.754) 21.994 (22.174) 21.983 (25.545)
distributions on the carbons of cyclopropyl ring C3C4C5 (Scheme 1) do not differ due to the absence of the electron withdrawing and/or electron donating groups at these positions (C3, C4 and C5). The geometrical parameters of 1p-X and 2p-X are calculated (Table 4 and Tables 5–7 in the supplementary section). The increasing trend of C2–C3 (R2,3) bond length, as a function of p-X in 1p-X is: 1p-NO2O1p-N(CH3)2O1p-CF3Z 1p-COOHO1p-OCH3O1p-OHO1p-CH3Z1p-BrZ1p-ClO1p-FZ 1P-H (Table 4). This may be attributed to the ability of X to inflict single bond character to C2–C3, through direct resonance. The size of the bond angle C1 –C 2–C 6
Chart 1. Hammet r value found as the slope of the curve (yZK0.81x C 22.66; correlation factor, R2Z0.97) for DE(1p-X)-(2p-X) vs. spara; where XZ–NO2, –F, –Cl, –Br, –CF3, –COOH, –NMe2, –OH and –Me.
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Table 3 B3LYP/6-31G* calculated total atomic Mulliken charges on carbon atoms, for substituted norbornadienes (1p-X), and their corresponding quadricyclanes (2p-X); where XZ–NO2, –F, –Cl, –Br, –CF3, –COOH, –NH2, –NMe2, –OMe, –OH and –Me Compound
Total atomic Mulliken charges on carbon atoms C1
C2
1p-NO2 1p-CF3 1p-COOH 1P-H 1p-F 1p-Cl 1p-Br 1p-CH3 1p-OH 1p-OCH3 1p-N(CH3)2
K0.10513 K0.10694 K0.10558 K0.10852 K0.10988 K0.10878 K0.10849 K0.10894 K0.11049 K0.11104 K0.11172
0.10706 0.10951 0.10999 0.11129 0.11220 0.11130 0.11085 0.11286 0.11371 0.11390 0.11511
2p-NO2 2p-CF3 2p-COOH 2P-H 2p-F 2p-Cl 2p-Br 2p-CH3 2p-OH 2p-OCH3 2-NH2 2p-N(CH3)2
K0.03055 K0.03534 K0.03377 K0.04001 K0.04032 K0.03784 K0.03771 K0.04045 K0.06159 K0.06239 K0.05386 K0.05486
K0.03400 K0.02660 K0.02784 K0.02014 K0.02033 K0.02265 K0.02342 K0.01912 K0.00356 K0.00200 0.00002 0.00051
C3
C4
C5
K0.0079 K0.0189 K0.0171 K0.0298 K0.0319 K0.0260 K0.0254 K0.0338 K0.0414 K0.0401 K0.0493
K0.07056 K0.07304 K0.07344 K0.07575 K0.07469 K0.07381 K0.07383 K0.07633 K0.07650 K0.07677 K0.07850
0.04241 0.03898 0.03832 0.03467 0.03636 0.03772 0.03774 0.03391 0.03370 0.03365 0.03085
0.04098 0.03703 0.03807 0.03329 0.03349 0.03529 0.03538 0.03247 0.03176 0.03037 0.02859
0.04132 0.03637 0.03594 0.03030 0.03197 0.03419 0.03424 0.02915 0.02841 0.02805 0.02475
K0.03965 K0.04342 K0.04379 K0.04771 K0.04614 K0.04476 K0.04471 K0.04838 K0.05175 K0.05241 K0.05290 K0.05362
0.01836 0.01401 0.01376 0.00892 0.01063 0.01233 0.01245 0.00798 0.00656 0.00530 0.00165 0.00089
0.03035 0.02245 0.02504 0.01533 0.01516 0.01834 0.01885 0.01377 0.01804 0.01693 K0.0028 K0.0038
0.07606 0.07008 0.07027 0.06350 0.06497 0.06757 0.06772 0.06215 0.06029 0.05933 0.05337 0.05242
0.01867 0.01499 0.01576 0.01205 0.01115 0.01296 0.01317 0.01125 0.01486 0.01611 0.02383 0.02428
(:A1,2,6) as a function of p-X is: 2p-N(CH3)2 O 2p-NH2 O 2p-OCH3 O 2p-OH O 2p-CH3 O 2 P-H O 2p-F O2p-ClO2p-Br O 2p-CF3O2p-COOHO2p-NO2 in 2p-X (Table 4). Decreasing of :A1,2,6 with the electron withdrawing groups may be due to the increasing of the s character of C2 in cyclopropyl ring through the direct resonance of C2 with the p-X-phenyl. Again, the analogous bond angles in C4–C3–C5 cyclopropyl ring (Scheme 1) do not vary as much, since no perturbations due to p-X-phenyl occurs in this ring. The size of dihedral angle :D3,2,8,13 in 1p-X is, as a function of p-X: 1p-N(CH3)2!1p-NH2!1p-Br!1p-NO2! 1p-OCH3!1p-OH!1p-CH3!1p-Cl!1p-COOH!1p-F!1p-CF3! 1P-H (Table 4). This trend may also be justified by considering the possibility of extension of conjugation through conglomeration of p-X-phenyl with CaC. Finally, this research is to serve those whose primary interest is to replace the fossil fuel and/or the nuclear energy with the most economical and very available solar energy. Hence, the scrutiny of the photochemical energy storage in the ground states of norbornadiene (1)/quadricyclane (2) system is carried out. Therefore, computations and/or measurement of the quantum yields of the scrutinized photolysis, which appears of interest to many workers [30–34], will be addressed elsewhere.
4. Conclusion The effects of both electron withdrawing and electron donating substituents are investigated on various positions
C6
C7
Table 4 Selected B3LYP/6-31G* calculated carbon-carbon bond (R2,3)/angstrom ˆ c,c,c)/degrees and dihedral angle (:D3,2,8,13)/ ˚ ), interatomic angles (A (A degree for substituted norbornadienes (1p-X), and quadricyclanes (2p-X); where XZ–NO2, –F, –Cl, –Br, –CF3, –COOH, –NH2, –NMe2, –OMe, –OH and –Me Compound
1p-NO2 1p-CF3 1p-COOH 1p-F 1p-Cl 1p-Br 1P-H 1p-CH3 1p-OH 1p-OCH3 1p-NH2 1p-N(CH3)2 2p-NO2 2p-CF3 2p-COOH 2p-F 2p-Cl 2p-Br 2P-H 2p-CH3 2p-OH 2p-OCH3 2p-NH2 2p-N(CH3)2
˚ ), interatomic Carbon–carbon bond (R2,3)/angstrom (A ˆ c,c,c)/degrees and dihedral angle (:D3,2,8,13)/ angles (A degree R2,3
A1,2,6
A4,3,5
D3,2,8,13
1.3484 1.3471 1.3478 1.3467 1.3469 1.3469 1.3467 1.3469 1.3470 1.3472 1.3477 1.3479 1.5528 1.5540 1.5536 1.5552 1.5546 1.5545 1.5551 1.5550 1.5599 1.5600 1.5655 1.5657
– – – – – – – – – – – – 58.685 58.702 58.747 59.126 59.043 59.014 59.046 59.094 59.458 59.522 59.989 59.999
– – – – – – – – – – – – 60.117 60.132 60.140 60.140 60.156 60.159 60.178 60.176 60.224 60.224 60.226 60.201
19.6608 20.7023 20.3748 20.61 20.1619 19.3027 21.0778 19.9491 19.7289 19.6682 18.0806 17.4645 16.8759 17.0823 17.0724 20.0759 19.7678 19.3027 19.4807 20.7477 32.3201 37.0113 93.8217 83.1235
M.Z. Kassaee, E. Vessally / Journal of Molecular Structure: THEOCHEM 716 (2005) 159–163
of norbornadiene (1p-X)/quadricyclane (2p-X) system. Electron donating substituents (p-X), attached at phenyl-C2 carbon, increase the solar energy storage. A Hammet r value of K0.81 is found for this process at HF/6-31G* and DFT/6-31G* levels of theory.
Acknowledgements We wish to express our special thanks to Dr Mahjoob, Mr Arshadi, Mr A.R. Bekhradnia, Ms M. Koohi and Mr S.M. Musavi, at Tarbiat Modares University, for their cordial cooperation in completing this research. Technical assistances of Mr A. Sayadi (at the Chemistry Department of Imam Hossein University), Mr M. Amanbaee and Ms Z. Farahani are most appreciated.
Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.theochem. 2004.12.006
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