- Email: [email protected]
Nanoscale molecular surface electron attachment
Chemical Physics Letters 445 (2007) 89–94 www.elsevier.com/locate/cplett
Nanoscale molecular surface electron attachment Abraham F. Jalbout b
a,*
, R. del Castillo a, L. Adamowicz
b
a Instituto de Quı´mica, Universidad Nacional Auto´noma de Me´xico, Mexico Department of Chemistry, The University of Arizona, Tucson, AZ, United States
Received 17 May 2007; in final form 28 June 2007 Available online 1 July 2007
Abstract In this work we have considered the ability of nanoscale molecular surfaces (around 2 nm in width) in trapping excess electrons. As previously reported, we suggested that molecular surfaces with hydrogen bonding networks (consisting of OH groups) on one side of the surface and hydrogen atoms on the opposite side were capable of forming stable dipole-bound anions. The increased dipole moments generated by the OH groups coupled to the partial positive charge of the hydrogen atoms creates charge pockets that are can trap excess electrons. We have extended the size of the surface to study the effect of electron localization on molecular surfaces. Ó 2007 Published by Elsevier B.V.
1. Introduction The design of novel electrons traps has been the topic of many research investigations for some time. These types of studies encompass the search for systems by which the internal dipole moments can be used to increase the attraction of excess electrons to a diffuse state of a molecular cluster or surface. The applications of this work are of widespread interest in the design of electrical conduits and in the formation of understanding how large scale molecular surfaces interact with extra charge. Recently, we have come across a set of unique geometrical arrangements which are composed of hydrogen fluoride (HF) subunits [1]. These structures were found to adopt interesting configurations that were capable of trapping excess electrons in-between their polar hydrogen bonds. These systems were among the first studies which suggested that small clusters could trap electrons in molecular ‘tweezer’ arrangements. It is known that systems of extended hydrogen bonding networks (as in the above mentioned example) can have adverse effects on various spectroscopic techniques [2]. Electron traps of this category
*
Corresponding author. Fax: +1 520 621 8047. E-mail address: [email protected] (A.F. Jalbout).
0009-2614/$ - see front matter Ó 2007 Published by Elsevier B.V. doi:10.1016/j.cplett.2007.06.127
as well as the progression of theoretical methods and applications has been the focus of a recent review that explains the importance and significance of trapping excess electrons in molecular frameworks [3]. The current focus of investigations by our research group is in the development of novel molecular surfaces which are capable of trapping excess electrons on molecular surfaces [4,5]. In these surfaces we proposed that molecular surfaces with hydrogen atoms on one side and OH groups on the opposite side formed stable dipole-bound anions. The reason for this has to do with the fact that the increase in dipole moment (which is a result of the uneven distribution of the OH groups) coupled to the partial positive charges of the hydrogen atoms creates pockets of charge capable of attracting excess electrons. Our computations suggest that the anions resulting from these systems are stable with respect to electron detachment and that as the number of OH groups increases as well as the size of the surface so does the value of the VDE (vertical electron detachment energy). This concept bears significance since experimental studies have shown [6,7] that extended surfaces are capable of undergoing ion-transfer chemistry, which is of direct relevance to the physical principle described herein. Additionally, experimental findings suggest [8–10] that charge transfer (CT) mechanisms on extended surfaces and films can lead to the absorption
90
A.F. Jalbout et al. / Chemical Physics Letters 445 (2007) 89–94
Scheme 1. General representation of the proposed molecular surfaces.
and formation of molecular systems. This therefore acts as a precursor for further theoretical exploration. While attempts to localize electron density on molecular surfaces has been briefly proposed [5] a definitive molecular arrangement has not been constructed. The aim of the present study is to extend the surfaces to nanodimensions, while maintaining the number of OH groups constant and localized in regions of the molecular framework. The reason behind this is that if we can localize electrons on distinct regions in molecular surfaces than they can expelled readily and used for charge conducting materials. A general representation of the configurations is presented in Scheme 1. As we can see the OH groups are in regions of the surface and are surrounded by cyclohexane rings. Additionally, we have shown that both HF [11] and H2O [12] are capable of solvating excess electrons on molecular surfaces, and this has been extended to our current model in the final stages of this investigation to test the efficacy of the suggested model. If electrons can be solvated on surfaces by polar molecules, then by using the current systems it can be possible for us to localize electrons on extended nanoscale molecular frameworks. This work should be of importance in future applications in the design of novel electron traps. 2. Calculations and discussion The quantum chemical calculations described in this study have been carried out with the GAUSSIAN 03 program codes [13]. Due to the fact that the systems under consideration are very large (between 174–208 atoms) the geometry optimizations were performed with the spin-unrestricted Hartree–Fock (UHF) method. Even calculations at this level of theory required vast supercomputer resources for long periods of time. From our recent work on molecular surfaces [4,11,12] the 3-21G* basis set was used for all computations, but since these systems are much larger, we must
moderate the level of theory due to limitations in computational resources, for that reason we use the UHF method coupled to a STO-3G* basis set. We also added six very diffuse Gaussian sp-shells with exponents equal to 0.01, 0.002, 0.0004, 0.00008, 0.000016, and 0.000032, and a p-shell with exponent 0.036 that have been placed away from the molecular framework of the system. The augmentation of the STO-3G* basis set and the addition of more basis functions is important, however, it is not necessary for the calculations presented herein. The functions added are so diffuse that the correct anion state will be located quite readily, and since no experimental data on these systems exists there is no reference of comparison. The VDE values are of qualitative significance and should be used to analyze relative energies. It is important to mention that the placement of the diffuse basis functions was also allowed to optimize during the course of the optimization procedure. The exact coordinates of the basis functions have been defined at the position by which the lowest unoccupied molecular orbital (LUMO) for the neutral species is the most negative in energy. Based on Koopmans theorem [3] the LUMO energy is an approximation to the vertical electron affinity. The HOMO energy of the corresponding anion is an approximation to its vertical electron detachment energy. Therefore, this principle has been obeyed throughout the calculations. As previously mentioned above, in Scheme 1 we depict the graphical depiction of the configurations studied in this work. In all situations structures are around 2.2 nm by 1.5 nm, ranging from 25 rings (species I) to 32 rings (species VIII). The number of OH groups added was eleven, which is what we previously reported [4,5] as the minimum number of OH groups needed to form stable dipole-bound anions on extended molecular surfaces. Additionally, the cyclohexane rings were added in order to localize the OH groups in distinct regions on the surfaces. These were the
A.F. Jalbout et al. / Chemical Physics Letters 445 (2007) 89–94
smallest possible configurations that permitted the electrons to localize on molecular surfaces and will be described in this section. Fig. 1 displays the HOMO molecular orbital plots of the structures studied in this work. Fig. 2 (as we will describe later) depicts an example of electron solvation on localized pockets in extended molecular surfaces by both the HF and H2O molecules. For the calculation of the solvated anions we have used the smallest surface (structure I) as the test case. The total energies, and VDE values for the structures studied herein are listed in Table 1 (where Ia, Ib are the VDE values of the HF, H2O solvated electron example). The first system that we will study is the 25 ring system which is depicted as I in this work. From the table we can see that the VDE is around 13.2 eV, which is relatively low due to the fact that the OH groups are surrounded by nonpolar cylcohexane rings. The most important feature to
91
extract from this is that these surfaces are capable of localizing electrons in charge pockets on extended molecular surfaces. While, the VDE values are small, they are positive and the electron is stable with respect to detachment. Fig. 1 demonstrates that the excess electron density appears to be directly influenced by the region of the OH groups. Increasing the number of rings by 1–26 results in structure II which has a VDE of around 11.6 meV. The decrease in the VDE is logical considering the fact that the dipole moment of the system decreases due to the fact that the cyclohexane rings are not polar and since the number of OH groups is constant the stability of the electron decreases. From our previous investigations [4] we have mentioned that as the size of the surface and the number of OH groups increases the VDE will also rise. However, since this is a linear relationship, if the number of OH groups is constant and the surface increases in size than
Fig. 1. UHF/STO-3G*X anion structures and HOMO plots for the nanoscale molecular surfaces.
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Fig. 1 (continued)
the VDE will decrease. The stability of the electron on the localized region of the surface is still stable and should be experimentally viable. As we can see from Fig. 1 the electron density is situated along the axis of the OH group concentration and again reinforces our assertions previously alluded to. If we increase the number of rings to 27 we obtain species III which has a VDE of around 11.53 meV. It appears that the VDE is similar to the previous structure and the electron density appears to be localized as a result of the hydrogen bonding OH network. For the next structure, species IV (which has 28 cyclohexane rings), the VDE is similar yielding a value of around 11.51 meV. It is an interesting observation that apart from the first structure the VDE values remains similar as we increase the size of the molecular surface. We therefore believe that the localiza-
tion of excess electrons in this configuration should be possible in larger molecular frameworks. For the next species (structure V) we can see that the VDE for structure is around 11.2 meV which is slightly lower than structure IV. This species has 29 rings and is the most stable conformer with the highest VDE for this arrangement of our molecular surfaces. From the figure again we can see that the excess electron occupies a dipole-bound anion state. Increasing the size of the surface to 30 cyclohexane rings (structure VI) we obtain a decrease in the VDE value to 10.96 meV. From the figure it is visible that while the extra rings have been added the electron density is localized along the axis of the OH hydrogen bonding region on the molecular surface. If the number of cyclohexane rings is now 31 (species VII) the VDE decreases to
A.F. Jalbout et al. / Chemical Physics Letters 445 (2007) 89–94
93
Fig. 2. UHF/STO-3G*X and HOMO depictions of the solvated AISE anions formed by HF and H2O on molecular surface I.
Table 1 Total energies in hartrees/particle (Eh) calculated at the UHF/STO-3G*X level of theory for the structures shown in Figs. 1 and 2 Structure
Number of rings
I Ia Ib II III IV V VI VII VIII
25 25 25 26 27 28 29 30 31 32
Anionianion (Eh) 3485.5551114 3584.0252724 3560.4139850 3600.1450931 3676.1529553 3752.1711812 3828.2012264 3942.7128072 4018.8137839 4094.7408237
Neutralianion (Eh) 3485.5546248 3584.0250216 3560.4139578 3600.1446678 3676.1525315 3752.1707581 3828.2008161 3942.7124046 4018.8134033 4094.7405557
VDE (meV) 13.24 6.82 7.40 11.57 11.53 11.51 11.16 10.96 10.35 7.29
Anionianion is the anion energy at its equilibrium geometry and neutralianion is the energy of the neutral structure at the anions geometry. The VDE is the vertical detachment energy (meV), which is the difference between the anionianion and the neutralianion energies. Note that a,b are the HF and H2O solvated AISE anions formed with structure I.
around 10.4 meV. From the HOMO plot we can see that this is a clear example of how the excess electron is attracted to the surface due to the field induced by the OH groups. The final system consists of 32 cyclohexane rings and is labeled as VIII in this work. From the table we can see that the VDE has reduced significantly to around 7.3 meV which can be attributed to the arrangement of the cyclohexane rings. We have tried other configurations but this is the lowest energy structure which leads to the highest VDE values. It is important to note that in species I–IV cyclohexane rings were added to one side of the surface, but in V–VIII they were added at the opposite side. In the later case, the addition of these rings caused a significant drop in the VDE to occur. While it is lower than the previous systems it still serves to demonstrate that the excess electron density is still localized along the plane of the molecular surface.
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A.F. Jalbout et al. / Chemical Physics Letters 445 (2007) 89–94
3. Summary In this work we have explored the possibility of extended molecular surfaces in trapping excess electrons in stable dipole-bound anion states. The surfaces considered in this work consisted of eleven OH groups, which was the minimum number of OH groups needed to form stable dipole-bound anions on larger molecular surfaces [4,5]. We extended the molecular surfaces to consist of 25–32 cyclohexane rings, in order to demonstrate the effect of electron localization more clearly. It was shown that due to the non-polarity of the cyclohexane rings extending the surface (while maintaining the number of OH groups constant) leads to decreases in the dipole moment and corresponding VDE values. However, the systems are still stable with respect to electron detachment and as the HOMO plots represent the electron density appears to localize along the axis by which the OH hydrogen bonded networks form on the surfaces. To further demonstrate the significance of the presented calculations we have computed solvated anion states on these surfaces. The results of these calculations are depicted graphically in Fig. 2 and listed in Table 1 (by which Ia, Ib are the VDE values of the HF, H2O solvated surfaces). We have chosen I to do these computations since it is the smallest surface and suffices to illustrate the effects of localized electron solvation. From the table we can see that the VDE values of the HF and H2O solvated surfaces are around 6.8 and 7.4 meV, respectively. The intermolecular separations between HF and H2O and the molecular sur˚ , respectively. The interface are around 11.97 and 11.1 A esting feature of these results is that the electrons are solvated (by HF and H2O) most profoundly along the region where the OH groups are situated in the molecular surfaces. Therefore, by using our localization of electrons approach it is possible to not only stabilize excess electrons but absorb molecules via solvated anion state formation. It is important to note that these co-called ‘Anions with Suspended Electrons’ or AISE’s for short [3,14] are metastable products formed from kinetic and thermodynamic effects.
While they are short lived species we believe that they are experimentally viable and should be considered as potential products. Since the presented cyclohexane systems resemble graphite frameworks (i.e. diamond) we can use this model to simulate charge conduction in complexes of practical importance. It is our belief that the study of these extended cyclohexane frameworks can also be useful when trying to model ion-chemistry at the surface. One of the most important features is that our scheme permits electrons to be localized at specific points in extended molecular surfaces. They can serve not only in the search for novel electrical conduits but also to add to the ‘tool box’ of molecular surface anion traps. Acknowledgements Special thanks are extended to DGSCA as well as UNAM for valuable financial and computational resources. References [1] A.F. Jalbout, C.A. Morgado, L. Adamowicz, Chem. Phys. Lett. 383 (2004) 317. [2] X.-F. Pang, H.-W. Zhang, A.F. Jalbout, J. Phys. Chem. Solids 66 (2005) 963. [3] A.F. Jalbout, L. Adamowicz, Adv. Quantum Chem. 52 (2007) 233. [4] A.F. Jalbout, L. Adamowicz, Mol. Phys. 19 (2006) 3101. [5] A.F. Jalbout, Int. J. Quantum Chem. 107, in press. [6] A. Bialonska, Z. Ciunik, Crys. Eng. Comm. 8 (2006) 66. [7] G. Klesper, F.W. Ro¨llgen, Surf. Sci. 422 (1999) 107. [8] J.F. Arenas, I. Lopez-Tocon, J.L. Castro, S.P. Centeno, M.R. LopezRamirez, J.C. Otero, J. Ram. Spectrosc. 36 (2005) 515. [9] K. Suzuki, H. Shiroishi, M. Hoshino, M. Kaneko, J. Phys. Chem. A 107 (2003) 5523. [10] A.E. Baber, S.C. Jensen, E. Sykes, H. Charles, J. Am. Chem. Soc. 128 (2006) 15384. [11] A.F. Jalbout, R. del Castillo, L. Adamowicz, Chem. Phys. Lett. 434 (2007) 15. [12] A.F. Jalbout, Mol. Phys. 105 (2007) 111. [13] M.J. Frisch et al., GAUSSIAN 03, Revision B.05, Gaussian Inc., Pittsburg, PA, 2003. [14] M. Gutowski et al., Phys. Rev. Lett. 88 (2002) 143001.
Nanoscale molecular surface electron attachment Abraham F. Jalbout b
a,*
, R. del Castillo a, L. Adamowicz
b
a Instituto de Quı´mica, Universidad Nacional Auto´noma de Me´xico, Mexico Department of Chemistry, The University of Arizona, Tucson, AZ, United States
Received 17 May 2007; in final form 28 June 2007 Available online 1 July 2007
Abstract In this work we have considered the ability of nanoscale molecular surfaces (around 2 nm in width) in trapping excess electrons. As previously reported, we suggested that molecular surfaces with hydrogen bonding networks (consisting of OH groups) on one side of the surface and hydrogen atoms on the opposite side were capable of forming stable dipole-bound anions. The increased dipole moments generated by the OH groups coupled to the partial positive charge of the hydrogen atoms creates charge pockets that are can trap excess electrons. We have extended the size of the surface to study the effect of electron localization on molecular surfaces. Ó 2007 Published by Elsevier B.V.
1. Introduction The design of novel electrons traps has been the topic of many research investigations for some time. These types of studies encompass the search for systems by which the internal dipole moments can be used to increase the attraction of excess electrons to a diffuse state of a molecular cluster or surface. The applications of this work are of widespread interest in the design of electrical conduits and in the formation of understanding how large scale molecular surfaces interact with extra charge. Recently, we have come across a set of unique geometrical arrangements which are composed of hydrogen fluoride (HF) subunits [1]. These structures were found to adopt interesting configurations that were capable of trapping excess electrons in-between their polar hydrogen bonds. These systems were among the first studies which suggested that small clusters could trap electrons in molecular ‘tweezer’ arrangements. It is known that systems of extended hydrogen bonding networks (as in the above mentioned example) can have adverse effects on various spectroscopic techniques [2]. Electron traps of this category
*
Corresponding author. Fax: +1 520 621 8047. E-mail address: [email protected] (A.F. Jalbout).
0009-2614/$ - see front matter Ó 2007 Published by Elsevier B.V. doi:10.1016/j.cplett.2007.06.127
as well as the progression of theoretical methods and applications has been the focus of a recent review that explains the importance and significance of trapping excess electrons in molecular frameworks [3]. The current focus of investigations by our research group is in the development of novel molecular surfaces which are capable of trapping excess electrons on molecular surfaces [4,5]. In these surfaces we proposed that molecular surfaces with hydrogen atoms on one side and OH groups on the opposite side formed stable dipole-bound anions. The reason for this has to do with the fact that the increase in dipole moment (which is a result of the uneven distribution of the OH groups) coupled to the partial positive charges of the hydrogen atoms creates pockets of charge capable of attracting excess electrons. Our computations suggest that the anions resulting from these systems are stable with respect to electron detachment and that as the number of OH groups increases as well as the size of the surface so does the value of the VDE (vertical electron detachment energy). This concept bears significance since experimental studies have shown [6,7] that extended surfaces are capable of undergoing ion-transfer chemistry, which is of direct relevance to the physical principle described herein. Additionally, experimental findings suggest [8–10] that charge transfer (CT) mechanisms on extended surfaces and films can lead to the absorption
90
A.F. Jalbout et al. / Chemical Physics Letters 445 (2007) 89–94
Scheme 1. General representation of the proposed molecular surfaces.
and formation of molecular systems. This therefore acts as a precursor for further theoretical exploration. While attempts to localize electron density on molecular surfaces has been briefly proposed [5] a definitive molecular arrangement has not been constructed. The aim of the present study is to extend the surfaces to nanodimensions, while maintaining the number of OH groups constant and localized in regions of the molecular framework. The reason behind this is that if we can localize electrons on distinct regions in molecular surfaces than they can expelled readily and used for charge conducting materials. A general representation of the configurations is presented in Scheme 1. As we can see the OH groups are in regions of the surface and are surrounded by cyclohexane rings. Additionally, we have shown that both HF [11] and H2O [12] are capable of solvating excess electrons on molecular surfaces, and this has been extended to our current model in the final stages of this investigation to test the efficacy of the suggested model. If electrons can be solvated on surfaces by polar molecules, then by using the current systems it can be possible for us to localize electrons on extended nanoscale molecular frameworks. This work should be of importance in future applications in the design of novel electron traps. 2. Calculations and discussion The quantum chemical calculations described in this study have been carried out with the GAUSSIAN 03 program codes [13]. Due to the fact that the systems under consideration are very large (between 174–208 atoms) the geometry optimizations were performed with the spin-unrestricted Hartree–Fock (UHF) method. Even calculations at this level of theory required vast supercomputer resources for long periods of time. From our recent work on molecular surfaces [4,11,12] the 3-21G* basis set was used for all computations, but since these systems are much larger, we must
moderate the level of theory due to limitations in computational resources, for that reason we use the UHF method coupled to a STO-3G* basis set. We also added six very diffuse Gaussian sp-shells with exponents equal to 0.01, 0.002, 0.0004, 0.00008, 0.000016, and 0.000032, and a p-shell with exponent 0.036 that have been placed away from the molecular framework of the system. The augmentation of the STO-3G* basis set and the addition of more basis functions is important, however, it is not necessary for the calculations presented herein. The functions added are so diffuse that the correct anion state will be located quite readily, and since no experimental data on these systems exists there is no reference of comparison. The VDE values are of qualitative significance and should be used to analyze relative energies. It is important to mention that the placement of the diffuse basis functions was also allowed to optimize during the course of the optimization procedure. The exact coordinates of the basis functions have been defined at the position by which the lowest unoccupied molecular orbital (LUMO) for the neutral species is the most negative in energy. Based on Koopmans theorem [3] the LUMO energy is an approximation to the vertical electron affinity. The HOMO energy of the corresponding anion is an approximation to its vertical electron detachment energy. Therefore, this principle has been obeyed throughout the calculations. As previously mentioned above, in Scheme 1 we depict the graphical depiction of the configurations studied in this work. In all situations structures are around 2.2 nm by 1.5 nm, ranging from 25 rings (species I) to 32 rings (species VIII). The number of OH groups added was eleven, which is what we previously reported [4,5] as the minimum number of OH groups needed to form stable dipole-bound anions on extended molecular surfaces. Additionally, the cyclohexane rings were added in order to localize the OH groups in distinct regions on the surfaces. These were the
A.F. Jalbout et al. / Chemical Physics Letters 445 (2007) 89–94
smallest possible configurations that permitted the electrons to localize on molecular surfaces and will be described in this section. Fig. 1 displays the HOMO molecular orbital plots of the structures studied in this work. Fig. 2 (as we will describe later) depicts an example of electron solvation on localized pockets in extended molecular surfaces by both the HF and H2O molecules. For the calculation of the solvated anions we have used the smallest surface (structure I) as the test case. The total energies, and VDE values for the structures studied herein are listed in Table 1 (where Ia, Ib are the VDE values of the HF, H2O solvated electron example). The first system that we will study is the 25 ring system which is depicted as I in this work. From the table we can see that the VDE is around 13.2 eV, which is relatively low due to the fact that the OH groups are surrounded by nonpolar cylcohexane rings. The most important feature to
91
extract from this is that these surfaces are capable of localizing electrons in charge pockets on extended molecular surfaces. While, the VDE values are small, they are positive and the electron is stable with respect to detachment. Fig. 1 demonstrates that the excess electron density appears to be directly influenced by the region of the OH groups. Increasing the number of rings by 1–26 results in structure II which has a VDE of around 11.6 meV. The decrease in the VDE is logical considering the fact that the dipole moment of the system decreases due to the fact that the cyclohexane rings are not polar and since the number of OH groups is constant the stability of the electron decreases. From our previous investigations [4] we have mentioned that as the size of the surface and the number of OH groups increases the VDE will also rise. However, since this is a linear relationship, if the number of OH groups is constant and the surface increases in size than
Fig. 1. UHF/STO-3G*X anion structures and HOMO plots for the nanoscale molecular surfaces.
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Fig. 1 (continued)
the VDE will decrease. The stability of the electron on the localized region of the surface is still stable and should be experimentally viable. As we can see from Fig. 1 the electron density is situated along the axis of the OH group concentration and again reinforces our assertions previously alluded to. If we increase the number of rings to 27 we obtain species III which has a VDE of around 11.53 meV. It appears that the VDE is similar to the previous structure and the electron density appears to be localized as a result of the hydrogen bonding OH network. For the next structure, species IV (which has 28 cyclohexane rings), the VDE is similar yielding a value of around 11.51 meV. It is an interesting observation that apart from the first structure the VDE values remains similar as we increase the size of the molecular surface. We therefore believe that the localiza-
tion of excess electrons in this configuration should be possible in larger molecular frameworks. For the next species (structure V) we can see that the VDE for structure is around 11.2 meV which is slightly lower than structure IV. This species has 29 rings and is the most stable conformer with the highest VDE for this arrangement of our molecular surfaces. From the figure again we can see that the excess electron occupies a dipole-bound anion state. Increasing the size of the surface to 30 cyclohexane rings (structure VI) we obtain a decrease in the VDE value to 10.96 meV. From the figure it is visible that while the extra rings have been added the electron density is localized along the axis of the OH hydrogen bonding region on the molecular surface. If the number of cyclohexane rings is now 31 (species VII) the VDE decreases to
A.F. Jalbout et al. / Chemical Physics Letters 445 (2007) 89–94
93
Fig. 2. UHF/STO-3G*X and HOMO depictions of the solvated AISE anions formed by HF and H2O on molecular surface I.
Table 1 Total energies in hartrees/particle (Eh) calculated at the UHF/STO-3G*X level of theory for the structures shown in Figs. 1 and 2 Structure
Number of rings
I Ia Ib II III IV V VI VII VIII
25 25 25 26 27 28 29 30 31 32
Anionianion (Eh) 3485.5551114 3584.0252724 3560.4139850 3600.1450931 3676.1529553 3752.1711812 3828.2012264 3942.7128072 4018.8137839 4094.7408237
Neutralianion (Eh) 3485.5546248 3584.0250216 3560.4139578 3600.1446678 3676.1525315 3752.1707581 3828.2008161 3942.7124046 4018.8134033 4094.7405557
VDE (meV) 13.24 6.82 7.40 11.57 11.53 11.51 11.16 10.96 10.35 7.29
Anionianion is the anion energy at its equilibrium geometry and neutralianion is the energy of the neutral structure at the anions geometry. The VDE is the vertical detachment energy (meV), which is the difference between the anionianion and the neutralianion energies. Note that a,b are the HF and H2O solvated AISE anions formed with structure I.
around 10.4 meV. From the HOMO plot we can see that this is a clear example of how the excess electron is attracted to the surface due to the field induced by the OH groups. The final system consists of 32 cyclohexane rings and is labeled as VIII in this work. From the table we can see that the VDE has reduced significantly to around 7.3 meV which can be attributed to the arrangement of the cyclohexane rings. We have tried other configurations but this is the lowest energy structure which leads to the highest VDE values. It is important to note that in species I–IV cyclohexane rings were added to one side of the surface, but in V–VIII they were added at the opposite side. In the later case, the addition of these rings caused a significant drop in the VDE to occur. While it is lower than the previous systems it still serves to demonstrate that the excess electron density is still localized along the plane of the molecular surface.
94
A.F. Jalbout et al. / Chemical Physics Letters 445 (2007) 89–94
3. Summary In this work we have explored the possibility of extended molecular surfaces in trapping excess electrons in stable dipole-bound anion states. The surfaces considered in this work consisted of eleven OH groups, which was the minimum number of OH groups needed to form stable dipole-bound anions on larger molecular surfaces [4,5]. We extended the molecular surfaces to consist of 25–32 cyclohexane rings, in order to demonstrate the effect of electron localization more clearly. It was shown that due to the non-polarity of the cyclohexane rings extending the surface (while maintaining the number of OH groups constant) leads to decreases in the dipole moment and corresponding VDE values. However, the systems are still stable with respect to electron detachment and as the HOMO plots represent the electron density appears to localize along the axis by which the OH hydrogen bonded networks form on the surfaces. To further demonstrate the significance of the presented calculations we have computed solvated anion states on these surfaces. The results of these calculations are depicted graphically in Fig. 2 and listed in Table 1 (by which Ia, Ib are the VDE values of the HF, H2O solvated surfaces). We have chosen I to do these computations since it is the smallest surface and suffices to illustrate the effects of localized electron solvation. From the table we can see that the VDE values of the HF and H2O solvated surfaces are around 6.8 and 7.4 meV, respectively. The intermolecular separations between HF and H2O and the molecular sur˚ , respectively. The interface are around 11.97 and 11.1 A esting feature of these results is that the electrons are solvated (by HF and H2O) most profoundly along the region where the OH groups are situated in the molecular surfaces. Therefore, by using our localization of electrons approach it is possible to not only stabilize excess electrons but absorb molecules via solvated anion state formation. It is important to note that these co-called ‘Anions with Suspended Electrons’ or AISE’s for short [3,14] are metastable products formed from kinetic and thermodynamic effects.
While they are short lived species we believe that they are experimentally viable and should be considered as potential products. Since the presented cyclohexane systems resemble graphite frameworks (i.e. diamond) we can use this model to simulate charge conduction in complexes of practical importance. It is our belief that the study of these extended cyclohexane frameworks can also be useful when trying to model ion-chemistry at the surface. One of the most important features is that our scheme permits electrons to be localized at specific points in extended molecular surfaces. They can serve not only in the search for novel electrical conduits but also to add to the ‘tool box’ of molecular surface anion traps. Acknowledgements Special thanks are extended to DGSCA as well as UNAM for valuable financial and computational resources. References [1] A.F. Jalbout, C.A. Morgado, L. Adamowicz, Chem. Phys. Lett. 383 (2004) 317. [2] X.-F. Pang, H.-W. Zhang, A.F. Jalbout, J. Phys. Chem. Solids 66 (2005) 963. [3] A.F. Jalbout, L. Adamowicz, Adv. Quantum Chem. 52 (2007) 233. [4] A.F. Jalbout, L. Adamowicz, Mol. Phys. 19 (2006) 3101. [5] A.F. Jalbout, Int. J. Quantum Chem. 107, in press. [6] A. Bialonska, Z. Ciunik, Crys. Eng. Comm. 8 (2006) 66. [7] G. Klesper, F.W. Ro¨llgen, Surf. Sci. 422 (1999) 107. [8] J.F. Arenas, I. Lopez-Tocon, J.L. Castro, S.P. Centeno, M.R. LopezRamirez, J.C. Otero, J. Ram. Spectrosc. 36 (2005) 515. [9] K. Suzuki, H. Shiroishi, M. Hoshino, M. Kaneko, J. Phys. Chem. A 107 (2003) 5523. [10] A.E. Baber, S.C. Jensen, E. Sykes, H. Charles, J. Am. Chem. Soc. 128 (2006) 15384. [11] A.F. Jalbout, R. del Castillo, L. Adamowicz, Chem. Phys. Lett. 434 (2007) 15. [12] A.F. Jalbout, Mol. Phys. 105 (2007) 111. [13] M.J. Frisch et al., GAUSSIAN 03, Revision B.05, Gaussian Inc., Pittsburg, PA, 2003. [14] M. Gutowski et al., Phys. Rev. Lett. 88 (2002) 143001.