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From geometrical features of the global minima, a growth mechanism for each series of the BnX clusters was established. The fundamental featuressuch as chemical bonding, electronic distribution, aromaticity, were probed using the analyses of canonical molecular orbital (CMO) in combination with other indices obtained from the electron localization function(ELF), nuclear independent chemical shift (NICS) etc
Quantitatively, different basic thermochemical properties of clusters such as total atomization energies, heats of formation, ionization energies, adiabatic and vertical detachment energies, binding energies and dissociation energies were calculated by using the CCSD(T)/CBS and Gn energies and compared to the available experimental data. Some general and important points emerge as follows.
There is an overall good agreement between the CCSD(T)/CBS and Gn (G3B3 and G4) methods in determining the energetic properties of boron-based clusters. The difference of heats of formation at 0K obtained by these methods is in the range of 0.0 5.0 kcal/mol. Both methods also show good agreement with available experimental data. These results give us confidence that in the lack of experimental data, and also with limited computer resources, the energetic properties of boron clusters can be reliably determined by using the Gn methods(Chapter 2).
Small pure boron clusters Bn with n up to 19 tend to exhibit planar and quasi-planar structures. While the B20 member has a three-dimensional tubular form, the 3D-structures found foreither B9 or B14 appear to be the unexpected cases where the sizes of boron rings are not large enough to hold the planar tendency. For these two structures, the identity of the lowest-energy structures is however not definitely established yet, and this can only be done using CCSD(T)/CBS computations in the future (Chapter 3).
Detachment of one electron from the neutrals tends to destabilize planar features of boron clusters, and the resulting cationic clusters Bn+ are thus favored tohave three-dimensional
structures at smaller sizes of n = 16 - 19. On the contrary, the anionic clusters Bn- with n up to 21 still exhibit planar shape. Interestingly, both B20- and B19- anionic clusters are found to exhibit aromatic andfluxional structures in which one small inner boron ring with a central B-atom is located inside one larger outer boronring. These novel features are expected to be found again for larger
anionic boron clusters. Additionally, geometries of small boron clusters considered reveal that they are constructed on the basis of triangularB3 units with a classical bonding motif of three-center two-electron (3c 2e) (Chapter 3).
One of the most intriguing features of smallboron clusters is their aromaticity. Based on the NICS calculations, our predictions show that most, if not all, closed-shell boron clusters have an aromatic character. Interestingly, while there is a good agreementbetween various indices for evaluating aromatic features of small boronclusters Bn+/0/- with n ≤ 13, the classical Hückel rule of (4N + 2) electrons is no longer valid for larger sizes.
In the present work, we proposed in Chapter 4 the concept of a new type of aromaticity, termed as disk aromaticity, to interpret the bonding features of B19- and B20-. Accordingly, a cyclic cluster will be disk aromatic when its delocalized valence π-electrons fully occupy thedegenerate eigenstates given from the model of a particle in a circularbox. This feature is also proven to be effective on other boron-carbon mixed clusters such as C5B11+/- and C6B120/2-. Additionally, the model is also successfully applied on polycyclic aromatic hydrocarbons (C24H12 and C20H10).
In Chapters 5 and 6, we also determined the global minima and growth mechanism of doped boron clusters BnSi and BnLi in both anionic and neutral states. Lithium tends to donate its unique valence electron to the Bn host and thus form the ionic complexes Liδ+-Bnδ-. Consequently, the
chemical bonding and aromaticity of BnLi0/- clusters are similar to those of Bn- and Bn2-, respectively. On the other hand, small silicon doped boron clusters BnSi with n ≤ 6 are constructed in the motif where replacement of one boron atom of Bn+1 by one silicon atom. In this motif, Si however prefers a peripheralposition of the ring and tends to connect with its two neighboring
B-atoms forming a bridge. BnSi is built up by doping Si on the symmetry axis of the Bn host where Si is bound to B-atoms of the ring and possesses a high coordination number. As a consequence, the chemical bonding andaromaticity features of small silicon doped boron clusters BnSi0/- are similar to those Bn+1- and Bn+12-, respectively.
The final project in this thesis relates to small boron oxides BnOm. The boron monoxidesBnO are formed by either condensing O on a BB edge of a Bn cycle as a η2-brigde or binding one BO boronyl group to a Bn-1 ring. The BO bond is particularly strong and its formation contributes to the overall stability of the new oxide. The balance between both factors is dependent on the inherent stability of the boron cycles.The boron dioxides are formed by incorporating the second O atom into the corresponding monoxides to form BO bonds. The anions BnOm- are constructed by binding the boronyl BO groups to the Bn-1 or Bn-2 rings to form the bonding motifof Bn-m(BO)m-. Interestingly, there is an analogy between geometries of the anions Bn-m(BO)m and those of corresponding Bn-mHm boron hydrides.
Similar observation is found for boron trioxides B3(BO)3 in neutral, anionic and dianionic states. The dianion B3(BO)32- has high symmetry geometry in which three boronyl BO groups are bound to a triangle B3 ring. The boron oxides interestingly conserve some of the properties of the
parent boron clusters such as the planarity and multiple aromaticity.
Generally, the theoretical studies reported in the present thesis point out the structural formation and growth mechanism of small boron based clusters. The energetic properties were obtained with high degree of confidence by using high accuracy computational methods. These predictions will be very useful for future experimental and theoretical investigations. In further work, investigations on the larger sized clusters closing the gap between B20 and B80, and also on bulk materials based on the boron element are highly desirable. These materials are promising candidates for possible applications in hydrogen storage, captures ofindustrial gases (CO, CO2, NO ), and as possible catalysts for chemical reactions.
Furthermore, with the presence of some good quality experimental photoelectron spectra, theoretical simulations of thephotoelectron spectra on the anionic boron Bn- clusters in order to confirm their ground states open an interesting avenue for further studies on the detailed electronic structure. Different spectroscopic signaturesincluding the vibrational (IR) spectra or the magnetic behaviors of open-shell ionic and neutral clusters also form a challenging subject of investigation where a close interaction between experiment and theory is necessary for further advances
One of the most important contributions of this work is the introduction of the novel concept of disk aromaticity which turns out to be effective in the determination of aromatic features of polycyclic compounds. This concept needs to be applied to a larger range of heteroatomic polycyclic compounds such as, for example, the sulflower C16S8 and its hetero-derivatives. We expect that diskaromaticity would become one of the basic chemical concept and be widely used by chemists.
Project
Computational Studies of Small Boron Based Clusters
From geometrical features of the global minima, a growth mechanism for each series of the BnX clusters was established. The fundamental featuressuch as chemical bonding, electronic distribution, aromaticity, were probed using the analyses of canonical molecular orbital (CMO) in combination with other indices obtained from the electron localization function(ELF), nuclear independent chemical shift (NICS) etc
Quantitatively, different basic thermochemical properties of clusters such as total atomization energies, heats of formation, ionization energies, adiabatic and vertical detachment energies, binding energies and dissociation energies were calculated by using the CCSD(T)/CBS and Gn energies and compared to the available experimental data. Some general and important points emerge as follows.
There is an overall good agreement between the CCSD(T)/CBS and Gn (G3B3 and G4) methods in determining the energetic properties of boron-based clusters. The difference of heats of formation at 0K obtained by these methods is in the range of 0.0 5.0 kcal/mol. Both methods also show good agreement with available experimental data. These results give us confidence that in the lack of experimental data, and also with limited computer resources, the energetic properties of boron clusters can be reliably determined by using the Gn methods(Chapter 2).
Small pure boron clusters Bn with n up to 19 tend to exhibit planar and quasi-planar structures. While the B20 member has a three-dimensional tubular form, the 3D-structures found foreither B9 or B14 appear to be the unexpected cases where the sizes of boron rings are not large enough to hold the planar tendency. For these two structures, the identity of the lowest-energy structures is however not definitely established yet, and this can only be done using CCSD(T)/CBS computations in the future (Chapter 3).
Detachment of one electron from the neutrals tends to destabilize planar features of boron clusters, and the resulting cationic clusters Bn+ are thus favored tohave three-dimensional
structures at smaller sizes of n = 16 - 19. On the contrary, the anionic clusters Bn- with n up to 21 still exhibit planar shape. Interestingly, both B20- and B19- anionic clusters are found to exhibit aromatic andfluxional structures in which one small inner boron ring with a central B-atom is located inside one larger outer boronring. These novel features are expected to be found again for larger
anionic boron clusters. Additionally, geometries of small boron clusters considered reveal that they are constructed on the basis of triangularB3 units with a classical bonding motif of three-center two-electron (3c 2e) (Chapter 3).
One of the most intriguing features of smallboron clusters is their aromaticity. Based on the NICS calculations, our predictions show that most, if not all, closed-shell boron clusters have an aromatic character. Interestingly, while there is a good agreementbetween various indices for evaluating aromatic features of small boronclusters Bn+/0/- with n ≤ 13, the classical Hückel rule of (4N + 2) electrons is no longer valid for larger sizes.
In the present work, we proposed in Chapter 4 the concept of a new type of aromaticity, termed as disk aromaticity, to interpret the bonding features of B19- and B20-. Accordingly, a cyclic cluster will be disk aromatic when its delocalized valence π-electrons fully occupy thedegenerate eigenstates given from the model of a particle in a circularbox. This feature is also proven to be effective on other boron-carbon mixed clusters such as C5B11+/- and C6B120/2-. Additionally, the model is also successfully applied on polycyclic aromatic hydrocarbons (C24H12 and C20H10).
In Chapters 5 and 6, we also determined the global minima and growth mechanism of doped boron clusters BnSi and BnLi in both anionic and neutral states. Lithium tends to donate its unique valence electron to the Bn host and thus form the ionic complexes Liδ+-Bnδ-. Consequently, the
chemical bonding and aromaticity of BnLi0/- clusters are similar to those of Bn- and Bn2-, respectively. On the other hand, small silicon doped boron clusters BnSi with n ≤ 6 are constructed in the motif where replacement of one boron atom of Bn+1 by one silicon atom. In this motif, Si however prefers a peripheralposition of the ring and tends to connect with its two neighboring
B-atoms forming a bridge. BnSi is built up by doping Si on the symmetry axis of the Bn host where Si is bound to B-atoms of the ring and possesses a high coordination number. As a consequence, the chemical bonding andaromaticity features of small silicon doped boron clusters BnSi0/- are similar to those Bn+1- and Bn+12-, respectively.
The final project in this thesis relates to small boron oxides BnOm. The boron monoxidesBnO are formed by either condensing O on a BB edge of a Bn cycle as a η2-brigde or binding one BO boronyl group to a Bn-1 ring. The BO bond is particularly strong and its formation contributes to the overall stability of the new oxide. The balance between both factors is dependent on the inherent stability of the boron cycles.The boron dioxides are formed by incorporating the second O atom into the corresponding monoxides to form BO bonds. The anions BnOm- are constructed by binding the boronyl BO groups to the Bn-1 or Bn-2 rings to form the bonding motifof Bn-m(BO)m-. Interestingly, there is an analogy between geometries of the anions Bn-m(BO)m and those of corresponding Bn-mHm boron hydrides.
Similar observation is found for boron trioxides B3(BO)3 in neutral, anionic and dianionic states. The dianion B3(BO)32- has high symmetry geometry in which three boronyl BO groups are bound to a triangle B3 ring. The boron oxides interestingly conserve some of the properties of the
parent boron clusters such as the planarity and multiple aromaticity.
Generally, the theoretical studies reported in the present thesis point out the structural formation and growth mechanism of small boron based clusters. The energetic properties were obtained with high degree of confidence by using high accuracy computational methods. These predictions will be very useful for future experimental and theoretical investigations. In further work, investigations on the larger sized clusters closing the gap between B20 and B80, and also on bulk materials based on the boron element are highly desirable. These materials are promising candidates for possible applications in hydrogen storage, captures ofindustrial gases (CO, CO2, NO ), and as possible catalysts for chemical reactions.
Furthermore, with the presence of some good quality experimental photoelectron spectra, theoretical simulations of thephotoelectron spectra on the anionic boron Bn- clusters in order to confirm their ground states open an interesting avenue for further studies on the detailed electronic structure. Different spectroscopic signaturesincluding the vibrational (IR) spectra or the magnetic behaviors of open-shell ionic and neutral clusters also form a challenging subject of investigation where a close interaction between experiment and theory is necessary for further advances
One of the most important contributions of this work is the introduction of the novel concept of disk aromaticity which turns out to be effective in the determination of aromatic features of polycyclic compounds. This concept needs to be applied to a larger range of heteroatomic polycyclic compounds such as, for example, the sulflower C16S8 and its hetero-derivatives. We expect that diskaromaticity would become one of the basic chemical concept and be widely used by chemists.
Date:1 Feb 2009 → 26 Jun 2012
Keywords:hydrogen storage and energy
Disciplines:Biochemistry and metabolism, Medical biochemistry and metabolism, Physical chemistry, Theoretical and computational chemistry, Other chemical sciences, Manufacturing engineering, Safety engineering
Project type:PhD project