DFT Study of Interactions of Carbenes with Boron …. Kakkar et al / Chemistry Journal (2011), Vol....

R. Kakkar et al / Chemistry Journal (2011), Vol. 01, Issue 01, pp. 9-20 ISSN 2049-954X

Available online at www.scientific-journals.co.uk 9

Research Paper

DFT Study of Interactions of Carbenes with Boron Nitride Nanotubes

Rita Kakkar* and Smriti Sharma

* Department of Chemistry, University of Delhi, Delhi-110 007, INDIA

E-Mail: [email protected] Abstract Single-walled boron nitride nanotubes are chosen as model reactants, and (10,0) and (6,6) are chosen as representatives of armchair and zigzag nanotubes, respectively, to study the interaction of carbenes of the type :CX2. It is found that, contrary to the case of carbon nanotubes, boron nitride tubes, particularly armchair BNNTs, do not show a propensity for cyclopropane ring formation. The SWBNNT (10,0) system with :CH2 forms an insertion product. Mulliken population analysis is also done to ascertain the charge distribution in the final product and to emphasize the effect of electronegativity on the functionalizations. The band gaps are also analysed and it is noted that the sidewall functionalisations by carbenes have a profound effect on the electronic properties of boron nitride nanotubes. The carbene adopts a triplet state for zigzag BNNTs, but remains in the singlet state for the armchair BNNTs. The bandgap is found to decrease with increasing tube diameter. Keywords: Boron Nitride Nanotubes, Zigzag, Armchair, Diameter, Adsorption, Carbene, Bandgap 1. Introduction After the discovery of carbon nanotubes, the existence of boron nitride nanotubes (BNNTs) was also predicted theoretically (Blase et al, 1994 and Rubio et al, 1994) based on the structural analogies between the parent materials for carbon nanotubes and boron nitride nanotubes, both being layered materials composed of layers of hexagonal lattices. Graphite has carbon atoms at all lattice points, while hexagonal boron nitride (h-BN) is composed of alternating boron and nitrogen atoms. These materials only differ in terms of their layer stacking. In h-BN, layers are arranged so that boron atoms in one layer are located directly on top of nitrogen atoms in the neighboring layer and vice versa, so that the hexagons lie on the top of each other. However, in the case of graphite, the stacking is slightly different, as the hexagons are offset and do not lie exactly on top of each other (Sichel et al, 1976 and Tatar et al, 1982) . The interlayer distances and lattice constants are comparable in both the cases, the interlayer distances being 3.35 Å for graphite and 3.33 Å for h-BN, while the lattice constants are 2.456 Å for graphite and 2.504 Å for h-BN (Ishigami et al, 2003).

BNNTs were successfully synthesized in 1995 (Chopra et al, 1995). Strictly speaking, fourteen years prior to this synthesis, the formation of hexagonal BN whiskers, now called bamboo-like BNNTs, was reported (Ishii et al, 1981). In addition to their structural similarity, BNNTs and CNTs have similar mechanical properties and thermal conductivities (Pouch et al, 1990; Blase et al, 1994 and Wilder et al, 1998). However, BNNTs are distinct in several key aspects. First, BNNTs are electrical insulators with a bandgap of 5.5 eV, basically independent of tube chirality and morphology (Blase et al, 1994), while CNTs may be metallic or semiconducting depending on the chirality and diameter (Wilder et al, 1998). This independence from tube chirality makes boron nitride nanotubes much more tractable; for example, by doping these tubes, it is conceivable to have devices on single BN tubes, which have diameters of the order of nanometers and lengths of the order of microns. Second, BNNTs are more chemically inert and structurally stable than CNTs.



Boron nitride is far more resistant to oxidation than carbon and therefore suited for high temperature applications in which carbon nanostructures would burn. Because of this, BNNTs could be used in the area of specific applications like NT protective shields for various nanomaterials encapsulated within, for example, metallic or semiconducting nanowires, nanorods, melts, or clusters, in particular, for those performing at high-temperatures and in hazardous environments. In spite of all this, the amount of successful research work performed on BNNTs has been negligibly low compared to that on CNTs (Golberg et al, 2007). There has been exponential growth for CNT research, but BNNT research follows a linear-like dependence. However, in recent years, the number of BNNT-related papers, especially theory-oriented works has begun to increase noticeably (Cao et al, 2009). Until now, much research effort has been placed on chemical functionalization of conventional CNTs. Chemical functionalization offers an effective route to enhance solubility of NTs in a given solvent and deter CNTs from association into bundles. Recently, several research works have been performed with respect to BNNT sidewall functionalization. In the first report on BNNT functionalization (Han et al, 2003), full coverage of BNNTs with semiconducting SnO2 nanoparticles 1–5 nm in size was obtained. After that, the development of soluble multi-walled boron nitride nanotubes (MWBNNTs) by amine-terminated oligomeric poly (ethylene glycol) surface groups was reported (Xie et al, 2005). Stearoyl chloride functionalized BNNTs have been synthesized through the interactions of COCl groups and amino groups on BNNT walls (Zhi et al, 2005a). In addition to covalent functionalization, so-called non-covalent functionalizations through wrapping BNNTs with a polymer have also been tried (Zhi et al, 2005c and 2006a). The functionalized BNNTs were perfectly soluble in many solvents. It has been suggested that functionalization may also purify BNNTs and tune their band structure (Zhi et al, 2006a). The effects of functionalizations of BNNT with NH3 and four other amino functional groups, i.e. NH2CH3, NH2CH2OCH3, NH2CH2COOH and NH2COOH, were also theoretically studied (Wu et al, 2004) using density functional calculations. In addition to long molecule chains including DNA (Zhi et al, 2007), BNNTs may be functionalized with various nanoparticles, for example, SnO2, Fe, or Au (Han et al, 2003 and Zhi et al, 2006a). Such particles may then serve as new catalysts for the secondary growth of branched wires on BNNT surfaces producing interesting heterojunctions and nanoarchitecture functionalizations. The range of applications (e.g., in optoelectronic devices) of these boron nitride nanotubes would be greatly extended if their bandgap can be tuned to

the desired values in a controlled way. Nanotube surface functionalization is a very nice method for doing so. First-principles density functional calculations have shown that BNNTs may become either p- or n-type doped, depending on the electronegativity of multifunctional molecules attached, and their energy gap may be adjusted from the UV to the visible optical range by varying the concentration of functionalized species (Zhi et al, 2005c). The bandgap was also found to decrease owing to intertube coupling and multiple charge transfer from the B atoms to N atoms in BNNT bundles with a honeycomb (in cross-section) arrangement of tubes. The value of the bandgap change is dependent on the arrangement of bundles and chirality of each BNNT within a bundle (Zheng et al, 2005a). Selective donor or acceptor states in multi-walled BNNTs may also be created when functionalizing them with dices (Huang et al, 2007). Very few studies have been done on functionalizations of boron nitride nanotubes with carbenes (Li et al, 2008 & Su, 2005) and none has tried to study the substituent effects on carbenes. In addition, a need was felt for a study of the effects of electronegativity on functionalizations of boron nitride nanotubes. 2. Computational Details The DMol3 (Delley, 2000) program package was used to carry out all calculations within the generalized gradient approximation (GGA) of the Perdew-Wang-91 (PW91) exchange-correlation potential functional (Perdew, 1992). Functionalization of both zigzag and armchair boron nitride nanotubes with :CX2 (where X= H, F, Cl, Br, I) was undertaken. The models selected for zigzag and armchair BNNT are (10,0) and (6,6) respectively, as shown in Figure 1. Hydrogen atoms, yielding B40N40H10 and B36N36 as formulas for zigzag and armchair nanotubes, respectively, terminated the dangling bonds. Interestingly, while the former structure was found to have dangling bonds only at one end, no hydrogens were required for the latter structure, unlike the case with the carbon nanotubes. The diameters of the zigzag and armchair nanotubes are comparable, i.e. 7.83 Å and 8.14 Å, respectively. 3. Results and Discussion 3.1. The (10,0) System At the uncapped end (Figure 1 a), the B-N bond distance is 1.424 Å, and the B-B distance decreases to 2.079 Å. The Mulliken charge on the nitrogens is -0.211, while that on the borons is 0.117. At the other end of the tube, the charges on nitrogen, boron and hydrogen are -0.213, 0.184 and 0.106, respectively. The Fermi energy for this system is calculated as -3.30 eV. Figure 2 displays the calculated



density of states for this system. It can be seen that the p orbitals contribute most to the states.

a

b

Figure 1. Optimized Structures for Different Models Used in this

Investigation (a) (10,0) BNNT, and (b) (6,6) BNNT

3.1.1. Bond Lengths and Adsorption Energies As in the case of the carbon nanotubes, the carbenes form cyclopropene rings, as shown in Figure 3. From Figure 3, it can be seen that the largest deformation seems to be in the case of :CH2. The lengths of the newly formed bonds, the original B-N bond lengths and the adsorption energies for each case are given in Table 1. The adsorption (or binding) energies of the carbenes adsorbed on BNNTs were calculated from the equation:

)(

22 :: CXBNNTCXBNNTads EEEE +−= + ……………(1)

Where

Eads : Total Adsorption Energy of Carbenes on BNNT,

2:CXBNNTE + ,EBNNT &2:CXE : Total Energy Of The Carbene

Adsorbed

BNNT, pristine BNNT, and the single carbene, respectively. The negative Eads values correspond to stable adsorption structures. The bond lengths in the cyclopropene ring are given in Table 1, from which it is clear that the B-N bond is disrupted in all cases except for :CH2 and :CCl2. Thus, insertion products are formed in the case of :CF2, :CBr2 and :CI2, and addition products for :CH2 and :CCl2. In fact, both the C-N and C-B bonds are weakest in the case of :CCl2 adsorption, while the B-N bond is strongest in this case. This does not agree with the fact that the adsorption energy is more negative for :CCl2

than that predicted from the general trend. The adsorption energies (Table 1) are also highest for these two cases. The magnitudes of the adsorption energies generally decrease with increasing carbene size, except for :CCl2 which shows a slight dip.

Table 1. Bond Lengths (Å) and Adsorption Energies (kcal/mol)

for :CX 2 attached to the (10,0) Boron Nitride Nanotube

Carbene B-C N-C B-N Adsorption Energy

:CH2 1.548 1.567 1.756 -159.9

:CF2 1.558 1.433 2.325 -106.6

:CCl2 1.575 1.605 1.707 -107.1

:CBr2 1.562 1.430 2.289 -45.5

:CI2 1.559 1.433 2.275 -18.9 Table 1 shows that the adsorption is stronger in this case, as compared with the CNTs. This is probably because the carbene lone pair can interact with the electron deficient boron, while its vacant orbital can interact strongly with the nitrogen lone pair. Even :CI2 forms a stable adsorption product. As compared to carbon nanotubes, where cyclopropane ring formation leads to a radial deformation, boron nitride nanotubes appear curly and out of plane. BNSWNTs have undulating, wavy, and poorly structured walls. Theoreticians assigned this to the fact that in a BN hexagonal sheet, the B and N atomic planes are slightly shifted in a direction perpendicular to the tube axis. In addition, it is thought that the partial ionic character of B–N bonding in a BN layered material may stabilize doubleor multi-layered NT morphologies owing to strong “lip–lip” interactions (Kwon et al, 1997; Nardelli et al, 1998) between adjacent layers. These interactions favor the placement of B atoms strictly above or below N atoms. In the homogenous, grapheme sheets, where chemical bonds of only one type are present, i.e. C–C, the interactions between layers are rather weak (van der Waals) and typically are not important for the NT growth. As can be seen from Figure 1, the bottom of the nanotubes, where there is no hydrogen termination, gets contracted. In fact, the B-B distance at that end reduces to 2.079 Å, pointing towards the possibility of addition at that end. We



have calculated the adsorption energies for adsorption at the unterminated end of the nanotube, and the results are presented in Table 2. It is obvious that these positions are more amenable to adsorption, since the adsorption energies are much higher.

Both :CF2 and :CBr2 show smaller adsorption energies than expected on the basis of a linear graph for the other three carbenes.

Table 2. Bond lengths (Å) and adsorption energies (kcal/mol)

for :CX 2 attached to the bottom of the (10,0) boron nitride nanotube

Carbene B1-C B2-C B1-B2 Adsorption Energy

:CH2 1.597 1.597 1.932 -270.7

:CF2 1.631 1.632 2.036 -196.9

:CCl2 1.612 1.616 2.003 -215.8

:CBr2 1.599 1.608 2.005 -171.3

:CI2 1.511 1.523 1.901 -178.2

Figure 4 shows the final structures of the adsorbed carbenes. It can be seen that, in the case of adsorption of :CI2, one of the C-I bonds breaks and the iodine atom forms a bridge with two opposite boron atoms. This is probably because of the longer C-I bond length, which brings the iodine atom within the range of interaction of the opposite boron atoms.

3.1.2. The SWBNNT (6,6) System Comprehensive literature survey regarding separation of armchair and zigzag boron nitride nanotubes indicates that no work has been done in this direction till date. This is

strange, since polydispersity and poor solubility of boron nitride nanotubes are major problems in BNNT chemistry. So research in this area will help us to deduce reagents that can be used to separate zigzag and armchair nanotubes. Keeping this in mind, the addition of armchair BNNT with carbenes was also studied. As a representative of the armchair tube, the (6,6) BNNT was chosen. The analysis of the data reveals that all the carbenes form insertion products (Figure 5), the B-N bond length increasing from 1.461 Ǻ to ~2.4 Ǻ, forming open structures accordingly. Figure 5 shows that :CF2 shows anomalous behaviour, the adsorption energy being smaller than expected on the basis of a linear trend.

As is evident from Table 1, the adsorption energies in this case follow the same pattern, but are much smaller than those of zigzag nanotubes, though trends for both of them show geometric similarity. Therefore, the determining factor for adsorption in this case is not the nature of the carbene but the nanotube structure. This opens up the possibility of use of carbenes as reagents for separating zigzag and armchair nanotubes. An analysis of Tables 1 and 2 reveals that, because of the large difference in adsorption energies, any of the carbenes can be used as a separating reagent. Compared to zigzag BNNT, these tubes show a higher propensity to insert, as is reflected by the B-N bond distances for :CH2 and :CCl2.

Figure 2. Calculated Density of States for the (10,0) System



As can be seen from Tables 1 and 3, these are in the range of 2.4 Å in the case of the (6,6) BNNT, but for the (10,0) case they are much less. This means that the sidewall of armchair nanotubes has a greater tendency of opening up by chemical modifications. Despite its simplicity, our approach can provide experimentalists with important insights into the factors controlling the functionalization of nanotube sidewalls.


for :CX 2 Attached to the (6,6) Boron Nitride Nanotube

Carbene B-C N-C B-N Adsorption Energy

:CH2 1.580 1.496 2.367 -104.9

:CF2 1.604 1.469 2.411 -39.4

:CCl2 1.607 1.479 2.399 -39.5

:CBr2 1.601 1.459 2.405 5.4

:CI2 1.599 1.406 2.406 46.5

Table 4 gives the bond lengths and adsorption energies. It can be seen that, in each case, an addition product is formed, since the B-N bond remains intact. Methylene, which shows the strongest adsorption, forms the weakest bond with nitrogen, as evidenced by the longest C-N bond length in this case. Almost the same trend is found as in the other cases, i.e. :CH2 shows the strongest adsorption and the adsorption energy decreases with increasing atomic number of X. However, :CF2 shows smaller adsorption energy than expected (Table 4). Although the adsorption energies are similar to those with the (6,6) BNNT for the other carbenes, :CBr2 and :CI2 show stronger adsorption. This is perhaps due to the curvature in the latter, which hinders effective σ-p overlap, especially for the heavier halogens. 3.2. Mulliken Charge Analysis 3.2.1. The (10,0) SWBNNT System As stated in the previous sections, two modes of adsorption were considered for the (10,0) system. For axial adsorption, except for :CI2, the charge on the boron atom in all the cases studied was found to be close to 0.5. Among the five systems, :CH2 and :CCl2 were found to behave anomalously with regards to adsorption energies.

This anomaly is manifested here too, as in these two cases the nitrogen on which adsorption takes place is more negatively charged and the halogen atoms are almost neutral. This is most surprising since chlorine is more electronegative than nitrogen (3.16 and 3.04, respectively, on the Pauling scale).


for :CX 2 Attached to the Unsaturated Boron Nitride Nanotube Analogue

Carbene B---C N---C B---N Adsorption

Energy :CH2 1.529 1.571 1.542 -100.3

:CF2 1.537 1.489 1.541 -34.2

:CCl2 1.544 1.504 1.515 -43.3

:CBr2 1.536 1.461 1.503 -35.0

:CI2 1.537 1.455 1.505 -27.3 The charge transfer from the (10,0) SWBNT to the carbine (0,018, 0.110, 0.131, 0.317 and 0.432, respectively, for X = H, F, Cl, Br and I) increases with the size of X. Tables 5 and 6 gives the Mulliken charges for functionalized and circumferential adsorption. In this case, the two halogen atoms lose their equivalency. The difference is largest for :CI2, where, as seen previously, one C-I bond breaks away. The high electronegativity of fluorine is manifested here also, as the carbon is positively charged only in this case.

Table 5. Mulliken Charges of Selected Atoms in Functionalized

(10,0) SWBNNT

Carbene B N C X

:CH2 0.531 -0.602 -0.118 0.068

:CF2 0.401 -0.515 0.564 -0.337

:CCl2 0.531 -0.596 -0.137 0.003

:CBr2 0.538 -0.475 0.141 -0.229

:CI2 0.553 -0.416 -0.126 -0.136

Table 6. Mulliken Charges of Selected Atoms in Circumferentially Functionalized (10,0) SWBNNT

Carbene B1 B2 C X1 X2

:CH2 0.388 0.394 -0.406 0.055 0.079

:CF2 0.375 0.380 0.301 -0.336 -0.342

:CCl2 0.424 0.444 -0.401 -0.021 -0.042

:CBr2 0.427 0.485 -0.089 -0.187 -0.241

:CI2 0.285 0.322 -0.283 -0.211 -0.042



a

b

c

d

e

Figure 3. Adsorption of, from Left to Right, Top to Bottom: :CH2, :CF2, :CCl2, :CBr 2, and :CI2 on (10,0) BNNT

a

b

c

d

e Figure 4. Adsorption Products at the Circumferential

Positions of the (10,0) BNNT



a

b

c

d

e

Figure 5. Insertion Products Formed on (6,6) SWBNTs

3.2.2. The (6,6) SWBNNT System The charge distributions are similar for the (6,6) SWBNNT and the B8N8 nanotubes (Tables 7 and 8).

The charge transfer to the carbene, however, is larger in the (6,6) SWBNNT than in the unsaturated analogue (Figure 6). Only :CH2 donates a small amount of charge. The rest of the carbenes, in which X is more electronegative, accept electron density from the nanotube.

Table 7. Mulliken Charges of Selected Atoms in Functionalized

(6,6) SWBNNT

Carbene B N C X

:CH2 0.414 -0.419 -0.155 0.078

:CF2 0.394 -0.481 0.526 -0.312

:CCl2 0.486 -0.415 -0.235 -0.003

:CBr2 0.521 -0.446 0.101 -0.197

:CI2 0.520 -0.387 -0.168 -0.080

Table 8. Mulliken Charges of Selected Atoms in Functionalized Analogue

Carbene B N C X

:CH2 0.602 -0.597 -0.119 0.087

:CF2 0.540 -0.672 0.628 -0.315

:CCl2 0.567 -0.592 -0.081 0.000

:CBr2 0.628 -0.607 0.246 -0.222

:CI2 0.642 -0.561 -0.027 -0.099

3.3. Bandgaps of BNNT 3.3.1. Band Gaps of Functionalized Boron Nitride

Nanotubes Insights into band gap engineering of nanotubes can be helpful in future nanodevices. In an effort to better understand the electronic structure of nanotube carbene chemistry, the band gaps were analyzed for all the nanotubes. In general, compared with the case in the isolated h-BN sheet, the σ–π rehybridization in the nanotube is stronger due to the curvature of the tube, which induces changes of the distributions of localized and delocalized electronic states on the tube and the change of the reactivity of the valence electrons. Contrary to expectations that the bandgaps are independent of tube chirality, we find significant differences in bandgaps of the two kinds of nanotubes investigated here. Taking first the zigzag (10,0) BNNT (Table 9), we found that the distribution resembles a singlet state, with 1.98 electrons in the one energy state, and 0.02 electrons in the next higher state placed 1.3 eV above it. However, the adsorbed carbene is in a triplet state, as seen from the fact that all the



carbene adsorbed systems have one electron each in two almost degenerate orbitals.

Table 9. Fermi and Orbital Energies (eV) of the Frontier Orbitals of the Axially Functionalized (10,0) SWBNNT

System Fermi Energy

Orbital Energy (Occupation)

SWBNNT (10,0)

-3.12 -4.88 (2.00)

-3.98 (1.98)

-2.68 (0.02)

+:CH2 -3.01 -5.17 (2.00)

-3.02 (1.04)

-3.00 (0.96)

-

+:CF2 -3.01 -5.22 (2.00)

-3.01 (1.01)

-3.01 (0.99)

-

+:CCl2 -3.08 -5.24 (2.00)

-3.09 (1.04)

-3.07 (0.96)

-2.02 (0.00)

+:CBr2 -3.15 -5.31 (2.00)

-3.15 (1.01)

-3.14 (0.98)

-2.35 (0.01)

+:CI2 -3.27 -5.21 (2.00)

-3.27 (1.00)

-3.26 (0.95)

-2.80 (0.06)

The electron distribution in the case of circumferential adsorption (Table 10) is similar. Table 11 gives the bandgaps for the (6,6) SWBNNT. Here the electron distribution is molecule-like. The energy of the highest

occupied molecular orbital (HOMO) goes up and that of the lowest unoccupied molecular orbital (LUMO)

decreases with increasing atomic number of X, with a result that the bandgap decreases marginally at first with increasing atomic number of X, and then more drastically for :CI2. The bandgap is large in magnitude.

Table 10. Fermi and Orbital Energies (eV) of the Frontier Orbitals of the Circumferentially Functionalized (10,0) SWBNNT

System Fermi Energy

Orbital Energy (Occupation)

SWBNNT (10,0)

-3.12 -4.88 (2.00)

-3.98 (1.98)

-2.68 (0.02)

+:CH2 -2.90 -5.11 (2.00)

-2.90 (1.01)

-2.89 (0.99)

+:CF2 -2.98 -5.21 (2.00)

-2.99 (1.02)

-2.98 (0.98)

+:CCl2 -2.94 -5.16 (2.00)

-2.96 (1.04)

-2.93 (0.96)

+:CBr2 -3.05 -5.19 (2.00)

-3.09 (1.16)

-3.00 (0.84)

-2.04 (0.00)

+:CI2 -3.12 -4.83 (2.00)

-3.47 (1.86)

-2.75 (0.12)

-2.40 (0.01)

Figure 6. Carbene Functionalized Unsaturated B8N8 Analogue



Table 11. Bandgaps (eV) of Functionalized (6,6) BNNTs

Carbene Energy of HOMO

Energy of LUMO

Band Gap

(6,6) BNNT

-6.52 -2.28 4.24

:CH2 -6.40 -2.28 4.12

:CF2 -6.59 -2.48 4.12

:CCl2 -6.48 -2.50 3.98

:CBr2 -6.20 -2.79 3.41

:CI2 -5.63 -3.34 2.20 Table 12 does a similar analysis for the B8N8 system. Here, the distribution of electrons is not so uniform. Occupation of electrons in the LUMO brings down its energy and the distribution becomes metallic.

Table 12. Fermi and Orbital Energies (eV) of the Frontier Orbitals of the Functionalized Analogue.

System Fermi Energy Orbital Energy (Occupation)

B8N8 -5.16 -5.64 (1.94)

-5.46 (1.80)

-4.90 (0.26)

- -

+:CH2 -4.95 -5.69 (1.99)

-5.43 (1.94)

-4.48 (0.06)

- -

+:CF2 -5.23 -6.12 (2.00)

-5.75 (1.96)

-4.73 (0.05)

- -

+:CCl2 -5.16 -6.26 (2.00)

-6.10 (2.00)

-5.68 (1.96)

-4.65 (0.05)

-

+:CBr2 -5.30 -6.30 (2.00)

-6.17 (2.00)

-5.90 (1.98)

-4.73 (0.03)

-

+:CI2 -5.23 -6.17 (2.00)

-5.98 (1.99)

-5.93 (1.99)

-5.79 (1.97)

-4.74 (0.05)

3.4. Effect of Diameter The effect of varying diameter was studied for (n,0) SWBNNTs for methylene adsorption, since the (10,0) nanotube-methylene system shows the maximum adsorption energy.

All nanotubes considered are of 8.52 Å length and n varies from 8 to 11. Table 13 gives the binding energy per BN unit. It is found that here, the stability of the nanotubes increases with increasing diameter. Table 14 gives the adsorption energy as a function of tube diameter. It can be seen that the adsorption energy is largest for the larger nanotubes and least for (9,0). Figure 7 shows the optimized structures of the BNNTs before and after functionalization. Considerable reconstruction is seen

to occur, even before adsorption, especially for the smaller diameter nanotubes. Also, the N-B-N forms a zigzag pattern.

Table 13. Binding Energy per Atom (kcal/mol) for Boron Nitride Nanotubes of Different Diameters (Å) Functionalized with Methylene

SWBNNT (n,0) Formula Diameter

Binding Energy per

BN unit

8,0 B32N32H8 6.26 -1386.3

9.0 B36N36H9 7.05 -1405.2

10,0 B40N40H10 7.83 -1410.1

11,0 B44N44H11 8.61 -1412.6


of Boron Nitride Nanotubes of Different Diameters Functionalized with :CH2

Nanotube

(n,0) B-C N-C B-N Adsorption Energy

8.0 1.567 1.566 1.621 -181.9

9,0 1.566 1.569 1.634 -71.9

10,0 1.547 1.566 1.755 -159.9

11,0 1.585 1.626 1.617 -210.1 The Mulliken charges on the functionalized BNNTs are given in Table 15. As expected, the borons carry positive charges and the nitrogens slightly larger negative charges. There is negligible charge transfer to the carbene moiety from the BNNT (0.004, 0.006, 0.018 and 0.006, respectively for (8,0), (9,0), (10,0) and (11,0) BNNTs.

Table 15. Mulliken Charges of Boron Nitride Nanotubes of

Different Diameter Functionalized with :CH2

BNNT (n,0)

B N C H1 H2

8,0 0.546 -0.576 -0.122 0.062 0.064

9,0 0.536 -0.582 -0.122 0.063 0.065

10,0 0.531 -0.602 -0.118 0.068 0.068

11,0 0.527 -0.579 -0.120 0.062 0.064

Table 16. Band Gaps (eV) of Nanotubes of Different Diameters

Functionalized with Carbenes SWCNT (n,0) HOMO LUMO Band Gap

8,0 -5.20 -2.02 3.18

9,0 -4.00 -1.94 2.06

10,0 -4.01 -1.90 2.11

11,0 -2.99 -1.69 1.30



Compared to the arm-chair CNTs (Table 11), the bandgap is larger for the zigzag BNNTs, but becomes smaller with increasing diameter (Table 16), and the decrease is quite steep, so that eventually the BNNTs are expected to display metallic properties. Although the data are not enough for a generalization to be made, these indicate an oscillating behaviour for the bandgap. 4. Conclusions Boron nitride nanotubes appear curly and out of plane. They have undulating, wavy, and poorly structured walls. For zigzag BNNTs, insertion products are formed in the case of :CF2, :CBr2 and :CI2, and addition products for :CH2 and :CCl2, but insertion products are formed in the case of all carbenes for armchair boron nitride nanotubes. Our analysis suggests that the sidewall of the armchair BNNTs has less tendency of opening up by chemical modifications compared to zigzag nanotubes. Methylene forms addition products with all zigzag boron nitride nanotubes. Carbenes can be used to separate armchair and zigzag boron nitride nanotubes, since carbenes show very high values of adsorption energy for zigzag BNNTs. The charge separation is large for BNNTs, due to the ionicity of boron nitride nanotubes. Surprisingly, the charge transfer values for armchair and zigzag nanotubes are comparable, although there is a slightly greater charge transfer in the case of the (10,0) BNNT. Only the (6,6) SWBNNT shows clearly defined HOMOs and LUMOs.

The bandgaps are also large in this case, explaining the smaller amount of charge transfer. The band gaps for BNNTs are generally believed to be independent of tube chirality. This is disproved by an analysis of the data in which the carbenes show appreciable change in bandgap values, both for armchair and zigzag nanotubes. This implies that there are differences in the bandgaps for small nanotubes, but, in the bulk, these differences may disappear. While the carbene remains in a singlet state on adsorption to (6,6) BNNTs, it changes to triplet character for adsorption on (10,0) BNNTs. Electron spin resonance studies may prove or disprove this prediction. In case this is true, separation of armchair and zigzag tubes on the basis of their magnetic properties may become a reality. The binding energy per BN unit of armchair BNNTs increases in magnitude exponentially with increasing tube diameter. The reactivity increases with tube diameter, except for the (9,0) tube, which shows anomalous behavior. The bandgap generally decreases with increasing tube diameter, though there is some oscillatory pattern for the bandgaps. Our observations show that, for narrow nanotubes, the effect of curvature can convert nanotubes expected to be semiconductors to metals and vice versa. Acknowledgement

Figure 7. Optimized Structures of BNNTs before (Left) and after (Right) Functionalization



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DFT Study of Interactions of Carbenes with Boron …. Kakkar et al / Chemistry Journal (2011), Vol....

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