3.1 Structure analysis and stability of pure C18 ring–
The overall idea of the present study is to find whether the ground state geometry is D18h cumulenic or D9h polyynic and also to see the impact of doping of B, N, and Li in C18 ring. By using DFT quantum simulation along with B3LYP functionals support the D18h symmetric minimum energy structure while the simulations using M062X/6-311 + + G(d) hybrid meta-GGA type functional result in a D9h polyynic ground state structure as reported in recent investigations [62–64]. The optimized minimum energy structures have shown in Fig. 1.
The results derived using the M062X/6-311 + + G(d) level of theory agree well with Parasuk et al. [62] who did ab-initio calculations and confirmed that the D9h polyynic structure is the minimum energy structure. The present simulation work accurately predict the ground state structures of the cyclo-18 ring and the structures obtained using M062X calculations are in well agreement with some previous theoretical and experimental reports [23, 34, 36]. So the reported work on Cyclo-C18 ring predict that the DFT theory with 6-311 + + G(d) type basis sets at M062X hybrid functional level will be very useful and can be used to obtain qualitative results for C4n + 2 carbon ring where n = 4.
Simulation software Gaussian also predicts quadrupole moments and higher multipole moments (through hexadecapole) to explore the idea about structural stability. The calculated dipole and quadrupole moments of D18h cumulenic and D9h polyynic forms are shown in Table-1.
Table 1
Dipole moments (in Debye) and quadrupole (in Debye-Angs) moments for cumulenic (D18h) and polyynic (D9h)
Polyynic (D9h)
|
Sr. No.
|
Moment
|
X
|
Y
|
Z
|
Total
|
1
|
Dipole
|
0.0009
|
0.0006
|
-0.0009
|
0.0014
|
2
|
Quadrupole
|
XX= -99.03
|
YY = -99.83
|
ZZ = -98.77
|
|
|
|
XY = 0.62
|
XZ = 0.06
|
YZ = -0.02
|
|
Cumulenic (D18h)
|
Sr. No.
|
Moment
|
X
|
Y
|
Z
|
Total
|
1
|
Dipole
|
0.0001
|
0.0016
|
0.000
|
0.0017
|
2
|
Quadrupole
|
-97.15
|
-99.07
|
-97.85
|
|
|
|
XY = 0.78
|
XZ = 0.34
|
YZ = -0.12
|
|
Quadrupole moment is the second order term in the expansion of the total electron distribution, and provides insight into its overall shape [63]. Denis J. [65] explored the stability on different molecules by using similar parameters. It was explained [65] that equal values of quadrupole parameters point towards the stability of the system. Similarly, here the values of XX, YY, and ZZ (quadrupole ) components are nearly equal which indicate a spherical distribution as shown in Table 1. On the other hand XY, XZ, and YZ indicate trans-axial distortion means stretching or compressing mode. In the present calculations, the values of XX, YY, and ZZ components of polynic are − 99.03, -99.83, and − 98.77 Debye-Angs respectively, very close to each other and similarly, the XY, XZ, and YZ components are 0.62, 0.06, and − 0.02 Debye-Angs respectively, less than the Cumulenic structure which indicates the stability of polyynic structure rather than Cumulenic.
3.2 Structure analysis and stability of C18 ring substituted with B, N, and Li –
Based on the quantum computational simulation, our next aim is to analyze the impact of B, N, and Li substitution on the electronic structures and other properties of C18 ring. Here again first the minimum energy structures are obtained upon full relaxation under the DFT framework. The optimized structure shown in Fig. 2, indicate that compared to C-C bond length of 1.27Å, the bond length of B-C, N-C, and Li-C becomes 1.43Å, 1.31Å, 2.03Å respectively. This elongation in bond length is just due to substitution of B, N, and Li in the C18 ring that modifies the spherical shape of the polyynic C18 structure.
Table 2
Dipole moments (in Debye) and quadrupole moments (in Debye-Angs) of B, N, and Li substitute polyynic (D9h)
B substituted Polyynic (D9h)
|
Sr. No.
|
|
X
|
Y
|
Z
|
Total
|
1
|
Dipole
|
0.50
|
0.27
|
0.10
|
0.5869
|
2
|
Quadrupole
|
XX= -100.07
|
YY = -98.94
|
ZZ = -97.88
|
|
|
|
XY = 2.7
|
XZ = -1.34
|
YZ = -0.47
|
|
N substituted Polyynic (D9h)
|
Sr. No.
|
|
X
|
Y
|
Z
|
Total
|
1
|
Dipole
|
-0.5534
|
0.7536
|
-0.0000
|
0.9350
|
2
|
Quadrupole
|
XX= -103.10
|
YY = -100.80
|
ZZ = -99.24
|
|
|
|
XY = 0.66
|
XZ = 0.00
|
YZ = 0.00
|
|
Li substituted Polyynic (D9h)
|
Sr. No.
|
|
X
|
Y
|
Z
|
Total
|
1
|
Dipole
|
-1.02
|
0.20
|
-0.0000
|
1.04
|
2
|
Quadrupole
|
XX = -83.99
|
YY = -102.82
|
ZZ = -99.10
|
|
|
|
XY = -3.6
|
XZ = 0.00
|
YZ = 0.00
|
|
Further to explain the stability of cyclo C17X (X = B, N, and Li) ring it can be clearly seen from the Table 2, that substitution of boron atom in C18 ring results in nearly equal values of within 1%)of XX, YY, and ZZ components of the Quadrupole moment, resulting in a structure with nearly spherical geometry. However, on substitutions of N and Li, The values of XX, YY, ZZ, are within 2% and 14%, thus explain the distrortion in the circular ring structure as shown in Fig. 2. The values of XY, XZ, and YZ components of the quadrupole moment which give a measure of the trans-axial distortion indicate that the stretching mode for N and Li substitutions will be distorted only in the XY directions as shown in Fig. 2.
3.3 Charge transfer mechanism, molecular orbitals, and contour map of charge distributions -
After calculation of the ground state electronic structure we analyzed atomic charges distribution using the natural bond orbital (NBO) analysis (Population = NPA) [55] and these atomic charges can be used to identify the electron transfer and spin distribution. The simulated values of NBO charges are listed in Table 3. It can be seen that B, N, and Li atoms are losing charge and it has transferred to C18 ring and making covalent bonds. Our calculated HOMO-LUMO plots also support the same as illustrated in Fig. 3(a-d). The charge population of present system is similar to the system studied by the Yingqian chen et al. [66], where it was found that the distributions of charge are most likely a result of the covalent bonding in the system. It is also noted that the distortion in the C18 clusters is mainly due to the charge distributions or transfer between C atom and the N or Li atom. As we can see, for example the bond length of C-C and C ≡ C are 1.27Å, and 1.28Å respectively, while the bond length of B-C, N-C, and Li-C changes to 1.31Å, 1.43Å, and 2.03Å respectively due to the charge transfer which point towards the distortion of C18 ring. The positive charges on B, N and Li and negative charges on the Cyclo-18 ring demonstrate the partial Coulomb interactions and also charge transfer from B, N, and Li to Cyclo-18 ring. It is also important to mention that the atomic charge redistribution in the doped system affects the energy of the system by changing the electronic states and make them useful for electronic and optical applications. The Charge transfer mechanism can also be seen by the HOMO-LUMO frontier molecular orbital which are formed by the orbital-orbital interaction of two bonding atoms. The alignments of these orbital are shown in Fig. 3. It can be seen that all the occupied MO’s have an equal pair of spin, giving net spin zero. As we discussed in charge transfer, the carbon ring is negatively charged predicted by natural population analysis due to the substitutions of B, N, and Li. Interestingly the LUMO molecular orbitals are localize on these cyclo-18 carbon atoms and this is due to the excessive accumulation of the negative charge on the C atoms transferred from the dopants. In Cyclo C18 system, the HOMO is an orbital formed from Py orbital’s from the carbon atoms while LUMO is formed from s orbitals of carbon atoms. The HOMO-LUMO gap is around 5.3 eV showing chemical stability of the pure C18 cyclo ring similar to the value predicted by P. Fabio [67].The doping of boron formed primarily Px, and Py orbital from the carbon and boron atoms as we can see HOMO, and LUMO image in Fig. 3(a-d). In the HOMO, the orbital’s have similar signs and so they combine to form a bonding π molecular orbital’s, while in the LUMO, they have up and down direction, indicating that they combine to form an antibonding π* molecular orbital’s. The same behavior can be seen for Li doped C18 ring as shown in Fig. 3b. Moreover, to verify the orbital charge density on the C18 and B, N, and Li doped C18 system, the contours map is plotted which allows one to display volumetric results in a selected plane as presented in the Fig. 3(a-d). A representation of electrostatic potential through total charge density “ρ(r)” in the plane of (0,0,1,0) has been shown. These contour lines give the information about the character of each type of bonding between the atoms [68]. The maxima occur at the nuclear sites and ρ(r) decays in a nearly spherical manner away from the nuclei. Comparing the electronic density contour lines, we found that the atoms share electrons leading with large covalent interactions. This analysis also predicts that C18 ring is charge gaining when doped with B, N and Li, which correlates well with the electro negativity difference between the species. The charge redistribution between C18 ring and dopant (B, Li, and N) illustrates that the electronic characteristics can be adjusted by changing the dopant atom and this knowledge can be exploited in the development of electronic devices [69].
Table 3
Natural population analysis to illustrate charge transfer in C18 and B, N, Li doped C18
Atom
|
No
|
Natural atomic charge difference in terms of e
|
|
|
Pure C18
|
B@C17
|
N@C17
|
Li@C17
|
C
|
1
|
-0.58
|
-0.56
|
0.30
|
-0.57
|
C
|
2
|
0.89
|
0.60
|
0.24
|
0.85
|
C
|
3
|
-0.28
|
0.61
|
-0.25
|
0.27
|
C
|
4
|
-0.01
|
0.59
|
-0.01
|
0.01
|
C
|
5
|
-0.00
|
0.06
|
-0.00
|
0.02
|
C
|
6
|
-0.29
|
-0.04
|
-0.15
|
-0.17
|
C
|
7
|
-0.00
|
0.02
|
0.17
|
0.30
|
C
|
8
|
-0.01
|
0.07
|
0.28
|
0.37
|
C
|
9
|
-0.27
|
-0.09
|
-0.17
|
-0.02
|
C
|
10
|
0.88
|
0.64
|
0.57
|
0.04
|
C
|
11
|
-0.24
|
0.14
|
-0.03
|
-0.02
|
C
|
12
|
-0.60
|
-0.44
|
-0.35
|
-0.21
|
C
|
13
|
-0.27
|
-1.67
|
0.32
|
-0.12
|
C
|
14
|
-1.91
|
-1.48
|
-1.73
|
-0.51
|
C
|
15
|
0.42
|
0.8
|
0.42
|
0.52
|
C
|
16
|
-0.50
|
0.83
|
0.32
|
-1.01
|
C
|
17
|
-0.00
|
0.28
|
1.55
|
0.25
|
C/B/N/Li
|
18
|
0.039
|
B = 1.62
|
N = 0.78
|
Li = 0.40
|
3.4 Density of States Analysis
In this section, we investigated the interaction mechanism and presented the electronic properties of B, N, and Li substitution in C18 ring through the total and partial density of states as illustrated in Fig. 4a. As VASP [56–59] code employs periodic boundary conditions, in order to avoid the interaction between adjacent periodic images, a vacuum of 15 Å was inserted in all directions and C18 was placed in the middle of the simulation box. We can observe that there are delocalized electronic states of C18 and doped C18 ring in the total density of states as shown Fig. 4a. The pure C18 ring exhibits a semiconductor nature. It can also be observed that substitution of B and Li change the system into metallic while N substitution makes band gap opening which may be due to the redistribution of charge modifying the electronic characteristics of the system.
When we substitute the B, N, and Li atom in C18 ring, the density of states of Li “s” orbital, B “2p” orbital and N “2p” at or below Fermi level decreases compare to isolated Li, B and N atom as displayed in Fig. 4(b). This indicates a charge transfer from B, N and Li atom to the isolated C18 ring. Here we can notice that in case of B substitution as given in Fig. 4b, B 2p orbital is mainly contributing near the Fermi level and the charge states are mainly accumulated in the valance band. Further the states at Fermi level show the metallic character of the system. On the other hand, substitution of N in isolated C18 ring shows that the charge states in DOS are mainly composed by N 2p orbital in the valence band region near the Fermi level as illustrated in the Fig. 4(b) while conduction band region is empty near Fermi level. For N-substituted C18 there is a gap at Fermi level indicating semiconductor character for N-C18 system, matching nicely with the work done on the DyMg system [70]. Metallic nature can be seen for Li substitution as there are finite electronic states at the Fermi level.
Table 4
Natural electron configuration for pure C18 ring and doped (B, N, and Li) C18 ring
Atom
|
Different Orbital’s
|
C
|
2S = 0.88
|
2P = 3.08
|
B
|
2S = 0.3
|
2P = 1.4
|
N
|
2S = 1.19
|
2P = 4.09
|
Li
|
2S = 0.30
|
2P = 0.54
|
To further investigate the redistribution of charge and their effect on DOS, we are providing natural electron configuration data analysis (Electron configuration of only outer orbtials to see interaction) as obtained by NPA [55]. Natural electron configuration shows distributions of electrons in atomic or molecular orbitals. As given in Table 4, the occupancies of atomic orbitals are non integer in molecular environment. Here mostly 2p orbital is contributing in the interaction process which also verified by the PDOS.
3.5 Optical Properties:
To investigate the optical response of C18 and C17X(X = B, N, and Li) we employed time dependent density functional theory. We found that the position of the adsorption peak in the UV-Vis spectra is related to its size and the electronic transition S0 S1 at 230 nm are of the HOMO-LUMO type for the pure C18 ring, matching well with the recent experimental work done by Kaiser et al. [23] as shown in ESI Fig. 1 (electronic supplementary information). It could find an application in drug delivery system [71]. When we substitute the B, N, and Li, the electronic states becomes more delocalized and it shifts to higher wavelengths at 360 nm, 410 nm, 430 nm respectively as shown in ESI Fig. 1(electronic supplementary information). Due to this shifting, the absorption spectrum shows a red-shift as shown in ESI (electronic supplementary information). These electronic transition have character of HOMO-LUMO + 1, and HOMO-LUMO type with significant oscillator strength indicating significant probability of occurrence. Shifting of wavelength after substituting the B, N, and Li is in the visible range which also shows the suitability of these doped C18 systems for photo-catalytic applications.