Structures, Stabilities, and Spectral Properties of Endohedral Borospherenes M@B40 0/- (M = H2, HF, and H2O).

The discovery of borospherene B40 leads to a new beginning for the study of boron chemistry and may lead to new boron-based nanomaterials. Based on density functional theory, the structures, electronic properties, infrared and Raman spectra, photoelectron spectra, and electronic absorption spectra of endohedral borospherenes M@B40 0/- (M = H2, HF, and H2O) are investigated. It is found that H2, HF, and H2O monomers can form stable endohedral borospherenes M@B40 0/- (M = H2, HF, and H2O). In addition, the calculated results indicate that the doped molecule at the off-center location can relax to the center location within the cage and the symcenter of the doped molecule is almost located in the center of the cage. Unlike endohedral metalloborospherene Ca@B40, which is a charge-transfer complex between Ca2+ and B40 2-, natural population analyses and chemical bonding analyses reveal that there is no significant charge transfer of the doped molecule. The calculated spectra indicate that doping of a molecule (H2, HF, or H2O) in borospherene B40 can change the photoelectron spectra and doping of a polar molecule (HF or H2O) in borospherene B40 can change the spectral properties. For instance, the addition of a molecule can increase infrared and Raman-active modes and cause a red shift or blue shift of electronic spectra. These spectral features can be compared with future experimental values of endohedral borospherenes M@B40 0/- (M = H2, HF, and H2O).

Introduction

After the discovery of C60,[1] boron clusters have been investigated by many researchers and research results show that boron clusters favor quasi-planar or planar structures.[2−14] Furthermore, a lot of experimental research works and theoretical calculations have been reported to investigate the electronic and structural properties of different boron carbide nanoclusters, boron clusters, tubular boron clusters, and boron sheets.[15−29] In 2014, a B40– cage with D2 symmetry was produced via laser vaporization.[30] The first B40– cage is named “borospherene”. This experimental research has aroused attention in boron clusters[31−48] and doped boron clusters[49−59] such as the experimental study of borospherene B39–;[31] structural and chemical properties of a B40 dimer and trimer,[32] doped boron clusters (MnB16– and CoB16–),[50,51] and anionic boron clusters B– (n = 11, 26–29, 33–34, 37–38);[33−39] structures and properties of borospherenes (B40+, B42+, B440/–, and B46);[40−44] structures and properties of metalloborospherenes (Ta@B23, Ag&B40, and Th@B38);[52,53] and studies of small doped boron clusters.[55−59]

The cavity of C60 with a diameter of 7.1 Å provides a suitable environment for encapsulating the monomer. Endohedral fullerenes based on C60 have attracted great attention due their potential applications in superconductivity and materials science.[60−63] The properties of the outer cage can be controlled by means of the inner small molecule or atom.[64] Based on the arc-discharge method, endohedral fullerenes encapsulating a nitrogen atom and rare gas have been synthesized.[65,66] In addition, based on the molecular surgical method, endohedral fullerenes M@C60 (H2O, HF, and H2) have been successfully synthesized.[64,67−69] Similarly, can endohedral borospherene also show new properties? The diameter of borospherene B40 is 6.2 Å, and it is about 1.0 Å smaller than that of C60 (7.1 Å). Similar to the endohedral M@C60, borospherene B40 can encapsulate a single atom or a small molecule to form endohedral borospherenes M@B40. Endohedral metalloborospherenes have been investigated by several theoretical studies.[70−77] Theoretical calculations of M@B40 (M = Ca and Sr) indicate that Sr@B40 possesses a perfect endohedral metalloborospherene structure with the Sr atom at the center, whereas the Ca atom is slightly off the cage center by 0.27 Å. Theoretical research works on endohedral M@B40 (M = Sc, Y, La, Na, and Ba) indicate that Sc, Y, Na, and La atoms all favor the off-center location in B40, whereas Ba favors the center location within the B40 cage.

It is worth noting that the hexagonal and heptagonal rings of B40 provide the possibility to encapsulate a single atom or a small molecule inside B40. In addition, the successful synthesis of (H2O)@C60, (HF)@C60, or (H2)@C60 provides the first light of morning to synthesize endohedral borospherenes M@B40. Herein, with the aim to obtain a quantum chemical prospect of endohedral borospherenes M@B400/– (M = H2O, HF, and H2), the comparative studies of electronic structures and spectral properties have been performed. The aim of the present work is to provide a theoretical research of endohedral borospherenes M@B400/– (M = H2O, HF, and H2) in which the H2O, H2, or HF monomer is encapsulated in the cage. The endohedral borospherenes M@B400/– (M = H2O, HF, and H2) studied in this paper are, of course, merely model systems, which may or may not be made experimentally. However, the calculated results of these model systems can provide useful data to assist further synthesis and applications of endohedral borospherenes M@B400/– (M = H2O, HF, and H2).

Computational Methods

Endohedral structures of M@B400/– (M = H2O, HF, and H2) were obtained by two steps. First, geometry optimizations were performed using the doped molecule (H2O, HF, or H2) as a probe to be placed in the cage. Considering the molecular symmetry, different positions and directions of the doped molecule in the cage were selected. The first geometry optimizations were performed using the PBE0, M06-2X, and TPSSh levels with the 6-31G basis set. After the first step, the last geometry optimizations and frequency analyses were performed at the PBE0/6-311+G* level. Electronic absorption spectra and photoelectron spectra (PES) were simulated with the time-dependent density functional theory (TD-DFT) calculations.[78] Adiabatic detachment energy (ADE) was calculated as the energy difference between the optimized anion and neutral endohedral structures. The vertical detachment energies (VDEs) were simulated via ΔSCF-time-dependent density functional theory (ΔSCF-TD-DFT).[30,31,78] Chemical bonding analyses were performed using the adaptive natural density partitioning (AdNDP) approach at the PBE0 level.[79] All computations were carried out using the Gaussian 09 software package.[80]

Results and Discussion

Structures and Electronic Properties

Figures S1–S3 (Supporting Information) present results of first geometry optimizations by placing the doped molecule in the cage with different positions and directions. Computed energy values of M@B400/– (M = H2, HF, and H2O) show that the endohedral structure I is lower in energy than other endohedral structures. For endohedral M@B400/– (M = H2 and HF), after geometry optimizations, the initial endohedral structures can relax to corresponding structures I–V. For endohedral H2O@B40 and H2O@B40–, after geometry optimizations, the initial endohedral structures can relax to corresponding structures I–IX. After geometry optimizations, the doped molecule at the off-center location can relax to the center location within the cage (the symcenter of the doped molecule is located almost in the cage center).

To gain further discussions about the endohedral borospherenes M@B400/– (M = H2, HF, and H2O), further structural optimizations and frequency analyses of endohedral structure I were performed. Optimized structures of endohedral borospherenes M@B400/– (M = H2, HF, and H2O) are displayed in Figure , and ground-state parameters are summarized in Table . It is worth noting that symmetries of M@B400/– (M = H2, HF, and H2O) reduce to D2 (H2@B400/–), Cs (HF@B400/–), and C2 (H2O @B400/–), respectively. Frequency calculations confirm the stabilities of these endohedral borospherenes M@B400/– (M = H2, HF, and H2O) by showing no imaginary frequencies. As given in Tables S1–S3, the bond length of the doped molecule shows no change compared to that of the single molecule. However, the B40 cage reduces the ∠HOH of the H2O molecule. Unlike Ca@B40, which is a charge-transfer complex between Ca2+ and B402–,[70] natural population analysis calculations (Tables S1–S3) reveal that there is no significant charge transfer of each doped molecule. However, as shown in Tables S1–S3, the B40 cage causes weak electronegativity of the doped molecule (the total charges on encapsulated H2O, HF, and H2 are negative) compared to that of the single electroneutral molecule (the total charges on single H2O, HF, and H2 are zero). The slight electronegativity of the doped molecule is the result of weak interaction between the outer borospherene cage and the inner molecule.

Figure 1

Structures of endohedral borospherenes M@B400/– (M = H2, HF, and H2O). (a) H2@B400/–, (b) HF@B400/–, and (c) H2O@B400/–.

Table 1

Symmetries, Dipole Moments (μ), Energy Gaps (Eg), Lowest Frequencies, and States of Endohedral Borospherenes M@B400/– (M = H2, HF, and H2O) and the B40 Cagea

	symmetry	μ/debye	Eg/eV	lowest frequency/cm–1	state
B40	D2d	0	3.13	170	1A1
H2@B40	D2	0	3.13	161	1A
H2@B40–	D2	0	1.31a	185	2B3
			2.98b
HF@B40	Cs	0.4732	3.10	165	1A′
HF@B40–	Cs	0.4693	1.40a	181	2A′
			2.98b
H2O@B40	C2	0.4176	3.10	129	1A
H2O@B40–	C2	0.2465	1.35a	129	2B
			2.97b

The markers a and b denote the α and β electrons, respectively.

The highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals (LUMOs) (see Figure S4) of endohedral M@B40 (M = H2O, HF, and H2) distribute in the B40 cage. The HOMO–LUMO energy gaps (Eg) of H2O@B40, HF@B40, and H2@B40 are 3.10, 3.10, and 3.13 eV, respectively. The Eg of B40 was simulated as 3.13 eV along with LUMO (−3.27 eV) and HOMO (−6.40 eV).[30] As shown in Table and Figure S4, Eg of borospherene H2@B40 is evaluated as 3.13 eV along with LUMO (−3.27 eV) and HOMO (−6.40 eV), which is the same as that of borospherene B40. However, encapsulation of HF lowers the LUMO orbital energy level by 0.03 eV and encapsulation of H2O lowers the HOMO and LUMO orbital energy levels by 0.02 and 0.05 eV, respectively. Overall, the addition of a H2O or HF molecule leads to a slight lowering of Eg of B40 by 0.03 eV. It indicates that the addition of a polar molecule (HF or H2O) can decrease the Eg and enhance the chemical activity of the B40 cage. Similar to endohedral fullerenes; the slight changes of energy gap for endohedral borospherenes are due to the weak interactions between the outer borospherene and the inner molecule.

To gain further insight into the stabilities of M@B40 (M = H2, HF, and H2O), we analyzed the chemical bonding in the closed-neutral endohedral borospherenes M@B40 (M = H2, HF, and H2O) via the AdNDP analyses,[79] which were performed using Multiwfn software version 3.4.[81] For borospherene B40, AdNDP reveals 40 3c–2e σ bonds, 4 5c–2e π bonds, 4 6c–2e π bonds, 8 6c–2e σ bonds, and 4 7c–2e π bonds on the B40 cage surface.[30] Similar to borospherene B40, for endohedral borospherenes M@B40 (M = H2, HF, and H2O), no 2c–2e bond is found on the B40 cage surface. However, encapsulation of object causes some changes in the bonding pattern of the cage. AdNDP analyses (see Figure a) reveal 1 2c–2e bond on the H2 and 40 3c–2e σ bonds on the 40 B3 triangles on the B40 cage surface. The remaining 20 bonds contain 12 π bonds and 8 σ bonds, which are readily classified into 3 sets: 8 5c–2e π bonds at the top and bottom of the cage (distributed symmetrically around the 2 6-membered rings), 4 6c–2e π bonds on the waist (each 6c–2e π bond on the 6 atoms between 2 7-membered rings, as shown in Figure S5b), and 8 7c–2e σ bonds (each 7c–2e σ bond on the quasi-planar close-packed B6 triangles plus an adjacent boron atom, as shown in Figure S5c). AdNDP analyses (see Figure b) reveal 3 lone pairs and 1 2c–2e bond on the HF and 40 3c–2e bonds on the surface of B40 cage. The remaining 20 bonds contain 12 5c–2e π bonds and 8 7c–2e σ bonds, which can be classified into 2 sets: 12 5c–2e π bonds (8 5c–2e π bonds distributed symmetrically around the 2 6-membered rings that are the same as the 5c–2e π bond distribution of H2@B40, and the other 4 5c–2e π bonds on the waist that are similar to the 4 6c–2e π bonds of H2@B40) and 8 7c–2e σ bonds that are the same as 8 7c–2e σ bonds of H2@B40. Single 5c–2e π bond and a 7c–2e σ bond of HF@B40 are shown in Figure S6. AdNDP analyses (see Figure c) reveal two lone pairs and 2 2c–2e bonds on the H2O and 40 3c–2e bonds on the surface of the B40 cage. The remaining 20 bonds contain 12 π bonds and 8 σ bonds, which are the same as those of H2@B40. Single 5c–2e π bond, 6c–2e π bond, and 7c–2e σ bond are shown in Figure S7. Overall, the 12 delocalized π bonds also cover the cage surface, which is similar to D2 B40. It is similar to D2 B40 in that there exists double (σ + π) delocalization of the electron clouds on the cage surface, which renders stability to the endohedral borospherene. Unlike Ca@B40,[70] the bonding patterns further reveal that there is no significant charge transfer. However, doping with the H2O, HF, or H2 molecule causes some changes in the bonding pattern on the cage surface, especially causing the eight 6c–2e σ bonds to disappear and simultaneously increasing eight 7c–2e σ bonds. Encapsulation of H2 and H2O increases four 5c–2e π bonds and simultaneously makes four 7c–2e π bonds to disappear. Encapsulation of HF increases eight 5c–2e π bonds and simultaneously makes four 7c–2e π bonds and four 6c–2e π bonds to disappear. Such changes in the bonding patterns further reveal that the perturbations of bonding patterns on the cage surface are due to the weak interactions between the outer borospherene and the inner molecule. AdNDP analyses reveal that doped molecules fail to bond effectively with atoms on the surface of the cage and their interactions show nonbonding properties. Their interaction mainly includes electrostatic interaction and the interaction caused by electron cloud overlap. According to the previous results, there is no significant charge transfer of the doped molecule, resulting in weak electrostatic interaction. Thus, the interaction caused by electron cloud overlap plays an important role in the interactions between the inner molecule and the outer borospherene cage.

Figure 2

Bonding patterns of endohedral borospherenes M@B40 (M = H2, HF, and H2O) from AdNDP analyses. (a) D2 H2@B40, (b) HF@B40, and (c) C2 H2O@B40. The occupation numbers are indicated.

Photoelectron Spectra

Photoelectron spectroscopy (PES) is a useful detection technique to understand the electronic structure of the cluster. Boron clusters and other clusters have been successfully probed via PES and theoretical calculations.[30,31] To facilitate future identifications of M@B40– (M = H2, HF, and H2O), the adiabatic detachment energies (ADEs) for endohedral borospherenes M@B40– (M = H2, HF, and H2O) were simulated, and then, we simulated the vertical detachment energies (VDEs) and PES for M@B40– (M = H2, HF, and H2O) via the TD-DFT method.[30,31,78] ADE of anionic endohedral borospherene represents the electron affinity of the corresponding neutral endohedral borospherene. The neutral endohedral borospherene with large electron affinity can easily capture an electron. ADEs of endohedral borospherenes M@B40– (M = H2, HF, and H2O) are 2.29 eV (H2@B40–), 2.35 eV (HF@B40–), and 2.35 eV (H2O@B40–). Among the endohedral borospherenes M@B40– (M = H2, HF, and H2O), H2@B40– and B40– have the same ADE (2.29 eV); however, HF@B40– and H2O@B40– have the same ADE (2.35 eV). The calculated results suggest that doping of polar molecules (HF and H2O) in B40 can cause a slight increase of the ADE.

The simulated photoelectron spectra (Figure ) of endohedral borospherenes M@B40– (M = H2, HF, and H2O) appear similar to the simulated spectrum of D2 B40–, except for the strong band at about 5.2 eV. The predicted photoelectron spectra show that H2@B40– and B40– have the same lowest first VDEs. For M@B40– (M = H2O, HF, and H2) and B40–, the energy gap between the first and second bands is about 1.85 eV. The first several bands of PES were used to distinguish boron clusters,[30,31] so it is important to discuss these bands. The first weak peaks originate from the simulated ground-state VDEs of H2@B40–, HF@B40–, and H2O@B40– at 2.39, 2.46, and 2.45 eV, respectively. The simulated ground-state VDEs of M@B40– (M = H2, HF, and H2O) originate from the detachment of the electron from the α singly occupied molecular orbital (α-SOMO). The second peak of each endohedral borospherene comes from the second and third simulated VDEs at 4.28 and 4.36 eV for H2@B40–, 4.28 and 4.37 eV for HF@B40–, and 4.30 and 4.37 eV for H2O@B40–. The second calculated VDE of each endohedral borospherene originates from the detachment of the electron from β-HOMO – 1, resulting in the first triplet state. The third calculated VDE of each endohedral borospherene originates from the detachment of the electrons from β-HOMO – 2. The 5th strong bands (5–5.4 eV) of these endohedral borospherenes largely consist of the 20th to 30th simulated VDEs. These calculated VDEs of each endohedral borospherene originate from the detachment of the electrons from α singly occupied molecular orbitals. The fifth band can be used to distinguish the endohedral borospherenes M@B40– (M = H2, HF, and H2O) and borospherene B40–.

Figure 3

Computed photoelectron spectra of borospherenes M@B40– (M = H2, HF, and H2O) and borospherene B40–. (a) D2 H2@B40–, (b) C HF@B40–, (c) C2 H2O@B40–, and (d) D2 B40–. The simulations were done by fitting the distributions of calculated vertical detachment energies at the PBE0 level with unit-area Gaussian functions of 0.05 eV half-width.

Figure a–c and PES of borospherene B40– (see Figure d)[30] indicate that encapsulation of a molecule hardly changes the first to third bands of borospherene B40–. However, the simulated results show that encapsulation of a polar molecule in borospherene B40– can increase the ground-state VDE compared to that of B40–. The simulated theoretical spectra in Figure are useful for the identification of endohedral borospherenes M@B40– (M = H2, HF, and H2O) by means of combined experimental and theoretical comparisons. Like borospherene B40–, if the photoelectron spectra of endohedral borospherenes M@B40– (M = H2, HF, and H2O) are measured, these simulated data are useful for the identification of endohedral borospherenes M@B40– (M = H2, HF, and H2O).

Infrared Spectra

Infrared spectra of endohedral borospherenes M@B400/– (M = H2, HF, and H2O) are displayed in Figure ; these spectral bands can be classified into three regions: low-frequency band (0–1000 cm–1), middle-frequency band (1000–1600 cm–1), and high–frequency band (160–4500 cm–1). The highest frequency of each endohedral borospherene is 4368 cm–1 for H2@B40, 4369 cm–1 for H2@B40–, 3704 cm–1 for HF@B40, 3591 cm–1 for HF@B40–, 3847 cm–1 for H2O@B40, and 3828 cm–1 for H2O@B40–. These vibrational modes come from the stretching vibrations of the doped molecule and are Raman-active modes. Among these vibrational modes, the vibrational modes at 4368 cm–1 (H2@B40) and 4369 cm–1 (H2@B40–) are IR-inactive modes and other four vibrational modes are infrared-active modes. However, intensities of the four IR-active modes are very weak. The lowest vibrational frequency for each endohedral borospherene is 161 cm–1 for H2@B40, 185 cm–1 for H2@B40–, 165 cm–1 for HF@B40, 181 cm–1 for HF@B40–, 129 cm–1 for H2O@B40, and 129 cm–1 for H2O@B40–. The lowest vibrational mode at 161 cm–1 (H2@B40) originates from the bending vibration of the doped molecule and is infrared inactive. The lowest vibrational mode at 185 cm–1 (H2@B40–) originates from the bending vibration of boron atoms and is also infrared inactive. The lowest vibrational modes at 165 cm–1 (HF@B40) and 181 cm–1 (HF@B40–) originate from the bending vibration of the doped molecule and boron atoms and are infrared active. The lowest vibrational modes at 129 cm–1 (H2O@B40) and 129 cm–1 (H2O@B40–) originate from the bending vibration of the doped molecule and are infrared active.

Figure 4

Computed infrared spectra of endohedral borospherenes M@B400/– (M = H2, HF, and H2O) by the PBE0/6-311+G* method. (a) D2 H2@B40, (b) D2 H2@B40–, (c) C HF@B40, (d) C HF@B40–, (e) C2 H2O@B40, and (f) C2 H2O@B40–. The disconnected shaded part represents the omitted frequency interval (without spectral data).

The sharpest peaks of H2@B40, H2@B40–, HF@B40, HF@B40–, H2O@B40, and H2O@B40– are at 1273, 1284, 1268, 1280, 1265, and 1277 cm–1, respectively. These strongest active modes of endohedral borospherenes M@B400/– (M = H2, HF, and H2O) come from stretching vibrations of boron atoms and are at about 1270 cm–1, which is almost same as that (at 1274 cm–1) of the B40 cage.[48] Due to the high D2 symmetry, B400/– cages have doubly degenerate vibrational modes. Encapsulation of a molecule leads to the lower symmetry of endohedral borospherene, and simulated results show that there are only nondegenerate vibrational modes for endohedral borospherenes M@B400/– (M = H2, HF, and H2O). The B40 cage encompasses 43 IR-inactive modes. However, the D2 H2@B40 encompasses 33 IR-inactive modes and the D2 H2@B40– encompasses 32 IR-inactive modes. In addition, other endohedral borospherenes M@B400/– (M = HF and H2O) have no IR-inactive modes. These simulated results indicate that encapsulation of a molecule can increase IR-active modes.

Figure a,c,e shows that endohedral borospherenes M@B40 (M = H2, HF, and H2O) have similar infrared spectra except for the region at 1600–4500 cm–1. In addition, Figure b,d,f shows that the main peaks for anionic H2O@B40– and HF@B40– are weaker compared to those of H2@B40–. It is worth noting that the infrared spectra of endohedral borospherenes M@B40 (M = H2, HF, and H2O) are quite similar to those of B40 (see Figure S8a).[48] However, Figure b,d,f indicates that only the spectral feature for endohedral borospherene H2@B40– is quite similar to that of the B40– cage (see Figure S8b). These spectral characteristics suggest that the polarity of the doped molecule has an effect on infrared spectra and also give some information about the identification and confirmation of endohedral borospherenes M@B400/– (M = H2, HF, and H2O).

Raman Spectra

Figure depicts the Raman spectra of endohedral borospherenes M@B400/– (M = H2, HF, and H2O), the sharpest peaks of H2@B40, H2@B40–, HF@B40, HF@B40–, H2O@B40 and H2O@B40– are at 1323, 1233, 1322, 1220, 1318 and 1105 cm–1, respectively. These strongest vibrational modes originate from stretching vibrations of boron atoms. Among the Raman-active modes, the typical radial breathing modes of H2@B40, H2@B40–, HF@B40, HF@B40–, H2O@B40 and H2O@B40– are at 180/429, 185/434, 192/429, 181/432, 193/432, and 198/438 cm–1, respectively, which are similar to that (at 170/427 cm–1) of the B40 cage.[82] The breathing modes are used to identify the hollow structures in nanotubes. The small change of typical radial breathing modes indicates that encapsulation of a molecule just induces a small change in the B40 configuration. The borospherene B40[47,48] has 14 Raman-inactive modes; however, there are 3 Raman-inactive modes in endohedral H2@B40 and other endohedral borospherenes have no Raman-inactive modes. Figures a,c,e and S6a show that endohedral M@B40 (M = H2, HF, and H2O) and B40 have similar spectral features. In addition, Raman spectra of endohedral H2@B40– are similar to that of dianion B40– (see Figure S9b). However, Raman spectra of endohedral HF@B40– and H2O@B40– are different to that of B40– (see Figure S9b). It indicates that doping of a polar molecule in the B40– cage can influence the Raman spectra of the B40– cage such as enhanced or weakened spectral bands.[48]

Figure 5

Computed Raman spectra of endohedral borospherenes M@B400/– (M = H2, HF, and H2O) by the PBE0/6-311+G* method. (a) D2 H2@B40, (b) D2 H2@B40–, (c) C HF@B40, (d) C HF@B40–, (e) C2 H2O@B40, and (f) C2 H2O@B40–. The disconnected shaded part represents the omitted frequency interval (without spectral data).

Raman spectra, as a supplement of infrared spectra, are useful for the basis of identification of endohedral borospherenes M@B400/– (M = H2, HF, and H2O). The simulated spectra of endohedral borospherenes M@B400/– (M = H2, HF, and H2O) show that some vibrational modes have strong infrared activity or strong Raman activity. However, some vibrational modes are weak (or inactive) infrared-active modes or weak (or inactive) Raman-active modes. Infrared activity is related to |∂μ⃗/∂Q|2;[83,84] here, Q is the normal-mode coordinate and μ⃗ is the dipole moment. Raman activity is related to the polarizability of the molecule. If the vibrational of a normal mode cannot cause a change of dipole moment, the information for this mode cannot be obtained in the infrared spectra. However, it may cause the change of polarizability and we can obtain the information in Raman spectra. The simulated Raman spectra can provide useful information to vibrational assignments and spectral interpretation.

Electronic Absorption Spectra

Finally, electronic absorption spectra of endohedral borospherenes M@B400/– (M = H2, HF, and H2O) were simulated, as displayed in Figures and 7 and 7. The B40 cage has only UV–vis spectral bands (see the black line in Figure );[47,48] similarly, one can observe only UV–vis spectral bands of endohedral borospherenes M@B40 (M = H2, HF, and H2O). However, the simulated results show that endohedral borospherenes M@B40– (M = H2, HF, and H2O) have several near infrared spectral bands between 800 and 4000 nm (see Figure ). Figure shows that endohedral borospherene H2@B40 and B40 cage have the almost the same spectral features; however, endohedral borospherenes HF@B40 and H2O@B40 have the same spectral features. The results show that doping of polar molecules (HF and H2O) in B40 can cause a slight red shift of electronic spectra. Red and blue lines in Figure represent the electronic spectra of H2@B40 and HF@B40; the largest excitation wavelengths are 537 and 543 nm for H2@B40 and HF@B40, respectively. The maximum absorption wavelength comes from the electron transitions from HOMO to LUMO. The green line in Figure represents the electronic spectra of H2O@B40. The maximum absorption wavelength is 543 nm that originates from the electron transitions from HOMO/HOMO – 1/HOMO – 2 to LUMO. The black line in Figure represents the electronic spectra of B40. The maximum absorption wavelength is 535 nm that originates from the electron transitions from HOMO to LUMO. The maximum band (500–600 nm) of B40 contains the first four excited states that originate from the electron transitions from HOMO to LUMO, HOMO – 1 to LUMO/LUMO + 1/LUMO + 2 coupled with HOMO – 2/HOMO – 3 → LUMO. The computed results indicate that the maximum band (500–600 nm) of each endohedral borospherene also contains the first four excited states that originate from the electron transitions of HOMO to LUMO, HOMO – 1 to LUMO/LUMO + 1/LUMO + 2 coupled with HOMO – 2/HOMO – 3 → LUMO. Other absorption bands of endohedral borospherenes M@B40 (M = H2, HF, and H2O) originate from the higher excited states that arise from the electron transitions of corresponding orbits.

Figure 6

Computed electronic absorption spectra of endohedral borospherenes M@B40 (M = H2, HF, and H2O) and borospherene B40. The red line represents D2 H2@B40, blue line represents C HF@B40, green line represents C2 H2O@B40, and black line represents D2 B40.

Figure 7

Computed electronic absorption spectra of endohedral borospherenes M@B40– (M = H2, HF, and H2O) and borospherene B40–. The red line represents D2 H2@B40–, blue line represents C HF@B40–, green line represents C2 H2O@B40–, and black line represents D2 B40–.

For the closed-shell endohedral borospherenes M@B40 (M = H2, HF, and H2O), the maximum excitation wavelength (minimum excitation energy) mainly originates from the electron transition from HOMO to LUMO. HOMO–LUMO energy gap (Eg) reflects the probability of the transition from the ground state to excited state. The molecule with larger Eg can induce larger excitation energy and is difficult to jump to the excited state. On the contrary, the molecule with smaller Eg can induce smaller excitation energy and can easily perform electron transition. The simulated values of Eg are 3.13, 3.10, and 3.10 eV for H2@B40, HF@B40, and H2O@B40, respectively. Encapsulation of a polar molecule (HF or H2O) in B40 can increase the probability of the transition. In addition, the same Eg of HF@B40 and H2O@B40 just reflects the same largest excitation wavelength (543 nm).

Figure presents the electronic spectra of endohedral borospherenes M@B40– (M = H2, HF, and H2O) and B40–. Like the neutral H2@B40, endohedral H2@B40–, and B40– have almost the same spectral features; however, the simulation results show that encapsulation of polar molecules (HF and H2O) can cause a significant blue shift of the maximum absorption band. The maximum absorption band (1500–4000 nm) of B40– comes from the first two excited states that originate from the transitions from α-SOMO to α-LUMO coupled with α-SOMO to α-LUMO + 1. Similarly, the computed results indicate that the maximum absorption band (1500–4000 nm) of each endohedral borospherene also comes from the first four excited states that originate from the transitions from α-SOMO to α-LUMO, coupled with α-SOMO to α-LUMO + 1. The calculated electronic spectra of endohedral borospherenes M@B400/– (M = H2, HF, and H2O) are useful for the electronic structure analysis.

Conclusions

The structures, electronic properties, Raman and infrared spectra, photoelectron spectra, and electronic spectra of endohedral borospherenes M@B400/– (M = H2, HF, and H2O) have been investigated by the DFT and TD-DFT methods. The calculated results suggest that H2, HF, and H2O molecules can form stable endohedral borospherenes M@B400/– (M = H2, HF, and H2O). In addition, structural research works suggest that the doped molecule at the off-center location can relax to the center location within the cage and the symcenter of the doped molecule is almost located in the cage center. Unlike endohedral metalloborospherenes Ca@B40, natural population analyses and chemical bonding analyses reveal that there is no significant charge transfer for the doped molecule. In addition, natural population analyses and chemical bonding analyses reveal the existence of weak interaction between the inner molecule and outer borospherene cage. The calculated spectra suggest that doping of a molecule (H2, HF, and H2O) in borospherene B40 can change the photoelectron spectra and doping of a polar molecule (HF and H2O) in borospherene B40 can change the electronic, Raman, and infrared spectra. For instance, the calculated photoelectron spectra show that doping of a molecule (H2, HF, and H2O) in borospherene B40 can lead to more bands. In addition, the addition of a molecule can increase Raman- and infrared-active modes and cause a red shift or blue shift electronic spectra. These simulated spectra can be compared with experimental data of M@B400/– (M = H2, HF, and H2O). The borospherenes M@B400/– (M = H2, HF, and H2O) are useful for the applications of borospherenes such as molecular device.