- Email: [email protected]
A simple method to generate B(N3)3
Chemical Physics Letters 419 (2006) 213–216 www.elsevier.com/locate/cplett
A simple method to generate B(N3)3 Fengyi Liu a,b, Xiaoqing Zeng b, Jianping Zhang b, Lingpeng Meng a, Shijun Zheng a, Maofa Ge *,b, Dianxun Wang a,b,c,*, Daniel Kam Wah Mok c, Foo-tim Chau *,c b
a Open Laboratory of Computational Quantum Chemistry, Hebei Normal University, Shijiazhuang, 050091, China State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Institute of Chemistry, The Chinese Academy of Sciences, Zhongguancun, Beijing, Haidian 100080, China c Department of Applied Biology and Chemical Technology, Hong Kong Polytechnic University, Hum Hom, Hong Kong, China
Received 22 March 2005; in final form 29 September 2005 Available online 15 December 2005
Abstract Boron triazide, B(N3)3, is first generated in gas phase by the heterogeneous reaction of BBr3 vapor with freshly synthesized AgN3 powder at room temperature. Identification of B(N3)3 has been achieved by combination of photoelectron spectroscopy with the outer valence GreenÕs function calculation. B(N3)3 is a planar molecule with C3h symmetry and has the first adiabatic ionization energy of 11.39 eV. Vibrational spacings 2200 ± 60 cm1 and 1250 ± 60 cm1 on the first and third bands match well to the N3 asymmetric stretch and the B–N stretching vibrations in the neutral B(N3)3 molecule, respectively. 2005 Elsevier B.V. All rights reserved.
1. Introduction Boron triazide, B(N3)3, has been known for more than 50 years. In 1954, Wiberg and Michaud were the first to report the synthesis of boron triazide B(N3)3 by the reaction of diborate with HN3 in an ether solution at low temperature [1]. In recent years, much attention has been paid on the properties of B(N3)3 since it is a potential high energy-density material (HEDM) and can also serve as precursors for boron nitride (BN) thin films [2–8]. Both hexagonal and cubic BN films have many important applications such as wide bandgap semiconductors and tribological coating [3]. Although cubic-boron nitride (c-BN) is only the second hardest material known so far, it is less reactive than diamond [8]. B(N3)3 can be a precursors as it decomposes spontaneously to form N2-rich hexagonal BN films. The advantage of this process is that no excessive energy input. Thus, far more perfect thin film can be formed as film damage caused by differential cooling or impact of high-energy species can be avoided. Coombe et al. reported *
Corresponding author. Fax: +86 1062559373. E-mail address: [email protected] (D. Wang).
0009-2614/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2005.11.082
a gas phase synthesis of B(N3)3 by reaction of HN3 with BCl3 to produce B(N3)3. This could be used to create the thin film. Yet corrosive HCl gas is also a product of the reaction [2]. In this Letter, we give a simple method for generating pure B(N3)3 in the gas phase. The photoelectron (PE) spectrum of the molecule is reported and the assignment of the spectrum is supported by the outer valence GreenÕs function (OVGF) calculation with 6-31+G(d) basis sets. 2. Experiment and theoretical calculations 2.1. Generation In order to realize the generation of some high-nitrogen content compounds at the low pressure and low temperature, new equipment consisting of the pumping systems and the temperature-controller was developed. The new equipment can be separately used for collecting new generated species under the controlled temperature (195 to 200 C). Of course, it can also be connected into the PE spectrometer as a new inlet system or other experimental spectrometer [9].
214
F. Liu et al. / Chemical Physics Letters 419 (2006) 213–216
First, the freshly prepared AgN3 powder [10] is loosely filled into the quartz reaction tube and supported on quartz wool to enlarge dispersion of AgN3. Second, after pumping the system to the pressure of 1 · 104 torr, B(N3)3 is generated by passing the BBr3 vapor at the pressure of 3 · 103 torr over the solid AgN3 (2 g). Caution: AgN3 is highly explosive, especially in its pure dried-powder, so appropriate safety precautions should be taken. 2.2. PES measurement The PE spectrum of B(N3)3 was recorded on a doublechamber UPS-II machine [11–13] which was built specifically to detect transient species at a resolution of about 30 meV as indicated by the Ar+ (2P3/2) photoelectron band. Experimental vertical ionization energies (Iv in eV) are calibrated by simultaneous addition of a small amount of argon and methyl iodide to the sample. BBr3 was bought by ACROS Company and its PE spectrum is the same with the literature [14]. 2.3. X-ray diffraction (XRD) An X-ray diffraction (XRD) experiment for the solid residue of the reactor tube was achieved on the Dmax/ 2500-PC [9]. 2.4. OVGF calculation To assign the PES bands of B(N3)3, the OVGF calculation with 6-31+G(d) basis sets based on the B3LYP optimized geometry has been performed for a minimum in energy on B(N3)3 with planar C3h symmetry. Vibrational frequency analysis was also done to compare with the vibrational spacing of different PE bands; the theoretical vibrational frequencies were scaled by a factor 0.9642 from the literature [15]. The vertical ionization energies (Ev) were calculated at the ab initio level according to CederbaumÕs outer valence GreenÕs function (OVGF) method [16], which includes the effect of electron correlation and reorganization beyond the Hartree–Fock approximation. The self-energy part was expanded up to third-order, and contributions of higher-orders were estimated by means of a renormalization procedure. All the above calculations were performed using GAUSSIAN98 Program [17].
Fig. 1 gives fully optimized geometry of B(N3)3 by B3LYP method at the 6-311+G(2df) basis sets, i.e. a minimum in energy with C3h symmetry. Prior to this work, computational efforts on B(N3)3 at the Hartree–Fock and MP2 levels of theory with the 6-31G(d) basis sets were firstly done by Mulinax et al. [2], as well as the calculated frequencies were compared to observed IR frequencies. The bond lengths and bond angles calculated in this work show good agreement with those reported in [2], with derivations no more than 1%. The PE spectrum of the product of the reaction (1) mentioned above is given in Fig. 2. Table 1 gives PES experimental vertical ionization energies (IP in eV), theoretical vertical and adiabatic ionization energies (Ev and Ea in eV) by OVGF and modified-DFT calculations, as well as the molecular orbital-ionized (orbital from which electron was ionized/removed) characters for B(N3)3. A good agreement between PES experiment and OVGF calculation for B(N3)3 with C3h symmetry can be clearly seen from Table 1. The first PES band of 10.30 eV with vibrational spacing 2200 ± 60 cm1 is designated as ionization of the electrons of the degenerate highest occupied molecular orbitals (HOMOs), orbital 34(2e00 ) and 33(2e00 ) in the B(N3)3 molecule, not only owing to its value is very close to computed data 10.322 eV (by OVGF/6-31+G(d)), 10.248 eV (at B3LYP/6-311++G(3df) level), but also it is an antibonding p orbitals with dominant contribution of the N3 groups, leading to equality of the vertical ionization energy (Ev) with the adiabatic ionization energy (Ea) on the band for the molecule. As seen in Table 2, vibrational spacing 2200 ± 60 cm1 is comparable to the calculation value 2254 cm1 (B3LYP/6-31+G(d)) for the first ionic state (X2A00 ) while is larger than the N3 asymmetric stretch frequency 2163 cm1 (corresponding theoretical value
3. Results and discussion Gas phase pure B(N3)3 is readily generated by the heterogeneous reaction of BBr3 vapor at the pressure of 3 · 103 torr with freshly synthesized AgN3 powder at room temperature: BBr3 ðgÞ þ AgN3 ðsÞ ! BðN3 Þ3 ðgÞ þ AgBr ðsÞ
ð1Þ
Identification for the product of reaction (1) was done by both PES experiment and OVGF calculation.
Fig. 1. Optimized geometric parameters for B(N3)3 by using density functional B3LYP, B3PW91 [in square brackets] and B3P86 (in parentheses) methods with 6-311+G(2df) basis sets.
F. Liu et al. / Chemical Physics Letters 419 (2006) 213–216
215
Table 2 Selected experimental and calculated frequencies for B(N3)3 v23/v24
v21/v22
Experiment PE in gas (this work) IR in gasa IR in matrixb
2200(±60) 2163a 2158b
1250(±60) 1360 1371
Calculation (B(N3)3) B3LYP/6-31 + G(d) B3LYP/6-311 + G(2df) MP2/6-31G(d)a
2296(1032)c 2282(1058) 2202 (841)
1439(973) 1431(987) 1451 (862)
Calculation ðBðN3 Þþ 3Þ B3LYP/6-31 + G(d) Assignment Bond/anti-bond
2338(470) Asymmetric N3 stretch Anti-bond
1125(54) B–N stretch Bond
a b c
Fig. 2. Full photoelectron spectrum of B(N3)3 molecule, in which the vibrational spacing of the third band at 11.80 eV is expanded. Table 1 Photoelectron ionization energies (IP in eV), computed ionization energies (Ev and Ea in eV) by the OVGF and modified-DFT calculation and molecular orbital-ionized characters for B(N3)3 IP (eV)
10.30
11.52
Ev (eV) OVGF/ 6-31+G(d)
DFT
10.322(0.901)
10.248
11.557(0.902)
Ea (eV)b
MO
Characterc
10.178
34(2e00 )
pN4–N5–N6, pN7–N8–N9 pN1–N2–N3 p0N4–N5–N6 ; p0N7–N8–N9 p0N1–N2–N3 ; p0N4–N5–N6 rB–N pB–N rN–N–N rN–N–N
a
33(2e00 ) 32(9e 0 ) 31(9e 0 )
11.80 12.49 15.45
11.737(0.901) 12.448(0.899) 16.131(0.859)
30(10a 0 ) 29(2a00 ) 28(8e 0 ) 27(8e 0 )
a The first vertical ionization energies (Ev) calculated by DFT at B3LYP/6-311++G(3df) level. b The first adiabatic ionization energies (Ea) calculated by B3LYP with 6-311++G(3df) basis sets. c p denotes the delocalized p orbital which are perpendicular to the molecular framework (see Fig. 3, MO-34 and MO-33), while p 0 is the Ôin planeÕ localized p orbital result from the p atom-orbitals of nitrogen inner one N3 group (see Fig. 3, MO-32 and MO-31).
2213 cm1) in azide groups binding to central boron atom in the neutral B(N3)3 molecule [2], because ionization of the electron of an antibonding orbital always leads to an increasing vibrational frequency on the ionic state compared with that of the neutral molecule. The second PES band of 11.52 eV results from ionization of the electrons of another pair of degenerate p orbi-
Ref. [2]: IR in gas. Ref. [8]: IR in argon matrices. Frequencies in cm1, followed by relative IR intensities in parentheses.
tals – the second HOMOs (SHOMOs) – orbital 32(9e 0 ) and 31(9e 0 ) in the molecule, because 11.52 eV is very close to the computed value of 11.557 eV and ionization of the electron of p orbitals generally leads to equality of the vertical ionization energy (Ev) with the adiabatic ionization energy (Ea) on the band. An unresolved structure of the second band shows that the SHOMOs are strong bonding orbitals, as shown in Fig. 3. The band with clear vibrational spacing 1250 ± 60 cm1 is considered as the third PES band of B(N3)3 molecule. Its overlapping with the band at 11.52 eV increases not only the band intensity of the side of high energy at 11.52 eV, but also the intensity of the vibrational components on the side of low energy of the third PES band. So the vertical ionization energy of the third band is determined to be 11.80 eV. This band is attributed from ionization of the electron of orbital 30(10a 0 ) which has dominant atomic contributions of both boron and nitrogen bound to boron atom in the molecule (see Fig. 3), not only owing to 11.80 eV is close to the computed value 11.737 eV, but also due to its vibrational spacing 1250 ± 60 cm1 is comparable to the B3LYP/6-31+G(d) predicated value 1084 cm1 for the ionic state (12A 0 ) and consistent with the B–N stretching vibrational frequency 1360 cm1 (corresponding computed value 1387 cm1) in the neutral B(N3)3 molecule [2], because ionization of the electron of the 30(10a 0 ) bonding orbital with dominant atomic contribution of the boron and nitrogen bound to boron atom leads to that the vibrational spacing 1250 ± 60 cm1 on the PES band is smaller than that (1360 cm1) of corresponding vibrational frequency in the neutral molecule. A long vibrational progression on the third band (four or more vibrational components) shows that the orbital 30(10a 0 ) of B(N3)3 is also a strong bonding orbital. The fourth PES band at 12.49 eV, close to the corresponding computed value 12.448 eV, comes from ionization of the electron of another deep-shell p orbital in the B(N3)3 molecule, because the band is narrow.
216
F. Liu et al. / Chemical Physics Letters 419 (2006) 213–216
Fig. 3. Characters of the six highest occupied molecular orbitals (HOMOs) of B(N3)3 molecule.
The band centered near 15.45 eV is very broad. This is considered as result of electron from several deep-shell orbitals in the B(N3)3 molecule (see Table 1). 4. Conclusion In summary, B(N3)3 was first generated in the gas phase by the heterogeneous reaction of BBr3 vapor at the pressure of 3 · 103 torr with freshly synthesized AgN3 powder at room temperature. Only gaseous product, B(N3)3 has been characterized on line by a combination of PES experiment and OVGF calculation at the 6-31+G(d) basis set. Good agreement between PES and OVGF calculations shows that B(N3)3 is a planar molecule with C3h symmetry and has the first adiabatic ionization energy of 11.39 eV, which results from ionization of the electron of the highest occupied molecular orbital (HOMO, 2e00 ) with dominant contribution of the N3 group in the molecule. The solid residue AgBr in the reactor tube by identification of X-ray diffraction (XRD) analysis and excellent agreement of vibrational spacings on the PES bands for only gaseous reaction product with previously experimental results, furthermore, support the formation of B(N3)3 in the reaction (1). Obviously, the introduction of a ready synthesis method for only gaseous product B(N3)3 might facilitates studies of its properties and applications to form the thin films. Acknowledgements This project was supported by Chinese Academy of Sciences (Contract No. KJCX2-SW-H8 and Hundred talents
fund) and the National Natural Science Foundation of China (Contract Nos.: 20477047, 20473094, 50372071, 20577052, 20503035). Xiaoqing Zeng thanks the Chinese Academy of Sciences for a scholarship during the period of this work. References [1] E. Wiberg, H. Michaud, Z. Naturforsch. 96 (1954) 497. [2] R.L. Mulinax, G.S. Okin, R.D. Coombe, J. Phys. Chem. 99 (1995) 6294. [3] J.J. Pouch, S.A. Alterovitz, Synthesis and Properties of Boron Nitride, Trans Tech, Aedermannsdorf, Switzerland, 1990. [4] R.H. Wentorf Jr., J. Chem. Phys. 36 (1962) 1990. [5] H. Saitoh, W.A. Yarbrough, Appl. Phys. Lett. 58 (1991) 2482. [6] D.J. Kester, R. Messier, Appl. Phys. Lett. 72 (1992) 506. [7] F. Qian, V. Nagabushnam, R.K. Singh, Appl. Phys. Lett. 63 (1993) 317. [8] I.A. Al-Jihad, B. Liu, C.J. Linnen, J.V. Gilbert, J. Phys. Chem. 102 (1998) 6220. [9] X.Q. Zeng, Ph.D. Thesis, Chinese Academy of Sciences, Beijing, China, 2004. [10] I.C. Tornieporth-Oetting, P. Buze, R. von P. Schleyer, T.M. Klapo¨tke, Angew. Chem., Int. Ed. 31 (1992) 1338. [11] X.J. Zhu, M.F. Ge, J. Wang, Z. Sun, D.X. Wang, Angew. Chem., Int. Ed. 112 (2000) 1940. [12] Z. Sun, J. Wang, X.J. Zhu, M.F. Ge, D.X. Wang, Chem. Eur. J. 14 (2001) 2995. [13] J. Wang, Z. Sun, X.J. Zhu, X.J. Yang, M.F. Ge, D.X. Wang, Angew. Chem., Int. Ed. 40 (2001) 3055. [14] P.J. Bassett, D.R. Lloyd, J. Chem. Soc. A (1971) 1551. [15] A.P. Scott, L. Radom, J. Phys. Chem. 100 (1996) 16502. [16] J.V. Ortiz, J. Chem. Phys. 108 (1998) 1008. [17] M.J. Frisch et al., GAUSSIAN98, Revision A.3, Gaussian Inc., Pittsburgh, PA, 1998.
A simple method to generate B(N3)3 Fengyi Liu a,b, Xiaoqing Zeng b, Jianping Zhang b, Lingpeng Meng a, Shijun Zheng a, Maofa Ge *,b, Dianxun Wang a,b,c,*, Daniel Kam Wah Mok c, Foo-tim Chau *,c b
a Open Laboratory of Computational Quantum Chemistry, Hebei Normal University, Shijiazhuang, 050091, China State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Institute of Chemistry, The Chinese Academy of Sciences, Zhongguancun, Beijing, Haidian 100080, China c Department of Applied Biology and Chemical Technology, Hong Kong Polytechnic University, Hum Hom, Hong Kong, China
Received 22 March 2005; in final form 29 September 2005 Available online 15 December 2005
Abstract Boron triazide, B(N3)3, is first generated in gas phase by the heterogeneous reaction of BBr3 vapor with freshly synthesized AgN3 powder at room temperature. Identification of B(N3)3 has been achieved by combination of photoelectron spectroscopy with the outer valence GreenÕs function calculation. B(N3)3 is a planar molecule with C3h symmetry and has the first adiabatic ionization energy of 11.39 eV. Vibrational spacings 2200 ± 60 cm1 and 1250 ± 60 cm1 on the first and third bands match well to the N3 asymmetric stretch and the B–N stretching vibrations in the neutral B(N3)3 molecule, respectively. 2005 Elsevier B.V. All rights reserved.
1. Introduction Boron triazide, B(N3)3, has been known for more than 50 years. In 1954, Wiberg and Michaud were the first to report the synthesis of boron triazide B(N3)3 by the reaction of diborate with HN3 in an ether solution at low temperature [1]. In recent years, much attention has been paid on the properties of B(N3)3 since it is a potential high energy-density material (HEDM) and can also serve as precursors for boron nitride (BN) thin films [2–8]. Both hexagonal and cubic BN films have many important applications such as wide bandgap semiconductors and tribological coating [3]. Although cubic-boron nitride (c-BN) is only the second hardest material known so far, it is less reactive than diamond [8]. B(N3)3 can be a precursors as it decomposes spontaneously to form N2-rich hexagonal BN films. The advantage of this process is that no excessive energy input. Thus, far more perfect thin film can be formed as film damage caused by differential cooling or impact of high-energy species can be avoided. Coombe et al. reported *
Corresponding author. Fax: +86 1062559373. E-mail address: [email protected] (D. Wang).
0009-2614/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2005.11.082
a gas phase synthesis of B(N3)3 by reaction of HN3 with BCl3 to produce B(N3)3. This could be used to create the thin film. Yet corrosive HCl gas is also a product of the reaction [2]. In this Letter, we give a simple method for generating pure B(N3)3 in the gas phase. The photoelectron (PE) spectrum of the molecule is reported and the assignment of the spectrum is supported by the outer valence GreenÕs function (OVGF) calculation with 6-31+G(d) basis sets. 2. Experiment and theoretical calculations 2.1. Generation In order to realize the generation of some high-nitrogen content compounds at the low pressure and low temperature, new equipment consisting of the pumping systems and the temperature-controller was developed. The new equipment can be separately used for collecting new generated species under the controlled temperature (195 to 200 C). Of course, it can also be connected into the PE spectrometer as a new inlet system or other experimental spectrometer [9].
214
F. Liu et al. / Chemical Physics Letters 419 (2006) 213–216
First, the freshly prepared AgN3 powder [10] is loosely filled into the quartz reaction tube and supported on quartz wool to enlarge dispersion of AgN3. Second, after pumping the system to the pressure of 1 · 104 torr, B(N3)3 is generated by passing the BBr3 vapor at the pressure of 3 · 103 torr over the solid AgN3 (2 g). Caution: AgN3 is highly explosive, especially in its pure dried-powder, so appropriate safety precautions should be taken. 2.2. PES measurement The PE spectrum of B(N3)3 was recorded on a doublechamber UPS-II machine [11–13] which was built specifically to detect transient species at a resolution of about 30 meV as indicated by the Ar+ (2P3/2) photoelectron band. Experimental vertical ionization energies (Iv in eV) are calibrated by simultaneous addition of a small amount of argon and methyl iodide to the sample. BBr3 was bought by ACROS Company and its PE spectrum is the same with the literature [14]. 2.3. X-ray diffraction (XRD) An X-ray diffraction (XRD) experiment for the solid residue of the reactor tube was achieved on the Dmax/ 2500-PC [9]. 2.4. OVGF calculation To assign the PES bands of B(N3)3, the OVGF calculation with 6-31+G(d) basis sets based on the B3LYP optimized geometry has been performed for a minimum in energy on B(N3)3 with planar C3h symmetry. Vibrational frequency analysis was also done to compare with the vibrational spacing of different PE bands; the theoretical vibrational frequencies were scaled by a factor 0.9642 from the literature [15]. The vertical ionization energies (Ev) were calculated at the ab initio level according to CederbaumÕs outer valence GreenÕs function (OVGF) method [16], which includes the effect of electron correlation and reorganization beyond the Hartree–Fock approximation. The self-energy part was expanded up to third-order, and contributions of higher-orders were estimated by means of a renormalization procedure. All the above calculations were performed using GAUSSIAN98 Program [17].
Fig. 1 gives fully optimized geometry of B(N3)3 by B3LYP method at the 6-311+G(2df) basis sets, i.e. a minimum in energy with C3h symmetry. Prior to this work, computational efforts on B(N3)3 at the Hartree–Fock and MP2 levels of theory with the 6-31G(d) basis sets were firstly done by Mulinax et al. [2], as well as the calculated frequencies were compared to observed IR frequencies. The bond lengths and bond angles calculated in this work show good agreement with those reported in [2], with derivations no more than 1%. The PE spectrum of the product of the reaction (1) mentioned above is given in Fig. 2. Table 1 gives PES experimental vertical ionization energies (IP in eV), theoretical vertical and adiabatic ionization energies (Ev and Ea in eV) by OVGF and modified-DFT calculations, as well as the molecular orbital-ionized (orbital from which electron was ionized/removed) characters for B(N3)3. A good agreement between PES experiment and OVGF calculation for B(N3)3 with C3h symmetry can be clearly seen from Table 1. The first PES band of 10.30 eV with vibrational spacing 2200 ± 60 cm1 is designated as ionization of the electrons of the degenerate highest occupied molecular orbitals (HOMOs), orbital 34(2e00 ) and 33(2e00 ) in the B(N3)3 molecule, not only owing to its value is very close to computed data 10.322 eV (by OVGF/6-31+G(d)), 10.248 eV (at B3LYP/6-311++G(3df) level), but also it is an antibonding p orbitals with dominant contribution of the N3 groups, leading to equality of the vertical ionization energy (Ev) with the adiabatic ionization energy (Ea) on the band for the molecule. As seen in Table 2, vibrational spacing 2200 ± 60 cm1 is comparable to the calculation value 2254 cm1 (B3LYP/6-31+G(d)) for the first ionic state (X2A00 ) while is larger than the N3 asymmetric stretch frequency 2163 cm1 (corresponding theoretical value
3. Results and discussion Gas phase pure B(N3)3 is readily generated by the heterogeneous reaction of BBr3 vapor at the pressure of 3 · 103 torr with freshly synthesized AgN3 powder at room temperature: BBr3 ðgÞ þ AgN3 ðsÞ ! BðN3 Þ3 ðgÞ þ AgBr ðsÞ
ð1Þ
Identification for the product of reaction (1) was done by both PES experiment and OVGF calculation.
Fig. 1. Optimized geometric parameters for B(N3)3 by using density functional B3LYP, B3PW91 [in square brackets] and B3P86 (in parentheses) methods with 6-311+G(2df) basis sets.
F. Liu et al. / Chemical Physics Letters 419 (2006) 213–216
215
Table 2 Selected experimental and calculated frequencies for B(N3)3 v23/v24
v21/v22
Experiment PE in gas (this work) IR in gasa IR in matrixb
2200(±60) 2163a 2158b
1250(±60) 1360 1371
Calculation (B(N3)3) B3LYP/6-31 + G(d) B3LYP/6-311 + G(2df) MP2/6-31G(d)a
2296(1032)c 2282(1058) 2202 (841)
1439(973) 1431(987) 1451 (862)
Calculation ðBðN3 Þþ 3Þ B3LYP/6-31 + G(d) Assignment Bond/anti-bond
2338(470) Asymmetric N3 stretch Anti-bond
1125(54) B–N stretch Bond
a b c
Fig. 2. Full photoelectron spectrum of B(N3)3 molecule, in which the vibrational spacing of the third band at 11.80 eV is expanded. Table 1 Photoelectron ionization energies (IP in eV), computed ionization energies (Ev and Ea in eV) by the OVGF and modified-DFT calculation and molecular orbital-ionized characters for B(N3)3 IP (eV)
10.30
11.52
Ev (eV) OVGF/ 6-31+G(d)
DFT
10.322(0.901)
10.248
11.557(0.902)
Ea (eV)b
MO
Characterc
10.178
34(2e00 )
pN4–N5–N6, pN7–N8–N9 pN1–N2–N3 p0N4–N5–N6 ; p0N7–N8–N9 p0N1–N2–N3 ; p0N4–N5–N6 rB–N pB–N rN–N–N rN–N–N
a
33(2e00 ) 32(9e 0 ) 31(9e 0 )
11.80 12.49 15.45
11.737(0.901) 12.448(0.899) 16.131(0.859)
30(10a 0 ) 29(2a00 ) 28(8e 0 ) 27(8e 0 )
a The first vertical ionization energies (Ev) calculated by DFT at B3LYP/6-311++G(3df) level. b The first adiabatic ionization energies (Ea) calculated by B3LYP with 6-311++G(3df) basis sets. c p denotes the delocalized p orbital which are perpendicular to the molecular framework (see Fig. 3, MO-34 and MO-33), while p 0 is the Ôin planeÕ localized p orbital result from the p atom-orbitals of nitrogen inner one N3 group (see Fig. 3, MO-32 and MO-31).
2213 cm1) in azide groups binding to central boron atom in the neutral B(N3)3 molecule [2], because ionization of the electron of an antibonding orbital always leads to an increasing vibrational frequency on the ionic state compared with that of the neutral molecule. The second PES band of 11.52 eV results from ionization of the electrons of another pair of degenerate p orbi-
Ref. [2]: IR in gas. Ref. [8]: IR in argon matrices. Frequencies in cm1, followed by relative IR intensities in parentheses.
tals – the second HOMOs (SHOMOs) – orbital 32(9e 0 ) and 31(9e 0 ) in the molecule, because 11.52 eV is very close to the computed value of 11.557 eV and ionization of the electron of p orbitals generally leads to equality of the vertical ionization energy (Ev) with the adiabatic ionization energy (Ea) on the band. An unresolved structure of the second band shows that the SHOMOs are strong bonding orbitals, as shown in Fig. 3. The band with clear vibrational spacing 1250 ± 60 cm1 is considered as the third PES band of B(N3)3 molecule. Its overlapping with the band at 11.52 eV increases not only the band intensity of the side of high energy at 11.52 eV, but also the intensity of the vibrational components on the side of low energy of the third PES band. So the vertical ionization energy of the third band is determined to be 11.80 eV. This band is attributed from ionization of the electron of orbital 30(10a 0 ) which has dominant atomic contributions of both boron and nitrogen bound to boron atom in the molecule (see Fig. 3), not only owing to 11.80 eV is close to the computed value 11.737 eV, but also due to its vibrational spacing 1250 ± 60 cm1 is comparable to the B3LYP/6-31+G(d) predicated value 1084 cm1 for the ionic state (12A 0 ) and consistent with the B–N stretching vibrational frequency 1360 cm1 (corresponding computed value 1387 cm1) in the neutral B(N3)3 molecule [2], because ionization of the electron of the 30(10a 0 ) bonding orbital with dominant atomic contribution of the boron and nitrogen bound to boron atom leads to that the vibrational spacing 1250 ± 60 cm1 on the PES band is smaller than that (1360 cm1) of corresponding vibrational frequency in the neutral molecule. A long vibrational progression on the third band (four or more vibrational components) shows that the orbital 30(10a 0 ) of B(N3)3 is also a strong bonding orbital. The fourth PES band at 12.49 eV, close to the corresponding computed value 12.448 eV, comes from ionization of the electron of another deep-shell p orbital in the B(N3)3 molecule, because the band is narrow.
216
F. Liu et al. / Chemical Physics Letters 419 (2006) 213–216
Fig. 3. Characters of the six highest occupied molecular orbitals (HOMOs) of B(N3)3 molecule.
The band centered near 15.45 eV is very broad. This is considered as result of electron from several deep-shell orbitals in the B(N3)3 molecule (see Table 1). 4. Conclusion In summary, B(N3)3 was first generated in the gas phase by the heterogeneous reaction of BBr3 vapor at the pressure of 3 · 103 torr with freshly synthesized AgN3 powder at room temperature. Only gaseous product, B(N3)3 has been characterized on line by a combination of PES experiment and OVGF calculation at the 6-31+G(d) basis set. Good agreement between PES and OVGF calculations shows that B(N3)3 is a planar molecule with C3h symmetry and has the first adiabatic ionization energy of 11.39 eV, which results from ionization of the electron of the highest occupied molecular orbital (HOMO, 2e00 ) with dominant contribution of the N3 group in the molecule. The solid residue AgBr in the reactor tube by identification of X-ray diffraction (XRD) analysis and excellent agreement of vibrational spacings on the PES bands for only gaseous reaction product with previously experimental results, furthermore, support the formation of B(N3)3 in the reaction (1). Obviously, the introduction of a ready synthesis method for only gaseous product B(N3)3 might facilitates studies of its properties and applications to form the thin films. Acknowledgements This project was supported by Chinese Academy of Sciences (Contract No. KJCX2-SW-H8 and Hundred talents
fund) and the National Natural Science Foundation of China (Contract Nos.: 20477047, 20473094, 50372071, 20577052, 20503035). Xiaoqing Zeng thanks the Chinese Academy of Sciences for a scholarship during the period of this work. References [1] E. Wiberg, H. Michaud, Z. Naturforsch. 96 (1954) 497. [2] R.L. Mulinax, G.S. Okin, R.D. Coombe, J. Phys. Chem. 99 (1995) 6294. [3] J.J. Pouch, S.A. Alterovitz, Synthesis and Properties of Boron Nitride, Trans Tech, Aedermannsdorf, Switzerland, 1990. [4] R.H. Wentorf Jr., J. Chem. Phys. 36 (1962) 1990. [5] H. Saitoh, W.A. Yarbrough, Appl. Phys. Lett. 58 (1991) 2482. [6] D.J. Kester, R. Messier, Appl. Phys. Lett. 72 (1992) 506. [7] F. Qian, V. Nagabushnam, R.K. Singh, Appl. Phys. Lett. 63 (1993) 317. [8] I.A. Al-Jihad, B. Liu, C.J. Linnen, J.V. Gilbert, J. Phys. Chem. 102 (1998) 6220. [9] X.Q. Zeng, Ph.D. Thesis, Chinese Academy of Sciences, Beijing, China, 2004. [10] I.C. Tornieporth-Oetting, P. Buze, R. von P. Schleyer, T.M. Klapo¨tke, Angew. Chem., Int. Ed. 31 (1992) 1338. [11] X.J. Zhu, M.F. Ge, J. Wang, Z. Sun, D.X. Wang, Angew. Chem., Int. Ed. 112 (2000) 1940. [12] Z. Sun, J. Wang, X.J. Zhu, M.F. Ge, D.X. Wang, Chem. Eur. J. 14 (2001) 2995. [13] J. Wang, Z. Sun, X.J. Zhu, X.J. Yang, M.F. Ge, D.X. Wang, Angew. Chem., Int. Ed. 40 (2001) 3055. [14] P.J. Bassett, D.R. Lloyd, J. Chem. Soc. A (1971) 1551. [15] A.P. Scott, L. Radom, J. Phys. Chem. 100 (1996) 16502. [16] J.V. Ortiz, J. Chem. Phys. 108 (1998) 1008. [17] M.J. Frisch et al., GAUSSIAN98, Revision A.3, Gaussian Inc., Pittsburgh, PA, 1998.