Theoretical study on the structures of boron–nitrogen alternant open chain compounds

Journal of Molecular Structure: THEOCHEM 715 (2005) 133–141 www.elsevier.com/locate/theochem

Theoretical study on the structures of boron–nitrogen alternant open chain compounds Jianguo Zhang, Qian Shu Li*, Shaowen Zhang The State Key Laboratory of Prevention and Control of Explosion Disasters, School of Science Beijing Institute of Technology, Beijing 100081, People’s Republic of China Received 13 July 2004; accepted 24 September 2004 Available online 21 December 2004

Abstract The Hartree–Fock HF/cc-pVDZ method, the density functional theory B3LYP/cc-pVDZ method and the Møller–Plesset MP2/cc-pVDZ method are employed to optimize the structures of a series of boron–nitrogen alternant open-chain compounds and their isomers. The results show that all the three methods can obtain reasonable structures. The relative stabilities of the isomers are compared based on the energies refined at the CCSD (T)/cc-pVTZ level of theory. The electronic properties of these compounds are also discussed. q 2004 Elsevier B.V. All rights reserved. Keywords: Boron; Nitrogen; Aminoborane; Open chain compounds; Ab initio; DFT method

1. Introduction Recently, the boron–nitrogen compounds have drawn the attention [1–14] of scientists due to their promising future in many applications, such as in the fields of conductingpolymers [15–17], the chemical vapor deposition (CVD) [18–22], the fuel cell and the hydrogen storage [23–26]. So far, due to its importance as a basic unit for complex aminoborane, most studies about boron–nitrogen compounds are concentrated on aminoborane, H2BNH2, the B–N analogue of ethylene. Besides the extensive experimental studies [27–31] on the determination of structure, detection of physical properties and reaction mechanism with other compounds, some theoretical investigations [32–36] were also carried out for H2BNH2. McKee [32] reported an ab initio study of the formation of H2BNH2 from the reaction of B2H6 with NH3 through 1,2 di-hydrogen elimination at the MP2/6-31G(d) level of theory. Ha [33] presented the results of ab initio SCF/6-31G** calculations for the aminoborane, diaminoborane and aminodifluoroborane. Then, Minyaev [34] and Mo [35] reported the reaction * Corresponding author. Tel./fax: C86 10 68912665. E-mail address: [email protected] (Q.S. Li). 0166-1280/$ - see front matter q 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.theochem.2004.09.062

paths and the theoretical analysis for the internal rotation in aminoborane with difference method of theories, respectively. Recently, Suresh [11] studied the conjugation involving nitrogen lone-pair electrons of some boron– nitrogen compounds with B3LYP/6-31G(d) level of theory. In 2001, Kiran [9] compared the parallel behavior between hydrocarbons and corresponding boron–nitrogen analogues at B3LYP/6-311CG** level and suggested that the protonation and methylation of boron–nitrogen compounds were coincident with corresponding hydrocarbons. However, knowledge about the structures and electronic properties of boron–nitrogen chain compounds are still quite limited. In the present study, we provide a systematic calculation on the structures and some electronic properties of smaller boron–nitrogen alternant open-chain compounds and their isomers (H2BNH2, H2BNHBH2, H2NBHNH2, H2BNHBHNH2, H2BHNBHNHBH2 and H2NBHNHBHNH2).

2. Computational methods In order to acquire reliable structures, we employ three sophisticated methods to optimize the geometries, namely,

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˚ , bond angles in deg) for 1–3, 4(a–b), 5(a–d) and 6(a–d) at the HF/cc-pVDZ (the first row, in normal font), B3LYP/ccFig. 1. Perspectives (bond lengths in A pVDZ (the middle row, in bold font) and MP2/cc-pVDZ (the last row, in italic font) levels of theory.

J. Zhang et al. / Journal of Molecular Structure: THEOCHEM 715 (2005) 133–141

135

Fig. 1 (continued)

the Hartree–Fock (HF) method [37], the DFT B3LYP method [38,39], and the second-order Møller–Plesset perturbation method (MP2) [40,41]. The basis set employed for optimization is Dunning’s correlation

consistent polarized valence double-zeta basis set [42–44], i.e. the cc-pVDZ basis set. The harmonic vibrational frequencies and infrared intensity are also predicted at the B3LYP/cc-pVDZ and MP2/cc-pVDZ

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Fig. 1 (continued)

levels of theory. To obtain more accurate energies, single point energy refinements are done at the CCSD(T) [45–48] (the coupled cluster with all single and double excitation and a quasi-perturbative treatment of connected triple

excitations) level of theory with the cc-pVTZ basis sets [42–44] (the cc-pVDZ basis set is adopted for several structures where the CCSD(T)/cc-pVTZ calculation is beyond the ability of our computer) on the HF, B3LYP

J. Zhang et al. / Journal of Molecular Structure: THEOCHEM 715 (2005) 133–141

137

Fig. 1 (continued)

and MP2 geometries. All calculations are performed using the GAUSSIAN-98 program packages [49].

energies of these conformations are listed in Tables 1 and 2. The harmonic frequencies and infrared intensities are showed in Table 3. The frontier molecular orbitals (FMOs) of the molecules are also analyzed. However, due to the limitation of the length of the article, the pictures of FMOs are not presented in this paper and will be available as supplementary materials.

3. Results and discussion The structures of the boron–nitrogen alternant open-chain compounds optimized at the HF/cc-pVDZ, B3LYP/ cc-pVDZ and MP2/cc-pVDZ levels of theory are shown in Fig. 1, in which structure 1 is NH2BH2; structures 2 and 3 are H2BNHBH2 and H2NBHNH2, respectively; structures 4(a– b), 5(a–d), and 6(a–d) are isomers of H2NBHNHBH2, H2NBHNHBHNH2 and BH2NHBHNHBH2, respectively. The total energies, zero point energies (ZPE) and relative

3.1. Geometries and stabilities From Fig. 1 it can be seen that the geometric parameters optimized at the HF/cc-pVDZ, B3LYP/cc-pVDZ and MP2/cc-pVDZ levels of theory are quite close. The average bond length predicted at the MP2 level of theory is slightly

Table 1 Total energies (E)a, zero-point energies (ZEP)b for the structure of 1–3, 4(a–b), 5(a–d) and 6(a–d) Species

1 2 3 4a 4b 5a 5b 5c 5d 6a 6b 6c 6d a b c

HF/cc-pVDZ EHF

ZEP

K81.49943 K106.78988 K136.58698 K161.88604 K161.88520 K216.97802 K216.97748 K216.97445 K216.97231 K187.18113 K187.18066 K187.17192 K187.16999

31.58(0) 39.57(0) 43.75(0) 52.35(0) 52.44(0) 64.77(0) 64.90(0) 65.23(0) 64.72(1) 60.45(0) 60.59(0) 60.55(0) 60.53(1)

CCSD(T)c E K81.88920 K107.28863 K137.21551 K162.62622 K162.62613 K217.72169 K217.72182 K217.71999 K217.71701 K188.03158 K188.03192 K188.02419 K188.02162

B3LYP/cc-pVDZ EB3LYP

ZEP

K82.04593 K107.51697 K137.45354 K162.93711 K162.93675 K218.35061 K218.35049 K218.34801 K218.34558 K188.41421 K188.41449 K188.40618 K188.40442

29.89(0) 37.60(0) 41.22(0) 49.57(0) 49.63(0) 61.13(0) 61.24(0) 61.60(0) 61.05(1) 57.40(0) 57.51(0) 57.45(0) 57.43(1)

CCSD(T)c E K81.88913 K107.28859 K137.21543 K162.62617 K162.62620 K217.72313 K217.72329 K217.72165 K217.71854 K188.03157 K188.03201 K188.02434 K188.02175

MP2/cc-pVDZ EMP2

ZEP

K81.76443 K107.12909 K137.01777 K162.39504 K162.39476 K217.65461 K217.65464 K217.65309 K217.64970 K187.76620 K187.76644 K187.75844 K187.75572

30.42(0) 38.20(0) 38.20(0) 50.23(0) 50.28(0) 61.81(0) 61.90(0) 62.35(0) 61.68(1) 58.14(0) 58.24(0) 58.24(0) 58.18(1)

CCSD(T)c E K81.88912 K107.28859 K137.21543 K162.62618 K162.62619 K217.72337 K217.72353 K217.72197 K217.71876 K188.03157 K188.03201 K188.02487 K188.02166

Total energies in Hartree. Zero-point energies in kcal/mol. The integers in parentheses are the number of imaginary frequencies (NIMAG). The single point calculations were performed at the CCSD(T)/cc-pVTZ level of theory except for 5(a–d) at the CCSD(T)/cc-pVDZ level of theory.

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Table 2 Relative energies (RE) (in kcal/mol) with ZPE corrections for the structure of 4(a–b), 5(a–d) and 6(a–d) Species

EHF/cc-pVDZ

ECCSD(T)//HF/cc-pVDZa

EB3LYP/cc-pVDZ

ECCSD(T)//B3LYP/cc-pVDZa

EMP2/cc-pVDZ

ECCSD(T)//MP2/cc-pVDZa

4a 4b 5a 5b 5c 5d 6a 6b 6c 6d

0.00 0.62 0.00 0.47 2.70 3.53 0.00 0.43 5.88 7.07

0.00 0.15 0.00 0.05 1.53 2.89 0.00 K0.07 4.74 6.33

0.00 0.29 0.00 0.18 2.10 3.07 0.00 K0.06 5.09 6.17

0.00 0.04 0.00 0.01 1.40 2.80 0.00 K0.17 4.59 6.19

0.00 0.22 0.00 0.08 1.49 2.96 0.00 K0.05 4.97 6.62

0.00 0.04 0.00 K0.01 1.42 2.76 0.00 K0.18 4.30 6.26

a

The single point calculations were made at the CCSD(T)/cc-pVTZ level of theory except for 5(a–d) at the CCSD(T)/cc-pVDZ level of theory.

larger than that predicted at the B3LYP level of theory. HF predicts the shortest bond length among the three methods. Species 1, 2 and 3 have C2V symmetry. The B–N bond ˚ at the B3LYP level of theory, which is length of 1 is 1.390 A the shortest B–N bond length among all the structures studied in the present paper due to its double bond nature, in contrast to the longer bond lengths in other structures caused from conjugation. The B–N bond lengths in species 2 and 3 ˚ at the B3LYP/cc-pVDZ level of are 1.423 and 1.414 A theory, respectively. Species 4a and 4b are of particularly interest because they are isoelectronic compounds with the cis- and transbutadiene [50], a kind of important chemical material. From Tables 1 and 2, we can find that the energy of 4a (the transBN butadiene) is close to that of 4b (the cis- structure) at all the levels of theory employed in this study. In particular, the refined energy at the CCSD(T) level of theory of 4a is only 0.04 kcal/mol lower than that of 4b based on the geometries optimized at B3LYP/cc-pVDZ and MP2/cc-pVDZ levels of theory. The terminal B–N bond lengths in structures 4a and 4b are slightly shorter than the middle B–N bond by ˚ , which are similar as in their hydrocarbon 0.050 A counterpart butadiene. Each B–N bond length is between the experimental boron–nitrogen double bond length of ˚ ) [30] and the boron–nitrogen single bond H2BNH2 (1.391 A ˚ ) [23,51]. The bond angles of B– length of H3BNH3 (1.580 A N–B and N–B–N are slightly larger than the ideal sp2 hybridization value of 1208. As shown in Fig. 1, we locate three stable structures of H2NBHNHBHNH2, namely 5a, 5b, and 5c. 5a and 5b are planar molecules which are in favor of forming the conjugation between the p orbitals of nitrogen and boron atoms. 5c, which has the C2 symmetry, is non-planar due to the repulsion between the terminal groups. The corresponding planar structure of 5c is 5d, which is a saddle point and has an imaginary frequency. The two B–N bond lengths in the middle are slightly longer than the terminal ones ˚ averagely. All the B–N bond lengths are in by 0.030 A ˚ . The bond angles of B–N–B are the range of 1.400–1.450 A slightly larger than that of N–B–N, but they are close to the ideal sp2 hybridization value of 1208. The energies of 5a

and 5b are almost the same at both CCSD(T)//B3LYP and CCSD(T)//MP2 levels of theory. The energy of 5c is about 1.40 kcal/mol higher than that of 5a and 5b. Structures 6a, 6b, 6c, and 6d are cis–trans isomers of H2BNHBHNHBH2. These structures are similar to the corresponding ones of 5a, 5b, 5c, and 5d except that the nitrogen atoms are replaced by boron atoms and vice versa. The energy of 6c is about 4.5 kcal/mol higher than that 6a and 6b, which is larger than the energy difference between 5c and 5a or 5b. This can be explained by the difference of repulsion energies between the terminal groups of 5c and 6c. In 5c, the central BNB bond angle is about 130 degree, which is larger than the central NBN bond angle of 123 degree in 6c. The difference of the central bond angle results in the larger repulsion energy of 6c than that of 5c. 3.2. Frequencies The harmonic vibrational frequencies and their infrared intensity of the stationary points are predicted at all the levels of theory mentioned above, which all yield real frequencies for the structures of 1–3, 4a–4b, 5a–5c and 6a– 6c and one imaginary frequencies for the structures of 5d and 6d. Thus, structures 1–3, 4a–4b, 5a–5c and 6a–6c are local minima. Structures 5d and 6d are the rotational translation states of the terminal groups of 5c and 6c, respectively. According to the data from Table 3, we can assign some important IR absorption bands. First, we can find the two vibrations (596 and 1004 cmK1) of B1 symmetry anticipated for the structure 1, aminoborane, corresponding to the out of plane deformation of the NH2 and BH2 subunits, which is in agreement with Dewar’s calculations [52] and Carpenter’s experiments results [27], but is conflict to the from the Gerry’s previous work [53]. The most intense absorption vibration for structure 1 predicted at B3LYP level of theory is the symmetric stretch of BH2 with B2 symmetry at 2647 cmK1. For structure 2, the lowest and highest frequencies are caused by the symmetric bent of B–N–B and the stretch of NH with A1 symmetry at 374 and 3551 cmK1, respectively.

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Table 3 Harmonic frequencies (cmK1), the code and integers in parentheses are the symmetry and infrared intensities (km/mol) for structures of 1–3, 4(a–b), 5(a–d) and 6(a–d) at the cc-pVDZ basis sets Species

Harmonic frequencies (cmK1) and infrared intensities (km/mol)a

1

B3LYP MP2 B3LYP

2

MP2 3

B3LYP MP2

4a

B3LYP MP2

4b

B3LYP

MP2

5a

B3LYP MP2

5b

B3LYP

MP2

5c

B3LYP

MP2

5db

B3LYP

MP2

6a

B3LYP

MP2 6b

B3LYP

MP2

6c

B3LYP

MP2

596(B1,165), 1004(B1,22), 1127(B2,30), 1353(A1,43), 1618(A1,60), 2571(A1,98), 2647(B2,170), 3573(A1,18), 3669(B2,22) 593(B1,185), 1027(B1,26), 1140(B2,36), 1379(A1,48), 1634(A1,67), 2623(A1,99), 2705(B2,178), 3635(A1,38), 3751(B2,34) 374(A1,2), 926(B1,52), 935(A1,6), 1034(B1,87), 1107(A1,23), 1181(B2,2), 1287(B2,91), 1297(A1,25), 1473(B2,324), 2576(B2,260), 2667(B2,43), 2675(A1,227), 3551(A1,14) 372(A1,2), 928(B1,72), 945(A1,6), 1051(B1,90), 1124(A1,27), 1194(B2,12), 1294(B2,78), 1328(A1,28), 1475(B2,380), 2628(B2,267), 2727(B2,47), 2735(A1,228), 3619(A1,26) 331(B1,351), 600(B1,2), 925(B1,7), 928(A1,3), 1184(A1,27), 1423(B2,162), 1605(A1,20), 1615(B2,234), 2587(A1,156), 3592(B2,32), 3593(A1,2), 3700(A1,13), 3701(B2,26) 282(B1,392), 609(B1,1), 929(A1,3), 944(B1,8), 1196(A1,33), 1439(B2,168), 1619(A1,22), 1633(B2,256), 2644(A1,165), 3658(B2,57), 3659(A1,6), 3780(A1,27), 3781(B2,28) 455(A 00 ,167), 613(A 00 ,2), 829(A 00 ,3), 844(A 00 ,67), 898(A 00 ,1), 1018(A 00 ,39), 1088(A 0 ,7), 1143(A 0 ,22), 1198(A 0 ,21), 1284(A 0 ,95), 1350(A 0 ,6), 1498(A 0 ,424), 1616(A 0 ,192), 2566(A 0 ,151), 2633(A 0 ,24), 2647(A 0 ,233), 3537(A 0 ,2), 3582(A 0 ,33), 3688(A 0 ,27) 430(A 00 ,188), 623(A 00 ,2), 829(A 0 ,3), 841(A 00 ,78), 902(A 0 ,1), 1038(A 00 ,41), 1101(A 0 ,9), 1156(A 0 ,30), 1210(A 0 ,27), 1282A 0 ,81), 1370(A 0 ,6), 1505(A 0 ,490), 1630(A 0 ,218), 2618(A 0 ,156), 2689(A 0 ,42), 2706(A 0 ,222), 3608(A 0 ,10), 3645(A 0 ,54), 3766(A 0 ,36) 152(A 00 ,4), 252(A 0 ,7), 352(A 00 ,2), 469(A 0 ,153), 632(A 00 ,16), 845(A 00 ,28), 935(A 00 ,36), 965(A 0 ,3), 992(A 0 ,9), 1009(A 00 ,39), 1147(A 0 ,11), 1182(A 0 ,6), 1310(A 0 ,28), 1409(A 0 ,227), 1474(A 0 ,23), 1607(A 0 ,172), 2557(A 0 ,147), 2604(A 0 ,150), 2628(A 0 ,180), 3577(A 0 ,18), 3590(A 0 ,23), 3700(A 0 ,35) 142(A 00 ,4), 257(A 0 ,6), 344(A 00 ,1), 446(A 00 ,178), 635(A 00 ,15), 849(A 00 ,40), 946(A 00 ,39), 967(A 0 ,4), 1000(A 0 ,9), 1030(A 00 ,39), 159(A 0 ,14), 1196(A 0 ,10), 1321(A 0 ,20), 1426(A 0 ,247), 1478(A 0 ,289), 1624(A 0 ,186), 2611(A 0 ,150), 2659(A 0 ,157), 2688(A 0 ,184), 2638(A 0 ,27), 3651(A 0 ,43), 3776(A 0 ,45) 113(B1,2), 181(A1,1), 386(B1,329), 539(B1,1), 732(B1,89), 937(B1,6), 1089(B2,2), 1157(B2,8), 1174(A1,48), 1262(B2,146), 1510(B2,636), 1610(B2,366), 1615(A1,33), 2606(B2,8), 2623(A1,241), 3585(B2,47), 3586(A1,3), 3692(B2,30), 3692(A1,12) 103(B1,2), 179(A1,1), 350(B1,366), 547(B1,5), 710(B1,96), 954(B1,6), 1098(B2,4), 1166(B2,7), 1184(A1,57), 1250(B2,132), 1515(B2,703), 1625(B2,404), 1629(A1,34), 2660(B2,8), 2676(A1,251), 3649(B2,79), 3650(A1,8), 3771(B2,32), 3771(A1,27) 122(A 00 ,7), 177(A 0 ,3), 389(A 00 ,57), 401(A 00 ,262), 560(A 00 ,5), 604(A 00 ,8), 696(A 00 ,68), 911(A 00 ,14), 925(A 0 ,4), 928(A 00 ,18), 994(A 00 ,2), 1114(A 0 ,7), 1135(A 0 ,5), 1208(A 0 ,33), 1312(A 0 ,46), 1418(A 0 ,245), 1494(A 0 ,466), 1606(A 0 ,229), 1614(A 0 ,135), 2578(A 0 ,254), 2584(A 0 ,75), 3570(A 0 ,5), 3585(A 0 ,27), 3596(A 0 ,21), 3693(A 0 ,22), 3704(A 0 ,25) 119(A 00 ,7), 181(A 0 ,3), 354(A 00 ,43), 366(A 00 ,308), 565(A 00 ,13), 611(A 00 ,9), 681(A 00 ,75), 923(A 0 ,4), 925(A 00 ,17), 942(A 00 ,15), 994(A 0 ,2), 1126(A 0 ,11), 1147(A 0 ,6), 1214(A 0 ,41), 1310(A 0 ,31), 1434(A 0 ,265), 1500(A 0 ,499), 1621(A 0 ,268), 1628(A 0 ,135), 2633(A 0 ,263), 2639(A 0 ,78), 3634(A 0 ,12), 3648(A 0 ,47), 3658(A 0 ,35), 3771(A 0 ,30), 3781(A 0 ,35) 171(A,3), 198(A,1), 207(B,10), 364(B,2), 421(B,217), 460(A,56), 585(B,7), 661(B,61), 665(A,6), 894(B,6), 923(B,60), 1093(B,3), 1129(B,4), 1152(A,9), 1370(B,151), 1435(A,160), 1446(B,383), 1600(B,96), 1612(A,185), 2579(B,277), 2586(A,114), 3584(B,7), 3586(A,26), 3603(A,21), 3689(B,53) 183(A,4), 197(B,11), 202(A,2), 359(B,3), 401(B,209), 467(A,81), 584(B,17), 652(B,73), 693(A,10), 895(B,4), 931(B,69), 1088(B,3), 1140(B,5), 1165(A,9), 1369(B,108), 1447(B,451), 1453(A,172), 1614(B,108),), 1625(A,192), 2635(B,291), 2641(A,115), 3638(B,12), 3638(A,44), 3661(A,32), 3759(B,74) 231i(A2,0), 189(B1,2), 349(B2,3), 398(B1,306), 599(B1,16), 672(B1,35), 883(B2,1), 931(B1,57), 1017(A1,4), 1098(B2,1), 1138(B2,2), 1149(A1,7), 1394(B2,200), 1443(A1,152), 1459(B2,414), 1601(B2,102), 1632(A1,200), 2575(B2,244), 2582(A1,136), 3594(A1,20), 3606(B2,24), 3615(A1,20), 3702(B2,33) 281i(A2,0), 171(B1,3), 341(B2,2), 360(B1,337), 596(B1,27), 667(B1,38), 876(B2,1), 941(B1,60), 1019(A1,4), 1100(B2,3), 1146(B2,2), 1162(A1,8), 1397(B2,141), 1463(A1,159), 1463(B2,520), 1614(B2,112), 1647(A1,220), 2630(B2,255), 2637(A1,142), 3651(A1,27), 3667(B2,30), 3677(A1,40), 3782(B2,49) 192(A1,4), 463(B1,2), 857(A1,4), 862(B1,119), 936(B1,11), 1027(B1,80), 1091(B2,6), 1117(B2,3), 1128(A1,41), 1206(A1,13), 1221(B2,114), 1297(B2,112), 1307(A1,22), 1498(B2,1070), 2578(B2,317), 2660(A1,49), 2664(B2,27), 2674(A1,263), 3532(A1,9) 188(A1,4), 467(B1,5), 850(B1,146), 864(A1,4), 948(B1,13), 1046(B1,82), 1105(B2,11), 1129(B2,6), 1140(A1,48), 1210(B2,122), 1216(A1,7), 1315(A1,29), 1324(B2,98), 1500(B2,1216), 2629(B2,326), 2715(A1,10), 2723(B2,33), 2729(A1,301), 3607(A1,27) 123(A 00 ,4), 193(A 00 ,5), 202(A 0 ,6), 354(A 00 ,2), 432(A 00 ,3), 850(A 00 ,3), 876(A 00 ,99), 938(A 0 ,8), 946(A 00 ,14), 976(A 0 ,3), 1016(A 00 ,37), 1027(A 00 ,50), 1118(A 0 ,25), 1161(A 0 ,1), 1189(A 0 ,8), 1266(A 0 ,188), 1304(A 0 ,25), 1331(A 0 ,5), 1472(A 0 ,171), 1495(A 0 ,811), 2573(A 0 ,130), 2577(A 0 ,149), 2640(A 0 ,69), 2648(A 0 ,156), 2660(A 0 ,206), 3548(A 0 ,17), 3570(A 0 ,22) 116(A 00 ,4), 187(A 00 ,5), 201(A 0 ,6), 351(A 00 ,3), 434(A 00 ,6), 852(A 00 ,2), 869(A 00 ,129), 950(A 0 ,8), 953(A 00 ,14), 981(A 0 ,3), 1035(A 00 ,2), 1045(A 00 ,47), 1132(A 0 ,28), 1172(A 0 ,9), 1199(A 0 ,9), 1261(A 0 ,181), 1321(A 0 ,19), 1349(A 0 ,2), 1474(A 0 ,189), 1495(A 0 ,928), 2625(A 0 ,143), 2629(A 0 ,144), 2692(A 0 ,81), 2707(A 0 ,165), 2719(A 0 ,193), 3619A 0 ,28), 3629(A 0 ,33) 251(B,14), 333(B,5), 822(B,46), 879(A,3), 917(B,16), 934(B,91), 1018(B,73), 1022(A,10), 1022(B,8), 1180(B,4), 1185(A,7), 1289(B,23), 1309(A,65), 1384(B,174), 1439(B,467), 499(A,247), 2578(B,132), 2582(A,268), 2597(A,60), 2663(B,257), 2675(A,27), 3559(A,12), 3559(B,31) 255(B,13), 334(B,7), 831(B,65), 888(A,4), 924(B,61), 934(B,58), 1026(B,41), 1041(B,41), 1045(A,13), 1188(B,2), 1195(A,14), 1300(B,36), 1322(A,49), 1390(B,122), 1442(B,561), 1498(A,292), 2630(B,127), 2634(A,253), 2652(A,91), 2721(B,274), 2731(A,19), 3619(A,21), 3620(B,43) (continued on next page)

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Table 3 (continued) Species

Harmonic frequencies (cmK1) and infrared intensities (km/mol)a

6db

B3LYP

MP2

a b

109i(A2,0), 249(B1,4), 275(B2,7), 331(B1,3), 836(A1,1), 836(B1,20), 922(B2,2), 961(B1,107), 1005(A1,6), 1029(B1,79), 1199(A1,9), 1295(B2,15), 1326(A1,93), 1401(B2,291), 1449(B2,504), 1515(A1,215), 2581(B2,221), 2581(A1,282), 2593(A1,3), 2683(B2,144), 2710(A1,87), 3553(A1,14), 3554(B2,25) 135i(A2,0), 250(B1,3), 261(B2,7), 320(B1,6), 838(A1,1), 847(B1,25), 933(B2,3), 963(B1,136), 1020(A1,7), 1049(B1,75), 1215(A1,20), 1309(B2,31), 1342(A1,80), 1404(B2,218), 1453(B2,651), 1517(A1,253), 2634(B2,222), 2635(A1,293), 2646(A1,2), 2742(B2,151), 2771(A1,89), 3611(A1,26), 3613(B2,32)

Only IR-active modes with intensity R1 km/mol are given. The strongest IR-mode was in bold style. There is only a image frequency (showed in the table in bold, italic and underline style) for the structure 5d and 6d.

Four strong B–H stretching bands of BH2 (two asymmetry at 2576, 2667 cmK1 and two symmetry at 2592, 2675 cmK 1 ) could definitely be assigned. The most intense absorption vibration corresponds to the asymmetric stretch of B–N–B framework at 1473 cmK1 with B2 symmetry. Structure 3 could be assigned at the same way as for structure 2 due to their similar geometry. But its most intense vibration is the N–H out-of-plane wage of NH2 at 331 cmK1 with B1 symmetry. A strong B–H stretch at 2587 cmK1 and four strong N–H stretches (two asymmetry at 3592, 3701 cmK1 and two symmetry at 3593, 3700 cmK1) can be designed. The results is in accordance with Ha’s results [33]. For structures 4a–4b, 5a–5c and 6a–6c, the most intensive vibration are all located in the range of 1400–1500 cmK1, and corresponds to the stretch of the boron–nitrogen framework. By analyzing the data of Table 3, we may divide the IR vibrations into three fields. The low frequencies band (!1600 cmK1) can be assigned to the bend of BH, the bend of NH, and the bend and stretch of boron–nitrogen framework. The medium (2500– 2700 cmK1) vibrations belong to the BH stretch. The high vibration (3500–3700 cmK1) might be assigned to the NH stretch. We can find that there is one imaginary frequency for structure 5d (231i) and 6d (109i). The vibrational modes of these imaginary frequencies point to the non-planar structures with the rotation of NH2 group and BH2 group.

orbitals of the alternant alkenes. For instance, structures 4a and 4b have the same p electrons as butadiene. Thus, the HOMO and LUMO of 4a and 4b are almost the same as those of butadiene except that the contribution from the B atom is different from the N atom in the former. The FMOs of 5c and 6c are somewhat confusing due to their C2 symmetry. However, if we examine the FMOs of their corresponding planar structures (5d and 6d), we can found their FMOs are the same as 5a(5b) and 6a(6b), respectively. The LUMO of structure 3 is a s orbital instead of a p orbital. This probably caused by the higher energies of its LUMO orbitals. Since structure 3 has four p electrons and only three p orbitals, the only unoccupied p orbital has higher energies than some s orbitals and thus makes its LUMO being a s orbital.

4. Summary In the present paper, we report the geometries, energies, harmonic vibrational frequencies and bonding of a series of boron nitrogen alternant open-chain compounds using ab initio and B3LYP methods. We analyze the vibrational frequencies and modes of these compounds. The energies of the isomers are also compared. We find the FMOs of these compounds are much similar to their conjugated alkene counterparts.

3.3. The frontier molecular orbitals The frontier molecular orbitals (FMO) of each structure are provided as supplementary material. All these orbitals are similar to their counterparts of the alternant alkenes. In the planar alternant B–N compounds, each boron and nitrogen atom exhibits sp2 hybrid. The boron atom will provide an unoccupied p orbital and the nitrogen atom will provide a doubly occupied p orbital to form the conjugated p orbitals. In fact, if the number of boron atom is equal to the number of nitrogen atom in the planar B–N compounds, the number of p orbitals and p electrons that form the conjugated p bonds will be equal to those of their alkene counterparts. Thus, it is expected that the FMOs of the alternant B–N compounds have similar conjugated p bonds to the alternant alkenes. With only one exception (LUMO of structure 3), all the FMOs of the planar B–N compounds have the same nodes as the corresponding conjugated p

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