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 conducting-

polymers [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 com-

pounds 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

0166-1280/$ - see front matter q 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.theochem.2004.09.062

* Corresponding author. Tel./fax: C86 10 68912665.

E-mail address: qsli@bit.edu.cn (Q.S. Li).

paths and the theoretical analysis for the internal rotation in

aminoborane with difference method of theories, respect-

ively. 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. How-

ever, 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,

Journal of Molecular Structure: THEOCHEM 715 (2005) 133–141

www.elsevier.com/locate/theochem

Fig. 1. Perspectives (bond lengths in A, 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/cc-

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) 133–141134

Fig. 1 (continued)

J. Zhang et al. / Journal of Molecular Structure: THEOCHEM 715 (2005) 133–141 135

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

Fig. 1 (continued)

J. Zhang et al. / Journal of Molecular Structure: THEOCHEM 715 (2005) 133–141136

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

Fig. 1 (continued)

J. Zhang et al. / Journal of Molecular Structure: THEOCHEM 715 (2005) 133–141 137

and MP2 geometries. All calculations are performed using

the GAUSSIAN-98 program packages [49].

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

Table 1

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

Species HF/cc-pVDZ CCSD(T)c

E

B3LYP/cc-pVDZ

EHF ZEP EB3LYP Z

1 K81.49943 31.58(0) K81.88920 K82.04593 29

2 K106.78988 39.57(0) K107.28863 K107.51697 37

3 K136.58698 43.75(0) K137.21551 K137.45354 41

4a K161.88604 52.35(0) K162.62622 K162.93711 49

4b K161.88520 52.44(0) K162.62613 K162.93675 49

5a K216.97802 64.77(0) K217.72169 K218.35061 61

5b K216.97748 64.90(0) K217.72182 K218.35049 61

5c K216.97445 65.23(0) K217.71999 K218.34801 61

5d K216.97231 64.72(1) K217.71701 K218.34558 61

6a K187.18113 60.45(0) K188.03158 K188.41421 57

6b K187.18066 60.59(0) K188.03192 K188.41449 57

6c K187.17192 60.55(0) K188.02419 K188.40618 57

6d K187.16999 60.53(1) K188.02162 K188.40442 57

a Total energies in Hartree.b Zero-point energies in kcal/mol. The integers in parentheses are the number oc The single point calculations were performed at the CCSD(T)/cc-pVTZ level

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.

3.1. 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

(a–d) and 6(a–d)

CCSD(T)c

E

MP2/cc-pVDZ CCSD(T)c

EEP EMP2 ZEP

.89(0) K81.88913 K81.76443 30.42(0) K81.88912

.60(0) K107.28859 K107.12909 38.20(0) K107.28859

.22(0) K137.21543 K137.01777 38.20(0) K137.21543

.57(0) K162.62617 K162.39504 50.23(0) K162.62618

.63(0) K162.62620 K162.39476 50.28(0) K162.62619

.13(0) K217.72313 K217.65461 61.81(0) K217.72337

.24(0) K217.72329 K217.65464 61.90(0) K217.72353

.60(0) K217.72165 K217.65309 62.35(0) K217.72197

.05(1) K217.71854 K217.64970 61.68(1) K217.71876

.40(0) K188.03157 K187.76620 58.14(0) K188.03157

.51(0) K188.03201 K187.76644 58.24(0) K188.03201

.45(0) K188.02434 K187.75844 58.24(0) K188.02487

.43(1) K188.02175 K187.75572 58.18(1) K188.02166

f imaginary frequencies (NIMAG).

of theory except for 5(a–d) at the CCSD(T)/cc-pVDZ level of theory.

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-pVDZ

a EMP2/cc-pVDZ ECCSD(T)//MP2/cc-pVDZa

4a 0.00 0.00 0.00 0.00 0.00 0.00

4b 0.62 0.15 0.29 0.04 0.22 0.04

5a 0.00 0.00 0.00 0.00 0.00 0.00

5b 0.47 0.05 0.18 0.01 0.08 K0.01

5c 2.70 1.53 2.10 1.40 1.49 1.42

5d 3.53 2.89 3.07 2.80 2.96 2.76

6a 0.00 0.00 0.00 0.00 0.00 0.00

6b 0.43 K0.07 K0.06 K0.17 K0.05 K0.18

6c 5.88 4.74 5.09 4.59 4.97 4.30

6d 7.07 6.33 6.17 6.19 6.62 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.

J. Zhang et al. / Journal of Molecular Structure: THEOCHEM 715 (2005) 133–141138

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

length of 1 is 1.390 A at the B3LYP level of theory, which is

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

are 1.423 and 1.414 A at the B3LYP/cc-pVDZ level of

theory, respectively.

Species 4a and 4b are of particularly interest because

they are isoelectronic compounds with the cis- and trans-

butadiene [50], a kind of important chemical material. From

Tables 1 and 2, we can find that the energy of 4a (the trans-

BN 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

0.050 A, which are similar as in their hydrocarbon

counterpart butadiene. Each B–N bond length is between

the experimental boron–nitrogen double bond length of

H2BNH2 (1.391 A) [30] and the boron–nitrogen single bond

length of H3BNH3 (1.580 A) [23,51]. The bond angles of B–

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 correspond-

ing 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

by 0.030 A averagely. All the B–N bond lengths are in

the range of 1.400–1.450 A. The bond angles of B–N–B are

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.

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 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)

MP2 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)

2 B3LYP 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)

MP2 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)

3 B3LYP 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)

MP2 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)

4a B3LYP 455(A00,167), 613(A00,2), 829(A00,3), 844(A00,67), 898(A00,1), 1018(A00,39), 1088(A 0,7), 1143(A0,22), 1198(A 0,21), 1284(A0,95),

1350(A0,6), 1498(A 0,424), 1616(A0,192), 2566(A 0,151), 2633(A 0,24), 2647(A 0,233), 3537(A 0,2), 3582(A0,33), 3688(A0,27)

MP2 430(A00,188), 623(A 00,2), 829(A0,3), 841(A 00,78), 902(A0,1), 1038(A00,41), 1101(A 0,9), 1156(A 0,30), 1210(A 0,27), 1282A0,81),

1370(A0,6), 1505(A 0,490), 1630(A0,218), 2618(A 0,156), 2689(A 0,42), 2706(A 0,222), 3608(A 0,10), 3645(A0,54), 3766(A0,36)

4b B3LYP 152(A00,4), 252(A 0,7), 352(A00,2), 469(A 0,153), 632(A 00,16), 845(A 00,28), 935(A00,36), 965(A 0,3), 992(A 0,9), 1009(A 00,39),

1147(A0,11), 1182(A 0,6), 1310(A0,28), 1409(A 0,227), 1474(A 0,23), 1607(A0,172), 2557(A 0,147), 2604(A 0,150), 2628(A0,180),

3577(A0,18), 3590(A0,23), 3700(A0,35)

MP2 142(A00,4), 257(A 0,6), 344(A00,1), 446(A 00,178), 635(A 00,15), 849(A 00,40), 946(A 00,39), 967(A0,4), 1000(A 0,9), 1030(A 00,39),

159(A0,14), 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(A0,157), 2688(A0,184),

2638(A0,27), 3651(A0,43), 3776(A0,45)

5a B3LYP 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)

MP2 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)

5b B3LYP 122(A00,7), 177(A 0,3), 389(A00,57), 401(A 00,262), 560(A 00,5), 604(A 00,8), 696(A00,68), 911(A 00,14), 925(A 0,4), 928(A00,18),

994(A00,2), 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(A0,229), 1614(A0,135),

2578(A0,254), 2584(A0,75), 3570(A 0,5), 3585(A 0,27), 3596(A 0,21), 3693(A 0,22), 3704(A 0,25)

MP2 119(A00,7), 181(A 0,3), 354(A00,43), 366(A 00,308), 565(A 00,13), 611(A00,9), 681(A 00,75), 923(A0,4), 925(A 00,17), 942(A 00,15),

994(A0,2), 1126(A 0,11), 1147(A0,6), 1214(A0,41), 1310(A0,31), 1434(A0,265), 1500(A 0,499), 1621(A 0,268), 1628(A 0,135),

2633(A0,263), 2639(A0,78), 3634(A 0,12), 3648(A 0,47), 3658(A 0,35), 3771(A 0,30), 3781(A 0,35)

5c B3LYP 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)

MP2 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)

5db B3LYP 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)

MP2 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)

6a B3LYP 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)

MP2 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)

6b B3LYP 123(A00,4), 193(A 00,5), 202(A 0,6), 354(A 00,2), 432(A00,3), 850(A 00,3), 876(A00,99), 938(A 0,8), 946(A 00,14), 976(A0,3),

1016(A00,37), 1027(A 00,50), 1118(A0,25), 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(A0,130), 2577(A 0,149), 2640(A 0,69), 2648(A 0,156), 2660(A 0,206), 3548(A0,17), 3570(A0,22)

MP2 116(A00,4), 187(A 00,5), 201(A 0,6), 351(A 00,3), 434(A00,6), 852(A 00,2), 869(A00,129), 950(A 0,8), 953(A 00,14), 981(A0,3),

1035(A00,2), 1045(A 00,47), 1132(A0,28), 1172(A0,9), 1199(A0,9), 1261(A0,181), 1321(A 0,19), 1349(A 0,2), 1474(A 0,189),

1495(A 0,928), 2625(A0,143), 2629(A 0,144), 2692(A 0,81), 2707(A 0,165), 2719(A 0,193), 3619A0,28), 3629(A 0,33)

6c B3LYP 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)

MP2 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)

J. Zhang et al. / Journal of Molecular Structure: THEOCHEM 715 (2005) 133–141 139

Table 3 (continued)

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

6db B3LYP 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)

MP2 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)

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

J. Zhang et al. / Journal of Molecular Structure: THEOCHEM 715 (2005) 133–141140

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.

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

orbitals of the alternant alkenes. For instance, structures 4aand 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 porbital. 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.

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