# Theoretical Investigation on Structure-Property Relationship of Asymmetric Clusters (CH3FBN3)n (n = 1– 6)

Authors: Deng-Xue Ma; Yao-Yao Wei; Yun-Zhi Li; Guo-Kui Liu; Qi-Ying Xia
Journal: Journal of Chemistry
 (2020)

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## Abstract

The structural, relative stability, electronic, IR vibrational, and thermodynamic properties of asymmetric clusters (CH3FBN3)n (n = 1–6) are systematically investigated using density functional theory (DFT) method. Results show that clusters (CH3FBN3)n (n = 2–6) form a cyclic structure with a B atom and a Nα atom binding together. Five main characteristic regions are observed and assigned for the calculated IR spectra. The size-dependent second-order energy difference shows that clusters (CH3FBN3)3 and (CH3FBN3)5 have relatively higher stability and enhanced chemical inertness compared with the neighboring clusters. These two clusters may serve as the cluster-assembled materials. The variations of thermodynamic properties with temperature T or cluster size n are analyzed, respectively. Based on enthalpies in the range of 200–800 K, the formations of the most stable clusters (CH3FBN3)n (n = 2–6) from monomer are thermodynamically favorable. These data are helpful to design and synthesize other asymmetric boron azides.


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## Body

## 1. Introduction

The extensive studies on boron azides are developed as these molecules have been shown to be good precursors for boron nitride (BN) film deposition [1]. The chemistry of boron azides commenced in 1954 with the synthesis of boron triazide B(N3)3 [2]; since then, it developed slowly. In 1963, Paetzold produced trimeric (Cl2BN3)3 from the reaction of LiN3 and BCl3 in CH2Cl2 solution [3]. The molecular structure of dichloroboron azide containing six-membered boron nitrogen heterocycle with diazo groups bounded to nitrogen atoms was determined by Müller in 1971 [4]. Boron dihalide azides (BX2N3)3 (X = F, Cl, or Br) from the reaction of BCl3 with trimethylsilyl azide in CH2Cl2 solution were also recorded by Wiberg et al. in 1972 [5]. Dehnicke reported the infrared spectra of the monoazide products, I2MN3 (M = B, Al, Ga), and observed the formation of oligomers of these species in 1978 [6]. Several alkyl and arylboron azides have been synthesized using similar methods. Oligomerization for Me2BN3 was established by 11B NMR spectroscopy in 1966 [7]. The chemistry of alkylboron azides was further investigated, and the development of this field has been reviewed by Paetzold, Fraenk, and coworkers [8–16].Although the chemistry of boron azides in the condensed phase has been extensively discussed in the literature, research studies of these species in the gas phase are rather limited. With the goal to use boron azides as the single-source precursors (SSP), knowledge of their gas phase stability and thermodynamic properties becomes essential. The structure of monomers Cl2BN3 and (CH3)2BN3 has been explored using ab initio calculation, and the thermodynamic stabilities of X2BN3 with respect to its dimerization and trimerization have been gained and discussed [17–20] from the theoretical view. Previous theoretical studies of the boron azides mostly focused on the symmetric boron azides.The effect for the break of the symmetry of substituted boron azides is hardly considered. Here, in accordance with previous theoretical studies on the asymmetric clusters of inorganic boron azides [21, 22], and experimental studies on the asymmetric clusters of organic gallane azides and aluminum azides (RR’MN3)n (M = Al, Ga; R = CH3; R’ = H, Cl, Br; n = 3−4) [23, 24], the structure, stability, IR spectra, and thermodynamic characteristics of the asymmetric clusters of organic boron azides (CH3FBN3)n (n = 1 – 6) were theoretically discussed in detail. These discussions will provide fundamental data and references for experimentalist to design and synthesize the novel boron azides in the future.


## 2. Computational Methods

As is well known, it is important to choose an appropriate basis set to give the accurate description of clusters’ structures and energies. Usually, a substantial size of basis set is required. However, the size of clusters studied in this work excluded the use of a very large basis set, and hence, all calculations were performed using the DFT-B3LYP method with the 6-31G∗ basis set [25, 26] via the Gaussian 09 program package with the default convergence thresholds [27]. To ensure the adequacy of this basis set, we also optimized the clusters with the 6-311 + G∗ basis set. As shown later on, results obtained from these two basis sets were similar except for slightly numerical differences. The energies of all clusters were also evaluated using the 6-311 + G∗ basis set. The initial configurations are searched by two ways: (1) by considering the numbers of known clusters (HClBN3)n (n = 1–6) in previous works [21] and (2) by placing -CH3 groups and F atoms at various substitutional sites (H or Cl) on the basis of optimized (HClBN3) geometries. No symmetry constraints were applied. Lots of rationally initial one-, two-, and three-dimensional configurations were built to seek the most stable structures. In this way, a large number of optimized isomers for the asymmetric clusters (CH3FBN3)n (n = 1–6) were obtained. Frequencies as well as their respective IR intensities were then calculated, and the lack of imaginary frequencies confirmed that the true minimum was obtained in each case. According to the previous study, the calculated frequencies were scaled uniformly by 0.96 to approximately correct the systematic overestimation [28].


## 3. Results and Discussion

### 3.1. Structures and Charge Distribution

A number of isomers are calculated at each size. Here, structures of clusters (CH3FBN3)n (n = 1 − 6) having the lowest energy were focused because all properties of clusters (CH3FBN3)n (n = 1–6) were calculated based on the lowest energy. Two stable structures of the monomer CH3FBN3 were obtained (connectivity: CH3FB−Nα−Nβ−Nγ) with a slight difference in geometrical parameters. The clusters (CH3FBN3)n (n = 2 – 6) are produced by the head-to-tail oligomerization of the CH3FBN3 monomers, which is a starting point for the oligomerizations. Two dimers, seventeen trimers, sixty-four tetramers, two hundred and fifty-six pentamers, and five hundred and thirty-six hexamers are obtained in this manner. Judged by the total energies, the most stable isomers at each size are labeled as 1, 2, 3, 4, 5, and 6 and shown in Figure 1. Obviously, B and Nα atoms easily bond together, and B–B and Nα–Nα bonds are not formed in the clusters (CH3FBN3)n (n = 2 − 6). In other words, the clusters (CH3FBN3)n (n = 2 − 6) contain cyclic (BNα)2n structures with alternating boron and α-nitrogen atoms.Figure 1
Structures of the asymmetric clusters (CH3FBN3)n (n = 1–6).The corresponding geometrical parameters are collected in Table1, and the data in parentheses are calculated results from the 6-311 + G∗ basis set. The results obtained from different basis sets are generally consistent. In detail, bond lengths (except for B−F bonds) from the 6-311 + G∗ basis set are slightly shorter than those from the 6-31G∗ basis set. This whole agreement shows that it is appropriate to choose the 6-31G∗ basis set to compute the titled systems. Therefore, in this work, we only report the geometrical parameters obtained with the 6-31G∗ basis set. For n = 1, the azide group in the monomer CH3FBN3 is slightly bent with a Nα–Nβ–Nγ angle of 173.1°. The calculated B–N bond length of 1.447 Å is between B–N single (1.58 Å) and double bonds (1.37 Å). The Nβ−Nγ bond length at 1.135 Å is shorter than the Nα−Nβ bond length at 1.240 Å. The Nβ−Nγ bond length is between N–N double (1.25 Å) and N–N triple bonds (1.10 Å). For n = 2 – 6, the computed B–N bond length of 1.601–1.638 Å possesses typical character of B–N single bond, which is similar to the B–N length of other covalent boron azides previously determined, such as (F2BN3)3 (1.616 Å) [19] and (C6F5B(N3)2)3 (1.60 Å) [12]. The B−C and B−F bond lengths are in the range of 1.585–1.596 Å and 1.363–1.381 Å, respectively. The computed structural parameters of the azide units in the clusters (CH3FBN3)n (n = 2 – 6) are 1.240–1.255 Å for Nα−Nβ bonds, 1.128–1.133 Å for Nβ−Nγ bonds, and 177.1–179.4° for Nα−Nβ−Nγ bond angle. It is obvious that the azide group is nearly linear. These results are perfectly consistent with those found in (Cl2BN3)3 (1.25 Å, 1.09 Å, 178°) [4], (F2BN3)3 (1.249 Å, 1.129 Å, 179.8°) [19], and (C6F5B(N3)2)3 (1.27 Å, 1.11 Å, 179°) [12]. Therefore, the calculation method in this work is reliable, and it gives credence to our computed structural parameters of clusters (CH3FBN3)n (n = 2 – 6).Table 1
Ranges of the bond lengths (Å) and bond angles (deg) for the most stable structures of the asymmetric clusters (CH3FBN3) (n) (n = 1 – 6) optimized at the DFT-B3LYP levela.
123456Nβ–Nγ1.1351.1331.128–1.1331.1331.128–1.1331.129(1.126)(1.125)(1.120–1.125)(1.121)(1.119–1.124)(1.120)Nα−Nβ1.2401.2401.243–1.2461.2481.246–1.2551.251(1.236)(1.235)(1.240–1.249)(1.246)(1.243–1.253)(1.250)Nα−B1.4471.6331.614–1.6381.601–1.6021.608–1.6371.606(1.442)(1.641–1.643)(1.613–1.632)(1.598–1.600)(1.603–1.627)(1.601–1.604)B−C1.5631.5851.589–1.5931.5911.591–1.5961.592(1.555)(1.579)(1.584–1.588)(1.587)(1.587–1.591)(1.588)B−F1.3411.3631.376–1.3811.3851.380–1.3861.387–1.388(1.349)(1.374)(1.388–1.393)(1.397)(1.387–1.398)(1.398–1.399)C−H1.093–1.0981.096–1.0981.094–1.0981.096–1.0981.093–1.0981.094–1.098(1.091–1.096)(1.094–1.096)(1.092–1.096)(1.094–1.096)(1.091–1.096)(1.093–1.096)B−Nα−B94.1124.7–125.2126.7–126.9126.3–128.2125.2–125.3(95.2)(123.9–125.0)(127.1)(125.6–131.2)(126.0–126.1)Nα−B−Nα85.9100.2–100.7102.9–103.1105.0–107.8106.2–106.2(84.8)(99.3–100.7)(103.3)(104.1–106.4)(106.6–106.7)Nα−Nβ−B121.3123.3.1–124.6117.6–119.6114.9–118.4115.5–118.7118.2–118.4(121.4)(125.6–125.8)(115.6–119.4)(118.0–118.6)(113.3–118.0)(117.4–117.5)Nα−Nβ−Nγ173.1177.1177.8–178.9178.3–178.4178.2–179.4177.6–177.8(173.3)(177.5)(178.1–179.1)(178.3–178.4)(178.3–179.4)(177.6–177.9)aThe data in parentheses are from 6-311 + G∗ basis set.Through the above discussions, it is obvious that Nβ−Nγ bond is shorter than Nα−Nβ bond in the clusters (CH3FBN3)n (n = 1 – 6). This can be interpreted as a higher bond order for the terminal N−N bond, showing a preformation of the N2 molecule. Moreover, the cluster size n has an important effect on the geometries. An obvious increase is shown for the length of B-Nα bond with cluster size n increasing from 1 to 2∼6 owing to the tension of the ring. However, B−Nα bond length shows little change with cluster size in the range of 2∼6, which fluctuates in the range of 1.601–1.638 Å. The Nα−Nβ bond lengths increase as the cluster size n increases from 1 to 5 and show little change from n = 5 to n = 6. Similarly, when cluster size n increases from 1 to 3, the B−C and B−F bond lengths increase. However, the change of B−C and B−F bond lengths is not obvious from n = 4 to n = 6. The Nβ−Nγ bond length shortens from 1.135 Å in cluster size with 1 to 1.129 Å in cluster size with 6 and tends to be constant around 1.130 Å with n = 3 – 6. Compared with monomer 1, the increasing bond lengths of Nα−Nβ, B−C and B−F bonds that are outside of rings show that N2 (Nβ−Nγ), -CH3, and F- groups can be easily removed to yield BN material.To give a deeper understanding on these structures, we calculate the charge distribution of the clusters (CH3FBN3)n (n = 1 – 6) as shown in Table 2. It can be seen that the charge distribution of CH3FBN3 molecule exhibits zwitterionic characteristics with the charge centers of N and B atoms, respectively. As for (CH3FBN3)n (n = 2 – 6), charge distributions are similar to those of CH3FBN3. However, because B atoms in these molecules all reach the high coordination, the characteristic of zwitterionic ion for these clusters is a problem needing more studies.Table 2
Atomic charge (e) of the most stable structure (CH3FBN3)n (n = 1 – 6).
123456Nα−0.5840−0.5922−0.5847−0.5885−0.5854−0.5918Nβ0.24470.27610.27600.27530.27040.2686Nγ−0.01750.02990.05120.06020.06080.0673B1.13191.08691.07211.07451.07731.0809F−0.4760−0.4973−0.5100−0.5174−0.5171−0.5227C−1.0830−1.0571−1.0545−1.0551−1.0568−1.0561H0.26270.25120.24990.25040.25030.2503


### 3.2. Relative Stabilities

All energies are displayed in Table3, and the data in parentheses are from the 6-311 + G∗ basis set. Obviously, the energies obtained at the B3LYP/6-31G∗ levels are similar to those obtained with the 6-311 + G∗ basis set. The use of larger basis sets has no significant influence on the binding energies, which again shows that the 6-31G∗ basis set is suitable for the clusters studied here. Thus, in this work, we only report the relative stability of the clusters (CH3FBN3)n with the 6-31G∗ basis set. The uncorrected binding energies (Eb), corrected binding energies (Eb-c), average corrected binding energy (Eb-c-ave), and the second-order energy difference (Δ2E) are calculated using the following equations:(1)Ebn=nECH3FBN3−ECH3FBN3n,Eb−cn=nECH3FBN3+0.96∗nZPECH3FBN3−ECH3FBN3n−0.96∗ZPECH3FBN3n,Eb−c−aven=Eb−cnn,Δ2En=ECH3FBN3n+1+0.96∗ZPECH3FBN3n+1+ECH3FBN3n−1+0.96∗ZPECH3FBN3n−1−2ECH3FBN3n−2∗0.96∗ZPECH3FBN3n,where E (CH3FBN3)n and E (CH3FBN3) are the total energies of the clusters (CH3FBN3)n (n = 2 – 6) and CH3FBN3, respectively; ZPE (CH3FBN3)n and ZPE (CH3FBN3) represent the zero point energies of the most stable clusters (CH3FBN3)n (n = 2 – 6) and CH3FBN3, respectively; and the 0.96 is a scaling factor [28].Table 3
Total energies (E), zero point energy (ZPE), uncorrected binding energies (Eb), corrected binding energies (Eb-c), average corrected binding energy (Eb-c-ave), and second-order energy difference (Δ2E) (unit: kJ · mol−1)a.
ClustersEEbZPEEb-cEb-c-aveΔ2E1−863544.11150.49(−863796.61)(149.36)2−1727098.4510.23306.954.502.25−29.27(−1727602.68)(9.45)(304.09)(4.30)(2.15)(−27.67)3−2590684.8952.56466.3638.2712.7618.76(−2591439.62)(49.78)(462.16)(36.26)(12.09)(16.97)4−3454250.8874.44624.0153.2713.32−3.75(−3455257.97)(71.51)(618.54)(51.26)(12.81)(−4.25)5−4317819.6799.12780.6772.0314.413.97(−4319078.66)(95.59)(772.93)(70.51)(14.10)(5.01)6−5181384.41119.75937.2486.8214.47(−5182894.97)(115.28)(927.98)(84.74)(14.12)aThe data in parentheses are from 6-311 + G∗ basis set.From Table3, the ratios of ZPE corrections to their binding energies Eb are large for the clusters (CH3FBN3)n (n = 2 − 6). Thus, it is necessary to carry out the ZPE corrections for the binding energies. For intuitive presentation, the calculated energy is also plotted as a function of cluster size n as shown in Figure 2. The curves of Eb (Figure 2(a)) and Eb-c (Figure 2(b)) are seen to increase monotonically with the augment of cluster size n, which means that clusters can continue to gain energy during the clusters’ growth process. The Eb-c-ave values increase sharply with the clusters size n from 2 to 3; then, it approaches to be stable around 13–14 kJ·mol−1 when cluster size n ≥ 3. Therefore, the geometrical structures of clusters (CH3FBN3)n (n = 1 − 6) tend to be stable when cluster size n ≥ 3.Figure 2
The uncorrected binding energiesEb (a), corrected binding energies Eb-c (b), average corrected binding energy Eb-c-ave (c), and the second-order energy difference Δ2E (d) of the asymmetric clusters (CH3FBN3)n (n = 1 − 6) versus the cluster size n.The relative stability of clusters can be also estimated through the Δ2E (n). According to the definition, clusters with positive Δ2E are more stable than those with negative Δ2E. A clear odd-even oscillation behavior is presented in Figure 2(d). The clusters (CH3FBN3)n (n = 1 − 6) at n = 3, 5 correspond to local maximal on the curve of Δ2E, indicating that they are relatively more stable than other clusters. This trend is in excellent agreement with that of the asymmetric clusters of the inorganic boron azides (HClBN3)n (n = 1 – 6) [21]. In particular, the trimer (CH3FBN3)3 possesses the highest stability among all clusters in terms of the largest value of Δ2E. The stable structure of six-member ring of trimer plays a vital role.


### 3.3. IR Spectrum

The IR spectrum is not only the basic property of compounds and effective method to analyze or identify substances, but also has a direct relationship with thermodynamic properties. However, there are no experimental data for the title compounds. Thus, it is necessary to use the theoretical method to calculate and predict IR spectra for both theoretical and practical reasons. Figure3 provides the calculated IR spectra of the clusters (CH3FBN3)n (n = 1 − 6) at the B3LYP/6-31G∗ level. Due to intrinsic complexity, it is difficult to assign all vibrational modes. Thus, we only analyze and discuss some typical vibrational modes.Figure 3
IR spectra of the asymmetric clusters (CH3FBN3)n (n = 1 − 6) with full width at half maxima of 20 cm−1.There are five main characteristic regions for clusters (CH3FBN3)n (n = 1 – 6) that correspond to the –N3 asymmetric and symmetric stretching, –CH3 stretching and in-plane bending vibrations, and the fingerprint region. As for monomer CH3FBN3, the –N3 asymmetric and symmetric stretching vibrations with intense peak lie at 2215 and 1360 cm−1, respectively. The modes in the range of 2923–3022 cm−1 are classified to the characteristic –CH3 stretching vibrations. The peak around 1440 cm−1 corresponds to the –CH3 in-plane bending vibrations. The peaks at less than 1210 cm−1 belong to the fingerprint region, which are mainly composed of wagging and scissoring of –CH3 and the stretching of B−F and B−C bonds. As can be seen from Figure 3, similar vibrational modes are observed in clusters (CH3FBN3)n (n = 2 – 6). The regions of 2917–3019, 2202–2243, 1331–1458, 1217–1308, and 0–1204 cm−1 are individually assigned to -CH3 stretching vibrations, -N3 asymmetric stretching vibrations, –CH3 in-plane bending vibrations, -N3 symmetric stretching vibrations, and fingerprint region. In previous studies, the N–N asymmetrical stretching vibration for Cl2BN3, Br2BN3, Me2BN3, (C6F5)2BN3 polymer, and NaN3 crystal were individually located at 2210–2219 [15], 2200–2215 [15], 2128 [7, 14], 2135–2209 cm−1 [11, 12], and 2128–2151 cm−1 [29] from experiments. Our calculated results are in accordance with those in crystal, which shows that the structures of N3 groups in clusters and in crystal are identical. Moreover, the obtained result is similar to that found in the clusters (HClBN3)n (n = 1 – 6) (2217–2250 cm−1) of DFT calculations [21].Furthermore, compared with monomer, the N3 asymmetric stretching vibration presents the blue-shifted phenomenon for clusters with n = 2 − 6. However, the N3 symmetric stretching vibration and the –CH3 stretching vibration show red-shifted trend. Meanwhile, in three characteristic regions of N3 asymmetric, N3 symmetric, and –CH3 stretching vibration, the number of vibration mode equals to that of azido groups and C–H bonds, respectively. For example, the monomer 1 has three bands at 2923, 2972, and 3022 cm−1 with C–H stretching vibration, and the trimer 3 has three bands at 2213, 2225, and 2240 cm−1 with N3 asymmetric stretching vibration, and the hexamer 6 has six bands at 1237, 1237, 1245, 1245, 1247, and 1255 cm−1 with N3 symmetric stretching vibration.


### 3.4. Thermodynamic Properties

Thermodynamic functions are important parameters for clusters to predict reactive property in a chemical reaction. According to vibrational analysis and statistical thermodynamic method, the standard molar heat capacity (Cp,mθ), standard molar thermal entropy (Smθ), and standard molar thermal enthalpy (Hmθ) of the asymmetric clusters (CH3FBN3)n (n = 1 – 6) in the range of 200–800 K are evaluated and tabulated in Table 4. It is obvious that the calculated thermodynamic functions increase with the temperature T raising. For more clear intuitive, taking monomer 1 as an example, the temperature-dependent relationships for Cp,mθ, Smθ, and Hmθ are expressed in formulas (2)–(4) and all are plotted in Figure 4(a). These correlations approximate to linear equations due to the small coefficients of T2; namely, the Cp,mθ, Smθ, and Hmθ increase linearly with the increase of temperature T. This can be understood from the fact that these three thermodynamic functions are dominated by the weak translational and rotational motions of the clusters at lower temperature, whereas the vibrational motion is intensified and makes more contributions to Cp,mθ, Smθ, and Hmθ at higher temperature. The same linear relationships are found for clusters (CH3FBN3)n with n = 2 – 6:(2)Cp,mθ=33.4650+0.2460T−1.1698×10−4T2,R2=0.9999,SD=0.3318,(3)Smθ=231.0291+0.4049T−1.4317×10−4T2,R2=0.9998,SD=0.9034,(4)Hmθ=−3.6904+0.0606T+6.401×10−5T2,R2=0.9999,SD=0.2784,where R2 represents the square of correlation coefficient and SD represents standard deviation.Table 4
Thermodynamic properties for the most stable clusters (CH3FBN3)n (n = 1 − 6) at different temperaturesa.
T200298.24005006007008001Cp,mθ77.7396.48113.47127.30138.62147.95155.71Smθ305.18339.77370.57397.42421.66443.75464.03Hmθ11.3019.8730.5842.6555.9670.3085.502Cp,mθ161.34206.04243.12271.97295.00313.71329.19Smθ409.57482.65548.58606.05657.75704.68747.61Hmθ19.8838.0060.9486.75115.14145.61177.783Cp,mθ242.25312.13369.54413.90449.12477.62501.12Smθ509.21619.50719.58807.00885.70957.141022.50Hmθ28.4755.8190.64129.90173.12219.50268.474Cp,mθ326.44420.82497.89557.25604.30642.35673.70Smθ584.95733.64868.53986.271092.191188.301276.18Hmθ37.1374.00120.94173.82231.98294.38360.235Cp,mθ411.59530.37627.05701.33760.11807.59846.71Smθ686.62874.081044.021192.261325.521446.381556.85Hmθ46.5693.03152.18218.74291.93370.39453.176Cp,mθ496.46639.39755.59844.78915.34972.331019.26Smθ788.601014.651219.481398.061558.561704.091837.08Hmθ55.96111.99183.27263.47351.61446.09545.74aUnits: T (K), Cp,mθ (J·mol−1·K−1), Smθ (J·mol−1 K−1), and Hmθ (kJ·mol−1).Figure 4
Variations of the thermodynamic functions (Cp,mθ,Smθ, and Hmθ) with the temperature (T) (a) for the monomer CH3FBN3 and the cluster size n (b) and for the asymmetric clusters (CH3FBN3)n (n = 1–6).
(a)(b)Moreover, the cluster size-dependent relationships forCp,mθ, Smθ, and Hmθ are expressed in formulas (5)–(7) and all are shown in Figure 4(b). All thermodynamic functions increase monotonically with the enlarged cluster size n; in other words, when one more CH3FBN3 is bound, the Cp,mθ, Smθ, and Hmθ increase with the average of 108 J·mol−1 K−1, 133 J·mol−1 K−1, and 18 kJ·mol−1, separately. This means that the contribution of monomer CH3FBN3 to the thermodynamic properties of cluster matches the cluster additivity:(5)Cp,mθ=−12.0847+108.4637n,R2=1.0000,SD=1.0282,(6)Smθ=211.0987+133.2237n,R2=0.9996,SD=8.0272,(7)Hmθ=1.0620+18.3966n,R2=0.9999,SD=0.5260.In addition, the theoretical enthalpies (ΔH) and Gibbs free energies (ΔG) of various oligomerizations from monomer CH3FBN3 in the range of 200–800 K are compiled in Table 5. The values of ΔH in the processes are negative except for the process 1 ⟶ 2 beyond 700 K, indicating that these oligomerization processes are exothermic. The ΔG at a given temperature is evaluated by the equation ΔG = ΔH − TΔS. The values of ΔG are all positive, which reveals the oligomerizations cannot occur spontaneously in the range of 200–800 K. There are no experimentally thermodynamic data available for the asymmetric clusters (CH3FBN3)n (n = 1 − 6), so the calculated thermodynamic functions and the obtained relationships of them with the temperature and cluster size n may help the experiment to further study the physical, chemical, and energetic properties of the asymmetric clusters (CH3FBN3)n (n = 1 − 6) or other asymmetric boron azides.Table 5
Enthalpy change and Gibbs free energy change of oligomerization at different temperaturesa.
T200298.24005006007008002(1) ⟶ (2)ΔH−7.22−6.24−4.72−3.05−1.280.512.28ΔG32.9452.4572.3091.35110.06128.48146.643(1) ⟶ (3)ΔH−43.70−42.07−39.37−36.32−33.03−29.67−26.30ΔG37.5777.11117.48156.31194.54232.21269.374(1) ⟶ (4)ΔH−61.34−58.75−54.65−50.05−45.13−40.09−35.04ΔG65.81127.69190.85251.66311.54370.60428.915(1) ⟶ (5)ΔH−81.97−78.35−72.75−66.54−59.90−53.14−46.36ΔG85.89167.51250.78330.88409.77487.52564.286(1) ⟶ (6)ΔH−98.66−94.05−87.03−79.25−70.97−62.53−54.08ΔG109.84211.20314.55413.98511.87608.36703.60aUnits: T (K), ΔH (kJ·mol−1), and ΔG (kJ·mol−1).


### 3.5. Aromatic Properties

Here, the aromaticity of the studied systems will be discussed. Because our systems are nonplanar, it is not suitable to use the criterion of nucleus-independent chemical shift (NICS) [30]. In order to find out whether a large π bond exists in our studied system, we use the newly proposed localized orbital locator (LOL)-π method [31] that can be used for studying nonplanar system to study the π bonds of our systems. The LOL-π is analyzed and visualized with the Multiwfn [32]. From Figure 5, there is π bond of NNNB in 1 and similar π bonds in 2-6. However, the π conjugation of 2–6 is not as good as that of 1. Meanwhile, Figure 5 also shows no large π bonds in all systems.Figure 5
LOL-π isosurface of different systems with the isovalue of 0.4.


## 3.1. Structures and Charge Distribution

A number of isomers are calculated at each size. Here, structures of clusters (CH3FBN3)n (n = 1 − 6) having the lowest energy were focused because all properties of clusters (CH3FBN3)n (n = 1–6) were calculated based on the lowest energy. Two stable structures of the monomer CH3FBN3 were obtained (connectivity: CH3FB−Nα−Nβ−Nγ) with a slight difference in geometrical parameters. The clusters (CH3FBN3)n (n = 2 – 6) are produced by the head-to-tail oligomerization of the CH3FBN3 monomers, which is a starting point for the oligomerizations. Two dimers, seventeen trimers, sixty-four tetramers, two hundred and fifty-six pentamers, and five hundred and thirty-six hexamers are obtained in this manner. Judged by the total energies, the most stable isomers at each size are labeled as 1, 2, 3, 4, 5, and 6 and shown in Figure 1. Obviously, B and Nα atoms easily bond together, and B–B and Nα–Nα bonds are not formed in the clusters (CH3FBN3)n (n = 2 − 6). In other words, the clusters (CH3FBN3)n (n = 2 − 6) contain cyclic (BNα)2n structures with alternating boron and α-nitrogen atoms.Figure 1
Structures of the asymmetric clusters (CH3FBN3)n (n = 1–6).The corresponding geometrical parameters are collected in Table1, and the data in parentheses are calculated results from the 6-311 + G∗ basis set. The results obtained from different basis sets are generally consistent. In detail, bond lengths (except for B−F bonds) from the 6-311 + G∗ basis set are slightly shorter than those from the 6-31G∗ basis set. This whole agreement shows that it is appropriate to choose the 6-31G∗ basis set to compute the titled systems. Therefore, in this work, we only report the geometrical parameters obtained with the 6-31G∗ basis set. For n = 1, the azide group in the monomer CH3FBN3 is slightly bent with a Nα–Nβ–Nγ angle of 173.1°. The calculated B–N bond length of 1.447 Å is between B–N single (1.58 Å) and double bonds (1.37 Å). The Nβ−Nγ bond length at 1.135 Å is shorter than the Nα−Nβ bond length at 1.240 Å. The Nβ−Nγ bond length is between N–N double (1.25 Å) and N–N triple bonds (1.10 Å). For n = 2 – 6, the computed B–N bond length of 1.601–1.638 Å possesses typical character of B–N single bond, which is similar to the B–N length of other covalent boron azides previously determined, such as (F2BN3)3 (1.616 Å) [19] and (C6F5B(N3)2)3 (1.60 Å) [12]. The B−C and B−F bond lengths are in the range of 1.585–1.596 Å and 1.363–1.381 Å, respectively. The computed structural parameters of the azide units in the clusters (CH3FBN3)n (n = 2 – 6) are 1.240–1.255 Å for Nα−Nβ bonds, 1.128–1.133 Å for Nβ−Nγ bonds, and 177.1–179.4° for Nα−Nβ−Nγ bond angle. It is obvious that the azide group is nearly linear. These results are perfectly consistent with those found in (Cl2BN3)3 (1.25 Å, 1.09 Å, 178°) [4], (F2BN3)3 (1.249 Å, 1.129 Å, 179.8°) [19], and (C6F5B(N3)2)3 (1.27 Å, 1.11 Å, 179°) [12]. Therefore, the calculation method in this work is reliable, and it gives credence to our computed structural parameters of clusters (CH3FBN3)n (n = 2 – 6).Table 1
Ranges of the bond lengths (Å) and bond angles (deg) for the most stable structures of the asymmetric clusters (CH3FBN3) (n) (n = 1 – 6) optimized at the DFT-B3LYP levela.
123456Nβ–Nγ1.1351.1331.128–1.1331.1331.128–1.1331.129(1.126)(1.125)(1.120–1.125)(1.121)(1.119–1.124)(1.120)Nα−Nβ1.2401.2401.243–1.2461.2481.246–1.2551.251(1.236)(1.235)(1.240–1.249)(1.246)(1.243–1.253)(1.250)Nα−B1.4471.6331.614–1.6381.601–1.6021.608–1.6371.606(1.442)(1.641–1.643)(1.613–1.632)(1.598–1.600)(1.603–1.627)(1.601–1.604)B−C1.5631.5851.589–1.5931.5911.591–1.5961.592(1.555)(1.579)(1.584–1.588)(1.587)(1.587–1.591)(1.588)B−F1.3411.3631.376–1.3811.3851.380–1.3861.387–1.388(1.349)(1.374)(1.388–1.393)(1.397)(1.387–1.398)(1.398–1.399)C−H1.093–1.0981.096–1.0981.094–1.0981.096–1.0981.093–1.0981.094–1.098(1.091–1.096)(1.094–1.096)(1.092–1.096)(1.094–1.096)(1.091–1.096)(1.093–1.096)B−Nα−B94.1124.7–125.2126.7–126.9126.3–128.2125.2–125.3(95.2)(123.9–125.0)(127.1)(125.6–131.2)(126.0–126.1)Nα−B−Nα85.9100.2–100.7102.9–103.1105.0–107.8106.2–106.2(84.8)(99.3–100.7)(103.3)(104.1–106.4)(106.6–106.7)Nα−Nβ−B121.3123.3.1–124.6117.6–119.6114.9–118.4115.5–118.7118.2–118.4(121.4)(125.6–125.8)(115.6–119.4)(118.0–118.6)(113.3–118.0)(117.4–117.5)Nα−Nβ−Nγ173.1177.1177.8–178.9178.3–178.4178.2–179.4177.6–177.8(173.3)(177.5)(178.1–179.1)(178.3–178.4)(178.3–179.4)(177.6–177.9)aThe data in parentheses are from 6-311 + G∗ basis set.Through the above discussions, it is obvious that Nβ−Nγ bond is shorter than Nα−Nβ bond in the clusters (CH3FBN3)n (n = 1 – 6). This can be interpreted as a higher bond order for the terminal N−N bond, showing a preformation of the N2 molecule. Moreover, the cluster size n has an important effect on the geometries. An obvious increase is shown for the length of B-Nα bond with cluster size n increasing from 1 to 2∼6 owing to the tension of the ring. However, B−Nα bond length shows little change with cluster size in the range of 2∼6, which fluctuates in the range of 1.601–1.638 Å. The Nα−Nβ bond lengths increase as the cluster size n increases from 1 to 5 and show little change from n = 5 to n = 6. Similarly, when cluster size n increases from 1 to 3, the B−C and B−F bond lengths increase. However, the change of B−C and B−F bond lengths is not obvious from n = 4 to n = 6. The Nβ−Nγ bond length shortens from 1.135 Å in cluster size with 1 to 1.129 Å in cluster size with 6 and tends to be constant around 1.130 Å with n = 3 – 6. Compared with monomer 1, the increasing bond lengths of Nα−Nβ, B−C and B−F bonds that are outside of rings show that N2 (Nβ−Nγ), -CH3, and F- groups can be easily removed to yield BN material.To give a deeper understanding on these structures, we calculate the charge distribution of the clusters (CH3FBN3)n (n = 1 – 6) as shown in Table 2. It can be seen that the charge distribution of CH3FBN3 molecule exhibits zwitterionic characteristics with the charge centers of N and B atoms, respectively. As for (CH3FBN3)n (n = 2 – 6), charge distributions are similar to those of CH3FBN3. However, because B atoms in these molecules all reach the high coordination, the characteristic of zwitterionic ion for these clusters is a problem needing more studies.Table 2
Atomic charge (e) of the most stable structure (CH3FBN3)n (n = 1 – 6).
123456Nα−0.5840−0.5922−0.5847−0.5885−0.5854−0.5918Nβ0.24470.27610.27600.27530.27040.2686Nγ−0.01750.02990.05120.06020.06080.0673B1.13191.08691.07211.07451.07731.0809F−0.4760−0.4973−0.5100−0.5174−0.5171−0.5227C−1.0830−1.0571−1.0545−1.0551−1.0568−1.0561H0.26270.25120.24990.25040.25030.2503


## 3.2. Relative Stabilities

All energies are displayed in Table3, and the data in parentheses are from the 6-311 + G∗ basis set. Obviously, the energies obtained at the B3LYP/6-31G∗ levels are similar to those obtained with the 6-311 + G∗ basis set. The use of larger basis sets has no significant influence on the binding energies, which again shows that the 6-31G∗ basis set is suitable for the clusters studied here. Thus, in this work, we only report the relative stability of the clusters (CH3FBN3)n with the 6-31G∗ basis set. The uncorrected binding energies (Eb), corrected binding energies (Eb-c), average corrected binding energy (Eb-c-ave), and the second-order energy difference (Δ2E) are calculated using the following equations:(1)Ebn=nECH3FBN3−ECH3FBN3n,Eb−cn=nECH3FBN3+0.96∗nZPECH3FBN3−ECH3FBN3n−0.96∗ZPECH3FBN3n,Eb−c−aven=Eb−cnn,Δ2En=ECH3FBN3n+1+0.96∗ZPECH3FBN3n+1+ECH3FBN3n−1+0.96∗ZPECH3FBN3n−1−2ECH3FBN3n−2∗0.96∗ZPECH3FBN3n,where E (CH3FBN3)n and E (CH3FBN3) are the total energies of the clusters (CH3FBN3)n (n = 2 – 6) and CH3FBN3, respectively; ZPE (CH3FBN3)n and ZPE (CH3FBN3) represent the zero point energies of the most stable clusters (CH3FBN3)n (n = 2 – 6) and CH3FBN3, respectively; and the 0.96 is a scaling factor [28].Table 3
Total energies (E), zero point energy (ZPE), uncorrected binding energies (Eb), corrected binding energies (Eb-c), average corrected binding energy (Eb-c-ave), and second-order energy difference (Δ2E) (unit: kJ · mol−1)a.
ClustersEEbZPEEb-cEb-c-aveΔ2E1−863544.11150.49(−863796.61)(149.36)2−1727098.4510.23306.954.502.25−29.27(−1727602.68)(9.45)(304.09)(4.30)(2.15)(−27.67)3−2590684.8952.56466.3638.2712.7618.76(−2591439.62)(49.78)(462.16)(36.26)(12.09)(16.97)4−3454250.8874.44624.0153.2713.32−3.75(−3455257.97)(71.51)(618.54)(51.26)(12.81)(−4.25)5−4317819.6799.12780.6772.0314.413.97(−4319078.66)(95.59)(772.93)(70.51)(14.10)(5.01)6−5181384.41119.75937.2486.8214.47(−5182894.97)(115.28)(927.98)(84.74)(14.12)aThe data in parentheses are from 6-311 + G∗ basis set.From Table3, the ratios of ZPE corrections to their binding energies Eb are large for the clusters (CH3FBN3)n (n = 2 − 6). Thus, it is necessary to carry out the ZPE corrections for the binding energies. For intuitive presentation, the calculated energy is also plotted as a function of cluster size n as shown in Figure 2. The curves of Eb (Figure 2(a)) and Eb-c (Figure 2(b)) are seen to increase monotonically with the augment of cluster size n, which means that clusters can continue to gain energy during the clusters’ growth process. The Eb-c-ave values increase sharply with the clusters size n from 2 to 3; then, it approaches to be stable around 13–14 kJ·mol−1 when cluster size n ≥ 3. Therefore, the geometrical structures of clusters (CH3FBN3)n (n = 1 − 6) tend to be stable when cluster size n ≥ 3.Figure 2
The uncorrected binding energiesEb (a), corrected binding energies Eb-c (b), average corrected binding energy Eb-c-ave (c), and the second-order energy difference Δ2E (d) of the asymmetric clusters (CH3FBN3)n (n = 1 − 6) versus the cluster size n.The relative stability of clusters can be also estimated through the Δ2E (n). According to the definition, clusters with positive Δ2E are more stable than those with negative Δ2E. A clear odd-even oscillation behavior is presented in Figure 2(d). The clusters (CH3FBN3)n (n = 1 − 6) at n = 3, 5 correspond to local maximal on the curve of Δ2E, indicating that they are relatively more stable than other clusters. This trend is in excellent agreement with that of the asymmetric clusters of the inorganic boron azides (HClBN3)n (n = 1 – 6) [21]. In particular, the trimer (CH3FBN3)3 possesses the highest stability among all clusters in terms of the largest value of Δ2E. The stable structure of six-member ring of trimer plays a vital role.


## 3.3. IR Spectrum

The IR spectrum is not only the basic property of compounds and effective method to analyze or identify substances, but also has a direct relationship with thermodynamic properties. However, there are no experimental data for the title compounds. Thus, it is necessary to use the theoretical method to calculate and predict IR spectra for both theoretical and practical reasons. Figure3 provides the calculated IR spectra of the clusters (CH3FBN3)n (n = 1 − 6) at the B3LYP/6-31G∗ level. Due to intrinsic complexity, it is difficult to assign all vibrational modes. Thus, we only analyze and discuss some typical vibrational modes.Figure 3
IR spectra of the asymmetric clusters (CH3FBN3)n (n = 1 − 6) with full width at half maxima of 20 cm−1.There are five main characteristic regions for clusters (CH3FBN3)n (n = 1 – 6) that correspond to the –N3 asymmetric and symmetric stretching, –CH3 stretching and in-plane bending vibrations, and the fingerprint region. As for monomer CH3FBN3, the –N3 asymmetric and symmetric stretching vibrations with intense peak lie at 2215 and 1360 cm−1, respectively. The modes in the range of 2923–3022 cm−1 are classified to the characteristic –CH3 stretching vibrations. The peak around 1440 cm−1 corresponds to the –CH3 in-plane bending vibrations. The peaks at less than 1210 cm−1 belong to the fingerprint region, which are mainly composed of wagging and scissoring of –CH3 and the stretching of B−F and B−C bonds. As can be seen from Figure 3, similar vibrational modes are observed in clusters (CH3FBN3)n (n = 2 – 6). The regions of 2917–3019, 2202–2243, 1331–1458, 1217–1308, and 0–1204 cm−1 are individually assigned to -CH3 stretching vibrations, -N3 asymmetric stretching vibrations, –CH3 in-plane bending vibrations, -N3 symmetric stretching vibrations, and fingerprint region. In previous studies, the N–N asymmetrical stretching vibration for Cl2BN3, Br2BN3, Me2BN3, (C6F5)2BN3 polymer, and NaN3 crystal were individually located at 2210–2219 [15], 2200–2215 [15], 2128 [7, 14], 2135–2209 cm−1 [11, 12], and 2128–2151 cm−1 [29] from experiments. Our calculated results are in accordance with those in crystal, which shows that the structures of N3 groups in clusters and in crystal are identical. Moreover, the obtained result is similar to that found in the clusters (HClBN3)n (n = 1 – 6) (2217–2250 cm−1) of DFT calculations [21].Furthermore, compared with monomer, the N3 asymmetric stretching vibration presents the blue-shifted phenomenon for clusters with n = 2 − 6. However, the N3 symmetric stretching vibration and the –CH3 stretching vibration show red-shifted trend. Meanwhile, in three characteristic regions of N3 asymmetric, N3 symmetric, and –CH3 stretching vibration, the number of vibration mode equals to that of azido groups and C–H bonds, respectively. For example, the monomer 1 has three bands at 2923, 2972, and 3022 cm−1 with C–H stretching vibration, and the trimer 3 has three bands at 2213, 2225, and 2240 cm−1 with N3 asymmetric stretching vibration, and the hexamer 6 has six bands at 1237, 1237, 1245, 1245, 1247, and 1255 cm−1 with N3 symmetric stretching vibration.


## 3.4. Thermodynamic Properties

Thermodynamic functions are important parameters for clusters to predict reactive property in a chemical reaction. According to vibrational analysis and statistical thermodynamic method, the standard molar heat capacity (Cp,mθ), standard molar thermal entropy (Smθ), and standard molar thermal enthalpy (Hmθ) of the asymmetric clusters (CH3FBN3)n (n = 1 – 6) in the range of 200–800 K are evaluated and tabulated in Table 4. It is obvious that the calculated thermodynamic functions increase with the temperature T raising. For more clear intuitive, taking monomer 1 as an example, the temperature-dependent relationships for Cp,mθ, Smθ, and Hmθ are expressed in formulas (2)–(4) and all are plotted in Figure 4(a). These correlations approximate to linear equations due to the small coefficients of T2; namely, the Cp,mθ, Smθ, and Hmθ increase linearly with the increase of temperature T. This can be understood from the fact that these three thermodynamic functions are dominated by the weak translational and rotational motions of the clusters at lower temperature, whereas the vibrational motion is intensified and makes more contributions to Cp,mθ, Smθ, and Hmθ at higher temperature. The same linear relationships are found for clusters (CH3FBN3)n with n = 2 – 6:(2)Cp,mθ=33.4650+0.2460T−1.1698×10−4T2,R2=0.9999,SD=0.3318,(3)Smθ=231.0291+0.4049T−1.4317×10−4T2,R2=0.9998,SD=0.9034,(4)Hmθ=−3.6904+0.0606T+6.401×10−5T2,R2=0.9999,SD=0.2784,where R2 represents the square of correlation coefficient and SD represents standard deviation.Table 4
Thermodynamic properties for the most stable clusters (CH3FBN3)n (n = 1 − 6) at different temperaturesa.
T200298.24005006007008001Cp,mθ77.7396.48113.47127.30138.62147.95155.71Smθ305.18339.77370.57397.42421.66443.75464.03Hmθ11.3019.8730.5842.6555.9670.3085.502Cp,mθ161.34206.04243.12271.97295.00313.71329.19Smθ409.57482.65548.58606.05657.75704.68747.61Hmθ19.8838.0060.9486.75115.14145.61177.783Cp,mθ242.25312.13369.54413.90449.12477.62501.12Smθ509.21619.50719.58807.00885.70957.141022.50Hmθ28.4755.8190.64129.90173.12219.50268.474Cp,mθ326.44420.82497.89557.25604.30642.35673.70Smθ584.95733.64868.53986.271092.191188.301276.18Hmθ37.1374.00120.94173.82231.98294.38360.235Cp,mθ411.59530.37627.05701.33760.11807.59846.71Smθ686.62874.081044.021192.261325.521446.381556.85Hmθ46.5693.03152.18218.74291.93370.39453.176Cp,mθ496.46639.39755.59844.78915.34972.331019.26Smθ788.601014.651219.481398.061558.561704.091837.08Hmθ55.96111.99183.27263.47351.61446.09545.74aUnits: T (K), Cp,mθ (J·mol−1·K−1), Smθ (J·mol−1 K−1), and Hmθ (kJ·mol−1).Figure 4
Variations of the thermodynamic functions (Cp,mθ,Smθ, and Hmθ) with the temperature (T) (a) for the monomer CH3FBN3 and the cluster size n (b) and for the asymmetric clusters (CH3FBN3)n (n = 1–6).
(a)(b)Moreover, the cluster size-dependent relationships forCp,mθ, Smθ, and Hmθ are expressed in formulas (5)–(7) and all are shown in Figure 4(b). All thermodynamic functions increase monotonically with the enlarged cluster size n; in other words, when one more CH3FBN3 is bound, the Cp,mθ, Smθ, and Hmθ increase with the average of 108 J·mol−1 K−1, 133 J·mol−1 K−1, and 18 kJ·mol−1, separately. This means that the contribution of monomer CH3FBN3 to the thermodynamic properties of cluster matches the cluster additivity:(5)Cp,mθ=−12.0847+108.4637n,R2=1.0000,SD=1.0282,(6)Smθ=211.0987+133.2237n,R2=0.9996,SD=8.0272,(7)Hmθ=1.0620+18.3966n,R2=0.9999,SD=0.5260.In addition, the theoretical enthalpies (ΔH) and Gibbs free energies (ΔG) of various oligomerizations from monomer CH3FBN3 in the range of 200–800 K are compiled in Table 5. The values of ΔH in the processes are negative except for the process 1 ⟶ 2 beyond 700 K, indicating that these oligomerization processes are exothermic. The ΔG at a given temperature is evaluated by the equation ΔG = ΔH − TΔS. The values of ΔG are all positive, which reveals the oligomerizations cannot occur spontaneously in the range of 200–800 K. There are no experimentally thermodynamic data available for the asymmetric clusters (CH3FBN3)n (n = 1 − 6), so the calculated thermodynamic functions and the obtained relationships of them with the temperature and cluster size n may help the experiment to further study the physical, chemical, and energetic properties of the asymmetric clusters (CH3FBN3)n (n = 1 − 6) or other asymmetric boron azides.Table 5
Enthalpy change and Gibbs free energy change of oligomerization at different temperaturesa.
T200298.24005006007008002(1) ⟶ (2)ΔH−7.22−6.24−4.72−3.05−1.280.512.28ΔG32.9452.4572.3091.35110.06128.48146.643(1) ⟶ (3)ΔH−43.70−42.07−39.37−36.32−33.03−29.67−26.30ΔG37.5777.11117.48156.31194.54232.21269.374(1) ⟶ (4)ΔH−61.34−58.75−54.65−50.05−45.13−40.09−35.04ΔG65.81127.69190.85251.66311.54370.60428.915(1) ⟶ (5)ΔH−81.97−78.35−72.75−66.54−59.90−53.14−46.36ΔG85.89167.51250.78330.88409.77487.52564.286(1) ⟶ (6)ΔH−98.66−94.05−87.03−79.25−70.97−62.53−54.08ΔG109.84211.20314.55413.98511.87608.36703.60aUnits: T (K), ΔH (kJ·mol−1), and ΔG (kJ·mol−1).


## 3.5. Aromatic Properties

Here, the aromaticity of the studied systems will be discussed. Because our systems are nonplanar, it is not suitable to use the criterion of nucleus-independent chemical shift (NICS) [30]. In order to find out whether a large π bond exists in our studied system, we use the newly proposed localized orbital locator (LOL)-π method [31] that can be used for studying nonplanar system to study the π bonds of our systems. The LOL-π is analyzed and visualized with the Multiwfn [32]. From Figure 5, there is π bond of NNNB in 1 and similar π bonds in 2-6. However, the π conjugation of 2–6 is not as good as that of 1. Meanwhile, Figure 5 also shows no large π bonds in all systems.Figure 5
LOL-π isosurface of different systems with the isovalue of 0.4.


## 4. Conclusions

We have systematically studied the structure, relative stability, IR, and thermodynamic properties of the asymmetric clusters (CH3FBN3)n (n = 1 – 6) at the DFT-B3LYP/6-31G∗. The B and Nα atoms tend to bond together in clusters (CH3FBN3)n (n = 2 − 6). The variation trend of geometrical parameters shows N2 (Nβ−Nγ), -CH3, and F- groups are easily eliminated and BN material is yielded. The calculated Δ2E of the asymmetric clusters (CH3FBN3)n (n = 1 − 6) exhibits a pronounced odd-even alternation phenomenon with cluster size n increasing, indicating that the cluster at n = 3 is more stable than other clusters. From monomer (n = 1) to clusters (n = 2 – 6), the N3 asymmetric stretching vibration presents blue-shifted trend while the N3 symmetric stretching vibration and –CH3 stretching vibration are red-shifted. The thermodynamic functions (Cp,mθ, Smθ, and Hmθ) of clusters (CH3FBN3)n (n = 1 – 6) show linear increase with the augment of both temperature T and cluster size n of clusters. Judged by the enthalpies in the range of 298.2–600 K, the calculated thermodynamic properties demonstrate that the formation of clusters (CH3FBN3)n (n = 2 − 6) from the monomer is thermodynamically favorable; however, the formation cannot occur spontaneously.



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