1.College of Physics and Materials Science, Henan Normal University, Xinxiang 453007, China 2.Department of Computing Technology, Henan Information Engineering School, Zhengzhou 450000, China
Fund Project:Project supported by the National Natural Science Foundation of China (Grant No. 61540016).
Received Date:18 February 2019
Accepted Date:02 April 2019
Available Online:01 June 2019
Published Online:05 June 2019
Abstract:According to density functional theory, in this paper we report a simulation result obtained by using the Gaussian09 package. Adopted in the calculation are an optimized Opt Freq and a base group of B3LYP/6-311g to simulate the absorption of 16 kinds of liquid crystal (LC) molecules of 4-(trans-4-n-alkylcyclohexyl) isothiocyanatobenzenes (CHBT) in a 0.1?5.0 terahertz band (THz). The results show that in the low terahertz band, the absorption is caused mainly by the vibration and rotation of the molecules. So for convenience, we present an novel analytical method of studying the influence of molecular moment of inertia and mass center of gravity shift on absorption. An important result is found that the length of the molecular alkyl chain can lead to different molecular mass, mass center of gravity and moment of inertia, which causes the rotation and vibration of the molecule to be different. These factors lead to the difference in terahertz wave absorption. In the 0.1?5.0 terahertz band, the molecules with 3?7 alkyl chain carbon atoms show a strong absorption. As a reference, reducing and increasing the carbon atoms in the alkyl chain will cause the molecules to reduce the absorption of terahertz waves . In the end, the calculated results are compared with the experimental results obtained from 10 molecules according to the reference data in a frequency range of 0.3?3.0 terahertz. It is found that in the low frequency band there exist some differences between the calculation results and the experimental measurements, in which the difference in the position of the absorption peak may originate from a hydrogen bond. Comparing the relative magnitudes of the absorption intensities, it is found that the experimental measurements are consistent with the calculated results, indicating that the absorption intensity comes from the absorption of dipole vibration and rotation, which demonstrates the positive significance of computational simulation. We look forward to the experimental measurements in the future, and correct the calculation methods and keywords as well as the parameters such as temperature calculation that is to be done in future work. As a theoretical basis, the calculation results can better reflect the absorption of molecular materials, and it is expected to provide useful suggestions for designing and synthesizing the liquid crystal molecules. Keywords:liquid crystal/ terahertz/ alkyl chains/ Gaussian
另一方面, 红外吸收谱的强度是分子振动跃迁概率的表征, 与分子振动时偶极矩的变化相关. 因此, 与基团固有偶极矩有关, 极性越强, 振动时偶极矩变化越大, 吸收谱峰值越高; 分子中的原子对称性好, 振动时偶极矩变化就小, 吸收谱峰值越弱. 液晶分子在低太赫兹波段的吸收同时又是分子转动的一个重要体现. 分子的转动取决于分子内原子质量分布和转动惯量. 因此, 为了分析0.1—5.0 THz波段吸收峰产生的原因, 我们计算了这些分子的偶极矩和转动惯量. 16个分子转动惯量和偶极矩的计算结果随烷基链长度的变化展示在图4和图5中. 从图4和图5可以看出, 分子的转动惯量随碳原子数目的增加而增大. 偶极矩只有n = 3, 5候有些异常, 可能是烷基链碳氢的增加对分子重心从苯环到环己烷的迁移产生的电荷中心的差异造成的. 其他分子的偶极矩整体差异不大, 但是微小差别显示出偶极矩的奇偶效应. 这说明分子的电荷分布随烷基链的增加变化较小, 显示烷基链对分子的极性贡献不大. 图 4 16个分子在其长轴的转动惯量值的变化趋势 Figure4. Trend of the rotational inertia of 16 molecules on their long axis.
图 5 16个分子在其长轴的总偶极矩值的变化趋势 Figure5. Trend of the total dipole moment of 16 molecules on their long axis.
影响分子振动和转动的因素主要考虑分子的重心和转动惯量, 因此, 需要讨论分子内的原子质量分布(如表1)和转动惯量对吸收的影响. 我们把整个分子分为NCS-Benzene-Cyclohexane-CnH2n+1几个部分. 根据各部分的原子质量之和, 确定分子的重心位置. 表1和图6展示了分子质量分布和重心位置的标定. 以n = 1为例, 经过计算发现, 分子的质量中心在靠近环己烷一端的苯环位置. 图 6 1CHBT的分子质量分布和重心位置标定 Figure6. Calibration of mass distribution and center of gravity for 1CHBT.
nCHBT
NCS/ g·mol–1
Benzene/ g·mol–1
NCS+ Benzene/ g·mol–1
Cyclohexane/ g·mol–1
NCS+ Benzene + Cyclohexane/g· mol–1
CnH2n+1/ g·mol–1
Cyclohexane + CnH2n+1/ g·mol–1
0CHBT
58
76
134
82
216
1
83
1CHBT
58
76
134
82
216
15
97
2CHBT
58
76
134
82
216
29
111
3CHBT
58
76
134
82
216
43
125
4CHBT
58
76
134
82
216
57
139
5CHBT
58
76
134
82
216
71
153
6CHBT
58
76
134
82
216
85
167
7CHBT
58
76
134
82
216
99
181
8CHBT
58
76
134
82
216
113
195
9CHBT
58
76
134
82
216
127
209
10CHBT
58
76
134
82
216
141
223
11CHBT
58
76
134
82
216
155
237
12CHBT
58
76
134
82
216
169
251
13CHBT
58
76
134
82
216
183
265
14CHBT
58
76
134
82
216
197
279
15CHBT
58
76
134
82
216
211
293
表116个分子各个组成部分的原子质量 Table1.Atom mass of each component of 16 molecules.