褐煤分子片段结构电子吸收光谱的理论研究

doi:10.7503/cjcu20150689

摘要/Abstract

摘要：

应用不同密度泛函计算了几种代表性化合物和褐煤分子片段结构的电子吸收光谱性质, 考察了结构修饰与杂环原子对电子光谱性质的影响. 计算结果表明, 杂化泛函B3LYP和HSEH1PBE较适合这类体系电子吸收光谱的计算, 预测的吸收光谱主要出现在近紫外区, 随着共轭环的增加, λ_max吸收移向可见光区. 当共轭支链基团通过C=C键和分子片段模型连接时, 在可见光区(460~480 nm)有非常强的吸收. 杂环原子的存在及其在五元环中的位置对电子吸收光谱有显著影响, 处在相对对称的位置时, 前线轨道的共轭程度较高, 导致λ_max吸收出现较显著的红移, 预测的这些分子片段光谱性质有助于理解褐煤的光响应性质.

关键词: 褐煤, 分子片段结构, 吸收光谱, 含时密度泛函理论

Abstract:

Electronic spectra of the lignite structural units and the effects of structural modification were explored by extensive density functional calculations. Based on calibration calculations on six representative compounds by different functionals, the feasibility of both B3LYP and HSEH1PBE for this kind of systems was validated. Predicted absorption spectra indicate that the main bands appear in near Ultraviolet-Visible region, while for the lignite structural unit with the extended conjugated components the λ_max absorption occurs in visible region. Note that there are quite strong absorptions in the visible region of 460—480 nm as the side conjugated chain is linked to the structural unit through CC. The types of hetero-atoms and their positions show remarkable effects on the electronic absorption features, and the frontier orbitals in the structural units with a symmetric hetero-atom are well delocalized, especially for LUMO, resulting in notable red-shifted absorption of λ_max. The predicted spectroscopic properties of the lignite structural units provide a basis to understand their features of light response.

Key words: Lignite, Structural unit, Absorption spectra, Time-dependent density functional theory

中图分类号:

O641

TrendMD:

胡薛, 孙铭骏, 张向飞, 曹泽星. 褐煤分子片段结构电子吸收光谱的理论研究. 高等学校化学学报, 2015, 36(11): 2292.

HU Xue, SUN Mingjun, ZHANG Xiangfei, CAO Zexing. Theoretical Study of Electronic Spectra of Lignite Structural Units^†. Chem. J. Chinese Universities, 2015, 36(11): 2292.

图/表 11

Fig.1 Six selected compounds for calibration of density functionals in calculation of electronic absorption spectra

Table 1 Predicted electronic absorptions(λmax/nm), oscillator strengths(f), and assignments for six compounds by different approaches

Compound	λ_max/nm	λ_max/nm(f), assignment^*(contribution to the excited state)
Compound	λ_max/nm	B3LYP	CAM-B3LYP	HSEH1PBE	ωB97X	M062X	ωB97XD
Ⅰ	400	430(0.13), H→L(70%)	380(0.18), H→L(70%)	422(0.13), H→L(70%)	359(0.21), H→L(70%)	377(0.17), H→L(70%)	379(0.19), H→L(70%)
Ⅱ	375	386(0.13), H→L(70%)	343(0.11), H→L(70%)	380(0.08), H→L(70%)	325(0.13), H→L(70%)	341(0.11), H→L(70%)	341(0.11), H→L(70%)
Ⅲ	258	266(0.13), H→L(60%)	250(0.08), H→L(50%)	260(0.10), H→L(50%)	245(0.07), H→L(47%)	249(0.09), H→L(53%)	251(0.08), H→L(53%)
Ⅳ	340	333(0.13), H→L(68%)	302(0.14), H→L(66%)	302(0.14), H→L(66%)	290(0.16), H→L(64%)	298(0.15), H→L(66%)	301(0.15), H→L(66%)
Ⅴ	238	241(0.13), H→L(69%)	226(0.42), H→L(68%)	226(0.44), H→L(68%)	221(0.39), H→L(67%)	223(0.46), H→L(69%)	226(0.42), H→L(68%)
Ⅵ	324	329(0.13), H→L(70%)	301(0.13), H→L(69%)	322(0.11), H→L(70%)	290(0.14), H→L(68%)	298(0.13), H→L(69%)	301(0.13), H→L(69%)

Fig.2 B3LYP-predicted absorption spectra and related frontier orbitals of the structural unit 1(inset)

Fig.3 Structural units containing furan, pyrrole, or thiophene component for the lignite fragment

Fig.4 Predicted electronic spectra of the structural units 1, 2b, 3b and 4b

Fig.5 Selected frontier orbitals in structural units 2b, 3b and 4b

Fig.6 Selected tricyclic structural units for the lignite fragment

Fig.7 Predicted electronic spectra of structural units 1, 5b, 6b, and 7b

Fig.8 Structural units 8a(A) and 9a(B) for the lignite fragment

Fig.9 Predicted electronic spectra of structural units 2b(a), 8a(b) and 8b(c)

Fig.10 Predicted electronic spectra of structural units 2b(a), 9a(b) and 9b(c)

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