Theoretical Study of Electronic Spectra of Lignite Structural Units†

doi:10.7503/cjcu20150689

Abstract

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

CLC Number:

O641

TrendMD:

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

Figures/Tables 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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