Chapter 9 紫外光谱

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Ultraviolet Spectroscopy
紫外光谱
9.1 Basics of Ultraviolet Spectroscopy

The UV involves the absorption of ultraviolet light
by a molecule causing the promotion of an
electron from a ground electronic state to an
excited electronic state. The transitions are
generally between a bonding or lone-pair orbital and
an unfilled non-bonding or anti-bonding orbital. The
wavelength of the absorption is then a measure of the
separation of the energy levels of the orbitals
concerned.
UV
UV-Vis
* > n* > * > n*
<200nm
200 to 800 nm
9.2 The UV-Vis spectrum and Beer’ laws

The UV-Vis spectrum
typically represents the
absorption of light as a plot
(Figure 3.2) of energy (as
wavelength, λ , in
nanometers, from E = hc/λ)
versus the intensity of
absorption (as absorbance,
A, or molar extinction
coefficient, ε).
Quantitative measurement (定量测定)
(IR, UV-vis, AAS, etc.)
9.3 Instrumentation







Instrument: spectrophotometer(分光光度计)
Stimulus: monochromatic light energy(单色光)
Analytical response: light absorption
Transducer(传感器): photocell
Data: electrical current
Data processor处理器: current meter(测流计)
Readout: meter scale(仪表)
photocell
Light Intensity
Current Meter
Current
Scale
动画(部件), P246
闪耀光栅
准直透镜
P247
动画grating and
monochromator
仪器进展:
透镜
传感器、检测器
动画PMT
动画pee
动画(紫外)
single beam (SB)
double-beam (DB)
double beam
9.4 Charge transfer transitions (P250)
电荷转移跃迁(荷移吸收光谱 )
电荷转移跃迁本质上属于分子内氧化还
原反应,因此呈现荷移光谱的必要条件是构
成分子的二组分,一个为电子给予体,另一
个应为电子接受体。
2+
+ Fe2+
3
N
N
N
N
3
Fe2+
A + D
hv
D +A -
(受电子体)(给电子体)
[Fe3+ SCN-] +h =
黄色
无色
O
四氯苯醌
血红色;490nm
[Fe2+SCN ]2+
Cl
Cl
Cl
Cl
O
深红色
六甲基苯
9.5 Applications:Foundamental variables
• The wavelength (or energy) of the radiation.
(辐射的波长;定性)
☆不同的物质只对不同的、特定波长的光有较强
的吸收或发射。(other wavelength, transparent)
• The amount of the radiation at .(辐射的强
度;定量)☆同一物质(溶液)不同浓度的吸
收曲线
c3 > c2 > c1
B
A
c3
c2
c1
λa
λb

a

Example 1(P7); Example 2
 Potassium dichromate(重铬酸盐) and
potassium permanganate have
overlapping absorption spectra in 1 M
H2SO4. K2Cr2O7 has an absorption
maximum at 440 nm, and KMnO4 has a
band at 545 nm (the maximum is actually
at 525 nm, but the longer wavelength is
generally used where interference from
K2Cr2O7 is less).
A mixture is analyzed by measuring
the absorbance at these two
wavelengths with the following
results: A440 = 0.405, A545 = 0.712 in
a 1-cm cell (approximate; exact
length not known). The
absorbances of pure solutions of
K2Cr2O7 (1.00 10-3 M) and
KMnO4 (2.00  10-4 M) in 1 M
H2SO4,
using the same cell gave the
following results: ACr,440 = 0.374,
ACr,545 = 0.009, AMn,440 = 0.019,
AMn,545 = 0.475. Calculate the
concentrations of dichromate
and permanganate in the sample
solution.
Interferences(干扰、影响因素)



Spectral interference(光谱干扰)
matrix 基体,基质,样品;
component 组成,组分
assay 特定的测定方法
Chemical interference(化学干扰)
Instrument interference(仪器干扰)
 定性分析(qualitative analysis确定未知
化合物)
缺点:吸收峰少
定性鉴别的依据→吸收光谱的特征(反映结构
中生色团和助色团的特性,不完全反映分
子特性;甲苯与乙苯:谱图基本相同;)
吸收光谱的形状、吸收峰的数目、吸收峰的位
置(波长)、吸收峰的强度、相应的吸光
系数max;(比较法、标准谱图库)
借助经验规则计算吸收峰波长;
1.比较下列化合物(a, b, c)的最大吸收波长
2.IR表明化合物C9H10O分别含有一个苯环、羰
基、亚甲基、甲基,但无法确定羰基的位置。
UV 显 示 有 三 个 吸 收 带 254nm(lgmax=4.1);
280nm(lgmax=3.1);320nm
(lgmax =1.9)。化合物结构是?
3. 某化合物在200-400nm范围内没吸收,该化合
物是芳香族;含共轭双键;醛;酮;羧酸
4. 某化合物分子式C5H8O,在UV图上有两个
吸收带(224,9750;314,38),结构是
CH3CH=CHCOCH3;CH2=CHCH2COCH3
CH3CH=CHCH2CHO;CH2=CH(CH2)2CHO
5. 某羰基化合物在近紫外和可见光区只产生
一个204nm (60)的弱吸收带。该化合物
可能是酯;醛酮;,-不饱和醛酮;,
-不饱和酯;
 在下列化合物中同时含有*、
n*跃迁的化合物是
三氯甲烷、丙酮、丁二烯、二甲苯
 在下列化合物中,那一个化合物能吸
收波长较长的辐射( )
苯、二甲苯、对氯代甲苯、萘
 某化合物分子式为C9H10O,
其图如右,指出发色团。
~320nm
230nm
270nm
360nm
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