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Overview of Vibrational Optical Activity
1.1Introduction to Vibrational Optical Activity Vibrational optical activity (VOA)is a new form of natural optical activity who early history dates back to the nineteenth century.We now know that the original obrvations of optical activity,the rotation of the plane of linearly polarized radiation,termed optical rotation (OR),or the differential absorption of left and right circularly polarized light,circular dichroism (CD),have their origins in electronic transitions in molecules.Not until after the establishment of quantum mechanics and molecular spectroscopy in the twentieth century was the physical basis of natural optical activity revealed for the first time.1.1.1Field of Vibrational Optical Activity
Vibrational optical activity,as the name implies,is the area of spectroscopy that results from the introduction of optical activity into the field of vibrational spectroscopy.VOA can be broadly defined as the difference in the interaction of left and right circularly polarized radiation with a molecule or molecular asmbly undergoing a vibrational transition.This definition allows for a wide variety of spectroscopies,as will be discusd below,but the most important of the are the forms of VOA associated with infrared (IR)absorption and Raman scattering.The infrared form is known as vibrational
circular dichroism,or VCD,while the Raman form is known as vibrational Raman optical activity,VROA,or usually just ROA (Raman optical activity).VCD and ROA were discovered experimentally in the early 1970s and have since blossomed independently into two important new fields of spectroscopy for probing the structure and conformation of all class of chiral molecules and supramolecular asmblies.
VCD has been measured from approximately 600cm À1in the mid-infrared region,into the hydrogen stretching region and through the near-infrared region to almost the visible region of the spectrum at 14000cm À1.The infrared frequency range of up to 4000cm À1is comprid mainly of fundamental transitions,while higher frequency transitions in the near-infrared are dominated by Vibrational Optical Activity:Principles and Applications ,First Edition.Laurence A.Nafie.Ó2011John Wiley &Sons,Ltd.Published 2011by John Wiley &Sons,Ltd.C O P Y R I G H T E D M A T E R I A L
2Vibrational Optical Activity
overtone and combination band transitions.ROA has been measured to as low as50cmÀ1,a distinct difference compared with VCD,but ROA is more difficult to measure beyond the range of fundamental transitions and is typically only measured for vibrational transitions below2000cmÀ1.VC
D and ROA can both be measured as electronic optical activity in molecules posssing low-lying electronic states, although in the ca of VCD it is appropriate to refer to the phenomena as infrared electronic circular dichroism,IR-ECD or IRCD,and electronic ROA,or EROA.
VCD and ROA are typically measured for liquid or solution-state samples.VCD has been measured in the gas pha and in the solid pha as mulls,KBr pellets andfilms of various types.When sampling solids,distortions of the VCD spectra due to birefringence and particle scattering need to be avoided. To date,ROA has not been measured in gas or diffu solids,but nothing precludes this sampling option,although technical issues may ari,such as sufficient Raman intensity for gas and competing particle scattering for diffu solids.
At prent,there is only one form of VCD,namely the one-photon differential absorption form, although recently,a cond manifestation of VCD,the differential refractive index,termed the called vibrational circular birefringence(VCB),has been measured.AVCB spectrum is the Kramers–Kronig transform of a VCD spectrum and is also known as vibrational optical rotatory dispersion(VORD).As we shall e,ORD is the oldest form of optical activity and the form of VOA that was sought in the 1950s and1960s before the discovery of VCD.By comparison,ROA is much richer in experimental possibilities.Becau one can consider circular(or linear)polarization differences in Raman scattering
intensity associated with the incident or scattered radiation,or both,in-pha and out-of-pha,there are four(eight)distinct forms of ROA.Further,for ROA there are choices of scattering geometry and the frequency of the incident radiation,both of which give ri to different ROA spectra. As a result,there is in principle a continuum of different types of VOA measurements that can be envisioned for a given choice of sample molecule.
Beyond this,many other forms of VOA are possible.One form is reflection vibrational optical activity,which would include VCD measured as specular reflection,diffu reflection or attenuated total reflection(ATR).In principle,VCD could also be measured influorescence.Becau fluorescence depends on the third power of the exciting frequency,infraredfluorescence VOA would be very weak relative to VCD and thus very difficult to measure.As withfluorescence in the visible and ultraviolet regions of the spectrum,fluorescence VCD could be measured in two forms,fluorescence detected VCD or circularly polarized emission VCD.In the former,one would measure all thefluorescence intensity resulting from the differential absorbance of left and right circularly polarized infrared radiation(VCD)or measure the difference in left and right circularly polarized infrared emission from unpolarized exciting infrared radiation.Finally,we note the various manifestations of nonlinear or multi-photon VCD,such as two-photon infrared absorption VCD.
In the ca of ROA there are a variety of different forms of VOA yet to be measured.One recently reported for thefirst time is near-infrared excited ROA.Other forms of ROA yet to be measured are ultraviolet resonance Raman ROA,surface-enhanced ROA,coherent anti-Stokes ROA,and hyper-ROA in which two lar photons generate an ROA spectrum in the region of twice the lar frequency.Second harmonic generation(SHG)ROA at two-dimensional interfaces has been measured,and attempts have been made to measure sum frequency generation(SFG)VOA,which is an interesting form of optical activity that depends on transition moments which ari in both VCD and ROA.
Another class of optical activity that has VOA content is vibronic optical activity.Here the source of optical activity is a combination of electronic optical activity(EOA)and VOA when changes to both electronic and vibrational states occur in a transition.This form of EOA–VOA aris in ECD whenever vibronic detail is obrved.The analogous form of ROA is either vibronically resolved electronic ROA or ROA arising from strong resonance with particular vibronic states of a molecule.
Finally,we consider other forms of radiation that may affect vibrational transitions in molecules.In particular,it is possible to create beams of neutrons that are circular polarized either to the left or to the right.This phenomenon has been considered theoretically,but experimental attempts at measure
ment have not been reported.Another common form of vibrational spectroscopy that does not involve photons as the source of radiation interaction is electron energy loss spectroscopy.This is esntially Raman scattering using electrons.If modulation between left and right circularly polarized electrons could be realized,then this could become a new form of VOA in the future.
1.1.2Definition of Vibrational Circular Dichroism
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VCD is defined as the difference in the absorbance of left minus right circularly polarized light for a molecule undergoing a vibrational transition.For VCD to be non-zero,the molecule must be chiral or el be in a chiral molecular environment,such as a non-chiral molecule in a chiral molecular crystal or bound to a chiral molecule.The definition of VCD is illustrated in Figure1.1for a molecule undergoing a transition from the zeroth(0)to thefirst(1)vibrational level of the ground electronic state (g)of a molecule.
More generally,we can define VCD for a transition between any two vibrational sublevels ev and ev0 of an electronic state e as:
VCD D A
永远不分离ðÞa
ev0;ev ¼A L
ðÞa ev0;evÀA R
ðÞa ev0;evð1:1Þ
where A L is the absorbance for left circularly polarized light and A R is the absorbance for right circularly polarized light.The superscript a refers to the vibrational mode,or modes,associated with the vibrational transition.The n of the definition of VCD is left minus right circularly polarization in conformity with the definition ud for electronic circular dichroism(ECD).The parent ordinary infrared absorption intensity associated with VCD,also referred to as vibrational absorbance(V A),is defined as the average of the individual absorbance intensities for left and right circularly polarized radiation,namely:
VA AðÞa
ev0;ev ¼1
2
A L
ðÞa ev0;evþA R
ðÞa ev0;ev
h i
ð1:2Þ
A L A R
(ΔA)a g1,g0 = (A L)a g1,g0 − (A R)a g1,g0
VCDqq申请账号
g
Figure1.1Energy-level diagram illustrating the definition of VCD for a molecule undergoing a transition from the zeroth to thefirst vibrational level of the ground electronic state
Overview of Vibrational Optical Activity3
The definitions of VCD and V A reprent the total intensity associated with a given
vibrational transition with the label a.Experimentally,one measures VCD and V A spectra as
bands in the spectrum that have a shape or distribution as a function of radiation frequency n,
which is expresd as f0aðnÞfor each vibrational transition.The reason for the prime will be explained in Chapter3.An experimentally measured VCD or V A spectrum is therefore related to
the defined quantities in Equations(1.1)and(1.2)by sums over all the vibrational transitions a in
the spectrum as:
D AðnÞ¼
X
衬衣英文a
ðD AÞa ev0;ev f0aðnÞð1:3Þ
AðnÞ¼
X
a
ðAÞa ev0;ev f0aðnÞð1:4Þ
From the expressions it can also be en that the original definitions of VCD and V A in Equations(1.1)and(1.2)reprent integrated intensities over the measured VCD,or V A,band of vibrational transition a by writing for example:
D A a ev0;ev¼
ð
a D AðnÞd n¼
ð
a
D A a ev0;ev f0aðnÞd n¼D A a ev0;ev
ð
a
f0
a
ðnÞd nð1:5Þ
where the last integral on the right-hand side of this expression is equal to1when a normalized bandshape of unit area is ud as:
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ð
a f0
a
ðnÞd n¼1ð1:6Þ
Experimentally,the V A intensities are defined by the relationship:
A nðÞ¼Àlog10I nðÞ=I0nðÞ
½ ¼«nðÞbCð1:7Þ
羽扇纶巾风袅袅where I nðÞis the IR transmission intensity of the sample,which is divided by the reference transmission spectrum of the instrument,I0nðÞ,usually without the sample in place.Normalization of the sample transmission by the reference spectrum removes the dependence of the measurement on the characteristics of the instrument ud for the measurement of the spectrum,namely throughput and spectral profile.The cond part of Equation(1.7)assumes Beer–Lambert’s law and defines the molar absorptivity of the sample,«nðÞ,where b and C are the pathlength and molar concentration in the ca of solution-pha samples,respectively.The experimental measurement of VCD is similar,but more complex than the definition of V A in Equation(1.7),and we defer description of this definition until Chapter6,when the measurement of VCD is described in detail. The definition of the molar absorptivity in Equation(1.7)yields a molecular-level definition of VCD intensity,D«nðÞ,which is free of the choice of the sampling variables pathlength and concentration. This is given by:
D«nðÞ¼D A nðÞ=ðeeÞbCð1:8Þ4Vibrational Optical Activity
where(ee)is the enantiomeric excess of the sample.The(ee)can be defined as the concentration of the major enantiomer,C M,minus that of the minor enantiomer,C m,divided by the sum of their concentrations,which is also the total concentration.
ðeeÞ¼C M ÀC m
C MþC m ¼C M
ÀC m
C
ð1:9Þ
The value of(ee)can vary from unity for a sample of only a single enantiomer to zero for a racemic mixture of both enantiomers,such that neither enantiomer is in excess.Thus we can write:
D«nðÞ¼D A nðÞ=bðC MÀC mÞð1:10ÞThis definition of VCD reprents a molecular-level quantity t
hat has been corrected for the pathlength and concentrations of both enantiomers.The intensity expresd as molar absorptivity of a VCD band
for vibrational transition a,D«
ðÞa
ev0;ev ,can be extracted from the experimentally measured molar
absorptivity VCD spectrum by integration over the VCD band of transition a,as:
D«
ðÞa ev0;ev¼
ð
a
D«ðnÞd nð1:11Þ
The quantity D«
ðÞa
ev0;ev can be compared directly with theoretical expressions of VCD intensity.
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A transition between vibrational levels parated by a single quantum of vibrational energy corresponds to a fundamental transition and is described by the superscript a for a particular vibrational mode in the definitions above.In the ca of higher level vibrational transitions,more than one vibrational quantum number is needed,such as ab for a a combination band of mode a and mode b,or2a for thefirst overtone of mode a.All fundamental transitions occur in the IR region below a frequency of4000cmÀ1and all vibrational transitions above that frequency in the near-infrared region involve only overtones and combination bands.
1.1.3Definition of Vibrational Raman Optical Activity
ROA is defined as the difference in Raman scattering intensity for right minus left circularly polarized incident and/or scattered radiation.There are four forms of circular polarization ROA.Energy-level diagrams are given in Figure1.2for a molecule undergoing a transition from the zeroth to thefirst vibrational level of the ground electronic state.The left-hand vertical upward-pointing arrows reprent the incident lar radiation,and the right-hand downward-pointing arrows reprent the sca
ttered Raman radiation.A Stokes Raman scattering process is assumed such that the molecule gains vibrational energy while the scattering Raman radiation is red-shifted from the incident lar radiation by the same energy.The initial andfinal states of the Raman-ROA transitions,g0and g1,are the same as tho in Figure1.1for V A-VCD transitions.The excited vibrational–electronic(ev)states of the molecule are reprented by energy levels above the energy of the incident lar radiation,which applies for the common ca in which the incident radiation has lower energy than any of the allowed electronic states of the molecule.
本金等于什么公式The original form of ROA is now called incident circular polarization(ICP)ROA.Here the incident lar is modulated between right and left circular polarization states,and the Raman intensity is measured at afixed linear or unpolarized radiation state.The cond form of ROA is called scattered circular polarization(SCP)ROA.In this form,fixed linear or unpolarized incident lar radiation is ud and the difference in the right and left circularly polarized Raman scattered light is measured.The third form of ROA is in-pha dual circular polarization(DCP I)ROA.Here the
Overview of Vibrational Optical Activity5
