Working Principle

VCD spectroscopy technique identifies the absorption difference between left and right circular radiation from the samples.

(1)

Unpolarized absorption can be formulated as:

(2)

\( I(\bar{\nu}) \) : intensity of the transmission by the sample \( I_0(\bar{\nu}) \) : intensity of the transmission without the sample \( \bar{\nu} \) : wavenumber frequency.

VCD can be defined in terms of molar absorptivity as: 

(3) 

where molar absortptivity is formulated in terms of pathlength b and concentration C:

(4) 

^[4]

Figure 2. Representation for interaction between sample and circularly polarized light. (Figure: Sabanur Mete)

The interaction between the optically active molecules and circularly polarized light responds with different intensities and absorption depending on enantiomer configuration. In chiral molecules, ΔA is not zero and this phenomenon is named circular dichroism. For non chiral molecules, the difference is zero which means they are not optically active such as in the case of isotropic and racemic molecules^[5]. 

Circularly polarized light absorption by optically isomer molecules are not characteristics of vibrational excitations. General formula of Rosenfeld analyzes the circular dichroism of randomly oriented molecules. Absorption and rotational strength are positively correlated to each other from initial state (a) to final state (b) as follows:

µe: electric dipole operator

µm: magnetic dipole operator

The relationship between electric and magnetic dipole operator for the k^th particle can be seen as follows:

q:charge

r: position operator

p: momentum operator

This formula demonstrates properties specific to VCD. For instance, µe and µm include distinct symmetry characters under symmetry operation. Rotational strength of their product is in odd character and changes its sign after the symmetry operation. Therefore, calculations will show VCD peak intensities with the same magnitude in opposite directions^[6].

Experimental Details

In this section, general experimental setup, sample preparation and reliability of the results are briefly described.

Equipment

VCD instruments include FT-IR set  up, interferometer, light source, optical elements for polarisation and detector as seen in Figure 3^[2].

Figure 3. Instrumentation setup for VCD. (Figure: Sabanur Mete)

VCD measurements are taken in the IR region down to approximately 700 cm^-1. Transmission limit of optical materials, source strength and detective sensitivity restraints this measurement range. VCD equipment is evolved by modification of dispersive IR and/or FT-IR integration into polarized light and detection of articulated intensities. Wire grid polarizers pass the linear polarized light beam. Moreover, they have high density wire arrays that enable higher polarization ratio for bigger wavenumbers. For a wider spectral range, it is possible to use ZnSe polarizers but it will result in loss in reflection. Photo elastic modulator (PEM), generates left and right circulatory polarized light with sine-wave modulation and is placed after the polarizer. Afterwards, the beam goes into the sample and the detector collects the signals. Usually the detectors include photoconducting diodes^[3].

Sample Preparation

VCD samples' transmission can be observed in liquid or solid such as pure solids in solutions, pellet form or in a suspension. Therefore, in order to obtain the optimum set up for VCD; liquid cells, window materials and spacers can be used. It is also possible to observe the effect of temperature on the change of conformation by using the temperature controlled liquid cells^[5]. There are different options for window materials such as barium fluoride (BaF₂) and calcium fluoride (CaF₂). Since barium fluoride has a solubility in water and light transmission range is until 800 cm^-1, it is recommended to use it as a plate when the sample is non-aqueous. On the other hand, calcium fluoride is not soluble in water and light transmission range is until 1100 cm^-1 so it should be used with aqueous solution samples^[2]. Collecting the VCD spectra for protein and peptides are similar to FT-IR but ∆A of the data is in smaller amplitude. Therefore in order to obtain VCD spectra longer time is needed for collecting the data^[3].

Reliability

Computational results show the VCD spectra since there are not solid experimental laws to express the sign of bands or intensity of the peaks. Even though there are efforts to formulate, they are not placed in general use. Since the development of empirical interpretation of VCD is still ongoing, quantum chemical methods are benefiting the improvement of the instrument^[6]. In order to minimize the thermal drift of the light source, it is placed away from the optical bench and main cover. Then, the light is collected to be polarized and go through the photoelastic modulator (PEM). Thus, this can prevent the generation of artifacts. Thermostatted elements in the instrumentation can help to prevent disturbance in the data^[2].

Case Study

Metal organic frameworks (MOFs), are cross linked polymers that are crystalline solids. They can generate pore systems in homogenous shapes and sizes and are made out of inorganic vertices with organic edges making them a hybrid system. They are important solid state materials in terms of gas storage, catalysis and separation of chemically different molecules. 

Földes et al,  reports a novel metal organic framework ,which includes  chiral carbocyclic sprained based linker synthesis with a solvothermal approach. MOF structure and synthesis was analysed using VCD among other techniques. The measurements were conducted in the 1800-800 cm^-1 range. KRS5 polarization filter with ZnSe PEM is used to produce circular polarization of the light. For the calibration purposes, CdS plate was used. To reduce the background noise, pure KBr pellets were used in order to decrease the artifacts of the light and the sample. 

As seen in Figure 4, it is possible to observe small negative peaks at around 1340 cm^-1 for both linker (b) and MOF (d). Also, around 1400 cm^-1, both of the structures display similar couple structures. Thus, VCD results indicate that linker and MOF have the same stereostructure. VCD was beneficial to demonstrate that carbocyclic linkers were embedded in metal organic framework without deactivating the optical properties or causing inversion of the symmetry^[7].

Figure 4.  (b) VCD spectra of linker  (d) VCD spectra of MOF. License CC BY 4.0