J. C. Dobrowolsk et al.
94
5.4.2.1 Vibrational Circular Dichroism (VCD)
vCd is an extension of the electronic Cd technique into the IR region of the electromagnetic field. the spectrum usually consists of numerous bands in the 800–
2000 cm
−1
region, which is the advantage of this method. Nevertheless, a disadvantage of vCd in comparison to ECd is the intensity, about four to five orders
of magnitude lower than that of the parent IR vibrational absorption spectra vs. ca.
three orders in the parent uv-vis spectra [69]. this explains the difficulties with
observation of vCd spectra before the era of Fourier-transform IR spectrometers.
this method has become popular for elucidating AC by correlation between the
experimental and calculated vCd spectra. usually, only fundamental vibrational
transitions are considered in the vCd spectra. As for the IR absorption spectra, the
vCd band frequency yields information on the energy of a particular vibrational
mode (the frequencies), but the vCd band intensity can take either a positive or
negative sign.
the first vCd measurements were published in 1972 by dudley et al. for cholesteric mesophase [70] and by Chabay, holzwarth and hsu for tartrate salt and inorganic salts [71–73]. the theoretical fundament for vCd theory was established by
Barron, Buckingham, and Raab in 1975 [74, 75]. Nafie et al. presented good quality vCd spectra of a number of chiral molecules in the liquid phase in 1973 [76].
the first FtIR-vCd spectrometer at Syracuse university was designed and built in
1979 [77], while the first commercially available vCd spectrometers appeared on
the market in the mid-1990s [55].
Experimental vCd measurements are usually carried out in IR transmitting solvents or, in high concentrations, in light or heavy water. the pioneering work by
Schlosser et al. [78] and henderson and Polavarapu [79] opened new possibilities
for experimental investigations using low-temperature matrix isolation vibrational
circular dichroism spectra (mI-vCd). Recently, a contemporary, experimental matrix isolation-vCd paper by tarczay et al. for (R)-2-amino-1-propanol [80] has
been published, and the vCd spectra of Ac-gly-Nhme and Ac-L-Ala-Nhme were
registered in matrices [81]. the vCd signals of water molecules interacting with
peptides were detected [82, 83].
Now, we briefly describe the basic terms involved in the calculations of vCd
spectra. For a more detailed description, we refer the interested reader to some
excellent reviews [55, 61, 68, 84, 85]. At the molecular level, for each mode, the
intensity of the vA band is proportional to the absolute square of the electric-dipole transition moment (Edtm) of the molecule called the dipole strength of the
transition. For the i-th mode, this is denoted by D
i
g g
1 0
, ( ), expressing the transition
from the ground electronic ( g) and ground vibrational state ( 0) (described by the
Ψ g 0 wave function) to the ground electronic ( g) and vibrationally excited state ( 1)
(described by the Ψ g1 wave function). the dipole strength is always positive as an
absolute square. the corresponding intensity of the vCd band of the i-th mode is
connected to the rotatory strength R
i
g g
1 0
, ( ): the imaginary part of the scalar product
of Edtm and the magnetic-dipole transition moment (mdtm).
94
5.4.2.1 Vibrational Circular Dichroism (VCD)
vCd is an extension of the electronic Cd technique into the IR region of the electromagnetic field. the spectrum usually consists of numerous bands in the 800–
2000 cm
−1
region, which is the advantage of this method. Nevertheless, a disadvantage of vCd in comparison to ECd is the intensity, about four to five orders
of magnitude lower than that of the parent IR vibrational absorption spectra vs. ca.
three orders in the parent uv-vis spectra [69]. this explains the difficulties with
observation of vCd spectra before the era of Fourier-transform IR spectrometers.
this method has become popular for elucidating AC by correlation between the
experimental and calculated vCd spectra. usually, only fundamental vibrational
transitions are considered in the vCd spectra. As for the IR absorption spectra, the
vCd band frequency yields information on the energy of a particular vibrational
mode (the frequencies), but the vCd band intensity can take either a positive or
negative sign.
the first vCd measurements were published in 1972 by dudley et al. for cholesteric mesophase [70] and by Chabay, holzwarth and hsu for tartrate salt and inorganic salts [71–73]. the theoretical fundament for vCd theory was established by
Barron, Buckingham, and Raab in 1975 [74, 75]. Nafie et al. presented good quality vCd spectra of a number of chiral molecules in the liquid phase in 1973 [76].
the first FtIR-vCd spectrometer at Syracuse university was designed and built in
1979 [77], while the first commercially available vCd spectrometers appeared on
the market in the mid-1990s [55].
Experimental vCd measurements are usually carried out in IR transmitting solvents or, in high concentrations, in light or heavy water. the pioneering work by
Schlosser et al. [78] and henderson and Polavarapu [79] opened new possibilities
for experimental investigations using low-temperature matrix isolation vibrational
circular dichroism spectra (mI-vCd). Recently, a contemporary, experimental matrix isolation-vCd paper by tarczay et al. for (R)-2-amino-1-propanol [80] has
been published, and the vCd spectra of Ac-gly-Nhme and Ac-L-Ala-Nhme were
registered in matrices [81]. the vCd signals of water molecules interacting with
peptides were detected [82, 83].
Now, we briefly describe the basic terms involved in the calculations of vCd
spectra. For a more detailed description, we refer the interested reader to some
excellent reviews [55, 61, 68, 84, 85]. At the molecular level, for each mode, the
intensity of the vA band is proportional to the absolute square of the electric-dipole transition moment (Edtm) of the molecule called the dipole strength of the
transition. For the i-th mode, this is denoted by D
i
g g
1 0
, ( ), expressing the transition
from the ground electronic ( g) and ground vibrational state ( 0) (described by the
Ψ g 0 wave function) to the ground electronic ( g) and vibrationally excited state ( 1)
(described by the Ψ g1 wave function). the dipole strength is always positive as an
absolute square. the corresponding intensity of the vCd band of the i-th mode is
connected to the rotatory strength R
i
g g
1 0
, ( ): the imaginary part of the scalar product
of Edtm and the magnetic-dipole transition moment (mdtm).