J. C. Dobrowolsk et al.
96
dALtoN [91] and PSI [96, 97] developed by the Ruud and Crawford laboratories,
respectively, capable of computing the chiroptical response and circular dichroism
spectra using the CC linear-response method. this method provides the best comparison to experimental gas-phase vCd spectra.
Practical simulation of VCD spectra A recent increase in the accessibility of the
vCd spectrometers resulted in intense development of quantum chemistry programs calculating vCd intensities directly from first principles. In the following
subsection, we will briefly discuss the practical aspects of the computational methods used for the calculation of the vCd spectra.
Chiroptical properties are sensitive to the approximation made in electronic
structure calculations. therefore, it is advisable to use the most accurate method
that can be afforded for a given molecule. the study of vCd of a chiral molecule requires several steps and begins with the determination of the molecular
structure.
1. the first step is to determine local minimum structures by a systematic conformational search or by molecular dynamics (md) or by monte Carlo (mC)
simulations. For the given optimised geometry obtained by quantum mechanical calculations, the harmonic force field and normal coordinates are then
found.
2. Calculation of the vCd spectra, within the quantum mechanical approaches,
as a second step, can be performed at different levels of theory. Currently, most
studies of vCd spectra employ the density Functional theory (dFt) approach.
these computations give reasonably accurate results at a moderate computational cost. of course, the simulated vCd spectra are sensitive to the dFt
functionals and basis sets [98]. hybrid functionals with a large fraction of hartree-Fock exchange (50 %) are often employed for these calculations. having
the structures and their relative energies for the different conformers, the vCd
spectrum has to be calculated for individual conformers. this means that the calculations of the derivatives of the electric-dipole transition moments (as for IR
absorption spectra) must be followed by the calculations of the magnetic-dipole
moments. the final rotational strengths are computed as products of both transition moments.
3. the third step is to obtain the final spectrum; thus, the rotational strength of the
spectrum of each conformer has to be scaled by a factor that reflects the relative
Boltzmann population N i of a given conformer at a given temperature T, calculated with the gibbs free energy difference ΔG i of the i-th conformer with respect
to the most stable conformer. Boltzmann averaging gives spectra that can be
compared with experimental results.
5.4.2.2 Vibrational Raman Optical Activity (ROA)
RoA is described by means of the absolute difference between the intensities of the
(inelastic) scattering of right (R) and left (L) circularly polarised incident light by
96
dALtoN [91] and PSI [96, 97] developed by the Ruud and Crawford laboratories,
respectively, capable of computing the chiroptical response and circular dichroism
spectra using the CC linear-response method. this method provides the best comparison to experimental gas-phase vCd spectra.
Practical simulation of VCD spectra A recent increase in the accessibility of the
vCd spectrometers resulted in intense development of quantum chemistry programs calculating vCd intensities directly from first principles. In the following
subsection, we will briefly discuss the practical aspects of the computational methods used for the calculation of the vCd spectra.
Chiroptical properties are sensitive to the approximation made in electronic
structure calculations. therefore, it is advisable to use the most accurate method
that can be afforded for a given molecule. the study of vCd of a chiral molecule requires several steps and begins with the determination of the molecular
structure.
1. the first step is to determine local minimum structures by a systematic conformational search or by molecular dynamics (md) or by monte Carlo (mC)
simulations. For the given optimised geometry obtained by quantum mechanical calculations, the harmonic force field and normal coordinates are then
found.
2. Calculation of the vCd spectra, within the quantum mechanical approaches,
as a second step, can be performed at different levels of theory. Currently, most
studies of vCd spectra employ the density Functional theory (dFt) approach.
these computations give reasonably accurate results at a moderate computational cost. of course, the simulated vCd spectra are sensitive to the dFt
functionals and basis sets [98]. hybrid functionals with a large fraction of hartree-Fock exchange (50 %) are often employed for these calculations. having
the structures and their relative energies for the different conformers, the vCd
spectrum has to be calculated for individual conformers. this means that the calculations of the derivatives of the electric-dipole transition moments (as for IR
absorption spectra) must be followed by the calculations of the magnetic-dipole
moments. the final rotational strengths are computed as products of both transition moments.
3. the third step is to obtain the final spectrum; thus, the rotational strength of the
spectrum of each conformer has to be scaled by a factor that reflects the relative
Boltzmann population N i of a given conformer at a given temperature T, calculated with the gibbs free energy difference ΔG i of the i-th conformer with respect
to the most stable conformer. Boltzmann averaging gives spectra that can be
compared with experimental results.
5.4.2.2 Vibrational Raman Optical Activity (ROA)
RoA is described by means of the absolute difference between the intensities of the
(inelastic) scattering of right (R) and left (L) circularly polarised incident light by