Spectrum visualization window

            This window provides visualization of vibrational or TDDFT spectrum. The "Broaden lines" checkbox enables Lorentzian or Gaussian broadening of spectral lines, which facilitates comparison of calculated spectrum with the experimental one. The "Interpolation coordinates" button allows one to customize the X coordinates of the calculated contour of broadened lines. The contour values can be shown and exported into another application by the "Export values" button.

            The contour of spectral lines determined by Lorentzian broadening is calculated by this formula:

            The Gaussian broadening is described by this formula:

Here λ=1/2 of spectral width on 1/2 height

            Chemcraft can visualize spectra from PCGAMESS output files with mcqdpt2 computations. In these spectra, the wavenumbers are calculated from E(MP2) energy difference between the lowest state and other states, and the intensities are calculated by this formula:

    Where TD - norm of radiative transition dipole moment between state #1 and other states, ∆E - energy difference between the state #1 and other states.

Raman intensities computation

By default, Gaussian and other QC software computes raman activities for vibrational spectra, which are not the intensities. Chemcraft can convert these activities into intensities, which can be directly compared with the experimental intensities. The following formula for conversion is used:

Here S_i are the raman activities. By default they are printed in A⁴/AMU in Gaussian, and Chemcraft converts their units into sm⁴/kg for further calculation. v_i is the frequency of the i_th band, v₀ is the frequency of the laser excitation line (of the incident laser), T is the temperature in K. The fundamental constants used for calculation are:

1 AMU (atomic mass unit)= 1.66053878 × 10^{- 27 kilograms}

h=6.62606896 x 10^-34 Joule*sec

c=2.99792458 x 10¹⁰ cm/sec

k=1.3806504 x 10-23 Joules/Kelvin  

Note that in Chemcraft the frequency of the laser excitation line is typed in cm^-1 (in same units as the vibrational frequencies are usually printed), not is nanometers. 

The formula for intensities calculation was taken from the following papers:

P.L. Polavarapu, Ab initio vibrational Raman and Raman optical activity

spectra, J. Phys. Chem. 94 (1990) 8106–8112.

V. Krishnakumar, G. Keresztury, T. Sundius, R. Ramasamy, Simulation of IR and

Raman spectra based on scaled DFT force fields: a case study of 2-

(methylthio)benzonitrile, with emphasis on band assignment, J. Mol. Struct.

702 (2004) 9–21.

D. Michalska, R. Wysokinski, The prediction of Raman spectra of platinum (II)

anticancer drugs by density functional theory, Chem. Phys. Lett. 403 (2005)

211–217.

Example of a paper which used this formula:

Igor Reva. Comment on ‘‘Density functional theory studies on molecular structure,

vibrational spectra and electronic properties of cyanuric acid’’. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy. 2015, 22 June 2015. http://dx.doi.org/10.1016/j.saa.2015.06.070

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