Spectroscopy from Ernst Schumacher

Spectros the spectroscopy package

Three examples from the NMR simulation package 'Spin' and 'NMRPlot':

ABX-spectrum Phoxirane

Acrylonitrile, type ABC.

'Wet' ethanol, 6 Protons

Program 'Spin' solves the Schrödinger equation of the given spinsystem of up to 8 protons. The Hamilton matrix is exactly composed and then diagonalized producing the spin eigenvalues (from which -> NMR transition frequencies) and eigenvectors (-> intensities of the allowed transitions within the spin system). With 'NMRplot' the spectrum is plotted by overlaying the calculated 'stick'-plot with a Lorentzian line shape. The chemical shifts for the envisaged frequency (or magnetic field strength) of the simulated NMR spectrometer and the spin-spin coupling constants are given as input. The listing of the Pascal source program explains all the steps of the calculation in detail The result is so good that the program is often used to simulate observed spectra in order to retrieve chemical shifts and spin coupling constants. This can be automatized with the 'NMRfit' program which starts with a trial set of these constants and then varies them in a 'least squares fit' between a measured and a simulated spectrum until the sum of the errorsquares reaches a minimum.
For an interactive tutorial about Fourier-Transform NMR and all its more subtle features, see suite of MathCad programs by Scott E. Van Bramer, Widener University, Chester, PA 19013.

Fouriertransform Infrared Spectroscopy

Fouriertransform Infrared Spectroscopy is a newer development. A 'white light' pulse is applied to a substance. This simultaneously excites all infrared active normal modes which remove their distinct resonant energy from the pulse. The decay of the pulse by these (and other) energy dissipating interactions is measured and an interferogram (against the unmodified white light) is produced (here simulated). By Fourier transformation of the observed time series the frequency-line shape signatures of the normal modes are retrieved from the signal. They can be plotted as ('power'-) spectrum. In order to understand the principles of this spectroscopy the simulation with program 'FTIR' easily allows to show what happens by manual input of the relevant parameters.

Generation and Visualization of Molecular Vibrations

'MolVib' generates molecular vibrations, given masses, molecular structure and force constant matrix from experiment or theory. It approximately solves the vibrational Schrödinger equation and produces vibrational eigenvalues (-> frequencies) and eigenfunctions (-> vibrational normal modes). It then allows to visualize the vibrations either as normal mode displacement vectors or as animations. Superposition of any two normal modes is possible. In order to understand the normal modes of polyatomic molecules their simulation is very important.

With the Quantum Chemical programs presented on another page you can compute Infrared- and Ramanspectra of molecules composed of several dozens of atoms and plot the spectra. They can be animated out of those applications with the help of clickable tools.