Software simulated spectra

Software for simulating spectra from assumed values of chemical shifts and coupling constants is widely available—usually supplied with NMR spectrometers and with many data processing programs. Many programs also use the simulated spectrum as the first step in an iterative process to fit the frequencies observed in an experimentally obtained spectrum. 

At the high end of the simulation spectrum considerable human effort is needed and can be motivated by the intrinsic scientific or technical nature of the problem. The software developed should have a long useful lifetime and overall efficiency and speed should be of primary importance [61]. However, the list of very promising parallel computer vendors that are no longer in business has been growing for each year during this decade. Despite this fact, software for programming parallel computers has made a number of important advances in... 

Modern spectrometers contain software that can calculate spectra fairly accurately for systems with three or four spins. Usually, a trial-and-error approach is first u.sed to estimate chemical shill s and coupling constants to produce a simulated spectrum to match the experimental spectrum. I lien, ehemieui shifts are varied until the widths and locations ol the multiplels approximately agree, rinally, coupling constanls or their sums and differences are varied until a suitable agreemeni between observed and simulated spectra is obtained. 

Fig. 22. Spectra subtraction for analysis of EPR hyperfine lines a Unresolved line simulated by the software WINEPR SimFonia b Overlap of the simulated spectrum and marine diesel spectrum at 383 K for subtraction of the unresolved line c Result of the spectra subtraction.

Figure 20-30 Effect of signal averaging on a simulated noisy spectrum. Labels refer to number of scans averaged. [From R. Q. Thompson, Experiments in Software Data Handling," J. Chem. 

Fig. 5 Electronic transitions of [Re(Cl)(CO)3(bpy)] calculated in vacuum (a), acetonitrile (b), simulated absorption spectra in vacuum (c, dashed) and acetonitrile (c, full), and experimental absorption spectrum measured in acetonitrile (d). It follows that successful TD-DFT simulation of the absorption spectrum and singlet CT states requires using hybrid functionals and continuum dielectric models for the solvent [11, 33], Calculation TD-DFT G03/PBE0, CPCM for MeCN. Simulation All calculated transitions included. Gaussian shapes (jwhm = 0.4 eV cm- ) of the absorption bands are assumed. Band areas are proportional to calculated oscillator strengths. Simulated using the GaussSum software. Reprinted with permission from [33]...

Most spectrometers today contain software that can calculate spectra for up to seven spins. The first step is a trial-and-error procedure of approximating the chemical shifts and coupling constants in order to match the observed spectrum through computer simulation. Chemical shifts are varied until the widths and locations of the observed and calculated mul-tiplets approximately agree. Then the coupling constants or their sums and differences are varied systematically until a reasonable match is obtained. This method is relatively successful for three and four spins, but is difficult to employ with larger systems. 

NMR-SIM can be seen as a software tool to analyse separately the effects on a NMR spectrum of different experimental variables such as spin system, pulse sequence and experimental parameters. The simulation program reduces the number of experimental parameters to a minimum to resemble as closely as possible a real experiment. This reduction in the number of variable parameters is a basic principle of scientific research because often the relationship between only a limited number of parameters can be interpreted unambiguously. The parameter reduction also speeds up calculation time. As illustrated in Fig. 4.1 the NMR spectrometer, represented by the magnet and computer, can be replaced by a computer plus software and the NMR sample by a defined theoretical spin system. Such an "ideal spectrometer" is the basis of the NMR-SIM or NMR experiment simulator software tool. 

W. Theiss, Scout 2 - Spectrum interpretation by simulation, M. Theiss - Hard-and Software for Optical Spectroscopy, Aachen, 2001. 

---