Gaussian broadening as chemical shift distribution in 1D and 2D
Created by: jtrebosc
This pull request propose 2 unrelated commits : 1. adding some tooltip on parameter labels for their description 2. Gaussian broadening is treated as position : unit can be switched to ppm In MQMAS and MQMAS Czjzek 2D models, the gaussian broadening is removed and replaced by sigmaCS which can be expressed as ppm as well.
Why :
In many samples, there is some chemical shift distribution that follow the normal distribution and can be described as gaussian broadening. Similarly lorentzian broadening has a physical meaning as it describes the T2 relaxation. Currently, the gaussian broadening proposed in ssnake MQMAS model only has a cosmetic function, except in the Czjzek MQMAS model where an additionnal sigmaCS is available.
Note that a "cosmetic" line broadening may be still required depending on the apodization used for processing the experimental data, but this should be a single parameter for all sites/models.
The first thing done is to express the gaussian broadening (or sigmaCS for MQMAS) in ppm as for chemical shift. The main advantage of using ppm unit is that it would allow to connect this physical parameter when fitting spectra recorded at different fields for example. The second is related to chemical shift distribution in MQMAS (without Czjzek) : I added a sigmaCS parameter in MQMAS model without Czjzek. Indeed, this should not lead to significant extra calculation and some nuclei can have sigmaCS but no significant Czjzek distribution character (BO3, small Cq sites).
In my opinion, 3QMAS line broadening should only expose one gaussian (sigmaCS) broadening parameter and 2 lorentzian parameters (for relaxation during t1 and t2). I’m not convinced that 5 parameters (sigmaCS, LB1, LB2, GB1, GB2) are needed as exposed in MQMASCzjzek model. Therefore I removed the Gauss2 and Gauss1 parameters.
This pull request significantly improves fitting of 3QMAS without Czjzek whenever CS distribution is present (that is often).