How to write a function of intensity as a function of frequency

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Sun, 09/18/2011 - 04:05 pm

Hello!

I'm extremely new to Igor and am carrying out an experiment to determine the rotational barrier in N,N - dimethylacetamide.

I have used Macnuts to fourier transform the data from the NMR machine and to carry out some phasing. This data was then exported and used to create a graph in Igor.

Now In order to determine the rate constant I need to create a function as described in the subject. Using this function I can fiddle with particular variables to make this graph match the graph from the macnuts data.

My problem is how to write out the following expression:

g(v) = Kτ(υA – υB)2 / [0.5.{(υA – υB) - υ}]2 + 4π2τ2.(υA – υB)2 .(υB-υ) 2

Where the 2's are squaring, (υA – υB) and τ are the variables to be fiddled with.
Multiplication is done with *, exponents with ^. Probably you want to do a curve fit. I entered your expression into the New Fit Function dialog (which you can get to from the Curve Fit dialog). The result was this function:
Function NMRPhase(w,u) : FitFunc
    Wave w
    Variable u

    //CurveFitDialog/ These comments were created by the Curve Fitting dialog. Altering them will
    //CurveFitDialog/ make the function less convenient to work with in the Curve Fitting dialog.
    //CurveFitDialog/ Equation:
    //CurveFitDialog/ f(u) = Kt*(uA-uB)^2/(((uA-uB)-u)/2)^2 + 4*pi*2*TT^2*(uA-uB)^2*(uB-u)^2
    //CurveFitDialog/ End of Equation
    //CurveFitDialog/ Independent Variables 1
    //CurveFitDialog/ u
    //CurveFitDialog/ Coefficients 3
    //CurveFitDialog/ w[0] = uA
    //CurveFitDialog/ w[1] = uB
    //CurveFitDialog/ w[2] = TT

    Variable Kt = <replace this with the actual number>

    return Kt*(w[0]-w[1])^2/(((w[0]-w[1])-u)/2)^2 + 39.4784176043574*w[2]^2*(w[0]-w[1])^2*(w[1]-u)^2
End

This isn't really very well written- repeated expressions so that they are computed only once. The number 39.4784176043574 is 4*pi^2.

Please pull down the Help menu and take a look at the Getting Started help. The Guided Tour that's the majority of Getting Started will introduce you to Igor's main concepts. It includes a couple of sections on curve fitting as well.

John Weeks
WaveMetrics, Inc.
support@wavemetrics.com

September 19, 2011 at 09:21 am - Permalink