A simplified version of the original Karplus equation is used to very quickly calculate the dihedral/torsion angle (in degrees) from a proton/1H NMR spectrum 3J (vicinal H-C-C-H) coupling constant (Hz) and vice-versa. The calculation is based on published material; see the info (i) dialogue for details of the equations and references. The calculations work best for rigid bicyclic systems, getting as close as one degree or better compared to the original H-C-C-H Karplus equation. The user can choose between a likely angle range between 0-90 degrees or 90-180 degrees for use as input or output. As an alternative to using two equations, and a variation of the Karplus equation, the Bothner-By equation is used to calculate 3J H-C-C-H dihedral angles. Finally, the user can enter their own empirically-derived parameters (especially relating to electronegativities and other data) and coupling constant or angle for use in modifications of the Karplus equation to frequently determine coupling constants and extended to including atoms other than hydrogen (C-13, N-15 etc.). This can be used for amino acid or protein analysis calculations. It will also allow you to negate these input values and save the input angle, constants (coefficients) A, B and C, output J and name (optional) to a file (dihedral.txt), with a notification, in your default Internal storage (sdcard) Dihedrals folder, which you can transfer (copy & paste) to external media with a program like File Commander. There is the option to round up the calculated coupling constant from that calculation.
Be aware that for a quartet splitting pattern, as an example (AX3 pattern, e.g. vicinal methyl protons) the average J is about 7 Hz and the calculation does not give their individual dihedral angles as they exist in different conformations with respect to the proton(s) they are coupled to.
Graph support (G) has been added to version 2, so that an attractive line graph (0-180 degrees) is produced on-screen for a modified Karplus equation using your determined constants A, B and C and is scrollable to see the complete range. The resolution can also be increased by using more points on the graph by a factor of five. Your device or 3rd party Apps can be used for screenshot capture to save the graph to your device.
Support for use of the Haasnoot-Altona equation has been added to version 3 to investigate the effects of electronegative substituents. This is intended for more advanced users, familiar with these equations and parameters choosing a likely dihedral angle to calculate the 3J coupling constant and the data can be saved.
This App requires no network connection to do the calculations; adjusting the font size can be achieved from your device settings and the App (to do the calculations) is scrollable.
Clicking on Dihedral angle or Coupling constant gives the result above it. Home (the centre button at the bottom of the screen) exits the application and Back (the left button) gives previous results. Data inputted out-of-range responds with NaN. Better support for user error has been added and a more realistic output to 4 d.p. in most calculations on the main screen activity.
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Feedback encouraged for future updates!