How to calculate molecular weight, exact mass, TPSA and logP
Tutorial · macOS and Windows
The Calculated Properties window shows the numbers you usually look up or calculate by hand, and updates as you draw.
Open the properties
- Draw the molecule, or insert it from SMILES.
- Open (Mac). On Windows the Properties pane is on the right of the window ().
- With several molecules on the canvas, choose one in the Molecule menu, or all of them combined.
What you get, for aspirin
Formula C9H8O4
Molecular weight 180.1583 g/mol
Monoisotopic mass 180.0423 Da
Heavy atoms / total 13 / 21
Degree of unsaturation 6
Rotatable bonds 3
TPSA 63.60 Ų
H-bond donors 1
H-bond acceptors 4
Fraction Csp³ 0.11
logP (Mannhold) 2.01
logP (Crippen) 1.04
Ionizable groups carboxylic acid, pKa ≈ 4.5
- Molecular weight uses average atomic masses: use it for weighing and stoichiometry.
- Monoisotopic mass uses the most abundant isotope of each element: compare it with high-resolution mass spectra.
- TPSA, logP, H-bond donors and acceptors are the usual drug-likeness descriptors (Lipinski's rule of five, Veber's rules).
- pKa and predicted NMR shifts are quick estimates, useful for a first look rather than a final answer.
For the full isotope pattern behind the monoisotopic mass,
see simulating a mass
spectrum.
Try it yourself. ChemPencil runs on macOS 12 and later and on Windows 10 and 11.