Semi-experimental equilibrium structure of pyrazinamide from gas-phase electron diffraction. How much experimental is it?

Abstract A semi-experimental equilibrium structure of free molecules of pyrazinamide has been determined for the first time using gas electron diffraction method. The refinement was carried using regularization of geometry by calculated quantum chemical parameters. It is discussed to which extent is the final structure experimental. A numerical approach for estimation of the amount of experimental information in the refined parameters is suggested. The following values of selected internuclear distances were determined (values are in A with 1 σ in the parentheses): r e (C pyrazine —C pyrazine ) av  = 1.397(2), r e (N pyrazine —C pyrazine ) av  = 1.332(3), r e (C pyrazine —C amide ) = 1.493(1), r e (N amide —C amide ) = 1.335(2), r e (O amide —C amide ) = 1.219(1). The given standard deviations represent pure experimental uncertainties without the influence of regularization.