[en] Based on classical and quantum mechanical calculations, we study the possibility of generating ultrashort X-ray bursts, free of attosecond frequency modulation (attochirp), with photon energies of the order of 1 keV and higher in Coulomb collisions of nuclei in an ensemble of diatomic molecules containing nuclei with different masses, driven by ultraintense IR laser pulses. The minimally attainable duration of the generated X-ray pulses is investigated as a function of intensity and wavelength of laser radiation. It is shown that the considered mechanism of nuclear collisions makes it possible to generate transform-limited X-ray pulses with a duration of about 1 as using modern petawatt laser systems. (paper)