Rotating & Translating Frame

CSP

Hi,
I'm calculating Normal Modes, and i have problem in separate the Rot./Trans. from the 3N DOF. I need help in generating vectors corresponding to rotational motion in Cartesian coordinates.
I tried to do it as in the next link: http://www.gaussian.com/g_whitepap/vib.htm
but the vectors that i created were not orthogonal (Vr1*Vr2 !=0).
can someone explain how to generate the orthogonal rot. vectors (base on the eigenvectors of the principal moment).

Thanks