The values of a, for which the points $A, B, C$ with position vectors $2\widehat i - \widehat j + \widehat k,\,\,\widehat i - 3\widehat j - 5\widehat k$ and $a\widehat i - 3\widehat j + \widehat k$ respectively are the vertices of a right angled triangle with $C = {\pi \over 2}$ are :