Arsenic containing heterocycles. II. Confirmation of the structure of benzo[a]‐9‐chlorophenarsazine 12‐chloride by 13C‐NMR spectroscopy and spin‐lattice relaxation time studies: Possible evidence for the contribution of an 75as‐13C mechanism to the relaxation of the arsenic bearing quarternary carbons in the phenarsazine ring system