The "silent CO": a new technique for calculating transition metal carbonyl force fields

The notion of a "silent CO group" (effectively an infinitely heavy CO group) is introduced to enable energy-factored force fields to be estimated accurately for molecules where there are fewer ν(CO) frequencies than force constants in the force field (viz. underdetermined force fields). The symmetry classes of molecules covered are the C_s tricarbonyls (e.g. Fe(CO)₃(diene) and fac-Re(CO)₃(L-L)X), C_2v tricarbonyls (e.g. mer-M(CO)₃(L)₃ M = Cr, Mo, W), C_3v tetracarbonyls (e.g. Fe(CO)₄(L)), C_2v tetracarbonyls (e.g. cis-M(CO)₄(L)₂ and Fe(CO)₄(L)) and C_4v pentacarbonyls (e.g. M(CO)₅(L) M = Cr, Mo, W and M(CO)₅(X) M = Mn, Re). It is a relatively simple matter to extend the method to types of molecules not directly considered in this paper.