As part of a global analyis of the infrared spectrum of chloromethane, the region of the fifth polyad involving the upper states of seven interacting bands 2υ6, υ2+υ3, υ3+υ5, 3υ3, υ2+υ6, υ5+υ6, 2υ3+υ6, from 1900 to 2600 cm-1 has been re-investigated. More than 20 000 transitions for each isotopomer 12CH335Cl and 12CH337Cl have been assigned and fitted with a standard deviation better than 4 10-4 cm-1 close to the experimental precision. The present paper is focussed on the determination of the ground state constants of 12CH337Cl by exploiting in a global theoretical model perturbation-allowed transitions already observed for 12CH335Cl by Di Lauro and Alamichel (J. Mol. Spectrosc. 81, 390-412 (1980)).