A complete vibrational analysis of liquid crystals 4-alkyl 4'-cyano biphenyls [CnH2n+1-C6H5-C6H5-CN, n=9, 10 and 11] is presented. The vibrational assignments are discussed in relation to their respective crystal structures. The vibrational bands in the CH stretching as well as CH2 wagging regions are effectively correlated to the crystal structures of the respective compounds. The ratios of IR and Raman intensities of the bands in the CH stretching region reveal the presence of disorder in the compounds.