Low-frequency vibrational modes and static vibrational hyperolarizabilities of long-chain molecules: polyenes and polyacetylene

A model for calculating vibrational frequencies of Transverse Acoustical Mo des (TAM) of all-trans polyenes of any length is presented. Based on the re sults obtained, the relevance of TAM modes in determining static vibrationa l polarizability and second vibrational hyperpolarizability of very long po lyenes (polyacetylene) in real materials is discussed. The major role of th e Effective Conjugation Coordinate (ECC) in determining the second vibratio nal hyperpolarizability is also presented and compared to that of the TAM m odes. The results obtained point out the limitations of calculations on isolated molecules when the behaviour of real materials in the low-frequency domain is under investigation. (C) 2000 Elsevier Science B.V. All rights reserved.