Modern Polymer Spect..

t
Order/Disorder in Chain Molecules and
Polymers
G Zerhi mid M. Del Zoppo
3.1 Introduction
At present the theoretical calculation of vibrational properties of polyatomic molecules
can be carried out with reasonable success with the help of large, fast computers
using filly automatic computing programs. Normal mode calculations have
developed along three main lines of thought. On the one hand, a major effort has
been made by the pioneers of vibro-rotational spectroscopy with the aim of determining
accurate ro-vibrational constants to determine harmonic and/or anharinonic
vibrational force fields and the derived physical properties [ 11. Once norinal
modes are known accurately (both fundamental frequencies and relative vibrational
displacements [2, 3]), they become the essential ingredients for the treatment of
vibrational intensities in infrared and scattering cross-sections in Raman [4-61 or
neutron-scattering experiments [7]. A huge effort along these lines has been made
by many authors in the past 50 years, with the pioneering works froin the group of
B.L.C. Crawford at Minnesota and of T. Shimanouchi at Tokyo having been followed
by many authors. The basic mathematics and the computing programs first
derived by Curtiss [S] and then by Schachtschneider and Snyder [9] and freely distributed
worldwide [ 101 have been reproduced with different names and are used
ubiquitously and on most occasions without due mention and thanks to the original
authors for their great efforts.
A second school of thought has actively pursued the idea of calculating vibrational
properties froin ‘ah initio’ quantum chemical calculations [ 1 I]. The underlying
concept is that if optimal atomic wavefunctions (and if large, fast computers)
are available all physical observables can be calculated crb initio, thus making
nonnal-mode calculations from empirical force constants obsolete. A major effort is
currently being made to assess critically the reliability of Lib initio calculations in the
prediction of spectroscopic observables [ 11-14].
A third group of workers has introduced the idea that vibrational force constants
derived from critically determined semiempirical potentials provide an easy way
to calculate vibrational properties, as well as many other structural and dynaniical
molecular properties. These semiempirical potentials are part of fully automated
coinputer packages offered commercially and used widely. For expressing judgment
on the reliability of these semiempirical potentials related to spectroscopic properties,
one matter is to provide a qualitative description of molecular dynamics