摘要:

1IntroductionSince the pioneering work by Snyder1–3and Schachtschneider,3vibrational spectroscopy of molecular chains of finite length such asn-alkanes, CH3–(CH2)n−2–CH3, has provided a challenging sequence of case studies from several viewpoints. A particularly interesting issue of concern is to establish a reliable assignment of observed CH vibrational modes in both infrared and Raman spectra. Moreover, the specific role of anharmonic (Fermi) resonances, involving overtone and combination CH bending modes, is still to be convincingly settled. Such couplings have been described with sensible accuracy in the Raman spectra of solid and liquid samples ofn-alkanes and of several polymers in the region of fundamental CH stretches (3000 cm−1).4–7It has been also shown that similar mechanisms could be considered to explain some minor features in the infrared spectrum of this same energy region.8,9More recently, the infrared spectra of the first CH stretching overtone region of n-butane, n-pentane, n-heptane and polyethylene (in both solid and liquid states) have been measured and reported.10In the present work, we adopt a one-dimensional algebraic (vibron) model to obtain, in terms of a unique set of parameters, energies and infrared intensities of both fundamental and first overtone modes of CH stretches ofn-alkanes (arbitraryn). The algebraic parameters are optimized to reproduce fundamental infrared spectra in the gas phase. The algebraic Hamiltonian leads to a very good agreement forv= 2 CH stretching modes as well. In particular, we show that anharmonic resonances arenotof dramatic relevance to obtain such an agreement, which is instead based on a careful calibration of the electric dipole algebraic representation. A further refinement of the computed spectra is however achieved by adding quite a typical 1 ∶ 2 anharmonic resonant term involving CH bending modes. Raman intensities will be addressed in a forthcoming paper.The use of an algebraic (vibron) model has some advantages over more traditional approaches, the most appealing one being related to the intrinsically anharmonic nature of the Morse basis which is the building block of this model. Such an aspect becomes important as soon as one addresses vibrational overtone/combination manifolds of levels, which is in fact a point of concern in the present work. Moreover, there are still controversial results concerning reliable values ofab initiodipole surfaces of carbon based molecular chains such asn-paraffins. Our approach, even if quite simple in these respects, can nonetheless give a fair contribution to such a discussion since it provides a straightforward way to compute local bond dipole operators, as already discussed elsewhere and shown in the following.

ISSN:1460-2733    

DOI:10.1039/b304717b

出版商:RSC    

年代:2003

数据来源: RSC