Evidence for solution-state nonlinearity of sp-carbon chains based on IR and Raman spectroscopy: violation of mutual exclusion.
Adamantyl-end-capped polyynes with chains of 4, 6, 8, 10, 12, 16, and 20 sp-hybridized carbons (C4-C20) have been synthesized and their IR and Raman spectra obtained. On the basis of violations of the mutual-exclusion principle between IR and Raman spectroscopy, spectral evidence demonstrates that these molecules possess a noncentrosymmetric molecular structure in both the solid and solution states. This premise is supported by X-ray crystallographic analysis of C12, which shows a bent, noncentrosymmetric structure in the solid state. Density functional theory (DFT) calculations for adamantyl-end-capped polyynes, in comparison with those for hydrogen-end-capped polyynes, show that the observed violation of mutual exclusion is independent of the end group of the polyyne chain (i.e., adamantyl versus H). The origin of these experimental spectroscopic observations is ascribed to the existence of dynamic contributions to molecular nonlinearity resulting from low-frequency skeletal bending vibrations of the chains and/or the existence of low-energy bent conformations of the polyyne chains, as DFT-optimized structures seem to suggest.
Lucotti
Andrea
A
Politecnico di Milano, Dipartimento di Chimica, Materiali e Ingegneria Chimica G. Natta Piazza Leonardo da Vinci 32, 20133 Milano, Italy.
Tommasini
Matteo
M
Fazzi
Daniele
D
Del Zoppo
Mirella
M
Chalifoux
Wesley A
WA
Ferguson
Michael J
MJ
Zerbi
Giuseppe
G
Tykwinski
Rik R
RR
D002244:Carbon / Q000737:chemistry*; D002625:Chemistry, Organic / Q000379:methods; D018360:Crystallography, X-Ray; D006859:Hydrogen / Q000737:chemistry; D008956:Models, Chemical; D008968:Molecular Conformation; D015394:Molecular Structure; D012984:Software; D012996:Solutions; D017550:Spectroscopy, Fourier Transform Infrared / Q000379:methods*; D013059:Spectrum Analysis, Raman / Q000379:methods*
D012996:Solutions; D002244:Carbon; D006859:Hydrogen