Published: March 08, 2011 r2011 American Chemical Society 2533 dx.doi.org/10.1021/jp111974d | J. Phys. Chem. A 2011, 115, 2533–2539 ARTICLE pubs.acs.org/JPCA Isotopic Study of New Fundamentals and Combination Bands of Linear C 5 ,C 7 , and C 9 in Solid Ar Eric Gonzalez, C. M. L. Rittby, and W. R. M. Graham* Department of Physics and Astronomy, Texas Christian University, Fort Worth, Texas 76129, United States 1. INTRODUCTION C n carbon clusters have been of spectroscopic interest for many years, particularly because of their roles in the chemistry of the interstellar medium and circumstellar shells. Understanding their structures and how they bond has been a challenge for theorists and experimentalists alike. In the first systematic experimental investigation Weltner and McLeod 1 reported infra- red fundamentals for C n species formed by trapping the products of graphite evaporation in Ne matrices. Later, the discovery of C 60 fullerene 2 stimulated renewed interest in carbon chains, rings, and clusters. Despite considerable work by many investi- gators focused on small C n species (n e 12), the frequencies of many vibrational fundamentals remain unmeasured. Linear C 5 has been extensively studied with several spectro- scopic techniques both in the gas phase and in matrices. 1,3-16 All the fundamentals have been assigned except for the ν 1 (σ g þ ) symmetric stretch. In the present work FTIR (Fourier transform infrared) measurements of frequencies and 13 C isotopic shifts coupled with DFT (density functional theory) calculations have enabled the assignment of the (ν 1 þ ν 4 ) combination band of linear C 5 . Since the ν 4 (σ u þ ) fundamental has been previously observed in an Ar matrix, 15 it has now been possible to obtain a frequency for the ν 1 (σ g þ ) fundamental. Linear C 9 has similarly been the subject of many spectroscopic studies, 1,3,11,14,17-24 but of the eight stretching fundamentals, the three symmetric modes, ν 1 (σ g þ ), ν 2 (σ g þ ), and ν 8 (σ g þ ), have yet to be observed. In the present study, the (ν 2 þ ν 7 ) combination band has been measured, and using the frequency of the ν 7 (σ u þ ) mode reported earlier in an Ar matrix, 24 a frequency for the ν 2 (σ g þ ) fundamental has been obtained. Earlier isotopic studies of carbon clusters trapped in Ar by Vala and co-workers have included measurements of combination bands of C 3 (ref 25) and C n (n= 5, 6, 7, 9). 26,27 The present work, with the observation of new combination bands of C 5 and C 9 , complements those studies. Extensive studies of linear C 7 in both the gas phase and in matrices. 28,29,26,30-38 have resulted in the assignment of all the infrared active, asymmetric stretching modes with the exception of ν 6 (σ u þ ), which is predicted to be very weak. The FTIR measurements and DFT calculations performed as part of the present study have enabled the assignment of this fundamental. 2. THEORY DFT calculations were performed using the Gaussian 03 suite 39 with the B3LYP 40-42 functional (Becke type 3 exchange and the correlation functional of Lee, Yang, and Parr) and cc- pVDZ basis set. Currently, this program suite does not include calculations of the anharmonic corrections to the fundamentals, combination bands, or overtones of molecules having a point group with degenerate irreducible representations. Since this is the case for linear molecules, in the present study of combination bands of C 5 and C 9 the chains have been slightly bent to give a distorted structure of C 2v symmetry. The global minima for the distorted structures are shifted respectively, only 2 Â 10 -3 and 4 Â 10 -4 kcal/mol from those of the linear structures, and the frequencies of the stretching modes of the distorted isomers differ by only ∼0.1 cm -1 from their linear counterparts. Using the C 2v structures, however, has permitted the calculation of anharmonic corrections for the fundamentals, combination bands, and overtones of vibrational transitions including stretch- ing modes only. Table 1 lists the harmonic fundamentals calculated for linear C 5 and the anharmonic fundamentals for the structure slightly distorted from the linear isomer. The anharmonic bending mode frequencies are not shown; a singularity occurs because of pairs of close lying energy levels, which result from the distorted struc- ture. Table 2 gives similar results calculated for the stretching fundamentals of C 9 . The isotopic shifts for the combination bands (ν 1 þ ν 4 ) of C 5 and (ν 2 þ ν 7 ) of C 9 , which are presented in Tables 3 and 4, were computed using the distorted structures. Received: December 16, 2010 Revised: February 6, 2011 ABSTRACT: A high yield of carbon chains has been produced by the laser ablation of carbon rods having 13 C enrichment. FTIR spectroscopy of these molecules trapped in solid Ar has resulted in the identification of two new combination bands for linear C 5 and C 9 . The (ν 1 þ ν 4 ) combination band of linear C 5 has been observed at 3388.8 cm -1 , and comparison of 13 C isotopic shift measurements with the predictions of density functional theory calculations (DFT) at the B3LYP/cc-pVDZ level makes possible the assignment of the ν 1 (σ g þ ) stretching fundamental at 1946 cm -1 . Similarly, the observation of the (ν 2 þ ν 7 ) combination band of linear C 9 at 3471.8 cm -1 enables the assignment of the ν 2 (σ g þ ) stretching fundamental at 1871 cm -1 . The third and weakest of the infrared stretching fundamentals of linear C 7 , the ν 6 (σ u þ ) fundamental at 1100.1 cm -1 , has also been assigned.