Theoretical Studies On The Structures And Spectroscopic Properties Of Carbon Chain Clusters

Carbonaceous molecules are widely present in interstellar and circumstellar mediums. They play an important role in the formation of stars, protoplanetary disks, planetesimals, and in the study of the origin of life. Most of these molecules are carbon clusters, including pure carbon clusters (C2, C3, etc.) and substituted carbon clusters (C8H, C2S, HC11N, etc.). Carbon clusters are also found to be present in hydrocarbon flames and other soot-forming systems. They are thought to be intermediates for many chemical reactions. Some kinds are reported to have extraordinary electronic properties. They are correlative in astrophysics, cosmic chemistry, combustion, molecular electronics and material science. So studies on them have attracted much attention.Carbon chain clusters can be divided into two types, cumulene and polyyne. However, carbon clusters have high reactivity, which are difficult to produce in laboratory. Computational chemistry doesn't need to synthesize or extract matter in laboratory, but can predict various properties of molecules. And the results usually show reliable agreement with the experiment. Quantum chemistry, as a part of computational chemistry, can do high accuracy calculations on small systems. A thorough understanding of the excited-state properties is important for material science, luminescence, and chemistry of the interstellar medium. As a result, the aim of this dissertation is to study the structures and electronic spectra of carbon chain clusters by quantum chemical calculations, and supply theoretical basis to experiments and astronomical observations. The selected systems are typical and extensive, including carbon chain cations C_NH⁺, isoelectronic carbon chains C_NS, C_NCl⁺, valence isoelectronic carbon chain anions C_2nO–, C_2nS–, C_2nSe–, and disubstituted neutral carbon chains PC_2nP. The computational methods are as follows: The geometries of the ground states are optimized with density functional theory.The vibrational frequencies and IR intensities are also calculated after the geometry optimization at the same level of theory. The geometries of the selected excited states are optimized with Complete Active Space Self Consistent Field (CASSCF) method. After the geometries are located, the vertical excitation energies, vertical emission energies and vertical detachment energies are computed with Complete Active Space Second order Perturbation Theory (CASPT2), which is known as one of the most accurate methods to calculate excited states. The corresponding transition dipole moments are calculated to obtain the oscillator strengths. For C_NS and C_NCl⁺, the single-triple transition probability is estimated by Spin Orbit Coupling Configuration Interaction (SOC-CI) approach.It reveals that theoretically predicted values show good agreement with the available observed values. The calculations not only give the assignment of the absorption peaks in previous experiments, but also predict other new peaks and the electronic spectra of the systems that haven't been observed in lab. Furthermore, many properties of such substituted carbon chain clusters exhibit parity effect. See below in detail:1. In general, for the carbon chains considered here, almost all the linear structures are stable. For C_NH⁺, C_NS and C_NCl⁺, when N is odd, the linear ground state is a singlet state X¹Î£⁺ with the electronic configuration as···π⁴σ2Ï€⁰ or···σ²Ï€⁴Ï€⁰; when N is even, the ground state is a triplet state X³Î£^- with the electronic configuration as···π⁴σ2Ï€²Ï€⁰. For valence isoelectronic species C_2nO^-, C_2nS^- and C_2nSe^-, the ground state is X2Πwith the electronic configuration as···π⁴σ2Ï€³Ï€⁰σ0. The PC_2nP polyynes have the singlet ground state X1Σg⁺ with an electronic configuration of···(Ï€_u)⁴(Ï€_g)⁴(Ï€_u)⁰(Ï€_g)⁰ (n-odd) or···(Ï€_g)⁴(Ï€_u)⁴(Ï€_g)⁰(Ï€_u)⁰ (n-even).2. As the chain size lengthens, C–X (X = H, O, S, Se) bond lengths in C_NH⁺, C_2nS, C_2nO–, C_2nS– and C_2nSe– decrease slightly; C–X (X = S, Cl) bond lengths in C_2nCl⁺, C_2n-1Cl⁺ and C_2n-1S increase slightly; C–X (X = P) bond lengths in PC_2nP keep almost the same. In C_NS, the C-C bonds have a cumulenic structure, and tend to be equalization. While in ions C_NH⁺, C_NCl⁺, C_2nO–, C_2nS– and C_2nSe–, the bond lengths of C-C bonds near the heteroatoms have a single-triple alternate bonding character. This acetylenic bonding may disperse the charge in such carbon clusters through conjugation interaction and it will stabilize the ionic clusters.3. The calculated IR frequencies show that the minimum bending frequencies are very small. With the chain lengthens, they decrease, and the corresponding intensities also decrease, nearly vanished. Such molecules are floppy.4. With the increase of n, the dipole moments increase, while the rotational constants decrease. In general, with the same number of carbon atoms, the dipole moments increase in the order of C_NCl⁺NH^+C_NSNO?NS?NSe?.5. Generally, the heteroatoms in C_NCl⁺, C_NH⁺ and C_NS bear positive charges, while in C_NO?, C_NS? and C_NSe?, they have negative charges. As N increases, the charges on the heteroatoms decrease. For all the chains considered here, the second atoms on both ends of all these carbon chains have negative charges, while the carbon atoms in the middle of the chains have positive charges.6. The predicted vertical excitation energies of C_NH⁺, C_NS and C_NCl⁺ have a similar pattern with odd-N and even-N respectively, and so do C_2nO^-, C_2nS^- and C_2nSe^-. In general, as the carbon chain lengthens, the vertical excitation energies to the same excited states decrease gradually, and the vertical emission energies exhibit similar behavior. The vertical emissions exhibit less red shift due to relaxation of the excited state with respect to their corresponding absorption spectra.7. SOC-CI calculations show that only the forbidden 2¹Î£⁺←X³Î£^- excitation for C_2nS and C_2nCl⁺ may take place. But for C_2n-1S and C_2n-1Cl⁺, the oscillator strengths for singlet-triplet transitions are generally very small and these excited triplet states in both isoelectronic series are less accessible by direct singlet-triplet transitions.8. The vertical transition energies (E_T) and ionization energies (IE) of PC_2nP are compared with those of linear NC_2nN, HC_2n⁺1N, NC_2nP and HC_2n+1P, and it is discovered that ET and IE of PC_2nP are the lowest, while those of NC_2nN are the highest. The former can be explained by the different electronic delocalization effect, and the latter can be explained by electronegativies of the heteroatoms. An explicit non-linear analytical expression between E_T (or IE) and n has been obtained, from which we can predict ET (or IE) with larger n.