Published: June 09, 2011 r2011 American Chemical Society 8119 dx.doi.org/10.1021/jp204844j | J. Phys. Chem. A 2011, 115, 8119–8124 ARTICLE pubs.acs.org/JPCA Singlet Excited States of Silicon-Containing Anions Relevant to Interstellar Chemistry Ryan C. Fortenberry and T. Daniel Crawford* Department of Chemistry, Virginia Tech, Blacksburg, Virginia 24061, United States ’ INTRODUCTION Recent interstellar detections of the C n H  anions for n = 4, 6, 8 (refs 15) and the related C n N  anions for n = 1, 3, 5 (refs 68) have shown that the interstellar medium (ISM) is rich with negatively charged molecular species. It has been suggested by Sarre and co-workers 9,10 that excited states of anions bound by dipole forces may play a role in the explanation of the longest standing problem in spectroscopy, that of the carriers of the diffuse interstellar bands (DIBs). Although one hypothesis currently under scrutiny 11 suggests that a few of these unattrib- uted interstellar absorption features, which stretch from the UV to near-IR and are observed from numerous astronomical sources, 12 may be explained by propadienylidene, the carriers for the vast majority of the remaining lines are completely unknown. Valence- or dipole-bound excited states of anions may help to explain some of these features. 10 Most anions are deionized upon exposure to radiation and have no excited states because their associated electron binding energies (eBEs) are on the order of millielectronvolts. 1315 However, a select few behave differently. As explained by Simons, 14 some anions may possess an excited state if the radical’s dipole moment is strong enough to bind the extra electron within the system after excitation just below the electron binding energy (eBE). This results in a so-called dipole-bound excited state or threshold resonance. Rarer still are those anions that also possess valence-bound excited states well below the eBE or dipole-bound state. In such states, the excited wave function is predominantly characterized by excitations into valence, non- diffuse virtual orbitals 14 and is, hence, independent of the need for a strong dipole moment. Few anions possess multiple excited states of both valence- and dipole-bound character, but one of the first found was the radical anion 7,7,8,8-tetracyanoquinodi- methane (TCNQ) 16 and, later, its tetra-fluorinated cousin. 17 Recent theoretical work in our group 18 used coupled cluster theory to examine fourteen closed-shell anions for their electronic excited-state properties. Two, CH 2 CN  and CH 2 CHO  , are known to exhibit singlet dipole-bound excited states 10,19 just below their eBEs. Our methods reproduced adiabatic eBEs for each of these known anions to within 0.06 eV and adiabatic excitation energies to within 0.05 eV. It was also shown that to adequately treat the diffuse nature of excited electrons, doubly-, triply-, and even quadruply-augmented basis sets are necessary. With this knowledge, six anions (CH 2 SiN  , SiH 2 CN  , CH 2 SiHO  , SiN  , CCOH  , and HCCO  ) were predicted to possess singlet dipole-bound excited states below their eBEs, and CH 2 SiN  also was predicted to have at least one valence-bound excited state below its dipole-bound state. The detection of silicon-containing compounds in the ISM is well documented, 20 and silicon is one of the more common atom types known to exist in the universe. 21 Our previous study 18 showed that CN  and C 3 N  , both of which are known to exist in the ISM, 6,8 do not possess singlet valence- or even dipole-bound excited states, 22 but SiN  may exhibit a singlet dipole-bound state just above a triplet valence state 23 below the eBE. Hence, the present work utilizes the same theoretical approach used previously to examine the excited states of the C 3 N  analogues with single silicon substitutions: SiCCN  , CSiCN  , and CCSiN  . Addition- ally, the SiNC and SiCN radicals are known to exist in the ISM, 24,25 and we also examine the excited states of the corresponding anions for these interstellar molecules in the same fashion. ’ COMPUTATIONAL DETAILS Following our previous approach, 18 the closed-shell anion computations involved the use of spin-restricted (RHF) 26 re- ference wave functions while open-shell computations of the neutral radicals were based upon spin-unrestricted (UHF) 27,28 Received: May 24, 2011 Revised: June 7, 2011 ABSTRACT: As the number of anions detected in the interstellar medium (ISM) increases, knowledge of their chemical properties is crucial in expanding our understanding of the chemistry of space. In this work we build on a previous study done in our group to examine the excited-state properties of five anions likely to exist in the ISM: SiCCN  , CSiCN  , CCSiN  , SiCN  , and SiNC  . Our coupled cluster results indicate that SiCCN  and SiNC  possess dipole-bound singlet excited states while SiCCN  also has one valence state and CCSiN  potentially has two. Nearly all of the associated transition energies fall within the visible to near-IR region of the electromagnetic spectrum, making them applicable to the study of phenomena such as the diffuse interstellar bands.