Copyright (c) 2009 All rights reserved. http://works.bepress.com/nancy_richardson Recent documents in Nancy A. Richardson en-us Sat, 20 Jun 2009 11:55:58 PDT 3600 http://works.bepress.com/nancy_richardson/2 http://works.bepress.com/nancy_richardson/2 Thu, 18 Jun 2009 12:26:53 PDT The singlet ground _X 1_+_ and excited _1_− , 1__ states of HCP and HPC have been systematically investigated using ab initio molecular electronic structure theory. For the ground state, geometries of the two linear stationary points have been optimized and physical properties have been predicted utilizing restricted self-consistent field theory, coupled cluster theory with single and double excitations _CCSD_, CCSD with perturbative triple corrections _CCSD_T__, and CCSD with partial iterative triple excitations _CCSDT-3 and CC3_. Physical properties computed for the global minimum _X 1_+HCP_ include harmonic vibrational frequencies with the cc-pV5Z CCSD_T_ method of _1=3344 cm−1, _2=689 cm−1, and _3=1298 cm−1. Linear HPC, a stationary point of Hessian index 2, is predicted to lie 75.2 kcal mol−1 above the global minimum HCP. The dissociation energy D0_HCP_X 1_+_→H_2S_+CP_X 2_+__ of HCP is predicted to be 119.0 kcal mol−1, which is very close to the experimental lower limit of 119.1 kcal mol−1. Eight singlet excited states were examined and their physical properties were determined employing three equation-of-motion coupled cluster methods _EOM-CCSD, EOM-CCSDT-3, and EOM-CC3_. Four stationary points were located on the lowest-lying excited state potential energy surface, 1_− →1A_, with excitation energies Te of 101.4 kcal mol−1_1A_ HCP_, 104.6 kcal mol−1_1_− HCP_, 122.3 kcal mol−1_1A_ HPC_, and 171.6 kcal mol−1_1_− HPC_ at the cc-pVQZ EOM-CCSDT-3 level of theory. The physical properties of the 1A_ state with a predicted bond angle of 129.5° compare well with the experimentally reported first singlet state _A 1A__. The excitation energy predicted for this excitation is T0=99.4 kcal mol−1_34 800 cm−1 , 4.31 eV_, in essentially perfect agreement with the experimental value of T0=99.3 kcal mol−1_34 746 cm−1 ,4.308 eV_. For the second lowest-lying excited singlet surface, 1_→1A_, four stationary points were found with Te values of 111.2 kcal mol−1 _21A_ HCP_, 112.4 kcal mol−1 _1_ HPC_, 125.6 kcal mol−1_2 1A_ HCP_, and 177.8 kcal mol−1_1_ HPC_. The predicted CP bond length and frequencies of the 2 1A_ state with a bond angle of 89.8° _1.707 Å, 666 and 979 cm−1_ compare reasonably well with those for the experimentally reported C 1A_ state _1.69 Å, 615 and 969 cm−1_. However, the excitation energy and bond angle do not agree well: theoretical values of 108.7 kcal mol−1 and 89.8° versus experimental values of 115.1 kcal mol−1 and 113°. Justin B. Ingels http://works.bepress.com/nancy_richardson/3 http://works.bepress.com/nancy_richardson/3 Thu, 18 Jun 2009 12:26:53 PDT Density functional theory (DFT) has been used to investigate the conformations and thermochemistry on the singlet and triplet potential energy surfaces (PES) of Cr2(CO)10 . The global minimum energy structure for the lowest singlet state of C2h symmetry is consistent with a model of two interacting Cr(CO)5 fragments in which one carbonyl in each fragment acts as an asymmetric four-electron donor bridging carbonyl, with chromium-chromium distances of 2.93 A ˚ (B3LYP) or 2.83 A ˚ (BP86). Avoiding a CrCr bond by incorporating four-electron donor CO groups in this way allows each chromium atom in singlet Cr2(CO)10 to attain the favored 18-electron configuration by using, in a simple picture of the bonding, only the six octahedral sp3d2 hybrids. The dissociation energy to two Cr(CO)5 fragments or to Cr(CO)6+Cr(CO)4 fragments is predicted to be 10 kcal mol_1. The lowest triplet state of Cr2(CO)10 is predicted to lie _10 kcal mol_1 above the singlet global minimum. In the case of triplet Cr2(CO)10 the lowest energy minima were found to be of C2 and C2h symmetry, with similar energies. The chromium-chromium distances in triplet Cr2(CO)10 were found to be shorter than those in the corresponding singlet structures, namely 2.81 (B3LYP) or 2.68 A ˚ (BP86) suggesting a s+2(1/2) p Cr=Cr double bond similar to the O=O bond in O2 or the Fe=Fe bond in the experimentally observed triplet state (Me5C5)2Fe2(m-CO)3 . Se Li http://works.bepress.com/nancy_richardson/1 http://works.bepress.com/nancy_richardson/1 Thu, 18 Jun 2009 12:26:52 PDT A range of highly correlated ab initio methods is used to predict the geometrical parameters of silicon cyanide (SiCN), silicon isocyanide (SiNC), and two transition states (2A' and 2A'') for the isomerization reaction transforming one to the other. Also predicted are dipole moments, rotational constants, and harmonic vibrational frequencies. At all levels of theory, the SiCN and SiNC molecules are predicted to have linear equilibrium structures. The SiNC isomer is found to lie 1.5 kcal/mol above the SiCN species at the coupled cluster (CC) with single, double, and full triple excitations (CCSDT) level of theory with the correlation-consistent polarized valence quadruple zeta (cc-pVQZ) basis set. These theoretical predictions complement the recent laboratory production of SiCN and SiNC and subsequent astronomical detection of SiCN in the envelope of the C Star IRC110216/CW Leo. The theoretical Be values of 5481 MHz (SiCN) and 6316 MHz (SiNC) at the CC with single, double, and iterative partial triple excitations (CCSDT-3) level of theory with the cc-pVQZ basis set are consistent with the experimental B0 values of 5543 MHz (SiCN) and 6397 MHz (SiNC). The transition states for the isomerization reaction SiCN->SiCN are found to proceed through the 2A' and 2A'' surfaces, which lie 20.9 and 21.8 kcal/mol above the SiCN minimum at the cc-pVQZ CCSDT-3 level of theory. The ground states of SiCN and SiNC radicals are subject to Renner-Teller interactions. At the CC with single, double, and perturbative triple excitations [CCSD(T)] level of theory, the Renner parameters and the averaged harmonic bending vibrational frequencies are determined to be 0.318 and 249 cm-1 for SiCN and 0.412 and 195 cm-1 for SiNC. Nancy A. Richardson