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Kanazawa Y, Tsuji H, Ehara M, Fukuda R, Casher DL, Tamao K, Nakatsuji H, Michl J. Electronic Transitions in Conformationally Controlled Peralkylated Hexasilanes. Chemphyschem 2016; 17:3010-3022. [DOI: 10.1002/cphc.201600633] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/14/2016] [Indexed: 11/08/2022]
Affiliation(s)
- Yuki Kanazawa
- SOKENDAI The Graduate University for Advanced Studies, Nishigonaka, Myodaiji Okazaki 444–8585 Japan
- Institute for Molecular Science and Research Center for Computational Science, Nishigonaka, Myodaiji Okazaki 444–8585 Japan
| | - Hayato Tsuji
- International Research Center for Elements Science (IRCELS) Institute for Chemical Research Kyoto University, Uji Kyoto 611-0011 Japan
- Department of Chemistry Faculty of Science Kanagawa University 2946 Tsuchiya Hiratsuka Kanagawa 259–1293 Japan
| | - Masahiro Ehara
- SOKENDAI The Graduate University for Advanced Studies, Nishigonaka, Myodaiji Okazaki 444–8585 Japan
- Institute for Molecular Science and Research Center for Computational Science, Nishigonaka, Myodaiji Okazaki 444–8585 Japan
- Elements Strategy Initiative for Catalysts and Batteries (ESICB) Kyoto University Kyoto 615–8520 Japan
| | - Ryoichi Fukuda
- Elements Strategy Initiative for Catalysts and Batteries (ESICB) Kyoto University Kyoto 615–8520 Japan
| | - Deborah L. Casher
- Department of Chemistry and Biochemistry University of Colorado at Boulder Boulder Colorado 80309-0215 USA
| | - Kohei Tamao
- International Research Center for Elements Science (IRCELS) Institute for Chemical Research Kyoto University, Uji Kyoto 611-0011 Japan
- RIKEN 2-1 Hirosawa Saitama 351-0198 Japan
| | - Hiroshi Nakatsuji
- Quantum Chemistry Research Institute Goryo Oohara 1–36, Nishikyo-ku Kyoto 615–8245 Japan
| | - Josef Michl
- Department of Chemistry and Biochemistry University of Colorado at Boulder Boulder Colorado 80309-0215 USA
- Institute of Organic Chemistry and Biochemistry Academy of Sciences of the Czech Republic Flemingovo nám. 2 16610 Prague Czech Republic
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2
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Su TA, Li H, Zhang V, Neupane M, Batra A, Klausen RS, Kumar B, Steigerwald ML, Venkataraman L, Nuckolls C. Single-molecule conductance in atomically precise germanium wires. J Am Chem Soc 2015; 137:12400-5. [PMID: 26373928 DOI: 10.1021/jacs.5b08155] [Citation(s) in RCA: 31] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
While the electrical conductivity of bulk-scale group 14 materials such as diamond carbon, silicon, and germanium is well understood, there is a gap in knowledge regarding the conductivity of these materials at the nano and molecular scales. Filling this gap is important because integrated circuits have shrunk so far that their active regions, which rely so heavily on silicon and germanium, begin to resemble ornate molecules rather than extended solids. Here we unveil a new approach for synthesizing atomically discrete wires of germanium and present the first conductance measurements of molecular germanium using a scanning tunneling microscope-based break-junction (STM-BJ) technique. Our findings show that germanium and silicon wires are nearly identical in conductivity at the molecular scale, and that both are much more conductive than aliphatic carbon. We demonstrate that the strong donor ability of C-Ge σ-bonds can be used to raise the energy of the anchor lone pair and increase conductance. Furthermore, the oligogermane wires behave as conductance switches that function through stereoelectronic logic. These devices can be trained to operate with a higher switching factor by repeatedly compressing and elongating the molecular junction.
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Affiliation(s)
- Timothy A Su
- Department of Chemistry, Columbia University , New York, New York 10027, United States
| | - Haixing Li
- Department of Applied Physics and Applied Mathematics, Columbia University , New York, New York 10027, United States
| | - Vivian Zhang
- Department of Chemistry, Columbia University , New York, New York 10027, United States
| | - Madhav Neupane
- Department of Chemistry, Columbia University , New York, New York 10027, United States
| | - Arunabh Batra
- Department of Applied Physics and Applied Mathematics, Columbia University , New York, New York 10027, United States
| | - Rebekka S Klausen
- Department of Chemistry, Columbia University , New York, New York 10027, United States
| | - Bharat Kumar
- Department of Chemistry, Columbia University , New York, New York 10027, United States
| | - Michael L Steigerwald
- Department of Chemistry, Columbia University , New York, New York 10027, United States
| | - Latha Venkataraman
- Department of Chemistry, Columbia University , New York, New York 10027, United States.,Department of Applied Physics and Applied Mathematics, Columbia University , New York, New York 10027, United States
| | - Colin Nuckolls
- Department of Chemistry, Columbia University , New York, New York 10027, United States
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Su TA, Li H, Steigerwald ML, Venkataraman L, Nuckolls C. Stereoelectronic switching in single-molecule junctions. Nat Chem 2015; 7:215-20. [PMID: 25698330 DOI: 10.1038/nchem.2180] [Citation(s) in RCA: 117] [Impact Index Per Article: 13.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/15/2014] [Accepted: 01/09/2015] [Indexed: 11/09/2022]
Abstract
A new intersection between reaction chemistry and electronic circuitry is emerging from the ultraminiaturization of electronic devices. Over decades chemists have developed a nuanced understanding of stereoelectronics to establish how the electronic properties of molecules relate to their conformation; the recent advent of single-molecule break-junction techniques provides the means to alter this conformation with a level of control previously unimagined. Here we unite these ideas by demonstrating the first single-molecule switch that operates through a stereoelectronic effect. We demonstrate this behaviour in permethyloligosilanes with methylthiomethyl electrode linkers. The strong σ conjugation in the oligosilane backbone couples the stereoelectronic properties of the sulfur-methylene σ bonds that terminate the molecule. Theoretical calculations support the existence of three distinct dihedral conformations that differ drastically in their electronic character. We can shift between these three species by simply lengthening or compressing the molecular junction, and, in doing so, we can switch conductance digitally between two states.
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Affiliation(s)
- Timothy A Su
- Department of Chemistry, Columbia University, New York 10027, USA
| | - Haixing Li
- Department of Applied Physics, Columbia University, New York 10027, USA
| | | | | | - Colin Nuckolls
- Department of Chemistry, Columbia University, New York 10027, USA
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4
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MacLeod MK, Michl J. From ordinary to blue emission in peralkylated n-oligosilanes: the calculated structure of delocalized and localized singlet excitons. J Phys Chem A 2014; 118:10538-53. [PMID: 25011390 DOI: 10.1021/jp504805y] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/18/2023]
Abstract
Excited singlet state structures believed to be responsible for the Franck-Condon-allowed and the strongly Stokes-shifted (blue) emissions in linear permethylated oligosilanes (Si(n)Me(2n+2)) have been found and characterized with time-dependent density functional (TD-DFT) methods for chain lengths 4 ≤ n ≤ 16. For chain lengths with n > 7, the S1 relaxed structures closely resemble the S0 equilibrium structures where all valence angles are tetrahedral and all backbone dihedral angles are transoid. At chain lengths with n < 8 more strongly distorted structures with one long Si-Si bond built from silicon 3p orbitals are encountered. The large Stokes shift is due more to a large destabilization of the ground state than the relaxation in the S1 excited state. For n = 7, both types of minima were located, exactly reproducing the borderline between the large-radius and the small-radius self-trapped excitons known from experiments.
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Affiliation(s)
- Matthew K MacLeod
- Department of Chemistry and Biochemistry, University of Colorado , Boulder, Colorado 80309-0215, United States
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Tsuji H, Fogarty HA, Ehara M, Fukuda R, Casher DL, Tamao K, Nakatsuji H, Michl J. Electronic transitions in conformationally controlled tetrasilanes with a wide range of SiSiSiSi dihedral angles. Chemistry 2014; 20:9431-41. [PMID: 25043859 DOI: 10.1002/chem.201403495] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/12/2014] [Indexed: 11/08/2022]
Abstract
Unlike π-electron chromophores, the peralkylated n-tetrasilane σ-electron chromophore resembles a chameleon in that its electronic spectrum changes dramatically as its silicon backbone is twisted almost effortlessly from the syn to the anti conformation (changing the SiSiSiSi dihedral angle ω from 0 to 180°). A combination of UV absorption, magnetic circular dichroism (MCD), and linear dichroism (LD) spectroscopy on conformationally controlled tetrasilanes 1-9, which cover fairly evenly the full range of angles ω, permitted a construction of an experimental correlation diagram for three to four lowest valence electronic states. The free chain tetrasilane n-Si4 Me10 (10), normally present as a mixture of three enantiomeric conformer pairs of widely different angles ω, has also been included in our study. The spectral trends are interpreted in terms of avoided crossings of 1B with 2B and 2A with 3A states, in agreement with SAC-CI calculations on the excited states of 1-7 and conformers of 10.
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Affiliation(s)
- Hayato Tsuji
- International Research Center for Elements Science (IRCELS), Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011 (Japan); Present address: Department of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku,Tokyo 113-0033 (Japan)
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Bande A, Michl J. Conformational dependence of sigma-electron delocalization in linear chains: permethylated oligosilanes. Chemistry 2009; 15:8504-8517. [PMID: 19658142 DOI: 10.1002/chem.200901521] [Citation(s) in RCA: 51] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
Abstract
The effects of sigma-electron delocalization on optical properties of saturated linear chains of permethylated oligosilanes are strongly conformation dependent. We analyze the origin of the conformational dependence of the energies of molecular orbitals and of electronic excitations in simple intuitively understandable terms by using a first-order approximation to the Hückel version of the "Ladder C" model. The analysis is supported by comparison with results of numerical calculations from time-dependent density functional theory, which agree well with experiment. To facilitate the comparison, a simple procedure has been developed that defines the overall and local fractional sigma and pi characters of a backbone molecular orbital and a fractional overall and local sigma sigma* and sigma pi* characters of an excited state for any conformation of a linear chain.
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Affiliation(s)
- Annika Bande
- Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO 80309-0215, USA
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Piqueras MC, Crespo R, Michl J. Interpretation of the electronic spectra of four disilanes. J Phys Chem A 2008; 112:13095-101. [PMID: 18821735 DOI: 10.1021/jp804677v] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Time-dependent density functional theory (TD-DFT/B3LYP(AC)/cc-pVTZ/cc-pVTZ/6-311G//MP2/cc-pVTZ/cc-pVTZ/6-31G**) has been used to compute vertical excitation energies and oscillator strengths of the six low-lying excited states of four peralkylated disilanes, hexamethyldisilane (1), hexa-tert-butyldisilane (2), 1,6-disila[4.4.4]propellane (3), and 1,7-disila[5.5.5]propellane (4). The results provide an accurate interpretation of the reported UV absorption spectra of 1-4 in solution, and for 1 also in the gas phase up to 62,000 cm(-1). The excellent agreement of the calculated with the available experimental energies and oscillator strengths, and with magnetic circular (MCD) and linear (LD) dichroism, gives us confidence that the method will be useful for dependable interpretation of the electronic spectra of longer oligosilanes. Although the disilane chromophore finds itself in quite different environments in 1-4, its fundamental characteristics remain the same, with one important exception. In all four compounds, the first valence excited state is due to an electron promotion from the sigma(1) HOMO to the pi(1)* orbital, and the second valence excited state to a promotion from the sigma(1) HOMO to the sigma(1)* orbital. Surprisingly, however, it is only in 2, which has an extraordinarily long SiSi bond, that the terminating sigma(1)* orbital is the sigma*(SiSi) antibond, as anticipated, and the sigma sigma* transition has the expected very high oscillator strength. In 1, 3, and 4, the sigma*(SiSi) antibonding orbital is high in energy and does not play any role in low-energy excitations. Instead, the terminating orbital of the sigma(1)sigma(1)* excitation is represented by Si-alkyl antibonds, combined symmetrically with respect to rotation around the SiSi axis and antisymmetrically with respect to operations that interchange the two Si atoms. The common assumption that the characteristic intense sigma sigma* transitions of longer peralkylated oligosilanes extrapolate to the lowest sigma sigma* transition in common peralkylated disilanes is incorrect, and only the weak sigma pi* transitions extrapolate simply.
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Affiliation(s)
- Mari Carmen Piqueras
- Departament de Química Física, Universitat de València, Dr. Moliner 50, E-46100 Burjassot, Spain
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Tachikawa H. Mechanism of Electron and Hole Localization in Poly(dimethylsilane) Radical Ions. J Phys Chem A 2007; 111:10134-8. [PMID: 17880048 DOI: 10.1021/jp072630u] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/23/2023]
Abstract
The mechanism of excess electron and hole localizations in radical ions of poly(dimethylsilane) (PDMS) has been investigated by means of molecular dynamics (MD) and extended Hückel methods. Oligo(dimethylsilane) composed of 100 monomer units of dimethylsilane, CH3(Si(CH3)2)(n)CH3 (n = 100), were used as a model of PDMS. Both wings of the oligomer were capped by a methyl group. First, the geometry of PDMS with a regular all-trans form was fully optimized by MM2+ energy gradient method. Next, the MD calculation was carried out for PDMS at 300 K. The structure of PDMS was gradually deformed as a function of simulation time, especially the dihedral angle of Si-Si-Si-Si backbone that was randomized. At time zero when the structure has the regular all-trans form, both the excess electron and hole were completely delocalized on the Si backbone of PDMS. After thermal activation, the localization of the electron and hole was found. The mechanism of the localization was discussed on the basis of theoretical results.
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Affiliation(s)
- Hiroto Tachikawa
- Division of Materials Chemistry, Graduate School of Engineering, Hokkaido University, Sapporo 060-8628, Japan. hiroto@eng. hokudai.ac.jp
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Strohmann C, Däschlein C, Kellert M, Auer D. Ein hoch enantiomerenangereichertes Lithiosilan durch selektive Spaltung einer Silicium-Phenyl-Bindung mit Lithium. Angew Chem Int Ed Engl 2007. [DOI: 10.1002/ange.200604636] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
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Strohmann C, Däschlein C, Kellert M, Auer D. A Highly Enantiomerically Enriched Lithiosilane by Selective Cleavage of a Silicon–Phenyl Bond with Lithium. Angew Chem Int Ed Engl 2007; 46:4780-2. [PMID: 17516593 DOI: 10.1002/anie.200604636] [Citation(s) in RCA: 26] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
Affiliation(s)
- Carsten Strohmann
- Institut für Anorganische Chemie, Universität Würzburg, Am Hubland, 97074 Würzburg, Germany.
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Sandström N, Piqueras MC, Ottosson H, Crespo R. Electronic excitations of 1,4-disilyl-substituted 1,4-disilabicycloalkanes: a MS-CASPT2 study of the influence of cage size. J Phys Chem A 2007; 111:2804-10. [PMID: 17388376 DOI: 10.1021/jp070010n] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
We present a multistate complete active space second-order perturbation theory computational study aimed to predict the low-lying electronic excitations of four compounds that can be viewed as two disilane units connected through alkane bridges in a bicyclic cage. The analysis has focused on 1,4-disilyl-1,4-disilabicyclo[2.2.1]heptane (1a), 1,4-bis(trimethylsilyl)-1,4-disilabicyclo[2.2.1]heptane (1b), 1,4-disilyl-1,4-disilabicyclo[2.1.1]hexane (2a), and 1,4-bis(trimethylsilyl)-1,4-disilabicyclo[2.1.1]hexane (2b). The aim has been to find out the nature of the lowest excitations with significant oscillator strengths and to investigate how the cage size affects the excitation energies and the strengths of the transitions. Two different substituents on the terminal silicon atoms (H and CH3) were used in order to investigate the end group effects. The calculations show that the lowest allowed excitations are of the same character as that found in disilanes but now red-shifted. As the cage size is reduced from a 1,4-disilabicyclo[2.2.1]heptane to a 1,4-disilabicyclo[2.1.1]hexane, the Si...Si through-space distance decreases from approximately 2.70 to 2.50 A and the lowest allowed transitions are red-shifted by up to 0.9 eV, indicating increased interaction between the two Si-Si bonds. The first ionization potential, which corresponds to ionization from the Si-Si sigma orbitals, is lower in 1b and 2b than in Si2Me6 by approximately 0.9 and 1.2 eV, respectively. Moreover, 1b and 2b, which have methyl substituents at the terminal Si atoms, have slightly lower excitation energies than the analogous species 1a and 2a.
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Affiliation(s)
- Niclas Sandström
- Department of Biochemistry and Organic Chemistry, Box 576, Uppsala University, 751 23 Uppsala, Sweden
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Piqueras MC, Michl J, Crespo R. MS-CASPT2 analysis of the UV thermochromism of octamethyltrisilane. Mol Phys 2007. [DOI: 10.1080/00268970500418448] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/24/2022]
Affiliation(s)
- M. C. Piqueras
- a Departament de Química Física , Universitat de València , Dr. Moliner 50 E-46100 Burjassot (Valencia), Spain
| | - J. Michl
- b Department of Chemistry and Biochemistry , University of Colorado Boulder , Colorado 80309-0215, USA
| | - R. Crespo
- a Departament de Química Física , Universitat de València , Dr. Moliner 50 E-46100 Burjassot (Valencia), Spain
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Crespo R, Piqueras MC, Michl J. Electronic excitation in a syn-tetrasilane: 1,1,2,2,3,3, 4,4-octamethyltetrasilacyclopentane. Theor Chem Acc 2007. [DOI: 10.1007/s00214-007-0246-1] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
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14
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The electronic structures of all trans form of permethyl-oligosilane radical cation with longer chain: A density functional theory study. J Organomet Chem 2006. [DOI: 10.1016/j.jorganchem.2006.08.090] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/23/2022]
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Fogarty HA, Ottosson H, Michl J. Calculation of Relative Energies of Permethylated Oligosilane Conformers in Vapor and in Alkane Solution. J Phys Chem B 2006; 110:25485-95. [PMID: 17165997 DOI: 10.1021/jp064643y] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
The geometries of 35 conformers of Me(SiMe2)nMe (n = 4, 1; n = 5, 2; n = 6, 3; n = 7, 4) were optimized at the MP2/VTDZ level, and CCSD(T) single-point calculations were done at three MP2/VTDZ conformer geometries of 1. The relative ground-state energies of the conformers of 1-4 in the gas phase were obtained from the MP2/VTDZ electronic energy, zero-point vibrational energy, and thermal corrections at 0, 77, and 298 K. Relative energies in an alkane solvent at 77 and 298 K were obtained by the addition of solvation energies, obtained from the SM5.42R model. The calculated energies of 26 of the conformers (n = 4-6) have been least-squares fitted to a set of 15 additive increments associated with each Si-Si bond conformation and each pair of adjacent bond conformations, with mean deviations of 0.06-0.20 kcal/mol. An even better fit for the energies of 24 conformers (mean deviations, 0.01-0.09 kcal/mol) has been obtained with a larger set of 19 increments, which also contained contributions from selected combinations of conformations of three adjacent bonds. The utility of the additive increments for the prediction of relative conformer energies in the gas phase and in solution has been tested on the remaining nine conformers (n = 6, 7). With the improved increment set, the average deviation from the SM5.42R//MP2 results for solvated conformers at 298 K was 0.18 kcal/mol, and the maximum error was 0.98 kcal/mol.
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Affiliation(s)
- Heather A Fogarty
- Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309-0215, USA
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Tachikawa H, Kawabata H. Structures and Electronic States of Permethyloligosilane Radical Ions with All-Trans Form Sin(CH3)2n+2± (n = 2−6): A Density Functional Theory Study. J Chem Theory Comput 2006; 3:184-93. [DOI: 10.1021/ct600163r] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/03/2023]
Affiliation(s)
- Hiroto Tachikawa
- Division of Molecular Chemistry, Graduate School of Engineering, Hokkaido University, Sapporo 060−8628, Japan
| | - Hiroshi Kawabata
- Venture Business Laboratory, Kyoto University, Kyoto 606−8501, Japan
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Fogarty HA, Imhof R, Michl J. Magnetic circular dichroism of peralkylated tetrasilane conformers. Proc Natl Acad Sci U S A 2004; 101:10517-22. [PMID: 15249672 PMCID: PMC489969 DOI: 10.1073/pnas.0403209101] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Magnetic circular dichroism (MCD) of five peralkylated tetrasilanes (1-5) conformationally constrained to angles ranging from nearly 0 degrees to 180 degrees and of the open chain tetrasilane Si(4)Me(10) (6) shows a clear conformational dependence and permits the detection of previously hidden transitions. In the tetrasilane CH(2)Si(4)Me(8) (1), with the smallest dihedral angle, comparison of MCD with absorption spectra reveals four low-energy electronic transitions. In the tetrasilanes 2-4, three distinct transitions are apparent. In tetrasilanes 5 and 6, MCD reveals the very weak transition that has been predicted to be buried under the first intense peak and to which the anomalous thermochromism of 6 and other short-chain oligosilanes has been attributed.
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Affiliation(s)
- Heather A Fogarty
- Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO 80309-0215, USA
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