Ankan Paul

Publications (14)43.7 Total impact

• Article: The geometry and electronic topology of higher-order charged Möbius annulenes.

  Chaitanya S Wannere, Henry S Rzepa, B Christopher Rinderspacher, Ankan Paul, Charlotte S M Allan, Henry F Schaefer, Paul v R Schleyer
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  ABSTRACT: Higher-order aromatic charged Möbius-type annulenes have been L(k) realized computationally. These charged species are based on strips with more than one electronic half-twist, as defined by their linking numbers. The B3LYP/6-311+G(d,p) optimized structures and properties of annulene rings with such multiple half-twists (C(12)H(12)(2+), C(12)H(12)(2-), C(14)H(14), C(18)H(18)(2+), C(18)H(18)(2-), C(21)H(21)(+), C(24)H(24)(2-), C(28)H(28)(2+), and C(28)H(28)(2-)) have the nearly equal C-C bond lengths, small dihedral angles around the circuits, stabilization energies, and nucleus-independent chemical shift values associated with aromaticity. The topology and nature of Möbius annulene systems are analyzed in terms of the torus curves defined by electron density functions (rho(r)(pi), ELF(pi)) constructed using only the occupied pi-MOs. The pi-torus subdivides into a torus knot for annulenes defined by an odd linking number (L(k) = 1, 3pi) and a torus link for those with an even linking number (L(k) = 2, 4pi). The torus topology is shown to map onto single canonical pi-MOs only for even values of L(k). Incomplete and misleading descriptions of the topology of pi-electronic Möbius systems with an odd number of half twists result when only signed orbital diagrams are considered, as is often done for the iconic single half twist system.
  The Journal of Physical Chemistry A 08/2009; 113(43):11619-29. · 2.95 Impact Factor
• Article: Characterization of the HSiN–HNSi system in its electronic ground state

  Maria C. Lind, Frank C. Pickard, Justin B. Ingels, Ankan Paul, Yukio Yamaguchi, Henry F. Schaefer
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  ABSTRACT: The electronic ground states ( 1Σ+) of HSiN, HNSi, and the transition state connecting the two isomers were systematically studied using configuration interaction with single and double (CISD) excitations, coupled cluster with single and double (CCSD) excitations, CCSD with perturbative triple corrections [CCSD(T)], multireference complete active space self-consistent field (CASSCF), and internally contracted multireference configuration interaction (ICMRCI) methods. The correlation-consistent polarized valence (cc-pVXZ), augmented correlation-consistent polarized valence (aug-cc-pVXZ) (X = T,Q,5), correlation-consistent polarized core-valence (cc-pCVYZ), and augmented correlation-consistent polarized core-valence (aug-cc-pCVYZ) (Y = T,Q) basis sets were used. Via focal point analyses, we confirmed the HNSi isomer as the global minimum on the ground state HSiN–HNSi zero-point vibrational energy corrected surface and is predicted to lie 64.7 kcal mol−1 (22 640 cm−1, 2.81 eV) below the HSiN isomer. The barrier height for the forward isomerization reaction (HSiN→HNSi) is predicted to be 9.7 kcal mol−1, while the barrier height for the reverse process (HNSi→HSiN) is determined to be 74.4 kcal mol−1. The dipole moments of the HSiN and HNSi isomers are predicted to be 4.36 and 0.26 D, respectively. The theoretical vibrational isotopic shifts for the HSiN/DSiN and HNSi/DNSi isotopomers are in strong agreement with the available experimental values. The dissociation energy for HSiN [HSiN( 1Σ+)→H(2S)+SiN(X 2Σ+)] is predicted to be D0 = 59.6 kcal mol−1, whereas the dissociation energy for HNSi [HNSi( 1Σ+)→H(2S)+NSi(X 2Σ+)] is predicted to be D0 = 125.0 kcal mol−1 at the CCSD(T)/aug-cc-pCVQZ level of theory. Anharmonic vibrational frequencies computed using second order vibrational perturbation theory are in good agreement with available matrix isolation experimental data for both HSiN and HNSi isomers root mean squared derivation (RMSD = 9 cm−1).
  The Journal of Chemical Physics 03/2009; 130(10):104301-104301-13. · 3.33 Impact Factor
• Article: Coupled cluster investigation on the low-lying electronic states of CuCN and CuNC and the ground state barrier to isomerization.

  Ankan Paul, Yukio Yamaguchi, Henry F Schaefer
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  ABSTRACT: The observation of several metal cyanides and isocyanides in interstellar space has raised much interest these molecules. Optimum molecular structures, harmonic vibrational frequencies, and dipole moments of the ground electronic states (X1Sigma+), triplet excited states, and open shell singlet excited states of CuCN and CuNC were determined using different levels of nonrelativistic and scalar relativistic (Douglas-Kroll) [Ann. Phys. 82, 89 (1979)] coupled cluster theory in conjunction with atomic natural orbital basis sets and correlation consistent basis sets. For the relativistic computations the specially contracted correlation consistent Douglas-Kroll (DK) basis sets were used. Moreover, barriers to isomerization from CuCN to CuNC were computed. The predicted structures of the X1Sigma+ state for CuCN are re(Cu-C)=1.826 A and re(C-N)=1.167 A, at the most sophisticated level of theory, the scalar relativistic DK-CCSD(T)/cc-pVQZ(DK) method. These results are in excellent agreement with the experimentally determined Cu-C bond length of 1.829 A and C-N bond distance of 1.162 A. At the same level of theory, the zero-point corrected barrier to isomerization from CuCN to CuNC is estimated to be 14.7 kcal mol(-1), and the cyanide is more stable than the isocyanide by 11.5 kcal mol(-1). For both CuCN and CuNC the 3Sigma+ state is the lowest lying excited electronic state. At the DK-CCSD/cc-pVQZ(DK) level of theory, the energetic ordering of excited states of CuCN and CuNC is X1Sigma+<a3Sigma+<b3Pi<2(1)Sigma+ approximately 3Delta<1Pi<1Delta. The variations of CN bond lengths in the optimized structures for the different electronic states and the CN stretching frequencies of the ground state and the excited states suggest that metal dpi to ligand pi charge transfer is insignificant, in contrast to previous results for isoelectronic NiCO.
  The Journal of Chemical Physics 11/2007; 127(15):154324. · 3.33 Impact Factor
• Article: The existence of secondary orbital interactions.

  Chaitanya S Wannere, Ankan Paul, Rainer Herges, K N Houk, Henry F Schaefer, Paul von Ragué Schleyer
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  ABSTRACT: B3LYP/6-311+G** (and MP2/6-311+G**) computations, performed for a series of Diels-Alder (DA) reactions, confirm that the endo transition states (TS) and the related Cope-TSs are favored energetically over the respective exo-TSs. Likewise, the computed magnetic properties (nucleus-independent chemical shifts and magnetic susceptibililties) of the endo- (as well as the Cope) TS's reveal their greater electron delocalization and greater aromaticity than the exo-TS's. However, Woodward and Hoffmann's original example is an exception: their endo-TS model, involving the DA reaction of a syn- with an anti-butadiene (BD), actually is disfavored energetically over the corresponding exo-TS; magnetic criteria also do not indicate the existence of SOI delocalization in either case. Instead, a strong energetic preference for endo-TSs due to SOI is found when both BDs are in the syn conformations. This is in accord with Alder and Stein's rule of "maximum accumulation of double bonds:" both the dienophile and the diene should have syn conformations. Plots along the IRC's show that the magnetic properties typically are most strongly exalted close to the energetic TS. Because of SOI, all the points along the endo reaction coordinates are more diatropic than along the corresponding exo pathways. We find weak SOI effects to be operative in the endo-TSs involved in the cycloadditions of cyclic alkenes, cyclopropene, aziridine, cyclobutene, and cyclopentene, with cyclopentadiene. While the endo-TSs are only slightly lower in energy than the respective exo-TSs, the magnetic properties of the endo-TS's are significantly exalted over those for the exo-TS's and the Natural Bond Orbitals indicate small stabilizing interactions between the methylene cycloalkene hydrogen orbitals (and lone pairs in case of aziridine) with pi-character and the diene pi MOs.
  Journal of Computational Chemistry 02/2007; 28(1):344-61. · 4.58 Impact Factor
• Article: The existence of secondary orbital interactions.

  Chaitanya S. Wannere, Ankan Paul, Rainer Herges, K. N. Houk, Henry F. Schaefer III, Paul von Ragué Schleyer
  Journal of Computational Chemistry. 01/2007; 28:344-361.
• Article: High electron affinities of bicyclo[n, n, 0]perfluoroalkanes

  Ankan Paul, Paul R. Schleyer, Henry F. Schaefer III
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  ABSTRACT: Electron attachment is an important contributor to the chemistry of perfluoroalkanes. The adiabatic electron affinities (AEAs) and vertical electron affinities (VEAs) of bicyclo[n, n, 0]perfluoroalkanes (n, n-BCPFA) (where n= 1, 2, 3, 4) were computed using hybrid density functionals with DZP++ (polarization and diffuse function augmented double-ζ) basis sets. The bicyclo[1, 1, 0]perfluorobutane (1,1-BCPFA) and bicyclo[2, 2, 0]perfluoro-octane exhibit exceptionally high electron affinities for a saturated perfluoroalkane at all the levels of theory (2.20 and 1.86 eV, respectively; KMLYP/DZP++ with zero-point corrections). The predicted zero-point corrected AEAs of the n, n-BCPFAs range from 1.40 to 2.20 eV with the KMLYP/DZP++ level of theory. The structural changes which occur over the different ring sizes are varied and are dictated by the mode of electron binding by the n, n-BCPFA. Spin density and SOMO plots reveal that 1,1-BCPFA and 2,2-BCPFA bind the electron in a C–C antibonding σ* orbital. The 2,2-BCPFA radical anion has two minima differing in their nature of binding the ‘extra electron’. One of the minima has the electron distributed around the cleaved bridgehead C–C bond, and the other has it dispersed over the entire carbon skeleton. The 4,4- and 5,5-BCPFA radical anions hold the unpaired electron in a C–F antibonding σ* orbital which is localized on an exceptionally long tertiary C–F bond. The 1,1-BCPFA radical anion exhibits an exceptionally long bridgehead C–C bond. The most stable minimum of the 2,2-BCPFA radical anion exhibits similar characteristics as the 1,1-BCPFA radical anion. The 3,3-BCPFA (both the cis and trans forms) and the trans form of the 4,4-BCPFA radical anion display exceptionally long tertiary C–F bonds.
  Molecular Physics 04/2006; 104(8):1311-1324. · 1.82 Impact Factor
• Article: Publisher’s Note: “The ground and two lowest-lying singlet excited electronic states of copper hydroxide (CuOH)” [ J. Chem. Phys. 123, 014313 (2005) ]

  Suyun Wang, Ankan Paul, Nathan J. DeYonker, Yukio Yamaguchi, Henry F. Schaefer
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  ABSTRACT: Abstract unavailable.
  The Journal of Chemical Physics 01/2006; 124(1):019901-019901-1. · 3.33 Impact Factor
• Article: The low-lying electronic states of nickel cyanide and isocyanide: A theoretical investigation.

  Ankan Paul, Yukio Yamaguchi, Henry F Schaefer, Kirk A Peterson
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  ABSTRACT: At different levels of coupled cluster theory optimum structures, energetics, and harmonic vibrational frequencies for several low-lying doublet and quartet electronic states of linear NiCN and NiNC were studied using four contracted Gaussian basis sets, ranging from Ni[6s5p4d2f], CN[4s3p2d] to Ni[8s7p5d3f2g1h], CN[5s4p3d2f1g]. The most reliable predictions were obtained with a relativistic Douglas-Kroll restricted open-shell-based coupled cluster method including singles, doubles, and perturbative triple excitations [DK-R/UCCSD(T)]. This level of theory was used in conjunction with correlation-consistent polarized valence Douglas-Kroll recontracted quadruple-zeta basis sets (cc-pVQZDK). The energetic ordering of the electronic states of NiCN is predicted to be 2delta < 2sigma+ < 2pi < 4delta < 4pi and that of NiNC is 2delta approximately 2sigma+ < 2pi < 4delta < 4pi < 4sigma-. Our theoretical investigation supports the assignment of the ground-state term symbol, the Ni-C stretching frequency, and the bending frequency for the ground electronic state of NiCN by Kingston et al. [J. Mol. Spectrosc. 215, 106 (2002)] and by Sheridan and Ziurys [J. Chem. Phys. 118, 6370 (2003)]. The predicted structure of the 2delta ground state of NiCN, r(e)(Ni-C) = 1.822 angstroms and r(e)(C-N) = 1.167 angstroms, at DK-R/UCCSD(T)/cc-pVQZDK shows excellent agreement with the experimentally determined Ni-C bond length of 1.826 A and less satisfactory agreement for the C-N bond length of 1.153 angstroms [J. Chem. Phys. 118, 6370 (2003)]. It is also concluded that the metal-to-ligand pi back donation is weak or negligible. Additionally, we found that on the 2delta surface the linear cyanide isomer lies lower in energy than the linear isocyanide isomer by 12.2 kcal mol(-1).
  The Journal of Chemical Physics 01/2006; 124(3):034310. · 3.33 Impact Factor
• Article: The peculiar trend of cyclic perfluoroalkane electron affinities with increasing ring size.

  Ankan Paul, Chaitanya S Wannere, Veronica Kasalova, Paul v R Schleyer, Henry F Schaefer
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  ABSTRACT: The adiabatic electron affinities (AEAs), vertical electron affinities (VEAs), and vertical detachment energies (VDEs) of cyclic perfluoroalkanes, c-C(n)F(2n) (n = 3-7), and their monotrifluoromethyl derivatives were computed using various pure and hybrid density functionals with DZP++ (polarization and diffuse function augmented double-zeta) basis sets. The theoretical AEA of c-C(4)F(8) at KMLYP/DZP++ is 0.70 eV, which exhibits satisfactory agreement with the 0.63 +/- 0.05 eV experimental value. The nonzero-point-corrected AEA of c-C(4)F(8) is predicted to be 0.41 eV at the CCSD(T)/aug-cc-pVTZ//MP2/aug-cc-pVTZ level of theory, which shows a slight deviation of 0.11 eV from the KMLYP estimated value of 0.52 eV for the same. With the zero-point correction from the MP2/6-311G(d) [Gallup, G. A. Chem. Phys. Lett. 2004, 399, 206] level of theory combined with the CCSD(T)/aug-cc-pVTZ//MP2/aug-cc-pVTZ result, the most reliable estimate of AEA of c-C(4)F(8) is 0.60 eV. c-C(3)F(6)(-), c-C(4)F(8)(-), and c-C(5)F(10)(-) are unusual in preferring planar to near planar ring structures. The ZPE-corrected AEAs of c-C(n)F(2n) increase from n = 3 (0.24 eV) to n = 5 (0.77 eV), but then dramatically fall off to 0.40 eV for both n = 6 and n = 7. All of the other functionals predict the same trend. This is due to a change in the structural preference: C(s)() c-C(6)F(12)(-) and C(1) c-C(7)F(14)(-) are predicted to favor nonplanar rings, each with an exceptionally long C-F bond. (There also is a second, higher energy D3d minimum for C(6)F(12)(-).) The SOMOs as well as the spin density plots of the c-PFA radical anions reveal that the "extra" electron is largely localized on the unique F atoms in the larger n = 6 and n = 7 rings but is delocalized in the multiatom SOMOs of the three- to five-membered ring radical anions. The computed AEAs are much larger than the corresponding VEAs; the latter are not consistent with different functionals. The AEAs are substantially larger when a c-C(n)()F(2)(n)() fluorine is replaced by a -CF(3) group. This behavior is general; PFAs with tertiary C-F bonds have large AEAs. The VDEs for all the anions are substantial, ranging from 1.89 to 3.64 eV at the KMLYP/DZP++ level.
  Journal of the American Chemical Society 12/2005; 127(44):15457-69. · 9.91 Impact Factor
• Article: Effects of fluorine on the structures and energetics of the propynyl and propargyl radicals and their anions.

  Raj K Sreeruttun, Ponnadurai Ramasami, Chaitanya S Wannere, Ankan Paul, Paul V R Schleyer, Henry F Schaefer Iii
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  ABSTRACT: [reaction: see text] The adiabatic electron affinity (EA(ad)) of the CH(3)-C[triple bond]C(*) radical [experiment = 2.718 +/- 0.008 eV] and the gas-phase basicity of the CH(3)-C[triple bond]C:(-) anion [experiment = 373.4 +/- 2 kcal/mol] have been compared with those of their fluorine derivatives. The latter are studied using theoretical methods. It is found that there are large effects on the electron affinities and gas-phase basicities as the H atoms of the alpha-CH(3) group in the propynyl system are substituted by F atoms. The predicted electron affinities are 3.31 eV (FCH(2)-C[triple bond]C(*)), 3.86 eV (F(2)CH-C[triple bond]C(*)), and 4.24 eV (F(3)C-C[triple bond]C(*)), and the predicted gas-phase basicities of the fluorocarbanion derivatives are 366.4 kcal/mol (FCH(2)-C[triple bond]C:(-)), 356.6 kcal/mol (F(2)CH-C[triple bond]C:(-)), and 349.8 kcal/mol (F(3)C-C[triple bond]C:(-)). It is concluded that the electron affinities of fluoropropynyl radicals increase and the gas-phase basicities decrease as F atoms sequentially replace H atoms of the alpha-CH(3) in the propynyl system. The propargyl radicals, lower in energy than the isomeric propynyl radicals, are also examined and their electron affinities are predicted to be 0.98 eV ((*)CH(2)-C[triple bond]CH), 1.18 eV ((*)CFH-C[triple bond]CH), 1.32 eV ((*)CF(2)-C[triple bond] CH), 1.71 eV ((*)CH(2)-C[triple bond]CF), 2.05 eV ((*)CFH-C[triple bond]CF), and 2.23 eV ((*)CF(2)-C[triple bond]CF).
  The Journal of Organic Chemistry 11/2005; 70(22):8676-86. · 4.45 Impact Factor
• Article: The ground and two lowest-lying singlet excited electronic states of copper hydroxide (CuOH).

  Suyun Wang, Ankan Paul, Nathan J DeYonker, Yukio Yamaguchi, Henry F Schaefer
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  ABSTRACT: Various ab initio methods, including self-consistent field (SCF), configuration interaction, coupled cluster (CC), and complete-active-space SCF (CASSCF), have been employed to study the electronic structure of copper hydroxide (CuOH). Geometries, total energies, dipole moments, harmonic vibrational frequencies, and zero-point vibrational energies are reported for the linear 1Sigma+ and 1Pi stationary points, and for the bent ground-state X 1A', and excited-states 2 1A' and 1 1A". Six different basis sets have been used in the study, Wachters/DZP being the smallest and QZVPP being the largest. The ground- and excited-state bending modes present imaginary frequencies for the linear stationary points, indicating that bent structures are more favorable. The effects of relativity for CuOH are important and have been considered using the Douglas-Kroll approach with cc-pVTZ/cc-pVTZ_DK and cc-pVQZ/cc-pVQZ_DK basis sets. The bent ground and two lowest-lying singlet excited states of the CuOH molecule are indeed energetically more stable than the corresponding linear structures. The optimized geometrical parameters for the X 1A' and 1 1A" states agree fairly well with available experimental values. However, the 2 1A' structure and rotational constants are in poor agreement with experiment, and we suggest that the latter are in error. The predicted adiabatic excitation energies are also inconsistent with the experimental values of 45.5 kcal mol(-1) for the 2 1A' state and 52.6 kcal mol(-1) for the 1 1A" state. The theoretical CC and CASSCF methods show lower adiabatic excitation energies for the 1 1A" state (53.1 kcal mol(-1)) than those for the corresponding 2 1A' state (57.6 kcal mol(-1)), suggesting that the 1 1A" state might be the first singlet excited state while the 2 1A' state might be the second singlet excited state.
  The Journal of Chemical Physics 08/2005; 123(1):014313. · 3.33 Impact Factor
• Article: Do Linear-Chain Perfluoroalkanes Bind an Electron?

  Ankan Paul, Chaitanya S. Wannere, Henry F. Schaefer III
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  ABSTRACT: The adiabatic electron affinities (AEAs), vertical electron affinities (VEAs), and vertical detachment energies (VDEs) of linear-chain perfluoroalkanes (PFAs), n-CnF2n+2 (n = 2−8) are predicted using carefully calibrated computational methods (Chem. Rev. 2002, 102, 231). Density functional theoretical methods and hybrid Hartree−Fock/density functional methods have been used with double-ζ-quality basis sets with polarization and diffuse functions, DZP++. Vibrational frequency analyses were performed to compute the zero-point energy corrections and determine the nature of the stationary points. The estimated adiabatic electron affinities of linear-chain PFAs (CnF2n+2), from n = 3 to n = 8, turn out to be appreciable, ranging from 0.26 to 0.58 eV (B3LYP/DZP++ method). The corresponding zero-point-corrected values are a bit larger, ranging from 0.39 to 0.71 eV. C2F6 is the only n-PFA exhibiting a negative adiabatic electron affinity. The trends in AEAs of the n-PFA show that the AEA increases with increasing chain length until n = 7 and then slightly decreases at n = 8. The VEAs of all the linear chain PFAs are negative. VEAs increase with increasing length of the linear-chain PFAs. The VDEs indicate that all the straight-chain PFA anions considered are bound with respect to electron loss. It was also observed that PFA molecules show enhanced AEAs when they are branched. The presence of tertiary C−F bonds in PFAs results in high AEAs compared to those of their straight-chain counterparts.
  09/2004;
• Article: The low-lying electronic excited states of NiCO.

  Lubos Horny, Ankan Paul, Yukio Yamaguchi, Henry F Schaefer
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  ABSTRACT: Highly correlated coupled cluster methods with single and double excitations (CSSD) and CCSD with perturbative triple excitations were used to predict molecular structures and harmonic vibrational frequencies for the electronic ground state X 1Sigma+, and for the 3Delta, 3Sigma+, 3Phi, 1 3Pi, 2 3Pi, 1Sigma+, 1Delta, and 1Pi excited states of NiCO. The X 1Sigma+ ground state's geometry is for the first time compared with the recently determined experimental structure. The adiabatic excitation energies, vertical excitation energies, and dissociation energies of these excited states are predicted. The importance of pi and sigma bonding for the Ni-C bond is discussed based on the structures of excited states.
  The Journal of Chemical Physics 08/2004; 121(3):1412-8. · 3.33 Impact Factor
• Article: Radicals Derived from Adenine:  Prediction of Large Electron Affinities with a Considerable Spread

  Francesco A. Evangelista, Ankan Paul, Henry F. Schaefer III
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  ABSTRACT: The electron affinities of five radicals of adenine minus a hydrogen atom are predicted using carefully calibrated (Chem. Rev. 2002, 102, 231) computational methods. Density functional theory (DFT) and hybrid DFT/Hartree−Fock functionals were used with double-ζ quality basis sets augmented with polarization and diffuse functions, DZP++. Vibrational frequency analyses were performed to compute zero-point energy corrections and to determine the nature of the stationary points. The energetic spacing of the five adenine-related radicals and five anions is predicted and found to be quite different between the radicals and anions. The electron affinities are found to be large, ranging from 0.9 to 3.2 eV. In all anions, the “last” electron displays little diffuse character. Breaking a N−H bond leads (experimentally corresponding to deprotonating the electron-oxidized adenine radical) to the most stable radical, which in turn has the highest electron affinity. Radicals and anions which are created at carbon centers are less stable with respect to those generated at nitrogen centers in the adenine framework.
  03/2004;