skip to main content

Explore Research Products in the NSF-PAR

  • NSF Public Access
  • Search Results
  • A Fermi resonance and a parallel-proton-transfer overtone in the Raman spectrum of linear centrosymmetric N4H+: A polarizability-driven first principles molecular dynamics study
Title: A Fermi resonance and a parallel-proton-transfer overtone in the Raman spectrum of linear centrosymmetric N4H+: A polarizability-driven first principles molecular dynamics study

We present molecular dynamics (MD), polarizability driven MD (α-DMD), and pump–probe simulations of Raman spectra of the protonated nitrogen dimer N4H+, and some of its isotopologues, using the explicitly correlated coupled-cluster singles and doubles with perturbative triples [CCSD(T)]-F12b/aug-cc-pVTZ based potential energy surface in permutationally invariant polynomials (PIPs) of Yu et al. [J. Phys. Chem. A 119, 11623 (2015)] and a corresponding PIP-derived CCSD(T)/aug-cc-pVTZ-tr (N:spd, H:sp) polarizability tensor surface (PTS), the latter reported here for the first time. To represent the PTS in terms of a PIP basis, we utilize a recently described formulation for computing the polarizability using a many-body expansion in the orders of dipole–dipole interactions while generating a training set using a novel approach based on linear regression for potential energy distributions. The MD/α-DMD simulations reveal (i) a strong Raman activity at 260 and 2400 cm−1, corresponding to the symmetric N–N⋯H bend and symmetric N–N stretch modes, respectively; (ii) a very broad spectral region in the 500–2000 cm−1 range, assignable to the parallel N⋯H+⋯N proton transfer overtone; and (iii) the presence of a Fermi-like resonance in the Raman spectrum near 2400 cm−1 between the Σg+ N–N stretch fundamental and the Πu overtone corresponding to perpendicular N⋯H+⋯N proton transfer.

  more » « less
Award ID(s):
1855583
NSF-PAR ID:
10473174
Author(s) / Creator(s):
; ; ; ;
Publisher / Repository:
Wiley
Date Published:
Journal Name:
The Journal of Chemical Physics
Volume:
157
Issue:
15
ISSN:
0021-9606
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. ; ; ( , International Journal of Quantum Chemistry)
    Abstract

    The exact expressions for the dipole, quadrupole, and octupoles of a collection ofNpoint charges involve summations of corresponding tensors over theNsites weighted by their charge magnitudes. When the point charges are atoms (in a molecule) theN‐site formula is an approximation, and one must integrate over the electron density to recover the exact multipoles. In the present work we revisit theN(N + 1)/2‐site point charge density model of Hall (Chem. Phys. Lett.6, 501, 1973) for the purpose of fitting ab initio derived multipole moment hypersurfaces using permutationally invariant polynomials (PIP). We examine new approaches in PIP‐fitting procedures for the dipole, quadrupole, octupole moments, and polarizability tensor surfaces (DMS, QMS, OMS and PTS, respectively) for a non‐polar CCl4and a polar CHCl3and show that compared to the primitiveN‐site model theN(N + 1)/2‐site model appreciably improves the relative RMSE of the DMS and does much more substantially so, by an order of magnitude, for the corresponding ones of QMS and OMS. Training datasets are obtained by sampling potential energies up to 18 000 cm−1above the global minima, generated by molecular dynamics simulations at the DFT B3LYP/aug‐cc‐pVDZ level of theory.

     
    more » « less
  2. ; ; ; ( , Physical Chemistry Chemical Physics)
    null (Ed.)
    We investigated the collision-induced dissociation (CID) reactions of a protonated Hoogsteen 9-methylguanine–1-methylcytosine base pair (HG-[9MG·1MC + H] + ), which aims to address the mystery of the literature reported “anomaly” in product ion distributions and compare the kinetics of a Hoogsteen base pair with its Watson-Crick isomer WC-[9MG·1MC + H] + (reported recently by Sun et al. ; Phys. Chem. Chem. Phys. , 2020, 22 , 24986). Product ion cross sections and branching ratios were measured as a function of center-of-mass collision energy using guided-ion beam tandem mass spectrometry, from which base-pair dissociation energies were determined. Product structures and energetics were assessed using various theories, of which the composite DLPNO-CCSD(T)/aug-cc-pVTZ//ωB97XD/6-311++G(d,p) was adopted as the best-performing method for constructing a reaction potential energy surface. The statistical Rice–Ramsperger–Kassel–Marcus theory was found to provide a useful framework for rationalizing the dominating abundance of [1MC + H] + over [9MG + H] + in the fragment ions of HG-[9MG·1MC + H] + . The kinetics analysis proved the necessity for incorporating into kinetics modeling not only the static properties of reaction minima and transition states but more importantly, the kinetics of individual base-pair conformers that have formed in collisional activation. The analysis also pinpointed the origin of the statistical kinetics of HG-[9MG·1MC + H] + vs. the non-statistical behavior of WC-[9MG·1MC + H] + in terms of their distinctively different intra-base-pair hydrogen-bonds and consequently the absence of proton transfer between the N1 position of 9MG and the N3′ of 1MC in the Hoogsteen base pair. Finally, the Hoogsteen base pair was examined in the presence of a water ligand, i.e. , HG-[9MG·1MC + H] + ·H 2 O. Besides the same type of base-pair dissociation as detected in dry HG-[9MG·1MC + H] + , secondary methanol elimination was observed via the S N 2 reaction of water with nucleobase methyl groups. 
    more » « less
  3. ; ; ; ( , Physical Chemistry Chemical Physics)
    null (Ed.)
    The challenges associated with the out-of-plane bending problem in multiply-bonded hydrocarbon molecules can be mitigated in quartic force field analyses by varying the step size in the out-of-plane coordinates. Carbon is a highly prevalent element in astronomical and terrestrial environments, but this major piece of its spectra has eluded theoretical examinations for decades. Earlier explanations for this problem focused on method and basis set issues, while this work seeks to corroborate the recent diagnosis as a numerical instability problem related to the generation of the potential energy surface. Explicit anharmonic frequencies for c-(CH)C 3 H 2 + are computed using a quartic force field and the CCSD(T)-F12b method with cc-pVDZ-F12, cc-pVTZ-F12, and aug-cc-pVTZ basis sets. The first of these is shown to offer accuracy comparable to that of the latter two with a substantial reduction in computational time. Additionally, c-(CH)C 3 H 2 + is shown to have two fundamental frequencies at the onset of the interstellar unidentified infrared bands, at 5.134 and 6.088 μm or 1947.9 and 1642.6 cm −1 , respectively. This suggests that the results in the present study should assist in the attribution of parts of these aromatic bands, as well as provide data in support of the laboratory or astronomical detection of c-(CH)C 3 H 2 + . 
    more » « less
  4. ; ; ( , Astronomy & Astrophysics)
    Context. Formaldehyde is a potential biogenic precursor involved in prebiotic chemical evolution. The cold conditions of the interstellar medium (ISM) allow H 2 CO to be reactive, playing a significant role as a chemical intermediate in formation pathways leading to interstellar complex organic molecules. However, gas-phase molecular formation mechanisms in cold regions of the ISM are poorly understood. Aims. We computationally determine the most favored gas-phase molecular formation mechanisms at local thermodynamic equilibrium conditions that can produce the detected amounts of H 2 CO in diffuse molecular clouds (DMCs), in dark, cold, and dense molecular clouds (DCDMCs), and in three regions of circumstellar envelopes of low-mass protostars (CELMPs). Methods. The potential energy surfaces, thermodynamic functions, and single-point energies for transition states were calculated at the CCSD(T)-F12/cc-pVTZ-F12 and MP2/aug-cc-pVDZ levels of theory and basis sets. Molecular thermodynamics and related partition functions were obtained by applying the Maxwell-Boltzmann quantum statistics theory from energies computed at CCSD(T)-F12/cc-pVTZ-F12 with corrections for zero-point energy. A literature review on detected abundances of reactants helped us to propose the most favorable formation routes. Results. The most probable reactions that produce H 2 CO in cold astrophysical regions are: 1 CH 2 + ⋅ 3 O 2 → 1 H 2 CO + O⋅( 3 P) in DMCs, ⋅ 3 CH 2 + ⋅ 3 O 2 → 1 H 2 CO + ⋅O( 3 P) in DCDMCs, and ⋅CH 3 + ⋅O( 3 P) → 1 H 2 CO + ⋅H in region III, ⋅CH 3 +⋅O( 1 D) → 1 H 2 CO + ⋅H in region II, and 1 CH 2 + ⋅ 3 O 2 → 1 H 2 CO + ⋅O( 3 P) in region I belonging to CELMPs. Conclusions. Quantum chemical calculations suggest that the principal carbonaceous precursors of H 2 CO in cold regions for the gas-phase are CH 2 (a 1 A 1 ), and ⋅CH 2 (X 3 B 1 ) combined with ⋅O 2 ( 3 Σ g ) and ⋅CH 3 ( 2 A ” ) + ⋅O( 3 P) / O( 1 D). Reactions based on more complex reagents yield less effective thermodynamics in the gas-phase H 2 CO molecular formation. 
    more » « less
  5. ; ; ; ( , ChemistrySelect)
    Abstract

    The structures of zinc carbene ZnCH2and zinc carbyne HZnCH, and the conversion transition states between them are optimized at B3LYP/aug‐cc‐pVTZ, MP2/aug‐cc‐pVTZ, and CCSD/aug‐cc‐pVTZ levels of theory. The thermodynamic energies with CCSD(T) method are further extrapolated to basis set limit through a series of basis sets of aug‐cc‐pVXZ (X=D, T, Q, 5). The Zn−C bonding characteristics are interpreted by molecular plots, Laplacian of density plots, the integrated delocalization indices, net atomic charges, and derived atomic hardness. On the one hand, the studies demonstrated the efficiency of DFT method in structure optimizations and the accuracy of CBS method in obtaining thermodynamic energies; On the other hand, the density analysis of CCSD/aug‐cc‐pVDZ density demonstrates that both the sharing interaction and ionic interaction are important in ZnCH2ad HZnCH. The3B1state of ZnCH2is the global minimum and formed in visible light, but its small bond dissociation energy (47.0 kcal/mol) cannot keep the complex intact under UV light (79.4–102.1 kcal/mol). However, the3Σ−state of HZnCH can survive the UV light due to the greater Zn−C dissociation energy (100.7 kcal/mol). The delocalization indices of Zn…C in both3B1of ZnCH2(0.777) and the3Σ−state of HZnCH (0.785) are close to the delocalization index of the single C−C bond of ethane (0.841), i. e. the nomenclature of Zinc carbene and Zinc carbyne is incorrect. The stronger Zn−C bond in the3Σ−state of HZnCH than in the3B1state of ZnCH2can be attributed to the larger charge separation in the former. It was found that the Zn−C bonds in related Zinc organic compounds were also single bonds no matter whether the organic groups are CR, CR2, or CR3. The ionic interactions were discussed in terms of the atomic hardness that were in turn related to ionization energy and electron affinity. The unique combination of covalent and ionic characteristics in the Zn−C bonds of organic Zinc compounds could be the origin of many interesting applications of organic Zinc reagents.

     
    more » « less
  • Citation Formats
  • MLA
  • APA
  • Chicago
  • BibTeX
  • Save / Share this Record
  • Send to Email