Evaluation of recent scaling free methods to predict phenol and phenoxyl radical vibrational properties—Guidelines for more complex models of lignin or protein moieties

Phenol and phenoxyl radical are simple models of neutral or oxidized states of important moieties in biomacromolecules such as lignin or proteins. The evaluation of methods to predict their vibrational properties provides a guideline for predictions based on more complex and adequate model compounds. Fundamental frequencies of phenol and phenoxyl radical are predicted from harmonic frequencies ν_h obtained from B3LYP calculations, and anharmonic corrections Δν_anh from HF, MP2 or B3LYP calculations. The complete basis set limit is estimated for ν_h and IR intensities. Basis set qualities are evaluated for these properties and for Raman activities. Δν_anh values compare well between methods, except that MP2 is for the basis sets used unreliable for out‐of‐plane modes. Correlation effects increase Δν_anh(HF) values by an average factor ～1.1, but the correlated values do not improve predicted fundamental frequencies. The B3LYP functional is inadequate for the phenoxyl radical. Errors of approximately 5–6 cm^?1 were obtained for the phenol in‐plane low‐frequency modes. For out‐of‐plane modes basis set dependencies are critical to a degree not previously recognized. The predictions appear mainly limited by the method(s) used to calculate ν_h and Δν_anh, and the basis set dependency of Δν_anh. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

Transition structures and energy barriers of pericyclic reactions in the CASSCF approach

Five energy hypersurfaces of the most examined pericyclic reactions have been investigated by using ab initio SCF , CASSCF , and the semiempirical AM 1 methods. The systems are H₄, H₆, C₃H₆, C₄H₄, and C₃OH₄. Stationary points and their sets of harmonic vibrational frequencies have been calculated by means of analytical gradient techniques in the frameworks of the respective approaches. ZPE corrected energy barriers are based on single-point calculations including dynamical correlation corrections by CAS -CI (SD )+DC , CASCEPA , or MP 2.     

Theoretical study of [Li(H2O)n]+ and [K(H2O)n]+ (n = 1−4) complexes

The geometries, successive binding energies, vibrational frequencies, and infrared intensities are calculated for the [Li(H₂O)_n]⁺ and [K(H₂O)_n]⁺ (n = 1?4) complexes. The basis sets used are 6-31G* and LANL 1DZ (Los Alamos ECP +DZ ) at the SCF and MP 2 levels. There is an agreement for calculated structures and frequencies between the MP 2/6-31G* and MP 2/LANL 1DZ basis sets, which indicates that the latter can be used for calculations of water complexes with heavier ions. Our results are in a reasonable agreement with available experimental data and facilitate experimental study of these complexes. © 1995 John Wiley & Sons, Inc.     

Basis set and correlation energy dependence of geometry and harmonic frequencies of difluoroethane,CHF2CH3

Optimum geometries and harmonic frequencies calculated at the Hartree–Fock and the MP2 level are reported for the fluorohydrocarbon CHF₂CH₃; basis sets employed range from STO-3G to 6-311G**. The significantly shortened C? C distance of 1.50 Å is reproduced already with the simplest split-valence basis set; the C? F distance of 1.36 Å on the other hand needs MP2 correction at least at the double-ζ or 6-311G* level. Symmetry coordinates defined in terms of internal coordinates are in qualitative agreement with available experimental evidence. Even the best basis set yields frequencies that differ from experimental (anharmonic) values by up to 200 cm^?1 indicating the well-known necessity of including higher-order force constants if quantitative agreement with experiment is to be achieved.     

An improved generator coordinate Hartree–Fock method applied to the choice of contracted Gaussian basis sets for first‐row diatomic molecules

Accurate Gaussian basis sets (18s for Li and Be and 20s11p for the atoms from B to Ne) for the first‐row atoms, generated with an improved generator coordinate Hartree–Fock method, were contracted and enriched with polarization functions. These basis sets were tested for B₂, C₂, BeO, CN⁻, LiF, N₂, CO, BF, NO⁺, O₂, and F₂. At the Hartree–Fock (HP), second‐order Møller–Plesset (MP2), fourth‐order Møller–Plesset (MP4), and density functional theory (DFT) levels, the dipole moments, bond lengths, and harmonic vibrational frequencies were studied, and at the MP2, MP4, and DFT levels, the dissociation energies were evaluated and compared with the corresponding experimental values and with values obtained using other contracted Gaussian basis sets and numerical HF calculations. For all diatomic molecules studied, the differences between our total energies, obtained with the largest contracted basis set [6s5p3d1f], and those calculated with the numerical HF methods were always less than 3.2 mhartree. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 78: 15–23, 2000     

New analysis of the 2ν4, ν4+ν6, 2ν6, ν3+ν4, ν3+ν6, ν1, ν5, ν2+ν4, 2ν3, ν2+ν6 and ν2+ν3 bands of formaldehyde H212C16O: Line positions and intensities in the 3.5 μm spectral region

This work is mainly motivated by the atmospheric importance of formaldehyde. The 3.5 μm region is indeed commonly used for the infrared detection of this molecule in the troposphere and the line parameters which are presently available in the atmospheric databases for H₂CO are of rather poor quality in this spectral range. Using New Fourier transform spectra recorded in LPMA and in GSMA it has been possible to perform an extensive study of the 2ν₄, ν₄+ν₆, 2ν₆, ν₃+ν₄, ν₃+ν₆, ν₁, ν₅, ν₂+ν_4, 2ν₃, ν₂+ν₆ and ν₂+ν₃ bands of formaldehyde. Combining these data with previous frequency and intensity measurements for the ν₃+ν_4, ν₃+ν₆, ν₁, ν_5, ν₂+ν_4, 2ν₃ and ν₂+ν₆ bands [L.R. Brown R.H. Toth and A.S. Pine J. Mol. Spectosc. 406–428 and references therein] and an adequate theoretical model, it proved possible to reproduce rather satisfactorily the experimental data and to generate a list of line positions and intensities for the 3.5 μm region. The Hamiltonian model accounts for the various Coriolis-type resonances and anharmonic resonances which perturb the energy levels of the 4², 4¹6¹, 6², 3¹4¹, 3¹6¹, 1¹, 5¹, 2¹4¹, 3², 2¹6¹, and 2¹3¹ vibrational states. This is also the case for the line intensity calculations, which allow one to reproduce satisfactorily the line-by-line intensity measurements as well as the integrated intensities available in the literature.     

FT-Raman and infrared spectra,r0 structural parameters,ab initio calculations and vibrational assignment for nitryl chloride

The FT-Raman spectra (2000-30 cm⁻¹) of liquid and solid nitryl chloride, ClNO₂, along with the infrared spectra (2000-80 cm⁻¹) of the gas and solid have been recorded. All six fundamentals are confidently identified and the potential energy distributions determined from the force fields obtained from ab initio calculations. Several different basis sets have been utilized to determine the harmonic frequencies and force constants which are compared to the previously reported valence force constants. Structural parameters have been calculated with these basis sets including electron correlation with MP2, MP3 and MP4 perturbation. The calculated equilibrium structural parameters are compared to the experimental r₀ structural parameters. The spectra of the solid indicate that there are at least two molecules per primitive cell. All of these results are compared to the corresponding quantities for some similar molecules.     

Ruling out harmonic couplings as a possible origin of an anomalous isotope effect on the ν5modes in H bonds

Measurements of the transition probabilities of all the IR bands of adipic acid crystals, polarized in the a, c plane and having frequencies in the range 1000–4000 cm^?1show that the anomalous value P_H/P_D= 2 found for the ratio of the transition probabilities of ν_s bands

and

cannot be due to a harmonic coupling of ν_s vibrations with other vibrations.     

The microwave spectrum of dichlorosilylene (SiCl2) in excited vibrational states

The microwave spectrum of dichlorosilylene in excited vibrational states has been measured in the millimetre- and submillimetre-wave regions. Rotational and centrifugal distortion constants were determined for the ν₁, ν₂, 2ν₂ and ν₃ excited states of the Si³⁵Cl₂ isotopic species and for the ν₂ and 2ν₂ states of Si³⁵Cl³⁷Cl. Analysis of the Coriolis resonance between the ν₁ and ν₃ states of Si³⁵Cl₂ yielded values of the D Coriolis interaction constant with F constrained, of two higher-order terms and also an accurate value [5.402338(95) cm⁻¹] of the energy difference between the two excited vibrational states. The rotational constants of Si³⁵Cl₂ in the first excited states of the three normal vibrations were combined with those of the ground vibrational state reported in a previous paper [M. Tanimoto et al., J. Chem. Phys. 91, 2102 (1989)] to obtain the equilibrium structure, harmonic and cubic/third-order anharmonic potential constants.     

Equilibrium structure and vibrational frequencies of the O4 molecule

SCF closed shell calculations were performed to determine the equilibrium structure and vibrational frequencies of the O₄ molecule by means of Payne's method and with the help of the molecule's symmetry coordinates. The equilibrium geometry corresponds to symmetry group D_2d with R = 1.505 Å and h = 0.094 Å. The vibrational frequencies are: ν₅(E) = 885.5 cm^?1, ν₃(B₁) = 1051.9 cm^?1, ν₁(A₁) = 1018.3 cm^?1, ν₄(B₂) = 880.3 cm^?1. The second vibrational coordinate (A₁) corresponds to a double-well potential. The first vibrational levels were calculated by a variational method.     

An ab initio calculation of symmetric bending and stretching vibrational states of the H3O+ and D3O+ ions

Calculations are reported for the symmetric bending and stretching vibrational states of H₃O⁺ and D₃O⁺ including coupling between these two modes. The calculations were carried out by using a potential surface calculated by the SCF CI method and expressed in terms of symmetric internal coordinates. The transition energy of the ν₂ (1^? ← 0⁺) inversion mode is found to be 985 cm^?1, which is comparable to the experimental value of 954.417 cm^?1 observed by Haese and Oka. The calculated inversion doubling of the lowest state is 51 cm^?1.     

Ab initio Study of Anharmonic Force Field and Spectroscopic Constants for Germanium Dichloride

Ab initio study of the equilibrium structure, spectroscopy constants, and anharmonic force field for several isotopomers of germanium dichloride (⁷⁰GeCl₂, ⁷²GeCl₂, and ⁷⁶GeCl₂) have been carried out at the MP2 and CCSD(T) levels of theory using cc-pVTZ basis set. The calculated geometries, rotational constants, vibration-rotation interaction constants, harmonic frequencies, anharmonic constants, quartic and sextic centrifugal distortion constants, cubic and quartic force constants are compared with experimental data. For small mass differences of the Ge isotopes, the isotopic effects for germanium dichloride are much weaker. The agreements are satisfactory for these two methods, but the deviations of CCSD(T) results are slightly larger than that of MP2, because of CCSD(T)'s inadequate treatment of electron correlation in hypervalent Cl atom.     

Ab initio configuration interaction study of the structure and magnetic properties of radicals and radical ions derived from group 13–15 trihydrides

The structures, characteristic vibrations and magnetic properties of two isoelectronic series of radicals and radical ions derived from group 13–15 trihydrides have been investigated by post-Hartree-Fock theoretical techniques. Møller-Plesset perturbation theory based on an unrestricted Hartree-Fock determinant has been employed to determine the structures and vibrational frequencies in the 9-electron series, BH⁻₃, CH₃, and NH⁺₃. These species are found to be planar. Spin density distributions and ionization energetics have been estimated using a variational configuration interaction procedure. A positive electron affinity for BH₃ has not been demonstrated. The effect of out-of-plane vibrations on the hyperfine coupling constants is determined at a similar level of theory. In the 17-electron series AlH⁻₃, SiH₃, and PH⁺₃, pyramidal structures are found by using and extended split-valence basis at the SCF level. The computed harmonic force field suggests that a tentative assignment of a matrix isolated infrared spectrum to SiH₃ is incorrect. This conclusion is reinforced by calculation of the vibrational intensity patterns. Hyperfine interaction tensors computed at the optimized geometries from the UHF wavefunction with a more complete polarized double-zeta basis set are in accord with experiment. Vibrational effects are estimated by averaging the UHF spin density over an energy surface determined by second-order perturbation theory. Corrections due to vibrations are smaller than in the carbon series and single-point configuration interaction calculations confirm the reliability of the UHF spin densities.     

Interaction of acetonitrile with alkaline metal cations: A density functional,coupled‐cluster,and quadratic configuration interaction study

A systematic quantum chemical study of CH₃CN and its CH₃CN?M⁺ 1:1 model adducts (M⁺∈{Li⁺, Na⁺}) is presented, with respect to binding energetics, structural and vibrational force field changes. Several gradient‐corrected density functional levels of theory were employed (of both “pure” and “hybrid” character), together with the coupled cluster including double substitutions from the Hartree–Fock determinant (CCD) and quadratic configuration interaction including single and double substitutions (QCISD) methods [with the rather large 6‐311G(d,p) basis set], and their performances compared. The binding energy decompositions according to the Kitaura–Morokuma scheme and the reduced variational space self‐consistent field (RVS‐SCF) method have shown that the electrostatic plus polarization interaction terms are primarily responsible for overall stabilization, while the charge‐transfer term is negligibly small and virtually identical for both adducts. The computed harmonic vibrational frequencies for acetonitrile correlate excellently with the experimental ones (r²>0.9998 for almost all cases, while for the BLYP level, r²=1). It is shown for the first time that the experimentally observed blue shifts of the ν_CN mode are caused even by formation of 1:1 adducts, contrary to the previously accepted opinions. The CCD and QCISD, as well as the BPW91 and BP86 levels of theory predict almost excellently the ν_CN mode blue shift upon adduct formation, while the BLYP and B3LYP levels perform significantly poorer. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001     

HF and MP2 calculations on CN−, N2, AlF,SiO, PN,SC, ClB,and P2 using correlated molecular wave functions

Contracted basis sets of double zeta valence quality plus polarization functions (DZP) and augmented DZP basis sets, which were recently constructed for the first‐ and second‐row atoms, are applied to study the electronic ground states of the diatomic molecules CN^?, N₂, AlF, SiO, PN, SC, ClB, and P₂. At the Hartree–Fock (HF) and/or Møller–Plesset second‐order (MP2) levels, total and molecular orbital energies, dissociation energies, bond lengths, harmonic vibrational frequencies, and dipole moments are calculated and compared with available experimental data and with the results obtained from correlation consistent polarized valence basis sets of Dunning's group. For N₂, calculations of polarizabilities at the HF and MP2 levels with the sets presented above are also done and compared with results reported in the literature. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2006     

Vibrational properties and DFT calculations of formamidine-templated Co and Fe formates

Experimental Raman and IR spectra of [NH₂-CH-NH₂][M(HCOO)₃] (M = Co, Fe), containing formamidinium cations [NH₂-CH-NH₂]⁺ (FMD⁺) were recorded at room temperature. In order to assign the vibrational modes corresponding to the FMD⁺ cation, the three-parameter hybrid B3LYP density functional method has been used with the 6-311G(2d,2p) basis to derive the vibrational wavenumbers (harmonic and anharmonic), infrared intensities and Raman scattering activities of formamidine molecule and FMD⁺ cation. The performed calculations revealed that protonation should affect most significantly the ν(CH), ρ(NH₂), ω(NH₂) and τ(NH₂) modes, which are expected to shift towards higher wavenumbers after protonation.     

Raman and resonance-Raman spectra of CsI/I−3

Raman and resonance-Raman spectra of the I^?₃ ion isolated within CsI crystals have been studied using 647 nm and 488 nm exciting radiation. Sample temperatures between 300 and 20 K have been used. Eleven overtones of the symmetric stretching mode (nν₁) have been observed in the resonance-Raman spectrum excited by the 488 nm Ar⁺ laser line. Bands centred at 153, 170, 264 and 304 cm^?1 have been assigned as ν₃, 2ν₂, ν₁+ν₃ and 2ν₃(Σ⁺) respectively. The remaining structure between the nν₁ lines has been assigned as due to combinations of these lines with the lattice vibrations of the CsI crystal.     

Disilene,silylsilylene and their cations

An accurate examination of features of the ground state surfaces of Si₂H₄ and Si₂H ₄ ⁺ is reported; they are compared to C₂H₄ and C₂H ₄ ⁺ . For the neutral species, accurate SCF calculations show disilene to be planar, but silylsilylene has the lower energy, whereas at the correlated (CI, MP2, MP3, MP4(SD)) levels disilene becomes trans bent and has the lower energy by 6 kcal/mol. In view of a recent theoretical suggestion that this value should be 23 kcal/mol, we have used large basis sets in these investigations. Our calculations cannot support this large value. Similar investigations are reported for the cation, where the planar disilene structure is predicted to be the most stable. It may be very slightly twisted at high accuracy CI, but it is much lower in energy than the silylsilylene structure. Vibrational frequencies and infra-red intensities are also reported. Theoretical photoelectron spectra of C and Si systems are presented and compared with experiment.     

A quantum chemical study of the infrared absorption intensities of the isolectronic C3v systems NH3, H3O+ and CH−3

The dipole moment derivatives and the infrared absorption intensities for the isoelectronic, isostructural species NH₃, H₃O⁺ and CH⁻₃, calculated by ab initio quantum methods within the double harmonic approximation, are reported. The calculations were performed at the SCF, CI and CPA″ levels of theory using basis sets of triple zeta+two polarization functions quality. For the ions H₃O⁺ and CH⁻₃, in the absence of adequate experimental information, the calculations are fully ab initio, since the equilibrium geometries as well as the force constants had to be computed. The applicability of the harmonic treatment to systems with inversion potentials is discussed, especially with regard to H₃O⁺. The dipole moment derivatives of the three systems show interesting, regular trends in accordance with the amount of electronic charge associated with the hydrogen atoms.