Harmonic Force Field Calculations Research Articles

Spectroscopic (FTIR, FT-Raman, 1H NMR and UV–Vis) and DFT/TD-DFT studies on cholesteno [4,6-b,c]-2′,5′-dihydro-1′,5′-benzothiazepine

The density functional theory calculations and spectroscopic characterization (FTIR, FT-Raman, 1H NMR and ultraviolet–visible) of the pharmaceutically active compound Cholesteno [4,6-b,c]-2′,5′-dihydro-1′,5′-benzothiazepine (CHBT) have been carried out. The recorded Vibrational, proton NMR and electronic spectra have been compared with the simulated one and they are found in well agreement. The bands appearing in FTIR and FT-Raman spectra are assigned with great accuracy using animated modes. The ultraviolet–visible (UV–Vis) absorption bands are assigned using molecular orbitals data obtained by TD-B3LYP/PCM calculations with 6-311G(d,p) basis set. Proton NMR signal peaks have been assigned using GIAO-B3LYP/6-311G(d,p)/PCM method. The hyperconjugative interactions and charge delocalization within the molecule have been studied using natural bond orbital (NBO) analysis. The non-linear optical (NLO) behavior of the present compound has been investigated using theoretical dipole moment, polarizability and first static hyperpolarizability values. The thermodynamic functions (entropy, heat capacity and enthalpy) obtained from harmonic force field calculations for the range of temperature 100–1000 K are reported. Moreover, highest occupied molecular orbital – lowest unoccupied molecular orbital (HOMO-LUMO) analysis, molecular electrostatic potential (MEP) diagram, global reactivity descriptors and condensed or atomic Fukui Functions have been also presented.

Structure and Vibrational Motion of Insulin from Raman Optical Activity Spectra

The Raman optical activity (ROA) spectroscopic technique has been applied in the past to many biologically relevant systems including peptides, proteins, sugars, and even viruses. However, theoretical interpretation of the spectra relies on lengthy quantum-chemical computations, which are difficult to extend to larger molecules. In the present study, ROA and Raman spectra of insulin under a range of various conditions were measured and interpreted with the aid of the Cartesian-coordinate tensor transfer (CCT) method. The CCT methodology yielded spectra of insulin monomer and dimer of nearly ab initio quality, while at the same time reproducing the experimental data very well. The link between the spectra and the protein structure could thus be studied in detail. Spectral contributions from the peptide backbone and the amino acid side chains were calculated. Likewise, specific intensity features originating from the α-helical, coil, β-sheet, and 3(10)-helical parts of the protein could be deciphered. The assignment of the Raman and ROA bands to intrinsic molecular coordinates as based on the harmonic force field calculation revealed their origin and degree of locality. Alternatively, the relation of the structural flexibility of insulin to the inhomogeneous broadening of spectral bands was studied by a combination of CCT and molecular dynamics (MD). The present study confirms the sensitivity of the ROA technique to some subtle static and dynamic changes in molecular geometry, and many previous ad hoc or semiempirical spectral-structure assignments could be verified. On the other hand, a limitation in longer-range tertiary structure sensitivity was revealed. Unlike for smaller molecules with approximately equal contributions of the electric dipole (α), quadrupole (A), and magnetic dipole (G') polarizabilities, only the electric dipolar polarization (α) interactions seem to dominate in the protein ROA signal. The simulations concern the largest molecule for which such spectra were interpreted by a priori procedures and significantly enhance protein folding studies undertaken by this technique.

An exploration of electronic structure and nuclear dynamics in tropolone: II. The à B12 (π∗π) excited state

The first excited singlet state of tropolone (A (1)B(2)) and the attendant pi(*)<--pi electronic transition have been examined computationally by applying several quantum chemical treatments built upon the aug-cc-pVDZ basis set, including time-dependent density functional theory (TDDFT/B3LYP), configuration interaction singles with perturbative corrections [CIS and CIS(D)], and equation-of-motion coupled-cluster schemes [EOM-CCSD and CR-EOMCCSD(T)]. As in the case of the X (1)A(1) ground state [L. A. Burns, D. Murdock, and P. H. Vaccaro, J. Chem. Phys. 124, 204307 (2006)], geometry optimization procedures and harmonic force-field calculations predict the electronically excited potential surface to support a global minimum-energy configuration of rigorously planar (C(s)) symmetry. Minimal Hartree-Fock (HF/CIS) and density-functional (DFT/TDDFT) approaches yield inconsistent results for the X (1)A(1) and A (1)B(2) manifolds; however, coupled-cluster (CCSD/EOM-CCSD) methods give fully relaxed proton-transfer barrier heights of DeltaE(pt) (X)=3296.1 cm(-1) and DeltaE(pt) (A)=1270.6 cm(-1) that are in accordance with the experimentally observed increase in vibrationless tunneling splitting upon electronic excitation. Detailed analyses show that this reduction in DeltaE(pt) stems from a variety of complementary factors, most notably an overall contraction of the proton-transfer reaction site (whereby the equilibrium O...O donor-acceptor distance decreases from 2.53 to 2.46 A) and a concomitant shortening of the intramolecular hydrogen bond. Further refinement of A (1)B(2) energies through single-point perturbative triples corrections [CR-EOMCCSD(T)] leads to 1316.1 cm(-1) as the best current estimate for DeltaE(pt) (A). Direct comparison of the lowest-lying out-of-plane torsional mode [nu(39)(a(2))] for X (1)A(1) and A (1)B(2) tropolone reveals that its disparate nature (cf. nu(39) (X)=101.2 cm(-1) and nu(39) (A)=42.0 cm(-1)) mediates vibrational-averaging effects which can account for inertial defects extracted by rotationally resolved spectroscopic measurements.

Factors Contributing to the Accuracy of Harmonic Force Field Calculations for Water.

An analysis of the major factors affecting the accuracy of harmonic force field computations of water is presented. By systematically varying the level of approximation in the basis set, treatment of electron correlation, core electron correlation, and relativistic correction, the underlying sources of error in the computation of harmonic vibrational frequencies for water were quantified. The convergence error due to wavefunction description with a cc-pVQZ basis set in the absence of electron correlation was 1.6 cm(-1), as determined from extending the Hartree-Fock computations to larger basis sets. The convergence error due to neglecting higher-order electronic correlation terms than are included at the CCSD(T) level using the cc-pVTZ basis set was estimated to be 4.7 cm(-1), as determined from frequency calculations up to CCSDTQ for water and literature results up to CCSDTQP for diatomic molecules. The convergence error due to omitting higher-order diffuse functions than included in aug-cc-pVQZ was found to be 3.7 cm(-1), as determined by adding more diffuse functions in larger basis sets. The error associated with neglecting core electron correlation effects (i.e., "freezing" core electrons) was 5.0 cm(-1) and with neglecting relativistic effects was 2.2 cm(-1). Due to a cancellation among these various sources of error, the harmonic frequencies for H2O computed using the CCSD(T)/aug-cc-pVQZ model chemistry were on average within 2 cm(-1) of experimentally inferred vibrational frequencies.

Absorption Spectra of Ground-State and Low-Lying Electronic States of Copper Nitrosyl: A Rare Gas Matrix Isolation Study

The reaction of ground-state Cu atoms with NO during condensation in solid argon, neon, and binary argon/neon mixtures has been reinvestigated. In addition to the ground-state already characterized in rare gas matrixes by its nu1 mode in reactions of laser-ablated Cu with nitric oxide, another very low lying electronic state is observed for CuNO in solid argon. Photoconversion and equilibrium processes are observed between the two lowest lying electronic states following photoexcitations to second and third excited states in the visible and near-infrared. The electronic spectrum of the CuNO complex was also recorded to understand the photoconversion processes. In solid neon, only the ground state (probably 1A') and the second and third excited states are observed. This suggests that interaction with the argon cage stabilizes the triplet state to make 1A' and 3A' ' states almost isoenergetic in solid argon. On the basis of previous predictions founded on DFT calculations on the very low lying 1A' and 3A' ', a mechanism is proposed, involving the singlet-triplet state manifolds. For these two lower and one higher electronic states, 14N/15N, 16O/18O, and 63Cu/65Cu isotopic data on nu1, nu2, and nu3 have been measured. On the basis of harmonic force-field calculations and relative intensities in the vibronic progressions, some structural parameters are estimated. The molecule is bent in all electronic states, with Cu-N-O bond angles varying slightly around 130 +/- 10 degrees , but the Cu-N bond force constants are substantially different, denoting larger differences in bond lengths.

Molecular structure, spectral constants, and fermi resonances in chlorine nitrate

Chlorine nitrate has two low-lying vibrational modes that lead to a series of Fermi resonances in the 9υ97υ7 family of levels that include the 92⇔71 and 93⇔7191 dyads and the 94⇔9271⇔72 and 95⇔9371⇔9172 triads. These states, along with the ground and 91 vibrational states, have been previously analyzed with millimeter and submillimeter wave spectroscopy and provide a substantial body of data for the investigation of these resonances and their impact on calculated spectroscopic constants and structural parameters. Due to fitting indeterminacies, these previous analyses did not include the main Fermi resonance interaction term. Consequently, the fitted rotational constants are linear combinations of the unmixed rotational constants of the basis vibrational states. In this paper, we have calculated the contributions of the Fermi resonances to the observed rotational constants in a model that determines the vibrational–rotational constants, the Fermi term and the mixing between interacting vibrational states, the cubic potential constant (φ997) that connects interacting levels through a Fermi resonance, and the inertial defects. These results agree with predictions from ab initio and harmonic force field calculations and provide further experimental information for the determination of the fundamental molecular properties of chlorine nitrate.

Molecular structure, spectral constants, and fermi resonances in chlorine nitrate

Chlorine nitrate has two low-lying vibrational modes that lead to a series of Fermi resonances in the 9 υ 9 7 υ 7 family of levels that include the 9 2⇔7 1 and 9 3⇔7 19 1 dyads and the 9 4⇔9 27 1⇔7 2 and 9 5⇔9 37 1⇔9 17 2 triads. These states, along with the ground and 9 1 vibrational states, have been previously analyzed with millimeter and submillimeter wave spectroscopy and provide a substantial body of data for the investigation of these resonances and their impact on calculated spectroscopic constants and structural parameters. Due to fitting indeterminacies, these previous analyses did not include the main Fermi resonance interaction term. Consequently, the fitted rotational constants are linear combinations of the unmixed rotational constants of the basis vibrational states. In this paper, we have calculated the contributions of the Fermi resonances to the observed rotational constants in a model that determines the vibrational–rotational constants, the Fermi term and the mixing between interacting vibrational states, the cubic potential constant ( φ 997) that connects interacting levels through a Fermi resonance, and the inertial defects. These results agree with predictions from ab initio and harmonic force field calculations and provide further experimental information for the determination of the fundamental molecular properties of chlorine nitrate.

The Co + CO Reaction: Infrared Matrix Isolation Study and Density Functional Calculations

The Co + CO reaction argon has been reinvestigated using deposition of ground-state reagents in solid argon. The Co + CO reaction products observed without activation energy are Co2CO, a van der Waals Co···CO complex, and the higher carbonyl species Co(CO)x. Infrared radiation at approximately 3000 cm-1, to promote Co into the first excited-state b 4F (3d84s1 configuration), converts the van der Waals complex to the chemically bound, most stable carbonyl form CoCO. A second irradiation with an approximately 580 nm wavelength (17000 cm-1) reconverts the chemically bound CoCO (2Δ) state into the Co···CO van der Waals (4Φ) state. The photoprocess is fully reversible. Isotopic data on ν1, ν2, ν3, 2ν1, ν1 + ν2, and ν1 + ν3 have been measured in the near- and far-infrared regions for the carbonyl form. This enables a complete harmonic force-field calculation based on a linear geometry, in agreement with all theoretical predictions. DFT calculations of the geometrical and electronic properties of CoCO complexes from the Co + CO reaction are also presented and compared to the experimental values. Reaction mechanisms are proposed and comparisons with bond force constants of NiCO and CuCO are also presented.

Infrared Spectrum of the Palladium Nitrosyl Complex Isolated in Solid Argon

The reaction of ground-state Pd atoms with NO during condensation in solid argon has been reinvestigated. The Pd(η1-NO) molecule, already characterized in reactions of laser-ablated Pd, is the only product observed for the reaction between one Pd atom and one nitric oxide molecule. Isotopic data on ν1, ν2, and ν3 have been measured in the mid- and far-infrared regions. These enable an independent estimate of the molecular shape on the basis of complete harmonic force-field calculations. The molecule is bent, with a Pd−N−O bond angle around 115 ± 15°, in basic agreement with theoretical predictions. This study provides a useful test for comparing the performance of various quantum mechanical methods for reproducing the spectroscopic properties. The bonding in Pd- and NiNO is analyzed using natural bonding orbital and topological methods.

Reactions of ground state Ti atoms with NO: insertion versus complexation. An IR matrix isolation study

The reaction of ground state Ti atoms with NO during condensation in solid argon has been reinvestigated. The NTiO molecule, already characterized in reactions of laser-ablated Ti, is the only product observed for the reaction between one Ti atom and one nitric oxide molecule. Isotopic data on ν 1, ν 2, ν 3, 2 ν 1 and 2 ν 2 have been measured in the mid- and far-infrared regions. This enables a complete harmonic force-field calculation based on a bent geometry, in agreement with the conclusions of the previous study. No evidence is found, however, of a metastable nitrosyl complex intermediate, as previously proposed. This study confirms that the insertion reaction proceeds directly from the ground electronic state reagents, with no or very little activation energy.

The vibrational spectrum of PdCO isolated in solid argon

The infrared spectrum of PdCO isolated in solid argon has been reinvestigated. Isotopic data on ν 1, ν 2, ν 3, 2 ν 1, ν 1+ ν 2 and ν 1+ ν 3 have been measured in the near- and far-infrared regions. This enables us to make a complete harmonic force-field calculation based on a linear geometry, in agreement with all theoretical predictions. Comparisons with bond force constants of NiCO and CuCO and the latest ab initio predictions of the PdCO vibrational frequencies are also presented.

The rotational spectrum of chloryl chloride, ClClO2, in its ground vibrational state

Rotational spectra of the four main isotopomers of ClClO2 which together span the quantum numbers 10⩽J⩽77 and 0⩽Ka⩽34 have been studied in selected regions between 10 and 417 GHz. The in situ syntheses using reactions between FClO2 and either HCl or BCl3 are described. Rotational and centrifugal distortion constants have been derived and used for structure and harmonic force field calculations. The quadrupole coupling constants for both Cl nuclei have been determined and the molecular dipole moment has been derived from low field Stark effect measurements in the submillimeter region. The results are discussed in relationship to published results from a matrix-isolation study, from theoretical calculations, and from studies of related molecules.

Far-infrared spectrum and structure of copper monocarbonyl isolated in solid argon

The vibrational spectrum of ground-state CuCO isolated in solid argon at low temperature has been reinvestigated. In addition to the strongly infrared (IR) absorbing mode, ν 1, already observed, the low-frequency metal–ligand stretching and bending modes ν 2 and ν 3 have been measured in the far-IR region. With the help of various isotopic substitutions using Cu 12C 16O , Cu 13C 16O and Cu 12C 18O and harmonic force-field calculations based on the assumption of linear and bent geometries, some of the molecular parameters (bond force constants and valence angle) can be estimated and compared with theoretical predictions and to those derived for NiCO.

Millimeter-Wave Spectrum of Fluorodiacetylene (H[sbnd]C[tbnd]C[sbnd]C[tbnd]C[sbnd]F)

The millimeter-wave spectrum of HCCCCF was observed in the ground state as well as in the low-lying vibrationally excited states ν7, ν8, ν9and also associated ν8+ ν9, 2ν9, and 3ν9states. The HCCCCF molecule was produced by a dc glow discharge in a flowing vapor of 2,3,4,5,6-pentafluorostyrene. Thel-type doubling constants determined are in good agreement with the values predicted from the harmonic force field calculation.

Complete vibrational spectrum of NiCO isolated in solid argon

The infrared spectrum of NiCO isolated in solid argon has been reinvestigated. Isotopic data on ν 1, ν 2, ν 3, 2 ν 1, ν 1+ ν 2 and ν 1+ ν 3 have been measured in the near- and far-infrared regions. This enables a complete harmonic force-field calculation based on the assumption of a linear geometry. Comparisons with bond force constants of Ni(CO) 4 and the latest ab initio predictions of the NiCO vibrational frequencies are also presented.

On the infrared spectrum of lithium-monoethylene complementary infrared argon matrix and normal coordinate analysis studies

The vibrational spectrum of ground state lithium-monoethylene isolated in argon at low temperature has been reinvestigated for various isotopic forms ( 6 Li 7 LiC 2 H 4 , 6 Li 7 Li 13 C 2 H 4 or 6 Li 7 LiC 2 D 4 ). Additional information concerning some five yet unobserved fundamentals have been gathered, and assignments are suggested based on harmonic force-field calculations. Comparisons with recent DFT theoretical calculations outline the need for improved models.

Fourier-transform microwave spectrum, structure, harmonic force field, and hyperfine constants of sulfur chloride fluoride, ClSF

The rotational spectrum of sulfur choride fluoride, ClSF, has been observed for the first time in the frequency range 8–26 GHz by means of a pulsed molecular jet Fourier-transform microwave spectrometer. The unstable sample molecule has been prepared using a pulsed electrical discharge in jets containing a mixture of SF6 and SCl2 in Ne. Besides the parent species 35Cl32S19F, the isotopomers 37Cl32S19F and 35Cl34S19F could be observed in natural abundance. Rotational constants and quartic centrifugal distortion constants as well as nuclear quadrupole coupling constants due to 35Cl and 37Cl and spin–rotation constants due to 35Cl, 37Cl, and 19F are given. The data were used for the determination of r0, rΔP, rs structural parameters. Additionally, the new data were used for the refinement of the molecular harmonic force field. Results from those harmonic force field calculations were applied in the evaluation of the ground-state average structure, rz, and the estimation of the equilibrium structure, re. The rs structure is r(SF)=160.653(162) pm, r(SCl)=199.437(65) pm, and ∠(ClSF)=100.732(81)°. The diagonal elements of the 35Cl/37Cl quadrupole coupling tensors have been obtained, and are interpreted in terms of the bonding at 35Cl/37Cl. Negative 19F spin–rotation constants suggest a close analogy of the electronic structures of ClSF and SF2.

Ab Initio Study of Hydrogen Abstraction Reactions

Ab initio electronic structure calculations have been carried out on six hydrogen abstraction reactions of the form, X−H + Y• → X• + H−Y, where X, Y = CH3, NH2, and OH. Geometric structures for the reactants, reactant complexes, transition states, product complexes, and products of each reaction have been gradient optimized at both the UMP2 and DFT(B3LYP) theory levels using the 6-311++G(2d,p) basis set. The character of each stationary state as a minimum or saddle point was determined by a harmonic force field calculation. PMP2 and CCSD(T) energies were also calculated at the UMP2 optimized geometries. CCSD(T) geometry optimizations were carried out for selected cases to resolve differences in results between UMP2 and DFT. The calculated reaction energies, barrier heights, and weak complex stabilities are compared to experiment, where possible, and among the different theory levels. The geometric structures and wave function properties are compared between the UMP2, CCSD(T), and DFT(B3LYP) methods. In general, all the methods predict reaction energies to within several kcal/mol of experiment. Calculated DFT(B3LYP) activation barriers are usually equal to or smaller than fitted Arrehenius equation activation energies (Ea), with the gap increasing as X and Y are more electronegative. The DFT(B3LYP) activation barrier for the CH4 + CH3• reaction is, therefore, in the best agreement with experiment. The UMP2, PMP2//UMP2, and CCSD(T)//UMP2 methods give activation energies that are somewhat greater than Ea, as expected. The geometric structures of the three exchange reaction (X ≠ Y) transition states (TS) generally agree reasonably well between the UMP2 and DFT(B3LYP) methods, with active site bond length differences reflecting variations in the calculated exothermicities of the reactions. The UMP2 and DFT(B3LYP) methods give different conformations for the NH3 + OH• transition state, and CCSD(T) endorses the DFT results. For the H2O + OH• reaction UMP2 gives the symmetric structure as an energy minimum. DFT(B3LYP) predicts the symmetric structure to be the TS, in agreement with the optimized CCSD(T) result. For the NH3 + NH2• reaction the UMP2 transition state is symmetric but with an unusually low imaginary reaction path frequency, while the DFT(B3LYP) frequency is reasonable. CCSD(T)//UMP2 and DFT(B3LYP) hydrogen-bonding energies for the reaction and product complexes are very similar and generally smaller than the UMP2 and PMP2//UMP2 results. The equilibrium geometric structures at the UMP2 and DFT(B3LYP) levels generally agree reasonably well. However, for the NH2···H−OH product complex DFT(B3LYP) gives a planar conformation, while UMP2 predicts a Cs nonplanar configuration.

Vibrational energy relaxation of HOD in liquid D2O

Molecular Dynamics simulation is used to study the vibrational relaxation of the first excited state of the O–H stretch for HOD dissolved in D2O. The technique applied is based on a Landau–Teller type formula, in which the solvent contribution is computed classically, while the quantum nature of the solute enters through the transition moments of the molecular normal modes. The experimental result for the relaxation time (≊8 ps) is accounted for, and the pathway to the ground state is determined. The relaxation proceeds through a sequence of intramolecular transitions initially facilitated by the solute internal anharmonicities. In particular, the anharmonicity allows an initial and rate-determining transfer to the first overtone of the HOD bend; a corresponding harmonic force field calculation in which this step is precluded yields a relaxation time that is three orders of magnitude larger. The excess energy is removed by the bath modes, which include rotations and translations of all molecules, including the solute. Relaxation by Coriolis coupling plays a minor but non-negligible role, while the centrifugal coupling contribution to the relaxation is negligible.

Aluminium, gallium and indium trihydrides. an IR matrix isolation and ab initio study

Aluminium, gallium and indium trihydrides have been synthesized in solid argon by two different ways. All monoalane, gallane and indane species can be produced by direct condensation of H or D atoms, produced in an hydrogen/argon discharge, and metal atoms (M). For monoalane, a second synthetic route is possible: insertion into a hydrogen molecule of electronically excited A1H * following irradiation in the (A 1Π←X 1Σ +) transition region (365 to 404 nm). Complete sets of isotopic substitions (MH 3, MH 2D, MHD 2, MD 3) are reported for each metal. Subsequent harmonic force field calculations justify the proposed complete assignments based on the D 3h symmetry geometries calculated at all computational levels, and demonstrate an increase in the M—H force constant in monogallane with respect to monoalane (2.15 versus 2.02 mdyne Å −1), while it experiences a sharp decrease in monoindane (1.81. mdyne Å −1). These conclusions meet very satisfactorily the results of high level ab initio calculations which predict shorter GaH bond distance than A1H or InH ones. As for the dihydride species, it can be shown that only DFT and CCSD methods are capable of accurately predicting the vibrational frequencies.