Gas-phase Vibrational Frequencies Research Articles

Computational description of quantum chemical calculations and pharmacological studies of the synthesized chalcone derivative: A promising NLO material

The intended chalcone, (E)-1-(4-aminophenyl)-3-(4-ethoxyphenyl)-prop-2-en-1-one (4A3EP), was synthesized in an alkaline medium by the Claisen–Schmidt condensation reaction of 4-aminoacetophenone with 4-ethoxybenzaldehyde. Spectroscopic analytical techniques such as UV–visible, FT-IR, FT-RAMAN, 1H NMR, and 13C NMR investigations were used to analyze the molecular structure of the title molecule. The optimized molecular structure of the chalcone in gas phase vibrational frequencies and associated vibrational assignments were theoretically studied and compared with experimental results using the B3LYP/6–311++G techniques. All the experimental results were found to be in line with the theoretical values. The non linear optical activity of the title compound was proved from the hyperpolarizability calculations. In addition, EHOMO (−5.9038 eV), ELUMO (−2.2833 eV), energy gap (3.6205 eV) and electrophilicity index (4.628) were calculated to explore the reactivity, stability and bio activity of the title compound. The molecular electrostatic potential map was generated in order to spot the electrophilic and nucleophilic sites in the title compound. Natural bond orbital analysis was investigated in order to forecast the stability and charge transfer tendency of a title molecule. FUKUI FUNCTIONS were also calculated using DFT. Its anti-inflammatory, anti-diabetic, and anti-oxidant activities were also investigated. A molecular docking model was used to study the ligand-protein binding interactions of a synthetic chalcone derivative with the main protease of SARS-CoV-2 (the PDB code is 6yb7).

Thermophysical properties of water using reactive force fields

The widescale importance and rich phenomenology of water continue to motivate the development of computational models. ReaxFF force fields incorporate many characteristics desirable for modeling aqueous systems: molecular flexibility, polarization, and chemical reactivity (bond formation and breaking). However, their ability to model the general properties of water has not been evaluated in detail. We present comprehensive benchmarks of the thermophysical properties of water for two ReaxFF models, the water-2017 and CHON-2017_weak force fields. These include structural, electrostatic, vibrational, thermodynamic, coexistence, and transport properties at ambient conditions (300K and 0.997g cm-3) and along the standard pressure (1bar) isobar. Overall, CHON-2017_weak predicts more accurate thermophysical properties than the water-2017 force field. Based on our results, we recommend potential avenues for improvement: the dipole moment to quadrupole moment ratio, the self-diffusion coefficient, especially for water-2017, and the gas phase vibrational frequencies with the aim to improve the vibrational properties of liquid water.

A Homoleptic KrF2 Complex, [Hg(KrF2 )8 ][AsF6 ]2 ⋅2 HF.

The reaction of Hg(AsF6 )2 with a large molar excess of KrF2 in anhydrous HF has afforded the first homoleptic KrF2 coordination complex of a metal cation, [Hg(KrF2 )8 ][AsF6 ]2 ⋅2 HF. The [Hg(KrF2 )8 ]2+ dication is well-isolated in the low-temperature crystal structure of its HF-solvated [AsF6 ]- salt, and consists of eight KrF2 molecules that are terminally coordinated to Hg2+ by means of Hg-F(KrF) bonds to form a slightly distorted, square-antiprismatic coordination sphere around mercury. The Raman spectrum of [Hg(KrF2 )8 ]2+ was assigned with the aid of calculated gas-phase vibrational frequencies. Computational studies indicate that both electrostatic and orbital interactions are important for metal-ligand bonding and provide insight into the geometry of the [Hg(KrF2 )8 ]2+ cation and the nature of noble-gas difluoride ligand bonding.

Electronic structure modeling of dinuclear copper(II)-methacrylic acid complex by density functional theory

A dinuclear centrosymmetric copper(II) complex with the formula [Cu(2)(mu-maa)(4)(maaH)(2)] has been synthesized and experimentally characterized by IR, electronic spectroscopy, and X-ray single-crystal diffractometry. Starting from experimental X-ray geometry and using antiferromagnetic singlet ground state, gas phase geometry optimization was performed by density functional hybrid (B3LYP) method with 6-31G(d) and LANL2DZ basis sets. Gas-phase vibrational frequencies and single point energy (SPE) calculations have been carried out at the geometry-optimized structure. Molecular electrostatic potential calculated at the optimized geometry and natural bond orbital analysis data have been extracted from SPE output. The gas-phase electronic transitions of the title complex were investigated by the time dependent-density functional theory (TD-DFT) approach with the same theory employing LANL2DZ basis set. Also the calculated UV-Vis based upon TD-DFT results and IR spectra were simulated for comparison with the experimental ones.

Slow photoelectron velocity-map imaging of the CnH− (n = 5–9) anions

High-resolution photoelectron spectra of the CnH− anions with n = 5–9 are acquired with slow electron velocity-map imaging (SEVI). Spectral features are assigned with the help of electronic structure calculations and Franck–Condon simulations. Well-resolved transitions to the linear 2Π and ã4Σ− neutral states are observed for species with an odd number of carbon atoms. For C6H− and C8H−, transitions to the 2Π neutral ground state and the low lying Ã2Σ+ excited state are observed. Precise electron affinities, term energies, fine structure splittings, and gas-phase vibrational frequencies are determined. The C5H−, C7H− and C9H− SEVI spectra are consistent with the anions having 3Σ− linear triplet ground states.

The molecular structure of selenium dibromide as determined by combined gas-phase electron diffraction–mass spectrometric experiments and quantum chemical calculations

The vapour over solid SeBr(4) at 10 degrees C was investigated with a combined gas-phase electron diffraction/mass spectrometric (GED/MS) method. The composition of the vapour derived from the mass spectra (43% SeBr(2), 56.7% Br(2) and 0.3% Se(2)Br(2)) was in agreement with the composition obtained from the analysis of the simultaneously recorded GED intensities (41(3)% SeBr(2), 59(3)% Br(2)). The GED study results in the following geometric parameters (r(g), angle(g) values with total uncertainties): Se-Br = 2.306(5) A and Br-Se-Br = 101.6(6) degrees . Most quantum chemical approximations (B3LYP, MP2, CCSD and CCSD(T) with relativistic effective core potentials and cc-pVTZ as well as aug-cc-pVTZ basis sets for the outer shells) overestimate the Se-Br bond length by 0.01 to 0.03 A. All methods reproduce the bond angle correctly, except for the B3LYP method. Gas phase vibrational frequencies estimated from experimental vibrational amplitudes agree well with those measured by Raman spectroscopy in acetonitrile solutions. All computational methods overestimate vibrational frequencies, especially that for the symmetric stretch vibration, by about or 8 to 13%.

Propargyl Radical: Ab Initio Anharmonic Modes and the Polarized Infrared Absorption Spectra of Matrix-Isolated HCCCH2

The propargyl radical has twelve fundamental vibrational modes, gamma(vib)(HCCCH2) = 5a1 [symbol: see text] 3b1 [symbol: see text] 4b2, and nine have been detected in a cryogenic matrix. Ab initio coupled-cluster anharmonic force field calculations were used to help guide some of the assignments. The experimental HC=:C-:CH2 matrix frequencies (cm(-1)) and polarizations are a1 modes--3308.5 +/- 0.5, 3028.3 +/- 0.6, 1935.4 +/- 0.4, 1440.4 +/- 0.5, 1061.6 +/- 0.8; b1 modes--686.6 +/- 0.4, 483.6 +/- 0.5; b2 modes--1016.7 +/- 0.4, 620 +/- 2. We recommend a complete set of gas-phase vibrational frequencies for the propargyl radical, HC=:C-:CH2 2 X (2)B1. From an analysis of the vibrational spectra, the small electric dipole moment, mu(D)(HCCCH2) = 0.150 D, and the large resonance energy (HCCCH2), roughly 11 kcal mol(-1), we conclude that propargyl is a completely delocalized hydrocarbon radical and is best written as HC=:C-:CH2.

Molecular structure and nicotinic activity of arecoline. A gas electron diffraction study combined with theoretical calculations

The molecular structure of arecoline (methyl 1,2,5,6-tetrahydro-1-methylnicotinate, ▪ has been determined by gas electron diffraction. Diffraction patterns were taken at about 370 K. Structural constraints for the data analysis were obtained from MP2/6-31G** calculations. Vibrational mean amplitudes and shrinkage corrections were calculated from the force constants obtained from the gas-phase vibrational frequencies and the B3LYP/6-31G** calculations. The electron diffraction data were well reproduced by assuming the mixture of four conformers. The determined structural parameters ( r g (Å) and ∠ α (°)) for the main conformer with 3 σ in parentheses are as follows: 〈 r g(N–C ring)〉=1.456(4); r g(N–C methyl)=1.451 (d.p.); r g(C C)=1.339(9); 〈 r g(C–C)〉=1.512(3); r g(O–C methyl)=1.434(5); r g(C(O)–O)=1.355 (d.p.); r g(C O)=1.209(4); the out-of-plane angle of the methyl group=50.3(23); ∠C ringN ringC ring=112.8(30); ∠N ringC ringC ring(H 2)=110.5(16); 〈∠C ringC ringC ring〉=118.4(5); ∠C CC(O)=116.8(7); ∠CC O=127.6(9); ∠CC–O=109.8(8), where the angle brackets denote averaged values and d.p. denotes dependent parameters. Fixing the abundances of the minor conformers, Ax-s- cis and Ax-s- trans, at the theoretical values (13% in total), those of the Eq-s- cis and Eq-s- trans conformers were determined to be 46(16) and 41(16)%, respectively. Here Ax and Eq denote the axial and equatorial directions of the N–CH 3 bond and s- cis and s- trans show the orientation of the methoxycarbonyl group expressed by the configuration of the C O and C C bonds. The N⋯O carbonyl distances of the Eq-s- cis and Ax-s- cis conformers are 4.832(13) and 4.874(16) Å, respectively. They are close to the N⋯N distance of the most abundant conformer of nicotine, 4.885(6) Å, suggesting that the Eq-s- cis and Ax-s- cis conformers have nicotinic activity.

The infrared fundamental intensities and polar tensor of CH 3NC

The molecular force field and polar tensor of methyl isocyanide have been determined from its gas phase vibrational frequencies and infrared intensities. Quantum chemical results from MP2(FC), B3LYP and quadratic configuration interaction calculation including single and double substitutions procedures using a 6-311++G(3d,3p) basis set have been used to determine the signs of the dipole moment derivatives with respect to the normal coordinates as well as estimate individual fundamental intensities of the overlapped v l– v 5 and v 3– v 6 band systems. Principal component graphical representations of the A 1 and E symmetry polar tensor elements were useful in determining preferred sets of tensor elements. The mean dipole moment derivative (GAPT charge) of the methyl carbon in CH 3NC, 0.347 e, is between the corresponding values in CH 3CN, 0.110 e, and CH 3F, 0.541 e. The mean dipole moment derivatives obtained here indicate the correct 1s methyl carbon ionization energy as 293.35 eV which is 0.98 eV higher than the corresponding ionization energy of the terminal atom.

Density function studies of peptides

Isotopic effects on vibrational frequencies and chemical shifts of N-methylacetamide (NMA) are of significance to the further development of multi-dimensional NMR and IR studies in protein structural biology. Density functional theory (DFT) has proven to be a powerful method for the calculation of molecular structure, conformation, and torsional barriers for small molecules. In the first part of the paper we demonstrate that DFT reproduces experimentally observed geometries, conformation, and torsional barriers before we proceed to a discussion of chemical shifts in NMA, whose accurate calculation has been made possible by sum-over-states density functional perturbation theory. There is good agreement between the calculated proton and carbon chemical shifts of NMA and the experimentally derived values of the same in DMSO-d6. We have used gradient corrected non-local functional BPW91/6-31 G(d,p) for the calculation of vibrational frequencies of NMA and the effect of 2H, 13C, 15N isotopic effects. The unscaled calculated vibrational frequencies of NMA are in agreement with the gas-phase vibrational frequencies of NMA obtained from electron diffraction of NMA at low temperatures in an inert matrix. The results presented in this paper auger well for the application of DFT to di-, tri, and larger peptides and we have such studies in progress in our laboratories.

Polarized Infrared Absorption Spectrum of Matrix-Isolated Methylperoxyl Radicals, CH3OO X̃ 2A‘ ‘

We have used a tandem pair of supersonic nozzles to produce clean samples of CH3OO radicals in cryogenic matrices. One hyperthermal nozzle decomposes azomethane (CH3NNCH3) to generate intense pulses of CH3 radicals, while the second nozzle alternately fires a burst of O2/Ar at the 20 K matrix. The CH3/O2/20 K argon radical sandwich acts to produce target methylperoxyl radicals: CH3 + O2 → CH3OO. The absorption spectra of the radicals are monitored with a Fourier transform infrared spectrometer. We report 10 of the 12 fundamental infrared bands of the methylperoxyl radical CH3OO, X̃ 2A‘ ‘, in an argon matrix at 20 K. The experimental frequencies (cm-1) and polarizations follow: the a‘ modes are 3032, 2957, 1448, 1410, 1180, 1109, 902, 492, while the a‘‘ modes are 3024 and 1434. We cannot detect the asymmetric CH3 rocking mode, ν11, nor the torsion, ν12. The infrared spectra of CH318O18O, 13CH3OO, and CD3OO have been measured as well in order to determine the isotopic shifts. The experimental frequencies, {ν}, for the methylperoxyl radicals are compared to harmonic frequencies, {ω}, resulting from a UB3LYP/6-311G(d,p) electronic structure calculation. Linear dichroism spectra were measured with photooriented radical samples in order to establish the experimental polarizations of most vibrational bands. The methylperoxyl radical matrix frequencies listed above are within ±2% of the gas-phase vibrational frequencies. A final set of vibrational frequencies for the CH3OO radical are recommended. See also http://ellison.colorado.edu/methylperoxyl.

Polarized Infrared Absorption Spectra of Matrix-Isolated Allyl Radicals

We have measured the polarized infrared absorption spectrum of the allyl radical, CH2CHCH2 (X) 2A2, in an argon matrix at 10 K. The experimental CH2CHCH2 frequencies (cm-1) and polarizations follow: a1 modes, 3109, 3052, 3027, 1478, and 1242; b1 modes, 983, 801, and 510; b2 modes, 3107, 3020, 1464, 1390, and 1182. Two modes (ν6 and ν18) are very weak and could not be detected; the lowest frequency a1 mode (the CH2−CH−CH2 bending mode ν7) is estimated to be beyond the wavelength range of our MCT infrared detector. Infrared absorption spectra of two deuterated isotopomers, CH2CDCH2 and CD2CDCD2, were recorded in order to compare experimental frequency shifts with calculated [UB3LYP/6-311-G(d,p)] harmonic frequencies. Linear dichroism spectra were measured with photooriented samples in order to establish experimental polarizations of most vibrational bands. True gas-phase vibrational frequencies were estimated by considering the gas-to-matrix shifts and matrix inhomogeneous line broadening. The allyl radic...

The infrared spectrum of the matrix-isolated phenyl radical.

We have measured the infrared absorption spectrum of C(6)H(5), /X (2)A(1), in an Ar matrix at 10 K. The experimental frequencies (cm(-)(1)) and polarizations follow. a(1) modes: 3086, 3072, 3037, 1581, 1441, 1154, 1027, 997, 976, 605; b(1) modes: 972, 874, 706, 657, 416; b(2) modes: 3071, 3060, 1624, 1432, 1321, 1283, 1159, 1063, and 587. Three different methods have been used for the production of the phenyl radicals. Infrared absorption spectra of five deuterated isotopomers, C(6)D(5), p-C(6)H(4)D, p-C(6)HD(4), o-C(6)H(4)D, and m-C(6)H(4)D, were recorded to compare experimental frequency shifts with calculated (UB3LYP/cc-pVDZ) harmonic frequency shifts. The use of CO(2) or NO as internal standards enabled the experimental determination of absolute infrared intensities. The linear dichroism was measured with photooriented samples to establish experimental polarizations of each vibrational band. True gas-phase vibrational frequencies were estimated by considering the gas-to-matrix shifts and matrix inhomogeneous line broadening. The phenyl radical matrix frequencies listed above are within +/-1% of the gas-phase vibrational frequencies. The C(6)H(5) frequencies from this paper supersede our earlier values reported in J. Am. Chem. Soc. 1996, 118, 7400-7401. See also: http://ellison.colorado.edu/phenyl.

Electronic spectroscopy and ab initio quantum chemical study of the Ã(1A″)−X̃(1A′) transition of CFBr

The Ã(1A″)−X̃(1A′) electronic transition of jet-cooled CFBr has been investigated by spectroscopic and ab initio theoretical methods. Laser induced fluorescence (LIF) excitation spectroscopy was used to explore the rovibronic levels in the à state, and dispersed fluorescence spectroscopy was used to study ground-state vibrations. Analysis of these spectra yielded gas-phase vibrational frequencies and anharmonicity constants in both electronic states. The computed ab initio vibrational frequencies in both X̃ and à states are in good accord with the experimental values. The Ã-state fluorescence lifetimes varied between 100 ns and 3 μs as a function of excited vibronic state. The highest lying levels displayed a shortened fluorescence lifetime, and some vibronic states that involved ν1 (the CF stretch) exhibited shortened lifetimes (300–500 ns) irrespective of the vibrational energy. Vibronic structure in the LIF spectrum disappeared for vibrational energy in excess of 2957 cm−1. Calculations of the Ã-state potential-energy surface show that it has a small barrier to dissociation to CF+Br with a barrier height in good accord with observed termination of fluorescence. The predicted photochemical pathway to production of CF+Br fragments was proven experimentally by detection of CF fragments. The photofragment excitation spectrum showed strong, increasingly broad vibronic structure at higher energies than the LIF spectrum. At lower energy, sharp but weaker vibronic structure was still evident, overlapping the LIF spectrum. There appears to be two photochemical mechanisms to produce CF+Br, one direct and one indirect. We estimate the height of the barrier to direct dissociation to lie 3250±150 cm−1 above the zero-point level of the à state. The asymptotic thermochemical dissociation limit is estimated to lie ⩾1100 cm−1 lower. The thermochemical bond dissociation energy for the C–Br bond in CFBr was thereby estimated to be Ediss⩽23 180 cm−1, which led to an estimate of the heat of formation for CFBr, ΔfH2980⩾86 kJ mol−1.

Properties of Closed-Shell, Octahedral, Multiply-Charged Hexafluorometallates MF63-, M = Sc, Y, La, ZrF62-, and TaF6-

Properties of multiply-charged hexafluorometallates MF63-, M = Sc, Y, La, ZrF62-, and TaF6-, have been examined at the SCF, MP2, and Becke3LYP levels of theory using the SBKJ pseudopotentials and extended one-electron basis sets. For MF63-, M = Sc, Y, La, our results suggest a different assignment of vibrational transitions than those quoted by Nakamoto1 and recorded by von Becker et al.2 For the series of isoelectronic complexes, we document that the discrepancy between the theoretical gas phase vibrational frequencies and those measured in condensed phase experiments increases as the Becke3LYP HOMO binding energies become more positive. The frequencies of stretching modes measured in condensed phase experiments are systematically higher than those calculated for the gas phase anions. We suggest that a partial charge transfer from the multiply-charged anions to countercations is responsible for the stiffening of vibrational modes in condensed phase environments. The vertical electron detachment energies were determined at the outer valence Green's function, MP2, and B3LYP levels. The binding energies of hexafluorometallates with respect to the energies of atomic ions were also calculated and only a small contribution of the electron correlation effects was found.

The ν1+ν3 combination mode of C3 in Ar and Kr matrices: Evidence for a bent structure

The ν1+ν3 combination mode of laser-ablated C3 has been observed in Ar and Kr matrices for all six possible 12C/13C isotopomers. The ν1 totally symmetric mode frequencies have been determined by difference with the known ν3 asymmetric stretching frequencies, corrected for anharmonicity effects. A comparison of certain isotopomer pairs reveals that C3 is bent in these matrices. A normal coordinate analysis using four force constants resulted in an excellent fit of the 12 observed frequencies (ν1 and ν3 for six isotopomers). From the fit, a value for the ν2 bending mode frequency of 82 cm−1 was obtained. This is shown to be in good agreement with the value obtained from the C3 phosphorescence spectrum. Finally, the molecular bending angle which was determined to be 160.0° (in Ar and Kr) is shown to match very well with the predicted value from ab initio theory (161.6°) and from a fitting of a Morse oscillator–rigid bender potential to gas phase vibrational frequencies (162.5°). The influence of the matrix environment on the vibrational force constants and frequencies of C3 is discussed.

The vibrational spectra of molecular ions isolated in solid neon. IV. NO+, NO−, ONNO+, and ONNO−

When a Ne:NO sample is codeposited at approximately 5 K with a beam of neon atoms that has been excited in a microwave discharge, photoionization and Penning ionization of the NO lead to the stabilization of both monomer and dimer ions. The most prominent infrared absorption, at 1619.2 cm−1, results from the reaction of NO− with NO or of an electron with (NO)2 to form the charge-delocalized trans-ONNO− species. The position of the totally symmetric NO-stretching absorption of trans ONNO− can be inferred from observation of a combination band. The infrared absorption of NO+ has been detected very close to the gas-phase band center. Other infrared absorptions have been tentatively assigned to NO−, cis ONNO−, trans ONNO, and the lowest energy isomer of ONNO+. Comparison of the gas-phase vibrational frequencies observed for the weakly bound species cis ONNO and N2O3 with the corresponding frequencies observed in this neon-matrix study suggests that the matrix shifts for the dimer ion absorptions in the mid-infrared probably amount to less than 1%.

MCSCF/CI ground state potential energy surface, dipole moment function, and gas phase vibrational frequencies for the nitrogen dioxide positive ion

The ground state potential energy surface for the nitrogen dioxide positive ion, NO+2X 1Σ+g(Σ+,A1,A′), has been scanned with a correlated wave function to obtain directly, for the first time, the gas phase equilibrium geometry, force constants, vibrational frequencies, and dipole moment function. The wave function for this scan was constructed from a double-zeta plus polarization one-electron basis with a 12 configuration MCSCF determination of the orbital basis for a full valence 1Σ+g configuration interaction expansion. The calculated equilibrium bond length is 1.12 Å. The vibrational frequencies are computed to be ν1=1514, ν2=679, and ν3=2614 cm−1 The present ab initio results differ significantly from crystalline spectroscopic studies and are, thus, the best values available for the gas phase vibrational frequencies. The dipole moment function is nonzero at the Σ+, A1, and A′ geometries included in the potential surface scan, and is obtained here to provide for the future a priori calculation of the infrared band intensities.

The infrared spectrum, Coriolis coupling, force constants, mean amplitudes and shrinkage effect for SiCl 4

The Coriolis coupling constant has been measured from the contours of the ν 4 band for SiCl 4, and improved gas phase vibrational frequencies have been obtained. The force field for the molecule has been calculated along with the mean amplitudes of vibration, Σ and L matrix elements, shrinkage effect and thermodynamic functions.