Equilibrium Molecular Structure Research Articles

Molecular Dynamics Investigation of Bent-Core Molecules on a Water Surface

Molecular dynamics simulations are carried out for bent-core molecules at water surfaces. The water surface is shown to alter the equilibrium molecular structure significantly by causing a different class of torsional states to become more favorable. The equilibrium structure is also altered by the substitution of chlorine atoms for hydrogen atoms on the central phenyl ring in that this substitution forces the bent core to remain in a single torsional state rather than be delocalized among several torsional states. The consequences of these structural changes on the chirality and packing of these molecules on water surfaces are discussed.

Gas-phase detection of HSOD and empirical equilibrium structure of oxadisulfane

We present the first gas phase spectra of singly deuterated oxadisulfane, HSOD, in its vibrational ground state. More than 100 transitions have been recorded with highest frequency accuracy using the Cologne Terahertz Spectrometer. The molecular parameters derived from a least squares fit analysis proof HSOD to be an almost accidental symmetric prolate top molecule with an asymmetry parameter κ=−0.9985. Spectra of c-type and weaker b-type transitions have been recorded in the range from 716 to 772 GHz. The ratio of the dipole moments μ c/ μ b=2.4(3) has been derived from measured line intensities. The c-type transitions are split by the tunneling motion of a hindered internal rotation, whereas b-type transitions show no splitting within the Doppler limited line profiles. We derived the equilibrium molecular structure of oxadisulfane, HSOH, from experimental values of the rotational constants A 0, B 0, and C 0 of HSOH, H 34SOH, DSOD, and HSOD. The equilibrium rotational constants A e, B e, and C e were derived by taking vibration–rotation interaction constants α r obtained from high-level ab initio calculations into account.

The Chiral Molecule CHClFI: First Determination of Its Molecular Parameters by Fourier Transform Microwave and Millimeter-Wave Spectroscopies Supplemented by ab Initio Calculations

The rotational spectrum of chlorofluoroiodomethane (CHClFI) has been investigated. Because its rotational spectrum is extremely crowded, extensive ab initio calculations were first performed in order to predict the molecular parameters. The low J transitions were measured using a pulsed-molecular-beam Fourier transform spectrometer, and the millimeter-wave spectrum was measured to determine accurate centrifugal distortion constants. Because of the high resolution of the experimental techniques, the analysis yielded accurate rotational constants, centrifugal distortion corrections, and the complete quadrupole coupling tensors for the iodine and chlorine nuclei, as well as the contribution of iodine to the spin-rotation interaction. These molecular parameters were determined for the two isotopologs CH35ClFI and CH37ClFI. They reproduce the observed transitions within the experimental accuracy. Moreover, the ab initio calculations have provided a precise equilibrium molecular structure. Furthermore, the ab initio molecular parameters are found in good agreement with the corresponding experimental values.

Network model for the viscoelastic behavior of polymer nanocomposites

A theoretical network model reproducing some significant features of the viscoelastic behavior of unentangled polymer melts reinforced with well dispersed non-agglomerated nanoparticles is presented. Nanocomposites with low filler volume fraction (∼10%) and strong polymer–filler interactions are considered. The model is calibrated based on results obtained from discrete simulations of the equilibrium molecular structure of the material. This analysis provides the statistics of the network of chains connecting fillers, of dangling strands having one end adsorbed onto fillers, and that of the population of loops surrounding each nanoparticle. The network kinetics depends on the attachment–detachment dynamics of grafted chains of various types and is modeled by using a set of convection equations for the probability distribution functions. The overall viscoelastic response depends strongly on the lifetime of the polymer–filler junctions. The largest reinforcement is observed at low strain rates and low frequency oscillations. A solid like behavior is predicted for systems in which the polymer molecules interact strongly with the nanoparticles, effect which is associated with the behavior of the network of bridging segments.

Molecular structure, conformational mobility, vibrational spectra, and thermochemistry of peroxyacetic acid: an ab initio and density functional study

The structure of the peroxyacetic acid (PAA) molecule and its conformational mobility under rotation about the peroxide bond was studied by ab initio and density functional methods. The free rotation is hindered by the trans-barrier of height 22.3 kJ mol–1. The equilibrium molecular structure of AcOOH (Cs symmetry) is a result of intramolecular hydrogen bond. The high energy of hydrogen bonding (∼46 kJ mol–1 according to natural bonding orbital analysis) hampers formation of intermolecular associates of AcOOH in the gas and liquid phases. The standard enthalpies of formation for AcOOH (–353.2 kJ mol–1) and products of radical decomposition of the peroxide — AcO· (–190.2 kJ mol–1) and AcOO· (–153.4 kJ mol–1) — were determined by the G2 and G2(MP2) composite methods. The O—H and O—O bonds in the PAA molecule (bond energies are 417.8 and 202.3 kJ mol–1, respectively) are much stronger than in alkyl hydroperoxide molecules. This provides an explanation for substantial contribution of non-radical channels of the decomposition of peroxyacetic acid. The electron density distribution and gas-phase acidity of PAA were determined. The transition states of the ethylene and cyclohexene epoxidation reactions were located (Ea = 71.7 and 50.9 kJ mol–1 respectively).

The use of ab initio anharmonic force fields in experimental studies of equilibrium molecular geometry

The use of ab initio methods has been investigated for obtaining physically meaningful anharmonic force fields applicable in structure analysis of molecules by electron diffraction. The quadratic and cubic force constants for the sample molecule SF6 chosen as a suitable test case were theoretically estimated and improved by an empirical scaling based on a quadratic force constant scale factors. It was confirmed that if theoretical calculations are made with well selected basis sets the accuracy of the individual values of the computed cubic constants established by reference to precise spectroscopic data is practically sufficient to experimentally determine the accurate equilibrium S–F distance and to theoretically estimate the amplitudes of vibration and the phase shift parameters for all internuclear distances. Calculations based on a Morse-like anharmonic model function were also performed for comparison.The present calculations show that determination of an accurate equilibrium molecular structure by electron diffraction is possible through the appropriate combination of experimental and theoretical data. The best equilibrium geometry results, if empirically scaled ab initio quadratic and cubic force constants are used in a regularizing algorithm developed earlier for the effective interaction of electron diffraction with vibrational and microwave spectroscopy techniques.

A theoretical study on the ionization of H2S with analysis of vibrational structure of the photoelectron spectra

Abstract Ab initio calculations were performed to study the molecular structures and the vibrational levels of the low-lying ionic states ( 2 B 1 , 2 A 1 and 2 B 2 ) of hydrogen sulphide. The equilibrium molecular structure and the vibrational analysis of these states are presented. The normal vibrational calculations at the RHF level and more extensive calculations at the SDCI level using the explicit vibrational Hamiltonians were used for the vibrational analysis. The theoretical ionization intensity curves including the vibrational structures of these ionic states are also presented and compared with the photoelectron spectrum. The results show that the global shape of the potential energy surface has to be taken into consideration in order to analyze the 2 A 1 and 2 B 2 states. A new assignment of the photoelectron spectra of H 2 S is proposed.

Millimeter-wave spectrum of germanium dichloride GeCl 2. Equilibrium structure and anharmonic force field

The millimeter-wave spectrum of germanium dichloride GeCl 2 produced in a glow discharge of germanium tetrachloride was observed for several isotopic species in the ground and vibrationally excited states, v 1, v 2, 2 v 2, and v 3. The analysis of the rotational spectra yielded the equilibrium molecular structure, r e = 2.169 452(15) A ̊ and θ e = 99.882 5(15)°, and the harmonic and anharmonic force constants up to the third order.

The anharmonic force field and equilibrium molecular structure of ketene

A comprehensive anharmonic vibrational analysis of isotopic ketenes has been performed on the basis of a complete ab initio quartic force field constructed by means of second-order Mo/ller–Plesset perturbation theory (MP2) and the coupled-cluster singles and doubles (CCSD) approach, augmented for structural optimizations by a contribution for connected triple excitations [CCSD(T)]. The atomic-orbital basis sets of the study entailed C,O(10s6p/5s4p) and H(6s/4s) spaces multiply polarized in the valence region to give QZ(2d,2p) and QZ(2d1f,2p1d) sets. An iterative anharmonic vibrational refinement of a limited set of quadratic scaling parameters on 27 fundamentals of H2CCO, HDCCO, D2CCO, and H2C13CO generates a final quartic force field which reproduces the empirical νi data with an average absolute error of only 1.1 cm−1. This force field yields a complete and self-consistent set of Coriolis (ζij), vibrational anharmonic (χij), vibration–rotation interaction (αi), and quartic and sextic centrifugal distortion constants, providing a critical assessment of the assorted spectroscopic constants determined over many years and also facilitating future computations of vibrational state densities for detailed tests of unimolecular dissociation theories. The harmonic frequencies ascertained for H2CCO (in cm−1), with associated anharmonicities in parentheses, are ω1(a1)=3202.2(−129.2), ω2(a1)=2197.2(−44.4), ω3(a1)=1415.2(−25.9), ω4(a1)=1146.0(−29.7), ω5(b1)=581.9(+7.1), ω6(b1)=502.6(+26.3), ω7(b2)=3308.2(−141.3), ω8(b2)=996.0(−17.9), and ω9(b2)=433.6(+5.0). The large positive anharmonicity for the ν6(b1) C=C=O bending mode, which is principally a Coriolis effect, warrants continued investigation. Explicit first-order treatments of the strong Fermi interactions within the (ν4,2ν5,ν5+ν6,2ν6) manifold reveal resonance shifts for ν4(H2CCO, HDCCO, D2CCO) of (−12.1, −10.0, +12.2) cm−1, in order. The experimental assignments for this Fermi tetrad are confirmed to be problematic. From high-precision empirical rotational constants of six isotopomers and the theoretical anharmonic force field, the equilibrium structure of ketene is derived: re(C=O)=1.160 30(29) Å, re(C=C)=1.312 12(30) Å, re(C–H)=1.075 76(7) Å, and θe(H–C–H)=121.781(12)°. A natural bond orbital (NBO) analysis shows that the unusually large methylene angle is attributable to extensive in-plane π delocalization.

A theoretical study on the first ionic state of benzene with analysis of vibrational structure of the photoelectron spectrum

Ab initio calculations have been performed to study the vibrational levels of the first ionic state (2E1g) of benzene. The equilibrium molecular structure and vibrational modes of the state are presented. The theoretical ionization intensity curve including the vibrational structure is also presented and compared with the photoelectron spectrum. A number of new assignments of the photoelectron spectra are proposed.

A theoretical study on ionization of ethylene with analysis of vibrational structure of the photoelectron spectra

A b initio calculations have been performed to study the vibrational levels of the low-lying ionic states, 2B3u , 2B3g, 2Ag, 2B2u, and 2B1u of ethylene. The equilibrium molecular structure and vibrational modes of these states are presented. The theoretical ionization intensity curve including the vibrational structure is also presented and compared with the photoelectron spectra of C2H4 and C2D4. A number of new assignments of the photoelectron spectra are proposed.

A theoretical study on the ionic states and the photoelectron spectra of difluoromethane (CH 2F 2)

Ab initio calculations have been performed to study the four low-lying ionic states, 2B 2, 2B 1, 2A 1 and 2A 2 of difluoromethane (CH 2F 2). The equilibrium molecular structure of these states is presented. The theoretical ionization intensity curve including the vibrational patterns is also presented and is compared with the photoelectron spectra of CH 2F 2 and CD 2F 2. A new assignment of the photoelectron spectra is proposed.

Perturbative corrections to basis incompleteness in molecular SCF calculations

A unified summary is presented of the mathematical approach developed by McDowell for employing perturbation theory to correct for basis-set incompleteness in ab initio SCF calculations. Revised expressions for the corrections to the wavefunction both in terms of orbitals and spin-orbitals are presented with explicit incorporation of the spin variables. Employing H2O as an example, we show that this approach is considerably more powerful for computing molecular energies with standard basis sets than was indicated by previous work. In particular at the higher levels of approximation it accurately reproduces the effect of polarization functions in sets such as 6-31G* and 6-31G**. The equilibrium molecular structure of H2O was also computed by this approach and found to give good accuracy. In each case perturbing functions coupled to both occupied and virtual orbitals are required for acceptable results.

Microwave spectrum, molecular structure, and force field of HBO

The HBO molecule was generated as a transient intermediate in the reaction of diborane with oxygen induced by a dc discharge. The J = 3 ← 2 transition was observed for H10BO and was combined with the spectra already reported to derive the rotational, centrifugal distortion, and nuclear quadrupole coupling constants for this species. The rotational and centrifugal distortion constants were determined also for D11BO, D10BO, H11B18O, and H10B18O. The measurements were extended to rotational transitions of H11BO and H10BO in the ν1, ν2, 2ν2, and ν3 states and to those of D11BO and D10BO in the ν2, 2ν2, and ν3 states. The vibrational frequencies and the rotational, vibration-rotation, l-type doubling, and centrifugal distortion constants thus determined for the six isotopic species were simultaneously analyzed to derive the equilibrium molecular structure and the harmonic and third-order anharmonic potential constants of the HBO molecule. The equilibrium bond lengths thus obtained are re(HB) = 1.166 67(41) Å and re(BO) = 1.200 68(10) Å, and the substituted, average, and effective structures were also calculated for comparison.

Difference-frequency laser spectroscopy of the ν1 and ν3 fundamental bands of H 2Cl +: Determination of the equilibrium molecular structure

The gas-phase spectra of the ν 1 and ν 3 fundamental bands of H 2Cl + have been observed with a difference-frequency laser spectrometer between 2525 and 2755 cm −1. The ions were generated in a hollow cathode discharge through a gas mixture of HCl and H 2. A simultaneous least-squares analysis of the ν 1 and ν 3 bands with an effective vibration-rotation Hamiltonian including Coriolis interaction between the v 1 = 1 and v 3 = 1 states yielded the band origins (2643.2200 and 2630.1404 cm −1 for the ν 1 and ν 3 of H 2 35Cl +, and 2641.4931 and 2628.1065 cm −1 for the ν 1 and ν 3 of H 2 37Cl +, respectively), the rotational constants, and the centrifugal distortion constants for the ground and excited states. The equilibrium molecular structure has been determined to be r e ( HCl) = 1.30412(17) A ̊ and θ e(HClH) = 94.243(27)° for H 2 35Cl + and r e ( HCl) = 1.30411(17) A ̊ and θ e(HClH) = 94.244(26)° for H 2 35Cl +.

Infrared spectroscopy of carbo-ions. III. ν3 band of methyl cation CH+3

The infrared spectrum of the degenerate ν3 vibration–rotation band of methyl cation CH+3, one of the most fundamental carbo-ions, has been observed and analyzed. The spectrum was observed in the frequency range of 3240–2960 cm−1 using the difference frequency laser system as the tunable coherent infrared source. A gas mixture of He:H2:CH4∼700:20:1 with a total pressure of ∼7 Torr was used for the ac discharge in an air-cooled and a water-cooled multiple inlet–outlet discharge tube. The velocity modulation method together with noise subtraction and the unidirectional multiple passing of the beam was used for high sensitivity. The spectral lines of CH+3 appeared amid many other lines of C2H+2 and C2H+3 but their widths and their response to chemical conditions gave a good initial clue for the identification. Over 200 absorption lines have been assigned to CH+3 and the isotopic species 13CH+3. The spectral pattern clearly shows that the equilibrium molecular structure is a planar, equilateral triangle as expected from Herzberg’s Rydberg spectrum of CH3 and from ab initio theory. The spectrum has been analyzed by using the symmetric rotor Hamiltonian in a degenerate vibrational state with various A1–A2 splittings and l resonance. The spectral lines have been least-squares fit with a standard deviation of 0.0058 cm−1, and led to 21 vibration–rotation constants of CH+3. Several analytical expressions have been derived among these constants to determine the structure and potential constants. Some perturbation has been noticed and discussed. Also quite a few lines which are definitely due to carbocations with one carbon atom have been left unassigned. The prominence of the CH+3 spectrum in hydrocarbon discharges confirms the importance of this species in the ion–molecule reaction scheme in the laboratory and in space which has already been noted by many people in the field. A simple discussion is given about the formation and destruction of CH+3 as well as other carbocations.

Detection and equilibrium molecular structure of a short-lived molecule, HBO, by microwave spectroscopy

The rotational constants, centrifugal distortion constants and nuclear quadrupole coupling constants were determined for the H 11BO and H 10BO species produced in the reaction of diborane with oxygen in a dc discharge plasma. The rotational transitions in the excited vibrational states, v 1, v 2, 2 v 2 and v 3, were also observed for H 11BO and H 10BO. The structure was determined as r e(H-B) = 1.1681(10) Å and r e (B-O) = 1.2004(3) Å with estimated incertainties in parentheses.

Determination of the equilibrium molecular structure of inverting molecules by microwave spectroscopy: Application to aniline

A procedure for calculating the partial equilibrium molecular structure from microwave data is described for molecules in which a large-amplitude internal motion occurs. It consists of measuring the rotational constants of two or more successive vibrational state rotators and correcting them for the vibrational contribution in accordance with a simple double minimum quadraticquartic potential function. The well-known Kraitchman relations are then applied in order to obtain the substitution structure. The method is justified by the coherence of the numerical results obtained from multiple substitutions.

ChemInform Abstract: Microwave Spectrum, Electric Dipole Moment, Internal Rotation Barrier, and Molecular Structure of Methyl Hydrodisulfide.

Abstract The rotational spectra of CH 3 SSH and CH 3 SSD have been investigated between 18 and 40 GHz. Rotational constants for both isotopic species and centrifugal distortion constants for the parent species have been fitted to the measured transition frequencies. The permanent electric dipole moment has been obtained from quantitative Stark effect measurements. Internal rotation splittings due to the methyl group have been evaluated and the barrier determined. The equilibrium molecular structure and internal rotation barriers of the SH group have been calculated with the ab initio method.

Microwave spectrum, electric dipole moment, internal rotation barrier, and molecular structure of methyl hydrodisulfide

The rotational spectra of CH 3SSH and CH 3SSD have been investigated between 18 and 40 GHz. Rotational constants for both isotopic species and centrifugal distortion constants for the parent species have been fitted to the measured transition frequencies. The permanent electric dipole moment has been obtained from quantitative Stark effect measurements. Internal rotation splittings due to the methyl group have been evaluated and the barrier determined. The equilibrium molecular structure and internal rotation barriers of the SH group have been calculated with the ab initio method.