Bond Dipole Model Research Articles

High-resolution infrared spectroscopy of supersonically cooled singlet carbenes: Bromomethylene (HCBr) in the CH stretch region.

First high-resolution spectra of cold (∼35K) singlet bromomethylene HCBr in the CH stretching (v1) region from 2770 to 2850cm-1 are reported using near quantum shot-noise limited laser absorption methods in a slit jet supersonic discharge expansion source. Three rovibrational bands are identified at high S/N (20:1-40:1) and rotationally assigned to (i) the CH stretch fundamental (v1) band X̃1,0,0←X̃0,0,0 and (ii) vibrational hot bands [X̃(1,1,0)←X̃(0,1,0) and X̃(1,0,1)←X̃(0,0,1)] arising from vibrationally excited HCBr populated in the discharge with single quanta in either the H-C-Br bend (v2) or C-Br stretch (v3) modes. Precision rotational constants are reported for a total of six states, with an experimentally determined CH stretch vibrational frequency (2799.38cm-1) in good agreement with previous low-resolution fluorescence studies [M. Deselnicu et al., J. Chem. Phys. 124(13), 134302 (2006)]. Detailed analysis of the fundamental v1 band highlights the presence of perturbations in the X̃1,0,0 level, which we tentatively attribute to arise from the nearby triplet state ã(0,0,1) through spin-orbit interaction or the multiple quanta X̃0,2,1 singlet state via c-type Coriolis coupling. Reduced-Doppler resolution (60MHz) in the slit-jet IR spectrometer permits for clear observation of a nuclear spin hyperfine structure, with experimental line shapes well reproduced by nuclear quadrupole/spin-rotation coupling constants from microwave studies [C. Duan et al., J. Mol. Spectrosc. 220(1), 113-121 (2003)]. Finally, the a-type to b-type transition intensity ratio for the fundamental CH stretch band is notably larger than that predicted by using a bond-dipole model, which from high level ab initio quantum calculations [CCSD(T)/PVQZ] can be attributed to vibrationally induced "charge-sloshing" of electron density along the polar C-Br bond.

Suppressed-Doppler slit jet infrared spectroscopy of astrochemically relevant cations: ν1 and ν4 NH stretching modes in NH3D+

A suppressed-Doppler (Δν = 180 MHz) infrared spectrum of monodeuterated ammonium ions (NH3D+) has been obtained for the ν1 (symmetric) and ν4 (degenerate) N-H stretch bands via direct absorption high resolution IR laser spectroscopy in a planar slit jet discharge expansion. The ion is efficiently generated by H3 + protonation of NH2D in a discharge mixture of H2/NH2D, with the resulting expansion rapidly cooling the molecular ions into low rotational states. The first high-resolution infrared spectrum of ν1 is reported, as well as many previously unobserved transitions in the ν4 rovibrational manifold. Simultaneous observation of both ν1 and ν4 permits elucidation of both the vibrational ground and excited state properties of the ion, including rigorous benchmarking of band origins against high-level anharmonic ab initio theory as well as determination of the ν1:ν4 intensity ratio for comparison with bond-dipole model predictions. Ground-state combination differences from this work and earlier studies permit the rotational constants of NH3D+ to be determined to unprecedented accuracy, the results of which support previous laboratory and astronomical assignment of the 10-00 pure rotational transition and should aid future searches for other rotational transitions as well.

Dipolar−Dipolar Interactions and the Crystal Packing of Nitriles, Ketones, Aldehdyes, and C(sp2)−F Groups

The Cambridge Structural Database is used to survey the relative orientations of neighboring intermolecular nitrile, carbonyl, or CF bonds in organic crystals. The observed orientations are directly compared to the distribution law which would be expected if these intermolecular interactions were governed solely by a purely electrostatic bond dipole−bond dipole effect. In the case of nitrile groups (and to a lesser extent carbonyl groups), at short distances (<3.6 Å) there is a good correspondence between the observed orientation in crystals and the orientations predicted by such a bond dipole model. In the case of the C−F bond, steric and other factors appear to play an important role. A comparison to ab initio calculations at the MP2 level for a dimer of carbon dioxide molecules substantiates these findings. The accuracy of a bond dipole model in rationalizing nearest neighbor orientations in crystals is in accord with literature bond dipole moments of the nitrile, carbonyl, and C−F groups, values which are, respectively, 3.6, 2.5, and 1.5 D.

The stretching vibrational overtone spectra of PH3: Local mode vibrational analysis, dipole moment surfaces from density functional theory and band intensities

The infrared spectra of PH3 molecule were recorded on a Bruker IFS 120HR Fourier transform spectrometer from 4000 to 9500 cm−1. The P–H stretching vibrational frequencies and intensities were derived from the experimental data. The Morse oscillator parameters De and α in the anharmonically coupled anharmonic oscillator local mode model were determined by the least-squares fitting with the observed vibrational band centers. The ab initio three-dimensional P–H stretching dipole moment surfaces were calculated by the density functional theory method. The dipole moment vectors were projected to three kinds of molecule-fixed reference systems. The corresponding dipole moment components were fitted to polynomial functions in terms of the P–H bond length displacements with the molecular symmetry taken into account. The absolute band intensities were obtained and then compared with the experimental data. The results showed that a proposed improved bond dipole model can predict the absolute band intensities within a factor of 2 for most of the observed transitions, indicating a reasonably good agreement.

The vibrational overtones of SiH4 isotopomers: experimental wavenumbers, assignment, ab initio dipole moment surfaces and intensities

The overtone spectra of SiHD3, SiH2D2 and SiH3D in the range 2000–9000 cm−1 have been measured up to 720000 Pa m using a Bruker 120HR interferometer. The transition frequencies and intensities are reported. The observed band centers are found to be in good agreement with the predictions from high order canonical Van Vleck perturbation theory (CVPT) calculations. A four-dimensional Si–H stretching dipole moment surface (DMS) has been determined from coupled cluster [CCSD(T)] methods and expanded using a bond dipole model and a Cartesian model. The band intensities of the Si–H and Si–D stretching transitions have been computed employing these expansions and a local mode force field optimized to the observed band centers. The calculations agree reasonably well with observations for most bands. The vibrational intensity anomaly in the third local mode manifolds is partly reproduced and is attributed to the cancellation of linear and quadratic terms in the DMS.

Local mode overtone intensities of SiH 4 and GeH 4 from a bond dipole model with an ab initio calculated dipole moment surface

The absorption intensities of the local mode overtones of SiH 4 and GeH 4 were investigated by a bond dipole model with ab initio calculated dipole moment surfaces, which were obtained by using the density functional method B3PW91 with a 6-311++G(3df, 2pd) basis set and then fitted to the Mecke type function. The calculated overtone intensities agree with the observed values.

Absorption Intensity of Stretching Overtone States of Silane and Germane

The band absorbance and line absorbance of R branch transitions are obtained from high-resolution Fourier transform measurement of silane and germane local mode overtones (n000), n = 1–5, under proper sample pressures and absorption lengths. The absorption coefficients of these overtones are derived by linear least-squares fitting. The relative intensities of overtones can be satisfactorily reproduced with the bond dipole model containing two adjustable parameters.

OH stretch overtone spectroscopy and transition dipole alignment of HOD

Photoacoustic spectroscopy in room-temperature H2O/HOD/D2O cells has been used to probe the 3νOH and 4νOH overtone bands in HOD via excitation with a single-mode (160 MHz) injection-seeded optical parametric oscillator (OPO) and pulsed dye laser, respectively. Transitions corresponding to HOD are isolated by comparison between H2O/HOD/D2O and pure H2O overtone spectra and analyzed with a Watson asymmetric top Hamiltonian. Least-squares fits yield the vibrational dependence of Av, Bv, and Cv rotational constants as well as a Birge–Sponer analysis of the vibrational overtone series for up to vOH=4. Relative intensities of a-type and b-type bands are reported for 3νOH and 4νOH which indicate that the transition dipole moment vector tilts away from the OH bond with increasing νOH excitation. This observation is qualitatively inconsistent with simple local-mode, bond–dipole model predictions of the vibrational transition moments. However, the data are in excellent agreement with full 3D quantum variational calculations based on the H2O potential of Polyansky et al. [J. Chem. Phys. 105, 6490 (1996)] and dipole moment surface of Gabriel et al. [J. Chem. Phys. 99, 897 (1993)]. These results highlight the relative importance of electrical versus mechanical anharmonicity effects in preparing highly excited vibrational states of HOD via laser overtone pumping.

Infrared absorption in amorphous silicon from ab initio molecular dynamics

We present an ab initio calculation of the infrared vibrational spectrum of amorphous silicon. The electron polarization, which is the main ingredient to obtain the infrared spectra, is evaluated using the recent formulation in terms of Berry’s phase, while the time evolution of the system is studied using Car–Parrinello molecular dynamics. Our results are in quantitative agreement with experimental data and also with the results of empirical calculations based on a bond-dipole model.

Dipole moment function and equilibrium structure of methane in an analytical, anharmonic nine-dimensional potential surface related to experimental rotational constants and transition moments by quantum Monte Carlo calculations

The pure rotational spectrum in the far-infrared and its absolute intensity in the vibrational ground state of CHD3 and CH3D, and the integrated band strength of the N=5 CH-stretching overtone of CHD3 in the near infrared to visible were measured by high-resolution interferometric Fourier transform techniques. The far-infrared data result in permanent electric dipole moments (‖μz0‖=(5.69±0.14)×10−3 D for CHD3, ‖μz0‖=(5.57±0.10)×10−3 D for CH3D), consistent with previous experimental data. The integrated N=5 overtone cross section is found to be (0.828±0.068) fm2. The overtone data are used, together with previous data, to derive a new, nine-dimensional, isotopically invariant dipole moment function for CH4 within the chromophore model for the CH chromophore in CHD3. With this function, the experimental data can be reproduced to an averaged factor of 1.2, in the best case. In the vibrational ground state, a nine-dimensional calculation of expectation values on a new, fully anharmonic potential surface was performed using the solution of the rovibrational Schrödinger equation by diffusion quantum Monte Carlo methods. The results for the rotational constants of several isotopomers, which include significant contributions from rovibrational interactions, indicate that the equilibrium CH bond length of methane is re=108.6 pm. The calculated value for the vibrationally averaged permanent dipole moment from these nine-dimensional vibrational quantum calculations, using the dipole moment function consistent with the analysis of the overtone bands, is μz0=−(6.6±0.4)×10−3 D for CHD3 (with positive z coordinate for the H atom) and μz0=(6.8±0.5)×10−3 D for CH3D (with positive z coordinate for the D atom) in essential agreement with the far-infrared rotational intensities. The sign could be determined unambiguously by comparison with ab initio data. We predict the permanent dipole moment of several further methane isotopomers. The polarity of the CH bond in methane is C−–H+, within our simple bond dipole model, but is discussed to be a model dependent (not purely experimental) quantity.

Fermi resonances in overtone spectra of bromoform, CHBr3

A curvilinear internal coordinate Hamiltonian is used to analyze Fermi resonances between CH stretching and bending vibrations in bromoform. Besides the usual Fermi resonance terms, cubic and quartic kinetic and potential energy terms in bending variables are included in the Hamiltonian. The eigenvalues are obtained variationally with a Morse oscillator basis set for the stretch and a two-dimensional harmonic oscillator basis set for the bend. The potential energy parameters are optimized with the nonlinear least squares method. A bond dipole model is used to calculate infrared absorption intensities. The model reproduces well the observed vibrational term values and infrared absorption intensities.

Fermi resonances between CH stretching and bending vibrations in CHF3, CHCl3, and (CF3)3CH

A simple vibrational curvilinear internal coordinate Hamiltonian is used to account for Fermi resonance interactions between CH stretching and bending vibrations in CHF3, CHCl3, and (CF3)3CH. The eigenvalues of the Hamiltonian are obtained variationally with a Morse oscillator basis set for the stretch and two-dimensional harmonic oscillator basis set for the bend. Five or six potential energy parameters are found to describe well the experimental CH stretching and bending overtone levels of the studied molecules. In the case of CHF3 the potential energy surface obtained is in good agreement with ab initio surfaces. A bond dipole model is used to calculate infrared absorption intensities for the transitions from the ground state to the totally symmetric excited states.

Representation of the molecular electrostatic potential by atomic multipole and bond dipole models

AbstractAb initio wave functions using the 6‐31G** basis set were calculated for a set of 24 organic molecules which included saturated and unsaturated hydrocarbons, fluoro compounds, hydroxyl and ether compounds, carboxylic acids, esters, amines, carbonyl compounds, amides, and N‐methyl amides. The electric potential on a grid within a van der Waals shell around each molecule was calculated directly from the wavefunctions. The electric potential values were modeled by placing multipoles up to quadrupoles at atomic sites. The electric potential was well fitted by a model which included atomic monopoles, dipoles, and quadrupoles. Also, the electric potential was modeled by placing dipoles at bond centers. This bond dipole was either allowed to point in any direction, or was restricted to the bond direction. In general the values shown for the monopoles and the values and directions for the bond dipoles were as expected from considerations of electronegativity and reactivity. For the general bond dipole model in a number of cases the direction of the bond dipole was not parallel to the bond direction. It is suggested that this is an artifact caused by the effects of lone pair electrons or electron delocalization on the model. The restricted bond dipole model fitted the electric potential about as well as the atomic monopole model. Atomic monopole values (net atomic charges) and bond dipole values for various atoms, functional groups, and bonds are discussed. Since bond‐dipole interaction energy has better long‐range convergence than monopole interaction energy, bond dipoles are a useful alternative to atomic monopoles.

Interbond coupling in HCN and DCN

The development of a new local-mode model for the analysis of the overtone spectra of complex polyatomic molecules is described and is tested here with the simple linear triatomics HCN and DCN. Morse oscillator potential functions are used for the CH(D) and CN bonds, together with a Morse-like interbond coupling term. The novelty lies in our treatment of this interbond coupling term, which we believe to be more realistic than the widely used model of harmonically coupled anharmonic oscillators. The Hamiltonian is transformed to a basis of harmonic oscillator states and treated by perturbation theory to third order to yield an effective vibrational Hamiltonian matrix which is block-diagonal in the total excitation quantum number. The model contains five parameters relating to the stretching potential-energy surface which are refined in least-squares fits to the available spectroscopic data. Also a simple bond dipole model is used to estimate the intensities of the overtone bands within a given manifold of CH and CN vibrational levels. Comparisons with the previously determined quartic anharmonic force field of HCN and recent variational calculations on an ab initio potential surface reveal that our model captures the essential features of the stretching potential surface which determine the band origins and integrated band intensities of the HCN and DCN stretching vibrational overtones. Some new data on the band centres and integrated intensities of DCN are presented.

The intensities of HCN overtone transitions from 12 600–18 400 cm−1

The absolute intensities of 12 visible overtone bands of HCN have been determined using intracavity photoacoustic detection and a long pathlength absorption cell. The relative intensities of nine of these weak overtone bands (from 14 600–18 400 cm−1) were obtained by using intracavity photoacoustic detection. The absolute intensity of the (0,0,5) band, obtained previously using a multipass cell with an optical pathlength×pressure of about 34 km Torr, was used to calibrate the photoacoustic response, and thus to obtain the absolute intensities for eight more HCN overtone bands. The absolute intensities of three HCN overtone bands between 12 600–13 800 cm−1 were measured using a multipass cell. Optical pathlengths×pressure from 10 to 15 km Torr were used for these three bands. The band origins in cm−1, the band labels (CN, bend, CH), and the intensities in cm/mol are:12 636, (0,0,4), 154(3); 12 658, (3,0,2), 9.7(3); 13 702, (2,0,3), 3.1(2); 14 654, (4,0,2), 8.0(4); 14 670, (1,0,4), 7.2(4); 15 552, (0,0,5), 17.5(4); 15 711, (3,0,3), 0.05(1); 16 164, (0,11,5), 0.047(8); 16 640, (5,0,2), 0.40(4); 16 674, (2,0,4), 1.7(1); 17 550, (1,0,5), 13.9(4); 18 377, (0,0,6), 2.4(2). This work represents the most complete study to date of the overtone intensities of a polyatomic molecule. We have shown that the typical method of intracavity laser power correction is likely to introduce serious errors in relative intensity measurement. We also have found evidence, namely the anomalously large intensity of the (1,0,5)–(0,0,0) transition, for the breakdown of the typically invoked bond dipole model for overtone and combination band intensities. As well as discussing the astromonical interest in predicting overtone intensities of HCN, we present a thorough comparison of the measured absolute overtone intensities with ab initio results.

A bond-origin-independent formulation of the bond dipole model of vibrational circular dichroism

A bond-origin-independent formulation of the bond dipole model of vibrational circular dichroism

Matrix isolation studies on alkali metal perrhenates: the shape and vibrational assignments of caesium perrhenate, CsReO4

The matrix i.r. and Raman spectra obtained from the vaporisation of potassium and caesium perrhenate show spectra characteristic of the molecular species MReO4 with C2v symmetry. Additional 18O enrichment experiments provide an unequivocal assignment for the νRC–O modes and it is also shown that the bond dipole model leads to a useful simulation of relative band intensities.

Excited stretching vibrations of water: the quantum mechanical picture

Quantum mechanical eigenvalues derived from a Morse oscillator basis are reported for stretching vibrational states of H2O with total quantum numbers ν = ν1 + ν3 = 0–8, on the Sorbie-Murrell potential. Spectroscopic absorption intensities for all interstate transitions were calculated in a bond dipole model. The dominant feature of the eigenvalue spectrum is a pattern of local mode doublets, such that linear combinations of the component wavefunctions for doublets with splittings less than 10 cm-1 are localized in the bond directions. The timescales for intramolecular energy transfer derived from these splittings increase along the most strongly localized progression from 0·5 ps at ν = 1 to 30 ns at ν = 8. This most localized progression is substantially the most strongly coupled in absorption to the ground state. The strongest transitions in general are subject to a bond excitation rather than a normal coordinate excitation selection rule Δn = ± 1. The relative intensities of particular pairs of transiti...