Signs Of Dipole Moment Derivatives Research Articles

Ab InitioCalculations and High-Resolution Spectroscopy of the Bending Pentad of SiH2D2in the 10–16 μm Region

The SiH2D2asymmetric top has nine vibrational modes, five of them forming a pentad strongly perturbed by Coriolis interactions. High-levelab initiocalculations of SiH2D2have been performed which yield numerous spectroscopic parameters related to the harmonic and anharmonic force fields. The bending pentad comprising ν4(A1), ν7(B1), ν5(A2), ν9(B2), and ν3(A1) has been studied by high-resolution Fourier transform spectroscopy; the region 600–1050 cm−1has been investigated with a resolution of ca. 4 × 10−3cm−1. Raman BOXCARS spectroscopy has been used for the infrared inactive ν5band. The Raman apparatus function was 0.0054 cm−1. Assignments of about 4000 transitions including all bands have been made, mostly employing ground state combination differences techniques, and a global fit has been performed. The fundamentals ν4(681.624 cm−1), ν7(742.640 cm−1), ν5(842.381 cm−1), ν9(859.750 cm−1), and ν3(942.741 cm−1) are strongly coupled by A-, B-, and C-type Coriolis interactions, andab initiopredictions of these interaction parameters were used to set up a network of interactions that was refined by the experimental data. The global standard deviation for the entire body of data is 7.1 × 10−4cm−1. Satisfactory synthetic spectra which are very sensitive to relative signs of dipole moment derivatives and Coriolis interaction constants were obtained with the guidance ofab initiocalculations. Finally, fair to good agreement of experimental andab initiocalculated molecular parameters was obtained. For the first time, a complete analysis of the pentad of SiH2D2in the 10–16 μm region has been carried out. A full set of rovibrational parameters is given for these five interacting levels, including first and second order Coriolis interaction constants.

Experimental determination of the sign of molecular dipole moment derivatives: an infrared—visible sum frequency generation absolute phase measurement study

We show that the measurement of the absolute phase of the infrared—visible sum frequency susceptibility of a monolayer of molecules of known orientation determines the relative sign of the dipole moment derivative and the polarizability derivative with respect to the normal mode. With the sign of the polarizability derivative well established, we determine the sign of the CH bond dipole moment derivative with respect to the CH stretch for the methyl group of an alkane and for methoxy.

The dipole moment derivatives and charge flux parameters from vibrational absorption intensities of ethylene oxide

The signs of dipole moment derivatives for ethylene oxide obtained from ab initio calculations using the 6-31G ∗ basis set are presented. From these signs and the experimental vibrational absorption intensity data, the atomic polar tensor elements and bond charge flux parameters were evaluated. These quantities were compared with those in the literature calculations employing approximate signs and force constants.

Interpretation of infrared intensities of ethylene by parametric methods

An interpretation of infrared intensities of ethylene and its deuterated derivatives is presented in this paper; different kinds of parameters, namely, molecular dipole derivatives, electro-optical parameters, polar tensors and effective charges, derived from infrared intensity data are compared. From this comparison and from a further comparison with the quantum mechanical calculations the most likely signs of dipole moment derivatives ∂M/∂Qi seem to be (+ −− ++); this choice of signs leads to a positive equilibrium bond dipole moment μCH0 (C−–H+) and to a decreasing of μCH when the bond is stretched. These results, however, need a further check on other molecules containing the vinyl group before being considered final. In this paper it is also shown that electro-optical parameters are very useful for interpretative purposes.

The determination of experimental signs of dipole moment derivatives: G sum rule analysis of allene

The determination of experimental signs of dipole moment derivatives: G sum rule analysis of allene

Experimental determination of relative signs of dipole moment derivatives: HCN and DCN

An alternative numerical analysis for the experimental determination of the relative signs of the dipole moment derivatives with respect to the normal coordinates is presented. Use is made of the polar tensor and effective charge equations and the G–intensity sum rule of Crawford. The relative signs are determined by sufficiently accurate individual infrared fundamental intensities of one molecule and only the sum of these intensities for an isotopically related molecule. The intensity data of HCN and DCN are used to illustrate the procedure. The as yet unreported values of the atomic polar tensors and effective charges of these molecules are also reported.

Infrared band intensities and polar tensors: Properties of effective charge ξα and its relation to the sign of dipole moment derivatives

Polar tensors for a variety of molecules, XY4, XY3, CH3X, and C2F6, are evaluated from infrared band intensities. The effective charges ξα for all atoms are reported. It is found that the difference in the ξα2 value of the central atom and the sum of the ξα2 values the terminal atoms is maximal for the preferred sign combination of dipole moment derivatives δμg/δQi in all the molecules considered. Making use of this property, we find it possible to determine unequivocally the sign of dipole moment derivatives with respect to normal coordinates δμg/δQi. It is noted that the ξα and ξαβ values follow the trend in electronegativity of atoms and ionic character of bonds, respectively, in a series of molecules.

Absolute signs of dipole moment derivatives in carbonyl sulphide

By comparison of i.r. intensity measurements and microwave Stark effect information, it is established experimentally that the absolute signs of the dipole moment derivative for carbonyl stretching in OCS is negative; that is ∂p/∂rCO=–1·5 eeCO where e is the charge on the proton and eCO is a unit vector from the carbon towards the oxygen atom, or, in more familiar, non-SI units the dipole moment gradient with respect to CO stretch is 7 D/A with the oxygen becoming more negative.

A simple model for predicting the sign of dipole moment derivatives in polyatomic molecules

A simple model for predicting the sign of dipole moment derivatives in polyatomic molecules