Stark effect permanent moment measurement

The ANI permanent dipole moment has not yet been well established. Earlier dielectric constant measurements (at a temperature of 459 K in the gas phase) provided a value of = 1.53 D62 (in units of Debye, ID = 3.33564 x 10 3° Cm). An earlier study using the Stark effect determination from the rotational spectroscopy reported a smaller value, /r = 1.15 0.02 D for C6H5NH2 and /r = 1.13 0.02 D for C6H5ND238. However, an independent MW study evaluated the p., component at 1.07 D and did not detect any h-and c-type transitions in the MW spectra37. More recent MW studies43 using also Stark effects in gas-phase electronic spectra pointed out that the values determined from MW spectra are actually the a-axis projection of the dipole moment. Therefore, the in-plane component of the dipole moment can be established as p.,(,Y0) = 1.13 D. 

Molecules with permanent electric dipoles have moments that can be measured by applying an electric field to remove the degeneracy seen in their rotational spectra (Chapter 2). This is called the Stark effect ... 

The Stark effect of a rotational transition of a specific vibronic state will only yield the permanent electric dipole moment and the anisotropy of the electric polarizability (an — aj.). In principle, both molecular parameters are functions of the intemuclear distance. The measured Stark effect yields, therefore, the vibrational average of the molecular parameter which in trun is represented as a power series expansion in (t) + 1/2) and J J + 1) ... 

Measmements of Stark splittings in mierowave and radiofrequeney speetra allow these eomponents to be determined. The main contribution to the dipole moment of the eomplex arises from the permanent dipole moment vectors of the monomers, whieh projeet along the axes of the complex aeeording to simple trigonometry (eosines). Thus, measurements of the dipole moment eonvey information about the orientation of the monomers in the complex. It is of course neeessary to take aeeount of effeets due to indueed dipole moments and to consider whether the effects of vibrational averaging are important. 

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