Dipole moments experimental determination

Because atomic charge is not a quantum mechanical observable, we must use some indirect method to calculate these values. Moreover, since we lack experimental results to guide us, other methods of validating our assignments must be devised. Accurate reproduction of some observable, whether experimentally measured (dipole moment) or determined directly from the wavefunction (electrostatic potential, molecular moments, etc.), increases our confidence in the reliability of the assigned charges. 

From a monopole population analysis of uracil, a dipole moment was determined.29 The result is shown in Figure 4. Note the error in the magnitude is 32 percent the estimated standard deviation in the angle is 12°. A similar monopole analysis for cyclotriborazane leads to a dipole moment of 3.0 D, which compares favorably with a different experimental value.30 The direction of the moment is along the threefold axis from the plane of the B atoms toward the plane of N atoms. The largest source of error in these dipole moment results is the error in the proton positions of the hydrogen atoms. 

Polarity is a molecular property. For polyatomic species, the net molecular dipole moment depends upon the magnitudes and relative directions of all the bond dipole moments in the molecule. In addition, lone pairs of electrons may contribute significantly to the overall value of ji. We consider three examples below, using the Pauling electronegativity values of the atoms involved to give an indication of individual bond polarities. This practice is useful but must be treated with caution as it can lead to spurious results, e.g. when the bond multiplicity is not taken into account when assigning a value of x - Experimental values of molecular electric dipole moments are determined by microwave spectroscopy or other spectroscopic methods. 

Remark Electric dipole moment function determined from experimental data in [87Huf]. ... 

TIk experimentally determined dipole moment of a water molecule in the gas phase is 1.85 D. The dipole moment of an individual water molecule calculated with any of thv se simple models is significantly higher for example, the SPC dipole moment is 2.27 D and that for TIP4P is 2.18 D. These values are much closer to the effective dipole moment of liquid water, which is approximately 2.6 D. These models are thus all effective pairwise models. The simple water models are usually parametrised by calculating various pmperties using molecular dynamics or Monte Carlo simulations and then modifying the... 

MO methods have been used to calculate dipole moments of each of the three ring systems (73MI50403, B-70MI50400). Calculated values for aziridine are somewhat higher (2.09-2.40 D) than the known experimental value (1.89 D). Dipole moment studies on a few simple aziridines have led to the determination of the preferred conformation of N-arylaziridines in solution and in the vapor state (71JCS(C)2104, 66DOK(169)839). For the 1-azirine system, no values have been determined experimentally, but values of 2.40-2.56 D for 1-azirine and 2.50-2.51 D for 2-azirine have been calculated (73MI50403). 

These can be determined experimentally to very high accuracy from the Stark effect and molecular beam studies. The experimental accuracy is far beyond the capabilities of ab initio studies. At the other extreme, the original route to these quantities was through studies of the dielectric polarization of species in solution, and there is currently interest in collision-induced dipole moments. In either case, the quantities deduced depend critically on the model used to interpret the experiment. 

Calculated (6-31G //3-21G) dipole moments were also compared with the corresponding experimental values determined in benzene at 25°C... 

For methods of determining dipole moments and discussions of their applications, see Exner, O. Dipole Moments in Organic Chemistry, Georg Thieme Publishers Stuttgart, 1975. For tables of dipole moments, see McClellan, A.L. Tables of Experimental Dipole Moments, vol. / W.H. Freeman San Francisco, 1963 vol. 2, Rahara Enterprises El Cerrito, CA, 1974. 

As mentioned earlier, heavy polar diatomic molecules, such as BaF, YbF, T1F, and PbO, are the prime experimental probes for the search of the violation of space inversion symmetry (P) and time reversal invariance (T). The experimental detection of these effects has important consequences [37, 38] for the theory of fundamental interactions or for physics beyond the standard model [39, 40]. For instance, a series of experiments on T1F [41] have already been reported, which provide the tightest limit available on the tensor coupling constant Cj, proton electric dipole moment (EDM) dp, and so on. Experiments on the YbF and BaF molecules are also of fundamental significance for the study of symmetry violation in nature, as these experiments have the potential to detect effects due to the electron EDM de. Accurate theoretical calculations are also absolutely necessary to interpret these ongoing (and perhaps forthcoming) experimental outcomes. For example, knowledge of the effective electric field E (characterized by Wd) on the unpaired electron is required to link the experimentally determined P,T-odd frequency shift with the electron s EDM de in the ground (X2X /2) state of YbF and BaF. 

Thiazyl monomer is a radical with one unpaired electron. It exhibits an IR band at 1209 cm-1. The experimental dipole moment is 1.83 0.03 D in the opposite direction to that in NO (p = 0.16 D). Much less is known about selenazyl monomer, SeN, but it has been characterized by infrared spectroscopy.36 The structure of a transition-metal complex [OsTp(NSe)Cl2] (Tp=hydrotris(l-pyr-azolyl)borate) has been determined.39... 

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