Gas-phase dipole moment

Compound Relative molar mass Melting point, C Boiling point, C Gas phase dipole moment, debyes... 

Karelson et al. [268] used the AMI D02 method with a spherical cavity of 2.5 A, radius to study tautomeric equilibria in the 3-hydroxyisoxazole system (the keto tautomer 13 is referred to as an isoxazolone). AMI predicts 13 to be 0.06 kcal/mol lower in energy than 14 in the gas phase. However, the AMI dipole moments are 3.32 and 4.21 D for 13 and 14, respectively. Hydroxy tautomer 14 is better solvated within the D02 model, and is predicted to be 2.6 kcal/mol lower in energy than 13 in a continuum dielectric with e = 78.4. Karelson et al. note, however, that the relative increase in dipole moment upon solvation is larger for 13 than for 14 (aqueous AMI dipole moments of 5.05 and 5.39 D, respectively). This indicates that the relative magnitude of gas-phase dipole moments will not always be indicative of which tautomer will be better solvated within a DO solvation approach — the polarizability of the solutes must also be considered. In any case, the D02 model is consistent with the experimental observation [266] of only the hydroxy tautomer in aqueous solution. 

The gas-phase dipole moment of 1,2,3-trioxolane was determined to be 3.43 D from microwave spectroscopic studies <88JA799l>. This compares with calculated values of 3.90 D and 4.03 D using SCF and MP2 methods, respectively <89JA2497>. The dipole moment of 1,3,2-dioxathiolane 2-oxide has been measured as 3.65 D <49DOK(69)4i> and 3.74 D <6IJA2105> (both in benzene), and that of 1,3,2-dioxathiolane 2,2-dioxide has been determined to be 5.64 D (in dioxane) <68JHC289>. These results are consistent with the compounds adopting nonplanar conformations. 

HF-level ab initio calculations have often been used for CSM calculations. Combined with good basis sets providing sufficient polarization functions, which are essential for good electrostatic properties, good correlations between the calculated and experimental gas-phase dipole moments can be achieved, but with a general overestimation by 16% (see Table 2.1). As a consequence, good HF parameterizations of CSMs have been reported, which compensate for the electrostatic overestimation by increased cavity radii. Nevertheless, since better DFT methods are available with about the same computational costs, and with more... 

The gas-phase dipole moment of arsabenzene was found to be 1.10D 45). In cyclohexane solution it was measured as 1.02D27). These values are typical of those found for tertiary arsines for trimethylarsine p = 0.86D76), for triethylarsine p = 1.04D77) and for triphenylarsine p = 1.23D78). No dipole moment data are available for stibabenzene and bismabenzene. However, based on trends shown in acyclic compounds 79), it is expected that they have smaller moments. 

Another very important hydrogen bond solvent is acetonitrile that is characterized by a strong gas phase dipole moment (3.92 D), which may lead to the formation of... 

Another way in which polarizability is included implicitly is in the value of the partial charges, qi, that are assigned to the atoms in the model. The charges used in potential energy models for condensed phases are often enhanced from the values that would be consistent with the gas-phase dipole moment, or those that would best reproduce the electrostatic potential (ESP)... 

Applications of the Born—Kirkwood-Onsager model at the ab initio level include investigations of solvation effects on sulfamic acid and its zwitterion,23i an examination of the infrared spectra of formamide and formamidic acid,222 and a number of studies focusing on heterocyclic tautomeric equilibria.222,232,233 a more detailed comparison of some of the heterocyclic results is given later. The gas phase dipole moment depends on basis set, and systematic studies of this dependence are available. Furthermore, the effects of basis set choice and level of correlation analysis have been explored in solvation studies as well,222,233 but studies to permit identification of particular trends in their impact on the solvation portion of the calculation are as yet insufficient. 

De Pablo et al. [83] find that on the vapor-liquid coexistence curve, p is consistently underestimated while p is overestimated at all temperatures. The same Authors obtain a better agreement, also for T, with the experimental data using SPC for the liquid and a modified SPC for the gas, with charges scaled down to reproduce the gas phase dipole moment. The alternative to simple charge scaling, as suggested by Strauch and Cummings [84], is the use of a polarizable model. 

The next section in this chapter provides a brief comparison of the dipole moment (magnitude and direction) for a set of simple alcohols. Experimental gas phase dipole moments are compared to ab initio and as molecular mechanics computed values. It is important to note that the direction of the vector dipole used by chemists is defined differently in classical physics. In the former definition, the vector points from the positive to the negative direction, while the latter has the orientation reversed. 

Debyes,but lower than the gas phase dipole moment of formamide, 3.7 Debyes. (16) The small increase in the amide dipole with the addition of water indicates that water mobility is hindered in the presence of the amide group. 

Figure 5.5 Excitation energies of the singlet electronic states of pNA in cyclohexane, 1,4-dioxane, and water compared to the gas-phase energies. Gas-phase dipole moments (p, Debye) are also shown the ground state dipole moment is 7.7 D. State assignments are given in C2v symmetry group.

An example of solvent-induced solvatochromic shifts (calculated at a characteristic snapshot from the MD trajectory for each solvent) on different electronic excited states is shown in Fig. 5.5. Inspection of this plot reveals that the electronic states with the dipole moments that are larger than the dipoles in the ground state (shown as solid red curves in Fig. 5.5) become increasingly stabilized (red-shifted) in polar solvents. For example, l Ai, l Bi, 2 Bi states, which dipoles are larger than in the ground state dipole (7.7 Debye), demonstrate systematic red shifts upon solvation. The red shift increases in more polar solvents (in the order of c-hexane, dioxane, and water). The most dramatic red shift is experienced by the experimentally observed l Ai charge-transfer state with the (gas-phase) dipole moment of 12.9 D. It is quite intriguing that this state (the lowest red state in Fig. 5.5) is only the third lowest excited state in the gas phase but becomes the lowest excited state in water. On the... 

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